Wood preservation facilities, 2013 technical recommendations: chapter 1

1. The Need for Wood Preservation

1.1 Introduction

Wood preservation is the pressure or thermal impregnation of chemicals into wood to a depth that will provide effective long-term resistance to attack by fungi, insects and marine borers. By extending the service life of timber, wood preservation reduces the harvest of valuable forestry resources, reduces operating costs in industries such as utility and railroads, and ensures safe conditions where timbers are used as support structures. In addition to industrial and commercial application, a significant part of the treated wood volume is used for residential construction to protect homeowners' investments and provide outdoor living spaces that are a desired part of the Canadian way of living.

The chemicals predominantly used in Canada for wood preservation are as follows:

• aqueous formulations of arsenic, copper and chromium; borate; or copper, in combination with organic pesticides
• pentachlorophenol in petroleum oil
• creosote and creosote/petroleum oil mixtures

1.2 Wood Deterioration

Timber is subject to several types of deterioration following its removal from the forest. Wood decaying fungi and insects drastically reduce the usefulness of unprotected lumber and other forest products. Decay reduces wood to its basic components: carbon dioxide and water. This may happen quite rapidly depending on the exposure conditions. As an example, untreated red pine posts were found to be serviceable for only 4.5 years, whereas creosote-treated posts lasted between 40 and 48 years under the same conditions (1). Similarly, railway ties used in North America would have an average life of five years without treatment, whereas treated ties under normal service conditions last in excess of 30 years. Protection is also needed against wood-boring insects. For example, termites are responsible for extensive damage to wood in storage and in service in southern Ontario and on the Pacific Coast. Marine structures, such as untreated dock pilings along North American coasts, can be destroyed by marine borers in less than one year. In contrast, properly preserved wood structures in marine waters are reported to perform for 30 to 45 years (2).

The main enemies of wood, and the wood destroyers of greatest commercial importance, are decay fungi. The growth of these fungi is dependent upon the temperature, moisture content, oxygen level and nature of the wood. Wood products such as construction lumber, railway ties, bridge timbers, fence posts and utility poles, are usually in direct contact with moist soil or in locations where moisture collects and cannot readily evaporate. When there is no practical means of controlling the moisture, oxygen level or temperature, the options for the protection of such wood products are limited to the application of chemicals, which prevent fungal growth by making the wood unsuitable as a food source. Simultaneously, the treatment can also protect the wood from other wood-destroying organisms, such as insects and marine borers.

1.3 Wood Preservation Chemicals

The preservation of wood by chemical means can be traced back over 4000 years, to the time when the Egyptians apparently used bitumen to treat wooden dowel-pins in the stonework of temples (3). During the Roman Empire, tar, linseed oil, oil of cedar and mixtures of garlic and vinegar were used for the preservation of wooden statues. Charring of wood surfaces and soaking in brine, alum, arsenic or copper salts were other methods used in Roman times and in the Middle Ages (4). Investigations to define alternative wood preservation agents were reported in the late 1600s. Efforts escalated during the 1800s, when economic considerations, prompted by the need for durable wooden ships and railway ties and trestles, spurred the search for effective preservatives and application methods (5). A review of the many chemicals and chemical formulations used historically can be found in the above references and in texts such as those written by Hunt and Garratt (6) and Wilkinson (7).

Creosote and the full-cell treatment process have been used since the early 1800s, whereas the empty-cell processes were introduced in the first decade of the 20th century. Pentachlorophenol and the water-borne arsenical preservatives became of commercial significance in Canada during the 1950s and 1970s respectively. Current research has yielded not only modifications to existing formulations and treatment technologies but also the introduction of new preservative chemicals. The voluntary withdrawal of chromated copper arsenate (CCA) from residential markets in 2003 prompted the introduction of new organometallic preservatives, namely alkaline copper quaternary (ACQ) and copper azole (CA-B).

The choice of wood preservatives depends upon the character of the wood to be treated, the anticipated service and the properties of the chemical or formulation. Wood preservation formulations must:

• protect against attacking organisms;
• be able to penetrate wood;
• remain in the wood for the length of the intended service;
• be chemically stable;
• be safe to handle;
• be economical to use;
• not weaken structural strength; and
• not cause significant dimensional changes within the wood.

Other factors that determine selection of chemicals or formulations include fire resistance, colour or odour, paintability, corrosiveness, electrical conductivity and environmental considerations.

In Canada the predominant wood preservative chemicals or formulations are as follows:

Aqueous based formulations for residential use:

• ACQ (alkaline copper quaternary) - ACQ-treated products include lumber for patios and fencing in residential construction.
• CA-B (copper azole) - CA-B-treated products include lumber for patios and fencing in residential construction
• Borates - Uses are for wood components in interior applications.

Aqueous based formulations for commercial & industrial use:

• CCA (chromated copper arsenate) - CCA-treated products include agricultural fence posts, foundation lumber and plywood, utility poles and construction timber.
• ACZA (ammoniacal copper zinc arsenate) - ACZA-treated products include marine structures and construction timbers. At the time of publication, this preservative was not in use in Canada.

Oil based formulations for commercial & industrial use:

• PCP (pentachlorophenol) - PCP-treated products include utility poles and cross-arms.
• Creosote - Uses include treatment of railway ties, utility poles for export, and pilings and timbers for marine applications.

The development of alternative chemicals for wood preservation is the subject of ongoing research. The use of alternative chemicals will depend on industry and safety evaluations, and on approval under the Pest Control Products Act (PCPA), administrated by Health Canada's Pest Management Regulatory Agency (PMRA).

1.4 The Value of Wood Preservation

In 2010, the Canadian wood preservation industry sold, in total, $874 million Canadian dollars worth of treated product (industrial and residential) (8). Controlled studies have shown that wood preservation enhances the lifetime utility of wood by a factor of 5 to 10 or more, depending on the species, end use and efficacy of the treatment. This emphasizes the considerable contribution that the wood preservation industry makes to forest conservation (9).

Substituting alternative materials (such as steel, concrete or plastics) for treated wood in industrial applications (not including residential) could increase material costs to users.