Superabsorbent polymer composites: does clay always improve properties? K. Kabiri • S. Hesarian • M. J. Zohuriaan-Mehr • A. Jamshidi • H. Boohendi • M. R. Pourheravi • S. A. Hashemi • H. Omidian • S. Fathollahi Received: 10 April 2011 / Accepted: 10 May 2011 / Published online: 19 May 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Clay is frequently incorporated to many mate- rials 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 superabsor- bents, particularly the residual monomer (RM), has been unnoticed in the academic literature. Here, we report preparation of a series of SAPCs via a conventional solu- tion polymerization of partially neutralized acrylic acid in the presence of common MMT clay (Na-montmorillonite). The products were characterized by FTIR spectros- copy, differential scanning calorimetry, thermogravimetric, thermomechanical, and rheometrical analyses. The RM content of the samples was determined by high perfor- mance 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. Introduction Superabsorbent polymer (SAP) hydrogels are lightly crosslinked three dimensional networks with the ability of absorbing and retaining huge amounts of water and aque- ous fluids [1, 2]. The applications of SAPs can be divided into hygienic and non-hygienic areas [2, 3]. The worldwide SAP production is *1.5 million tons per year. Majority of the materials is consumed in hygienic uses such as baby diaper and feminine napkin [1, 2]. The practical features of an ideal SAP material can be stated as absorption capacity and rate, swollen gel strength (as a measure of the absorbency under load), non-toxicity, price, etc. [1, 2]. Meanwhile, particularly for the hygienic grade SAPs, high gel fraction and low content of the residual monomer (RM) acrylic acid is one of the most K. Kabiri (&)  M. J. Zohuriaan-Mehr  A. Jamshidi  H. Boohendi  S. A. Hashemi Department of Color, Resin and Surface Coatings, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965-115, Tehran, Iran e-mail: k.kabiri@ippi.ac.ir; kouroshkabiri@yahoo.com S. Hesarian  M. R. Pourheravi Department of Chemistry, Payam Nour University, P.O. Box 97, Abhar, Iran H. Omidian College of Pharmacy, Nova Southeastern University, 3200 South University Drive, Fort Lauderdale, FL 33328, USA S. Fathollahi Central Laboratories, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965-115, Tehran, Iran 123 J Mater Sci (2011) 46:6718–6725 DOI 10.1007/s10853-011-5627-0