Traditionally phenol and formaldehyde containing adhesives are used in wood products such as plywood, oriented strand board and laminates. There is a drive from society and wood product manufacturers to find bio-based and safe alternatives to these oil-based and toxic adhesive components.
Annually around 60 million tons of alkali lignin is produced worldwide as a by-product of the pulping industry. More than 98% is currently combusted for energy. As an aromatic biopolymer, lignin is an attractive renewable source of aromatic chemicals and polymers.
The use of lignin as phenol substitute in phenol formaldehyde resins is one of the most potential application for lignin. However, the reactivity of lignin is limited compared to phenol and typically relatively low proportions of phenol can be replaced. Unlike phenol, lignin is already crosslinked and most of the reactive sites are methoxylated, preventing the crosslinking reactions with formaldehyde.
VTT has developed a technology that produces unique highly reactive lignin from black liquor. During reactive lignin production, significant demethylation and demethoxylation take place, increasing lignin reactivity enabling high phenol replacement levels also with hardwood lignin. Reactive lignin has up to five times more reactive sites compared to commercial lignins. Therefore, reactive lignin enables high phenol replacement levels in resins. Reactive lignin made of both softwood and hardwood has been tested in LPF resin synthesis at substitution levels of 50 and 70%. Higher reactivity and faster curing compared to alkali lignins were detected during the resin synthesis.
The use of bio-based resins plays a major role in lowering the environmental impacts of wood products. Adhesives have significant impact on CO2 footprint of engineered wood products. The CO2 footprint of reactive lignin is only around 20% that of phenol.
Hanne Wikberg, Taina Ohra-Aho, Tiina Liitiä