Quantifying hydrolytic degradation of poly(ethylene terephthalate glycol) under storage conditions and for fused filament fabrication mechanical properties

Abstract

Poly(ethylene terephthalate glycol) (PETG) is a promising polyester for additive manufacturing (AM) applications. However, an important question is its thermal and hydrolytic stability under storage conditions. The current work investigates which measuring technique is most suited to qualitatively answer this question, completing the field for polyester stability analysis by focusing on more amorphous structures. In combination with regression analysis under lab accelerated conditions to practically estimate characteristic degradation time scales and currently lacking (overall) Arrhenius parameters, a distinction is made between techniques dealing with molecular and larger scale properties, distinguishing between Fourier Transfer Infrared (FTIR) spectroscopy, size exclusion chromatography (SEC), rheological measurements, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It is shown that hydrolytic scission is dominant over thermal scission, and mainly affects the lower mass region of the molar mass distribution. However, the degradation sensitivity can only be assessed using the SEC number average molar mass, and a more pronounced qualitative sensitivity is only obtained applying rheological analysis, with an extrapolated very low half-life time under water-saturated conditions of ca. 3 days in the Newtonian region and of 2 days in the shear thinning region. It is further highlighted that hydrolytic degradation influences the tensile properties of parts fabricated with fused filament fabrication, which is an important AM technique.