Technology | What Factors are related to the Properties of Polyurethane Flexible Foam

Why are there so many types of flexible polyurethane foams and so many applications? This is due to the variety of production raw materials, so that the properties of the flexible polyurethane foams made are also different. Then, the raw materials used for flexible polyurethane foams What effect does the nature of the finished product have?

1. Polyether polyol

As the main raw material for producing flexible polyurethane foam, polyether polyol reacts with isocyanate to form urethane, which is the skeleton reaction of foam products. If the amount of polyether polyol is increased, the amount of other raw materials (isocyanate, water and catalyst, etc.) is reduced, which is easy to cause cracking or collapse of the polyurethane flexible foam products. If the amount of polyether polyol is reduced, the obtained flexible polyurethane foam product will be hard and the elasticity will be reduced, and the hand feeling will be bad.

2. Foaming agent

Generally, only water (chemical foaming agent) is used as the foaming agent in the manufacture of polyurethane blocks with a density greater than 21g/cm3, and low boiling points such as methylene chloride (MC) are used in low-density formulations or ultra-soft formulations. Compounds (physical blowing agents) act as auxiliary blowing agents.

As a blowing agent, water reacts with isocyanate to form urea bonds and release a large amount of CO2 and heat. This reaction is a chain extension reaction. The more water, the lower the foam density and the stronger the hardness. At the same time, the cell pillars become smaller and weaker, which reduces the bearing capacity, and is prone to collapse and cracking. In addition, the consumption of isocyanate increases, and the heat release increases. It is easy to cause core burning. If the amount of water exceeds 5.0 parts, a physical foaming agent must be added to absorb part of the heat and avoid core burning. When the amount of water is reduced, the amount of catalyst is correspondingly reduced, but the density of the obtained flexible polyurethane foam is increased.

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Auxiliary blowing agent will reduce the density and hardness of polyurethane flexible foam. Since the auxiliary blowing agent absorbs part of the reaction heat during gasification, the curing rate is slowed down, so it is necessary to appropriately increase the amount of catalyst; at the same time, because the gasification absorbs part of the heat, the danger of core burning is avoided.

3. Toluene diisocyanate

Polyurethane flexible foam generally chooses T80, that is, a mixture of two isomers of 2,4-TDI and 2,6-TDI with a ratio of (80±2)% and (20±2)%.

When the isocyanate index is too high, the surface will be sticky for a long time, the compressive modulus of the foam body will increase, the foam network structure will be coarse, the closed cell will increase, the rebound rate will decrease, and sometimes the product will crack.

If the isocyanate index is too low, the mechanical strength and resilience of the foam will be reduced, so that the foam is prone to fine cracks, which will eventually cause the problem of poor repeatability of the foaming process; in addition, if the isocyanate index is too low, it will also It will make the compression set of the polyurethane foam larger, and the surface of the foam is prone to feel wet.

4. Catalyst

1. Tertiary amine catalyst: A33 (triethylenediamine solution with a mass fraction of 33%) is generally used, and its function is to promote the reaction of isocyanate and water, adjust the density of the foam and the opening rate of the bubble, etc., mainly to promote foaming reaction.

If the amount of tertiary amine catalyst is too much, it will cause the polyurethane foam products to split, and there will be pores or bubbles in the foam; if the amount of tertiary amine catalyst is too small, the resulting polyurethane foam will shrink , closed cells, and will make the foam product bottom thick.

2. Organometallic catalyst: T-9 is generally used as an organotin octoate catalyst; T-9 is a gel reaction catalyst with high catalytic activity, and its main function is to promote the gel reaction, that is, the later reaction.

If the amount of organotin catalyst is appropriately increased, a good open-cell polyurethane foam can be obtained. Further increasing the amount of organotin catalyst will make the foam gradually tighter, resulting in shrinkage and closed cells.

Reducing the amount of tertiary amine catalyst or increasing the amount of organotin catalyst can increase the strength of the polymer bubble film wall when a large amount of gas is generated, thereby reducing the phenomenon of hollowing or cracking.

Whether the polyurethane foam has an ideal open-cell or closed-cell structure mainly depends on whether the gel reaction speed and the gas expansion speed are balanced during the formation of the polyurethane foam. This balance can be achieved by adjusting the type and amount of tertiary amine catalyst catalysis and foam stabilization and other auxiliary agents in the formulation.

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