GB2463930A

GB2463930A - Auxetic monofilaments

Info

Publication number: GB2463930A
Application number: GB0817871A
Authority: GB
Inventors: David Skertchly
Original assignee: Global Composites Group Ltd
Current assignee: Global Composites Group Ltd
Priority date: 2008-10-01
Filing date: 2008-10-01
Publication date: 2010-04-07
Anticipated expiration: 2028-10-01
Also published as: GB2463930B; GB0817871D0

Abstract

A monofilament which displays auxetic behaviour, wherein the monofilament comprises a low modulus component and a high modulus component, each extending longitudinally along the monofilament such that a cross-section of the monofilament comprises a region of a material having a low modulus and a region of a material having a high modulus, the position of the high modulus region in the cross-section varies along the length of the fibre, and the auxetic behaviour occurs due to a change in the relative transverse positions of the high and low modulus components when a strain is applied to the monofilament.

Description

I

AUXETIC MONOFILAMENTS

Background

The current application relates to auxetic monofilaments, and in particular to auxetic monofilaments manufactured by extrusion.

The Poisson's ratio of a material is a measure of its expansion or contraction in a direction perpendicular to an applied strain. Materials with a positive Poisson's ratio contract in a direction perpendicular to an applied tensile strain whereas materials having a negative Poisson's ratio expand in a direction perpendicular to an applied tensile strain. Materials having a negative Poisson's ratio are known as auxetic materials.

Auxetic materials have been proposed in various forms, including foams, composite materials and fibres. W02004/088015 proposes an auxetic fibre formed by intertwining first and second filaments in a helical pattern. When such a structure is stretched longitudinally, realignment of the fibres may cause a transverse expansion of the fibre. The fibres proposed are, however, complex to manufacture and relatively fragile due to the multiple filament structure.

There is therefore a requirement for a robust, easy to manufacture, auxetic monofi lament.

Summary

The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not an extensive overview of the disclosure and it does not identify key/critical elements of the invention or delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

There is provided a monofilament which displays auxetic behaviour, wherein the monofilament comprises a low modulus component and a high modulus component, each extending longitudinally along the monofilament such that a cross-section of the monofilament comprises a region of a material having a low modulus and a region of a material having a high modulus, the position of the high modulus region in the cross-section varies along the length of the fibre, and the auxetic behaviour occurs due to a change in the relative transverse positions of the high and low modulus components when a strain is applied to the monofilament.

Optional features of the monofilament are set out in the claims.

There are also provided methods of manufacturing the monofilaments described herein.

Description of the drawings

Embodiments of the present invention will now be further described, by way of example, with reference to the drawings, wherein:-Figure 1 shows a cross-section of a monofilament according to an embodiment of the invention; Figure 2 shows a side view of a monofilament according to an embodiment of the invention in a relaxed state; and Figure 3 shows a side view of a monofilament according to an embodiment of the invention in a stretched state.

Detailed description

The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present example may be constructed or utilized. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example.

However, the same or equivalent functions and sequences may be accomplished by different examples.

A monofilament having auxetic properties is described. The auxetic properties are inherent and do not rely on particular wrapping or knitting steps to acquire that characteristic.

Figure 1 shows a cross-section of an auxetic monofilament according to an embodiment of the current invention. Monofilament 10 comprises two sections, a low modulus section 11 and a high modulus section 12. The circumferential position of the high modulus section 12 rotates around the low modulus section 11 along the length of the fibre such that it forms a helix around the monofilament along the length of that monofilament. The overall monofilament is, however, in this embodiment, straight and does not follow a helical pattern.

When the monofilament of Figure 1 is stretched the high modulus section 12 moves laterally such that its overall length increases to accommodate the stretch. The low modulus section 11 is pushed laterally by the high modulus section 12. As the magnitude of stretch is increased, the deformation increases such that the high modulus section 12 tends towards a direct line between the two ends of the monofilament 10.

As the high modulus section 12 deforms laterally towards the direct line, the low modulus section 11 is pushed away from that direct line. The monofilament 10 is therefore, at each point along its length, no longer centred on the direct line. The direct line actually lies between the physical centre of the monofilament 10 and the location of the high modulus section 12 at each point along the monofilament's length. Since the high modulus section 12 forms a helix in the relaxed state, the transverse direction of the offset of the physical centre from the direct line varies along the monofilament's length.

The diameter of the space occupied by the monofilament along its length is therefore increased compared to the relaxed state. The monofilament therefore has a negative Poisson's ratio and thus has auxetic properties.

Figure 2 shows a schematic side-view of the monofilament 10 of Figure 1 in the relaxed state. The high modulus section 12 forms a helix around the monofilament which is formed mostly of low modulus material 11.

Figure 3 shows a schematic side-view of the monofilament 10 of Figure 1 in a stretched state, to the same scale as Figure 2. The dashed line 30 represents the average centre line of the monofilament, towards which the high modulus section 12 is tending. The dotted lines 31 represent the size of the monofilament in the relaxed state shown in Figure 2. As can be seen the average radius over length of the monofilament has increased by the distance 32.

The Poisson's ratio of the monofilament is affected by a number of physical parameters of the monofilament, including the relative modulus values of the high and low modulus elements, the centres of area of the high and low modulus elements, and the pattern of the high modulus element. For example, a ratio of 2:1 between the modulus of the high and low modulus sections may provide a monofilament with a useful negative Poisson's ratio.

As will be appreciated by the skilled person the performance of a particular design can be ascertained using well-known modelling or experimental techniques.

The Poisson's ratio of the filament is defined by the shape change which occurs when the high modulus section tends towards the direct line. That shape change is in turn defined by the shape of the high modulus section along the length of the monofilament in the relaxed state. For example, the above example utilised a helical arrangement of the high modulus element which results in a shape-change in both transverse axes. The monofilament thereby has a negative Poisson's ratio in both of the transverse axes. In further embodiments different shapes may be utilised for the high modulus section resulting in different behaviour. For example, a sine-wave pattern, in which the high-modulus section lies on a single plane and passes through the centre of the low modulus section would lead to the monofilament only deforming in one axis, and thereby the fibre would have a differential Poisson's ratio between the two axes, being negative in the axis of deformation and being defined by the conventional material properties in the other axis. As will be appreciated by the skilled person, these are only examples and that by tailoring the shape and dimensions of the path of the high modulus section the behaviour of the monofilament can be defined.

The monofilaments described in relation to Figure 1 may be utilised in any application where a filament having a negative Poisson's ratio is required. For example, the filaments may be utilised in woven or knitted fabrics to provide a fabric having a negative Poisson's ratio.

The monofilaments described herein are particularly suitable for manufacture using thermoplastic or thermosetting plastics in an extrusion process. In particular the monofilaments may be manufactured in a single extrusion step using a piggyback striping extruder. In such machines, a die is fed with two materials such that the extruded product comprises the two constituents in a pattern defined by the configuration of the die. It will be apparent to the skilled person how to design and manufacture a die suitable for the production of the monofilaments described herein. The die design will depend upon the pattern of the high modulus material along the length of the monofilament.

The spiral pattern of the high modulus element of the monofilament shown in Figures 1 and 2 may be produced by rotation of the die during extrusion, or by twisting of the monofilament after extrusion and prior to winding onto a storage bobbin.

In an alternative embodiment, the high modulus element may be provided by a high modulus fibre within a low modulus material. For example, a carbon fibre may be formed in a helix, or other appropriate shape as discussed above, within a filament of low modulus material. Such a monofilament may be formed using an extrusion technique wherein the fibre is introduced in the extruder through the die, or is laid into the extruded monofilament on its exit from the die.

In order to allow the monofilament to be attached at its ends, the high modulus element may extend beyond the end of the low modulus element.

The pattern of the high modulus element is not necessarily consistent along the length of the monofilament, but may vary to provide varying behaviour along the length of the monofilament.

As will be appreciated, the description of the monofilaments described above as having a negative Poisson's ratio are references to the expansion of the outer diameter of the helix, or other shape, formed by the monofilament when a strain is applied. It is not intended to imply that the actual monofilament dimensions change to provide the auxetic behaviour. Rather, the negative Poisson's ratio is achieved by a rearrangement of the relative positions of the high and low modulus sections when a longitudinal strain is applied to the monofilament, which leads to a change in the shape taken by the monofilament. The negative Poisson's ratio is therefore provided by an increase in the space occupied by the monofilament due to rearranging into a helical, or other, non-straight shape. References to the Poisson's ratio of the monofilaments should therefore be read in an appropriate manner.

The shape of the high modulus region shown in the cross-section of Figure 1 is for example only, and as will be appreciated, various shapes of the high modulus region may be appropriate.

The description has been given in relation to circular cross-section filaments, but as will be appreciated the construction and methods described herein are equally to various cross-section shapes of filament.

It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. It will further be understood that reference to an' item refers to one or more of those items.

Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments of the invention. Although various embodiments of the invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.

Claims

Claims 1. A monofilament which displays auxetic behaviour, wherein the monofilament comprises a low modulus component and a high modulus component, each extending longitudinally along the monofilament such that a cross-section of the monofilament comprises a region of a material having a low modulus and a region of a material having a high modulus, the position of the high modulus region in the cross-section varies along the length of the fibre, and the auxetic behaviour occurs due to a change in the relative transverse positions of the high and low modulus components when a strain is applied to the monofilament.
2. A monofilament according to claim 1, wherein the high modulus component is arranged in a helix.
3. A monofilament according to any preceding claim, wherein the monofilament forms a helix when a longitudinal strain is applied to the high modulus component.
4. A monofilament according to claim 1, wherein the position of the high modulus region in the cross-section varies in two axes perpendicular to the longitudinal axis of the monofilament along the length of the monofilament.
5. A monofilament according to claim 1, wherein the position of the high modulus region in the cross-section varies in only a single axis perpendicular to the longitudinal axis of the monofilarnent along the length of the monofilament.
6. A monofilament according to any preceding claim, wherein the modulus of the high modulus component is twice the modulus of the low modulus component.
7. A monofilament according to any preceding claim wherein the monofilament comprises a thermoplastic or a thermosetting plastic.
8. A monofilament according to any preceding claim, wherein the high modulus element is a fibre.
9. A monofilament according to claim 6 wherein the fibre is a carbon fibre.
10. A method of manufacturing a monofilament according to any of claims 1 to 9, comprising the steps of extruding a high modulus component and a low modulus component through a die to form a monofilament.
11. A method of manufacturing a monofilament according to claim 10, wherein the extrusion is performed utilising a piggy-back extruder.
12. A method of manufacturing a monofilament according to claim 11, wherein the position of the high modulus region in the cross-section of the monofilament is varied by movement of part of the die.
13. A method of manufacturing a monofilament according to claim 10 or 11, wherein the position of the high modulus region in the cross-section of the monofilament is varied by twisting the monofilament as it leaves the die.