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Forestry 485

Learn about the chemistry, curing process, and applications of polyurethane and epoxy adhesives in the forestry industry. Understand their strengths and limitations for bonding wood substrates. Discover the importance of moisture and fillers in enhancing adhesive performance.

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Forestry 485

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  1. Forestry 485 Lecture 3-3: Polyurethane and Epoxy Adhesives

  2. Polyurethane Adhesives • Chemistry is same as for MDI • Consists of two components: • “A” side=isocyanate, containing MDI or TDI • “B” side=polyol=hydroxyl-containing molecules • “A” and “B” are kept separate physically or chemically (e.g., by emulsion) until use • Mixing of iso and polyol results in chemical reaction

  3. Polyurethane cure • When iso and polyol are combined, a “urethane bridge” or urethane chemical linkage is created: R-N=C=O + HO-R’ R-NH-C-OR’ Urethane “bridge”

  4. Polyurethane cure • Iso reacts readily with water and should also react with wood substrates • Reaction with moisture in air and/or from wood speeds cure process • Excess moisture may result in liberation of carbon dioxide, resulting in foaming of the adhesive (“blowing” reaction) • Cure/setup of adhesive is generally rapid

  5. Epoxy adhesives • 100% solids; no solvent loss upon cure, resulting in low shrinkage of adhesive • See p. 90 of Marra (Reading 3-3A) and class blackboard notes for example of chemical structure • Contains two components: Epoxy plus catalyst or reactive hardener • Forms an insoluble, infusible, thermoset resin

  6. Epoxy hardeners • Many classes of compounds used, depending on application, including: • Polyamines • Polyamides • Polysulfides • Urea resins • Phenolic resins • Acid anhydrides • Some of these compounds are toxic; ALWAYS read and follow label directions!

  7. Other components of epoxies • Fillers are used to increase gap-filling ability • Example materials include titanium dioxide and ferric oxide • Fillers may be up to 50% by weight • Epoxies are generally used for specialty applications due to high cost • Wood-epoxy bonds are not durable in long-term exterior exposure due to variation in dimensional change of substrate and adhesive; failure occurs at wood-epoxy interface.

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