Exterior finishes may catch the eye, but interior coatings must also fulfil aesthetic and functional needsAutomotive interior coatings have two main functions: to provide a quality decorative finish and to be warm to the touch, helping occupants to feel ‘at home’ in their vehicle. These coatings are mostly applied to plastic and leather substrates; a small amount of real wood is used in some premium vehicles, but in general, plastic-based materials with coatings which look like wood have taken its place in vehicle interiors. As detailed below (see boxes), there have been a number of recent developments in interior coatings, notably ultraviolet (UV) coatings, the combined direct coating and skinning process – arguably the most significant recent development in one-shot and in-mould coating – and physical vapour deposition (PVD).
The finish of interior parts, especially plastic parts fitted in and around the cockpit, is crucial to both the perceived and the actual quality of the vehicle. In addition to the sensory aspects, both visual and haptic (tactile), coatings must fulfil a number of functional requirements. These include protecting the components from scratches, weathering and UV rays, as well as products such as cosmetics and cleaning agents, which frequently come into contact with vehicle interiors. In response to the use of such harmful materials, and the need to maintain the quality of interior parts in service, the industry now sees UV coatings as a crucial technology.
In parallel with the rising popularity of UV coatings, the industry has seen the emergence of two-component paint systems which allow suppliers to coat parts that are made of polypropylene (PP) and thermoplastic polyoelfein (TPO) without the need for either pre-treatment or a primer. A further development has been that of a filler-free paint process which eliminates intermediate drying stages, lowers energy consumption by as much as 20% and also reduces the factory footprint. The first car companies to use the two-component system for painting PP and TO parts without flame treatment were General Motors and Toyota. Meanwhile, the solvent-free, 100% UV-curing clearcoat process for coating acrylonitrile butadiene styrene (ABS) was first approved by Renault, Volkswagen and Daimler.
In addition to enhancing the visual quality and feel of vehicle interiors, coatings play an important role in keeping these components clean and eliminating inadvertent noises; a 2014 quality survey by market research company JD Power found that squeaking or other sounds from interior parts was one of the most significant irritations highlighted by consumers.
In terms of keeping the parts clean, the main issue is how easily seat materials scuff and soil, affecting light-coloured interiors in particular; blue-dye transfer from jeans was identified in the survey as the key problem for seats. One solution has been developed by Dutch company Stahl, which has developed a coating that protects pale leather and synthetic surfaces. It has also developed PolyMatte, a material which adds to the luxurious feel of the leather treated with Stahl’s coatings.