Polyacrylonitrile fibres have been converted commercially to carbon fibres for a number of years. A known process is described in US Patent 3,533,743, in which the acrylonitrile fibre is oxidised prior to carbonisation and graphitisation. The acrylonitrile fibre is typically placed under tension during the oxidation phase of the process so that essentially carbon fibre is made from continuous filament acrylonitrile fibre. Continuous filament carbon fibre is used as reinforcement in composite materials for the manufacture of high-strength/low-weight composites, used for example in fishing rods, tennis racquets and racing car body parts. Where the carbon fibres are used in short lengths for discontinuous fibre reinforcement, these are obtained by chopping the continuous filament into short lengths.

Fibrillated acrylic fibres are disclosed, for example, in EP-A-0,265,762.

The present invention provides fibrillated acrylic fibres which are suitable for oxidation in the form of staple fibres, and which are copolymers of acrylonitrile and an unsaturated carboxylic acid.

Such fibres are particularly suitable for oxidation to form fibrillated oxidised acrylic fibres. Preferably the fibres are copolymers of acrylonitrile and itaconic acid or methacrylic acid. Preferably the fibre contains 0.5-20 itaconic acid groups. The copolymer fibre may comprise a

further monomer such as methylacrylate, methylmethacrylate or vinylacetate, or any combination thereof. Typically the copolymer contains 3-8% of the further monomer. The staple fibres have a length of about 4-8mm and are fibrillated to a Canadian Freeness value of less than 700 and more typically 350-500.

The fibre has a decitex value of between 0.6 and 6.0 decitex, preferably about 1.2-1.7 decitex.

Also according to the invention there is provided a method of manufacture of oxidisable acrylic fibre in which a staple acrylic fibre is made by copolymerisation of acrylonitrile and an unsaturated carboxylic acid, and is fibrillated in the gel state prior to drying.

The staple fibre is cut to a length in the range of 4- 8mm, preferably 6mm.

Further monomers such as methyl acrylate, methyl methacrylate or vinyl acetate may be added to the copolymerisation.

Preferably the acrylic fibre comprises 0.5-2% itaconic acid, 3-8% methyl acrylate, and 90-96.5 of acrylonitrile.

The itaconic acid or methacrylic acid may be added in the form of free acid or as a sodium or amine salt.

The invention will hereinafter be described by way of example and with reference to the accompanying drawings in which:- Figure 1 is a schematic drawing of various stages in the production of fibrillated polyacrylonitrile fibres, and Figure 2 is a photograph of the fibrillated staple fibres made according to the present invention in Example B after 240 minutes'refining.

A reaction mixture containing 93.5o acrylonitrile, 5°Õ methyl acrylate and 1.5o itaconic acid (°Õ by weight) is mixed in a feed mix vessel (11) with sodium thiocyanate, accelerators and initiators and is then polymerised in solution in a reactor (12). The polymerised polyacrylonitrile copolymer is then wet spun at a spinning cell (13) into a dilute solution of sodium thiocyanate having a specific gravity of 1.052, stretched, washed and cut (14) into staple fibre having a length in the range 4- 12mm, preferably 6mm-8mm.

The polyacrylonitrile is spun and drawn to produce fibre having a decitex value in the range 0.6-6.0 decitex, preferably about 1.2-1.7 decitex.

The staple fibre is then mechanicallv fibrillated in its gel state (never-dried) as a dispersion in the range 0.5-3t fibres by weight in water. The fibre is processed in a suitable fibrillating apparatus (15) such as a disc refiner e. g. a Sprout-Waldron type 105 available from Sprout-Waldron & Co., Muncy, Pennsylvania, USA. The fibrillating apparatus basically comprises a pair of disc- like plates about 30cm in diameter and spaced apart by a variable distance of between 0.2 and 3.75 mm. One plate is fixed and the other plate rotates at a speed of about 3,000 revolutions per minute.

The dispersion is continuously recycle through the apparatus for a number of cycles to increase fibrillation.

In the examples which follow, the refining time is given as a total refining time.

The through-put through the disc refiner can be varied from 50 litres per minute up to 900 litres per minute.

The fibrillated loose staple fibre is then passed into an oven (16) for drying, and is then suitable for oxidation.

The fibre is opened out (17) and then passed into an oven for oxidation. Oxidation may take place with the opened

fibres spread thinly on trays. The polyacrylonitrile (PAN) fibre may be held at 225°C for two hours to produce a fibrillated oxidised PAN fibre containing about 8% of added oxygen (by weight).

Such fibres may be used as reinforcing fillers in other compounds.

The degree of staple fibre fibrillation varied with the processing time as detailed below in the fibrillation tests.

Fibrillation Test 1 The fibrillation obtained over different refining time periods is expressed as water retention and imbibition value measured in accordance with DIN 53814. The results are given in Table 1 and Table 2 below. The degree of fibrillation was determined for the following examples in which 6mm staple fibre was dispersed in 600 litres of water which was continuously passed through a disc refiner at a throughput of 500 litres per minute, with the two plates set 1. Omm apart. Table 1 Water Imbibition RefiningtimeWaterImbibition % (mins)ExampleAExampleB ExampleC 0 106 106 110 30 111 119 60 111 117 131 90 129 121 120 116 143 115 150 161 114 180 168 179 120 210 200 120 240 181 223 121 300 200 360 240 Samples A and B were 1.2 decitex Polyacrylonitrile fibres processed in 1.3% and 2% dispersions respectively, and Sample C was a 4.3 decitex fibre processed in a 2°õ dispersion.

Table 2 Residual Water Content ResidualWaterContent o RefiningTimeExampleAExampleB ExampleC 0115115 125 240mins508 533

Fibrillation Test II Acrylic fibre (6g, staple length 6mm) from Example B and demineralised water (2 1) were placed in the bowl of the standard disintegrator described in TAPPI Standard T-205 om- 88, and disintegrated (simulating valley beating) until the fibre was well-dispersed. Suitable disintegrators are available from Messmer Instruments Limited, Gravesend, Kent, UK and from Buchel van de Korput BV, Veemendaal, Netherlands. The Canadian Standard Freeness (CSF) of the fibre in the resulting slurry or stock was measured according to TAPPI Standard T227 om. 97 and recorded in ml.

The stock was divided into two 1 1 portions for measurement of CSF and the two results averaged.

The results of CSF value versus refining time are given in Table 3 below.

Table 3 Canadian Standard Freeness RefiningtimeCSF(ml) (min) 0 60692 120646 240358 The results of the water imbibition and Canadian Standard Freeness test show a correlation between refining time and increased fibrillation. An increase in fibre content in the dispersion being processed also produces an increase in fibrillation over a given process time.