Non-Toxic, Non-Flammable, –80 ºC Phase Change Materials

The objective of this effort was to develop a non-toxic, non-flammable, –80 °C phase change material (PCM) to be used in NASA’s ICEPAC capsules for biological sample preservation in flight to and from Earth orbit. A temperature of about –68 °C or lower is a critical temperature for maintaining stable cell, tissue, and cell fragment storage.

Within this technical effort, two phase change fluids were developed with melting onset at –85 °C and –61 °C, and latent heat of fusion of 100 and 136 J/mL, respectively. The experimental results indicate good repeatability of the freeze/thaw cycle, compatibility with high-density polyethylene, thermal stability, and flashpoints exceeding 100 °C. Based on the individual components, the phase change fluids are expected to have low acute toxicity.

There are several types of phase change materials (PCMs) that can be considered for the preservation of biological samples at a temperature of about –68 ºC. These include hydrated salts in water, non-hydrated or weakly hydrated salts in water, non-electrolyte aqueous solutions, and pure, non-aqueous fluids. However, none of these options resulted in a –80 °C freezing point stable PCM during the freeze/thaw cycling. Therefore, a non-aqueous mixture was formulated yielding a pseudo melting point plateau, adequate stability over numerous thermal cycles, and suitable latent heats. The product is a non-aqueous base fluid with a somewhat tailored freezing point, and latent heats on the order of 100 J/mL for the –85 °C PCM.

The fluid developed is an organic solution with adequate resistance to biological growth, compatible with HDPE (high density polyethylene) plastic, and is characterized by a negative freezing expansion ratio. The negative expansion ratio during freezing will allow for the ICEPAC modules to be completely filled with PCM material as compared to previously used fluids requiring a 20-mL air bubble (within a 120-mL capsule). This translates to a 20-percent increase in cooling capacity for a given latent heat. Furthermore, the specific gravity of the PCM is on the order of 0.92 g/mL, making it lighter than an aqueous- based solution per ICEPAC capsule.

This work was done by J. Michael Cutbirth of Mainstream Engineering Corp. for Johnson Space Center. MSC-24460-1

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