A Comparison of Two Effective Approaches to Accelerated Weathering & Light Stability Testing

The Need for Testing:

Weathering and light exposure are important causes of damage to coatings, plastics, inks, and other organic materials. This damage includes gloss loss, fading, yellowing, cracking, peeling, embrittlement, loss of tensile strength, and delaminating.

Even indoor lighting and sunlight through window glass can degrade some materials like pigments and dyes, causing fade and color change. For many manufacturers, it is crucial to formulate products that can withstand weathering and light exposure. Accelerated weathering and light stability testers are widely used for research and development, quality control and material certification. These testers provide fast and reproducible results.

Two Different Approaches:

In recent years, low-cost and easy to use laboratory testers have been developed, including the UV-3 Accelerated Weathering Tester (ASTM G154) and the XTC Chamber (ASTM G155). This paper will explore the ways in which these two testers differ, including emission spectra and method of moisture simulation.

The inherent strengths and weaknesses of each tester will be discussed, including purchase price and operating costs. Guidelines will be given for which tester is generally recommended for a particular material or application.

Historical Perspective:

While it is clear that weatherability and light stability are important for many products, the best way to test is sometimes controversial. Various methods have been used over the years. Most researchers now use natural exposure testing, the xenon arc, or the UV-3 Weathering Tester. Natural exposure testing has many advantages: it is realistic, in-expensive, and easy to perform. However, many manufacturers do not have several years to wait and see if a “new and improved” product formulation is really an improvement. The xenon arc and UV-3 are the most commonly used accelerated testers. The two testers are based on completely different approaches. The xenon test chamber reproduces the entire spectrum of sunlight, including ultraviolet (UV), visible light, and infrared (IR). The xenon arc is essentially an attempt to reproduce sunlight itself, from 295 nm - 800 nm.

The UV-3, on the other hand, does not attempt to reproduce sunlight, just the damaging effects of sunlight that occur from 300 nm - 400 nm. It is based on the concept that, for durable materials exposed outdoors, short-wave UV causes the most weathering damage.

Which is the better way to test? There is no simple answer to this question. Depending on your application, either approach can be quite effective. Your choice of tester should depend on the product or material you are testing, the end-use application, the degradation mode with which you are concerned, and your budgetary restrictions. To understand the differences between the XTC chamber and the UV-3, it is necessary to first look more closely at why materials degrade.