Corn husk Fibers Reinforced Polyester Composites: Tensile Strength Properties, Water Absorption Behavior, and Morphology

The effects of fiber content and immersion time in the water on the tensile, morphology and water absorption properties of composites made from corn husk fiber/polyester have been studied. Composite made with a variety of different fiber contents namely: 20%, 25%, 30%, 40%, 50% and 60% respectively. All composite specimens were immersed in water for 24 h and 72 h. The effects of fiber content and time of immersion of composites in water have been determined by examining the nature of tensile strength, water absorption behavior, morphology. The results demonstrated that after soaked in water for 24 h and 72 h, the water absorption properties of the composites increased with increasing fiber content. The tensile strength and modulus of elasticity of composites tend to increase from 20% to 30% fiber content after immersed for 24 h, and then decrease with increasing fiber and soaking time because the interface between fiber and polyester becomes weak. These results suggest that corn husk fiber composites could have the potential to use as decking, siding, and exterior windows.


Introduction
Composites made from natural fibers and thermoset resins are increasingly developing in the polymer industry, specifically as a substitute for wood for outdoor products under wet environments. The advantages of hydrophobic resins have been to protect natural fibers and increase the durability of the final product; therefore they are successfully used in structures such as decking, docks, and exterior windows, etc [1] that are directly in contact with water.
Although natural fiber composites are widely used in many industries, long-term performance and durability are still not comprehensively understood. The fibers and adhesives are inevitable from changes and hostile environmental conditions. Water diffusion in composites and polymer adhesives is considered as one of the main reliability problems for the performance of composites.
Composites made from corn fiber are increasingly interesting to study and their properties still need to be developed. Some researchers have reported the best properties of corn husk fiber composites (CHF) with a polymer matrix. CHF composites with polyester matrices have a sound absorption coefficient of 0.8-0.9 at a frequency of 2 kHz. They also reported the tensile and Young's moduli of CHF composites around 18.81 MPa-25.73 MPa [2]. The ability to absorb sound from CHFpolypropylene composites is superior to jute-polypropylene [3]. Cornhusk fiber plastic composites had the highest flexural and tensile strengths of 46.10 MPa and 26.58 MPa respectively [4]. 5% CHF ICET4SD IOP Conf. Series: Materials Science and Engineering 722 (2020) 012035 IOP Publishing doi:10.1088/1757-899X/722/1/012035 2 composites showed deformability than 0%-8% CHF with low methoxy pectin (LMP) films [5]. From this previous study, it was agreed that other properties associated with corn husk fiber composites are still very limited.
As materials to be applied to a structure under a wet environment, the absorbed moisture will cause changes in the polymer microstructure, and degradation in their mechanical, thermo-physical, and chemical characteristics [6][7][8]2]. The effect of moisture or water exposition on mechanical, morphology, and water absorption of composites is very important to be studied and to explain the performance of composites in wet environments.
Therefore, this study aims to explore the properties of corn husk fiber composites in water immersion. The effects of the CHF content on water absorption behaviour, tensile strength properties, and morphology were investigated.

Materials.
Cornhusk has been obtained from the Pagesangan market, Mataram, Indonesia (see in Fig 1a). Corn husk selected on the outside; to maintain uniformity. The average length and width of corn husk is 13.5 cm-15.2 cm. The polyester resin (PE) has a density of 1.2 g/cm 3 , the tensile strength and a tensile modulus of 8.8 kg/mm 2 and 500 kg/mm 2 , respectively, and elongation of 2.3%.

Extraction of fibers
They are immersed in fresh water for 10 days to undergo decay (Fig. 1b). The fibers were taken using a wooden comb with teeth diameter of 0.02 mm; to maintain fiber uniformity, and dried under the sun's heat (Fig. 1c).

Alkaly treatment of fibers
The prepared CHFs were immersed in NaOH 8% for 2 h (Fig. 2a). They are washed and rinsed with fresh water and repeated three times, and then followed by drying under the sun (Fig. 2b), then dry CHFs (Fig. 2c) are stored in a platics storage box.

Preparation of composites
CHF with a fiber length of 4 cm was prepared. A mixture of polyester and catalyst is poured into a mold that filled CHFs with different volume fraction (see in Table 1). Then the mold is closed and pressed at 5MPa for 4 min (temperature 175 o C), followed by cooling at ambient temperature. The composite is removed from the mold and ready for testing.
Where, N1 and N2 show the dry weight (g), and the weight after time t (g).
Measurement of the percentage swellability evaluated using equation 2 [8]: Where, x and y are composite volumes after and before immersion.

2.5.2.
Tensile strength test. The specimen prepared was according to ASTMD3039 standard [11] used a Tensilon RTG-1310 that operated at a speed and load cell of of 5 mm/min and 5 kN respectively. 2.5.4. Scanning electronic microscopy, SEM. In this test, the fracture surfaces of the specimen were characterized by SEM Inspect-S50type at 18mA and 10 kV.

Water absorption and swelling analysis
The water absorption capacity of composite as display in Fig. 3. The nature of water uptake increases with an increasing amount of fiber content and immersion time of the composite. During 24 h -72 h period, the polyester resin demonstrated negligible water uptake and the CHF induced significant water uptake. Maximum water uptake is obtained from polyester composites with a volume fraction of 60% CHF (NC60) of 5.62% at 72 h immersion. A possible reason for this behavior might be because CHF shows tendency to absorb water higher than polyester (hydrophobic). The presence of lumens, defects, fissures at the interface, hydrogen bonds in fibers, and micro crevices in the matrix can cause the composite to absorb water [13,14]. Hence, the water uptake increases with more CHFs content.
Conversely, composites with low CHF content have better interface adhesion which reduces the interface width between fibers and reduces water uptake through this part to the interior of the composite [15]. It was noted that fiber adhesion/strong interface can help reduce water hygroscopicity, reduce penetration, hence avoiding deterioration in the mechanical performance of composites [16][17][18]. This also answers the reason why the ability to absorb water from NC20 is lower than other samples. This result has been confirmed by mechanical test results.
Typical swelling data for all composites displayed in Fig. 4, which shows that CHF/polyester swelling increases with increased water absorption, and thus the rate of swelling changes increases with immersion time. The effect of CHF on the polyester ratio on swelling thickness can also be explained by the difference in water uptake between CHF-polyester (see discussion on composite water absorption). Thickness swelling is affected by water uptake and change due to the same mechanism as water uptake.  From Fig. 5a shows that the strength of composites with a 20 -30% fo fiber content (NC20, NC25, and NC30) at the 24 h immersion stage tends to increase due to the strong bonding interface between polyester and CHF, and after being soaked in water for 72 h, the strength of the composite tends to decrease with the increasing number of CHFs. This is indicated that the amount of water absorbed in the composite has caused the interface bond between CHF and resin to be quite weak; as a result, the tensile and modulus of elasticity (MOE) of the composite tend to decrease with longer immersion time. A sharp decrease in tensile strength and Young's modulus values was also seen in composites with 40 -60% fiber content (see NC40, NC50, and NC60 specimens). This drastic decrease is indicated that when CHFs content was increased, the matrix is no longer evenly distributed and many CHFs overlap one another, resulting in bad bond at the interface, causing the composite strength to be small. The same tendency behaviour is also found in the modulus of elasticity of the composite (Fig.  5b). The Young's moduli demonstrated a gradual increase, its value increased up to 30% CHF content then decreased; it is attributed to the flow of polyester which increased the bond strength and the composite strength.

Tensile strength analysis
Furthermore, the NC20, NC25, and NC30 composites have increasing strain values (seen in Fig.  5c), this means that there is an opposite response to the large tensile load received, which is indicated by the effect of internal shifts at the atomic level in the composite material so that the composite increases in length thus the strain produced to be high. Conversely, the low strain value is due to the opposite response given by the composite to the small tensile load received.  Fig. 6 shows the flexural strength of the composite which tends to be the same as the tensile strength. From Fig. 6 Figure 6. Flexural strength of CHF/polyester composites cornhusk fiber composites.

SEM
Morphology of the fractured surface of specimen composite in tensile is shown in Figs. 7 and 8. After immersed for 24 h (seen in Figs 7a, 7b, and 7c), it was observed that the composite display the interfacial bonding between the CHFs -PE was high and strong. Localized bunch of CHFs is shown, which indicates the good dispersion of CHFs within the polyester, and the fracture occurred at the CHFs itself. This shows that the stress was well propagated between CHFs-polyester, resulting in enhanced flexural and tensile strength in response to stress. The composite with higher fibers content (seen in Figs. 7d-f) appears to be dominated by fibers breakage. The interfacial fracture accompanied by cross-section damage of the CHFs, resulting in decreased tensile strength. Figs. 8a, 8b, and 8c shows a crack running through the CHF, and this an indication of the lack of stress-transfer from polyester to CHFs. Figs 8d, 8e, and 8f, it was found that composite had a damage area interface between CHF and PE is loose. The interfacial fracture is demonstrated by CHF crosssection damage, resulting in decreased tensile, and flexural strength.