Timber Preservation

“Complete sapwood penetration is always achievable”

George Beasley
Production Manager

The Need To Treat With Preservatives

As with most softwoods, New Zealand pine is not a naturally durable species and its use in New Zealand for structural purposes has gone hand-in-hand with the development of an efficient wood preservation industry.

Unlike many traditional softwoods the sapwood of New Zealand pine is very permeable to wood preservatives, particularly in the radial direction. Complete penetration of the sapwood is always achievable, resulting in very extensive service lives for such commodities as electric power or telecommunications transmission poles, marina poles, ground retention etc. Such total penetration with preservatives is rarely achieved with other softwood species.

Chemicals For Preservative Treatment

To a large degree, in-service exposure conditions dictate the types of preservative one can use to treat New Zealand pine. Our own products use CCA.

Copper-Chrome-Arsenate (CCA)

Freshly steamed poles exiting the conditioning plant.

Freshly steamed poles exiting the conditioning plant.

CCA has universally been found to be a very effective wood preservative. It is very suitable for treatment of New Zealand pine which will be used in moderate or high decay hazard environments. Once the solution is in the wood, complex chemical reactions occur which firmly bind CCA to the wood, making it extremely resistant to leaching out.

In all tests controlled by the New Zealand Forest Research Institute (NZFRI), performance has been equal to or has exceeded that expected of naturally durable species from throughout the world. Long term experience has been gained through extensive field testing of a wide range of treated products.

Processes have been developed to accelerate this fixation process to minimise or even eliminate the possibility of environmental contamination associated with the use of CCA.

However, where environmental or health legislation has forced restrictions on lumber treated with CCA, there are alternative formulations which are ideally suited for treatment of New Zealand pine. Under our current R & D programmes Croft Timber is currently experimenting with several alternatives among which is Ammonium Copper Quaternary (ACQ).

Preservative Treatment Processes

An important feature of New Zealand pine is that it can be treated easily.Most of our product is APM treated.

Not only is the sapwood of New Zealand pine easy to treat, but the relatively small amount of heartwood present can sometimes be treated as well. Research has shown that penetration of preservative into heartwood is improved by high-temperature drying or by steamconditioning before treatment.

Poles exiting the treatment plant.

Poles exiting the treatment plant.

At Croft Timber Company, all roundwood and timber specified for critical end use is steam conditioned.With this process the whole of the thin walled ray parenchyma is removed creating uninterrupted pathways for easy radial movement of the preservation solution.

Not only is this method most effective in achieving ultimate penetration levels but it is also the fastest and this has been key to Croft Timber successfully meeting the high demands of export and shipping deadlines.

Preservative Treatment for Specific End-Use Conditions

There are a number of ways of writing standards or specifications for preservative treatment.Most common are Commodity Standards (e.g. USA), Process Specifications (UK), and Hazard Class Specifications (New Zealand,Australia).

With hazard class specifications, the nature of the biodegradation risk (decay, wood-boring insects or termites) is first determined from the wood exposure conditions (e.g. indoors, protected from the weather, outdoors, in ground contact) and the preservative retention and penetration into the wood are varied to reduce the risk of biodegradation to an acceptable level.

In New Zealand, roundwood (posts and poles), sawn lumber, and plywood are treated to the following six hazard class levels. Preservative treatment requirements are generally equivalent to or exceed those of other countries which have formal wood preservation standards.

H1.1 – Sawn lumber used in situations continuously protected from the weather.The purpose of preservative treatment is to protect against attack by wood-boring insects.H1.2 – Sawn lumber and plywood used in interior situations where there is a risk of moisture content conducive to decay. LOSP is the main preservative used.H3.1 – Sawn lumber and plywood which will be used in exterior situations but not in contact with the ground where periodic wetting might occur. LOSP is the main preservative used.H3.2 – As for 3.1 but for exposed exterior situations and more critical end uses. CCA is the main preservative used.H4 – Sawn lumber, roundwood and plywood used in ground contact in non-critical situations. CCA and ACQ are used in New Zealand for wood in this category.

H5 – Sawn lumber, roundwood and plywood used in ground contact with extreme decay hazard or critical end-use requires greater protection – mainly for house foundation piles and transmission poles. CCA and ACQ are approved for this use. Preservative retentions are 33% higher than those of Hazard Class H4.

H6 – Sawn lumber and roundwood used in land based environments.