Industrial automotive polymeric coatings, or varnishes, are often exposed to harsh environmental conditions yet they are expected to maintain a high gloss finish for at least five years. Following a strong demand from the automobile industry, their customers and suppliers (paint manufacturers), a project was set up to evaluate new test methods for characterising polymeric topcoat materials in terms of significant parameters.
Industrial Polymer Coatings
One of the primary functions of industrial polymer coatings is to protect the underlying substrate material. Automotive coatings are subjected to daily and seasonal fluctuations of temperature and humidity and are exposed to environmental contaminants such as acid rain and salt. Additional hazards include car washes, road grit and gravel.
Mar resistance characterises the ability of the coating to resist damage caused by light abrasion. The difference between mar and scratch resistance is that mar is related only to the relatively fine surface scratches which spoil the appearance of the coating. Mar resistance depends on a complex interplay between viscoelastic or thermal recovery, yield or plastic flow, and fracture. Polymers are challenging because they exhibit a range of mechanical properties from near liquid through rubbery materials to brittle solids. The mechanical properties are rate and temperature dependent and viscoelastic recovery can cause scratches to change with time.
Nano Scratch Tester
Previous work has focussed on the problems involved with characterisation of such complex coatings, but a dedicated instrument designed for standard testing has not previously existed. To this end, a project has now been underway for some time between all interested parties and after extensive testing and development, the outcome has been that the Nano Scratch Tester (NST) from CSM Instruments has been accepted as the preferred method for assessing scratch resistance of automobile coatings.
This article features some of the most interesting results from this project and shows the versatility of the NST in measuring the mechanical properties of a range of topcoats.
Figure 1. Nano Scratch Tester results for progressive load (0 - 20 mN) measurements on two different polymer varnish topcoats (A and B). The penetration depth (Pd) during scratching and the residual depth (Rd) after scratching are presented for both samples. Optical micrographs show the onset of plastic deformation (left) and the extent of deformation at maximum load (right). Measurements were made with a 2 μm radius diamond stylus.
Figure 2. Residual scratch depth (corresponding to an applied load of 5mN) measured as a function of gloss retention for a series of unweathered topcoat samples. The level of gloss was measured with a glossmeter.
The study consisted of selecting 10 different materials which are all high bake (140°C) polymer topcoats. They consist of a pigmented basecoat (thickness = 12μm) which is applied first, followed by the clearcoat (thickness = 45 - 50μm), after which both are baked together. Two different colours were selected for the basecoat pigment (red and black) in order to investigate whether this had any influence on the scratch performance.
The base ingredients of each sample material consisted of one or more components of acrylic, melamine, urethane, silane and carbamate. The samples can be considered as the present state-of-the-art in automobile polymeric topcoats and were sourced from three different suppliers. The samples were evaluated as ‘unweathered’ and ‘weathered’, the latter consisting of 400 hours in a Xenon Arc weatherometer using borosilicate inner and outer filters. Such filters give a better equivalent spectra to sunlight and 400 hours is approximately equal to 3 months in service. This is also the period of time in which customers provide the manufacturer with feedback on the quality of their vehicles.