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

It is preferred that the polyol component comprises one or more high functionality polyols particularly ones having a functionality of at least 5, more preferably having a functionality of from 5 to 8. Initiators for such polyols include, for example, pentaerythritol, sorbitol, sucrose, glucose, fructose or other sugars, and the like. It is preferred that the initiator molecule has a functionality of 6, and in particular is sorbitol. Such higher functional polyols will have an average hydroxyl number from about 200 mg KOH/g to about 850 mg KOH/g, preferably from about 300 mg KOH/g to about 770 mg KOH/g.

The high functional polyol preferably such polyols will generally comprise from 10 to 90% by weight of the total amount of polyol present, more preferably from 25 to 75% and yet more preferably from 40 to 60% by weight.

The high functional polyol can be a polyether polyol or a polyester polyol. However it is preferred that it is a polyether polyol. The polyether polyol is usually a polyoxypropylene, a polyoxyethylene or combination thereof, either as a block copolymer or a random copolymer. Polyoxypropylene polyols are particularly preferred.

The polyol component preferably comprises one or more other polyols. It is preferred to include at least one low functionality polyol along with the high functionality polyol. A low functionality polyol is one having a functionality from 2 to 4, preferably from 2 to 3. Preferred starters include glycerine and propylene glycol

The low functional polyols can be polyether polyols or polyester polyols. However it is preferred to use polyether polyols. The polyether polyol is usually a polyoxypropylene, a polyoxyethylene or combination thereof, either as a block copolymer or a random copolymer, with polyoxypropylene being preferred.

The polyol or polyol composition will generally have a hydroxyl number of from 100 mg KOH/g to 1,200 mg KOH/g. Preferably the hydroxyl number is from 100 mg KOH/g to 500 mg KOH/g and more preferably from 110 mg KOH/g to 300 mg KOH/g.

The low functionality polyol or polyols are preferably present in an amount of from 5 to 60 % by weight of the total amount of polyol present. Preferably, the low functionality polyol are present in an amount of from 15 to 55 % by weight, more preferably from 20 to 50 % by weight.

Low functionality polyols are generally known and are described in such publications as High Polymers, Vol. XVI; "Polyurethanes, Chemistry and Technology", by Saunders and Frisch, Merscience Publishers, New York, Vol. I, pp. 32-42, 44-54 (1962) and Vol. II, Pp. 5-6, 198-199 (1964); Organic Polymer Chemistry by K. J. Saunders, Chapman and Hall, London, pp. 323-325 (1973); and Developments in Polyurethanes, Vol. I, J.M. Burst, ed., Applied Science Publishers, pp. 1-76 (1978). Representative of suitable polyols include polyester, polylactone, polyether, polyolefin, polycarbonate polyols, and various other polyols. If desired, the polyol formulation may also contain copolymer polyols such as those of styrene/acrylonitrile (SAN), polyisocyanate polyaddition products (PIPA) or polyurea polyols (PHD).

In preferred embodiments, additional polyols can also be present in the polyol component. Suitable polyols include polyols formed from mixtures of initiators such as a high functionality starter and a lower functionality starter such as glycerin to give co-initiated polyols having functionality of from 4.5 to 7 hydroxyl groups per molecule and preferably a hydroxyl equivalent weight of 100 mg KOH/g to 200 mg KOH/g. Such components are preferably present in an amount of from 10 to 30 % by weight of the total amount of polyol present.

While it is generally preferred to use a polyol or polyol component having a low viscosity for ease of processing, the process conditions of the present invention allow use of a polyol formulation having a viscosity of 3000 mPa.s or greater (measured at 25°C.) without the auxiliary blowing agent. Polyol formulations having higher viscosity result in higher viscosities of the system formulation. It is believed that a higher viscosity of the system formulation hinder the drainage of liquids in the cell structure during foam rise giving smaller cell size which aids in getting lower lambda values with good retention over time.

The polyols form the bulk of the polyol formulation. It is preferred that the polyol component comprises from 40 to 95 weight percent of the polyol formulation, preferably from 60 to 95 weight percent and more preferably from 70 to 95 weight percent. The balance of the polyol formulation is made up of catalysts, cross-linkers, chain extenders, surfactants, fillers and other additives.

In a preferred embodiment of the present invention, there is provided one or more catalysts. Polyurethane catalysts provide three main purposes, namely to act as curing catalysts, blowing catalysts and trimerization catalysts. It is preferred that the catalytic package provides at least two of the curing catalyst, blowing catalyst and trimerization catalyst. It is further preferred that all three catalyst types are present.


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