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«In the field of automotive coatings the colour harmony between add on components and the car body is a topic of enduring importance. Significant ...»

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Challenges of today and of the future 17

In the field of automotive coatings the colour harmony between add on components and the

car body is a topic of enduring importance. Significant progress has been achieved within

the last few years by various measures, like the limited tolerance of colour quadrants, the

implementation of colour measurement systems and by the adaptation of paint application techniques for plastic parts and car body. One further option is the standardization of basecoat formulations for car body and plastic parts. However, with every new colour the usefulness of the existing concepts for colour harmony is put to the test again.

Concerning the technical coating properties, the main issue of adhesion is neither solved for all plastic substrates nor is it well understood on a scientific base. To secure paint adhesion on polyolefin’s is the biggest challenge, moreover as they gain more and more market share. For automotive OEM coatings, scratch resistance is also a well known demand, and meanwhile it has established itself also in the field of coating add on plastic components.

1.3.2 Reduction of emissions/low emission coatings Many of the larger Tier One component suppliers have recently switched to waterborne coatings, including the adaptation of specialized humidity and temperature controlled processes for their application. However, the shift from conventional solventborne to low-VOC clear coats is a task still waiting to be solved for many fields of application. Besides waterborne materials, another option for clear coats is UV curing systems. Material development has left the state of basic research and development and finds evaluation underway in many market segments, with degrees of success depending on the operational area. Powder coatings are only suitable for plastics with low thermal expansion, so that parts can overcome higher cure temperatures without loss of shape. It is not expected that in major operational areas like the automotive supplier parts business, powder coatings would substitute the classical wet paints in the foreseeable future.

1.3.3 Increase of efficiency/profitability Dispensing with coating layers is one option by which producers of automotive supplier parts reduce costs. Positive experiences have been made with the primer-less coating of bumper housings. However an important demand of this approach is a high plastic surface quality. So in cases of difficult substrates with lower surface quality, for example with some highly filled fibre reinforced plastics, one may not be successful without the use of a primer.

If the substrate quality demands are fulfilled, as in the case of some coating processes for bumper-housings, this concept may work. Here a two layer paint body is realized with success. It consists of a (waterborne) basecoat and clear coat. In other cases, the requirements of colour harmony and intrinsic colour of the substrate, in combination with a low hiding power of the basecoat, might afford the use of a coloured primer.

Dispensing with coating layers in its most extreme way has been practiced by decorating mass coloured plastic parts utilizing just a clear coat. By this method straight shades can be realized in an elegant way, as it is done with body panels, like the “Smart” car.

An extension of clear-coated mass-coloured plastics on other car models would afford the realization of effect colours. But the mass colouring of plastics in this special case is confronted with the high demand of colour and effect harmony on the car. As the pigmentation of the plastic bulk naturally does not result in an orientation of effect pigments similar to that of a paint spray application, up to now it has not prevailed as a successful concept for coating automotive assembly parts.

18 Introduction In the search of alternative coating concepts a completely different direction is followed by foil technology. In connection with the possibility to combine several car body parts to one module, foil technology promises to be particularly cost efficient by reducing the vertical range of manufacture. Meanwhile there are numerous foil concepts, which will be further addressed in Chapter 4, and have found limited introduction into the market. Current applications are limited to extensive body parts of moderately complex geometry. One typical and appropriate application for foil concepts is the car roof that can be partitioned in rigid, movable, well hiding and transparent modules. One might hope and expect that foil technology will master its challenges and continue to gain in popularity.

1.3.4 Design One of the major trends currently observed in the market is individualization. The wish for colour and effect is considered as the driving force for an ongoing market growth for plastic coatings, especially for vehicles. Also in business areas like commercial electronics, represented by objects such as mobile phones, notebooks, etc. which are styling trends are influenced by individual colour design. Today these aesthetic needs can be realized at best by advanced coating technologies. This is particularly evident in coatings with interference and pearl effect, which develop their optical performance, such as colour flop effects, only by the interaction of the coating material and the application process.

One area, in which the trend for individualization can be developed in a strong way, is automotive interior coating. The great variety of customers needs can be at best be satisfied by many materials. The colour of plastic, leather and textile materials have to harmonize with each other. In this context the coating of automotive interior parts is necessary, because coating allows the most flexible differentiation of the optical design. A large variety of colours in the automotive interior can only be realized with plastic coatings today, but innovative foil concepts like insert film moulding (IFM) have already found its place in the market.

Three dimensional shaped plastic parts, being illuminated by the application of an alternate voltage, can be manufactured by back injection moulding of a transparent foil that is then coated with a thin electroluminescent layer [5].

In general one predicts a great future for switchable coating systems. It may also be assumed that there will be further examples for the innovative connection of switchable coating systems with plastic processing, allowing expanding marketplace acceptance when design and function of industrial products are desired and/or needed.

1.4 References [1] The ChemQuestGroup, Inc., www.radtech.org/An Overview of the North American Plastic Coatings.pdf, 2003 [2] U. Hoffmann, Journal für Oberflächentechnik (JOT) 46, 11 (2006), 56 [3] S. Kirchmeyer, L. Brassat, Kunststoffe 95, 10 (2005), 202 [4] H. Rost, Kunststoffe 95, 10 (2005), 209 [5] E. Foltin, G. Wießmeier, Journal für Oberflächentechnik (JOT) 3 (2004), 34 Survey of plastic materials 19

2 Plastics in coating technology

2.1 Survey of plastic materials 2.1.1 Classification by technical criteria According to DIN 7724, plastics can be classified into three groups: thermoplastic, elastomeric and thermosetting (duroplastic) materials.

The basic characteristics of these three groups of materials as well as their international abbreviations according to DIN EN ISO 1043-1, set in brackets, shall be presented here briefly.

Thermoplastic polymers are not cross-linked, more or less soluble and can be softened or molten reversibly. Materials belonging to this largest group of polymers are the so-called standard plastics like polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyvinylchloride (PVC). Also many of the so-called technical plastics belong to this group, examples are polyesters like polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), polyamides (PA), blends of polystyrene- (ASA, ABS, SB, SBS), polycarbonate (PC) and its blends (PC/PBT, PC/ABS), and polymethyl methacrylate (PMMA).

Elastomers are widely cross-linked polymers, showing a rubber-like at ambient temperature. They are generally not soluble in organic solvents, but swellable, and can be softened by heating, but then they don’t have a plastic character and therefore cannot be moulded easily. Examples are natural or synthetic rubber (NR, BR), silicon rubber (SI), some polyurethanes (PUR) and fluorine rubber (FKM).

Thermosetting (duroplastic) polymers are densely cross-linked materials that are hard and brittle at ambient temperature. They are not soluble or swellable in most organic solvents, and upon heating they soften only slightly. Therefore, thermosets exposed to elevated temperatures are not easily processed and show some level of decomposition. Examples for thermosetting polymers are cross-linked epoxy- (EP), unsaturated polyester- (UP), phenolPF), melamine- (MF) and urea resins (UF).

2.1.2 Economical and coating related significance Over the past 50 years, world production of plastics has increased on average 10 % annually, and in 2008 reached 245 Million tons [1].

Wilke/Ortmeier: Coatings for Plastics © Copyright 2012 by Vincentz Network, Hanover, Germany ISBN: 978-3-86630-859-6 20 Plastics in coating technology Figure 2.1: Worldwide production of plastics in 2004 Source: Verband Kunststofferzeugende Industrie, from [1] Figure 2.1 presents the share of market segments on a worldwide basis in 2004 and Table

2.1 shows those plastic substrates being most significant for coatings, and their fields of application.

Roughly half of the worldwide consumption of plastics accounts for polyolefins. This group is represented mainly by polyethylene (PE), which has the biggest share and by polypropylene (PP), showing a strong increase in market share (6 % per year on a worldwide basis, 12 % in china), resulting from its use for high value and cheap mass articles. It is expected that polyolefins will dominate the market even more in the future, because of their yet not fully explored high tech-potential, the ongoing price pressure on industrial products, and due to the constraints of recycling. About 70 % of all automotive plastic materials are based on polypropylene [2].

Pure and coated polyolefins are important materials for the packaging industry. For example, foils of polyethylene and polypropylene are coated with barrier layers and hollow pieces

–  –  –

are printed for decorative purposes. Flexible PP-blends like TPO have a strong position in the car industry). The share of TPO in plastics used for automobiles is about 20 %. Examples of items are deep drawn foils for dashboards and injection moulded bumper fascia.

Efficiency, recyclability and safety issues (passive pedestrians safety) cause an increasing trend towards replacing other common plastics with polyolefins.

Another standard plastic, that is processed to foils and sheet flooring, and frequently coated afterwards is polyvinyl chloride (PVC). The group of polystyrenes (PS) and foamed, expanded polystyrenes (EPS), since years ranking forth in worldwide production, is mainly applied in the packaging and building industry. Television cabinets of polystyrene were among the first plastic objects that were coated industrially. Other fields of applications for painted polystyrene are hi-fi systems and computer housings. As elastified polystyrene-type plastics like copolymers with butadiene (ABS) and acrylester (ASA) suit often better, today they have more significance as substrate for coatings. They have claimed a firm position in the field of consumer electronics and as a blend with polycarbonate (PC+ABS, PC+ASA) in automobile parts. Examples for coated parts from those blends are exterior mirror housings, licence plates and consoles. A reinforced ABS/PA-blend is in use for online-coatable fenders.

Thermoplasts like polycarbonate (PC), polybutylene- and polyethylene terephthalate (PBT resp. PET) and polyamide (PA) are referred to as technical plastics. With a production share of around 9 % of the world market they rather belong to the small volume plastics today, for which on the other hand the highest production growth rates of 6 to 8.5 % are forecasted.

Technical plastics are popular substrates in the coatings technology. Polycarbonate is applied as material for discs and sight panes, where they have to be equipped with a thin coating layer protecting against UV light and scratches. An important application of PBT is as a blend with PC in car bodies, for example for air-inlet grilles. PET is used for coated multilayer films for food packaging (snacks), coated polyamide (PA) for example in household appliances like washing machines. Glass fibre reinforced polyamides are applied especially for tool housings, vehicle door handles, rear handles and wheel covers.

The blend of PA with the heat stable polyphenylen oxide (PPO) has a strong position for automobile fenders. In most cases, except for bumpers and fenders, large surface parts for automobiles and also for aircrafts and boats are made from duroplastic fibre reinforced polymers. Those fibre composites can be composed of cured unsaturated polyesters- (UP) or epoxy resins (EP) and glass fibres (GF) or carbon fibres (CF). One variant of glass fibre reinforced plastics that is of significance for large automotive vehicle parts is SMC (Sheet Moulding Compound). This material consists basically of glass fibre mat impregnated with UP-resin, equipped with fillers, thickeners, catalysts, further additives and rolled up. After cutting an appropriate sheet from the roll, it is inserted into the press form and cured thermally to the desired part.

Polyurethanes also are applied as substrates in coatings. By reaction of polyoles with isocyanates in a form, PUR-RIM (Reaction-Injection-Moulding) parts and reinforced types (PUR-RRIM) can be obtained. Front and side fascia of automobiles have been manufactured of this group of materials.

Nanocomposites present a group of polymer materials, which today is intensely investigated in research and development. It is the high ratio of surface and volume of nanoparticles that promises extreme physical properties, compared with those of normal scaled particles. Examples of nano-scaled materials, applied for reinforcing plastics, are sol-gelmaterials, nano-scaled fillers, carbon nanotubes and nano-layer silicates [3].

22 Plastics in coating technology 2.1.3 Causes for the increasing importance of plastic Why is it, that plastic has become an interesting material for industrial and consumer

goods? The responsible reasons for this can be summarized as follows:

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