What is rubber vulcanisation?
Vulcanisation is commonly used throughout the rubber industry and occupies an important position in rubber chemistry. Vulcanisation of rubber is the transformation of plastic raw rubber into an elastic and dimensionally stable product through chemical cross-linking between rubber molecules. The physical properties of the vulcanised rubber are stable and the temperature range is extended. The ability to vulcanise (cross-link) rubber molecular chains depend on their structure.
Unsaturated diene rubbers (e.g. natural rubber, SBR and NBR) contain unsaturated double bonds in their molecular chains and can be cross-linked with sulfur-yellow, phenolic resins, organic peroxides, etc. by substitution or addition reactions. Saturated rubbers are generally cross-linked with free radicals of a certain energy (e.g. organic peroxides) and high-energy radiation, etc. Rubber containing special functional groups (e.g. chlorosulphonated polyethylene, etc.) is cross-linked by specific reactions of various functional groups with established substances, e.g. the sulphonamide group in rubber is cross-linked by reaction with metal oxides and amines.
Rubber vulcanisation principle
Rubber becomes soft when heated, hard and brittle when cold, not easy to shape, easy to wear, easily soluble in organic solvents such as petrol, with double bonds within the molecule, easy to start addition reactions, easy to age.
In order to improve the performance of rubber products, the production of raw rubber to carry out a series of processing processes. Under certain conditions, the raw rubber in the rubber material and the vulcanising agent chemical reaction, so that the linear structure of the macromolecules cross-linked into a three-dimensional network structure. Thus, the rubber material has high strength, high elasticity, high wear resistance, corrosion resistance and other excellent properties. This process is called rubber vulcanisation.
The vulcanisation process is generally divided into four stages, induction – pre-cure – normal cure – over cure. To achieve this reaction, energy must be applied to bring it up to a certain vulcanisation temperature, and then the rubber is held within that vulcanisation temperature range to complete the entire vulcanisation reaction.
Factors affecting vulcanisation: vulcanisation pressure, vulcanisation temperature, vulcanisation time.
1、Vulcanisation pressure
To fill the mould with rubber, prevent bubbles or lack of rubber; to improve the denseness of the rubber; to enhance the adhesion strength of the rubber to the fabric or metal; to help improve the physical and mechanical properties of the rubber (e.g. tensile, abrasion resistance, flexural resistance, aging resistance, etc.).
It is usually decided according to the plasticity of the compounding rubber and the specific conditions of the specimen (product) structure.
2、Vulcanization temperature
The vulcanization temperature directly affects the speed of vulcanization reaction and the quality of vulcanization. The vulcanisation temperature can be increased to speed up the vulcanisation process, but the high temperature can easily cause the molecular chain of the rubber to crack, resulting in a reduction in the physical and mechanical properties of the vulcanisation, so the vulcanisation temperature should not be too high.
The suitable vulcanization temperature should be based on the rubber formula, which mainly depends on the type of rubber and vulcanization system.
3、Vulcanisation time
Vulcanisation time is determined by the rubber formula and the vulcanisation temperature. For a given rubber material, in a certain vulcanisation temperature and pressure conditions, there is a most suitable vulcanisation time.
The choice of suitable vulcanisation time can be determined by the vulcanisation instrument (process positive vulcanisation time).