SYNOPSISThermal analysis of controlled-release formulations of fenthion, prepared by its entrapment in cross-linked carboxymethylcellulose and carboxymethylcellulose-gelatin matrices was carried out by thermogravimetric analysis and under accelerated storage conditions. Thermograms of samples of sodium carboxymethylcellulose, cross-linked polymer samples, and the formulations were studied. The thermogravimet,ric analysis showed improved heat resistance and retention of fenthion or its decomposition products in the formulated samples. The decomposition maximum of technical-grade fenthion was 260°C and the corresponding weight loss was about 90%, whereas fenthion or its decomposition products appear to be retained up to temperatures ranging from 400 to 500°C in the formulations. The half-life of fenthion in the mechanical mixture with the carrier polymer was found to range from
I NTRO D UCTl 0 NThe controlled-release formulations of various larvicides were developed by the physical entrapment in polymer matrices of plastics and rubbers. Incorporation of insecticides in nonbiodegradable polymer matrices resulted in the residual loss of the larvicide and its biomagnification in the Fenthion ( O,O-dimethyl-O-4-methylthio-m-tolylphosphorothioate) is widely used as a mosquito larvicide and its low persistence in the polluted aquatic habitats necessitates repeated application at short interval^.^ Therefore, studies were undertaken in this laboratory to develop controlled-release formulations of larvicides using environmentally compatible polymers that undergo biodegradation following the device exhaustion.Controlled-release monolithic formulations of fenthion were prepared using a chemically modified cellulose derivative, sodium carboxymethylcellulose (NaCMC), as the entrapping matrix followed by ionotropic cross-linking with cupric or ferric ions under ambient conditions of temperature and pH.5-7 Formulations were also prepared with an interactive polymer gelatin in combination with carboxymethy l~e l l u l o s e .~~~ The controlled-release matrices thus obtained were found to release the larvicide for a prolonged duration following, more or less, a constant release rate.Thermogravimetric analysis (TGA) is a useful technique to study the thermal resistance of the incorporated active agents in polymersg and the characterization of pesticide metal complexes." The storage stability of pesticide formulations is an important parameter to assess their suitability for large-scale production and applications. It is influenced by both the inherent stability of the active agent and the formulation The storage stability tests are generally performed under accelerated conditions of temperat~re.~ The present study deals with the thermal analysis of controlled-release formulations of fenthion by thermogravimetric analysis and under accelerated storage conditions and the results are compared with the mechanical mixture of the carrier polymers and fenthion.