Mechanical behavior of walnut (Juglans L.) shell particles reinforced bio-composite
-
Vinay K. Singh
Abstract
In the present work walnut particle reinforced composite material was developed. Ten wt%, 15 wt%, 20 wt% and 25 wt% (weight percentage) of walnut particles were mixed with epoxy resin (CY-230). Scanning electron microscopy (SEM) shows that the walnut particles were well dispersed in the epoxy resin matrix. Addition of walnut particles increased the modulus of elasticity of the bio composite. Addition of walnut particles in bio composite decreased the ultimate strength both in compression and tension. However, addition of walnut particles in bio composite increased the hardness. Flexural modulus of elasticity also increased with increasing walnut particles weight percentage, whereas flexural strength and strain decreased with increased weight percentage of walnut particles.
1 Introduction
Composite is a material formed with two or more components, combined as a macroscopic structural unit with one component as a continuous matrix, and other as reinforcements with significantly different physical or chemical properties, which remain separate and distinct on a macroscopic level within the finished structure. Normally, the matrix is the material that holds the reinforcements together and has lower strength than the reinforcements. Most commercially produced composites use a polymer matrix material called as resin solution [1].
Composite resin technology has continuously evolved since its introduction by Bowen [2] as a reinforced Bis-GMA system. A major breakthrough in composite technology was the development of photo-curable resins [3]. Continued development resulted in materials with reduced particle size and increased filler loading that significantly improved the universal applicability of light-cured composite resins [4].
As epoxy resins are a good solvent they are widely used in industrial applications because of their high mechanical and adhesion characteristics and chemical resistance together with their curability in a wide range of temperatures without the emission of any volatile byproducts. The properties of epoxy-based organic/inorganic filled composites can be finely tuned by an appropriate choice of the structures of epoxy pre-polymer and hardener, type and amount of inorganic filler. The composites have many advantages over traditional silica powder or inorganic mineral filled materials, including lower cost, lighter weight, environmental friendliness and recyclability.
With growing environmental awareness, ecological concerns and new legislation, bio particle reinforced plastic composites have received increasing attention during recent decades. Particleboards are of very recent origin. The important aspect that has impacted favorably on the development of these composite materials is the possibility of incorporating waste agro-waste (agricultural residues including stalks of most cereal crops, rice husks, coconut fibers, bagasse, maize cobs, peanut shells, and other wastes) product and recycled plastics with the advantage of a positive eco-environmental impact. Due to a worldwide shortage of trees and environmental awareness, research on the development of composite preparations using various waste materials is being actively pursued [5–7]. Based on a literature search [6, 8–10], among the possible alternatives, the development of composites using agricultural byproducts or agro-waste materials are currently the center of attention.
Particleboards are among the most popular materials used in interior and exterior applications such as floor, wall and ceiling panels, office dividers, bulletin boards, cabinets, furniture, counter tops and desk tops [11]. The production of particleboard can be related to the decided economic advantage of low cost raw wood material, inexpensive agents and simple processing. Therefore, agro-waste instead of wood is widely used in the manufacturing of particleboard. Among the raw materials are almond shell [12], wheat straw [13], bamboo [14], cotton seed hulls [15], flax shiv [16], rice straw-wood [17], vine prunings [18], coir pith [19] and wood flour [20]. Polymers such as urea-formaldehyde, phenol-formaldehyde, melamine formaldehyde, polyethylene and polyvinylidene are commonly used as binders. Urea formaldehyde is the most economic and useful adhesive among these binders.
The aim in the preset investigation with the objectives was to develop a composite material containing different percentages of walnut particle as the filler material and investigate the mechanical behavior of different composites.
2 Materials and methods
2.1 Matrix material
2.1.1 Epoxy resin CY-230
Epoxy resin is widely used in industrial applications because of its high strength and mechanical adhesiveness characteristic. It is also a good solvent and has good chemical resistance over a wide temperature range. Araldite CY-230 purchased from M/s Petro Araldite Pvt. Limited (Chennai, India) was used in the present investigation.
2.1.2 Hardener HY951
Hardener HY-951 purchased from M/s Petro Araldite Pvt. Limited (Chennai, India) was used as the curing agent. In the present investigation 8 wt% hardener HY-951 with epoxy resin (CY-230) was used in all the material developed. The weight percentage of hardener used in the present investigation was as per recommendation of Singh and Gope [21].
2.2 Reinforcing element
2.2.1 Walnut particles
The walnut particles are residues widely generated in high proportions in the agro-industry by the grinding of walnut shell. It is generally light to dark brown in color. The walnut shells are underutilized, renewable agricultural material. In the present study weight fraction (Vf) of walnut particles varied from 10–25. Walnut particle purchased from Allied Buss., Haldwani, India.
2.3 Optimization of weight percentage
2.3.1 Hardener (HY-951)
According to Misra and Singh [22] the per cent elongation, yield strength and Young modulus reached the maximum at 8 wt% of hardener (HY-951) when mixed with resin (CY-230). Therefore in the present study 8 wt% of HY-951 has been used.
2.3.2 Walnut particles
It was mixed with the resin up to the limits and the flow-ability of the mixture was maintained for the purpose of pouring the mixture into the vertical mould. No compression load was applied in this arrangement. The size of the walnut particles was controlled by sieving with ASTM 40 and ASTM 80.
2.4 Method
Epoxy resin (CY-230), hardener (HY-951), and walnut particles with different weight percentages were used. Different weight percentage (wt%) of walnut particles (15, 20, 25, 30 wt%) and epoxy resin were mixed by mechanical stirring at 3000 rpm. Based on the curing curve [23], the solution obtained by mixing of walnut particles with resin was kept in the furnace at a temperature of 90±10°C for 2 h [21]. At intervals of 30 min the solution was taken out of the electric furnace and remixed by a mechanical stirrer at the same speed. After 2 h the whole solution was taken out and allowed to cool to 45°C. When a temperature of 45°C was attained the hardener HY-951 (8 wt%) was mixed immediately [21]. Due to the addition of hardener a highly viscous solution was obtained which was remixed at high speed by the mechanical stirrer. The viscous solution so obtained was poured into different moulds for sample preparation. Tensile, compression and bending tests were conducted on a 100 kN servo hydraulic universal testing machine (ADMET, USA) under displacement mode of control of 1 mm/min. The results are presented and discussed in subsequent sections.
3 Results
3.1 Density
Density is one of the most important properties of the particle board material. The density of walnut particles reinforced composite for various weight percentages along with density of epoxy resin are presented in Table 1.
Density of walnut particle reinforced composite.
| S. no. | Walnut particle (10 wt%) (g/cm3) | Walnut particle (15 wt%) (g/cm3) | Walnut particle (20 wt%) (g/cm3) | Walnut particle (25 wt%) (g/cm3) | Epoxy (g/cm3) |
|---|---|---|---|---|---|
| 1 | 1.169 | 1.161 | 1.159 | 1.157 | 1.179 |
| 2 | 1.172 | 1.167 | 1.164 | 1.156 | 1.184 |
| 3 | 1.168 | 1.163 | 1.159 | 1.157 | 1.186 |
| Mean | 1.168 | 1.161 | 1.159 | 1.156 | 1.179 |
| SD | 0.0020 | 0.0031 | 0.0029 | 0.0006 | 0.0036 |
Table 1 reveals that increase in weight percentage of reinforced particles, i.e., the walnut particles in the resin solution decreases the density. This decrease in density of 25 wt% is about 1% of 10 wt%. The decrease in density can be related to the fact that the walnut particles are light but occupy a substantial amount of space. Hence there is a general decrease in the density of all the composite materials with regard to the epoxy resin.
3.2 Water absorption capacity
Water absorption capacity is another crucial factor to be taken into account when considering the effect of water on the composite material developed. The soaking period is 24 h taken as constant for all combinations of material. The effect is presented in Table 2.
Water absorption capacity.
| S. no. | Walnut particle (10 wt%) | Walnut particle (15 wt%) | Walnut particle (20 wt%) | Walnut particle (25 wt%) | Epoxy resin |
|---|---|---|---|---|---|
| 1 | 0.554% | 0.573% | 0.581% | 0.613% | 0.543% |
| 2 | 0.557% | 0.579% | 0.583% | 0.629% | 0.549% |
| 3 | 0.548% | 0.569% | 0.582% | 0.619% | 0.546% |
| Mean | 0.553% | 0.573% | 0.582% | 0.620% | 0.546% |
| SD | 0.000045 | 0.00005 | 0.00001 | 0.00008 | 0.00003 |
The effect of water absorption was important in case the material that has been developed when used for applications comes in contact of water. The water absorption capacity was found to be higher for 25 wt% of walnut particle reinforced composite as compared with lower weight percentage of walnut particles. This substantial increase with regard to the epoxy resin could be because the walnut particles here have maximum capacity for water absorption compared to the resin particles.
3.3 Scanning electron microscope (SEM)
The state of dispersion of particles into the resin matrix plays a significant role with regard to the mechanical properties of the composite. In the present investigation SEM was carried out on LEO435V6 instrument and voltage was kept 20 kV for bio composite containing different weight percentage of walnut particles to evaluate the particle size, particle matrix interface and dispersion of walnut particles in the epoxy resin matrix.
Figure 1(A) and 1(B) show the SEM micrographs of different bio composite material investigated in the present work. In all cases, good dispersion of walnut particles in the resin matrix has been observed. Figure 1(A) and 1(B) show the SEM micrograph of composite containing 10 wt% and 25 wt% of walnut particles, respectively. It is seen in the figures that walnut particles are well dispersed in the epoxy resin matrix in a preferred orientation.
(A) 10 wt% of walnut particles. (B) 25 wt% of walnut particles.
Hence, from the above micrographs it is can be concluded that due to uniform dispersion of walnut particles in epoxy resin, a remarkable effect on the mechanical properties may be obtained.
3.4 Mechanical properties
3.4.1 Tensile stress-strain curve
The mechanical properties of the walnut particles filled epoxy resin bio composite materials were determined by a 100 kN ADMET Servo hydraulic Universal Testing Machine at 1 mm/min strain rate under displacement control mode. The tensile stress-strain curve for walnut particles reinforced composite materials containing 10 wt%, 15 wt%, 20 wt% and 25 wt% of walnut particles reinforced composite is shown in Figure 2. All tests were conducted as per ISO in 100 kN Servo hydraulic Universal Testing Machine. Brittle behavior can be seen in the stress strain diagram due to addition of walnut particles in the epoxy resin matrix for all weight percentages of walnut particles. However, it is seen that beyond 10 wt% of walnut particle the stress strain behavior does not increase, therefore there is no improvement in load bearing capacity.
Stress-strain diagram under tension for different wt% of walnut particles.
3.4.2 Tensile properties
Tensile tests were carried out at strain rates of 1 mm/min. The properties of the walnut particle of 10, 15, 20 and 25 wt% reinforced composite are presented in Table 3.
Tensile properties of the composite materials.
| Property | Walnut particle (10 wt%) | Walnut particle (15 wt%) | Walnut particle (20 wt%) | Walnut particle (25 wt%) |
|---|---|---|---|---|
| Ultimate tensile strength (MPa) | 163.00 | 119.00 | 114.00 | 104.00 |
| % Elongation in length | 8.49 | 7.29 | 6.93 | 6.85 |
| Modulus of elasticity (MPa) | 2013.00 | 1388.00 | 1333.00 | 1328.00 |
The results of the ultimate tensile strength, percentage elongation in length and modulus of elasticity are shown in the Table 3 for strain rate of 1 mm/min. Remarkable differences can be seen on the ultimate tensile strength of the bio composite material between 10 wt% and over 10 wt% of walnut particles. It can be noticed that for all specimens the ultimate tensile strength is highest for the 10 wt% of walnut reinforced composite and is 163 MPa. Also, 10 wt% of walnut reinforced composite is shown as maximum percentage elongation from amongst the composite materials. It is seen that addition of walnut particles significantly affects the ultimate strength and percentage elongation. The ultimate tensile strength and the modulus of elasticity of 10 wt% of walnut board are almost 1.37 and 1.45 times higher than 15 wt% walnut board, 1.43 and 1.51 times higher than 20 wt% walnut board and 1.57 and 1.52 times higher than 25 wt% walnut board. It is true for all particulate composite material; no material can be fabricated which has more ultimate strength from matrix material if reinforced material is mixed at macro level. These behaviors are also shown in Figure 3.
Variation of ultimate tensile strength, modulus of elasticity and elogation for different weight percentage of walnut reinforced composite.
On the basis of results obtained the effect of weight fraction (Vf) on modulus of elasticity and ultimate strength are shown in Equations 1 and 2 with a correlation coefficient greater than 0.99.
3.4.3 Compressive strength
The compressive strength properties of the walnut particle filled epoxy resin composite materials were determined by 100 kN ADMET Servo controlled Universal Testing machine at 1 mm/min strain rate under displacement control mode.
The results of the compressive test are shown in Table 4. All tests were conducted under displacement control mode. Stress strain diagram obtained from compressive test is shown in Figure 4.
Compressive properties of the composite materials.
| Property | Walnut particle (10 wt%) | Walnut particle (15 wt%) | Walnut particle (20 wt%) | Walnut particle (25 wt%) |
|---|---|---|---|---|
| UTS (MPa) | 261.00 | 231.00 | 191.00 | 135.00 |
| % Reduction in length | 49.95 | 46.61 | 45.69 | 31.48 |
| Modulus of elasticity (MPa) | 1578.00 | 1668.00 | 2321.00 | 2391.00 |
Stress-strain diagram under compression for different wt% of walnut particles.
A remarkable difference can be noticed in the value of the compressive strength with different weight percentage composition of walnut particle. It can be noticed that addition of walnut particle improves the modulus of elasticity of composite materials. It is found that ultimate compressive strength of 10 wt% of walnut is about 261.0 MPa. But increase in weight percentage of walnut particles, the ultimate strength decreases considerably. Hence, taking into consideration the requirement and the cost effectiveness various composition of the reinforced material can be taken. Variation in ultimate strength, percentage reduction in length and modulus of elasticity with respect to different weight percentage walnut reinforced composite are shown in Figure 5.
Ultimate strength for different weight percentage of walnut reinforced composite.
On the basis of results obtained the effect of weight fraction (Vf) on modulus of elasticity and ultimate strength are shown in Equations 3 and 4 with a correlation coefficient >0.9.
3.4.4 Hardness
As known, hardness implies a resistance to indentation, permanent or plastic deformation of material. In a bio composite material, filler weight fraction significantly affects the hardness value of the hybrid composite material. Hardness values measured on the Rockwell M-scale showing the effect of weight percentage of walnut particles on the hardness values of hybrid composite are presented in Table 5. Variation of hardness with walnut particles weight percentage is shown in Figure 6.
Rockwell hardness values on M-scale for various filled hybrid composites.
| S. no | Walnut (10 wt%) | Walnut (15 wt%) | Walnut (20 wt%) | Walnut (25 wt%) | Resin |
|---|---|---|---|---|---|
| 1 | R-63 | R-67 | R-77 | R-90 | R-57 |
| 2 | R-64 | R-65 | R-81 | R-87 | R-55 |
| 3 | R-61 | R-66 | R-79 | R-89 | R-58 |
| 4 | R-60 | R-68 | R-80 | R-91 | R-57 |
| 5 | R-63 | R-64 | R-78 | R-92 | R-55 |
| Mean | R-62.2 | R-66 | R-79 | R-89.8 | R-56.4 |
| SD | 1.6431 | 1.5811 | 1.5811 | 1.9235 | 1.3416 |
Hardness (MRH) for different weight percentage of walnut reinforced composite.
It is found that hardness of neat epoxy resin (CY-230 +8 wt% of HY-951) is 56.4 MRH. The hardness of the fabricated composite made of epoxy resin and 25 wt% is the maximum and is 89.8 MRH. The hardness increases with increase in walnut particles weight percentage. Figure 7 shows that with increasing of hardness, ultimate strength in compression as well tension deceases and material behaved in a brittle manner.
Variation of ultimate strength with hardness (MRH).
The present result shows that a linear relation between hardness and ultimate strength in tension and compression exists. The following correlation between hardness and ultimate strength has been developed (Equations 5 and 6) with a correlation coefficient >0.9, where H is hardness in MRH scale.
3.4.5 Flexural strength
The flexural strength of the walnut particle filled epoxy resin composite materials were determined by 100 kN ADMET make servo controlled universal testing machine at 1 mm/min strain rate under displacement control mode using three point bend test. The results are presented in Table 6.
Flexural strength properties for resin and composites materials.
| Properties | Walnut particle (10 wt%) | Walnut particle (15 wt%) | Walnut particle (20 wt%) | Walnut particle (25 wt%) |
|---|---|---|---|---|
| Flexural modulus (MPa) | 1360.0 | 1450.0 | 1500.0 | 1560.0 |
| Flexural stress (MPa) | 769.0 | 614.0 | 603.0 | 439.0 |
| Flexural strain | 0.057 | 0.042 | 0.040 | 0.028 |
As depicted by the test data, amongst the composite materials developed the 25 wt%, walnut reinforced composite shows the best results with regard to the flexural modulus of elasticity (1560 MPa) and also it is better than 10 wt% walnut reinforced composites with regard to the flexural modulus of elasticity. But flexural stress and flexural strain was found to be higher for 10 wt% walnut filled composites as compared with others investigated in this report.
4 Conclusions
Epoxy bio composites reinforced with walnut particles were prepared. Such bio composites were experimentally characterized by means of microscopy, tensile, compression, hardness and bending test. Remarkable changes in the mechanical properties have been noticed due to addition of walnut particles in bio composite. Addition of walnut particles increased the hardness, which is very important property for particles board with sustainable tensile and compressive properties.
Acknowledgments
The author expresses his gratitude and sincere thanks to Department of Science and Technology, India, for providing finance to carry out this research work smoothly.
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- Original articles
- Triticale straw and its thermoplastic biocomposites
- Morphological and mechanical properties of chemically treated municipal solid waste (MSW)/banana fiber and their reinforcement in polymer composites
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