30
Polyoxymethylene Handbook
Scrivener Publishing
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Beverly, MA 01915-6106
Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
Polyoxymethylene Handbook
Edited by
Sigrid Lüft l, Visakh P.M., and Sarath Chandran
Structure, Properties, Applications and Th eir Nanocomposites
Copyright © 2014 by Scrivener Publishing LLC. All rights reserved.
Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem,
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Cover design by Russell Richardson
Library of Congr ess Cataloging-in-Publication Data:
ISBN 978-1-118-38511-1
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
v
Contents
Preface xiii
1 Polyoxymethylene: State of Art, New Challenges
and Opportunities 1
Sigrid Lüft l and Visakh P.M.1.1 Scope 21.2 History 21.3 Commercial Signifi cance 7References 13
2 Polymerization and Manufacture of Polyoxymethylene 21
Johannes Karl Fink2.1 Introduction 212.2 Monomers 22
2.2.1 Formaldehyde 222.2.2 Trioxane 24
2.3 Comonomers 252.3.1 Ethylene Oxide 252.3.2 Propylene Oxide 252.3.3 Tetrahydrofuran 262.3.4 Dioxane Derivates 272.3.5 Branching Agents 272.3.6 Chain Transfer Agents 28
2.4 Polymerization and Fabrication 282.4.1 Anionic Polymerization 312.4.2 Cationic Polymerization 312.4.3 Catalysts in Detail 332.4.4 Undesired Side Reactions 342.4.5 Terminators 342.4.6 Homopolymers 36
vi Contents
2.4.7 Copolymers 372.4.8 Removal of Residual Monomers 402.4.9 Special Compositions 42
2.5 Special Additives 442.5.1 Antioxidants 442.5.2 Impact Modifi ers 45
References 46
3 Polyoxymethylene Additives 53
Emmanuel Richaud3.1 Introduction 533.2 Antioxidants 54
3.2.1 Strategy of Stabilization to Inhibit Th ermal Oxidation 54
3.2.2 Case of Hydroperoxide Reducers in POM 543.2.3 Case of Phenolic Antioxidants in POM 55
3.3 Compounds Reacting with Secondary Reaction Products 59
3.4 UV Stabilization 603.4.1 Generality on UV Stabilization 603.4.2 Performances of UV Stabilizers in POM 63
3.5 Impact Modifi er 653.6 Nucleating Agent 67
3.6.1 Th eoretical Recall on Crystallization of Polymers 67
3.7 Pigments and Dyes 723.7.1 Mineral Pigments 733.7.2 Organic Pigments and Dyes 743.7.3 Conclusion 74
3.8 Flame Retardants 753.8.1 Principles of Combustion 753.8.2 Strategies for Improving Flame Resistance 773.8.3 Examples of Flame Retardants in POM 77
3.9 Antistatic Agents 793.10 Lubricating Agents 803.11 Fillers 82
3.11.1 Eff ect of Fillers on Th ermal Properties 823.11.2 Eff ect of Fillers on Mechanical Properties 84
3.12 Processing Aids 90References 91Appendix 3.1: List of Stabilizers 100
Contents vii
4 Polyoxymethylene Processing 107
Kinga Pielichowska4.1 Introduction 1074.2 Injection Molding 1094.3 Melt Extrusion 1164.4 Solid-State Extrusion 1184.5 Extrusion Assisted by Supercritical Carbon Dioxide 1204.6 Blow Molding 1214.7 Others Methods 123
4.7.1 Melt Blowing 1234.7.2 Compression Molding 1244.7.3 Rolling 1254.7.4 Sintering 1264.7.5 Spinning 129
4.8 Highly Oriented Products 1324.9 Recycling of Production Waste 1364.10 Finishing and Machining of POM 1384.11 Conclusions 141References 142
5 Polyoxymethylene Applications 153
Lidia Tokarz, Slawomir Pawlowski and
Michal Kedzierski5.1 Introduction 1535.2 Automotive Industry, Mechanical Engineering 1565.3 Electrical and Electronic Industry, Fancy Goods 1575.4 Medical Applications 1585.5 Future Trends 160References 160
6 Structure and Morphology of Polyoxymethylene 163
Maria Raimo6.1 Introduction 1636.2 Crystalline Structure of POM: Orthorhombic
and Hexagonal Phases 1656.3 Crystal Structure Determination 1706.4 Morphology of Orthorhombic and
Hexagonal POM 1736.5 Morphology of Rubber-Modifi ed POM 1796.6 Structure-Properties Relationships 181References 186
viii Contents
7 Crystal Structure and Crystallization Behavior of
POM and its Microscopically-Viewed Relation with the Physical
and Th ermal Properties on the Basis of X-ray
Scattering, Vibrational Spectroscopy and Lattice
Dynamical Th eory 193
Kohji Tashiro7.1 Introduction 1947.2 Crystal Structure Analysis of POM 195
7.2.1 Synchrotron X-ray Diff raction Analysis 1967.2.2 Wide-Angle Neutron Diff raction Analysis 1987.2.3 (29/16) Helical Model 2017.2.4 Electron Diff raction of Whisker 203
7.3 Vibrational Spectra of POM 2047.4 Structural Evolution in Isothermal Crystallization 2077.5 Microscopically-Viewed Mechanical
Property of POM 2167.6 Conclusions 223Acknowledgements 224References 224
8 Physical Properties of Polyoxymethylene 227
Johannes Karl Fink8.1 Introduction 2278.2 Density 2288.3 Hardness 2308.4 Heat Capacity 2318.5 Melt Flow 231
8.5.1 Tribological Properties 2358.5.2 Whiskers 235
8.6 Water Absorption 2358.7 Gas Permeability 236
8.7.1 Permeability to Fuels 2378.8 Specifi c Absorption 238
8.8.1 Polychlorinated Biphenyls 238References 239
9 POM Mechanical Properties 241
Fahmi Bedoui and Bruno Fayolle9.1 Short Term Properties 242
9.1.1 Elastic Properties 2429.1.2 Properties at Failure 245
Contents ix
9.2 Long-Term Properties 2499.2.1 Fatigue Properties 2499.2.2 Chemical Degradation 250
9.3 Conclusion 252Acknowledgement 253References 253
10 Th ermal Properties and Flammability
of Polyoxymethylene 257
Vasiliki-Maria Archodoulaki and Sigrid Lüft l 10.1 Glass Transition and Melting Temperature 25710.2 Coeffi cient of Linear Th ermal Ex pansion 26010.3 Th ermal Conductivity and Specifi c Heat 26010.4 HDT and Vicat 26110.5 Th ermo-Oxidative Degradation Behavior and Aging 26110.6 Testing of Long-Term Heat Aging 26610.7 Flammability 26710.8 Hot Sterilization 270References 271
11 Chemical Resistance of Polyoxymethylene 277
Sigrid Lüft l and Emmanuel Richaud11.1 Intoduction 27711.2 Degradation and Oxidation Mechanisms in POM 278
11.2.1 Depolymerization 278 11.2.2 Acidolysis 279 11.2.3 Th ermal Oxidation 280 11.2.4 Photothermal Oxidation 282 11.2.5 Conclusion on Polyacetal Degradation
Mechanims 28311.3 Resistance to Chemicals 283
11.3.1 Th eoretical Recall on Polymer Interaction with Solvents 283
11.3.2 Typical Resistance of POM in Presence of Alkalis, Acids or Solvents 285
References 295
12 Th e Electrical Response of Polyoxymethylene (POM) 301
D.A. Wasylyshyn12.1 Introduction 301
x Contents
12.2 Interactions between POM and Electromagnetic Waves 302
12.2.1 Dielectric Response of POM 302 12.2.2 Eff ects of Water on the Dielectric
Response of POM 305 12.2.3 Dielectric Properties of POM-Based
Composites 30712.3 Interactions between POM and Arc Plasma 313
12.3.1 Low Energy Applications 314 12.3.2 High Energy Applications 316
References 318
13 Electrical and Optical Properties of Polyoxymethylene 321
Natamai Subramanian Muralisrinivasan13.1 Introduction 32113.2 Electrical Properties 322
13.2.1 Conductivity 322 13.2.2 Dielectric Properties 322 13.2.3 Dielectric Loss Factor 323 13.2.4 Microwave Dielectric Measurements 323 13.2.5 Resistivity 327 13.2.6 Applications – Electrical Properties 327
13.3 Optical Properties 327References 329
14 Nanocomposites of Polyoxymethylene 331
Agnieszka Leszczyńska and Krzysztof Pielichowski14.1 Introduction 33114.2 Preparation and Structure of POM Nanocomposites
with Diff erent Nanoadditives 332 14.2.1 Methods of Fabrication of POM-Based
Nanocomposites 332
14.2.2 Modifi cation of Nanoparticles Towards Preparation of POM-Based Nanocomposites 335
14.2.3 Preparation and Structure of POM-Based Nanocomposites 339
14.3 Properties of Polyoxymethylene-Based Nanocomposites 347 14.3.1 Solid-State Phase Transitions and Polymer
Morphology in Polyoxymethylene Nanocomposites 347
Contents xi
14.3.2 Mechanical Behavior of POM-Based Nanocomposites 354
14.3.3 Th ermal Stability of POM Modifi ed with Nanofi ller 356
14.3.4 Tribological Properties and Wear Resistance of POM-Based Nanocomposites 364
14.3.5 Electrical Conductivity of Polyoxymethylene-Based Nanocomposites 373
14.3.6 Rheological Properties of Polyoxymethylene Nanocomposites 376
14.4 POM Blends as Matrices in Nanocomposite Materials 37614.5 POM Nanostructures – Electrospun POM Nanofi bers 38114.6 Applications of POM-Based Nanocomposites
and Future Trends 38514.7 Conclusions 386List of acronyms 387References 388
15 Future, Environmental Impact and Suppliers 399
Takashi Iwamoto and Junzo Masamoto15.1 Introduction 40015.2 Developments and Specialty Resins 400
15.2.1 Performance Improvement by Copolymerization of Polyoxymethylene and Polymer Alloy Technology 400
15.2.2 Technology for Improving Heat Stability 41215.3 Safety (Regulation and Approvals) 42115.4 Environmental Impact 42415.5 Suppliers and Commercial Grades 42615.6 Future 426References 432
Index 435
xiii
Preface
Polyoxymethylene Handbook: Structure, Properties, Applications and Th eir Nanocomposites summarizes many of the recent technological and research accomplishments in the area of polyoxymethylene (POM). Included in the book are presentations on state of art in the area, polyoxymethylene struc-ture, morphology, processing and applications, and new challenges and opportunities. Also discussed are the polymerization and manufacture of polyoxymethylene and various types of additives. Other topics covered are the structure and crystallization behavior of POM and its thermal, physi-cal, mechanical, fl ame retardant, chemical, electrical and optical prop-erties. Th e environmental impact of POM is also addressed. As the title indicates, the book emphasizes various aspects of polyoxymethylene and their nanocomposites for the scientifi c community.
Th is handbook serves as an up-to-date record on the major fi ndings and observations related to polyoxymethylene. It is intended to serve as a “one stop” reference resource for important research accomplishments on the subject. Th e various chapters in this book are contributed by prominent researchers from industry, academia and government/private research laboratories across the globe. Th e handbook is a very v aluable reference source for university and college faculties, professionals, post-doctoral research fellows, senior graduate students, and researchers from R&D lab-oratories working in the area of polyoxymethylene.
Chapter 1 is an overview of the state of art in the area and also presents new challenges and opportunities for polyoxymethylene and their nano-composites. An overview of the polymerization and manufacture of poly-oxymethylene is presented in Chapter 2. Its author explains monomers and discusses other topics such as comonomers, polymerization, homo-polymerization, copolymerization, block copolymers and terpolymers. In Chapter 3, several diff erent types of additives used for the synthesis and manufacturing of polyoxymethylene are discussed. Th ere is an analysis of many types of agents such as UV stabilizers, impact modifi ers, nucleating
xiv Preface
agents, pigments, fl ame retardants, antistatic and lubricating agents, fi llers and other additives
Th e main focal point of Chapter 4 is the diff erent types of processing methods such as injection molding, extrusion, and blow molding, among others, for polyoxymethylene and their nanocomposites. Also reported in this chapter are topics related to POM products such as highly oriented products, recycling of production waste, machining, assembling of mold-ings and semi-fi nished parts. A survey on applications of polyoxymeth-ylene is provided in Chapter 5. Many explanatory topics and subtopics are used by the chapter’s authors such as automotive, electronic and consumer applications, white goods, mechanical engineering, plumbing and water installation. A discussion about the structure and morphology of poly-oxymethylene is presented in Chapter 6. Th is chapter mainly discusses the crystalline structure of POM; orthorhombic, hexagonal phases, crystal structure determination, morphology of orthorhombic, hexagonal POM, morphology of rubber-modifi ed POM and structure-property relation-ships. Microscopically-viewed crystal structure and crystallization behav-ior of POM are discussed in Chapter 7. In this chapter the author’s review of physical and thermal properties is based on X-ray scattering, vibrational spectroscopy and lattice dynamical theory. Th e physical properties of poly-oxymethylene homopolymers and copolymers are discussed in Chapter 8. Among the topics covered are density, hardness, heat capacity, melt fl ow behavior and other tribological properties, as well as water absorption and gas permeability
In Chapter 9, the mechanical properties of polyoxymethylene are pre-sented in two major parts. Th e fi rst part is devoted to short-term mechani-cal properties such as elastic properties as a function of temperature and morphology aspects, and failure properties and the diff erent strategies to improve them. Th e intent of the second part is to present long-term prop-erties such as creep behavior and modifi cations induced by aging, which lead to the embrittlement of POM, thereby limiting its lifetime. Th ermal properties and fl ammability are discussed in Chapter 10. Among the top-ics are glass transition, melting temperature, coeffi cient of linear thermal expansion, thermal conductivity, thermo-oxidative aging, and testing of long-term heat aging.
Chemical resistance is discussed in Chapter 11. Th e authors explain the diff erent types of chemical changes and chemical properties of poly-oxymethylene. Included in the many topics and subtopics are degradation and oxidation mechanisms in polyoxymethylene, depolymerization, ther-mal oxidation, photothermal oxidation, polyacetal degradation mecha-nism, resistance to chemicals, theoretical recall on polymer interaction
Preface xv
with solvents and typical resistance of polyoxymethylene in the presence of alkalis, acids or solvents. Chapter 12 explains the electrical response of polyoxymethylene (POM) homopolymer. Th is chapter includes vari-ous discussions about interactions between POM, electromagnetic waves, dielectric response of POM, eff ects of water on dielectric response of POM, dielectric properties of POM-based composites, interactions between POM, arc plasma, low energy applications and high energy applications. Diff erent aspects of electrical and optical properties are explained in Chapter 13 such as conductivity, dielectric properties, dielectric loss factor, microwave dielectric measurements and resistivity applications.
Next, Chapter 14 on nanocomposites of polyoxymethylene explores the subject using many subtopics such as methods of fabrication of POM-based nanocomposites, modifi cation of nanoparticles towards preparation of POM-based nanocomposites, preparation and structure of POM-based nanocomposites, properties of polyoxymethylene-based nanocomposites, POM blends as matrices in nanocomposites and POM nanostructures-electrospun POM nanofi bers. Finally, the future developments in POM are discussed in Chapter 15 along with its environmental impact and suppliers; in particular, specialty resins, performance improvement by copolymeriza-tion of polyoxymethylene and polymer alloy technology, technology for improving heat stability, high viscosity (high molecular weight) POM, safety (regulations and approvals), and commercial grades of POM.
Lastly, the editors would like to express their sincere gratitude to all the contributors of this book who provided excellent support throughout the successful completion of this venture. We are grateful to them for the com-mitment and the sincerity they have shown towards their contributions to the book. Without their enthusiasm and support, the compilation of a book would not have been possible. We would like to thank all the review-ers who have taken their valuable time to make critical comments on each chapter. We also thank the publisher Wiley-Scrivener for recognizing the demand for such a book and for realizing the increasing importance of the area of polyoxymethylene and their nanocomposites, and also for starting such a new project on which not many other publishers have yet to put their hands.
Sigrid Lüft lVisakh P. M.
1
1
Polyoxymethylene: State of Art, New Challenges and Opportunities
Sigrid Lüft l1,* and Visakh P.M.2
1Vienna University of Technology; Institute of Materials Science and Technology,
Vienna, Austria2School of Chemical Sciences and Centre for Nanoscience and
Nanotechnology
Mahatma Gandhi University, Kerala, India
AbstractPolyoxymethylene (POM) is a semicrystalline polymeric material belonging to
engineering thermoplastics because of its low friction and wear characteristics
and its excellent balance of mechanical properties and chemical resistance to
most solvents, chemicals and fuels at room temperature. Hence, such polymers
are used to serve as an alternative to metals. As formaldehyde was initially the
raw material used in the manufacturing of commercial POM it is also referred to
as polyformal or polyacetal. Among the polymers obtained from the polymeriza-
tion of aldehydes, POM is the only one that could reach commercial signifi cance.
Polyoxymethylene resins have traditionally covered homopolymers (POM-H) and
copolymers (POM-C), and more recently ter- and block-copolymers have been
developed. Today, most of the POM resins manufactured worldwide are used in
the production of automotive parts followed by electrical and electronic parts, and
industrial parts. Hence, at present most of the POM is manufactured and con-
sumed in Asia. However, besides Asia, manufacturing sites with high production
capacities are also located in Germany and the USA.
Keywords: Polyacetal, engineering thermoplastic, automotive industry, produc-
tion capacity
*Corresponding author: s.lueft [email protected]
Sigrid Lüft l, Visakh P.M., and Sarath Chandran, Polyoxymethylene Handbook, (1–20)
2014 © Scrivener Publishing LLC
2 Polyoxymethylene Handbook
1.1 Scope
Because of its low friction and wear characteristics, excellent balance of mechanical properties and chemical resistance to most solvents, chemicals and fuels at room temperature, polyoxymethlene (POM) is an engineer-ing thermoplastic material whose typical applications as of today include cover sliding and guiding elements, gears, housing parts, brewing units of espresso machines, loudspeaker grills, fastener release buttons, impel-lers, springs, chains, screws, nuts, insulators, coils, fi ttings, zippers; parts of pumps, valve bodies, televisions, windows, locks, door lock systems, hinges, joint covers, insulin pens, powder inhalers, etc.
In principle, polyacetal resins can be processed by all methods suitable for thermoplastics. However, the most important methods are injection molding and extrusion. Lightly crosslinked grades can be used to produce hollow moldings by blow molding. Extrusion applies to the manufacture of semi-fi nished parts like fi lm, sheet, rods, pipes, and profi le sections, most of which are further machined by means of cutting tools to form fi nished parts. Even the most complicated moldings can be mass produced very economically by injection molding.
In the past the demand in this resin has increased steadily by 5–7% per year.
1.2 History
Polyoxymethylene (POM) dates back to 1859 when the Russian chem-ist Aleksandr Mikhailovich Butlerov published his fi ndings on working with 1,2-diiodoethane. He identifi ed a structure which he thought was composed of C
4H
4O
4 and called it “dioxymethylene” [1]. Th e synthe-
sized dioxymethylene was white and had a melting point of 152 °C, it was odorless at room temperature but developed a pungent smell upon heat-ing. Th e beginning of a thermooxidative degradation could be noticed at 100 °C, but a complete gasifi cation occurred only beyond the melting point. Further, when the dioxymethylene was boiled in water, ether or alcohol for a short time no dissolution could be observed. However, stor-age at 100 °C in water in a sealed glass container for a few hours resulted in its complete dissolution. Of course at that time nobody knew what to do with the synthesized dioxymethlene and it remained an object of laboratory studies for a while. Later on, Butlerov became one of the fi rst chemists to systematically study polymerization reactions, and in partic-ular reactions with formaldehyde. Synthetic polymers were niche prod-ucts only and hardly known, mostly as rubber materials. Further, neither
Polyoxymethylene: State of Art, New Challenges 3
knowledge on how to process this new kind of material as plastics nor appropriate processing technologies existed.
At the beginning of the 20th century, Auerbach and Barschall suc-ceeded in synthesizing polyoxymethylene by polymerizing aqueous formaldehyde solutions with sulphuric acid. Th ey obtained polyoxymeth-ylenes that exhibited a diff erent resistance against diverse chemical agents, specifi cally in view of the resulting degradation behavior. Th is led at the time to a categorization of polyoxymethylenes diff erent from that of today; polyoxymethylenes were separated into a-polyoxymethylene, b-polyoxymethylene, g-polyoxymethylene and d-polyoxymethlylene. Th e a-polyoxymethylene corresponds to a polyoxymethylene with both ends composed of OH-groups (polyoxymethylene dihydrate), while the b-polyoxymethylene corresponds to a sulphuric acid ester of the poly-oxymethylene dehydrate; the name g-polyoxymethylene is assigned to a polyoxymethlylene dimethyl ether. Th e d-polyoxymethlylene is in fact a g-polyoxymethylene having adjacent C-C-bonds because of a rearrange-ment of some formaldehyde molecules [2–7].
In the 1920s, Hermann Staudinger and his coworker started extensive investigations on polyoxymethylenes since they thought that they could use them as a kind of model to derive the structure of more complex cellulose [8]. As a result, a series of up to 40 publications from 1925 to 1971 dealing with the diff erent properties they found in the course of their work with these formaldehyde-derived polymers was published from the 14th com-munication on under the serial title “Mitteilung über Polyoxymethylene” (Communication on Polyoxymethylenes) in diff erent journals at the begin-ning, but later on specifi cally in Die Makromolekulare Chemie and today in Macromolecular Chemistry and Physics.
At the beginning of their works on the polymerization of formaldehyde to polyoxymethylene Staudinger and his coworker could obtain poly-mers having a polymerization degree in the range of 50 to 100 only, but with these low molar mass polymers a considerable gain in knowledge on polymer structures and polymerization know how could be generated. Specifi cally, the role of the polymer chain end groups on the degradation behavior could be elucidated. Later on the workgroup could synthesize a polyoxymethylene diacetate with a melting point around 150 °C to 170 °C [6, 7, 9–12]. In those days it was also found that even small amounts of hydroxyl ions are able to catalyze the cleavage of the polyoxymethylene hydrate chain into formaldehyde, while sodium hydroxide solutions do not deteriorate polyoxymethylene dimethyl ether [13, 14].
Th e working group of Staudinger was primarily focused on deter-mining the structure of polymeric materials as previously mentioned.
4 Polyoxymethylene Handbook
Unfortunately, polyoxymethylene with higher molar mass could hardly be dissolved in those days without using harsh conditions (e.g., temperature in the range of the melting and beyond) that eventually caused degrada-tion in this material. Further, with the drawback of thermal instability due to the OH-group at the end of the macromolecular chain, POM was not a promising candidate for commercial exploitation [12, 15, 16].
However, in the 1940s E.I. Du Pont de Nemours & Co. in Wilmington, Delaware (USA) started extensive research on a process to obtain pure formaldehyde, and later on a resin derived from it. As a result, Robert N. McDonald could synthesize polymers that had a higher thermal stability than previous polyoxymethylenes. Ultimately, in 1954 F.C. Starr submit-ted a patent application “process for the polymerization of formaldehydes employing metal carbonyl initiators” and obtained the patent in February 1956, while MacDonald received his patent for “Polyoxymethylene” in October 1956 [17–19]. Hence, DuPont announced the production of a commercial POM-H with the trade name Delrin®. By the end of the 1950s the fi rst production plant of Delrin with a capacity of ca. 11.000 metric tons (20 million pounds) per year was installed at Parkersburg, West Virginia, and production started in 1960 [19, 20] (Fig. 1.1).
With their commercialization, POM-H polyacetals, became the focus of other research institutions and chemical companies, and as a result several papers dealing with polymerization, structure and thermal stability were
E.I.Du Pont de
Nemour
Delrin®Ultraform GmbH
Ultraform®
Ticona
GmbH
Hostaform®
Zaklady Azotowe in
Tarnów-Mościce S.A
Tarnoform®
1960
Polyplastics
Co., Ltd.
Duracon®
Celanese
Corporation
Celcon®
Asahi Kasei
Chemicals
Corporation
Tenac®
Mitsubishi Gas
Chemicals Company
Iupital®Asahi Kasei
Chemicals
Corporation
Tenac® C
PTM Engineering Plastics
(Nantong) Co., Ltd.
Duracon® Tepcon®
1970 1980 1990 2000 2010
Korea Engineering
Plastics Co., Ltd.
Kepital®
Asahi-Du Pont POM
(Zhangjiagang) Co., Ltd.
Tystron®
Figure 1.1 Manufacturing and commercialization of POM resins from 1960 to present.
Polyoxymethylene: State of Art, New Challenges 5
published, specifi cally in the former Soviet Republic, Japan and Germany [e.g., 21–59] from 1960–1970. Despite extensive research on the polymer-ization of other higher aldehydes [16, 20], none of this polymers made it out of the laboratory or reached a commercial signifi cance as POM.
Further, in 1960 another chemical company in the USA, the Celanese Corporation, applied for a patent disclosing a process to manufacture a polyoxymethylene copolymer by polymerizing trioxane and cyclic ethers in cyclohexane using boron fl uoride etherate as a catalyst. Th e copolymer obtained by this process exhibited higher thermal stability than the homo-polymer. Th e Celanese Corporation received the patent in 1962 [60] and called the copolymer Celcon®.
In 1961, the Celanese Corporation and Hoechst AG of Germany signed a joint venture to create Ticona Polymerwerke GmbH in Kelsterbach (Germany) for the purpose of producing and marketing a POM-C under the trade name Hostaform®. Th e production of Hostaform started in 1963. Moreover, in 1964 the Celanese Corporation signed a joint venture with Daicel Chemical Industries, Ltd. (Current name: Daicel Corporation) of Japan and founded Polyplastics Co., Ltd., to manufacture POM-C in Japan under the trade name of Duracon®. Later on in 1987, Hoechst took over the Celanese Corporation, which was then merged with the American Hoechst Corporation to form the Hoechst Celanese Corporation in the US. One year aft er that event, Hoechst Celanese was involved in a new joint venture uniting Hoechst Celanese, Hoechst AG, Polyplastics (the Chang Group of Taiwan and Hoechst Taiwan) to create TEPCO (Taiwan Engineering Plastics Company). Finally, in 1999 aft er the restructuring of Hoechst AG, the Celanese AG as well as Ticona were spun off . Today Ticona represents the high-performance plastic materials business part of the Celanese Corporation [20, 61].
Th e German companies BASF AG and Degussa AG decided in 1971 to found Ultraform GmbH to manufacture a POM-C (Ultraform®) that con-tains oxybutyl-units from the comonomer [20, 62, 63].
Th e Asahi Kasei Chemicals Corporation of Japan was begun in May 1972 with the production and sales of homopolymer resins (Tenac®) and in December 1985 for copolymer resins (Tenac®C). Asahi Kasei is at present the worlds only producer of both POM-H and POM-C with a capacity of 44.000 t/a (20.000 t/a homopolymer, 24.000 t/a copolymer) [64].
A further European polyacetal producer, the Polish Zakłady Azotowe in Tarnów-Mościce S.A., started with the production of a POM-C (Tarnoform®) in Tarnów in 1976 [20].
A third Japanese company, the Mitsubishi Gas Chemical Company,was launched in 1981 with the production of a POM-C (Iupital®) in Yokkaichi.
6 Polyoxymethylene Handbook
Th e capacity of the plant was 10.000 t/a [20]. Th e polyacetal business was outsourced to a separate company in 1994 with the founding of the Mitsubishi Engineering-Plastics Corporation. Moreover, in 1987 Mitsubishi Gas Chemical Company, Inc., Mitsubishi Corporation in Korea and Tongyang Nylon Co., Ltd. (Hyosung Corp. since 1998) established Korea Engineering Plastics Co., Ltd. (KEP) as a joint venture. Th e fi rst commercial production began in 1988 under the brand name Kepital®. Th e production capacity of 65.000 t/a in 1998 covered about 10% of the world consumption in polyacetal resins [65].
For the purpose of manufacturing and selling POM resins in China, in December 2001 a joint venture of Polyplastics Co., Ltd., Mitsubishi Gas Chemical Company, Inc., Korea Engineering Plastics Co., Ltd., and Ticona PTM Engineering Plastics (Nantong) Co., Ltd. was founded [66].
A joint venture between DuPont China Holding Company and Asahi Kasei Chemicals Corporation was established on August 8, 2002 which founded the Asahi-DuPont POM (Zhangjiagang) Co., LTD., located in Jiangsu Zhangjiagang Yangtze River International Chemical Industrial Park. Th e polyacetals are called Tystron® and their production started in 2004 in a plant with an initial capacity of 20.000 t/a [67].
A further joint venture between Celanese Corporation and Saudi Basic Industries Corporation (SABIC) announced in April 2010 the construc-tion of a 50.000 t/a POM production facility in Saudi Arabia in the frame of their National Methanol (Ibn Sina) co-operation started in 1981. Th e key feedstock methanol for synthesizing POM is to be produced at Ibn Sina. Th e facility was envisaged to go on-stream by 2013 [68, 69], but it seems that it will only begin commercial operation in 2015 [70].
Besides the typical POM-H and POM-C resins, POM block copolymers were commercialized by Asahi Chemical in 1987 [71]. According to ref. [72], block copolymers with oxymethylene segments can be obtained in diff erent ways. In the so-called chain-transfer copolymerization, trioxane is polymerized in the presence of oligomers or polymers with end groups allowing a chain transfer with the growing oxymethylene-cation. Th is pro-cess leads in dependence of the number of reactive end groups to diblock- or multiblock-copolymers.
Another process uses oxymethylene blocks carrying reactive end groups that can link to other polymer blocks via polycondensation or polyaddi-tion reactions [72].
In the Asahi process, formaldehyde is polymerized in the presence of a functional polymer with the structure R(X)
m-OH with an active hydro-
gen atom (Fig. 1.2). In this formula, R is conventionally a long-chain alkyl group and X an oxyalkylene group. Th e functional group on the acetal
Polyoxymethylene: State of Art, New Challenges 7
block copolymer can be varied, e.g., the use of an elastomeric functional polymer leads to a toughened acetal block copolymer [71].
Further POM terpolymers have been developed that can be obtained, e.g., by cationic polymerization of trioxane and a substantially equal molar mixture of α,α- or α,β-isomers of glycerol formal or their ester-functionalized derivatives. Th erefore, the resulting terpolymer carries side-chain functionality (i.e., hydroxyl or ester functional groups pendent from the backbone of the polymer) in contrast to simply blended block copoly mers (Fig. 1.3). Th is provides greater functionality, since such acetal terpolymers may be reacted with other resins and/or chemical moieties so as to enable synthesis of acetal block copolymers with desired chemical and/or physical properties [73].
In comparison with a conventional trioxane-ethylene oxide copolymer, the functionalized terpolymer shows an increased thermal stability above 200 °C. Th e crystallinity is greater than about 40% [73].
1.3 Commercial Signifi cance
Polyoxymethylene as an engineering thermoplastic is used for a huge number of applications ranging from big functional parts in the automo-tive and mechanical engineering industry to tiny precision parts in drug delivery systems. Since the beginning of commercialization in the 1960s, POM has been developed as a plastic material used specifi cally in technical applications with high demands on mechanical properties as well as good dimensional stability and sliding properties (Table 1.1).
Th e current POM market is supplied by six major manufacturers that provide about 70% of the worldwide production (Table 1.2). Further, POM manufacturers with lower market shares are, for instance, Mitsubishi Engineering-Plastics Corporation (Tokyo, Japan, POM-C Iupital®), Asahi Kasei Chemicals Corporation (Tokyo, Japan, POM-H and POM-C Tenac®),
n CH2O + R(X)mOH R(X)m—O(CH
2O)n—H
Figure 1.2 Chemical reaction to polymerize an acetal block copolymer.
Figure 1.3 Structural formula of an acetal terpolymer [73].
–(CH2O)
m(CH
2 CH–O)
n– –– – – – –
CH2
–
X–
X
(CH2
CH–CH2
O)p
X ... hydroxyl or ester radical; m,n,p ... integers
such as that m+n+p is between 5 to 20.000
Table 1.1 Typical properties of POM resins (without consideration of special types).
Advantages Disadvantages
• Excellent short-term mechanical
properties in terms of high tensile
strength, rigidity and toughness
• Poor thermal stability without
appropriate stabilizer system for
POM-H
• Excellent long-term mechanical
properties like low tendency to
creep and fatigue
• Poor chemical resistance to strong
acids, bases and oxidizing agents,
and to certain metal ions
• Good mechanical and electrical
properties up to 140 °C
(short-term) and 90 °C (long-term)
• Reduced resistance to hot water
(specifi cally in the presence of
chlorine)
• Good mechanical properties over
a large temperature range even at
low temperatures down to –40 °C
• Can be stained by strong
discoloring food (tea, coff ee, red
wine)
• Excellent chemical resistance
against most chemicals, organic
solvents and fuels at room
temperature
• Flammable without fl ame
retardants because of the high
oxygen content in the polymer
chain
• Not susceptible to environmental
stress cracking
• Excellent dimensional stability
• Release of toxic gases (e.g.,
formaldehyde) when subjected to
high temperature or burning
• Low permeability to many
chemicals, fuels and gases
• Low water absorption
• Diffi cult to bond or decorate
without surface preparation
because of the low surface energy
and polarity
• Excellent tribological proper-
ties like low static and dynamic
coeffi cient of friction, and high
resistance to abrasion
• For outdoor applications
appropriate UV-stabilizers are
needed, further POM is sensitive to
radiation
• High hardness in comparison with
other thermoplastics
• Glossy and smooth molded
surfaces
• Special types are required for
medical and food applications,
further not recommended as
implant material by diff erent
manufacturers
• Can be processed in general on all
standard processing machinery for
thermomplastics
Polyoxymethylene: State of Art, New Challenges 9
or Zakłady Azotowe w Tarnowie-Mościcach S.A. (Tarnów, Poland, POM-C Tarnoform®) [74]. But due to ongoing formation and dissolution of joint ventures among the diff erent POM manufacturers it becomes diffi cult to estimate the real market shares of the diverse companies.
POM resins are produced in different grades with various degrees of polymerization resulting in different properties. In general the individual grades differ from one another in their flow behavior (melt mass-flow rate) and in the type and concentration of additives used. Thus, POM resins can usually be separated into the following categories [75–81]:
• Basic or standard grades: Th ese grades diff er primarily in their melt fl ow rate to meet the requirements of diff erent processing methods and conditions.
• Grades with improved slip/wear properties: Th ese grades are modifi ed with special additives (e.g., MoS
2, graphite, PTFE,
chemical lubricants, mineral fi ller, etc.) which enhance slip properties and/or abrasion resistance.
• Reinforced grades: Th ese grades typically contain glass fi bers or glass spheres, some special grades with carbon fi bers are also on the market. Depending on the type and content
Table 1.2 Major manufacturers of POM and their market share in 2011 [74].
Company Site, Country Resin type, Trade
name
Market
share
Ticona GmbH Kelsterbach,
Germany
POM-C Hostaform®
Celcon®
20–21%
Polyplastics Co., Ltd. Tokyo, Japan POM-C Duracon®
Tepcon®
17–18%
E.I. Du Pont de
Nemour & Co.
Wilmington, USA POM-H Delrin® 15–16%
Korea Engineering
Plastics
Seoul, Republic of
Korea
POM-C Kepital® 7–9%
BASF SE Ludwigshafen,
Germany
POM-C Ultraform® 6–8%
Asahi-DuPont POM
(Zhangjiagang)
Co., Ltd.
Zhangjiagang,
China
POM-C Tystron®
POM-H Delrin®
6–8%
10 Polyoxymethylene Handbook
of fi ller they diff er from the basic grades in their higher ultimate tensile strength and/or higher rigidity.
• High-impact or toughened grades: Th ese resins are blends with elastomers (e.g., rubber, thermoplastic polyurethane) in order to obtain a higher impact strength than the basic grades.
• Special grades: Th is group includes all grades which cannot be assigned to one of the above groups, e.g., grades with special additives against weathering or chemical agents like fuels, electrical conductive grades, medical grades, low emission grades, etc.
Th e price for basic grade pellets currently ranges from 900 US$/t to 1800 US$/t; special grades can reach prices >100.000 US$/t. Recycling POM (purity 92–96%) can be obtained at a price ranging from 800 US$/t to 1.000 US$/t depending on the grade and amount of contaminants. Semi-fi nished parts (rods, sheets, boards) from basic grades may be purchased at a price ranging from 1.000 US$/t to 6.000 US$/t, while the price range of semi-fi nished parts from special grades is 4.000 US$/t to >100.000 US$/t.
A continuous increase in the consumption of POM over the years can be observed (Fig. 1.4). Average growth rates in the range of 5–7% are reported [74, 82–85] over the period depicted in Figure 1.4. However, in
Figure 1.4 Worldwide consumption of POM resins over the last 30 years based on data
from refs. [63,74,82–95].
800.000
700.000
600.000
500.000
400.000
300.000
200.000
100.000
1980 1985 1990 1995 2000
PO
M c
on
sum
pti
on
in
me
tric
to
ns/
ye
ar
Year
2005 2010 2015
900.000
Polyoxymethylene: State of Art, New Challenges 11
2001 a drop in the demand in POM resins from 610.000 t/a to 560.000 t/a resulted because of diffi culties in the automotive industry [86]. Further, the economic crisis of 2008 followed by its global economic slow down in 2009 is refl ected in the consumption of POM resins during this period. Nonetheless, since 2010 the demand for POM has again increased as the global economy is recovering, and specifi cally in China an extraordinary rise in consumption in plastics can be noticed in comparison to other countries.
Th e major consumer of POM is the automotive industry followed by the electronic industry and consumer goods: automotive industry 27%, elec-tronics 21%, consumer goods 16%, industrial applications 11%, fl uid han-dling 7%, home appliance 4%, other 14% on a worldwide scale, but a clear diff erence in the repartition of the several applications can be seen in the three main regions involved in the major consumption of POM (Fig. 1.5) in 2009.
In Europe about 38% of the processed POM belongs to the automotive sector [63, 95–97]. Here the whole product range of POM is currently used, e.g., basic grades, fi ber reinforced grades, mineral fi ller containing grades, impact modifi ed grades, UV resistant grades, easy fl owing grades for thin-walled parts, low-friction and low-wear grades, fuel resistant grades, low odor grades, etc.
Figure 1.5 Consumption of the POM resins by regions based on data from [63].
Other
14%
Other
19%
Other
6%Home appliances
3%
Home appliances
6%
Fluid handling
4%
Home appliances
6%Automotive
24%
Automotive
38%
Automotive
24%
Electronics
31%
Asia USA
Europe
Electronics
10%
Electronics
6%
Fluid handling
5% Fluid handling
15%
Industry
4%
Industry
15%
Industry
25%
Consumer goods
19%
Consumer goods
8%
Consumer goods
18%
12 Polyoxymethylene Handbook
Typical applications are in component parts for safety belt buckles, loudspeaker grills, fuel systems, snap fi ts, gears for the movement of wing mirrors, fan wheels, etc.
A special POM grade enables the manufacture of indoor components having a metallic appearance. Th is grade can be used for door pulls or other components that usually require a laborious procedure to obtain an aesthetic and robust metallic fi nishing [63].
To meet the market demand in POM the production capacities of the diff erent manufacturers are constantly expanded and new manufacturing sites are constructed. In 2006, Ticona GmbH settled with the Frankfurt Airport to relocate their production facility in Kelsterbach, as it was an obstacle to the expansion of the airport. Hence, Ticona had to move its manufacturing plant to the Höchst Industrial Park. On this occasion, in 2011 a production plant was installed with an increased capacity of 40% (to 140.000 t/a) in view of the increasing demand for polyacetals [98].
Other manufacturers enlarged their production capacities specifi cally in Asia to respond to the increasing demand in automotive parts made of POM in this part of the world. In particular between 1998 and 2001 the production capacities have been expanded. Th is is particularly true for the Asian region that nowadays holds a capacity share beyond 50% of the world production capacity in POM resins [86] (Fig. 1.6). Th e production of polyplastics in Japan is currently at 100.000 t/a, in Kuanam (Malaysia) it will be raised by 90.000 t/a. Further, the company owns production facili-ties with a capacity of 20.000 t/a in Taiwan and 30.000 t/a in Malaysia. Th e joint venture with PTM Engineering Plastics in Nantong (China) gave access to an additional production capacity of 60.000 t/a. Mitsubishi Gas Chemical enlarges the production capacity up to a scale of 80.000 t/a to 100.000 t/a in its production sites in China and Th ailand [74].
For the future it is expected that the growing purchasing power in Brazil, Russia, India and China (BRIC countries) will lead eventually to an increased consumption in automotive parts as well as consumer goods and home appliances, the major applications of POM resins. Hence, the shift in the division of markets from Europe and USA towards Asia will be more pronounced. As a result the enlargement of production capacities for POM resins in the Asian region will continue in order to meet the demands in this part of the world [74].
In the past the enlargement of the product range in POM resins has been the focus of the manufacturer. Nowadays, the current products are optimized in view of their properties. Nevertheless, new products have been presented to the market. Ticona has developed a POM resin that can be detected by ultraviolet (UV) light. Th e purpose of MT (medical
