Harnessing of Waste Rubber Crumb and Development of Sustainable Hybrid Composite Using Kenaf (Hibiscus cannabinus) for Structural Applications

ABSTRACT The present work aims at harnessing the rubber crumb in polymer matrix composite for structural purposes. The hybrid composites were made utilizing the hand layup method using rubber crumb with varying weight percentages of 1.5, 3, and 5. The physio-mechanical characteristics of hybrid composites were compared to the unfilled kenaf epoxy composite. The results showed that adding a rubber crumb helps to offset the disadvantage of natural fibers absorbing water, as the water absorption of composites filled with 5 wt% rubber crumb was decreased by 2.4 times when compared to unfilled composites. The inclusion of rubber crumb, on the other hand, increased the density of the composites. When compared to the unfilled composite, the tensile and flexural strength of the composite with 3 wt% rubber crumb was shown to be superior, increasing by 24.5% and 36.83%, respectively, and the impact strength of composite containing 5 wt% rubber crumb was enhanced by 54.33%. The VIKOR technique, which is part of the MADM methodology, is successfully used to pick the optimal configuration of the suggested hybrid composites and it indicates that composite with 5 wt% of rubber crumb is the optimum weight percentage that can be conveniently used in the proposed composite.


Introduction
In comparison to their traditional isotropic equivalents, recent innovations have elevated the use of composite materials. Synthetic fibers are used in almost all engineering applications to minimize the density of the finished product (Luo et al. 2018). Artificial fibers, on the other hand, have recently lost favor with marketers and engineers due to environmental and energy concerns . Natural fibers provide a number of advantages, including environmental friendliness, lower cost, low density, adequate specific strength, stiffness, and toughness (Mahesh, Harausampath, and Mahesh 2021), and are extensively employed in production of interior parts of automobiles (Thomas et al. 2011). Natural fibers might be a feasible alternative to synthetic fibers in sacrificial structural applications (La Mantia and Morreale 2011;Kulkarni 2021a, 2021b).
Kenaf looks to be the most promising natural fiber among the several natural fibers available owing to the superior properties it exhibits compared to other natural fibers. Kenaf strengthened composites can therefore be employed in a variety of technical applications (Salleh et al. 2014). Hybrid composites, on the other hand, have arisen as a way to achieve a compromise between cost, biodegradability, and characteristics. Furthermore, their decreased production costs increase their application in a variety of settings (Negi et al. 2019).
The idea of integrating particles and fibers into polymers has fascinated researchers. Adding filler particles to a fiber-reinforced matrix enhanced modulus while lowering material costs and weakening the structure, according to research (Weidenfeller, Höfer, and Schilling 2004). Waste tires have become a big problem for many countries as the transportation sector continues to develop. Rubber Crumb is a recycled rubber produced from automotive and truck scrap tires. Rubber crumb does not have enough mechanical strength on its own, thus it must be mixed with a matrix material to make a solid structure. According to physio-mechanical characterization, adding waste tire rubber crumb to epoxy increased the hardness, tensile strength, and water absorption capabilities of polymer matrix composites (Jena, Nayak, and Satapathy 2020).
When at least two alternative materials are available for a particular application, selecting the best one necessitates careful consideration. Multi-attribute decision-making (MADM) techniques can help with this. Various areas such as retails, logistic networks, engineering, and so on use MADM tools to provide realistic results.
Although there has been a lot of work done on putting rubber crumbs into concrete (Dong et al. 2019(Dong et al. , 2020, there has been relatively little work done on using rubber crumbs as filler polymer matrix composites. Thus, the current work focuses on proposing a composite using waste tire in the form of rubber crumb as a filler with varying weight percentages in a natural fiber (kenaf) reinforced polymer matrix hybrid composite, evaluating the mechanical properties of the proposed composite, and selecting the best configuration using the MADM approach.

Materials and methods
The present section describes the selection of materials, manufacturing methods used and characterization techniques.

Materials
The suggested composites are made from kenaf, rubber crumb, and epoxy resin in this study. Kenaf, was sourced from Go Green Products in Chennai, India. The matrix material was L12 epoxy from Yuje enterprises in Bangalore, India, mixed with K6 hardener in a 10:1 weight ratio. Rubber crumbs with a mesh size of 40 were procured from local sources in Bangalore, India, and employed as fillers in hybrid composites.

Composite preparation
The hand layup method was used to create the suggested composites. The mat was trimmed to a 300 mm by 300 mm size. A mechanical stirrer was used to mix the L12 epoxy and K6 hardener in a 10:1 weight ratio. To make a laminate with a thickness of 3 mm, three layers of kenaf fiber were employed. To make the laminate removal easier, the mold was coated with wax. The kenaf mat was put on the mold after being soaked in the L12 epoxy and K6 hardener mixture. To remove the extra resin, a hand roller was used. This process was done for each of the fabric's three layers. The top mold plate was put after the third layer. The whole thing was kept under compression using compression molding machine. To make the hybrid composites, the rubber crumbs were fully combined with the epoxy and hardener mixture. The rest of the operation is the same as before. Table 1 lists the specifics of the suggested composites.

Physio-mechanical characterization
The theoretical and experimental densities of the suggested composites were determined using the rule of mixing and the Archimedes principle, respectively. Equation 1 was used to compute the void content of the composite.
where ρ t is the theoretical density and ρ e is the experimental density. The composite's water absorption behavior was evaluated using the ASTM D 570-98 standard. After 300 h, the percentage of water absorption was calculated using Eq. 2.
where w f is the final weight of the sample after the test and w i is the initial weight of the sample before the test. The ASTM standards ASTM D3039/D3039 M-17 was used for the tensile test, ASTM D6110-18 for Charpy impact test and ASTM D7264/D7264 M − 15 for a flexural test.

MADM approach
Many researchers have utilized the VIKOR technique to solve complicated issues with many and contradictory criteria. This method compares the proximity measure from the ideal alternative and ranks the alternatives to find a compromise solution for a problem with competing criteria. The entropy method calculates the weights of the various criteria for use in the VIKOR method.
D ¼ x 11 x 12 x 13 x 21 x 22 x 23 x 31 x 32 x 33 x 14 x 24 x 34 2 4 3 5 (3) Decision matrix is shown in Eq. 3 which is further normalized to get normalized matrix using Eq. 4.
where i = 1, 2, . . ., m and j = 1, 2 . . ., n Weights for each criteria were found using entropy method using Eq. 5 to calculate the index's proportion "P ij ," and Eq. 6 to find index's entropy "E j ." where k was calculated using Eq. 7 where m indicates the number of alternatives. Equation 8 was used to calculate the entropy weight "w j " of index "j." The standardized value of weight "v ij " was calculated using Eq. 9 and the standardized weighted normalized matrix was built.
. . . v À n for maximization problems (11) . . . v À n for minimization problems Equations 12 and 13 were used to calculate the utility and regret measures for each non-dominated solution, respectively where S i ; R i ε [0,1], represents the greatest and worst possible outcomes. Equation 14 was used to determine the VIKOR index, with the lower VIKOR index being the preferred alternative.
where α is a weighting factor between 0 and 1, in most cases, is set to 0.5. Table 2 shows a bird's eye perspective of the acquired data during the physio-mechanical characterization of the suggested composites.

Physical properties
The density of the generated composite is mostly governed by the void content of the composite.
Equation (1) is used to compute the void content of the composite. Natural fiber lumens act as void fillers in composites, increasing the void content (Shuhimi et al. 2016). The inclusion of a rubber crumb lowers void content because the filler is packed into the void, as seen in Table 2. The inclusion of a rubber crumb, on the other hand, increases the density of the suggested composite.
One of the most severe disadvantages of natural fiber-reinforced composites is moisture absorption. As a result, the moisture absorption qualities of such composites are crucial. The water absorption of KE composites is higher than hybrid composites. The hydrophilic nature of kenaf accounts for this behavior. The water absorption of the hybrid composites decreases when the rubber crumb is introduced to the composite because rubber is hydrophobic. Also, addition of rubber crumb fills up the void content and also deposits on the fibers covering them and thereby reducing the exposure of fiber to water. The composite absorbs less water when additional rubber crumb is added.

Mechanical properties
To establish the tensile strength of the suggested composites, tensile tests were performed. The variance in tensile strength of the suggested composites. The inclusion of rubber crumb increases tensile strength up to a percentage of rubber crumb of 3 wt%. The addition of more rubber crumb reduces the tensile strength of the material. KERC3 has the highest maximum tensile strength among the hybrid composites, which is 24.5% more than the KE composites. The acquired results are consistent with the trend found in the literature (Verma, Negi, and Kumar Singh 2018). Rubber crumb with a concentration of up to 3 wt% fills cavities in the composite, assisting weight transmission. Agglomeration occurs if the rubber crumb is added in excess of 3 wt% as shown in Figure 1.
As a result, the interfacial connection between the matrix and the filler is poor. As a result, the tensile stress transmission rate between them is exceedingly slow, resulting in early specimen failure.
The flexural strength of KERC3 is the strongest of all the suggested composites, as can be observed. As 3 wt% rubber crumb is added, the flexural strength increases by 36.83% when compared to KE. This might be because the rubber crumb fills the voids in the suggested composites and increases their flexibility. Also, epoxy being a material with capability to adhere to many materials gets properly bonded with fibers and fillers. The woven mat is used as a reinforcing fiber in the present study. This woven mat has a gap between warp and weft which will be filled by the rubber crumb reinforced epoxy, resulting in enhanced flexural strength. The inclusion of more rubber crumb than 3 wt% reduces flexural strength due to agglomeration, allowing the fracture to spread even further. By increasing the amount of filler to 5% wt, the strength dropped down consequently. This can be defined by the fact that overloading the resin phase by filler will increase the viscosity of resin phase that subsequently causes internal porosities in this phase and reduce its strength. On the other hand, the viscous resin is not capable to wet the fibers properly that leads to a reduction in strength properties. The acquired results are consistent with the trend found in the literature (Verma, Negi, and Kumar Singh 2018). However, even if the flexural strength of KERC5 (5 wt%) is lower than that of KERC3, it is still superior than KE composite since the flexural strength of JERC5 is 16.25% more than that of KE composite. This demonstrates that the inclusion of rubber crumb improves flexural strength.
The impact strength of the suggested composites is determined using the Charpy impact test. KERC5 exhibits highest impact strength and the impact strength of composites vary in the order KERC5>KERC3>KERC1.5>KE. The impact strength of JERC5 is 54.33% higher compared to KE composite. It is well known that flexible materials are better energy absorbers than stiffer materials (Mahesh, Joladarashi, and Kulkarni 2021b;Mahesh, Joladarshi, and Kulkarni 2019). Rubber is a compliant material that enhances flexibility when incorporated in a composite. The inclusion of rubber crumb as filler in the proposed composites, apart from filling up voids in the composites, also induces flexibility in the composite due to the compliant nature of the rubber. This makes the rubberfilled composites absorb more impact energy than non-filled composites. The rubber crumb, which serves as a stress concentrator and causes shear yielding in the matrix, is responsible for the increased impact strength. These rubber crumbs also cause the matrix to become more flexible.

MADM
To choose the best composite from the offered composites, the MADM method is used. The performance defining criteria and their implication are presented in Table 3.
The decision matrix generated as a result of the testing findings is shown in Table 4.  Higher the better Standard deviation is not taken into consideration for the normalized matrix in Table 5 since normalizing makes it easier to compare the various values of the characteristics gathered experimentally. This is similar to Chauhan and colleagues' technique (Chauhan et al. 2017) The calculated weight using entropy method is presented in Table 6. The positive and negative ideal solutions are generated and summarized in Table 7 after the weights are determined using the entropy approach.
S i and R i values are found out and are reported in Table 8. Finally, the possibilities are ranked using the VIKOR index. Table 8 shows the VIKOR index and rating for each choice. The option with the lowest VIKOR index receives rank 1, and so on. Table 8 shows that the VIKOR index for KERC5 is the lowest compared to its counterparts. The MADM approach named VIKOR used in the present study indicates that the composite with 5 wt% of rubber crumb gives the lower VIKOR index indicating that it is the optimum composition of the rubber crumb that can be used in Kenaf reinforced composite considering all the physical and mechanical attributes considered in the present study. KERC5 appears as the optimal configuration for the criteria covered in this study, according to the VIKOR technique.

Fractography
The suggested composites' varied mechanisms of failure may be assessed using SEM. The different types of failure discovered in the suggested composites are depicted in Figure 2. Fiber breakage, fiber pull out, and matrix cracking are shown to be the major causes of composite failure. The matrix breaking causes the composite to fail first, followed by fiber pullout and fracture. Damage mechanisms discovered are characteristic of brittle composites.
The insufficient bonding between the fibers and the matrix leads to fiber pullout. Fracture mechanics of composites suggests that the stress is initially borne by the fibers and matrix in the composite system, when the matrix fractures, the fibers behave to retain its apparent ductility by behaving as crackstoppers/arrestors to slowdown the material's catastrophic failure until the occurrence of fiber fracture. As the applied stress increases, the weak primary cell wall collapses, and the cohesion of cells occurs, leading to fiber failure. Cracks in the matrix are an indication of brittle failure and is considered as one of the major failure mechanisms. Voids due to fiber pullout can be seen in Figure 2. The extent of poor adhesion between fibers and the matrix can be judged by the gap between the fiber and the matrix.
The obtained results are in accordance with the trend available in Verma, Negi, and Kumar Singh (2018). Addition of rubber crumb up to 3 wt% of rubber crumb results in filling up of voids in the composite and thereby helping in load transfer. Further addition of the rubber crumb beyond 3 wt% could result in agglomeration. This leads to weak interfacial bonding between matrix and filler. Thereby, the tensile and flexural stress transfer rate between them proceeds at a very low rate, resulting in early failure of the specimen.  It is well known that flexible materials are better energy absorbers compared to stiffer materials (Mahesh, Joladarashi, and Kulkarni 2021b;Mahesh, Joladarshi, and Kulkarni 2019). Rubber is a compliant material which enhances the flexibility when incorporated in a composite. Inclusion of rubber crumb as filler in the proposed composites, apart from filling up of voids in the composites, also induces the flexibility in the composite due to compliant nature of the rubber. This makes the rubberfilled composites absorb more impact energy compared to non-filled composites. This enhancement in the impact strength is due to the addition of rubber crumb which acts as stress concentrator and creates a shear yielding in matrix. Additionally, these rubber crumbs also induce flexibility in the matrix.

Potential industrial application
The proposed composites prove their potentiality for structural applications, especially for automobile bumpers. The main target for sustainable product life planning in the automotive industries requires maximum recovery of non-biodegradable materials from end-of-life vehicles through recycling. The natural fiber-reinforced composites (NFRCs) are light in weight and biodegradable. Most of the experimental car components produced used NFRCs in non-structural parts, such as door trim panels, seat fillers, seatbacks, dashboards, and many interior parts, in structural applications like seat frames and steering components that 15-20% of vehicle weight. The results obtained from this study support that kenaf and rubber crumb are potential materials for such applications, and it is promising material for further development of more components.
The proposed composites exhibit better mechanical properties compared to hybrid polymer matrix composite reinforced with areca nano filler and coir (Mahesh, Mahesh, and Puneeth, 2020). The comparison of the mechanical properties of the proposed composites with that of the composite available in literature shows that the tensile strength of proposed KERC 3 composite is 40.54 MPa as opposed to 27 MPa of the Areca nano filler and coir composite. Also, the flexural strength of proposed KERC3 composite is 80.65 MPa as opposed to 64.23 MPa of the Areca nano filler and coir composite. Similarly, the proposed composite KERC5 exhibits an highest impact strength of 27.41 kJ/m 2 as opposed to 19 kJ/m 2 of the Areca nano filler and coir composite. This reveals that the mechanical properties of the proposed composites are better compared to the arecanut nano-filler and coir reinforced composites.

Conclusions
The advantages of waste rubber crumb as a filler in natural fiber-reinforced hybrid composites for diverse structural purposes are investigated in this study. The following assumptions are based on the findings of this study: • Rubber crumb, a waste product of discarded tyres, can be utilized as filler in polymer-based composites, resulting in better waste management. • It has been discovered that the lumens contained in natural fibers cause water absorption in composites made only of kenaf. The hybrid composites (KERC1.5, KERC3 and KERC5) absorb less water than the KE composites. The addition of a rubber crumb helps to overcome the disadvantage of water absorption by natural fiber alone reinforced polymer matrix composites. • It is found that with the addition of rubber crumb, the density of the composites gets increased, with KERC5 exhibiting 13.3% more density compared to KE. • The addition of rubber crumb showed a positive effect on mechanical properties of the proposed composites with tensile and flexural strength enhanced up to the addition of 3 wt% of rubber crumb. However, in the case of impact strength, continuous enhancement in impact strength is seen.
• Rubber crumb with a concentration of up to 3 wt% fills cavities in the composite, assisting weight transmission. Additional rubber crumb additions may cause agglomeration, resulting in poor interfacial adhesion between matrix and filler. As a result, the tensile stress transmission rate between them is exceedingly slow, resulting in early specimen failure. • The suggested composites' flexural strength is in the order KERC3>KERC1.5>KERC5>KE. The inclusion of rubber crumb improves flexural strength, showing that it is beneficial. When the rubber crumb content is raised beyond 3%, instead of filling in the gaps in the composite, the rubber crumbs begin to aggregate, resulting in agglomeration. • Impact strength of the proposed composites vary in the order KERC5>KERC3>KERC1.5>KE.
The inclusion of a rubber crumb works as a stress concentrator and causes matrix shear yielding, resulting in improved impact strength of the suggested composites. • The damage processes discovered are similar to those of brittle composites. • The MADM approach named VIKOR used in the present study indicates that the composite with 5 wt% of rubber crumb gives the lower VIKOR index indicating that it is the optimum composition of the rubber crumb that can be used in a kenaf-reinforced composite considering all the physical and mechanical attributes considered in the present study.