Superabsorbent polymer composites: does clay always improve properties?

Clay is frequently incorporated to many materials including superabsorbent polymers (SAPs) to improve their properties. Superior properties have been reported for the SAP composite (SAPC) materials comparing to their clay-free counterparts. However, study of the effect of clay on some of very important requirements of superabsorbents, particularly the residual monomer (RM), has been unnoticed in the academic literature. Here, we report preparation of a series of SAPCs via a conventional solution polymerization of partially neutralized acrylic acid in the presence of common MMT clay (Na-montmorillonite). The products were characterized by FTIR spectroscopy, differential scanning calorimetry, thermogravimetric, thermomechanical, and rheometrical analyses. The RM content of the samples was determined by high performance liquid chromatography. It was found that the clay had unfavorable effects on the crosslinking polymerization process. This fact was observed as declining mechanical strength of the SAPCs in both dried and swollen states, increased swelling capacity, decreased gel fraction, and increased RM content. For instance, RM of clay-free sample was 740 ppm which was continuously increased with level of the clay incorporated. It surprisingly reached to ~34,000 ppm at clay content of 12%. The undesirable function of clay was attributed to inactivation and barrier effects of clay incorporated to the polymerization medium. These unwanted effects were more pronounced at high clay content. However, overall thermostability of SAPCs was improved comparing to the non-composite counterpart. It was concluded that such conventionally prepared SAPCs, in spite of the previously reported claims, could not be suitable candidates for hygienic applications, particularly those prepared with high clay percentages.