Forestry 485 Lectures 3-5 and 3-6: Natural Adhesives/Adhesives from Renewable Resources

Classes of “Natural” Materials for Adhesives Carbohydrate CarbohydrateStarchCellulose Protein Protein Plant-based (e.g., soybeans) Animal-based Protein from hides, bones, sinew Protein from hides, bones, sinew Liquid (“hide” glue) Solid (“hot animal glues) Blood Blood Casein (milk) Casein (milk) Lignocellulosic Extracts Lignocellulosic ExtractsTanninsLignin

Historical Note Prior to the 1940’s, wood adhesives were almost exclusively made from “natural” sources Petroleum-based synthetics have since displaced most of the “natural” or “renewable” sources On-going research, economic, and environmental factors may bring renewables back into greater usage

Contemporary Renewables used in Wood Adhesives A few renewable adhesives are currently in use, including: Lignin Lignin Tannins Tannins Plant protein (mainly soybean mean or soybean protein) Plant protein (mainly soybean mean or soybean protein)

Lignin Patents from the late 1800’s described use of spent sulfite lignin as a wood and paper adhesive Despite much research, use of “technical lignin” worldwide remains relatively low (about 20% used for dispersants, oil- drilling muds, pelletizing materials, adhesive additives; remainder burned in pulp & paper factories)

Lignin Lignin is polyphenolic; could expect to use in manner similar to phenol The condensation reactions of lignin are less effective than phenol due to: Less free reaction sites on aromatic nuclei Less free reaction sites on aromatic nuclei Lower overall reactivity Lower overall reactivity Practical outcome: Higher press temperatures Higher press temperatures Longer press times Longer press times

Plant-based Tannins as Adhesives Tannins: Plant-derived extractives, typically obtained from bark or sometimes the wood of certain tree species These polyphenolic compounds were historically used for “tanning” leather Two classes: Hydrolyzable tannins Hydrolyzable tannins Condensed tannins Condensed tannins >90% of world production of commercial tannin

Condensed Tannins Condensed Tannins are “natural polymeric flavonoids” They are very reactive Reaction is 10-50x greater than phenol Reaction is 10-50x greater than phenol Cannot make tannin resoles; those with methylol groups will condense very rapidly with other tannin nuclei, resulting in poor stability and shelf life Cannot make tannin resoles; those with methylol groups will condense very rapidly with other tannin nuclei, resulting in poor stability and shelf life

Condensed tannin adhesives Have been used industrially since early 1970’s, particularly in S. Africa and Australia Applications: Exterior plywood, gluelaminated members, finger- jointing, particleboard Exterior plywood, gluelaminated members, finger- jointing, particleboard Tannin-formaldehyde adhesives are obtained by hardening of natural polymeric flavonoids via polycondensation with formaldehyde

Condensed tannin adhesives Since tannins are so reactive, the formaldehyde component is typically added at point of use Formaldehyde sources Paraformaldehyde Paraformaldehyde Hexa Hexa Urea-formaldehyde Urea-formaldehyde Liquid and spray-dried powder have indefinite shelf life Liquid and spray-dried powder have indefinite shelf life

Soy-based adhesives On-going research at ISU has led to two U.S. patents on the use of soybean meal (containing protein and carbohydrates) crosslinked with PF and other synthetics