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Rheological properties of colloidal gels from Hevea brasiliensis latex

De Oliveira Reis G., Menut P., Vaysse L., Bonfils F., Sanchez C.. 2013. In : Ed. L. Rovigatti. International Soft Matter Conference 2013: Book of poster abstracts, September 15-19, 2013, Rome, Italy. s.l. : s.n., p. 222-222. International Soft Matter Conference 2013, 2013-09-15/2013-09-19, Rome (Italie).

Hevea brasiliensis latex and natural rubber are nowadays mostly produced in south east asia. This raw material is used to prepare a large diversity of products, vehicule tyre being the most important one (70% of natural rubber market). Because of its exceptional properties, such as high green strength, low internal heat build-up or building tack, synthetic elastomers cannot be used to replace it in certain applications. Therefore, and for few decades already, natural rubber keeps its more than 40% share in world elastomer market. Though the natural rubber latex is used for manufacturing products for more than a century, the native structural organization of latex but also the changes of this organization during the processes used to transform latex into natural rubber need to be clarified. Natural rubber latex is a complex colloidal suspension composed of poly(cis-1,4-isoprene) particles, and others natural components dispersed or dissolved in an aqueous phase called serum. Latex particles exhibit a core-shell structure in which a poly-isoprenic core is surrounded by a mixed layer of proteins and phospholipids. At basic pH, the surface layer is negatively charged, ensuring the repulsion between particles and therefore the stability of the suspension. If the pH decreases, the stability is lost and aggregation is observed. Formation and growth of fractal aggregates leads to the formation of a colloidal gel at high enough particle volume fraction. In this study we probed the rheological properties of final gels within a large range of latex volume fractions. We used a commercial stable prevulcanized natural rubber latex diluted in TRIS buffer and acidified with glucono-delta-lactone (GDL), the hydrolysis of which initiates a progressive and homogeneous acidification of the suspension. The fractal structure of these gels was investigated by rheology with a couette geometry.. A power law relationship between equilibrium G' modulus and latex volume fraction was observed. Plotting the stress-strain relationship observed during a strain sweep allowed determining the limit of linearity, which decreases with volume fraction, a characteristic of a strong-link regime. The aggregation kinetics of latex rubber were evaluated to be in the DLCA regime (Diffusion Limited Cluster Aggregation) with a fractal dimension (Df ) of 1.8 using the model of Wu and Morbidelli. 1 In a large range of volume fractions, gel showed evidence of a strain hardening at high strain. This phenomenon was characterized by a non-linear increase in mechanical moduli (G' and G") before failure. This phenomenon was analyzed in terms of amplitude, reversibility and stress bearing. A power law dependence of the relative amplitude of the strain hardening with latex volume fraction was demonstrated.

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