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.     

Measuring adhesion to poly(olefins): The role of adhesion promoter and substrate

The use of thermoplastic poly(olefins) in the automotive industry continues to proliferate due to their decreased cost and weight and increased recyclability in comparison to thermoplastics such as poly(carbonate) alloys or poly(urethanes). An attribute that continues to hamper the widespread introduction of thermoplasticpoly(olefins), in particular thermoplastic olefin (TPO, a blend of impact copolymer and elastomer), into additional automotive components, however, is its poor surface wettability and adhesion. Adhesion promoter formulation, both in terms of resin composition and solvent variation, has been known to influence the adhesive propensity of topcoats when analyzed by typical tests such as peel strength. It has long been disputed, however, that peel strength is not a true measure of paint adhesion since it artificially introduces a film between the paint and the adhesion promoter to enable one to perform the test. In contrast, this paper discusses the use of a newly developed in-situ adhesion test, described as compressive shear delamination (CSD), to quantify the adhesive/cohesive propensity of coatings to a variety of TPO substrates. The effect of solvent type and chlorinated poly(olefin) (CPO) adhesion promoting resin on the adhesion/cohesion of topcoats to TPO is described. Chlorinated poly(olefin) type, followed by solvent variation, was shown to have the most significant impact on the adhesion/cohesion of topcoats. This newly described CSD protocol for determining the weak link in painted plastic may have a significant impact on the choice of topcoat, adhesion promoting primer formulation, and substrate in particular automotive applications. Presented at the 78th Annual Meeting of the Federation of Societies for Coatings Technology, on October 16–20, 2000, in Chicago, IL. Dearborn, MI 48121 Santa Barbara, CA Evansville, IN     

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.     

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.     

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.     

Investigation of the “Surface” and “Interphase” Composition of Adhesion Promoter/Thermoplastic Olefin Systems: The Effect of Adhesion Promoter Bake Temperature

The interfacial chemistry of model systems consisting of two adhesion promoting primers and a single Thermoplastic Olefin (TPO) substrate was examined. Two commercial adhesion promoter (AP) materials were applied to a commercially-available TPO material and either flash dried at room temperature or baked at 100°C. The surface composition of the AP films and TPO substrate, and the interfacial compositions of the AP/TPO systems were characterized using x-ray photoelectron spectroscopy (XPS). The AP films studied were based upon a chlorinated polyolefin (CPO). For one adhesion promoter film (AP-1), no chlorine was present at the surface suggesting a nonhomogeneous system. For the second adhesion promoter film (AP-2), the surface composition was about 15% CPO and 85% AP matrix. No changes in AP surface composition were evident for the different bake conditions for either AP. Interfacial compositions of the room temperature flashed materials were found to be very similar for both AP/TPO systems, with CPO being present for each and at similar concentrations. Interfacial compositions for the baked materials were also similar for the two systems, although the level of CPO at the interface increased for both the AP-1 and AP-2 relative to the unbaked materials. The relative increases observed were 46% and 41% for the AP-1 and AP-2 systems, respectively. The increase in the relative concentration of CPO at the interface with bake temperature suggests that there is a stronger interaction between the AP and TPO. The implication of these data is that a baked AP should result in a more robust paint system with respect to AP/TPO adhesion.     

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.     

The synthesis,characterization and properties of silicone adhesion promoters for addition-cure silicone rubber

Two types of polysiloxanes, GVPMS and GVHMS, were synthesized through nonhydrolytic sol-gel reaction from vinyltrimethoxysilane(VTMS), 3-glycidoxypropyltrimethoxysilane(KH560) and diphenylsilandiol(DPSD) or hydroxyl silicone oil, respectively. The chemical structures were characterized by Fourier transform infrared (FTIR) spectroscopy, ¹H nuclear magnetic resonance (¹H-NMR) spectroscopy and gel permeation chromatography (GPC). GVPMS and GVHMS were used as adhesion promoter for addition-cure silicone rubber. It was found that the adhesion promoters not only greatly improved adhesion strength but also had excellent compatibility with silicone rubber. Among them, GVPMS gave better adhesion strength in silicone rubber. The shear strength of silicone rubber reached 1.70 MPa with addition of 1.5 phr GVPMS, which was about 317% higher than that of silicone rubber without the adhesion promoter. Meanwhile, the shear strength of silicone rubber reached 1.14 MPa with addition of 2.0 phr GVHMS, which was 183% higher than that of silicone rubber without the adhesion promoter. Optical property test results revealed that the refractive indexes of GVPMS and GVHMS were 1.5185 and 1.4133 respectively and both of them had good compatibility with silicone rubber, which satisfied the application demand of high-refractive and low-refractive electron encapsulation. Thermal resistance test and SEM results further proved that adhesion promoter could significantly increase the adhesion between the copper substrate and silicone rubber, in which GVPMS had a better performance. Oxidation treatment experiment further explained the mechanism that adhesion promoter functioned as a bridge, linked silicone rubber and copper substrate through chemical bonds.     

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.     

Thermodynamics of polymer-paper adhesion: a review

A review of studies of polymer-paper adhesion illustrates the thermodynamic nature of the bondability of polymers to plain, uncoated paper surfaces. The bond strength depends strongly on the chemical nature of the polymer surface and on that of the fibrous paper surface. Adhesion to paper may be characterized indirectly through thermodynamic analysis of the paper substrate, or directly through paper laminate or adhesion tape peel testing. The need for adequate paper adhesion is emphasized, particularly for some of the newer printing processes (electrophotographic and thermal imaging). It is concluded that some of the indirect methods of adhesion characterization (surface energetics analysis via contact angle measurements or the inverse gas chromatography technique) may serve to characterize paper adhesion in these processes.     

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.     

Mechanical effects in peel adhesion test

—Mechanical effects in the peel strength of a thin film have been studied both experimentally and theoretically. It has been found that the adhesion strength measured by the peel test is a practical adhesion (an engineering strength per unit width) and does not represent the true interface adhesion strength. The measured value may represent a multiplication of the true interface adhesion and other work expended in the plastic deformation of the thin film. The contribution of the latter to the peel strength is found to be, sometimes, of the order of 100 times higher than the former. It is found that the major controlling factors in the peel strength are the thickness, Young's modulus, the yield strength, the strain hardening coefficient of the film, and the compliance of the substrate as well as the interface adhesion strength. Even though the true interface adhesion strength is the same, a higher peel strength is obtained if the film is thinner or more ductile under the test conditions reported in this paper. The same effect can be obtained if the substrate is thinner in the case where the substrate is a soft elastic material, or if the substrate is thicker in the case where the substrate is a rigid material.     

Effect of curing agent on interfacial adhesion in acrylic polymer-based laminates

The effect of crosslinking on interfacial adhesion between an acrylic elastomer and poly(methyl methacrylate) has been studied using a 90° peel test. Elastomers were master-batched with a 1 : 10 sulfur/sodium mixture. The compounded elastomer was then bonded with poly(methyl methacrylate) by in situ curing at various temperatures. Variations in the curing affect both the mechanism of adhesion and separation. The relationship between peel strength and crosslink density is found to be P = kM_c. Crosslinking at relatively low temperatures produced a partially crosslinked elastomer that leads to high peel strengths. When crosslinked at 180°C, the acrylic elastomer was completely cured, and the peel strength decreased by more than 80%. This is consistent with an optimum level of crosslinking required for peel strength. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1277–1284, 1998     

Enhancement of polymer film adhesion using acid-base interactions determined by contact angle measurements

Quantitative correlations among surface chemical composition, acid-base thermodynamics, adhesion strength, and locus-of-failure are demonstrated. Four types of functional Teflon surfaces were prepared: two acidic (containing hydroxyl and carboxyl groups), and two basic (containing acetyl and dinitrobenzoate groups). X-Ray photoelectron spectroscopy (XPS) and attenuated total reflection infrared (ATR-IR) spectroscopy were used to characterize the molecular structure of the surface region. Contact angle adsorption isotherms were determined using phenol as an acidic probe and tetrahydrofuran (THF) as a basic probe. The carboxylated surface had a higher molar ?H^ab with basic THF than the hydroxylated surface, and neither surface had any interaction with the acidic phenol probe. The acetylated surface behaved as a base, interacting with phenol but not with THF, while the dinitrobenzoyl surface had both acidic and basic character. Adhesion tests were carried out in the 180° peel mode using post-chlorinated poly(vinyl chloride) as a model acidic adhesive between pairs of each type of film. The two surfaces with basic character had significant peel strengths, while the two acidic surfaces had very low peel strengths. Scanning electron microscopy (SEM) of the basic failure surfaces showed significant plastic deformation of the Teflon polymer, while the acidic failure surfaces showed no deformation. XPS analysis of the failure surfaces confirmed interfacial failure for the acid-acid pairs, and bulk FEP failure for the acid-base pairs. These results demonstrate directly and quantitatively the enhancement of adhesive bond strength through acid-base interactions.     

Characterizing interfacial structure of TPO/CPO/TPO adhesive joints by PFM-AFM and SEM

The influence of annealing and EBR component in injection-molded thermoplastic polyolefin (TPO) plaques on adhesion strength of CPO to TPO was investigated by a lap shear test. The TPO was fabricated as a blend of highly crystalline isotactic polypropylene (iPP) and low crystalline poly(ethylene-butene) impact modifier (EBR28). The CPO was a maleated chlorinated polypropylene containing 21.8 wt% Cl. High resolution pulsed force mode-atomic force microscopy (PFM-AFM) combined with the image analysis was used to characterize the interfacial properties of the lap shear joints. Based on PFM-AFM stiffness images, a “transition zone” with a width on the order of 600-1500 nm was observed between CPO and the TPO substrate that may play an important role in affecting the adhesion strength. This zone exhibits enhanced stiffness after annealing at 120 °C. The PFM-AFM images further show that the interface between iPP and CPO without annealing is very sharp and the interface between TPO and CPO without annealing is wider than the interface between iPP and CPO. Annealing (120 °C/20 min) leads to broadening of the interface between TPO and CPO. The thickness of the interface in lap shear joints was obtained from the z-directional line profiles of the stiffness maps. The fracture surface morphology was revealed by scanning electronic microscope (SEM), which showed that the fracture structure varied with both the addition of EBR28 in TPO plaques and the annealing condition. Finally, a correlation of interfacial properties to adhesion was obtained: higher stiffness in the transition zone coupled with a thicker interface resulted in stronger adhesion and cohesive failure within the CPO and TPO. In the case of CPO/iPP, the narrow interface and absence of a clearly defined transition zone correlated with interfacial failure between these components.     

Improved cathodic disbondment performance of polyethylene blends

Cathodic disbondment (CD) performance of a range of modified polyethylenes (PE) compression molded on to steel plates at 320°C is reported. Adhesion strength was measured by the 90° peel test and good dry adhesion strength was obtained for all modified polyethylene materials and blends, as well as for the neat polymer. It is shown that dry bond strength does not correlate with CD performance. Initial results of wet peel tests of samples in various concentrations of NaOH are presented where it is observed that for samples with improved wet adhesion strength, CD performance was also improved. Surface polarity was determined from contact angle measurements, and it is shown that increased surface polarity of the coating was not the only determinant for improved CD performance. Inorganic fillers such as talc were also found to improve CD performance by changing the bulk properties, with little measurable change in polarity. Some mechanistic aspects of CD performance are also discussed.     

Synthesis of a boron-containing adhesion promoter for improving the adhesion of addition-cure silicone rubber to PPA

Abstract

In this study, a boron-containing adhesion promoter PhBSiO with a high refractive index and MeBSiO with a low refractive index were synthesized by a non-hydrolytic sol-gel reaction. The chemical structures of the synthesized adhesion promoters were firstly characterized by infrared spectroscopy (FT-IR) and gel permeation chromatography (GPC). Then, the effects of adhesion promoters on the bonding properties of the cured silicone rubber were analyzed and the compatibility of the adhesion promoters to the silicon rubber was further studied through light transmittance detection. The boron-containing silicone adhesion promoters significantly enhanced the bonding strength of the cured silicone rubber to poly-phthalamide (PPA). When the addition amounts of PhBSiO and MeBSiO was 3.0 phr, the shear strength between silicone rubber and PPA respectively reached 0.761?MPa and 0.809?MPa, which were 145% and 161% higher than that of silicone rubber without adhesion promoter. In addition, the adhesion mechanism of the boron-containing adhesion promoter was investigated. The microscopic morphology of the fractured surface of the cured silicone rubber further demonstrated that boron atoms in the adhesion promoters significantly improved the adhesion strength between silicone rubber and PPA.     

重防腐涂料用新型附着力促进剂

环氧涂料由于其优良的抗腐蚀性能和机械性能,被广泛应用于海洋涂料和重防腐涂料.在这些应用中,环氧涂料通常作为底漆直接施工在各种不同的金属底材上.当金属底材的表面处理、施工条件不一样时,要使涂料与底材获得良好的附着力并不容易.使用附着力促进助剂能有效地改善涂料对金属底材的附着力,从而提高涂层的抗腐蚀性能.本文介绍了为改善海...     

New heat-resistant polytriazole adhesives: investigation of adhesion of polytriazole resins to metals

Crosslinkable polytriazole resins (CPTs) were synthesized by 1,3-dipolar cycloaddition reaction between azide and alkyne monomers. Adhesion properties of CPTs and an epoxy resin to different metal substrates (copper, iron and aluminium) were evaluated in terms of the tensile lap shear strength. CPTs to aluminium and iron substrates have higher adhesion strength than those to copper substrates. The effect of temperature and humidity on adhesion strength has been investigated. The resin CPT-3 has the best heat resistance among CPTs, whose retentions of adhesion strength are approximate 80% at 150?°C and 70% at 180?°C to all the substrates. CPTs have high adhesion properties at high temperature and humidity as compared with an epoxy resin.     

Using heterogeneous silane patterns to maintain adhesion and decrease water penetration into epoxy/aluminum interfaces

Silanes are used for surface modification to improve dispersion, bind biomaterials, improve adhesion, etc. Their use in improving adhesion in composite materials is widespread, and for these systems there is a growing need for both increased adhesion performance and resistance to water penetration across polymer/oxide interfaces. The present work reports adhesion results obtained using patterned, binary combinations of adhesion-promoting and non-adhesion-promoting silanes patterned onto an oxide surface. The effects of pattern shape, texture (feature size) and the fractional coverage of the adhesion promoter are explored for the bonding of epoxy matrices to aluminum oxide surfaces using combinations of γ-glycidoxypropyltrimethoxysilane (GPS), an adhesion promoter, and either octadecyltrichlorosilane (ODTS) as a hydrophobic non-adhesion promoter, or vinyltrimethoxysilane (VMS) as a water scavenger. In addition to dry tests, samples are submerged in boiling water for 48 h, and in 50°C water for 4 h to determine if water penetrated into the interface, thereby reducing the adhesion. Climbing drum peel tests reveal that heterogeneity of silane primer substrates can influence the adhesion and increase the durability to water penetration. Adhesion enhancement is attributed to the blocking of lateral diffusion of water by barriers due to the presence of the hydrophobic silane. Results show that due to exposure to water at 50°C for 4 h, the adhesion is reduced by 46% in the homogeneous samples but only by 20% in the heterogeneous samples. In contrast, the use of VMS was ineffective in preventing the loss of adhesion due to water penetration.