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Polymer materials include plastics, rubber, fibers, films, adhesives and coatings.
Due to their many potential properties superior to traditional structural materials, they are more and more widely used in the field of military and civilian products.
The polymer material has light weight, high strength, good corrosion resistance and good protection performance, and is widely used in aviation, automobile, ship, basic construction, military supplies and other fields.
But in the processing, storage and use process, due to the light, heat, oxygen, water, high-energy radiation, chemical and biological corrosion combination of internal and external factors, such as the chemical composition and structure of the polymer material can produce a series of changes, a corresponding bad physical properties, such as hair hard, brittle, sticky, discoloration, loss of strength, etc.,
This phenomenon is known as the aging of polymer materials.
The aging nature of polymer materials refers to the change of physical or chemical structure, which shows that the properties of the materials gradually decline and lose their due use value.
The aging failure of polymer materials has become one of the key problems that restrict the further development and application of polymer materials.
aging
Due to the different varieties of polymers, the use of different conditions, so there are different aging phenomenon and characteristics.
For example, agricultural plastic film after the sun and rain discoloration, brittle, transparency decline;
Aviation plexigron with a long time after the appearance of silver, transparency decline;
After long use, the elasticity of rubber products decreases, becomes hard, cracking or becomes soft and sticky;
After long use of coatings, loss of light, powder, bubbles, peeling and so on.
The phenomenon of aging can be summed up in the following four changes:
1. Change of appearance
The appearance of stains, spots, silvering, cracks, frosting, powdering, viscosity, warping, fisheye, wrinkling, shrinkage, burning, optical distortion and optical color change.
2. Changes in physical properties
Including solubility, swelling, rheological properties and cold resistance, heat resistance, water permeability, air permeability and other performance changes.
3. Change of mechanical properties
Tensile strength, bending strength, shear strength, impact strength, relative elongation, stress relaxation and other properties.
4. Changes in electrical properties
Such as surface resistance, volume resistance, dielectric constant, electric breakdown strength and so on.
Aging factors
The physical properties of polymer materials are closely related to their chemical structure and aggregation structure.
Chemical structure is the long chain structure of macromolecules connected by covalent bonds. Aggregated structure is the spatial structure arranged and piled up by many macromolecules by molecular forces, such as crystalline state, amorphous state and crystal-amorphous state.
The intermolecular forces that maintain the aggregated structure include ionic, metallic, covalent and van der Waals forces.
Environmental factors will lead to the change of intermolecular force, even the breaking of chain or the shedding of some groups, which will eventually destroy the aggregation structure of the material and change the physical properties of the material.
There are two kinds of factors that affect the aging of polymer materials: internal factors and external factors.
Internal factors
1. Chemical structure of the polymer
The aging of polymer is closely related to its own chemical structure. The weak bond of the chemical structure is easy to be affected by external factors and break into free radicals.
This radical is the starting point for the free radical reaction.
2. Physical form
Some of the molecular bonds in a polymer are ordered and some are disordered.
Ordered molecular bonds can form crystalline regions, while disordered molecular bonds are amorphous regions. The morphology of many polymers is not uniform, but semi-crystalline, with both crystalline regions and amorphous regions. The aging reaction starts from the amorphous regions.
3, three-dimensional consolidation
The stereoscopicity of a polymer is closely related to its crystallinity.
In general, structured polymers have better aging resistance than atactic polymers.
4. General molecular weight and its distribution
The molecular weight of the polymer has little relationship with aging, but the distribution of molecular weight has great influence on the aging properties of the polymer. The wider the distribution, the easier the aging, because the wider the distribution, the more end groups, the easier the aging reaction.
5, trace metal impurities and other impurities
Polymer in the process, and metal contact, may be mixed with trace metal, or in the polymerization, some residual metal catalyst, which will affect the automatic oxidation (aging) initiation.
External factors
1. The influence of temperature
As the temperature increases, the movement of polymer chain increases. Once the dissociation energy of chemical bond is exceeded, the thermal degradation or group shedding of polymer chain will be caused. At present, there are a lot of literature reports on the thermal degradation of polymer materials.
When the temperature decreases, the mechanical properties of materials are often affected.
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
Humidity on polymer materials can be attributed to the influence of moisture on the material of the swelling and dissolving effect, make maintain polymer molecular inter-atomic forces change the state of aggregation structure, state of aggregation of undermining the material, especially for the crosslinking of amorphous polymers, the influence of humidity is extremely obvious, will make the polymer material disintegration of swelling and even the state of aggregation,
So that the performance of the material is damaged;
For crystalline plastics or fibers, the effect of humidity is not obvious because of water penetration limitations.
3. The influence of oxygen
Oxygen is the main cause of aging of polymer materials. Because of the permeability of oxygen, crystalline polymer is more resistant to oxidation than amorphous polymer.
Oxygen first attacks the weak links on the main chain of polymer, such as double bond, hydroxyl group and hydrogen group or atom on tertiary carbon atom, forming peroxide radical or peroxide of polymer, and then causes the fracture of the main chain at this position. In serious cases, the molecular weight of polymer decreases significantly, the glass transition temperature decreases, and the polymer becomes viscous.
In the presence of some initiators or transition metals that are easy to decompose into free radicals, the oxidation reaction tends to be intensified.
4, light aging
Whether the polymer is irradiated by light will cause the breakage of molecular chain depends on the relative size of light energy and dissociation energy and the sensitivity of the chemical structure of the polymer to light wave.
Because of the existence of ozone layer and atmosphere on the surface of the earth, the wavelength range of the solar light that can reach the ground is 290nm ~ 4300nm, and the light wave energy is greater than the dissociation energy of the chemical bond only in the ultraviolet region, which will cause the fracture of the polymer chemical bond.
For example, the uv wavelength of 300nm ~ 400RIM can be absorbed by the polymer containing carbonyl group and double bond, and make the macromolecular chain break, change the chemical structure, and make the material performance become poor;
Polyethylene terephthalate has a strong absorption of ultraviolet light at 280nm, and the degradation products are mainly CO, H, CH.
Polyolefin containing only C-C bonds do not absorb ultraviolet light, but in the presence of a small number of impurities, such as carbonyl, unsaturated bonds, hydrogen peroxide groups, catalyst residues, aromatic hydrocarbons and transition metal elements, can promote the photooxidation of polyolefin.
5, the influence of chemical media
Chemical medium can only play a role when it penetrates into the polymer material. These effects include the covalent bond and the secondary bond.
The action of covalent bond is shown as chain breaking, crosslinking, addition or the combination of these actions, which is an irreversible chemical process.
Although the destruction of secondary bond by chemical medium does not cause the change of chemical structure, the aggregation structure of the material will change, so that its physical properties will change accordingly.
Environmental stress cracking, dissolution cracking, plasticizing and other physical changes are typical manifestations of chemical media aging of polymer materials.
The method of eliminating dissolution cracking is to eliminate the internal stress of the material. Annealing after the material forming is conducive to eliminating the internal stress of the material.
Plasticizing is in the liquid medium and polymer material continuous contact occasions, polymer and small molecular medium interaction between the interaction between part instead of polymer interaction, so that the polymer chain segment is easy to move, shown as glass transition temperature decreases, the strength, hardness and elastic modulus of the material decreases, elongation at break increase.
6. Biological aging
As plastic products in the processing process almost all use a variety of additives, so often become a source of mold nutrients.
When the mold grows, it absorbs the nutrients on the surface and inside the plastic and becomes mycelium. Mycelium is a conductor, so the insulation of the plastic drops, the weight changes, and the serious peeling will occur.
Organic acids and toxins are contained in the metabolites of mold growth, which will make the surface of plastic sticky, discoloration, brittle, smooth and other phenomena, and also make long-term contact with the people of this mold rot plastic with disease.
Polysaccharide and their modified natural polymer compounds with general means such as blending modification of plastics can be made into biodegradable disposable film, sheet, container, foaming products, etc., its wastes can exist in the natural environment of amylase and so on polysaccharide of natural polymer decomposition enzyme, hydrolysis step by step into a small molecular compound,
And eventually break down into pollution-free carbon dioxide and water and return to the biosphere.
Based on these advantages, starch as a representative of the polysaccharide natural polymer compound is still an important component of degradable plastics.
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