Composites that grow in fields

A small sample of agricultural straw is passed round the room, hand to hand, while the speaker explains that, unlikely as it might seem, this material is key to a new form of reinforced plastic.

The speaker is Marek Radwanski from Ekotex, a Polish company that grows flax, from the stems of which it extracts natural fibres that can be used as reinforcement in ‘biocomposites’ – reinforced plastics wholly or partly derived from plants. The event is a late September symposium on biocomposites held at Smithers Rapra, Shawbury – appropriately deep inside Shropshire’s (UK) agricultural countryside. All the presenters today are from organisations taking part in BIOCOMP, an integrated project for small to medium-scale enterprises (SMEs) supported by the European Commission through the EU’s sixth framework programme. The project commenced in 2005 and the seminar approximately coincides with its finish.

Natural fibres

The agricultural nature of the reinforcements under discussion impresses itself still further as more samples illustrate the various stages of planting, harvesting, retting, scutching etc that extract from the raw straw cellulosic fibres having significant mechanical and other properties. Samples of unidirectional tows and nonwoven mats made with the fibres provide the persuasive clincher to the idea that one can actually grow useful fibre reinforcements in fields.

Radwanski makes it abundantly clear that obtaining bio-fibres in this way requires the application of agricultural science, including appropriate techniques for fertilisation, seeding, chemical protection of plants and harvesting, along with favourable growing conditions and seasonal factors. Once the flax is harvested, it has to be de-seeded, after which comes the critical retting process, used to remove nature’s pectin ‘gum’ that holds the fibres together. Dew retting the natural way in the fields takes three to six weeks, explaining why the Institute of Natural Fibres in Poznan has investigated osmotic retting as a potentially faster alternative. Once the pectin has been removed, fibres are recovered by scrutching, a mechanical process that yields short fibres for making into tows and long fibres that can be used in technical textiles.

Fibres can be cut to lengths to suit particular client uses. For instance, cleaning a flax tow, carding it to produce 18-30 ktex slivers then cutting the slivers to lengths of 1 mm, 2 mm, 4 mm etc serves to prepare the fibres for injection moulding. Fibre properties can be improved by further processes such as boiling, bleaching and plasma treatment.

Expounding the virtues of natural fibres as compared with E-glass, Radwanski cites density about half, low cost, lower embodied energy and the fibres’ non-abrasive nature. Natural fibres (NFs) can, he declares, be recycled, are a source that is renewable, and are carbon neutral since carbon dioxide (CO2) emitted during production is reabsorbed by new plant growth. They pose no risk to human health when fibre particles are inhaled,and at end of life they are biodegradable. A downside is that, unlike glass, NFs are hydrophilic and can, if left unprotected, absorb moisture, swelling in the process.

Although concentrating on flax, the speaker also mentions the role of hemp since this too can be grown in Europe, though it is somewhat more difficult to cultivate. On the other hand, hemp requires less soil than flax and has greater resistance to drought. Fibres from flax and hemp – also other plants such as jatropha, ramie, coir, sisal and kenaf – can be used in thermosets and thermoplastics.

Resins too

Resins, too, can be grown in fields, as Hans Hydockx, business development manager with TransFurans Chemicals bvba (TFC), Belgium, made clear in a presentation that followed. TFC starts one of its core activities with bagasse, a biomass waste product from sugar cultivation, and uses it to produce furans-based resins for exploitation in composites, modified wood, polymer concrete and other products. Furans is a basic ring-configured organic chemical that gives rise to a number of process streams.

TFC produces thousands of tonnes per year of furfural, a hemi-cellulose, which is then converted using a high-pressure steam process into furfuryl alcohol, a key platform chemical from which resins can be obtained. Although TFC exploits bagasse, other agro wastes containing the pentosan (C5) form of sugar can be used; these include corn cobs, oat hills, flax shivers, cotton seed hulls, almond husks, beech and birch wood, hazelnut shells and others. From furfuryl alcohol chemists can, by the use of various co-reactants and additives, derive natural thermoset resin that can be used with various organic and inorganic fibres to form composites. The biomatrix, which is notably fire and chemical resistant, can also be used with rockwool fibres in horticultural growing substrates, in wood to make a durable wood that can resist fungal and microbial attack, and in other products. One fire and chemical-resistant plastic made from it is Furolite, a resin which, when NF reinforced, results in an entirely plant-derived composite.

Furolite thermoset resin can be reinforced with natural, glass and carbon fibres plus natural and mineral fillers to produce engineering materials. It can, says Hoydockx, be used with hand lay-up, spray up, prepreg, vacuum infusion, pultrusion and filament winding techniques. Composites can be made by spraying the resin into NF mat that is then compression moulded into a finished part, or by passing a NF textile through a resin bath.

The speaker explained that the resin adheres well to a range of fibres and has been used in prototype biocomposite floor modules, trunks, head liners and similar parts for passenger cars. Advantages include solvent-free part production with no harmful volatile organic compounds (VOCs), easy machining with negligible abrasion of cutting tools, reduced reliance on oil feedstock and of course, the fact that the composite is made entirely from renewable resources. Cycle times of below 60 seconds at temperatures of around 180°C make the system suitable for automatic high-volume production. Physical properties are said broadly to equal those of NF-reinforced polyester composites, while high resistance to corrosion means that the resin can usefully substitute for epoxy in corrosive organic acid environments.

Bo Madsen from the Materials Research Department of the Risø National Laboratory for Sustainable Energy at the Technical University of Denmark conceptually brought the two composite phases, fibres and matrix, together in his consideration of biocomposite properties. Warming to his subject, he reminded us that nature has optimised plant fibres over millions of years and declared that biocomposites are both affordable and immune to oil price escalation. In subsequent discussion, he expressed his belief that biocomposites have a bright future.

Madsen told delegates that plant-derived, cellulosic fibres, such as the bast fibres from flax and hemp, can provide good properties, but that there is a natural variability within a plant population, and even within individual plants according to where in the plant the fibres are taken from. The most useful fibre bundles are found in the outer stem layers just beneath the epidermis. Fibres from inside the stem are shorter and trail off into the shortest shives which, from the composite point of view, are useless. Dislocations and discontinuities within fibres can degrade properties, as can voids and lumen – space within the fibres. Flax and hemp have low lumen whereas wood fibres can be up to 50% lumen. Other influential factors include degree of crystallinity, fibre density and volume fraction, plus the cross sectional area and shape of the fibre bundles.

Cellulose is itself nature’s sophisticated composite, said Madsen, with micro fibrils made up of complex polymer chains arranged in various alignments, all within nature’s hemicellulose or lignin matrix. Generalised properties for natural fibres include density of around 1.5 g/cm3, strength of 300-900 MPa and stiffness 30-70 GPa, compared with 2.6 g/cm3, 3500 MPa and 70 GPa respectively for E-glass. In terms of composite properties, plant fibre composites typically have lower stiffness per volume but higher stiffness per weight. Thus for a given material stiffness, plant fibre composites need larger volume, but will weigh less than a comparable glass fibre composite. Plant fibre composites also excel in stiffness per cost (based on costs of plant and glass fibre preforms – nonwoven and strand mat). Overall, argued Madsen, new-generation biocomposites can be viable alternatives to traditional composites in selected applications.

Embedding the fibres into resins such as polypropylene (PP), polyethylene teraphthalate (PET) and polylactic acid (PLA) provides complete dual-phase biocomposites that are suitable for efficient manufacturing processes. Production articles can, for instance, be made by interleaving films of PLA (made from fermented corn starch) and nonwoven jute mat layers in a stack and then compression moulding the result, all in a single continuous manufacturing step. A fibre volume fraction of around 40% results in a biocomposite having a stiffness of some 9 GPa and strength of 100 MPa. Another manufacturing method is to co-mingle the two phases by filament winding the fibre with thermoplastic yarn.

As with conventional composites, properties are highly dependant on fibre alignment, fibre volume and weight fractions, but also on porosity. Risø researchers have devised a mathematical tool for modelling the properties of biocomposites. The result differs from ‘standard’ models (ie. for standard composite materials) in allowing for the porosity (and hence moisture absorption) of natural fibres that can have a deleterious effect on tensile stiffness and other mechanicals.

How green is your biocomposite?

Environmental factors were the concern of Frank Markert, also from the Risø campus of the Technical University of Denmark, who outlined the role of life cycle analysis (LCA) in determining how ‘green’ any given biocomposite is. LCA is a systematic cradle-to-grave approach to assessing the environmental impacts of a material during extraction, manufacture, service life and disposal. It takes into account such factors as CO2 release; pollution of soil, surface and ground water; effects on human health – including occupational health and accidents; energy and resource consumption; generation of waste; energy release from end-of-life incineration, and so on. For a biocomposite, for instance, LCA can be useful in evaluating the effects of pesticides and fertilisers and for taking note of various environmental indicators such as eutriphication (depletion of oxygen in surface water) resulting from flax or hemp retting. As examples, Markert identified some of the impacts of flax retting and showed how, environmentally, the production of PLA using biomass feedstock and wind energy compares well with traditional methods of corn fermention.

The speaker declared that LCA amounts to a green way of thinking that is not just ‘eco point scoring’ but yields real environmental benefits. It can reveal the best ways of producing a material or item consistent with minimum inputs and environmental effects. There are now established standards for LCA, the latest ISO/TS14048 having been published in 2006.

Applications

Brendan Weager, technical manager of NetComposites UK, shifted the focus to biocomposite applications, while highlighting his company’s consolidation role within BIOCOMP. In discussing the desirability of replacing oil-based composites with plant-derived substitutes, he re-emphasised earlier assertions by other speakers that the mechanical properties of biocomposites can compare well with those of glass reinforced plastics (GRP), in some instances exceeding them. There remain, however, issues around aspects of availability, quality consistency and degradation through moisture absorption. Even so, there is great interest in the potential for biocomposites, especially given the growing socio-economic imperatives, exemplified by legislation such as Europe’s end-of-life directives and the way these will affect automotive and other industries.

As a result, we now see emerging engineering applications for biocomposites. Examples include vehicle door and head liners and other interior semi-structural parts. Car maker Lotus, for instance, uses a natural fibre/polyethylene matrix composite in plastic profiles that resemble wood in appearance.

Waeger foresaw potential for thermoplastic biopolymers such as polyhydroxylbutyrate (PHB), PLA and starch, as well as thermosets based on oils from crops such as linseed, castor and sugar (furans). In the case of full dual-phase biocomposites (fibres and resin matrix), low impact resistance has been a mechanical weakness but work by the Transfercenter for Polymer Technology (TCKT) in Austria has shown that addition of impact modifiers can improve this quality. TCKT has also shown that PLA/flax and hemp composites can be compounded using co-rotating twin-screw extruders. Inputting short fibres (less than 2 mm) to the extruder can result in composites with fibre volume fractions of up to 35%, but using NF/PLA in pelletised form results in higher throughput and hence fractions of up to 50%. Samples of NF/PLA plastics on display during the seminar included edge protectors for furniture packaging, PLA/flax and PLA/wood footplates for operators in machine shops, and a PLA/flax box with walls just 1 mm thick. Simple geometric profiles can be manufactured in volume.

TCKT also showed, as part of its BIOCOMP commitment, that conical twin-screw extrusion can be used to produce items having more complex three-dimensional geometries. Flax/PLA and wood/PLA decking, flooring, railings and other products can be extruded using short fibres, while longer fibres lend themselves to production of semi-structural items such as foot rests in Smart cars. Producing a composite by spraying a bioresin onto a NF mat and then hot compression moulding the result can yield useful structural items such as door panels. This continuous one-shot process lends itself to high volume production, cycle time being less than 1 minute, using tooling and other production facilities that would typically exist already in a vehicle manufacturing facility. Long-fibre composites can also be produced by the Direct Long Fibre Thermoplastic (DLFT) process which involves the in-line compounding of fibres and resin coupled with injection moulding of the intended part.

A panel passed round for delegate inspection at the seminar was designed for an ambulance and derives stiffness both from the NF/furans composite used and the panel’s three-dimensional geometry. This item was produced by hand lay-up into a mould and then consolidating the composite under vacuum. In tests, the panel performed fairly well against a GRP equivalent, withstanding high maximum stresses – albeit tensile strength was lower. Fastener pull through and bolt bearing resistance were, however, comparable.

Crop-based resins can substitute for formaldehyde, the main resin binder used in medium density fibreboard (MDF), a wood substitute. The National Institute of Wood in Romania has demonstrated this by producing an environmentally-friendly bio-MDF inner door whose properties compare well with those of standard formaldehyde-based MDF.

Early days

These are early days for biocomposites but, from these and other examples, the speaker drew positive and encouraging conclusions. In summary, Weager emphasised that BIOCOMP has shown that NF/PLA compounds can be injection moulded and extruded successfully into viable products, and that NF/furans composites compare well with conventional glass-based equivalents, while additionally offering superior fire and chemical resistance. The fact that crop oil-based resins can replace formaldehyde in MDF products raises other market prospects, said Weager. All in all, he concluded, the results bode well for future commercial exploitation of biocomposites, particularly in commodity and automotive sectors where there is pressure to reduce environmental impact, weight and cost.

Further research in which NetComposites is involved include COMBINE, the UK’s £1 million Comingled Biomaterials from Nature programme running from 2007 to 2009; BIOCONSTRUCT, a €10 million European 7th Framework programme aimed at delivering complex structural and multifunctional parts from enhanced wood-based composites, running from this year to 2012; and NATEX, another FP7 project, worth €5 million, to further the state-of-the-art in natural aligned fibres and textiles for use in structural composite applications, on-going from 2008 to 2012.

Overall the seminar, as well as airing results from BIOCOMP, showed that biocomposites look like being more than just a straw in the wind and that 21st Century drivers are hastening their development and exploitation.