Synthesis and characterization of low-temperature polyamide 6 (PA6) copolyamides used as hot melt adhesives and derived from the comonomer of novel aliphatic diamine bis(2-aminoethyl) adipamide and adipic acid

Abstract A novel aliphatic PA6 copolyamide (PA6(BAEA/AA))-based hot melt adhesive was synthesized from e-caprolactam with the organic salt of bis(2-aminoethyl) adipamide (BAEA) and adipic acid (AA), with BAEA/AA content ranges from 5 to 15 mol%. The chemical structures of BAEA diamine, BAEA/AA organic salt, PA6(BAEA/AA) copolyamides, and neat PA6 were obtained using FT-IR and 1H NMR. Determined by the comonomer compositions, the relative viscosity (ηr) and molecular weight (Mw) of the PA6(BAEA/AA) were from 2.29 to 2.74 and from 14,530 to 19,500 g mol−1, respectively. Adding the BAEA/AA structure strongly affected the crystallization behavior and thermal properties of the copolyamides. DSC results revealed that the melting temperature (Tm) of the copolyamides declined as the mole% of BAEA/AA increased in the range 5–15 mol%, and a TGA test revealed that the thermal decomposition of the copolyamides was similar to that of neat PA6. The Tm, glass transition temperature (Tg), and thermal stability (Td5%) with a BAEA/AA salt content of 10 mol% had the lowest values of 162.2, 40.3, 354.6 °C, respectively, while the fusion enthalpy (ΔHm) was lowest at 15 mol% (21.2 J g−1). Most remarkably, PA6(BAEA/AA) exhibits much better mechanical properties such as tensile strength (33.6–52.6 MPa) and elongation at breakage (114.7–584.5%), than the PA6 homopolymer. The peel strength of PA6 reached a maximum value of 53.4 N cm−1 at 10 mol% of BAEA/AA salt, and this value is better than those of commercial hot melt adhesives.