STUDY ON THE FOAMABILITY, MORPHOLOGICAL STRUCTURE AND PRESSURE-RELIEF PERFORMANCE OF AMMONIA-FREE NATURAL RUBBER LATEX FOAM FOR SHOE INSOLES APPLICATIONS

Abstract

Ammonia-free natural rubber (AFNR) latex is a newly developed commercial grade of natural rubber (NR) latex concentrate aimed to address environmental and health concerns related to ammonia in the NR latex industry. This study investigates the viability of using AFNR latex to produce latex foam shoe insoles and compares it with that of traditional high-ammonia NR (HANR) latex. The Dunlop foaming process was used to produce both types of latex foams, each with a wet foam density of 0.2 gcm^-³. The study assessed the effect addition of varying levels (0, 0.5, 1.0 and 2.0 phr) of potassium oleate, a foaming agent, on the foamability of the latex. The findings indicate that AFNR latex exhibits lower foamability compared to HANR latex, although increasing the potassium oleate level enhances the foamability of AFNR latex. Additionally, the results show that AFNR latex foam requires a longer time to get a gel during the gelling process compared to HANR latex. A cross-sectional view using FESEM reveals that both AFNR and HANR latex foams exhibit open-cell structures. FESEM images also show smoother, thicker skin at the bottom compared to the upper part of both samples, with AFNR having 60 µm at the bottom and 38 µm at the upper and HANR having 52 µm at the bottom and 37 µm at the upper. The difference in skin structure contributes to the variation in Shore F hardness values but does not show a significant difference in pressure-relief performance. Despite differences in foamability and skin structure, AFNR latex shows comparable pressure-relief performance to HANR latex, indicating its potential for application in latex foam shoe insoles.