Starch-based completely biodegradable polymer materials

Starch is a natural polymer which possesses many unique properties and some shortcoming simultaneously. Some synthetic polymers are biodegradable and can be tailor-made easily. Therefore, by combining the individual advantages of starch and synthetic polymers, starch-based completely biodegradable polymers (SCBP) are potential for applications in biomedical and environmental fields. Therefore it received great attention and was extensively investigated. In this paper, the structure and characteristics of starch and some synthetic degradable polymers are briefly introduced. Then, the recent progress about the preparation of SCBP via physical blending and chemical modification is reviewed and discussed. At last, some examples have been presented to elucidate that SCBP are promising materials for various applications and their development is a good solution for reducing the consumption of petroleum resources and environmental problem. Keyword: biodegradable polymers, starch, biopolymer, preparation, application eXPRESS Polymer Letters Vol.3, No.6 (2009) 366–375 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2009.46 2. Structure and properties of starch Starch is mainly composed of two homopolymers of D-glucose [8]: amylase, a mostly linear αD(1, 4’)-glucan and branched amylopectin, having the same backbone structure as amylose but with many α-1, 6’-linked branch points (Figure 1). There are a lot of hydroxyl groups on starch chains, two secondary hydroxyl groups at C-2 and C-3 of each glucose residue, as well as one primary hydroxyl group at C-6 when it is not linked. Evidently, starch is hydrophilic. The available hydroxyl groups on the starch chains potentially exhibit reactivity specific for alcohols. In other words, they can be oxidized and reduced, and may participate in the formation of hydrogen bonds, ethers and esters [12]. Starch has different proportions of amylose and amylopectin ranging from about 10–20% amylose and 80–90% amylopectin depending on the source [13]. Amylose is soluble in water and forms a helical structure [14]. Starch occurs naturally as discrete granules since the short branched amylopectin chains are able to form helical structures which crystallize. Starch granules exhibit hydrophilic properties and strong inter-molecular association via hydrogen bonding formed by the hydroxyl groups on the granule surface. Owing to its hydrophilicity, the internal interaction and morphology of starch will be readily changed by water molecules, and thereby its glass transition temperature (Tg), the dimension and mechanical properties depend on the water content. Tg of native starch can be as low as 60 to 80°C when the weight fraction of water is in the range 0.12 to 0.14, which allows starch to be successfully injection moulded to obtain thermoplastic starch polymers in the presence of water [15]. On the other hand, the hydrophilicity of starch can be used to improve the degradation rate of some degradable hydrophobic polymers, which will be shown in 3.1.1. Starch is totally biodegradable in a wide variety of environments. It can be hydrolyzed into glucose by microorganism or enzymes, and then metabolized into carbon dioxide and water [16]. It is worth noting that carbon dioxide will recycle into starch again by plants and sunshine. Starch itself is poor in processability, also poor in the dimensional stability and mechanical properties for its end products [17]. Therefore, native starch is not used directly. 3. Preparation of starch-based biodegradable polymers To improve the properties of starch, various physical or chemical modifications of starch such as blending, derivation and graft copolymerization have been investigated. 367 Lu et al. – eXPRESS Polymer Letters Vol.3, No.6 (2009) 366–375 Figure 1. Molecular structure of starch 3.1. Physical blends 3.1.1. Blend with synthetic degradable polymers At first, starch was adopted as a filler of polyolefin by Griffin [18] and its concentrations is as low as 6–15%. Attempts to enhance the biodegradability of the vinyl polymers have been investigated by incorporating starch to a carbon-carbon backbone matrix [19]. In all these cases starch granules were used to increase the surface area available for attack by microorganisms. However, such a system is partially biodegradable and not acceptable from an ecological point of view. Thus, the blends of starch and polyolefin will not be mentioned any more in this article. To prepare completely biodegradable starch-based composites by this strategy, biodegradable polymers are assumed. Usually, the components to blend with starch are aliphatic polyesters, polyvinyl alcohol (PVA) and biopolymers. The commonly used polyesters are poly(β-hydroxyalkanoates) (PHA), obtained by microbial synthesis, and polylactide (PLA) or poly(ε-caprolactone) (PCL), derived from chemical polymerization. The goal of blending completely degradable polyester with low cost starch is to improve its cost competitiveness whilst maintaining other properties at an acceptable level [20, 21]. PLA is one of the most important biodegradable polyesters with many excellent properties and has been widely applied in many fields, especially for biomedical one. PLA possesses good biocompatibility and processability, as well as high strength and modulus. However, PLA is very brittle under tension and bend loads and develops serious physical aging during application. Moreover, PLA is a much more expensive material than the common industrial polymers [22]. Many efforts have been made to develop PLA/ starch blends to reduce total raw materials cost and enhance their degradability. The major problem of this blend system is the poor interfacial interaction between hydrophilic starch granules and hydrophobic PLA. Mechanical properties of blends of PLA and starch using conventional processes are very poor because of incompatibility [23]. In order to improve the compatibility between hydrophilic starch granules and hydrophobic PLA, glycerol, formamide, and water are used alone or combined as plasticizers to enhance the dispersion and the interfacial affinity in thermoplastic starch (TPS)/ PLA blends. In the presence of water and other plasticizers including glycerol, sorbitol, urea, and formamide [24], the strong intermolecular and intramolecular hydrogen bonds in starch can be weakened. To improve the compatibility between PLA and starch, suitable compatibilizer should be added. Besides, gelatinization of starch is also a good method to enhance the interfacial affinity. Starch is gelatinized to disintegrate granules and overcome the strong interaction of starch molecules in the presence of water and other plasticizers, which leads to well dispersion [25, 26]. The glass transition temperature and mechanical properties of TPS/PLA blend depend on its composition and the content of plasticizer as well (Table 1), indicating the compatibility between PLA and TPS is low but some degree of interaction is formed [26]. PCL is another important member of synthetic biodegradable polymer family. It is linear, hydrophobic, partially crystalline polyester, and can be slowly degraded by microbes [27–29]. Blends between starch and PCL have been well documented in the literatures [30–35]. The weakness of pure starch materials including low resilience, high moisture sensitivity and high shrinkage has been overcome by adding PCL to starch matrix even at low PCL concentration. Blending with PCL, the 368 Lu et al. – eXPRESS Polymer Letters Vol.3, No.6 (2009) 366–375 Table 1. Thermal and mechanical properties of thermoplastic starch/polylactide (TPS/PLA) blends athe content of glycerol and water in TPS1 and TPS2 are 18 and 12, 10 and 16 wt% respectively bTg value is not found in the literature Content of TPS [wt%] Tg [°C] Tensile Strength [MPa] Elongation at Break [%] PLA TPS 100 (TPS1)a – 10 3.4 152.0 090 (TPS1) 47 NFb 2.9 48.8 075 (TPS1) 53 NF 4.8 5.7 100 (TPS2)a – 43 19.5 2.8 090 (TPS2) NF NF 14.1 1.3 075 (TPS2) NF NF 12.0 0.9