Rubber, with plastic, fiber and known as the three synthetic materials, is the only highly scalable and excellent flexibility of the polymer material. The excellent performance of rubber makes it an excellent material for industrial shock absorption, sealing, flexing, abrasion, corrosion, insulation and bonding. It is widely used in aerospace, automobile, aircraft tires and medical and health fields.
However, rubber is also an unsaturated olefin polymer. According to the South and South rubber and plastics technical staff, rubber is characterized by rubber and its products in the processing, storage and use of the process vulnerable to their own or external factors, so that the structure or component damage, affecting the use of value and application The This phenomenon is known as the rubber industry, rubber aging (Rubber Aging). Natural rubber aging phenomenon varied, such as raw rubber or product surface sticky, brittle, hardened, cracked, delaminated and long mold; solubility, rheology, heat resistance and cold resistance changes; Electrical constant, surface resistance and conductivity changes.
Acrometer for everyone to introduce the phenomenon of aging rubber, aging and prevention of some of the small knowledge, so that we can more rational understanding of the phenomenon of rubber aging, pay attention to rubber, semi-finished products, finished anti-aging prevention methods.
First, what is the aging of rubber
The aging of rubber is an irreversible process of change, its essence is the molecular structure of rubber in the weather, physical, chemical and biological factors such as the role of oxidative degradation reaction or structural reaction. The degradation of the rubber molecules causes the average relative molecular weight to decrease, the strength decreases and the performance decreases. The structural reaction is further branched and crosslinked, and the strength of the rubber is decreased, the surface hardens and cracks are lost. Rubber in the process of aging is mainly based on degradation or structural, mainly depends on the structure of the rubber itself and the external conditions of these two factors. In order to prevent the deterioration of rubber aging, to extend its service life, in addition to the use of rubber products, maintenance, usually in the rubber with the processing should be added to the corresponding protective agent, such protective agent called antioxidant.
The types of rubber aging are divided into: oxygen aging, ozone aging, fatigue oxidation, oxygen aging and subdivided into thermal oxygen aging and catalytic oxidation (can be divided into photocatalytic oxidation and valence metal ion catalytic oxidation).
Second, the cause of rubber aging
Zhongnan rubber and plastics technical personnel to introduce the cause of rubber aging is divided into internal and external causes.
Internal factors: the key is the type of rubber, molecular structure and performance.
External factors: divided into physical factors, chemical factors and other factors. Physical factors: heat, light, mechanical stress (deformation), radiation, etc .; chemical factors: oxygen, ozone, variable metal ions, chemical media, etc .; other factors: such as the role of microorganisms.
1, rubber aging resistance, first depends on the rubber varieties, molecular structure. Where unsaturated rubber (which is diene rubber) are not heat-resistant oxidation of the elastomer, where the saturated rubber has excellent resistance to aging. Natural rubber and some general synthetic rubber contains 150 to 250 double bonds per 1000 carbon atoms. These parts are the most prone to chemical reactions, so their thermal and oxygen aging properties are not good, especially natural rubber. The curse of rubber thermal oxygen aging lies in its unsaturation. While the structure tends to saturation of silicone rubber, fluorine rubber, butyl rubber, ethylene propylene rubber, polyurethane rubber, acrylic rubber, etc. are heat aging aging of the elastomer.
2, caused by the external factors of rubber aging is very complex, in which oxygen, ozone, heat, light and mechanical stress is the main cause of rubber aging, and thermal aging is the culprit of rubber aging.
Oxygen in the rubber with the rubber molecules free radical chain reaction, molecular chain fracture or excessive cross-linking, causing changes in rubber properties. Oxidation is one of the important reasons for the aging of rubber.
Ozone chemical activity is much higher than oxygen, more destructive, it is also the molecular chain fracture, but the role of ozone on the rubber with the rubber deformation or not different. When used as a deformed rubber (mainly unsaturated rubber), there is a crack that is perpendicular to the direction of the stress, the so-called "ozone cracking"; when the rubber is deformed, only the oxide film is formed on the surface without cracking.
Increasing the temperature can cause thermal cracking or thermal crosslinking of the rubber. Rubber heated to a certain temperature will be degraded. For example, natural rubber more than 200 degrees Celsius began to break down low molecular substances.
But the basic role of heat or activation. Increase the rate of oxygen diffusion and activation of oxidation, thereby accelerating the rate of oxidation of rubber, which is a common phenomenon of aging - thermal oxygen aging. In the oxygen under the participation of a variety of rubber aging results are not the same. Natural rubber and isoprene rubber and butyl rubber in the process of thermal oxygen aging is mainly macromolecule chain fracture, softening sticky; and styrene butadiene rubber, butadiene rubber, nitrile rubber and EPDM rubber in the thermal oxygen The process of aging is mainly broken after the product intertwined, destroyed the rubber elastic structure, so it becomes hard and brittle
Rubber thermal stability depends mainly on its chemical composition and structure, silicone rubber and fluorine rubber thermal stability than general rubber good. In general rubber, butadiene rubber thermal stability of the best, followed by styrene butadiene rubber.
The shorter the light, the greater the energy. The rubber is the destruction of the role of high energy UV. In addition to UV can directly lead to the rubber molecular chain of the fracture and cross-linking, the rubber due to the absorption of light energy generated free radicals, trigger and accelerate the oxidation chain reaction process. Ultraviolet light plays the role of heating; light of its other characteristics (and thermal effect is different) is mainly in the rubber surface, the high rate of plastic samples, both sides will appear reticular cracks, the so-called " "
5) mechanical stress
Under the repeated action of mechanical stress, the rubber molecular chain will break into free radicals, causing oxidation chain reaction, the formation of chemical process. Mechanical fracture molecular chain and mechanical activation oxidation process. Which can prevail, depending on the conditions in which it is located. In addition, under the action of stress easily lead to ozone cracking.
The role of water in two ways: the rubber in the humid air rain or soak in the water, easy to damage, it is due to the rubber in the water-soluble substances and water groups and other ingredients are dissolved by water, especially in water immersion and Atmospheric exposure of the alternating effect, will accelerate the destruction of rubber; but in some cases the moisture on the rubber can not afford to damage, and even delay the role of aging.
The role of the rubber there are chemical medium, variable metal ions, high-energy radiation, electricity and biological.
3, when the use of conditions, the choice of raw rubber has been timed, rubber aging resistance depends on the choice of curing system and antioxidant varieties and dosage.
Third, the rubber anti-aging measures
As the factors that cause the aging of rubber products are different, it should be based on different aging mechanism to take the appropriate anti-aging measures, mainly physical protection and chemical protection law.
1, physical protection law
Try to avoid rubber and a variety of aging factors, such as the use of rubber and plastic blends, surface coating or treatment, plus light shielding agent, plus paraffin and so on. The treatment of the rubber surface is an important means to prevent the aging of the rubber. For example, the TiO2 film can be selected to cover the surface of the natural rubber. Since the TiO2 film can improve the aging resistance of the natural rubber, it can effectively prevent or delay the aging of the natural rubber The In addition, the plasma surface of the implementation of plasma oxygen treatment, can effectively change the rubber surface of the water repellency, and enhance the anti-aging rubber function, play to prevent or delay the role of natural rubber aging.
2, chemical protection law
1) Add anti-aging agent
Free radical reaction is the main cause of rubber aging, for this reason, we can take the addition of antioxidants and other measures to prevent the rubber free radical reaction, and stop the rubber free radical chain reaction, anti-aging agent can produce peroxide, and decomposition Into a stable compound, so that the internal structure of the rubber in a stable state, inhibit the phenomenon of aging rubber, which play a role in protecting the rubber.
According to the function of antioxidants, rubber antioxidants mainly free radical inhibitors, light stabilizers, anti-ozone agents, harmful metal inhibitors, peroxide decomposition agent and other antioxidants. According to their protective behavior can be divided into ordinary antioxidants, long-term antioxidants, in situ or migratory antioxidants, pollution and non-polluting antioxidants. In addition, due to the role of antioxidant or performance depends on its structure, different replacement base, which can produce different effects, so anti-aging agent according to the type of replacement base, can be divided into phenols, thiourea, Amines and dithiocarbamates.
Different types of anti-aging agents, there are different properties, its resistance to ozone, heat resistance, resistance to harmful metal aging and resistance to flex cracking will be different, so you can choose according to different performance antioxidant species. In addition, the antioxidant also has the characteristics of additive, therefore, in practical application, we can choose several different performance of the antioxidant, the implementation of the merger, to achieve the purpose of rubber anti-aging.
In addition, the antioxidant may be added to the rubber raw material or may be added during the processing of the rubber product to retard or prevent the aging of the rubber.
Actively adding substances to prevent or delay the aging of the rubber to continue, such as adding amine or phenolic chemical antioxidant. Anti-aging is mainly from the reduction of vulcanized rubber polymer chain on the unsaturated chain of the main, for example, with a number of chain modification, the use of special chemical reactions, so that unsaturated bonds saturated, so that the molecular structure of the level to be improved Aging, delay aging and extend its service life.
In the rubber, plastic and other processing process to add antioxidants, for example, in the vulcanized rubber formula to add UV stabilizer, anti-mildew agent, heat stabilizer, add to prevent ozone or oxygen caused by aging antioxidant.
2) add other substances to prevent the aging of rubber
In addition to adding antioxidants, you can also add some other anti-aging effect of the material, for example, in natural rubber, add aluminum powder. Natural rubber in the addition of aluminum powder, after the emergence of thermal aging, can still maintain good mechanical properties, and natural rubber in the addition of aluminum powder, its microwave irradiation, ozone and heat and other external interference play a stronger resistance , Natural rubber and therefore have a strong anti-aging. The addition of ethylene thiourea (ETU) to chloroprene rubber can also play a very good anti-aging effect. The more ethylene thiourea is added, the stronger the anti-aging properties of chloroprene rubber.
With the continuous development of science and technology, nanotechnology has been rapid development, in the anti-aging rubber, nano-technology plays a very important role. As a dispersed phase, nanoparticles can produce strong interface, large specific surface area and scale effect of the interaction, and can be adsorbed or chemical bonds and other components between the implementation of synergies, the rubber material to play a role in dispersion, the rubber Material composite nanocomposite rubber material, compared with ordinary rubber, nano-composite rubber material with many new excellent performance, greatly improving the anti-aging properties of rubber materials.
Fourth, the choice of anti-aging agent points 1
, The coloring property of the antioxidant (pollution)
Amine antioxidant coloring, will make the rubber coloring. and will be transferred to the rubber and contact with the material, as well as pollution, therefore, in particular, need to be noted. When exposed to hot, sunlight, it will become benzoquinone structure, and significant discoloration. Amine antioxidant coloring (pollution) are different.
2, the solubility of antioxidants in rubber
The solubility of the antioxidant in the rubber, in general, can be seen to show whether the frost, the greater the solubility, the more difficult to spray. In general, the solubility of the amine antioxidant in polar rubber (NBR, CR, etc.) is higher. Due to the different chemical structure of the antioxidant, the mutual solubility is also different, and the substitution of alkyl is better than that of 8101NA (IPPD).
3, the heat loss of antioxidants
Considering the stability of the antioxidant, it is important to understand its own heating reduction (volatility). In the same compound, the greater the molecular weight, the smaller the heating loss, CD (carbodiimide) and white white antioxidant even at high temperatures is also difficult to volatilize. So it has good stability.
4, the amount of antioxidant in the rubber residue
The antioxidant added to the rubber is gradually reduced due to volatilization and oxidation. 6C is oxidized and becomes benzoquinone. Recently, a research report said that the benzoquinone structure due to heat and rubber reaction, but also to restore the original structure.
5, the environmental protection of antioxidants
6, the synergistic effect of antioxidants
Under normal circumstances, phenols, amines antioxidant as the first antioxidant, not alone, often and phosphorus, sulfur antioxidant (second antioxidant) and use, the use of BHT (butylated hydroxytoluene) and phosphorus Class and sulfur class synergistic effect, to prevent aging is a conventional practice. In addition, in order to improve the heat resistance, it is possible to achieve the purpose of preventing aging by using the acid-base interaction between the first antioxidant and the second antioxidant MBI (mercaptobenzimidazole).
7, the migration of antioxidants
Easy to migrate antioxidants, recommended for NR, SBR, BR, EPDM, CR; and difficult to migrate, can be used for NBR, CSM, CPE, IIR, Cl-IIR and other rubber. The reason is that the number of polar groups in the polymer, the level of secondary transformation point, the molecular weight of the antioxidant used, the structure and coloring, the degree of crosslinking of the polymer and so on. In the polymer (rubber), if the content of the polar group is high, it is necessary to select the antioxidant having a high secondary transition point. For antioxidants, the greater the molecular weight, the molecular branch structure, the migration of the smaller pollution.