Natural Fiber Eco-Composites
G. Bogoeva-Gaceva,1 M. Avella,2 M. Malinconico,2 A. Buzarovska,3 A. Grozdanov,3 G. Gentile,4 M.E. Errico4 1
Faculty of Technology and Metallurgy, University St Cyril and Methodius, 1000 Skopje, Republic of Macedonia 2
Institute for Chemistry and Technology of Polymers, ICTP-CNR, 80078 Pozzuoli, Naples, Italy
3
Faculty of Technology and Metallurgy, University St. Cyril and Methodius, 1000 Skopje, Republic of Macedonia 4
Institute for Chemistry and Technology of Polymers, ICTP-CNR, 80078 Pozzuoli, Naples, Italy
The natural fiber (NF) reinforced composites, so called eco-composites, are subject of many scientific and research projects, as well as many commercial programs. The growing global environmental and social concern, high rate of depletion of petroleum resources, and new environmental regulations have forced the search for new composites and green materials, compatible with the environment. The aim of this article is to present a brief review of the most suitable and commonly used biodegradable polymer matrices and NF reinforcements in eco-composites, as well as some of the already produced and commercialized NF eco-composites. POLYM. COMPOS., 28:98 –107, 2007. © 2007 Society of Plastics Engineers
INTRODUCTION One of the major environmental problems we are facing today is the plastic waste problem. The tremendous production and use of plastics in every segment of our life has increased the plastic waste in huge scales. The waste disposal problems, as well as strong European regulations and criteria for cleaner and safer environment, have directed great part of the scientific research to eco-composite materials that can be easily degraded or bio assimilated [1, 2]. As a term, eco-composite is usually used to describe composite material with environmental and ecological advantages over conventional composites. By definition, an eco-composite may contain natural fiber (NF) and natural polymer. More-
Correspondence to: Dr. A. Grozdanov; e-mail:
[email protected] Contract grant sponsor: EU FP6 INCO Program; contract grant number: INCO-CT-2004 –509185. DOI 10.1002/pc.20270 Published online in Wiley InterScience (www.interscience.wiley.com). © 2007 Society of Plastics Engineers
POLYMER COMPOSITES—2007
over, eco-composites could be a combination of NF and biodegradable polymer matrix. Using the last terminology, i.e. biodegradable polymers, the number of polymer matrices that could be used in eco-based composite formulations is significantly increased. The research field of biodegradable polymers is still in its early stages, but is growing in popularity every day. Currently, a lot of polymer biodegradable matrices have appeared as commercial products, offered by various producers. The high price of these materials is the main restriction in their use (Table 1), in spite of their unique physical and chemical properties [3, 4]. The aim of this article is to present a brief review of the most commonly used biodegradable polymers and NFs in ecocomposite materials. BIODEGRADABLE MATRICES IN ECOCOMPOSITES The general division of biodegradable polymers, widely accepted in numerous literature surveys, classifies these polymers into three categories, based on whether they polymerize biologically or synthetically: ● ● ● ●
Biosynthetic Semi-biosynthetic Chemo-synthetic polymers Biosynthetic are actually natural polymers, produced from natural sources. Starch, as one of the commonly used natural polymers, offers limited substitute for petroleum plastics. Starch is a polysaccharide that can be found in numerous renewable resources as corn, potato, rice, and others [5]. Starch is the least expensive biopolymer. It becomes thermoplastic when properly plasticized with water or other plasticizers. Starch formulations can be processed by all common methods, used for synthetic
TABLE 1. Average cost of biodegradable polymers and some traditional thermoplastic polymers [3, 4]. Polymer
(eur/kg)
biodegradable polymers
(eur/kg)
HDPE LLDPE PP PS PVC
0.92 0.82 0.80 1.08 0.78
PLA Starch Polyesters BAK PCL
3–4 2–4 3.5–5 5.58 8.3
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resins (extrusion, injection molding, thermoforming, etc.). It can be used in its granular form, or as biodegradable filler [6]. The main shortcomings, related to this material, are low impact resistance, strong water absorption, and very poor mechanical properties (lack of interfacial adhesion). To improve some of the properties, starch is usually blended with other thermoplastic polymers [7, 8]. In one of the earliest uses of starch in plastics, it was blended with PE, with 20 – 80% of starch content. Starch can be blended with polyvinyl alcohol, as well: the blends are thermoplastic and easily processible. Starch-based blends biodegrade at rates that depend on composition and crystallinity. The blends of starch with aliphatic polyesters, especially poly(-caprolactone) (PCL), were widely studied [9 –11]. The mechanical properties of starch as a polymer matrix could be also significantly improved in the presence of NFs as reinforcements [12]. It was shown that the tensile strength in eco-composites is strongly dependent on fiber content. Vazques et al. have studied the properties of biodegradable composites based on PCL/starch blend matrix, known under the commercial name MaterBi-Z and NFs [13]. They showed significant increase in elastic modulus (E) up to 700 MPa and mechanical strength () up to 15 MPa of the composites when compared with the polymer matrix (E ⫽ 37 MPa and ⫽ 7.3 MPa, respectively). Starch is usually surface treated to improve the compatibility between the starch and the synthetic polymer. For example, different surface treatments of starch with in situ -caprolactone or ␦-valerolactone monomers are proposed [14]. The role of the monomer is to promote covalent grafting of the polyester chains into starch, resulting in a material with good interfacial adhesion between the two components. Other possibility to overcome the poor mechanical properties of the starch is to obtain starch graft copolymers having thermoplastic branches [15]. As a material, starch has very low decomposition temperature, and cannot be simply melted. For these reasons this natural polymer is often chemically modified by partial or complete esterification of the hydroxyl groups in the side chain. The modified commercial product is known as SCONACELL A [16]. Cellulose is another natural polymer. It differs in some respect from other polysaccharides produced by plants. Its molecular chain is very long, consisting of only one repeating unit, glucose, and occurs in crystalline state [17]. Cellulose plastic has been a commercial product for many years. The usefulness of cellulose as a starting material can be also extended by chemical modification to methyl cellulose (MC), hydroxypropylmethyl cellulose
DOI 10.1002/pc
TABLE 2. polymers.
Thermal and mechanical properties of some biodegradable
Polymer
Tg (°C)
Tm (°C)
Tensile strength (MPa)
PLA PCL PHB PHB-V PP
54 ⫺62 5 2 to ⫺8 ⫺10
170 57 171 162–102 176
28–48 16 40 28 38
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●
Tensile modulus (GPa)
El. break (%)
3.5 0.4 3.5 0.9 1.7
6 80 7 15 40
(HPMC), hydroxypropyl cellulose (HPC), and carboxymethyl cellulose (CMC). These cellulose ether films, cast from aqueous or aqueous-ethanol solutions of MC, HPMC, HPC, and CMC, tend to have moderate strength; are resistant to oils and fats; and are flexible, transparent, odorless, tasteless, water-soluble, moderate barriers to moisture and oxygen [17]. MC films provide an excellent barrier against migration of fats and oils. HPC is the only edible and biodegradable cellulose-derived polymer that is thermoplastic and, therefore, capable of injection molding and extrusion. Development of cellulose plastics as a matrix in bio-composites requires properties such as percent elongation, flexibility, and impact behavior. The chemical, physical, mechanical, and thermal properties of cellulose are also influenced by its modifications. Mohanty et al. used various eco-friendly plasticizers to obtain a suitable polymer matrix for eco-composite application [18]. They have shown at the same time that the plasticizer content can improve stiffness and toughness properties of the cellulose plastics. Cellulose could be found in eco-composites more as a fiber, than as a polymer matrix [19]. Polyhydrohyalkanoates belong to the biosynthetic class of polymers, produced in the nature by fermentation of sugars or lipids [20, 21]. More than a 100 different monomers can be combined within this family to give materials with extremely different properties. Polyhydroxy butyrate (PHB), polyhydroxy valerate, and their copolymers P(HB-V) are the commonly used polymer matrices in numerous eco-composites [22, 23]. The interest for these polymers has grown, since they are biodegradable, and at the same time have very similar mechanical and thermal properties to polyolefins (propylene (PP) and PE) (Table 2). PHB is highly crystalline, and prior to its melting has higher decomposition level, resulting in a very brittle material. These properties could be easily controlled by the content of hydroxy valerate in the P(HB-V) copolymers. The resulting copolymers are less crystalline and more processible [24]. Depending on the HV content, PHB-V can display physical properties and processing behavior resembling PE or PP, and can range from brittle plastic to an elastomer. P(HB-V) properties can be further modified with additives. P(HB-V) has substantial water resistance, greater than most polysaccharides and proteins. P(HB-V) is thermoplastic and can be processed by injection molding, extrusion, blow molding, film and fiber forming as well as a variety of coating and lamination techniques. POLYMER COMPOSITES—2007
99
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100
The second class of biodegradable polymers, i.e. semibiosynthetic polymers, are polymers in which the monomer unit is produced naturally or by a fermentation process, and then the polymerization procedure is a classical synthetic procedure. A representative of the semi-biosynthetic polymers is polylactic acid (PLA). This polymer is characterized by its transparency, humidity, and oil resistance. It can be found in the D,L PLA or L PLA form. The first form of PLA is an amorphous polymer, while the second one is a semi-crystalline polymer. PLA is polyester synthesized from LA. Condensation polymerization of LA generally produces a low-molecular-weight polymer, which is then treated with coupling reagents that act as chain extenders to give high molecular-weight (Mw) PLA. Recently, a catalyzed condensation polymerization has been found to give high Mw PLA, when carried out in a high boiling point solvent at reduced pressure. Alternatively, the low-Mw prepolymer is depolymerized to produce lactide (the cyclic dimmer of LA), and then metalcatalyzed ring opening polymerization of lactide is performed to produce high-Mw PLA with good mechanical properties. Its mechanical properties can be modified by varying its Mw and its crystallinity. Its properties can also be modified by copolymerization of the LA with caprolactone or glycolic acid. PLA resins are being developed and marketed through a number of commercial efforts for different applications. PLA is thermoplastic, and can be processed by most common methods, including fiber spun. As a monomer, lactic acid can be produced in chiral forms by fermentation of corn and other agricultural resources [25]. The mechanical properties of PLA have been extensively studied as a biomaterial in the medicine, but only recently it has been used as a polymer matrix in eco-composites [26]. Its applicability and use in eco-composites is still limited by its high price when compared with other biodegradable polymers. Xia et al. [27] investigated the use of PLA resin reinforced with kenaf fibers for the interior parts of its Prins hybrid car. In 2002, Cargill-Dow LLC started up a commercial polylactide plant, with the aim of production of PLA fibers for textiles and nonwovens, PLA film packaging applications, and rigid thermoformed PLA containers [28]. The chemosynthetic polymers are another large class of degradable polymers, and are actually a family of polyesters. PCL is one of the representatives of this class of polymers. PCL is hydrophobic, commercially available thermoplastic aliphatic polyester with good mechanical properties. It is prepared by ring opening polymerization of -caprolactone. The degradable synthetic polyesters, polyglucolic acid, and PCL are susceptible to hydrolytic degradation, because the ester groups are separated by CH2 groups, which impart some flexibility to the chain. Even when they are in highly crystalline forms, the flexibility allows penetration of water molecules [25]. Polyester amides that can be found commercially (BAK 1095 and BAK 2195) belong to the chemo-synthetic biodegradable polymers as well. These polymers commercially appeared in 1995 by Bayer AG. Mohanty et al. have reported biodegradable jute/BAK 1095 composites [29]. Novel eco-friendly bio-composites based on BAK have been developed recently [30]. It was shown that the POLYMER COMPOSITES—2007
TABLE 3. Tensile properties of polyolefine composites with 20% of natural fibers.
Composite PP/Bamboo PP/sugar cane LDPE/short sisal LDPE/treat sisal
●
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●
Modulus (MPa)
Elongation at break (%)
Strength (MPa)
2500 600 453 926
/ 8.0 10 12
16 17 12.5 16.5
efficiency of BAK reinforcement is strongly dependent on the surface modification, but it is also completely biodegradable. For these reasons, Bayer AG has recently quit the production of BAK, since there is no economical justification for continuing this costly effort. The poly(itaconic acid esters) could also be an important polymers, because of the fact that itaconic acid is not only an oil-based product, but it can also be obtained by fermentation process of molds and various carbohydrates. Poly(dicyclohexyl itaconate) is especially interesting polymer, with properties that could be significantly modified when blended with other thermoplastic polymers [31, 32] It can be concluded that the last group of biodegradable polymers, i.e. chemo-synthetic, are generally polyesters. A common feature of all polyesters is the very reactive ester functional group, which could have significant role in tailoring the interface between the polymer matrix and NFs as reinforcement. Another polymer, although not biodegradable, that could be found as a polymer matrix in numerous eco-friendly composites is PP. As an oil-based product, PP could not be classified as a biodegradable polymer, but by introducing thermo-sensitive catalysts to increase the degradability, PP takes an important place in eco-composite materials. For example, Mohanty et al. have demonstrated that the NF reinforced PP composites have potential to replace glass-PP composites [33]. It has also been reported that PP can be effectively modified by maleic anhydride. Maleic anhydride provides polar interactions and covalently links PP to the hydroxyl groups of the cellulose fiber [34]. Special attention has been paid to the modification of fibers with specific chemical treatments to improve the adhesion with polyolefin matrices. Table 3 shows some of the mechanical properties reported in the literature for several thermoplastic matrices, reinforced with NFs.
NATURAL FIBERS AS REINFORCEMENTS FOR ECO-COMPOSITES In the last decade, there is a growing interest in NF reinforced composites because of their high performance in terms of mechanical properties, significant processing advantages, chemical resistance, and low cost/low density ratio [13, 35, 36]. On the other hand, for environmental reasons, there is an increased interest in replacing reinforcement materials (inorganic fillers and fibers) with renewable organic materials. DOI 10.1002/pc
TABLE 4.
Basic properties of some natural fibers [40].
Fiber
Density (g/cm3)
Elongation at break (%)
Fracture stress (MPa)
Young modulus (GPa)
Cotton Jute Flax Hemp Sisal Bamboo Soft wood Ramie
1.5 1.3–1.46 1.4–1.5 1.48 1.2–1.5 0.8 1.5 1.5
7.0–8.0 1.5–1.8 2.7–3.2 1.6 2.0–2.5 / / 3.6–3.8
287–597 393–800 345–1500 270–900 511–700 391–1000 1000,0 400–938
5.5–12.6 10–30 10–80 20–70 3.0–98 48–89 40,0 44–128
NFs represent environmentally friendly alternatives to conventional reinforcing fibers (glass, carbon, kevlar). Advantages of NF over traditional ones are low cost, high toughness, low density, good specific strength properties, reduced tool wear (nonabrasive to processing equipment), enhanced energy recovery, CO2 neutral when burned, biodegradability. Because of their hollow and cellular nature, NF perform as acoustic and thermal insulators, and exhibit reduced bulk density [35, 37]. Depending on their performance when they are included in the polymer matrix, lignocellulosic fibers can be classified in three categories: (1) wood flour particulate, which increases the tensile and flexural modulus of the composites, (2) fibers of higher aspect ratio that contribute to improve the composites modulus and strength when suitable additives are used to regulate the stress transfer between the matrix and the fibers, (3) long NFs with highest efficiency amongst the lignocellulosic reinforcements. The most efficient NFs have been considered those that have high cellulose content coupled with a low microfibrile angle, resulting in high filament mechanical properties. Depending on their origin, NFs can be grouped into seed, bast, leaf, and fruit qualities. The bast and leaf (hard fiber) types are the most commonly used in composite applications [38, 39]. Examples of bast fibers include hemp, jute, flax, ramie, and kenaf. Leaf fibers include sisal, pineapple, leaf fiber, banana. The characteristic properties of NF vary considerably, depending not only on their origin, but also on the quality of plants location, the age of the plant, and the preconditioning [38]. The strength characteristics (Tables 4 and 5) of NF depend on the properties of the individual constituents, the TABLE 5.
Fiber Sisal Flax E-Glass Kevlar Carbon (standard)
DOI 10.1002/pc
fibrillar structure, and the lamellae matrix [40]. For an understanding of the mechanical properties and durability of fibers, the amount of major structural constituents of the fibers, cellulose, hemicellulose, and lignin should be known [41]. Chemical composition and structural parameters of some NFs are given in Table 6. NFs exhibit considerable variation in diameter, along with the length of individual filaments (Table 7). The quality and overall properties of fibers depend also on factors such as size, maturity, and processing methods adopted for the extraction of fiber [42]. The properties such as density, electrical resistivity, ultimate tensile strength, and initial modulus are related to the internal structure and chemical composition of fibers. NFs are complex in their chemical structure; generally they are lignocellulosic, consisting of helically wound cellulose microfibrils in an amorphous matrix of lignin and hemi-cellulose. Mechanical properties are determined mainly by the cellulose content and microfibrillar angle. Young’s modulus of NF decreases with the increase of diameter. A high cellulose content and low microfibril angle are desirable properties of a fiber to be used as reinforcement in polymer composites [41] (Table 6). The mechanical properties of NF are also significantly related to the degree of polymerization of cellulose in the fiber. The cells of flax fiber consist mostly of pure cellulose, being cemented as fascicle bundles by means of noncellulosic incrusting such as lignin, hemicellulose, pectin, protein or mineral substances, resins, tannins, dyers, and a small amount of waxes and fat [38, 39]. A mature flax cell wall consists of about 70 –75% cellulose, 15% hemicellulose, and pectic materials. Cellulose is a natural polymer with high strength and stiffness per weight, and it is the building material of long fibrous cells. Selective removal of noncellulosic compounds constitutes the main objective of fiber chemical treatment. Both the hemicellulosic and pectin materials play important roles in fiber bundle integration, fiber bundle strength, and individual fiber strength as well as water absorbency, swelling, elasticity, and wet strength. The production of individual fibers without the generation of kink bands will generate fibers with much higher intrinsic fiber strength, which is very useful for composite application [41, 42]. Desirable properties for fibers include excellent tensile strength and modulus, high durability, low bulk density,
Mechanical properties of natural fibers when compared with conventional reinforcements [40].
Specific gravity (g/cm3)
Tensile strength (GPa)
Tensile modulus (GPa)
Specific strength (GPa/g cm3)
Specific modulus (GPa/g cm3)
Cost ratio
1.20 1.20 2.60 1.44 1.75
0.08–0.5 2.00 3.50 3.90 3.00
3–98 85 72 131 235
0.07–0.42 1.60 1.35 2.71 1.71
3–82 71 28 91 134
1 1.5 3 18 30
POLYMER COMPOSITES—2007
101
TABLE 6.
Chemical composition and structural parameters of natural fibersa [41].
Fiber
Cellulose (%)
Hemi-cellulose (%)
Lignin (%)
Extractives (%)
Ash (%)
Pectin (%)
Wax (%)
Microfibril/spiral angle (°)
Moisture content (%)
Jute Flax Hemp Kenaf Sisal Cotton
61–71 71–78 70.2–74.4 53–57 67–78 82.7
13.6–20.4 18.6–20.6 17.9–22.4 15–19 10–14.2 5.7
12–13 2.2 3.7–5.7 5.9–9.3 8–11 /
/ 2.3 3.6 3.2 / /
/ 1.5 2.6 4.7 1 /
0.2 2.2 0.9 / 10 /
0.5 1.7 0.8 / 2.0 0.6
8.0 10.0 6.2 / 20.0 /
12.6 10.0 10.8 / 11.0 /
a Literature data on chemical composition of NF have been inconsistent, and for this reason the range of chemical constituents given in this table should be viewed with caution.
good moldability, and recyclability. NFs have an advantage over conventional reinforcement fibers in that they are less expensive (Table 8), abundantly available from renewable resources, and have a high specific strength. Application of long NFs instead of short wood-fibers, such as flax, kenaf, and sisal, is reasonable in automobile sector because of the specific modulus, close to that of glass-reinforced composites. Until now, NFs were used in composites with unsaturated polyesters, epoxides, and polyurethanes. Among them epoxy resins are characterized with the best adhesion toward NFs. The composites based on polyurethane matrices and sisal are used for production of door panels in automobile industry; they are characterized with increased strength and sufficient stiffness to preserve their shape during exploitation. Thermoplastic polymers have significant viscosity, and therefore the fibers wetting is difficult. On the other hand, increasing the temperature to lower the viscosity could cause some undesirable effects, such as damage and destruction of the NFs. The application of appropriate compatibilizers in these systems is practically unavoidable, since otherwise the fibers will start to behave as filler, instead of reinforcement. A well-known example is maleic anhydride modified polypropylene in PP matrix, causing a significant improvement of the mechanical properties of the material [43]. Natural Fiber’s Modification Some of the disadvantages and limitations of NFs, when used as reinforcement for composites, are related to the lack of proper interfacial adhesion, poor resistance to moisture TABLE 7. Fiber Cotton Flax Hemp Juta Straw Kenaf
102
Dimensions of some natural fibers [40]. Average length (mm)
Width (mm)
10–60 5–60 5–55 1.5–5 1–3.4 2.6–4
0.02 0.012–0.027 0.025–0.050 0.02 0.023 0.018–0.024
POLYMER COMPOSITES—2007
absorption, limited processing temperature to about 200°C, and low dimensional stability (shrinkage, swelling). The fiber/matrix interface plays an important role in the physical and mechanical properties of composites. To improve the interfacial properties, NFs are subjected to chemical treatments such as dewaxing, mercerization, bleaching, cyanoethylation, silane treatment, benzoylation, peroxide treatment, isocyanate treatment, acrylation, acetylation, latex coating, steam-explosion [37, 38, 44]. Research on “a cost-effective” modification of NFs is necessary, since the main market attraction of eco-composites is the competitive cost of NFs. Surface modification of NF prior to their use in composite materials is also needed to facilitate fiber dispersion and induce bond formation between the fiber and the polymer matrix. The fiber-to-fiber interaction in NF, resulting from intermolecular hydrogen bonding, usually restricts the dispersion of fibers in the polymer matrix. On the other hand, the use of surface treatments has the disadvantage of increasing the cost of the final product. Chemical modification can be defined as a chemical reaction between some reactive constituents of the NF and chemical reagent, with or without a catalyst, to form a covalent bond between the two. As the NFs result from the chemistry of cell wall components, modification of the chemistry of cell wall polymers can change the basic properties of a fiber. The chemicals to be used for chemical modification must be capable of reacting with lignocellulosic hydroxyls under neutral, mild alkaline, or acid conditions at temperatures below 150°C. The chemical system should be simple and capable of swelling the structure to facilitate penetration. The complete molecule should react quickly with the lignocelluloses components yielding stable chemical bonds, and the treated lignocelluloses must still possess the desirable properties of untreated lignocellulosics. Therefore, the fiber strength should not be reduced, TABLE 8. fibers.
Comparison of the price of some synthetic and natural
Fiber Cost (USD $/kg) Modulus Cost (GPa kg/$)
Carbon Steel Glass Sisal 200 2.0
30 6.7
Jute
Coir
3.25 0.36 0.30 0.25 21.5 41.7 43.3 20.0
DOI 10.1002/pc
no change in color, good electrical insulation properties should be retained, and the hydrophobic nature of reagent should be selected accordingly.
3. Another effective method of surface chemical modification of NFs is graft copolymerization. Optimized vinyl grafted NFs, consisting of orderly arrangement of grafted segments, act as compatible reinforcing fibers with several resin systems to obtain better fiber–matrix adhesion of the resulting eco-composites [38]. 4. Isocyanate has a functional group ONACAO, which is very susceptible to reaction with the hydroxyl group of cellulose and lignin in the fibers, and forms strong covalent bonds, thereby creating better compatibility with the polymer matrix in the composites [Eq. (3)]. Kokta et al. have studied the performance of isocyanate as a coupling agent. Isocyanates provided better interaction with thermoplastics resulting in superior properties. Isocyanates could act as a promoter, or as an inhibitor of the interaction [45].
1. Alkali treatment of NFs, also called mercerization, is the usual method to produce high quality fibers. By removing the natural and artificial impurities, alkali treatment leads to fibrillation of the fiber bundle into smaller fibers. In other words, alkali treatment reduces the fiber diameter and thereby increases the aspect ratio [38, 42]. Therefore, the development of a rough surface topography and enhancement in aspect ratio offer better fiber– matrix interface adhesion and an increase in mechanical properties. Alkali treatment increases surface roughness, resulting in better mechanical interlocking and the amount of cellulose exposed on the fiber surface. This increases the number of possible reaction sites and allows better fiber wetting [38]. As a result of alkali treatment, the following reaction [Eq. (1)] takes place:
Fiber™OH ⫹ NaOH 3 Fiber™O™Na⫹ ⫹ H2O
(1)
Moreover, alkali treatment influences the chemical composition of the fibers, the degree of polymerization, and molecular orientation of the cellulose crystallites, due to the removal of cementing substances such as lignin and hemicellulose. Consequently, mercerization has lasting effect on the mechanical behavior of fibers, especially on their strength and stiffness. Several studies conducted on alkali treatment reported that mercerization led to increase of the amount of amorphous cellulose at the expense of crystalline cellulose, and removal of hydrogen bonding in the network structure [37, 38]. 2. Acetylation of NFs is a well-known esterification method of introducing plasticization to cellulosic fibers. Acetylation has been extensively applied to wood cellulose to stabilize the cell wall, improving dimensional stability and environmental degradation. One of the modification techniques employed by the Okura Company in Japan was to produce esterified woods, which would be molded into plastic sheets by hot pressing [38].
(3) 5. Peroxide-Induced adhesion in cellulose fiber-reinforced thermoplastic composites has attracted the attention of various researchers because of easy processability and improvement in mechanical properties [35]. Sapieha et al. have found that addition of a small amount of benzoyl peroxide or dicumyl peroxide to cellulose–polymer (LLDPE) systems during the processing improved the composite mechanical properties [45]. The improvement of mechanical properties is attributed to the peroxide-induced grafting of polyethylene onto cellulose surfaces, following the reaction [Eq. (4)]:
(4)
RO.OCelluloseOH 3 ROOHOCellulose.
Fiber™OH ⫹ (H3C™CO)2O (2)
According to Rowell [43], the hydroxyl groups that react with the reagent are those of lignin and hemicelluloses (amorphous material), while the hydroxyl groups of cellulose (crystalline material), being closely packed with hydrogen bonds, prevent the diffusion of the reagent and thus result in very low extents of reaction. It has been shown that esterification improves the dispersion of lignocellulosic maDOI 10.1002/pc
FiberOOH⫹RONACAO 3 FiberOOOCO™NH™R
ROOOR 3 2RO.
Acetylation is based on the reaction of cell wall hydroxyl groups of lignocellulosic materials with acetic or propionic anhydride at elevated temperature (usually without a catalyst) [Eq. (2)]:
3 FiberOO™CO™CH3 ⫹ CH3COOH
terials in a polymer matrix, as well as the dimensional stability and interface of the final composites [38, 40, 43].
6. Several authors have investigated the effect of silane coupling agent on the interface performance of NF reinforced composites. The fiber-surface silanization resulted in better interfacial load transfer efficiency, because of the increased adhesion. Hydrogen and covalent bonding mechanisms could be found in the NF-silane system [Eq. (5)]. It was assumed that the hydrocarbon chains provided by the silane application influenced the wettability of the fibers, thus improving the chemical affinity to the polymer matrix. Silane treatment also enhanced the tensile strength of the composite [45], minimized the effect of moisture on composite properties, increased the adhesion, and thereby the composite strength.
OH Fiber™OH ⫹ R™Si(OH)3 3 Fiber™O™ Si ™R OH POLYMER COMPOSITES—2007
(5)
103
7. Acrylation treatment, maleated polypropylene/maleic anhydride treatment, and titanate treatment of cellulosic fibers have also been reported [46]. The acrylation treatment resulted in high strain values of the composites. The composites’ ability to withstand the applied flexural stress is manifested by higher strain values, which indicate the elastic nature of the material. Maleic anhydride grafted polypropylene (MAPP) has been widely used as a coupling agent or as a compatibilizer in NF-reinforced polypropylene composites. The treatment of NFs with MAPP copolymer provides covalent bonds across the interface. Through such a treatment, the surface energy of the fibers is increased, thereby providing better wettability and high interfacial adhesion. Many other compounds (such as chromium complexes and titanates) can be used as coupling agents. The processing of composites with titanate coupling agents found that the deposition of a monolayer of organ functional titanate eliminated the water of hydration, thus enhancing the dispersion and compatibility at the interface.
Concerning the second drawback, moisture absorption, NFs are hygroscopic in nature, and they absorb or release moisture depending on environmental conditions. Major limitations of using NFs in durable composite applications are their high moisture absorption and poor dimensional stability (swelling). Fiber swelling can lead to micro cracking of the composites and degradation of their mechanical properties. This problem can be overcome by treating these fibers with suitable chemicals to decrease the hydroxyl group in the fibers. According to Stamboulis et al. (2000), moisture absorption and swelling of treated flax fiber composites is ⬃30% lower than that of composites with untreated flax fibers [47]. Strong intermolecular fiber/matrix bonding decreases the rate of moisture absorption in ecocomposites. It is difficult to eliminate totally the moisture absorption without using expensive surface barriers on the composite surface.
ECO-COMPOSITES In this respect, NF-reinforced composites (so-called ecocomposites) are subject of many scientific and research projects as well as many commercial programs [48, 49]. The growing global environmental and social concern, high rate of depletion of petroleum resources, and new environmental regulations have forced the search for new composites and green materials that are compatible with the environment, since the preexisting technology and machinery for blending, forming, and processing of these composites offer easy and cost-effective processes, so the market for eco-composites seems to be promising and realizable for double-digit growth in the near future [49 –51]. Generally, production procedures for NF composites (with certain adjustments) are quite similar to those for the production of fiber glass composites. During the processing, temperature must not exceed 200°C, and the retention time of the material exposed to high temperatures should not be 104
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too long, to avoid destruction of the fibers. Very common technologies for NF composite materials are resin transfer molding, vacuum injection molding, structural reacting injection molding, injection molding, and compression molding. NFs are used in the form of nonwoven textile, fabrics, mat (NMT-mat of thermoplastic matrices and NFs), SMC, and BMC for compression molding, and hybrid mats (NF/ PP, for example) and sometimes as UD-reinforcing fibers [52]. Daimler Chrysler has finished the so-called EXPRESS process for thermoplastic matrix/NFs composites that actually represents compression under pressure of NFs mat. Thermoplastic matrices (in melt) are introduced into the mold through the extruder, and mat layers are alternately situated in the mold after each loading of polymer melt together with the additives [53]. It is very important to note that the cycle time of one NMT is comparable with the cycle time for production of thermoplastics reinforced with long fibers. (LFT-D/NF) [54]. LFT-D process is developed for direct introduction of glass fibers in polymer melt of PP. LFT-D/NF represents process of reinforcement of thermoplastic polymers with direct introducing of long NFs, resulting in long reinforced compound that could be processed and pressed in one stadium. During the processing of NFs, various composites production procedures result in materials with various properties, because of the different degrees of fiber damages and distribution. The influence of fiber length on the mechanical properties could be explained by the fact that long fibers tend to bend, entangle, and scratch during the processing and pressing procedures. This causes a decrease of the effective fiber lengths below the critical length (lc) in certain direction, thus resulting in deterioration of the mechanical properties [55]. The German institute of agricultural engineering from Potsdam has presented a simple technology, suitable for processing unretted hemp, flax, coir, and other bast fiber plants into composites for heat and sound insulation panels, furniture boards, and mats for the automotive industry. A capacity of up to 3 t/h of raw materials input (with fiber yields efficiency of up to 29%) ensures economical and profitable fiber production and marketing [50]. Tipco Industries from India launched a commercial production of TIPWOOD®50EX eco-friendly building material, applicable for door panels, trims, office cabinet, marine flooring, furniture, mono-block chairs, and various artificial wood product [51]. The US company Phenix ™ Biocomposites LLC uses wheat straw in its Biofiber ™ composites, and sunflower hulls in its Dakota Burl composites, both intended for furniture applications [48]. Significant research work on eco composites has been done at the German Aerospace Center (DLR). Recently, Riedel and Nickel have reported on the state-of-the art and future perspectives of eco composites, mainly based on UD DOI 10.1002/pc
laminates of hamp and flax fibers in thermoplastics (PHBBiopol, PCL-Capa, Starch based-SCONACELL A) [16]. Takagi and coworkers have reported composites based on modified starch resin, hemp, and bamboo fibers [56]. Unidirectional composites of hemp fiber and starch-based resin have tensile strength in the range of 200 MPa, making them useful for structural applications. NEC Corp. (NEC, Tokyo, Japan) and UNITIKA (Tokyo, Japan) announced March 2006 the joint development of kenaf fiber-reinforced PLA [57]. More recent research, carried out by both companies, involved further improvement of the characteristics of the kenaf fiber/PLA composites, to allow its application in cell phones. Its moisture resistance and the fall impact durability were improved by using the latter’s commercialized PLA “TERRAMAC”. Another group of research-attractive eco-composites are plant fiber based soy protein composites. Lodha and Netravali have prepared composites of chopped ramie fibers and soy protein isolate, with significant strength improvement [58]. Nam and Netravali announced UD ramie fiber/soy protein concentrate composites (65% of fiber content) with strength of 275 MPa, suitable for indoor applications in housing and transportation [48]. Chaba and Netravali have worked on fabrication and characterization of green UD composites, based on flax yarn and crosslinked soy flour resin. The composites specimens exhibited fracture stress and Young’s modulus of 259.5 MPa and 3.71 GPa, respectively, and flexura strength of 174 MPa in longitudinal direction [48]. These properties seem to be sufficient for considering these green composites for indoor structural applications. Phoenix bio-composites commercialized fully degradable Environ ® composite, based on soy flour with recycled paper. These eco-composite materials could be used for furniture and architectural nonstructural applications [58]. Chiellini’s research group from the University of Pisa reported their research on injection molded eco-composites based on corn and orange fibers and poly(vinyl alcohol) [59]. These composites showed small changes in their mechanical properties after conditioning at variable relative humidity, and even after complete soaking in water. Composites, tested after storage for 1 year at 50% relative humidity and 23°C, exhibited mechanical properties similar to those of freshly prepared materials. Significant research efforts have been spent on ecocomposites based on recyclable polymer with NFs. Currently, the widely favored PP is used for great number of recyclable eco-composites. Visteon and Technilin developed their own flax/PP material R-Flax®, based on a lowcost fiber. Taking into account very high specifications from Opel, which include critical safety requirements, R-Flax ® can be used for interior items (door panels), where its aesthetic qualities can even add to its consumer appeal. Tech-Wood International from the Netherlands announced its Tech-Wood ® eco-composite, aimed for construction elements [48]. Tech-Wood ® eco-composite material contains 70% pine-wood fibers and 30% compatibilized PP. The textile Institute from Aachen, Germany, reported use of DOI 10.1002/pc
recycled fibers (including carpet trimmings) in the automobile industry [60]. Headliners based on flax-PP were shredded, granulated, and processed. Old carpet material was sorted into two layers using IR spectroscopy and image analysis, and PP secondary fibers were extracted from the shredded material with a recovery rate of about 50%. These secondary fibers were then used with flax fibers to obtain a hybrid flax-PP yarn by friction spinning. The final products were obtained by thermoforming. Composite mechanical properties were satisfactory, as were product odor assessments. The headliner is the costliest part of a vehicle interior, and it is perhaps questionable whether or not the process is economical [60]. NFC Wageningen UR from Germany has announced a patent and commercial production of NF reinforced PP composite granules, GreenGran NF30, 50, 70, by extrusion technology [61]. Because of its high NF content, GreenGran NF30, 50, 70 granules have better insulating properties (heat, sound), better flame retardancy properties, improved dimension stability at higher temperatures, and no sharp edges after a car crash (safety requirements for automotive interior parts). Yang et al. have studied the possibility of using lignocellulosic rice-husk flour (RHF) as the reinforcing filler in polyolefine composites [62]. They have designed RHF/ Polypropylene composites with four levels of filler loading (10, 20, 30, and 40 wt%). The results of tensile test performed at six levels of temperatures and various crosshead speed have shown that tensile strength of the composites slightly decreased as the filler load increased. Applying the method used in the wood-based panel industry, composite insulation boards were produced with rice straw [63]. Composite boards with specific gravity of 0.8 have slightly better bending modulus than wood particle board (as a control board) at a rice straw content of 10 wt%, and show no differences from the control boards at a 20 wt% rice straw level. At the last Eco-comp 2005 conference, prof. Nishino from Kobe University, Japan, reported on “all-cellulose” composites, prepared by partial dissolving of cellulose fiber surfaces [64]. By optimizing the immersing conditions of cellulose fibers into solvent, the fibers (with partially dissolved surfaces) have been unified by compression, followed by drying. These all-cellulose composites showed excellent mechanical and thermal properties [64]. In the framework of ECO-PCCM project, supported by the EU FP6-INCO program, eco-composites based on PLA, PHBV, and PP reinforced with different NFs are aimed for construction panels and different elements for eco-houses [65], and some of the results were presented at Eco-composites 2005 [66]. CONCLUSION The future opportunities for further research on ecocomposites involve two basic routes. One is being conducted to develop new pathways to synthesize inexpensive biodegradable polymers with improved mechanical and POLYMER COMPOSITES—2007
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thermal properties. The second field of research is on a “cost effective” modification of NFs, since the main market attraction of eco-composites is the competitive cost of NFs. The reports, related to the comparison of LCA assessment of NF composites with glass fiber reinforced composites, have found that NF composites are superior in the specific automotive industry applications, first of all due to the lower weight of NF composites. Further evaluation of the eco performance of NF composites and LCA studies should provide further confirmation of their “green” character [67].
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