Blended-fibre filter material.

0001This invention is a filter material of blended fibres. The filter material may find application in dust helmets, respirators, suction cleaners and air-conditioning systems as a disposable filter element, and even as a duster.

0002In air filters, a low resistance to air flow is desirable, coupled with, of course, a high filtration efficiency (= a high probability that a particle borne in the air flow through the filter will be captured by the filter). These properties can be met by a filter material having an open structure but carrying electric charge, to attract dust particles. Such a filter material is described in British Patent Specification 384052. wherein wool fibres are carded with resin particles. In this filter material, the action during manufacture of rubbing the wool against the resin imparts a persistent negative charge to the resin and an induced positive charge to the wool. These charges attract dust particles.

0003It would be desirable to find an air filter with a high filtration efficiency, which is reasonably priced, which has a low resistance to air flow and which is non-flammable.

0004According to the present invention, a filter material comprises a blend of (i) clean polyolefine fibres and (ii) clean fibres of a substituted addition polymer, preferably a polymer comprising one or more substituted hydrocarbons; the substituent may be -CN or a halogen (preferably fluorine or chlorine), and the hydrocarbon may be a polyolefine, preferably polyethylene or polypropylene. The fibres (ii) could thus contain for example one or more of chloroethylene units, for example -CHCl-CH₂- or -CCl₂-CH₂-, or acrylonitrile units -CH₂-C(CN)H-, or tetrafluoroethylene units -CF₂-CF₂-, and could thus be for example acrylic, polyvinyl chloride, polyvinylidene chloride, vinyl chloride - vinylidene chloride copolymer, chlorinated polyvinyl chloride, polytetrafluorethylene or (preferably) modacrylic. By "clean" we mean that the fibre has no coating of lubricant or anti-static agent, or that any such coating was removed before blending, for example by scouring with a non-ionic detergent. By "modacrylic" we mean a copolymer comprising from 35 to 85 weight percent acrylonitrile units and preferably having the balance made up substantially of other addition-polymer-forming units, preferably vinyl ester or halogenated hydrocarbon such as vinyl chloride or vinylidene chloride.

0005The polyolefine fibres (i) may be polyethylene, ethylene-propylene copolymer, or (preferably) polypropylene.

0006The modacrylic is preferably Kanekalon (trade mark) by Kanegafuchi Chem. Co. and Kanekalon Co. of Japan or Teklan (trade mark) by Courtaulds. Kanekalon comprises 55 weight % vinyl chloride. Teklan comprises 50 weight % of vinylidene chloride and under 2 weight of a third monomer for providing dye sites. The balance in both cases is substantially acrylonitrile. The modacrylic preferably comprises at least 40, more preferably at least 44, most preferably at least 50 weight % acrylonitrile.

0007The ratio of fibre (i) to fibre (ii) is preferably from 30:70 to 80:20 by surface area, more preferably from 40:60 to 70:30.

0008The fibres are preferably finer than l00 micrometres. preferably from l0 to 25 micrometres and preferably the fibre (i) cross-sectional area does not differ from the fibre (ii) crosssectional area by more than a factor of 3. Staple length is preferably from 30 to 200 mm, more preferably 40 to l00 mm. The fibres (i) or (ii) or both are preferably crimped.

0009Preferably the material is a felt, preferably made by carding the fibres into a fleece and needling them. The carding is preferably performed by passing the material over a conventional two-swift card, or to substantially the equivalent extent by any alternative means. (The carding is likely to impose a practical limit on the fine-ness of usable fibres. The finer the fibres, the more efficient the filter but the higher the resistance to air flow.) The carding step must not be allowed to contaminate the fibres with anti-static agent or lubricant.

0010The invention will now be described by way of example.

0011The filter material is a mixture of two different crimped synthetic polymer fibres carded into a fleece and then needled to form a felt. The fibres are well mixed, and they became electrically charged during carding. The electric charge that they hold is critical in effecting the capture of small dust particles by the material.

0012The first fibre is polypropylene 20 micrometres thick, which in commerce is 2.8 decitex (n tex = n g/km). This may be e.g. round or trilobal. Polypropylene from various manufacturers has been tried, and found to be fairly consistent. Pigmented polypropylene is acceptable. Commercially produced polypropylene fibres normally have on them a "spin finish", which is a mixture of lubricant and anti-static agent. Polypropylene can be made finish-free quite easily, but usually it is not. This finish is removed before the fibres are carded. Any of the usual processes of textile scouring could be used, such as detergent, alkali or solvent scouring; scouring with a non-ionic detergent is quite effective provided it is thorough. If the fibres are moderately clean the filter will be moderately good. Then the fibre is well rinsed.

0013The second fibre is modacrylic l8 micrometres thick, in particular Teklan (described above) of 3.5 decitex. Dyed modacrylic fibre is acceptable but undyed modacrylic fibre is preferred. Commercial modacrylic fibre cannot in practice be made without a "spin finish", which (as with polypropylene) is a mixture of lubricant and antistatic agent. This finish is removed before the fibres are carded. Any of the usual processes of textile scouring could be used, such as detergent, alkali or solvent scouring; scouring with a non-ionic detergent is quite effective provided it is thorough. If the fibres are moderately clean the filter will be moderately good. Some benefit may be obtained from an alkali scour or chemical leaching. Acid scouring is not recommended.

0014These fibres are carded using a two-swift card, each swift having four worker-stripper units, in weight proportions 60 polypropylene: 40 modacrylic, i.e. to give surface areas roughly in the ratio 2:l polypropylene: modacrylic, into a fleece. The carding causes both fibres to acquire electric charge, which, after falling during the next day, thereafter remains at a fairly constant level for several months. The fleece has a mass of l.5 kg per square metre.

0015The fleeces are lightly needled. using a needle-punching loom, at 8mm penetration, 25 needlings/cm², to form a felt. Depending on the end use, where a stiffer or denser felt is wanted, heavier needling may be preferable.

0016The fibre processing machines (i.e. card and needle-loom) are kept clean to avoid contaminating the fibres, especially with spin finish from previous batches. Although not strictly essential, preferably all-metal card clothing is used, and if the needle-loom has boards with wooden backs, a new board should be used. Card clothing is usually contaminated with oil, and this must be removed.

0017The fibres may be (coarsely) mixed using a blender before the carding. The blender should likewise be kept clean. The carding completes the mixing, but if this is inadequate laterally across the fleece, a cross-laying stage can be interposed between the swifts, as is known practice.

0018The resulting filter material has low flammability, and may find use in dust helmets, disposable respirators and elsewhere.

0019The performance of the filter depends on its packing fraction (proportion of its volume occupied by fibres). Thus, the penetration of aerosols in general decreases (i.e. the filter is more effective) and the pressure drop increases (i.e. air flow is more impeded) as the packing fraction is increased at constant filter mass per unit area.

0020At a given packing fraction and mass per unit area, filters made from finer fibres are more effective than those made from coarser, but they also have a higher resistance to airflow. The choice of fibre diameter is finally governed by the purpose for which the filter is intended, but it is likely that the most useful will be made from the finest fibres that can be carded without any difficulty.

0021A 'standard' aerosol (to B.S.4400 of l969) at a fixed filtration velocity (0.28 m s⁻¹) was used on filters according to the invention on polypropylene/wool filters, and on filters made entirely from polypropylene, the respective filters being matched in a first series of comparative tests as regards pressure drop across them (l00 Pa at the filtration velocity), and being matched in a second series as to mass per unit area (l.5 kg m⁻²).

0022In the first series of tests (constant pressure drop), the penetration of the 'standard' aerosol was 0.07% through the filter of the invention. 0.40% through the polypropylene/wool filters. and l0.2% through the pure polypropylene filter. In the second series of tests (constant filter mass per unit area), the penetration of the 'standard' aerosol through the filter of the invention was 0.032%, through the polypropylene/wool filter was 0.3l% and through the pure polypropylene filters was 7.0%.

0023In a further test, the filter of the invention was encapsulated in a plastic filter cartridge of a type normally used in approved respirators. The cartridge was a short cylinder of approximately l3mm depth and 70 mm diameter. The filter was subjected to a test conforming in all essential features to that part of the British Standard BS 209l test for respirators which comprises measurement of 'standard' aerosol penetration at a volume flow rate of 30 litres/minute through the filters, and measurement of the pressure drop across the filter at a flow rate of 85 litres/minute through the filter.

0024According to the Standard, a filter is rated as Class A if the aerosol penetration is no greater than l0% and the pressure drop is no greater than 200 Pa, and it is rated as Class B if the penetration is no greater than 2% (different criteria are applied to filters of different types; 2% penetration is applied to resin wool filters; it is the most rigorous and the most appropriate for the filter of the invention) and the pressure drop no greater than 320 Pa.

0025l.5 g of the material of the invention encapsulated in this way gave a penetration of 0.9% and a pressure drop of 63 Pa, and 3.0 g of the material of the invention so encapsulated gave a penetration of 0.3% and a pressure drop of l90 Pa. Both filters thus satisfied each class of the Standard.

0026Sufficient time has not elapsed for the stability of the charge on the filters described above to be conclusively established, but from previous experience it is considered that a substantial increase in aerosol penetration is likely in the first 24 hours after manufacture, followed by a much slower increase. Penetration may double in the first 24 hours; from the 24 hour value it may increase by a factor of l.5 in one month and l.7 in one year. The filters used in the previous comparison tests were all therefore at least one day old.