Physical aging and evolution of mechanical properties of additively manufactured polyethylene terephthalate glycol

Abstract

Near net shape manufacture via material extrusion (MEX) of thermoplastics tends to rely on glassy amorphous polymers to avoid stresses generated from crystallization. Polyethylene terephthalate glycol (PETG) has emerged as a high performance, low-cost feedstock for MEX. Crystallization in PETG is suppressed by the inclusion of cyclohexane dimethanol (CHDM) as a comonomer, but CHDM increases the segmental flexibility that can accelerate physical aging. Repeated thermal cycling during MEX printing can accelerate physical aging. Here, we investigate the aging of three commercial PETG filaments with different CHDM content. Thermal analysis demonstrated increased aging as the CHDM content in the PETG increased. Aging of additively manufactured PETG demonstrated that the process path during printing leads to a distribution of aging behavior. The thermal history is spatially dependent, leading to differences in how the PETG ages between layers and location within a printed layer. Additionally, this aging in the MEX printed PETG induced changes to the ultimate tensile stress and elongation at break that are dependent on the filament source. Accelerated aging based on time–temperature superposition demonstrated embrittlement of the printed PETG after the equivalent of 1 year of aging at 25 °C in all cases. However, an unusual increase in both strength and ductility after aging for the equivalent of 30 days of aging at 25 °C was observed with some PETG. Although print conditions are commonly optimized for mechanical performance, long-term aging behavior needs to be understood to ensure reliability of additively manufactured durable goods through their expected lifetime.