Advanced spectroscopy, microscopy, diffraction and thermal analysis of polyamide adhesives and prediction of their functional properties with solid-state NMR spectroscopy

Abstract

Adhesives are an essential class of industrial polymers with applications ranging from pressure-sensitive adhesives to hot-melt adhesives used for repairing conveyor belts in mines. The composition and homogeneity of a polyamide-based hot-melt adhesive (HMA) was revealed with attenuated total reflection (ATR)-FTIR and solid-state NMR spectroscopy. Analysis of the polyamide shows that it is obtained through sustainable manufacturing based on dimer acids. ATR-FTIR showed incorporation of an abrasion-resistant additive on the surface of the HMA but was unsuitable for other additives such as carbon black or an antistatic agent. Quantitative ¹³C NMR spectroscopy revealed heterogeneity in the distribution of an antistatic agent in the HMA, which was supported by observations with differential scanning calorimetry (DSC). ¹H NMR relaxation and two-dimensional wideline separation (2D-WISE) NMR revealed differences in the molecular dynamics of functional groups in the polyamide resin and the additives dispersed in the resin matrix. ¹H T₂ relaxation revealed that the molecular mobility of the least mobile and moderately mobile components increased with increasing temperature and antistatic agent content. 2D-WISE NMR revealed a phase separation in the base resin matrix and plasticization of the whole sample at very high antistatic agent content. ¹H T₂ relaxation showed possible correlations with mechanical properties such as Young's modulus and Shore A hardness and weaker correlations with adhesive properties such as T-peel strength. This shows the suitability of NMR to assist product innovation through the design of better-performing HMAs or of HMAs for application in different climatic conditions.