Overview

Many American hardwoods can, in practice, be used externally if the correct design, detailing and construction techniques have been adopted. Coupled with this procedure is the proper use of pre-treatments, end grain sealers and the selection of a coating system, suitable for the particular exposure conditions. In order to maintain the protective coating “envelope”, an on-going maintenance programme is recommended.

All wood species, including American hardwoods, vary in their natural resistance to wood-destroying fungi. The long term performance of American hardwoods can readily be achieved by correctly selecting from a range of preventative measures.

The first stage is to ensure an “in-service” moisture content below 20%. Wood destroying fungi cannot grow at this level of moisture content.

As the second stage, consideration should be given to the selection and use of naturally durable wood species. Apart from white oak, walnut, cherry and sassafras, most other commercial American hardwoods are classed as non durable.

Finally, where the “in-service” moisture content will exceed 20% or where the inherent natural durability of the wood is insufficient, pre-treatment with chemicals (preservatives) that are effective against wood-destroying fungi and/or insects is advisable. It is possible to obtain the required penetration and retention levels for many American hardwoods. Recent developments in the field of pre-treatment processes have made this possible.

Relevant European Standards

EN 350-2 : 1994… lists the natural durability of solid wood to wood-destroying fungi for selected species. A five class system is used to define the resistance of heartwood:

Durability	Description
1	very durable
2	durable
3	moderately durable
4	slightly durable
5	not durable (includes all sapwood)

Cross referencing the relevant hazard class contained within EN 335-2 : 1992 is recommended.

Substrate behaviour

To realise the optimum performance of American hardwoods, an understanding of the behaviour that individual species have on both the pre-treatment and coating system is required.

The principle influences are the natural wood extractives contained within the timber substrate. Some of the many properties they impart are colour, resistance to wood-destroying fungi and the uptake or release of moisture.

As all American hardwoods vary in colour, so will their respective natural properties. This balance of properties can have a marked effect on the finished appearance of an opaque coating. As the natural colour of American hardwoods is soluble (to some degree) in either water, alcohols or organic solvents, this can impact on several pre-treatment or coating processes.

Using American red oak as an example; variations in the degree of both penetration and retention can be experienced when an organic solvent preservative is used, resulting in variable drying of the pre-treatment and the subsequent coating material. The effects of overcoating should be considered, particularly if a water-borne coating system was specified, as this could affect adhesive performance and discolour the finish.

Pre-treatments

Wood preservatives are classified according to their efficacy in particular environments. The level of treatment is further classified by penetration into the timber and the retention or loading in a specified zone (irrespective of pre-treatment method or wood species). In this move towards ”results”, as opposed to “process” orientated standards in Europe, an awareness of the latest technologies entering the marketplace, such as solvent free preservatives, are important.

Examples of preservative types

Organic Solvent (e.g. Acypetacs Zinc)
Vapour Phase Treatments (e.g. Boron)

Methods of application

Penetrating Treatment Process (examples are diffusion treatments, double-vacuum and vacuum pressure methods).
Superficial Application Process (examples are brush and spray techniques and short-time immersion (dipping) processes in wood that normally has only a few minutes contact time with the preservative).

The Preservative Pre-treatment “Blueprint”

the active ingredient must be effective against the target organism(s)
the active ingredient must be permanent
it should have the required penetration and retention values
it should be safe
it should be easy to use
it should have a minimal effect on the timber’s properties, for example: overcoating, moisture content, residual odours, gluing, and handling

Influences On The Absorption Of Preservatives

Timber varies in its permeability to liquids, whether it is water or a wood preservative. Therefore, to be effective against a particular target organism, a degree of penetration (and subsequently, retention) is required.

Currently, there are nine penetration (minimum depth) classes, P1to P9(reference: EN 351-1 : 1995). The ability to meet a penetration class will depend on the treatability of the timber species concerned.

A four class system of treatability is currently in use (reference EN 350-2 : 1994) and is based upon general observations associated with penetrating treatment processes. The treatability classification cannot be precisely defined. In treatment, irregular penetration can often be expected.

Coating systems

Coating manufacturers are having to recognise and anticipate the increasing pressure being exerted to limit the levels of certain Volatile Organic Compounds (VOCs) in their products.

Due to the growth in environmental awareness, legislation has and will be enacted that will result in a continual reduction in the solvent content of surface coatings.

The legislation that will be introduced into the marketplace will influence the trend towards high solids, low solvent systems and will continue to gain momentum over the next 10 years, as solvent emissions in Western Europe are likely to come under further pressure for limitation by legislation.

Information on individual species and the influence of surface coatings

This section provides information on five commercially important American hardwood species; ash, hard maple, red oak, white oak and tulipwood. For all species the colour will vary depending upon their exposure to light. The appearance of a coating system will be strongly influenced by the original timber colour. The extent of this influence can be established by the utilisation of a trial area. All species samples shown in this publication were treated with Danish oil.

Section prepared by Geoff Taylor, Technical Consultant, Akzo Nobel Woodcare (UK).

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