CA2260343A1

CA2260343A1 - Production of rigid foams based on isocyanate

Info

Publication number: CA2260343A1
Application number: CA002260343A
Authority: CA
Inventors: Eva Baum; Udo Rotermund; Michael Reichelt; Ludwig Jung
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1998-02-13
Filing date: 1999-02-11
Publication date: 1999-08-13
Also published as: DE19805879A1; EP0936240A2; EP0936240A3

Abstract

Rigid foams based on isocyanate are produced by reacting a) organic and/or modified organic polyisocyanates with b) at least one relatively high molecular weight compound containing at least two reactive hydrogen atoms and, if desired, c) low molecular weight chain extenders and/or crosslinkers in the presence of d) carbon dioxide as sole or additional blowing agent, e) foam stabilizers, f) catalysts and, if desired, g) further auxiliaries and/or additives, wherein at least one organosilicon compound having a relative ethylene oxide/propylene oxide weight ratio of over 70/30 is used as foam stabilizer, with the total Si content being greater than or equal to 5% by weight, based on the weight of the foam stabilizer or stabilizers used. The isocyanate-based rigid foams produced in this way can be used as insulation material.

Description

Production of rigid foams based on isocyanate The present invention relates to a process for producing rigid foams based on isocyanate, in which carbon dioxide is used as sole or additional blowing agent and at least one organosilicon compound is used as foam stabilizer, and also the use of these rigid foams as insulation material.
Rigid foams based on isocyanate, in particular polyurethane (PUR) and isocyanurate foams, have been known for a long time and are used predominantly for insulation against heat or cold, e.g. in refrigeration appliances, in building and construction, in hot water storage tanks and long-distance heating pipes. A summary overview of the production of such foams is given, for example, in the Kunststoff-Handbuch, Volume VII, "Polyurethane", 1st edition 1966, edited by Dr. R. Vieweg and Dr. A. Hochtlen, and 2nd edition, 1983, and also 3rd edition, 1993, edited by Dr. G.
Oertel, (Carl Hanser Verlag, Munich).
The foaming of PUR foams with addition of COz can be carried out by the gas loading process or by the liquid metering process.
There are already various machine manufacturers which offer such metering units. These processes can also be employed in the case of rigid foam systems, but with the disadvantage that the cell structure is destroyed when the amount of COz added increases. The cell diameters become larger and less uniform. The frothing effect forms large voids. The coarsening of the cells and the void structure :results in loss of the good insulating properties too.
The machine manufacturers are at present attempting to alleviate.
these deficiencEa by means of structural changes, by developing a new generation of mixing heads or various techniques for introducing the C02 (e. g. 18th Synthetic Foam Conference 1997 (Polyurethan-Formteile grenzenlos innovativ), Wiesbaden, R.
Apenburg et al j Cannon Group "Polyurethan mit Fliissig-COZ
sch~umen", pager 213-241).
The other way of: improving the COz compatibility and the degree of loading of the raw material component is a task for the raw material manufacturers.

In the technical journal "Plastics Technology" 09/1996, pages 29-35 (Sherman L.M. "Surfactants & Catalysts Are Matched To Ozone-Friendly Urethane Foams"), the influences of raw materials on the foaming of flexible PUR foam using liquid C02 are discussed. It is established that it is not the selection of stabilizer, but an appropriate machine technique in the C02 foaming process which leads to good results. Traditional stabilizers are said to be completely suitable.
In a lecture at Utech 96 (Noakes C.W. / Dow Europe S.A. and Casagrande G. / Dow Italia S.p.A. "Liquid C02 Blown Foam For Automotive Flexible Moulding"), various flexible foam formulations comprising silicone stabilizers are presented without these and their effects being described.
WO 97/10277 describes the use of various stabilizers. It is established that stabilizers having an alkylene oxide content of less than 37% b;y weight are used for producing flexible PUR block foams foamed using inert gases and their ethylene oxide content is from 20 to 6~7~ by weight. However, these stabilizers do not give void-free and homogeneous rigid foams by the C02 foaming technique.
It is an object of the present invention to produce foams based on isocyanate, in particular rigid PUR foams, having homogeneous, fine and void-free cell structures using the C02 foaming technique with the aid of suitable raw material formulations.
We have found that this object is surprisingly achieved by using organosilicon compounds having a relative ethylene oxide/propylene oxide weight ratio of over 70/30 as foam stabilizer, with the total Si content being greater than or equal.
to 5~ by weight.
The present invention accordingly provides a process for producing rigid foams based on isocyanate by reacting a) organic andior modified organic polyisocyanates with b) at least one: relatively high molecular weight compound containing at least two reactive hydrogen atoms and, if desired, c) low molecular weight chain extenders and/or crosslinkers in the presence of d) carbon dio:!cide as sole or additional blowing agent, e) foam stabi:Lizers, f) catalysts and, if desired, g) further au~ciliaries and/or additives, wherein at least one organosilicon compound having a relative ethylene oxide/propylene oxide weight ratio of over 70/30 is used as foam stabilizer, with the total Si content being greater than or equal to 5% by weight, based on the weight of the foam stabilizer or stabilizers used.
The invention further provides for the use of the isocyanate-based rigid foams produced in this way as insulation material.
According to th~s invention, at least one organosilicon compound having a relative ethylene oxide/propylene oxide weight ratio of over 70/30 is used as foam stabilizer. The ethylene oxide/propylene oxide ratio is preferably over 90/10, particularly preferably 100/0. In the latter case, this means that no propylene oxide is used.
The total Si content of the foam stabilizer is greater than or equal to 5% by vaeight, preferably greater than or equal to 10% by weight. As silicon-containing foam stabilizers for the production according to the: present invention of rigid foams based on isocyanate, prei:erence is given to using compounds of the structural formula (R1)3--Si-O S1 O Si-O Si (R5)3 I I

n I
O
I
CHZ
CHZ
x I

O
y m In this formula, R1, RZ, R4 and R5 are linear or branched alkyl radicals having from 1 to 3 carbon atoms and R3 is (CHz)P where p = 1, 2, 3, 4) 5 or 6. The ratio of x to y is greater than 70/30 and the total S:i content is greater than or equal to 5~ by weight.
In a particular:Ly advantageous embodiment, the commercially available silicon-containing foam stabilizers from Goldschmidt, for example Tegostab B 8423 or Tegostab B 8466, and Air Products, for example Dabc:o~ DC 5103, DabcoO DC 5357 or DabcoO OS 340, are used, individua::ly or in admixture.
Mixtures of one or more organosilicon compounds can be used as foam stabilizer:;. Apart from organosilicon compounds, it is also possible to make: concomitant use of further foam stabilizers customary in PUF: chemistry, as are mentioned below by way of example. If further foam stabilizers customary in PUR chemistry are used) it is necessary according to the present invention for the Si content of the total foam stabilizer mixture to be within the above-described limits in order to achieve the desired result.
Any further foam stabilizers employed are used in an amount of at 5 most from 0.5 to 3~ by weight, based on the total weight of the components (b), (f) and, if used, (c) and (g).
The rigid foams based on isocyanate are produced by reacting a) organic and/or modified organic polyisocyanates with b) at least or..e relatively high molecular weight compound containing at least two reactive hydrogen atoms and, if desired, c) low molecular weight chain extenders and/or crosslinkers in the presence of d) carbon dioxide as sole or additional blowing agent, e) foam stabilizers, f) catalysts and, if desired, g) further auxiliaries and/or additives, in a manner knovun per se .
To produce the :rigid foams based on isocyanate by the process of the present invention, use is made, with the exception of the foam stabilizers (e), of the starting materials customary in PUR
chemistry, abou~~ which the following may be said by way of example:
Suitable organic and/or modified organic diisocyanates and/or polyisocyanates (a) are the aliphatic, cycloaliphatic) araliphatic and preferably aromatic polyfunctional isocyanates known per se.
Specific examplEa are: alkylene diisocyanates having from 4 to 12 carbon atoms in the alkylene radical, e.g. dodecane 112-diisocyanai:e, 2-ethyltetramethylene 1,4-diisocyanate, 2-methylpentamel:hylene 1,5-diisocyanate, tetramethylene 1,4-diisocyanatE: and preferably hexamethylene 1,6-diisocyanate, cycloaliphatic diisocyanates such as cyclohexane 1,3- and 1,4-diisocyanate, and also any mixtures of these isomers, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), hexahydrotolylene 2,4- and 2,6-diisocyanate and also the corresponding isomer mixtures, dicyclohexylmethane 4,4'-, 2,2'-and 2,4'-diisocyanate and the also the corresponding isomer mixtures and preferably aromatic diisocyanates and polyisocyanates such as tolylene 2,4- and 2,6-diisocyanate (-TDI) and the corresponding isomer mixtures, diphenylmethane 4,4'-, 2,4'-and 2,2'-diisocyanate (-MDI) and the corresponding isomer mixtures, mixtures of 4,4'- and 2,2'-MDI, polyphenylpolymethylene polyisocyanates, mixtures of 4,4'-, 2,4'- and 2,2'-MDI and polyphenylpolymethylene polyisocyanates (crude MDI) and mixtures of crude MDI and TDI. The organic diisocyanates and polyisocyanates can be used individually or in the form of their mixtures.
Use is frequently also made of modified polyfunctional isocyanates, i.e. products which are obtained by chemical reaction of organic diisocyanates and/or polyisocyanates.
Examples which may be mentioned are diisocyanates and/or polyisocyanates containing ester, urea, biuret, allophanate, isocyanurate and preferably carbodiimide, uretdione and/or urethane groups. Specific examples of suitable modified isocyanates are: organic, preferably aromatic polyisocyanates containing urethane groups and having NCO contents of from 33.6 to 15~ by weight, preferably from 31 to 21% by weight, based on the total weight, for example 4,4'-MDI modified with low molecular weight diols, triols, dialkylene glycols, trialkylene glycols or polyoxyalkylene glycols having molecular weights of up to 6000, in par~~icular molecular weights up to 1500, modified 4,4'- and 2,4'-IrIDI mixtures or modified crude MDI or 2,4- or 2,6-TDI, with examples of dialkylene or polyoxyalkylene glycols .
which can be usE:d individually or as mixtures being: diethylene glycol, dipropy:Lene glycol, polyoxyethylene, polyoxypropylene and polyoxypropylene-polyoxyethylene glycols, triols and/or tetrols.
Also suitable a~.e prepolymers containing urethane groups and having an NCO content of from 25 to 3.5~ by weight, preferably from 21 to 14o by weight, or pseudoprepolymers having an NCO
content of from 35 to 14% by weight, preferably from 34 to 22~ by weight) based on the total weight, prepared by reacting diols, oxyalkylene glyc:ols and/or polyoxyalkylene glycols having molecular weight, of from 62 to 6000, preferably the polyether polyols and/or polyester polyols described below, with TDI, 4,4'-MDI, MDI isomer mixtures and/or crude MDI, e.g. at from 20 to 110°C, preferably from 50 to 90°C. Other modified isocyanates which have been found to be useful are liquid polyisocyanates containing carbodiimide groups and/or isocyanurate rings and having NCO contents of from 33.6 to 15~ by weight, preferably from 31 to 21~ by weight) based on the total weight, e.g. those based on MDI isomers and/or TDI.
The modified polyisocyanate can be mixed with one another or with unmodified organic polyisocyanates, such as, for example, 2,4'-and/or 4,4'-MDI, crude MDI, 2,4- and/or 2,6-TDI.
Organic polyisocyanates which have been found to be particularly useful and are therefore preferably employed are: TDI, 1~I, crude MDI and their mixtures or mixtures of modified organic polyisocyanate,s containing urethane groups and having an NCO
content of from 33.6 to 15~ by weight, in particular those based on TDI, 4,4'-M1~I, MDI isomer mixtures or crude MDI, and in particular crude MDI having an 1~I isomer content of from 30 to 80~ by weight, preferably from 30 to 55% by weight.
Suitable relatively high molecular weight.compounds containing at least two reaci:ive hydrogen atoms (b) are compounds which have two or more reactive groups selected from among OH groups, SH
groups, NH groups, NH2 groups and CH-acid groups, e.g. (3-diketo groups, in the molecule. Use is advantageously made of those having a functionality of from 2 to 8, preferably from 2 to 6, and a molecular- weight of from 300 to 8000, preferably from 400 to 4000.
Compounds which have been found to be useful are, for example, polyetherpolyamines and/or preferably polyols selected from the group consisting of polyether polyols, polyester polyols, polythioether F~olyols, polyesteramides, hydroxyl-containing polyacetals anc', hydroxyl-containing aliphatic polycarbonates or mixtures of at least two of the polyols mentioned. Preference is given to using polyester polyols and/or polyether polyols. The hydroxyl number of the polyhydroxyl compounds is generally from 100 to 850 and preferably from 200 to 600.
Suitable polyester polyols can be prepared, for example, from organic dicarboxylic acids having from 2 to 12 carbon atoms, preferably aliphatic dicarboxylic acids having from 4 to 6 carbon atoms, and polyhydric alcohols, preferably diols, having from 2 to 12 carbon atoms, preferably from 2 to 6 carbon atoms. Examples of suitable dicarboxylic acids are: succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, malefic acid, fumaric acid, phthalic acid, isophthalic acid and terephthalic acid. The dicarboxylic acids can be u~;ed either individually or in admixture with one another. In place of the free dicarboxylic acids, it is also possible to uss: the corresponding dicarboxylic acid derivatives such as dicarboxylic esters of alcohols having from 1 to 4 carbon atoms or dicarboxylic anhydrides. Preference is given to using dicarboxylic acid mixtures of succinic, glutaric and adipic acids in weight ratios of, for example, 20 - 35 : 35 - 50 . 20 - 32, and in particular adipic acid. Examples of dihydric and polyhydric alcohols) in particular diols, are: ethanediol, diethylene glycol, 1,2- or 1,3-propanediol, dipropylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,10-decanediol, glycerol and trimethylolpropane. Preference is given to using ethanediol, dieahylene glycol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexaned.iol. It is also possible to use polyester polyols derived from lactones, e.g. s-caprolactone, or hydroxycarboxylic acids, e.g. cc~-r~ydroxycaproic acid.
To prepare the polyester polyols, the organic, e.g. aromatic and preferably aliphatic, polycarboxylic acids and/or derivatives and polyhydric 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 monoxide) helium, argon, etc., in the melt at from 150 to 250°C, preferably from 180 to 220°C, under atmospheric or subatmospheric pressure to the desired acid number which is advantageously less than 10, preferably less than 2. According to a preferred embodiment, the esterification mixture is polycondensed at the abovementioned temperatures to an acid number of from 80 to 30, preferably from 40 to 30, under atmospheric pressure and subsequently under a pressure of less than 500 mbar) preferably from 50 to 150 mbar. Suitable esterification catalysts are, for example, iron, cadmium, cobalt, lead, zinc, antimony, magnesium, titanium and tin catalysts in the form of metals, metal oxides or metal salts. However, the polycondensation can also be carried out in the liquid phase in the presence of diluents and/or entrainers such as benzene, toluene, xylene or chlorobenzene to azeotropically distil off the water of condensation.
To prepare the polyester polyols, the organic polycarboxylic acids and/or derivatives and polyhydric alcohols are advantageously polycondensed in a molar ratio of 1:1 - 1.8, preferably 1:1.05 - 1.2. The polyester polyols obtained preferably have a functionality of from 2 to 4, in particular from 2 to 3, anti a molecular weight of from 300 to 3000, in particular from 400 to 600.

However, polyo7.s used are particularly preferably polyether polyols which are prepared by known methods, for example from one or more alkylene oxides having from 2 to 4 carbon atoms in the alkylene radical by anionic polymerization using alkali metal hydroxides such as sodium or potassium hydroxide or alkali metal alkoxides such as sodium methoxide, sodium or potassium ethoxide or potassium iaopropoxide as catalysts with addition of at least one initiator molecule containing from 2 to 8, preferably from 2 to 6, reactive hydrogen atoms in bound form, or by cationic polymerization using Lewis acids such as antimony pentachloride, boron fluoride etherate, etc., or bleaching earth as catalysts.
For specific applications, monofunctional initiators can also be incorporated into the polyether structure.
guitable alkyls:ne oxides are, for example, 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 oxide:. can be used individually, alternately in succession or a.s mixtures. Examples of suitable initiator molecules are: water, organic dicarboxylic acids such as succinic acid, adipic acid, phthalic acid and terephthalic acid, aliphatic and aromatic, u.nalkylated, N-mono-alkylated, N,N- and N,N'-di.alkylats:d diamines having from Z to 4 carbon atoms in the alkyl radical, e.g. unalkylated, monoalkylated and dialkylated ethylenediamine, diethylenetriamine, triethylenetetramine, 1,3-propylenedi.amine) 1,3- or 1,4-butylenediamine, 1,2-, 1,3-, 1,4-, 1,5- and 1,6-hexamethylenediamine, phenylenediamine, 2,3-, 2,4- and 2,6-tolylenediamine and 4,4'-, 2,4'- and 2,2'-diaminodiphenylmethane. Further suitable initiator molecules are: alkanolami.nes such as ethanolamine, N-methylethanolamine and N-ethylethanola.mine, dialkanolamines such as diethanolamine, N-methyldiethanolamine and N-ethyldiethanolamine, and trialkanolaminea such as triethanolamine, and ammonia. Preference is given to using polyhydric, in particular dihydric and/or trihydric, alcohols such as ethanediol, 1,2- and 2,3-propanediol, diethylene glycol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, glycerol, trimethylolpropane, pentaerythritol, sorbitol and sucrose.
The polyether F~olyols, preferably polyoxypropylene polyols and polyoxypropyler..~e-polyoxyethylene polyols, have a functionality of preferably front 2 to 6 and in particular from 2 to 4 and molecular weights of from 300 to 8000, preferably from 400 to 6000 and in particular from 1000 to 5000, and suitable polyoxytetramet.hylene glycols have a molecular weight up to about 3500.

Further suitable polyether polyols are polymer-modified polyether polyols, for example graft polyether polyols, in particular those based on styrene and/or acrylonitrile which are prepared by in situ polymerization of acrylonitrile) styrene or preferably 5 mixtures of styrene and acrylonitrile, e.g. in a weight ratio of from 90:10 to 10:90, preferably from 70:30 to 30:70, advantageously in the abovementioned polyether polyols using methods similar to those given in the German patents 1111394, 1222669 (US-A-3304273, 3383351, 3523093), 1152536 (GB 1040452) 10 and 1152537 (GH 987618), and also polyether polyol dispersions which contain as disperse phase, usually in an amount of from 1 to 50~ by weight, preferably from 2 to 25o by weight: e.g.
polyureas, polyhydrazides, polyurethanes containing bound tertiary amino groups and/or melamine and are described, for example, in EP-B-011752 (US-A-4304708), US-A-4374209 and DE-A-3231497.
Like the polyester polyols, the polyether polyols can be used individually or in the form of mixtures. They can also be mixed with the graft polyether polyols or polyester polyols or the hydroxyl-containing polyesteramides, polyacetals, polycarbonates and/or polyetherpolyamines.
Suitable hydro~:yl-containing polyacetals are, for example, the compounds which can be prepared from glycols such as diethylene glycol and trie:thylene glycol, 4,4'-dihydroxyeahoxydiphenyldimethylmethane or hexanediol and formaldehyde. ~~uitable polyacetals can also be prepared by polymerization of cyclic acetals.
Suitable hydro~:yl-containing polycarbonates are those of the type known per se which can be prepared, for example, by reacting diols such as 1,3-propanediol, 1,4-butanediol and/or 1~6-hexanediol, diethylene glycol, triethylene glycol or tetraethylene crlycol with diaryl carbonates, e.g. diphenyl carbonate, or phosgene.
The polyesteramides include, for example, the predominantly linear condensates obtained from polybasic, saturated and/or unsaturated carboxylic acids or their anhydrides and polyfunctional saturated and/or unsaturated aminoalcohols or mixtures of po7.yfunctional alcohols and aminoalcohols and/or polyamines.

Suitable polyetherpolyamines can be prepared from the abovementioned polyether polyols by known methods. Examples which may be mentioned are the cyanoalkylation of polyoxyalkylene polyols and subsequent hydrogenation of the nitrile formed (US-A-3267050) or the partial or complete amination of polyoxyalkylene polyols using amines or ammonia in the presence of hydrogen and catalysts (DE-A-1215373).
The rigid foams based on isocyanate can be produced with or without concomitant use of chain extenders and/or croslinkers (c). However, the addition of chain extenders, crosslinkers or, if desired, mixtures thereof can prove to be advantageous for modifying the mechanical properties, e.g. the hardness. Chain extenders and/or crosslinkers used are water and also diols and/or triols having molecular weights of less than 400, preferably from 60 to 300. Suitable chain extenders/crosslinkers are, for example, aliphatic, cycloaliphatic and/or araliphatic diols having from 2 to 14, preferably from 4 to 10, carbon atoms, e.g, ethylene glycol, 1,3-propanediol, 1,10-decanediol, o-, m-, p-dihydroxycyclohexane, diethylene glycol, dipropylene glycol and preferably 1,4-butanediol, 1,6-hexanediol and bis(2-hydroxyethyl)hydroquinone, triols such as 1,2,4-, 1,3,5-trihydroxycyclohexane, glycerol and trimethylolpropane, and low molecular weight hydroxyl-containing polyalkylene oxides based on ethylene oxide and/or 1,2-propylene oxide and the abovementioned diols and/or triols and/or pentaerythritol, sorbitol and sucrose as initiator molecules.
The chain extenders, crosslinkers or mixtures thereof are advantageously used in an amount of from 0 to 20% by weight, preferably from 2 to 8% by weight, based on the weight of the component (b).
As blowing agent (d), use is made according to the present invention of carbon dioxide, either alone or as additional blowing agent. 'The physical metering and the loading of the components with C02 is carried out in a known way. Details may be found in relevant specialist journals, for example in "Kunststoffe" 6/97, pages 722-727 (H. Klahre et al:
PUR-Schaumstoffe mit homogener Zellstruktur) and "Plastverarbeiter" 7/97, pages 46/47 (Perfekt geschaumt). Some information regarding the introduction of C02 is given below.
Apart from C02, it is possible to use further blowing agents which are generally k;aown from polyurethane chemistry.

Apart from chlorofluorocarbons (CFCs), whose use is being greatly restricted or completely stopped for ecological reasons, use can be made of, in particular, aliphatic and/or cycloaliphatic hydrocarbons having up to 12 carbon atoms, in particular butanes, pentanes cnd c~lclopentane, lower monofunctional alcohols, acetals such as methylal, or else partially halogenated hydrocarbons (HCFCs) as alternative blowing agents. Furthermore, it is advantageous to use perfluoro compounds such as perfluoroalkanes, preferably n-perfluoropentane, n-perfluorohexane, n-perfluoroheptane and n-perfluorooctane, as co-blowing agent.
The highly fluorinated and/or perfluorinated.hydrocarbons are usually emulsi:Eied in the polyol component. If emulsifiers are employed, use :is made, for example, of oligomeric acrylates which contain bound polyoxyalkylene and fluoroalkane radicals as side groups and have a fluorine content of from about 5 to 30% by weight. Such p:coducts are sufficiently well known from polymer chemistry.
The physical b:Lowing agents can be used individually or in any mixtures with one another, as pure isomers or as isomer mixtures.
They are usual:Ly added to the polyol component of the system.
However, they can also be added to the isocyanate component or, as a combination, both to the polyol component and to the isocyanate component.
The amount of blowing agent or blowing agent mixture used is from 1 to 25% by weight, preferably from 1 to 20 % by weight, in each case based on the components (b) to (g).
Furthermore, ii. is possible and customary to add water in an amount of from 0.5 to 15% by weight, preferably from 1 to 5% by weight, based on the formative components (b) to (g), as blowing agent. This ut:Llizes the blowing action of the COz generated by the isocyanate~-water reaction. The addition of water can be combined with 1=he use of the other blowing agents described.
As foam stabil:Lzers (e), use is made according to the present invention of the above-described organosilicon compounds, if desired in admixture with further foam stabilizers customary in PUR chemistry. Details of further foam stabilizers which may be used can be found in the specialist literature, for example the monograph by J.H. Saunders and K.C. Frisch "High Polymers" Volume XVI, Polyureth<~nes, Parts 1 and 2, Interscience Publishers 1962 and 1964, and also the above-cited Kunststoff-Handbuch, Volume VII, "Polyurethane".

Catalysts (f) used for producing foams based on isocyanate are, in particular, compounds which strongly accelerate the reaction of the compounds containing reactive hydrogen atoms, in particular hydroxyl groups, of the components (b). (c) and (e) with the organic, modified or unmodified polyisocyanates (a).
Suitable catalysts axe organic metal compounds, preferably organic tin compounds such as tin(II) salts of organic carboxylic acids, e.g. tin(II) acetate, tin(II) octoate, tin(II) ethylhexanoate and tin(II) laurate, and the dialkyltin(IV) salts of organic carboxylic acids, e.g. dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate and dioctyltin diacetate. The organic metal compounds are used alone or preferably in combination with strongly basic amines. Examples which may be mentioned are amidines such as 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, tertiary amines such as triethylamine, tributylamine, dimethylbenzylamine, N-methylmorpholine, N-ethylmorpholine, N-cyclohexylmorpholine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetramethylbutanediamine, N,N,N',N'-tetramethylhexane-1,6-diamine, pentamethyldiethylenetriamine, bis(dimethylaminoethyl) ether, bis(dimethylaminopropyl)urea, dimethylpiperazine, 1,2-dimethylimidazole, 1-azabicyclo(3,3.0]octane and preferably 1,4-diazabicyclo[2.2.2]octane, and alkanolamine compounds such as triethanolamine, triisopropanolamine, N-methyldiethanolamine and N-ethyldiethanolamine and dimethylethanolamine.
Further suitable catalysts are:
tris(dialkylaminoalkyl)-s-hexahydrotriazines, in particular tris(N,N-dimethylaminopropyl)-s-hexahydrotriazine, tetraalkylammonium hydroxides such as tetramethylammonium hydroxide, alkali metal hydroxides such as sodium hydroxide and alkali metal al:koxides such as sodium methoxide and potassium isopropoxide, a:nd also alkali metal salts of long-chain fatty acids having from 10 to 20 carbon atoms and possibly lateral OH
groups. Preference is given to using from 0.001 to 5% by weight, in particular from 0.05 to 3% by weight, of catalyst or catalyst combination, based on the weight of the formative components (b) to (g) .
If desired, further auxiliaries and/or additives (g) can be incorporated into the reaction mixture for producing the polyurethane foams. Examples which may be mentioned are flame retardants, surface-active substances) cell regulators, fillers, dyes, pigments, hydrolysis inhibitors, fungistatic and bacteriostatic substances.

Suitable flame retardants are, for example, tricresyl phosphate, tris(2-chloroet:hyl) phosphate, tris(2-chlororopyl) phosphate, tetrakis(2-chloroethyl) ethylenediphosphate, dimethyl methanephosphonate, diethyl diethanolaminomethylphosphonate and also commercial. halogen-containing flame retardant polyols. Apart from the abovementioned halogen-substituted phosphines, it is also possible t:o use inorganic or organic flame retardants such as red phosphorus, hydrated aluminum oxide, antimony trioxide, arsenic oxide, ammonium polyphosphate and calcium sulfate, expandable graF~hite or cyanuric acid derivatives such as melamine, or mixtures of at least two flame retardants such as ammonium polyphosphates and melamine and also, if desired, maize starch or ammonium polyphosphate, melamine and expandable graphite and/or aromatic or aliphatic polyesters for making the polyisocyanate polyaddition products flame resistant. Additions of melamine have been found to be particularly effective here. In general, it has been found to be advantageous to use from 5 to 60 parts by weight, preferably from 10 to 50 parts by weight, of the flame retardants mentioned per 100 parts by weight of the formative components (b) to (g).
Further details regarding the abovementioned and other starting materials may be found in the above-cited specialist literature.
To produce the rigid foams based on isocyanate, the components (a) to (g) are reacted in such amounts that the equivalence ratio of the NCO groups of the component (a) to the sum of the reactive hydrogen atoms of the components (b), (e), (f) and, if used, (c) and (g) is 0.85 - 1.75:1, preferably 1.0 - 1.3:1. If the rigid foams based on isocyanate contain at least some bonded isocyanurate gr~~ups. a ratio of said components of 1.5 - 60:1, preferably 3 - 'B : 1, is usually employed.
The isocyanate-based rigid foams produced by the process of the present invention are advantageously produced by the one-shot process, for example by means of the high-pressure or low-pressure technique in open or closed molds, for example metal molds. The continuous application of the reaction mixture to suitable conveyor belts for producing foam blocks is also customary.
It has been found to be particularly advantageous to employ the two-component method and to combine the formative components (b), (c), (d), (e), (f) and, if used, (g) to form a polyol component, also referred to as component A, and to use the formative component (a) and, if desired, blowing agent (d) as isocyanate component, also referred to as component B. The starting components are mixed at from 15 to 90°C, preferably from 20 to 60°C and in particular from 20 to 35°C, and introduced into the open mold or under atmospheric or superatmospheric pressure into 5 the closed mold. Mixing can be carried out, for example, mechanically by means of a stirrer or a stirring screw. The mold temperature is advantageously from 20 to 110°C, preferably from 30 to 60°C and in particular from 45 to 50°C.
10 The introduction of C02 as blowing agent for foaming is carried out in a known manner. The following 2 process routes have been found to be particularly useful:

15 1- Loading the polyol component with C02 gas at pressures of from 4 to 15 bar, preferably from 6 to 10 bar, to a C02 content which is below the frothing limit of the reaction mixture and needs to be determined for the respective system as a function of the raw material compo:~ition of the components. After loading, the polyol component is mixed with the isocyanate component in a mixing head and introduced into the mold or applied to a belt for foaming.
2. Liquid COZ as additional component is, in the amounts determined as described above, either metered via a nozzle directly into the main components in the mixing head or introduced by means of a metering unit into a static mixer immediately upstream of the mixing head and homogeneously mixed into the polyol component which is then conveyed to the mixing head.
The rigid PUR foams produced by the process of the present invention have a density of from 0.02 to 0.75 g/cm3, preferably from 0.025 to 0.24 g/cm3 and in particular from 0.03 to 0.1 g/cm3.
They are particularly suitable as insulation material for refrigeration appliances, in building for the insulation of cold stores, warehouses, long-distance heating pipes, containers and the like.
The present invention is illustrated by the Examples below.

Example 1 (Com;parison) The rigid foam formulation was made up using the stabilizer Tegostab B 8863 (Goldschmidt) having an ethylene oxide/propylene oxide weight r~3tio of 44/S6, an Si content of 3% by weight, based on the weight of the stabilizer used, and a hydroxyl number of 34 mg KOH/g:
A component:
55.5% by weight of a polyhydroxyl compound prepared from sucrose, glycerol and propylene oxide and having a hydroxyl number of 423 mg KOH/g, 20.0% b5~ weight of a Br-, Cl- and P-containing flame rsaardant mixture having a Br content of 34% by weight, a C1 content of 11% by weight, a P content of 3~c by weight and a hydroxyl number of 210 mg KOH/g, 20.0% by weight of trischloropropyl phosphate, 1.0% by weight of tris(dimethylaminopropyl)-s-he.xahydrotriazine, 0.5% by weight of triethylamine (TEA), 2.0% by weight of water and 1.0% by weight of Si stabilizer Tegostab B 8863 (Goldschmidt) B component:
Polyisocyanate Lupranat0 M 50 (BASF), viz. a mixture of diphenylmethane diisocyanate and polyphenyl polyisocyanates having an NCO content of 31.5% by weight and a viscosity of S50 mPas at 25°C.
Mixing ratio:
Component A 100 parts by weight Component B 117 parts by weight Results after machine foaming using C02 gas and liquid C02 (analogous resu:Lts are obtained by both methods):

C02 addition, '~ by weight 0 0.8 1.2 based on comp. A

Foam density, g/1 62 52 50 Thermal conductivity, mW/mK23.9 23.3 not measurable*

Cell size, Eun 190/250**190/290 not measurable*

Number of voids per 400 0 10 57 cm2 * no measurement was possible because of an inhomogeneous cell structure and large voids ** perpendicu:.ar/parallel to the direction of foaming The foam had an increasingly poorer homogeneity and void-containing cell structure with increasing C02 adddition for both C02 methods. The rigid foam could not be utilized.
Example 2 Example 2 was carried out using the same raw materials and the same mixing ratio as in Example 1, but the stabilizer Tegostab B
8863 was replaced by a mixture of stabilizers Tegostab B 8466 (G°ldschmidt) a;nd DabcoO OS 340 (Air Products), each in an amount of 0.5% by weight. The EO/PO ratio of the stabilizer mixture was 81/19%, the Si content was 7% by weight, based on~the weight of the stabilizer mixture used, and the hydroxyl number was 65 mg KOH/g.
Results after m<~chine foaming using C02 gas and liquid C02:
C02 addition, % by weight 0 0.8 1.2 based on comp. A

Foam density, g/1 61 52 50 Thermal conductivity, mW/m~K23.3 23.3 23.7 Cell size, Eun 180/240 190/250 220/260 Number of voids per 400 0 2 20 cm2 The ridid foam was more fine-celled and had fewer voids than that in Example 1.

Example 3 Example 3 was :Likewise carried out using the same raw materials and the same mixing ratio as in Example 1, but the stabilizer Tegostab B 886:3 was replaced by the stabilizer Dabco~ DC 5357 (Air Products) in an amount of 1.0% by weight. The EO/PO ratio was 100/0%, ths: Si content was 16% by weight, based on the weight of the stabili::er used, and the hydroxyl number was 50 mg KOH/g.
Results after machine foaming using C02 gas and liquid C02:
C02 addition, ~~ by weight,0 0.8 1.2 based on comp. A

Foam density, ~~/1 62 52 51 Thermal conductivity, mW/m~K23.3 23.0 23.2 Cell size, Eun 160/170 180/200 180/230 Number of voids per 400 0 0 0 cm2 The rigid foam was fine-celled and continued to have a homogeneous cell structure after C02 addition. Despite the frothing effect at 1.2% of C02, there was no formation of voids in the foam.
Example 4 The following e:~ample demonstrates a rigid foam formulation using cyclopentane as additional blowing agent. The stabilizer used was a mixture of Tec~ostab B 8423 and DabcoO DC 5103 having an EO/PO
weight ratio of 74/26, an Si content of 8% by weight and a hydroxyl number of 91 mg KOH/g:
A component:
54.0% by weight of a polyhydroxyl compound prepared from sorbitol and propylene oxide and having a hydroxyl number of 490 mg KOH/g, 28.0% by weight of a polyhydroxyl compound prepared from ~:ucrose and diethylene glycol and having a r.ydroxyl number of 400 mg KOH/g, 5.3% b~y weight of dipropylene glycol, 1.9% by weight of glycerol, 2.1% by weight of a catalyst mixture of dimethylcyclohexylamine and pentamethyldiethylene-triamine, 0.8% by weight ofwater, 0.9% by weight ofSi stabilizer Tegostab B
8423, 0.5% by weight ofSi stabilizer DabcoO DC5103 and 6.5% by weight ofcyclopentane B component:
Polyisocyanate Lupranat~ M 20 (BASF), viz. a mixture of diphenylmethane diisocyanate and polyphenyl polyisocyanates having an NCO content of 31.5% and a viscosity of 220 mPas at 25°C.
Mixing ratio:
Component A 100 parts by weight Component B 143 parts by weight Results after machine foaming using liquid C02:
C02 addition, '% by weight 0 0.6 1.2 based on comp. A

Foam density, g/1 57.5 51 49.5 Thermal conducaivity, mW/mK21.2 21.4 21.6 Cell size, E.tm 307/320**270/290 290/330 Number of voids per 400 0 5 10 cm2 ** perpendicular/parallel to the direction of foaming The foam had a finer cell structure with increasing COZ addition.
Void formation was significantly less than in Example 1. .
Example 5 Example 5 was carried out using the same raw materials and the same mixing ratio as in Example 4, but the stabilizer mixture was replaced by the: stabilizer Dabco~ DC 5357 (Air Products). The Si content of the stabilizer based on 100% by weight of EO was thus increased to 16% by weight.
Results after machine foaming using liquid COZ:

C02 addition, 9's by weight,0 0.6 1.2 based on comp. A

Foam density, ~~/1 57.5 51 49 Thermal conductivity, mW/mK20.8 21.0 21.2 5 Cell size, Eun 190/220 240/260 270/310 Number of voids per 400 0 1 5 cm2 10 The rigid foam was more fine-celled and had fewer voids than that in Example 4.

Claims

1. A process for producing rigid foams based on isocyanate by reacting a) organic and/or modified organic polyisocyanates with b) at least one relatively high molecular weight compound containing at least two reactive hydrogen atoms and, if desired, c) low molecular weight chain extenders and/or crosslinkers in the presence of d) carbon dioxide as sole or additional blowing agent, e) foam stabilizers, f) catalysts and, if desired, g) further auxiliaries and/or additives, wherein at least one organosilicon compound having a relative ethylene oxide/propylene oxide weight ratio of over 70/30 is used as foam stabilizer, with the total Si content being greater than or equal to 5% by weight, based on the weight of the foam stabilizer or stabilizers used.

2. A process as claimed in claim 1, wherein at least one compound of the structural formula is used as foam stabilizer, where R1, R2, R4 and R5 are linear or branched alkyl radicals having from 1 to 3 carbon atoms and R3 is (CH2)p where p = 1, 2, 3, 4, 5 or 6, the ratio of x to y is greater than 70/30 and the total Si content is greater than or equal to 5% by weight.

3. A process as claimed in claim 1, wherein the foam stabilizer is used in an amount of from 0.5 to 3% by weight, based on the total weight of the components b), d), e), f) and, if used, c) and g).

4. The use of the isocyanate-based rigid foams produced as claimed in claim 1 as insulation material.