Abstract
Analyzing the development of cohesive strength of polymeric diphenylmethane diisocyanate (pMDI) with the help of the small-scale test method proposed in the standard (ASTM-D7998-15) is rarely used, as resulting lap-shear strength values are lacking in informative value. Up to now, the gained strength values have been dramatically below the ones typically achieved with most standard wood adhesives, while the performance in a panel product is at least of equal quality. In order to address this discrepancy and to fill the gap of the lacking method to properly investigate curing behavior of pMDI adhesives, a modification of the specimen geometry for pMDI adhesive analysis is proposed, resulting in meaningful tensile shear strength values also when using pMDI as adhesive. Applying this modified specimen geometry enables investigating relevant processing parameters, such as the effect of press time, press temperature and wood moisture content on the tensile shear strength development using pMDI. The strength development was found to be positively affected by all three factors significantly. Furthermore, the calculated reactivity index showed a decrease in activation energy of 20% with increasing wood moisture content. Based on the results gained, it can be concluded that both, wood moisture and temperature, are crucial factors to accelerate curing of pMDI. Even so the principle influence of the exemplary selected parameters has already been shown earlier, the new methodology opens up new possibilities in investigating the curing behavior of pMDI.
Highlights
The development of cohesive strength of a thermosetting adhesive usually takes place during the curing process under the influence of temperature, pressure and time, whereby the pressing time strongly depends on the properties of the binder
In order to reach the maximum observable tensile shear strength level of approximately 6–7 N/mm2 of the configuration used, 10-min press time was necessary at a wood moisture content of 16% and a press temperature of 110 °C, while only half of the time (5 min) was needed when press temperature was increased to 130 °C at the same moisture content level
The increase in specimen size compared to conventional ASTM-D7998-15 (2015) specimens led for the first time to similar lap joint strength values when using polymeric diphenylmethane diisocyanate (pMDI) to the ones typically observed with urea formaldehyde (UF) and phenol formaldehyde (PF) adhesives
Summary
The development of cohesive strength of a thermosetting adhesive usually takes place during the curing process under the influence of temperature, pressure and time, whereby the pressing time strongly depends on the properties of the binder (reactivity of the system, molar ratio, amount of hardener, solid content, additives, degree of condensation, etc.). To examine the mechanical strength development as a result of the chemical curing of an adhesive, the automated bonding evaluation system (ABES) proposed by Humphrey (1990) has been shown to be a powerful and versatile method. The tenuity enables a nearly isothermal evaluation of the bonding strength development. Derived from the measured values, Humphrey and Shan (1989) defined the reactivity index (Ri), which shows the relationship between the strength development rate to the press temperature and can be further used for simulation models
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