The Next Generation in Weatherable Hardcoats for Polycarbonate
Polycarbonate is used in many automotive applications due to its excellent properties of impact resistance, high heat distortion temperature and clarity. When used to replace glass or steel, polycarbonate also brings the added benefit of design freedom deriving from injection molding and low part weight. These benefits come at the cost of poor abrasion resistance, weatherability and chemical resistance. To meet application requirements for the first polycarbonate headlamps in the mid 1980s, thermally cured silicone abrasion-resistant hardcoats were developed to augment performance. Coated headlamps were able to meet the Department of Transportation’s minimum requirement of three years’ Florida and Arizona weatherability, as called out in SAE J576 JUL91 (see Table 1).1 In 1989, GE Silicones introduced AS4000 hardcoat and primer SHP401. This system (referred to as AS4000) extended coating delamination in Florida and Arizona testing well beyond the SAE minimum requirements, making it one of the leading coatings in use for headlamps even today, over a decade later. The success of AS4000 led to its expansion into other applications such as sunroofs and pillar posts, which was begun by Lexamar Corp. in 1989.
Assessing Needs
The first part of the new product design was to determine what properties were Crucial to Quality (CTQs). Six-sigma based customer needs mapping revealed that a 10-year weatherable coating was desirable. Features of conventional thermal cure hardcoats that could not be lost in the new development were abrasion resistance, transparency and chemical resistance. In addition, the unspoken CTQs — those expected by the customer as a given — were properties such as ease of application and retained impact resistance.