Influence of Aging Time on the Structural Changes of Cassava Thermoplastic Starch
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Influence of Aging Time on the Structural Changes of Cassava Thermoplastic Starch José H. Mina1, Alex Valadez2, Pedro J. Herrera-Franco2 and Tanit Toledano2 1
Facultad de Ingeniería de la Universidad del Valle, Escuela de Ingeniería de Materiales. Grupo de Materiales Compuestos. Email: [email protected] 2
Unidad de Materiales, Centro de Investigación Científica de Yucatán A.C. (CICY) Calle 43# 130, Chuburná de Hidalgo, Mérida Yucatán México.
ABSTRACT In this work the change in the structural properties of cassava (manihot sculenta Crantz) thermoplastic starch (TPS) under controlled environment (humidity and temperature) was studied. Fourier Transform Infrared spectroscopy (FTIR) and X-ray diffraction (XRD) results showed an evident increasing in the amorphous phase of the TPS regarding the native starch. There was a relative decrease of the band at 1047 cm-1 associated to crystalline structure of starch compared to the amorphous peak at 1022 cm-1. The X-ray diffraction patterns confirmed the increment of the amorphous phase in the TPS samples. Likewise the X-ray diffraction patterns shows evidence of residual type C crystallinity and the formation of a new crystalline phase type VH due to the orientation induced in plasticization process. In first stage of conditioning the tensile yield stress drops from 7.5 drops to 0.5 MPa and the break strain increases 1000%. At the same time it seems that the crystallinity of the samples increases as was evidenced by the gradually increasing of the FTIR band at 1047 cm-1. In a second stage, the yield stress increases, the break strain drops and the crystallinity continue growing steadily. These findings suggest that coexist two phenomena simultaneously in the samples. A phenomenon of re-crystallization (retrogradation) that tends to make the material more stiff and a process of plasticization that tends to softening it. It seems that the latter mechanism predominates in the first stage, at short times, and the former in the second stage, at older times. Keywords: Polymer, Macromolecular structure, X-ray diffraction (XRD), Scanning electron microscopy (SEM), Stress/strain relationship. INTRODUCTION Recent research efforts in the area of polymeric materials have focused on the development of biodegradable materials obtained from renewable resources such as starch, proteins, hydroxyalkanoates, etc., which are liable to complete biodegradability when subjected to composting conditions [1-3]. Nowadays, due to worldwide decrease of oil production, the search for new materials from renewable resources becomes crucial to substitute traditional synthetic polymers. Among these materials brings out the thermoplastic starch (TPS) since the starches are the most abundant and cheapest materials from renewable resources [1, 4]. Thermoplastic starch (TPS) is recognized as an environmentally-friendly alternative for replacing conventional synthetic plastics, mainly in packing and wrapping applications. Starch consists of two major components: amylose, a mostly linear Į-D(1,4)- glucan and
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