WO1989002948A1 - Fire resistant pile fabrics - Google Patents

Abstract

A process is provided for the production of fire resisting pile fabrics by the conversion of readily flammable precursor pile fabrics, which comprise entirely of polyacrylonitrile fibres in the pile, into pile fabrics which then consist substantially of so called oxidised polyacrylonitrile fibres in the pile. This is effected by subjecting the pile fibres of the precursor pile fabric to the hot drawing action of a rapidly rotating heated and fluted roller followed by the heating of the pile fabric in air at a temperature of 225 5 degrees Celsius for a period of from 5 hours to 7 hours. Pile fabrics so treated incandesce under incident flame. There is little smoke release or fibre melting. The excellent initial properties of thermal insulation and resistance to attack by microorganisms are retained. Pile fabrics so treated retain these desirable properties following exposure to severe elemental conditions and following repeated field maintenance procedures such as washing and dry-cleaning.

Description

Fire resistant pile fabrics

This invention relates to fire resistant fabrics and to a process for the production of fire resistant fabrics.

With the removal of asbestos based products from many end-uses there is considerable interest in the development of textile fabric alternatives, which will provide a good level of protection to persons and property in fire risk situations not only from the heat and flames but also from the even more dangerous smoke release, which occurs during the heating and eventual combustion of conventional textile materials and other materials with which they are in contact, for example upholstery foam blocks.

It is known from early work on the production of carbon fibres from precursor polyacrylonitrile fibres in the hot drawn molecular aligned condition that there is an intermediate temperature range in the heating process, which if maintained for a sufficiently long period of time will provide fibres, which show a high degree of fire resistance and with very low smoke release when a flame impinges on the fibres. The precise mechanism of their formation is not fully understood and various molecular structures have been proposed for the heated form which call for a high degree of molecular alignment and crystallinity within the fibres. Such fibres are generally known as oxidised polyacrylonitrile fibres. However, such fibres are brittle when subjected to combined twisting, bending, compressive and tensile forces experienced by textile fibres in general during their conventional processing into slivers, yarns and fabrics made from them. High fibre cost per unit of weight, fibre breakage and slippage due to lack of crimp after hot drawing, leading to processing difficulties and waste has restricted the textile use of such fibres and carbon fibres to blends with other fibres, which provide a degree of support to the former fibres during fibre to yarn to fabric textile conversion processes. However, the very low smoke release property of the oxidised polyacrylonitrile fibres and carbon fibres is largely offset by the presence of these other fibres.

Many fire resistant fabric types and combined fabric laminations have been proposed and developed. In general, such fabrics are flat fabrics and are thin in section. Thus, although good in flame retarding effect they provide poor thermal insulation and the risk of skin burns to wearers of garments made from these fabrics is high. This necessitates the use of bulky air entrapping layers between the outer protective fabric and the skin or the item to be protected. Such insulation has been provided by waddings of equally expensive non-flammable fibres or by pile fabrics of flame retardant fibres combined to the outer protective fabric. Smoke release during burning is then still a significant problem.

It is also known that due to a high level of air entrapment animals are provided with a good degree of thermal insulation by their furs, and similarly animal furs as well as some man-made pile fabrics provide a good level of protection to human beings in low temperature environments, and also upon very short exposure to high temperatures until the pile fibres themselves combust. It is further known that during the fabric finishing sequence of natural furs and some man-made fibre pile fabrics, a special process is necessary to improve the aesthetics of the fibrous pile surface by removing the natural or imparted fibre waviness generally known as fibre crimp. Pile surface fibres so straightened and then sheared to equalise surface unlevelness present an aesthetically pleasing feel and appearance, which is essential to satisfy the appeal of the article to the customer. This operation is commonly known as rotary polishing.

The apparatus is commercially available and has been so ^f«r many years. In its basic form the apparatus consists essentially of an endless belt, which supports the pile fabric with the pile uppermost and across its entire width. The apparatus can be used to treat the pile of hand held short pieces of pile fabric, or long production lengths of pile fabric via an array of fabric spreading, tensioning and driving rollers. The transporting belt is capable of being raised so that the pile of the pile fabric which it supports is brought into an adjustable arc of pressure contact with a rapidly rotating roller, which is provided with a series of hard edged flutes around its periphery and across its working surface width.

The pile fibres across the entire width of the pile fabric are effectively flicked by the rapidly rotating flute edges. Since most fibres have one of their ends firmly anchored in the pile fabric technical ground, they are caused to undergo drawing, and hence molecular readjustment, in a direction parallel to their long axis. Those fibres which are not held in the pile fabric prior to this treatment are simultaneously removed from the pile via a ducted exhaust system. The natural or imparted fibre waviness or crimp is effectively removed by one or more such treatments and thereafter the pile fibres remain in the straightened condition. Such fluted rollers are usually electrically heated internally across their working widths in order to assist the straightening effect especially in thermoplastic pile fibres. The linear speed of the pile fabric transporter can be adjusted, as can be the surface speed of the roller and the surface temperature of the roller and its flutes, to enable an optimum lustrous finish to be obtained for each of a wide range of pile fabric pile fibre heights, fibre diameters and pile fibre weights per square metre.

The use of rotary polishing within the finishing routine of pile fabrics improves their aesthetics by removing the naturally occuring waviness (crimp) from the fibres in the pile when they are- of animal origin, or artificially imposed crimp when they are of synthetic origin. Fibres without crimp are virtually impossible to process into pile slivers and yarns, because crimp provides the necessary inter-fibre friction and cohesion to aid processing. Without crimp they are too slippery to handle satisfactory.

The production of oxidised polyacrylonitrile fibres and from them carbon fibres, entails taking, as the first step, continuous filament fibres and stretching them, whilst heated, to beyond their yield point and almost to their limit of plastic flow. Thereafter, these precursor fibres are heated under conditions of time and temperature. Fibres so produced and then chopped into discreet lengths are permanently very straight and cannot satisfactorily be "crimped", and hence are virtually impossible to process into the pile fabrics unless they are supported and diluted by crimped fibres such as wool eg; the Glenoit U.S. patent No. 4513042. In the converted condition such fibres are extremely expensive and also not only 'slippery' to process but also brittle leading to waste.

According to the invention I provide a pile fabric having a technical ground and a pile in which the pile consists entirely of oxidised polyacrylonitrile fibres. Also according to the invention I provide a process for the production of a fire resisting pile fabric which comprises subjecting the pile of a pile fabric having a technical ground and a pile, in which the pile consists entirely of polyacrylonitrile fibres, to heating and drawing or stretching action and the pile fabric so treated is subsequently heated to a temperature of 200 C to 250 C until the pile is in an oxidised condition.

A heated fluted roller may be used to achieve the stretching of the pile fibres of preformed pile fabrics beyond their yield point and up to their limit of plastic flow thereby effectively by-passing the problems of slipperyness and brittleness. Pre- stretching perse prior to heating is necessary to ensure adequate tensile properties in the finished fibre.

The pile fabrics exhibit excellent flame retardancy, very low smoke release, no melting, and retain their intrinsically good thermal insulation properties and resistance to attack by micro-organisms.

Thus by bringing together the widely disparate industrial techniques of oxidised polyacrylonitrile and carbon fibre production with pile fabric lustring by means of rotary polishing, for the first time provides an industrial process by which it is possible to produce a wide range of pile fabric types and constructions, which consist of oxidised polyacrylonitrile fibre in the pile. The pile fabrics so produced can be used as produced or can be given further conventional treatments in order to improve water and/or oilrepellancy for particular end-use requirements, for example clothing and tents, and/or coated or printed with pigmented or un-pigmented flame- retarding resins on the non-pile side if so required, or combined with a second non-inflammable fabric on the non-pile side if so required.

Thus for the first time unitary and base fabrics can be produced which combine the most desirable properties of excellent thermal insulation, light in weight with relatively high thickness, flexibility, high resistance to fire, very low smoke release, non-melting and high resistance to attack by micro-organisms, together with a ready acceptance of additional chemical finishes for example to improve water and/or oil repellancy.

In some constructions the fabrics are also reversible in that the high degree of fire resistance, low smoke release, non-melting and thermal insulation are retained irrespective of on which side of the fabric the heat source impinges. This desirable combination of properties is not lost when the fabric is subjected to washing and dry cleaning procedures, or additionally subjected to repeated exposure to severe environmental weather conditions. The colour of the polyacrylonitrile fibre component of the pile fabrics, which undergo this process of the present invention, becomes black during the final heat treatment stage of this process.

Polyacrylonitrile is a synthetic linear polymer in which the chief repeating unit is CH. ■CH

CN

The pile can be produced by tufts of fibre, or sliver, or slubbing, or roving, or yarn either alone or in combinations of these, introduced into or on to the technical ground for the purpose, that stand up from the body of the fabric.

The pile can be inserted into a preformed technical ground fabric by a technique known as tufting or can be formed by the weaving of technical ground yarns with the simultaneous formation of pile tufts from sliver and/or slubbing and/or roving and/or yarn; or by the warp or weft knitting of technical ground yarns with the simultaneous formation of pile tufts from sliver and/or slubbing and/or roving, and/or yarn; or by forming tufts from silver and/or slubbing and/or roving and/or yarn and then bonding them to an adhesive coated technical ground fabric; or by applying fibres in a high-voltage electrostatic field to an adhesive coated technical ground fabric by a technique known as flocking.

A tuft can be an i; - J; - U; or W shaped length of fibre, sliver, slubbing, roving or yarn of which the leg or legs form the pile of the fabric.

A technical ground yarn may be any yarn which is used to weave or to knit around the pile tufts thereby providing anchorage and support to the tufts as well as forming the basis or body of the pile fabric itself from which the pile stands up. For the purpose of the present invention such yarns should be made from fibres which will not only withstand the conditions of the process of this present invention but will also withstand those conditions to which the pile fabric processed by the present invention will be subjected during specific end use conditions. A technical ground fabric may be any type of pre anufactured fabric for example woven, knitted or bonded into which silver and/or slubbing and/or roving and/or yarn is inserted by tufting, or attached by bonding or flocking to form tufts thereby providing anchorage and support to the tufts as well as forming the basis or body of the pile fabric itself from which the pile stand up. For the purpose of the present invention such fabrics should be made from fibres which will not only withstand the conditions of the process of this present invention but will also withstand those conditions to which the pile fabric processed by the present invention will be subjected during specific enduse conditions.

For example, technical ground yarns and technical ground fabrics can be based on polyester fibres for some less critical end uses, whilst for example but not exclusively, the use of glass fibres or aramid fibres either alone or in their various combinations would be necessary when more severe end use conditions are envisaged for the pile fabrics which are to be produced by the process of this invention. A technical backcoating may be used to coat the non-pile side of the pile fabric prior to its treatment by the process of this invention. Such backcoatings can be emulsions or solutions of synthetic polymer resins which are applied by roller or by spray onto the non-pile side of the precursor pile fabric. On removing the water or solvent the polymers form a cured film which provides better fibre anchorage and fabric flatness and width stability for presentation to the subsequent process of this present invention. Hot-melt polymers can also be used to achieve the same ends. The type of technical back coating used is chosen so as not to reduce the fire resistance of the non-pile side of the pile fabric after its subsequent treatment according to the process of this present invention.

Shearing may be used, this being a conventional intermediate finishing process for pile fabrics, which is designed to cut the surface of pile fabrics to a uniform height. Patterned shearing may also be used to provide areas, of different pile heights across the width and along the length of the pile fabric surface. Shearing may be used to improve the presentation of the pile and hence the effectiveness of the hot drawing action of the rotary polishing process which follows it. The present invention is applicable to a wide range of pile fabric types whose const uctional parameters may be altered to provide a very wide range of fabric qualities and constructions. Such parameters are fibre diameter, fibre weight per unit of length, fibre length, technical ground yarns and fabrics and in a wide range of pile heights, pile densities, weights per unit of area, and technical back-coatings, all selected to satisfy the technical end-use requirements for the fabrics which are produced by the process of the present invention and articles made from such fabrics either before or after the process of this invention.

Pile fibres which are stretched to near the limit of plastic flow such as occurs during a rotary polishing operation are known as drawn fibres. Cold drawing can occur if the fluted cylinder is intentionally not heated. Hot drawing occurs if the fluted roller is heated. If the fluted roller is heated to a sufficiently high temperature the polyacrylonitrile pile fibres with which it is in momentary contact are thereby heated to beyond their second order transition temperature and plastic flow and molecular orientation take place more readily. In this latter condition inter-molecular re-structuring and bond formation is more readily achieved, as well then as intra-molecular restructuring, to form the so called oxidised form during the subsequent prolonged heating stage of the process of this present invention.

Hot drawn precursor polyacrylonitrile fibre yarns may also be used as the technical ground yarn or to make the technical ground fabric for the polyacrylonitrile pile fibre pile fabrics of the present invention.

A specific embodiment of the invention will now be described for one of the many types of pile fabrics to which the invention applies by way of example with reference to the accompanying drawing in which:-

Figures la and lb are elevation and plan views showing the relatively disorganised state of the pile fibres of a precursor pile fabric and the likely condition at the molecular level of a single fibre within it.

Figures 2a and 2b are elevation and plan views showing the more organised condition of the pile fabric following preliminary preparatory processes and the likely condition at the molecular level of a single fibre within it.

Figures 3a and 3b are elevation and plan views showing the action of the rotary polishing step of the two step process on the organisation and straightening of the pile fibres of the precursor pile fabric and the likely condition at the molecular level of a single fibre within it.

Figure 4 shows the chemical nature of a single polyacrylonitrile fibre after the rotary polishing step of the two-step process and before the subsequent prolonged heating step of the process.

Figure 5 shows likely chemical structures of a single fibre after both the rotary polishing step and the prolonged heating step of the process .

Figure 6 shows a glove made of the material of the present invention.

Figure 7 shows the flame-barrier and heat insulating effects of the glove of Figure 6 with the pile side outermost, which is made from fabric treated by the process.

Figure 8 shows the flame-barrier and heat insulating effect of a glove with the pile side innermost, which is made from fabric treated by the process . Referring to the drawing a pile fabric is knitted in tubular form on a circular multi-feed weft knitting machine fed with previously prepared pile slivers comprising entirely of undyed polyacrylonitrile fibres supplied as 5 denier with a mean fibre length of 27mm, at a sliver weight of approximately I6gm per linear metre and combined by the knitting action of the machine with a technical ground yarn composed of polyester fibre.

After slitting open the knitted tube along its length the opened out precursor pile fabric is placed in open width onto a parallel series of moving pins which guide the fabric under widthways tension and pile-side uppermost over the surface of a roller which rotates in a bath of acrylic polymer resin emulsion in water at approximately 25? of solids content and which deposits a technical backcoating onto the pile fibres, which appear on the non-pile side of the precursor pile fabric, in order to bind the fibres into the ground of the pile fabric. This is achieved by continuing to guide the backcoated precursor pile fabric through an oven with hot air circulation systems whereby the water is removed from the technical back coating to deposit a dry polymer film at the approximate rate of 60 grammes per square metre of precursor pile fabric. ^fv-f- backcoated fabric 10 is subjected to preparatory shearing of the pile. It is recognised that pile fabric preparatory processes such as steaming, brushing, shearing may be carried out in part before backcoating as well as after backcoating in a variety of combinations and sequencies according to the type of pile fabric being processed and the finished appearance desired. For the purpose of the present example the precursor pile fabric is sheared by way of preparation of the pile 11 in readiness for its presentation to the first part of the two part process which is the subject of this invention. The precursor pile fabric so prepared 11 is now subjected to the first part of this two part process of the present invention by subjecting the pile side of the prepared pile fabric to the hot drawing action of the rapidly rotating heated and fluted roller of a rotary polishing machine. Shearing and rotary polishing are repeated until the desired pile drawing effect 12 is obtained either by using one shearing machine and one rotary polishing machine more than once on the same length of fabric or by providing a series of rotary polishing machines with intermediate shearing for the continuous single passage and multiple treatment of one length of pile fabric.

The hot drawn readily flammable pile fabric is now ready for the second stage of the process in which the hot drawn and prepared pi le fabr ic 12 with polyacrylonitrile fibre in the pile 13 is converted to the oxidised form of polyacrylonitrile fibre in the pile 14.

At this point the fabric so prepared 12 (which may be cut into panels or made-up into preformed articles) is placed in a oven which is provided with close control over temperature. The required minimum temperature of 200 degrees Celsius (e.g. 200 to 250 C) is thereafter maintained for a minimum period of 4 hours (e.g. 4 to 8 hours) during which the polyacrylontrile fibres become black, fabric relaxation in length and width takes place as well as some weight loss as the oxidised form of polyacrylonitrile is realised in the pile of the fabric 14. Heating to about 220 degrees centigrade for 5 hours is satisfactory. Generally 225 + 5 degrees C for a period of 5 to 7 hours is adequate. After cooling, the fabric is flexible and strong. A glove made from this fabric 14 incandesces under incident flame when the pile is outermost 15, and with a hand wearing this glove it is possible to extinguish a burning domestic gas ring at full throttle and to pick up a domestic hand iron by its heated platten when at full scale temperature without in either case any feeling of discomfort other than a slight sweating of the palm of the hand within the glove. There is very- little smoke release and the glove is reusable. With the pile of the glove innermost 16 that part of the pile which appears on the non-pile side of the glove under which the technical ground yarn is buried also incandesces under incident flame with no discomfort to the skin of the hand within the glove. However, heat conduction takes place from the outside to the inside surfaces of the fabric of the glove and after very few seconds the heat transfer is such that the polyester technical ground yarn reaches its softening temperature and the fabric technical ground structure is then easily stretched and then broken. More heat resistant fibres must be used for technical ground yarns and fabrics in order to obtain full reversibility.

When the fabric of the glove 15 is subjected to dry cleaning and to washing cycles followed by drying the above observations are unchanged.

When the glove is further subjected to alternating freezing and thawing cycles there is no effect on the observed properties given above. Burying the glove in soil under warm and wet conditions also had no effect on the observed properties given above after three weeks burial. Both the pile side of the fabric and the non-pile side of the fabric are very readily wetted by water and by oi l .

The fabric produced according to the invention has, in addition to the excellent fire resistant property, qualities of extremely low smoke release, resistance to fibre melting and excellent retention of thermal insulation. It also has a resistance to attack by micro-organisms .

The fabrics so produced and articles made from them may be further treated with water-repelling agents or oil and water-repelling agents to improve their unitary fabric performance in hostile weather conditions and working environments such as are encountered on ships, oil-rigs, and battleground environments.

In another example of this invention prior to the stretching action on the pile, and subsequent heating to oxidise the pile fibre, the pile fibres are secured during tufting, weaving, knitting, bonding or flocking, by a technical ground made of fibres, which will withstand the subsequent heating to oxidise the pile fibre, as well as even more severe end-use temperatures, for example glass, aramid, or hot drawn continuous filament polyacrylonitrile yarn.

In another example of this invention the technical ground is composed of previously hot drawn continuous filament polyacrylonitrile yarn. Tufted, woven, knitted, bonded and flocked pile-fabrics made with this foregoing polyacrylonitrile yarn in the technical ground and with polyacrylonitrile fibre in the pile are then subjected to the stretching action on the pile whereafter the pile fabric with prestretched polyacrylonitrile technical ground together with the stretched polyacrylonitrile fibres of the pile are subjected to heating to effect oxidation of both the polyacrylonitrile fibres of the technical ground and the polyacrylonitrile fibres of the pile.

In another example of this invention the foregoing pile fabrics with oxidised polyacrylonitrile in both the technical ground and in the pile may be converted by the further heating of such structures at carbonising temperatures (eg. 1200-1400 degrees C.) and conditions into useful carbon structures to act alone or as binders for resins. When used as binders for resins such carbonised fibre structures being in an orderly array in three dimensions thus provide lightweight, rigid structures of very high strength and resistance to thermal degradation.

Fabrics produced in the present invention are not only extremely, uniquely and intrinsically useful when used as produced in fire risk situations as low smoke release, non-melting, flame barrier panels to increase escape times, but they are also amenable to various additional fabric treatments such as water and oil repellancy as well as to reverse side coatings or secondary fabric combinations. Such fabrics will find many end-uses when taken as produced and then made-up into articles to provide for example flame-barrier low smoke panels, protective clothing, hoods, gloves, socks, curtains, carpets, blankets, upholstery foam- block covers and the like in domestic, industrial, commercial, institutional, military and space fire-risk and hot contact risk situations as well as filtration media for hot oil, hot chemical liquids, and hot gases. When additionally treated, such fabrics will form the basis of all-weather fire protection apparel, awnings, tents and protective covers, industrial protective clothing, military protective clothing, such as gloves, socks and suits.

Claims

1. A pile fabric having a technical ground and a pile in which the pile consists entirely of oxidised polyacrylonitrile fibres.

2. A pile fabric as claimed in claim 1 treated or combined with a flame retarding resin.

3. A pile fabric as claimed in Claims 1 and 2, combined with another non-flammable fabric.

4. A pile fabric as claimed in Claim 2, wherein the pile is on one side only of the technical ground and the resin is on the non-pile side of the fabric.

5. A pile fabric as claimed in any of Claims 1 to 4, wherein the technical ground also consists entirely of oxidised polyacrylonitrile fibres.

6. A pile fabric as claimed in Claim 1, wherein the fabric is resistant to flame on both sides.

7. A pile fabric as claimed in any of Claims 1 to 6, which is treated with a water repelling agent or oil and water repelling agents.

8. A pile fabric as claimed in any one of claims 1-7, whenever the technical ground comprises oxidised polyacrylonitrile fibres.

9. A pile fabric as claimed in any one of the claims 1-7, whenever the technical ground comprises polyester, glass or aramid fibres.

10. An article of clothing, curtain, blanket, carpet, upholstery cover made from a pile fabric as claimed in any of claims 1 to 9.

11. A process for the production of a fire resisting pile fabric having a technical ground and a pile, in which the pile consists entirely of polyacrylonitrile fibres, comprising subjecting the pile to a heating and stretching action and subsequently heating the pile fabric to a temperature of 200 degrees centigrade to 250 degrees centigrade until the pile is in an oxidised condition.

12. A process as claimed in Claim 11, wherein the pile of the pile fabric is heated and stretched by subjecting it to the action of a rotating fluted heated roller .

13. A process as claimed in Claim 11 or 12 wherein the heating at a temperature of 200 degrees centigrade to 250 degrees centigrade is maintained for at least four hours.

14. A process as claimed in Claim 11, 12 or 13, wherein an article of clothing or industrial article is made from the fabric after heating and stretching the pile but before heating the article to a temperature of 200 degrees centigrade to 250 degrees centigrade.

15. A process as claimed in any one of Claims 11- 14, wherein prior to the stretching action of the pile fibres continuous filament polyacrylonitrile yarn for the technical ground is previously subjected to heating for plasticising and stretching, whereafter the technical ground together with the stretched fibres of the pile are subjected to heating to effect oxidisation of the fibres of the technical ground and pile.

16. A process as claimed in Claim 15, wherein the stretched polyacrylontrile filament is tufted, bonded, woven, knitted or flocked together with the pile of polyacrylonitrile fibres whereafter the pile fibres are stretched and subsequently heating of both technical ground and pile fibres is effected for oxidisation.

17. A process as claimed in any one of claims 11- 14, wherein prior to the stretching action and subsequent heating to oxidise the pile fibres, the pile fibres are secured during weaving or knitting to a technical ground make of polyester, glass or aramid.

18. A process as claimed in Claim 15 or 16, wherein the pile and technical ground both in the form oxidised polyacrylonitrile are converted by the heating of such a structure at a temperature of from 1200 degrees centigrade and 1400 degrees centigrade into a carbon structure.