Heterogeneous knitted fabric comprising metal fibres

The heterogeneous knitted fabric comprises metal fibres and other fibres. The fabric has a single knitted layer structure in which yarns with a high metal fibre content are situated towards one surface and yarns with a low metal fibre content are situated towards the other surface.

The present invention relates to a relatively thin, heterogeneous knitted
 fabric comprising metal fibres and other fibres in which, more
 specifically, the yarn composition of the zone bordering its one surface
 differs from that bordering its other surface. Provided that the other
 fibres can also resist extreme temperatures, the knitted fabrics can be
 used for applications in very cold or in very hot environments.
 Custom-made heterogeneous knitted fabrics comprising metal fibre yarns are
 already known from the applicant's patent WO 94/01373. The knitting
 pattern may hereby differ in predetermined zones over their surface,
 and/or through the thickness of the cloth. Patent WO 94/01372 also
 describes the application of this sort of heterogeneous fabric as
 separation cloth in the moulding of plate glass at high temperature. There
 is also an ever-increasing demand, in such production and in other
 applications, for relatively thin heterogeneous knitted fabrics comprising
 metal fibres that combine a relatively high air permeability with some
 measure of insulating capacity. The weight of such fabrics will preferably
 be kept to a minimum, despite the necessary proportion of contained metal
 fibres.
 The heterogeneous knitted fabric comprising metal fibre according to the
 present invention attempts to meet this requirement by providing a plain
 flat knitted structure in which yarns having a high metal fibre content
 are present near the one surface, while yarns having a low metal fibre
 content are present near the other. The expression "high metal fibre
 content" is here to be understood to mean at least 30% metal fibres by
 volume, preferably more than 40% by volume, and even yarns consisting of
 100% metal fibre. "Low metal fibre content" is here to be understood to
 mean less than 12% metal fibre by volume and, preferably, less than 7% by
 volume. The low-content yarn may even consist 100% of other fibres. These
 may be carbon, glass, basalt, ceramic or synthetic fibres. The knitted
 fabric generally weighs less than 2000 g/m.sup.2 and, preferably, will be
 of a weight of less than 1500 g/m.sup.2.
 In order to procure the best possible fabric density (air permeability),
 the yarns bordering one surface, and those bordering the other surface
 (see overleaf) will preferably be oriented in parallel--according to the
 same knitting pattern--and be contiguous with each other. They behave, so
 to speak, as twin yarns, and can be plain flat knitted, as weft thread on
 a circular knitting machine or on a flat knitting machine. The machine
 separation can, depending amongst other things on yarn thickness, be
 optionally between 5 and 30 gauge for round knitting and between 5 and 20
 gauge for flat knitting. The yarns having a high and those having a low
 metal fibre content are thus simultaneously drawn in for knitting towards
 the same needle, each placed under sufficient tension to form a flat-knit
 fabric (single stitch layer) in which one of the yarns always passes in
 front of the other, known as plating technique. The yarns with a low metal
 fibre content are usually smoother than those with a high metal fibre
 content. This facilitates the flat knitting operation.
 The yarns with a high metal fibre content are, preferably, staple fibre
 yarns in which the metal fibres, for example stainless steel fibres, have
 a diameter of between 4 .mu.m and 50 .mu.m. The yarns on the opposite side
 of the cloth may be filament yarns of 100% glass, basalt or carbon or
 synthetic filaments with a diameter below 40 .mu.m. Suitable synthetic
 filaments include polyaramid or polyimide filaments (Kevlar (R)), brand
 name of du Pont de Nemours, Twaron (R), brand name of AKZO/ENKA). Spun
 yarns, whether or not twisted yarn consisting of one or more sorts of
 fibre, are equally suitable.
 During the flat knitting process, one type of yarn (with high metal fibre
 content) can be used as well as another (with little or no metal fibres)
 so as to produce a fabric with zones presenting knitting patterns over its
 surface with differing weight per m.sup.2, density (air permeability),
 stretchability or electrical conductivity. The fabric may also contain a
 fine interlining yarn, e.g., to reinforce the fabric as described in the
 applicant's Belgian patent appli-cation 97/00614. Finally, a single layer
 fabric zone as described above can be incorporated locally within a
 surroun-ding--e.g. double layer--knitted structure on a flat knit-ting
 machine. This then produces a knitted fabric containing the relatively
 thin heterogeneous knitted fabric over part of its surface. The cloth is
 thus heterogeneous through its thickness as well as over its surface.

EXAMPLE
 By way of example, one embodiment of the knitted fabric according to the
 present invention and its application as a separation cloth in the
 compression moulding of plate glass will now be explained in further
 detail. The invention is not, however, limited to this or to analogous
 embodiments. The cloth was fabricated at a pitch of 12 gauges on a Stoll
 CMS 440 flat knitting machine to a single Jersey structure without thin
 interlining stiffening yarn. After removal of all brightening agents from
 the cloth, the cloth thickness was approximately 1 mm. The cloth counted
 62.5 stitches per cm.sup.2 and had a weight of 980 g/m.sup.2. This cloth
 displayed an air permeability (l/h/10 cm.sup.2) of 4164 and was therefore
 suitable as lining for the male section of the compression mould. The
 glass fibre surface of the cloth was in contact with the mould, the metal
 fibre outer surface thus forming the contact with the glass plate during
 moulding.
 In this separation cloth, the spun (staple fibre) high metal-fibre-content
 yarns consisted of 100% Bekinox type AISI 316 L fibres with a diameter of
 12 .mu.m and measuring 133 tex. The low metal-fibre-content yarn consisted
 of a 100% glass filament yarn measuring 136 tex.
 If a heterogeneous knitted fabric according to the present invention forms
 a burner membrane for gas burners, coarse yarns are used. Separation on
 the knitting machine can be set at, e.g., 5. A 100% metal filament yarn as
 described in the applicant's patent WO 97/04152 is fitted at the gas
 outlet side (burner face) of the membrane.
 The layer fitted at the gas supply side of the cloth may be, e.g., 100%
 glass yarn or ceramic yarn. The heterogeneous knitted fabric according to
 the present invention can also be used as a heat shield against oven heat
 or as electromagnetic shielding cloth. Polishing cloth can also be
 fabricated according to the invention, with abrasion-resistant metal fibre
 yarns on the polishing side. It is further possible to imbed the
 heterogeneous knitted fabric in a plastic matrix. The proportion of other
 fibres will then preferably comprise synthetic fibres capable of combining
 with the plastic of the mould.