Hello,

It depends on the treatment you apply on your plant fiber as well as the treatment parameters. A number of treatments are available for imparting hydrophobicity on plant fibers, namely, alkalization, bleaching, acetylation, silane treatment and graft polymerization. These treatments may affect mechanical properties positively or negatively in the short term. However, their long term effects tend to be positive as they preserve the fibers by avoiding swelling, rotting, biomicrobial attack, etc that take place in the presence of water.

Below is a list of treatments that are applied on fibers taken from (Yılmaz, N.D. “Agro-Residual Fibers as Potential Reinforcement Elements for Biocomposites”, Chapter 11, in “Lignocellulosic Polymer Composites: Processing, Characterization and Properties”, Ed. Thakur, V.K., Wiley -Scrivener, USA, ISBN 978-1-118-77357-4, s. 233-270. ,2015)

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Some chemical treatments may act as removing extra-cellulosic materials. Others,

like those utilizing silane or maleated agents, provide a surface coating via a chemical bond reducing the hydrophilicity of the fiber and improving the adhesion to the polymer matrix [15]. Chemical treatmens lead to decrease in variation of properties, as Moniruzzaman et al. [73] reported for okra fibers. They also reported that different chemical treatments lead to further separation of okra fibrils, which hold loosely together after retting.

11.7.2.1 Alkalization

Alkaline treatment (mercerization) is one of the most common chemical methods

applied to plant fibers which are used as reinforcement in biocomposites. Alkalization partially removes lignin, wax and oils covering the surface of fiber cell wall [3]. Thus, alkalization improves the surface adhesion of fiber to the polymeric matrix with the removal impurities which results in a rough surface topography [8].

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11.7.2.2 Acetylation

The acetylation treatment decreases the hydrophilic nature of the plant fibers. During the acetylation treatment the hydroxyl groups in the cellulosic fiber are replaced with acetyl functional groups. The decrease in hydrophilicity results in lower moisture intake and stronger interfacial bonding [8]. De Rosa et al. [15] obtained lower water content in acetylated okra fibers compared to untreated ones. Hill and Khalil [72] reported that the acetylation treatment enhanced the  bioresistance of coir and oil palm fiber-reinforced polyester composites.

11.7.2.3 Silane Treatment

By forming stable covalent bonds, the silane treatment would improve the contact

angle against water by increasing the hydrophobicity. The mechanism of this treatment can be explained as a silane chemical reacts with water and forms a silanol and an alcohol. With the moisture present, the silanol reacts with the hydroxyl groups of the cellulose and bonds itself to the cell wall [8].

Decrease in water absorption behavior is reported to occur with silanizing of banana stem and bunch fibers [26]. Ganan et al. [26] reported an increase in contact angle and decrease in surface free energy and polar component. They found silane deposition on fibers surface according to Fourier transform infrared spectrometry (FTIR) analysis.

11.7.2.4 Bleaching

Bleaching causes the fiber parameters to become more uniform [5]. Due to removal of some extra-cellular materials, the degree of chrystallinity increases [28], fibers acquire a whiter color and the inherent yellowness is decreased. Fibers lose some weight and become finer [12]. Increase in water absorption was reported to occur with bleaching of okra bast fibers. This might be due to the removal of hydrophobic substances which exposes hydrophilic cites [5]. Contrarily, the water absorption of banana fibers was reported to decrease from 61% to 45% for 70 hour water immersion upon sodium hypochlorite bleaching [28].

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11.7.2.5 Enzyme Treatment

The utilization of enzymes in the natural fiber modification field is rapidly increasing.

This trend may be due to the environmentally-friendly nature of enzyme treatments

as the catalyzed reactions are very specific and the performance is very focused [3].

Several enzymes have been used in order to enhance the effectiveness of extracting fibers from the agro-residue or fine-tune the properties of extracted fibers such as lowering their diameter. Xylanases, cellulases, pectinases are the enzyme types that have found more use in agro-residual fiber modification [12,13,38].

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11.7.2.7 Graft Copolymerization

The compatibility between the hydroxyl group containing polar cellulose fibers and

apolar polymer matrices may not allow for production of high mechanical performance biocomposites. In order to overcome this difficulty, a third material, a coupling agent can be introduced that modifies the interface morphology and the acid-base reactions in the interface, the surface energy and wetting efficiency [8].[...]

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Best Regards,

Nazire

Plant  fibers are covered by silicon film will be more hydrophobic, and there thickness is rise too. It is possibly this method will permit to increase a tensile strength of pineapple fibers. 

Sincerely yours, 

T. Ulyanova