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42 MATERIALS MAY/JUN 2026 FDM ASIA | www.fdmasia.com

ercanozay and the springback of compressed particles or fibres.
This phenomenon results in a permanent increase in
thickness during wetting redrying cycles, indicating the release
of accumulated compression stresses and reflecting fundamental
structural damage to the panel.
Repeated wetting and drying cycles cause progressive

damage to bonding points, leading to fatigue failure even when
individual cycles do not exceed the immediate bond strength.
An increase in moisture content has pronounced effects on
the mechanical performance of wood based panels.
Conditioning from an oven dry state to a moisture content
smaller particles can improve internal bond (IB) strength by of approximately 15–20 percent generally results in a 30–50
increasing contact area and reducing voids. However, smaller percent reduction in modulus of rupture (MOR), while modulus
particles may also result in lower adhesive availability per unit of elasticity (MOE) exhibits a comparable, though slightly less
surface area, which can negatively affect bonding. pronounced, decrease.

Therefore, the effect of particle size on IB depends on IB strength is consistently the most moisture sensitive
the surface specific adhesive amount. The geometry and property, with strength losses frequently exceeding 60–70
orientation of an element affect the surface area for bonding percent following moisture conditioning or aging treatments.
and swelling stresses. This pronounced sensitivity reflects the vulnerability of inter
The type of adhesive used is another critical factor in particle or inter fibre bonds to swelling induced stresses and
determining moisture resistance. Phenol formaldehyde (PF) bond line degradation.
adhesives perform better than urea formaldehyde (UF)

adhesives when exposed to moisture, while panels bonded Aging Methods
with polymeric methylene diphenyl diisocyanate (pMDI) are According to Kojima et al., thickness swelling is an important
highly moisture resistant. durability indicator because it reflects the cumulative effects
UF and PF adhesives mainly rely on mechanical adhesion of springback, bond line failure, and micro crack formation in
mechanisms, such as adhesive penetration into the wood mat formed wood based panels, and shows strong correlations
structure and mechanical interlocking after curing. with the degradation of mechanical properties.
In contrast, pMDI adhesives have been shown to provide Panels with poor bond integrity and limited resistance to
superior bonding performance and moisture resistance, partly swelling induced stresses exhibit rapid increases in thickness
due to stronger interactions with wood polymers through swelling, often exceeding 15–20 percent under accelerated

covalent linkages and favourable absorption. aging or severe moisture exposure.
pMDI generally requires lower adhesive contents than PF In contrast, panels with well developed, continuous bonding
or UF to achieve equivalent or superior bond strength, which networks exhibit low thickness swelling, typically below three
is important when considering performance differences under to four percent, even after severe laboratory aging or long
moisture exposure. term outdoor exposure.
Water absorption in wood based panels causes swelling This clear distinction in thickness swelling behaviour has
due to the hygroscopic expansion of the cell walls of wood been consistently linked to differences in IB retention and the