1 / 17

Forestry 485

Forestry 485. Lecture 3-1: Urea and Phenol Formaldehyde Adhesive Resins. Thermosetting Resins for Wood Composites. Panel Products - three primary resins in use in North America: Urea formaldehyde (water resistant bond), used exclusively for interior applications

anika
Download Presentation

Forestry 485

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Forestry 485 Lecture 3-1: Urea and Phenol Formaldehyde Adhesive Resins

  2. Thermosetting Resins for Wood Composites Panel Products - three primary resins in use in North America: Urea formaldehyde (water resistant bond), used exclusively for interior applications Phenol formaldehyde (fully exterior, waterproof bond), used almost exclusively for exterior application and/or structural products Polymeric MDI (water resistant to water proof), used mainly as a core resin in OSB

  3. Urea Formaldehyde (UF) • Synthesis • Cure chemistry

  4. UF Synthesis • UF is an “amino resin” or “amine-formaldehyde” class of adhesive • UF is synthesized by the reaction of urea and formaldehyde to obtain stable, but reactive intermediates • Storage stability due to reduced reactivity of intermediates in alkaline conditions (>pH 7)

  5. UF Synthesis Mild alkali (pH>7) • pH is used to control reaction rate. Occurs very rapidly under acidic conditions. • Production of the reactive intermediates is monitored and controlled by viscosity. • Note methylolation of urea residue.

  6. UF Synthesis, continued • Other intermediates, such are dimethylol urea, are also formed • Again, viscosity is monitored to indicate the stage of the synthesis. Increasing viscosity is correlated to polymer growth; this indicates how “advanced” the resin is • pH is maintained above 7 to slow polymerization and thereby increase storage life

  7. UF Cure Chemistry • Polymerization of the reaction intermediates • Results from chemical reaction to build polymer chain length and molecular weight • Polymerization is characterized as a “condensation reaction” • Resin changes from low to high viscosity liquid and ultimately to solid • Resin hardening corresponds to development of mechanical strength of adhesive bond

  8. UF Condensation Reaction: Favored by acidic conditions Condensation may belinear or crosslinked Result: Cured network (cross-linked) molecular structure

  9. UF Condensation • Controlled by: • Temperature (150 oC cure temperature) • Time • pH (>7, stable; <7, condensation) • Reaction accelerated by “curing agent” or “hardener” • Mineral acid (e.g., HCl) or salts (e.g., ammonium chloride) See Reading 3-1c • Ratio of formaldehyde to urea • Molar ratio varies from just >1.0 to approx. 1.5 • Lower molar ratios favored in contemporary formulations to reduce formaldehyde emissions. Most are <1.1 • Formaldehyde scavengers may be used (see Reading 3-1d)

  10. Phenol formaldehyde (PF) adhesive resins • Two major types: • Resole (cross-linking thermosetting resin) • Novolac (resin with some thermoplastic properties) • Synthesis • Condensation reactions (resin cure)

  11. PF Synthesis: Resoles • Two stages: • Methylolation of phenol to produce methylol phenols. First stage, or “A” stage, is alkaline catalyzedfor the synthesis of resoles • Alkaline A stage, P/F ratio=1:1 to 1:3 (i.e., molar EXCESS of formaldehyde) • B stage is the condensation of intermediates to produce water insoluble, but fusible products. Reaction is monitored by viscosity and terminated prior to complete polymerization.

  12. Resole “A” Methylolation may occur at ortho, meta, or para position.

  13. Resole “B” Note that an excess of formaldehyde is used. This promotes full methylolation of the phenol moieties.

  14. Resole “C” (curing) Process cure temperature typically 190-200 oC. ONLY heat is needed to cure. Three-dimensional cross-linked structure is formed. Water insoluble.

  15. Novolac Synthesis • P/F ratio is > 1.0 (note error in figure on p. 78 of Marra; in fact, note somewhat unorthodox means of representing P:F ratio on p. 77 & 78) • Acid A stage, P/F ratio=1:1 to 1:0.6 • Few methylols are formed; results in linear, rather than cross-linked structure. • Has indefinite shelf life. • Soluble, (partially) thermoplastic. • Hardener in the form of additional formaldehyde is needed for cure.

  16. Novolac synthesis Formaldehyde donors for cure: Formalin Paraformaldehyde Hexamethylenetetramine (“hexa” hardener)

  17. Resole A-stage is a low molecular weight “impregnating” resin (use for making Impreg, Compreg, and laminating materials) B-stage is a bonding (adhesive) resin Needs ONLY heat to polymerize Continues to polymerize in storage (limited shelf life) Novolac Fewer methylol groups, thus much less cross-linking (linear structure) Formaldehyde must be supplied to cure, along with heat Resin is soluble and thermoplastic Has an indefinite shelf life May be supplied in sheet or flake form (dry) Resole vs. Novolac

More Related