Nano-lignocellulose from recycled fibres in coatings from aqueous and ethanolic media: effect of residual lignin on wetting and offset printing quality

Abstract

Nano-lignocellulose (NLC) and lignin-free nanocellulose (nano-holocellulose, NHC) were used in paper coating to investigate their effect on coating layer quality and offset printing. The NLC was produced by microfluidisation of unbleached secondary fibres while the reference NHC was obtained from the same fibre source after lignin removal (OHEPH bleaching), following the same mechanical process. TEMPO-mediated oxidation of the fibres prior to microfluidisation was applied to increase the electrostatic charge and hydrophilicity of the nanofibrils. The coatings, displaying given surface morphology and energy, were applied on Kraft, printing-grade papers at three grammage levels. The structure of the coated and uncoated (reference) papers were accessed (SEM and AFM) and IGT printing was carried out to determine the print density, print gloss, rub-off resistance, surface energy, roughness, ink transfer, dry pick resistance, water interference and set-off. The results highlight the important effect of residual lignin or type of nanocellulose on the coating layer and the development of offset printing properties. It was observed that roughness was a key factor leading to a deterioration of the print properties, predominantly affecting the NLC coating. Considering the lower hydrophilicity of NLC, an alternative dispersion with water-alcohol mixtures is proposed. By using this dispersing medium, tailorable surface coverage, surface smoothness, ink acceptance and improved printability was achieved. We show that under these conditions and compared to NHC, NLC is equally effective as a coating layer.

Coupling Nanofibril Lateral Size and Residual Lignin to Tailor the Properties of Lignocellulose Films

“…The roughness results agree with Rojo et al who also used hot pressing, but as would be expected, are in contrast with the results of Bian et al who allowed drying without hot pressing. These findings are corroborated by our earlier work in which coatings of lignocellulose fibrils were allowed to air‐dry and developed greater roughness than similar fibrils, which did not contain lignin. It was concluded that the surface roughness was closely related to the hydrophobicity of lignin.…”

“…The procedure was described by Imani et al In brief, 2 g (dry weight) of unbleached fibers (see Table S1, Supporting Information, for chemical composition analyses) was first dispersed at 1% (w/v) solids content in 200 cm 3 distilled water, to which sodium bromide (NaBr) and TEMPO were added in sequence (1.0 and 0.1 mmol g −1 based on the dry weight of fibers, respectively). The mixed suspension was stirred at 700 min −1 using an Ultra Turrax mixer (IKA, T 25 digital) for 5 min.…”

“…When the drop touched the films surface, its image was recorded with a CAM 2008 software. The average droplet receding contact angle, θ r, of each of four pure probing liquids (water, formamide, diiodomethane, and ethylene glycol) was measured (Table ) . An average of five separate analyses on the same film was used, and the resulting surface energy components (dispersive γ LW and polar γ AB ) and total solid surface energy (γ Tot ) evaluated (Table ).…”

“…Reported studies on cellulose nanofibrils (CNF), derived from different cellulose sources including bleached wood fiber, cotton, and agricultural residues show a yet untapped opportunity for uses in diverse applications such as in reinforcement of nanocomposites with thermoplastic and thermoset polymers, membranes, coatings, packaging, and film materials . Large scale production of CNF is possible by the combined effects of chemical, enzymatic, and mechanical treatments such as grinding, high‐pressure homogenization, and microfluidization …”

Using micro- and nanofibrillated cellulose as a means to reduce weight of paper products: A review

“…Before reaching a definitive conclusion, more research should focus on how the presence of lignin alters the performance and reinforcement capabilities of the MNFC in targeted papermaking applications. For example, there is evidence suggesting that even though lignin-containing MNFC could be suitable as a bulk additive in papermaking, it would make a poor coating additive compared to lignin-free MNFC (Imani et al 2019a).…”