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Ramie (Boehmeria nivea (L.) Gaud.) and Spanish Broom (Spartium junceum L.) fibres for composite materials: agronomical aspects, morphology and mechanical properties
Industrial Crops and Products 11 (2000) 145 – 161 www.elsevier.com/locate/indcrop
Ramie (Boehmeria ni6ea (L.) Gaud.) and Spanish Broom (Spartium junceum L.) fibres for composite materials: agronomical aspects, morphology and mechanical properties L.G. Angelini a,*, A. Lazzeri a, G. Levita b, D. Fontanelli b, C. Bozzi c a
Dipartimento di Agronomia e Gestione dell’Agroecosistema, Uni6ersity of Pisa, Via S. Michele degli Scalzi 2, 56124 Pisa, Italy b Dipartimento di Ingegneria Chimica, Uni6ersity of Pisa, Via Diotisal6i 2, 56100 Pisa, Italy c Stazione Sperimentale Cellulosa, Carta, Fibre tessili Vegetali ed Artificiali, P.zza L. da Vinci, 20133 Milano, Italy Accepted 8 October 1999
Abstract The agronomic characteristics of Ramie and Spanish Broom were investigated for seven years in the pedoclimatic conditions of Central Italy. The chemical, physical and mechanical properties of these fibres were also examined in order to evaluate the feasibility to use them in composite materials. Results demonstrate that Ramie grown in the temperate environment can be harvested three times a year. The yield of green Ramie plants per annum was almost 10 000 g m − 2. The fresh Ramie plant is composed, by weight, of 30% green leaves and 70% green stems. The yield dry fibre is almost 3% of the green stems, giving a total yield of 200 g m − 2. Spanish Broom can reach a fresh biomass yield of 4000 g m − 2 per annum, represented by 53% of long slender terete green branches which constitutes the economic products. The dry yield per annum was about 1800 g m − 2 with a dry branchlets yield of 900 g m − 2. Ramie and Spanish Broom cortical fibres are multiple elementary fibres (ultimates) arranged in bundles. In Ramie, the elementary fibres are bound by gums and pectins, while in Spanish Broom they are bound together by lignin. Both species showed a thick secondary cell wall indicating a high cellulose content. Ramie ultimate fibres are flattened and irregular in shape, while those of Spanish Broom are more regular in shape. The diameter of the ultimates varies from 10 to 25 mm in Ramie, while the Spanish Broom ultimates ranges from 5 to 10 mm; the diameter of the whole bundle is about 50 mm for both species. Ramie fibre showed a content of lignin, pentosans and extractives lower than Spanish Broom. Both fibres had a high content of cellulose which, on the base of X-ray analysis, was evaluated to be in excess of 70%. Ramie and Spanish Broom fibres had tensile strength of 950 MPa and 700 MPa, respectively. The elastic moduli were : 65 and : 20 GPa, respectively, which well compare with the modulus of E-glass fibres (70 – 90 GPa). The strength of the fibre-matrix interface was measured using the single filament fragmentation technique and an epoxy resin as the polymer matrix. Values for carbon and glass fibres in the same resin were also measured for comparison. The interface strength for the vegetable fibres was higher than that of carbon and glass, likely due to a mechanical lock mechanism. These values confirm both fibres as potential replacement for man made fibres in composite materials. © 2000 Elsevier Science B.V. All rights reserved.
* Corresponding author. Tel.: + 39-50-599111; fax: + 39-50-540633. E-mail addresses: [email protected] (L.G. Angelini), [email protected] (G. Levita) 0926-6690/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 6 6 9 0 ( 9 9 ) 0 0 0 5 9 - X
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Keywords: Boehmeria ni6ea; Spartium junceum; Spanish Broom; Ramie; Fibre; Mechanical properties; Interface strength; Composite materials
1. Introduction Application of vegetable products in various branches of industry will benefit from their intrinsic biocompatibility and biodegradability. Industries all over the European Community are looking for raw material for replacing artificial fibres in composite materials (CMs) to alleviate problems related with CMs disposal at the end of the technical life. Vegetables fibres could be a viable alternative to man-made fibres (especially asbestos and glass fibres) at least in applications in which the overall performance, evaluated in term of life cycle analysis, has to take into account the final disposal. A number of suitable plant fibres can be successfully grown in Italy, including Ramie (Boehmeria ni6ea (L.) Gaud.), a member of the Urticaceae, and Spanish Broom (Spartium junceum L.) for which promising agronomic results have been obtained (Oggiano et al., 1997). Ramie is a perennial plant native to China, Japan, and the Malay Peninsula, where it has been used as a textile fibre for centuries due to its excellent fibre (Kirby, 1963; Wood and Angus, 1974; Batra and Bell, 1975). The fibres, obtained from the outer part of the stem, are the longest and one of the strongest fine textile fibres (Dempsey, 1975; Jarman et al., 1978). Other advantages of this fibre is the resistance to bacteria, mildew, and insect attack. Its strength slightly increases when wet (Fontanelli, 1998). Spartium junceum (belonging to the Leguminosae family) is a perennial shrub, wide-spread throughout the Mediterranean area (Munz and Keck, 1973; Pignatti, 1982) where it naturally occurs in hilly soils, contributing to lower erosion and risks of nutrient leaching. This plant is somewhat adapted to alkaline and salty soils. The name Spartium is from the Greek word denoting ‘cardage’, in allusion to the use of the plant. By macerating the twigs a good fibre was obtained, which was made into thread and cords and a
coarse sort of cloth. It was cultivated on a large scale in southern Italy around the thirties; but later it fell out of favour. Recently there has been a revival of interest in Spanish Broom as a possible source of raw materials to be used in CMs for automobile applications. The agronomic characteristics of Ramie and Spanish Broom were investigated for seven years in the pedoclimatic conditions of Central Italy. In order to evaluate the feasibility to use Ramie and Spanish Broom fibres in composite materials, the morphology and chemical composition together with physical and mechanical properties of these fibres were also examined. 2. Materials and methods
2.1. Agronomic aspects A 7-year field trial was set up at the Department of Agronomy of the University of Pisa (about 43°N; 10°E; 3 m elevation) on a deep silt loam soil (sand 15.5%; silt 65.5%; clay 18.0%; organic matter 1.15%; pH 8.1; total nitrogen 1.3%; assimilable P2O5 35 mg kg − 1; exchangeable K2O 165 mg kg − 1). The soil was characterised by a water table rather superficial with a deph of 120 cm during the driest season. The soil displayed the following hydrological characteristics: field capacity, 27.3% dw, wilting point 9.4% dw. Meteorological conditions during the seven years of study are shown in Table 1. The warm-season perennial species used in this evaluation were Ramie, a genotype from Palermo Botanical garden, and Spanish Broom, one local ecotype. Spanish Broom was transplanted at the end of April 1992, while Ramie was transplanted at the end of March 1993 (Table 2). Stump sprouting for Spanish Broom and rhizome cuttings for Ramie were used. During the establishment year, plants were cut at the end of the growing season to allow the vegetative regrowth in the second year.
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The experiment was laid out in a randomised block design with four replicates. Plot size was 48 m2. In Ramie they consisted of 16 rows, each 6 m long, with an interrow and interplant spac-
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ing of 0.5 m. For Spanish Broom a plant density of 2 plants m − 2 (0.1× 0.5 m) was adopted and each plot consisted of 8 rows, each 6 m long.
Table 1 Climatic conditions during trials in 1992–1998 and 1918–1982 mean values Years / Months
J
Rainfall (mm) 1992 45.0 1993 2.6 1994 58.2 1995 58.4 1996 2.8 1997 118.0 1998 73.6 1918–1982 89.0 Temperature min. ( °C) 1992 2.9 1993 2.2 1994 4.4 1995 0.5 1996 5.3 1997 4.5 1998 3.6 1918–1982 2.8 Temperature max ( °C) 1992 11.9 1993 12.1 1994 13.7 1995 11.1 1996 12.6 1997 13.7 1998 12.9 1918–1982 11.2
F
M
A
M
J
J
A
S
O
N
D
15.0 2.1 21.4 101.2 77.5 21.3 56.9 80.0
17.5 32.2 2.0 56.8 16.1 21.1 49.0 79.0
82.5 65.1 123.3 136.4 83.1 55.0 49.0 77.5
101.8 13.2 22.3 91.1 49.7 25.0 24.2 67.2
79.4 3.9 62.4 40.6 44.5 58.4 44.8 46.4
18.8 0.0 1.5 2.1 12.2 7.2 1.7 25.0
60.6 34.1 0.8 8.2 98.1 0.0 8.8 42.0
115.7 130.2 192.3 108.1 91.7 45.9 98.2 83.7
191.4 166.0 91.4 71.6 114.5 37.5 95.3 134.2
50.6 78.7 122.9 25.4 224.6 82.4 42.2 118.0
87.3 36.0 84.1 88.6 122.6 95.6 56.6 99.0
1.6 −0.7 3.1 3.7 2.0 7.5 2.6 2.3
4.8 2.2 6.9 3.2 4.1 4.6 3.0 5.3
7.5 7.6 7.5 8.1 8.7 5.7 3.0 8.1
12.3 11.5 12.5 11.6 11.9 12.0 10.5 11.6
14.9 16.2 15.3 13.4 15.5 16.6 14.4 15.0
17.2 16.2 18.7 18.9 17.1 16.5 16.3 17.2
18.9 19.1 20.1 17.9 17.2 19.1 17.8 17.0
15.0 15.7 16.9 14.4 13.1 15.9 14.1 14.8
12.9 9.5 11.6 11.8 10.9 11.1 11.2 11.0
8.9 6.3 9.9 5.9 9.2 8.1 4.9 6.9
4.9 6.4 5.3 5.2 4.9 4.1 1.5 3.7
13.4 13.3 12.7 13.3 11.3 10.8 15.0 12.7
15.6 13.9 17.6 13.8 14.3 17.9 15.8 15.1
17.5 17.4 17.7 18.0 17.9 17.4 15.8 18.3
24.0 22.8 22.5 21.1 21.6 23.2 23.3 22.4
24.3 27.9 26.1 23.8 27.0 25.7 26.4 26.1
27.5 28.2 31.1 29.8 28.3 28.0 28.8 29.1
29.7 31.3 31.0 28.6 28.8 30.8 29.4 29.2
25.8 25.9 26.1 23.9 23.7 27.7 24.9 26.2
20.5 18.0 21.8 23.1 21.1 21.6 21.1 21.3
17.3 14.5 18.4 15.6 16.5 16.1 14.1 15.8
12.7 14.2 12.5 12.8 12.4 13.1 10.5 12.0
Table 2 Dates of vegetative regrowth and of harvest for Ramie and Spanish Broom at Pisa in the different years of growtha Year
1992 1993 1994 1995 1996 1997 1998
Ramie
Spanish Broom
VR
C1
C2
18 01 26 01 18 01
– 21 10 12 09 08
08 24 19 05 18 15
April March February March February March
June July June July June
C3
August August September September September September
21 18 – 02 25 –
October October December November
VR
H
10 03 04 03 06 27 26
– 11 18 22 05 24 29
May April April April April March March
September October October November October September
a VR, Vegetative regrowth; H, Harvest; Ramie was transplanted on 23 March 1993 and Spanish Broom on 23 April 1992; C1, C2, C3, first, second, third cutting.
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In order to evaluate maximum crop yield, plants were maintained in optimum water supply conditions. All plots received the same amount of N, P, and K: 100 Kg ha − 1 per year. The nitrogen dose was split into two equal pre-planting and late-spring applications. During the second and the third growing seasons, plots received only 50 Kg ha − 1 of N in a single dose at the end of winter. Plots were kept weed free by hand hoeing. The establishment year was not considered and crops were tested starting from the second growing season. Screening of these species for phenological and biometrics characteristics as well as for above-ground biomass and stem production was carried out from 1993 to 1998. Cycle length was measured as the number of days from vegetative regrowth to harvest. Biometrics and productive determinations were performed on a minimal area of 6 m2 in the inner part of each plot. According to Iyengar and Bhujang (1961) Ramie harvests were usually accomplished when the lower part of the stem was turning brown. Spanish Broom was harvested only once in autumn and plants were cut at 15 cm from the soil. Aerial dry matter was separated in the different plant organs. The useful part was represented by stems without cymes and leaves for Ramie and by new branches for Spanish Broom. After harvest, all plants in the plots were cut 10 – 15 cm above ground for allowing uniform vegetative regrowth. Both species were harvested by hand. Spanish Broom new branches and Ramie dry stalks, with leaves and cymes removed, were decorticated using a small decorticating machine in order to remove the outer bark/epidermis and the bast from the woody core of the stems. Several stalks were passed through fluted crushing rollers at the entrance to the machine. Stalks were thereby constrained as their full length was decorticated in one pass through the machine. During this operation the cortex, comprising the bast and outer bark, was removed from the stem. The cortex was scraped to remove most of the outer bark and the parenchyma in the bast and, for Ramie, some of the gums and pectin. After decortication the fibres were hand brushed for make them suitable for physical and mechanical testing.
2.2. Morphological, chemical and mechanical characterisation Fibre bundles were confined in small plastic sleeves and then cross-sectioned. Scanning microscopy (SEM) was carried out on gold coated cross-sections. X-Ray patterns were recorded using Ni-filtered Cu–K radiation from a Siemens 500 D diffractometer equipped with a scintillator counter and a linear amplifier. Fourier-transform infrared spectroscopy (FTIR) spectra were obtained with a Bruker IFS 66-FTIR spectrometer on samples dispersed in KBr; 32 scans were accumulated for each sample, with a resolution of 4 cm − 1. In accordance with the TAPPI OM 250 method, the lignin content was determined as the sum of insoluble and soluble lignin, the latter being determined spectrophotometrically at 205 nm. Pentosan content was determined according to the TAPPI T 223 hm 84 and ash content according to the TAPPI 15 OS 58 method. The TAPPI 284 OM 82 method was used to assess the extractives content and the UNI 8282 method to determine the degree of cellulose polymerisation in cupriethylenediamine (CED) after delignifying the material with sodium chlorite.
2.2.1. Fibre strength The tensile properties of selected filaments were determined with an Instron 1185 (load cell 10 N) at the cross-head speed of 2 mm/min at room temperature (2092°C) and 709 5% relative humidity. Since the diameter of filaments, particularly for Ramie, was not uniform, selection of suitable samples was made with the help of a low magnification microscope; the diameter for each filament was taken at different places with the help of a precision gauge meter and the average value was used. Diameters were found to vary in the range 40–60 mm. Unlike man made fibres for composites, Ramie and Spanish Broom fibres are not circular. The diameter referred to in this work is that of the circle of the same area as the vegetable filaments.
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The elastic modulus (E) was measured by the slope of the conventional stress-strain curves taking the distance between grips as the gauge length. To measure the strength of fibres different gauge lengths were used, in the range 10 – 50 mm; a minimum of 50 filaments was taken for each gauge length to give data statistical meaning.
2.2.2. Single fibre composite (SFC) tests A silicon rubber mould was used to make dogbone shaped single fibre coupons (approximately 60 mm long, 10 mm wide, 1 mm thick). Filaments were selected as to assure their diameters were similar (:50 mm). The epoxy resin was a bifunctional bisphenol-A type with an epoxy equivalent of :195 (Epikote 828 by Shell). The hardener was p-amine-dicycloexyl-methane, used at the content of 25% by weight. Resin and hardener were intimately mixed at room temperature and freed from air bubbles by degassing at 50°C for 10 min in a vacuum oven. The mould containing the filaments was also equilibrated at 50°C prior to resin pouring. The casts were cured at 70°C for 2 h and postcured at 100°C for 3 h. The coupons were slowly strained in an Instron tensile machine (1185) at the crosshead speed of 0.02 mm/min. The fragmentation of the fibre was observed by means of a microscope attached to the machine, at magnification 40, both in natural and polarised light. The fragment lengths were measured by the help of a calibrated eyepiece. Tests were repeated with identical coupons to get at least 100 fragments for collecting a reasonable number of fragments. The interfacial strength of HS-Carbon fibres and of E-Glass fibres was measured, for comparison, using the same epoxy resin as the matrix.
3. Results and discussion
3.1. Agronomic data Ramie and Spanish Broom crops growth ceased during the winter months. In spite of its tropical origin Ramie plant resists well the winter low temperatures-such as the minimum temperatures
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below 0°C experienced in 1993 and 1995 winterthanks to the protection offered to rootstocks by a layer of dead leaves. Both species did not seem to withstand prolonged waterlogging. Soils with poor drainage and high water retention must be avoided, but at the same time the soil must have a good water retaining capacity. Spanish Broom and Ramie are perennials, and the production cycle is up to 20 years (Trotter, 1941; Bruno, 1951; Jarman et al., 1978), however this estimates is not supported by experimental evidence. In Ramie vegetative regrowth generally began at the end of February (Table 2) and stems are sent up from underground rootstocks; during the following months the stems elong rapidly with few branches. The species shows a good competition toward weeds due to a rapid interrow closure (30 days after regrowing). Stems ranged from 1.2 to 1.6 m high and about 0.8 to 1.2 cm in diameter, depending on the climatic conditions experienced in the different years. Stems were cut during the growing season and therefore the rootstocks send up new shoots, and consequently several crops can be taken from the plant each year. According to Bally (1957), the first crop after planting is very poor and unsuitable for fibre production. In each year, the time of harvest is very important; if delayed the stems become lignified and it is difficult to remove the fibre. First-crop Ramie was usually harvested before flowering (June) and the second crop during the peak of flowering (September). In each case the correct harvesting time was when the stem bases had just turned brown (Iyengar and Bhujang, 1961). Results demonstrate that Ramie grown in Central Italy under temperate environment can be harvested three times a year. The third cutting is usually performed during late autumn and therefore is often at risk for the low temperatures and rainy conditions during this period. During 1993–1998 trials, three crops were taken from the plant each year, with the exception of 1995 and 1998 when the low temperatures of November (below the long-period mean value, see Table 1) and/or rain distribution did not allow accomplishment of the last autumn harvest. The annual total yield of green Ramie plants per hectare, as 1994–1998 mean value (establishment
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Fig. 1. Total dry yield (a) and stem dry yield (b) of Ramie and Spanish Broom in the different growing seasons and as overall mean (establishment year not included). Values are the mean of four replications and bars correspond to the mean standard error.
Fig. 2. Partition of Ramie stem dry yield among first, second and third cutting in the different years of the production cycle. The establishment year (1993) was not included in the mean value.
year was not included), is about 10 000 g m − 2. The fresh Ramie plant was composed, by weight, of 70% green stems and 30% green leaves and
cymes. The yield of dry Ramie plants per hectare per annum was 1905 g m − 2 (Fig. 1a) with a dry stem yield of 1202 g m − 2 (Fig. 1b). The dry Ramie plant was composed by weight of 62% dry stems and 38% dry leaves and cymes. The final product was obtained mainly from the first and second harvest, which gave 93% of the yearly production as dry stems (Fig. 2). The crop reaches its full productivity in the year after planting; in the following years Ramie production depended on climatic conditions rather than on the age of the crop. Rainfalls during the summer months strongly affected the yield and higher production were observed during the most rainy summer seasons such as 1996 (735 mm from vegetative regrowth, VR, until third cutting, C3) and 1994 (457 mm from VR to C3). According to Iyengar and Bhujang (1961), Ramie can be harvested in the tropics three or four times a year when grown under favourable conditions; however, with supplementary irrigation and fertiliser application more than six harvests a year are possible (Dempsey, 1975). According to Dempsey (1975) the number of crops is reduced to two or three annually as the climate becomes more temperate. Although Ramie can be harvested more frequently in the tropics yields per crop are lower, annual yields per hectare in the tropic and in the temperate zones are consequently similar. The yield dry fibre was almost 3% (Dempsey, 1975; Jarman et al., 1978) of the green stems, giving a total fibre yield of about 200 g m − 2 as 1994–1998 mean value. In Spanish Broom vegetative regrowth generally began at the beginning of April (Table 2). Plants reached up to 2 m high, with long, slender, leafless or few-leaved, green, rushlike branchlets. The above ground plants were cut at the end of the growing season (Table 2). This crop performed well after the establishment year (1992), reaching in 1993 a total fresh biomass of about 4700 g m − 2 corresponding to 2200 g m − 2 dry yield (Fig. 1a). The dry yield was composed of 56% new branches, representing the economic yield. When established, Spanish Broom can reach a fresh biomass yield about of 4000 g m − 2 per annum as 1993–1998 mean value, repre-
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sented by 53% of long slender terete green branches. The dry yield per annum was about 1800 g m − 2 (Fig. 1a) with a dry branchlets yield of 900 g m − 2 (Fig. 1b). The moisture content of branchlets averaged 60%, while Ramie stems presented over 80% of moisture. The useful dry yield to total fresh biomass ratio was higher in Spanish Broom than in Ramie (Fig. 3) and this fact is important for the necessity to storage raw material with low moisture content and to maximise the marketable products. Spanish Broom was disease resistant, exception given by a fungal attack (tracheomycosis) in the fourth growing season, that decreased the yield. This species appeared also drought and heat tolerant; moreover it is a nitrogen-fixing plant and therefore could be cultivated on marginal lands due to its low input requirements.
3.2. Morphological and chemical aspects Ramie and Spanish Broom cortical fibres are multiple elementary fibres (ultimates) arranged in bundles. In Ramie, the elementary fibres are bound by gums and pectins (Jarman et al., 1978; Batra, 1981), while in Spanish Broom they are
151
bound together by lignin (Trotter, 1941; Fontanelli, 1998). Bundles of ultimates are clearly visible in SEM micrograph (Fig. 4a, b): Spanish Broom contained many more ultimates than Ramie. Both species showed a thick secondary cell wall indicating a high cellulose content, a defined lumen sometimes collapsed (Morton and Hearle, 1993). The diameter of the ultimates varied from 10 to 25 mm in Ramie, while the Spanish Broom ultimates ranged from 5 to 10 mm. Ramie ultimate fibres (Fig. 4a) are flat and irregular in shape, with a thick cell wall, and taper to rounded ends, while Spanish Broom fibres (Fig. 4b) are more regular in shape, as observed also by Trotter (1941). The primary cell wall is often lignified and this aspect is responsible of the low hygroscopicity of the fibres. The longitudinal SEM view of the Ramie and Spanish Broom bundles (Fig. 5a, b), shows irregularities and defects. In Ramie the analysis revealed cross markings or dislocation characteristics of flax fibres. The influence of these defects on the fibres mechanical characteristics is discussed later. The chemical composition of the whole stem (Table 3) showed a significant difference between
Fig. 3. Stem dry yield to total fresh yield ratio for Ramie and Spanish Broom in the different growing seasons. Values are the mean of four replications and bars correspond to the mean standard error.
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tallinity reported in Table 3 is given by the ratio to the area of the (002) peak to the intensity of the amorphous background. The FTIR spectra of bundles are shown in Fig. 7a, b: both fibres showed the same absorption bands with significant differences in intensity and width at half height. The intensity of signal at 1373 cm − 1 (lignin), related to the signal intensity at 2916 cm − 1 as the internal standard, was lower in Ramie fibre than in Spanish Broom (10.8 and 17.0, respectively), in agreement with the chemical results. The signal of the non-cellulosic components at 1734 cm − 1 (hemicellulose esters and pectic substances) appeared slightly more pronounced for Ramie than for Spanish Broom; due to the prevailing pectic substances in Ramie (Jarman et al., 1978; Batra, 1981). Another very important difference between the species is the polymerisation degree that was about 3000 in Ramie and about 1700 in Spanish Broom (Table 3).
Fig. 4. Cross section micrography of Ramie (a) and Spanish Broom (b) fibres.
the two species. Ramie has an higher lignin content and a lower level of pentosans and extractives than Spanish Broom. More interesting is the difference in the compositional analysis of the two fibres (Table 3). Ramie fibre showed a content of lignin, pentosans and extractives lower than Spanish Broom. Both fibres had a high content of cellulose, no less than 70%. The highest X-ray index of crystallinity obtained for Ramie, Table 3, confirms this. In fact, while cellulose is a highly crystalline polymer, hemicelluloses and lignin are amorphous. In the diffractograms in Fig. 6, three peaks are shown, at : 22° and 14 – 18°, two of them almost superimposed, corresponding to the (002), (101) and (10-1) planes of cellulose. The index of crys-
3.2.1. Fibre stiffness and strength Both types of fibres were very rigid when loaded in tension. The load-elongation diagrams were almost linear up to fracture (Fig. 8). Occasionally, irregularities were observed in the curves which were attributed to (a) failure of some of the individual fibrils, of which filaments are made up, prior to the final cumulative rupture; and (b) internal rearrangement of fibre subunits under the action of the tensile load. The mean elastic moduli were 65 9 18 GPa and 21.5 95 GPa for Ramie and Spanish Broom, respectively. These values place the examined fibres among the stiffest vegetable fibres such as, for example, cotton, pineapple and sisal. Although the standard deviation for Ramie was greater than that for Spanish Broom, the coefficients of variability, i.e. the standard deviation to the mean value ratio, were substantially the same for both fibres. The modulus of Ramie was very high; it approached that of E-glass fibres ( :72 GPa) making this fibre very attractive for mechanical purposes. Although the stiffness of Spanish Broom is 1/3 than that of Ramie, it is yet greater than that of most rigid non-oriented polymers (1–3 GPa) making it possible to stiffen commodity plastics such as, e.g. polyolefins.
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The brittle behaviour of both fibres allowed their strength to be analysed in terms of Weibull’s statistics. Due to the intrinsic variability of properties that characterises natural products, the tensile data of vegetable fibres are rather variable. The situation, on the other hand, appears to be
153
the same with man-made fibres, too. Broad distributions in tensile strength of fibres is usually attributed to flaws or defects that can be naturally exist or be introduced during handling or processing or, finally, resulting from surface ageing. It is widely accepted that these defects are
Fig. 5. Longitudinal view of Ramie (a) and Spanish Broom (b) fibres obtained by SEM.
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Table 3 Composition (%), degree of polymerisation and cristallinity of Ramie and Spanish Broom dry stems and fibres Compounds
Ramie Stem
Lignin acid soluble Lignin acid insoluble Total lignin Pentosans Extratives Ash Degree of polymerisation X-ray index of cristallinity (%)
0.9 20.6 21.5 10.5 8.1 5.1
Spanish Broom Fibre
Stem
0.2 1.3 1.5 7.0 1.0
0.7 13.7 14.4 15.3 16.2 3.6
3000 58
Fibre 0.3 6.2 6.6 9.5 3.3 1700 52
Fig. 6. X-ray diffractogram of Ramie and Spanish Broom fibres.
the main cause of premature failure of the fibre under tensile load (Curtin, 1994). Since the occurrence of flaws is random in nature, the tensile strength is to be characterised by a statistical model, the most widely used being the Weibull’s distribution function (Weibull, 1951). In the twoparameter model, the cumulative probability of failure Pn(s), i.e. the fraction of filaments having tensile strength not exceeding s, is given by:
a
Pn (s)= 1− e − l(s/g)
(1)
where a and g are the parameters that characterise the fibre, s is the stress at break and l is the gauge length. Eq. (1) can be cast as: f(Pn,l)=ln[ln(1− Pn(s)) − 1]− ln l =a ln s − a ln g
(2)
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Fig. 7. FTIR Spectra of Ramie (a) and Spanish Broom (b) fibres.
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Fig. 7. (Continued)
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so that a plot of f(Pn(s), l) versus ln(s) is linear; a and g are thus obtained by the slope and the intercept, respectively. Fig. 9 gives the plots of f(Pn, l) versus ln(stress at break), at 50 mm gauge lengths, for both types of filaments. Similar plots for the other gauge lengths were also drawn. Once a and g are known, the mean fibre strength, sm, at a given gauge length can be calculated by the following equation: log sm(l)=a − 1 log(l) + log(g) +log[G(1 + a)a − 1]
(3)
Where G is the complete Gamma function. A plot of log sm versus log l is again expected to be
Fig. 8. Typical stress-strain diagrams for Spanish Broom and Ramie filaments (at 10 mm gauge length).
Fig. 9. Distribution of fibre strength (according to Eq. (2)).
Fig. 10. Comparison of the influence of gauge length on strength of Spanish Broom, Ramie, carbon and glass fibres (according to Eq. (3)).
linear. The mean tensile strength at the gauge lengths required in the fragmentation analysis is experimentally inaccessible and is evaluated by extrapolation of such plots. Fig. 10 gives the plots of log(mean stress) versus log (gauge length) for Ramie and Spanish Broom fibres. The solid line in each figure represents the regression line. In both cases it is observed that the fibre strength increases with decrease of gauge length. The strength of Ramie fibres was rather high, in the range 800–1000 MPa. These values, that agree with those found by Ho¨ck (1995), are not far from the strength of E-glass; only flax is reported to be stronger (Flemmin et al., 1995). Spanish Broom fibres are somehow weaker; the strength varied, in fact, from 400 to 700 MPa. No data on this fibre are known in literature for comparison. Both elastic modulus and strength data are large enough for present fibres to be utilised as reinforcing means of low stiffness matrices such as non oriented glassy or crystalline polymers (typical strength in the range 20–80 MPa), provided the transverse properties are not of primary importance. It is interesting to compare the properties of Ramie and Spanish Broom fibres and of the two most important synthetic types, E-glass and carbon (Fig. 10). As expected, the artificial fibres are stronger than the vegetable counterparts whose
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strength, however, approaches the GPa range. It has to be noted that a comparison made on the ground of specific properties would appreciably reduce the distance between glass (specific gravity : 2.6 g/ml) and the two natural fibres (specific gravity : 1.45 g/ml). This confirms that both types of vegetable fibres can stay beside artificial reinforcements, at least in non-structural applications. It is also interesting to note that the slopes of lines in Fig. 10, are substantially the same for all fibres. This was somehow stunning since one would expect natural fibres to exhibit a much wider variability and a more pronounced effect of filament length on fracture stress. All fibres appeared to be very similar in this respect, instead. To be mentioned is however the fact that glass fibres are isotropic in nature, whereas the vegetable filaments, due to the alignment of cellulose micro fibrils along the axis, are not. Moreover, vegetable filaments may be split if high transverse stress are applied. All this would adversely affect the properties of unidirectional composites but might have positive effect on the impact behaviour. It is interesting to note that the elongation at break, oB, of both fibres was influenced by the gauge length the same way the strength was (Fig. 11). The behaviour of fibres was almost linear so that sB : EoB. Since there is no influence of gauge length on E (fluctuation of modulus values was
independent of gauge length), it follows that the gauge length dependence of strength has to be paralleled by that of elongation.
3.2.2. Interface strength The chief function of the interface in composite materials is to transmit stresses from the weak polymer matrix to the high strength fibres. The stress transfer efficiency depends on the mechanical properties of the matrix, the load bearing capacity of the fibre and the strength of the fibre-matrix interface. A good adhesion is also required to prevent environmental agents from impairing the interface. In case of lignocellulosic fibres the degradation caused by water at the interface is of primary concern because the fibres are highly hygroscopic. The assessment of interface soundness is consequently of primary importance. Out of the several methods devised for characterising the stress transmission capability across the interface, the most elegant is the single fibre composite (SFC) test. This technique, first proposed by Kelly and Tyson (1965), has been widely used to study the interfacial adhesion of synthetic fibres (Fraser et al., 1983; Di Benedetto and Lex, 1989; Di Benedetto, 1991; Curtin, 1994; Levita et al., 1997) and only recently extended to the natural fibres (Van Den Oever and Bos, 1998). When a single fibre coupon is loaded in tension, fragmentation of the fibre occurs, Fig. 12, provided the ultimate elongation of the matrix is higher than that of the fibre. Fragmentation continues until all the segments are shorter than a critical length. Beyond this point, the stress transfer is no longer high enough to cause fibre breakage. The maximum shear stress the interface can bear (t), is given by the following equations: t=
Fig. 11. Influence of gauge length on elongation at break of Spanish Broom and Ramie.
scd 3scd = 2lc 8lm
(4)
where sc is the strength of the fibre at the critical length (lc), lm is the observed mean fragment length and d is the fibre diameter. The results from SFC tests are collected in Table 4. It is interesting to note that the present data of the critical length is close to the pull-out fibre length reported by Wollerdorfer and Bader (1998) for Ramie and other vegetable fibres.
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159
Fig. 12. Multiple fragmentation in a single fibre coupons loaded in tension (view in polarised light). Table 4 Diameter (d), mean fragment length (lm), critical length (lc), fibre strength (sc) and interface strength (t) for Ramie, Spanish Broom, carbon and glass fibresa Fibre type
d (mm)
lm (mm)
lc (mm)
sc (MPa)
t(MPa)
Ramie Spanish Broom Carbon Glass
:50 :50 :7 :11
0.35 0.32 0.55 0.34
0.47 0.43 0.73 0.46
1480 1325 7430 4740
79 77 36 56
a
All fibres embedded in the same resin and tested in identical conditions.
It has to be pointed out that the assumptions on which the SFC method is based upon are the brittle behaviour of the fibres (requirement fully fulfilled by present fibres) and that the probability to find defects along the filaments only depends on the gauge length. The latter assumption is satisfied in artificial fibres whose diameter is fairly constant. In the case of natural fibres there can be differences in diameter among filaments. A difference in diameter brings about a change in surface extension (even at constant gauge length) that scales with the square of diameter. When comparing strength data pertaining to different gauge lengths, one has either to limit the diameter vari-
ability or to assure that diameters are evenly distributed. The method usually adopted in the Weibull’s analysis for determining sc at fibre lengths of 1 mm, or less, is the extrapolation of the log sm versus log l plots. The fragment length distribution, i.e. plots of cumulative probability versus fragment length, are obtained from the SFC tests. From such plots, lm at P(l) = 0.5 were obtained and from Eq. (4) the t values were calculated. A point of weakness of this approach is the fact that the strength of fibres are determined at gauge lengths in the 101 mm order of magnitude and estimated in the 10 − 1 mm order of magnitude. It
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is doubtful such a long extrapolation would accurately estimate sc. Although doubts can be cast on the real meaning of t values, particularly when, as in the present case, they exceed the yield strength of the matrix, they can be safely used to compare the interfacial properties of similar components (Di Benedetto and Lex, 1989; Di Benedetto, 1991; Levita et al., 1997). High t values are generally taken as indicative of good adhesion between resin and fibre. One of the reasons for the efficiency of epoxy resins as adhesives is the formation of polar groups (-OH) that strongly interact with high surface energy solids. A high concentration of -OH groups, on the other hand, characterises the surface of cellulosic materials so that a strong interface can readily develop. Besides chemistry, other effects that may contribute to the stress transfer mechanism are: (1) the irregularity of the surface; and (2) the variation of fibre diameter. The simple stress analysis embodied in equation 4 assumes the fibres to be circular. As shown in Fig. 5a,b, the actual fibres have rather rough surfaces so that the real extension of the interface is higher than computed assuming a circular cross-section. The values of t should accordingly be lowered. A further contribution to the shear strength of the interface comes from the longitudinal fluctuation of diameter because in the region in which the diameter changes the shear component of the stress acting upon the interface decreases and normal components develop. This provides an effective locking mechanism since the fibre would be held in place even in the absence of adhesion.
4. Conclusions Establishment of a stable offtake of plant fibres will provide new perspectives for arable farming and contribute to a solution of the agricultural surplus. In the selection of fibre crops that can be grown for producing fibres for industrial uses, some agronomic aspects have been taken into consideration such as adaptability, yield potential and stability. Some excellent and positive agronomic attributes are found in Ramie and Spanish Broom. In contrast to other fibre crops, Spanish
Broom proved to be a low-input crop, growing well on marginal and hilly soils where it can contribute to lower risks of erosion. Although the normal habitats of Ramie are tropical, it was possible to cultivate this species successfully under the temperate climate of central Italy. Besides, the environmentally friendly cultivation of the crops concerning the demand of fertilizers and pesticides, both are very interesting in an era of restrictions for use of agricultural chemicals, implemented in most EU countries. The Ramie and Spanish Broom fibres were brittle with load-elongation diagrams almost linear up to failure. The elastic modulus of Ramie was 65 GPa, therefore similar to E-glass. For Spanish Broom the elastic modulus was 22 GPa, about one order of magnitude greater than that of glassy polymers. The strength varied from : 900 MPa for Ramie to : 650 MPa for Spanish Broom; in both cases the strength decreased with increasing fibre length. The distribution of failure strength obeyed the Weibull’s model. The influence of filament length on strength, related with the defect population, was found to be the same for Ramie, Spanish Broom, carbon and glass fibres. The interface strength between vegetable fibres and an epoxy resin (t: 80 MPa) was higher than that of carbon and glass (t= 36–56), likely due to a mechanical lock mechanism. The results support the idea that vegetable fibres, due to the high values of stiffness and strength, can replace, at least partially, man-made fibres applications in which high mechanical performance is not the primary concern but considerations on the environmental impact are to be included.
Acknowledgements The authors gratefully acknowledge Dr L. Ceccarini and Dr F. Tommaso for their expert technical assistance. This work was partially supported by the Italian Ministry of Agriculture and Forestry (PRisCA Project) and by the University of Pisa.
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References Bally, W., 1957. Ramie. Ciba Rev. 11 (123), 2–31. Batra, S.K., 1981. Other long vegetable fibres. In: Lewin, M., Pearce, E.M. (Eds.), Handbook of Fibre Science and Technology, vol. 4. Marcel Dekker, New York, pp. 727–808. Batra, S.K., Bell, C., 1975. On the efficient utilization of natural (vegetable) fibers. A Technology Adaptation Program Report. Massachusetts Institute of Technology, Cambridge, MA. Bruno, F., 1951. La ramia prodotta in Sicilia, grandemente apprezzata all’estero, ha varcato i confini della patria. Estratto dal Bollettino di Studi ed Informazioni del Giardino Coloniale di Palermo. vol. 20, pp. 1-7. Curtin, W.A., 1994. Determining fiber strength versus gage length. Polym. Compos. 15 (6), 474. Dempsey, J.N., 1975. Fibre Crops. The University Presses of Florida, Gainesville, p. 457. Di Benedetto, A.T., Lex, P.J., 1989. Evaluation of surface treatments for glass fibers in composite materials. Polym. Eng. Sci. 29 (8), 543. Di Benedetto, A.T., 1991. Measurement of the thermomechanical stability of interphases by the embedded single fibre test. Compos. Sci. Technol. 42, 103. Flemmin, M., Ziegmann, G., Roth, S., 1995. Faserverbundbauweisen — Fasen und Matrices. Springer, Berlin, p. 202. Fontanelli, D., 1998. Proprieta` meccaniche e morfologia di fibre di Spartium junceum L. (Ginestra). Thesis, Department of Chemical Engineering, University of Pisa (Italy). Fraser, W.A., Ancker, F.H., Di Benedetto, A.T., Elbirli, B., 1983. Evaluation of surface treatments for glass fibers in composite materials. Polym. Compos. 4 (4), 238. Ho¨ck, P., 1995. Versta¨rkung von Prolypropylen durch Flaschfasern auf Gleichdralldoppelschneckenextrudern. Augustinus Buchhandlung, Aachen.
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Iyengar, R.L.N., Bhujang, K.S., 1961. Heard of Ramie fibre. Ind. Farm. 11 (8), 12 – 14. Jarman, C.G., Canning, A.J., Mykoluk, S., 1978. Cultivation, extraction and processing of Ramie fibre: a review. Trop. Sci. 20 (2), 91 – 116. Kelly, A., Tyson, W.R., 1965. Tensile properties of fibre-reinforced metals: copper/tungsten and copper/molybdenum. Mech. Phys. Solids. 13, 329 – 337. Kirby, R.H., 1963. Vegetable fibres. World Crops Books, Leonard Hill, London, p. 464. Levita, G., Di Landro, L., Marchetti, A., 1997. Interface strength in composites having epoxy matrix toughened with reactive rubber. Plast. Rubber Compos. Process. Appl. 26 (6), 250. Morton, W.E., Hearle, J.W.S., 1993. Physical Properties of Textile Fibres, third ed. The Textile Institute, Manchester. Munz, P.A., Keck, D.D., 1973. A California Flora and Supplement. University of California Press, Berkeley. Oggiano, N., Angelini, L., Cappelletto, P., 1997. Pulping and paper properties of some fibre crops. Ind. Crop. Prod. 7, 59 – 67. Pignatti, S., 1982. Flora d’Italia, vol. 3. Edagricole, Bologna. Trotter, A., 1941. La Ginestra. Arte della Stampa, Roma, pp. 61 – 71. Van Den Oever, M.J.A., Bos, H.L., 1998. Critical fibre length and apparent interfacial shear strength of single flax fibre polypropylene composites. Adv. Comp. Lett. 7 (3), 81 – 85. Weibull, W., 1951. J. App. Mech. (ASME) 18, 293 Cited in Hertzberg, R.W., 1996. Deformation and Fracture Mechanics of Engineering Materials, Wiley, New York. Wollerdorfer, M., Bader, H., 1998. Influence of natural fibres on the mechanical properties of biodegradable polymers. Ind. Crop. Prod. 8, 105 – 112. Wood, I.M., Angus, J.F., 1974. A review of prospective crops for the Ord irrigation area. II Fibre Crops. CSIRO Austr. Div. Land Use Res. Tech. Paper 36, 1 – 27.
Ramie (Boehmeria ni6ea (L.) Gaud.) and Spanish Broom (Spartium junceum L.) fibres for composite materials: agronomical aspects, morphology and mechanical properties L.G. Angelini a,*, A. Lazzeri a, G. Levita b, D. Fontanelli b, C. Bozzi c a
Dipartimento di Agronomia e Gestione dell’Agroecosistema, Uni6ersity of Pisa, Via S. Michele degli Scalzi 2, 56124 Pisa, Italy b Dipartimento di Ingegneria Chimica, Uni6ersity of Pisa, Via Diotisal6i 2, 56100 Pisa, Italy c Stazione Sperimentale Cellulosa, Carta, Fibre tessili Vegetali ed Artificiali, P.zza L. da Vinci, 20133 Milano, Italy Accepted 8 October 1999
Abstract The agronomic characteristics of Ramie and Spanish Broom were investigated for seven years in the pedoclimatic conditions of Central Italy. The chemical, physical and mechanical properties of these fibres were also examined in order to evaluate the feasibility to use them in composite materials. Results demonstrate that Ramie grown in the temperate environment can be harvested three times a year. The yield of green Ramie plants per annum was almost 10 000 g m − 2. The fresh Ramie plant is composed, by weight, of 30% green leaves and 70% green stems. The yield dry fibre is almost 3% of the green stems, giving a total yield of 200 g m − 2. Spanish Broom can reach a fresh biomass yield of 4000 g m − 2 per annum, represented by 53% of long slender terete green branches which constitutes the economic products. The dry yield per annum was about 1800 g m − 2 with a dry branchlets yield of 900 g m − 2. Ramie and Spanish Broom cortical fibres are multiple elementary fibres (ultimates) arranged in bundles. In Ramie, the elementary fibres are bound by gums and pectins, while in Spanish Broom they are bound together by lignin. Both species showed a thick secondary cell wall indicating a high cellulose content. Ramie ultimate fibres are flattened and irregular in shape, while those of Spanish Broom are more regular in shape. The diameter of the ultimates varies from 10 to 25 mm in Ramie, while the Spanish Broom ultimates ranges from 5 to 10 mm; the diameter of the whole bundle is about 50 mm for both species. Ramie fibre showed a content of lignin, pentosans and extractives lower than Spanish Broom. Both fibres had a high content of cellulose which, on the base of X-ray analysis, was evaluated to be in excess of 70%. Ramie and Spanish Broom fibres had tensile strength of 950 MPa and 700 MPa, respectively. The elastic moduli were : 65 and : 20 GPa, respectively, which well compare with the modulus of E-glass fibres (70 – 90 GPa). The strength of the fibre-matrix interface was measured using the single filament fragmentation technique and an epoxy resin as the polymer matrix. Values for carbon and glass fibres in the same resin were also measured for comparison. The interface strength for the vegetable fibres was higher than that of carbon and glass, likely due to a mechanical lock mechanism. These values confirm both fibres as potential replacement for man made fibres in composite materials. © 2000 Elsevier Science B.V. All rights reserved.
* Corresponding author. Tel.: + 39-50-599111; fax: + 39-50-540633. E-mail addresses: [email protected] (L.G. Angelini), [email protected] (G. Levita) 0926-6690/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 6 6 9 0 ( 9 9 ) 0 0 0 5 9 - X
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Keywords: Boehmeria ni6ea; Spartium junceum; Spanish Broom; Ramie; Fibre; Mechanical properties; Interface strength; Composite materials
1. Introduction Application of vegetable products in various branches of industry will benefit from their intrinsic biocompatibility and biodegradability. Industries all over the European Community are looking for raw material for replacing artificial fibres in composite materials (CMs) to alleviate problems related with CMs disposal at the end of the technical life. Vegetables fibres could be a viable alternative to man-made fibres (especially asbestos and glass fibres) at least in applications in which the overall performance, evaluated in term of life cycle analysis, has to take into account the final disposal. A number of suitable plant fibres can be successfully grown in Italy, including Ramie (Boehmeria ni6ea (L.) Gaud.), a member of the Urticaceae, and Spanish Broom (Spartium junceum L.) for which promising agronomic results have been obtained (Oggiano et al., 1997). Ramie is a perennial plant native to China, Japan, and the Malay Peninsula, where it has been used as a textile fibre for centuries due to its excellent fibre (Kirby, 1963; Wood and Angus, 1974; Batra and Bell, 1975). The fibres, obtained from the outer part of the stem, are the longest and one of the strongest fine textile fibres (Dempsey, 1975; Jarman et al., 1978). Other advantages of this fibre is the resistance to bacteria, mildew, and insect attack. Its strength slightly increases when wet (Fontanelli, 1998). Spartium junceum (belonging to the Leguminosae family) is a perennial shrub, wide-spread throughout the Mediterranean area (Munz and Keck, 1973; Pignatti, 1982) where it naturally occurs in hilly soils, contributing to lower erosion and risks of nutrient leaching. This plant is somewhat adapted to alkaline and salty soils. The name Spartium is from the Greek word denoting ‘cardage’, in allusion to the use of the plant. By macerating the twigs a good fibre was obtained, which was made into thread and cords and a
coarse sort of cloth. It was cultivated on a large scale in southern Italy around the thirties; but later it fell out of favour. Recently there has been a revival of interest in Spanish Broom as a possible source of raw materials to be used in CMs for automobile applications. The agronomic characteristics of Ramie and Spanish Broom were investigated for seven years in the pedoclimatic conditions of Central Italy. In order to evaluate the feasibility to use Ramie and Spanish Broom fibres in composite materials, the morphology and chemical composition together with physical and mechanical properties of these fibres were also examined. 2. Materials and methods
2.1. Agronomic aspects A 7-year field trial was set up at the Department of Agronomy of the University of Pisa (about 43°N; 10°E; 3 m elevation) on a deep silt loam soil (sand 15.5%; silt 65.5%; clay 18.0%; organic matter 1.15%; pH 8.1; total nitrogen 1.3%; assimilable P2O5 35 mg kg − 1; exchangeable K2O 165 mg kg − 1). The soil was characterised by a water table rather superficial with a deph of 120 cm during the driest season. The soil displayed the following hydrological characteristics: field capacity, 27.3% dw, wilting point 9.4% dw. Meteorological conditions during the seven years of study are shown in Table 1. The warm-season perennial species used in this evaluation were Ramie, a genotype from Palermo Botanical garden, and Spanish Broom, one local ecotype. Spanish Broom was transplanted at the end of April 1992, while Ramie was transplanted at the end of March 1993 (Table 2). Stump sprouting for Spanish Broom and rhizome cuttings for Ramie were used. During the establishment year, plants were cut at the end of the growing season to allow the vegetative regrowth in the second year.
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The experiment was laid out in a randomised block design with four replicates. Plot size was 48 m2. In Ramie they consisted of 16 rows, each 6 m long, with an interrow and interplant spac-
147
ing of 0.5 m. For Spanish Broom a plant density of 2 plants m − 2 (0.1× 0.5 m) was adopted and each plot consisted of 8 rows, each 6 m long.
Table 1 Climatic conditions during trials in 1992–1998 and 1918–1982 mean values Years / Months
J
Rainfall (mm) 1992 45.0 1993 2.6 1994 58.2 1995 58.4 1996 2.8 1997 118.0 1998 73.6 1918–1982 89.0 Temperature min. ( °C) 1992 2.9 1993 2.2 1994 4.4 1995 0.5 1996 5.3 1997 4.5 1998 3.6 1918–1982 2.8 Temperature max ( °C) 1992 11.9 1993 12.1 1994 13.7 1995 11.1 1996 12.6 1997 13.7 1998 12.9 1918–1982 11.2
F
M
A
M
J
J
A
S
O
N
D
15.0 2.1 21.4 101.2 77.5 21.3 56.9 80.0
17.5 32.2 2.0 56.8 16.1 21.1 49.0 79.0
82.5 65.1 123.3 136.4 83.1 55.0 49.0 77.5
101.8 13.2 22.3 91.1 49.7 25.0 24.2 67.2
79.4 3.9 62.4 40.6 44.5 58.4 44.8 46.4
18.8 0.0 1.5 2.1 12.2 7.2 1.7 25.0
60.6 34.1 0.8 8.2 98.1 0.0 8.8 42.0
115.7 130.2 192.3 108.1 91.7 45.9 98.2 83.7
191.4 166.0 91.4 71.6 114.5 37.5 95.3 134.2
50.6 78.7 122.9 25.4 224.6 82.4 42.2 118.0
87.3 36.0 84.1 88.6 122.6 95.6 56.6 99.0
1.6 −0.7 3.1 3.7 2.0 7.5 2.6 2.3
4.8 2.2 6.9 3.2 4.1 4.6 3.0 5.3
7.5 7.6 7.5 8.1 8.7 5.7 3.0 8.1
12.3 11.5 12.5 11.6 11.9 12.0 10.5 11.6
14.9 16.2 15.3 13.4 15.5 16.6 14.4 15.0
17.2 16.2 18.7 18.9 17.1 16.5 16.3 17.2
18.9 19.1 20.1 17.9 17.2 19.1 17.8 17.0
15.0 15.7 16.9 14.4 13.1 15.9 14.1 14.8
12.9 9.5 11.6 11.8 10.9 11.1 11.2 11.0
8.9 6.3 9.9 5.9 9.2 8.1 4.9 6.9
4.9 6.4 5.3 5.2 4.9 4.1 1.5 3.7
13.4 13.3 12.7 13.3 11.3 10.8 15.0 12.7
15.6 13.9 17.6 13.8 14.3 17.9 15.8 15.1
17.5 17.4 17.7 18.0 17.9 17.4 15.8 18.3
24.0 22.8 22.5 21.1 21.6 23.2 23.3 22.4
24.3 27.9 26.1 23.8 27.0 25.7 26.4 26.1
27.5 28.2 31.1 29.8 28.3 28.0 28.8 29.1
29.7 31.3 31.0 28.6 28.8 30.8 29.4 29.2
25.8 25.9 26.1 23.9 23.7 27.7 24.9 26.2
20.5 18.0 21.8 23.1 21.1 21.6 21.1 21.3
17.3 14.5 18.4 15.6 16.5 16.1 14.1 15.8
12.7 14.2 12.5 12.8 12.4 13.1 10.5 12.0
Table 2 Dates of vegetative regrowth and of harvest for Ramie and Spanish Broom at Pisa in the different years of growtha Year
1992 1993 1994 1995 1996 1997 1998
Ramie
Spanish Broom
VR
C1
C2
18 01 26 01 18 01
– 21 10 12 09 08
08 24 19 05 18 15
April March February March February March
June July June July June
C3
August August September September September September
21 18 – 02 25 –
October October December November
VR
H
10 03 04 03 06 27 26
– 11 18 22 05 24 29
May April April April April March March
September October October November October September
a VR, Vegetative regrowth; H, Harvest; Ramie was transplanted on 23 March 1993 and Spanish Broom on 23 April 1992; C1, C2, C3, first, second, third cutting.
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In order to evaluate maximum crop yield, plants were maintained in optimum water supply conditions. All plots received the same amount of N, P, and K: 100 Kg ha − 1 per year. The nitrogen dose was split into two equal pre-planting and late-spring applications. During the second and the third growing seasons, plots received only 50 Kg ha − 1 of N in a single dose at the end of winter. Plots were kept weed free by hand hoeing. The establishment year was not considered and crops were tested starting from the second growing season. Screening of these species for phenological and biometrics characteristics as well as for above-ground biomass and stem production was carried out from 1993 to 1998. Cycle length was measured as the number of days from vegetative regrowth to harvest. Biometrics and productive determinations were performed on a minimal area of 6 m2 in the inner part of each plot. According to Iyengar and Bhujang (1961) Ramie harvests were usually accomplished when the lower part of the stem was turning brown. Spanish Broom was harvested only once in autumn and plants were cut at 15 cm from the soil. Aerial dry matter was separated in the different plant organs. The useful part was represented by stems without cymes and leaves for Ramie and by new branches for Spanish Broom. After harvest, all plants in the plots were cut 10 – 15 cm above ground for allowing uniform vegetative regrowth. Both species were harvested by hand. Spanish Broom new branches and Ramie dry stalks, with leaves and cymes removed, were decorticated using a small decorticating machine in order to remove the outer bark/epidermis and the bast from the woody core of the stems. Several stalks were passed through fluted crushing rollers at the entrance to the machine. Stalks were thereby constrained as their full length was decorticated in one pass through the machine. During this operation the cortex, comprising the bast and outer bark, was removed from the stem. The cortex was scraped to remove most of the outer bark and the parenchyma in the bast and, for Ramie, some of the gums and pectin. After decortication the fibres were hand brushed for make them suitable for physical and mechanical testing.
2.2. Morphological, chemical and mechanical characterisation Fibre bundles were confined in small plastic sleeves and then cross-sectioned. Scanning microscopy (SEM) was carried out on gold coated cross-sections. X-Ray patterns were recorded using Ni-filtered Cu–K radiation from a Siemens 500 D diffractometer equipped with a scintillator counter and a linear amplifier. Fourier-transform infrared spectroscopy (FTIR) spectra were obtained with a Bruker IFS 66-FTIR spectrometer on samples dispersed in KBr; 32 scans were accumulated for each sample, with a resolution of 4 cm − 1. In accordance with the TAPPI OM 250 method, the lignin content was determined as the sum of insoluble and soluble lignin, the latter being determined spectrophotometrically at 205 nm. Pentosan content was determined according to the TAPPI T 223 hm 84 and ash content according to the TAPPI 15 OS 58 method. The TAPPI 284 OM 82 method was used to assess the extractives content and the UNI 8282 method to determine the degree of cellulose polymerisation in cupriethylenediamine (CED) after delignifying the material with sodium chlorite.
2.2.1. Fibre strength The tensile properties of selected filaments were determined with an Instron 1185 (load cell 10 N) at the cross-head speed of 2 mm/min at room temperature (2092°C) and 709 5% relative humidity. Since the diameter of filaments, particularly for Ramie, was not uniform, selection of suitable samples was made with the help of a low magnification microscope; the diameter for each filament was taken at different places with the help of a precision gauge meter and the average value was used. Diameters were found to vary in the range 40–60 mm. Unlike man made fibres for composites, Ramie and Spanish Broom fibres are not circular. The diameter referred to in this work is that of the circle of the same area as the vegetable filaments.
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The elastic modulus (E) was measured by the slope of the conventional stress-strain curves taking the distance between grips as the gauge length. To measure the strength of fibres different gauge lengths were used, in the range 10 – 50 mm; a minimum of 50 filaments was taken for each gauge length to give data statistical meaning.
2.2.2. Single fibre composite (SFC) tests A silicon rubber mould was used to make dogbone shaped single fibre coupons (approximately 60 mm long, 10 mm wide, 1 mm thick). Filaments were selected as to assure their diameters were similar (:50 mm). The epoxy resin was a bifunctional bisphenol-A type with an epoxy equivalent of :195 (Epikote 828 by Shell). The hardener was p-amine-dicycloexyl-methane, used at the content of 25% by weight. Resin and hardener were intimately mixed at room temperature and freed from air bubbles by degassing at 50°C for 10 min in a vacuum oven. The mould containing the filaments was also equilibrated at 50°C prior to resin pouring. The casts were cured at 70°C for 2 h and postcured at 100°C for 3 h. The coupons were slowly strained in an Instron tensile machine (1185) at the crosshead speed of 0.02 mm/min. The fragmentation of the fibre was observed by means of a microscope attached to the machine, at magnification 40, both in natural and polarised light. The fragment lengths were measured by the help of a calibrated eyepiece. Tests were repeated with identical coupons to get at least 100 fragments for collecting a reasonable number of fragments. The interfacial strength of HS-Carbon fibres and of E-Glass fibres was measured, for comparison, using the same epoxy resin as the matrix.
3. Results and discussion
3.1. Agronomic data Ramie and Spanish Broom crops growth ceased during the winter months. In spite of its tropical origin Ramie plant resists well the winter low temperatures-such as the minimum temperatures
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below 0°C experienced in 1993 and 1995 winterthanks to the protection offered to rootstocks by a layer of dead leaves. Both species did not seem to withstand prolonged waterlogging. Soils with poor drainage and high water retention must be avoided, but at the same time the soil must have a good water retaining capacity. Spanish Broom and Ramie are perennials, and the production cycle is up to 20 years (Trotter, 1941; Bruno, 1951; Jarman et al., 1978), however this estimates is not supported by experimental evidence. In Ramie vegetative regrowth generally began at the end of February (Table 2) and stems are sent up from underground rootstocks; during the following months the stems elong rapidly with few branches. The species shows a good competition toward weeds due to a rapid interrow closure (30 days after regrowing). Stems ranged from 1.2 to 1.6 m high and about 0.8 to 1.2 cm in diameter, depending on the climatic conditions experienced in the different years. Stems were cut during the growing season and therefore the rootstocks send up new shoots, and consequently several crops can be taken from the plant each year. According to Bally (1957), the first crop after planting is very poor and unsuitable for fibre production. In each year, the time of harvest is very important; if delayed the stems become lignified and it is difficult to remove the fibre. First-crop Ramie was usually harvested before flowering (June) and the second crop during the peak of flowering (September). In each case the correct harvesting time was when the stem bases had just turned brown (Iyengar and Bhujang, 1961). Results demonstrate that Ramie grown in Central Italy under temperate environment can be harvested three times a year. The third cutting is usually performed during late autumn and therefore is often at risk for the low temperatures and rainy conditions during this period. During 1993–1998 trials, three crops were taken from the plant each year, with the exception of 1995 and 1998 when the low temperatures of November (below the long-period mean value, see Table 1) and/or rain distribution did not allow accomplishment of the last autumn harvest. The annual total yield of green Ramie plants per hectare, as 1994–1998 mean value (establishment
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Fig. 1. Total dry yield (a) and stem dry yield (b) of Ramie and Spanish Broom in the different growing seasons and as overall mean (establishment year not included). Values are the mean of four replications and bars correspond to the mean standard error.
Fig. 2. Partition of Ramie stem dry yield among first, second and third cutting in the different years of the production cycle. The establishment year (1993) was not included in the mean value.
year was not included), is about 10 000 g m − 2. The fresh Ramie plant was composed, by weight, of 70% green stems and 30% green leaves and
cymes. The yield of dry Ramie plants per hectare per annum was 1905 g m − 2 (Fig. 1a) with a dry stem yield of 1202 g m − 2 (Fig. 1b). The dry Ramie plant was composed by weight of 62% dry stems and 38% dry leaves and cymes. The final product was obtained mainly from the first and second harvest, which gave 93% of the yearly production as dry stems (Fig. 2). The crop reaches its full productivity in the year after planting; in the following years Ramie production depended on climatic conditions rather than on the age of the crop. Rainfalls during the summer months strongly affected the yield and higher production were observed during the most rainy summer seasons such as 1996 (735 mm from vegetative regrowth, VR, until third cutting, C3) and 1994 (457 mm from VR to C3). According to Iyengar and Bhujang (1961), Ramie can be harvested in the tropics three or four times a year when grown under favourable conditions; however, with supplementary irrigation and fertiliser application more than six harvests a year are possible (Dempsey, 1975). According to Dempsey (1975) the number of crops is reduced to two or three annually as the climate becomes more temperate. Although Ramie can be harvested more frequently in the tropics yields per crop are lower, annual yields per hectare in the tropic and in the temperate zones are consequently similar. The yield dry fibre was almost 3% (Dempsey, 1975; Jarman et al., 1978) of the green stems, giving a total fibre yield of about 200 g m − 2 as 1994–1998 mean value. In Spanish Broom vegetative regrowth generally began at the beginning of April (Table 2). Plants reached up to 2 m high, with long, slender, leafless or few-leaved, green, rushlike branchlets. The above ground plants were cut at the end of the growing season (Table 2). This crop performed well after the establishment year (1992), reaching in 1993 a total fresh biomass of about 4700 g m − 2 corresponding to 2200 g m − 2 dry yield (Fig. 1a). The dry yield was composed of 56% new branches, representing the economic yield. When established, Spanish Broom can reach a fresh biomass yield about of 4000 g m − 2 per annum as 1993–1998 mean value, repre-
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sented by 53% of long slender terete green branches. The dry yield per annum was about 1800 g m − 2 (Fig. 1a) with a dry branchlets yield of 900 g m − 2 (Fig. 1b). The moisture content of branchlets averaged 60%, while Ramie stems presented over 80% of moisture. The useful dry yield to total fresh biomass ratio was higher in Spanish Broom than in Ramie (Fig. 3) and this fact is important for the necessity to storage raw material with low moisture content and to maximise the marketable products. Spanish Broom was disease resistant, exception given by a fungal attack (tracheomycosis) in the fourth growing season, that decreased the yield. This species appeared also drought and heat tolerant; moreover it is a nitrogen-fixing plant and therefore could be cultivated on marginal lands due to its low input requirements.
3.2. Morphological and chemical aspects Ramie and Spanish Broom cortical fibres are multiple elementary fibres (ultimates) arranged in bundles. In Ramie, the elementary fibres are bound by gums and pectins (Jarman et al., 1978; Batra, 1981), while in Spanish Broom they are
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bound together by lignin (Trotter, 1941; Fontanelli, 1998). Bundles of ultimates are clearly visible in SEM micrograph (Fig. 4a, b): Spanish Broom contained many more ultimates than Ramie. Both species showed a thick secondary cell wall indicating a high cellulose content, a defined lumen sometimes collapsed (Morton and Hearle, 1993). The diameter of the ultimates varied from 10 to 25 mm in Ramie, while the Spanish Broom ultimates ranged from 5 to 10 mm. Ramie ultimate fibres (Fig. 4a) are flat and irregular in shape, with a thick cell wall, and taper to rounded ends, while Spanish Broom fibres (Fig. 4b) are more regular in shape, as observed also by Trotter (1941). The primary cell wall is often lignified and this aspect is responsible of the low hygroscopicity of the fibres. The longitudinal SEM view of the Ramie and Spanish Broom bundles (Fig. 5a, b), shows irregularities and defects. In Ramie the analysis revealed cross markings or dislocation characteristics of flax fibres. The influence of these defects on the fibres mechanical characteristics is discussed later. The chemical composition of the whole stem (Table 3) showed a significant difference between
Fig. 3. Stem dry yield to total fresh yield ratio for Ramie and Spanish Broom in the different growing seasons. Values are the mean of four replications and bars correspond to the mean standard error.
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tallinity reported in Table 3 is given by the ratio to the area of the (002) peak to the intensity of the amorphous background. The FTIR spectra of bundles are shown in Fig. 7a, b: both fibres showed the same absorption bands with significant differences in intensity and width at half height. The intensity of signal at 1373 cm − 1 (lignin), related to the signal intensity at 2916 cm − 1 as the internal standard, was lower in Ramie fibre than in Spanish Broom (10.8 and 17.0, respectively), in agreement with the chemical results. The signal of the non-cellulosic components at 1734 cm − 1 (hemicellulose esters and pectic substances) appeared slightly more pronounced for Ramie than for Spanish Broom; due to the prevailing pectic substances in Ramie (Jarman et al., 1978; Batra, 1981). Another very important difference between the species is the polymerisation degree that was about 3000 in Ramie and about 1700 in Spanish Broom (Table 3).
Fig. 4. Cross section micrography of Ramie (a) and Spanish Broom (b) fibres.
the two species. Ramie has an higher lignin content and a lower level of pentosans and extractives than Spanish Broom. More interesting is the difference in the compositional analysis of the two fibres (Table 3). Ramie fibre showed a content of lignin, pentosans and extractives lower than Spanish Broom. Both fibres had a high content of cellulose, no less than 70%. The highest X-ray index of crystallinity obtained for Ramie, Table 3, confirms this. In fact, while cellulose is a highly crystalline polymer, hemicelluloses and lignin are amorphous. In the diffractograms in Fig. 6, three peaks are shown, at : 22° and 14 – 18°, two of them almost superimposed, corresponding to the (002), (101) and (10-1) planes of cellulose. The index of crys-
3.2.1. Fibre stiffness and strength Both types of fibres were very rigid when loaded in tension. The load-elongation diagrams were almost linear up to fracture (Fig. 8). Occasionally, irregularities were observed in the curves which were attributed to (a) failure of some of the individual fibrils, of which filaments are made up, prior to the final cumulative rupture; and (b) internal rearrangement of fibre subunits under the action of the tensile load. The mean elastic moduli were 65 9 18 GPa and 21.5 95 GPa for Ramie and Spanish Broom, respectively. These values place the examined fibres among the stiffest vegetable fibres such as, for example, cotton, pineapple and sisal. Although the standard deviation for Ramie was greater than that for Spanish Broom, the coefficients of variability, i.e. the standard deviation to the mean value ratio, were substantially the same for both fibres. The modulus of Ramie was very high; it approached that of E-glass fibres ( :72 GPa) making this fibre very attractive for mechanical purposes. Although the stiffness of Spanish Broom is 1/3 than that of Ramie, it is yet greater than that of most rigid non-oriented polymers (1–3 GPa) making it possible to stiffen commodity plastics such as, e.g. polyolefins.
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The brittle behaviour of both fibres allowed their strength to be analysed in terms of Weibull’s statistics. Due to the intrinsic variability of properties that characterises natural products, the tensile data of vegetable fibres are rather variable. The situation, on the other hand, appears to be
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the same with man-made fibres, too. Broad distributions in tensile strength of fibres is usually attributed to flaws or defects that can be naturally exist or be introduced during handling or processing or, finally, resulting from surface ageing. It is widely accepted that these defects are
Fig. 5. Longitudinal view of Ramie (a) and Spanish Broom (b) fibres obtained by SEM.
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Table 3 Composition (%), degree of polymerisation and cristallinity of Ramie and Spanish Broom dry stems and fibres Compounds
Ramie Stem
Lignin acid soluble Lignin acid insoluble Total lignin Pentosans Extratives Ash Degree of polymerisation X-ray index of cristallinity (%)
0.9 20.6 21.5 10.5 8.1 5.1
Spanish Broom Fibre
Stem
0.2 1.3 1.5 7.0 1.0
0.7 13.7 14.4 15.3 16.2 3.6
3000 58
Fibre 0.3 6.2 6.6 9.5 3.3 1700 52
Fig. 6. X-ray diffractogram of Ramie and Spanish Broom fibres.
the main cause of premature failure of the fibre under tensile load (Curtin, 1994). Since the occurrence of flaws is random in nature, the tensile strength is to be characterised by a statistical model, the most widely used being the Weibull’s distribution function (Weibull, 1951). In the twoparameter model, the cumulative probability of failure Pn(s), i.e. the fraction of filaments having tensile strength not exceeding s, is given by:
a
Pn (s)= 1− e − l(s/g)
(1)
where a and g are the parameters that characterise the fibre, s is the stress at break and l is the gauge length. Eq. (1) can be cast as: f(Pn,l)=ln[ln(1− Pn(s)) − 1]− ln l =a ln s − a ln g
(2)
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Fig. 7. FTIR Spectra of Ramie (a) and Spanish Broom (b) fibres.
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Fig. 7. (Continued)
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157
so that a plot of f(Pn(s), l) versus ln(s) is linear; a and g are thus obtained by the slope and the intercept, respectively. Fig. 9 gives the plots of f(Pn, l) versus ln(stress at break), at 50 mm gauge lengths, for both types of filaments. Similar plots for the other gauge lengths were also drawn. Once a and g are known, the mean fibre strength, sm, at a given gauge length can be calculated by the following equation: log sm(l)=a − 1 log(l) + log(g) +log[G(1 + a)a − 1]
(3)
Where G is the complete Gamma function. A plot of log sm versus log l is again expected to be
Fig. 8. Typical stress-strain diagrams for Spanish Broom and Ramie filaments (at 10 mm gauge length).
Fig. 9. Distribution of fibre strength (according to Eq. (2)).
Fig. 10. Comparison of the influence of gauge length on strength of Spanish Broom, Ramie, carbon and glass fibres (according to Eq. (3)).
linear. The mean tensile strength at the gauge lengths required in the fragmentation analysis is experimentally inaccessible and is evaluated by extrapolation of such plots. Fig. 10 gives the plots of log(mean stress) versus log (gauge length) for Ramie and Spanish Broom fibres. The solid line in each figure represents the regression line. In both cases it is observed that the fibre strength increases with decrease of gauge length. The strength of Ramie fibres was rather high, in the range 800–1000 MPa. These values, that agree with those found by Ho¨ck (1995), are not far from the strength of E-glass; only flax is reported to be stronger (Flemmin et al., 1995). Spanish Broom fibres are somehow weaker; the strength varied, in fact, from 400 to 700 MPa. No data on this fibre are known in literature for comparison. Both elastic modulus and strength data are large enough for present fibres to be utilised as reinforcing means of low stiffness matrices such as non oriented glassy or crystalline polymers (typical strength in the range 20–80 MPa), provided the transverse properties are not of primary importance. It is interesting to compare the properties of Ramie and Spanish Broom fibres and of the two most important synthetic types, E-glass and carbon (Fig. 10). As expected, the artificial fibres are stronger than the vegetable counterparts whose
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strength, however, approaches the GPa range. It has to be noted that a comparison made on the ground of specific properties would appreciably reduce the distance between glass (specific gravity : 2.6 g/ml) and the two natural fibres (specific gravity : 1.45 g/ml). This confirms that both types of vegetable fibres can stay beside artificial reinforcements, at least in non-structural applications. It is also interesting to note that the slopes of lines in Fig. 10, are substantially the same for all fibres. This was somehow stunning since one would expect natural fibres to exhibit a much wider variability and a more pronounced effect of filament length on fracture stress. All fibres appeared to be very similar in this respect, instead. To be mentioned is however the fact that glass fibres are isotropic in nature, whereas the vegetable filaments, due to the alignment of cellulose micro fibrils along the axis, are not. Moreover, vegetable filaments may be split if high transverse stress are applied. All this would adversely affect the properties of unidirectional composites but might have positive effect on the impact behaviour. It is interesting to note that the elongation at break, oB, of both fibres was influenced by the gauge length the same way the strength was (Fig. 11). The behaviour of fibres was almost linear so that sB : EoB. Since there is no influence of gauge length on E (fluctuation of modulus values was
independent of gauge length), it follows that the gauge length dependence of strength has to be paralleled by that of elongation.
3.2.2. Interface strength The chief function of the interface in composite materials is to transmit stresses from the weak polymer matrix to the high strength fibres. The stress transfer efficiency depends on the mechanical properties of the matrix, the load bearing capacity of the fibre and the strength of the fibre-matrix interface. A good adhesion is also required to prevent environmental agents from impairing the interface. In case of lignocellulosic fibres the degradation caused by water at the interface is of primary concern because the fibres are highly hygroscopic. The assessment of interface soundness is consequently of primary importance. Out of the several methods devised for characterising the stress transmission capability across the interface, the most elegant is the single fibre composite (SFC) test. This technique, first proposed by Kelly and Tyson (1965), has been widely used to study the interfacial adhesion of synthetic fibres (Fraser et al., 1983; Di Benedetto and Lex, 1989; Di Benedetto, 1991; Curtin, 1994; Levita et al., 1997) and only recently extended to the natural fibres (Van Den Oever and Bos, 1998). When a single fibre coupon is loaded in tension, fragmentation of the fibre occurs, Fig. 12, provided the ultimate elongation of the matrix is higher than that of the fibre. Fragmentation continues until all the segments are shorter than a critical length. Beyond this point, the stress transfer is no longer high enough to cause fibre breakage. The maximum shear stress the interface can bear (t), is given by the following equations: t=
Fig. 11. Influence of gauge length on elongation at break of Spanish Broom and Ramie.
scd 3scd = 2lc 8lm
(4)
where sc is the strength of the fibre at the critical length (lc), lm is the observed mean fragment length and d is the fibre diameter. The results from SFC tests are collected in Table 4. It is interesting to note that the present data of the critical length is close to the pull-out fibre length reported by Wollerdorfer and Bader (1998) for Ramie and other vegetable fibres.
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Fig. 12. Multiple fragmentation in a single fibre coupons loaded in tension (view in polarised light). Table 4 Diameter (d), mean fragment length (lm), critical length (lc), fibre strength (sc) and interface strength (t) for Ramie, Spanish Broom, carbon and glass fibresa Fibre type
d (mm)
lm (mm)
lc (mm)
sc (MPa)
t(MPa)
Ramie Spanish Broom Carbon Glass
:50 :50 :7 :11
0.35 0.32 0.55 0.34
0.47 0.43 0.73 0.46
1480 1325 7430 4740
79 77 36 56
a
All fibres embedded in the same resin and tested in identical conditions.
It has to be pointed out that the assumptions on which the SFC method is based upon are the brittle behaviour of the fibres (requirement fully fulfilled by present fibres) and that the probability to find defects along the filaments only depends on the gauge length. The latter assumption is satisfied in artificial fibres whose diameter is fairly constant. In the case of natural fibres there can be differences in diameter among filaments. A difference in diameter brings about a change in surface extension (even at constant gauge length) that scales with the square of diameter. When comparing strength data pertaining to different gauge lengths, one has either to limit the diameter vari-
ability or to assure that diameters are evenly distributed. The method usually adopted in the Weibull’s analysis for determining sc at fibre lengths of 1 mm, or less, is the extrapolation of the log sm versus log l plots. The fragment length distribution, i.e. plots of cumulative probability versus fragment length, are obtained from the SFC tests. From such plots, lm at P(l) = 0.5 were obtained and from Eq. (4) the t values were calculated. A point of weakness of this approach is the fact that the strength of fibres are determined at gauge lengths in the 101 mm order of magnitude and estimated in the 10 − 1 mm order of magnitude. It
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is doubtful such a long extrapolation would accurately estimate sc. Although doubts can be cast on the real meaning of t values, particularly when, as in the present case, they exceed the yield strength of the matrix, they can be safely used to compare the interfacial properties of similar components (Di Benedetto and Lex, 1989; Di Benedetto, 1991; Levita et al., 1997). High t values are generally taken as indicative of good adhesion between resin and fibre. One of the reasons for the efficiency of epoxy resins as adhesives is the formation of polar groups (-OH) that strongly interact with high surface energy solids. A high concentration of -OH groups, on the other hand, characterises the surface of cellulosic materials so that a strong interface can readily develop. Besides chemistry, other effects that may contribute to the stress transfer mechanism are: (1) the irregularity of the surface; and (2) the variation of fibre diameter. The simple stress analysis embodied in equation 4 assumes the fibres to be circular. As shown in Fig. 5a,b, the actual fibres have rather rough surfaces so that the real extension of the interface is higher than computed assuming a circular cross-section. The values of t should accordingly be lowered. A further contribution to the shear strength of the interface comes from the longitudinal fluctuation of diameter because in the region in which the diameter changes the shear component of the stress acting upon the interface decreases and normal components develop. This provides an effective locking mechanism since the fibre would be held in place even in the absence of adhesion.
4. Conclusions Establishment of a stable offtake of plant fibres will provide new perspectives for arable farming and contribute to a solution of the agricultural surplus. In the selection of fibre crops that can be grown for producing fibres for industrial uses, some agronomic aspects have been taken into consideration such as adaptability, yield potential and stability. Some excellent and positive agronomic attributes are found in Ramie and Spanish Broom. In contrast to other fibre crops, Spanish
Broom proved to be a low-input crop, growing well on marginal and hilly soils where it can contribute to lower risks of erosion. Although the normal habitats of Ramie are tropical, it was possible to cultivate this species successfully under the temperate climate of central Italy. Besides, the environmentally friendly cultivation of the crops concerning the demand of fertilizers and pesticides, both are very interesting in an era of restrictions for use of agricultural chemicals, implemented in most EU countries. The Ramie and Spanish Broom fibres were brittle with load-elongation diagrams almost linear up to failure. The elastic modulus of Ramie was 65 GPa, therefore similar to E-glass. For Spanish Broom the elastic modulus was 22 GPa, about one order of magnitude greater than that of glassy polymers. The strength varied from : 900 MPa for Ramie to : 650 MPa for Spanish Broom; in both cases the strength decreased with increasing fibre length. The distribution of failure strength obeyed the Weibull’s model. The influence of filament length on strength, related with the defect population, was found to be the same for Ramie, Spanish Broom, carbon and glass fibres. The interface strength between vegetable fibres and an epoxy resin (t: 80 MPa) was higher than that of carbon and glass (t= 36–56), likely due to a mechanical lock mechanism. The results support the idea that vegetable fibres, due to the high values of stiffness and strength, can replace, at least partially, man-made fibres applications in which high mechanical performance is not the primary concern but considerations on the environmental impact are to be included.
Acknowledgements The authors gratefully acknowledge Dr L. Ceccarini and Dr F. Tommaso for their expert technical assistance. This work was partially supported by the Italian Ministry of Agriculture and Forestry (PRisCA Project) and by the University of Pisa.
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