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This invention relates to novel copolymerizable flame retardants and to
polyesters wherein at least a portion of the repeating units are derived
from difunctional carboxylic acids containing a halogenated thiophene ring
structure. This invention further relates to flame retardant polyesters
and resins which exhibit excellent clarity, color and stability in those
applications where the bulk polymer, the final product, or both are
exposed to elevated temperatures.
With the increasing use of saturated and unsaturated polyesters, as in the
fiber industry for example, the flame retardant characteristics of these
materials have become increasingly important. In the face of increasingly
strict governmental regulations, efforts are being made to develop flame
retardant additives for polymers and resins which will improve their flame
retardant characteristics without adversely affecting other desirable
properties of the final composition, such as by imparting unwanted color
to the polymer or, even more importantly, degrading the mechanical
characteristics of the polymer so as to decrease its effectiveness for a
given end use.
In addition to good performance, the cost of the flame retardant is an
important consideration for commercial processes wherein thousands of
pounds of polymer are produced.
A variety of antimony compounds, e.g., antimony trioxide, in combination
with a halogen source, e.g., a halogenated hydrocarbon, impart flame
retardancy to many polymer compositions without interfering with their
stability either in the environment in which they are to be used or during
their preparation. A major shortcoming of antimony compounds is their
adverse effect on the tensile properties of polymeric materials when the
compounds are employed at concentration levels required to attain an
adequate degree of flame retardancy.
Many otherwise useful flame retardants are sufficiently volatile and/or
unstable that a substantial portion is lost at the elevated temperatures
encountered during formulating and shaping of polyester compositions, many
of which are processed at temperatures above about 250.degree.C.
It is well known that flame retardant properties can be imparted to
polyesters by employing one or more halogenated di- or polyfunctional
compounds as comonomers. Useful comonomers include halogen-containing
diols and/or dicarboxylic acids which contain aliphatic, cycloaliphatic,
or aromatic hydrocarbon structures. The aliphatic compounds are relatively
unstable, with the result that some halogen is evolved during processing
of the polymer, which may impart an undesirable coloration to the final
product.
Chlorinated compounds, as a class, are less than satisfactory because of
the high concentration of these compounds required to obtain effective
amounts of free chlorine at flame temperature.
Monomers wherein the bromine is bonded to an aliphatic or cycloaliphatic
hydrocarbon radical are considerably less stable than the corresponding
chlorinated derivatives. A portion of the available bromine is often
liberated during processing of the polymer, resulting in discoloration of
the final product. At the other extreme, the bond between bromine and an
aromatic hydrocarbon radical is so strong that only a relatively small
portion of the total available bromine is evolved even at flame
temperatures. Inordinately high concentrations of these compounds are
therefore required to obtain adequate flame retardancy.
One objective of this invention is to provide effective flame retardants
that can be incorporated into the backbone structure or side chains of
saturated and unsaturated polyesters.
A second objective is to provide a class of flame retardants which are
stable at temperatures encountered during processing of polyesters, yet
which decompose readily at flame temperatures to liberate an effective
amount of bromine or chlorine.
It is also an objective of this invention to provide flame retardants which
do not adversely affect the physical properties of a polyester.
It has now been found that acceptable to excellent flame-retardant
properties can be imparted to synthetic polyesters by incorporating into
the structure of the polymer an effective amount of a halogenated
thiophene mono- or dicarboxylic acid. The resultant polyesters can be
employed as flame retardants for normally flammable polymers.
STATEMENT OF THE INVENTION
This invention concerns novel, flame retardant polyesters containing
repeating units selected from the group consisting of:
##SPC1##
wherein X represents a chlorine or bromine atom and m represents the
integer 1 or 2.
For brominated derivatives of the present thiophene dicarboxylic acids, an
effective concentration of acid is equivalent to between about 6 and 15%
bromine, based on polymer weight, which corresponds to between about 12
and 30% by weight of a dibrominated thiophene dicarboxylic acid.
Adequate flame retardance is obtained with greater than 12 and up to about
20% by weight of chlorine, based on the dichlorinated thiophene
dicarboxylic acid. Since flame retardancy appears to be directly related
to halogen concentration, it is evident that a proportionately larger
concentration of flame retardant is requiring using mono-halogenated
thiophene dicarboxylic acids.
DETAILED DESCRIPTION OF THE INVENTION
The flame-retardant polyesters of this invention are derived, at least in
part, from mono- or dihalogenated thiophene dicarboxylic acids. The
dicarboxylic acids exhibit one or more of the following formulae
##SPC2##
wherein X represents a chlorine or bromine atom, and m represents the
integer 1 or 2.
PREPARATION OF HALOGENATED THIOPHENE DICARBOXYLIC ACIDS
Halogenated 2, 5-thiophene dicarboxylic acids corresponding to formula I
hereinabove are conveniently prepared by reacting the corresponding
2,3,4,5-tetrahalo- or 2,3,5-trihalothiophene (prepared by the halogenation
of thiophene) with a stoichiometric amount or a slight excess of
organolithium compound at low temperatures and in the presence of a cyclic
or linear ether as the diluent. The resultant organolithium derivative is
then reacted with carbon dioxide using procedures described in the prior
art.
The reaction is believed to follow the course outlined below for the
tetrabrominated derivative:
##EQU1##
Thiophene-3, 4-dicarboxylic acids can be prepared as described in U.S. Pat.
No. 2,744,917 from the corresponding formyl succinate and phosphorus
pentasulfide, followed by hydrolysis of the resultant ester.
A method for preparing thiophene-2, 4-dinitrile is described in U.S. Pat.
No. 3,162,650. The nitrile is readily hydrolyzed to the corresponding acid
using known procedures.
INCORPORATION OF HALOGENATED THIOPHENE DICARBOXYLIC ACIDS INTO POLYMERS
The dicarboxylic acids of this invention can be incorporated into the
structure of synthetic polyesters derived from dicarboxylic acids and
either diols or dihydric phenols.
Polyesters are prepared by reacting one or more aliphatic or aromatic
dicarboxylic acids or suitable derivatives thereof, such as anhydrides or
esters of the acids prepared using volatile monohydric alcohols containing
between 1 and 5 carbon atoms, with one or more alkyl, aralkyl or
cycloalkyl diols containing between 2 and 12 carbon atoms. Suitable acids
contain between 2 and 18 carbon atoms and include oxalic, malic, maleic,
butanedioic, hexanedioic, octanedioic, and the isomeric cyclohexanedioic
acids, the three isomeric phthalic acids, trimellitic acid, and the
isomeric naphthalene dicarboxylic acids. Suitable diols include ethylene
glycol, propylene glycol, 1, 4-butanediol, 1, 6-hexanediol and the
isomeric cyclohexanediols. A di- or bis- phenol can be substituted for all
or a portion of the glycol. If a cross-linked polymer is desired, one or
more compounds containing three or more acid or hydroxyl groups is
employed in place of a portion of one of the corresponding difunctional
reagents.
Polyesters are also prepared from a hydroxyacid or the corresponding cyclic
lactone.
While it is possible to prepare saturated polyesters of relatively low
molecular weight by a direct esterification reaction between the free acid
and a diol, it is usually preferable to obtain the polymer by a
transesterification reaction between the diol and an ester derived from
the desired dicarboxylic acid and a relatively low boiling monohydric
alcohol containing between 1 and 4 carbon atoms. Methyl esters are usually
employed for this purpose. A mixture of dicarboxylic acid esters,
including one or more derived from the present thiophene dicarboxylic
acids, are reacted with one or more diols at a temperature between
150.degree. and about 200.degree.C. under an inert atmosphere. For the
lower boiling glycols, such as ethylene glycol, it is desirable to employ
a considerable excess of the glycol, for example 2.2 moles of glycol per
mole of dicarboxylic acid ester. The resultant bis(hydroxyalkyl) ester is
polymerized at temperatures between 200.degree. and 250.degree.C. The
pressure in the reaction vessel is gradually reduced, usually to less than
1 mm. of mercury, as the excess glycol is distilled. The partial vacuum is
maintained until the polyester attains the desired molecular weight.
A variety of well known catalysts are usually employed for both the ester
interchange and polycondensation reactions. Suitable ester interchange
catalysts include weak bases such as the carbonates, alkanoates, hydrides
and alkoxides of sodium, lithium, zinc, magnesium and titanium. Catalysts
for the polycondensation reaction of the bis(hydroxyalkyl) ester include
antimony trioxide and the titanates such as tetraisopropyl titanate.
Unsaturated polyester resins are generally of relatively low molecular
weight prior to curing. These polymers are prepared from a saturated acid,
such as one of the isomeric phthalic acids or adipic acid, an unsaturated
acid, such as maleic or fumaric acid, and a glycol using procedures
described hereinbefore. One of the present thiophene dicarboxylic acids is
included in the reaction mixture. The resin is crosslinked by reacting the
double bonds of the unsaturated acid residue with a vinyl monomer, usually
styrene, in the presence of a free radical source such as an organic
peroxide.
The relative concentration of unsaturated acid residues in the polymer
chains is based on the degree of crosslinking desired, which is, in turn,
determined by the physical properties to be exhibited by the cured resin.
Procedures for compounding and curing the unsaturated polyester resins in
the presence of free radical sources are well known in the art. The
conventional methods should not require significant modification due to
the presence of a halogenated thiophene dicarboxylic acid residue in the
resin. The mixture of polyester vinyl monomer and free radical source is
formed into the desired shape or coated on a substrate. Some free radical
sources will initiate polymerization, or cross-linking, of the resin at
ambient temperature while others must be heated slightly. The
concentration of free radical source is usually between 0.5 and 20%, based
on the weight of the resin. The rate of cure can be varied by the use of
appropriate accelerators and inhibitors that are well known in the art.
Curing unsaturated polyesters is an exothermic reaction. If it is desired
to reduce the peak temperature reached during the exotherm this can be
achieved by using appropriate inhibitors, which also increase cure time,
or by using .alpha.-methylstyrene, which is an effective exotherm
depressant yet does not significantly increase the cure time or adversely
affect the physical properties of the cured resin.
The novel compounds of this invention can also be reacted with preformed
polyesters that contain free hydroxyl radicals or other radicals which
will react with carboxylic acids. These radicals are introduced by use of
a stoichiometric excess of glycol or by employing trifunctional hydroxy
compounds, such as glycerol, as one of the alcohol components and
adjusting the stoichiometry such that the final polyester contains 2 moles
of carboxyl radicals for every three moles of trifunctional alcohol. This
will prevent or minimize crosslinking.
Detailed procedures for preparing representative saturated and unsaturated
polyesters are contained in a text entitled "Preparative Methods of
Polymer Chemistry" by W. R. Sorenson and T. W. Campbell, published by
Interscience Publishers (New York, 1961). The chlorinated or brominated
thiophene dicarboxylic acids of this invention can be incorporated into
the polyester using the procedures of Sorenson and Campbell, substituting
one or more acids of this invention for a portion or all of the specified
saturated or aromatic dicarboxylic acids. Little if any variation from the
disclosed reaction conditions should be required. For a solution
polymerization it may be necessary to find a solvent or mixture of
solvents that dissolves all of the monomers.
Polymers containing the halogenated thiophene carboxylic acids of this
invention can be processed at temperatures above 180.degree.C. without
undergoing significant discoloration. By comparison, a polyester prepared
using tetrabromophthalic anhydride, a commercial flame retardant, at an
equal bromine concentration, is appreciably discolored (Gardner color
number of about 7) following a short exposure to a temperature of about
190.degree.C.
The minimum concentration of repeating units or side chains derived from
the halogen-containing thiophene dicarboxylic acids of this invention
required to impart useful levels of flame retardancy to a given polymer
composition is most conveniently expressed in terms of the percent by
weight of halogen (either bromine or chlorine) present for a given weight
of polyester, which is in turn a function of the particular polymer being
flame retarded. These values are well known in the art, and are relatively
constant regardless of the source of the halogen, i.e., whether it be the
compounds of this invention, a low molecular weight halogenated
hydrocarbon or an inorganic halide. Polyesters usually require at least 6%
by weight of bromine or 12% of chlorine to exhibit an acceptable level of
flame retardancy. The foregoing minimum concentrations of halogen can
often be reduced up to 50% by the addition of known synergists such as
antimony oxide or other antimony compounds. It will be understood that the
degree of flame retardancy imparted to a given polymer is in most
instances directly proportional to the concentration of halogen available
at flame temperatures. Incorporating more than about 50% by weight of the
compounds of this invention into a polymer is usually undesirable because
the cost of the thiophene dicarboxylic acid may be considerably higher
than that of the other monomers. In addition, the presence of the
thiophene-based dicarboxylic acid in excessive amounts may adversely
affect the physical properties of the polyester.
METHODS FOR MEASURING FLAME RETARDANCY
The Limiting Oxygen Index (L.O.I.) value provides a useful criterion for
determining the flammability of various materials. The method employed to
determine this value for polymer samples is described in ASTM Test Method
No. D-2863-70. The test samples are placed in a vertically oriented Pyrex
glass tube, approximately 3.5 inches (8.9 centimeters) in diameter and
17.7 inches (45.0 centimeters) in height, which has a bed of glass beads
disposed at the bottom thereof and a holder located about 6 inches (15
cm.) above the glass beads, by means of which the samples are maintained
in a vertical position. The gas is introduced at the bottom of the tube by
allowing it to diffuse through the glass beads. The gas is pure nitrogen,
pure oxygen, or a specified mixture of the two. The flow of gas is
controlled and monitored by means of suitable valves and flow meters. Once
the desired gas atmosphere has been established in the tube, a flame is
touched to the test sample and the minimum oxygen concentration required
to support combustion is noted. The limiting oxygen index is then
calculated using the following formula:
##EQU2##
wherein [O.sub.2 ] represents the minimum oxygen concentration required to
support combustion and [N.sub.2 ] represents the corresponding
concentration of nitrogen.
Specimens exhibiting a limiting oxygen index value less than 21.0 will burn
readily in air while a limiting oxygen index greater than 21.0 indicates
that the sample will burn sluggishly, if at all, in ambient air.
The following examples represent preferred embodiments of this invention
and should not be interpreted as limiting the scope thereof.
EXAMPLE 1
Preparation of 3,4-dibromo-2,5-thiophene dicarboxylic acid.
Into a reaction vessel equipped with a mechanically driven agitator, water
cooled reflux condenser, nitrogen inlet, and addition funnel were placed
775 g. (1.94 moles) of 2,3,4,5-tetrabromothiophene and 2.5 liters of
anhydrous diethyl ether. The resultant suspension was stirred and cooled
to 0.degree.C., at which time 2 moles of butyl lithium (as a 22% by weight
solution in hexane) were added over a period of about 0.5 hour while
maintaining the cooled reaction mixture under a nitrogen atmosphere.
Following completion of the addition, the reaction mixture was poured into
a vessel containing a large stoichiometric excess of dry ice (solid carbon
dioxide), and allowed to warm to ambient temperature. An excess of water
was added, followed by sufficient aqueous hydrochloric acid to completely
precipitate the free acid. The crude acid was purified by dissolving it in
an aqueous solution of sodium hydroxide, treating the resultant solution
with activated charcoal and acidifying the liquid phase using sulfuric
acid (about 95% H.sub.2 SO.sub.4).
The dried solid product weighed 627 g. (99% yield) and did not melt below
300.degree.C. Elemental analysis yielded the following results, expressed
as weight percent:
FOUND CALCULATED
______________________________________
Bromine 47.5 48.8
Sulfur 9.45 9.7
______________________________________
The neutralization equivalent, i.e., the equivalent weight of the acid, was
166. The theoretical value for the diacid is 165.
EXAMPLE 2
Preparation of a Polyester Using 3,4-dibromo-2,5-thiophene Dicarboxylic
Acid
An unsaturated polyester was prepared by combining 140 g. (0.42 mole) of
3,4-dibromo-2,5-thiophene dicarboxylic acid with 105 g. (1.4 mole) of
propylene glycol and 82.5 g. (0.85 mole) of maleic anhydride in a reactor
equipped with a mechanically driven agitator, water cooled reflux
condenser, nitrogen inlet, and a trap for removing liquid returning from
the reflux condenser. The reaction mixture was heated to a temperature
between 100.degree.-200.degree.C. for about 3 hours, at which time
evolution of water, a by-product of the reaction, was no longer observed.
The acid number of the polymer was 40. The reaction mixture was then
cooled to a temperature of 120.degree.C., at which time 0.1 g. of
hydroquinone and 120 g. (1.15 mole) of styrene were added. The mixture was
cooled to 50.degree.C. with stirring to obtain a homogeneous solution.
100 grams of the resultant polymer was cured by adding 1 g. of a 60% (by
weight) solution of methyl ethyl ketone hydroperoxide in dimethyl
phthalate and 0.4 g. of a 6% by weight solution of cobalt naphthenate
(based on cobalt) in a liquid hydrocarbon.
The final polymer exhibited a Limiting Oxygen Index (L.O.I.) value of 23.9.
A second polymer prepared as described in this example, with the exception
that the 3,4-dibromo-2,5-thiophene dicarboxylic acid was replaced by an
equimolar amount of maleic anhydride, exhibited an L.O.I. value of 19.0,
indicating that it burns readily in air.
EXAMPLE 3
This example demonstrates the preparation of a saturated polyester using
one of the present flame retardants.
A mixture containing 25 g. of 3,4-dibromo-2,5-thiophene dicarboxylic acid,
23.9 g. of ethylene glycol and 0.0012 g. of zinc diethoxide was heated for
30 minutes with stirring at 160.degree.C. under a nitrogen atmosphere. A
0.0065 g. portion of sodium silicate nonahydrate was added and heating
continued for an additional 30 minutes, at which time 0.013 g. of antimony
tris-butoxide was added. The pressure within the reaction vessel was then
gradually reduced. Ethylene glycol distilled and was collected over a
period of about 2.5 hours, at which time the temperature of the reaction
mixture was increased to 220.degree.C. and maintained for 1.5 hours. A
brown, brittle polymer was obtained upon cooling of the reaction mixture.
The polymer was soluble in hot o- dichlorobenzene or N,N-
dimethylformamide and precipitated upon cooling of the solution.
Although this invention has been disclosed by reference to specific
examples, it will be apparent to those skilled in the art that various
modifications and changes may be made thereto which fall within the scope
of this invention.
* * * * *
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