Lignin-Based Sunscreens:State-of-the-Art, Prospects and Challenges

This review covers the latest developments and challenges in the field of broad-spectrum sunscreens and how sunscreens based on lignin address their requirements in terms of sunlight protection, antioxidants, and preservatives

Oxidative activation of wood fibers for the manufacture of medium-density fiberboard (MDF)

The aim of this thesis work was to study the possibilities of radical formation in wood fiber surfaces to enable direct fiber-to-fiber adhesion by radical-based reactions in the manufacture of fibreboard, particularly medium-density fiberboard (MDF). The fibers were produced by defibration at high temperatures. Radical formation was achieved by treatment with laccase, treatment with Fenton's reagent, or gamma-irradiation. High-temperature defibration was found to cause cleavage of interunit beta-aryl ether linkages of lignin, resulting in formation of mechanoradicals and phenolic hydroxyl groups. The proportion of water-extractable low-molecular weight lignin and hemicelluloses present in the fibers increased with an increase in defibration temperature. In the laccase treatment of fibers in water suspension, much more radicals were formed in hardwood than in softwood fibers for fibers produced at equal temperature. Radical formation increased with increasing defibration temperature. The treatment of fibers with Fenton's reagent in water suspension resulted in the formation of similar numbers of radicals in hardwood and softwood fibers. Radical formation increased with increasing defibration temperature but not so drastically as with laccase treatments. Also the gamma-irradiated fibers contained large numbers of radicals. Their content increased as a function of increasing defibration temperature and was higher for hardwood than for softwood fibers. The internal bond strength (IB) of fiberboards made from fibers treated with laccase or Fenton's reagent in the defibrator blowline improved with increasing defibration temperature. The IB of boards made from laccase-treated fibers correlated with the number of radicals formed in the fibers on laccase treatment in water suspension, indicating that adhesion in the boards was largely due to reactions of radicals on the fiber surfaces. The IB of boards made from fibers treated with Fenton's reagent also correlated with the radical content of the fibers, but this relationship was not as strong as with the laccase treatments. This suggests that bonding mechanisms other than radical coupling may have contributed significantly to adhesion. Gamma-irradiation of fibers before their fabrication into boards resulted in a marked increase in board IB, indicating that radicals play a significant role in the adhesion of boards made from gamma-irradiated fibers.reviewe

Improved Fire Retardancy of Cellulose Fibres via Deposition of Nitrogen-Modified Biopolyphenols

Driven by concerns over the health and environmental impacts of currently used fire retardants (FRs), recent years have seen strong demand for alternative safer and sustainable bio-based FRs. In this paper, we evaluated the potential of nitrogen-modified biopolyphenols as FRs for cellulosic natural fibres that could be used in low-density cellulose insulations. We describe the preparation and characterisation of nitrogen-modified lignin and tannin containing over 10% nitrogen as well as the treatment of cellulose pulp fibres with combinations of lignin or tannin and adsorption-enhancing retention aids. Combining lignin or tannin with a mixture of commercial bio-based flocculant (cationised tannin) and anionic retention chemical allowed for a nearly fourfold increase in lignin adsorption onto cellulosic pulp. The nitrogen-modified biopolyphenols showed significant improvement in heat release parameters in micro-scale combustion calorimetry (MCC) testing compared with their unmodified counterparts. Moreover, the adsorption of nitrogen-modified lignin or tannin onto cellulose fibres decreased the maximum heat release rate and total heat release compared with cellulose reference by 15–23%. A further positive finding was that the temperature at the peak heat release rate did not change. These results show the potential of nitrogen-modified biopolyphenols to improve fire-retarding properties of cellulosic products

New insights into the chemical activation of lignins and tannins using K₂CO₃—a combined thermoanalytical and structural study

Engineering of activated carbons (ACs) through chemical activation of organic precursors has been extensively studied for a wide variety of biopolymers, biomasses, wastes and other fossil-based precursors. Despite huge efforts to engineer evermore performant and sustainable ACs, “searching-for-the-best-recipe” type of studies are more the rule than the exception in the published literature. Emerging AC applications related to energy and gas storage require strict control of the AC properties and a better understanding of the fundamentals underlying their engineering. In this study, we provide new insights into the K2CO3 chemical activation of plant-based polyphenols—lignins and tannins—through careful thermoanalytical and structural analyses. We showed for the the first time that the reactivity of polyphenols during K2CO3 chemical activation depends remarkably on their purity and structural properties, such as their content of inorganics, OH functionalities and average molecular weight. We also found that the burn-off level is proportional to the K2CO3/lignin impregnation ratio (IR), but only within a certain range—high impregnation ratios are not needed, unlike often reported in the literature. Furthermore, we showed for the first time that the K2CO3 chemical activation of different carbon surfaces from lignins and tannins can be modelled using simple global solid-state decomposition kinetics. The identified activation energies lay in the range of values reported for heterogenous gas-carbon surface gasification reactions (O2-C, H2O-C, or CO2-C) in which the decomposition of C(O) surface complexes is the common rate-limiting step.</p

Tannin-Based Microbicidal Coatings for Hospital Privacy Curtains

The goal of this study was to develop a sustainable, tannin-based option for silver-based and other current antimicrobial solutions for hospital privacy curtains. Commercial tree-derived tannins were characterized and their in vitro antibacterial properties against Staphylococcus aureus and Escherichia coli were determined. Hydrolysable tannins showed greater antibacterial efficacy than condensed tannins but differences in antibacterial efficacy between any of the tannins could not be attributed to their functional group content or molar mass. Outer membrane disruption was not a significant factor in antibacterial efficacy of tannins against E. coli. In a hospital field study, draw patches coated with hydrolysable tannins and affixed to privacy curtains reduced total bacteria count by 60% over eight weeks compared to their matching uncoated reference sides. In a follow-up laboratory study with S. aureus, very light spraying with water improved contact between bacteria and coating, enhancing the antibacterial effect by several orders of magnitude.publishedVersionPeer reviewe

Industrial scale evaluation of cationic tannin as a binder for hardboard

Hardboards (HBs) (wet-process high-density fibreboards) were made in an industrial trial using a binder system consisting of cationic mimosa tannin and laccase or just cationic tannin without any thermosetting adhesive. The boards displayed superior mechanical strength compared to reference boards made with phenol–formaldehyde, easily exceeding the European standards for general-purpose HBs. The thickness swell of most of the boards was slightly greater than the standards would allow, so some optimisation is required in this area. The improved board properties appear to be mainly associated with ionic interactions involving quaternary amino groups in cationic tannin and negatively charged wood fibres rather than to cross-linking of fibres via laccase-assisted formation and coupling of radicals in tannin and fibre lignin

Isolation and purification of high-molecular weight hemicelluloses from radiata pine wood chips prior to thermo-mechanical pulp (TMP) production

A novel thermo-mechanical pulping (TMP) process has been developed to produce a by-product rich in high-molecular weight (MW) hemicelluloses, a potential raw material for barrier coatings and films. This process uses prehydrolysis to solubilise the hemicelluloses followed by chip compression to separate the soluble material from the wood matrix. The pressate from the chip compression stage was dark-coloured and had a high content of high-MW hemicelluloses and lignin. However, isolating the high-MW material from the pressate directly by ultrafiltration was not feasible because of membrane fouling by dissolved lignin and wood extractives, while coloured impurities are undesirable for many potential applications. To solve these problems the pressate was purified using XAD adsorbent resin to remove low-MW lignin, extractives and colour. Ultrafiltration of the purified pressate yielded lightly coloured high-MW hemicellulose with a low content of lignin and well suited for barrier films and coatings