Production of CMCs from Non-Conventional Precursor Materials

Aside from conventional plant-based cellulosic precursors, some waste materials of the textiles industries (e.g., knitted rag, cotton linters) as well as regularly used household and office products (e.g., office waste papers, paper sludge, waste textiles, etc. are available almost free of charge, which can be utilized for the synthesis of CMCs. Production of CMC and its various composites for numerous applications from these waste materials reduces the production cost and plays a vital role in preventing environmental pollution.

For instance, cotton linters are a waste product of cotton cleaning factories. In 1954, Ott et al.  found that almost 3–5% of cotton fibers are wasted as cotton linters, which contain a high percentage of cellulose (i.e., about 90%) in their raw states. Hence, they can synthesize different grades of CMC products for numerous applications. A few years back, Jahan et al. (2007) reported a facile synthesis of water-soluble CMC from cotton linters that demonstrated an appreciable DS value with high viscosity. In another study, Hivechi et al. (2015) reported a cost-effective, environmentally friendly process for the synthesis of CMC from cotton linters using ultrasonic and microwave radiation that facilitated the conventional approach (i.e., alkylation-etherification) of CMC production.

Fakrul Alam and Mondal (2012)synthesized carboxymethylated cellulose from the knitted rag, which is a common waste material of almost all textile industries and contains a high percentage of ɑ-cellulose (e.g., 95–98%). To investigate the effect of multiple carboxymethylation steps on the CMC quality and grade, the authors conducted consecutive seven-step carboxymethylation using the same experimental conditions and chemicals listed in the Supplementary Materials (Table S2). Multiple carboxymethylation steps of the crude cellulose demonstrated high yield (%) of CMC, DS, molecular weight, and water solubility. More specifically, in the first carboxymethylation, the DS value and yield of CMC obtained were only 0.91 and 360%, respectively. In contrast, after repeating the process seven times, these values reached up to 2.84 and 1494%, respectively.

Recently, Li et al. (2020)synthesized CMC from office waste papers to fabricate an environmentally friendly, low-cost, crust-dust suppressant. However, a few years earlier, Joshi and his co-workers (2015)demonstrated the feasibility of CMC synthesis from mixed office waste papers after deinking and pulping them. It is worth mentioning that deinking is an ancient process for reusing waste papers. The ink particles are removed from fiber surfaces either by chemical treatment methods such as alkali boiling. The dispersed ink particles are removed from the fiber suspensions by the floatation method]. Therefore, a thousand tons of waste paper daily obtained from various offices and industries can be utilized as a plentiful precursor of CMC. Table S2 in the Supplementary Materials assembles some of the most promising research works based on the synthesis of CMCs from such waste cellulosic materials. We have summarized their grave essences in brief.