By Alderson, Andy; Alderson, Kim
Auxetics are extraordinary materials that become fatter when stretched and thinner when compressed. Andy and Kim Alderson of Bolton University introduce us to this exotic world and some of the applications for textiles made with these materials.
What might you do with a textile that, rather than becoming thinner, exhibits the unusual behaviour of increasing in thickness when stretched (Figure 1) or becoming thinner when compressed? Perhaps it could be a medical suture or a fibrous reinforcement in a composite-in both cases it could be envisaged that this unusual property would lock the fibre, filament or yarn into place when it’s placed under a tensile load (Figure 2). It might be made into a so- called “smart” bandage-capable of releasing a useful (such as an anti-inflammatory, anti-odour or anti-bacterial) agent from within the pores of the filaments making up the structure (Figure 3) or a breathable fabric demonstrating increased porosity variation due to the high volume change associated with this unusual behaviour.
In fact, materials with this counterintuitive property exist and are known as auxetics’ (from the Greek word auxetos meaning “that which grows”). Here we briefly review the current state-of-the art in auxetics and report on recent progress towards the development of auxetic technical textiles. We shall see that these materials have other benefits too and so the applications you are already thinking of will, excuse the pun, expand as we proceed through the article.
The range of materials and structures that exhibit auxetic behaviour is, perhaps,greater than might be expected when first encountering the concept. In fact, a variety of naturally occurring auxetics are now known (including, for example, certain types of skin(ii,ii)) and auxetic forms of the four major classes of materials (metalse;, ceramics(v), polymers(vi,vii) and compositesv(viii,ix)) have been made or discovered. Further, the auxetic effect is known to arise from material features acting from the molecular level, for example auxetic silica(x), all the way to the macroscale, such as the graphite core structures in certain nuclear reactors(xi) (Figure 4, page 30).
Interest in auxetics is due to the effect itself, and because the behaviour leads to enhancements in a range of other properties including:
Figure 1: Counter intuitively, auxetics are materials that increase in thickness when stretched or become thinner when compressed.
Figure 2: In a fibre, filament or yarn, the unusual properties of auxetics can be exploited in different ways; for instance, to lock the textile in position when it is subjected to a tensile load in, say, a composite matrix.
Figure 3: Alternatively, the auxetic might be used in a so- called “smart” bandage-one capable of releasing a useful (such as an anti-inflammatory, anti-odour or anti-bacterial) agent from within the pores of the filaments making up the structure.
Figure 4: The auxetic effect is observed in material features acting from the molecular level, for example auxetic silica, all the way to the macroscale, such as the graphite core structures in found certain nuclear reactors.
* increased resistance to impact(xii) (Figure 5);
* energy absorption(xiii);
* fracture toughness(xiv);
* the ability to form doubly curved (domed) shapes(xv) (Figure 6);
* porosity variation with applied strain(xvi).
Consequently, potential applications for auxetics include, for instance, civil engineering applications (seismic structures), aerospace, automotive and marine applications (lightweight curved body parts), and chemical engineering and pharmaceutical applications, exploiting the porosity variation to entrap and/or release material (such as drug molecules and volatile compounds) within the pores of the auxetic.
How is the auxetic effect achieved?
The auxetic effect is achieved through the interplay between the internal structure of a material and how it deforms. The classic example is to consider a honeycomb deforming by hinging of the walls of the honeycomb cells (Figure T). In the conventional hexagonal honeycomb the alignment of the cell walls along the direction of stretch results in a narrowing of the honeycomb cells and, therefore, conventional (non-auxetic) behaviour (Figure 7a). However, converting the structure of the honeycomb from the conventional hexagon to a re-entrant (or bow-tie) hexagon (Figure 7b) clearly shows an opening of the cells as the honeycomb is stretched, leading to auxetic behaviour. Figure 7 also demonstrates the entrapment and release capabilities of auxetics for potential use in sieving and controlled delivery applications.
Figure 5 (above left): increased resistance to impact and Figure 6 (above right): the ability to form doubly curved (domed) shapes.
The first example of a synthetic auxetic occurred in the late 1980s when Roderic Lakes developed a route to convert conventional open-cell polyurethane foams into an auxetic form(xvii). Through a combination of triaxial compression and heat treatment of the foam, Lakes was able to perform the three-dimensional (3D) equivalent of converting a conventional hexagonal cell structure to a re-entrant hexagonal cell structure.
At around the same time, Ken Evans was discovering that a commercially available form of expanded polytetrafluoroethylene (PTFE) exhibited auxetic behaviour6, and that in this case the effect arose due to structural features at the microscale, in contrast the foams where it was a macroscale effect. We have subsequently worked with Ken Evans to reproduce the microstructure observed in PTFE in other microporous polymers – ultra-high molecular weight polyethylene (UHMWPE)7, polypropylene (PP)(xviii) and nylon(xix) – and to develop models to understand the deformation mechanisms giving rise to the auxetic effect in these polymers(xx,xxi).
Figure 7: Auxetic effects result from the interplay between tfie internal structure of a material and how it deforms.
Textile developments-monofilaments
The batch-processing route developed to produce auxetic microporous polymers was based on powder metallurgical techniques of compaction, sintering and extrusion, and resulted in cylindrical specimens of the order of 1-1.5cm in diameter and a few centimetres in length. Our group at Bolton University has successfully extended this knowledge to develop a continuous partial melt extrusion process to produce polymeric monofilaments displaying auxetic behaviour(xxii). To date we have made auxetic PP22, polyester(xxiii) and nylon23 monofilaments with diameters in the range of 0.14-1 mm.
The processing route produces filaments having a microstructure of interconnected surface-melted powder particles, and so the mechanical properties, including the auxetic effect, arise due to structure and deformation mechanisms at the microscale, rather than at the molecular level as in conventional filaments extruded from a fully molten polymer. Consequently, the stiffness and strength are not yet sufficient for load-bearing applications.
Nevertheless, we have been able to use the filaments to perform proof-of-concept studies to confirm, for example, the potential for auxetic filaments in sutures or fibre-reinforced composites having enhanced resistance to the fibres being pulled out (Figure 2, page 29); Figure 8 shows the results of comparative single-fibre tests(xxiv) on samples in which a single filament has been embedded within an epoxy resin and then the free-end of the filament subjected to a tensile load to extract it using a mechanical testing machine. Auxetic and non-auxetic PP filaments, having diameters and Young’s moduli equal to within 2% between the filaments, were tested.
The results shown in Figure 8 are the average of several tests and clearly demonstrate the auxetic specimen can sustain more than twice the maximum load of the non-auxetic specimen, as well as requiring more than three times the energy (denoted by the area under the curve) to extract the filament from the resin. This not only confirms the concept of enhanced anchoring behaviour, but also suggests that, while the current filaments may not be appropriate loadbearing constituents within fibre-reinforced composites, they do have clear potential as energy-absorbing components within these structures.
Figure 8: Comparative single-fibre tests on samples of auxetic and nonauxetic polypropylene filaments, having diameters and Young’s moduli equal to within 2%, show that auxetic specimens can sustain more than twice the maximum load of the non-auxetic ones, as well as requiring more than three times the energy (denoted by the area under the curve) to extract the filament from the resin.
Figure 9: Scientists at Exeter University have adopted an alternative approach to the development ofauxetic textiles. They have produced a multifilament construction in which a high- stiffness filament is wrapped helically around a thicker, low- stiffness filament. Neither of the constituent filaments are required to be auxetic.
Work is in progress at Bolton University to demonstrate:
* further enhancements in applications employing auxetic filaments;
* to improve the other physical p\roperties of the filaments;
* to produce other polymers in auxetic filamentary form;
* to develop new routes to produce auxetic monofilaments.
The current process has also been adapted to successfully produce auxetic PP films (about 0.15 mm in thickness).
Ultimately, auxetic monofilaments will be achieved where the auxetic effect occurs at the molecular scale, leading to the production of monofilaments having the high strength and stiffness required for load-bearing applications. Recent work by synthetic chemists such as Anselm Griffin at the Georgia Institute ofTechnologyxxv and Stephen Moratti at the University of Cambridge is extending the earlier attempts at designing molecular-level auxetics by engineers such as ourselves and Ken Evansxxvl.
With the current rate of progress in synthetic chemistry, and the increased understanding of mechanisms leading to auxetic behaviour at the molecular levelxxv”, it is likely that the first synthetic molecular-level auxetic material will be produced in the near future.
Auxetic multifilaments
Ken Evans and Patrick Hook at the University of Exeter have adopted an alternative approach to the development of auxetic technical textiles. They have produced a multifilament construction in which a high-stiffness filament is wrapped helically around a thicker, low-stiffness filament (Figure 9)xxviii. Neither of the constituent filaments are required to be auxetic. The overall multifilament construction exhibits auxetic behaviour upon stretching due to straightening of the high-stiffness filament causing the lower stiffness filament to helically wrap around it.
These multifilament constructions can be produced using existing textile machinery, such as wrap spinning,for instance.
Combining two of these multifilaments in an appropriate manner leads to further development of an auxetic structure and the Exeter University team has used this approach to produce an aramid-nylon multifilament yarn that is now moving the technology towards load- bearing applications.
Stretching into the future
The developments in auxetic monofilaments and multifilaments clearly demonstrate potential in a host of applications. The suture and fibre-reinforced composite applications have already been referred to above. However, imagine what might be achievable if we can produce auxetic yarns having other useful properties; for example, a conductive auxetic yarn would have sensor and actuator potential for use in monitoring aerospace and civil engineering structures, and as a synthetic biomaterial where a high volume change stimulated via electrical signals is required (such as muscle tissue).
Also it is possible to consider employing one of the Bolton University monofilaments within the Exeter University multifilament yarns to produce a hierarchical structure in which the benefits due to auxetic functionality exist at two different length scales (monofilament microstructure and multifilament macrostructure).
The possibility of creating textile structures (from either or both auxetic and non-auxetic filaments or yarns) in which the fabric structure creates the auxetic effect would have interesting possibilities in, for example, bandages for compression therapy (where the bandage would react to compress swelling of the limb while also improving breathability where and when needed).
The use of auxetic filaments, yarns or fabric structures to deliver active agents is another as yet largely unexplored area, but could lead to intelligent textiles having anti-inflammatory, anti- odour, or drug-release capabilities (see Figure 3, for example).
The energy absorption enhancements of auxetics lead to the possibility of developing personal protective equipment and clothing (from bulletproof vests to equipment for sports) which are both lighter and/or stronger as a result of incorporating auxetic textiles. Not only that, but the novel double-curvature characteristics of auxetics (Figure 6) would lead to increased comfort in these often cumbersome protectors, leading to improved wearer compliance, which can be critical in cases where the user is elderly or infirm.
How are such developments likely to be achieved? In the UK, there are two university spin-out companies, each with the sole purpose of commercializing intellectual property relating to auxetic materials developments:
* AuxeticTechnologies Ltd (working with the Bolton University group);
* Auxetix Ltd (working with the Exeter University group).
These companies are already engaging with prospective commercial partners to develop the relationships necessary to take the technologies to market. There is also the Auxetic Materials Network (AuxetNet)xxix,funded by the Engineering and Physical Sciences Research Council (EPSRC) and hosted by Bolton University, which includes six academic partners (including Bolton and Exeter) and eight commercial partners (including Auxetix Ltd). The commercial partners are active in steering the research towards commercialization via this network.
It is clear then that the efforts described here should ensure that we are driving towards the realization of truly intelligent and technical textiles for the near future.
TechniTex
This is the second in a series of articles based on the TechniTex Core Research programme (see also, Technical Textiles International, May 2005, pages 19-22). For further information on the activities of TechniTex, see Technical Textiles International, October 2004, pages 7-10, or contact: Brian McCarthy, TechniTex Faraday Ltd.
Tel: +44-161 -306-8500. Email: [email protected]; Internet: www.technitex.org
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Further information
The authors gratefully acknowledge the provision of material relating to auxetic multifilaments by Dr Patrick Hook (Auxetix Ltd), and funding for the monofilament developments from the EPSRC, Bolton and Bury Chamber’s Business Link, Du Pont and Auxetic Technologies Ltd.
Andy Alderson, Centre for Materials Research and Innovation, The University of Bolton, Deane Road, Bolton BL3 SAB, UK.
Tel: +44-1204-903513. Fax: +44-1204-370916.
E-mail: [email protected];
Kim Alderson. Tel:+44-1204-903519.
E-mail: [email protected]
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