ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия А, 2012, том 54, № 2, с. 302-313
ОБЗОРЫ
УДК 541.64:547.458.61
THERMOPLASTIC STARCH BLENDS: A REVIEW OF RECENT WORKS1 © 2012 г. Mosab Kaseem, Kotiba Hamad, and Fawaz Deri
Department of Chemistry, Faculty of Science, Laboratory of Materials Rheology (LMR), University of Damascus, 31513 Damascus, Syria
e-mail: mosabkaseem@yahoo.com Received June 5, 2011 Revised Manuscript Received August 23, 2011
Abstract — The aim of this review is to discuss the recent developments in thermoplastic starch blends. Starch has been considered as an excellent candidate to partially substitute synthetic polymer in packaging, agricultural mulch and other low-cost applications. Recently, the starch granules were plasticized using different plasticizers under heating and shearing, giving rise to a continues phase in the form of a viscous melt which can be processed using traditional plastic processing techniques, such as injection molding and extrusion. This kind of starch composites is called thermoplastic starch. Unfortunately, thermoplastic starch presents some drawbacks, such as low degradation temperatures, which make it difficult to process, poor mechanical properties and high water susceptibility. Much work has been carried out to overcome these drawbacks, including the combination of thermoplastic starch with other polymers, aiming at lowering the cost and enhancing the biodegradability of the final product.
INTRODUCTION
Rising oil prices and increased activity in regards to environmental pollution prevention have pushed research and development of biodegradable plastics, where the development of plastics by using renewable resources which are naturally biodegradable and their possibility of combining their biodegradability with cost reduction and market needs have been the object of intensive academic and industrial researches.
Starch is one of the most promising natural polymers to be abundant, cheap and biodegradable. Starch offers a possible alternative to the traditional nonbiodegradable polymers, especially in short life-time application and when their recycling is difficult and/or not economical. Starch consists of two major components: amylose, a mostly linear a-D(1-4)-glucan and amylopectin, an a-D-(1-4) glucan which has a-D(1-6) linkages at the branch point. The linear amylose molecules of starch have a molecular weight of 0.2— 2 million, while the branched amylopectin molecules have molecular weights as high as 100—400 million. Starch is unique among carbohydrates because it occurs naturally as discrete granules. This is because the short branched amylopectin chains are able to form helical structures which crystallize. Starch granules exhibit hydrophilic properties and strong intermolecular association via hydrogen bonding due to the hydroxyl groups on the granule surface. The melting point of native starch is higher than the thermal decomposition temperature: hence the poor thermal stability of native starch and the need for conversion to
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starch-based materials with a much improved property profile.
Starch was used as natural filler in traditional plastics (PE, PP, etc.) [1—10] and particularly in polyole-fins; it helps in cost reduction and rising biodegrad-ability of resultant composite as well as in increasing the rigidity (high modulus) [3] of the material. However, the starch concentration at which useful product can be obtained is therefore limited to a low range after which the materials properties suffer dramatically. The overall disintegration of these materials is achieved by the use of transition metal compounds, soluble in the thermoplastic matrix, as prooxidant additives which catalyses the photo and thermo-oxidative process.
Recently, starch was used as a main component in polymer blends, it was applied in plastic rather than in the native form, where the starch granules were plasti-cized by using plasticizers under heating, giving rise to a continues phase in the form of a viscous melt which can be processed using traditional plastic processing techniques [11], such as injection molding and extrusion. This kind of starch composites is called thermoplastic starch (TPS). Several plasticizers have been used for plasticization process to convert starch into TPS to be used in polymer blends such as glycerol [12—21], water [21, 22], urea [23—25], formamide [22, 24—26], ethylenebisformamide [27—29], sorbitol [27, 30], citric acid [31], N-(2-hydroxyethyl)forma-mide [32] and amino acids [20, 33]. Water is more effective as a plasticizer than glycerol, but the most used plasticizer in TPS preparation is glycerol due to its high boiling point, availability, and low cost. Figure 1 shows the familiar plasticizers as a percentage for glycerol (100% glycerol).
Plasticizer, %
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80
60
40
20
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Fig. 1. Some familiar plasticizers.
Blends of TPS with traditional polymers were prepared in efforts to obtain new materials with low cost and high biodegradability. This work gives a brief review most of the recent development in TPS blends.
TPS/PE BLENDS
PE is one of the most important traditional polymers, it has excellent chemical resistance and is not attacked by acids, bases, or salts. The other characteristics of polyethylene which have led to its widespread use are low cost, easy process ability, excellent electrical insulation properties, toughness and flexibility even at low temperatures, freedom from odor and tox-icity, reasonable clarity of thin films, and sufficiently low permeability to water vapor for many packaging, building, and agricultural applications.
Pierre et al. [34] processed and characterized TPS/PE (LDPE and LLDPE) blends, glycerol was used as a plasticizer. It was found that the modulus decreased as expected with addition of TPS in the TPS/LDPE blends. Also it was found that the blends containing 22% TPS in LDPE and 39% TPS in LLDPE maintained high elongation even at these high loading. The morphology showed the incompatibility in TPS/PE blends.
Bikiaris and Panayiotou [35] prepared TPS/LDPE blends; two different polyethylene-g-maleic anhydride copolymers containing 0.4 and 0.8 mol% anhydride groups were used as reactive compatibilizers. It was found that the compatibilized blends have only a slightly lower biodegradation rate compared to the un-compatibilized blends.
Jang et al. [36] studied mechanical properties and morphology of modified TPS/ HDPE blends, HDPE was chemically modified to enhance the compatibility
with TPS. It was found that the compatibility of TPS/HDPE was improved by increasing HDPE-g-glycidyl methacrylate content.
In another work, Sailaja and Chanda [37] used HDPE-g-maleic anhydride as a compatibilizer for TPS/HDPE blends. It was found that the mechanical properties of the blends were improved by the addition of HDPE-g-maleic anhydride. Sailaja et al. [38] studied the effect of epoxy functionalized compatibilizer on the mechanical properties of TPS/LDPE blends, poly(ethylene-co-glycidyl methacrylate) (PEGMA) was used as a compatibilizer. It was found in this study that mechanical properties of the TPS/LDPE were improved when PEGMA was used as a compatibilizer. Sailaja and Chanda [39] used of poly(ethylene-co-vi-nyl alcohol) as a compatibilizer in TPS/ LDPE blends. It was found that the impact strength of the blends was improved by the addition of a compatibilizer even with a high TPS loading of 40 and 50%.
In the study of Gonzalez et al. [40], the influence of melt drawing on the morphology of one- and two-step processed TPS/LDPE blends were examined. It was found that the blends prepared in the one-step extrusion process show increased levels of anisotropy as a consequence of a combination of coalescence and particle deformation during melt drawing. These data indicated that the one-step processing of TPS/LDPE blends can be used to generate a wide range of highly elongated morphological structures.
Gonzalez et al. [41] prepared high performance TPS/LDPE blends under particular one-step extrusion conditions, they found that the extrusion process and the controlled deformation of the TPS phase yields an important improvement in the elongation at break of TPS/LDPE blends as a function of composition, and they studied the role of the morphology on
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Polyethylene
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Starch-MA-grafted PE blend
Fig. 2. Simplified reaction scheme showing: (a) addition of maleic anhydride to PE and (b) reaction of maleated PE with starch, reported by Kalambur and Rizvi [43].
+
the mechanical properties of TPS/LDPE blends. They observed that elongation at break and Young's modulus of TPS/LDPE blends decreased as TPS content increased. They also demonstrated that the reduction of elongation at break and Young's modulus was more dramatic in the case of blends having spherical morphology than those composed by fiber-like particles. As observed in these work TPS/LDPE blends having spherical morphology resulted in larger reduction of elongation at break and Young's modulus than those showing elongated particles.
Wang et al. [42] prepared compatible TPS/LLDPE blends by one-step reactive extrusion in a single-screw extruder and maleic anhydride was added to improve the compatibility between TPS and LLDPE. Rheo-logical, mechanical, morphological and thermal properties of the prepared blends were determined. It was found that the blends with maleic anhydride had better mechanical, morphological and thermal properties than the blends without MA. Figure 2 illustrates a simplified reaction scheme of
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