Bonding of recycled fibres with urea-formaldehyde resins

The work aimed to study the influence of replacing virgin wood fibres by recycled fibres, made from recovered medium-density fibreboards (MDF) by thermo-mechanical pulping, on the physical-mechanical properties and formaldehyde release of urea-formaldehyde (UF)-bonded MDF. It was found that replacement of 33% virgin fibres by recycled fibres decreased significantly the thickness swelling and water absorption of the UF-bonded MDF and mitigated the formaldehyde release. However, supplanting 33% of virgin fibres by recycled fibres had a negative impact on the internal bond strength. UF-bonded MDF with higher amounts of recycled fibres (67 and 100%) could not be produced without applying the hybrid bonding technology, i.e. adding small quantities of polymeric diphenylmethane diisocyanate to the UF resin. MDF made from 100% recycled fibres showed significantly lower thickness swelling values as well as a drastic drop in the formaldehyde release compared with boards made from virgin wood fibres.