CA1102988A

CA1102988A - Polymeric materials

Info

Publication number: CA1102988A
Application number: CA286,596A
Authority: CA
Inventors: Lionel B. Adams; Keith V. Lovell; Gordon Partridge; Barbara J. Ringrose
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1976-09-14
Filing date: 1977-09-13
Publication date: 1981-06-16
Also published as: DE2741439C2; FR2364241B1; FR2364241A1; US4143218A; DE2741439A1; JPS5356271A; JPS6028851B2; GB1581641A

Abstract

ABSTRACT OF THE DISCLOSURE

A method of treating a hydrophilic polymer film is described in which the film is immersed at a temperature greater than 80°C, and preferably greater than the crystalline melting point of the polymer, in a liquid capable of swelling the polymer. Polymers to which this method can be applied include graft copolymers of for example polyethylene, polypropylene, polytetrafluoroethylene, and nylon with acrylic or methacrylic acid. The films when treated have decreased electrical resistance and increased hydroxyl ion permeability and are then useful as battery separators and in other semi permeable membrane applications.

Description

The present invention is concerned with rnethods of heat treatment of polymers and wi.th pol.ymers so heat treated, and is particularly concerned with graft copolymers.
Graft copolymers are well kno~l and preparative metnods are well doclimented, ~or example in "GraEt Copolymers" by H A Battaerd and G W rrregear (Vol 16 o~ Polymer Reviews) published in 1967 by Interscience, "Radiation Chemistry of Po:lymeric Systems" edited by A Chapiro (Vol 15 of High Polymers) published in 1962 by Interscience, and "Modern Plastics", ~ (1957) page 171~176~ They are conveniently prepared by polymerisation of a monomer in the presence of a polymeric substrate using initiation means, generally irradiation, which generates free radical sites on the backbone of the existing polymer which initiate copolymerisation of the monomer. A cer-tain amoun-t of cross-linking and homopolymerisation may also take place.
Amongst the most important of these gra~t copolymers are those in which a hydrophobic baokbone, eg polyethylene7 is grafted with a hydrophilio monomer, eg acrylic acid, resulting in materials having uses as semi-permeable membranes in dialysis sepa.rators9 in water purification appara-tus by the phenomena o~ ultra~iltra-tion or reverse osmosisr and as anolyte-catholyte separators in electrochemical systems. ~he use of these materials in primary and seoondary alkaline and acid batteries is -to replace the long es-tablished and somewhat unsatis~actory cellulosic separator as indicated in "Zinc-Silver Oxide Batteries" edited by A Fleisher and J Lander and published by J Wiley ~ Sons in 1971~ (s0e partioularly page 271)~

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In accordance ~lith -the present invention a film of hydrophilic polymeric material i6 immersed at an elevated temperature, for example in excess of 80C, in a liquid treatment medium as defined hereafter capable of swelling the hydrophilic polymeric ma-terial after which -the liquid treatment medium iB
removed by quenching and washing in cold water or dilute alkaline solution. It has been discovered that the treated material may be quenched in either dilute alkali or in distilled water at about room temperature or indeed it may be quenched to about room temperature in air, after which it is normally washed, for example in dilute alkali.
Preferably the treatmen-t temperature i8 above the crystalline melting point of the hydrophilic polymeric material and the optimum temperature for any particular treatment will vary depending upon the identity of the hydrophilic polymeric material being treated and the properties desired in the ~inal material.
Suitable hydrophilic polymeric materials include graft copolymers in which a hydrophilic comonomer is grafted on to a hydrophobia polymer backbone or base polymer, sulphonated polymers, and polymers containing quaternary ammonium groups~
for example aontaining pyridine or pyrrolidone groups.

In a preferred embodiment of the invention the hydrophilic polymeric material is a gra~t copolymer on any of the polyole~ins or copolyolefins, for example polyetheylene (low, medium or high density~, polypropylene, poly 4-methy~pentene-1 (and copolymers of these polyolefins), products of other vinyl polymerisations1 for example polyvinylacetate and polyvin~lalcohol as well as the halogen containing polymers or copolymers, ~or example poly~inylchloride, chlorin~ rubbers, an~ polytetrafLuorethylene7 polychlorofluozcet.~ylere, products of condensation polymerisa-tions, for example poly~nides or copolymides, ~e -the nylons,or the saturated and the un-satur~ted polyesters, or a mixture of any of these polymers. The hydrophilic comonomer may be any reactive polar vinyl monomer selected from acrylic acid, methacrylic acid, other ethylenic carboxylic acids, for example itaoonic acid, ethylenic carboxylic acid amides, for example acrylamide and methacrylamide, and ethyleniccarboxylic acid amines, for example butylamine acrylate, and organic bases capable of quatern~sation for example vinylpyridine or vinylpyrrolidoneO Of these acrylic acid and methacrylic acid are preferred and acrylic acid is the most commonly used.
These graft copolymers may be polymerised by any suitable method although the preferred technique is radiation initiated grafting with high energy radiation, for example gamma photons or accelerated electrons. The copolymers may include additives such as filler particles or other additives such as thermal or oxidation stabilisers, dyes1 and pigments.
The hydrophi]ic polymsric material should be adequately cross-linked to ensuretheintegrity of the film during treatment. In certain instanceæ, for example graf-t copolymers using acrylic acid as the comonomer7 sufficient cross-linldng is normal]y introduced during the grafting procesæ. If sufficient cross-linking is not present in the polymer then it may be introduced, for example by irradiation, or possibly by chemical meanæ using divinylbenzene, for examplet as cross~linking agent. The pre~ence of cross-linking may be checked by a conventional gel -test in a solvent. In -this way the mechanical integrity of the film of hydrophilic polymeric material may be ensured during the reao-tion.

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Ad~antageously the liquid trea-tmen-t medium is a ~ydro~yl containing organic liquid and suitable liquids include alkanols, for example octanol or decanol, polyh~ydric alcohols, for e~ample glycerol, mono-, di-, and tri-ethylene glycoll and polye-thylene g~lycol and although the liquid treatment medium is normally substa~tiall~ non-a~ùeous may also include azeotropic mixtures of alkanols and polyhydric alcohols containing less than about 20% by weight of water.
The preferred heat treatment medium is glycerol. The heat treatment medium may include up to 5% molar of an alkali or al~aline earth metal hydroxide and potassium hydroxide is preferred.
The object of the process of the present invention is to ~mprove certain properties of the film of hydrophilic polymeric material so as to make it more useful as a ba-ttery separator material. The process produces ~ilm material which tends to have a decreased electrical resistance, increased hydroxyl ion permeability, increased rate of we-tting out (as defined hereinafter~
and in increased equilibrium water conten-t (as defined hereinafter) when compared with the untreated film.
~ he optimum time of heat treatmen-t varies with the values of the various parameters employed, eg temperature of treatmen-t bath, iden-tity of heat treatment medium and it is believed that in each instance there is a particular time after which no improve~ent takes place. The optimum time tends to be measured in minutes rather then hours in most instances.
It is believed that the present process achieves its beneficial effects by relieving s-tresses and strains in the molecular structure through increasing molecular movement and permitting reorientation and rearrangemen-t of the molecular structure. X-ray crystallography shows that there is no marked reduction in total bulk crystalline content although -there is a marked reduc-tion in the degree of preferred molecular orientation originating in the base film from which the copolymer was made. rrhe process is usually accompanied by changes in the dimensions of the material in length, breadth and thiokness, furtrler indication of a relaxation and reordering of struc-ture.
It has been discovered that no chemical interaction occurs between the oopolymer and the treatmen-t medium in the process of the present invention as determined by spectrophotometric analysis before and after treatment; neither is there a weight loss of copolymer during processing indicating that the process is not a leaching ou-t treatment producing pores or voids in the material by removal of matter. rrhe foregoing discussion of the postulated reaction mechanism is not to be cons-trued as defining the extent or scope of the invention in any way.
rrhe present invention will now be described by way of exa~ple only with reference -to the following Examples~
rrhe degree of grafting as used in the following is defined as (WF -WI)/w~ x 100%

where W~ is the weight of -the copolymer and WI is the weight of the original polymer fil~ and the equilibrium water content of the , ,: : .
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' :'' , ' :' ' . ' copolymer fil~ is defined as (WW -W3)/T~D x 100%

whe~e Ww is the weight of -the sample after having been equilibrated in distilled water for 20 hours and WD is the weight of the same sample af-ter drying in an oven in the presen¢e of silica gel for 20 hours a-t ~5C.
~ he electrical resistivity of the material is de-ter~ined by the method outlined on page 258 of "Alkaline Storage Batteries" by Falk & S~lkind published by J Wiley & Sons in 1969 in conjunction with a Wayne-E~rr Au-tobalance bridge using 30% aqueous Potassium hydroxide solution at 25C as electrolyte.
Wetting-out rate vPlues refer to the time taken for the resistivity of the sample to fall from infini-ty at the moment of introduction of the elctrolyte to a value 10% above the final equilibrium valueO It is a measure of -the rate of absorption of electrolyte by -the separa-tor and is related to porous structure and hydrophilic nature of the separator~
The hydroxyl ion permeability is measured at th0 rate at which hydroxyl ions migrate across the separator, at 25C, from a cell compartment con-taining a 10 molar aqueous potassium hydroxide solution to a cell compartment containing initially only distilled water. ~he rate is measured titrimetrically using ~ hydrochloric acid with phenolphthalein as indicator.

2~

A 12~ thick low density polyethylene film (supplied by ~ritish Visqueen Ltd) having a size of 5 feet by 1 foot was rolled up with a single ply ~lue Tissue paper interle~ving and placed in a glass tube 22 inches x 1.5 inches, which was then filled with 500 ml of a 25% by volume aqueous soltuion of acrylic acid and containlng 4g/l of Analar ferrous sulphate.
The tube and contents were e~acuated by water pump for 20 minutes at the end of which time it was back filled with nominally o~ygen-free nitrogen and sealed. The tube was irradiated with gamma rays from a Co source at 20C to a total absorbed dose of 6 x 105 rad at a dose rate of 3.5 x 104 rad per hour. The graft copoly~erised LDPE film was removed from the tube, washed three times with distilled water and air dried in a hot air oven, Thc ~eight Or copolymerised aorylic acid was found to be 31.~o by weight, being an horogeneous graf-t.
The copolymer was subjected to heat treatment by immersion in a bath o~ Glycerol A.R. (ex ~DH) at 122C, for 5 minutes.
Following i~mersion the film wPs quenched in 0.1N aqueous pOtaSSiUm hydroxide solution at 25 C for 10 minutes and then air dried at 45C.
_ The properties defined above ~re measured and the resul~s listed in Table 1 below.
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qlA13LE 1 --- ~ -- ~ -- l Electriczl Hydroxyl ionl Equilivrium Wetting-out reJ~is~ivity permeability water content rate ~cm ) (~ KHl ~i~(secs.) cm nlin _____~___ ~ .- l Before heat 0.08 0.39 72 72 treatmen-t ______~
After heat 0.03 0.53 135 12 treatment ___ ~__ __ .

A 38~ thick low density polyethylene film (supplied by Metal ~ox Co Ltd) was graft copolymerised to a level of 32.5~ with acrylic acid in the manner of example 1.
The heat treatment was carried out by immersion in Glycerol A.R.
at 120C for 4 minutes followed by quenchin~ and drying as in example 1.
The properties were determined as abo~e and are listed in Table 2 below.

~ABLE 2 -- Electrioal Hydr yl ion E uilibriD Wettir~out resistiv~ty permeability water con-tent rate (~1~ cm ) (ml ~ KOH 1 ~, (secs.
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~efore heat0.18 0.12 70 44 treatment ~ ....... ~__ After heat O 09 0 20 105 68 treatment . .
--_____ __ ~__ ..

A 19~ high density polythene film ~supplied by Dickinson Robinson Ltd) was copolymerised with acrylic acid to a level of 28.3~, by the technique of exarnple 1.

The copolymer was subsequently heat treated by immersion in Clycerol A.~. at 120C for 5 minutes~ quenched in dilute alkali and dried.

The properties were determined as above and are listed in Table 4 below.

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Electrical Hydroxyl ion Equilibrium ~etting-outi resist~vity permeability water content rate cm ) (ml ~ K0~ 1 % (secs~) cm min _~ _____. ~_ before heat 0.12 0.29 48 560 treatment After _____________ ___________ _ heat 0.12 0.27 62 100 treatment ~ r ___ ~ ~_ ~ .
A 25~ polytetrafluorethylene film (supplied by Polypenco Ltd) was grafted, in a similar manner to exarr.ple 1~ with acrylic acid to a level of 12. ~.
The copolymer filrn was heat treated by immersion in Glycerol A.R~
at 120 C for 4 minutes, followed by quenching in dilute alkali and air drying. ~he properties were determined as a~ove and are listed in Table 4 below.

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~ ~_ , ...... _ ,_ _ . , Electrical Hydroxyl ion Equilibrium Wettin~-out Resist2vity Permeability water content rate ~r~ cm ) ( m~~ n~1) % (secs) . _ . _ ~efore hea-t 0.18 ~ 0.18 29 127 treatment _ _ . . .... _ .

After heat 0.18 O.20 31 9o treatment , .
EX~PLE ~
A 4 ~ thick ~ilm ~ polyamide (GRILAMID L25 Nylon 12) film (supplisd by Grilon Plastics Ltd) was gra~t copolymerised to a level of 41.5%
with Acrylic Acid in the manner o~ example 1.
The heat treatment was ~arrled out by immersion in Glycerol AR at 1gO~C for 5 minutes followed by quenching and drying as in example 1 and the properties determined and listed in Table 5 below.
; TABLE 5 _ ~ ~__ _ Electrical ~ydroxyl ion Eq~ilibri= Netting out Resist~nce permeabili~ watsr content rate ( n~cm ~ ~mlN ~ Hc~ % ( 6ecs ~ _ _ _ . ____ . _ Before heat 0~16 0.10 26 425 treatment __ ~ _ _ _ _ .
Af~er heat 0.04 0~60 915 15 treatment :
_ _ _ ~ _r _ _. A _ ___._ _ -- T ~ _ __ *

Trade Mark 11 .

A 32 ~ thick cas-t polypropylene film (supplied by Shorko Films Ltd) was graft copolymerised to a level of 33.5~0 wi-th Acrylic acid in the manner of example 1.
~ he heat treatment was carried out by immersion in glycol AR at 190C ~or 5 minu-tes followed by quenching and drying as in example 1 to gi~e material having the properties listed in ~able 6 below.
TA~LE 6 _ . _ _ __.
Electrical Hydroxyl ion Equilibrium Wetting out Resist~nce permeabili-ty2 water content rate ( Q.cm ) min ) % (secs) _ . . _ ~ ~ __ ___ ~efore heat 0.11 0.25 27 120 treatment Ai~er _ _ ~
heat 0.08 0.37 1~2 45 treatment ~ ~ ~ _ A 19 ~ thick cast film of high densi-ty polyethylene ~supplied by Dickenson Robinson Group Ltd, Packaging Division) was graft copolymerised to a level of 55.3% with Acrylic Acid, The heat treatment waæ carried out by immersi~n in glycerol AR at 190C for 5 minutes followed by quenching a-nd drying as in e~ample 1 to give material ha~ing the propterles listed in ~able 7 belo~

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Blectrioal Hydroxyl ion Equilibrium Wetting out resista~ce permeability water content rate ( n,cm ) cm~~min 1) % (secs) _._ , . ~ . _ Before heat 0.04 o.46 43 20 treatment Aftsr heat 0.02 0.52 207 10 treatment ~

EXA ~
A 38 ~ thick cast low density polyethylene (supplied by Dixons Ltd) was gra~t copolymerised to a level of 43.8% with Methacrylic Acid in the manner described in Example 10 The heat treatmen-t, quenching and drying was carried out in the manner o~ example 1 to yield material having the properties set out in ~able 8 below.

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Electrical Hydroxyl ion Equilibri~n ~et-ting out : resista~ce permeability water content rate : ( Q.cm ) cm~Zmln~l) % (secs) Before heat 2.0 0~01 15 ~ 1000 treatmsnt Aft=r heat 0~8 0.07 : 81 ~ 1000 treatment ~ . ., ~

, -, ', EXAM~
l~is example illustrates three difference methods of quenching the material after -the heat treatment of the present invention.
A 25 ~ lhick film of low density polye-thylene film (supplied by ~ritish Visqueen Ltd) was graft copolymerised to a level of 34.8 with acrylic acid as described in Example 1.
q`he heat treatment was carried out by immersion in glyoeral A~ at 120C for 5 minutes ~ollowed by one of the following quenching trea-tmen-ts:-a quenohed in 0.1N aqueous po-tassium hydroxide solution at 25C ~or 10 minutes followed by drying in air at ~5 C.
b quenched in distilled water a-t 20C for 10 minutes followed by d~ying in air at 45C.
c Air cooled in contact with glycerol ~R from 120 C to 25 C
followed by washing in 001N aqueous KOH solution at 25C. Air dried at 45C~
q`he properties of the products were determined and are listed in Table 9 below.

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Electrical Hydroxyl ion E~ilibriumWetting out Sample resis-ta~ce permeability water contentrate ( n .cm ) cm~2min~1) % (8ec8) _ _ _ ~efore hea-t 0.11 0.20 25 145 trea-tment After ~ _ _ _ - ~ _ quenching 0. o6 o. 37 88 44 treatment (a) A~ter ~ . ~ ~ _ quenching 0, o6 o. 44 68 47 treatment (b l A~ter ~ _ _ _ _ _ quenching 0.05 0. 40 94 41 treatmen-t(c ~ __ As will be seen all -three methods of quenching give similar results but generally method (a) is pre~erred.

This example illustrates the use of heat treatment media other than glycerol.
A 25 ~ thick low densi-ty polyethylene film (supplied by British Visqueen ~td) was gra~t copolyerised to a level of 34.8~ with Acrylic Acid in -the manner of example 1 and samples were subjected to each of the following heat treatments:-(a1 Immersion in a bath of diethylene glycol (supplied by Koch Light Laboratories) at 120C for 5 min~tes Immersion in a bath o~ polyethylene glycol (Code No 200 supplied by BDH Ltd) a-t 120C for 5 minutes.
After the heat treatment the materials were then quenched and dried as described in Example 1 and the proper-ties determined to give the results ~uoted in Table 10 below.

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~lectrical Hydro~yl ion Equilibrium Wetting out resista~ce permeability water content rate Sample (n .cm )(ml N KOH % (secs) . cm~~min~1 ) Before heat 0.11 0.20 25 145 treatment Aftte-r ' . _ _ treatment 0.05 0.50 113 18 (a) he~t _ _ __ _ _ treatment 0.05 0.49 116 17 and que~ch _ _ EX~PLE 11 This example illustrates the addition of alkali metal hydr~xide particularly pstaæsium hydroxide to the heat treatment medium.
A 12 ~ thick low density polyethylene film (ex British Visqueèn Ltd) was grafted copolymerised to a level of 31.2% with Acrylic Acid as in example 1.
The heat treatment was carried out by immersion in Glycerol AR
in ~hich had been dissolved 5~0 weight for volume of Analar (ex ~DH Ltd3 potassium hydroxide, for 5 minutes at 122 C, followed by quenching ~nd drying as in example 1 and the porperties of the material are givenlin Table 11 below.

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q'ABl.E 1 1 .
Electrical Hydroxyl ion Equilibri.um Wet-ting out resista~ce premeability Wa-ter content rate (Q ,cm ) (ml N KOH % (secs) cm~Z'min 1 ) After _ _______________ __ _ treatment 0.03 0.53 135 12 A~ter ~ ~. ._ . _ treatment 0.02 o.58 140 10 in Glycerol : & 5~0 w/v KOH
_ ~___ ... . - . . ~
This illustrates a marginal irnprovement in the properties of the film with the addition of potassium hydroxide to the heat treatment `; medium.

This Example illustrates the effect of temperature upon the process of the present inventionO

~a) A 19 ~ thick cast film if high density polyethylene (supplied by Dickenson Robinson Group Ltd, Packaging Division) was graft copolymerised to a level of 5503% with Acrylic Acid similarly to Example 1.
Heat treatment was oarried out in a bath of Glycerol AR at temeratures of 120C and 190C for 5 minutes~ Quenchir~ and drying were carried out in the manner of example 1 and the properties of the products are given in Table 12 below.

' ' ", ' ' : ' ' '' ' _ _ Electrical ~ydFoxyl ion Equilibrium Wetting out resista~ce permeability water conten-t rate ( n.cm ) (ml N KOH % (secs) cm min ~_ _ Before heat 0.04 0.46 43 20 treatment . _ __ _ .
After treatmOent 0.03 0.51 78 13 _ . ~ ~ , .-......... ~ _ _ After -treatmgnt 0.02 0-52 207 10 at 190 C
. , ~b) A ~2 ~ -thick cast polypropylene ~ilm tsupplied by Shorko Films Ltd) was graft copolymerised to a level o~ 33.5~ with Acrylic Acid in the manner of example 1.
Heat treatmen-t was carried out in a ba-th of Glycerol ~R at temperatures of 120C and 190C for 5 minutes. Quenching and drying were carried out as in example 1 and -the propertias of the products are given in Table 13 below.
~A~LE 13 . ; ~ ___ ___ ~_ Electrical H~droxyl ion Equilibrium Wetting out resista~ce permeability water content rate ( Q.cm ) (ml N KOH % (secs) . cm- min-1) Before heat 0.11 0.25 : 27 120 treatmsn-t _ ~_ ___ ~ _ ~_ : After heat 0 16 0.21 79 180 treatm8nt ~
. : _~ ___ ~ __ .~ After ea O, o8 o. 37 142 45 t~=atment 8~

~c) A 40 ~ thick polyamide (Grilanid L25 Nylon 12) supplied by Grilon Plastic~ Ltd was graft copolymerised to a level of 41.5% with Acrylic Acid in the manner of E~ample 1.
Heat trea-tment was carried out in a bath of Glycerol AR at temperatures of 120C and 190C for 5 minutes. Quenching and drying were carried out as in Example 1 and the properties of the products are given in Table 14 below9 .
Electrical ~ydroxyl ion Equilibrium Wetting out resistanoe permeability water content rate (n .cm2) (ml~Nmin~1) % (secs) ,_ _ ~ _ _ _ . ~
Before heat 0~16 0.10 26 425 treatment After _ _ _ _ heat 0.20 0020 64 580 at 120C

After _ __ _______________ heat 0.04 o.60 915 15 treatment at 190C

The bass polymers used in this example (a) cast high density polyethylene~
(b) cast polypropylene, and (c) nylon 12 have crystalline melting points of 127C, 159 C, and 175C respectively. The crystalline melting points of the respective graft copoly~ers are 127C, 157C, and 174C. The results quoted in Tables 12 to 14 indicate that the best imProvement in properties is obtained w~en the heat treat~ent temperature is above the crystalline melting point o~ ~he copolymer. It has been found that below the crystalline melting point, the results are variable.
* Trade ~ark .

Claims

1. A method of treating a film of hydrophilic polymeric material selected from graft copolymers in which a hydrophilic comonomer is grafted on to a hydrophobic base polymer, sulphonated polymers, and polymers containing a quaternary ammonium group, which includes the steps of a. immersing, at a temperature in excess of 80°C, the said film of polymeric material in a liquid treatment medi?
consisting of a liquid which is mainly organic and contains hydroxyl groups or ions and is capable of swelling the said hydrophilic polymeric material, and b. removing the liquid treatment medium by quenching the said film to about room temperature and washing the said polymeric film in a wash liquid selected from cold water or dilute alkaline solution.

2. A method as claimed in claim 1 and wherein the temperature of the liquid treatment medium is above the crystalline melting point of the said hydrophilic polymeric material.

3. A method as claimed in claim 1 and wherein the liquid treatment medium is selected from alkanols, polyhydric alcohols, ethylene glycols and polyethylene glycols.

4. A method as claimed in claim 3 and wherein the heat treatment medium is selected from glycerol, diethylene glycol and polyethylene glycol.

5. A method as claimed in claim 3 and wherein the liquid treatment medium also contains up to 5% molar of an alkali metal hydroxide or alkaline earth metal hydroxide.

6. A method as claimed in claim 5 and wherein the alkali metal hydroxide is potassium hydroxide.

7. A method of treating a film of hydrophilic polymeric material which includes the steps of a. immersing at a temperature in excess of 80°C, the said film of polymeric material in a liquid treatment medium consisting of a liquid which is mainly organic and contains hydroxyl groups or ions and is capable of swelling the said hydrophilic polymeric material, and b. removing the liquid treatment medium by quenching the said film to about room temperature and washing the said polymeric film in a wash liquid selected from cold water or dilute alkaline solution, wherein the hydrophilic polymeric material is a graft copolymer in which a hydrophilic comonomer is grafted on to a hydrophobic base polymer, and the said base polymer being selected from polyolefins, copolyolefins, condensation polymers, and mixtures thereof, and the hydrophilic comonomer is selected from ethylenic carboxylic acids, ethylenic carboxylic acid amides, ethylenic carboxylic acid amines, and organic bases capable of quaternization.

8. A method as claimed in claim 7 and wherein the said base polymer is selected from low, medium and high density polyethylene, polypropylene, poly-4-methylpentene-1, polyvinylacetate, polyvinyl-alcohol, polyvinylchloride, chlorinated rubber, polytetrafluoro-ethylene, polychlorofluoroethylene, polyamides, copolyamides, saturated and unsaturated polyesters, and mixtures and copolymers thereof.

9. A method as claimed in claim 7 and wherein the said hydrophilic copolymer is selected from acrylic acid, methacrylic acid, itaconic acid, acrylamide methacrylamide, butylamine acrylate; vinyl pyridine and vinyl pyrrolidone.

10. A method as claimed in claim 7 and wherein the said base polymer is selected from low, medium and high density polyethylene, polypropylene, polytetrafluoroethylene and polyamides and the said hydrophilic comonomer is selected from acrylic acid and methacrylic acid.

11. A method as claimed in claim 7 and wherein the temperature of the liquid treatment medium is above the crystalline melting point of the said hydrophilic polymeric material.

12. A method as claimed in claim 7 and wherein the liquid treatment medium is selected from alkanols, polyhydric alcohols, ethylene glycols and polyethylene glycols.

13. A method as claimed in claim 12 and wherein the heat treatment medium is selected from octanol, decanol glycerol, diethylene glycol and polyethylene glycol.

14. A method as claimed in claim 12 and wherein the liquid treatment medium also contains up to 5% molar of an alkali metal or alkaline earth metal hydroxide.

15. A battery separator comprising the product of the process of claim 1.

16. A process for decreasing the electrical resistance, increasing the hydroxyl ion permeability, increasing the rate of wetting out and increasing the equilibrium water content of a cross-linked polymer film which comprises immersing a graft copolymer film on which the grafted copolymer has been prepared by grafting a hydrophilic comonomer on to a hydrophobic base polymer, sulphonated polymer or a polymer containing a quaternary ammonium group at a temperature above 80°C in a polyhydric alcohol, cooling the film to about room temperature and washing the film with cold water or a dilute aqueous alkaline solution.