Coatings of Polymers and Plastics part 15 doc

Automotive Plastic Coatings in Europe 339 T ABLE 8 Application Parameters for Waterbased Basecoats First spraying operation Metallic basecoats approximately 6–8 µm dry film thickness Air speed 0.4 to 0.6 m/s Relative air humidity 60 +/− 5% Room temperature 23 +/− 3°C Intermediate flash-off Approximately 2 min. @ 23 +/− 3°C Second spraying operation Metallic approximately 6–8 µm dry film thickness Air speed 0.5 m/s +/− 0.1 m/s Relative air humidity 60 +/− 5% Room temperature 23 +/− 3°C Air lock (Approximately 3 to 5 min.) Final flash-off Approximately 6 min. at 60°C (drier rated for 80°C) on work piece 2 +/− 0.5 m/s Cooling The substrate in the discharge lock of the blowing tunnel with fresh air to 30°C for the processing of waterborne paints. A case-to-case examination has to be made as to whether integration into existing painting lines is possible. However, the paint supply system and the application technology must always be adapted. The most widely used pneumatic application process, manual and/or automatic processing, is described in detail as an example. In general, the parameters also apply to electrostatic processing. Further details are given in the next para- graph. The effect of the humidity (see processing range) is more crucial with waterborne basecoats than with conventional systems. Even after a 10-minute flash-off period there is still too much residual solvent in the paint film under T ABLE 9 Pneumatic Spray Application Parameters Hand spray gun, stainless steel DeVilbiss JGV-563 St Automatic Gun, alternatively DeVilbiss AGMD, ABB, Behr Nozzle (∅ mm) 1.1/1.4 Air cap (DeVilbiss) 765/789/797 Sprayer air pressure (bar) dynamic 4–5 5–6 Material flow rate (ml/min.) 200–400 200–400 Object distance (mm) 250–300 250–300 Booth temperature (°C) 23 +/− 323+/− 3 Relative air humidity (%) 60 +/− 560+/− 5 (best spraying conditions) Down draft (m/s) 0.5 +/− 0.1 0.5 +/− 0.1 340 Gruner and Reinhart normal booth conditions and therefore a special evaporation tunnel is needed. In the actual painting of plastics a combination of infrared drying and subse- quent blowing off with heated air has not proved successful for the forced evaporation of waterborne basecoats and instead pure convection drying is recom- mendable. Figure 9 shows a temperature curve for this basecoat clearcoat application and bake process. Over the last years, several new concepts for the integration of waterborne basecoats into existing paint shops have been introduced. Due to the three- dimensional, complicated geometry and poor temperature stability of many plastic parts, development of high-temperature radiation dryers (like infrared [IR] and ultraviolet [UV]) could not be introduced. With the exception of some middle- or long-wave infrared heaters, and also a few catalytic gas dryers (for smaller and more flat parts) in Great Britain and in France, process engineers and plastic coaters studied alternate dryers that operated at low temperature and worked on the principle of condensation. For these types of systems, closed air circulation is needed and the relative humidity must be less than five grams per kilogram of air. Processing temperatures are approximately 40°C. These conditions offer the benefit of a shorter process and the typical cooling time period that is required after drying is not necessary. F IG .9 Temperature curve for waterbased basecoat–waterbased clearcoat. Automotive Plastic Coatings in Europe 341 In view of the increasing significance of painting plastic surfaces electrostatically, this process is explained in more detail. Electrostatic painting has been the main standard method for coating metallic substrates for many decades. The advantage of this technique is the low loss of paint mist compared with purely pneumatic spraying processes. As a result, it is a cost-effective method of delivering paint to a part. Particularly more and more color harmony is required between the car body and the plastic parts. This can be very challenging because most off-line application methods are not at all similar to that of the car-body paint lines. In an attempt to improve this color harmony, conveyor systems have been built to hang and carry the plastic parts in car-body position through the plastic paint line. Also the application concept with the typical electrostatic atomizer is in use, similar to that used with the car body. Examples of robot types used in Europe are ABB and Fanuc. Electrostatic units operate on two principles, the purely electrostatic method and those methods with additional auxiliary energy for paint atomization. One thing that all processes have in common, however, is the fact that electrostatically charged paint droplets when sprayed are transported to the grounded part under the action of the electrostatic field and deposited on the job. Electrostatic application on plastics is a little more sensitive than the process used on the car body, as the typical plastic part is not inherently conductive. For optimum transfer efficiency the experience is, that a specific system resistance has to be less than one Mega-Ohm. Best results are measured in a combination of waterborne basecoat with an underlying layer of conductive primer. The high conductivity of the wet waterborne paint increases the transfer efficiency higher than normal. Electrostatic painting using high-rotation atomization is characterized by a considerably higher transfer efficiency (24,25) compared with pneumatic atomization. Spraying is performed purely mechanically at the bell edge. This process promises to exhibit the highest efficiency for coating plastic parts. Un- der the pilot plant conditions at DuPont, paint utilization of up to 60 percent was attained on bumpers whereas in a conventional pneumatic plant approximately only 30 percent is achieved. In practice, these figures are considerably lower when applied through electrostatic application owing to poor grounding. In spite of these advantages, the use of electrostatic high-rotation sprayers has for a while only been partially implemented for the application of “effect” basecoats. Normally, variations in color and a different flop behavior compared with pneumatic atomization occurs, which would clearly become noticeable as an optical flaw in the repair of parts without electrostatics in series production or in the field. The color and effect deviations that are usually seen are attributed to the different atomization and transport conditions of the paint droplets, the different 342 Gruner and Reinhart evaporation behavior of the solvents contained in the atomized droplets and the different kinetics of the droplets when they impact on the surface being painted. These differences induce an alignment of the aluminum platelets or other effect pigments causing the color deviation (see Fig. 10). However, in spite of this limitation, currently between 50 and 70 percent of the dry film thickness of effect basecoats can be applied electrostatically without any loss of optical quality. To apply as much as possible, ideally up to 100 percent of the basecoat, through electrostatical application requires close cooperation of all partners throughout the whole development process, starting with the OEM stylists. Surface tension, viscosity, and the paint thickness influence the droplet size distribution and the average droplet diameter just as much as the angular velocity, diameter, and specific design details of the sprayer in conjunction with the paint throughput. In this context, the very low electrical resistance of the water in water-thinnable basecoats deserves special attention. If the paint is supplied from a closed-circuit pipe, the high voltage present at the spraying head owing to the paint column created can be discharged into the entire supply system. The possibility of completely interrupting the paint column with optimum safety by means of intermediate replenishing tanks operated in isolation was previously used with waterborne fillers in automobile painting. Figure 11 shows a typical replenishing tank system. As a replenishing tank located in the voltage cascade and requiring a relatively large space was needed for each color, F IG .10 Aluminum- or mica-flake orientation in HR-bell and pneumatic spray. Automotive Plastic Coatings in Europe 343 F IG .11 Intermediate replenishing tank system, ohmic insulated for electrostatic waterbased basecoat application. (Courtesy of: LACTEC GmbH, Rodgau, Germany.) 344 Gruner and Reinhart it was not possible to transfer such a solution to basecoats with their wide variety of shades. At present, the standard solution for basecoats is therefore still the concept of high-rotation atomization with external charging. In this case, the paint is merely sprayed by high rotation. The electrostatic charge is created in a second stage by the air ions attaching themselves to the paint droplets in the high- voltage field between the external charging electrodes and the object being painted. The field geometry, voltage, shaping air, air velocity, and air humidity in the booth must be set in relation to each other so that return-spray effects, which could lead to contamination of the electrodes, are avoided. 7.3 Clearcoats Due to the plastics and molding conditions used by the European automotive industry, the maximum curing condition for bumpers, grilles, side claddings and most other plastic items is 90°C (194°F). Therefore, since the early 1980s isocyanate curing 2K clearcoats have been used. Initially, highly flexible, polyester resins were needed in the backbone to avoid deterioration of the low- temperature impact resistance of the painted part. A cryogenic polishing technique, using liquid nitrogen, was carried out to touch up any of these parts when and if defects were seen. More recently, clearcoats have been developed with built-in flexibility providing good low-temperature impact, which can be pol- ished at ambient temperature. In terms of resin chemistry, the clearcoats are based on a hydroxy-functional polyester and acrylic-resin blend. The polyester is responsible for providing the high flexibility at low temperature. The hardener is based on hexamethy- lene diisocyanate (HDI) trimer. Ultraviolet absorbers (UVA), and hindered amine light stabilizers (HALS) are additives added to absorb UV light, protect the basecoat pigments and to quench free radicals that could deteriorate and decompose the backbone resins. To meet the need for very robust products, new products are continually being developed that may be applied at low cost through high pressure spray guns or through high-efficiency bells with little risk of popping, nonuniform film build, or even sagging over a wide range of film builds. High skills are needed in the formulation chemist. Formulation tools are needed to build struc- tural viscosity to avoid sagging “in the booth,” thixotropy is needed to avoid sagging in the “flash off” zone, and temperature-induced viscosity is needed to control the film in the oven on vertical areas of the molded part. The typical product used on the European continent is a medium solids 2K clearcoat. In the United Kingdom however, local authorities require products with VOCs less than 420g/l. Clearly, these higher solid products have limited use compared to their medium solid counterparts when highly effective applica- Automotive Plastic Coatings in Europe 345 tion with smooth orange peel, even at low film build, is needed. In addition to solvent-based clearcoats, water-based clearcoats have also been developed and these trial products are under evaluation for industrial use (26). Figure 12 shows a typical application of a waterbased clearcoat. 8 BODY PARTS To a growing extent, the European automotive industry uses plastic and thermosetting materials for body applications. Two materials, SMC and PPO/PA are dominating in this area. It was in the early 1980s that SMC first appeared on middle volume vehicles and since then it has been used to a higher or lower extent, depending on the automotive OEM. Typical application for these materials are tailgates, an early example being the Audi Avant. Today, SMC plays an increasing role in this area and a number of variations of this technology are currently used. A SMC trunk lid is assembled on DaimlerChrysler’s S-Klasse Coupe. This part is precoated with an in-mold coating (IMC) and a black conductive primer. It is assembled to the body and passed through a cathodic electrodeposition tank, with no coating adhering to the SMC part. The black conductive primer allows for electrostatic application of primer surfacer and topcoat along with the automobile body. Renault (VelSatis) and Volvo (V70 station wagon) tailgates are coated completely off-line, without the use of IMC, and are coated with a dual primer system consisting of highly conductive primer and a light gray, F IG .12 Application of waterbased clearcoat. A few minutes after application, the wet film changes from “milky” to “transparent”. 346 Gruner and Reinhart nonconductive primer on top. Renault also topcoats SMC fenders that are primed with a conductive powder coating. Despite many improvements in SMC technology, the major problem of popping has not been completely eliminated. Porosity, due to gas inclusions mainly on the edges and wherever the part has been stressed, limits the scope of many SMC applications. To cope with this inherent porosity best, two “theo- ries” are followed. The first is to have a process starting with high temperatures of the primer bake. In this early stage, any pores popping out can be filled with putty. Due to the lower temperatures used later in topcoat bake, a minimum of pores are assumed to pop due to a sealing effect of the paint film. The primers used in this method are 140°C melamine-cured systems. More recently, according to the second theory, the overall process to coat SMC should be performed at low temperature. Low-bake 2K primers have been commercialized to effec- tively reduce the amount of visible porosity. In increasing volume, hard polyurethane (PU) composites are commonly used for body applications, like hardtops. Typically, a reinforced molding cov- ered by a 3 mm skin, is coated off-line. In addition to thermosetting materials, highly temperature-resistant thermoplastic polymers have been introduced to this body technology. A particularly suitable thermoplastic material for body components is the blend of PPO/PA. The structure is a matrix with spherical domains due to the noncompatibility of the polymers. As the PA phase is semi- crystalline, shrinkage phenomena of the plastic also have to be considered. When using the part for “on-line coating” processes, a topcoat cure of 140°C (285°C) or higher has to be taken into account with mold dimensions. Typically in these cases, the provider of the primed part is requested to use 140°C primer bake to preshrink the part for optimal body fit when assembled to the vehicle. A few examples illustrate as with SMC parts, for PPO/PA body components, many different coating options are used and no general coating process has been established. DaimlerChrysler’s A-Classe has a Noryl tailgate that is completely coated off-line in body color. The fenders of the same car however, are primed using a conductive primer and are assembled “after e-coat” for on- line coating. This is accomplished using a functional-surfacer, water-based basecoat and powder slurry clearcoat. Other fenders like that on the Audi A2, are coated completely off-line. For Renault’s Scenic and Clio models, black conductive PPO/PA is used so that the part can be coated on-line after passing through the electrocoat bath. 9 INTERIOR AUTOMOTIVE COATINGS Interior design of automotive vehicles in Europe today is an important factor in their sales success. The design stylists try to create a complete harmony of the interior trim using a combination of leather, woven fabrics, wood, and plastic Automotive Plastic Coatings in Europe 347 surfaces. In our perception, plastics don’t meet the stylist’s wish for idle or luxurious materials and hence the trend is to apply a coating finish to make the plastic surface more appealing. A wide range of finishes is at disposition of the designer. Traditional “low-gloss coatings” can make plastic components made of different materials or molded by different techniques, look alike and offer a elegant finish. Espe- cially when in dark colors, they eliminate or minimize reflection in the under window area of the car’s interior. Lighter colors are the trend for components below this area where low-gloss finishes can look comparable to natural materials. Additional highlight styling elements are high-gloss metallic or pearlescent coated trim moldings. Galvanic metallizing of ABS-plastics in chrome, brushed aluminium, or other special effects are used and usually coated with a special clearcoat to protect the finish. Alternatively, chrome effect basecoats are available. For even greater design variation, techniques like Cubic  for almost any graphic and colored pattern or picture have been developed and special clearcoats are also needed to support the appearance of the finish with respect to gloss, smoothness, and light refraction. Functional coatings combine both an attractive design with functional needs, for example laserable coatings to label buttons, dis- plays, or special paints to provide physical properties to material surfaces. The automotive stylist has to focus on more than just what is seen with the human eye, he has to appeal to the other senses. Antisqueak coatings can be applied to avoid unwanted noise that can be created when adjacent plastic surfaces rub against one another. Interior coatings can add an idle smell to the component and also give the perception of the surfaces when touched by the hand. Based on the human experience with natural matters, a range of high elasticity materials with certain friction to our fingertips is pleasantly perceived and coatings providing this effect are called softcoatings. Very often, manual operation elements like gear knobs, door handles, hand brakes, and radio knobs are given soft coatings. More and more all surfaces within the reach of the driver and passenger now have this soft finish including middle consoles, dash- board inlets, armrests, and airbag covers. According to simple model considerations, soft coatings can be accurately described in terms of their elasticity modulus and frictional resistance. When moving our fingertips along the surface of a soft coating, a minimal shift of the film surface versus the lower face bonded to the substrate occurs and can be “felt.” What we feel in physical terms is the sheer modulus of the film. Sheer modulus is related to tensile modulus and, using Poisson’s constant (P), can be transformed into the other using the following equation: Tensile Modulus = 2(1 + P) Shear Modulus Eq. (1) Hence, dynamic tensile measurements are suitable to characterize paint materials in terms of a number of factors such as a “storage factor” and a “loss 348 Gruner and Reinhart factor.” Both of these factors describe the energy of distortion that can be recov- ered or lost by heat formation when a paint film is sheared. Other available data that may be obtained includes “time lag” of the periodically applied shear stress and shear strain. Both these are temperature influenced and can be seen in Fig- ure 13. According to the shear model, thicker films exhibit better soft effects because a defined shear stress gives more shear strain. Typical soft coatings are applied in the range of 40 to 60µm. Additives and pigments can influence the type of soft effect we feel without dramatically changing the sheer modulus and the friction of our fingers to the paint film is responsible for this. When testing textured surfaces rather than smooth ones, things can get quite complicated and on these surfaces, qualifica- tion by trained test personnel is the only way to consistently characterize soft coats. In a somewhat simplified characterization, different types of soft-touch coatings can be represented in terms of resin shear modulus and frictional behavior of the film surface (see Fig. 14). For better quantification of the latter, “artificial” fingertips for test purposes are currently under evaluation. For soft finishes, solventbased and waterbased coatings are available both in middle and northern Europe, with the waterbased widely dominating the market. The chemical basis of waterbased finishes is based on a 2K polyol-isocyanate. Suitable polyols are special aqueous polyurethane dispersions. The isocyanate is provided in liquid form and usually contains a small amount of suitable solvents. Mixing to get the paint ready to spray can be done in small lots as most of these 2K materials exhibit a pot life of about one hour at ambient temperature. However, it is best to apply soft coatings through 2K automated mix equipment with modified statical mixers. These waterbased soft paints adhere to many plastics. Clearly ABS domi- nates in the interior trim market. In cases where polypropylene blends are used, a consistent pretreatment by flaming or fluorination is necessary to get adequate adhesion. For the application of waterbased soft paints, some pertinent data is given in Table 10. Soft coatings mostly are solid colors like black, gray, beige, blue, and others. In addition to this, certain metallic effects such as pearlescent effect textures can be provided. Also, soft clearcoats over special metallic basecoats also have become available recently. In addition to standard applications, special soft coatings exist that offer a wide range of options. For example, infrared reflecting coats have been pat- ented (27), and are available for dashboards to help reduce component temperatures that often reach 90°C, especially in dark colors. If these high temperatures can be reduced, the dashboards can be constructed much easier, cheaply, and at less weight. Antibacterial soft coatings have recently been offered for door handles, steering wheels, and gearknobs of rental cars. In addition, flame-retardant [...]... analysis of a softcoating film Tensile Stress: 1 cps/sinus, Tension: 1% 350 Gruner and Reinhart FIG 14 Representation of softtouch coatings in terms of shear modulus and frictional resistance coatings can greatly contribute to the safety of passengers in modern vehicles when accidents do occur 10 FUTURE DEVELOPMENTS AND SUMMARY To meet emerging ecological legislation needs for cost reduction and high-quality... for the Application of Waterbased Soft Coatings Equipment Stainless steel, suitable plastics Relative humidity Spray booth temperature Spray gun nozzle Atomization pressure Flash-off Oven cure 50–70% 21–28°C 1.6 mm 5 bar 8 min @ 26°C 40 min @ 80°C Automotive Plastic Coatings in Europe 351 moldings are needed, further replacement of metal by SMC and new reinforced and “sandwich-type” plastics is expected... to the study of toughness in thermaoplastics Plastic and Rubber Processing and Applications 6:67–79, 1986 3 Official Journal of the European Communities L85, 29.03.1999 4 T May Umweltmanagement im Lackierbetrieb Vincentz Verlag, 1997 5 T May Deutsche Loesemittelverordnung beschlossen JOT 10:8–11, 2001 6 E Seitl, F Altendorfer Innovative Kunststoffe fuer den Stossfaengerbereich Kunststoffe 84 Jahrgang... needs for cost reduction and high-quality standards, plastic coatings in Europe will change on a number of fronts The material of choice will remain for large components in the car body’s impact area, but the range of commercial grades will widen At both ends of the flexural modulus scale, new products will be introduced, highly crystalline at the high end and rubber-like blends at the low end For body... Coalescence, 153 Coatings: alternates, 279, 297 antisqueak, 347 appearance, 159 automotive market, 295–296 basecoats, 189–190 and body seals, 311 and bumpers, 322 clearcoats, 190 desirable features, 2 exterior, 7 requirements, 298 functions, 294 hindered amine light stabilizers, 190 in-mold, 268, 345 interior requirements, 298 low gloss, 347 multigrain, 312 nonpolluting, 265 (see also Nonpolluting coatings) off-line,... coatings) off-line, 318, 331 one-component, 191–193 on-line, 318, 331, 346 physical properties, 158 of polycarbonate, 329 Index [Coatings] powder, 153 precolored plastics, 62 radiation cured, 149 surface: appearance, 62 migration, 159 trends, 299 two-component, 193–195 ultraviolet absorbers, 190–191 waterborne, 152 Co-injection molding, 67 Colorants, 282 Color harmony, 341 Compatibility, 96, 131, 272–273... Fukata, T Kaneko Plasma treatment for painting of polypropylene 352 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Gruner and Reinhart bumper exterior body panel development International Congress and Exposition, Detroit, MI, 1985, pp 65–72 TA Wilde, H Gruenwald, H Bickmann Plasma am laufenden Band JOT Heft 11: 18–24, 1992 S Fischer, K Jesch Kunststoffteile im Innen- und Aussenbereich fluorieren... technique for pretreatment of TPO, but will be enhanced by complementary methods for special purposes Plasma polymerization has the potential to provide both adhesion and an electrical conductivity, but today the technique is in an early immature phase Waterbased primers are already being used today for TPO and other plastics These primers have been improved and formulated to offer adhesion even on unflamed... Problemloesungen beim Lackieren von Polypropylen Blends Kunststoffe 82 Jahrgang 9, 1992, pp 4–7 8 M Osterhold, K Armbruster Oberflaechenspannungs- und ESCA-Messungen an vorbehandelten Kunststoffproben Farbe und Lack 97:780–783, 1991 9 M Hill Die Beflammung als kostenguenstiges Vorbehandlungsverfahren fuer Kunststoffe Erfolgreich Lackieren von Kunststoffen Congress Praxis Forum Oberflaechentechnik Bad Nauheim,... body and glazing seals, 315 coated fabrics, 309, 315 coextruded films, 314 door trim panels, 305–308 floor: modules, 308 systems, 315 instrument panels, 301 molded-in effects, 313 patterns, 312 skins, 303, 312, 314 slush molding, 304–305 soft trim fabrication, 301–303 trim, 252 vacuum forming, 303 Isocyanate, 258 356 Legislation: ELV, 300 PVC, 300 Light microscope, 222, 234 Liquid crystalline polymers, . perceived and coatings providing this effect are called softcoatings. Very often, manual operation elements like gear knobs, door handles, hand brakes, and radio knobs are given soft coatings. More and. soft coatings are applied in the range of 40 to 60µm. Additives and pigments can influence the type of soft effect we feel without dramatically changing the sheer modulus and the friction of. mechanical analysis of a softcoating film. Tensile Stress: 1 cps/sinus, Tension: 1%. 349 350 Gruner and Reinhart F IG .14 Representation of softtouch coatings in terms of shear modulus and frictional