Fabrication and characterization of latex foam shoe insoles made from novel ammonia-free natural rubber latex concentrate

The presence of ammonia as a preservative in natural rubber (NR) latex concentrate induces a variety of problems in the NR latex foam manufacturing process, consequently leading to inconsistency in the quality of the end products. To overcome these issues, a novel ammonia-free NR (AFNR) latex concentrate that uses palm oil-based polymeric surfactants as an alternative preservative has been developed. This study explores the feasibility of using AFNR latex to produce latex foam shoe insoles. In this investigation, the AFNR latex was foamed into extra-high-density (EHD) and high-density (HD) foams using the Dunlop batch foaming process. The study found that HD latex foam is softer than EHD latex foam, with the Shore F hardness of HD latex foam and EHD latex foam being 57 and 77, respectively. The study also discovered that the peak pressure value and rebound resilience properties of HD latex foam shoe insoles, which were fabricated at 7 mm thickness, are 41% and 6% lower than those of EHD latex foam, respectively. Additionally, EHD latex foam and HD latex foam have different morphological characteristics, with EHD latex foam having a thicker cell wall, smaller pore size, and being less porous compared to HD latex foam. Taking these properties into consideration, HD foam could be the ideal choice for shoe insole applications, such as sports shoes.