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.     

Diffusion of adhesion promoter (CPO) into polypropylene/ethylene–propylene (PP/EP) copolymer blends: mechanism

Chlorinated polypropylene (CPO) is commonly used as an adhesion promoter for paint on polypropylene/ ethylene-propylene copolymer blends (PP/EP). An adhesion mechanism frequently proposed in the literature is the CPO diffusion into the substrate, leading to molecular entanglement. It is shown here that CPO and PP/EP are immiscible. On the other hand, the use of radiolabelled CPO allowed quantification of CPO diffusion into PP/EP substrates as a function of several parameters and under conditions which simulate industrial practice. Diffusion requires substrate swelling by the solvent. This explains that it increases with the EP content of the material, with the solvent ability to swell the substrate, and with the thickness of the paint. For baked samples, diffusion passes through a maximum when the delay between CPO application and baking is varied. This is due to a balance between two processes working in opposite directions, i.e. substrate swelling and solvent evaporation.     

Influence of glass mechanical strengthening on the adhesion properties of poly(vinyl butyral) films to a float glass surface

The effects of various methods of mechanical strengthening of glass on the adhesion properties of poly(vinyl butyral) (PVB) film to a float glass surface were investigated. The mechanisms of the influence of the strengthening processes on the adhesion properties were analyzed. The influence of different types of pretreatment of the glass surface on the adhesion of the polymer films was also considered. It was shown that ion-exchange strengthening followed by treatment with an alkaline water solution provided the best combination of high mechanical strength of glass and good adhesion of the PVB films to the glass surface. Metal-oxide coatings on float glass increased the mechanical strength of glass but decreased the adhesion strength between the polymer and glass. The adhesion of PVB to the metal-oxide layers was determined not only by the chemical composition of the layers, but also by the method of layers formation, the type of glass surface pretreatment, and the nature of the intermediate layer between the metal-oxide layer and the glass surface.     

Photo-Initiated Lamination of Polyethylene (PE) and Poly(ethylene terephthalate) (PET)

Besides plasma-based processes, photo-initiated surface modifications have an interesting potential for adhesion promotion. This is of special interest with applications ranging from classical finishing to composites. Photo-chemical processes using continuous UV sources — monochromatic as well as broad band — are based on radical activation and ensuing reaction with the atmosphere. Achievable effects are addition of atoms — e.g., introduction of oxygen (photo-oxidation) resulting in increased surface energy — or grafting of functional groups. Both have certain potentials for adhesion promotion in a physico-chemical way. Based on the fundamental scheme of these processes — i.e., a photon-initiated radical reaction at the substrate-atmosphere interface — a direct 'inter-linking' of coating polymer and substrate is presented in this paper. The principal idea is to apply a thin layer of coating polymer on the substrate and irradiate this composite system at certain UV wavelengths. Given a low absorption of the radiation by the thin coating and — at the same time — a high absorption by the substrate, the radiation will penetrate the coating layer and generate radicals at the interface, which will induce cross-linking between the coating polymer and substrate. It is shown that for the example of laminates of polyethylene (PE) film on fabrics made of poly(ethylene terephthalate) (PET), extremely high adhesion strenghths are achieved without any use of additional adhesion promoters.     

Surface modification of films of various high temperature resistant thermoplastics

The influence of different surface treatments on the physical and chemical surface properties of poly(etheretherketone) (PEEK), poly(phenylenesulfide) (PPS) and a liquid crystal polymer (LCP) was studied. For all the three polymers, the adhesion strength of an adhesively-bonded copper foil could be increased significantly by a chemical etching process using chromic sulphuric acid or a low pressure air-plasma treatment. However, for LCP the enhancement of adhesion by the surface treatments was lower than for the other polymers. Peel tests were employed for determining the adhesion strength of the copper foil. The physical surface properties were investigated by laser scanning microscopy (LSM). Contact-angle measurements and X-ray photoelectron spectroscopy (XPS) provided detailed information on the chemical surface properties. The detailed XPS analyses revealed different chemical mechanisms of the surface treatments depending on the polymer investigated. In all cases an incorporation of oxygen containing groups by the surface treatments was found to be responsible for a better adhesion of the copper foil on the treated polymer films compared to the untreated.     

The role of interfacial interactions and loss function of model adhesives on their adhesion to glass

The measurement of adhesion and the evaluation of influencing factors are of great scientific and technological importance. There are two distinct viewpoints on adhesion: (i) surface chemistry, and (ii) fracture mechanics. For elucidation of the relative importance of mechanical properties in the bonding of adhesives, the strength of adhesion between model adhesives and glass plates was measured by the wedge cleavage (WC) test method. Copolymers of methyl methacrylate (MMA) with n-butyl acrylate (nBA) and methyl methacrylate with styrene (S) were prepared as model adhesives. The results show that in MMA-nBA copolymers, by increasing the amount of nBA, both the loss function and the adhesion energy of the adhesives increase. However, by increasing the amount of nBA above a certain level, the adhesion strength begins to decrease. In this situation, the cohesive strength of the adhesive dominates the failure mechanism. On the other hand, a decrease in adhesion was expected upon increasing the amount of styrene in the poly(styrene-co-methyl methacrylate) adhesive, because methyl methacrylate, an interactive monomer with glass, is replaced by a non-interacting styrene monomer, while the loss function of the adhesive is almost constant. But our practical adhesion measurement technique was not sensitive enough to detect this adhesion loss.     

Coating solvent effects producing adhesion to molded plastic parts

While adhesion is of paramount importance to ensure durability and quality of a coating system, solvents have long been known to affect coating adhesion to molded plastics, including polyolefins. Varions research group in their characterizations of interfaces and/or adhesion to plastics, have produced much understanding of the role played by solvents in adhesion to plastics. However, the 'goodness' of a coating solvent for the plastic, defined as a small Flory interaction parameter χ12, was not indicative of the quality of the coating adhesion to the molded plastic substrates under consideration. For instance, coating adhesion to either polypropylene or ethylene–styrene interpolymer substrates, as affected by the coating solvent, was poorly related to the solvent–polymer miscibility, the magnitude of solubility parameter mismatch, or the ability of the solvent to swell the polymer bulk, except where such swelling produced a topographical change to the polymer surface. Though the topographical change was different for polypropylene compared to the interpolymer surface, if a coating solvent induced little or no change in the topography of a molded plastic then little or no coating adhesion to the plastic was observed. Polymer surface crystallinity reduced the amount of topographical change caused by a solvent and, likewise, reduced adhesion of a coating containing the solvent. The alteration of topographical features by solvent provides an adhesion mechanism and an explanation for the reported formation of diffuse interfaces and how a coating solvent could expose sub-surface domains within a polymer substrate.     

Diffusion of an adhesion promoter (chlorinated polypropylene) into polypropylene/ethylene–propylene copolymer (PP/EP) blends: method of quantification

The adhesion between an organic coating and a polymer substrate may depend on the diffusion of organic coating constituents into the substrate. The diffusion of chlorinated polypropylene (CPO) into polymer substrates was quantified by using radiolabelled CPO and monitoring its dissolution subsequent to its application. The CPO was ³H-radiolabelled by reduction with NaB³H₄ in tetrahydrofuran. The characterization of native and labelled CPO by size exclusion chromatography and differential scanning calorimetry showed that the reaction did not degrade the resin. Several substrates and CPO application conditions were used in order to create a wide range of CPO-substrate interfaces. The measurement of the progressive dissolution of radiolabelled CPO showed a clear-cut differentiation between the CPO bound by the substrate and the CPO simply deposited as a film, and provided a minimum estimation of the amount which was retained through macromolecular entanglement or diffusion. A larger amount of bound CPO was found in PP/EP than in PP, in agreement with a broader CPO concentration profile (several μm) at the surface of the former, as observed by ToF-SIMS imaging.     

The effect of surface properties on the adhesion of modified polychloroprene used as adhesive

The adhesion properties of polychloroprene can be improved by addition of such materials as piperylene–styrene co-polymer (PSC), VeoVa-10 polymer, VeoVa-11/methyl methacrylate/2ethylhexyl acrylate co-polymer (VeoVa-11/MMA/2EHA) and poly(vinyl acetate) waste (wPVAc). Here, the relationship between adhesion properties and surface tension of polychloroprene was investigated. Contact angle measurements have been used to study the effects of nature and content of polymeric additives on the adhesion and surface properties of polychloroprene. Low-surface-tension VeoVa-10 polymer has the tendency to migrate to the surface of polychloroprene; thus, adhesion is determined mainly by this additive property. Enrichment of polychloroprene film bottom layer by the additive was observed using high-surface-tension PSC and wPVAc. In this case, the adhesion properties of polychloroprene depend on the interactions at the interface. Adhesion properties of polychloroprene were found to depend not only on compatibility between adhesive components, but also on compatibility between the adherend and the adhesive.     

Adhesion Improvement of Powder Coating on Medium Density Fibreboard (MDF) by Thermal Pre-treatment

In the powder coating of non-conventional substrates such as engineered wood, natural fibre composites or synthetic polymers, several technological problems must be resolved which result from the substantial differences between such coating substrates and metallic ones. Unlike metals, non-conventional carrier materials show low temperature stability, much rougher and more irregular surface texture, large dependence of their dimensional stability on the moisture content of the surrounding environment and significantly less electrical conductivity. Hence, when powder technology is transferred from coating metals to coating fiberboards, for example, the surfaces of the boards need to be engineered in order to provide ideal adhesion for the coating layer. One major problem is the lack of electrical conductivity. Since the powder coatings are preferably applied using electrostatic spraying equipment, sufficient electrical conductivity of the surface is a major requirement and the correct moisture content plays an important role as well. In the present study, a pre-heating process was used to improve the powder application during powder coating of medium density fibreboards (MDFs). Electrical resistance, treatment temperature and moisture content were systematically studied to better understand the complex physical mechanisms leading to an improvement in powder application by such a pre-heating process. To this end, a new sophisticated procedure was developed to measure the electrical resistance (surface and core resistance) during pre-heating. The results show, that the electrical resistance of MDF is influenced by board temperature and moisture content. Moreover, it is confirmed that pre-heating proves to be an efficient method to improve the powder application onto non-conventional substrates.     

Preparation and characterization of a novel conducting nanocomposite blended with epoxy coating for antifouling and antibacterial applications

In this study, novel epoxy-based paint was synthesized to be applied on carbon steel. The composition of the paint mainly contains epoxy mixed with an electronically conductive polymer, polyaniline (PANI), alone and combined with its nanocomposite derivation containing ZnO nanorods as an additive. The antifouling properties of the paint applied on carbon steel were investigated. The conductive nanocomposite was synthesized by an in situ chemical oxidative method of aniline in the presence of ZnO nanorods and then well characterized. The antifouling behavior was evaluated for 9 months in the Caspian Sea and Persian Gulf. Results revealed that epoxy/PANI–ZnO nanocomposite coating can prevent accumulation of marine macroorganisms on the coated panel. In addition, the epoxy coating comprising PANI–ZnO nanocomposite as well as the epoxy/ZnO coating exhibit significant antibacterial characteristics against (E. coli and S. epi). We interpret the antifouling and antibacterial behavior of the paint with (i) the presence of emeraldine salt structure in PANI which develops a surface pH in a range of 4–5 preventing the adhesion of microorganisms on the surface and (ii) the antibacterial and antifouling properties of zinc oxide nanorods that occurred by the production of hydrogen peroxide on the surface of the coating.     

Adhesion and corrosion resistance properties of coatings obtained from modified low-molecular-weight polystyrenes

In this study, as a continuation of our previous studies, chemical modification of low-molecular-weight polystyrenes (PSs) was carried out with various functional group modifiers: epichlorohydrin (ECH), maleic anhydride (MA) and acetic anhydride (AA), in a single stage using a cationic catalyst. It was determined that the amounts of the functional groups bound to the structure of the polymer depended on the molecular weight of the polymer used, and more functional groups were bound to the lower-molecular-weight PSs. It was found that the coating properties (adhesion properties and resistance to aggressive conditions) of the functional group containing PS to the metal surface depended on the structure and the amount of the functional groups bound to the aromatic ring of the polymer. In addition, it was observed that the PS modified with MA and ECH having carboxyl- and epoxy-groups in their aromatic rings had higher adhesion, as well as higher corrosion resistance properties. Various functional groups bound to the aromatic ring of the polystyrene and their amounts were determined by spectral and chemical analysis methods.     

Effects of contact time,humidity, and surface roughness on the adhesion hysteresis of polydimethylsiloxane

Dynamic contact mechanics experiments have been performed on small polydimethylsiloxane (PDMS) lenses and several substrates in both ambient air and in dry nitrogen. The experimental results are analyzed with the Johnson-Kendall-Roberts theory. While the theory adequately describes the approach data, it is unable to account for the large hysteresis observed upon retraction. Adhesion hysteresis is shown to scale with the roughness of the substrate, the hydrophilicty of the substrate, the time of contact, and the ambient humidity. The experimental results also demonstrate that this method is sensitive to changes in the surface energy of the substrate. The cumulative adhesion hysteresis is quantified and is shown to be largest for rough, hydrophilic substrates in relatively high humidity and smallest for smooth substrates in dry nitrogen. The origin of the hysteresis is analyzed by considering favorable interfacial bonding resulting from water-mediated bonding between the substrate and oxygen atoms in the PDMS backbone or other polar species on the polymer surface. Capillary forces are also postulated to contribute to the cumulative adhesion hysteresis.     

The effect of vinyl alcohol content on adhesion performance in poly(vinyl butyral)/glass systems

The effect of the concentration of vinyl alcohol (VOH) groups in commercial poly(vinyl butyral) (PVB) on the adhesion of the polymer to glass beads in particle-filled composites is investigated. Both the optical single particle composite (SPC) and acoustic emission (AE) techniques were used to determine the applied uniaxial tensile stress at which a single imbedded particle was detached from the matrix. The results for the failure stress from the two techniques are consistent with one another and show that the adhesion strength is improved substantially as the vinyl alcohol content increases. The improved adhesion performance is attributed to an increase in condensation of hydroxyls within the polymer matrix and acid–base interactions between the hydroxyl groups on the bead's surface and those in the polymer. The AE results also provide details of the mechanism of the debonding event in cases of both weak and strong adhesion.     

Al/PET adhesion: role of an excimer laser pretreatment of PET prior to aluminum thermal evaporation

The modifications induced on PET, poly(ethylene terephthalate), by an excimer laser irradiation performed below the polymer ablation threshold were studied. The laser treatment of the polymer before aluminum thermal evaporation improved the Al-PET adhesion. The enhancement of the metal-polymer interface strength is attributed to surface morphological changes rather than to chemical modifications of the PET surface. Surface oxidation under UV irradiation was found to degrade the adhesion.     

Collagen immobilization onto the surface of artificial hair for improving the tissue adhesion

To improve the bonding ability of artificial hair towards soft tissue, type I atelocollagen was immobilized onto the hair surface. The artificial hair used was made of a poly(ethylene terephthalate) monofilament. Following photo-induced graft polymerization of a hydrophilic monomer onto the surface of artificial hair, collagen was complexed with the graft chains. Poly(acrylic acid) was selected as the polymer to be grafted onto the artificial hair because this synthetic polymer exhibited the greatest ability to form an interpolymer complex in solution with collagen among the three anionic polymers poly(acrylic acid), poly(2-acrylamido methylpropane sulfonic acid), and sodium poly(styrene sulfonic acid). When the surface of the poly(ethylene terephthalate) film used as a model substrate was grafted with poly(acrylic acid), the surface density of the collagen immobilized by interpolymer complexation was found to increase with increasing surface density of the graft chains. Immobilization of collagen onto the filament surface was confirmed by surface analysis with X-ray photoelectron spectroscopy and transmission electron microscopy. It was shown that in vitro degradation of the collagen immobilized onto poly(ethylene terephthalate) was suppressed by crosslinking the collagen molecules with glutaraldehyde. Cell culture tests revealed that L-cells were attached well to the surface of collagen-immobilized artificial hair. The surface-modified hairs were implanted percutaneously in the scalp of a human volunteer. Neither infection nor rejection of the hair filaments was observed after 1 year of implantation. It was found that the number of collagenimmobilized filaments remaining fixed in the scalp after 3 years of implantation was significantly larger than that of untreated filaments. These results indicate that surface-modified artificial hair is highly biosafe and shows excellent tissue adhesion.     

The role of the polymer/metal interphase and its residual stresses in the critical strain energy release rate (Gc) determined using a three-point flexure test

The critical strain energy release rate (G _c), the residual stresses (σ), Young's modulus (E), and the practical adhesion, characterized by ultimate parameters (F_max or d_max), of organic layers made of DGEBA epoxy monomer and IPDA diamine hardener were determined. The prepolymer (DGEBA-IPDA) was deposited both as thick coatings and as a mechanical stiffener onto degreased aluminum alloy (5754) or chemically etched titanium alloy (Ti-6Al-4V). During the three-point flexure test used as a practical adhesion test [this test is also called the double cantilever adhesion test (DCAT)], the failure may be regarded as a special case of crack growth. To understand the real gradient properties of the interphase, substrate, and bulk polymer properties, a three-layer model was developed for quantitative determination of the critical strain energy release rate (G_c). The particular characteristic of this model was to consider the residual stresses developed within the entire three-layered system, leading to an intrinsic parameter representing the practical adhesion between a polymer and a metallic substrate. Moreover, to determine the residual stresses generated in such three-layer systems, the gradient of interphase mechanical properties was considered. The maxima of residual stress intensities are found at the interphase/substrate interface, leading to an adhesional (interfacial) failure that is experimentally observed. The determination of the critical strain energy release rate by the three-point flexure test (DCAT) shows that residual stresses cannot be neglected. A comparison between the results obtained from the three-point flexure test (DCAT) and those obtained by the tapered double cantilever beam (TDCB) test is presented.     

Coating of untreated polypropylene with halogen free aqueous materials

Substrates made out of polypropylene (PP) or it’s blends with ethylene-propylene rubbers (PP-EPR) which are typically used for exterior automotive parts are either pretreated before they are coated or the primary coating layer consists of or at least comprises a chlorinated polyolefin (CPO) as an adhesion promoter — most often applied as an organic solution. The presentation describes the approaches for the development of a halogen free aqueous adhesion promoter.

While chemical bonding via catalytic oxidation of the unactivated C---H bonds as well as via insertion of a nitrene into the C---H bonds proved to be ineffective, excellent adhesive strengths similar to those known from the state-of-art could be obtained via physical interactions with polyacrylic grafted polyolefins which have been applied as aqueous dispersions.

The correlation of adhesive strengths with process parameters and analytical results obtained from model systems leads to the assumption that the adhesion is effected by amphiphilic polymers interacting with the outermost polypropylene at the surface rather than by penetrating deeply into the substrate.     

Analysis of adhesion in an aggressive environment of a protective paint coating on an aluminium alloy surface treated by air cold plasma

2024 aluminium alloy is commonly used for the fuselage of aircraft or helicopters because of its high strength. The fuselage bears the weight of the movers of the aircraft and the oil or petrol tanks. It works where the climatic conditions are very harsh, i.e. very low temperature and salt fog, and in presence of aggressive chemical agents. The aluminium fuselage needs to be protected from water, heat and aggressive chemical agents, such as hydrocarbons, lubricating oils or salt fog, by a paint coating. It is important that the adhesion between the paint and the aluminium alloy remains strong for a long time. A complex and critical process is industrially used to enhance both wettability and adhesion of paint to the aluminium alloy. Cold plasma represents an efficient, non-polluting and economical alternative to clean and activate aluminium surfaces. The present work describes the possibility to use an aluminium alloy, pre-treated by cold plasma and then protected by a paint coating, in the presence of water or high temperature, as well as aggressive environmental conditions. Adhesion between the paint and the aluminium surface was characterized by a set of mechanical and physical-chemical tests. Many experimental tests were carried out in order to understand the interaction between plasma and workpiece material. The results obtained by the cold plasma treatment and the commonly used industrial process have been compared. Also, the nature of the relationship between wettability and adhesion was investigated. The results show that cold plasma allows to eliminate cleaning by a solvent and to reduce the quantity of Alodine 1200^® or XPD^® primer applied and, therefore, to reduce the use of polluting substances and the weight of the whole helicopter or aircraft.     

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.