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MATERIALS 49
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strength after severe accelerated aging, confirming that This study evaluated three industrial exterior grade panels—
PF adhesives, when properly formulated, are well suited two PF bonded particleboards (PB1: three layer; PB2: single
for exterior applications. layer) and one pMDI bonded MDF—after six accelerated aging
This is reassuring for the industry given the ongoing cycles (ASTM D1037 12). The main conclusions are as follows:
search for formaldehyde free alternatives. 1. PB2 (single layer fine particle board) demonstrated superior
• Directional anisotropy is panel specific. PB2 showed significant overall performance with the lowest residual thickness

orientation effects (25% higher MOR and 35% higher MOE swelling (0.49%) and high retention of MOR (67%), MOE
in the parallel direction under non aged condition), while (56–64%), and IB (75%).
MDF remained isotropic. It is strongly recommended for demanding outdoor
Product designers and fabricators should consider panel environments requiring both dimensional stability and
orientation when cutting panels for load bearing exterior structural integrity.
applications to optimise performance. 2. PB1 (three layer board) showed intermediate performance
• For MDF manufacturers aiming to develop exterior grade (Residual TS: 1.58%; MOR retention: 66–74%; IB retention:
products, improving the moisture resistance of the fibre 71%).
adhesive interface is critical. It is suitable for semi protected exterior applications.

Strategies may include increasing adhesive coverage, 3. MDF performed poorly under unprotected accelerated aging,
using more moisture resistant adhesives (such as pMDI, as with excessive thickness swelling (5.43%) and very low IB
used here, which alone was insufficient for IB retention), or retention (30%).
incorporating water repellent additives. Unfinished MDF is not recommended for unprotected
The poor IB retention of MDF (30%) despite pMDI bonding outdoor use. However, with effective surface coatings, its
suggests that fibre morphology plays a dominant role that bending properties (MOR and MOE retention above 50%)
adhesive chemistry alone cannot overcome without additional may still be acceptable.

treatments. 4. Directional anisotropy varied by panel type. PB2 showed
significant orientation effects (higher MOR and MOE in the
Conclusions parallel direction), while MDF remained isotropic.
All panels were evaluated in an unfinished condition, whereas Panel orientation should be considered in product
in practical applications they are typically coated. Future design for load bearing exterior applications.
research should investigate the effectiveness of various 5. Practical guidance for the wood industry: Single layer fine
protective coatings (paints, laminates, overlays) in improving particle particleboard is a viable and potentially superior
the durability of these panels, particularly for MDF. alternative to three layer structures for exterior siding.
Additionally, long term outdoor exposure studies should be PF adhesives provide adequate durability for exterior

conducted to correlate accelerated aging results with real world particleboards. For MDF, improving fibre adhesive interface
performance, thereby validating material selection guidelines moisture resistance remains a key challenge.
for specific service conditions. Future research should investigate the protective effects
Microstructural characterisation (e.g., particle orientation of surface coatings and correlate accelerated aging results
analysis using X ray microtomography) would help confirm with long term outdoor exposure to further validate material
the mechanisms behind PB2’s exceptional dimensional stability selection guidelines for specific service conditions. FDM
and the anisotropy observed in PB1 and PB2. ENQUIRY NO. 3301