Polyurethane flexible foam refers to flexible polyurethane foam, which is a kind of flexible polyurethane foam with certain elasticity. It is the most used polyurethane product in polyurethane products. Polyurethane soft foam is mostly open-cell structure, with low density, good elastic recovery, sound absorption, ventilation, heat preservation and other properties. It is mainly used as cushion material for furniture, mattress, vehicle seat cushion, etc. The soft foam is used as filter material, sound insulation material, shockproof material, decorative material, packaging material and heat insulation material.
Polyurethane flexible foam can be divided into different types according to different classification standards:
A. According to the degree of softness and hardness, that is, the different load-resistant performance, polyurethane flexible foam can be divided into ordinary flexible foam, super-soft foam, high-loading flexible foam and high-resilience flexible foam, among which high-resilience flexible foam and high-loading flexible foam Foam is generally used in the manufacture of seat cushions, mattresses, etc.
B. According to the different production processes, polyurethane soft foam can be divided into block soft foam and molded soft foam. Block soft foam is produced by a continuous process to produce large-volume foam, and then cut it into foam of the desired shape. The molded soft foam is a foam product that directly mixes the raw materials and then injects it into the mold to foam into the desired shape through the gap method.
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 are the effects of the nature of the finished product? The answer will be given below.
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.
In addition, the average functionality of the polyether polyol also affects the properties of the obtained polyurethane flexible foam material. In the case of the same functionality, the larger the molecular weight of the polyether polyol, the lower its reactivity, but the tensile strength, elongation and resilience of the obtained polyurethane flexible foam products are significantly improved; in the equivalent value In the case of the same (molecular weight/functionality), if the functionality of polyether polyol increases, the reactivity will be enhanced, the reaction rate will be relatively accelerated, the degree of cross-linking of the resulting polyurethane will increase, and the hardness of the foam will increase, but the elongation of the material will increase. has declined. Therefore, Luoyang Tianjiang Chemical New Materials Co., Ltd. suggested that the production of polyurethane flexible foam materials should select polyether polyols with an average functionality of more than 2.5. If the average functionality of polyether polyols is too low, the obtained polyurethane foams The recovery after compression is poor.
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.
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)%.
The actual amount of isocyanate=[0.1554×(acid value of polyol polymer+hydroxyl value)+9.667×water%]×isocyanate index. The isocyanate index is usually controlled between 1.03-1.10. When the isocyanate index increases within a certain range, the hardness of the foam increases, but after reaching a certain point, the hardness no longer increases significantly, while the tear strength, tensile strength and elongation all decrease.
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. At the same time, due to the continuous reaction of unreacted TDI, the calorific value is increased, and the exothermic time and curing time are prolonged, sometimes up to several hours. This will keep the center temperature of the foam at a high temperature for a long time, which will easily cause coking and core burning in the center of the polyurethane block.
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 lead to the problem of poor repeatability of the foaming process; in addition, if the isocyanate index is too low, 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
A. 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.
B. Organometallic catalyst: T-19 is generally used as an organotin octoate catalyst; T-19 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 too much, it will lead to too fast gelation speed, increase in viscosity, change in resilience and air permeability, and easily cause closed-cell phenomenon; if the amount of organotin catalyst is too small, it will cause condensation Insufficient glue, resulting in splitting during the foaming process, cracks on the edge or top of the foam, and de-blanking and burrs. 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.
5. Foam stabilizer (silicone oil)
Foam stabilizer is a kind of surfactant, which can make polyurea disperse well in the foaming system, play the role of "physical cross-linking point", and can obviously improve the early viscosity of polyurethane foam mixture and avoid foam cracking.
On the one hand, the foam stabilizer has an emulsification effect, which can enhance the mutual solubility between the components of the foam material. It can also make the air dispersed in the raw material easier to nucleate during the stirring and mixing process, which is helpful for the generation of fine bubbles, adjusting the size of the foam pores, controlling the cell structure, and improving the foaming stability. In addition, it can effectively prevent Problems such as collapse and rupture of the cells make the foam wall elastic and control the pore size and uniformity of the foam. Experts from Luoyang Tianjiang Chemical Industry summarized the functions of foam stabilizers as follows: stabilize the foam in the initial stage of foaming, prevent the foam from merging in the middle stage of foaming, and connect the cells in the later stage of foaming. Generally, the more foaming agent and POP are used, the more silicone oil is used.
If the amount of foam stabilizer is too much, the elasticity of the foam wall will increase in the later stage, and the cells will be fine and not easy to rupture, but it is easy to cause closed cells; if the amount of foam stabilizer is too small, the foam will burst and collapse after starting. Foam, the pores of the foam are large and it is easy to bubble and so on.
6. The influence of temperature
The foaming reaction of polyurethane increases as the temperature of the material increases, which can cause core burning and fire hazards in sensitive formulations. The temperature of the polyol and isocyanate components is generally controlled constant. When foaming, the foam density decreases and the material temperature increases accordingly. The same formula, the same material temperature and high temperature in summer, the reaction speed is accelerated, resulting in a decrease in foam density and hardness, an increase in elongation, and an increase in mechanical strength. In summer, the isocyanate index can be appropriately increased to correct the decrease in hardness.
7. The influence of air humidity
When the humidity increases, the hardness decreases due to the reaction of the isocyanate group in the foam with the moisture in the air, so the amount of isocyanate can be appropriately increased during foaming. If it is too large, it will cause the curing temperature to be too high and cause heartburn.
8. The influence of atmospheric pressure
The atmospheric pressure of the environment during the foaming process will also affect the properties of the obtained polyurethane foam products to a certain extent. The higher the pressure, the higher the density of the finished product; on the contrary, the lower the pressure, the lower the density of the finished product. For example, using the same formulation, foaming at higher altitudes results in a lower density foam product.
Finally, we would like to remind you to pay attention to the following points:
A. In the formation process of foamed plastic products, the gel reaction and the foaming reaction occur at the same time, but there is a competitive relationship between the reactions. In general, the foaming reaction speed is greater than the gelation reaction speed.
Gel Reaction - Carbamate formation reaction (ie reaction of isocyanate groups with hydroxyl groups).
Foaming reaction - refers to the reaction involving water, forming urea and generating bubbles.
Whether polyurethane foam has an ideal open-cell or closed-cell structure mainly depends on whether the gel speed and gas expansion speed are balanced during foam formation. This balance can be achieved by adjusting the types and amounts of tertiary amine catalysts and foam stabilizers in the formulation.
B. The number of bubbles formed in the foaming system and the size of the cells in the foam depend on the effect of the external nucleating agent. The more the nucleating agent, the more bubbles are generated, and the smaller the cells are.
Nucleating agent is a substance that can cause bubble formation, such as fine solid particles in the system, liquid, foam stabilizer or fine bubbles originally dissolved in the material, including air or nitrogen dissolved in polyols and isocyanates, Carbon dioxide, foam stabilizer, carbon black and other fillers. These substances can cause the gas to generate more bubbles in the material, and the more stable the bubbles, the finer the pores.
C. The length of the milking time will also affect the properties of the finished polyurethane foam to a certain extent. The longer the milking time is, the more conducive to the growth of large bubbles. Therefore, in order to reduce the generation of large bubbles, the amount of catalyst can be appropriately increased, which can shorten the milking time, and fine-cell foam can be obtained due to the competition between the gel reaction and the bubble formation reaction.
