Material aging

There is also exposure to chemical substances in the environment

Is it mainly the oxidation caused by contact with substances in the environment that breaks the chemical bonds?

This post was last edited by Wang Genrong on 2017-7-2 08:35. I. Introduction to aging phenomena Polymer materials include plastics, rubbers, fibers, films, adhesives, and coatings, among others. Plastics, synthetic fibers, and synthetic rubbers, which are known as the three major synthetic polymer materials, are being used increasingly in various fields such as aviation, automobiles, ships, infrastructure construction, and military equipment. However, during processing, storage, and use, polymer materials undergo degradation under the combined action of internal and external factors such as light, heat, water, chemical, and biological erosion. This results in a gradual decline in their properties, leading to a partial or complete loss of their utility; this phenomenon is known as aging. The aging of polymer materials has become a very important issue; the actual damage it causes is much more severe than people might imagine. Especially under harsh environmental conditions, it often leads to premature failure of equipment and significant loss of materials. This results not only in substantial economic losses and waste of resources but also in environmental pollution due to the degradation of these materials. II. Causes of aging The main reason for aging is the presence of weaknesses within the structure or components that can lead to aging, such as unsaturated double bonds, peroxides, branches, carbonyl groups, hydroxyl groups at the ends, and so on. External or environmental factors mainly include sunlight, oxygen, ozone, heat, water, mechanical stress, high-energy radiation, electricity, industrial gases (such as carbon dioxide, hydrogen sulfide, etc.), seawater, salt spray, molds, bacteria, insects, and so on. Environmental influence factors: Temperature. As temperature rises, the movement of polymer chains intensifies; once it exceeds the dissociation energy of chemical bonds, it can lead to the thermal degradation of polymer chains or the detachment of functional groups. The effect of humidity: The impact of humidity on polymer materials can be attributed to the swelling and dissolution of the materials caused by water, which alters the intermolecular forces that maintain the aggregated structure of the polymers, thereby disrupting this aggregated state. This effect is particularly pronounced in uncross-linked amorphous polymers, where humidity can cause the polymers to swell or even have their aggregated structure broken down, resulting in a deterioration of the material’s properties. In the case of crystalline plastics or fibers, the effect of humidity is not as significant due to restrictions on water penetration. The effect of oxygen: Oxygen is the main cause of aging in polymer materials. Due to its ability to penetrate, crystalline polymers are more resistant to oxidation than amorphous polymers. Oxygen first attacks the weak points in the polymer backbone, such as double bonds, hydroxyl groups, hydrogen atoms on tertiary carbon atoms, and other such groups or atoms, thereby forming polymer peroxyl radicals or peroxides. This leads to the breakdown of the backbone at those locations; in severe cases, the molecular weight of the polymer decreases significantly, its glass transition temperature drops, and the polymer becomes more viscous. In the presence of initiators that are prone to decomposing into radicals or transition metal elements, the oxidation reaction tends to intensify. Whether exposure to light causes the breakage of molecular chains in photo-aged polymers depends on the relative magnitude of light energy and dissociation energy, as well as the sensitivity of the polymer’s chemical structure to light waves. Due to the ozone layer and atmosphere present on the Earth’s surface, the range of wavelengths of solar light that can reach the ground is between 290nm and 4300nm. Only the light waves in the ultraviolet region have energy greater than the dissociation energy of chemical bonds, and they can cause the breaking of these chemical bonds in polymers. The effect of chemical mediators: Chemical mediators can exert their effects only when they penetrate into the interior of polymer materials. These effects include those on covalent bonds and those on secondary bonds. The effect of covalent bonds manifests as the breaking, cross-linking, or addition of polymer chains, or a combination of these processes – it is an irreversible chemical reaction. Although the destruction of secondary bonds by chemical agents does not lead to changes in the chemical structure, it does alter the aggregated structure of the material, thereby changing its physical properties. When polymer materials subject to biological aging are exposed to an environment for an extended period of time, and given the high genetic variability of microorganisms, these microorganisms gradually evolve enzymes capable of breaking down and utilizing such polymers, thereby using them as a carbon source or energy source for growth. Although the degradation rate is very low, this potential hazard does exist. However, for certain polymer packaging materials, it is desired that they be rapidly biodegraded after use. III. Preventive measures: Polyesters, polyacetals, polyamides, and polysaccharide polymers can undergo hydrolysis when exposed to water in the presence of acid or base catalysts. In areas with severe air pollution and frequent acid rain, the use of such polymer materials is restricted. If a waterproof film can be applied to the surface of such materials, hydrolytic aging can be reduced or even prevented. During polymer processing, oxygen is present, along with amine antioxidants, phenolic antioxidants, sulfur-containing organic compounds, and phosphorus-containing compounds. These substances can react rapidly with peroxide radicals, thereby terminating the chain reaction sooner. During the processing of materials, photoaging can be prevented through the addition of light stabilizers, thereby avoiding the aging and degradation of the materials. Based on their mechanism of action, such light stabilizers include light shields, ultraviolet absorbers, quenchers, and free radical scavengers.

Sun exposure, rain, large fluctuations in operating conditions, chronic corrosion, and biochemical reactions – that’s about it