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Hydro and thermal stability of die drawn wood polymer composites in comparison to solid wood
Composites Science and Technology 72 (2012) 1436–1442
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Hydro and thermal stability of die drawn wood polymer composites in comparison to solid wood I.A. Połec´ a,⇑, P.J. Hine a, M.J. Bonner a, I.M. Ward a, D.C. Barton b a b
School of Physics & Astronomy, IRC in Polymer Science and Technology, University of Leeds, Leeds LS2 9JT, UK School of Mechanical Engineering, University of Leeds, UK
a r t i c l e
i n f o
Article history: Received 1 February 2012 Received in revised form 30 April 2012 Accepted 6 May 2012 Available online 19 May 2012 Keywords: A. Short-fibre composites Wood powder B. Thermal properties Hygrothermal effect E. Die drawing
a b s t r a c t Both isotropic and oriented wood polymer composites (WPC) based on 40% w/w of a softwood powder/ hardwood powder and polypropylene (PP), together with solid pieces of wood, were subjected to water immersion and thermal expansion tests. Although generally die drawing increased the amount of water absorbed by the WPC by about 2-fold when compare to isotropic WPC, the oriented WPC exhibited extremely high hydro-dimensional stability. The values of the longitudinal and transverse swelling/shrinkage of the WPC oscillated only between 0 and 2.3% compared to values of between 4 and 14% for the solid woods. Incorporation of soft/hard wood powders into PP also substantially decreased its thermal expansion coefficient a in both the isotropic and the oriented states. This extremely positive effect was enhanced by increasing the draw ratio. In the longitudinal direction, a decreased from about 80 106 °C1 (for the isotropic PP) to 5 106 °C1 for the highly drawn PP filled with softwood. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction One of the most important properties taken into account during developing any new material is the dimensional stability of a product in a relatively large range of environmental conditions. Generally, high dimension resistance is desirable for any structural material. It is especially important not only when the material is to be used in conjunction with another material but also when the material is a composite or a blend of two or more components. Water (moisture) and temperature are the two factors which influence both properties and dimension of a polymer composite most considerably. During absorption, water molecules firstly come into the free spaces within the matrix i.e. the voids. At the same time water molecules penetrate and diffuse along the matrix/particle interface and are transported inside the material [1]. If compatibility between a filler and a polymer is insufficient the former is substantially exposed to water accelerating the process. Water may plastizise polymer chains and, penetrating the structure, may deteriorate interface between components leading to a decrease in mechanical and physical properties of the whole system. The negative effect is especially important for composites containing any organic plant-like components e.g. wood powders. This is mainly due to interaction of water with the plant fibres through OH groups and hydrogen bonding. Water absorbed by WPC causes so called hydrothermal ageing [2,3], i.e. it reduces ⇑ Corresponding author. E-mail address: [email protected] (I.A. Połec´). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.05.019
the overall properties, especially modulus, yield strength and glass transition temperature as reported by many groups [2–5]. A polymer material exposed to temperature variations can undergo several phenomena resulting in dimensional changes: (1) true thermal expansion (2) shrinkage, and (3) creep, all of which are seen as significant disadvantages. One of the most effective ways to overcome this problem is, as reported by many authors e.g. [6–8], incorporation of fillers into a polymer. This feature is mainly ascribed to the substantial differences in the thermal expansivity of the components due to which, when the temperature fluctuates, thermal stresses develop near the filler/polymer interface. On cooling, each component shrinks but the shrinkage of the matrix is restrained by the filler, thus setting up compressive stresses across the interface. When the composite is heated, the matrix expands more than the particles, and if the interface is capable of transferring the induced stresses, the expansion of the matrix is reduced. The situation is complicated when the materials show anisotropy in their properties. It was found [9–11] that highly oriented PE and PP exhibited very low, even negative, thermal expansion in the direction of polymer chain orientation a||. In the opposite direction a\, the thermal expansion of these polymers was large and positive. Despite the extensive studies on water absorption of WPCs, little attention has been paid to their thermal expansion as well as to their dimensional stability. It therefore seemed desirable to measure these two parameters and compare with the solid wood. This paper results from a continuing collaboration between the School of Physics and Astronomy and the School of Mechanical Engineering at the University of Leeds, directed at exploring the
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application of hydrostatic extrusion and die drawing for the production of high stiffness polymers in engineering systems. Although most of these studies have been focussed on unfilled polymers, there has been some research on polymers filled with glass [12,13] and hydroxyapatite [14,15]. Due to the market needs and our keen interest, WPC was another material class which was investigated. Our previous publications [16,17] focused on mechanical properties and their modelling. In the current paper we would like to contribute more understanding of WPCs providing experimental data on their hydro- and thermal stability. 2. Experimental details 2.1. Materials and composite manufacturing BP polypropylene pellets (PP) were used as the matrix for two types of commercially available wood powders (WP): Lignocel C120 (C120) and Lignocel HB120 (HB120) supplied by Rettenmaier GmbH, Germany. Two types of WPC, denoted C120PP and HB120PP, containing 40% w/w WP were blended using a co-rotating twin-screw extruder. Details on the component characteristics and the manufacturing process are reported in [16].
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stage, the materials were drawn at a temperature of 150 °C. In the second stage, the temperature was increased to 160 °C and 165 °C for C120PP and HB120PP respectively. For both draw ratios, a draw speed of 20 mm/min was used. 2.4. Water absorption and dimension stability Cylindrical samples of the unfilled and filled PP for each of the isotropic and oriented states, approximately 5 mm long, were dried in a vacuum oven at 103 ± 1 °C for 24 h. The samples were then placed in a glass vial and left for 12 h to cool. Following this, the samples were immersed in distilled water at room temperature. For the first 2 weeks, the measurements were taken in 2day intervals. Subsequently, the samples were measured once or twice a month. Before weighing, the samples were wiped on a paper tissue to eliminate the excess of water on the sample surfaces. The change in mass of the samples, to an accuracy of 0.001 mg, and their dimensions, to an accuracy of 0.01 mm, were measured. The percentage of absorbed water was calculated using the following equation:
MC ¼
m1 m0 100% m1
ð1Þ
where MC is the moisture content, m0 is the initial mass of the sample and m1 is the mass of the samples after immersing in water.
2.2. Sample preparation The samples were prepared from the compounded pellets by a compression moulding technique according to details reported in [16]. These isotropic samples were then machined to a tapered cylindrical shape suitable for use with the die drawing technique. 2.3. Die drawing Die drawing [18] is a technique for the solid deformation of polymers. After reaching thermal equilibrium, a polymer billet heated close to its melting point is pulled through a heated die by applying a tensile force to the billet at the die exit. The deformation imposed on a material is defined by the nominal draw ratio (RN: ratio of initial cross-sectional area to die exit cross-sectional area) and the actual draw ratio (RA: ratio of initial cross-sectional area to final product cross-sectional area). Die drawing of the filled and unfilled PP was carried out to nominal draw ratios of 3 (RN = 3) and 10 (RN = 10). For the reasons described in [16], to reach RN of 10, the materials drawn to the lower RN had to be redrawn in a second stage operation. In the first
2.5. Coefficient of linear thermal expansion Thermal expansion experiments were carried out using equipment developed in our laboratory [9]. As shown in Fig. 1, the device has two separate stations designed to measure samples with different geometries: the fibres or tapes in tension and the solid samples in compression. Thermal expansion measurements were made by ramping the temperature from 5 to 35 °C at a rate of 2 °C/min, dwelling for 10 min and then ramping the sample at the same rate back to 5 °C. The related change in the length sample DL was recorded and the coefficient of thermal expansion (a) was calculated using the general equation:
a¼
DL LDT
ð2Þ
where L is the original length of the sample and DT is temperature change. A LVDT (linear variable differential transducer) and a Pico voltage data logger were employed to measure the changes in length
Fig. 1. Sketch of a thermal expansion device: (a) the compression part, (b) the tension part.
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DL. A calibration of the system was performed by measuring the dependence of the output voltage on the displacement of the central core. The software registered the variations in voltage as a function of time caused by changes in the applied temperature. The sample was allowed to thermally equilibrate before testing. Each sample was measured over three whole cycles i.e. heating and cooling as one cycle, resulting in an average value of the thermal expansion coefficient at 20 °C. All samples were conditioned at 65% RH for 2 weeks prior to measurements following the guidelines of ASTM D1037-99 by placing them in a box containing saturated magnesium acetate tetrahydrate which gives an atmosphere at the required relative humidity. 2.5.1. Thermal expansion in compression – filled and unfilled polypropylene The thermal expansion coefficient (a) of the PP and its composites at various orientation levels was measured using the compression mode of the thermal expansion apparatus, a schematic sketch of which is shown in Fig. 1a. The cylindrical sample, approximately 5 mm in length, was placed on a low thermal expansion alloy (Invar) cemented to the bottom of a silica tube. The top of the tube was linked with the bottom of a LVDT, the measurement accuracy being 0.1 lm. The moving element of the LVDT was connected to the top of the sample via a long fused silica rod. The silica rod applied a light pressure to the sample. The temperature of the nitrogen gas in the silica tube was controlled by a programmable temperature controller. To establish a precisely, the thermal expansion of the invar platform without the sample was measured first and subtracted from the measurements taken with the sample in place.
2.5.2. Thermal expansion in tension – solid wood The tension device schematically illustrated in Fig. 1b was used to measure the thermal expansion of the solid wood. This mode of the apparatus is specially designed to measure fibres and other types of materials with a small thermal expansion such as solid wood [19]. In order to amplify the measurement, the wood samples were much longer than those measured on the compression side of the device. The samples, approximately 60 mm long and 5 mm wide, were suspended inside a silica tube. The temperature inside the tube was controlled by a heated liquid nitrogen gas. The LVDT was attached to the bottom end of the sample. The instrumental error was reduced to a minimum by constructing the majority of the rig with invar. Heat transfer was reduced by separating the transducer from the heated section by stainless steel rods. 3. Results and discussion 3.1. Water absorption and hydro dimensional stability 3.1.1. Isotropic materials The average percentage of the water absorbed by isotropic filled and unfilled PP as a function of immersion time is shown in Fig. 2a. Incorporation of hydrophilic fibres into the hydrophobic polymer is generally associated with an increase in water absorption of the composite material as confirmed for both C120PP and HB120PP composites. After soaking in a water bath for about 20 days at room temperature, the absorption speed decreased radically indicating close to saturation conditions. In total, after 58 days, the C120PP composites absorbed approximately 10% of their weight in water but the HB120PP composites appreciably less. The influence of
Fig. 2. Water absorption and corresponding dimensional changes for the isotropic PP and its composites with C120 and HB120: (a) isotropic stage, (b) drawn at RN of 3, (c) drawn at RN of 10.
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the type of the wood powders used on the quantity of the water absorbed is not very clear. There are two feasible explanations for this behaviour: methodological and structural. Although the composites were conditioned under the same conditions, it cannot be ensured that they had possessed initially the same amount of water. Structurally, hardwood is more complicated than softwood and is commonly assumed to be more porous than softwoods [20]. It also possesses more OH groups and thus has a higher potential than softwood to attract and retain water [21]. However, many authors [3–5] observed that composites containing hardwood powder generally absorbed a lower amount of water compared to the softwood composites. There might be three reasons for such behaviour: (1) pre-existing structural water (bound water and free water); on average hardwood generally possesses more of this than softwood so absorbs relatively less, (2) mechanical properties of these fillers; this data indicated that the composites filled with the hardwood generally had higher tensile and flexural properties. These findings are consistent with the results published in our previous paper [16], (3) density; when density increases, the number of paths conducting water through the structure has to be reduced. However the results of our investigation did not confirm this hypothesis. There was only a marginal difference in the densities between isotropic C120PP and HB120PP composites, 1049 ± 3 kg/m3 and 1026 ± 26 kg/m3 respectively, as reported in [16]. The porous structure of a plant fibre is naturally designed to retain water. The OH groups present on the surface of cellulose fibres attract water and are strongly attached by hydrogen. In addition, each cellulose fibre contains a lumen at its centre [20] which acts as a water tank. In a composite system, however, due to the micro-scale diameter of the lumens and most pores, the highly viscous PP melt blocked the vast majority of lumens and pores, retarding or preventing further water penetration. This may be the reason why both isotropic C120PP and HB120PP exhibited very small dimensional swelling in both longitudinal and transverse directions as shown in Fig. 2a. These results suggest that the wood particles were well encapsulated by the PP.
3.1.2. Drawn materials For the die drawn materials, the amount of water absorbed is a function of three variables: immersion time, draw ratio and the filler; Fig. 2b and c. Draw ratio exhibited a major impact on the level of absorbed water as expected, since it affects the porosity of these materials. The void fraction for the composites drawn at RN of 3 at 150 °C was 26 ± 5% and 23 ± 7% for C120PP and HB120PP respectively. An increase in RN to 10 and draw temperature to 165 °C caused a reduction in the void fraction of the composites to a level of 6 ± 1% and 5 ± 1% for C120PP and HB120PP respectively. In comparison with the isotropic materials, the composites drawn to RN of 3 revealed a 3-fold increase in the amount of absorbed water while their density decreased by approximately 36% (details in [16]). For C120PP, a 20% rise in density caused by drawing to RN of 10 resulted in a reduction of water absorption by only 5%. For HB120PP, the drop was 25%. It has to be noted that the composites drawn to RN = 10 (presented in Fig. 2c) were oriented at different temperatures: HB120PP at 165 °C and C120PP at 160 °C. For this reason it proved difficult to establish exactly how the type of the wood particles used influenced the water absorption of the highly oriented wood composites. On the basis of these results for the isotropic and partially oriented composites, it can be concluded that the type of wood powder exhibited some influence on the water absorption. However, for the composites drawn to RN of 10, it is more likely that draw temperature had a higher impact than the wood species. As reported in [16] the composites possessed the same density (about 850 kg/m3) when drawn at 165 °C. However, there is no clear
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evidence that HB120 did not contribute to the greater reduction in water absorption. In order to determine the dimensional stability of C120PP and HB120PP at different orientation levels, the diameter (transverse direction) and the length (longitudinal direction) of the samples were also measured. Although the results presented in Fig. 2a–c are rather scattered, the anisotropy in the dimensional change is noticeable; all the materials shrank in the longitudinal direction and swelled in the transverse direction. The dimensional changes were however comparatively small and the shrinkage of the composites could have been caused by relaxation of the oriented PP. The composites shrank more than the unfilled polymer which might be a result of lower orientation of the PP surrounding the particles. The most striking result to emerge from this data is that although the oriented composites revealed large water absorption, the dimension stability of the materials was very high. This suggests that the die drawn composites behaved as a sponge. The water penetrating the composite structure was stored in the empty spaces between the polymer fibres. Some water must have been absorbed by the wood fillers due to their chemical affinity but the size of the analysed samples was only slightly affected. Independently of draw ratio, both composites shrank marginally by up to 0.3% along the draw direction. The highly oriented C120PP was the exception; its shrinkage increased to about 2.2%. In the opposite direction, the composites drawn to RN of 3 swelled by 1.7%. With increasing deformation ratio, swelling of C120PP was reduced slightly although the distance between the softwood fillers and the oriented polymer chains was much more reduced. HB120PP drawn to RN of 10 swelled on average by 2.5%. Again, this might be attributed to draw temperature, and thus density. As shown in our previous paper, [16], for the systems analysed, the oriented polymer chains are the main filaments which transfer the stress across the composite materials. The C120 and HB120 particles did not act as a reinforcing element but as a filler. Therefore it is expected that the mechanical properties of the drawn materials will not be affected by water uptake. If this hypothesis is correct, die drawing gives a potential solution for the significant problem of swelling due to moisture uptake and it would be a very important benefit in developing these materials. 3.1.3. Solid wood To determine whether the composites here absorbed higher quantity of water and swelled more than solid wood, representative wood samples were kept in the same water bath as the composite materials under the same conditions. During the first days of immersion, as Fig. 3a illustrates, the solid woods absorbed the largest quantity of water. The wood absorbed water in the same manner as the composites. The beech (which represents the hardwood) absorbed a lower amount of water than fir and spruce (the softwoods). This might be explained by the differences in density. The beech is the densest (706 ± 5 kg/m3) and the spruce is the lightest (414 ± 9 kg/m3) of the solid woods investigated. Because wood fibres interact with water not only at the surface like synthetic fibres but also in the bulk, 2 dimensional swelling was also observed as shown in Fig. 3b and c. The results confirmed, as could be expected, that the solid wood absorbed much more water leading to more significant swelling than the WPC. 3.2. Coefficient of linear thermal expansion 3.2.1. Isotropic unfilled and filled PP The linear thermal expansion coefficients (a) of the unfilled and filled PP in both the isotropic and die drawn stages measured in compression in longitudinal a|| and transverse direction a\ are presented in Table 1. It can be seen that the wood particles, with smaller thermal expansion (due to high thermal stability of the
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components but also may be affected by the mechanical properties of the filler and its geometry. This assumption is especially important when the one of these constituents, e.g. the WP, is sensitive to changes in external environmental conditions, which may affect its mechanical and thermal properties. This hypothesis found confirmation in the experiments curried out by Hatta et al. [22] who reported that a|| of short fibre wood composites was reduced with increasing tensile modulus of the filler (the drier the wood the higher its tensile modulus). This was because a compressive stress of higher magnitude could develop along the axis of the short fibres with higher tensile modulus. For the same reason, higher aspect ratio of short fibres might cause higher magnitude of the compression stress. Thus, a|| decreases with an increase of the aspect ratio. This could explain the lower a|| for C120PP; C120 particles had higher aspect ratio than HB120 as reported in our previous article from this series [17]. 3.2.2. Drawn materials The structure of a die drawn polymer is perceived as periodic crystalline regions connected by oriented amorphous regions. The amorphous areas provide the major portion of the thermal expansion [9] and therefore drawing increases the thermal stability of the overall structure. The thermal expansion of oriented polymers shows anisotropy as reported previously in e.g. [9,11,23,24] to a degree strongly dependent on the deformation ratio. Our results, Fig. 4a and b, confirm this phenomenon. As RA increased, a|| decreased rapidly and a\ increased slightly. Choy et al. [23,24] proposed that the reduction in a|| in oriented polymers is ascribed to the presence of the highly stiff intercrystalline bridges. These bridges constrain the expansion of the amorphous phase along the draw direction. The increasing orientation of the molecules in the amorphous region as draw ratio increases gives rise to more severe constraints on the expansion of the amorphous
Fig. 3. Water absorption (a) and corresponding dimensional changes for the solid wood measured in tangential (b) and radial (c) directions.
Table 1 Linear thermal expansion coefficients of the isotropic PP, C120PP and HB120PP.
PP C120PP HB120PP
a|| (106 °C1)
a\(106 °C1)
98.7 ± 0.5 70.0 ± 1.3 78.6 ± 0.1
98.2 ± 2.9 73.4 ± 1.8 73.7 ± 1.4
cellulose skeleton) than PP, are indeed a suitable material for reducing the thermal expansion of the unfilled PP in both measured directions. This 30% decrease in expansivity of both the composites, together with the increase in their tensile moduli as reported previously in [16], may suggest that there are some interactions between the wood particles and the matrix at small strain (<0.5%). Otherwise, if there is insufficient bonding between the constituents, thermal stresses do not develop at the interface and, as pointed out by Wang and Kwei [7], the thermal expansion coefficient of the composite will be the same as that of the polymer matrix. Holliday and Robinson [6] and Wang and Kwei [7] have implied that the development of those internal thermal stresses at the interface might depend not only on the thermal expansion of the
Fig. 4. Changes in thermal expansion coefficient (a) with increasing RA and draw temperature: (a) parallel to the draw direction a||, (b) normal to the draw direction a\.
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regions. Hence, a|| will reach the negative thermal expansion of the polymer crystal ac|| along the draw direction. However, this hypothesis had to be rejected after showing [9,25,26] that the longitudinal thermal expansion of drawn polymers can be far lower than the thermal expansion of their crystal structure. An extensive study on the thermal expansion of oriented pure polymers was undertaken by Orchard et al. [9,11,27]. They suggested that a decrease in the thermal expansion of the oriented polymers in the draw direction with increasing deformation ratio is not simply due to the negative expansion coefficient of the crystalline region but also relates to the frozen- in entropic internal stresses in the amorphous regions developed during manufacture. A further step in our investigation was to establish the effect of draw ratio on the values of a|| and a\ for the die drawn WPC. The results obtained are presented in Fig. 4a and b. The expansion behaviour of C120PP and HB10PP followed the behaviour of the die drawn unfilled PP in both directions; a|| decreased drastically and a\ increased slightly with increasing RA. The results showed that for low draw ratio, the incorporation of the C120 and HB120 particles into the PP produced an overall significant reduction in both a|| and a\. The data indicated also that C120 reduced both a|| and a\ more effectively than HB120. However, there is no evidence to conclude that the difference was caused by the morphological differences between these two wood powders and or, what is more likely, due to the varying porosity of the two composite materials. As shown previously in [16], the density of the die drawn HB120PP and C120PP is a function of the draw ratio and the draw temperature. To recapitulate, all HB120PP composites were denser than C120PP. Hatta and co-workers [22] and Thomason and Groenewoud [28] reported that a was significantly reduced by the existence of microvoids in the matrix, thus by a drop in the density of the materials. As mentioned above, when the composite is subjected to a temperature rise, a thermal expansion mismatch strain is induced. This gives rise to compressive and tensile stresses in the matrix and the particle respectively. If microvoids are dispersed in the matrix, the microvoids are subjected to the matrix compressive stresses, resulting in the shrinkage of the void volume and hence, the overall reduction in a. For both the C120PP and HB120PP composites drawn to RN of 10, the value of a\ increased marginally but it was still significantly lower in comparison to the unfilled PP. It was difficult to establish which of C120 and HB120 is more effective in reducing the expansion of the pure PP especially since the composites were drawn at different temperature, and temperature has a profound impact on the structure, properties and hence the thermal expansion of the final material. The materials drawn at higher temperature generally speaking are less oriented and consequently their thermal expansivity is larger. The changes in a|| of the composites induced by an increase in RN exhibited an opposite trend. Although HB120PP was oriented at higher temperature than C120PP, it exhibited lower a|| suggesting the expansivity of the composite is dominated either by the HB120 powder or by the density/porosity. It is unlikely that the composition of the wood particles had any effect on this behaviour. According to characteristics given by the filler supplier listed [16], both the wood powders contain similar amount of cellulose (46%) and non-cellulosic components (51%). Therefore it can be concluded that the composite density, which is related to draw temperature, is the main parameter dictating the expansion properties of the C120PP and HB120PP composites. 3.2.3. Solid wood The results of the investigations for three species of solid wood, namely, spruce, fir and beech are showed in Fig. 5. Dimensional changes of solid wood depend on three parameters: moisture level, temperature and the wood structure. The influence of moisture is much greater than that of temperature [29]. This is because a
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Fig. 5. Variation in the thermal expansion coefficients of the solid woods with species testing direction and thermal treatment.
change in temperature causes changes in moisture content, and the resulting swelling and shrinking are much greater than the effects of thermal expansion. According to Dinwoodie [29], about two thirds of the cellulose substance is in some degree crystalline and about one third amorphous; however others [21,30,31] assume that only 5–10% is truly amorphous. The high level of crystallinity provides, as for the die drawn polymers, the low magnitude of the thermal expansivity in the fibre direction. Nishiyama et al. [32] and Hori and Wada [33,34] studied cellulose crystallography. They reported that the cellulose crystal is thermally stable up to 180 °C (a in the c axis is in the order of 107 /K), and above this temperature it increases gradually. The stability is attributed to strong covalent bonds linking each glucose, (and thus cellulose) chains in the crystal c-axis direction. This is also reinforced by interamolecular hydrogen bonds [35–37]. The longitudinal thermal expansion a||, although substantially different between species, can be up to 5 106 °C1 [21,29,38,39]. a\ can be an order of magnitude higher than a||. This phenomenon is even more complex and is not fully understood because many factors and parameters interact. In general, the large positive transverse thermal expansion is ascribed to both the amorphous regions and the non-cellulosic components in a real wood (plant) which are mixed up with the cellulose and occupy empty spaces between and within microfibrils. Their reaction on temperature change is unknown due to superimposition of water on the relationship and the substantial differences within and between species. The results given by Weatherwax and Stamm [39] and Tsoumis [20] indicated a connection between the thermal expansion of wood and its density; the former increased with increasing the density. a of the wood analysed in this study followed this finding. The density of the wood samples established by simply dividing the samples mass by their volumes was as follows: 414 ± 9, 636 ± 5 and 706 ± 5 kg/m3 for spruce, fir and beech respectively. This corresponds with the trend for a\, namely the higher density of the solid wood, the higher its a\.
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Fig. 5 shows measured values of a|| and a\ for some typical solid wood samples, measured in the same way as the WPC. Although these may not be identical to the wood powders used, they do indicate that the woods show anisotropic behaviour and confirm the literature. The values of a|| are below 5 106 °C1 and ten times lower than the typical values of a\. The solid wood samples did all show an interesting behaviour, in that different values were measured in heating and cooling. Although we attribute this to changes in water moisture, further experiments would be required for clarification. 4. Conclusions The hydro- and thermal dimensional stabilities of the isotropic and anisotropic wood polymer composites containing 40% w/w soft/hard wood powders were measured. The results for unfilled and filled PP were determined at each processing stage, together with the solid wood as a comparison (which was immersed in a water bath at room temperature for 58 days). All the composites absorbed a much lower amount of water than the solid woods; the isotropic composites absorbed only approximately 10% water whereas the woods absorbed between 50 and 70%. For the composites drawn to draw ratio of 3 this absorption increased up to 30%. Redrawing to draw ratio of 10 resulted in a reduction in water absorption of only 5–25%. The increase in the draw temperature from 160 °C to 165 °C for the composites drawn to draw ratio 10 reduced the water content to 20%. Despite the significant water uptake, the die drawn composites showed only marginal dimensional swelling during this period of time. It is suggested that the die drawn composites behave as a sponge with the water penetrating the composite structure by being stored in the voids formed around the wood particles during die drawing. If this hypothesis is correct, and if the mechanical properties are not affected by water, die drawing would give a potential solution for this extremely important problem of swelling in the WPC field. It is likely that this is correct because it has been shown that the wood particles, in the system analysed, acted as a filler rather than as a reinforcement and the oriented polymer chains are the main filaments transferring the stress across the material. Another significant finding to emerge from this study is that the C120 and HB120 particles, with their own very small thermal expansion coefficient a, substantially decreased the a of the pure PP in both the isotropic and the oriented states. This extremely positive effect in the drawing direction was enhanced by increasing the draw ratio. In the longitudinal direction, a decreased from about 80 106 °C1 to 5 106 °C1 for the isotropic and highly drawn C120PP respectively. The HB120PP composites exhibited the same tendency. In the transverse direction, the coefficient of thermal expansion for the die drawn composites was much larger although the beneficial impact of the wood fillers, especially of the softwood C120, remained. In conclusion the die drawn WPC combine improved mechanical, very low swelling in water and better dimensional stability compared to isotropic WPC. References [1] Zhang JH, Zhan MS. Visual experiments for water absorbing process of fiberreinforced composites. J Compos Mater 2004;38(9):779–90. [2] Gauthier R, Joly C, Coupas AC, Gauthier H, Escoubes M. Interfaces in polyolefin/ cellulosic fiber composites: chemical coupling, morphology, correlation with adhesion and aging in moisture. Polym Compos 1998;19(3):287–300. [3] Lin QF, Zhou XD, Dai G. Effect of hydrothermal environment on moisture absorption and mechanical properties of wood flour-filled polypropylene composites. J Appl Polym Sci 2002;85(14):2824–32. [4] Bledzki AK, Faruk O. Wood fibre reinforced polypropylene composites: effect of fibre geometry and coupling agent on physico-mechanical properties. Appl Compos Mater 2003;10(6):365–79.
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Compos Sci Technol 2010;70(1):53–60. [18] Coates PD, Ward IM. Drawing of polymers through a conical die. Polymer 1979;20(12):1553–60. [19] Simpson W, TenWolde A. Wood handbook – Wood as an engineering material. U.S. Department of Agriculture, Forest Service, Forest Product Laboratory; 1999. [20] Tsoumis T. Science and technology of wood: structure properties utilization. New York, London: Chapman & Hall; 1991. [21] Stamm AJ. Wood and cellulose science. New York: The Ronald Press Company; 1964. [22] Hatta H, Takei T, Taya M. Effects of dispersed microvoids on thermal expansion behavior of composite materials. Mat Sci Eng A - Struct 2000;285(1–2): 99–110. [23] Choy CL, Chen FC, Young K. Negative thermal-expansion in oriented crystalline polymers. J Polym Sci Part B – Pol Phys 1981;19(2):335–52. [24] Choy CL, Chen FC, Ong EL. Anistropic Thermal-expansion of oriented crystalline polymers. Polymer 1979;20(10):1191–8. [25] Ward IM, Wilczynski AP. Bounds for the elastic-constants of a unidirectional fiber composite – a new approach. J Mater Sci 1993;28(7):1973–7. [26] Bozec YL, Kaang S, Hine PJ, Ward IM. The thermal-expansion behaviour of hotcompacted polypropylene and polyethylene composites. Compos Sci Technol 2000;60(3):333–44. [27] Gibson AG, Ward IM. Thermal-expansion behavior of hydrostatically extruded linear polyethylene. J Mater Sci 1979;14(8):1838–42. [28] Thomason JL, Groenewoud WM. The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene 2. Therm Properties Compos Part A – Appl Sci Manuf 1996;27(7):555–65. [29] Dinwoodie JM. Wood nature’s cellular, polymeric fibre-composite. London: The Institute of Metals; 1989. [30] Skaar C. Concise encyclopedia of science and technology of wood & woodbased materials. Oxford: Pergamon; 1989. [31] Sjöström E. Wood chemistry, fundamentals and application. New York, London: Academic Press; 1981. [32] Nishiyama Y, Sugiyama J, Chanzy H, Langan P. Crystal structure and hydrogen bonding system in cellulose 1(alpha), from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 2003;125(47):14300–6. [33] Hori R, Wada M. The thermal expansion of wood cellulose crystals. Cellulose 2005;12(5):479–84. [34] Hori R, Wada M. The thermal expansion of cellulose II and IIIII crystals. Cellulose 2006;13(3):281–90. [35] Langan P, Nishiyama Y, Chanzy H. A revised structure and hydrogen-bonding system in cellulose II from a neutron fiber diffraction analysis. J Am Chem Soc 1999;121(43):9940–6. [36] Wada M. Lateral thermal expansion of cellulose I-beta and IIII polymorphs. J Polym Sci Part B – Pol Phys 2002;40(11):1095–102. [37] Langan P, Sukumar N, Nishiyama Y, Chanzy H. Synchrotron X-ray structures of cellulose I-beta and regenerated cellulose II at ambient temperature and 100 K. Cellulose 2005;12(6):551–62. [38] Dinwoodie JM. Timber: its nature and behaviour. 2nd ed. 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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Hydro and thermal stability of die drawn wood polymer composites in comparison to solid wood I.A. Połec´ a,⇑, P.J. Hine a, M.J. Bonner a, I.M. Ward a, D.C. Barton b a b
School of Physics & Astronomy, IRC in Polymer Science and Technology, University of Leeds, Leeds LS2 9JT, UK School of Mechanical Engineering, University of Leeds, UK
a r t i c l e
i n f o
Article history: Received 1 February 2012 Received in revised form 30 April 2012 Accepted 6 May 2012 Available online 19 May 2012 Keywords: A. Short-fibre composites Wood powder B. Thermal properties Hygrothermal effect E. Die drawing
a b s t r a c t Both isotropic and oriented wood polymer composites (WPC) based on 40% w/w of a softwood powder/ hardwood powder and polypropylene (PP), together with solid pieces of wood, were subjected to water immersion and thermal expansion tests. Although generally die drawing increased the amount of water absorbed by the WPC by about 2-fold when compare to isotropic WPC, the oriented WPC exhibited extremely high hydro-dimensional stability. The values of the longitudinal and transverse swelling/shrinkage of the WPC oscillated only between 0 and 2.3% compared to values of between 4 and 14% for the solid woods. Incorporation of soft/hard wood powders into PP also substantially decreased its thermal expansion coefficient a in both the isotropic and the oriented states. This extremely positive effect was enhanced by increasing the draw ratio. In the longitudinal direction, a decreased from about 80 106 °C1 (for the isotropic PP) to 5 106 °C1 for the highly drawn PP filled with softwood. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction One of the most important properties taken into account during developing any new material is the dimensional stability of a product in a relatively large range of environmental conditions. Generally, high dimension resistance is desirable for any structural material. It is especially important not only when the material is to be used in conjunction with another material but also when the material is a composite or a blend of two or more components. Water (moisture) and temperature are the two factors which influence both properties and dimension of a polymer composite most considerably. During absorption, water molecules firstly come into the free spaces within the matrix i.e. the voids. At the same time water molecules penetrate and diffuse along the matrix/particle interface and are transported inside the material [1]. If compatibility between a filler and a polymer is insufficient the former is substantially exposed to water accelerating the process. Water may plastizise polymer chains and, penetrating the structure, may deteriorate interface between components leading to a decrease in mechanical and physical properties of the whole system. The negative effect is especially important for composites containing any organic plant-like components e.g. wood powders. This is mainly due to interaction of water with the plant fibres through OH groups and hydrogen bonding. Water absorbed by WPC causes so called hydrothermal ageing [2,3], i.e. it reduces ⇑ Corresponding author. E-mail address: [email protected] (I.A. Połec´). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.05.019
the overall properties, especially modulus, yield strength and glass transition temperature as reported by many groups [2–5]. A polymer material exposed to temperature variations can undergo several phenomena resulting in dimensional changes: (1) true thermal expansion (2) shrinkage, and (3) creep, all of which are seen as significant disadvantages. One of the most effective ways to overcome this problem is, as reported by many authors e.g. [6–8], incorporation of fillers into a polymer. This feature is mainly ascribed to the substantial differences in the thermal expansivity of the components due to which, when the temperature fluctuates, thermal stresses develop near the filler/polymer interface. On cooling, each component shrinks but the shrinkage of the matrix is restrained by the filler, thus setting up compressive stresses across the interface. When the composite is heated, the matrix expands more than the particles, and if the interface is capable of transferring the induced stresses, the expansion of the matrix is reduced. The situation is complicated when the materials show anisotropy in their properties. It was found [9–11] that highly oriented PE and PP exhibited very low, even negative, thermal expansion in the direction of polymer chain orientation a||. In the opposite direction a\, the thermal expansion of these polymers was large and positive. Despite the extensive studies on water absorption of WPCs, little attention has been paid to their thermal expansion as well as to their dimensional stability. It therefore seemed desirable to measure these two parameters and compare with the solid wood. This paper results from a continuing collaboration between the School of Physics and Astronomy and the School of Mechanical Engineering at the University of Leeds, directed at exploring the
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application of hydrostatic extrusion and die drawing for the production of high stiffness polymers in engineering systems. Although most of these studies have been focussed on unfilled polymers, there has been some research on polymers filled with glass [12,13] and hydroxyapatite [14,15]. Due to the market needs and our keen interest, WPC was another material class which was investigated. Our previous publications [16,17] focused on mechanical properties and their modelling. In the current paper we would like to contribute more understanding of WPCs providing experimental data on their hydro- and thermal stability. 2. Experimental details 2.1. Materials and composite manufacturing BP polypropylene pellets (PP) were used as the matrix for two types of commercially available wood powders (WP): Lignocel C120 (C120) and Lignocel HB120 (HB120) supplied by Rettenmaier GmbH, Germany. Two types of WPC, denoted C120PP and HB120PP, containing 40% w/w WP were blended using a co-rotating twin-screw extruder. Details on the component characteristics and the manufacturing process are reported in [16].
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stage, the materials were drawn at a temperature of 150 °C. In the second stage, the temperature was increased to 160 °C and 165 °C for C120PP and HB120PP respectively. For both draw ratios, a draw speed of 20 mm/min was used. 2.4. Water absorption and dimension stability Cylindrical samples of the unfilled and filled PP for each of the isotropic and oriented states, approximately 5 mm long, were dried in a vacuum oven at 103 ± 1 °C for 24 h. The samples were then placed in a glass vial and left for 12 h to cool. Following this, the samples were immersed in distilled water at room temperature. For the first 2 weeks, the measurements were taken in 2day intervals. Subsequently, the samples were measured once or twice a month. Before weighing, the samples were wiped on a paper tissue to eliminate the excess of water on the sample surfaces. The change in mass of the samples, to an accuracy of 0.001 mg, and their dimensions, to an accuracy of 0.01 mm, were measured. The percentage of absorbed water was calculated using the following equation:
MC ¼
m1 m0 100% m1
ð1Þ
where MC is the moisture content, m0 is the initial mass of the sample and m1 is the mass of the samples after immersing in water.
2.2. Sample preparation The samples were prepared from the compounded pellets by a compression moulding technique according to details reported in [16]. These isotropic samples were then machined to a tapered cylindrical shape suitable for use with the die drawing technique. 2.3. Die drawing Die drawing [18] is a technique for the solid deformation of polymers. After reaching thermal equilibrium, a polymer billet heated close to its melting point is pulled through a heated die by applying a tensile force to the billet at the die exit. The deformation imposed on a material is defined by the nominal draw ratio (RN: ratio of initial cross-sectional area to die exit cross-sectional area) and the actual draw ratio (RA: ratio of initial cross-sectional area to final product cross-sectional area). Die drawing of the filled and unfilled PP was carried out to nominal draw ratios of 3 (RN = 3) and 10 (RN = 10). For the reasons described in [16], to reach RN of 10, the materials drawn to the lower RN had to be redrawn in a second stage operation. In the first
2.5. Coefficient of linear thermal expansion Thermal expansion experiments were carried out using equipment developed in our laboratory [9]. As shown in Fig. 1, the device has two separate stations designed to measure samples with different geometries: the fibres or tapes in tension and the solid samples in compression. Thermal expansion measurements were made by ramping the temperature from 5 to 35 °C at a rate of 2 °C/min, dwelling for 10 min and then ramping the sample at the same rate back to 5 °C. The related change in the length sample DL was recorded and the coefficient of thermal expansion (a) was calculated using the general equation:
a¼
DL LDT
ð2Þ
where L is the original length of the sample and DT is temperature change. A LVDT (linear variable differential transducer) and a Pico voltage data logger were employed to measure the changes in length
Fig. 1. Sketch of a thermal expansion device: (a) the compression part, (b) the tension part.
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DL. A calibration of the system was performed by measuring the dependence of the output voltage on the displacement of the central core. The software registered the variations in voltage as a function of time caused by changes in the applied temperature. The sample was allowed to thermally equilibrate before testing. Each sample was measured over three whole cycles i.e. heating and cooling as one cycle, resulting in an average value of the thermal expansion coefficient at 20 °C. All samples were conditioned at 65% RH for 2 weeks prior to measurements following the guidelines of ASTM D1037-99 by placing them in a box containing saturated magnesium acetate tetrahydrate which gives an atmosphere at the required relative humidity. 2.5.1. Thermal expansion in compression – filled and unfilled polypropylene The thermal expansion coefficient (a) of the PP and its composites at various orientation levels was measured using the compression mode of the thermal expansion apparatus, a schematic sketch of which is shown in Fig. 1a. The cylindrical sample, approximately 5 mm in length, was placed on a low thermal expansion alloy (Invar) cemented to the bottom of a silica tube. The top of the tube was linked with the bottom of a LVDT, the measurement accuracy being 0.1 lm. The moving element of the LVDT was connected to the top of the sample via a long fused silica rod. The silica rod applied a light pressure to the sample. The temperature of the nitrogen gas in the silica tube was controlled by a programmable temperature controller. To establish a precisely, the thermal expansion of the invar platform without the sample was measured first and subtracted from the measurements taken with the sample in place.
2.5.2. Thermal expansion in tension – solid wood The tension device schematically illustrated in Fig. 1b was used to measure the thermal expansion of the solid wood. This mode of the apparatus is specially designed to measure fibres and other types of materials with a small thermal expansion such as solid wood [19]. In order to amplify the measurement, the wood samples were much longer than those measured on the compression side of the device. The samples, approximately 60 mm long and 5 mm wide, were suspended inside a silica tube. The temperature inside the tube was controlled by a heated liquid nitrogen gas. The LVDT was attached to the bottom end of the sample. The instrumental error was reduced to a minimum by constructing the majority of the rig with invar. Heat transfer was reduced by separating the transducer from the heated section by stainless steel rods. 3. Results and discussion 3.1. Water absorption and hydro dimensional stability 3.1.1. Isotropic materials The average percentage of the water absorbed by isotropic filled and unfilled PP as a function of immersion time is shown in Fig. 2a. Incorporation of hydrophilic fibres into the hydrophobic polymer is generally associated with an increase in water absorption of the composite material as confirmed for both C120PP and HB120PP composites. After soaking in a water bath for about 20 days at room temperature, the absorption speed decreased radically indicating close to saturation conditions. In total, after 58 days, the C120PP composites absorbed approximately 10% of their weight in water but the HB120PP composites appreciably less. The influence of
Fig. 2. Water absorption and corresponding dimensional changes for the isotropic PP and its composites with C120 and HB120: (a) isotropic stage, (b) drawn at RN of 3, (c) drawn at RN of 10.
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the type of the wood powders used on the quantity of the water absorbed is not very clear. There are two feasible explanations for this behaviour: methodological and structural. Although the composites were conditioned under the same conditions, it cannot be ensured that they had possessed initially the same amount of water. Structurally, hardwood is more complicated than softwood and is commonly assumed to be more porous than softwoods [20]. It also possesses more OH groups and thus has a higher potential than softwood to attract and retain water [21]. However, many authors [3–5] observed that composites containing hardwood powder generally absorbed a lower amount of water compared to the softwood composites. There might be three reasons for such behaviour: (1) pre-existing structural water (bound water and free water); on average hardwood generally possesses more of this than softwood so absorbs relatively less, (2) mechanical properties of these fillers; this data indicated that the composites filled with the hardwood generally had higher tensile and flexural properties. These findings are consistent with the results published in our previous paper [16], (3) density; when density increases, the number of paths conducting water through the structure has to be reduced. However the results of our investigation did not confirm this hypothesis. There was only a marginal difference in the densities between isotropic C120PP and HB120PP composites, 1049 ± 3 kg/m3 and 1026 ± 26 kg/m3 respectively, as reported in [16]. The porous structure of a plant fibre is naturally designed to retain water. The OH groups present on the surface of cellulose fibres attract water and are strongly attached by hydrogen. In addition, each cellulose fibre contains a lumen at its centre [20] which acts as a water tank. In a composite system, however, due to the micro-scale diameter of the lumens and most pores, the highly viscous PP melt blocked the vast majority of lumens and pores, retarding or preventing further water penetration. This may be the reason why both isotropic C120PP and HB120PP exhibited very small dimensional swelling in both longitudinal and transverse directions as shown in Fig. 2a. These results suggest that the wood particles were well encapsulated by the PP.
3.1.2. Drawn materials For the die drawn materials, the amount of water absorbed is a function of three variables: immersion time, draw ratio and the filler; Fig. 2b and c. Draw ratio exhibited a major impact on the level of absorbed water as expected, since it affects the porosity of these materials. The void fraction for the composites drawn at RN of 3 at 150 °C was 26 ± 5% and 23 ± 7% for C120PP and HB120PP respectively. An increase in RN to 10 and draw temperature to 165 °C caused a reduction in the void fraction of the composites to a level of 6 ± 1% and 5 ± 1% for C120PP and HB120PP respectively. In comparison with the isotropic materials, the composites drawn to RN of 3 revealed a 3-fold increase in the amount of absorbed water while their density decreased by approximately 36% (details in [16]). For C120PP, a 20% rise in density caused by drawing to RN of 10 resulted in a reduction of water absorption by only 5%. For HB120PP, the drop was 25%. It has to be noted that the composites drawn to RN = 10 (presented in Fig. 2c) were oriented at different temperatures: HB120PP at 165 °C and C120PP at 160 °C. For this reason it proved difficult to establish exactly how the type of the wood particles used influenced the water absorption of the highly oriented wood composites. On the basis of these results for the isotropic and partially oriented composites, it can be concluded that the type of wood powder exhibited some influence on the water absorption. However, for the composites drawn to RN of 10, it is more likely that draw temperature had a higher impact than the wood species. As reported in [16] the composites possessed the same density (about 850 kg/m3) when drawn at 165 °C. However, there is no clear
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evidence that HB120 did not contribute to the greater reduction in water absorption. In order to determine the dimensional stability of C120PP and HB120PP at different orientation levels, the diameter (transverse direction) and the length (longitudinal direction) of the samples were also measured. Although the results presented in Fig. 2a–c are rather scattered, the anisotropy in the dimensional change is noticeable; all the materials shrank in the longitudinal direction and swelled in the transverse direction. The dimensional changes were however comparatively small and the shrinkage of the composites could have been caused by relaxation of the oriented PP. The composites shrank more than the unfilled polymer which might be a result of lower orientation of the PP surrounding the particles. The most striking result to emerge from this data is that although the oriented composites revealed large water absorption, the dimension stability of the materials was very high. This suggests that the die drawn composites behaved as a sponge. The water penetrating the composite structure was stored in the empty spaces between the polymer fibres. Some water must have been absorbed by the wood fillers due to their chemical affinity but the size of the analysed samples was only slightly affected. Independently of draw ratio, both composites shrank marginally by up to 0.3% along the draw direction. The highly oriented C120PP was the exception; its shrinkage increased to about 2.2%. In the opposite direction, the composites drawn to RN of 3 swelled by 1.7%. With increasing deformation ratio, swelling of C120PP was reduced slightly although the distance between the softwood fillers and the oriented polymer chains was much more reduced. HB120PP drawn to RN of 10 swelled on average by 2.5%. Again, this might be attributed to draw temperature, and thus density. As shown in our previous paper, [16], for the systems analysed, the oriented polymer chains are the main filaments which transfer the stress across the composite materials. The C120 and HB120 particles did not act as a reinforcing element but as a filler. Therefore it is expected that the mechanical properties of the drawn materials will not be affected by water uptake. If this hypothesis is correct, die drawing gives a potential solution for the significant problem of swelling due to moisture uptake and it would be a very important benefit in developing these materials. 3.1.3. Solid wood To determine whether the composites here absorbed higher quantity of water and swelled more than solid wood, representative wood samples were kept in the same water bath as the composite materials under the same conditions. During the first days of immersion, as Fig. 3a illustrates, the solid woods absorbed the largest quantity of water. The wood absorbed water in the same manner as the composites. The beech (which represents the hardwood) absorbed a lower amount of water than fir and spruce (the softwoods). This might be explained by the differences in density. The beech is the densest (706 ± 5 kg/m3) and the spruce is the lightest (414 ± 9 kg/m3) of the solid woods investigated. Because wood fibres interact with water not only at the surface like synthetic fibres but also in the bulk, 2 dimensional swelling was also observed as shown in Fig. 3b and c. The results confirmed, as could be expected, that the solid wood absorbed much more water leading to more significant swelling than the WPC. 3.2. Coefficient of linear thermal expansion 3.2.1. Isotropic unfilled and filled PP The linear thermal expansion coefficients (a) of the unfilled and filled PP in both the isotropic and die drawn stages measured in compression in longitudinal a|| and transverse direction a\ are presented in Table 1. It can be seen that the wood particles, with smaller thermal expansion (due to high thermal stability of the
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components but also may be affected by the mechanical properties of the filler and its geometry. This assumption is especially important when the one of these constituents, e.g. the WP, is sensitive to changes in external environmental conditions, which may affect its mechanical and thermal properties. This hypothesis found confirmation in the experiments curried out by Hatta et al. [22] who reported that a|| of short fibre wood composites was reduced with increasing tensile modulus of the filler (the drier the wood the higher its tensile modulus). This was because a compressive stress of higher magnitude could develop along the axis of the short fibres with higher tensile modulus. For the same reason, higher aspect ratio of short fibres might cause higher magnitude of the compression stress. Thus, a|| decreases with an increase of the aspect ratio. This could explain the lower a|| for C120PP; C120 particles had higher aspect ratio than HB120 as reported in our previous article from this series [17]. 3.2.2. Drawn materials The structure of a die drawn polymer is perceived as periodic crystalline regions connected by oriented amorphous regions. The amorphous areas provide the major portion of the thermal expansion [9] and therefore drawing increases the thermal stability of the overall structure. The thermal expansion of oriented polymers shows anisotropy as reported previously in e.g. [9,11,23,24] to a degree strongly dependent on the deformation ratio. Our results, Fig. 4a and b, confirm this phenomenon. As RA increased, a|| decreased rapidly and a\ increased slightly. Choy et al. [23,24] proposed that the reduction in a|| in oriented polymers is ascribed to the presence of the highly stiff intercrystalline bridges. These bridges constrain the expansion of the amorphous phase along the draw direction. The increasing orientation of the molecules in the amorphous region as draw ratio increases gives rise to more severe constraints on the expansion of the amorphous
Fig. 3. Water absorption (a) and corresponding dimensional changes for the solid wood measured in tangential (b) and radial (c) directions.
Table 1 Linear thermal expansion coefficients of the isotropic PP, C120PP and HB120PP.
PP C120PP HB120PP
a|| (106 °C1)
a\(106 °C1)
98.7 ± 0.5 70.0 ± 1.3 78.6 ± 0.1
98.2 ± 2.9 73.4 ± 1.8 73.7 ± 1.4
cellulose skeleton) than PP, are indeed a suitable material for reducing the thermal expansion of the unfilled PP in both measured directions. This 30% decrease in expansivity of both the composites, together with the increase in their tensile moduli as reported previously in [16], may suggest that there are some interactions between the wood particles and the matrix at small strain (<0.5%). Otherwise, if there is insufficient bonding between the constituents, thermal stresses do not develop at the interface and, as pointed out by Wang and Kwei [7], the thermal expansion coefficient of the composite will be the same as that of the polymer matrix. Holliday and Robinson [6] and Wang and Kwei [7] have implied that the development of those internal thermal stresses at the interface might depend not only on the thermal expansion of the
Fig. 4. Changes in thermal expansion coefficient (a) with increasing RA and draw temperature: (a) parallel to the draw direction a||, (b) normal to the draw direction a\.
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regions. Hence, a|| will reach the negative thermal expansion of the polymer crystal ac|| along the draw direction. However, this hypothesis had to be rejected after showing [9,25,26] that the longitudinal thermal expansion of drawn polymers can be far lower than the thermal expansion of their crystal structure. An extensive study on the thermal expansion of oriented pure polymers was undertaken by Orchard et al. [9,11,27]. They suggested that a decrease in the thermal expansion of the oriented polymers in the draw direction with increasing deformation ratio is not simply due to the negative expansion coefficient of the crystalline region but also relates to the frozen- in entropic internal stresses in the amorphous regions developed during manufacture. A further step in our investigation was to establish the effect of draw ratio on the values of a|| and a\ for the die drawn WPC. The results obtained are presented in Fig. 4a and b. The expansion behaviour of C120PP and HB10PP followed the behaviour of the die drawn unfilled PP in both directions; a|| decreased drastically and a\ increased slightly with increasing RA. The results showed that for low draw ratio, the incorporation of the C120 and HB120 particles into the PP produced an overall significant reduction in both a|| and a\. The data indicated also that C120 reduced both a|| and a\ more effectively than HB120. However, there is no evidence to conclude that the difference was caused by the morphological differences between these two wood powders and or, what is more likely, due to the varying porosity of the two composite materials. As shown previously in [16], the density of the die drawn HB120PP and C120PP is a function of the draw ratio and the draw temperature. To recapitulate, all HB120PP composites were denser than C120PP. Hatta and co-workers [22] and Thomason and Groenewoud [28] reported that a was significantly reduced by the existence of microvoids in the matrix, thus by a drop in the density of the materials. As mentioned above, when the composite is subjected to a temperature rise, a thermal expansion mismatch strain is induced. This gives rise to compressive and tensile stresses in the matrix and the particle respectively. If microvoids are dispersed in the matrix, the microvoids are subjected to the matrix compressive stresses, resulting in the shrinkage of the void volume and hence, the overall reduction in a. For both the C120PP and HB120PP composites drawn to RN of 10, the value of a\ increased marginally but it was still significantly lower in comparison to the unfilled PP. It was difficult to establish which of C120 and HB120 is more effective in reducing the expansion of the pure PP especially since the composites were drawn at different temperature, and temperature has a profound impact on the structure, properties and hence the thermal expansion of the final material. The materials drawn at higher temperature generally speaking are less oriented and consequently their thermal expansivity is larger. The changes in a|| of the composites induced by an increase in RN exhibited an opposite trend. Although HB120PP was oriented at higher temperature than C120PP, it exhibited lower a|| suggesting the expansivity of the composite is dominated either by the HB120 powder or by the density/porosity. It is unlikely that the composition of the wood particles had any effect on this behaviour. According to characteristics given by the filler supplier listed [16], both the wood powders contain similar amount of cellulose (46%) and non-cellulosic components (51%). Therefore it can be concluded that the composite density, which is related to draw temperature, is the main parameter dictating the expansion properties of the C120PP and HB120PP composites. 3.2.3. Solid wood The results of the investigations for three species of solid wood, namely, spruce, fir and beech are showed in Fig. 5. Dimensional changes of solid wood depend on three parameters: moisture level, temperature and the wood structure. The influence of moisture is much greater than that of temperature [29]. This is because a
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Fig. 5. Variation in the thermal expansion coefficients of the solid woods with species testing direction and thermal treatment.
change in temperature causes changes in moisture content, and the resulting swelling and shrinking are much greater than the effects of thermal expansion. According to Dinwoodie [29], about two thirds of the cellulose substance is in some degree crystalline and about one third amorphous; however others [21,30,31] assume that only 5–10% is truly amorphous. The high level of crystallinity provides, as for the die drawn polymers, the low magnitude of the thermal expansivity in the fibre direction. Nishiyama et al. [32] and Hori and Wada [33,34] studied cellulose crystallography. They reported that the cellulose crystal is thermally stable up to 180 °C (a in the c axis is in the order of 107 /K), and above this temperature it increases gradually. The stability is attributed to strong covalent bonds linking each glucose, (and thus cellulose) chains in the crystal c-axis direction. This is also reinforced by interamolecular hydrogen bonds [35–37]. The longitudinal thermal expansion a||, although substantially different between species, can be up to 5 106 °C1 [21,29,38,39]. a\ can be an order of magnitude higher than a||. This phenomenon is even more complex and is not fully understood because many factors and parameters interact. In general, the large positive transverse thermal expansion is ascribed to both the amorphous regions and the non-cellulosic components in a real wood (plant) which are mixed up with the cellulose and occupy empty spaces between and within microfibrils. Their reaction on temperature change is unknown due to superimposition of water on the relationship and the substantial differences within and between species. The results given by Weatherwax and Stamm [39] and Tsoumis [20] indicated a connection between the thermal expansion of wood and its density; the former increased with increasing the density. a of the wood analysed in this study followed this finding. The density of the wood samples established by simply dividing the samples mass by their volumes was as follows: 414 ± 9, 636 ± 5 and 706 ± 5 kg/m3 for spruce, fir and beech respectively. This corresponds with the trend for a\, namely the higher density of the solid wood, the higher its a\.
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Fig. 5 shows measured values of a|| and a\ for some typical solid wood samples, measured in the same way as the WPC. Although these may not be identical to the wood powders used, they do indicate that the woods show anisotropic behaviour and confirm the literature. The values of a|| are below 5 106 °C1 and ten times lower than the typical values of a\. The solid wood samples did all show an interesting behaviour, in that different values were measured in heating and cooling. Although we attribute this to changes in water moisture, further experiments would be required for clarification. 4. Conclusions The hydro- and thermal dimensional stabilities of the isotropic and anisotropic wood polymer composites containing 40% w/w soft/hard wood powders were measured. The results for unfilled and filled PP were determined at each processing stage, together with the solid wood as a comparison (which was immersed in a water bath at room temperature for 58 days). All the composites absorbed a much lower amount of water than the solid woods; the isotropic composites absorbed only approximately 10% water whereas the woods absorbed between 50 and 70%. For the composites drawn to draw ratio of 3 this absorption increased up to 30%. Redrawing to draw ratio of 10 resulted in a reduction in water absorption of only 5–25%. The increase in the draw temperature from 160 °C to 165 °C for the composites drawn to draw ratio 10 reduced the water content to 20%. Despite the significant water uptake, the die drawn composites showed only marginal dimensional swelling during this period of time. It is suggested that the die drawn composites behave as a sponge with the water penetrating the composite structure by being stored in the voids formed around the wood particles during die drawing. If this hypothesis is correct, and if the mechanical properties are not affected by water, die drawing would give a potential solution for this extremely important problem of swelling in the WPC field. It is likely that this is correct because it has been shown that the wood particles, in the system analysed, acted as a filler rather than as a reinforcement and the oriented polymer chains are the main filaments transferring the stress across the material. Another significant finding to emerge from this study is that the C120 and HB120 particles, with their own very small thermal expansion coefficient a, substantially decreased the a of the pure PP in both the isotropic and the oriented states. This extremely positive effect in the drawing direction was enhanced by increasing the draw ratio. In the longitudinal direction, a decreased from about 80 106 °C1 to 5 106 °C1 for the isotropic and highly drawn C120PP respectively. The HB120PP composites exhibited the same tendency. In the transverse direction, the coefficient of thermal expansion for the die drawn composites was much larger although the beneficial impact of the wood fillers, especially of the softwood C120, remained. In conclusion the die drawn WPC combine improved mechanical, very low swelling in water and better dimensional stability compared to isotropic WPC. References [1] Zhang JH, Zhan MS. Visual experiments for water absorbing process of fiberreinforced composites. J Compos Mater 2004;38(9):779–90. [2] Gauthier R, Joly C, Coupas AC, Gauthier H, Escoubes M. Interfaces in polyolefin/ cellulosic fiber composites: chemical coupling, morphology, correlation with adhesion and aging in moisture. Polym Compos 1998;19(3):287–300. [3] Lin QF, Zhou XD, Dai G. Effect of hydrothermal environment on moisture absorption and mechanical properties of wood flour-filled polypropylene composites. J Appl Polym Sci 2002;85(14):2824–32. [4] Bledzki AK, Faruk O. Wood fibre reinforced polypropylene composites: effect of fibre geometry and coupling agent on physico-mechanical properties. Appl Compos Mater 2003;10(6):365–79.
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