Natural rubber latex foam production using air microbubbles: Microstructure and physical properties

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

“…The cell coalescence and lack of cell homogeneity can result in structural collapse, affecting both the physical and mechanical properties of cellular materials [11] . This poor morphology was reported in other studies of NRLF produced using the Dunlop method [10] , [16] . When foaming processes of polymeric cellular materials were carried out in a multi-phase system, cell nucleation is nonuniform [26] .…”

“…This causes the foams to have higher compressibility degree under the indentation force, resulting in lower indentation hardness and vice versa. According to the standard classification (ISO 2439), foam materials with hardness values lower than 100 N can be classified as soft foams [10] . Hence, NRLFs under the ultrasonic dissipation process with irradiation time less than 5 min can be classified as soft foams.…”

“…The foam properties, e.g., flexibility, density, indentation hardness, compression set, thermal insulation, mechanical properties and energy absorption, also depend on these cellular architectures [8] . Therefore, to obtain desired properties, several studies have attempted to control the microstructure of rubber foams using various techniques, such as a microbubbling technique [10] , a microwave processing technique [11] , electron beam irradiation [12] and a filler-assisted technique [13] , [14] . However, morphological control in thermoset foams (rubber foams) is still a challenge and appears more complex than that in thermoplastic foams since they are comprised of several phases and must be nearly simultaneously foamed and vulcanized [12] , [14] .…”

“…The Dunlop and Talalay processes are two well-known NRLF production methods. The Talalay method provides softer NRLFs with greater flexibility than the Dunlop method [10] . However, the Dunlop method is commonly used as it is convenient, dependable, cost-effective, organic solvent-free and more energy-efficient [15] , [16] .…”

“…After vulcanization, the foam is cleaned and moisture eliminated from the finished product [16] . To the best of our knowledge, the Dunlop method does not involve any technical step for controlling the microstructure quality of the foam, even though the microcellular structure can be an important parameter that greatly influences physico-mechanical properties of rubber foams [8] , [10] , [11] , [12] . In other words, NRLFs prepared by the Dunlop method still require some production techniques that allow manipulation their cellular orientation.…”