Production of rigid polyurethane foams having a low thermal conductivity

Rigid polyurethane foams having a low thermal conductivity are produced by reacting PA1 a) organic and/or modified organic polyisocyanates with PA1 b) at least one compound containing at least 2 reactive hydrogen atoms in the presence of PA1 c) blowing agents, PA1 d) catalysts and, if desired, PA1 e) auxiliaries and/or additives, wherein graphite is added to the component a) and/or b).

The present invention relates to a process for producing rigid 
polyurethane, hereinafter referred to as PU for short, foams from 
formative components known per se in the presence of graphite and the use 
of these rigid PU foams for filling hollow spaces in refrigeration 
appliances or heating elements with foam and also as insulation material 
for composite elements. 
The production of composite or sandwich elements which are composed of a 
rigid PU foam and at least one covering layer of a rigid or elastic 
material, e.g. paper, plastic films, metal sheets, glass nonwovens, 
particleboards, etc., is known. Also known is the filling of hollow spaces 
in household appliances such as refrigeration appliances, for example 
refrigerators or freezer chests, with rigid PU foam as insulation material 
or cladding hot water storage tanks with such foam. In order to avoid 
defects in the foam, the foamable PU reaction mixture has to be introduced 
into the hollow space to be insulated within a short time. Low-pressure or 
preferably high-pressure machines are customarily used for filling hollow 
spaces of such items with foam. 
It is known that heat- and cold-insulating rigid PU foams suitable for this 
purpose can be produced by reacting organic polyisocyanates with one or 
more compounds containing at least two reactive hydrogen atoms, preferably 
polyester polyols and/or polyether polyols, customarily with concomitant 
use of chain extenders and/or crosslinkers, in the presence of blowing 
agents, catalysts and, if desired, auxiliaries and/or additives. If the 
formative components are selected appropriately, rigid PU foams having a 
low thermal conductivity and good mechanical properties can be obtained in 
this way. 
A summary overview of the production of rigid PU foams and their use as 
covering or preferably core layer in composite elements and also their use 
as insulating layer in cooling or heating engineering has been published, 
for example, in Polyurethane, Kunststoff-Handbuch, Volume 7, 1st Edition 
1966, edited by Dr. R. Vieweg and Dr. A. Hochtlen, and 2nd Edition 1983, 
edited by Dr. Gunter Oertel, Carl Hanser Verlag, Munich, Vienna. 
As blowing agents, use has been made in the past of chlorofluorocarbons 
(CFCs) which, owing to the known ozone problems, have more recently been 
replaced by hydrochlorofluorocarbons (HCFCs). Since even HCFCs have a 
certain ozone degradation potential, halogen-free blowing agents are being 
increasingly used. Typical representatives of this group of substances are 
n-pentane and cyclopentane. For technical reasons, foaming is not carried 
out using pentane alone, but CO.sub.2 which is formed in situ by the 
reaction of NCO groups with water is employed as co-blowing agent. The 
disadvantage of the pentanes is their somewhat poorer insulation 
performance compared with CFCs. Reduction of the thermal conductivity of 
such foams is therefore an important problem. 
The thermal conductivity of a foam is made up of 3 contributions: thermal 
conductivity of the gas phase, thermal conductivity of the matrix and 
thermal conductivity by radiation. Since blowing agent and matrix are 
fixed, the reduction of the radioactive component of the thermal 
conductivity is the most important possible method in practice. 
It is known that the radioactive component of the thermal conductivity of 
foams can be reduced by addition of carbon black (JP 5 7147-510, DE 3 629 
390, U.S. Pat. No. 4,795,763, U.S. Pat. No. 5,137,930, U.S. Pat. No. 
5,149,722, U.S. Pat. No. 5,192,607, JP 6 228 267, JP 7 082 402, U.S. Pat. 
No. 5,397,808, U.S. Pat. No. 5,565,497, EO-A-338 131, WO 94/13721, WO 
95/10558). However, carbon black in PU foam has 2 great disadvantages: a) 
handling, in particular the dispersion of the very fine material in the 
starting components, is difficult and requires complicated precautionary 
measures and b) the relatively hard carbon black particles have, in the 
long term, an abrasive effect on the carefully ground pistons of the 
mixing heads of PU metering machines. 
It is an object of the invention to reduce the thermal conductivity of 
rigid PU foams while overcoming the disadvantages resulting from the use 
of carbon black. 
We have found that this objective is achieved by using graphite instead of 
carbon black as additive. This gives a better reduction in the thermal 
conductivity of PU foams and, owing to its physical nature, graphite does 
not have the abovementioned adverse processing properties of carbon black. 
The present invention accordingly provides a process for producing rigid PU 
foams having a low thermal conductivity by reacting 
a) organic and/or modified organic polyisocyanates with 
b) at least one compound containing at least 2 reactive hydrogen atoms and, 
if desired, 
c) blowing agents 
d) catalysts and, if desired, 
e) auxiliaries and/or additives, 
wherein the foams further comprise graphite. 
The graphite content of the PU foam is preferably 1-20% by weight, in 
particular 1-10% by weight, based on the weight of the foam. Lower 
contents do not sufficiently lower the thermal conductivity while higher 
contents can lead to damage to the foam framework. 
It is particularly advantageous if at least 50% of the graphite has a 
particle size of less than 20 .mu.m. 
To produce the rigid PU foams by the process of the present invention, use 
is made of formative components known per se about which the following 
details may be given. 
a) Suitable organic polyisocyanates are the aliphatic, cycloaliphatic, 
araliphatic and preferably aromatic polyfunctional isocyanates known per 
se. 
Specific examples are: alkylene diisocyanates having from 4 to 12 carbon 
atoms in the alkylene radical, for example dodecane 1,12-diisocanate, 
2-ethyltetramethylene 1,4-diisocyanate, 2-methylpentamethylene 
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 
(isophorone diisocyanate), hexahydrotolylene 2,4- and 2,6-diisocyanate and 
also the corresponding isomer mixtures, dicyclohexylmethane 4,4'- and 
2,2'-diisocyanate and also the corresponding isomer mixtures and 
preferably aromatic diisocyanates and polyisocyanates such as tolylene 
2,4- and 2,6-diisocyanate and the corresponding isomer mixtures, 
diphenylmethane 4,4'-, 2,4'- and 2,2,'-diisocyanate and the corresponding 
isomer mixtures, mixtures of diphenylmethane 4,4'- and 2,4'-diisocyanates, 
polyphenylpolymethylene polyisocyanates, mixtures of diphenylmethane 
4,4'-, 2,4'- and 2,2'-diisocyanates and polyphenylpolymethylene 
polyisocyanates (crude MDI) and mixtures of crude MDI and tolylene 
diisocyanates. The organic diisocyanates and polyisocyanates can be used 
individually or in the form of mixtures. 
Use is frequently also made of modified polyfunctional isocyanates, ie. 
products which are obtained by partial chemical reaction of organic 
diisocyanates and/or polyisocyanates. Examples which may be mentioned are 
diisocyanates and/or polyisocyanates containing ester, urea, biuret, 
allophanate, carbodiimide, isocyanurate and/or urethane groups. Specific 
examples 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, 
diphenylmethane 4,4'-diisocyanate or tolylene 2,4- or 2,6-diisocyanate 
modified, for example, with low molecular weight diols, triols, dialkylene 
glycols, trialkylene glycols or polyoxyalkylene glycols having molecular 
weights of up to 1500, where examples of dialkylene or polyoxyalkylene 
glycols which can be used individually or as mixtures are: diethylene 
glycol, dipropylene glycol, polyoxyethylene, polyoxypropylene and 
polyoxypropylene-polyoxyethylene glycols or triols. Also suitable are 
prepolymers containing NCO groups and having NCO contents of from 25 to 9% 
by weight, preferably from 21 to 14% by weight, based on the total weight, 
prepared from the polyester polyols and/or preferably polyether polyols 
described below and diphenylmethane 4,4'-diisocyanate, tolylene 2,4- 
and/or 2,6-diisocyanates or crude MDI. Further 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, eg. those based on diphenylmethane 4,4'-, 2,4'- and/or 
2,2'-diisocyanate and/or tolylene 2,4- and/or 2,6-diisocyanate. 
The modified polyisocyanates can, if desired, be mixed with one another or 
with unmodified organic polyisocyanates such as diphenylmethane 2,2'- 
and/or 4,4'-diisocyanate, crude MDI, tolylene 2,4- and/or 
2,6-diisocyanate. 
Organic polyisocyanates which have been found to be particularly useful and 
are therefore preferably employed for producing the rigid PU foams are: 
mixtures of tolylene diisocyanates and crude MDI or mixtures of modified 
organic polyisocyanates containing urethane groups and having an NCO 
content of from 33.6 to 15% by weight, in particular those based on 
tolylene diisocyanates, diphenylmethane 4,4'-diisocyanate, diphenylmethane 
diisocyanate isomer mixtures or crude MDI and in particular crude MDI 
having a diphenylmethane diisocyanate isomer content of from 30 to 80% by 
weight, preferably from 30 to 55% by weight. 
b) Suitable compounds containing at least two reactive hydrogen atoms (b) 
are preferably polyhydroxyl compounds having a functionality of from 2 to 
8, preferably from 3 to 8, and a hydroxyl number of from 150 to 850, 
preferably from 350 to 800, and also, if desired, chain extenders and/or 
crosslinkers. 
Examples of polyhydroxyl compounds which may be mentioned are polythioether 
polyols, polyesteramides, hydroxyl-containing polyacetals, 
hydroxyl-containing aliphatic polycarbonates and preferably polyester 
polyols and polyether polyols. Mixtures of at least two of the 
polyhydroxyl compounds mentioned can also be employed as long as these 
have an average hydroxyl number in the abovementioned range. 
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, maleic acid, fumaric acid, phthalic 
acid, isophthalic acid and terephthalic acid. The dicarboxylic acids can 
be used either individually or in admixture with one another. In place of 
the free dicarboxylic acids, it is also possible to use the corresponding 
dicarboxylic acid derivatives such as dicarboxylic monoesters or diesters 
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 acid 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, diethylene 
glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol or mixtures of at 
least two of the diols mentioned, in particular mixtures of 
1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol. It is also possible to 
use polyester polyols derived from lactones, eg. .epsilon.-caprolactone, 
or hydroxycarboxylic acids, eg. .omega.-hydroxycaproic acid. 
To prepare the polyester polyols, the organic, eg. 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 gases such as nitrogen, carbon dioxide, helium, argon, etc., in the 
melt at from 150 to 250.degree. C., preferably from 180 to 220.degree. 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. Examples 
of suitable esterification catalysts are 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 for 
azeotropically distilling off the water of condensation. 
To prepare the polyester polyols, the organic polycarboxylic acids and/or 
derivatives and polyhydric alcohols are polycondensed, advantageously in a 
molar ratio of from 1:1-1.8, preferably 1:1.05-1.2. 
The polyester polyols obtained preferably have a functionality of from 2 to 
3 and a hydroxyl number of from 150 to 400 and in particular from 200 to 
300. 
However, the polyhydroxyl compounds 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 isopropoxide 
as catalysts and at least one initiator molecule which contains from 2 to 
8, preferably from 3 to 8 reactive hydrogen atoms in bonded form or by 
cationic polymerization using Lewis acids such as antimony pentachloride, 
boron fluoride etherate, etc., or bleaching earth as catalysts. 
Suitable alkylene 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 oxides can be used 
individually, alternately in succession or as 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, unalkylated, N-monoalkylated, N,N- and 
N,N'-dialkylated diamines having from 1 to 4 carbon atoms in the alkyl 
radical, for example unalkylated, monoalkylated and dialkylated 
ethylenediamine, diethylenetriamine, triethylenetetramine, 
1,3-propylenediamine, 1,2- or 1,4-butylenediamine, 1,2-, 1,3-, 1,4-, 1,5- 
and 1,6-hexamethylenediamine, phenylenediamines, 2,3-, 2,4- and 
2,6-tolylenediamine and 4,4'-, 2,4'- and 2,2'-diaminodiphenylmethane. 
Further suitable initiator molecules are: alkanolamines such as 
ethanolamine, diethanolamine, N-methylethanolamine and 
N-ethylethanolamine, N-methyldiethanolamine and N-ethyldiethanolamine and 
triethanolamine, and ammonia. 
Preference is given to using polyhydric, in particular trihydric and/or 
higher-hydric alcohols such as ethanediol, 1,2- and 1,3-propanediol, 
diethylene glycol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, 
glycerol, trimethylolpropane, pentaerythritol, sorbitol and sucrose. 
The polyether polyols have a functionality of preferably from 3 to 8 and in 
particular from 3 to 6 and hydroxyl numbers of preferably from 300 to 850 
and in particular from 350 to 800. 
Further suitable polyether polyols are melamine-polyether polyol 
dispersions as described in EP-A-23 987 (U.S. Pat. No. 4,293,657), 
polymer-polyether polyol dispersions prepared from polyepoxides and epoxy 
resin hardeners in the presence of polyether polyols as described in DE 29 
43 689 (U.S. Pat. No. 4,305,861), dispersions of aromatic polyesters in 
polyhydroxyl compounds as described in EP-A-62 204 (U.S. Pat. No. 
4,435,537) or DE-A 33 00 474, dispersions of organic and/or inorganic 
fillers in polyhydroxyl compounds as described in EP-A-11 751 (U.S. Pat. 
No. 4,243,755), polyurea-polyether polyol dispersions as described in 
DE-A-31 25 402, tris(hydroxyalkyl) isocyanurate-polyether polyol 
dispersions as described in EP-A-136 571 (U.S. Pat. No. 4,514,526) and 
crystallite suspensions as described in DE-A-33 42 176 and DE-A-33 42 177 
(U.S. Pat. No. 4,560,708), where the disclosures in the patent 
publications mentioned are incorporated by reference into the present 
patent description. 
Like the polyester polyols, the polyether polyols can be used individually 
or in the form of mixtures. They can also be mixed with the abovementioned 
dispersions, suspensions or polyester polyols and with the 
hydroxyl-containing polyesteramides, polyacetals and/or polycarbonates. 
Suitable hydroxyl-containing polyacetals are, for example, the compounds 
which can be prepared from glycols such as diethylene glycol, triethylene 
glycol, 4,4'-dihydroxyethoxy-diphenyldimethylmethane or hexanediol and 
formaldehyde. Suitable polyacetals can also be prepared by polymerization 
of cyclic acetals. 
Suitable hydroxyl-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 glycol with diaryl carbonates, eg. 
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 amino alcohols or mixtures of 
polyfunctional alcohols and amino alcohols and/or polyamines. 
The rigid PU foams can be produced with or without use of chain extenders 
and/or crosslinkers. However, the addition of chain extenders, 
crosslinkers or, if desired, mixtures thereof can prove to be advantageous 
for modifying the mechanical properties. Chain extenders and/or 
crosslinkers used are preferably alkanolamines and in particular diols 
and/or triols having molecular weights of less than 400, preferably from 
60 to 300. Examples of suitable chain extenders/crosslinkers are 
alkanolamines such as ethanolamine and/or isopropanolamine, 
dialkanolamines such as diethanolamine, N-methyldiethanolamine, 
N-ethyldiethanolamine, diisopropanolamine, trialkanolamines such as 
triethanolamine, triisopropanolamine and the addition products of ethylene 
oxide or 1,2-propylene oxide and alkylenediamines having from 2 to 6 
carbon atoms in the alkylene radical, eg. 
N,N'-tetra(2-hydroxyethyl)ethylenediamine and 
N,N'-tetra(2-hydroxypropyl)ethylenediamine, aliphatic, cycloaliphatic 
and/or araliphatic diols having from 2 to 14, preferably from 4 to 10, 
carbon atoms, eg. 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- or 
1,3,5-trihydroxy-cyclohexane, glycerol and trimethylolpropane and low 
molecular weight hydroxyl-containing polyalkylene oxides based on ethylene 
oxide and/or 1,2-propylene oxide and aromatic diamines such as 
tolylenediamines and/or diamino-diphenylmethanes and also the 
abovementioned alkanolamines, diols and/or triols as initiator molecules. 
If chain extenders, crosslinkers or mixtures thereof are employed for 
producing the rigid PU foams, they are advantageously used in an amount of 
from 0 to 20% by weight, preferably from 2 to 5% by weight, based on the 
weight of the compounds containing at least two reactive hydrogen atoms. 
c) Blowing agents used are the customary physical blowing agents such as 
alkanes, alkenes, cycloalkanes, esters, ethers, ketones, acetals, 
fluoroalkanes, hydrofluorochloroalkanes, etc. Specific examples are: 
butane, n-pentane, iso-pentane, cyclopentane, cyclohexane, methyl formate, 
ethyl formate, methyl acetate, ethyl acetate, methyl ethyl ether, diethyl 
ether, acetone, formaldehyde dimethyl acetal, tetrafluoroethane, 
difluorochloromethane or 1,1,1-dichlorofluoroethane. Naturally, the 
physical blowing agents can also be used as mixtures. A combination of 
physical blowing agents and water, ie. CO.sub.2 which is formed in the 
reaction of water with isocyanate, is advantageous and preferred. 
d) Catalysts (e) used are, in particular, compounds which greatly 
accelerate the reaction of the hydroxyl-containing compounds of the 
component (b) with the polyisocyanates. Suitable catalysts are organic 
metal compounds, preferably organic tin compounds such as tin(II) salts of 
organic carboxylic acids, eg. tin(II) acetate, tin(II) octoate, tin(II) 
ethylhexanoate and tin(II) laurate, and the dialkyltin(IV) salts of 
organic carboxylic acids, eg. dibutyltin diacetate, dibutyltin dilaurate, 
dibutyltinmaleate 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'-tetramethyl-butanediamine 
or N,N,N',N'-tetramethylhexanediamine, 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 dimethyl-ethanolamine. 
Further suitable catalysts are: 
tris(dialkylaminoalkyl)-s-hexahydrotriazines, in particular 
tris(N,N-dimethylaminopropyl)-s-hexahydrotriazine, tetraalkylammonium 
hydroxide such as tetramethylammonium hydroxide, alkali metal hydroxides 
such as sodium hydroxide and alkali metal alkoxides such as sodium 
methoxide and potassium isopropoxide and 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 2% by weight, of catalyst or catalyst 
combination, based on the weight of the component (b). 
e) If desired, auxiliaries and/or additives (e) can also be incorporated 
into the reaction mixture for producing the rigid PU foams. Examples which 
may be mentioned are surface-active substances, foam stabilizers, cell 
regulators, fillers, dyes, pigments, flame retardants, hydrolysis 
inhibitors, fungistatic and bacteriostatic substances. 
Suitable surface-active substances are, for example, compounds which serve 
to aid the homogenization of the starting materials and may also be 
suitable for regulating the cell structure. Examples which may be 
mentioned are emulsifiers such as the sodium salts of castor oil sulfates 
or of fatty acids and also amine salts of fatty acids, eg. diethylamine 
oleate, diethanolamine stearate, diethanolamine ricinoleate, salts of 
sulfonic acids, eg. alkali metal or ammonium salts of dodecylbenzene- or 
dinaphthylmethanedisulfonic acid and ricinoleic acid; foam stabilizers 
such as siloxane-oxalkylene copolymers and other organopolysiloxanes, 
ethoxylated alkylphenols, ethoxylated fatty alcohols, paraffin oils, 
castor oil or ricinoleic esters, Turkey red oil and peanut oil and cell 
regulators such as paraffins, fatty alcohols and dimethylpolysiloxanes. 
Oligomeric polyacrylates having polyoxyalkylene and fluoroalkane radicals 
as side groups are also suitable for improving the emulsifying action, the 
cell structure and/or stabilizing the foam. The surface-active substances 
are usually employed in amounts of from 0.01 to 5 parts by weight, based 
on 100 parts by weight of the component (b). 
Fillers, in particular reinforcing fillers, are, for the purposes of the 
present invention, the customary organic and inorganic fillers, 
reinforcing materials, weighting agents, agents for improving the abrasion 
behavior in paints, coatings, etc, known per se. Specific examples are: 
inorganic fillers such as siliceous minerals, for example sheet silicates 
such as antigorite, serpentine, hornblends, amphiboles, chrysotile, talc; 
metal oxides such as kaolin, aluminum oxide, aluminum silicate, titanium 
oxides and iron oxides, metal salts such as chalk, barite and inorganic 
pigments such as cadmium sulfide, tinsulfide and also glass particles. 
Examples of suitable organic fillers are: melamine, rosin, 
cyclopentadienyl resins and graft polymers. 
The inorganic and organic fillers can be used individually or as mixtures 
and are advantageously incorporated into the reaction mixture in amounts 
of from 0.5 to 50% by weight, preferably from 1 to 40% by weight, based on 
the weight of the components (a) and (b). 
Suitable flame retardants are, for example, tricresyl phosphate, 
tris(2-chloroethyl) phosphate, tris(2-chloropropyl) phosphate, 
tris(1,3-dichloropropyl) phosphate, tris(2,3-dibromopropyl) phosphate and 
tetracis(2-chloroethyl)ethylene diphosphate. 
Apart from the abovementioned halogen-substituted phosphates, it is also 
possible to use inorganic flame retardants such as red phosphorus, 
hydrated aluminum oxide, antimony trioxide, arsenic oxide, ammonium 
polyphosphate and calcium sulfate 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, starch for making the 
rigid PU foams produced according to the invention flame resistant. In 
general, it has been found to be advantageous to use from 5 to 50 parts by 
weight, preferably from 5 to 25 parts by weight, of the abovementioned 
flame retardants or mixtures for each 100 parts by weight of the 
components (a) to (c). 
Further details regarding the abovementioned other customary auxiliaries 
and additives may be found in the specialist literature, for example the 
monograph by J. H. Saunders and K. C. Frisch "High Polymers", Volume XVI, 
Polyurethanes, Parts 1 and 2, Interscience Publishers 1962 or 1964, or the 
Kunststoff-Handbuch, Polyurethane, Volume VII, Carl-Hanser-Verlag, Munich, 
Vienna, 1st, 2nd and 3rd Editions, 1966, 1983 and 1993. 
According to the present invention, graphite is used as additive for 
reducing the thermal conductivity. Graphite can be added in an amount of 
up to 20% by weight, preferably from 1 to 10% by weight, based on the 
foam. Particularly useful forms of graphite are finely milled graphite 
grains which have a particle distribution in which at least 50% is less 
than 20 .mu.m, preferably less than 10 .mu.m. 
It is advantageous to disperse the graphite with stirring in the formative 
component (a) and/or in the formative component (b) before producing the 
foam by mixing the components. 
To produce the rigid PU foams, the organic, unmodified or modified 
polyisocyanates (a), the relatively high molecular weight compounds 
containing at least two reactive hydrogen atoms and, if desired, chain 
extenders and/or crosslinkers (b) are reacted in such amounts that the 
equivalence ratio of NCO groups of the polyisocyanates (a) to the sum of 
the reactive hydrogen atoms of the component (b) is 0.85-1.25:1, 
preferably 0.95-1.15:1 and in particular about 1.0-1.10:1. If the foams 
containing urethane groups are modified by the formation of isocyanurate 
groups, for example to increase the flame resistance, it is usual to 
employ a ratio of NCO groups of the polyisocyanates (a) to the sum of the 
reactive hydrogen atoms of the component (b) of 1.5-10:1, preferably 
1.5-6:1. 
The rigid PU foams can be produced batchwise or continuously by the 
prepolymer method or preferably by the one-shot method with the aid of 
known mixing equipment. 
It has been found to be particularly advantageous to employ the 
two-component process and to combine the formative components (b), (c), 
(d) and, if used, (e) as the polyol component (A component) and to use the 
organic polyisocyanates and, if desired, blowing agents (c) as component 
(B). 
The starting components are mixed at from 15 to 90.degree. C., preferably 
from 20 to 35.degree. C., and introduced into an open, heated or unheated 
mold in which the reaction mixture is allowed to foam essentially without 
application of pressure to avoid a compacted surface zone. To form 
composite elements, one side, advantageously the reverse side, of a 
covering layer is coated with the foamable reaction mixture, eg. by 
casting or spraying, and the mixture is allowed to foam and cure to give 
the rigid PU foam. 
The rigid PU foams produced by the process of the present invention 
preferably have densities of from 20 to 50 g/l. 
The rigid PU foams are preferably used as thermally insulating intermediate 
layer in composite elements and for filling hollow spaces in refrigeration 
appliance housings, in particular for refrigerators and freezer chests, 
with foam and as outer cladding of hot water storage tanks. The products 
are also suitable for insulating heated materials, as engine covering and 
as sheathing for pipes.

EXAMPLES 
In all examples, foams were produced in a wooden mold having the dimensions 
20.times.20.times.20 cm. To measure the thermal conductivity, specimens 
having the dimensions 20.times.20.times.5 cm were sawn from the foam block 
and measured in a customary laboratory instrument (model: Lambda-Control 
from Hesto/6070, Langen). Since, in practice, the gas-phase composition of 
the foams and thus the thermal conductivity changes on storage as a result 
of CO.sub.2 diffusing out and air diffusing in, the specimens were heated 
at 70.degree. C. for 7 days before the measurement. Experience has shown 
that the CO.sub.2 /air exchange is concluded after this time and constant 
thermal conductivity values are obtained. Measurements were carried out 
both parallel and perpendicular to the foaming direction, since 
free-foamed PU foams normally display anisotropy and the thermal 
conductivities in the two directions are different. 
The following 5 types of graphite were used (manufacturer: Graphitwerk 
Kropfmuhl AG). 
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Particle size 
Manufacturer's distribution 
Type designation % C (minimum) (50% value) 
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A AF 96-97 8.5-11.0 .mu.m 
B AFspez. 99.5 6.0-8.5 
C UF 4 96-97 5.5-7.0 
D UF 4 99.5 5.5-7.0 
E UF 2 96-97 4.0-5.5 
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To produce the foams, a polyol premix having the following composition was 
prepared. 
78 parts by weight of a polyether polyol having a hydroxyl number of 400 mg 
KOH/g and prepared by anionic polyaddition of 1,2-propylene oxide onto 
sucrose. 
15 parts by weight of a polyether polyol having a hydroxyl number of 105 mg 
KOH/g and prepared by anionic polyaddition of 1,2-propylene oxide onto 
dipropylene glycol. 
2.5 parts by weight of a foam stabilizer based on silicone (Polyurax.RTM. 
SR 321 from UCC) 
2.0 parts by weight of N,N-dimethylcyclohexylamine 
The respective blowing agent was added to the polyol premix and graphite 
was subsequently stirred in. The dispersions obtained were mixed with 
Lupranat.RTM. M20 (mixture of diphenylmethane diisocyanate and 
polyphenylpolymethylene polyisocyanate, NCO content 31% by weight) and 
allowed to foam. The composition of the reaction mixtures of the 
individual examples, the measured thermal conductivity values after 
CO.sub.2 /air exchange has been completed and also the reduction in the 
thermal conductivity values achieved by graphite addition are shown in the 
table. 
TABLE 1 
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Example 
1 7 
(Comparison) 2 3 4 5 6 (Comparison) 8 
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Polyol premix 
97.5 97.5 
97.5 
97.5 
97.5 
97.5 
97.5 97.5 
Water 2.2 2.2 2.2 2.2 
2.2 2.2 2.5 
2.5 
n-Pentane -- -- -- -- -- 
-- 11 11 
Cycopentane 
9 9 9 
9 9 9 -- 
-- 
Graphite A -- 3 -- -- -- 
-- -- 3 
Graphite B 
-- -- 3 -- 
-- -- -- -- 
Graphite C -- -- -- 3 -- 
-- -- -- 
Graphite D -- -- -- -- 3 
-- -- -- 
Graphite E -- -- -- -- -- 
3 -- -- 
Foam density (g/l) 28.4 28.6 29.6 29.4 
29.5 28.8 25.4 
6.1 
Thermal conductivity 29.3 27.6 27.4 27.2 
26.7 27.8 29.7 
28.1 
parallel to the foaming 
direction (mW/mK) 
Thermal conductivity -- 1.7 1.9 2.1 
2.6 1.5 -- 
1.6 
reduction (mW/mK) 
Thermal conductivity 21.8 21.0 21.0 21.1 
20.6 21.3 22.1 
21.5 
perpendicular to the 
foaming direction (mW/mK) 
Thermal conductivity -- 0.8 0.8 0.7 
1.2 0.5 -- 
0.6 
reduction 
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