GB2448159A

GB2448159A - Co-monomer grafted polymer

Info

Publication number: GB2448159A
Application number: GB0706563A
Authority: GB
Inventors: Susan Christine Waring; Jacqueline Anne Horsfall; Keith Victor Lovell
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-04-02
Filing date: 2007-04-02
Publication date: 2008-10-08
Also published as: GB0706563D0

Abstract

A co-monomer grafted polymer is prepared by the steps of activating a polymer, such as polyolefins, fluorinated and per-fluorinated polymers, copolymers and terpolymers, to produce free radicals, by radiation provided by gamma rays, X rays, ultraviolet radiation, plasma irradiation or beta radiation and contacting the activated polymer with an emulsion that includes mixed monomers, selected from styrene, trifluorostyrene, alpha methyl styrene, divinylbenzene, chloromethyl styrene, chlorostyrene, vinyl pyridine, acrylic acid, methacrylic acid, vinyl pyrrolidone, vinylacetate, trifluorovinylacetate, methyltoluene, emulsifiers, chain transfer agents and water. The temperature is raised in the range 40-90{C to initiate the grafting reaction. The activated polymer is in contact with the emulsion for sufficient time to affect a desired grafting yield.

Description

* 2448159

Description

Emulsion grafting This present invention relates to the process of grafting unsaturated monomers. It relates to a process of forming a monomer-grafted polymer for membranes which may be further functionalised. Such membranes may be used in various applications e.g. electrodialysis, filtration and energy systems such as redox cells and batteries but principally in fuel cells.

The present invention may be applied to microporous polymers and other structural forms such as non-woven fabrics, tubes, powders, pellets, sheets and/or films.

The techniques for the grafting of unsaturated monomers to a polymer are described in US Patent numbers 3,481,848, 4,012,303, 433,473, 4,605,685, 6,225,368, EP Patent No. 05203 and GB Patent No. 1237293/2001, The recognised techniques involve the steps of activation of the polymer, which is the generation of free radicals on the polymer backbone, followed by reaction of the activated polymer with the unsaturated monomers, producing a grafted polymer. The activation step can be carried out simultaneously known as mutual grafting, or sequentially which has been referred to as pre-irradiation grafting. The subject area has been reviewed by B. Gupta and G.G. Scherer, Chimia 48 (1994), 127-137.

US Pat. No. 3,481,848 describes a method of radiation graft polymensation of vinyl compounds onto cellulose where the cellulose is pre-irradiated in air and reacted with a vinyl monomer in an emulsified system in which alumina or alumina-silica mixture is used as a catalyst in order to deactivate OH radicals. Without the catalyst a large quantity of homopolymer is formed. The method is applied to cellulose membranes and would be unsuitable for many polymers which may undergo degradation.

US Pat. No. 4,012,303 discloses a process for the simultaneous irradiation grafting of trifluorostyrene to an inert polymeric film base. In this instance the polymer is activated by irradiation while immersed in an organic solution of the monomer. The process has a number of disadvantages. The monomer may be prone to significant homopolymerisation in preference to grafting to the polymeric film base, since it too is being activated simultaneously. Secondly this process uses an organic solvent to solubilise the monomer is environmentally disadvantageous. Thirdly this process is slow taking 400 hours to achieve graft levels of 30%.

US Pat. No. 4,339,473 also describes a simultaneous radiation grafting process which uses water as the solvent and includes homo-and/or co-polymerisation retardants. Therefore the problem of bomopolymensation is reduced and the use of an organic solvent avoided. This process is limited to grafting monomers which are hydrophilic monomers which are substantially soluble in water.

US Pat. No. 4,605,685 discloses a sequential process for the irradiation grafting of trifluorostyrene to an inert polymer film base. This process involves activation of the polymer film base by irradiation using beta radiation prior to contact with the monomer. If stored at sufficiently low temperatures the active polymer may remain stable for up to two months.

GB-A-1237293 discloses an emulsion of monomer in water for the grafting a polymer by thermal polymerisation. This process involves simultaneous thermal activation of the polymer and reaction with an emulsion of the monomer in water. The disclosure makes no mention of homopolymerisation which is likely to be a problem for such systems. It is thought that in order to alleviate the problem of homopolymerisation the process operates at very low concentrations of monomer resulting in low levels of grafting.

EP-A-0526203 discloses a separation membrane composed of microporous polyethylene having fine pores substantially filled with a graft polymer. The membrane is obtained by bringing the microporous polyethylene, having radicals formed by plasma irradiation, into contact with an emulsion comprising a water-insoluble monomer, a surface active agent and water.

US Pat. No. 6,225,368 discloses a monomer-grafted, cross-linked polymer activated by radiation, quenching to crosslink the activated polymer, followed by irradiation activation for contacting the unsaturated monomer. This process prefers beta radiation rather than gamma irradiation and the primary aim is cross-linking.

It is an object of this invention to provide a process for grafting mixed monomers which does not require an organic solvent and which does not result in significant homopolymerisation, but which produces a uniform co-graft of monomers througjiout the polymer within a reasonable reaction time, at a low reaction temperature. The aim of this invention is to produce uniformly grafted polymers by an environmentally friendly process. Many monomers do not graft readily in organic or aqueous media and require an excess of monomer or extreme conditions to achieve useful yields. The aqueous emulsion process which is described herein enables graft yields similar to those achieved by mutual irradiation in an organic system, but with much lower concentrations of monomers.

The present invention also provides a process for the preparation of a polymer grafted in the presence of a co-monomer acting as a spacer in an aqueous emulsion.

The process comprises the steps of (I) Activating the polymer by irradiation (ii) Preparing an aqueous emulsion of co-monomers, emulsifier, chain transfer agent and water (iii) Contacting the activated polymer and emulsion at temperature between 40-90 C for a time sufficient to effect the required graft yield.

The process of the present invention maybe used to graft unsaturated monomers to a large number of polymers, copolymers or terpolymers formed from hydrocarbon and halogenated compounds and poiy alcohols, for example, but not limited to, those known as polyethylene, polypropylene, polytetrafluororethylene, polyhexafluoropropylene (HFP), tetrafluoroethylene-hexafluoropropylene (FEP), tetrafluoroethylene-propylene copolymer, tetrafluoroethylene-ethylene copolymer (ETFE), hexafluoropropylene-ethylene copolymer, polyvinylidene halides, such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride; vinylidene fluoride tetrafluoroethylene copolymer (PVDF-TFE), polyvinyl fluoride, perfluoroalkoxy copolymer (PFA), polyisobutylene; acrylic polymers, such as polyacrylate, polymethacrylate, polyethylacrylate; vinyl halide polymers, such as polyvinyl chloride; fluoropolymers such as chiorotrifluoroethylene and polyvinyl ethers, such as polyvinyl methyl ether; polyacrylonitrile; polyvinylketones; polyvinyl aromatics; and polyvinyl esters, such as polyvinylacetate.

Additionally the present process may be applied to the grafting of unsaturated monomers to microporous substrates comprised of any of the above ranges of polymers.

The process of the present invention maybe used to graft a large number of unsaturated monomers. Preferably the unsaturated monomers are selected from the group consisting of vinyl benzyl halides, styrene and substituted styrenes such as trifluorostyrene, alpha methyl styrene, chioromethyl styrene and chiorostyrene, divinylbenzene, vinyl pyridine, acrylic acid, methacrylic acid, vinyl pyrrolidone, vinyl pyridine, vinylacetate, trifluorovinylacetate, and methyltoluene and mixtures thereof The process is furthermore suited to the grafting of unsaturated monomers containing functional groups wherein said functional group is selected from optionally-substituted aiyl and heteroaryl groups, carboxylic acid, carboxylic acid derivatives, amines, amine derivatives, inorganic acid, sulphate, hydroxy and substituted alkyl, cycloalkyl and cycloheteroalkyl groups and protected versions of any of the aforesaid.

It is acknowledged that some of the above monomers are soluble in water, however, the use of the process as described in the present invention is still of advantage by virtue of improving the uniformity and yield of the copolymerisation process.

The monomer content of the emulsion can be between 0.5-50% by weight. More preferably the monomer component should be between 2-30% by weight and advantageously between 2- 15% by weight. The emulsifiers used in the present invention are selected from alkyl sulphates, alkyl aromatic sulphonates, ethoxylated fatty alcohols, fatty acid ester, and mixtures thereof The emulsifier component is preferably present in the amount 0.0 1-10% by weight. The emulsions are purged using nitrogen in order to remove oxygen which is known to be deleterious to the grafting reactions. The emulsions have been shown to be relatively stable at room temperature can be stored in a refrigerator at 4 C prior to use.

The radiation for activation of the polymer maybe provided by gamma rays, X-rays, UV light, plasma irradiation or beta radiation. Gamma irradiation is the preferred source of irradiation and the total dose for the activation step depends on the polymer to be grafted but is typically in the range of 0.1-1 5OkGy. Preferably the radiation dose range is from I to 150 kGy, more preferably 10 to 50 kGy.

The irradiated polymer has been shown to remain active for up to a year stored at temperatures in the range -60 C to 0 C. Therefore the grafting step does not have to be carried out immediately following activation. The stability of activated polymers is discussed in U.S. Pat. No. 4,605,685.

It is preferred that the grafting stage is carried out at a temperature from 40 to 100 C, more preferably from 45-90 C. At lower temperatures the rate of radical-radical coupling is slower and the stability of the emulsion is therefore greater, leading to a higher overall percentage graft but longer reaction time. The reaction time is also a function of the polymer and unsaturated monomers involved. The grafting step is preferably carried out over 0.5 to I 5hrs, more preferably over 3 to 6 hours.

Where the copolymer formed by the described process is to be used as an anionic exchange membrane in an electrochemical cell, the process may additionally comprise of the step of aminating the co-grafted polymer. This provides a membrane which is selective to anions.

Preferably the amination step comprises of contacting the grafted polymer with tnmethylamine (TMA) triethylamine (TEA), N,N,N-trimethylbis(hexamethylene)triamine, and other tertiary, quaternary amines, di-amines and tri-amines as well as their salts.

The present invention is illustrated but not limited, by the typical procedure described below: The polymer film was irradiated by Co6 gamma radiation in air to a total dose between of 30-lOOkGy. The irradiated films may be stored below -18 C for up to a year or used immediately. In preparation for grafting the film was cut into strips 6x1 1cm long and placed in a re-sealable glass vessel.

An emulsion comprising the chosen monomers, emulsifier, chain transfer agent and deionised water was made using a Janke & Kunkel I1KA Labortnik Ultra-turrax T25 homogeniser. A typical formulation comprised 0.5-10 % mixed monomers, 0.01-1% dodecylbenzenesulphonic acid-sodium salt, 0.01-1% 2-Methyl-2-propene-1 -sulphonic acid-sodium salt, and 80-90% deionised water. The emulsification process typically lasted for 5-10 minutes and the resulting emulsion was decanted into the vessel containing the activated film which was then sealed and purged with nitrogen for 2 hours.

The vessel was placed in a water bath at 80 C, preferably a shaker bath. After the desired reaction time of between 0.5-10 hours, the film was removed from the vessel and rinsed to remove any residual monomers. The grafted film was then rinsed with toluene or acetone, washed in deionised water at 80-90 C for up to 5 hours before being dried to constant weight.

At this point and according to end use, the dried film could be subjected to further treatments, for example amination. The resulting functionalised film was rinsed in water before being boiled in deionised water and equilibrated in NaOH and dried.

In the examples which follow, the copolymer yield is expressed by the formula: Degree of grail (yield) = GWF-GWI / GWF xlOO GW = Initial weight of polymer GWF = Final weight of polymer The present invention will now be described with reference to the following specific and comparative examples.

Comparative Examples I to 6 Samples of LDPE 50p.m, LDPE 125.tm, and HDPE 40iim were cut into strips 11cm x 6cm long and irradiated using cobalt 60 gamma radiation to a dose of lOOkGy. The samples were placed in I 50m1 glass bottles and the monomer emulsions added, according to the respective compositions and concentrations given below. The bottles were sealed and purged with nitrogen gas for 2hrs and thereafter placed in a shaker water bath 50-90 C for 5 hours. The samples were then recovered and rinsed in a small quantity of toluene at 50 C for 3hrs. The films were wiped with acetone before being rinsed in water and then placed in a water bath at 80-90 C for 5hours, after which they were patted dry and dried in an oven at 40 C to constant weight.

The percentage yields for the respective example numbers were: 1. LDPE (501.tnl) 5%VBC 6.9% 2. LDPE(1251tm) 5%VBC 1.7% 3. HDPE (4OJLm) 5%VBC 5.0% 4. LDPE (50pm) 3%VBCI2% Styrene 19.4% 5. LDPE (125t.tm) 3%VBC/2% Styrene 9.4% 6. HDPE (4Opm) 3%VBCI2% Styrene 9.1% This shows that the composition of the copolymer is influenced by the composition of the emulsion used in its preparation and that the use of mixed monomer systems increases the graft yield compared with the use of single monomer compositions.

Example 7

A sample prepared according to Example 4 was treated by immersion in 45% aqueous trimethylamine for 16 hours at 20 C. The resulting functionalised film was rinsed in delonised water, boiled in deionised water for 1 hour, equilibrated in 6M NaOH for 16 hours before the ionic conductivity in 6M NaOH was measured as a resistance using a Wayne Kerr AC Impedance Bridge, model no. B642.

The resistance value was 1. 55Qcm2.

Comparative Examples 8 to 10 Samples of LDPE 50tm were cut into strips 11 cm x 6cm long and irradiated using cobalt 60 gamma radiation to a dose of I OOkGy. The samples were placed in I SOml glass bottles and the monomer emulsions added, according to the respective compositions and concentrations given below. The reaction process was thereafter carried according to that outlined in

examples 1 -6 above.

The percentage yields for the respective example numbers were: 8. LDPE (50ini) 4%VBC/0% Styrene 6.9% 9. LDPE (50.tm) 4%VBC/3% Styrene 20.1% 10. LDPE (50p.m) 4%VBC/4% Styrene 19.8%

Examples 11 to 12

Samples prepared according to Example 8, 9 and 10 were treated by immersion in 45% aqueous trimethylamine for 16 hours at 20 C. The resulting flinctionalised films were rinsed in deionised water; boiled in deionised water for 1 hour, equilibrated in 6M NaOH for 16 hours before ionic conductivities in 6M NaOH were measured as a resistance using a Wayne Kerr AC impedance Bridge, model no. The resistance values were as follows: 11. LDPE (50tm) 4%VBCIO% Styrene 498Qcm2 12. LDPE (50tm) 4%VBCI3% Styrene 0.48 =cm2 13. LDPE (SOpm) 4%VBCI4% Styrene 0.74acm2 This shows that the ionic conductivity (ac resistance) of the membranes is strongly influenced by the composition of the copolymer.

Claims

Claims What is claimed is: 1. A process for the preparation of a graft

copolyrner from an organic monomer singly or in combination in an aqueous emulsion by the process steps of (i) Activating the polymer to produce free radicals.

(ii) Preparing an aqueous emulsion of co-monomers, emulsifiers, chain transfer agents, and water.

(iii) Contacting the activated polymer with the aqueous emulsion and raising the temperature to initiate the grafting reaction.
2. A process according to claim I where the polymer is selected from the group consisting of polyolefins, fluorinated and per-fluorinated polymers, copolymers and terpolymers.
3. A process according to claim 1 wherein the polymer is selected from the group consisting of polyethylene, polypropylene, polyhexafluoropropylene (HFP), tetrafluoroethylene-propylene copolyrner, tetrafluoroethylene-ethylerie copolymer (ETFE), hexafluoropropylene-ethylene copolymer; polyvinylidene halides, such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride; vinylidene fluoride-tetrafluoroethylene copolymer (PVDF-TFE), polyvinyl fluoride, perfluoroalkoxy copolymer (PFA), polyisobutylene; acrylic polymers, such as polyacrylate, polymethylmethacrylate, polyethylacrylate; vinyl halide polymers, such as polyvinyl chloride; fluoropolymers such as polytetrafluoroethylene (PTFE), chlorotrifluoroethylene and fluorinated ethylene-propylene (FEP); polyvinyl ethers, such as polyvinyl methyl ether; polyacrylonitrile; polyvinylketones; polyvinyl aromatics; and polyvinyl esters, such as polyvinylacetate.
4. A process according to claim 1 wherein the unsaturated monomer is selected from the group consisting of styrene, trifluorostyrene, alpha methyl styrene, divinylbenzene, chioromethyl styrene, chiorostyrene, vinyl pyridine, acrylic acid, methacrylic acid, vinyl pyrrolidone, vinyl pyndine, vinylacetate, trifluorovinylacetate, and methyltoluene and mixtures thereof. Unsaturated monomers containing functional groups wherein said functional group is selected from optionally-substituted aryl and heteroaryl groups, carboxylic acid, carboxylic acid derivatives, amines, amine derivatives, inorganic acid, sulphate, hydroxy and substituted alkyl, cycloalkyl and cycloheteroalkyl groups and protected versions of any of the aforesaid.
5. A process according to claim 1 wherein the monomer components of the emulsion are present in an amount from 0.5 to 45% by weight.
6. A process according to claim I wherein the monomer components of the emulsion are present in an amount from 0.5 to 25% by weight.
7. A process according to claim I wherein the monomer components of the emulsion are present in an amount from 0.01 to 15% by weight.
8. A process according to claim I wherein the emulsifier is selected from alkyl sulphates, alkyl sulphonates, alkyl aromatic suiphonates, ethoxylated fatty alcohols, fatty acid esters or mixtures thereof
9. A process according to claim I wherein the emulsifier components of the emulsion are present in an amount from 0.01 to 15% by weight.
10. A process according to claim I wherein the emulsifier components of the emulsion are present in an amount from 0.01 to 10% by weight.
11. A process according to claim I wherein the emulsion is prepared by a process which comprises the steps of: (a) combining emulsifier with water (b) adding chain transfer agents (c) adding monomers (d) homogenising
12. A process according to claim 1 wherein the total radiation dose for polymer activation is in the range of from 1-150 kGy.
13. A process according to claim I wherein the total radiation dose for polymer activation is in the range of from 30-100 kGy.
14. A process according to claim 1 wherein the radiation dose is provided by gamma rays, x rays, ultraviolet radiation, plasma irradiation or beta radiation.
15. A process according to claim I wherein the copolymerisation reaction is carried out at a temperature in the range of from 40-90 C.
16. A process according to claim I wherein the reaction is carried out at a temperature in the range of from 60-85 C.
17. A process according to claim 1 wherein the reaction is carried out for a period of time in the range from 0.5-lOhours.
18. A process according to claim 1 wherein the reaction is carried out for a period of time in the range from 3-8hours.
19. A process according to claim I additionally comprising the step of further functional isation.
20. A process according to claim 19 wherein the functionalisation step comprises contacting the grafted polymer with TMA, TEA. N,N,N-trimethylbis(hexamethylene)triamjne, and other tertiary, quaternary amines, diamines and tnamines as well as their salts.
21. A process as claimed in claim 1 and subsequent claims wherein the polymer is in the form of a membrane suitable for use in an electrochemical cell.
22. A process as claimed in claim I and subsequent claims wherein the polymer is in the form selected from the group comprising threads, non-woven fabrics, tubes, powders, pellets, sheets and films