CA2259588A1 - Preparation of rigid pu foams having small cell diameters and the use thereof - Google Patents

Abstract

A process for the preparation of fine-cell rigid polyurethane foams by the reaction of a) polyisocyanates with b) compounds having at least two reactive hydrogen atoms in the presence of c) expanding agents, d) catalysts and, optionally, e) auxiliaries and/or additives, where the expanding agents c) used are perfluoro compounds which are emulsified in at least one of the components a) or b), and water and air are removed from the components a) to e) prior to conversion to the polyurethane.

Description

BASF Aktienges~llschaft 970857 O.Z. 0050/48762 Preparation of rigid PU Foams Having Small Cell Diameters and The Use Thereof The present invention relates to a process for the preparation of rigid polyurethane foams - referred to below as rigid PU foams -having small cell diameters and containing known constituents, in the presence of perfluoroalkanes acting as expanding agents, and to the use of these rigid PU foams for foam-filling cavities in refrigerating units or heating elements and also as an insulating material for composite structures.
The preparation of composite or sandwich structures composed of a rigid PU foam and at least one surface layer of a rigid or ela-stic material such as paper, plastic sheet, plate metal, glass 'fiber non-woven, chip board and the like, is known. Foam-filling of cavities in domestic appliances such as refrigerating units, for example refrigerators or freezers, or in hot water tanks, using rigid PU foam as thermal insulator, is also known. In order to avoid foam defects, the foamable PU reaction mixture must be filled into the cavity that is to be insulated within a short time. Foam-filling such cavities is usually carried out using low-pressure machines or, preferably, high-pressure machines.
High-temperature and low-temperature insulating rigid PU foams suitable for this purpose can be prepared, as is well known, by the reaction of organic polyisocyanates with one or more com-pounds of higher molecular weight and containing at least two re-active hydrogen atoms, preferably polyester polyalcohols and/or polyether polyalcohols, usually using low-molecular chain-exten-ders and/or cross-linking agents in the presence of expanding agents, catalysts and, optionally, auxiliaries and/or additives.
With a good choice of constituent components, rigid PU foams ha-ving low thermal conductivity and good mechanical properties can be made.
One method of insulating with rigid PU foams involves the use of open-cell foams packed in an evacuated impermeable material.
An overview of methods of preparing rigid PU foams and the use thereof as surface layers or, preferably, cores of composite structures and also the use thereof as insulating layers in coo-ling or heating engineering applications has been published for example in Polyurethanes, Kunststoff-Handbuch, Vol. 7, 1st Edi-tion 1966, edited by Dr. R. Vieweg and Dr. A. Hochtlen, 2nd Edi-BASF Aktiengesellschaft 970857 O.Z. 0050/48762 tion 1983, edited by Dr. Gunter Oertel, and 3rd Edition 1993, edited by Dr. Gunter Oertel, Carl Hanser Verlag, Munich, Vienna.
Since the purpose of the rigid PU foams in such applications is primarily to have a heat-insulating effect, low thermal conducti vity is the most :important property of the foam. One way of redu-cing the thermal conductivity of a foam is to reduce the size of the cells. when operating with open-cell foams under reduced pressure the cell diameter of the foams is again a decisive fac-for for acquiring the required vacuum. Foams having small cell diameters are therefore an important object of research.
According to the classical foam theory, so-called nucleating seeds are formed when the starting components are mixed, into which seeds the expanding agent diffuses. For a given foam den-sity, the number of these nuclei determines the size of the cell diameters. In conventional, water-expanded rigid foam systems ha-ving a density of from 30 to 40 g/L the concentration of the nu-clei is from 106 to 10~ nuclei/g of foam, which gives cell diame-ters of from approximately 200 to 300 Vim.
All attempts to increase the number of the nuclei to a noticeable extent by classical methods of homogeneous and heterogeneous nu-cleation, have provided only slight improvements.
A considerable improvement was achieved by foams containing per-fluoro compounds, incorporated by emulsification, such as are described in EP-A 351,614 for example. Cell diameters around 100 ~m have been achieved. The precise mechanism of the nucleating action is not known but it is suspected that it is caused by low interfacial tension.
we have now found that foams having much smaller cell diameters can be prepared when the following procedure is adopted:
A mixture of polyalcohols, catalysts, foam stabilizers and optionally other additives, usually referred to as the poly-alcohol component or component A, and a polyisocyanate (com-ponent B) are dried and rendered air-free by evacuation, whe-reupon a perfluoroalkane is incorporated in component A and/
or B in known manner by emulsification and components A and B
are then mixed and allowed to foam.
There are obtained, surprisingly, foams having cell diameters of less than 50 Vim. This is fully unexpected, since it would have been thought that the elimination of air, which is regarded as being very important for the formation of nuclei according to BASF Aktiengesellschaft 970857 O.Z. 0050/48762 prevailing expert opinion, would have completely suppressed nu-cleation, resulting in the formation of foams having only very coarse cells.
In another embodiment of the invention) foaming of the mixture of components is carried out in a mold evacuated prior to or during foaming. In this way it is possible to reduce the density of the foams obtained to a considerable extent.
Thus the present invention relates to a process for the prepara-tion of finely cellular rigid PU foams, particularly rigid PU fo-ams having cell diameters of less than 50 (.im and thus exhibiting low thermal conductivity, by the reaction of a) organic and/or modified organic polyisocyanates with b) at least one compound having at least two reactive hydrogen atoms in the presence of c) expanding agents, d) catalysts and, optionally, e) auxiliaries and/or additives, Which is characterized in that the expanding agents (c) used are perfluoro compounds incorporated in (a) and/or (b) to (e) by emulsification, the starting components (a) to (e) having been previously rendered free from water and air by heating in vacuo.
Another embodiment of the invention consists in carrying out foa-ming in vacuo in a sealed foam mold.
The constituents used in the preparation of the rigid PU foams by the process of the invention are components known per se, which are described below in detail.
a) Suitable organic polyisocyanates are a11 known aliphatic, cy cloaliphatic, araliphatic and, preferably, aromatic polyfunc tional isocyanates.

BASF Aktiengesellschaft 970857 O.Z. 0050/48762 Specific examples thereof are the following: alkylene diiso-cyanates containing from 4 to 12 carbon atoms in the alkylene radical such as dodecane-1,12-diisocyanate) 2-ethyltetrame-thylene-1,4-diisocyanate, 2-methylpentamethylene-1,5-diiso-cyanate, tetramethylene-1,4-diisocyanate and preferably hexa-methylene-1,6-diisocyanate; cycloaliphatic diisocyanates such as cyclohexane-[1,3 and 1,4]-diisocyanates and also arbitrary mixtures of these isomers, 1-isocyanato-3,3,5-trime-thyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate), hexahydrotoluylene-[2,4 and 2,6]-diisocyanates and the cor-responding isomer mixtures, dicyclohexylme-thane-[4,4', 2,2' and 2,4']-diisocyanates and the correspon-ding mixtures of isomers and preferably aromatic diisocyana-tes and polyisocyanates such as toluylene-[2,4 and 2,6]-dii-socyanates and the corresponding mixtures of isomers, diphe-nyl-[4,4', 2,4' and 2,2']-methanediisocyanates and the cor-responding mixtures of isomers, mixtures of diphenylme-thane-[4,4' a:nd 2,4']-diisocyanates, polyphenyl-polymethylene polyisocyanates, mixtures of diphenylme-thane-[4,4', 2,4' and 2,2']-diisocyanates and polyphenyl-po-lymethylene polyisocyanates (crude MDI) and mixtures of crude MDI and toluylene diisocyanates. The organic diisocycanates and polyisocyanates can be used alone or intermixed.
Frequently use is made of so-called modified polyfunctional isocyanates, ie products obtained by partial chemical conver-sion of organic diisocycanates and/or polyisocyanates. As ex-amples thereof there may be mentioned diisocycanates and/or polyisocyanates containing ester, urea, biuret, allophanate, carbodiimide, isocyanurate and/or urethane groups. As exam-ples thereof there may be mentioned urethane group-containing organic, preferably aromatic, polyisocyanates having NCO con-tents of from 33.6 to 15 wt%, preferably from 31 to 21 wto, based on the total weight, for example with low-molecular diols, triols, dialkylene glycols, trialkylene glycols or poly(oxyalkylene glycol)s having molecular weights of up to 1,500, modified diphenylmethane-4,4'-diisocyanate or toluy-lene-[2,4 or 2,6]-diisocyanate, examples of the dialkoxylene glycols or poly(alkoxylene glycol)s, which may be used singly or intermixed, being diethylene glycol, dipropylene glycol, poly(oxyethylene glycol), poly(oxypropylene glycol) and poly-oxypropylene-polyoxy(ethylene glycol) or diethylene triol, dipropylene triol, poly(oxyethylene triol), poly(oxypropy-lene triol) and polyoxypropylene-poly(oxyethylene triol).
Also suitable are NCO group-containing prepolymers having NCO
( contents of from 25 to 9 wt%, preferably from 21 to 14 wto, based on the total weights prepared from the polyester poly-BASF Aktiengesellschaft 970857 O.Z. 0050/48762 alcohols and/or, preferably, polyether polyalcohols described below and diphenylmethane-4,4'-diisocyanate, mixtures of di-phenylmethane-[2,4' and 4,4']-diisocyanates, toluy-lene-[2,4 and/or 2,6]-diisocyanates or crude MDI. The follo-5 wing have been found to be useful: liquid polyisocyanates containing carbodiimide groups and/or isocyanurate rings and having NCO contents of from 33.6 to 15, preferably from 31 to 21 wto, based on the total weight, based on for example di-phenylmethane-[4,4', 2,4' and/or 2,2']-diisocyanates and/or toluylene-[2,4 and/or 2,6]-diisocyanates.
The modified polyisocyanates may be mixed, if desired, either with each other or with unmodified organic polyisocyanates such as diphenylmethane-[2,4' or 4,4']-diisocyanate, crude MDI and toluylene-[2,4 and/or 2,6]-diisocyanate.
The following organic polyisocyanates have proven to be par-ticularly useful and are therefore preferably used for the -preparation of the rigid PU foams: mixtures of toluylene dii-socyanates and crude MDI or mixtures of modified urethane group-containing organic polyisocyanates having an NCO con-tent of from 33.6 to 15 wto, particularly those based on to-luylene diisocyanates, diphenylmethane-4,4'-diisocyanate, mixtures of diphenylmethane diisocyanate isomers or crude MDI
and particularly crude MDI having a content of diphenylme-thane diisocyanate isomers of from 30 to 80 wt%, preferably from 30 to 55 wto.
b) Suitable compounds (b) having at least two reactive hydrogen atoms are) for example, compounds of high molecular weight, preferably polyhydroxyl compounds having a hydroxyl value of from 150 to 8S0, preferably from 350 to 800.
As examples thereof there may be mentioned polythioether po-lyalcohols, polyesteramides, poly(oxymethylene)s containing hydroxyl groups, aliphatic polycarbonates containing hydroxyl groups and preferably polyester polyalcohols and polyether polyalcohols. Use may also be made of mixtures of at least two of the specified polyhydroxyl compounds, provided said mixtures have an average hydroxyl value within the aforemen-tioned range.
Suitable polyester polyalcohols can be prepared, for example, from organic dicarboxylic acids containing from 2 to 12 car-bon atoms, preferably aliphatic dicarboxylic acids containing from 4 to 6 carbon atoms and polyhydric alcohols, preferably diols containing from 2 tp 12 carbon atoms, preferably from 2 BASF Aktiengesellschaft 970857 O.Z. 0050/48762 to 6 carbon atoms. Examples of suitable dicarboxylic acids are succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, decanedioic acid, malefic acid, fumaric acid, phthalic acid, isophthalic acid and terephtha-lic acid. The dicarboxylic acids can be used either singly or intermixed. Instead of the free dicarboxylic acids use may be made of corresponding dicarboxylic derivatives such as monoe-sters or diesters of dicarboxylic acid with alcohols contai-ning from 1 t.o 4 carbon atoms or dicarboxylic anhydrides. Di-carboxylic acid mixtures of succinic, glutaric and adipic acids in proportions of, say, 20-35:35-50:20-32 parts by weight, and particularly adipic acid are preferably used. Ex-amples of dihydric and polyhydric alcohols, particularly diols, are ethanediol, diethylene glycol, pro-pane-[1,2 or 1,3)-diol, dipropylene glycol, butane-1,4-diol, pentane-1,5-diol, hexane-1,6-diol, decane-1,10-diol, glycerol and trimethylol propane. We prefer to use ethanediol, diethy-lene glycol, butane-1,4-diol, pentane-1,5-diol, he-xane-1,6-diol or mixtures of at least two of said diols, par-ticularly mixtures of butane-1,4-diol, pentane-1,5-diol and hexane-1,6-diol. Use may also be made of polyester polyalco-hols of lactones, for example s-caprolactone or hydroxycarbo-xylic acids, for example w-hydroxyhexanoic acid.
To prepare the polyester polyalcahols, the organic, for exam-ple aromatic and preferably aliphatic, polycarboxylic acids and/or derivatives thereof and polyhydroxylic alcohols can be polycondensed in the absence of catalysts or, preferably, in the presence of esterification catalysts, advantageously in an atmosphere of inert gas such as nitrogen, carbon dioxide, helium, argon and the like, in the melt at temperatures of from 150~ to 250~C, preferably from 180~ to 250~C optionally under reduced pressure, until the desired acid value is achieved, this being advantageously lower than 10, preferably lower than 2. In a preferred embodiment the esterification mixture is polycondensed under standard pressure at the afo-rementioned temperatures, until an acid value of from 80 to 30, preferably from 40 to 30, is obtained, and then under a pressure of less than 500 mbar, preferably from 0 to 150 mbar. Suitable esterification catalysts are for example catalysts containing iron, cadmium, cobalt, lead, zinc, anti-mony, magnesium, titanium or tin in the form of metals, metal oxides or metal salts. Alternatively, however, the polycon-densation can be carried out in the liquid phase in the pre-sence of diluents and/or entraining agents such as benzene, i BASF Aktiengesellschaft 970857 O.Z. 0050/48762 toluene, xylene or chlorobenzene, to effect azeotropic remo-val of the water of condensation.
The preparation of polyester polyalcohols comprises polycon-densation of organic polycarboxylic acids and/or derivatives thereof and polyhydroxylic alcohols advantageously in a molar ratio of from 1:1 to 1:1.8, preferably from 1:1.05 to 1:1.2.
The resulting polyester polyalcohols possess a functionality of, preferably, from 2 to 3 and a hydroxyl value of from 150 to 400 and particularly of from 200 to 300.
In particular however, the polyhydroxyl compounds used are polyether po:Lyalcohols, which are prepared from one or more alkylene oxides containing from 2 to 4 carbon atoms by known processes, for example by anionic polymerization with alkali metal hydroxides such as sodium or potassium hydroxide or al-kali metal alkoxides such as sodium methoxide, sodium or po-tassium ethoxide or potassium isopropylate as catalysts and at least one primer containing from 2 to 8, preferably from 3 to 8, attached reactive hydrogen atoms or by cationic polyme-rization using Lewis acids such as antimony pentachloride, boron fluoride etherate and the like, or bleaching earth as catalysts.
Examples of suitable alkylene oxides are tetrahydrofuran, 1,3-propylene oxide, [1,2 or 2,3]-butylene oxide, styrene oxide and preferably ethylene oxide and 1,2-propylene oxide.
The alkylene oxides can be used singly, consecutively, or as mixtures. Examples of suitable primers are water, organic di-carboxylic acids such as succinic acid, adipic acid, phthalic acid and terephthalic acid, aliphatic and aromatic, optio-nally N-monoalkyl-substituted, N,N-dialkyl-substituted and N,N'-dialkyl-substituted diamines containing from 1 to 4 car-bon atoms in the alkyl group, such as optionally monoalkyl-substituted and dialkyl-substituted ethylenediamine, diethy-lenetriamine, triethylenetetramine, 1,3-propylenediamine, [1,3 or 1,4]-butylenediamine, [1,2, 1,3, 1,4, 1,5 and 1,6]-hexamethylenediamines, phenyle-nediamines, [2,3, 2,4 and 2,6]-toluylenediamines and [4,4', 2,4' and 2,2']-diaminodiphenylmethanes, Other suitable primers are alkanolamines such as ethanola-mine, diethanolamine, N-methyl and N-ethyl ethanolamines, N-methyl and N-ethyl diethanolamines and triethanolamine and ammonia.

BASF Aktiengesellschaft 970857 O.Z. 0050/48762 Polyhydric) particularly trihydric and higher alcohols such as ethanediol, propane-[1,2 and 1,3]-diols, diethylene gly-col, dipropylene glycol, butane-1,4-diol, hexane-1,6-diol, glycerol, trimethylol propane, pentaerythritol, sorbitol and sucrose are preferably used.
The polyether polyalcohols possess a functionality preferably ranging from 3 to 8 and particularly from 3 to 6, and hydro-xyl values preferably ranging from 300 to 8S0 and particu-larly from 350 to 800.
Other suitable polyether polyalcohols are melamine/polyether polyalcohol dispersions as described in EP-A 23,987 (US-A
4,293,657), polymer/polyether polyalcohol dispersions, prepa-red from polyepoxides and epoxy resin curing agents in the presence of polyether polyalcohols as described in DE-A
2,943,689 (US-A 4,305,861), dispersions of aromatic poly-esters in polyhydroxyl compounds as described in EP-A 62,204 (US-A 4,435,537) or DE-A 3,300,474, dispersions of organic and/or inorganic fillers in polyhydroxyl compounds as descri-bed in EP-A 1.1,751 (US-A 4,243,755), polyurea/polyether poly-alcohol dispersions as described in DE-A 3,125,402, tris(hy-droxyalkyl)isocyanurate polyether polyalcohol dispersions described in DE-A 3,342,176 and DE-A 3,342,177 (US-A
4,560,708), which published patents are to be regarded as part of the present patent description by reference.
Like the polyester polyalcohols, the polyether polyalcohols can be used individually or intermixed. Furthermore they can be mixed with the aforementioned dispersions, suspensions or polyester pol.yalcohols and also polyesteramides, poly(oxyme-thylene)s containing h~~droxyl groups and/or polycarbonates.
Examples of suitable poly{oxymethylene)s containing hydroxyl groups the compounds obtainable from glycols such as diethy-lene glycol, triethylene glycol, 4,4'-dihydroxyethoxydiphe-nyldimethylmethane, hexanediol and formaldehyde. In addition, suitable poly(oxymethylene)s can be prepared by polymeriza-tion of cyclic acetals.
Suitable polycarbonates exhibiting hydroxyl groups are the known types such as can be prepared, for example, by the re-action of diols such as propane-1,3-diol, butane-1,4-diol and/or hexane-1,6-diol, diethylene glycol, triethylene glycol or tetraethylene glycol with diaryl carbonates, for example diphenyl carbonate, or phosgene.

BASF Aktiengesellschaft 970857 O.Z. 0050/48762 The polyesteramides include, for example, the predominantly linear condensates formed from polybasic saturated and/or un-saturated carboxylic acids or their anhydrides and amino al-cohols or mixtures of polyhydroxylic alcohols and amino alco-hols and/or polyamines.
Those polyhydroxyl compounds which have proven to be particu-larly useful and are therefore preferably used are mixtures, which advantageously comprise, based on 100 parts by weight:
b1) From 0 to 95 parts by weight, preferably from 20 to 80 parts by weight of a polyether polyalcohol primed with sucrose and having a hydroxyl value of from 300 to 500, preferably from 350 to 450, based on 1,2-propylene oxide or 1,2-propylene oxide and ethylene oxide, b2) From 0 to 15 parts by weight, preferably from 5 to 15 parts by weight of a polyether polyalcohol primed with sorbitol and having a hydroxyl value of from 400 to 600, preferably from 450 to 550, based on 1,2-propylene oxide or 1,2-propylene oxide and ethylene oxide, b3) From 0 to 20 parts by weight, preferably from 5 to 15 parts by weight of a polyether polyalcohol primed with an amine and having a hydroxyl value of from 700 to 850, preferably from 750 to 800, based on 1,2-propylene oxide and b4) From 0 to 60 parts by weight, preferably from 5 to 40 parts by weight, of a polyether polyalcohol having a hy-droxyl value of from 400 to 600, preferably from 450 to 550, based on 1,2-propylene oxide or 1,2-propylene oxide and ethylene oxide, prepared using a mixture of sucrose and triet:hanolamine in a ratio, by weight, of from 1:2 to 2:1, as primers.
The rigid PU foams can be prepared with or without the use of chain extending and/or cross-linking agents. However, the ad-dition of chain-extenders, cross-linking agents or if desired mixtures thereof may prove to be advantageous for modifica-tion of the mechanical properties The chain extending and/or cross-linking agents used are preferably alkanolamines and particularly diols and/or triols having molecular weights be-low 400, preferably from 60 to 300. Suitable alkanolamines are for example ethanolamine and/or isopropanolamine, dialka-nolamines such as diethanolamine, N-methyl and N-ethyl die-thanolamines, diisopropanQlamine, trialkanolamines such as BASF Aktiengesellschaft 970857 O.Z. 0050/48762 triethanolamine, triisopropanolamine and the addition pro-ducts of ethylene oxide or 1,2-propylene oxide and alkylene~
diamines containing from 2 to 6 carbon atoms in the alkylene radical such as N,N'-tetra(2-hydroxyethyl)ethylenediamine and 5 N,N'-tetra(2-hydroxypropyl)ethylenediamine, aliphatic, cy-cloaliphatic and/or araliphatic diols containing from 2 to 14) preferably from 4 to 10 carbon atoms such as ethylene glycol, propane-1,3-diol, decane-1,10-diol, o-, m- and p-di-hydroxycyclohexanes, diethylene glycol, dipropylene glycol 10 and preferably butane-1,4-diol, hexane-1,6-diol and bis(2-hy-droxyethyl)hydroquinone, triols such as [1,2,4 and 1,3,5]-trihydroxycyclohexanes, glycerol and trime-thylol propane and low-molecular hydroxyl group-containing poly(alkylene oxides based on ethylene oxide and/or 1,2-pro-pylene oxide and aromatic diamines such as toluylenediamines and/or diaminodiphenylmethanes and also the aforementioned alkanolamines, diols and/or triols as primers.
If chain-extenders, cross-linking agents or mixtures thereof are used for the preparation of the rigid PU foams, they will be employed in amounts ranging from 0 to 20 wto, preferably from 2 to 5 wto, based on the weight of the polyhydroxyl com-pound.
c) The expanding agents used can be theoretically a11 perfluoro compounds that are difficultly soluble or insoluble in the constituent components (a) to (e), and are liquid at room temperature. Perfluoroalkanes are preferred because they are commercially available. For example, use is made of perfluo-ropentane, perfluorohexane, perfluoroheptane, perfluorooctane or a mixture thereof. In addition, mixtures of perfluorobu-tane or perfluoropropane with perfluorohexane, perfluorohep-tane or perfluorooctane can be used, provided the mixing ra-tio is adjusted such that the mixtures are liquid at room temperature.
d) The catalysts (d) used are, in particular, compounds which greatly accelerate the reaction of the hydroxyl group-contai-ning compounds of component (b) with polyisocyanates. Suita-ble compounds are organic metal compounds, preferably organic tin compounds such as tin(II) salts of organic carboxylic acids, for example tin (II) acetate, tin (II) octoate, tin (II) ethylhexoate and tin(II) laurate and the dialkyltin(IV) salts of organic carboxylic acids, for example dibutyltin diace-tate, dibutyltin dilaurate, dibutyltin maleate and dioctyltin diacetate. The organic metal compounds are used alone or, preferably, together with, strongly alkaline amines. Specific BASF Aktiengesellschaft 970857 O.Z. 0050/48762 examples thereof are amidines such as 2,3-dime-thyl-3,4,5,6-tetrahydropyrimidine, tertiary amines such as triethylamine, tributylamine, dimethylbenzylamine, N-methyl, N-ethyl and N-cyclohexyl morpholines, N,N,N',N'-tetramethyle-5 thylenediamine, N, N, N', N'-tetramethylbutanediamine or N,N,N',N'-tetramethylhexanediamine, pentamethyldiethylene-triamine, tetramethyldiaminoethyl ether, bis(dimethylamino-propyl) urea, dimethylpiperazine, 1,2-dimethylimidazol, 1-azabicyclo-(3,3,0)-octane and preferably 1,4-diazabi-10 cyclo-(2,2,2)-octane and alkanolamine compounds such as trie-thanolamine, triisopropanolamine, N-methyl and N-ethyl die-thanolamines and dimethyl ethanolamine.
Other suitable catalysts are tris(dialkylaminoalkyl)-s-hexa-15 hydrotriazines, particularly tris(N,N-dimethylaminopropyl)-s-hexahydrotriazine, tetraalkylammoniumhydroxides such as te-tramethylammoniumhydroxide, alkali metal hydroxides such as sodium hydroxide and alkali metal alkoxides such as sodium methoxide and potassium isopropylate and also alkali metal 20 salts of long-chain fatty acids containing from 10 to 20 car-bon atoms and optionally lateral hydroxyl groups. We prefer to use from 0.001 to 5 wt%, particularly from 0.05 to 2 wto, of catalyst or catalyst combination, based on the weight of component (b).
e) Auxiliaries and/or additives (e) can be optionally incorpora-ted in the reaction mixture used for the preparation of the rigid PU foams. Specific examples thereof are surfactants, foam stabilizers) cell regulators, fillers, colorants, pig-ments, flame retardants, antihydrolysis agents and fungista tically and bacteriostatically effective substances.
Examples of suitable surfactants are compounds which are ca-pable of supporting homogenization of the starting materials and may also serve to control the cellular structure. Speci-fic examples are emulsifiers such as the sodium salts of sul-fated castor oil or of fatty acids and also salts of fatty acids with amines, for example diethylamine oleate, diethano-lamine stearate, diethanolamine ricinoleate, salts of sulfo-nic acids, for example alkali metal or ammonium salts of do-decylbenzenedisulfonic acid or dinaphthylmethanedisulfonic acid and ricinoleic acid; foam stabilizers, such as poly(si-loxane-co-alkoxylene)s and other organopolysiloxanes) ethoxy-lated alkylphenols, ethoxylated fatty alcohols, paraffinic oils, esters of castor oil or ricinoleic acid, Turkey-red oil and peanut oil and cell regulators such as alkanes, fatty al-cohols and dimethyl polys~loxanes. Furthermore, oligomeric BASF Aktiengesellschaft 970857 O.Z. 0050/48762 polyacrylates containing polyoxyalkylene radicals and fluoro-alkane radicals as side-groups are suitable for improving the emulsifying action, the cellular structure and/or the stabi-lity of the foam. The surfactants are usually employed in amounts of from 0.01 to 5 parts by weight, based on 100 parts by weight of component (b).
The fillers, particularly those having a reinforcing effect, include conventional organic and inorganic fillers, reinfor-cing agents, weighting agents, agents for improving the anti-scuff properties of paints, coating compositions, etc.. As specific examples thereof there may be mentioned inorganic fillers such as siliceous minerals, for example lamellar si-licates such as antigorite, serpentine, hornblendes, amphibo-les, chrisotile, talcum) metal oxides such as kaolin, alumi-num oxides, aluminum silicate, titanium oxides and iron oxi-des, metal salts such as chalk, heavy spar and inorganic pig-ments such as cadmium sulfide and zinc sulfide, and glass particles. Examples of suitable organic fillers are melamine, colophony, cyclopentadienyl resins and graft polymers.
The inorganic and organic fillers can be used individually or as mixtures and are incorporated in the reaction mixture ad-vantageously in amounts of from 0.5 to 50 wto, preferably from 1 to 40 wt%, based on the weight the components (a) and (b) .
Examples of suitable flame retardants are tricresyl phos-phate, tris(2-chloroethyl) phosphate, tris-(2-chloropropyl) phosphate, tris(1,3-dichloropropyl) phosphate, tris(2,3-di-bromopropyl) phosphate and tetrakis(2-chloroethyl)ethylene diphosphate.
Apart from the aforementioned halogen-substituted phosphates, use may also be made of inorganic flame retardants such as red phosphorus, hydrous aluminum oxide, antimony trioxide, arsenic oxide, ammonium polyphosphate and calcium sulfate or cyanuric derivatives such as melamine or mixtures of at least two flame retardants such as ammonium polyphosphates and me-lamine and optionally starch for flameproofing the rigid PU
foams manufactured by the process of the invention. It has been generally found to be advantageous to use from 5 to 50 parts by weight, preferably from 5 to 25 parts by weight, of the said flame retardant or mixture thereof for every 100 parts by weight of components (a) and (b).

BASF Aktiengesellschaft 970857 O.Z. 0050/48762 More detailed statements on the other conventional auxiliaries and additives mentioned above are to be found in the technical literature, for example in the monograph by J.H. Saunders and K.C. Frisch "High Polymers", Vol. XVI, in Polyurethanes, Parts 1 and 2, Interscience Publishers 1962 or 1964, or in Kunststoff-Handbuch, Polyurethanes, Vol. VII, Carl-Hanser-Verlag, Munich, Vienna, 1st and 2nd Editions, 1966 and 1983.
To prepare the rigid PU foams, the optionally modified organic polyisocyanates (a) and the compounds of higher molecular weight containing at least two reactive hydrogen atoms (b) are caused to react in amounts such that the equivalent ratio of NCO groups of polyisocyanates (a) to the total number of reactive hydrogen atoms in component. (b) is from 0.85:1 to 1.25:1, preferably from 0.95:1 to 1.15:1 and more preferably from approximately 1.0:1 to 1.10:1. If the urethane group-containing foams are modified by the formation of isocyanurate groups) for example to increase their flame resistance, a ratio of NCO groups in the polyisocyaw nates (a) to the total number of reactive hydrogen atoms in com-ponent (b) of from 1.5:1 to 10:1, preferably from 1.5:1 to 6:1 is usually maintained.
The rigid PU foams can be prepared batchwise or continuously by a prepolymerizing process or, preferably, by a one-shot process using known mixing devices.
It has been found to be particularly advantageous to operate using a two-component process in which constituents (b), (d) and (e) are combined to form component A and organic polyisocyanates or modified polyisocyanates (a) are used as component B.
Components A and B are separately degassed and dewatered by gentle heating (40~ to 50~C) and evacuation (<500 mbar). The per-fluoro compound is then incorporated in A or B, but preferably in A, by emulsification, with stirring. During this operation care must be taken to ensure that air is not entrapped in any great amount. The finished emulsion may have to be re-evacuated for a short period to remove any such excess air.
The resulting emulsion of perfluoro compound and component A is mixed with component B at a temperature of from 15~ to 90~C, pre-ferably from 20~ to 35~C, and placed in an optionally temperature-controlled mold, in which the reaction mixture is allowed to foam. To make a composite structure, the reverse side of a sur-face layer is advantageously coated with the foamable reaction BASF Aktiengesellschaft 970857 O.Z. 0050/48762 mixture, for example by pouring or spraying, and the latter is allowed to foam and harden to form a rigid PU foam.
In one embodiment of the invention the mold is evacuated follo wing the introduction of the mixture of components, so that foa ming takes place under reduced pressure. The lower the pressure used during foaming, the lower the foam density obtained and the higher the permissible boiling point of the perfluoro compound used. The pressure in the mold used should therefore be <1000 mbar but cannot be specified generally, since it is gover-ned by the boiling point of the perfluoro compound used and by the desired bulk density of the foamed article.
The densities of the rigid PU foams prepared by the process of the invention are preferably below 200 g/L. The foams are useful as intermediate heat-insulating layers in composite structures and for foam-filling cavities in casings for refrigerating ele-ments, particularly casings for refrigerators and freezers, and as jackets for hot water tanks and district heating pipelines.
The products are also suitable for the insulation of heated mate-rials and for use as engine covers and pipeline sleeves. Another application involves the use of an open-cell variant as foamed plastic in vacuum insulation panels or in other evacuated insula-ting elements.
Examples Examples 1 and 2 Component A was prepared from the following ingredients:
100 parts by weight of a polyether polyalcohol having a hy-droxyl value of 400 mg of KOH/g, prepared by anionic po-lyaddition of 1,2-propylene oxide to glycerol as primer 3 parts by weight of a polysiloxane-based foam stabilizer (Polyurax~ SR 321 sold by UCC) 0.3 part by weight of dibutyltin dilaurate.

The mixture was degassed and dewatered for 1 hour at 70~C in va-cuo. Following cooling, 10 parts by weight of perfluoropentane were incorporated by emulsification with vigorous stirring.
A

. BASF Aktiengesellschaft 970857 O.Z. 0050/48762 Component B was a mixture of diphenylmethane diisocyanates and polyphenyl-polymethylene polyisocyanates (crude MDI having an NCO
content of 31 wt%). It was likewise degassed by heating in vacuo.

Components A and B were mixed in a ratio, by weight, of 100:108 in conventional manner and were allowed to foam in an aluminum mold. The mold was capable of being evacuated, so that foaming was carried out under standard pressure (1.050 mbar) in one test 10 and under reduced pressure (600 mbar) in a second test.
There were obtained foams having very fine cells. Their characte-ristics are listed in Table 1.

15 Table 1 Ex. Pressure in foam Densit of foam Average cell diameter mold (g/L) (ym) (mbar) Example 3 Various amounts of perfluoropentane were incorporated in compo-nent A described in Examples 1 and 2, by emulsification, and foa-ming was then carried out as described in Examples 1 and 2. There were obtained foams having the following characteristics (Table 2) .
3 o Tabie 2 Content of PF-pentaneDensity of foam (g/L)Average cell diameter (% in foam) (gym) 2.2 289 63 4.4 159 54 3 6.6 116 62 8.8 102 59 Example 4 5 parts by weight of perfluorohexane and 5 parts by weight of perfluoropentane were incorporated, by emulsification, in 103 parts by weight of degassed and dewatered component A in a manner similar to that described in Example 1, and the emulsion obtained was caused to react with crude MDI in a ratio of 100:95 and allo-v BASF Aktiengesellschaft 970857 O.Z. 0050/48762 wed to foam. There was obtained a foam having a density of 195 g/L and an average cell diameter of 68 Vim.
parts by weight of perfluoropentane were emulsified in a de-5 gassed mixture of 108 parts by weight of crude MDI and 3 parts by weight of Polyurax~ SR 321. The emulsion was combined with a de-gassed mixture of 100 parts by weight of the polyether polyalco-hol described in Example 1 and 0.3 parts by weight of dibutyltin dilaurate and allowed to foam. There was obtained a foam having a 10 density of 142 g/L and an average cell diameter of 55 Vim.
Example 6 Component A
91.3 parts by weight of the polyether polyalcohol described in Example 1 3 parts by weight of foam stabilizer (Polyurax~ SR 321) 2.5 parts by weight of N,N-dimethylcyclohexylamine 4.2 parts by weight of water 100 parts by weight of component A were mixed with 163 parts by weight of crude MI7I and allowed to foam. There was obtained a foam having a density of 41 g/L and an average cell diameter of 231 Vim.
Example 7 (Comparative Example) Component A
100 parts by weight of the polyether polyalcohol described in Example 1 3 parts by weight of foam stabilizer (Polyurax~ SR 321) 2.5 parts by weight of N,N-dimethylcyclohexylamine 3.8 parts by weight of water 9 parts by weight of perfluorohexane were incorporated in the mixture by emulsification. The emulsion obtained was then caused to react with 158 parts by weight of crude MDI in conventional BASF Aktiengesellschaft 970857 O.Z. 0050/48762 manner and allowed to foam. There was obtained a foam having a density of 40 g/L and an average cell diameter of 109 Vim.

a

Claims (12)

1. A process for the preparation of a fine-cell rigid polyurethane foam by the reaction of a) a polyisocyanate with b) a compound having at least two reactive hydrogen atoms in the presence of c) an expanding agent, d) a catalyst and, optionally, e) an auxiliary and/or additive, wherein the expanding agent c) used is a perfluoro compound, which is emulsified in at least one of the components a) or b), and water and air are removed from the components a) to e) prior to conversion to the polyurethane.

2. A process as defined in claim 1, wherein the removal of water and air is carried out prior to the introduction of the expanding agent into the components a) and/or b).

3. A process as defined in claim 1, wherein the removal of water and air is carried out by heating the components a) to e).

4. A process as defined in claim 1, wherein the removal of water and air is carried out by heating the components a) to e) in vacuo.

5. A process as defined in claim I, wherein the removal of water and air is carried out by heating the components a) to e) to temperatures of from 40~ to 50~C under a pressure of less than 500 mbar.

6. A process as defined in claim 1, wherein the removal of water and air is carried out by evacuating the foam mold following the introduction of the foam components.

7. A process as defined in claim 1, wherein as the perfluoro compound used is a perfluoroalkane.

8. A process as defined in claim 7, wherein the perfluoroalkane is liquid at room temperature.

9. A process as defined in claim 7, wherein the perfluoroalkane used is perfluoropentane, perfluorohexane, perfluorooctane or a mixtures of at least two of said perfluoroalkanes.

10. A process as defined in claim 7, wherein the perfluoroalkane used is a mixture of perfluorobutane and/or perfluoropropane with perfluorohexane, perfluoroheptane and/or perfluorooctane.

11. A fine-cell rigid polyurethane foam whenever prepared by the process defined in claim 1.

12. A method of using a fine-cell rigid polyurethane foam as defined in claim 11 as an insulation material.