Catalyst
Amine: generally use A33 its role is to promote the reaction of isocyanate and water, adjust the foam density, bubble opening rate, etc., mainly to promote the foaming reaction.
More amine: the foam products appear split, there are holes or bubble eyes in the foam.
Less amine: the foam appears shrinkage, closed cell, and the bottom of the issued foam products is thick.
Tin: generally use stannous octoate T-9; T-19 is a gel reaction catalyst with high catalytic activity, mainly to promote gel reaction, i.e. late reaction.
Tin more: gelling fast, viscosity increases, poor resilience, poor permeability, resulting in closed hole phenomenon. If appropriately increase its dosage can get loose good open-cell foam, further increase the dosage so that the foam gradually become tight, so that shrinkage, closed-cell.
Tin less: insufficient gel, the foaming process causes splitting. There is cracking at the edge or top, and dehiscence, burr phenomenon.
Reducing the amine or increasing the tin can increase the polymer bubble film wall strength when gassing occurs in large quantities, thus reducing hollowing or cracking.
Whether a polyurethane foam has a desirable open or closed cell structure depends primarily on the balance between the rate of gel reaction and the rate of gas] expansion during foam formation. This balance can be achieved by adjusting the type and amount of tertiary amine catalysts and additives such as foam stabilizers and other additives in the formulation.
Foam stabilizers (silicone oils)
Foam stabilizer is a kind of surfactant, which can make the polyurea well dispersed in the foaming system, play the role of “physical cross-linking point”, and can obviously improve the early viscosity of the foam mixture to avoid cracking foam. On the one hand, it has emulsifying effect, so that the mutual solubility between the components of the foam material is enhanced, on the other hand, after adding silicone surfactant, it can reduce the surface tension of the liquid r, the free energy required to increase the gas dispersion is reduced, so that the air dispersed in the raw material is easier to nucleate in the process of mixing and stirring, which can help the generation of small bubbles, adjust the size of foam pores, control the structure of the pores, and improve the stability of foaming; prevent the deflation and rupture of pores, and avoid the cracking of bubbles. It can adjust the size of foam pores, control the structure of foam pores, and improve the stability of foaming; prevent the foam pores from deflating and rupturing, make the foam wall elastic, and control the pore size and uniformity of foam. It stabilizes the foam at the early stage of foaming, prevents the foam from bubbling in the middle of foaming, and connects the foam holes at the late stage of foaming. Generally speaking, the more foaming agent and POP dosage, the more silicone oil dosage.
More dosage: make the late foam wall elasticity increase, should not rupture, bubble hole fine. Cause closed cell.
Low dosage: foam rupture, collapse bubble after foaming, larger pore size, easy and bubble.
Influence of temperature
The foaming reaction of polyurethane is accelerated with the rise of material temperature, which will cause the danger of core burning and fire in sensitive formulations. Generally control the temperature of polyol and isocyanate components remain unchanged. When foaming foam density decreases material temperature increases accordingly. The same formula, material temperature is the same summer temperature, reaction speed accelerated, resulting in a decrease in foam density, hardness, elongation increased, mechanical strength increased. In summer, the TDI index can be increased appropriately to correct the decline in hardness.
Influence of air humidity
Humidity increases, due to the isocyanate base part of the foam reacts with the moisture in the air, resulting in a decrease in hardness, so the TDI dosage can be increased appropriately when foaming. When it is too large, it will cause high maturation temperature causing heartburn.
Influence of atmospheric pressure
The same formula, foaming at high altitude, the density of foam products is small.
Note:
In the process of foam formation, gel reaction and foaming reaction occur at the same time, but there is competition between the reactions.
reactions compete with each other, and generally the speed of the foaming reaction is greater than the speed of the gel reaction.
Gel reaction – the formation reaction of urethane, (reaction with -OH)
Foaming reaction – refers to a reaction involving water to form urea and produce gas bubbles
Nucleating agents – substances that cause the formation of bubbles, such as fine solid particles in the system, liquids
Foam stabilizers or fine bubbles originally dissolved in the material, etc.; including air or nitrogen, carbon dioxide, foam stabilizers, carbon black and other fillers dissolved in polyols and isocyanates. However, the gas in the material to produce more bubbles; stabilization and the more the generation of bubble holes will be finer.
The number of bubbles formed in the foaming system and the size of the bubble holes in the foam depends on the role of the addition of nucleating agents; more nucleating agents, more bubbles, smaller bubble holes.
When the temperature rises, the solubility of the gas in the liquid decreases, thus more bubbles are formed or the previous start grows. Longer emulsion time favors the growth of large bubbles.
Increasing the amount of catalyst shortens the emulsification time, and fine-cell foams can be obtained due to the competing reactions of gelation and bubble formation.
Whether the foam has a desirable open or closed cell structure depends mainly on the balance between the gelation rate and the gas expansion rate during foam formation. This balance can be achieved by adjusting the type and amount of tertiary amine catalysts and additives such as foam stabilizers in the formulation.
Translated with DeepL.com (free version)
