Abstract
Epoxy-bamboo long natural fibres composites were prepared by hand lay-up method to study the influence of fibre's treatment on their characteristics. SEM showed increased surface roughness due to the removal of amorphous compounds from the treated fibre as confirmed by the disappearance of certain peaks in FTIR spectroscopy. Resultantly, ∼52% increase in crystallinity was observed by XRD analysis. TGA results also showed the effectiveness of the treatment as mass loss for treated fibres was ∼15% lesser than the untreated fibres. Moreover, thermogravimetric analysis of composites showed highest mass loss in untreated fibres composites and lowest in neat epoxy. The DSC-DTA curves indicated an early start of initiation temperature in treated fibre composite. Tension tests showed ∼12% and ∼16% increase in tensile strength and modulus respectively for treated fibres composite due to increased interfacial strength caused by fibre treatment. Treatment also resulted in decreased impact energy and water absorption level.
Keywords
Introduction
Natural fibres are in high demand in the field of composites because of their abundant availability, biodegradability and cost effectiveness. Major attraction for using natural fibres is to decrease the harmful effects caused to the environment by the synthetic fibres [1-3]. Along with less harmful effects, natural fibres show good enhancement in strength and toughness when incorporated inside the polymer matrices which is why they are replacing synthetic fibres for medium-strength applications. Different natural fibres exist in nature with properties different from each other, and from all of the naturally existing green fibres, bamboo fibres have very good strength [4]. Bamboo fibres have also shown low density, high young's modulus and its growing cycle is also continuous. These advantages of bamboo fibres over other natural fibres provide the solid reason for using bamboo fibres in medium strength applications. Natural fibres can be extracted by different methods which can be chemical or mechanical. In chemical processes, alkali immersion method is the most common whereas steam explosion method is the most common mechanical method for fibres extraction [5].
Apart from having numerous advantageous characters, bamboo fibres also have demerits of absorbing water because of its hydrophilic nature which can significantly change the physical properties of the composite with hydrophobic abilities [6]. Additionally, if the bamboo fibres are used in as extracted form and without any surface modification, it produces loose interfacial bonding with the matrix materials because of hindrance caused by dust, impurities and other unfavourable functional species present on the fibres surface [7-9]. Overall, undesirable characteristics of the natural fibres not only cause decrement in mechanical properties but also affect the dimensional stability of the composite [10-12]. However, these adverse effects can be minimised by tailoring the fibre's surface by chemical treatments. Surface treatment of fibres not only causes the strong interfacial bond but also helps in the removal of amorphous compounds from the fibres which in return enhance the mechanical as well as thermal properties of the fibres and the composite [13].
Many types of research have been done on various chemical treatments where treatment caused improvement in mechanical properties of the composites. Hand lay-up, surface selective dissolution method, vacuum infusion process are commonly used methods for composite preparation. Each method has its advantages and depends on the type of application, e.g. for short fibres composite selective dissolution method is of high importance and gives high strength and modulus values to the composite [14]. The main purpose of treating fibres is to detach the impurities already present on the fibre surface and to change the surface morphology. Surface treatment is also beneficial in the removal of lignin and hemicellulose content from the fibres and hence increases the fibre adhesion with the matrix [15]. Most common chemical treatments are alkali, isocyanates, silane, acetylation and benzoylation. They can significantly improve the mechanical performance of natural fibres reinforced polymer-based composites, but most of these chemicals have some environmental concerns. Isocyanates are the leading cause of occupational asthma [16]. Silane comes under the list of highly reactive gases that cause significant fire [17]. Acetylation and Benzoylation are also hazardous to health due to their toxicity apart from being cost effective [18, 19]. Sodium hydroxide (NaOH) is the most commonly used alkali for most of natural fibres, especially bamboo. NaOH treatment removes the fatty or gummy substances attached on the fibre surface and hence increases the fibre/matrix contact area. Manalo et al. [20] examined the improvement in properties of the composite reinforced with bamboo fibres after the fibres were treated with different concentrations of NaOH. The effect of modification depends on treatment time, concentration of the solution and temperature at which treatment is performed [20-22]. Similarly, Lu et al. [23] studied the treatment effect of silane and sodium hydroxide on mechanical properties of bamboo fibres composite and found the enhancement in tensile strength. Buson et al. [24] investigated the acetylation and alkalisation effect on physical and mechanical performance of the bamboo composite where significant change in properties was observed. To avoid the expensive chemical treatment methods Tanpichai et al. [25] studied the process of fibre extraction and chemical treatment which consumes less energy as compared to other traditional methods.
In this study, Na2CO3 has been used for treating the natural long bamboo fibres. Although a few types of research have reported the use of Na2CO3 and NaHCO3 as modification agents but these were employed on coir, flax and sisal natural fibres [26-29]. These studies report that NaHCO3 treatment helps in the improvement of mechanical properties of coir and sisal fibres reinforced polymer composite by removing the hemicellulosic compounds from the fibres [30-32]. Because of the mildly alkaline nature of sodium bicarbonate, it needs a long treatment time when compared to other alkaline media (NaOH). Na2CO3 is comparable to NaOH in terms of fibres treatment as it is also effective in removing impurities and changing the fibre's texture which is the cause of increment in mechanical properties. Sodium carbonate also overcomes some major problems linked with alkaline pretreatments such as corrosion [33]. Sodium carbonate treatment has several advantages over sodium bicarbonate which includes it's eco-friendly nature and reduction in treatment time of fibres [9, 34]. In a previous study by the authors, the effect of sodium carbonate was studied on short bamboo natural fibres embedded in polyester matrix where significant enhancement in mechanical properties was observed [9]. Whereas the current work is focused on studying the behavior of alkali-treated long bamboo fibres in epoxy matrix. Furthermore, the effect of treatment on crystallinity of the fibres and on the thermal properties of the composite was also investigated. Experimental techniques involving tensile, impact, FTIR, TGA, XRD, DSC and water absorption tests were used to examine the dependence of sodium carbonate treatment on composite properties. So overall, this study provides information for the development of chemically treated eco-friendly and cost-effective long bamboo fibres reinforced polymer composite by exploring their mechanical and physical properties.
Materials and methods
Materials
Bambusa arundincenia specie of bamboo plant (8 years of age) is obtained from a local market in Topi, Pakistan. Epoxy and curing agents were purchased from Nobel Trading Company, Pakistan. NaOH (Sigma Aldrich, USA) (purity 97%) and Na2CO3 (BDH England) (purity 99%) used in the study were obtained from City Scientific Store Islamabad, Pakistan.
Extraction, Na2CO3 treatment and characterisation of bamboo fibres
Bamboo nodes were separated from the obtained bamboo slabs and were cut into strips of 30 cm long and 2–3 mm wide. Chemical extraction route was employed for fibres extraction in which bamboo strips were immersed in 3 wt. % solution of NaOH for 10 h at a temperature of 70°C. Strips were removed from the solution and washed with distilled water followed by manual extraction of fibres. Manually extracted fibres were chemically treated with 2.5, 5 and 10 wt. % of Na2CO3 solutions for 3, 6, 12 and 24 h at room temperature. Treated fibres were then washed and oven dried at a temperature of 50°C for 24 h [33].
To find the average diameter, fibres were analysed in the optical microscope Olympus BHM. About 30 different fibres from each batch were taken and the diameter was measured at five different positions to get the average value of diameter. For understanding the surface morphology of the fibres, SEM Philips (XL 30) with an accelerating voltage of 20 kV was used. Treatment also produced chemical changes in the fibres. Therefore, to analyse the chemical structure, virgin bamboo fibres and chemically treated bamboo fibres were analysed in FTIR-SHIMADZU IR Tracer 100 (resolution of 4 cm−1, wave number up to 400–4000 cm−1). Thermal properties of the fibres were also investigated by TGA under argon atmosphere using TAQ-500. Approximately 10 mg of bamboo fibres were taken and analysed at temperature up to 600°C with the heating rate of 10°C min−1. To observe the change in crystallinity, X-ray diffraction (XRD) technique was employed. Pressure of 111 MPa, scan speed of 0.5° min−1 and step size of 0.02° was kept in XRD. To find the crystallinity of the fibres before and after treatment, Segal empirical method [35] was used. Segal formula is also given in Equation (i).
Production and characterisation of composite
For composite preparation, first epoxy resin and hardener were mixed in 1:2 ratio with the help of an overhead stirrer. The mixture was then degassed for 15 min to remove any entrapped air bubbles inside the mixture induced due to stirring. Then the composites with variable fibre loading (10, 20, 30 wt. %) were prepared using hand lay-up process in steel molds. Different molds were used according to the sample size requirements mentioned in ASTM standards for tensile test (ASTM D3039), impact test (ASTM D6110-10) and water absorption test (ASTM D570-98). The molds were priorly greased for easy removal of cured composites at the end. Alternate layers of epoxy and long fibres were added to secure uniform distribution of both components. In the first step epoxy is added as a first layer followed by bamboo fibres layers. To ensure proper alignment of the fibres, fibres were first fixed from both ends. Latterly, second layer of epoxy and fibres were added. In the last step, curing was done by closing the mold from the top and the composites were allowed to cure at room temperature for 24 h.
As the control of the fibre content inside the composite is important to be controlled at a point where maximum mechanical strength can be achieved. Therefore, fibres were weighed before adding them inside the epoxy matrix. Similarly, weight of the epoxy before the mixing is also measured. Using the weight/weight of fibres and epoxy, composite with specific fibre content can be prepared [36].
Density of the prepared composites was also measured by Archimedes' principle as per ASTM standard D792 [37, 38]. To determine the thermal properties of the treated and untreated fibre composites, simultaneous thermal analyser (STA8000 by Perkin Elmer USA) was used with argon inert atmosphere. For a better understanding of fractured surfaces of the composite and their relationship with mechanical properties, SEM analysis was performed on an SEM EVO15 by Zeiss. Mechanical properties of the composites were evaluated by tensile test and impact test. The tensile testing machine INSTRON 5567 was used to carry out the tension tests (ASTM D3039). Charpy impact tests were performed on a SHIMADZU impact testing machine with a capacity of 1.5 kg.f.m (ASTM D6110-10). Resistance towards water absorption was analysed by water absorption tests as per ASTM D570-98. Difference in weight before and after immersion indicated the ability of the composite to absorb water.
Results and discussion
Microscopic analysis of the fibres
Diameter analysis in an optical microscope shows a little distribution in diameter (0.7–1.1 mm) as seen in Figure 1(a,b) which is mainly because of the non-uniform cut procedure. Histogram for giant bamboo fibres diameter is also plotted as a function of frequency which is shown in Figure 1(c). As observed, average diameter lies in the range of 0.8–0.9 mm. Fibre diameter is an important variable and has huge influence on mechanical properties because fibres with less diameter provide more area of contact with the matrix [39].
Measured diameter and histogram of extracted bamboo fibres.
The treated and untreated fibres were also analysed under SEM to study the surface features and to see their effects on the mechanical behavior ultimately. Figure 2 explains why increase in treatment time causes decrease in fibres strength [9]. Actually, the SEM micrographs of untreated fibres show the presence of impurities [23] as seen in Figure 2(a). However, some parts of the fibre show a partially cleaned surface which is due to NaOH treatment done for the fibre extraction. NaOH treatment also helped in dislodging the impurities from the fibres surface. Impurities presence on the surface does not allow the strong contact formation between fibres and matrix and hence decrease the interfacial strength. Sodium carbonate which acts as alkali makes the fibre surface rough by removing the gummy substances, lignin and hemicellulose from the fibres. Roughness has a strong relationship with treatment time and is directly proportional to it. Mechanism in the creation of a rough surface involves the interaction of OH- ions with the fibres. Na2CO3 when dissolved in water releases Na+ and CO3 2− ions. Carbonate ions cause the release of OH- ions from the water. Hydroxyl bonds present in cellulose have high affinity towards OH- ions and their interaction disrupts the hydrogen bonding of the cellulose. Disruption of hydrogen bonds is the main cause of creation of rough surface on the fibre.
SEM micrographs of (a) before extraction, (b) Na2CO3 treated with 6 h, (c) 12 h, (d) 24 h.
Figure 2(b) shows the 6 h treated fibre with a clean and rough surface. The creation of rough surface is because of early removal of cuticle from the fibres. As treatment time is increased up to 12 and 24 h, surface roughness starts to increase further due to complete removal of cuticle from the fibres as shown in Figure 2(c,d). With an increase in treatment time, fibres also start degrading and degradation level increases as treatment time reaches up to 24 h. Extreme degradation of fibres at 24 h of treatment time is the main reason of the decrease in mechanical strength.
FTIR analysis of the raw and treated fibres
Raw and Na2CO3 treated fibres were chemically analysed using FTIR to find the difference in chemical structure created by the treatment. Figure 3 shows the spectrums of treated and untreated bamboo fibres. Cellulosic peaks are similar to the research done by Shih [40]. Labelled peak at 1633 cm−1 corresponds to the lignin content. Lignin peak for treated fibres has far less transmittance as compared to untreated fibres which confirms the involvement of Na2CO3 in removal of amorphous lignin phase from the fibres.
FTIR spectra of raw and treated fibres.
Peak at 3407 cm−1 is for stretched hydroxyl groups present in cellulose and hemicellulose. As discussed earlier alkali treatment resulted in exclusion of hemicellulose content which is why decrease in intensity of peak 3407 cm−1 was observed for treated fibres [41]. Peak at 2918 cm−1 is for C–H bond vibration present in hydrocarbons [42]. A noticeable difference is the disappearance of the peak at 1718 cm−1. This peak is due to the presence of hemicellulose in bamboo fibres. The disappearance of peak at 1718 cm−1 confirms that Na2CO3 also eliminated hemicellulose from the fibres and thus rearranged the fibre structure [43]. Peak appears at 1159 cm−1 corresponds to typical cellulose peak whereas the last peak at 1032 cm−1 belongs to C–O and C–C stretching vibration peak of hydrocarbons present in the fibre [44].
Thermogravimetric analysis and measurement of crystallinity index using XRD
TGA was used to analyse the thermal stability of the fibres. It measures the weight loss at various temperatures occurred through water removal and fibre decomposition [42]. Figure 4 shows the thermal behavior of the fibres. Total weight loss in bamboo fibres is completed in three steps. Weight loss in the first step is due to water removal up to 100°C, second step is the decomposition of cellulose and hemicellulose whereas third stage involves the decomposition of lignin [45]. Lignin has a low rate of decomposition which starts at room temperature and proceeds till the highest temperature [46].
TGA plot of treated and untreated fibres.
Weight loss for treated and untreated bamboo fibres.
Second stage involved degradation of hemicellulose and cellulose which occurs at a temperature range of 160–360°C. Hemicellulose present in bamboo fibre degrades at lower temperature (180–280°C) as compared to cellulose whose degradation temperature is higher (250–350°C) [42]. Second stage in the current TGA graph shows the hemicellulose degradation followed by degradation of the cellulose. It has also been discussed in previous sections that Na2CO3 treatment causes removal of amorphous hemicellulose from the fibres which is why treated fibres showed less weight loss (approx. 50%) than untreated fibres (approx. 65%). Degradation temperature of cellulose also has some significance and for untreated fibres it appears to be at 360°C but for treated fibres it is shifted to 325°C displaying the decrease of thermal stability by Na2CO3 treatment. From FTIR spectra, it is not possible to guess if the cellulose I is regenerated into cellulose II but decrease in thermal stability of fibres may be attributed to the fact that cellulose I may not has transformed into cellulose II. The presence of cellulose II can be related to the high thermal stability as it presents more number of hydrogen bonds [47-49]. Some studies also showed the cellulose I conversion to cellulose II when NaOH was used as treatment media which is because of higher pH of NaOH solution. Low pH of Na2CO3 is therefore responsible for incomplete conversion of cellulose I into cellulose II. It is also reported in the literature that the use of 10 wt. % of Na2CO3 as surface treatment for coir fibres was not sufficient to regenerate cellulose II [33]. A decrease in thermal stability has also been observed from various studies involving NaHCO3 solution as a fibre treatment media [27, 33]. Third and the final stage above 360°C is related to the decomposition of lignin. Broad temperature range at the end of each curve corresponds to tough decomposition of lignin thus causing a slow decomposition rate. Alkali treatment also leads to removal of lignin which is why mass loss for treated fibres is nearly equal to 34% whereas for untreated fibres it is 46%.
The thermogravimetric curves were also analysed to estimate the relative removal of cellulose, hemicellulose and lignin contents and the results are presented in Table 1. Data shows that from 160 to 360°C decomposition of holocellulose (hemicellulose and cellulose) occurs and % age weight loss in untreated fibres corresponding to this region is 65% whereas treated fibres showed a weight loss of 50%. From these obtained values (65% and 50%), it can be estimated that out of the total 100% holocellulose (65%) present inside the fibres, sodium carbonate treatment can successfully remove approximately 23% (relative percentage removal = 65–50/65 × 100) of the holocellulose. Similarly, when lignin removal was calculated above 360°C, it comes out to be 46% and 34% for untreated and treated fibres respectively. Comparing both the values it can be concluded that treatment releases 26% (relative percentage removal = 46–34/46 × 100) of total lignin present inside the fibres. Therefore, it can be said that sodium carbonate is a very good treatment medium that helps in the removal of approx. 25% of amorphous compounds from the fibres thus increasing the interfacial strength of the fibres with the matrix. From literature chemical composition of Bambusa arundincenia showed presence of 60% of cellulose, 5% of hemicellulose and 35% of lignin [50]. It should be noted that the chemical compositions of the treated and untreated fibres were not determined but the obtained results from FTIR and TGA are in correspondence with the already published results [33].
Figure 5 shows the XRD patterns of untreated and treated bamboo fibres. Major peaks for both types of fibres were observed at diffraction angles of 26.4° and 18.3° indicating the crystal planes of 002 and 101. The presence of corresponding planes is also attributed to the crystalline phase of the cellulose I. The presence of cellulose I also confirms the statement that cellulose I has not completely transformed into cellulose II but still there's an increase of crystallinity which is because of the removal of amorphous compounds from the fibres. Removal of amorphous compounds in returns increases the crystallinity of the treated fibres [51]. The crystallinity index of the treated fibres, calculated by using Equation (i), came out to be higher than that of untreated fibres. Untreated bamboo fibres showed a crystallinity index of 22.86 whereas treated fibres depict the crystallinity index of 48.56. An increase of crystallinity as a result of treatment is also related to elimination of amorphous compounds such as hemi-cellulose, lignin, and waxes from the fibres. Close packing of cellulose chains after treatment may also be attributed to an increase of crystallinity. In short two factors which played an important role in increasing the crystallinity of fibres after treatment are: (i) structural rearrangements occurred in crystalline regions i.e. cellulose and (ii) removal of amorphous material i.e. lignin, hemicellulose, and other gummy substances.
XRD patterns of (a) untreated bamboo fibre and (b) treated bamboo fibre.
Figure 5 also indicated how treatment affects the crystallinity of the fibres and it can be seen with the increase of treatment time crystallinity slightly increases which could be related to further removal of non-cellulosic compounds such as lignin, waxes and hemicellulose. After 6 h of treatment, the crystallinity index further increases up to 1% due to better packing of cellulose chains. The effect of better packing of cellulose chains diminishes because fibres also start degrading at 12 and 24 h of treatment time thus decreasing the strength of the fibres [52].
Density measurement and thermal analysis of treated and untreated fibres composites
The density of the treated and untreated fibre composites was measured by Archimedes principle. Results indicated that treated and untreated composites have the same value of density and the average value of density is 1.013 g cm−3. Minute differences in density value can be attributed to the presence of voids or trapped air [53]. Table 2 shows density values of treated and untreated fibres composites. For better understanding of the thermal behavior of the prepared composites simultaneous thermal analysis (TGA, DSC, DTA) was performed and obtained curves are shown in Figure 6. Overall, a similar trend is observed for all the composites even though they have different nature of reinforcements. Figure 6(a) demonstrates the TGA curve for all three types of composites prepared. It can be seen that the weight loss is the lowest for simple epoxy and highest for untreated fibres reinforced epoxy composite. Low mass loss in epoxy is due to the absence of any reinforcement. Prominent difference is observed in between epoxy and fibre composite, epoxy starts to lose its weight nearly at 300°C whereas epoxy reinforced with bamboo fibres start to lose weight at much lower temperature of 225–250°C. Obtained results confirms that reinforcing bamboo fibre into epoxy resin decreases the thermal stability of the composite as bamboo fibres have lower thermal degradation temperature of almost 220°C as compared to clean epoxy, which starts to degrade above 300°C. In TGA of fibres, a decrease of thermal stability of treated fibres can clearly be seen when degradation temperature shifted from 360°C to 325°C. The effect of decrement in thermal stability is also observed in composites at a temperature range of 450°C to 500°C. Previous studies also indicated that bamboo degradation depends on holocellulose and lignin content present whereas epoxy degradation is due to radical chain mechanism [54]. After 450°C, the entire composite started turning into ash. Changes in weight loss of the composites are also compared and it has been concluded that the composite with untreated fibres showed highest mass loss whereas composite reinforced with treated fibres showed less percentage of weight loss. These results further confirm the involvement of sodium carbonate in removal of hemicellulose and lignin from the fibres and hence lower weight loss was obtained in composites reinforced with treated fibres. These results are in good agreement with a previous study done on polyester matrix composites [9].
TGA, DSC and DTA plots of simple epoxy, treated fibres composites and untreated fibres composite. Measured density of treated and untreated fibre composites.
Figure 6(b,c) present DSC and DTA analysis of the neat epoxy, treated and untreated bamboo fibres reinforced polymer composites. DSC curves for the composites reinforced with untreated and treated fibres showed the results in favor of TGA. As stated above, sodium carbonate treatment causes a decrease of thermal stability of fibres due to reduction in the formation of hydrogen bonds which also initiates the thermal decomposition of treated fibres composite at lower temperature than that of untreated fibres composite. Thermal decomposition region is in between 200°C and 450°C and it belongs to the decomposition of matrix, hemicellulose and cellulose from the fibres. Peak appearing above 450°C is related to the combustion of material and it can be seen that treated fibres composite showed lower combustion temperature when compared with untreated fibres composite. DSC curves also showed the presence of two exothermic peaks. It has also been observed that the presence of reinforcement has a strong dependence on the shape of the peaks. The DTA curves presented in Figure 6(c) show the results in compliance with the DSC and TGA curves. Three main peaks can be seen where first peak corresponds to the removal of water at 100°C whereas the next two peaks in the range of 300°C to 450°C related to degradation of composite.
Mechanical properties evaluation of the epoxy-bamboo composites
To find out the optimum concentration of sodium carbonate and treatment time, composites with different concentrations (2.5, 5, 10 wt. %) and treatment times (3, 6, 12, 24 h) have been prepared. A treatment time has considerably affected the value of tensile strength as presented in Figure 7. With the increase of treatment time, strength of the composite significantly decreased which is because of fibres degradation that starts to occur as treatment exceeds by 6 h which is in accordance with the published conclusions [9]. The SEM micrographs taken as the function of treatment time, as presented in Figure 2 and discussed in the microscopy section, also support this aspect that why the increase in treatment time causes a decrease in fibres strength. So, examining the obtained results, 5 wt. % of Na2CO3 concentration with 6 h of treatment time was chosen for further analysis.
Tensile strength of composites as the function of treatment time and concentration of the Na2CO3 solution.
After choosing suitable treatment time, composites with variable wt. % (0%, 10%, 20% and 30%) were also made. Figure 8 (a,b) represents the tensile strength and modulus of treated and untreated composite with variable fibre loading. As seen in the start, the value of tensile strength was low due to less fibre content but as soon as fibre content reaches 20 wt. %, tensile strength and modulus increased owing to good dispersion of fibres inside the matrix. With further increase in fibre content up to 30 wt. %, decrements in properties were observed which is because of poor dispersion of fibres within the matrix, as epoxy is not sufficient to coat all the fibres and hence negatively affects the load transfer mechanism. Poor dispersion of fibres was the main cause of defects formation. Created defects behaved as a crack initiation site and were the reason for composite failure at low stress. Increased mechanical strength of the composites due to sodium carbonate treatment is in accordance with the already reported studies on use of alkali as a chemical treatment medium [20, 33]. The strength of the fibres and composite itself highly depends on fibres and solution interaction. Sodium carbonate interaction with fibres involves the breakage of bonds present between cellulose and hemicellulose along with the formation of new hydrogen bonds in cellulose chains. The formation of new bonds is a very important factor in giving the fibres high strength [27, 51].
Tensile strength (a) and Young's modulus (b) of composite with variable wt.% of fibres.
SEM analysis of fractured tensile samples showed long length of fibre pull out in untreated fibres composite whereas treated fibres composite has short length of fibre pull out. As stated before, sodium carbonate treatment removes amorphous substances from the fibre which makes the surface rough. The creation of rough surface provides mechanical interlocking between fibres and matrix which act as strength enhancement mechanism [33]. Figure 9(a) shows the fractured cross-section of the untreated composite with long fibre pull out. Fibre pull out length depicts the extent of sliding between fibres and matrix. The absence of rough surface in untreated fibres does not allow the fibres to provide sufficient interlocking within the matrix and hence fibres slide easily. Slipping of fibres leads to the composite failure at much lower stress as compared to treated fibre composite. When cross-section of Na2CO3 treated fibres composite was observed in SEM, length of the fibres being pulled out is less as shown in Figure 9(b) indicating the prominent involvement of fibres in overall strength of the composite. Sodium carbonate treatment caused the removal of amorphous compounds which in return produced rough texture on the fibre surface. Rough surface interlocks the fibres tightly to adhere strongly with the matrix. Small length of fibre pull-out indicated the strong interfacial bond between two components which is why fibre sliding inside the matrix is difficult hence composite experienced high stress value. Less fibre pull-out also directed towards more involvement of fibres in bearing the high load.
SEM micrographs of 20 wt. % of (a) untreated fibres composite and (b) treated fibres composite.
A decrease in hydrophilic nature of fibres is also accountable for the improvement of mechanical properties because with the decrease of hydrophilic nature of fibres their wettability with hydrophobic matrix is increased and hence the improvement of mechanical properties was observed. The change of hydrophilic behavior was also due to transformation in spatial orientation of the cellulose [33].
Comparison of tensile strengths of epoxy and polyester matrices composites reinforced with 20 wt. % short and long bamboo natural fibres.
Charpy impact values of treated and untreated composite specimens with variable fibre content were also measured and are plotted in Figure 10. Impact strength of fibres reinforced composite greatly depends on wetting behavior of fibres towards matrix, fibre friction with the matrix and interfacial bond between the matrix and reinforcement. Analysing these factors, impact energy absorbed by the composite is strongly dependent on fibre pull-out [55, 56]. As seen in Figure 10, untreated fibre composite shows more impact energy value as compared to treated fibre composite which is because of loosely bound untreated fibres inside the matrix resulting in absorption of more energy. As stated before, untreated fibre surfaces can easily slide within the matrix therefore, absorbed energy is high. Reason for low impact energy value for treated fibre composite was the difficult sliding of fibres because of mechanical interlocking between two components which restricts the fibres to be pulled out easily. In this research, composite reinforced with 20 wt. % of fibres showed a decrease in impact value from 0.59 to 0.51 kJ m−2 indicating the prominent involvement of chemical treatment in impact strength.
Charpy impact value of treated and untreated fibre composites as the function of fibres content.
Water absorption analysis of epoxy-bamboo composites
Water absorption characteristics of composite have detrimental effects on dimensional and mechanical properties of the composite [57, 58]. Therefore, it is very important to study water absorption in any composite. When a composite is in a moist or water environment, water is absorbed through various phenomena, i.e. water intake from micro cracks inside the matrix, transport of molecules through capillary action to the fibre/matrix interface and movement of molecules in between polymer chains [59, 60]. As the entire composites were prepared under the same condition which is why interfacial strength plays a vital role.
Figure 11 represents water absorption behaviour of treated and untreated fibres composite. As seen with the increase of fibre content, water absorption also increases which is because of the higher amount of cellulose present. Cellulose is hydrophilic and can interact with water easily. When composites with treated fibres were tested under the same conditions, less water intake was absorbed indicating the decrease of hydrophilic nature of fibres by alkali treatment. Same behavior was observed when abaca fibres were treated with NaOH [41]. As discussed above, water absorption also depends on the interfacial bond between fibres and matrix therefore when interfacial bond is weak, a large number of water molecules can easily reach the interface. Na2CO3 treatment changed the surface energy and surface topography of the fibres thus increasing the wettability of reinforcement towards matrix which is why treated fibres showed formation of strong bond with the matrix and strong interfacial bond played a vital role in protecting composite from absorbing large quantity of water [57].
% water absorbed by the composites as a function of treatment and fires content.
Conclusion
Epoxy-treated bamboo long fibres composites were produced where eco-friendly sodium carbonate treatment of long bamboo natural fibres helped in the removal of lignin and hemicellulose from the fibres and in return improvement in interfacial bonding between fibres and epoxy was observed. FTIR analysis showed the change in chemical structure as certain peaks disappeared after treatment which is a confirmation of the removal of lignin and cellulosic content from the fibres. As per TGA analysis of the fibres, mass loss for treated fibres was approx. 50% at a temperature range of 160°C to 360°C whereas mass loss for untreated fibres was nearly 65% which is because of the abundant amount of hemicellulose present in raw fibres. XRD analysis indicated the increase of crystallinity of the fibres when treated with sodium carbonate owing to the removal of amorphous compounds by the chemical treatment. Thermal analysis (TGA, DTA, DSC) of the neat epoxy, untreated composite and treated fibres composites showed significant difference in weight loss, initiation of thermal decomposition and combustion temperature. Variation in thermal characteristics indicated the dependence of reinforcement on thermal properties of the composite.
As for mechanical properties, maximum tensile strength of 88 MPa was achieved for composite reinforced with 20 wt. % treated fibres. Chemical treatment of fibres also leads to lowering of impact strength from 0.59 to 0.51 KJ m−2 because of good interfacial bond which prevents fibre sliding. Decrease in impact energy value was found to be dependent on fibre pull-out i.e. when fibres pull-out is less, composite has absorbed less energy. Water absorbed by the composite also decreased when treated fibres were reinforced inside epoxy and the reason was again the formation of strong interfacial bond which caused a 1.5% decrease in water absorption.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the author(s).