GB2184737A - Manufacture of fluorinated copolymers - Google Patents

Abstract

In a process for the preparation of copolymers of vinylidene difluoride (VDF) and trifluoroethylene (TrFE) the VDF and TrFE are subjected to bulk polymerisation in the presence of an organic peroxide or azo compound and in the substantial absence of oxygen, at a temperature of from 110 to 170 DEG C and at elevated pressure. The copolymers generally consist essentially of VDF and TrFE wherein the VDF and TrFE are present in molar ratios of from 50:50 to 75:25. The resultant copolymers are soluble in common organic solvents and exhibit piezoelectric and pyroelectric properties without the need for stretching.

Description

SPECIFICATION Manufacture of fluorinated polymers This invention relates to the manufacture offluorinated polymers and in particular to a process for the preparation of copolymers of vinylidene difluoride (VDF) and trifluoroethylene (TrFE).

Homopolymers of VDF have been used widely as an anticorrosion lining for chemical equipment and more recently have been used in electronic equipment. The latter uses of polyvinylidene difluoride (PVDF) have arisen from the discovery of the piezoelectric and pyroelectric properties of one particular crystal form of PVDF. PVDF can exist in two principal crystal forms (alpha and beta phases) but it is onlythe beta phase which has pronounced piezoelectric and pyroelectric properties. PVDF is a thermoplastic material and when extruded from a melt the polymer is essentially in the alpha form. The alpha crystalline form can, however, be converted conventientlyto the beta phase by mechanical orientation of the polymer. This can readily be carried out by stretching of a film of the polymer, either uniaxially or biaxially.The piezoelectric and pyroelectric properties are developed by subjecting the stretched film to poling. Although PVDF has found app lications which makes use of the above electrical properties, (e.g. as transducers), it has the disadvantage that the beta crystalline phase is less stable than the alpha and tends to revert to the alpha form attempera- tures in the region of 10000. Also, PVDF is insoluble in most common solvents which limits the ability to process the material.

Recent work has shown that copolymers of vinylidene difluoride and trifluoroethylene also possess piezoelectric and pyroelectric properties when subjected to polarisation, see the article by Furukawa and Johnson published in the Journal of Applied Physics 52 (2 February 1981) page 940. This paper reports the investigation of the electrical properties of a copolymer of VDF and TrFE containing 55 mol.% of VDF and 45 mol.% of TrFE. It was found that this polymer had advantages over PVDF in thatfirstlythe crystal form in which it is obtained from a melt possesses piezoelectric and pyroelectric properties on polarisation and that this crystal form appears to be its most stable crystalline form of the copolymer.Secondly, VDF - TrFE copolymers are soluble in several common solvents including acetone, methyl ethyl ketone (MEK), dimethyl formamide (DMF), tetrahydrofuran (THF), ethyl acetate, dioxan, and dimethyl acetamide..

The standard process for production of PVDF is by emulsion polymerisation usually initiated by a water soluble peroxy compound, e.g. persulphates, and in the presence of a chain transfer agent. Patents relating to PVDF production also recommend emulsion polymerisation for preparation of VDF copolymers with other fluorine-containing monomers, see, for example, U.S. Patent No.4,360,652 and European PatentApplication publication No.0073295. However, our investigations have indicated that emulsion polymerisation is not suitable for producing VDF-TrFE copolymers having the desired electrical properties. This may be because in emulsion polymerisation, the polymerisation takes place in the aqueous phase and water soluble materials such as surfactants and water soluble initiators are employed which are difficult to removefrom the vinyl polymer.

We have now discovered that VDF-TrFE copolymers having very advantageous electrical properties can be produced buy a bulk polymerisation process.

According to the present invention there is provided a processforthe preparation of a copolymer ofvinylidene difluoride (VDF) and trifluoroethylene (TrFE) which comprises subjecting VDF and TrFE to bulk polymerisation in the presence of an organic free radical generating initiator, such as an organic peroxide, ata temperature of from about 110 to 17000 and under elevated pressure and wherein the TrFE is reacted in a molar amount which is at least about 20% of the VDF.

Preferably, the VDF is present in at least equal molar proportions with the TrFE and preferred proportions ofVDFto TrFE range from about 40-60 to 80-20. The most preferred proportions are in the range of VDFto TrFE of about 50-50 to 75-25. The most pronounced piezoelectric and pyroelectric properties appear two be attained in copolymers having as VDF content of about 50 to 55%. On the other hand, copolymers containing about 65 to 70% of VDF appearto retain these properties better at high operating temperatures.

The process is carried out in the absence of oxygen and this involves flushing the apparatus free from oxygen with an inert gas such as nitrogen prior to introduction of the reactants. It also appears two be importans to exclude water.

The reaction is carried out at elevated pressure preferably at a pressure of at least about 100 atmospheres and commonly in the range of about 100 to 140 atmospheres. Atypical reaction temperature is about 100to 140 C.

Suitable initiators are compounds which generate free radicals in a temperature range of from about 80 C to 250 C and have a half life at such temperatures in the range of 10 minutes to 10 hours, preferably 1 to4 hours. Examples are organic peroxides and azo compounds. We prefer two employ substantially pure initiators, i.e. without solvent or other additive. Preferred initiators are ditertiary butyl peroxide (available as Trigonox B) and butyl hydroperoxide (availabie as Trigonox A80) and hexamethyl tetroxa-cyclononane (otherwise known as H.M.C.N.).

In the practice of the invention, the initiator is added to a stainless steel autoclave and the gaseous monomers VDF and TrFE are measured using a graduated tube cooled in liquid nitrogen and then distilling the monomer in turn into the autoclave while similarly cooling the autoclave in liquid nitrogen. The detailed procedure was as follows using a stainless steel 50 ml. autoclave fitted with a glandless stirrer, pressure gauge and thermocouple. The autoclave was linked to a vacuum manifold including a glass graduated tube for measuring the volume of liquified and solidified monomers by volume.

The initiator choice (normally ditertiary butyl peroxide available from Akzo Chemie as Triganox B)was placed in the base of the autoclave which was assembled and connected to the manifold. The system was evacuated and purged with oxygen-free nitrogen and the process repeated at least six times. The system was once more evacuated,the autoclave and pressure vessel sealed and VDF orTrFE admitted to the system and condensed in a graduated glass tube. The glass tube wasthen heated to evacuate the monomer which was then condensed in the pressure vessel which was then sealed and removed for weighing. Finally, the pressure vessel was reconnected to the manifold and the monomer released into the system and condensed in the autoclave.The autoclave was then sealed and the same procedures used to transfer the second monomer to the glass tube and thence to the pressure vessel for measurement by volume and weight. The autoclave was then reconnected and the second monomer condensed into it.

The autoclave was then sealed and heated to the reaction temperature. Stirring was commenced when room temperature was reached. Temperature and pressure were recorded during the course ofthe reaction.

As the monomers were converted to polymers the pressure in the autoclave dropped. This was taken to indicatethatthe polymerisation was nearing completion. In some runs, stirring became difficult asthe visco- sity increased close to completion of polymerisation. Although stirring could be continued when operating at higher reaction temperatures, e.g. in the vicinity of 1700C, it did not seem to essential to maintain stirring towards the end of the reaction.

Table 1 indicates the proportion of reactants during eight runs. In general, the reaction temperature was between about 120 and 180"C. The pressure in the reactor varied between about 25 atmospheres to 150.

Normally, the initial pressure was in the region of 100 atmospheres and was reduced to around 40 at the end of the reaction. A reaction time of about 10 hours ways generally found to be satisfactory. Table 2 indicates a series of reaction temperature pressure and reaction duration procedures which were followed. The product in each case was soluble in MEK and acetone and solubility in these solvents couid be used to remove impurities. On reducing the pressure in the reactor, unreacted monomer was distilled off and measured.

Table 2 indicates in the last column the amount of unreacted monomer.

Using polystyrene as a standard, the weight average molecular weight of the resulting polymers ranges from about 180,000 to about 500,000. However, from measurements of number average molecularweights and weight average molecularweights, there appears two be a fairlywide distribution of molecularweights, the poly disperse factor being generally in the range or ofabout4 to 6.

Products produced as described above were analysed by NMR and these showed a close correlation between the calculated percentage of VDF in the polymer compared with the initial monomer used.

Polymer produced in accordance with the processes ofthe invention may be extruded or injection moulded or cast into films from a solution. Samples after poling were fond to exhibit pyroelectric and piezoelectric propertieswhich, in the case ofthe latter properties, are significantly greaterthan the corresponding values for PVDF. Details offilms cast from the copolymers are given after Tables 1 and 2 below.

TABLE 1 Quantity Run No.

1 2 3 4 5 6 7 8 vol VDF 20.0 20.0 10.0 22.0 20.8 21.2 22 23 wt VDF(x) 26.6 26.6 13.3 29.3 27.7 30.2 29.3 30.6 %wt VDF 51.2 50.8 51.2 51.6 64.6 61.2 64.1 62.7 vol TrFE 15.4 15.6 7.7 15.6 9.2 11.8 10 11 wt TrFE(y) 25.4 25.7 12.7 27.4 15.2 19.1 16.4 18.2 %wt TrFE 48.8 49.2 48.8 48.4 35.4 38.7 35.9 37.3 mole % x::y 57.4/42.6 57/43 57.4/42.6 57.7/42.5 70/30 67/33 69.4/30.6 68.5/31.5 initiator Trigonox Trigonox HMCN Trigonox Trigonox Trigonox Trigonox Trigonox type A80 A80 30AL B B B B B Trigonox A80 11 drops B Initiator vol. 18 drops 20 drops 28 drops 11 drops 22 drops 22 drops 22 drops 22 drops A80 Initiator .1g Trig.B wt. 0.2g 0.2g 0.25g .1g trig 0.2g 0.2g 0.2g 0.2g A80 TABLE 2 Reaction Temperature Pressure (Atmospheres) Time Hours %wt Programe monomer unreacted Maximum Final (C) To completion To end of Reaction 1 jr/130, 1hr 135 C, 1/2hr 141 1/2hr 141, 3/4hr 145, 1/2hr 153, 11hr 160C, 6 1/2 hr 170C 134 45(172) 10 hr 10 hr 30 4 1/2hr 130, 1/2hr 140,5hr 150 1/2 hr 180C 120 40 11 hr 6-8 hr 20 21 hr @ 126C 108 24 21 hr 12-20 7 33 hr @ 126C 114 33 18 7-15 10 21 hr @ 126 105 18 21 14 6 Electrical Properties The electrical properties ofthe copolymers were evaluated by casting films of the copolymers from solution in an organic solvent, e.g. a ketone such as acetone or M.E.K. Solutions having a suitable viscosityfor casting using a spreader bartechnique can be prepared by dissolving 10 to 20 grams of copolymer in 100mis of M.E.K., e.g. 15 grams per 100ml of M.E.K. Films can be cast in conventional fashion onto a suitableflat substrate, from which the dried film can be released using a spreader spaced to provide a wet film thickness of about 100 m. The cast films were dried in airat room temperature for 15 minutes and subsequently oven dried at about 60 C.

Poling of the dried films can be achieved by vacuum evaporation of an aluminium coating onto each side of the film and attaching wires to the aluminium electrodes so formed using a conductive elastomer. The aluminium coated films can be poled by conventional techniques using a silicone oil bath. Electrical fields of various strengths can be applied at different temperatures by heating the oil bath. Generally, field strengths of 1 5 to 35 MVm-1 are satisfactory. For copolymers having VDF contents of about 50 mole per cent, satisfac- tory piezoelectric properties can be induced in the films at a temperature in the region of 70 to 75 C. In the case of copolymers containing higher propertious of VDF, higher poling temperatures up to about 100 C can be employed.However, there may be a relationship between poling field strength and temperature in that at lowertemperatures higher poling fields can be employed without electrical breakdown occurring. Similarly, at highertemperatures poling fields may need to be correspondingly lower.

The piezoelectric properties of cast films of copolymers produced in accordance with this invention were determined using conventional techniques (see G.M.Sessler: J. Acoust Soc. Am 70(6) 1981 pages 1596-1608 and paper by K.J.Humphrey et al presented to the International Symposium of Applications of Ferroelectrics (ISAF) June 1986).

The results are given in the following Table 3 which also gives figuresforthe homopolymer of VDFafter uniaxial stretching. The measurements for PVDF are taken from the Sessler paper and the measurements given for the VDF:TrFE copolymers in accordance with this invention were made by K.J. Humphrey et al and also appear in the K.J. Humphrey et al paper.

TABLE 3

PVDF Funiaxially 56:44 70:30 stretched) VDF: TrFE VDF: TrFE Tc( C) > melting temp. 70 99 tan 8 0.02 0.021 0.025 Er 11 ~ 11 10.7 d31 (pCN-1) 17.9 21.4 17.1 d32(pCN-1) 0.9 21.4 17.1 (1) H (pCN-1) -8.3 -9.2 -6.4 333 (pCN-1) -27.1 -5.2 40.6 p ( cm-2K-1) 27 20 In the above Table, Tc = Curie temperature d31 and d32 are longitudinal and transverse piezoelectric coefficients, respectively.

dH= hydrostatic piezoelectric coefficient d33 = piezoelectric thickness coefficient.

p = pyroelectriccoefficientat2OC As can be seen the piezoelectric properties are generally superiorto those of the PVDFfilm. Moreover, the 70:30VDF:TrFE copolymer has superior temperature stability and retains its piezoelectric properties on extended storage.

The pyroelectric coefficient of the 56:44 VDF:TrFE copolymer at room temperature is slightly lowerthan that of the VDF homopolymer. However, the use of VDF:TrFE copolymers may nevertheless be advantag emus for pyroelectric applications because the copolymer does not require two be stretched and this precludes the use of the hompolymer in applications which require forms otherthan films.

Claims (11)

1. A process for the preparation of a copolymer of vinylidene difluoride (VDF) and trifluoroethylene (TrFE) which comprises subjecting VDF and TrFE to bulk polymerisation in the presence of an organic, free radical generating initiator at a temperature from about 110 to 170 C and under elevated pressure and wherein the TrFE is reacted in a molar amount which is at least about 20% of the VDF.

2. A process according to claim 1 in which the VDF and TrFE are present in the polymerisation mixture in the molar proportions offrom 40:60to 80:20.

3. A process according to claim 2 in which the VDF and TrFE are present in the molar proportions offrom 50:50 to 75:25.

4. A process according to any one of the preceding claims which is carried out in the substantial absence of oxygen.

5. A process according to any one of the preceding claims which is conducted at a pressure between about 25 and 200 atmospheres.

6. A process forthe preparation of VDF :TrFE copolymers substantially as described with reference to the Examples.

7. Copolymers of VDF:TrFE when prepared by the process claimed in any one of the preceding claims.

8. Asolution of a copolymerof VDF:TrFE, produced by bulk polymerisation in an organic solvent wherein the copolymer consists essentially of VDF and TrFE in the molar ratio of from 50:50 to 75:25.

9. A solution according to claim 8 in which the solvent is a ketone.

10. A method offorming a copolymerfiim having piezoelectric properties which comprises casting on a surface a film of a solution as claimed in claim 8 or 9 and, afterthe solvent has evaporated, subjecting thefilm to poling in an electrical field.

11. AVDF:TrFE copolymerfilm having piezoelectric properties whenever produced by the method claimed in claim 10.