Thin plastic film colorimetric sensors for carbon dioxide: effect of plasticizer on response

Different plasticizers, including phosphate-, phthalate-and adipate-based types were used in the creation of a range of colorimetric plastic film sensors for CO2. The different types of plasticizer used in the formulation of a colorimetric plastic film sensor for CO2 affect the response and recovery times of the sensor differently. An effective plasticizer was taken as one that decreased the response and recovery times of the final film sensor when exposed to an alternating atmosphere of 0–5% CO2. On this basis, the most efficient plasticizers appeared to be phosphate-based, followed by phthalate-and adipate-based plasticizers. This trend appears to reflect the degree of the polymer-plasticizer compatibility. Increasing the amount of plasticizer in the film formulation decreased the response and recovery times of the sensor dramatically. The sensitivity of the film sensor towards CO2 appears to decrease with increasing plasticizer effectiveness; thus, the general order of film CO2 sensitivity with respect to plasticizer type was found to be adipate > phthalate > phosphate. In general, the response of the optical films towards CO2 was found to be temperature sensitive [typically, ΔH=–(44–55) kJ mol–1]. The phosphate-based plasticized films appear to be less temperature sensitive than the other plasticized films, and 2-ethylhexyl diphenylphosphate appears particularly effective in this respect (ΔH=–18.5 kJ mol–1).