ADHESION MECHANISMS FOR AUTOMOTIVE PLASTIC PARTS Dr. Marcos Fernandes de Oliveira 1 1 DuPont do Brasil S.A. - São Paulo Brazil Contacting the author: marcos-fernandes.oliveira@bra.dupont.com marcosfernandes1@yahoo.com.br Keywords: plastic, adhesion, CPO, Chlorinated Polyolefin, OEM Abstract The plastics usage in OEM applications continues to grow, due to their excellent features regarding flexibility, high-impact and strength. Today in automotive applications, plastics components are approximately 10% of the total vehicle weight, offering ductility, corrosion resistance and increased styling capability (RYNTZ, 2006). In order to improve the appearance and provide aesthetic value, the plastic surface must be coated with systems with good adhesion. This adhesion is not easy to reach, due to the very low surface energy the plastics components have. Due to these characteristics, several pretreatment methods are used today to increase this surface energy, providing better adhesion. These methods vary from oxidation as plasma or flame and the applying of adhesion promoters as CPO (Chlorinated Polyolefin). The aim of this work is to show how the different surface treatment and adhesion promoters based on CPO (Chlorinated Polyolefin) can improve the anchoring process on automotive plastic surfaces. 1. Introduction The importance of plastics to industry today is very high and the industries such automotive (OEM) had increased the application on several car parts. This trend has been influenced, among many reasons to reduce weight and to improve the car design, due the infinite molding possibilities. About 40 basic types of plastics are used to produce an automobile today, and while 40% of exterior plastics are molded-in-color and unpainted, automotive manufactures decide to paint in order to improve the aesthetics, the weather durability or the chemical and mechanical resistance (RYNTZ, 2006; PAUL, 2002). As example it is possible to mention the polycarbonate used as lens on car lights. Despite the polycarbonate is very resistant, it scratches easily, so it is import to spray a clear coat on this plastic to improve the scratch resistance. Another good examples are the effect colors. Until now it is not possible to cast the plastic parts with metallic and mica pigments at good color match with the car body. From the several plastic types used by car manufactures, we can mention the main examples and where they are used: PP/EPDM (polypropylene / ethylene propylene diene monomer rubber) in bumpers, ABS (Acrylonitrile Butadiene Styrene) in instrumental panels and grids, PP (polypropylene) in interior door handles and interior door panels, nylon (diamine+dicarboxylic acid) or glass fiber with mineral extender in gas tanks cap and hubcaps, noryl (blends of PPO = polyphenylene oxide resin and polystyrene) in mud guards and PBT (polybutylene terephthalate) + fiber glass or + nylon+fiber glass in exterior door handles. Due to the deformation the plastics can undergo at high temperatures, the coatings applied must crosslink at low or room temperature. Usually the process temperature applied by automakers for plastics is around 60 C 80 C and for these conditions, the polyurethanes (PUR) coatings are the best options. In Figure 1 is illustrated the typical PUR reactions with isocyanate. Examples of hydroxyl resins used for this purpose are polyester, acrylic or alkyd polymers. The urethane bond formed in this reaction is very strong providing good resistance and appearance since the substrate preparation is suitable. Due to the typical lower surface energy on plastics, some pretreatments are necessary. These pretreatments usually starts with 1
alkaline cleaning followed by methods that can increase the surface energy as oxidation or adhesion promoters apply (LAWNICZAK, J.; WILLIAMS, K.A.; GERMINARIO, 2005). Isocyanate Hidroxyl Resin N = C = O OH CH Room Temperature Isocyanate Hidroxyl Resin H N - C = O O CH Poliurethane System Urethane Bond Figure 1 Typical PUR resin reaction with isocyanate hardener. 2.Surface Tension and Surface Energy Surface tension will directly influence a coatings s ability to wet out, to adhere and to penetrate the structure of a plastic surface (ISEGHEM, 1998). To understand well this statement, first it is necessary to see how different liquids with different surface tensions behave on surfaces with different surface energy. These phenomena are illustrated in Figure 2 showing the two classical wetting situations. Case #1 shows a poor wetting process that occurs when the liquid is not able to spread on the surface. This condition is due to the lower surface energy of the substrate which acts repelling the liquid that has high surface tension. This condition can result on paint incompatibility with surface, poor adhesion work, poor leveling and surface defects (e.g.cratering). The case # 2 shows a better wetting condition, where the liquid now, with lower surface tension on a substrate with higher surface energy is able to spread. As results a good interaction between the paint and the substrate can be achieved and a good adhesion work and good leveling as well. contact angle Case # 1 Poor wetting process δ substrate < δ coating/paint Case # 2 Good wetting process δ substrate > δ coating/paint Figure 2 - Different wetting possibilities. For both cases, the contact angle between the liquid and substrate are important parameters to design a good coating system. Based on this examples it is possible to conclude that as higher the substrate surface energy, higher the paint spreading and the compatibility interfaces, providing good adhesion. The contact angle helps to identify if the liquid is compatible or not 2
with the solid surface. This is particular important to plastic coatings, due to the lower surface energy that plastic surfaces present. According to surface tension theory the total spreading, with contact angle close to zero could represent the better wetting condition for a coating. It is illustrated in Figure 3. SURFACE TENSION - LIQUID / SOLID INTERFACE γ S γ L θ γl. cos θ γ L = Liquid Surface Tension líquid air γ I = γ LS SOLID / LIQUID INTERFACE solid γ S γ I = γ L. cos θ + = γ L. cos θ - γ I γ S Spreading work is defined as W S = γ L (cos θ - 1) θ Zero cos θ 1 W S Zero Total spreading Figure 3 Surface tension theory, where the contact angle 0 represents the better wetting condition for a coating. 3.Adhesion Process Improvement Besides the mentioned lower surface energy, other characteristics can interfere on the painting process on plastics. These facts are related to their poor solubility or lower attacking by the most organic solvents. Antistatic additives or mold release remains on surface can prejudice the coating process too. The low polarity on plastic substrate is also a problem (TSUTSUI, K.; IWATA, A.; IKEDA, S, 1989). During several years, some methods had been developed to increase the plastics surface energy. Usually the process starts with a cleaning based on alkaline de-greasing products, de-ionized water and isopropyl alcohol. On next steps chemical process (acid), oxidation (flame, plasma) or adhesion promoter (CPO) can be applied on substrates. The chemical process uses acids to convert smooth hydrophobic polymer surfaces to rough hydrophilic surfaces by dissolution of amorphous regions (PAUL, 2002). Chromic acid is the most widely used however, the environmental restriction is compelling industry to find alternatives. The oxidation process by flame basically consists to contact the polymer substrate with a flame at 800 C for few seconds. The heat generates chemical groups from the plastic surface (e.g. OH, COOH, C=O) which react with the chemical groups from the coating (e.g. NCO, OH), building chemical bonds between the coating and substrate. This conditions provide good adhesion work because occurs a good interaction between the interfaces. The flame process is illustrated on Figure 4. 3
NCO Coating with functional groups OH 2500 cm/sec FLAME 800 C Hidroxyl OH COOH Carboxyl C = O Carbonyl 100 mm PLASTIC SUBSTRATE Figure 4 Flame treatment showing the interaction between the chemical groups from the polymer and the coating. The plasma treatment is another oxidation process caused by an electrical field under ionized atmosphere on plastic surface. The power of electric field can vary from 25 100 W at 15 90 sec. The plasma process helps to generate chemical groups in similar way the flame method and at same time, improving the wetting process because increase the surface energy (ATAEEFARD et al, 2008; AKUTSU et al, 2000; MAN, 2003). The Figure 5 shows how some atmospheres containing different gases can influence the surface topography on a polyethylene sample. The changes on contact angle and surface energy increasing are seen in Figure 6. Example: Polyethylene different atmospheres Argon Oxygen Non-treatment Nitrogen CO 2 Figure 5 Surface modification on polyethylene plastic surface at different atmospheres after plasma treatment. (ATAEEFARD, M. et al, 2008.). 4
Contact Angle θ Surface Energy Better Surface Energy Increases according the new lower contact angles Figure 6 Decreasing in contact angle and surface energy increasing on polyethylene plastic surface at different atmospheres after plasma treatment. (ATAEEFARD, M. et al, 2008). Besides the already mentioned treatments, the use of Chlorinated Polyolefin (CPO) had shown good results regarding adhesion properties and good interaction with plastic surface. The typical structure of CPO is illustrated below in Figure 7. Figure 7 Typical CPO structural formula The good adhesion work provided by CPO is explained by the chlorine properties. Chlorine has been shown to have a unique combination of large electron affinity and minimal atomic radius, yielding near maximum London force development ability with a high electron density per volume (PAUL, 2002). According Linus Pauli equation, shown here as eq(1), Cl has electronegativy number equal 3.15 while another elements vary from 0.6 to 2.85. n = valence number r = covalent radius c = atom charge 0.31( n + 1± c) χ = + 0.50 r eq(1) 5
Basically CPO works as a link between the coating and plastic interface, interacting with both. While the organic fraction of CPO molecule bonds with the coating, chlorine is attracted by any polar groups on plastic surface. Studies conducted by several authors (TANG; MARTIN, 2002; RYNTZ; BRITZ, 2001; MIRABELLA; DIOH, 2000; MA, WINNIK, 2005; RYNTZ, 2005) with PP/EPDM blends (usually used for car bumpers) had shown that the CPO performance depends on good diffusion trough the polymer matrix in order to reach the elastomer phase. This diffusion is controlled by the temperatures and by the solvent used to prepare the CPO solution and aromatic solvents as solvesso 100 and xylene has shown better diffusion than ketones. When the CPO reaches the elastomer phase, a swelling process occurs and the elastomer phase migrates to top PP surface matrix. The swelling breaks the PP crystal structure increasing the surface energy and providing good adhesion work. Figure 7 illustrates the swelling process and Figure 8 the microscopic evidence of these steps. PP matrix Solvent CPO + Heat swelling / interaction Elastomer (rubber) Rubber migration to top PP layer Figure 8 Elastomer swelling process followed by migration due to the CPO diffusion trough the PP/EPDM matrix. (unbaked) CPO diffusion through the PP/EPDM (baked 45 min @ 120 C) Figure 9 SEM photos showing the CPO diffusion trough PP/EPDM before and after baking. (TANG; MARTIN, 2002). 6
4.Conclusions The very low energy and surface tension are the main reasons that are so difficult to spray coatings on plastics. The flame process can be used to generate chemical groups on plastic surface, improving the reaction between plastic and the coating. The plasma process besides the chemical groups generation is able to modify the plastic surface, increasing the surface energy. Different atmospheres can produce different levels of roughness. The CPO resins are able to promote adhesion on PP/EPDM substrates, since a good diffusion occurs. The CPO diffusion through the PP/EPDM substrate will be better at higher temperatures and with appropriate solvents. The rubber fraction swelling is the CPO diffusion indication. Some studies suggest that the CPO helps to rubber migration to PP layer, breaking the PP crystal structure and improving the adhesion. 5. References AKUTSU, K.; IWATA, A.; IRIYAMA, Y. Surface Modification of Polymeric Films by Atmospheric Plasma Treatment. Journal of Photopolymer Science and Technology, v.13, n.1, 2000, p.75-78. ATAEEFARD, M.; MORADIAN, S.; MIRABEDINI, M.; EBRAHIMI, M.; ASIABAN, S.Surface Properties of Low Density Polyethylene upon Low-Temperature Plasma Treatment with Various Gases. Plasma Chem Plasma Process, 28, 2008, p.377-390 ISEGHEM, L.C.V. Important concepts on coatings plastics from a formulator s perpective, Modern Paint and Coatings, Feb.1998,p.30-38. LAWNICZAK, J.; WILLIAMS, K.A.; GERMINARIO. Characterization of Adhesion Performance of TopCoats and Adhesion Promoters on TPO Substrates. JTC Researh, v.2, n. 5, Jan.2005, p.399-405. MAN, A.B.G. Low Temperature Plasma Treatment as an Effective Method for Surface Modification of Polymeric Materials High Energy Chemistry. v.37, n.1, 2003, p.17-23. MIRABELLA, F.M., DIOH N. Theoretical Analysis and Experimental Characterization of the TPO/Adhesion Promoter/Paint Interface of Painted Thermoplastic Polyolefins (TPO). Polymer Engineering and Science, v.40, n.9, 2000, p.2000-2006. MA, Y.; WINNIK, M.A.; Surface and Interface Characterization of Chlorinated Polyolefin Coated Thermplastic Polyolefin. JTC Researh, v.2, n. 5, Jan.2005, p.407-416 PAUL, S. Painting of Plastics: new challenges and possibilities. Surface Coatings Internation Part B: Coatings Transactions, v.85, B2, jun.2002, p.79-86. RYNTZ, R.A.; BRITZ, D. Measuring Adhesion to Poly(olefins): The role of Adhesion Promoter and Substrate. Journal of Coating Technology, v.73, n.921, Oct.2001 p.107-115. RYNTZ, R.A. Attaining Durable Painted Plastic Components. JTC Researh, v.2, n. 5, Jan.2005, p.351-360. RYNTZ, R.A. Bring back the steel? The growth of plastics. JCT Research, v.3, n. 1, 2006. p.3-14. 7
TANG, H; MARTIN, D.C. Microstructural studies of interfacial deformation in painted thermoplastic polyolefins (TPOs). Journal of Materials Science, 37, 2002, p.4783-4791 TSUTSUI, K.; IWATA, A.; IKEDA, S. Plasma Surface Treatment of Polypropylene- Containing Plastics. Journal of Coatings Technology, v.61, n.776, Sep, 1989,p.65-72. 8