Forestry 485Forestry 485

Lecture 3-1: Urea and Phenol Lecture 3-1: Urea and Phenol Formaldehyde Adhesive ResinsFormaldehyde Adhesive Resins

Thermosetting Resins for Wood Thermosetting Resins for Wood CompositesComposites

Panel Products - three primary resins in use Panel Products - three primary resins in use in North America:in North America:Urea formaldehyde (water Urea formaldehyde (water resistantresistant bond), used bond), used

exclusively for interior applicationsexclusively for interior applications

Phenol formaldehyde (fully exterior, Phenol formaldehyde (fully exterior, waterproofwaterproof bond), used almost exclusively for exterior bond), used almost exclusively for exterior application and/or structural productsapplication and/or structural products

Polymeric MDI (water resistant to water proof), Polymeric MDI (water resistant to water proof), used mainly as a core resin in OSBused mainly as a core resin in OSB

Urea Formaldehyde (UF)Urea Formaldehyde (UF)

SynthesisSynthesis

Cure chemistryCure chemistry

UF SynthesisUF Synthesis

UF is an “amino resin” or “amine-UF is an “amino resin” or “amine-formaldehyde” class of adhesiveformaldehyde” class of adhesive

UF is synthesized by the reaction of urea UF is synthesized by the reaction of urea and formaldehyde to obtain stable, but and formaldehyde to obtain stable, but reactive intermediatesreactive intermediates

Storage stability due to reduced reactivity Storage stability due to reduced reactivity of intermediates in alkaline conditions of intermediates in alkaline conditions (>pH 7)(>pH 7)

UF SynthesisUF SynthesisMild alkali

(pH>7)

pH is used to control reaction rate. Occurs pH is used to control reaction rate. Occurs very rapidly under acidic conditions.very rapidly under acidic conditions.

Production of the reactive intermediates is Production of the reactive intermediates is monitored and controlled by viscosity.monitored and controlled by viscosity.

Note methylolation of urea residue.Note methylolation of urea residue.

UF Synthesis, continuedUF Synthesis, continued

Other intermediates, such are dimethylol Other intermediates, such are dimethylol urea, are also formedurea, are also formed

Again, viscosity is monitored to indicate the Again, viscosity is monitored to indicate the stage of the synthesis. Increasing viscosity stage of the synthesis. Increasing viscosity is correlated to polymer growth; this indicates is correlated to polymer growth; this indicates how “advanced” the resin ishow “advanced” the resin is

pH is maintained above 7 to slow pH is maintained above 7 to slow polymerization and thereby increase storage polymerization and thereby increase storage lifelife

UF Cure ChemistryUF Cure ChemistryPolymerization of the reaction intermediatesPolymerization of the reaction intermediates Results from chemical reaction to build polymer Results from chemical reaction to build polymer

chain length and molecular weightchain length and molecular weight Polymerization is characterized as a Polymerization is characterized as a

“condensation reaction”“condensation reaction” Resin changes from low to high viscosity liquid Resin changes from low to high viscosity liquid

and ultimately to solidand ultimately to solid Resin hardening corresponds to development Resin hardening corresponds to development

of mechanical strength of adhesive bondof mechanical strength of adhesive bond

UF Condensation Reaction: Favored by acidic UF Condensation Reaction: Favored by acidic conditionsconditions

Result: Cured network (cross-linked) molecular structure

Condensation may be linear or crosslinked

UF CondensationUF CondensationControlled by:Controlled by: Temperature (150 Temperature (150 ooC cure temperature)C cure temperature) TimeTime pH (>7, stable; <7, condensation)pH (>7, stable; <7, condensation)

Reaction accelerated by “curing agent” or “hardener”Reaction accelerated by “curing agent” or “hardener” Mineral acid (e.g., HCl) or salts (e.g., ammonium chloride) See Mineral acid (e.g., HCl) or salts (e.g., ammonium chloride) See

Reading 3-1cReading 3-1c Ratio of formaldehyde to ureaRatio of formaldehyde to urea

Molar ratio varies from just >1.0 to approx. 1.5Molar ratio varies from just >1.0 to approx. 1.5Lower molar ratios favored in contemporary formulations to Lower molar ratios favored in contemporary formulations to reduce formaldehyde emissions. Most are <1.1reduce formaldehyde emissions. Most are <1.1Formaldehyde scavengers may be used (see Reading 3-1d)Formaldehyde scavengers may be used (see Reading 3-1d)

Phenol formaldehyde (PF) Phenol formaldehyde (PF) adhesive resinsadhesive resins

Two major types:Two major types: ResoleResole (cross-linking thermosetting resin) (cross-linking thermosetting resin) NovolacNovolac (resin with some thermoplastic (resin with some thermoplastic

properties)properties)

SynthesisSynthesis

Condensation reactions (resin cure)Condensation reactions (resin cure)

PF Synthesis: ResolesPF Synthesis: Resoles

Two stages:Two stages: Methylolation of phenol to produce methylol phenols. Methylolation of phenol to produce methylol phenols.

First stage, or “A” stage, is First stage, or “A” stage, is alkaline catalyzedalkaline catalyzed for the for the synthesis of resolessynthesis of resoles

Alkaline A stage, P/F ratio=1:1 to 1:3 Alkaline A stage, P/F ratio=1:1 to 1:3 (i.e., molar (i.e., molar EXCESS of formaldehyde)EXCESS of formaldehyde)

B stage is the condensation of intermediates to B stage is the condensation of intermediates to produce water insoluble, but fusible products. produce water insoluble, but fusible products. Reaction is monitored by viscosity and terminated Reaction is monitored by viscosity and terminated prior to complete polymerization.prior to complete polymerization.

Resole “A”Resole “A”

Methylolation may occur at ortho, meta, or para position.

Resole “B”Resole “B”

Note that an excess of formaldehyde is used. This promotes full methylolation of the phenol moieties.

Resole “C” (curing)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.

Novolac SynthesisNovolac Synthesis

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

Novolac synthesisNovolac synthesis

Formaldehyde donors for cure:

Formalin

Paraformaldehyde

Hexamethylenetetramine (“hexa” hardener)

Resole vs. NovolacResole vs. Novolac

ResoleResole A-stage is a low molecular A-stage is a low molecular

weight “impregnating” resin weight “impregnating” resin (use for making Impreg, (use for making Impreg, Compreg, and laminating Compreg, and laminating materials)materials)

B-stage is a bonding B-stage is a bonding (adhesive) resin(adhesive) resin

Needs ONLY heat to Needs ONLY heat to polymerizepolymerize

Continues to polymerize in Continues to polymerize in storage (limited shelf life)storage (limited shelf life)

NovolacNovolac Fewer methylol groups, Fewer methylol groups,

thus much less cross-thus much less cross-linking (linear structure)linking (linear structure)

Formaldehyde must be Formaldehyde must be supplied to cure, along with supplied to cure, along with heatheat

Resin is soluble and Resin is soluble and thermoplasticthermoplastic

Has an indefinite shelf lifeHas an indefinite shelf life May be supplied in sheet or May be supplied in sheet or

flake form (dry)flake form (dry)