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Polymer Science
Biodegradable polymers are those which can degrade into water and carbon dioxide under normal environmental conditions through microbial action, providing compost as a simple and sustainable disposal option.
- biodegradable polymers
- films
- degradation
1. Introduction
Polymeric films attract a significant degree of interest because of their widespread use in several industrial applications, with particular reference to packaging. Over the last decades, the large use of oil-derived polymers, due to their good mechanical, thermal and barrier properties, as well as their low cost, has led to significant problems in term of production and accumulation of wastes [1,2]. Rising concern about the reduction of waste coming from plastic packaging has encouraged both academia and industry to engage in research focused on polymers coming from natural resources, biodegradable or compostable [3,4,5,6]. Bio-based polymers are those obtained from natural resources, while biodegradable polymers are those which can degrade into water and carbon dioxide under normal environmental conditions through microbial action, providing compost as a simple and sustainable disposal option [7,8,9,10], and all of these can be regarded to as bioplastics. For instance, in the case of food packaging, polymers combining renewable sources and biodegradable nature are preferred [11,12].
The replacement of traditional packaging requires the new biodegradable packaging to guarantee comparable levels of performance and cost [13]. However, bio-based films often show disappointing mechanical, thermal and barrier properties [14]. Therefore, new strategies for the development of materials with suitable properties to replace traditional materials for packaging have attracted many research efforts [15,16,17].
Several biodegradable polymers were developed and many of them showed good potential for several applications. Among them, poly-lactic acid (PLA) probably attracted the greatest interest for several applications; in fact, being bioresorbable and biocompatible, this polyester is mainly employed in biomedicine for the preparation of tissue-engineering scaffolds, drug-delivery devices, biosensors, and is considered one of the most promising for food-packaging applications [18]. It is versatile, compostable, recyclable [19,20,21,22], highly transparent, with a high molecular weight, high resistance to water and good processability; on the other hand, its mechanical and thermal properties are still not suitable for several applications [23,24].
Among the polyesters, poly-caprolactone (PCL) has traditionally found applications in the fields of biomedicine and food packaging. With respect to PLA, it presents lower stiffness and tensile strength but higher stretchability and better processability. Moreover, the temperature required for its melt compounding (below 100 °C) allows it to be incorporated in a broader spectrum of natural additives for food packaging, especially those prone to thermal degradation/deactivation [25].
Poly-butylene adipate-co-terephthalate (PBAT) belongs to biodegradable aliphatic-aromatic polyesters and, thanks to its high ductility, it finds applications in agricultural or food packaging materials and was recently considered as one of the candidate materials for blending with PLA [26,27].
Another promising biodegradable polymer is the MaterBi® or, better, the MaterBi® family, including a series of bioplastics coming from modified starch and/or synthetic biodegradable polyesters [28,29]. It found applications in several fields due to good mechanical properties, thermal stability, barrier properties, as well as good processability and compostability [30,31,32]. Another recent example of biodegradable/compostable polymer is the Bioflex® family, typically based on biodegradable polymers such as PLA and thermoplastic copolyesters, such as PBAT [33,34,35,36,37].
2. Degradation of Biodegradable Polymer-Based Systems
Some studies are available regarding the numerous degradation paths of bioplastic-based systems.
Biodegradable polyesters are prone to several degradation mechanisms that may occur depending on the processing conditions. Hydrolysis of ester linkages is a water-induced degradation mechanism, whose rate and extent depends on water concentration, pH, eventual presence of acid or base catalyst, morphology of the polymer and temperature [42]. Two common degradation pathways are typically proposed for PLA: hydrolytic chain scission (a) and main chain scission, i.e., β-C-H hydrogen transfer (b). Hydrolytic degradation in PLA can take place during melt processing or in aqueous media. In the former case, it can be activated by the presence of moisture at high temperatures, in the latter, hydrolytic reactions display pH-dependent kinetics, being particularly faster under alkaline conditions. In both cases, the degradation determines a dramatic molecular weight reduction and can be easily monitored by spectroscopic measurement of –OH and –COOH groups, since the degradation products are hydroxyl- or carboxyl- terminated PLA. In the case of β-C-H hydrogen transfer, instead, carboxylic acid end groups and vinyl esters are formed. Moreover, at high temperatures (above 200 °C), the main degradation mechanism of PLA is trans-esterification, which leads to the formation of cyclic oligomers.
Similarly, PBAT undergoes hydrolytic degradation owing to the cleavage of ester linkages, which requires the presence of water. However, in the case of PBAT, H2O can even react with the carbonyl groups located in proximity of benzene rings. β-C-H hydrogen transfer reactions are supposed to occur randomly even in PBAT backbone [42].
PCL substantially follows the same mechanisms as those previously discussed for PLA and PBAT, although its higher crystallinity degree results in slower degradation [43].
Hydrolytic degradation of poly(α-hydroxyl) esters can proceed by either surface or bulk degradation pathways, with these latter being regulated by mass transfer phenomena (diffusion) and kinetics of chemical reactions [43]. If the rate of hydrolytic chain scission is faster than that of water diffusion into the polymer bulk, the hydrolysis occurs only at the polymer–liquid interface, thus resulting in the progressive thinning of the sample, whose bulk properties, in terms of molecular weight and crystallinity, remain unaltered. Otherwise, when water diffusion is faster than hydrolytic reactions, hydrolysis takes place randomly throughout the entire polymer bulk. This aspect leads to an overall and uniform decay of molecular weight. Oligomers and monomers that are formed diffuse out, thus causing gradual erosive phenomena until achieving the equilibrium between diffusion and chemical kinetics. Whether this equilibrium is hindered, the accumulation of -OH and -COOH terminated byproducts may trigger an internal autocatalysis that accelerates the bulk degradation with respect to that of the outer layers. In this case, a bimodal distribution of molecular weights would be observed, with a degraded inner core and a less altered skin. As the oligomers become small enough to diffuse throughout the structure, this latter tends to be hollowed.
Enzymatic biodegradation of PCL and PLA is shown to be extremely fast when accomplished by outdoor living organisms (bacteria and fungi), typically present in the soil, whereas biodegradation in the human body, for instance, due to the lack of suitable enzymes, is extremely slow, ranging from 6–12 months for PLA to 2–4 years for PCL, depending on starting crystallinity and molecular weight [44]. In fact, bio-resorbability of these polymers involves a two-stage mechanism: hydrolysis of ester groups occurs in the first stage, while the intracellular digestion is carried out by macrophages only in a subsequent stage, that is when the polymer molecular weight is low enough, and crystallinity degree is extremely high [45].
3. Recycling of Systems Based on Biodegradable Polymers
The use of bioplastic-based packaging allows significant advantages to be obtained in terms of environmental impact related to the entire life cycle of the product [57,58,59,60]. However, this can be further improved if the post-consumption recycling of bioplastic-based systems is carried out [61,62,63,64,65,66,67].
The idea to recycle biodegradable polymers, currently used for several applications [13,65,68,69] may sound odd, considering that polymers are regarded as a sustainable alternative for oil-derived polymers since they come from renewable resources and since they are biodegradable/compostable; however, there are several reasons to suggest that recycling of bioplastics is a sensible strategy. These are mainly related to the growing industrial demand and to the fact that recycling is crucial to the reduction of non-renewable resources consumption (including the energy demand linked with their production). Furthermore, some commercial bioplastics do not undergo severe degradation under normal conditions, and the disposal of bioplastic-made items leads to the disposal of valuable raw secondary materials [70,71,72,73].
Moreover, in view of the rising interest towards polymer nanocomposites, due to their improved mechanical thermal and barrier properties [79,80], some researchers have recently studied the of recycling biobased nanocomposites [19,63,81]. With particular reference to PLA, it is known that this polymer undergoes thermodegradation during melt processing [76,82] and, therefore, its recyclability is strictly related to the actual extent of thermodegradative phenomena during melt processing [76,83,84]. For instance, Tesfaye et al. [81] investigated the effect of silk nanocrystals (SNC) on thermal and rheological properties of PLA subjected to repeated extrusion operations; they found that the presence of SNC slows down the thermal degradation of PLA. Peinado et al. [63] studied the effects of extrusion on the rheological and mechanical properties of PLA filled with nanoclays; they found that, although both PLA and nanocomposites show viscosity decreases after each processing step, there was no major reduction of the mechanical performance.
It was generally observed that upon reprocessing cycles, the properties of biodegradable-based nanocomposites depends on to two opposite effects: (i) chain scission due to thermo-mechanical degradation, and (ii) filler dispersion effect resulting from multiple processing. Although the latter phenomenon may prevail at low reprocessing cycles, the former proves to be dominant at higher cycles. Among possible strategies were proposed the reactive extrusion with chain extenders or branching agents [42] or using stabilizers [85]. However, even a combination of chain extenders and stabilizers may be involved to increase the recyclability and reprocessability of biodegradable polymers.
This entry is adapted from the peer-reviewed paper 10.3390/polym11040651
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