Durability of treated PUR-bonded wood

By and - , Build 185

A project examining use of polyurethane adhesives for laminated structural timber elements in local conditions has shown acceptable performance.

Figure 1: Areas of delamination visible in two different samples after ageing.
Figure 2: The similar FTIR spectra of PUR adhesive (top) and wood (bottom) after ageing at 65°C and 100% RH indicate no significant degradation after 9 months.

ENGINEERED WOOD PRODUCTS (EWPs) rely on adhesive bonds for their structural performance. In the article PUR-bonded wood products in Build 162, we reported on the collaborative BRANZ and University of Auckland research project assessing the performance of one-component polyurethane (PUR) adhesives for laminated structural timber elements.

The project was initiated in response to an industry need for technical information on the durability of preservative-treated radiata pine bonded with PUR adhesives in New Zealand’s environmental conditions.

Range of PUR adhesives tested

The study considered the durability of different PUR adhesive formulations. Radiata pine was used for the test samples, and these were either untreated or preservative treated with chromated copper arsenate (CCA) or micronised copper azole (MCA).

Test specimens were bonded with different PUR adhesives and compared to samples bonded with traditional resorcinol-formaldehyde (RF) adhesive. Solid timber samples were also included in the study to provide a performance reference for the adhesive bonded samples.

Bond performance was evaluated using a range of tests including conventional delamination and shear strength testing. Fourier transform infrared (FTIR) spectroscopy provided chemical information complementing the mechanical test data, while scanning electron microscopy was used to assess adhesive penetration into the wood.

The effects of temperature and moisture on the adhesive bonds were investigated through a comprehensive experimental programme comprising both accelerated and natural ageing of samples as well as ambient and wet testing (see Table 1).

Table 1 Table 1: Summary of experimental test conditions and ageing methods

More delamination in latewood

Standard delamination test results showed that untreated and preservative treated samples met the delamination requirements of AS/NZS 1328.1:1998 Glued laminated structural timber – Performance requirements and minimum production requirements. However, interesting differences were noted between areas of earlywood (wood formed at the beginning of the growth season) and latewood (wood formed later in the growing season).

While minor delamination was associated with RF bondlines and PUR bondlines in earlywood areas, more delamination was observed with PUR bondlines in latewood areas (see Figure 1).

Greater delamination appeared to be related to less adhesive penetration into the timber and when there was a higher percentage of latewood adjacent to the bondline. The orientation of the annular growth rings was also found to be important. Samples with alternating growth ring orientation and outer laminations with the pith side facing inwards showed more delamination.

Figure 1: Areas of delamination visible in two different samples after ageing.

Opportunities for mill and manufacture

Controlling the angle of growth rings is likely to be difficult to achieve in practice for sawmills since this is constrained by cutting logs for maximum yield. Adjusting ring orientation is included in European Standard EN 14080 but is not a production requirement in New Zealand.

It also presents a problem for timber manufacturers during fabrication, especially when the timber boards are cut down into smaller pieces to remove defects before finger-jointing.

Despite the challenges, developing an automated process that optimises growth ring angle and orientation may be worth considering in the context of delivering improved product performance.

Adhesives resistant to degradation

Ageing of the samples led to a reduction in strength, but this was found to be generally comparable across all adhesives. FTIR spectroscopy of the PUR adhesives before and after ageing did not reveal any notable changes in their chemical structure, suggesting that the PUR adhesives were resistant to degradation under the test conditions (see Figure 2). However, deterioration because of heat or moisture over a longer timeframe cannot be excluded.

Under wet conditions, the shear strength for all bonded and solid specimens dropped significantly, although they were all still within a comparable range. Wet samples had a moisture content of 120–150% when tested. Shear strength reduction when wet was greatest for the treated PUR-bonded samples.

Figure 2: The similar FTIR spectra of PUR adhesive (top) and wood (bottom) after ageing at 65°C and 100% RH indicate no significant degradation after 9 months.

More work on effect of preservative treatments

A general comparison of the data suggested that CCA-treated timber was more challenging to bond. Based on an ageing model fitted to the results, the CCA-treated and untreated specimens had the highest and lowest degradation rates, respectively, for shear strength retention.

However, preservative treatment was not found to affect the penetration behaviour of the adhesives observed by the scanning electron microscopy, nor did FTIR spectroscopy indicate any detrimental effect of preservatives on the bondline.

The untreated and preservative-treated timber used in this study was sourced from a range of sawmills. Combined with the inherent variability of wood properties, further investigation is still required to establish the effect of preservative treatments on PUR bond performance.

There is no evidence from this study that would raise concerns around the use of PUR adhesives for service class 3 bonding radiata pine preservative treated with CCA or MCA to hazard class H3.2. Further work would be required to assess the performance of bonding preservative treated timber with higher retention levels and when exposed to in-ground conditions.

Note

This work was the basis of a PhD in Engineering awarded to Zubin Karami by the University of Auckland and was supported by the Building Research Levy. 

Full results, including those from the wider structural adhesives research project, will be available later in the year. For more information, contact Dr Catherine Nicholson at [email protected]

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Figure 1: Areas of delamination visible in two different samples after ageing.
Figure 2: The similar FTIR spectra of PUR adhesive (top) and wood (bottom) after ageing at 65°C and 100% RH indicate no significant degradation after 9 months.

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