Recycling of Paper Waste for Conversion into Cellulose, Cellulose Derivatives, Cellulose Composites, Nanocellulose, and Nanofibrillated Cellulose
Cellulose is the major constituent of paper, paperboard, card stock, and the main ingredient of textiles made from cotton, linen, and other plant fibres (Conte 2016).
Studies of the transformation of paper waste into cellulose, cellulose derivatives, cellulose composites, nanocellulose, and nanofibrillated cellulose occupy a crucial and special place among the directions of its processing. Waste paper is particularly attractive as feedstock for isolation of cellulose fibres due to high content of cellulose and to the fact that cellulose fibres from paper waste exhibit high flexibility and good mechanical properties (Conte 2016).
Generally, the isolation of the cellulosic components of the municipal solid wastes is obtained through the transformation under heat and pressure of the whole cellulose biomass into a fairly uniform material that is separated from most of the metals, plastics, textiles, and glass by vibratory or trommel screening. Some of these contaminants are removed from the wet or dry material by stoner processing (Conte 2016).
Cellulose and Cellulose Derivatives Produced from Waste Paper
The extracted cellulose from waste paper has been often used as a component with valuable properties in different composite systems, despite the fact that sophisticated equipment and reagents are required in some cases. Zhang et al. recycled old newspaper (ONP) as a typical waste paper to prepare natural fibre composites with capping agent maleic anhydride grafted high-density polyethylene (MAPE) (Zhang W. et al. 2019). The dried ONP strips were treated with methyltrichlorosilane for hydrophobization at 60°C, then washed to pH neutral, and dried at 120°C. The treated strips mixed with MAPE pellets in high-speed mixer at 100°C were subjected to further extrusion-pelletization processes to obtain the pelletized composites. ONP/ MAPE composites prepared with fibres modified with 4% (v/w) MTCS showed the best mechanical properties and satisfactory water-resistance properties.
Guo et al. studied the production of recycled cellulose fibres from waste paper (newsprint fibres and kraft fibres) using ultrasonic wave processing (Guo et al. 2015). It was shown that the ultrasonic cavitation effect was feasible for the preparation of the secondary fibres. The fibres exhibited high values of water absorption and surface area. Due to these properties, the recycled cellulose fibres after processing fulfilled several technical indexes; therefore, they could be considered as a filling material for used in cement-based materials.
Cellulosic fibres can be prepared from different sources of recycled waste papers (newspapers, magazines, and cardboard). Hospodarova et al. showed that unbleached recycled cellulosic fibres w'ere obtained right after repulping in the raw state w'ithout further treatment (Hospodarova et al. 2018). These fibres were grey coloured and contained 80% of cellulose. In the amount of 0.5% of the weight of other components (fillers and binders), they were used as an additive in cement composites together with other components. The cellulose fibres positively affected the physical properties of fibre/cement composites.
Cellulosic waste materials, such as waste printing paper and cardboard boxes, can be successfully converted into nanostructured SiO, ceramics and carbon/SiO, nanocomposites by submersing cellulosic materials into silica sol, followed by calcination at 550°C in air and nitrogen, respectively (Pang et al. 2011). Waste paper was cut into smaller pieces and underwent the initial maceration process to disperse the cellulosic fibres. After that, the macerated waste paper fibres were ground into powdery form using a blender. Once residual lignin had been removed, conversion of cellulosic samples into nanostructured ceramics was performed by handling in silica sol. To obtain the nanostructured SiO, ceramics with defined and specific microstructure, the samples were heated at 550°C and then cooled. This method provided a cost- effective synthesis approach for the preparation of nanostructured ceramics and nanocomposites.
One more application for recycled cellulose fibres was suggested in the paper of Ma and co-workers (Ma et al. 2016). High-strength fibres have been prepared using cellulosic waste, A4 copy paper sheets, cardboard (fluting board mill), and the ionic liquid l,5-diazabicyclo[4.3.0]non-5-ene-l-ium acetate as a solvent. Cellulosic waste materials have been dissolved, and solutions with visco-elastic properties, suitable for dry-jet wet fibre spinning, have been obtained. The resulting samples were cost- efficient fibres with high tensile strength and Young’s modulus. The advantage of the process was that carbohydrates were preserved almost entirely. Bleaching was possible but not necessary as the resulting colour could serve as a natural dye in the garment production.
Liu et al. studied the possibility to recycle mixed office waste paper, which is an available and inexpensive source of high-quality bleached chemical fibre with improved drainability (Liu et al. 2012). Firstly, the waste paper was deinked and the pulp suspensions were prepared by disintegrating to separate the fibres completely and in the same time without considerable changing of their structure. Then, the pulp suspensions were warmed and enzyme endoglucanase was added to the suspensions. The optimal pretreatment conditions were improved on the basis of the simulated model of enzymatic reaction. The obtained lignocellulose fibres had a high surface area and showed a positive impact on the drainability of cardboard. Enzymatic hydrolysis of the waste paper fractions with different mesh (from 80 to 180) was also supplied using enzymatic treatments with commercial cellulase ‘Celluclast’ or cellobiase enzyme ‘Novozyme’ (Li et al. 2015). Waste paper pulp was treated with enzymes for 96 h and after adding of sodium acetate buffer the solid was extracted and dried. The BET specific area of the obtained pulps was the highest for the mixed fraction (80-180 mesh); however, the presence of lignin and ash in the fraction with 180 mesh inhibited the effect of hydrolysis.
Hasan and Sauodi extracted pure cellulose amounting 17.4%, 20%, and 18.2%, respectively, from agricultural and industrial waste sources: rice husk, waste office paper, and sugar cane via fast and simple technique (Hasan and Sauodi 2014). Cellulose from waste paper exhibited crystallinity 47.7% and FTIR confirmed that it was of a high purity.
The cellulose extracted from waste paper can be derivatized to its esters and other compounds. Unlit obtained carboxymethylcellulose (CMC) with commercial grade from the recycled newspaper (Unlii 2013). Cellulose was recovered from newspapers under oxidative alkaline conditions and the recovery was determined as 75-90% (w/w) of starting material. Degree of substitution of CMC was between 0.3 and 0.7% and 84-94% CMC content.
All of the above in this chapter is only a part of the existing data on the conversion of waste paper to cellulose, its derivatives, and composites. However, this shows that the methods of obtaining these materials are very diverse and differ both in complexity of implementation and in their cost. In addition, they have not been exhausted and new advanced technologies are likely to be developed in the future to produce valuable cellulose-containing materials.
