Abstract
Cellulose-based films can potentially replace non-biodegradable plastic films in various applications such as food packaging. In this work we produced and studied films made of mixtures of chemical pulps and catalytically oxidized microfibrillated cellulose. The films were prepared on a support which was then soaked in solutions of CaCl2 and MgCl2 to exchange the sodium ions originally present in the film to divalent metal cations. We assumed that the electrostatic interaction of the anionic pulp fibers and the fibrils with Ca2+ and Mg2+ would promote internal bonding of the fiber-fibril network that would then reflect positively on the film properties. The immersion of the wet film into aqueous CaCl2 or MgCl2 solidified the film with time. When the solidified films were dried with an excess of the salt, elastic, skin-like materials were formed. Rewetting in water and redrying the materials produced paper-like films with improved mechanical properties in comparison with films prepared without the divalent cation salts. SEM imaging of the fracture surfaces provided support for the increased internal film strength by the divalent cations. The new knowledge on their role could be utilized in tailoring cellulosic film properties for specific uses.
Highlights
Renewable and biodegradable polymers, such as cellulose, starch, chitosan, and hemicelluloses have recently been studied for many applications (Fang et al 2000; Xu et al 2005; Durango et al 2006; Nakagaito and Yano 2008; Escalante et al 2012)
In this paper we report on fixing of wet films of hardwood kraft pulps and oxidized microfibrillated cellulose (MFC) (Na? form) with calcium and magnesium salts as a means to improve the strength of the sheets
The sodium counter ions in MFC/ pulp are substituted for Ca2? or Mg2? by the actions of diffusion, ion exchange and the Donnan effect that favors bonding of divalent cations on the anionic MFC/pulp
Summary
Renewable and biodegradable polymers, such as cellulose, starch, chitosan, and hemicelluloses have recently been studied for many applications (Fang et al 2000; Xu et al 2005; Durango et al 2006; Nakagaito and Yano 2008; Escalante et al 2012). Wet films of HT BHKP/MFC were immersed in 0.3 M MgCl2 or 0.3 M CaCl2 solutions for 3 h before freeze drying of the films. Form) and pulp is first spread on a support and immersed in an aqueous solution of a divalent metal salt, such as CaCl2 or MgCl2.
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