Tribology analysis of MMT nanoclay alkali-treated coconut sheath reinforced hybrid composite

Abstract

This research paper focuses on the tribology analysis of MMT - Montmorillonite nanoclay alkali-treated coconut sheath reinforced hybrid composite. The study aims to analyze the mechanical properties of coconut sheath reinforced polymer composites as compared to traditional synthetic fibers. The specific impact of MMT clay on the material’s mechanical properties is also considered. The experimental method involves the use of compression molding for fabrication, and various treatments are applied to the coconut sheath to improve its mechanical properties. The microstructure, tensile, flexural, and impact characterization of the specimens are analyzed. The results indicate that alkali-treated coconut sheath outperforms untreated coconut sheath in terms of surface quality. Additionally, the addition of MMT clay improves the bonding and surface area coverage, resulting in better mechanical properties. However, the brittleness of the treated coconut sheath specimen increased, reducing its energy absorption in impact tests. Overall, the study highlights the potential of coconut sheath as a natural fiber reinforcement for polymer composites and the impact of MMT clay on its mechanical properties.

Introduction

Many researchers have focused their efforts on using biodegradable materials in many engineering applications because of substantial developments in materials science.^1–3 Materials must be biodegradable and have considerable mechanical qualities to be considered as an eco-friendly alternative for functional applications. ⁴ Natural fibers (NFs) can be used to reinforce polymers because they possess the properties of high strength, non-toxic, cheap cost, widespread availability, reusability, recyclability and minimum environmental impact.^5,6 They are currently widely used as a reinforcement in a variety of polymer composite products. Natural fibers such as banana, sisal, hemp, flax, bamboo, ramie, pineapple crown fibers, and jute have been used as polymer reinforcement.^7–12

Because of two factors, natural fibers are frequently employed in hybridization with synthetic fibers. Natural fibers are mechanically inferior to glass, carbon, and Kevlar fibers for two reasons: first, they have poor mechanical qualities, and second, they are hydrophilic.^13–15 These problems are resolved by the hybridization process, which also results in certain environmentally friendly and long-lasting materials. The stacking order and manufacturing method used have a considerable influence on the mechanical characteristics of hybrid composites. The mechanical characteristics of hybrid composites having varied stacking sequences of coconut leaf sheath/jute/glass fibers were investigated by Bharath et al. ¹⁶ These composites were made using the hand layup process.

Andrzej et al. ¹⁷ investigated the chemical, physical and surface aspects of barley husk and coconut shell reinforced polypropylene composites. The major aim of the research was to investigate the possibilities of grain byproducts like barley husk and coconut shell as thermoplastic reinforcements, either alone or in combination with wood fibers. Balaji et al. ¹⁸ used the compression molding method to manufacture polymer composites made of vinyl ester (VE) reinforced with 15% coconut particles (CP) by weight and nanoclay at various percentage combinations. The research work explored better mechanical properties due to increased fiber to resin bonding. Excellent tensile, flexural, and impact strength was exhibited at 5% combination of the nanoclay material with 15% CP.

Mithil et al. ¹⁹ examined the mechanical characteristics of the phenolic resin-based coir/glass fiber composites. The findings revealed that the tensile characteristics improved with increase in the fiber content. Composites with a higher percentage of glass fiber exhibited increased mechanical properties. The mechanical characteristics of coir/glass fiber composites were studied by Harish et al. ²⁰ Scanning electron micrographs of fracture surfaces were utilized to assess the interfacial characteristics of coir/epoxy and were compared to glass fibers with the coir fiber as the reinforcement and the rubber as the matrix. Sankar et al. ²¹ analyzed and presented the combined impact of Montmorillonite nanoclay addition and fiber surface chemical treatments on the mechanical and tribological characteristics of polyester composites reinforced with Palmyra fruit fiber. Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is 2:1 clay, meaning that it has two tetrahedral sheets sandwiching a central octahedral sheet. The particles are plate-shaped with an average diameter of approximately one μm. Members of this group include saponite. Montmorillonite is the main constituent of the volcanic ash weathering product, bentonite.

Most research articles reported the mechanical properties of natural fiber reinforced hybrid composites, but only few works have been done on the impact of stacking sequence on alkali-treated coconut leaf sheath (CLS)/glass fiber MMT nanoclay reinforced hybrid composites, according to the existing literature.^22–25 As a result, the current research focuses on features of hybrid composites such as tensile strength, impact strength flexural strength and microstructure analysis.

Experimental method

The mold is made up of three different metal parts: one is the bottom portion, which has a smooth flattened surface finish, and a wider width than the others, the other is a hollow region in the shape of a rectangle, as shown in the Figure 1, and the third is a projection of the same size as the hollow one. A through hole is drilled on all four sides of the corner so that an insertion pin may be used to keep the mold parts together. Finally, using compression force, a specimen with a thickness of 3 mm was obtained. The molecular weight of the specimen was recorded after the completion of the process, to determine the weight percentage. The type of compression molding machine used was a DEEPAK POLYPAST COMPRESSION MOLDING MACHINE. The mold is made up of three different metal parts with a through hole drilled on all four sides of the corner so that an insertion pin may be used to keep the mold parts together. The being width of the bottom portion of the mold is wider than the others with a smooth flattened surface finish. The second metal part is a hollow region in the shape of a rectangle. The third is a projection of the same size as the hollow one. After all, production isn't a tedious process. We've divided coconut sheath into several groups, as listed in Table 1.

Figure 1.

Fabrication process—(a) Mold preparation, (b) Mold setting, (c) Composite preparation on mold, and (d) Fiber specimen.

Table 1.

Types of specimens and processing methods.

Sl. No	Specimen	Process
1	UT WC & UT WOC	Untreated coconut sheath with MMT clay and without MMT clay
2	TC WC & TC WOC	Treated coconut sheath with MMT clay and without MMT clay
3	UT WH WOHC	Untreated coconut sheath with hybrid varieties without MMT clay
4	UT WH WHC	Untreated coconut sheath with hybrid varieties and with MMT clay
5	TC WH WHC	Treated coconut sheath with hybrid varieties and with MMT clay

Impurities like scales, depositions etc., are commonly found in natural coconut sheath which requires NaOH treatment. This method necessitates the use of surface modification techniques such as acid treatment, base treatment, and fiber surface pre-coating, among others.

Untreated coconut sheath without MMT clay

The size of the unprocessed coconut sheath is 300 × 125 mm. Six layers of coconut sheath were stacked on top of one another, with weight applied to straighten the fiber. 300 gm of resin is mixed thoroughly with 4.5 mL of catalyst and accelerator using a glass rod and manual stirring process. A steel mold was used to build the composite to the necessary size of 300 × 125 × 3 mm³. Resin was applied to the fiber mat in such a manner that it ensured the wetting of the entire surface of the mat. Steel mold was used to fabricate the composite to the required size of 300 × 125 × 3 mm³. After that, the mold is placed in a compression molding machine with a pressure of 150 kg/cm² to completely shut the mold. The curing process was continued at room temperature for another 4 h.

Untreated coconut sheath with hybrid (with MMT and without MMT clay)

The two UTC sheaths are 300 × 12.5 (mm) in size. The four glass fibers are placed in a UTC sheath of the same size. 300 gm of resin is mixed well with 4.5 mL of catalyst and accelerator. Two UTC coconut sheaths are inserted one after the other in the mold, followed by four glass fibers. Resin is poured such that, it is coated thoroughly to the whole surface of the fiber. After allying the resin, the mold is placed in a COMPRESSION MOLDING MACHINE. To get a thickness of 3 mm, we need to crush the mold at a pressure of 150 Kg/cm².

The mold is taken out of the molding machine after 3 hours. To prevent water absorption, MMT clay was mixed with resin. MMT clay was used in a proportion of 2%. The technique outlined above is used to create a sample of untreated coconut sheath NGG (UTC WC NGG). Both compounds are now ready to be employed. The catalyst is first taken and poured into a cup containing pure resin, as shown in the diagram. The resin is then spread by adding an accelerator. It’s important to note that we must start building right away after adding the two chemicals because the resin may set up quickly.

The layers are piled one on top of the other and the resin is applied simultaneously. It's important to make sure the Coconut Sheath is properly wetted by applying matrix on both sides. The specimen is encased in mold, and the top layer is covered before being placed in the molding machine. The pressure applied to the specimen must be restricted below 150 kgf/cm² to prevent the squeezing of the matrix (i.e., adding modest increment pressure slowly to the specimen). The matrix must be cured for 4–5 h before the material is removed.

Treated coconut sheath with MMT clay

This technique is similar to the Untreated Coconut Sheath process, with the exception that the Coconut Sheath is washed in NaOH to remove pollutants, as previously stated. 40 g of NaOH should is mixed with 1 L of water in a tub. The NaOH is added to the water and mixed with a stirrer until it is completely dissolved. Typically, NaOH in pellet form is employed in laboratories, as shown in the figure. The Coconut Sheath is placed in it after NaOH has been dissolved. To eliminate surface contaminants, the layer is wiped softly and gently. The timer is set for 1 h to ensure that it is thoroughly immersed. The Coconut Sheath was rinsed in water, and then placed in Owen or ambient conditions to dry. The 300 gm of resin is mixed thoroughly with 4.5 mL of catalyst and accelerator.

The treated coconut sheaths are inserted one by one in the mold, and resin is applied on each coconut sheath, till the resin is fully coated to the entire surface of the fiber. After applying the resin, the mold was placed in a DEEPAK POLYPAST COMPRESSION MOLDING MACHINE. The mold is crushed to a thickness of 3 mm by applying 150 bar pressure. The mold was taken out of the compression machine after 3 hours. To prevent water absorption, MMT clay was mixed the resin. MMT clay was used in a proportion of 2%.

Treated coconut sheath with hybrid (with MMT and without MMT clay)

The coconut sheaths are treated with the NaOH to remove the cellulose content and waxy layer. Four glass fibers are treated with sodium hydroxide at the same time to promote bonding between them. 40 g of NaOH is mixed with one litre of water in a tube, and the six coconut sheaths are submerged in the solution for 1 hour. After 1 hour, the coconut sheaths are taken out from the beaker, cleaned in distilled water, and dried in the hot sun. The 300 gm of resin is mixed well with 4.5 mL of catalyst and accelerator.

The treated coconut sheath is inserted one by one in the mold, together with four glass fiber, and resin is poured over the coconut sheath and glass fiber. The entire surface of the fiber is thoroughly coated with resin. After applying the resin, the mold was placed in a DEEPAK POLYPAST COMPRESSION MOLDING MACHINE. Then the mold is crushed at a pressure of 150 bar, to achieve a thickness of 3 mm. The mold is removed from the machine after 3 hours. To prevent water absorption, MMT clay was mixed with resin. MMT clay was used in a proportion of 2%.

Figure 2 shows the images of Alkali treatment of coconut sheath and stirrer processing unit. The machine is equipped with a special attachment to ensure constant rpm (rotation per minute). In addition, it includes a special feature that enables it to rotate in both clockwise and anticlockwise orientations. The stirrer is set to 500 r/min. The purpose of the stirrer machine is to uniformly distribute the MMT clay in the matrix Figure 3 shows the oven processing unit and the compression moulding machine.

Figure 2.

(a) Alkali treatment of coconut sheath and (b) Stirrer process.

Figure 3.

(a) Oven processing (b) Compression molding machine.

Result and discussion

Microstructure analysis

Figure 4 shows the microstructure of the treated nature and Glass fiber without clay and untreated glass fiber without clay.

Figure 4.

Microstructure analysis of (a) and (b) Treated nature fiber without clay (c) and (d) Treated glass fiber without clay (e) and (f) Untreated glass fiber without clay.

The surface topology of treated nature fiber with clay is shown in Figure 5(a) and (b), which explores an uneven surface due to globular protrusions. The SEM micrographs of Treated nature and Glass fiber with clay and untreated glass fiber with clay are exhibited in Figure 5(a)–(f). Figure 5(c) and (d) depict surface topological changes of glass fiber treated with clay. Untreated fibers with clay cause removal of globular protrusions on the surface, resulting in the formation of many voids. These spaces enable mechanical contact between the fiber and the polyester matrix. The interaction of the fiber surface with the polyester matrix is also improved because the effective surface area of the fibers is exposed. As can be seen in Figure 5, the untreated fiber with clay exhibits a rough surface topography (b). These gaps help the alkali-treated fibers become more hydrophilic. Figure 5(c) shows the SEM image of an untreated glass fiber of clay, with the clay forming a dome on the fiber surface. The fiber covers part of the clay voids on the surface, reducing the amount of moisture entering through the voids.

Figure 5.

Microstructure analysis of (a) and (b) Treated nature fiber with clay (c) and (d) Treated glass fiber with clay (e) and (f) Untreated glass fiber with clay.

Tensile characterization

The tensile specimens for study are shown in Figure 6(a). The specimens produced under all the four types of treatments are compared, and the tensile strength of TC WC is observed to be higher, compared to the others and is depicted in Figure 6(b). Because of the alkali treatment, the sheath’s silky or waxy layer is removed, and the addition of MMT clay covers the greatest surface area of the coconut sheath, tightly interlocking the fiber and resin. We conclude that the Alkali-treated coconut sheath is a better substitute for other treatments.

Figure 6.

(a) Tensile specimen, (b) Nature fiber tensile characterization, (c) Hybrid fiber with clay tensile characterization, and (d) Hybrid with and without clay tensile characterization.

In all hybrid combinations, the Figure 6(c) clearly illustrates that TCWC has a larger value than UTWC. Glass fiber is mixed with natural fiber in this example, and the glass fibers are alkali-treated. TCWC NNG, for example, has an entry-level performance; its tensile strength is higher than that of UTWC NNG. With an increase in the glass fiber mix, the breaking load or tensile strength for all the TC WC HYBRID specimens increased as well. According to the experimental results, the TCWC HYBRID has a higher tensile strength and is a better substitute for the UTWC HYBRID. The tensile strengths of UT WOC, UTWC and TC WC are compared and are shown in Figure 6(d). The tensile strength of UT WC & TC WC is considerably lowered when the glass fibers in MMT Clay are increased (GGN and GNG). The tensile strength of the TC WC is higher compared to the other two specimens, and the reason is explained in the final chapter.

Flexural characterization

The specimens for studying the flexural characterization are shown in Figure 7. Figure 7(b) shows a comparison of treatment kinds based on flexural strength. It should be noticed that UTWC and TCWC have more strength than UTWOC and TCWOC. As previously stated, when MMT clay is applied to a specimen, the whole surface area is covered, resulting in improved bonding.

Figure 7.

(a) Flexure specimen, (b) Nature fiber flexure characterization, (c) Hybrid fiber with clay flexure characterization, and (d) Hybrid with and without clay flexure characterization.

Figure 7(c) demonstrates that flexural strength of UTWC has a greater value than UTWOC. The presence of the MMT clay present in UTWC enhances the bonding process. The specimen with pure glass fiber (GGG), on the other hand, exhibits a reverse result, because when MMT clay is added to glass fibers, it tends to slide layer by layer slowly as the manufacturing process progresses. As a result, the bonding area is not as strong as UTWOC.

The TCWC has higher flexural strength compared to UTWOC and UTWC and is shown in Figure 7(d). Except for the pure glass fiber combinations, the MMT clay improves flexural strength. We may deduce that the flexural strength of all TC WC manufactured samples is higher compared to the UTWOC and UTWC for all the specimens except for the GGG specimen. TCWC has higher flexural strength compared to UTWC and UTWOC is because the addition of MMT clay to the carbon fibers in TCWC improves the bonding process, resulting in a stronger structure.

Impact characterization

Figure 8 shows the specimens used for the impact test. The amount of energy absorbed by WOC and WC specimens in both treated and untreated conditions during the impact test is shown in Figure 8(b). The amount of energy absorbed by UTWOC specimen is higher, when compared to the TCWOC specimen. The reason is that the TC WOC specimen becomes brittle due to the addition of MMT clay. Figure 8(c) illustrates the comparison of the impact energy of hybrid UTWOC and UTWC variants, with respect to the Stacking sequences.

Figure 8.

(a) Impact specimen, (b) Nature fiber impact characterization, (c) Hybrid fiber with clay impact characterization, and (d) Hybrid with and without clay impact characterization.

The results of values for Hybrid variations of UTWOC and UTWC cannot be concluded since they are randomly oriented. The variation of the impact energy with respect to the stacking sequences for the hybrid UTWOC, UTWC, and TCWC variants is shown in Figure 8(d). Since the experiments exhibited an irregular variation in the results for the hybrid UTWOC, UTWC, and TCWC variants, it was difficult to conclude. The experiments revealed the impact strength of all impact energy combinations. In all circumstances, the untreated specimen without clay yields a better outcome. TCWOC becomes brittle due to the addition of MMT clay is that the clay particles in the resin matrix can create stress points, leading to micro cracks and a decrease in ductility.

Conclusions

Based on the experimental characterization of MMT nanoclay alkali-treated coconut sheath reinforced hybrid composite, the following conclusions are arrived at

• The treated Coconut Sheath outperforms Untreated Coconut Sheath. Due to the alkali treatment, enhanced surface quality (surface roughness) was explored due to the removal of the waxy layer in the treated coconut sheath.

• The addition of MMT clay improves the characteristics a little more. MMT clay is nano-sized, and when mixed with a matrix, it covers the most surface area possible, allowing for stronger bonding.

• The brittleness of the treated coconut sheath specimen increased which in turn reduced the absorption of energy. Untreated Coconut Sheath has greater energy absorption in impact tests.