Forestry 485Forestry 485

Lectures 3-5 and 3-6: Natural Lectures 3-5 and 3-6: Natural Adhesives/Adhesives from Adhesives/Adhesives from

Renewable ResourcesRenewable Resources

Classes of “Natural” Materials for Classes of “Natural” Materials for AdhesivesAdhesives

CarbohydrateCarbohydrateStarchStarchCelluloseCellulose

ProteinProteinPlant-based (e.g., soybeans)Plant-based (e.g., soybeans)Animal-basedAnimal-based

Protein from hides, bones, sinewProtein from hides, bones, sinewLiquid (“hide” glue)Liquid (“hide” glue)Solid (“hot animal glues)Solid (“hot animal glues)

BloodBlood Casein (milk)Casein (milk)

Lignocellulosic ExtractsLignocellulosic ExtractsTanninsTanninsLigninLignin

Historical NoteHistorical Note

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

Contemporary Renewables used in Contemporary Renewables used in Wood AdhesivesWood Adhesives

A few renewable adhesives are currently A few renewable adhesives are currently in use, including:in use, including: LigninLignin TanninsTannins Plant protein (mainly soybean mean or Plant protein (mainly soybean mean or

soybean protein)soybean protein)

LigninLignin

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

LigninLignin

Lignin is polyphenolic; could expect to use Lignin is polyphenolic; could expect to use in manner similar to phenolin manner similar to phenolThe condensation reactions of lignin are The condensation reactions of lignin are less effective than phenol due to:less effective than phenol due to: Less free reaction sites on aromatic nucleiLess free reaction sites on aromatic nuclei Lower overall reactivityLower overall reactivity

Practical outcome:Practical outcome: Higher press temperaturesHigher press temperatures Longer press timesLonger press times

Plant-based Tannins as AdhesivesPlant-based Tannins as Adhesives

Tannins: Plant-derived extractives, Tannins: Plant-derived extractives, typically obtained from bark or sometimes typically obtained from bark or sometimes the wood of certain tree speciesthe wood of certain tree speciesThese polyphenolic compounds were These polyphenolic compounds were historically used for “tanning” leatherhistorically used for “tanning” leatherTwo classes:Two classes: Hydrolyzable tanninsHydrolyzable tannins Condensed tanninsCondensed tannins

>90% of world production of commercial tannin>90% of world production of commercial tannin

Condensed TanninsCondensed Tannins

Condensed Tannins are “natural polymeric Condensed Tannins are “natural polymeric flavonoids”flavonoids”

They are very reactiveThey are very reactive Reaction is 10-50x greater than phenolReaction is 10-50x greater than phenol Cannot make tannin resoles; those with Cannot make tannin resoles; those with

methylol groups will condense very rapidly methylol groups will condense very rapidly with other tannin nuclei, resulting in poor with other tannin nuclei, resulting in poor stability and shelf lifestability and shelf life

Condensed tannin adhesivesCondensed tannin adhesives

Have been used industrially since early 1970’s, Have been used industrially since early 1970’s, particularly in S. Africa and Australiaparticularly in S. Africa and Australia

Applications:Applications: Exterior plywood, gluelaminated members, finger-Exterior plywood, gluelaminated members, finger-

jointing, particleboardjointing, particleboard

Tannin-formaldehyde adhesives are obtained by Tannin-formaldehyde adhesives are obtained by hardening of natural polymeric flavonoids via hardening of natural polymeric flavonoids via polycondensation with formaldehydepolycondensation with formaldehyde

Condensed tannin adhesivesCondensed tannin adhesives

Since tannins are so reactive, the Since tannins are so reactive, the formaldehyde component is typically formaldehyde component is typically added at point of useadded at point of useFormaldehyde sourcesFormaldehyde sources ParaformaldehydeParaformaldehyde HexaHexa Urea-formaldehydeUrea-formaldehyde

Liquid and spray-dried powder have Liquid and spray-dried powder have indefinite shelf lifeindefinite shelf life

Soy-based adhesivesSoy-based adhesives

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