Forestry 485 lecture 3 1

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 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 Urea Formaldehyde (UF) Synthesis Cure chemistry UF Synthesis UF is an “amino resin” or “amineformaldehyde” 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) 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 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 to slow polymerization and thereby increase storage life 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 UF Condensation Reaction: Favored by acidic conditions Condensation may be linear or crosslinked Result: Cured network (crosslinked) molecular structure UF Condensation Controlled by:    Temperature (150 oC cure temperature) Time pH (>7, stable; 1.0 to approx 1.5 Lower molar ratios favored in contemporary formulations to reduce formaldehyde emissions Most are 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 Novolac synthesis Formaldehyde donors for cure: Formalin Paraformaldehyde Hexamethylenetetramine (“hexa” hardener) Resole vs Novolac 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 crosslinking (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) ... 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