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.

Triple Threat: Light, Temperature, and Moisture

Most weathering damage is caused by three factors: light, high temperature, and moisture.

Any one of these factors may cause deterioration. Together, they often work synergistically to cause more damage than any one factor alone.

Light

Spectral sensitivity varies from material to material. For durable materials, like most coatings and plastics, short-wave UV is the cause of most polymer degradation. However, for less durable materials, such as some pigments and dyes, longer wave UV and even visible light can cause significant damage.

High Temperature

The destructive effects of light exposure are typically accelerated when temperature is increased. Although temperature does not affect the primary photochemical reaction, it does affect secondary reactions involving the by-products of the primary photon/electron collision. A laboratory weathering test must provide accurate control of temperature, and it usually should provide a means to elevate the temperature to produce acceleration.

Moisture

Dew, rain, and high humidity are the main causes of moisture damage. Our research show that objects stay wet outdoors for a
surprisingly long amount of time each day (8-12 hours daily, on average). Studies have shown that condensation in the form of dew is responsible for  most outdoor  wetness. Dew is more damaging than rain because it  remains  on the material  for a  long time, allowing significant moisture absorption. Of course, rain can  also be very  damaging  to some  materials. Rain  can  cause  thermal  shock  , a  phenomenon  that occurs , for example, when  the  heat  that builds up in an automobile over  the  course  of  a  hot summer day is rapidly dissipated by a sudden  shower. Mechanical erosion  caused  by the scrubbing action of rain can also degrade materials such as wood coatings because it wears away the surface, continually exposing fresh material to the damaging  effects of sunlight. For  indoor  materials , the major  effect  of  humidity is often  the physical stress  caused   by   the material   trying   to  maintain   moisture  equilibrium  with  its surroundings. The greater  the  range  of humidity  the  material  is exposed to, the greater the overall stress. Although  indoor products, such as textiles and inks, may only be exposed to moisture in the form of humidity, it can also be an important factor in the degradation of outdoor materials. Outdoors, the  ambient  relative  humidity  (RH)  will  affect the  speed  at  which a wet material dries. The UV-3 and the XTC chamber each reproduce light temperature, and moisture in different ways.

Practical Considerations

Of course, no matter how informative and realistic a piece of testing equipment is, it will not be practical if it is too expensive to purchase or operate. That is why purchase price, operating costs, and maintenance are critical issues, and must be weighed against the benefits of owning a tester.

Purchase Price

In general, the UV-3 is more economical than the xenon arc chamber. The XTC Chamber may cost three times as much as the UV-3, depending on the size of the unit and the features.

3D Test Specimens

The XTC allows more flexibility in terms of the types of specimens that can be mounted. It accommodates flat panels as well as 3D parts, test tubes, and petri-dishes. The UV-3’s standard test sample holders were designed for flat, relatively thin panels or specimens.

Ease of Use and Maintenance

Both the UV-3 and the XTC chamber are easy to use and easy to maintain. Both testers are completely automated and can operate continuously, 24 hours per day, seven days per week. Automatic shutdown timers allow tests to finish at any time that is specified. Calibration is also simple with the patented Auto Cal system and calibration radiometers. Calibration is accomplished with a keystroke as the instrument automatically measures the lamp output and automatically adjusts the on-board control system accordingly.

Test specimens and lamps all stay in place during the procedure. The Xenon Test Chamber and the UV

Accelerated Weathering Tester are both designed to be user-friendly. The XTC's front access and the UV-3's double sided access make lamp loading simple, and specimens easy to mount.

Maintenance Costs

Both the UV-3 and XTC chamber offer relatively low maintenance costs. XTC annual lamp costs are significantly higher than the UV-3. XTC electrical costs are also higher. Additionally, ordinary tap water can be used in the UV-3, whereas the XTC requires pure, de-ionized water. In summary, the UV-3 annual operating costs are considerably less than those of XTC.

Technical Summary

The Right Tester for the Right Job Deciding on the right weathering or light stability device can be confusing, especially if you’re new to this type of testing. Which is the better for you? Below are some general guidelines. As with any generalization, there may be exceptions to the rule.

The UV-3 is better in the short-wave UV

The UV-3 with UVA-340 lamps provides the best available simulation of sunlight in the critical short-wave UV region. Short-wave UV typically causes polymer degradation such as gloss loss, strength loss, yellowing, cracking, crazing, embrittlement, etc. In addition, the UV-3’s fluorescent UV lamps are spectrally stable, with very little change in the SPD over time. This enhances reproducibility and repeatability.

The XTC is a better match with sunlight in the long-wave UV and visible spectrum

Long-wave UV and even visible light can cause fade and color change in pigments and dyes. Where color change is the issue, the XTC is usually recommended. The XTC, using Window Glass filters, is also generally better than the XTC for testing indoor products.

The UV-3 is better at simulating the effects of outdoor moisture

The UV-3’s condensation system (100% RH) is more aggressive and realistic than the XTC’s water spray and humidity control systems. Deeply penetrating moisture may cause damage such as blistering in paints.

The XTC is better for controlling humidity

The XTC can control relative humidity. This can be an important feature for humidity sensitive materials like many textiles and inks. High humidity can cause color shift and uneven dye concentrations.

A Two Tier Approach

Because many researchers are concerned with polymer degradation, moisture degradation and color change, a two-tier testing program is often the best approach. Many manufacturers get cost-effective results by using the UV-3 Accelerated Weathering Tester for polymer degradation and a XTC Xenon Test Chamber for color change.

List of Instruments for Starting a Paint Lab

Suggested list of instruments to start up a quality control laboratory for paint testing: