The Acidification Effect on The Stability of Natural Latex Particles and Their Coagulation Rheological Properties

Abstract

Natural Rubber Latex (NRL) particles contain poly-isoprenes chains, small amounts of varying minerals and covered by membranes of proteins and phospholipids. They carry an overall negative charge responsible for the overall stability of rubber particles suspended in the aqueous serum. However, upon acidification of NRL, these electrostatic charges are neutralised, resulting in the aggregation of the particles and subsequent formation of a colloidal network (gel). Glucono-δ-Lactone (GDL) is used, at 25oC, to study the aggregation stability of vulcanized NRL and two native NRL suspensions sourced from Cameroun and Cambodian. These latex dispersions were dialysed against Tris buffer (pH8.5) to ensure the same starting pH. In brief, Transmission Electron Microscopy (TEM) micrographs showed that the natural rubber latex particles particles have a spherical shape with varied sizes. This was confirmed by light scattering methods. Diffusing-Wave spectroscopy (DWS) was used on dilute vulcanised NRL samples and showed that the critical aggregation pH to be around pH5.5. The DWS measurements also showed that it possible to superpose the aggregation behaviour by plotting the increase in apparent size as a function of change in pH. Rheological measurements including small and large oscillatory were performed. Small deformation rheology showed that the acidified samples behaved as weak gels and that the final elastic modulus G' follows a power law behaviour with the latex concentration. This observations holds for all the NRL samples investigated. This study showed that it is possible to model acidified NRL systems using a theory developed for colloidal gel. In addition, large deformation measurements unravelled the strain-hardening behaviour of these systems. A comparison is drawn between NRL dispersion and milk systems. It is believed that the knowledge developed in these two areas, which are believed to be different, can be used to further understand these systems.