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Forestry 485

Forestry 485. Lectures 3-5 and 3-6: Natural Adhesives/Adhesives from Renewable Resources. Classes of “Natural” Materials for Adhesives. Carbohydrate Starch Cellulose Protein Plant-based (e.g., soybeans) Animal-based Protein from hides, bones, sinew Liquid (“hide” glue)

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Forestry 485

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

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

  3. 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

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

  5. 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)

  6. 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 • Lower overall reactivity • Practical outcome: • Higher press temperatures • Longer press times

  7. 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 • Condensed tannins • >90% of world production of commercial tannin

  8. Condensed Tannins • Condensed Tannins are “natural polymeric flavonoids” • They are very reactive • 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

  9. 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 • Tannin-formaldehyde adhesives are obtained by hardening of natural polymeric flavonoids via polycondensation with formaldehyde

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

  11. 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

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