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Method of molding rigid polyurethane foams
Updated: 2013-10-22 14:40 Source: PUWORLD share:

The present invention relates to a method of molding rigid polyurethane foam. Polyurethane foam molding are conventionally manufactured by introducing a polyurethane reactive mixture containing a blowing agent into a mold cavity, the blowing agent being released in the course of the polyaddition reaction between the isocyanate and isocyanate-reactive components in the mixture, causing the reactive mixture to foam and fill the cavity.

A physical blowing agent is typically used in the production of the foam. The physical blowing agents used are all greenhouse gases to a greater or lesser extent. The fluorinated hydrocarbons are more damaging greenhouse gases than the hydrocarbons. The physical blowing agents may also be flammable, and therefore require specialist equipment in the factory to limit the risk of fire and/or explosion. Accordingly, it has been known previously to try to produce a foam using only a chemical blowing agent, such as water (See for example "Development of all water-blown polyurethane rigid foam for housing insulation" by J.Goto, K. Sasaki, S. Mashiko, Y.Kataoka, Y. Kambara and I.Ohki or "novel polyol for all water-blown rigid polyurethane foams" by Y. Miyamoto, K. Harada, C, Suzuki, H.Sato and H.Wada). Foams produced using only chemical blowing agents like water have various problems including an uneven density of the polyurethane, poor dimensional stability, high thermal conductivity and long demolding time that limits their application and processability, especially for medium to high foam thickness due to the high reaction exothermicity.

Manufacturing processes and characteristics of rigid polyurethane molded foams, including those used in appliances, are well known. See for example, Polyurethane Handbook by G. Oertel et al., 2nd edition, Hanser Publishers, 1993. The polyurethane foaming mixture is generally injected into the mold cavity at atmospheric pressure. Difficulties can be encountered in completely filling a mold cavity and in producing pieces which are uniform in density.

To aid in the flow of material into a cavity, the reduction in the pressure of the mold cavity is proposed for specific applications, see for example U.S. Patents 3,970,732 and 5,972,260 . WO2007/058793 teaches a method of forming a rigid polyurethane foam at reduced pressure using a physical blowing agent. It is an object of the present invention to produce a rigid polyurethane foam without the use of a physical blowing agent. It is a further object of the present invention to provide a composition which is particularly suited to forming a rigid foam without the use of a physical blowing agent.

In a first aspect of the present invention, there is provided a method of making a molded rigid polyurethane foam comprising: injecting into a closed mold cavity a reaction mixture at a packing factor of 1.03 to 1.9, wherein the mold cavity is under a pressure of from 300 to 950 mbar, wherein the reaction mixture comprises:

1.     a) an organic polyisocyanate;

2.     b) a polyol composition;

3.     c) a catalyst;

4.     d) optionally auxiliary substances and/or additives; and

5.     e) a chemical blowing agent component in an amount of from 1 to 5 weight percent based on the total weight of components b) to e), the chemical blowing agent component comprising at least one chemical blowing agent,

wherein the chemical blowing agent component is the sole blowing agent. Suitable chemical blowing agents include water and formic acid, with water being particularly preferred.

In a second aspect, the present invention is a foam produced according to the method of the first aspect.

In a third aspect, there is provided a composition for producing a rigid polyurethane foam comprising the reaction product of:

6.     a) an organic polyisocyanate;

7.     b) a polyol composition, wherein the polyol composition comprises:

§  b1) a polyether polyol having a functionality of from 5 to 8

§  b2) a polyether polyol having a functionality of from 2 to 4;

8.     c) at least two of a curing catalyst, a blowing catalyst and a trimerization catalyst;

9.     d) at least one silicone surfactant; and

10.  e) water, in an amount of from 2.5 to 3.5 weight percent based on the total weight of components b) to e),

wherein water is the sole blowing agent.

 

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