Polymer Composition

The present invention relates to polymer compositions, and the production and use of such compositions.

Polymeric materials are now extensively used for many purposes and in recent years polymers have become available which have high melting or softening points which make them suitable for use at higher temperatures than are attainable using the longer established polymeric materials. Polyarylether derivatives such as polyaryletherketones are examples of such polymers which, in addition to having high service temperatures have other desirable properties such as electrical insulation characteristics. Polyaryletherketones have good electrical insulating properties and high melting or softening temperatures. Some polymers of this type can be used as en electrical insulator continuously at temperatures in excess of 200.degree. C., for example up to about 250.degree. C. However it will be appreciated that the period of use at elevated temperature is dependent on the temperature and decreases as the temperature is increased. Although the polyaryletherketones have got properties which make them suitable for use as high temperature electrical insulators, many of these polymers have a high flexural modulus and hence may not be entirely suitable for use as a wire coating if the coated wire requires pliability. It has been found that certain additives, for example fluorinated ethylene-propylene copolymers and ethylene-tetrafluoroethylene copolymers, can improve the pliability of the polymer. However, when compositions containing these additives are used for wire coating, the coating shows reduced coherency and had a tendency to crack or even fall apart.

According to the present invention there is provided a polyaryletherketone composition which has a pliability which is such that a copper wire of mean diameter 1 mm, when coated with the composition to give a coating of 0.25 mm average thickness, shows a recoil, after bending through 90.degree., of not more than 40.degree..

Pliability is determined by bending annealed, coated wires over a 6 mm diameter mandrel. The load applied is increased until it is sufficient to bend the coated wire through 90.degree.. The amount of recoil is measured after removing, from the coated wire, the minimum load required to bend the coated wire through 90.degree.. For convenience hereafter, the foregoing procedure will be referred to as the "recoil test".

We have found that the amount of recoil is influenced by the load applied in order to bend the coated wire. Accordingly, the amount of recoil is measured using the minimum load to bend the coated wire through 90.degree.. The recoil test is effected at ambient temperature conditions, which are typically from 15.degree. C. up to 30.degree. C. Depending on the composition, the load required to bend the coated wire through 90.degree. may be one kilogramme or greater. However, with some compositions in accordance with the present invention the minimum load is less than one kilogramme and may be 0.5 kilogramme or even less. Preferred compositions provide coherent coatings on the wire and are suitable for use at temperatures in excess of 200.degree. C. for continuous periods of many weeks.

We have found that compositions having these properties are produced by the addition of boron nitride to the polyaryletherketone.

Thus, according to a further aspect of the present invention there is provided a polymer composition which comprises a mixture of a polyaryletherketone with boron nitride.

The composition may include other components which are suitable for incorporation into a polyaryletherketone. Such other components include, for example, inorganic and organic fibrous fillers such as glass fibre, carbon fibre and poly-paraphenylene terephthalamide fibre; organic and inorganic fillers such as polytetrafluoroethylene, graphite, mica, talc and vermiculite; and stabilisers such as phosphates.

It is preferred that the total proportion of additives (including boron nitride) is at least 1%, and does not exceed 70%, by weight of the composition and especially does not exceed 50% by weight of the composition. The composition preferably contains from 5 up to 30% by weight of boron nitride relative to the total weight of the composition. Compositions in accordance with the present invention include compositions consisting essentially of 70 to 95% by weight of the polyaryletherketone and correspondingly 30 to 5% by weight of boron nitride.

The polyaryletherketone component of the polymer composition may be a polymer which contains the repeat units ##STR1## or the repeat units ##STR2## alone or in conjunction with each other and/or in conjunction with other repeat units.

The other repeat units which may be present in the polyaryletherketone are typically of the general formula ##STR3## where A is a direct link, a sulphur atom, a divalent hydrocarbon radical or a group

in which

Q is --CO--, --SO.sub.2 -- or --O--;

Q.sup.1 is --CO--, --SO.sub.2 -- or --O--;

Ar is a divalent aromatic radical; and

n is 0, 1, 2 or 3.

In the formula III, if the group A is a divalent hydrocarbon radical it is typically a phenylene group or a dimethylmethylene group (--C(CH.sub.3).sub.2). The aromatic radical Ar is preferably a divalent aromatic radical selected from phenylene, biphenylylene or terphenylylene.

We prefer that the polyaryletherketone is a crystalline polymer containing the repeat units I. Especially preferred are such polymers which are tough and have an inherent viscosity of at least 0.7, as is described in more detail in our European Patent Publication No. 001 879. Inherent viscosity is measured at 25.degree. C. on a solution of the polymer in concentrated sulphuric acid of density 1.84 g cm.sup.-3, said solution containing 0.1 g of polymer per 100 cm.sup.3 of solution. As an alternative to inherent viscosity, an indication of the average molecular weight of the polymer may be obtained by measuring the melt viscosity of the polymer. Preferred polymers have a melt viscosity of at least 0.1 KNs/m.sup.2, which corresponds to an inherent viscosity of 0.7. The melt viscosity is measured using a ram extruder fitted with a 3.175 mm.times.0.5 mm die and operating at 400.degree. C. and a shear rate of 1000 s.sup.-1.

The boron nitride is preferably of at least 99% purity. Preferably the boron nitride is in the form of a powder of small particle size, for example having a weight average particle size of not more than 10 micrometers, particularly not more than 5 micrometers.

The polymer composition may be made by admixture of the two components plus any additional optional components in a suitable mixing machine to effect particle or melt blending. More specifically, the polyaryletherketone and boron nitride, in the form of dry powders or granules or mixtures thereof, can be mixed together using a suitable solids blending technique such as tumble blending or a high speed mixer. The blend thus obtained may be extruded into a lace which is chopped to give granules. The granules can be used to produce shaped articles by the use of a suitable forming operation, for example injection moulding or extrusion, to give a shaped article.

The shaped article obtained from the polymer composition is preferably an electrical conductor on which the polymer composition is the insulating layer or is a sheath for one or more insulated conductors. If desired, the polymer composition can be used both as the insulating layer and the outer sheath.

The formation of the insulating layer or outer sheath can be effected using standard wire coating conditions with the exception that the extruder is operated at a higher temperature than is usual with most plastics materials, for example at a barrel temperature of at least 340.degree. C. and preferably with a die temperature which is at least 375.degree. C., and which may be in excess of 400.degree. C.

Wires insulated with a polymer composition of polyaryletherketone and boron nitride are more pliable than wires insulated with the polyaryletherketone only.

Some of the compositions of the present invention have been found to have a lower coefficient of friction and a higher limiting PV value (as defined in Notes to Table Seven) than either the polymer alone or a blend of the polymer and the same proportion by weight of poly-tetrafluoroethylene. Such a combination of properties makes the composition suitable for use in bearing applications. Thus, the polymer compositions may also be used as a bearing or as a bearing liner.

Thus, as a further aspect of the present invention there is provided a plain bearing formed of a polymer composition as herein described or comprising a metal backing having a lining formed of a polymer composition as herein described.

If the composition is used to form a plain bearing this may be done by injection moulding the composition onto the bearing. Alternatively, the composition may be extruded into the form of a rod, the centre of which is machined out to form the bearing.

The composition may also be used as a bearing liner on a metal backing. The metal backing may be any suitable metal such as, for example, aluminium alloy or steel with a suitable surface layer to assist in bonding the lining to the backing. The lining may be a thick coating of 0.75 to 35 mm in thickness but may be an overlay, for example of 10 to 30 micrometers, applied to a metal bearing lining. The polymer composition is preferably applied to the metal backing as a sheet. The polymer composition may be extruded to form a sheet and the sheet issuing from the extruder then bonded to the metal backing as a second stage of a single operation. Alternatively, a preformed sheet of the polymer composition is pressure bonded or roll-bonded to a heated metal backing. The temperature of bonding the polymer composition to the metal backing is preferably sufficiently high that the composition can flow under the conditions of bonding to the backing and typically the temperature is at least 350.degree. C. The coated backing is preferably allowed to cool slowly from the temperature of bonding to ambient temperature to permit the polyaryletherketone component of the polymeric composition to attain a high crystallinity, for example at least 30%.

The polymer composition also has improved burning characteristics compared to the respective polymer alone. More specifically, the polymer composition has a higher oxygen index compared to the respective polymer alone.

The present invention is illustrated by the following examples and test results.

Boron nitride (obtained from Borax Consolidated Borides Division, Lake Road, Newport Industrial Estate, Gwent, NPT OSR, Great Britain) and polyetheretherketone (`Victrex` (Registered Trade Mark) PEEK aromatic polymer 450 P grade, obtainable from Imperial Chemical Industries PLC) were dry mixed by tumble blending for 15 minutes in the proportions of 15% by weight of boron nitride and 85% by weight of polyetheretherketone.

The blend was then formed into granules by extrusion into a lace which was cooled by passing into a water bath and the lace was then cut to form granules. The extrusion was effected using a single screw Plaston extruder having a 3.8 cm diameter screw with a length to diameter ratio of 24:1 and having the following temperature profile from the hopper end to the die 345.degree. C., 360.degree. C., 365.degree. C., 370.degree. C. and 380.degree. C. The die was of 5 mm diameter and the extruder was operated at a rate of about 5 kg/hour.

The procedure of Example 1 was repeated using different proportions of boron nitride to give compositions containing 5% by weight of boron nitride (Example 2) and 10% by weight of boron nitride (Example 3).

The composition of Example 1 was coated onto a 1 mm diameter copper wire using an 18 mm Bone extruder fitted with a Betol designed tubing wire coating die with total diameter of 1.5 mm. The wire was passed through the die at a rate of about 15 m/minute and the extruder was operated at a rate of about 1.6 kg/hour to give a coating of average thickness 0.25 mm. The die temperature for wire coating was about 400.degree. C.

For the purposes of comparison, samples of the copper wire were also coated with the polymer (polyetheretherketone) alone.

The coated wires were then subjected to tests of continuous use temperature, solvent resistance and pliability and the results of these tests are set out in Tables One to Three respectively.

Injection moulded samples of the compositions of Examples 1 to 3 were subjected to limiting oxygen index, smoke density, toxic gas emission and flammability tests and also to thermo-gravimetric analysis. The results are set out in Tables Four to Six. Specimens were cut from discs 11.4 cm diameter and 3.2 mm thick, injection moulded using an injection moulding machine operating at 380.degree. C. with a mould at about 150.degree. C.

ND means that the proportions of these gases were below the lower limits of detection of the test, that is <1 ppm of SO.sub.2 and <2 ppm of HCN.

Examination of the PEEK+BN sample of Example 1 which had been exposed in the NBS smoke chamber for 20 minutes showed a pronounced intumescent effect and a hard char.

A quantity of granules obtained by Example 1 was subjected to thermo-gravimetric analysis between 200.degree. and 500.degree. C. using a temperature rise of 5.degree. C. per minute. The total weight loss from the polyetheretherketone/boron nitride composition produced by the process of Example 1 was about 2% up to a temperature of about 520.degree. C. (PEEK shows a total weight loss of about 3% under the same test conditions).

Test specimens of about 10 mm.times.10 mm.times.3.0(.+-.0.5) mm in size, cut from the injection moulded discs, were subjected to tribilogical testing. The test results are set out in Tables Seven and Eight.

The compositions of Examples 2 and 3 were coated onto 1 mm diameter copper wire in the manner described previously in respect of the composition of Example 1. For the purposes of comparison, the copper wire was also coated with different grade of the polymer (polyetheretherketone) alone.

The tests described previously and reported in Tables One to Three were repeated and the results of these additional tests are set out in Tables Nine to Eleven.