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Creep and impact properties of wood fibre–polypropylene composites: influence of temperature and moisture content
Composites Science and Technology 64 (2004) 693–700 www.elsevier.com/locate/compscitech
Creep and impact properties of wood fibre–polypropylene composites: influence of temperature and moisture content Andrzej K. Bledzki*, Omar Faruk Institut fu¨r Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Mo¨nchebergstr. 3, D- 34109 Kassel, Germany Received 22 December 2002; accepted 13 July 2003
Abstract Wood fibre reinforced polypropylene composites of different fibre content (40, 50 and 60% by weight) have been prepared and wood fibres (hard and long fibre) were treated with compatibiliser (MAH-PP) to increase the interfacial adhesion with the matrix to improve the dispersion of the particles and to decrease the water sorption properties of the final composite. Results indicated that impact properties were affected by moisture content. The Charpy impact strength decreased and maximum force was increased with increasing of moisture content. With the addition of MAH-PP (5% relative to the wood fibre content), damping index decreased around 145% for hard wood fibre–PP composites at 60 wt.% wood fibre content. Long wood fibre–PP composites showed more impact resistance than hard wood fibre–PP composites. Short term flexural creep tests were conducted to investigate the creep behaviour of wood fibre–PP composites. Three experimental parameters were selected: the addition of compatibiliser, temperature and wood fibre content. The addition of MAH-PP, increased creep modulus that means reduced the creep. The extent of creep resistance (creep modulus and creep strength) decreased with increasing temperature. It was also found that wood fibre content has a great effect on creep resistance which is increased with increasing wood fibre content. # 2003 Elsevier Ltd. All rights reserved. Keywords: B. Creep; B. Impact behaviour; Wood fibre–polypropylene composites
1. Introduction With recent advancements in the science and technology of wood fibre and plastic composites and the parallel increasing industrial interest in advanced wood fibre and plastic materials, such as in construction, building and automotive components, the subject of viscoelasticity and environmental effect have recently gained strong momentum in the realm of process engineering and applications. Thermoplastics and corresponding composites are sensitive to changes in the environment and their mechanical properties may vary considerably with environmental conditions. Variations in temperature and moisture are always encountered by these materials in service. Polymeric matrices filled with wood and natural fibres offer a high specific stiffness and strength, flexibility during processing with no harm to the equipment, low density, and low cost per volume basis [1]. * Corresponding author. Fax: +49-561-804-3692. E-mail address: [email protected] (A.K. Bledzki). 0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00291-4
Unfortunately, the frequent incompatibility between polar wood fibre and non-polar polymeric matrices affects the degree of dispersion of the fibres in the matrix and the overall homogeneity of the composite structure. To overcome these obstacles, various types of remedies have been suggested [2–6]. The use of compatibiliser for treating the fibres prior to, or as an addition in the compounding step results in improved mechanical properties [7,8]. Most of synthetic polymers absorb moisture in a humid atmosphere and when they are immersed in water. Wood fibre is a natural structure made of cellulose fibres which contains numerous hydroxyl groups that are strongly hydrophilic. The rate at which water is absorbed by a composite depends on many variables including fibre type, matrix, temperature, the difference in water distribution within the composite, reaction between water and the matrix, among others [9]. Then, wood fibre reinforced polymers can take up a high amount of water, which generally causes a reduction in mechanical properties [10]. Both the rate of water pickup and the total amount of moisture absorbed depend
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on the chemical structure of the resin and crosslinking agent together with the temperature and the relative humidity (RH) [11]. Moisture content at a given relative humidity can have a great effect on the biological performance of a composite made of those fibres. Table 1 shows the equilibrium moisture content of some common natural and wood fibres which indicates that a composite made from pennywort fibres would have much greater moisture content at 90% relative humidity than would a composites made from bamboo fibres. The pennywort product would be much more prone to decay as compared to the bamboo product. Aranguren et al. [13] investigated the dependence of the mechanical properties of wood flour–polyester composites dependence on the moisture content at 60 and 90% relative humidity. Results indicated that mechanical properties (compression and bending tests) were severely affected by moisture content and it was found that this was a reversible effect, because the original values of the compression properties were recovered after drying. The review report [14] described the effect of water absorption, moisture content and swelling on natural and wood fibre reinforced composites. In spite of the recent progress in processing methods and improving the mechanical performance of wood fibre reinforced composites, limited experimental results are available on creep. Creep, the deformation over time of a material under stress, is one characteristic of wood fibre reinforced polymer composites that has resulted in poor performance in certain applications. Creep and creep rupture (time dependent failure) consideration are essential if wood fibre thermoplastic composites are to have long-term loading applications. Creep of polymers occurs because of a combination of elastic deformation and viscous flow, commonly known as viscoelastic deformation [15]. Creep in thermoplastics is a complex phenomenon, which depends both on material properties (molecular orientation, crystallinity, etc.) [16] and external parameters (applied stress, temperature and humidity). In general, the impact behaviour of composites laminates in engineering applications is of main importance. Table 1 Equilibrium moisture content of some natural and wood fibres [12] Fibre
Bamboo Bagasse Jute Aspen Southern pine Water hyacinth Pennywort
Equilibrium moisture content at 27 C 30% RH
65% RH
90% RH
4.5 4.4 4.6 4.9 5.8 6.2 6.6
8.9 8.8 9.9 11.1 12.0 16.7 18.3
14.7 15.8 16.3 21.5 21.7 36.2 56.8
Structure of the textiles, fibre type and content, matrix ductility and void content of the composites are the main structural parameters which affect the impact behaviour [17]. The impact resistance is the ability of a material and its structure to survive impact induced damages during an impact event. The objective of this study was to determine the effects of the addition of compatibiliser, wood fibre content and temperature on short term creep properties of wood fibre–polypropylene composites and to determine the influence of the moisture content on the impact properties of the composites.
2. Experimental 2.1. Materials Polypropylene (type Stamylan P17M10) was supplied from DSM, Gelsenkirchen, Germany. The wood fibres were standard hard wood fibre (Lignocel HBS 150-500) with particle size 150–500 mm, supplied by J. Rettenmaier & So¨hne GmbH+Co. and long wood fibre (particle size 4–25 mm) obtained from Johnson Controls, Lu¨neburg, Germany. Micrographs (Fig. 1) show the difference of fibre length for both wood fibres. A compatibilizer maleic anhydride polypropylene copolymer (Licomont AR 504 FG) was used with the intension of improving the mechanical properties of composites and it obtained from Clariant Corp., Frankfurt, Germany. 2.2. Compounding and processing Wood fibre was initially dried at 80 C in an air-circulating oven for 24 h and then mixed with matrix and coupling agent in a high speed mixer (Henschel, type HM40 KM120). Wood–polymer composites (WPC) were prepared in the agglomerate form by mixing different amount of wood fibres (40, 50 and 60%), coupling agent (5% of the wood fibre content) and relative amounts of polypropylene (PP).The WPC granules were then dried 80 C for 24 h (water content approximately 0.2%) before moulding. Test samples were prepared by injection moulding of the dried agglomerate at melting temperature 150– 180 C and mould temperature of 80–90 C.The mechanical properties were investigated at room temperature (23 C) at a relative humidity of 50%. Charpy impact test (EN ISO 179) was carried out with 10 unnotched samples per series. In each series standard deviation ( < 15%) was used to measure Charpy impact energy. To measure the impact characteristics values, the specimens were tested by using a low-velocity falling weight impact tester (EN ISO 6603-2) at room temperature in
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Fig. 1. Micrographs of hard wood fibre (a) and long wood fibre (b) (magnification 12.5:1).
non-penetration-mode. The impactor had a mass of 0.75 kg and the impact energy was 0.96 Joules. The results of the impact test can be described by two separate issues (Fig. 2): (a) Force–deflection curve: the force–deflection curve refers to all the materials behaviours including the damage initiation defined by the first significant drop of the force. (b) Characteristic values: loss energy (Wv) as a measure of dissipated energy and strain energy (Ws) as a measure of the stored energy, and the damping index (*) as ratio of loss energy to strain energy. Determination of moisture content was followed DIN 52375. The volume of the samples was calculated from the measurement of the dimensions ( 0.01 mm) of each specimen. The determination of moisture content and volume were performed on at least three specimens for each sample.
2.3. Moisture sorption Humid environments were prepared by an air-conditioned oven (Weiss, WKI 340) and specimens were stored there for 72 h at 60 and 90% relative humidity. To ensure the final humidity content, composite specimens were weighed after every 3 h. 2.4. Creep test Short term creep tests were performed with flexural creep equipment according to standard procedures (EN ISO 899-2). Standard injection moulded flexural specimens were used for the creep test. For temperature testing, the creep devices were placed inside an insulated chamber. A small fan and light bulbs were placed inside the chamber for air circulation and heating. A temperature controller was used to regulate the test temperatures of 23 (room temperature), 40 and 60 C within 1 C accuracy. The creep devices and specimens were preconditioned for two hours at each test temperature. The creep properties (creep modulus and creep strength) were measured with a increasing of time up to 180 min as a deformation of the composites.
3. Results and discussion 3.1. Moisture content
Fig. 2. Typical impact force–deflection curve for natural reinforced polymer composites including definition of the characteristic values used.
The final moisture content of hard wood and long wood fibre reinforced polypropylene composites in 60 and 90% RH environments are shown in Table 2 respectively. Long wood fibre reinforced composites are much more hygroscopic than the hard wood fibre reinforced composites which is nearly two times more at the wood fibre content 60 wt.%. It seems that the fibre length and geometry plays a role for different amount of moisture expansion in the wood fibre–PP composites.
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Table 2 Equilibrium moisture content of wood fibre-PP composites at dry, 60% RH and 90% RH atmospheres Sample
Moisture content (%) Wood fibre 40 wt.%
HW+PP HW+PP+MAH-PP LW+PP LW+PP+MAH-PP
Wood fibre 50 wt.%
Wood fibre 60 wt.%
Dry
60% RH
90% RH
Dry
60% RH
90% RH
Dry
60% RH
90% RH
0.22 0.21 0.80 0.47
0.50 0.45 1.23 1.04
0.75 0.71 1.37 1.04
0.31 0.29 1.03 0.64
0.77 0.56 1.42 1.07
1.14 0.77 2.06 1.10
0.57 0.41 1.1 0.64
0.78 0.59 1.60 1.25
1.19 0.86 2.18 1.29
With the increasing of wood fibre content, moisture content also increases in the both types of wood fibre– PP composites. Besides, the modification of the wood fibre with MAH-PP 5% reduces its hygroscopicity (around 75% for hard wood–PP composites at 90% RH at wood fibre content 60 wt.%) because a considerable amount of accessible OH groups disappeared to become firmly bonded to the polar carboxylic groups of the maleic acid. Figs. 3 and 4 summarize the resulting volume expansion due to water sorption in 60 and 90% RH atmospheres for the both types of wood fibre–PP composites with different fibre content. As expected, samples air conditioned at 90% RH showed higher volume expansion than those at 60% RH. In both cases, volume increment increases with the increasing of wood fibre content which is corresponds to the highest filler percentage loaded into the matrix. Moreover, results indicates that, modified with compatibiliser (MAH-PP), wood fibre–PP composites exhibit a lower volume expansion which is at best decreased 35% for long wood fibre–PP composites at 60% wood fibre content. Besides, the volume increment of all composites is rather important due to the indication of the composites dimensional stability.
Fig. 3. Volume increment of hardwood fibre–PP composites at different relative humidity atmospheres.
Fig. 4. Volume increment of long wood fibre–PP composites at different relative humidity atmospheres.
3.2. Charpy impact strength The results of Charpy impact strength are shown in Figs. 5 and 6. Both parts of the figure show that the Charpy impact strength of the composites, with and without compatibiliser, decreases with the increasing of wood fibre content.
Fig. 5. Charpy impact strength of hardwood fibre–PP composites at different relative humidity atmospheres.
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Fig. 6. Charpy impact strength of long wood fibre–PP composites at different relative humidity atmospheres.
The Charpy impact strength increases with the addition of MAH-PP 5%. With the increasing of moisture content from room humidity to 90% RH in both with and without MAH-PP wood fibre–PP composites, the Charpy impact strength decreases not a significant way. It indicates that, a short time moisture expansion (72 h) of the composites couldn’t affect the Charpy impact strength significantly. 3.3. Impact properties The force–deflection curve refers to associate damage initiation by the first significant change in the slope of the curve. Figs. 7 and 8 shows the impact resistance (maximum force) of both types of wood fibre–PP composites with MAH-PP5% at wood fibre content 60 wt.%. In both cases, maximum force increases with the increasing of equilibrium moisture content and long wood fibre–PP composites shows a greater maximum force than hard wood fibre–PP composites. The damping index for all samples was calculated by taking the ratio of dissipated energy (loss energy) to the stored energy (strain energy) to measure the impact characteristic values. The loss energy involves energy
Fig. 7. Impact resistance of hard wood fibre–PP composites at different relative humidity atmospheres (wood fibre content 60 wt.%).
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Fig. 8. Impact resistance of long wood fibre–PP composites at different relative humidity atmospheres (wood fibre content 60 wt.%).
which based on irreversible deformations, energy dissipation due to creation of matrix cracks and their propagation, delaminations and finally fibre fracture. Figs. 9 and 10 shows the damping index for both types of wood fibre–PP composites. For hard wood fibre–PP composites, damping index increases with the increasing of wood fibre content and decreasing with the addition of MAH-PP5%. Fig. 9 also illustrates that, with addition of MAH-PP5% at wood fibre content 60%, damping index decreased highest around 145%. With the increasing of equilibrium moisture content, the damping index decreases for all samples at a certain value which is not very significant. For long wood fibre– PP composites follows the similar trend to those of the hard wood fibre–PP composites except with the increasing of wood fibre content, the damping index decreases. 3.4. Creep properties To investigate the effect of the addition of compatibiliser, creep tests were performed on both untreated and treated composites with MAH-PP. These composites prepared have a constant wood fibre content (40
Fig. 9. Damping index of hard wood fibre–PP composites at different relative humidity atmospheres.
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Fig. 10. Damping index of long wood fibre–PP composites at different relative humidity atmospheres.
Fig. 12. Creep modulus of hard wood fibre–PP composites at different temperatures.
wt.%). Fig. 11 shows the creep modulus of the composites made of untreated and treated wood fibres at temperature 60 C. The treated specimens showed higher creep modulus than untreated specimens. That means, MAH-PP treated composites showed lower creep which was also observed for jute fibre reinforced PP composites [18]. Fig. 11 illustrates that in both cases, creep modulus decreases very fast at the initial position with the increasing of time and then decreases exponentially. The addition of compatibiliser usually improves fibre dispersion in matrix resins and facilitates the interfacial adhesion between the fibres and polymer matrix. The improvement of strength properties of wood fibre–PP composites with the addition of MAH-PP has been already reported. Fig. 12 shows the creep modulus of the hard wood– PP composites containing 40 wt.% wood fibre content under different temperatures. All curves show that creep modulus decreases with an increase in time and temperature. The creep modulus at 60 C was around 1800 MPa after 180 min, which was nearly 65% lower that of room temperature modulus after the same duration. Thermoplastic materials are typically softened by elevated temperatures. As a result, higher temperatures
normally reduce the creep modulus of matrix-dominant composites. The effect of wood fibre content on the creep response of the composites is shown in Figs. 13 and 14. For both wood fibre–PP composites, the creep test was performed at 40 C temperature level. Both hard wood and long wood fibre–PP composites containing wood fibre content 40 wt.% showed lowest creep modulus. When
Fig. 11. Creep modulus of hard wood fibre (40 wt.%) – PP composites with and without compatibiliser at 60 C.
Fig. 14. Creep modulus of long wood fibre–PP composites with different wood fibre content (temperature 40 C).
Fig. 13. Creep modulus of hard wood fibre–PP composites with different wood fibre content (temperature 40 C).
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response of the wood fibre reinforced PP composites were investigated in this study. This was also conducted to experimentally determine the effects of the addition of a compatibiliser and wood fibre content on short term creep characteristics of wood fibre–PP composites. The following points can be drawn from these studies.
Fig. 15. Creep strength of hard wood fibre–PP composites with compatibiliser at different temperatures after 180 min deformation.
The moisture content increases in composites with the increase of wood fibre content. Compatibiliser MAH-PP5% reduced the hygroscopicity (around 75%) for hard wood fibre-PP composites. Long wood fibre reinforced composites are much more hygroscopic than the hard wood fibre reinforced composites which is near two times more at the wood fibre content 60 wt.%. Maximum force increases with the increasing of moisture content. Damping index decreased around 145% with the addition of MAH-PP5% for hard wood fibre–PP composites at wood fibre content 60 wt.%. The creep modulus of wood fibre–PP composites increased around 165% with increasing wood fibre content 40–60 wt.%. Creep modulus and creep strength decreases with increasing temperature.
References Fig. 16. Creep strength of long wood fibre–PP composites with compatibiliser at different temperatures after 180 min deformation.
wood fibre content increases 40–60 wt.%, the creep modulus increases significantly to 165% for both wood fibre–PP composites and decreased 15–20% after 180 min deformation. In comparison between two wood fire–PP composites, long wood fibre–PP composites shows better creep modulus than hard wood fibre–PP composites which is around 50%. May be wood fibre length plays a role into interaction between wood fibre and polypropylene matrix. Figs. 15 and 16 show the creep strength after 180 min of both wood fibre–PP composites with the addition of compatibiliser at different temperatures. Figures illustrate that with the increasing of wood fibre content, the creep strength shows a relatively higher tendency and simultaneously a lower tendency with the increasing of temperature.
4. Conclusions The influence of moisture content on the impact behaviour and the effect of temperature on creep
[1] Felix M, Gatenholm P, The nature of adhesion in composites of modified cellulose fibers polypropylene. J Appl Polym Sci 1991; 42:609–20. [2] Hon DNS, Chao WY. Composites from benzylated wood and polystyrenes: their processability and viscoelastic properties. J Appl Polym Sci 1993;50:7–11. [3] Maldas D, Kokta BV. Influence of phthalic anhydride as a coupling agent on the mechanical behavior of wood fiber-polystyrene composites. J Appl Polym Sci 1990;41:185–94. [4] Yap MGS, Que YI, Chia LHL. Dynamic mechanical analysis of tropical wood–polymer composites. J Appl Polym Sci 1991;43: 1999–2004. [5] George J, Bhagwan SS, Prabhakaran N, Thomas S. Short pineapple leaf fiber reinforced low density polyethylene composites. J Appl Polym Sci 1995;57:843–54. [6] Dos NL, Elawday MM, Monsour SH. Impregnation of white pine wood with unsaturated polyesters to produce wood–plastic combinations. J Appl Polym Sci 1991;42:2589–94. [7] Felix M, Gatenholm P. Formation of entanglements at brushlike interfaces in cellulose–polymer composites. J Appl Polym Sci 1993;50:699–708. [8] Bledzki AK, Huque M, Faruk O. Physico-Mechanical studies of wood fibre reinforced composites. Polym-Plast Technol Eng 2002;41(3):435–51. [9] Springer GS. Environmental Effects on Composites Material. Westport, (CT): Technomic; 1981. [10] Karmaker C, Hoffmann A, Hinrichsen G. Influence of water uptake on the mechanical properties of jute fiber-reinforced polypropylene. J Appl Polym Sci 1994;54:1803–7. [11] George J, Bhagawan SS, Thomas S. Effects of environment on
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the properties of low-density polyethylene composites reinforced with pineapple-leaf fibre. Comp Sci Tech 1998;58:1471–85. [12] Rowell RM, Han JS, Rowell JS. Characterization and factors affecting fiber properties. In: Natural polymers and agrofibres composites. San Carlos (Brazil); 2000. [13] Marcovich NE, Reboredo MM, Aranguren MI. Dependence of the mechanical properties of woodflour-polymer composites on the moisture content. J Appl Polym Sci 1998;68:2069–76. [14] Bledzki AK, Sperber VE, Faruk O. Natural and Wood Fibre Reinforcement in Polymers. Rapra Review Reports 2002;13(8):1– 144.
[15] Park BD, Balatinez JJ. Short term flexural creep behavior of wood-fiber/polypropylene composites polymer composites 1998; 19(4):377–82. [16] Pomeroy CD. Creep of engineering materials. Cambridge (UK): Heffers; 1978. [17] Bledzki AK, Gassan J, Zhang W. Impact properties of natural fiber-reinforced epoxy foams. Journal of Cellular Plastics 1999; 19(6):550–62. [18] Bledzki AK, Gassan J. Influence of fiber surface treatment on the creep behavior of jute fiber-reinforced polypropylene. Journal of Thermoplastic Composite Materials 1999;12:388–98.
Creep and impact properties of wood fibre–polypropylene composites: influence of temperature and moisture content Andrzej K. Bledzki*, Omar Faruk Institut fu¨r Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Mo¨nchebergstr. 3, D- 34109 Kassel, Germany Received 22 December 2002; accepted 13 July 2003
Abstract Wood fibre reinforced polypropylene composites of different fibre content (40, 50 and 60% by weight) have been prepared and wood fibres (hard and long fibre) were treated with compatibiliser (MAH-PP) to increase the interfacial adhesion with the matrix to improve the dispersion of the particles and to decrease the water sorption properties of the final composite. Results indicated that impact properties were affected by moisture content. The Charpy impact strength decreased and maximum force was increased with increasing of moisture content. With the addition of MAH-PP (5% relative to the wood fibre content), damping index decreased around 145% for hard wood fibre–PP composites at 60 wt.% wood fibre content. Long wood fibre–PP composites showed more impact resistance than hard wood fibre–PP composites. Short term flexural creep tests were conducted to investigate the creep behaviour of wood fibre–PP composites. Three experimental parameters were selected: the addition of compatibiliser, temperature and wood fibre content. The addition of MAH-PP, increased creep modulus that means reduced the creep. The extent of creep resistance (creep modulus and creep strength) decreased with increasing temperature. It was also found that wood fibre content has a great effect on creep resistance which is increased with increasing wood fibre content. # 2003 Elsevier Ltd. All rights reserved. Keywords: B. Creep; B. Impact behaviour; Wood fibre–polypropylene composites
1. Introduction With recent advancements in the science and technology of wood fibre and plastic composites and the parallel increasing industrial interest in advanced wood fibre and plastic materials, such as in construction, building and automotive components, the subject of viscoelasticity and environmental effect have recently gained strong momentum in the realm of process engineering and applications. Thermoplastics and corresponding composites are sensitive to changes in the environment and their mechanical properties may vary considerably with environmental conditions. Variations in temperature and moisture are always encountered by these materials in service. Polymeric matrices filled with wood and natural fibres offer a high specific stiffness and strength, flexibility during processing with no harm to the equipment, low density, and low cost per volume basis [1]. * Corresponding author. Fax: +49-561-804-3692. E-mail address: [email protected] (A.K. Bledzki). 0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00291-4
Unfortunately, the frequent incompatibility between polar wood fibre and non-polar polymeric matrices affects the degree of dispersion of the fibres in the matrix and the overall homogeneity of the composite structure. To overcome these obstacles, various types of remedies have been suggested [2–6]. The use of compatibiliser for treating the fibres prior to, or as an addition in the compounding step results in improved mechanical properties [7,8]. Most of synthetic polymers absorb moisture in a humid atmosphere and when they are immersed in water. Wood fibre is a natural structure made of cellulose fibres which contains numerous hydroxyl groups that are strongly hydrophilic. The rate at which water is absorbed by a composite depends on many variables including fibre type, matrix, temperature, the difference in water distribution within the composite, reaction between water and the matrix, among others [9]. Then, wood fibre reinforced polymers can take up a high amount of water, which generally causes a reduction in mechanical properties [10]. Both the rate of water pickup and the total amount of moisture absorbed depend
694
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on the chemical structure of the resin and crosslinking agent together with the temperature and the relative humidity (RH) [11]. Moisture content at a given relative humidity can have a great effect on the biological performance of a composite made of those fibres. Table 1 shows the equilibrium moisture content of some common natural and wood fibres which indicates that a composite made from pennywort fibres would have much greater moisture content at 90% relative humidity than would a composites made from bamboo fibres. The pennywort product would be much more prone to decay as compared to the bamboo product. Aranguren et al. [13] investigated the dependence of the mechanical properties of wood flour–polyester composites dependence on the moisture content at 60 and 90% relative humidity. Results indicated that mechanical properties (compression and bending tests) were severely affected by moisture content and it was found that this was a reversible effect, because the original values of the compression properties were recovered after drying. The review report [14] described the effect of water absorption, moisture content and swelling on natural and wood fibre reinforced composites. In spite of the recent progress in processing methods and improving the mechanical performance of wood fibre reinforced composites, limited experimental results are available on creep. Creep, the deformation over time of a material under stress, is one characteristic of wood fibre reinforced polymer composites that has resulted in poor performance in certain applications. Creep and creep rupture (time dependent failure) consideration are essential if wood fibre thermoplastic composites are to have long-term loading applications. Creep of polymers occurs because of a combination of elastic deformation and viscous flow, commonly known as viscoelastic deformation [15]. Creep in thermoplastics is a complex phenomenon, which depends both on material properties (molecular orientation, crystallinity, etc.) [16] and external parameters (applied stress, temperature and humidity). In general, the impact behaviour of composites laminates in engineering applications is of main importance. Table 1 Equilibrium moisture content of some natural and wood fibres [12] Fibre
Bamboo Bagasse Jute Aspen Southern pine Water hyacinth Pennywort
Equilibrium moisture content at 27 C 30% RH
65% RH
90% RH
4.5 4.4 4.6 4.9 5.8 6.2 6.6
8.9 8.8 9.9 11.1 12.0 16.7 18.3
14.7 15.8 16.3 21.5 21.7 36.2 56.8
Structure of the textiles, fibre type and content, matrix ductility and void content of the composites are the main structural parameters which affect the impact behaviour [17]. The impact resistance is the ability of a material and its structure to survive impact induced damages during an impact event. The objective of this study was to determine the effects of the addition of compatibiliser, wood fibre content and temperature on short term creep properties of wood fibre–polypropylene composites and to determine the influence of the moisture content on the impact properties of the composites.
2. Experimental 2.1. Materials Polypropylene (type Stamylan P17M10) was supplied from DSM, Gelsenkirchen, Germany. The wood fibres were standard hard wood fibre (Lignocel HBS 150-500) with particle size 150–500 mm, supplied by J. Rettenmaier & So¨hne GmbH+Co. and long wood fibre (particle size 4–25 mm) obtained from Johnson Controls, Lu¨neburg, Germany. Micrographs (Fig. 1) show the difference of fibre length for both wood fibres. A compatibilizer maleic anhydride polypropylene copolymer (Licomont AR 504 FG) was used with the intension of improving the mechanical properties of composites and it obtained from Clariant Corp., Frankfurt, Germany. 2.2. Compounding and processing Wood fibre was initially dried at 80 C in an air-circulating oven for 24 h and then mixed with matrix and coupling agent in a high speed mixer (Henschel, type HM40 KM120). Wood–polymer composites (WPC) were prepared in the agglomerate form by mixing different amount of wood fibres (40, 50 and 60%), coupling agent (5% of the wood fibre content) and relative amounts of polypropylene (PP).The WPC granules were then dried 80 C for 24 h (water content approximately 0.2%) before moulding. Test samples were prepared by injection moulding of the dried agglomerate at melting temperature 150– 180 C and mould temperature of 80–90 C.The mechanical properties were investigated at room temperature (23 C) at a relative humidity of 50%. Charpy impact test (EN ISO 179) was carried out with 10 unnotched samples per series. In each series standard deviation ( < 15%) was used to measure Charpy impact energy. To measure the impact characteristics values, the specimens were tested by using a low-velocity falling weight impact tester (EN ISO 6603-2) at room temperature in
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Fig. 1. Micrographs of hard wood fibre (a) and long wood fibre (b) (magnification 12.5:1).
non-penetration-mode. The impactor had a mass of 0.75 kg and the impact energy was 0.96 Joules. The results of the impact test can be described by two separate issues (Fig. 2): (a) Force–deflection curve: the force–deflection curve refers to all the materials behaviours including the damage initiation defined by the first significant drop of the force. (b) Characteristic values: loss energy (Wv) as a measure of dissipated energy and strain energy (Ws) as a measure of the stored energy, and the damping index (*) as ratio of loss energy to strain energy. Determination of moisture content was followed DIN 52375. The volume of the samples was calculated from the measurement of the dimensions ( 0.01 mm) of each specimen. The determination of moisture content and volume were performed on at least three specimens for each sample.
2.3. Moisture sorption Humid environments were prepared by an air-conditioned oven (Weiss, WKI 340) and specimens were stored there for 72 h at 60 and 90% relative humidity. To ensure the final humidity content, composite specimens were weighed after every 3 h. 2.4. Creep test Short term creep tests were performed with flexural creep equipment according to standard procedures (EN ISO 899-2). Standard injection moulded flexural specimens were used for the creep test. For temperature testing, the creep devices were placed inside an insulated chamber. A small fan and light bulbs were placed inside the chamber for air circulation and heating. A temperature controller was used to regulate the test temperatures of 23 (room temperature), 40 and 60 C within 1 C accuracy. The creep devices and specimens were preconditioned for two hours at each test temperature. The creep properties (creep modulus and creep strength) were measured with a increasing of time up to 180 min as a deformation of the composites.
3. Results and discussion 3.1. Moisture content
Fig. 2. Typical impact force–deflection curve for natural reinforced polymer composites including definition of the characteristic values used.
The final moisture content of hard wood and long wood fibre reinforced polypropylene composites in 60 and 90% RH environments are shown in Table 2 respectively. Long wood fibre reinforced composites are much more hygroscopic than the hard wood fibre reinforced composites which is nearly two times more at the wood fibre content 60 wt.%. It seems that the fibre length and geometry plays a role for different amount of moisture expansion in the wood fibre–PP composites.
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Table 2 Equilibrium moisture content of wood fibre-PP composites at dry, 60% RH and 90% RH atmospheres Sample
Moisture content (%) Wood fibre 40 wt.%
HW+PP HW+PP+MAH-PP LW+PP LW+PP+MAH-PP
Wood fibre 50 wt.%
Wood fibre 60 wt.%
Dry
60% RH
90% RH
Dry
60% RH
90% RH
Dry
60% RH
90% RH
0.22 0.21 0.80 0.47
0.50 0.45 1.23 1.04
0.75 0.71 1.37 1.04
0.31 0.29 1.03 0.64
0.77 0.56 1.42 1.07
1.14 0.77 2.06 1.10
0.57 0.41 1.1 0.64
0.78 0.59 1.60 1.25
1.19 0.86 2.18 1.29
With the increasing of wood fibre content, moisture content also increases in the both types of wood fibre– PP composites. Besides, the modification of the wood fibre with MAH-PP 5% reduces its hygroscopicity (around 75% for hard wood–PP composites at 90% RH at wood fibre content 60 wt.%) because a considerable amount of accessible OH groups disappeared to become firmly bonded to the polar carboxylic groups of the maleic acid. Figs. 3 and 4 summarize the resulting volume expansion due to water sorption in 60 and 90% RH atmospheres for the both types of wood fibre–PP composites with different fibre content. As expected, samples air conditioned at 90% RH showed higher volume expansion than those at 60% RH. In both cases, volume increment increases with the increasing of wood fibre content which is corresponds to the highest filler percentage loaded into the matrix. Moreover, results indicates that, modified with compatibiliser (MAH-PP), wood fibre–PP composites exhibit a lower volume expansion which is at best decreased 35% for long wood fibre–PP composites at 60% wood fibre content. Besides, the volume increment of all composites is rather important due to the indication of the composites dimensional stability.
Fig. 3. Volume increment of hardwood fibre–PP composites at different relative humidity atmospheres.
Fig. 4. Volume increment of long wood fibre–PP composites at different relative humidity atmospheres.
3.2. Charpy impact strength The results of Charpy impact strength are shown in Figs. 5 and 6. Both parts of the figure show that the Charpy impact strength of the composites, with and without compatibiliser, decreases with the increasing of wood fibre content.
Fig. 5. Charpy impact strength of hardwood fibre–PP composites at different relative humidity atmospheres.
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Fig. 6. Charpy impact strength of long wood fibre–PP composites at different relative humidity atmospheres.
The Charpy impact strength increases with the addition of MAH-PP 5%. With the increasing of moisture content from room humidity to 90% RH in both with and without MAH-PP wood fibre–PP composites, the Charpy impact strength decreases not a significant way. It indicates that, a short time moisture expansion (72 h) of the composites couldn’t affect the Charpy impact strength significantly. 3.3. Impact properties The force–deflection curve refers to associate damage initiation by the first significant change in the slope of the curve. Figs. 7 and 8 shows the impact resistance (maximum force) of both types of wood fibre–PP composites with MAH-PP5% at wood fibre content 60 wt.%. In both cases, maximum force increases with the increasing of equilibrium moisture content and long wood fibre–PP composites shows a greater maximum force than hard wood fibre–PP composites. The damping index for all samples was calculated by taking the ratio of dissipated energy (loss energy) to the stored energy (strain energy) to measure the impact characteristic values. The loss energy involves energy
Fig. 7. Impact resistance of hard wood fibre–PP composites at different relative humidity atmospheres (wood fibre content 60 wt.%).
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Fig. 8. Impact resistance of long wood fibre–PP composites at different relative humidity atmospheres (wood fibre content 60 wt.%).
which based on irreversible deformations, energy dissipation due to creation of matrix cracks and their propagation, delaminations and finally fibre fracture. Figs. 9 and 10 shows the damping index for both types of wood fibre–PP composites. For hard wood fibre–PP composites, damping index increases with the increasing of wood fibre content and decreasing with the addition of MAH-PP5%. Fig. 9 also illustrates that, with addition of MAH-PP5% at wood fibre content 60%, damping index decreased highest around 145%. With the increasing of equilibrium moisture content, the damping index decreases for all samples at a certain value which is not very significant. For long wood fibre– PP composites follows the similar trend to those of the hard wood fibre–PP composites except with the increasing of wood fibre content, the damping index decreases. 3.4. Creep properties To investigate the effect of the addition of compatibiliser, creep tests were performed on both untreated and treated composites with MAH-PP. These composites prepared have a constant wood fibre content (40
Fig. 9. Damping index of hard wood fibre–PP composites at different relative humidity atmospheres.
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Fig. 10. Damping index of long wood fibre–PP composites at different relative humidity atmospheres.
Fig. 12. Creep modulus of hard wood fibre–PP composites at different temperatures.
wt.%). Fig. 11 shows the creep modulus of the composites made of untreated and treated wood fibres at temperature 60 C. The treated specimens showed higher creep modulus than untreated specimens. That means, MAH-PP treated composites showed lower creep which was also observed for jute fibre reinforced PP composites [18]. Fig. 11 illustrates that in both cases, creep modulus decreases very fast at the initial position with the increasing of time and then decreases exponentially. The addition of compatibiliser usually improves fibre dispersion in matrix resins and facilitates the interfacial adhesion between the fibres and polymer matrix. The improvement of strength properties of wood fibre–PP composites with the addition of MAH-PP has been already reported. Fig. 12 shows the creep modulus of the hard wood– PP composites containing 40 wt.% wood fibre content under different temperatures. All curves show that creep modulus decreases with an increase in time and temperature. The creep modulus at 60 C was around 1800 MPa after 180 min, which was nearly 65% lower that of room temperature modulus after the same duration. Thermoplastic materials are typically softened by elevated temperatures. As a result, higher temperatures
normally reduce the creep modulus of matrix-dominant composites. The effect of wood fibre content on the creep response of the composites is shown in Figs. 13 and 14. For both wood fibre–PP composites, the creep test was performed at 40 C temperature level. Both hard wood and long wood fibre–PP composites containing wood fibre content 40 wt.% showed lowest creep modulus. When
Fig. 11. Creep modulus of hard wood fibre (40 wt.%) – PP composites with and without compatibiliser at 60 C.
Fig. 14. Creep modulus of long wood fibre–PP composites with different wood fibre content (temperature 40 C).
Fig. 13. Creep modulus of hard wood fibre–PP composites with different wood fibre content (temperature 40 C).
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response of the wood fibre reinforced PP composites were investigated in this study. This was also conducted to experimentally determine the effects of the addition of a compatibiliser and wood fibre content on short term creep characteristics of wood fibre–PP composites. The following points can be drawn from these studies.
Fig. 15. Creep strength of hard wood fibre–PP composites with compatibiliser at different temperatures after 180 min deformation.
The moisture content increases in composites with the increase of wood fibre content. Compatibiliser MAH-PP5% reduced the hygroscopicity (around 75%) for hard wood fibre-PP composites. Long wood fibre reinforced composites are much more hygroscopic than the hard wood fibre reinforced composites which is near two times more at the wood fibre content 60 wt.%. Maximum force increases with the increasing of moisture content. Damping index decreased around 145% with the addition of MAH-PP5% for hard wood fibre–PP composites at wood fibre content 60 wt.%. The creep modulus of wood fibre–PP composites increased around 165% with increasing wood fibre content 40–60 wt.%. Creep modulus and creep strength decreases with increasing temperature.
References Fig. 16. Creep strength of long wood fibre–PP composites with compatibiliser at different temperatures after 180 min deformation.
wood fibre content increases 40–60 wt.%, the creep modulus increases significantly to 165% for both wood fibre–PP composites and decreased 15–20% after 180 min deformation. In comparison between two wood fire–PP composites, long wood fibre–PP composites shows better creep modulus than hard wood fibre–PP composites which is around 50%. May be wood fibre length plays a role into interaction between wood fibre and polypropylene matrix. Figs. 15 and 16 show the creep strength after 180 min of both wood fibre–PP composites with the addition of compatibiliser at different temperatures. Figures illustrate that with the increasing of wood fibre content, the creep strength shows a relatively higher tendency and simultaneously a lower tendency with the increasing of temperature.
4. Conclusions The influence of moisture content on the impact behaviour and the effect of temperature on creep
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