Thursday, 4 April 2013

Designer materials: Helping nature?

Trying to keep up with advances in science and technology to support my teaching and consultancy in building sustainability, is getting harder and harder. I don't think it's just my age; rather there has been a change in the whole paradigm.

It would be fair to say that we have arrived in an era where it is increasingly possible to say that you want a particular performance out of a building element, and legitimate to expect that a brand-new material can be tailored to meet that required performance.

Putting it that way makes it sound like the materials are necessarily terribly high-tech – like the novel steel alloy conjured up for the Birdsnest arena for the Beijing Olympics. Or some glazing or other surface, typically invoking the prefix 'nano'.

The reality is much more prosaic.  A good case in point is what has happened with timber and timber products.

Back in the 1960s it was recognised in Australia that continued exploitation of native hardwoods for building was unsustainable, in large part because of their slow growth. Substitution with fast-growing introduced softwoods itself raised a number of other sustainability issues. Chief amongst these on the building site (as distinct from the source forests) was limited durability. The response at the time was pressure impregnation with copper chrome arsenate (CCA), trademarked as ‘tanalised’.  Initially only used to treat timbers for external use, compounds in CCA preservative are toxic to people, with arsenic a known carcinogenic.  The threat of fatal consequences in the case of fire inhibited the use of these treated timbers in house framing.

Around the turn of the century, a new product, generically known as 'blue pine' was developed, using sprayed or dipped synthetic pyrethroids, that provide termite protection when used within house frames for at least 25 years. The synthetic pyrethroids, Permethrin and Bifenthrin are commonly used for head lice treatments and fly sprays and so pose very little or no risk to humans during handling and whilst in place in house framing applications. Because framing timber can be treated in line in mill production, it is so much more cost effective than past systems, that it has become ubiquitous in individual dwelling construction.

Because it is a renewable resource, with the additional benefit of sequestering carbon, other frontiers for use of timber were also beckoning. Engineered timber products had been pioneered with lamination for discrete members (Gluelam), and plywood for panels. Stranded boards and particle boards evolved to overcome difficulties with moisture resistance, and later a bad reputation for off-gassing from the adhesives used. More recently, we have seen an explosion of developments including laminated veneer lumber (LVL), culminating in reconstructed timber products that obscure the boundaries between sticks and boards, and indeed between structure and cladding. Cross Laminated Timber (CLT), which has a structural strength comparable to the traditionally used concrete and steel, has recently been successfully used to build the world's tallest timber apartment block in Melbourne.

But Blue Pine is blue, even if only light blue, and not quite up to meeting our desire for durable timber finishes where appearance is a critical criterion. Nor are the reconstructed timber products. What was desired was a sustainably grown timber, where nature has been given a helping hand to overcome its disadvantages.

As far as I can tell, that is how we came to acetylated wood modification, a process which increases the amount of 'acetyl' molecules in wood, thereby changing its physical properties. The process protects wood from rot by making it "inedible" to most micro-organisms and fungi, without making it toxic. It also greatly reduces the wood's tendency to swell and shrink, making it less prone to cracking and ensuring that, when painted (or more likely coated with a translucent finish to celebrate the timber), it requires dramatically reduced maintenance.

But I didn't set out to write a short story of timber products in order to sound like I am selling you one. It came about only as an illustration of my earlier point:
These days there is not a lot of hope of maintaining a comprehensive overview of available material choices – the required skill appears to have become the ability to define what you want in words suitable for a Google search if the material already exists, or for a brief to the material scientists and manufacturers, if it doesn't.  The expert knowledge on the other hand, appears to have shifted to reading the environmental credentials of the products.
So, to complete my example, here are two key graphs from reports made available by the manufacturers of Accoya wood.  The first shows embodied greenhouse gas per unit volume comparisons with common building materials, calculated on a 'cradle to gate' basis.
The second shows the same information, from a different source (TU Delft), when applied to an actual building component, in this case a window frame.
Both graphs raise immediately interesting questions:
  • In the first one, the product seems to have counter-intuitively better credentials than directly competitive untreated timbers.  The explanation turns out to be that one of the waste products of the acetylation process is acetic acid, which is a valuable feedstock in other industries.  Therefore, the GHG of the process is split between two products, the Accoya wood, and the acetic acid.  
  • In the second graph, the question is how come the figures are negative for Accoya (and for sustainably grown meranti)– in other words the use of the products is benign to the environment?  The answer turns out to be how sequestered CO2 is treated in the calculation.  Because of their likely long life installed in a building, these timber window frames lock away carbon long enough to outweigh the carbon cost of the production.

If you are up to it, you now should read:
Cradle to Grave Carbon Footprint Assessment for Accoya® Wood and its applicationsand if you are truly committed:
Greenhouse Gas Emissions Assessment for Accoya® Wood – Public Version
and
Accoya® Wood 2012 cradle-to-gate carbon footprint update

2 comments:

Vince said...

I was quite interested in what sort of greenhouse gas emissions or potentially hazardous chemicals might be either inputs or by-products in the industrial production of acetic anhydride (the chemical used in the acetylation process) so I had a bit of a read through the cradle to grave carbon-footprint analysis. There wasn’t much on the acetic anhydride in particular, but I did find something that really grinds my gears.

On page 10 it says that it is assumed that in the end of life stage, the wood will be combusted as an alternative to fuel. From a limited point of view, using a waste product as fuel in an attempt to replace fossil fuels as a source of energy is an admirable goal. It must also be noted, however, that wood itself is an organic carbon compound, and as such when it is burnt it produces Carbon Dioxide and, where Oxygen supply to the reaction is insufficient, Carbon Monoxide.

This is fundamental to a critical understanding of the argument of what is “sustainable” by which the argument for the replacement of fossil fuels is made. I believe that the greenhouse effect and global warming are issues sufficiently well understood that I do not have to go into detail about how they work. What should be plainly obvious is that the production of Carbon Dioxide in the end of life phase of a product whose principal argument for environmental sustainability is carbon sequestration is not the most ideal of situations.

What frustrates me about the way the carbon footprint analysis is used is that there is absolutely no need to embellish the performance of the Accoya products. The stated greenhouse gas emissions offset of the Accoya products are around -13kg during the end of life phase, and contributes to an average total emissions of -13kg. Compare this total to that of Aluminium (132.51kg), PVC/Steel (115.98kg) and unsustainably sourced Red Meranti (314.71kg). Even without the supposed end of life gains the Accoya product is by far and away a better option in terms of its carbon footprint.

Push this even further with the understanding that the acetylation process gives the fast-growing softwoods material properties much closer to hardwoods, making them more suitable for structural purposes and therefore in direct competition with Steel and Aluminium and we have a very strong argument for the wider application of this product.
This is clearly an excellent product with great potential for the future of the construction industry. It just disappoints me that they always have to try to make it seem even better than it is.

Steve King said...

Good points! Thanks Vince.
Steve