Mold and Core Sands in Metalcasting: Chemistry and Ecology : Sustainable Development [1st ed.] 9783030532093, 9783030532109

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Mariusz Holtzer Angelika Kmita

Mold and Core Sands in Metalcasting: Chemistry and Ecology Sustainable Development

Mold and Core Sands in Metalcasting: Chemistry and Ecology

Mariusz Holtzer • Angelika Kmita

Mold and Core Sands in Metalcasting: Chemistry and Ecology Sustainable Development

Mariusz Holtzer AGH University of Science and Technology Krakow, Poland

Angelika Kmita AGH University of Science and Technology Krakow, Poland

Translated by Janina Pawlikowska-Czuba

ISBN 978-3-030-53209-3    ISBN 978-3-030-53210-9 (eBook) https://doi.org/10.1007/978-3-030-53210-9 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Foundry industry is a key factor in the economy of several countries. At the same time, foundry operations, by generating harmful gases and vast amounts of waste, are increasing environmental pollution. During the last 15–20 years, the negative effect of foundry was more evident when molding sands with chemically bound binders were introduced. This plight was forced by customers who required castings of improved surface quality, higher dimensional accuracy, decreased walls thickness, and better efficiency from foundries. This led to a wide-scale application of castings in fields such as motorization, space and armaments industries. On the other hand, due to increasing societal pressure and administrative requirements, foundries are forced to employ effective technologies for molding sands using binding materials. For example, using geopolymer binders or water glass–based inorganic binders hardened by physical factors, e.g., temperature. However, these inorganic binders have limited applications and depend on dimensions of cores and molds, temperatures of pouring metals, or matrix types. Molding sands with bentonite (green sands) and with additions generating lustrous carbon will be still dominating the production of iron castings. However, it seems that some more time is required to produce molding sands with chemically bound binders or with bentonite (green sands) with limited or without harmful carbon additives. A good casting depends on two elements: proper sand mold and liquid metal with desired properties and composition. The quality of these two elements determines the quality of the obtained casting. Compared with research carried out for melting of metals, inoculation, refining, or spheroidizing of metal alloys, research concerning chemical and physicochemical processes and gases evolved during hardening or destruction of molding sands under high temperature. To a certain degree, the book should fill this gap. In addition, harmful effects of molding and core sands, their historical trail, their preparation, processes of making molds and cores, pouring liquid metal into molds, and knocking out castings are discusses in this book. Simultaneously, readers’ attention was drawn to environmental impacts of spent foundry sands. Authors also attempt to accurately describe reactions occurring in sands during their hardening and thermal degradation, but this was not always possible due to high complexity of these processes. Several v

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Preface

relevant research and their investigation results for molding and core sands – regarding their harmful effects for employees and the environment  – are mentioned in this book. As molding sands use chemically bound binders, authors decided to include a short course on organic chemistry and the bases of the polymerization process. We expect this book will allow its readers to understand better about phenomena and processes in casting production, especially related to molding sands. Therefore, novel – not much harmful – binders for molding and core sands are introduced in this book. We hope this book will be a useful source of information for students, scientists, academicians, research scholars, and foundry engineers. Special thanks are directed toward MSc Joanna Kuciakowska and Dr Agnieszka Roczniak for their help in the preparation of this book for printing. We would like to express our gratitude to the reviewers Prof. Józef Dańko and Prof. Andrzej Baliński, who provided their valuable comments and suggestions for this book. Krakow, Poland, 08.2020 

Mariusz Holtzer Angelika Kmita

Abbreviations

AC Amorphous carbon ADV Acid demand value AFS American Foundryman Society AOP Advanced oxidation process Ar Aromatic group BAT Best Available Techniques BMGVs Biological monitoring guidance values BOELV Occupational exposure BS Benzene sulfonic acid BTEX Benzene, toluene, ethylobenzene, xylenes CAS Chemical Abstracts Service (Registry Number) CAAA Clear Air Act Amendments CERP Casting Emissions Reduction Program CT Coagulation threshold DMEA Dimethylethylamine DMPA Dimethylpropylamine DMIPA Dimethylisopropylamine DNA Deoxyribonucleic acid DSC Differential scanning calorimetry EU European Union EPS Expanded polystyrene FA Furan (resin) FC Fugacit carbon FMP Full-mold process FNB Furan no-bake FRC Free radical curing FTIR Fourier transform infrared spectroscopy GC/MS Gas chromatography/mass spectrometry GC Gas chromatography HAP Hazardous air pollutant HMTA Hexamethylenetetramine vii

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Abbreviations

IARC International Agency of Research on Cancer IOELV Occupational exposure limit values IUPAC International Union of Pure and Applied Chemistry KS Sulfuric acid LAc Lactic acid LC Lustrous carbon LCF Lustrous carbon formers LFP Lost-foam process LOI Loss on ignition MAK limit Maximal concentration of chemical substances at the work place MDI Methylene diphenyl diisocyanate MMA Methyl methacrylate NDS Occupational limit NDSCH Exposure limit NDSP Highest permissible threshold concentration OSHA PEL Permissible exposure limits OTS o-Toluenosulfonic acid PAHs Polycyclic aromatic hydrocarbons PCDD Polychlorodibenzodioxine PCDF Polychlorodibenzofuranes PCB Polychlorinated biphenyls PCL Polycaprolactam R Alkyl group SFS Spent foundry sand PEVA Polyethylene vinyl acetate PF-FA Phenol-formaldehyde–furfuryl alcohol PGA Poly(glycolic acid) PLA Poly(lactic acid) ppm parts per million PTS p-Toluene sulfonic acid PU Phenolic urethane PUCB Phenolic urethane cold-box PUNB Phenolic urethane no-bake REACH Registration, Evaluation, Authorization and Restriction of Chemicals R–FA Resorcinol–furfuryl alcohol RNA Ribonucleic acid RP Rapid prototyping RT Rapid tooling RM Rapid manufacturing STEL Short-term exposure limits TCLP Toxicity characteristic leaching procedure TDI Toluene-2,4-diisocyanate TEA Triethylamine TEOS Tetraethyl orthosilicate

Abbreviations

TGA Thermogravimetric analysis TLV Threshold limit value TLV–TWA Threshold limit value–time-weighted average TLV–STEL Threshold limit value–short-term weighted average TLV–C Threshold limit value–ceiling limit TMA Trimethylamine TWA Time-weighted average US EPA United States Environmental Protection Agency UF–FA Urea-formaldehyde–furfuryl alcohol UF–PF–FA Urea-formaldehyde–phenol resin–furfuryl WEL–TWA Workplace exposure limits–time-weighted average WEL–SHORT Workplace exposure limits–short-term weighted average VOC Volatile organic compounds XSA Xylene sulfonic acid

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Contents

  1 Introduction����������������������������������������������������������������������������������������������    1 References��������������������������������������������������������������������������������������������������   10 Part I Organic Chemistry   2 General Principles of Organic Chemistry ��������������������������������������������   15 2.1 Organic Compounds: Structure and Bonding ��������������������������������   15 2.2 Organic Chemical Reactions: Classifying��������������������������������������   16 2.3 Hydrocarbons����������������������������������������������������������������������������������   18 2.3.1 Alkanes ����������������������������������������������������������������������������   18 2.3.2 Alkenes ����������������������������������������������������������������������������   23 2.3.3 Alkynes ����������������������������������������������������������������������������   25 2.3.4 Cycloalkanes��������������������������������������������������������������������   27 2.3.5 Cycloalkenes��������������������������������������������������������������������   29 2.3.6 Aromatization and Benzene����������������������������������������������   30 2.4 Hydrocarbons with Functional Groups������������������������������������������   36 2.4.1 Alcohols and Phenols ������������������������������������������������������   36 2.4.2 Ethers��������������������������������������������������������������������������������   45 2.4.3 Aldehydes and Ketones����������������������������������������������������   46 2.4.4 Carboxylic Acids��������������������������������������������������������������   48 2.4.5 Esters of Carboxylic Acids ����������������������������������������������   52 2.4.6 Nitrogen Organic Compounds������������������������������������������   56 2.4.7 Sulfuric Organic Compounds ������������������������������������������   61 2.5 Polymers and Polymerization ��������������������������������������������������������   62 2.5.1 Polymers and their Properties������������������������������������������   62 2.5.2 Polymerization: Kinds and Mechanisms��������������������������   65 2.5.3 Inorganic Polymers ����������������������������������������������������������   72 2.5.4 Inorganic–Organic and Organo-Metallic Polymers����������   73 2.5.5 Degradable Polymers��������������������������������������������������������   74 2.5.6 Natural Polymers (Biopolymers)��������������������������������������   78 References��������������������������������������������������������������������������������������������������   82 xi

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  3 Synthetic Resins ��������������������������������������������������������������������������������������   83 3.1 Alkyd Resins����������������������������������������������������������������������������������   83 3.2 Phenol–Formaldehyde Resins��������������������������������������������������������   84 3.3 Phenol–Urethane Binder System����������������������������������������������������   97 3.4 Furan Resins������������������������������������������������������������������������������������  101 3.5 Urea–Formaldehyde Resins������������������������������������������������������������  105 References��������������������������������������������������������������������������������������������������  107 Part II Organic Binder Systems of Molding and Core Sands   4 Aggregate Molding Materials ����������������������������������������������������������������  113 4.1 Silica Sand��������������������������������������������������������������������������������������  113 4.2 Zircon Sand������������������������������������������������������������������������������������  118 4.3 Olivine Sand������������������������������������������������������������������������������������  121 4.4 Chromite Sand��������������������������������������������������������������������������������  122 4.5 Other Minerals Used as Matrices����������������������������������������������������  123 4.6 Synthetic Sands������������������������������������������������������������������������������  125 Appendix 4.1��������������������������������������������������������������������������������������������   127 References��������������������������������������������������������������������������������������������������  127   5 Division of the Molding and Core Sands: Criteria ������������������������������  129 References��������������������������������������������������������������������������������������������������  143   6 Cold-Setting Processes (No-Bake)����������������������������������������������������������  145 6.1 Furan Acid Catalyzed����������������������������������������������������������������������  145 6.2 Phenolic–Urethane No-Bake System (PUNB) ������������������������������  162 6.3 Resol Ester Hardened (ALPHASET)����������������������������������������������  168 6.4 Alkyd Oil, No-Bake������������������������������������������������������������������������  178 References��������������������������������������������������������������������������������������������������  180   7 Gas-Hardened Processes (Cold-Box) ����������������������������������������������������  185 7.1 SO2 Hardened Furan Resins (HARDOX) ��������������������������������������  185 7.2 Amine Hardened Phenolic–Urethane Cold-Box (PUCB)��������������  186 7.3 Alkaline Phenolic Methyl Formate Hardened (BETASET)������������  198 7.4 CO2 Hardened Alkaline Phenolic����������������������������������������������������  200 7.5 SO2 Hardened Epoxy/Acrylic (Free Radical Curing) (FRC)����������  202 References��������������������������������������������������������������������������������������������������  203   8 Heat Curing Processes ����������������������������������������������������������������������������  205 8.1 Linseed Oil Oven Bake ������������������������������������������������������������������  205 8.2 Warm-Box Process��������������������������������������������������������������������������  206 8.3 Hot-Box Process, Phenolic or Furan-Based������������������������������������  208 8.4 Croning Process (Shell Process) ����������������������������������������������������  212 References��������������������������������������������������������������������������������������������������  214

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Part III Inorganic Binder Systems of Molding and Core Sands and Protective Coatings   9 Sodium Silicate Molding Sands��������������������������������������������������������������  219 9.1 Sodium Silicate: Structure and Physical–Chemical Properties������  221 9.2 Hardening of Molding Sands with Sodium Silicate ����������������������  225 9.2.1 Molding Sands Hardened by Chemical Agents����������������  225 9.2.2 Molding Sands Hardened by Physical Factors ����������������  229 9.2.3 Sodium Silicate Modification ������������������������������������������  234 References��������������������������������������������������������������������������������������������������  239 10 Green Sands����������������������������������������������������������������������������������������������  243 10.1 General Characteristics of Green Sands ����������������������������������������  243 10.2 Bentonite: Structure and Properties������������������������������������������������  245 10.3 Additions Generating Lustrous Carbon������������������������������������������  251 10.4 Water and its Role in Green Sands�������������������������������������������������  257 10.5 Thermophysical Properties of Green Sands������������������������������������  258 10.6 Rebounding and Reclamation of Green Sands ������������������������������  260 10.7 Limitations of Negative Influences of Green Sands on the Environment and Work Conditions������������������������������������������������  267 References��������������������������������������������������������������������������������������������������  274 11 Other Molding and Core Sands with Inorganic Binders ��������������������  277 11.1 Geopolymer Binder������������������������������������������������������������������������  277 11.2 Water-Soluble Cores ����������������������������������������������������������������������  279 References��������������������������������������������������������������������������������������������������  282 12 Protective Coatings for Mold and Core Sands��������������������������������������  285 References��������������������������������������������������������������������������������������������������  292 Part IV Special Technologies of Making Molds and Cores 13 Alternative Methods Using in Mold and Core Technologies����������������  297 13.1 Evaporative-Pattern Casting������������������������������������������������������������  297 13.1.1 Materials for Evaporative-Pattern Casting������������������������  298 13.1.2 Coats for Evaporative Patterns������������������������������������������  301 13.2 Lost Foam Process (LFP)����������������������������������������������������������������  302 13.3 Full-Mold Process (FMP) ��������������������������������������������������������������  305 13.4 Ceramic Shell����������������������������������������������������������������������������������  306 13.4.1 Shaw’s Method ����������������������������������������������������������������  307 13.4.2 Investment Casting������������������������������������������������������������  309 13.5 Vacuum Molding (V-Process) ��������������������������������������������������������  314 13.6 Frozen Mold������������������������������������������������������������������������������������  316 13.7 Rapid Prototyping Technology ������������������������������������������������������  318 13.8 Natural Polymer-Based Composite Binders ����������������������������������  321 13.8.1 α-Starch-Based Binders����������������������������������������������������  321 13.8.2 Collagen-Based Composite Binders ��������������������������������  322

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13.8.3 Binder Based on Polysaccharide��������������������������������������  324 13.9 Synthetic Polymer-Based Composite Binders��������������������������������  324 References��������������������������������������������������������������������������������������������������  326 Part V Environmental Aspects Mold and Core Technologies 14 Influence of the Technology of Molding and Core Sands on the Environment and Working Conditions: Summary������������������������������  333 14.1 Emission of Harmful Substances from Molding and Core Sands ����������������������������������������������������������������������������������������������  333 14.2 Influence of Spent Molding and Core Sands on the Environment������������������������������������������������������������������������������������  337 14.3 After Reclamation Dusts and their Influence on the Environment������������������������������������������������������������������������������������  342 References��������������������������������������������������������������������������������������������������  345 Index������������������������������������������������������������������������������������������������������������������  347

About the Authors

Mariusz Holtzer  Ph.D., is currently Full Professor at AGH-University of Science and Technology, Faculty of Foundry Engineering, Krakow, Poland. Professor M. Holtzer graduated from the Jagiellonian University, Department of Chemistry, Krakow. His research activities are focused on the following areas: physical chemistry of metallurgical and foundry phenomenon on the interface of the liquid metal – ceramic material, the construction of metallurgical slags, issues related to limiting the negative impact of foundry industry on the environment, waste management, new ecological binders for molding and core sands, hazard evaluation of materials used in foundry, and modern analytical methods such as Py/GC/MS, TG/MS, and TG/DTG/FTIR/MS. Professor M. Holtzer has participated in 16 scientific and industrial projects as project manager or a coworker. He has edited over 8 books and published over 300 scientific papers. He has coauthored over 14 patents (US and EU) and patent applications. He has authored two monographs: Metallurgical and Foundry Processes of Iron Alloys: Physicochemical Fundamentals (2013) (in Polish) and Microstructure and Properties of Ductile Iron and Compacted Graphite Iron Castings: The Effects of Mold Sand/Metal Interface Phenomena (2015) (in English).

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About the Authors

Angelika Kmita  is currently an Assistant Professor at AGH-University of Science and Technology, Academic Centre for Materials and Nanotechnology, Krakow, Poland.  Her research activities are focused on the following areas: (a)

Synthesis of ferrite nanoparticles with controlled morphology and size; (b) Synthesis of magnetic fluids (ferrofluids); (c) Synthesis of multifunctional hybrid nanocomposites; (d) Examination of the magnetic hyperthermia of the aqueous solutions of nanoparticles; (e) Investigation of rheological properties of materials; (f) Modification of binders for molding and core sands by nanoparticles; (g) Topics related to environmental protection problems in the foundry industry. Dr A. Kmita has participated in 11 scientific and industrial projects (as project manager or co-worker). She has co-authored over 40 scientific publications (e.g., in Inorganic Chemistry, Composites Part B Engineeing, Applied Thermal Engineering, Journal of Materials Chemistry C, Materials, Nanoscale etc.), and has co-authored three monographs as well as three patents.

List of Figures

Fig. 1.1

Sources of emission of compounds from the HAP group in the iron alloy foundry. (Adapted from Ref. [12])�������������������������� 4

Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 Fig. 2.5 Fig. 2.6

Schematic division of hydrocarbons������������������������������������������������ 19 The characteristic reaction of aromatic hydrocarbons��������������������� 32 Oxidation of aromatic hydrocarbons������������������������������������������������ 33 Derivatives of carboxylic acid���������������������������������������������������������� 52 Basic division of polymers, adapted from [15]�������������������������������� 63 Dependence of the polymerization degree on the conversion degree of the chain polymerization (curve 1) and gradual polymerization (curve 2). (Adapted from Ref. [16])������������������������ 64 Thermal decomposition of dibenzoyl peroxide�������������������������������� 67 Formation of polystyrene����������������������������������������������������������������� 68 The anionic polymerization course at the example of styrene��������� 69 Schematic presentation of the poly(lactic acid) formation by the polymerization method with opening the lactide ring����������� 75 Schematic presentation of the poly(lactic acid) formation by the polycondensation of lactic acid��������������������������������������������� 75 Curves of the thermal decomposition of selected polymers (PP, polypropylene; PE, polyethylene; PS, polystyrene; PVC, polyvinylchloride) [16]�������������������������������������������������������������������� 77

Fig. 2.7 Fig. 2.8 Fig. 2.9 Fig. 2.10 Fig. 2.11 Fig. 2.12

Fig. 3.1 Fig. 3.2

Fig. 3.3 Fig. 3.4 Fig. 3.5

Polymerization of alkyd resin with using phthalic acid anhydride, glycerine, and fatty acid������������������������������������������������� 84 Scheme representing polycondensation of phenol and formaldehyde to obtain various products of this reaction, depending on the reaction conditions. (Reprinted by permission from Ref. [12])��������������������������������������������������������������� 88 Schematic presentation of the polymerization process course of novolak and resol phenol resins��������������������������������������� 88 Scheme of the phenol and formaldehyde connection���������������������� 88 Scheme of the formation of hydroxyl derivatives in the reaction of phenol with formaldehyde��������������������������������������������� 89 xvii

xviii

Fig. 3.6 Fig. 3.7 Fig. 3.8 Fig. 3.9 Fig. 3.10 Fig. 3.11 Fig. 3.12 Fig. 3.13 Fig. 3.14 Fig. 3.15 Fig. 3.16 Fig. 3.17 Fig. 3.18 Fig. 3.19 Fig. 3.20 Fig. 3.21 Fig. 3.22 Fig. 3.23 Fig. 3.24 Fig. 3.25 Fig. 3.26 Fig. 3.27 Fig. 4.1 Fig. 4.2 Fig. 4.3

List of Figures

Condensation of hydroxyl derivatives���������������������������������������������� 89 Formation of the ether bond as the result of condensation of hydroxyl derivatives������������������������������������������������������������������������� 89 Possibility of forming methylene bridge (−CH2−) in ortho- or para- position aromatic ring. (Adapted from Ref. [19])������������������� 90 Formula of hexamethylenetetramine and the structure of cross-linked novolak resin (formation of methyleneamine bridges). The hardening process occurs in two stages��������������������� 91 Cross-linking of novolak resin in the hexamine presence. (Reprinted by permission from Ref. [20])��������������������������������������� 92 Structure of cross-linked phenol–formaldehyde resol resin������������� 93 Possible reactions occurring at the thermal degradation of PF resin. (Adapted from Ref. [31])������������������������������������������������������� 96 Polyurethane synthesis��������������������������������������������������������������������� 98 General equation of forming the urethane group. (Reprinted by permission from Ref. [12])���������������������������������������������������������� 99 Examples of the most often applied aromatic diisocyanates����������� 99 Scheme of the synthesis of diisocyanates (without phosgene)������ 100 Raw materials for production of furan resins��������������������������������� 101 Obtaining of furfuryl alcohol and furan from raw plant materials. (Adapted from Ref. [50])����������������������������������������������� 102 World utilization of furfuryl alcohol in 2015. (Reprinted by permission from Ref. [51])������������������������������������������������������������� 102 Self-­condensation reaction of furfuryl alcohol������������������������������ 103 Reaction of self-condensation of furfuryl alcohol in the acidic environment������������������������������������������������������������������������������������ 103 Structure of phenol–formaldehyde modified by furfuryl alcohol and hardened by p-­toluenesulfonic acid. (Adapted from Refs. [55, 56])������������������������������������������������������������������������ 104 Transition from furfuryl alcohol to a highly cross-linked furan matrix in furan binder. (Adapted from Refs. [58–60])��������� 105 Two steps in formation of urea–formaldehyde resin���������������������� 106 Formation of mono- and dimethylol ureas in the reaction of urea with formaldehyde (first stage)���������������������������������������������� 106 Polycondensation of methyl derivatives of urea with a formation of macroparticles connected by methylene bridges. (Adapted from Ref. [65])��������������������������������������������������������������� 106 Formation of a linear polymer�������������������������������������������������������� 107 Typical grain size distribution of silica sand. (Adapted from Refs. [1, 3])������������������������������������������������������������������������������������ 115 Thermal linear expansion characteristic of sands for molds and cores. (Reprinted by permission from Ref. [5])���������������������� 116 Thermal volumetric expansion characteristics of sands for molds and cores. (Reprinted by permission from Ref. [5])����������� 116

List of Figures

Fig. 5.1 Fig. 5.2 Fig. 5.3 Fig. 5.4 Fig. 5.5

Fig. 6.1 Fig. 6.2 Fig. 6.3 Fig. 6.4 Fig. 6.5 Fig. 6.6

Fig. 6.7 Fig. 6.8 Fig. 6.9

Fig. 6.10

xix

Fractions of green sand molding in technologies applied for cast iron production. (Reprinted by permission from Ref. [2])������������������������������������������������������������������������������������������ 130 Typical pathway of hardening curve of self-setting sands (Tw. work time; Ts, strip time; Tc, casting time; Tmax, time to achieve the maximum strength). (Adapted from Ref. [12])����������� 135 Technologies applied in EU for producing molds and cores (a) and technologies of producing cores with chemically bonded sands (b). (Adapted from Refs. [1, 15])���������������������������� 137 Air emission sources during preparing of sands and producing castings. (Reprinted by permission from Ref. [26])����������������������� 141 Comparison of gas evolution rates of sands with organic and inorganic binders and for green sands: Sinotherm®, alkaline phenolic resole resin; Kalltharz X 850®, furan resin; Geopol® and Rudal®, inorganic binders. (Reprinted by permission from Ref. [29])���������������������������������������������������������������������������������������� 142 Thermal decomposition of p-toluenesulfonic acid [8]������������������� 149 Polymerization reaction of furan resin������������������������������������������� 150 Influence of a sulfur content in a binder on its amount in a reclaim: (red line, low-sulfur binder; black line, high-sulfur binder). (Reprinted by permission from Ref. [13])������������������������ 152 Influence of nitrogen content in a binder on its amount in a reclaim: (red line, low-­nitrogen binder; black line, high-nitrogen binder). (Reprinted by permission from Ref. [13])������������������������ 152 Overlay reaction pathways for the formation of aromatic compounds in pyrolysis of the furan binder cured with p-xylene sulfonic acid. (Adapted from Refs. [16–18])������������������ 154 Scheme formation of aromatic compounds from the PAHs group by pyrolysis process (Reprinted by perrmission from Ref. [17]) Hydrogen–abstraction/acetylene–addition (HACA) mechanism accelerate the formation polycyclic aromatic hydrocarbons (PAHs) like pyren from naphthalene����������������������� 156 Structure of phenol–urethane binder���������������������������������������������� 162 Differences in structures of binders applied in the PUNB and PUROLiTe technology. (Reprinted by permission from Ref. [37])���������������������������������������������������������������������������������������� 164 Emission of VOC and formaldehyde from molding sand with polyurethane (PUNB) and PUROLiTe binder during making cores by means of shooting machine (0.8% of resin and 0.8% of activator). (Reprinted by permission from Ref. [37])���������������� 167 Leaching of organic compounds into the environment from spent sands (PUNB and PUROLiTe) during their storing (0.8% of resin and 0.8% of activator); limits for individual classes of wastes valid in Germany. (Reprinted by permission from Ref. [37])������������������������������������������������������������������������������� 167

xx

Fig. 6.11 Fig. 6.12 Fig. 6.13 Fig. 6.14 Fig. 6.15 Fig. 6.16

Fig. 7.1 Fig. 7.2 Fig. 7.3 Fig. 7.4 Fig. 7.5 Fig. 7.6 Fig. 7.7

Fig. 7.8 Fig. 7.9 Fig. 7.10 Fig. 8.1 Fig. 8.2

List of Figures

Hydrolysis reaction of ester in a basic environment���������������������� 170 Reaction of the ester (hardener/catalyst) with −CH2O group from the resin��������������������������������������������������������������������������������� 171 Quinone methide is an active intermediate substance, facilitating hardening of alkaline phenol resin������������������������������� 171 General scheme of the reaction of phenol–formaldehyde resin with ester���������������������������������������������������������������������������������������� 172 Structure of polymerized alkaline phenol–formaldehyde resin hardened by esters�������������������������������������������������������������������������� 175 Py/GC/MS chromatogram obtained as a result of the “flash” pyrolysis of the commercial resin used in the ALPHASET technology at 700 °C [72]�������������������������������������������������������������� 178 Structural formulae of amines applied in the cold-box process����� 187 Components of binder for cold-box technology���������������������������� 189 Toluene diisocyanates used in cold-box technology���������������������� 189 Reaction of the cold-box process with using polyisocyanate�������� 190 Hardened polyurethane binder������������������������������������������������������� 190 Solvents applied in the cold-box technology in a historical perspective. (Adapted from Refs. [13, 14])������������������������������������ 194 Comparison of odor emissions from the cold-box technology, measured at the hood stack outlet in the aluminum alloys foundry. Both technologies (traditional and TEOS) contained equivalent amounts of a binder. (Reprinted by permission from Refs. [23, 24])������������������������������������������������������������������������ 195 Structure cold-box resin of the 5th generation. (Adapted from Ref. [13])������������������������������������������������������������������������������� 195 Reactions during curing of the mold sands (BETASET process). (Adapted from Ref. [3])����������������������������������������������������������������� 199 Resin cross linking after CO2 introduction (resole–CO2 process). (Adapted from Ref. [3])����������������������������������������������������������������� 201

Fig. 8.5

Structure of cross-linked of unsaturated fatty acids����������������������� 206 Comparison of volumes of gases emitted at a temperature of 800 °C from molding sands prepared according to the hot-box and warm-box technologies. (Reprinted by permission from Ref. [3])��������������������������������������������������������������� 207 Scheme of the hot-box process: core making. (Adapted from Ref. [7])������������������������������������������������������������������������������������������ 209 Scheme of the curing of PF novolak resin in the hot-box plus process (HTMA – hexamethylenetetramine)��������������������������������� 210 Scheme of the curing of furfuryl resin in the hot-box process������� 211

Fig. 9.1 Fig. 9.2

Preparation of sodium silicate. (Adapted from Refs. [1, 18])�������� 221 The reaction of hydrolysis of sodium silicate�������������������������������� 221

Fig. 8.3 Fig. 8.4

List of Figures

Fig. 9.3 Fig. 9.4 Fig. 9.5 Fig. 9.6 Fig. 9.7 Fig. 9.8 Fig. 9.9 Fig. 9.10 Fig. 9.11 Fig. 9.12

Fig. 9.13 Fig. 9.14 Fig. 9.15 Fig. 9.16 Fig. 9.17

Fig. 10.1 Fig. 10.2

xxi

Reaction of sodium silicate activation. (Adapted from Refs. [1, 18])���������������������������������������������������������������������������������� 222 The condensation reaction by dehydration. (Adapted from Refs. [1, 18])���������������������������������������������������������������������������������� 222 Increase of sodium silicate particles by multiple condensation – formation of spatially branched chains. (Adapted from Refs. [1, 18])���������������������������������������������������������� 222 Decomposition reaction of sodium silicate as a result of the siloxane (Si–O–Si) bond hydrolysis. (Adapted from Refs. [1, 18])������������������������������������������������������������������������������������������������� 223 The reaction of deactivation of sodium silicate. (Adapted from Refs. [1, 18])�������������������������������������������������������������������������� 223 Stages of monomer polymerization and particle formation according to R. K. Iler [19]������������������������������������������������������������ 223 A simplified model of the micelle. (Adapted from Ref. [21])������� 224 Scheme sol ↔ gel transition����������������������������������������������������������� 225 Changes of the final compression strength (Rctk) at the temperature increase: molding sand with sodium silicate hardened by CO2. (Adapted from Refs. [3, 35])���������������������������� 227 Mechanism of bonding of hydrated sodium silicate in the esterification process according to [3] (a) ester hydrolysis, (b) dissociation of acetic acid, and (c) reaction of acetic anions with sodium silicate. (Adapted from Refs. [3, 38])������������ 228 Changes of the final compression strength (Rctk) at the temperature increase: molding sand with sodium silicate hardened by esters. (Adapted from Refs. [3, 35])�������������������������� 229 INOTEC® binder composition. (Adapted from Ref. [41])������������� 231 Comparison of the condensate volume for the CORDIS® binder with other binders (temperature of liquid aluminum alloy: 700 °C). (Reprinted by permission from Ref. [13])������������� 232 Schematics of bond formation by AWB® binder. (Reprinted by permission from Ref. [15])�������������������������������������������������������� 234 Comparison of knocking out properties (knockout work Lw value) of molding sands with sodium silicate modified by alcoholic solutions of nanoparticles [49]��������������������������������������� 237 Crystal structure of montmorillonite. (Adapted from Refs. [11, 12])������������������������������������������������������������������������������������������������� 246 (a) A silica tetrahedron in which the central silicon ion is coordinated to four oxygen ions (bright balls are oxygen atoms and black balls are silicon cations). (b) An alumina octahedron in which the central aluminum ion is coordinated to six hydroxyls (black balls are hydroxyls, and small gray balls are cations with octahedral coordination Al, Fe, Mg). (Adapted from Ref. [13])������������������������������������������������������������������������������� 247

xxii

Fig. 10.3 Fig. 10.4 Fig. 10.5

Fig. 10.6 Fig. 10.7 Fig. 10.8 Fig. 10.9

Fig. 10.10 Fig. 10.11 Fig. 10.12 Fig. 10.13 Fig. 10.14 Fig. 10.15

Fig. 11.1 Fig. 11.2 Fig. 12.1

List of Figures

Cis- and trans isomers in the octahedral structure of bentonite. (Adapted from Ref. [13])��������������������������������������������������������������� 247 Structure of montmorillonite, 1 Å = 10−8 m. (Reprinted by permission from Ref. [14])������������������������������������������������������������� 248 (a) Oxidation atmosphere in the mold cavity before pouring with molten metal. (b) Emission of lustrous carbon from gaseous phase on cooler surfaces. (Reprinted by permission from Ref. [14])������������������������������������������������������������������������������� 253 Formation of benzene from sea coal dust (pyrolysis, dehydration, and condensation). (Adapted from Ref. [28])����������� 253 Reactions in result of which the reducing atmosphere is formed in a mold���������������������������������������������������������������������������� 253 Influence of salt (NaCl) on the bentonite activity. Salt deactivates electrostatic bonds of bentonite. (Reprinted by permission from Ref. [14])������������������������������������������������������������� 258 Structure of the heated surface of a green sand mold, against a steel casting, and the forms of silica (after Sosman R.R. 1927) with solid lines denoting stable states and broken lines denoting unstable states. (Reprinted by permission from Ref. [4])�������������� 259 Schematic presentation of distribution of devices in the foundry using green sands. (Reprinted by permission from Refs. [16, 43])�������������������������������������������������������������������������������� 261 Bridge between sand grains in green sands. (Reprinted by |permission from Ref. [14])������������������������������������������������������������ 263 Dependence of the water boiling temperature on the outside pressure������������������������������������������������������������������������������������������ 264 Molding sand preparation plant with vacuum mixer-cooler. (Reprinted by permission from Refs. [16, 47])������������������������������ 265 Comparison of green sands, with no carbon, with sea coal, |and with lignite. (Reprinted by permission from Ref. [3])������������ 267 Effect of increasing lignite: sea coal ratio on (a) benzene emissions and (b) toluene emissions. (Reprinted by permission from Ref. [3])��������������������������������������������������������������������������������� 270 Diagram of the structure of geopolymer: tetrahedrons SiO44− and AlO45−. (Adapted from Ref. [3])���������������������������������������������� 278 Results of pollutants (BTEX and PAHs) measurement during pouring, comparison of organic and inorganic binder systems. (Reprinted by permission from Ref. [9])��������������������������������������� 280 Comparison of the surface quality of iron casting with and without protective coatings at different metallostatic pressures ( “O”, samples without protective coating; “1,” samples with commercial protective coating; “2” and “3” – samples with experimental protective coating). (Reprinted from Ref. [13])������� 290

List of Figures

Fig. 13.1 Fig. 13.2 Fig. 13.3

Fig. 13.4

Fig. 13.5 Fig. 13.6 Fig. 13.7 Fig. 13.8 Fig. 13.9 Fig. 13.10 Fig. 13.11 Fig. 13.12 Fig. 13.13 Fig. 13.14 Fig. 13.15 Fig. 13.16

xxiii

Formation of polystyrene��������������������������������������������������������������� 298 Formation of polymethyl methacrylate������������������������������������������ 298 TGA and DTG curves of polystyrene in 1, air and 2, nitrogen atmosphere. Ta, temperature of the polymer decomposition beginning and Tmax, maximum temperature on the DTA curve, in which the polymer decomposition rate is the highest. An exothermic peak also appears at this temperature. (Reprinted by permission from Ref. [6])���������������������������������������������������������� 300 FTIR spectra of volatile products of styrene thermal decomposition in the air atmosphere (decomposition temperature: 1, 300 °C; 2, 400 °C; 3, 500 °C; 4, 600 °C). (Reprinted by permission from Ref. [6])��������������������������������������� 301 Main decomposition products of the polystyrene macro molecules. (Adapted from Ref. [11])��������������������������������������������� 302 The lost foam process (LFP). (Reprinted by permission from Refs. [20, 21])�������������������������������������������������������������������������������� 303 Structure of ethyl silicate (ethylene groups are bound with silicon atom by oxygen bridges)���������������������������������������������������� 308 Scheme of the investment casting process. (Reprinted by permission from Refs. [20, 21])����������������������������������������������������� 310 Stages of vacuum forming. (Reprinted by permission from Refs. [21, 53])�������������������������������������������������������������������������������� 315 Schematic illustration of the differential pressure mold freezing method. (Reprinted by permission from Ref. [60])��������� 317 High-temperature stress-strain curves of cores manufactured with binder containing α-starch. (Reprinted by permission from Ref. [75])������������������������������������������������������������������������������� 322 Hot distortion test for cores manufactured with several types of binders. (Reprinted by permission from Ref. [75])������������������� 323 Scheme manufacturing PLA as a polymerization result of lactide��������������������������������������������������������������������������������������������� 325 Scheme manufacturing PLA by polycondensation lactic acid������� 325 Lactic acid production on an industrial scale��������������������������������� 325 Preparation of polycaprolactone���������������������������������������������������� 325

List of Tables

Table 1.1 Table 1.2

The largest producers of castings in the world (data for 2018)���������� 2 Indicative exposure limit values applied in Poland and other countries for compounds emitted in casting process�������������������������� 6

Table 2.1

Classification of organic compounds on the bases of functional groups����������������������������������������������������������������������������������������������� 17 Table 2.2 Simplified formulas, names, and symbols of abbreviations of the selected hydrocarbon groups������������������������������������������������������������ 20 Table 2.3 The alkanes homologous series�������������������������������������������������������� 23 Table 2.4 The homologous series of alkenes���������������������������������������������������� 24 Table 2.5 The homologous series of alkynes��������������������������������������������������� 26 Table 2.6 The homologous series of cycloalkanes������������������������������������������� 28 Table 2.7 The homologous series of cycloalkenes������������������������������������������� 29 Table 2.8 Physical properties and structures of aromatic hydrocarbons from BTEX group���������������������������������������������������������������������������� 34 Table 2.9 Physical properties and the structure of the selected substances from PAHsa group����������������������������������������������������������������������������� 37 Table 2.10 Examples of aliphatic and aromatic ethers��������������������������������������� 46 Table 2.11 Examples of carboxylic acids (monocarboxylic acids)�������������������� 50 Table 2.12 Comparison of the gradual and chain polymerization��������������������� 72 Table 3.1 Table 3.2 Table 3.3

Properties of phenol [6]�������������������������������������������������������������������� 85 Properties of formaldehyde [6]�������������������������������������������������������� 86 Processes of molds and cores production in which phenol–formaldehyde resin is used�������������������������������������������������� 95

Table 4.1

Recommended chemical composition of silica sand applied in castings������������������������������������������������������������������������������������������� 114 Recommended physicochemical properties of silica sands applied in foundries������������������������������������������������������������������������ 115 Grain composition of typical silica sands applied in Germany and Great Britain���������������������������������������������������������������������������� 117 Basic properties of sands use for making of molds and cores������� 119 Chemical composition of zircon sand (mas.%)������������������������������ 121

Table 4.2 Table 4.3 Table 4.4 Table 4.5

xxv

xxvi

List of Tables

Table 4.6 Table 4.7 Table 4.8 Table 4.9 Table 4.10 Table 4.11

Chemical composition of typical olivine sand (mas.%)����������������� 121 Grain analysis of olivine sand (mas.%)������������������������������������������ 122 Chemical composition of typical chromite sand (mas.%)�������������� 123 Chemical composition of silica sand and Fe–Cr slag (%) [19]������ 124 Chemical composition of CERABEADS® synthetic sand (%)����� 125 Grain size distribution of CERABEADS® synthetic sand������������ 126

Table 5.1

Fractions of individual technologies of making molding and core sands in the foundry industry of EU and the USA����������������� 131 Resins applied for production of molds and cores as well as the possibility of their using for the given casting alloy���������������������� 136 Consumption of various resins, catalysts, hardeners, and additions at preparing chemically bond sands�������������������������������� 138 History of mold and core making processes���������������������������������� 140 Emission coefficients during thermal decomposition of selected organic binders������������������������������������������������������������������������������� 141 Emission of dangerous substances from molding sands (total dust, PAHs, B(a)P)������������������������������������������������������������������������� 142

Table 5.2 Table 5.3 Table 5.4 Table 5.5 Table 5.6 Table 6.1 Table 6.2

Table 6.3 Table 6.4 Table 6.5

Table 6.6 Table 6.7 Table 6.8 Table 6.9

Hardeners used in molding sands with furan resins����������������������� 147 Emission of compounds of the BTEX and HAP groups from molding sands with furan resin (containing 25% of free furfuryl alcohol) hardened by various hardeners (the sand sample was poured with cast iron of a temperature of 1350 °C)���������������� 157 Concentration of PAHs which released during the thermal decomposition of molding sands with furan resin with different contents of free furfuryl alcohol����������������������������������������������������� 157 Analyses of gases emitted from molds made of molding sands with furan resin hardened by sulfonic acids������������������������� 158 Emission of BTEX gases from molding sands made on matrices with various fractions of the reclaim (100SP, matrix, 100% of fresh sand; 50SP50R, 50% of fresh sand +50% of reclaim; 100R, matrix, 100% of reclaim) (sand composition: urea–formaldehyde resin modified by furfuryl alcohol, 1.0%; hardener, 0.5%)������������������������������������������������������������������������������ 159 Emission of PAHs from molding sands made on matrices with various fractions of the reclaim������������������������������������������������������ 159 Effect of reclaim addition on leaching of substances from molding sand with furan resin�������������������������������������������������������� 161 Esters used as hardeners in the ALPHASET process and their properties���������������������������������������������������������������������������������������� 169 Influence of the amount of ester hardener of a medium hardening rate (glycerol triacetate +1,4 butyrolactone) on the tensile strength of molding sand from the ALPHASET technology�������������������������������������������������������������������������������������� 170

List of Tables

xxvii

Table 6.10 Obtained levels of molding silica sand reclamation, based for selected technologies [64]�������������������������������������������������������������� 172 Table 6.11 Comparison of physical properties of fresh olivine sand, its reclaim after mechanical and thermal reclamation������������������������ 174 Table 6.12 Emission of gases from molding sand with alkaline resol resin (ALPHASET) and with furan resin������������������������������������������������ 176 Table 6.13 Characteristic of the molding sand before pouring with molten metal����������������������������������������������������������������������������������������������� 176 Table 6.14 Emission of compounds from the BTEX group released during pouring of the cores from ALPHASET technology with cast iron and aluminum alloy AK 11����������������������������������������������������� 176 Table 6.15 Emission of compounds from the PAH group (sample of molding sand pouring by cast iron, matrix – quartz sand) (sampling according to the methodology described in [69])���������� 177 Table 6.16 List of the identified gases released during pyrolysis, at 500 and 700 °C, from commercial resin used in the APLHASET technology [72]������������������������������������������������������������������������������ 179 Table 6.17 Results of the analysis of gases from the BTEX group released from molding sand with alkyd binder (alkyd resin + polyisocyanate) (Commercial resin: A, B, and C come from different manufacturers) during semi-technical scale tests������������ 180 Table 6.18 Results of the PAHs analysis in gases emitted from the molding sand (calculated on 1 kg of molding sand) with resin A after pouring by cast iron at the temperature 1350 °C��������������� 180 Table 7.1 Table 7.2 Table 7.3 Table 7.4 Table 7.5 Table 7.6 Table 7.7 Table 8.1 Table 8.2 Table 9.1

Properties of tertiary amines [2, 4, 7, 8]����������������������������������������� 190 Chronological development of new generation of the cold-box process, with taking into account the environment protection (Huttenes-Albertus)������������������������������������������������������������������������ 193 Emission BTEX from cold-box technology����������������������������������� 196 Emission of PAHs from cold-box technology�������������������������������� 197 Physical chemical properties of resin used in BETASET technology�������������������������������������������������������������������������������������� 199 Emission of BTEX from resole–CO2 process, pouring with molten cast iron and aluminum alloy��������������������������������������������� 202 Emission of PAHs (resole–CO2 process), poured with molten cast iron������������������������������������������������������������������������������������������ 203 Properties of urea–furfuryl resin and a catalyst applied in the hot-box technology [13]����������������������������������������������������������������� 211 Emission of gases from molding sand made in the hot-box technology (PF resin + hardener + HMTA)����������������������������������� 212 Comparison of the tensile strength of molding sands with sodium silicate (inorganic binder) thermally hardened and molding sands with organic binders in dependence on hardening time and humidity���������������������������������������������������������� 220

xxviii

Table 9.2

List of Tables

Parameters of cores made by various technologies������������������������ 233

Table 10.1 Recommended properties of green sands used in the cast iron foundry depending on the molding technique�������������������������������� 245 Table 10.2 Properties of green sands applied in various parts of the world����� 245 Table 10.3 Properties of typical foundry bentonites used in various countries and mold sands with these bentonites����������������������������� 249 Table 10.4 Substitutes of sea coal dusts����������������������������������������������������������� 256 Table 10.5 Comparison of work rates before and after the replacement of part of the coal dust mixture of graphite (savings of materials 14%, reduction of benzene emission >50%)���������������������������������� 268 Table 10.6 Emission of gases from green sand with addition of different lustrous carbon formers and content of BTEX������������������������������� 272 Table 10.7 Concentration of compounds from the PAH group in gases emitted from the green sands and a substance forming lustrous carbon (semi-industrial tests)��������������������������������������������������������� 272 Table 10.8 Concentrations of substances from the BTEX group in gases emitted during the thermal decomposition of a mixture of sea coal dusts and synthetic resin, in dependence of the temperature (decomposition in the air atmosphere) (Counted over for 1 g of sample)������������������������������������������������������������������������������������������� 272 Table 11.1 Volumes of gases and concentrations of substances from the BTEX group (mg/kg molding sands)��������������������������������������������� 279 Table 12.1 Properties of some materials applied as matrix of coatings for cores and molds������������������������������������������������������������������������������ 288 Table 13.1 Emissions from aluminum alloys foundry using LFP technology and green sand (g/1 mg of liquid aluminum alloy). (Adapted from Refs. [21, 27])�������������������������������������������������������� 307 Table 13.2 Composition of the sand mold in which furan resin is substituted by PLA or PCL������������������������������������������������������������ 323 Table 14.1 List of substances identified in the air in cast iron and cast steel foundries during the casting production����������������������������������������� 334 Table 14.2 Harmfulness of molding sands produced by various technologies������������������������������������������������������������������������������������ 336 Table 14.3 The chemically analysed PAH compounds in spent foundry sands (mg kg−1)������������������������������������������������������������������������������� 339 Table 14.4 Investigation results of the elution of spent molding sands originated from mold poured with Al alloy (T = 730 °C) (Sample 1) and cast iron (T = 1400 °C) (Sample 2) as well as the limit elution values, determined for wastes other than dangerous and inert, which are not municipal wastes, allowed to be stored in storage yards for wastes different from dangerous and inert wastes and for inert wastes (mg/kg of d.m.)������������������������������������������������������������������������������� 341

List of Tables

xxix

Table 14.5 Basic properties of the after reclamation dusts originated from the mechanical reclamation of molding sands with various chemically bound binders�������������������������������������������������� 343 Table 14.6 Energetic properties and the technical analysis of dusts from the mechanical reclamation process of molding sands with resins (marking of dusts the same as in Table 14.5)����������������������� 344

Chapter 1

Introduction

The foundry industry constitutes a huge and complex system of an essential influence on the economic development, social development and the environment condition, recycling application, waste utilization, and the environment improvement [1–4]. The intensively developing foundry industry consumes large amounts of natural resources, energy, and metals as well as generates significant amounts of gases and solid wastes, which influence the natural environment and work conditions. Therefore the adjustment of foundry plants to the sustainable development strategy is the necessary condition of the foundry industry development [1, 4, 5]. Managers of foundry plants should realize that the responsibility for the environment and for financial results is not mutually excluding. Taking care of the environment and introducing more environment-friendly changes can cause the production increase and its cost decrease, improve the recognition of the company name, strengthen relations with customers, and improve profitability of the company. By implementing the proper management of materials and wastes and applying recycling, the foundry plants can matter greatly in a sustainable consumption of natural resources. It means the necessity of taking into consideration – in strategic decision-­ makings and production operations – the mutual dependence between the environment and social and economic aspects. The sustainable development is based on innovative operations being the effect of research, i.e., of science [6]. This means that the science is extremely important in the implementing process of sustainable development rules. This mainly concerns working out of new innovatory sustainable technologies, which will contribute to materials saving and efficient energy consumption, thus influencing favorably the environment. The world casting production of all kinds of alloys was equal to 109.8 million Mg in the year 2017, and in 2018 this production increased to 112.7 million Mg, i.e., by 2.6%. Moreover, ten countries were producing 87% of castings (Table 1.1). Regardless of higher and higher expenditures on the work safety and better devices and securities, quite often health threats and even life hazards occur in foundry plants. During the whole production process, the employees are endangered by hazardous and noxious factors related, among others, to the emission of © Springer Nature Switzerland AG 2020 M. Holtzer, A. Kmita, Mold and Core Sands in Metalcasting: Chemistry and Ecology, https://doi.org/10.1007/978-3-030-53210-9_1

1

2

1 Introduction

Table 1.1  The largest producers of castings in the world (data for 2018) Country China India USA Japan Germany

Production (Mg) 49,350,000 13,399,682 10,756,492 5,575,417 5,432,999

Country Russia Mexico South Korea Brazil Italy

Production (Mg) 4,200,000 2,909,461 2,516,800 2,283,379 2,261,196

Adapted from Ref. [7]

hazardous and dangerous chemical substances, air pollution, high temperatures, noises, vibrations, and electromagnetic field. Chemical substances especially of organic origin, present in the atmosphere, are hazardous for the employees. The emission of inorganic dangerous substances occurs mainly during melting and cleaning processes of castings, and mostly these substances are metal oxides. Hazardous substances at workplaces can be gaseous, liquid, or solid. Their dangerous operations can take place by skin contact, breathing, or eating with food. The main source of dust and gaseous contaminations is operations of mold pouring with liquid metal, mold cooling, and casting knocking out. In the case of molding sands with bentonite, these operations generate up to 90% of contaminations from the hazardous air pollutants (HAPs) group [8–10]. According to REACH1 the binders applied in mold and core production can be classified by their chemical composition [11]. The REACH aim is the assurance of the high level of the human health and environment protection against chemical hazards, promotion of alternative testing methods, increased competitiveness and innovativeness, and ensuring the free passage of substances within the EU countries. The REACH renders the industry responsible for the evaluation and management of the risk caused by chemical substances and for supplying the information concerning their safety. Simultaneously the EU can require additional tests of highly hazardous substances. The scheme of the casting production process, with marked operations during which the danger of the HAP emission occurs, is presented in Fig. 1.1. The USA, on the basis of the Clear Air Act Amendments (CAAA) Title III, controls the emission of 189 toxic chemical compounds encompassed by the common name HAPs, known or suspected to be carcinogenic, occurring in various industries and considered dangerous for the health and environment. 40 substances from this list were occurring in the air emitted from the foundry plant, and 90% of them constituted hydrocarbons. Such compounds as benzene, toluene, and xylenes dominated among them (approx. 80%)2). The fraction majority in generating these compounds has the pyrolysis of additions containing carbon (in the case of sands

 REACH is the acronym of Registration, Evaluation, Authorization and Restriction of Chemicals.  Cast iron foundry having cupolas can constitute the potential source of polychlorodibenzodioxines (PCDD), polychlorodibenzofuranes (PCDF), and polychlorinated biphenyls (PCB). However, the emission coefficients of these compounds from cupolas are much lower than the emission coefficients from incinerating plants, secondary production of non-ferrous metals, and the steel or pig iron production [38–41]. 1 2

1 Introduction

3

with bentonite) and binders (based on organic substances) applied for cores [13– 16]. For the realization of this Act in the foundry industry, the Casting Emissions Reduction Program (CERP) was developed. This Program exists from 1994 and is financed by the US Department of Defense. It gathers representatives from the American Foundry Society (AFS), the Society of Foundry Materials Suppliers, and the Department of Defense. They closely cooperate with the representatives of universities, foundry industry, suppliers of foundry materials, and the US Environmental Protection Agency (US EPA). The main aim of the CERP program is the evaluation of materials, equipment, and processes applied in the casting production, with regard to their negative influence on the environment. More than 80% of castings are produced in expendable molds prepared from sand-based matrices with additions of the proper binder. The main emission sources in the casting production process are operations related to molding and core sands3.Certain amounts of dangerous substances are emitted already at the preparation stage of molding and core sands and at making molds and cores. Solvents, amines, and volatile organic compounds (VOCs), often of unpleasant smells, should be mentioned in this place. The polymer formed at the production of phenol–formaldehyde resin can contain not reacted monomers of phenol and formaldehyde. Moreover, certain additions are applied to binders to improve their humidity resistance [17]. However, the majority of harmful substances are formed during the mold pouring with molten metal, as the result of the high-temperature influence on binders or organic-type additions. In dependence on the oxygen content in the atmosphere and temperature, the effect of pyrolysis, cracking, gasification, or combustion can occur. Due to these processes, chemical compounds which were not present in initial materials can be formed. These substances can evaporate from molds after finishing their pouring process, during mold cooling or casting knocking out, especially from cooler parts of cores or molds, which were not subjected to the high-temperature influence. They can also condense on matrix grains and remain in sands, from where they can be eluted to the environment during spent sands storages or released at successive operations. In dependence on the applied resin (phenol–formaldehyde, furan, furfuryl–urea, alkyd) under the temperature influence, such substances as furfuryl alcohol, formaldehyde, and phenol, and from the BTEX group (benzene, toluene, ethylbenzene, xylnes), and also polycyclic aromatic hydrocarbons (PAHs), can be formed and released. In addition, for the molding sand hardening, the gaseous factors such as CO2, SO2, or amines are often used. The emission potential of cores is higher than that of molds. Therefore creating the high-quality market of foundry materials, safe for employees and of a small negative influence on the environment, is necessary [10, 18–24]. The atmospheric contaminations in ferrous alloys foundry plants constitute complex mixtures of dusts, fumes, gases, and steams generated during various operations. The solid fraction of contaminations is called the “ferrous foundry particulate” (FFP). The FFP composition is very variable since it depends on the performed production processes and applied materials (especially dangerous is the respirable fraction of crystalline silica). The evaluation of the occupational exposure to chemical factors occurring in the work environment is mostly based on measuring or estimating the concentrations of these factors in the air and comparing them

4

1 Introduction

Fig. 1.1  Sources of emission of compounds from the HAP group in the iron alloy foundry. (Adapted from Ref. [12])

with the assumed criteria. These criteria, also called hygienic standards and allowable values determine the permissible concentrations of chemical substances in the air, in dependence on the averaging period to which they are related, known as threshold limit value (TLV) [25]. Two time intervals are applied: long term (8 hours) and short term (15 minutes). There are three kinds of TLV indicators, which are equivalents of the Polish indicators: NDS (occupational limit), NDSCh (exposure limit), and NDSP (the highest permissible threshold concentration) • TLV–TWA (time-weighted average): the average exposure during 8 work hours/ day and 40 work hours/week (equivalent to NDS). • TLV–STEL (short-term weighted average): the exposure not longer than 15 minutes, repeated not often than four times a day, at maintaining at least 60 minutes break in between the exposure periods (equivalent to NDSCh). • TLV–C (ceiling limit): the exposure value which cannot be exceeded in any moment (equivalent to NDSP). TLV indicators are related to chemical substances as well as to physical factors (noise, vibration, ionizing radiation, heat, and cold). TLV for chemical substances is expressed in ppm (parts per million), for gases, and mg/m3, for solid particles (dusts, fumes).

1 Introduction

5

For recalculating of these units – in the case of gases – the simple equation is used (Eq. 1.1):



ppm =

24.45 mg × m 3 molecular mass

(1.1)

In Great Britain the workplace exposure limits (WELs) determine the concentration of dangerous substances as the time-weighted average (TWA). Two time intervals are applied: long term (8 hours) and short term (15 minutes) (Table 1.2). In the European Union the workplace state is determined by several directives: Directive (EU) 2017/164 [26] and Directive (EU) 2017/2398 [27]. Two kinds of standard values are determined in these directives for chemical substances: indicative occupational exposure limit values (IOELV) and binding occupational exposure limit values (BOELV). The IOELV values for 123 chemical substances and BOELV values for 10 chemical substances are determined in these directives. For substances, for which the BOELV values are determined: • Member countries must determine their equivalent domestic values, which can be on the same or lower level but not higher than the values determined in the European Union. Indicative occupational exposure allowable values according to various systems (WEL–TWA, WEL–SHORT, TLV–Stel, TLV–TWA and TLV–C, NDS, NDSCh, NDSP [28]) for selected chemical substances, which were found at individual stages of the casting production process, are given in Table 1.2. The presence of unpleasant smells (e.g., amine) decreases the work comfort; however due to that, an earlier discovery of harmful substances in the foundry atmosphere is possible (the smell threshold of substances is lower than the toxicity threshold). Binding materials for foundry sands will be rather changing by the evolutionary and not the revolutionary way. More and more strong regulations concerning the environment protection and growing social pressure are causing foundry plants to either look for new, more friendly, technologies of molding and core sand preparation or try to improve and modify the already applied ones, to limit the emission of harmful substances and unpleasant smells during preparations of sands, mold pouring, cooling, and casting knocking out. These elements decide  – to a significant degree – on the foundry sector image. Simultaneously, this is forcing producers of foundry materials to develop such binders which will meet these requirements. The example of this trend can be molding sands with bentonite and substances generating lustrous carbon, substituting sea coal dusts [29–32]. Investigations concerning molding and core sands will be carried out in three basic directions: the environment protection, technology (among others, work time of sands, prolongation of sands storage time, higher dimensional accuracy of castings, castings quality improvement), and the production efficiency. The market of molding sands was for many years dominated by sands with bentonite (green sands) and sands with water glass hardened at the room temperature.

CAS number(5) 7664-417 Benzo(a)pyrene 50-32-8 Benzene 71-43-21 Hydrogen cyanide 74-90-8 26,447Diisocyanate methylene diphenyl – 40-5 mixed isomers 91-08-7 Toluene-2-6diisocyanate – mixed isomers Carbon disulfide 75-15-0 Nitrogen dioxide 10,10244-­0 Sulfur dioxide 7446-095 Carbon dioxide 124-38-9 Ethylbenzene 100-41-4 Ethyltoluene 25,55014-5 Phenol 108-95-2

Compounds Ammonia

5 –

– 0.09

0.021

– 1.5 2.7 27,000 400 – 16

0.007

12.5 0.7

1.3

9000 200 100

7.8

–

– – –

– –

–

–

–

28

TLV

2

5000 100

0.5

5 0.5

1 0.9

1

1

4.5

ppm 35

7.8

4

(4)

0.5

5 0.5

1 0.9

6

2

1

1

7.8

4

9150 15,000 441 125

1.3

15 0.96

3.25 1 4.5

Carc, Sk Sk

16

(continued)

Sk

27,400 552 Sk

2.7

1.91

5

WELs (8) (G. Britan) Short-term Long-term exposure limit exposure limit (15 min reference (8 h reference period) Comments period) mg/ ppm m3 ppm mg/m3 25 18 35 25

21,400 5000 552 100

2.7

1.91

5

mg/m3 25

TLV–STEL (15 min reference period)

9150 1500 441 125

1.3

15 0.96

3.25 1

TLV–TWA(3) (8 h reference NDSP period) mg/ ppm (7) m3 (6) 25 18

0.002 1.6 1 0.03

mg/m3 14

WEL (Poland) NDSCh NDS (8 h (30 min reference reference period) period)

Table 1.2  Indicative exposure limit values applied in Poland and other countries for compounds emitted in casting process

6 1 Introduction

Carbon monoxide Styrene Toluene Trimethylamine Triethylamine Sulfur trioxide

Formaldehyde Phosgene 2-Furaldehyde Glutaraldehyde Cresol Xylene –o, −m, −p or mixed isomers Adipic acid – inhalable fraction Methyl methacrylate Methanol Methylamine Naphthalene Methyl formate Methyl acetate Tetraethyl orthosilicate Hydrogen sulfide 14

7

7783-064 630-08-0 100-42-5 108-88-3 75-50-3 121-44-8 7446–119

23 50 100 12 3 1

300 300 15 50 200 1468 –

100 100 5 20 100 179 44

80-62-6 67-56-1 74-89-5 91-20-3 107-31-3 179-20-9 78-10-4

1217 100 200 24 9 3

10

5

124-04-9

0.74 0.16 25 0.6 – –

0.37 0.08 10 0.4 22 100

50-00-1 75-44-5 98-01-1 111-30-8 95-48-7 95-47-6

–

–

–

–

–

– – –

–

– – –

– –

20 100

30

50 200 5

200

2 0.02 2

25 430

35

125 734 44

266

2.5 0.08 8

100 250

200

100 400

250

2 0.06 5

117 1080

232

210 1468

333

2.5 0.25 20

2

20 100 50

5

100 200 5

50 200

2 0.02 2 0.05 0.05 50

8

23 430 191

7

250 734 44

208 266

2.5 0.08 8 0.2 0.2 220

4

100 250 100

10

150 40

100 250

2 0.06 5 0.05 0.05 100

17

117 1080 384

14

375 1468

416 333

2.5 0.25 20 0.2 0.2 441

(continued)

Sk

Sk

Sk

Sen Sen Sk BMGV

1 Introduction 7

14,80860-7 14,46446-1

0.1

CAS number(5) mg/m3

WEL (Poland) NDSCh NDS (8 h (30 min reference reference period) period) TLV–TWA(3) (8 h reference NDSP period) mg/ ppm (7) m3 (6)

TLV

ppm

mg/m3

TLV–STEL (15 min reference period) (4)

WELs (8) (G. Britan) Short-term Long-term exposure limit exposure limit (15 min reference (8 h reference period) Comments period) mg/ ppm m3 ppm mg/m3

Adapted from Refs. [26–28, 37] Annotations BMGVs Biological monitoring guidance values Carc Capable of causing cancer and/or heritable genetic damage Sen Capable of causing occupational asthma Sk Can be absorbed through the skin. The assigned substances are those for which there are concerns that dermal absorption will lead to systemic toxicity (1) Inhalable fraction: It is the fraction of total airborne particles that enters the body through the nose and/or mouth during breathing. This fraction corresponding to particles with aerodynamic diameter (dae) ≤100 μm) is relevant to health effects anywhere in the respiratory tract such as rhinitis, nasal and lung cancer, and systemic effects (2) Respirable fraction dust: It is the fraction that penetrates to the deep lung where gas exchange takes place. The particle sizes of respirable dust are up to 10 microns (3) TWA: Time-weighted average (4) STEL: Short-term exposure limits (5) CAS: Chemical Abstracts Service (Registry Number) (6) mg/m3: milligrams/cubic meter of air (7) ppm: parts per million (ml/m3) (8) WELs: Workplace exposure limits (EU Directive 2009/161/EU)

Crystalline silica Quartz Cristobalite – respirable fraction

Compounds

Table 1.2 (continued)

8 1 Introduction

1 Introduction

9

Processes of sand binding by means of furan and phenol resins, hardened at a room temperature, were developed in the 1950s of the twentieth century. At the end of the 1960s, the cold-box process for various resins was developed. The development of sands with water glass, hardened by drying technique, started in the middle of the 1990s. Efforts are undertaken to use these sands, generally counted to sands with inorganic binders (however, apart from water glass, these sands contain certain organic additions for their properties improvement), as substitutes of widely applied sands with organic binders. Molding sands with organic binders have several advantageous features, which previous sands did not have. This concerns better efficiency, higher reliability, and better mechanical properties. Foundry plants of aluminum and copper alloys were the first plants in which sands with organic binders were substituted by sands with water glass thermally hardened. The main reasons of changing cores with organic binders (hot-box, warm-box, cold-box) for cores with water glass constituted difficulties in removing these cores from the mold after casting (too low overheating degree in the case of aluminum alloys). In addition, sands with water glass are considered to be environmental-friendly and not toxic. Drawbacks of sands with water glass such as difficulty in knocking out, low elasticity, or weak reclaimability can be improved by means of various additives or modifiers (collagen, starch, dextrin). Several producers of foundry sand binders developed their own technologies, based on water glass thermally hardened, of trade names CORDIS®, AWB®, INOTEC®, and based on geopolymer of trade name GEOPOL®. However, not every mold or core can be produced by this technology. It concerns especially large molds (of a few and a dozen or so tons) and large cores. At the development of new kinds of binders, the attention should be directed to the following properties: • • • • • • •

Minimizing of binder additions. Low binder viscosity. Possibility of long-term storage of cores. Low level of unpleasant smells. Low emission during sand preparations and mold producing. Easy reclamation of spent sands. Low level or a lack of nitrogen in a binder, especially when sands are used for cast steel castings. • Low level of sulfur in a binder, when sands are used for spheroidal cast iron castings. According to the assumed rule, the best way of reduction of contaminations is their elimination at a source. In the case of foundries, the most efficient method of VOC reduction is the substitution of traditional binders – based on petroleum industry products  – by less harmful binders based either on inorganic polymers (e.g., geopolymers) or on natural polymers. In order to satisfy this task, the hybrid binder consisting of alkaline silicate with the addition of, e.g., hydrolyzed collagen was developed. This new binder – of a low VOC emission – allowed to decrease significantly amounts of VOC released every year by the foundry industry [33–35].

10

1 Introduction

The evaluation of harmfulness of sands applied for molds and cores contains two basic elements: • Emissivity of harmful gases and dusts during such operations as preparations of sands, their molding, making cores, mold pouring with liquid metal, mold cooling, casting knocking out, and spent sand reclamation. • Possibility of the elution of dangerous substances from spent sands into the environment during these sands’ storage or functional utilization. Molding sands with binders, from which harmful substances are not eluted, can be utilized in other fields, due to which their storage is avoided [36]. Therefore, at the evaluation of the given sand influence on the environment, these two problems should be also taken into account. Only when such investigations were performed the total evaluation of the given sand harmfulness would be achieved.

References 1. Holtzer M (2018) Adjusting the foundry plants operations to the sustainable development strategy – the success condition. Foundry J Polish Foundrymen’s Assoc 68:20–21 2. Patange GS, Khond MP, Chaudhari NV (2012) Some studies and investigations of foundry wastes for sustainable development. Int J Ind Eng Res Dev 3:51–57 3. Wadhwa R (2015) Sustainable manufacturing in SMEs: Technology options. Int J Comput Sci Issues 12:107–112 4. Guangkuo A (2014) The establishment of sustainable development capacity of the foundry industry index evaluation system. 71st World Foundry Congr. Bilbao, Spain, p 19–21 5. Engelhardt T (2010) New concepts to reduce the emission from green sand. Fonderie Magazine (7) : 24−36 6. Garbarz B (2008) Advances in iron and steel technologies meeting the principles of sustainable development. Pr Inst Metal Żelaza 60:1–7. (in Polish) 7. Census of World Casting Production (2018) Mod Cast 2019 December 24–5 8. Elliehausen HJ, Knecht U, Maa β-Rühl B (1984) Schdstofbelastung durch formsande einer Eisengieβerei. Gentner Verlag, Stuttgart 9. Palmer WG, Scoott WD (1981) Lung cancer in ferrous foundry workers: a review. Amer Ind Hyg Assn J 42:329–340 10. LaFay VS, Neltner S, Carroll D, Couture DJ (2010) Know the environmental impact of your additives. Mod Cast 10:27–29 11. Regulation (EC) no 1907/2006 of the european parliament and of the council of 18 december 2006 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) 12. Crandell GR, Schifo J, Mosher G (2006) CERP Organic HAP emission measurements for iron foundries and their use in development of an AFS HAP guidance document. Trans Am Foundry Soc 114:1–17 13. Kumar R, Abhishek MK, Fuller A, Bosco M, Rego JV (2017) Study on mechanical properties of bio based and inorganic binders for the preparation of core in metal casting Energy and Power. 7:136–141 14. Glowacki CR, Crandell GR, Cannon FS, Voi RC, Clobes JK, Furness JC et al (2003) Emissions studies at a test foundry using an advanced oxidation-clear water system. Am Foundry Soc Trans 3:22

1 Introduction

11

15. Wang Y, Cannon FS, Salama M, Goudzwaard J, Furness JC (2007) Characterization of hydrocarbon emissions from green sand foundry core binders by analytical pyrolysis. Environ Sci Technol 41:7922–7927 16. Technikon L (2000) US Army Task N256 Ashland Core Binder Replacement 17. ASK Chemicals (2016) Newest technology platform for cold box binders. Foundry Trade J 190:240–242 18. Riberio MG, Filho WR (2006) Risk assessment of chemicals in foundries: the International Chemical Toolkit pilot-project. J Hazard Mater 136:432–437 19. Humfrey CDN, Levy LS, Faux SP (1996) Potential carcinogenicity of foundry fumes: a comparative in vivo-in vitro study. Food Chem Toxicol 34:1103–1111 20. Scarbel P, Bats CE, Griffin J (2006) Effect of mold and binder formulation on gas evolution when pouring aluminum casting. AFS Trans 114:435–445 21. Bobrowski A, Holtzer M (2009) Assessment of environmental influence of bentonite and lustrous carbon carrier – in an ascpect of gases emission. Arch Foundry Eng 9:21–24 22. Teles MT, Delerue-Matos C, Alvim-Ferraz M (2005) Determination of free furfuryl alcohol in foundry resin by chromatographic techniques. Anal Chim Acta 537:47–51 23. Kubecki M, Holtzer M, Grabowska B, Bobrowski A (2011) Development of method for identification of compounds emitted during thermal degradation of binders used in foundry. Arch Foundry Eng 11:125–130 24. Ji S, Wan L, Fan Z (2001) The toxic compounds and leaching characteristics of spent foundry sands. Water Air Soil Pollut 132:347–364 25. Gromiec JP, Czerniak S (2002) Polish and worldwide criteria for assessing exposure to chemicals: procedures and applications. Occup Med (Chic Ill) 53:53–59 26. Directive (EU) 2017/164 - indicative occupational exposure limit values 27. Directive (EU) 2017/2398 on the protection of workers from the risks related to exposure to carcinogens or mutagens at work 28. Regulation of the Minister of Family, Labour and Social Policy of June 12-th 2018, concerning the highest allowable concentrations and intensities of harmful for health factors in the work environment 2 Dz. U. 2018, item 128 (in Polish) 29. Holtzer M, Dańko J, Dańko R (2007) Possibilities of formation of dioxins and furans in metallurgical processes as well as methods of their reduction. Metalurgija 46:285–290 30. Holtzer M (2003) Directions of development of molding and core sand with organic binders. Arch Foundry Eng 3:189–196 31. Wolff T, Steinhaeuser T (2004) AWB – an environment friendly core production technology. Giesserei 91:80–84 32. Howden JD (2014) Green sand “less is best” a more sustanable philosophy for change. Proceed. Mater. 71st World Foundry Congr., Bilbao, Spain, pp 19–21 33. Wang J, Fan Z, Wang H (2007) An improved sodium silicate binder modified by ultra-fine powder materials. China Foundry 4:26–30 34. Fan G, Gu Z, Yang L, Li F (2009) Nanocrystalline zinc ferrite photocatalysts formed using the colloid mill and hydrothermal technique. Chem Eng J 155:534–541 35. Kmita A, Drożyński D, Roczniak A, Gajewska M, Marciszko M, Górecki K et  al (2018) Adhesive hybrid nanocomposites for potential applications in molding sands technology. Compos Part B Eng 146:124–131 36. Holtzer M, Dańko R, Kmita A (2016) Influence of a reclaimed sand addition to molding sand with furan resin on its impact on the environment. Water Air Soil Pollut 227:1–12 37. EH40/2005 (2018) Workplace exposure limits. Third edition, published in 2018 by The Stationary Office 38. Holtzer M, Danko J, Danko R (2007) Possibilities of formation of dioxins and furans in metallurgical processes as well as methods of their reduction. Metalurgija 46:285–290 39. Lv P, Zheng M, Liu G, Liu W, Xiao K (2011) Estimation and characterization of PCDD/Fs and dioxin-like PCBs from Chinese iron foundries. Chemosphere 82:759–763

12

1 Introduction

40. Grochowalski A, Lassen C, Holtzer M, Sadowski M, Hudyma T (2007) Determination of PCDDs, PCDFs, PCBs and HCB emissions from the metallurgical sector in Poland. Environ Sci Pollut Res 14:326–332 41. Suhr M, Klein G, Kourti I, Gonzalo MR, Santonja GG, Roudier S, et al. (2015) Best Available Techniques (BAT) Reference Document for the Production of Pulp, Paper and Board

Part I

Organic Chemistry

Chapter 2

General Principles of Organic Chemistry

2.1  Organic Compounds: Structure and Bonding Structure of Organic Compounds Organic chemistry is the chemistry of carbon compounds. Carbon is the basic component of organic compounds, but a majority of these compounds contain also hydrogen, nitrogen, oxygen, sulfur, phosphorus, chlorine, or other elements. Carbon, as the element of group IV, can share four valence electrons and form four strong atomic bonds. In addition, carbon atoms can join with each other forming long chains and rings [1]. Atoms can join into molecules, since particles are more stable (has lower energy) than individual atoms of which they were formed. Therefore, when the chemical bond is formed, the energy is always released from the system. However, when the chemical bond decomposes, the energy must be supplied to the system. Chemical bonds can have different characters. The organic compound structure consists of a hydrocarbon core (chain or ring) or of a chain or ring with built-in heteroatoms, constituting the particle backbone and linked with this backbone substituents and functional groups [2–4]. Substituent is an atom or a group of atoms with which the hydrogen atom was substituted in a chain or ring. Functional group is an atom or a group of atoms linked with the parent structure. Functional groups decide on the chemical properties of organic compounds (therefore multiple bonds are considered to be functional groups). One, two, three, or more functional groups can occur in an organic compound. Since the distinction between two notions, substituent and functional groups, is difficult, it can be assumed that – from the point of view of the chemical reaction – substituents are atoms or groups of atoms not taking part in the given reaction, while functional groups are those which are taking part in the reaction. Types of Bonds in Organic Compounds Ionic Bonds © Springer Nature Switzerland AG 2020 M. Holtzer, A. Kmita, Mold and Core Sands in Metalcasting: Chemistry and Ecology, https://doi.org/10.1007/978-3-030-53210-9_2

15

16

2  General Principles of Organic Chemistry

Presence of eight electrons in the outer shell (i.e., noble gas structure) provides especially stable structure of the element. Several elements of the main group of the periodic table are striving to obtain such structure either by giving electrons (forming cations) or by taking electrons (forming anions). Ions formed in this way are joining with each other, due to the electrostatic attraction, forming compounds of A+B− type; this is an ionic bond. Atomic Bonds Not all elements, including also carbon, are able to form ionic bonds. It results from the fact that atoms of such elements require too much energy for giving or taking electrons, in order to achieve the noble gas configuration. As an example, carbon has the electronic configuration 1s22s22p2, which means that in order to obtain the stable noble gas configuration, it must give or take four electrons, achieving either the helium (He 1s2) or neon (Ne 1s22s22p6) structure. Therefore carbon is joining with other elements by common electron pairs forming single, double, and triple bonds. Such bonds are called atomic bonds (covalent bonds). This is a symmetric bond because two binding electrons are equally used by two equal carbon atoms. A majority of chemical bonds are neither fully ionic nor fully atomic since binding electrons are more attracted in the direction of one of the atoms (more electronegative), forming the polarized atomic bond [5].

2.2  Organic Chemical Reactions: Classifying Classifying in Respect of Structural Changes There are four general types of organic reactions, which involve structural changes in substrates [1]. Addition reaction – two substrates are joined together forming one new product. Substitution reaction – substrates are exchanging fragments of their particles forming new products. Elimination reaction – single substrate disintegrates into two products. Rearrangement reaction – in a single substrate, the rearrangement of bonds occurs. Classifying in Respect of the Reaction Type Oxidation and Reduction Reactions Carbon atoms have various oxidation degrees, from −4 (e.g., CH4) to +4 (e.g., CO2), depending on substituents. The oxidation in inorganic chemistry is defined as a loss of one or more electrons by the given atom while the reduction as obtaining of one or more electrons. The problem whether the given carbon atom was reduced or oxidized during the reaction is determined in organic chemistry by the change of the number of C–H bonds and the change of bonds of carbon with more electronegative elements (N, O, F, Cl, Br, I, S). Bonds between carbon atoms are omitted. Such calculation should be performed for each carbon atom, which is changing during the reaction.

2.2 Organic Chemical Reactions: Classifying

17

Table 2.1  Classification of organic compounds on the bases of functional groups Class of chemical compound Carboxylic acid

Functional group

Group name in Prefix Carboxyl-

Suffix

Sulfonic acid

Sulfo-

Ester

R-Oxycarbonyl-

-an R

Nitrile

Cyano-

-nitrile

Alcohol Phenol

−O − H

HydroksyEther

Ether

Amine Alkenes Alkynes Alkanes

Aldehyde Ketone

C − NH2 −C = C− −C ≡ C− −C − C− −CH2OH− CH3− CH3O− C6H5O− −CH2− −C = O

-amine

Methylol group Methyl group Methoxy group Phenoxy group Methylene group Carbonyl group

Adapted from Refs. [2, 6]

• When the number of C–H bonds increases and/or the number of bonds of carbon with more electronegative elements decreases, it means that such carbon atom was reduced (i.e., it is on a lower oxidation degree). • When the number of C–H bonds decreases and/or the number of bonds of carbon with more electronegative elements increases, it means that such carbon atom was oxidized (i.e., it is on a higher oxidation degree).

18

2  General Principles of Organic Chemistry

Classifying Based on Functional Groups Organic compounds can be grouped, taking into account their structural features and related to them similar physical and chemical properties. Compounds included into the given group are characterized by functional groups (Table 2.1). Functional groups are the sites of chemical reactions in organic compounds.

2.3  Hydrocarbons Hydrocarbons are compounds which contain only carbon and hydrogen. Atoms can be joined together by single, double, or triple bonds in chains of an arbitrary length or in rings of various sizes. Hydrocarbons are divided into aliphatic (chain), cyclic, and aromatic hydrocarbons. Among aliphatic and cyclic hydrocarbons, there are saturated hydrocarbons, i.e., alkanes or cycloalkanes, and unsaturated hydrocarbons. Unsaturated hydrocarbons contain alkenes (with double joints), alkynes (with triple joints), and alkadienes (with two double joints) as well as cycloalkenes, cycloalkynes, and cycloalkadienes, respectively (Fig.  2.1). Carbon obtains +4 valence in organic compounds. The selected substances from individual hydrocarbon groups are shown in Table 2.2.

2.3.1  Alkanes Alkanes contain saturated hydrocarbons in which carbon atoms are joined with each other by single bonds only. According to the binding systematic of IUPAC1 [7], chains of carbon atoms in particles of alkanes can be both normal and branched, but cannot form closed loops. The general molecular formula of alkanes is of a form: CnH2n + 2. The alkanes group, put in order according to the carbon chain length, constitutes the alkanes homologous series. This series is as follows (all names of alkanes have at the end -ane) (Table 2.3) [6]. The first four saturated hydrocarbons of the homologous series are gases. Alkanes having from 5 to 15 carbon atoms in a particle are liquids, while the ones in which the number of carbon atoms exceeds 15 are solids. Natural sources of saturated hydrocarbons are natural gas, crude oil, fossil wax, and paraffin. Natural gas is a mixture of gaseous hydrocarbons. Its main component is methane (50–98%) and then ethane, propane, and butane. Crude oil contains alkanes (30–80%) and various amounts of cycloalkanes and aromatic hydrocarbons.

 IUPAC – International Union of Pure and Applied Chemistry.

1

2.3 Hydrocarbons

19

Fig. 2.1  Schematic division of hydrocarbons

Alkanes can be obtained by the thermal decomposition of carboxylic acid salts. For example, methane is obtained by heating sodium acetate (Eq. 2.1): hν

CH 3 COONa + NaOH → CH 4 + Na 2 CO3



(2.1)

Saturated hydrocarbons can be also obtained by catalytic hydrogenation of unsaturated hydrocarbons or by electrolysis of carboxylic acid salts. Alkanes are highly hydrophobic, do not mix with water, are not wetted by water, and are not soluble in water. However, they easily dissolve in nonpolar, weakly polar, and moderately polar organic solvents. These substances are easily flammable. Gaseous hydrocarbons form explosive mixtures with the air. Alkanes are weakly reactive, but under certain conditions can violently react, e.g., in a substitution reaction, when the hydrogen atom is substituted by another atom, the most often by halogen (X) (Eqs. 2.2 and 2.3): hν



R − H + X 2 → R − X + HX

(2.2)

HC ≡ CH + Br2 → CHBr = CHBr ethine 1, 2 dibromoethene

(2.3)

Bromination:



CHBr = CHBr + Br2 → CHBr2− CHBr2

1, 2 dibromoethene 1,1, 2, 2 tetrabromoethane Reactions of alkanes with oxygen:

CH2

CH

EtheneH2C

PropeneH3C

CH2

Alkyl groups of unsaturated aliphatic hydrocarbons Hydrocarbon

Propane CH3CH2CH3

Ethane CH3CH3

Alkyl groups of aliphatic hydrocarbons Hydrocarbon Methane CH4

H3C

CH2

H3C

H2C

C

CH

CH

CH2

CH

CH

Simplified pattern group

CH2

Isopropenyl, 1-methylvinyl

Prop-2en-1-yl

Propenyl

Name of group Ethyl, vinyl

Isopropyl group, (i-propyl)

Propylene group, (Pr)

CH2CH2CH3 CH3CHCH3

Ethylene group

Ethylidyne group

Ethyl group

Methyne group Ethyl group, (Et)

Methylene group

Group name Methyl group, (Me)

CH2CH2

CH3CH2 CH3CH CH3C

CH2

CH3

Simplified formula

Table 2.2  Simplified formulas, names, and symbols of abbreviations of the selected hydrocarbon groups

20 2  General Principles of Organic Chemistry

CH2

Naphthalene

Benzene

6

7

5

8

4

1

3

2

Aryl groups of aromatic hydrocarbons Hydrocarbon

2-Butene

CH3 CH CH CH3 CH EthyneHC C CH PropyneH3C

ButeneH3C CH2 CH

CH2

C

CH2

C

CH

CH CH2

Simplified pattern group

HC

CH3CH

H3C CH2 CH

β-Naphthyl (C10H7-)

α-Naphthyl (C10H7-)

o-, m-, p-phenyl

Name of group Phenyl

Propen-2-en-1

Ethyne

But-2-en-1-yl

But-1en-yl

(Continued)

2.3 Hydrocarbons 21

2

CH

CH2CH3

1

Adapted from Adapted from Ref. [2]

Styrene

Ethylbenzene

Toluene

CH3

Table 2.2   (Continued)

CH2

CH2

CH2

CH2

CH

CH3

CH

C

HC

CH2

Styrene

β-Phenyl (C6H5-phenyl group

α-Phenyl (C6H5-phenyl group)

Benzylidene group

Benzylidene group

Benzyl group

22 2  General Principles of Organic Chemistry

2.3 Hydrocarbons

23

Table 2.3  The alkanes homologous series Hexane C6H14 Heptane C7H16 Octane C8H18 Nonane C9H20 Decane C10H22

Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 Pentane C5H12 Methane CH4 Ethane CH3–CH3 Propane CH3–CH2–CH3 Butane CH3–CH2–CH2–CH3

• Complete burning (Eq. 2.4): CH 4 + 2O2 → CO2 + 2H 2 O + 890 kJ / mol



(2.4)

• Semi-burning (Eq. 2.5): 2CH 4 + 3O2 → 2CO + 4H 2 O



(2.5)

Under the influence of high temperatures, alkanes are decomposing. Alkanes, in a similar fashion as other hydrocarbons, indicate the isomeric effect. In this case it is the chain isomer, being various chain designs of isomeric particles. Isomerism – the effect of an occurrence of two or more compounds of the same molecular formula, but of various molecular structures, e.g.,

H

H

H

H

H

C

C

C

C

H

H

H

H

n-butane C4H10

H

H H HH C H H

C

C

C

H

H

H

H

2-methylpropane Isobutane C4H10

2.3.2  Alkenes General formula of alkenes is CnH2n, where n means the number of carbon atoms in a chain. Alkenes have the double bond between carbon atoms (C=C). This bond cracks during the addition reaction. Due to the fact that alkenes have double bond in their chain, they are unsaturated hydrocarbons. They decolor bromine water in the addition reaction.

24

2  General Principles of Organic Chemistry

CH2 = CH2 ethene CH2 = CH − CH2 − CH3 butene

CH2 = CH − CH3 propene CH2 = CH − CH2 − CH2 − CH3 pentene

Table 2.4  The homologous series of alkenes

C2H4 ethene C3H6 propene C4H8 butene C5H10pentene C6H12 hexene

C7H14 heptene C8H16 octene C9H18 nonene C10H20 decene

Names of alkenes are formed from the appropriate alkanes names by changes of their endings (−ane turns into -ene). The homologous series of alkenes is as follows (Table 2.4): In the case of unsaturated hydrocarbons occurs the isomerism, related to the double bond placement in the carbon chain, i.e., the so-called double bond isomerism (Eq. 2.6): 1 2 3 4 CH 2 = CH − CH 2 − CH 3 but   1 en 1 2 3 4 CH 3 − CH = CH − CH 3 but 2 en



(2.6)

and geometric isomerism (Eq. 2.7):

H

H C

C

C

H3C

H

CH3

cis-but-2-ene

C

H3C

CH3 H

trans-but-2-ene

(2.7)

The compound having substituents at the same side of the double bond is called cis-, while the isomer with substituents at the reverse side is called trans- (trans compounds are more stable). Alkenes containing 1 to 4 carbon atoms in their chain are gases, containing from 5 to 18 carbon atoms are liquids, while those having at least 18 carbon atoms are solids. Alkenes are obtained on the industrial scale by cracking of light petrol. They can be obtained in the laboratory, e.g., by dehydration of ethyl alcohol (Eq. 2.8):



CH 3 − CH 2 − OH → ethyl alcohol

CH 2 = CH 2 + H 2 O ethene

(2.8)

2.3 Hydrocarbons

25

Alkenes are highly reactive, which is caused by the double bond presence. These substances easily react in: • Addition reaction of, e.g., hydrogen (Eq. 2.9): CH 2 = CH 2 + H 2 = CH 3 − CH 3 ethene ethane



(2.9)

• Polymerization reaction (Eq. 2.10): n CH 3 = CH − CH 3 → ( −CH − CH − CH 3 − )n propene



(2.10)

• Burning reaction of ethene (Eqs. 2.11 and 2.12):

C2 H 4 + 3O2 = 2CO2 + 2H 2 O complete burning

(2.11)



C2 H 4 + 2O2 = 2CO + 2H 2 O non complete burning

(2.12)

2.3.3  Alkynes The general molecular formula for alkynes is CnH2n−2, where n means the carbon atoms number in a chain. Alkynes analogically as alkenes are unsaturated hydrocarbons, which means that they are also easily taking part in the addition reaction. Since they have the triple bond (−C ≡ C−), they are more reactive than alkanes and alkenes. They are unstable and subjected to several natural processes. Their names are formed by changing the ends and adding the number of the carbon atom at which the triple bond occurs (Table 2.5). Ethine, i.e., acetylene, can be obtained: • By hydrolysis of calcium carbide (Eq. 2.13):

CaC2 + 2HOH → HC ≡ CH + Ca ( OH )2



(2.13)

• By pyrolysis of light hydrocarbons, mainly methane (this is highly endothermic reaction) (Eq. 2.14): 1500° C



2CH 4 → HC ≡ CH + 3H 2 + 377 kJ Alkynes are not water soluble and undergo reactions:

• Addition of hydrogen, halogen, or water: Partial hydrogenation (Eq. 2.15):

(2.14)

H

H

C

C

H

C

H

H

C3H4 propyne

C

C2H2 ethine

C

H

Table 2.5  The homologous series of alkynes

H

H

C

C C H

C H

H

H

C H

C

H

C5H8 1-pentyne

C

H

C4H6 1-butyne

C

H

H

C

H

H

H

26 2  General Principles of Organic Chemistry

2.3 Hydrocarbons

27 H2

HC ≡ CH → H 2 C = CH 2 ethine catalyst ethene



(2.15)

Complete hydrogenation (Eq. 2.16): H2

H 2 C = CH 2 → CH 3 − CH 3 ethene catalyst ethane



(2.16)

• Polymerization (Eq. 2.17): n CH ≡ CH → ( −CH = CH − )n ethine



(2.17)

2.3.4  Cycloalkanes Hydrocarbon chains can create rings forming cyclic substances. Cycloalkanes are saturated hydrocarbons, which particles contain either one or more non-substituted rings or rings connected with side chains of saturated hydrocarbons. Cycloalkanes contain repeatable elements: – CH2 –, which means that their molecular formula can be presented as CnH2n. The simplest, single-ring cycloalkanes form the homologous series of compounds of an increasing ring size. Their names are formed from names of appropriate alkanes, with added prefix cyclo- (Table 2.6). Cycloalkanes can be obtained, among others, by hydrogenation of aromatic hydrocarbons (high pressure, increased temperature, catalyst: Ni) (Eq. 2.18): + H2 benzene (C6H6)



cat. 2.5 MPa 200°C cyclohexane (C6H12)

(2.18)

or by cyclization (closing of ring) of two substituted chain compounds (Eq. 2.19):

H2C



CH2

Cl

CH2

Cl

Zn, Na+

1,3 – dichloropropane

CH2 H2C

CH2

+ 2HCl cyclopropane

(2.19)

28

2  General Principles of Organic Chemistry

Table 2.6  The homologous series of cycloalkanes Compound Cyclopropane

Cyclobutane

Cyclopentane

Chemical formula C3H6

Structural formula

CH2

C4H8

C5H10

H2 C

CH2

H2 C

CH2

H2 C

CH2

H2C

CH2

H2C

CH2 CH2

Cyclohexane

H 2C

C6H12

H 2C

CH2 CH2

H 2C

CH2

Cycloalkanes are hydrophobic, form explosive mixtures with the air, and are easily volatile. In respect of chemistry, simple cycloalkanes containing five or more carbon atoms in a ring are similar to alkanes: • They are oxidizing. • They react with chlorine and bromine (Eq. 2.20): + Cl2



cyclopropane C3H6

hv

– Cl + HCl chlorocyclopropane C3H5Cl

(2.20)

• They are dehydrogenizing (aromatized) (catalyst: Pt, 500 °C) (Eq. 2.21):

2.3 Hydrocarbons

29

-3 H2



C6H12 cyclohexane

(2.21)

C6H6 benzene

Cycloalkanes are applied as solvents and in chemical syntheses.

2.3.5  Cycloalkenes Cycloalkenes are built of rings containing one double bond C=C (Table 2.7). The general formula of cycloalkenes is CnH2n−2. They are liquids insoluble in water. Their chemical properties are similar to the ones of alkenes. Since they are unsaturated compounds, they are taking part in addition reactions (Eq. 2.22): Br + Br2



cyclohexane

Table 2.7  The homologous series of cycloalkenes

Br 1,2-dibromocyclohexane

Compound Chemical formula Cyclopropene C3H4

Cyclobutene

Cyclopentene

(2.22)

Structural formula

CH2 HC

CH

C4H6

H2C

CH2

CH

C5H8

HC HC

CH

H2C

CH2 CH2

Cyclohexene

C6H10

HC H2C H2C

CH CH2 CH2

30

2  General Principles of Organic Chemistry

These compounds are used as solvents and in organic syntheses.

2.3.6  Aromatization and Benzene Aromatic hydrocarbons have chains closed into rings. In between carbon atoms, the single and double bonds alternately occur. It means that they are quasi-unsaturated compounds. In contrast with alkenes, arenes can have a few double bonds. A special structure of these hydrocarbons causes that they do not always behave as saturated compounds, neither do they have an unsaturated character. Their bond structure causes that they are very stable and their reactivity is significantly different than the reactivity of other unsaturated hydrocarbons [3]. The simplest arene is benzene, which molecule can be presented as follows (Kekule’s formula) (Eq. 2.23): H H

H

C C

C

C H



C

H

C H

(2.23)

Many aromatic compounds have both common and IUPAC names. These common names are shown below their UPAC names (Eq. 2.24):

CH



CH2

vinylbenzene (styrene)

OH

benzol (phenol)

H3C

CH3

isopropylbenzene (2.24) (cumene)

(however such form is not used in the case of successive aromatic hydrocarbons). Benzene homologues are organic compounds containing benzene ring and one or a few saturated hydrocarbon chains, e.g. (Eq. 2.25),

2.3 Hydrocarbons

31

C2H5

CH3

benzene

methylbenzene (toluene) C6 H5 − CH3

C6 H6



ethylbenzene C6 H5 − CH2 − CH3

(2.25)

Xylenes o-, m-, and p- are isomers, differing by mutual positions of methylene groups (Eq. 2.26): 1 6

2

1 6

4

3

1 2

6 5

5

5

CH3

CH3

CH3 CH3

4

3

CH3

4

2

3

CH3 1,2-dimethylbenzene o-xylene

1,3-dimethylbenzene m-xylene

1,4-dimethylbenzene p-xylene

(2.26)

Structural isomers are presented below (Eq.  2.27): 2-methylphenol (o-cresol), 3-methylphenol (m-cresol), and 4-methylphenol (p-cresol). They have similar chemical properties, but differ slightly in melting and boiling points:

OH

OH

OH

OH

CH3

CH3 CH3 benzol (phenol)

o-cresol (2-methylphenol)

m-cresol (3-methylphenol)

p-cresol (2.27) (4-methylphenol)

Some compounds, which apart from carbon atoms, have in their rings atoms of other elements can have aromatic character (aromatic heterocyclic compounds) (Eq. 2.28):

32

2  General Principles of Organic Chemistry NO2

H2SO4

+ HNO3

H2SO4

SO3H + H2O

FeCl3

Cl

+ SO3

+ Cl2

+ H2O

AlCl3

+ CH3Cl

+ CH3COCl

sulfonation

+

HCl

chlorination

+

HCl

alkylation

CH3

COCH3 + HCl

AlCl3

nitration

acylation

Fig. 2.2  The characteristic reaction of aromatic hydrocarbons



.. .. N

.. O ..

N

N ..

N ..

H pyrrole

furan

pyridine

pyrimidine

N

.. quinoline

(2.28)

Benzene and the basic aromatic compounds are obtained, in large amounts, from the crude oil distillation, from gas pitch (obtained during the coal carbonization2), and from ethyne (acetylene) (Eq. 2.29):

2  Hard coal is a mixture composed mainly of the network of, similar to benzene, rings connected with each other. When coal is heated to a temperature of 1000 °C without the air access, the thermal decomposition occurs and the mixture of compounds, possible to be distilled – known as gas pitch – is obtained.

2.3 Hydrocarbons

33

Fig. 2.3  Oxidation of aromatic hydrocarbons

CH3

COOH

oxidation

toluene

benzoic acid

CH3

COOH

oxidation

CH3 o-xylene

CH HC HC



CH

CH

COOH orthophthalic acid

500 °C

CH

acetylene (ethine)

benzene

(2.29)

Aromatic hydrocarbons are liquids or solids, not water soluble. Hydrocarbons containing more than one aromatic ring are usually solids, often of a high boiling point and melting temperature. A large group of these compounds is cancerous or mutagenic. Aromatic hydrocarbons are applied in production of high-octane petrol, solvents, chemical compounds, and medicines as well as in organic syntheses. Electrophilic Substitution The bond structure in arenes cause that they are very stable, and the ring reactivity is relatively low. The characteristic reaction of aromatic hydrocarbons is the electrophilic substitution in the aromatic ring (Fig. 2.2): In alkyl derivatives of benzene, there are aliphatic (side chains) and aromatic fragments (rings). Reactions can occur in both parts of a particle. However in practice, reactions of electrophilic substitution in benzene alkyl derivatives and in other aromatic hydrocarbons occur always only in rings. Oxidation of Aromatic Hydrocarbons At first, side chains will be oxidized without disturbing the aromatic system. It is possible to obtain benzoic acid from toluene and dicarboxylic acids from xylene (Fig. 2.3).

106.17

106.17

C6H4(CH3)2

C6H4(CH3)2

C6H4(CH3)2

o-Xylene

m-Xylene

p-Xylene

106.17

106.17

C6H5C2H5

Ethylbenzene

Molecular mass [g/ mol] 78.11

92.14

Chemical formula C6H6

C6H5CH3

Structural formula

Toluene

Compound Benzene

Table 2.8  Physical properties and structures of aromatic hydrocarbons from BTEX group

0.861

0.861

0.880

0.867

0.873

Density at 20 °C [g/ cm3] 0.874

136.2

144

138

−95

−25

−48

138.3

110.6

−95

13

Boiling point [oC] 80

Melting point [oC] 5.5

34 2  General Principles of Organic Chemistry

2.3 Hydrocarbons

35

2.3.6.1  Aromatic Hydrocarbons from BTEX Group Out of aromatic hydrocarbons, two groups of substances, which usually occur in the casting production  – mainly in molding sands with bentonite and with additions containing carbon or in chemically bound molding sands – will be discussed. BTEX is an acronym, related to the small group of aromatic hydrocarbons: benzene, toluene, ethylbenzene, and xylene (Table 2.8). These are the so-called volatile organic compounds (VOCs)3 . They are obtained from the crude oil treatment and are applied mainly as solvents. All these substances are harmful for health and can negatively influence the central nervous system. . Benzene (carcinogenicity 1A can cause cancer, while mutagenicity 1B can cause genetic disorders) is a volatile, colorless, flammable liquid. It enters organisms by airways, but can be also – to a limited degree – absorbed by the skin and from the alimentary canal. It is a dangerous substance due to its flammability as well as toxicity (inhaling benzene causes pathological blood changes, which can lead to leukemia [8]). Benzene toxic influences cause narcotic effects in the central nervous system. Benzene poisoning, in its initial phase, causes sensitization of mucous membranes of airways and eyes. However, the strongest symptoms are the result of benzene influencing the central nervous system. Other symptoms of chronic benzene poisoning are disturbances of the alimentary canal and hematopoietic necrosis (lack of appetite, headaches, drowsiness, excitability). Toluene (carcinogenicity 3  – toxic for reproduction) is a colorless liquid of a characteristic smell, twice less volatile than benzene. Toluene is absorbed by airways and in a liquid state also by the skin. Its toxic influence is similar to the benzene influence. However, toluene influences more strongly the nervous system and significantly weaker the hematopoietic system. It causes different changes than benzene, e.g., it does not cause leukemia. Acute poisonings by toluene can cause headaches, imbalances, nausea, vomiting, and loss of consciousness. Chronic poisonings are manifested by mucous membrane irritations, throat inflammation, and dizziness. Ethylbenzene is mainly absorbed by airways. It irritates mucous membranes of the eyes, nose, and airways. At high concentrations it can influence the central nervous system. Dizziness and loss of consciousness can sometimes occur. A long contact with the skin can cause its inflammation and/or allergic effects. Xylenes (o-xylene, m-xylene, p-xylene) are colorless liquids, flammable, and harmful for the human organisms. Xylenes are absorbed by the respiratory system, from the intestinal tract and through a skin. They have irritating and depressive effects on the central nervous system. At high concentrations they have narcotic effects causing cardiac rhythm disturbances, loss of consciousness, and death. At a lower poisoning level, the person is tired, is dizzy, and has difficulty breathing. Substance vapors irritate mucous membranes of the eyes and throat. Disturbances of the digestive tract such as loss of appetite, vomiting, and diarrhea also occur. 3  According to the Directive 2004/42/EC (Paints Directive) [28], the name volatile organic compound (VOC) means any organic compound of the boiling temperature lower than, or equal, 250 °C, measured under the standard pressure, being 101.3 kPa.

36

2  General Principles of Organic Chemistry

2.3.6.2  Polycyclic Aromatic Hydrocarbons (PAHs) The numerous groups of aromatic hydrocarbons contain condensed systems of benzene rings. Among them, the numerous and dangerous group constitute polycyclic aromatic hydrocarbons (PAHs) of a ring structure characterized by similar physical–chemical properties [9]. PAHs – in a pure state – occur as colorless, white, bright yellow, or bright green crystals. These substances are slightly soluble in water, while much better soluble in organic solvents. More than 100 various PAHs are known. The most often 17 PAHs occur in the environment. Their physical–chemical properties are presented in Table 2.9. PAHs are mainly formed in pyrolysis of organic substances occurring in several industrial processes and also under conditions of not complete burnings (e.g., forest fires, waste incineration, car exhausts, cigarette fumes). A majority of PAHs occur as vapors or aerosols in the air. PAHs from the air are depositing on dust particles of equivalent grain diameters, being approximately 0.5 nm, and together with them are falling down on soils, vegetables, and surface waters. PAH substances of a lower molar mass have certain volatility, while substances of a higher molar mass are solids. These substances do not occur as individual ones but always as mixtures. When in the environmental test one substance from this group is found, it means that other PAH substances are also present. PAH substances occur in all elements of the human environment: in the air, water, soil, and food. Thus, the exposure to their influence is universal. They can reach the human body during eating, by inhaling, and through a skin. At the occupational exposure, the main way of absorbing PAHs is the respiratory system into which less volatile substances, including benzo(a)pyrene, enter as aerosols adsorbed on dust particles (most often respirable). At the environmental exposure, the main way of absorbing PAHs is the gastrointestinal tract. These substances as the air contaminants arouse more and more interests since some of them are highly carcinogenic or mutagenic (Table  2.9). The most dangerous substance from the PAH group is benzo(a)pyrene, since it is widespread and highly carcinogenic.

2.4  Hydrocarbons with Functional Groups 2.4.1  Alcohols and Phenols Alcohols are organic compounds in which molecules of one or more hydrogen atoms are substituted by hydroxyl functional group –OH. This –OH group can be connected either to carbon from the aliphatic chain in the hybridization degree sp3 (aliphatic alcohols) or to carbon from the side aliphatic chain in the aromatic ring (aromatic alcohols).

152.20

166.22

C12H8

C13H10

Fluorene

154.21

Molecular mass [g/mol] 128.16

Acenaphthylene

Chemical formula C10H8

C12H10

Structural formula

Acenaphthene

Aromatic hydrocarbona Naphthalene

Table 2.9  Physical properties and the structure of the selected substances from PAHsa group

114.8

92.5

93.4

Melting point [oC] 80.2

295

280

279

3

3

(continued)

Boiling point Carcinogenicity [oC] Group (IARC) 217.9 2B

2.4 Hydrocarbons with Functional Groups 37

202.26

202.26

C16H10

C16H10

Pyrene

178.24

Molecular mass [g/mol] 178.22

Fluoranthene

Chemical formula C14H10

C14H10

Structural formula

Phenanthrene

Aromatic hydrocarbona Anthracene

Table 2.9 (continued)

151.0.2

107.8

99.2

Melting point [oC] 215

404

384

340

3

3

3

Boiling point Carcinogenicity [oC] Group (IARC) 339.9 3

38 2  General Principles of Organic Chemistry

252.32

252.32

C20H12

C20H12

Benzo(a)pyrene

Benzo(b) fluoranthene

Molecular mass [g/mol] 228.28

228.29

Chemical formula C18H12

C18H12

Structural formula

Benzo(a)anthracene

Aromatic hydrocarbona Chrysene

168

176.5

84

Melting point [oC] 258.2

357

495

437.6

2B

1

2B

(continued)

Boiling point Carcinogenicity [oC] Group (IARC) 448 2B 2.4 Hydrocarbons with Functional Groups 39

252,0.32

C20H12

C20H12

Benzo(k) fluoranthene

Benzo(j) fluoranthene

252.32

Molecular mass [g/mol] 252.32

Structural formula

Chemical formula C20H12

Aromatic hydrocarbona Benzo(e)pyrene

Table 2.9 (continued)

166

217

Melting point [oC] 177.5

490

480

2B

2B

Boiling point Carcinogenicity [oC] Group (IARC) 492 3

40 2  General Principles of Organic Chemistry

a

PAH identified as probable human carcinogens by the EPA [29]

C22H12

Indeno(1,2,3-cd) pyrene

Chemical formula C22H14

C22H12

Structural formula

Benzo(g, h, i) perylene

Aromatic hydrocarbona Dibenzo(a, h) anthracene

276.34

276.34

Molecular mass [g/mol] 278.35

163.6

278

Melting point [oC] 269.5

536

>500

2B

3

Boiling point Carcinogenicity [oC] Group (IARC) 524 2A 2.4 Hydrocarbons with Functional Groups 41

42

2  General Principles of Organic Chemistry

Phenols are compounds in which hydroxyl –OH group is connected directly to carbon from the aromatic ring. This structural difference is the reason of different chemical properties of alcohols and phenols: R R′

C

OH HO

R″

phenol

alcohols

where R′; R″ – alkyl groups. Aliphatic Alcohols Alcohols are classified as primary, secondary, or tertiary in dependence on the number of organic groups connected to the carbon atom with OH group:

Alcohols can be considered the derivatives of alkanes, in which the hydrogen atom was substituted by –OH group, or the derivatives of water formed as the result of exchanging the hydrogen atom with alkyl residue:

Names of alcohols (alcanols), according to the IUPAC, are formed by adding the ending -ol to the name of the parent hydrocarbon, which is considered hydrocarbon of the longest normal carbon chain, containing also the carbon atom connected with –OH group. The numeration is selected in such way as to have the carbon atom connected with –OH group of the smallest possible number. Thus: • In the group of monohydric alcohols:

2.4 Hydrocarbons with Functional Groups

43

• In the group of polyhydric alcohols:

Aromatic Alcohols By the substitution of the hydrogen atom of the alkyl radical of any aliphatic alcohol by the aryl radical, the aromatic alcohol is obtained: CH3 CH2

OH

CH

-phenylethyl alcohol

benzyl alcohol

CH2

OH

CH2

OH

-phenylethyl alcohol

Alcohols: α-phenylethyl alcohol and β-phenylethyl alcohol are isomers. Phenols Phenols are hydroxylic derivatives of benzene. Names of phenols according to the IUPAC system are formed of the prefix “hydroxy-” and the hydrocarbon name. Positions of substituents are marked by proper numbers: OH

OH

OH

OH

CH3

CH3 Phenol

2-methylphenol o-cresol

3-methylphenol m-cresol

CH3 4-methylphenol p-cresol

44

2  General Principles of Organic Chemistry

Alcohols and phenols have significantly higher boiling points as compared with other substances of similar molecular masses. This is due to hydrogen bonds forming in a liquid state [10, 11]. The positively charged hydrogen atom from -OH group of one particle is attracted by the negatively charged oxygen atom of another particle. This weak attraction between particles keeps them together, and in result the aggregates of R–O–H particles are formed. These intermolecular influences must be overcome by a particle freeing itself from the liquid state and entering into the vapor state. Therefore the boiling point increases. The same effect occurs in the case of water. The solubility of alcohols and phenols in water is decided by the solvent interaction with the polar hydrophilic –OH group and with nonpolar, hydrophobic hydrocarbon fragment of a particle. In the case of short carbon chains (to C4), the hydrophilic character prevails, and monohydric alcohols are mixed with water in every ratio. Along with the chain length increase (above C4), water solubility of alcohols decreases. Simultaneously, the particle polar character gives alcohols properties of good solvents of organic and inorganic substances. Alcohols and phenols have acidic properties; however they are not acids, because their water solutions are not acidic. Acidic properties of these substances are seen, among others, in their reactions with metals (Eq. 2.30):



2 R − O − H + 2 Me → 2 R − O − Me + H 2 alcohol metal alcoholate or or phenol ( e.g.,Li,Na,Mg,Al ) metal phenolate

(2.30)

Alcoholates belong to strong bases (stronger than hydroxides of alkali metals). Phenols indicate higher acidity than alcohols and are stronger acids than water, but weaker than carboxylic acids. Different acidity of alcohols and phenols is related to a different stability of substrates and dissociation products of these substances (Eq. 2.31):

(2.31)



Basic properties of alcohols and phenols are revealed in esterification reactions, in which these substances react with acids or their derivatives (in the case of phenols). That time the hydrogen atom from –OH group is changed by the group of atoms originated from an acid. Alcohols are susceptible to oxidation. As a result of the primary alcohols, oxidation aldehydes (or carboxylic acids) are obtained (Eq.  2.32), while oxidation of secondary alcohols leads to the formation of ketones (Eq. 2.33):

45

2.4 Hydrocarbons with Functional Groups

R

CH2

OH

[O] [H]

primary alcohol

R

C

O

[O] [H]

H

aldehyde

R CH

OH

R

[O]

R

O

C

OH

carboxylic acid

(2.32)

R C R

O + H 2O

(2.33)

secondary alcohol

ketone



2.4.2  Ethers Ethers can be considered to be water derivatives as well as alcohol derivatives. Two hydrogen atoms in water are substituted by an alkyl or aryl group, while in alcohols one hydrogen atom in -OH group is substituted by a radical, which means that two organic groups are connected with the same oxygen atom: R–O–R. The bond system C–O–C occurs in ethers, but not all substances containing this structural element are counted to ethers.

When both substituents connected with oxygen  – in ethers  – are the same (R = R′), ethers are called symmetric ethers, and when they are different (R ≠ R′), ethers are called mixed ethers. In dependence on the character of substituents, there are chain ethers, cyclic ethers, aliphatic saturated and non-saturated ethers, and aromatic and aliphatic–aromatic ethers. Ethers belonging to individual groups are shown in Table 2.10. Due to the presence of O atom in their molecules, ethers can form hydrogen bonds with substances containing H atoms, e.g., with water. Ethers, due to their polarity and ability to form hydrogen bonds, are good solvents of several organic substances. Ethers are well soluble in strong mineral acids, forming unstable oxonium salts. The common feature of all ethers is their alkalinity, being the result of the free electron pair presence at the oxygen atom. Ethers are chemically passive compounds, and they do not react with a majority of chemical reagents.

46

2  General Principles of Organic Chemistry

Table 2.10  Examples of aliphatic and aromatic ethers CH3 − O − CH3 C2H5 − O − C2H5 dimethyl ether diethyl ether

CH3CH2 − O − CH = CH2 ethyl vinyl ether

O

O - CH3

CH3

CH3

C

O

H phenyl methyl ether

phenyl ether

phenyl isopropyl ether

When working with ether, a special caution should be exercised since ether forms explosive mixture with the air.

2.4.3  Aldehydes and Ketones Aldehydes – organic substances having carbonyl group connected with at least one hydrogen atom. Such group is also called the aldehyde group. Carbonyl group  – functional group occurring in several types of organic substances, consisting of a carbon atom connected by double bond with an oxygen atom.

carbonyl group

C

O

C

O

(Ar)R aldehyde group H

O H

C H

methanal (formaldehyde)

O

H

where: R - hydrogen atom or alkyl group (R) or aromatic group (Ar)

O CH3C

CH3(CH2)2C H

ethanal

butanal

O CH

benzaldehyde

2- naphthalenecarboaldehyde

O

CH3

H

CH3CHC

O H

2-methylpropanal

2.4 Hydrocarbons with Functional Groups

47

Ketones  – substances containing carbonyl group connected with two carbon atoms. general formula for ketone:

(Ar) R1 C

O where: R1, R2 - alkyl group Ar - aromatic group

(Ar) R2

Names of ketones end by ending with -ne. O

CH3

C

CH2

O

CH3

C

C

O

H3C CH3 propanone

butanone

benzophenone

Properties of the carbonyl group, which is highly reactive, decide on chemical reactions of aldehydes and ketones. Aldehydes and ketones (up to four carbon atoms) are fully or partially dissolved in water. They dissolve in a majority of organic solvents. Aldehydes and ketones are very weak bases. Oxidation of Aldehydes and Ketones Aldehydes are easily oxidized to carboxyl acids, according to the formula (Eq. 2.34): O

O C

CH3

CH3

[O]

C2H5

OH

(2.34)

propanoic acid

propanone



C

Oxidation occurs easily under an influence of such substances as Na2Cr2O7, KMnO4, and HNO3. Aldehydes undergo slow oxidation also under an influence of atmospheric oxygen. Ketones do not subject to influences of a majority of oxidizers but undergo fission reactions under an influence of hot alkaline solution of KMnO4. C–C bond, at the carbonyl group, undergoes tearing, and carboxylic acids of shortened carbon chains are formed (Eq. 2.35):

O CH3

C

CH3

K2Cr2O2

4H2SO4

propanone

O CH3

C

acetic acid

H

O H

C

K2SO4 OH

formic acid

Cr2(SO4)3

4H2O

(2.35)

48

2  General Principles of Organic Chemistry

Hydrogenation of Aldehydes and Ketones Aldehydes and ketones are hydrogenated (reduction of carbonyl group) in the presence of catalysts such as palladium, activated platinum, and nickel. Hydrogenation of aldehydes leads to the formation of primary alcohols, while hydrogenation of ketones leads to secondary alcohols. As an example (Eq. 2.36a, 2.36b):

(2.36a)



(2.36b)



Lower aldehydes, especially formaldehyde and acetaldehyde, are characterized by a significant inclination to polymerization, in which particles of the backbone – built of alternately repeating carbon and oxygen atoms – are formed. In dependence on conditions, formaldehyde is polymerizing to linear products of various molecular masses or forming ring-trimmer called trioxane (Eq. 2.37). O (CH2O)3

H2C

CH2 O

O CH2



formaldehyde

(2.37)

1,3,5, - trioxane

2.4.4  Carboxylic Acids Connection of carbonyl group with hydroxyl one forms the carboxylic group characteristic for carboxylic acids:

2.4 Hydrocarbons with Functional Groups

49

O C OH carboxylic group A mutual influence of carbonyl and hydroxyl group, situated at one carbon atom, is the reason that properties of carboxylic acids are neither the sum of ketone and alcohol properties nor the sum of aldehyde and ketone properties (Table 2.11). Carboxylic acids have boiling points much higher than other compounds of similar molecular masses. This temperature increase is the result of the fact that in the liquid state, particles of carboxylic acids are strongly associated due to intermolecular hydrogen bonds. These bonds are much stronger in acids than in alcohols. Aliphatic acids, to C4 (e.g., formic, acetic) are well water soluble. As far as the hydrophobic acid radical increases, the water solubility decreases. Chemical Properties of Carboxylic Acids Carboxylic acids in water solutions are dissociating, according to the formula (Eq. 2.38): O R



O H2O

C OH

R

H3O+

C O-

(2.38)



Carboxylic acids influence metals causing their oxidation and the evolution of free hydrogen (Eq. 2.39):

2CH 3 COOH + Zn → ( CH 3 COO )2 Zn + H 2

(2.39)



They are reacting with hydroxides, oxides, and metal carbonates (Eq.  2.40a, 2.40b, 2.40c):

CH 3 COOH + NaOH → CH 3 COONa + H 2 O



2CH 3 COOH + HgO → ( CH 3 COO )2 Hg + H 2 O



2CH 3 COOH + CaCO3 → ( CH 3 COO )2 Ca + H 2 O + CO2

(2.40a) (2.40b)



(2.40c)

Carboxylic acids are weaker acids than mineral ones, however stronger than carbonic acid. Carboxylic acids, apart from a few exceptions, are resistant to oxidizers. Heating of some carboxylic acids causes their decomposition and evolution of carbon dioxide (decarboxylation reaction) (Eq. 2.41a, 2.41b):

OH

OH

OH

COOH

OH

(CH2)3(COOH)2 glutaric acid

(CH2)2(COOH)2 succinic acid

nicotinic acid

N

O

propionic acid

CH2

naphthoic acid

acetic acid

CH3

hydroxybenzoic acid Dicarboxylic acids

OH

COOH

benzoic acid

O

formic acid

H

C

CH3

C

C

OH

O

O

O

Table 2.11  Examples of carboxylic acids (monocarboxylic acids)

OH

(CH2)4(COOH)2 adipic acid

cyclopentanecarboxylic acid

H

C

O

50 2  General Principles of Organic Chemistry

C6H4(COOH)2 phthalic acid

C6H4(COOH)2 terephthalic acid

O

HO (COOH)2 oxalic acid

OH

O

2.4 Hydrocarbons with Functional Groups 51

52

2  General Principles of Organic Chemistry

(2.41a) O C C



OH

ΔT

CH3

C

O 3-phenyl-3-oxopropanoic

CO2

CH3

O 1-phenyl-ethanone

(2.41b)

When −OH group in carboxylic acid is substituted by other group, the following can be obtained: acid chlorine (−Cl), acid anhydride (−OCOR), amide (−NH2), or ester (−OR) (Fig. 2.4).

2.4.5  Esters of Carboxylic Acids Esters constitute the group of substances, derivatives of carboxylic acids in which hydrogen atom is substituted by the ester group.

O C OH R carboxylic acid

O

O

C

R

Cl R acid chloride

O R

C

C O R anhydride

Fig. 2.4  Derivatives of carboxylic acid

amide

O

O R

C cster

C

OR

NH2

2.4 Hydrocarbons with Functional Groups

53

O C O Ester group

Ester is formed in the reaction of carboxylic acid with alcohol. This is a reversible reaction, which  – after a longer time  – achieves the state of equilibrium (Eq. 2.42): O R

O

H R1

C

OH

R

H2O

C

OH



(2.42)

OR1

acid

alcohol

water

ester

This reaction is catalyzed by strong mineral acid (e.g., sulfuric), which simultaneously binds water formed in this reaction. A majority of aliphatic and aromatic carboxylic acids undergo esterification (Eq. 2.43):

O

O +

C CH3 acetic acid

OH

OH + O

CH2 CH3

CH2 CH2

+

O

= phenol

CH2 CH2

CH2

CH3

+

H2O

ester: butyl acetate O

OH +

benzoic acid

C CH3

=

butyl alcohol OH

H+

=

O

(2.43)

phenyl benzoate

The hydrolysis reaction belongs to the most important transformations of esters. It can occur as: • Reversible acidic hydrolysis (Eq. 2.44):

(2.44)



• Irreversible basic hydrolysis (saponification) where carboxylic acid salt and alcohol are products (Eq. 2.45):

54

2  General Principles of Organic Chemistry O

O R

+

C

NaOH

R

OR1



+

C

R1OH

ONa

(2.45)



When alcohol and inorganic acid take part in the reaction, inorganic ester is formed (Eq. 2.46): O CH3

CH2

HNO3

CH2 OH

CH3

N O

O

(2.46) Esters of dicarboxylic acids

CH₃-OOC-(CH₂)₃-COO-CH₃

dimethyl ester of glutaric acid

C2H5- OOC-(CH₂)2- COO-C₂H₅.

dimethyl ester of succinic acid

CH₃-OOC-(CH₂)4-COO-CH₃

dimethyl ester of adipic acid

Esters are very weak bases, only slightly stronger than aldehydes and ketones. They also have a weak acidic character. Ester in reaction with ammonia is turning into amide and alcohol (Eq. 2.47):



R − COO − R ′ + NH 3 → R − CO − NH 2 + R ′ − OH ester ammonia amide alcohol

(2.47)

Fats – are esters of higher aliphatic carboxylic acids and trihydroxide alcohol (glycerin). The general formula is as follows (acid radicals R1 ≠ R2 ≠ R3) (Eq. 2.48):

55

2.4 Hydrocarbons with Functional Groups



(2.48) Examples of higher fatty acids: Saturated:  Palmitic acid CH3(CH2)14COOH  Stearic acid CH3(CH2)16COOH  Arachic acid CH3(CH2)18COOH

Unsaturated:  Oleic acid CH3(CH2)7CH = CH(CH2)7COOH  Linoleic acid CH3(CH2)4CH = CHCH2CH=CH(CH2)7COOH

An example of obtaining solid fat is given below:

CH2

OH

CH

OH

CH2

OH

glycerol

3C15 H31COOH

palmitic acid

CH2

OCOC15H31

CH

OCOC15H31

CH2

OCOC15H31

H

3H2O

glycerol tripalmitate (solid fat)

Waxes – are esters of saturated fatty acids (often containing more than 20 carbon atoms) and alcohols of not branched long chains (up to more than 30 carbon atoms). The general formula of waxes:

56

2  General Principles of Organic Chemistry

2.4.6  Nitrogen Organic Compounds Amines Amines can be considered derivatives of ammonia, in which one or a few hydrogen atoms are substituted by alkyl (R) or aryl (Ar) radicals. Amines are classified on the bases of the number of substituents connected with nitrogen atom. Orders of amines: H H

R1

N

H

N

H

H H

ammonia

primary amine

R1 N

R4

R1 R3

N

R2 secondary amine

R1

R2 tertiary amine

N R2

R3

quaternary amine

Aliphatic Amines H3C–NH2 CH3–CH2–NH2 H3C–NH–CH3 CH3–CH2–NH–CH3

Methylamine (a primary amine) Ethylamine (a primary amine) Dimethylamine (a secondary amine) Ethylmethylamine (a secondary amine) Trimethylamine (a tertiary amine)

Aliphatic amines form salts with organic and inorganic acids (Eq. 2.49):

RNH 2 + H + = RNH 3 + ( alkyl ammonium cation )



(2.49)

They undergo electrolytic dissociation in water (Eq. 2.50):

RNH 2 + H 2 O = RNH 3 + + OH −

(2.50)

2.4 Hydrocarbons with Functional Groups

57

Primary and secondary amines undergo alkylation and secondary and tertiary amines are formed (Eq. 2.51): RNH 2 + R – X = R 2 NH 2 X



( ammonium salt )

(2.51)

X – halogen or acid radical of sulfuric or sulfonic acid. Primary and secondary amines are sensitive to the influence of various oxidative substances. Amines have relatively low boiling points. Aliphatic amines are better water soluble than alcohols. Amines of low molecular masses have characteristic smells, similar to the fish smell. Amines are widely present in nature. Trimethylamine occurs in animal tissues. Nicotine is a known substance contained in tobacco. Cocaine is a well-known stimulant. Aromatic Amines

aminobenzene dimethylaniline naphthylamine. Amines have basic properties, in a similar fashion as ammonia, in addition to which aliphatic amines indicate higher basic properties than ammonia while aromatic amines weaker basic properties than ammonia. Acid Amides Acid amides are derivatives of carboxylic acids, in which –OH group from the carboxyl group is substituted by –NH2 group:

O

O R

R

C O

H

C N

H

H carboxylic acid

amide of carboxylic acid

Acid amides are divided into I, II, and III order (R1 and R2  – alkyl and aryl substitute):

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2  General Principles of Organic Chemistry

O

O R1

R1

C

C N

H

N

O R1

C

H

N

R2

H Primary amide

R3

R2

secondary amide

tertiary amide

Examples of acid amides: CH3

O

C

C H

O

O

amide of formic acid

C NH2

H

NH2

O

C

H

N

amide of acetic acid

OH

NH2

amide of benzoic acid amide of salicylic acid

Amides undergo hydrolysis, both acidic and basic. Hydrolysis in the acidic environment under an influence of strong inorganic acids (Eq. 2.52): CH3

O

C

NH2

amide of acetic acid

H2SO4

H2O

CH3

C

O

NH4HSO4

O

sulfuric acid (VI)

acetic acid

acidic ammonium sulfate (VI)

(2.52)

Hydrolysis in the basic environment under an influence of alkali metals (Eq. 2.53): CH3

C

O NH2

amide of acetic acid

NaOH

CH3

C

O

ONa sodium hydroxide

sodium acetate

NH3

(2.53)

ammonia

Urea Amide derivatives of carbonic acid constitute the separate group. Carbonic acid, due to the presence of two carboxyl groups in its particle, can form organic derivatives, e.g., urea (diamide of carbonic acid):

2.4 Hydrocarbons with Functional Groups

HO

59

O

O

C

C OH

carbonic acid

H2N

NH2

urea (carbamide)

Urea is a colorless crystalline substance and well water soluble giving solution of neutral pH. It undergoes hydrolysis: • In an acidic environment under an influence of strong inorganic acids (Eq. 2.54):

(2.54)



• In the basic environment under an influence of alkali metals (Eq. 2.55):

(2.55)



As the result of the condensation of two urea particles, the diurea particle called biuret, which contains peptide bond, is formed (Eq. 2.56):

(2.56) Nitro Compounds Nitro compounds are organic compounds containing nitro group −NO2, in which the nitrogen atom is directly connected with the carbon atom. Aliphatic nitro compounds are divided into primary, secondary, and tertiary compounds in dependence of the order of carbon with which NO2 group is connected.

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2  General Principles of Organic Chemistry

General formulas of aliphatic nitro compounds of I, II, and III order:

Examples of nitro compounds:

Nitro compounds are usually difficult to dissolve in water. Their solubility decreases when molecular mass increases. Aromatic nitro compounds are highboiling liquids or solids insoluble in water. Nitro compounds are reduced to primary amines [4, 9]. The most important representative of nitro compounds is 2,4,6-trinitrotoluene, i.e., C6H2(NO2)3CH3. It is obtained by two-stage nitration of toluene by a mixture of sulfuric and nitric acids. Nitriles Nitriles are organic substance derivatives of hydrogen cyanide, containing nitrile group –C ≡ N . Their general formula is R –C ≡ N, where R means either alkyl or aryl. Lower nitriles are quite well water soluble:

In acidic or basic environments, nitriles hydrolyze into carboxylic acids (Eq. 2.57):

R − CN + 2H 2 O → RCOONH 4



RCOONH 4 + HCl → RCOOH + NH 4 Cl

(2.57)

2.4 Hydrocarbons with Functional Groups

61

Nitriles subjected to reduction form primary amines: R − CN + 2H 2 → R − CH 2 − NH 2



2.4.7  Sulfuric Organic Compounds Sulfuric acids are substances containing sulfo group SO2OH.  This group has a highly acidic character, of a power comparable with a power of inorganic acids. One compound can contain a few sulfo groups forming polysulfuric acid.

SO3H

SO3H

SO3H H3C

benzenesulfonic acid

CH3

CH3

p-toluenesulfonic acid

m-toluenesulfonic acid

SO3H

o-toluenesulfonic acid

Aliphatic sulfonic acids are obtained, among others, in reaction of sodium sulfite with alkali chlorides (Eq. 2.58): R − Cl + Na 2 SO3 → R − SO2 OH



(2.58)

Aromatic sulfonic acids are obtained by direct sulfonation by concentrated sulfuric acid or oleum. Sulfonation reaction of toluene by sulfuric acid (Eq. 2.59):

CH3

CH3 H2SO4

toluene

H2O SO3H p-toluenosulfonic acid

(2.59)

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2  General Principles of Organic Chemistry

Evolved water dilutes sulfuric acid and thereby influences the reaction inhibition. Therefore this reaction is often performed at temperatures above 100 °C. A specific meaning of sulfonic acids is based on the fact that sulfonic group can be easily substituted by other functional groups. At melting sodium or potassium salts of aromatic acids with strong hydroxide (NaOH), the exchange of the sulfonic group into hydroxyl one occurs. In result phenol and sodium sulfate are formed (Eq. 2.60):

C6 H 5 – SO3 Na + NaOH → C6 H 5 OH + Na 2 SO3

(2.60)

2.5  Polymers and Polymerization 2.5.1  Polymers and their Properties Polymer is a high-molecular substance, built of organic or inorganic macromolecules. It consists of a large number of joined together basic structural units – monomers. The number of monomers can be from 10,000 to 1,000,000. In general, the polymerization reaction can be written as:



nA + nB → A·B·A·B·A·B·A → [ A B ] p monomer monomer polymer mer

where p – polymerization degree. Macromolecules can be joined together by means of covalent, hydrogen, ionic, or coordinate bonds. The kind of bonds connecting macromolecules decides on physical–chemical, mechanical, processing, and functional properties of polymers [12]. Polymerization degree – number of monomers (including also the chain beginning and end) contained in one macromolecule. On account of their origin, polymers are divided into [13, 14] (Fig. 2.5): • Natural polymers (biopolymers) are high-molecular substances produced by organisms: proteins (keratin, collagen, natural silk, casein), nucleic acids (DNA, deoxyribonucleic acid, and RNA, ribonucleic acid), natural rubbers (polyisoprene, natural rubbers of vegetative origin), and polysaccharides (e.g., cellulose, starch, glycogen, chitin, chitosan, pectin). • Synthetic polymers are polymers chemically produced of monomers. • Modified polymers are natural or synthetic polymers, which surface structure or the whole mass was changed by means of chemical or physical way. The term plastics determines material, in which, apart from the basic component, polymer, additional components (fillers, plastifiers, stabilizers, dyes, and others) are scattered.

2.5 Polymers and Polymerization

63

Fig. 2.5  Basic division of polymers, adapted from [15]

In respect of their physical structure, polymers can be divided into: • Thermoplastic polymers: of a linear or branched structure (crystalline or amorphous), usually soluble in organic solvents, are melting and flowing, and can be heated (below the degradation temperature) and cooled several times. A majority of polymerizing materials belong to thermoplasts and – in addition – polyamides, polycarbonates, polysulfones, thermoplastic derivatives of cellulose, and polyesters. • Thermosetting polymers and chemically cured polymers (duroplasts): occur only in the glass state, have a high cross-linking degree, and are insoluble in organic solvents. They do not melt and flow and, when heated above a thermosetting temperature, undergo usually a thermal degradation. The most important representatives of this group of materials are phenol–formaldehyde resins and amino plastics. Phase transformations of polymers, the so-called secondary transformations: occur at the determined temperature, characteristic for the given polymer. Thus, there is: • Brittle temperature Tb – temperature of transition from the brittle glass state into glass state with forced elasticity.

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2  General Principles of Organic Chemistry

• Flow temperature Tf – temperature of transition from the elastic state into plastic state (and vice versa). • Glass temperature Tg – temperature of transition from the glass state into elastic (and vice versa). • Melting temperature Tm (melting is the phase transformation of the first kind) – temperature of transition from the crystalline state into liquid state. Polymers are produced by: • Chain polymerization (previous name: polyaddition) – it means joining of several monomer particles into one macromolecule, without emitting side products. • Gradual polymerization (previous name: polycondensation) – it means joining of several particles containing reactive groups into one macromolecule with emitting low-molecular side products (e.g., HCl, H2O, etc.). • Synthesis of macromolecules by living organisms  – gradual polymerization which is much slower than the chain polymerization. • Chemical modification of natural or synthetic polymers. The kind and properties of the formed polymer depend mainly on the applied monomer. As the result of the polymerization of two functional monomers, containing in their particles either one double bond or two reactive groups, the linear polymers, which can melt and dissolve in solvents of a similar polarity, are formed (Fig. 2.6).

Fig. 2.6  Dependence of the polymerization degree on the conversion degree of the chain polymerization (curve 1) and gradual polymerization (curve 2). (Adapted from Ref. [16])

2.5 Polymers and Polymerization

65

2.5.2  Polymerization: Kinds and Mechanisms Polymerization is the chemical process in the result of which the polymer is obtained from monomers. This process occurs by multiple repetitions of a simple chemical reaction, which constitute the so-called polyreaction. Synthetic polymers are divided, in dependence on their synthesis process, into polymers formed according to the mechanism [5, 15] of: • Chain polymerization (chain or additive polymers): Each reaction of chain elongation in the chain polymerization is related to joining only one monomer particle. Its main property is the occurrence of the reactive place (called the active center) in a particle – able to join the substrate – generally at the chain end [17]:

−M ∗ + M → −M − M ∗

The created, reactive intermediate product attacks the next particle with C=C bond, and the new reactive product is formed, which attacks the next monomer particle, etc. The active center constitutes the most often radical, cation, anion, or a strongly polarized covalent bond. On the industrial scale, by means of the chain polymerization can be obtained, among others, polystyrene, polyethylene, and polyvinylchloride. Generally, monomers with multiple bonds (double, triple) or reacting with the ring opening undergo the chain polymerization. The chain polymerization is characterized by a lack of a direct dependence between the polymerization degree and the conversion degree of the monomer functional groups. The following stages can be singled out in the chain polymerization process [18]: • Initiation of the polymerization. • Increase of the polymer chain (propagation). • Chain ending (termination). Initiation reaction (formation of active centers) – the monomer particle is supplied with energy (e.g., by heating or lightening) necessary for its activation (e.g., for a double bond breaking). Sometimes a special chemical substance, the so-called initiator which undergoes disintegration, is added to the system. As the result, an unstable intermediate product is formed (radical or ion). It connects itself with the monomer particle and starts the chain increasing period leading to macromolecule formations. The polymer chain increase can be stopped either by destruction of active centers or by transferring the activity from the macromolecule being formed into the monomer or solvent particle (the so-called chain transfer). Such chain transfer causes an increase of the next macro-radical, while the polymerization rate is not changed. Usually reactions of the chain ending or chain transfer occur spontaneously, due to various chemical reactions with solvent, random contaminations,

66

2  General Principles of Organic Chemistry

oxygen, or growth controllers introduced on purpose. The chain process is finally ended in the reaction absorbing a radical, e.g., the recombination reaction. In dependence on the element initiating the chain polymerization process, the following chain polymerizations are singled out: • • • • •

Free radical polymerization. Cationic polymerization. Anionic polymerization. Coordination polymerization. Polymerization with a ring opening.

Radical Polymerization Radical polymerization is the most useful industrial method of obtaining  – on a large scale – different kinds of polymers. More than 50% of all plastics are produced by this method. The radical polymerization mechanism contains several elementary processes, as the result of which a polymer is obtained from a monomer [15]. Initiation of Polymerization Initiation of the radical polymerization reaction occurs by means of free radicals (R•), which after joining with a monomer particle (M) form initiation radicals (RM•):

R • + M = RM•

These radicals react with successive monomer particles causing a fast growth of polymer macro-radicals (RMn + 1)

RM• + nM = RM n +1

Radicals are electrically neutral fragments of particles with an individual unpaired electron. They are usually unstable, but are characterized by a high chemical activity. Reactions with a participation of radicals can lead either to forming of other radicals, i.e., new particles with an unpaired electron, or to a decay of the radical active center. The radical sources constitute mainly initiator particles, which undergo thermal or photolytic dissociation. Free radicals are also formed in redox reactions. The radical polymerization of styrene consists of the following stages: (a) Radical formation of dibenzoyl peroxide, which at temperatures 40–90  °C undergoes thermolysis to benzoyl radical and then to phenyl radical [10, 15, 18, 19]. Both radicals are taking part in the polymerization (Fig. 2.7). (b) Beginning of the chain formation: benzoyl and phenyl radicals formed in the decomposition of the initiator are joining the monomer particle in the double bond place, forming a new radical. New radicals initiate the chain growth reaction, with various rates depending on their activity (Eq. 2.61).

67

2.5 Polymers and Polymerization . C

O

O

O

C

2

C

O.

+ 2 CO2

2

O

O

dibenzoyl peroxide

benzoyl radical

phenyl radical

Fig. 2.7  Thermal decomposition of dibenzoyl peroxide

(c) R

CH2

Radical



CH

R

CH

CH2

(2.61)

Styrene



where R·– initiator radical. Free radicals, being initiators, can be also created in a photochemical process. This process is based on absorption of the light energy quant by a monomer particle and its transition into the excited state. A main advantage of photochemical reactions is their total independence from a temperature. Sometimes the free radical polymerization can be thermally initiated without using initiators. (d) Chain growth, i.e., a propagation, is based on successive joining monomer particles with the growing macro-radical. The chain growth rate depends on monomer reactivity, radical reactivity, as well as viscosity of polymerizing mixture. The chain growth rate is very high (polymer particle of 10,000 monomer particles is formed in 1 s) (Eq. 2.62): R

CH2

CH

CH2

CH

R

CH2

CH

CH2

CH

(2.62)



(e) The chain growth end in the radical polymerization process can occur due to: • Recombination of radicals  – the chain growth is stopped by a mutual recombination reaction of two growing chains, whereas the formed macromolecule has its molecular mass equal to the sum of molecular masses of both radicals (Eq. 2.62):

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2  General Principles of Organic Chemistry

2R

R

CH2

CH

CH2

CH2

CH

•

R

•

CH + R

CH2

CH

CH

CH2

R

R

CH2

CH

CH2

CH

•

R

• Disproportioning reaction of macro-radical causing the hydrogen atom transfer from one macro-radical to the other: R

•

CH2

CH

CH2

CH + R

R

CH2

CH

CH

•

CH2

CH + R

CH

CH2

CH2

CH

CH

•

CH2

CH2

(2.63) • The chain activity transfer caused by the collision with no active particle being in the environment of the reaction (Fig. 2.8). Ionic Polymerization Catalysts of the ionic polymerization constitute appropriate anions or cations. Their catalytic activity is the acceleration of reactions. They are temporally joining with the reagents but do not enter into the final product.

Fig. 2.8  Formation of polystyrene

CH

CH2

H cat.

n

H

H

C

C

H

H n

styrene

polystyrene

69

2.5 Polymers and Polymerization

Anionic Polymerization Initiators of the anionic polymerization are alkali metals (Li, Na, K), hydrides of alkali metals, and ions of alkali metals with aromatic organic compounds (e.g., sodium naphthalene, sodium benzene) [13, 15]. The anionic polymerization course is shown in Fig. 2.9, the example of styrene, where sodium amide NaNH2 is its initiator (reaction proceeds in liquid ammonia) [18]. Sodium amide dissociates, according to a reactions (Fig. 2.9):

Stage III – the growth end – negative macroion is neutralized by proton or the charge is transferred on a monomer or solvent: Cationic Polymerization The cationic polymerization constitutes the process of a monomer or a mixture of monomers transferring into a polymer, according to the cationic mechanism, in the presence of catalysts.

H2N

CH2

C6H5

H2 N

CH2

CH2

CH

C6H5

CH2 m

O

.

Na

O

C6H5

m

CH

CH

CH2 C6 H5

Fig. 2.9  The anionic polymerization course at the example of styrene

NaNH2

H

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2  General Principles of Organic Chemistry

Catalysts of this reaction are sulfuric acid, perchloric acid, Lewis acids, or Friedel–Crafts catalysts such as AlCl3, BF3, and TiCl4. The activity of these last catalysts increases in the presence of coinitiators (co-catalysts), i.e., substances being the source of protons (H+ ions). As co-catalysts can be applied: water, hydrochloric acids (e.g., HClO4). An excess of a coinitiator leads to a premature chain end. The cationic polymerization process of isobutylene with BF3 as a catalyst and water as the initiator is shown below [15]. During the first stage, the substance which dissociates into ions is formed (Eq. 2.64): BF3 + H 2 O → [ BF3 • H 2 O ] ↔ H + + [ BF3 OH ]

−



(2.64)



The formed H+ proton is joining the monomer and initiates the reaction:

CH3 H

CH3

H2C = C

C

CH3

O

CH3

During the chain increase, the positive macroion is created (Eq. 2.65):

CH3

CH3 H3 C

O

C

O

CH2

C

CH3

CH3 C CH3

C

C

CH2

C CH3

O

CH3

n CH2

C CH3

CH3

CH3 CH2

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH2

C

O

CH3

(2.65)

The reaction end occurs because the proton splits, due to a decreasing activity of large ions. This causes the positive ion regeneration and ending of the polymerization process. Monomers polymerizing according to the cationic mechanism are, among others, isobutylene, styrene, and vinyl ethers. Gradual Polymerization (Condensation Polymers) The gradual polymerization (previous names: polycondensation or polycondensation polymerization) is the synthesis method, alternative in relation to the chain polymerization. There are two basic kinds of gradual polymerization: polycondensation and polyaddition. The characteristic feature of polycondensation is

2.5 Polymers and Polymerization

71

obtaining, apart from polymers, low-molecular products such as H2O and HCl. Such effect does not occur in the polyaddition reaction. In macromolecules of the created product, the main chain contains, apart from carbon, atoms of other elements such as oxygen, nitrogen, phosphor, boron, or silica [19]. In contrast to the chain polymerization, the gradual polymerization occurs – as the name implies – gradually. Temporary, stable products able to be separated are formed in each stage of this process. The gradual polymerization is characterized by the lack of the chain ending reaction. The macromolecule chain is growing slowly. Its kinetics depends on temperature, removal rate of low-molecular side products, as well as the amount and character of the catalyst (usually on the hydrogen ion concentration). In the gradual polymerization, a macromolecule increases gradually due to direct reactions of functional groups occurring in substrates and later at the end of a growing chain. Larger and larger particles, formed successively from substrates, can join with each other and with substrates:

n A R A + n B R B → A R n B + n AB

where: AB = H2O, HCl. By the gradual polymerization method, the following polymers are produced, on the industrial scale, polypropylene, polyethylene, polystyrene, and vinyl polychlorinated. Monomers (substrates) in the polymerization process constitute the most often substances containing unsaturated bonds (mainly C=C). Comparison of the gradual and chain polymerization is presented in Table 2.12. The gradual polymerization is a very important technological process of obtaining several, widely applied, polymers, e.g., polyesters, polyamides, phenolic and urea resins, or silicones (Table  2.3). Some of them found the application in the foundry industry as synthetic resin binders. To this group belong phenol–formaldehyde, urea–formaldehyde, and furan resins. Also biopolymers, such as nucleic acids and proteins, are produced by living organisms in the gradual polymerization. Nearly all polymers which are obtained in the chain polymerization can be also obtained by means of the gradual polymerization. The gradual polymerization is a multiple condensation reaction of functional groups, which – at the process initiation moment – are present in a monomer and then at the ends of growing chains: for example polyester is formed as a result of a condensation reaction with a participation of carboxylic and hydroxyl functional groups (polyesterification). The necessary condition of the polymer formation is the presence of at least two functional groups in each particle. Coordination Polymerization The coordination polymerization is a new type of synthesis of macromolecular substances leading to achieving polymers of regular spatial structures. This is a catalytic process, based on the formation of coordination connections between a catalyst

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2  General Principles of Organic Chemistry

Table 2.12  Comparison of the gradual and chain polymerization Chain polymerization Monomer contains functional groups (e.g., double bonds) requiring the activation Initiation of polymerization is necessary Polymerization product of a large molecular mass is obtained already during initial polymerization stages There is a lack of side products Chemical composition of a polymer and monomer is the same Polymerization degree is equal to the quotient of molar masses of a polymer and monomer Polymerization process is usually irreversible High purity of a monomer is required Time of the polymerization reaction significantly influences its yield but only slightly the molar mass

Gradual polymerization Monomer contains functional groups able to a direct reaction Initiation of polymerization is not necessary Polymerization product of a large molecular mass is obtained only at high conversion degrees Side products, e.g., water, occur Chemical composition of a polymer and monomer is different Polymerization degree is characterized by the polymer average molar mass Polymerization process is usually reversible Number of functional groups in a monomer is important Time of the reaction is essential in obtaining polymers of a high molar mass

Adapted from Ref. [13]

and monomer. Catalysts of the Ziegler–Natty type (TiCl4, TiBr4, VCl3, CoCl2) are widely applied in the industry. The coordination polymerization mechanism can differ, in dependence on the applied coordination catalyst. When metal halide – in which metal is at a higher oxidation degree – will be used in the catalytic complex synthesis, the coordinationradical polymerization will occur. This polymerization is caused by the free radical formation in the reduction reaction (Eq. 2.66) [15, 19]:



TiCl 4 + AlR 3 → RTiCl3 + AlR 2 Cl RTiCl3 → R • + TiCl3 R • + nM → polymer Free radicals “R•” initiate the polymerization process.

2.5.3  Inorganic Polymers Inorganic polymers are as follows [15]: • Hard materials (diamond, boron carbide B4C, silicon carbide SiC).

(2.66)

2.5 Polymers and Polymerization

73

• Fibers (glass, asbestos). • Structural materials (window glass, Portland cement, aluminum silicate). • Mineral fillers (metal oxides, soot, graphite). In the case of polymers built of silicon, phosphor, and boron, which constitute the skeleton of the main chain, it can be assumed that these are inorganic–organic polymers. Silicon carbide ( (β-SiC carborundum) is a crystalline polymer completely inorganic. It has a low density and is thermally stable up to 1600 °C. It is very hard and highly mechanically resistant.

2.5.4  Inorganic–Organic and Organo-Metallic Polymers Inorganic–organic and organo-metallic polymers have the following advantageous properties: • Density and stability similar to organic polymers. • Strength and electricity conduction comparable to metals. • Low density, high thermal resistance, as ceramics. These polymers do not have disadvantages of: • Organic polymers (low resistance to oxygen, organic solvent, high temperature). • Metals (high density, susceptibility to corrosion). • Ceramics (brittleness, difficult processing). Polysiloxanes (silicones) contain – in the main chain – siloxane (− Si - O – Si –) group, in which silica atoms are joined by covalent bonds with oxygen atoms. Hydrogen atoms or hydrocarbon molecules can be connected to silica atoms.

O

R Si R

siloxan structure, R – substitutes: − H; − CH3, − C2H5. Polysilanes have silane (–Si – Si –) group in their main chain. Any substitutes can be connected to silicon atoms.

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2  General Principles of Organic Chemistry

H3C

CH3 Si

H3C

CH3

tetramethylsilane The most important organo-metallic polymers are coordination polymers of ferrous, wolfram, chromium, molybdenum, zirconium, etc.

2.5.5  Degradable Polymers A large production of plastics as well as impossibility of a total usage of their wastes is the reason that their significant part is located on dumping grounds. In order to limit the amount of stored wastes and to relieve the environment from excessive amounts of wastes, studies on the development of such plastics which – after being used  – would undergo degradation during storage in the natural environment are carried out. Since the degradation of polymers can occur under an influence of radiation or microorganisms or enzymes, the degradable polymers can be divided into [19]: • Biodegradable polymers. • Photodegradable polymers. • Composites of biodegradable polymers. Biodegradable polymers are natural polymers or polymers obtained by modifications of natural polymers as well as polymers produced by various methods of chemical synthesis and in biotechnological processes. Biodegradable plastics are manufactured either of renewable raw materials (poly(lactic acid) (LAc) – polylactide (PLA), poly(glycolic acid) (PGA), modified starch) or of raw materials originated from the crude oil processing (polycaprolactone (PCL)). A significant advantage of biodegradable polymers is a possibility of the utilization of spent products and other wastes by means of the composting method where they are subjected to influences of various microorganisms. The polymer biodegradation process is a very complex process and can occur in many ways. It can be assumed that the biodegradation process occurs in two stages [20]. • Stage 1. Depolymerization of macromolecules into shorter chains (material fragmentation and weakening of its cohesiveness), mainly as a result of mechanical operations caused by microorganisms. This stage occurs usually out of organism due to the polymer chain size and insolubility of several polymers.

2.5 Polymers and Polymerization

75

• Stage 2. Represents the mineralization process, when sufficient amounts of small oligomers are formed and transported to cells where they are subjected to bioassimilation by microorganisms, leading  – in effect  – to a partial mineralization. The biodegradation can occur according to two schemes, in dependence on the oxygen presence: • Aerobic biodegradation (in an oxygen presence). • Anaerobic biodegradation (at a lack of oxygen). The final product of the biodegradation process is the biomass (organic matter mass) with emissions of water and gases, such as CO2, CH4, and NH3. The complete biodegradation or mineralization occurs when there are no residues, i.e., when the initial material is totally transformed into gases or salts. These changes cause worsening of physical and mechanical properties. A majority of synthetic polymers are not biodegradable. The following factors decide the biodegradation course and rate: microorganism kind, material type, and environmental conditions, which are the most important (temperature, pH, humidity, oxygen, and light access) [14]. Poly(lactic acid) (PLA) is a completely biodegradable polymer. The initial substrate for its synthesis is lactic acid (LAc), obtained in the fermentation process of carbohydrates originated from potatoes, corn, and sugar beet. Two methods of obtaining poly(lactic acid) are used: • Polymerization with opening the cyclic ring of lactide (Fig. 2.10). • Polycondensation of lactic acid (Fig. 2.11).

Fig. 2.10  Schematic presentation of the poly(lactic acid) formation by the polymerization method with opening the lactide ring

HO H3C

O OH

lactic acid

–H2O

CH3

O HO

O

O CH3

O

O

OH nCH

3

polylactic acid

Fig. 2.11  Schematic presentation of the poly(lactic acid) formation by the polycondensation of lactic acid

76

2  General Principles of Organic Chemistry

Photodegradable Polymers Polymers are aging with time, which is revealed by the degradation process consisting of a molecular mass decrease, changes of the chemical structure, and in consequence changes of physical–chemical properties. The factor causing these changes is often UV radiation originated, e.g., from the sunlight. Under natural conditions, different physical factors are important in the polymer photodegradation: sunlight, temperature, humidity, and atmospheric components (contaminations). When the air is present, secondary reactions occur with oxygen, and then the polymer photooxidation occurs. Photodegradable polymers, among which the most important are polyolefins, are mainly applied in the production of packages. Partially Biodegradable Materials One of the ways of obtaining thermoplastic biodegradable materials is the physical modification of polyolefins or other vinyl polymers by mixing them with natural biodegradable polymers. These natural polymers applied for modifying synthetic polymers are cellulose, lignin, starch, alginates, and chitin. In the case of synthetic polymers, their degradation constitutes the process due to which macromolecules are disintegrated into smaller fragments, which worsens polymer properties. A special case of the polymer degradation is depolymerization leading to the monomer formation. The degradation of polymers occurs under an influence of physical, chemical, or biological factors. Physical factors causing the degradation of polymers are increased temperature, ionizing radiation (X-ray radiation), sunlight, ultrasounds, and mechanical forces occurring during grinding and rolling. On account of the initiation way, the degradation processes caused by physical factors can be divided into thermal, mechanical, and photochemical degradation [21, 22]. The thermal degradation is caused by a temperature. In the majority of cases, polymers are thermally stable up to a temperature of approximately 200 °C. At a temperature of approximately 1000 °C, they disintegrate into small fragments of the type of free radicals, free ions and H2, CO, CO2, etc. The thermal sensitivity of organic substances is caused by covalent bonds. Two phenomena are effects of chemical changes occurring when polymers are heated: chemical bonds C–C in the main chain or in side chains are ruptured, which is revealed by decreasing of a molar mass and low-molecular gaseous products are emitted. Curves of the thermal disintegration of the most important multimolecular substances are presented below (Fig. 2.12). It is seen from these curves positions that the thermal degradation of the majority of polymers starts already at temperatures 150–200 °C and ends below 400 °C, with the exception of thermally stable phenolic polyester and urea resins. Disintegration of these resins ends only at temperatures 600–800 °C, in addition to which a high content of coke in pyrolysis products is characteristic. The pyrolysis course of polyvinylchloride is interesting. The process of the hydrogen chloride 100% removal from PVC occurs at a temperature below

2.5 Polymers and Polymerization

77

Fig. 2.12  Curves of the thermal decomposition of selected polymers (PP, polypropylene; PE, polyethylene; PS, polystyrene; PVC, polyvinylchloride) [16]

200 °C, and in the case of mixtures containing PVC, the process of hydrogen chloride emission can precede the relevant pyrolysis. Some polymers, e.g., polymethyl methacrylate or polystyrene, undergo depolymerization during heating. Then it is possible to recover – at least partially – the monomer [23]. The characteristic product of burning polymers, based on organic aromatic substances, especially polystyrene, is soot. It was found that amounts of soot can be limited when the polystyrene burning process is carried out in weakly oxidizing atmosphere. In dependence on the amount of oxygen, the soot oxidation to CO and CO2 occurs. During burning of polymers containing nitrogen atoms in macromolecules, e.g., polyamides, polyurethanes characteristic of yellow fumes formed by nitrogen oxides, the so-called NOx, are emitted. A serious threat to the life constitutes hydrogen cyanide (HCN) formed at burning of polyacrylonitrile. Generally the majority of macromolecular substances undergo unordered destruction during pyrolysis, usually with emitting volatile substances of small molecular masses, such as aliphatic hydrocarbons of various chain lengths, aromatic hydrocarbons, carbon oxide and dioxide, hydrogen chloride, water, and hydrogen. In dependence of the supplied oxygen and process temperature, coke can be obtained as a solid residue. The mechanical degradation concerns macroscopic effects occurring in polymers under the influence of tensile forces. The photochemical degradation occurs under an influence of visible light or ultraviolet and leads to physical and chemical changes of polymer. The photochemical degradation is especially essential from practical reasons. Plastic products being in general use are usually exposed to sunlight. This light contains a wide radiation range, including visible and ultraviolet ranges, causing disadvantageous changes of polymer properties.

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2  General Principles of Organic Chemistry

Factors causing the chemical degradation are mainly oxygen and ozone. Molecular oxygen, present in the air, facilitates the degradation process of polymers at relatively low temperatures, already at a room temperature. Even more easily this reaction proceeds under an influence of atomic oxygen or ozone. The reaction of ozone or oxygen with the polymer containing a double bond, in which the polymer is degraded into aldehydes and acids, is applied for producing degradable polymers. Plastics obtained in such a way are degrading under natural conditions and can be utilized after being used. This process can proceed also under an influence of acids, bases, or corrosive agents. The biodegradation process proceeds under an influence of biological agents: bacteria, enzymes, and fungi. This process is based on the disintegration of plastics, after a determined time and after finishing its usage. All natural macromolecular products, even wood and ivory, can be degrading to small particles; when producing them, organisms die. A composition and character of disintegrating substances allows for using them by other organisms in order to produce energy or for synthesis of new substances, including also biopolymers. In exceptional cases, natural biodegradation processes are leading to forming such final products as coal, peat, and hydrocarbons, which can be considered to be completely degraded biopolymers [16, 24]. The biodegradation process can be anaerobic, progressing without the air access – less efficient and less common in nature, leading to formation of methane and other simple hydrocarbons – or aerobic at the air access. The final products of aerobic biodegradation are carbon dioxide and water, sometimes mineral salts. In between individual degradation types, there are strong connections. Usually a few types occur simultaneously. Simultaneous operations of light, oxygen, and other atmospheric agents or simultaneous operations of heat, mechanical forces, and oxygen are typical examples of such situations.

2.5.6  Natural Polymers (Biopolymers) Biopolymers are macromolecular substances produced by organisms. They can be divided into groups [15]: • • • •

Nucleic acids. Peptides, proteins. Polysaccharides (cellulose, starch, collagen, chitin, and chitosan). Natural rubbers.

Examples of Natural Polymers Cellulose Cellulose (fiber) is a natural polymer, from the polysaccharide group, produced by photosynthesis. Cellulose constitutes the basic component of plants, which provides elasticity and stability to cell walls and protects against water losses. This polysaccharide is built of D-glucose mers, connected by β-glycoside bonds at carbon atoms: C1 and C4. Each heterocyclic ring of cellulose contains:

2.5 Polymers and Polymerization

79

• One primary alcoholic group (–CH2OH). • Two secondary alcoholic groups (–OH). CH2OH O

H

OH

H

H

OH

O

H

H O

H

H

OH

H

HO

O

H

H OH

CH2OH

CH2OH

CH2OH

H

H

O

OH

H

H

OH

H n

O OH

H OH

H

H

OH

H

cellulose formula (n approx. 2000) The average polymerization degree of cellulose equals 500–10,000. The natural raw material containing the highest amount of cellulose is cotton, followed by linen and hemp. Cellulose, regardless of a large number of hydroxyl (–OH) groups, does not dissolve in water and organic solvents. Cellulose is swelling in water due to forming hydrogen bonds with free hydroxyl groups; it dissolves in HCl, H2SO4, HF, tin chloride, and bismuth chloride. Cellulose is anisotropic polymer and durability of fibers is ten times higher along the fiber axis than in the transverse direction. Starch Starch is a natural biopolymer from the polysaccharide group. It constitutes a mixture of two kinds of polyglucans [25]: • Amylose of an average polymerization degree: P = 5000. • Amylopectin of an average polymerization degree: P = 50 000. Typical starch contains 20–30% of amylose. Starch has a globular structure and occurs in plants in a form of grains of sizes: 0.2–100 μm. Starch is produced by plants in the process of carbon oxide assimilation, (IV) CO2. It is deposited in cells, as the so-called reserve material. Thus, the following process occurs in plants (glucose under an influence of enzymes is turning into starch): hv CO2, H2O

glucose, oxygen chlorophyl enzyme

starch

80

2  General Principles of Organic Chemistry

Water solubility of starch depends on a temperature. It does not solve in cold water, while strongly bulge in hot water: 95–85 °C [19]. By substituting a part of hydroxyl groups by ester or ether group, the modified starch is obtained. The most important starch property is its ability to hydrolysis. Hydrogen ions and enzymes, called amylase, are catalysts of the starch hydrolyses. The scheme of the hydrolysis reaction is as follows (Eq. 2.67):

( C6 H10 O5 )n starch

HCl

+ nH 2 O →

hydrolysis

x 300 °C 7% 0.5 ppm 2 ppm

This is the first aldehyde from the group of aliphatic aldehydes. Its melting temperature equals −92  °C while a boiling point −19.1  °C (Table  3.2). Formaldehyde is well soluble in ethanol, chloroform, and water. Formaldehyde is a very reactive substance, since the carbonyl group in its particle has no carbon substitutes, and due to that the carbon atom in this group easily condensates. Therefore formaldehyde very easily forms polyoxymethylene (paraformaldehyde) in polycondensation reaction. Aqueous solutions of formaldehyde, which are applied in the resin production, are unstable especially at low temperatures and indicate a growing tendency to precipitating of paraformaldehyde. Therefore commercial grades of this substance are stabilized by various amounts of methyl alcohol (up to 15%). Formaldehyde belongs to the group of dangerous chemical substances and is classified as the proven carcinogenic for humans. Formaldehyde is easily absorbed into organisms by airways. It irritates conjunctivas and mucous membranes. It has an acute and chronic toxic activity. A connection between an exposure to formaldehyde and leukemia morbidity is also noticed [7, 8]. Methylene Glycol This substance is formed in a reaction of formaldehyde with water. Since this reaction is reversible, products containing methylene glycol can – under certain conditions  – release formaldehyde. Both substances occur in equilibrium and under various conditions easily change one into the other. The changing rate depends on a temperature, pH, concentration, and presence of other substances. Phenol–Formaldehyde Resins Phenol resins are one of the oldest resins commercially produced by means of polymerization. The first information concerning them provided Von Baeyer in the year 1872. The first patent concerning producing phenol–formaldehyde resins dates back the year 1907 and belongs to Leo H. Baekeland.

3.2 Phenol–Formaldehyde Resins

87

Phenol resins are thermo-hardened polymers of a high resistance to chemical agents, but of a low resistance to dynamic loads and mechanical operations. Phenol resins are resistant to self-ignition; they generate small amounts of fumes and are relatively cheap. In order to improve thermal properties of phenol resins, phenol– formaldehyde resins are applied [9]. Phenol–formaldehyde resins (phenoplasts) belong to the group of condensation resins, widely applied in the industry. They are used, among others, for a production of friction and abrasive materials, coatings of molds and cores for the foundry industry, electro-insulating products, thermo-insulating products, products of mineral wool and glass fibers, laminar plastics, binders applied in furniture industry, glues, paints, and products of general use [8]. Phenol–formaldehyde resins are synthetic thermo-hardened polymers resulting in the polycondensation reaction of phenols with aldehydes, usually with formaldehyde. Water is the by-product of this reaction. Since this reaction without a catalyst is very slow, the catalyst is always added. It can be either acid or base. These resins are solvable in organic solvents and can be melted. The kind of the product depends on the catalyst and molar ratio of reagents. Polymerization of Phenol and Formaldehyde Phenol and formaldehyde are raw materials for production of phenol–formaldehyde resins for the needs of the foundry industry. In aqueous solution, formaldehyde is in equilibrium with methylene glycol (CAS 463-57-0) (hydrated formaldehyde):

The polycondensation reaction of phenol with formaldehyde is catalyzed by hydrogen ions as well as by hydroxyl ions. This process can be carried out in an aqueous solution. Phenol with formaldehyde reaction is highly exothermic [10, 11]. Depending on the environment pH, in which this reaction occurs, and on the mutual fraction of each reagent (phenol and formaldehyde), one of two types of resins are obtained: novolak resin (in acidic environment, at phenol surplus) or resol resin (in basic environment, at formaldehyde surplus) (Fig.  3.2). Since novolak resin does not contain reactive groups, it requires an additional cross-linking agent and heat in order of hardening [10, 12–16]. The polymerization reaction course of both kinds of phenol resins can be written as (Fig. 3.3): The phenol polymer formed in polymerizations of resol and novolak resins is the same. The only difference constitutes the fact that the polymer obtained from resol resin does not contain hexamethylenetetramine, which can release nitrogen or ammonia. In the first stage of the polycondensation of phenol with formaldehyde, the electrophilic connection of the formaldehyde particle into phenol group in ortho- or para- position occurs with a simultaneous rearrangement of the hydrogen atom with oxygen atom (Fig. 3.4) [17, 18].

3  Synthetic Resins

88 OH

OH CH2

CH2OH

base F/P 1

OH

CH2

CH2OH

O

+ CH2O

resol resin

CH2 OH CH2OH

acid F/P 1

OH

OH CH2 novolak resin

OH

Fig. 3.2  Scheme representing polycondensation of phenol and formaldehyde to obtain various products of this reaction, depending on the reaction conditions. (Reprinted by permission from Ref. [12])

Fig. 3.3  Schematic presentation of the polymerization process course of novolak and resol phenol resins Fig. 3.4  Scheme of the phenol and formaldehyde connection

Hydroxymethyl (−CH2OH) group can be connected in the position ortho- or para- into OH- group in the phenol particle. Then mono-, two-, and tri-­hydroxyphenol derivatives are formed (Fig. 3.5). These derivatives are condensed, and in result oligomers with methylene –CH2− bonds are formed, and formaldehyde is evolved (Fig. 3.6). The obtained substances can react with each other via hydroxymethylene groups, connecting themselves by the ether –CH2−O−CH2− bond (Fig. 3.7). Reactions of this type are further occurring providing in result phenol–formaldehyde resin.

3.2 Phenol–Formaldehyde Resins

89

Fig. 3.5  Scheme of the formation of hydroxyl derivatives in the reaction of phenol with formaldehyde

Fig. 3.6  Condensation of hydroxyl derivatives

Fig. 3.7  Formation of the ether bond as the result of condensation of hydroxyl derivatives

Novolak Resins In the acidic environment (pH = 4–7), the initial reaction between methylene glycol and phenol (at a small surplus of phenol) is as follows: OH HO

CH2

OH

OH

CH2

H and CH2

methylene glycol (0.8 M)

OH

OH2

phenol methylphenol (1M) pH < 5 ; 95 oC

OH2 H2O

90

3  Synthetic Resins

CH2

+

OH

OH2

and

OH

OH CH2

+

OH CH2

+

ortho-ortho +

H2O + H+

and

OH2

ortho-para

OH CH2

+

H2O + H+

OH and para-para CH2 OH

+

H2O + H+

OH

Fig. 3.8  Possibility of forming methylene bridge (−CH2−) in ortho- or para- position aromatic ring. (Adapted from Ref. [19])

This reaction produces methyl phenols, which are very active in an acidic environment and react very fast with free phenol (Fig. 3.8). As the reaction result the methylene bridge (−CH2−) is formed in the ortho- or para- position of the aromatic ring of phenol. The para- position is nearly twice more reactive than ortho-, but there are twice more positions ortho- (two in the phenol particle), and in consequence fractions of bridges ortho–ortho, para–para, and ortho–para are, in approximation, the same [19]. Further branching occurs since the reaction can take place in three places of each ring. This reaction stops, when formaldehyde is exhausted and that time often remains app. 10% of not reacted phenol (this is the so-called free phenol). Since novolak resin contains only methylene (−CH2−) groups but does not contain free reactive hydroxymethylene (−CH2−OH) groups, heating of this resin – as the only factor – does not cause further reactions and cross-linking. Thermoplastic resin of a linear structure and not cross-linked is obtained. This resin cannot be further hardened without an additional cross-linking agent. Therefore novolak resins are called two-stage resins. It is necessary to supply the system with an additional portion of formaldehyde. Hexamethylenetetramine, known as HMTA or hexamine (Fig. 3.9), can be the cross-linking factor. This substance, at a temperature of 160 °C, decomposes into ammonia and formaldehyde.

3.2 Phenol–Formaldehyde Resins

91

OH OH N N N

HN

N

CH2

CH2

H2 C H2 C

N

CH2

OH

HO OH Fig. 3.9  Formula of hexamethylenetetramine and the structure of cross-linked novolak resin (formation of methyleneamine bridges). The hardening process occurs in two stages



( CH 2 )6 N 4 + 6H 2 O = 6CH 2 O + 4NH3



Novolak resin + HMTA + heat → cross − linked polymer + H 2 O

HMTA hardens resins by means of further cross-linking and polymerization of particles, up to the state of a high melting temperature (methyleneamine bridges are formed) (Fig. 3.9). However, the only presence of HMTA does not cause the resin hardening. Heating of the system is necessary. Hardening by means of HMTA is based on reactions, at a temperature above 150 °C, of active places (free ortho- or para- positions in a chain) with partially decomposing HMTA, according to the scheme (Fig. 3.10) [20]: HMTA dissolves relatively well in water while weaker in methanol or ethanol. It easily hydrolyzes into amino methyl substance. Its aqueous solution has weak basic properties of pH = 7–10. Novolak resin is a solid thermoplastic material of properties not changing with time. As it is shown by the newest studies [20, 21], novolak resins during their storage indicate only negligible changes in chemical composition, caused mainly by a partial hardening. Hardened resins also indicate only small changes with time. Novolak resins are amorphous (they do not crystallize) and thermoplastic. At a room temperature they are solids, which soften and flow at temperatures between 65 and 105 °C. These resins are soluble in several organic, polar solvents (e.g., alcohols, acetone), but insoluble in water and aromatic hydrocarbons. At heating, novolak resins are melting but are not condensing further (because they do not contain reactive hydroxymethyl (−CH2OH) groups). Generally, at temperatures up to 250 °C, novolaks are not chemically changing. Resol Resins Resol resins are obtained in the basic environment and at a surplus of formaldehyde over phenol (the most often: formaldehyde/phenol  =  1.5:1 and 2.5:1). Industrial

92

3  Synthetic Resins

Fig. 3.10  Cross-linking of novolak resin in the hexamine presence. (Reprinted by permission from Ref. [20])

synthesis of resol resins is often carried out in the presence of catalysts based on ammonia or tertiary amine, or NaOH [22, 23]. Advantages of resol resins, produced in the presence of trialkyl-amines, are longer gelling time, smaller amount of ashes, and resistant to hydrolysis. An addition of urea in the final production stage of the resol-type resin improves its viscosity and decreases production costs. Resol resins, contrary to novolak resins, have unreacted hydroxymethylene (−CH2OH) groups, and phenol particles are connected by methylene (−CH2−) bridges, as well as by dimethylene ether (−CH2−O−CH2−) bridges. The polycondensation process of phenol and formaldehyde can be presented as follows. Phenol reacts with methylene glycol forming hydroxymethylphenol (phenol with one or a few hydroxymethylene (−CH2OH) groups), when indirectly phenyl– alcohol is formed (reaction of electrophilic joining of aldehyde):

At a temperature above 110  °C, methylene glycol totally dissociates into formaldehyde and water. Methyl phenol molecules react with each other via hydroxymethylene groups, either connecting themselves by dimethylenether −CH2−O−CH2− bond:

or methylophenol particles are reacting with phenol particle, connecting themselves by methylene −CH2− bond:

3.2 Phenol–Formaldehyde Resins

93

In benzene ring positions, marked X, there is still a possibility of reaction. In case of resol resins, where is a surplus of formaldehyde, there is a sufficient amount of reactive hydroxymethylene (−HO−CH2−) and dimethylene ether (− CH2−O−CH2−) groups to complete the polymerization, without introducing a hardening addition, and providing in effect phenol–formaldehyde resin (Fig. 3.11). Therefore resol resins are called one-stage resins. By controlling the phenol amount ratio to the formaldehyde amounts, pH, temperature, catalyst kind, and reaction time, it is possible to obtain resins of various properties [17]. The polymerization reaction of resol resin occurs even at a room temperature, however much slower. The typical average number of molecular mass units in normal resol resin equals from 200 to 450. The cross-linking degree of resin decides on its properties, such as temperature of softening under load, stiffness, etc. Phenol–formaldehyde resol resins are liquid or solid thermo-reactive products, which color  – yellow or red  – depends on the applied catalyst. Liquid resol resins have on average app. two benzene rings in a molecule while solid resol resins three to four rings. Resol resins are to a certain degree water soluble, while they are well soluble in alcohols and acetone. However, after hardening they are completely losing their solubility. During storing resol resins are changing into the infusible and insoluble state. Heating is accelerating this process. In a temperature of 150  °C the hardening time equals app. 50 s. Organic and inorganic substances can be applied as catalysts of hardening phenol–formaldehyde resins. Resol-type resins hardened in the

CH2

CH2

C

O

OH

OH

OH

OH

OH H

CH2OH

CH2

CH2

CH2 C

CH2OH HOCH2

OH

O CH2OH

CH2

OH HO

CH2

O

CH2

CH2

H

CH2

CH2

CH2

OH

OH CH2

C H

Fig. 3.11  Structure of cross-linked phenol–formaldehyde resol resin

CH2

CH2OH

94

3  Synthetic Resins

presence of organic catalysts are characterized by a high resistance to humidity and high mechanical strength [24, 25]. Some resols contain latent acid catalysts (which are becoming active, e.g., after heating), which generate moderately strong acids [6]. This can be, e.g., (NH4)2SO4 or biphenyl hydrogen phosphite. The application of latent catalysts allows to widen the resin utilization range – since it prolongs its liquidity time. Several additions are applied in the production process of PF resol-type resins. These additions are introduced in order to extend the storage time, widen applicability, or control the hardening rate. The urea addition in the final phase of the resol resin synthesis is a usual practice [26]. It not only reduces costs of resins but also decreases their viscosity and thus influences their other properties. The application of tertiary amines R3N as catalysts extends a gelling time, decreases ashes amounts, and improves a resistance to hydrolysis [22]. Methanol is usually added at the beginning of the synthesis to control the heat effect of the polymerization process. At pH 4–6 the hardening process of phenol–formaldehyde resoltype resin is slower than at pH  8 and higher, but significantly slower than at pH 1–3. Important differences between phenol–formaldehyde resins of novolak and resol type: • Resol resins are produced with basic catalyst while novolak resins with acidic catalyst. • Resol resins have hydroxymethyl HO−CH2− groups, while novolak resins do not have these groups. • Resol resins have a short storage time, while novolak resins can be stored for a very long time. • Resol resins release water during hardening, while novolak resins release ammonia (by decomposition of hexamine, a binder component). • Novolak resins are characterized by more than twice better dimensional stability than resol resins. • Resol resins are usually liquids, while novolak resins are solids. PF resins are characterized by a high resistance to creep and to deformations under loads, by a low thermal conduction and are good insulators. They are resistant to chemical agents (resistant to influences of weak acids, weak bases, hydrocarbons, and detergents), have low tendency for water absorption (from 0.03% to 1.75% – resol resins are more humidity resistant), and have high dimensional accuracy and stability [27]. Water in a powder resol resin plays an important part in the curing. In the initial curing stages, water acts as diluent and retards the curing. At the higher conversion, water acts as plasticizer. Phenol–formaldehyde resins, both novolak and resol type as well as their mixtures with other resins: phenol/polyurethane and phenol/urea resins are widely applied in the foundry industry as components of binders for molding and core sands. Processes of productions of molds and cores are listed in Table 3.3.

3.2 Phenol–Formaldehyde Resins

95

Table 3.3  Processes of molds and cores production in which phenol–formaldehyde resin is used Phenolic resin Novolak Resol Resol Resol

Hardener/catalyst Hexa (HMTA) Acid + heat Polyisocyanate/amine Liquide ester Gas ester (methylformate) Carbon dioxide

Process Croning process Hot-box process Polyurethane cold-box process ALPHASET process MF: cold-box process BETASET CO2 – resole process

Reprinted by permission from Ref. [12]

Harmfulness of Phenol–Formaldehyde Resins The reaction of phenol with formaldehyde is highly exothermic [28]. During processing of resins, these substances are hardened due to a high temperature influence (from 160 to 250 °C). In this process the mixtures of harmful substances of various chemical character and various toxicity degrees are released into the atmosphere at workplaces. The source of these substances are resins themselves, substances modifying their properties, and certain additions. An exposure of employees to chemical substances can occur also during the preparation of mixtures and forming products since that time resins are being cross-linked. Phenol and formaldehyde are in a free state in not hardened resin. Qualitative and quantitative composition of emitted harmful substances depends on the resin kind and on conditions under which the hardening occurs, i.e., temperature, pressure, and time of their operations, as well as on the surface of produced elements. Hexamine added to novolak resins can be decomposing and emitting ammonia, under conditions of hardening. On the other hand, formalin (saturated aqueous solution of formaldehyde) applied as a raw material for the production of phenol–formaldehyde resins can be the source of the methanol presence in the air on the workplaces; 12% of this alcohol is added into formalin as a stabilizer [29, 30]. A problem is the release of phenol into the atmosphere during curing. Typical levels of free phenol in the phenol–formaldehyde resin are in the range of 5–15%. One method of reducing the free phenol level in the base phenol–formaldehyde resin is to increase the amount of formaldehyde in the resin during manufacturing. Thermal Degradation Thermostability of thermo-hardened polymer to a significant degree depends on its structure and cross-linking density. The polymer stability is an important property. The thermal degradation of phenol–formaldehyde resin starts from cracking of bonds between aromatic rings and methyl (−CH2−) bridges (Fig. 3.12) [31]. When hydrogen atoms are connecting with the formed radicals, phenol and its derivatives are created. Products formed during the thermal degradation of phenol–formaldehyde resin are forming compounds of condensed aromatic rings, e.g., fluorine [32, 33]. Investigations concerning the mechanism of the thermal degradation of PF resins [34–36] suggest that their thermal decomposition occurs according to the autoxidation process, which combines three stages [24–26]. Moreover, at considering the

96

3  Synthetic Resins

OH

OH +

OH

OH

OH

CH2

OH CH + H2O

OH

OH

OH CH2

CH2 +

2

2 OH

2

OH CH2

CH2

2

Fig. 3.12  Possible reactions occurring at the thermal degradation of PF resin. (Adapted from Ref. [31])

PF resin pyrolysis, two ways of heating should be taken into account, which will simulate effects occurring in a mold: “slow” and “flash.” The “flash” pyrolysis takes place in the initial period during a mold pouring with molten metal, while “slow” pyrolysis occurs in molding sand layers being at a significant distance from a casting. The heating method and character of the atmosphere, in which the process occurs, have the influence on the kind and amount of evolving substances. In case of “slow” pyrolysis, the thermal destruction of PF resin runs in three main stages. In the first stage, the additional cross-linking between remained unreacted functional groups in hardened PF resin occurs. Along with a temperature increase from a room temperature to 200 °C, the resin mass loss is mainly caused by releasing moisture, dehydration, as well as releasing substances of a low molecular weight. Water releasing is a result of condensation reaction between the remaining hydroxymethyl −CH2OH and phenol −OH group. Water removal can lead to forming new cross-linking [37, 38]. 5% of unreacted phenol can be in liquid resol resin while up to