3.1. Alkali treatment

Modification of natural fiber conducting with alkali including both the NaOH & KOH is one of the most common and effective treatments thus it has been frequently applied technique indeed in which the maximum attention goes to the sodium hydroxide (NaOH) particularly due to its availability, lower cost, and higher activity (Fig. 4(b)). However, in a broad sense, this treatment has covered four different types of practices, like (a) keeping a constant concentration of NaOH for a fixed period of time [[81], [82], [83]], (b) applying fluctuating concentration of NaOH for a fixed period of time [84], (c) maintaining a constant concentration of NaOH solution where the t time periods set as a fluctuating range [[85], [86], [87]] and (d) A setup in which both of the concentration and times are different [88]. The methods denoted by b & c are the furthermost commonly used modification methods which are generally conducted to determine the optimum conditions concerning the subjected natural fiber amendment/modification. According to technique (b) the subjected fibers could be modified with (2–5) % NaOH soln hanging the reaction temperature nearly at 23 °C as constant, in which the proportion of the fiber and liquor was 1:20 which allowed to remove the hemicellulose and other fatty and waxy materials from the internal structure of the individual fiber. Thus they might be nullified, cleaned and dried completely [84,89]. In the case of setup (c) the exposed natural fibers have been treated with 5 % NaOH soln (usually 5 % is ideal for most of the natural fibers) while the reaction times fluctuate inter between the range of (i.e. 15–90 min) [85,87,90]. After that, the obtained primarily modified fibers could be washed several times with DI water, followed by the dropwise addition of 0.1 N HCl for the complete removal of the excess amount of impurities essentially [86,88,91].

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Fig. 4. Different types of Chemical modification of natural plant fibers including (a) Scouring, (b) Alkali treatment, (c) Bleaching, (d) Acid hydrolysis, (e) Extracted CNC from banana rachis fibers, (f) Graft copolymerization of Agave cantala leaf fibers, the figures are reproduced with permission [33,34,58,92].

3.2. Modification by acetic acid (CH₃ COOH) solution

The acetic acid solution with different concentrations such as 5 %, 10 % additionally 15 % (w/v)) has generally been implied to modify the subjected natural cellulosic fibers with a view to eliminating the adhered hemicellulose plus other fatty and waxy fragments from the peripheral surface of the corresponding fibers by maintaining the temperature nearby 25 °C for a time period of about 2 h. After that 0.1 % NaOH solution is usually used to neutralize the fibers, then washed with fresh water and kept in an electric oven at 100 °C for 24 h to dry completely [93].

3.3. Modification by silane (SiH4)

Silane has been moderately applied as a coupling agent to treat the peripheral surface of the exposed natural plant fiber because it is considered a multifunctional molecule resulting in a suitable modified surface enrich fiber for further use indeed that have been stayed by the earlier literature [[94], [95], [96], [97]]. Such as aminopropyl triethoxy also vinyltrimethoxysilane are the two regularly used silanes that is applied to provide steadfast modification of plant fibers/cellulosic fibers [[98], [99], [100]]. When silane treatment is being conducted on subjected fibers, it is required to mix some extent of aminopropyl triethoxy also vinyltrimethoxysilane silanes per an ethanol water mixture maintaining the ratio around (60:40) then it has been kept hanging for 60 min with pH 4, by slow addition of weak acetic acid for a good result. According to some earlier literature, it is high time to submerge the raw natural fibers into the above freshly prepared solution whereas the time has been set around 2 h, then it is allowed to dry overnight at 60 °C in an electric oven [[101], [102], [103]].

3.4. Modification by benzoyl peroxide (C14H10O4)

To conduct this particular treatment the raw fibers have generally been submerged into 6 % benzoyl peroxide with acetone for sharply 30 min. After that the treated fibers could be washed with distilled water several times and exposed to open air for sundry properly [104,105].

3.5. Modification by potassium permanganate (KMnO4)

Chemical modification of natural fibers by potassium permanganate (KMnO4) has been conducted to increase their thermal stability, tensile strength, stiffness and in some cases to change their macromolecular and crystallographic structure which stated by previous literature [[106], [107], [108]]. Regarding this treatment, the raw fibers should have been submerged into 0.5 % KMnO4 solnwith acetone for around 30 min then washed several times and dried in an electric oven or open air for better observation [104].

3.6. Modification by stearic acid ((CH3(CH2)16COOH))

A soln of sharply 1 % stearic acid into ethyl alcohol was particularly applied for this treatment and the raw plant fibers has normally been placed slowly into the freshly prepared solution with continuous stirring by a magnetic stirrersetup. Then the preliminary modified fibers are later dried at about 80 °C for a time period of sharply 45 min undeniably [104,105].

3.7. Modification by seawater

This is a very much simple additionally cost-effective method which has been employed moderately to modify the plant fibers. Initially, one thing is important the pH and salinity of the seawater which must need to be watched, after that the raw fibers would be submerged in it for up to 30 days and lastly, the obtained fibers could be washed with fresh water then dried at ambient temperature [103,109,110].

3.8. Modification by cellulose powder ((C6H10O5)n)

The modification of plant fibers by contact with cellulose powder is a very recent trend which assumes that the fibers have been soaked individually into a steel container which should contain (2–10) % cellulosic pulp. Notable that it should be prepared inside the flow of hot distilled water, designed for up to 30 min. Furthermore, the obtained treated fibers could be dried at around 70 °C for 3 h roughly and stored for further study. However, this particular treatment had been conducted due to the distinguishable enrichment/improvement in the mechanical as well as the water/moisture absorption properties of the modified natural fibers [111,112].

3.9. Modification by polymer coating technology

It has been stated that polymer coating might be able to improve the adhesion capacity of the natural fibers whenever it is conducted with the matrix of {a(12 %) + b(1 %) + c(86 %) + d(1 %)} which has usually been applied for 3 h at 82 °C by maintaining 14,000 rpm, and lastly, the fibers could be filtered and dried for overnight at 60 °C [113]. Here, mixture (a) is made from (acrylic acid (46 %), water (42 %), styrene (8 %), itaconic acid (1.5 %), plus alkyl diphenyl oxide disulfonate (2.5 %)), (b) is made from (sodium persulphate (7.5 %) and water (92.5 %)), (c) is for fresh DI water and (d) is for anionic surfactant. Meanwhile, the mechanism advised that the existing carboxylic acid group in the polymeric system has the responsibility for better adhesion of the natural fiber because it reacts with the available hydroxyl group which belongs to the subjected natural fiber indeed. Besides this, it has also been hypothesized that, there is a greater chance of mechanical interlocking additionally physical interaction with the matrix and the polymer coated fibers, during compounding. This treatment had been performed due to improve the physical, surface as well as water/moisture absorption properties of modified natural fibers [114].

3.10. Modification by scouring

To remove impurities like dirty materials, and gummy substances from the natural fibers scouring process has intensively been used which is generally carried out by surface-active agents, i.e, soda and detergents. Rahman and co-workers had conducted and reported on it where the authors firstly cut the okra bast fibers and immersed them into a freshly prepared surfactant solution that contained 5 mg Na2CO3 plus 5 gm detergent per liter of water maintaining the ratio of fiber and liquor at 1:50 while the temp was 60 °C and the process conducted for 30 min (Fig. 4(a)). Later the fiber was washed successively by distilled water then dried at (100–105)°C in an electric oven for 2 h [58].

3.11. Modification by bleaching agent

Bleaching is one of the most important and effective techniques that has been widely used to modify natural fibers to remove impurities including lignin or something like that. However, in this process the fibers were dried at 90 °C and treated with 0.2 % of sodium chlorite (NaClO2) solution for 90–120 min at 90 °C temp with a pH value exactly around 4 that could be controlled by the slow addition of acetic acid. Later, the fiber was washed with fresh water and dried in open air then stored for further study [58]. Another report has been stated by Jayaramudu and co-workers which was conducted with calcium hypochlorite Ca(ClO)2 for 45 min. The obtained fiber was then washed with DI water and dried for overnight at 80 °C in an electric oven [30]. The diagram of a bleached banana rachis fiber has shown by Fig. 4(c) for better understanding.

3.12. Modification by acid hydrolysis

Acid hydrolysis has recently been considered the most innovative technique to modify the natural fiber stuff which is widely used to hydrolyze carbohydrates (such as natural fiber, cellulose, obtained from plants) into more smaller molecules that allow access to a range of renewable chemicals and materials such as crystalline nano-cellulose (CNC), nano-fiber (NF), microcrystallinecellulose (MCC), etc. that has widely been used to fabricate nanocomposite, nano-film, bioplastic, biomaterials, nano-filter, nanocomposite membrane, etc. [33,34,58,115,116]. However, Rahman and co-worker have recently carried out their investigation on banana rachis fiber to produce CNC where the authors conducted double acid hydrolysis by 60 % H2SO4 solution on banana rachis fiber after primarily modifying by bleaching firstly they cut the bleached fiber as small as possible then the considered cellulosic fiber stuff were immersed into the freshly prepared 60 % H2SO4 solution at 45 °C under continuous magnetic stirring on a hot plate for 30 min while the ratio of fiber to the solution was maintaining nearly 1:10. After 30 min, meanwhile the colour of the mixture had been turned into brownish (Fig. 4(d)) the hydrolysis reaction was stopped by adding 6 fold excess ice cool water into the mixing tank truly. Later the magnetic stirring was conducted again for another 30 min then the resulting mixture was cooled down to room temperature and centrifuged. The solid fraction had been washed off by DI water until obtained a neutral pH [35]. The hydrolysis was carried out again following the above procedure. Finally, the Nano-cellulose suspension was obtained by stirring the solid fraction with the required amount of water. The newly generated suspension was stored in the refrigerator at 4 °C or immersed into ethanol (Fig. 4(e)) for further study.

3.13. Modification by graft copolymerization technique

The prominent technique namely graft-copolymerization recently attracted the attention of researchers all over the world due to its outstanding knock with respect to the socioeconomically additionally bulk scale industrial point of view against the existing conventional methods of secondary modification of the natural fiber [117]. Conversely, in the case of the grafting technique, there has been an initiator (small amount) together with the monomer (HEMA or other vinyl) used while the primarily modified fiber could be immersed in DI water (Fig. 4(f)). The most important influencing parameters like the concentration of monomer, the concentration of initiator, reaction time and reaction temperature) should have to control properly otherwise it could hamper/reduce the chance of optimum grafting yield and efficiency by creating the homo-polymer into the existing copolymerization system that should not be desired regarding the plant fibers modification indeed [118,119]. The grafting yield percent and grafting efficiency could be measured by the following equations [92]:(1)$\frac{{\mathrm{}}_{\mathrm{}}{\mathrm{}}_{\mathrm{}}}{{\mathrm{}}_{\mathrm{}}}\mathrm{}$(2)$\frac{{\mathrm{}}_{\mathrm{}}{\mathrm{}}_{\mathrm{}}}{{\mathrm{}}_{\mathrm{}}}\mathrm{}$Here, W0, W1, and W2 represents the weight of raw fibers, weight of grafted products and weight of applied monomer correspondingly [[120], [121], [122]].

Some schematic diagram regarding the chemical modification of natural plant fibers has been given in Fig. 4.

4.1. Surface modification by plasma treatment

To eradicate the adhered impurities from the peripheral surface of the exposed natural fibers plasma modification technique has successfully been used for better results [139,140]. Oliveira and co-workers, delivered a stout review concerning the adaptation of the banana tree rachis fibers by conducting the treatment with dielectric barrier discharge (DBD) plasma in which a semi-industrial pattern instrument had been applied with a view to perform the experiments in ambient environments [138].

4.2. Surface modification by vacuum ultraviolet irradiation (VUV)

This treatment has been considered a comparatively new approach by which adhered impurities could be removed successfully from the subjected surface of natural fiber. In an attention-grabbing work, Kato and co-workers have reported on an interesting work addressing the surface oxidation of the exposed natural fibers by UVU technique. Nevertheless, to conduct this modification treatment, first of all the raw fibers could be placed into a stainless steel chamber which has a length of 140 mm as well as a diameter of 35 mm. During the experimental session a high energy radiations lower than 200 nm had been implied to generate the irradiation process. The trials were executed at room temperature by maintaining the pressure at about 2.5 Torr whereas the source lamp was Xe KsR-2A8 and equipped with MgF2 window, that actually 30 mm left from the sample holder. The photodiode could be measured at an intensity of 3 × 1015 photons/(s cm2) while irradiated with radiations of 147 nm particularly [143].

4.3. Surface modification by conducting ozone treatment

In the case of this treatment the ozone or oxygen-fluorine gas has successfully been applied to purify additionally to improve the peripheral surface of the considered natural fibers such types of significant works have stated by Kato and co-workers in which the authors have clarified the detailed process that permits to generate surface oxidation reaction onto the cellulosic fibers by means of ozone. This individual method adopts that the naturally occurring cellulosic nonwoven form of fiber stuffs could be exposed to ozone gas at 20 °C, maintaining the flow rate of gas about 50 L/h. But notable that the reaction times varies from 5 min to 9 h. The corresponding fiber stuffs that has usually been exposed to treat was washed systematically by distilled water in order to eradicate any adsorbed ozone fraction onto the surface of the treated fibers, then they were sent in a vacuum electric oven for drying at 60 °C overnight [143].

4.4. Surface modification by conducting corona treatment

Belgacem and co-workers already have directed some experimentations by using corona treatment to detect the improvements concerning the amendment of the peripheral surface of the natural cellulosic fibers for further study. Where the experimental set up had been comprised 2 plane electrodes constructed by aluminum besides a quartz plate that acts as a dielectric spacer. Around 1 g of cellulosic fibers was positioned in the concerning corona cell with 5 cm3volume and conducted for 1 min by maintaining a potential of about 15 kV as well as a frequency of 60 Hz at 25 °C while the relative humidity was 50 % [144,145,147].

4.5. Surface modification by conducting γ -ray treatment

This treatment has recently been marked as an innovative and interesting surface modification technique which has usually been conducted to improve the overall tensile property of the subjected natural fibers by increasing their hydrophobic nature resulting an impressive enhancement of tensile strength of the fiber as well as the composites which has been fabricated by theses treated fibers. However, Toth and co-workers for the first time had been carried out this particular treatment on cotton-cellulose and Kabir and coworkers have been reported a work on jute fiber to enhance its mechanical properties [149,151]. Where, first of all the cotton cellulose could be treated by NaOH solution (1–6 mol dm−3) this was followed by TMAH (1–3 mol dm−3). However, the neutralization of the primarily treated fiber stuff is very important then drying. And finally, the subjected fibers were post-processed with 5 kGy/h dose rate maintained by a Co60 γ source in exposed air [148,150].

4.6. Surface modification by conducting laser treatment

A pretty interesting and innovative but limitedly used surface modification technique namely laser treatment was reported by Mizoguchi and co-workers, which had been conducted on natural cellulosic fibers by an excimer laserirradiation system to improve the quality of fiber stuff. There are three types of excimer lasers (such as ArF (193 nm), KrF (248 nm) and XeCl (308 nm) that have generally been used for irradiation treatment. Meanwhile the Lambda-Physik LPX210i (of ArF and KrF laser) too LambdaPhysik EMG102MSC (of XeCl laser) have been applied for the treatment of exposed fiber stuff due to their surface modification with an ambient condition indeed. Here one thing is important that the pulse width consists of 20 ns of ArF, 23 ns of KrF and 14 ns of XeCl depending upon the types of the individual lasers, the frequency has generally been used as 1 Hz. Where Neon gas has been considered as a buffer gas. To obtain decent focus from the laser beam there is a concavo-convex lens has been used that allows the adjustment regarding the laser fluence which usually been measured by a joulemeter and an oscilloscope. The raw fibers were positioned in the sample holder exactly between the lens and joule meter. The irradiation area is (0.24–1.20) cm2 while the influence of irradiation fluency lies between (100–500 mJ/cm2) and the pulses number were (1–100) on the fiber surface structure might be analyzed [154].

5.1. Density measurement

To measure the density of the subjected natural fibers, it has been early need to dry the fibers completely in a non-hygroscopic desiccator for 48 h which holds calcium chloride. After that, the fibers were impregnated with toluene for 2 h to eradicate the existing microbubbles that belong to the fiber stuff. Then the fibers were cut into a desired length of about 5–10 mm and placed into a pycnometer which has the responsibility to determine the density of the fibers [62,76,155,156]. By using the following mathematical formula the density of plant fiber (ρ) could be effectively calculated-(3)$\mathrm{}\frac{{}_{\mathrm{}}{}_{\mathrm{}}}{{}_{\mathrm{}}{}_{\mathrm{}}{}_{\mathrm{}}{}_{\mathrm{}}}{\mathrm{}}_{}$Here, m1, m2, m3, and m4 represent the weight of the blank pycnometer (in kg), pycnometer loaded with chopped fibers (in kg), pycnometer loaded with toluene (in kg), and pycnometer loaded with chopped fibers and toluene solution (in kg) correspondingly [[157], [158], [159]].

5.2. Measurement of diameter

In exercise, the diameter of the addressing natural fiber has normally been measured by using a digital micrometer or by a microscope (including both the OM and SEM). The application of the digital micrometer which allows to measure the diameter of the subjected individual fiber with an accuracy around 0.001 mm, or else, measurements through an air wedge (±0.001 mm) could be an alternative technique [155,160].