Properties of corn husk–reinforced polyester composites

Composites made from corn fibre are increasingly interesting to study and their properties still need to be developed, informs Dr N Gokarneshan.

Composites made from natural fibres 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 fibres and increase the durability of the final product; therefore they are successfully used in structures such as decking, docks, and exterior windows, etc. that are directly in contact with water. Although natural fibre composites are widely used in many industries, long–term performance and durability are still not comprehensively understood. The fibres 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 fibre are increasingly interesting to study and their properties still need to be developed.

Some researchers have reported the best properties of corn husk fibre 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. The ability to absorb sound from CHFpolypropylene composites is superior to jute–polypropylene. Cornhusk fibre plastic composites had the highest flexural and tensile strengths of 46.10 MPa and 26.58 MPa respectively. 5 per cent CHF composites showed deformability than 0 per cent–8 per cent CHF with low methoxy pectin (LMP) films. From this previous study, it was agreed that other properties associated with corn husk fibre 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. 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 fibre composites in water immersion.

The effects of the CHF content on water absorption behaviour, tensile strength properties, and morphology were investigated.

1. Technical details

      Average length and width of 13.5 cm–15.2 cm is used. The polyester resin (PE) hCorn husk fibres having has a density of 1.2 g/cm3 , the tensile strength and a tensile modulus of 8.8 kg/mm2 and 500 kg/mm2 , respectively, and elongation of 2.3 per cent.

The following procedures have been followed

• Extraction of fibres
• Alkali treatment of fibres
• Preparation of composites
• Characterization
• Water absorption and swelling tests
• Tensile srength test
• Flexural test
• Scanning electron microscopy

Material preparation, a. Corn husk, b. Extration of CHFs, and c. CHF raw.

Alkali treatment process, a. CHFs immersion in NaOH8 per cent for 2 h, b. The drying process of CHFs, and c. CHFs after alkali treatment

1.1. Water absorption and swelling analysis The water absorption capacity of composite as display in Figure 3. The nature of water uptake increases with an increasing amount of fibre 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 per cent CHF (NC60) of 5.62 per cent at 72 h immersion.

Water absorption of corn husk fibre/polyester composites

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 fibres, and micro crevices in the matrix can cause the composite to absorb water. 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 fibres and reduces water uptake through this part to the interior of the composite. It was noted that fibre adhesion/strong interface can help reduce water hygroscopicity, reduce penetration, hence avoiding deterioration in the mechanical performance of composites. 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 Figure 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.

Swelling of polyester/corn husk fibre composites.

1.2. Tensile strength analysis

It has been found that the strength of composites with a 20 – 30 per cent of fibre 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 per cent fibre 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. The Young’s moduli demonstrated a gradual increase, its value increased up to 30 per cent CHF content then decreased; it is attributed to the flow of polyester which increased the bond strength and the composite strength. Furthermore, the NC20, NC25, and NC30 composites have increasing strain values, 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.

1.3. Flexural strength analysis

It has been found that the flexural strength of the composite tends to be the same as the tensile strength. It is observed that the average flexural strength value of the composite after immersed in water for 24 h varies from 36.495 MPa to 35.650 MPa, and after immersed for 72 h the flexural strength varies from 30.301 MPa to 32.370 MPa. The flexural strength trend is seen to increase with increasing CHF content from 20 per cent to 25 per cent, and subsequently decreasing. Maximum flexural strength is obtained in the composite after soaking 24 h with 25 per cent fibre content and 75 per cent PE resin of 42.739 MPa. The detected increase in term of tensile and bending behaviour was related to interfacial bonding between the CHF-polyester, and the modification of single corn husk fibres. After 72 h of soaking, the flexural strength of the composite is known to decrease. For example, NC25 with a 25 per cent fibre content have a higher flexural strength value (42.194 MPa) when compared to NC50 specimen which have a flexural strength of only around 36.192 MPa. This decrease was maybe due to the higher level of brittleness of the incorporation of CHF into the PE.

1.4. SEM

Morphology of the fractured surface of specimen composite in tensile is shown in Figs.5 and 6. After immersed for 24 h (seen in Figures 5a, 5b, and 5c), 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 fibres content (seen in Figures 5d–f) appears to be dominated by fibres 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 6d, 6e, and 6f, it was found that composite had a damage area interface between CHF and PE is loose. The interfacial fracture is demonstrated by CHF cross– section damage, resulting in decreased tensile, and flexural strength.

SEM photos, (a) NC20, (b). NC25, c. NC 30, d. NC40, e. NC50, and f. NC60 after water immersed for 24 h.

SEM images, (a) NC20, (b). NC 25, c. NC 30, d. NC40, e. NC50, and f. NC60 after water immersed for 72 h.

1. Conclusion

An experimental investigation of the behavior of tensile strength, morphology and water absorption from CHF–based composites under the water environment was carried out. The water uptake and swelling properties of the composites increase with an increasing amount of fibre content and soaking time. Consequently, the tensile and bending strength of the composite to decreased. The maximum tensile strength, and young’s moduli are obtained from composites with 30 per cent fibre content (NC30) after 24 h water immersed, and then decreases. SEM images display the interfacial fracture accompanied by cross–section damage of the CHFs. Composites based CHF are suitable as an alternative material for decking, siding, exterior windows, and doors.

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About the author:

Dr N Gokarneshan is from the Department of Textile Chemistry, SSM College of Engineering, Tamil Nadu.