Surface Treatments of Natural Fibres in Fibre Reinforced Composites: A Review

Several studies have been conducted to evaluate the effect of the addition of an alkali to a fibre-reinforced composite. This method is also known as mercerisation, where a natural fibre is immersed in each concentration of aqueous sodium hydroxide (NaOH) at a given temperature for a given time. This procedure increases surface roughness, allowing for better mechanical interlocking, and exposes cellulose to the surface [20]. The chemical reaction for this treatment for this process is given by Equation (1) [21]; (1) Fibre -OH+NaOH→Fibre -O -Na+H2O {\text{Fibre}}\;{\text{ -OH}} + {\text{NaOH}} \to {\text{Fibre}}\;{\text{ -O}}\;{\text{ -Na}} + {\text{H}}_2 {\text{O}}

One specific study on alkalisation was by Oushabi et al. [9], who evaluated how it affects the morphological, mechanical, and thermal properties of date palm fibres. Sodium hydroxide (NaOH) of different concentrations; 0 wt%, 2 wt%, 5 wt%, and 10 wt% were used to treat the date palm fibres for an hour at 25 °C. They were then rinsed by purified water and oven-dried at 80°C for 24 hours. 5 wt% NaOH treatment was able to remove more surface impurities hemicellulose and lignin than 2 wt% NaOH treatment whilst not damaging the fibre surface, as would occur with 10 wt% NaOH. Furthermore, this treatment improved the tensile strength of the date palm fibre by 76%, with a reported value of 460 MPa at 5 wt% of NaOH. Thermogravimetric analysis showed decomposition of the treated date palm fibres at about 310°C, while raw date palm fibre showed decomposition of hemicellulose, cellulose, and pectin at 290°C and secondary decomposition of lignin between 425°C and 475°C. Secondary decomposition was not present when the date palm fibre had been treated, hence alkali treatment improved thermal stability.

In another report by Lassoued et al. [11], who also studied date palm fibres, particularly their thermochemical behavior before and after alkalisation, fibres were treated with NaOH of various concentrations 0.5%, 0.75%, 1%, 2%, and 5 % in time intervals of 2, 5, 7, and 24 hours. Residual sodium hydroxide was then removed from the fibres using distilled water, and then they were dried at room temperature and atmospheric pressure. Thermogravimetric analysis showed that untreated fibres underwent a mass loss of 48% from a temperature of 550 °C, while treated fibres stabilised from 700 °C with a mass loss of 59%. The greater mass loss of the treated Tunisian palm fibres is postulated to be due to alkaline treatment improving their thermal stability. This is caused by the elimination of hemicellulose and lignin present in the outside layer of the date palm fibre. Alkali fibre treated with 2% sodium hydroxide for 2 hours had a tensile strength of 242.2MPa, which is an improvement from raw fibre, which had a tensile strength of 175 MPa. Treatment with 5% concentrations of NaOH and 2% of NaOH for 5-hour, 7-hour and 24-hour time periods had a tensile strength less than 175MPa, which showed that cellulose, which is important in giving the fibre better mechanical properties, gets damaged, thus inhibiting it from doing so. The improvement of tensile properties after NaOH treatment is consistent with researches performed by Ariawan et al, 2020 [23] on a Salacca zalacca fibre reinforced high density polyethylene (HDPE) composite and by Ganapathy et al. [24] on aerial roots of Banyan tree fibre used as reinforcement in fibre reinforced plastics.

This chemical treatment is performed after the fibre has been pre-treated with an alkali. It is usually performed using benzoyl chloride and benzoyl peroxide, which reduces moisture absorption and improves the thermal stability of a given fibre [14]. The chemical reactions below show these two-part treatments, where Equation (2) [25] shows the alkali pre-treatment and Equation (3) [26] the benzoylation treatment ; (2) Fibre -OH+NaOH→Fibre- O− Na++H2O {\text{Fibre}}\;{\text{ -OH}} + {\text{NaOH}} \to {\text{Fibre- }}\;{\text{O}}^ - \;{\text{Na}}^{\text{ + }} + {\text{H}}_2 {\text{O}} (3) Fibre- O− Na++Cl-Cl(=O)−C6H5→Fibre−O-C(=O)−C6H5+NaCl \matrix{{{\text{Fibre- }}\;{\text{O}}^ - \;{\text{Na}}^ + + {\text{Cl-Cl}}\left( { = {\text{O}}} \right) - {\text{C}}_6 {\text{H}}_5 \to } \cr {{\text{Fibre}} - {\text{O-C}}\left( { = {\text{O}}} \right) - {\text{C}}_6 {\text{H}}_5 + {\text{NaCl}}} \cr }

This procedure was performed by Safri et al, [27] when they investigated how adding glass fibre and chemically treating sugar palm composites by means of benzoylation treatment affected their properties by performing a dynamic mechanical analysis of the composite. They reported that benzoylation treatment affected the loss modulus of the composite by reducing energy loss during material deformation. Furthermore, the storage modulus was increased in the benzoyl treated composites. Therefore, benzoylation treatment enhances fibre-matrix interfacial bonding.

In another benzoylation research by Izwan et al. [28], the effects of benzoyl treatment on sugar palm fibres that had been pre-treated using sodium hydroxide were studied. The sugar palm fibre specimen was immersed in a mixture of benzoyl chloride and 10% NaOH for three different immersion times of 10, 15, and 20 minutes. They observed a reduction in diameters in the treated sugar palm fibre, which pointed to the removal of the amorphous layer that is present in the fibre surface, and this subsequently enhanced the interfacial adhesion that would occur between the fibre and polymer. Optimum tensile strength was observed for the benzoyl treated specimen after 15 minutes, which had the highest tensile strength and modulus of 173.99 MPa and 6.64 GPa, respectively. This is because it had the highest cellulose content, due to more hemicellulose and lignin being removed in the chemical treatment. However, longer immersion time led to a reduced tensile strength because cellulose was damaged during the process. The increase in tensile strength of the benzoyl treated natural fibre reinforced composite from raw fibre is correlated to the study by P. Madhu et al. [6] performed on Agave americana fibre.

This treatment is used to remove lignin left on the surface of the fibre after it has gone through alkali treatment. This is done by immersing the natural fibre in a mixture of hydrogen peroxide and sodium hydroxide (H₂O₂ + NaOH) or a mixture of sodium chlorite (NaClO₂) and acetic acid (CH₃COOH) [14].

Kumneadklang [29] characterised cellulose fibre harvested from oil palm frond biomass. Sodium hydroxide pellets of 98% concentration were used in alkaline treatment. Two methods of cellulose fibre isolation were used: without pressure for 2 hours and at 90–100 °C, and under a pressure of 7 bar and 150 °C for an hour. In both methods, fibres were immersed in various NaOH solutions of 5, 10, and 15 %wt. The fibres were cleansed in distilled water to get rid of residual lignin and hemicellulose, and then dried. The fibre was subsequently bleach treated with 10% hydrogen peroxide at 90-100 °C for an hour. The surface morphology of the treated fibres showed that the surface roughness was reduced because impurities had been removed during these treatments. Treated oil palm fibre under pressure had the highest crystallinity at 77.78% due to the elimination of hemicelluloses and lignin. Thermal analysis showed the thermal properties of the oil palm fibre specimens. Raw oil palm fibre had a residual mass of 12.12% from the onset temperature of 263.5°C to a maximum of 365.3°C, Oil palm fibre treated with 15% NaOH without pressure had a residual temperature of 8.42 °C from the onset temperature of 265.8 °C to a maximum of 364.8 °C, and oil palm fibre treated with NaOH under pressure had a residual mass of 1.93 % from the onset temperature of 317.1 °C to a maximum of 366.8 °C. This study attributed the removal of hemicellulose and lignin from the samples to the improvement in thermal properties.

Improved thermal stability and higher cellulose content was confirmed by Johar [30] after treating industrial rice crops with the bleaching process. The chemical composition of the untreated rice husk fibres was found to be 35wt% cellulose, 33wt% hemicellulose, 23wt% lignin and 25wt% silica ash. Thermogravimetric analysis (TGA) was used to study the thermal stability of the rice husk fibres by observing the fibres’ crystallinity index. This was done by the continuous heating of the fibre to 900 °C. The untreated rice husk fibre had a crystallinity index of 46.8%. The fibres were then pre-treated using alkali solution of 4wt% NaOH at the reflux temperature for 2 hours, and next rinsed with distilled water, which process was repeated two more times. The crystallinity index after alkali treatment was 50.2%. After this stage, the chemical composition of the rice husk fibres was found to be 57wt% cellulose, 12wt% hemicellulose and 21wt% lignin. The fibres were bleached using a buffer solution of acetic acid, 1.7wt% aqueous sodium chlorite and distilled water at a reflux temperature of 100–130 °C for 4 hours. The chemical composition after bleaching was found to be 96wt% of cellulose and a negligible composition of lignin, hemicellulose, and silica ash. The crystallinity index after bleaching was 56.5%. The improvement in the crystallinity of the fibres meant that the bleached fibres had the highest thermal stability as compared to the untreated fibres and those that had only been alkali treated. Colour changes observed on the rice husk fibres were consistent with the composition results. The untreated rice husk fibres were brownish and then changed to a brownish orange after alkali treatment. Bleaching treatment further changed their colour to completely white, which was due to the removal of non-cellulosic components from the fibre as well as the hemicellulose, lignin, and waxy layers from the rice husk fibres. Consistent with the results of Kumneadklang [29], the smoothness of the fibre surface after bleaching treatment was observed. This is postulated to be due to the removal of the protective layer of waxes and pectin which exist on the fibre surface.

This treatment is usually preceded by alkaline treatment or bleaching treatment for better results. Acetylation treatment is a chemical reaction that introduces the acetyl function into a natural fibre [14]. This is done by applying acetic anhydride and acetic acid to the fibres via the esterification method, where the hydroxyl group (-OH) present in the natural fibre is replaced by an acetyl group (CH₃CO) [31]. This reaction can be conveyed as Equation (4) [21]; (4) Cellulose-OH+CH3 -CO−O−CO−CH3→Cellulose- O - CO−CH3+CH3COOH \matrix{{{\text{Cellulose-OH}} + {\text{CH}}_3 \;{\text{ -CO}} - {\text{O}} - {\text{CO}} - {\text{CH}}_3 } \cr { \to {\text{Cellulose- }}\;{\text{O}}\;{\text{ - }}\;{\text{CO}} - {\text{CH}}_3 + {\text{CH}}_3 {\text{COOH}}} \cr }

Senthilraja [32] studied the effect of acetylation on the mechanical behaviour and durability of palm fibre vinyl-ester composites. The palm tree interior trunk part was crushed and used to prepare fibre specimens. The palm fibres were immersed in vinyl ester of: 2%, 4%, 6%, 8%, and 10% concentrations, and the composite was cured at 80 °C and 2.6 MPa. Results gathered from this study showed that the mechanical behaviour and durability of the composite improved due to the chemical treatment with acetyl chloride. This is because acetylation treatment reduces the moisture absorption of the fibres, thus improving interfacial adhesion between them and the matrices.

Fitch-Vargas et al. [33] studied the properties of an acetylated sugarcane fibre-reinforced starch-based bio-composite. The sugarcane fibre underewent alkaline and bleaching treatment before acetyl treatment with sodium hydroxide and hypochlorite sodium solution. Acetylation of the fibres was done by immersing them in a 4% acetic acid solution for an hour.

The fibres were subsequently rinsed by distilled water and oven-dried at 60 °C for 24 hours. The treated fibres showed better mechanical properties, lower water affinity, and better thermal stability than untreated fibres. This was due to the presence of hydrophobic acetyl groups in the sugarcane fibres. Fourier Transform Infrared (FTIR) analysis corroborated this. These results are consistent with what Senthilraja [32] reported.

Silane treatment is a coupling agent that fills micropores present in the natural fibres’ surface and in so doing strengthening interfacial adhesion [21]. A coupling agent forms links between the natural fibre reinforcement and matrix to improve interfacial adhesion [34]. The chemical reaction for silane treatment is shown in Equation 5 [35].

Mohammad Asim et al. [36] investigated the impact of using silane treatment on the fibre-matrix bond of kenaf and pineapple leaf fibres. Two treatments were prepared of 2% NaOH, 6% silane in distilled water and 6% NaOH, 2% silane to treat kenaf and pineapple leaf fibre. The scanning electron micrograph showed the existence of impurities on the surfaces of both the kenaf and pineapple leaf fibre. Treatment of these fibres made them smoother than untreated fibres. The smoothness of the surfaces is caused by the silane treatment, and the removal of lignin and hemicellulose from the fibre surface was caused by the alkali treatment. Smaller, uniform fibres were found in treated fibres, because of the fact that the chemicals used removed the amorphous outer layer present in the untreated fibres. Results showed that the 2% silane, 6% NaOH treatment gave better results. Asim concluded this investigation by saying that silane treatment improves the hydrophilicity of pineapple leaf fibre and kenaf fibre.

Orue et al. [37] studied how silane treatments affect poly(lactic acid)/sisal fibre composites. The fibre was pre-treated with alkali treatment before silane was applied. The alkali pre-treatment was done in 2 steps: firstly, immersing the raw sisal fibres in 2wt% NaOH solution for 12 hours at room temperature to swell them and then immersing the fibres in 7.5 wt% NaOH for 90 minutes at 100 °C. The fibres were then washed in distilled water and oven-dried at 100°C for 12 hours to prepare for the silane treatment. The sisal fibres were then soaked in water for 3 hours while being sonicated to enhance silane accessibility to cellulose hydroxyl groups. Results showed a decrease in the density of the fibres, caused by the removal of lignin, hemicellulose, and impurities that exist in the sisal fibre surface. Morphological analysis of the composites showed improved interfacial adhesion in the composite, which was caused by the removal of the amorphous layer.

Liu et al. [38] investigated the influence of silane treatment on corn stalk fibre reinforced polymer composites. Silane coupling agents in water and absolute ethyl alcohol of 1 wt%, 5 wt%, 9wt% and 13wt% concentrations were used to immerse the corn stalk fibre specimen. The silane-treated corn stalk fibres were then rinsed in distilled water to remove residual silane coupling before being oven-dried for 24 hours at 90 °C. The polymer matrix was dried in a heat-treatment tank at 80 °C for 24 hours, mixed with treated corn stalk fibre, and hot pressed to make a composite. The silane solution treatment was found to be effective in reducing the water absorption of the fibre. The removal of lignin and hemicellulose present in the fibre surface enhanced the corn stalk fibre’s hydrophobicity and matrix-reinforcement bond.

This technique is whereby plasma is produced from a vacuum chamber through ionisation of a gas and is used to modify a fibre surface [39]. As a result of this method, there is less chemical consumption and waste produced, and the production time is lower compared to the conventional chemical treatments discussed [40]. Plasma treatment leads to the etching of surface layers of the fibres, enhancing interlocking with the matrix [41].

Zhou et al. [43] was able to experiment on ramie fibres with polypropylene to find out how atmospheric pressure plasma affects their interfacial shear strength. The fibres were pre-treated by immersing them in ethanol for 10 minutes, and then plasma treatment was conducted using an atmospheric pressure plasma jet system at 20 °C and 65% relative humidity. The treatment times selected were 8, 16 and 24 s. The fibres that were plasma treated for 24 s gave an optimal interfacial shear strength improvement of 46%. The authors postulated that the plasma treatment duration must be sufficiently brief for maximum surface grafting and long enough to enhance interlocking of the fibre with the polypropylene matrix.

The effect of cold plasma treatment on polyethylene/kapok composite interfacial adhesion was investigated by M. Macedo et al. [44]. Kapok fibres were treated with a voltage in the range of 400–500V, oxygen gas flowrate of 10cm³/min, a reactor pressure of 1.5 mbar, and an exposure time of an hour before the composite was fabricated. The tensile strength of the polypropylene was not significantly affected when the fibres were added. Morphological analysis showed a good adhesion between the polyethylene and kapok fibres. The authors conclude that the improvement was due to the plasma treatment.