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TRANSFORMING WOOD INTO TMP FIBRES FOR MDF
The primary manufacturing step in the production of MDF is termed Thermo Mechanical Pulping (TMP). This process is used to convert solid wood chips into fibres, explains Dr Rob Elias of the BioComposites Centre, Bangor
T
he TMP process originated from the pulp and paper sector and was driven by a
demand to reduce the amount of chemicals and water used to make pulps for paper and hardboards. The TMP process combines heat (thermo) with mechanical energy to generate wood fibres for medium density fibreboard manufacture. To make MDF boards the fibres are then resinated, dried and hot pressed to make MDF panels.
The TMP process uses heat generated from high pressure steam. This heat is used to soften the wood feedstock prior to mechanical processing in the refiner. Wood chips are softened in a pressure vessel or digestor until they become “plastic” like. The softened wood chips are then fed from the digestor into a mechanical refiner. The refiner consists of 2 discs, one is stationary and one is rotating at high speed. Here, the wood chips are passed between the discs and the mechanical action of the discs breaks the chips down into their constituent fibres. To assist this process the surface of each disc has a pattern of raised grooves that are designed to cut and crush the wood chips and separate them into fibres, see figure 1.
Fibre quality is controlled in the TMP process through careful optimisation of the steam pressure, the plate gap and the wood chip throughput. As a rule of thumb, fibres for MDF production are normally produced using high steam pressures, normally 8 bar, and with a temperature of 160°C. Under these conditions the softening of the wood chips is optimum in the area of the fibre known as the middle lamella. This is the lignin-rich layer that joins or “glues” the wood cells together. The softened wood chips when refined, split at this layer giving a furnish that consists of fibres and fibre bundles. The ratio of fibre-to-fibre bundles is known as the shive content. This is a measure of the fibre quality. If the shive content is too high the furnish will not be suited for
applications where surface smoothness is important e.g. deep machining applications. Equally, too much refining can reduce the fibre length and result in fines or dust. Dusty short fibres don’t overlap as well when pressed into panels and therefore the mechanical performance of boards with high fines content can be poor. In the EU the majority of MDF is made from softwood but over the years research has focussed on alternative feedstocks. In the late 90s the BioComposites Centre worked closely with industry to assess the use of agricultural residues such as empty fruit bunch fibres from oil palm plantations, miscanthus, bagasse and rice straw. As there are differences in the chemical composition and morphology of these feedstocks, the TMP process had to be adapted. Pressure, temperature and disc designs were optimised, and this research demonstrated that it was possible to make MDF from a variety of different crop residues.
So, what is next for the sector? Increasing virgin wood and energy costs combined with technical issues of using crops continues to drive the MDF industry to look at alternative feedstocks and waste materials will offer a solution. Waste material uses less energy during conversion to fibre, thereby offering significant savings. The regulatory position demanding incorporation of recycled content in products is tightening and end users are no longer accepting of non-recyclable products. IKEA for example has stated that they will only use renewable and recyclable materials in their production processes by 2030. Globally over 111 million m3
of MDF
were produced in 2021. A lot of this MDF soon enters the waste stream because it is often used for products with relatively short lifespans. To exploit this opportunity, technologies for recycling this MDF waste are now being commercialised. There will soon be more to MDF production than just TMP! ●
Above: Technology to create MDF fibres WBPI | April/May 2024 |
www.wbpionline.com
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