Ultrasonic-assisted Dunlop method for natural rubber latex foam production: Effects of irradiation time on morphology and physico-mechanical properties of the foam

An ultrasonic-assisted technique was first introduced for the production of natural rubber latex foam (NRLF). The flexible elastomeric foam was formed by a liquid–solid state transformation in an aqueous media. The aim of the current research was to provide a novel strategy for fabricating NRLF using ultrasonication and the Dunlop method, as well as to simultaneously utilize irradiation events to achieve the desired foam properties. NRLFs were exposed to ultrasonication at 25 kHz at the beginning of the gelling process. The effects of irradiation times of 0, 1, 3, 5 and 7 min on the morphology, foaming behaviors, physical properties and mechanical performance of NRLFs were investigated. The results revealed that using ultrasonic irradiation, unfoamed regions and a bimodal structure, which seem to be microstructural defects in conventional NRLF, could be completely eliminated. However, excessive irradiation times of 5 min and longer appeared to affect the physico-mechanical properties of the foams in terms of transient cavitation and the unfavorable physicochemical effects of ultrasonic vibrations. As a result, the optimal ultrasonic irradiation time was found to be 3 min. Using this irradiation duration, a foam with the suitable microcellular structure achieved the most desirable properties, such as its expansion ratio (7-fold increase), foam porosity (85.7%), compression recoverability (98.7%), and tensile strength (307.3 kPa). Moreover, the foam still maintained its characteristic soft nature (hardness less than 100 N) with an indentation hardness of 71.9 N. Therefore, ultrasonic treatment introduced to the conventional Dunlop method is a potentially feasible technique since it improves the morphology and the physico-mechanical properties of NRLFs.