Abstract
The layer-by-layer (LbL) technology was used to adsorb polyelectrolyte multilayers consisting of cationic polyethylenimine (PEI) and anionic sodium hexametaphosphate (SHMP) onto cellulose fibers in order to enhance the flame-retardancy and tensile strength of paper sheets made from these fibers. The fundamental effect of PEI molecular mass on the build-up of the multilayer film was investigated using model cellulose surfaces and a quartz crystal microbalance technique. The adsorption of a low (LMw) and a high molecular weight (HMw) PEI onto cellulose fibers and carboxymethylated (CM) cellulose fibers was investigated using polyelectrolyte titration. The fibers were consecutively treated with PEI and SHMP to deposit 3.5 bilayers (BL) on the fiber surfaces, and the treated fibers were then used to prepare sheets. In addition, a wet-strength paper sheet was prepared and treated with the same LbL coatings. Thermal gravimetric analysis of LbL-treated fibers showed that the onset temperature for cellulose degradation was lowered and that the amount of residue at 800 °C increased. A horizontal flame test and a vertical flame test were used to evaluate the combustion behavior of the paper sheets. Papers prepared from both cellulose fibers and CM-cellulose fibers treated with HMw-PEI/SHMP LbL-combination self-extinguished in a horizontal configuration despite the rather low amounts of adsorbed polymer which form very thin films (wet thickness of ca. 17 nm). The tensile properties of handsheets showed that 3.5 BL of HMw-PEI and SHMP increased the stress at break by 100% compared to sheets prepared from untreated cellulose fibers.
Highlights
Cationic polyelectrolytes play an important role in papermaking as they improve the retention of fines and added mineral fillers
It has been shown that the saturation adsorption of weak cationic polyelectrolytes used in paper making, such as polyethylenimine (PEI), onto cellulose fibers and silica is significantly affected by the molecular weight, concentration of solution, pH and salt concentration used during the adsorption (Alince et al 1996; van de Cellulose (2018) 25:2691–2709
To quantitatively determine the build-up of the LbLs, polyelectrolyte multilayers composed of cationic PEI and anionic sodium hexametaphosphate (SHMP) were formed on the model cellulose surfaces
Summary
Cationic polyelectrolytes play an important role in papermaking as they improve the retention of fines and added mineral fillers. They have been used to increase fiber interactions in fiber/fiber joints to increase paper strength. Wagberg et al investigated the adsorption of cationic polyelectrolytes onto cellulose-rich fibers and the effects of the porous structure of the cell wall (with pores ranging from 1 to 30 nm) (Wagberg and Hagglund 2001), and it has been shown that the adsorbed amount (i.e., mg/g fiber) is highly dependent on the molecular mass of the polymer (Alince 1990; Haggkvist et al 1998; Stone et al 1968; van de Ven and Alince 1997; Wagberg and Hagglund 2001). It has been shown that the saturation adsorption of weak cationic polyelectrolytes used in paper making, such as polyethylenimine (PEI), onto cellulose fibers and silica is significantly affected by the molecular weight, concentration of solution, pH and salt concentration used during the adsorption (Alince et al 1996; van de Cellulose (2018) 25:2691–2709
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