The composite materials when exposed to the environment or water absorb moisture. The absorption of the moisture results in the expansion as we have seen already. This also affects the degradation of the mechanical as well as thermal properties.

 The moisture content in a polymeric composite is given in terms of percent of moisture by weight. Hence, to measure the moisture content a sample is weighed at the ambient conditions. Then the sample is dried and weighed again.  The difference in these two weights per unit weight of the dry sample gives the weight change due to moisture content. Figure 8.1 shows a qualitative variation in weight change over time due to two different drying conditions for a polymer composite. From the figure it can be seen that the drying in vacuum increases the desorption rate significantly. That is why the vacuum ovens are used in laboratory for drying the specimens.