Effect of surface modification on mechanical properties of filature silk waste and nanoclay filler-based polymer matrix composite

Abstract

Natural fibers have been attracting researchers and engineers as an alternative reinforcement of synthetic fibers in polymer composites due to their low cost, availability from natural resources, satisfactory high modulus and tensile strength, and biodegradability. Filature silk waste (FSW) is the remnant part of the cocoons which is produced during the silk forming process. The current study focuses on the comparison of tensile properties between untreated filature silk waste reinforced epoxy-based composite (UTFSWREC), 2 wt% alkali-treated filature silk waste reinforced epoxy-based composites (TFSWREC) and 2 wt% alkali-treated filature silk waste reinforced epoxy nanocomposites (TFSWRENC). The tensile properties showed that Young’s modulus of composites increases with surface modification of fiber and further enhances with nanoclay filler. TFSWREC and TFSWRENC displayed a higher tensile modulus than UTFSWREC. Scanning Electron Microscopy (SEM) showed the removal of the sericin layer from the surface of fiber, which resulted in the separation of fibrils and further resulted in the enhancement of the mechanical properties. FTIR analysis confirmed that intermolecular bonding improves with the chemical treatment and further refined with nanoclay filler addition.

Keywords

Filature silk waste alkali treatment nanoclay tensile properties green composite

Introduction

The research field of composite study has progressed rapidly in the past few decades. Their properties like low density, corrosion-resistant, lightweight, high strength, and stiffness resulted in selecting composite material as an effective alternative to metals and alloys. Synthetic fiber like glass, aramid, and carbon as reinforcement in composites is harmful to the environment because of their higher pollutant and greenhouse emission, which led researchers to explore abundantly available natural fibers for the development of natural fiber-reinforced composites. Today’s manufacturing industry mainly focuses on utilizing sustainable material as raw material to manufacture their products. Natural fiber-reinforced composites are emerging materials used to manufacture door panels, interiors, and seat backs in automobiles and aerospace industries.¹ The classification of natural fibers is broadly divided into two main categories: plant-based natural fiber and animal-based natural fiber.² There are many research accomplishments under plant-based natural fiber like flax,³ bamboo,³ hemp,⁴ jute,⁵ sisal,⁶ coconut,⁷ kenaf,⁸ hibiscus sabdariffa⁹ and abaca.¹⁰ Fibers like human hair,¹¹ cow hair,¹² spider silk,¹³ and silk¹⁴ come under animal-based natural fibers which are mainly made of proteins. Silk fiber has an advantage over the plant fiber due to its orderly fiber properties, high fiber strength, linear fibrils type, and relatively better toughness. Silk is mainly made of two proteins: silk fibroins and sericin. Silk fibroin is present in aqueous solution in the glands of insects, during spinning an arc movement of silkworm’s head leads to an increase in the silk proportion in the emulsion and due to the elongation stresses an insoluble thread is produced.¹⁵ Silk is obtained from bombyx mori cocoons in a length range between 750 and 1500 m with a sericin layer over the fiber’s surface. Sericin acts as an adhesive between the layers of fiber in multilayer cocoons.¹⁶ Silks are fibrous proteins containing a high amount of amino acid residues, which are repetitive in sequence.¹⁷ Each silk is made up of twisted bunches of fibrils of diameter 20–25 nm, which gives strength to the fiber in the same way as the smaller twisted strands strengthen the rope.¹⁸ Since silk is made of proteins, it has been utilized in various biomedical applications like tissue engineering, wound healing, and drug delivery vehicles. On an average, around 1 million tons of raw silk is produced worldwide from which around 33,000 tons is produced by India only. Processed silk is far more expensive than plant fiber. However, with the development in fabric industries, plenty of waste silk (kibisu) is produced during the reeling process, which is available at a very low cost.¹⁹ Although these damaged silks that are commercially available at a very low cost, their utilization are rare in actual engineering problems. However, several recent studies show that this tough protein fiber can endow solutions for modern engineering problems. Waste silk reinforced polymer composites have shown better mechanical properties than other natural fiber-reinforced composites.^20-22 These researches focus on waste silk ‘chindi’ which is left out parts of the processed silk. The current study focuses on the effect of filature silk waste as reinforcement on the epoxy-based composite’s mechanical properties. Filature silk waste, which is locally known as kibisu is the remnant part of the cocoons after removing fine silk produced during the reeling process. Every year, textile industries produce a large amount of silk waste, which is just dumped aside without knowing its real value. In India only, 600 tons of FSW is produced every year. Filature silk waste has shown significantly better mechanical properties than other plant fibers, but unfortunately, not much research has been done on this waste miracle material up to till date.

The main challenge faced during the reinforcement of natural fiber in polymer composite is the lack of interfacial bonding between fiber and matrix and high liquid retention of natural fiber. These limitations limit the natural fiber to replace conventional synthetic fiber. Surface modification can be a viable method to solve this problem.^23-28 Various treatments have been investigated for fiber surface modification like alkali, saline,²⁹ acetylation, and benzoylation. However, the most commonly used chemical treatment is alkali treatment, mainly due to its easy availability, low cost and simplicity to use.^30-33 Many investigations have been carried out regarding alkali treatment with several natural fibers like coconut, sisal and flax with different weight percentages. Observations from these investigations showed that alkali (NaOH) treatment had improved resultant composite’s mechanical properties. During composite manufacturing, adding only a small amount of filler material can alter the matrix’s physical and mechanical properties in composites. From the past few years, nanoclay has attracted researchers to improve the mechanical performance of composites. Montmorillonite (MMT) clay contains amino groups which react with epoxy and improves the adhesion between matrix and fiber; this leads to a significant improvement in mechanical properties of composites.^34-40 The surface area of nanoparticles is more than their volume, which increases the area of interface between matrix and nanoparticles in the composite.⁴¹ Therefore, in the current study, FSW is surface modified with a 2 wt% NaOH solution for 1 h to increase the interfacial adhesion between fiber and matrix. Further, to increase the mechanical properties of composite 2 wt% of nanoclay is added to the matrix. Effect of alkali treatment on waste silk reinforced composite and addition of nanoclay in waste silk reinforced composite is observed.

Materials and method

Materials

In this study, FSW was used as a reinforcement supplied by Aliya silk traders (India). Average diameter and length of the silk were observed as 1.3 mm and 2 m, respectively. The density of the silk was observed to be 1.41 mg/mm³. Epoxy LY 556 and hardener HY 951 were used as the matrix material provided by CFW Enterprises (India) and utilized in a weight ratio of 10:1 as prescribed by the dealer. The properties of epoxy and hardener are shown in Tables 1 and 2, respectively. Nanoclay SC3000 Cloisite was utilized as a filler material provided by Ultra Nano Tech (India).

Table 1.

Properties of LY 556 epoxy resin.

Aspect (visual)	Clear, white, transparent liquid
Color (Gardener, ISO 4630)	<2
Epoxy index (ISO3001)	5.30–5.45 (Eq/kg)
Viscosity at 25°C	10–12 (Pa-s)
Density at 25°C	1.15–1.20 (mg/mm³)
Flash point	>200°C

Table 2.

Properties of LY 556 epoxy resin.

Aspect (visual)	Clear liquid
Color	<2
Viscosity at 25°C	0.01–0.02 (Pa-s)
Density at 25°C	1.20–1.25 (mg/mm³)
Flash point at 110°C	°C

Surface modification

The fibers were chopped into a length of 10 mm then washed thoroughly with double distilled water for two to three times to remove the dust and contamination and then dried at the room temperature for 24 h. The fibers then went through alkali treatment previous to the fabrication of composites to increase the interfacial adhesion between fiber and matrix. Chopped fibers were first treated with 2 wt% NaOH solution (alkali treatment) for 1 h at room temperature, pH value of the NaOH solution was measured as 9. Alkali-treated fibers were then washed thoroughly with the daily used detergent to remove the dirt or any other remaining impurities. A pH value of 9 was observed in the detergent solution. Double distilled water was used to wash the detergent washed fibers. The washing with double distilled was done until a pH value of 7 was achieved. Washed fibers were then dried in an oven at 50°C for 24 h. These dried fibers were then utilized for the composite preparations. A weight reduction of 33% was observed in the treated fibers after drying.

Preparations

Fabrication of composites was done by the hand-lay-up technique. The received filature silk waste was in a highly tangled condition as shown in Figure 1, so for the manufacturing convenience randomly oriented short-fiber reinforcement approach was opted for the preparation of composites. Samples were prepared according to ASTM standard D638 for tensile specimens. Epoxy LY 556 and Hardener HY 951 was mixed in a weight ratio of 10:1, and further 2 wt% untreated fibers and 2 wt% treated fibers were added separately into the matrix to fabricate UTFSWREC and TFSWREC. The prepared solution was then cascaded into the aluminium mold and allowed to cure for 24 h in normal room conditions. Treated filature silk waste reinforced epoxy nanocomposite (TFSWRENC) was fabricated with the addition of 2 wt% nanoclay to the epoxy and then heated for 30 min at 1000 rpm in a magnetic stirrer. The prepared mixture was then ultra-sonicated for 3 h. Ultra-sonication was done to reduce the formation of globular structures of nanoclay in the composite. After 3 h of sonication at 60°C, the respective ratio of hardener was added to the epoxy-nanoclay solution followed by treated fiber. The prepared mixture was then poured into the mold and allowed to cure for 24 h. All prepared samples were post cured for 4 h at 70°C in a muffle furnace to increase the composite’s cross-link density. Figure 2 shows the schematic block diagram of the fabrication process and the mold dimensions.

Figure 1.

Tangled filature silk waste.

Figure 2.

(a) Schematic of the fabrication process, (b) Mold dimensions (in mm).

Scanning electron microscopy

The samples were tested on SEM to analyze the effect of NaOH treatment on fiber and on the mechanical property of fiber-reinforced composite. Surface morphology of untreated and alkali-treated fiber, as well as the fracture surfaces of tensile specimens, were obtained by Joel Scanning Electron Microscope 6610LV (magnification: 5× to 300,000×, Resolution: 3 nm@30 kV). Before the SEM study, the samples were coated with a thin layer of gold to minimize the electron charge aggregation.

Tensile testing

Prepared dog bone samples were tested for tensile strength by Tinius Olsen digital universal testing machine (model H50KS, capacity: 50 kN) at the room temperature. Throughout the test, the crosshead speed was kept at 1 mm/min and with a load cell of 25 kN. Each tensile test was conducted according to ASTM standard D638. A set of three samples each of UTFSWREC, TFSWREC and TFSWRENC were prepared. An average value was obtained for the tested samples, specimen image of UTFSWREC composite is shown in Figure 3.

Figure 3.

Tensile specimen.

Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy was utilized to test the unique atomic mark of materials, permitting close assessment of a sample’s synthetic make-up. Every chemical compound absorbs radiation at a particular frequency and therefore, by identifying these frequencies and intensities, one can identify the given specimen’s chemical structure. FTIR was done on the fabricated specimens by Bunker platinum ATR Tensor II spectrometer. Three samples; UTFSWREC, TFSWREC and TFSWRENC were analyzed for intermolecular bonding by varying wavenumber from 500 cm⁻¹ to 4000 cm⁻¹.

Result and discussion

Scanning electron microscopy

Surface morphology of untreated and treated filature silk waste are shown in Figure 4. SEM image in Figure 4 (a) shows that the untreated fiber consists of a non-uniform cluster of fibrils adhered to each other by the sericin layer. The untreated fiber also consists of impurities, which can be dirt particles or wax (see Figure 4(b)) on the surface of the fiber. The effect of alkali treatment on the filature silk waste can be seen from Figure 4(c) and (d). These SEM images indicate that the impurities and the sericin layer were removed due to its diffusibility in NaOH, which led to the separation of fibrils. The alkali treatment ensued in the change of chemical interaction at the fiber-matrix interface, which resulted in a better adhesion between matrix and fiber. Furthermore, this separation of fibrils increased the surface area which caused an increase in the contact area between fiber and matrix. This increased contact area provided better interfacial adhesion between fiber and matrix, enhancing the strength of treated filature silk waste reinforced epoxy composite (TFSWREC). Figure 4(d) also shows that after alkali treatment the fibrils have become uniform in structure with a decrease in the fiber’s diameter. This decrease may be due to the eradication of alleged components.

Figure 4.

SEM images of filature waste silk: (a) and (b) untreated fiber, (c) and (d) 2 wt% alkali-treated fiber.

Figure 5 shows the surface fracture of UTFSWREC, TFSWREC and TFSWRENC. From SEM image of UTFSWREC (see Figure 5(a) and (b)), it can be seen that the fiber is pulled out from within the matrix during the tensile testing, resulting in the formation of voids near the sides of matrix and fiber interface. The formation of voids is due to the lack of interfacial adhesion between matrix and fiber, resulting in the decrease of mechanical properties of the composite. The fracture surface of TFSWREC (see Figure 5(c)) shows that the fibrils of the fiber are much more firmly adhered to the matrix. This firm adhesion between matrix and fiber is due to the eradication of impurities and sericin layer from the surface of fiber, which allowed the matrix to fill the gaps in between the fibrils presented in treated fiber. The resulted interfacial adhesion increased the mechanical properties of TFSWREC than UTFSWREC. Figure 5(d) and (e) show the surface fracture of TFSWRENC after the tensile test. It is observed from these images that the adhesion between matrix and the fiber is relatively strong than UTFSWREC and TFSWREC. The adhesion between matrix and fiber became much more robust than UTFSWREC and TFSWREC with the addition of a small amount of nano clay. This strong interfacial adhesion between fiber and matrix increased the Young’s modulus and ultimate tensile strength of nanocomposite.

Figure 5.

SEM image of fracture surface of composite: (a) and (b) UTFSWREC, (c) TFSWREC and (d) and (e) TFSWRENC.

Tensile testing

The comparative analysis of stress and strain of UTFSWREC, TFSWREC and TFSWRENC is shown in Figure 6. From the stress-strain graph, it can be observed that the ultimate tensile strength increases with alkali treatment and further increases with the addition of nanoclay filler. Many of the specimens have shown brittle behavior and had the maximum stress at 20.5 MPa.

Figure 6.

Stress-strain curve of tensile specimens of: UTFSWREC, TFSWREC and TFSWRENC.

Figure 6 demonstrates that the ultimate tensile strength is around 10.12 MPa for the UTFSWREC with a failure strain of around 0.68% strain percentage. The decrease in strain percent can be due to the presence of the sericin layer which restrict the interfacial bonding between matrix and fiber. Due to this weak bonding, the fiber could not provide much resistance to the applied load, which resulted in the lower tensile strength at a lower tensile strain. This diminution in ultimate tensile strength resulted in a decrease in Young’s modulus. The resulted decrease in Young’s modulus made the composite less stiff and more favorable to deformation. Neat resin showed Young’s modulus and ultimate tensile strength as 2.35 GPa and 9 MPa, respectively. Addition of only 2 wt% nanoclay increased the Young’s modulus and ultimate tensile strength to 2.9 GPa and 12 MPa, respectively. An increase of 18% in Young’s modulus and 25% in tensile strength shows that the addition of nanoclay increased the mechanical properties significantly. After alkali treatment, the failure strain percent was observed at 0.9% with a 13.22 MPa ultimate strength. A significant increase in strain percent (32%) was observed after the alkali treatment. This increase can be due to the removal of the sericin layer (see Figure 4(c)) from the fiber’s surface. Furthermore, ultimate strength of the alkali-treated composite was increased with the addition of nanoclay filler with an increase in failure strain percentage. The addition of nanoclay filler observed an increase of 12% in Young’s modulus. The increase in ultimate strength indicates that the amino group present in nanoclay reacted with the epoxy matrix which gave rise to higher interfacial adhesion between matrix and fiber. From the bar graph in Figure 7(a) and (b), it can be perceived that the tensile strength of TFSWREC and TFSWRENC are much more than UTFSWREC. The Young’s modulus and tensile strength of UTFSWREC were 1.48 GPa and 10.12 MPa, respectively. After alkali treatment Young’s modulus and tensile strength were observed to be 1.565 GPa and 13.22 MPa, justifying that the surface modification contributed to an increase of 6% and 30% in Young’s modulus and tensile strength. Further, the prepared specimens of TFSWRENC showed Young’s modulus and tensile strength as 1.73 GPa and 20.5 MPa, respectively. It signifies that accumulation of only 2 wt% of nanoclay leads to a 12% and 55% increase in Young’s modulus and tensile strength. These results indicate that after assimilating nanoclay, the interfacial adhesion between fiber and polymer matrix got intensified which further improved the composite’s mechanical properties. The tensile strength of the composite mainly depends on fiber orientation, fiber length and interfacial adhesion between fiber and matrix. The filature silk waste was randomly oriented to manufacture epoxy-based composite. According to composite matrix theory, the tensile property of composite is strongly influenced by the strength and fiber length of natural fiber which is being reinforced. The increase in fiber strength and interfacial adhesion between matrix and fiber will intend to decrease the length of short fiber for a compelling shift of stress.

Figure 7.

Comparison of mechanical properties in terms of (a) Young’s modulus, and (b) ultimate strength.

Fourier-transform infrared spectroscopy (FTIR)

FTIR analysis, a non-destructive method to analyze intermolecular bonding in organic, polymeric and inorganic materials. Spectroscopy of filature silk waste reinforced composite with the effect of alkali treatment of fiber and nanoclay as filler is demonstrated in Figure 8. The spectrum band between 1580 and 1630 cm⁻¹ is assigned to the C–H bond stretching which is of oxirane ring and N–H bonding stretching. The mentioned band represents the aromatic behavior of epoxy resin. The peaks at 824.66 cm⁻¹ represent stretching of the oxirane group and absorption peak at 1456.378 cm⁻¹ allotted to the CH₃ symmetrical curve of the epoxy ring. A broad band of –OH group was identified at a peak of 2921.323 cm⁻¹ for UTFSWREC and this peak shifted to 2959.911 cm⁻¹ for TFSWREC and further intensity of peak increased to 2961.42 cm⁻¹ for TFSWRENC. From this, it can be depicted that a decrease in hydrogen bonding furthermore increases the –OH concentration. It leads to a decrease in the hydrophilicity of filature silk waste, which intends exposed further –OH groups to react with epoxy matrix. The peak at 1230.56 cm⁻¹ of UTFSWREC represents the C=O stretching of the aromatic ring of sericin, and with the alkali treatment, this peak shifted to 1235 cm⁻¹ and 1240 cm⁻¹ for TFSWREC and TFSWRENC, respectively. From the above results, it can be observed that the sericin was removed from the fiber surface with alkali treatment.

The chemical formula of epoxy and sericin is given below:

Epoxy LY-556⁴²

Structural formula of sericin⁴³

Figure 8.

FTIR spectra analysis of (a) UTFSWREC, (b) TFSWREC, (c) TFSWRENC.

Conclusion

Mechanical properties of filature silk waste reinforced composites in terms of tensile modulus (GPa) and tensile strength (MPa) were enhanced by a 2 wt% NaOH treatment. The alkali treatment observed an increase of 6% in Young’s modulus and 30% in tensile strength. The significant improvement in the mechanical properties of composite is due to the better interfacial adhesion between matrix and fiber. Alkali treatment provides the separation of individual fibers through which the matrix can flow efficiently resulting in the enhancement of mechanical properties. Adding a small amount (2 wt%) of nanoclay elevated the mechanical properties significantly. An increase of 12% in Young’s modulus and a 55% increase in tensile strength was observed from the tensile test. It is mainly due to the presence of an amino group in nanoclay which reacts with the epoxy matrix to form a firm bonding between fiber and matrix. SEM images of FSW indicated that the sericin layer was removed from the silk surface, which resulted in the separation of fibrils. The enhancement of mechanical properties for the tensile test is due to this separation of fibrils. However future works are required to analyze the effect of fiber orientation and fiber weight percentage on the mechanical and thermal properties of FSW reinforced polymer composite. FTIR analysis further supported the data by showing the increase in wavenumber from UTFWREC to TFSWRENC. Increase in wavenumber represents that the intermolecular bonding increases with the alkali treatment and further increases with nanoclay filler. Filature silk waste composites have shown better mechanical properties than hemp and flax fiber-reinforced composites. These results show that FSW could be utilized as an alternative to traditional natural fibers. Today’s automobile industry focuses on increasing efficiency with a minimal cost, and hence, they are utilizing composites as their raw material for the manufacturing of automobile body parts. Hemp and flax fiber-reinforced composites are currently used for door panels and seat back in the automobile industry and this filature silk waste could be a viable replacement for these parts.

Footnotes

Acknowledgments

The authors sincerely acknowledge a partial financial assistance received from Department of Science and Technology, India under project number DST/TDT/AMT/2017/026.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Ravi Kant

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