ENERGY-ABSORBING, FORMABLE FIBROUS COMPOSITIONS

Abstract: The present invention relates to a fibrous composition comprising a thermally heat shrinkable fibre structure having in at least a proportion of the structure a substantially homogeneous content of a conducting material sufficient to impart limited conducting properties to the composition.

Description: ENERGY-ABSORBING, FORMABLE FIBROUS COMPOSITIONS DESCRIPTION The present invention relates to fibrous compositions and has particular reference to fibrous compositions having functional properties. Functional layers of the fibrous compositions disclosed herein are particularly useful in absorbing energy in the microwave and radio frequency ranges. Functional materials of this kind are generally well known and most of the materials hitherto employed are based on polyurethane or vinyl compounds which, when over heated, decompose and are destroyed. Thus, the microwave absorbing capacity of existing materials is strictly limited by the low use temperatures of the polyurethane and vinyl compounds. A need exists for high temperature non-flammable materials which, because of the nature of the materials themselves would tend to have greater capacity. According to one aspect of the present invention, therefore, there is provided a fibrous composition comprising a thermally shrinkable fibre and a substantially homogeneous content of a high temperature, electrically conducting material sufficient to impart limited conducting properties to the structure. In one aspect of the invention, the electrically conducting material may be selected from certain conductive carbon or metallic fibres. The thermally shrinkable fibre itself may be any flame resistant fibre, but is preferably a polyimide fibre such as that commercially available under the trade name "P84" supplied by Messrs. Lenzing Aktiegesellschaft of Austria. In accordance with the present invention, the composition of the functional layer may contain, but is not limited to 0.1 to 50% by weight of conducting material. The shrinkable fibre may be staple fibre and the conducting material may, in a further aspect of the invention, be carbon fibre present in an amount of 0.1 to 50% by weight. In a typical embodiment of the invention, the composition may be in the form of a paper-like sheet. These materials may be used to line, for example, a test chamber. The invention further includes a fibre structure comprising a layer of a fibrous composition in accordance with the present invention and at least one layer of a batt of compatible, heat-shrinkable, fibre needled thereto to form a non-woven fibrous structure. The non-woven fibrous structure may have been subjected to heat to densify the structure itself. This supporting fibrous structure serves to contribute structural integrity and rigidity to the overall structure and also serves to space functional layers at certain predetermined depths. In another aspect of the present invention, it may provide a non-woven fibrous structure comprising a heat shrinkable fibre layer together with a functional material substantially homogeneously distributed through said layer with at least one layer of a compatible heat shrinkable material needled thereto and subsequently densified to encapsulate the functional material within the structure. The invention further includes a method of locating a functional material within a heat shrinkable fibre structure which method comprises forming a functional fibre structure containing a substantially homogeneous distribution of said functional material therein, in the desired concentration, completing the formation of a fibre structure by surrounding or laminating the functional fibre structure with compatible fibre structures, needling the structures together to produce a unitary whole and subjecting the structure so formed, to densification by the application of heat whereby the functional structure is fixed within the densified structure per se . In accordance with the present invention, therefore, it is possible to provide fibre structures having a layer of a functional material located therein. A particular aspect of the present invention provides for a rigidified product which may be shaped to form a structural member. To achieve the fibre structure in accordance with the present invention, the structure should include a major proportion of heat shrinkable fibres, some of these fibres being disposed in discrete fibres groups, said structure being capable of heat treatment to produce a structure of increased density in which the density of the fibre groups is greater than that of the remainder of the structure. In this case, the density of the structure after heat treatment is non-uniform and in particular, may have a plurality of longitudinal elements therein, each element comprising a group of said fibres oriented in a plane and densified by heat treatment. Typically, the fibre structure may be a non-woven felt such as a batt layer or may comprise a series of layers of separated fibres. The fibre structure, when it comprises several layers of separated fibres may comprise several layers of batt material laminated together prior to shrinkage. The structures also may be laminated through thermal bonding or the use of adhesives. Fibres for use in the present invention are preferably polyimide fibres having the general structure:- in which n is an integer greater than 1 and R is selected from one or more of These fibres are particularly useful for practising the invention in that by heat treatment, they permit the production of shaped articles of high tensile strength, high heat resistance, good flame-retardant properties and relatively low density. On exposure to open flames in case of a fire, the fibres decompose with the production of gases of only very low optical density and low toxicity. The fibres indicated above for use in accordance with the present invention exhibit high fibre shrinkage. This results in the generation of a considerable force during shrinkage and results in the production of cohesive bonds between individual fibres at their contact points. These cohesive bonds when formed, impart additional structural integrity, high stability and tensile strength to the shaped article. The functional composite in accordance with the present invention may be shaped by holding the composite against a shaping surface and thereby subjecting the structure to heat and/or pressure. This may be carried out prior to shrinkage of the fibres or after shrinkage of the fibres. Where the fibre structure is caused to have a number of fibres extending generally transverse to the plane of the fibre structure layer, then by conforming such a structural layer against a shaping surface and subjecting the material to a heat treatment, if the material is constrained against a forming surface or constrained against shrinkage in two dimensions, the only dimension in which the material is free to shrink is in the third dimension, namely substantially perpendicular to the plane of the structure. This means that the transverse fibres are capable of almost free shrinkage thereby markedly increasing their density relative to the open fibrous web surrounding. Thus, at the completion of the heat treatment, a structure has been produced which has a slightly densified surface due to any surface heating together with densified pillars or elements within the material extending substantially transverse to the surface thereof. This results in a substantial stiffening and an increase in the compressional strength of the material. The formation of groups of fibres within the material can be effected by, for example, needling or hydro-entangling. Where a layer is to be needled, each structural layer may be needled from either one side or both sides either simultaneously or in succession. The size of the structural element formed within the layer during the heat shrinkage step may be controlled fairly precisely by the size and nature of the needles employed in the needling operation. The more fibres that are re-oriented transverse to the plane of the material, the greater is the transverse rigidity after densification. The extent of the formation of the structural elements of pillars within the material may thus be controlled by the number of penetrations. Thus, when needling, by increasing the density of needling, it is possible to enhance the compression modulus of the layer transverse to the plane of the fibre structure sample. Large transverse elements may be provided by employing extra large needles or a combination or large needle size and design of bar structure at an extremity thereof. The invention particularly provides, therefore, a rigidified structure which may incorporate or include within it a functional layer composite in accordance with the present invention. The preparation of the functional layer composite may be accomplished by first forming a paper-like sheet composed of polyimide P84 fibre and a small amount of conductive fibre such as carbon. The staple length of the polyimide fibre can be any convenient length useful for wet laid, non-woven preparation. The carbon or graphite fibres should be typically within the range of 0.1 to 0.5 inches, preferably 0.125 to 0.3 inches in length. The composition of the paper sheet may contain 0.1 to 50% by weight, preferably 0.1 to 15% by weight of carbon fibre in the functional layer. The sheet may typically be made by slurrying the appropriate amount of P84 fibre and carbon fibre in distilled water and forming a sheet by standard wet laid non-woven procedure, for example, a vacuum application to a forming wire upon which the slurry is poured. The carbon containing sheet is then layered between batts of shrinkable polyimide fibre and needled to form a non-woven structure. The application of heat causes the non-woven structure to densify forming a rigid functional layer composite. The density, rigidity and shape of the final structure can be controlled by restraining the non-woven or holding the non-woven against a shaping surface during the heat shrinking process in the manner described above. Following is a description by way of example only and with reference to the accompanying informal drawings of a method of carrying the invention into effect. In the drawings:- Figure 1 is a photomicrograph section of a rigidified structure in accordance with the present invention having a functional layer composite therein. - EXAMPLE 1 A mixture of 3.968g of 2.5mm P84 staple fibre (yarn size 1.7 dtex) and 0.032g of 0.125 inch conductive carbon fibres was slurried in approximately 10 litres of water 5 . and formed into a sheet on a PET forming fabric in an 8" x 8" sheet mold. A second piece of forming fabric was used to cover the wet laid sheet and the sheet was dewatered with a laboratory nip press and drum dryer. The wet laid sheet was removed from the forming fabrics 10 and placed between two P84 fibre batts. Each batt comprised 2.2 dtex fibre of two inch length, the fibre being high shrinkage P84 staple fibre supplied by Messrs. Lenzing Aktiengesellschaft of Austria. Each batt weighed 7oz/yd 2 . The assembled batts and wet 15 laid sheet were needled to an approximate needle density of 1000 penetrations per square inch and a thickness of approximately 0.25". The needled structure was clamped between two rigid 20 metal frames with 8" x 8" square openings. The resultant needled structure was given a heat treatment of one hour at 630°F in a forced convection oven. After cooling, a relatively rigid, flat panel was cut from the frames. The panel had a thickness of 0.10" and - - ~ a density of 15 lb/ft . The carbon fibre content of the functional layer was 0.8% by weight. The photograph of Figure 1 shows a sheet structure of two layers of batt material with clear evidence of the needling and the rigidified structural element traversing a layer of dark, i.e. carbon composite material.

Claims: CLAIMS 1. A fibrous composition comprising a thermally heat shrinkable fibre structure having in at least a proportion of the structure a substantially homogeneous content of a conducting material sufficient to impart limited conducting properties to the composition. 2. A composition as claimed in claim 1 wherein the conducting material is selected from conductive carbon. 3. A composition as claimed in claim 1 wherein the heat shrinkable fibre is any flame resistant heat shrinkable fibre. 4. A composition as claimed in claim 3 wherein the flame resistant fibre is a polyimide fibre. 5. A composition as claimed in claim 1 wherein the conducting material is a conductive carbon present in an amount of 0.1 to 50% by weight in the functional layer. 6. A composition as claimed in claim 1 wherein the fibre is a staple fibre. 7. A composition as claimed in claim 6 wherein the conducting material is conductive carbon present in an amount of 0.1 to 50% by weight in the functional layer. 8. A composition as claimed in claim 1 in the form of a paper sheet. 9. A surface having a coating of a composition in accordance with claim 1. 10. A fibrous structure comprising a layer of a fibrous composition as claimed in claim 1 and at least one layer of a non-woven compatible, heat-shrinkable fibre needled thereto. 11. A fibre structure as claimed in claim 10 characterised in that the non-woven fibrous structure has been subjected to heat to densify the structure. 12. A structure as claimed in claim 10 wherein the conducting material is selected from conductive carbon fibre or certain conductive metallic fibres. 13. A structure as claimed in claim 10 wherein the heat shrinkable fibre is any flame resistant heat shrinkable fibre. 14. A structure as claimed in claim 13 wherein the flame resistant fibre is a polyimide fibre. 15. A structure as claimed in claim 10 wherein the conducting material is present in an amount of 0.1 to 50% by weight in the functional layer. 16. A structure as claimed in claim 10 wherein the fibre is a staple fibre. 17. A structure as claimed in claim 15 wherein the conducting material is conductive carbon present in an amount of 0.1 to 50% by weight. 18. A structure as claimed in claim 10 in the form of a paper sheet. 19. A surface having a coating of a structure in accordance with claim 10. 20. A non-woven fibrous structure comprising a heat shrinkable fibre layer together with a functional material substantially homogeneously distributed through said layer with at least one layer of a compatible heat shrinkable material needled thereto and subsequently densified to encapsulate the functional material within the structure. 21. A structure as claimed in claim 20 wherein the conducting material is selected from conductive carbon. 22. A structure as claimed in claim 20 wherein the heat shrinkable fibre is any flame resistant heat shrinkable fibre. 23. A structure as claimed in claim 22 wherein the flame resistant fibre is a polyimide fibre. 24. A structure as claimed in claim 20 wherein the conducting material is present in an amount of 0.1 to 50% by weight. 25. A structure as claimed in claim 20 wherein the fibre is a staple fibre. 26. A structure as claimed in claim 24 wherein the conducting material is carbon present in an amount of 0.1 to 50% by weight. 27. A structure as claimed in claim 20 in the form of a paper sheet. 28. A surface having a coating of a structure in accordance with claim 20. 29. A method of locating a functional material within the heat shrinkable fibre structure which method comprises forming a functional fibre structure containing a substantially homogeneous distribution of said functional material therein, in the desired concentration, completing the formation of a fibre structure by surrounding or laminating the functional fibre structures with compatible fibre layers, needling the structures together to produce a unitary whole and subjecting the structure so formed, to densification by the application of heat whereby the functional structure is fixed within the densified structure per se . 30. A method as claimed in claim 29 in which the fibre structure is a non-woven felt comprising several layers of material laminated together prior to shrinkage. 31. A method as claimed in claim 29 wherein the lamination is effected using an adhesive. 32. A method as claimed in claim 29 wherein on completion of the heat treatment, the structure is produced which has densified elements or pillars within the materials extending in the direction of needling, thereby providing substantial stiffening of the structure and an increaέe in compressional strength of the material.

Inventors: DITARANTO FRANK ANTHONY, FINN RICHARD JOSEPH, KRAMER CHARLES EDWIN, DITARANTO, FRANK, ANTHONY, FINN, RICHARD, JOSEPH, KRAMER, CHARLES, EDWIN
Applicants: ALBANY INT CORP, ALBANY INTERNATIONAL CORP.
Application year: 1990
Main classification: D04H1
IPC codes: B32B5, D04H1, H05K9