1. Molecular structure of rubber: Rubber also belongs to polymer compounds, with a relative molecular weight of several hundred thousand. The molecular size of polymer compounds is not fixed, and the relative molecular weight usually refers to their average relative molecular weight. The relative molecular weight has a significant impact on the strength of rubber. Generally, only when the molecule is large enough to show a certain level of strength, the aging resistance of polymers decreases with the increase of polymerization degree (an indicator of polymer molecular size). However, during the plastic refining process, rubber molecules will break and the relative molecular weight will decrease, which can increase plasticity and improve processing conditions.
From the geometric shape of rubber molecular chains, they can be divided into three types: linear, branched, and cross-linked (also known as body type). Different geometric shapes have completely different physical properties. Rubber has strong flexibility because the carbon atoms in the C-C chain of rubber molecules can rotate around chemical bonds.
2. The aggregation state of rubber refers to the state in which rubber molecules gather together. Aggregated states can be divided into three states: solid, liquid, and gas. Rubber has liquid properties because its molecular segments can move more freely like low molecular liquids. Rubber also has solid properties because, from the perspective of the entire macromolecular chain, its molecular chains cannot be relatively displaced and can maintain a certain shape and greater strength. Rubber also has gas properties, such as an increase in elastic modulus with temperature and heat generation during stretching, just like heat generation during gas compression due to energy conversion.
3. The viscoelasticity of rubber is characterized by good elasticity. In addition, it also exhibits certain characteristics of viscous liquids, mainly manifested in the linear development of deformation over time, just like viscous liquids, after being subjected to force. Therefore, rubber is considered a viscoelastic material, resulting in a series of viscoelastic phenomena such as creep, stress relaxation, and internal friction.
4. Although rubber has high elasticity, its forming and processing, such as molding, mixing, extrusion, or rolling, all require deformation in a flowing state. Therefore, the forming process inevitably involves the fluidity of rubber. Viscosity is an important parameter that characterizes the fluidity of liquids. The viscosity of rubber is different from that of plastic. Plastic can greatly reduce the viscosity of the melt by increasing temperature, while the viscosity of rubber is less affected by temperature and mainly depends on the relative molecular weight. Reducing the relative molecular weight can reduce viscosity and elasticity, which will be beneficial for the molding and processing of rubber.
5. The vulcanization of rubber occurs under heating conditions, where the raw rubber in the rubber compound undergoes a chemical reaction with the vulcanizing agent. The cross-linking process of the rubber macromolecules from a linear structure to a three-dimensional network structure is called vulcanization, and the vulcanized rubber is called cured rubber. After vulcanization, the physical, mechanical, and other properties of the rubber material have been significantly improved.
The changes in rubber properties during the vulcanization process are the result of changes in molecular structure. Unvulcanized raw rubber is a linear structured macromolecule with independent molecular chains that exhibit high plasticity, high elongation, and solubility. After vulcanization, rubber macromolecules generate transverse chains between molecular chains to form a spatial network structure. Therefore, in addition to the secondary bond force between molecules, there is also a primary bond force at the junction of molecules. Therefore, cured rubber has higher tensile strength, lower elongation, and greater elasticity than raw rubber. In the production of rubber products, vulcanization is the last processing step.
With the development of production, the concept of vulcanization has also made new progress. Vulcanizing agents and high temperature are no longer necessary conditions for vulcanization. Some special rubber materials can be vulcanized at lower temperatures, or even at room temperature. It is also possible to use physical methods without adding vulcanizing agents in the rubber material, such as using γ X-rays) for crosslinking.
