Factors of polymer weathering

The aging of natural and artificial polymeric materials has long time been acknowledged as it is a natural and universal phenomenon (metals, glass, minerals and other inorganic materials age as well). Among the environmental parameters influencing the degradation of polymeric materials, it is daylight, combined with the effect of temperature, moisture and atmospheric oxygen, that acts as the main parameters of stress for outdoor weathering.

Introduction

The components of the weather responsible for the deterioration of most materials are non-ionizing radiation, atmospheric temperature and its cycles, moisture in its various forms, wind and the normal constituent gases and atmospheric pollutants. Although the ultraviolet (UV) portion of solar radiation is mainly responsible for initiating weathering effects, the visible and near-infrared portions can also contribute to the weathering processes. Colored materials are susceptible to visible radiation, and near-infrared radiation can accelerate chemical reactions by raising the temperature. The other factors act synergistically with solar radiation to significantly influence the weathering processes. All weather factors, including the quality and quantity of sunlight, vary with geographic location, time of day and year, and climatalogical conditions. Therefore, in order to understand and predict the effect of weather on materials, data are required on each factor that may contribute to degradation.

Solar Radiation

Physical changes resulting from exposure to the environment are initiated by chemical bond breaking reactions caused by the absorbed light, either through direct or indirect processes. Chemical bond breaking is a prerequisite to any chemical reaction, and chemical reactions are a prerequisite to observable or measurable physical changes. Other weather factors mainly promote weathering through their influence on the secondary reactions which follow the breaking of bonds. Degradation of most materials exposed to outdoor conditions is caused mainly by the ultraviolet portion of solar energy, with the shortest wavelengths often having the greatest effect. Therefore, variations in both the quantity and quality of ultraviolet in both the direct solar beam and diffuse sky radiation are important factors in the design and evaluation of weathering tests.

Temperature

The temperature of materials exposed to solar radiation has a significant influence on the effect of the radiation. The destructive effects of light are usually accelerated at elevated temperatures as a result of the increased rate of secondary reactions, with reaction rates about doubling with each 10°C rise; this may not be true of all materials but is often found with polymers. At high temperatures molecules have greater mobility. Therefore, the rate of oxygen diffusion increases and free-radical fragments formed in primary photochemical processes are more readily separated. Thus, the chance of recombination is reduced and secondary reactions are promoted. Reactions may take place at higher temperatures that occur at a very low rate or not at all at lower temperatures.

In the presence of sunlight the surface temperature of an object is usually considerably higher than the temperature of the air. Solar absorptivity is closely related to color, varying from about 20% for white materials to 90% for black materials; thus samples of different colors will reach different on-exposure temperatures. Because the thermal conductivity and heat capacity of polymeric materials are generally low, much higher temperatures can be obtained on the surface than in the bulk of the material. Therefore, both the surface temperatures of the samples, produced largely by infrared radiation absorption which varies by material color, and ambient air temperature and its fluctuations during exposure do play a role.

Diurnal and seasonal variations occur in solar radiation. Temperature cycling can cause mechanical stress, particularly in composite systems consisting of materials with widely differing temperature coefficients of expansion. Temperature and its cycles are also closely linked with water in all of its forms. Drops in temperature can cause water to condense on the material as dew, a rise in temperature causes evaporation, and sudden rainfall can cause thermal stress.

Moisture

Moisture can take the form of humidity, dew, rain, snow, frost or hail, depending on the ambient temperature. Moisture, in combination with solar radiation, contributes significantly to the weathering of many materials. This is due both to the mechanical stresses imposed when moisture is absorbed or desorbed and to the chemical participation of moisture in the chemical evolution (and in some instances physical effects such as impact). The span of time over which the precipitation occurs and the frequency of wetness are more important in the weathering of materials than the total amount of precipitation. The mechanical stresses induced by freeze/thaw cycling can cause structural failures in some systems, or accelerate degradation already initiated.

Moisture participates both physically and chemically in degradation. Water absorption by synthetic materials and coatings from humidity and direct wetness is a diffusion controlled process. This hydration of the surface layers produces a volume expansion which places mechanical stress on the dry subsurface layers. A following drying out period signifies a desorption of water. The drying out of the surface layers would lead to a volume contraction; the hydrated inner layers resist this contraction, leading to surface stress cracking. This oscillation between hydrated and dehydrated states may result in stress fractures. Because of diffusion rates in organic materials, it may takes weeks or months to reach a moisture equilibrium.

The chemical effects of moisture can be seen in the chalking of titanium dioxide (TiO2) pigmented coatings and polymers; the anatase form is particularly sensitive to wavelengths below about 405 nm while the rutile forms absorb energy above that wavelength. Chalking results from the degradation of the binding material resulting in a release of the TiO2 pigment particles. These particles form a dull layer on the surface which may be wiped off. Experience shows that chalking is strongest where more water is available on the surface; little to no chalking occurs in dry atmospheres. TiO2 is a semiconductor where electron transitions from the valence band to the conduction band result from the absorption of light at wavelengths in the near UV range, below 400 nm. Ultraviolet radiation causes electron hole pairs to be created in the TiO2 lattice. These react with the hydroxide groups on the surface and the Ti4+ ions. Hydroxyl and perhydroxyl radicals are formed through the conversion of oxygen and a water molecule whereby the TiO2 surface again resumes the initial form and acts as a catalyst for continued activity, thus repeating the chalking cycle. The hydroxide and perhydroxyl radical then cause oxidative decomposition of the binder with the subsequent release of TiO2 particles.

Atmospheric Oxygen

Photooxidation accounts for most polymer failures that occur during outdoor exposure. It results from the effects of solar radiation in combination with oxygen. Oxygen can promote degradation in several ways. Free radicals, formed as a result of the cleavage of chemical bonds by solar radiation, react with oxygen to form peroxy radicals that initiate a series of radical chain reactions. The destructive effect of the radiation is multiplied manifold by propagation of bond breakage and the formation of hydroperoxides that further absorb solar ultraviolet radiation. This cascade effect results in an auto-acceleration of the weathering process, and may partially account for the general non-linearity of the weathering response to radiant exposure.

In addition to the reactions of oxygen in its normal ground state, some reactions of oxygen are due to the excited singlet state, a highly reactive form of the molecule. Singlet oxygen is responsible for the rapid deterioration of materials, particularly those with conjugated unsaturation such as natural rubber and synthetic elastomers. It is formed when triplet oxygen, the normal ground state, reacts with sensitizers, such as certain dyes and ketones, excited by radiation to their triplet states. Oxygen also increases the amount of solar radiation absorbed by conjugated unsaturated hydrocarbons through formation of a complex with these materials.

The extent of photochemical reactions involving oxygen differ in the inner and outer layers of both aromatic and aliphatic polymers due to their dependence on the diffusion of oxygen through the polymer. Photooxidation is significantly reduced at depths beyond which oxygen penetrates. Studies of the degradation profiles of low density polyethylene (LDPE), polymethylmethacrylate (PMMA) and polyvinylchloride (PVC) show that photooxidation was higher at the front and back surfaces than in the interior bulk of the material. Because ultraviolet radiation is not strongly absorbed by these materials, a considerable amount of the radiation incident on the front surface is transmitted to the back surface where it initiates photooxidation.

Secondary factors of weather

Ozone is produced by short wavelength (110 nm - 220 nm) UV photolysis of oxygen in the upper atmosphere. The photochemical reaction of nitrogen oxides and hydrocarbons from automobile exhausts is another source. Ozone plays a dual role in weathering. The concentrated layer in the upper atmosphere absorbs the short wavelength (≤300 nm) ultraviolet radiation emitted by the sun and thus plays a critical role in protecting terrestrial objects from this actinic radiation. Ozone is also a powerful oxidant and reported to react rapidly with elastomers and other unsaturated polymers. Ozonolysis typically results in stiffening and cracking, particularly under mechanical stress. However, the contribution of ozonolysis reactions to the overall photooxidation process is still subject to controversy.

Atmospheric pollutants (e.g., sulfur dioxide, nitrogen oxides, hydrocarbons, etc.), in combination with solar radiation, can also be responsible for severe damage. Acid-base induced chemical changes may also be responsible for much pollution-caused damage. Unsaturated alkyl and aromatic compounds may act as catalysts in the photooxidation of polymers. In the presence of sulfur dioxide and oxygen, ultraviolet radiation causes crosslinking of polyethylene and polypropylene and is responsible for the rapid loss of color in pigmented coatings.

Moisture, in combination with temperature, may also promote microbial growth. Mold, mildew and other microbiological and botanical agents may play a significant role in material degradation, particularly in tropical and subtropical climates, although they may not be generally thought of as weathering factors.