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filler of thermoplastic composite materials
Composites: Part A 38 (2007) 369–377 www.elsevier.com/locate/compositesa
Full exploitation of Cannabis sativa as reinforcement/filler of thermoplastic composite materials P. Mutje´, A. Lo`pez, M.E. Vallejos, J.P. Lo´pez, F. Vilaseca
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Grup Lepamap, Departament d’Enginyeria Quı´mica, Universitat de Girona, Avda. Lluis santalo, s/n, Girona 17071, Spain Received 12 August 2005; received in revised form 16 March 2006; accepted 18 March 2006
Abstract Hemp strands and cane straw of hemp have been used as reinforcement and filler of polypropylene composites obtained by injection moulding. The aim of the work was to improve the tensile properties of hemp composites and make them more similar to those obtained with glass fibre composites. For this reason, maleated polypropylene (MAPP) was used as compatibility agent in hemp strand and hemp straw composites. MAPP decreases the hydrophobic nature of PP matrix and enhances the dispersion and the adhesion at interface between both constituents. The hydrophilic property of material’s surface was determined by colloidal titration. The length and diameter of the fibre reinforcement during processing were analysed and their aspect ratio calculated. The tensile results obtained showed that the mechanical properties of hemp strand composites can amount to 80% of the mechanical properties of glass fibre composites. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: B. Strength; B. Wettability; E. Injection moulding; Hemp
1. Introduction One of the current aims of the European Union is to reduce excess of foodstuff coming from traditional crops. Every year, the maintenance cost for the rural population, which is needed for the territorial equilibrium, is increasing and has turned unsustainable if the cost per capita is considered. The governmental authorities, therefore, tend to support the production of those crops able to confer additional value to the final products. The crop of hemp (Cannabis sativa), therefore, is becoming an alternative to the traditional ones for the production of animal and human foodstuff. Awareness of hemp’s potential is increasing rapidly and large ranges of hemp products are now becoming available. Hemp has recently found use for the production of specialty papers and also as reinforcement for composite materials. Hemp is one of the highest yielding and least
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Corresponding author. Tel.: +34 9724 18438; fax: +34 9724 18399. E-mail address: [email protected] (F. Vilaseca).
1359-835X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2006.03.009
intensive crops, which grows in temperate countries. It is highly self-compatible so that there is no need for crop rotation. Like other lignocellulosic fibres, hemp is biodegradable and environmentally friendly, and its Young’s modulus is one of the highest of natural fibres [1]. Currently, the market cost of hemp strands provided to industry is from 0.3 to 0.4 € kg1. The price might increase to 0.5 € kg1 if the amount of cane straw was set below 5%, or to 2 € kg1 if bleached hemp fibres were desired. Therefore, if the market cost of E glass fibre is taken into consideration (1.15 € kg1), hemp strands might be preferred for reinforcement in composites instead of ultimate hemp fibres. The use of natural fibres as reinforcement of composite materials is being one of the most important targets in materials research today. The main advantages of natural fibres are their low density, good availability and non-abrasive nature, and their high specific properties, their biodegradable characteristics and low cost of production. The leading geographic market in the use of natural fibre-based composites is Europe [2].
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The material gathered after the hemp is harvested constitutes of 33% hemp strands, 52% cane straw and 15% of waste. Fig. 1 shows a scanning electron microphotograph of a hemp stem. While the hemp strands may be used for different applications, the hemp straw use restricts to the construction of insulating materials and as bedding for animals. This is why the use of hemp straw as reinforcement/filler for composite materials has been proposed in this study. Many investigators have reported the chemical composition of natural fibres, however, there is a wide variation reported for each type of fibre. In general, the bast type fibres have higher cellulose contents and a few have very low lignin content. It is accepted that the hemp fibre is constituted by 67% of cellulose, 16% of hemicellulose and 8% of lignin [3]. The different chemical and physical characteristics of natural fibres results in a wide range of differenced in the properties and applications. Thus, while bast fibres have found to be particularly suited to composite applications, leaf fibres have found use in specialty paper requiring high strength. Annual bast fibres like hemp are known for their outstanding intrinsic properties and have the potential to compete with glass fibres as reinforcing agents in plastics. However, the use of natural fibre reinforced composites is restricted due to their inherent high moisture absorption capacity, thermal instability during processing, poor wettability and poor adhesion towards synthetic resins. This surface characteristic of natural fibres is important as many of the applications of the fibres are related to the surface features. Because of the hydrophilic surface properties of natural fibre constituents, chemical or physical modifications are usually applied to impart bonding and adhesion affinity
Fig. 1. SEM microphotograph of a hemp stem.
to polymeric matrices, and dimensional stability. The use of compatibilizers [4], surface modification techniques such as alkali treatments [5–7], acetylation [5], graft co-polymerisation [8] or the use of maleic-anhydride–polypropylene copolymer (MAH–PP) [9,10] has been reported to overcome the incompatible surface polarities between the natural fibre and polymer matrix. Several research studies have shown the effectiveness of hemp fibres reinforcing epoxy resins [11], polyester resins [6], phenolic resins [12] and thermoplastic matrices [13–15]. Among the thermoplastic matrices, polypropylene (PP) is one of the commodity thermoplastics with greater properties like low density, high vicat softening point, good surface hardness, good flex life, scratch resistance, abrasion resistance and very good electrical properties [16]. In this study, hemp strand and hemp straw PP composites were obtained by injection moulding. In order to improve the fibre/matrix adhesion, PP modified with maleic anhydride has been used as polymeric matrix. The objectives of this work were to obtain various mechanical properties of hemp strand reinforced PP composites and to determine the effect MAPP while trying to approach those mechanical properties to that obtained in glass fibre reinforced PP composites.
2. Experimental 2.1. Materials – Hemp strands comprising 25% of hemp straw supplied by Agrofibra S.L. (Puigreig, Spain) were used as cellulosic reinforcement for polypropylene composites. The initial fibre length for the hemp strands was 20–30 cm. The hemp strands were chopped in a blade mill to a nominal length of 10 mm and the hemp straw removed by floatation in a floatation cell at low consistency (1%) and low rotor speed (500 rpm) for 20 min. The isolated hemp fibre bundles were dried in a Dycometal oven at 80 °C for 24 h before used as reinforcement. – Hemp straw was also supplied by Agrofibra S.L. and used as filler of polypropylene composites. The hemp straw is a sub-product in the process to obtain hemp strands and its initial length was between 0.5 and 1 cm. The hemp straw was used directly as filler/reinforcement of polypropylene matrix after drying. – E glass fibres from Vetrotex (Chambe´ry Cedex, France) were supplied by Maben SL (Banyoles, Spain). The average length of glass fibres was around 3.3 mm and contained a sizing agent for compatibilization. – Polypropylene homopolymer Isplen PP 090 G2M from Repsol-YPF was used as thermoplastic matrix. The PP was a fluid injection-moulding grade with a melt flow index of 30 g/10 min (according to ISO 1133 standard) to facilitate the dispersion and processability of the composite material.
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– Modified MAPP Epolene G 3015 supplied by Eastman Espan˜a S.L. (Capelle aan den IJssel, The Netherlands) was used to improve the interface adhesion and compatibility fibre/matrix. – Xylene from Sigma Aldrich Chemie (Steinheim, Germany) was used as polypropylene solvent.
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40 wt% of reinforcement, although referred to the amount of PP. Tensile properties of hemp composites were compared to those obtained from polypropylene homopolymer reinforced with 20, 30 or 40 wt% of E glass fibre following the same procedure. Fig. 2 gives a flow chart of sample preparation and characterization. 2.3. Physico-mechanical properties
2.2. Preparation of composite samples Composite materials from polypropylene homopolymer, reinforced with hemp strands or hemp straw, were obtained using a heated roll mixer from IQAP LAB SL (Roda de Ter, Barcelona, Spain) at 180 ± 5 °C for 10 min. Blends comprising 20, 30 and 40 wt% of reinforcing material and 0, 2, 4, 6 or 8 wt% of MAPP with respect to the fibre content were obtained. After melt mixing, the blends were granulated in a blade mill from Agrimsa (St. Adria del Besos, Barcelona, Spain) provided with a 10 mm mesh and moulded in a injection-moulding machine (Type 35-Mateu & Soler SA, Barcelona, Spain). A steel mould according to ASTM D3641 standard was used and the injection conditions (screw speed and injection speed) were varied to produce samples with good surface appearance and to minimize reduction of the aspect ratio of the reinforcement. The samples were conditioned according to ASTM D618 standard before testing. For each composite blend, 10 specimens were tested. The effect of the coupling agent MAPP to the mechanical properties of the non-reinforced homopolypropylene was also studied. In this case, the percentage of MAPP was equivalent to those used in composites reinforced at 20, 30 and
2.3.1. Tensile properties The tensile test was carried out using an Instron 1122 universal testing machine according to ISO 527 standard. 2.3.2. Colloidal titration Polarity of hemp fibres hemp straw and polypropylene was evaluated through colloidal titration with methyl-glycol chitosan (MGCh). The cationic demand of fibre or polymer suspensions was calculated using the colloidal titration technique developed by Terayama [17] and later applied by different research teams [18,19]. Four milliliters of MGCh from Wako Chemicals GmbH (Neuss, Germany) was added to 25 ml suspension of finely powdered substrates at 1% consistency. After magnetic stirring for 1 min, the suspension was centrifuged for 15 min at 3000 rpm. Afterwards, 10 ml of the above suspension was submitted to titration with the standard potassium polyvinyl sulphate (PVSK) solution (N/400 concentration, fa activity) from Wako Chemicals GmbH (Neuss, Germany) and cationic polymer not fixed on the substrate was measured. O-Toluidine blue from Sigma Aldrich Chemie (Steinheim, Germany) was used as indicator. Polarity results are given in leq. of MGCh per gram of substrate, where polar substrates will show higher cationic polymer absorption than non-polar substrates. 2.4. Scanning electron microscopy (SEM) analysis The tensile fractured surfaces of the composites were used for morphological study using scanning electron microscopy. A Zeiss DSM 960 microscope operating at 30 kV was used and the samples were glued onto tube, gold sputtered and dried prior to study. 2.5. Optical microscopy A DMRXA optical microscope from Leica was used to analyse the fibre dimensions after processing. For fibre size analysis, the composites were dissolved in xylene at 137 °C for 3 days and fibres were dried under vacuum at 80 °C. Fibres size was analysed by means of optical microscopy and Sigma Scan Pro image analyser. 2.6. Differential scanning calorimetry (DSC)
Fig. 2. Flow chart for the general procedure and characterization methods.
Differential scanning calorimetry (DSC) was performed using a DCS 820 from Metler Toledo. Around 15 mg of sample was placed in a DSC cell. Each sample was heated
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from 30 to 200 °C at a heating rate of 10 °C min1. The melting temperature (Tm) was taken as the peak temperature of the melting endotherm and the enthalpy of melting (DHm) was calculated from the area of the melting endotherm. The degree of crystallinity for PP and PP/MAPP blends were evaluated as the ratio between the enthalpy of melting of the sample and that of the theoretical 100% crystalline polypropylene. 2.7. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) FTIR spectra of PP homopolymer, MAPP and PP/ MAPP blends were recorded using a Mattson Satellite spectrometer equipped with a MKII Golden Gate Reflection ATR System. Each spectrum was recorded by co-adding 64 scans at 4 cm1 optical resolution within the range 600–4000 cm1. The ATR-FTIR technique is capable of probing functional groups present both above and just below the top molecular layer of flat surfaces.
domains of polymeric blends in relation with the melting heat of non-reinforced PP. The values from DSC analysis collected in Table 2 shows no correlation between MAPP composition and the degree of crystallinity, although vc was higher for mixtures at 1.5 wt% of MAPP. The presence of carbonyl and ester groups in PP/MAPP blends were evidenced by FTIR spectra were peaks at 1775 and 1725 cm1 in Fig. 3 indicated the characteristic absorption bands of maleic residue in the polymeric mixture. Afterwards, composites reinforced with hemp strands, hemp straw and glass fibre tensile were obtained and their tensile properties evaluated. Table 3 shows the tensile properties for hemp strand composites. In general, the hemp strands strengthened the tensile properties of PP homopolymer, without any compatibility agent. Thus, the tensile stress increased up to 7% for composites at 20 wt% of reinforcement and up to 19% for composites at 30 or 40 wt% of reinforcement and without MAPP. This phenomenon cannot be explained by fibre-matrix affinity as the surface polarity
3. Results and discussion Blends from non-reinforced PP and MAPP were obtained following the procedure described and their thermal and mechanical properties evaluated. The percentages of MAPP were those needed to reproduce the amount of MAPP used for the production of hemp/PP composites at 20, 30 and 40 wt% of reinforcement and comprising 0, 2, 4, 6 and 8 wt% of MAPP. Table 1 shows the tensile properties of non-reinforced PP/MAPP blends. As expected, the tensile properties of non-reinforced homopolymer PP containing up to 5.33% of MAPP did not vary significantly compared to those observed for non-reinforced PP homopolymer. However, slightly higher tensile stress and lower tensile strain were observed when the amount of MAPP was between 1.5 and 2.5 wt% with respect to PP. It is worth noting that the Young’s modulus was always at least 7.5% higher for any PP/MAPP blend and was 17% higher when the amount of MAPP was 1.5 wt% with respect to PP. DSC studies of PP homopolymer and PP/MAPP mixtures were also performed. The degree of crystallinity (vc) was determined considering the melting heat of crystalline
Table 2 Temperatures of the calorimetric transitions of PP and PP/MAPP blends obtained from DSC curves: melting temperature (Tm) and degree of crystallinity (vc) MAPP (%)
Tm (°C)
vc (%) (DH 100% ¼ 204 J/g) m
0.00 0.50 1.00 1.50 2.00 2.57 3.43 4.00 5.30
162.67 163.83 163.83 163.33 164.33 163.10 164.50 163.00 162.17
53.7 58.6 59.0 61.7 57.9 57.8 59.4 61.2 52.0
Table 1 Tensile properties of PP/MAPP blends MAPP (%)
0.00 0.50 1.00 1.50 2.00 2.57 3.43 4.00 5.30 a
Stress
Strain
Young’s modulus
(MPa)
ra
(%)
r
(MPa)
r
27.5 27.2 27.6 28.8 28.4 28.7 27.9 27.6 26.7
0.50 0.42 0.32 0.40 0.37 0.79 0.19 0.23 0.15
9.3 9.0 8.8 8.4 8.8 8.6 8.8 8.9 9.1
0.21 0.27 0.20 0.25 0.23 0.23 0.16 0.20 0.20
1134 1222 1240 1334 1280 1317 1289 1220 1227
57.8 55.5 56.0 40.3 74.4 80.0 47.7 31.5 29.2
r: Standard deviation.
Fig. 3. FTIR spectra for MAPP, PP and PP with 5 wt% of MAPP.
P. Mutje´ et al. / Composites: Part A 38 (2007) 369–377 Table 3 Tensile properties of hemp strand composites Reinforcement (%)
MAPP (%)
Stress
Table 4 Polarity of raw materials (leq. of cationic polymer per gram of material)
Strain
Young’s modulus
(MPa)
ra
(%)
r
(MPa)
r
0 20 20 20 20 20
0 0 2 4 6 8
27.56 29.51 30.73 33.32 34.68 36.25
0.50 0.65 0.54 0.38 0.41 0.90
9.28 3.43 3.31 3.63 3.96 3.91
0.21 0.08 0.06 0.19 0.08 0.05
1135 2133 2052 2143 2146 2162
58 59 59 150 92 51
30 30 30 30 30
0 2 4 6 8
32.86 37.21 43.43 42.36 39.80
0.45 1.04 0.99 1.54 0.92
3.11 3.13 3.72 3.52 3.36
0.09 0.20 0.21 0.25 0.16
2618 2658 2605 2598 2627
208 81 152 84 113
40 40 40 40 40
0 2 4 6 8
32.84 44.08 48.81 48.68 46.20
0.88 1.46 1.85 1.11 2.04
2.92 3.45 3.74 3.72 3.45
0.17 0.17 0.10 0.23 0.19
3498 3491 3504 3470 3554
281 183 100 129 98
a
373
r: Standard deviation.
between hemp strands and PP matrix in absence of MAPP is different. However, other factors such as mechanical anchoring could explain the reinforcement effect found for hemp strands composites. Polarity results obtained by colloidal titration of hemp strands, hemp straw E fibreglass and PP are shown in Table 4. The results illustrate the hydrophobic behaviour of both fibreglass and PP, in contrast with the hydrophilic nature of hemp strands and cane straw of hemp. The wettability of hemp derivates in the polypropylene matrix was low due to its high surface energy, but this did not impede the enhancement of tensile strength of hemp strand reinforced composites. The wettability affected the compatibility between the matrix and the reinforcement. This was found acceptable because there are other factors involved in vegetal fibres influencing the compatibility [20], such as surface morphology and porosity.
Material
Polarity (leq. MGCh/g)
Hemp strand fibres Cane straw of hemp Polypropylene Fibreglass
114.7 ± 8.15 179.2 ± 13.50 6.5 ± 0.77 11.3 ± 0.85
The irregular surface morphology of hemp strands might improve the inter-diffusion of PP matrix and could relate to the increasing tensile stress observed for composites reinforced with hemp strands. Actually, the SEM microphotographs of Fig. 4 evidenced a surface roughness of hemp strands bundles (before extrusion process with the heated roll mixer) and individual hemp fibre (after extrusion process) that could explain the tensile stress improvement obtained when reinforcing PP homopolymer with hemp strands. In addition, the medium viscosity of the polymer could assist the good dispersion and wettability between both constituents. In the absence of MAPP, the percentages of reinforcement higher than 30 wt% did not allow fibres to be incorporated efficiently into the matrix, as evidenced by that composite at 40 wt% of hemp strands with the same tensile stress than those at 30 wt%. Hemp strands composites comprising MAPP compatibility agent gave significantly better tensile stress results, and the tensile stress obtained for hemp strand composites was 31.5%, 57.5% and 77.1% higher than that of non-reinforced PP for composites reinforced at 20, 30 and 40 wt% of hemp strands respectively. However, the effect of MAPP was different at 20 wt% than at 30 or 40 wt% of reinforcement. While at 20 wt% the tensile stress simply increased when increasing the amount of MAPP, a maximum was observed at 30 or 40 wt%. The maximum occurred at 4 wt% of MAPP addition. This fact could be explained if the percentage of MAPP was referred to the matrix content, instead to the fibre content. Thus, whereas at 20 wt% of hemp strands and 8% of MAPP with respect the fibre content represents a 2% of MAPP with respect
Fig. 4. SEM microphotographs for hemp strand bundles (a) and individual hemp fibre (b).
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to the matrix, at 30% and 40 wt% of hemp strands it represents a 3.43% and 5.33% of MAPP with respect to the matrix, respectively. As observed in the tensile properties of PP/MAPP blends, higher tensile stress were found for those composites containing amounts of MAPP between 1.5 and 2.5 wt% with to the matrix. Therefore, for composites at 20 wt% of reinforcement, the occurrence of MAPP was to increase the tensile stress when increasing the amount of MAPP. Enhancements up to 22.8% were obtained when the percentage of MAPP was 8%, compared to composites at 20 wt% of reinforcement without MAPP. For composites at 30 and 40 wt% of hemp fibre, the tensile stress improved up to 32% and 49%, respectively, when the MAPP percentage was 4%. Afterwards, higher MAPP concentrations results in slight decreasing of tensile stress, compared to those obtained at 4% of MAPP. Consequently, the presence of MAPP improved the tensile stress of hemp fibre composites if the composition of MAPP was between 1.5 and 2.5 wt% referred to PP matrix. This fact could be explained due to better dispersion and wettability of the reinforcement within the PP matrix. It is also worth noting that blends at 1.5% of MAPP (referred to PP) gave higher degree of crystallinity. The fractured surfaces from tensile test samples were evaluated by SEM analysis. Fig. 5 shows the microphotographs for composites at 40 wt% of hemp strands without (a) and with 8% of MAPP (b). Better dispersion and adhesion between the matrix and hemp strands could be observed for those composites containing MAPP. If the breaking strain is evaluated, it can be observed that tensile strain decreased notably for hemp strand/PP composites without MAPP. A strong decrease was observed for higher hemp strand content in PP composite. The strain decreased by 63% for hemp strand composites at 20 wt% and by 68% for hemp strand composites at 40 wt%. The presence of MAPP did not affect significantly the tensile strain. In general, composites containing MAPP show somewhat higher strain than those without MAPP, but the effect was not comparable due to the hemp strand
reinforcement. As for the tensile stress, the strain tended to increase with the MAPP content for composites reinforced at 20 wt%. It did show a maximum at 4% of MAPP for composites reinforced at 30 or at 40 wt%. A similar maximum of strain was observed at each percentage of reinforcement, which was 3.9% at 20 wt%, 3.72% at 30 wt% and 3.74% for composites at 40 wt%. The reinforcement with hemp strands also improved the stiffening of the PP composites. Significant improvements of Young’s modulus were observed with the hemp fibre content, which increased by 88% for composites at 20 wt% of reinforcement, by 130% for composites at 30 wt% of reinforcement and by 208% for composites at 40 wt% of reinforcement. However, the Young’s modulus was not influenced by the presence of MAPP. The better dispersion due to the MAPP only altered 1.5% the Young’s modulus value, no matter the reinforcement percentage. Generally, it is accepted that the modulus increases with the percentage of reinforcement, and that this depends on the degree of dispersion of the reinforcement in the matrix. The wettability produces slight improvements that contribute to the better dispersion of the reinforcement in the matrix [10,21]. The tensile results can also be explained considering the aspect ratio of hemp strand reinforcement. The processing caused a diminishing of fibre dimensions as shown in Table 5. The initial fibre length and diameter of hemp strands and glass fibres were similar to fibre bundles. The fibre length of both hemp strands and glass fibres were Table 5 Fibre dimensions throughout the processing
Initial Heated roll mixer Injection moulding
Length (lm)
Diameter (lm)
L/D ratio
Hemp strands
Glass fibre
Hemp strands
Glass fibre
Hemp strands
Glass fibre
10000 600 295
4000 480 330
200 41 17
1000 11 11
– 14.6 17.3
– 43.6 30
Fig. 5. SEM microphotographs for composites at 40 wt% of hemp strands without (a) and with 8% of MAPP (b).
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shortened throughout the blending and the injection processes. The diameter reduction of hemp strands and glass fibres was due to the fibre individualization caused by the shearing during the blending process. Once individualized, while the injection process decreased the hemp fibre diameter, the glass fibre diameter remained constant at 11 lm. Then, the L/D ratio for hemp or glass fibre reinforcement could be calculated. It is worth noting that the L/D ratios were kept above 10, which is considered the minimum aspect ratio for the good strength transmission for any reinforcement. Aspect ratios superior than 10 also explain the good tensile results for the hemp strand composites. Because, one of the aims of the work was the complete use of hemp plant derivates, composites filled with cane straw of hemp were prepared. Due to their poor intrinsic mechanical properties, the hemp straw should be considered as filler for polymeric matrices rather than reinforcement. In general, the incorporation of mineral fillers into polymeric matrices is done for economic reasons. It is well known that such incorporation improves the thermal behaviour, the stiffness and dimensional stability of composites. Nevertheless, the tensile strength decreases significantly. For this reason, the optimization of the stiffening/ strengthening ratio is one of the most interesting research targets [22,23]. The tensile properties for PP and PP/MAPP composites filled with hemp straw are shown in Table 6. As expected, the stiffness of the PP homopolymer improved with the incorporation of hemp straw. Young’s modulus increased by 43.6% for composites with 20 wt% of hemp straw, by 79.7% for composites comprising 30 wt% of hemp straw and by 119% for composites comprising 40 wt% of hemp straw. The use of the compatibility agent MAPP did not affect significantly the Young’s modulus value in every case. The presence of MAPP only altered the Young’s Table 6 Tensile properties of PP and PP/MAPP composites reinforced with hemp straw Reinforcement (%)
MAPP (%)
Stress (MPa)
r
Strain (%)
r
Young’s modulus (MPa)
r
0 20 20 20 20 20
0 0 2 4 6 8
27.56 26.38 27.67 27.79 28.63 27.62
0.50 0.38 0.48 0.72 0.40 0.49
9.28 4.27 4.19 4.33 4.22 4.25
0.21 0.39 0.27 0.38 0.23 0.32
1135 1630.28 1582.35 1605.67 1701.63 1629.98
58 58.93 160.57 57.56 36.07 78.28
30 30 30 30 30
0 2 4 6 8
27.31 27.99 28.93 30.44 30.04
0.40 0.23 0.52 0.49 0.69
3.57 3.19 3.38 3.75 3.93
0.16 0.24 0.21 0.22 0.29
2040.24 2140.29 2048.06 1995.87 2135.11
67.29 44.47 46.24 64.34 79.80
40 40 40 40 40
0 2 4 6 8
26.95 30.06 32.57 32.91 32.28
0.76 0.12 0.12 0.61 0.26
3.16 3.09 3.62 3.51 3.72
0.35 0.63 0.61 0.27 0.32
2485.28 2474.44 2499.05 2541.24 2426.39
180.50 112.17 156.13 108.11 133.14
375
modulus value by 7.5%. The stiffness increase was followed by a reduced breaking strain, by 54%, 61.5% and 66% for composites at 20, 30 and 40 wt% respectively. Most important was however that the incorporation of hemp straw did not decrease the tensile strength significantly. The incorporation of 20 wt% of hemp straw decreased by 4.2% the tensile strength of unfilled PP. This decrease was lower for the composites at 30 or 40 wt% of hemp straw. In every case, the use of MAPP improved the tensile strength of unfilled PP and the enhancement was by 3.8% for composites at 20 wt% of hemp straw, by 10.4% for composites at 30 wt% and by 19.4% for composites at 40 wt% of hemp straw. These are remarkable results and attract attention to the cane straw of hemp for the great value of its final products. The cane straw of hemp used as filler of PP/ MAPP matrix not only increased the elastic modulus but also the tensile strength of the final composites. In this sense, it can be said that the stiffening/strengthening ratio was improved with the incorporation of hemp straw as filler and with MAPP as compatibility agent. Finally, polypropylene composites comprising 20, 30 and 40 wt% of glass fibre were prepared and their tensile properties evaluated. In this case, the compatibility agent MAPP was avoided because of the hydrophobic nature of glass fibre surface (shown in Table 4). Table 7 show the tensile properties of glass fibre PP composites. As expected, higher tensile properties were obtained for glass fibre composites. Thus, the tensile strength and the Young’s modulus of un-reinforced PP increased by 120% and 280% respectively for composites comprising 40 wt% of glass fibre. On the other hand, it is worth noting that hemp strand composites at the same fibre percentage achieved by 80% the tensile properties provided by the glass fibre reinforcement. The hemp straw composites could only attain 50% the tensile stress and 57% the Young’s modulus of fibreglass composites. Fig. 6 shows a comparison of tensile properties between the reinforcements. Hemp strand and hemp straw composites at 40 wt% of reinforcement containing 4% of MAPP were chosen because of the high tensile properties obtained in those cases. It is important to pay attention to the intrinsic properties of the reinforcements. It is accepted that Young’s modulus of E glass fibre is about 70 GPa and its tensile strength between 2000 and 3500 MPa [24]. The Young’s modulus measured for hemp strands was 25–30 GPa and its tensile strength was in the range of
Table 7 Tensile properties of PP composites reinforced with glass fibre Reinforcement (%)
0 20 30 40
Stress
Strain
Young’s modulus
(MPa)
r
(%)
r
(MPa)
r
27.56 50.94 57.45 61.04
0.50 0.81 0.34 0.73
9.28 3.07 2.96 2.28
0.21 0.08 0.20 0.12
1135 2896.7 3257.7 4324.5
58 104.9 195.7 159.3
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Fig. 6. Comparison of tensile stress and Young’s modulus for glass fibre PP composites, hemp strand PP composites containing 4 wt% of MAPP and with hemp straw PP composites containing 4% of MAPP.
450–800 MPa. The intrinsic properties of hemp straw were not evaluated. Although high tensile properties are expected for glass fibre composites, according to the mixture’s rule, similar modulus can be obtained for hemp strand composites by increasing the volume fraction of the reinforcement. It is in this sense worth noting that hemp strands composites at 40 wt% gave superior Young’s modulus than glass fibre composites at 30 wt%. 4. Conclusions The amount of MAPP should be in the range of 1.5– 2.5 wt% with respect to the PP matrix to improve the tensile properties of PP composites comprising up to 40 wt% of reinforcement. At these percentages, composites comprising MAPP gave higher tensile stress than those without MAPP due to the enhanced properties at interface and to the better dispersion of the reinforcement. Hemp strand composites at 40 wt% gave tensile stress values 4, 16 and 20% lower than glass fibre composites at 20, 30 and 40 wt% of reinforcement respectively. This is of special interest when using hemp strands to replace glass fibre as reinforcement in composite materials. The Young’s modulus was a function of the type and percentage of reinforcement but was independent of the percentage of compatibility agent MAPP. Young’s modulus of hemp strand composites at 40 wt% was higher than Young’s modulus of glass fibre composites at 30 wt%. Hemp straw composites gave low tensile properties. However, the tensile stress of hemp straw composites showed a slight increase for composites containing MAPP which means that hemp straw could be used as filler. Moreover, the Young’s modulus of hemp straw composites was higher than that of the PP matrix and achieved 70% of the Young’s modulus of hemp strand composites. In general, it can be said that hemp plant derivates, hemp strands and hemp straw, can be used as reinforcement or filler for polymeric matrices, respectively, and give composites with remarkable tensile properties at very attractive economic costs.
References [1] Manitoba Industrial Hemp Association Market Opportunity for Industrial Hemp Fibre-based Products, December 2000. Available from: http://www.gov.mb.ca/agriculture/crops/hemp/bko07s01.html. [2] (a) Oliver Al, Joynt H. Industrial Hemp Fact Sheet, British Columbia Ministry of Agriculture and Food. Kamloops, British Columbia, Canada, April 1999, 20 p. (b) Karus M, Kaup M. Use of natural fibres in the German automotive industry. Available from: http:// mojo.calyx.net/~olsen/HEMP/IHA/jiha6209.html. [3] Young RA. Utilization of natural fibers: characterization, modification and applications. In: Leao AL, Carvalho FX, Frollini E, editors. Lignocellulosic-plastics composites. Brasil: USP; 1997. [4] Rana AK, Manda A, Mitra BC, Jacobson R, Rowell R, Banerjee AN. Short jute fiber-reinforced polypropylene composites: Effect of compatibilizer. J Appl Polym Sci 1997;69:329–38. [5] Cyras VP, Vallo C, Henny JM, Vazquez A. Effect of chemical treatment on the mechanical properties of starch-based blends reinforced with sisal fibre. J Compos Mater 2004;38(16):1387–99. [6] Bledzki AK, Fink H-P, Specht K. Unidirectional hemp and flax EPand PP-composites: influence of defined fiber treatments. J Appl Polym Sci 2004;93:2150–6. [7] Aziz SH, Ansell MP. The effect of alkalisation and fibres alignment on the mechanical and thermal properties of kenaf and hemp bast fibre composites: Part 1—Polyester resin matrix. Compos Sci Technol 2003;64:1219–30. [8] Mishra S, Mishra M, Tripathy SS, Nayak SK, Mohanty AK. Graft copolymerization of acrylonitrile on chemically modified sisal fibers. Macromol Mater Eng 2001;286(2):107–13. [9] Mohanty S, Nayak SK, Verma SK, Tripathy SS. Effect of MAPP as a coupling agent on the performance of jute-PP composites. J Reinf Plast Comp 2004;23:625–37. [10] Karmaker AC, Youngquist JA. Injection moulding of polypropylene reinforced with short jute fibers. J Appl Polym Sci 1996;62:1147–51. [11] Hautala M, Pasila A, Pirila¨ J. Use of hemp and flax in composite manufacture: a search for new production methods. Compos Pt—A Appl Sci Manuf 2004;35:11–6. [12] Ozturk S. The effect of fibre content on the mechanical properties of hemp and basalt fibre reinforced phenol formaldehyde composites. J Mater Sci 2005;40:4585–92. [13] Wielage B, Lampke T, Utschick H, Soergel F. Processing of naturalfibre reinforced polymers and the resulting dynamic-mechanical properties. J Mater Process Technol 2003;139:140–6. [14] Keller A. Compounding and mechanical properties of biodegradable hemp fibre composites. Compos Sci Technol 2003;63:1307–16. [15] Vilaseca F, Lopez A, Llauro X, Pelach MA, Mutje P. Hemp strands as reinforcement of polystyrene composites. Chem Eng Res Des 2004;82(A11):1425–31.
P. Mutje´ et al. / Composites: Part A 38 (2007) 369–377 [16] Brydson JA. Plastic materials. 3rd ed. London: Newnes Butterworths; 1975 [chapter 11]. [17] Terayama H. Method of colloid titration (a new titration between ions). J Polym Sci 1953;8(2):243–53. [18] Rouger J, Mutje´ P. Correlation between the cellulose fibres beating and the fixation of a soluble cationic polymer. Brit Polym J 1984;16(2):83. [19] Carrasco F, Mutje´ P, Pe`lach MA. Control of retention in papermaking by colloid titration and zeta potential techniques. Wood Sci Technol 1998;32:145. [20] Felix J. Enhancing interactions between cellulose fibres and synthetic polymers. PhD Thesis, Chalmers University of Technology, Goteborg, 1993.
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[21] Colom X, Carrasco F, Page`s P, Can˜avate J. Effects of different treatments on the interface of HDPE/lignocellulosic fibre composites. Compos Sci Technol 2003;30:145. [22] Thio YS, Aragon AS, Cohen RE. Role of interfacial adhesion strength on toughening polypropylene with rigid particles. Polymer 2004;45:3139. [23] Wu G, Wen B, Hou S. Preparation and structural study of polypropylene–talc gradient materials. Polym Int 2004;53(6): 749. [24] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Prog Polym Sci 1999;24:221–74.
Full exploitation of Cannabis sativa as reinforcement/filler of thermoplastic composite materials P. Mutje´, A. Lo`pez, M.E. Vallejos, J.P. Lo´pez, F. Vilaseca
*
Grup Lepamap, Departament d’Enginyeria Quı´mica, Universitat de Girona, Avda. Lluis santalo, s/n, Girona 17071, Spain Received 12 August 2005; received in revised form 16 March 2006; accepted 18 March 2006
Abstract Hemp strands and cane straw of hemp have been used as reinforcement and filler of polypropylene composites obtained by injection moulding. The aim of the work was to improve the tensile properties of hemp composites and make them more similar to those obtained with glass fibre composites. For this reason, maleated polypropylene (MAPP) was used as compatibility agent in hemp strand and hemp straw composites. MAPP decreases the hydrophobic nature of PP matrix and enhances the dispersion and the adhesion at interface between both constituents. The hydrophilic property of material’s surface was determined by colloidal titration. The length and diameter of the fibre reinforcement during processing were analysed and their aspect ratio calculated. The tensile results obtained showed that the mechanical properties of hemp strand composites can amount to 80% of the mechanical properties of glass fibre composites. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: B. Strength; B. Wettability; E. Injection moulding; Hemp
1. Introduction One of the current aims of the European Union is to reduce excess of foodstuff coming from traditional crops. Every year, the maintenance cost for the rural population, which is needed for the territorial equilibrium, is increasing and has turned unsustainable if the cost per capita is considered. The governmental authorities, therefore, tend to support the production of those crops able to confer additional value to the final products. The crop of hemp (Cannabis sativa), therefore, is becoming an alternative to the traditional ones for the production of animal and human foodstuff. Awareness of hemp’s potential is increasing rapidly and large ranges of hemp products are now becoming available. Hemp has recently found use for the production of specialty papers and also as reinforcement for composite materials. Hemp is one of the highest yielding and least
*
Corresponding author. Tel.: +34 9724 18438; fax: +34 9724 18399. E-mail address: [email protected] (F. Vilaseca).
1359-835X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2006.03.009
intensive crops, which grows in temperate countries. It is highly self-compatible so that there is no need for crop rotation. Like other lignocellulosic fibres, hemp is biodegradable and environmentally friendly, and its Young’s modulus is one of the highest of natural fibres [1]. Currently, the market cost of hemp strands provided to industry is from 0.3 to 0.4 € kg1. The price might increase to 0.5 € kg1 if the amount of cane straw was set below 5%, or to 2 € kg1 if bleached hemp fibres were desired. Therefore, if the market cost of E glass fibre is taken into consideration (1.15 € kg1), hemp strands might be preferred for reinforcement in composites instead of ultimate hemp fibres. The use of natural fibres as reinforcement of composite materials is being one of the most important targets in materials research today. The main advantages of natural fibres are their low density, good availability and non-abrasive nature, and their high specific properties, their biodegradable characteristics and low cost of production. The leading geographic market in the use of natural fibre-based composites is Europe [2].
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The material gathered after the hemp is harvested constitutes of 33% hemp strands, 52% cane straw and 15% of waste. Fig. 1 shows a scanning electron microphotograph of a hemp stem. While the hemp strands may be used for different applications, the hemp straw use restricts to the construction of insulating materials and as bedding for animals. This is why the use of hemp straw as reinforcement/filler for composite materials has been proposed in this study. Many investigators have reported the chemical composition of natural fibres, however, there is a wide variation reported for each type of fibre. In general, the bast type fibres have higher cellulose contents and a few have very low lignin content. It is accepted that the hemp fibre is constituted by 67% of cellulose, 16% of hemicellulose and 8% of lignin [3]. The different chemical and physical characteristics of natural fibres results in a wide range of differenced in the properties and applications. Thus, while bast fibres have found to be particularly suited to composite applications, leaf fibres have found use in specialty paper requiring high strength. Annual bast fibres like hemp are known for their outstanding intrinsic properties and have the potential to compete with glass fibres as reinforcing agents in plastics. However, the use of natural fibre reinforced composites is restricted due to their inherent high moisture absorption capacity, thermal instability during processing, poor wettability and poor adhesion towards synthetic resins. This surface characteristic of natural fibres is important as many of the applications of the fibres are related to the surface features. Because of the hydrophilic surface properties of natural fibre constituents, chemical or physical modifications are usually applied to impart bonding and adhesion affinity
Fig. 1. SEM microphotograph of a hemp stem.
to polymeric matrices, and dimensional stability. The use of compatibilizers [4], surface modification techniques such as alkali treatments [5–7], acetylation [5], graft co-polymerisation [8] or the use of maleic-anhydride–polypropylene copolymer (MAH–PP) [9,10] has been reported to overcome the incompatible surface polarities between the natural fibre and polymer matrix. Several research studies have shown the effectiveness of hemp fibres reinforcing epoxy resins [11], polyester resins [6], phenolic resins [12] and thermoplastic matrices [13–15]. Among the thermoplastic matrices, polypropylene (PP) is one of the commodity thermoplastics with greater properties like low density, high vicat softening point, good surface hardness, good flex life, scratch resistance, abrasion resistance and very good electrical properties [16]. In this study, hemp strand and hemp straw PP composites were obtained by injection moulding. In order to improve the fibre/matrix adhesion, PP modified with maleic anhydride has been used as polymeric matrix. The objectives of this work were to obtain various mechanical properties of hemp strand reinforced PP composites and to determine the effect MAPP while trying to approach those mechanical properties to that obtained in glass fibre reinforced PP composites.
2. Experimental 2.1. Materials – Hemp strands comprising 25% of hemp straw supplied by Agrofibra S.L. (Puigreig, Spain) were used as cellulosic reinforcement for polypropylene composites. The initial fibre length for the hemp strands was 20–30 cm. The hemp strands were chopped in a blade mill to a nominal length of 10 mm and the hemp straw removed by floatation in a floatation cell at low consistency (1%) and low rotor speed (500 rpm) for 20 min. The isolated hemp fibre bundles were dried in a Dycometal oven at 80 °C for 24 h before used as reinforcement. – Hemp straw was also supplied by Agrofibra S.L. and used as filler of polypropylene composites. The hemp straw is a sub-product in the process to obtain hemp strands and its initial length was between 0.5 and 1 cm. The hemp straw was used directly as filler/reinforcement of polypropylene matrix after drying. – E glass fibres from Vetrotex (Chambe´ry Cedex, France) were supplied by Maben SL (Banyoles, Spain). The average length of glass fibres was around 3.3 mm and contained a sizing agent for compatibilization. – Polypropylene homopolymer Isplen PP 090 G2M from Repsol-YPF was used as thermoplastic matrix. The PP was a fluid injection-moulding grade with a melt flow index of 30 g/10 min (according to ISO 1133 standard) to facilitate the dispersion and processability of the composite material.
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– Modified MAPP Epolene G 3015 supplied by Eastman Espan˜a S.L. (Capelle aan den IJssel, The Netherlands) was used to improve the interface adhesion and compatibility fibre/matrix. – Xylene from Sigma Aldrich Chemie (Steinheim, Germany) was used as polypropylene solvent.
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40 wt% of reinforcement, although referred to the amount of PP. Tensile properties of hemp composites were compared to those obtained from polypropylene homopolymer reinforced with 20, 30 or 40 wt% of E glass fibre following the same procedure. Fig. 2 gives a flow chart of sample preparation and characterization. 2.3. Physico-mechanical properties
2.2. Preparation of composite samples Composite materials from polypropylene homopolymer, reinforced with hemp strands or hemp straw, were obtained using a heated roll mixer from IQAP LAB SL (Roda de Ter, Barcelona, Spain) at 180 ± 5 °C for 10 min. Blends comprising 20, 30 and 40 wt% of reinforcing material and 0, 2, 4, 6 or 8 wt% of MAPP with respect to the fibre content were obtained. After melt mixing, the blends were granulated in a blade mill from Agrimsa (St. Adria del Besos, Barcelona, Spain) provided with a 10 mm mesh and moulded in a injection-moulding machine (Type 35-Mateu & Soler SA, Barcelona, Spain). A steel mould according to ASTM D3641 standard was used and the injection conditions (screw speed and injection speed) were varied to produce samples with good surface appearance and to minimize reduction of the aspect ratio of the reinforcement. The samples were conditioned according to ASTM D618 standard before testing. For each composite blend, 10 specimens were tested. The effect of the coupling agent MAPP to the mechanical properties of the non-reinforced homopolypropylene was also studied. In this case, the percentage of MAPP was equivalent to those used in composites reinforced at 20, 30 and
2.3.1. Tensile properties The tensile test was carried out using an Instron 1122 universal testing machine according to ISO 527 standard. 2.3.2. Colloidal titration Polarity of hemp fibres hemp straw and polypropylene was evaluated through colloidal titration with methyl-glycol chitosan (MGCh). The cationic demand of fibre or polymer suspensions was calculated using the colloidal titration technique developed by Terayama [17] and later applied by different research teams [18,19]. Four milliliters of MGCh from Wako Chemicals GmbH (Neuss, Germany) was added to 25 ml suspension of finely powdered substrates at 1% consistency. After magnetic stirring for 1 min, the suspension was centrifuged for 15 min at 3000 rpm. Afterwards, 10 ml of the above suspension was submitted to titration with the standard potassium polyvinyl sulphate (PVSK) solution (N/400 concentration, fa activity) from Wako Chemicals GmbH (Neuss, Germany) and cationic polymer not fixed on the substrate was measured. O-Toluidine blue from Sigma Aldrich Chemie (Steinheim, Germany) was used as indicator. Polarity results are given in leq. of MGCh per gram of substrate, where polar substrates will show higher cationic polymer absorption than non-polar substrates. 2.4. Scanning electron microscopy (SEM) analysis The tensile fractured surfaces of the composites were used for morphological study using scanning electron microscopy. A Zeiss DSM 960 microscope operating at 30 kV was used and the samples were glued onto tube, gold sputtered and dried prior to study. 2.5. Optical microscopy A DMRXA optical microscope from Leica was used to analyse the fibre dimensions after processing. For fibre size analysis, the composites were dissolved in xylene at 137 °C for 3 days and fibres were dried under vacuum at 80 °C. Fibres size was analysed by means of optical microscopy and Sigma Scan Pro image analyser. 2.6. Differential scanning calorimetry (DSC)
Fig. 2. Flow chart for the general procedure and characterization methods.
Differential scanning calorimetry (DSC) was performed using a DCS 820 from Metler Toledo. Around 15 mg of sample was placed in a DSC cell. Each sample was heated
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from 30 to 200 °C at a heating rate of 10 °C min1. The melting temperature (Tm) was taken as the peak temperature of the melting endotherm and the enthalpy of melting (DHm) was calculated from the area of the melting endotherm. The degree of crystallinity for PP and PP/MAPP blends were evaluated as the ratio between the enthalpy of melting of the sample and that of the theoretical 100% crystalline polypropylene. 2.7. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) FTIR spectra of PP homopolymer, MAPP and PP/ MAPP blends were recorded using a Mattson Satellite spectrometer equipped with a MKII Golden Gate Reflection ATR System. Each spectrum was recorded by co-adding 64 scans at 4 cm1 optical resolution within the range 600–4000 cm1. The ATR-FTIR technique is capable of probing functional groups present both above and just below the top molecular layer of flat surfaces.
domains of polymeric blends in relation with the melting heat of non-reinforced PP. The values from DSC analysis collected in Table 2 shows no correlation between MAPP composition and the degree of crystallinity, although vc was higher for mixtures at 1.5 wt% of MAPP. The presence of carbonyl and ester groups in PP/MAPP blends were evidenced by FTIR spectra were peaks at 1775 and 1725 cm1 in Fig. 3 indicated the characteristic absorption bands of maleic residue in the polymeric mixture. Afterwards, composites reinforced with hemp strands, hemp straw and glass fibre tensile were obtained and their tensile properties evaluated. Table 3 shows the tensile properties for hemp strand composites. In general, the hemp strands strengthened the tensile properties of PP homopolymer, without any compatibility agent. Thus, the tensile stress increased up to 7% for composites at 20 wt% of reinforcement and up to 19% for composites at 30 or 40 wt% of reinforcement and without MAPP. This phenomenon cannot be explained by fibre-matrix affinity as the surface polarity
3. Results and discussion Blends from non-reinforced PP and MAPP were obtained following the procedure described and their thermal and mechanical properties evaluated. The percentages of MAPP were those needed to reproduce the amount of MAPP used for the production of hemp/PP composites at 20, 30 and 40 wt% of reinforcement and comprising 0, 2, 4, 6 and 8 wt% of MAPP. Table 1 shows the tensile properties of non-reinforced PP/MAPP blends. As expected, the tensile properties of non-reinforced homopolymer PP containing up to 5.33% of MAPP did not vary significantly compared to those observed for non-reinforced PP homopolymer. However, slightly higher tensile stress and lower tensile strain were observed when the amount of MAPP was between 1.5 and 2.5 wt% with respect to PP. It is worth noting that the Young’s modulus was always at least 7.5% higher for any PP/MAPP blend and was 17% higher when the amount of MAPP was 1.5 wt% with respect to PP. DSC studies of PP homopolymer and PP/MAPP mixtures were also performed. The degree of crystallinity (vc) was determined considering the melting heat of crystalline
Table 2 Temperatures of the calorimetric transitions of PP and PP/MAPP blends obtained from DSC curves: melting temperature (Tm) and degree of crystallinity (vc) MAPP (%)
Tm (°C)
vc (%) (DH 100% ¼ 204 J/g) m
0.00 0.50 1.00 1.50 2.00 2.57 3.43 4.00 5.30
162.67 163.83 163.83 163.33 164.33 163.10 164.50 163.00 162.17
53.7 58.6 59.0 61.7 57.9 57.8 59.4 61.2 52.0
Table 1 Tensile properties of PP/MAPP blends MAPP (%)
0.00 0.50 1.00 1.50 2.00 2.57 3.43 4.00 5.30 a
Stress
Strain
Young’s modulus
(MPa)
ra
(%)
r
(MPa)
r
27.5 27.2 27.6 28.8 28.4 28.7 27.9 27.6 26.7
0.50 0.42 0.32 0.40 0.37 0.79 0.19 0.23 0.15
9.3 9.0 8.8 8.4 8.8 8.6 8.8 8.9 9.1
0.21 0.27 0.20 0.25 0.23 0.23 0.16 0.20 0.20
1134 1222 1240 1334 1280 1317 1289 1220 1227
57.8 55.5 56.0 40.3 74.4 80.0 47.7 31.5 29.2
r: Standard deviation.
Fig. 3. FTIR spectra for MAPP, PP and PP with 5 wt% of MAPP.
P. Mutje´ et al. / Composites: Part A 38 (2007) 369–377 Table 3 Tensile properties of hemp strand composites Reinforcement (%)
MAPP (%)
Stress
Table 4 Polarity of raw materials (leq. of cationic polymer per gram of material)
Strain
Young’s modulus
(MPa)
ra
(%)
r
(MPa)
r
0 20 20 20 20 20
0 0 2 4 6 8
27.56 29.51 30.73 33.32 34.68 36.25
0.50 0.65 0.54 0.38 0.41 0.90
9.28 3.43 3.31 3.63 3.96 3.91
0.21 0.08 0.06 0.19 0.08 0.05
1135 2133 2052 2143 2146 2162
58 59 59 150 92 51
30 30 30 30 30
0 2 4 6 8
32.86 37.21 43.43 42.36 39.80
0.45 1.04 0.99 1.54 0.92
3.11 3.13 3.72 3.52 3.36
0.09 0.20 0.21 0.25 0.16
2618 2658 2605 2598 2627
208 81 152 84 113
40 40 40 40 40
0 2 4 6 8
32.84 44.08 48.81 48.68 46.20
0.88 1.46 1.85 1.11 2.04
2.92 3.45 3.74 3.72 3.45
0.17 0.17 0.10 0.23 0.19
3498 3491 3504 3470 3554
281 183 100 129 98
a
373
r: Standard deviation.
between hemp strands and PP matrix in absence of MAPP is different. However, other factors such as mechanical anchoring could explain the reinforcement effect found for hemp strands composites. Polarity results obtained by colloidal titration of hemp strands, hemp straw E fibreglass and PP are shown in Table 4. The results illustrate the hydrophobic behaviour of both fibreglass and PP, in contrast with the hydrophilic nature of hemp strands and cane straw of hemp. The wettability of hemp derivates in the polypropylene matrix was low due to its high surface energy, but this did not impede the enhancement of tensile strength of hemp strand reinforced composites. The wettability affected the compatibility between the matrix and the reinforcement. This was found acceptable because there are other factors involved in vegetal fibres influencing the compatibility [20], such as surface morphology and porosity.
Material
Polarity (leq. MGCh/g)
Hemp strand fibres Cane straw of hemp Polypropylene Fibreglass
114.7 ± 8.15 179.2 ± 13.50 6.5 ± 0.77 11.3 ± 0.85
The irregular surface morphology of hemp strands might improve the inter-diffusion of PP matrix and could relate to the increasing tensile stress observed for composites reinforced with hemp strands. Actually, the SEM microphotographs of Fig. 4 evidenced a surface roughness of hemp strands bundles (before extrusion process with the heated roll mixer) and individual hemp fibre (after extrusion process) that could explain the tensile stress improvement obtained when reinforcing PP homopolymer with hemp strands. In addition, the medium viscosity of the polymer could assist the good dispersion and wettability between both constituents. In the absence of MAPP, the percentages of reinforcement higher than 30 wt% did not allow fibres to be incorporated efficiently into the matrix, as evidenced by that composite at 40 wt% of hemp strands with the same tensile stress than those at 30 wt%. Hemp strands composites comprising MAPP compatibility agent gave significantly better tensile stress results, and the tensile stress obtained for hemp strand composites was 31.5%, 57.5% and 77.1% higher than that of non-reinforced PP for composites reinforced at 20, 30 and 40 wt% of hemp strands respectively. However, the effect of MAPP was different at 20 wt% than at 30 or 40 wt% of reinforcement. While at 20 wt% the tensile stress simply increased when increasing the amount of MAPP, a maximum was observed at 30 or 40 wt%. The maximum occurred at 4 wt% of MAPP addition. This fact could be explained if the percentage of MAPP was referred to the matrix content, instead to the fibre content. Thus, whereas at 20 wt% of hemp strands and 8% of MAPP with respect the fibre content represents a 2% of MAPP with respect
Fig. 4. SEM microphotographs for hemp strand bundles (a) and individual hemp fibre (b).
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to the matrix, at 30% and 40 wt% of hemp strands it represents a 3.43% and 5.33% of MAPP with respect to the matrix, respectively. As observed in the tensile properties of PP/MAPP blends, higher tensile stress were found for those composites containing amounts of MAPP between 1.5 and 2.5 wt% with to the matrix. Therefore, for composites at 20 wt% of reinforcement, the occurrence of MAPP was to increase the tensile stress when increasing the amount of MAPP. Enhancements up to 22.8% were obtained when the percentage of MAPP was 8%, compared to composites at 20 wt% of reinforcement without MAPP. For composites at 30 and 40 wt% of hemp fibre, the tensile stress improved up to 32% and 49%, respectively, when the MAPP percentage was 4%. Afterwards, higher MAPP concentrations results in slight decreasing of tensile stress, compared to those obtained at 4% of MAPP. Consequently, the presence of MAPP improved the tensile stress of hemp fibre composites if the composition of MAPP was between 1.5 and 2.5 wt% referred to PP matrix. This fact could be explained due to better dispersion and wettability of the reinforcement within the PP matrix. It is also worth noting that blends at 1.5% of MAPP (referred to PP) gave higher degree of crystallinity. The fractured surfaces from tensile test samples were evaluated by SEM analysis. Fig. 5 shows the microphotographs for composites at 40 wt% of hemp strands without (a) and with 8% of MAPP (b). Better dispersion and adhesion between the matrix and hemp strands could be observed for those composites containing MAPP. If the breaking strain is evaluated, it can be observed that tensile strain decreased notably for hemp strand/PP composites without MAPP. A strong decrease was observed for higher hemp strand content in PP composite. The strain decreased by 63% for hemp strand composites at 20 wt% and by 68% for hemp strand composites at 40 wt%. The presence of MAPP did not affect significantly the tensile strain. In general, composites containing MAPP show somewhat higher strain than those without MAPP, but the effect was not comparable due to the hemp strand
reinforcement. As for the tensile stress, the strain tended to increase with the MAPP content for composites reinforced at 20 wt%. It did show a maximum at 4% of MAPP for composites reinforced at 30 or at 40 wt%. A similar maximum of strain was observed at each percentage of reinforcement, which was 3.9% at 20 wt%, 3.72% at 30 wt% and 3.74% for composites at 40 wt%. The reinforcement with hemp strands also improved the stiffening of the PP composites. Significant improvements of Young’s modulus were observed with the hemp fibre content, which increased by 88% for composites at 20 wt% of reinforcement, by 130% for composites at 30 wt% of reinforcement and by 208% for composites at 40 wt% of reinforcement. However, the Young’s modulus was not influenced by the presence of MAPP. The better dispersion due to the MAPP only altered 1.5% the Young’s modulus value, no matter the reinforcement percentage. Generally, it is accepted that the modulus increases with the percentage of reinforcement, and that this depends on the degree of dispersion of the reinforcement in the matrix. The wettability produces slight improvements that contribute to the better dispersion of the reinforcement in the matrix [10,21]. The tensile results can also be explained considering the aspect ratio of hemp strand reinforcement. The processing caused a diminishing of fibre dimensions as shown in Table 5. The initial fibre length and diameter of hemp strands and glass fibres were similar to fibre bundles. The fibre length of both hemp strands and glass fibres were Table 5 Fibre dimensions throughout the processing
Initial Heated roll mixer Injection moulding
Length (lm)
Diameter (lm)
L/D ratio
Hemp strands
Glass fibre
Hemp strands
Glass fibre
Hemp strands
Glass fibre
10000 600 295
4000 480 330
200 41 17
1000 11 11
– 14.6 17.3
– 43.6 30
Fig. 5. SEM microphotographs for composites at 40 wt% of hemp strands without (a) and with 8% of MAPP (b).
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shortened throughout the blending and the injection processes. The diameter reduction of hemp strands and glass fibres was due to the fibre individualization caused by the shearing during the blending process. Once individualized, while the injection process decreased the hemp fibre diameter, the glass fibre diameter remained constant at 11 lm. Then, the L/D ratio for hemp or glass fibre reinforcement could be calculated. It is worth noting that the L/D ratios were kept above 10, which is considered the minimum aspect ratio for the good strength transmission for any reinforcement. Aspect ratios superior than 10 also explain the good tensile results for the hemp strand composites. Because, one of the aims of the work was the complete use of hemp plant derivates, composites filled with cane straw of hemp were prepared. Due to their poor intrinsic mechanical properties, the hemp straw should be considered as filler for polymeric matrices rather than reinforcement. In general, the incorporation of mineral fillers into polymeric matrices is done for economic reasons. It is well known that such incorporation improves the thermal behaviour, the stiffness and dimensional stability of composites. Nevertheless, the tensile strength decreases significantly. For this reason, the optimization of the stiffening/ strengthening ratio is one of the most interesting research targets [22,23]. The tensile properties for PP and PP/MAPP composites filled with hemp straw are shown in Table 6. As expected, the stiffness of the PP homopolymer improved with the incorporation of hemp straw. Young’s modulus increased by 43.6% for composites with 20 wt% of hemp straw, by 79.7% for composites comprising 30 wt% of hemp straw and by 119% for composites comprising 40 wt% of hemp straw. The use of the compatibility agent MAPP did not affect significantly the Young’s modulus value in every case. The presence of MAPP only altered the Young’s Table 6 Tensile properties of PP and PP/MAPP composites reinforced with hemp straw Reinforcement (%)
MAPP (%)
Stress (MPa)
r
Strain (%)
r
Young’s modulus (MPa)
r
0 20 20 20 20 20
0 0 2 4 6 8
27.56 26.38 27.67 27.79 28.63 27.62
0.50 0.38 0.48 0.72 0.40 0.49
9.28 4.27 4.19 4.33 4.22 4.25
0.21 0.39 0.27 0.38 0.23 0.32
1135 1630.28 1582.35 1605.67 1701.63 1629.98
58 58.93 160.57 57.56 36.07 78.28
30 30 30 30 30
0 2 4 6 8
27.31 27.99 28.93 30.44 30.04
0.40 0.23 0.52 0.49 0.69
3.57 3.19 3.38 3.75 3.93
0.16 0.24 0.21 0.22 0.29
2040.24 2140.29 2048.06 1995.87 2135.11
67.29 44.47 46.24 64.34 79.80
40 40 40 40 40
0 2 4 6 8
26.95 30.06 32.57 32.91 32.28
0.76 0.12 0.12 0.61 0.26
3.16 3.09 3.62 3.51 3.72
0.35 0.63 0.61 0.27 0.32
2485.28 2474.44 2499.05 2541.24 2426.39
180.50 112.17 156.13 108.11 133.14
375
modulus value by 7.5%. The stiffness increase was followed by a reduced breaking strain, by 54%, 61.5% and 66% for composites at 20, 30 and 40 wt% respectively. Most important was however that the incorporation of hemp straw did not decrease the tensile strength significantly. The incorporation of 20 wt% of hemp straw decreased by 4.2% the tensile strength of unfilled PP. This decrease was lower for the composites at 30 or 40 wt% of hemp straw. In every case, the use of MAPP improved the tensile strength of unfilled PP and the enhancement was by 3.8% for composites at 20 wt% of hemp straw, by 10.4% for composites at 30 wt% and by 19.4% for composites at 40 wt% of hemp straw. These are remarkable results and attract attention to the cane straw of hemp for the great value of its final products. The cane straw of hemp used as filler of PP/ MAPP matrix not only increased the elastic modulus but also the tensile strength of the final composites. In this sense, it can be said that the stiffening/strengthening ratio was improved with the incorporation of hemp straw as filler and with MAPP as compatibility agent. Finally, polypropylene composites comprising 20, 30 and 40 wt% of glass fibre were prepared and their tensile properties evaluated. In this case, the compatibility agent MAPP was avoided because of the hydrophobic nature of glass fibre surface (shown in Table 4). Table 7 show the tensile properties of glass fibre PP composites. As expected, higher tensile properties were obtained for glass fibre composites. Thus, the tensile strength and the Young’s modulus of un-reinforced PP increased by 120% and 280% respectively for composites comprising 40 wt% of glass fibre. On the other hand, it is worth noting that hemp strand composites at the same fibre percentage achieved by 80% the tensile properties provided by the glass fibre reinforcement. The hemp straw composites could only attain 50% the tensile stress and 57% the Young’s modulus of fibreglass composites. Fig. 6 shows a comparison of tensile properties between the reinforcements. Hemp strand and hemp straw composites at 40 wt% of reinforcement containing 4% of MAPP were chosen because of the high tensile properties obtained in those cases. It is important to pay attention to the intrinsic properties of the reinforcements. It is accepted that Young’s modulus of E glass fibre is about 70 GPa and its tensile strength between 2000 and 3500 MPa [24]. The Young’s modulus measured for hemp strands was 25–30 GPa and its tensile strength was in the range of
Table 7 Tensile properties of PP composites reinforced with glass fibre Reinforcement (%)
0 20 30 40
Stress
Strain
Young’s modulus
(MPa)
r
(%)
r
(MPa)
r
27.56 50.94 57.45 61.04
0.50 0.81 0.34 0.73
9.28 3.07 2.96 2.28
0.21 0.08 0.20 0.12
1135 2896.7 3257.7 4324.5
58 104.9 195.7 159.3
376
P. Mutje´ et al. / Composites: Part A 38 (2007) 369–377
Fig. 6. Comparison of tensile stress and Young’s modulus for glass fibre PP composites, hemp strand PP composites containing 4 wt% of MAPP and with hemp straw PP composites containing 4% of MAPP.
450–800 MPa. The intrinsic properties of hemp straw were not evaluated. Although high tensile properties are expected for glass fibre composites, according to the mixture’s rule, similar modulus can be obtained for hemp strand composites by increasing the volume fraction of the reinforcement. It is in this sense worth noting that hemp strands composites at 40 wt% gave superior Young’s modulus than glass fibre composites at 30 wt%. 4. Conclusions The amount of MAPP should be in the range of 1.5– 2.5 wt% with respect to the PP matrix to improve the tensile properties of PP composites comprising up to 40 wt% of reinforcement. At these percentages, composites comprising MAPP gave higher tensile stress than those without MAPP due to the enhanced properties at interface and to the better dispersion of the reinforcement. Hemp strand composites at 40 wt% gave tensile stress values 4, 16 and 20% lower than glass fibre composites at 20, 30 and 40 wt% of reinforcement respectively. This is of special interest when using hemp strands to replace glass fibre as reinforcement in composite materials. The Young’s modulus was a function of the type and percentage of reinforcement but was independent of the percentage of compatibility agent MAPP. Young’s modulus of hemp strand composites at 40 wt% was higher than Young’s modulus of glass fibre composites at 30 wt%. Hemp straw composites gave low tensile properties. However, the tensile stress of hemp straw composites showed a slight increase for composites containing MAPP which means that hemp straw could be used as filler. Moreover, the Young’s modulus of hemp straw composites was higher than that of the PP matrix and achieved 70% of the Young’s modulus of hemp strand composites. In general, it can be said that hemp plant derivates, hemp strands and hemp straw, can be used as reinforcement or filler for polymeric matrices, respectively, and give composites with remarkable tensile properties at very attractive economic costs.
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