Transparent Sheets
Weathering stability & Yellowness

Light weight and high design flexibility make transparent plastic sheets attractive for use as “organic glass” in many different applications e.g. noise barriers, green houses, sport arenas, sky domes, solar panels or bus stop shelters. In addition, rigidity and impact resistance of acrylic (PMMA) and polycarbonate (PC) sheets were optimized expanding its usage for safety and architectural glazing as well as for automotive, aircraft, yacht or caravan applications. Depending on the application, the transparency requirements will be very different and need to be objectively controlled – often within very tight specification.

Weathering stability

Outdoor applications like noise barriers, solar panels or automotive glazing are expected to reach a lifetime of several decades. Harsh weather conditions affect the stability and the transparency of plastic glazing. Today a wide range of plastics is available from photo stable types to UV stabilized polymers. Nevertheless, natural and artificial, accelerated weathering tests are necessary to evaluate the impact of heat, UV light and humidity to the products quality. Therefore, transmission haze and gloss are measured after certain exposure periods.

The following example shows the haze results of different polycarbonate samples after artificial weathering.

As the degradation is usually not homogenous within one sample a statistical evaluation of several measurements is highly recommended. Additionally, only samples of the same thickness should be compared.

Yellowness

Another critical aspect of weathering is the tendency of polymers to change its color mainly turning yellowish, which is typically caused by a reduced transmission in the blue spectral range. Yellowing is evaluated by measuring the color in reflection mode with a white background. It is recommended to always use the same backing and only samples of the same thickness.

The spectro2guide – a portable spectrophotometer which easily can be used at the outdoor weathering station, measures the spectral distribution and the CIELab color coordinates. Often only the yellowness index according to ASTM YI E313 or YI D1925 is used as a quick check.

In the righthand example, different modifications of polycarbonate were exposed to natural weathering for several years.