Characterization of adhesion performance of topcoats and adhesion promoters on TPO substrates

Adhesion of organic coatings to thermoplastic olefin (TPO) substrates in automotive applications has been an issue for makers of automotive parts since TPO was first used in exterior applications, primarily fascia. A widely used technique for assuring paint adhesion to TPO is the use of adhesion promoter primers based on chlorimated polypropylenes (CPO). Much research has been focused on understanding the forces involved at the interfaces of substrate, adhesion promoter, and topcoats resulting in the adhesion or the loss of adhesion in various environmental conditions. This study correlates the adhesion performance of CPO and nonchlorinated adhesion promoters (NCPO) as measured by peel strength with properties observed through microscopy techniques. Adhesion performance of CPOs, NCPOs, and blends are quantified through the use of 90° and 180° peel strength studies. Surface characteristics of adhesion promoters applied over a TPO substrate and cured at various temperatures are examined through the use of atomic force microscopy (AFM).     

How do chlorinated poly(olefins) promote adhesion of coatings to poly(propylene)?

The mechanism by which chlorinated poly(olefin) (CPO) primer coatings promote adhesion of paints to poly(propylene) and thermoplastic poly(olefins) (TPO) has been examined by surface characterization techniques including electron spectroscopy for chemical analysis (ESCA), time-of-flight secondary ion mass spectrometry (ToFSIMS) and transmission electron microscopy (TEM). The coatings, their interfacial failures, and taper-cut cross sections were studied, using both waterborne and solventborne CPO primers. The results were then correlated with peel strength and crosshatch adhesion tests. CPO primers do not penetrate deeply into the poly(olefin) substrates, but are quite mobile following application of the topcoat. Solventborne CPO's generally showed adhesive failure at the CPO/poly(olefin) interface when dried at ambient temperatures. Test results are also reported for waterborne CPO adhesion promoters.     

The relationship between clearcoat permeability and adhesion promoter penetration to the gasoline resistance of painted TPO substrates

Adhesion of topcoats to a variety of painted thermoplastic olefin (TPO) substrates, varying in the ratio of poly(propylene) to elastomer components, was determined in the presence of gasoline. Adhesion to TPO substrates was achieved through the use of chlorinated polyolefin (CPO) adhesion promoters. The adhesion promoter utilized in this study was a solventborne thermoplastic CPO, the penetration of which into the TPO substrate was monitored through the use of fluorescent tagging and subsequent optical microscopy. The topcoats utilized consisted of both one-component (1K) melamine crosslinked systems as well as two-component (2K) isocyanate crosslinked systems. Ultimate adhesion of the coatings in the presence of gasoline was found to be directly proportional to the depth of the CPO adhesion promoter diffusion into the substrate as well as the resistance of the clearcoat to gasoline permeation. Methods of analysis and supporting data are presented.     

The influence of compositional variations in compounded thermoplastic olefins (TPOs) on the physical and mechanical attributes of injection molded plaques

Compounded TPO continues to make inroads into automotive applications because of the excellent price/performance balance reached in this thermoplastic substrate. Thermoplastic olefin (TPO), a blend of elastomer and poly(olefin), achieves its balance of properties through the choice of compounded ingredients. The injection molding conditions through which the desired plastic part is achieved are known to also influence the attained properties of the blend. In this paper, the influence of poly(olefin), namely poly(propylene) homopolymer, and elastomer utilized in the compounded blend, in conjunction with molding properties used to produce plaques, are studied as they relate to the physical and mechanical properties achieved. Paint adhesion and friction induced paint damage resistance of coated plaques are shown to be directly related to poly(propylene) molecular weight and elastomer crystallinity. Molding conditions, mainly influenced through the shear induced injection molding process, are also correlated.     

The effects of thermoplastic poly(olefin) (TPO) morphology on subsequent paintability and thermal shock performance

Adhesion to thermoplastic olefin (TPO) substrates is strongly influenced by the type and amount of solvent contained within paint applied. Morphological changes in the TPO substrate are accomplished in the presence of solvent from the topcoat and vary depending upon paint bake times and temperatures. These morphological changes at and near the surface of TPO affect not only the paint adhesion to the substrate but also the cohesive integrity of the painted plastic composite. This paper attempts to delineate the influence of paint and paint processes on the adhesion/cohesion and mechanical properties of coated TPO parts, in particular, the performance of 2K topcoated TPO substrates under thermal shock conditions. It was found that the most important attribute contributing to thermal shock resistance of painted TPO parts was the bake temperature of the topcoat. A temperature of 250 °F in either the adhesion promoter bake or the topcoat bake is necessary to afford acceptable thermal shock performance. It is postulated that the rearrangement of poly(propylene) crystallites at the uppermost surface of the TPO under a 250 °F bake accounts for the increased cohesive strength of the painted composite.     

Theoretical analysis and experimental characterization of the TPO/adhesion promoter/paint interface of painted thermoplastic polyolefins (TPO)

Exterior and interior automotive applications of TPO (thermoplastic polyolefin) resins, which are, often, composed of a paint coating over an injection‐molded TPO, have increased interest in the surface chemistry and physics of TPOs. Specifically, the interface system composed of base‐coat paint/adhesion promoter/TPO is of primary importance in controlling the paint adhesion to the TPO. The major, active component in the adhesion promoter is a chlorinated polypropylene (CPO). A theoretical model based on phase thermodynamics and diffusion kinetics resulted in a prediction that the TPO/CPO interface should have a lower bound thickness of about 11 nm and an upper bound of about 400 nm. A battery of experimental strategies to characterize this interface system was discussed. Techniques used were transmission electron microscopy (TEM), atomic force microscopy (AFM) and scanning transmission X‐ray microscopy (STXM). The near‐surface morphology of both unpainted and painted, injection molded TPO plaques exhibited ethylenepropylene rubber particles close to the surface, i.e. within the first 0.1–0.8 micrometer of the TPO surface, and no “overlayer” of transcrystalline polypropylene at the surface of the TPO. Each of these microscopic methods showed that the adhesion promoter/TPO interface was very sharp. The thickness of this interface was measured with respect to the interdiffusion of the CPO and TPO by STXM. The STXM measurements yielded an apparent interface thickness between the adhesion promoter and TPO of 340 ± 80 nm. This was in good agreement with the theoretical predictions.     

Correlating thermodynamic and mechanical adhesion phenomena for thermoplastic polyolefins

Polyolefins are used over a wide range of industries due to their low cost and adaptable mechanical properties. However, their low surface energy makes fabricating composites and applying coatings challenging. Therefore, various surface treatments have been utilized to enhance their adhesion properties. In this paper, the surface energies of various thermoplastic polyolefins (TPO) have been measured via Inverse Gas Chromatography (IGC). These surface energy values were correlated to mechanical adhesion testing of the painted polyolefins. The adhesive integrity of the painted TPO was determined by applying a comprehensive-shear load to the material. Higher surface energies measured by IGC lead to increased adhesion with the paint. The surface energies also correlate with TPO crystallinity, as determined by microhardness testing of the unpainted TPO. Presented at the 2006 FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA, on November 1–3, 2006.     

Resistance of paint coatings to multiple solid particle impact: effect of coating thickness and substrate material

A novel technique has been developed to determine the resistance of paint coatings to multiple solid particle impact (i.e. solid particle erosion). The effect of paint layer thickness on erosion resistance was evaluated for two acrylic automotive topcoats. These coatings displayed a two-stage response to erosion. Initially, their thickness was reduced progressively, but once a critical thickness was reached the remaining coating was removed by individual impacts. A simple model is proposed to describe this behaviour. A new measure, specific erosion resistance, which takes account of the coating thickness, is defined to allow coatings with different thicknesses to be compared and has been applied to several industrially sprayed automotive clearcoats on both steel and polymer (TPO) substrates. The clearcoats exhibited significantly higher specific erosion resistance when applied to polymer substrates.     

Polyester adhesion to chromium-coated steel for poly(ethylene terepthalate) and an isophthalate copolymer

The performance of PET [poly(ethylene terephthalate)] and a copolymer of PET has been tested as an adhesive for chromium-coated steel. Crystallinity in the polyesters is found to limit adhesion, probably by restricting chain mobility. Lamination temperatures above the melting point of each polyester give the best adhesion. Degradation of the polyesters at yet higher temperature resulted in both reduced polymer cohesion and adhesion. Optimum bonding to steel was obtained at lamination temperatures between 230 and 275°C for the copolyester and between 280 and 300°C for PET. The standard laminate compression time was 15 min at 50 kg/cm². Adhesion was evaluated by the ASTM T-peel test. Assessments were made by both the peel energy and the peak load for peel.     

2003 Roy W. Tess Award

Conclusions Mechanical interlocking of topcoat with the nonpolar TPO surface can be achieved through the use of an adhesion promoter, namely a chlorinated poly(olefin). The type of CPO used, in addition to the types of solvents and heat effects used, can substantially influence the degree of adhesion/cohesion obtained within the CPO.TPO system. Heat histories, TPO molding variations, CPO types, including solvent and resin variations, and topcoat (basecoat/clearcoat) chemistries were all found to influence the adhesion/cohesion of the painted TPO assembly. Surface damage resistance was found to mirror the effects of adhesion as described earlier. Control of the interphase formed between the TPO substrate and the subsequent topcoat layers becomes increasingly important if one wishes to maintain damage resistance within the painted composite. Testing methodology development, namely “gouge” chip, abrasion, and scratch resistance, is paramount in predicting performance under specified loads. Through interpretation of data received in the various testing methodologies, the mechanical properties of the topcoat/substrate combination may be varied to obtain the performance required in a variety of applications. The Roy W. Tess Award in Coatings is presented annually by the Division of Polymeric Materials: Science and Engineering (PMSE) in recognition of outstanding contributions to coatings science and technology. Funded by a grant to the Division by Dr. and Mrs. Roy W. Tess, the purpose of the award is to encourage interest and progress in coatings and recognize significant contributions to the field. Dr. Rose Ryntz, Manager and Staff Technical Fellow with Visteon Corporatiom, Dearborn, MI, received the award from Dr. Paul Valint, Jr., Chair of the PMSE Division in September 2003 during the 226th meeting of the American Chemical Society in New York, NY. Dr. Ryntz's award address followed the Award Symposium. The following papers were presented at that symposium.     

Effects of surface modification by oxygen plasma on peel adhesion of pressure‐sensitive adhesive tapes

Surfaces of poly(isobutylene) (PIB) and poly(butylacrylate) (PBA) pressure‐sensitive adhesive tapes were treated by oxygen plasma, and effects of surface modification on their adhesive behavior were investigated from the viewpoint of peel adhesion. The peel adhesion between PIB and PBA pressure‐sensitive adhesive tapes and stainless steel has been improved by the oxygen plasma treatment. The surface‐modification layer was formed on PIB and PBA pressure‐sensitive adhesive surfaces by the oxygen plasma treatment. The oxygen plasma treatment led to the formation of functional groups such as various carbonyl groups. The treated layer was restricted to the topmost layer (50–300 nm) from the surface. The GPC curves of the oxygen plasma‐treated PBA adhesive were less changed. Although a degradation product of 1–3% was formed in the process of the oxygen plasma treatment of the PIB adhesive. There are differences in the oxygen plasma treatment between the PIB and PBA adhesives. A close relationship was recognized between the amount of carbonyl groups and peel adhesion. Therefore, the carbonyl groups formed on the PIB and PBA adhesive surfaces may be a main factor to improve the peel adhesion between the PIB and PBA adhesive and stainless steel. The peel adhesion could be controlled by changing the carbonyl concentration on the PIB and PBA adhesive surfaces. We speculate that the carbonyl groups on the PIB and PBA adhesive surface might provide an interaction with a stainless steel surface. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1392–1401, 2000     

Polyethylene/polyurethane blends for improved paint adhesion

Polyolefins have low free surface energy that prevents good wettability of adhesives or paint emulsions to their surface. This work shows that adhesion of olefin block copolymers (OBC) to a polyurethane-based paint can be significantly improved by blending thermoplastic polyurethane (TPU) into OBC. Furthermore, blend morphologies near the paint/polymer interface, and surface compositions of injection molded plaques, were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) in order to explore the underlying mechanism of paint adhesion to TPU/OBC blends. It was found that for 35 wt% and 25 wt% TPU loading, the top paint layer is well-attached at the interface, whereas for 15 wt% loading, there was incomplete wetting of the paint, and a gap between the polymer substrate and paint was apparent. XPS and SEM gave consistent results demonstrating that outermost surface composition of TPU in these blends is slightly higher than in the bulk. It is speculated here that, during painting and the subsequent drying step, polyurethane chains from the paint diffuse into the blend substrate and entangle with TPU in the blend. The entanglement between paint and substrate generates a physical link that provides adhesion.     

A variable radius roll adhesion test (VaRRAT) suitable for measuring the adhesion of paint to metal

Early trials and analysis of a new adhesion test are discussed. The test is designed for measuring the adhesion of paint to deformable steel sheets as used in building, automotive, and other cladding applications, and does not require detailed knowledge of the paint mechanical properties. A stiff overlay, such as an epoxy resin, is applied to the coating, and the steel substrate is peeled away using a roll of well-defined radius to which the steel substrate is constrained. The propagation of a crack within the paint or at some interface in the paint/metal system depends mostly on the mechanical properties and thickness of the overlay and the radius of the constraining roll. The test is shown to discriminate better than existing practical adhesion tests between paints of expected differing adhesion/cohesion, but also presents some inconsistencies that require further work to resolve. BHP Institute of Steel Processing and Products, Wollongong NSW 2522, Australia.     

Thermal properties and adhesion strength of amorphous poly(α‐olefins)/styrene–ethylene–butylene copolymer/terpene hot‐melt adhesive

The thermal stability and adhesion properties, such as lap‐shear strength of hot‐melt adhesives were obtained from amorphous poly(α‐olefins) and thermoplastic rubber [styrene–ethylene–butylene copolymer (SEBS)] blends. The addition of SEBS increased the toughness and viscosity and decreased the lap‐shear strength of the hot‐melt adhesive. Terpene tackifier resin offered enhanced lap‐shear strength; this was more effective when combined tackifier resin was added on the hot‐melt adhesive. Only a small amount of wax and antioxidant affected the thermal stability and lap‐shear strength of the hot‐melt adhesive. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Plasma treatment and adhesion properties of a rubber-modified polypropylene

In this study, cold, glow-discharge plasmas were used as a pretreatment method for the lacquering of rubber-modified polypropylene plates. This type of material is also referred to as thermoplastic polyolefins (TPOs). The effects of plasma treatments at radio- and microwave frequencies (RF and MW) and in combined MW-RF modes were studied, as were the effects of plasma power-to-gas flow (P/F) ratios and of discharges in oxygen, nitrogen, air, argon, and hydrogen. Surface characterization was carried out by contact angle measurements with water as the wetting liquid, and by XPS analyses. The adhesion between a two-component polyurethane (PUR) lacquer and plasma-treated TPO plates was evaluated by 180°-peel testing. The wettability of TPO surfaces was not affected by the plasma frequency or the P/F ratio, while the influence of the discharge gas was noticeable. Furthermore, no correlation between wettability and peel force could be found. Instead, lacquer adhesion was shown to be highly dependent on the P/F ratio and on the choice of discharge gas. The peel forces were found to be in the range of 0.1-35 N/15 mm, and the locus of failures was shown (by visual inspection or by XPS analysis) invariably to be in the TPO substrate. Electromagnetic radiation, most likely vacuum-ultraviolet (VUV) emission (<200 nm), was proposed to be a critical factor in plasma treatments. Attributed to VUV radiation was the creation of radicals in the TPO substrate; these lead to severe chain scission reactions and thereby govern the cohesive strength of the near-surface region of the substrate.     

Adhesion to Plasma-Modified LaRC-TPI II. Effect of Plasma Treatment on Peel Strength

LaRC-TPI, an aromatic thermoplastic polyimide, was exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. A 180[ddot] peel test with an acrylate-based pressure sensitive adhesive tape as an adherend was utilized to study the interactions of the plasma-treated polyimide surface with another polymeric material. The peel strengths of the pressure sensitive adhesive tape on the plasma-treated LaRC-TPI fell below the level of the non-treated controls, regardless of the plasma treatment used. Failure surface analysis by XPS revealed the presence of polyimide on the pressure sensitive adhesive failure surface, indicating failure in the plane of a weak boundary layer created by plasma treatment. The removal of the weak boundary layer by a solvent rinse restored the peel strength to the level of the control. Comparison with tape adhesion peel strengths of oxygen plasma-treated high density polyethylene showed that the physical condition of a polymer surface following plasma treatment plays an important role in determining the level of adhesion which can be achieved.     

Adhesion properties of a thermoplastic olefin elastomer grafted with maleic anhydride under ozone

The adhesion properties of a thermoplastic olefin elastomer (TPO), surface-grafted with maleic anhydride (MA) under ozone exposure, were analysed. TPO samples were dip-coated with maleic anhydride (MA) that was mixed with one of the initiators benzophenone (BP), benzoyl peroxide (BPO), or azoisobutyronitrile (AIBN) before being exposed to ozone. The adhesion properties of the TPO samples after ozone exposure were analysed by attenuated total reflection infrared spectroscopy (ATR-IR), lap shear tests and contact angle measurements. ATR-IR spectra confirmed that the amount of grafted MA increased with the ozone exposure time and changed with the type of initiator used. The grafted MA greatly improved the lap shear strength (LSS) of the TPO in TPO/epoxy adhesive/steel laminates. The LSS also depends on the initiator applied. The total and the polar component of the surface energy of the MA-grafted TPO increased after ozone exposure, following the order AIBN < BPO < BP, while the lap shear strength increased in the order of BP < AIBN < BPO after long time ozone treatment. The specimen grafted using BPO as initiator has the greatest LSS.     

The effects of filler size on the properties of thermoplastic polyolefin blends

The effects of filler size on the properties of a thermoplastic polyolefin (TPO) blend were examined by using wollastonite and talc with particle sizes ranging from 1.2 to 40 μm. While addition of filler produced significant changes in the mechanical properties of the blend, filler size affected only impact strength. However, filler size, filler coating, and injection speed had a major effect on the surface properties of the blend. Faster injection produced denser “shear zone layers” which exhibited better scratch resistance and poorer paint adhesion than those obtained with slower injection. Scartch resistance and paint adhesion also decreased with increasing filler particle size. Filler coatings altered the scratch and adhesion properties of the polypropylene (PP) blends.     

Improvement of the adhesion of continuously manufactured multi-material joints by application of thermoplastic adhesive film

The development of tailored lightweight solutions is especially required for the support of the structural change from combustion to electric driven vehicles, due to the still challenging weight of the currently used lithium-ion batteries. The manufacturing of tailored multi-material structures consisting of steel and carbon fibre reinforced thermoplastics (CFR-TP) within a continuous roll forming process is a promising approach to combine innovative lightweight design and large-scale production capability.Nevertheless, fusion bonding as joining technology for steel and carbon fibre reinforced thermoplastics within a continuous process is still challenging because of not achieving sufficient joint strength. The application of adhesive films, which can be easily integrated in a roll forming line by coil-coating processes, seems to be promising to increase the adhesion between metal and CFR-TP. Therefore, a thermoplastic adhesive multilayer-film, consisting of functionalized polymers in different thermoplastic layers, is used within this study. The most important challenge consists in the even heating and consolidation of the multi-material structure within the limited joining time in a continuous fusion bonding process.To enable a variation in joining time, the existing test set-up is extended to a reversible, continuous process. The manufactured hybrid specimens as well as high pressure blasted references without adhesive film application are tested mechanically by climbing drum peel test to determine the production-related influence. In addition, micrographs of specimens are evaluated analytically by scanning electron microscopy.The examinations show the significant improvement of adhesion between steel and CFR-TP by application of thermoplastic adhesive multilayer-film. Lower steel temperatures in combination with reversing operation result in peel resistances almost comparable to sequential process.     

Adhesion of CPO onto high modulus TPO: Lap-shear tests in conjunction with microscopy studies of the fracture surface structure

A lap-shear test was employed to investigate the failure mechanism of a chlorinated polyolefin (CPO) coating on a high-modulus thermoplastic olefin (TPO) substrate fabricated as a blend of a highly crystalline Ziegler-Natta isotactic polypropylene (iPP) and a crystalline metallocene poly(ethylene-butene) (9 wt% butene, EB9) impact modifier. The CPO was a chlorinated polypropylene containing 20 wt% Cl. The results showed that the fracture strength increased with increasing EB9 content in TPO blends. They also showed that the presence of xylene vapor during the bake step improved the adhesion between CPO and iPP itself (by 40%), but had a much smaller effect for the TPOs. Optical and transmission electronic microscopy images revealed a well-defined skin layer approximately 230 μm thick at the mold surface of the injection molded substrates. For the 25 wt% EB9 blend (TPO₂₅), this skin layer consists of thin fibers of EB trapped in a transcrystalline iPP matrix, with crystalline lamellae propagating from the matrix across the EB9 domains. Laser scanning confocal fluorescence microscopy (LCFM) and scanning electron microscopy images of iPP/CPO/iPP samples indicate that failure occurred close to the interface between the CPO and the iPP substrate, and, during fracture, the CPO layer maintained its original thickness. For the TPO/CPO/TPO sandwich samples, the fracture surfaces themselves were much rougher than that between CPO and iPP. Substantial deformation of the CPO layer was seen in the fractured samples, and failure was due primarily to cohesive fracture of the CPO in the region adjacent to the TPO substrate. From the perspective of newly introduced environmental regulations restricting aromatic hydrocarbons in automotive coatings, the most important result was the strong adhesion between CPO and TPO₂₅, with little difference between the samples exposed to xylene vapor and those not exposed to xylene.