According to Raoult's law, the pressure of liquid vaporization in absolute space is determined by the amount of liquefiable substance. Under normal pressure, each mole of liquid can be completely vaporized to give an atmospheric pressure of 22.4l space during the experiment of counter pressure cooking. We often think that the temperature has been accurately controlled, there is enough back pressure when cooling down, or there is often a bag burst phenomenon.
It must be because the pressure inside the bag is higher than that outside the bag.
1) First of all, the water is put into the boiling bag according to the regulations, and the air is vented and sealed. The air is not drained. During the heating process before 100 ℃, the expansion of boiling bag is normal. When the water is sealed at room temperature, the pressure inside and outside the bag is balanced. When the temperature is heated to 98 ℃, the liquid water itself will expand, and the air in the bag will expand. At this time, part of the water vapor generated will occupy part of the space, leading to expansion. At this time, the pressure in the bag will increase. The burst of bag is the creep phenomenon of polymer material in the sealing part under this thermal state.
2) Before heating to 100 ℃, the change of the cooking bag is the same as that of the boiling bag. In the cooking pot, when the heating temperature exceeds 100 ℃, the pressure inside the bag is always higher than the pressure outside the bag. Therefore, the pressure of the inner bag must be completely higher than that of the outer bag during the vacuum heating process.
3) Generally, rcpp or SCPP can be produced by binary copolymer, and the thickness must be greater than 60um. As a kind of high-temperature cooking CPP, it should not be combined with Homo polypropylene (PPH), but should be combined with copolypropylene (block copolymer PP-B) or blending modified polypropylene granular film. Because CPP made of Homo polypropylene film will be embrittled after high temperature cooking, it is easy to break from the seal, resulting in leakage of contents, and the marks at the vacuum point are easy to leak, and the oil resistance is very poor. However, copolymeric or blending CPP is better than homopolymer CPP because of its high crystallinity, low temperature resistance and impact resistance.
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