Gel, also known as jelly, refers to a transparent jelly obtained by gelation of a lyophilic sol and a sputum sol. A typical example is gelatin. Although a large amount of liquid is present inside the gel, fluidity is lost. The so-called gel in rubber refers to a solvent-insoluble jelly present in the solid dry glue. However, this situation is limited to synthetic rubber, but not in natural rubber. When the gel-containing raw rubber is masticated, the frozen material is converted into an elastic body having a network structure, so that various compounding agents cannot enter, so that subsequent processing cannot be performed.
(1) Reasons for gel formation
Gels are more common in copolymerized synthetic rubbers which are butadiene-based, such as SBR.NBR, etc., but are substantially absent in natural rubber. Gels generally occur during the polymerization of monomers in the synthesis of synthetic rubber. Taking styrene-butadiene rubber as an example, it is limited to a thermal polymerization process with a polymerization temperature of 50 ° C, and low-temperature polymerization at 5 ° C means that no gel exists. There are two reasons for the gel:
1 Higher polymerization temperature The polymerization accelerates as the polymerization temperature increases, while the trans-1,4 structure decreases sharply, and the network structure increases greatly, causing gelation. At lower temperatures, the opposite is true. For example, in the polymerization of styrene-butadiene rubber, when the polymerization temperature is raised from 5 ° C ± 50 ° C (that is, from low-temperature polymerization to high-temperature polymerization), the branch reaction rate Accelerate, while the linear reaction slows down, speeding up gel formation.
2 Monomer conversion rate control improper When the polymerization reaches a certain conversion rate, it will promote the disproportionation and cross-linking reaction in the system to predominate, so that the gel content and Mooney viscosity increase. In summary, monomer conversion exceeds the appropriate range and can also result in gel formation.
(2) Adverse effects of gel on rubber properties
1 brings difficulties to mastication
As the amount of gel increases, the Mooney viscosity of the green rubber also increases. The heat-polymerized styrene-butadiene rubber has a rubbery Mooney viscosity of usually more than 100, and the frictional heat generation is also sharply increased, making the mastication and subsequent processes difficult.
2 Due to the three-dimensional network structure formed by the branching in the raw rubber, the portion occupied by the gel becomes a closed region. Various compounding agents cannot enter even under the action of mechanical shearing force, forming a blank area. That is to say, the part of the gel that is dominant can neither be vulcanized nor protected by a protective system.
1 The use of thermoplastics is a measure used to deal with hot polystyrene-butadiene rubber gels in the early 1960s, that is, the action of chemical chain scission to release the gel under the action of heat/oxygen. Although it was a cure, it was an effective response at the time. Specific practices include:
a. First use a crusher to cut the hot butyl benzene raw rubber into a wire and put it in the dish.
b. The tray is sent to the heating tank, and the chemical chain breaking is performed under the action of heat/oxygen for 550-165 ° C and 0.5-0.6 MPa high temperature and high pressure for 50 min.
c. After the cans are cooled, the tablets are crushed.
1 Polymerization technology is improved, the polymerization temperature is controlled at 5 ° C, and the gel is solved from the source. This is the general soft styrene butadiene rubber.