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Preface

“If I have seen a little further, it is by standing on the shoulders of giants.” (Isaac Newton, 1676) The excellent original book “Powder Coatings, Chemistry and Technology” written by Tosko Aleksandar Misev and later updated by Peter Gillis de Lange has served for my generation as a sort of bible on its sector. Therefore, it has been a great honor while at the same time a tremendous challenge to write a third edition of this book. Due to this respect the original text is maintained where possible, but altered and extended if recent scientific findings made it necessary.

The Stone Age did not end due to a lack of stones. New technologies replace the old ones; novel developments change the look of the world, sometimes at an incredibly fast speed. Just to name a few recent incidents since the last print of the book: The rise of China, severe worldwide financial crises, the changed way of communicating (e.g. smart phones, social media) and radical governmental regulations (e.g. REACH). Of course, these developments had influences on the world of powder coatings, too. Consolidation, relocation and customization are some of the effects. Replacement of toxic ingredients (e.g. TGIC), use of energy saving curing methods and the extension of powder coatings into new application fields are the interconnected technological changes that are covered in this book.

This book is directed to anybody who is involved in developing, producing, testing and marketing of powder coatings, raw materials or application equipment.

What is new compared to the second edition? More than 5,000 recent articles and patents concerning powder coatings have been evaluated and 250 of those have been referenced in this book to ensure that it illustrates the current state-of-the-art. Highlighted core terms and a significant extended index should help finding the desired topic in a shorter time. A list of powder coating related web addresses will enable the reader to locate additional relevant information at the push of a button. Product and company names have been updated as much as possible. Plus more than 30 new photos, diagrams and drawings complete this revised and updated third edition.

I would like to thank Werner Grenda for valuable discussions, Dr. Corey King for corrections of the manuscript and Dr. Michael Ringel for his contribution regarding REACH. Many thanks to the two dozens companies for the excellent additional photos they provided for the 3_rd edition of this book.

1Introduction

1.1Historical background

Two thousand five hundred years ago the great Greek philosopher Thales of Miletus (624 to 556 BC), who was dubbed as the “father of science”, was the first to discover that amber stone when rubbed attract other objects. The Greek word for amber,

(electron), is the origin of electrostatic forces, which are used nowadays for almost 90% of all powder coating applications.

The appearance of powder coatings is often associated with the ecological and energy related events of the late 1960’s and early 1970’s. The famous Rule 66 which was brought in by The Town Council of Los Angeles in 1966 was the first legislative act regulating the environmental aspects of the coatings. Later on similar regulations were introduced in most of the industrially developed countries.

Although the history of powder coatings has been strongly influenced by environmental aspects, first developments in the field began in the 1940’s with a simple flame spray application process. Early in 1950’s powdered PVC was successfully applied by Gemmer in a fluidized bed process on a preheated metal surface ^[1]. A patent application for Gemmer’s invention was filed in Germany in 1953 and the patent was issued in 1955. Very soon the fluidized bed technique for application of thermoplastic powders including polyethylene and nylon powder coatings was well established in the USA.

In the late 1950’s the first thermosetting powder coatings appeared on the market, mainly as a result of the research work done by Shell Chemicals. The target was development of superior protective (“functional”) organic coatings for the company’s own underground natural gas and oil pipelines. The first systems were relatively simple physical dry blends of epoxy resins, hardeners and pigments dispersed by ball milling techniques. Due to a considerable degree of heterogeneity, the application results were rather inconsistent.

The hot melt mixing methods of the present day for production of powder coatings were preceded by a technique that employed liquid epoxy resins and hardeners. The homogeneous liquid binder/crosslinker blend was prereacted until partially cured (“B stage”) solid material was obtained, which was finely ground in the next step. The completely cured “C stage” was obtained by stoving the “B stage” powders at high temperatures. A drawback of this technique was the lack of reproducibility and difficult control of the process ^[2].

Hot melt compounding on a heated twin roller mill or in a heated Z-blade mixer was already a step forward in the development of thermosetting powder coatings, but the immense cleaning problems, created by the fast(er) curing powder coatings, have almost completely excluded the Z-blade mixer and of course the twin roller mill from the machines (extruders) used to produce contemporary powder coatings. However, Z-blade mixers are still used for batch-wise production of thermoplastic powder coatings, where chemical reactivity does not play a role.

Extrusion methods for production of thermosetting powder coatings, which are in current use, were developed in the Shell Chemical Laboratories in England and The Netherlands in the period 1962 to 1964. In 1962 the first decorative epoxy/dicyandiamide (DICY) powder coatings, produced by Wagemakers (now DuPont) in Breda, and soon followed by Libert in Ghent and Van Couwenberghe in Le Havre, appeared on the European market. Also in 1962 SAMES in France developed the first equipment for electrostatic spraying of organic powder coatings. This made a considerable contribution to the success of decorative thermosetting powder paint, since for the first time coatings with an acceptable “thin” layer thickness could be applied.

One serious drawback of epoxy/DICY thermosetting powder coatings is their sensitivity to attack by UV light. Exposed to sun they chalk and deteriorate rapidly. This, followed by poor yellowing resistance restricted their use mainly to protective coatings or decorative coatings for interior use where resistance towards yellowing was not a prime target.

Attempts to overcome these problems led to polyester/melamine systems which were introduced in 1970 by Scado BV and UCB in The Netherlands and Belgium ^{[3, 4]}. Almost at the same time Hüneke reported that the gloss retention and yellowing resistance of powder coatings based on blends of epoxy and polyester resins is considerably improved compared to pure epoxy powders ^[5].

The real breakthrough in the area was made in 1971 by Scado BV in The Netherlands. It was discovered that coatings with exceptionally good decorative properties can be obtained by hot melt compounding of carboxyl functional polyester resins and epoxy compounds in the form of bisphenol A resins (polyester/epoxy hybrids) or triglycidyl isocyanurate (TGIC systems) ^[6]. Shortly after, in 1972 a weathering resistant powder coating system produced by VP-Landshut in Germany was used for protection of aluminum extrusions and claddings in outdoor architecture in Switzerland ^[7].

At the same time in 1971 Bayer AG and BASF AG in Germany each offered thermosetting acrylic powder coatings to the market. Although later on attempts to commercialize the acrylic systems failed in Europe and the USA, they were widely accepted in Japan for outdoor use.

In the 1980’s polyurethane powder coatings established a solid position in the American market and in Japan with a marginal market share in Europe.

Other developments in the area include a wider acceptance of PVDF powder coatings ^[8] mainly used for monumental architectural objects and ethylene chlorotrifluoroethylene (ECTFE) powders for protective purposes ^[9].

The development of new powder coating systems was quickly followed by improvements in the production and application equipment. Although the melt extrusion method, “borrowed” in the1960’s from the plastic industry, remained almost the only process used for powder production, contemporary plants are changing substantially and very often use a continuous concept of powder paint production.

Developments in the application equipment led to the introduction of so-called frictional tribo guns. The frictional method of electrical charging of powder coatings has reduced considerably the Faraday cage effect, improving the penetration of powder coatings into recessed areas during spraying. These developments were followed by corresponding efforts to overcome the inefficient tribo chargeability of polyester (including hybrid) powder coatings, which at this moment dominate the market ^[10–13].

Reclaiming of powder during application is an advantage that leads to almost 100 % utilization of the material. This creates, however, serious problems with respect to the color change in the spraying booths. The producers of application equipment have made considerable efforts to speed up the color change. This resulted in the development of systems that allow color change up to 5 minutes which is quite close to the liquid systems ^[14]. But modern equipment suppliers have their doubts and say, that the shortest possible time for color changing is 10 to 15 minutes.

The automotive market is one of the most demanding concerning the protective and decorative characteristics of the coating. The lack of good flow was an inborn weakness of powder systems and the orange peel effect was one of the major concerns. Other problems encountered when powder paints were used as automotive body coatings were inconsistent transfer efficiency, color change time, color contamination, difficulties to reach areas around the doors, and under the hood and trunk areas. Together with relatively high curing temperatures these were the main reasons for powder coatings not being successful competitors to the wet paints for car body finishing. Therefore, for a long time all applications have been limited to underbody and interior trim components and later on to exterior trim parts and steel and aluminum car wheels and the car manufacturers remained reluctant to use powder as a full-body coating.

However, Honda in Japan began in the 1980’s with the application of a powder coating primer-surfacer on the car bodies, followed by Fiat in Italy. The car manufacturers in the USA were more careful, regarding the speed of introduction of full-body primer-surfacers. General Motors began in 1982 with a powder primer-surfacer on small pick-up trucks and built up experience and know-how, which was implemented in the early 1990’s to passenger cars. Miller and Kerr ^[15] summarize the qualities and advantages provided by powder primer-surfacers as follows: overall surface smoothness (improving the appearance of the succeeding layers), no VOC emission, chip and corrosion resistance for the entire vehicle body, almost 100 % utilization and no waste disposal problems.

Especially the combination of the high utilization rate and the elimination of paint sludge, collected in the spraying booths, make powder primer-surfacers very interesting paints from an economical point of view, and can be considered as one of the major driving forces for a “revolution” in the automotive industry. The following examples may illustrate it: In the period 1991 to 1995 General Motors and Chrysler introduced full-body powder primersurfacers in more than 10 production sites for passenger cars. Some years later Eurostar Works in Austria (a joint-venture of Chrysler and Steyer-Daimler-Puch) introduced powder primer-surfacers as full-body anti-chip coatings for their passenger cars ^[16]. These developments in the 1990’s secured and consolidated the position of the powder primer-surfacer at the automotive producers in the USA. This is confirmed by the fact, that in 2000 more than 10,000 car bodies were coated daily with powder primer-surfacers at both GM and Daimler-Chrysler ^[17]. This trend is now followed in Europe.

The automotive clear topcoat was considered for years as a too difficult field for powder coatings to enter in. But the ecological and economical advantages offered by powder coatings, and, last but not least, the proven success with the powder primer-surfacer, initiated extensive research work at both coating producers and car manufacturers. General Motors, Ford and Chrysler have formed the so-called Low-Emission Paint Consortium (LEPC) to jointly develop new technologies for application of powder clear topcoats on automotive bodies ^{[18, 19]}. BMW, Volvo, Audi and Renault are or were among the European car manufacturers, experimenting with powder clear topcoats over metallic base coats.

BMW is the first car manufacturer in the world to use powder clear topcoat in standard production. Up to the end of 2000 powder clear topcoats has been commercially utilized in BMW’s German plant for over 500,000 cars ^[20]. Since 2007 the main supplier of powder clear coats, PPG and BASF, delivered 800 t to BMW each year.

1.2Market situation and powder economics

The different powder coating systems that have been developed in the last 40 years employ most of the polymers used in the conventional solventborne coatings. At the same time the continuing market growth considerably lowered production costs. Combined with sharp price increases of solvents after the “oil shock” in the 1970’s this resulted in powder coating prices comparable with those of the wet coatings. Therefore powder coatings today are recognized not only as environmentally friendly systems, but also as materials that can compete successfully in price with the solvent and waterborne coatings.

Next to environmental and price aspects, quality is an even more important element, especially in high-tech areas of use, where powder coatings are commonly used. In this respect, powder coatings satisfy many of the most stringent requirements by the end users, failing only in cases where an extremely good flow of the coating is expected. Therefore the answer to the question “why powder?” contains the popular “Four E’s” introduced 1986 by Bocchi ^[21]:

Even a fifth E could be added for efficiency: no need get rid of a solvent to cure. Comparing the relative importance of these factors, it can be said, that at the moment, the high quality and the economic aspects perhaps contribute more to the acceptance of powder coatings by industrial users, than the regulatory compliances alone.

The market shares of powder coatings compared to other types of coating materials are not yet large. However, in a relatively mature field like the paint industry, powder coatings are among the few still enjoying a considerable growth in volume per year. According to available marketing data the world-wide market for powder coatings was 290 kt in 1990, 900 kt in 2001 ^[22], and about 2000 kt in 2010 ^[23] as shown in Figure 1.1, representing a total value of roughly 8 billion € ($ 10 billion). The annual growth rate, which showed double digits (more than 10 %) up to 1991, has been decreased in recent years to 4.5 to 10 % (varying by region), due to an economic recession in the nineties and a serious financial crisis in 2009.

Western Europe, where the powder coatings were invented, shows the lowest annual growth rate in the last 10 years, but the highest market share in industrial coatings (18 %), when compared to North America (15 %) and Japan (15 %) ^[24]. Inefficient production sides are recently rationalized and eliminated in Western Europe and reopened in lower-cost emerging regions like China. Table 1.1 based on data of SRI Consulting ^[24] gives an indication of the production of powder coatings in different geographical areas. China accounts for 700 kt in 2007, but it is believed that only half of it meets western quality (and price!) standards. The Chinese impetus is not restricted on production volumes. 2004 only 12 % of the patent applications in the field of powder coatings were filed in Chinese, 33 % in English and 30 % in Japanese. This picture is reversed in 2011: Now China is the formal innovation leader with 37 % patent applications, followed by English (27 %) and Japanese (18 %). It has to be mentioned, though, that similar to the powder coatings not all of the Chinese patents meet the high western standard, yet.

The various types of powder coatings commonly used in different geographical locations differ considerably. For example the three major market areas in the world, Europe, USA and Asia, have developed completely different systems for exterior use. In Europe, systems based upon carboxyl functional polyester resins/ß-hydroxy alkylamides (HAA) – which have replaced TGIC almost totally in the last couple of years – are the most popular, in the USA polyurethanes with aliphatic polyisocyanates as crosslinkers have dominated for many years, but ever since 1998 they are competing with the polyester/TGIC/HAA/PT910 (TGIC alternatives) types, while in Japan powder coatings with glycidyl functional acrylic resins as binders and dibasic acids as crosslinkers and polyurethanes are market leaders. TGIC systems are not at all used in Japan ^[25]. For coatings for indoor applications the socalled hybrids, based on carboxylated polyesters and epoxy resins, are the most popular, both in Europe and the USA. Table 1.2 presents average figures for the market shares of five types of powder coatings ^[23]. The different regional consumption of these competitive powder coating systems are accounted to historical reasons, differences in prices and availability of resins, toxicological issues, governmental regulations and an unequal intensity of sunlight which the coatings are exposed to ^[24].

More than 1000 powder coating producers are active world-wide, most of them serving a limited geographical area. The three largest are responsible for 30 % of the global market if China is excluded. These three multinationals are AkzoNobel, DuPont and Rohm and Haas. In China there are about 600 powder coating formulators, but the majority of them are very small.

Table 1.1: The world market for powder coatings by region for 2000 and 2007 ^[24]

The metal furniture industry was one of the first areas invaded by powder coatings, and it is steel that is the important substrate. Hybrids are the main type used for this purpose, while for outdoor furniture (garden furniture for example) polyester/TGIC (and its alternatives) or polyurethanes are the common systems.

The industry of domestic appliances is an important consumer of powder coatings. Appliance applications in total represent about 14 % of the market for powder coatings world-wide ^[26]. More than half of this market is located in Asia Pacific, since the appliance production itself has been shifted largely in this region. In Europe hybrids are the main type used since a mild orange peel effect is acceptable. In the U.S. a smooth finish is preferred and therefore polyurethanes are much more common. Typical products of this industry that are coated with powder coatings are food freezers and refrigerators, water heaters, toasters, washing machine lids, domestic cookers, electric and gas heaters, etc.

Architectural application is still the most important market for powder coatings, holding a market share of 36 %. Aluminum plays a special role in this field. Powder coatings show a penetration of roughly 30 %, whereas anodizing accounts for 55 %. In 2007 45 % of the powder coated architectural aluminum has been located in Europe, 34 % in Asia Pacific and only 3 % in North America. In U.S. coil coating panels are used predominantly, due to cost reasons ^[24].

The automotive market accounts for one third of all powder coating applications. The major part of this is claimed by primer-surfacer, a coating to protect the electrocoat primer and to provide a smooth surface for the basecoat. Primer-surfacer resins are usually made by acrylic/styrene/methyl methacrylate mixtures and cured by dodecanoic diacid (DDDA). But ß-hydroxyalkylamide (HAA) based systems and polyurethanes are used, too, due to superior chip resistance. The system, that has been chosen by all car producers, for automotive topcoats is a blend of an epoxy functional acrylic copolymer containing glycidyl methacrylate (GMA) as co-monomer, and DDDA or its (poly)anhydride as crosslinker. Very positive experiences are reported with such acrylic clear topcoats ^[27]. In 2007 almost half of the powder world market for automotive has been located in Europe, one third in North America ^[24].

The market growth of exterior powder coatings has continued to expand due to the wide acceptance of polyester/HAA systems in Europe and polyurethanes in the USA. Exterior coatings held a market share of 9 % of all powder coatings in 2008 ^[26].

Another market for powder coatings is in corrosion resistant types for e.g. pipes and reinforcement bars (rebars) holding a share o about 6 %. One third of this amount was produced in China, one fourth in the rest of Asia Pacific in 2007 ^[24]. This is the area where pure (“fusion-bonded”) epoxies dominate. In Europe the rebar market has been largely ignored, but its importance from a technical point of view has been recognized and this is an area where a considerable growth can be expected.

When powder coatings economics is compared with the other VOC (volatile organic compound) compliance coating options, one has to take into account all of the expenses connected with the coating process. On-line operating advantages reported for powder coatings compared with other environmental friendly systems are an application utilization of the material from 95 % up to 99 %; 30 % less energy consumption (than with conventional lowsolids enamels), combined with a reduction of labor expenses of 40 to 50 %; reduction of waste material accounts for almost 90 % and there are approximately four to six times fewer rejects due to surface defects ^[28].

Figures with respect to the economy of the powder coatings process have been published by the PCI ^[28] in 1999. The major cost factor for powder coating formulations is raw materials which make up for two thirds of the total costs. In powder coating applications of medium sized job shops the powder coating itself attributes only for 20 % of total costs, the main factor is labor (40 %) ^[24].

Recent developments in application equipment that make the clean – out of the line easier and the change of the color faster and the trend in developing powders with lower curing temperatures and higher reactivity, which allows higher line speeds thus saving energy, make the economics concerning powder coatings even more attractive. The “Four E’s” – excellence of finish, ecology, economy and energy – are attributes the powder coatings future can certainly rely upon.

1.3REACH

No chemical legislation ever before has influenced the research, the production and the sales of coating raw materials as fundamentally as REACH – The chemicals legislation of the European Community. Therefore a short description of REACH will follow in this chapter.

REACH (Regulation EC (No) 1907/2006) is the chemicals legislation of the European Community (EC) which came into force in 2007. REACH consists of three main legislative processes: registration, evaluation and authorization of chemicals.

Registration process

All substances which are produced within the EC in quantities of at least 1 metric ton per year have to be registered by the EC manufacturer. Furthermore all substances which have to be imported in quantities of at least 1 metric ton have to be registered by the EC importer. These substances are either imported as pure substances, as substances in mixtures or as monomers covalently bound in imported polymers. A registration is also needed for certain substances contained in imported articles. For registration a Technical Dossier has to be submitted to the European Chemicals Agency (ECHA). The Technical Dossier contains information regarding substance identity, uses of the substance and information about physicochemical, toxicological and eco-toxicological properties. For this purpose available studies should be used, either own studies or studies that have to be shared between the registrants after receipt of a Letter of Access from the study owner. If no study or other alternative data (e.g. read across, qualitative or quantitative structure-activity relationships) are available, physicochemical, toxicological and eco-toxicological studies have to be conducted. With regard to animal welfare, multiple testing of a substance has to be avoided, i.e. studies with vertebrates shall be conducted only once. The registration costs can rise up to several hundred thousands of Euros depending on the substance properties, the annual quantity of the substance, data availability and number of registrants.

Furthermore a Chemical Safety Report has to be submitted to ECHA for substances registered in quantities of at least 10 metric tons per year and per registrant. The Chemical Safety Report consists of an exposure assessment and a risk assessment, which has to be conducted for hazardous substances based on their physicochemical, toxicological and eco-toxicological properties.

A simplified registration is allowed for intermediates which are produced and used under strictly controlled conditions.

Transition periods are granted, if the substance is pre-registered by the potential registrant. Within the transition periods the potential registrant can produce or import the pre-registered substances without registration. The transition period for substances produced or imported in quantities of at least 1000 metric tons per year as well as for substances with carcinogenic, mutagenic or reprotoxic properties (≥1 ton/year) and for substances that are dangerous to the environment (R 50/53 substances ≥100 tons/year) expired in 2010. The transition period for substances produced or imported in quantities of at least 100 metric tons per year ends in 2013 and for substances produced or imported in quantities of at least 1 metric ton per year the transition period ends in 2018.

There are substances that are exempted from REACH registration. Whereas article 2 of the REACH regulation lists substances that are regulated by other legislations, annexes IV and V of the regulation include substances that are exempted from registration (e.g. natural substance such as glucose, carbon dioxide, nitrogen, crude oil).

Polymers are exempted from registration as well but the monomers that are used to manufacture the polymer have to be registered either by the producer of the monomer, by the EC importer of the monomer or by the EC importer of the polymer. Since powder coating raw materials are often polymers, powder coating have a natural advantage over other environmentally compliant coatings systems, e.g. UV-curing formulations.

Substances produced or imported for product and process orientated research and development (PPORD) are exempted from registration for at least five years. However the manufacturer or importer has to notify the substance to ECHA.

Evaluation process

The evaluation process will be conducted by ECHA and the national competent authorities of the Member States. Within the dossier evaluation process, ECHA checks the registration documents for compliance and evaluates the testing proposals that are included in the Technical Dossier. The substance information and the chemical safety assessment are evaluated by ECHA in cooperation with the competent authorities of the different Member States.

Authorization process

The authorization process aims to control risks from Substances of Very High Concern (SVHC) and to replace the substances by suitable alternatives substances or to develop alternative technologies. Therefore, the uses of these substances have to be authorized by the European Commission.

The following categories of substances are considered to be Substances of Very High Concern:

Substances of the mentioned categories will be listed in Annex XIV. If a substance is listed in this annex, it can be used only, if an authorization dossier was submitted and approved by the European Commission.

More detailed information about REACH and current activities of ECHA is available at the ECHA homepage (http://echa.europa.eu).

1.4References

[2] Harris, S.T., The Technology of Powder Coatings, Portcullis Press Ltd., Redhill, England, 1976

[8] Meiyden, B. van der, in Thermoset Powder Coatings, Ed. John Ward, FMJ International Publications Ltd, Redhill, 1989, p.54

[10] Misev, A.T., Binda, P.H.G., Hardeman, G., to Stamicarbon BV, NL Pat. 8.800.640, 1988

[13] Hoechst AG, EP 315 082, 1987; Hoechst AG, EP 315 083, 1987; Hoechst EP 315 084, 1987

[23] Grenda, W., Spyrou, E., Congress Papers European Coatings Congress (ECS), Nuremberg, 113, 2011

[24] Linak, E., Kishi, A., Yang, V., Thermosetting Powder Coatings, Specialty Chemicals SRI CONSULTING, 2008

[25] Rankl, F-J., (CEPE), Proc. Pulverlack Kongress, Hamburg, Sep.’96, p. 6, 1996

[26] Moran, B., Market research report, Powder Coatings: Materials, Coatings, Technologies, CHM051A, BCC research, 2009

[28] Powder Coating – The Complete Finisher’s Handbook, Edited by the PCI, p. 301–304, 2nd Ed. 1999

2Thermoplastic powder coatings

The first powder coatings produced were based on thermoplastic polymers which melt at the application temperature, and solidify upon cooling. Several factors such as relatively simple methods of manufacturing and application, no involvement of complicated curing mechanisms, raw materials that in many cases belong to commodity polymers, acceptable properties for many different applications etc. contributed to the popularity of these coatings in the market very soon after their appearance in the beginning of the 1950’s. At the same time, however, weaknesses such as high temperature of fusion, low pigmentation level, poor solvent resistance and bad adhesion on metal surfaces necessitating the use of a primer can be listed. These problems inherent to the thermoplastic powder coatings were successfully overcome later on by the thermosetting powders which very quickly took the largest part (90 %) of the market.

Despite the disadvantages, thermoplastic powder coatings can offer some distinguished properties. Some of them possess excellent solvent resistance (polyolefins), outstanding weathering resistance (polyvinylidene fluoride), exceptional wear resistance (polyamides), a relatively good price/performance ratio (polyvinyl chloride) or high aesthetic appearance (polyesters). These properties combined with the simplicity of the system, created a considerable market share for thermoplastic powder coatings.

Thermoplastic coatings are offered in a variety of performance classes (Figure 2.1). Besides the engineering and commodity polymers, which will be discussed in the following chapters, there are high performance polymers, e.g. PEEK (polyether ether ketones).

PEEK is a fully aromatic, semi-crystalline thermoplastic polymer with a maximum crystallinity of 48 %. It is a member of the class of polyaryl ether ketones (PAEK) high performance polymers. PEEK has a glass transition point of 143 °C and a melting range of ca. 340 °C. Characteristics of the polymer include a very high heat resistance and therefore service temperature, high rigidity, low water absorption, high hardness, good strength, low sliding friction, excellent chemical and hydrolysis resistance, low flammability, very low emission of smoke and toxic fumes during burning and its good electrical characteristics. PEEK offers one of the highest resistances against radiation among polymers.

Due to its high price PEEK is normally only used in applications where special properties are needed. Examples include valves, cable insulation, bearings, pump parts and sealings. Evonik Industries offers industrial grade PEEK under the trade name “Vestakeep”.

2.1Vinyl powder coatings

Two binders are used for the manufacture of the so-called vinyl powder coatings; polyvinyl chloride (PVC) and polyvinylidene fluoride (PVDF). On the basis of their polymer nature both powder coatings can be included in the same group, although they differ considerably in their performance. While PVC powder coatings are predominantly intended for indoor application because of their limited outdoor durability, PVDF powder coatings are among the best coating systems with respect to their weathering resistance.

Vinyl polymers belong to a group of resins having a vinyl radical as the basic structural unit. Polyvinyl chloride and copolymers of vinyl chloride are the most significant members of this group being among the first thermoplastics to be applied by powder techniques.

2.1.1PVC powder coatings

Polyvinyl chloride powder coatings were introduced on the market in the time when the thermosetting powder coatings were in very early stages of development. PVC based coatings offered many advantages over the other thermoplastic materials available as binders for coating production. These coatings have very good resistance to many solvents, which is a rather poor characteristic of the thermoplasts, combined with resistance towards water and acids. They have excellent impact resistance, salt spray resistance, food staining resistance, and good dielectric strength for electrical applications.

Polyvinyl chloride (–CH₂–CHCl–)_n is one of the cheapest polymers produced by the industry on a large scale. Its basic properties include chemical and corrosion resistance, good physical strength and good electrical insulation. Polyvinyl chloride (PVC) is by nature a brittle polymer, but the flexibility of the material can be easily adjusted by using an appropriate amount of a suitable plasticizer.

Polymerization of vinyl chloride into PVC homopolymer or its co-polymerization with different co-monomers is carried out by a free radical mechanism. Most PVC resins are produced by emulsion or suspension polymerization of vinyl chloride in an aqueous system containing an emulsifying agent or suspension stabilizer. However, bulk and solution polymerization processes are also carried out on an industrial scale.

Emulsion polymerization of vinyl chloride can be performed in both a batch and continuous way. The reaction temperature is maintained between 40 and 50 °C by cooling the reactor in order to remove the heat developing during the polymerization. The reaction medium is de-ionized water containing enough proper surfactant to obtain a stable emulsion. The initiators used are peroxides soluble in water, such as hydrogen peroxide or different persulphates. Since the monomer itself is a gas at room temperature, the polymerization is performed under pressure in autoclaves. The pressure in the reactor falls as the polymerization proceeds. After reaching conversion of ca. 90 %, the content of the reactor is discharged and the unpolymerized vinyl chloride is recovered.

Suspension polymerization of vinyl chloride is an important process in the commercial manufacture of PVC. In principle it is a batch process, although attempts have been made to develop a continuous technique for suspension polymerization of PVC. The polymerization is carried out by first charging to the reactor the required amount of de-ionized water and adjusting the pH depending on the suspending agent used and then the dispersing agent and the initiator. The monomer is charged after sealing the reactor and evacuating the oxygen. The polymerization reaction is carried out under pressure at 40 to 60 °C, controlling the temperature by appropriate cooling of the reaction mass. Although a great amount of research has been done in this area, it is interesting to note that the compositions of the reaction mass do not differ substantially from those used in the very early days of development of suspension PVC. The differences are mainly limited to the choice of the suspending agent, which is in most cases polyvinyl alcohol obtained by saponification of polyvinyl acetate, gelatin, methyl cellulose and copolymers of vinyl acetate with maleic anhydride. The initiator is water insoluble peroxide, such as lauryl peroxide, or azobisisobutyronitrile.

The type of suspending agent plays a very important role in obtaining primary particles with high porosity. Gelatin normally produces glassy spherical particles which have poor plasticizer absorption characteristics, whereas polyvinyl alcohol gives particles of a porous nature which readily absorb plasticizers to give dry powder blends. Thus, a patent of Air Products and Chemicals, Inc. ^[1] discloses a process for suspension polymerization of vinyl chloride giving a polymer specially suitable for production of powder coatings. The use of an excessive amount of secondary suspending agent causes two effects necessary for the critical powder coating application. Firstly, it reduces the size of the primary particles within the polyvinyl chloride grain, thus raising the surface to volume ratio and allowing plastication of the primary particles by the plasticizer. Secondly, very high porosity is gained, allowing complete and uniform plasticization of the resin grain in its entirety.

Solution polymerization of vinyl chloride is almost exclusively used for manufacturing copolymers containing vinyl acetate. The co-monomers are dissolved in a suitable solvent such as cyclohexane or n-butane, and the polymerization is carried out at 40 to 60 °C catalyzed by a free radical initiator which is soluble in the reaction mass. The copolymer begins to precipitate after a certain molecular mass is reached, which depends on the co-monomers’ ratio, polymerization temperature and type of solvent. The last step of the process includes filtering of the final product and then washing to remove the residual diluent and any traces of organic peroxide which would have a detrimental effect on the heat stability.

The bulk polymerization of vinyl chloride has been developed by Pechiney-St. Gobain, and plays an important role in the commercial production of PVC. Although bulk polymerization is associated with a homogeneous system, this is not the case with the bulk polymerization of vinyl chloride; namely, at very early stage of the reaction the polymer formed precipitates from the reaction medium in a form of insoluble material dispersed in the monomer. The system is therefore heterogeneous for a significant portion of the whole conversion. Since the polymer precipitates from the monomer, and there are no solvents present in the system, the concentration of the monomer available for polymerization remain constant with time. Therefore, at constant temperature the average molecular weight of the polymer obtained is apparently independent of the monomer conversion.

Preface

Contents