Thermal and ionizing radiation induced degradation and resulting formulation and performance of tailored poly(lactic acid) based hot melt adhesives

Polylactic acid (PLA) was studied for variation in material properties after step-wise thermal degradation at 255 °C in air. It was found that melting temperature, Tm, and enthalpy of fusion,ΔHf, decreased systematically with corresponding decreases in molecular weight. Radiation induced degradation via Co-60 (1.17 and 1.33 MeV γ-photons) produced equivalent but not the same trends for Tm and ΔHf. For both degradation pathways, differential scanning calorimetry (DSC) revealed a clear transition from one well-defined melting peak in the control case, to two separate regions growing broader with increased exposure resulting in the polymer becoming almost completely amorphous after one hour. PLA based formulations with and without plasticizers as hot melt adhesives were devised for deriving tailored properties. Comparable to, or superior performance to an EVA based industry benchmark adhesive was attainable in terms of hardness, elasticity, shear strength, set time, viscosity, and fiber tearing on corrugated cardboard. In a novel, in-situ application mode using quenched-annealed PLA to join steel dowels, bond strengths of 42.9 MPa (6200 psi) were attainable; potentially offering the community a new method for joining structures.