Wood Guide

Introduction

Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants.

It is an organic material – a natural composite of cellulose fibres that are strong in tension and embedded in a matrix of lignin.

Aside from water, wood has three main components.

1. Cellulose – A crystalline polymer derived from glucose, which constitutes about 41-43%

2. Hemicellulose – around 20% in deciduous but nearer 30% in conifers.

3. Lignin – 27% in coniferous wood and 23% in deciduous wood.

These three components are interwoven, by direct covalent linkages.

The Chemical composition of wood varies from species to species but is approximately 50% carbon, 42% oxygen, 6% hydrogen, 1% nitrogen and 1% other elements.

Hard Versus Soft

It is common to classify wood as either softwood or hardwood.

The wood from conifers (e.g. Pine) is called softwood and the wood from dicotyledons (usually broad-leaved trees e.g. Oak) is called Hardwood. These names are a bit misleading as hardwoods are not necessarily hard and softwoods are not necessarily soft. The well-known balsa (a hardwood) is actually softer than any commercial softwood. Conversely some softwoods (e.g. Yew) are harder than many hardwoods.

Heartwood / Sapwood

Heartwood is wood that as a result of a naturally occurring chemical transformation has become more resistant to decay.

Sapwood is the younger, outermost wood in the growing tree. It is living wood, and its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season.

Growth Rings

Tree rings grow under the bark, and the bark is pushed out while the tree is growing. The inner part of a growth ring is formed early in the growing season, when growth is fast and is known as early wood. The outer portion is the late wood and is denser than early wood. Many trees in places with hot summers and cold winters make one growth ring a year.

The greater the proportion of late wood the greater the density and strength.

Water Content

Water occurs in living wood in three locations namely.

• In the cell walls
• In the protoplasmic content of the cells
• As free water in cell cavities and spaces

Wood that is thoroughly air-dried retains 8-16% of the water in the cell walls.

The general effect of the water content upon the wood is to render it softer and more pliable.

Specific Gravity

The single most revealing property of wood as an indicator of wood quality is specific gravity as both pulp yield and timber strength are determined by it.

Extractives

Aside from the structural polymers wood contains a large variety of non-structural constituents, composed of low molecular weight organic compounds called extractives.

These compounds contribute to various physical and chemical properties of the wood, such as wood colour, fragrance, durability, acoustic properties, adhesion and drying.

Most extractives are lipophilic and only a small percentage are water soluble unfortunately this small percentage can be absorbed through the paint film as a stain.

Application

A good paint job can be expected to last up to 10 years.

Paint on external wood will erode gradually, however premature failure is quite common.

The usual reasons are:

1. Excessive and prolonged substrate moisture levels.

2. Poor preparation and product selection

3. Joinery Design

4. Fungal attack

5. Migration of water-soluble extractives

6. UV degradation

7. Mildew – a form of stain fungi

All coatings systems require a regular maintenance schedule to prevent premature failure. Translucent finishes require more frequent maintenance than opaque finishes.

Dimensional stability of a species has a significant impact on joinery and cladding life span.

For stability heartwood should be preferable to sapwood for given species.

Modified timber such as Accoya, Acetylated wood, and Thermowood (steam dried) are becoming more popular because of their excellent dimensional stability.

Micro-porous or breathable paints

The concept of micro porosity stemmed from the need to control and reduce the build-up of moisture within wood joinery to below 25%. Generally accepted as the point at which timber starts to become susceptible to attack from wood destroying organisms.

It can be argued that most paints are micro-porous to a certain extent.

It is a common misconception that micro-porous coatings can allow water to escape from painted wood without allowing it to enter.

If a coating is permeable, it will allow moisture vapour to cross in both directions’ dependant on the relative vapour differential which exists on both sides of the paint film.

Many of the arguments in favour of using micro-porous paints can be dismissed by careful detailing and good design which can go a long way to excluding water uptake in the first place. One can make a case for using a permeable paint system as a measure to offset or delay the inevitable consequences of bad design.

However, one can make an equally valid argument for using less permeable paint systems, because of their greater properties of extensibility, they can effectively prevent moisture penetration and resist the onset of premature cracking. Food for thought.

Timber Treatment

Wood easily degrades without some sort of preservative or modification.

Creosote was the first wood preservative to gain industrial importance more than 150 years ago. It is still used today for the protection of industrial timber where a long service life is essential.

Chromated copper arsenate (CCA) is a preservative that was very common for many decades. However, like creosote, concerns have been raised in the last decade over environmental and health issues, therefore preventing their use for residential or domestic construction.

Safer chemical preservatives are now available. These include.

• Alkaline copper Quaternary (ACQ)
• Copper Azoles (C.A.B and CA-C)
• Copper Naphthenate
• Light organic preservatives (LOSP)

Wood treated with modern chemicals is generally safer to handle given appropriate handling precautions and personal protection measures.

Modified Wood

Chemical Modification at the molecular level can greatly improve dimensional stability and durability. Two processes in particular have become very popular over the last decade.

a. Acetylation ( Accoya )

b. Heat treatment ( Thermowood )

Preservative Application

The first attempts to protect wood from decay and insect attack consisted of either brushing, rubbing, or soaking preservative onto the surface of the wood.

However, penetration was minimal. Consequently, resistance to attack was poor.

Newer application processes in which the treatment is carried out in closed cylinders with applied pressure or vacuum have greatly improved absorption and control of the preservative.

Whilst modern preservation methods give excellent control and penetration, properties such as adhesion and stain resistance can be compromised by the addition of additives such as wax, resin, and dyes. Therefore, we do not recommend our products are applied to wood which has had water repellent preservative treatment applied.

All BEDEC products can be applied to suitable wood.

Barn Paint is a water based self-priming and self-undercoating acrylic paint designed for the quick application to external surfaces, including wood. This coating provides a tough and highly durable finish which protects external buildings from sunlight, rain and severe weather while being flexible enough to move with the underlying surface without cracking.

MSP can be applied to internal, as well as external surfaces and like Barn Paint exhibits excellent durability and flexibility.

Extra-Flex Masonry paint can be applied to most surfaces, including wood, where significant movement of the substrate is a concern.

When painting porous wood which has not been previously painted, we recommend the first coat of the paint is diluted by 30% with clean water followed by two (or more) undiluted topcoats. This helps the paint penetrate the wood and provide greater protection and adhesion.

Treated timber should be left to weather for 6 months before painting.

Staining

Staining of painted wood can occur when water soluble extractives (such as tannins) within the wood are dissolved and leached from the wood by water. The water then moves to the paint surface, evaporates, and leaves the extractives behind as a reddish-brown stain.

All woods, stain to a greater or lesser extent. As well as allowing the wood to weather before applying a surface coating the application of a stain blocking agent such as BEDEC ALL PRIME or BEDEC STAIN BLOCK will reduce the extent of staining.

The following table describes popular wood varieties and their properties. The higher the degree of water solubles then the greater the risk of staining.

However, penetration was minimal. Consequently, resistance to attack was poor.

Newer application processes in which the treatment is carried out in closed cylinders with applied pressure or vacuum have greatly improved absorption and control of the preservative.

Whilst modern preservation methods give excellent control and penetration, properties such as adhesion and stain resistance can be compromised by the addition of additives.

Modified Woods List

Timber List