Mechanical Properties and Failure Analysis of Short Kenaf Fibre Reinforced Composites Processed by Resin Casting and Vacuum Infusion Methods

Abstract

Modified and unmodified short kenaf fibre reinforced epoxy composites were processed with different short fibre lengths and fibre concentrations by resin casting (RC) and vacuum-assisted resin infusion (VARIM) methods. Three types of kenaf fibres were reinforced in epoxy polymer, namely, untreated kenaf fibre, mercerised and nanoclay-infused kenaf fibres. The mechanical properties such as tensile, flexural and impact properties of composites were studied. Nanoclay infused kenaf fibres have shown better tensile, flexural and impact properties than those of untreated and mercerised fibres. The composites processed by VARIM possess improved tensile and flexural properties when compared with RC composites, whereas the impact properties were better in RC composites than those of VARIM processed composites. The results showed that the mechanical properties of composites depend on the short fibre length and fibre concentration, irrespective of the processing conditions. Improved water barrier properties were also obtained in nanoclay-treated banana fibre composites.

Keywords

Natural fibres Kenaf fibres Surface treatment Nanoclay Resin casting Vacuum infusion Mechanical properties

1. Introduction

Natural fibre-reinforced composites have attracted considerable research attention during the past 1.5 decade as a possible alternative material to synthetic glass and carbon fibre reinforced composites^1–3. A significant improvement has been observed in this material in terms of comparable specific properties, light weight and with reduced-cost^4–5. The improvement was achieved by physical and chemical modification of natural fibres resulting in increased fibre properties, fibre-matrix adhesion and compatibility^6–7. Several types of natural fibres such as sisal, kenaf, hemp, flax and banana. fibre reinforced composites were developed with improved thermal, mechanical and barrier properties in various thermoset and thermoplastic composites^8–10. Kenaf fibre based composites research has attracted recent attention in South Africa due to their recent spike in the cultivation of kenaf (Hibiscus cannabinus L) plant from which the fibres are extracted. A typical kenaf fibre consists of ∼44–57% cellulose, 20–23% hemicellulose, 15–19% lignin and 2–10% of moisture and other extractives such as pectin and amorphous waxy phases. Cellulose is the main load bearing phase of the natural fibres, and much of structural application of natural fibres arises from this cellulose phase^11–13. As kenaf fibre consists of comparable level of cellulosic phase to that of commonly used natural fibres (sisal, coir, ramie), their potential in engineering and commodity application can be realised and achieved.

Kenaf fibre reinforced composites were processed by various techniques depending upon the type of the polymer matrix used. Thermoplastic polymers reinforced with kenaf fibres were processed by melt blending, injection moulding and extrusion techniques. Whereas thermoset polymers reinforced with kenaf fibres were commonly processed by resin transfer moulding, casting, hand lay-up and compression moulding techniques or combination of these processing techniques^14–16. The chemical or physical modification of kenaf fibre affects the processing and properties of composites. Improved fibre-matrix adhesion, stress transfer and thermal stability were observed when the fibres were treated with silane chemical^17–19. Mercerised fibre resulted in reduced fibrillation angle with increased modulus and tensile properties. The mercerised fibres were also found to be effective in removing non-cellulosic phase of the fibre such as lignin, amorphous wax. This modification in fibre causes effective processing and dispersion of fibres in the matrix^20–22.

The objective of this work was to infuse the nanoclay particles into the kenaf fibres. In recent years, nanoparticles were impregnated into the natural fibres using silane and NaOH (mercerised) chemical methods, in order to have superior mechanical, thermal and physical properties^23–26. Montmorillonite (MMT) type of nanoclays consisting of alumina-silicate nanolayers can be effectively infused into fibres using chemical treatments due to their compatible fibre hydroxyl group^27–28. The hard nanolayers of clay serve as a barrier medium and improve liquid and gas barrier properties along with thermal and mechanical properties^28–29. However, the effects of nanoclay treatment on the processing, fibre concentration and their properties in composites were not understood in detail. To understand this effect, nanoclay impregnated kenaf fibres were incorporated into an epoxy polymer matrix and studied in this work. The composites were processed by resin casting (RC) and vacuum assisted resin infusion moulding (VARIM) methods, with varying fibre weight content (25 wt.%, 50 wt.% and 75 wt.%). The nanoclay impregnated kenaf fibre composite properties were also compared with untreated (UT) and mercerised (NaOH) treated kenaf fibre reinforced composites. The outcome of this study will help to understand the effect of nanoclay impregnation in kenaf fibres, with respect to processing, fibre concentration and fibre treatments. The effects of nanoclay impregnation in kenaf fibres composites on tensile, flexural, impact and water barrier properties were also evaluated.

2. Experimental Details

2.1 Raw Materials

Kenaf fibres extracted from the stem section of plant were supplied by Agricultural Research Council of South Africa. Na⁺ montmorillonite (MMT) clay was obtained from Southern Clay Products, Inc. USA, supplied with the trade name Na⁺ Cloisite. The diglycidyl ether of bisphenol-A (DGEBA) based epoxy resin (LR-20) and cyclic aliphatic amine based epoxy hardener (LH-281) were obtained from AMT Composites, Durban, South Africa. All the chemicals used for mercerisation treatment were obtained from Merck Chemicals, South Africa.

2.2 NaOH Treatment of Fibres (Mercerisation)

Initially, 1 mol/L of NaOH solution was taken and equivalent weight of 5 cm long untreated kenaf fibre to that of NaOH was soaked in solution for 4 h. The soaked fibres were then taken out and rinsed in 0.01 mol/L dilute acetic acid solution for 30 min. Following this procedure, the fibres were washed, and air dried at room temperature (RT) for 48 h.

2.3 NaOH/Clay Treatment of Fibres

In this treatment, mercerised fibres were infused with MMT clays by shear-induced mixing. Initially 750 mL of distilled water was taken in a beaker and placed on a temperature controlled magnetic stirrer (Heidolph MR Hei: Standard, Labotec, South Africa) and heated at 80°C. 10 g of clay was then gently introduced into the water medium and the clay/water solution was stirred for 0.5 h at 80°C with 500 rpm. Following this, 10 g of short kenaf fibres were introduced into the clay/water solution and stirring was continued for another 4 h at 80°C with 500 rpm. The stirred fibre-clay solution was then placed in an ultrasonic agitation bath (MRC laboratory Equipment, UK. Model: DC-150 H, operating at 40 KHz with ultrasonic power 150 W) for 1 h. The nanoclay treated fibres were then removed from the ultrasonic bath and air-dried for 48 h.

2.4 Composite Processing

Composites were prepared by two processes, namely, resin casting (RC) and vacuum assisted resin infusion moulding (VARIM) methods. In both processing methods, three types of kenaf fibre-reinforced epoxy composites were prepared, namely, untreated (UT) fibre, NaOH treated and nanoclay treated kenaf fibre-reinforced composites. In each composite type, fibre content of 25 wt.%, 50 wt.% and 75 wt.% was varied and processed. For each fibre content in the composites, fibre lengths of 10 mm, 20 mm, 30 mm and 40 mm were varied and examined.

In the RC process, kenaf fibre-reinforced epoxy composite was prepared by mixing technique.

In this method, a known weight of epoxy resin (100 g) was taken in a beaker. Following this, the desired amount of short kenaf fibre with chosen length and concentration was added into the resin bath. The fibre-resin was mixed in an electric shear mixer at 1000 rpm for 30 min at RT. Hardener was then added to the resin-fibre mixture for curing purposes. The resin to hardener weight ratio was kept at 3:1, as per the supplier's manual. The resin and hardener mixture was stirred using a glass rod for about 3 minutes before the mixture was cast in closed glass moulds at room temperature. Two 30 cm x 30 cm x 3 mm square glass plates were used as moulds to fabricate composite casting. A 3 mm rubber gasket was placed between the glass plates and fastened by clips. The rubber gasket covered the three sides of the mould and the remaining side was used to pour the resin/ hardener-fibre mixture through the runner attached to the mould. Wax was used as a mould release agent and was applied to the faces of glass plates and rubber gasket for easy removal of the cast product. The cured epoxy fibre composite was removed after two days. The final cured composite was obtained in the form of 27 cm x 27 cm x 3 mm flat sheet.

VARIM process of composite preparation involved two steps; first step was the preparation of randomly oriented non-woven short kenaf fibre mat and second step was resin infusion into the fibre mat. Three types of kenaf fibre mats were prepared, namely, untreated, NaOH and nanoclay treated fibres, at various fibre lengths each ranging from 10 mm to 40 mm. To prepare fibre mat, 10 g of short kenaf fibres were randomly spread in an open-end flat die steel plate with dimension of 18 cm x 30 cm. The randomly spread short fibres were then compressed by air pressure assisted die set-up A compressive pressure of 8 bars was applied on the randomly dispersed fibres for 1 h at R T. The fibre mat was then infused with epoxy resin (mixture of resin and hardener at a ratio of 1:0.3 wt.% respectively) at 2 bars pressure. The resin infused fibre mat was left for curing and the cured sampled were removed after two days. The concentration of the fibre in the composite (25 wt.%, 50 wt.% and 75 wt.%) was varied by changing the resin content during vacuum infusion.

2.5 Characterisation

The structural analysis of untreated and treated fibre series was studied by Scanning Electron Microscopy (SEM). The surface morphology and chemical composition of gold sputter coated (Quorum-150 R ES model thin film coating equipment) longitudinal fibre specimen was examined by Energy Dispersive X-ray (EDX) analysis using Zeiss Environmental SEM (ESEM: model EVO HD 15) operating at controlled atmospheric conditions at 20 k V. The fracture surface morphology of fibre-reinforced composites was also examined by ESEM.

2.6 Testing

Tensile and 3-point flexural tests of composites was carried out using MTS-UTM machine (Model LPS 304 – 424708 series) as per ASTM D638-76 and ASTM D790 methods respectively. Un-notched Izod impact test was carried out using table-top pendulum swing impact tester (Hounsfield impact machine, model H10-3) as per ASTM D4812 method. In all these tests, 3 test samples were randomly chosen and the average value was considered for the results and discussion.

Water absorption tests of all the composite series were conducted to study the barrier properties. Three composite test specimens with dimensions 10 (length) x 10 (breadth) x 3 (thick) mm³ were fully dipped and tested in a distilled water bath medium at RT. Water-dipped samples were removed at different times, wiped on a paper towel to remove surface water, and weighed with an electronic balance until the increase in weight of water in the specimen reached an equilibrium (W_e) level (i.e. no more water uptake by test sample). The W_e value is expressed in terms of % water mass uptake in Equation 1:

W_{e} = \frac{W_{t} - W_{i}}{W_{i}} \times 100

(1)

where W_e is the mass of water uptake at equilibrium (i.e. no more water uptake by specimen), W_t is the mass of water soaked sample at time (t) and W_i is the initial mass of the sample before placing in the water medium.

3. Results & Discussion

3.1 Structure and Morphology of Fibres

Figure 1 shows SEM images of the longitudinal morphology of untreated and treated kenaf fibres. Untreated fibre ( Figure 1a ) shows a discontinuous phase attached in a continuous fibre backbone. The discontinuous phase was distributed throughout the fibre surface. This phase consisted of amorphous waxy or lignin phases^30–31. The NaOH-treated fibre ( Figure 1b ) shows the absence of discontinuous phase and occurrence of fibrillation (breakage of large fibre into small fibrils) as indicated by arrow marks. The removal of the discontinuous phase suggests the elimination of lignin and amorphous phase from the fibre. The alkaline (NaOH) chemical treatment is effective in removing lignin and other non-cellulosic phase of the natural fibres^32–33. The fibrillation by NaOH treatment increases the fibre aspect ratio (length/diameter ratio) and eliminates imperfections. Therefore treated fibres resulted with increased mechanical properties such as strength and modulus^34–35. The discontinuous phase of UT fibre also has low temperature stability and removal of these phases by NaOH treatment results in improved thermal properties of the fibre^36,37. Furthermore, NaOH treatment also increases the net cellulose content (which is the structural bearing phase of the natural fibres) by effectively removing non-cellulosic phases such as lignin, amorphous and waxy phases and positively imparts structural stability to the fibres and composites.^{38, 39} Nanoclay-treated fibre ( Figure 1c ) also shows the absence of discontinuous phase and presence of fibrillation. Additionally, nanoclay treated fibre shows the presence of nanoclay particles, which are distributed throughout the fibre surface. A few selected nanoparticles on the fibre surface are indicated by arrow marks. The nanoclays were seen as a bright spherical particle like phase impregnated into the fibres and dispersed uniformly in the fibre. The uniformity of dispersion was examined by mathematical parameter degree of dispersion (D_p), as per our earlier procedure⁴⁰. The (D_p) of nanoclay particles in the fibres was 0.89, which indicates good dispersion of particles in the fibre. The hard ceramic phase nanoclay particle is expected to improve the fibre-matrix interface properties, mechanical and barrier properties of fibres and composites.

Figure 1

SEM of longitudinal image of (a) untreated, (b) NaOH treated and (c) nanoclay treated kenaf fibres

To study the elemental phases and chemical composition of the untreated and treated fibres, EDX analysis on the fibre surface was carried out, and the results are shown in Figure 2 and Table 1 . The EDX results indicate that the major elemental composition of all the fibre series are due to C and O elements which form bulk of the cellulose, hemicellulose and other non-cellulosic polymer structure. Nanoclay treated fibre also shows the presence of Al and Si elements in nanoclay treated fibre, which is due to the MMT clay phase (Alumina and Silica layered structure). MMT consist of nanosized layers of aluminosilicate sheets in which alumina sheets sandwiched between two silicate sheets^41,42. The EDX results confirm the presence of nanoclay particles by showing the presence of Al and Si elements which arose from the alumina-silica nanosheets of the clay. The stoichiometric result shows about 2 wt.% of nanoclay particles impregnated into the fibre.

Figure 2

EDX spectrum of (a) untreated, (b) NaOH treated and (c) clay treated kenaf fibres

Table 1

EDX elemental analysis of untreated and treated fibres

Element	Untreated fibre	NaOH treated fibre	Nanoclay treated fibre
Element	Wt. %	Wt. %	Wt. %
C	27.25	26.97	25.98
O	72.65	72.2	71.17
Na	–	0.05	1.02
Mg	0.02	0.31	0.22
Al	–	–	0.42
Si	–	–	0.84
Cl	–	0.04	–
K	0.02	0.19	0.02
Ca	0.04	0.2	0.02
Fe	–	–	0.09
Cu	0.02	0.04	0.07
Mo	–	–	0.14
Total	100	100	100

3.2 Tensile Properties of the Composites

Figure 3 shows tensile stress-strain curves of 25 wt.% untreated kenaf fibre-reinforced epoxy composites prepared by RC ( Figure 3a ) and VARIM ( Figure 3b ) methods at various fibre lengths. The results indicated that the tensile values (strength, modulus and elongation at break) changes as a function of fibre lengths. Tensile value attains a maximum increase at certain length (critical length) of fibre and lower ones at other fibre lengths. The results show that a maximum increase of tensile values was obtained at 30 mm fibre length. At the critical fibre length optimised and stable network composite structure could have been obtained⁴³. The stress transfer, fibre-matrix interface and adhesion properties could be optimum at 30 mm fibre length. Higher fibre length (>30 mm) could have induced entanglement and bending of fibres, which reduces effective fibre-matrix stress transfer. On the other hand, low fibre length (<30 mm) might not have enough length to carry the load⁴³. The effect of fibre length on the tensile properties is similar for both processing (RC and VARIM) of composites. However, VARIM processed composites resulted in increased tensile values when compared with RC composites at their respective fibre lengths and content. This effect was examined by studying the fracture surface of VARIM and RC tensile tested composite samples, as shown in Figure 4. Figure 4a shows VARIM processed composites showing a void-free matrix phase, whereas Figure 4b of RC processed composite shows voids (indicated by arrow marks), which possibly resulted in reduced tensile properties. The voids could have acted as stress concentrators and induced crack propagation at lower stress values, resulting in poorer tensile properties. The fracture surface image result also suggests that the distribution of fibres in VARIM processed composites is better than that in RC processed composites.

Figure 3

Tensile stress-strain curves of 25 wt.% untreated kenaf fibre reinforced composites at various fibre lengths processed by (a) RC and (b) VARIM methods

Figure 4

SEM of 25 wt.% untreated 30 mm fibre length kenaf fibre composites processed by (a) VARIM and (b) RC methods

Table 2 shows the effect of fibre content on the tensile properties of RC and VARIM processed composites, measured at the critical fibre length. The result indicated that the fibre content affects the tensile properties of composites. The maximum improvement of properties was obtained at 50 wt.% irrespective of processing condition and the type of fibre. Figure 5 shows 25 wt.% and 75% wt.% UT fibre reinforced composites processed by VARIM method. At higher fibre content (75 wt.%), fibre agglomeration in matrix phase was observed. Possibly due to agglomeration, fibre-fibre interaction weakened the stress transfer by poor wetting and affects the dispersion. Therefore, the crack initiation and propagation is much easier and lowers strength. At low fibre concentration (25 wt.%), fibres might not have enough critical concentration to bear the effective load and resulted with reduced tensile properties.

Figure 5

SEM of VARIM processed (a) 50 wt.% and (b) 75 wt.% UT kenaf fibre reinforced composites

Table 2

Tensile properties of RC and VARIM processed composites

Fibre content	Fibre type	30 mm fibre length composites
		RC processed composites			VARIM processed composites
		Strength, MPa	Modulus, GPa	Strain, %	Strength, MPa	Modulus, GPa	Strain, %
25 wt.%	UT	22.1 ± 3.1	3.3 ± 0.1	4.9 ± 0.4	27.8 ± 3.3	3.9 ± 0.3	5.3 ± 0.6
	NaOH	24.3 ± 2.7	3.4 ± 0.1	4.9 ± 0.4	29.1 ± 3.2	4.0 ± 0.3	5.4 ± 0.6
	NaOH/clay	26.1 ± 3.1	3.5 ± 0.1	5.1 ± 0.4	32.3 ± 3.1	4.1 ± 0.2	5.7 ± 0.5
50 wt.%	UT	29.3 ± 3.7	4.2 ± 0.2	6.3 ± 0.3	36.5 ± 3.0	4.7 ± 0.3	6.6 ± 0.6
	NaOH	32.1 ± 3.2	5.1 ± 0.1	7.1 ± 0.3	38.1 ± 3.3	5.3 ± 0.4	7.3 ± 0.7
	NaOH/clay	35.1 ± 3.3	6.3 ± 0.2	7.7 ± 0.4	41.3 ± 3.2	6.9 ± 0.6	6.6 ± 0.6
75 wt.%	UT	27.6 ± 2.6	4.4 ± 0.2	6.8 ± 0.2	38.6 ± 3.5	5.4 ± 0.4	6.9 ± 0.5
	NaOH	29.4 ± 2.1	4.4 ± 0.2	7.3 ± 0.4	41.6 ± 3.7	5.6 ± 0.6	7.4 ± 0.7
	NaOH/clay	32.1 ± 3.4	4.6 ± 0.2	8.2 ± 0.3	43.7 ± 3.8	7.1 ± 0.5	8.4 ± 0.8

Among the composite series, nanoclay treated fibre reinforced composites shows highest tensile values when compared with NaOH and untreated fibres composites. The nanoclay infusion possibly improves the fibre-matrix adhesion and fibre properties resulting with the maximum increase of the tensile properties. The high modulus of nanoclay (∼160 GPa) with high purity and dispersion effect possibly resulted in a maximum increase in tensile properties^40–42.

3.3 Flexural Properties of the Composites

The effects of fibre content and type on flexural properties of composites at the critical fibre length were studied, and the results are shown in Table 3 . The outcome is similar to that of the tensile properties results, in which a maximum level of improvement of flexural properties was obtained at 50 wt.% fibre content. The improper dispersion of fibres, fibre-fibre interaction and lesser load transfer and concentration could have affected the flexural properties of other fibre content (25 wt.% and 75 wt.%) composites. The VARIM processed composites show higher flexural properties than RC processed composites due to improved fibre dispersion, wetting and lesser voids. The nanoclay treated fibre reinforced composites show a greater increase in flexural properties than the other fibre types.

Table 3
Flexural properties of RC and VARIM processed composites

Fibre content Fibre type 30 mm fibre length composites

RC
VARIM

Strength, MPa Modulus, GPa Strain, % Strength, MPa Modulus, GPa Strain, %

25 wt.% UT 27.0 ± 4 4.6 ± 0.5 1.9 ± 0.9 38.9 ± 6 4.8 ± 0.4 1.7 ± 0.1

NaOH 46.9 ± 5 6.5 ± 0.5 2.1 ± 0.8 55.6 ± 7 7.7 ± 0.4 1.9 ± 0.1

NaOH/clay 50.8 ± 4 6.7 ± 0.6 2.3 ± 0.3 61.4 ± 7 8.1 ± 0.5 2.2 ± 0.3

50 wt.% UT 53.7 ± 4 8.0 ± 0.7 2.4 ± 0.4 69.4 ± 6 8.6 ± 0.4 2.1 ± 0.2

NaOH 55.0 ± 6 9.2 ± 0.5 2.7 ± 0.5 69.0 ± 4 9.1 ± 0.7 2.2 ± 0.1

NaOH/clay 58.1 ± 5 11.0 ± 0.5 2.5 ± 0.3 71.9 ± 8 11.4 ± 0.7 2.4 ± 0.2

75 wt.% UT 50.5 ± 6 7.7 ± 0.4 2.7 ± 0.3 67.9 ± 6 9.9 ± 0.8 2.5 ± 0.3

NaOH 42.0 ± 7 8.1 ± 0.9 2.8 ± 0.4 76.1 ± 8 8.0 ± 0.4 2.6 ± 0.4

NaOH/clay 61.7 ± 8 9.0 ± 0.7 2.9 ± 0.4 85.7 ± 5 13.5 ± 1.1 2.8 ± 0.3

Fibre content	Fibre type	30 mm fibre length composites
25 wt.%	UT	27.0 ± 4	4.6 ± 0.5	1.9 ± 0.9	38.9 ± 6	4.8 ± 0.4	1.7 ± 0.1
NaOH	46.9 ± 5	6.5 ± 0.5	2.1 ± 0.8	55.6 ± 7	7.7 ± 0.4	1.9 ± 0.1
NaOH/clay	50.8 ± 4	6.7 ± 0.6	2.3 ± 0.3	61.4 ± 7	8.1 ± 0.5	2.2 ± 0.3
50 wt.%	UT	53.7 ± 4	8.0 ± 0.7	2.4 ± 0.4	69.4 ± 6	8.6 ± 0.4	2.1 ± 0.2
NaOH	55.0 ± 6	9.2 ± 0.5	2.7 ± 0.5	69.0 ± 4	9.1 ± 0.7	2.2 ± 0.1
NaOH/clay	58.1 ± 5	11.0 ± 0.5	2.5 ± 0.3	71.9 ± 8	11.4 ± 0.7	2.4 ± 0.2
75 wt.%	UT	50.5 ± 6	7.7 ± 0.4	2.7 ± 0.3	67.9 ± 6	9.9 ± 0.8	2.5 ± 0.3
NaOH	42.0 ± 7	8.1 ± 0.9	2.8 ± 0.4	76.1 ± 8	8.0 ± 0.4	2.6 ± 0.4
NaOH/clay	61.7 ± 8	9.0 ± 0.7	2.9 ± 0.4	85.7 ± 5	13.5 ± 1.1	2.8 ± 0.3

The mechanism of flexural properties increase in nanoclay treated fibre-reinforced composites was examined by fracture surface analysis of the flexural samples. Figure 6 shows the flexural fracture surfaces of different types of fibre-reinforced composites (25 wt.% fibres) processed by RC method. The matrix fracture surfaces of all the types of fibre-reinforced composites reveal a similar type of failure. The matrix failure occurred by brittle fracture with a cleavage type fracture surface. All the composites fibre fracture surfaces also revealed fibre breaking, fracture or pull-out. However, the fibre-matrix interface shows a different type of failure pattern among the composites. In untreated fibre composites ( Figure 6a ), fibre-matrix interface were clearly de-bonded during failure and indicate weak fibre-matrix interface. This fibre-matrix de-bonding could have resulted in crack initiation and propagation into the matrix phase at lower stress values. Propagation of the crack front from the fibre-matrix interface was clearly observed, and is shown by an arrow mark. In NaOH-treated ( Figure 6b ) and nanoclay-treated ( Figure 6c ) fibre composites, the fibre-matrix bond is better than that of untreated composites, as fibre-matrix interface cracking was not observed. The elimination of discontinuous fibre surface phase could possibly have induced better fibre-matrix adhesion in the treated fibres. The matrix resin might have covered the entire fibre surface during curing due to the absence of micro-level discontinuous phase. This would have eliminated the possibilities of voids/porosities at the fibre-matrix interface, resulting in improved bonding of the treated fibre-reinforced composite. The nanoclay infused fibre composites shows further improved fibre-matrix adhesion than that of NaOH-treated fibre-reinforced composites. Nanoclay-treated fibre-reinforced composites show some scratch marks (indicated by arrow marks) at the fibre-matrix interface, and this could be due to the nanoclays. During fibre pull-out, nanoclays might have resisted the fibre pull-out by scratching the matrix surface. The scratching of nanoclays along the fibre-matrix interface would have absorbed further loading and resulted in improved failure and tensile properties.

Figure 6

Flexural fracture surface of RC processed 25 wt.% kenaf fibre reinforced with (a) untreated fibre, (b) NaOH treated fibre and (c) nanoclay treated fibre

3.4 Impact Properties of Composites

Table 4 shows the impact properties of the composite series at corresponding critical fibre lengths. Impact strength of composites is dependent upon the type of fibre and its content in the matrix polymer. Impact properties are at a maximum for 50 wt.% fibre-reinforced composites and decreased at other concentrations due to poor fibre dispersion, fibre-fibre adhesion and ineffective stress transfer. The impact strengths of all the composite series (irrespective of fibre type and concentration) were higher than that of the neat epoxy matrix polymer. The kenaf fibre is a porous cellulosic structure which serves as an effective energy absorption medium during impact⁴⁴, and hence resulted in increased impact strength. The results also indicated that the RC processed composites had higher impact strengths than VARIM processed composites. As the fibre-matrix interface adhesion is poor in the RC processed composites due to poor fibre distribution, the fibres tend to debond easily during impact and create new surfaces. This process induces energy absorption capacity of the composites with increased surface area and impact properties. This phenomenon is also observed at the impact fracture surface of the RC and VARIM processed composites. Figure 6 shows the SEM image of impact fracture surface of 25 wt.% kenaf fibre composites processed by VARIM ( Figure 6a ) and RC ( Figure 6b ) methods. The distribution of fibres in RC composites appears to be more crowded than that of VARIM processed composites. These fibres can be more easily pulled out from the crowded fibre distribution. This new surface can absorb energy during impact. The impact result also indicated that the nanoclay treated fibre resulted in maximum improved impact strength over untreated and NaOH treated fibre composites. Figure 7 shows higher magnification of the fibre-matrix interface fracture of untreated and nanoclay treated kenaf fibre composites. The smoother fibre-matrix interface fracture (as shown by arrow mark in Figure 7a ) indicates a weaker fibre-matric interface, and would have resulted in the reduced impact strength of untreated fibre-reinforced composites. Figure 7b shows the clay particles scratch mark debris at the fibre-matrix interface (shown by arrow mark) which would have caused fibre pull-out at higher impact strength. Possibly the presence of clay could have induced a crack arresting mechanism at the fibre-matrix interface.

Figure 7

Impact fracture surface of 25 wt.% untreated kenaf fibre reinforced processed by (a) VARIM, (b) RC method

Table 4

Impact properties of the RC and VARIM processed composites at their critical length

Fibre content	Fibre type	RC processed composites	VARIM processed composites
Fibre content	Fibre type	Strength, MPa	Strength, MPa
0 wt.%	Neat epoxy	9.8 ± 0.6	–
25 wt.%	UT	12.0 ± 1.1	10.5 ± 1.6
	NaOH	11.0 ± 1.3	10.6 ± 1.7
	NaOH/clay	13.0 ± 1.5	11.3 ± 1.1
50 wt.%	UT	28.0 ± 1.8	24.7 ± 1.8
	NaOH	30.0 ± 2.3	26.3 ± 1.9
	NaOH/clay	32.0 ± 3.3	29.1 ± 1.1
75 wt.%	UT	26.0 ± 2.7	23.1 ± 1.8
	NaOH	27.0 ± 2.8	24.6 ± 1.5
	NaOH/clay	29.1 ± 3.3	25.3 ± 1.8

3.5 Water Barrier Properties of Composites

Natural fibres are prone to higher water absorption due to their hydrophilic and porous structure^45,46. This effect reduces their service life applications and properties. The influence of nanoclay treatment on the fibre surface is expected to improve the water barrier properties of kenaf fibre-reinforced composites. The nanolayered clays protect the fibre as a barrier medium by resisting water uptake. The alumina-silica nanolayers are hard ceramic phase material, which resist water molecules to penetrate into the fibres^47,48. The effect of clay treatment on water barrier properties is shown in Figure 9 and Table 5 . Figure 9 shows the water uptake curves of the 50 wt.% untreated and treated kenaf fibre composites processed by VARIM method. The water absorption curves shows that as time increases, the water mass content in the composite also increases and after a certain period of time, the water uptake becomes constant. The time at which the water uptake becomes constant will be called the equilibrium water mass uptake (W_e). Table 5 shows the W_e values of all the composite series. Clay treated kenaf fibre composites resulted in decreased W_e when compared with untreated and NaOH treated fibre composites. Clay infusion in the fibre has resulted in 32 to 53% reduced W_e _in the composites series, compared with untreated fibre-reinforced composites. Table 5 shows that the W_e increase is proportional to the fibre content in both the RC and VARIM processed composites. The processing method of composites also affects the water mass uptake, since increased W_e was observed in RC composites when compared with VARIM processed composites, for all the fibre type and fibre contents. Possibly the presence of voids would have resulted in increased water mass uptake of the RC processed composites.

Figure 8

Impact fracture surface around (a) untreated and (b) nanoclay treated kenaf fibre reinforced composites

Figure 9

Water mass uptake of 50 wt.% untreated and treated kenaf fibre composites

Table 5

Equilibrium water mass uptake of VARIM and RC processed composite series at critical fibre length

Fibre content	Fibre type	W_e (%)
Fibre content	Fibre type	VARIM	RC
25 wt.%	Untreated	2.7	2.9
	NaOH treated	2.1	2.3
	Clay treated	1.5	1.8
50 wt.%	Untreated	3.6	3.8
	NaOH treated	3.0	3.3
	Clay treated	1.7	2.1
75 wt.%	Untreated	4.1	4.6
	NaOH treated	3.3	4.0
	Clay treated	2.3	3.1

4. Conclusions

The objective of this work was to understand the effect of nanoclay treatment on the fibre and fibre reinforced composites, comparing UT and NaOH-treated fibre and fibre reinforced composites. The effect of nanoclay on processing, mechanical and water barrier properties were examined. EDX result showed that ∼2 wt.% of clay particles were infused into the fibre. The results indicated that the clay addition improved the mechanical and barrier properties of the composites; however, the level of improvement depended upon the method of composite preparation (VARIM and RC) and fibre content. The critical fibre length and concentration was found to be 30 mm and 50 wt.% respectively. Nanoclay-treated kenaf fibre composites resulted in 3–20% increased tensile strength, 6–50% increased tensile modulus and 0–22% tensile elongation when compared with untreated kenaf fibre composites, at various fibre contents and processing methods. Likewise, 3–88% increased flexural strength; 16–70% increased flexural modulus and 7–30% increased flexural strain values were observed in nanoclay treated fibre reinforced composites when compared with untreated kenaf fibre composites, at various fibre contents and processing methods. Similarly, 7 to 15% increased impact strength was observed in nanoclay treated kenaf fibre composites over untreated kenaf fibre composites, for various fibre contents and processing methods. Nanoclay addition resulted in improved water barrier properties, with 32–53% reduction of W_e. The outcome of this work suggested that the most effective mechanical and water barrier properties of nanoclay treated kenaf fibre reinforced composites can be achieved by selecting a suitable processing method, fibre length and concentration.

Footnotes

Acknowledgment

This research work is supported by South African Council of Scientific & Industrial Research (CSIR) -Biocomposites Centre of Competence (BCoC), project reference number SIIGBC3.11214.02100.02170.

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