Xiao-Fei Ma, Jiu-Gao Yu
School of Science, Tianjn University, Tianjin 300072, P.R.China
Much effort [1-3] has recently been made to develop biodegradable materials because of the worldwide environment and resources problems resulted from petroleum-derived plastics. Starch, a natural renewable polysaccharide obtained from a great variety of crops, is one of the promising raw materials for the production of biodegradable plastics [4].
The development of starch-based materials, which have developed for 30 years, approximately passes through three phases: starch/polyolefin blends, starch/ biodegradable polymer (such as polyester) blends and thermoplastic starch (TPS).
1 starch/polyolefin blends
The study on starch/polyolefin blends firstly appeared in the Griffin 1972 patent [5], which induced much research on the compatibility of hydrophilic starch and hydrophobic polyolefin. The compatibility between starch and polyolefin can be improved by introducing coupling agent, reactive compatibilizer and unreactive compatibilizer.
However, it is thought that starch/polyolefin blends are not the exactly biodegradable materials. When these blends are disposed in the environment, however, the micro-organisms consume only the starch leaving the polyolefin (for example, LDPE) intact. No significant decrease in the molecular weight of the LDPE in such blends has been detected. The only advantage in the incorporation of starch into LDPE is that after starch consumption the blend ends up in a form full of holes. This offers an increased surface area on which the micro-organisms can attack the polymer matrix. It is found that certain micro-organisms can degrade paraffins with molecular weight up to 450, like C32H66. The molecular weight of polyolefin, such as LDPE, is between 200, 000 and 500, 000. That means that for biodegradation to be possible, a chemical degradation must take place to reduce the molecular weight to values where microbial attack can occur. [6] Polyolefin degradation can be accomplished either by UV irradiation or by thermal or thermo-oxidative degradation.
2 starch/biodegradable polyester blends
The decomposition of polymers involves bond breaking in the main chain and hence a significant decrease in molecular mass. The main reaction that facilities bond breaking in nature is hydrolysis, which is catalyzed by enzymes (i.e. hydrolases) in thecase of biopolymers. [7] By contrast, the decomposition of polyolefin is rarely enzymatically catalyzed. It is obviously important for biopolymers to consisting of hydrolyzable functional groups. Many aliphatic polyesters are the biodegradable polymers with many interesting properties, including good mechanical properties, thermoplastic processability, thermal and chemical resistance. However, the commercialization of aliphatic polyesters has been slowed by their relatively high cost. [8] Therefore, many researches focus on the addition of low cost starch to polyesters to obtain biodegradable starch/polyester blends with improved cost competitiveness, while maintaining good mechanical properties and processability. Polyesters, which are sued in starch/polyester blends, mainly include polycaprolactone (PCL), polylactides (PLA), poly (hydroxybutyrate-co- valerate) (PHBV), polybutylene succinate adipate (PBSA), poly(hydroxy ester ether) (PHEE), poly(ester amide) (PEA) and so on. Some commercial starch/polyester blends have appeared. Mater-Bi Z class, produced by Novamont, is made of TPS and PCL mainly for films and sheets. [9] The compatibility amelioration of hydrophilic starch and hydrophobic polyester will definitely extend the application scopes of starch/polyester blends.
3 thermoplastic starch (TPS)
Starch presents two different polysaccharides: the linear (1,4)-linked α-D-glucan amylose and highly (1,6)-branched α-D-glucan amylopectin. Commonly, amylopectin takes part in the formation of crystalline structure, but amylose not [10]. Nature starch is usually about 15-45% crystallinity. [11] Under the action of high temperature and shear, starch can be processed into a mouldable thermoplastic, a material known as thermoplastic starch (TPS). [12] During the thermoplastic process, water contained in starch and other plasticizers play an indispensable role, [13] because the plasticizers could form the hydrogen bonds with starch, take the place of the strong action between hydroxy groups of starch molecules, and make starch display the plasticization. In most literatures for thermoplastic starch, polyols as plasticizers are usually used such as glycerol [12,14,15], glycol [16], sorbitol [17] and sugars [18]. This kind of TPS is thought to tend to re-crystallization (retrogradation) after being stored for a period of time and retrogradation embrittles TPS. Urea is shown to prevent retrogradation. It is, however, a high melting solid with little internal flexibility and hence urea-plasticized TPS become rigid and brittle. [19] The retrogradation of either starch or plasticizers will inhibit the application of TPS.
To prepare TPS with retrogradation resistance and good mechanical properties, we [20] chose some plasticizers containing amide groups, such as formamide, acetamide and urea. Formamide-plasticized TPS (FPTPS) has a good break strain but poor break stress [21], while Urea-plasticized TPS (UPTPS) has a good break stress but poor break strain. The effects of water contents on mechanical properties of TPS show that FPTPS has a good flexibility at water contents of 13% similar to 35% [22], and the strain of UPTPS increases up to 65% at high water content (42%). Fourier Transform infrared (FT-IR) spectroscopy shows that a small percent of urea could react with starch at thegiven TPS processing conditions. The other plasticizers can not react with starch. The order of the hydrogen bond-forming abilities with starch was urea > formamide > acetamide > polyols, confirmed by B3LYP chemical computation. The hydrogen- bonding interaction in 1:1 complexes formed between plasticizers (urea, formamide, acetamide or glycerol) and starch are respectively 14.167 Kcal/mol, 13.795 Kcal/mol, 13.698 Kcal/mol and 12.939 Kcal/mol. Glycerol-plasticized TPS (GPTPS), FPTPS, acetamamide-plasticized TPS (APTPS) and UPTPS are tested at three typical humidities using X-ray diffractometry. Compared to glycerol, urea and formamide can effectively restrain the retrogradation of TPS, and acetamide can keep the re-crystalline of original APTPS. The properties of TPS mainly rely on the hydrogen bond-forming abilities between plasticizers and starch molecules.
We [23] also analyze the hydrogen bond interaction between plasticizer and starch in GPTPS and FPTPS. FT-IR reveales that the oxygen of C-O-C group participates in the hydrogen bond with plasticizers, and the hydrogen of C-O-H group in starch formed the hydrogen bond with plasticizers rather than the oxygen of C-O-H group. The excessive plasticizers will form hydrogen bond within each other and weaken the interaction between plasticizers and starch. This difference of the properties between GPTPS and FPTPS is attributed to starch interaction with the plasticizer.
Since formamide is a good solvent for urea, a combination of urea and formamide may be a better plasticizer for starch than either alone. In order to restrain the retrogradation and improve the mechanical properties of TPS, formamide and urea were used as a mixed plasticizer to prepare urea and formamide-plasticized TPS (UFPTPS). [24] Formamide together with moisture in native starch can effectively prevent urea from being solidified; consequently this novel fixed plasticizer ameliorated mechanical properties of TPS with suppressing the retrogradation.
4 Others
Unfortunately, the properties of TPS are not satisfied with some applications such as packaging. TPS has two main disadvantages when compared to most plastics currently in use, i.e. it is mostly water-soluble and has poor mechanical properties. Blending it with biodegradable polymers, adding crosslinking agents or adopting the multiplayer technique, as mentioned above in this paper, may improve water resistance and mechanical properties of TPS. Another arresting approach is the use of fibers or silicate-aluminate (i.e. clay, montmorillonite) as reinforcement for TPS.
4.1 starch/fiber blends
The fibers described in the literature for this intention are cellulose micro-fibrils [25], natural fibers [26] and commercial regenerated cellulose fibers [27]. Unlike biodegradable polyester, when natural fibers are mixed TPS their mechanical properties are obviously improved, for the chemical similarity of starch and plant fibers provide a good compatibility. And a significant improvement of water resistance by adding commercial cellulose fibers [27] or micro-fiber [28] is obtained. This behavior is related to the hydrophobic character of the cellulose fibers in comparison to starch hydrophilicproperty. Besides, these authors [25,29] show an improved thermal stability due to a higher and longer rubbery plateau.
4.2 starch/silicate-aluminate blends
In the recent decade, the polymer/ silicate-aluminate (i.e. MMT and clay) nanocomposites develop rapidly, and the methods of polymer intercalation and intercalative polymerization resolve the problems of the dispersion and interface in the preparation of polymer nanocomposites. [30] Currently, the matrixes of nanocomposities are mainly the synthesized polymers, such as polyolefin, polyamide and polyester, [31-33] however the study on the natural polymers is few. Park et al [34,35] prepared the TPS/Clay nanocomposites. However an intercalation of TPS in the silicate layer requires a large quantity of plasticizers for improving the TPS fluid enough into the silicate layer. The superfluous plasticizers weaken the mechanical properties of the TPS/Clay nanocomposites.
In our labs, we modify montmorillonite (MMT) with citric aicid, and prepare citric aicid-activated MMT (CMMT), the interbed space of which is extended. The UFPTPS/CMMT nanocomposites are obtained by melt-intercalation. When the content of the mixed plasticizers composed of urea/formamide (2:1) is only wt. 30%, TPS can intercalate the extended interbed space of MMT. When CMMT content is wt.10%, the mechanical testing indicates that the tensile stress of the nanocomposites reaches 25MPa, and the tensile strain reaches 101%, Youngs modulus increases from the 47MPa of pure UFPTPS to 620MPa of UFPTPS/CMMT nanocomposites, and breaking energy increases from 1.99N·m to 2.49 N·m. The stress-strain curves of the typical polymer at glassy state are shown in the mechanical testing of UFPTPS/CMMT nanocomposites. UFTPS/CMMT nanocomposites occur the typical tough fracture, so-called the breaking after yield, which are so different from the curves of the TPS matrix. In contrast to pure UFPTPS, the nanocomposites also have better thermal stability and water resistance.
5 Conclusion
This review indicates that research efforts have focused on the compatibility, the mechanical properties and the cost of the biodegradable starch-based blends. Today it is still very difficult to completely take the place of the conventional polymeric materials with the biodegradable starch-based materials. Fortunately, there is now much research effort being expended worldwide, both fundamental and applied. Therefore, it may be assumed that most of the present problems will be solved within a reasonable time. Gradual application of legislative and consumer pressure is beginning to initiate real commercial interest in this area. This will inevitably continue and the rate of development will parallel this enhanced commercial awareness.
However, further research within different areas of the biodegradable starch-based blends, for example, environmental legislation, processing technology, and compatibility studies must be initiated before the biodegradable starch-based blends can be used on a large scale.
References (Omitted)
論文來源：1st International Conference on Technology and Application of Biodegradable Polymers and Plastics,October,2004
