ReviewGreen composites: A review of material attributes and complementary applications
Introduction
As global societies continue to grow, increasing emphasis is being placed on ensuring the sustainability of our material systems. Topics such as greenhouse gas emissions, embodied energy, toxicity and resource depletion are being considered increasingly by material producers. Some of this practice is being driven by regulations (particularly in Europe as a result of legislation such as the end of life vehicle directive [1]), but increasingly, anecdotal evidence would suggest consumers are also demanding improved environmental credentials from the products they consume. Improving the sustainability of our material systems will require not just the development of new sustainable materials, but also the increased application of existing green materials.
One existing class of materials with good environmental credentials are green composites. Green composites are defined, in this work, as biopolymers (bio-derived polymers) reinforced with natural fibres. More specifically, this work will only look at the subset of green composites that are commonly considered as being biodegradable (counter intuitively, not all biopolymers are biodegradable), as defined by an appropriate standard (EN 13432 [2], EN 14995 [3]).
There are several recently published reviews on green composites, but unlike those, this work is not application specific, nor does it present the detailed chemistry of natural fibre and biopolymer enhancement. Instead, this work provides guidelines for engineers and designers on the appropriate application of green composites. For a detailed review of aspects relating to the materials science of green composites or their application in the automotive and construction sectors, the reader is referred to [4], [5], [6], [7], [8].
The initial part of this review provides a concise summary of the major material attributes of green composites. Significant results from literature are presented, as well as techniques and prospects for future advances. Discussions of the relative merit of each attribute are included, with Ashby charts (Fig. 2, Fig. 3, Fig. 4) being employed to allow easy comparison of the relative mechanical, environmental and economic properties of green composites compared to other materials.
These material attributes are then used to define a series of complementary application attributes. For example, one attribute of green composites is their tendency to absorb water and degrade, a complementary application attribute would be: limited exposure to moisture. As a result, a list is developed that – combined with the mechanical properties of green composites – can be used as a guide for their successful application. Applications that maximise the advantages of green composites, minimise the disadvantages and are obtainable with the necessary mechanical performance are then investigated, before general conclusions are made.
Section snippets
Natural fibres
Natural fibres are generally classed as either vegetable or animal. Vegetable fibres are principally composed of cellulose, whilst animal fibres are composed of proteins [6]. Vegetable fibres are the most commonly used in composite applications and, as such, are the primary focus of this work. The cell structures of natural fibres are relatively complicated, with each fibre being a composite of rigid cellulose microfibrils embedded in a soft lignin and hemicellulose matrix. This structure is
Mechanical properties
Fig. 2 shows two Ashby plots, where the densities of a variety of materials are plotted against tensile strength and Young’s modulus on logarithmic scales. An Ashby plot facilitates easy comparison of materials for differing design criteria. The dashed lines in Fig. 2 are guidelines for minimum weight tie, beam and plate design. By this it is meant that from any material that falls on the same guideline – or on any line parallel to these – a structure, classed as either a tie, beam or plate can
Defining complementary applications
Green composites are often touted for application in the automotive [4] and construction [5] industries. The potential for green composites to have a positive environmental impact when applied to these industries is large, as carbon can be effectively sequestered in these products for many years. Green composites can also provide weight saving and vibration damping of benefit to the automotive sector. However, excluding non-structural, interior applications, the material properties required in
Short life-span products
Short life-span products are typically thought of as those that are disposable, such as plastic cutlery and packaging. So called commodity plastics such as polyethylene, polystyrene, polypropylene, and polyvinyl chloride are used heavily in packaging causing several environmental concerns due to their non-biodegradability. Bio-composites that incorporate a biodegradable polymer comparable to the price and material performance of commodity polymers may well be a solution to the problem [7], [31].
Conclusion
This review has provided a concise summary of the major material attributes of green composites. These include: good specific – but variable – mechanical properties, good environmental credentials (renewable, biodegradable, low embodied energy, non-toxicity), low cost, high water absorption, low durability and biocompatibility. As work continues to improve the attributes of green composites, particular care must be taken to ensure the inherent green characteristics of these materials are not
Acknowledgments
The authors gratefully acknowledge the support of the EPSRC under its ACCIS Doctoral Training Centre grant, EP/G036772/1.
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