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wood flour thermoplastic composites: Manufacturing and mechanical characterization
Composites: Part A 42 (2011) 649–657
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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Hybrid recycled glass fiber/wood flour thermoplastic composites: Manufacturing and mechanical characterization Marco Valente, Fabrizio Sarasini ⇑, Francesco Marra, Jacopo Tirillò, Giovanni Pulci Department of Chemical Engineering Materials Environment, Sapienza-Università di Roma, Via Eudossiana 18, 00184 Rome, Italy
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Article history: Received 20 October 2010 Received in revised form 29 January 2011 Accepted 5 February 2011 Available online 26 February 2011 Keywords: A. Polymer-matrix composites (PMCs) A. Hybrid A. Recycling B. Mechanical properties
a b s t r a c t Hybrid thermoplastic composites from wood flour and recycled glass fibers were manufactured through a two-step process involving a kinetic mixer and a compression molding machine. To evaluate the effect of recycled glass fibers, hybrid composites containing virgin glass fibers were also manufactured and tested. Mechanical properties of the composites including flexural modulus and strength, hardness as a function of temperature, screw withdrawal resistance and water absorption behavior were studied. The flexural modulus and hardness were found to increase as a function of increasing wood flour and glass fiber content, whilst the flexural strength and screw withdrawal resistance decreased as a function of increasing wood flour content, even though a positive effect of the addition of glass fibers was found. The recycled glass fibers showed comparable behavior to that of the virgin ones. A morphological analysis of hybrid composites to study the interfacial interactions was carried out by scanning electron microscopy. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Wood plastic composites (WPCs) are relatively new materials which are gaining considerable attention during last years. This is mainly due to a renewed interest in environmental protection. The use of wood, a natural and renewable resource, can help to reduce the carbon footprint of plastics and their proportion in municipal solid waste. Moreover, WPCs are potentially recyclable and can be considered sustainable materials, as the wood can be obtained from sawdust or scrap wood products as well as the plastic, which can be mainly derived from consumer and industrial recycling efforts. The present generation of WPCs comprises a very broad range of materials, from those manufactured from 100% post-consumer waste to those containing pulped wood and engineering resins. Common polymers used in WPCs include polyethylene (low and high density) [1–4], polypropylene [5–8], polyvinyl chloride [9–11], polystyrene [12,13], acrylonitrile–butadiene–styrene [14], nylon [15,16]. Currently, large volumes of WPCs are manufactured from recycled polymers, however virgin materials are used when demanding properties are needed. Current applications for WPCs are in diverse fields, including windows and door frames, interior panels in cars, railings, decking, cladding and fences. The increasing industrial acceptance of WPCs is due to their low moisture absorption, resistance to biological attack, dimensional stability and to a combination of high specific stiffness and strength, less abrasion during processing, low density and low ⇑ Corresponding author. E-mail address: [email protected] (F. Sarasini). 1359-835X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2011.02.004
price with respect to mineral fillers. An important issue in WPC manufacturing is represented by the addition of compatibilizers in order to improve compatibility between the polar wood and the non-polar polymers. A non optimal compatibility is often accompanied by significantly reduced properties, especially impact and tensile strength. In this regard, many approaches have been described in the literature aiming to improve the interfacial adhesion between the organic fillers and the polymer matrix, among which one of the most promising is the addition of maleic anhydride grafted polyolefins as compatibilizers [17–20]. A possible solution to enhance the mechanical properties of natural fiber reinforced composites (including WPCs), can be represented by the hybridization with inorganic fillers [21–27]. Hybridization may offset the disadvantages of one component by the addition of another fiber. In this regard, this paper aims at evaluating the effect of short glass fibers on wood flour thermoplastic composites. Glass fibers are known to be a suitable reinforcement for polymers. An effective way to enhance the performance of glass fiber while minimizing cost and landfill disposal of thermoset composites would greatly improve the economic attractiveness of WPCs. A possible approach to this issue would be using glass fibers which have been recovered from post-industrial waste. This paper reports the results of a project investigating the possibility of using glass fibers obtained from thermoset composites in combination with wood flour reinforced low density polyethylene (LDPE) and polypropylene (PP) composites. For comparison purposes, hybrid composites containing virgin glass fibers were also manufactured and tested. The effect of recycled glass fiber contents on water absorption behavior, mechanical properties (flexural strength and
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modulus), screw withdrawal resistance and microstructure of the hybrid composites was addressed. 2. Materials and methods 2.1. Materials Two thermoplastic polymers were used as matrix materials, namely low density polyethylene (LDPE) and polypropylene (PP), which were supplied by ExxonMobil Chemical. LDPE LD 250 has the following properties: a MFI of 5 g/10 min (190 °C/2.16 kg), a density of 0.916 g/cm3, a flexural strength of 15 ± 1.21 MPa, a flexural modulus of 460 ± 47 MPa, a Shore D hardness of 48 ± 0.37 (at room temperature), whilst PP was a PP1042 with a MFI of 1.9 g/ 10 min (230 °C/2.16 kg) and a density of 0.9 g/cm3, a flexural strength of 23 ± 1.43 MPa, a flexural modulus of 1355 ± 170 MPa, a Shore D hardness of 59 ± 0.68 (at room temperature). Wood flour (WF) of hardwood beech was kindly supplied by La.So.Le. Est Srl – Italy (Figs. 1 and 2). The sieve analysis (Fig. 2c) showed that most of the wood particles fall in the 35–60 mesh sizes, with the corresponding diameter ranging between 500 and 250 lm, respectively. Both virgin E-glass fibers (chop length = 3 mm) and recycled ones (Figs. 3 and 4) were used as reinforcement. The recycled fibers were obtained from mechanical recycling of glass fiber reinforced polyester composites coming from the automotive industry. The details of the mechanical grinding process and plant have been published elsewhere [28]. For comparison purposes, samples of Ipê wood (labeled as I) were used for water absorption tests. Ipê is a tropical wood often used in outdoor applications (decking). 2.2. Composite manufacturing The manufacturing process is shown schematically in Fig. 1. A compression molding machine was used to mold the square plates (200 mm 200 mm 10 mm) from which the samples for the characterization were cut. Prior to processing, both polymers and WF were oven dried at 65 °C and 105 °C for 24 h in order to remove moisture. A kinetic mixer was used to blend the polymer and the
reinforcements (wood flour and glass fibers). Kinetic mixing is a batch polymer processing technique that uses high shear and rapid rotational motion (3000 rpm) to create frictional heat sufficient to volatilize moisture and melt thermoplastics. Polymer and fillers are fed to the mixing chamber, rapidly (3 min) brought to melt temperature of polymers, mixed and then discharged as a thoroughly homogenized molten mass. As the temperature increased up to a preset value (130 °C and 160 °C for LDPE and PP, respectively), the chamber door was opened and the molten mass was transferred to the compression molding. A summary of the samples manufactured and tested are reported in Table 1. 2.3. Mechanical properties measurements The flexural properties (strength and modulus) were measured in four point bend tests at room temperature using a Zwick/Roell Z010 equipped with a 10 kN load cell. The tests were performed in accordance with ASTM D 7264. The test parameters were as follows: crosshead speed of 2 mm/min, support span length of 140 mm. The specimens were obtained from the square plates with the following dimensions: 200 mm 30 mm 10 mm (L W t). Strain gauges were used to measure the strain for the evaluation of elastic modulus. Five specimens of each sample were tested and the average results are reported. Hardness measurements (Shore D) were performed according to ASTM D 2240, using a Shore Model S1 Digital Durometer by Instron. The hardness value for each sample was calculated as the average of 30 measurements at five different temperatures, namely 0, 23 (room temperature), 40, 60 and 80 °C (±2 °C). The screw withdrawal test (ASTM D 1037) determines the load required to pull a screw from the panel specimen. A photograph of the screw (major diameter = 5 mm) used is shown in Fig. 5a. The threaded length was 18 mm. The screw was hand-driven perpendicular to the face of the panel (18 mm) into each specimen immediately before testing. A 4 mm diameter pilot hole was drilled into each specimen. Two panels were glued together in order to achieve the required specimen thickness. For the withdrawal tests, the specimens were 80 mm in length, 30 mm in width and 20 mm in thickness. Three screws for each composite type were tested. The screws were withdrawn at a uniform rate of speed (2 mm/min) un-
Fig. 1. Manufacturing process of the hybrid composites.
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Fig. 2. (a and b) SEM images of WF at different magnifications; (c) sieve curve.
Fig. 3. Photograph (left) and SEM (right) micrograph of virgin glass fibers.
Fig. 4. Photograph and SEM images of recycled glass fibers at different magnifications.
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Table 1 Wood plastic composite formulations (percent by weight). Composite sample code
LDPE content
PP content
Wood flour content
Glass fibers content
Recycled glass fibers content
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20
80 65 50 35 45 40 60 50 60 50 45 40 – – – – – – – –
– – – – – – – – – – – – 80 65 50 40 45 40 45 40
20 35 50 65 45 40 30 30 30 30 45 40 20 35 50 60 45 40 45 40
– – – – 10 20 10 20 – – – – – – – – 10 20 – –
– – – – – – – 10 20 10 20 – – – – – – 10 20
Note: LDPE, PP, W, V and R codes were used for low density polyethylene, polypropylene, wood flour, virgin glass fibers and recycled glass fibers, respectively.
Fig. 5. (a) Screw used in the withdrawal test; (b) holding apparatus and (c) experimental set-up.
til maximum load was recorded at room temperature (23 ± 2 °C). As shown in Fig. 5b and c, a specific screw holding apparatus was designed and used during the tests.
again after 24 h. The percentage increase in weight (W) during immersion was calculated as follows:
Wð%Þ ¼ 2.4. Water absorption tests Water absorption tests were performed in accordance with ASTM D 570. Three specimens were oven dried at 105 °C for 24 h. The conditioned specimens were immersed in distilled water for 2 h and 24 h at a temperature of 23 ± 2 °C. At the end of the preset time, the specimens were removed from the water, all surface water was wiped off with a cloth, and then were weighed. After 2 h immersion, the specimens were replaced in water and weighed
mt m0 100 m0
where m0 and mt are the conditioned and wet weight (at time t), respectively. 2.5. Scanning electron microscopy (SEM) The fracture surfaces of the flexural test specimens were investigated using a scanning electron microscope (Philips XL40). All specimens were sputter coated with gold prior to examination.
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3. Results and discussion 3.1. Morphology of WF, virgin and recycled glass fibers Fig. 2a and b shows SEM micrographs of the wood flour used in this study. As can be seen in Fig. 3, wood flour is comprised of fiber bundles rather than individual wood fibers with small aspect ratios (usually 1–5 [29]). Though the low aspect ratio can reduce the reinforcing ability, mechanical performance of the composite is sufficient for many applications. In fact, the use of wood fibers in polymer composites still greatly lags behind that of wood flour because of greater cost and increased processing difficulties when using traditional plastics processing methods. The virgin glass fibers are short fibers traditionally used in composites, as can be seen in Fig. 3. Recycled glass fibers appear to be completely different from the virgin ones, as can be clearly seen in Fig. 4. They appear to be heavily entangled and they do not seem to suffer from evident surface damage. However, it is noted the presence of some residual resin coming from the previous mechanical recycling operation. The entanglement could affect to a great extent both the processing and mechanical properties of the resulting hybrid composites.
3.2. Mechanical properties of composites A summary of hardness data is presented in Table 2. As for wood flour composites, hardness of polymer matrix is found to slightly increase with increasing WF content, both for LDPE and PP, as in [30]. The increase is more evident at higher temperatures, where the polymer suffer from a softening effect with increasing temperature, whilst WF is not affected by temperatures as high as 80 °C. The presence of glass fibers causes a further improvement of hardness especially at higher temperatures, whilst a significant difference between virgin and recycled fibers is not observed, thus confirming the positive role of recycled glass fibers. The values for PP-based composites are comparatively higher than those of LDPEbased ones, because of higher hardness and stiffness of polypropylene matrix. As a whole, the presence of both WF and glass fibers is beneficial as regards the hardness of the thermoplastic polymers investigated, especially at high temperature where they are able to counteract effectively the polymer softening.
The flexural strength and stiffness of the composites measured using four point bending tests are shown in Figs. 6–9 for LDPE- and PP-based composites, respectively. As a general comment, the flexural strength of the composites increases with decreasing wood content in the matrix. The addition of glass fibers does not cause an improvement of flexural strength compared with the neat polymer, even though an enhancement of strength occurred for hybrid composites in comparison with those having higher amount of wood flour (65 wt.%). Nevertheless, this increase is not sufficient to attain the strength values of composites reinforced with the smallest amount of wood flour (LDPE80W20 and PP80W20). This trend is confirmed by the fact that lowering the wood flour content while keeping constant the glass fibers content resulted in composites characterized by comparable flexural strength (LDPE50W30V20 and LDPE50W30R20). This is quite a common feature for filled thermoplastic composites [31], especially for composites reinforced with lignocellulosic fillers [1,30,32,33]. In particulate filled and short fiber reinforced composites the dominant micromechanical deformation process is the debonding of the phases. Debonding stress depends on particle size, on the stiffness of the matrix and on interfacial adhesion [34]. The low aspect ratio of wood flour particles makes them prone to debonding: this problem can only be addressed by improving adhesion through the use of coupling agents [33,35]. The decrease in flexural strength is therefore to be ascribed to a poor interface between polymer and reinforcement (WF and glass fibers) which makes ineffective the transfer of stress from the matrix to the fibers, thus not allowing a full exploitation of the reinforcement. The recycled glass fibers provided strength values which are comparable with those of the virgin ones, thus highlighting that recycling operation did not affect the reinforcing efficiency of the fibers, even though a suitable compatibilizing agent is needed between glass and thermoplastic polymer [36,37]. The flexural modulus of the composites exhibited a different behavior, increasing with reinforcement content. This trend is even more marked with the addition of glass fibers, both virgin and recycled. This is in agreement with the literature [1,29,32,33]. Modulus is usually less dependent of interfacial adhesion than strength is [33], and at the low strains involved in modulus calculations the reinforcement did not exhibit considerable debonding. Adding fillers to a polymer restrains the movements of its chains, thus increasing the stiffness but significantly decreasing the ductility and toughness of the composites, which was noted
Table 2 Shore D hardness of hybrid composites. Composite sample code
Temperature (°C) 0
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20 a
63.32 66.22 67.06 66.49 66.95 68.22 67.25 67.62 67.74 67.12 66.67 67.36 75.22 75.52 76.63 76.89 78.85 79.37 77.32 78.74
(1.55a) (1.22) (1.35) (1.12) (1.96) (1.79) (1.81) (1.94) (1.32) (1.32) (1.62) (1.19) (3.29) (3.02) (2.35) (1.38) (1.84) (1.02) (1.65) (1.94)
Room temperature
40
55.16 56.59 57.89 58.93 59.86 62.64 59.01 59.49 59.38 60.07 59.47 61.38 66.71 67.83 68.02 68.31 68.60 70.40 68.47 71.83
44.37 45.06 48.55 50.62 51.21 53.26 49.64 50.13 48.63 49.18 49.38 50.87 52.43 58.33 59.76 60.32 61.74 63.32 60.77 61.39
(0.87) (1.62) (1.08) (1.29) (1.65) (1.43) (1.64) (1.96) (0.99) (1.02) (1.15) (1.07) (2.24) (1.18) (1.81) (1.62) (1.12) (1.82) (1.84) (2.29)
Values are average of 30 measurements and values in parentheses are standard deviations.
60 (0.91) (1.98) (2.37) (1.56) (1.09) (1.12) (1.20) (2.04) (1.80) (1.34) (1.74) (2.62) (3.04) (1.35) (1.48) (1.02) (1.76) (1.26) (1.95) (1.34)
36.00 37.86 44.15 44.54 45.81 48.28 45.32 47.34 45.23 46.31 45.94 47.28 49.20 50.98 52.12 54.05 56.00 57.90 54.97 56.45
80 (0.86) (1.31) (1.33) (1.78) (1.68) (2.03) (1.28) (1.69) (1.26) (1.58) (1.18) (1.09) (1.07) (1.54) (1.63) (2.21) (2.37) (1.89) (1.55) (2.17)
29.92 32.98 34.08 36.74 38.56 40.65 37.23 38.12 36.96 37.18 37.25 39.84 44.63 45.84 47.82 51.44 52.54 54.19 51.83 52.61
(1.35) (2.19) (2.06) (1.95) (2.04) (1.84) (1.23) (1.47) (2.01) (1.84) (1.76) (1.65) (1.71) (1.27) (1.67) (2.90) (1.78) (1.09) (1.18) (1.83)
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Fig. 9. Flexural modulus of PP-based composites.
Fig. 6. Flexural strength of LDPE-based composites.
Table 3 Screw withdrawal resistance of composites.
a
Fig. 7. Flexural modulus of LDPE-based composites.
Composite sample code
Screw withdrawal resistance (N/mm)
COVa (%)
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20
91.63 91.92 57.16 48.09 65.27 76.91 85.85 93.06 75.04 81.66 63.34 71.79 206.11 193.21 145.54 133.60 182.46 146.87 153.44 142.28
8.60 2.92 4.45 8.25 5.91 9.12 7.04 8.75 9.23 5.97 2.63 10.05 6.15 4.44 4.45 9.52 11.23 10.04 9.36 10.83
COV = coefficient of variation.
Table 4 Water absorption of hybrid composites. Composite sample code
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20 I
Fig. 8. Flexural strength of PP-based composites.
Water absorption (%) 2h
24 h
0.20 (0.01a) 0.54 (0.03) 1.45 (0.01) 3.18 (0.44) 1.21 (0.09) 1.13 (0.14) 0.35 (0.01) 0.46 (0.03) 0.31 (0.01) 0.61 (0.06) 1.31 (0.01) 1.97 (0.40) 0.22 (0.03) 0.52 (0.03) 0.89 (0.04) 2.60 (0.08) 1.29 (0.11) 1.95 (0.01) 1.56 (0.22) 1.46 (0.19) 10.79 (1.84)
0.38 (0.01) 1.03 (0.03) 3.25 (0.17) 7.16 (0.64) 2.85 (0.22) 2.56 (0.25) 0.66 (0.01) 0.83 (0.05) 0.57 (0.02) 1.16 (0.05) 2.79 (0.07) 4.61 (0.96) 0.32 (0.01) 0.90 (0.04) 1.43 (0.10) 4.51 (0.12) 2.29 (0.11) 3.30 (0.19) 2.66 (0.46) 2.37 (0.20) 27.22 (2.16)
a Values are average of three replicates and values in parentheses are standard deviations.
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also in this work [1,30,32,33]. As for the flexural stiffness, virgin glass fibers show a much better effect than the recycled ones, especially for LDPE-based composites. This could be ascribed to some damage experienced by the recycled fibers due to the recycling process. Results for screw withdrawal resistance are given in Table 3. In order to take into account any difference in specimen’s thickness, the values are presented in force per unit of embedded screw length (N/mm). As shown in Table 3, the screw withdrawal resistance ranges from 48.09 N/mm to 91.92 N/mm and 133.60 N/mm to 206.11 N/mm for LDPE-and PP-based wood flour composites, respectively. It can be seen that composites with higher wood flour content exhibit lower screw strength, which is in agreement with results of other authors [38,39]. It is worth noting that the variation of fiber content up to 35 wt.% does not seem to influence the screw withdrawal resistance of composites, both for LDPE and
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PP. The presence of glass fibers has a positive role on the screw strength of hybrid composites, offsetting the decrease caused by increasing wood flour content. This is particularly true for LDPEbased composites (as can be seen, for instance, by comparing LDPE50W50 and LDPE50W30V20 or LDPE45W45V10 and LDPE40W40V20). The effect produced by the recycled glass fibers is similar, even though a slightly better behavior is observed for the virgin ones. Nevertheless, the use of recycled glass fibers appears to be promising. The variability in withdrawal resistance, as measured by the coefficient of variation, was rather low, ranging from 2.63% to 11.23%. 3.3. Water absorption behavior Results of water absorption are given in Table 4. Water absorption increases with increasing wood content in the composites, a
Fig. 10. SEM micrographs of fracture surfaces for: (a) LDPE50W50, (b) and (h) LDPE45W45V10, (c) PP50W50, (d) PP45W45V10, (e) and (f) LDPE60W30V10, (g) PP40W40R20.
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trend that is found for both polymers and for both 2 h and 24 h water immersion tests, but it was nevertheless very low as compared with the control samples (Ipê wood), because the matrix polymers are hydrophobic, whereas the control samples are hydrophilic. With the increase in wood content there are more sites for water absorption (free hydroxyl groups in cellulose and hemicelluloses). Water absorption in composites can be mainly ascribed to the presence of lumens and hydrogen bonding sites in the wood flour, and the gaps at the interface between matrix and reinforcement [40]. Virgin glass fibers caused an increase in water absorption especially for PP-based composites. It is thought that this can be ascribed to the poor interface between fillers and matrix, since the use of compatibilizers proved to be successful [1,41]. The effect of recycled glass fibers is comparable to that of the virgin ones. 3.4. Morphology of fracture surfaces As a whole, SEM micrographs (Fig. 10a–h) show that a good and homogeneous dispersion of the fillers, both WF and glass fibers, has been achieved through the use of the two step manufacturing process. The compounding step was also able to allow an adequate level of dispersion of the recycled glass fibers, regardless of their high entanglement. This is a confirmation of the effectiveness of the manufacturing process used. Only at higher fiber contents some localized agglomerations of fibers occurred. From micrographs is evident that debonding and pull-out dominate the fracture surface, thus confirming the poor interfacial bonding as proposed in the discussion on the mechanical properties. Both glass fibers (virgin and recycled) (Fig. 10f–h) and wood flour (Fig. 10a–d) show a clean surface and finite gaps near the interfacial region are present, indicating a poor adhesion between filler and matrix. These features provide an explanation of the decrease in strength and increase in water absorption. 4. Conclusions In this work hybrid wood flour/recycled glass fiber thermoplastic composites were manufactured using a kinetic mixer and a compression molding machine. The morphology of the composites showed that a quite uniform dispersion of the fillers was achieved. The addition of wood flour and recycled glass fibers significantly increased the flexural modulus but decreased the flexural strength, a decrease only partially offset by the presence of glass fibers. This has been ascribed to the poor interfacial bonding, as confirmed by SEM analysis of fracture surfaces. In wood flour composites, water absorption increased with growing filler content, though remaining negligible when compared with that of solid wood. The introduction of recycled glass fibers in hybrid composites provided an improved water absorption behavior if compared to that of WPCs with equal amounts of filler. Screw withdrawal resistance, an important engineering property for the potential applications of these materials, has also been evaluated. The results showed that composites with higher wood flour content exhibited lower screw strength and that the resistance was relatively unaffected by wood flour content up to 35 wt.%. Here again, the presence of recycled glass fibers provided some improvement in withdrawal resistance over wood flour composites. This study demonstrates that hybridization of WPCs with recycled glass fibers can be successfully achieved and that the obtained composites show a promising combination of properties for industrial applications. This result can offer a suitable solution to disposal problems of thermoset composites at end-of-life and a way to reduce polymer-to-filler ratio while retaining satisfactory properties for cosmetic or semistructural applications. Another significant outcome is that interfa-
cial bonding needs to be optimized in order to fully exploit the combined potential of both reinforcement agents with a view to increasing the mechanical properties. As a general conclusion, morphological and mechanical characterization confirm that the manufacturing procedure adopted represents a cost-effective and fast solution to the fabrication and homogenization of highly entangled blend of polymers and fillers. This process can be successfully used in the broader case of composites reinforced with recycled fillers, even though an optimization of the process parameters for a better dispersion is needed. References [1] Adhikary KB, Pang S, Staiger MP. Dimensional stability and mechanical behaviour of wood–plastic composites based on recycled and virgin highdensity polyethylene (HDPE). Composites Part B 2008;39:807–15. [2] Marcovich NE, Villar MA. Thermal and mechanical characterization of linear low-density polyethylene/wood flour composites. J Appl Polym Sci 2003;90:2775–84. [3] Grubbström G, Holmgren A, Oksman K. Silane-crosslinking of recycled lowdensity polyethylene/wood composites. Composites Part A 2010;41:678–83. [4] Bouafif H, Koubaa A, Perré P, Cloutier A. Effects of fiber characteristics on the physical and mechanical properties of wood plastic composites. Composites Part A 2009;40:1975–81. [5] Lee S-Y, Yang H-S, Kim H-J, Jeong C-S, Lim B-S, Lee J-N. Creep behavior and manufacturing parameters of wood flour filled polypropylene composites. Compos Struct 2004;65:459–69. [6] Dányádi L, Móczó J, Pukánszky B. Effect of various surface modifications of wood flour on the properties of PP/wood composites. Composites Part A 2010;41:199–206. [7] Ichazo MN, Albano C, González J, Perera R, Candal MV. Polypropylene/wood flour composites: treatments and properties. Compos Struct 2001;54:207–14. [8] Kim J-W, Harper DP, Taylor AM. Effect of wood species on the mechanical and thermal properties of wood–plastic composites. J Appl Polym Sci 2009;112:1378–85. [9] Jiang H, Kamdem DP. Development of poly(vinyl chloride)/wood composites. A literature review. J Vinyl Addit Technol 2004;10:59–69. [10] Matuana LM, Kamdem DP. Accelerated ultraviolet weathering of PVC/woodflour composites. Polym Eng Sci 2002;42:1657–66. [11] Joshi PS, Marathe DS. Mechanical properties of highly filled PVC/wood-flour composites. J Reinf Plast Compos 2010;29:2522–33. [12] Maldas D, Kokta BV, Raj RG, Sean ST. Use of wood fibres as reinforcing fillers for polystyrene. Mater Sci Eng A 1988;104:235–44. [13] Lisperguer J, Bustos X, Saravia Y. Thermal and mechanical properties of wood flour–polystyrene blends from postconsumer plastic waste. J Appl Polym Sci, in press. doi: 10.1002/app.32638. [14] Yeh S-K, Agarwal S, Gupta RK. Wood–plastic composites formulated with virgin and recycled ABS. Compos Sci Technol 2009;69:2225–30. [15] Lu JZ, Doyle TW, Li K. Preparation and characterization of wood-(nylon 12) composites. J Appl Polym Sci 2007;103:270–6. [16] Chen J, Gardner DJ. Dynamic mechanical properties of extruded nylon–wood composites. Polym Compos 2008;29:372–9. [17] Maldas D, Kokta BV. Role of coupling agents on the performance of wood flour filled polypropylene composites. Int J Polym Mater 1994;27:77–88. [18] Lai SM, Yeh FC, Wang Y, Chan HC, Shen HF. Comparative study of maleated polyolefins as compatibilizers for polyethylene/wood flour composites. J Appl Polym Sci 2003;87:487–96. [19] Bengtsson M, Oksman K. The use of silane technology in crosslinking polyethylene/wood flour composites. Composites Part A 2006;37:752–65. [20] Kazayawoko M, Balatinecz JJ, Matuana LM. Surface modification and adhesion mechanisms in woodfiber–polypropylene composites. J Mater Sci 1999;34:6189–99. [21] Sreekala MS, George J, Kumaran MG, Thomas S. The mechanical performance of hybrid phenol–formaldehyde-based composites reinforced with glass and oil palm fibres. Compos Sci Technol 2002;62:339–53. [22] Rozman HD, Hazlan A, Abubakar A. Preliminary study on mechanical and dimensional stability of rice husk–glass fiber hybrid polyester composites. Polym-Plast Technol Eng 2004;43:1129–40. [23] Rozman HD, Tay GS, Kumar RN, Abusamah A, Ismail H, Mohd Ishak ZA. Polypropylene–oil palm empty fruit bunch–glass fibre hybrid composites: a preliminary study on the flexural and tensile properties. Eur Polym J 2001;37:1283–91. [24] Jiang H, Kamdem P, Bezubic B, Ruede P. Mechanical properties of poly(vinyl chloride)/wood flour/glass fiber hybrid composites. J Vinyl Addit Technol 2003;9:138–45. [25] Mishra S, Mohanty AK, Drzal LT, Misra M, Parija S, Nayak SK, et al. Studies on mechanical performance of biofibre/glass reinforced polyester hybrid composites. Compos Sci Technol 2003;63:1377–85. [26] Rizvi GM, Semeralul H. Glass–fiber-reinforced wood/plastic composites. J Vinyl Addit Technol 2008;14:39–42. [27] Arbelaiz A, Ferna´ndez B, Cantero G, Llano-Ponte R, Valea A, Mondragon I. Mechanical properties of flax fibre/polypropylene composites. Influence of
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Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Hybrid recycled glass fiber/wood flour thermoplastic composites: Manufacturing and mechanical characterization Marco Valente, Fabrizio Sarasini ⇑, Francesco Marra, Jacopo Tirillò, Giovanni Pulci Department of Chemical Engineering Materials Environment, Sapienza-Università di Roma, Via Eudossiana 18, 00184 Rome, Italy
a r t i c l e
i n f o
Article history: Received 20 October 2010 Received in revised form 29 January 2011 Accepted 5 February 2011 Available online 26 February 2011 Keywords: A. Polymer-matrix composites (PMCs) A. Hybrid A. Recycling B. Mechanical properties
a b s t r a c t Hybrid thermoplastic composites from wood flour and recycled glass fibers were manufactured through a two-step process involving a kinetic mixer and a compression molding machine. To evaluate the effect of recycled glass fibers, hybrid composites containing virgin glass fibers were also manufactured and tested. Mechanical properties of the composites including flexural modulus and strength, hardness as a function of temperature, screw withdrawal resistance and water absorption behavior were studied. The flexural modulus and hardness were found to increase as a function of increasing wood flour and glass fiber content, whilst the flexural strength and screw withdrawal resistance decreased as a function of increasing wood flour content, even though a positive effect of the addition of glass fibers was found. The recycled glass fibers showed comparable behavior to that of the virgin ones. A morphological analysis of hybrid composites to study the interfacial interactions was carried out by scanning electron microscopy. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Wood plastic composites (WPCs) are relatively new materials which are gaining considerable attention during last years. This is mainly due to a renewed interest in environmental protection. The use of wood, a natural and renewable resource, can help to reduce the carbon footprint of plastics and their proportion in municipal solid waste. Moreover, WPCs are potentially recyclable and can be considered sustainable materials, as the wood can be obtained from sawdust or scrap wood products as well as the plastic, which can be mainly derived from consumer and industrial recycling efforts. The present generation of WPCs comprises a very broad range of materials, from those manufactured from 100% post-consumer waste to those containing pulped wood and engineering resins. Common polymers used in WPCs include polyethylene (low and high density) [1–4], polypropylene [5–8], polyvinyl chloride [9–11], polystyrene [12,13], acrylonitrile–butadiene–styrene [14], nylon [15,16]. Currently, large volumes of WPCs are manufactured from recycled polymers, however virgin materials are used when demanding properties are needed. Current applications for WPCs are in diverse fields, including windows and door frames, interior panels in cars, railings, decking, cladding and fences. The increasing industrial acceptance of WPCs is due to their low moisture absorption, resistance to biological attack, dimensional stability and to a combination of high specific stiffness and strength, less abrasion during processing, low density and low ⇑ Corresponding author. E-mail address: [email protected] (F. Sarasini). 1359-835X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2011.02.004
price with respect to mineral fillers. An important issue in WPC manufacturing is represented by the addition of compatibilizers in order to improve compatibility between the polar wood and the non-polar polymers. A non optimal compatibility is often accompanied by significantly reduced properties, especially impact and tensile strength. In this regard, many approaches have been described in the literature aiming to improve the interfacial adhesion between the organic fillers and the polymer matrix, among which one of the most promising is the addition of maleic anhydride grafted polyolefins as compatibilizers [17–20]. A possible solution to enhance the mechanical properties of natural fiber reinforced composites (including WPCs), can be represented by the hybridization with inorganic fillers [21–27]. Hybridization may offset the disadvantages of one component by the addition of another fiber. In this regard, this paper aims at evaluating the effect of short glass fibers on wood flour thermoplastic composites. Glass fibers are known to be a suitable reinforcement for polymers. An effective way to enhance the performance of glass fiber while minimizing cost and landfill disposal of thermoset composites would greatly improve the economic attractiveness of WPCs. A possible approach to this issue would be using glass fibers which have been recovered from post-industrial waste. This paper reports the results of a project investigating the possibility of using glass fibers obtained from thermoset composites in combination with wood flour reinforced low density polyethylene (LDPE) and polypropylene (PP) composites. For comparison purposes, hybrid composites containing virgin glass fibers were also manufactured and tested. The effect of recycled glass fiber contents on water absorption behavior, mechanical properties (flexural strength and
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modulus), screw withdrawal resistance and microstructure of the hybrid composites was addressed. 2. Materials and methods 2.1. Materials Two thermoplastic polymers were used as matrix materials, namely low density polyethylene (LDPE) and polypropylene (PP), which were supplied by ExxonMobil Chemical. LDPE LD 250 has the following properties: a MFI of 5 g/10 min (190 °C/2.16 kg), a density of 0.916 g/cm3, a flexural strength of 15 ± 1.21 MPa, a flexural modulus of 460 ± 47 MPa, a Shore D hardness of 48 ± 0.37 (at room temperature), whilst PP was a PP1042 with a MFI of 1.9 g/ 10 min (230 °C/2.16 kg) and a density of 0.9 g/cm3, a flexural strength of 23 ± 1.43 MPa, a flexural modulus of 1355 ± 170 MPa, a Shore D hardness of 59 ± 0.68 (at room temperature). Wood flour (WF) of hardwood beech was kindly supplied by La.So.Le. Est Srl – Italy (Figs. 1 and 2). The sieve analysis (Fig. 2c) showed that most of the wood particles fall in the 35–60 mesh sizes, with the corresponding diameter ranging between 500 and 250 lm, respectively. Both virgin E-glass fibers (chop length = 3 mm) and recycled ones (Figs. 3 and 4) were used as reinforcement. The recycled fibers were obtained from mechanical recycling of glass fiber reinforced polyester composites coming from the automotive industry. The details of the mechanical grinding process and plant have been published elsewhere [28]. For comparison purposes, samples of Ipê wood (labeled as I) were used for water absorption tests. Ipê is a tropical wood often used in outdoor applications (decking). 2.2. Composite manufacturing The manufacturing process is shown schematically in Fig. 1. A compression molding machine was used to mold the square plates (200 mm 200 mm 10 mm) from which the samples for the characterization were cut. Prior to processing, both polymers and WF were oven dried at 65 °C and 105 °C for 24 h in order to remove moisture. A kinetic mixer was used to blend the polymer and the
reinforcements (wood flour and glass fibers). Kinetic mixing is a batch polymer processing technique that uses high shear and rapid rotational motion (3000 rpm) to create frictional heat sufficient to volatilize moisture and melt thermoplastics. Polymer and fillers are fed to the mixing chamber, rapidly (3 min) brought to melt temperature of polymers, mixed and then discharged as a thoroughly homogenized molten mass. As the temperature increased up to a preset value (130 °C and 160 °C for LDPE and PP, respectively), the chamber door was opened and the molten mass was transferred to the compression molding. A summary of the samples manufactured and tested are reported in Table 1. 2.3. Mechanical properties measurements The flexural properties (strength and modulus) were measured in four point bend tests at room temperature using a Zwick/Roell Z010 equipped with a 10 kN load cell. The tests were performed in accordance with ASTM D 7264. The test parameters were as follows: crosshead speed of 2 mm/min, support span length of 140 mm. The specimens were obtained from the square plates with the following dimensions: 200 mm 30 mm 10 mm (L W t). Strain gauges were used to measure the strain for the evaluation of elastic modulus. Five specimens of each sample were tested and the average results are reported. Hardness measurements (Shore D) were performed according to ASTM D 2240, using a Shore Model S1 Digital Durometer by Instron. The hardness value for each sample was calculated as the average of 30 measurements at five different temperatures, namely 0, 23 (room temperature), 40, 60 and 80 °C (±2 °C). The screw withdrawal test (ASTM D 1037) determines the load required to pull a screw from the panel specimen. A photograph of the screw (major diameter = 5 mm) used is shown in Fig. 5a. The threaded length was 18 mm. The screw was hand-driven perpendicular to the face of the panel (18 mm) into each specimen immediately before testing. A 4 mm diameter pilot hole was drilled into each specimen. Two panels were glued together in order to achieve the required specimen thickness. For the withdrawal tests, the specimens were 80 mm in length, 30 mm in width and 20 mm in thickness. Three screws for each composite type were tested. The screws were withdrawn at a uniform rate of speed (2 mm/min) un-
Fig. 1. Manufacturing process of the hybrid composites.
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Fig. 2. (a and b) SEM images of WF at different magnifications; (c) sieve curve.
Fig. 3. Photograph (left) and SEM (right) micrograph of virgin glass fibers.
Fig. 4. Photograph and SEM images of recycled glass fibers at different magnifications.
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Table 1 Wood plastic composite formulations (percent by weight). Composite sample code
LDPE content
PP content
Wood flour content
Glass fibers content
Recycled glass fibers content
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20
80 65 50 35 45 40 60 50 60 50 45 40 – – – – – – – –
– – – – – – – – – – – – 80 65 50 40 45 40 45 40
20 35 50 65 45 40 30 30 30 30 45 40 20 35 50 60 45 40 45 40
– – – – 10 20 10 20 – – – – – – – – 10 20 – –
– – – – – – – 10 20 10 20 – – – – – – 10 20
Note: LDPE, PP, W, V and R codes were used for low density polyethylene, polypropylene, wood flour, virgin glass fibers and recycled glass fibers, respectively.
Fig. 5. (a) Screw used in the withdrawal test; (b) holding apparatus and (c) experimental set-up.
til maximum load was recorded at room temperature (23 ± 2 °C). As shown in Fig. 5b and c, a specific screw holding apparatus was designed and used during the tests.
again after 24 h. The percentage increase in weight (W) during immersion was calculated as follows:
Wð%Þ ¼ 2.4. Water absorption tests Water absorption tests were performed in accordance with ASTM D 570. Three specimens were oven dried at 105 °C for 24 h. The conditioned specimens were immersed in distilled water for 2 h and 24 h at a temperature of 23 ± 2 °C. At the end of the preset time, the specimens were removed from the water, all surface water was wiped off with a cloth, and then were weighed. After 2 h immersion, the specimens were replaced in water and weighed
mt m0 100 m0
where m0 and mt are the conditioned and wet weight (at time t), respectively. 2.5. Scanning electron microscopy (SEM) The fracture surfaces of the flexural test specimens were investigated using a scanning electron microscope (Philips XL40). All specimens were sputter coated with gold prior to examination.
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3. Results and discussion 3.1. Morphology of WF, virgin and recycled glass fibers Fig. 2a and b shows SEM micrographs of the wood flour used in this study. As can be seen in Fig. 3, wood flour is comprised of fiber bundles rather than individual wood fibers with small aspect ratios (usually 1–5 [29]). Though the low aspect ratio can reduce the reinforcing ability, mechanical performance of the composite is sufficient for many applications. In fact, the use of wood fibers in polymer composites still greatly lags behind that of wood flour because of greater cost and increased processing difficulties when using traditional plastics processing methods. The virgin glass fibers are short fibers traditionally used in composites, as can be seen in Fig. 3. Recycled glass fibers appear to be completely different from the virgin ones, as can be clearly seen in Fig. 4. They appear to be heavily entangled and they do not seem to suffer from evident surface damage. However, it is noted the presence of some residual resin coming from the previous mechanical recycling operation. The entanglement could affect to a great extent both the processing and mechanical properties of the resulting hybrid composites.
3.2. Mechanical properties of composites A summary of hardness data is presented in Table 2. As for wood flour composites, hardness of polymer matrix is found to slightly increase with increasing WF content, both for LDPE and PP, as in [30]. The increase is more evident at higher temperatures, where the polymer suffer from a softening effect with increasing temperature, whilst WF is not affected by temperatures as high as 80 °C. The presence of glass fibers causes a further improvement of hardness especially at higher temperatures, whilst a significant difference between virgin and recycled fibers is not observed, thus confirming the positive role of recycled glass fibers. The values for PP-based composites are comparatively higher than those of LDPEbased ones, because of higher hardness and stiffness of polypropylene matrix. As a whole, the presence of both WF and glass fibers is beneficial as regards the hardness of the thermoplastic polymers investigated, especially at high temperature where they are able to counteract effectively the polymer softening.
The flexural strength and stiffness of the composites measured using four point bending tests are shown in Figs. 6–9 for LDPE- and PP-based composites, respectively. As a general comment, the flexural strength of the composites increases with decreasing wood content in the matrix. The addition of glass fibers does not cause an improvement of flexural strength compared with the neat polymer, even though an enhancement of strength occurred for hybrid composites in comparison with those having higher amount of wood flour (65 wt.%). Nevertheless, this increase is not sufficient to attain the strength values of composites reinforced with the smallest amount of wood flour (LDPE80W20 and PP80W20). This trend is confirmed by the fact that lowering the wood flour content while keeping constant the glass fibers content resulted in composites characterized by comparable flexural strength (LDPE50W30V20 and LDPE50W30R20). This is quite a common feature for filled thermoplastic composites [31], especially for composites reinforced with lignocellulosic fillers [1,30,32,33]. In particulate filled and short fiber reinforced composites the dominant micromechanical deformation process is the debonding of the phases. Debonding stress depends on particle size, on the stiffness of the matrix and on interfacial adhesion [34]. The low aspect ratio of wood flour particles makes them prone to debonding: this problem can only be addressed by improving adhesion through the use of coupling agents [33,35]. The decrease in flexural strength is therefore to be ascribed to a poor interface between polymer and reinforcement (WF and glass fibers) which makes ineffective the transfer of stress from the matrix to the fibers, thus not allowing a full exploitation of the reinforcement. The recycled glass fibers provided strength values which are comparable with those of the virgin ones, thus highlighting that recycling operation did not affect the reinforcing efficiency of the fibers, even though a suitable compatibilizing agent is needed between glass and thermoplastic polymer [36,37]. The flexural modulus of the composites exhibited a different behavior, increasing with reinforcement content. This trend is even more marked with the addition of glass fibers, both virgin and recycled. This is in agreement with the literature [1,29,32,33]. Modulus is usually less dependent of interfacial adhesion than strength is [33], and at the low strains involved in modulus calculations the reinforcement did not exhibit considerable debonding. Adding fillers to a polymer restrains the movements of its chains, thus increasing the stiffness but significantly decreasing the ductility and toughness of the composites, which was noted
Table 2 Shore D hardness of hybrid composites. Composite sample code
Temperature (°C) 0
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20 a
63.32 66.22 67.06 66.49 66.95 68.22 67.25 67.62 67.74 67.12 66.67 67.36 75.22 75.52 76.63 76.89 78.85 79.37 77.32 78.74
(1.55a) (1.22) (1.35) (1.12) (1.96) (1.79) (1.81) (1.94) (1.32) (1.32) (1.62) (1.19) (3.29) (3.02) (2.35) (1.38) (1.84) (1.02) (1.65) (1.94)
Room temperature
40
55.16 56.59 57.89 58.93 59.86 62.64 59.01 59.49 59.38 60.07 59.47 61.38 66.71 67.83 68.02 68.31 68.60 70.40 68.47 71.83
44.37 45.06 48.55 50.62 51.21 53.26 49.64 50.13 48.63 49.18 49.38 50.87 52.43 58.33 59.76 60.32 61.74 63.32 60.77 61.39
(0.87) (1.62) (1.08) (1.29) (1.65) (1.43) (1.64) (1.96) (0.99) (1.02) (1.15) (1.07) (2.24) (1.18) (1.81) (1.62) (1.12) (1.82) (1.84) (2.29)
Values are average of 30 measurements and values in parentheses are standard deviations.
60 (0.91) (1.98) (2.37) (1.56) (1.09) (1.12) (1.20) (2.04) (1.80) (1.34) (1.74) (2.62) (3.04) (1.35) (1.48) (1.02) (1.76) (1.26) (1.95) (1.34)
36.00 37.86 44.15 44.54 45.81 48.28 45.32 47.34 45.23 46.31 45.94 47.28 49.20 50.98 52.12 54.05 56.00 57.90 54.97 56.45
80 (0.86) (1.31) (1.33) (1.78) (1.68) (2.03) (1.28) (1.69) (1.26) (1.58) (1.18) (1.09) (1.07) (1.54) (1.63) (2.21) (2.37) (1.89) (1.55) (2.17)
29.92 32.98 34.08 36.74 38.56 40.65 37.23 38.12 36.96 37.18 37.25 39.84 44.63 45.84 47.82 51.44 52.54 54.19 51.83 52.61
(1.35) (2.19) (2.06) (1.95) (2.04) (1.84) (1.23) (1.47) (2.01) (1.84) (1.76) (1.65) (1.71) (1.27) (1.67) (2.90) (1.78) (1.09) (1.18) (1.83)
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Fig. 9. Flexural modulus of PP-based composites.
Fig. 6. Flexural strength of LDPE-based composites.
Table 3 Screw withdrawal resistance of composites.
a
Fig. 7. Flexural modulus of LDPE-based composites.
Composite sample code
Screw withdrawal resistance (N/mm)
COVa (%)
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20
91.63 91.92 57.16 48.09 65.27 76.91 85.85 93.06 75.04 81.66 63.34 71.79 206.11 193.21 145.54 133.60 182.46 146.87 153.44 142.28
8.60 2.92 4.45 8.25 5.91 9.12 7.04 8.75 9.23 5.97 2.63 10.05 6.15 4.44 4.45 9.52 11.23 10.04 9.36 10.83
COV = coefficient of variation.
Table 4 Water absorption of hybrid composites. Composite sample code
LDPE80W20 LDPE65W35 LDPE50W50 LDPE35W65 LDPE45W45V10 LDPE40W40V20 LDPE60W30V10 LDPE50W30V20 LDPE60W30R10 LDPE50W30R20 LDPE45W45R10 LDPE40W40R20 PP80W20 PP65W35 PP50W50 PP40W60 PP45W45V10 PP40W40V20 PP45W45R10 PP40W40R20 I
Fig. 8. Flexural strength of PP-based composites.
Water absorption (%) 2h
24 h
0.20 (0.01a) 0.54 (0.03) 1.45 (0.01) 3.18 (0.44) 1.21 (0.09) 1.13 (0.14) 0.35 (0.01) 0.46 (0.03) 0.31 (0.01) 0.61 (0.06) 1.31 (0.01) 1.97 (0.40) 0.22 (0.03) 0.52 (0.03) 0.89 (0.04) 2.60 (0.08) 1.29 (0.11) 1.95 (0.01) 1.56 (0.22) 1.46 (0.19) 10.79 (1.84)
0.38 (0.01) 1.03 (0.03) 3.25 (0.17) 7.16 (0.64) 2.85 (0.22) 2.56 (0.25) 0.66 (0.01) 0.83 (0.05) 0.57 (0.02) 1.16 (0.05) 2.79 (0.07) 4.61 (0.96) 0.32 (0.01) 0.90 (0.04) 1.43 (0.10) 4.51 (0.12) 2.29 (0.11) 3.30 (0.19) 2.66 (0.46) 2.37 (0.20) 27.22 (2.16)
a Values are average of three replicates and values in parentheses are standard deviations.
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also in this work [1,30,32,33]. As for the flexural stiffness, virgin glass fibers show a much better effect than the recycled ones, especially for LDPE-based composites. This could be ascribed to some damage experienced by the recycled fibers due to the recycling process. Results for screw withdrawal resistance are given in Table 3. In order to take into account any difference in specimen’s thickness, the values are presented in force per unit of embedded screw length (N/mm). As shown in Table 3, the screw withdrawal resistance ranges from 48.09 N/mm to 91.92 N/mm and 133.60 N/mm to 206.11 N/mm for LDPE-and PP-based wood flour composites, respectively. It can be seen that composites with higher wood flour content exhibit lower screw strength, which is in agreement with results of other authors [38,39]. It is worth noting that the variation of fiber content up to 35 wt.% does not seem to influence the screw withdrawal resistance of composites, both for LDPE and
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PP. The presence of glass fibers has a positive role on the screw strength of hybrid composites, offsetting the decrease caused by increasing wood flour content. This is particularly true for LDPEbased composites (as can be seen, for instance, by comparing LDPE50W50 and LDPE50W30V20 or LDPE45W45V10 and LDPE40W40V20). The effect produced by the recycled glass fibers is similar, even though a slightly better behavior is observed for the virgin ones. Nevertheless, the use of recycled glass fibers appears to be promising. The variability in withdrawal resistance, as measured by the coefficient of variation, was rather low, ranging from 2.63% to 11.23%. 3.3. Water absorption behavior Results of water absorption are given in Table 4. Water absorption increases with increasing wood content in the composites, a
Fig. 10. SEM micrographs of fracture surfaces for: (a) LDPE50W50, (b) and (h) LDPE45W45V10, (c) PP50W50, (d) PP45W45V10, (e) and (f) LDPE60W30V10, (g) PP40W40R20.
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trend that is found for both polymers and for both 2 h and 24 h water immersion tests, but it was nevertheless very low as compared with the control samples (Ipê wood), because the matrix polymers are hydrophobic, whereas the control samples are hydrophilic. With the increase in wood content there are more sites for water absorption (free hydroxyl groups in cellulose and hemicelluloses). Water absorption in composites can be mainly ascribed to the presence of lumens and hydrogen bonding sites in the wood flour, and the gaps at the interface between matrix and reinforcement [40]. Virgin glass fibers caused an increase in water absorption especially for PP-based composites. It is thought that this can be ascribed to the poor interface between fillers and matrix, since the use of compatibilizers proved to be successful [1,41]. The effect of recycled glass fibers is comparable to that of the virgin ones. 3.4. Morphology of fracture surfaces As a whole, SEM micrographs (Fig. 10a–h) show that a good and homogeneous dispersion of the fillers, both WF and glass fibers, has been achieved through the use of the two step manufacturing process. The compounding step was also able to allow an adequate level of dispersion of the recycled glass fibers, regardless of their high entanglement. This is a confirmation of the effectiveness of the manufacturing process used. Only at higher fiber contents some localized agglomerations of fibers occurred. From micrographs is evident that debonding and pull-out dominate the fracture surface, thus confirming the poor interfacial bonding as proposed in the discussion on the mechanical properties. Both glass fibers (virgin and recycled) (Fig. 10f–h) and wood flour (Fig. 10a–d) show a clean surface and finite gaps near the interfacial region are present, indicating a poor adhesion between filler and matrix. These features provide an explanation of the decrease in strength and increase in water absorption. 4. Conclusions In this work hybrid wood flour/recycled glass fiber thermoplastic composites were manufactured using a kinetic mixer and a compression molding machine. The morphology of the composites showed that a quite uniform dispersion of the fillers was achieved. The addition of wood flour and recycled glass fibers significantly increased the flexural modulus but decreased the flexural strength, a decrease only partially offset by the presence of glass fibers. This has been ascribed to the poor interfacial bonding, as confirmed by SEM analysis of fracture surfaces. In wood flour composites, water absorption increased with growing filler content, though remaining negligible when compared with that of solid wood. The introduction of recycled glass fibers in hybrid composites provided an improved water absorption behavior if compared to that of WPCs with equal amounts of filler. Screw withdrawal resistance, an important engineering property for the potential applications of these materials, has also been evaluated. The results showed that composites with higher wood flour content exhibited lower screw strength and that the resistance was relatively unaffected by wood flour content up to 35 wt.%. Here again, the presence of recycled glass fibers provided some improvement in withdrawal resistance over wood flour composites. This study demonstrates that hybridization of WPCs with recycled glass fibers can be successfully achieved and that the obtained composites show a promising combination of properties for industrial applications. This result can offer a suitable solution to disposal problems of thermoset composites at end-of-life and a way to reduce polymer-to-filler ratio while retaining satisfactory properties for cosmetic or semistructural applications. Another significant outcome is that interfa-
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