Sustainable castor oil-based semi-aromatic copolyesters with high strength and low hydrolysis

Abstract

Hot-melt adhesives are solid adhesives that are converted to a melting state for application to substrates and on cooling efficiently sets up the bond. Due to low or no volatile organic compound emissions, hot-melt adhesives are extensively used in various applications. Poly(trimethylene terephthalate- co-adipate) copolymers containing dimethyl terephthalate, adipic acid, and 1,3-propanediol along with varying amounts of castor oil were synthesized by one-step melt polymerization as hot-melt adhesives. The structure, composition, and thermal properties of these samples were analyzed, and all samples showed excellent thermal stability with Td-5% values above 300°C. Differential scanning calorimetry measurements showed tunable melting temperatures of the copolymers in the range of 80–180°C, which increased with the content of dimethyl terephthalate and decreased with increasing amounts of castor oil. The incorporation of castor oil as soft segments enhanced the flexibility of the molecular chain, indicating an increment in the elongation rate while decreasing the stress at the break and glass transition temperatures. Rheological examinations showed significant increments in the viscosity at lower angular frequencies with the introduction of castor oil; this was attributed to the entanglement caused by the multifunctional groups of castor oil with dangling chains in the segments. As the angular frequency increased, the viscosities of the samples decreased almost linearly, revealing good applicability in the melting process of the copolymers, which presented controllable melting viscosities under different process conditions. The T-peel test was used to investigate the adhesion properties of the copolymers, and the copolymer with 50/50 molar ratio of dimethyl terephthalate/adipic acid with 3 mol% castor oil PT(T50A50)-C3 exhibited excellent binding capacity between the copolymer and poly(ethylene terephthalate) fabric with a peel strength of 2624.5 N m−1. Moreover, PT(T50A50)-C3 shows low moisture absorption, good hydrolysis resistance, and relatively low hardness, which indicates its high potential for application in hot-melt adhesives on polyester fabrics with high strength, good tactile quality, and long service life.