FIGURE 5 Microscope images of composites: (a) SCG/PP,(b) NSCG/PP, and (c) ASCG/PP
3.3.2 Water contact angle and Water absorption of SCG/PP composites
The water contact angle is one of the methods to characterize the hydrophilicity of materials. Figure 6 (a) is the result of the water contact angle. It shows the contact angle of PP is 84.37°, exhibiting weak hydrophilicity. The contact angle of the SCG/PP composite is 79.7°, which is smaller than that of PP. The water contact angle of NSCG/PP and ASCG/PP composites are 84.14° and 87.31°, respectively, which is close to or greater than that of PP. The contact angle of SCG/PP is more hydrophilic. It may be due to the poor dispersion of SCG in PP, resulting in high surface roughness and surface energy of the material, so the water wettability is stronger. After heat treatment, the dispersion of SCG in PP is uniform, the interface compatibility is better, and the relative probability of defects is small.
The water absorption capacity of plastic is closely related to its service life, so we test the water absorption rate of the samples, as shown in Figure 6 (b). It is worth noting that SCG/PP has the highest water absorption rate of 1.41%, followed by NSCG/PP with 1.2%, and ASCG/PP has the lowest water absorption rate of 0.9%. The hydrophobicity of the filler itself will affect the water absorption of the composite. With the addition of filler, the water absorption of the composites increased significantly. In addition, the water absorption for composites is closely related to the interfacial gap and defect. The material with better interface compatibility has less water absorption. The above data prove that SCG with air atmosphere heat treatment is more advantageous as a filler in solving the problem of interface compatibility. Food packaging materials should have good ability to block water. The stronger the hydrophobicity and the weaker the water absorption capacity, the more valuable it is in the preparation of finished products.