The Question of Durability25 November 2019
Our guest expert, Dr Simon Curling FIMMM, project officer at the BioComposites Centre in Bangor discussed decay and durability
The problem with wood is that it can rot! In a previous column I discussed the importance of thinking about biological durability when developing products (WBPI June/July 2018). As durability tests take months to conduct, as ever in science, asking the right questions can save both time and money.
This column discusses the questions that need to be asked when thinking about durability, and how this can aid product development. The first question requiring an answer is, what is the end use of the panel? This determines the hazards that the panel will face in service. Current testing standards use the use class system that categorise product testing requirements based on exposure criteria. Simply put, the lowest hazard class (UC1) is an interior dry location; moving through to possible wetting (UC2); exterior use above ground (UC3); exterior ground contact or submerged in fresh water (UC4) and finally submerged in salt water (UC5).
The next question is what challenges the wood panel will face in that environment? Fungi and insects are the two main groups that are an issue, depending on location. Looking at the fungal side, in an interior situation mould and staining fungi are the main issue, with decay fungi (brown rot and white rot) becoming a threat as exposure to moisture and wetting increases. If ground contact is involved, or wetting is prolonged, then soft rot fungi also need to be addressed.
Having decided what the hazards are, how is this tested? Initially panels are tested in a lab environment using a number of standard methods, for example: ASTM D3273 or EN 152 for moulds, ENV12028 and ASTM 1413 for decay fungi and DD ENV 807 for soft rot fungi. However, before biological testing, other factors may need to be taken into account, with one of the main issues being the glues and resins used.
Off-gassing from boards can impact the results of a decay test in a closed vessel so an ageing regime is used to counter the effect. Both treated and untreated boards can be aged either by leaching (EN84) or by evaporative heating (EN73) to give potentially more indicative results.
Next comes the actual biological test. All follow a broadly similar course with small samples being exposed to the test fungi for a set length of time (such as 16 weeks for a decay test). Exposure may be to single or multiple fungi in sterile test or exposure to an active soil for soil bed tests. Assessment is generally by mass loss or by an estimation of the level of growth on the material, although some non-standard tests use variables such as moisture or mechanical strength. Performance is usually compared to that of reference samples of timber whose performance in service is generally well known. Most of the fungal testing standards were developed with wood preservative testing in mind and then adapted to cover natural durability of timber. Now there is further research into adapting the methods for use with modified wood and wood products.
Insects, such as termites and borers, are also a threat to panels. A number of test methods exist (generally designed to test the preservatives) based on the insect threat, for example: EN117, EN118 and ASTM D3345 (termites), EN 20 (powderpost beetle), EN21 and EN49 (Anobium, ie ‘woodworm’), EN46, EN47 and EN1390 (longhorn beetle). The procedure for beetle tests consists of exposing the product to the larvae (or placing the larvae within the product) and waiting to see the survival rates of the insects. For termites the assessment is based on the level of damage to the material. In marine environments marine borers can be tested for using the EN275 standard.