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Ecological composite materials: is there room in the industry for more “green” materials?

17 March 2021
Article by Carla Gomes, project manager, Marta Martins and Susana Sousa, INEGI researchers in the area of ​​composite materials and structures.

Synthetic materials were born from the need to design products with better properties than the so-called "natural" materials have. The synergistic properties obtained when fibers, dies and other materials are combined, make composite products able to have several functionalities simultaneously, and thus, they are used in a wide range of sectors1, 2, 3. Currently, it is common to use composite materials in the automotive, aeronautics and space sectors, but also in sports and industrial applications.

However, the advent of new environmental concerns in society, legislative changes, and the reinforcement of ‘green marketing’ observed in the last decade, has led the industry to reflect the paradigm of sustainability. Awareness that led to an increase in interest in the use of composite materials with natural fibers (instead of synthetic ones), and recyclable or naturally originated resins, as a way to develop economical, sustainable, light and flexible composites.

In recent years, INEGI has also been aware of the need to reduce the ecological footprint of different sectors, having collaborated with several companies in the development of sustainable solutions using fiber composite materials, and natural and / or recyclable resins.

In general, depending on the constituents of nature, composites composed of these materials can be classified as partially ecological (when one of them comes from non-renewable resources) or green (when all are obtained through renewable resources)1, 4. Thus, there is a greater interest in replacing carbon fibers, glass or aramid with fibers of natural origin, such as flax or hemp, in a substitution that can be total or partial.

Use of sustainable materials raises challenges for the scientific-technological community

But how to provide these materials with renewable and biodegradable characteristics? In response to this question, natural fibers appear as a possible solution. Its functional and economic viability, however, is not yet at the ideal level.

These fibers are mainly of vegetable origin (essentially composed of cellulose) or animal origin (based on proteins). Vegetable fibers are the most commonly used in composites, and can be obtained through the stems, leaves or seeds of various plants. The cellular structures of these fibers are complex, as each one has a set of rigid cellulose microfibers incorporated in a matrix of lignin and hemicellulose. For a mechanical failure to occur, sufficiently high energy is required to unroll all microfibers, and then cause them to rupture.

The industry has shown interest in application with jute, linen, hemp and ramie fibers, due to their mechanical properties2,5,6. However, natural fibers can have different compositions from batch to batch, this being the main disadvantage of this reinforcement material.

Other potential drawbacks include high water absorption, low impregnability and eventual incompatibility with the matrix. In fact, it is common practice for fibers to undergo a previous treatment to make them compatible with the composite matrix, but naturally these materials and processes are established for commonly used matrices such as epoxide, polyester, vinyl ester, among others. These factors usually limit the large-scale production of composites with natural fibers1, 2.

Regarding matrices of biological origin, these can also be of vegetable or animal origin. The most commonly used in composites are poly (lactic acid) (PLA), polyhydroxybutyrate (PHB), polysaccharides, cellulose, alginate, chitin and hyaluronate 2, 3, 4.

However, due to its decomposition nature, use on finishing surfaces is quite problematic, especially in applications with a long service life and which do not have additional treatments or coating. Another disadvantage of these matrices is their high cost (for example, the PLA biological origin polymer is 1.5 times more expensive than a synthetic polypropylene resin), which makes them less accessible to the industry, even for large scale productions. scale. Among other disadvantages of these resins are their fragility, thermal behavior, high permeability, low melt viscosity for further processing, among others 2, 3, 4.

New opportunities and scientific advances make the use of natural fibers a reality

But not everything is negative.

Nature has given us an abundance of sustainable and renewable resources. Fibers and natural resins are an attractive option for industries to meet socio-economic and environmental challenges, although, for now, only in parts with little mechanical stress. Currently, the use of these composites is already a reality, especially for non-structural elements, in several sectors, namely in transport, health, sport, construction and design 1, 2, 6.

In this context, INEGI's work in the NEXMENT and NOPROMAT projects stands out, where the challenge was to create eco-efficient solutions for the construction sector. During the NEXMENT project, concepts of temporary modular houses were created with the application of recycled materials, one of the proposed solutions consisting of thermoplastic matrices with expanded cork agglomerate cores. The NOPROMAT project, on the other hand, focused on the development of products for the sanitary ceramics industry, namely the creation of production processes for washbasins and taps using new "natural" materials, such as natural stone residues, coconut fiber, among others.

Even so, typically more demanding sectors have also been investing in the search for answers to this challenge. In the aeronautical sector, the LIFE and ECO-COMPASS projects can be highlighted, which counted on the participation of INEGI specialists.

Within the scope of the LIFE project, the solutions developed for the interior of the aircraft were oriented towards the use of natural, light and comfortable materials, namely cork and natural leather. The ECO-COMPASS project, which consisted of a collaboration between China and Europe, also contemplated the development of multifunctional ecological composites, with linen fibers and bio resins for application in secondary and interior aircraft structures.

Among the benefits of investing in ecological composites, it should be noted that the use of these materials has the potential to create new employment opportunities in rural and less developed regions, thus helping to achieve sustainable development goals. The inclusion of fibers and resins of natural origin in other sectors, in addition to conventional ones, may boost and encourage the market in relation to them, and may contribute to revitalize the agricultural and textile sector in a sustained manner 2,7.

Searching for sustainable alternatives motivates future developments

Extending the production and use of structural technical composites with fibers and matrices of natural origin requires, however, an adaptation of the legislation currently applicable to synthetic composites, as well as standardizing the way in which we characterize their properties for the elaboration of more appropriate technical sheets. This characterization enhances the creation of methodologies for simulating these materials, increasing confidence, from designers to consumers when using these materials.

To this end, the scientific community has a crucial role with regard to the dissemination of studies on the optimized combination of fibers and matrices of natural base for the production of composites on an industrial scale 2, 7. There may also be a need to change the processes currently used, which will translate into investment costs for industries, but which can be easily overcome if the use of these materials proves beneficial in some application sectors.

These are the challenges of the future in the search for sustainable, low-cost alternatives with similar or improved properties compared to those already used. Continuing this effort is essential to reduce the industry's environmental footprint, and INEGI will continue to work to be an active part of this change.

1 Sousa, S.P.B., Ribeiro, M.C.S., López, M.M., Barrera, G.M., Ferreira, A.J.M. (2014). Mechanical behaviour analysis of polyester polymer mortars reinforced with luffa fibres. In: CLB-MCS 2014 – Congresso Luso-Brasileiro de Materiais de Construção Sustentáveis, 5-7 março 2014, Guimarães, Portugal, Vol. 2, 331-338

2 Peças, P., Carvalho, H., Salman, H., & Leite, M. (2018). Natural fibre composites and their applications: a review. Journal of Composites Science, 2(4), 66.

3 Mann, G. S., Singh, L. P., Kumar, P., & Singh, S. (2020). Green composites: A review of processing technologies and recent applications. Journal of Thermoplastic Composite Materials, 33(8), 1145-1171.

4 Peças, P., Carvalho, H., Salman, H., & Leite, M. (2018). Natural fibre composites and their applications: a review. Journal of Composites Science, 2(4), 66.

5 Koronis, G., Silva, A., & Fontul, M. (2013). Green composites: A review of adequate materials for automotive applications. Composites Part B: Engineering, 44(1), 120-127.

6 Dicker, M. P., Duckworth, P. F., Baker, A. B., Francois, G., Hazzard, M. K., & Weaver, P. M. (2014). Green composites: A review of material attributes and complementary applications. Composites part A: applied science and manufacturing, 56, 280-289.

7 Muthu, S. S. (Ed.). (2018). Green Composites: Sustainable Raw Materials. Springer.

A version of this article was originally published at revista interPLAST.