Because many potentially volatile compounds are contained in plastic resin, It is unlikely  that quantify water content of the material by simply heating a sample, measuring the resulting weight loss, leads to the conclusion that the entire weight loss is water !

Figure 1 shows the results of a PY-MS test performed on a nylon material.Each peak in the graph represents a compound that was evolved from the material. One of these will be moisture (water) and the rest are something else. Because water is one of the lower-molecular-weight compounds that will be contained in a polymer, it will be one of the first to evolve; therefore, it is likely to be one of the peaks captured in the first 1-2 minutes of the scan. The peak heights represent relative abundance and it is fairly easy to see that when the test time approaches 30 minutes, most of the volatile material has been captured. It is equally easy to see that the single peak that represents water accounts for only a small percentage of the total material that is collected.