Methods for improving rubber hardness

1 Basic cooperation

1.1 Raw rubber system

The formula of high hardness rubber often needs a lot of filling and reinforcing agents. The raw rubber should be selected with low Mooney viscosity. For example, EPDM should choose a brand with low Mooney and high ethylene content; NBR can choose brands with low Mooney and high acrylonitrile content. Sometimes, according to the needs of other properties, the combination of high and low Mooney gum can be selected. After a large amount of high Mooney gum is filled, the viscosity increases too much, which brings difficulties to rubber mixing.

1.2 Reinforcement filling system

Reinforcing with carbon black and white carbon black is the best choice. Both have good comprehensive performance and strong ability to increase hardness. Among them, fine carbon black has the best effect. However, high hardness rubber requires a lot of filling. If only fine carbon black is used, serious problems will occur in rubber mixing, vulcanization and other processes. Therefore, the combination of fine carbon black and large carbon black has the best effect, such as N330/N550. Other weak reinforcing fillers should not be used as much as possible. Because these fillers have a weak ability to increase hardness, more fillers are needed to achieve the target hardness than carbon black and white carbon black, resulting in a decline in the overall performance of the rubber compound and the process performance.

1.3 Plasticizing and softening system

High hardness rubber requires a large amount of filling and reinforcing agent, resulting in process problems such as rubber mixing and vulcanization. An appropriate amount of plasticizer should be added, although the plasticizer will reduce the hardness of rubber. The selection of plasticizers should be based on solid plasticizers, such as solid coumarone. Commercially available flow aids can also improve the process performance, such as flow exhaust agent, dispersant, etc.


1.4 Curing system

Increasing the crosslinking density of rubber compound is beneficial to improving the hardness of rubber compound. For sulfur curing system, increasing the amount of accelerant, especially TMTD accelerant, is beneficial to improve the hardness. The increase of vulcanization dosage can also improve the crosslinking density and the hardness of the rubber compound. For example, when the sulfur dosage reaches 30-50 parts by mass, hard rubber can be prepared. However, when the sulfur dosage and the accelerator dosage are both increased, it is easy to produce frosting, and the dosage needs to be balanced. In addition, for the sulfur curing system, due to the high filling of the high hardness rubber compound, the heat generation of mixing and curing process is increased, which is easy to cause scorching. Therefore, for the sake of safety, it is also beneficial to cooperate with the anti-coking agent, such as CTP. For peroxide curing system, increasing peroxide dosage can improve hardness, but it should not be too high. Multiple peroxide curing agents can be selected and used together. In addition, adding crosslinking aid can significantly improve the crosslinking density of rubber compound. For example, for DCP curing agent, TAIC, PDM (HVA-2), TAC, PL-400 (TMPTMA) can be added, and of course, sulfur can also be selected as crosslinking aid.

2 Other cooperation

Increasing the amount of carbon black is a classic method to obtain high hardness vulcanizates, but in fact, the amount of carbon black has a certain limit. When the amount of carbon black exceeds 85 phr, it is not obvious to continue to increase the hardness. Generally, it is difficult to obtain vulcanizates with a hardness (Shore A) greater than 90 by using carbon black alone. Even if it can be achieved, the processing performance of the rubber compound is also poor. Plasticizers must be added to the formula, and the processing performance is good after adding plasticizers, However, it is not conducive to increase the hardness of products. At present, the effective stiffening method is to select additives that can participate in the vulcanization reaction or have some chemical interaction with macromolecules to provide some structural factors that can improve the hardness.

2.1 High styrene resin

In general, the styrene content in styrene butadiene rubber is 23.5 ± 1%, and the content of more than 40% is called high styrene rubber (HSR), and the combined styrene 80 ± 10% is styrene butadiene resin. High styrene resin (HSR) is a styrene-butadiene copolymer with a mass fraction of 50% to 90% of styrene. Because of the high content of styrene, HSR has good aging resistance, high hardness and rigidity. The hardness of rubber compound can be improved by blending HSR with rubber. Due to the high content of styrene in its composition, the wear resistance, aging resistance, hardness, rigidity, tear strength and constant elongation strength of its products are greatly improved; Because it contains double bonds, it can be sulfurized with sulfur and has flexibility under high hardness. High-styrene rubber has excellent rheological properties and processing properties. After calendering, the shrinkage rate is small. It can be used as a reinforcing agent for hard rubber plate and rubber floor. Its combined dosage is generally 40 ± 10%.

2.2 Homopolyester

Homopolyester, also known as low polyester, is an oligomer of methacrylate. Homopolyester is a viscous liquid with different viscosities. Its liquid state is a temporary plasticizer during processing, and it also participates in curing reaction during curing, giving vulcanizate a high cross-linking density. The chemical structure of oligoester is characterized by unsaturated double bonds at the molecular end. In the presence of peroxide initiator, oligoester and rubber produce co-crosslinked bonds, which increases the cross-linking density of vulcanizate, thus greatly increasing the constant elongation stress and hardness of vulcanizate. With the increase of the content of double bonds in the oligomer molecule, the number of cross-linked bonds formed will also increase, and its tensile stress and hardness will also increase.

The neat polyester can increase the crosslinking density of the rubber compound, but when the crosslinking density is too high, the mesh chain can not be uniformly loaded, and it is easy to concentrate on the local mesh chain, so that the effective mesh chain number is reduced. Therefore, when the amount of neat polyester is too large, the tensile strength and elongation at break and other properties will be greatly reduced because of the high crosslinking density.

2.3 Unsaturated carboxylate

Unsaturated carboxylate is a newly developed new type of reinforcing filler, which shows different reinforcing effect and characteristics from traditional reinforcing agent carbon black and white carbon black. The general formula of unsaturated carboxylates can be expressed as: Mn+(RCOO -) n, where M is the metal ion with valence n, R is the unsaturated hydrocarbon group β- Sodium, magnesium and zinc salts of phenylacrylic acid, methacrylic acid (MAA), maleic acid (MA), etc. When it is added to rubber as a reinforcing filler, it can participate in the cross-linking reaction during vulcanization due to its double bond, so as to improve the cross-linking density and hardness of rubber compound.

Compared with traditional carbon black reinforcement, unsaturated carboxylate reinforced rubber has the following characteristics:

1. It has high strength in a wide range of hardness;

2. With the increase of the amount of unsaturated carboxylate, the viscosity of the rubber compound has little change and has good processability;

3. At high hardness, it still has high elongation;

4. High elasticity and low heat generation;

5. With the addition of unsaturated carboxylate, the compression set of vulcanizate increased;

6. Good adhesion with metal.

The addition of unsaturated carboxylate will increase the viscosity of the rubber compound, especially the tendency of the rubber compound to stick to the mold, which will cause trouble to the curing process, and its dosage should not be too large.

2.4 Liquid rubber

Liquid rubber is the oligomeric product of various rubbers. Due to its low molecular weight and liquid state, it can be used instead of plasticizer to avoid the decrease of rubber hardness caused by the use of plasticizer. Common products include liquid cis-1,4-polybutadiene rubber, liquid ethylene-propylene rubber, liquid nitrile rubber, etc. Liquid rubber plays a role of softening agent in mixing, vulcanization and other processing processes, improving the processability, and after vulcanization, it participates in the cross-linking reaction as rubber, improving the cross-linking density, which is conducive to improving the hardness of rubber compound.

2.5 Rubber and plastic

The rigidity of plastic is greater than that of rubber. When used with rubber, the stiffness of rubber can be improved, and the hardness of rubber can be improved after vulcanization. Common rubber and plastic blends include EPDM/PP, NBR/PVC, etc. Due to the fact that rubber and plastics cannot produce co-crosslinking bonds during vulcanization, as well as the type of plastics selected, and the difference in mixing ratio and mixing process will cause micro-phase separation and other reasons, although the hardness of the rubber compound is significantly improved, other properties of the rubber compound will be affected to varying degrees, so it is carefully selected according to the product technical requirements.

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