4.1. Poly(lactic Acid)
Poly(lactic acid) (PLA) is at present a largely used bio-based and compostable packing material due to its good processability and biocompatibility. PLA is mainly composed of lactic acid (2-hydroxy propionic acid) and contains a pendent methyl group on the alpha carbon atom, which gives rise to a specific structure [20
]. Lactic acid is a three-carbon organic acid: one terminal carbon atom is part of an acid or carboxyl group; the other terminal carbon atom is part of a methyl or hydrocarbon group; and a central carbon atom having an alcohol carbon group attached. It exists in two optically active isomeric forms. One is known as L-lactic acid, and is the biologically important isomer, and the other, its mirror image, is D-lactic acid (Figure 2
PLA is a thermoplastic material that can be processed by injection molding, blow molding, thermoforming, and extrusion [21
]. Semi-crystalline PLA is preferred over the amorphous polymer when higher mechanical properties are desired. Semi-crystalline PLA has an approximate tensile modulus of 3 GPa, tensile strength of 50–70 MPa, flexural modulus of 5 GPa, flexural strength of 100 MPa, and an elongation at break of about 4%. PLA is industrially compostable and its degradation is dependent on time, temperature, molecular weight, crystallinity, impurities, and catalyst concentration [22
]. PLA films have better ultraviolet light barrier properties than low density poly(ethylene) (LDPE). Commercial PLA is a homopolymer of poly(L-lactic acid) (PLLA) or copolymer of poly(D,L-lactic acid) (PDLLA), produced from L-lactic acid and D,L-lactic acid. The L-isomer constitutes the main fraction of PLA derived from renewable sources since the majority of lactic acid from biological sources exists in this form.
PLA has four optical enantiomers: PLLA, PDLA, PDLLA (racemic), and meso-PLA. The semi-crystalline PLLA is the most widely used. The glass transition temperature of PLLA is 50–60 °C, the crystallization temperature is 90–140 °C, and the melting temperature is 150–180 °C. Compared with common thermoplastics, PLLA has better mechanic properties like tensile strength, but the crystallization rate is extremely slow which limits the application in high temperatures. One of the efficient methods to improve the crystallization behavior of PLLA is adding nucleating agents, which have several advantages: providing more nucleation sites, shortening the induction time for crystallization, and enhancing the tensile strength of polymer. Inorganic particles (like nano-CaCO3
) are often used to improve the mechanical properties and lower the cost of polymer materials [23
], but they can also be added in small amounts as nucleating agents to improve the crystallization behavior of polymers [24
PLA has good potentialities for use in rigid packaging because of its good rigidity and mechanical resistance. However, the high brittleness can limit its use, hence several strategies have been used for improving PLA toughness, such as the blending with other polyesters, like poly(butylene adipate-co-terephthalate) (PBAT) [27
], behaving like elastomers. In the industrial production of rigid containers usually PLA is processed in the melt and then rapidly cooled below its glass transition temperature. As the crystallization of PLA is slow, final products (such as injection molded or blow molded containers) are essentially amorphous. The increase of temperature above PLA glass transition during the further processing steps (e.g., in the packaging of hot products) or during the use of the material, can enable cold crystallization, resulting in dimension instability and deformation of the items. In addition, an increase in brittleness and change in optical properties can be observed as consequence of crystallinity increase. Thus, the use of proper nucleating agents, allowing increasing the crystallization rate of PLA during the rapid cooling is particularly interesting on a technological point of view. Nucleated PLA results stabilized and its optical and mechanical properties are not modified as a consequence of heating in the temperature range of its glass transition [24
In the case of filling PLA bottles with a hot liquid, the use of a material having an improved mechanical resistance above the PLA glass transition can be considered. This is the case of PLA blends with polycarbonate (PC), that can also be obtained by peculiar reactive processing techniques, as explained by Phuong et al. [30
] and Gigante et al. [32
] added cellulosic fibers to PLA/PC blends to further improve their elastic modulus and investigated models to express mechanical properties as a function of composites composition, keeping also into account the peculiar waviness of cellulosic fibers [32
]. The disadvantage of PLA/PC blends is the not complete biodegradability, as PC is not biodegradable. PLA blends containing cellulose acetate are even very promising for rigid packaging and fully biodegradable depending on the cellulose acetate acetylation degree and of its content in the blend [33
]. Recently, composites consisting of PLA reinforced with plasticized cellulose acetate were obtained by extrusion reporting an improvement of toughness with respect to raw PLA, while maintaining a high value of Young’s Modulus [34
Use of plasticizers is also a good strategy to improve PLA ductility and toughness, and it is fundamental in flexible packaging formulations (Table 2
). Plasticizers will decrease PLA glass-transition temperature (Tg) resulting in a lower stress at yield and higher elongation at break at room temperature. These conditions are required for improving flexibility of films and sheets. Acetyl Tributyl Citrate [35
], triacetine [36
] and oligoethers [26
], oligo lactic esters [24
], and oligo adipic esters [36
] resulted in efficient plasticizers for PLA and its blends. Interestingly, if the plasticization was carried out in the presence of poly(Butylene adipate-co-terephthalate) (PBAT) as polymeric additive, a preferential migration of the acetyl tributyl citrate in the PBAT phase was observed [36
] suggesting the necessity of selecting a plasticizer on the basis of its preferential affinity with the PLA matrix.
As both rigid and flexible packaging is generally used for liquid or pasty cosmetic, the barrier properties of PLA-based formulations must be particularly high. Usually barrier properties can be improved thanks to the use of inorganic additives. Among the inorganic additives, layered silicates, such as montmorillonite (MMT), appear to be effective fillers to improve the overall performances of PLA system even at low concentration (1–5 wt %) [37
]. However, to reach this improvement, a high degree of clay dispersion that highly depends on the adopted preparation method, and on the compatibility between the polymer matrix and the clay is needed [38
]. Jorda-Beneyto et al. [40
] used two organo-modified clays for food contact applications to produce hydrophobically-modified montmorillonite, to obtain better compatibility between the biopolymer and the filler (nanoclay). The fillers were used to reinforce PLA bottles, and the results were compared with conventional PLA bottles. The use of modified clay in PLA bottles was found to lead to an improvement in mechanical and barrier properties. Finally, cytotoxicity tests were conducted with organo-modified clays using two different cell lines but both clays showed different cytotoxicity profiles. Further studies showed different degrees of cytotoxicity or mutagenicity as a function of clay type. Non-organophilic clay (sodium clay) did not show any cytotoxicity or mutagenicity [41
These studies showed that nanoclays cannot be used currently in food packaging, but these nanocomposites were investigated for being used in the cosmetic field through the European FP7 BioBeauty project. In this project both tube and pot for cosmetics were prepared by extrusion and injection molding, respectively, using nanocomposites based on PLA and nanoclay. The potential hazards that may arise from the use of these PLA-based nanocomposites for cosmetic packaging applications were assessed by the study addressing the dermal toxicity of components that may migrate from PLA nanocomposites into cosmetic formulations.
An experimental approach was designed to test the biocompatibility of nanocomposites and their potential to release migration extracts that could be cytotoxic or phototoxic towards human HaCaT keratinocyte skin cells or cause skin irritation in the EpiDermTM human skin model, according to OECD TG 439. Overall findings from these studies provided valuable information showing that PLA nanocomposites developed within the BioBeauty project [42
] can be used safely in the cosmetic packaging industry and meet regulatory requirements. The shelf life of cosmetic products filled in these packages have been evaluated though lipid oxidation according to the Thiobarbituric acid reactive substances (TBARS) method during the accelerated oxidation experiment and it could be observed that cosmetics remained quite stable in terms of oxidation up to day 20. From that time, the cosmetics packed in the different packaging systems underwent different degrees of oxidation over time. It seems that the short shelf life resulted in the main limit for these packagings, especially for tubes, having a minor thickness.
The durability of PLA-based packaging in the presence of a cosmetic based on water and oil can be much affected by its tendency to hydrolyze. The kinetic of hydrolysis can much influence the shelf life of packed products.
Andrzejewska et al. [43
] demonstrated that water, also containing salts to mimate biological fluids, determined an increase in the mass of PLA of about 1 week because of water absorption on time lower than 1%. The properties were not much affected by the presence of water-based fluids. It was found that the material does not undergo rapid degradation in the environment corresponding to the “in vivo” conditions.
The situation seemed different in the case of water/ethanol solutions [44
]. Hydrolytic degradation of poly(lactic acid) (PLA-C) and PLA-nanocomposite produced with organo-modified montmorillonite at 5 wt% (PLA-OMMT) were investigated in pure water, 50% and 95% ethanol solutions at 40 °C. The nanoclay did not affect the hydrolytic degradation but it doubled the sorption of ethanol into the film because of its access into nanoclay galleries. Hydrolysis of PLA-C and PLA-OMMT was related to the release of LA during exposure. The clay released from PLA-OMMT films during hydrolysis in 50% ethanol was 0.58% wt. at 90 days of the initial amount of nanoclay in the PLA film.
Changes in PLA-based material composition can affect the degradation behavior of PLA. Zhang et al. [45
] investigated the effect of nanocrystalline cellulose (NCC) and poly(ethylene glycol) (PEG), both very hydrophilic, on the hydrolytic degradation behavior of poly(lactic acid) (PLA) bio-nanocomposites compared with that of neat PLA, under specific environmental condition, namely at 37 °C in a pH 7.4 phosphate buffer medium for a time period up to 60 days. The results showed that the presence of hydrophilic NCC and PEG significantly accelerated the hydrolytic degradation of PLA, which was related to the rapid dissolution of PEG causing easy access of water molecules to the composites and initiating fast hydrolytic chain scission of PLA. The thermal degradation temperatures of the nanocomposites slightly decreased due to the poor thermal stability of NCC in comparison with that of the neat PLA. After degradation, the thermal stability of the separated PLA from nanocomposites significantly decreased.
Despite the act that PLA-based composites can be suitable for packaging cosmetics, it is necessary to make specific tests considering the specific cosmetic composition, the required shelf life and final goals the cosmetic aims to achieve. Migration of substances from the container to the cosmetic, can in fact slightly alter its content.
Polyhydroxyalkanoates (PHAs) are gaining attention among biodegradable polymers due to their promising properties such as high biodegradability in different environments, not just in composting plants, and versatility. Indeed, PHAs can be formulated and processed for use in many applications, including packaging, molded goods, paper coatings, nonwoven fabrics, adhesives, films, and performance additives [46
PHAs polymers are naturally produced by bacteria in general cultivated biomass. They can be processed to make a variety of useful products, where their biodegradability and naturalness are quite beneficial in particular for application in single-use packaging and. Poly(3-hydroxybutyrate) (PHB) is a homopolymer of 3-hydroxybutyrate and is the most widespread and best characterized member of the PHAs family (Figure 3
This family of polymers presents interesting properties for packaging such as a low water vapor permeability comparable to Low Density Poly(ethylene) (LDPE) [48
]. PHB is used in bulk shrink packaging and flexible intermediate bulk containers for food packaging [50
Copolymers of hydroxybutyrate and hydroxyvalerate, including poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV), have thermoplastic properties similar to polypropylene (PP), good mechanical properties and are commercially marketed [50
]. Thus, the presence in the chains of 3HV or 4HB co-monomers results in considerable changes in mechanical properties. The ratio of co-monomer addition is directly proportional to the toughness and inversely proportional to the stiffness and tensile strength. PHAs can be used as alternatives for several traditional polymers, since they exhibit similar chemical and physical characteristics [52
Although PLA is produced on a higher scale and is currently less expensive than PHAs, some characteristics make PHAs more advantageous for application in contact with skin such as the lower greenhouse gas emissions [53
], the very high biocompatibility [54
PHAs have also an excellent biodegradability, many aerobic and anaerobic microorganisms (bacteria, cyanobacteria, and fungi) may degrade PHAs in several environments: in soil, in industrial/domestic compost, in fresh water, and in various marine ecosystems both as raw material [55
] than as polymeric matrix in bio-composites [57
These properties make the PHA-based materials very promising for being used in applications where environmental concern and biocompatibility are fundamental. However, there is not much work in literature about their durability and possible shelf life when used as packaging [58
], thus research on these materials is still ongoing, and the positive perspective predicts their future application as packaging even for cosmetic products.
Cellulose and starch derivatives are the most utilized polysaccharides for packaging production, and more recently even chitosan and chitin have been proposed in particular for the production of active packaging due to their anti-microbial activity. Cellulose is the most abundantly occurring natural polymer on earth. It consists of glucose monomer units that are joined together via β-1, 4 glycosidic linkages, which enable cellulose chains to form strong inter-chain hydrogen bonds (Figure 4
Raw cellulose is highly hydrophilic and consequently is not suitable for packaging of easily perishable goods, since it presents poor barrier to moisture, moreover the crystalline structure and relatively low thermal degradability makes cellulose a brittle material [59
]. The most used cellulose derivatives are polysaccharides composed of linear chains of β (1–4) glycosidic units with methyl, hydroxypropyl or carboxyl substituents such as hydroxypropyl cellulose, hydroxypropyl methylcellulose, carboxymethylcellulose, or methyl cellulose. Even these materials have a low barrier to moisture [60
], and can be used for packaging just in multi layers materials or in composite materials [61
Starch, is a polysaccharide composed of amylose (20–30%) and amylopectin (70–80%) (Figure 5
), and is primarily derived from cereal grains such corn (maize) or wheat.
Other commonly used sources are tapioca, potato, and rice. To be processable, starch must be modified by either plasticization, blending with other materials, genetic or chemical modification, or combining different approaches [62
]. Starch-based materials available on the market consist of blends of starch and other polymers, such as poly(ethylene-co-vinyl alcohol), poly(vinyl alcohol), or polycaprolactone. These starch-based thermoplastic materials have found wider industrial applications ranging from extrusion applications, injection molding, blow molding, film blowing, and foaming [63
]. The resistance to water or water vapor is, however, very limited because water swells the starch fraction of the blends, as observed for films prepared using wheat starch [65
]. Hence the starch is partially released. Starch based packaging containers or films are thus suitable only for not hygroscopic dry products.
Chitosan is a cationic polysaccharide derived from the deacetylation of chitin and it is of animal (crustacean shells) or vegetal origin (fungal mycelium) (Figure 6
In general, chitosan is characterized by a lack of toxicity, biodegradability, film-forming capacity, anti-fungal and antioxidant properties, and good barrier properties of chitosan films to oxygen [66
]. For these reasons, chitosan is largely used in many applications such as coatings to fruit and vegetables [67
], meat [68
], cheese [69
], and sea-food [70
]. Chitosan anti-fungal properties were largely investigated alone and in combination with other antioxidants and anti-fungal substances, such as essential oils [71
]. Considering these active properties, chitosan can be applied as a coating on a PLA film in order to produce flexible packaging with biodegradability and antioxidant functionalities to protect perishable products [72
]. The presence of anti-microbial coatings based on natural biopolymers that can also improve cells vitality and their regeneration [73
], can suggest the possibility of transforming some general hygienic products in more sophisticated products promoting health and beauty of skin. Chitin and chitosan can be considered an optimum basis for this product, because of their anti-microbial and skin regenerative properties [74
]. Moreover, an added value for the cosmetic packaging would be the capability of increasing the products’ shelf-life. Even chitin nanofibrils and their derivatives, also in the presence of active molecules, were kept into account for preparing bioplastic active materials and surfaces [75
] that can have potential application in the cosmetic field [79