Formula design scheme for high temperature resistant rubber products

The high temperature resistance of rubber products is the most common property among the special properties of rubber. High temperature resistant rubber products refer to products that should maintain normal physical and mechanical properties such as elasticity, strength, elongation and hardness for a long time when used under high temperature conditions. The essential reason for the stable performance of rubber in this case is that it can resist the influence of factors such as oxygen, ozone, corrosive chemicals, high energy radiation and mechanical fatigue at high temperatures, the rubber molecular structure does not undergo significant changes and damage, and can maintain good performance.

1 Selection of rubber varieties

The heat resistance of rubber products mainly depends on the variety of rubber used. Therefore, when designing the formula, the choice of raw rubber should be considered first. The heat resistance of rubber is manifested in the fact that the rubber has a higher viscosity flow temperature, higher thermal decomposition stability and good chemical stability. The viscosity flow temperature of rubber depends on the polarity of the rubber molecular structure and the rigidity of the molecular chain. The greater the polarity and rigidity, the higher the viscosity flow temperature. The polarity of rubber molecules is determined by the polar groups and molecular structure they contain. The rigidity of the molecular chain is also related to the regularity of the polar substituents and the spatial structure arrangement. Introducing cyano, ester, hydroxyl or chlorine atoms, fluorine atoms, etc. into rubber molecules will improve heat resistance. The thermal decomposition temperature of rubber depends on the chemical bond properties of the rubber molecular structure. The higher the chemical bond energy, the higher the bond energy of macromolecular chains such as silicone rubber and polytin siloxane, so they have superior heat resistance.

The chemical stability of rubber is an important factor in heat resistance, because under high temperature conditions, some chemical substances will promote the corrosion of rubber and reduce heat resistance if they come into contact with oxygen, ozone, acid, alkali and organic solvents. Chemical stability is closely related to the molecular structure of rubber. Butyl rubber, ethylene propylene rubber and chlorosulfonated polyethylene with low unsaturation have excellent heat resistance. In addition, if there is an aromatic structure connected by a single bond on the main chain, the molecular chain will also promote structural stability by virtue of the conjugation effect. The heat resistance of various rubbers is shown in Table 1.

Table 1 Heat resistance of various rubbers

Several commonly used heat-resistant rubbers are as follows.

Nitrile rubber The use temperature of nitrile rubber does not exceed 150℃. What is rare is that at this temperature, it still has excellent oil resistance and is often used to manufacture rubber parts in aircraft or oil wells. The heat resistance of nitrile rubber increases with the increase of acrylonitrile content, but too high acrylonitrile content is prone to thermal cross-linking and deterioration of physical and mechanical properties. Therefore, it is advisable to use nitrile rubber with a medium acrylonitrile content of about 30%.

Chloroprene rubber W type chloroprene rubber has better heat resistance than G type chloroprene rubber. They have better heat resistance, which is largely related to their sulfur-free vulcanization system. Increasing the amount of zinc oxide in the formula can increase the degree of crosslinking, and the antioxidant MB also has a good effect on improving heat resistance.

Butyl rubber Butyl rubber is a good heat-resistant rubber. The lower the unsaturation, the better the heat resistance. The use of phenolic resin vulcanization system formula can obtain better heat resistance, and the heat resistance of benzoquinone dioxime vulcanization is also much better than the sulfur vulcanization system. Chlorobutyl rubber not only has excellent heat resistance but also has good process performance.

Chlorosulfonated polyethylene Chlorosulfonated polyethylene has a use temperature of 130-160°C, stable performance, and good ozone resistance, chemical corrosion resistance and combustion resistance. It can maintain the original performance of the rubber after continuous aging at 149°C for two weeks.

Silicone rubber Silicone rubber is one of the most heat-resistant rubbers currently available and can be used for a long time at 250°C. Its biggest disadvantage is its poor physical and mechanical properties.

Fluororubber There are many types of fluorine-containing rubbers. Currently, the fluorine rubbers used for high temperature resistance are mostly Kel-F and Viton. Type A, the latter can be used for a long time at about 300℃, and has the characteristics of chemical corrosion resistance.

The heat resistance of EPDM rubber mainly depends on its unsaturation and the third monomer. The heat resistance of EPDM rubber with very low unsaturation is better than that of EPDM rubber. The thermal aging behavior of the two in air is completely different. The degradation of EPDM rubber is dominant, while the cross-linking of EPDM rubber is dominant. The influence of the third monomer, based on the change of physical properties, its heat resistance decreases in the following order: 1,4-hexadiene < ethylidene norbornene < dicyclopentadiene. As the content of the third monomer and propylene in EPDM rubber increases, its heat resistance decreases.

Acrylic rubber The heat resistance of acrylate rubber is higher than that of nitrile rubber and lower than that of fluororubber. The long-term (1000h) use temperature is 170℃, and the short-term (70h) The use temperature can be increased to 200℃. In the process of heat aging, cross-linking reaction usually dominates, which increases the tensile stress and hardness, and reduces the tensile strength and elongation at break. However, some acrylic rubbers degrade during heat aging. Various types of acrylic rubbers have little difference after aging at 150℃ for 70h. At 200℃, the vulcanized rubber based on Hycar401 ethyl acrylate rubber has the best heat resistance. The heat resistance of ethylene methyl acrylate rubber (trade name Vamac) developed by Dupont Corporation of the United States is second only to fluororubber and silicone rubber. Under the heat aging conditions of 150℃×4300h, 170℃×1000h, 177℃×670h, 191℃×240h, and 200℃×168h, the reduction of its elongation at break is not less than 50%.

2 Vulcanization system

For some olefin rubbers, such as EPDM, NBR or butyl rubber, the heat resistance of peroxide or resin vulcanization system is superior to that of sulfur vulcanization system. Among the sulfur vulcanization systems, the heat resistance of sulfur carrier vulcanization is the best. Others such as quinone oxime vulcanization system can also make the vulcanized rubber have good heat resistance. The above reasons are all related to the thermal stability of the cross-linking bonds in the vulcanized rubber network. Resin cross-linking can give the rubber the best heat resistance, and this vulcanization system has a more significant effect in butyl rubber. Peroxide cross-linking makes the generated cross-linking bonds more stable, thus making the vulcanized rubber have higher heat resistance. Different vulcanization systems will produce different cross-linking bond structures. Generally, C-C cross-linking bonds have the best heat resistance, followed by single sulfur bonds, and polysulfide bonds have the worst heat resistance. In order to obtain heat-resistant cross-linking bonds, low sulfur high-promoting systems, effective vulcanization systems, peroxide vulcanization systems or other sulfur-free vulcanization systems should be used. The design features of various vulcanization systems are shown in Table 2.

Table 2 Design features of various vulcanization systems

When using peroxide vulcanization system for formula design, attention must be paid to the appropriate amount of peroxide. If the amount is too much, the crosslinking density will be too high and the performance will decrease; if the amount is insufficient, the crosslinking density will decrease, resulting in a decrease in heat resistance. In addition, when vulcanized with peroxide, the physical and mechanical properties of the vulcanized rubber are low, especially the hot tear strength is low, and special attention should be paid to molded products. Different rubber heat-resistant vulcanization systems have different characteristics. In addition to the above-mentioned vulcanization systems, the following better systems can also be selected: chloroprene rubber should use metal oxide vulcanization system, nitrile rubber should use cadmium magnesium vulcanization system, butyl rubber should use resin vulcanization system, etc., which can give the product better heat resistance.

3 Protection system

Heat-resistant rubber must use high-efficiency heat-resistant antioxidants, which can significantly improve the heat-resistant aging effect of rubber. Practice shows that different rubbers should use different antioxidants. For example, butyl rubber has no significant effect when using amine antioxidants, but phenol antioxidants (such as antioxidant 2246, dialkylphenol sulfide and 4,4'-methylenebis 6-tert-butyl o-cresol, etc.) significantly improve its heat resistance. Heat-resistant rubber must use high-efficiency heat-resistant antioxidants. Commonly used heat-resistant antioxidants for various rubbers are shown in Tables 3.

Table 3 Commonly used heat-resistant antioxidants for general-purpose rubber

In order to reduce the fatigue damage caused by repeated deformation of rubber at high temperature, antioxidants with good fatigue resistance can be used together with heat-resistant antioxidants, and the dosage of antioxidants is 1.5% to 2.0% of the raw rubber. The use of heat stabilizers, such as stannous chloride, antimony trioxide, pentamethylene thiuram tetrasulfide, hepatocyanine and a small amount of phenolic antioxidants, can also improve the thermal stability of the rubber.

4 Filling system

Generally, organic fillers have better heat resistance than carbon black. Among inorganic fillers, white carbon black, active zinc oxide, magnesium oxide, aluminum oxide and silicate are more suitable for heat-resistant matching.

5 Effect of softener

Generally, the relative molecular mass of softeners is low, and they are easy to volatilize or migrate and seep out at high temperatures, resulting in increased hardness and reduced elongation of vulcanized rubber. Therefore, the heat-resistant rubber formula should use varieties with good thermal stability and low volatility at high temperatures, such as petroleum oils with high flash points, polyester plasticizers with large relative molecular weight and high softening point, and some low relative molecular weight oligomers such as liquid rubber. Heat-resistant nitrile rubber is best softened by coumarone resin, styrene-indene resin, polyester and liquid nitrile rubber. Chlorosulfonated polyethylene rubber can use esters, aromatic oils and chlorinated paraffin. The heat resistance is better when chlorinated paraffin is used as a softener. For heat-resistant butyl rubber, it is recommended to use coumarone resin in an amount not exceeding 5 parts by mass, and 10 to 20 parts by mass of vaseline or paraffin oil, mineral rubber and petroleum asphalt resin can also be used. Ethylene propylene rubber usually uses cyclopentane oil and paraffin oil as softeners.