What light initiates, temperature accelerates, moisture precipitates

If radiation can get into a material, it potentially can cause it to change. But does this mean that black paint will degrade in the sun because it is absorbing nearly all wavelengths of visible light? The answer to that question lies in understanding the chemical nature of the paint and which wavelengths of radiation will cause this paint to degrade.

Earth’s annual solar energy balance.

The three main environmental stress factors of weathering are solar radiation (light energy), temperature, and water (moisture). But it is not just “how much” of each of these factors ultimately causes degradation to materials, because different types of solar radiation, different phases of moisture, and temperature cycling have a significant effect on materials on exposure.

These factors – along with secondary considerations, such as airborne pollutants, biological phenomena and ‘acid rain’ – act together to cause weathering.

Radiant energy that comes from the sun is made up of photons that travel through space as waves. The solar radiation that reaches Earth’s surface consists of wavelengths between 295 and 3,000 nano-meters. A nano-metre (nm) is one billionth (1x10-9) of a metre.

Wavelengths between 295 and 400 nm are considered the ultra-violet (UV) portion of the solar spectrum, making up to 7 % of the total radiation. Ozone in the stratosphere absorbs and essentially eliminates all radiant energy below 295 nm.

Visible light (the radiation the human eye can detect) is between 400 and 800 nm, making up just over half of the solar spectrum. About 40 % of the radiation from the sun is contained in the infra-red portion of the solar spectrum beyond 800 nm.

Irradiance can be defined as the radiant flux incident on a surface per unit area, commonly expressed in W/m2 (Watts per square metre). For this parameter, it is necessary to indicate the spectral range in which the measurements were taken or for which the values were calculated.

If we turn our attention to narrow wavelength intervals, we obtain the spectral irradiance, measured in W/(m2nm). For weathering tests, the concept of radiant exposure, which is the time integral of (spectral) irradiance, maybe more important, stated in J/m2 (Joules per square metre).

Most radiant exposures are measured in either kJ/m2 or MJ/m2 to convert this energy into numbers to which we can more easily relate.

Understanding radiation

Direct radiation is radiant energy that reaches Earth’s surface directly from the sun, excluding the scattered radiation of the atmosphere. Diffuse radiation is a component of radiant energy that has been scattered by the atmosphere, and therefore, reaches exposed surfaces at all angles.

One easy way to understand direct and diffuse radiation is if we imagine a tall building. The direct light causes the sharp shadow of the building. Diffuse radiation creates general ambient light, allowing us to see if we are standing in the shadow of that building.

In addition, there is radiation that reflects off Earth’s surface (sometimes referred to as albedo radiation) that also reaches the surface of a wood specimen being tested.

The amount of radiation reflected off Earth’s surface is dependent on the ground

Covering: bare rocks, sand or gravel will reflect much more radiant energy than a grass-covered surface. Water or snow will reflect an even greater amount of radiant energy.

The ratio between direct and diffuse radiation reaching Earth’s surface is strongly influenced by atmospheric conditions. Water vapour (humidity) and pollution will increase the amount of radiant energy found in the diffuse component.

A desert climate has a much higher percentage of radiant energy in the direct component than a sub-tropical climate. This occurs because there is much less water vapour in the desert than in a sub-tropical climate.

By contrast, a location with higher levels of pollution dramatically reduces the amount of direct radiant energy. Shorter wavelengths of radiation are more likely to be scattered than long wavelengths, which makes the sky appear blue.

Therefore, the percentage of UV in the direct component will always be less than that of total solar radiation.

Water vapour

Because of the high water vapour content in a sub-tropical climate, about 50% of the UV radiation is diffused on clear days. Many days are not clear, which results in an even greater percentage of radiation in the diffuse component. To

maximise solar radiation, specimens in a sub-tropical climate should be exposed at a tilt angle close to horizontal, such as 5°.

Conversely, a desert climate would have a greater percentage of UV radiation in the direct component (75%). This means that the most radiant exposure over the course of one calendar year would be on specimens that were near the latitude angle of the exposure site, although other angles such as 5°, 45° and 90° are common.

Seasonal variability exists in both sub-tropical and desert environments. The amount of variation depends on the exposure angle and climate, especially the atmospheric conditions that cause different ratios of direct and diffuse radiation.

Effect of energy

While radiant exposure is an important factor in understanding the degradation of materials or determining the length of weathering, it really tells us only half the story. Radiant exposure tells only how much radiation has been deposited onto the surface of a material.

It says nothing about how much of that radiation has been absorbed by the material or what effect, if any, it has. According to the Grotthus-Draper principle, absorption of radiation by any component of the system is the first necessary event for photochemical reaction occurrence.

The molecular structures that constitute different polymers are susceptible to radiation they might absorb at sites called chromophores. Following another basic principle of degradation, the amount of energy absorbed by a molecule must exceed the bond energy to cause degradation.

Energy balance

Simply put, if the absorbed radiation has more energy than the energy holding the molecular structure together, polymeric bonds may be broken, and degradation will begin. Note, however, that not all absorbed photons result in bond breakage, and that the energy required for photo-chemical reactions is unrelated to thermal bond energy.

We know that shorter wavelengths contain higher amounts of energy. Therefore, it is now easy to understand, when we are discussing the durability of a material, why UV, as the shortest wavelength region of radiation to reach Earth’s surface, is the most important part of sunlight.

The result of the degradation characteristics of a material as a result of radiation is dependent on:

•        quality and quantity of radiant energy deposited onto the material

•        wavelengths of radiation absorbed by the material

•        whether or not that absorbed radiation has enough energy to cause a chemical change, which could lead to material degradation.

We have spent time discussing the changes that occur to a polymer as a result of short wavelength radiation. Colour changes, however, are only partly related to changes in the polymer matrix, such as yellowing. Often, they are caused by changes in the pigment or dyestuff used.

Both pigments and dyes absorb wavelengths in the visible range (otherwise, they wouldn’t be coloured!) and are also damaged by UV-A radiation and visible light. From a customer’s perspective, colour change is undoubtedly one of the most important parameters when evaluating a material’s performance.

Temperature changes

The temperature of materials exposed to solar radiation has an influence on the effect of the radiation. In polymer degradation it is often said that light initiates, temperature accelerates, and moisture participates. Temperature often determines the rate of subsequent secondary reaction steps in polymer degradation.

These secondary reactions can be often qualified using the Arrhenius equation. A general rule of thumb in chemistry assumes that reaction rates approximately double with each 10°C rise in temperature, although this doesn’t usually apply to solid-state materials, and may not be seen when measuring physical or appearance changes.

Also, thermo-chemical reactions that may be initiated at higher temperatures may not occur at all or at a very low rate at lower temperatures.

The temperature of a material exposed to natural sunlight is a function of ambient temperature, solar absorptivity and irradiance, and surface conductance and emissivity. In the presence of sunlight, the surface temperature of an object is usually considerably higher than the temperature of the air.

Solar absorptivity in both the visible and infra-red regions is usually closely related to colour, varying from about 20% for white surfaces to over 90% for black surfaces. Thus, materials of different colours will reach different temperatures on exposure.

As a result of different surface temperatures, mildew and other biological growth will form and accumulate at different rates on materials of different colours. White, or lighter coloured materials, tend to “grow” more mildew than darker coloured materials.

Weathering can be both chemical and physical in nature. Temperature cycling can induce mechanical stresses, which can lead to non-chemical degradation such as cracking and crazing.

Ambient air temperature, evaporation rates, and the convective cooling from the surrounding air during exposure all play a role in the temperature of a material, and therefore, influence degradation rates.

Water effects

Water is one of the substances in our environment that is everywhere, whether in the form of humidity, rain, dew, snow, or hail. All materials used outdoors are exposed to these influences.

There are two ways in which water affects materials. Water absorption by synthetic materials and coatings from humidity and direct wetness are examples of physical effects. As the surface layers absorb moisture, a volume expansion is produced that places stress on the dry sub-surface layers.

Following a drying-out period, or desorption of water, the surface layers will lead to a volume contraction. The hydrated inner layers resist this contraction, leading to surface stress cracking. This fluctuation between hydrated and dehydrated states may result in stress fractures.

The freeze-thaw cycle is another physical effect. Because water expands when it freezes, absorbed moisture in a material causes expansion and stresses that cause peeling, cracking and flaking in coatings.

Rain, which periodically washes dirt and pollutants from the surface, has an effect on the long-term rate of deterioration that is determined more by its frequency than its amount. When rain strikes an exposed surface, evaporation cools the surface rapidly, which may cause physical degradation to a material. Frozen rain, or hail, may also cause physical degradation.

Secondary effects

Gases and pollutants in the atmosphere, especially in the form of ‘acid rain’, may cause entirely new reactions. In highly industrialised areas, acid rain is the primary element driving the weathering process that affects a wide range of materials.

Blowing dirt and dust may have effects on the weathering process without reacting with the actual molecular structure of the material. These effects include the screening of UV radiation from the materials by dirt, which absorbs the UV portion of the spectrum. Semi-permanent “varnishes” can form on the surface of exposed materials in certain climates.

When considering the roles that solar radiation, temperature, moisture and secondary effects play on products, we must realise that these factors work together to degrade materials.

The synergistic effects of the main factors of weathering vary, depending on materials being exposed. Even small changes to a product’s formulation, such as the addition of stabilisers, flame retardants, fillers, etc., will change the degradation characteristics of that material.

The use of recycled material, impurities in the polymer matrix, and the characteristics of product processing are additional variables in weathering performance. That is why it is safe to say that a complete understanding of the effect of weathering factors on every material will never be achieved.

– Courtesy: Atlas Material Testing Technology LLC, Mount Prospect, Illinois, US (www.atlas-mts.com).

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