Page 636 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Applications 603

successfully to characterize thermal behavior of power resins displaying two
exothermic peaks. Finally, the isothermal method can provide a means of
determining a reaction type.
Yin, et al., [45] studied the use of TMA to determine the variation of
physical properties of UF, MUF, and PMUF thermosetting resins in-situ in
wood joints. The TMA analysis was carried out on a three-point flexion
from 25° to 220°C at 10°C/min heating rate and a constant applied force of
0.3 N or at a dynamic force of 0.2 N. Some of the tests were performed at
60, 80, 100, and 120°C. Depending on the rate of the adhesive hardening,
the measured times varied between 5 and 80 minutes.
They reported that the three adhesives/wood joints behave in a
similar manner. The relative elastic modulus versus temperature of the
joints are shown in Fig. 11. According to the authors, three distinct zones
st
are observed on the TMA curves. The low temperature zone (1 zone) is
characterized by a low relative elastic modulus, the middle zone shows a
sudden and marked increase of the modulus value, and in the higher
rd
temperature zone (3 ), a slow decrease is observed. It was concluded that
before gelling the adhesive behaves as a liquid, hence, a low elastic modulus
as it cannot transfer the stress between the two wood layers. On gelling, a
tri-dimensional structure started to form, transforming the adhesive from a
liquid to a rubbery state increasing the relative elastic modulus of the
adhesive and, thus, the wood joint. The beginning of the second zone can
be considered as the gel temperature.
The results indicated that the final hardening appears like a second
order transition and that the definition of hardening temperature is compli-
cated because hardening occurs within either a wider or a narrower
temperature range, which varies with the technique. Therefore, Yin, et
al., [45] defined the hardening temperature (T ) as the one at which the rate of
V
increase of the elastic modulus attains its maximum.
From the TMA results, Yin, et al., [45] concluded that non-isother-
mal and isothermal TMA analysis is a useful tool for the determination of
the variation of different chemical properties of thermosetting wood
adhesives directly in the wood joint during the cross-linking and hardening
process of the adhesives. Furthermore, an increase in the mechanical
resistance of a bonded joint during adhesive hardening can be clearly
correlated with the degree of resin conversion. In this respect, gelling,
hardening, vitrification temperatures, and the temperature at which the
modulus attains its maximum can be determined by TMA.