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composed of 1–4-linked b -N-acetyl- D -glucosamine (Fig. 1; Rinaudo 2006). Chitin is not water soluble but its N-deacetylated derivate, chitosan, is soluble in water at slight acidic conditions. Chitosan is the only pseudonatural cationic polysaccharide, with a pK a of about 6.3–6.7, and it also contains a high percentage of nitrogen (Ravi Kumar 2000; Rinaudo 2006). Chitosan is biocompatible, biodegradable and non-toxic and it is used in many application such as food, cosmetics, textiles, pharmaceuticals and nanomaterial (Ravi Kumar 2000; Rinaudo 2006; Shukla et al. 2013). The aim of this study was to enhance the tensile strength, tensile stiffness and strain at break of handsheets after restrained and unrestrained drying by spray application of different available polysac- charides onto wet handsheets. Model films from the polysaccharides were also prepared and tested in order to gain information about their mechanical properties.
2011). It is also beneficial if the additives are hydrophilic and able to form hydrogen bonds. Dry strength agents are generally water-soluble polymers, meaning that the effectiveness of the additives is largely governed by the retention of the additive to the fiber network. Recent studies have been able to circumvent the reliance on retention to some extent by performing spray additions of various additives onto already formed, wet paper sheets (Lindqvist et al. 2014; Khakalo et al. 2014; Vishtal and Retulainen 2014a; Vishtal et al. 2015). This methodology has previously been used with agar, gelatin, and combi- nations of gelatin and agar to enhance paper proper- ties. Spray additions of agar and gelatin in combination with unrestrained drying have been shown to result in high strain at break of paper. In the current study, some additional polysaccharides were selected for spray addition. Alginates are anionic unbranched block co-poly- mers of b - D -mannosyluronic acids and a - L -gulosy- luronic acids linked 1–4. The blocks can be either homopolymers of either mannosyluronic or gulosy- luronic units or a heteropolymer of both units (Fig. 1; Gacesa 1988; BeMiller and Whistler 1996). The conformation of these units are different; blocks of mannosyluronic acids will therefore form a ribbon- like structure while blocks of gulosyluronic units will form a buckled chain. Alginate readily forms hydro- gels and are used as stabilizers, gelling agents, thickening agents or as films or fibers. Guar gum is extracted from the pods of the guar plant ( Cyamopsis tetragonoloba ) (Chudzikowski 1971; BeMiller and Whistler 1996; Mudgil et al. 2014). This polysaccharide have a linear backbone of 1–4-linked b - D -mannosyranose units with 1–6-linked a - D -galactopyranos as side groups. The ratio of mannose to galactose is about 1.6–1.8:1 (Fig. 1; BeMiller and Whistler 1996; Mudgil et al. 2014). Guar gum is water soluble and has a very high molar mass. Guar gum is used as additives in e.g. food, pharmaceuticals, paper, textile, explosives, oil well drilling and cosmetics industry (Mudgil et al. 2014). Guar gums can easily be modified and cationic guar gum (guar hydroxypropyltrimonium chloride) is used in e.g. cosmetics as an emulsifier. Chitin is, after cellulose, the second most produced polysaccharide annually in the world. Chitin is found in the support material of crustaceans, as shrimp and crabs, and insects. It is a polysaccharide that is
Materials and methods
Materials
ECF-bleached, once-dried softwood kraft pulp was obtained from a Finnish pulp mill. Alginic acid sodium salt from brown algae (alginate) was obtained from Sigma–Aldrich. According to the supplier, the alginate consisted of approximately 61% mannosy- luronic units, and 39% gulosyluronic units, with 19–25% carboxyl groups, and an approximate M w of 240 kDa. The alginate powder was directly dissolved in distilled water to a concentration of 1 wt% with the aid of boiling and agitation. Chitosan (medium molecular weight) was obtained from Sigma–Aldrich. According to the supplier, the M w of the chitosan was 190–310 kDa, and the degree of deacetylation was 75–85%. The cationic charge density of the chitosan was 4.7 meq/g, determined by polyelectrolyte titration using a particle charge analyzer (Mu¨tek PCD 03, Mu¨tek Analytic GmbH, Germany; BTG, Switzerland). The chitosan powder was dissolved in 1–1.5% acetic acid to a concentration of 1 wt% with the aid of boiling and agitation. Cationic guar gum (guar hydroxypropy- ltrimonium chloride) was obtained from Making Cosmetics, USA. The reported viscosity of a 1% solution (20 rpm, 20 C) was 3000–4000 cP, accord- ing to the supplier. The charge density of the cationic guar gum was 0.8 meq/g, determined by
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