Rubber foam bottom is a bottom fabric made of natural or synthetic rubber with closed or open cell foam materials. Half a century ago, people extensively used this material for sneakers, athletic shoes, travel shoes, release shoes, casual shoes, and more. It has excellent elasticity, good tear resistance, aging resistance, corrosion resistance, electrical insulation and other properties.

The three most basic stages of rubber foaming in internal mixer mixing are: wetting dispersion kneading

The most widely used application of rubber foam is still in shoe soles, followed by automotive tires.

What are the principles and influencing factors of rubber foaming:

The principle of producing rubber sponge by solid rubber foaming is to add foaming agent or auxiliary foaming agent to the selected rubber material. At the vulcanization temperature, the foaming agent decomposes and releases gas, which is surrounded by the rubber material to form foam holes, causing the rubber material to expand and form sponge. The main factors that determine and affect the pore structure are: the gas generation rate of the foaming agent, the diffusion rate of gas in the rubber material, the viscosity of the rubber material, and the vulcanization rate. The most critical factor is the matching of the gas generation rate of the foaming agent, the gas generation rate, and the vulcanization rate of the rubber material.

1. Gas generation and decomposition rate of foaming agent The gas generation of foaming agent refers to the volume of gas released by the complete decomposition of foaming agent per unit mass under standard conditions, measured in mL/g. The decomposition rate of foaming agent refers to the amount of gas released per unit time from the decomposition of a certain mass of foaming agent at a certain temperature. Since the polymer itself does not change the decomposition mechanism of the foaming agent, it is not necessary to measure the gas generation of the foaming agent in the polymer. Usually, the foaming agent is placed in an inert dispersant (such as DOP or mineral oil) at a certain temperature and heated for a period of time to collect the released gas. The relationship curve between the gas volume (converted to the volume under standard conditions) and the heating time is plotted (as shown in Figure 6-1). The decomposition rate of the foaming agent at that temperature can be obtained from the slope of the curve. The total volume of gas completely decomposed is divided by the mass of the foaming agent to obtain the gas generation rate of the foaming agent. Different types of foaming agents have different particle sizes, temperatures, and decomposition rates. In general, foaming agents with low decomposition temperatures have a faster decomposition rate; For the same foaming agent, with small particle size and high temperature, its decomposition rate is fast. The gas generation and decomposition rate of foaming agents affect the size and structure of the foam pores. A large gas generation rate leads to a fast decomposition rate, resulting in larger foam pores and a higher probability of opening. Analysis of the matching between the decomposition rate of foaming agents and the vulcanization rate of rubber materials.

The compatibility between the two affects the generation and structure of bubbles. If there is a significant difference in the decomposition rate of the foaming agent or the vulcanization rate of the rubber material, it cannot be combined and foamed. The decomposition curve of the foaming agent matches the vulcanization curve of the rubber compound, where A is the burning point and D is the positive vulcanization point. Before point A, the rubber material has not yet crosslinked. If the foaming agent decomposes at this time (curve 1), the released gas will easily escape from the low viscosity rubber material.

(AB segment) decomposition (curve 2), due to the cross-linking of the rubber material, the viscosity of the rubber material increases but remains low, and the pore walls are weak and prone to rupture, forming an open pore structure; If foaming occurs in the middle stage of hot vulcanization (BC section) (curve 3), due to the appropriate vulcanization of the rubber material, the viscosity is high, the pore wall is strong and not easy to break, and there are more closed cell structures generated; In the later stage of hot vulcanization (CD stage) foaming (curve 4), the rubber material has mostly cross-linked, with high viscosity. The gas generated by the decomposition of the foaming agent is difficult to diffuse, and is bound by the cross-linked network to form a closed cell structure with small pores; If foaming occurs at or after point D (curve 5), the rubber material has already been fully crosslinked and the viscosity is too high to foam.

To achieve a match between the two and make better rubber products, the key is to choose the type of foaming agent and the vulcanization system of the rubber material. There are two specific methods: one is to select a foaming agent with a decomposition temperature that is suitable for the vulcanization temperature, and then adjust the vulcanization speed of the rubber material based on the decomposition rate of the foaming agent at that vulcanization temperature. If a slow acting accelerator and other accelerators are used in a vulcanization system, the amount of accelerator can be adjusted to adjust the vulcanization speed; The second is to select the type and appropriate particle size of foaming agent based on the vulcanization rate when the vulcanization system is determined. The particle size of foaming agent is also one of the most important factors determining the decomposition rate of foaming agent. The decrease in particle size leads to an increase in the specific surface area of the particles, which improves the thermal conductivity efficiency and accelerates the decomposition rate. Therefore, the balance between the decomposition rate of the foaming agent and the vulcanization rate of the rubber material can be adjusted by selecting the appropriate particle size of the foaming agent. In addition, strict control of the particle size distribution of the foaming agent is the key to obtaining uniformly hooked pores. The average particle size of foaming agent AC is between 2 and 15 microns, with different particle size ranges of foaming agent AC.