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Abstract

Accelerated laboratory weathering testing can contribute to the development of more eco-friendly coatings and, at the same time, reduce material cost. Unrealistic testing, however, can lead to unnecessary overdosing of additives versus required service life. If a more realistic sun simulation — aligned to material spectral sensitivities — is used in the accelerated test, it can help develop more eco-friendly coatings via optimization of required UV stabilizer levels.

Introduction

Weathering is the adverse response of a material or product to climate, often causing unwanted change or premature product failure. Typical visible phenomena are delamination, color fading, color change, cracking, gloss loss and chalking. 

UV stabilizers and other types of light stabilizers are needed to make coatings long-lasting; however, companies need to use them responsibly.

An ongoing challenge for the industry is the protection of coatings against three weathering factors: light, heat and temperature. One decisive aspect of weathering is solar radiation, which initiates coating degradation and, consequently, affects the substrate.¹

Each polymer and additive is sensitive to one or more wavelength ranges in the solar spectrum. Not only is the energy-richest part of the solar spectrum — the UV-B range ca. 295nm-320nm — important, but the remaining UV-A range — 320nm-380nm — is also a concern, as it can excite chemical bonds of polymers, such as polyurethanes, and ultimately separate them.

Colors interact with the visible spectrum, which can contribute to color fading. Further, every polymer reacts with the infrared part of the solar spectrum. Surface temperatures increase and, consequently, acceleration of the degradation takes place in this particularly critical area.

Figure 1: International Reference Sun: CIE No.85 (Table 4) and the impact of the spectral parts, ultraviolet (UV), visible (VIS) and infrared (IR), on coatings²

Accelerated Laboratory Testing

Modern accelerated laboratory test equipment, such as full-spectrum test chambers based on filtered xenon-arc technology, offer valuable support to research and development. The enhanced control of test conditions, especially the match to natural sunlight, enables improved studies of the coating degradation process and optimizes stabilizer choice and usage level.

In photodegradation studies, the spectral sensitivity is of interest, as it is important to know which wavelength of radiation is the most degrading for a particular material.³ Table 1 presents a selection of polymeric materials alongside their change of properties due to sensitivity toward certain wavelength ranges and initiated photodegradation.⁴
 

Table 1: Spectral sensitivity of polymeric materials 

Figure 2 shows the spectral sensitivity of a high-density polyethylene material (see HDPE, Table 1) in light conditions created with a test chamber using a sunshine carbon arc light source. This light source, compared with the match to natural sunlight, in the UV wavelength range 295nm-400nm, is shown to be particularly unrealistic. Use of such or similar unrealistic light could test far stronger than nature in accelerated laboratory testing because of the wavelengths in the ranges below ca. 295nm. Overdosing of UV stabilizers would, therefore, be a high risk, and transformations of test results toward expected product service lifetimes would be inaccurate and likely longer than needed.

Figure 2: Spectral sensitivity of HDPE[5] and spectrum comparison reference sun CIE No.85 (Table 4) and Sunshine Carbon Arc

Figure 3 shows the spectral sensitivity of a polyurethane material (see Polyurethanes, Table 1) and the light conditions created with a test chamber using a UV fluorescent light source. Compared with the match to natural sunlight, this light source is realistic in the UV cut-on at ca. 295nm but decidedly unrealistic in the UV wavelength range 370nm-400nm.

Figure 3 Spectral sensitivity of polyurethanes and spectrum comparison of reference sun CIE No.85 (Table 4) and UV fluorescent UVA-340 lamp

Further, light sources like UVA-340 fluorescent lamps do not emit visible or infrared wavelengths, decisive for realistic heat build-up of materials — colored ones in particular. Using this light for accelerated laboratory testing would risk causing the material to test weaker with applied long-wavelength UV-A radiation but stronger than nature when applying unrealistic temperatures (e.g. 60 C). Overdosing of UV stabilizers would be a consequential risk. Expected product service lifetimes would be calculated inaccurately, negatively impacting cost.

Figure 4 shows the test of the same polyurethane material and the light conditions created with a test chamber using a filtered xenon source. This light source is realistic regarding the match to natural sunlight, not only in the UV cut-on at ca. 295nm, but also in the VIS and IR wavelength range. Thus, filtered xenon can be regarded as today’s best simulation of natural sunlight.

By using xenon light sources, two decisive preconditions of weathering testing are met. First, no other degradation paths than initiated by natural sunlight in the end-use environment will be initiated. Second, heat build-up of colored products will be respected realistically. Both preconditions are decisive to reliably interpret accelerated laboratory weathering test results while translating them into real service lifetimes in their targeted end-use environment.

Figure 4: Spectral sensitivity of polyurethanes and spectrum comparison of reference sun CIE No.85 (Table 4) and filtered xenon with Atlas Daylight filters

CONCLUSION

Accelerated laboratory weathering done with light sources that produce unrealistic sunlight, such as carbon arc, or are incomplete with regard to the full spectrum of natural sunlight, such as UVA-340 fluorescent lamps, should be either avoided or carefully validated. Otherwise, there is risk of over- or underdosing of UV stabilizers, which does not lead to eco-friendly coating products. Xenon provides realistic sunlight, including UV, VIS and IR wavelengths. The risk of overdosing UV stabilizers is minimized and the development of eco-friendly as well as cost-effective coatings becomes possible.