## 4 L-MMCF Manufacturing Process
### Pulping Process
DP = M/162, with the cellulose molar mass repeat unit being estimated at 162 g/mol in a cellulose/1-butyl-3-methylimidazol chloride (BmimCI) solution [85,86]. Note that the actual DP may be higher than the measured value due to short-term oxidative damage (i.e., depolymerisation) to the pulp when exposed to the highly alkaline organic solvents during the viscometry analysis [87]. It may be possible to obtain more accurate measurements by initially removing reactive carbonyl groups via reduction with sodium borohydride (NaBH\({}_{4}\)). The kappa number indicates the amount of lignin present in the pulp as well as providing information on delignification between pulp processing steps [36]. According to TAPPI/ANSI T236 om-22, the lignin percentage can be calculated as follows [81]:  
\[\text{Lignin level (\%) = Kappa number }\times 0.15\]  
However, caution needs to be taken when performing this calculation for non-wood pulps such as hemp biomass due to variation in hexenuronic acid content; therefore, it is recommended that the conversion factor be determined once both lignin content and kappa number have been evaluated separately for non-wood pulps [88]. For L-MMCF, the kappa number should be below 5 as a high lignin content can affect the solubility of cellulose in the NMMO solvent [24, 25, 26]. Feedstocks for PHK undergo prehydrolysis in a hot, weak acid solution (e.g., deionised water at 150-180 \({}^{\circ}\)C for 60 min; 0.5% H\({}_{2}\)SO\({}_{4}\) at 150 \({}^{\circ}\)C for 90 min) prior to the Kraft pulping process to remove the hemicellulose content [62,71,82,89]. During prehydrolysis, acetyl groups are liberated via auto-hydrolysis, creating an acidic solution, which is opposite of the highly alkaline Kraft pulping process [18,82]. The acid condition reduces the molecular size of hemicellulose, allowing it to be easily removed from the pulp [71]. Prehydrolysis results in a higher alpha-cellulose pulp than just Kraft pulping alone, which is ideal for the dissolving pulp stage. However, acid prehydrolysis can result in the hydrolysis of the alpha-cellulose, causing a decrease in DP, so careful monitoring is required to ensure higher yield pulps [71,90,91]. Additionally, under acidic conditions, lignin can self-condense or react with phenol, making it more difficult to remove lignin content (i.e., delignification) during the pulping process (i.e., cooking) [82]. To avoid lignin condensation, the prehydrolysis pulp is neutralised by rinsing with water; it is then dried into a sheet for pulping. After prehydrolysis, the pulp is chipped up and soaked in a solution containing sodium hydroxide (NaOH) and sodium sulphide (Na\({}_{2}\)5) (i.e., white liquor), resulting in delignification due to the alkaline conditions and removal of extractives via saponification (i.e., Kraft pulping) [17,60]. The pH of the liquor remains greater than 13 for the duration of the treatment (2 to 4 h at 170 \({}^{\circ}\)C to 180 \({}^{\circ}\)C) [38]. This process removes 80-90% of the lignin as the lignin swells and separates into fragments (i.e., breakage of C-O-C lignin link), which are then dissolved as carboxylate or phenolate ions; this process also removes 100% of the wood extractives, residual hemicellulose, and approximately 10% of cellulose due to reduced DP [38]. Cooking times vary depending on feedstock chip size, with sawdust and smaller chips cooking faster than larger chips. Cooking times are also adjusted based on the initial characterisation, specifically the lignin and hemicellulose content. As an example, hemp past fibres reached an optimal kappa number (5) with a cooking time of 53 min and a temperature of 160 \({}^{\circ}\)C; hurd reached this kappa number with a cooking time of 105 min and a temperature of 170 \({}^{\circ}\)C [25,26]. As shown in Table 2, the lignin content in hurd is higher than in past fibre, requiring a longer cooking time. After cooking, the resulting black liquor is drained to remove dissolved lignin from the pulp in the form of dissolved sodium salts.