Abstract
This study aims to optimize strong acid hydrolysis-based production of cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) from pre-extracted and fully bleached kraft pulp of loblolly pinewood, the most abundant and commercially significant softwood species in southeastern United States. The effect of four parameters, including acid concentration, temperature, duration and pulp particle size, on the yield and properties of CNCs was investigated using the central composite design (CCD) of response surface methodology (RSM) for process optimization. While CNC yield was significantly affected by acid concentration and hydrolysis temperature and was adequately explained by an empirical model, none of the characteristic properties of CNCs, including crystallinity index, surface charge and particle size, displayed any strong correlation to the process parameters within the experimental ranges tested. At different hydrolysis severities, we not only analyzed the waste streams to determine the extent of holocellulose degradation, but also evaluated the properties of leftover partially hydrolyzed pulp, called cellulosic solid residues (CSR), to gauge its potential for CNF production via mechanical fibrillation. Conditions that maximized CNC yields (60% w/w) were 60% acid concentration, 58 °C, 60 min and 40 mesh particle size. Twenty percent (w/w) of the pulp was degraded under these conditions. On the other hand, conditions that maximized CSR yields (60% w/w) were 54% acid, 45 °C, 90 min and 20 mesh particle size, which also produced 15% CNCs, caused minimal pulp degradation (< 5%) and imparted sufficient surface charge such that CSR was easily microfluidized into CNFs. Therefore, the strong acid hydrolysis process could be tuned to maximize the production of cellulose nanocrystals and nanofibers and obtain two products with different properties and applications through the process optimization.
Highlights
Growing demands of the world population are necessitating that we gradually reduce our dependence on nonrenewable petroleum-based materials and transition to Kandhola et al Bioresour
Of the 26 experimental runs conducted in our study, both Cellulose nanocrystal (CNC) and cellulosic solid residues (CSR) were recovered in only 13 runs, whereas for the rest, the CSR fraction was below recoverable limits
Depending on the severity of reaction conditions, CNC yields varied between 0% and 52%, and CSR yields varied between 0% and as high as 85% (Table 3)
Summary
CNCs are rigid, rodlike crystals, with diameters in the range of 5–20 nm and lengths in the range of 200–500 nm, characterized by high aspect ratio, low density, high tensile strength and stiffness, high surface area and modifiable surface chemistry (Habibi et al 2010). CNFs have similar diameters but can extend up to a few micrometers in length, resulting in much higher aspect ratios and formation of flexible web-like network structures (Jonoobi et al 2015). Wood is the most widely and cheaply available raw material with high cellulose content (Brinchi et al 2013; Jonoobi et al 2015; Lee et al 2014; Sacui et al 2014)
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