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Polymer materials include plastics, rubber, fiber, film, adhesive and coating. Because of their many potential properties superior to traditional structural materials, they are increasingly used in the military and civilian fields. Polymer materials are light in weight, high in strength, good in corrosion resistance, and have good protective properties. They are widely used in aviation, automobiles, ships, infrastructure, military products and other fields.
However, during processing, storage and use, due to the combined effects of internal and external factors such as light, heat, oxygen, water, high-energy radiation, chemical and biological erosion, the chemical composition and structure of polymer materials will undergo a series of changes, and the physical properties will also deteriorate accordingly, such as hardening, stickiness, brittleness, discoloration, loss of strength, etc. This phenomenon is called the aging of polymer materials. The essence of polymer material aging refers to the changes in physical structure or chemical structure, which is manifested as the gradual decline in the performance of the material and the loss of its due use value.
The aging and failure of polymer materials has become one of the key issues restricting the further development and application of polymer materials.
Aging phenomenon
Due to different polymer varieties and different use conditions, there are different aging phenomena and characteristics. For example, agricultural plastic film changes color, becomes brittle, and has reduced transparency after being exposed to the sun and rain; aviation plexiglass develops silver streaks and has reduced transparency after long-term use; rubber products lose elasticity, harden, crack, or become soft and sticky after long-term use; paint loses gloss, powders, bubbles, and peels after long-term use. The aging phenomenon can be summarized into the following four changes:
1. Changes in appearance
Stains, spots, silver streaks, cracks, frost, powdering, stickiness, warping, fish eyes, wrinkles, shrinkage, scorching, optical distortion, and changes in optical color.
2. Changes in physical properties
Including changes in solubility, swelling, rheological properties, and cold resistance, heat resistance, water permeability, and air permeability.
3. Changes in mechanical properties
Changes in properties such as tensile strength, bending strength, shear strength, impact strength, relative elongation, and stress relaxation.
4. Changes in electrical properties
Such as changes in surface resistance, volume resistance, dielectric constant, and electrical breakdown strength.
Aging factors
The physical properties of polymer materials are closely related to their chemical structure and aggregate structure. The chemical structure is a long chain structure of macromolecules connected by covalent bonds, and the aggregate structure is a spatial structure in which many macromolecules are arranged and stacked by molecular forces, such as crystalline, amorphous, and crystalline-amorphous.
The intermolecular forces that maintain the aggregate structure include ionic bond forces, metallic bond forces, covalent bond forces, and van der Waals forces. Environmental factors can cause changes in intermolecular forces, even chain breakage or the shedding of certain groups, which will eventually destroy the aggregate structure of the material and change the physical properties of the material. There are usually two factors that affect the aging of polymer materials: internal factors and external factors.
Internal factors
1. Chemical structure of polymers
The aging of polymers is closely related to their own chemical structure. The weak bond parts of the chemical structure are easily affected by external factors and break to become free radicals. This free radical is the starting point for initiating free radical reactions.
2. Physical form
Some of the molecular bonds of polymers are arranged in order, while others are disordered. Orderly arranged molecular bonds can form crystalline areas, and disorderly arranged molecular bonds are amorphous areas. The morphology of many polymers is not uniform, but semi-crystalline, with both crystalline and amorphous areas. The aging reaction starts from the amorphous area.
3. Stereoscopic regularity
The stereoscopic regularity of a polymer is closely related to its crystallinity. Generally, regular polymers have better aging resistance than random polymers.
4. Molecular weight and its general distribution
The molecular weight of a polymer has little to do with aging, but the distribution of molecular weight has a great influence on the aging performance of the polymer. The wider the distribution, the easier it is to age, because the wider the distribution, the more end groups, and the easier it is to cause aging reactions.
5. Trace metal impurities and other impurities
When polymers are processed, they come into contact with metals, and trace metals may be mixed in, or some metal catalysts may remain during polymerization, which will affect the initiation of auto-oxidation (ie, aging).
External factors
1. The influence of temperature
As the temperature rises, the movement of polymer chains intensifies. Once the dissociation energy of chemical bonds is exceeded, it will cause thermal degradation of polymer chains or group shedding. At present, there are a large number of literature reports on the thermal degradation of polymer materials; when the temperature decreases, the mechanical properties of the material are often affected. The critical temperature points closely related to mechanical properties include glass transition temperature T, viscous flow temperature Tf and melting point Tm. The physical state of the material can be divided into glass state, high elastic state and viscous flow state.
2. The influence of humidity
The influence of humidity on polymer materials can be attributed to the swelling and dissolution of water on the material, which changes the intermolecular forces that maintain the aggregate structure of the polymer material, thereby destroying the aggregate state of the material. Especially for non-cross-linked amorphous polymers, the influence of humidity is extremely obvious, which will cause the polymer material to swell or even disintegrate in the aggregate state, thereby damaging the performance of the material; for crystalline plastics or fibers, due to the existence of water penetration restrictions, the influence of humidity is not very obvious.
3. The influence of oxygen
Oxygen is the main cause of aging of polymer materials. Due to the permeability of oxygen, crystalline polymers are more resistant to oxidation than amorphous polymers. Oxygen first attacks the weak links on the main chain of polymers, such as double bonds, hydroxyl groups, hydrogen groups or atoms on tertiary carbon atoms, forming polymer peroxyl radicals or peroxides, and then causes the main chain to break at this position. In severe cases, the molecular weight of the polymer decreases significantly, the glass transition temperature decreases, and the polymer becomes sticky. In the presence of certain initiators or transition metal elements that are easily decomposed into free radicals, there is a tendency to intensify the oxidation reaction.
4. Photoaging
Whether the polymer is exposed to light and causes the molecular chain to break depends on the relative size of light energy and dissociation energy and the sensitivity of the polymer chemical structure to light waves. Due to the presence of the ozone layer and the atmosphere on the surface of the earth, the wavelength range of sunlight that can reach the ground is between 290nm and 4300nm. Only the light waves in the ultraviolet region have light wave energy greater than the chemical bond dissociation energy, which will cause the chemical bonds of polymers to break.
For example, ultraviolet wavelengths of 300nm to 400nm can be absorbed by polymers containing carbonyl groups and double bonds, causing macromolecular chains to break, chemical structures to change, and material properties to deteriorate; polyethylene terephthalate has a strong absorption of 280nm ultraviolet rays, and the degradation products are mainly CO, H, and CH; polyolefins containing only C-C bonds have no absorption of ultraviolet rays, but in the presence of a small amount of impurities, such as carbonyl groups, unsaturated bonds, hydroperoxide groups, catalyst residues, aromatic hydrocarbons, and transition metal elements, they can promote the photooxidation reaction of polyolefins.
5. Influence of chemical media
Chemical media can only play a role when they penetrate into the interior of polymer materials. These effects include covalent bonds and secondary bonds. The effect of covalent bonds is manifested as chain scission, crosslinking, addition, or a combination of these effects of polymer chains. This is an irreversible chemical process; although the destruction of secondary bonds by chemical media does not cause changes in the chemical structure, the aggregate structure of the material will change, causing corresponding changes in its physical properties.
Physical changes such as environmental stress cracking, dissolution cracking, and plasticization are typical manifestations of chemical medium aging of polymer materials.
The way to eliminate dissolution cracking is to eliminate the internal stress of the material. Annealing after the material is formed is conducive to eliminating the internal stress of the material. Plasticization is when the liquid medium is in continuous contact with the polymer material. The interaction between the polymer and the small molecule medium partially replaces the interaction between the polymers, making the polymer chain segments easier to move, which is manifested as a decrease in the glass transition temperature, a decrease in the strength, hardness and elastic modulus of the material, and an increase in the elongation at break.
6. Biological aging
Since plastic products almost all use a variety of additives during the processing process, they often become a source of nutrition for mold. When mold grows, it absorbs nutrients on the surface and inside of the plastic and becomes mycelium, which is also a conductor, thus reducing the insulation of the plastic, changing its weight, and peeling in severe cases. The metabolites of mold growth contain organic acids and toxins, which will make the surface of the plastic sticky, discolored, brittle, and reduce the smoothness, and will also cause people who have long-term contact with such moldy plastics to contract diseases.
Polysaccharide natural polymers and their modified compounds can be processed into degradable disposable films, sheets, containers, foam products, etc. through blending and modification with general plastics. Their waste can be gradually hydrolyzed into small molecular compounds through the intervention of polysaccharide natural polymer decomposition enzymes such as amylase that are widely present in the natural environment, and finally decomposed into pollution-free carbon dioxide and water, returning to the biosphere. Based on these advantages, polysaccharide natural polymer compounds represented by starch are still an important component of degradable plastics.
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