POLYPROPYLENE/STARCH BLENDS: STUDY OF THERMAL AND MORPHOLOGICAL PROPERTIES

Abstract

SUMMARY Polypropylene/starch blends were prepared using starch of different botanical source. The influence of starch type and starch content on thermal properties and morphology of polypropylene/starch blends was studied. Corn and potato starches were used as starch source. The starch content in the blends was 7 and 15 wt-%. Characterization of PP and PP/starch blends by (DSC), (TGA), and (SEM) was carried out to obtain information on miscibility among PP and starch. The starch domains exhibited characteristic voids be due to thermal degradation.Two types of PP, PP 1 poly(propylene- co -ethylene), random copolymer, and PP 3 poly(propylene- co -ethylene), heterophasic copolymer were studied Keywords: Polypropylene, starch, blends, characterization INTRODUCTION Blends are a mixture of two or more polymers obtained by melting or in solution. Its objective is the preparation of a material that posses the combination of properties of their components, not compassable by separated. The development of a blend with an adequate behavior, especially with respect to their mechanical and physical properties, depends on the interfacial adhesion among the phases. A compatible blend is that in which the dispersed or minority phase is found homogeneously distributed in the matrix or majority phase and in which the adhesion among the same is strong. In this type of blends, a force applied is transmitted among the components of the phases (1).Polypropylene (PP), a synthetic polymer is an economical material that offers a combination of outstanding physical, mechanical, thermal, chemical properties that are not found in any other thermoplastic polymer (2)The great number of polyolefinic products has caused harmful consequences to the nature due to the low degradation rates reached by the traditional polyolefins, e.g. low-density polyethylene (LDPE) and isotactic polypropylene (iPP).By this way, researchers have studied the incorporation of biodegradable polymers, such as the starch, to the traditional polyolefins, which is justified by the presence of hydrolyzable groups in the natural polymer that are able to induce the biodegradation of the polyolefins (3-5).Ramis et al. (6) evaluated the biodegradability of iPP/starch blends under simulated soil and observed the materials decomposition under nitrogen and oxygen atmosphere, by different mechanisms of degradation. The biodegradation affected the starch but not the iPP matrix.When dealing with starch-derivative materials, this last can present a thermoplastic characteristic (commonly called thermoplastic starch or TPS) when incorporating glycerin to it, acting as a plasticizer (7).Moreover, the use of glycerin, as described by Carvalho (7) can increase the fragmentation rate of the material (morphological structure), obtaining though materials with many different characteristics just by varying the amount of glycerin on the starch.In general, it is said that in order to let the polyolefins more susceptible to degradation, some kind of modification is necessary, such as modifications on the crystallinity degree (8), chemical degradation and reduction on the molar mass of polymers to create short molecules that are capable of being absorbed by microorganisms (9).The inclusion of such materials allows products traditionally produced from non-renewable and environmentally persistent petroleum-based materials, to be produced from materials made from renewable resources which may also provide a degree of degradability to the final product. Starch, in particular, has been used since the 1970s as a filler in plastics (10) and has recently been plasticized and extruded with traditional plastics (11) used as a baked foam for thin walled applications (12), and used as packaging foams (13). Dry granular starch and baked starch, which is pressed and molded, have limited processability and can be used in only a limited range of applications. Plasticized starch, however, is more versatile and can be blended with various polymeric materials for numerous applications.Starch is an abundant, naturally occurring polysaccharide that is obtained from various plant sources, such as corn, potato, rice, and cassava [14]. Starch is a semi-crystalline polymer comprised of glucose monomer units. its granular form is comprised of linear amylose or branched amylopectin macromolecules with amylose content of ranging from 20-30% (see Scheme 1). Dry granular starch, by itself, cannot be processed like a plastic; however, it can be heated and blended with several different small polar molecules (water, glycerol) or polar oligomers (polyols), giving a thermoplastic material, generally called (PLS). This process, known as gelatinization, breaks up the granular structure of starch by disrupting hydrogen bonding between adjacent glucose molecules and essentially destroys its crystallinity [15].