1、 Basic reaction and data
The formation of polyurethane soft foam includes 2 basic reactions: foaming reaction and polymerization reaction (also called gel reaction).
Foaming reaction: the reaction of isocyanate and water to produce double-substituted urea and carbon dioxide. The reaction formula is as follows.
2R-N=C=O+HOH→R-NH-CO-NH-R+CO2↑
If TDI is used, according to the above reaction equation, it is concluded that 1 part of water reaction requires 9.67 parts (about 9.7 parts of TDI)
The carbon dioxide released becomes the nucleus of the bubble, which expands the reaction mixture to obtain a foam with an open-hole structure.
Polymerization reaction: The hydroxyl group in the polyether reacts with the isocyanate in a stepwise polymerization reaction to form a carbamate. The reaction formula is as follows.
R=N=C=O+R′-OH→R-NH-COO-R′
If you use TDI, according to the above reaction formula, it is concluded that 100 parts of hydroxyl value 56 of polyether reaction requires 8.68 parts (about 8.7 parts of TDI)
2、Polyol
Domestic block bubble production using 3 functional degrees, molecular weight 3000 (hydroxyl value 56) or 3500 (hydroxyl value 48, with less) of soft foam polyether. Such as my company’s JH-3031K, JH-3050D, JH-3500, etc.
3、Multi-isocyanate
The main use of toluene diisocyanate (TDI), TDI industrial products are mainly three kinds: pure 2, 4TDI (or TDI100), TDI80/20, TDI65/35. TDI80/20 production costs are the lowest, is the most widely used in industrial use of the largest variety.
TDI molecular weight of 174, which contains two isocyanate group (-N = C = O) molecular weight of 84, so the TDI isocyanate content of 48.28%.
The amount of TDI has a great influence on the bubble performance, in the foam formulation, the degree of TDI excess is expressed by isocyanate index, isocyanate index is the ratio of the actual amount and the theoretical calculation of the amount. In the production of soft foam, the general index is 105-115 (100 is equal to the theoretical calculation) (or write 1.05-1.15, 1 is the theoretical calculation), in this range, the TDI index increases, the foam hardness increases, tear strength becomes smaller, tensile strength becomes smaller, elongation at break becomes smaller. If the TDI index is too high, large holes and closed holes will be formed, and the maturation time is too long, which will also cause foam heartburn; if the TDI index is too low, cracks will be easily produced, poor resilience, poor strength and large permanent deformation in compression.
4、Foaming agent
The reaction between water and TDI to produce carbon dioxide is the main blowing agent used in soft foam, increasing the amount of water in the formula will increase the content of urea, increase the hardness of foam, reduce the density of foam, and reduce the load capacity of foam. But TDI reacts with water to produce large amount of heat, and the amount of water is too high, which will make the foam appear to be heartburn or burning.
Dichloromethane is a physical foaming agent, its boiling point 39.8 ℃, is a non-flammable gas, can be vaporized in the foam, reduce the foam density, hardness also decreased. Add the amount of methylene chloride, that is, to prevent the phenomenon of heartburn in the foam, but also to ensure that not too much methylene chloride and too much heat away on behalf of the impact of foam maturation, but also to ensure that the physical properties of the foam is not significantly reduced, so the amount of methylene chloride has a limit.
The foaming efficiency of water and methylene chloride (i.e., the same volume of gas produced) has the following empirical relationship.
1 part water = 9 parts methylene chloride
Foaming index: that is, the formula, based on 100 parts of polyether ratio, the number of blowing agents used.
Foaming index = water + methylene chloride / 9
The approximate relationship between the foaming index and density is 100/foaming index, which is the formula when no filler is added to the formula. When adding filler, the density is larger than that calculated by this formula.
5、Catalyst
The main role of catalyst is to regulate the speed of foaming reaction and gel reaction, so that it is in a good balance.
Triethylene diamine (A33, 33% of the content of dipropylene glycol or propylene glycol solution) is the most important tertiary amine catalyst agent in the production of soft foam, it has 60% efficacy for promoting the reaction between isocyanate and water, that is, the foaming reaction, 40% for promoting the reaction between hydroxyl and isocyanate, that is, the gel reaction.
Bis(2,2-dimethylaminoethyl) ether (A-1) is a general-purpose tertiary amine catalyst for soft foam. 80% of its efficacy is used to promote the foaming reaction and 20% is used to promote the gelation reaction. It is mostly used in combination with triethylenediamine.
Improper amount of amine catalyst has a great impact on the products, too much amine will cause (1) short reaction time, the starting viscosity increases too fast, foaming is smoking big. (2) foam cracking. Too little amine dosage will cause the starting speed to be too slow and affect the foaming height.
Stannous octanoate is the most commonly used organotin catalyst, it is extremely easy to hydrolyze and oxidize in the polyether mixture containing water and tertiary amine catalyst.
The lower the density of the foam, the narrower the adjustable range of stannous octanoate, the tin dosage affects the foam as follows.
Too little: foam cracking.
Too much: the viscosity increases quickly, the foam forms closed pores and shrinks, forming bottom skin and side skin.
6、Foam stabilizer (also called silicone oil)
Foam stabilizer can reduce the surface tension of the foam system mixture, so as to stabilize the bubble hole, prevent the foam from collapsing, and control the size and uniformity of the throwing hollow.
The amount of silicone oil from the minimum amount to the appropriate degree, you can get a good open-hole foam. When the dosage is high the foam closed pore rate increases. Its dosage and density relationship is as follows.
Foam density (Kg / cubic meter) 100 parts of polyether silicone oil dosage
35 0.7-1.0
25 0.8-1.2
20 1.0-1.5
Less than 20 1.5-2.0
7、Other influencing factors
In addition to the formula, the process parameters, environment, etc. have some influence on the foam performance.
Raw material temperature: in a more normal ambient temperature (20-28 degrees), the raw material temperature control at 25 ± 3 degrees, preferably within plus or minus 1 degree. There is also control in the range of 28-30 degrees.
The effect of increasing or decreasing raw material temperature on the rate of foaming and gelation reaction is not the same. The increase in temperature increases the value of polymerization reaction much more than the increase in foaming reaction. The catalyst has to be adjusted for temperature changes.
For the same recipe, using the same amount of foaming agent, the foam density is also related to the altitude, which is significantly lower when in high altitude areas.
