During hot pressing a synthetic resin binder (adhesive)
is usually added to glue the fibres together to form
a composite material. For several decades the adhesives
used for the manufacture of MDF and high-density fibreboard
(HDF) have been formaldehyde-based resins such
as urea–formaldehyde (UF), urea–melamine–formaldehyde
(UMF), and phenol–formaldehyde (PF).