Comparative assessment of miscibility and degradability on PET/PLA and PET/chitosan blends
Graphical abstract
Introduction
The long-lasting petroleum polymers have been widely used provoking that the waste of this kind of polymers takes a very long time to be broken down. Nowadays, this indiscriminate use of petroleum-based polymers has caused a big pollution problem [1], [2]. To reduce this problem, it has been used biodegradable polymers from renewable sources like collagen, keratin, gluten, milk proteins, soy proteins, polysaccharides like starch, cellulose derivatives, chitosan, alginate, carrageenan, pectins. These biodegradable polymers have a short-lifetime because of are ideal for short-time applications such as; disposable packages, agricultural mulches, horticultural pots, etc [3], [4], [5], [6]. They are also naturally degradable when disposed in the environment. Despite its advantages, many of these kinds of polymers exhibit poor thermal stability, low steam and gas barrier and low mechanical properties, making them unsuitable for other applications [7], [8]. Therefore, the general trend is to combine the mechanical, barrier and thermal properties of petroleum based polymers with the biodegradability properties of renewable polymers, resulting in the production of polymeric materials with controlled lifetime. The designed materials must be resistant during their use and must have short time degradation at the end of their useful life [4], [9].
The most favorable packaging material for disposable soft drink bottles is polyethylene terephthalate (PET), a kind of semicrystalline, thermoplastic polyester with high strength and transparency properties as well as excellent barrier properties. Unfortunately, most of these beverage bottles are used only once and then discarded, which inevitably generates serious environmental problems (white pollution) [10]. Therefore, recycling the discarded PET polymer along with obtaining biodegradable PET-based blends are efficient approaches to reduce the resources consumption and to protect the environment at the same time [11]. The recycling of post-consumer packaging materials into direct food contact packaging applications was not possible, because of the lack of knowledge about the contamination of packaging polymers during first use or recollection. However, for PET the situation is much favorable: due to its inert character, recycling technologies have been developed to establish a bottle-to-bottle recycling of post-consumer PET bottles [12].
On the other hand, most biodegradable polymers are thermoplastics (e.g. poly(lactic acid), poly(hydroxyalkanoate), poly(vinyl alcohol)) [9]. Among them, poly(lactic acid) (PLA) is a bio-based polymer obtained from renewable sources mainly from corn and starch [13]. PLA is an aromatic polyester and has several applications, for example, is used for films, extrusion-thermoformed containers and medical applications for tissue engineering, bone reconstruction and controlled delivery systems [14]. The use of PLA in beverage bottles is limited due to its poor oxygen barrier and low mechanical properties [15]. Another interesting biodegradable polymer is chitosan, a biopolymer derived from chitin, a natural compound from crustacean shells; chitosan has the ability to form semipermeable films and, in recent years, the efforts have been intensified to develop chitosan films and its application in food packing [16]. Biodegradable copolymers of PET and aliphatic polyesters have been synthesized, such as poly(lactic acid), poly(β-hydroxyalkanoate), poly(ε-caprolactone) and poly(butylene succinate) in order to obtain a degradable polymer with a faster degradation rate [17], [18]. Additionally, physical mixtures of conventional and biodegradable phases have been studied [19]. To our best knowledge, few researches are focused in determine the effects on physicochemical, structural and morphological properties as well as degradation time of blends PET/PLA or PET/Chitosan. In this work, the issue has been investigated from different perspectives and the results are discussed in the terms of the quantities of biodegradable polymers that were added in two different matrixes commercial PET and recycled PET during extrusion process.
Section snippets
Materials and processing
During the first set of experiments, it was used a commercial PET (CLEARTUF®-MAX2, lot no. 1008-03219) provided as pellets by M&G Polymers Company whereas in the second step recycled PET (R-PET) was obtained from discarded bottles after they were washed, dried and cut into flakes. The polylactic acid pellets, PLA-2002D (containing 4.4 wt-% in average of isomer D), (batch: YA0828b131) and chitosan (low molecular weight, with a deacetylation degree ⩾75%) were purchased from NatureWorks LLC, USA
Characterization of as-prepared polymer blends after extrusion
Since, the final properties of the materials strongly depend on the induced microstructure which can be governed by the complex thermo-mechanical history; it is of prime interest to observe and to understand the development of the crystalline phase. Fig. 1a–c shows X-Ray diffraction patterns of the polymer blends with chitosan or PLA starting from commercial PET and recycled PET at different weight ratios. As a reference, it is also presented the diffractograms of raw materials. The commercial
Conclusions
Blends of commercial and recycled PET/PLA and -/Chitosan were prepared by extrusion process at 250 °C to analyze the miscibility and degradability as a function of biopolymer content. The following conclusions can be drawn:
After extrusion process, XRD patterns and infrared spectra analysis showed a weak interaction between PET matrixes and biodegradable polymers suggesting type secondary bonds by hydrogen bridges or by electrostatic forces. In independence of the amount of biodegradable
Acknowledgements
D. Palma-Ramírez is grateful for her postgraduate fellowship to CONACYT, COFAA and SIP-IPN. The authors are also grateful for the financial support provided by CONACYT through the CB2009-132660 and CB2009-133618 projects and to IPN through SIP 2014-0164 and 2014-0992 projects and SNI-CONACYT. The authors also thank M.E.A.E. Rodríguez-Salazar and ROMFER SA CV industries for their technical support.
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