Manufacturing Technique and Mechanical Property Evaluations of
Polypropylene/Short Coir/Short Glass Fiber Wood Plastic Composites
Jia-Horng Lin1, 2, 3, b, Zheng-Ian Lin1, Jin-Mao Chen4, Cha-Chi Fan1 and
Ching-Wen Lou4, a
1
Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials,
Feng Chia University, Taichung City 40724, Taiwan, R.O.C.
2
School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan, R.O.C.
3
Department of Fashion Design, Asia University, Taichung 41354, Taiwan, R.O.C.
4
Institute of Biomedical Engineering and Materials Science, Central Taiwan University of Science
and Technology, Taichung 40601, Taiwan, R.O.C.
a
cwlou@ctust.edu.tw, bjhlin@fcu.edu.tw
Keywords: wood plastic composites (WPC), polypropylene (PP), short coir (SC), short glass fiber
(SGF), coupling agent, interfacial adhesion.
Abstract. This study uses a single screw extruder to make short coir (SC), an agricultural waste,
high-strength short glass fiber (SGF), and polypropylene (PP) into pellets, which are then made into
wood plastic composites (WPC) on an injection machine. During the process, maleic anhydride
grafted polypropylene (PP-g-MA) is used as a coupling agent to improve the interfacial adhesion
between coir and PP as well as between SGF and PP. A tensile strength test, flexural strength test,
and Izod impact strength test are performed on the samples to examine the mechanical properties.
The experiment results show that when the content of SGF increases from 3 wt% to 12 wt%, the
tensile strength increases from 26.08 MPa to 36.68 MPa, and flexural strength increases from 36.01
MPa to 49.91 MPa, but the Izod impact strength decreases from 286.16 J/m to 218.14 J/m. In
addition, the addition of 2 wt% of PP-g-MA improves the interfacial adhesion between matrices (PP)
and reinforcement (SC or SGF), thereby fortifying the mechanical properties of the composites.
Introduction
WPC, known as a green composite made of plant fibers and plastic, can be a replacement for
wood and plastic; furthermore, the high availability of it also eases the problems of wood deficiency
and severe waste plastic pollution. WPC is characterized by chemical resistance, good mechanical
properties, low cost, ease of processing, and recyclability, and as a result it is massively used in
construction industry and interior decorating field.
PP is one of the commonly-used polymers and is characterized as easily-processed and low-cost,
and possesses good mechanical properties. The biggest setback of PP is its low impact strength,
which limits its practical application. To cope with this disadvantage, this study uses impact
copolymer PP. Coir contains a great amount of cellulose and thus has excellent mechanical
properties, water resistance, and heat resistance. It is biodegradable and does no harm to the
environment. Furthermore, it has high toughness and is able to substitute for synthetic fibers and
serves as reinforcement in composites [1-2]. Due to a high tensile strength, low elongation, high
modulus, and good dimensional stability, and being flame, heat and chemical resistant, glass fibers
are commonly used as a reinforcement in polymeric materials. Thermoplastic composites that are
reinforced with glass fibers have high tensile strength, high elongation, and a low cost [3], and have
been widely used in the automotive, construction, and appliance manufacturing industries.
Being composed of polymeric matrices, WPC requires a good interfacial adhesion between
matrices and plant fibers or glass fibers because interfacial adhesion plays a decisive role in
providing good mechanical properties. As testified by the results of previous studies, surface
modification and the addition of coupling agent for interfacial adhesion are both helpful for a better
interfacial adhesion [4-5]. In this study, melt-blending process is used for the preparation of
PP/SC/SGF WPC, and then the influence of the content of the reinforcement (SC and SGF) as well
as the addition of coupling agent (PP-g-MA) on the mechanical properties of WPC is examined.
Experimental
Materials
PP (k8009, Formosa Chemicals & Fiber Corporation, Taiwan, R.O.C.) is an impact copolymer
polypropylene with a tensile strength of 30.06 MPa, an elongation at break of 415 %, a flexural
strength of 32.86 MPa, and Izod impact strength is NB at 23 °C. SGF (Taiwan Glass Ind., Corp.,
Taiwan, R.O.C.) is treated with silane coupling agent and has a 3.2-mm length and a 13-µm
diameter. SC is treated with alkali solution with a concentration of 17.5 wt% for 1 hour and has a
4±2-mm length and 1-mm diameter. PP-g-MA is a homopolymer polypropylene with MFI of 30~35
g/10 min.
Experimental Procedure
2 wt% PP-g-MA serves as a coupling agent. The addition proportion of SC and SGF varies as
12/3, 9/6, 6/9, and 3/12 wt% to mix with a specified content of PP to form pellets on a single screw
extruder (SEVC-45, RE-PLAST Extruder Corp., Taiwan, R.O.C.) at a screw speed of 24 rpm. The
temperatures of three barrels in the extruder are 190, 200, and 210 °C, and the temperature of its die
is 220 °C. After being dried at 70 °C for 8 hours, the pellets are then made into samples by an
injection molding machine (Ve-80, VICTOR Taichung Machinery Works Co., Ltd., Taiwan, R.O.C.).
The temperatures of three barrels in this machine are 190, 200, and 210 °C, and the temperature of
its nozzle is 220 °C. Finally, samples are tested for tensile, flexural, and impact strength.
Tests
Tensile Strength Test
An Instron (Instron 5566, US) measures the tensile strength of the samples as specified in ASTM
D638. The distance between grips is 25 mm, the tensile speed is 5 mm/min, and the number of
samples is 5.
Flexural Strength Test
Five samples measuring 127.0 mm × 12.7 mm × 3.2 mm are tested for flexural strength with an
Instron (Instron 5566, US) that has a 50-mm support span, at a test speed of 2 mm/min, as specified
in ASTM D790. The flexural strength is calculated with the equation (1).
𝟑𝐏𝐋
𝛔𝒇𝐦𝐚𝐱 = 𝟐𝐛𝐝𝟐
(1)
Where σfmax refers to flexural strength (MPa), P refers to the load (N), L refers to the support
span (mm), b refers to the width of the sample (mm), and d refers to the thickness of the sample
(mm).
Izod Impact Strength Test
The impact strength of samples is tested with an Izod tester (CPI, ATLAS, US) as specified in
ASTM D256. Five samples measuring 63.5 mm × 12.7 mm × 3.2 mm are with a 45° V-shape cut.
Results and Discussion
Tensile Strength
Figure 1 shows that an increment of SGF from 3 to 12 wt% increases the tensile strength of
WPC from 26.08 MPa to 39.68 MPa. SGF is a rigid material, the strength of which is greater than
that of SC. Increasing content of SGF results in its greater volume in the matrices, and thereby
absorbing higher force than matrices do. In other words, the force applied to matrices share is
reduced, which means SGF provides WPC with reinforcement. The addition of PP-g-MA also
heightens the tensile strength, and this increase is ascribed to the improved interfacial adhesion. The
coupling agent betters the interfacial adhesion between matrices (PP) and reinforcement (SC and
SGF), resulting in a greater tensile strength.
Figure 1. Tensile strength of PP/SC/SGF WPC.
Flexural Strength
Figure 2 shows that the increment of SGF (from 3 to 12 wt%) contributes to a higher flexural
strength of WPC (from 36.01 to 49.91 MPa). This increase is due to the inherent strength of SGF,
which is higher than that of SC, and the flexural strength increases as a result of a greater content of
SGF. In addition, flexural strength also increases after PP-g-MA is added. PP-g-MA improves the
interfacial adhesion between matrices and reinforcement, resulting in a greater flexural strength,
which shows the same trend as tensile strength in Figure 1.
Izod Impact Strength
Figure 3 shows that an increment from 3 to 12 wt% of SGF causes a decrease in impact strength
of WPC, which descends from 286.16 J/m to 218.14 J/m. SC is softer than SGF, and the
deformation of fibers in WPC can absorb impact energy. Therefore, the content of SGF is inversely
proportional to the impact strength. On the other hand, the addition of PP-g-MA benefits the impact
strength of WPC. Shown in Figure 4 (a-b), the anhydride groups of this coupling agent not only are
grafted onto the molecular chains of PP, but also create the interfacial bonding together with
hydroxyl group of SC and SGF along the compounding process, and thereby yielding greater
impact strength.
Figure 2. Flexural strength of PP/SC/SGF WPC.
Figure 3. Impact strength of PP/SC/SGF WPC.
a
b
Figure 4. Bonding reaction between PP-g-MA and a) SC or b) SGF.
Conclusion
This study successfully produces WPC with satisfactory mechanical properties by using SC and
SGF to fortify PP. A high content of SGF leads to an 38 % increase in tensile strength, a 36 %
increase in flexural strength, but a 31 % decrease in impact strength. The addition of PP-g-MA (2
wt%) positively promotes the overall mechanical properties of WPC, indicating that a proper
amount of coupling agent betters the interfacial adhesion between PP and SC or SGF.
Acknowledgement
The authors would especially like to thank National Science Council of the Taiwan, for
financially supporting this research under Contract NSC 102-2622-E-035-029-cc3.
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