Plastics are used for many varied applications in packaging, ranging from sterile storage of medical and pharmaceutical goods, to extending the shelf life of food and protecting sensitive products from damage. Polymers, like poly (ethylene terephthalate) (PET) or polyvinyl chloride (PVC), have a wide, extended use due to their excellent mechanical, chemical and thermal properties. However, their non-renewable nature and lower recyclability are their main disadvantages that need to be dealt with.
Biodegradable polymer materials have been proposed as potential and suitable replacements for traditional plastics as they present good processing properties and a much lower environmental impact [1
]. Among all the biodegradable polymers produced from renewable resources, polyhydroxyalkanoates (PHAs) have a very wide range of properties and applications [2
]. They are synthesized intracellularly by bacteria from agricultural raw materials as a source of carbon and energy. Poly (3-hydroxybutyrate-co
-3-hydroxyvalerate) (PHBV) is among the most popular of the PHAs [4
]. This thermoplastic polyester mainly consists of hydroxybutyrate (HB), along with hydroxyvalerate (HV) units randomly repeated throughout the polymer chain. The physical properties of PHBV vary with the increase of the HV content, which can be controlled by the carbon source supplied during biosynthesis [5
]. Among its properties, PHAs exhibit intermediate oxygen and water barrier properties compared to some petroleum based polymers and feature viscosity properties suitable for the melt processes, such as injection, moulding or extrusion [6
Similarly, polylactide (PLA) is an aliphatic polyester that can be obtained from agricultural resources as corn starch or sugarcane. Its synthesis is a multistep process which starts from the production of lactic acid, with an intermediate step with the formation of lactide and ending with the polymerisation reaction [7
]. Currently, PLA is the first commercially used polymer produced from renewable resources [8
]. It is biodegradable, recyclable and compostable [9
]. Furthermore, it is easily processable on standard equipment used for traditional plastics with a better thermal processability with respect to other biodegradable polymers [10
]. It has, however, some disadvantages, such as poor toughness, slow degradation rates, a hydrophobic character and the lack of reactive side-chain groups [11
The pristine PLA and PHBV can be blended with natural fibres to improve their weak mechanical properties and extend their range of applications [3
]. These fibres are strong, light in weight, abundant, non-abrasive, non-hazardous and inexpensive and act as excellent reinforcing agents for bioplastics [12
]. Lignocellulosic fibres like jute, sisal, coir, and pineapple have been used as reinforcements in polymer matrices. Among them, sisal is of particular interest as it provides high impact strength besides having moderate tensile and flexural properties compared to other lignocellulosic fibres [13
]. Sisal has been used as reinforcement for high density polyethylene [14
], polypropylene [15
] and polyester [16
] among others.
Overall, bioplastics account for nearly 300,000 metric tons of the plastic market (1% of the 181 million metric tons of synthetic plastics produced worldwide) [11
]. PLA is the leader in the biodegradable polymers production market with a share of 5.1% in the total production of 5778 million tons in 2016, followed by 2.5% accounted for by PHA biodegradable polymers [17
]. This market is growing by 20–30% each year [18
] and, following this trend, polymeric waste is also expected to potentially increase. Consequently, its management, once their end of lifetime use is achieved, must be evaluated [19
Among the different end-of-life scenarios, recycling should be the first option in a circular economy strategy. In most cases, recycled products lose their quality with respect to the initial material. The compliance of the standards established by traditional fossil-derived plastics is crucial to ensure the applicability of biodegradable polymers and their recycled products in an industrial level [20
]. The mechanical recycling of biodegradable polymers has been mainly reported for poly(hydroxybutyrate) [21
], poly(caprolactone) [22
] and poly(lactide) [23
]. Furthermore, recycling of blends with polycarbonate or HDPE, or biodegradable polymers, such as starch, have been studied [25
]. A slight decrease of the mechanical properties of pristine PHBV [27
] and PLA [28
] have been shown after several cycles as a result of successive melting or extrusion processing.
Biodegradable polymers can, after a long useful life and after being recycled a maximum number of times, be used in energy recovery systems (i.e., thermal recycling). The generation of heat (or electricity) by incineration is currently the most applied option in Europe [17
]. The study of the thermal decomposition of materials stands out as the main basis for the correct tuning of facilities for energy recovery purposes. In this sense, PLA/sisal and PHBV/sisal have proved their suitable behaviour for thermal recycling [30
Within this framework, the main aim of this work is to study the mechanical recycling process of pure polylactid acid (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and their sisal-reinforced biocomposites. The mechanical and thermal properties of the new samples at different recycling cycles are discussed and compared with the initial materials in order to ascertain the maximum recycling steps that biocomposites could undergo without losing their main mechanical and thermal properties.
The changes induced by reprocessing in terms of the mechanical and thermal properties of PHBV, PLA and their biocomposites were investigated and compared to the unrecycled biocomposites. The mechanical and thermal analysis were performed after each of the three reprocessing cycles to evaluate the potential properties decay along the process.
The tensile modulus increased for PLA biocomposites, whereas it was hardly influenced by recycling for PHBV biocomposites. The tensile strength and deformation at the break decreased notably after the first cycle in all cases. Although all the biocomposites became more brittle with recycling, the properties were conserved along until the third cycle indicating a promising recyclability of these materials. In particular, as evaluated through the dynamic mechanical tests, the samples of PHBV were less affected by the deterioration after the recycling processes than the samples of PLA. For the PLA specimens, it also seemed that the presence of the sisal fibres reduced, at least partially, the embrittlement of the material.
Glass transition, crystallization and melting temperatures were not highly affected by recycling suggesting a good reprocessability of the biocomposites. Moreover, the processing conditions lied in the same range as those for conventional plastics which would facilitate potential joint valorization techniques by adapting already existing recycling units.