GB2324758A

GB2324758A - Adaptive composite fibre

Info

Publication number: GB2324758A
Application number: GB9708979A
Authority: GB
Inventors: Colin Dawson; Julian Francis Vincent
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1997-05-03
Filing date: 1997-05-03
Publication date: 1998-11-04
Anticipated expiration: 2017-05-03
Also published as: GB2324758B; GB9708979D0

Abstract

A yarn or fabric which adjusts its permeability in response to changes in the surrounding environment (eg. temperature or humidity) comprises a composite fibre having a compliant absorptive component 1, which changes volume in response to changes in its surrounding environment, and a less absorbent support component 2, which is arranged to constrain volume changes in the absorptive component and thereby alter the composite fibre's configuration. Preferably the support component is provided by one or more stiff fibrils 2 which are helically wound around or embedded within an absorptive matrix 1. In alternative embodiments, the absorptive and support components may comprise separate elongate strands bonded together (Fig 1a) or a uniform fibre may be chemically treated so as to produce within the fibre both absorptive and support components (Fig 1b). The fibre is suitable for use in foul weather clothing, food packaging and wound dressings.

Description

ADAPTIVE YARN The present invention relates to biomimetic adaptive materials which can alter their shape or configuration on application of an external stimulus. In particular, the invention relates to fibres or yarns and fabrics made therefrom which adapt in response to conditions in the local environment such as changes in temperature, relative humidity or exposure to a liquid or gas.

Known breathable fabrics utilise the biological concept of pores.

An impermeable, insulating fabric is provided with small holes which are suitably sized to allow diffusion of water vapour through the fabric whilst retaining its insulation properties. In improved designs, the outer surface of the fabric is provided with relatively small "pores" whilst the internal surface of the fabric defines a larger air chamber below the "pore" which has been found to assist in the diffusion of water vapour through the fabric.

However, such breathable fabrics are not responsive to their surrounding environment. The "pores" remain open even when the surroundings and the wearer are cold. In such circumstances, the enhanced insulating properties of an impermeable, insulating fabric without perforations may be desirable. Furthermore, the effectiveness of the fabric is closely dependent upon the number and position of the "pores". A fabric with too few pores which are poorly positioned will not allow the diffusion of sufficient water vapour from the skin making a garment produced from the fabric uncomfortable for the wearer. Too many "pores" in the fabric will degrade its insulation properties.

A fabric with pores" that close on immersion in water is also known. Domed air pockets are provided across the surface of the fabric, each air pocket having a small hole at its apex. The holes allow diffusion of vapour from the wearer, via the domed air pockets, into the surroundings. However, when the fabric is immersed in water, the water pressure acts on the domes and tends to close the holes, thereby maintaining the thermal integrity of the insulation.

Although the "pores" in this type of fabric respond to an external stimulus, this response is limited to closing under the direct action of water pressure. Thus the "pores" will close only if submerged in water. They are unresponsive to other changes in the surrounding environment. Again, the efficacy of this fabric will be dependent upon the number and position of the "pores". Thus, a problem with this type of fabric is that it has to be constructed with suitably sized and positioned pores which is both difficult and time consuming.

Accordingly, the present invention provides a composite fibre comprising a compliant absorptive component, which changes volume in response to changes in its surrounding environment, and a less absorbent support component, which is arranged to constrain volume changes in the absorptive component and thereby alter the composite fibre's configuration.

Composite fibres capable of responding to a range of temperatures or liquids and gases can be produced by choosing an appropriate material as the absorption component. Where the absorption component is hygroscopic, the composite fibre will react to the level of water vapour in its surroundings.

Such composite fibres can be used to produce a textile using techniques common in the art. The textile can be produced by knitting or weaving the composite fibres together, provided the weave or knit is sufficiently open to allow the fibres to change their configuration. As the relative humidity of the surroundings change, the composite fibres change shape and the properties of the textile alter as the composite fibres become more or less closely packed together. This gives the textile desirable properties such as variable Xbreathabilityt mediated by changes in relative humidity. The advantage of such a material over known breathable fabrics is its ability to alter its permeability in response to changes in its surrounding environment. Such a textile has many applications, including foul weather clothing, food packaging and wound dressings.

The composite fibres operate on the same principles as adaptive materials in nature, such as ovuliferous scales on pine cones which can move in response to changes in relative humidity (RH), thereby allowing seeds to be distributed with maximum effectiveness.

This movement is achieved by the interaction of two types of cells, both composed of cellulose and lignin. In both types of cell, stiff cellulose microfibrils are arranged in a helical configuration, surrounded by a matrix material containing lignin.

However, in one cell type, the acute angle of the microfibril helix to the longitudinal axis of the cell is small and thus these cells are restricted from expanding by the microfibrils. In the other cell type, the acute angle of the microfibril helix to the longitudinal axis of the cell is relatively large and the cells are able to expand in response to changes in the relative humidity of their surroundings. The two cell types interact in much the same way as a bimetallic strip and are located above and below the bracts in pine cones to cause the bracts to move in response to changes in relative humidity.

In one embodiment of the invention, the two components of the composite fibre are arranged in a form analogous to a bi-metallic strip. This arrangement can be manufactured by co-extruding two fibres with the desired properties such that they are tightly bonded together. Alternatively, the arrangement can be manufactured by chemical or physical treatment of part of a uniform fibre such that the treated portion of the fibre forms the support component and the untreated portion forms the absorption component or vice versa.

The composite fibres may be provided in a straight arrangement. In the presence of the particular liquid or gas to which the absorption component reacts, this component will try to increase in length but will be restricted by the support component to which it is tightly bonded. To accommodate the strain differences between the two components, the composite fibre bends. If the level of the particular liquid or gas in the environment falls, the absorption component will tend to shorten and the composite fibre bends in the other direction. Thus, the composite fibre changes its configuration in response to changes in the surrounding environment.

This change of configuration can be amplified by constructing the composite fibre in a spiral arrangement. If the fibre is manufactured by co-extruding the two component parts, this can be achieved by increasing the rate of extrusion of one of the components relative to that of the other component. The components can be arranged such that the spiral either increases length and decreases width, or vice versa, in response to changes in the surrounding environment. The way in which the spiral adapts (i.e.

whether it increases or decreases in length) will depend upon which of the component parts of the fibre is positioned on the external surface of the spiral.

In an alternative embodiment of the invention, the absorption component of the composite fibre is provided by a strand of suitable absorptive material. At least one stiff fibril is either helically wound around the absorptive matrix or embedded within the absorptive matrix in a helical configuration. When the environment around the fibre contains the fluid to which the absorptive matrix reacts, the absorptive matrix increases in volume. The fibrils absorb little or no fluid and restrict the expansion of the absorptive matrix in certain directions.

The winding angle of the fibril to the longitudinal axis of the composite fibre determines whether the fibre contracts or expands in the presence of the relevant fluid. The fibril winding will tend towards an optimum angle at which the absorptive matrix attains its maximum volume. For example, if the optimum angle is approximately 540 and the fibril is wound at 300 or less to the longitudinal axis of the composite fibre, the fibre will contract in the presence of the fluid. If the fibril is wound at an acute angle of 600 or more to the longitudinal axis of the composite fibre, the fibre will extend in the presence of the fluid.

As outlined by Harris in "The mechanical behaviour of composite materials", in "The mechanical properties of biological materials" (1980) published by Cambridge University Press, composite theory shows that for two cylinders with the same volume fraction ratio of stiff fibres to compliant matrix, the winding angle (0) of the stiff fibres to the longitudinal axis of the cylinder will determine the Youngs modulus of the cylinder. If the matrix is the absorptive component of the composite cylinder, the winding angle (0) will also determine the direction and extent to which the cylinder changes dimension in the presence of the fluid absorbed by the matrix. A cylinder with a low winding angle (0) will expand primarily in the transverse direction and a cylinder with a high winding angle (o) will swell primarily in the longitudinal direction of the cylinder..

In the arrangement described above, the absorptive matrix may either be provided as a solid mass or in the form of a hollow cylinder. The fibril or fibrils may either be wound around the absorptive matrix or embedded therein. Where the arrangement incorporates a plurality of fibrils, the points at which the fibrils cross must be free to move either in shear or pivotally to enable the angle of the fibril windings to the longitudinal axis of the composite fibre to change when the fibre is exposed to the relevant fluid.

The mechanical properties of the composite fibre can be tailored by the choice of fibril winding angle and the quantity of absorptive matrix material used. The smaller the quantity of matrix material or the further the fibril winding angle from the optimum angle, the quicker the response to the relevant liquid or gas or to changes in temperature. Different matrix materials may be chosen to provide the required response characteristics. Known polymers or gells which absorb different liquids and gases would provide a suitable absorptive matrix as would some natural fibres.

Three embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein Fig 1. Shows a composite fibre according to one embodiment of the invention with the absorption and support components provided in a straight configuration Fig 2. Shows the composite fibre of Fig. 1 provided in a spiral configuration Fig 3. Shows a composite fibre according to another embodiment of the invention, comprising an absorbent matrix around which two stiff fibrils are helically wound.

The composite fibre shown in Fig.l may be manufactured in a number of ways. Fig. la shows a composite fibre 3 which is manufactured by co-extruding an absorbent fibre 1 with a support fibre 2 which has a lower coefficient of hygroscopic expansion but a higher Youngs modulus than the absorbent fibre 1. Alternatively, as illustrated in Fig. lb, a fibre of uniform properties 4 is chemically or physically treated on one side to provide a composite fibre having an absorbent component 5 and a support component 6.

In creating a composite fibre according to either of the methods outlined above, it is possible to ensure that the composite fibre assumes a straight configuration at a particular desired relative humidity by: a) With the co-extruded fibre shown in Fig. la, choosing a suitable combination of fibres with different Youngs moduli, coefficient of hygroscopic expansion and depth ratio.

b)With the chemically or physically treated fibre shown in Fig.

lb, ensuring that the chemical or physical treatment is applied for a predetermined amount of time or under conditions which yield a composite fibre with the desired combination of Youngs modulus, coefficient of hygroscopic expansion and depth ratio.

Fig. 2 shows a composite fibre 3 manufactured in a spiral arrangement to amplify changes in its configuration, arising from the presence of the particular fluid to which the fibre is designed to react. Fig. 2b shows the composite fibre 3 in its normal configuration (for example, at a predetermined relative humidity) . Fig. 2a shows the composite fibre 3 in the configuration adopted when the relative humidity increases and Fig. 2c shows the composite fibre 3 in the configuration adopted when the relative humidity decreases. As shown, the spiral composite fibre 3 increases in length and decreases in width as the relative humidity decreases and vice versa. By rearranging the absorbent and support components of the composite fibre 3 within the spiral arrangement, it is possible to create a spiral which decreases in length and increases in width as relative humidity decreases and vice versa.

Fig. 3 shows a composite fibre comprising an absorbent matrix 1, provided in cylindrical form, and two stiff fibrils 2 which are helically wound around the absorbent matrix 1. The acute angle of the fibril winding to the longitudinal axis of the composite fibre determines whether the fibre increases or decreases in length in the presence of the fluid which the absorbent matrix is selected to absorb. The points at which the fibrils 2 cross must be allowed to move, either in shear or pivotally about the cross points.

Any of the composite fibres described above can be knitted, woven or incorporated into a fabric in some other way. In the presence of the fluid to which the absorbent component reacts or on temperature changes in the surrounding environment, the composite fibres will alter their configuration and thereby become more or less closely packed together, changing the physical properties of the fabric.

Claims

1. A composite fibre comprising a compliant absorptive component, which changes volume in response to changes in its surrounding environment, and a less absorbent support component, which is arranged to constrain volume changes in the absorptive component and thereby alter the composite fibre's configuration.

2. A composite fibre as claimed in claim 1 wherein the absorptive and support components are provided by separate elongate strands which are tightly bonded together.

3. A composite fibre as claimed in claim 2 wherein the separate elongate strands are co-extruded.

4. A composite fibre as claimed in claim 1 wherein a uniform fibre is chemically treated to alter the physical properties of part of the fibre, thereby providing the absorptive and support components.

5. A composite fibre as claimed in any of the preceding claims provided in a spiral arrangement to amplify changes in the fibre's configuration.

6. A composite fibre as claimed in claim 1 wherein the support component is provided by one or more fibrils which are helically wound at an angle to the longitudinal axis of the fibre such that the absorptive component is restricted to a change in length in response to changes in its surrounding environment.

7. A composite fibre as claimed in claim 6 wherein the absorptive component comprises a matrix material in which the fibrils are embedded.

8. A composite fibre as claimed in claim 6 wherein the absorptive component comprises and elongate core about which the fibrils are wound.

9. A composite fibre as claimed in claims 6 to 8 wherein two fibrils are wound in a double helix.

10. A composite fibre as claimed in claims 6 to 9 wherein the angle of the fibril winding to the longitudinal axis of the fibre is an acute angle of 600 or more.

11. A composite fibre as claimed in claims 6 to 9 wherein the angle of the fibril winding to the longitudinal axis of the fibre is 30 or less.

12. A composite fibre as claimed in any of the preceding claims wherein the absorptive component absorbs a fluid from its surrounding environment.

13. A composite fibre as claimed in claim 11 wherein the absorptive component is provided by a hygroscopic material such that the composite fibre responds to the relative humidity of its surrounding environment.

14. A composite fibre as claimed in any of the preceding claims wherein the support component is substantially non-absorbent.

15. A composite fibre as hereinbefore described with reference to the accompanying drawings.

16. A responsive material comprising a plurality of composite fibres as claimed in any of the preceding claims arranged such that the material changes its permeability in response to its surrounding environment.