Moisture detection in polymer composites

An important example of sample change is the ingress of liquids in contact with the object surface. Already small amounts of solvent may invoke major changes in material properties that can be detected by mobile unilateral NMR sensors. In this work the water uptake in a polymer composite was followed as a function of the absorption time. A 3 mm thick sample was initially dried in an oven and then immersed in water at room temperature. A series of profiles was measured as a function of the time of exposure to water (Figure a). The profiles were obtained by scanning the sample with a resolution of 200 µm and assigning to each position the amplitude resultant from the addition of the echoes acquired during a solid-echo train. The vertical scale in Fig. 6 was converted into water content (% in weight) via calibration. It is important to stress that the variation in the signal intensity in the profiles is due to a lengthening of the T2eff of the material, which is influenced by the presence of water. It allows the quantification of tiny amounts of water having NMR signals smaller than the detectable limit. The results show that the technique can be used to follow the penetration front of solvents into hard materials. The sensitivity of the technique is high enough even to detect the moisture of the sample due to the ambient humidity. Figure (b) compares the profile of the dried sample with the one of a sample exposed for a long time to the humidity of air at room temperature. The profile reveals that the sample contains 1% moisture distributed almost uniformly over its thickness.

 

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