The performance characteristics of polylactic acid (PLA) as an active food packaging film can be highly influenced by the incorporation of active agents (AAs) into PLA, and the type of processing technique. In this review, the effect of processing techniques and the addition of natural AAs on the properties related to PLA performance as a packaging material are summarized and described through a systematic analysis, giving new insights about the relation between processing techniques, types of AA, physical–mechanical properties, barriers, optical properties, compostability, controlled release, and functionalities in order to contribute to the progress made in designing antioxidant and antimicrobial PLA packaging films. The addition of AAs into pla films affected their optical properties and influenced polymer chain reordering, modifying their thermal properties, functionality, and compostability in terms of the chemical nature of AAs. The mechanical and barrier performance of PLA was affected by the AA’s dispersion degree and crystallinity changes resulting from specific processing techniques. In addition, hydrophobicity and AA concentration also modified the barrier properties of PLA. The release kinetics of AAs from PLA were tuned, modifying diffusion coefficient of the AAs in terms of the different physical properties of the films that resulted from specific processing techniques. Several developments based on the incorporation of antimicrobial and antioxidant substances into PLA have displayed outstanding activities for food protection against microbial growth and oxidation.
Previous works have already compiled the description of antioxidant and antimicrobial PLA film developments due to the relevance of PLA in the design of bioderived active packaging [ 23 , 24 , 25 ]. In this framework, this review was focused on summarizing the effect of processing techniques and the addition of AAs on the main physical and compostability properties of PLA in a critical and comprehensive manner. Additionally, their functionalities for food packaging applications and their mass-transfer properties for AA release were analyzed. From the described features, this review gives new insights on the relation between processing techniques and the type of AA, physical–mechanical properties, compostability, and controlled release from active PLA packaging films in order to contribute to the progress of designing antimicrobial and antioxidant PLA packaging films with desired characteristics for specific food packaging applications.
Furthermore, the functionality analysis of these active PLA packaging films has essentially been determined through in vitro assays following standard procedures and methods [ 21 , 22 ]. The effectiveness of AA-modified PLA films depends on the release kinetics of the AA from the polymer to the food, which results from the mass transfer phenomena that involve the equilibrium and kinetic processes in both food and packaging material. Although few studies have dealt with the assessment of the release kinetics of AAs from PLA films, results have shown that the release process was dependent on the processing technique and the AA’s chemical nature [ 12 , 15 ].
The addition of an AA to PLA changed its physical properties, i.e., its barrier, mechanical, thermal, and optical properties. Several works have reported that AAs that exhibited nucleating and/or plasticizing properties sometimes modify the crystallinity degree of PLA and its physical–mechanical characteristics, compostability, and mass transfer properties. In this context, several studies have been carried out to evaluate the effect of AAs, such as thymol, eugenol, and some natural extracts, on the thermal, optical, barrier, mechanical, and biodegradability properties of PLA [ 17 , 18 , 19 , 20 ].
An interesting research area in PLA food packaging is the development of active release systems [ 8 ]. Active release packaging is an innovative concept, which involves the interaction between package and food through the release of active substances from the polymer matrix to the food surface in order to prolong shelf life or enhance its safety and/or sensory properties, while simultaneously preserving the quality. One of the biggest challenges of active food packaging is providing superior protection against microbiological deterioration and oxidative reactions compared to conventional food packaging. Incorporating AAs inside the polymer matrix films allows for their functionalization. In this context, the primary deterioration process of the packaged food product, such as the oxidation process, or bacterial and fungal incidence, determines the selection of the active agent. Natural extracts and essential oils (EOs) from plants or their derivatives have been used mainly to develop antimicrobial and antioxidant activities due to their recognition as safe [ 9 ]. The characteristics of EOs as food ingredients or as part of the packaging have recently been reviewed [ 10 , 11 ]. AAs have been incorporated into the polymer structure during the film formation through melt-blending processes, such as extrusion and compression-molding processes [ 12 ], or through other techniques, such as solvent-casting [ 13 ] and electrospinning processes [ 14 ], where PLA was previously dissolved, and lower temperatures were needed. An alternative strategy to develop active PLA packaging films has also been carried out by incorporating thermosensitive compounds into the PLA matrix by a supercritical carbon dioxide (scCO 2 ) impregnation process [ 15 ]. The feasibility, advantages, and limitations of the current processing techniques used to develop active PLA packaging films have recently been reviewed [ 16 ].
Environmental pollution generated from the accumulation of plastic waste is undoubtedly one of the most urgent environmental problems that needs to be addressed. This fact originates mainly due to the increase in disposable plastic products principally used in the food packaging sector, whose degradation time can extend several hundred years. An alternative to these long-lasting synthetic plastics is biodegradable materials, where polylactic acid (PLA) has become the most commonly used environmentally friendly bioplastic because it is commercially available, compostable, and can be processed by conventional melting extrusion [ 1 , 2 ]. PLA is a biopolymer that has received increasing attention in the food packaging industry in recent decades due to its sustainability, biocompatibility, biodegradability, and safety [ 3 , 4 ]. PLA is a linear aliphatic thermoplastic polymer produced by the polymerization of L- and D- lactic acid isomers, which are obtained through the fermentation of starch from renewable sources such as potatoes, sugarcane, and corn [ 5 ]. Meanwhile, at an industrial level, PLA is primarily produced through the polycondensation of lactic acid and/or the ring-opening polymerization of lactide [ 6 ]. Due to its high transparency, easy processability, resistance to chemicals, and good and smooth appearance, PLA has been processed for industrial film production, mainly by extrusion, casting, and blow-molding. Nevertheless, PLA packaging films have also been obtained through other techniques such as compression molding, solvent casting, and electrospinning, among others [ 7 ].
Color and opacity are two of the main optical properties to be evaluated during the development of active films, since noticeable changes in these parameters could negatively affect the consumer’s acceptability towards their application as food package materials [26]. PLA can be produced with high transparency, and mono- or several multilayer PLA films maintained this character regardless of the processing technique [7,27,28].
Regarding color changes on PLA-based films, the intrinsic color of an AA and/or the polymer were the main factors that produced such variation, and it was generally expressed as ΔE. A slight change in ΔE value of ±0.5 can be recognized by humans as a different color, therefore producing a rejection of the food which was packaged. Furthermore, this effect can be intensified if ΔE is higher than a value of 2.0 [29]. On the other hand, the transparency of films has been related to the capability of light transmission through the material, which is typically measured by spectrophotometric methods at visible wavelength values between 500 and 600 nm. The opacity and transparency are inversely proportional parameters and depend on the film’s thickness [13,30]. Maintaining the film’s transparency after incorporating an AA is highly preferred in active packaging design when being able to observe the food is desirable. On the other hand, the increase in the opacity of PLA packaging films can be significant for avoiding light-induced degradation processes, such as fat oxidation and pigment degradation. Opacity has been attributed to the differences in the refractive indices of the AA and PLA matrix, which modify the light scattering and absorption through the film and, therefore, the light transmittance [30].
Some works have shown a color change in PLA films when the AA has been directly added (non-encapsulated AA). For example, the incorporation of Allium ursinum extract (AU) (wild garlic) into solvent-cast PLA films displayed ΔE values of 4.4 and 41.4 when AU was added at 0.5 and 5 wt% in PLA, respectively, resulting in transparent and yellowish films [31]. Likewise, the intrinsic color of freeze-dried orange peel extract directly added to PLA, or previously encapsulated with either pectin or β-cyclodextrin (CD) through spray drying, and subsequently incorporated in the polymeric matrix, modified the color of PLA films obtained by hot pressing [32]. Freeze-dried extract and spray-dried powder at concentration values between 0.25 and 2 wt% were loaded into PLA films. Researchers reported a more intense orange coloration of PLA as the concentration of the extracts were increased.
Similarly, certain polymers used in PLA-based bilayer films or in AAs incorporated in some of the layers have also produced a color variation of the films. The application of a zein coating loaded with quercetin at 5 wt% onto an extruded PLA film changed its color, but maintained its transparency [33]. The resulted bilayer films reflected the characteristic yellowish tone from both zein and quercetin incorporation. The PLA/zein bilayer displayed ΔE values of 1.95, which was increased to 5.36 when quercetin at 5.0 wt% was incorporated into the zein layer. Likewise, a recent study about PLA-based bilayers evidenced an increase in ΔE value toward yellowness when a gelatin layer loaded with different concentrations of epigallocatechin gallate (EGCG) (0–12 wt%) was incorporated into PLA through compression molding [34]. Furthermore, the films showed a good transparency. The incorporation of umbelliferone (UMB) also produced a red-yellowish coloration in mono- (PLA) and bilayer (PLA/PLA) films obtained through extrusion and compression molding techniques [35]. The active PLA films showed high ΔE values with respect to their corresponding monolayer (ΔE = 23.9) or bilayer (ΔE = 58.4) controls. On the other hand, the low miscibility between AAs and the polymeric matrix due to their different chemical characteristics can intensify the opacity of the films [36,37]. This effect can be intensified at high AA loads since a good dispersion of AA into PLA films is hindered. This fact results in the formation of particles larger than the visible wavelength, blocking the light path and increasing the opacity of films. A study about an active PLA film loaded with clove essential oil at 15 and 30 wt%, and graphene oxide nanosheets at 1 wt%, clearly evidenced this effect [13]. The increase in the loading of EO reduced the light transmittance, and this reduction was intensified by the presence of graphene oxide nanosheets. Researchers attributed this drop to the interruption of light passage by the nanosheets and to a bad dispersion of oil droplets in the polymer matrix. Likewise, a work based on PLA active films loaded with β-cyclodextrin/2-nonanone inclusion complexes at 5 and 10 wt% also showed a similar behavior. The incorporation of inclusion complexes increased the opacity of PLA active films up to 3-fold due to the presence of agglomerates which scattered light and increased opacity [37]. The immiscibility between the inclusion complexes and PLA increased the film discontinuities and films’ opacity, and this effect increased as the concentration of the inclusion complex containing AA increased.
Semi-crystalline polymers, such as PLA, show several thermal transitions when heated as endothermic processes, such as glass and melting transitions, and exothermic processes, such as thermo-oxidative degradation, crosslinking reactions, and crystallization. Furthermore, heating also induced the cold crystallization of PLA, which is associated with the transition from an amorphous to a crystalline phase, including the rearrangement of imperfect crystals. Heating also induced polymer changes from a glassy solid state to a viscoelastic rubbery state, with greater mobility above the glass transition temperature (Tg) associated with the amorphous zones. Subsequently, the crystals collapse at the melting temperature (Tm). The values of these temperatures enable the evaluation of the conditions of processability of PLA films for different applications.
Thermal parameters of PLA and active PLA films reported by several works are summarized in . PLA presented the glass transition at around 60 °C and the melting transition at between 145 and 170 °C. Lower Tg values were reported for PLA obtained by electrospinning or solvent casting, possibly due to the presence of residual solvent that acts as a plasticizer. These thermal parameters depended on several factors associated with the grade of PLA (L- and D-lactide content, melt flow index, molar mass, and dispersity index) and the processing conditions [31,38,39]. The thermal transitions of PLA, manifested as heat flow changes with the temperature variation, were detected through differential scanning calorimetry (DSC) analysis. Moreover, the crystallinity has been calculated by the ratio between the melting heat of the PLA sample and the melting heat of 100% crystalline PLA [40,41]. The effect of the processing technique on the PLA’s thermal properties has been evaluated by the first DSC heating process. In this context, significant differences have mainly been found between extruded and electrospun PLA films obtained using virgin PLA of the same grade. The Tg of PLA films obtained by electrospinning tended to be at lower values (40 °C) [14] than the Tg of PLA films obtained through extrusion (61 °C) which can be associated with the residual solvent in the PLA [37]. Meanwhile, lower crystallinity has been reported for electrospun PLA films than extruded PLA films, as a result of the inhibition of the polymer chain order due to fast cooling produced by solvent evaporation and the large surface area of the nanofibers [42].
PLA’s thermal parameters also depended on the processing conditions and/or measurement parameters of the DSC analysis, which could be responsible for the differences found for the Tm and Xc of extruded PLA [37,42].
Regarding PLA films obtained by electrospinning, lower values of Tg and Tm (40 °C and 137 °C) were reported for PLA films obtained using CHCl3:DMF at 7:3 volume ratio [14] instead of CHCl3:DMF at 1:1 solution (53 °C and 153 °C), using PLA with the same grade [42]. These results suggest a higher volatility of chloroform-inhibited chain ordering during fiber formation when CHCl3:DMF ratio 7:3 was used, so the polymeric film was more amorphous, and therefore flexible, with a lower melting temperature. Furthermore, it is important to highlight that different amounts of residual solvent in the fibers could cause different plasticization effects and Tg values. Similarly, solvent volatility and casting conditions strongly influenced the thermal parameters of cast PLA films [44,45].
Thermal parameters of active PLA films reported by several works are summarized in . The incorporation of AAs has modified the thermal properties and crystallinity of PLA in terms of the AA chemical structure. The PLA–AA interactions increased Tg, and caused a reduction of the crystallinity due to the changes in the mobility of polymeric chains during the processing of PLA-curcumin by electrospinning [14]. This effect was attributed to hydrogen-bond interactions between PLA and the curcumin. Furthermore, the curcumin caused PLA to cold crystallize during first heating process, promoting a reordered structure with a unique melting transition [14]. Conversely, some AAs exerted a plasticizing effect on the PLA chains by lowering their glass transition and melting temperatures, as observed by incorporating ungeremine into PLA by electrospinning [46,47].
Nonetheless, the changes in PLA crystallinity depend on the AA concentration. Radusin et al. (2019) [31] reported that wild garlic at low concentration exerted a nucleating effect on cast PLA films due to the good dispersion of this antimicrobial compound. Conversely, the formation of amorphous zones was favored at higher AA concentrations due to polymer-chain-ordering disruption through steric hindering [31].
Furthermore, it is highlighted that the molecular structure of the AA plays a fundamental role in the formation of crystals by processing techniques involving stretching. For example, a 14% reduction in PLA crystallinity was reported, with the addition of 20 wt% thymol, in PLA films obtained by extrusion and biaxial stretching. In contrast, the degree of crystallinity increased by 7.3% when R-(−)-carvone was added using the same technique at the same nominal concentration, considering that final concentration in the films was reduced to lower than 50 wt% of the initial concentration loaded. These results suggested that orientation- and strain-induced crystallization in films containing R-(−)carvone could be reached [48].
On the other hand, some works have evaluated the thermal properties of active PLA films after erasing the thermal history of the polymer through a second heating process to study the effect of the concentration and chemical structure of the AA, which modifies the PLA chain ordering. A drop of 3 °C in the Tg of cast PLA films was reported, due to the incorporation of bergamot essential oil at 9 wt%, and a 6 °C reduction due to clove oil added at the same concentration [49]. Meanwhile, a more significant temperature drop in the Tg of extruded PLA films was reported when the concentration of thymol in the PLA films was increased from 5 wt% (5 °C) to 20 wt% (26 °C) [38].
The thermal stability of PLA was modified by the incorporation of AAs in terms of their molecular structure. Gavril et al. (2019) [50] reported a drop of 10 °C and 13 °C in temperature at the maximum degradation rate of PLA films (Td = 345 °C) obtained by melt mixing with lemon balm leaves and sage at 3 wt%, respectively. Some active PLA films obtained by different types of processing have evidenced two degradation processes: (i) the first stage, corresponding to the decomposition of the AA; and (ii) the second stage, associated with PLA degradation at higher temperatures [15,49,51]. Nonetheless, antioxidant compounds such as cinnamaldehyde (CIN) and curcumin have improved the thermal stability of PLA [14,42]. Moreover, the encapsulation of AAs such as carvacrol, thymol, and 2-nonanone in β-cyclodextrin and pectin, has effectively inhibited the degradation of the EO contained in PLA matrices [37,52].
PLA is a rigid low-stretch polymer with mechanical properties comparable to those of poly(ethylene terephthalate) (PET) and polystyrene (PS), although PLA is more brittle and less impact-resistant [53]. Mechanical properties depend on PLA architecture based on L, D, and meso-lactide proportions, molar mass, and processing conditions [54]. The ratio of lactide isomers influenced the thermal properties and the crystallinity, and therefore, the flexibility of the PLA film.
The mechanical performance of PLA is commonly evaluated by tensile testing that reveals its main mechanical parameters: (i) Young’s modulus (YM): the slope of the elastic-linear zone in the stress vs. strain curve and the measuring of the stiffness; (ii) tensile strength (TS): the maximum force the material can withstand; and (iii) elongation at break (EB): material extensibility.
shows the tensile parameters of active PLA films obtained through different processing techniques. In general, the YM, TS, and EB values of PLA films were reported as between 2800–3400 MPa, 40–65 Mpa, and 1–6%, respectively. Exceptionally, the YM of solvent-cast PLA films (approx. 1700 MPa) has shown a value approximately 50% lower than the YM values of PLA films obtained through melting processes ( ). Similarly, electrospun PLA films have exhibited lower stiffness and tensile strength depending on the fiber diameter, and high levels of deformation at break (approx. 60%) ( ). This fact may be associated with the lower crystallinity degree of electrospun PLA films compared with extruded PLA films [14,37,42].
The effect of the addition of AAs on the mechanical properties of PLA films depended on the molecular structure of the AA, the processing technique, and the conditions. shows the mechanical properties of recent active PLA developments. The changes in the mechanical properties of PLA films were greater as AA concentration increased. In general, YM values of PLA films decreased with AA incorporation. This fact is mainly attributed to a plasticizer effect of the AA, which was located between PLA chains. This increased the free volume, reduced chain entanglements, and caused greater chain mobility and flexibility. On the contrary, when chitosan was added to PLA, this AA acted as a reinforcing filler and restricted the segmental chain movement of PLA. The stiffness of the film increased due to the rigidity of these polysaccharide chains and the generated interactions between PLA and chitosan [55]. These interactions could be hydrogen bonding between PLA (carbonyl or end hydroxyl groups) and hydroxyl, and amine/acetamide groups of chitosan [56]. Nonetheless, the Young modulus of PLA films with chitosan at concentrations above 10 wt% decreased due to the formation of chitosan agglomerates [55]. Moreover, TS variation was attributed to the concentration and dispersion of the AA in the PLA matrix. TS of the films increased with chitosan loaded at 5 wt%, but decreased gradually with up to 15 wt% of chitosan. This gradual reduction was associated with the formation of chitosan aggregates surrounded by a weak interface that acted as stress concentrator lowering tensile strength [55].
Other works have reported that the AA addition has led to a reduction in TS values, possibly due to lower PLA–PLA intermolecular forces generated by the intercalation of AA molecules between the polymer chains ( ) [13]. Meanwhile, some studies have reported increases in TS values for active PLA films. For example, increases of 95% and 48% were reported when AU extract (wild garlic) and propyl gallate were incorporated in cast PLA films and in the core of a coaxial PLA/PLA fiber, compared to the uniaxial PLA fiber, respectively [31,57]. Furthermore, in the case of PLA fibers, TS depended on the fiber diameter which varied with the PLA solution’s viscosity, and therefore with the AA concentration. In this context, Liu et al. (2018) [58] reported a decrease in the TS values of PLA films due to the addition of tea polyphenols, which decreased PLA solution viscosity and thus reduced the fiber diameter.
Regarding the elongation at break of active PLA films, several works have reported a plasticizing effect of the AA due to its intercalation between the polymer chains that caused higher molecular mobility and film deformation ( ). In extrusion-stretching processes, the ductility of PLA films was increased according to the chemical structure and concentration of the AA. Martins et al. (2018) [60] reported the EB’s increase of blown extruded PLA films by 9.6% and 36% due to the addition of green tea (GT) extract at 1.0 and 2.0 wt%, respectively. Similarly, Boonruang et al. (2016) [48] reported that thymol and R-(−)-carvone increased the extensibility of PLA films obtained by extrusion-biaxial stretching (BOPLA). Although both AAs presented a plasticizing effect, the magnitude of the EB variation depended on the changes in crystallinity of the resulted films according to the chemical structure of the AA. Furthermore, thymol exhibited a dramatically more significant plasticizing effect for injection-molded PLA than PLA films obtained by other techniques, such as melt blending, hot pressing, extrusion, and biaxial stretching. Celebi and Gunes (2018) [38] reported that the EB of injection-molded PLA containing 20 wt% of thymol was 144 times higher than that of the neat PLA, while Boonruang et al. (2016) [48] reported a 3.6-fold increase at the same thymol nominal concentration compared to neat PLA films processed through extrusion stretching. This comparison is made based on the initial AA load added to PLA and does not consider the loss of AA associated with each process, which is unknown for the injection-molded PLA.
Nonetheless, some studies have reported that EB values decrease for active PLA films due to different factors. Vasile et al. (2019) [55] pointed out that the addition of chitosan at 6 wt% reduced the ductility of PLA films melt-blended with polyethylene glycol (PEG) as a plasticizer. However, the simultaneous addition of chitosan and rosemary ethanolic extract drastically increased the extensibility of PLA by up to 52% due to a synergistic effect and interactions between both components. On the other hand, it was observed that incorporating Allium extract at 2 wt% reduced the EB value of extruded PLA films by approximately 16% due to the addition of these AA produced films with lower thickness and homogeneity [51]. Similarly, the incorporation of merkén, a Chilean spice containing antioxidant activity, decreased EB values due to discontinuities in the extruded PLA films [43].
Barrier properties, including permeability to gases—such as O2, CO2, and aroma compounds—and water vapor transmission are crucial parameters in the development of food packaging films. The permeability of polymers results from the sorption process of permeant molecules followed by their diffusion through the polymeric matrix [63]. Specifically, because of the semi-crystalline nature of PLA, their barrier properties are closely affected by the film crystallinity, since the diffusion mechanisms of these molecules occur through the amorphous regions [64], as shown in a.
Open in a separate windowThe barrier properties of PLA films were extensively studied, and in general terms, these values were medium [54]. Specifically, CO2 permeability coefficients, PCO2, for PLA (approx. 0.7 × 10−17–1.1 ×10−17 kg m m−2 s−1 Pa−1) were lower than for crystal polystyrene, PS (1.55 × 10−16 kg m m−2 s−1 Pa−1), but higher than those for polyethylene terephthalate, PET (1.7 × 10−18–3.17 × 10−18 kg m m−2 s−1 Pa−1) [54]. The oxygen permeability coefficient, PO2, follows the same trend with values around 20 times lower than those published for PS, but higher than those reported for PET. These values depended on temperature and were ranged between 1.2 × 10−18–4 × 10−18 kg m m−2 s−1 Pa−1 [54]. The effect of the relative humidity (RH) on oxygen barrier properties were also analyzed, and the values followed a similar behavior to the PET polymer. Although oxygen diffusion coefficients increased with water activity, PO2 values did not change significantly [67]. The incorporation of AAs and plasticizers affected the barrier properties of PLA since permeability values depended on intrinsic factors, such as PLA architecture (L, D isomeric composition), crystallinity degree, thickness, free volume, and chain stiffness. On the other hand, extrinsic factors included processing technique and conditions, e.g., temperature and relative humidity [68]. It is essential to consider the difficulty in establishing correlations and comparisons between different active PLA films, because many investigations did not include data on properties such as thickness or crystallinity, and because the units of permeability were not standardized. Nonetheless, summarizes some works that present the effect of AA addition on the moisture and oxygen permeability (PH2O and PO2) of active PLA films obtained through different processing techniques. Several works evidenced that the variation in oxygen permeability was highly dependent on the concentration of AA and the crystallinity degree of active materials. The crystallinity degree decrease due to the addition of AAs was proportional to the increase of gases and moisture permeability values in active PLA films [13,43].
When AAs were added at low concentrations, barrier properties were not generally affected, and in some cases, they were improved. PO2 values of solvent-cast PLA films containing allyl isothiocyanate at 5 wt% decreased from 3.77 × 10−6 to 2.3 × 10−6 m3 m−2 s−1 [44]. On the other hand, extruded PLA films containing thymol and carvone at concentrations between 10 and 20 wt% presented gas permeability behaviors dependent on the concentration and the type of these AAs. As shows, the addition of both AAs at 10 wt% improved the oxygen and carbon dioxide barrier properties of the extruded PLA films. Nevertheless, PO2 and PCO2 values were increased greatly at higher concentrations. Active PLA films with thymol at 20 wt% revealed the highest value of PO2, because thymol acted as a plasticizer and rearranged the polymer chains layers, lowering the degree of crystallinity and increasing the free volume in the polymer matrix. On the other hand, PLA film containing 20 wt% of carvone manifested the highest PCO2 value due to the higher solubility of CO2 in this AA [48]. Generally, incorporating AAs at high concentrations (approx. 10–30 wt%) entailed an increase in oxygen permeability due to the increase of the O2 diffusivity in the film. The increase in oxygen diffusion at high AA concentrations could also be a result of the increase in the free volume of the polymer in the interphase between the AA and the PLA matrix, probably due to the formation of discontinuous phases [69].
The same tendency was observed by solvent-cast PLA films containing AU extract at 0.5 and 5 wt%, whose PO2 values were influenced by AU concentration and relative humidity (RH). In dry conditions (0% RH), the PLA barrier properties were improved slightly by the addition of AU at both concentrations. Higher crystalline content in active PLA films positively influenced its barrier properties. Nevertheless, in humid conditions (50% RH), both active films presented different behaviors. PLA with a low AU concentration exhibited lower oxygen permeability than that of pure PLA, while PO2 significantly increased at a high AU concentration [31]. In this case, it was evidenced that, unlike pure PLA, RH affected the gas barrier properties of active PLA films, principally when the AA displayed moderate hydrophilicity due to the interaction between the AA and water vapor.
The incorporation of plasticizers, such as tributyl citrate (TBC), also affected the barrier properties of PLA films in some developments [44]. The water vapor permeability values of PLA film increased from 2.06 × 10−6 to 2.38 × 10−6 kg m−2 s−1 with the incorporation of TBC at 7.5 wt%, due to the free volume generated by the addition of plasticizer [44].
In conclusion, the addition of AAs did not present a unique tendency in their effect on the oxygen barrier performance of active PLA films. The gas permeability would be dependent on the AA concentration and the crystallinity of the resulted active films, but also on the hydrophilic character of the AA, and even the relative humidity.
With regard to water vapor barrier properties, in general, although PLA is a polar material, this polymer presented interesting water vapor permeability coefficients (WVPC) [54]. The effect of temperature and RH in two extruded PLA films was analyzed, and evidenced that WVPC was maintained reasonably stably as RH was increased. Meanwhile, a decrease in WVPC values was observed as the temperature was increased [54]. In this study, the reported PLA WVPC values ranged from 2.2 × 10−14 to 1.48 × 10−14 kg m m−2 s−1 Pa−1 when temperature increased from 10 to 37.8 °C, while PET and PS films exhibited values of approximately 1.1 × 10−15 and 6.7 × 10−15 kg m m−2 s−1 Pa−1, respectively (at 25 °C).
b summarizes some results presented in various developments of active PLA films obtained through different processing techniques, with the intention of observing the effect of incorporating an AA on WVPC values. b also shows the increase or decrease in WVPC results, expressed as % with respect to the neat PLA. In general, no specific trend in WVPC of PLA was found with the incorporation of AAs, nor by using a specific processing technique. In some cases, WVPC values of PLA depended on the AA type and their concentrations. The addition of an AA at low concentrations has resulted in either a WVPC increase or decrease with respect to the neat PLA. As shows, incorporating GT at 1 and 2 wt% has improved the water barrier properties of extruded PLA films, decreasing WVPC values from 9.31 × 10−7 to 6.06 × 10−7 kg m−2 s−1 [60]. Meanwhile, as b shows, an extruded PLA film containing 5 wt% of merkén, a Chilean spice with antioxidant properties, exhibited an increase of WVPC from 2.5 × 10−15 to 3.83 × 10−15 kg m m−2 s−1 Pa−1 [43]. Although merkén contains hydrophobic antioxidant compounds, such as rutin, myricetin, and quercetin, this spice consists of high levels of organic matter content, since it is made from dry and crushed Chilean peppers. This organic fraction possibly hindered the miscibility of antioxidant compounds with PLA. Merkén components at both 3 and 5 wt% decreased PLA crystallinity (see ) and increased its free volume; therefore, the polymer chain mobility and the molecular diffusivity were increased. Moreover, the formation of agglomerates could also have influenced permeation due to the generation of spaces in the interfacial zones. Thus, the effect of the AA’s addition on PLA crystallinity and its dispersion into the polymer matrix are crucial factors to be considered [43].
When the AA was incorporated at high concentrations, an increase or decrease of permeability with respect to the neat PLA was also displayed. Different Eos, such as bergamot, lemongrass, rosemary, and clove oil were incorporated into PLA at 9 wt% through the solvent-casting process. All samples revealed an increase of WVPC values from 1.06 × 10−14 kg m m−2 s−1 Pa−1 to 1.7–1.9 × 10−14 kg m m−2 s−1 Pa−1 ( b) [49]. Conversely, some studies also showed improved water vapor barrier properties of active PLA films ( b). Solvent-cast active PLA films containing propolis extract and powdered raw propolis at 8.5 and 13 wt% manifested lower WVPC values (approximately 1.36 × 10−14 kg m m−2 s−1 Pa−1) than PLA control film (2.39 × 10−14 kg m m−2 s−1 Pa−1), and the improvement was proportional to propolis concentration, probably due to the high hydrophobicity of these AAs [61]. Recently, cast PLA films containing thymol, kesum, and curry at 10 wt% evidenced a decrease in WVPC values from 4.35 × 10−18 kg m−1 s−1 Pa−1 to 3.97 × 10−18, 3.83 × 10−18 and 3.85 × 10−18 kg m−1 s−1 Pa−1, respectively [72]. Although Mohamad et al. (2020) stated that the increased barrier to the diffusion of water molecules into the polymeric matrix was attributed to the strong interactions between PLA and the AA through hydrogen bonding, the hydrophobicity of these AAs also had a barrier effect against water vapor permeability. This fact was in accordance with works that had also evidenced an increase in the contact angle of PLA by the addition of hydrophobic AAs [73].
Although the analysis of the barrier properties of active electrospun PLA films is not very common, Radusin et al. (2019) demonstrated that incorporating AU extract at 10 wt% through electrospinning decreased the WVPC values by approximately 25% (from 9.2 × 10−14 to 6.9 × 10−14 kg m m−2 s−1 Pa−1) (see b) [65]. In general, electrospun active PLA mats are expected to display lower barrier properties than PLA films obtained through other techniques due to the action of their higher surface area, fibrillary morphology, and higher porosity [74].
The study of the barrier properties of PLA films after processing with high pressure-CO2 has also been very rare. The impact of high-pressure CO2 on water vapor barrier properties was analyzed and revealed an increase in WVPC more significant than expected, due to scCO2 acting as plasticizer and being physically intercalated between polymer chains, increasing the sites for sorption of water molecules [75]. 30 μm-thick PLA films presented an initial WVPC coefficient of 5.1 × 10−6 kg m−1 s−1 Pa−1 and, after CO2 sorption at 25 bar, WVPC increased to 7.2 × 10−6 kg m−1 s−1 Pa−1.
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