U.S. patent application number 10/770475 was filed with the patent office on 2004-08-12 for process for the high-pressure polymerization of 1,1-difluoroethylene.
Invention is credited to Senninger, Thierry.
Application Number | 20040158015 10/770475 |
Document ID | / |
Family ID | 8854660 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040158015 |
Kind Code |
A1 |
Senninger, Thierry |
August 12, 2004 |
Process for the high-pressure polymerization of
1,1-difluoroethylene
Abstract
The invention relates to a continuous process for the
manufacture of PVDF homopolymer or copolymer. The optional
comonomer comprises a vinyl group polymerized by free radicals, and
comprises at least one fluorine atom, a fluoroalkyl group or a
fluoroalkoxy group, directly attached to this vinyl group. The
invention also relates to PVDF homopolymers made by processes of
the invention. The invention further relates to a process for
deoxygenating a flow.
Inventors: |
Senninger, Thierry;
(Hayange, FR) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Family ID: |
8854660 |
Appl. No.: |
10/770475 |
Filed: |
February 4, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10770475 |
Feb 4, 2004 |
|
|
|
09963892 |
Sep 26, 2001 |
|
|
|
6723812 |
|
|
|
|
Current U.S.
Class: |
526/255 ;
526/250 |
Current CPC
Class: |
C08F 14/22 20130101;
C08F 2/02 20130101; C08F 14/22 20130101 |
Class at
Publication: |
526/255 ;
526/250 |
International
Class: |
C08F 114/18 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2000 |
FR |
0012191 |
Claims
1. A process for deoxygenating a flow comprising at least one
fluoro monomer selected from VF2 and monomers comprising a vinyl
group polymerizable by free radicals and at least one fluorine
atom, a fluoroalkyl group or a fluoroalkoxy group, directly
attached to this vinyl group, said process comprising placing this
flow in contact with (i) a sufficient amount of a catalyst whose
active sites are elements belonging to groups 8 to 11 of the
Periodic Table of the Elements and (ii) for a period which is
sufficient to obtain the desired oxygen content.
2. A process according to claim 1, wherein the support for the
active sites is a mineral of alumina, silica, a zeolite or an
aluminosilicate.
3. A process according to claim 2, wherein the catalyst comprises
0.05-5% by weight of the active element.
4. A process according to claim 1, wherein the temperature is
0-200.degree. C.
5. A process according to claim 1, wherein the temperature is
50.degree. C.-100.degree. C.
6. A process according to claim 1, wherein the active element is
copper or palladium.
7. A process according to claim 1, wherein the flow of deoxygenated
VF2 and of optional comonomer comprises less than 5 ppm of
oxygen.
8. A process according to claim 1, wherein the oxygen content is
less than 1 ppm.
9. A flow comprising less than 5 ppm of oxygen, and at least one
fluoro monomer selected from VF2 and monomers comprising a vinyl
group self polymerizable by free radicals, wherein the monomer
comprises at least one fluorine atom, a fluoroalkyl group or a
fluoroalkoxy group directly attached to this vinyl group.
10. A flow according to claim 9, comprising less than 1 ppm of
oxygen.
11. A PVDF, optionally comprising from 0 to 50% of comonomers,
having a melt flow index, measured at 230.degree. C. under a 5 kg
load, of greater than 50 g/10 min according to ASTM D-1238.
12. A PVDF according to claim 11, having a melt flow index, of
greater than 100 g/10 min according to ASTM D-1238.
13. A PVDF according to claim 11, having a melt flow index, of
greater than 200 g/l 0 min according to ASTM D-1238.
14. A PVDF according to claim 11, having a melt flow index, of
greater than 400 g/10 min according to ASTM D-1238.
15. A PVDF homopolymer with a level of defects, measured by
fluorine NMR, of greater than 6%.
16. A PVDF homopolymer according to claim 15, with a level of
defects, measured by fluorine NMR, of greater than 7%.
17. A PVDF homopolymer with an elastic modulus (at 23.degree. C.,
according to ASTM D-1708) of between 1020 and 650 MPa.
18. A PVDF homopolymer according to claim 17, with an elastic
modulus (at 23.degree. C., according to ASTM D-1708) of less than
1000 MPa.
19. A PVDF homopolymer according to claim 17, with an elastic
modulus (at 23.degree. C., according to ASTM D-1708) of less than
900 MPa.
20. A PVDF homopolymer according to claim 17, with an elastic
modulus (at 23.degree. C., according to ASTM D-1708) of less than
800 MPa.
21. A PVDF homopolymer according to claim 17, with an elastic
modulus (at 23.degree. C., according to ASTM D-1708) of less than
700 MPa.
22. A PVDF with an Mw/Mn ratio of 1.5-1.9.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a continuous process for
the high-pressure polymerization of 1,1-difluoroethylene (VF2) to
give polyvinylidene fluoride (PVDF).
BACKGROUND OF THE INVENTION
[0002] The polymerization of 1,1-difluoroethylene is currently
carried out industrially in an aqueous medium either in emulsion or
in suspension. This type of polymerization thus corresponds to
batchwise processes.
[0003] In the case of the emulsion, it is necessary to add
emulsifiers to the polymerization medium in order to stabilize the
PVDF latex particles formed. The emulsifiers must be removed in a
subsequent step in order to ensure that the polymer is in a
suitable purity. Furthermore, certain emulsifiers which are used in
the emulsion process are perfluoro molecules. Studies have shown
that these molecules have a tendency to accumulate in aquatic
flora. Although considered as harmless, nothing is as yet known
regarding the long-term impact of this bioaccumulation.
[0004] In the case of the suspension, protective colloids are
incorporated in order to stabilize the PVDF particles.
[0005] Another process is currently under investigation in many
laboratories. This continuous process differs from
emulsion/suspension processes in that it is carried out in
supercritical CO.sub.2 (ScCO.sub.2) without stabilizing additive.
During the polymerization, the PVDF which is insoluble in the
ScCO.sub.2 precipitates and forms a powder. However, it is
necessary to add a powder-treatment step in order to convert it
into PVDF granules since this type of process does not make it
possible to obtain a powder of controlled and narrow particle size
(as with the emulsion process).
[0006] Patent FR-A-1 260 852 discloses the polymerization of VF2
under pressure either in the presence of a neutral reaction medium
such as water or in the absence of a reaction medium. According to
the first form, deionized and deoxygenated water are loaded into an
autoclave, followed by a peroxide and VF2, the proportion of
peroxide being 0.8 g per 35 g of VF2. The autoclave is closed and
heated, the pressure establishes at values of about 40 to 60 bar,
and the reaction time is about 18 hours. Next, the autoclave is
cooled and PVDF is recovered therefrom. According to the second
form, the process is performed as in the first form, but without
introducing water, the pressure and the reaction time being the
same. It is stated that this second form lends itself to a
continuous operation in which the autoclave is connected to a
source of VF2 under pressure such that the fresh VF2 enters the
autoclave when the conversion into polymer takes place. According
to a variant, catalyst may be added continuously or in batchwise
mode. This prior art is based on a fatal pressure (autogenous
pressure) generated by the volume of the autoclave, the reagents
loaded in and the temperature. The fact that it can be made
continuous by injecting fresh VF2 to compensate for the VF2 which
is polymerized has nothing to do with a process in which the
reaction pressure is determined by the pressure supplied by the VF2
injection pumps. Nothing is stated regarding the oxygen content of
the VF2.
[0007] U.S. Pat. No. 2,435,537 discloses a process similar to the
previous one but still in the presence of water, 50 parts per 40
parts of VF2, and the pressure may be 1000 bar. All the examples
are in batchwise mode in an autoclave. It is stated that the
process may be performed continuously; however, it is recommended
always to use an inert medium such as water to disperse the
catalyst and control the reaction by dissipating the heat. It is
stated that oxygen has a harmful effect on the polymerization, but
the oxygen content of VF2 is not specified and nothing is stated
regarding the means for reducing it.
[0008] Patents FR-A-2 650 593 and FR-A-2 689 134 disclose processes
for the high-pressure synthesis either of copolymers of VF2 and of
ethylene or copolymers of VF2 and of fluoroacrylates.
[0009] Patent WO 98/28351 discloses the continuous polymerization
of VF2 in supercritical CO.sub.2 (75.degree. C.-276 bar). PVDF
copolymers may thus be manufactured. In one example, the flow rate
of fluoro monomers is 200 g/h for a CO.sub.2 flow rate of 518
g/h.
SUMMARY OF THE INVENTION
[0010] A high-pressure process exists, in which the VF2 is
converted into PVDF solely under the effect of pressure and traces
of a peroxide. The high-pressure polymerization makes it possible
to overcome the problems mentioned in the other processes cited
above:
[0011] the process does not require stabilizing additives
(protective colloids or surfactants),
[0012] no treatment of the powders.
[0013] Another advantage of the high-pressure process is that it
offers better production efficiencies than the continuous process
in ScCO.sub.2.
[0014] The present invention relates to a process of this type.
Particularly, a continuous process for preparing PVDF homopolymer
or copolymer has now been found, the comonomer being a fluoro
monomer rather than an acrylate, in which no organic solvent or
water is used and which is not in ScCO.sub.2 medium.
[0015] The invention is a continuous process for the manufacture of
PVDF homopolymer or copolymer, the comonomer being chosen from
compounds containing a vinyl group capable of being opened by the
action of free radicals in order to polymerize, and which contains,
directly attached to this vinyl group, at least one fluorine atom,
a fluoroalkyl group or a fluoroalkoxy group in which:
[0016] (a) a flow of VF2, of optional comonomer and of radical
initiator is introduced into a reactor maintained at a pressure of
between 300 bar and 3000 bar, the reactor containing essentially
VF2, an optional comonomer and PVDF;
[0017] (b) a flow of reaction mixture is removed from the reactor
for step (a) and introduced into a separator;
[0018] (c) molten PVDF is recovered in the separator and purged
continuously;
[0019] (c1) the flow of PVDF from step (c) is optionally introduced
into a device to place it in the form of granules;
[0020] (d) VF2 and optionally comonomer are recovered in the
separator and recycled into step (a).
[0021] According to one advantageous form of the invention, the
fresh VF2, the other portion being recycled, which is introduced
into step (a) contains less than 5 ppm of oxygen, preferably less
than 1 ppm and better still between 0.1 and 0.8 ppm.
[0022] The process of the invention advantageously comprises an
additional step consisting in deoxygenating the fresh VF2 before
introducing it into step (a).
[0023] This step consists in placing the flow of VF2 in contact
with (i) a sufficient amount of a catalyst whose active sites are
elements belonging to groups 8 to 11 of the Periodic Table of the
Elements and (ii) for a time which is sufficient to obtain the
desired oxygen content.
[0024] The invention also relates to this isolated step of
treatment of VF2 to reduce its oxygen content. This step
advantageously precedes a process which is different from the
preceding one in which the VF2 is polymerized or copolymerized.
[0025] This deoxygenation may also apply to mixtures of VF2 and of
one or more comonomers and also to the VF2 comonomers alone.
[0026] The invention also relates to, as a product, VF2, the
mixture of VF2 and of comonomer or the comonomer alone (or mixture
of comonomers) containing less than 5 ppm of oxygen, advantageously
less than 1 ppm and better still between 0.1 and 0.8 ppm.
[0027] The present invention also relates to a PVDF with an Mw/Mn
ratio of between 1.5 and 1.9.
[0028] The process of the invention has many advantages:
[0029] there is no water or organic solvent: it is a clean process,
not requiring the recycling/treatment of water or the recycling of
solvent;
[0030] it is a <<dry>> process not requiring the
removal of water or solvent from the polymer: it is less expensive
in energy terms;
[0031] the polymer is cleaner since it is not soiled by the
presence of surfactants or other additives;
[0032] there is no use of perfluoro surfactants which may
bioaccumulate in the environment: the process is environmentally
friendly;
[0033] there is no handling of powder as in the ScCO.sub.2 process:
this entails a simplification of the process, with no cyclones to
be added, no step of uptake of the powder to convert it into
granules, no problem of powder explosion and no problem of
electrostatics;
[0034] the production efficiencies are better than in the
ScCO.sub.2 process (as demonstrated in Example 1).
[0035] As regards the proportions of VF2 and of comonomer in the
PVDF and thus the proportions of fresh VF2 and of fresh comonomer
introduced into step (a), the proportion by weight of comonomer is
advantageously between 0 and 50% and preferably between 0 and
30%.
[0036] Examples of comonomers which may be mentioned are vinyl
fluoride; trifluoroethylene; chlorotrifluoroethylene (CTFE);
1,2-difluoroethylene; tetrafluoroethylene (TFE);
hexafluoropropylene (HFP); perfluoro(alkyl vinyl) ethers such as
perfluoro(methyl vinyl) ether (PMVE), perfluoro(ethyl vinyl) ether
(PEVE) and perfluoro(propyl vinyl) ether (PPVE);
perfluoro(1,3-dioxole); perfluoro(2,2-dimethyl-1,3-dioxole) (PDD);
the product of formula CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)OCF.sub-
.2CF.sub.2X in which X is SO.sub.2F, CO.sub.2H, CH.sub.2OH,
CH.sub.2OCN or CH.sub.2OPO.sub.3H; the product of formula
CF.sub.2.dbd.CFOCF.sub.2CF.sub- .2SO.sub.2F; the product of formula
F(CF.sub.2).sub.nCH.sub.2OCF.dbd.CF.su- b.2 in which n is 1, 2, 3,
4 or 5; the product of formula R.sub.1CH.sub.2OCF.dbd.CF.sub.2 in
which R.sub.1 is hydrogen or F(CF.sub.2).sub.z and z is 1, 2, 3 or
4; the product of formula R.sub.3OCF.dbd.CH.sub.2 in which R.sub.3
is F(CF.sub.2).sub.z-- and z is 1, 2, 3 or 4;
perfluorobutylethylene (PFBE); 3,3,3-trifluoropropene and
2-trifluoromethyl-3,3,3-trifluoro-1-propene. Several comonomers may
be used.
[0037] The reactor contains at least 90% by weight of VF2, one (or
more) optional comonomer and PVDF. There is no organic solvent,
water or ScCO.sub.2.
[0038] The reactor pressure is advantageously between 500 and 3000
bar and preferably between 1500 and 2500 bar. The reactor has a
volume such that the residence time is advantageously between 1 min
and 1 h.
[0039] As regards the radical initiator, the product is known per
se. Suitable radical initiators which may be used comprise
tert-butyl perpivalate, t-butyl hydroperoxide, cumene
hydroperoxide, diisopropylbenzene hydroperoxide, di-t-butyl
peroxide, t-butylcumyl peroxide, dicumyl peroxide,
1,3-bis(t-butylperoxy-isopropyl)benzene, acetyl peroxide, benzoyl
peroxide, isobutyryl peroxide, bis(3,5,5-trimethyl)hexanoyl
peroxide and methyl ethyl ketone peroxide. The initiator may be
dissolved in a solvent; examples which may be mentioned are heptane
and isododecane.
[0040] The flow rate of initiator is advantageously between 2 ppm
and 1000 ppm by weight relative to the flow rate of fresh VF2 or of
the fresh VF2 and fresh comonomer together, and preferably between
2 ppm and 100 ppm.
[0041] The flow rate of VF2 and of optional comonomer is adjusted
to compensate for the production of PVDF, and the temperature is
adjusted by the reactor-cooling system. The flow rate of initiator
is adjusted to obtain a conversion of the monomers into PVDF. The
temperature is advantageously maintained between 50.degree. C. and
300.degree. C. and preferably between 90.degree. C. and 220.degree.
C. The advantage of working at a temperature above 150.degree. C.
to obtain low crystallinity is explained later.
[0042] The apparatus used can be the same as that in which the
synthesis of low-density polyethylene (LDPE) is carried out. This
apparatus is known.
[0043] The autoclave and tubular processes both form part of the
so-called "high-pressure" polymerization processes and a person
skilled in the art immediately knows what it involves. These two
processes involve the high-pressure radical-mediated polymerization
of ethylene, at pressures generally of between 100 MPa and 350 MPa
and at temperatures above the melting point of the polyethylene
being formed. The tubular process involves the polymerization in a
tubular reactor. A tubular reactor comprises cylinders whose inside
diameter is generally between 1 cm and 10 cm and whose length is
generally from 0.1 km to 3 km. In a tubular reactor, the reaction
medium is driven at high linear speed, generally of greater than 2
metres per second and short reaction times, which may be, for
example, between 0.1 min and 5 min.
[0044] The pressure in a tubular reactor may be, for example,
between 200 MPa and 350 MPa.
[0045] The autoclave process involves polymerization in an
autoclave whose length/diameter ratio generally ranges from 1 to 25
in the case of a single-zone reactor. In the case of a
multiple-zone reactor, the ratio of the length of each zone to the
diameter generally ranges from 0.5 to 6, it being understood that
the reaction medium flows in the direction of the length. The
pressure in an autoclave reactor may be, for example, between 100
MPa and 250 MPa.
[0046] It would not constitute a departure from the context of the
invention to add to the reactor or at the outlet a product for
promoting flow. By way of example, this product may be
supercritical CO.sub.2, but this has nothing to do with a
polymerization in ScCO.sub.2. Specifically, the amount added is
from about 0 to 10% by weight relative to the mass of fluoro
monomers and of PVDF contained in the reactor.
[0047] As regards the oxygen content of the VF2, it is generally
known that the presence of oxygen may have an appreciable influence
on radical-mediated polymerizations, whether they are carried out
in emulsion, suspension or bulk, at low pressure (P<500 bar) or
at high pressure (P>500 bar). Specifically, the oxygen
participates in the radical-mediated processes and can act either
as initiator or as polymerization inhibitor. Reference may be made
to Chemical Reviews 1991, 91 (2), 99-117 for further details
regarding the exact mechanisms involving the O.sub.2 species.
However, it was not known that VF2 contained amounts of oxygen
which could impair its polymerization, whichever process is
used.
[0048] For example, when VF.sub.2 is manufactured by a process of
cracking 1-chloro-1,1-difluoroethane:
CH.sub.3--CF.sub.2Cl.fwdarw.HCl+VF.sub.2
[0049] This process does not make it possible to obtain VF.sub.2
containing less than 5 ppm of residual oxygen. Generally, the
VF.sub.2 obtained from this process contains between 10 ppm and 15
ppm of oxygen, which is much too high for polymerization, in
particular at high pressure. It is thus desirable to have available
a simple and economical single operation for removing the residual
oxygen in the VF.sub.2 and possibly in the comonomers.
[0050] The catalyst used is a solid catalyst composed of active
sites dispersed on a mineral support, optionally containing
promoters whose role is to increase the chemical activity of the
catalyst. The active sites are elements belonging to groups 8-11 of
the Periodic Table.
[0051] The solid catalyst is in the form of granules of varied
shapes (cylinders, flakes, beads, etc.). The support for the active
sites is preferably mineral. It may be alumina, silica, zeolite or
aluminosilicate or any other support known to those skilled in the
art of heterogeneous catalysis. The catalyst contains between 0.05%
and 5% by weight of the active element.
[0052] In order to ensure optimum catalytic activity, the support
is preferably porous. The catalyst has a pore volume of between
0.001 ml/g and 1000 ml/g, preferably between 0.01 ml/g and 100
ml/g. The specific surface area of the catalyst makes it possible
to assess the catalyst's capacity to expose its active sites to the
flow of reagents. The specific surface area is expressed in
m.sup.2/g; the active surface area is between 1 m.sup.2/g and
10,000 m.sup.2/g, preferably between 1 m.sup.2/g and 1000
m.sup.2/g.
[0053] A magnitude, known as the space velocity, may be defined,
which relates the flow rate of gas to the amount of solid catalyst.
This magnitude may be expressed in Nm.sup.3/m.sup.3/h or in kg/kg
catalyst/h, and is between 0.01 kg/kg/h and 1000 kg/kg/h,
preferably between 0.1 kg/kg/h and 100 kg/kg/h.
[0054] The process is useful for reducing the oxygen content from
100 ppm to less than 5 ppm and advantageously to less than 1 ppm
and more particularly from 30 ppm down to 0.2 ppm or 0.8 ppm.
[0055] The process may be performed at any pressure, which has no
effect on such small contents. The advantage of a high pressure is
that the apparatus for placing the VF2 in contact with the catalyst
is more compact. The pressure at which the VF2 to be deoxygenated
is available is usually used.
[0056] As regards the temperature, it may be between 0.degree. C.
and 200.degree. C. However, it is advantageously between 50.degree.
C. and 100.degree. C.
[0057] Preferentially, the element is copper. When the element is
copper, the removal of oxygen is based on the following chemical
equations:
2Cu+O.sub.2.fwdarw.2 CuO
4Cu+2O.sub.2.fwdarw.Cu.sub.2O
[0058] The removal of oxygen is carried out by simply passing the
monomer(s) through a cartridge. The catalyst contained in the
cartridge is preferably a solid catalyst containing the elements
copper or palladium. On the industrial scale, given the larger flow
rates by volume of monomer which it is necessary to treat, the
cartridge is replaced with apparatus of larger volume. This may be,
for example, a column or an assembly of two or more columns
functioning in series or in parallel. If the columns function in
parallel, a step of catalyst regeneration may take place while
another column carries out the deoxygenation of the monomer.
[0059] Another solution for deoxygenating the monomer consists in
combining with the VF.sub.2 container a deoxygenating cartridge
directly linked to the container. The deoxygenation thus takes
place in semi-continuous mode each time the monomer is removed from
the container.
[0060] This type of catalyst is already known and is used to treat
the neutral gases (argon, nitrogen) of laboratory glove boxes. This
is necessary, for example, for the handling of organometallic
compounds which are oxygen-sensitive (for example organometallic
aluminum derivatives). These catalysts are also used to remove the
traces of oxygen from gaseous monomers containing only carbon and
hydrogen, for example ethylene or propylene. Specifically, in order
to polymerize ethylene efficiently using catalysts of Ziegler-Natta
type, it is necessary to have available monomers containing minute
traces of oxygen.
[0061] The catalyst of the type R3-15 T5x3 sold by BASF.RTM. may be
mentioned for example.
[0062] This deoxygenation is useful in the high-pressure PVDF
preparation process described above, but also in the other PVDF
preparation processes.
[0063] Case of the high-pressure polymerization according to the
main process of the present invention:
[0064] The bulk polymerization of 1,1-difluoroethylene to give PVDF
is a polymerization carried out at high pressure, and requires the
removal of all traces of oxygen from the fluoro monomer in order to
avoid an untimely polymerization in the reactor or even in the
pumps during the compression phase. Specifically, the oxygen may
act as radical initiator in the same way as the organic radical
initiators which are intentionally added to the polymerization
medium. The presence of oxygen thus impairs the working of the
process and also its safety.
[0065] For example, when the bulk polymerization of VF.sub.2 is
carried out at 1850 bar, it is necessary to inject the equivalent
of 7 ppm of pure tert-butyl perpivalate (sold under the reference
code LUP 11 by Atofina). However, if the monomer contains 10 ppm of
residual oxygen, this is equivalent to an amount of LUP 11 of 54
ppm, i.e. much more than is required to carry out the
polymerization. If the oxygen content is lowered to 1 ppm of
residual oxygen, the equivalent amount of LUP 11 is 5.4 ppm.
[0066] Case of the Emulsion Polymerization
[0067] In the case of the emulsion polymerization of VF.sub.2, an
excessive amount of oxygen may retard the polymerization or even
prevent it from taking place. Tests have shown that at and above an
oxygen content of 20 ppm, the polymerization is totally
inhibited.
[0068] When the polymerization of VF.sub.2 takes place in emulsion
with initiation with ammonium persulphate, chain ends are
terminated with fragments of the initiator. These ends are fragile
and on heating give sulphuric acid which degrades the polymer.
Similarly, the use of percarbonates as initiators leads to the
formation of aldehydes as decomposition by-products. These
aldehydes may give rise to colorations of the polymer when this
polymer is extruded or converted at high temperature. Consequently,
PVDF prepared in emulsion is proportionately more stable the lower
the amount of initiator required. There is thus a need to reduce
the residual oxygen content in order to use as little initiator as
possible.
[0069] For example, when the emulsion polymerization of VF2 is
carried out, 1 g of potassium persulphate per 10 kg of PVDF is
frequently used, which is equivalent to 7.7.times.10.sup.-7 mol of
radical anions per gram of PVDF. Now, if the monomer contains 10
ppm of residual oxygen, this is equivalent to a theoretical amount
of 4.3.times.10.sup.-7 mol of O.degree. radicals per gram of PVDF
(for a final solids content of about 40%). Without prejudging the
inhibition mechanism, it is found that the concentrations are of
the same order of magnitude. The potential advantage of having a
VF2 containing an amount of residual oxygen of less than 1 ppm,
especially for the VF2 added during the polymerization and which
does not undergo the initial degassing procedure, may thus be
appreciated.
[0070] As regards PVDF of high melt flow index, another aspect of
the invention relates to the possibility of obtaining PVDFs (or
corresponding copolymers) of high melt flow index (MFI).
Specifically, it is known that there is a limit to the production
of PVDF of high melt flow index by emulsion or suspension
processes.
[0071] The process disclosed in the present invention is
particularly flexible as regards the production of these products
of high melt flow index. The expression "melt flow index" means the
mass of resin flowing through a die in a given time, at a given
temperature and under a given weight. It is thus a fully
standardized measurement. In the case of PVDF or its copolymers,
the following standard is applied: the measurement is carried out
at 230.degree. C., under a 5 kg load, through a die 2.09 mm in
diameter. PVDFs with an MFI value of greater than 50, 100, 200 or
even 400 g/10 min (at 230.degree. C./5 kg according to ASTM D-1238)
are prepared.
[0072] The molecular masses are controlled by injecting transfer
agents into the reactor. The same transfer agents as for the
production of polyethylene may be used. Highly efficient transfer
agents exist, which are well known in the processes for producing
low-density polyethylene. They are molecules containing labile
hydrogen atoms. Mention may be made, for example, of alcohols,
aldehydes, in particular propanal or butanal, or even alkanes or
alkenes, for example butane, propylene, heptane or isododecane.
[0073] These PVDFs of high melt flow index are useful for making
coatings. The present invention also relates to, as products, these
PVDFs having these MFIs.
[0074] As regards PVDF of low crystallinity, the emulsion or
suspension processes are carried out in the presence of water,
which necessarily limits the polymerization limit temperature. This
is generally between 80.degree. C. and 100.degree. C. It is an
advantage to be able to work at high pressure, without water, since
it is easy to polymerize VF2 at temperatures above 150.degree. C.
At these temperatures, the number of defects present in the polymer
chains increases, which has an effect on the crystallinity of the
polymer and thus also on its melting point. The larger the number
of defects, the more the crystallinity decreases and the more the
melting point decreases.
[0075] The expression "inversion defect" means any combination of
the type --CH.sub.2--CF.sub.2--CF.sub.2--CH.sub.2-- along the
polymer chain (this also being known as the head-head combination
as opposed to the head-tail combination of the type
--CH.sub.2--CF.sub.2--CH.sub.2--CF.sub.2--). The number of defects
may be measured by the fluorine NMR. The number of inversion
defects is thus generally given as a percentage.
[0076] The level of inversion defects may be between 5% and 15% and
for example greater than 6 or 7%. The present invention also
concerns, as products, these PVDF homopolymers having these levels
of inversion defects.
[0077] Another way of measuring the crystallinity of the PVDF is to
measure the melting point. It is thus an advantage to be able
easily to obtain PVDFs with a melting point of less than
162.degree. C. and advantageously between 162 and 135.degree.
C.
[0078] This temperature may be for example 155.degree. C., indeed
less than 150 or 145.degree. C. The present invention also
concerns, as products, these PVDFs with these melting points.
[0079] Another way of measuring the crystallinity of the PVDF is to
measure the elastic modulus. In an entirely advantageous manner, it
has been noted that the PVDFs prepared under high pressure had low
moduli, giving rise to the possibility of using these products in
Kynar Flex.RTM. applications (that is to say VF2-HFP copolymers).
The elastic modulus at 23.degree. C. according to ASTM D-1708 may
be of between 1020 and 650 MPa. The modulus may be for example less
than 1000, 900, 800 or even 700 MPa. The present invention also
concerns, as products, these PVDF homopolymers with these elastic
moduli.
[0080] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilize the
present invention to its fullest extent. The following preferred
specific embodiments are, therefore, to be construed as merely
illustrative, and not limitative of the remainder of the disclosure
in any way whatsoever.
[0081] In the foregoing and in the following examples, all
temperatures are set forth uncorrected in degrees Celsius; and,
unless otherwise indicated, all parts and percentages are by
weight.
[0082] The entire disclosure of all cited applications, patents and
publications, and of corresponding French application 00/12191,
filed Sep. 26, 2000, is hereby incorporated by reference.
EXAMPLES
[0083] High-pressure polymerization examples.
Example 1
[0084] The high-pressure polymerization is carried out in a 100
cm.sup.3 steel single-zone autoclave reactor equipped with a
collecting separator and a stirrer, and thermostatically maintained
at 180.degree. C. The reactor is fed continuously with
1,1-difluoroethylene with the aid of two pumps connected in series.
The monomer first passes through a cartridge containing a copper
catalyst in order to remove all trace of residual oxygen. The 1st
pump compresses the monomer from 50 bar to 300 bar and feeds the
second pump which brings the pressure from 300 bar to 1850 bar. The
flow rate of the monomer is 4400 g/h. Its temperature at the
reactor inlet is 74.degree. C. Its oxygen content, measured using
an oxymeter, is 0.6 ppm.
[0085] A solution of tert-butyl perpivalate (sold as a solution in
isododecane by Atofina under the brand name LUP 11-M-75) diluted in
heptane is also introduced into the reactor at a flow rate of 41
cm.sup.3/h. The flow rate of pure LUP 11 is then, after
calculation, 1.5.times.1 0-2 g/h.
[0086] On decomposing, the initiator initiates the polymerization
of the 1,1-difluoroethylene, which heats up the reaction mixture.
The temperature in the reaction mixture is then 199.degree. C. The
polymer is subsequently recovered by decompression in expansion
vessels. The mass of PVDF recovered makes it possible to determine
the polymerization yield. The melt index is measured using a melt
indexer at 230.degree. C. under a 5 kg load according to ASTM
D-1238.
[0087] conversion: 9.4%
[0088] MFI [230.degree. C.; 5 kg]=6 g/10 min
[0089] The improved production efficiency of the high-pressure
process compared with the process in supercritical CO.sub.2 may be
appreciated by calculating the polymerization rate R.sub.p: 1 R p =
m VF 2 .times. conversion V
[0090] with
[0091] {dot over (m)}.sub.VF.sub..sub.2: flow rate by mass of
VF.sub.2 in g/s
[0092] V: volume of the reactor (litres)
[0093] For our example:
[0094] V=0.1 litre
[0095] {dot over (m)}.sub.VF.sub..sub.2=4400 g/h=1.22 g/s
[0096] conversion=9.4%
[0097] R.sub.p=1.15 g/Ls
[0098] In the case of the polymerization in ScCO.sub.2, the
polymerization rate may be calculated in an identical manner on the
basis of the examples given in patent WO 98/28351. For example,
this gives:
[0099] V=0.8 litre
[0100] {dot over (m)}.sub.VF.sub..sub.2=1.9 g/min=0.032 g/s
[0101] conversion=24%
[0102] R.sub.p=0.0096 g/L s
[0103] The process disclosed in this patent is thus 120 times as
fast as the process disclosed in the patent mentioned above.
Example 2
[0104] Example 1 is repeated under the following conditions:
[0105] pressure=950 bar
[0106] flow rate of 1,1-difluoroethylene=5.5 kg/h
[0107] flow rate of LUP 11=41.times.10.sup.-2 g/h
[0108] temperature of the reaction mixture=210.degree. C.
[0109] conversion=14%
[0110] MFI [230.degree. C.; 5 kg]=200 g/10 min
[0111] melting point=161.3.degree. C.
Example 3
[0112] Example 1 is repeated under the following conditions:
[0113] pressure=1450 bar
[0114] flow rate of 1,1-difluoroethylene=5.5 kg/h
[0115] flow rate of LUP 11=7.4.times.10.sup.-2 g/h
[0116] temperature of the reaction mixture=195.degree. C.
[0117] conversion=10.8%
[0118] MFI [230.degree. C.; 5 kg]=7 g/10 min
[0119] melting point=158.4.degree. C.
[0120] The molecular masses are determined by steric exclusion
chromatography in solution in DMF.
[0121] Mn=171,000 g/mol
[0122] Mw=290,000 g/mol
[0123] Mw/Mn=1.7
[0124] It will be noted that the polydispersity (1.7) of the
polymer produced is lower than that which is generally encountered
in the PVDFs commercially available (2-3 instead) and which are
produced by emulsion/suspension. The product was evaluated at
23.degree. C. according to ASTM standard D-1708 and compared with
the Kynar.RTM. 740 grade:
1 TABLE 1 Example 3 Kynar 740 threshold 43 54 stress (MPa)
threshold 9.0 8.3 elongation (%) tensile stress 52 47 (MPa) elastic
932 1160 modulus (MPa)
[0125] It is found that, despite a smaller modulus, the product of
Example 3 has mechanical properties (in particular threshold
stress) that are comparable with Kynar 740. Kynar.RTM. 740 is a
PVDF homopolymer.
Example 4
[0126] Example 1 is repeated with the following conditions:
[0127] pressure=1850 bar
[0128] flow rate of 1,1-difluoroethylene=4.3 kg/h
[0129] temperature of the reaction mixture=189.degree. C.
[0130] conversion=9.6%
[0131] MFI [230.degree. C.; 5 kg]=8 g/10 min
[0132] melting point=158.degree. C.
[0133] Mn=152,000 g/mol
[0134] Mw=320,000 g/mol
[0135] Mw/Mn=2.1
[0136] level of defects (measured by .sup.19F NMR)=6.6%
[0137] threshold stress=46 MPa
[0138] threshold elongation=8.3%
[0139] tensile stress=45 MPa
[0140] modulus=1000 MPa
Examples of Deoxygenation
[0141] The 1,1-difluoroethylene used feeds a high-pressure
polymerization reactor. The monomer is pumped using two
high-pressure pumps in series which compress the monomer in two
steps: pump 1 from 40 bar to 300 bar, pump 2 from 300 bar to 1900
bar.
[0142] The monomer first passes through a cylindrical steel
cartridge (dimensions: length 630 cm, diameter 168 cm) containing a
copper-based catalyst sold by BASF under the brand name R 3-11
(mass of catalyst: 2000 g). The metal cartridge is surrounded by an
electrical system for heating the catalyst. The residual oxygen
content at the cartridge inlet and outlet is measured using an
EC180 oxymeter from Hermann Moritz, to assess the efficacy of the
catalyst.
Example 5
[0143] flow rate of 1,1-difluoroethylene: 4500 g/h, i.e. 2.25
kg/h/kg of catalyst
[0144] temperature of the cartridge: 22.degree. C.
[0145] pressure in the cartridge: 40 bar
[0146] inlet O.sub.2 content: 16 ppm
[0147] outlet O.sub.2 content: 15 ppm
Example 6
[0148] flow rate of 1,1-difluoroethylene: 4500 g/h, i.e. 2.25
kg/h/kg of catalyst
[0149] temperature of the cartridge: 50.degree. C.
[0150] pressure in the cartridge: 40 bar
[0151] inlet O.sub.2 content: 16 ppm
[0152] outlet O.sub.2 content: 0.6 ppm
[0153] These two examples show that the deoxygenation of
1,1-difluoroethylene can be carried out efficiently and that the
efficacy of the catalyst is improved by heating the cartridge.
[0154] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0155] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention
and, without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *