CA2107427A1

CA2107427A1 - Heat curable, expandable one-component polyurethane composition

Info

Publication number: CA2107427A1
Application number: CA002107427A
Authority: CA
Inventors: Joachim Werner; Ulrich Liman; Walter Meckel
Original assignee: Individual
Current assignee: Bayer AG
Priority date: 1992-10-05
Filing date: 1993-09-30
Publication date: 1994-04-06
Also published as: ES2097958T3; EP0591766A1; MX9306144A; EP0591766B1; BR9304123A; DE4233346A1; KR940009280A; JPH06192565A; DE59305322D1

Abstract

HEAT CURABLE, EXPANDABLE ONE-COMPONENT POLYURETHANE COMPOSITION
ABSTRACT OF THE DISCLOSURE
Heat curable, expandable one-component polyurethane compositions are described. These compositions comprise a) an isocyanate prepolymer which is flowable at room temperature, and is obtained by the reaction of an organic polyisocyanate with a polyhydroxyl compound;

b) a solid, finely divided polyhydroxyl compound which has an average at least two hydroxyl groups per molecule, is insoluble in the prepolymer phase a), and is finely distributed in the prepolymer phase a);
and c) a solid, finely divided blowing agent which is finely distributed in the prepolymer phase a);

with the proviso that the decomposition temperature of the blowing agent c) is not significantly above the curing temperature of the composition.
These polyurethane compositions may also comprise d) auxiliary agents and additives such as pigments, catalysts, stabilizers, flow improvers, thickeners, drying agents and foam regulators.

Description

21~ 7 l~ 2 7 Mo-3954 LeA 29,372 HEAT CURABLE, EXPANDABLE ONE-COMPONENT POLYURETHANE COMPOSITION
BACKGROUND OF THE INYENTION
The present inYention relates to heat curable, expandable one-component polyurethane compositions comprising a dispersion of pulverulent solid blowing agent and a solid polyhydroxyl compound in an isocyanate polyurethane prepolymer. These polyurethane compositions exhibit great storage stability and, in the cured state, high thermal stability. The invention also relates to the use of these compositions for the production of foamed or at least cellular moldings.
It is well known to foam up polyurethane (PU~ rigid foam inside cavities, e.g. for the construction of motor vehicles or ships ~see Becker/Braun, Kunststoff-Handbuch, Volume 7, "Polyurethane", 2nd Edition, Carl Hanser Verlag, Munich, Vienna, 1983, pages 320 et seq). In this process, the liquid two-component foam formulation is injected into the cavity from dosing devices and the two components react inside the cavity and expand. This entails the risk of the reaction mixture escaping or leaking out from parts which are not firmly sealed.
This can only be kept within limits by a process of "frothing".
In the construction of car bodies, filling the cavities with foam ;s preferably carried out after lacquering. This prevents the foam from shrinking or becoming detached during stoving of the lacquer. Expulsion or outflow of the foam mixture in the hereinabove described process is particularly disturbing.
It is also known to use high melting hydroxyl compounds in systems containing isocyanate groups for the preparation of heat curable compositions. Thus, the use of starch for the preparation of cross-linked PU elastomers is described, for Le A 2~ 372-Forei~n Countries 2~

example, in U.S. Patent 2,908,657, and the use of such systems as adhesives and sealing materials, or for coatings using pentaerythritol as a hydroxyl component is described, for example, in U.S. Patents 3,488,302, 4,390,678 and 4,412,033, and German Offenlegungsschrift 3,734,340.
The use of a pulverulent mixture of a powdered NCO-PU
prepolymer which is solid up to 70~C, in comb;nation with a pulverulent material which splits off water at an elevated temperature (e.g. boric acid) as heat curable composition for o the production of PU foams is described, for example, in U.S.
Patent 3,280,04~. The preparation of this pulverulent mixture, however, requires an extensive procedure of grinding the prepolymer and various operations of mixing solids. European Patent 392,171 also describes foamable, heat curable polyureth-anes based on isocyanate-PU prepolymers and materials which split off water when heated (e.g. CaS04 x H20). In order to achieve sufficient stability in storage, the particles of the added hardener which splits off water must be rendered inert before dispersion. This can be done, for example, by a reaction with monoisocyanates, or by enveloping the particles in inert materials, for example, thermoplasts such as polyethylene, polyamides or polyurethanes. However, this method also requires additional expensive operating steps.
In view of the above information, it would be advantageous to be able to use a one-component formulation without elaborate dosing devices, e.g. for filling cavities with foam, witho~t the risk of the foaming mixture leaking out or being forced out of the cavity. It would also be advantageous to be able to carry out the foaming before lacquering, especially in the construction of car bodies. To minimize the cost of this process, it would also be advantageous to be able to carry it out in only a few, simple steps.

7~2~

DESCRIPTION OF THE INVENTION
It has now been found that this problem may be solved by means of a heat-curable, expandable one-component polyurethane composition comprising a~ an isocyanate prepolymer which is flowable at room temperature, and is obtained by the reaction of one or more organic polyisocyanates with one or more polyhydroxyl compounds;

b) a solid, finely divided polyhydroxyl compound having an average of at least two hydroxyl groups per molecule, and which is insoluble in the prepolymer phase a) and is finely divided therein; and c) a solid, finely divided blowing agen~ which is finely distributed in prepolymer phase a~;

with the proviso that the decomposition temperature of the 20 blowing agent c) is not significantly higher than the curing temperature of the polyurethane composition.
By the phrase "flowable at room temperature", it is meant that the isocyanate prepolymer, i.e. component a), must be either a liquid or at least pasty at room temperature. Pasty 25 iS defined as flowable under shear stress, i.e. stirrable in a conventianali stirrer apparatus.
These heat-curable, expandable one component polyurethane composition may additionally comprise d) auxiliary agents and additives such as9 for example, p;gments, catalysts, stabil;zers, flow improvers, thickeners, drying agents and foam regulators'.
Suitable organic polyisocyanates for the preparation of the isocyanate prepolymer, i.e. component a) according to the invention, include the aliphatic, aromatic or cycloaliphatic 7~27 polyisocyanates, or mixtures thereof. The term ~aliphatic isocyanates"
includes compounds in which the isocyanate groups are attached to saturated carbon atoms. The following are examples of suitable polyisocyanates: 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 4,4'-methylene-bis-(cyclohexyl-isocyanate), 2,4'~ and 4,4'-diphenylmethane diisocyanate and mixtures thereof, 2,4- and 2,6-tolylene diisocyanate and mixtures thereof, 1,4-phenylene diisocyanate, and 1,5-naphthylene diisocyanate.
o Polyisocyanates having two NCO groups per molecule are preferably used, although higher functional polyisocyanates are also suitable, provided the resulting prepolymer a) remains flowable at room temperature. If this ;s ensured, the h;gher functional isocyanates known in the lacquer and coating industry, for example, may also be used or at least included as a proportion of isocyanates. These include polymers and polymer/monomer mixtures of diphenylmethane diisocyanate, biurets, trimethylolpropane adducts, and trimers (i.e.
isocyanurates) of the above-mentioned diisocyanates.
The polyhydroxyl compounds for the preparation of the isocyanate prepolymer used as component a) according to the invent;on are preferably glycols having an average of two hydroxyl groups9 the molecular mass thereof being preferably up to about 6000.
Suitable polyhydroxyl compounds include, for example, hydroxy functional polyesters, polycarbonates, polyester carbonates, polyethers, polyether carbonates~ polyacetals, polyacrylates, polybutadienes, polyester amides, and polythioethers. Amino functional polyethers, such as those described in U.S. Patent 4,724,252 and German Offenlegungsschrift 3,713,858 are also suitable. It is preferred that these polyhydroxyl compounds contain an average of two isocyanate reactive groups per molecule. Higher functional compounds may be also used.
However, the use of higher functional compounds makes it is necessary to ensure that the prepared isocyanate prepolymer a) remains flowable at room temperature. This can be done, if ~ ~ ~ 7L~

necessary, by the addition of monofunctional compounds to the higher functional polyhydroxyl compounds.
Suitable polyethers for the preparation of the isocyanate prepolymer include, for example, those obtained by ring opening polymerization of propylene oxide or ethylene oxide in the presence of one or more compounds containing active hydro~en, or by ring opening polymerization of tetrahydrofuran.
If the end products are required to b~ resistant ~o light, it is preferred that polyesters, polycarbonates or polyester carbonates are used as the polyhydroxyl compounds. Suitable polyester polyols may be obtained, for example, by the condensation of one or more dicarboxylic acids, their anhydrides or diesters, with one or more low molecular weight glycols. So~e examples of suitable dicarboxylic acids include succinic acid, adipic acid, suberic acid, aromatic dicarboxylic acids such as phthalic acid, isophthalic acid or terephthalic acid, and the corresponding partially or per-hydrogenated compounds. Examples of suitable low molecular weight glycols include ethylene glycol, 1,2- and 1,3-propylene glycol, 1,4-and 2,3-butanediol, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, cyclohexane dimethanol, diethylene glycol, and dipropylene glycol. Polyesters obtained by the polymerization of lactones such as ~ caprolactone are also suitable. Suitable al;phatic hydroxyl functional polycarbonates may be obtained, for example, by the reactio~ of the above-mentioned low molecular weight glycols with, for example, diarylcarbonates or cyclic carbonates such as propylene carbanate. The low molecular weight glycols exemplified hereinabove may also be at least proportionally used as a polyhydroxyl compound for the pre- .
paration of the isocyanate prepolymer, i.e. component a).
When choosing from the aforementioned polyhydroxyl compounds for the preparation of isocyanate prepolymer a), those which impart flowability to the prepolymer at room temperature are suitable for the invention. Polyether polyols and polyester polyols which are either liquids or have melting points below 80C and ~7~

glass transition temperatures below 0C are particularly suitable. Particularly preferred are Polyethers or -esters with an Rverage molecular weight of above 500 g/mole or mixtures thereof.
The prepolymers a) used according to the invention are prepared by the method well known to the PU chemist, for example, reacting a stoichiometric excess of one or more polyisocyanates with one or more polyhydroxyl compounds. The reaction is preferably carried out at a temperature of about 80 to 100C with stirring and in the presence of an inert gas, e.g. nitrogen. The ratio of NCO groups to OH groups in the o reactants is generally above 1.05:1, preferably from 1.05:1 to 10:1, and most preferably from 1.5:1 to 3:1. Catalysts may also be used for the reaction. However, catalysts generally reduce the storage stability of the expandable polyurethane composition prepared according to the present invention.
Compounds suitable for use as the solid, finely divided polyhydroxyl compounds, i.e. component b), have at least two OH
groups per molecule, and are virtually completely insoluble in the flowable prepolymer at least at temperatures below the curing temperature of the prepared polyurethane compos~tion.
These polyhydroxyl compounds can be either finely dispersed or suspended in the isocyanate prepolymer. It is preferred that these polyhydroxyl compounds have more than two OH groups per molecule, and melting points in the range of the curing temperature or higher. The melting point is preferably above 100C, and most preferably above 150C. The particle size of the solid9 finely divided polyhydroxyl compound b) should be less than 1 mm, and preferably less than 100 ~m.
Starch, cane sugar (i.e. sucrose) and sugar alcohols such as, for example, mannitol and sorbitol, are examples of suitable solid, finely divided polyhydroxyl compounds b).
Finely divided powders of synthetic macromolecules containing hydroxyl groups are also suitable for use as component b).
These includP, for example, hydrolysed ethylene/vinyl acetate copolymers. Tris-(2-hydroxyethyl)-isocyanurate, for example, is also suitable as component b). Pentaerythritol (2,2-bis-(hydroxymethyl)-1,3-propanediol), a crystalline, colorless and odorless tetrafunctional alcohol which ;s obtained, for example, by the condensation of formaldehyde with acetaldehyde, and commercially available in a finely ground form with a particle size below 50 ~m (e.g. Degussa AG, Frankfurt/M.) is preferably used as component b). The melting point of this compound is from 260 to 263C.
Dimers and trimers of pentaerythritol, or esters derived from pentaerythritol, or its oligomers or mixtures of such esters, for example, are also preferred compounds for component b).
The substances used as solid, finely divided blowing agents c) which are f;nely d;str;buted in the isocyanate prepolymer phase a) are pulYerulent blowing agents which are solid at room temperature (i.e. 25C), have an average particle size of from about 1 to about 300 ~, preferably up to 30 ~, and are preferably ;nsoluble ;n prepolymer phase a) but can be f;nely d;spersed or suspended within the prepolymer phase a).
Chem;cal compounds wh;ch decompose w;th;n a particular, preferably very narrow temperature interval with a h;gh gas yield are su;table for use as the blow;ng agent c). The decompos;t;on temperature of the blowing agent must be adapted to the processing and cur;ng temperature of whichever expandable PolYurethane comDos;tion accordinq to the invention are employed. Preferred are blowing agents whi~h decompose at t~mpe- I
ratures of from 100 to 200 C. The blowing agent should not undergo any undesirable reactions with any of the otner components ~l.e.
a), b) and d~) which are used in the process. Furthermore, the decomposit;on products resulting from the thermal decomposition of the blowing agents should not pose potential health r;sks or hazards, and should not adversely affect the thermostab;lity and mechanical properties of the foamed or cellular poly-urethane moldings, or cause them to bleed or discolor.
Suitable examples of solid blowing agents which at least partly fulfil the above requirements include azo compounds such as, for example, azoisobutyric acid nitrile, azodicarbonamide S~ ~ ~7 ~27 (also known as azo-bis-formamide) and barium azodicarboxylate;
substituted hydrazines such as, for example, diphenyl-sulfone-3,3'-dis-sulfohydrazide, 4,4'-oxy-bis-(sulfohydrazide), trihydrazinotriazine and aryl-bis-(sulphohydrazide~;
semicarbazides such as, for example, p-tolylenesulfonyl semicarbazide or 4,4'-oxy-bis-(benzenesulfonyl-semicarbazide);
triazoles such as, for example, 5-morpholyl-1,2,3,4-thiatri-azole; N-nitroso compounds such as, for example, N,N'-dinitroso-pentamethylene tetramine or N,N-dimethyl -N,N'-dinitrosoterephthalamide; benzoxazines such as, for example isatoic acid anhydride; and blowing agent compositions such as, for example, mixtures of sodium bicarbonate and citric acid. Among these compounds, the azo compounds and hydraz;nes have proved to be particularly suitable. The solid blowing agents according to the invention may be used as individual compounds or as mixtures.
Azodicarbonamide, which is part;cularly preferred for the present invention, is obtainable commercially in various specified average particle sizes. Those having an average 20 particle size below 100 ~m are preferred, and those having an average particle size below 50 ~m are particularly preferred.
The decomposition temperature of azodicarbonamide is from 205 to 215C. The particle size influences the decomposition temperature and speed of decomposition. The decomposition temperature and speed may be adjusted by means of special decomposition catalysts (which are commonly referred to as "kickers"). Commercially obtainable azo d;carbonamide preparations which decompose at temperatures of from 150 to 200 C are particularly preferred Special catalysts which do not significantly reduce the stability in storage may be used according to the present invention for adjusting the cur;ng temperature of the polyurethane composition according to the invention, for accelerating curing at a given temperature, and for adapting the curing temperature to the decomposition temperature of the S~17~l g blowing agent. Suitable catalysts of this type includ~, for example, the metal salts of fatty acids containing more than 11 carbon atoms and having melting points above 100C as described in German Offenlugungsschrift 3,734,340. It is preferred to use zinc stearate (melting point 11~C)~ the catalytic activity of which is described e.g. in U.S. Patent 4,119,594, and which is preferably in a finely divided form wi~h particle sizes below 100 ~m, and in quantities of up to 1%, most preferably from 0.1 to 0.5%.
o Auxiliary agents and additives such as, for example, inorganic or organic pigments, dyes, antioxidants, UV
stabilizers, flow improvers, plasticizers, etc. may also be used as components of the heat-curable, expandable one component polyurethane composition of the invention.
The foam regulators known per se to the polyurethane chemist, e.g. foam stabilizers such as polyethersiloxanes, may be used to influence the nature of the foam, such as its cell structure or size.
So-called "chemical thickeners" such as diamines, e.g.
4,4'-methylene-bis-(cyclohexylamine) or diethyl tolyl diamine, may be added to the polyurethane compositions according to the invention in quantities of up to about 10 parts by weight, and preferably up to 5 parts by weight, based on 100 parts of the total mixture, to adjust the flow properties so that the 25 formation of urea groups increases the viscosity of the polyurethane composition without causing solidification~ Inert inorganic fillers, e.g. heavy spar, aluminium oxide, talc or vapour phase silicas (e.g. Aerosils~ of Degussa Company) may also be added. Thus, for example, the flow properties of these compositions during curing may be adjusted, whereby, for 30 example, the as yet not completely cured and foamed mixture may advantageously be prevented from flowing out of the cavity.
At the same time, the sedimentation of components b) and c) which have been incorporated by dispersion is prevented. For this purpose, hydrophobicized vapor phase silicas such as 2'7 Aerosil~ R202 of Degussa Company are stirred into the mixture in quantities of from 0.02 to 10%, preferably from ~.02 to 4%, (based on final composition), or kneaded into the mixture in suitable apparatus. The quantity of these fillers to be used is calculated on the basis 5 that the resultant expandable polyurethane composition should have a viscous to pasty consistency at room temperature (i.e.
23 C), i.e. that the composition should still be flowable under shear stress,~
but not necessarily flowable without shear stress.
Preparation of the expandable polyurethane composition of the invention ;s preferably carried out as a "one-shot reaction" in a conventional stirrer apparatus. If the resulting e~pandable polyurethane composition has a pasty consistency, it may be advantageous to use a kneading apparatus. The reaction of the polyisocyanates and polyhydroxyl compoùnds to form the flowable isocyanate prepolymer a~ is preferably carried out at 50 to 15 80C until the isocyanate value is constant. Components b), c), and d) which have, optionally, been freed from adhering moisture, for example, by heating under vacuum, are then added with stirring, optionally with kneading, and homogeneously 20 distributed in the prepolymer phase. The temperature at this stage should not exceed 70C. The optional curing catalyst, e.g. zinc stearate, and other additives which sharply lncrease the viscosity, e.g. the above-mentioned gas phase silicas (e.g.
Aerosils), are preferably the last component incorporated into the mixture. The resultant heat-curable, expandable polyurethane composition should have a viscous to pasty consistency at room temperature (23C). The viscosity can be adjusted, for example, by addition of a suitable quantity of inorganic fillers as described here;nabove, after the addition of components b) and c).
The expandable polyurethane composition may be filled, for example, into vats or cartridges from which it can be dosed at temperatures not above 70C, e.g. by means of vat pumps or under pressure. As an alternative method which is advantageous for foaming inside cavities, the expandable polyurethane composition may be enclosed in flexible plastic containers such ~7~2~

as polyethylene tubes or bags, e.g. they may be sealed therein.
These "containers ready for use" may be stored, for example, in moisture-tight containers and sent to the user in this form.
Curing of these polyurethane compositions with expansion iS preferably carried out at temperatures of from 100 to 200C, most preferably from 150 to 200C. Curing can be performed, for example, in a hot a;r oven or by microwave treatment.
The expandable polyurethane compositions may advan-tageously be used for cavity foaming in the construction of car bodies. This can be done, for example, by introducing these polyurethane compositions into the cavity at room temperature in the form of "containers ready for use". For example, they may be welded ;nto polyethylene bags, so that leakage of liquid from cavities which slope downward is advantageously avoided, and the contents can be adapted to the geometry of the cavity due to their consistency of a viscous liquid or paste. Curing may then be brought about at temperatures of from 150 to 200C
over a period of from 10 to 15 minutes, e.g. in the course of a lacquering process, wherein the bag which contains the expandable polyurethane bursts open due to the volumetric expansion and a cross-linked, foamed or at least cellular molding is produced which fills the cavity and adheres to the walls.
The expandable polyurethane composition may thus advantageously be used for cavity foaming, for example, for heat or soùnd insulation, and sound or vibration damping.
These compositions are also suitable as foamable adhesive, for example, in sandwich elements used in the building industry.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.

2~7~2~7 EXAMPLES
The starting materials and additives listed below are used in the Examples which follow:

IPPD commercially obtainable stabilizer against oxidation (isopropy~phenyl-phenylenediamine) Pentaerythritol very finely ground pentaerythritol powder (Degussa AG, Frankfurt/M.), average particle size below 50 ~m Azodicarbonamide pulverulent azodicarbonam;de preparation decomposing at about 150-160 C (Bayer AG), average particle size about 5 ~m Aerosil R 202 hydrophobicized vapor phase silica (Degussa AG, Frankfurt /M.).

Example 1 500 Parts by weight of a polypropylene glycol with OH
number 112 and 175 parts by weight of 2,4-tolylene diisocyanate are reacted under a nitrogen atmosphere at 80C with stirring until the isocyanate value is constant (calculated 6. 2% NCO).
The temperature is lowered to 60C. The following are added with stirring:

3.38 parts by weight of IPPD9 34 parts by weight of pentaerythritol, 33.4 parts by weight of azodicarbonamide and 3.35 parts by weight of Zn stearate.

The viscous mixture is poured into polyethylene bags about 100 ml in capacity equipped with pressure seals, and the bags are sealed and the contents left to cool.

30 .

To produce a cross-linked foam, the sealed bag containing the mixture which is highly viscous at room temperature (25C) is kept in a hot air oven at 190C for 15 minutes. As the contents expand so that the polyethylene bag bursts, an 5 elastic, foamed moulding having a gross density of about 0.4 g/cm3 is obtained. The foam obtained does not soften or melt below 250C.
Example 2 1000 Parts by weight of a polypropylene glycol with OH
number 56 and 250 parts by weight of 4,4'-diphenylmethane-diisocyanate are reacted under a nitrogen atmosphere with stirring at 80C until the isocyanate value is constant (calculated 3.4% NCO). The temperature is lowered to 60C.
The following are added with stirring:

6.25 parts by weight of IPPD, 34.0 parts by weight of pentaerythritol, 62.3 parts by weight of azodicarbonamide and 6.25 parts by weight of Zn stearate.

35 Parts by weight of Aerosil R 202 are then stirred in, with the result that the mixture thickens considerably but remains pourable at 60C.
The viscous mixture is poured into polyethylene bags of 25 about 100 ml capacity equipped with pressure seals and the bags are sealed and the contents left to cool.
To produce a cross-linked foam, the sealed bag containing the mixture which has a waxy, kneadable cDnsistency at room temperature (25C) is kept in a hot air oven at 190C for 15 minutes. As the contents expand and the polyethylene bag 30 bursts, an elastic, foamed molding having a gross density of about 0.4S g/cm is obtained. The foam does not soften or melt below 250C.
To simulate foaming lnside a cavity, a polyethylene bag filled with the mixture is introduced into a commercially ~7~?~

available aluminium cartridge (diameter 5 cm, height 20 cm) and the cartridge is heated in a hot air oven at 190C for 15 minutes with its opening sloping downwards by about 30. As the mixture foams up, the foam tears open and melts the polyethylene bag and fills the cartridge. No ~oaming mixture leaks out.
ExamDle 3 500 Parts by weight of a polypropylene glycol with OH
number 112 and 258 parts by weight of a polyisocyanate mixture of the diphenylmethane series having an NCO value of 32.5% and containing 90% of diphenylmethane diisocyanate isomers (the remainder being higher functional polyisocyanates) which in turn consist to an extent of about 90% of 4,4'-diphenylmethane diisocyanate are reacted at 80C under a nitrogen atmosphere with stirring until the isocyanate value is constant (calculated 5.5% NCO). The temperature is lowered to 60C.
The following are added with stirring:

3.8 parts by weight of IPPD, 34 parts by weight of pentaerythritol, 38 parts by weight of azodicarbonamide and 3.~ parts by weight of Zn stearate.

20 Parts by weight of Aerosil R 202 are then stirred in, with the result that the mixture undergoes considerable thickening but remains pourable at 60C.
The viscous mixture is poured into polyethylene bags of about 100 ml capacity fitted with pressure seals and the bags are sealed and left to cool.
To produce a cross-linked foam, the sealed bag conta1ning the mixture, which is waxy and kneadable at room temperature (25C), is kept in a hot air oven at 190C for 15-minutes.
The polyethylene bag is torn open by the expansion of the mixture and an expanded, elastic, foamed molding having a gross ~7~7 density of about 0.45 g/cm3 is obtained. The foam produced does not escape or melt below 250C.
Example 4 1000 Parts by weight of a neopentyl glycol/hexanediol polyadipate with OH number 56 and 222.3 parts by weight of isophorone diisocyanate are reacted under a nitrogen atmosphere at 90C with stirring until the NCO value is constant (calculated 3.4% NCO). The temperature is lowered to 60C.
The following are added with stirring:

34 parts by weight of pentaerythritol, 61.1 parts by weight of azodicarbonamide and 6.25 parts by weight of Zn stearate.

18 Parts by weight of Aerosil R 202 are then stirred in so that the mixture becomes very thick but remains pourable at 60C.
The viscous mixture is poured into polyethylene bags having a capacity of about 100 ml and equipped with pressure seals and the bags are sealed and left to cool.
To produce a cross-linked foam, the sealed bag containing the mixture, which is waxy and kneadable at room temperature (25C), is kPpt in a hot air oven at 190C for 15 minutes. An elastic, foamed molding which tears open the polyethylene bag as it expands and has a gross density of about 0.4 g/cm3 is obtained. The foam produced does not escape or melt below 250C.
Although the invention has been described in detail in the foregoing ~or the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing ~rom the spirit and scope of the invention except as it may be limited by the claims.

Claims

1. A heat-curable, expandable one component polyurethane composition comprising a) an isocyanate prepolymer which is flowable at room temperature, and is obtained by the reaction of an organic polyisocyanate with a polyhydroxyl compound;

b) a solid, finely divided polyhydroxyl compound having an average of at least two hydroxyl groups per molecule and which is insoluble in the prepolymer phase a) and finely distributed therein; and c) a solid, finely divided blowing agent which is finely distributed in the prepolymer phase a);

with the proviso that the decomposition temperature of said blowing agent c) is not significantly higher than the curing temperature of the composition.

2. The composition of Claim 1, wherein said polyurethane agent composition additionally comprises d) auxiliary agents and additives.

3. The composition of Claim 1, wherein said polyhydroxyl compound b) is a pulverulent pentaerythritol having a particle size below 1 mm.

4. The composition of Claim 1, wherein said blowing agent c) is a pulverulent azodicarbonamide preparation having a particle size below 100 µm and a decomposition temperature in the range of from 140 to 210°C.

5. The composition of Claim 1, wherein said organic polyisocyanate used in the reaction with said polyhydroxyl compound to form said isocyanate prepolymer a) is selected from the group consisting of diphenylmethane-4,4'-diiso-cyanate, tolylene diisocyanate, and mixtures thereof.

6. The composition of Claim 1, wherein said organic polyisocyanate used in the reaction with said polyhydroxyl compound to form said isocyanate prepolymer a) is selected from the groups consisting of hexamethylene-1,6-diisocyanate, isophorone diisocyanate, 4,4'-methylene-bis-(cyclohexyl-isocyanate), and mixtures thereof.

7. The composition of Claim 1, wherein said polyhydroxyl compound used in the reaction with said organic polyisocyanate to form said prepolymer a) is selected from the group consisting of a polyether polyol having an average molecular weight above 500, a polyester polyol having an average molecular weight above 500, and mixtures thereof.

8. In a process for the production of a cross-linked foamed molding by curing a polyurethane composition at elevated temperature, the improvement wherein said polyurethane composition corresponds to the expandable one-component polyurethane composition of Claim 1.