As we all know, thermoplastic polyurethane materials TPU due to high wear resistance, high elasticity, fatigue resistance, chemical resistance, etc. Many advantages, so in many industries, a wide range of applications, including the sole market is particularly impressive. However, the TPU sole also has some shortcomings. How to improve the shortcomings of the TPU sole and make the TPU sole have many advantages and advantages, which has become a major research topic in the current TPU sole industry.
For TPU foam material, the foaming system is the same as the general EVA foaming system, which is formed by peroxide bridge and then foamed with chemical foaming agent. A foamed material of independent foam. The timing of the bridging and the decomposition of the blowing agent are the most important. The viscosity of each rubber material can be processed after heat melting. In the foaming theory, the faster the bubble expands, the shorter the stabilization time. At this time, the control of the hot melt viscosity of the rubber material is extremely important. If it is too high, it will not foam. If it is too low, the gas will overflow from the plastic.
When the TPU foaming material is foamed outside the mold, the bridging degree is linear with the melt viscosity of the colloid. If the foaming is earlier than the sulfurization (bridge) Too much, the surface of the product may be easily discharged due to the discharge of gas or even cause the foamed product to rupture. If the addition of sulfur occurs too early than the foaming, the viscosity of the colloidal bridge will increase, so that the foaming is not easily performed, and the foaming is insufficient. Therefore, for TPU foaming, the combination of foaming rate and bridging rate is extremely important. It is recommended to present the state of foaming after bridging, but the bridging and foaming time should not be too large. The finished product has the best performance.
General rubber plastic foam molding can be roughly divided into:
Mechanical foaming method:The mechanical foaming method is to make the gas evenly mixed into the rubber by means of the strong stirring of the machine. Bubbles are formed in the plastic melt, and the bubbles generated in this process are easily lost. Therefore, a bubble stabilizer is required for mechanical foaming.
Physical foaming method: A large amount of bubbles are formed by the change of the physical state of the foaming agent in the resin. The general physical foaming agent can be divided into: an inert gas system, a low boiling point liquid system, etc. Currently, a physical foaming agent of a low boiling point liquid system is generally used.
Chemical foaming method: Chemical foaming agent undergoes chemical changes in heat, which decomposes and produces gas, which promotes rubber plastics. Polymer foaming. Representative chemical blowing agents include azo compounds (ADCA, AZDN), nitroso compounds (Nitriso), inorganic compounds (sodium bicarbonate, sodium carbonate) and hydrazines (OBSH, TSH, BSH). The foaming agent can be used alone or in combination.
When using chemical blowing agents, it should be noted that it is suitable for non-crystalline polymers. The decomposition temperature of the blowing agent should be higher than the polymer temperature. °C or so. If the decomposition temperature is too high, the activator (zinc oxide, urea, etc.) can be used to lower the decomposition temperature of the blowing agent. For crystalline polymers, the decomposition temperature of the blowing agent should be about 10 ° C higher than the activation temperature of the crosslinking agent. Decomposition temperature and gas generation are the main basis for the selection of chemical blowing agents.
The traditional foaming method is mostly compression molding (flat foaming). Due to the cumbersome process, it has been developed into injection foam molding at this stage. Although the level of injection foam molding technology is relatively high, considering the economic benefits (about 45% efficiency) and mold design, it will greatly reduce manufacturing costs, product design and functionalization, so it has been A large number of traditional molding foam molding technology has been replaced. In addition, TPU foaming materials are highly compatible with various rubbers and plastics, and can be formulated with different characteristics and functions depending on the formulation of the substrate. This is also the main advantage of the existing foaming materials. One.