Improved method for analyzing cohesive strength development of pMDI

Abstract

AbstractAnalyzing 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.