
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13464-0
10.1016/j.heliyon.2024.e37433
e37433
Research Article
Effect of wood varnish coating on the water absorption and mechanical properties of jute fiber reinforced epoxy composites
Sajib Md Abu Rayan a
Islam Md Shariful msislam@me.kuet.ac.bd
b⁎
Arifuzzaman Md b
Shoily Safina Kamal a
a Department of Chemical Engineering, Khulna University of Engineering & Technology, Khulna, 9203, Bangladesh
b Department of Mechanical Engineering, Khulna University of Engineering & Technology, Khulna, 9203, Bangladesh
⁎ Corresponding author. msislam@me.kuet.ac.bd
04 9 2024
30 9 2024
04 9 2024
10 18 e3743315 7 2024
3 9 2024
3 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Natural fiber-reinforced composites are becoming popular day by day because of their low cost and renewable nature. However, a major concern of these composites is water absorption. To minimize the water absorption in natural fiber-reinforced composites, wood varnish coating was used in this paper to investigate the effect of this coating on water absorption and mechanical properties of jute fiber-reinforced composites. Four types of composites were fabricated using the hand lay-up technique, among them on Type 1 no coating was used, Type 2 was surface coated, Type 3 was fiber coated and Type 4 was both fiber and composite surface coated. Water absorption, tensile, bending, short beam shear, and impact tests were performed on the composite specimens according to their respective ASTM standards to see the effect of coating. Both fiber and fracture surface morphology were observed using Scanning Electron Microscope. It was found that the water absorption was significantly reduced for Type 2 (50.31 %) and Type 4 (56.01 %) specimens. However, the tensile strength and bending strength were reduced by 74.17 %, and 72.71 % for Type 3 and 77.07 % and 84.24 % for Type 4 composites while for Type 2, they were slightly increased (7.26 % and 1.13 %). The ILSS of Type 3 and Type 4 were reduced by 69.34 % and 58.07 % respectively while it was reduced by only 4.49 % for Type 2 composite. On the other hand, the impact energy absorption was increased by 85.32 % for Type 4 composite. The findings of this study suggest that Type 2 composite (only composite surface coating) has the overall best performance and wood varnish coating has the potential to reduce the water absorption on natural fiber-reinforced composites.

Graphical abstract

Image 1

Highlights

• Wood varnish coating has significantly reduced the water absorption (WA).

• Fiber becomes more brittle after applying wood varnish coating.

• Fiber coating significantly reduced composite mechanical properties.

• Composite surface coating reduces WA without impacting mechanical properties.

Keywords

Jute fiber
Coating
Water absorption
Wood varnish
Mechanical properties
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pmc1 Introduction

Natural fiber-reinforced composites (NFRC) are being widely used in construction [[1], [2], [3], [4]], aerospace [[5], [6], [7], [8]], naval [[9], [10], [11], [12]], nuclear [[13], [14], [15]], and automobile industries [16,17]due to their high stiffness to weight, strength, biodegradability [18,19], low cost, and renewable source. However, one of the major concerns while using NFRCs is their moisture or water absorption during their service in a moist environment or when in contact with water. Natural fibers are hydrophilic and absorb moisture/water which causes problems like thickness swelling, degradation of mechanical properties, and weak interface between the fiber and matrix which limits its application in the actual service.

To overcome this concern, researchers have tried to minimize water absorption by incorporating different fillers in the matrix [20,21], physical treatment of fibers [22,23], chemical treatment [24,25], coating on fiber [26,27], etc. Liu and Tisserat [26] investigated the effect of acrylated epoxdized soyabean oil (AESO) coating on DDGS/flax mat and found that water absorption has significantly reduced without affecting the mechanical properties. Prasad et al. [28] used nano TiO2 coating on flax fiber reinforced coating and found that the coating improved the mechanical properties and reduced the water absorption of the composite. Gapsari et al. [29] investigated the effect of AESO coating on timoho fiber and found that AESO coating improves the adhesion between the fiber and the matrix as well as improves the thermal stability of the composites. Londhe et al. [30] investigated the effect of acrylic paint coating on jute fiber composite and found that the acrylic paint coating can reduce the water and moisture absorption at different pH media. PLA coating on jute fiber composite was studied by Gupta [31] where he showed that PLA coating on jute fiber could enhance the mechanical and water absorption properties of jute fiber-reinforced polyester composites. The effect of adhesive tape coating was investigated by Hu et al. [32] and found that adhesive tape coating on jute fiber composite can significantly reduce the moisture uptake. Other coating materials studied by different researchers includes PHB [33], timoho fiber [29], starch [34], magnesium hydroxide [35], PLA [36], PVA [37] etc.

Researchers are continuously investigating different process parameters and materials to increase the hydrophobicity of different materials. Jiang et al. [38] prepared MXene-PZN through the polymerization phosphazene on the surface of MXene and found that the flame retardancy and mechanical properties of epoxy-based MXene-PZN composite have been significantly improved. Wang et al. [39] prepared the superhydrophobic surface by using laser etching and electron beam irradiation. The macroscale superlubricity was investigated by Han et al. [40]. Li et al. [41] prepared bio-inspired super hydrophobic composite films by spraying method.

Based on the above discussion it is evident that researchers have used different coating material to improve resistance to water absorption on NFRC. However, the effect of wood varnish coating on water absorption and mechanical properties of NFRC remained unexplored. Wood varnish coating is used to give the wooden furniture a glossy surface as well as protect them from scratch, and moisture or water. This is typically hydrophobic and repels the water particles when come in contact. Therefore, in this paper, the potential of wood varnish coating on jute fiber-reinforced composites (JFRC) is explored in terms of water absorption and mechanical properties. Jute fiber was chosen as the natural fiber due to its wide availability in the Indian subcontinent especially in Bangladesh [42,43]. It is also cheap in this region and mostly used natural fiber in NFRCs. Towards that, JFRCs were fabricated with and without the coating and mechanical properties of the JFRC as well as water absorption is studied. The remaining portion of the manuscript is organized as follows: section 2 describes the details of materials used along with the methodology; section 3 represents the results with appropriate discussions followed by the conclusion.

2 Materials and methodology

2.1 Materials

Epoxy resin (LY556) with hardener (HY951) was bought from the local market which was used as the matrix material. The density of the epoxy was 1.20 g/cm3. Woven jute fabric with an aerial density of 280 GSM was purchased from Sonali Jute Mills, Khulna. NaOH pallets were bought from Khulna Scientific Store, Khulna. The wood varnish used in this paper was collected from the local market and the brand name was ‘ROXY paints 777 synthetic clear varnish’. The resin-based wood varnish is hydrophobic and used in wood furniture to minimize water and moisture absorption.

2.2 Chemical treatment of jute fabric

Initially, the collected jute fabric was cleaned in distilled water to remove any dirt and impurities on the surface of the fabric. The jute fabric was then dried in sunlight for 6 h. Then the jute fabric was dipped into a previously prepared 5 % NaOH solution for 4 h and manually stirred occasionally. The concentration of NaOH solution was used at 5 % because several studies reported that [44,45] this concentration of NaOH solution is optimum for treating natural fiber. The jute fabric was then washed in tap water several times to remove any trace of the NaOH from the fabric. Finally, the treated jute fabric was dried in sunlight for 6 h followed by 2 h of drying at 120 °C in an electric oven.

2.3 Coating on jute fabric

The NaOH-treated jute fabric was dipped into wood varnish for 8 h and stirred manually from time to time to facilitate the coating of the fabric properly. The coated fabric was then dried in sunlight for 6 h followed by oven drying at 80 °C for 24 h. The coated jute fabric was used later for the fabrication of composites.

2.4 Fabrication of composites

Four different types of composites were fabricated to investigate the effect of wood varnish coating on water absorption and mechanical properties. Table 1 shows different types, among them on Type 1 no coating was applied on both fabric and composite (which also serves as the control sample) surface while for Type 2 the fabric was not coated but the composite surface was coated. In Type 3, the fabric was coated but the composite surface was not coated and finally, in Type 4, both the fabric and the composite surface were coated.Table 1 Types of composite fabricated with coating.

Table 1Types of composites	Fiber coating	Surface coating	
Type 1	No	No	
Type 2	No	Yes	
Type 3	Yes	No	
Type 4	Yes	Yes	

Four layers of treated/coated jute fabric were used in composite fabrication. The hand lay-up method was used in the fabrication process which is an easy and effective way to fabricate fiber-reinforced composite [46,47]. Two glass plates were used as a mold in this process. At first, the resin and hardener were mixed at a ratio of 10:1 as suggested by the manufacturer. The glass plates were sprayed with a silicon mold release to ease the removal of the composite. The first layer of jute fabric was placed on a mold plate and a previously prepared resin/hardener mixer was applied to the fabric with a brush. To remove any air bubbles, a grooved roller was used. The resin/hardener mixture was similarly applied to the subsequent layers of fabric. Once the application of resin on all four layers was complete, then the other mold plate was placed on the top layer. A mild load of 20 kg was applied and left the setup for curing. The composite was cured at atmospheric conditions (26 °C and 75 % relative humidity) for 24 h and after removal from the mold plate, it was post-cured for at least 72 h before testing.

For type 2, and 4 the fabricated composite panels were brushed with wood varnish coating on one surface and dried for 6 h in sunlight. Then the other surface was brushed and dried similarly. Finally, the coated composite was dried in an oven at 80 °C for 24 h. The fabricated composite was cut to the required size for different testing.

2.5 Water absorption test

The water absorption test of the specimens was conducted following the ASTM D570 standard. In this case, square specimens of dimension 50 mm × 50 mm were taken. The specimens were heated at 120 °C for 2 h in an electric oven to remove any moisture in the specimen and then cooled to room temperature before measuring the initial mass. The initial mass of all the specimens was measured using a laboratory scale with a precision of 0.001 g and then the specimens were completely submerged in distilled water. The mass of the specimens was measured every 6 h for the first three days of immersion and then measured once every day. While measuring the mass of the submerged specimens, they were taken out from the water and gently wiped with a cotton cloth to remove the surface water followed by measuring the mass. The water absorption was calculated using equation (1)(1) Waterabsorption=M−M0M0×100%

where, M is the mass of the specimens at a given time in g while M0 is the initial mass of the specimen in g.

2.6 Mechanical test

Different mechanical tests were conducted on the fabricated composites to characterize them. The testing methods for each mechanical test are described in the following subsections. At least four specimens were tested for each type of specimen to ensure the reliability of the test results.

2.6.1 Tensile test

Tensile test was conducted on all types of composite specimens in a universal testing machine (Shimadzu AGXV-300KN, Japan) following ASTM D3039 standard. Rectangular coupon-type specimens were cut from the fabricated panel to a dimension of 300 ± 1 mm × 25 ± 0.5 mm. The loading rate was maintained at 2 mm/min. The load-displacement response was recorded through the data acquisition software (Trapeziumx-V) which was later utilized to calculate various properties. The tensile test setup is shown in Fig. 1 (a). The maximum stress on the stress-strain diagram was considered as the tensile strength and the slope of the initial linear portion of the stress-strain diagram represents the modulus of elasticity. The Tensile Toughness (TT) was calculated from the area under the stress-strain diagram from equation (2) [48].(2) Tensiletoughness,TT=∫0εtσtdεt

where, σt is the tensile stress in MPa and εt is the tensile strain in mm/mm. The unit of TT was found in MPa from equation (2).Fig. 1 (a) Tensile, (b) bending, and (c) short beam shear test setup.

Fig. 1

2.6.2 Bending test

Three-point bending test was conducted on the specimens to investigate the bending properties of composites using the same universal testing machine as shown in Fig. 1 (b). For the bending test, ASTM D7264 standard was followed with a loading rate of 2 mm/min. The specimen dimension for the bending test was 100 ± 1 mm × 12.5 ± 0.5 mm and a span length to thickness ratio of 32:1 was maintained for all types of specimen. The bending strength (BS), Bending Modulus (BM), and Bending Toughness (BT) were calculated using equations (3), (4), (5) respectively while the bending stiffness was taken from the slope of the initial linear portion of the load-displacement diagram.(3) Bendingstrength,σf=3PLbt2

(4) Bendingmodulus,Ef=L3m4bt3

(5) Bendingtoughness,BT=∫0δbpdδb

Where, P is the maximum bending load in the load-displacement curve in N, p is the bending load in N, L is the support span length in mm, b is the specimen width in mm, t is the specimen thickness in mm, m is the slope of the initial linear portion of the load-displacement diagram in N/mm, δb is the displacement in mm. The units of the BS, BM, and BT was found in MPa, MPa, and mJ respectively from these equations.

2.6.3 Short beam shear test

The Short beam shear (SBS) test was conducted on the specimens to measure the Inter Laminar Shear Strength (ILSS) of the composite according to ASTM D2344 standard with a loading rate of 1 mm/min. The specimen dimension for the SBS test was 25 ± 0.5 mm × 12.5 ± 0.5 mm and the span to thickness was maintained 4:1 as recommended in ASTM D2344. Fig. 1 (c) shows the SBS test setup. The ILSS of the tested specimens were calculated using the following equation (6) [49],(6) ILSS=0.75Pmb×t

where, ILSS is the interlaminar shear strength in MPa, Pm is the maximum force on the load-displacement curve from the SBS test in N, b is the specimen width in mm and t is the specimen thickness in mm.

2.6.4 Impact test

A charpy impact test was conducted on the composites to measure the impact resistance of the fabricated composites on a Charpy impact test machine. ASTM D256 standard was followed for the impact test. The specimen dimension was 63.5 ± 0.5 mm × 12.5 ± 0.5 mm and the mass of the hammer was 20 kg.

2.7 Morphological observation

A Scanning Electron Microscope (SEM) was used to observe the NaOH-treated and wood varnish-coated fiber surface. JEOL (JCM-7000) SEM was used for morphological analysis of both the fiber surface as well as fracture surface of the specimens with Gold coating. The scanning was performed on high vacuum mode with an acceleration voltage of 5.0 kV for the fiber surface and 10.0 kV for the fracture surface.

3 Results and discussion

3.1 Water absorption

All four types of composites studied in this paper were fabricated in the same way except for the coating to investigate the effect of fiber and surface coating on water absorption. Fig. 2 (a) shows the typical variation of water absorption in all types of composites over immersion time (days). It is observed in this figure that the water absorption rapidly increases initially and then reaches saturated condition after approximately 6 days for Type 1, Type 3, and Type 4. However, Type 2 composite took a longer time (15 days) to reach saturated condition. To capture the rapid weight gain in the initial period, the weight of each sample was measured every 6 h for the first three days and then the weights were measured in 24-h intervals.Fig. 2 (a) Weight of the composite as a function of time, and (b) percentage of water absorption.

Fig. 2

The surfaces of Type 2 composite were coated with water-resistant wood varnish and the water diffusion occurs only through the edges exposed to water. Both the fiber and the matrix absorb water for Type 2 composite. On the other hand, Type 4 composite reaches saturation earlier than Type 2 composite although the surfaces of both composites are coated. However, in the Type 4 composite, the fibers were also coated causing resistance to water ingress into the fiber and the composite reached saturation condition earlier than the Type 2 composite through the water absorption by the matrix micropores only. Therefore, the time required to reach saturation for Type 2 composite is greater than Type 4 composite because of the water absorption by the fibers. The water absorption of all composite types is shown in Fig. 2 (b) after fifteen days of water immersion. It is observed from this figure that the maximum water absorption was 4.91 % for Type 1 composite. This water absorption in the composite can be attributed in three ways [50]: firstly, diffusion of water molecules into the microgap of the polymer chain, secondly, the water transport within the interface of fiber and the matrix, and thirdly, the water absorption by the fibers. For Type 1 composite, all three modes of water absorption could be present which resulted in the maximum water absorption among all composite types investigated. For Type 2, Type 3, and Type 4 composites, the water absorption is reduced by 50.31 %, 15.07 %, and 56.01 % compared to Type 1 composites. The reason for the reduction in water absorption of Type 2 composite is that the coating was applied to the surface of the composite in this type and this coating hinders the diffusion of water molecules through the surfaces of the composites to the microgap inside the polymer chain of the epoxy matrix. In the case of Type 3 composite, where the fiber coating was applied but the surface coating was not applied, the water absorption is greater than Type 2 but less than Type 1. This is because the surface coating was not applied in Type 3 and the water molecules were diffused into the microgap of the polymer chain and in the fiber matrix interface. However, the coating on the fiber prevents the water molecule from coming in contact with the fiber surface and ultimately reduces the water absorption compared to Type 1. In the case of Type 4 composite where both fiber and composite surface coating were applied, the coating prevents the water molecule from penetrating inside the composite and also prevents the fibers from coming in contact with the water molecule that's why a maximum reduction on the water absorption for Type 4 was found. Comparing the water absorption of Type 1 with Type 3 and Type 2 with Type 4, it is seen that the fiber coating has less influence on the water absorption. On the other hand, comparing the water absorption of Type 1 with Type 2 and Type 3 with Type 4, it appears that the surface coating significantly affects the water absorption.

3.2 Tensile properties

Fig. 3 (a) shows the typical stress-strain diagram of all types of composites investigated in this paper. The Tensile Strength (TS) and Tensile Modulus (TM) were calculated using the stress-strain diagram and shown in Fig. 3 (b) and Table 2. It is observed from Fig. 3 (b) and Table 2 that the TS of Type 2 composite is highest among the four types. The TS for Type 1 was found to be 17.62 MPa and for Type 3, and 4, it was reduced by 62.50 %, and 77.07 % respectively. The TS of Type 2 composite is 18.90 MPa which is 7.26 % higher compared to Type 1 composite. The slight increase in the TS for Type 2 is because the surface of the Type 2 composite was coated and this coating created a thin hard and brittle layer on the composite surface which contributed to the load-bearing ability of the composite. A significant reduction in tensile strength of Type 3 and Type 4 composites was found, in which cases, the fibers were coated. The difference in the tensile strength of Type 3 and Type 4 composites is insignificant. The reason for such a big reduction in tensile strength may be related to the damage of fibers during the coating process and the brittleness of the coating material. Once the fiber is damaged, during the tensile test, premature failure of the fibers takes place and the load is mainly carried by the matrix only. The contribution of fibers in the development of strength is eliminated resulting in a considerably lower tensile strength.Fig. 3 (a) Stress-strain diagram, (b) tensile strength/modulus, (c) elongation at break, (d) tensile toughness of different types of specimens.

Fig. 3

Table 2 Comparison of tensile and bending properties of different composites.

Table 2Sample Type	TS (MPa)	TM (GPa)	EoB (%)	TT (mJ)	BS (MPa)	BM (GPa)	B.Stiff. (%)	BT (mJ)	
Type 1	17.62 ± 1.34	5.28 ± 1.22	0.46 ± 0.07	180.77 ± 28.73	50.64 ± 3.96	3.91 ± 0.05	6.62 ± 0.09	155.99 ± 24.42	
Type 2	18.90 ± 1.72	5.50 ± 0.95	0.40 ± 0.08	127.58 ± 7.72	51.21 ± 2.23	3.95 ± 0.04	6.64 ± 0.83	122.15 ± 12.40	
Type 3	4.55 ± 1.06	1.98 ± 0.40	0.26 ± 0.07	32.59 ± 15.68	13.82 ± 2.58	1.96 ± 0.06	3.38 ± 0.59	29.46 ± 6.26	
Type 4	4.04 ± 1.41	1.87 ± 0.33	0.24 ± 0.06	27.63 ± 15.71	7.98 ± 0.97	1.97 ± 0.04	3.40 ± 0.24	9.65 ± 0.69	

The modulus of elasticity for Type 1 composite was found 5.28 GPa and it is increased by 4.17 % for Type 2 composite which is coated on the surface. However, for Type 3 and 4, the TM is decreased by 62.50 % and 64.58 % respectively compared to the Type 1 composite. The reason for a slight increase in TM for Type 2 is that the composite surface had a thin layer of coating which slightly contributed to the load-bearing capacity of the composite. On the other hand, as the fibers were damaged and became more brittle in the case of Type 3 and 4 due to coating, the TM has been reduced significantly compared to Type 1 and Type 2. No significant variation on the TM of Type 3 and Type 4 is noticed.

Fig. 3 (c) shows the elongation at break for all types of composites. The elongation at break for Type 1 was found to be 0.46 % while it was reduced by 13.04 %, 43.48 %, and 47.83 % respectively for Type 2, Type 3, and Type 4. The significant reduction in the elongation at break also indicates the damage and the brittleness of the fibers after coating.

The tensile toughness is shown in Fig. 3 (d) and it is observed from this figure that the toughness was significantly decreased for Type 2, Type 3, and Type 4 composites compared to Type 1. The toughness depends on the load carrying capacity of the constituent materials in the composites and the load transfer between the fibers. Since a thin brittle coating on the surfaces of Type 2 composite was introduced the overall brittleness of the composite is increased causing a lower elongation at break as seen in Fig. 3 (c). Although the strength and modulus of Type 2 composite is comparable with Type 1, because of the lower elongation at break, it showed lower toughness. On the other hand, for Type 3 and Type 4 composites, the premature fiber failure caused lower strength, modulus and elongation at break which results in a significantly lower toughness.

3.3 Bending properties

The typical load-displacement diagrams from the three-point bending test are shown in Fig. 4 (a). Fig. 4 (b) and Table 2 show the Bending Strength (BS) and Bending Modulus (BM) of all the composite. It was found that the BS and BM of Type 1 composite (both fiber and composite surface were uncoated) were 50.64 MPa and 3.91 GPa respectively. The BS of Type 3 and Type 4 composites is reduced by 72.71 % and 84.24 % respectively while the BS for Type 1 was increased slightly compared to Type 1 composite. The BM of Type 3, and Type 4 were reduced by 49.87 % and 49.62 % respectively but the BM of Type 2 composite was slightly increased. So, it can be said that the surface coating did not influence the bending strength and modulus significantly for Type 2 composite. However, the surface coating of the Type 4 composite caused a degradation of the bending strength although the bending modulus is not influenced (see Fig. 5 (b)). The reason behind the significant reduction in the BS and BM for Types 3, and Type 4 is that after the coating on the fibers, they were damaged causing degradation in the load-bearing capacity and became brittle, and brittle fracture was observed which is discussed in a later section (section 3.6 morphological analysis). A similar observation i.e. a decrease in the BS was also observed in the case of jute fiber coated with PHB [33].Fig. 4 (a) Stress-strain diagram, (b) bending strength/modulus, (c) stiffness, (d) bending toughness of different types of specimens.

Fig. 4

Fig. 5 ILSS of different composites.

Fig. 5

The stiffness is a measure of the resistance of a structure against deformation. The stiffness of all types of composites was calculated from the slope of the load-displacement curve and shown in Fig. 4 (c) and Table 2. It was found that the stiffness of type 2 composite remains unchanged (0.30 % increase) while for types 3, and 4 it was reduced by 49.87 % and 49.62 % respectively compared to type 1 composite (6.62 N/mm).

The bending toughness of the composites is plotted in Fig. 4 (d) and given in Table 2. The toughness of Type 2, Type 3, and Type 4 composites was reduced by 21.69 %, 81.11 %, and 93.79 % respectively compared to Type 1 (155.99 mJ). The reason for the reduction in the toughness in Type 2 composite is the brittleness of the surface coating causing a lower strain at break. The reason for lower toughness of Type 3 and Type 4 composite is the brittleness of coating material and the damage in the fibers causing lower strength, modulus and strain at break as discussed in section 3.1. The failure mechanism is further analyzed in section 3.6 based on the fracture surface morphology.

3.4 Interlaminar shear strength (ILSS)

The interlaminar shear strength (ILSS) of different types of composites is shown in Fig. 5 and also listed in Table 3. It is observed that the ILSS of Type 1 composite is 55.26 MPa which is the highest among the four types. For Type 2 (the composite surface was coated) a marginal reduction of 4.49 % is observed compared to Type 1. However, a significant reduction in the ILSS is observed where the fibers were coated (Type 3 and Type 4). For Type 3 specimens, the ILSS is reduced by 69.34 % while it is 58.07 % for Type 4 specimens. The reduction in the ILSS could be due to the poor adhesion between different layers [51] of the composites after the coating as well as the premature failure of the fibers due to shear stress. The lower ILSS of composites due to fiber coating is not uncommon as similar results were also found in the case of polyamide coating of Kevlar fiber in the epoxy matrix [52], and polyamide coating of glass fabric in polyester resin [53].Table 3 ILSS and impact energy absorbed for different composites.

Table 3Specimen Type	ILSS (MPa)	Impact Energy Absorption (J)	
Type 1	55.26 ± 9.37	2.125 ± 0.217	
Type 2	52.78 ± 11.38	3.125 ± 0.217	
Type 3	16.94 ± 0.791	2.375 ± 0.415	
Type 4	23.17 ± 10.18	3.938 ± 0.108	

3.5 Impact test result

Fig. 6 shows the impact energy absorbed during the fracture of different composites which is also listed in Table 3. It is observed from this figure that the maximum impact energy absorbed during failure is for the Type 4 specimen while the minimum is for the Type 1 specimen. The impact energy absorbed for the Type 1 specimen is 2.125 J and it is increased by 47.05 %, 11.16 %, and 85.32 % for Type 2, Type 3, and Type 4 composites respectively compared to Type 1. It is noticed that only the surface coating (Type 2) can significantly increase the impact energy while only fiber coating (Type 3) also improves the impact resistance but comparatively lower proportions compared to the surface coating. The reason for the improvement in impact energy for Type 2 composite is that the coating applied on the composite surface creates a thin hard layer that requires additional energy to fracture. Type 3 composite where only the fiber was coated also shows some improvement in the impact energy despite significantly lower bending toughness. Note that the nature of loading is different for quasi-static bending and impact. In the case of quasi-static loading gradual breakage of fibers takes place while for impact loading the fracture takes place within a short time. Therefore, for Type 3 composite, the coating material on the surfaces of fibers contributed to the energy absorption causing a higher energy absorption during fracture compared to Type 1 composite. The reason for the highest improvement in Type 4 is that in Type 4 specimens, both the fiber and the composite surface were coated which combinedly contributed to the improvement of impact resistance.Fig. 6 Impact energy absorption.

Fig. 6

Based on the findings of the coated composites' mechanical properties, the coating on the fiber significantly decreases the TS, TM, BS, BM, ILSS, etc. However, if the composite's surface is coated with wood varnish, the water absorption is significantly reduced without affecting the mechanical properties of the composite. So it will be wise to coat the composite surface instead of the fibers for the best result if wood varnish is used as the coating material.

3.6 Morphological analysis

Fig. 7 (a) and (b) show the SEM images of the uncoated fiber while Fig. 7 (c) and (d) show the SEM images of coated fiber used as reinforcement in the composite. It is to be remembered that all the jute fibers underwent a 5 % NaOH treatment for 24 h and were later used in the composite with or without applying coating on them. Fig. 7 (c), (d) shows the presence of the coating material on the fiber surface. It should be noted that the coating thickness was not possible to control properly as the jute fabric was dipped into the coating for 24 h and then dried. The coating thickness could be controlled by varying the concentration of the coating material. However, in this work 100 % wood varnish coating was used and not diluted or the concentration was not varied.Fig. 7 SEM images of jute fibers (a), (b) uncoated fiber, and (c), (d) coated fiber.

Fig. 7

Fig. 8 shows the SEM images of the fracture surface of coated and uncoated fiber-reinforced composite under bending load. For Type 1 and Type 2 composites where the fibers were not coated, the presences of long fibers on the fracture surface (red circles) as seen in Fig. 8 (a) and (b) indicates that the dominating failure mode was fiber pullout. Matrix cracking (yellow circle) and fiber breakage (green arrows) were also observed. The phenomena of fiber pullout for the uncoated composite consumes more energy due to the friction while pullout of the fiber from the matrix occurs. That is why these composites (Type 1 and Type 2) have higher toughness compared to the coated fiber composites. This observation matches with the toughness shown in Fig. 4 (d). However, for the fiber-coated composites (Type 3 and Type 4) as shown in Fig. 8(c) and (d), the dominating failure mode was fiber breakage (green arrows) although some matrix cracking and fiber pullout were also observed. However, the pullout length of fibers is significantly smaller compared to uncoated fiber composites supporting smaller elongation/strain at break as shown in Fig. 4, Fig. 5. Moreover, the coated fiber-reinforced composite fails rapidly under bending load and consumes very less energy compared to the uncoated fiber-reinforced composite and eventually has less toughness as evident in Fig. 4 (d).Fig. 8 SEM images of fracture surfaces (a), (b) uncoated fiber-reinforced composite, and (c), (d) coated fiber-reinforced composite.

Fig. 8

4 Conclusions

In this paper, the different types of JFRCs were fabricated with or without fiber/composite surface coating to reduce water absorption in JFRCs. Along with the water absorption, mechanical tests such as tensile, bending, SBS, and impact tests were conducted to investigate the influence of wood varnish coating on these properties. The major outcome of this investigation can be summarized as follows-• The application of wood varnish as a coating on fiber or composite surfaces can significantly reduce the water absorption in the JFRC. The percentage of water absorption was reduced by 50.31 % for Type 2 composite (only composite surface coated) and 56.01 % for Type 4 composite (both fiber and composite surface coated).

• The tensile strength and modulus slightly increased for Type 2 while the elongation at break and toughness decreased compared to Type 1 as the coating induced fiber damage and brittleness. For Type 3 and Type 4, all the tensile properties were decreased significantly.

• No significant change was found in the bending strength and modulus of Type 2 composite although the toughness was decreased by 21.69 % compared to Type 1 composite. However, all the bending properties of Type 3 and Type 4 composites were reduced significantly.

• The ILSS of Type 2 composite slightly decreased (4.49 %), while for Type 3, and Type 4 it was reduced by 69.34 % and 58.07 % respectively compared to Type 1.

• A significant increase in the impact energy absorption was found for Type 2 and Type 4 composites (47.05 % and 85.32 % respectively) compared to Type 1. This indicates that the composite surface coating can significantly increase the impact energy.

Data availability

Data will be made available on request.

CRediT authorship contribution statement

Md Abu Rayan Sajib: Writing – original draft, Visualization, Methodology, Data curation. Md Shariful Islam: Writing – review & editing, Supervision, Investigation, Formal analysis, Conceptualization. Md Arifuzzaman: Writing – review & editing, Validation, Investigation, Formal analysis. Safina Kamal Shoily: Writing – original draft, Visualization, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors acknowledge the partial financial support from Khulna University of Engineering & Technology, Bangladesh for this work.
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