Moisture Influence on Viscoelastic and Thermal Properties of Additively Manufactured Syntactic Foams

Abstract

Syntactic foams are lightweight structures extensively used in marine and aerospace applications as core materials due to their inherent buoyant nature and energy absorption capability. In addition to structural applications, syntactic foams find applications in microelectronics. However, exposure to moisture for a prolonged period will mediate moisture ingression into syntactic foams that will eventually degrade the structure and cause catastrophic failure. Despite its importance, most of the past research has focused primarily on experimental studies of mechanical property degradation of syntactic foams. In contrast, time-dependent property degradation due to moisture exposure is lacking. Such behavior of additively manufactured syntactic foams is not known. To address this knowledge gap, this paper focuses on elucidating the impact of moisture absorption on the viscoelastic and thermal properties of syntactic foams, particularly Thermoplastic Polyurethane (TPU) based syntactic foams. In this paper TPU based syntactic foams with 20% and 40% reinforced glass micro balloons (GMBs) are additively manufactured using selective laser sintering. To that end, the viscoelastic and thermal properties are measured in both pristine and water-aged syntactic foams. Dynamic mechanical analysis is performed at temperatures from -50 oC to 100 oC at 0.1% strain and 1 Hz. It was found that glass transition temperature increases with an increase in GMB volume fraction due to the effect of reinforcement, which hinders the polymer chain movement at the interface. The peak damping factor, which is representative of ???, is reduced by 5% in neat TPU and TPU + 20% GMB after the exposure due to plasticizing effect in TPU whereas no change is found in TPU+40% GMB foams. This is attributed to the majority of the moisture absorption occurring at the interface between GMB and matrix. The thermal conductivity of the neat TPU and TPU+20% GMB is increased by 8% after the samples are exposed to moisture due to the formation of thermal bridges when water molecule reacts with TPU matrix, however, it slightly decreased in TPU+40% GMB due to insufficient formation of thermal bridges as a result of insufficient matrix and hindrance from the reinforcement. Our results demonstrate that the thermal properties of the syntactic foams can be tailored based on our knowledge of their response to moisture and exposure time.