11.3.3.1 Cellulose/Pulp

The obtained cellulose has a high amount of cellulose, high proportion of para-crystalline and amorphous cellulose and lower DP as compared to the native material. Lower DP of the obtained material can improve the enzymatic hydrolysis due to the two factors [52]: (1) increasing the amount of the reducing ends of cellulose chain; and (2) making cellulose more amenable to enzymes. Cellulose with short chains allows it to be attacked by enzymes more easily because they form weaker networks rather than strong hydrogen bonding [19, 53]. This suggests that the cellulose-rich residue is amenable to the enzymatic deconstruction for the production of ethanol. Generally, high conversion of cellulose to glucose can be achieved up to 90-100% for most softwood and hardwood after ethanol fractio­nations [20, 45]. For instance, the ethanol/water treated B. davidii has been subjected to enzymatic hydrolysis applying cellulose (Cellucast 1.5 l) and b-glucosidase (Novozm 188). The data indicates that high conversion of cellulose to glucose up to 98% was achieved under the optimal conditions [39].

Most studies show that lignin removal enhances the enzymatic deconstruction of cellulose, since lignin inhabits cellulose activity [54-57]. While reducing the lignin content from 30 to 19%, the enzymatic hydrolysis was enhanced hugely, whereas further reduction in the lignin content to 9%, only negligible increase of enzymatic hydrolysis was observed [39]. An exception was the treatment of B. davidii, decreasing the lignin content of the sample did not increase the enzymatic hydrolysis. It seems that other factors influenced the enzymatic hydrolysis in addition to lignin content [39].

It is generally believed that amorphous cellulose is more easily attacked than crystalline cellulose [55]. A comparative study conducted by Jeoh et al. [58] showed that amorphous cellulose exhibited high enzymatic hydrolysis as compared to crystalline cellulose, due to formation of more extensive bonding between the reducing ending groups of amorphous cellulose and cellulase (Trichoderma reesei). With respect to enzymatic hydrolysis of the ethanol pretreated B. davidii, it was considered that a low CrI value of 0.55 was already efficient for hydrolysis, and further reduction of the value did not afford additional benefits for hydrolysis [39].

Compared to the conventional chemical pulping process, the obtained pulp from ethanol fractionation has a higher yield, easier bleachability and comparable pulp properties.

Poplar was subjected to ethanol pulping to optimize the process by varying the ethanol concentration, pulping time, pulping temperature and usage of catalyst (H2SO4). Even using 0.02% acid catalyst, the obtained pulp yield and viscosity were lower than the acceptable level; therefore, acid catalyst should not be added. This was due to the serious degradation of carbohydrates in an acid medium. Under optimal conditions, i. e., cooking at 180°C for 90 min with 50% ethanol, pulp was obtained with yield around 45%, viscosity 892 ml/g and kappa number 67 [59].

A significant feature of the ethanol/water produced pulp is that the pulp is easy to be bleached even with rather high kappa number. It has been reported that ethanol aspen pulp of a kappa number 30 was bleached to 81-86% ISO brightness applying a chlorine bleaching sequence (CEH) and to an higher brightness of 90% ISO via a chlorine dioxide bleaching (DED) sequence, whereas ethanol birch pulp of a kappa number 40 was bleached to 83% ISO brightness with a CEH bleaching sequence [60]. With respect to the ability of delignification in oxygen delignifi — cation process, hardwood ethanol pulp showed more extensive delignification extent than the corresponding kraft pulp. A delignification up to 75% was achieved without a significant reduction of pulp viscosity, and pulp was bleached to a brightness level greater than 92% ISO after either an ECF sequence or a totally chlorine free (TCF) sequence [61]. On the contrary, delignification extent of kraft pulp in oxygen delignification stage is below 50% to avoid the extensive degra­dation of carbohydrates.

It has been reported that ethanol pulp was also suitable for alkaline extraction and alkaline oxygen delignification [62]. Reduction of residual lignin prior to bleaching by alkaline extraction can reduce the amount of bleaching chemicals thus reducing the environmental impact of the bleaching process. After the wheat straw ethanol pulp with kappa number around 60 was extracted with 1% NaOH aqueous solution for 1 h, a large proportion of lignin was removed from the fiber [62].

However, an increase of alkaline concentration resulted in an increase of the lignin concentration on the fiber surface due to the enhanced adsorption of the dissolved lignin back on the fiber surface, similar to the phenomenon observed in alkaline extraction of kraft pulp.

Sugar bagasse pulps produced from ethanol/water organosolv process were used to produce carboxymethyl cellulose [63]. In this process, the acid-catalyzed ethanol pulp (prepared with 0.02 mol/l sulfuric acid at 160°C for 1 h) was bleached with sodium chlorite, and then was used to prepare carboxymethyl cel­lulose (CMC). The CMC yield was 35% (based on the pulp) with substitution up to 0.70 groups CH2COONa per unit of glucose residue.

Surface modification of cellulose fractionated from ethanol/water was conducted by heterogeneous esterification with octadecanoyl and dodecanoyl chloride [64]. After esterification, the modified cellulose showed strong reduction in the values of the polar parameter cpp, i. e., 4.4 and 1.8 for dodecanoyl and octadecanoyl cellulose, as compared to a high value of 35.7 for the original ethanol extracted cellulose. Since the esterified cellulose had a good fiber/matrix interfacial compatibility and low moisture uptake, it was a potential feedstock as reinforcing elements used in composite materials.

Sisal (Agave sisalana) ethanol pulp prepared from cordage residues was used as reinforcement to cement-based composites [65], and the prepared pulp/cement composites could further combine with polypropylene (PP) fibers. Compared to that added by kraft pulp, the composites with the addition of ethanol pulp showed lower modulus of rupture (MOR), limit of proportionality (LOP) and toughness. However, the performance of ethanol pulp reinforced composites was improved through a further optimization of pulping process. After 100 aging cycles (without fast carbonation), the ethanol pulp composites showed lower water absorption and apparent voids volume than that combined with PP.