Freeze 12 is 20% R142B and 80% R134a. The R142B is added to make the R134a compatible with your system. The problem with such blends is that in the event of a leak, the composition of the blend changes because one component will leak at a different rate than the other due to differing molecule sizes between the two refrigerants. In the case of Freeze 12, the R142b will leak out first, leaving you with a system filled with R134a, which will cause compressor seizure because of the incompatibility with the mineral oil compressor lubricant that is used in R12 systems.
