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Review

Green composites: A review of material attributes and complementaryapplications

Michael P.M. Dicker , Peter F. Duckworth, Anna B. Baker, Guillaume Francois, Mark K. Hazzard,Paul M. Weaver Advanced Composites Centre for Innovation and Science (ACCIS), University of Bristol, Queens Building, University Walk, Bristol BS8 1TR, UK

a r t i c l e i n f o

Article history:Received 11 June 2013Received in revised form 1 October 2013Accepted 18 October 2013Available online 25 October 2013

Keywords:A. FibresA. Polymermatrix composites (PMCs)B. Environmental degradationB. Mechanical properties

a b s t r a c t

Despite thelargenumber of recent reviews on green composites, limited investigationhas taken place intothemost appropriate applications forthese materials. Green compositesare regularlyreferredto as havingpotential uses in the automotive and construction sector, yet investigation of these applications revealsthat they are often an inappropriate match for the unique material attributes of green composites. Thisreview provides guidelines for engineers and designers on the appropriate application of green compos-ites. A concise summary of the major material attributes of green composites is provided; accompaniedby graphical comparisons of their relative properties. From these considerations, a series of complemen-tary application properties are dened: these include applications that have a short life-span and involvelimited exposure to moisture. The review concludes that green composites have potential for use in anumber of applications, but as with all design, one must carefully match the material to the application.

2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Natural fibres. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Biopolymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Attributes of green composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Mechanical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Variable fibre properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Renewability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Low embodied energy and CO 2 emissions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.6. Low cost. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. High natural fibre water absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.8. Poor durability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.9. Non-toxicity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10. Biocompatibility and bioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3.11. Fibre degradation at elevated temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2864. Defining complementary applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285. Complementary applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.1. Short life-span products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Sporting equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Biomedical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1359-835X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesa.2013.10.014

Corresponding author. Tel.: +44 (0) 117 33 15768.E-mail address: [email protected] (M.P.M. Dicker).

Composites: Part A 56 (2014) 280289

Contents lists available at ScienceDirect

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1. Introduction

As global societies continue to grow, increasing emphasis isbeing placed on ensuring the sustainability of our material sys-tems. Topics such as greenhouse gas emissions, embodied energy,toxicity and resource depletion are being considered increasinglyby material producers. Some of this practice is being driven by reg-

ulations (particularly in Europe as a result of legislation such as theend of life vehicle directive [1]), but increasingly, anecdotal evi-dence would suggest consumers are also demanding improvedenvironmental credentials from the products they consume.Improving the sustainability of our material systems will requirenot just the development of new sustainable materials, but alsothe increased application of existing green materials.

One existing class of materials with good environmental cre-dentials are green composites. Green composites are dened, inthis work, as biopolymers (bio-derived polymers) reinforced withnatural bres. More specically, this work will only look at thesubset of green composites that are commonly considered as beingbiodegradable (counter intuitively, not all biopolymers are biode-gradable), as dened by an appropriate standard (EN 13432 [2],EN 14995 [3] ).

There are several recently published reviews on green compos-ites, but unlike those, this work is not application specic, nor doesit present the detailed chemistry of natural bre and biopolymerenhancement. Instead, this work provides guidelines for engineersand 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 con-struction sectors, the reader is referred to [48] .

The initial part of this review provides a concise summary of themajor material attributes of green composites. Signicant resultsfrom literature are presented, as well as techniques and prospectsFig. 1. Structural constitution and arrangement of a natural vegetable bre cell [9] .

10 0

10 1

10 2

10 0

10 1

10 2

10 3

103

104

103

104

Fig. 2. Density specic mechanical properties. Dashed lines indicate constant material performance for tie stiffness E /q and strength r /q , beam stiffness E 1/2 /q and strengthr

2/3 /q and plate stiffness E 1/3 /q and strength r 1/2 /q . Green composite properties from [76] (Long ax slivers/Randy PL-1000 PLA resin, 0.610.72 bre volume fraction), [77](10 mm length kenaf/PLA 3051D resin, 0.20.4 bre weight fraction, only exural strength and modulus reported), [78] (Woven jute fabric/soy resin, 0.40.6 bre weightfraction), [79] (10 mm NaOH treated jute bres/starch resin injection moulded, 0.10.3 bre weight fraction), [80] (Abaca bre pellets/PLA injection moulded, 0.3 bre weightfraction), [81] (Jute bre pellets/PLA injection moulded, 0.3 bre weight fraction), [82] (Flax bres/PLA and PHB resins, 0.3 bre weight fraction, lm stacking compression

moulding), [83] (Flax bres/PLA resin, 0.3 bre weight fraction, lm stacking compression moulding). (For interpretation of the references to colour in this gure legend, thereader is referred to the web version of this article.)

M.P.M. Dicker et al./ Composites: Part A 56 (2014) 280289 281


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for future advances. Discussions of the relative merit of each attri-bute are included, with Ashby charts ( Figs. 24 ) being employed toallow easy comparison of the relative mechanical, environmentaland economic properties of green composites compared to othermaterials.

These material attributes are then used to dene 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 exposureto moisture. As a result, a list is developed that combined withthe mechanical properties of green composites can be used as aguide for their successful application. Applications that maximisethe advantages of green composites, minimise the disadvantagesand are obtainable with the necessary mechanical performanceare then investigated, before general conclusions are made.

2. Materials

2.1. Natural bres

Natural bres are generally classed as either vegetable or ani-mal. Vegetable bres are principally composed of cellulose, whilstanimal bres are composed of proteins [6]. Vegetable bres are

the most commonly used in composite applications and, as such,are the primary focus of this work. The cell structures of natural -bres are relatively complicated, with each bre beinga compositeof rigid cellulosemicrobrilsembedded in a soft lignin and hemicellu-lose matrix. This structure is illustrated in Fig. 1; where it can beseen that themicrobrils arehelically wound along thehollow breaxis. Mechanical failure requires a large amount of energy to uncoilthese spirally oriented brils [9]. These vegetable bres can be

100

101

102

103

100

101

102

100

101

102

103

100

101

102

103

Fig. 3. Material production embodied energy specic mechanical properties,expected accuracy 10% [13] . Dashed lines indicate constant material performancefor tie stiffness E /H and strength r /H , beam stiffness E 1/2 /H and strength r 2/3 /H and

plate stiffness E 1/3 /H and strength r 1/2 /H . (For interpretation of the references tocolour in this gure legend, the reader is referred to the web version of this article.)

101

100

101

102

100

101

10

2

100

101

102

103

101

100

101

102

Fig. 4. Cost specic mechanical properties [13] . Dashed lines indicate constantmaterial performance for tie stiffness E /C and strength r /C , beam stiffness E 1/2 /C andstrength r 2/3 /C and plate stiffness E 1/3 /C and strength r 1/2 /C . (For interpretation of

the references to colour in this gure legend, the reader is referred to the webversion of this article.)

282 M.P.M. Dicker et al./ Composites: Part A 56 (2014) 280289


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harvested from the stems, leaves, or seeds of various plants [5],which classies them as a distinctly renewable resource. Table 1provides an overview of natural bre properties and composition.

2.2. Biopolymers

As with natural bres, there is a wide range of biodegradablebiopolymers [10] . These are derived from a variety of renewablesources and include both thermosetting and thermoplastic poly-mers. However, if these thermosets are formed by polymerisationwith synthetic monomers as many are they are no longer100% green materials [11,12] . A review of the current state of development of both thermosetting and thermoplastic biopoly-mers, highlighting their respective biodegradability is presentedin [6] .

Bio-thermoplastics are commonly used in green composites andwill be highlighted in this work. These include poly(lactic acid)(PLA), polyhydroxybutyrate (PHB), polysaccharides of plant origin;cellulose and alginate, animal origin; chitin and of microbe origin;hyaluronate. Thermoplastic starch is another polysaccharide com-monly used as a matrix for green composites, it is comprised of both linear regions, that form helical structures, and branched re-gions. Proteins such as collagen (gelatin) and albumin are also bio-polymers. Table 2 provides an overview of some of these polymers.

3. Attributes of green composites

3.1. Mechanical properties

Fig. 2 shows two Ashby plots, where the densities of a variety of materials are plotted against tensile strength and Youngs moduluson 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 thisit 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 be designed with equal weight and equivalentstiffness or strength [13] . Ashby plots are also used in this paperto examine embodied energy and cost, here the dashed lines relateto structures with equal embodied energy or cost and equivalentstiffness or strength. Such a plot allows for a comparative evalua-tion of the mechanical properties of green composites to be made.The plots also provide a convenient way of reporting the mechan-ical properties of the wide range of green composites reviewed inthis work.

In the interests of fair comparison, the majority of the proper-ties presented in these plots are extracted from the 2012 editionof Materials and the Environment by Ashby [13] (excluding the val-ues for biopolymers and of course the green composites). Valuesfor the green composites are extracted from several different re-search papers (references in the caption of Fig. 2); due to the oftenlimited nature of the information provided in these, much of thedensity data for green composites is calculated from reportedweight/volume fractions, with constituent densities used fromAshby [13] . It should be noted that the attribute values used for

natural bres do not always align exactly with those reported inother publications (although they are similar). In addition, the plot-ted ellipses in these charts are made within a bounding box, de-ned by the extremes of the reported values. Thus one mustallow for some error when considering these plots, but they aredeemed to be acceptable for the desired purpose of comparison.

It can be seen in Fig. 2 that natural bres and glass bres arecomparable in terms of specic stiffness and specic strength.However, a wide gap appears when comparing the mechanicalproperties of composite materials constructed from each of thesebres. This disparity between the properties of the raw materialand those of the manufactured composite has several root causes;in part it is due to the use of low volume fractions of short, una-ligned, reinforcement in green composites. However, the chemical

incompatibility between the hydrophilic natural bre and hydro-phobic polymer matrix is also responsible as it leads to poor bre

Table 1

Summary of bre properties, values primarily taken from [13] , with additional values for bre length, diameter, composition and all values for bamboo sourced from[5,8,22,31,8486] .

Fibre type Density(kg/m 3)

Price(USD/kg)

Youngsmodulus(GPa)

Tensilestrength(MPa)

Elongation(%)

Length(mm)

Diameter(l m)

Moisturecontent(wt.%)

Cellulose(wt.%)

Hemi-cellulose(wt.%)

Lignin(wt.%)

Synthetic Carbon HS 18001840 124166 225260 44004800 0.0


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wetting which produces an inferior interface and encourages breagglomeration. Overcoming this issue is the subject of much re-search which is touched upon in the following paragraphs.

Fibre alkali treatment, also referred to as mercerisation (seeSection 3.7 .) is both a common and effective method for reducingwater absorption and improving the bre/matrix adhesion. Theprocess improves the capacity for chemical interaction between

the matrix and bres, while allowing for better mechanical inter-locking through rougher topography and larger numbers of indi-vidual brils [14] . The treatment can also dissolve hemicellulose;the most hydrophilic part of natural bre structures which contrib-utes little to the strength [15] .

In an alkali treatment study by De et al. the tensile strength of agrass/phenolic composite improved 52% after alkali treatment [16] .In the work of Van de Weyenberg et al. the effect of alkali treat-ment on a unidirectional ax/epoxy composite was investigated.An average increase in strength of 30% was found in the materialbre direction, while an increase of 138% was found in the trans-verse (matrix dominated) direction [14] . This large increase inthe matrix dominated property compared to the bre directionshows that alkali treatment has a large impact, specically, uponbre/matrix bonding.

Despite such work, structural applications of green compositeshave been extremely limited. Further work is needed to translatethe good specic mechanical properties of the natural bres intothose of a composite material. However, care should be taken toensure any chemical modication of the bres in order to achievethis does not adversely affect the inherent environmental creden-tials of these materials.

3.2. Variable bre properties

Due to natural bres being obtained from natural sources, theysuffer from natural variability in properties, including bre shape,length and chemical composition. Crop variety, seed density, soilquality, fertilisation, eld location, bre location on the plant, cli-mate and harvest timing are all factors that induce variation inthese properties [5]. This variability can be seen in the value rangesreported in Table 1 and seen graphically in Fig. 2. Variability is amajor issue to be resolved if these materials are to be relied uponin structural applications where failure is unacceptable and thusmust be accurately predicted. Attempts have been made to modelthe size, shape and tensile properties through statistical analysis of individual bres [17,18] , but few consider modelling of lamina andlaminates, where three dimensional discontinuities such as micro-beads may cause variation. Fibre lengths being tested in mostgreen composites are relatively small (up to around 300 mm) com-pared with higher performance man-made bres. Although this re-duces the chance of bres containing critical aws which reducethe materials tensile strength [19,20] , their non-uniform length

and shape leads to increased variability [4] . To effectively utilisethe properties of brous composites, bre length should be maxi-mised and bre direction aligned with the applied load. Increasedbre alignment also allows for higher bre volume fractions to beachieved [20] .

3.3. Renewability

The majority of traditional polymer matrix materials are de-rived from non-renewable petroleum which is formed from bio-mass over the course of 10 6 years; yet when consumed as plasticproducts or fuel, it is usually converted into CO 2 within 110 years[8] . Thus the use of this distinctly nite and often volatilelypriced resource is unsustainable. This is a large part of the incen-

tive for pursuing green composites where both the reinforcementand matrix materials are derived from plants usually in the time

span of less than a year [8]. Using renewable resources in this way;whereby the rate of CO 2 sequestered is balanced with the rate of consumption, contributes signicantly to developing carbon neu-tral materials.

3.4. Low embodied energy and CO 2 emissions

Values for the embodied energy and CO 2 emissions of materialscan be highly subjective [13] , yet are increasingly useful measuresby which the environmental impact of a material can be deter-mined. In addition, it provides an indicator of the materials costsensitivity to energy prices and emission regulations. In this work,we highlight embodied energy for the materials primary produc-tion only. This is the most signicant component of the materialslife-cycle energy ow (as opposed to processing) and eliminatessome of the variability/uncertainty regarding system boundariesand how the material may eventually be disposed of (for example,one could gain energy back through incineration with energycapture).

Ashby plots, as presented in Section 3.1 can also be applied tothe comparison of material environmental credentials. Weaveret al. show how such plots can be used to aid material selectionfor reduced environmental impact, demonstrated through a casestudy of refrigerator insulation [21] . Fig. 3 presents plots of embod-ied energy vs. mechanical properties. Unlike CO 2 gures, embodiedenergy is independent of assumptions of the greenhouse gas inten-sity of electricity generation. The values in Fig. 3 are mostly fromAshby [8] and thus are suitably comparable. Similar techniqueswere used to calculate the embodied energy from volume/weightfractions, as for density, in Fig. 2. Here we are using only theembodied energy to produce the constituent materials, not anythat is associated with creating the composite, nor that associatedwith any bre treatment. This is a signicant limitation of theanalysis.

In Fig. 3 it can be seen that a large number of natural bres haveboth an energy specic stiffness and strength advantage comparedto synthetics, which still holds when they are implemented as rein-forcements in polymers. In absolute terms, the embodied produc-tion energy in synthetic bres is in the order of 10 times greaterthan natural bres, whilst synthetic composites require aroundve times more energy for production than green composites.

Looking at a wider range of results in the literature, natural breproduction has been reported to involve 2025% [22] , 3040% [23]and 5055% [24] the energy of synthetics (glass bres). For greencomposites more generally, it is reported by Martin and Ramani[24] that a saving of 20 MJ/kg is achievable, with an associatedreduction of 1 kg of CO 2/kg, and an overall environmental impactreduction of 20%. Pervaiz and Sain [25] investigated a polypropyl-ene (PP) composite reinforced with hemp and found that an energysaving of 50 MJ/kg (3 kg of CO 2/kg) was possible by replacing a

glass bres component with a bre weight fraction of 0.3, with ahemp bre component with a 0.65 bre weight fraction.

Clearly there is an environmental benet in regards to energyuse and CO 2 emissions that can be achieved through the use of green composites.

3.5. Biodegradability

Materials are dened as biodegradable if they degrade throughthe actions of living organisms; this denition is often widened toinclude degradation through non-enzymatic hydrolysis [26] . Natu-ral bres are inherently biodegradable, as are many polymers thedegradation rate of which is dictated by the chemistry along itsbackbone [26] . For example, polyanhydrides and polyesters both

degrade through hydrolysis but at signicantly different rates;0.1 h and 3.3 years respectively [27] , whilst polyethers are



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non-biodegradable since they do not contain a hydrolysable bond.By making copolymers from polymers with different degradationrates, this property can be tailored to the application [28,29] .

Biodegradation is a desired quality since it prevents accumula-tion of solid waste, which is a major consideration for compositematerials in general and hinders their use in products with a lim-ited service life [8,30] . Green composites could allow composite

materials to enter new markets such as these, as their biodegrad-ability offers a serious advantage over synthetic composites. Forexample, pure PLA will degrade to carbon dioxide, water and meth-ane, within 2 years, whereas petroleum-based plastics requirehundreds of years [15] .

3.6. Low cost

Fig. 4 plots the production cost of a variety of materials againsttensile strength and Youngs modulus. As with Figs. 2 and 3 , themajority of values are taken from [13] . Green composite valuesare calculated from the reported volume/weight fractions and thusdo not consider the cost of creating the composite itself, only theconstituent elements.

It can be seen from Fig. 4 that natural bres have a clear costadvantage over synthetic bres. However, biopolymers can beslightly out performed by their synthetic counterparts, althoughthis situation is improving. In 2000 it was regarded that the costof biopolymers was too high to sustain the industry [31] ; sincethen, petroleum prices have risen and biopolymer productionmethods have improved. The result of this economic change hasbeen quite signicant, for example, adjusting for ination, the costof PP has increased by 111%, while the cost of PLA has fallen 73%[13,31] . Despite this turnaround in cost, it is remarked in [32] , thatlinking performance with cost is still a difcult task. Thus, althoughwork continues to improve the properties of biopolymers, the costeffectiveness of green composites depends heavily on the bre vol-ume fraction used.

Despite natural bres being currently inexpensive, two addi-tional points should be considered. The rst is that the bre treat-ment and processing techniques required for adequateperformance add additional cost to green composites [22] . The sec-ond point relates to the nature of the natural bre industry; in thatthese are materials with a young market for their supply and use.As such, there is great uncertainty around the future price of thesematerials. The North American market for natural bres is pro- jected to grow from USD 155 million in 2000 to USD 1.38 billionby 2025 [8] . Whether this growth leads to shortages and price in-creases, or the development of a more mature and robust produc-tion industry remains to be determined.

3.7. High natural bre water absorption

With three alcohol groups per glucan repeating unit, cellulose isa highly hydrophilic molecule and this property is imparted to nat-ural bres comprised of cellulose [22] . The manufacturing issuesthis raises were discussed in Section 3.1 . However, a further resultof this property is that of water absorption of the nished compos-ite material. Water absorption causes bre swelling which leads todelamination, surface roughening and a subsequent loss of strength of the material reported at up to 31% [5]. The moist envi-ronment can also facilitate the growth of fungus and bacteriawhich leads to rotting, this will be discussed in Section 3.8 wherethe durability of green composites is presented.

In addition, bio-derived polymers also have a tendency to ab-sorb more water than their synthetic counterparts, compounding

the problem. It was found by Masoodi and Pillai [33] that for 40%(by mass) jute/epoxy composites, water absorption stabilised at

17.5% (by mass of total specimen); this value jumped to 26% whena bio-derived epoxy was used instead.

The effect of moisture on the mechanical properties of naturalbres was investigated in detail by Symington et al. [34] . This workconcluded that moisture plays a signicant role in inuencing themechanical properties of natural bres. Whilst the tensile strengthof bres such as kenaf, jute and abaca centre around similar values

when at room temperature/humidity conditions compared tobeing fully soaked, bres such as ax and coir undergo a notabledecrease. Hemp was so degraded it was unable to be tested, raisingconcerns around its stability when exposed to various environ-mental conditions. In this work abaca was found to have the high-est capacity to absorb moisture (164% by weight), which, especiallyconcerning for the performance of a composite, led to volumeswelling of 245.85%.

Signicant research has been undertaken into improving theproperties of natural bres through chemical modications to theirsurface, this work is presented and reviewed in a number of pa-pers, including [22,35,36] . These reviews suggest that the mostcommon and efcient chemical modication method for reducingmoisture absorption is alkali treatment (also referred to as mer-cerisation), which has been used to treat almost all natural breswith successful results (although quantiable results are difcultto nd). Exposure of the bres to alkali solutions (most commonlyKOH or NaOH) causes dissolution of the noncellulosic cementingsubstances; hemicellulose and lignin ( Fig. 1) which reduces mois-ture absorption by the bre [34] . The alkali treatment must becarefully controlled, as too long an exposure will degrade the struc-tural cellulose [34] . Other issues with the process include the cre-ation of high pH waste products which compromises theenvironmental credentials of green composites [5] .

One alternative to alkali treatment is the Duralin process which,by utilising steam to degrade and remove the hemicellulose andlignin, is more environmentally friendly [37,38] . In an investigationby Stamboulis and Baillie involving ax bres, the maximum mois-ture content (by mass) after exposure to 100% humidity was re-ported at 14.33% for a Duralin treated sample, compared with42.58% for an untreated specimen [38] . Other benets of the Dur-alin process include better bre dimensional and temperature sta-bility, better resistance to fungal attack and generally improvedmechanical properties. However, the high temperatures requiredadds signicant energy costs to the material which again compro-mises the environmental credentials of green composites [5] .

Material coatings are another method that can be used to pre-vent water absorption. But this is not 100% effective [39] and is of-ten only a short to medium term solution.

3.8. Poor durability

As one would expect from biodegradable materials, green com-

posites have limited durability. Exposure to environmental condi-tions can lead to a rapid degradation of the material. Accuratelypredicting the lifetime of green composites is a major challengeto their widespread implementation [8] .

One concerning cause of degradation in green composites is thegrowth of fungus and bacteria. Stamboulis and Baillie observedfungal growth on ax bres after just three days exposure to mois-ture [38] . In the work of Nadali et al. a bagasse/polypropylene com-posite was exposed to rainbow fungus ( Coriolus versicolor ): after4 months a 3050% reduction in bending strength, bending modu-lus and hardness was observed [40] . Singh and Gupta show thatweathering over a two year period of a jute/phenolic composite re-sulted in colour fading, resin cracking, bulging, brillation andblack spots [41] . In addition, signicant reductions in mechanical

properties were logged: for example, exural strength reducedby 22%. Similar results were reported in [42,43] .



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3.9. Non-toxicity

Natural bres are generally non-toxic [6] , providing scope formanufacturing composites with no or heavily reduced humanhealth hazards and environmental damage throughout their lifecycle (production, processing, use and disposal). A life cycleassessment by Corbiere et al. of biobre pallets compared to glassbre equivalents found reductions of: human toxins (43%), carcin-ogenic substances (63%) and heavy metals (71%) [44] . However,as shown in the work of Alves et al. bre treatment can havean effect on the level of respiratory irritants contained in theresulting composite [45] .

3.10. Biocompatibility and bioactivity

A biocompatible material exists in harmony within a biologicalsystem, whilst a bioactive material facilitates a (in this instance benecial) biological response from the system through a directinteraction [26,46] . Non-toxicity is a fundamental requirement of a product for in vivo use and should the material biodegradein situ, it is important that the by-products also meet these criteria.

Here the hydrophilicity of green composites allows cellularinteractions; cellulose, collagen, starch, chitin and PLA have alldemonstrated biocompatibility [26,47] .

3.11. Fibre degradation at elevated temperatures

Natural bres can begin to degrade at temperatures exceeding170200 C, limiting the range of suitable applications, matrix sys-tems and processing methods [8,22,36] . However, for manufactur-ing of green composites with a bio-derived polymer matrix (suchas PLA) this has not proved a limiting factor, as these materialshave low processing temperatures [5] .

4. Dening complementary applications

Green composites areoften touted for application in theautomo-tive [4] and construction [5] industries. The potential for green com-

posites to have a positive environmental impact when applied tothese industries is large, as carbon can be effectively sequesteredin these products for manyyears.Green composites canalsoprovideweight saving and vibration damping of benet to the automotivesector. However, excluding non-structural, interior applications,the material properties required in automotive and constructionuses are not always the best match for the currently obtainableattributes of green composites (variable breproperties, poor dura-bility). Similarly, the attributes of green composites do not makethem a potential substitute for glass bre reinforced composites(GFRP); not only are their absolute mechanical properties inferior,but their water absorption tendencies may exclude them from usein the vast array of wet GFRP applications (boats, kayaks, piping,tanks, etc.). Even if designs and manufacturing techniques for the

structural intensive GFRPapplications suchas windturbineblades could be modied to make use of the good specic mechanical

properties of green composites, great challenges would still ob-struct their implementation in the form of bre property variabilityandlow durability. Inlightof theseobservations a list hasbeencom-piled ( Table 3 ) that attempts to match the material attributes of green composites with complementary application attributes.

This list should be thought of as a guide for the successful appli-cations of green composites. Applications that maximise theadvantages of green composites (environmental), minimise thedisadvantages (durability) and are obtainable with the mechanicalperformance offered. This paper does not want to rule out the useof green composites in any application, it simple tries to better pro-mote the use of green composites in a wider array of applications,beyond the already heavily investigated areas of automotive andconstruction. In the following sections relevant current, developingand future applications of green composites are discussed.

5. Complementary applications

5.1. Short life-span products

Short life-span products are typically thought of as those thatare disposable, such as plastic cutlery and packaging. So calledcommodity plastics such as polyethylene, polystyrene, polypropyl-ene, and polyvinyl chloride are used heavily in packaging causingseveral environmental concerns due to their non-biodegradability.Bio-composites that incorporate a biodegradable polymer compa-rable to the price and material performance of commodity poly-mers may well be a solution to the problem [7,31] .

However, short life-span products are not only those items thatwe consider disposable, they are also items such as consumer elec-tronics for example, smart phones. With such items, changes instyle and improvements in technology can quickly lead to a prod-uct becoming obsolete, despite it still being functional [48] .Although research on product life-spans is limited [49] , anecdotalevidence would suggest that as consumers become more involvedin the fast paced development of electronics, the rate of productobsolescence is increasing; coupled to a reduction in product life-spans. A study into discarded products in the UK from 1993 to

1998 found that computers, telephones, faxes, radios, stereos, CDplayers, mobile phones, pagers and toys had a mean age of sixyears or under [49] . The very fact that some of these items canno longer be purchased and there is no mention of smart phones would suggest that the mean age of discarded items in the UK to-day would be substantially less.

More recently, the European Consumer Centre has reportedaverage life-spans for smart phones of just 20 months, not fromfailure or planned obsolescence, but rather style obsolescence[48] . The same report found average life-spans for rst, secondand third generation iPods of just 18 months before battery failure[48] . Such products like many other modern designs for consumerelectronics turn out to be more cost effective to replace rather thanrepair or replace the damaged subcomponent.

The electronics company NEC has been working with greencomposites since 2004 when they used a PLA/kenaf composite in

Table 3

Summarised green composite attributes, matched to complementary application attributes.

Material attributes Application attributes

Excellent weight specic stiffness Good weight specic strength Weight critical vehicles/products (transport, mobile electronics, sport equipmentVariable bre properties Non-safety critical/low required reliability applicationsRenewable resource Low embodied energy Biodegradable Short life-span product (disposable and high obsolescence rate products)Non-toxic Childrens toys, consumer handled items, hobbyist built itemsBiocompatible Medical devices and implants

Low cost Competitive consumer productsHigh water absorption Dry use productsPoor durability Short life-span products, limited exposure to harsh environments



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dummy cards in personal computers [50] . In 2006 NEC developedan eco phone with this material and in 2007 began using somegreen composites in the housing of their personal computers [51] .

The green credentials of mobile phones are an increasingly pop-ular area of development. US mobile phone retailer Sprint declaredin January 2012 that it would require all phones it sells to meet aminimum environmental standard [52] . While a June 2012 report

by Juniper Research has stated that 392 million green handsets(those with at least 50% recycled content and made without certainhazardous chemicals) will be shipped by 2017 [53] .

Clearly phones, and electronics in general, are a potential appli-cation for green composites due to the high rates of obsolescencealong with their limited exposure to moisture during use.

Toys are another potential short life-span application for greencomposites. Although no current examples of biodegradable greencomposite toys were found excluding wooden toys the com-pany Sprigwood manufactures toys from cellulose reinforced recy-cled plastics [54] . This shows that there is both consumer andbusiness interest in using materials with improved environmentalcredentials in toy applications. The non-toxic attributes of greencomposites could also be highly desirable in toy applications par-ticularly for very young children.

From these considerations, we conclude that consumer elec-tronics and toys are potential applications for green compositessince they would complement their renewable, low embodied en-ergy, biodegradability and low durability properties.

5.2. Sporting equipment

The varying properties of natural bres and their limited abso-lute strength present a challenge for their use in load bearing appli-cations. The issues associated with maintaining this strength whenformed into a composite material are another obstacle for this typeof application. However, the weight specic properties particu-larly stiffness of these bres is excellent, thus there is potentialand incentive for them to be used in applications where mechani-cal properties are important. Sporting equipment may be a goodstarting point since failure (resulting from material variation ordegradation) is less likely to cause serious injury or expensiveproperty damage than more critical applications.

Despite this, sporting equipment applications to date have in-volved either the use of hybrid composites (natural and syntheticbres combined) and/or non-biodegradable matrix polymers,negating one of the materials principal benets biodegradability.Examples included ax reinforced snowboards [55,56] , ax andcarbon (25:75%) reinforced tennis rackets [57] and ax and carbon(80:20%) reinforced bicycle frames [58] . In these applications theax bres impart enhanced performance, since they provide supe-rior vibration dampening compared to those of carbon or glass. It isclaimed that this leads to snow boards that can navigate uneven

terrain more uidly and with greater speed [55] , tennis racketsthat give added comfort when striking the ball [57] , and bicyclesthat absorb micro-shocks for superior comfort [58] . Althoughmany of these claims may be based more on marketing than engi-neering, the concept is aligned with one of the messages of this re-view; that the most successful green composite applications willbe those that complement the inherent material attributes.

5.3. Biomedical applications

The hydrophilicity of green composites facilitates interactionswith other hydrophilic surfaces and substances such as living celltissue. This property of bioactivity, coupled with their biocompat-ibility and biodegradability, distinguishes green composites from

their synthetic counterparts for use in biomedical applications.By opening up this eld to composites, materials can be produced

which offer signicant advantages over those traditionally used orthose of just a single component [59,60] :

Superior material properties such as strength and toughnesswithout sacricing weight,

The ability to tailor other properties such as, Biodegradation kinetics,

Cell permeability, The possibility of incorporating other materials within the cellsupport such as growth factors or nutrients [61] ,

Shapability [62] .

One biomedical application for which green composites havedemonstrated these advantages is the interdisciplinary eld of tis-sue engineering; here a material scaffold acts as an extra cellularmatrix to which cells adhere and grow. Specically, green compos-ite materials have shown promise as scaffolds for soft tissuegrowth: Highly tailorable three-dimensional composite structuresof poly(lactic- co-glycolic acid) (PLGA)/collagen have demonstratedsuccess in regeneration of articular cartilage [62] . The biodegrada-tion of PLGA is easily tailorable through the monomer ratio [63]and the by-products produced are naturally present in the bodyso this material poses minimal toxicity risk [61] . Similarly, by usingthese two polymers in their homopolymer form a highly tailorablescaffold for cartilage tissue growth has been produced. Here, a -brous mesh of poly(glycolic acid) (PGA) was embedded within apoly(lactic acid) (PLA) matrix, and an increase in the proportionof the matrix was found to lead to an increase in both stiffnessand degradation rate [64] .

Cellulose has been studied in a range of biomedical applications[6569] . Bacterially sourced cellulose (from Gluconacetobacter xyli-nus ) has been successfully incorporated into a PLA matrix and thematerial was touted for biomedical applications [70] , as have cellu-lose/chitosan composites [71] .

Human mesenchymal stem cells (MSCs) can be seeded onto ascaffold and directed to differentiate down a desired cell lineageto grow tissues or organs. A trachea has been successfully grownusing this technique and the subsequent transplant proved groundbreaking since the procedure allowed the patient an immunosup-pressant-free life, unlike normal transplantees [72] . The tracheawas grown ex vivo on a donated extracellular matrix and differen-tiation was induced using growth factors.

Cell differentiation canbe inuenced by a number of factors, oneof which is the stiffness of the scaffold material [73] . It can be envi-sioned thatby using a green composites bioactivity to induce differ-entiation, the tissuegrowth could occur in vivo without theneed forgrowth factors [73] . A composite of cellulose and silk was found toinitiate bioactively chondrogenic differentiation of human MSCs,and lead to the deposition of cartilage matrix material in vitro.The hydroxyl and amide functional groups were found to be impor-

tant factors along with the materials stiffness, all of which are tail-orable through the proportions of the two materials used [74] .

Biopolymers are already widely used in medical applications[75] ; however it is only recently that natural bres have beenincorporated into these systems to yield new green compositematerials which offer unique performance and functionality[60,70,71] . This eld is very much in its infancy and with the realadvances being made, it could one day offer some truly life chang-ing medical treatments.

6. Conclusion

This review has provided a concise summary of the major mate-

rial attributes of green composites. These include: good specic but variable mechanical properties, good environmental creden-



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tials (renewable, biodegradable, low embodied energy, non-toxic-ity), low cost, high water absorption, low durability and biocom-patibility. As work continues to improve the attributes of greencomposites, particular care must be taken to ensure the inherentgreen characteristics of these materials are not undermined.

This review investigates a series of potential application forthese materials. Applications such as consumer electronics, sport-

ing equipment and medical devices that have been deemed to havecomplementary attributes to those of green composites have beenpresented. These are only examples of potentials applications andare by no means meant to represent an exhaustive list of greencomposite applications. Instead, they simply demonstrate the mes-sage of this review; that the most successful green compositeapplications will be those that complement their inherent materialattributes. Green composites have potential for use in a number of applications, but as with all design, one must carefully match thematerial to the application.

Although the information presented in this review does notdelve into the details of green composite material science, it ishoped it does allow engineers and designers to have a better graspof the most appropriate applications for green composites. It isanticipated that this will lead to the increased application of greencomposites, ultimately leading to an improvement in the sustain-ability of our material systems.

Acknowledgments

The authors gratefully acknowledge the support of the EPSRCunder its ACCIS Doctoral Training Centre grant, EP/G036772/1.

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