Rose A. Ryntz

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.

Scratch resistance behavior of model coating systems

Coatings are often subjected to physical deformations caused by car wash brushes, tree limbs, keys, fingernails, and the like, which may result in mechanical abrasion. The resistance of the coating to scratches imposed by such mechanical abrasions has been studied through the utilization of laboratory tests such as nanoindentors, crockmeters, scanning probe microscopes, and taber abraders. Little emphasis, however, has been placed on the influence of coating attributes on measured scratch resistance. In this study we attempt to relate the effect of coating glass transition temperature, crosslink density, and crosslink type in a series of formulated waterborne polyurethane dispersion clearcoats on resultant scratch resistance. Methods utilized to impart the scratches, e.g., scanning probe microscopy, weighted fingernail, and crockmeter, as well as related coating physical properties measured, e.g., viscoelastic behavior, hardness, and tensile strength, are discussed. The scratch resistance of model coating systems analyzed was found to be dependent upon the base resin Tg, which affected the surface hardness of the coating and the toughness of the crosslinked network, as measured by the method of essential work. Coatings that exhibited both hard surfaces and tough-elastic network integrity afforded the optimized scratch resistance behavior.

Coatings of polymers and plastics

Overview of the automotive plastics market, Susan J. Babinec and Martin C. Cornell plastics processing, Steven D. Stretch formulating plastics for paint adhesion, Dominic A. Berta polymers for coatings for plastics, J. David Nordstrom performance and durability testing, Philip V. Yaneff painting problems, Clifford K. Schoff recycling of automotive plastics, Rose A. Ryntz alternatives to coatings for automotive plastics norm, Kakarala and Thomas Pickett trends in coatings for automotive plastics and rubber in North America and Europe, Robert Eller automotive plastic coatings in Europe, Hans Christian Gruner and Klaus-Werner Reinhart.

Scratch resistance behavior of automotive plastic coatings

The sensitivity of automotive coatings, particularly coatings for plastics, to scratching has been a growing concern among automakers. Scratching may result from such predelivery events as polishing of minor defects embedded in the paint, or post-delivery events such as car wash bristles, dirt embedded under a cloth utilized in polishing the car, tree branches, and the like. Warranty cannot separate out which event is the more prevalent (e.g., predelivery or post-delivery to the customer) form of damage on plastics. Data available on coatings for metal, however, does suggest that isocyanate-based crosslinked systems perform more poorly than their melamine-based crosslinked counterparts when exposed to in-plant (predelivery) handling. This work attempts to correlate the scratch resistance behavior of coatings of plastics, both in their “green state” (right out of the oven, less than one week post-cure time) and in their infancy in the field (simulated 250 kJ Xenon arc Weather-ometer), to surface attributes such as toughness, hardness, and elasticity. Functional carbamate-melamine crosslinked one-component coatings and functionalized silane-melamine crosslinked one-component coatings appear to out-perform selected two-component coatings, which in turn outperform one-component hydroxyl functional acrylic or polyester melamine crosslinked coatings. Material attributes such as surface hardness, toughness (as measured through the method of essential work), and the ability to recover from an applied load are most important in the ability of the coating to resist damage.

Use of a scanning probe microscope to measure marring mechanisms and microhardness of crosslinked coatings

A scanning probe microscope (SPM) was equipped with a high-modulus probe to indent coating surfaces when normal force is applied. A method for measuring microhardness with this probe is described. The high-modulus probe was also used to mar coating surfaces under controlled conditions by application of normal force plus lateral motion. Dimensions of the mars were measured by conventional scanning probe microscopy. The data were analyzed in terms of a “three response, two mechanism model” of marring in which three types of responses of polymeric materials: elastic, plastic deformation, and fracture, are measured. Of the three responses, only plastic deformation and fracture result in marring, and the two mechanisms can be quantified. A fourth quantity which combines plastic deformation and fracture is suggested as a method of comparing “micro mar resistance” of materials under specified conditions. Three crosslinked polymeric coatings were studied in detail. Two had hard crusts of material near their surfaces that responded quite differently than the bulk of the material.

Coating adhesion to low surface free energy substrates

Abstract With the increased usage of plastics in the automotive industry, the science of polymer (paint) to polymer (plastic) adhesion has gained increased recognition. This paper attempts to delineate the basics to attaining adhesion to plastics and to describe the current means by which adhesion to low surface free energy plastics is attained. Adhesion to very low surface free energy substrates, such as thermoplastic olefins (TPOs; blends of poly(propylene) and rubber), is most often accomplished through the use of a surface pretreatment. The surface pretreatment, for example plasma discharge, flaming, chemical etching, or solvent degreasing, modifies the chemistry of the plastic surface mainly through oxidation and surface roughening. The change in polarity of the surface then affords greater adhesion of subsequent topcoats. Adhesion to low surface free energy plastics can also be attained through the use of adhesion promoters. The adhesion promoter most commonly consists of chlorinated poly(olefin) dissolved in a nonpolar solvent. When applied to the low surface free energy plastic, the solvents can swell and diffuse into the surface. Mechanical interlocking with rubber domains below the surface accounts for afforded adhesion. Modern techniques available to the coatings chemist are described in relation to adhesion achieved. New chemistries developed to replace chlorinated poly(olefins) are reviewed.

Degradation modes of crosslinked coatings exposed to photolytic environment

The objective of this study is to assess the degradation modes of crosslinked coatings exposed to photolytic environments. Three model crosslinked coatings were exposed in various ultraviolet environments. Atomic force microscopy and Fourier transform infrared spectroscopy were used in following nanoscale physical and chemical degradation during exposures. Results indicated that photodegradation of crosslinked coatings is a spatially localized (inhomogeneous) process in which nanometer-sized pits are initially formed; these pits deepen and enlarge with exposure. A conceptual model is proposed to explain the inhomogeneous degradation mode. The model proposes that nanosize “hydrophilic” domains are dispersed randomly with the highly crosslinked units. These hydrophilic domains, which are energetically preferred, comprise polar, unreacted and partially polymerized molecules, chromophores, and other additives. Photodegradation initiates at degradation-susceptible hydrophilic domains spreading to surrounding areas contiguous with the initiation site.

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.

Visco-elastic visco-plastic analysis of scratch resistance of organic coatings

The sensitivity of automotive coatings to scratching, particularly coatings for plastics has been a growing concern among automakers. Scratching may result from such predelivery events as polishing minor defects embedded in the paint, or postdelivery events such as car wash bristles, dirt embedded under a cloth utilized in polishing the car, tree branches, and the like. Warranty cannot separate out which event is the more prevalent (e.g., predelivery or postdelivery to the customer) form of damage on plastics. Data available on coatings for metal, however, does suggest that isocyanate-based crosslinked systems perform more poorly than their melamine-based crosslinked counterparts when exposed to in plant (predelivery) handling. Coatings on plastics, while lower in modulus than coatings on steel, are still subject to scratch events, albeit they have a greater tendency to “self-heal” once scratching events have occurred. This work attempts to correlate the scratch resistance behavior of three different one-component hydroxyl-functional acrylic or polyester-acrylic-melamine-crosslinked systems. Relationships between the tensile and indentation properties of the coatings were found to be related to the initial warranty data of the coating systems. Most importantly, it was apparent that the critical depth to fracture of the flexible coatings may be the relevant indicator of field performance for coatings on plastics. For metals, the relevant indicator to field performance is often only critical load to fracture, negating the influence of indentation depth. This variance is important to consider since coatings on plastic are often softer and more resilient to surface abrasions than their counterparts on metal.

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.