Enhancing product information and materials verification
Scientists have been investigating several methods of testing thermally modified wood to see if it could be used to demonstrate product compliance.
The topic of materials substitution has once again been in the spotlight with well-reported issues of delays in materials supply and ballooning costs dominating media and causing concern for the New Zealand construction industry.
The problem with materials substitution
In the case of consented substitution, replacement of one product for another of comparable performance is entirely appropriate and legitimate. But problems arise when unauthorised substitution leads to an inferior material being used or its use has unintended consequences for compliance with other requirements around durability, weathertightness, fire safety and so on.
In the article Verifying materials compliance, we reported on a project aiming to chemically identify the composition of materials for the purpose of identifying non-compliant product. Since then, the Building Amendment Act 2021 has introduced new minimum information requirements for building products.
Using our expertise in Fourier transform infrared (FTIR) spectroscope (see Are materials the real deal? and Structural adhesive durability) and other analytical techniques, our attention turned to applying spectroscopy. We examined the potential for this technology to supply a materials verification component to the information already held for a particular building product.
Spectroscopic analysis of thermally modified timber
Four categories of materials were chosen for investigation in this project:
- Fire materials – detection of compositional differences.
- Sealants – effect of UV and accelerated ageing.
- Membranes – effect of UV and accelerated ageing.
- Thermally modified timber – ability to establish modification temperature.
This article focuses specifically on the results of our investigation into thermally modified timber to provide a practical illustration of how spectroscopic data could be used to enhance product information and materials verification.
All timber products are susceptible to the damaging effects of weathering and microbial attack. The radiata pine that is typically utilised in Aotearoa New Zealand is not durable enough to be used in its natural state and must be protected in some way. This is typically achieved through the application of chemical preservatives.
As an alternative, thermal modification aims to achieve improved timber durability without the need for chemical application. It involves heating wood in a controlled environment to produce irreversible changes in wood properties that confer the required durability characteristics for specific end-use applications such as decking and cladding.
Commercial processes vary widely amongst manufacturers – for example, in terms of modification temperature and duration as well as according to the species of wood.
Following thermal modification, chemical changes occur in the wood structure because of the degradation of some constituents and formation of new compounds. These chemical changes can be tracked spectroscopically.
Correlation of chemical changes to specific processing conditions for a particular wood species may then enable independent verification of the modification temperature and subsequent demonstration of product compliance – for example, with durability requirements.
The testing process
To investigate this, commercially produced radiata pine samples modified at temperatures of 210°C, 220°C and 230°C were used for testing and compared with unmodified samples (see Figure 1).
These temperatures were chosen based on typical processing conditions for Aotearoa products and to determine if samples could be distinguished within a narrow modification temperature range. Samples were modified in a European laboratory based on commercial technology used in Aotearoa.
Figure 2 shows FTIR spectra from solid samples of untreated and thermally modified radiata pine. The overlap of these spectra mean that these samples cannot be distinguished from one another. However, by applying a well-established statistical method known as principal component analysis (PCA) to the FTIR data, samples may be separated based on underlying chemical differences.
PCA is routinely used to transform large, complex datasets such as those generated from spectroscopic analysis into much smaller sets of data known as principal components (PCs) that are more easily interpreted.
The first PC (PC1) explains most of the difference between samples but is affected by factors such as the colour of the material, so subsequent PCs such as PC2 and PC3 are also considered when interpreting chemical differences between samples. In this way, PCA enables subtle chemical differences to be picked up that cannot be seen in the raw FTIR spectra.
PCA of the FTIR data from thermally modified samples showed clustering of samples based on modification temperature (see Figure 3). However, a significant area of overlap remained.
A literature review of previous research in this area suggested that a technique complementary to FTIR spectroscopy, known as near infrared (NIR) spectroscopy, could improve our ability to distinguish between the three groups of thermally modified samples. Figure 4 shows that this was indeed the case, with PCA of the NIR dataset clearly differentiating between samples based on modification temperature.
The results
These laboratory-scale findings suggest that a spectroscopic-based methodology offers a non-destructive, fast, reliable and relatively simple means of verifying the degree of thermal modification of wood and a potential pathway for demonstrating product compliance. Subsequent proof-of-concept testing with more samples would need to be undertaken in an industry setting to establish practical application.
Given the diversity of processing conditions and wood species used in the manufacture of thermally modified timber, further work would also be required to validate the findings from this project for products manufactured under a range of different conditions and with other timber species.
Note
This research was supported by the Building Research Levy and a Victoria University Summer Research Scholarship. Full results of the work, including materials other than the thermally modified timber, will be available soon. For more, contact [email protected]
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