Process for rigid polyurethane foams

Details Process for rigid polyurethane foams Process for rigid polyurethane foams

1999-03-30

2002-08-13

Sergent, Rabon (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C428S069000, C428S071000, C428S076000, C428S317500, C428S318400, C521S114000, C521S115000, C521S116000, C521S117000, C521S118000, C521S128000, C521S129000, C521S130000, C521S163000, C521S164000, C521S166000, C521S167000, C521S174000, C521S176000

Reexamination Certificate

active

06433032

ABSTRACT:

This invention relates to a process for the preparation of open-celled rigid polyurethane or urethane-modified polyisocyanurate foams, to foams prepared thereby, to the use of these foams in evacuated insulation panels and to certain novel polyisocyanate compositions useful in the process.
Rigid polyurethane and urethane-modified polyisocyanurate foams are in general prepared by reacting the appropriate polyisocyanate and polyol in the presence of a blowing agent. They can be open- or closed-celled.
One use of open-celled rigid polyurethane or urethane-modified polyisocyanurate foams is as a thermal insulation medium, for example, in evacuated insulation panels used in the construction of refrigerated storage devices.
Evacuated insulation panels generally comprise a low thermal conductivity filler material (such as open-celled polyurethane foam) and a vessel formed of a gastight film enveloping said filler, the whole being evacuated to an internal pressure of about 5 mbar or less and then hermetically sealed.
Open-celled polyurethane foams suitable as low thermal conductivity filler material for evacuated insulation panels can advantageously be produced by reacting an organic polyisocyanate with an isocyanate-reactive material comprising at least one isocyanate-reactive cyclic compound, as described in, for example, EP-A-498628, EP-A-498629, EP-A-419114, EP-A-662494, WO 95/15355, WO 95/02620, WO 96/25455, WO 96/32605, WO 96/36655, WO 98/54239 and GB 2324798, all incorporated herein by reference.
General descriptions of the construction of evacuated insulation panels and their use in thermal devices can be found in U.S. Pat. Nos. 5,066,437, 5,032,439 and 5,076,984 and European Patent Publications Nos 434266, 434225 and 181778, all incorporated herein by reference as well as the references mentioned therein.
EP 547515 describes a method for producing open cell rigid polyurethane foam for use in a vacuum insulating material by reacting polymethylene polyphenylisocyanate prepolymer with polyol at an NCO/OH equivalent ratio of 1.3 to 3.0 using water as blowing agent. The prepolymer is obtained by reacting polymethylene polyphenylisocyanate (p-MDI) with a polyol (oxypropylene based) and has an amine equivalent of 140 to 200 (which corresponds to an NCO value of 21 to 30%). The obtained open cell rigid polyurethane foam has a fine cell structure due to the presence of the p-MDI prepolymer and no scorching due to the index range employed.
EP 581191 describes a method for producing an open cell rigid polyurethane foam for use in vacuum insulating material by reacting a polyol with a polymethylene polyphenyl polyisocyanate prepolymer derived from a monol by use of a substitute for trichlorofluoromethane as blowing agent. The use of a prepolymer leads to a very fine cell structure of the contained foam.
The present Applicant has now developed an improved process for the preparation of open-celled rigid polyurethane and urethane-modified polyisocyanurate foams derived from certain polyisocyanate compositions comprising specific prepolymers (i.e. reaction products of a stoichiometric excess of a polyisocyanate and an isocyanate-reactive material).
Accordingly the invention provides a process for the preparation of an open-celled (semi-)rigid polyurethane or urethane-modified polyisocyanurate foam by reaction of a polyisocyanate composition with a polyfunctional isocyanate-reactive composition under foam-forming conditions, characterised in that the polyisocyanate composition has a free NCO-value of 21 to 30% by weight and comprises the reaction product of a stoichiometric excess of an organic polyisocyanate and an isocyanate-reactive material comprising a polyether polyol having a number average molecular weight between 1000 and 10000 and an average nominal hydroxyl functionality of from 2 to 6 and containing from 10 to 50% by weight of oxyethylene units.
The process of the invention is suitable for the preparation of open-celled rigid polyurethane or urethane-modified polyisocyanurate foams having improved cell-opening and finer cells. Closed cell contents of below 5% for the core of the foam are generally obtained by using the process of the present invention. Generally the closed cell content is below 2%, even more generally below 1%.
The open-celled foams according to the invention are characterised by excellent thermal insulation properties with no degradation of mechanical properties and are therefore particularly suitable for thermal insulation purposes.
Foam blocks at a thickness of greater than 30 cm and even greater than 45 cm can be prepared without scorching or splitting of the foam by using the process of the present invention.
Prepolymers of the claimed range of NCO values lead to lower exotherms within the foam thereby reducing the scorching of the foam and the tendency to split. Preferably the NCO value of the polyisocyanate composition is between 23 and 29 wt %, more preferably between 24 and 26 wt %.
The viscosity of the polyisocyanate composition is preferably below 2000 cps, more preferably below 1500 cps at 25° C. to allow easy processing.
The use of a polyfunctional isocyanate-reactive material to prepare the prepolymer is preferred over the use of a monofunctional isocyanate-reactive material. The use of prepolymers derived from monofunctional isocyanate-reactive material (such as a monohydric alcohol, preferably having a molecular weight of at least 340, for example a polyalkylene glycol monoalkyl ether of molecular weight 350 to 750) still leads to splitting of the foam especially in the production of thick foam blocks.
The isocyanate-reactive material which is reacted with a stoichiometric excess of an organic polyisocyanate to form the reaction product which is present in the polyisocyanate composition (hereinafter called the ‘isocyanate-reactive material’) is preferably a polyether polyol having a number average molecular weight between 1000 to 6000 and an average nominal hydroxyl functionality of from 2 to 4 and an oxyethylene content of between 10 and 50%. Preferably the polyether polyol has a molecular weight between 3000 and 5000, a functionality between 2.5 and 3.5 and an oxyethylene content of between 15 and 35%.
The term “average nominal hydroxyl functionality” is used herein to indicate the average functionality (number of hydroxyl groups per molecule) of the polyol composition on the assumption that this is the average functionality (number of active hydrogen atoms per molecule) of the initiator(s) used in their preparation although in practice it will often be somewhat less because of some terminal unsaturation.
Suitable polyether polyols for use in the isocyanate-reactive material have been fully described in the prior art and include reaction products of alkylene oxides, for example ethylene oxide and/or propylene oxide, with initiators containing from 2 to 6 active hydrogen atoms per molecule. Suitable initiators include water and polyols, for example ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, cyclohexane dimethanol, resorcinol, bisphenol A, glycerol, trimethylolpropane, 1,2,6-hexanetriol, triethanolamine, pentaerythritol and sorbitol; polyamines, for example ethylene diamine, tolylene diamine, diaminodiphenylmethane and polymethylene polyphenylene polyamines; and aminoalcohols, for example ethanolamine and diethanolamine, and mixtures of such initiators.
Preferred polyether polyols are poly(oxyethylene-oxypropylene) polyols containing preferably 10 to 50%, and more preferably 15 to 35% by weight, based on the total weight of the polyol, of oxyethylene groups.
Preferably, at least 50%, more preferably at least 75%, of these oxyethylene groups are present at the end of the polyether polyol.
If the oxyethylene content of the polyether polyol used in preparing the prepolymer is below 10 wt % the foam cell texture becomes coarse; if the oxyethylene content of the polyether polyol is above 50 wt % the closed cell content of the foam becomes too high.
In the preparation of the reaction product of the organic polyisocyanate

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