Weathering refers to changes in material properties that result from exposure to the radiant energy present in sunlight, in combination with heat (including temperature cycling), and water in its various states, predominately as humidity, dew, and rain. These three elements of outdoor exposure – sunlight, heat, and water - are referred to as the “Forces of Weathering.” Weathering degradation is typically initiated by sunlight, and can be accelerated by heat and affected by the presence of water.

A closely-related concept is lightstability, or lightfastness, which refers to a material’s ability to withstand two of the three forces of weathering - sunlight and heat. Lightstability testing does not involve liquid water, and often is used to test textiles and indoor materials. Q-Lab's introduction to Sunlight, Weathering, and Lightstability explains further the similarities and differences between them.

Weathering and lightstability can result in many different types of material degradation, including yellowing, cracking, crazing, color change and fading, delamination and adhesion loss, warping, embrittlement, blistering, and loss of tensile strength. The image below shows a number of different types of material property change resulting from exposure to outdoor conditions.

Weathering and light degradation affect a broad range of materials and industries – effectively, any products placed in service outdoors or otherwise exposed to the elements. This includes paints and coatings, plastics, packaging, textiles and fabrics, home and consumer goods, additives and colorants, architectural and building materials, inks and printing materials, electronics, and an array of automotive components. Browse our weathering industries pages and case studies for examples of weathering durability testing, or check out this general testing guide.

Forces of Weathering

When we talk about sunlight as one of the forces of weathering, we are usually concerned with ultraviolet (UV) light. Although it represents only about 7% of the sun’s total energy, UV is responsible for virtually all polymer degradation. The character of the sunlight received by products outdoors depends on a number of factors – time of year, time of day, latitude, cloud cover, and altitude. Additionally, window glass filters out some shortwave UV light, depending on its thickness and tint, resulting in a less harsh light spectrum than materials outdoors. The spectral sensitivity of materials is also critical – for materials degradation to occur, the material must be able to absorb the incident light, and the light must have enough energy to cause chemical bonds to break. 

Heat, both in the form of high temperatures and thermal cycling, can accelerate outdoor property changes. Photochemical reactions caused by incident sunlight are not usually simple one-step reactions. Although the primary reactions caused by sunlight are not affected by heat, secondary reactions often are. In some cases, a 10 °C increase in temperature can double the rate of some chemical reactions in solution. Thermal cycling outdoors can change a material's surface temperature from 75 °C to 25 °C in a matter of minutes, causing physical stress on assemblies and coatings. 

Water can affect materials degradation in a number of ways. Water can affect chemical reactions – both accelerating reactions in solution and leading to an increase in oxygen transport. Water can also cause physical effects like erosion, absorption, freeze/thaw stress, thermal shock, and material loss from impact.

Water can be present in the form of dew (condensation), relative humidity, and rainfall. Of these, dew is by far the largest source of outdoor moisture. Materials outdoors can be wet for over 12 hours a day in many locations, so designing for outdoor performance means designing for an aqueous environment.

The synergistic effects of sunlight, heat, and water are what result in weathering of materials; sunlight and heat resistance determine a material’s lightfastness. How can we make sure our products will last outdoors? The answer is: weathering testing.