The effect of vulcanization on the structure and properties of rubber

In the production process of rubber products, vulcanization is the last processing step. In this process, the rubber undergoes a series of complex chemical reactions, changing from a linear structure to a body-shaped structure, losing the plasticity of the mixed rubber and having the high elasticity of cross-linked rubber, thereby obtaining excellent physical and mechanical properties, heat resistance The performance, solvent resistance and corrosion resistance improve the use value and application range of rubber products.

Before vulcanization: linear structure, intermolecular interaction by van der Waals force;

Properties: great plasticity, high elongation, and solubility;

During vulcanization: the molecule is initiated, and a chemical cross-linking reaction occurs;

After vulcanization: network structure, intermolecular with chemical bonds;

structure:

(1) chemical bond;

(2) The position of the cross-linking bond;

(3) degree of cross-linking;

(4) Cross-linking;

performance:

(1) Mechanical properties (constant elongation strength. Hardness. Tensile strength. Elongation. Elasticity)

(2) Physical properties

(3) Chemical stability

Changes in the properties of rubber:

Taking natural rubber as an example, with the increase of vulcanization degree:

(1) Changes in mechanical properties

Elasticity. Tear strength. Elongation strength. Tear strength. Hardness increase; elongation. Compression deformation. Fatigue heat generation reduction

(2) Changes in physical properties

The air permeability and water permeability are reduced and cannot be dissolved, only swelling and heat resistance can be improved

(3) Changes in chemical stability

Improved chemical stability

 reason:

a. The cross-linking reaction makes the chemically active groups or atoms no longer exist, making the aging reaction difficult to carry out

b. The network structure hinders the diffusion of low molecules, making it difficult for rubber free radicals to diffuse

Selection and determination of rubber vulcanization conditions:

1. Vulcanization pressure

(1) Pressure needs to be applied when rubber products are vulcanized. The purpose is to:

a. Prevent the rubber from producing bubbles and improve the compactness of the rubber;

b. Make the rubber material flow and fill the mold to make products with clear patterns

c. Improve the adhesion between each layer (adhesive layer and cloth layer or metal layer, cloth layer and cloth layer) in the product, and improve the physical properties (such as flexural resistance) of the vulcanizate.

(2) Generally speaking, the selection of vulcanization pressure should be determined according to the product type, formula, plasticity and other factors.

(3) In principle, the following rules should be followed: the plasticity is large, and the pressure should be smaller; the product thickness, the number of layers, and the complex structure should be larger; the pressure of thin products should be smaller, and even normal pressure can be used

The methods of vulcanization and pressure are as follows:

(1) The hydraulic pump transfers the pressure to the mold through the flat vulcanizer, and then transfers the pressure to the rubber compound from the mold

(2) Directly pressurized by vulcanizing medium (such as steam)

(3) Pressurized by compressed air

(4) Injection by injection machine

2. Vulcanization temperature and curing time

The vulcanization temperature is the most basic condition for the vulcanization reaction. The level of vulcanization temperature can directly affect the vulcanization speed, product quality and economic benefits of the enterprise. High vulcanization temperature, fast vulcanization speed and high production efficiency; otherwise, the production efficiency is low.

Increasing the vulcanization temperature will cause the following problems;

(1) Causes the cracking of the rubber molecular chain and the vulcanization reversion, resulting in a decrease in the mechanical properties of the rubber compound

(2) Reduce the strength of textiles in rubber products

(3) The scorch time of the rubber compound is shortened, the filling time is reduced, and the product is partially lacking in glue.

(4) Because thick products will increase the temperature difference between the inside and outside of the product, resulting in uneven vulcanization