Note: Descriptions are shown in the official language in which they were submitted.
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.
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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~
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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
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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
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(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
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(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
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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
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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.
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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 .
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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
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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
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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.
