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The use of bark particles in urea-formaldehyde (UF) resin adhesives has become an emerging strategy in composite panel manufacturing. By adjusting the size of bark particles, manufacturers can modify the bonding efficiency, mechanical performance, and environmental characteristics of wood-based panels. Understanding these effects is critical to achieving optimal balance between strength, cost-efficiency, and sustainability.
Bark is a by-product of wood processing that contains natural compounds such as lignin, tannins, and extractives. These compounds interact chemically with UF resin, influencing both adhesion and curing behavior. Depending on their dimensions, bark particles can serve as fillers, partial substitutes for wood flour, or reactive modifiers that improve the bonding interface. The particle size distribution plays a key role in how effectively the resin wets and adheres to the substrate.
The particle dimensions of bark directly affect the internal bonding strength and modulus of rupture of UF-bonded panels.
Fine Bark Particles (below 0.2 mm) Fine bark particles distribute uniformly in the adhesive matrix, promoting strong cohesion. Their small size enhances resin penetration into wood fibers, leading to higher internal bond strength (up to 0.5 MPa improvement observed in some studies). However, excessive fineness can increase resin viscosity and reduce workability during hot pressing.
Medium Bark Particles (0.2–0.5 mm) Medium-sized bark particles balance mechanical reinforcement and resin flow. They provide micro-mechanical interlocking that enhances the modulus of elasticity without sacrificing adhesive spreadability. Panels made with this fraction often exhibit stable strength values with lower formaldehyde emissions.
Coarse Bark Particles (above 0.5 mm) Coarse bark fragments create discontinuities within the adhesive layer. They act as stress concentrators, decreasing bending strength and dimensional stability. The gaps formed around large particles can result in incomplete wetting and uneven curing, leading to localized delamination under moisture exposure.
The hydrophilic nature of bark depends on its extractive composition and particle surface area. Smaller particles expose more reactive hydroxyl groups, increasing water absorption potential, while larger particles contribute to internal void formation.
| Particle Size | Water Absorption (24 h) | Thickness Swelling (24 h) | Resin Consumption |
|---|---|---|---|
| < 0.2 mm | High | Moderate | High |
| 0.2–0.5 mm | Medium | Low | Medium |
| > 0.5 mm | Low | High | Low |
The table shows that medium bark particles achieve the best balance between dimensional stability and resin economy. Excessively fine particles may lead to denser but more water-sensitive panels, whereas coarse particles reduce uniformity and increase swelling.
Bark particle dimensions also influence the curing kinetics of UF adhesives. Fine bark increases the available surface area for heat transfer during hot pressing, leading to faster gelation and improved polymer network formation. Conversely, coarse bark acts as a thermal barrier, slowing the curing process and requiring longer press times or higher temperatures to reach complete polymerization. Differential scanning calorimetry (DSC) tests typically show a curing temperature increase of 5–8 °C when coarse bark is used compared with fine particles.
Using bark as a filler can reduce formaldehyde emissions, particularly with medium or fine particle sizes. Bark tannins and phenolic compounds react with free formaldehyde, forming stable chemical bonds that trap volatile molecules. Panels incorporating 10–15 % bark powder often show emission reductions of 25–35 % compared to standard UF-bonded panels. However, oversized bark particles may not participate effectively in this reaction, resulting in less improvement.
Selecting the appropriate bark particle dimension depends on production objectives:
For high-strength furniture-grade panels, fine particles below 0.2 mm are preferred to maximize bonding density.
For general-purpose panels or flooring substrates, medium fractions of 0.3–0.4 mm offer a balance between performance and cost.
For lightweight or acoustic applications, larger particles may be used intentionally to introduce controlled porosity, though at the expense of mechanical strength.
Process parameters such as resin content, press temperature, and moisture level must also be adjusted accordingly to maintain consistent curing and bonding outcomes.
Bark particle size is a critical factor influencing the mechanical, physical, and environmental properties of UF resin-bonded panels. Finer particles enhance bonding and reduce emissions, while coarser particles lower density and mechanical strength. By tailoring bark particle dimensions to the intended panel application, manufacturers can optimize performance while promoting sustainable resource utilization.
