Lignin is a complex natural polymer and it is one of the main constituent of the lignocellulosic biomass. Moreover, it is a bio-renewable material and it is available in large amounts as by-product from the forest industry. Lignin-based hydrogels with high swelling capabilities were prepared by crosslinking poly (methyl vinyl ether co-maleic acid) and different technical lignins in ammonium and sodium hydroxide solutions. The produced hydrogels showed a wide range of water absorption capacities varying from 13 to 130 g of water per 1 g of sample. It was observed that the higher the water uptake the poorer mechanical performance, as evaluated in terms of storage and loss modulus (G' and G″, respectively) of the materials. Methylene blue (MB) was used as a model dye to evaluate the adsorption and release capabilities of the lignin hydrogels. Results suggested that these hydrogels showed a high MB removal efficiency, which ranged from 12 to 96%. On the contrary, the percentages of MB released depended on the negative surface charge of the hydrogels, showing values which ranged from 0.06 to 0.35%. Thus, these materials have potential to be used as adsorbents for the removal of organic dyes from waste water.
The substitution of phenol by lignin in phenol-formaldehyde (PF) resins is one of the most promising end uses of lignin valorization. Lignin from grasses and softwood has been the focus of the studies in this field as they present a higher number of theoretical reactive sites for resin synthesis. Herein we examined the composition and chemical reactivity of “less-reactive” hardwood lignin fractions and their performance in PF resins, synthesized by substituting 50 wt% of the phenol with lignin. Before resin synthesis, the samples were hydroxymethylated and the maximum formaldehyde consumption was recorded. By doing so, we observed that hardwood fractions consumed formaldehyde close to the theoretical calculation, whereas the reference softwood lignin consumed only about ¼ of the theoretical value. In the resin synthesis, we added formaldehyde to the formulation according to the measured maximum formaldehyde consumption. Thus, low values of free formaldehyde in lignin-PF (LPF) resins were achieved (<0.23%). Moreover, the resin bonding strength displayed similar performance irrespective of whether the LPF resins were made with softwood or hardwood lignin (range of 3.4–4.8 N mm−2 at 150°C and 45–480 s of press time). Furthermore, we concluded that hardwood kraft lignins present no disadvantage compared to softwood lignins in PF resin applications, which have significant practical implications.
Technical
lignins are widely available as side streams from pulping
and biorefining processes. The aromatic structure of such lignins
could be exploited in coating formulations to provide antioxidant
or UV-blocking functionalities to packaging films. In this study,
six technical lignins sourced from different plant species by given
isolation/modification methods were compared for their composition,
molar mass, and functional groups. The lignins were then used to prepare
thin spin-coated films from aqueous ammonia media. All the lignins
formed ultrathin (<12 nm), smooth (roughness < 2 nm), and continuous
films that fully covered the solid support. Most of the films contained
nanometer-sized particles, while those from water-insoluble lignins
also presented larger particulate features, which likely originated
from macromolecular association during solvent evaporation. These
latter films had water contact angles (WCAs) between 40 and 60°,
corresponding to a surface energy of 42–48 mJ/m
2
(determined by Zisman plots). For comparison, the water wettability
measured on lignin pellets obtained by mechanical compression tracked
closely with the WCA obtained from the respective thin films. Considering
the widely diverse chemical, molecular, and structural properties
of the tested lignins, comprehensively documented here by using a
battery of techniques, the solubility in water was found to be the
most important and generic parameter to characterize the thin films.
This points to the possibility of developing lignin coatings with
predictable wetting behavior.
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