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This paper describes the latest industrial applications for natural-fibre-reinforced composite materials. It also presents some lines of university research, various industrial products, and possible applications in the aeronautical sector, starting from non-structural components for ultralight aircraft.
(Published on January-February 2009 – JEC Magazine #46)
PROF. CLAUDIO SCARPONI, DEPARTMENT OF AEROSPACE ENGINEERING AND ASTRONAUTICS, THE SAPIENZA UNIVERSITY OF ROME
Growing environmental concerns have sparked renewed interest in the development of natural-fibre composites (NFC) and biodegradable materials, which could be the new bio-friendly alternative to fibre-reinforced plastics. The many advantages of these materials include specific mechanical properties, good thermal, acoustic and electrical insulating properties, low density, reduced tool wear, excellent formability, and safe crash behaviour (no splintering). They are non-toxic, bio-/environment-friendly, and low-cost materials that are easily available and help promote healthier, safer work places. As a consequence, natural fibres (NF) could be used as a lowcost reinforcement alternative to glass fibres. Considering that it is possible to use the same processes, tools, labour, equipment, controls and know-how, an easy substitution is possible at reasonable cost.
Varying mechanical properties
Table 1 reports the main mechanical characteristics of NF, as compared with glass fibres. Unsurprisingly, significant variations are observed, obviously connected to biological differences among plants, climatic history, type of cultivation, geographic area, and fibre extraction technology. As a consequence, differences can be found in batches coming from the same area and brands that differ only in the year of production. It is particularly important to consider that mechanical properties can differ greatly as a function of whether elementary fibres, bundles or only a thread are being tested (degree of twist and fibre length are also important parameters). Difference between fibres and bundles are shown in Figure 1.
Table 2 provides data on the main geographical areas producing natural fibres, with relative unit costs. The low cost of the material and the fact that developing countries are major producers of many of these fibres explain the growing interest there for these natural materials.
The International Conference on Innovative Natural-Fibre Composites for Industrial Applications, which was organized by the Sapienza University of Rome in October 2007, gathered participants from all over the world: Europe (France, Germany, Great Britain, the Netherlands and Lithuania), Africa (Kenya and Tanzania), North America (Texas, USA), South America (Brazil and Argentina), Asia (China, India, Bangladesh and Japan), and the Southwest Pacific (New Zealand).
*percent of fibre’s growing area in the world
**approx values; strongly dependent from: period, country and stock size
A long history
Natural-fibre composites are by no means new materials. Here are a few major examples, just from the 20th century:
Nowadays, NF are successfully used in the automotive industry, reducing weight up to 30%. Usually, around 10-15 kg NF are used per vehicle. One of the most recent examples is the Lotus Eco Elise, which was presented at the British International Motor Show in July 2008. This car is built with sustainable materials (such as hemp, eco wool or sisal), using green technologies and water-based paint solutions. It weighs 32 kg less than the standard Elise car.
From 1999 to 2005, the use of NF (excluding wood and cotton) in the German automotive industry increased from 9,600 to 19,000 metric tons. This fast growth clearly illustrates the advantages and potential of bio-composites. Figure 2 shows applications in a Mercedes E-Class.
Natural fibres are also attractive for civil-engineering applications such as prefabs, floor tiles, panels or wall materials – particularly in developing countries – because of their low cost, easy availability, reduced construction times and low- polluting emissions: around 50% of the UK’s CO2 emissions comes from civil constructions.
A step-by-step approach in aeronautical construction
As far as aeronautical structures are concerned, the situation is radically different for several reasons:
As a consequence, the substitution with a new material has a high impact on the costs and times required by the certification procedures. The process is usually long and costly, due to testing and numerical simulations.
For these reasons, the University of Rome’s Dipartimento di Ingegneria Aerospaziale and Dipartimento di Ingegneria Chimica e dei Materiali decided to proceed slowly, step by step, to introduce NFC in aeronautical construction. The researchers focused on four main research lines in order to improve/determine:
In-depth studies were conducted first on jute (and jute/glass hybrid laminates) and later on hemp, processed with thermosetting and thermoplastic matrices. The aim was to achieve good chemical/physical and mechanical characterization, focusing on the differences and the reproducibility of mechanical properties among various production batches/stocks. A wide array of tests were performed, ranging from traction, compression and bending to impact.
SEM analysis was used to investigate the microstructure and interfacial properties. The internal damage status of specimens tested at different loads was detected by acoustic emission and ultrasound technologies based on reflection.
Additional research is required to study the basic materials, the chemical-physical compatibility between fibres and matrix, and the interface to improve fibre-matrix adhesion. A number of chemical treatments improving adhesion are described in literature. Some of them were successfully tested, producing higher performance than untreated materials.
More information: www.diaa.uniroma1.it/docenti/ c.scarponi/conferenza2009/fibrenaturali09.html
RTM (Resin Transfer Moulding) is considered as a particularly suitable manufacturing technology for aeronautical construction, also because of its considerable cost/benefits advantages as compared with traditional vacuum bagging and autoclave technologies.
The next step will involve electromagnetic characterization (for potential future applications in airplane radomes) and mechanical tests involving time-ambient variables, such as wet, hot and hot-wet tests.
First, the idea is to introduce NFC in the construction of nonprimary structural parts for ultralight aircraft (the certification procedures are not applicable in this case) and to deal with “certified aviation” later. Thanks to their low weight and good insulating properties, NFC could be used for interiors components like hatboxes, seats, tables or sidewall panels, and for non-structural exterior applications such as radomes, fairings or wing-fuselage junctions.
To demonstrate the recent applications of such materials, several examples were presented at the above-mentioned conference in Rome, including a hemp/glass rudder prototype for an ultralight airplane (Figure 3), a composite boat hull, chairs, lamps, tables, seats, casings, boards, road signs, train interior parts, NFreinforced honeycomb cores, and more (see Figure 4).