December 22, 2023 Longchang Chemical

Polymeric materials include plastics, rubber, fibers, films, adhesives and coatings. Because they have many potential properties superior to those of traditional structural materials, they are increasingly used in the field of military and civilian products.
However, in the process, storage and use, due to light, heat, oxygen, water, high-energy radiation, chemical and biological erosion and other internal and external factors, the chemical composition and structure of polymer materials will undergo a series of changes in the physical properties will be changed accordingly, such as hardness, viscosity, brittle, discoloration, loss of strength, and so on, this phenomenon is the aging of polymer materials.
The nature of polymer material aging is the physical structure or chemical structure changes, manifested as a gradual decline in the performance of the material, and lose its due use value. The aging failure of polymer materials has become one of the key issues limiting the further development and application of polymer materials.

Aging phenomenon
Due to the different varieties of polymer materials, the use of different conditions, so there are different aging phenomena and characteristics. For example, agricultural plastic film after sun and rain after the occurrence of discoloration, brittle, decreased transparency; aviation plexiglass with a long time after the emergence of silver grain, decreased transparency; rubber products after a long time after the decline in elasticity, hardening, cracking or soft, sticky; paint after a long period of time after the occurrence of loss of light, chalking, bubbles, flaking, and so on.
Aging phenomenon is summarized in the following four changes:
1, appearance changes
Appearance of stains, spots, silver, cracks, spray frost, chalking, sticky, warping, fish-eye, wrinkles, shrinkage, burnt, optical aberrations and optical color changes.
2, physical property changes
Including solubility, swelling, rheological properties, as well as cold, heat, water permeability, air permeability and other performance changes.
3, mechanical property changes
Tensile strength, bending strength, shear strength, impact strength, relative elongation, stress relaxation and other performance changes.
4, changes in electrical properties
Such as surface resistance, volume resistance, dielectric constant, electrical breakdown strength and other changes.

Aging factors
The physical properties of polymer materials have a close relationship with their chemical structure, aggregation state structure.
The chemical structure is the long chain structure of macromolecules connected by covalent bonds, and the aggregated structure is the spatial structure of many macromolecules arranged and stacked up by intermolecular forces, such as crystalline, amorphous, crystalline – amorphous state. The intermolecular forces that maintain the aggregate structure include ionic bonding, metallic bonding, covalent bonding, and van der Waals forces.
Environmental factors can lead to changes in intermolecular forces, or even chain breakage or the shedding of certain groups, which will ultimately destroy the aggregation state structure of the material and change the physical properties of the material. There are usually two kinds of factors affecting the aging of polymer materials: intrinsic factors and extrinsic factors.

Intrinsic factors
1, the chemical structure of the polymer
Polymer aging and its own chemical structure is closely related to the weak bond of the chemical structure of the site is susceptible to the influence of external factors to break into free radicals. This free radical is the starting point of the free radical reaction.
2、Physical form
Some of the molecular bonds of polymers are ordered and some are disordered. Orderly arrangement of molecular bonds can form a crystalline area, disorderly arrangement of molecular bonds for the amorphous area, many polymers are not uniform, but semi-crystalline state, both crystalline and amorphous areas, the aging reaction first from the amorphous area.
3, three-dimensional normalization
The three-dimensional integration of polymers and its degree of crystallinity has a close relationship. Generally speaking, regular polymers have better aging resistance than random polymers.
4、Molecular weight and its distribution
Generally speaking, the molecular weight of polymer has little relationship with aging, but the distribution of molecular weight has great influence on the aging performance of polymer, the wider the distribution, the easier it is to aging, because the wider the distribution, the more end groups, the easier it is to cause aging reaction.
5, trace metal impurities and other impurities
Polymer in the processing, and metal contact, may be mixed with trace metals, or in the polymerization, residual metal catalysts, which will affect the automatic oxidation (i.e., aging) of the initiating role.

External factors
1, the effect of temperature
Temperature increases, the polymer chain movement intensifies, once more than the dissociation energy of the chemical bond, it will cause thermal degradation of polymer chain or group shedding, the thermal degradation of polymer materials have a large number of literature reports; temperature reduction, often affecting the mechanical properties of the material. Closely related to the mechanical properties of the critical temperature point, including the glass transition temperature, viscous flow temperature and melting point, the physical state of the material can be divided into glass, high elastic state, viscous flow state.

2, the effect of humidity
The effect of humidity on polymer materials can be attributed to the swelling and dissolution of water on the material, so as to maintain the structure of the polymer material aggregation state of the intermolecular forces change, thus destroying the aggregation state of the material, especially for non-crosslinked amorphous polymers, the effect of humidity is extremely obvious, will make the polymer material swelling and even aggregation state disintegration, so that the performance of the material is damaged; for the crystallization of plastic or fiber, due to the existence of moisture infiltration limitations, the effect of humidity is not obvious. For crystalline form of plastics or fibers, due to the existence of water penetration restrictions, the effect of humidity is not very obvious.
3, the influence of oxygen
Oxygen is the main cause of aging polymer materials, due to the permeability of oxygen, crystalline polymers more amorphous polymers resistant to oxidation. Oxygen first attack on the weak links in the polymer main chain, such as double bonds, hydroxyl, tertiary carbon atoms on the hydrogen and other groups or atoms, the formation of polymer peroxyl radicals or peroxides, and then in this part of the main chain caused by the fracture of the severity of the polymer molecular weight dropped significantly, the glass transition temperature is reduced, so that the polymer becomes viscous, in the presence of some easy to decompose into free radicals of the initiator or the transition metal elements, there is a tendency to aggravate the oxidation reaction. Oxidation reaction trend.
4、Light aging
Polymer irradiated by light, whether or not to cause molecular chain fracture depends on the relative size of the light energy and dissociation energy and the sensitivity of the chemical structure of the polymer to light waves. Due to the presence of ozone layer on the surface of the earth and the atmosphere, can reach the ground of the sun’s light wavelength range of 290 ~ 4300nm, the light wave energy is greater than the dissociation energy of the chemical bond is only the ultraviolet region of the light wave, will cause the polymer chemical bond rupture.
For example, the ultraviolet wavelength of 300 ~ 400nm, can be absorbed by polymers containing carbonyl and double bond, and make the macromolecular chain fracture, chemical structure change, and make the material performance deterioration; polyethylene terephthalate 280nm of ultraviolet light has a strong absorption, the degradation product is mainly CO, H, CH; only contains C-C bond polyolefin on the ultraviolet light absorption, but in the presence of a small amount of impurities, such as carbonyl, unsaturated, and the degradation product, the polyolefins are not absorbed, but in the presence of a small amount of impurities. However, in the presence of a small number of impurities, such as carbonyl, unsaturated bonds, hydroperoxide groups, catalyst residues, aromatic hydrocarbons and transition metal elements, can promote the photo-oxidation reaction of polyolefins.
5, the influence of chemical medium
Chemical agents only penetrate into the interior of polymer materials to play a role, these roles include the role of covalent bonds and the role of subvalent bonds of two categories. The role of covalent bonds manifested as polymer chain breakage, crosslinking, addition or a combination of these roles, which is an irreversible chemical process; chemical media on the destruction of the secondary valence of the bond although it does not cause a change in the chemical structure, but the aggregation of the structure of the material will be changed, so that the physical properties of the corresponding changes.
Physical changes such as environmental stress cracking, solvation cracking, plasticizing, etc., are typical manifestations of chemical media aging of polymer materials.
The method of eliminating solvation cracking is to eliminate the internal stress of the material, and annealing after the molding process of the material is conducive to the elimination of the internal stress of the material. Plasticization is in the liquid medium and polymer materials in continuous contact with the occasion, polymer and small molecule interactions between the medium partially replaces the interaction between the polymer, so that the polymer chain segments are easier to move, manifested as a reduction in the glass transition temperature, the material’s strength, hardness and elastic modulus decreased, the elongation at break increased, and so on.
6, biological aging
As plastic products in the process of processing almost all use a variety of additives, and thus often become a source of mold nutrients. Mold growth absorbs nutrients on the surface and inside the plastic and become mycelium, mycelium is a conductor, thus making the insulation of the plastic decreased, weight changes, and when there will be a serious peeling. The metabolites of mold growth contain organic acids and toxins, which will make the surface of the plastic appear sticky, discoloration, brittleness, gloss reduction and other phenomena, and will also make long-term contact with the moldy plastic people infected with disease.
Polysaccharide natural macromolecules and their modified compounds can be processed into biodegradable disposable films, sheets, containers, foamed products and so on by means of blending and modification with general-purpose plastics, and their wastes can be hydrolyzed into small molecule compounds step by step through the intervention of polysaccharide natural macromolecule decomposition enzymes such as amylase, which are widely found in the natural environment, and eventually decomposed into non-polluting carbon dioxide and water, which can be returned to the biosphere. Based on these advantages, polysaccharide natural polymer compounds represented by starch are still an important part of degradable plastics

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