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Glyoxal Thermal Degradation of Solid Waste UF Resin

Urea formaldehyde resin (UF resin) is the dominant adhesive for plywood, MDF, and particleboard, with annual global consumption exceeding 14.9 million tons. In daily manufacturing, large volumes of solid UF resin waste are inevitably generated due to improper operation, long-term storage sedimentation, incomplete curing, and batch production errors.

Cured solid UF waste is hard to biodegrade; direct landfill causes formaldehyde leaching and soil pollution, while incineration creates toxic exhaust and high disposal costs. Based on the reversible polycondensation reaction mechanism of UF resin, glyoxal-assisted thermal degradation is a low-cost, green recycling technology that breaks cured solid UF into low-molecular liquid resin precursors.

This article systematically analyzes the physical and molecular characteristics of glyoxal pyrolysis products, tests curing performance, and verifies plywood bonding performance with flour tackifier modification, providing a full industrial recycling workflow for wood adhesive manufacturers.

Full Glyoxal Thermal Degradation of Solid Waste UF Resin

Raw Materials & Standard Test Equipment

  • Raw materials: Industrial solid waste UF resin blocks, 40% glyoxal aqueous solution, food-grade flour tackifier, poplar veneer (1.5mm thickness)
  • Test instruments: DSC differential scanning calorimeter, FT-IR spectrometer, ESI-MS mass spectrometer, universal shear testing machine
  • Testing standards: GB/T14074 (adhesive performance), GB/T17657 (plywood mechanical test)

Step-by-Step Degradation Workflow

  1. Crush solid waste UF resin into small pieces; weigh 3 g of solid waste for each test group.
  2. Mix solid UF with 40% glyoxal solution at mass ratios 1:5, 1:7, 1:9, and soak for 30 minutes.
  3. Adjust the mixed solution pH below 3 with a weak acid catalyst.
  4. Oil bath heating at 85°C, constant temperature thermal degradation for 5 hours
  5. Cool naturally to obtain dark yellow acidic liquid pyrolysis products.
  6. Blend flour tackifier at 5–35% mass ratio for adhesive modification before plywood production.

Physical Properties of Glyoxal Degradation Liquid (Pyrolysis Products)

Three glyoxal dosage groups show regular changes in solid content, viscosity, and appearance:
Glyoxal:Solid UF RatioSolid ContentViscosity (mPa·s)pHVisual State
1:558%170.93Thick dark yellow paste
1:755%81.63Moderate viscosity liquid
1:954%71.953Thin watery solution
Key trend: Higher glyoxal addition continuously reduces solid content and viscosity. All three groups have solid content matching standard industrial UF resin, which theoretically supports direct gluing application. However, low-viscosity groups (1:7, 1:9) suffer severe glue penetration on veneers and insufficient initial tack without tackifier modification.

Molecular Structure Characterization of Degraded UF resin Products

FT-IR Infrared Spectrum Analysis

All three degradation liquids show nearly identical infrared absorption curves, confirming consistent functional group composition with fresh commercial UF resin:
  • Broad peak at 3500 cm⁻¹: Abundant hydroxyl (-CH₂OH) groups
  • 2920 cm⁻¹, 1450 cm⁻¹: N-H, C-H stretching vibration of urea units
  • 1610 cm⁻¹: Carbonyl C=O from urea and glyoxal co-reaction
  • 1070–1230 cm⁻¹: Ether and methylene bridge bonds
Minor peak intensity differences reflect varying degrees of reaction under different glyoxal dosages; no new harmful functional groups were generated during thermal degradation.

ESI-MS Molecular Weight Distribution (1:5 Optimal Group)

Mass spectrum test proves degraded fragments concentrate within 1000 u molecular weight, mainly distributed at 127, 163, 217, 309, 463 Da, consistent with conventional UF oligomer distribution:
  • 79 Da: Residual hydrated glyoxal monomer
  • 127 Da: Dihydroxymethyl urea core fragment
  • 163/217 Da: Methylene-bridged urea-glyoxal copolymers
The degradation reaction successfully breaks high-molecular cured UF crosslinked networks into reusable low-molecular adhesive prepolymers.

Curing Characteristics of Waste UF Degradation Liquid

DSC thermal analysis identifies a single concentrated curing exothermic peak for all samples, indicating uniform polymer composition and stable crosslinking behavior during hot pressing:
  • 1:5 group curing peak: 113.5°C (lowest curing temperature, highest crosslink efficiency)
  • 1:7 group curing peak: 131.8°C
  • 1:9 group curing peak: 142.2°C
Conclusion: 1:5 glyoxal ratio delivers the lowest curing temperature, reducing hot press energy consumption in mass production. 160°C hot press temperature fully completes crosslink reaction for all recycled adhesives.

Plywood Bonding Performance: Raw Degraded Liquid vs Flour Modified Adhesive

Test plywood specification: 3-layer poplar veneer, glue spread 150 g/m², hot press 160°C / 1.5 MPa / 5 min, test dry shear strength and 24 h cold water wet shear strength (Class II plywood standard ≥0.7 MPa).

Performance of Unmodified Degradation Liquid

Directly using waste UF resin degradation liquid as glue fails industrial standards:
  • Dry strength only 0.63–1.02 MPa, wet strength below 0.7 MPa for all groups
  • Thin 1:9 liquid causes excessive penetration, resulting in wet strength as low as 0.43 MPa
    Low viscosity lacks sufficient cohesive force and forms a loose cured film with poor hydrolysis resistance.

Optimized Flour Tackifier Modification Data

Add food-grade flour at 5%,10%,15%,20%,35% proportions to each degradation liquid, test bonding strength:

1:5 Glyoxal Group (Highest Base Viscosity)

  • Peak performance at 20% flour: Dry strength 1.94 MPa, Wet strength 1.49 MPa
  • Far exceeds GB/T 9846 Class II plywood threshold

1:7 Glyoxal Group

  • Optimal flour dosage 20%: Dry 1.33 MPa, Wet 1.34 MPa

1:9 Glyoxal Group

  • Optimal flour dosage 20%: Dry 1.22 MPa, Wet 0.97 MPa
Universal rule: 20% flour addition achieves maximum dry & wet bonding strength for all three degradation liquids. FT-IR comparison of cured films proves starch hydroxyl groups undergo co-crosslinking with UF oligomer amino/hydroxymethyl groups, forming denser, waterproof crosslinked networks that not only raise viscosity but permanently improve water resistance.

Industrial Production Recommendations for Waste UF Recycling

Optimal Degradation Formula & Process

  1. Solid waste UF: 40% glyoxal mass ratio = 1:5 (balanced solid content, low curing temperature, highest base bonding potential)
  2. Thermal degradation condition: pH ≤3, 85°C constant temperature, 5 h holding time
  3. Tackifier dosage: 20% food flour mixed evenly at room temperature before gluing
  4. Hot press parameter: 160°C, 1.5 MPa, 5 min holding time, same as standard UF production

Factory Cost & Environmental Advantages

  1. Circular economy benefit: Eliminate solid UF waste disposal fees, reduce raw resin purchasing volume.
  2. Low-cost modifier: Edible flour is cheap and widely available, no expensive chemical additives
  3. Green degradation agent: Glyoxal replaces strong acid/hydrothermal oxidants, no toxic waste discharge
  4. Product compliance: Modified recycled adhesive meets national plywood mechanical and environmental standards

Existing Limitations & Optimization Tips

  1. Storage limit: Recycled glue shelf life shorter than fresh UF; prepare batches for same-day use
  2. High glyoxal ratio (1:7/1:9) only suitable for thick veneers to avoid penetration
  3. Flour over 25% increases glue solid content excessively, leading to brittle cured film and reduced panel toughness.

FAQ

Q1 What solvent is best for thermal degradation of solid waste UF resin?

A: 40% aqueous glyoxal solution is the ideal choice. It hydrolyzes cured UF crosslinked networks under mild 85°C heating, generating low-molecular reusable oligomers without destroying core adhesive functional groups.

Q2 What flour addition ratio delivers the best plywood bonding strength?

A: 20% flour mass fraction relative to the degradation liquid is the optimal dosage for all glyoxal ratios, significantly boosting viscosity and forming crosslinked structures to enhance water resistance.

Q3 Does glyoxal recycled UF glue meet national plywood standards?

A: Yes. Under a 1:5 glyoxal ratio + 20% flour modification, plywood wet shear strength reaches 1.49 MPa, fully satisfying GB/T 9846 Class II plywood requirements.

Q4 Can wood panel factories adopt this waste UF recycling process on existing lines?

A: No equipment overhaul required. A degradation reactor can be added as an auxiliary unit, and hot-pressing parameters remain consistent with conventional UF production.

Q5 Why does unprocessed waste UF degradation liquid have poor water resistance?

A: Low viscosity leads to a loose cured polymer film with abundant hydrophilic gaps. Flour participates in co-condensation to build compact, hydrophobic crosslink networks, fundamentally solving hydrolysis defects.

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