The use of urea–formaldehyde (UF) resins with lower contents of free formaldehyde in the board industry has led to products with
very low emissions of formaldehyde. This study gives a detailed account of the influence on the mechanical and physical properties of the
particleboard using UF resins with different mole ratios of formaldehyde:urea within the range 0.97–1.27. The mole ratio influenced not
only the thickness swelling (TS) and water absorption (WA) but also the internal bond strength (IB) and the modulus of rupture (MOR)
if the manufacturing process was optimized. The investigation revealed clearly that at mole ratios of formaldehyde giving the emission
class El the MOR, IB, TS, and WA of particleboard deteriorated. Compensation for the deterioration could be provided through a
higher dosage of resin or through the modification of UF resin. However, both alternatives meant that the product would be more
expensive.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Mechanical and physical properties; Mole ratio; Particleboard; Urea–formaldehyde resin
1. Introduction
The intensive debate about formaldehyde throughout
the last 10 years has been caused primarily by resins of the
urea–formaldehyde (UF) type. Undoubtedly, the debate
has expedited developments towards UF resin with a lower
content of free formaldehyde all over the world. This has
led to the introduction of new UF resin types by
manufacturers. In the particleboard industry, there has
been a gradual switch to UF resins with lower mole ratios
of formaldehyde to urea (F/U).
Moreover, the rate and extent of formaldehyde emission
from particleboard is influenced by a large number of
parameters including mole ratio of F/U, resin level, catalyst
level and composition, moisture content and distribution
prior to pressing, board post-treatment, and duration of
storage before use [1–4]. One of the most important factors
in this area is the mole ratio of formaldehyde to urea in UF
resin. The formaldehyde content of UF resins has been
gradually lowered over the years because it is one of the
most effective ways of reducing formaldehyde emission.
The emission of formaldehyde from particleboard decreased
as the mole ratio falls, but unfortunately, the other
physical and mechanical properties were influenced negatively
at the same time [5–7]. At the mole ratios that give
the desired low formaldehyde emission with pure UF
resins, that is, at mole ratio of F/Uo1.1, the transverse
tensile strength became lower and thickness swelling
increased. It proves difficult to find published studies
dealing with the role of the mole ratio that cover the entire
range of relevant measurable properties of particleboard,
including comparative formaldehyde measurements with
different methods. If some of this information is lacking, it
is difficult to make correct and comparable assessments.
In view of all this, it was decided to carry out an
investigation that is broader than normal study dealing
with how the mole ratio of UF resins affects the mechanical
and physical properties of particleboard. The boards,
which were manufactured under fully controlled production
conditions, were examined with all normal standard
tests. The concepts of El c