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

This review is the third part of a series of reviews on hydrophobization of lignocellulosic materials, a relevant topic nowadays, due to the need to replace fossil fuel-based materials. The review provides an overview of the hydrophobization of lignocellulosic materials by polymer adsorption, and both chemical and radiation-induced grafting of polymers. While adsorbed polymers are only attached to the surfaces by physical interactions, grafted polymers are chemically bonded to the materials. Radiation-induced grafting is typically the most environmentally friendly grafting technique, even though it provides little control on the polymer synthesis. On the other hand, controlled radical polymerization reactions are more complex but allow for the synthesis of polymers with elaborated architectures and well-defined properties. Overall, a wide range of contact angles can be obtained by polymer adsorption and grafting, from a slight increase in hydrophobicity to superhydrophobic properties. The choice of modification technique depends on the end-use of the modified material, but there is a clear trend towards the use of more environmentally friendly chemicals and processes and the grafting of polymers with complex structures. Graphical abstract

Lignocellulosic materials with hydrophobic properties are of great interest for developing sustainable products that can be used in various applications such as packaging, water-repellent and self-cleaning materials, oil and water separation or as reinforcements in biocomposite materials. The hydroxyl functional groups present in cellulose provide the possibility to perform various chemical modifications to the cellulosic substrates that can increase their hydrophobicity. This review is the second part of a comprehensive review on hydrophobization of lignocellulosic materials and summarizes the recent advances in the chemical modification of such substrates. The methods described in this review can provide changes in the hydrophilicity of the materials that range from a small decrease in the initial hydrophilicity of the substrate (contact angles below 90°) to superhydrophobic properties (contact angles above 150°). Additional attention has been paid to whether the modification is limited to the surface of the substrate or if it occurs in the bulk of the material. We also discuss hydrophobized cellulose material applications in packing and oil/water purification. Graphical abstract

We have in this paper investigated how water sorbs to cellulose. We found that both cellulose nanofibril (CNF) and cellulose nanocrystal (CNC) films swell similarly, as they are both mainly composed of cellulose. CNF/CNC films subjected to water at 0.018 kg/m3 at 25 °C and 39 °C, showed a decrease in swelling from ~ 8 to 2%. This deswelling increased the tensile index of CNF-films by ~ 13%. By molecular modeling of fibril swelling, we found that water sorbed to cellulose exhibits a decreased diffusion constant compared to bulk water. We quantified this change and showed that diffusion of sorbed water displays less dependency on swelling temperature compared to bulk water diffusion. To our knowledge, this has not previously been demonstrated by molecular modeling. The difference between bulk water diffusion (DWW) and diffusion of water sorbed to cellulose (DCC) increased from DWW − DCC ~ 3 × 10–5 cm/s2 at 25 °C to DWW − DCC ~ 8.3 × 10–5 cm/s2 at 100 °C. Moreover, water molecules spent less successive time sorbed to a fibril at higher temperatures.