Recently, development of biodegradable, ecofriendly polymer composite materi­als has been focused towards monocomponent, all-cellulose composites32. As it was previously mentioned, cellulose is one of the most abundant renewable and biodegradable biopolymer resources with high mechanical performance. In their cell walls, the spirally oriented cellulose plays the role of reinforcements in a soft hemi-cellulose and lignin matrix. All-cellulose composites were produced based on the original concept of self-reinforced composites, a composite with a matrix and reinforcement from the same polymer, which has been primary developed for ther­moplastic high-density polyethylene33. In this new type of so-called self-reinforced composites, the interfacial bonding problems are circumvented by the use of cel­lulose for both the reinforcement and the matrix. These composites have exhibited significant prospects as bio-based and biodegradable materials that have excellent mechanical properties. Because cellulose does not exhibit a melting point, it must be dissolved in order to aid in processing. Fiber surface selective dissolution of aligned cellulose fibers has been employed in the solvent by controlling the immersion time. Since the cell wall of the natural fibers is build of several layers, the surface layer of the fibers can be partially dissolved and transformed into the composite matrix phase. The remaining fiber cell cores maintain their original structure and impact a reinforcing effect to the composite. Due to the fact that both the fiber and matrix phases of this cellulose composite are from the same origin, and they are chemically identical, a strong interfacial adhesion could be expected between them33.

In this procedure, activated fibers are immersed in lithium chloride/N, N-dimeth- ylacetamide (LiCl/DMAc) for a specified immersion times. The fibers are then re­moved from the solvent and the partially dissolved fibers start to gel. Finally, this fiber-incorporated gel are coagulated in a nonsolvent system to extract the DMAc and LiCl, and then dried under vacuum. The cell wall of a cellulose fibers is consti­tuted by a number of layers; therefore, the surface layer of the fibers can be partially dissolved and transformed into the matrix phase of the composites, whereas the
undissolved part of the fibers, preserve the original structure, thus imparting the reinforcing effect to the composite.

Currently, several kinds of solvent systems have been used to dissolve cellulose, such as lithium chloride/N, N-dimethylacetamide (LiCl/DMAc), dimethyl sulfox­ide (DMSO)/tetrabutylammonium fluoride, NH3/NH4SCN, NaOH/urea, ionic liq­uids, PEG/NaOH, etc.34 All-cellulose composites have been prepared by dissolving pretreated cellulose pulp and then impregnation of the cellulose solution into the aligned fibers followed by coagulation in methanol and drying. Examples of starting materials are pulp,35 filter paper36- and long fibers37. Nishino et al. have prepared all­cellulose composites from pure cellulose and ramie fibers in LiCl/DMAc32. Duch — emin et al. have studied the effect of dissolution time and cellulose concentration on the crystallography of precipitated cellulose, using microcrystalline cellulose (MCC) as a model material38. The results of their work have contributed to a further understanding of the phase transformations that occur during the formation of all­cellulose composites by partial dissolution.

All-cellulose composites have been obtained, as an example, from aligned ra­mie fibers32. Due to the high fiber volume fraction in these composites (up to 80%), the tensile strength of these uniaxially reinforced all-cellulose composites has been found as high as 480-540 MPa. A similar approach has been also used to prepare random all-cellulose composites from filter paper36. As concerning the preparation methodology, by increasing the immersion time of cellulose fibers, larger fractions of the fiber skin are dissolved to form a matrix phase. Therefore, an improvement of interfacial adhesion has been observed by increasing the dissolution time. In the case of aligned ramie fibers, longitudinal tensile tests have shown that an immersion time of 2 h is the optimum processing condition to produce all-cellulose composites. In these conditions, it has been found that the amount of fiber surface selectively dissolved to form the matrix phase. This is adequate to provide sufficient interfacial adhesion to the composite, whereas the undissolved fiber cores retain their original structure and strength.

Lu et al.39 have published the results of their work on all-cellulose composites prepared by molding slightly benzylated sisal fibers. In contrast to plant fiber/syn — thetic polymer composites, water resistance of the current composites was greatly increased as characterized by the insignificant variation in the mechanical properties of the composites before and after being aged in water. They have found that sisal/ cyanoethylated wood sawdust and sisal/benzylated wood sawdust all-composites exhibit mechanical properties similar to those of glass fiber reinforced composites39. Physical heterogeneity in these all-plant fiber composites was favorable for the in­terfacial interaction. Biodegradability of the self-reinforced sisal composites was also followed. They found that in the case of enzymolysis aided by cellulose, the degradation rate of the composites gradually slowed down due to the hindrance of the lignin, which cannot be hydrolyzed by cellulose39.

A. Grozdanov et al. have worked on all-cellulose composites based on cotton textile fabrics, prepared by partial fiber surface dissolution in lithium chloride dis­solved in A, A-dimethylacetamide (LiCl/DMAc)40. Two different parameters have been studied: (i) the influence on type of scouring (alkaline or enzymatic) and (ii) the cotton textile preforms (knits, woven). In order to improve the interface and pro­tect against fiber degradation for the all-cellulose composites, alkaline scouring was performed by using 4% NaOH for 60 min treatment at 100°C. Enzymatic scouring was done with two conditions: alkaline pectinase-BioPrep3000 L at 55°C for 30 min and acid pectinase-NS 29048 at 45°C for 30 min2829. All-cellulose composites with ~90-95% fiber volume fraction were successfully prepared by using solutions of 3 (wt./v) cellulose concentrations in 8% (wt./v) LiCl/DMAc for impregnation of cotton textile preforms. Characterization protocols of the obtained all-cellulose composites have included FTIR, SEM, TGA/DTA, 13C-NMR, mechanical tests and Dp-determination.

It was found that a dissolution time of 24 h lead to bio-based materials with the best overall mechanical performance, since this time allowed dissolution of a suf­ficient amount of the fiber surface to obtain good interfacial bonding between fibers, while keeping a considerable amount of remaining fiber cores that provide a strong reinforcement to the composite. Characteristic mechanical curves of the studied all­cellulose composites based on various treated cotton woven fabrics are presented in Fig. 10.4. The measured data for the maximum load and deformations are presented in Table 10.7. Comparison of the mechanical performances of all-cellulose com­posites based on alkali — and enzyme-treated cotton-woven preforms has shown that the treatments can effectively improve the mechanical strength of the composites. The higher values for the mechanical strength were obtained for the all-cellulose composites based on enzyme-treated cotton-woven preforms. Han et al.34 have con­firmed the same effect. Moreover, they found that with an increase of the immersion time from 1 to 3h, the values of the mechanical strength sharply increased.

Although the biocomposites based on enzyme — and bleach-treated preforms have shown the best mechanical properties, alkali-treated cotton-woven preforms have shown higher lateral crystalline indices in the obtained composites compared to enzyme treated ones. Crystalline indices for the studied all-cellulose composites, obtained as a ratio of the FTIR bands at 1430 cm-1 (CH2 symmetric band) and 898 cm-1 (Group Clfrequency: — CH2=C-R) are presented in Table 10.8. The results obtained for the crystalline indices confirmed there were not significant changes in the crystalline structure of the cotton based composites.

The crystalline structure was studied also by 13C-NMR spectroscopy. The ob­tained results are shown in Table 10.9. The crystalline fraction Xc was calculated by deconvolution of the spectra in the 80-90 ppm region, according to the following equation:

I88 5 and I83 5 are the intensity of the peaks assigned to the crystalline and amorphous fraction, respectively.

No significant changes in cellulose structure were evidenced, as all the spectra are very similar (almost identical) (Fig. 10.5). For all of the studied samples, it is evident that the fiber core was not damaged and the fibers kept their structural and strength performance.

Characteristic SEM microphotographs of the morphology in the obtained all­cellulose composites are shown in Figs. 10.6 and 10.7. Figure 10.6 show morphol­ogy of alkali-treated cotton all-cellulose composites, while in Fig. 10.7, morphology of enzyme (alkali pectinase) treated cotton performs are shown. SEM images pro­vide direct information regarding the interfacial bonding of the studied all-cellulose composites based on alkali and enzyme treated cotton performs confirming where

good fiber-fiber adhesion was registered in both types of cotton performs. For alkali scoured performs, progressive build-up of covering thermoplastic films around the fibers were found, while for enzymatic scoured performs bonding bridges were reg­istered between two fibers.

Therefore, it can be affirmed that all-cellulose composites based on various treated cotton fiber performs show very interesting mechanical properties and rep­resent a new class of bio-derived and biodegradable composite materials with inter­esting possibilities of future development.