Abstract
Ammoxidation of pine kraft lignin in aqueous 5 wt % ammonia affords a novel type of phenol substitute that significantly accelerates resole synthesis and curing as demonstrated for 40 wt % phenol replacement. Compared to non-ammoxidized lignin, which already shortens significantly the cooking time required to reach a resole viscosity of 1000 Pa·s (250 vs. 150 s) and reduces the typical curing B-time by about 25% at 100 °C, the use of ammoxidized lignin has an even more pronounced impact in this respect. Activation of lignin by Fenton-type oxidation prior to ammoxidation further boosts both synthesis and curing of the resole. This is presumably due to the intermediary formation of polyvalent cross-linkers like N,N,N-tris (methylol) trimethylene triamine triggered by saponification of a larger fraction of nitrogenous moieties present in such a treated lignin (ammonium salts, amide-type nitrogen, urea) and reaction of the released ammonia with formaldehyde. Except for the fact that phenol replacement by ammoxidized lignin results in a somewhat less brittle cured adhesive polymer and higher elastic modulus, the aforementioned acceleration in curing could no longer be observed in the presence of wood, where a significantly delayed wood-adhesive bond formation was observed for the lignin-containing adhesives as evident from the automated bonding evaluation system.
Highlights
Phenol formaldehyde (PF) resins are comparatively inexpensive commodity plastics and adhesives that feature good moisture, in addition to chemical and heat resistance, and have found application in a vast variety of fields including engineered wood products, such as plywood, laminated veneer lumber, glue laminated timber, compact laminates or binders for mineral-based insulation boards [1,2]
Size exclusion chromatography confirmed a pronounced increase of the molecular weight ammoxidized pine kraft lignin
Phenol replacement by ammoxidized lignin surprisingly translated into further reduced ultimate strength (APK-lignin phenol formaldehyde (LPF): 3.96 N·mm−2; Fenton-oxidized and subsequently ammoxidized pine kraft lignin (FAPK)-LPF: 3.93 N·mm−2) which contrasts the results of differential scanning calorimetry and B-time measurements at a first glance but could be explained by more rapid network formation due to the significantly higher molecular weights of both types of ammoxidized
Summary
Phenol formaldehyde (PF) resins are comparatively inexpensive commodity plastics and adhesives that feature good moisture, in addition to chemical and heat resistance, and have found application in a vast variety of fields including engineered wood products, such as plywood, laminated veneer lumber, glue laminated timber, compact laminates or binders for mineral-based insulation boards [1,2]. As the percentages of the principal lignin-forming building blocks vary to a large extent depending on the plant species, the macromolecular features of lignin follow this diversity as evident from the wide range of average molecular weight, branching, steric demand as well as pattern and abundance of functional groups [7]. These difficulties of working with lignin in material science are amplified by other factors including the different technical lignin isolation procedures which lead to comprehensive chemical alteration of the biopolymer in terms of de-polymerization; changes in functional group pattern; and even undesired re-polymerization in pulp production. Thermal and mechanical properties of the obtained adhesives were evaluated based on the time to reach the B-stage of curing (B-time), differential scanning calorimetry (DSC), tensile shear strength development of adhesive bonds as a function of hot pressing time using an automated bonding evaluation system (ABES), and nanoindentation of cured adhesive polymers
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