Publications des agents du Cirad


Study of the mechanism of acid coagulation of Hevea latex and of the rheological properties of resulting gels

De Oliveira Reis G.. 2015. Montpellier : Université de Montpellier, 165 p.. Thèse de doctorat -- Biochimie, chimie et technologie des aliments.

Natural rubber latex is mainly composed by rubber particles whose core is made of cis-1,4-polyisoprene and the shell of non-isoprene compounds. It stability in the tree is provided by the negative charge of the shell due to the presence of proteins and lipids. Natural rubber latex coagulates after acidification to form a colloidal gel that is the first step of its transformation into natural rubber. In this PhD thesis, we studied this acid-induced aggregation and gelation and the properties of the resulting gels. For this, a commercial natural rubber latex composed by rubber particles (1 µm in diameter), was used. Aggregation was induced by hydrolysis of a compound called glucono-delta-lactone (GDL). The aggregation was followed by different techniques based on light scattering (turbidimetry, DWS, SLS). Gelation was followed in situ by rheology. Then, the rheological properties of colloidal gels formed at pH 4 were characterized in the linear and non-linear regimes. Dependence to the volume fraction and the pH of the onset of aggregation was observed. The non-redispersion of aggregates proves the strong interactions between rubber particles. Three different state behaviors (stable solution, phase separation and gel) can be predicted from a state diagram pH-volume fraction that was established over 5 weeks. The gel state exhibited a fractal structure, characterized by a power law relationship between the elastic modulus G' and the volume fraction. At high deformations, an irreversible reorganization under stress resulted in an irreversible strain hardening of the material. We observed by rheo-ultrasound that above 50 % deformation, a homogenous destructuration occurred in the gel.

Mots-clés : hevea brasiliensis; latex; coagulation; gélification; acidification; caoutchouc; rhéologie; propriété rhéologique; Échographie; gel (matériau); État colloïdal; france; brésil

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