Thermal-induced recovery of small deformations and degradation defects on epoxy coating surface

Abstract

Surface defects, which are dependent on the surface properties, determine the appearance, toughness, and other properties of coatings; thus, changes to the morphology of a damaged epoxy coating surface were investigated on annealing. Changes in mechanically produced indentations and scratches with annealing indicate a surface transition temperature about 10 degree lower than the measured bulk glass transition temperature (T g ). Polymer molecules at a free surface, even in a crosslinked coating, may have higher mobility, and thus allow different relaxation activity from the bulk material. The eventual extent of deformation is a function of the annealing temperature and time. Results showed that the deformation profile diminished, driven by viscoelastic deformation and surface energy; the effect of structural relaxation will be further studied. When the surface features were generated by photodegradation, the roughness increased initially when temperature was increased, possibly due to phase coarsening, and then the roughness diminished during extended annealing.

An Overview of the Scratch Resistance of Automotive Coatings: Exterior Clearcoats and Polycarbonate Hardcoats

“…For those coatings that contain nanoparticles primarily at the surface, the long-term resistance to scratching is not known. Morphological changes that occur during weathering may reduce the effectiveness of this approach [17]. Also, it has been shown that forces during scratch events can reach upwards of 35 N [5], a thin nanoparticle/resin layer will likely not be able to survive these loads, resulting in deep scratches of similar morphology to those seen in standard thermoset systems.…”

“…The links between nano-scratch behavior and the chemical composition of the hardcoat or PECVD layer are currently unknown, but would provide a fertile area of research to guide the improvement of hardcoat performance. To further understand the correlation between haze and scratch resistance/morphology, Wyko 3D optical profilometry (used in previously published work [5,17]) was conducted on the annealed glass, standard hardcoat, and PECVD glass layer samples that were exposed to 1000 cycles of Taber abrasion. 3D profiles of the resulting damage to the three tested systems are shown in Figure 7.…”

Reduced indentation recovery temperature at the surface of a crosslinked epoxy coating in humid conditions

“…The enhancement of dynamics at a surface and the confinement effect at polymer-substrate interface can change T g relative to its bulk value, over a length scale of the order of 100 nm from the interface. 2 In prior work on crosslinked epoxy coatings, 3 the softening transition temperature, measured by the relaxation of roughness caused by photodegradation or nanoindentations at the surface of the material, was found to be significantly lower than the bulk T g . Barring accidents of curing chemistry, typical crosslinking processes involve evaporation of solvent and constrained migration of monomers, which cause the accumulation of chain ends and limit the crosslink density near the surface.…”

“…Thus, for a number of reasons one might anticipate T g of the material at the surface of a crosslinked polymer to be less than in the bulk, as seems to be the case. 3 It is natural to think about the likely response of a coating at a particular ambient temperature by referring to its T g , but there may be a discrepancy between T g as it might be easily (and conventionally) measured in the bulk, and the value of T g that occurs where the performance is determined, i.e., at the surface.…”

The effect of weathering and thermal treatment on the scratch recovery characteristics of clearcoats

“…Morphological changes that occur during weathering may reduce the effectiveness of this approach. 8 Clearly, scratches that penetrate deep into the coatings system will not be aided by the nanoparticles approach.…”