Building in natural strengths - natural fibre composite materials
Chartered Architect John Hutchinson takes a look at the development of strong natural fibre composite materials and their potential commercial application in the construction industry.
Matrix polymers reinforced with natural fibres were around before the more familiar synthetic fibre reinforced polymers that now constitute a significantly greater part of the composites industry worldwide. The latter superseded the early versions of natural fibre composite after World War Two as a result of their more reliable performance and less complicated, cheaper processing.
The now 10-year-old EU directive on the recycling of motor vehicles opened up new opportunities for natural fibre composites. This is because they are much easier to recycle at the end of their life than glass, aramid or carbon-reinforced polymers. Door panels, rear parcel shelves and dashboards can be made by injecting heat-softened pellets of chopped fibres encapsulated with recycled olefin plastic into heated moulds. However, their higher cost relative to synthetic composites means these moulded products have not appeared in other markets. They are also incapable of taking structural load or being subjected to wetting and drying cycles over a long period.
Unreliable structural properties
Natural fibre composites are thought to be unsuitable for structural applications for several reasons. Natural strong plant fibres are susceptible to damage during growth, harvesting, decortication (separation of the strong fibres from the plants stem), mat or yarn manufacture and during composite production. Every site of damage is a location for potential structural failure. Temperate zone strong fibres from plants can also contain nodes produced during growth that generate susceptibility to compression failure in the finished composite.
If these problems were not enough, there are fundamental chemical and physical incompatibilities between hydrophilic organic fibres and hydrophobic matrix resins. Coupling agents can be used, but they increase both cost and embodied energy. In addition, their presence in the composite can cause difficulties with recycling. Like wood, there is a risk that a natural fibre composite section carrying structural load could succumb to ‘deflection creep’ over time.
Until now, it has been assumed that natural fibre composites would not be suitable for applications in building construction. This is a pity, because they possess good and useful material properties that are not available with the customary materials, for example low density, high thermal resistance and dimensional stability in relation to temperature change. Provided the fibres are permanently isolated from moisture by the matrix resin, there would also be dimensional stability in relation to changes in relative humidity.
Would it be possible to produce natural fibre composite sections that can be relied upon to be strong enough to withstand modest structural loads in secondary framing elements in buildings? This was the task that NATCOM, the academic industry consortium funded by the Technology Strategy Board, set itself at its commencement four years ago.
Passive House Institute, based in Germany, is an independent research body that carries out research and development into highly efficient energy use. The Institute has been clear about the basic principles for zero-carbon building. Material specification must be restricted to materials with low thermal conductivity wherever possible (low lambda value). Buildings must be resistant to infiltration by external air, which implies best possible air-tightness at panel perimeters and framing joints (low psi value). And finally, so-called thermal-bridging through insulation layers must be reduced to the practical minimum.
Two strong materials came into construction in the late 19th Century – mild steel made in a Bessemer converter and cement-bound concrete reinforced with mild steel. Both are carbon emitters during manufacture. At the moment, best practice results in 1.65-1.70t of CO2 becoming airborne for every tonne of steel made from ore. The equivalent figure for cement production is 0.80-0.85t of CO2 per tonne. In contrast, materials obtained from plants capture CO2 from the air by recent photosynthesis. When specified for construction, they can capture and store it as organic carbon inside a building.
Thermal conductivity of mild steel is 54W/mK. Stainless steel is lower, at around 15W/mK. Aluminium alloys used in rain-screen or curtain walling support structures have much higher thermal conductivity at 130-170W/mK. The polyamide ‘thermal break’ included with these support systems is too thin to have any dramatic effect. The thermal conductivity of concrete is usually more than 1.20W/mK. Steel and reinforced concrete have the same linear coefficient of thermal expansion, at 12x10-6m/mK. The figure for aluminium alloy is at least 18x10-6m/mK. Movement around panel joints and framing is likely to induce fatigue failure in mastic or other sealants well before the end of the building’s life, in which case air will enter through the resulting fissures.
1: A large number of rovings leads to an increase in the feed angle to the pultruder. Natural fibre rovings tended to break or fray.
2: Lower tax of natural rovings gave rise to a large increase in the number of creels feeding the pultruder.
3: Pultrusion of a window frame section in natural fibre composite
NATCOM set out to make a natural fibre composite able to withstand modest uniformly distributed loads across short spans in secondary framing elements reliably for the lifetime of a building (assumed to be 60 years). It did this by optimising all the processes at every stage, from crop harvest to product manufacture. In order to impart some fire resistance, obviate risk of deflection creep and maximise strength and stiffness, it chose commercial thermo-setting polyester matrix resin at the highest possible fibre/volume fraction. Pultrusion was adopted as the manufacturing technique, not only because it produces lineal sections suitable for framing with expenditure of minimal labour, but also because it optimises fibre/volume fraction and consequent material stiffness. Protein fibre in the form of non-scoured but pressure-washed coarse wool at a grease content of three percent (too high for textile applications) was included in the proportion of 10%. The remaining reinforcement consisted of the two temperate-zone strong fibres – hemp and flax.
The wool content was intended to improve thermal resistance. Solid protein fibres have around half the thermal conductivity of solid ligno-cellulosic fibres. The presence of wool also improves impact resistance (wool is not a strong fibre, but it is highly elastic) and fire performance.
Yarn formulation proved challenging, because temperate-zone strong plant fibres tend to be short. The fibres cannot be twisted in yarns, but must be aligned in the warp direction, and in any case, twisting would impede wetting and reduce the strength of the composite. The aligned fibres were retained in the yarn by wrap-spinning with polyethylene terephthalate thread. Early yarns were torn and ruptured by the pultrusion puller, so the wrap density had to be increased and the yarn weight reduced to 1,000Tex, which is one fifth of the weight of glass roving. However, the technique needs refining as the greatly increased number of creels needed to feed the yarn into the pultrusion machine could have an adverse effect on product cost.
Material testing results
The structural properties for the NATCOM composite, as pultruded in the form of a commercial window sill section, were determined by limited testing at the premises of the industrial partner, Exel Composites UK Ltd by its engineer, Majeed Al-Zubaidy. The results were –
- Tensile strength: 133 MPa
- Young’s Modulus: 13.5GPa
- Flexural Modulus: 19GPa
This suggests NATCOM has succeeded in creating a new species of strong material, suitable for secondary framing applications in construction. The provisional ascertainment of lambda is 0.12W/mK, which is almost the same as dry softwood. Linear coefficient of thermal expansion appears to be low at 3.5-4.0x10-6m/m/K. Both the latter results are tentative, and many more materials tests will be needed to validate them. The density of the composite is averaging 1,200kg/m3, which is less than two thirds the density of a glass-reinforced pultrusion (around 1,900kg/m3). With the exception of wood, the NATCOM composite has a far lower density than other building materials capable of taking load. In terms of performance-to-weight, the NATCOM composite competes favourably with steel and aluminium alloy. Compared with C24 stressgraded softwood, which has a maximum permitted tensile load capability of 7.5MPa, the NATCOM material would have at least twice the performanceto- weight, despite the imposition of a very conservative safety factor of four.
Composites for building construction
If these test results prove to be consistent, natural fibre composite would offer a solution to the problem of thermal bridging through deep insulation layers in modern, highly insulated buildings. The composite would be dimensionally stable in relation to changes in both temperature and relative humidity. Therefore, the risk of air leakage through joints would remain low for the lifetime of the building. At present, there would be an increase in cost arising from substitution of conventional materials by natural fibre composite sections. However, wherever it could be proved that minimising thermal bridging offered great improvement in the lifetime energy consumption performance of a building, the additional cost should be considered a wise investment. There is little doubt that mass production would impart huge cost benefits. Pultrusion is a process that lends itself very readily to mass production.
The NATCOM consortium is confident that its prototype formula for pultruded secondary structural sections offers solutions to technical problems intrinsic to the construction of zero-carbon buildings that, until now, have not been available to architects.
NATCOM Partnership. Project leader: Matthew Tipper, Tel: +44 (0)113 343 3790. Project Manager: John Hutchinson. Tel: +44 (0)20 7841 1980. Email: firstname.lastname@example.org