Preparation of cellular polyurethanes

A process for the production of cellular polyurethanes by reacting PA0 a) at least one organic and/or modified organic polyisocyanate, PA0 b) at least one relatively high-molecular-weight compound containing at least two reactive hydrogen atoms, if desired PA0 c) low-molecular-weight chain extenders and/or crosslinking agents, in the presence of PA0 d) blowing agents, PA0 e) catalysts, and, if desired, PA0 f) auxiliaries and/or additives, wherein the blowing agent d) is at least one acetal.

The present invention relates to a process for the preparation of cellular 
polyurethanes, in particular polyurethane moldings having a compacted 
peripheral zone and a cellular core, known as structural foams. 
Polyurethane structural foams have been known for some time and have a 
variety of applications, for example as shoe soles or as safety parts in 
motor vehicles. 
They are usually prepared by reacting organic polyisocyanates and/or 
modified organic polyisocyanates with relatively highly functional 
compounds containing at least two reactive hydrogen atoms, for example 
polyoxyalkylene-polyamines and/or preferably organic polyhydroxyl 
compounds, in particular polyetherols having molecular weights of, for 
example from 300 to 10,000, and, if desired, chain extenders and/or 
crosslinking agents having molecular weights of up to about 400 in the 
presence of catalysts, blowing agents, auxiliaries and/or additives. Their 
preparation has been described in a number of documents. A review of the 
production of polyurethane structural foams (moldings having a compacted 
peripheral zone and a cellular core) is given, for example, in 
Kunststoff-Handbuch, Volume VII, Polyurethane, 1st Edition 1966, edited by 
Dr. R. Vieweg and Dr. A. Hochtlen and 2nd Edition, 1983, edited by Dr. G. 
Oertel (Carl Hanser Verlag, Munich), and in Integralschaumstoffe by Dr. H. 
Piechota and Dr. H. Rohr, Carl-Hanser-Verlag, Munich, Vienna, 1975. These 
processes are also described in DE-A-16 94 138 (GB 1,209,243), DE-C-1 955 
891 (GB 1,321,679) and DE-B-1 769 886 (U.S. Pat. No. 3,824,199). 
The blowing agents usually used in the past were chlorofluorocarbons. 
However, since these compounds degrade the ozone layer, they can no longer 
be used as blowing agents for polyurethane foams. Various alternatives 
have been proposed. For example, DE-A-42 10 404 describes the use of 
partially halogenated hydrocarbons, known as HCFCs or HFCs, as blowing 
agents for polyurethane foams. Although these compounds are less harmful 
to the ozone layer than chlorofluorocarbons, they are still ecologically 
unacceptable and must be replaced. 
U.S. Pat. No. 5,210,103 proposes the use of volatile silicones and water as 
blowing agent for polyurethane structural foams. DE-A-25 44 560 describes 
a blowing agent mixture for polyurethane structural foams which contains 
HCFCs, acetone and diethyl ether. 
U.S. Pat. No. 5,283,003 describes a blowing agent combination for 
polyurethane foams which comprises methylene chloride, methyl formate and 
pentane. 
U.S. Pat. Nos. 5,189,074 and 5,075,346 describe the use of tert-butyl 
methyl ether as blowing agent for polyurethane foams, it also being 
possible for the ether to be introduced into the reaction mixture via a 
polymer. 
TEDA and TOYOCAT NEWS, June 1994, Vol. 1, describes the use of 
1,3-dioxolane as blowing agent for rigid polyurethane foams. 
However, all these have disadvantages. Thus, blowing agents which work with 
elimination of carbon dioxide frequently result in low quality of the 
peripheral zone, and undesired pore formation occurs. Flammable blowing 
agents, in particular pentane, require complex safety precautions during 
foam production. 
It is an object of the present invention to find a blowing agent for 
polyurethane structural foams which does not damage the environment, is 
easy to handle without particular safety precautions and gives 
polyurethane structural foams having good quality, in particular having a 
uniform surface. 
We have found that, surprisingly, this object is achieved by using acetals 
as blowing agents for polyurethane structural foams. 
Accordingly, the present invention provides a process for the preparation 
of cellular polyurethanes by reacting 
a) at least one organic and/or modified organic polyisocyanate, 
b) at least one relatively high-molecular-weight compound containing at 
least two reactive hydrogen atoms, if desired 
c) low-molecular-weight chain extenders and/or crosslinking agents, 
in the presence of 
d) blowing agents, 
e) catalysts, and, if desired, 
f) auxiliaries and/or additives, 
wherein the blowing agent d) is at least one acetal. 
Particular advantages are achieved by the novel use of acetals as blowing 
agents in the production of polyurethane structural foams, ie. 
polyurethane foams having a compacted peripheral zone and a cellular core. 
Accordingly, the present invention provides in particular a process for the 
production of polyurethane structural foams 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 crosslinking agents, 
in the presence of 
d) blowing agents, 
e) catalysts, and, if desired, 
f) conventional auxiliaries and/or additives, 
in a closed mold with compaction, wherein the blowing agents (d) are 
acetals, if desired in combination with other blowing agents. The acetals 
employed have a boiling point in the range from 40.degree. to 140.degree. 
C., preferably from 40.degree. to 120.degree. C., in particular from 
40.degree. to 90.degree. C. 
A particularly suitable blowing agent for the novel process is methylal, 
but other preferred representatives are dimethoxyethane, diethoxymethane, 
diethoxyethane, 1,3-dioxolane and 2-methyl-1,3-dioxolane. The acetals can 
be employed alone, in mixtures with one another or together with 
conventional blowing agents. 
The acetals are used, in particular, in an amount of from 0.1 to 20% by 
weight, based on the weight of components a) to f). For polyurethane 
structural foams, the acetals are preferably employed in an amount of from 
0.2 to 8% by weight. It is possible to use the acetals alone, in mixtures 
with one another or in mixtures with other blowing agents which are 
conventional in polyurethane chemistry. 
For example, the acetals can be used together with water, aliphatic and/or 
cycloaliphatic hydrocarbons or partially halogenated hydrocarbons as 
blowing agents. 
Preference is given to mixtures of from 0.1 to 20% by weight of at least 
one acetal and from 0.1 to 5% by weight of water, to mixtures of from 0.1 
to 20% by weight of at least one acetal and from 0.1 to 15% by weight of 
at least one aliphatic and/or cycloaliphatic hydrocarbon, and to mixtures 
of from 0.1 to 20% by weight of at least one acetal and from 0.1 to 15% by 
weight of at least one partially halogenated hydrocarbon. The % by weight 
data in each case again refer to the total weight of components a) to 
f). If water is used as co-blowing agent, the water usually present in the 
polyols must be taken into account. 
For specific areas of application, for example for flexible, low-density 
polyurethane foams, larger amounts of blowing agent can also be employed. 
The blowing agent or blowing agent mixture is usually added to the polyol 
component, but it can also be added to the isocyanate component for 
certain applications. 
The following details apply to the starting components used for the novel 
process: 
a) Suitable organic polyisocyanates are the aliphatic, cycloaliphatic, 
araliphatic and preferably aromatic polyisocyanates known per se. 
The following may be mentioned as examples: alkylene diisocyanates having 
from 4 to 12 carbon atoms in the alkylene moiety, such as 1,12-dodecane 
diisocyanate, 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 any 
desired mixtures of these isomers, 
1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), 2,4- 
and 2,6-hexahydrotolylene diisocyanate, and the corresponding isomer 
mixtures, 4,4'-, 2,2'- and 2,4'-dicyclohexylmethane diisocyanate and the 
corresponding isomer mixtures, and preferably aromatic diisocyanates and 
polyisocyanates, eg. 2,4- and 2,6-tolylene diisocyanate and the 
corresponding isomer mixtures, 4,4'-, 2,4'- and 2,2'-diphenylmethane 
diisocyanate and the corresponding isomer mixtures, mixtures of 4,4'- and 
2,2'-diphenylmethane diisocyanates, polyphenyl-polymethylene 
polyisocyanates, mixtures of 4,4'-, 2,4'- and 2,2'-diphenylmethane 
diisocyanates and polyphenyl-polymethylene polyisocyanates (crude MDI), 
and mixtures of crude MDI and tolylene diisocyanates. The organic 
diisocyanates and polyisocyanates may be employed individually or in the 
form of mixtures. 
Frequently, modified polyisocyanates are also used, ie. products which are 
obtained by chemical reaction of organic diisocyanates and/or 
polyisocyanates. Specific examples are ester-, urea-, biuret-, 
allophanate-, carbodiimide-, isocyanurate-, uretdione- and/or 
urethane-containing diisocyanates and/or polyisocyanates. Individual 
examples are urethane-containing organic, preferably aromatic, 
polyisocyanates containing from 33.6 to 15% by weight, preferably from 31 
to 21% by weight, of NCO, based on the total weight, for example 
4,4'-diphenylmethane diisocyanate, 4,4'- and 2,4'-diphenylmethane 
diisocyanate mixtures or crude MDI or 2,4- and/or 2,6-tolylene 
diisocyanate, in each case modified by means of low-molecular-weight 
diols, triols, dialkylene glycols, trialkylene glycols or polyoxyalkylene 
glycols having molecular weights of up to 6000, in particular up to 1500, 
specific examples of di- and polyoxyalkylene glycols, which can be 
employed individually or as mixtures, being diethylene, dipropylene, 
polyoxyethylene, polyoxypropylene and polyoxypropylene-polyoxyethylene 
glycols, -triols and/or -tetrols. NCO-containing prepolymers containing 
from 25 to 3.5% by weight, preferably from 21 to 14% by weight, of NCO, 
based on the total weight, and prepared from the polyester- and/or 
preferably polyether-polyols described below and 4,4'-diphenylmethane 
diisocyanate, mixtures of 2,4'- and 4,4'-diphenylmethane diisocyanate, 
2,4- and/or 2,6-tolylene diisocyanates or crude MDI are also suitable. 
Furthermore, liquid polyisocyanates containing carbodiimide groups and/or 
isocyanurate rings and containing from 33.6 to 15% by weight, preferably 
from 31 to 21% by weight, of NCO, based on the total weight, eg. based on 
4,4'-, 2,4'- and/or 2,2'-diphenylmethane diisocyanate and/or 2,4- and/or 
2,6-tolylene diisocyanate, have also proven successful. 
The modified polyisocyanates may be mixed with one another or with 
unmodified organic polyisocyanates, eg. 2,4'- or 4,4'-diphenylmethane 
diisocyanate, crude MDI or 2,4- and/or 2,6-tolylene diisocyanate. 
Organic polyisocyanates which have proven particularly successful and are 
therefore preferred for use in the production of cellular elastomers are: 
NCO-containing prepolymers having an NCO content of from 25 to 9% by 
weight, in particular based on polyether- or polyester-polyols and one or 
more diphenylmethane diisocyanate isomers, advantageously 
4,4'-diphenylmethane diisocyanate and/or modified, urethane 
groups-containing organic polyisocyanates having an NCO content of from 
33.6 to 15% by weight, in particular based on 4,4'-diphenylmethane 
diisocyanate or diphenylmethane diisocyanate isomer mixtures: those which 
are preferred for the production of flexible polyurethane foams are 
mixtures of 2,4- and 2,6-tolylene diisocyanates, mixtures of tolylene 
diisocyanates and polyphenyl-polymethylene polyisocyanate or in particular 
mixtures of the abovementioned prepolymers based on diphenylmethane 
diisocyanate isomers and crude MDI (polyphenyl-polymethylene 
polyisocyanate) having a diphenylmethane diisocyanate isomer content of 
from 30 to 80% by weight. The relatively high-molecular-weight compounds 
containing at least two reactive hydrogen atoms are expediently those 
having a functionality of from 2 to 8, preferably from 2 to 4, and a 
molecular weight of from 300 to 10,000, preferably from 1000 to 6000. 
Success has been achieved using, for example, polyetherpolyamines and/or 
preferably polyols selected from the group comprising the 
polyether-polyols, polyesterpolyols, polythioether-polyols, 
polyesteramides, hydroxyl-containing polyacetals and hydroxyl-containing 
aliphatic polycarbonates or mixtures of at least two of said polyols. 
Preference is given to polyester-polyols and/or polyetherpolyols. The 
hydroxyl number of the polyhydroxyl compounds is generally from 20 to 120, 
preferably from 27 to 60. 
Suitable polyester-polyols may 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 may 
be used either individually or mixed with one another. The free 
dicarboxylic acids may also be replaced by the corresponding dicarboxylic 
acid derivatives, for example dicarboxylic acid esters of alcohols having 
1 to 4 carbon atoms or dicarboxylic anhydrides. Preference is given to 
dicarboxylic acid mixtures comprising succinic acid, glutaric acid and 
adipic acid in ratios of, for example, from 20 to 35:35 to 50:20 to 32 
parts by weight, and in particular adipic acid. Examples of dihydric and 
polyhydric alcohols, in particular diols, are ethanediol, diethylene 
glycol, 1,2- and 1,3-propanediol, dipropylene glycol, 1,4-butanediol, 
1,5-pentanediol, 1,6-hexanediol, 1,10-decanediol, glycerol and 
trimethylolpropane. Preference is given to ethanediol, diethylene glycol, 
1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol. Furthermore, 
polyester-polyols made from lactones, eg. .epsilon.-caprolactone or 
hydroxycarboxylic acids, e.g. .omega.-hydroxycaproic acid, may also be 
employed. 
The polyester-polyols may be prepared by polycondensing the organic, eg. 
aromatic and preferably aliphatic polycarboxylic acids and/or derivatives 
thereof and polyhydric alcohols without using a catalyst or preferably in 
the presence of an esterification catalyst, expediently in an inert gas 
atmosphere, eg. nitrogen, carbon monoxide, helium, argon, inter alia, in 
the melt at from 150.degree. to 250.degree. C., preferably from 
180.degree. to 220.degree. C., at atmospheric pressure or under reduced 
pressure until the desired acid number, which is advantageously less than 
10, preferably less than 2, is reached. In a preferred embodiment, the 
esterification mixture is polycondensed at the abovementioned temperatures 
under atmospheric pressure and subsequently under a pressure of less than 
500 mbar, preferably from 50 to 150 mbar, until an acid number of from 80 
to 30, preferably from 40 to 30, has been reached. 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 may also be carried out in 
the liquid phase in the presence of diluents and/or entrainers, eg. 
benzene, toluene, xylene or chlorobenzene, for removal of the water of 
condensation by azeotropic distillation. The polyester-polyols are 
advantageously prepared by polycondensing the organic polycarboxylic acids 
and/or derivatives thereof with polyhydric alcohols in a molar ratio of 
from 1:1 to 1.8, preferably from 1:1.05 to 1.2. 
The polyester-polyols obtained preferably have a functionality of from 2 to 
4, in particular from 2 to 3, and a molecular weight of from 480 to 3000, 
preferably from 1000 to 3000. 
However, the polyols used are in particular polyether-polyols prepared by 
conventional processes, for example by anionic polymerization using alkali 
metal hydroxides such as sodium hydroxide or potassium hydroxide, or 
alkali metal alkoxides, such as sodium methoxide, sodium ethoxide, 
potassium ethoxide or potassium isopropoxide as catalysts and with 
addition of at least one initiator molecule containing from 2 to 8, 
preferably 2 to 4, reactive hydrogen atoms in bound form or by cationic 
polymerization using Lewis acids, such as antimony pentachloride, boron 
fluoride etherate, inter alia, or bleaching earth as catalysts, from one 
or more alkylene oxides having from 2 to 4 carbon atoms in the alkylene 
moiety. Examples of suitable alkylene oxides are tetrahydrofuran, 
1,3-propylene oxide, 1,2- and 2,3-butylene oxide, styrene oxide and 
preferably ethylene oxide and 1,2-propylene oxide. The alkylene oxides may 
be used individually, alternatively one after the other 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, unsubstituted or N-mono-, N,N- and 
N,N'-dialkyl-substituted diamines having from 1 to 4 carbon atoms in the 
alkyl moiety, such as unsubstituted or mono- or dialkyl-substituted 
ethylenediamine, diethylenetriamine, triethylenetetramine, 
1,3-propylenediamine, 1,3- and 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. 
Other suitable initiator molecules are alkanolamines, eg. ethanolamine, 
N-methyl- and N-ethylethanolamine, dialkanolamines, eg. diethanolamine, 
N-methyl- and N-ethyldiethanolamine, and trialkanolamines, eg. 
triethanolamine, and ammonia. Preference is given to polyhydric alcohols, 
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 and 
pentaerythritol. 
The polyether-polyols, preferably polyoxypropylene- and 
polyoxypropylene-polyoxyethylene-polyols, advantageously have a 
functionality of from 2 to 4 and molecular weights of from 300 to 10,000, 
preferably from 1000 to 6000, in particular from 1500 to 5000, and 
suitable polyoxytetramethylene glycols have a molecular weight of up to 
approximately 3500. 
Other suitable polyether-polyols are polymer-modified polyether-polyols, 
preferably graft polyether-polyols, in particular those based on styrene 
and/or acrylonitrile and prepared by in situ polymerization of 
acrylonitrile, styrene or preferably mixtures of styrene and 
acrylonitrile, for example in a weight ratio of from 90:10 to 10:90, 
preferably from 70:30 to 30:70, expediently in the abovementioned 
polyether-polyols by a method similar to that of German Patents 11 11 394, 
12 22 669 (U.S. Pat. Nos. 3,304,273, 3,383,351 and 3,523,093), 11 52 536 
(GB 1,040,452) and 11 52 537 (GB 9,876,618), and polyetherpolyol 
dispersions which contain, as the disperse phase, usually in an amount of 
from 1 to 50% by weight, preferably from 2 to 25% by weight, for example 
polyureas, polyhydrazides, polyurethanes containing tertiary amino groups 
in bound form, and/or melamine and described, for example, in EP-B-011 752 
(U.S. Pat. No. 4,304,708), U.S. Pat. No. 4,374,209 and DE-A-32 31 497. 
Like the polyester-polyols, the polyether-polyols can be used individually 
or in the form of mixtures. Furthermore, they may be mixed with the graft 
polyether-polyols or polyester-polyols and the hydroxyl-containing 
polyesteramides, polyacetals, polycarbonates and/or polyetherpolyamines. 
Examples of suitable hydroxyl-containing polyacetals are the compounds 
which can be prepared from glycols, such as diethylene glycol, triethylene 
glycol, 4,4'-dihydroxyethoxydiphenyldimethylmethane, hexanediol and 
formaldehyde. Suitable polyacetals can also be prepared by polymerizing 
cyclic acetals. 
Suitable hydroxyl-containing polycarbonates are those of a conventional 
type, which can be prepared 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 polyester-amides include, for example, the predominantly linear 
condensates obtained from polybasic, saturated and/or unsaturated 
carboxylic acids or anhydrides thereof and polyhydric, saturated and/or 
unsaturated antino alcohols, or mixtures of polyhydric alcohols and amino 
alcohols and/or polyamines. 
Suitable polyether-polyamines can be prepared by known methods from the 
abovementioned polyether-polyols. Mention may be made by way of example of 
the cyanoalkylation of polyoxyalkylene-polyols followed by hydrogenation 
of the resultant nitrile (U.S. Pat. No. 3,267,050), or the partial or full 
amination of polyoxyalkylene-polyols using amines or ammonia in the 
presence of hydrogen and catalysts (DE 12 15 373). 
c) The moldings having a compacted peripheral zone and a cellular core can 
be produced with or without the use of chain extenders and/or crosslinking 
agents. However, it may prove advantageous, in order to modify the 
mechanical properties, for example the hardness, to add chain extenders, 
crosslinking agents or, if desired, mixtures thereof. Examples of chain 
extenders and/or crosslinking agents are diols and/or triols having a 
molecular weight of less than 400, preferably from 60 to 300. Examples are 
aliphatic, cycloaliphatic and/or araliphatic diols having from 2 to 14 
carbon atoms, preferably from 4 to 10 carbon atoms, eg. ethylene glycol, 
1,3-propanediol, 1,10-decanediol, o-, m- and p-dihydroxycyclohexane, 
diethylene glycol, dipropylene glycol and preferably 1,4-butanediol, 
1,6-hexanediol and bis(2-hydroxyethyl)hydroquinone, triols, such as 
1,2,5-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 as initiator molecules. 
For the production of cellular elastomer moldings and structural foams, 
secondary aromatic diamines, primary aromatic diamines, 3,3'-di- and/or 
3,3',5,5'-tetraalkyl-substituted diaminodiphenylmethanes can be used as 
chain extenders or crosslinking agents in addition to the abovementioned 
diols and/or triols or mixed therewith. 
Examples which may be mentioned of secondary aromatic diamines are 
N,N'-dialkyl-substituted aromatic diamines, which are unsubstituted or 
substituted on the aromatic ring by alkyl groups, having 1 to 20, 
preferably 1 to 4, carbon atoms in the N-alkyl radical, eg. 
N,N'-di-sec-pentyl-, N,N'-di-sechexyl-, N,N'-di-sec-decyl- and 
N,N'-dicyclohexyl-p- and -m-phenylenediamine, N,N'-dimethyl-, 
N,N'-diethyl-, N,N'-diisopropyl-, N,N'-di-sec-butyl- and 
N,N'-dicyclohexyl-4,4'-diaminodiphenylmethane and 
N,N'-di-sec-butylbenzidine. 
d) The blowing agents (d) used according to the invention are, as described 
above, acetals, which can be employed individually, in mixtures with one 
another or in mixtures with other co-blowing agents. 
e) The catalysts used to produce the moldings having a compacted peripheral 
zone and a cellular core are, in particular, compounds which greatly 
accelerate the reaction of the reactive hydrogen atoms, in particular 
hydroxyl-containing compounds of component (b) and, if used, (c) with the 
organic, modified or unmodified polyisocyanates (a). Suitable compounds 
are organometallic compounds, preferably organotin compounds, such as 
tin(II) salts of organic carboxylic acids, eg. tin(II) acetate, tin(II) 
octanoate, tin(II) ethylhexanoate and tin(II) laurate, and dialkyltin(IV) 
salts of organic carboxylic acids, eg. dibutyltin diacetate, dibutyltin 
dilaurate, dibutyltin maleate and dioctyltin diacetate. The organometallic 
compounds can be employed alone or preferably in combination with highly 
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-methyl-, N-ethyl-, 
N-cyclohexylmorpholine, N,N,N',N'-tetramethylethylenediamine, 
N,N,N'N'-tetramethylbutanediamine, 
N,N,N',N'-tetramethyl-1,6-hexanediamine, pentamethyldiethylenetriamine, 
tetramethyldiaminoethyl ether, bis(dimethylaminopropyl)urea, 
dimethylpiperazine, 1,2-dimethylimidazole, 1-aza-bicyclo-[3.3.0]octane, 
and, preferably, 1,4-diazabicyclo[2.2.2]octane and alkanolamine compounds 
such as triethanolamine, triisopropanolamine, N-methyl- and 
N-ethyldiethanolamine and dimethylethanolamine. 
Other suitable catalysts are: tris(dialkylaminoalkyl)-s-hexahydrotriazines, 
in particular tris(N,N-dimethylaminopropyl)-s-hexahydrotriazine, 
tetraalkylammonium hydroxides, such as tetramethylammonium hydroxide, 
alkali metal hydroxide, such as sodium hydroxide, and alkali metal 
alkoxides, such as sodium methoxide and potassium isopropoxide, and alkali 
metal salts of long-chain fatty acids having 10 to 20 carbon atoms and, if 
desired, pendant OH groups. 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 formative component (b), are preferably used. 
f) Auxiliaries and/or additives (f) can also be incorporated into the 
reaction mixture for the production of the moldings having a compacted 
peripheral zone and a cellular core. Examples which may be mentioned are 
surfactants, foam stabilizers, cell regulators, fillers, dyes, pigments, 
flameproofing agents, hydrolysis-protection agents and fungistatic and 
bacteriostatic substances. Examples of suitable surfactants are compounds 
which serve to support homogenization of the starting materials and may 
also be suitable for regulating the cell structure of the plastics. 
Specific examples are emulsifiers, such as the sodium salts of castor oil 
sulfates, or of fatty acids and salts of fatty acids with amines, for 
example diethylamine oleate, diethanolamine stearate and diethanolamine 
ricinoleate, salts of sulfonic acids, eg. alkali metal salts or ammonium 
salts of dodecylbenzene- or dinaphthylmethanedisulfonic acid and 
ricinoleic acid; foam stabilizers, such as siloxane-oxyalkylene copolymers 
and other organopolysiloxanes, oxyethylated alkylphenols, oxyethylated 
fatty alcohols, paraffin oils, castor oil esters, ricinoleic acid esters, 
Turkey red oil and groundnut oil, and cell regulators, such as paraffins, 
fatty alcohols and dimethylpolysiloxanes. Suitable compounds for improving 
the emulsification action, the cell structure and/or stabilizing the foam 
are furthermore the abovementioned oligomeric acrylates containing 
polyoxyalkylene and fluoroalkane radicals as side groups. The surfactants 
are usually used in amounts of from 0.01 to 5 parts by weight, based on 
100 parts by weight of components (b) to (f). 
Examples which may be mentioned of suitable blowing agents are: products of 
the reaction of fatty acid esters with polyisocyanates, salts of 
amino-containing polysiloxanes and fatty acids, salts of saturated or 
unsaturated (cyclo)aliphatic carboxylic acids having at least 8 carbon 
atoms and tertiary amines, and, in particular, internal release agents, 
for example carboxylates and/or carboxamides, prepared by esterification 
or amination of a mixture of montanic acid and at least difunctional 
alkanolamines, polyols and/or polyamines having a molecular weight of from 
60 to 400 (EP-A-153 639) or mixtures of organic amines, metal salts of 
stearic acid and organic mono- and/or dicarboxylic acids or anhydrides 
thereof (DE-A-36 07 447). 
For the purposes of the present invention, fillers, in particular 
reinforcing fillers, are conventional organic and inorganic fillers, 
reinforcing agents, weighting agents, agents for improving the abrasion 
behavior of paints, coating compositions etc known per se. Specific 
examples are inorganic fillers, such as silicate minerals, for example 
phyllosilicates, such as antigorite, serpentine, hornblends, amphiboles, 
chrysotile, talc; metal oxides, such as kaolin, aluminum oxides, titanium 
oxides and iron oxides, metal salts, such as chalk, barytes and inorganic 
pigments, such as cadmium sulfide, zinc sulfide and glass, inter alia. 
Preference is given to kaolin (china clay), aluminum silicate and 
co-precipitates of barium sulfate and aluminum silicate, and natural and 
synthetic fibrous minerals, such as wollastonite, metal fibers and in 
particular glass fibers of various lengths, which may have been sized. 
Examples of suitable organic fillers are: charcoal, melamine, collophony, 
cyclopentadienyl resins and graft polymers, and cellulose fibers, 
polyamide fibers, polyacrylonitrile fibers, polyurethane fibers and 
polyester fibers based on aromatic and/or aliphatic dicarboxylic acid 
esters, and in particular carbon fibers. The inorganic and organic fillers 
may be used individually or as mixtures and are advantageously introduced 
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 components 
(b-f). 
Examples of suitable flameproofing agents are tricresyl phosphate, 
tris(2-chloroethyl) phosphate, tris(2-chloropropyl) phosphate, 
tetrakis(2-chloroethyl)ethylene diphosphate, dimethyl methanephosphonate, 
diethyl diethanolaminomethyl phosphonate and commercially available 
halogen-containing flameproofing polyols. 
In addition to the abovementioned halogen-substituted phosphates, it is 
also possible to use inorganic or organic flameproofing agents, such as 
red phosphorus, aluminum oxide hydrate, antimony trioxide, arsenic oxide, 
ammonium polyphosphate and calcium sulfate, expandable graphite or 
cyanuric acid derivatives, eg. melamine, or mixtures of two or more 
flameproofing agents, eg. ammonium polyphosphate and melamine, and also, 
if desired, corn starch or ammonium polyphosphates, melamine and 
expandable graphite and/or, if desired, aromatic polyesters, in order to 
flameproof the polyisocyanate polyaddition products. In general, it has 
proven expedient to use from 5 to 50 parts by weight, preferably from 5 to 
25 parts by weight, of said flameproofing agents per 100 parts by weight 
of formative components (b-f). 
Further details on the other conventional assistants and additives 
mentioned above can be obtained from the specialist literature, for 
example from the monograph by J. H. Saunders and K. C. Frisch in High 
Polymers, Volume XVI, Polyurethanes, Parts 1 and 2, Interscience 
Publishers 1962 and 1964 respectively, or Kunststoff-Handbuch, 
Polyurethane, Volume VII, Carl-Hanser-Verlag, Munich, Vienna, 1st and 2nd 
Editions, 1966 and 1983. To produce the moldings having a compacted 
peripheral zone and a cellular core, the organic polyisocyanates (a), the 
relatively high-molecular-weight compounds (b) containing at least two 
reactive hydrogen atoms and, if used, chain extenders and/or crosslinking 
agents (c) are reacted in such amounts that the equivalence ratio between 
the NCO groups of the polyisocyanates (a) and the total number of reactive 
hydrogen atoms in components (b) and, if used, (c) and, if water is used 
as blowing agent, also in the water is from 0.85 to 1.25:1, preferably 
from 0.95 to 1.15:1, in particular from 1 to 1.05:1. If the polyurethane 
structural foams contain at least some bonded isocyanurate groups, a ratio 
between the number of NCO groups in the polyisocyanates (a) and the total 
number of reactive hydrogen atoms in components (b) and, if used, (c) of 
from 1.5 to 20:1, preferably from 1.5 to 8:1 is used. 
The moldings having a compacted peripheral zone and a cellular core are 
advantageously produced by the one-shot process, for example using the 
high-pressure method or the low-pressure method in a closed mold, for 
example a metallic mold. These procedures are described, for example, by 
Piechota and Rohr in Integralschaumstoffe, Carl-Hanser-Verlag, Munich, 
Vienna, 1975. 
It has proven particularly advantageous to use the two-component method and 
to combine components (b), (d), (e) and, if used, (c) and (f) in component 
(A) and to use the organic polyisocyanates, the modified polyisocyanates 
(a) or the mixture of said polyisocyanates and, if desired, the blowing 
agent (d) as component (B). 
The starting components are mixed at from 15.degree. to 90.degree. C., 
preferably at from 20.degree. to 60.degree. C., in particular at from 
20.degree. to 45.degree. C., and introduced into the open or closed mold 
at atmospheric pressure or superatmospheric pressure. The mold temperature 
is expediently from 20.degree. to 110.degree. C., preferably from 
30.degree. to 60.degree. C., in particular from 45.degree. to 55.degree. 
C. 
Moldings having a compacted peripheral zone and a cellular core produced by 
the novel process have a density of from 0.2 to 1.0 g/cm.sup.3, preferably 
from 0.3 to 0.7 g/cm.sup.3, in particular from 0.3 to 0.55 g/cm.sup.3. 
The polyurethane moldings produced by the novel process are distinguished 
over those of the prior art, in particular water-blown ones, by a 
significantly better, flaw-free and smooth surface. There is significant 
formation of the cellular core and the compacted peripheral zone. The 
acetals used according to the invention as blowing agents are readily 
compatible with the other constituents of the polyol component, so that no 
inhomogeneities or separations occur. Their use requires no particular 
safety precautions. 
The novel process can be used both for flexible structural foams, as used, 
for example, as shoe soles and safety parts in automobiles, and for rigid 
structural foams, known as thermosetting foams.

The invention is described in greater detail, with reference to the 
examples below. 
EXAMPLE 1 (comparison) 
100 parts by weight of polyol component comprising 67.40 parts by weight of 
a trifunctional polyether alcohol having a hydroxyl number of 35 mg of 
KOH/g (Lupranol.RTM. 2045 from BASF AG), 11.65 parts by weight of a 
difunctional polyether alcohol having a hydroxyl number of 29 mg of KOH/g 
(Lupranol.RTM. 2043 from BASF AG) 13.00 parts by weight of a polymeric 
polyether alcohol having a hydroxyl number of 24 mg of KOH/g 
(Pluracol.RTM. 973 from BASF AG), 6.0 parts by weight of ethylene glycol, 
1.1 parts by weight of an amine catalyst (Lupragen.RTM. N 201 from BASF 
AG) and 0.85 part by weight of water, were introduced into a sealable mold 
together with 56.4 parts by weight of a prepolymer based on 
diphenylmethane diisocyanate, carbodiimide-modified diphenylmethane 
diisocyanate and polyphenyl-polymethylene polyisocyanates having an NCO 
content of 26.3% by weight, and the components were reacted to give a 
polyurethane structural foam. 
EXAMPLE 2 
100 parts by weight of a polyol component comprising 66.40 parts by weight 
of Lupranol.RTM. 2045, 11.00 parts by weight of Lupranol.RTM. 2043, 13.00 
parts by weight of Pluracol.RTM. 973, 6.00 parts by weight of ethylene 
glycol, 1.1 parts by weight of Lupragen.RTM. N 201, 0.60 part by weight of 
water and 1.9 parts by weight of methylal were reacted with 52.0 parts by 
weight of the prepolymer from Example 1 as described in Example 1. 
EXAMPLE 3 
100 parts by weight of a polyol component comprising 63.40 parts by weight 
of Lupranol.RTM. 2045, 11.20 parts by weight of Lupranol.RTM. 2043, 13.00 
parts by weight of Pluracol.RTM. 973, 6.00 parts by weight of ethylene 
glycol, 1.1 parts by weight of Lupragen.RTM. N 201, 0.40 part by weight of 
water and 4.9 parts by weight of methylal were reacted with 48.0 parts by 
weight of the prepolymer from Example 1 as described in Example 1. 
EXAMPLE 4 
100 parts by weight of a polyol component comprising 66.40 parts by weight 
of Lupranol.RTM. 2045, 11.00 parts by weight of Lupranol.RTM. 2043, 13.00 
parts by weight of Pluracol.RTM. 973, 6.00 parts by weight of ethylene 
glycol, 1.1 parts by weight of Lupragen.RTM. N 201, 0.60 part by weight of 
water, 1.60 parts by weight of methylal and 0.30 part by weight of 
1,3-dioxolane were reacted with 51.8 parts by weight of the prepolymer 
from Example 1 as described in Example 1. 
EXAMPLE 5 
100 parts by weight of a polyol component comprising 65.9 parts by weight 
of Lupranol.RTM. 2045, 11.00 parts by weight of Lupranol.RTM. 2043, 13.00 
parts by weight of Pluracol.RTM. 973, 6.00 parts by weight of ethylene 
glycol, 1.1 parts by weight of Lupragen.RTM. N 201, 0.60 part by weight of 
water, 1.60 parts by weight of methylal, 0.50 part by weight of 
1,3-dioxolane and 0.3 part by weight of dimethylacetal were reacted with 
51.8 parts by weight of the prepolymer from Example 1 as described in 
Example 1. 
EXAMPLE 6 (comparison) 
100 parts by weight of a polyol component comprising 65.9 parts by weight 
of Lupranol.RTM. 2043, 16.3 parts by weight of a trifunctional polyether 
alcohol having a hydroxyl number of 27 mg of KOH/g (Lupranol.RTM. 2042 
from BASF AG), 3.1 parts by weight of Lupranol.RTM. 2045, 10.7 parts by 
weight of 1,4-butanediol, 2.0 parts by weight of ethylene glycol, 1.1 
parts by weight of Lupragen.RTM. N201 and 0.9 part by weight of water were 
reacted with 75.3 parts by weight of the prepolymer from Example 1 as 
described in Example 1. 
EXAMPLE 7 
100 parts by weight of a polyol component comprising 65.9 parts by weight 
of Lupranol.RTM. 2043, 15 parts by weight of Lupranol.RTM. 2042, 3.1 parts 
by weight of Lupranol.RTM. 2045, 10.7 parts by weight of 1,4-butanediol, 
2.0 parts by weight of ethylene glycol, 1.1 parts by weight of 
Lupragen.RTM. N201, 0.6 part by weight of water and 1.6 parts by weight of 
methylal were reacted with 69.7 parts by weight of the prepolymer from 
Example 1 as described in Example 1. 
The Shore hardness of the moldings was measured in accordance with DIN 53 
505, the density of the molding as a whole, of the core zone and of the 
peripheral zone was measured in accordance with DIN (53420) and the 
proportion of closed cells in the peripheral zone was determined by 
microscopic analysis. The results are shown in the table. 
TABLE 
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Density 
Density peripheral 
Shore core zone zone 
Ex. hardness [kg/m.sup.3 ] 
[kg/m.sup.3 ] 
Surface 
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1 A 78 400 750 irregular, holes 
2 A 78 410 960 smooth, closed 
3 A 77 390 920 smooth, closed 
4 A 79 385 930 smooth, closed 
5 A 78 405 900 smooth, closed 
6 A 50 455 780 irregular, holes 
7 A 53 463 920 smooth, closed 
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