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polypropylene composites
Composites Science and Technology 63 (2003) 1247–1254 www.elsevier.com/locate/compscitech
Effects of fibre treatment on wettability and mechanical behaviour of flax/polypropylene composites Guillermo Cantero, Aitor Arbelaiz, Rodrigo Llano-Ponte, In˜aki Mondragon* Materials and Technologies Group, Departmento Ingenierı´a Quı´mica y Medio Ambiente, Escuela Ingenierı´a Te´cnica Industrial, University Paı´s Vasco/Euskal Herriko Unib, Avda. Felipe IV, 1 B. 20011 Donostia-San Sebastia´n, Spain Accepted 21 February 2003
Abstract Chemical treatment of natural reinforcements can enhance their adhesion to polymer matrices. This work reports the effects of different treatments on the fibre–matrix compatibility in terms of surface energy and mechanical properties of composites. The composites were compounded with two kinds of flax fibres (natural flax and flax pulp) and polypropylene. The applied treatments were maleic anhydride (MA), maleic anhydride-polypropylene copolymer (MAPP) and vinyl trimethoxy silane (VTMO). The treatment effects on the fibres have been characterised by Infrared Spectroscopy. Two techniques have been used to determine the surface energy values: the Dynamic Contact Angle method for the long flax fibres and the Capillary Rise method for the irregular pulps. The use of different methods involves a small discordance in the wettability values. Nevertheless, the three treatments reduce the polar component of the surface energy of the fibre. Composites containing MAPP-treated did the highest mechanical properties, whilst the MA and VTMO-treated fibre gave similar values to that for the untreated ones. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: B. Mechanical properties; Flax–polypropylene composites; Fibre treatments; Contact angle measurements
1. Introduction Natural fibres have proved to be a suitable reinforcement material for composites thanks to a combination of good mechanical properties and environmental advantages. This fact is especially remarkable in the case of the flax fibre, which is a renewable raw material with high specific strength and modulus and low density as well [1–4]. The main limitations of these fibres are the hydrophilic nature, the low thermal resistance and the quality inconstancies [5–7]. The final mechanical behaviour of a composite material depends to a great extent on the adhesion between the reinforcing fibre and the surrounding matrix. The adhesion between two different materials is a function of several factors, among which are the surface roughness and surface polarity [8,9]. Measurements of surface energies can give an idea of the interfacial adhesion between fibre and matrix.
* Corresponding author. Fax: +34-943-471097. E-mail address: [email protected] (I. Mondragon).
The goal of this work is to relate the mechanical behaviour of natural fibre composites with the surface energy values of chemically treated flax fibres. Composites have been performed with polypropylene and two different kinds of flax fibres: a natural flax fibre and a flax pulp obtained by natural flax cooking. For improving the adhesion between the hydrophilic flax fibre and the hydrophobic polypropylene, the fibres have been treated with maleic anhydride (MA), maleic anhydride-polypropylene copolymer (MAPP) and vinyl trimethoxy silane (VTMO). The fibre treatments have been characterised by Fourier transform infrared spectroscopy (FTIR). Two different techniques have been used to determine the surface energy values because shape differences of each kind of fibre used. Tensile and flexural tests of the composites have been carried out. Nevertheless, this is a preliminary investigation of the influence of chemical treatments on fibre/matrix compatibilization. At the moment, our group is working in combined treatments, improving fibre strength and interphase at the same time. This is possible by applying a physical treatment, such as mecerization, followed by a chemical one, as analysed in this report.
0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00094-0
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G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
2. Experimental 2.1. Materials The thermoplastic matrix used for the preparation of composites was a commercially available polypropylene (Eltex P HV200 from Solvay; MFI=10 g/10 min). As reinforcement two different kinds of flax fibres were used. The first one was a natural flax fibre (NF) supplied by Finflax (Finland) and obtained by a retting process of flax plants. The second one was a flax pulp (FP) supplied by Celesa (Spain) and reached by flax fibre cooking, in a process similar to kraft pulping. This pulp has a 5 wt.% shives content and a permanganate index of 9.4. Maleic anhydride (MA), from Cepsa, maleic anhydride-polypropylene copolymer, MAPP Epolene E-43, and vinyl trimethoxy silane (VTMO), from Degussa-Hu¨ls, were used as coupling agents. Epolene E-43, kindly supplied by Eastman Chemical, has a low molecular weight (Mn=3900, Mw=9100), 0.934 g/ml density and acid number of 45. 2.2. Fibre treatments The maleic anhydride (MA) has been applied in a 5 and 10 wt.% with respect to the fibre weight. The fibre was esterified during 25 h with MA dissolved in boiling acetone (T=50 5 C) with a fibre:solvent ratio of 1:25 (w/v). Thereafter, the fibre was washed several times in cold acetone and distilled water. Finally, the fibre was dried in an oven at 105 C for 24 h at least before any testing [10]. A 5 and 10 wt.% MAPP with respect to the fibre weight have also been used. For this treatment, MAPP was dissolved in boiling xylene (T=150 5 C) with a fibre:solvent ratio of 1:25 (w/v). Then, the fibre was soaked in the solution for 5 or 6 min. As in the former case, the fibre was washed and strongly dried [10,11]. For silanization, VTMO was dissolved in acidified water (pH=3.5) during 10 min to get a better functionalization. Then, the fibre was added and maintained for 1 h in the solution for obtaining a 2.5 wt.% silane with respect to the fibre weight [12]. A Perkin-Elmer 16 PC FTIR spectrophotometer was used to obtain the IR spectra of treated fibres. Pure powdered potassium bromide (KBr) was used as a reference substance. Small quantities of samples were dispersed in dry KBr and further ground to a fine mixture in a mortar, before pressing to form KBr pellets for analysis. 2.3. Specimen preparation Test specimens containing 30 wt.% flax fibre have been formed by extrusion mixing and injection moulding. For extrusion, a conical twin-screw extruder, Haake Rheomex CTW 100, was used. The rotor speed was 20 rpm and temperature varied between 180 and 185 C. For injection moulding, a Battenfeld Plus 250 injection
machine was used, working with 1057 bar injection pressure and 175 C. Test samples for flexural and tensile tests were moulded. 2.4. Surface energy characterisation The contact angle values have been obtained using a Kru¨ss K 12 tensiometer. Two different techniques have been used to determine the surface energy values because of the morphology of each kind of fibre. The dynamic contact angle method for the long fibres of natural flax and the polypropylene matrix is based in the Wilhelmy equation; and the capillary rise method for the irregular flax pulps is based in the Washburn equation. The use of different methods involves a small discordance in the wettability values. Surface energy values have been calculated by the Owens–Wendt equation: h 1=2 p p 1=2 i ð1 þ cosÞL ¼ 2 Ld Sd þ L S where is the contact angle of the sample with the test liquid; dL, pL, and L are disperse, polar and total surface energy of the test liquid; and dS, Sp are disperse and polar components of the surface energy of the tested sample. 2.5. Mechanical characterisation Three-point bending tests according to ASTM D-790M standard and tensile tests according to ASTM D-638M standard were carried out using a universal mechanical testing machine Instron, model 4206. For each test and composite, a minimum of six samples were tested.
3. Results and discussion 3.1. Infrared spectroscopy analysis Before any other testing, treated fibres have been analysed by FTIR spectroscopy. Fig. 1 compares the IR spectra of the natural flax fibres (NF) before and after treatments. The zone between 1800 and 1600 cm1, related with the formation of new ester groups between hydroxyl groups from cellulose and the applied agents, is the most interesting area of the spectra [5]. It is remarkable the presence of an intense peak in the NF spectra at about 1735 cm1 associated to carbonyl (C¼O) stretching of acetyl groups of hemicellulose in the natural flax [6,10]. MA treatment leads to a more intense peak at about 1735 cm1, due to the esterification reaction. MAPP treatment also produces a new ester group, which appears at 1740 cm1. Fig. 2 shows the effect of the same treatments on the flax pulps (FP), with similar results. It is remarkable the absence of the hemicellulose band at about 1735 cm1 in the FP spectra. The cooking process suffered by FP removes the
G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
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Fig. 1. FTIR spectra of treated natural flax fibres (NF).
Fig. 2. FTIR spectra of treated flax pulps (FP).
main amount of hemicellulose. VTMO treatment produces a narrow peak around 1650 cm1. It is worth noting for both flax fibres used that the shape of the 3700–300 cm1 broad band changes for MA treatment thus indicating a variation on the ratio of intermolecular to intramolecular OH bonds.
3.2. Surface energy measurements Figs. 3 and 4 show the effects of the fibre treatments on the contact angles of natural flax and flax pulp with the test liquids. The treatments increase the contact angle of NF with water, therefore decreasing its polar-
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Fig. 3. Effect of fibre treatments on Wilhelmy contact angles of natural flax fibres.
Fig. 4. Effect of fibre treatments on Washburn contact angles of flax pulps.
ity. However, in the case of FP the contact angle stays practically constant with treatments because the initial contact angle of the untreated pulp is very high, though the used Washburn method can also have some influence on these values. While contact angle with bromonaphtalene decreases in a similar way in both cases, contact angle with ethylenglycol varies in an irregular way. Wilhelmy technique used for NF seems to show lower values than Washburn one used for FP. Figs. 5 and 6 compare the effect of treatments on the polar component and on total surface energy of treated natural flax fibres and flax pulps. MAPP seems to be the
optimum treatment for both flax fibre and pulp because it reaches a polar component similar to that for PP, which is strongly non-polar. VTMO treatment also produces a high decreasing of the polar component. 3.3. Mechanical properties of the composites Figs. 7 and 8 show the effect of fibre treatments on the flexural properties of composites made with natural flax fibres and flax pulps. MAPP functionalization seems to be the only treatment leading to strength improvement, especially on FP. Flexural modulus of both kinds of
G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
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Fig. 5. Effect of fibre treatments on the polar component of natural flax fibres and pulps.
Fig. 6. Effect of fibre treatments on total surface energy of natural flax fibres and pulps.
fibres is better than that for neat polypropylene because the fibres have a higher stiffness than polymer. Composites containing NF are always stiffer than those containing FP because of the own nature of these fibres. Treatments vary the modulus values, especially for natural flax fibres. MA treatment produces the highest modulus values on NF, being less effective on FP. This fact could be related to the variations on the OH absorption bands above shown for this treatment. Flexural strengths of all composites are slightly higher than that for neat polypropylene. Although their modulus is much lower, FP composites strength is similar to
that for NF composite, being even higher with MAPP treatment. This fact is possibly related to a better fibre/ matrix interphase because of the absence of hemicellulose in FP. Polypropylene chains of the MAPP smooth the different surface energy values of matrix and reinforcement fibre, helping to achieve a better wetting of fibres in the melted polymer, and as a result it improves the interfacial adhesion [6,7]. There are many studies concerning the action of the MAPP, and most of them show high increases (from 25 to 100%) on mechanical properties of the MAPP-treated composites [13,14]. In this study MAPP is less effective than other
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Fig. 7. Effect of fibre treatments on flexural modulus of composites made with natural flax fibres and pulps.
Fig. 8. Effect of fibre treatments on flexural strength of composites made with natural flax fibres and pulps.
authors reported, maybe because of the way the chemical was applied. VTMO treatment produces a slight improvement on the strength, whilst MA does not. These results are according to surface energy obtained values. Not any treatment has a clear effect on NF composite strength, which stays practically constant. Consequently, it can be concluded that treatments are not effective on NF, possibly because the present hemicellulose does not allow a close contact between cellulose on the fibre and polypropylene matrix. Figs. 9 and 10 report the effect of fibre treatments on the tensile performance of composites made with natural flax fibre and flax pulp. Treatments enhance the tensile stiffness of the untreated fibre composites in dif-
ferent rates, following similar trends to the flexural modulus. Tensile strength of all the analysed composites is lower than that for polypropylene because not any treatment reaches an optimum adhesion between fibre and matrix. Therefore, a better interphase optimisation is still necessary. Treatments do not have a clear influence on NF composite strength, being all strength values in the same range. As for flexural properties, composites made with MAPP treated FP show the highest tensile strength. As it has been above explained, the absence of hemicellulose in FP allows a better esterification of the hydroxyl groups of cellulose, thereby leading to a stronger interphase than that for NF composites.
G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
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Fig. 9. Effect of fibre treatments on tensile modulus of composites made with natural flax fibres and pulps.
Fig. 10. Effect of fibre treatments on tensile strength of composites made with natural flax fibres and pulps.
4. Conclusions Wettability by PP matrix of both flax fibres and pulps can be improved by the action of chemical treatments, due to the reduction of the polar components of their surface energies. MAPP treatment produces the best surface energy values for all percentages. Composites made with a 10 wt.% MAPP treated fibre have the
highest flexural and tensile strength, according with the surface energy values. Its effect is especially remarkable for flax pulps. The treatment effect is not so evident on the modulus values, having MA and MAPP treated composites similar stiffness. In order to study the influence of the amorphous constituents of flax fibres on the final behaviour of composites, work is in progress with mercerized flax fibres.
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Acknowledgements The reported work forms part of a bigger Brite project (GRD1-1999-10951-ECOFINA) founded by the European Community.
References [1] Van de Velde K, Kiekens P. Die Angew Makromol Chem 1999; 272:87–93. [2] Baille CA, Zafeiropoulos N. J Mater Sci 2001;36:2107–31. [3] Gassan J, Gutowski VS, Bledzki K. Macromol Mater Eng 2000; 286:132–9. [4] Bledzki AK, Gassan J. Prog Polym Sci 1999;24:221–74.
[5] Kokta BV, Raj RG, Daneault C. Polym-Plast Technol Eng 1989; 28:247–59. [6] Joseph PV, Joseph K, Thomas S. Comp Sci Technol 1999; 59:1625–40. [7] Felix JM, Gatenholm P. J Appl Polym Sci 1991;42:609–20. [8] Oksman K, Clemons C. J Appl Polym Sci 1998;67:1503–13. [9] Woodhams RT, Thomas G, Rodgers DK. Polym Eng Sci 1984; 24:1166–71. [10] Kazayawoko M, Balatinecz JJ, Woodhams RT. J Appl Polym Sci 1997;66:1163–73. [11] Raj RG, Kokta BV, Nizio JD. J Appl Polym Sci 1992;45:91–101. [12] Karnani R, Krishnan M, Narayan R. Polym Eng Sci 1997; 37:476–87. [13] Coutinho FMB, Costa THS, Carvalho DL. J Appl Polym Sci 1997;65:1227–35. [14] Yuan X, Zhang Y, Zhang X. J Appl Polym Sci 1999;71:333–7.
Effects of fibre treatment on wettability and mechanical behaviour of flax/polypropylene composites Guillermo Cantero, Aitor Arbelaiz, Rodrigo Llano-Ponte, In˜aki Mondragon* Materials and Technologies Group, Departmento Ingenierı´a Quı´mica y Medio Ambiente, Escuela Ingenierı´a Te´cnica Industrial, University Paı´s Vasco/Euskal Herriko Unib, Avda. Felipe IV, 1 B. 20011 Donostia-San Sebastia´n, Spain Accepted 21 February 2003
Abstract Chemical treatment of natural reinforcements can enhance their adhesion to polymer matrices. This work reports the effects of different treatments on the fibre–matrix compatibility in terms of surface energy and mechanical properties of composites. The composites were compounded with two kinds of flax fibres (natural flax and flax pulp) and polypropylene. The applied treatments were maleic anhydride (MA), maleic anhydride-polypropylene copolymer (MAPP) and vinyl trimethoxy silane (VTMO). The treatment effects on the fibres have been characterised by Infrared Spectroscopy. Two techniques have been used to determine the surface energy values: the Dynamic Contact Angle method for the long flax fibres and the Capillary Rise method for the irregular pulps. The use of different methods involves a small discordance in the wettability values. Nevertheless, the three treatments reduce the polar component of the surface energy of the fibre. Composites containing MAPP-treated did the highest mechanical properties, whilst the MA and VTMO-treated fibre gave similar values to that for the untreated ones. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: B. Mechanical properties; Flax–polypropylene composites; Fibre treatments; Contact angle measurements
1. Introduction Natural fibres have proved to be a suitable reinforcement material for composites thanks to a combination of good mechanical properties and environmental advantages. This fact is especially remarkable in the case of the flax fibre, which is a renewable raw material with high specific strength and modulus and low density as well [1–4]. The main limitations of these fibres are the hydrophilic nature, the low thermal resistance and the quality inconstancies [5–7]. The final mechanical behaviour of a composite material depends to a great extent on the adhesion between the reinforcing fibre and the surrounding matrix. The adhesion between two different materials is a function of several factors, among which are the surface roughness and surface polarity [8,9]. Measurements of surface energies can give an idea of the interfacial adhesion between fibre and matrix.
* Corresponding author. Fax: +34-943-471097. E-mail address: [email protected] (I. Mondragon).
The goal of this work is to relate the mechanical behaviour of natural fibre composites with the surface energy values of chemically treated flax fibres. Composites have been performed with polypropylene and two different kinds of flax fibres: a natural flax fibre and a flax pulp obtained by natural flax cooking. For improving the adhesion between the hydrophilic flax fibre and the hydrophobic polypropylene, the fibres have been treated with maleic anhydride (MA), maleic anhydride-polypropylene copolymer (MAPP) and vinyl trimethoxy silane (VTMO). The fibre treatments have been characterised by Fourier transform infrared spectroscopy (FTIR). Two different techniques have been used to determine the surface energy values because shape differences of each kind of fibre used. Tensile and flexural tests of the composites have been carried out. Nevertheless, this is a preliminary investigation of the influence of chemical treatments on fibre/matrix compatibilization. At the moment, our group is working in combined treatments, improving fibre strength and interphase at the same time. This is possible by applying a physical treatment, such as mecerization, followed by a chemical one, as analysed in this report.
0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00094-0
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G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
2. Experimental 2.1. Materials The thermoplastic matrix used for the preparation of composites was a commercially available polypropylene (Eltex P HV200 from Solvay; MFI=10 g/10 min). As reinforcement two different kinds of flax fibres were used. The first one was a natural flax fibre (NF) supplied by Finflax (Finland) and obtained by a retting process of flax plants. The second one was a flax pulp (FP) supplied by Celesa (Spain) and reached by flax fibre cooking, in a process similar to kraft pulping. This pulp has a 5 wt.% shives content and a permanganate index of 9.4. Maleic anhydride (MA), from Cepsa, maleic anhydride-polypropylene copolymer, MAPP Epolene E-43, and vinyl trimethoxy silane (VTMO), from Degussa-Hu¨ls, were used as coupling agents. Epolene E-43, kindly supplied by Eastman Chemical, has a low molecular weight (Mn=3900, Mw=9100), 0.934 g/ml density and acid number of 45. 2.2. Fibre treatments The maleic anhydride (MA) has been applied in a 5 and 10 wt.% with respect to the fibre weight. The fibre was esterified during 25 h with MA dissolved in boiling acetone (T=50 5 C) with a fibre:solvent ratio of 1:25 (w/v). Thereafter, the fibre was washed several times in cold acetone and distilled water. Finally, the fibre was dried in an oven at 105 C for 24 h at least before any testing [10]. A 5 and 10 wt.% MAPP with respect to the fibre weight have also been used. For this treatment, MAPP was dissolved in boiling xylene (T=150 5 C) with a fibre:solvent ratio of 1:25 (w/v). Then, the fibre was soaked in the solution for 5 or 6 min. As in the former case, the fibre was washed and strongly dried [10,11]. For silanization, VTMO was dissolved in acidified water (pH=3.5) during 10 min to get a better functionalization. Then, the fibre was added and maintained for 1 h in the solution for obtaining a 2.5 wt.% silane with respect to the fibre weight [12]. A Perkin-Elmer 16 PC FTIR spectrophotometer was used to obtain the IR spectra of treated fibres. Pure powdered potassium bromide (KBr) was used as a reference substance. Small quantities of samples were dispersed in dry KBr and further ground to a fine mixture in a mortar, before pressing to form KBr pellets for analysis. 2.3. Specimen preparation Test specimens containing 30 wt.% flax fibre have been formed by extrusion mixing and injection moulding. For extrusion, a conical twin-screw extruder, Haake Rheomex CTW 100, was used. The rotor speed was 20 rpm and temperature varied between 180 and 185 C. For injection moulding, a Battenfeld Plus 250 injection
machine was used, working with 1057 bar injection pressure and 175 C. Test samples for flexural and tensile tests were moulded. 2.4. Surface energy characterisation The contact angle values have been obtained using a Kru¨ss K 12 tensiometer. Two different techniques have been used to determine the surface energy values because of the morphology of each kind of fibre. The dynamic contact angle method for the long fibres of natural flax and the polypropylene matrix is based in the Wilhelmy equation; and the capillary rise method for the irregular flax pulps is based in the Washburn equation. The use of different methods involves a small discordance in the wettability values. Surface energy values have been calculated by the Owens–Wendt equation: h 1=2 p p 1=2 i ð1 þ cosÞL ¼ 2 Ld Sd þ L S where is the contact angle of the sample with the test liquid; dL, pL, and L are disperse, polar and total surface energy of the test liquid; and dS, Sp are disperse and polar components of the surface energy of the tested sample. 2.5. Mechanical characterisation Three-point bending tests according to ASTM D-790M standard and tensile tests according to ASTM D-638M standard were carried out using a universal mechanical testing machine Instron, model 4206. For each test and composite, a minimum of six samples were tested.
3. Results and discussion 3.1. Infrared spectroscopy analysis Before any other testing, treated fibres have been analysed by FTIR spectroscopy. Fig. 1 compares the IR spectra of the natural flax fibres (NF) before and after treatments. The zone between 1800 and 1600 cm1, related with the formation of new ester groups between hydroxyl groups from cellulose and the applied agents, is the most interesting area of the spectra [5]. It is remarkable the presence of an intense peak in the NF spectra at about 1735 cm1 associated to carbonyl (C¼O) stretching of acetyl groups of hemicellulose in the natural flax [6,10]. MA treatment leads to a more intense peak at about 1735 cm1, due to the esterification reaction. MAPP treatment also produces a new ester group, which appears at 1740 cm1. Fig. 2 shows the effect of the same treatments on the flax pulps (FP), with similar results. It is remarkable the absence of the hemicellulose band at about 1735 cm1 in the FP spectra. The cooking process suffered by FP removes the
G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
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Fig. 1. FTIR spectra of treated natural flax fibres (NF).
Fig. 2. FTIR spectra of treated flax pulps (FP).
main amount of hemicellulose. VTMO treatment produces a narrow peak around 1650 cm1. It is worth noting for both flax fibres used that the shape of the 3700–300 cm1 broad band changes for MA treatment thus indicating a variation on the ratio of intermolecular to intramolecular OH bonds.
3.2. Surface energy measurements Figs. 3 and 4 show the effects of the fibre treatments on the contact angles of natural flax and flax pulp with the test liquids. The treatments increase the contact angle of NF with water, therefore decreasing its polar-
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Fig. 3. Effect of fibre treatments on Wilhelmy contact angles of natural flax fibres.
Fig. 4. Effect of fibre treatments on Washburn contact angles of flax pulps.
ity. However, in the case of FP the contact angle stays practically constant with treatments because the initial contact angle of the untreated pulp is very high, though the used Washburn method can also have some influence on these values. While contact angle with bromonaphtalene decreases in a similar way in both cases, contact angle with ethylenglycol varies in an irregular way. Wilhelmy technique used for NF seems to show lower values than Washburn one used for FP. Figs. 5 and 6 compare the effect of treatments on the polar component and on total surface energy of treated natural flax fibres and flax pulps. MAPP seems to be the
optimum treatment for both flax fibre and pulp because it reaches a polar component similar to that for PP, which is strongly non-polar. VTMO treatment also produces a high decreasing of the polar component. 3.3. Mechanical properties of the composites Figs. 7 and 8 show the effect of fibre treatments on the flexural properties of composites made with natural flax fibres and flax pulps. MAPP functionalization seems to be the only treatment leading to strength improvement, especially on FP. Flexural modulus of both kinds of
G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
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Fig. 5. Effect of fibre treatments on the polar component of natural flax fibres and pulps.
Fig. 6. Effect of fibre treatments on total surface energy of natural flax fibres and pulps.
fibres is better than that for neat polypropylene because the fibres have a higher stiffness than polymer. Composites containing NF are always stiffer than those containing FP because of the own nature of these fibres. Treatments vary the modulus values, especially for natural flax fibres. MA treatment produces the highest modulus values on NF, being less effective on FP. This fact could be related to the variations on the OH absorption bands above shown for this treatment. Flexural strengths of all composites are slightly higher than that for neat polypropylene. Although their modulus is much lower, FP composites strength is similar to
that for NF composite, being even higher with MAPP treatment. This fact is possibly related to a better fibre/ matrix interphase because of the absence of hemicellulose in FP. Polypropylene chains of the MAPP smooth the different surface energy values of matrix and reinforcement fibre, helping to achieve a better wetting of fibres in the melted polymer, and as a result it improves the interfacial adhesion [6,7]. There are many studies concerning the action of the MAPP, and most of them show high increases (from 25 to 100%) on mechanical properties of the MAPP-treated composites [13,14]. In this study MAPP is less effective than other
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Fig. 7. Effect of fibre treatments on flexural modulus of composites made with natural flax fibres and pulps.
Fig. 8. Effect of fibre treatments on flexural strength of composites made with natural flax fibres and pulps.
authors reported, maybe because of the way the chemical was applied. VTMO treatment produces a slight improvement on the strength, whilst MA does not. These results are according to surface energy obtained values. Not any treatment has a clear effect on NF composite strength, which stays practically constant. Consequently, it can be concluded that treatments are not effective on NF, possibly because the present hemicellulose does not allow a close contact between cellulose on the fibre and polypropylene matrix. Figs. 9 and 10 report the effect of fibre treatments on the tensile performance of composites made with natural flax fibre and flax pulp. Treatments enhance the tensile stiffness of the untreated fibre composites in dif-
ferent rates, following similar trends to the flexural modulus. Tensile strength of all the analysed composites is lower than that for polypropylene because not any treatment reaches an optimum adhesion between fibre and matrix. Therefore, a better interphase optimisation is still necessary. Treatments do not have a clear influence on NF composite strength, being all strength values in the same range. As for flexural properties, composites made with MAPP treated FP show the highest tensile strength. As it has been above explained, the absence of hemicellulose in FP allows a better esterification of the hydroxyl groups of cellulose, thereby leading to a stronger interphase than that for NF composites.
G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
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Fig. 9. Effect of fibre treatments on tensile modulus of composites made with natural flax fibres and pulps.
Fig. 10. Effect of fibre treatments on tensile strength of composites made with natural flax fibres and pulps.
4. Conclusions Wettability by PP matrix of both flax fibres and pulps can be improved by the action of chemical treatments, due to the reduction of the polar components of their surface energies. MAPP treatment produces the best surface energy values for all percentages. Composites made with a 10 wt.% MAPP treated fibre have the
highest flexural and tensile strength, according with the surface energy values. Its effect is especially remarkable for flax pulps. The treatment effect is not so evident on the modulus values, having MA and MAPP treated composites similar stiffness. In order to study the influence of the amorphous constituents of flax fibres on the final behaviour of composites, work is in progress with mercerized flax fibres.
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G. Cantero et al. / Composites Science and Technology 63 (2003) 1247–1254
Acknowledgements The reported work forms part of a bigger Brite project (GRD1-1999-10951-ECOFINA) founded by the European Community.
References [1] Van de Velde K, Kiekens P. Die Angew Makromol Chem 1999; 272:87–93. [2] Baille CA, Zafeiropoulos N. J Mater Sci 2001;36:2107–31. [3] Gassan J, Gutowski VS, Bledzki K. Macromol Mater Eng 2000; 286:132–9. [4] Bledzki AK, Gassan J. Prog Polym Sci 1999;24:221–74.
[5] Kokta BV, Raj RG, Daneault C. Polym-Plast Technol Eng 1989; 28:247–59. [6] Joseph PV, Joseph K, Thomas S. Comp Sci Technol 1999; 59:1625–40. [7] Felix JM, Gatenholm P. J Appl Polym Sci 1991;42:609–20. [8] Oksman K, Clemons C. J Appl Polym Sci 1998;67:1503–13. [9] Woodhams RT, Thomas G, Rodgers DK. Polym Eng Sci 1984; 24:1166–71. [10] Kazayawoko M, Balatinecz JJ, Woodhams RT. J Appl Polym Sci 1997;66:1163–73. [11] Raj RG, Kokta BV, Nizio JD. J Appl Polym Sci 1992;45:91–101. [12] Karnani R, Krishnan M, Narayan R. Polym Eng Sci 1997; 37:476–87. [13] Coutinho FMB, Costa THS, Carvalho DL. J Appl Polym Sci 1997;65:1227–35. [14] Yuan X, Zhang Y, Zhang X. J Appl Polym Sci 1999;71:333–7.