Although the use of materials from biological sources is very promising as a re­inforcing material in polymer matrices, some challenges must be overcome con­sidering their dispersity and hydrophobicity. Therefore, the suspension stability of nanocrystalline and nanofibrillated celluloses in water and in other organic solvents is an important aspect for the composite preparation considering a large amount of apolar solvent available.

Nanofibrillated cellulose (NFC) refers to cellulose fibers that have been fibril — lated to achieve agglomerates of cellulose microfibril units. Those materials have nanoscale (less than 100 nm) diameter and typical length of several micrometers.68 The interest in nanofibrillated cellulose (NFC) has increased notably over recent de­cades mainly because its high mechanical reinforcement ability or barrier property in bionanocomposites or in paper applications, respectively. For the first applica­tion, the possibilities to interact with different polymer in matrices can be increased if different functions and/or their contents can be added in its surface. For Missoum and coauthors,68 the two main nanofibrillated cellulose drawbacks that are associ­ated with its mechanical properties are the high number of hydroxyl groups and the high hydrophilicity, which limits its uses for several applications. For both cases, the surface modification is recommended in order to reduce the number of hydroxyl groups and to increase their compatibility with hydrophobic polymer in matrices. Missoum and coauthors made a complete and recently review of nanofibrillated cellulose with focus on surface modification such as physical adsorption, molecular grafting or polymer grafting.

According to a recent review,68 the surface characteristics of the NFC depend on the raw material, the process of pretreating the material, technology of production of NFC and surface modification technique itself. Thus, to obtain NFC features and predefined hydrophobicity and dispersibility, it is essential that the raw materi­als and production steps such as pretreatment, mechanical treatment and surface modification, are previously specified. In summary, wood pulp bleached Kraft or sulfite are most often used as a starting material for the production of NFC but also nonwood fibers have been reported. About the devices, it is common among then the use of high pressure and strong mechanical shearing to fibrillate the fibers

Dhar and coauthors,69 changed nanocrystalline cellulose (NCC) surface from negative to positive by using surfactant, tetradecyl trimethyl ammonium bromide (TTAB). They observed that the addition of electrolyte or high amount of the surfac­tant the degree of phase separation in NCC suspension was reduced and the suspen­sion became more stable. Cationically modified NCC was also studied by Zaman et al.70 NCC, obtained from sulfuric acid hydrolysis of wood cellulose fibers, was ren­dered cationic by grafting with glycidyltrimethyl ammonium chloride (GTMAC). They found that the cationic surface charge density of NCC can be increased by controlling the water content of the reaction system. The optimum water content was found to be 36 wt.% for aqueous based media and 0.5 water to DMSO volume ratio for aqueous-organic solvent reaction media. As Dhar et al., Zaman and coau­thors also found that the cationically modified NCC was well dispersed and stable in aqueous media due to enhanced cationic surface charge density.

Lu, Askeled and Drzal71 studied the effect of surface modification of microfibril — lated cellulose (MCC) in the mechanical properties of composites with epoxy resin matrix. Three different coupling agents were employed to modify a sample of Kraft pulp microfibrillated cellulose from a mix of wood: 3-aminopropyltriethoxy si­lane, 3-glycidoxypropyltrimethoxysilane, and a titanate. The surface modification changed the character of microfibrillated cellulose from hydrophilic to hydropho­bic, maintaining the crystallinity of the material. Among the coupling agents, the titanate showed the most hydrophobic surface. Both treated and untreated materials were easily incorporated into the resin by using acetone as solvent. Better and stron­ger adhesion between the microfibrils and the epoxy polymer matrix was observed for the treated fibers, which resulted in better mechanical properties of the compos­ite materials.

In Zaman et al. study,72 hydrophilic surface finishing agent (glycidyl tri-methyl ammonium chloride) that contains nanocrystalline cellulose (NCC) was used to modify the quality characteristic of the polyethylene terephthalate (PET) fabric, coating durability, moisture regain, and wettability. The results showed that the surface properties of the fabric changed from hydrophobic to hydrophilic after the treatment, and the cationic NCC-containing textile surface finish showed superi­or adhesion onto the cationic dye able (anionic) PET surface over the unmodified NCC. Furthermore, the cationic textile surface finish was capable of withstanding multiple washing cycles.

Non-modified and modified (grafting of n-octadecyl isocyanate) sulfated nano­crystalline cellulose from Luffa cylindrical fibers were used to verify the effect of both NCCs on the glass transition temperature, melting point and degree of crys­tallinity of polycaprolactone (PCL) matrixes.73 The nanoparticles showed an aver­age length and diameter around 242 and 5.2 nm, respectively, with an aspect ratio around 46. The degree of crystallinity was further increased when using modified nanoparticles. Mechanical tests showed an increase of the modulus of the nanocom­posites upon addition of L. cylindrica nanocrystals. This effect was more marked for modified nanoparticles and probably partly due to the increased crystallinity of the PCL matrix. Moreover, chemical grafting promotes the more homogeneous disper­sion of nanocrystals within the PCL as shown by the significant improvement of the elongation at break compared to unmodified nanoparticles.

KEYWORDS

Bimaterials

Chemical Modification

Lignocellulosics

Polymer

Thermal, and Physical Properties Wood