In the last issue I mentioned the high moisture contents found in living trees and therefore in any fresh wood that is used to make wood based panels. Whatever the panel type and whatever the origin of the wood, that wood must be dried at some point in the production.

The wood in living trees has a high moisture content because of the transport of sugars and nutrients in the sap. Essentially, water enters the tree via the root system and departs through the leaves by the process of transpiration.

The fibreboard market is dominated by MDF-type products. Such products are normally made via the ‘dry process’, which means that the fibres are dried prior to being formed into a mattress that is subsequently hot-pressed. MDF is, however, a relative newcomer to the market compared to other fibreboards as the very first fibreboard products were made using the ‘wet process’, which is similar to paper making, more than a century ago.

A chamber method differs from the gas analysis method described last time in that formaldehyde emission is measured in a controlled atmosphere that is close to typical ambient conditions in a room.

As a reader of WBPI you are probably aware of much of the debate and data surrounding formaldehyde emission from wood based panels. You might not, however, be familiar with the test methods that are used to determine formaldehyde emission rates, so I thought it would be useful to give an overview of the main tests used to obtain emission measurements.

Manufacturers offer a wide range of solid colours, photo-finishes of marble, leather and wood grain. These can be particularly effective when pressed onto a panel with an etched platen that mimics the roughness of the real material being simulated. A vast number of textured finishes can be combined with many different colours and so the complete range is enormous – and growing all the time. This flexibility has contributed to the success of particleboard furniture as it has given designers the opportunity to use their imaginations to the full.

Human beings are very visual animals in that a lot of their understanding of the world around them comes from what they see.
The invention of the microscope in the early 17th Century opened up a new world, the observation of which paved the way for significant advances in biology, geology and medical science. The resolution possible with  light microscopy is limited to around 200 nano metres (2x10-7 metres) because of the wavelength of light. This resolution is sufficient to understand the anatomy of wood cells, but is not sufficient to see the  complex detail of the ultra-structure of the cell walls – that is, the layers of fibrils that make up the cell walls.

In the last issue I discussed panel cooling, which raised a few questions about mat temperature during hot pressing and so in this issue I intend to discuss what happens inside the mat during hot pressing.I have already touched on this subject in this column before. Those of you who have carefully kept previous issues can read my views on how steam affects the maximum panel width and density profile of hot-pressed products in WBPI 24(1):60 and 24(2):56.
The core layer of a panel is mainly heated by the penetration of steam generated in the surface layers of the mat. Consequently, the maximum temperature of the core is limited to the steam pressure inside the mat up until the point that the mat is completely dry, then conduction and convection mechanisms can raise the core temperature to that of the platens. Of course, this never happens in a commercial press because the press cycles are too short.

Acommon step in the manufacture of all wood based panels (WBP), except cement bonded particleboards, is hot pressing. Panels bonded with amide adhesives, eg UF or MUF, must be cooled after they have been taken out of the press.

Of course, the pressures used to manufacture most types of plywood are significantly lower than those needed for particle- and fibre-based products. However, the pressures are still high. Consequently, when the press opens, a panel of any type might stick to one of the press platens, causing a hold-up in the production.
The risk of this can be minimised by using release agents. External release agents are those applied either to the platens or to the surface of the panel, whereas internal release agents are mixed with the panel binder.
Oils and fats, when applied to cake tins and baking trays, act as release agents when cooking. They reduce the contact between the uncooked ingredients and the metal, thus reducing the potential for mechanical interlocking. As all cooks know, this does not work 100% of the time and in addition, the release agent must be applied each time the cooking utensil is used.
Applying a release agent for each panel is not a practical or cost-effective method in a factory situation. Modern release agents therefore consist of bi-polar molecules that are molecules which have a positive and a negative end. These molecules will arrange themselves so that their most compatible end will be orientated to the substrate.
For example, a long-chain carboxylic acid has a hydrophobic tail and a hydrophilic head so when applied to a metal surface the molecules will naturally orientate so that their hydrophobic, that is waxy, tails point outwards. The metal will therefore obtain a waxy finish to which hydrophilic substances such as wood and formaldehyde based adhesives do not readily stick.
Release agents applied to new platens will help maintain the smoothness of the platens for longer. This will be apparent on the surface of the pressed panels. For many panels this is of minor importance because the surface is removed in the following sanding/calibration step. However, there are panels where sanding is not used, so the release agent is then very effective.
Release agents also help keep the platens clean and the platens then provide better heat transfer. This will lead to cost savings through reduced energy usage and, possibly, higher production capacity.
I use a release agent on the caul plates of our laboratory press, not because of the high volumes, but because the small panel size we use often results in excess adhesive being exuded on to the plates and the release agent then helps in the cleaning process.

When discussing timber construction, the question of fire resistance often enters the conversation. In many ways, wood is an excellent material in a fire situation because its properties are predictable and, most importantly, it does not melt as metal beams are prone to do. Wood does burn and so in certain situations it cannot be used unless it is treated and/or protected in some way.
Practically every country in the world has developed a set of regulations concerning fire protection and the use of materials in construction. Invariably there are differences in test methods and classification systems, so a construction product made and used in one country cannot automatically be used in another unless there is some form of reciprocal agreement over fire regulations.
The European Commission recognised that national fire regulations inadvertently created trade barriers within the European Union and therefore developed a harmonised system of classification, testing and requirements of products in given situations.
Classification using test data from reaction to fire tests outlines the criteria and specifies the test methods used to classify products.
The classification system contains six classes (A1, A2, B, C, D and E), where A1 is the most fire-resistant. There is a second set of classes – A1fl through to Efl – for flooring products. The two sets are very similar in terms of criteria, however.
In addition, information must be given about smoke production for all products and the potential to form flaming droplets or particles for non-flooring products that are in classes A2 through to E. The smoke-forming grades are denoted s1 or s2 for all end-uses, where products in s1 produce the least smoke, plus there is a third category, s3, for non-flooring products.
For flaming droplets the categories are d0, d1 or d3, where d0 indicates that no flaming particles are formed within the parameters of the test.
This all sounds very complicated, but, for wood based panels it is not because plywood, OSB, particleboard and MDF all have the same EU fire class of Dfl-s1 for flooring products and D-s2,d0 for all other uses. It is a shame that the smoke categories are different, because I think it adds complication to a complex system. It is caused by the fact that there are only two smoke categories for flooring products.
The fire ratings of a panel can be improved by incorporating fire retardant chemicals during its manufacture, or applying them subsequently. If the CE mark of a panel indicates that its fire rating is better than D, then it must either have been treated in some way or is a cement-bonded particleboard, which has a classification of B-s1,d0.

There are some shelves in my garden shed that provide a clear and rather extreme example of the phenomenon known as ‘creep’. I hasten to add that I did not make this particular shelf!
Wood is a visco-elastic material (last discussed in WBPI 22(3):48) which means that it exhibits both elastic behaviour and plastic behaviour, depending on the test conditions and the level of applied stress.
Elastic behaviour is observed if relatively low loads are applied for short periods of time and plastic behaviour is seen when the load is applied for a long time. Time is the most crucial factor as creep can be observed in wooden objects even when subject to low loads but over long periods of time. The ability to observe the deformation is dependent on the accuracy of the measurement system.
The rate of deformation changes with time; initially the object, a shelf for example, will deform rapidly and then at an ever-decreasing rate. Creep occurs whatever types of forces are applied, eg tension, compression, bending, torsion, etc.
Greater deformations are observed at a given load at high wood moisture contents, which is often explained by the ‘lubrication’ of movement between wood polymer chains by water. This is relatively easy to comprehend, but what is less easy to understand is that wood creeps even faster when its moisture content changes. So a shelf which is subjected to a series of high and low humidities (cold rainy days followed by hot dry days) will deform more than a shelf in a constant high humidity! This phenomenon is known as ‘mechanosorption’.
My poor shelf is therefore an excellent example of mechanosorptive creep which I will fondly keep until it breaks.

A theme running through several presentations made at COST E49’s recent one day conference in Istanbul was the use of heat treatment to improve the dimensional stability and water-resistance of different wood based
panels. The idea is not a new one as it has been used by the hardboard industry since the 1950’s.
Heat treatment is also applied to solid wood. Examples of commercial products include: Retiwood; PlatoWOOD; and Termawood. Although there are differences in the techniques applied, the basic principal is the same in that the wood is heated to high temperature in an oxygen-depleted atmosphere. The temperature used will depend on the degree of modification of the wood, but it should exceed 200°C and temperatures of 230 to 240°C are common. The risk of fire around these temperatures is high and so a reduced oxygen level is an essential safety requirement. However, a low-oxygen environment also alters the chemistry of the modification reactions occurring in the wood polymers.
A heat treatment process will chemically modify wood without the addition of chemicals. In other words, some of the polymers and molecules present in the wood will be changed through a wide variety of chemical mechanisms. The upshot is that heat-treated wood (HTW) does not have the same chemistry as non-treated wood.
The potential advantages of using heat-treated wood for a panel manufacturer include: enhanced dimensional stability; greater bio-resistance; and lower density.
All three of these benefits are related to the fact that HTW adsorbs much less water than non-treated wood. For example, the fibre-saturation point of HTW is about 15%, which is around half that of untreated wood and so the swelling is also about half. Fungi and bacteria need water in order to attack wood. If the moisture content of wood can be kept below 18%, then it is not normally attacked. It can be concluded that when HTW is not in ground contact it should be very resistant to fungi and insects.
The lower density of HTW is partly due to its lower moisture content and partly to the loss of various organic compounds which evaporate or are degraded during the heat treatment process. This could be an advantage to panel manufacturers in that there is a market advantage for
products which have a lower density at a given
mechanical performance.
Of course, all processes have some disadvantages. Heat treatment causes significant darkening of the wood, a reduction in the mechanical properties and especially toughness, an increase in costs and reduced resin efficiency. However, there is the potential to apply a mild heat treatment to particles and fibres since they must be dried anyway. Perhaps, existing manufacturing lines could be modified to obtain some of the benefits without markedly increasing costs.
I wait to hear from anyone who is attempting to do this.

A theme running through several presentations made at COST E49’s recent one day conference in Istanbul was the use of heat treatment to improve the dimensional stability and water-resistance of different wood based
panels. The idea is not a new one as it has been used by the hardboard industry since the 1950’s.
Heat treatment is also applied to solid wood. Examples of commercial products include: Retiwood; PlatoWOOD; and Termawood. Although there are differences in the techniques applied, the basic principal is the same in that the wood is heated to high temperature in an oxygen-depleted atmosphere. The temperature used will depend on the degree of modification of the wood, but it should exceed 200°C and temperatures of 230 to 240°C are common. The risk of fire around these temperatures is high and so a reduced oxygen level is an essential safety requirement. However, a low-oxygen environment also alters the chemistry of the modification reactions occurring in the wood polymers.
A heat treatment process will chemically modify wood without the addition of chemicals. In other words, some of the polymers and molecules present in the wood will be changed through a wide variety of chemical mechanisms. The upshot is that heat-treated wood (HTW) does not have the same chemistry as non-treated wood.
The potential advantages of using heat-treated wood for a panel manufacturer include: enhanced dimensional stability; greater bio-resistance; and lower density.
All three of these benefits are related to the fact that HTW adsorbs much less water than non-treated wood. For example, the fibre-saturation point of HTW is about 15%, which is around half that of untreated wood and so the swelling is also about half. Fungi and bacteria need water in order to attack wood. If the moisture content of wood can be kept below 18%, then it is not normally attacked. It can be concluded that when HTW is not in ground contact it should be very resistant to fungi and insects.
The lower density of HTW is partly due to its lower moisture content and partly to the loss of various organic compounds which evaporate or are degraded during the heat treatment process. This could be an advantage to panel manufacturers in that there is a market advantage for
products which have a lower density at a given
mechanical performance.
Of course, all processes have some disadvantages. Heat treatment causes significant darkening of the wood, a reduction in the mechanical properties and especially toughness, an increase in costs and reduced resin efficiency. However, there is the potential to apply a mild heat treatment to particles and fibres since they must be dried anyway. Perhaps, existing manufacturing lines could be modified to obtain some of the benefits without markedly increasing costs.
I wait to hear from anyone who is attempting to do this.

In February of this year a storm hit the Aquitaine region in South-west France causing extensive damage, particularly to the forests. It is thought as much as 40 million m3 of trees were felled – equivalent to about five years of normal harvesting. Many trees broke mid-stem so this wood will have to be used for panel products, paper manufacture or energy generation. Others broke at the root base so, hopefully, can be sawn.
The huge quantity of wood on the ground relative to the capacity of saw and panel mills in the area to use it, especially with the current market situation, implies that the vast majority of the wood must be stored in some way.
Logs left unprotected will soon be attacked by bacteria, sap-stain, fungi and, later, insects. Also, logs will split as they dry due to differential shrinkage in the tangential and radial directions, reducing the potential yield and value of the wood.
Clearly, the damaged trees must be cleared from the forest to minimise the risk of disease and fire.
The most practical solution is to collect the logs together in several large storage areas and keep them wet with water sprays. Inevitably there is a cost: Land rent, spraying systems, cutting logs to length, transporting and stacking them, and water.
Previous experience has shown that an efficient water spraying system needs about 15 m3/hour for 4,000m3 of stacked logs. If it is assumed that half of the wood on the ground were to be stacked, then the water requirement would be 75,000m3/hour, or 655 million m3/year. This is equivalent to 20 lakes the size of Derwent Water (for UK residents), slightly less than the volume of Lac de Biscarosse in the Aquitaine region (for French residents) and just over half of Lake George (for US residents and I’ll stop there!); in other words, a lot of water. This could turn out to be a limiting factor.
Water pollution could also become an issue, due to the ‘run-off’ from the stacks fouling nearby waterways. There will be some loss of wood
quality too, because many bacteria, and some fungi, especially Armillaria mellea, can thrive in the water-logged conditions.
If water storage is chosen, then it needs to be done quickly if it is to be efficient. Spring is in the air and with the warmer temperatures, bacteria and fungi will colonize fallen trees quickly.
Unfortunately, this situation is not new to the Aquitaine region because a similar storm hit just under 10 years ago and so, in some ways fortunately, there are people who know how to minimise the effects of this natural disaster.

During the Christmas holidays, my wife’s mother told me about her visit to the Solar boat museum next to the Great Pyramids of Giza.
The Solar boat was built more than 4,600 years ago to ferry the body of Pharaoh Khufu to its resting place. It was then dismantled and buried in a pit on the southern side of the pyramid, where it rested until discovered in 1954.
The Solar boat was painstakingly reassembled and finally installed in a purpose-built museum in 1968. All 1,224 pieces of cedar and acacia that make up the boat are either keyed or stitched together with natural fibre rope; there are no nails, screws or glue.
This is fascinating, but the most impressive aspect is that this wooden boat has survived in an almost perfect state for all these years. No special treatments had been applied to the wood – it was preserved because of the dry, warm climate.
Water is the key. Without it many chemical reactions that are part of the normal breakdown processes of natural materials slow significantly or stop altogether.
Therefore, if we want to make long-lasting wood based composites, and by long-lasting I mean hundreds of years if kept indoors or tens of years if outside, then we need to either ensure somehow that the wood in the product always remains dry, or, reduce wood’s affinity for water.
An impermeable coating is unlikely to work or be practical because of the need to cut, shape, screw and nail composite products. All these processes would break the seal and allow water to enter and once it is in, the impermeable coating will trap the water in the product and thereby accelerate its breakdown.
A chemical modification of the wood so that it becomes hydrophobic and self-sticking would be very interesting.
Particles and fibres have high surface area to volume ratios so if their surfaces were modified so that they were hydrophobic, but self-sticking, then composites with a very uniform level and distribution of modified material could be made. There has been much progress in the chemical modification of wood over many years, but a one-shot commercial process has still to be developed.
Even today’s particleboard and fibreboard can last a long time. A study conducted by the Buildings Research Establishment in the UK showed that particleboard which had been stored for 40 years had the same mechanical properties as when the samples were originally tested. So if, by some chance, a piece of particleboard made today were to be found some 4,600 years later, will our successors be intrigued by the intricate way we laid down each particle?

Wood and wood products are known to help regulate the humidity in buildings because they are hygroscopic; when there is high humidity in a room, wood products will absorb water and swell. Conversely, they desorb when humidity is low.
The relative humidity (RH)
of air depends on temperature and pressure. In this article I only want to cover RH at atmospheric pressure.
Air at 20°C and 100% RH will contain 14.6g of water vapour per kg. So if the air in a room has an RH of 50%, and it is at 20°C, the air will have 7.3g of water vapour per kg of air.
Everyone has seen the effects of wood shrinking and swelling. Many assume wood products swell in winter and shrink in summer. This is generally true of exterior timbers, but the opposite is true indoors, because people keep room temperature around 20°C, so rooms are cool in summer and warm in winter, relative to the outside.
As I write this article, it is 6°C and 90% RH outside. Thus the amount of water in the outside air is 5.5g/kg. As the air from outside enters, it is warmed to 20°C and its RH falls to about 38%. This is a dry atmosphere so the wood products in my room are probably drying out, slowly.
My furniture and floors drying out means the water lost
raises the RH in the room. RH affects our perception of comfort. Wood products provide a useful regulation of RH.
In my current research on reducing formaldehyde emission from panels I have been investigating emission from glue-free plywood-type products to try to differentiate between formaldehyde from the glue and the wood.
I am beginning to conclude that the formaldehyde found in wood does not necessarily originate from the wood but from the water in the wood. Formald-
ehyde is very soluble in water and in ambient conditions prefers to be dissolved in water rather than to be a gas.
If formaldehyde is dissolved in the bound water of wood, when we measure its release the formaldehyde observed may not originate from chemical reactions within the wood but simply be driven out of solution.
If true, then wood products are sinks for formaldehyde and not necessarily sources.
I am confident that wood products cannot only regulate RH but also the concentration of other water-soluble compounds, like formaldehyde, in
the air.

It is my experience that people often use the term “physical property” when they are actually referring to a mechanical property.
A mechanical property of a material is any of its strength characteristics, such as compression and internal bond strength, or screw withdrawal, or bending stiffness, to name but a few.
In order to measure a mechanical property, one must deform the material – often to destruction – if an ultimate characteristic such as bending strength is required.
Physical properties on the other hand are those such as thermal conductivity, swelling, surface roughness, density and so on. These can be measured without deforming or changing the material.
What about swelling, which changes the dimensions of a product and sometimes irreparably? Well, it is the test that alters the product and not the measurement, which is a measure of thickness, or length, neither of which alter the product.
There is also an overlap with chemistry. For example a panel colour, which is a physical property, may be due to the species of wood used, the colour of which is largely determined by the quantity and type of extractives present, which is a chemical property.
Or the colour may vary from one shift to another because of different temperatures in the dryer or defibrator, among other reasons which may cause chemical changes in the wood.
I mention this topic because COST Action E49, a European network of researchers and professionals with an interest in wood based panels that I have the honour to chair, is organising a one-day conference dedicated to physical properties of panel products. We are expecting presentations on topics like: acoustic performance, dimensional stability, fire resistance, thermal conductivity/insulation, density, machinability, surface roughness, colour assessment, coatability and printability. These properties are vital for certain end-uses of panels, but are often side-lined in conferences.
There is also the possibility for manufacturers and researchers to demonstrate equipment they have developed to measure physical properties. The conference will be held in Istanbul on April 28/29, 2009 so you have plenty of time to submit a presentation or exhibit to the organisers. Information about the conference can be found at www.COSTE49.org.

Wood plastic composites (WPC) are made from plastic and, normally, a fine wood flour. Sometimes wood fibres are used, as are non-wood plants like hemp, flax, jute, Kenaf and rice husk. The flour is typically between 250 and 420µm in diameter and can be hardwood or softwood species. The wood normally makes up 50-70% by weight of a WPC. The flour is significantly cheaper than the plastic so there is a temptation to maximise the flour in the final product. The problem is that high wood loadings reduce dimensional stability and long term durability; particularly important to WPC because they are sold as low maintenance and long-term weather resistant.