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Description  |
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The invention relates to a process for producing foams based on
polyisocyanates, in particular polyurethane foams and polyisocyanurate
foams. The production of such foams is known and has been described, for
example, in Kunststoff-Handbuch [Plastics Handbook], Volume VII,
Polyurethane [Polyurethanes], Carl Hanser Verlag, Munich, Vienna (1983),
pages 246 to 331, and also in EP-Al-0,077,964, EP-Al-0,334,059 and German
Auslegeschrift 1,694,138 (=British Patent 1,209,243).
In Ullmanns Enzyklopadie der technischen Chemie [Ullmann's Encyclopedia of
Industrial Chemistry] (1980), Volume 19, pages 301 to 341, the raw
materials which can be used and the possible processes for producing rigid
polyurethane foams are described in summary.
Further relevant references can be found in Kirk-Othmer, Encycl. of Chem.
Technology, 3rd edition, Volume 11 (1980), pages 87-89, and Volume 23
(1983), pages 576-607.
The usual blowing agents for polyurethanes are carbon dioxide--which is
generated by addition of water during the production of the polyurethanes
from polyisocyanates and compounds having reactive hydrogen--and/or
so-called "physical blowing agents", namely readily volatile organic
substances such as acetone, ethyl acetate, halogen-substituted alkanes
such as methylene chloride, chloroform, ethylidene chloride, vinylidene
chloride and dichlorodifluoromethane, and also butane, hexane, heptane or
diethyl ether. The applicability of fluorinated hydrocarbons for producing
heat-insulating polyurethane foams is known, for example, from German
Patent 1,111,381. Inorganic blowing agents, for example air, CO.sub.2 or
N.sub.2 O, can also be used. Further details on the use of blowing agents
are described in Kunststoff-Handbuch, [Plastics Handbook], Volume VII,
Carl-Hanser-Verlag, Munich (1966), for example on pages 108 and 109, 453
to 455 and 507 to 510.
Physical blowing agents for polyurethane foams must meet certain demands.
Good miscibility of the blowing agents with the usual raw materials is
necessary, but they should be insoluble in the polyurethane being formed,
in order to ensure good material quality of the foams. Furthermore,
because of the heat of reaction which arises during foaming and can lead
to a temperature rise up to about 200.degree. C. when large-volume parts
are foamed up, high thermal stability of the blowing agent is expected. In
addition, the blowing agents will preferably be non-flammable. In the
course of the development of polyurethane foam technology, all these
demands led to the use of fluorinated chlorocarbons (CFC), especially
trichlorofluoromethane (CFC 11), as physical blowing agents.
However, the CFCs are now suspected of damaging the ozone layer around the
Earth. It is therefore necessary to dispense with the use of these
compounds as soon as possible and, instead, to use other substances, which
have no potential for ozone damage, as physical blowing agents.
The abandonment of CFCs as blowing agents in favor of CO.sub.2 which--as
mentioned above--is formed by addition of water during the production of
polyurethanes from polyisocyanates, is under discussion. Although this
method is acceptable for some foams, it causes great disadvantages,
especially in the case of rigid foams, because the foams produced in this
way have an increased thermal conductivity and hence a lower heat
insulation capacity than the foams produced with the aid of CFCs.
Surprisingly, it has now been found that fluorinated ethers are suitable in
the same way as CFCs for producing foams based on polyisocyanates and that
the thermal conductivity of the rigid foams produced with them is
substantially lower than that of those formed with CO.sub.2. In
particular, it has been found that these fluorinated ethers and CO.sub.2
can be used simultaneously as blowing agents, a considerable improvement
in the heat insulation effect being achieved even with a relatively small
proportion of fluorinated ethers (and a correspondingly high CO.sub.2
proportion and therefore a high water content in the rigid foam
formulation).
The invention relates to a process for producing foams based on
polyisocyanates by reacting polyisocyanates, compounds having at least two
hydrogen atoms reactive towards isocyanate groups, blowing agents and, if
appropriate, further additives, which comprises using a blowing agent
which is composed to the extent of at least 10 mol % of one or more
fluorinated ethers of the formula
C.sub.a H.sub.b F.sub.c --O--C.sub.d H.sub.e F.sub.f
where
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a = 1-6 d = 1-2
b = 1-12 e = 0-5
c = 1-12 f = 0-5.
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Preferably, at least 20 mol %, especially 50-80 mol %, of the blowing agent
is composed of one or more of the said fluorinated ethers. However, the
blowing agent can in principle also be composed of these ethers to the
extent of 100 mol %. However, if--which in general will be the case--not
all the blowing agent is composed of these ethers, then the remainder is
composed of one of the abovementioned conventional blowing gases.
Preferably, the remainder then is composed at least partially of CO.sub.2,
which is formed by addition of a suitable quantity of water during the
reaction of the polyisocyanates to give the foams. A "suitable" quantity
of water is here a quantity which forms the desired proportion of
CO.sub.2. Particularly preferred is a blowing gas which is composed only
of one or more of the said ethers and CO.sub.2 (formed by addition of
water), i.e. that the "remainder", mentioned above, of the blowing gas is
composed only of CO.sub.2.
Amongst the fluorinated ethers of the formula I, those are preferred for
which
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a = 1-5 d = 1-2
b = 1-6 e = 1-3
c = 5-10 f = 2-4.
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Those are particularly preferred for which
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a = 1-3 d = 1-2
b = 1-4 e = 1-3
c = 1-6 f = 2-4.
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The preparation of the fluorinated ethers is described in Ullmann's
Encyclopedia of Industrial Chemistry, Volume A 11 (1988), pages 349-389,
in particular page 367; A. M. Lovelace et al., Aliphatic Fluorine
Compounds (1958); H. Liebig and K. Ulm, Herstellung und Anwendung
aliphatischer Fluorverbindungen [Preparation and use of aliphatic fluorine
compounds] II, Chemiker-Zeitung (1976), pages 3-13.
The invention also relates to foams which are based on polyisocyanates and
which are obtainable by the above process.
When the said fluorinated ethers are used, the hitherto usual foam raw
materials can be used and, as stated above, the proportions of water or
conventional physical blowing agent can be greatly reduced or even
eliminated altogether.
Polyisocyanates suitable for the process according to the invention are the
aliphatic, cycloaliphatic and aromatic diisocyanates or polyisocyanates
usual for this purpose. 2,4- and 2,6-toluyl diisocyanate, diphenylmethane
diisocyanate, polymethylenepolyphenyl isocyanate and mixtures thereof are
preferred. Polyisocyanates which contain carbodiimide groups, urethane
groups, allophanate groups, isocyanurate groups, urea groups or biuret
groups and which are termed "modified polyisocyanates" and "isocyanate
prepolymers", can also be used.
The polyisocyanates are reacted with compounds which contain at least two
hydrogen atoms reactive towards isocyanate groups, for example compounds
containing hydroxyl groups and based on polyethers, polyesters and amines,
and also compounds having amino groups and/or carboxyl groups and/or thiol
groups. As a rule, these compounds have 2-8 hydrogen atoms reactive
towards isocyanates.
The catalysts used in this reaction are, as usual, tertiary amines which,
if desired, can also contain hydrogen atoms active towards isocyanate
groups, and/or organic metal compounds, preferably tin salts of carboxylic
acids.
In addition, surface-active additives such as emulsifiers and foam
stabilizers are in general also used. The emulsifiers are, for example,
salts of fatty acids. Polyethersiloxanes are frequently used as foam
stabilizers.
Examples 1-7 which follow explain the invention. They relate to typical
rigid foam formulations with different proportions of various fluorinated
ethers. In the comparison example, however, exclusively CO.sub.2 formed
from water is used as the blowing agent. When the fluorinated ethers
according to the invention are also used in addition to CO.sub.2, a marked
reduction in the thermal conductivity of the foams is found even at
relatively small ether proportions of, for example, 25 mol %, relative to
the total quantity of blowing agent. This effect even increases on
prolonged storage.
The fluorinated ethers are also suitable for foaming up flexible foams
having an open-cell structure and for producing foam moldings having a
porous core and a compact skin, according to German Auslegeschrift
1,694,138 (corresponding to British Patent 1,209,243).
The properties of the foams produced in the examples which follow are
indicated in the table after the examples.
COMPARISON EXAMPLE
(use of CO.sub.2 as the sole blowing agent)
85 g of sucrose/propylene oxide polyether of OH number 380, 15 g of
ethylenediamine/propylene oxide polyether of OH number 480, 1 g of foam
stabilizer (type DC 193 from Dow Corning Corp.), 1.5 g of
dimethylcyclohexylamine and 3.8 g of water were intimately mixed for 15
seconds by means of a stirrer at 2500 revolutions, then intimately mixed
for 10 seconds with 192 g of crude diphenylmethane diisocyanate (MDI of
usual commercial quality) and subsequently cast into a paper mold. Foaming
of the mixture started after about 15 seconds and was complete after about
75 seconds. This gave a rigid foam having the properties indicated in the
table.
EXAMPLE 1
The procedure of the comparison example was followed, but with the
difference that 2.0 g of dimethylcyclohexylamine, 2.0 g of water, 15 g of
1-hydro-tetrafluoroethyl methyl ether (CHF.sub.2 --CF.sub.2 --O--CH.sub.3)
and 165 g of MDI were used.
EXAMPLE 2
The procedure of Example 1 was followed, but the proportion of water was
increased to 3.0 g and the proportion of MDI was increased to 180 g, and
the 15 g of 1-hydrotetrafluoroethyl methyl ether were replaced by 10 g of
2-hydrohexafluoropropyl methyl ether (CF.sub.3 --CHF--CF.sub.2
--O--CH.sub.3).
EXAMPLE 3
The procedure of Example 2 was followed, but the proportion of water was
increased to 3.4 g and the proportion of MDI was increased to 186 g.
Instead of 2-hydrohexafluoropropyl methyl ether, 4.5 g of
1-hydrotetrafluoroethyl ethyl ether (CHF.sub.2 --CF.sub.2 --O--CH.sub.2
--CH.sub.3) were used.
EXAMPLE 4
15 g of qlycerol/ethylene oxide polyether of OH number 750, 50 g of
sucrose/propylene oxide polyether of OH number 490, 15 g of ethylene
diamine/propylene oxide polyether of OH number 480, 20 g of
tetrabromophthalate diol of OH number 220 (PHT-4-diol from Great Lakes
Chemical), 20 g of trichloroethyl phosphate, 1 g of foam stabilizer, 1.5 g
of dimethylcyclohexylamine, 1 g of water, 28 g of 1-hydro-tetrafluoroethyl
methyl ether (CHF.sub.2 --CF.sub.2 --)--CH.sub.3) and 147 g of MDI were
foamed up as in the comparison example.
EXAMPLE 5
The procedure of Example 4 was followed, but instead of 15 g only 10 g of
glycerol/ethylene oxide polyether of OH number 750 and 55 g of
sorbitol/glycerol/propylene oxide polyether of OH number 560 were used
instead of 50 g of sucrose/propylene oxide polyether of OH number 490, and
the 1-hydrotetrafluoroethyl methyl ether blowing agent was replaced by 40
g of 1,1-difluoro-2,2-difluoroethyl 1,1,1-trifluoroethyl ether (CHF.sub.2
--CF.sub.2 --O--CH.sub.2 --CF.sub.3).
EXAMPLE 6
15 g of a polyether obtained from ethylenediamine and equal proportions of
ethylene oxide and propylene oxide and having an OH number of 630 were, as
in Example 1, mixed and foamed up with 45 g of sorbitol/glycerol/propylene
oxide polyether of OH number 560, 20 g of glycerol/propylene oxide
polyether of OH number 160, 20 g of tetrabromophthalate diol of OH number
220 and 20 g of trichloroethyl phosphate as well as 1.0 g of foam
stabilizer, 1.0 g of dimethylcyclohexylamine, 2.2 g of water, 13 g of
1,1-difluoro-2,2-difluoroethyl ethyl ether (CHF.sub.2 --CF.sub.2
--O--CH.sub.2 --CH.sub.3) and 137 g of MDI.
EXAMPLE 7
The procedure of Example 4 was followed, but the 28 g of
1-hydrotetrafluoroethyl methyl ether were replaced by 35 g of
1,1,1-trifluoro-2-fluoro-3,3-difluoropropyl methyl ether (CF.sub.2
--CHF--CF.sub.2 --O--CH.sub.3).
TABLE
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Properties of the foam
Bulk Thermal
Water/fluor-
density conductivity
inated ether
(kg/ (23.degree. C., mW/(m .times. K))
molar ratio
m.sup.3)
after 1 day
after 6 weeks
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Comparison
100/0 37 25.8 34.7
example
Example 1
49/51 36 23.0 26.4
Example 2
75/25 37 22.8 26.4
Example 3
86/14 34 23.9 29.0
Example 4
21/79 32 22.1 23.8
Example 5
22/78 26 21.0 21.9
Example 6
58/42 32 23.1 26.3
Example 7
22/78 35 21.3 22.1
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