WO2014140457A1

WO2014140457A1 - Process for free-radical grafting of unsaturated monomers onto fluoropolymers

Info

Publication number: WO2014140457A1
Application number: PCT/FR2014/050509
Authority: WO
Inventors: Anthony Bonnet; Jean-Jacques Flat; RAMFEL (Epouse WIEGERT), Barbara; Sébastien QUINEBECHE; Livie LIENAFA; MONGE (Epouse DARCOS), Sophie; Bruno Ameduri; Jean-Jacques Robin
Original assignee: Arkema France; Universite Montpellier Ii, Sciences Et Techniques
Priority date: 2013-03-12
Filing date: 2014-03-06
Publication date: 2014-09-18
Also published as: FR3003258A1; FR3003258B1

Abstract

The present invention relates to a process for free-radical grafting of one or more unsaturated monomers, at least one being functional, onto fluoropolymers. The process according to the invention consists of a first step during which a nitroxide, used alone or with a peroxide, acts as a free-radical scavenger, followed by a step of grafting said unsaturated monomers. This novel route for modifying fluoropolymers using chemistry which is less aggressive and less destructive makes it possible to obtain fluoropolymers which are more reactive, in a reproducible manner, on an industrial scale and under more favourable economic conditions.

Description

METHOD FOR RADICAL GRAFTING OF UNSATURATED MONOMERS ON FLUORINATED POLYMERS

Field of the invention

The present invention relates to a radical grafting process of one or more unsaturated monomers, at least one being functional, on previously unirradiated fluoropolymers. The invention also relates to the grafted fluoropolymers thus obtained and their various applications. Technical background

The chemical modification of the fluoropolymers makes it possible to improve their properties, in particular their adhesion and their compatibility with other polymers and / or with non-polymeric materials. Indeed, fluorinated polymers, for example those based on vinylidene fluoride CF ₂ = CH ₂ (VDF) such as PVDF (polyvinylidene fluoride) are known to offer excellent properties of mechanical stability, a very high chemical inertness as well as good resistance to aging.

However, this chemical inertness of fluoropolymers makes it difficult to paint, bond or associate them with other materials, except for certain alkyl poly (meth) acrylates such as poly (methyl methacrylate) (PMMA). ), thus limiting the possibilities of producing multilayer structures by coextrusion.

To overcome this drawback, various methods have been proposed in which a reactive function is attached to a fluoropolymer. This fluorinated polymer thus modified can be more easily bonded or combined with other materials.

A first type of process consists in grafting an unsaturated monomer onto a fluorinated polymer previously irradiated with a beam of electrons, ions, X-rays, gamma rays, etc. These beams are very energetic and cut indifferently the C-C, C-H or C-F bonds, which leads to the partial destruction of the sample with appearance of hydroxide functions, peroxides and / or hydroperoxides on the surface of the polymer.

The grafting can be carried out in three stages by controlled radical polymerization, in particular after functionalization of the hydroxide groups (ATRP or NMP) or by polymerization initiated by heat or radiation. The Applicant had already proposed in document EP 1 484 346 to mix in the molten state a fluoropolymer and a functional monomer which is to be grafted onto said fluoropolymer, then irradiating the mixture obtained in divided form such as granules, to obtain the grafting of the functional monomer on the fluoropolymer. This radiation grafting process uses energy-intensive techniques and requires very specific installations.

The publication of Holmberg S. et al. in Macromolecules, 37 (26), 2004, pages

No. 9909-9915 discloses fluorinated graft polymers obtained by a process combining a prior step of irradiating the polymer with an electron beam, with a styrene radical polymerization step or copolymerization of styrene and maleimides controlled by a nitroxide.

WO 2008/090257 discloses field effect transistors comprising a polymer membrane having a region having a proton conductivity, said membrane being manufactured by a process combining the grafting of styrene on pre-irradiated PVDF whose free macro -radicals are blocked by nitroxides, with a sulfonation step leading to graft copolymers PVDF-g-poly (styrene sulfonic acid) s.

Another type of grafting process consists of treating the fluoropolymer with a base, such as sodium hydroxide. PVDF is a stable polymer that is resistant to acids but not to bases. In the presence of sufficiently strong bases, hydrogen atoms are torn off creating on the surface of PVDF unsaturations and functions epoxide and ketone which are reduced in hydroxyl function. These functions are then used to immobilize ATRP or RAFT type initiators on the polymer to enable controlled radical polymerization to be initiated. The functionalisation with sodium hydroxide has the disadvantage of passing through a dehydrofluorination step which generates hydrofluoric acid (dangerous) and which is difficult to control.

A third grafting method uses ozone to introduce peroxide functions on the surface of the fluoropolymer. Ozone treatment is often followed by exposure to air that allows the formation of hydroperoxide functions. This process however remains difficult to implement on an industrial scale. In addition, the exposure times are generally long; the reaction is difficult to control and few sites are created.

Another type of grafting method uses a plasma treatment of the fluoropolymer. With this mode of activation, all types of covalent bonds CH, CF and CC are cut indifferently in the chain of the fluoropolymer, creating radicals. Peroxide and hydroperoxide type functions are thus formed at the polymer surface. The grafting can then be carried out by conventional or controlled radical polymerization (ATRP, NMP, RAFT, etc.) or by thermally or radiation-initiated polymerization. This technique is very aggressive towards the fluoropolymer and requires a number of important steps (activation, functionalization and finally polymerization). In addition, the plasma treatment is difficult to perform on powders and is generally applied to parts prepared by plasturgical processes.

There is therefore a real need to develop an alternative fluoropolymer grafting method, which overcomes the aforementioned drawbacks so as to obtain more reactive fluoropolymers reproducibly, on an industrial scale and in more economical conditions. favorable.

The present invention proposes to develop a new way of modifying previously unirradiated fluorinated polymers by a simpler chemistry and process, less expensive in energy, less aggressive and less destructive than those described in the prior art.

It has now been found that, for the grafting of fluorinated polymers, in particular PVDF, the use of nitroxides makes it possible to tear off atoms (proton or possibly fluorine), thus leading, subsequently, to grafting reactions with a control of the length of the grafts.

Summary of the invention

The subject of the present invention is a process for grafting one or more unsaturated monomers, at least one being functional, on a non-irradiated fluoropolymer, said process consisting of a first initiation step with a nitroxide-type radical species or derivative (nitroxide, alkoxyamine, aminyls), used alone or in the presence of a peroxide type initiator, followed by a step of grafting said unsaturated monomers. Advantageously, the grafting process according to the invention takes place in solution or in the molten state.

The method which is the subject of the present invention has the advantage of not requiring specific and expensive equipment and of being reproducible, which makes it possible to apply it efficiently on an industrial scale. The grafted fluoropolymer structures obtained by the process of the invention have numerous applications, particularly in the field of filtration membranes (also including desalination membranes of seawater) but also for fuel cell membranes and electrolytes. polymers or binders used in lithium ion battery components. They are also used as additives to allow better miscibility, or improve the adhesion properties, emulsifiers, or the hydrophilicity of unmodified PVDF. Other characteristics and advantages will emerge from the detailed description of the grafting process according to the invention which follows.

Detailed description of the invention

The process according to the invention consists in the radical grafting of one or more unsaturated monomers, at least one being functional, on the chains of a fluorinated polymer such as PVDF, initiated by a nitroxide used alone or in combination with a peroxide. selected from benzoyl peroxide, tert-butyl peroxide, lauryl peroxide and hydroxycumyl peroxide.

The reactions take place in solution or in the molten state.

When the process is conducted in the molten state, it consists in introducing the polymer (in the form of granules or in another form), the unsaturated monomer or monomers, at least one being functional, the nitroxide optionally accompanied by a peroxide in a "reactor" known to those skilled in the art for conducting the process known as "reactive extrusion". By way of examples of such "reactors", mention may be made of co-rotating double-screw extruders, co-kneaders or internal mixers. The mixing is carried out at temperatures between 180 and 250 ° C while remaining below the degradation start temperature of the fluoropolymer.

When the process is carried out in solution, the solvent used is selected from N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethylacetamide (DMAc).

The unsaturated monomer or monomers are chosen from vinyl monomers, styrene monomers, (meth) acrylic monomers, (meth) acrylamides, N-vinylpyrrolidone, and other couples / co-monomer systems (styrene / maleic anhydride and others).

By "functional monomers" is meant any unsaturated monomer carrying a chemical function capable of conferring the expected application property on the fluoropolymer (adhesion, hydrophilicity, hydrophobicity, crosslinking, acidity for fuel cell membranes, conduction of lithium ions ...).

According to one embodiment, one of the unsaturated monomers carries a function of cyclic anhydride, epoxide, carboxylic acid, carbonate, oligo (oxyethylene) such as PEG, or trialkoxysilane type. This makes it possible to obtain functionalized grafted fluoropolymers which have adhesion properties and compatibility with other improved polymers with respect to the same grafted but non-functionalized polymers.

By way of examples of these functional monomers, mention may be made of:

unsaturated acids such as acrylic acid, methacrylic acid and their esters;

functional silanes such as vinyltrialkoxysilane, vinylmethyldialkoxysilane, vinyldimethylalkoxysilane;

unsaturated carboxylic acid anhydrides such as maleic anhydride or itaconic anhydride;

unsaturated monomers bearing epoxide, carbonate, hydroxyl, amine, sulphonate, sulphonamide or phosphate functions.

The nitroxide used as radical scavenger is capable, on the one hand, of initiating the grafting reaction after tearing off the hydrogen (or possibly fluorine) atoms of the fluoropolymer chain, and on the other hand, of controlling the length of the grafts during the grafting reaction. Advantageously, the nitroxide used in the process according to the invention is chosen from (2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide) (DPAIO) and its derivatives. Other hydrogen-abstracting nitroxides such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 2,2,5-tri-methyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO), N- (2-methylpropyl) -N- (1-diethylphosphono-2,2-dimethylpropyl) -N-oxyl (SGI), phthalimide-N-oxyl (PINO), alkoxyamines or aminyls or the like can be used.

Preliminary work showed that nitroxides (TEMPO and DPAIO) used alone were very poor proton removers in solvents other than NMP. On the other hand, in NMP (or DMF), it is possible to extract protons (or possibly fluorine) in the presence or absence of a peroxide or hydroperoxide. This result demonstrates the effect of the solvent on the nitroxide grafting: parallel tests in N-methyl pyrrolidone and DMF have shown that grafting is much more efficient in NMP. This then implies a participation of the solvent in the grafting mechanism. Moreover, the modification of the PVDFs is due to an intrinsic characteristic of the PVDF oligo-mothers used: method of preparation, molar mass but especially inversion rate. The oligomers were used as models so as to characterize the products obtained and to provide evidence of an effective grafting of polymers on PVDF. The results obtained can be extrapolated to any PVDF-type polymer, regardless of its molar mass. The preferred solvent during the peeling and grafting step is NMP but other solvents can be used. According to one embodiment, the nitroxide used is DPAIO which gave the best molar levels of grafting in the presence of NMP.

The nitroxide modified PVDF, PVDF-g-DPA10, was then used as a macro-initiator for the polymerization of one or more unsaturated monomers, at least one of them being functional. The NMR / DOSY (Diffusion Order Spectroscopy) analysis of the samples (after dialysis) made it possible to show that the DPAIO is grafted onto the PVDF and that, subsequently, the unsaturated monomer also grafts onto the oligomer. The following examples illustrate the invention without limiting it.

Example 1 Synthesis of Model Molecules by ITP

PVDF oligomers were prepared by radical transfer polymerization of iodine (ITP). dehalogenation

+

ΛΙΚΝ

The polymerization is carried out in acetonitrile at 74 ° C. in a Hastelloy reactor equipped with a manometer, introduction valves and a mechanical stirrer. The initiator used is tert-butylperoxypivalate (TBPPI) and the iodinated transfer agent is 1-iodoperfluorohexane C ₆ F ₁₁ (RFI). The molar monomer / transfer agent / initiator ratio is 100/1/0, 1. After the polymerization, two fractions are recovered: a first sulfur fraction and a second fraction insoluble in acetonitrile. The fractions are then precipitated in hexane and then characterized by 1 H NMR and ¹⁹ F. The crude product is then reduced by action of tributyltin hydride in order to eliminate end-iodine. The PVDF is then fractionated by successive precipitations: the fraction dissolved in acetonitrile is dissolved in acetone while the fraction insoluble in acetonitrile is dissolved. in cold DMF. Each fraction is characterized by 1 H NMR and ¹⁹ F. The NMR spectra show that the peak at 3.7 ppm has disappeared confirming the total elimination of end-iodine. The ¹⁹ F NMR spectrum also confirms this with the disappearance of the peaks at -109 and -39 ppm as well as an increase in that at -107 ppm characteristic of the CF ₂ -CH ₂ -H end of chain and the presence of a byte doublet centered at -114.8 ppm.

The results of the characterization by 1H and ¹⁹ F SEC are shown in Table 1:

steric exclusion chromatography (SEC) - THF detection RI, standards PS ^{b size} exclusion chromatography - DMF + 1% LiBr detection RI, standards

PMMA

Table 1. Results of the characterization of synthesized PVDFs.

EXAMPLE 2 Grafting of Nitroxide on PVDF Oligomers In a second step, these oligomers reacted with DPA10 alone or in the presence of a peroxide-type initiator made it possible to demonstrate the binding of DPAIO on the polymer chain. It is accompanied by a decrease in the rate of head-to-head sequences which could indicate that these two phenomena are linked. The modified PVDF is purified by cryo-distillation of the solvent and then by successive precipitations in hexane. The dried product, under a high vacuum, is characterized by 1 H NMR, ¹⁹ F and steric exclusion chromatography (CES) RI and UV detection according to the nature of the nitroxide used. In some tests, benzoyl peroxide has also been used as a proton scavenger (ti / ₂ = lh at 90 ° C).

The data relating to the grafting of DPA10 onto the PVDF oligomers prepared as described in Example 1, in the absence and in the presence of benzoyl peroxide, are shown in Table 2.

Table 2. Characteristics of modified PVDF in the presence of nitroxide type

DPAIO. All these modified PVDF oligomers were then used as macroamines during the polymerization of an unsaturated monomer.

Example 3 Polymerization of Styrene from Nitroxide-grafted PVDF Only one example in the literature describes the grafting of polystyrene onto

PVDF but no evidence is given as to the effective grafting of nitroxide and PS graft onto PVDF (Macromolecules 2004, 37, (26), 9909-15). Moreover, in this particular case, the PVDF is irradiated beforehand (expensive and aggressive technique), which causes the partial destruction of the polymer.

In the present invention, polystyrene (PS) chains have actually been grafted onto the nitroxide-modified PVDF oligomers. The results of these tests are summarized in Table 3. For each test, a "blank" was made (ie a thermal homopolymerization of styrene) in order to differentiate in the samples the thermally formed PS homopolymer from graft copolymer PVDF-g-PS. eq of

styrene

Macro-initiator molar%

Name of Nature of by

Input PVDF-g- Purification of styrene macroinitiator sample report

grafted nitroxide / VDF on the ground

VDF

9a LL 303 LL 300 PVDF-g-DPAIO 1 dialysis 11

9b LL 309 LL 300 PVDF-g-DPAIO 5 Dialysis 25

10a LL 304 LL 301 PVDF-g-DPAIO 1 Dialysis 20

10b LL 310 LL 301 PVDF-g-DPAIO 5 Dialysis 50

11 LL 305 LL 302 PVDF-g-DPAIO 1 precipitation -

12 LL 315 LL 313 PVDF-g-DPAIO 6 precipitation -

13 LL 326 LL 314 PVDF-g-DPAIO Precipitation -

16 LL 345 LL 335 PVDF-g-DPAIO 5 Dialysis 11

Table 3. Characteristics of PVDF-g-PS graft copolymers

The styrene polymerization products from these macroamines were first purified by precipitation in hexane, to remove ungrafted DPA10 and traces of solvent, then by dialysis in THF with a regenerated cellulose membrane ( MWCO 25000). The degree of grafting increased from 11 to 25 mol% / VDF when the amount of starting monomer is multiplied by 5 (according to entries 9a and 9b of Table 3). This indicates that the homopolymerization of styrene remains the majority. The dialyzed products were also characterized by NMR / DOSY and thermogravimetric analysis (TGA) which confirmed that the PS is well grafted to the PVDF and that the dialyzed product is pure.

Claims

A method of grafting one or more unsaturated monomers, at least one being functional, onto a non-irradiated fluoropolymer, said process consisting of: a step during which a nitroxide used alone or with a peroxide acts as a radical scavenger, followed by a step of grafting said unsaturated monomers.

2. Process according to claim 1, in which the reactions take place in solution, the solvent used being chosen from N-methyl-2-pyrrolidone, dimethylformamide, dimethylsulfoxide or dimethylacetamide.

3. The process according to claim 1, wherein the reactions take place in the molten state.

4. Method according to one of claims 1 to 3 wherein said fluoropolymer is poly (vinylidene fluoride).

5. Method according to one of claims 1 to 4 wherein said unsaturated monomer is selected from vinyl monomers, styrenic, (meth) acrylic monomers, (meth) acrylamides and N-vinylpyrrolidone.

6. Process according to one of claims 1 to 5 wherein the nitroxide is selected from 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide, the 2,2,6,6- tetramethylpiperidine-1-oxyl, 2,2,5-tri-methyl-4-phenyl-3-azahexane-3-nitroxide, N- (2-methylpropyl) -N- (1-diethylphosphono-2,2-dimethylpropyl) ) -N-oxyl, phthalimide-N-oxyl, alkoxyamines and aminyls.

7. Method according to one of claims 1 to 6 wherein the peroxide is selected from benzoyl peroxide, tert-butyl peroxide, lauryl peroxide and hydroxycumyl peroxide.

8. Method according to one of claims 1 to 7 wherein the initiation step takes place in the presence of 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide and benzoyl peroxide. .

9. Process according to one of claims 1 to 8 wherein said functional unsaturated monomer carries a cyclic anhydride, carboxylic acid, epoxide, trialkoxysilane, hydroxyl, carbonate or oligo (oxyethylene) type function.

10. grafted fluorinated polymer obtainable by the method according to one of claims 1 to 9.

1. Use of a grafted fluoropolymer obtained according to one of claims 1 to 9 in the field of membranes filtration or desalinization of seawater.

12. Use of a grafted fluoropolymer obtained according to one of claims 1 to 9 in the field of fuel cell membranes or as a binder for lithium ion batteries.

13. Use of a grafted fluoropolymer obtained according to one of claims 1 to 9 as an additive for PVDF.