High density

Mechanical Properties of Bio-Composite Natural Rubber/High Density Polyethylene/ Mengkuang Fiber (NR/HDPE/MK)

Mechanical Properties of Bio-Composite Natural Rubber/High Density Polyethylene/ Mengkuang Fiber (NR/HDPE/MK) Mohd Razi Mat Piah1,2, Azizah Baharum1,2 *, and Ibrahim Abdullah1,2

Polymer Research Center (PORCE), Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor 2 School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor 1

Summary The use of mengkuang fiber (MK) as a filler in NR/HDPE/MK bio-composite was investigated. The suitability of MK as reinforcing filler was studied in terms of the mechanical properties and phase morphology formed. Melt-blending was performed using an internal mixer (Haake Rheomix 600). The processing parameters identified were 135 °C temperature, 45 rpm rotor speed and 12 min processing time. The optimum MK loading in 40/60 of NR/HDPE blend was obtained at 20 wt.% with the tensile strength, tensile modulus, and impact strength of 12.4 MPa, 377 MPa and 17.2 kJ/m2, respectively. These results showed an enhancement in tensile strength of 10.7% compared to the unfilled NR/HDPE blend. The maximum impact strength obtained at 5% fiber loading was 23.5 kJ/m2, which was 7.8% higher compared to unfilled NR/HDPE blend. The FESEM micrographs showed good adhesion between MK fiber and matrix. These mechanical properties enhancements proved the suitability of mengkuang fiber as potential reinforcing filler in NR/HDPE blend.

Keywords: Bio-composite, Mengkuang fiber, Natural fiber, TPNR, Melt-blending

1. Introduction Environmental friendly bio-composite can be produced by replacing glass fibers or inorganic fibers with various types of cellulose fibers6. Natural fiber offers several advantages such as good mechanical properties, thermal stability, and low processing density2,9. Mengkuang fiber can be obtained from the leaves of mengkuang tree, which has the scientific name of pandanus atrocarpus5,17. This plant grows in wet areas such as rivers, swamps and lakes. Mengkuang leaves had been used as a handicraft resources and it is still in use current times. Therefore, mengkuang is believed to possess strong, tough and durable fibers, which is suitable for use in research because its utilization is not fully explored. In this study, the suitability of mengkuang fiber in NR/HDPE blend was studied

in terms of mechanical properties such as tensile strength and impact strength. The morphological examination was performed on fracture of tensile test specimens of NR/HDPE/MK biocomposite to investigate the interface interactions. The NR/HDPE blend or thermoplastic natural rubber (TPNR) at a ratio of 40/60 was used, and it was categorized as a thermoplastic elastomer (TPE) 1,3,12,20. The NR/ HDPE blend has good compatibility as natural rubber and HDPE have almost similar viscosity22. Natural rubber (NR) has been used to improve ductility and impact strength of brittle polymers. Meanwhile, HDPE has been used to improve mechanical strength, such as stiffness, tensile strength, and resistance to chemicals12,21. In

*Corresponding author: [email protected]

Smithers Information Ltd., 2016

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Polymers & Polymer Composites, Vol. 24, No. 9, 2016

addition, thermoplastic have good processing properties such as low melting temperature (not exceeding 230 °C) and able to withstand repeated processing. This is imperative because degradation of its natural fibers will incur high processing cost and weak mechanical strengths12,23. Most lignocelluloses will burn or degrade at temperature of 240 °C2,13. Generally, bio-composite reinforced natural fibers were influenced by the nature of natural fibers such as fiber structure, thermal stability, fiber size, fiber loading, distribution and orientation. In addition, processing method and natural fiber surface treatment also affect the bio-composite m e c h a n i c a l p r o p e r t i e s 7,9,13,24. According to Ahmad1, mechanical properties are affected by the material’s ability to transfer stress exerted from the matrix to fiber particles. Efficiency of stress transfer will determine the high tensile strength3. Homogeneous dispersion of natural fiber will affect the stiffness and determined the tensile

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Mohd Razi Mat Piah, Azizah Baharum, and Ibrahim Abdullah

modulus of the material. Impact strength indicates the energy transfer from matrix to filler particles that related to particle dispersion and matricfiber interactions. Morphological examination was conducted to assess the state of the interaction between matrix and mengkuang fiber via the fracture of surface specimen.

2. Materials and Methods 2.1 Materials Natural rubber (NR), grade SMR-L, was purchased from the Malaysian Rubber Board (MRB) and high density polyethylene (HDPE) with the density of 0.95 g/cm 3 from Etilinas Polyethylene Malaysia Sdn. Bhd. Mengkuang leaves obtained locally were processed in our research laboratory at Universiti Kebangsaan Malaysia.

2.2 Preparation of MK Fibers Mengkuang leaves were cut into 2 cm long pieces and dried under sunlight for 4 days. The dried leaves were ground using a fiber cutting machine (model CM). The MK powder was washed thoroughly with water until the dark green solution became clear. MK fiber was soaked in water for 24 hours. After immersion, MK fiber was dried in an oven at 60 °C for 24 hours. Next, MK fiber was ground to micrometer size using a continuous automatic hammer mill (model DF-15). The product was washed thoroughly with running water, soaked in water for 24 hours and dried in an oven at 60 °C before sieving to sizes of 126-250 µm using a sieve (Retsch Test Sieve, model ZM200).

2.3 Preparation of Biocomposite The blending of NR/HDPE/MK biocomposites were done in an internal mixer (Haake Rheomix 600) with a capacity of 60 g. Blending parameters identified were temperature of 135 °C, rotor speed of 45 rpm, and overall

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mixing time of 12 min. NR was added first and followed by MK after the first minute and HDPE in the third minute. NR/HDPE/MK bio-composite obtained was hot pressed into sheets of 1 mm and 3 mm thickness at a temperature and pressure of 145 °C and 70 kgf/cm2, respectively. MK compositions in NR/ HDPE blends were varied from 0%, 5%, 10%, 20%, and 30%.

2.4 Infrared Spectroscopy Analysis The surface structure of the MK fibers was characterized by FourierTransform Infrared (FTIR). The sample was prepared in potassium bromide (KBr) pellet and analyzed in the absorption mode in the wavenumber range of 500–4000 cm-1 (Perkin- Elmer, GX model)

2.5 Fiber Length Distribution The fiber length distributions of MK fibers before melt-blending with NR and HDPE were determined using optical microscope (Olympus CH20 Model). The MK fiber was sieved to 250 – 125 µm. The sample distribution area was limited to specific size. Then, three samples distribution area was taken randomly. The fiber length was calculated under optical microscope. Then, the mean of each fiber lengths were calculated and a bar chart was plotted to compare the percentage distribution of MK fiber lengths.

2.6 Characterization and Mechanical Testing Tensile test was performed using a universal testing machine (Instron model 5566) according to ASTM D412 standard. Crosshead speed and load cell were at 50 mm/min and 1 kN, respectively. 1 mm-thick dumbbellshaped specimens were cut using a dumbbell cutter JIS K-6251-6 (model DMK-1000-D). The thickness of the specimen was measured using a digital caliper (0.01 mm). Impact test was conducted using a pendulum digital universal fractoscope machine (model

Ceast 6545/000), in accordance to ASTM D256 and a load capacity of 2 J. The test was performed on specimens with a dimension of 65 mm × 12 mm × 3 mm. Each specimen was made with a 1 mm depth notch and immersed in liquid nitrogen for 35 s prior to the impact test. Morphological examination was conducted using Field Emission Scanning Electron Microscope (SUPRA 55VP model). The investigation was performed on the fractured surface of the tensile test samples. The samples were coated with approximately 4 nm platinum layer using sputter coated before the scan was performed.

3. Results and Discussion 3.1 FTIR Spectroscopy Analysis FTIR analysis of mengkuang fiber shows similar functional groups of common natural fibers such as kenaf, flax and jute. The carbonyl group from hemicellulose is identified at 1735 cm-1 13. The hemicellulose is an amorphous polymer with shorter polymer chains than cellulose those are physically bonded with lignin to form plant supporting system2,13. A broad peak at 3326 cm-1 is due to free hydroxyl group that was highly abundant in lignocellulosic fibers and contributing to their hydrophilic nature2,7,13. The lignocellulosic fibers was mainly composed of cellulose, hemicellulose and lignin. The lignin is hydrophobic in nature which is important in plant water transportation system. The other components are also presence in lignocellulose fiber such as pectin, wax, and lipid2,13. The peak around 1054 cm-1 is representing the stretching of C-O from methoxyl and hydroxyl groups in lignocellulose fiber. The other absorption bands was provided in Table 1.

3.2 Tensile Strength The effect of MK fiber loading on the tensile strength of NR/HDPE/MK bio-



composite is shown in Figure 2. The tensile strength of NR/HDPE blend increases with increasing MK fiber loading up to 20%. A further increase in MK fiber loading to 30% reducing the tensile strength. Therefore, the most optimum fiber loading is at 20% MK, with the highest tensile strength of 12.4 MPa. This also shows a significant improvement up to 10.7% higher than unfilled NR/HDPE blend. The enhancement of tensile strength indicated a strong matrix-fiber interphase adhesion that later on was shown on morphological study in Figure 6a-d and Figure 7a-b. These strong adhesions were believed to promoting a better stress-transfer ability in NR/HDPE/MK bio-composites. The stress transfer capability will increase via increasing the matrixfiber interphase interlocking. The optical microscopic study (Figure 8) shows that MK fibers posses high fiber length that contributing to their high aspect ratio (length/diameter) which is important in promoting good surface interactions and increasing matrix-fiber adhesion2,7,8. The porous structures of MK fiber allowed the matrix resins to penetrated into fiber vascular structures that providing more surface area for good matrix-fiber interactions as shown in Figure 7a-b. In addition, the MK fibers morphology as shown in Figure 5a-c also providing a rough and jagged structure those increasing matrix-fiber adhesion. Thus, good matrix-fiber interphase interaction will promoting the efficient in stress transfer and give rise to high tensile strength3. However, the compatibility of matrixfiber is still low due to high hydrophilic nature of MK and hydrophobic nature of matrix. The surface modification is needed to reduce the hydrophilic nature and moisture absorption of MK fiber in order to improve matrix-fiber interphase interaction. However, at 5%wt. MK, there is a slight decrease in tensile strength, which is due to dilution effect.

Table 1. FTIR Analysis of mengkuang fiber Wave number (cm-1)

Functional group

3326

O-H stretching

2915

C-H stretching

1735

C=O stretching

1655

C=C stretching of water absorption

1054, 1330

C-O stretching

609

C-C stretching

Figure 1. FTIR spectrum of mengkuang fiber

Figure 2. Effect of MK fiber loading on the tensile strength

According to Ho10, the incorporation of hydrophilic fibers in polymers leads to heterogeneous systems whose properties are inferior due to poor adhesion between fibers and matrices. Therefore, these debonded fibers dilute the matrix content and act as flaws, which reduce the effective cross-sectional area and finally produce poor mechanical strength. Another factor is low homogeneity of NR with


HDPE, which is the matrix was only mechanically bonded to each other12. A slight reduction in tensile strength is observed at 30%wt. MK due to low dispersion and increasing agglomeration of MK fibers in NR/ HDPE matrix. Similar problems were also observed in other natural fibers such as kenaf, jut, rami, and etc.1. Typically, the formation of agglomerates will

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affect the distribution of fibers in NR/ HDPE blend, which produce micro pores that become the breaking point for the materials16. The establishment of the focal points (breaking points) weakened the mechanical properties due to interferences of stress-transfer at the matrix-fiber interfaces. Thus, the disturbance caused NR/HDPE/MK bio-composites to be easily broken. Figure 3 shows effects of fiber loading on the tensile modulus. The tensile modulus decreases when MK fibers were added to NR/HDPE blend and almost unchanged with the increase in fiber loadings. The vascular structures of MK fiber is believed to be the factor that reducing the rigidity of fiber as shown in Figure 7a-b. Hornsby et al.11 reported that most vegetable fibers such as corn fiber and sugarcane were composed of vascular bundles that contains xylem, floem, fibers of bundle sheath and parenchyma cells. These structures have lower rigidity than cellulose microfibrils. The only enhancement in tensile modulus of 561 MPa is obtained at 10%wt. MK, which was increased by 5.6% compared to the unfilled NR/HDPE blend. This enhancement was influenced by the homogeneous distribution of fibers in the NR/HDPE blend. Thus, good fiber distribution will affect the rigidity of NR/HDPE/MK bio-composite. The matrix rigidity is higher at 0% MK due to better homogeneity of NR with HDPE, which increases the cohesion and adhesion of the materials. According to Ibrahim and Dahlan12, viscosity of the component was important to produce a well-blended material with higher homogeneity and good mechanical properties. The micrograph of Figure 5d shows good homogeneity between NR and PE.

loading up to 20% and 30% MK reduced the tensile modulus due to the softer material than its original matrix. There are two factors that influence the rigidity of a material, which are the shorter fiber length comparing to the original fiber and the matrix breakdown. A shorter fiber will reduce the rigidity of the NR/HDPE/ MK bio-composite due to increasing agglomeration because of the increased surface area and more hydroxyl groups will interact with each other. The second factor is the matrix breakdown due to low interface interaction. The incorporation of MK fiber led to lower intermolecular interactions at NR and HDPE interfaces. Hence, the NR/ HDPE/MK bio-composite became softer and reducing the tensile modulus.

Figure 4 shows effects of MK fiber loadings on impact strength. The impact strength is enhanced by the addition of MK fibers in NR/HDPE blend. The increasing in the impact strength of 23.5 kJ/m2 and 23.0 kJ/m2 are observed at 5%wt. MK and 10%wt. MK, respectively. The interaction between matrix and fiber is the main factor contributing to the increased impact strength. Another factor is good fiber dispersion in the NR/HDPE blend that promoting good interphase interaction and lowering the fiber-fiber agglomeration. A strong matrix-fiber adhesion allows stress exerted on matrixs to be efficiently transfer to filler particles. Thus, increasing the load bearing ability of a composite1. Basically, at 5% wt. MK produced

Figure 3. Effect of the fiber loading on the tensile modulus

Figure 4. Effect of MK fiber loading on the impact strength

Factors influencing the interaction of matrix-fibers can be categorized into mechanical binding sites, intermolecular forces, and chemical bondings18. Intermolecular attractions greatly affect the tensile modulus of a material. The increase in filler

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the highest impact strength because the NR/HDPE/MK bio-composite’s internal structure is more compact due to good dispersion and lower agglomeration among MK fibers. The decrease in impact strength was due to increasing fiber ends at higher fiber loading such as 30%wt. MK. The fiber ends causing the distortion of stress-transfer ability that led to poor impact strength. In addition, increasing fiber loading led to greater moisture absorption and greater dispersion problem those lowering the mechanical strengths of a composite.

NR/HDPE matrix. A few fine lines and curls with almost uniform size showed the homogeneity of NR/HDPE blend. Dahlan et al.4 reported that the wrinkles structure shows good homogeneity between NR and PE.

The micrograph of Figure 6a shows a strong matrix-fiber interface interaction at 5%wt. MK fiber loading. A strong adhesive interaction was observed at MK fiber and the entire surface of MK fiber was almost wetted by NR/HDPE

Figure 5. FESEM micrographs (1000×) of MK fibers with different conditions: (a) first MK fiber surface, (b) second MK fiber surface, (c) third MK fiber surface, (d) matrix NR/HDPE surface

3.3 Morphological Observation The morphological examination was conducted using Field Emission Scanning Electron Microscope (FESEM), there are three types of MK fiber surfaces were identified. The first surface consisted of two distinct layers as shown in micrograph of Figure 5a. A uniform shape with small pores in the middle is a top laminar layer. The second layer below the first layer, shows fibrous structure in parallel arrangement. These fibrous structures called microfibrils8. These parallel fibrous layers wrapped together by a laminar layer in the two MK microfibrils.

Figure 6. FESEM micrographs (1000×) of bio-composite at different MK fiber loadings: (a) 5% fiber loading bio-composite interface and (b) the surface fracture of 5% MK fiber loading (c) 20% fiber loading bio-composite interface and (d) the surface fracture of 20% MK fiber loading

The second surface was made of rough and jagged surface as shown in micrograph of Figure 5b. This fiber surface affects its ability as filler in the NR/ HDPE blend. In addition, parallel microcracks were observed in the MK fiber wall. These factors contributed to the surface cohesion and adhesion, which enhanced the mechanical properties of bio-composite. The micrograph 5c shows the third surface of MK fiber, which was covered by a thick laminar layer. The existence of small fragments is believed as the effect of fiber fractions. The micrograph of Figure 5d shows the matrix surface after tensile fractured. The visible stretch marks were uniformly observed on the surface of


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matrix. However, the micrograph 6b which also contain 5%wt. MK fiber loading shows the fracture occurred at the fiber interface. The MK fiber surface experienced a breakdown or pull-out effect of the fiber wall. The bonding forces seemed to be smaller than matrix-fiber interaction, hence, under severe pressure the laminated layer give way. As a result, the material’s tensile strength, lower than unfilled NR/HDPE blend. This result shows a similar trend to the dilution effect of 5%wt. MK fiber loading in NR/HDPE matrix. An intensive matrix-fiber interface interaction at 20%wt. MK fiber loading was also observed in micrograph of Figure 6c. The fiber was fully covered by the matrix, which shows a high surface wetted and a strong adhesive interaction to the matrix. This intensive matrix-fiber interface interaction indicated the suitability of MK as reinforcing filler in NR/HDPE blend of 40/60 ratios. This observation also corresponds to the tensile and impact test, which showed a significant improvement in both tensile strength and impact strength of NR/HDPE/ MK bio-composite. Furthermore, micrograph 6d also shows MK fiber fracture at 20% fiber loading, which was observed after the tensile test was performed. The fiber broke and partially dislodged from NR/HDPE matrix. The migrographs 6a-d proved an intensive interaction between matrix and fiber. However, a partially dislodge MK fiber indicated that under severe pressure the laminated layer give way. Thus, the investigation proved that mengkuang fiber breaks down easily to the smaller particles. The MK structure was also observed in the preparation of MK fiber, which had identified that mengkuang fiber was made up of laminated layers of fibrils that wrapped together by thin laminar.

fiber and sugarcane were composed of vascular bundles those contains xylem, floem, fibers of bundle sheath and parenchyma cells. Li et al.15 reported that sisal ribbon fibers composed of xylem fibers and vascular bundles. These structures allowed better surface interactions by providing a larger area of matrix-fiber adhesions.

3.4 Fiber Length Distribution The fiber length is an important factor to determine the fiber aspect ratio (length/diameter). High fiber length will increase the aspect ratio. The matrix-fiber interface interactions increase due to high fiber aspect ratio those promoting better mechanical strengths such as tensile strength and impact strength2,7,13. Figure 8 shows MK fiber lengths distribution of optical

microscope. The highest fiber length obtained was 610-909 µm around 38.8% distribution. The longest MK fiber length was 1210-2200 µm with 14.1% distribution. And the shortest MK fibers length was 310-609 µm with 24.8% distribution. The highest aspect ratio is 17.6. Kyosov14 reported that low aspect ratio is lower than 10.

4. Conclusions The use of MK fibers as fillers had been proven to improve the mechanical properties (tensile strength, tensile modulus and impact strength) of NR/HDPE blend. The optimum MK fiber loading was identified at 20% MK with the tensile strength, tensile modulus and impact strength of 12.4 MPa, 377 MPa and 17.2 kJ/m2,

Figure 7. FESEM micrographs of NR/HDPE/20%MK bio-composite impact fracture surface (a) MK break surface at 20 %wt. fiber loading with magnification 1000×(b) resin filled into MK cellular tube at 20% wt. fiber loading with magnification 2500×

Figure 8. Optical microscope analysis of MK fiber length

The micrograph 7a-b shows the internal structures of MK fiber penetrated by matrix resins. Hornsby et al.11 reported that most vegetable fibers such as corn

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respectively. 30% MK fiber loading reduces the mechanical properties due to the formation of agglomerate. The highest tensile modulus obtained was 561 MPa at 10% MK, and the highest impact strength obtained was 23.5 kJ/m2 at 5% MK. The Mechanical properties studied showed a significant improvement when MK fiber was used as reinforcement filler in NR/HDPE blend. Therefore, this study proved the suitability of mengkuang fiber as reinforcing filler in the blending of NR/ HDPE/MK bio-composite.

Acknowledgements The authors would like to acknowledge Universiti Kebangsaan Malaysia and the Ministry of Science Technology and Innovation (MOSTI) for sponsoring the research under grant no. 03-0102-SF1000.

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