kinetic response of thermosetting adhesive systems to heat

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Figure 3, A close-up of the bond forming and testing zone of ABES (the cooling head is retracted and was not used in the reported research) Adhesive was metered onto the end-most 5 mm of pairs of Maple veneer strips measuring 0.7 mm thick, 20 mm wide and 117 mm in length (conditioned to 8 % moisture content). Adherend pairs where mounted in ABES with an overlapping area of 100 mm² and pressed together at 1.2 N/mm². Bonds were pressed at temperatures of 95°C, 105°C, 115°C and 125°C. The wood was especially thin in order to hasten the transfer of heat from the precisely temperature-controlled pressing blocks to the bonding interface. In this way, near-isothermal adhesive conditions (±1°C) were attained within 12 seconds following press closure. Measured bond strength accumulation for all pressing times in excess of 12 seconds could therefore be attributed to the target temperature. After each pre-selected pressing time, bond strength was tested almost instantaneously in shear mode (ABES is digitally controlled and pneumatically driven). All bonds were therefore tested while still at the forming temperature. The effect of testing temperature on the strength of variously cured bonds (where the cooling head is activated immediately following press-opening and before bond pulling) is the subject of a separate study (Kim and Humphrey, 2002; Kim, 2002). Further details of the ABES testing approach, including issues of shear stress-correction, heat transfer, cooling, and bond analysis may be found in a publication (Humphrey, 1999). Temperature effects on bond strength development: A typical family of isothermal bond strength development curves is shown in Figure 4a (for formulation #3.) It is convenient here to sub-divide the curves into three portions. During the first section no significant bond strength occurs (although tack may be quantified when the high-sensitivity mode of ABES is activated.) Once bonding begins, bond strengths rise at a near-constant rate, before, in the third stage, the increase of bond strength decreases and the curves level off until they reach their maximum values. The initial delay in the onset of bond strength development may be caused by a short period of increasing temperature combined with a loss of energy due to evaporation of water within the glue line. The adhesive may also still be in the fully liquid state (and even sustain a temporary decrease in viscosity upon initial temperature rise.) The second and near-linear stage is likely directly attributable to the polymerization reaction of the adhesive systems by chainextension and cross-linking processes.
It is the regressed gradient of this dominant portion of the data that will be used here to derive reactivity indices and activation energy values for the adhesive systems. The decreased-rate stage may be due both to a slowing of the polymerization reaction of the adhesive combined with the onset of cohesive failure of the adherend on a micro-scale. The adherends begin to fail by fiber pull-out and therefore ultimate strength values of the fiber-adhesive interface itself may be masked. Taking the slope of the linear section of each bond strength curve from Figure 4a (excluding the initial delay and the leveling-off section at advanced pressing times) the regressed isothermal bond strength rate can be plotted versus temperature (Figure 4b). Shear Strength [N/mm²] 5,0 4,0 3,0 2,0 1,0 125°C 115°C 105°C 95°C 0,0 0 60 120 180 Pressing Time [s] 240 300 360 0 90 100 110 Pressing Temperature [°C] 120 130 a) b) Figure 4. a) A representative family of bond strength development curves for four temperature levels (for adhesive encoded UF#3), and b) Regressed isothermal bond strength development rate (for the middle linear stage) versus temperature for all four adhesive systems. Isothermal Shear Strength Development [kPa/s] 240 180 120 60 UF #2 UF #3 UFm #4 UFm #5 Repetition of the above analysis for all four adhesive systems enables differences in their responsiveness to temperature to be explored. These differences are evident in Fig 4b. The higher formaldehyde content appears to correspond with faster bonding rates, while increasing amounts of melamine reduces the rates. For each adhesive system a linear correlation can be derived by plotting the natural logarithm of the regressed isothermal bond strength development rate against the reciprocal of absolute temperature (Figure 5). ln k [kPa/s] 5,5 5,0 4,5 4,0 3,5 3,0 UF #2 UF #3 UFm #4 UFm #5 2,5 2,50 2,55 2,60 2,65 2,70 2,75 1 / absolute Temperatur [°K*10 -3 ] Figure 5. ABES-derived Arrhenius plots (natural logarithm of regressed bond strength rate vs. the reciprocal of absolute temperature) for each of the four adhesive systems.
Page 1 and 2: KINETIC RESPONSE OF THERMOSETTING A
Page 3: Exothermic Heat [0,5 W/g] UF#2 UF#3
Page 7 and 8: Activation Energy Index 1,00 0,95 0
Page 9 and 10: Shear Strength [N/mm²] 2,0 1,5 1,0
Page 11: Kissinger H.E. (1957): Reaction kin

kinetic response of thermosetting adhesive systems to heat

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