Medium density fibreboards (MDF) are currently not recycled after service life, but various publications report on recycling by the disintegration of MDF using various techniques and the properties of obtained recovered fibres (RF). In this study, the main aim was to put back RF into the MDF manufacturing process as closed-loop recycling using repeated thermo-hydrolytic disintegration. Compared to previous studies, the focus was on the recycling of MDF with a relatively low F:U molar ratio (1.11). Urea–formaldehyde-bonded MDF with a target density of 700 kg m⁻³ was subjected to thermo-hydrolytic disintegration in an autoclave using only water at 95 °C for 20–30 min. Afterwards, the properties of RF and virgin fibres (VF), of MDF produced thereof and the composition of the disintegration water (DW) were determined. The nitrogen content (NC) revealed that RF contained about 30% of the initially applied UF. The pH of the DW hardly changed during recycling and it contained considerable amounts of reducing sugars. Using RF did not result in higher formaldehyde emissions than VF. Compared to earlier studies using a higher formaldehyde content (higher F:U ratio), MDF bonded with modern UF resins can be disintegrated under clearly milder disintegration conditions with respect to temperature and time. The properties of recycled MDF were similar to those of reference MDF; up to 100% RF could be used without severely deteriorating the strength and increasing formaldehyde emissions from these panels.

In most countries, fibreboards are not recovered after utilization but burned for energy production. This study aims at recovering fibres from industrial fibreboards and reusing them as reinforcement elements in wood polymer composites (WPC). Recovered fibre (RF) material was generated by the thermo-hydrolytic disintegration of medium and high density fibreboards bonded with urea-formaldehyde resin. Various formulations of RF and polypropylene were used with or without the addition of the coupling agent to manufacture WPC using a co-rotating extruder. Test specimens were produced via injection moulding whereby those containing ‘virgin’ fibres served as a reference with respect to mechanical and physical properties. WPC formulations containing RF and ‘virgin’ fibres exhibited similar results, but composites containing RF exhibited improved mechanical and water-related properties, especially without coupling agent. The study indicates that recovered fibres are suitable to produce WPC with very similar physico-mechanical properties as those from ‘virgin’ fibres.

A thermo-hydrolytic disintegration process qualifies as a promising option for recycling the waste MDF and preserving the fibrous morphology of the recovered lignocellulosic fibre material. This study aims to include a drainage process between the thermo-hydrolytic disintegration and the further utilization of the recovered fibres (RF) obtained using a screw press for removing the disintegration water (DW). In this context, the chemical properties of the RF (pH, nitrogen content, formaldehyde emissions) and the DW (pH, formaldehyde, reducing sugars and equivalents and nitrogen contents) were analyzed. Moreover, the RF material was utilized to produce recycled MDF panels, solely containing the RF (100%) and hence supplanting 50% of the virgin fibres (VF). The recycled MDF portrayed significant reductions in the internal bond strength (IB), and flexural properties (MOR, MOE): in the case of MDF made from 100% recycled fibres, about half the strength was reduced, and in the case of MDF made from 50% recycled fibres, the strength was reduced by 20-25%. The Thickness swelling (TS) of the recycled MDF panels was similar, while the water uptake (WA) was higher than that of the original MDF. The recycled MDF panels also exhibited a higher content of formaldehyde and emission. The findings recommend the application of a screw press process for prompter drainage of the RF and to utilize the RF obtained in combination with the VF to achieve adequate mechanical properties rather than using the RF separately for the manufacturing of the recycled MDF panels.