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.

It is dynamic in that visitors to the site can vote for the research topics proposed; the number of votes received can change the list order. In addition, votes cast some time ago have a lower weighting than those cast recently. Consequently, the list order changes regularly.
The top three research topics at the time of writing are: understanding formaldehyde emission from panels; ecological glues for panel manufacture; lightweight panels.
I have covered formaldehyde emission in previous columns and the second topic is also linked to the first, so I will address the third topic in this issue. Lightweight panel is the term usually used to describe a product that has relatively thin panels for faces, which are joined together by a low density core. The core material is most commonly a paper-based honeycomb, but can equally be made from aluminium honeycomb, low density wood such as balsa or cork, and rigid plastic or metal foams. These products are used extensively in the making of furniture – especially for boat, camper van and caravan building, where weight is of considerable importance.
These lightweight panels are macro-composites in that they are made by combining several different materials. The utopia sought after by panel manufactures is to make truly low-weight products in one step.
The lowest density MDF I have seen on sale is around 500kg/m3 – I would be happy to hear from anyone who makes something lighter! However, this is still rather heavy compared to most of the macro-composites described above.
Throughout the panel manufacturing sector there has been a tendency to reduce panel weight in order to reduce manufacturing costs and to address the market desire for panels that are easier to handle, cut and shape.
MDF, OSB and particleboards are pressed to high densities in order that the small droplets of glue applied to the wood elements have a chance of sticking two or more elements together. If they were not pressed together then too many of the droplets would harden without forming a bridge between particles and the product would be weak. Consequently, when weight is reduced we normally reduce the board’s mechanical properties too.
In theory, if we could perfectly align the particles in a mattress, like a jigsaw, then we could make strong panels that have the same density as the original raw material and with very little adhesive. Although this is not possible (yet!), careful attention to particle size distribution in the furnish, together with excellent resin distribution, has enabled some manufacturers to lower their product density without detriment to mechanical properties.
The reductions in density may be only 10- 15kg/m3 but this adds up to a lot of material saved over the course of a year – for example around 6,000 tons for a modern, high-capacity production line of 400,000m3/year.
Clearly the light-weight panel is the way to go.

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.