SYNTHESIS AND CHARACTERIZATION OF BIODEGRADABLE STARCH-CLAY MATERIALS Ing. Jaromír Dlouhý West Bohemia University Univerzitni 8, 306 14 Czech Republic ABSTRACT This paper presents results from research of biodegradable polymer material, which could replace i.e. polyethylene in packaging applications. However, the biggest disadvantages of starch-based materials are their low strength, low water stability and finally their ageing. Reinforcement of various kinds of thermoplastic starch and its ageing are discussed in this article. Reinforcement was performed by adding of clay mineral montmorillonite. Montmorillonite structure can be delaminated into nanosized particles reinforcing strongly thermoplastic starch matrix. The ageing of the thermoplastic starch causes change of mechanical properties and can limit life time of thermoplastic starch products. KEYWORDS Thermoplastic starch, montmorillonite, clay nanocomposite, mechanical properties INTRODUCTION Thermoplastic starch is biodegradable material from renewable resources. It is composed of starch and plasticizer and can be processed as well as any other thermoplastic material, i.e. by melt extrusion conducted in this research program. Thermoplastic starch has potential to replace conventional oil-based polymers in disposable products with short life time, especially in packaging industry. There are several kinds of commercially produced packaging products from biodegradable polymers (polyvinylalkohol, polykaprolakton), but their big disadvantage is their high cost. The aim of presented research is to develop material comparable with low density polyethylene in terms of mechanical properties and price. Pure starch is a brittle semicrystalline polymer at common temperature. It cannot be processed due to high glass transition temperature and melting temperature. Therefore, it is necessary to add plasticizer to lover these temperatures and make starch processable and elastic. Tensile strength of thermoplastic starch is highly dependant on plasticizer content - the more plasticizer in starch, the lower tensile strength and the higher resistance to brittle crack propagation. Thermoplastic starch reinforcement by montmorillonite can enhance tensile strength significantly without considerable increase of brittleness. Moreover, biodegradable polymer matrix is reinforced by natural mineral, so that bidegradability is not affected. Thermoplastic starch is amorphous after extrusion, but starch itself is semicrystalline polymer. Thus, crystallization take place in material during time after extrusion. This ageing is called retrogradation because increasing amount of crystalline phase usually leads to tensile strength increase with corresponding decrease of elastic and plastic deformability. This phenomena can shorten significantly life time of possible products due to mechanical properties change. Therefore, mechanical properties were measured 10 days and one year after preparation of materials. Starch structure Starch is biopolymer of glucose consisting of two phases: linear amylose and branched amylopectin (Fig. 1). Potato starch was used in presented research. Under common conditions, starch is brittle semicrystalline polymer. Its glass transition temperature T g is well above temperature of thermal decomposition, so that pure starch is unprocessable by common technologies used in plastic industry. Pure starch does not have mechanical properties suitable for its application as a plastic material for common use.
Fig. 1 Structure of starch; linear amylose (up) and branched amylopectine (right). Source [1]. Thermoplastic starch Plasticization of the starch is one way to transform starch into a material with usable properties. Plasticized starch is usually called as a thermoplastic starch (TPS). Generally, starch plasticizers are polar organic substances, which are miscible with starch (glycerol, dimethylsulfoxide, formamide and many other). For example, addition of 30wt.% of glycerol lowers T g of the potato starch to approx. 0 C (T g of pure starch is around 250 C [2]) and forms elastic and well processable TPS. The most effective plasticizer of starch is water, but its content in starch cannot be stabilized. It changes with temperature and humidity of surrounding atmosphere trying to reach an equilibrium state. There are two undesirable features of TPS which disables its application: low water stability and low strength. TPS is soluble in hot water and swells in cold water as well as pure starch. Tensile strength of TPS with 30wt.% of glycerol is around 4 MPa, whereas packaging foils made of LDPE has tensile strength typically around 16 MPa. Final product should be a low-cost biodegradable material, available in large amounts. Glycerol, as significant compound, meets all these requirements Starch/clay composite Reinforcement of TPS by addition of filler is the easiest way to enhance mechanical properties. Clay minerals are ideal filler, because they are abundant and low cost. Montmorillonite (MMT) is clay mineral used in presented research. It is main component of bentonite clays. MMT has, as well as all clay minerals, layered structure, composed of 1 nm thick platelets stacked into crystalline lattice. Uniqueness of MMT structure consists in its ability of exfoliation. MMT can be dispersed into individual nanometric platelets for instance in water, but also in polymer matrix compatible with MMT. That is the principle of formation of polymer/clay nanocomposites. Actually, there are three degrees of clay dispersion (Fig. 2). Degree of nanocomposite formation is usually determined by XRD which determines interlamellar distance d 001 of MMT. Original interlamellar distance of used MMT was 12Å. Increasing distance d 001 points to polymer a) MMT particle b) c) Fig. 2 Schematic view on polymer/mmt composite structure a) conventional particle composite (MMT in form of crystalline particles, d 001 = 12 Å), b) intercalated nanocomposite (penetration of polymer chains into interlamellar spaces MMT increasing d 001 ), c) ideally exfoliated nanocomposite. Polymer chains
penetration into interlamellar spaces. There is not any diffraction maximum of MMT in case of ideally delaminated clay nanocomposite, because crystalline structure is completely dissipated. MMT delamination ability depends strongly on clay/polymer compatibility. Natural MMT is hydrophilic material dispersible in water, which ensures good compatibility with also hydrophilic TPS. Clay minerals are also natural and environmentally-friendly materials. Chemical modifications Water stabilization and further strengthening of TPS is achievable by chemical modification of starch. There were used also two kinds of chemically modified starches (Perlbond 930, Perlbond 994) in this research, except of native potato starch. The chemical modification consists in various degree of cationization of starch. There are ammonium cationic ligands, chemically bonded to the starch polymer chain. Degrees of cationization D (i.e. ratio of cationized chain units to total number of chain units) was 0,04 (Perlbond 930) and 0,08 (Perlbond 994). Cationized starches were chosen because of anionic character of MMT platelets. Electroneutral native starch chain can be bonded to the clay filler only by weak Van der Waals interactions or hydrogen bonds. But cationic starch can be bonded much strongly by ionic bond. EXPERIMENTAL Extrusion processing was carried out by laboratory single-screw extruder Brabender PLASTI CORDER. Before extrusion, TPS or its composite had to be prepared in a kneader Werner&Pfleinderer LDUK 05. Starch powder was mixed together with appropriate amount of plasticizer (and with MMT powder in case of TPS/MMT composite preparation) and plasticized in the kneader at 90 C for 10 minutes. TPS (or its composite) was then get out of the kneader, cut into small pieces of size approx. 5 mm and fed into extruder. Temperature of extruder barrel was 140 C, temperature of flat die was 120 C. A ribbon 50 mm wide and 0,4 mm thick was extruded. Ribbons were cut into strips 20 mm wide and 160 mm long for tensile test according to norm ČSN EN ISO 527-3. Tensile test was carried out 10 days after extrusion and one year after extrusion for all materials, at laboratory temperature 20 C. Length of the specimen was 100 mm, velocity 50mm/min. Degree of MMT delamination was determined by XRD analyses. RESULTS AND DISCUSSION Mechanical properties of TPS and TPS/MMT composites are listed in Table 1 and Table 2. Degree of cationization: native strch DS = 0, Perlbond 930 DS = 0,04, Perlbond 994 DS = 0,08. Table 1 Mechanical properties of extruded TPS (70 wt.% starch, 30wt.% glycerol). Tensile strength [MPa] native starch 2.7 ± 0.1 5.8 ± 0.3 Perlbond 930 1.4 ± 0.1 4.6 ± 0.2 Perlbond 994 0.7 ± 0.1 3.1 ± 0.3 Elongation at break [%] native starch 67.5 ± 6.6 50.3 ± 8.4 Perlbond 930 88.4 ± 15.3 33.1 ± 4.2 Perlbond 994 192.6 ± 41.3 37.8 ± 3.2 Table 2 Mechanical properties of extruded TPS/MMT composites (66,5wt.% starch, 28,5wt.% glycerol, 5wt.% MMT). Tensile strength [MPa] Elongation at break [%] native starch 4.8 ± 0.1 6.2 ± 0.2 Perlbond 930 3.7 ± 0.1 5.0 ± 0.1 Perlbond 994 2.2 ± 0.2 3.6 ± 0.4 native starch 39.0 ± 3.1 21.3 ± 2.8 Perlbond 930 68.6 ± 3.0 27.4 ± 1.9 Perlbond 994 130.1 ± 13.4 41.4 ± 8.0
Tensile strength decreases with increasing degree of cationization. Reinforcement of TPS matrix by MMT is much more significant in fresh materials after one year is difference between tensile strength of neat TPS and TPS/MMT composite much smaller. Elongation of all measured samples was almost completely elastic. Elastic deformation relaxed after break for several minutes. Relaxation velocity was fastest immediately after break, than was steadily decreasing. All samples restored after 10 minutes their original shape, plastic deformation was in all cases less than 5%. XRD analyses XRD of TPS/MMT composite based on starch Perlbond 930 showed strong peak at 14.9Å, which is probably intelamellar distance d 001 of MMT with monolayer of starch molecular chains in interlamellar spaces. Thus, a penetration of starch into MMT structure occurred but significant amount of MMT retained its crystallic structure. It was not delaminated into individual platelets, but only intercalated by starch macromolecules. Diffraction maximum of MMT filler (original distance of pure MMT was 12Å). Diffraction maximums of semicrystalline TPS matrix. Fig. 3 XRD spectrum of TPS/MMT composite (TPS contains70wt.% of Perlbond 930 and 30wt.% of gylcerole). CONCLUSION AND RECOMMENDATIONS Thermoplastic starches and their clay composites were prepared by melt extrusion. XRD analyses showed that MMT delamination in thermoplastic starch, if occurs, is far from complete delamination. Significant amount of MMT is only intercalated by starch chains. I is proved by increase of MMT interlamellar spacing in composite compared to interlamellar spacing of neat MMT. All measured thermoplastic starches were reinforced successfully by 5 wt.% MMT. Thermoplastic starch underwent during one year after preparation significant change in mechanical properties. All prepared thermoplastic starches enhanced their tensile strength at least by factor of two. Their composites did not change tensile strength so rapidly. Reinforcing effect of MMT is more significant for samples 10 days after extrusion. Cationization of starch does not lead to strengthening of TPS/MMT composite. Ionic interaction between cationic starch and MMT filler is probably not crucial to improvement of mechanical properties.
Further research will focus on native potato starch and its another chemical modifications (crosslinking for strengthening of TPS matrix and copolymerization with another polymer for hydrophobization). ACKNOWLEDGMENTS Author would like to thank all people who supported this research, especially to supervisor Doc. Ing. Petr Duchek CSc. Thanks belongs also to Doc. Ing. Antonín Kuta CSc. and Ing. Drahomír Čadek for enabling sample extrusion at VŠCHT. Finally, author want to thank the Ministry of Industry of Czech Republic for financial support (grant under TIP programme No. FR-T1/566). REFERENCES [1] Rose E.: Clay Nanocomposites in Biodegradable Starch Plastics, Brisbane: University of Queensland, 2001 [2] Janssen L. Moscicki L.: Thermoplastic starch. Weinheim: WILLEY-VCH Verlag GmbH, 2009