Starch based/Montmorillonite Nanocomposites

LILIANA MANFREDI,; VIVIANA CYRAS,; MIN-TAN TON-THAT; ANALIA VAZQUEZ

Zalakaros, HUNGRIA

Simposio; Eurofillers 2007; 2007

Starch is a biodegradable polymer and it is produced in abundance from many renewable resources. As a consequence the starch is easily available and very cheap.    Starch is a semicrystalline polymer stored in granular form as a reserve in certain plants.  It is composed of repeating a-D-glucopyranosyl unit: amylose and amylopectin.  The amylose is linear, the repeating units are linked by a (1-4) linkages, the amylopectin has a a (1-4) linked backbone and about 5% a (1-6) linked branches.     In native form, the starch granules are insoluble in cold water and most uses involve a heating treatment in the presence of excess of water.  Below a critical temperature (about 600C), that is the so-called gelatinization temperature, starch granules absorb water and undergo swelling to many times their original size.  This process is attributed to the diffusion of amylose outside the granule.  Beyond this critical temperature, the swollen starch granules can undergo a disruption into smaller aggregates or particles, and result in a gelatinized starch. The thermoplastic starch or plasticized starch is obtained after disruption and plasticization of native starch, with water and another plasticizer, such as glycerol. This polymer could be use in packaging where good barrier and thermal properties are required.   The nanocomposites obtained by the addition of low percentages of clay to polymers exhibited an improvement in the properties such as barrier, thermal and oxidative when compared with traditional composites. This improvement was reached when the clay is exfoliated and the mayor problem in preparing these composites is to separate the layers of the clay because they are initially agglomerated.                 The aim of this work is to improve the barrier and thermal properties of the starch by the addition of low percentages of clay. Nanocomposites were obtained from potato starch and montmorillonite (Cloisite® Na+) with glycerol as plasticizer. On one hand, the starch was gelatinized with water and glycerol. On the other hand, the clay was sonicated in water during 10 min. Then the gelatinized starch and different percentages of clay (2, 3 and 5% w/w) were mixed and sonicated during 30 min. The nanocomposites were prepared by casting and the films obtained were dried in an oven at 450C, until constant weight. The dispersion of the clay in the films was analysed by X-Ray diffraction. It was observed that the 001 diffraction peak of clay was shift to lower angles in the nanocomposites patterns. Also this peak appeared as a broad band for the nanocomposites with 5% w/w of clay. These results indicated that there was an increment in the interlayer distance of the clay and they provided strong evidence that the clay nanolayer formed a disordered intercalated structure without reach a complete exfoliation. An improvement in the thermal resistance of the starch with the addition of clay was observed by means of thermogravimetric analysis. This effect was due to the fact that the clay acts as heat barrier enhancing the overall thermal stability of the system.             The absorption of humidity by the nanocomposites was measured in an environment with a 75% of constant relative humidity. It was observed that the maximum of water uptake was reduced by the addition of clay to the starch. This could be related with the cristallinity of the system.