Eco-Friendly Polyurethane Reactive Hot-Melt Adhesive Derived from Poly(ε-caprolactone-co-lactic acid) Diols

Due to the exhaustion of fossil resources and the degradation of the environment, there is an increasing utilization of eco-friendly materials as substitutes for petroleum-derived materials. In this work, a series of poly(ε-caprolactone-co-lactic acid) diols were synthesized via one-pot ring-opening polymerization with different lactic acid (LA)/ε-caprolactone (CL) feed ratios. Subsequently, the eco-friendly diols were mixed with polyether diol, 4,4′-diphenylmethane diisocyanate (MDI), and other additives to produce polyurethane reactive hot-melt adhesives (PURHAs). The enhancement of LA/CL feed ratios correspondingly led to an increase in the glass transition temperature (Tg) of the eco-friendly polyester diols. The differential scanning calorimetry (DSC) data demonstrated that PCL-diol had the highest crystallinity, and the polarizing optical microscopy (POM) observations revealed the smallest spherulite size in PCLA28-diol. Thermogravimetric analysis (TGA) results indicated that the 5 wt % loss temperature (T5) of PURHAs surpassed 265 °C. Tensile testing showed that the elongation at break of PURHAs containing PCL segments increased by at least eight times compared to PURHA without PCL segments. Lap-shear testing illustrated that a PURHA with a higher LA/CL ratio exhibits higher green strength. In addition, PUHRA-PCLA28 derived from PCLA28-diol and PUHRA-PCL derived from PCL-diol possessed better tensile and lap-shear strengths. Notably, the lap-shear strengths of PUHRA-PCLA28 and PUHRA-PCL on a PC substrate reached impressive values of 8.85 and 8.05 MPa, respectively. These bio-based PURHAs synthesized from eco-friendly polyester diols with different LA/CL ratios show potential applications in electronic appliances, the automotive industry, and construction materials.