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Rapid Development of Wet Adhesion between Carboxymethylcellulose Modified Cellulose Surfaces Laminated with Polyvinylamine Adhesive Emil Gustafsson,†,‡,§ Robert Pelton,*,§ and Lars Wågberg†,‡ †
Wallenberg Wood Science Center, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡ Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden § Department of Chemical Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada S Supporting Information *
ABSTRACT: The surface of regenerated cellulose membranes was modified by irreversible adsorption of carboxymethylcellulose (CMC). Pairs of wet CMC-modified membranes were laminated with polyvinylamine (PVAm) at room temperature, and the delamination force for wet membranes was measured for both dried and never-dried laminates. The wet adhesion was studied as a function of PVAm molecular weight, amine content, and deposition pH of the polyelectrolyte. Surprisingly the PVAm−CMC system gave substantial wet adhesion that exceeded that of TEMPO-oxidized membranes with PVAm for both dried and never-dried laminates. The greatest wet adhesion was achieved for fully hydrolyzed high molecular weight PVAm. Bulk carboxymethylation of cellulose membranes gave inferior wet adhesion combined with PVAm as compared to CMC adsorption which indicates that a CMC layer of the order of 10 nm was necessary. There are no obvious covalent crosslinking reactions between CMC and PVAm at room temperature, and on the basis of our results, we are instead attributing the wet adhesion to complex formation between the PVAm and the irreversibly adsorbed CMC at the cellulose surface. We propose that interdigitation of PVAm chains into the CMC layer is responsible for the high wet adhesion values. KEYWORDS: wet adhesion, wet strength, cellulose, polyvinylamine, carboxymethyl cellulose, polyelectrolyte
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groups8 and (2) the irreversible adsorption of CMC onto cellulose by treatment at high temperature and in the presence of calcium ions.9 Herein we study the wet adhesive properties of adsorbed CMC together with PVAm and compare the pretreatment to carboxymethylation and TEMPO oxidation. Anionic CMC can, despite the expected electrostatic repulsion, be irreversibly adsorbed onto negatively charged cellulose.9 The adsorption is significantly improved by a low degree of substitution (DS) of the CMC, high temperature, and high ionic strength during the adsorption.10 Divalent Ca2+ has been demonstrated to be especially effective. 9,11 The mechanism for the interaction between CMC and the cellulose surface is not yet fully understood, but it has been suggested to be a result of cocrystallization of the CMC onto the fibers due to the strong chemical similarity of the biopolymers.9 The Ca2+ ions that facilitate the adsorption by screening the negative charges on cellulose and CMC have also been claimed to form complexes with the CMC which are adsorbed on the cellulose surface.11 The adsorbed CMC layer is highly hydrated which
INTRODUCTION Poor adhesion between wet lignocellulosic surfaces is one of the challenges influencing the use of forest based materials both in traditional paper/packaging applications and as replacements for nonrenewable materials. In all types of applications polymeric adhesives are required to create significant wet cellulose-to-cellulose adhesion.1 Currently, polyamideamine epichlorohydrin (PAE) resins are widely used and are often added as polyelectrolyte complexes with carboxymethylcellulose (CMC).2 With drying and heating, PAE cross-links and grafts onto carboxyl groups in CMC and on fiber surfaces, giving moisture resistant fiber−fiber joints.3,4 Other mechanisms, such as polymer mixing across interfaces,5 can also be of large importance for the wet strength development, especially for initial wet strength, but have neither been considered nor quantified. In an effort to replace PAE, we have previously demonstrated that high molecular weight polyvinylamine (PVAm) gives excellent wet adhesion if the fibers are slightly oxidized to give aldehyde groups that, with drying, covalently couple to the primary amines on PVAm.6,7 In this case, the oxidation serves to activate the cellulose surfaces for adhesion. Two other approaches to cellulose surface activation have been described: (1) carboxymethylation to give surface carboxyl © XXXX American Chemical Society
Received: May 12, 2016 Accepted: August 23, 2016
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DOI: 10.1021/acsami.6b05673 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces has been shown both on cellulose fibers12 and on cellulose model surfaces using quartz crystal microbalance.13 CMC modification of fibers has been demonstrated to improve dry strength of paper in both lab12 and pilot scale.14 Laine et al. has shown that irreversible adsorption of CMC improves the effectiveness of cationic PAE additives. This was attributed to increased PAE adsorption and cross-linking reaction with the high concentration of carboxyl groups on the CMC.15 We have previously also shown that carboxymethylcellulose (CMC) in combination with PVAm can improve the wet adhesive strength both in the form of polyelectrolyte complexes16 and through layer-by-layer deposition.17 It was however concluded that the CMC itself did not improve the strength; it merely assisted in adsorbing more adhesive cationic PVAm onto the cellulose. However, the cohesive strength of the CMC−PVAm complex was strong enough to give improved wet adhesion as compared to a case where only PVAm was used but no real molecular explanation to this could be given based on the achieved results. The standard approach for evaluating wet strength enhancing polymers is to prepare paper sheets from surface modified fibers and to evaluate the wet strength of the sheets. However, using this method, it is hard to evaluate underlying mechanisms, due to the complex mechanics of fiber network failure and the fact that the amount of polymer adsorbed to the fibers cannot be controlled. In an effort to isolate the adhesion phenomena from papermaking and testing, we have developed a model setup based on wet peel delamination of two regenerated cellulose films that can be surface modified and where the amount of applied polymer can be controlled.6,18 In this work the wet strength of dried laminates was evaluated as well as the initial wet strength as a function of water content for laminates that had never been dried.
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Following the CMC attachment the membranes were washed with water until the conductivity was
