Evaluation of VIPs after mild artificial aging during 10 years: Focus on the core behavior

The prediction of the long term performance of VIPs remains challenging. To improve the forecast, the evaluation of VIPs aged for very long periods can help significantly. This study reports the characterization method which was implemented on VIPs after an artificial aging of 10 years in the laboratory, at room temperature in two different relative humidities: quite low and high (23 °C at 33 and 80 %RH). The aim is to evaluate the aging of the fumed silica core thanks to the detailed study of the hygrothermal and structural evolutions of the core material. The evaluation reveals that the silica core has been partly aged at high relative humidity (80 %RH), as highlighted by: (i) the moisture content at equilibrium which is not so high as the moisture content that could be reached by short-term additional aging of the sole silica at high humidity levels, (ii) the evolution of the specific area (decrease of only several percents). For the VIP aged at relatively high humidity, the water sorption isotherm indicates that the moisture content inside the VIP corresponds to a humidity level of 44 %: in comparison with the permeation at the beginning of the accelerated aging, the WVTR decreased approximately by a factor 2 (humidity gradient from 80 to 44 %). Furthermore, thanks to the follow-up on the weight and internal pressure of the VIPs, the permeances of the barrier laminate to water vapor and air are also estimated.