Clearcoats, the outermost automotive coating, are subject to damage caused by numerous elements, including environmental fallout; exposure to ultraviolet radiation from sunlight; exposure to high relative humidity at high temperature; and defects made by impact from small, hard objects resulting in chipping. Topcoats can be formulated to reduce so-called scratch and mar, and environmental etch. Scratch and mar refers to damage from impact, rubbing or abrasion that produces visible scratches or marring that sometimes can be rubbed out. Environmental etch is a term applied to a type of exposure degradation that is characterized by spots or marks on or in the finish of the coating that often cannot be rubbed out.
To be commercially successful, a coating should provide as many favorable characteristics as possible. Accordingly, it is most preferable to produce a coating that has an optimum mix of characteristics with regard to various forms of damage resistance. For example, it is desirable to provide an increase in scratch and mar protection without hampering the environmental etch protection. A number of coatings systems have been optimized to provide a favorable combination of these properties. However, because the systems represent a compromise, usually one property has been at least partially sacrificed to increase the other.
This article reviews recent developments in coatings technology that meet topcoat scratch and mar resistance expectations of OEMs in India. We discuss recent developments in coating technologies using resins and additives to improve scratch/mar resistance of automotive polymeric coatings.
The technology used in OEM clearcoats has been based on acrylic polyols, which are crosslinked with melamine formaldehyde resins in a stoving cycle of 30 min at 130 °C – 140 °C. Such clearcoats are limited in weight spray solids (50% weight) for two reasons:
1. Higher solids formulations would need either lower-molecular-weight acrylic polyols and/or lower-TG compositions, which would not develop sufficient mechanical (hardness) properties and/or crosslink density (solvent/chemical resistance); 2. Lower-molecular-weight melamines (monomeric), which are fully alkylated, do give higher solids formulations but need ‘external’ strong acid catalysis.
Evaluation of Different Scratch Effects
When an automotive OEM looks for coatings with high-value perceived surfaces, many different test methods are used to describe the visibility of a scratch. A short overview to summarize different scratch effects follows:2
Scratch− a type of friction-induced damage in which a sharp object cuts the surface of a polymeric material.
Abrasion− a phenomenon caused by the mechanical action of rubbing, scraping or erosion.
Mar− friction-induced damage in which the material surface is compressed, causing a marking that changes the appearance or gloss.
Writing effect− a soft scratching (writing) with a dull object (like a fingernail) on the coating surface. The load used for the writing is lower than the scratch, abrasion and mar.
The above are only examples of a few noticeable effects that constitute the different scratch effects. All ideal polymeric materials should cope well with all types of induced damage mentioned above.
Resin Approach to Improve Mar Resistance
The physics of marring is complex. Different authors use different terms to describe the phenomena involved. Various models have been proposed to describe what happens to a viscoelastic material when a hard object is drawn over its surface. One such model classifies the response of the material as elastic, plastic or fracture. Since the elastic response recovers essentially instantaneously, only plastic deformation and fracture lead to marring. Although simplistic, this model has the advantage that the three responses can be measured quantitatively by scanning probe microscopy or with a nanoindenter. Most coatings exhibit a mixture of responses.
Efforts are underway to relate mar resistance to the chemical structure of the coatings, but relatively few systematic studies have been published. In general, MF crosslinked acrylic clearcoats are more resistant than isocyanate crosslinked coatings (urethanes) to marring, but MF crosslinked coatings have poorer environmental etch resistance. MF crosslinked polycarbamates are an exception, combining etch and mar resistance. Since urethanes generally have superior abrasion resistance, it is surprising that they have inferior mar resistance. This might be explained by differences between bulk and surface properties. A study of clearcoat marring by a scanning probe microscope indicated that acrylic polyurethane had a thin layer of deformable plastic material on its surface, whereas an acrylic MF clearcoat had a layer of elastic material.
Two strategies are available for designing coatings with exceptional mar resistance:3
• They can be made hard enough that the marring object does not penetrate far into the surface; or
• They can be made elastic enough to recover after the marring stress is removed.
If the hardness strategy is chosen, the coating must have a minimum hardness. However, such coatings may fail by fracture. Film flexibility is an important factor influencing fracture resistance. Use of 4-hydroxybutyl acrylate instead of 2-hydroxyethyl acrylate in an acrylic resin crosslinked with MF resin gave improved results, as did use of a polyol-modified hexamethylene diisocyanate isocyanurate instead of isophorone diisocyanate isocyanurate in crosslinking urethane coatings. Courter4 proposes that maximum mar resistance will be obtained with coatings having as high a yield stress as possible without being brittle. In this way, high yield stress minimizes plastic flow, and avoidance of brittleness thereby minimizes fracture. Courter’s paper4 provides a good review of attempts to relate bulk mechanical coating properties to their mar resistance, but these studies have not led to a broadly applicable theory of marring. This is understandable, since the mechanical properties near a coating’s surface are likely to be quite different from the mechanical properties of the bulk material.
A further problem related to mar resistance is metal marking. When a metal edge is rubbed across a coating, a black line is sometimes left on the coating where metal has rubbed onto the coating’s surface. Metal marking usually occurs with relatively hard coatings. The problem can be reduced or eliminated by reducing the surface tension of the coating, so the coefficient of friction is low and the metal slips over the surface. The various approaches using the resin/polymer technique follow.
Perfluoropolyether (PFPE) Resins for High Solids
Formulations containing PFPE resins and mixed hexamethylene diisocyanate/isophorone diisocyanate HDI/IPDI-blocked isocyanates have shown better scratch resistance properties. Fluorinated films showed excellent stain-release properties.5
Incorporating Slip Agents
When incorporating the slip materials in the resin or polymer itself, these materials act to lubricate the surface to provide an anti-scratch effect and also reduce scratch whitening.2 However, there are several disadvantages related to uncontrolled migration, including exudation on the surface of the parts, stickiness after aging/weathering and weatherability is poor.
The common systems currently used in the coating industry are often limited in their balance between chemical and scratch or mar resistance. If one of the properties is optimized, the coatings do not show an acceptable level of performance in the second field, and vice versa. Theα-silanes6 are promising candidates for solving this problem because of their potential to provide highly crosslinked systems with crosslink density being related to scratch or mar resistance, along with the benefit of the pronounced acid resistance of the formed siloxane bonds.
Colloidal nanosilica particles have been used to improve scratch and mar resistance of waterborne epoxy coatings by directly blending. To enhance the compatibility of nanosilica particles within the polymer matrix, nanosilica particles were first modified with 3-glycidoxypropyl-trimethoxysilane (GPTMS).7
Relative to unmodified nanosilica, GPTMS-modified nanosilica particles can improve the scratch and mar resistance more significantly and reduce the transparency and gloss of waterborne epoxy coatings less significantly.
Adding nanoparticles to a polymer matrix depends on the molecular structure of the polymer backbone, size and amount of nanoparticles added, particle dispersion throughout the coating, structure and functionality of the nanoparticles.
Silicone and Vinyl Compounds
A mar-resistant coating system was obtained by screening combinations of silicone and vinyl compounds. It was found that binary systems, such as the g-glycidoxypropyltrimethoxysilane-glycidyl methacrylate system, were excellent not only in mar resistance but also in adhesion to base resin polymers.8 These coatings were also good in weather resistance due perhaps to good adhesion to the base resin.
Traditionally, silicone polyether technology has been used to impart slip, wetting, leveling and defoaming. These copolymers have typical surfactant features that are hydrophobic and hydrophilic in nature. The ratio of the hydrophobic/hydrophilic segments is important to achieve the required compatibility balance. A high hydrophobic content may lead to de-wetting defects, such as fish eyes and craters. On the other hand, a high hydrophilic content can increase the solubility of the copolymer so that there is no driving force for accumulation at the coating/air interface during the drying process.
Carbinol-functional silicone resins have been recently reported in the literature.9 Moving from a linear fluid to a rigid, three-dimensional resin brings improvements in mar resistance without adversely affecting the slip or gloss level of the coating. Carbinol-functional resins show improved mar resistance compared to non-functional resins, but this must be balanced with phenyl functionality to ensure compatibility in the coating. Further work needs to be done in this area to determine performance improvements by reacting carbinol resins with organic binders.
Polyurethanes Synthesized Using Specialty Isocyanates
Polyurethane coatings formulated with specialty isocyanates are the most appropriate systems to obtain excellent mar resistance. Specialty isocyanates are premium-priced isocyanates that can be used to produce polyurethane polymers with unusual properties. Examples of these isocyanates are Desmodur TH, N and HH. They are adducts. Others are monomeric isocyanates. The molecular structure of these products is shown in Figure 1. Coatings systems based on polyol reacted with specialty isocyanates like XDI give excellent mar resistance and at the same time do not exhibit any gloss reduction.
Polyesters Reacted with Polyurethanes
The polyisocyanate ester compound is a branched material having at least two ester linkages, at least four urethane linkages further from the center of the compound compared to the ester linkages, and at least one terminal isocyanate group for each urethane linkage that may be blocked. The compound can be prepared by reaction of a polyol having at least two hydroxyl groups with a carboxylic acid having one carboxylic acid group and at least two hydroxyl groups to form a hydroxyl-functional ester product. The hydroxyl-functional ester product is then reacted with a polyisocyanate in which the isocyanate groups have different reactivities. The reaction with the polyisocyanate is carried out under conditions so that only one of the isocyanate groups is substantially reactive with the hydroxyl groups of the ester product of the first stage. Because each of the isocyanates is monofunctional under the reaction conditions, the product avoids building viscosity. In this way, the highly functional isocyanate can be used as a crosslinker without the increase in viscosity experienced with polyisocyanate crosslinkers of the existing technologies. It has been shown that this type of polymeric backbone gives excellent mar resistance.11
Modified Amino Resin Crosslinkers
In a new development, crosslinkers containing amino resin cores that are substituted with at least one silicon-containing group (silicon-containing functionality) and unsaturation (olefin functionality) compound per molecule have been shown to provide increased scratch and mar resistance.12
These silicon-containing crosslinkers tend to migrate to the surface of the coating composition. Hence their concentration is greater in the liquid/air interface (at the surface of the coating) than in the bulk portion of the coating composition. The olefin functionality on the novel crosslinker is available to participate in the radiation-curing reaction by undergoing 1,2 additions to itself under radical initiation. Thus, radiation curing forms a crosslinked network of silicon-containing crosslinking agents with an enhanced concentration at the surface of the coating. It is believed that this results in the increased scratch and mar resistance of the resulting coatings. At the same time, the alkoxyalkyl or alkylol groups of the crosslinker react with the resins and anchor the crosslinking agent in the coating. It is believed that this minimizes the problems of migration that otherwise tend to be observed with silicone additives and coatings.
Scratch and Mar Resistance Improvement in Plastic Components
The need for improved scratch and mar resistance for TPOs and other polyolefins used in automotive applications is well known. Advanced materials based on PP continue to be the first choice for automotive interior and exterior components due to major advantages such as low density, easy processability and good cost/performance balance.13 Most of the currently used PP/TPO materials in automotive applications are standard grades with low to average scratch resistance. Commonly used techniques for the surface coating of the TPO are organic coatings or laminates.
Plastics such as PVC, PC/ABS and ionomer have superior scratch resistance as compared to PP and TPO. These products generally have higher cost and have inferior weatherability than PP and TPO. PC/ABS can only be used in interior applications without painting.
Technologies currently applied, based on improving the performance of the TPO itself, are the use of coatings with special fillers (e.g., nanoclay) and slip additives or silicon-based additives.2
Epoxy Resin Technologies
One U.S. Patent14 describes a mar- and etch-resistant film-forming composition containing a polyepoxide, a polyacid curing agent and an additive amount effective to improve mar resistance of a solution polymer of an ethylenically unsaturated monomer component containing a polymerizable alkoxy silane monomer. The film-forming composition is particularly advantageous as a clear topcoat over a pigmented basecoat in a color-plus-clear automotive coating system.
Apart from the above specific technologies, general strategies of increasing crosslink density by use of higher-functionality oligomers and/or larger amounts of crosslinking agents have been used to improve abrasion resistance. In addition, the technique of using acrylic resins synthesized using acrylonitrile monomer has also been employed for mar resistance improvement.