It was proved that the properties of the natural fiber composites depend on con­stituents (fiber/matrix) and their interfacial bonding (Agoudjil et al. 2011; Al-Khanbashi et al. 2005; Huda et al. 2008; Kalia et al. 2011b). The interfacial

bonding between the reinforcing natural fibers and the polymer matrix in the composite has a vital role in determining its final mechanical properties. That is, the reinforcement efficiency depends upon the stress transfer between the matrix and fibers. Most of the thermoplastic polymers are nonpolar, hydrophobic com­pounds whereas natural fibers are polar, hydrophilic ones. Because of this inher­ent dissimilarity, the NFRPC are usually not compatible and interfacial adhesion in these composites tends to be poor. Therefore, the surfaces of the natural fibers are usually treated via different means in order to improve the interfacial bonds between the fibers and the matrix. Several methods of surface modification by physical, chemical, or mechanical means have been investigated to improve the fiber properties and to increase the bond ability as well as the wettability between the cellulosic fibers and the polymer matrix. Such used methods of treatment are tabulated in Table 1.5.

Several studies had proved the efficiency of such surface treatments on the date palm fibers for different polymer matrices when treated with suitable means of treatments particularly the chemical one (Abdal-hay et al. 2012; Al-Khanbashi et al. 2005; Alsaeed et al. 2012; Sbiai et al. 2010; Shalwan and Yousif 2014). The effect of different treatment process on the data palm fiber was investigated by sev­eral studies (Alawar et al. 2009) where different concentrations of alkali treatment were used to modified the fiber surface (Alawar et al. 2009), a range from 0.5 to 5 %, and acid treatment with 0.3, 0.9, and 1.6 N were used and performed at 100 °C for 1 h. Results demonstrated that the surface morphology was improved. NaOH treated fibers showed an increase in tensile and considerable advancement in sur­face morphology. On the other hand, fibers treated with hydrochloric acid were found to be unfavorable due to its negative impact on tensile strength and surface morphology. Microscopic examinations were demonstrated the effectiveness of the chemical treatment on the date palm fiber as can be shown in Fig. 1.9 where untreated fibers are demonstrated having a weak outer layer that can prevent strong bonding with the polymer matrix in one hand, and another treated one where the weak outer layer was removed through the treatment process which can lead to stronger bonding with the matrix.

It is worthy to note that the proper treatment conditions like the suitable solution type, concentration, and time can dramatically enhance the mechanical properties of the fiber. The effect of the chemical treatment of the date palm fiber on its mechanical

Table 1.5 Physical and chemical treatments methods for cellulosic natural fibers

Physical treatments processes change the fiber’s structural and surface properties; thereby influence the mechanical bonding with the matrix. Such treatments involve surface fibrillation, electric discharge (corona, cold plasma), etc. The Cold plasma method is a very effective one that can clean the bio-fibers and modify its surface imparting different functional groups and changes in surface energies. Steam treatment of the natural fiber can be performed by applying of high pressure steaming, where heating at high temperatures and pressures are preformed, then mechanical disruption by violent discharge or explosion is usually used (Kalia et al. 2011b; Arbelaiz et al. 2005)

Natural fibers are highly polar-owing to the hydroxyl groups. Such groups are readily available for chemical bonding (hydrogen bonding) with compatible polymer matrices and physical interlocking (wetting) with the nonpolar matrices. Several chemical treatments were investigated having potential to remove both waxes and oils from the fiber’s surface in one hand, and to make it rough, and stop the water uptake

Alkaline treatment: Alkaline treatment (mercerization) is one of the most popular chemical treatments of natural fibers. The major modification done by this treatment is the disruption of hydrogen bonding in the network structure, herewith increasing surface roughness. Alkaline treatment by adding of aqueous sodium hydroxide (NaOH) to natural fiber can remove a certain amount of lignin, wax, and oils covering the external surface of the fiber cell wall, depolymerizes cellulose, and exposes the short length crystallites (Mohanty et al. 2001; Nouira and Frein 2014). Thus, alkaline processing can directly affect the cellulosic fibril, the degree of polymerization, and the extraction of lignin and hemi cellulosic compounds (Li et al. 2007; Osman 1983)

Liquid ammonia treatment: Liquid ammonia has the ability to penetrate quickly to the interior of cellulose fibers, forming a complex compound after the rupture of hydrogen bonds. This ability of ammonia is due to its low viscosity and surface tension. In addition, the relatively small molecule of ammonia is able to increase the distances between cellulose chains and penetrate crystalline regions. Therefore, the liquid ammonia treatment can change the original crystal structure of cellulose I into cellulose III. Then, after hot water treatment, cellulose III changes into cellulose I again (Pickering 2008; Rachini et al. 2012). This type of treatment results in deconvolution and smoothing of the natural fibers’ surface. At the same time, the fiber cross-section becomes round and lumens decrease (Kozlowski and Wladyka-Przybylak 2004; Rowell et al. 1997)

Silanization: Silane is a chemical compound with chemical formula of SiH4. Silanization of cellulosic fibers can minimize the disadvantageous effect of moisture on the properties of fiber composites in one hand and can increase the adhesion between the fiber and its polymer matrix. Thus, upgrading the composite strength, proper conditions like the type of silane used, its concentration in solution, time and temperature of fiber silanization, in addition to the moisture content, can determine the effectiveness of the silanization (Li et al. 2007; Osman 1983)

Table 1.5 (continued)

Graft copolymerization: Grafting copolymerization can be used to modify the surface of the natural fibers. The cellulose is treated in this process with an aqueous solution containing selected ions and is exposed to a high-energy radiation. After that, cellulose molecules crack and radicals are formed (Bledzki and Gassan 1997; Salem et al. 2008). Afterwards, the radical sites of the cellulose are treated with a suitable solution usually compatible with the polymer matrix type like, vinyl monomer, acrylonitrile, and methyl (Kalia et al. 2011b; Arbelaiz et al. 2005)

Esterification: Esterification is a popular chemical treatment method usually involves the reactions with organic acids or anhydrides. Many esters are possible to be used depending on the nature of the organic acid involved in the reaction. Esters containing 1-4 carbon atoms are formate, acetate, propionate, and butyrate; laurate has 12 carbon atoms and stearate has 18 carbon atoms. Maleate and fumarate are esters of dicarboxylic acids containing double bonds in the carbon chain. Such treatment modification can alter polarization of the fibers to make them more compatible to nonpolar matrix (Kalia et al. 2011b; Arbelaiz et al. 2005) Acrylation and maleic anhydride treatment and treatment with

isocyanates: Other types of chemical natural fiber’s treatment can be performed by Acrylation and Maleic Anhydride Treatment and Treatment with Isocyanates to enhance the mechanical properties of cellulose fiber silanization (Arbelaiz et al. 2005; Kalia et al. 2011b; Li et al. 2007; Osman 1983)

properties is demonstrated in Fig. 1.10 where proper treatments demonstrate a dra­matically enhancement of the tensile strength as well as the Young’s modulus of the fiber. Stress/strain diagrams of date palm fibers treated with different NaOH concen­tration can be shown in Fig. 1.11.

1.4

Matrices for Date Palm Fibers

The composites’ shape, appearance, as well as the environmental and overall dura­bility are dominated by the matrix type whereas fillers carry most of the structural loads hereby they provide macroscopic stiffness and strength. Polymers domi­nated the market of the commodity plastics with about 80 % consuming materials
based on nonrenewable resources (Faruk et al. 2012). Due to the public awareness of the environment as well as the limited fossil fuel resources, alternative matrices of the conventional petroleum-based ones are emphasized by different governing and industrial sectors. Therefore bio-based plastics that consist of renewable resources have experienced a revival in the past few decades. Polymers and their composites have recently emerged in wide different applications of modern indus­tries due to their desirable properties like: low weight, low cost, recyclability, biodegradability, availability, and high specific properties (Alves et al. 2010; Faruk et al. 2012).

The petrochemical-based matrices such as thermoplastics and thermoset were extensively investigated for natural fiber composites. Thermoplastics such as Polyethylene (PE) (Alawar et al. 2009), polypropylene (PP) (Rachini et al. 2012), polystyrene (PS) (Singha and Rana 2012), and PVC (polyvinylchloride) (Huang et al. 2012) were used as polymeric base of the natural fiber composites, whereas Phenol formaldehyde (Zhang et al. 2012), Polyester (Al-Khanbashi et al. 2005), Epoxy resin (Shalwan and Yousif 2014), and Vinyl esters (Huo et al. 2012) were widely used as thermosets matrix.

Due to the undesirable properties as well as the technical drawback of natural fibers such as high moisture absorption and anisotropic characteristics (Arbelaiz et al. 2005), and the low permissible processing temperature, proper polymer matri­ces have to be selected for a particular fiber type to avoid the possibility of any lig- nocellulosic degradation and to prevent volatile emissions that could hurt composite characteristics (Rowell et al. 1997). Different mechanical properties, deformations, thermal analysis, degradability, weather resistance, and thermo-mechanical proper­ties of different composites were studied (Abdal-hay et al. 2012; Abu-Sharkh and Hamid 2004; Agoudjil et al. 2011; Al-Khanbashi et al. 2005; Alawar et al. 2009; Dehghani et al. 2013; Ibrahim et al. 2014). In these studies, researchers used date palm fibers with different matrix such as Polypropylene, Polyester, Epoxy, High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE), Polyester, and ethylene terephthalate. Moreover, date palm fibers were used with other types of fibers to make completely biodegradable hydride natural fiber composites like flax fibers and starch-based composites (Ibrahim et al. 2014). A typical SEM micro­graph of fracture surface of date palm fiber/polyester composite is shown in Fig. 1.12 using raw date palm fiber.

Generally, it was reported that using date palm fibers with different polymer types can enhance the beneficial desired characteristic of the composites like the tensile strength, Young’s modulus, flexural strength and modulus, thermal and acoustical properties (Abdal-hay et al. 2012; Al-Kaabi et al. 2005; Al-Khanbashi et al. 2005; Shalwan and Yousif 2014) which can with no doubt demonstrate the effectiveness and competitiveness of the date palm fibers to be used in different natural fiber composites for wide industrial applications (Al-Oqla and Sapuan 2014). Data of a single fiber pull out treated date palm fiber/Epoxy composite with different NaOH concentrations to determine the maximum stress is shown in Fig. 1.13.

Fig. 1.12 Atypical SEM micrograph of fracture surface of date palm fiber/ polyester composite. From Al-Kaabi et al. (2005)