Effect of rice husk (treated/untreated) and rice husk ash on fracture toughness of epoxy bio-composite

2.1 Materials and manufacturing

Rice husk obtained from a local mill was thoroughly washed and sun dried to remove moisture. Thereafter it was grounded to sieve size of 125 microns. Moulds were prepared using perpex sheets of 10 mm thickness. Commercially available resin CY-231 was used as the matrix material along with hardener HY-951 as the curing agent. Samples with different concentration of rice husk were prepared. The weight percentage of rice husk was kept as 10, 20, 30 and 40. Sample with alkali treated rice husk was also prepared. For this purpose NaOH was used. Rice husk flour was soaked in 8% volume/volume NaOH solution for 5 hours and then was thoroughly washed with distilled water to remove any traces of NaOH. After this the flour was sun dried to remove moisture.

For casting rice husk flour was mixed with the resin and well stirred and then kept in a furnace for 2 h at 110^∘C. The mixture was allowed to cool down to 40^∘C and then hardener was mixed and after stirring for a minute the mixture was poured into the moulds using the hand lay-up technique [28]. The ratio of resin and hardener was kept as 9:1. The casting was allowed to stand for 24 h under gravity afterwhich the moulds were opened and various specimens were prepared. Silicone grease was applied on the inner walls of the mould for easy removal of the casting.

2.2 Preparation of rice husk ash

Different processes have been used by different researchers to obtain silica from rice husk. The process which has been utilised here as proposed [26] can be summarized in following steps:

1. Rice husk was washed with water to remove any foreign material.

2. Hydrochloric acid solution of 0.4 ML⁻¹ was prepared than 100 g cleaned husk was mixed in 1 L of prepared acid solution and boiled at 100–105^∘C for 30–45 min. After the reaction, the acid was completely removed from the husk by washing with tap water.

3. It was then dried in an oven at 110^∘C for 3–5 h in oven.

4. The treated husk burned in an electric furnace at 600^∘C for 6 h; silica was obtained as white ash. The shape of the silica is similar to the shape of the husk but smaller in size.

2.3 Fracture toughness specimen

The estimation of fracture parameters is essential for service life evaluation. Failure of most in service materials starts with crack nucleation and propagation so it becomes important from structural integrity point of view. Fracture toughness of a material is its resistance to crack extension. The fracture toughness test standards ASTM E399-19 ASTM D5045-20 were developed to determine the value of plane strain fracture toughness at or near to the onset of crack initiation. Six types of conventional fracture test specimens are permitted in ASTM fracture test standards. These include compact tension (C(T)) specimen, single edge-notched bend (SEN(B)) specimen in three-point bending, middle-cracked tension (M(T)) panel, disk-shaped compact tension (DC(T)) specimen, arc-shaped tension (A(T)) specimen, and arc-shaped bend (A(B)) specimen.

In this case the SENB specimen has been employed to evaluate fracture toughness of various specimens. The SENB specimen was chosen because of simple design and loading condition, cheap preparation procedure, convenient testing set up and the ability of introducing complete mode mixities. Here we have only focused on the mode I stress intensity factors.

The SENB specimen used for testing is shown in Figure 1 where W = width of the specimen, L = span length, B = thickness and “a” is the crack length. The crack is made normal to the base and at the centre of the specimen. The crack was made using an electrical saw. A sharp pre-crack is introduced in the bend specimen by lightly tapping a sharp new razor blade of thickness 0.2 mm into the tip of the mechanical slit or notch.

Figure 1

SENB specimen

Figure 2

Following equation was used to obtain critical stress intensity factor in mode I

Using the fracture load obtained from each specimen, the critical stress intensity factor was calculated. Figure 2(a) & 2(b) represent the sample and fracture specimen.

Fracture toughness tests are conducted at room temperature in a servo hydraulic universal test system ADMET, USA make.

3.1 Effect of RH addition

Figure 3 represents the fracture toughness with varying concentration of rice husk in comparison to pure epoxy. It can be seen that the fracture toughness increases due to addition of rice husk up to 20% filler loading. The fracture toughness for pure epoxy is 1.028MPa and it increases to 1.275 MPa showing a remarkable increase of around 25% for 10 wt% composite. For 20 wt% composite the increase was even more significant at 159%. This can be attributed to the prober adhesion between the fibre and the matrix and there is a proper transfer of load to the fibres.

Figure 3

Fracture toughness for different filler loading

However, further increase in rice husk content results in decrease in fracture toughness it however remains greater than pure epoxy. Fracture toughness is basically the resistance by a material to crack growth and when rice husk particles are added to the epoxy resin they form a barrier to crack propagation and hence fracture toughness increases. But at higher level of filler loading the toughness reduces because at higher filler loading there are voids created in the material and also rice husk tends to agglomerate as can be seen in the SEM images. Still the fracture toughness is considerably higher than the pure epoxy up to 40% filler loading. Further to observe the effect of fibre treatment on fracture toughness the filler level was chosen as 20%, value at which fracture toughness was highest.

3.2 Effect of NaOH treatment

To improve the adhesion between fibre and the matrix material, RH was pre-treated with NaOH solution. The concentration of NaOH solution was also varied and its influence on various properties has been observed. In the above section it has been seen that 20 wt% is the optimized filler loading and so all the composites fabricated from now on are based on 20% RH. In a study it was seen that the mechanical strength with alkali treatment improved up to 8% NaOH solution beyond which there was an excessive deterioration of fibres and strength decreased [29]. The tests have therefore been conducted on 8% NaOH solution.

From Figure 4 which shows the comparative fracture toughness for epoxy, 20 wt% reinforced rice husk and 8% NaOH treated rice husk it can be seen that although the fracture toughness improves due to treatment, the increase was not as significant. The increase due to treatment compared to untreated fibre was 3.12% and compared to pure epoxy was 167%. As has been explained in the previous section the reason behind the toughening due to particle addition is the barrier to crack propagation so treatment per se does not have much impact on the toughening mechanism since the reinforcement quantity was same at 20%; however the limited improvement in toughening parameter was due to improved adhesion between the matrix and the particles in which the nature of hydrophilic fibres changes to hydrophobic and hence an improved adhesion occurs.

Figure 4

Comparison of fracture toughness of pure epoxy, untreated 20 wt% reinforced and 8% alkali treated (20 wt%) reinforced composite

The effect of RHA addition on the fracture toughness is shown in Figure 5. For studying RHA’s influence on fracture toughness its value has been compared with pure epoxy, 20 wt% untreatedRHcomposite, 20 wt%treated RH composite. The fracture toughness corresponding to 1% Si has been taken. As can be seen RHA addition increases the fracture toughness from 2.746 to 2.952 MPa an increase of around 7.5%. The increase in toughness can be due to the dispersion of second reinforcement in the matrix due to which the barrier to crack extension improves and for the crack to propagate it either has to change the course or more load has to be applied.

Figure 5

Comparison of fracture toughness of pure epoxy, 20 wt% untreated RH composite, 8% treated (20 wt%) and 1% Si reinforced composite

Figure 6 shows the load deflection curve for fracture specimens of composites with different compositions. It can be seen that for 20 and treated 20 wt% reinforcement mode of crack extension is plastic in nature and therefore there is an increase in fracture toughness. For RHA reinforced composite it can be seen that the load bearing capacity of the material has increased due to proper dispersion of small sized silica particles and therefore acts as barrier to crack extension. For 10 wt% reinforcement also the mode of failure is plastic in nature thereby increasing the fracture toughness compared to pure epoxy. While for 30 and 40 wt% it can be seen that mode of failure is purely elastic and so the mode I fracture toughness decreases compared to 10 and 20 wt.% reinforcement of pure RH.

Figure 6

Load-deflection curve for various fracture specimens

3.3 SEM tests

Scanning electron microscopy was conducted on the different samples at G.B. Pant University of Agriculture and Technology.

Figures 7–9 show the SEM images for 20 wt% untreated RH, 20 wt% 8% NaOH treated and 20 wt% 8% NaOH treated+1%Si composites. From the images it is clear that fibre treatment improves the adhesion between the fibre and the matrix imparting greater strength to the composite. It can also be observed that addition of 1% Si results in a smoother surface of the bio-composite. The little roughness on the surface of second sample is reduced imparting higher fracture toughness to the resulting composite.

Figure 7

Scanning electron micrograph for (CY-230 + 10 wt% HY-951+ 20 wt% untreated rice husk

Figure 8

Scanning electron micrograph for (CY-230 + 10 wt% HY-951+ 20 wt% treated Rice Husk (8% NaOH solution)

Figure 9

Scanning electron micrograph for (CY-230 + 10 wt% HY-951+ 20 wt% treated Rice Husk (8% NaOH solution) +1% RHA)

3.4 TGA analysis

Thermal analysis of the specimen was conducted at Institute Instrumentation Centre, Indian Institute of Technology, Roorkee. For thermal analysis the samples were prepared in powder form by crushing them and the weight of samples were near 10.5 mg and the base material was alumina powder, which was mixed with composite samples in equal amount and medium in which the tests were conducted was air flowing at 20 ml/min. The rate of change of temperature was 10^∘C/min and range of temperature was room temperature to 1000^∘C.

Figure 10 shows thermo gram of neat epoxy with 10 wt% of hardener. Decomposition of this material has been accomplished under two stages ranging from 356^∘C to 510^∘C with corresponding rate of decomposition ranging 1.27 mg/min to 0.72 mg/min. Prior to 200^∘C, the weight loss of 7.63% may be attributed to the expulsion of the moisture, low molecular mass molecules and volatile matter associated with the material. The maximum rate of decomposition of 1.27 mg/min was observed at 356^∘C. Such decomposition has been supported with the heat of fusion of −2.487 J/mg centred in the temperature range of 279^∘C to 511^∘C with DTA signal of 28.4 μV to 109.2 μV. The decomposition of the material had been concluded at 536^∘C leaving char residue 3% of initial weight.

Figure 11 shows thermo gram of epoxy reinforced with 20 wt% of RH. Decomposition of this material has been accomplished under three stages ranging from 197^∘C to 301^∘C and 556^∘C corresponding rate of decomposition ranging 0.25 mg/min, to 0.54 mg/min and 0.34 mg/min. Prior to 200^∘C, the weight loss of 11.89% may be attributed to the expulsion of the moisture, low molecular mass molecules and volatile matter associated with the material. The maximum rate of decomposition of 0.54 mg/ min was observed at 301^∘C. Such decomposition has been supported with the heat of fusion of −1.09 J/mg centred in the temperature range of 302^∘C to 545^∘C with DTA signal of 51.3 μV to 44.1 μV. The decomposition of the material had been concluded at 700^∘C leaving char residue 4.887% of initial weight.

Figure 10

Thermal analysis of neat epoxy

Figure 11

Thermal analysis of 20 wt% RH reinforced epoxy

Figure 12 shows thermo gram of epoxy reinforced with 20 wt% of treated RH and 1% RHA. Decomposition of this material has been accomplished under three stages ranging from 192^∘C to 304^∘C and 529^∘C corresponding rate of decomposition ranging 0.17 mg/ min, to 0.47 mg/ min and 0.30 mg/ min. Prior to 200^∘C, the weight loss of 11.34% may be attributed to the expulsion of the moisture, low molecular mass molecules and volatile matter associated with the material. The maximum rate of decomposition of 0.47 mg/ min was observed at 304^∘C. Such decomposition has been supported with the heat of fusion of −4.52 J/mg centred in the temperature range of 305^∘C to 522^∘C with DTA signal of 44.5 μV to 45.1 μV. The decomposition of the material had been concluded at 700^∘C leaving char residue 9.462% of initial weight.

Figure 12

Thermal analysis of 20 wt% RH (8% NaOH treated) + 1 wt% RHA reinforced epoxy