Preliminary Study of Polypropylene/Sawdust Green Composite
Ching-Wen Lou1, Ching-Wen Lin2,a, Ting-Ting Li3, Jin-Mao Chen4, Chen-Hung
Huang5 and Jia-Horng Lin4,6,b
1
Institute of Biomedical Engineering and Material Science, Central Taiwan University of Science
and Technology, Taichung 406, Taiwan.
2
Department of Fashion Design, Asia University, Taichung 41354, Taiwan.
3
School of Textiles, Tianjin Polytechnic University, 63 Chenglin Road, Hedong District, Tianjin,
300160, China.
4
Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials,
Feng Chia University, Taichung City 407, Taiwan.
5
Department of Aerospace and Systems Engineering Feng Chia University, Taichung City 407,
Taiwan.
6
School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan.
Corresoonding email: achingwen@asia.edu.tw, bjhlin@fcu.edu.tw
ABSTRACT
In this study, sawdust and polypropylene (PP) are melt-blended and injection-molded to form the
wood-plastic composite (WPC).The WPC is then tested in terms of mechanical properties and
compared with control groups of pure PP plate and PP/glass fiber (PP/GF) composite. In the tensile
test, the WPC displays a tensile strength of 25-27 MPa, regardless of whether the sawdust content is
5, 10, or 15 wt%. Pure PP composite has a tensile strength of 30 MPa; PP/GF composite with 15
wt% glass fibers has a tensile strength of 57 MPa. In the bending test, the WPC displays a flexural
strength of 44-45 MPa as the sawdust content does not influence the bending strength. PP/glass
fiber composite yields a bending strength of 85 MPa when the content of glass fiber is 15 wt%.
WPC is 5% lighter than PP/glass fiber composite.
Keywords: natural fiber, wood-plastic composite, melt-blending, injection molding, mechanical
properties
INTRODUCTION
Plastic features light mass, high strength, corrosion resistance, and easy workability; hence, it
plays an important role in all fields of economic construction and has become a foundational
industry. In the 70s, composites that were mainly composed of cellulose fiber and plastic were
developed and called wood-plastic composite (WPC). At the early stage of development, WPC was
not largely used. Not until the 90s, has WPC stepped into a fast-growing phase [1]. Polypropylene
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(PP) is one of the five pervasive plastics and a thermoplastic polymer that has great mechanical
properties, dimensional stability, and easy workability. It can be used in various manufacturing
processes, such as injection molding, extrusion, and flow molding, and thus is widely used in
containers, automobiles, and household products [2].
WPC is a novel green composite to replace wood. WPC has the advantages of both plastic and
wood. Like plastic, WPC is moisture resistance, mold resistant, and easily processed. Like wood,
WPC is eco-friendly, easily cut and adhered, and it has a natural texture. Thus, WPC is largely used
in various fields, such as construction, packaging, transportation, and garment industries. In 2004,
global consumption of WPC reached one million tons, and the yearly growth rate exceeded 15% [3,
4].
In recent years, many institutes have begun to explore WPC. However, in most studies, WPC
is produced with polyvinyl chloride (PVC) and polyethylene (PE) as a matrix resin for extrusion
and sheet molding. As few studies use recycled PP and injection molding, this study uses PP and
sawdust as primary materials, which are then melt-blended and injection-molded into PP/sawdust
WPC. The resulting WPC is then tested for mechanical properties and compared with pure PP plate
and PP/GF composite.
EXPERIMENTAL
Material
Sawdust is provided by a local wood factory. PP at injection grade (K1030, Formosa
Chemical & Fibre Corporation, R.O.C.) has a specific gravity of 0.9 and a melting index of 3.0 g/10
min (230 ℃/2.16kg). Glass fiber (202P) is provided by T.G.I. Co., Ltd.
Preparation of Sample
PP/GF Composite
Glass fibers (5, 10, or 15 wt%) are melt-blended with PP on a single-screw extruder machine and
injection molding machine, forming the PP/glass fiber composite. The temperature of the first,
second, and third tanks of the single-screw extruder machine are set at 190, 190, and 200℃, and a
screw speed of 24 rpm; the injection molding machine has its first, second, and third tanks at 210
℃ and a die at 220 ℃. PP/glass fiber composite is thus denoted by GF5, GF10, or GF15; “GF”
refers glass fiber and the digit after “GF” refers to the weight ratio of glass fiber.
PP/Sawdust WPC
Sawdust (5, 10, or 15 wt%) is melt-blended with PP on a single-screw extruder machine and
injection molding machine, forming the PP/sawdust composite. The temperature of the first, second,
and third tanks of the single-screw extruder machine are set at 175 ℃, and a screw speed of 26 rpm;
the injection molding machine has its first, second, and third tanks at 190 ℃ and a die at 200 ℃.
PP/Sawdust Composites are denoted by S5, S10, or S15
Test
Tensile Test
This test is performed as specified in ASTM D638. The distance between the upper and lower
fixture is 25 mm; the extension rate is 5 mm/min; and five pieces of samples from each
specification are tested.
Impact Test
ASTM D256 is followed in the impact test. Samples have a V-shape notch with a depth of
0.25R±0.5 mm. The sample size is 63.5×12.85×3.2 mm, and five samples from each specification
are tested.
Bending Test
This three-point bending test is performed according to ASTM D790 with an Instron 5566 (US).
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The test speed is 1.3mm/min; span length is 50 mm; sample size is 80×12.85×3.2 mm; and five
samples from each specification are evaluated.
RESULTS AND DISCUSSION
Tensile Test
Figure 1 shows that PP/GF composite
exhibits a greater tensile strength than that
of pure PP plate, indicating that glass fiber
is capable of reinforcing the composite.
However, the tensile strength of PP/sawdust
composite is far less than that of PP/GF
composite and is even lower than that of
pure PP plate. The possible explanation is
that PP has a hydrophobic group.
Conversely, sawdust has a hydrophilic
group. Due to poor compatibility, blending
Figure 1. Tensile strength of pure PP
PP and sawdust leads to a decrease in the
plate, PP/GF composite, and PP/sawdust
tensile strength of the resulting composite,
WPC
and the tensile strength decreases with an
increase in sawdust.
Figure 2. Flexural strength of pure PP plate,
PP/GF composite, and PP/sawdust WPC.
Figure 3. Impact strength of pure PP plate,
PP/GF composite, and PP/sawdust WPC.
Figure 2 shows that the bending strength of PP/GF composite increases with an increase in the
content of glass fibers; in addition, PP/GF composite also demonstrates a greater bending strength
than pure PP plate. In addition, the bending strength of PP/sawdust composite does not increase
significantly, regardless of whether the amount of sawdust is 5, 10 or 15 wt%. It is possibly due to
sawdust that is incompatible with PP. When sawdust is 5 wt%, the incompatibility between sawdust
and PP exceeds the maximum that stress and strain that it can bear, thus the bending strength of
PP/sawdust composite does not increase, regardless of whether the sawdust content is 5, 10, or 15
wt%.
Figure 3. Impact strength of pure PP plate, PP/GF composite, and PP/sawdust WPC. Figure 3
shows that PP/GF composite displays a greater impact strength than pure PP plate; additionally,
with 10 wt% glass fiber, PP/GF composite displays the optimum impact strength. Just like its tensile
strength and bending strength, the impact strength of PP/sawdust WPC does not show a clear
tendency to increase or decrease. This is due to the poor compatibility between sawdust and PP;
thus an improvement of the compatibility between sawdust and PP can largely heighten the
mechanical property of the resulting WPC.
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Specimen Weight
Table 1. Specimen weight of pure PP plate, PP/GF composite, and PP/sawdust WPC.
additives
content
(wt%)
PP
GF
5wt%
GF
10wt%
GF
15wt%
Sawdust
5wt%
Sawdust
10wt%
Sawdust
15wt%
average
value
(g)
5.47133
5.70833
6.04367
6.17933
5.693
5.72667
5.886
Standard
deviation
(g)
0.03729
0.00379
0.08536
0.00681
0.08143
0.00551
0.00173
Table 1 shows that for both PP/GF composite and PP/sawdust WPC of the same volume, the
specimen weight increases with an increase in the additive content. PP/sawdust WPC is the most
lightweight; in comparison with composites with the same mechanical properties and volume, more
lightweight composite adds more convenience to its usage.
CONCLUSION
According to the experimental results, the mechanical properties of PP/sawdust WPC are measured
lower than that of PP/GF composite. For example, with the addition of 15 wt% glass fiber, the
resulting PP/GF composite displays a greater tensile strength, going from 30 to 57.6 MPa.
Conversely, with the addition of 15 wt% sawdust, the resulting WPC displays a decrease in tensile
strength, going from 30 MPa to 25.3 MPa. This decrease is ascribed to incompatibility between
sawdust and PP. The goal of current studies emphasizes lightweight composite, and sawdust is more
lightweight than glass fiber. Hence, improving the compatibility between PP and sawdust and
making the mechanical properties of WPC greater than that of PP/GF composite can further bring
industrial progress.
ACKNOWLEDGE
This work would especially like to thank National Science Council of the Taiwan, for financially
supporting this research under Contract NSC 100-2621-M-035-001.
REFERENCE
[1] S.L. Jin and L.J. Cao: Resource Development & Market, Vol. 24 (2008), p. 651-653.
[2] C.C. Kuo: Central Taiwan University of Science and Technology, Master Thesis (2010).
[3] C. Clemons: Forest Products Journal, Vol. 52, No. 6 (2002), p.10-18.
[4] T. Okamoto: Association for Promoting Advanced Science and Technology of Forest Resources
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Utilization, Vol. 42 (2002), p.11-15.
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