Cellulose (2019) 26:959–970
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Furthermore, FCM can be used to measure the fluorescence of particles at different wavelengths, which is very useful when combined with addition of selective dyes to the samples. In the field of pulping and papermaking, FCM has so far been used to analyze colloidal wood pitch, bacteria, coated broke, precip- itated oxalate, various filler particles, and polyelec- trolyte complexes (Va¨ha¨salo et al. 2003; Lindberg et al. 2004; Va¨ha¨salo and Holmbom 2005; Strand et al. 2013; Ha¨a¨ra¨ et al. 2014; Strand et al. 2018). With FCM techniques, it is possible to analyze particles in suspension directly without complicated pre-treat- ments. The strengths of the FCM technique are short analysis times, as well as insight into the behavior and interactions of particle populations in aqueous suspensions. The aim of this study was to obtain new insights about CNCs in suspension by FCM analyses. Raw materials that are available in large quantities world- wide, i.e. different kraft pulps, were chosen for this CNC study. An additional aim was to determine if eucalyptus, birch, or softwood kraft pulp was most suited for continued CNC production.
previously published procedure (Strand et al. 2018). The lipophilic extractives from thermomechanical pulp from Norway spruce were extracted with hexane. Colloidal wood pitch emulsions were prepared from the hexane extracts according to a previously pub- lished procedure (Sundberg et al. 1996b).
Methods
Analysis of carbohydrates
Analysis of carbohydrates in the dry kraft pulps was performed through methanolysis, silylation, and gas chromatographic analysis (GC) (Sundberg et al. 1996a). Resorcinol was added as internal standard for quantification. The analysis of sugar units was conducted with a long column GC (HP-1, 25 m 9 0.20 mm) with split injection, equipped with a flame ionization detector (FID). The methodology gave information mainly about the amount of non- crystalline carbohydrates in the fiber samples. The determined sugar unit contents of the different kraft pulps are presented in Table 1.
Preparation of cellulose nanocrystals (CNCs)
Materials and methods
CNCs were prepared according to previously pub- lished methodology, with some modifications, and the concentration of acid during hydrolysis was chosen accordingly (Revol et al. 1994; Dong et al. 1998; Beck-Candanedo et al. 2005). Grinding of the differ- ent kraft pulps was performed with a laboratory-sized mill (IKA MF 10). The kraft pulp powders were freeze-dried for a few days. 5 o.d. g of the freeze-dried pulp was weighed into a glass bottle, and 250 mL of 64 wt % H 2 SO 4 was added to the pulp. The pulp suspension was agitated by magnetic stirring for 30 min at room temperature in order to homogenize the mixture. The temperature of the suspension did not increase, since the pulp was completely dry when the H 2 SO 4 was introduced. The glass bottle was sealed and placed in a 45 C water bath for 25 min. The glass bottle was shaken manually continuously during this time, in order to further homogenize the mixture. The hydrolysis was stopped after 25 min by diluting the suspension 10 9 with distilled water. Secondary aggregation of particles took place slowly in the diluted suspension. The suspension was allowed to sediment over night at room temperature.
Materials
ECF-bleached, once-dried softwood kraft pulp was obtained from a Finnish pulp mill. The softwood kraft pulp was prepared from a mixture of Nordic pine and spruce. ECF-bleached, once-dried birch kraft pulp was obtained from a Finnish pulp mill. ECF-bleached, once-dried eucalyptus kraft pulp was obtained from a pulp mill in Uruguay. Concentrated, analytical reagent grade sulfuric acid was obtained from Fisher Scientific UK. TheH 2 SO 4 was diluted with distilled water to 64 wt % and was allowed to cool down prior to use. Nile red was obtained from Tamro (Vantaa, Finland). The nile red was directly dissolved in methanol to a concentration of 10 ppm. A commercially prepared and available cationic starch (Raisabond 15) was obtained from Chemigate, Finland as a pre-boiled slurry. Sodium carboxymethyl cellulose, M w 250, degree of substitution 0.9, was obtained from Acros Organics. Polyelectrolyte complexes (PECs) were prepared from the cationic starch and CMCs at a polycation to polyanion ratio of 5.5:1 according to a
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