As a major component of plant cell walls, cellulose is found in virtually all types of plants, and typically 40—50 wt% of wood, 40—55 wt% of bamboo, 75—80 wt% of hemp, and 90—99 wt% of cotton are cellulose. Chemically, cellulose is a polysaccharide consisting of D-glucose units linked by в (1—4) glycosidic bonds. Cellulose has been widely used in a variety of industrial products such as pulp/paper, filters, textile, etc. More recently cellulose nanocrystals (CNC) have been used as adsorbent and polymer reinforcement [117,118]. Direct use of cellulose as high performance materials (such as filters, films, and polymer composites—cellulose acetate PVC sheets, etc.) has been facing a long-lasting challenge due to its relatively high crystallinity and high glass phase transition temperature (Tg > 200°C), and hence poor processability . Esterification of cellulose to cellulose acetate (CA) has demonstrated an effective chemical modification measure for reducing cellulose’s crystallinity and enhancing its processability and compatibility [120,121]. CA is produced by esterifying cellulose with acetic anhydride under acidic conditions through two-step reactions involving the full esterification of cellulose with excess acetic
Scheme 8.2 The reaction scheme of production process for cellulose acetate.
anhydride (commonly catalyzed by sulfuric acid) and the partial hydrolysis of the resultant ester groups to a desired acetyl value (degree of substitution, DS), as illustrated in Scheme 8.2.
CA has the following outstanding properties, although depending on the esterification extent (or DS): (1) nontoxic nature; (2) derived from renewable resources; (3i) low cost; and (4) biodegradable. CA has found a wide range of applications in the fields of coatings, films, membrane separation, textiles, and cigarette industries owing to its low cost, high toughness, moderate flux, and high salt rejection properties [122,123], as well as great transparency and biodegradability. For instance, CA membrane is ideal in filtration where maximal product recovery is critical, such as in protein and enzyme filtration, biological fluid sterilization, and tissue culture media sterilization [124,125]. CA filtration membranes are composed of pure cellulose acetate that is internally supported by an inert polyester web. This web gives each membrane exceptional strength to prevent cracking, tearing, breaking, and distortion when handled. The resulting membrane has dimensional stability that can withstand autoclaving or steam sterilizing, leaving the membrane unaffected at temperatures up to 135°C. The exceptional dimension stability and low binding characteristics of CA filtration membranes provides higher throughputs than competitive offerings and reduces the amount of filter changes needed during solution filtering. Recently, CA film has been used in various photographic elements because it is tough and flame retardant. In addition, CA film is frequently used in the liquid crystal display devices as a protective film of a polarizing plate or a color filter, where liquid crystal controls transmission of light according to the voltage generated by the electrodes in the pixels .
Due to its improved processability and compatibility [120,121], it is expected that CA can be compounded with other synthetic polymers (such as PE, PP, PC, or PVC) in manufacture of biocomposites or FRC, although not much work has been reported by far.