2.5. Characterization Methods

Several analytical procedures were used to determine the chemical composition of raw CS in terms of both major (polysaccharides and lignin) and minor components: ash–TAPPI T 211 om-02, 2002 [60]; hot water extractives—TAPPI T 207 om-88 [61]; organic solvent extractives T 204 cm-97 [62]; acetone extractives (AE)—TAPPI T280 pm-99 standard (2000) [63]. While acid-insoluble lignin (AIL) and acid-soluble lignin (ASL) were determined using the sulfuric acid two stages hydrolysis method specified by NREL/TP-510-42618 method [64], the major polysaccharide components (cellulose and hemicelluloses) of the biomass and the obtained papermaking fiber were determined following an adapted procedure of that described by Sluiter et al. [65]. The adaptation involved neutralizing the hydrolysate from a G3 crucible filter to pH 5.6 before HPLC analysis.

The carbohydrates present in liquor samples were analyzed after they were treated with 4% sulfuric acid (60 min at 121 °C) according to NREL (LAP) TP-510-42623 [66]. Samples of 60 to 80 mg were suspended in 5 mL of 1 M NaOH and vigorously shaken for at least 30 min to facilitate dissolution. The complete hydrolysis was achieved by treating the samples with 4% sulfuric acid for 60 min at 121 °C. The acid treatment completes the hydrolysis of the polymeric carbohydrates extracted during HAE. Following hydrolysis, the samples were neutralized, and the concentration of monosaccharides was determined using HPLC.

The glucan, xylan, and arabinan content of the liquor were added to determine the total amount of polysaccharides (PStot). The conversion of monomer concentrations to their corresponding polymer concentrations was realized considering the ratio between the molecular weight of the anhydro-sugar unit and sugar unit (162/180 = 0.9 for C6 sugars and 132/150 for C5 sugars).

The HC recovery yield (HCRY) was then calculated using Equation (1) as the ratio between the number of polysaccharides in recovered crude HC (o.d.) and the number of polysaccharides theoretically determined in either extraction or black liquor.

where MS_i (%) is the content of individual polysaccharides (glucan, xilan, arabinan) content in the HC sample; m_HC is the mass of the HC sample, in g; C_MSi is the concentration of the individual sugar in the analyzed sample, in g/L; V is the volume of liquor sample, in L.

The solid extraction yield (SY, %) was calculated using Equation (2).

where SY (%) represents the solid yield; m_i is the o.d. weight of the initial CS biomass; m_i. is the o.d. weight of the CS after the treatment or treatment sequence.

The degree of polymerization of HC was proven by using viscosity data as described in literature [67]. In brief, the samples were dissolved in a 0.04 M cupriethylenediamine (CED) solution, and the intrinsic viscosity [η] was determined at 25 °C. The Staudinger–Mark–Houwink equation of xylan in CED is (Equation (3)) [67,68,69]:

The obtained pulps’ DP was established after determining the intrinsic viscosity (Equation (4)) in 0.5 M CED solution [70].

Following washing and refining (beating), the obtained pulps were transformed into paper sheets that were subjected to analysis of tensile strength (ISO 1924:2008) [58] and burst strength (ISO 2758:2014) [59].

The temperature and duration of extraction can be combined in a single parameter, the severity factor (SF), to reduce the total number of experiments. The SF is defined as the combination of extraction time and the temperature (Equation (5)) [71,72].

where τ is the processing time at selected temperature T.