In light of these goals, plastic parts present an attractive alternative. Plastics are relatively light and can have a potentially lower impact on the environment, since weight reduction typically leads to improved fuel efficiencies. Additionally, the versatility of plastic part processing provides the industry with design choices that support product differentiation for consumers. Plastics such as polycarbonate (PC) and polyacrylate demonstrated these benefits when they were selected to replace glass in headlight (Figure 1) and rear lenses.
However, plastics also present a major challenge to the automotive industry and its suppliers. Despite its many desirable features (light weight, impact resistance, transparency, flame resistance and design flexibility), plastic is susceptible to three potential shortcomings: 1) degradation when exposed to weather conditions (UV, heat, water) for long periods of time, 2) scratching or marring with little force and 3) susceptibility to chemical and/or solvent attacks. Any of these potential deficiencies may lead to product failures and/or consumer complaints that the plastic part looks old too soon - consumers expect vehicles to maintain their gloss. As a result, while plastics offer many desirable features for the automotive industry and its suppliers as alternatives for heavier parts, there is no “perfect material” available yet to address the fact that the part needs to last in the car over a long period and still look like new.
Hardcoat AdvancesAlthough thermally cured silicone hardcoats have long been available in the marketplace, more demanding performance requirements have led to the development of a two-component silicone hardcoat system consisting of a hardcoat topcoat and a basecoat.* This thermal-cure silicone coating system can provide exceptional protection from ultraviolet light exposure, excellent abrasion resistance and chemical resistance. The system typically produces a high-performance protective hardcoat for polycarbonate that imparts excellent protection for parts, exceeding the protective value of other coating systems currently available on the market.
In general, the synthesis and thermal cure process of a silicone hardcoat can be summarized as shown in Figure 3. The final product of this reaction process is a highly crosslinked silicone resin/nanosilica composite that, when used in conjunction with the thermal cure silicone basecoat, typically has excellent adhesion to plastic (such as polycarbonate) and high resistance to abrasion and scratching. Patented UV absorbers provide the UV screening properties of the coating system. These UV absorbers are among the most photo-stable UV absorbing molecules currently known, and their use in the advanced thermal cure silicone coating system provides the coating with its exceptional performance characteristics.
A Durable, High-Gloss FinishThe high-gloss appearance of the advanced hardcoat system can be maintained long-term due to its excellent abrasion resistance and superior chemical resistance. More importantly, this new hardcoat system appears to have the potential of lasting two to three times longer than other commercially available hardcoat systems. The performance characteristics of the new system are summarized in Table 1.
The new hardcoat system is already finding use in the automotive industry. Additional applications are likely as automakers seek new ways to reduce vehicle weight without compromising durability.
For more information, visit www.momentive.com.
Thanks to LexaMar Corp., Boyne City, MI and USi Inc., Rockford, TN, for their assistance with this article.
*The new hardcoat system is supplied as a topcoat (AS4700) and basecoat (SHP470) from Momentive Performance Materials, Wilton, CT.
†Lexan is a trademark of SABIC Innovative Plastics IP.
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