1. Introduction
Today, plastic consumption has increased exponentially at an estimated global annual production rate of almost 400 million tons [
1]. Bio-based, biodegradable and compostable plastics currently represent 1% of the global European plastics market. An overall 5–8% growth of these more environmentally friendly plastics between 2020 and 2025 is expected in Europe, where the packaging sector makes up 40% of plastic production and is the leader in market share [
2].
The current socioeconomic system is based on a linear economy, which implies the generation of waste, energy outflow and noticeable degradation of the environment However, plastic containers also have good qualities; they are lighter and more durable compared to other options [
3,
4,
5].
Since 3 July 2021, when the SUP (Single-Use Plastic) directive EU 2019/904 was implemented, a legislative movement toward compostable plastics has developed throughout Europe, aligned with the “Green Deal” of 2020 [
6,
7], with the final goal of climate neutrality by 2050. In Spain, this European directive was transposed on 8 April 2022 with Law 7/2022 [
8,
9] on waste and contaminated soils for a circular economy. Article 28 of this regulation is aimed at clarifying the different types of plastics related to packaging and their classification in order to proceed with selective recycling (
Figure 1).
The trend toward packaging plastics that are in consonance with the concepts of circular economy is encouraged, not only by European legislation but also by different international policies around the world, including China, the USA, India, Japan, Malaysia, Indonesia, Canada and others [
10].
To align the current packaging films with the new regulations, there is a necessity for the development of novel films that meet the criteria of being biodegradable, compostable, and in full compliance with all legal mandates [
11].
The term biodegradable implies that the material can be decomposed by microorganisms, but not necessarily that it is compostable [
12]. A compostable material is considered when the compost it produces is of good quality and is safe for use as a fertilizer for plants without damaging the cultivated soil [
13].
Evidently, biodegradability and compostability depend, to a great extent, on the conditions and environment in which the material is degraded. This shares similarities with other plastic materials [
14]. Variables such as the type of microorganisms existing in the soil, temperature, moisture and others can substantially affect the degradation process. Plastics that are biodegradable in a composting industrial plant, where the conditions are aggressive, cannot be biodegradable in regular conditions, as when deposed in a composting container in a garden [
15].
The use of films that present features such as biodegradability can enormously reduce their environmental impact. The use of such films for applications with high consumption in agriculture could help to decrease the presence of microplastics in soil. It is estimated that biodegradable and compostable plastics could absorb approximately 50% of global production [
16].
However, being biodegradable a desired feature in these plastics, can in many cases also mean a decrease in storage time, an uncontemplated decrease in mechanical properties or unexpected changes in visual appearance. For this reason, an assessment of the degradation processes that take place in such materials is very important. At the same time, a better knowledge of the changes in the properties of the material would be achieved when the microstructural processes, degradation reactions and their relation to the macroscopic effects were properly researched [
17].
Previous works have already adopted this approach, studying films based on starch [
18]. Starch, because of its low price, biodegradability and availability, is considered a good substitute for synthetic plastics for some applications. Interesting data on the biodegradation of aliphatic–aromatic copolyesters and their ecotoxicology have also been published [
19]. Derivatives of polylactic acid are also considered for packaging [
20]. In the context of food packaging, a methodology for scoring and comparing environmental effects has also been developed [
21]. A general classification of some plastics according to their biodegradability and composition is presented in
Figure 2.
In this work, a commercial packaging film of known composition, obtained by finely tuned extrusion combining technology and materials, accredited according to the previously cited regulations, is chosen as the subject of study.
The formulation is based on a blend of polybutylene adipate terephthalate (PBAT), commercial starch compound, commercial PLA compound and mineral charge.
These polymers are compostable and have been drastically increasing in consumption in recent years. Companies such as BASF, Biotec and Novamont have developed several products related to these materials.
To observe Directive 94/62/CE, the films must pass tests and meet the following criteria: (i) no heavy metals and less than 50% volatile solids; (ii) 90% biodegradability; (iii) fragments of degraded material must be smaller than 2 × 2 mm within 2 weeks and (iv) the resulting compost must be non-ecotoxic.
PBAT has been used in the production of blown films, and similar polymers have been studied in format blown films, blends with montmorillonite, blends polylactide (PLA)/PBAT and PLA blends [
22,
23,
24,
25]. The PLA/poly(butylene adipate-co-terephthalate) blends have also been studied from the point of view of biodegradation [
26,
27,
28].
Nevertheless, not many studies on the biodegradability of real commercial blends are available in the literature, and most of them are related to the research of the degradation of the materials from the point of view of their effects on the environment. There is a lack of knowledge on the effects of the biodegradability of the new polymeric films regarding their usability. If the biodegradable films are called to replace traditional synthetic films, the following questions need to be answered: can the biodegradable polymers be stored before use without compromising their final properties? How can the storage conditions and composition affect the degradation? Will the bags stored typically for months still be in good condition to be used by consumers?
In this manuscript, the commercial films based on a commercial starch compound and PLA compound + mineral charge cited above have been subjected to degradation in real storage conditions. The changes in the samples have been monitored by the determination of their mechanical properties, (according to ISO 527 [
29], ISO 7765-1 [
30], ISO 6383-2 [
31]), by thermal analysis, (DSC) and by FTIR. In films, degradation may appear in the form of discoloration or yellowness. For this reason, a colorimetric study of the samples, initially transparent and colorless, has also been performed.
The primary objectives were twofold: firstly, to assess the packaging’s suitability under storage conditions one year after production while establishing an optimized “best before” date for these products in the market. Secondly, to comprehend the microstructural alterations occurring in the samples that subsequently influence macroscopic properties through the application of FTIR and thermal analysis. This approach enhanced our understanding of the material and paved the way for tailoring films to meet specific application requirements in the future.
2. Materials and Methods
2.1. Film Composition
In order to achieve the complex properties necessary for these films, a combination of selected materials and a special technology have been applied. The films are made using a tri-layer assembly, where the external layers are composed of a biocompostable fossil-based material and the internal layer is composed of commercial starch and PLA compounds (
Figure 3). This type of structure, obtained via coextrusion, combines the desired biodegradability with the mechanical properties required for the final use.
The formulation of the final product is shown in
Table 1.
The material in the central layer is, as mentioned above, a commercial blend starch compound. It is a plasticizer-free and GMO-free thermoplastic material that is based on natural starch and suitable for processing via blown-film extrusion to produce items that are completely biodegradable and compostable according to EN 13432 [
32]. It has been supplied by a European company. Starch is a combination of amylose and amylopectin, two very similar polysaccharides [
33].
In the starch, the content of amylopectin comprises between 70% and 80% by weight, independent of the size of the granules.
This starch compound has a density of 1.26 g/cm3 and an MFI (190 °C/5 kg) of 8.40 g/10 min. The PLA compound used is also provided by the same supplier; it has a density of 1.24 g/cm3 and MFI (190 °C/2.16 kg) of 4.10 g/10 min. The PBAT has a density of 1.22 g/cm3 and MFI (190 °C/2.16 kg) of 3.70 g/10 min. The multilayer film was produced with a coextruder Reifenhäuser Blown Film Polyrema (Reifenhäuser Blown Film GmbH & Co., Troisdorf, Germany) with extruders type 60F/70F/60F–30D V3 blow-up ratio 3.90 and average thickness of 14 microns.
2.2. Film Degradation
The film has been stored under standard conditions in Zaragoza, Spain (latitude: 41°39′38″ N–longitude: 1°0′15″ O) from 1 February 2021 (reference sample) to 31 January 2022. The meteorological parameters are presented in
Table 2 and have been supplied by AEMET (Spanish Agency of Meteorology).
2.3. Film Characterization
The mechanical properties of the samples were measured monthly over the course of 12 months. The tests performed were impact, tensile strength and tearing.
The impact properties were evaluated using a dart test according to ISO 7765-1 with Metrotec equipment (Metrotec, Lezo, Spain) in order to characterize the fracture behavior against impact dart loads. Enough distance was kept between successive tests in order to ensure no influence from previous impacts. The total mass was adjusted with the staircase method that applies to the mentioned standard, whose minimum objective is 10 breaks. The conditions were as follows: impact radius 8 cm, dart weight 25 g/50 g/100 g, added weights 5 g/15 g/30 g/45 g/60 g/90 g/100 g, minimal number of breaks, as said above, was 10.
Tensile strength and elongation at break of the samples were tested according to the standard ISO 527, considering the orientation of the test tube, using an IDM testing machine (0301N208) IDM Test (Ingeniería y Desarrollo de Máquinas, S.L. San Sebastian, Spain) with a cell load capacity of 250 N. Tensile tests were performed at a cross-head speed of 500 mm/min. Direct extension measurements were conducted periodically using an extensometer with sensor arms. 16 replicates of the test are carried out in each direction analyzed, with a constant width of the specimen of 15 mm and a distance of 50 mm between the grips.
The Elmendorf tear properties of all blown films were measured using an IDM Elmendorf DEA-80 Tear tester IDM Test (Ingeniería y Desarrollo de Máquinas, S.L. San Sebastian, Spain) following the standard ISO 6383-2. Two film sections of 76 mm × 63 mm were cut with a sample cutter from each test film produced and their thickness measured. A 20 mm slit was made at the center of the edge perpendicular to the direction being tested. Eight replicates of each test were performed in each direction.
A colorimetric study has also been performed on the samples, which were originally transparent and colorless. The measurements were made with a PCE-CSM-2 colorimeter (PCE Instruments, Meschede, Germany), determining the tristimulus values of 16 stacked layers of samples on a white surface. In order to evaluate the differences, the L*a*b* CIELAB color space is taken as a reference. The three coordinates of CIELAB represent the lightness (L* = 0 black and L* = 100 white), the balance red/green (a* green = negative; red = positive) and position yellow/blue (b*, blue = negative; yellow = positive). The asterisks (*) after L*, a*, and b* are pronounced stars and are part of the full name to distinguish L*a*b* from Hunter’s Lab. [
34]
The calorimetric analysis was performed using a Mettler DSC1 calorimeter (Columbus, OH, USA) calibrated using indium (heat flow calibration and temperature calibration) and zinc (temperature calibration) standards. Samples of approximately 10 mg of the mass were deposited in 40 µL aluminum pans in an air atmosphere to test their performance. The sample is heated at a rate of 10 °C/min to 180 °C to erase the thermal history of the processing. After that, it is cooled to 30 °C to determine crystallinity and left for 5 min for stabilization; after that, it is heated again to 180 °C to determine melting temperature.
The chemical structure of the compound-degraded samples was determined using FTIR analysis performed by means of a Spectrum Two spectrometer from Perkin Elmer (Waltham, MA, USA). The device had an ATR attachment with a diamond crystal.
Spectra were registered at a 2 cm−1 resolution and 40 scans in the range of 500–3500 cm−1, in which the compound signals related to different deformation bands can be observed.
To ensure the reproducibility of our results, we conducted multiple measurements for each test. We performed impact testing until at least 10 breaks occurred. Tensile strength and elongation at break were assessed using 16 specimens in each direction. Tearing tests were replicated eight times for each sample. Colorimetric measurements were obtained by measuring 15 stacked film layers. Calorimetric analysis was performed following standard procedures, with one sample of each type. Spectra were obtained via ATR, averaging 40 scans per spectrum. In the figures presented, the standard deviation is represented by error bars associated with each data point.
4. Conclusions
Analyzing the results of mechanical properties, it seems that the proposed biocompostable material, in regular storage conditions without any extreme weathering environments, can be altered in its mechanical properties during a period of one year. This may raise concerns about the conservation of these films pre-use and also recommends providing a caducity date for these materials.
From the point of view of color change (very important in the case of packaging), the degradation shows no visible modification of the material.
Eventually, the final properties will still fulfill the requirements. The film is useful after this test. The main issue here is that while synthetic films were stored without any other considerations about caducity, the data show that perhaps, in more aggressive environments, the biocompostable films could be altered quite significantly.
The microstructural studies performed with FTIR and DSC show that the main degradation occurs in the PBAT and, concretely, in the aliphatic chains.
As in other polymers, the main degradation phenomena involved are the breaking of chains (aliphatic parts of the PBAT), the formation of double bonds and the apparition of oxygenated products of the degradation. If the degradation period is long (or intense) enough, the formed double bonds can undergo crosslinking reactions that increase the fragility of the polymer.
As a consequence of this research, a new avenue of investigation emerges concerning the environmental conditions under which these films can be stored and the potential establishment of conditions for their preservation.
Moreover, considering the global context and taking into account current consumer preferences and their concerns [
39], we will propose a new study with the aim of examining the fragmentation and degradation of the film in various scenarios: (a) the final product made from biocompostable materials exposed to outdoor conditions, (b) the final product composed of biocompostable materials submerged in water and (c) the final product constructed from biocompostable materials buried in the ground.