Aging phenomenon of polymer materials (plastic products)

Polymer materials include plastics, rubber, fibers, films, adhesives and coatings. Because they have many potential properties better than traditional structural materials, they are used more and more widely in the field of military and civilian products.

However, in the process of processing, storage and use, due to the combined effect of 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, physical properties will also change accordingly, such as hard, sticky, brittle, discoloration, loss of strength, etc., this phenomenon is the aging of polymer materials.

The essence of aging of polymer materials refers to the change of 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. Aging failure of polymer materials has become one of the key problems that limit the further development and application of polymer materials.

Aging phenomenon

Due to the different varieties of polymer materials and different conditions of use, there are different aging phenomena and characteristics. For example, agricultural plastic film after the sun and rain occur discoloration, brittleness, transparency decline; Aviation plexiglass after use for a long time silver pattern, transparency decline; The elasticity of rubber products decreases, hardens, cracks or becomes soft and sticky after long-term use; Paint after long-term use, loss of light, powder, bubble, peeling, etc.

The aging phenomenon can be summarized in the following four changes:

1. Appearance changes

Stains, spots, silvers, cracks, frosting, powdering, stickiness, warping, fisheye, wrinkling, shrinking, scorching, optical distortion, and optical color changes occur.

2. Physical properties change

Including solubility, swelling, rheological properties and cold resistance, heat resistance, water permeability, air permeability and other performance changes.

3, mechanical properties change

Tensile strength, bending strength, shear strength, impact strength, relative elongation, stress relaxation and other properties change.

4, electrical performance changes

Such as surface resistance, volume resistance, dielectric constant, breakdown strength and other changes.

Aging factor

The physical properties of polymer materials are closely related to their chemical structure and aggregation state structure.

The chemical structure is a long chain structure of macromolecules connected by covalent bonds, and the aggregation structure is a spatial structure of many macromolecules arranged and piled up by intermolecular force, such as crystalline, amorphous, crystal-amorphous. The intermolecular forces that maintain the aggregate structure include ionic bond force, metallic bond force, covalent bond force and van der Waals force.

Environmental factors will lead to the change of intermolecular forces, even the break of the chain or the fall off of some 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.

Intrinsic factor

1. Chemical structure of polymer

The aging of polymers is closely related to their chemical structure, and the weak bond of the chemical structure is easily affected by external factors to break and become free radicals. This free radical is the starting point of radical reactions.

2. Physical form

Some of the molecular bonds of the polymer are ordered and some are disordered. The ordered molecular bonds can form crystalline regions, and the disordered molecular bonds are amorphous regions. The shape of many polymers is not uniform, but semi-crystalline, with both crystalline and amorphous regions. The aging reaction begins from the amorphous region.

3, three-dimensional integration

The stereointegration of polymer is closely related to its crystallinity. In general, structured polymers have better aging resistance than random polymers.

4, molecular weight and its distribution

In general, the molecular weight of the polymer has little relationship with aging, and the distribution of molecular weight has a great impact 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, the easier it is to cause aging reaction.

5, trace metal impurities and other impurities

When the polymer is processed, it is necessary to contact with the metal, and it may be mixed with trace metals, or in the polymerization, some metal catalysts remain, which will affect the initiation of automatic oxidation (that is, aging).

External factor

1. The influence of temperature

When the temperature increases, 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, thermal degradation of polymer materials has been extensively reported. The decrease of temperature often affects the mechanical properties of materials. The critical temperature points closely related to mechanical properties include glass transition temperature, viscous flow temperature and melting point. The physical state of the material can be divided into glassy 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, so that the intermolecular forces that maintain the aggregation structure of polymer materials change, thus destroying the aggregation state of the material. Especially for non-crosslinked amorphous polymers, the influence of humidity is extremely obvious, which will cause the swelling and even the aggregation state disintegration of polymer materials, thus damaging the performance of the material. For the crystalline form of plastics or fibers, the effect of humidity is not very obvious due to the existence of water penetration limitations.

3. The effect of oxygen

Oxygen is the main cause of aging of polymer materials. Due to the permeability of oxygen, crystalline polymer is more resistant to oxidation than amorphous polymer. Oxygen first attacks the weak links on the polymer main chain, such as double bonds, hydroxyl, hydrogen and other groups or atoms on the tertiary carbon atom, forming polymer peroxyradicals or peroxides, and then causes the break of the main chain in this part. In severe cases, the molecular weight of the polymer decreases significantly, the glass transition temperature decreases, and the polymer becomes viscous. In the presence of some initiators or transition metals that are easily decomposed into free radicals, the oxidation reaction tends to be intensified.

4, light aging

Whether the polymer is irradiated by light can cause the fracture of molecular chain depends on the relative size of light energy and dissociation energy and the sensitivity of polymer chemical structure to light wave. Due to the existence of the ozone layer and the atmosphere on the earth's surface, the wavelength range of solar light that can reach the ground is 290 ~ 4300nm, and the light wave energy is greater than the dissociation energy of chemical bonds only in the ultraviolet region, which will cause the fracture of polymer chemical bonds.

For example, the ultraviolet wavelength of 300 ~ 400nm can be absorbed by polymers containing carbonyl groups and double bonds, and the macromolecular chain is broken, the chemical structure is changed, and the material properties are deteriorated; Polyethylene terephthalate has strong absorption of 280nm UV, and the degradation products are mainly CO, H and CH. Polyolefin containing only C-C bonds has no UV absorption, but in the presence of a small amount of impurities, such as carbonyl groups, unsaturated bonds, hydroperoxide groups, catalyst residues, aromatics and transition metal elements, it can promote the photooxidation reaction of polyolefin.

5, the influence of chemical media

The chemical medium can only play a role if it penetrates into the interior of the polymer material, and these roles include the role of covalent bonds and the role of secondary bonds. The action of covalent bond is manifested as chain breaking, crosslinking, addition or the combination of these effects, which is an irreversible chemical process. Although the destruction of the secondary valence bond by the chemical medium does not cause the change of chemical structure, the aggregate structure of the material will change, and its physical properties will change accordingly.

Environmental stress cracking, dissolution cracking, plasticizing and other physical changes are typical manifestations of chemical aging of polymer materials.

The method of eliminating dissolution cracking is to eliminate the internal stress of the material, and annealing after the molding of the material is conducive to eliminating the internal stress of the material. Plasticizing is in the case of continuous contact between the liquid medium and the polymer material, the interaction between the polymer and the small molecule medium partly replaces the interaction between the polymer, so that the polymer chain segment is easier to move, which is manifested as the glass transition temperature is reduced, the strength, hardness and elastic modulus of the material is decreased, and the elongation at break is increased.

6. Biological aging

Since plastic products almost all use a variety of additives in the processing process, they often become a nutrient source of mold. When mold grows, it absorbs the nutrients on the surface and inside of the plastic and becomes mycelium, which is also a conductor, so that the insulation of the plastic is reduced, the weight changes, and the severe peel will occur. The metabolites of mold growth contain organic acids and toxins, which will make the surface of the plastic sticky, discoloration, brittleness, and reduced finish, and will also cause long-term contact with this mouldy plastic.

Polysaccharide natural polymers and their modified compounds can be processed into degradable disposable films, sheets, containers, foaming products, etc. by means of blending modification with general plastics. The waste can be gradually hydrolyzed into small molecular compounds by the intervention of amylase and other polysaccharide natural polymer decomposition enzymes widely existing in the natural environment. And eventually break down into pollution-free carbon dioxide and water and return to the biosphere. Based on these advantages, the polysaccharide natural polymer compounds represented by starch are still an important part of degradable plastics.