Pii: s0266-3538(00)00101-9

Composites Science and Technology 60 (2000) 2037±2055 Sisal ®bre and its composites: a review of recent developments Yan Li, Yiu-Wing Mai *, Lin Ye Centre for Advanced Materials Technology (CAMT), Department of Mechanical & Mechatronic Engineering J07, The University of Sydney, Sydney, NSW 2006, Australia Received 4 December 1998; received in revised form 4 April 2000; accepted 19 April 2000 Sisal ®bre is a promising reinforcement for use in composites on account of its low cost, low density, high speci®c strength and modulus, no health risk, easy availability in some countries and renewability. In recent years, there has been an increasing interest in ®nding new applications for sisal-®bre-reinforced composites that are traditionally used for making ropes, mats, carpets, fancy articles and others. This review presents a summary of recent developments of sisal ®bre and its composites. The properties of sisal ®bre itself, interface between sisal ®bre and matrix, properties of sisal-®bre-reinforced composites and their hybrid composites have been reviewed. Suggestions for future work are also given. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: B. Electrical property; B. Interface; B. Mechanical property; B. Surface treatment; Ageing; Degradation; Natural±®bre composite; Sisal mostly extracted from the periphery of the leaf. They have a roughly thickened-horseshoe shape and seldom Sisal ®bre is one of the most widely used natural ®bres divide during the extraction processes. They are the most and is very easily cultivated. It has short renewal times commercially useful of the sisal ®bre. Ribbon ®bres occur and grows wild in the hedges of ®elds and railway tracks in association with the conducting tissues in the median [1]. Nearly 4.5 million tons of sisal ®bre are produced line of the leaf. Fig. 1 shows a cross-section of a sisal leaf every year throughout the world. Tanzania and Brazil and indicates where mechanical and ribbon ®bres are are the two main producing countries [2].
obtained [3]. The related conducting tissue structure of Sisal ®bre is a hard ®bre extracted from the leaves of the ribbon ®bre gives them considerable mechanical the sisal plant (Agave sisalana). Though native to tropi- strength. They are the longest ®bres and compared with cal and sub-tropical North and South America, sisal mechanical ®bres they can be easily split longitudinally plant is now widely grown in tropical countries of during processing. Xylem ®bres have an irregular shape Africa, the West Indies and the Far East [3]. A sketch of and occur opposite the ribbon ®bres through the con- a sisal plant is shown in Fig. 1 and sisal ®bres are nection of vascular bundles as shown in Fig. 2. They are extracted from the leaves.
composed of thin-walled cells and are therefore easily A sisal plant produces about 200±250 leaves and each broken up and lost during the extraction process.
leaf contains 1000±1200 ®bre bundles which is com- The processing methods for extracting sisal ®bres posed of 4% ®bre, 0.75% cuticle, 8% dry matter and have been described by Chand et al. [2] and Mukherjee 87.25% water [1]. So normally a leaf weighing about and Stayanarayana [1]. The methods include (1) retting 600 g will yield about 3% by weight of ®bre with each followed by scraping and (2) mechanical means using leaf containing about 1000 ®bres.
decorticators. It is shown that the mechanical process The sisal leaf contains three types of ®bres [3]: yields about 2±4% ®bre (15 kg per 8 h) with good mechanical, ribbon and xylem. The mechanical ®bres are quality having a lustrous colour while the retting pro- cess yields a large quantity of poor quality ®bres. After extraction, the ®bres are washed thoroughly in plenty of * Corresponding author. Tel.: +61-2-9351-2290; fax: +61-2-9351- clean water to remove the surplus wastes such as chlor- E-mail address: [email protected] (Yiu-Wing Mai).
ophyll, leaf juices and adhesive solids.
0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 Fig. 2. Cross-section of a ribbon-®bre bundle.
reinforced with spirally oriented cellulose in a hemi-cel- lulose and lignin matrix. So, the cell wall is a composite structure of lignocellulosic material reinforced by helical micro®brillar bands of cellulose. The composition of the external surface of the cell wall is a layer of lignaceous material and waxy substances which bond the cell to its adjacent neighbours. Hence, this surface will not form a strong bond with a polymer matrix. Also, cellulose is a hydrophilic glucan polymer consisting of a linear chain of 1, 4-b-bonded anhydroglucose units [9] and this large amount of hydroxyl groups will give sisal ®bre hydro- philic properties. This will lead to a very poor interface between sisal ®bre and the hydrophobic matrix and very poor moisture absorption resistance.
Though sisal ®bre is one of the most widely used nat- ural ®bres, a large quantity of this economic and renewable resource is still under-utilised. At present, sisal ®bre is mainly used as ropes for the marine and agriculture industry [1]. Other applications of sisal ®bres Fig. 1. (a) A sketch of sisal plant and the cross-section of a sisal leaf [3]; (b) photograph of a sisal plant.
include twines, cords, upholstery, padding and mat making, ®shing nets, fancy articles such as purses, wall The chemical compositions of sisal ®bres have been hangings, table mats, etc. [10]. A new potential applica- reported by several groups of researchers [4±7]. For tion is for manufacture of corrugated roo®ng panels example, Wilson [4] indicated that sisal ®bre contains that are strong and cheap with good ®re resistance [11].
78% cellulose, 8% lignin, 10% hemi-celluloses, 2% During the past decade (1987±1998), the identi®cation waxes and about 1% ash by weight; but Rowell [5] of new application areas for this economical material found that sisal contains 43±56% cellulose, 7±9% lignin, has become an urgent task. The use of sisal ®bre as a 21±24% pentosan and 0.6±1.1% ash. More recently, reinforcement in composites has raised great interest Joseph et al. [6] reported that sisal contains 85±88% and expectations amongst materials scientists and engi- cellulose. These large variations in chemical composi- neers. The major studies on sisal ®bres carried out dur- tions of sisal ®bre are a result of its di€erent source, age, ing this 10-year period can be broadly divided into the measurement methods, etc. Indeed, Chand and Hashmi following topics: [7] showed that the cellulose and lignin contents of sisal vary from 49.62±60.95 and 3.75±4.40%, respectively, . Properties of sisal ®bres: Mechanical, thermal and depending on the age of the plant.
dielectric properties of sisal ®bre have been studied The length of sisal ®bre is between 1.0 and 1.5 m and in detail. X-ray di€raction, IR, TG, SEM, DSC, the diameter is about 100±300 mm [8]. The ®bre is actu- DMA, etc., have been used to determine the char- ally a bundle of hollow sub-®bres. Their cell walls are acteristics of sisal ®bre and provide theoretical Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 support for processing and application of this ulus per unit cost), it (41.67 GPakg/$) is almost the best next to jute (43.33 GPakg/$) amongst all the synthetic . Interface properties between sisal ®bre and matrix: and cellulosic ®bres.
The main purpose here is to modify the ®bre- surface structure by using chemical and thermal treatment methods in order to enhance the bond strength between ®bre and matrix and reduce Generally, the strength and sti€ness of plant ®bres water absorption of sisal ®bre.
depend on the cellulose content and the spiral angle . Properties of sisal-®bre-reinforced composites: The which the bands of micro®brils in the inner secondary matrix used in sisal-®bre-reinforced composites cell wall make with the ®bre axis. That is, the structure include thermoplastics (polyethylene, polypropylene, and properties of natural ®bres depend on their source, polystyrene, PVC, etc.), thermosets (epoxy, polye- age, etc. [12]. The tensile properties of sisal ®bre are not ster, etc.), rubber (natural rubber, styrene±buta- uniform along its length [3]. The root or lower part has diene rubber, etc.), gypsum and cement. The low tensile strength and modulus but high fracture e€ects of processing methods, ®bre length, ®bre strain. The ®bre becomes stronger and sti€er at mid- orientation, ®bre-volume fraction and ®bre-sur- span and the tip has moderate properties.
face treatment on the mechanical and physical Table 2 shows the properties of sisal ®bres as reported properties of sisal-®bre-reinforced composites have by di€erent researchers. Note that except for the struc- been studied. Also, several theoretical models are ture and properties of the natural ®bre itself, experi- given to predict the properties of the composites.
mental conditions such as ®bre length, test speed, etc., . Sisal/glass-®bre-reinforced hybrid composites: To all have some e€ects on the properties of natural ®bres take advantage of both sisal and glass ®bres, they have been added conjointly to the matrix so that Mukherjee and Satyanarayana [1] studied the e€ects an optimal, superior but economical composite of ®bre diameter, test length and test speed on the ten- can be obtained. The hybrid e€ect of sisal/glass sile strength, initial modulus and percent elongation at ®bres on the mechanical properties have been stu- the break of sisal ®bres. They concluded that no sig- died and explained.
ni®cant variation of mechanical properties with change in ®bre diameter was observed. However, the tensile Papers published between 1987 and 1998 related to strength and percent elongation at the break decrease sisal ®bres are listed in Table 1. It can be seen that while Young's modulus increases with ®bre length. With research interest has changed from the ®bre itself to increasing speed of testing, Young's modulus and tensile sisal-®bre-reinforced composites and hybrid composites.
strength both increase but elongation does not show The study of interface between sisal ®bre and matrix, any signi®cant variation. However, at a test speed of however, remains an important topic.
500 mm/min, the tensile strength decreases sharply.
These results have been explained in terms of the inter- nal structure of the ®bre, such as cell structure, micro- 2. Properties of sisal ®bre ®brillar angle (20±25), defects, etc. In rapid mechanical testing, the ®bre behaves like an elastic body, i.e. the crystalline region shares the major applied load result- ing in high values of both modulus and tensile strength.
Compared to synthetic ®bres, the price of sisal ®bre When the testing speed decreases, the applied load will (0.36 US$/kg) is very low [3]. It is about one-ninth of be borne increasingly by the amorphous region. How- that of glass ®bre (3.25 US$/kg) and one-®ve hundredth ever, at very slow test speeds, the ®bre behaves like a of carbon ®bre (500 US$/kg). For speci®c price (mod- viscous liquid. The amorphous regions take up a major Number of papers published in the period (1987±1998) related to sisal ®bresa Properties of sisal ®bres 6 [15±17,20±22] Interface between sisal ®bre and matrix Properties of sisal-ibre composites 13 [3,11,34±37,54, 57±59,61,62,64] 11 [26,32,33,42±44, 51±53,55,56] Sisal/glass hybrid composites a Data taken from Compendex (Computerised Engineering Index).
Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 Properties of sisal ®bres reported by di€erent researchers Density (kg/m3) Moisture content (%) Tensile strength (MPa) Tensile Modulus (GPa) Maximum strain (%) Diameter (mm) Reference portion of the applied load giving a low ®bre modulus shows the results of sisal ®bres of di€erent age at dif- and a low tensile strength. But at very high strain rates ferent temperature.
(500 mm/min), the sudden fall in tensile strength may For electrical applications, the dielectric properties of be a result of the presence of imperfections in the ®bre sisal ®bre at di€erent temperature and frequency have causing immediate failure.
also been studied [16]. Increase of frequency decreases In Ref. [1], the micro®brillar angle and number of the dielectric constant "0 value, while increase of tem- strengthening cells in the sisal ®bres did not show any perature increases "0 at all frequencies. Increasing the appreciable variation in ®bre diameter. Hence, no plant age shifts the dissipation factor ( tan ) peak to appreciable change in values of Young's modulus and higher temperature. These phenomena were explained tensile strength were observed. As the test length on the basis of structural changes. Water absorbed by increases, the number of weak links or imperfections sisal ®bres has OHÿ anions which act as dipoles. Other also increases, thus resulting in reduction in tensile than OHÿ anions, there are several impurities and ions strength. However, with increasing ®bre length, sisal on the ®bre. These dipoles and ions contribute to the "0 o€ers a higher resistance to applied stress as a result of and tan  behaviours of sisal ®bres. At low frequencies, the involvement of more oriented cellulosic ®bres. This high "0 and tan  values in sisal ®bre are caused by the probably also accounts for the higher modulus of the dipolar contribution of absorbed water molecules. "0 ®bres at longer test lengths. The reason for such beha- values at intermediate frequencies are the result of con- viour by the characteristics of natural ®bres such as tributions from space charge polarisation. At high fre- multi-cellular structures, visco-elastic nature and non- quencies, the contribution of polarisation of absorbed uniform structural inhomogeneity.
water molecules and space charge decreases and elec- Chand et al. [12] reported the e€ects of testing speed tronic and atomic polarisation becomes operative.
and gauge length on the mechanical properties of other Increase in temperature a€ects the mobility of ions and kinds of natural ®bres (sun-hemp ®bres). Their results consequently changes the ionic contributions. The tem- support the ®nding of Ref. [1], though the magnitudes perature dependence of the dielectric behaviour of sisal are much lower than those of sisal ®bres. (For example, ®bre is shown in Fig. 3.
when the gauge length is 50 mm and testing speed is 50 Yang et al. [17] used IR, X-ray di€raction and TG to mm/min, the tensile strength of sisal ®bre is 759 MPa.
study the e€ect of thermal treatment on the chemical However, for sun-hemp ®bre, the tensile strength is only structure and crystallinity of sisal ®bres. They concluded that the IR spectrum did not change below 200C The mechanical properties of sisal ®bres obtained treatment while density and crystallinity increased. This from di€erent age at three di€erent temperature were means that the chemical structure of sisal ®bres will not investigated by Chand and Hashmi [15]. The tensile change below 200C while the degree of crystallinity can strength, modulus and toughness (de®ned as energy be increased and hence the density. There was a slight absorption per unit volume) values of sisal ®bre weight loss (2%) below 200C probably caused by the decrease with increasing temperature. The relative e€ect evaporation of water absorbed by sisal ®bres, sub- of plant age on these mechanical properties is less pro- stances of low boiling point and others that can be minent at 100C than at 30C. This is attributed to the decomposed below this temperature. However, the large more intense removal of water and/or other volatiles (at amounts of cellulose, semi-cellulose and glucans were 100C) originally present in the ®bres, which otherwise not lost. Also, they found that thermal decomposition act as plasticising agents in the chains of the cellulose of sisal ®bre could be divided into three stages. The macromolecules. It is, however, noted that at 80C both thermal behaviours are essentially identical for heat tensile strength and modulus decrease with age of the treatment between 150 to 200C. Hence, thermal treat- plant. This trend is di€erent to testing at 100C. Table 3 ment of sisal ®bre can be carried out below 200C.
Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 Comparisons of mechanical properties of sisal ®bres with di€erent age at di€erent temperature [15] Toughness per unit volume (MJ/m3) Tensile strength (MPa) Tensile modulus (GPa) Results of TG of treated and untreated sisal ®bresa,b t1 (C) w1 (%) t2(C) w2 (%) t3 (C) w3 (%) 150C/4 h treated 200C/0.5 h treated 61 a Three-stage thermal decomposition [17].
b ti ith stage decomposition temperature; wi ith stage percentage weight remnant.
able levels by better wetting and chemical bonding between ®bre and matrix.
3.1. Treatment of sisal ®bre surface Most previous studies were focused on ®bre-surface treatment methods and the resultant e€ects on the phy- sical and mechanical properties of di€erent ®bre-matrix composite systems. But what will happen to the ®bres after being treated? Several investigators [20±22] have studied the surface morphology, mechanical and degra- dation properties of the treated ®bres.
Fig. 3. Variation of (a) dielectric constant "0 and (b) dissipation factor Yang et al. [20] studied the relationship of surface tan  with temperature in 2-year-old plant ®bres [17].
modi®cation and tensile properties of sisal ®bres. Their modi®cation methods include: alkali treatment, H2SO4 treatment, conjoint H2SO4 and alkali treatment, benzol/ Table 4 shows the TG results of untreated and heat- alcohol dewax treatment, acetylated treatment, thermal treated sisal ®bres.
treatment, alkali-thermal treatment and thermal-alkali treatment. The results are summarised in Table 5.
Thermal treatment (at 150C for 4 h) seems to be the 3. Interface modi®cations most desirable method in terms of strength and mod- ulus properties because of the increased crystallinity Interfaces play an important role in the physical and (from 62.4% for untreated to 66.2% for 150C/4 h mechanical properties of composites [18,19]. The treated) of sisal ®bres. When the temperature reaches hydroxyl groups which occur throughout the structure 200C, the tensile properties will drop greatly as a result of natural ®bres make them hydrophilic, but many of the degradation of ®bres. Other treatments increase polymer matrices are hydrophobic so that sisal-polymer the ductillity of sisal ®bres substantially but decrease the composites have poor interfaces. Also, the hydrophilic sisal ®bres will absorb a large amount of water in the To improve the moisture-resistance, Chand et al.
composite leading to failure by delamination. Adequate acetylated sisal ®bre and studied its tensile strength [23].
adhesion across the interface can be achieved at desir- It was shown that acetylation could reduce the moisture Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 . Coupling agent 3: E€ect of treatment methods on tensile properties of sisal ®bres [20] Treatment methods Tensile strength Tensile modulus Elongation Acetic acid+alkali . Coupling agent 4: content from 11 to 5.45%. However, the tensile strength of acetylated sisal ®bre was reduced from 445 to 320 MPa caused by the loss of the hemi-cellulose in the ®bre during acetylation.
Surface modi®cation of sisal ®bre using coupling agents was also studied by Singh et al. [24]. Sisal ®bres were cut into short lengths (6 cm) and washed with dis- tilled water to remove water solubles from the ®bre surface completely. After being air-dried they were The hydroxyl groups attached to the glucose units of heated to 80C for 8 h to remove excess surface moist- the cellulose will react with these coupling agents in the ure. Then these ®bres were treated by dipping into the presence of moisture by substituting the right part of the coupling-agent solution and stirring slowly for 0.5 h.
dashed lines shown on the chemical structures of the After that, the treated ®bres were washed with solvent coupling agents.
to remove compounds not covalently bonded to the The e€ects of these coupling agents on the moisture ®bres and then kept at 80C in an oven for 4 h to con- content in sisal ®bre are obtained and discussed. It is stant weight.
clear that moisture absorption of surface-treated ®bres The four coupling agents used were: N-substituted has been reduced signi®cantly by providing hydro- methacrylamide (coupling agent 1); gamma-methacry- phobicity to the surface via long-chain hydrocarbon loxypropyl trimethoxy silane (coupling agent 2); neo- attachment. In addition, these coupling agents penetrate the cell wall through surface pores and deposit in the (coupling agent 3); and neopentyl(diallyl)oxy, triacryl inter®brillar regions and on the surface, restricting fur- zironate (coupling agent 4). Their chemical structures ther ingress of moisture.
and possible interactions with sisal are as follows: 3.2. Treatment of ®bre/matrix interfaces . Coupling agent 1: To make good use of sisal-®bre reinforcement in composites, ®bre-surface treatment must be carried out to obtain an enhanced interface between the hydrophilic sisal ®bre and the hydrophobic polymer matrices.
Modi®cations of interfaces between sisal ®bre and polyester, epoxy, polypropylene, etc., have been studied.
Both mechanical and moisture absorption resistance properties can be improved. These results are described in the following sub-sections.
. Coupling agent 2: 3.2.1. Sisal/polyester composites The properties of sisal-®bre-reinforced polyester composites can be improved when sisal ®bres were sui- tably modi®ed with surface treatment [24]. The mod- i®cation methods have been discussed in Section 3.1.
In the work by Singh et al. [24], it was explained that the modi®ed interphase' is much less sti€ than the resin Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 matrix and provides a deformation mechanism to reduce 3.2.2. Sisal/epoxy composites interfacial stress concentration [25]. Further, it may also For the interface between sisal ®bres and epoxy, prevent ®bre/®bre contacts, hence removing the sources Bisanda and Ansell [8] adopted silane treatment meth- of high stress concentrations in the ®nal composites.
ods to improve the adhesion and moisture resistance The e€ect of ®bre-surface treatment on the mechan- properties. The ®bres were ®rst dewaxed in 2000 ml ical properties under wet environment has also been solution of benzene and alcohol (methylated spirit), ratio studied in [24]. The results in Fig. 4 clearly show that by 1:1, by soaking batches of about 200 g of sisal ®bres, improving interfacial adhesion the moisture-induced 300 mm long, in a sealed glass vessel. The ®bres were degradation of composites can be reduced. Treated ®bre soaked for 24 h, rinsed in alcohol and distilled water.
composites absorb moisture at a slower rate than the These ®bres were then mercerized by soaking in a 0.5 N untreated counterparts, probably because of the forma- solution of sodium hydroxide, for about 72 h, rinsed in tion of a relatively more hydrophobic matrix interface distilled water and dried.
region by co-reacting organo-functionality of the cou- To study the moisture resistance of composites, the pling agents with the resin matrix. Though signi®cant ®bres were treated using silane that had been diluted to reductions in tensile strength (30±44%) and ¯exural 5% in methylated spirits. A 0.1 M solution of ceric strength (50±70%) were observed for both untreated ammonium nitrate (CAN) was used as catalyst. The and surface-treated sisal composites (Table 6 and Fig.
®bres were treated in batches by soaking about 150 g of 4), the strength retention of surface treated composites the mercerized ®bres in 1500 ml of the silane/CAN is higher than that of composites containing untreated solution (2:1), for about 24 h at room temperature. The sisal ®bres. Fig. 4 also shows that N-substituted metha- ®bres were then rinsed in distilled water and dried.
crylamide treated sisal-®bre-reinforced polyester compo- The reaction mechanisms are as follows. First, silane sites generally exhibit better mechanical properties under reacts with water to form a silanol and an alcohol: dry and wet conditions. It should, however, be noted in Table 6 that with sisal ®bres treated by N-substituted methacrylamide and silane have relatively lower void NH2…CH2†3Si…OC2H5†3 ‡ 3H2O contents compared to other ®bre-treated composites.
ˆ …HO†3Si…CH2†3NH2 ‡ 3…C2H5OH† Then, in the presence of moisture, the silanol reacts with the hydroxyl groups attached to the glucose units (G) of the cellulose molecules in the cell wall, thereby bonding itself to the cell wall with further rejection of NH2…CH2†3Si…OH†3 ‡ H2O ‡ GOH ˆ NH2…CH2†3Si…OH†2OG ‡ 2H2O It is found that the treatment of sisal ®bres in silane, Fig. 4. E€ect of wet environment on tensile strength of sisal/polyester preceded by mercerization, provides improved wettabil- composites (®bre content 50 vol.%) for 35 days: 1, untreated; 2, N- substituted methyacrylamide treated; 3, silane treated; 4, titanate lity, mechanical properties and water resistance of sisal- treated; 5, zirconate treated. RH, relative humidity [24].
epoxy composites. The results are shown in Table 7.
E€ect of surface treatments of sisal ®bres on the properties of sisal/polyester compositesa [24] Zirconate treated methacrylamide treated Tensile strength (MPa) Tensile modulus (GPa) Energy to break (MJ/m2) 105 Flexural strength (MPa) Flexural modulus (GPa) a Fibre content  50 vol%.
Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 Mechanical and physical properties of sisal/epoxy compositesa [8] Compressive strength Flexural strength Water absorbed (%) a Fibre volume fraction: 0.4.
Mercerization has greatly improved resin pick-up, or initially alkali treated and then grafted with vinyl acid in wettability, of the ®bres because of the increase in den- an acid medium under N2 atmosphere with a grafting sity of the composite. It is believed that, by providing rate of 5.7%. The mechanical behaviour, water-resis- additional sites of mechanical interlocking, this treat- tance and ¯exible fracture morphology of short sisal/ ment leads to improvement of interfacial bonding, phenol formaldehyde composites have all been mea- hence promoting resin/®bre interpenetration at the sured. The results show that cohesiveness between rigid interface. The high hydrophobic resin pick-up could sisal ®bre and brittle phenol formaldehyde can be also account for the reduction in water absorption and improved when sisal ®bre is treated with silane so that hence improved mechanical properties under wet con- the mechanical properties of the composites can be increased. Water-resistance of the composites can be Though treatment of sisal ®bres in silane preceded by improved after being treated by grafting and silane mercerization produces very little change in the mechanical properties of dry composites, mechanical performance of wet composites, and hence water resis- 3.2.4. Sisal/polyethylene composites tance, can be improved. The treatment in 100% silane Owing to the increasing use of thermoplastics, sisal produces ®bres that are almost hydrophobic. This may ®bre reinforced-thermoplastics have become increas- be a result of improved interfacial bonding arising from ingly important. Joseph et al. [6] reported the e€ect of the use of the silane. Water molecules at the interface chemical treatment on the tensile properties of short- tend to replace the resin-®bre covalent bond by weaker sisal-®bre reinforced-polyethylene (PE) composites (both hydrogen bonds, hence silane plays an important role in randomly and unidirectionally oriented) and analysed reducing water absorption in cellulosic-®bre-reinforced the mechanisms of di€erent treatment methods.
The treatment methods and their mechanisms are: Sisal ®bres have a central hollow region, the lumen, which gives access to water penetration by capillarity, a. Alkali treatment: Alkali treatment can remove especially when composites have high ®bre content. So, natural and arti®cial impurities and produce a although silane treatment can create a hydrophobic rough surface topography. In addition, alkali ®bre surface, it is not possible to prevent water from treatment leads to ®bre ®brillation, i.e. breaking entering the composite by capillary action, as long as down the ®bre bundle into smaller ®bres. This the ®bre ends are exposed. It is recommended that, for increases the e€ective surface area available for practical purposes, it may be necessary to seal o€ the wetting by the matrix resin. Hence, increasing the external surfaces by water-repellents so as to keep water ®bre aspect ratio caused by reduced ®bre diameter uptake in the composite to a minimum.
and producing a rough surface topography o€er better ®bre/matrix interface adhesion and increase 3.2.3. Sisal/phenol formaldehyde composites in mechanical properties.
Yang et al. [26] using NaOH, silane (3-aminopropyl- b. Isocyanate treatment: The hydrophilic nature of triethoxy silane), chemical grafting and thermal treat- sisal ®bres can be reduced by treating the ®bres ment methods studied the e€ects on sisal/phenol surface with urethane derivative of cardanol formaldehyde composites. Alkali and thermal treatment (CTDIC) because of the linkage of the long chain methods have already been discussed in Section 3.1 [20].
structure of CTDIC to the cellulosic ®bres. This For silane treatment, the ®bre was ®rstly treated by makes sisal ®bres compatible with the PE matrix, alkali and then immersed in the silane alcohol solution.
thus resulting in a strong interfacial bond between After that, it was air-dried and followed by heating at these two constituents with improved mechanical 100C. While for chemical grafting the sisal ®bre is Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 c. Peroxide treatment: The function of peroxide . Sisal-®bre-reinforced thermosets treatment is that it can graft PE on to the cellulose . Sisal-®bre-reinforced thermoplastics surface. The grafting reaction is a result of the . Sisal-®bre-reinforced rubbers peroxide-initiated free radical reaction between the . Sisal-®bre-reinforced cement and gypsum PE matrix and cellulose ®bres as shown below: The number of papers published since 1987 are listed in Table 8. It is shown that the most widely used matrix RO: ‡ PE-H ! ROH ‡ PE: for sisal-®bre-reinforced composites has changed from gypsum and cement to rubber and polyethylene. Ther- RO: ‡ Cellulose-H ! ROH ‡ Cellulose: mosetting matrices such as epoxy and polyester have also been used for sisal-®bre-reinforced composites.
PE: ‡ Cellulose: ! PE-Cellulose 4.1. Sisal-®bre-reinforced thermoset matrices The tensile strength of the composites increases with peroxide concentration up to a critical level depending on The most widely used thermosetting matrix reinforced the ®bre content and then remains constant. The exis- by natural ®bres is polyester [27±30]. Compression tence of a critical concentration of peroxide suggests that moulding is the most widely used and convenient the peroxide-initiated grafting reactions terminate when method to make these composites, whether the ®bre is the ®bres are covered with grafted PE and excess peroxide long or short. The tensile and impact properties of this causes some crosslinking of the PE molecules themselves.
kind of composites have been obtained by Sanadi et al.
d. Permanganate treatment: Permanganate is thought It is shown that the tensile strength and elastic mod- to induce grafting reactions between sisal ®bres ulus of the composites containing up to 40% ®bre- and PE matrix [6]. Joseph et al. have attempted to volume fraction (Vf) increase linearly with Vf in good explain the initiating graft copolymerisation but agreement with the rule of mixtures. The work of the conditions for this to occur are quite critical fracture, as determined by Izod impact test, also and cannot be ful®lled easily. (It is also dicult to increases linearly with Vf. Analysis of the energy- justify the di-valency of H in reactions given in absorption mechanisms during impact fracture shows [6].) Instead, oxidisation between permanganate that ®bre pull-out and interface fracture are the major and sisal ®bre will take place. So, the mechanism contributors to the high toughness of these composites.
for improvement of the interfacial properties by This result indicates that sisal ®bres have potential to permanganate treatment is that the permanganate produce inexpensive materials with high toughness.
roughens the ®bre surface and produces mechan- Comparison of the impact properties of di€erent nat- ical interlocks with the matrix. Hence, the inter- ural ®bre-reinforced composites, including sisal, pine- facial bonding between permanganate treated sisal apple, banana and coir shows that sisal-®bre composites ®bre and matrix is improved.
possess the highest impact toughness owing to the opti- mal micro-®brillar angle of the ®bre (21 for sisal, 12 The permanganate concentration used is a critical factor for the mechanical properties of the composite. It is observed that the tensile strength reaches a maximum Number of papers related to sisal ®bres published in the period (1987± at a permanganate concentration of 0.055% and then decreases sharply with further increase in permanganate 1987±1990 1991±1994 1995±1998 concentration. This is caused by the degradation of cel- lulosic ®bres at high permanganate concentration.
Tensile properties of these sisal/PE composites with [6, 44,51±52,67±68] di€erent treatment methods were compared. It appears that the increase in properties as a result of these treat- ments are in the order: DCP (dicumyl peroxide) 4. Properties of sisal-®bre-reinforced composites According to the types of matrices used in sisal-®bre- reinforced composites, they can be divided into the following categories: a Data taken from Compendex (Computerised Engineering Index).
Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 for banana, 14 for pineapple and 45 for coir). It has of these composites rather than sisal-®bre-reinforced been proven by Gordon and Jeronimidis [31] that the thermosets. Such properties include mechanical, envir- toughness of composites increases with the micro- onmental, electrical and dynamic.
®brillar angle of the ®bres and reaches a maximum at 15±20. It will then decrease with increasing angle. The 4.2.1. Processing methods optimal micro-®brillar angle of sisal ®bre (21) leads to The mixing methods so far used are: melt mixing and better impact resistance with a work of fracture of 98.7 solution mixing. In melt mixing, the ®bre is added to a KJ/m2 when the ®bre-volume fraction is 50%. For the melt of thermoplastics and mixing is performed using a same volume fraction of pineapple ®bres, this is 79.5 mixer at a speci®ed temperature and speed for a speci- KJ/m2; for banana and coir ®bres, they are 51.6 and ®ed time. Then the mix is taken out from the mixer 43.5 KJ/m2, respectively. When compared to synthetic while hot and is extruded using an injection moulding ®bre composites, the speci®c impact work of fracture machine as long and thick rods.
for the natural ®bre composites is not much worse. The In the solution mixing method, the ®bres are added to speci®c work of fracture (ie toughness per unit density) a viscous solution of thermoplastics in a solvent in a of 60% volume fraction sisal ®bre/polyester composites stainless-steel beaker with a stainless-steel stirrer. The is 115 KJ mÿ2/g, while for ultra-high-modulus poly- temperature is maintained for some time and the mix ethylene (UHMPE) and E-glass ®bres, these values are transferred to a ¯at tray and kept in a vacuum oven to 125 and 165 KJ mÿ2/g, respectively.
remove the solvent. The solution-mixing procedure Rong et al. studied the e€ect of ®bre pre-treatment avoids ®bre damage that normally occurs during blend- and water absorption on the impact properties of sisal ing of ®bre and thermoplastics by melt-mixing [34].
®bre reinforced polyester and epoxy matrices [32,33].
Generally, randomly oriented sisal-®bre-reinforced Three ®bre-surface treatment methods including alkali composites are prepared by standard injection moulding treatment, coupling agent treatment and heat treatment of the blends. Oriented sisal composites are processed were used. They indicated that ®bre-surface treatment by aligning the long extruded rods with compression has a strong e€ect on the impact behaviour of the com- posites and the e€ects are di€erent for di€erent matri- Polyethylene is the most widely used thermoplastics ces. It is observed that ®bre pull-out is the major matrix [34±37]. For other natural ®bre composites, contributor to the energy absorption. Increased ®bre- polypropylene is also a major matrix material [38±41].
tensile strength promoted by thermal treatment can Recently, sisal ®bre-reinforced polystyrene and PVC increase the impact performance of the composites.
have also been studied [42,43].
Water absorption in sisal ®bre composites is mainly caused by the ®bres and leads to a very poor interface 4.2.2. Properties of sisal ®bre reinforced polyethylene between the sisal ®bre and the matrix. Di€erent matrix 4.2.2.1. Mechanical properties. Joseph et al. [35] studied systems have di€erent interface characteristics. Gen- the tensile properties of short sisal ®bre/polyethylene erally, water absorption in sisal/polyester composite is composites in relation to processing methods and the two to three times that in sisal/epoxy composite and this e€ects of ®bre content, length and orientation (see leads to their di€erent impact properties. For sisal/epoxy Tables 9 and 10). As expected, the tensile properties composite, the impact strength improves with water show a gradual increase with ®bre length reaching a absorption as a result of an acceptable level of interface maximum at about 6 mm (12.5 MPa) and then decrease debonding, but for sisal/polyester composites, the impact (e.g. 10.24 MPa at 10 mm). Unidirectional short ®bres strength decreases through the complete destruction of achieved by extrusion enhance the tensile strength and the interface.
elastic modulus of the composites along the axis of ®bre From the viewpoint of interface enhancement, alignment by more than two-fold compared to ran- Bisanda and Ansell [8] studied the mechanical and phy- domly oriented ®bre composites.
sical properties of sisal/epoxy composites and Yang et Di€erent processing methods lead to di€erent extent al. [26] investigated sisal/phenol formaldehyde compo- of ®bre damage, di€erent ®bre-length distribution and sites. The properties of these two composite systems hence di€erent mechanical properties. The e€ect of ®bre have already been discussed in Sections 3.2.2 and 3.2.3.
length on the mechanical properties can be explained by the fact that long ®bres tend to bend or curl during 4.2. Sisal-®bre-reinforced thermoplastics matrices moulding. This causes a reduction in the e€ective length of the ®bre below the critical ®bre length in a particular In recent years, sisal-®bre-reinforced thermoplastics direction and hence a decrease of mechanical properties.
composites have gained much more interest among The experimental tensile properties of short sisal- materials scientists and engineers than thermosets ®bre-reinforced LDPE with di€erent ®bre-volume frac- because of their low cost and recyclable properties. Many tions have been compared with existing theories of papers have now been published to study the properties reinforcement [44]. The models selected were series and Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 Variation of tensile properties of melt mixing composite (MMC) and solution mixing composite (SMC) with ®bre content [35]a Elongation at break Elongation at break a Average ®bre length: 5.8 mm; values given in parentheses are the properties of unidirectionally aligned ®bre composites.
c ˆ x…MmVm ‡ Mf Vf † ‡ …1 ÿ x† Variation of tensile properties of solution mixing composite (SMC) with repeated extrusion of the blends [35]a Elongation at break c ˆ x…TmVm ‡ Tf Vf † ‡ …1 ÿ x† where x is a parameter which determines the stress transfer between ®bre and matrix. It is always assumed that x is determined mainly by ®bre orientation, ®bre a Average ®bre length: 5.8 mm, ®bre content: 20 wt.%.
length l and stress ampli®cation e€ect at the ®bre ends.
This model is in fact a combination of the parallel and series models.
parallel [45], Hirsch [46], Halpin-Tsai [47], modi®ed Halpin-Tsai [48], Cox [49] and modi®ed Bowyer and . Halpin-Tsai model [47] Bader [50] models. These models are given below for . Parallel and series model [45]: Mc ˆ MfVf ‡ MmVm Tc ˆ TfVf ‡ TmVm where A is a factor determined by ®bre geometry, ®bre distribution and ®bre-volume fraction.  and  account for the relative moduli and strength of ®bre and matrix, . Modi®ed Halpin-Tsai equation [48] where M, T and V are Young's moduli, tensile strength and volume fraction, respectively. The subscripts c, m and f represent composite, matrix and ®bre, respec- . Hirsch's model [46] Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 where lc is the critical ®bre length.
Experimental results and theoretical values obtained from the above models were compared. It is observed that all models except the series and parallel models The modi®ed Halpin-Tsai equation includes the max- show reasonable agreement with experimental data of imum packing fraction, max, of the reinforcement. max longitudinally oriented composites, especially at low equals 0.785 for square arrangement of ®bres, 0.907 for ®bre-volume fractions. But, the Hirsch and modi®ed hexagonal array of ®bres and 0.82 for random packing Bowyer and Bader equations show the closest agree- of ®bres.  and  are given by Eqs. (9) and (10), ment with test results. However, agreement with the respectively, and depends on the particle packing experimental data of randomly oriented composites fraction. l=d is the ®bre aspect ratio.
cannot be accurately predicted with these two models as shown in Figs. 5 and 6. All theoretical models indicate that the tensile properties of short-®bre-reinforced com- posites strongly depend on ®bre length, ®bre volume fraction, ®bre dispersion, ®bre orientation and ®bre/ c ˆ Mf Vf …1 ÿ matrix interfacial strength.
4.2.2.2. Dynamic mechanical properties. The e€ects of ®bre length, orientation, volume fraction and ®bre sur- face treatment on the dynamic mechanical properties of sisal-®bre-reinforced PE including storage modulus, loss modulus and damping characteristics have also been where r is the radius of the ®bre, Gm is the shear mod- studied [34]. It is found that addition of 10% of short ulus of matrix, Af is the area of the ®bre and R is the sisal ®bres into LDPE increases the storage moduli and centre-to-centre distance of the ®bres. For hexagonally loss moduli of the composites, levelling o€ at higher volume fraction. This is believed to be caused by the increasing ®bre-to-®bre interaction at high volume frac- R ˆ …p †1 tions resulting in poor dispersion. Both storage and loss moduli decrease with increase of temperature. The damping properties of the composites decrease with For square packed ®bres addition of ®bres and are strongly in¯uenced by ®bre orientation. The storage and loss moduli of randomly oriented composites were intermediate between those of longitudinally and transversely oriented composites.
The in¯uence of ®bre length indicates that a critical . Modi®ed Bowyer and Bader's model [50] length of 6 mm is needed to obtain maximum dynamic moduli. This suggests that a critical length exists for Bowyer and Bader's model indicates that the tensile maximum stress transfer between ®bre and matrix. The strength of short-®bre-reinforced thermoplastic compo- storage and loss moduli of the isocyanate-treated com- sites is the sum of contributions from sub-critical and posites are higher than those of the untreated composites super-critical ®bres and that from the matrix.
as a result of the improved ®bre/matrix interface adhe- sion. Some dynamic mechanical results are shown in Mc ˆ MfK1K2Vf ‡ MmVm Figs. 7 and 8.
4.2.2.3. Electrical properties. The electrical properties of Tc ˆ TfK1K2Vf ‡ TmVm sisal-®bre-reinforced LDPE have been studied with respect to the e€ects of frequency, ®bre content and where K1 is the ®bre orientation factor and K2 is the ®bre length on the dielectric constant, volume resistivity ®bre length factor. If ®bre length l > lc, and dielectric loss factor [51]. The dielectric constant increases steadily with increasing ®bre content for all frequencies in the range 1 to 107 Hz. It is also noted that dielectric constant decreases with increase of ®bre length If ®bre length l < lc, and frequency. Maximum dielectric constant values are Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 Fig. 5. Variation of experimental and theoretical tensile properties of Fig. 8. Variation of loss moduli with temperature for untreated (ran- randomly oriented composites as a function of ®bre-volume fraction domly oriented) sisal/LDPE composites at di€erent ®bre content.
[44] ( Ð ) strength (- - - -) modulus.
Fibre length=6 mm, frequency=10 Hz [34].
dielectric constants of the latter composites with fre- quency and ®bre content are smaller as a result of their lower interfacial polarisation. The dielectric constant, dielectric loss factor and electrical conductivity of 25% sisal/LDPE can be increased considerably by adding 5% carbon black. The electrical conductivity of hydro- phobic LDPE can also be improved by mixing it with hydrophilic ligno-cellulosic ®bres. Solution mixing technique has no adverse e€ects. Finally, it is important to mention that a 25% sisal/LDPE composite contain- ing 5% carbon black can be used in anti-static applica- tions to dissipate static charges.
Fig. 6. Variation of experimental and theoretical tensile properties of randomly oriented composites as a function of ®bre length [44]. For keys, see Fig. 5.
4.2.2.4. Ageing properties. The environmental proper- ties of sisal-®bre composites are very important because, as a natural ®bre, sisal ages and causes degradation. The e€ects of aging on the physical and mechanical proper- ties of sisal-®bre-reinforced polyethylene composites have been studied [52]. The tensile properties and dimensional stability are evaluated under two di€erent ageing conditions: one is by immersing samples in boil- ing water for 7 h under atmospheric pressure; and the other is by heating the samples at 70C in an air circu- lating oven for 7 days. Both cardanol derivative of tol- uene di-isocyanate (CTDIC) treated and untreated sisal- ®bre-reinforced composites have been studied. The age- ing properties of the sisal composites are also compared Fig. 7. Storage moduli and mechanical-loss factor versus temperature to those of glass-®bre composites aged under identical for untreated (randomly oriented) sisal/LDPE composites at di€erent conditions. It is concluded that CTDIC-treated compo- ®bre content. Fibre length=6 mm, frequency=10 Hz [34].
sites showed better mechanical properties and dimen- sional stability as compared to untreated composites as obtained at low frequencies. Sisal/LDPE composites of 1 a result of the existence of an e€ective interfacial bond mm ®bre length and 30% ®bre content have the highest between ®bre and matrix. Better dimensional stability is values of dielectric constants at all frequencies. The o€ered by glass/LDPE composite because of the volume resistivity values decrease with increase of fre- hydrophobic nature of the glass ®bre. With suitable quency and ®bre content, i.e. the electric conductivity of ®bre-surface treatment, mechanical properties such as composites are greater than neat LDPE. When com- strength and elastic modulus of sisal/LDPE composites pared to glass/LDPE composites, the same trend in elec- can be improved to comparable levels as those of glass/ trical properties is observed, but the changes of Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 4.2.3. Properties of sisal ®bre-reinforced polystyrene leads to the poor immersion of ®bres in the PVC matrix.
Also, both treated and untreated sisal/PVC composites For other thermoplastics matrices, Manikandan et al.
have quite good moisture resistance.
[43] studied the tensile properties of short sisal-®bre- reinforced polystyrene composites. Untreated and ben- 4.3. Sisal-®bre-reinforced rubber matrix zoylated sisal ®bres were used to produce the compo- sites and the in¯uences of ®bre length, ®bre content, Table 8 shows that rubber is the second most widely ®bre orientation and ®bre benzoylation were investi- used matrix for sisal ®bre composites behind poly- ethylene [53±59]. Rubber matrices include natural rub- Variation in ®bre length produces no considerable ber and styrene±butadiene rubber. The main research change in the modulus of the composites but gives areas concern the e€ect of ®bre length, orientation, maximum tensile strength (25 MPa) at a ®bre length of loading, type of bonding agent and ®bre/matrix inter- about 10 mm (aspect ratio=82). This critical ®bre action on the properties of the composites which include length is quite di€erent to the sisal/PE composite which mechanical properties, rheological behaviour, thermal is 6 mm. Table 11 shows the e€ect of ®bre-volume frac- ageing, gamma-radiation and ozone resistance.
tion on the mechanical properties. There is an initial Experimental results show that, for best balance of reduction in tensile strength at Vf ˆ 10% followed by properties, the ®bre length is about 6 mm. This is the an increase to Vf ˆ 20% and remains constant at even same as the sisal/PE composites. Orientation e€ects are higher Vf. These results are also di€erent to sisal/PE as expected. Addition of short sisal ®bres to rubber o€ers composites which conform to the rule of mixtures. The good reinforcement, which can be further strengthened orientation e€ects on the mechanical properties of sisal/ by a suitable coupling agent such as a resorcinol/heca PS composites are also given in Table 11.
bonding system.
A two-stage stress relaxation has been observed in 4.2.4. Properties of sisal-®bre-reinforced PVC composite acetylated sisal-®bre-reinforced natural rubber compo- Yang et al. [42] studied sisal/PVC composites with sites. Initial relaxation occurs at short times (200 s), and respect to the e€ects of ®bre and matrix modi®cation, second-stage relaxation takes much longer to complete.
processing parameters on the mechanical and water The initial mechanism is a result of the ®bre/rubber resistance properties. Their main objective is to obtain attachments and the latter to the physical and chemical the best processing parameters and interface modi®ca- relaxation processes of the natural rubber molecules.
tion to make novel sisal/PVC composites. To make good The relaxation is in¯uenced by the coupling agent indi- use of sisal ®bre and PVC, it is important to improve the cating that the ®bre/rubber interface is involved. Gum interface so that better mechanical properties of the vulcanite shows only one relaxation process, the rate of composite can be obtained. But, unfortunately,their which is almost independent of the strain level. For the results show that thermal treatment, acetylation and composite without a coupling agent, the rate of relaxa- coupling agent improve neither the interface nor the tion increases with strain level and vice versa. The initial mechanical properties. On the contrary, the untreated rate of the stress-relaxation process diminishes after sisal-®bre-reinforced PVC composite possesses better ageing (at 70 and 100C for 4 days).
mechanical properties. These results have been explained Varghese et al. [57] studied the e€ect of acetylation by the small ®bre-volume fraction (18.5%) of their and bonding agent on the ageing properties of sisal-®bre- composites and the melting processing method that reinforced natural rubber composites which include Tensile properties of sisal-polystyrene composites as a function of ®bre content and ®bre orientation [43]a Fibre Content (wt%) Ultimate tensile strength (MPa) Young's modulus (MPa) Elongation at break (%) a Fibre length was 6 mm. The values in parentheses give the properties of untreated ®bre composites. L, longitudinal; T, transverse; R, randomly Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 thermal ageing, gamma radiation and ozone resistance.
®re behaviour of sisal short-®bre-reinforced gypsum High ®bre-volume fraction shows better resistance to using specially designed testing equipment. Though ageing, especially with ®bre-surface treatment. Fibre gypsum itself has good combustion-resistance, with orientation is also found to reduce the extent of degra- increasing water reduction as a result of increasing dation under these ageing conditions. Increasing the temperature, there is a progressive shrinkage process dosage of gamma radiation was found to increase the that promotes surface ®ssuration. Adding sisal ®bres extent of the ageing process.
into the gypsum matrix increases the ®re insulating per- formance and delays the occurrence of surface ®ssura- 4.4. Sisal-®bre-reinforced gypsum and cement matrices Swift [66] studied sisal/cement composites and their For applications as building materials, sisal-®bre- potential for rural Africa. He studied the mechanical reinforced gypsum and cement composites have long properties including ¯exural, energy absorption and been studied before 1994 [60±65]. The majority of these pointed out that a composite material formed by sisal works are focused on interface, mechanical, ®re and ®bre and cement is suitable for applications in several environmental properties and their applications.
structures. For example, cladding walls to produce Bessell and Mutuli [65] studied the interfacial bond earthquake-resistant adobe structures for houses, roof- strength of sisal/cement composites using a tensile spe- ing sheets and tiles, grain storage bins and water ducts.
cimen containing a single ®lament in the brittle matrix.
The crack spacing in the matrix was measured and used 4.5. Other matrix systems to evaluate the ®bre/matrix bonding strength. It is shown that the interfacial bond of sisal/cement compo- Bisanda and Ansell [3] used cashew nut shell liquid site is lower than that of other composites because sisal (CNSL) as a matrix to make sisal-®bre-reinforced com- ®bre absorbs moisture from cement thus leading to a posites. CNSL is a natural monomer blend that has very poor interface.
been condensation polymerized with formaldehyde The following aspects of sisal-®bre-reinforced cement in the presence of an alkaline catalyst to produce a or gypsum composites have also been studied pre- thermosetting resin. It can be used to bind sisal ®bres to viously. For example, Hernandez et al. [61] studied the produce a cheap and useful composite. The resin is Variation of mechanical properties with volume fraction of GRP in sisal/glass hybrid composites [68]ab Volume fraction of ®bres Elongation at break a SRP, sisal-®bre-reinforced plastics; GRP, glass-®bre-reinforced plastics.
b Values in parentheses are properties of randomly oriented composites.
Properties of 50:50 sisal/glass hybrid composites containing alkali-treated and untreated sisal ®bre [68]a Tensile strength (MPa) Young's modulus (MPa) Elongation at break (%) Tear strength (N/mm) Hardness (Shore D) a L, longitudinal oriented; R, random oriented.
Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 thermally stable up to 230C and is further cross-linked when exposed to simulated sunlight. The plain woven mats of mercerized sisal ®bre when impregnated with Sisal ®bre is an e€ective reinforcement of polymer, CNSL/fomaldehyde resin produced plain and corru- rubber, gypsum and cement matrices. This has created a gated laminated composites showing mean tensile range of technological applications beyond its tradi- strength of 94.5 MPa and Young's modulus of 8.8 GPa tional usage as ropes, carpets, mats, etc.
[3]. It is recommended that these low-cost corrugated The mechanical and physical properties of sisal ®bre panels can be used for roo®ng applications as a not only depend on its source, position and age which result of their adequate crush bending strength (13.9 will a€ect the structure and properties, but also depend on the experimental conditions, such as ®bre diameter, gauge length, strain rate and test temperature.
Fibre-surface treatment can improve the adhesion 5. Sisal and synthetic hybrid-®bre composites properties between sisal ®bre and matrix and simulta- neously reduce water absorption. Such methods include: Reinforcement by two or more ®bres in a single (a) silane and other coupling agents to introduce long matrix leads to hybrid composites with a great diversity chain structures onto the sisal ®bre to change its of material properties. It appears that the behaviour of hydrophilic characteristics; (b) peroxide to promote hybrid composites is simply a weighted sum of the indi- grafting reactions; (c) permanganate and alkali to vidual components so that there is a more favourable increase the roughness of ®bre surface hence increasing balance of properties in the resultant composite mate- the surface area available for contact with the matrix; rial. Sisal and glass ®bres are one good example of and (d) thermal treatment.
hybrid composites [67±69] possessing very good com- Di€erent matrix systems have di€erent properties.
bined properties.
The mechanical and physical properties of sisal-®bre- For sisal/glass-®bre-reinforced LDPE hybrid compo- reinforced composites are very sensitive to processing sites, the e€ects of ®bre orientation, composition and methods, ®bre length, ®bre orientation and ®bre-volume ®bre-surface treatment on the mechanical properties have been studied and the results are shown in Table 12.
Sisal and glass ®bres can be combined to produce Owing to the superior properties of glass ®bres the hybrid composites which take full advantage of the best mechanical properties of the hybrid composites increase properties of the constituents. Almost all the mechanical with increasing volume fraction of glass ®bres. Positive' properties show positive' hybrid e€ects.
or negative' hybrid e€ect is de®ned as larger' or smal- ler' than the properties calculated from the rule of mix- tures of the two constituent ®bre-reinforced composites.
A positive hybrid e€ect has been observed for all mechanical properties except elongation at break. This From this review, it is clear that chemically treated or e€ect is a consequence of increased ®bre dispersion and modi®ed sisal-®bre-reinforced composites are potential orientation with increasing volume fraction of glass structural materials as a result of their good mechanical, environmental and economic properties. The following The e€ect of chemical modi®cation (alkali treatment) areas of research are, however, needed to realise wider of sisal ®bres on the mechanical properties of a 50:50 applications of sisal ®bres in engineering: sisal/glass hybrid composite is shown in Table 13 and the improvement is generally less than 10%. Water absorption of the composite is reduced from 11.6 to . Sisal-textile-reinforced composite is an important 3.1% compared to the non-hybridized sisal-®bre com- area in which little work has been done [3]. Sisal ®bres can be woven into textile preforms and Yang et al. [69] studied the mechanical and interface impregnated with resins by resin transfer moulding properties of sisal/glass-®bre-reinforced PVC hybrid (RTM) or resin ®lm infusion (RFI) to make composites before and after immersion in water. It is superior but more economical composites.
found that there exists a positive' hybrid e€ect for the . Microstructure of the interface between sisal ®bre ¯exural modulus and unnotched impact strength but a and matrix still needs to be investigated and the negative' hybrid e€ect for the ¯exural strength. It is interfacial properties should be studied with more suggested that the negative' hybrid e€ect is caused by rigorous single ®bre pullout and fragmentation the poor interface between sisal, glass ®bres and PVC tests [70]. The relationship between interface and matrix. They also suggested that water will have a det- bulk composite properties should be established.
rimental e€ect on the ®bre/matrix interface leading to . Fracture toughness and fracture mechanisms of reduced properties.
sisal-®bre composites do not seem to have been Y. Li et al. / Composites Science and Technology 60 (2000) 2037±2055 studied in any depth in previous published works.
on the tensile properties of short sisal ®bre-reinforced poly- This is important if new improved materials are to ethylene composites. Polymer 1996;37:5139±49.
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[7] Chand N, Hashmi SAR. Metals Materials and Processes . Mechanical properties of sisal-®bre composites [8] Bisanda ETN, Ansell MP. The e€ect of silane treatment on the measured from tests quite often disagree with the mechanical and physical properties of sisal±epoxy composites.
rule of mixtures. A full explanation can only be Composites Science and Technology 1991;41:165±78.
obtained if the interface strength and the failure [9] Li H, Zadorecki P, Flodin P. Cellulose ®bre±polyester composites mechanisms are known. Further work is needed with reduced water sensitivity. (1) Chemical treatment and particularly to explain hybrid' e€ects in sisal/glass mechanical properties. Polymer Composites 1987;8:199±207.
[10] Chand N, Sood S, Rohatgi PK, Sayanarayana KG. Resources, structure, properties and uses of natural ®bres of madhya pra- . Economical processing methods must be devel- desh. Journal of Scienti®c and Industrial Research 1984;43:489± oped for the composites because of the very low price of sisal ®bres. The relationship between [11] Bisanda ETN. The manufacture of roo®ng panels from sisal ®bre mechanical properties and processing methods reinforced composites. Journal of Materials Processing Technol- should be established.
[12] Chand N, Satyanarayana KG, Rohatgi PK. Mechanical char- . New applications should be found for sisal-®bre- acteristics of sunhemp ®bres. Indian Journal of Textile Research based composites. Hybrid ®bre composites with sisal and other ®bres rather than glass may open [13] Hornsby PR, Hinrichsen E, Tarverdi K. Preparation and prop- up new applications. For example, from the eco- erties of polypropylene composites reinforced with wheat and ¯ax straw ®bres. Part 1, ®bre characterisation. Journal of Materials nomics point of view, sisal ®bres may be hybri- dised with carbon or aramid ®bres to reduce the [14] Satyanarayana KG, Sukumaran K, Mukherjee PS, Pavithran C, costs of these expensive ®bres reinforced compo- Pillai SG. Natural ®bre-polymer composites. Cement & Concrete sites whilst maintaining their good mechanical [15] Chand N, Hashmi SAR. Mechanical properties of sisal ®bre at . Recycling (including burning) characteristics and methods of sisal-®bre-reinforced composites are [16] Chand N, Joshi SK. Temperature dependence of dielectric beha- important aspects of this new material but there viour of sisal ®bre. Journal of Materials Science Letters are very few published data to date. Recycling is an attractive future research direction that will [17] Yang GC, Zeng HM, Zhang WB. Thermal treatment and ther- mal behaviour of sisal ®bre. Cellulose Science and Technology provide socio-economic bene®ts.
[18] Zhou LM, Mai Y-W, Ye L, Kim JK. Techniques for evaluating interfacial properties of ®bre-matrix composites. Key Engineering [19] Kim JK, Zhou LM, Mai Y-W. Interfacial debonding and ®bre pull-out stresses. Part III. Interfacial properties of cement matrix Y. Li would like to thank the University of Sydney composites. Journal of Materials Science 1993;28:3923±30.
for an Overseas Postgraduate Research Scholarship [20] Yang GC, Zeng HM, Li JJ, Jian NB, Zhang WB. Relation of (OPRS) and an International Postgraduate Award modi®cation and tensile properties of sisal ®bre. Acta Scien- (IPA). Y.-W. Mai also thanks the Australian Research tiarum Naturalium Universitatis Sunyatseni 1996;35:53±7.
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