Cellulose (2018) 25:3781–3795
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FCM has so far been used to analyze colloidal wood pitch, bacteria, coated broke, precipitated oxalate, and filler particles (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). With FCM techniques, it is possible to analyze particles in suspension directly without complicated pre-treatments. The strengths of the FCM technique are short analysis times, as well as insight into the behavior and interactions of particle populations in aqueous suspension. The aim of this work was to gain additional knowledge about PECs formed from commonly used papermaking additives, by analysis with flow cytom- etry. The light scattering properties of these particles might reveal important information about their prop- erties and structure. The additives chosen for the study were different cationic starches, polyDADMAC and carboxymethyl cellulose (CMC). Cationic starches are the most commonly used additive in papermaking (Fornue et al. 2011). Studies have shown that CMC greatly increases both the strength and stiffness of paper, which makes it a very interesting papermaking additive (Beghello et al. 1997; Strand et al. 2017). An additional aim of this work was to determine the hydrophobicity of PECs, and to study the time dependent stability of different PEC mixtures, since contact times between polyelectrolytes in papermak- ing can vary between fractions of a second to several hours.
polyDADMACs were received as viscous solutions with dry contents of about 26% and molar masses of 100–200 and 200–350 kDa, respectively. The poly- DADMAC was diluted with distilled water to a concentration of 0.1% under agitation. Poly(vinyl sulfate) potassium salt (KPVS) was obtained from Wako. The KPVS salt was dissolved in deionized water under agitation. Hexadimethrine bromide (poly- brene) was obtained from Sigma. The polybrene was dissolved in deionized water under agitation. Nile red was obtained in dry form from from Sigma-Aldrich, USA. The Nile red was dissolved in methanol to a concentration of 10 ppm. A variety of commercially available mineral par- ticles was obtained. Prismatic precipitated calcium carbonate (PCC p) was obtained from Specialty Minerals, New York, USA, as a slurry, which was diluted with pH-adjusted distilled water (pH 8). Scalenohedral precipitated calcium carbonate (PCC s) was obtained from Specialty Minerals Nordic, Anjalankoski, Finland as a slurry, which was diluted with pH-adjusted distilled water (pH 8). Commer- cially available ground calcium carbonate (GCC) was obtained as a slurry, and was diluted with distilled water. Kaolin was obtained from Imerys Minerals, Par Cornwall, UK, in powder form and was dispersed in distilled water. Three different grades of commercially available bentonites were obtained from Clariant Produkte, Germany, and dispersed in distilled water. Cellulose nanocrystals (CNC) were prepared by H 2 SO 4 hydrolysis of birch kraft pulp, with subsequent purification steps (Beck-Candanedo et al. 2005). Cellulose nanofibrils (CNF) were produced by TEMPO oxidation of birch kraft pulp followed by mechanical defibration (Shibata and Isogai 2003).
Materials and methods
Materials
Three different cationic starches were received from Chemigate as pre-boiled slurries; Raifix 01015 SW, Raifix 25035, and Raisabond 15. The cationic starch slurries were diluted with distilled water to 0.1% solutions under agitation. Sodium carboxymethyl cellulose (CMC), M w 700 kDa and DS 0.80–0.95, was obtained from Aldrich as dry powder. Addition- ally, three 250 kDa CMCs of varying DS were obtained from Acros Organics as dry powders. The different CMCs were dissolved in distilled water to a concentration of 0.1% with the aid of agitation and boiling, until clearly dissolved. Two different polydi- allyldimethylammonium chloride (PolyDADMAC) solutions were obtained from Aldrich. The
Methods
Charge titration
The anionic or cationic charge of the different polyelectrolytes was analyzed by polyelectrolyte titration with a particle charge detector (Mu¨tek PCD 03, Mu¨tek Analytic GmbH, Germany; BTG, Switzer- land) coupled to a titration device (736 GP Titrino, Metrohm Ltd., Switzerland). The cationic polyelec- trolytes were titrated with KPVS, and the anionic polyelectrolytes were titrated with polybrene with an M w of approximately 8 kDa (Sundberg et al. 2000).
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