Selecting the Right Material to Meet Critical Performances in Weatherable Automotive Applications

When General Motors redesigned the stylish Camaro, it visualized windshield wipers hidden by flush-fitting covers that match the color of the cowling. As an exterior component, the covers had to keep their appearance during years of exposure to solar ultraviolet (UV) radiation.

The material selected had to simultaneously meet many demanding criteria :

UV resistance was critical so the GM 848 Black covers would not fade over time.

To minimize wind noise, the material needed high dimensional stability (0.005 in tolerence).

Rigid enough yet flexible enough to withstand assembly and blade changes.

Accommodate ultrasonic welding and withstand temperatures from -40° to +120°F

The material needed to withstand exposure to ozone, salt spray, and auto fluids.

The long, slender profile of the wiper covers demanded an easy-flowing resin.

Aminimum of shrinkage and warpage in molding.

Weatherable Celanex 6500 BK225 combines advantage of PBT & PET
The need for impact strength, stiffness, and the ability to yield a fine finish led Hoechst Technical Polymers (HTP) engineers to suggest a 30% glass- and mineral-filled Celanex PBT/PET (polybutylene and polyethylene terephthalate) grade with custom UV stabilizers. This blend provides both PBT’s good molding characteristics, so just a single gate is needed, as well as PET’s fine, resin-rich surface. The mineral filler improved impact strength and dimensional stability. Results formulated the product to withstand 2,500 kJ/m2 while preserving moldability and mechanical properties. The weatherable resin developed is now a standard grade – Celanex 6500 BK225.

The study found that the polyester grades tested performed at least as well as two nylon grades commonly used in exterior auto components. Significantly, the polyesters had better gloss retention than the nylons. Another test series found excellent mechanical property retention in two grades of Celanex after prolonged exposure to ultraviolet light.

Table 1: Color Difference and Gloss Change
(After a two-year exposure to sunlight in South Florida following SAE J1976)

Product Name DL* DE* Gloss Retention (%) Celanex 6500 BK225 0.43 0.43 41.8 Nylon A 0.17 0.9 12 Nylon B 1.26 1.4 22.3

* In CIE Lab units. DL defines how lightness/darkness changed as a result of the test. DE provides an overall measure of color change.