A number of MDF plants are currently utilising production waste and used boards in quantities of 3-5%, whereby the material is pulped together with the wood chips. The results to date indicate that without making alterations to the pulping process and manufacturing parameters, a further increase in the amount of recycled fibre board utilised during manufacture would lead to a deterioration of the mechanical and hygric properties of the resulting MDF. Investigations into the utilisation of waste paper (newspapers, magazines and corrugated packaging) demonstrate that solely mechanically defibrated waste paper can be added (up to around 30%) to the thermomechanically obtained wood fibres without the mechanical properties of the MDF being significantly affected. The waste paper actually had a positive influence on the thickness swelling and water absorption of the MDF.
A further raw material which, until now, has not been industrially utilised in the manufacture of MDF is straw. The results of a research project prove that through the implementation of extruder technology, the gluability of straw fibres can be considerably improved in comparison to amino resins. As cereal straw has a relatively low bulk density, its utilisation in MDF can lead to a reduction in the bulk density of the entire board. The inclusion of short-fibre processed straw fibres could improve the surface properties of the MDF.
A joint research project aims to develop a process for the specific application of fibrous pulp from used wood-based materials, waste paper and annual plants (straw). For this, industrially-proven technology from assorted branches of industry (extruder technology, waste paper processing techniques and MDF manufacturing technology) are combined. Through the addition of selected separately-processed secondary raw materials to the MDF production process, the varying fibre characteristics can be specifically utilised for the board properties (e.g. thickness swelling, sorption behaviour, surface properties). Furthermore, this enables the possibility of tailoring the binding material and the binding material outlay to the various pulps, thereby saving binding material.
Up until now, pine wood, fibre boards and MDF have been thermomechanically (TMP) and chemo-thermomechanically (CTMP) processed in refiners and extruders in order to produce pulp. The CTMP demonstrated a lower splinter content than the corresponding TMP. Additionally, pulps from MDF demonstrated longer fibres and a higher fibre wall thickness as well as a lower proportion of fine dust compared to pulps from particle boards. Pine TMP demonstrated a relatively high splinter content, longer fibres and a lower proportion of fine dust compared to TMP from particle boards and MDF.
Furthermore, pulps were produced unpressurized from rape straw and wheat straw using a paper refiner (50-70°C), with and without alkaline pre-treatment. Rape straw produced a longer and stiffer fibre than wheat straw. Pre-treating the rape straw with alkali (sodium hydroxide) resulted in increased fibre length and a reduced energy consumption.
MDF was previously manufactured in accordance with the Blender process, in which 30% of the pine TMP is substituted by refiner pulps from rape and wheat straw as well as UF resin particle boards and MDF. The physical and technical properties of the resulting MDF demonstrate no significant differences compared to MDF from 100% pine and pine TMP. Boards with 30% MDF pulp actually demonstrated a considerably reduced thickness swelling and water absorption following immersion in water. The formaldehyde release from the boards produced using pulps from MDF and particle boards was lower than that of the MDF produced from pine TMP and the boards with pulp from wheat or rape straw. Boards with 30% MDF pulp had the lowest surface roughness.
The MDF facility at the WKI has been expanded to include the possibility of the input of non-glued or dry-glued pulp from various and variously-processed raw materials directly in the flash dryer. Tests concerning the integration of the separately-prepared pulps into the existing manufacturing process will be carried out in the near future.
Institute of Forest Botany
Dept. of Molecular Wood Biotechnology and Technical Mycology
Prof. Dr.-Ing. Edmone Roffael
Telephone: +49 (0) 551 / 391-9978
Papiertechnische Stiftung Heidenau (PTS)
Dipl.-Ing. Gert Bär
Telephone: +49 (0) 3529/55 16 88
AiF via iVTH