Secondary Vulcanisation
Role: When silicone rubber is vulcanised with peroxide, the decomposition of peroxide triggers the reaction of polymer, which generates low molecular compounds (e.g. benzene and benzoic acid, etc.) present in the rubber that will affect the mechanical properties of the rubber. Secondary vulcanisation can improve the silicone rubber in the first stage of heating after moulding, its crosslink density is not enough, to make it further vulcanisation reaction to increase the density of silicone rubber. Tensile strength, resilience, hardness, degree of swelling, density, thermal stability, mechanical properties and compression permanent deformation properties are greater than the first vulcanisation has improved.
The purpose of secondary vulcanisation is to further cross-link the rubber products, improve the mechanical properties and compression permanent deformation properties of rubber products;
If the secondary vulcanisation is not carried out, the performance of the silicone rubber may be affected to some extent, and better products may not be obtained. The parameters of primary vulcanisation and secondary vulcanisation may be different, which is related to the actual operation!
Rubber one-stage vulcanisation is mainly to make the products get shaped, and then the half vulcanisation state and get shaped products are concentrated for two-stage vulcanisation. It shortens the time of vulcanisation and improves the production efficiency of the vulcanising machine.
Secondary vulcanisation, also called post vulcanisation or two-stage vulcanisation, refers to the rubber products to reach a certain degree of vulcanisation, remove the heat source to continue to heat the vulcanisation process.
Broadly speaking, rubber products vulcanised to a certain extent, even if not heated, it can still be in the role of residual heat, or in the process of product storage, continue to deepen the vulcanisation process, or in the dynamic role of the phenomenon of crosslinking continues. Of course, it also includes the process of continuing to heat for the second stage of vulcanisation;
The purpose of secondary vulcanisation is to further cross-linking of rubber products, improve the mechanical properties of rubber products and compression permanent deformation properties;
The method of secondary vulcanisation is oven hot air vulcanisation or vulcanising tank hot air secondary vulcanisation;
At present, many rubber seals or rubber products production units using secondary vulcanisation technology to produce rubber sealing products, technology is also changing with the times, constantly updated, constantly being optimised!
Commonly used in peroxide vulcanisation of nitrile products, peroxide vulcanisation of ethylene propylene products, acrylates, hydrogenated nitrile, silicone rubber, fluorine rubber and fluorosilicone rubber vulcanisation.
Rubber products in the selection of peroxide vulcanisation, peroxide differentiation triggered polymer reaction, the generation of low molecular compounds present in the rubber will affect the mechanical function of rubber. Not to mention the rubber (peroxide) in the first stage of heating after moulding, its crosslinking density is not, to make it further vulcanisation reaction to add density of rubber products. Strength, resilience, hardness, degree of swelling, density, thermal stability are greater than the first vulcanisation has improved. If the second vulcanisation is not carried out, perhaps the production of rubber function has been affected to a certain extent, do not get very good function of the goods, the parameters of the first vulcanisation and the parameters of the second vulcanisation can be different, which is related to the actual operation.
In the secondary vulcanisation, peroxide has actually been very little, mainly by the role of oxygen to increase cross-linking. Therefore, the oven should be equipped with a blower, and the rubber should not be stacked too close.
Fluorine rubber vulcanisation of a section of the release of low molecular substances such as HF, will hinder the rubber crosslinking, corrosion of metal parts, so also need a second vulcanisation. In addition, the fluorine rubber may also occur cyclisation reaction to produce fluorine-containing base ring, so that its heat resistance is further improved.
Natural rubber (NR) vulcanisation temperature recommendations: the most reasonable range: 140 ~ 150 ℃, the maximum does not exceed 160 ℃;
Curing temperature recommendation for cis-butadiene rubber (BR), isoprene rubber (IR) and chloroprene rubber (CR): most reasonable range: 150～160℃, the highest not exceeding 170℃;
Styrene-butadiene rubber (SBR), nitrile rubber (NBR) vulcanisation temperature recommendations: vulcanisation temperature can be used above 150 ℃, but the maximum does not exceed 190 ℃;
Butyl rubber (IIR), ethylene propylene diene rubber (EPDM) vulcanisation temperature recommendations: the most reasonable range of 160 ~ 180 ℃, the maximum does not exceed 200 ℃;
Hot vulcanisation of silicone rubber (SR), fluorine rubber (FR) vulcanisation temperature recommendations: the most reasonable range of 170 ~ 180 ℃, if the need for secondary vulcanisation, the secondary vulcanisation temperature control in 200 ~ 230 ℃, the time according to the requirements of the process to control the process in the range of 4 ~ 24H or less. The secondary vulcanisation process below 5mm thickness formulated by a manufacturer (for reference only)
Rubber secondary vulcanisation common problems
First, rubber products cracking bubbles product reasons: volatile components discharged too quickly or shaped vulcanised products hidden inside the air Solution: 1, adjust the gradual heating conditions, slow down the heating rate 2, check the refining of the rubber, remove air bubbles
Second, the compression deformation of the causes: insufficient vulcanisation, rubber material formula selection of improper solutions: 1, into the oven to extend the vulcanisation time 2, increase air circulation 3, reduce the amount of products into the 4, check the oven temperature 5, the selection of compression deformation of the rubber material.
Third, the hardness is too high cause: baking heating vulcanisation excessive solution: 1, check the temperature of each part of the oven 2, check and inspect the temperature control system
Fourth, the hardness is too low cause: volatile gases are not excluded from the oven Solution: 1, check the ventilation in the oven 2, check the oven temperature 3, reduce the number of silicone rubber products into the number of
Fifth, sticky with black spots: product causes: volatile gases local concentration of rubber degradation
Solution:
1. Slow down the heating speed, timely discharge of volatiles
2. Check the air circulation
3. Extend the vulcanisation time to fully eliminate volatile gases.
Primary vulcanisation, secondary vulcanisation and tertiary vulcanisation refer to the three stages of vulcanisation.
1. Fluidity: the primary vulcanisation compound has good fluidity, the secondary vulcanisation compound has relatively poor fluidity, and the tertiary vulcanisation compound begins to cross-link and lose fluidity.
2. Degree of cross-linking: the degree of cross-linking of the primary vulcanisation is low, the degree of cross-linking of the secondary vulcanisation is relatively general, and the degree of cross-linking of the tertiary vulcanisation reaches the best.
3. Physical properties: the physical properties of the first vulcanisation are weak, the physical properties of the second vulcanisation gradually become better, and the third vulcanisation reaches a comprehensive balance of properties.