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This invention relates to functional poly(perfluoroalkylene oxides) and
their preparation. In another aspect, it relates to fluorine-containing
polyurethanes, and their preparation. In another aspect, it relates to a
solid propellant comprising said polyurethanes as a binder therefor. In
another aspect, it relates to an admixture of a hydroxy-terminated
poly(perfluoroalkylene oxide) and polyisocyanate, said admixture being
useful as a structural adhesive or solid propellant binder.
BACKGROUND OF THE PRIOR ART
Polyurethanes have been prepared in the past by reacting
hydroxyl-containing hydrocarbon polymers, such as poly(oxypropylene)
triols, with aliphatic or aromatic diisocyanates. Such prior art
polyurethanes, though widely useful for many applications, do not have the
low temperature flexibility, tensile and elongation, and the hydrolytic,
thermal, and oxidative stability required for many other applications
(such as solid propellant binders and other uses in the aerospace
industry). Recently, fluorine-containing polyurethanes have been disclosed
in the art which do have some thermal and oxidative stability, but they
and their methods of preparation suffer from a number of disadvantages or
limitations, as discussed below.
In Vysokomolekulyarnye Soedineiya Vol. (A) 9, No. 11, p. 2482 (1967) and
Jour. of Polymer Sci. Part A-1, Vol. 5, p. 2757 (1967), non-rubbery
fluorine-containing polyurethanes are disclosed as being prepared by
either the reaction of chloroformate derivatives of hydroxy compounds with
fluorine-containing diamines (which reaction evolves corrosive,
bubble-forming hydrogen chloride) or the reaction of fluorine-containing
hydroxy compounds (rather than prepolymers) with aliphatic diisocyanates.
These polyurethanes have a high ratio of urethane groups to the total
weight of the polymer, and consequently a low fluorine content. NASA
Publication No. SP-5901 (01), p. 14 (1968), published by NASA's Office of
Technology Utilization, discloses fluorine-containing polyurethanes,
having pendant --CF.sub.3 groups in the backbone, prepared by reaction of
an excess of aliphatic diisocyanate with hydroxyl- and fluorine-containing
prepolymers having hydroxyl functionalities typically less than two, using
undesirably high reaction temperatures. Though these prior art
fluorine-containing polyurethanes do have some thermal and oxidative
stability, they do not have very low temperature flexibility -- a property
which is highly desirable where such products are used, for example, as
low temperature adhesives and propellant binders.
BRIEF DESCRIPTION OF INVENTION
Briefly, the polyurethanes of this invention comprise urethane linkages and
perfluoroalkylene ether backbone units of the formua --CF.sub.2
O--(CF.sub.2 CF.sub.2 O).sub.m (CF.sub.2 O).sub.n --CF.sub.2 -- where m
and n designate randomly distributed perfluoroethyleneoxy and
perfluoromethyleneoxy backbone subunits, respectively, the ratio m/n being
0.2/1 to 5/1, preferably 0.5/1 to 2/1 or 0.5/1 to 3/1. These polyurethanes
are rubbery polymers of high molecular weight with glass transition
temperatures less than -78.degree.C. (and consequently flexible at
extremely low temperature) and can be crosslinked. Typically, the
polyurethanes have a molecular weight of at least 1100, preferably at
least 5000, and frequently as high as 2,000,000 or more. They also have
excellent tensile strength and elongation and highly useful degrees of
thermal, oxidative, and hydrolytic stability. The low temperature
flexibility of the polyurethanes is exceptional, the polyurethanes being
flexible at the temperature of dry ice (ca -80.degree.C.) and lower. Such
exceptional flexibility can be expressed in terms of glass transition
temperature (Tg): the polyurethanes of this invention have a Tg of less
than -78.degree.C., and some species have a Tg as low as -125.degree.C. By
comparison, commerical fluoro elastomers, such as vinylidene
fluoride/perfluoropropylene copolymers, have Tg values only as low as
-30.degree.C. Polytetrafluoroethylene has a Tg of +117.degree.C., and
oxygen-containing fluorinated polymers such as polytetrafluoroethylene
oxide, --(CF.sub.2 CF.sub.2 O).sub.n --, is a solid at room temperature
with a melting point of +37.degree.C. (see U.S. Pat. No. 3,355,397). The
lowest Tg of any known fluoro elastomer, viz., a perfluoroalkylene oxide
of the formula --(CF.sub.2 CF.sub.2 OCF.sub.2 CF.sub.2).sub.n --, is
-65.degree.C. [see Polymer Letters, Vol. 6, pp. 335-340 (1968)].
The fluorine-containing polyurethanes of this invention can be prepared by
reacting polyisocyanates with linear, hydroxy-terminated,
poly(perfluoroalkylene oxide) prepolymers, preferably methylol-terminated
prepolymers, having glass transition temperatures (Tg) lower than
-78.degree.C. The polyisocyanates preferred in preparing these
polyurethanes are the conventional aliphatic or aromatic polyisocyanates,
or fluorine-containing aliphatic diisocyanates.
The linear hydroxy-terminated poly(perfluoroalkylene oxides), useful as
prepolymers in the preparation of the above-described polyurethanes of
this invention, also have exceptionally low glass transition temperatures
of less than -78.degree.C. They can be prepared, for example, by reduction
of their ester-, carboxy-, or acyl halide-terminated precursors, or
prepared by reaction of said ester- or acyl halide-terminated precursors
with amino alcohols, said precursors and their preparation being described
in Italian Pat. No. 817,809. Hydroxy terminated poly(perfluoroalkylene
oxides) having more than two terminal hydroxy groups can be prepared, for
example, by reacting the dimethylol-terminated compound with up to two
molar equivalents of 2,3-epoxy-1-propanol in the presence of a basic
catalyst. Such polyhydroxy derivatives are useful as compatible
cross-linking components in the preparation of urethane polymers.
The hydroxy-terminated prepolymer and polyisocyanate reactants can be
admixed to form a pourable homogeneous solution which can be cast in a
mold and heated to effect crosslinking and solidification at relatively
low temperature, e.g. 60.degree.-80.degree.C., or even room temperature if
a catalyst is used, without the evolution of volatile byproducts, to form
a shaped article, such as an O-ring, gasket, etc., having excellent low
temperature flexibility and other desirable properties. Solid rocket
propellants of the composite type can be prepared in a conventional
manner, using the said solution of prepolymer and polyisocyanate to form a
high density thermoset binder for the propellant; however, it is
unnecessary to use a plasticizer in order to obtain a workable propellant
mixture or a cured propellant with low temperature flexibility when the
prepolymer of this invention is used in the binder.
Alternatively, the polyurethanes of this invention can be prepared by
reaction of a diol or polyol with an isocyanate-terminated
poly(perfluoroalkylene oxide) containing said --CF.sub.2 --(CF.sub.2
CF.sub.2 O).sub.m (CF.sub.2 O).sub.n --CF.sub.2 -- units.
DETAILED DESCRIPTION OF THE INVENTION
The linear hydroxy-terminated poly(perfluoroalkylene oxide) reactants or
prepolymers used in this invention are preferably those of the general
formula:
R--CF.sub.2 O--(CF.sub.2 CF.sub.2 O).sub.m (CF.sub.2 O).sub.n --CF.sub.2
--R' I
where R and R' are hydroxy-substituted organic radicals, such as
hydroxy-substituted aliphatic or hydroxy-substituted aromatic radicals, or
R and R' are preferably methylol, --CH.sub.2 OH, or --C(O)N(R")CH.sub.2
CH.sub.2 OH (where R" is hydrogen, lower alkyl, e.g. methyl, or ethylol,
--CH.sub.2 CH.sub.2 OH), and m and n designate randomly distributed
perfluoroethyleneoxy and perfluoromethyleneoxy backbone units, the ratio
m/n being 0.2/1 to 5/1, preferably 0.5/1 to 3/1 and typically 0.7/1 to
1.6/1. The number average molecular weight, M.sub.n, is in the range of
500 to 10,000 or 20,000 or higher, preferably 800 to 5000 or 15,000. The
glass transition temperature, Tg, of these prepolymers, as well as the
polyurethanes prepared therefrom, are much lower than any known
fluorine-containing polymers, and in general are lower than -78.degree.C.
and preferably lower than -90.degree.C., and can be as low as
-125.degree.C., the higher the oxygen-to-fluorine content in the
prepolymer, the lower the glass transition temperature. (The "glass
transition temperature" of a polymer is that temperature above which a
polymer is soft or rubbery, that is, flexible, and below which it is a
hard and brittle glass; such temperature is generally determined by
differential thermal analysis, "DTA", or changes in coefficient of
expansion.) The prepolymers are generally clear, colorless liquids at room
temperature, with low bulk viscosity (e.g., 125 cps at 27.degree.C.)
properties which are advantageous in using these materials.
Generally, the hydroxy-terminated prepolymer will be a mixture of such
compounds having different backbone or chain lengths. Representative
prepolymers useful in this invention to form polyurethanes, and coming
within the scope of general Formula I above, are the following:
Hoch.sub.2 --cf.sub.2 o--(cf.sub.2 cf.sub.2 o).sub.m (CF.sub.2 O).sub.n
--CF.sub.2 --CH.sub.2 OH, II
hoch.sub.2 ch.sub.2 n(h)c(o)--cf.sub.2 o--(cf.sub.2 cf.sub.2 o).sub.m
(CF.sub.2 O).sub.n --CF.sub.2 --C(O)N(H) CH.sub.2 CH.sub.2 OH, III
hoch.sub.2 ch.sub.2 n(ch.sub.3)c(o)--cf.sub.2 o--(cf.sub.2 cf.sub.2
o).sub.m (CF.sub.2 O).sub.n --CF.sub.2 --C(O)N(CH.sub.3)CH.sub.2 CH.sub.2
OH, IV
(hoch.sub.2 ch.sub.2).sub.2 nc(o)--cf.sub.2 o--(cf.sub.2 cf.sub.2 o).sub.m
(CF.sub.2 O).sub.n --CF.sub.2 C(O)N(CH.sub.2 CH.sub.2 OH).sub.2, V
hoch.sub.2 --cf.sub.2 o--(cf.sub.2 cf.sub.2 o).sub.m (CF.sub.2 O).sub.n
--CF.sub.2 CH.sub.2 OCH.sub.2 CH(OH)CH.sub.2 OH VI
hoch.sub.2 ch(oh)ch.sub.2 och.sub.2 --cf.sub.2 0--(cf.sub.2 cf.sub.2
o).sub.m (CF.sub.2 O).sub.n --CH.sub.2 OCH.sub.2 CH(OH)CH.sub.2 OH, VII
and mixtures thereof.
The simplest methylol-terminated poly(perfluoroalkylene oxide) prepolymers,
illustrated by Formula II above, are the preferred prepolymers to be used
in this invention because of the greater low temperature flexibility and
hydrolytic stability of the polyurethanes prepared therefrom.
The novel methylol-terminated poly(perfluoroalkylene oxides) of this
invention can be prepared by reduction of their ester-terminated
precursors, such as the lower alkyl esters, e.g.
CH.sub.3 OOC--CF.sub.2 O(CF.sub.2 CF.sub.2 O).sub.m (CF.sub.2 O).sub.n
CF.sub.2 --COOCH.sub.3 VIII
where m and n, and the ratio m/n, are as defined above for Formula I. Such
esters can be reduced to the methylol prepolymers by various reduction
precedures, such as catalytic hydrogenation in the presence of a copper
chromium oxide catalyst (see U.S. Pat. Nos. 2,911,444 and 3,314,987), but
preferably they are reduced in the presence of a complex metal
borohydride, such as sodium borohydride, NaBH.sub.4, the reduction being
carried out in an inert solvent, such as tetrahydrofuran, diglyme, or
dioxane, and at reflux temperatures. Generally the reaction product will
be a mixture of methylol-terminated poly(perfluoroalkylene oxides) of
different chain length. If desired, such a mixture can be fractionated,
e.g. by distillation, chromatography, selective extraction, and other
techniques, to obtain individual fractions of more limited molecular
weight distribution.
Instead of reducing the ester-terminated precursors, the corresponding
carboxy- or acyl halide-terminated precursors can be reduced, preferably
with LiAlH.sub.4, to form the methylol-terminated prepolymers. The
ethylol-substituted amide-terminated prepolymers illustrated by Formulas
III, IV and V above, can be readily prepared by reacting said diester
precursors VIII, or said acyl halide precursors, with a corresponding
ethanol amine, i.e. HOCH.sub.2 CH.sub.2 NH.sub.2, HOCH.sub.2 CH.sub.2
NHCH.sub.3, and (HOCH.sub.2 CH.sub.2).sub.2 NH, respectively.
The diester precursors VIII, themselves, and the other precursors mentioned
above, and their preparation are disclosed in Italian Pat. No. 817,809
(e.g., Example 11 thereof).
Where the prepolymers have terminal hydroxy-substituted aliphatic groups
other than methylol (i.e., prepolymers other than those like Formulas II
to V), then can be prepared, for example, by reacting said ester- or acyl
halide-terminated precursors with higher aliphatic amino alcohols, such as
propanol amine or .omega.-aminoundecyl alcohol. The hydroxy-substituted
aromatic-terminated prepolymers can be prepared, for example, by reacting
said acyl halide-terminated precursors with aminophenols, such as
m-aminophenol, hydroxyalkyl-substituted aromatic amines, such as
hydroxyethylaniline, or hydroxyalkyl-substituted aralkyl amines, such as
1-methyl-1-hydroxymethylbenzylamine. Said acyl halide-or ester-terminated
precursors can be converted to other hydroxy-substituted aliphatic or
aromatic prepolymers with methylene or substituted methylene linkages
between the polymer backbone and the hydroxy-substituted terminal groups.
For example, by reacting the acyl halide-terminated precursor with
potassium iodide to produce an iodide-terminated intermediate, then
reacting the latter with ethylene to produce an iodoethylene- or
iodopolyethylene-terminated intermediate, and then saponifying the latter.
As another example, said acyl halide- or ester-terminated precursors can
be reacted with organometallic compounds, such as ethyl magnesium bromide
or mixed methyl and isopropyl magnesium bromides, to produce prepolymers
with terminal groups that characterize the prepolymers as tert- or
sec-alcohols. As another example, the acyl halide- or ester-terminated
precursors can be reacted with excess polyols, such as neopentylglycol, to
produce 3-hydroxy-2,2-dimethyl propyl ester terminal groups. The
methylol-terminated prepolymers of Formula II can be converted to other
hydroxy-substituted aliphatic or aromatic terminated prepolymers with
ester or ether linkages in the terminal groups. For example, reaction of
the methylol-terminated prepolymer with cyclic esters, such as
beta-propiolactone to produce hydroxypropionates; or by reaction with
ethylene oxide and/or 1,2-propylene oxide to produce oxyalkylene or
polyoxyalkylene diols.
In the event such hydroxy-substituted aliphatic- or aromatic-terminated
prepolymers are used rather than he methylol-terminated prepolymers (of
Formulas II to V), the aliphatic or aromatic portions of such terminal
groups should preferably be less than 15 to 20 weight percent of the
prepolymer, and usually contain less than about 12 carbon atoms, in order
to retain the desired thermal stability, low temperature flexibility,
andother properties imparted to the prepolymers by the perfluoroalkylene
ether backbone. Said aliphatic and aromatic portions of said terminal
groups should not contain any active hydrogen atoms more reactive with the
isocyanato groups of the polyisocyanates than the hydroxy substituent;
however, said aliphatic and aromatic portions can contain other
substituents which are non-reactive with said isocyanato group.
The polyisocyanates which are admixed and reacted with the
hudroxy-terminated prepolymers can be conventional aliphatic or aromatic
polyisocyanates. Representative of these polyisocyanates which can be used
include: benzene-1,3-diisocyanate; benzene-1,4-diisocyanate; hexamethylene
diisocyanate; toluene-2,4-diisocyanate; toluene-2,5-diisocyanate;
diphenylmethane-4,4'-diisocyanate; diphenyl-4,4'-diisocyanate;
2-chloropropane-1,3-diisocyanate; 6-chloro-2,4,5-
trifluorobenzene-1,3-diisocyanate;
diphenyl-3,3'-dimethoxy-4,4'-diisocyanate; naphthalene-1,5-diisocyanate;
pentamethylene diisocyanate; tetramethylenediisocyanate; octamethylene
diisocyanate; dimethylene diisocyanate; propylene-1,2-diisocyanate;
benzene-1,2,4-triisocyanate; toluene-2,3-diisocyanate;
diphenyl-2,2'-diisocyanate; naphthalene-2,7-diisocyanate;
naphthalene-1,8-diisocyanate; toluene-2,4,6-triisocyanate,
benzene-1,3,5-triisocyanate; benzene-1,2,3-triisocyanate;
cyclohexane-1,3,5-triisocyanate; toluene-2,3,4-triisocyanate;
polymethylene polyphenyl isocyanate; and the like.
Another class of polyisocyanate which can be used in this invention are
fluorine-containing aliphatic ether or non-ether diisocyanates of the
general formulas:
Ocn--ch.sub.2 --(c.sub.x F.sub.2x --O--C.sub.y F.sub.2y --.sub.z CH.sub.2
--NCO IX
ocn--ch.sub.2 --(c.sub.x F.sub.2x --.sub.z CH.sub.2 --NCO X
where x and y are integers of 1 to 8, and z are integers of 1 to 12,
preferably 1 to 8. These diisocyanates can be named as .alpha.,
.omega.-bis (1,1-dihydroisocyanates). Preferred subclasses of these
diisocyanates are those of the general formulas:
Ocn--ch.sub.2 (cf.sub.2).sub.a O(CF.sub.2).sub.b --CH.sub.2 --NCO XI
ocn--ch.sub.2 (cf.sub.2).sub.a O(CF.sub.2).sub.b O(CF.sub.2).sub.c CH.sub.2
--NCO XII
ocn--ch.sub.2 [(cf.sub.2).sub.a O(CF.sub.2).sub.b ].sub.c CH.sub.2 --NCO
XIII
ocn--ch.sub.2 (cf.sub.2).sub.a CH.sub.2 NCO XIV
where a, b, and c are integers each preferably in the range of 1 to 8, the
sum of which in each such formula is preferably 16 or less. Representative
fluorine-containing diisocyanates of this type which can be used include
CF.sub.2 (CH.sub.2 NCO).sub.2, (CF.sub.2 CH.sub.2 NCO).sub.2, CF.sub.2
(CF.sub.2 CH.sub.2 NCO).sub.2, (CF.sub.2 CF.sub.2 CH.sub.2 NCO).sub.2,
CF.sub.2 (CF.sub.2 CF.sub.2 CH.sub.2 NCO).sub.2, (CF.sub.2 CF.sub.2
CF.sub.2 CH.sub.2 NCO).sub.2, CF.sub.2 (CF.sub.2 CF.sub.2 CF.sub.2
CH.sub.2 NCO).sub.2 (CF.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 NCO).sub.2,
O(CF.sub.2 CH.sub.2 NCO).sub.2, O(CF.sub.2 CF.sub.2 CH.sub.2 NCO).sub.2,
O(CF.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 NCO).sub.2, OCNCH.sub.2
(CF.sub.2).sub.2 O(CF.sub.2).sub.4 CH.sub.2 NCO, OCNCH.sub.2 CF.sub.2
OCF(CF.sub.3)CH.sub.2 NCO, (OCNCH.sub.2 CF.sub.2 OCF.sub.2).sub.2,
OCNCH.sub.2 (CF.sub.2 OCF.sub.2).sub.4 CH.sub.2 NCO, OCNCH.sub.2 (CF.sub.2
OCF.sub.2).sub.8 CH.sub.2 NCO, OCNCH.sub.2 (CF.sub.2 CF.sub.2 OCF.sub.2
CF.sub.2).sub.4 CH.sub.2 NCO, OCNCH.sub.2 CF.sub.2 O(CF.sub.2).sub.4
OCF.sub.2 CH.sub.2 NCO, OCNCH.sub.2 CF (CF.sub.3)OCF.sub.2
CF(CF.sub.3)O(CF.sub.2).sub.5 OCF(CF.sub.3)CF.sub.2 OCF(CF.sub.3)CH.sub.2
NCO, and the like. These fluorine-containing diisocyanates are
particularly useful in that they are more soluble in the
hydroxy-terminated prepolymers than the aliphatic or aromatic
polyisocyanates, allowing a more rapid attainment of homogeneity, and the
resultant polyurethanes have higher thermal stability.
The above fluorine-containing aliphatic diisocyanates can be prepared by
reacting phosgene with the corresponding fluorine-containing diamine or
diamine hydrochloride precursors which have general formulas like those of
Formulas IX and X above, except that inplace of isocyanate groups (--NCO)
there are amine groups (--NH.sub.2), or amine hydrochloride groups
(--NH.sub.2.NCl). Preparation of the diisocyanate is preferably carried
out by dissolving the diamine precursor in a solvent, such as
tetrahydrofuran, diglyme, or chlorobenzene. Alternatively, a slurry of the
diamine hydrochloride in these solvents can be prepared. In any event, the
phosgene is bubbled through the solution or slurry at a suitable
temperature, e.g. 0.degree. to 120.degree.C. for a period of time
sufficient to get the desired conversion. For this purpose, the course of
phosgenation can be followed by running infrared spectral analysis on
withdrawn samples of the reaction mixture. When the infrared spectrum
shows the conversion of most of the intermediate carbamoyl chloride to
diisocyanate, the solvent is stripped, and the residue distilled to obtain
the diisocyanate product.
The oxydiamine precursors themselves can be prepared by reduction of the
corresponding diamide or dinitrile precursor with a reducing agent, such
as lithium aluminum hydride.
Another class of polyisocyanates which can be used in this invention are
isocyanate-terminated poly(perfluoroalkylene oxides) containing said
--CF.sub.2 O--(CF.sub.2 CF.sub.2 O).sub.m (CF.sub.2 O).sub.n --CF.sub.2 --
units (hereinafter abbreviated R.sub.fo), such as
##STR1##
The preparation of these isocyanate-terminated poly(perfluoroalkylene
oxides) is described in said copending application Ser. No. 70,540.
The polyurethanes of this invention can be crosslinked materials,
possessing a three-dimensional network. The degree of crosslinking can be
varied from a low degree of crosslinking, e.g. one crosslink per 500,000
molecular weight units of polymer, to a high degree of crosslinking, e.g.
one crosslink per 1000 molecular weight units of polymer. However, a
highly crosslinked polyurethane will have a higher glass transition
temperature than may be desired, and it may be higher than -78.degree.C.
There are several ways in which the crosslinking of the polyurethanes of
this invention can be achieved. The preferred manner of achieving
crosslinking of the polyurethanes based on the hydroxy-terminated
prepolymer is by reacting difunctional hydroxy-terminated prepolymer with
diisocyanate and either triisocyanate or triol, which results both in
chain extension of the preplymer and crosslinking by urethane formation.
Another manner of achieving crosslinked polyurethanes is by reacting the
difunctional hydroxy-terminated prepolymer with a small excess (e.g. 2 to
30%) of diisocyanate, heating to a moderate temperature, e.g. 25.degree.
to 80.degree.C., to effect chain extension by urethane formation, and then
heating to a higher temperature, e.g. 60.degree. to 150.degree.C., to
effect crosslinking by allophanate formation. Another manner of achieving
crosslinking is by reacting a mixture of di- and tri-functional
hydroxy-terminated prepolymers with diisocyanate. Another manner of
crosslinking is to react the difunctional hydroxy-terminated prepolymer
with an excess of diisocyanate to produce isocyanato-terminated
prepolymer, and reacting the latter with triols (to form urethane
linkages) or effecting its trimerization by adding a conventional
trimerization catalyst, such as a tertiary amine, e.g. triethylamine or
N-methylmorpholine, alkali metal alkoxide, and other strong bases (to
effect isocyanurate formation). These various techniques for effecting
crosslinking are generally known in the art [e.g. see "Polyurethanes:
Chemistry & Technology", Vol. I, by Saunders and Frisch, published by
Interscience Publishers, N.Y. (1962)], and further discussion will be
omitted in the interest of brevity.
Regardless of the type of polyisocyanate and crosslinking technique used,
the amount of polyisocyanate to be used generally will be sufficient to
provide a mole ratio of NCO/OH in the range of 0.8/1 to 1.5/1. A
stoichiometric amount of the polyisocyanate will be sufficient to form
polyurethanes which are rubbery and are useful, e.g. as binders; less than
stoichiometric, will give softer polyurethane elastomers having greater
elongation under stress, and more than the stoichiometric will generally
give harder polyurethanes with higher strengths.
As mentioned above, the polyurethanes of this invention can be rubbery
crosslinked polymers, and as such are insoluble in all non-reactive
solvents, e.g. alcohols, ketones, esters, hydrocarbons, and halogenated
solvents. As an example, they are insoluble in and relatively unaffected
by jet fuels, and are particularly useful in this respect as sealants for
fuel tanks and as gaskets and O-rings, and in structural adhesives which
come into contact with such fuels. Unlike their prepolymers, the
polyurethanes are insoluble in fluorinated ether solvents, such as FC-75,
but swell therein. The polyurethanes are of relatively high density, which
means that when they are used as binders for solid propellants, the
density impulse thereof is enhanced. The exceptionally low Tg of these
polyurethanes is another property which enhances their use as such
binders, as well as their use in cryogenic adhesives and structural
metal-to-metal adhesives for airframes where low temperature flexibility
is required. Generally, the higher the number average molecular weight,
M.sub.n, of the hydroxy-terminated prepolymer used in making the
polyurethane, the lower the Tg. (M.sub.n as used herein is determined by
vapor phase osmometry.) Their hydrolytic stability is excellent and means
that they can be used in the form of shaped articles, such as gaskets,
seals, etc., which are subject to moisture contact during use or which
come into contact with aqueous solvents or water. They also have useful
degrees of thermal and oxidative stability, tensile strength and
elongation, and do not support combustion in air.
The hydroxy-terminated prepolymer and polyisocyanate, and crosslinking
polyol where used, can be admixed to form a homogeneous solution. In order
to get this homogeneous solution, it may be necessary to heat the mixture,
e.g. 1-2 hours at 60.degree.-100.degree.C., and get a partial reaction
between the components. The addition of catalyst, such as
N-methylmorpholine, conventionally used in making polyurethanes from
polyols, will speed the attainment of homogeneity. Alternatively,
cosolvents, such as 1,2-dimethoxyethane, tetrahydrofuran, and fluorinated
solvents, can be used to form a solution, the solvent thereafter being
stripped. The reaction mixture can be poured or cast in a mold of desired
shape, and the material heated to effect curing or crosslinking. Curing
temperatures in the range of 25.degree. to 125.degree.C. will be useful in
general, depending on the particular polyisocyanate and prepolymer, and
whether curing catalysts are used. Higher temperatures, e.g. up to
200.degree.C., are not necessary, and may result in decomposition of the
urethane linkages. The use of curing catalyst, such as tertiary amines,
e.g., N-methyl morpholine, ferric acetyl acetonate, stannous octoate, and
di-n-butyl tin diacetate, etc., in catalytic amounts (generally 0.005 to 1
wt. %, preferably 0.01 to 0.5 wt. %, based on the weight of prepolymer),
will enable the use of lower curing temperatures, e.g. 25.degree. to
40.degree.C. In any case, the optimum curing temperature and duration of
cure, can be determined empirically by simple routine tests.
In some applications of the polyurethane of this invention, it may be
desirable to use a plasticizer to facilitate the mixing or compounding of
the prepolymer with other materials, such as fillers, e.g. diatomaceous
earth, or propellant ingredients, e.g. oxidizer, fuel, etc. However, the
use of plasticizers is not essential, especially where the polyurethanes
of this invention are used as binders for solid rocket propellants.
The polyurethane products of this invention can be used as high density
binders in solid rocket propellants of the castable composite type. In
such application, the general procedure used involves blending the
admixture or solution of methylol-terminated prepolymer and polyisocyanate
with energetic fuel (e.g. aluminum powder), and/or propellant oxidizer
(e.g. ammonium perchlorate), and other conventional propellant additives,
shaping the resulting mixture in the form of a grain by means of casting
the mixture in a mold, and then heating the shaped grain at elevated
temperatures to effect the crosslinking of the prepolymer by the
diisocyanate to form a finished grain.
Generally, the propellant oxidizer will be an inorganic oxidizing salt,
such as the ammonium, alkali metal, and alkaline earth metal salts of
nitric, perchloric, and chloric acids. Mixtures of these oxidizing salts
can also be used. Ammonium nitrate and ammonium perchlorate are the
preferred oxidizers for use in the solid propellant compositions of this
invention. Other applicable oxidizers representatively include sodium
nitrate, potassium perchlorate, strontium chlorate, lithium chlorate,
calcium nitrate, barium perchlorate, and the like. In the preparation of
the propellant compositions, the oxidizers are powdered to sizes generally
in the range of from 1 to 300 microns average particle size, preferably in
the range between 20 and 200 microns. The propellant fuel will be a
powdered or finely divided metal, such as aluminum or boron, e.g. with a
particle size in the range of 20 to 200 microns.
The amount of solid oxidizer and fuel (e.g. powdered aluminum) employed
will usually be a major proportion of the total composition, and is
generally in the range between 50 and 85 percent by weight of the total
mixture. The binder in the propellant composition will usually be a minor
proportion of the total composition, and is generally in the range between
15 and 50 percent by weight of the total mixture.
The propellant compositions of this invention can also contain various
other conventional compounding ingredients, such as antioxidants, wetting
agents, curing agents, metal oxides, reinforcing agents, powdered metals,
and the like. The finished "propellant" usually contains these other
compounding ingredients, and the quoted term will be used generically
herein to cover the mixture of the fluorocarbon polymer with these other
ingredients, unless otherwise noted.
The propellant composition of this invention can be formed into a grain
having any desired shape or geometry, such as grains of the internal,
external, and internal-external burning types, and geometries which
provide progressive, neutral, or degressive modes of burning.
Further details on the use of polyurethanes of this invention as propellant
binders will be omitted in the interest of brevity, since the physical and
manipulative steps in preparing solid propellants is well-known in the art
(see, for example U.S. Pat. No. 3,050,423).
Objects and advantages of this invention are further illustrated by the
following examples, but the particular materials and amounts thereof
recited in these examples, as well as other conditions and details, should
not be construed to unduly limit this invention.
EXAMPLE 1
This example describes the preparation of a methylol-terminated
poly(perfluoroalkylene oxide) reactant used in this invention.
Powdered lithium aluminum hydride (1.9 g., 0.05 mole) was added to 120 ml.
of dry diethyl ether in a 500 ml., three-necked flask fitted with a
mechanical stirrer, a reflux condenser fitted with a Drierite drying tube,
and a gas-inlet tube, and the mixture stirred 4 hrs. under dry nitrogen.
Fifty ml. of an ether solution of the methyl diester precursor (see
Formula VIII above, where M.sub.n was 1800 and m/n was 1.4/1) was added to
the stirred solution of lithium aluminum hydride at a rate sufficient to
maintain a gentle reflux. After all the ester had been added, the
resulting mixture was heated at reflux overnight. Anhydrous methyl alcohol
(20 ml.) was added to decompose the excess hydride, followed by addition
of dilute sulfuric acid (37 g. of 36N H.sub.2 SO.sub.4 in 100 ml. of
water). The aqueous and organic layers were separated and the aqueous
layer extracted 4 times with diethyl ether, and the resulting ether
fractions and the organic layer combined and dried over calcium sulfate.
The calcium sulfate and ether were removed from the combined ether
fractions, yielding 32.5 g. of water-white, liquid methylol-terminated
poly(perfluoroalkylene oxide), HOCH.sub.2 --CF.sub.2 O--(CF.sub.2 CF.sub.2
O).sub.m (CF.sub.2 O).sub.n --CF.sub.2 --CH.sub.2 OH, which was found to
have a M.sub.n of about 1800, a hydroxyl equivalent weight of 975 .+-. 50,
and a T.sub.g of -107.degree.C.
EXAMPLE 2
In this example, another methylol-terminated poly(perfluroalkylene oxide)
was prepared following the procedure of Example 1, using a similar methyl
diester precursor (M.sub.n of 3000, m/n = 1.25/1) except that Freon 113
trichlorotrifluoroethane was used for extraction instead of diethyl ether.
The preparation yielded 40 g. of water-white, liquid methylol-terminated
fluoro polymer, HOCH.sub.2 --CF.sub.2 O--(CF.sub.2 CF.sub.2 O).sub.m
(CF.sub.2 O).sub.n --CF.sub.2 --CH.sub.2 OH, which was found to have a
M.sub.n about 3000, and hydroxyl equivalent weight of 1550 .+-. 50.
EXAMPLE 3-7
In these examples, a number of polyurethanes of this invention were
prepared using the methylol-terminated prepolymers of Example 1 (M.sub.n
1800) or Example 2 (M.sub.n 3000).
In each sample, 5,46 g. of the methylol-terminated prepolymer and 0.08 g.
of a 3 wt. % solution of N-methylmorpholine in acetone were mixed in a 10
ml. beaker. To the resulting solution, 0.34 g. of an isocyanate mixture
was added, made up of 33 wt. % cyclohexane triisocyanate and 67 wt. % of
hexamethylene diisocyanate. After mixing initially at 25.degree.C., the
mixture was heated for about 1 hr. at 80.degree.C. and then cast in a mold
in the form of a bar and heated at 80.degree.C. overnight, to produce a
gelled polyurethane product which became tack-free in 48 hrs. After 4 days
at 80.degree.C., the cured polyurethane was removed from the mold and its
physical properties determined. (In two of the runs, Example 5 and 7, the
bar was further cured for 24 hrs. at 125.degree.C. before removal from the
mold.) Results are summarized in Table I below:
TABLE I
__________________________________________________________________________
Example
3 4 5 6 7
__________________________________________________________________________
Prepolymer, M.sub.n
3000 3000 3000 1800 1800
NCO/OH equiv. ratio
1.15 1.36 1.39 1.12 1.36
Cure in hrs at: 80.degree.C.
96 96 24 96 24
125.degree.C.
-- -- 24 -- 24
Hardness, Shore A-2
5 12 15 20 30
Density, g/cc.
1.79 1.79 1.78 1.76 1.73
T.sub.g by DTA.sup.a, .degree.C.
-112 to
-112 to
-113 to
-102 to
-101 to
-104 -103 -104 -92 -90
Tensile strength.sup.b, psi
50 65 80 60 135
Elongation, %
440 280 260 150 185
TGA.sup.c -Temp. at 10%
375.degree.C.
324.degree.C.
322.degree.C.
304.degree.C.
312.degree.C.
Weight Loss
__________________________________________________________________________
.sup.a "DTA" means the T.sub.g was determined by differential thermal
analysis.
.sup.b Tensile strengths given are averages of two specimens, determined
at break.
.sup.c "TGA" means weight loss was determined by thermogravimetric
analysis.
Small strips of the polyurethanes of Examples 3 and 6 were immersed in a
trichloroethylene-dry ice bath and cooled to bath temperature
(-78.degree.C.). The cooled strips could be repeatedly bent into U and S
shapes without breaking, showing that they were flexible at this
temperature and that they had T.sub.g values of less than -78.degree.C. By
contrast, a strip of high molecular weight polyurethane (T.sub.g =
-55.degree.C.) made from HOCH.sub.2 (CF.sub.2 CF.sub.2 OCF.sub.2
CF.sub.2).sub.n CH.sub.2 OH and hexamethylene diisocyanate became hard
when cooled in the bath and broke into several pieces when bent.
EXAMPLE 8
A polyurethane was prepared by mixing 2.39 g. of the prepolymer of Example
1 with 0.41 g. of tetrafluorophenylene diisocyanate in a 5 cc. beaker at
25.degree.C. and then further heated for about 21/4 hrs. at 80.degree.C.
with frequent mixing. The resulting mixture was cast in a mold to form a
bar, the cast sample gelling in 4-5 hrs. at 80.degree.C. and becoming
tack-free overnight. The cure of the sample bar was completed by heating
at 125.degree.C. for 24 hrs. The resulting cured polyurethane elastomer
was found to have a density of 1.88 g/cc Shore A-2 hardness of 50, T.sub.g
of -100.degree. to -86.degree.C., a tensile strength of 965 psi, and an
elongation of 850%.
EXAMPLE 9
A polyurethane was prepared by mixing 0.74 g. of the prepolymer of Example
1 with 0.015 g. of a 3% solution of dibutyl tin diacetate in acetone,
followed by the addition of 0.097 g. of toluene diisocyanate. The mixture
was heated at 80.degree.C. with frequent stirring, and the product gelled
in about 1 hr. to form a tough polyurethane elastomer.
EXAMPLE 10
A solution was prepared by mixing in a 10 cc. vial 0.05 g. of the
prepolymer of Example 1, 0.009 g. of a 3 wt. % solution of
N-methylmorpholine in acetone, 0.080 g. of polymethylene
polyphenylisocyanate (PAPI), and 1.5 ml. of 1,2-dimethoxyethane solvent.
The solution became clear after mixing for 10 min. at 40.degree.C., and
mixing was continued overnight at this temperature. The solvent was then
removed from the mixture under reduced pressure, producing an opaque tan
liquid which gelled in 20 min. at 80.degree.C., and then cured for 4 days
at 80.degree.C. resulted in a tough polyurethane elastomer, having a
T.sub.g of -104.degree. to -92.degree.C.
EXAMPLE 11
In this example, an ethylol-substituted, amide-terminated prepolymer was
prepared by slowly adding and stirring 0.9 g. of ethanol amine to 10.2 g.
of a methyl diester precursor like that used in Example 1 with M.sub.n =
1400 and m/n = 1.55/1. After stirring the mixture for 1 hr., infrared
analysis indicated complete conversion of the ester precursor to the
amide-terminated prepolymer. The reaction mixture was dissolved in 125 ml.
diethyl ether, washed with 3-10 ml. portions of water, and dried over
calcium sulfate. Removal of the calcium sulfate and ether yielded 9.5 g.
of the pale yellow prepolymer having the structure shown by Formula III
above.
EXAMPLE 12
Amide-terminated prepolymer (4.2 g.) of Example 11 was mixed with 0.5 g. of
hexamethylene diisocyanate, and then heated at 80.degree.c. with frequent
mixing, producing a homogeneous mixture in 1 hr., and a tough strong
flexible tack-free polyurethane in 48 hrs.
EXAMPLE 13
Amide-terminated prepolymer (4.9 g.) of Example 11 was mixed with 0.78 g.
of toluene diisocyanate, then heated for 2 hrs. at 80.degree.C. to produce
a homogeneous mixture which gelled in 61/2 hrs. and was tack-free in 24
hrs., the resulting polyurethane elastomer being tough, and firm.
EXAMPLE 14
An ethylol-substituted, amide-terminated prepolymer was prepared by mixing
4.2 g. of the same ester precursor used in Example 11 with 0.47 g. of
2-(methyl amino)ethanol. The resulting product was dissolved in 50 ml.
diethyl ether, washed with two 15 ml. portions of 5% aqueous hydrochloric
acid, followed by washing with three 10 ml. portions of water, and dried
over calcium sulfate. Removal of the calcium sulfate and ether yielded 4
g. of pale yellow prepolymer having the structure shown in Formula IV
above.
EXAMPLE 15
One-half g. of amide-terminated prepolymer of Example 14 was mixed with
0.078 g. of toluene diisocyanate, and heated at 80.degree.C. with frequent
stirring, the mixture becoming homogeneous in 30 min., gelling in less
than 24 hrs. and becoming tack-free in 48 hrs., the resulting polyurethane
elastomer being flexible and firm.
EXAMPLE 16
This example illustrates the use of the polyurethane of this invention as a
structural adhesive.
Amide-terminated prepolymer (0.66 g.) of Example 11 was mixed with 0.098 g.
of toluene diisocyanate. After 2 hrs. at 80.degree.C., the homogeneous
mixture was spread on a 3.5 .times. 1.5 inch acid-etched aluminum panel
and another aluminum panel pressed thereover to form a 1/2 inch overlap
shear specimen, the assembly then being cured at 80.degree.C. for 2 hrs.
The cured specimen was then tested on an Instron machine at 23.degree.C.,
the bond failing only after application of a shear force of 1200 psi.
EXAMPLE 17
This example illustrates the use of the polyurethane of this invention as a
binder for a cast propellent.
Methylol-terminated prepolymer (0.41 g.) of Example 1 was mixed with 0.009
g. of a 3 wt. % solution of N-methylmorpholine in acetone, and 0.059 g. of
the isocyanate mixture used in Example 3. The resulting mixture became
homogeneous in 35 min. at 80.degree.C., and it was mixed with 0.49 g. of
aluminum powder and 0.956 g. of ammonium perchlorate. The resulting
propellent mixture was then pressed in a mold and heated for 2 days at
80.degree.C. to form a bar of cured solid propellent which was firm and
flexible and burned with a bright flame when ignited.
EXAMPLE 18
A methyl diester precursor (see Formula VIII), M.sub.n = 1940, m/n = 0.7/1,
was fractionated by precipitation and the molecular 25 weight distribution
of each fraction determined by vapor phase osmometry. Results are shown
below.
TABLE II
______________________________________
Fraction M.sub.n Wt. %
______________________________________
1 1090 12
2 1800 17
3 2000 7
4 2350 10
5 2850 15
6 3000 14
7 3350 8
8 3750 8
9 4650 6
10 7830 2
11 >7830 1
______________________________________
The above diester (M.sub.n = 1940) was reduced following the procedure of
Example 2 and the resulting methylol-terminated prepolymer was used to
prepare a polyurethane following the procedure of Example 3, using 2.61 g.
of the prepolymer, 0.028 g. of catalyst solution (3 wt. %
N-methylmorpholine in acetone), and 0.254 g. of a mixture of 80 wt. %
hexamethylene diisocyanate and 20 wt. % cyclohexane triisocyanate, and
mixing these materials at 80.degree.C. The mixture became homogeneous in
30 min., gelled in 3 hrs., and was tack-free in 24 hrs. The product was
then further cured 24 hrs. at 80.degree.C. and 24 hrs. at 125.degree.C.,
and the fully cured product had a Shore A-2 hardness of 15, a tensile
strength at break of 70 psi, an elongation at break of 200%, and a density
of 1.8 g./cc.
A sample of the above polyurethane elastomer was heated in air at about
150.degree.C. and weight loss determined by repeated weighings. Results
are shown in Table III.
TABLE III
______________________________________
Time, hrs. Wt. Loss, %
______________________________________
24 0.8
72 1.2
165 1.4
260 1.9
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