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Cellulose (2020) 27:7215–7225
such as a halide salt of quaternary ammonium or a phosphonium cation along with a hydrogen bond donor (HBD) (e.g., urea, glycerol, or ethylene glycol), to form a mixture that exhibits a notably lower melting point than either HBA or HBD. In the present work, a DES system based on choline chloride and urea (Singh et al. 2012) was used as a pretreatment to liberate CNF from recycled boxboard using mechanical grinding. The CNF obtained from a secondary cellulose source were further used as reinforcements in board sheets. CNF produced directly from recycled board without any chemical treatments were used as the reference additives. Nanopapers were produced from the CNF to evaluate their strength properties. Moreover, the work evaluated the optimal grinding level in CNF produc- tion to maximize the energy consumption and costs of manufacturing CNF for use as board reinforcements.
CNF have both amorphous and crystalline compo- nents, and they form of a web-like structure (Lavoine et al. 2012). The mechanical fibrillation process causes perma- nent changes in the cellulose fiber structure, and it increases the bonding ability of cellulose by modify- ing the morphology and reducing the size of the fibers (Kamel 2007; da Costa Correia et al. 2016). Thus, some previous studies have shown that CNF can notably improve the mechanical properties of paper or board. It has been found that tensile strength and elastic modulus can be improved significantly (Erik- sen et al. 2008; Hii et al. 2012; Sehaqui et al. 2013; Gonza´lez et al. 2013; Missoum et al. 2013; Djafari Petroudy et al. 2014; Hietala et al. 2016). High tensile strength and tensile stiffness contribute to the stacking strength of corrugated paperboard by reducing the risk of box wall bulging; thus, they are desired properties of board applications. The global increase in packaging board and fiber products consumption has yielded a large amount of potential secondary cellulose raw materials that con- tribute to 25–40% of municipal solid waste (Nour- bakhsh and Ashori 2010). Recycling these fiber sources and their use as raw materials for new sustainable products can preserve forest resources and minimize other environmental impacts. More- over, fibers from recycled paper and packaging are relatively affordable and widely available. Therefore, they offer an appealing source for the production of packaging materials and novel green materials, such as CNF. The use of bio-based and recycled fiber sources to create sustainable packaging materials to replace plastics derived from fossil oil resources promotes the emerging trend of mitigating the carbon footprint of materials. The successful liberation of nanofibrils require typically rigorous mechanical treatments due to the strong hydrogen-bonded structure of cellulosic mate- rials. Thus, numerous different chemical pretreat- ments have been used to loosen the rigid structure of cellulose. Deep eutectic solvents (DESs) belong to the most promising group of novel green chemicals to enable efficient CNF production (Selka¨la¨ et al. 2016; Li et al. 2017, 2018; Sirvio¨ 2018; Ojala et al. 2018). DESs can be derived from biodegradable and readily available green compounds that have a low toxicity (Sirvio¨ et al. 2015). DESs are typically synthesized by complexation of the hydrogen bond acceptor (HBA),
Materials and methods
Raw material and chemicals
Chemicals used
Urea (97%) and choline chloride ( [ 98%) for DES were purchased from Borealis (Austria) and Algry Quimica (Spain), respectively. All chemicals were used as delivered, without any further purification. In the dilutions and CNF production, deionized water was used throughout the experiments.
Raw material
Recycled boxboard was obtained directly from board- container collections, and it was used as the raw material for the board sheets and the production of the CNF. The boxboard was first pulped without any additional chemicals using a Kenwood Chef Titanium XL pulper (UK) with a nominal rotor power of 1700 W, which has an operating principle similar to that of the Hobart pulper, at a consistency of 15% using a temperature of 45 C. Pulper using a planetary type mixing during pulping procedure. The mixer and pulping bowl were manufactured in stainless steel. Pulping time was adjusted to 10 min and rotor speed 2 (i.e., * 250 rpm). After pulping, the recycled box- board pulp was washed and screened using a Somerville screen (Lorentzen and Wettre, Sweden).
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