Organic hemp and flax shiv composites
Composites are materials made from two main constituents with significantly different physical and/or chemical properties. One of the constituents is usually an organic matrix, i.e. polymeric material. In the case of organic hemp and flax shive composites the polymeric materials can be either bio-based, e.g. polylactide (PLA), or not, e.g. polyethylene (PE). Depending on the proportion among the matrix (polymeric material) and shives, as well as the origin of the matrix, those composites may or may not be considered as bio-composites.
The specificity of the hemp and flax shive composites is their charges, i.e. shives. Indeed, they are not fibres (either mineral (glass fibres) or vegetable (flax, hemp)), ordinarily used to reinforce polymeric matrix, but their wastes. Indeed, shives are woody parts of plants, in opposition to bast fibres – the noble part of the fibrous plants. Also, originally “shive” corresponded to the woody part of flax fibres, whereas the specific name for hemp-fibre wastes is “hurd” . However, nowadays the term “shive” used to designate both, flax and hemp, woody residues. Also, contrarily to flax, which fibre is very valuable, hemp is sometimes used for building applications after grinding of the whole plant, without any distinction between bast fibre and its hurd. Finally, tows, sometimes also designed as short fibres non-usable in textile industry, form a mixture with purely woody parts, and this is also called by the generic “shive” name.
Therefore, the particularity of the hemp and flax shive composites is that those are generally not noble materials with outstanding characteristics. On the contrary, the utilisation of shives (or hurds) is often justified by their lower cost, compared to the bast fibres. This explains that those composites are rarely “pure” hemp or flax shive composites, but merely flax or hemp fibre composites added with shives (hurds) to lower their price, or agglomerations of more or less small size shive particles with organic glue to produce cheap building materials.
Thus, the main applications of hemp and flax shive composites in building are insulation materials, pressboards and recently, fabrics (Figure 1).
2. Production technologies
The production of insulation materials and pressboards based on hemp and flax organic composites induces several stages of preparation (Figure 2). We can notice that some of these steps are similar in both cases, i.e. the primary decortication of fibres and the impregnation of shives with the organic matrix. The specificity of the production of insulation materials is the carding step. This step consists in ravelling and ventilating to make a lightweight composite.
The production of the pressboards, in the contrary, requires thermo-pressing, i.e. agglomeration of shives with the organic binder to make cohesive pressboards and to confer them better mechanical properties.
The main differences among the same type of materials and their production technologies consist on the ratio of shives used, the nature of the organic matrix (PLA, PE, etc.), the temperature and the duration of the thermo-bonding or, respectively, thermo-pressing steps.
As the fabrics are rather new materials based on hemp and flax shive composites, their production technologies are not yet fully disclosed, even if they follow the main steps of the press-boards preparations .
3. Use of products
The remarkable thermal and acoustic insulation properties of the materials based on hemp and flax shive organic composites make their usage widespread in the building industry. However, the main usage remains for the external insulation. This includes lining solid walls on the inside, their timber-frame and insulation between joists. Also, the usages for the roofs and attics insulations can consist of blowing between trusses and rafters, among others. The interior insulations comprise unrolling on the floors, load bearing, separating and injection into cavities .
The pressboards, on the contrary, are mainly used for partition walls and doors. However, due to their unsightly aspect, most of the time the doors made of a core based on shives materials are embellished by an external layer made of fibres.
Product characteristics – Durability
As outlined above, hemp and flax shive organic composites are relatively new building materials, therefore only little information is available on their durability. The main studies in the field were oriented on the water absorbance regulation and the end of life (composting) of these materials.
In terms of hygro-regulation, due to the natural ability of hemp and flax to absorb ambient moisture and to gradually return it, the hemp and flax based materials generate a comfortable and healthy indoor climate. The negative side of it is that those materials present a low resistance to water vapour diffusion, typically µ < 2. Therefore, sometimes the hardening of the composites to increase their resistance to water degradation is required, namely by MgO-cement as binding agent .
The behaviour at the end of life, specially composting, when biodegradable matrix is used, was also studied . The investigations concerned the impact of lignocellulosic charges on the evolution of PLA biocomposites during ageing in compost (at 30°C) or water (at 20°C), during 6 and 12 months, respectively. Various blends containing 0, 10, 20 and 50% shives were tested. It has been confirmed that flax shives lead to significantly higher water absorption equilibria, up to 18 wt % for composites containing 50 % of those charges. Hydrolytic degradation and abundant development of microorganisms occurred during both methods of ageing. Finally, according to the results, flax shives showed a great impact and may promote degradation phenomena of PLA biocomposites, thus limiting their potential to be used as insulation materials.
Exploration of the potential of the use of hemp and flax shive organic composites as building materials is not new. Indeed, since the beginning of the XXth century there are patents dealing with such type of materials - . However, the increased concern for our environment, the improvement of technics in the building industry and the will to improve their sustainability by the utilization of waste materials have induced a higher implication of research activities in both patents and academic literature since the 1990s .
The main concerns of the recent studies on hemp and flax shive composites are their length, water absorbability and fire resistance.
The introduction of charges in the polymeric matrix, i.e. production of composites, is meant for the improvement of the intrinsic polymeric properties . On the other hand, it has been abundantly documented that the mechanical properties of the obtained materials are directly correlated with the size of the incorporated hemp and flax shives, therefore it has been one of the major concerns of the field - . It has been thus demonstrated that the shorter the shives are the better the strength of the composite is, the optimal length being around 7.3 mm . However the impact of the length of shives on thermal properties was not confirmed19.
Water absorbability is another issue faced by the hemp and flax shive organic composites. Therefore the influence of the proportion of the shive fraction on the water uptake was studied. It was thus established that the increased proportion of shives (from 0 to 62.5 %) is concomitant with the increased water uptake of the composite13. Also, the length of the shives remains an important parameter for the water uptake considerations, the shortening of the shives contributes to the lowering of the water absorbability by the composite19.
As the fire resistance is one of the weak points of the hemp and flax shive organic composites, several studies concerned its remediation. Thus, Wang et al., have successfully incorporated maleic anhydride-grafted polyethylene (MAPE) and have observed the better compatibilization of hemp hurds and high-density polyethylene (HDPE) and the flame retardancy of the composite . Whereas, Lazko et al. studied the incorporation of several fire retardants by fixing them directly on shives. The best result was obtained while using MMB (borate melamine) as flame retardant. Adding 10 % of MMB (w/w) was shown to be sufficient to ensure the immediate extinction of the flame once the heat source is removed. For MMB content from 20 to 30 %, HRR peak (heat flux) is lowered by 50 % and the time of ignition is increased by six times compared to a reference made without flame retardant. Unfortunately, the addition of fillers, i.e. fire retardants, induces a decrease of the mechanical properties of the material.
Finally, the increased competitiveness in the globalized market also pushes to the increased intellectual protection of the obtained formulations. Thus, even slight improvements in the fields of acoustic and thermic insulation properties or the modulation of the organic matrix used were reported - .
From the purely scientific point of view one can regret that the comparisons between the obtained materials are generally made in a rather unfair way, and it is difficult to find comparisons between the most used, petroleum- or mineral fibres-based materials and the hemp and flax shives organic composites.
The recent realization that “green” or “bio-based” does not necessarily meant “sustainable” has launched an important demand for the LCA analysis of diverse products , including building materials. However, in this particular case the situation is mainly complicated by the complexity and abundance of different materials needed for the building construction. For example, if the use of a sustainable insulation material requires the use of a non-sustainable linking agent, would it make the whole building construction sustainable? Therefore, for buildings, a general appreciation of the whole construction is prefered.
However, a recent study of LCA on insulating materials was published . The authors stressed the point that the main issue appeared to be the bio-binder used. Indeed, it has to be first dried, then transported and then re-humidified again. A better solution would be the transportation of the wet material, however this would launch transportation problem. Finally, the on-site production seems to be one of the better recommendations to make.
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