Surface modification of cellulose fibers with layer-by-layer self-assembly of lignosulfonate and polyelectrolyte: effects on fibers wetting properties and paper strength

To convert the hydrophilic cellulose fiber into hydrophobic, multilayers composed of cationic polyacrylamide (CPAM) and lignosulfonate (LS) were constructed on cellulose fiber surface using layer-by-layer (LBL) self-assembly technique. The presence of CPAM/LS multilayers were validated by zeta potential, X-ray photoelectron spectroscopy and atomic force microscopy (AFM). It was found that potential of fiber surface inversed after deposition of each layer, the contents of characteristic elements (i.e. S and N) of CPAM/LS multilayers increased with increasing bilayer number, furthermore, the calculated surface LS content increased linearly as a function of bilayers. AFM phase images indicated that the cellulose microfibrils on fiber surface were gradually covered by LS granules, resulting in an increase in fiber surface roughness as self-assembly proceeded. The wetting properties of modified cellulose fibers were detected by dynamic contact angle measurement. The results showed that the initial water contact angle gradually increased and the attenuation rate of the contact angle gradually decreased with the number of bilayers, suggesting that the controllable hydrophobicity of cellulose fiber can be achieved depending on the number of bilayers. It also showed that the polyelectrolyte presented in the outermost layer significantly influenced the wetting properties of cellulose fibers, and a higher hydrophobicity was observed when LS was in the outermost layer. Moreover, tensile strength test was performed on the handsheet prepared from LBL modified fibers to evaluate the effect of CPAM/LS multilayers on strength property of cellulose fiber networks. The tensile index of handsheet prepared from fibers modified with a (CPAM/LS)₅ multilayer increased by 12.4% compared with that of handsheet prepared from original fibers. The print density of handsheet increased with the number of bilayers, suggesting that printability of the handsheet was improved by constructing CPAM/LS multilayers on cellulose fiber surface. This strategy will have a positive impact and potential application value in printing process control of cellulose fiber-based products.