@Carla I think for your four questions:
1 - To recharge the (65f/18C) cooling packs, they need to be warmed? Or refrigerated/frozen?
Answer: To recharge the cooling packs with a melting point of 65°F (18°C), they need to be cooled below this temperature. The packs absorb heat as they melt, providing cooling. To recharge them, you need to solidify the material again by cooling it below 65°F. This can be done using ground temperature in many regions, as Ben mentioned, or by refrigeration if necessary. The key is to get the temperature below the melting point so that the material can crystallize and be ready to absorb heat again when it melts.
2 - To make them colder, still (like for keeping ice cream frozen for a long drive, or keeping a temp-sensitive food cold, for a hot-day picnic or camping trip), you'd add more table salt?
Answer: Yes, adding more table salt to the mixture would lower the melting point, making the PCM colder. This follows the principle of freezing point depression in solutions. By increasing the concentration of salt, you can create a PCM with a lower melting point, suitable for keeping items colder than the original 65°F formulation. This absolutely would be useful for applications like keeping ice cream frozen or maintaining lower temperatures for food storage during hot weather conditions.
3 - If #2's a yes, any clue as to (based on his same 5C water/1C Sodium sulfate recipe) would the 1/4C = 30° extra cooling stay the same, so that an increase to 1/2C would equal a 60° cooler product, bringing it to 35°, and so on? (This would be perfect, for things you want very cold, but not frozen! )
Answer: While the relationship between salt concentration and freezing point depression isn't perfectly linear, it follows a predictable pattern based on the equation ΔT = Kf * m * i. For Ben's PCM, starting with the original recipe (5 cups water, 1 cup sodium sulfate, 1/4 cup table salt) at 65°F (18°C), we can estimate that doubling the salt to 1/2 cup might lower the temperature to around 52°F (11°C). Further increases would have diminishing effects: 3/4 cup salt might reach ~43°F (6°C), and 1 cup salt approximately 36°F (2°C). However, these are theoretical estimates... and the actual results may vary due to the complex interactions between sodium sulfate and sodium chloride in the solution. The effect is more pronounced at lower concentrations and becomes less effective as you add more salt, potentially affecting the PCM's heat absorption capacity and phase change behavior.
4 - If #3's a yes, any idea at what point on added table salt, you'd reach a point of diminishing return?
Answer: The point of diminishing returns for adding table salt would depend on several factors, including the solubility limit of salt in the solution and the eutectic point of the mixture. Generally, you'd reach diminishing returns when adding more salt no longer significantly lowers the melting point or begins to negatively impact the PCM's performance. This could occur when the solution becomes saturated with salt, or when the increased salt concentration starts to interfere with the crystallization process of the sodium sulfate. To determine this precisely, you'd need to conduct a series of experiments, gradually increasing the salt concentration and measuring the resulting melting point and heat absorption capacity of the PCM.
