Effects of isomer ratio on pMDI resin reactivity and oriented strandboard properties

The effect of varying 2,4′-MDI to 4,4′-MDI isomer ratio in diphenylmethane diisocyanate (pMDI) adhesives was investigated. In addition to probing resin cure kinetics, oriented strandboard panels were produced to investigate the effect of isomer ratio on panel properties. With one exception, differential scanning calorimetry results showed a trend of increasing activation energy with increasing 4,4′-MDI content - the opposite of what was predicted. Results of internal bond testing indicate that increasing 4,4′-MDI content gave higher internal bond strength, but no correlation was evident between resin and panel density, MOR, MOE, or 24-h thickness swell. It is important to note that isomer ratio was not the only variable within the resin series; oligomer content and hence viscosity increased as 4,4′-MDI content increased, which complicated the analysis.