Properties and sorption studies of polyethylene oxide–starch blended films
Graphical abstract
Introduction
Blending of polymers is an effective way for the development of novel polymeric materials with desired properties, low cost, less time consuming and easy processability. The blends between biopolymer and synthetic polymers are of particular significance, because they can combine biocompatibility with good processability and mechanical resistance and also can be used as biomaterials. In view of the recent thrust on eco friendly packaging materials (Weiping et al., 2005, Youg and Yoshio, 2004), one such interesting blend happens to be PEO/starch blend. PEO has been extensively used in biomedical field because of its excellent hydrophilic nature, high viscosity, ability to form hydrogen bonds with ether oxygen, and biocompatibility. It also finds applications in different aspects of industry including cosmetics (Hassouna et al., 2007, Suneela et al., 2005). Studies by Kondo and Sawatari (1994) have established that the primary hydroxyl group on cellulose and methyl celluloses can form hydrogen bond together with oxygen in PEO. Tuncer, Serkan, Mehmet, and Olgun (2005) have reported hydroxyl groups on sodium alginate which can also form a hydrogen bond to the ether oxygen in PEO. Similarly, hydroxyl group on starch can also form a hydrogen bond to the ether oxygen in PEO. Therefore, PEO as a suitable polymer blended with starch was consequently selected for these studies.
Today, starch a naturally occurring polymer next to cellulose, with its versatile properties, such as aqueous dispersion, bonding ability, formation of films, etc., has become the most important hydrocolloid for various uses (Ramesh & Tharanathan, 2003). At present, starch has been considered as a low-cost alternative to synthetic plastics in production of disposable plastics (Bastioli, 2001) and also totally biodegradable at various environmental conditions. In particular, packaging and agricultural mulching industries, starch based films have significant potential to replace synthetic films such as polyethylene (Siddaramaiah, Mruthyunjaya, Ramaraj, & Joong, 2008; Youg & Yoshio, 2004). In the last few decades there have been many attempts to blend starch with biodegradable synthetic polymers like LDPE (BaldevRaj et al., 2003, Eleni et al., 1997, Ioannis et al., 1998), poly (caprolactone) [PCL] (Koenig & Huang, 1995), poly(hydroxy butyrate) [PHB], poly (hydroxybutyrate-co-hydroxyvalerate) [PHBV] (Imam et al., 1995, Kontis et al., 1995, Shogren, 1995), poly(lactic acid) [PLA] (Jacobsen & Fritz, 1996) and poly (vinyl alcohol) (Jayasekara et al., 2003, Siddaramaiah et al., 2004) which have become the objective of many studies. The first important commercial application of starch has been the blending of polyethylene (PE). It was assumed that starch would accelerate the degradation of PE, but PE is virtually non-biodegradable. Currently, plastic films used in agriculture mulch are made with low-density polyethylene (LDPE) containing transition metal compounds soluble in the thermoplastic matrix and about 6–15% starch. However, the degradation duration is still high and will take a few years (Siddaramaiah et al., 2008).
An important role of packaging film is to reduce exchange of water between the product and the environment (Coupland, Shaw, Monahan, Oriordan, & Osullivan, 2000). The barrier property of such (packaging) films depends on both molecule diffusion coefficient and solubility of water in the matrix (McHugh & Krochta, 1994). The moisture-sorption isotherm is a means to characterize the water absorption property of the film, which in turn is transmitted to the product inside. Moisture sorption is important for predicting stability and quality changes during packaging and storage of food products. Equations for modeling water sorption isotherms are of special interest in many aspects of food preservation by dehydration. The sorption isotherms obtained from experimental data facilitate estimation of equilibrium moisture content, which is necessary to predict the hydrophilic properties of the film. The equilibrium moisture content is a quantitative measure in the practice of food storage, packaging and drying. Many mathematical relations have been proposed in literature to model hygroscopic equilibrium data related to food systems as well as bio-films. Chirife and Iglesias (1978) reviewed a number of isotherm equations for food and food products. BaldevRaj et al., 2002, BaldevRaj. et al., 2003 have studied sorption isotherm of LDPE/starch and PVA/starch blended films respectively. The uses of PEO/starch blends as a packaging material have better prospects when compared with LDPE/starch blends since LDPE is non-biodegradable. In this study, our focus was on the preparation of PEO and starch blends and to investigate their mechanical, barrier, thermal and optical properties along with surface morphology and sorption characteristics.
Section snippets
Materials
Polyethylene oxide (PEO) of molecular weight (∼200 kDa) was procured from Sigma Aldrich, St. Louis, MO, USA and corn starch was obtained from M/S Riddhi Siddhi, Gluco Bio Ltd. Gokak, Karnataka (India) respectively. All other chemicals were of analytical grade and obtained from M/s SD Fine Chemicals, Bangalore, Karnataka (India).
Extrusion process for PEO/starch films
A series of PEO/starch blends, namely 100/0, 90/10, 80/20, 70/30, 60/40 and 50/50 were extruded, using mixing extruder (LE-075, Custom scientific instrument). Rotor and
Chemical characterization by FTIR spectroscopy
FTIR analysis of blended films were carried out to detect any shift that could be attributed to weak interactions between two polymers. The FTIR spectra of PEO, starch and PEO/starch blended films in the wave number range of 4000–400 cm−1 are depicted in Fig. 1. In the spectrum of PEO, (–O–H) stretching at 3514.4 cm−1, (–C–H) stretching of methylene groups at 2894 cm−1 and –C–H stretching, bending and rocking vibrations at 1464 cm−1, 1340 cm−1 and 958 cm−1 were observed respectively. The
Conclusion
In summary, the PEO/starch blended films showed several interesting characteristics. The tensile strength, %ɛ, and heat seal strength were decreased, whereas, WVTR and haze values increased as the starch content in the blended films increased from 0 to 40%. These films with high WVTR values can be used for fresh produce to control moisture evaporation and enhances their shelf life. Moisture-sorption isotherms on PEO/starch films are essential in determining the sorption influence of PEO and
Acknowledgement
The authors wish to express their gratitude to Dr. V. Prakash, Director, CFTRI, Mysore, India, for the opportunity and encouragement for this investigation. Mr. R. S. Jagadish expresses his gratitude and sincere thanks to the JSSATE, Noida and JSS Maha Vidya Peetha, Mysore, India, for granting QIP fellowship.
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