WO1992008826A1

WO1992008826A1 - Metal-coated fibres

Info

Publication number: WO1992008826A1
Application number: PCT/GB1991/002040
Authority: WO
Inventors: James Zorab; Graham Athey
Original assignee: James Zorab; Graham Athey
Priority date: 1990-11-16
Filing date: 1991-11-18
Publication date: 1992-05-29
Also published as: AU8918091A; GB9024910D0

Abstract

Metal-coated fibres are produced by unwinding a roll of uniaxially oriented polymer film, metallising the film, rewinding the film, and unwinding the resulting roll of metallised film through a fibrillation zone in which the film is deformed and split so as to cause separation of the film into a fibrillar network, which may be further opened by carding or the like. The resulting material comprises fibrils of denier 1 to 50 each having parallel flat surfaces, one or both of which is metallised, the thickness and width each being 5 to 100 micrometres with the average width being no more than four times the thickness.

Description

Metal-Coated Fibres

The present invention is concerned with metal-coated fibres (the term "metal- coated" encompassing alloy-coated and metal oxide coated in addition to coated within the pure metal).

There is a growing need for fibres having all or part of their surface covered with "metal coatings". The main reason for this metal coating is to provide a low emissivity surface which has the ability to reflect electromagnetic radiation.

There are several known types of fibrous material with metal coatings, amongst which are continuously coated fabrics, needle-punched metallised fibres, metal- coated fibres, and tape-like elements produced from metal-coated film.

Continuously coated fabrics have a generally woven fabric base with a continuous coating of metal. Woven fabrics however have very rough surfaces and metallised woven fabrics are therefore very inefficient reflectors for radiant heat; furthermore such fabrics are substantially air-impermeable and therefore cannot "breathe".

Needle-punched metallised fibres are produced by aluminising non-woven substrates and which are then needle-punched with polyester wadding to provide a measure of loft. Again the metallised surfaces are rough and are not efficient in reducing radiant heat transfer. In addition they are inextensible and when used as a filling for clothing do not give sufficient stretch.

Metal coated fibres are manufactured along conventional lines of melt spinning by extrusion through spinnerets. The drawn fibre is then subjected to a chemical treatment to apply a fine metal coating along its entire surface. This process is tortuous, time consuming and expensive. Products made via this route account for only 1-2 hundred tonnes per annum, even though the coated fibres have highly desirable and useful textile properties. Tape-like elements produced from metal-coated film are produced by vacuum coating of plastics film with aluminium and then slitting or shredding the film into elements of width typically of the order of 1 mm, although widths as low as 0.05 to 0.1mm have been reportedly produced by a slitting process in Japanese patent specification 58/31134 (Yasuoka). When the film is slit, generally into continuous lengths, the tape can be woven to produce a fabric with a low emissivity surface (as described in British patent specification 2216067). Whilst this process is considerably cheaper than the production of metal-coated fibres, it does not result in the creation of a conventional fabric. The resulting fabrics have a harsh handle, poor drape, low extensibility and suffer from a high level of noise when flexed. These tapes cannot be used as filling fibres.

When the film is shredded, generally on a machine such as an industrial shredding machine, the resulting tapes (as described in, for example, British patent specification 1605045) have an intense high frequency crimp along their length and serrated edges. However, such tapes are harsh, have low bulk, poor drape, low extensibility and suffer from a high level of noise when flexed. They do not have typical fibre properties and cannot be handled on conventional textile equipment or subsequently blended with other textile fillings. In addition when the material is cut using conventional cutting techniques the fabric sheds fibre at the edge where it is cut.

We have now devised a metal-coated fibre material which has many of the advantages of metal-coated preformed fibres, without the uneconomic production costs, and substantial improvement in textile fibre properties over the tape-like elements produced by slitting or shredding metal-coated film.

According to the invention, there is provided a method of producing metal- coated fibres, which comprises providing a roll of uniaxially oriented polymer film, unwinding and metallising said film, re-winding said film to produce a roll of said metallised film, and unwinding said roll through a fibrillation unit in which said film is deformed and split so as to cause separation of the film into a network of elongate fibrils.

The deformation in the fibrillation unit may be twisting (for example, as described in British patent specification 1040663) or surface striation (for example, as described in "Fibre Technology: From Film to Fibre" by Hans A Krassig, published by Dekker ( 1984)). Such surface striation typically involves passing the film under tension against needles or pins provided on a rotating roller, to cause rupture of the film longitudinally (in the machine direction) but without lateral separation or splitting until after the latter has passed downstream of the roller. Such fibrillation is, of course, well known for non-metallised polymer films, where the film is fed in a continuous production run from the extruder to the fibrillation unit and it is one of the perceived advantages of the fibrillation process that it can be operated as an integral part of a continuous operation. It is therefore surprising that an advantageous and commercially valuable product can be achieved even though a substantial departure is made from the conventional fibrillation process, in winding up the uniaxially oriented material for further processing, unwinding the film for further processing (metallising), re¬ winding the film and then unwinding again for fibrillation.

The resulting fibrils may be converted into fabric via any of the multitudinous textile operations: the fibrils have all the hallmarks of conventional textile fibre, i.e. handle, softness, stretch, drape, bulk, elasticity and aesthetics. This process has the major advantage of being a fundamentally low cost production route for metal-coated fibres.

The fibrils have characteristic parallel-sided cross-section as opposed to a circular cross-section of conventional fibre.

According to the further aspect of the invention therefore, there is provided a fibrillar material comprising a plurality of elongate fibrils having substantially flat opposed surfaces, at least one surface having a metal coating thereon, the thickness of said fibrils being 5 to 100 micrometres and a width of 5 to 100 micrometres, the width being no more than four times the thickness, and the fibres having a denier of 1 to 50.

The film is typically of a linear, substantially non-cross-linked polymer, such as a polyolefin, PVC, polyester, polyamide, polyethersulphone or polyetheretherketone (PEEK). A preferred polymer is a polyolefin, especially a propylene polymer (which may be a homopolymer or an ethylene-propylene copolymer with a minor proportion of ethylene). The^' polyolefin is preferably polypropylene with a melt flow index of approx. 8-10 gms per 10 minutes according to ASTM D1238.

The polymer is typically extruded through an oblong die or circular die, water quenched and then stretched in the direction of the extrusion machine (in the case of an oblong die) or blown (in the case of a circular die) to a ratio of between 4: 1 and 10: 1 using hot ovens to soften the film during the process. The resulting film can typically have a thickness of from 5 micrometres up to 100 micrometres; a film thickness of about 25 micrometres is preferable. The film width may typically be up to 2.2m. The film exiting from the extruder may be corona discharge treated or flame treated (taking care not to apply too much heat and thereby de-orienting the molecules) to activate its surface. This treatment improves the adhesion of the metal applied to the film and can enable the fibres to be washed when the latter are used as textile material.

The metal or metal oxide may be applied using known vacuum metallising or sputter coating techniques to a thickness of up to 500 Angstroms which generally gives an optical density in excess of 2.5. Aluminium is one preferred metal for metallising the film in the method according to the invention: other suitable metals include, by way of example, copper, nickel, cobalt, zinc, cadmium, gold and platinum. Some such metals have catalytic properties, and the resulting metallised fibrils may therefore provide an inexpensive way of producing a voluminous structure having a catalytic surface.

The metallised film may then be fibrillated with a very high pin density - typically 20 to 50 pins per linear inch depending on the resulting textile denier required. The tow which is produced can be stretch-broken to produce fibrils of from 30 to 100mm in length. This stretch break also induces bulk in the fibre which is permanent and which serves to provide loft after the fibre has been processed.

Alternatively, if stretch-broken fibres are not required the crimp-free tow may be passed through a mechanical crimper (such as a one inch crimper) at a minimum pressure setting. A metal fibre finish (0.5% Fylube AR) may be applied to the fibre prior to crimping to lubricate the tow and alleviate fibre damage. The crimped tow is then generally cut to 75mm length. The resulting cut fibre (produced from a 25 micron film in the method according to the invention) typically has an average decitex of 15.0 decitex, about 10 crimps per inch and a percentage crimp of 20-25 % .

Subsequent processing of the fibrillar material according to the invention, for example on a textile card, can reduce the fibre length to circa. 50mm and decitex to 12 and in some cases decitex of 3.0 to 12.0. The reduction effect is due to further fibrillating which occurs when the fibre is "worked" on textile opening machinery.

Preferred features of the method according to the invention, and the resulting fibrillar material, will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a metallisation unit for use in the method according to the invention;

Figure 2 is a schematic diagram of a needle roller fibrillation unit for use in the method according to the invention;

Figure 3 is a schematic impression of a fibrillar network obtained according to the invention;

Figure 4 is a schematic impression of the cross-sectional structure of the fibrils in the network; and

Figure 5 is a schematic impression showing the fibrils lengthwise, after they have been subjected to stretch-breaking.

Referring to Figure 1, there is a vacuum chamber 1 provided with an outlet 2 to a vacuum pump (not shown). Within the chamber 1 is a roll 3 of uniaxially oriented film 4, which is passed successively around a pair of guide rollers 5,6, then around a cooled drum 7, then around a series of three guide rollers 8,9, 10 onto a take-up mandrel 11 on which the film is wound to produce a wound roll 12.

Just below cooled drum 7 is an aperture and shutter assembly 13, through which metal from source 14 can impinge on the film surface under vacuum deposition conditions.

Referring to Figure 2, the roll of metallised film 20 (which corresponds to wound roll 12 from Figure 1) is unwound, past an inlet trio of rollers 21 ,22,23, over a heated roller 24 and then over a pinned fibrillation drum 25. The fibrillar network leaving drum 25 passes to stretch-breaking station 26 and then to can coiler unit 27 in which tow is laid in a can 28.

Referring to Figure 4, it will be seen that the fibrils have essentially parallel faces, with the thickness (the spacing between opposed faces) being generally no more than twice the width (the dimension along the face).

Referring to Figure 5, it will be seen that the stretch-broken fibrils have tapered ends; they are not parallel as would be the case with slit films. Furthermore they have continuous variation of their dimensions along the length thereof. The present invention is further illustrated by the following Examples.

EXAMPLE 1

A one metre wide 25 micron clear polypropylene film was extruded through an oblong die and simultaneously uniaxially drawn at a ratio of 7: 1. The resulting uniaxially oriented film was wound into a roll of length approx. 20km. The roll was then taken to a metallising unit and unwound in a vacuum metallising chamber, in which one surface only of the film was activated by corona discharge. A 350 Angstrom coating of aluminium was applied to the one activated surface by vacuum deposition.

The resulting metallised film was wound onto a take-up mandrel, and the resulting roll was then taken to a fibrillation unit, in which the film was drawn at a rate of 20cm per second over a needle roller fibrillation unit having a pin density of 40 pins per linear inch, with the non-metallised surface in contact with the tips of the pins, which drawing was such that the surface of the film was striated but not split until downstream of the pins. The resulting 25 K/tex, crimp-free tow was processed through a can coiler into a tow container.

The 25 K/tex tow was then passed through a conventional one inch mechanical crimper at a minimum pressure setting. A metal fibre finish was applied prior to the crimper (0.5% Fylube AR.) to lubricate the tow and alleviate fibre damage. The crimped tow was then cut to 75mm length.

The cut fibre was found to have an average decitex of 15.0 decitex per filament, a crimp of 10.0 crimps per inch and a percentage crimp of 20-25 per cent.

Subsequent processing of this fibrillar material on a textile card resulted in a reduction in fibre length to circa. 50mm and decitex to circa. 12.0. decitex because further fibrillation occurred when the fibre was "worked" on textile opening machinery.

The resulting fibrils can be blended in any proportion with wool, cotton or polyester fibre and the blend can be carded and made into blankets on standard blanket- making machinery. EXAMPLE 2

40% of the metal-coated tow obtained in Example 1 was blended with 40% 5.6 decitex polyester fibre tow and 20% polyester thermal bond fibre tow prior to crimping and cutting.

The fibre blend was then carded and cross-lapped and converted into a 120 gsm wadding. The wadding was thermally bonded at 120°C using a through-air oven. The wadding was suitable for use in fib refill applications.

EXAMPLE 3

A 25 K/tex tow was produced as in Example 1 then processed on a stretch- break machine to give a sliver of 20grms per linear metre.

The sliver was blended in a ratio of 1 to 20 with standard textile sliver and then converted into textile yarn using standard textile procedures. The yarn was suitable for use in knitted and woven garments where the metallised fibre component produces a fancy or fashion effect.

EXAMPLE 4

Fibre produced as in Example 1 was blended in an amount of 1 % with 99% 5.6 decitex polyester fibre. The blend was carded, cross-lapped and needle punched into a 80 to 130gsm nonwoven fabric. The resulting fabric was suitable for the filtration of materials which are subject to the production of antistatic charges.

EXAMPLE 5

25 K/tex crimpfree tow produced as in Example 1 was precision cut to 0.2mm flock fibre, which was applied as a surface pile to a woven base fabric using standard flocking techniques to give a material having a surface layer pile of reflective and electrically conducting fibres. EXAMPLE 6

Example 1 was repeated except that Ti0₂ was included in the polypropylene and a vinyl lacquer applied to the film at lgsm/nr. The resulting fibrils could be washed at 100°C substantially without metal loss.

EXAMPLE 7

Example 1 was repeated, except that the drawn polypropylene film was activated on both sides using corona discharge. Subsequent film vacuum metallised on both surfaces.

EXAMPLE 8

Crimp-free tow produced as in Example 1 was precision cut to 6mm lengths. The crimp-free fibre was blended with 95-99% cellulose paper making fibres in an aqueous slurry. The slurry was cast into a wet laid non-woven for currency and security paper end uses.

EXAMPLE 9

A thermally bonded blend of metallised fibres produced as in Example 2 was calendered or "hot rolled" to reduce the loft of the wadding by up to 90% . Because the cross-section of these fibres is not circular but rectangular this has the effect of orienting the surfaces of the fibres and at the same time concentrating them at the surface of the wadding itself. The loft of the wadding was further reduced by "hot rolling" to form a "non-woven" fabric.

EXAMPLE 10

A tow of metallised fibres produced as in Example 1 was "stretch-broken" and converted into a "top" and then blended with other textile fibres such as polyester or polypropylene to form a "sliver" . This "sliver" can then be knitted and brushed to produce deep pile and fleece fabrics.

Claims

CLAEVIS:

1. A method of producing metal-coated fibres, which comprises providing a roll of uniaxially oriented polymer film, unwinding and metallising said film, re-winding said film to produce a roll of said metallised film, and unwinding said roll through a fibrillation unit in which said film is deformed and split so as to cause separation of the film into a network of elongate fibrils.

2. A method according to claim 1 , wherein said film is metallised on one face only.

3. A method according to claim 1 or 2, wherein said polymer is predominantly linear and substantially non-crosslinked.

4. A method according to claim 3, wherein said polymer comprises a polyolefin.

5. A method according to claim 4, wherein said polyolefin is polypropylene, preferably with a melt flow index of approx. 8-10 gms per 10 minutes according to ASTM D1238.

6. A method according to any of claims 1 to 5, wherein said oriented film has been stretched in the longitudinal direction in an amount of 4: 1 to 10: 1.

7. A method according to any of claims 1 to 6, wherein the film is corona discharge treated or flame-treated prior to metallisation.

8. A method according to any of claims 1 to 7, in which the network is subsequently opened by processing on textile machinery.

9. A fibrillar material comprising a plurality of elongate fibrils having substantially flat opposed surfaces, at least one surface having a metal coating thereon, the thickness of said fibrils being 5 to 100 micrometres, the width thereof being 5 to 100 micrometres, and on average no more than four times the thickness, and the fibrils having a denier of 1 to 50.