Hydrophobisation of lignocellulosic materials part I: physical modification

Abstract

This review is the first part of a comprehensive review of hydrophobisation of lignocellulosic materials. The purpose of this review has been to compare physical hydrophobisation methods of lignocellulosic materials. We have compared molecular physical adsorption with plasma etching and grafting. Adsorption methods are facile and rely upon the simple mixing or coating of the substrate with the hydrophobing agent. However, none of the surfactant-based methods reviewed here reach contact angles above 90°, making them unsuitable for applications where a high degree of hydrophobisation is required. Nevertheless, surfactant based methods are well suited for compatibilising the lignocellulosic material with a hydrophobic matrix/polymer in cases where only a slight decrease in the hydrophilicity of the lignocellulosic substrate is required. On the other hand, wax- and lignin-based coatings can provide high hydrophobicity to the substrates. Plasma etching requires a more complex set-up but is relatively cheap. By physically etching the surface with or without the deposition of a hydrophobic coating, the material is rendered hydrophobic, reaching contact angles well above 120°. A major drawback of this method is the need for a plasma etching set-up, and some researchers co-deposit fluorine-based layers, which have a negative environmental impact. An alternative is plasma grafting, where single molecules are grafted on, initiated by radicals formed in the plasma. This method also requires a plasma set-up, but the vast majority of hydrophobic species can be grafted on. Examples include fatty acids, silanes and alkanes. Contact angles well above 110° are achieved by this method, and both fluorine and non-toxic species may be used for grafting. Graphical abstract

Hydrophobization of lignocellulosic materials part II: chemical modification

“…As discussed in Rodríguez-Fabià et al (2022a), water swelling is a diverse concept, that is characterized by several different methods. The aim of hydrophobization is typically to reduce swelling kinetics (e. g. the time it takes to sorb water) as well as the maximal amount sorbed.…”

“…In the review Hydrophobization of lignocellulosic materials part I: physical modification (Rodríguez-Fabià et al 2022a), physical hydrophobization methods were described. It was clear that both hydrophobicity and superhydrophobicity could be obtained, although the contact angles (CAs) were measured over a short period of time.…”

“…The purpose of this review is to further discuss various molecular modifications that yield hydrophobic cellulose derivatives. As these methods all represent covalent modifications, they are evaluated as being more physically stable compared to adsorption and plasma methods discussed in the previous review (Rodríguez-Fabià et al 2022a).…”

The influence of temperature on cellulose swelling at constant water density

“…However, other factors, such as the films' porosity, density, or the crystals' alignment, can influence their mechanical properties. The mechanical properties of CNC 10,11 or CNF 12 films can be improved by chemical modification or decreasing crystallinity.Water swelling has a negative impact on cellulosic material applications in packaging 13,14 and as composite additives 15 . It is thus important to investigate such behaviour in neat cellulosic materials, to better be able to tune or reduce water swelling.Studies in this paper were performed with constant water present, and varying temperature.…”

“…Water swelling has a negative impact on cellulosic material applications in packaging 13,14 and as composite additives 15 . It is thus important to investigate such behaviour in neat cellulosic materials, to better be able to tune or reduce water swelling.…”