Note: Descriptions are shown in the official language in which they were submitted.
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WO 96/15178 PCT/E95/04339
Description
Oxidation of sulfur-cont~;n;ng polymers by NO2/N2O4
The in~ention relates to the oxidation of sulfur-con-
t~;n; ng polymers by NO2/N2O4 as oxidizing agent.
Sulfur-cont~in;ng polymers such as poly(arylene thio-
ethers) have long been known. Because of their high heat
distortion temperature and good chemical resistance,
these polymers are used for highly stressed components.
However, in some applications, higher material require-
ments are made. Specifically, an increase in the glass
transition temperature of the polymers is frequently
desirable. It is described that by a polymer-analogous
oxidation of poly(phenylene sulfide) in acetic acid by
concentrated nitric acid for 24 hours at 0 to + 5~C, a
conversion to poly(phenylene sulfoxide) is said to be
possible (US 3,303,007). "Polymer-analogous" means the
con~ersion of one polymer into another. However, the
properties quoted for the polymer formed indicate that no
polymer having a sulfur/oxygen ratio of 1 to 1 was
obtained, since otherwise the values for the heat distor-
tion temperature would have to be higher. Disadvantages
of the process described are firstly the long reaction
time necessary, secondly the possibility of electrophilic
addition and thirdly the acid attack o~ the strong
mineral acid on the thioether bond in the case of long
reaction times (degradation reactions).
It is known that NO2 and its dimer N2O4 are in chemical
equilibrium with each other (~ollemann-Wiberg, Lehrbuch
der Anorganischen Chemie, ~Textbook of inorganic
chemistry], Walter de Gruyter & Co. Berlin 1964, 70th
Edition, pp. 238/239).
2 NO2 s N2O~ + 13.9 kcal
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Depending on the temperature and concentration, a greater
or lesser portion of the NO2 is present as N2O4. At a
concentration of more than 99 % NO2/N2O4 in a closed
system, at 27~C 80 % of the nitrogen dioxide is present
5 as N2O4 and at 50~C 60 % is still present as N2O4. Not
until 135~C is only 1 % N204 still present.
The object of the invention is to develop processes for
the oxidation of sulfur-cont~;n;ng polymers such as
poly(arylene sulfides) which require substantially
shorter reaction times at mild reaction temperatures. The
use of poly(arylene sulfides) having relatively high
molecular weights as starting material is also desirable.
The object was achieved by the use of NO2/N204 as
oxidizing agent, the proportion of N2O4 being determined
by the reaction temperature (see above).
The e~uilibrium system NO2/N2O4 used for the oxidation is
termed N204 below.
The invention relates to a process for the oxidation of
a sulfur-cont~;n;ng polymer in which the sulfur-con-
2 0 t~; n; ng polymer is brought into contact with NO2 or N2O4.
The oxidation of the sulfur-cont~;n;ng polymer
N2O4 according to the invention leads with high selec-
tivity to the formation of sulfoxide bridges. This is all
the more surprising since N2O4 is an extremely strong
oxidizing agent. By appropriate use of the amount of N2O4
and the choice of reaction duration it is possible either
to convert the sulfur bond completely into the sulfoxide
bond or to achieve any desired partial oxidation of the
sulfur bonds in the sulfur-cont~;n;ng polymer. A hetero-
geneous oxidation or partial oxidation of the sulfur-
cont~;n;ng polymer can proceed fully in and right through
a material or only at the surface of a material. A
heterogenous oxidation right through is achieved all the
more easily, the finer is the material. Particularly
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suitable materials for oxidation right through are
ultrathin or ultrafine particles, fibers, films, moldings
or coatings. In this case a highly uniform oxidation is
achieved. Ultrathin or ultrafine denotes a diameter or a
thickness of up to 50 ~m, preferably 1 to 30 ~m, in
particular 5 to 25 ~m. The invention therefore further
relates to particles, fibers, films, moldings or coatings
o~ poly(arylene sulfoxide) ha~ing a diameter or a thick-
ness of up to 50 ~m.
The ultrathin or ultrafine poly(arylene sulfoxide) parts
can be produced by oxidation of appropriately ~;m~ncioned
poly(arylene sulfide) parts and feature a very high
homogeneity of composition or a very uniform degree of
oxidation, by which means, for example, the mechanical,
physical and chemical properties are improved. Ultrathin
or ultrafine poly(arylene sulfoxide) parts show, for
example, because of their homogeneity, improved heat
stability and chemical resistance.
For example, woven fabrics or no~woveLls made of ultrathin
poly(arylene sulfoxide) fibers are suitable for pro~ ;ng
filters for high-temperature applications.
Ultrafine particles of poly(arylene sulfoxide) are
particularly suitable for producing foamed poly(arylene
~ulfide). Poly(arylene sulfoxide) as a blowing agent for
poly(arylene sulfide) is described in German Patent
Application P 44 28 737.2 of 15th August 1994 with the
title "Poly(arylene sulfide) foams and process for their
production", which is incorporated herein by reference.
The expression sulfur-contA;n;ng polymers includes
polymer~ which contain at least one arylene thioether
unit (-AR-S-; Ar: arylene) or an aliphatic thioether unit
(-Alk-S-; Alk: alkylene), eg. poly(arylene thioethers) or
polysulfides. The arylene groups can comprise m~nsnnclear
or polynuclear aromatics. The arylene groups comprise at
least one 5- or 6-membered ring which can contain one or
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more heteroatoms and can be unsubstituted or substituted.
Heteroatoms are, eg., nitrogen or oxygen, substituents
are, eg. linear or branched alkyl groups. The sulfur-
containing polymers can also contain, apart from sulfur
bridges (-S-), sulfoxide groups (-SO-) or sulfone groups
(-SO2-). Arylenes are, for example, phenylene, biphenyl-
ene (-C6X4-C6H4-) or naphthylene which can be monosubsti-
tuted ~r polysubstituted. Substituents are, for example,
straight-chain, cyclic or branched C1-C20-hydrocarbon
radicals, such as C1-C10-alkyl radicals, eg. methyl,
ethyl, n-propyl, isopropyl, n-butyl, t-butyl or n-hexyl,
or C6-C14-aryl radicals, eg. phenyl or naphthyl; halide,
sulfonic acid, amino, nitro, cyano, hydroxyl or carboxyl
groups.
Preferred sulfur-containing polymers are poly(arylene
thioethers), also termed poly(arylene sulfide), in
particular poly(phenylene sulfide).
In the oxidation of poly(arylene sulfide) by N2O4,
poly(arylene sulfoxides) are formed. Poly(arylene sul-
foxides) are polymers which contain at least one arylenesulfoxide unit (-Ar-SO-; Ar = arylene radical,
SO = sulfoxide group).
Poly(arylene sulfides), in particular poly(phenylene
sulfide), may be prepared on the basis of the reaction of
dihalogenated aromatics with sodium sulfide according to
~n~o~ns and HILL. Poly(phenylene sulfide) and its prepa-
ration are described in ''u~ nn~s Encyclopedia of
Industrial Chemistry, Volume A21, B. Elvers, S. Hawkins
and G. Schulz (Eds.), VCH, Weinheim-New York 1992, pp.
463-472), which is incorporated herein by reference. The
synthesis of sulfone-cont~;n;ng poly(arylene sulfides) is
described in Chimia 28 (1974) 567, which is incorporated
herein by reference.
A suitable sulfur-cont~;n;ng polymer is, for example, one
ha~ing a mean molecular weight Mw of 4000 to 400,00Q,
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- preferably 10,000 to 150,000, in particular 25,000 to
100,000, determined by GPC.
The mean particle size D50 of the polymers used is
generally in the range of about 5 x 10-7 to about
5 x 10-2 m, preferably about 10-5 to about 10-3 m, and in
particular about 10-5 to about 2 x 10~4m.
The sulfur-cont~in;ng polymer is generally present in
solid form at the beg;nn;ng of the oxidation reaction. It
can also be used in dissolved form or in suspension.
When dissolved sulfur-cont~;n;ng polymer is used, the
reaction with N2O4 is a homogeneous oxidation reaction.
When solid or suspended sulfur-cont~;n;ng polymer is
used, the reaction with N204 is a heterogeneous oxidation
reaction.
The suspension medium for the sulfur-cont~;n;ng polymer
is generally a chemically inert sol~ent. Thus, liquids
can be used in which the reaction product is insoluble,
for example acetic acid, l,1-dichloroethane, dichloro-
methane or 1,1,2-trichloro-1,2,2-trifluoroethane. Xow-
ever, it is also possible to use liquids in which theoxidation product dissolves, such as dichloroacetic acid,
trichloroacetic acid or trifluoroacetic acid.
The reaction temperatures are generally in the range from
minus 40 to + 100~C, preferably from minus 5 to 80~C. The
reaction time required depends on the supply of N2O4 and
the type of reactor chosen and is generally 1 minute to
5 hours, preferably 15 minutes to 120 minutes, and in
particular 15 minutes to 60 minutes. When the reaction
conditions are optimized, it is possible to achieve
reaction times below one minute.
The oxidation according to the invention is carried out,
for example, in an atmosphere comprising N2O4 having a
concentration greater than 40 mol %, preferably greater
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than 60 mol %, and in particular greater than 70 mol %.
The reaction can be carried out in a reactor whose
temperature can be controlled, statically or with stir-
ring, at atmospheric pressure or under pressure.
Oxidation by N204 can be performed by liquid N204.
In the oxidation of a sulfide group to sulfoxide, N204 is
reduced to N203. By a subsequent reaction with atmos-
pheric oxygen, this N203 can be reoxidized back to N204
and thus reco~ered, so that the process described here
can be carried out highly cost-effectively.
The reoxidation of N203 to N204 can also take place, for
example, in a loop reactor in which the N204 is circu-
lated; in the reaction loop it oxidizes the sulfur-
containing polymer and in the return loop is brought into
contact with oxygen and reoxidized to N204.
It is also possible to carry out the reaction with
substoichiometric amounts of N204 if oxygen or air is
blown through the reaction solution. In this case, the
N203 formed in the reaction is immediately reoxidized to
N204 in the reaction mixture and can thus oxidize a
plurality of sulfide groups to sulfoxide. The N204 used
under such reaction conditions acts as an oxygen transfer
agent (catalyst). Consequently, the degree of oxidation
of the polymer can be set specifically by the reaction
duration.
After oxidation is completed, the N204 can be expelled
from a reactor, eg. from a solution or suspension, by a
vigorous air stream or oxygen stream and condensed out in
a cooling apparatus and thus recovered.
In addition, it is possible to carry out the reaction
with a substoichiometric amount of N204 in an autoclave
with an oxygen overpressure. The N203 formed in the
oxidation is here immediately reoxidized to N204.
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When a sol~ent or suspension medium is used, the reaction
can be carried out either discontinuously in a stirred
tank (batch process) or in a continuous reactor, for
example a reactor cascade, comprising a plurality of
pots, or a flow tube. When a reactor cascade is used, the
reaction products can, for example, be combined with the
solvent or dispersion medium in the first cascade pot,
the reaction and oxygen addition can take place in the
second cascade pot and the N204 can be expelled when
oxidation is completed in a third cascade pot.
The N204 (after the reoxidation with oxygen) and, if
appropriate, the solvent or suspension medium, can be
used for further reactions either in a discontinuous or
in a continuous reaction course, so that in the oxidation
reaction all components can be used again and no residues
cont~m;n~ting the environment occur.
The degree of oxidation of the polymer which can be
achieved depends on the precise reaction conditions.
When air or oxygen is added during the reaction, because
of the immediate reoxidation of the N203 formed to N204,
higher degrees of oxidation can be achieved than corres-
pond to the stoichiometry of the N204 used.
The oxidation products of the reaction of N204 with a
sulfur-contA;n;ng polymer are in many cases soluble in
dichloroacetic acid and can be processed therefrom in
conventional proce~ses.
The oxidized or partially oxidized sulfur-contA;n;ng
polymers obtained according to the invention may either
be processed thermoplastically or may be further pro-
cessed, for example, by customary sintering processes,but this is dependent on their melting points. The first
group, i.e. the compounds which can be processed thermo-
plastically, can be converted into moldings and func-
tional components by processing methods customary for
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thermoplastics, eg. injection molding or extrusion. The
molding compounds can also contain known pulverulent
fillers such as chalk, talc, clay, mica and/or fibrous
reinforcing agentsr such as glass fibers and carbon
fibers, whiskers, and other conventional additives and
processing aids, eg. lubricants, release agents, antioxi-
dants and W stabilizers. Such components are used a~
heavy-duty functional components, for example in aircraft
and automobile construction and in chemical plant con-
struction.
The second group, ie. the polymers which can be processedby sintering processes, are used in functional components
having high temperature and chemical stress.
The compounds obtained according to the invention can,
moreover, also be added in powder mixtures to other high-
temperature thermoplastics, eg. PPS or liquid-crystal
polymers. If this polymer mixture is rapidly heated in a
mold, the sulfoxide bridges are reduced to sulfide, gas
evolution occurring. This gas evolution leads to foaming
of the thermoplastics used.
In the examples, Tg is the glass transition temperature,
Tm is the melting point.
Examples
1. In a reaction vessel, 3 g of a poly(phenylene
sulfide) (PPS) powder (Mw 30,000) having a mean particle
diameter of 500 x 10~6m were reacted in 5 ml of N2O4 at a
temperature of minus 40~C for 2 hours.
After the reaction, the polymer powder was separated off
from the N2O4 and dried. In the infrared spectrum and
also in the ESCA spectrum, only the exclusive formation
of sulfoxide groups could be observed.
Tg: 240~C, Tm: 2 370~C (decomposition).
The sulfur/oxygen ratio of the polymer obtained was 1:1
in the ESCA analysis, ie. the S bridges of the PPS used
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were oxidized to SO groups.
2. In a reac~ion vessel, 3 g of a poly~phenylene
sulfide) (PPS) powder (Mw 30,000) having a mean particle
diameter of 500 x 10~6m were reacted in 5 ml of N2O4 at a
temperature of 50~C for 1 hour.
After the reaction, the polymer powder was separated off
from the N204 and dried. In the infrared spectrum and
also in the ESCA spectrum, likewise, only the exclusive
~ormation of sulfoxide groups could be observed.
Tg: 240~C, Tm: ' 370~C (decomposition).
The sulfur/oxygen ratio of the polymer obtained was 1:1
in the ESCA analysis.
3. Poly(phenylene sulfoxide) (PPSO)
In a reaction vessel, 108 g of a poly(phenylene sulfide)
(PPS) powder (Mw 30,000) having a mean particle diameter
of 20 x 10~6m were dispersed in 500 ml of dichloroacetic
acid at a temperature of 10~C. With good stirring, 40 ml
of N2O4 were added dropwise in the course of 40 minutes.
The temperature of the reaction mixture was increased to
40~C and an air current of 50 l/min was blown through for
80 minutes. A homogenous brownish solution resulted. The
N2O4 was then expelled with a more vigorous air current
(100 l/min), the solution was precipitated in water, and
the product was filtered off by suction, washed with
water and dried. In the infrared spectrum and also in the
ESCA spectrum, only the exclusive formation of sulfoxide
groups could be observed.
Tg: 240~C, Tm: ' 370~C (decomposition).
The sulfur/oxygen ratio of the polymer obtained was 1:1
in the ESCA analysis.
4. Poly(phenylene sulfoxide) (PPSO)
A dispersion of poly(phenylene sulfide) (PPS) powder
(Mw 30,000) having a mean particle diameter of 20 x 10~6m
in acetic acid and N2O4 dissolved in acetic acid was
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continuously added to a reactor cascade comprising three
reaction vessels. The temperature of the first cascade
pot was 40~C and that of the second and third pots was
50~C. In the second pot, an air current of 50 l/min was
additionally passed through, and in the third cascade pot
an air stream of 100 l/min was passed through. The mean
residence time of the overall cascade was approximately
90 minutes. After passage through the cascade, the
polymer powder was filtered off by suction, washed with
water, separated off and dried. In the infrared spectrum
and also in the ESCA spectrum, the formation of sulfoxide
groups could be observed.
Tg: 240~C, Tm: ~ 370~C (decomposition).
The sulfur/oxygen ratio of the polymer obtained was
1:0.75 in the ESCA analysis.
5. Production of ultrathin poly(arylene sulfoxide)
fibers
The production of ultrathin poly(phenylene sulfide)
fibers is described, for example, in EP 283 520 Bl
(Example 4, page 8), which is incorporated herein by
reference.
Ultrathin poly(arylene sulfide) or poly(phenylene
sulfide) fibers are treated with N2O4 similarly to
Examples 1, 2 or 4.
Ultrathin poly(arylene sulfoxide) or poly(phenylene
sulfoxide) fibers are obtained.
Examples 1 and 2 show that different reaction tempera-
tures give the same results.
