kinetic response of thermosetting adhesive systems to heat

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Temperature effects on internal mat characteristics during testing In order to resolve the role of the adhesive in affecting the shear strength of compressed fiber samples, a number of fiber samples were compressed in the absence of adhesive, and the results compared with corresponding samples pressed with adhesive. Clearly, the interplay of mechanisms which influence the structure and properties of compressed fiber networks is highly complex; it is beyond the scope of the present discussion and will be the subject of future publications. Such mechanisms are, however, assuredly influenced by fiber type and juxtaposition, transient moisture and temperature conditions, and the magnitude and duration of applied compressive stress. Figure 8 shows representative shear resistance values of non-resinated miniature fiber discs. Pressing temperature appears to affect measured values. This is no doubt attributable to the interplay of mechanisms listed above. Shear resistance does appear to decrease with increasing pressing temperature, while almost no influence can be detected with increasing pressing time. Under pressing conditions of low temperature and high specimen density, a decreasing trend of shear resistance is, however, evident; this might be attributable to thermal softening of cell wall material. Shear Strength [N/mm²] 1,2 0,8 0,4 0,0 125°C; 600kg/m³ 125°C; 500kg/m³ 100°C; 400kg/m³ 125°C; 400kg/m³ 150°C; 400kg/m³ 0 20 40 60 Pressing Time [s] 80 100 120 Figure 8. Shear Strength of non-resinated miniature fiber discs pressed at different temperatures and densities. Temperature Effects On Shear Strength Development Of Resinated Samples The shear strength development of resinated miniature fiber discs pressed at 100°C, 125°C and 150°C are presented in Figure 9. These initial results show a strong correlation between measured shear strength and pressing time (hollow symbols on Fig. 9). Solid symbols in Fig. 9 are data generated by offsetting the non-resinated shear strength values from the corresponding resinated values. As one may expect, inferred adhesive shear strength values increase with increasing pressing temperature and pressing time.
Shear Strength [N/mm²] 2,0 1,5 1,0 0,5 150°C 125°C 100°C 0,0 0 50 100 150 200 250 Pressing Time [s] Figure 9 - Shear strength development of resinated fiber discs at 400 kg/m³ (hollow dots are measured data while filled ones are the inferred adhesive contribution to shear strength) CONCLUSIONS Two methods, one physico-chemical (DSC) and one mechanical (ABES), have been presented to characterize adhesive systems (UF and UF-m) with different molar ratios and different melamine content. The kinetic response of thermosetting adhesives to heat has been described in terms of reactivity indices and activation energy values (following Kissinger’s method for the former). With the ABES approach, thermosetting adhesives have been cured at different temperatures (95°C-125°C) and subsequently destructively tested in shear mode. Veneer strips are used as wooden adherends. The reactivity index of the adhesive systems have been determined by plotting the natural logarithm of regressed isothermal bonding rates against the reciprocal of absolute temperature. Taking activation energy indices of all tested adhesives a satisfying conformity can be seen when comparing ABES and DSC. In the second part of the study, a method has been summarized which may be used to determine bond strength development of fibrous composite material. Miniature MDF-like fiber discs have been formed and subsequently tested in shear mode. Initial results are encouraging: resinated wood fibrous material pressed under controlled conditions of temperature, load and density have been presented. Internal mat characteristics have to be addressed when analyzing absolute strength values. Compared to ABES, which is a reliable method to determine strength of partially cured bonds, the modified ABES approach makes it possible to explore the micromechanical behavior of resinated wood fiber mats.
Page 1 and 2: KINETIC RESPONSE OF THERMOSETTING A
Page 3 and 4: Exothermic Heat [0,5 W/g] UF#2 UF#3
Page 5 and 6: It is the regressed gradient of thi
Page 7: Activation Energy Index 1,00 0,95 0
Page 11: Kissinger H.E. (1957): Reaction kin

kinetic response of thermosetting adhesive systems to heat

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