Heating matters

Like all chemical reactions, the polymerisation rate of an adhesive is affected by temperature, so heating a particle mattress or fibre web will accelerate the curing of glue within it. The production capacity and, therefore, profitability of a panel factory is directly dependent on the minimum press cycle that can be achieved. Saving two or three seconds per cycle could increase production overall by 1%.

You might think that raising the press temperature would be a simple way to increase production. Unfortunately, it is not as easy as that. Raising the temperature above 200°C begins to discolour the panel surface, especially if the panel is quite thick, thus requiring a longer press time. If the panel is to be sanded then this discolouration is less important because it is removed. So why not raise press temperature to, say, 300°C?

Leaving aside the technical difficulties of building and operating a press that would operate at these temperatures, the higher temperature is unlikely to give the benefits that you might imagine. The graph below shows how the temperature changes in a single-layer particleboard mattress when pressed at 200°C. The thermocouple placed on the top surface of the mattress quickly reaches the platen temperature once the platen makes full contact with the top of the mattress (about 45 seconds - yes our press closes slowly). The thermocouple just below the surface also heats up quickly, but the high thermal insulation properties of wood prevent it from reaching 200°C, even after 10 minutes of pressing.

The other two thermocouples show quite different temperature curves. Both show a relatively rapid rise to just over 100°C, followed by a stable period lasting about two minutes for the thermocouple at 6mm from the surface, and around four minutes for the thermocouple at the mid-point of the panel thickness. This is because the central part of the mattress is heated by steam. Even though the temperature of steam is 100°C at atmospheric pressure, it is at 106°C inside the laboratory panel because of the gas pressure in the mattress. The pressure is caused by the small escape paths in the mattress relative to the volume of steam present.

Raising press temperature to 300°C would provide little or no benefit because the bulk of the mattress is heated by steam and its temperature is pressure, rather than platen, dependent. We could consider increasing mattress pressure but, if we did, then the steam would cause the panel to delaminate when we opened the press.