Introduction

At present, the primary role for the organizations and research community is to substitute environmentally friendly materials in applications without compromising the strength requirements. Hence, material scientists are working towards maximizing their full potential to produce new biomaterials as a viable alternative to synthetic materials. Natural fibers such as banana, coconut sheath, sisal, etc. have been extensively used as reinforcement in the composite materials [1]. In addition to the usage of natural fibers, biopolymers are also used by many researchers for developing biodegradable polymer-based composites. For example, polybutylene succinate (PBS), polylactic acid (PLA), polyhydroxyalkanoates (PHAs), etc. are used for many potential applications [2]. Hence to preserve the green environment, in this present work, bio-based epoxy was selected as a polymer matrix and reinforced with hemp and sisal fiber mats. 56% of the molecular structure of chosen bio-epoxy comes from the plant-based source; hence they are classified into the bio-based resin.

Natural fibers are beneficial in terms of low cost, easy handling, biodegradability, abundant availability, high strength to weight ratio [3, 4]. Nevertheless, the designers are hesitant to use the natural fibers in main load-bearing outdoor applications owing to their inferior performance under the influence of moisture and temperature prevailing in the atmosphere [5]. The strength and stiffness of materials’ could degrade by having exposure to moisture, UV radiation, heat, and a combination of these parameters [6].

When natural fiber-reinforced composites (NFRC) are exposed to the UV radiation, the surface chemistry of the composites changes due to photodegradation or photocatalysis [7, 8]. It is essential to identify the (i) nature (ii) composition and (iii) structure of the fibers to understand the degradation of the NFRC. It is also to be noted that the degradation of NFRC’s in an outdoor application has a strong influence on many factors like (i) microorganism activities (ii) temperature (iii) ultraviolet radiation and (iv) moisture [9,10,11,12]. If the composites are intended for prolonged usage, these are to be tested under realistic service environments. For example, for long-term testing, the duration would be a couple of months or hundreds to thousands of years. Testing the composites for prolonged periods is usually not feasible, which would delay the development of products. Hence, accelerated weathering or aging methods could be the right choice to analyze the composites [5, 13]. During the accelerated aging, the test specimens would be exposed in UV radiation with controlled moisture and temperature. This method may be the fastest technique and more convenient method to imitate the natural weathering processes.

Krittirash et al. [14] performed the accelerated weathering test on sisal/kenaf/bio-based epoxy matrix composites. It was reported that the tensile strength, elongation at break, and impact strength of the hybrid composites were found to be reduced. Nevertheless, the tensile modulus of all the hybrid composites was found to be increased. Another study also illustrated that Young’s modulus of lignocellulosic fiber/polylactic acid composites was improved after the accelerated weathering [15]. In another work, coir fiber and pineapple leaf fiber reinforced in polylactic acid composites having higher biodegradability properties [2]. According to another study, mechanical properties of hemp fiber/polylactic acid composites were reduced after 250–1000 h of accelerated weathering. However, the impact strength was found to be increased [6]. Researchers studied the tensile and flexural properties of hemp fiber/polybenzoxazin composites by varying the accelerated weathering parameters such as temperature and relative humidity. All composites showed a decreasing trend by increasing the parameters [16].

Similarly, in other studies, researchers found that the flexural strength and modulus of wood/high-density polyethylene composites significantly reduced by exposure of moisture [17]. Abu-Sharkh et al. [18] revealed that insignificant changes were observed in tensile properties of date palm fiber/polypropylene composites by the weathering test as compared to the unweathered composites. It was due to the degradation and formation of polypropylene carbonyl groups, which were more compliant with the surface of cellulose fiber for improving interfacial adhesion. Chee et al. [19] analyzed the thermal stability of bamboo/kenaf/epoxy matrix composites by comparing before and after accelerated weathering results. They indicated that the hybrid composites with a ratio of 70:30 of bamboo: kenaf fibers had the best thermal stability when compared to the remaining combinations; also, these hybrids showed a balance resistance to the environmental effects. Yorseng et al. [14] found that a similar temperature of decomposition was observed when comparing before and after weathered composites with kenaf/sisal/bio-based epoxy resin hybrid composites.

In this work, an attempt was made to preserve the green environment; the bio-based epoxy resin was selected as the matrix. The bio-based epoxy was selected based on its interesting characteristics, which would permit extensive use as a resin for sisal and hemp fibers in many applications such as hot or cold press, bonding, casting, low to high-performance composites, etc. Among the natural fibers, the sisal and hemp fiber mats were used as reinforcement. These fibers could be used in many structural applications owing to non-abrasive, low density, and eco-friendly nature.

To date, works on the accelerated weathering effects of natural fiber-reinforced bio-based epoxy resins are limited. Thus, in this study, pure and hybrid bio-based epoxy composites reinforced with the sisal and hemp fiber mats were subjected to the accelerated weathering test. Their influence on the mechanical, thermal, and microstructural changes of the composites was assessed.

Materials and Methods

Materials

Hemp fiber mat and sisal fiber mat were purchased from Nirmala Industries, Telangana, India. The physical and mechanical properties of the hemp and sisal fibers are given in Table 1. The SR greenpoxy 56 and SD surf clear used as matrix and hardener, respectively. Both the bio-based epoxy resin and the hardener were procured from Sicomin company, France. The essential properties of the matrix and hardener are given in Table 2.

Table 1 Physical and mechanical properties of sisal fiber and hemp fiber [20,21,22,23]
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Table 2 Properties of bio-based epoxy resin and hardener
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Composite Preparation

The sisal and hemp fibers were reinforced with the bio-based epoxy resin under laboratory conditions. The fiber mats were produced into composite laminates by hand layup technique followed by the hot press machine identical to the first phase of published work [24].

Accelerated Weathering

The ultraviolet irradiation of bio-based epoxy resin, SSSS, HHHH, HSSH, and HSHS was performed by using Q-Sun Xenon test chambers (Q-Lab, USA). The weathering test was performed according to the ASTM G155-13 standard (Cycle-1) [25]. The specimens were placed inside the test chamber subjected to UV radiation (for 1.42 h) followed by UV radiation with water spray (for 0.18 h). The air and the black panel temperatures were set at 48 °C and 63 °C, respectively. The test chambers are fitted with the solar eye irradiance control system. The choice of control point selected as 0.35 Wm−2 @340 nm. The test was continuously performed without any interruption for a total of 2222 h ~ (1 year) (outdoor). The specimens after the test were further characterized to examine their properties as an effect of accelerated weathering.

Characterization

Tensile Testing

The ASTM D3039M-17 standard [26] was used to determine the tensile properties of the bio-based epoxy resin composites. An Instron 3382 universal testing machine with a load cell of 1000 kg was used. Initially, the composite samples were cut for the dimension of 120 × 20 × 3 mm, and then the samples were loaded between the two tensile holds, whereas the gauge length was fixed as 50 mm.

Further, these samples were subjected to a crosshead speed of 5 mm/min. The force (N) vs. elongation (mm) was measured using stretching the composite samples. The tensile test was conducted for five different samples, and the tensile properties were calculated using the average value of the measurements.

Flexural Testing

The ASTM D790-17 standard [27] was used to determine the flexural properties of the sisal/hemp fiber-reinforced composites with dimensions of 120 × 20 × 3 mm. At first, the test specimens were held on the two supports in a flat-wise position and was loaded by loading nose. The loading nose was located in the center between the two supports. The span to depth ratio of the specimen was 16:1. All the composite samples were deflected until the outside surface was ruptured. The flexural test was conducted for five different specimens, and the average value of the measurements was reported.

Impact Testing

The ASTMD 256–10 standard [28] was used to investigate the impact test of bio-based epoxy composites. Zwick/Roell HIT 5.5 impact tester was employed in the analysis of impact testing of composites with dimensions of 63 × 13 × 3 mm, without a notch. At first, the composite sample was held vertically like a cantilever beam, and the sample was broken by using a single swing of the pendulum. The impact results were reported in terms of energy observed per cross-sectional area of the test specimen. The impact test was conducted for five different specimens, and the average value of the measurements was reported.

Thermogravimetric Analysis (TGA)

The thermal stability of sisal/hemp/bio-based epoxy resin hybrid composites was performed by using a thermogravimetric analyzer with a high-temperature furnace (METTLER TOLEDO). The temperature range was fixed from 25 to 600 °C, and a dynamic N2 environment was used with a flow rate of 60 mL/min. At first, the composite sample of 5–15 mg was placed successfully in alumina crucible with lids (70µL). Then the temperature was raised to bring intense heating. Finally, weight loss (%) vs. temperature (°C) was plotted from the TGA curve.

Scanning Electron Microscopy (SEM)

Scanning electron microscopy (FEI, Quanta 450, Austria) was used to understand the failure mechanisms of sisal/hemp fiber-reinforced bio-based epoxy composites. Before visualizing the images, the specimens were prepared by gold coating.

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectra of sisal/hemp fiber-reinforced bio-based epoxy composites before and after weathering was obtained by using IRTracer-100, Shimadzu, in the wavenumber ranging from 500 to 4000 cm−1.

Results and Discussion

Tensile Testing

The tensile properties of the sisal/hemp fiber reinforced composites before and after accelerated weathering are presented in Fig. 1a–c.

Fig. 1
figure 1

Effects of accelerated weathering on a tensile strength; b tensile modulus and c elongation at break of composites. BW before weathering, AW after weathering, BW previously published values [24]

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From Fig. 1a, it could be observed that the weathered hemp (HHHH) and sisal (SSSS) composites showed higher tensile strength. However, the differences in the tensile strength was marginal before and after weathering. This is because both the fibers had equivalent tensile strength (sisal fiber had a tensile strength of 530–640 MPa, while the hemp fiber had a tensile strength of 690 MPa [21, 22]) and an equal number of fiber layers in the bioepoxy laminate. It is interesting to note that tensile strength of the unweathered hybrid composites (HSSH and HSHS), which were in a similar range to that of the pure composites, displayed inferior strength after weathering. The percentage of decrease in tensile strength for the weathered composites compared to unweathered samples of HSSH and HSHS are 7% and 14%, respectively. The physical difference between the sisal and hemp fibers corresponding to the fiber aspect ratio and their compatibility within the matrix could have allowed the photodegradation of the epoxy matrix [29, 30]. The photodegradation may cause the generation of cracks within the composite and this crack may act as a nucleating site for new cracks when the load is applied [31]. Both sisal and hemp fibers are hydrophilic, and the differences in fiber aspect ratio between them cause uneven thickness swelling within the matrix. This may result in stress built up at the fiber/matrix interface resulting in loss of adhesion between the fiber and matrix, and hence a drop in tensile strength is expected [32].

Figure 1b, c illustrates the Young’s modulus and elongation at break of the composites before weathering and after weathering, respectively. Interestingly, the weathered composites possessed higher tensile modulus and elongation at break than the unweathered composites. The UV irradiation and water spraying lead to the chain crosslinking and embrittlement of the bio-based epoxy resin, which is believed to have caused the increase in Young’s modulus and elongation at break [33]. Additionally, the elongation at break values of the composites was higher than that of the bio-based epoxy resin. This is because the accelerated weathering may cause embrittlement of the bio-based epoxy resins due to chain crosslinking caused by the photodegradation [33]. Trapped water molecules inside the composite specimens also act as a plasticizer leading to higher elongation at break [32].

SEM micrographs of the tensile fractured surfaces of the weathered composite specimens are shown in Fig. 2a–d, respectively. No significant difference in failure behavior was observed due to the accelerated weathering of pure and hybrid composites. Typical tensile failures such as matrix cracking and fiber pull-outs can be noticed. It is believed that the matrix cracking is the first mode of generating damages in the polymer matrix composites followed by fiber pull-out. Hence, the occurrence of the tensile failure is ascribed to the matrix cracking, fiber/matrix debonding, and fiber pull-outs.

Fig. 2
figure 2

SEM of tensile fractured surfaces of composites: a HHHH; b SSSS; c HSSH; d HSHS

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Flexural Testing

Flexural stress–strain curves, flexural strength, and flexural modulus of the neat resin, pure and hybrid composites are shown in Fig. 3a–c.

Fig. 3
figure 3

a Flexural stress–strain curve and b flexural strength and, c flexural modulus of composites. BW before weathering, AW after weathering, BW previously published values [34]

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Weathering was found to be detrimental to the flexural properties of the composite. Both the pure and hybrid composites were found to have substantially lower flexural strength and modulus than the unweathered composites. The degradation in the flexural strength and modulus was severe in the hybrid composite specimens when compared to the unweathered composites. The main factors that could affect the flexural properties of weathered composites are (i) the swelling and deswelling effect (ii) the plasticization (iii) the photochemical degradation and (iv) temperature [35, 36]. The application of accelerated weathering may result in the penetration of UV light into the epoxy matrix, which may cause the degradation of the epoxy matrix and the debonding of the fibers. Also, water absorption of the epoxy matrix during accelerated aging may result in swelling and plasticization [35]. In the case of composites, there is a tendency of moisture absorption by the composites during the accelerated weathering test due to the hydrophilic groups present in the fibers. The hydrophilic groups such as OH and COOH present in the fibers can form hydrogen bonding with moisture present in the environmental chamber resulting in the diffusion of water molecules into the composites. The moisture may also diffuse into the composites through the porous structure of the fiber and also through the fiber polymer interface [37]. Note that the absorbed moisture may desorb during the condensation cycle in the weathering test. Thus, the composites may swell and deswell multiple times during the weathering test; this may cause load yielding, especially at the fiber-matrix interface, and even tear the fibers leading to fracture and hence the flexural strength decreases [33].

Impact Strength

The impact strength of composite specimens before and after the accelerated weathering test is presented in Fig. 4.

Fig. 4
figure 4

Effects of accelerated weathering on the impact strength of composites. BW before weathering, AW after weathering, BW previously published values [34]

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Pure sisal, hemp, and hybrid composites displayed superior impact strength before and after weathering compared to the neat resin. The observed results were in good agreement with Islam et al. [32]. In a composite material, the energy dissipated from the impact on the specimen is carried by the fibers embedded within the matrix [38]. Thus, fibers in the composites helped to outperform neat resin in case of impact before and after weathering.

Recent studies have indicated that weak interfacial bonding between the unidentical fiber layers in the hybrid composite with less fiber compatibility absorbed more impact energy and possessed better impact resistance [39,40,41,42]. A similar trend was observed for the hybrid composites proving the incompatibility between the sisal and hemp fiber.

Figure 5a–d shows the SEM micrographs of impact fractured surfaces of HHHH, SSSS, HSSH, and HSHS composites exposed to accelerated weathering. Fiber pull-outs and matrix cracking could be seen in the microstructure.

Fig. 5
figure 5

SEM of impact fractured surfaces of composites: a HHHH; b SSSS; c HSSH and d HSHS

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Visual Appearance

A major problem in using polymeric matrix composites is the color fading or color change when exposed to outdoor conditions over a while [19, 33]. The photographs of composite specimens before and after are illustrated in Fig. 6a–f.

Fig. 6
figure 6

Various stages of visual changes observed for sisal/hemp fiber reinforced hybrid composites: a before weathering; b–e intermediate hours during the accelerated weathering composites; f post the accelerated weathering

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During the accelerated weathering test, the composites shown in Fig. 6 were subjected to different temperatures such as 63 °C (black panel: uninsulated) and 48 °C (air temperature) under the humidity of 30% simultaneously. In this study, initially, the unweathered composites were bright, and as it was exposed to accelerated weathering, a color change could be noticed for pure and hybrid fiber-reinforced composites. The absorbed moisture from the water spray and photodegradation of the matrix was due to the UV irradiation produced these visual changes along with the physical and chemical changes in the fiber and matrix. Nevertheless, the composite specimens subjected to accelerated weathering did not lose their integrity as visible from the smooth surface (Fig. 6f). No signs of erosion or material removal were observed. These findings are in agreement with the observation reported by Dubey and Pang in their study [43].

Thermogravimetric Analysis

Degradation effects of accelerated weathering on the pure and hybrid composites were evaluated from the thermogram and derivative thermogram obtained from TGA presented in Fig. 7a, b.

Fig. 7
figure 7

TGA plots a Thermogram and b derivative thermogram curves of composites. AW after weathering

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Weathered composites underwent a 2-stage decomposition process similar to the unweathered composites. Initial weight loss happened approximately at 100 °C owing to the removal of moisture. The initial weight loss is followed by complex secondary reactions and the formation of volatile products [44, 45]. Major weight loss occurred in the thermal degradation temperature between 250 and 305 °C with the onset degradation temperature around 295 °C. It was ascribed to the degradation of bio-based epoxy backbone, hemicellulose, and cellulose. Further, an increase in temperature results in degradation at 340 °C due to the degradation of HCN bonds in epoxy resin [46]. The degradation at 400 °C is due to the decomposition of lignin and carbonaceous residue [31, 47,48,49], as shown in Fig. 7. The thermal degradation temperature of the neat resin and weathered composite specimens were tabulated in Table 3.

Table 3 Thermal degradation temperature of the weathered composites from TGA
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Comparing the weathered composite data in Table 3 with that of the unweathered data [34], it was clear that differences in the onset, inflection, and endset thermal degradation temperatures were negligible. This is also shown in the thermogram (Fig. 8a) and thermal degradation temperatures (Fig. 8b) of the unweathered and weathered HSHS composite. Minimal change in degradation temperature of the hybrid composite specimens exposed to accelerating weathering was also reported by Krittirash et al. [14] for kenaf/sisal/bio-based epoxy composites, Techawinyutham et al. [50] for polylactic acid based composites and Harikrishnan et al. [31] for the liquid blends/bio-based epoxy composites.

Fig. 8
figure 8

a Thermogram of unweathered and weathered HSHS hybrid compositAWe. b comparison of degradation temperature of unweathered and weathered HSHS hybrid composite. BW before weathering, after weathering, BW previously published values [34]

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The char residue obtained from the TGA is an indication of thermal stability in a composite. In Fig. 9, it could be noticed that the weathered composites had slightly lower residue than the unweathered composites. This signifies the influence of UV irradiation and water spray on the thermal stability of the pure sisal, hemp, and hybrid composites.

Fig. 9
figure 9

Comparison of the residue of sisal/hemp fiber reinforced hybrid composites. BW before weathering, AW after weathering

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Fourier Transform Infrared Spectroscopy

The FTIR spectra of the bio-based epoxy matrix and sisal/hemp fiber-reinforced composites with different layering sequences of both the unweathered and weathered composites are shown in Fig. 10a, b. The observed characteristics peaks corresponding to the constituents of the bio-based epoxy, and unweathered sisal/hemp hybrid composite is tabulated in Table 4.

Fig. 10
figure 10

FTIR spectra a Before and after weathering of neat resin, pure sisal, and hemp composite and b Before and after weathering of sisal and hemp-based hybrid composite. BW before weathering, AW after weathering

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Table 4 Characteristic peaks observed from the FTIR spectra of bio-based epoxy and unweathered sisal/hemp hybrid composite [51,52,53,54,55,56,57,58,59,60]
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The influence of water spray and UV irradiation on the sisal, hemp, and hybrid composite is visible from their FTIR spectra shown in Fig. 10a, b. Weathered composites had similar characteristic peaks as unweathered composites. However, the intensity of the reported characteristic peaks declined for weathered composite specimens and the neat resin. All the characteristic peaks of the bio-based epoxy matrix and composites exposed to accelerated weathering were found to have lower intensity. In particular, the characteristic peaks corresponding to the hydroxyl group tend to get broader after weathering. Both the natural fibers and bio-based epoxy resin absorb UV irradiation and causes photodegradation. The photo-oxidation and hydrolysis induce the changes in the physical, chemical, and mechanical properties of the polymer and natural fibers subjected to accelerated weathering [9].

Conclusion

The effect of accelerated weathering with UV irradiation and water spray on the pure sisal (SSSS), hemp (HHHH), and hybrid composites with two different layering sequences (HSSH and HSHS) were evaluated. The severity of accelerating weathering was pronounced in the hybrid composite. Following were the observations from the study:

  • The tensile strength of the weathered hybrid composites was found to decrease by 7% and 14%, respectively. An increase in the Young’s modulus and elongation at break for the weathered composite specimens reflects the matrix plasticization behavior.

  • The weathering was detrimental to the exposed composite specimens under bending. The flexural strength and modulus were considerably lower for the weathered specimens.

  • Weathered composites specimens possessed better impact resistance than the unweathered specimens. Incompatibility between the sisal and hemp fibers and interfacial weakening of the weathered composites was evident from their improved energy absorption capacity.

  • Physical and chemical changes to the composites were apparent from the visual appearance and FTIR spectra. Weathered specimens appeared to be faded after exposed to 2222 h of UV irradiation and water spray. Reduction in intensity, broadening and coalesce of the characteristic peaks in FTIR spectra indicates the microstructural changes to the composite specimens exposed to weathering.

Hybrid bio-based epoxy composites with core/outer layer arrangement (HSSH) and intercalated arrangement (HSHS) though affected by accelerated weathering displayed superior energy absorption capability. Thus, the hybrid composites could be used in outdoor applications requiring impact resistance.