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Natural fibres have been used universally from the highest antiquity to the middle of the 20th century. The car industry is where the use of natural fibres as polymer reinforcement has been generalized the most, by far.
(Published on May-June 2006 – JEC Magazine #25)
BY GÉRARD MOUGIN, DIRECTEUR GÉNÉRAL D’AFT PLASTURGIE, AFT PLASTURGIE
This industry has intensified its demand for fibres since 1996, with significant peaks: for example, a more than 90% increase from 1999 to 2000. Many European models now commonly use thermoplastics and natural fibres for interior and structural parts. One model actually uses 5-10kg of natural fibre per vehicle! In 2000, the European car industry consumed 28,300 tons of natural fibre, which breaks down into 20,000 tons of flax, 3,700 tons of jute and 3,500 tons of hemp. The use of natural fibres is growing at a slower pace now, however, particularly in France. This appears to be due to preconceived ideas or perhaps to some asyet unsolved problems that manufacturers have been having.
Availability is one of the first problems you encounter with the use of natural fibres. The low cost of these products can be easily cancelled out if they must be transported over long distances. Flax is cultivated and used a lot in the more humid areas of Northern Europe, whereas jute, sisal or kenaf are found in the more arid parts of the world. Hemp has an advantage over other plants in that it is easily cultivated pretty much everywhere from Canada to South Africa, including Europe and China. Within their respective areas of production, all of these fibres are comparably priced.
Durability of supply
The future of a natural product is always a question of its availability over time, and more particularly of the ability of its producers to meet the demand, especially in an upbeat market. This is not really a problem for natural fibres, however, because for all plants considered, there are enough cultivable areas in all the production countries to follow market trends. To keep up with the growing market, you must not only increase the surface cultivated, but also create facilities for processing the plants so the fibres can be used. Such facilities are relatively costly.
The complex nature of natural fibres
The composition of natural fibres is not the same, even when they are of the same species – it varies not only from one type of fibre to another, but also as a function of how and where the plant is cultivated. The first constraint in producing natural-fibrereinforced composites is the ability to control plant-related parameters and to influence the properties of the final composite.
Over a period of several years, AFT Plasturgie worked with hemp producers and the French National Institute for Agricultural Research (INRA) on a study supported by the French Environment and Energy Management Agency (ADEME). The study consisted in examining the influence of genetic variety, the method and zone of culture, and the method of harvesting and defibering on the properties of the composites.
Table 1: tensile properties of fibres.
From FINK, WEIGEL IAP Potsdam - GEIGER ICT Pfinztal - BUSCH IWM Halle.
Thanks to the knowledge gained from this study, AFT can guarantee products of constant quality to its customers.
Natural fibres and polymers
Cellulose, the main component of natural fibres, is completely incompatible with the majority of polymers. This fact is illustrated in photo 1, for a traditional hemp/PP compound. AFT Plasturgie has developed an original thermo-mechanical process that allows anchoring the fibre in the polymeric matrix mechanically through fibrillation at the surface of the mother fibre (Photo 2).
Thanks to this process, it is now possible to obtain natural fibres with mechanical properties close to those of glass fibres.
Natural fibres are not inert materials and they react with their environment under certain conditions.
Moisture: Natural fibres can absorb a certain amount of water, a phenomenon that many consider as a degradation of the fibres.
In a study, AFT showed that a part reinforced with 30% hemp fibres immersed in water absorbed approximately 7% of its weight in water. On the other hand, under standard hygroscopicity and temperature conditions, after saturation the part releases all the water absorbed, and no hysteresis can be observed in the sorption/ desorption curves.
The mechanical properties of the parts decrease slightly during the absorption of water, although the original properties are recovered as soon as the water is released. One can thus produce parts which absorb and then slowly release active substances like bactericides, aromas, etc.
Temperature: Because of the nature of its components, a natural fibre cannot be brought up to temperatures higher than 220-230°C without involving a degradation of its components. Thus, composites containing natural fibres cannot be processed at temperatures higher than 230°C.
Odour: Some plastics converters complain about a strong odour generated by fibres during and after the processing of composites. This phenomenon is the consequence of an excessive rise in temperature. Excessive heat causes a carbonization of pectins and other weak components of fibres. To avoid this phenomenon, which can cause a clogging of tools, it is necessary to observe the recommended processing conditions. AFT has developed an additive that eliminates odours as soon as they appear.
Natural fibre advantages
Natural fibres can offer a large number of advantages when they are processed in compliance with the above-mentioned rules.
A compound reinforced with natural fibres is not only low-cost, lowdensity and abrasion resistant, it also offers better dimensional stability and an absence of toxicity.
Furthermore, when compared to a compound reinforced with glass fibres, it has significant advantages from an environmental point of view.
Table 2: for the production of one kilogramme of fibres.
Moreover, hemp can store 0.79 kg CO2 per kg of fibres and release 10 MJ/kg if incinerated at end of life.
Finally, it should be noted that hemp fibres are sufficiently flexible to withstand several mixing procedures with viscous plastic in a screw/barrel unit. Because their length, and therefore their L/D ratio, is preserved, the reinforcement properties are not ultimately affected by successive recycling steps.
Isotropy of the properties
Glass fibres are oriented in material flows. A complex part with varying thickness and/or direction of flow will thus have different mechanical properties in the various directions, but also differential shrinkage, and eventually post-moulding warpage.
In AFT’s original process, hemp fibres are not directed in flows, so we obtain parts with isotropic mechanical properties.
In addition to this advantage, the random orientation of hemp fibres blocks shrinkage in all the directions. This has many advantages, including: - total absence of shrink marks on thick parts, so blowing agents are not necessary; - good geometrical stability even at high temperature, so there is no post-moulding deformation; - the possibility to release the parts from the mould at high temperature, leading to as much as 30% cycle time savings; - very low shrinkage.
The future of natural-fibre composites
The use of natural fibres as reinforcement in thermoplastic or thermoset composites is bound to develop rapidly and extensively.
However, as seen above, the industrial development of natural fibres in plastic processing requires the perfect control of a number of parameters.
The fibre network
To ensure the quality of composites containing natural fibres, you have to control the design, the production and the processing of fibres.
To this end, AFT has built a perfectly structured network that is based on the best skills at each stage, from seed production all the way to the plastic converter, including: the ITC (Institut Technique du Chanvre): this technical hemp institute gives the entire network the benefit of its technical and economic training on hemp; the FNPC: this cooperative has universally recognized skills in seed selection; the CCPSC: this cooperative specialises in seed multiplication; INTERVAL, NOURICIA and LCDA: these cooperatives are hemp producers and AFT’s main shareholders.
One difficulty with producing composites containing natural fibres is being able to anticipate and keep up with fast-growing demand. We have seen that the cultivable surfaces are sufficient for the production of fibres. The main problem is with the output of composites. AFT Plasturgie’s current capacity is 5,000 tons of compounds and 3 million m of non-woven mats.
As this capacity is clearly insufficient for the projected market growth, we have launched “AFT Composites”, a project to build a production unit with a 40,000-ton capacity, in 2006. The facility should be operational in early 2007.
AFT has worked for a number of years to design composites, based on the use of polymers synthesized from natural products and with the aim of producing environmentally sound materials.
In 2006, in cooperation with research centres and industrial companies, AFT launched a large project for the production of biocomposites within the framework of the Champagne- Ardennes competitive cluster.