Polylactic Acid-Based Film Modified with Nano-Ag-Graphene-TiO<sub>2</sub>: New Film versus Recycled Film

Anca Peter; Camelia Nicula; Anca Mihaly Cozmuta; Goran Drazic; Antonio Peñas; Stefania Silvi; Leonard Mihaly Cozmuta

doi:10.1155/2023/9937270

Polylactic Acid-Based Film Modified with Nano-Ag-Graphene-TiO<sub>2</sub>: New Film versus Recycled Film

Peter, Anca;Nicula, Camelia;Mihaly Cozmuta, Anca;Drazic, Goran;Peñas, Antonio;Silvi, Stefania;Mihaly Cozmuta, Leonard 2023-11-25 00:00:00 Hindawi Advances in Polymer Technology Volume 2023, Article ID 9937270, 15 pages https://doi.org/10.1155/2023/9937270 Research Article Polylactic Acid-Based Film Modiﬁed with Nano-Ag-Graphene-TiO : New Film versus Recycled Film 1 2 1 2 Anca Peter , Camelia Nicula , Anca Mihaly Cozmuta , Goran Drazic , 3 4 1 Antonio Peñas , Stefania Silvi , and Leonard Mihaly Cozmuta Technical University of Cluj Napoca, Faculty of Sciences, Victoriei 76, 430072, Baia Mare, Romania National Institute of Chemistry, Hajdrihova 19 P.O. Box 660 SI-1001, Ljubljana, Slovenia Andaltec, Pol. Ind. Cañada de la Fuente Vílches s/n, 23600, Martos-Jaén, Spain University of Camerino, School of Bioscience and Veterinary Medicine, Gentile III da Varano, MC 62032, Camerino, Italy Correspondence should be addressed to Anca Peter; [email protected] Received 11 August 2023; Revised 26 October 2023; Accepted 30 October 2023; Published 25 November 2023 Academic Editor: Jun Ling Copyright © 2023 Anca Peter et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The increase in the polymer-based materials needs has induced along the waste accumulation, thus argued higher interest in recycling. The study aims to assess the structural, morphological, mechanical resistance, physical–chemical and biochemical characteristics, as well as the preservative role during the curd cheese storage of a recycled polylactic acid (PLA)-based ﬁlm modiﬁed with Ag-graphene-TiO nanostructured composite, obtained by recovering the composite from the used ﬁlm, followed by its incorporation in new PLA. The breaking load of the recycled ﬁlm was 24% lower than that of the new ﬁlm and 10% higher than of the neat PLA. Differential scanning calorimetry (DSC) showed changes of the recycled PLA’s surface tension and crystallization degree in a greater extent than in the newly prepared ﬁlm, revealing better incorporation of the recovered composite into the PLA matrix. Fourier transformed infrared spectroscopy showed the formation of C–O–Ti bridges between composite and PLA both in new and recycled ﬁlm. Oxygen transmission rate (OTR) of the new and recycled ﬁlm decreased by 33% and 45%, respectively, in comparison with reference PLA. The curd cheese was successfully stored in the recycled packag- ing; the organoleptic characteristics of cheese wrapped in recycled ﬁlm were superior in comparison with the new ﬁlm. The variation of fat and protein contents and mass loss was the lowest when the recycled ﬁlm was used as packaging material. The study successfully showed the possibility to recover and recycle the used PLA-based ﬁlms modiﬁed with inorganic nanocomposites. reduces its persistence in the environment and, consequently, 1. Introduction its impact. However, their effectiveness, as food packaging The sharp and constant increase of waste from nondegrad- materials, whether are not modiﬁed with active compounds, able plastics is a cruel reality, with negative effect over the is limited. For example, Ramos et al. [2] showed that the environmental components. Almost 448 million tons of non- presence of thymol and nano-Ag modiﬁes the thermal, opti- degradable plastics are discarded annually [1], which has cal, and barrier properties of polylactic acid (PLA) and prompted the need to ﬁnd new, efﬁcient, and fast solutions enhances its disintegration rate, in a higher extend, than in to reduce the plastic environmental impact. Thus, different the case of pristine PLA. Zabidi et al. [3] have obtained PLA approaches are under study at lab, pilot, and industrial scale, modiﬁed with cellulose and thymol/curry essential oil and which are mainly focused on two strategies: (1) using biode- they have demonstrated the antimicrobial and preservative gradable materials and (2) recycling. activity of the active ﬁlms during the tomato storage in com- Concerning biodegradable materials, their main advan- parison with the nonmodiﬁed PLA. Peter et al. [4] showed tage is the short degradability period (a few months), which that the best preservative role of PLA during the curd storage 2 Advances in Polymer Technology was achieved when activating with a content of the active (Ag- andeliminatedinthe form of CO and the composite materials graphene-TiO ) composite (in the range of 0.5–3wt%). More- based on TiO suffer reduced structural changes. Herein, we 2 2 over, the study reveals that the mechanical resistance was report a study aimed to recycle a modiﬁed PLA ﬁlm with Ag, enhanced by 30% by composite addition. Chi et al. [5] have graphene, and TiO composite and to establish the variations in demonstrated the preservative activity of PLA modiﬁed with structure, morphology, mechanical properties, and preservative essential oil, nano-Ag, and nano-TiO during the mango stor- activity during the curd cheese storage for a newly prepared and age in comparison with that of neat PLA by delaying the recycled ﬁlm. ﬁrmness loss, the color, total acidity, and vitamin C 2. Materials and Methods depreciation. The recycled materials are generating lower carbon foot- 2.1. Materials. TEOS, HNO , phenolphthalein, anhydrous potas- print than the pristine material and are providing unique sium sulfate (K SO ), selenium (Se), hydrochloric acid (HCl), 2 4 opportunity to obtain materials with high added value from and potassium bromide (KBr) were acquisitioned from Merck traditionally materials considered wastes, such as used food (Germany). Ninety-eight percent sulfuric acid (H SO )solution 2 4 packaging. Peter et al. [4] showed that the preservative role of was purchased from Lach-Ner, Czech Republic. Anhydrous the PLA-based ﬁlm for curd packaging, after the second use, ethanol (C H OH), AgNO , sodium hydroxide (NaOH), 35% 2 5 3 remained unchanged when the content of the active compos- hydrogen peroxide (H O ), boric acid (H BO ), and salicylic 2 2 3 3 ite into the PLA was 0.5 wt%. However, whether the percent of acid (C H O ) were purchased from Chemical Company 7 6 3 the active agent increased to 3 wt%, the active role of the (Romania). Ultrapure water was used for the gels preparation reused ﬁlm decreased to 85% from the maximum capacity and was prepared by using an equipment for ultrapure water of the new ﬁlm. Geetha et al. [6] showed that TiO –SO 2 4 Thermo Fisher Scientiﬁc (USA) device. Graphene powder was catalysts can be successfully used to piperidine synthesis GP500 Graphene Nanoplatelets from GrapheneTech (Spain). even ﬁve times, the process yield decreasing only by 2%. PLA NatureWorks 4043D (USA) has been used. The molecular Menon et al. [7] showed that the photocatalytic removal of weight of PLA 4043D from GPC is 154.378 g/mol [12]. estrogenic compounds over the TiO –ZnO nanocomposite is efﬁcient even after three cycles under visible irradiation with- 2.2. Preparation of the Nano-Ag-Graphene-TiO Composite. out loss in activity. The preparation procedure is described in our previous study Plavec et al. [1] showed that PLA-PHB blend can be [4]. The titanium dioxide sol was prepared by the sol–gel successfully recycled and the mechanical and thermal prop- method in acidic catalysis by TEOS, ultrapure water, anhy- erties were not negatively affected. Yan et al. [8] have dem- drous ethanol, and 63 wt% HNO . The molar ratio of the onstrated that CeO –TiO nanoarrays could be successfully reactants was TEOS:water:ethanol:HNO = 1 : 3 : 20 : 0.08. An 2 2 3 three times reused and also recycled in the water sterilization amount of 0.1 wt% graphene powder was, subsequently, process by keeping the same efﬁcacy. Liu et al. [9] reported added under continously magnetic stirring. The obtained black gels were immersed in 0.015 M AgNO solution for the recycling capacity of TiO -based ﬂocs modiﬁed with Al 2 3 and carbonate ions, having petal-like mesoporous structures 24 hr and were dried at 60°C and heat treated at 500°C for 2 hr. A black powder was obtained. and relatively high surface area and photoactivity. Khodanazary and Mohammadzadeh [10] and Izadi et al. 2.3. Preparation of the New Nano-Ag-Graphene-TiO -PLA [11] obtained active packaging based on PLA modiﬁed with Film. The nano-Ag-TiO -graphene composite was used to whey protein and ZnO nanoparticles [10] and with Thymus develop the Ag-graphene-TiO -PLA ﬁlm. A mixture of com- daenensis Celak, essential oil, beta-cyclodextrin, and nano-Ag, posite 0.5 wt% and PLA grains was prepared by using the respectively [11], that have been used successfully for ﬁsh and internal mixer equipment at 170°C. The homogenate was, beef meat storage. The modiﬁed PLA demonstrated a syner- subsequently, cooled down at room temperature. Then, the gistic effect in retarding the microbial growth and is preserv- mixture was extruded with a single screw extruder and the ing the organoleptical characteristics of the ﬁsh meat. obtained ﬁlament was chopped with a pelletizer in order to obtain Moreover, the efﬁcient entrapment of the essential oil in composite-PLA grains (∼2-mm diameter). Subsequently, the beta-cyclodextrin extended the shelf life of beef meat. composite-PLA ﬁlm was prepared by blow extrusion at 180°C. The novelty of this study lies in the fact that, at present, in the specialized literature, the information about the recyclability 2.4. Storage of the Curd Wrapped in the Nano-Ag-Graphene- of materials based on PLA is extremely low. The importance of TiO -PLA Film. The nano-Ag-graphene-TiO -PLA ﬁlm was 2 2 the inorganic composite recycling consists of the fact that the used for the curd storage according to the experimental con- precursors used for the Ag-graphene-TiO composite’sprepa- ditions described in our previous studies [4, 13]. The curd ration, namely titanium tetraisopropoxide (TEOS), silver nitrate cheese was homemade prepared according to the traditional (AgNO ), anhydrous ethanol, and nitric acid (HNO ), are very recipe used in the rural households from Northwestern 3 3 toxic. In this regard, the possibility of the inorganic composite’s Romania. An amount of 100 g curd cheese was wrapped in recycling will reduce the harmful effect of these substances both a 40 cm/40 cm sheet of nano-Ag-graphene-TiO -PLA ﬁlm over the researcher/worker that is preparing the composite and and the obtained packages were stored under UV-illuminated personal from the company that supplies these reagents. The refrigerator at 4°C. Samples were periodically analyzed in recycling procedure was applied due to the fact that it is known terms of organoleptic and physical–chemical changes (mass that by heat treatment at 500°C, the organic matter is charred loss, titratable acidity, dry mass, fat and protein contents). Advances in Polymer Technology 3 The organoleptic changes were monitored according was calculated by using the Kubelka–Munk equation. The to the Romanian Quality Standard for fresh cheese thermal stability of the composite was determined by thermo- SR3664:2008. Appearance, consistency, color, ﬂavor, gravimetric analysis (TGA)/differential scanning calorimetry and taste were evaluated and compared with the accepted (DSC). The photoactivity of the composite was carried out characteristics. during the photodegradation of salicylic acid under UV light illumination. The speciﬁcation of the abovementioned proce- Mass loss was determined by monitoring the difference dures is described in detail in our previous study [4]. of the package mass between day 0 (beginning) and 21 (ending) of the storage experiment. 2.8. Characterization of New and Recycled Nano-Ag-Graphene- Titratable acidity was determined by cheese titration TiO -PLA Film. The scanning electron microscopy (SEM) was with solution 0.1 N NaOH, in the presence of phenol- performed according to the method described in our previous phthalein 1%, until the pink color persisted at least 30 s. study [4]. The STEM measurements were performed using a The fat content was determined by using the solvent JEOL ARM 200CF STEM Cs-corrected system equipped with extractor VELP Scientiﬁca and anhydrous ethanol was a JEOL Centurio EDXS SDD spectrometer. The operating used as solvent. The working temperature was 210°C. voltage was 80 kV. A Gatan Quantum ER Dual EELS system The ethanolic solution obtained after extraction was was used to obtain the EELS spectra and to estimate the sam- dried in a desiccator until reaching the constant mass. ple thickness. Grammage, thickness, breaking length, tensile, tear, burst- The protein content was determined by Kjeldahl method ing, and folding resistance were performed according to our by using the VELP Scientiﬁca system. A mixture of 2 g previous study [14]. The grammage is the mass of the unit cheese, 7 g anhydrous potassium sulfate, 5 mg selenium, area of paper or paperboard determined by a speciﬁc test 7mL 98% H SO solution, and 5 mL 35% H O was 2 4 2 2 method. The breaking length was determined by using Elec- prepared for digestion, which took place at 420°C for tronic Testing System Instron 4411, according to Romanian 30 min. The distillation was performed in the presence ISO 1924-2:2009 standard. The bursting resistance was per- of 35% NaOH. The distillate was combined with 25 mL formed by using the Frank-Bursting Strength Tester 18530 4% H BO . The ﬁnal titration was performed with 0.2 N 3 3 F000 and according to Romanian SR EN ISO 2758:2015 stan- HCl in the presence of Tashiro indicator when a color dard. The folding endurance was determined by using the change from green into purple occurred. The protein Schopper type equipment according to the Romanian stan- content was calculated in mg of nitrogen in 1 g of cheese. dard SR ISO 5626:1996. DSC was performed according to our previous study 2.5. Recycling of the Nano-Ag-Graphene-TiO -PLA Film. The [14]. The degree of crystallinity (X ) was determined as fol- used nano-Ag-graphene-TiO -PLA packaging ﬁlm was cleaned lows: from the curd waste, washed three times with ultrapure water, rinsed for 1 min in anhydrous ethanol, and dried in air for 24 hr. ðÞ ΔH − ΔH m CC X ¼ × 100; ð1Þ The cleaned ﬁlm was calcinated in Carbolite furnance at 500°C ΔH° for 4 hr, the temperature increasing rate being 2°C/min. The removal of organic matter occurred during calcination and, where ΔH is melting enthalpy (J/g), ΔH is enthalpy of m CC ﬁnally, the inorganic composite (black powder) was obtained. cold crystallization (J/g), and ΔH° is enthalpy of crystalline Then, the recovered composite was included in new PLA fusion (i.e., 100% for PLA) (93 J/g) [15]. ﬁlm, according to the procedure presented in section “Prep- Oxygen transmission rate (OTR) was performed by cou- aration of the new nano-Ag-graphene-TiO -PLA ﬁlm.” lometric sensor method. The ﬁlm samples were cut using the template provided with the equipment. Before preparation, 2.6. Storage of Curd Wrapped in the Recycled Nano-Ag- the ﬁlm samples were conditioned in a desiccator containing Graphene-TiO -PLA Film. The recycled nano-Ag-graphene- anhydrous calcium chloride for a minimum of 48 hr. The TiO -PLA ﬁlm was used for the curd storage in identical ﬁlm sample was ﬁxed in the middle of test chamber to sepa- conditions with those presented in the section “Storage of rate the chamber into upper room and lower room. When the curd wrapped in the nano-Ag-graphene-TiO -PLA ﬁlm.” oxygen and nitrogen ﬂow in upper and lower rooms, respec- 2.7. Characterization of the Nano-Ag-Graphene-TiO Composite. tively, the oxygen molecules penetrate the ﬁlm sample into The nano-Ag-graphene-TiO composite was characterized as the lower room and the coulometric sensor system detects follows. Morphology was established by optical and electronic and analyzes the oxygen content and calculates the OTR. microscopy (scanning transmission electron microscopy When testing the container, oxygen is released outside and (STEM)-high-angle annular dark ﬁeld (HAADF)). Structure nitrogen inside of the container. The time required to reach was determined by performing Fourier transformed infrared steady state for OTR in sample was 64 hr. spectroscopy (FTIR) and X-ray diffraction (XRD) and by FTIR, ash content, pH, electrical conductivity, and anti- determining elemental composition by energy-dispersive oxidant activity were performed according to the procedure X-ray spectroscopy (EDXS). The optical analysis was evalu- detailed in a study by Peter et al. [14]. Water vapor perme- ated by performing UV–vis spectroscopy, and the gap energy ability (WVP) and grease permeability were determined 4 Advances in Polymer Technology according to the procedure described in a study by Peter observed at 60.2 Æ 2.2°C for neat PLA, at 58.1 Æ 3.3°C for et al. [4]. new ﬁlm, and at 59.7 Æ 4.8°C for the recycled ﬁlm. An exo- The barrier properties against UV–vis light was deter- thermic peak was formed at 121.9 Æ 9.2°C for the neat PLA, mined by using the PerkinElmer Lambda 35 Spectrophotom- at 117.6 Æ 8.1°C for the new ﬁlm, and at 95.2 Æ 3.4°C for the eter [9]. The experiment was performed in triplicate, and the recycled ﬁlm, due to the crystallization process of PLA, caus- opacity was determined by using the formula as follows: ing the reordering of the macromolecular chains from the PLA’s matrix and leading to mobility after thermal treatment Abs at 600 nm [16–18]. The endothermic peak corresponding to the melt- ð2Þ OpacityðÞ a:u: at 600 nm=mm¼ ; ing process is visible at 150.5 Æ 4.4°C for neat PLA, 148.1 Æ 7.9°C for the new ﬁlm, and at 178.6 Æ 10.2°C for the recycled ﬁlm. where Abs (absorbance) at 600 nm and x is ﬁlm thick- The incorporating Ag-graphene-TiO composite nano- ness (mm). particles slightly decreases the PLA’s T from 60.2 to 58.1°C in the new ﬁlm and to 59.7°C in the recycled ﬁlm when 2.9. Statistical Analysis. The characterization experiments, as compared to neat PLA (Table 2 and Figures S1–S3). This is well as those corresponding to the curd storage, were per- explained by the PLA chain shrinkage assigned by the inter- formed in triplicate and the values were reported as mean, by connectivity with the composite particles [19]. Similar results using the Excel program. The level of signiﬁcance was deter- were obtained by Deghiche et al. [17] and Nomai et al. [19]. mined by using the Statistica 7.0 software (StatSoft, Inc., The crystallization temperature (T ) of neat PLA and the CC Tulsa, USA) and one-way analysis of variance (ANOVA— new ﬁlm was similar (121.9 and 117.6°C, respectively), Tukey’s test). whereas that of the recycled ﬁlm decreased to 95.2°C. The crystallization degree increases in the new and recycled ﬁlms 3. Results and Discussion when compared to neat PLA because the composite nano- 3.1. Characterization of the New and Recycled Nano-Ag- particles act as nucleation centers [17]. It is a considerably Graphene-TiO -PLA Film 2 difference of X between the new and recycled ﬁlm, explained by the highly crystallized recovered composite, that induces a 3.1.1. Visual Appearance and Morphology of the Films. The more considerable crystallization in the recycled ﬁlm as in visual appearance of the new and recycled ﬁlms is shown in the new ﬁlm. There are differences in T and melting tem- cc Table 1. The image of the new ﬁlm clearly highlights the perature (T ), respectively, between the newly prepared ﬁlm presence of composite particles, having different dimensions, and the recycled ﬁlm. Moreover, the new ﬁlm has T and T cc m unevenly dispersed in the PLA matrix. The macroscopic values closer to the unmodiﬁed PLA than the recycled ﬁlm. image of the recycled ﬁlm is similar to that of the newly These differences are due to the elementary composition of prepared ﬁlm, that is, the composite particles distributed the newly prepared ﬁlm different than that of the recovered nonuniformly in the matrix and having different dimensions ﬁlm (Figures S1–S3). Also, differences between the compos- are observed. ite structure from new and recycled ﬁlm can be achieved in Table 1 also shows the SEM images of the new and the FTIR spectra (Figure S4). It was previously mentioned recycled samples. The observations are similar to those in that the recovered composite contains only traces of carbon, macroscopic analysis, meaning that composite particles with due to the heat treatment applied during the recovery, which dimensions of 10–20 nm in diameter are heterogeneously demonstrates that the graphene has been degraded. The distributed in the network. No difference in the morphology almost identical values of T determined for neat PLA and cc as a result of the recycling procedure could be observed. The new composite PLA demonstrate that the new Ag-graphene- STEM microscopy performed in our previous study [4] TiO composite did not induce signiﬁcant structural changes showed that the Ag-graphene-TiO composite contains tita- of the polymer matrix because of the strong PLA’ssurface nia and silver particles with approximately 20 and 5 nm in tension [20]. The signiﬁcant decrease in the T value (increase diameter, respectively, dispersed in the graphene matrix. cc in T ) achieved for recycled ﬁlm demonstrates that the com- The elemental analysis of the recycled and the new ﬁlm m position of the recovered material is structurally different from and of the neat PLA performed by EDXS analysis, as shown that of the new one and induced modiﬁcations in the PLA’s in Table 1, reveals that the difference of element content surface tension and crystallization in a greater extent than in between the new and recycled ﬁlm is statistically nonsigniﬁ- the newly prepared composite. Thus, it is inferred that the cant (one-way ANOVA—Tukey’s test). In contrast, the ele- recovered composite is better incorporated into the PLA mental composition of the newly prepared composite differs matrix than the new prepared one. signiﬁcantly from that of the recovered composite (Figure 1) in terms of carbon content. The explanation is the degrada- 3.1.3. FTIR Spectroscopy of the Films. There are characteristic tion occurred by carbon during the thermal treatment. signals corresponding to PLA, TiO , and graphene in the FTIR spectra of the new and recovered PLA-based ﬁlms, 3.1.2. Thermal Transitions of the Films (DSC). Figures S1–S3 show the DSC thermograms of the new and recycled PLA- showing the formation of the chemical connections between the PLA and the composite (Figure S4 and Table 3). The signals based and neat PLA ﬁlms in which the thermal transitions of −1 PLA are visible. The glass transition temperature (T ) was at 2,973, 1,750, 1,451, 1,182–1,190, 1,081, and 867–689 cm g Advances in Polymer Technology 5 Æ Æ Æ Æ Æ Æ TABLE 1: Images and elemental composition of the new and recycled nano-Ag-graphene-TiO -PLA ﬁlms. New Ag-graphene-TiO -PLA Recycled composite PLA Neat PLA Visual appearance SEM image a a a C(wt%)52 5 58 1 60 1 a a a O (wt%)47 4 42 2 40 1 a a Ti (wt%) 0.2 0.01 0.11 0.01 – a a Ag (wt%) 0.1 0.01 0.1 0.01 – In each line, mean values with superscript letter ( ) are statistical nonsigniﬁcant different at p<0:01 (one-way ANOVA—Tukey’s test). 6 Advances in Polymer Technology OTi C Ag New composite Recovered composite FIGURE 1: Elemental composition of the new and recovered Ag-graphene-TiO nanocomposite. TABLE 2: Thermal properties of the ﬁlms. Sample T (°C) T (°C) T (°C) ΔH (J/g) ΔH (J/g) Xc g CC m CC m a a a a a a Neat PLA 60.2 Æ 2.2 121.9 Æ 9.2 150.5 Æ 4.4 17.0 Æ 0.7 17.12 Æ 5.1 0.129 Æ 0.001 a b b b a b New ﬁlm 58.1 Æ 3.3 117.6 Æ 8.1 148.1 Æ 7.9 15.1 Æ 1.1 17.2 Æ 5.3 2.258 Æ 1.7 b c c c b c Recycled ﬁlm 59.7 Æ 4.8 95.2 Æ 3.4 178.6 Æ 10.2 21.2 Æ 1.9 32.2 Æ 3.9 11.827 Æ 4.8 T , glass transition temperature; T , crystallization temperature; T , melting temperature; ΔH , enthalpy of cold crystallization; ΔH , melting enthalpy. In g CC m CC m a,b,c each line, mean values with superscript letters ( ) are statistical nonsigniﬁcant different at p<0:01 (one-way ANOVA—Tukey’s test). TABLE 3: Speciﬁc signals and description from the FTIR spectra. −1 Peak (cm ) New ﬁlm Recycled ﬁlm Neat PLA Description 2,973 √√ √ Stretching vibration of aliphatic methyl groups from PLA [17] 1,750 √√ √ Stretching vibration of the C═O bond from the ester group from PLA [17, 21] 1,451 √√ √ 1,395 – √ – Vibration of ─Ti─O─C bonds between recovered composite and PLA 1,382 √ – √ ─O─C─O─ stretching from PLA [22] 1,368 – √ – Vibration of ─Ti─O─C bonds between recovered composite and PLA 1,303 √ (small) √ – Stretching of ─Ti─O─C included in the PLA matrix 1,182–1,190 √√ √ ─C─O─ stretching from PLA [17] 1,127 and 1,044 √ (shoulder) √ – Presence of composite into the PLA matrix 1,081 √√ √ Vibration of ─O─CH─CH from PLA [17] 952 √ (small) √√ (small) Presence of composite into the PLA matrix 867 √√ √ Vibration of C─C bond from the ─C─COO─ in PLA [21] Vibration of 751 √√ √ Ti─O─Ti and C─O─Ti [23] 689 √√ √ −1 found in all FTIR spectra are characteristic to the vibration of peaks at 1,395 and 1,368 cm found in the FTIR spectrum speciﬁc bonds in the PLA skeleton [17, 21]. of recovered ﬁlm. The peaks at 1,303, 1,127, 1,044, and −1 Speciﬁc signals assigned by the composite presence can 952 cm are small in the FTIR spectrum of the new ﬁlm −1 be found at 1,382, 1,303, 1,127, 1,044, and 952 cm and are but become stronger in that of the recovered ﬁlm. These assigned by the stretching of the Ti─O─C connections peaks are assigned to the interconnection of PLA with the [22, 23]. Moreover, the formation of these C─O─Ti bonds TiO network and the formation of C─O─Ti bridges [23] −1 is also demonstrated by the signals in the range of 600–900 cm and the different signal intensities demonstrate the different that overlaps with those of PLA. structures of the recovered composite in comparison with The FTIR spectra of the new ﬁlm and of the recycled ﬁlm the new one. are different in the regions at 1,368–1,395, 1,303, 1,127, These observations are supported by the DSC results, −1 −1 1,044, and 952 cm . Thus, the peak at 1,382 cm from which showed that the recycled composite, due to its differ- the FTIR spectra of new ﬁlm and of PLA is splitted in two ent compositions in comparison with the new one, altered Element content (%) Advances in Polymer Technology 7 TABLE 4: Characteristics of the new and recycled nano-Ag-graphene-TiO -PLA ﬁlms. Characteristic New ﬁlm Recycled ﬁlm Neat PLA 2 a a a Grammage (g/m 39.6 Æ 8.54 41.66 Æ 5.68 40.95 Æ 4.24 a a b Thickness (µm) 79 Æ 2.57 75 Æ 0.87 23 Æ 1.68 a b c Breaking length (%) according to SR EN ISO 1924-2:2009 4.11 Æ 0.45 6.19 Æ 0.42 15.28 Æ 2.45 a b a Tearing resistance (mN) according to SR EN ISO 1974:2012 120 Æ 5.46 190 Æ 4.23 100 Æ 3.61 a b c Slashing resistance (kPa) according to SR EN ISO 2758:2015 167 Æ 5.89 207 Æ 4.28 135 Æ 5.63 a b a Folding resistance (no.) according to SR ISO 5626:1996 3,484 Æ 21 3,998 Æ 35 3,452 Æ 42 3 2 a b c Oxygen transmission rate (OTR) (cm /m × day) 480 Æ 156 393 Æ 89 719 Æ 91 a a b Opacity (a.u. at 600 nm) 0.328 Æ 0.013 0.304 Æ 0.013 0.921 Æ 0.56 a a b Ash (%) 0.49 Æ 0.0045 0.4 Æ 0.088 0.03 Æ 0.001 a a a pH 5.86 Æ 0.04 5.72 Æ 0.089 5.91 Æ 0.09 a b c Electrical conductivity (µS/cm) 26.43 Æ 3.15 33.3 Æ 7.23 13.2 Æ 5.82 10 a b c Water vapor permeability (WVP) (g/s m Pa) × 10 6.903 Æ 0.85 7.98 Æ 1.05 3.13 Æ 0.85 Grease permeability (%)0 0 0 a b c Antioxidant activity (%) 13.3 Æ 0.24 10.15 Æ 0.37 14.57 Æ 0.26 a b c 20°C 5.54 Æ 0.54 0.4 Æ 0.01 3.52 Æ 0.57 Solubility in simulant A (distilled water) (%) a b c 4°C 8.6 Æ 0.79 2.4 Æ 0.08 3.99 Æ 0.42 a b c 20°C 5.54 Æ 0.42 12.1 Æ 1.47 3.36 Æ 0.42 Solubility in simulant B (acetic acid 3%)(%) a b c 4°C 7.81 Æ 1.23 6.2 Æ 0.21 1.91 Æ 0.21 a b c 20°C 3.8 Æ 0.22 1 Æ 0.04 3.61 Æ 0.47 Solubility in simulant C (ethanol 65%)(%) a b c 4°C 3.5 Æ 0.24 5.9 Æ 0.99 4.28 Æ 0.98 a,b,c In each line, mean values with different letters ( ) are signiﬁcantly different at p<0:01 (one-way ANOVA—Tukey’s test). the PLA’s surface tension and induced more efﬁcient chemi- new bonds arising between the PLA network and the com- cally connections with the PLA chain. posite particles, proven by the FTIR spectroscopy analysis, i.e., O─C─O─ and ─Ti─O─C─PLA bonds (Figure S4 and 3.1.4. Physical–Chemical and Biochemical Characteristics of Table 3), which physical hydrogen bonds are added to [25]. the Films. The characteristics of the new and recycled ﬁlm in All of these bonds restrict the mobility of PLA chains, comparison with neat PLA are shown in Table 4. The differ- increase its strength, and ensure the matrix’s rigidity. This ence between the two ﬁlm types and neat PLA in terms of explains why the elongation at break of the modiﬁed ﬁlm is grammage, pH, and fat permeability is statistically nonsig- much lower in comparison to that of the neat PLA. niﬁcant. However, the opacity and the ash content variation OTR of the modiﬁed ﬁlms has decreased considerably by are also statistically nonsigniﬁcant when comparing the new 33% (in the new ﬁlm) and 45% (in the recycled ﬁlm), respec- ﬁlm with the recycled ﬁlm. tively, compared to PLA (Table 4). Among the modiﬁed In contrast, the difference of mechanical characteristics, ﬁlms, OTR of the recycled ﬁlm is 18.1% lower than of the OTR, WVP, solubility in food simulants, electrical conduc- new ﬁlm. This is correlated with the mechanical resistance tivity, and antioxidant activity is statistically signiﬁcant in a characteristics, i.e., the recycled ﬁlm is least permeable for quantum higher than 99% (p<0:01, one-way ANOVA— oxygen and the most mechanically resistant due to the com- Tukey’s test) between new and recycled ﬁlms. paction of the composite PLA network in a greater extent The largest tear elongation was determined for neat PLA, than in the case of the new prepared ﬁlm (Table 4). Subse- 73% and 59% higher than for the newly prepared ﬁlm and for quently, this is a result of different chemical compositions the recycled ﬁlm, respectively. When comparing the modi- and structures of the recovered composite when compared to ﬁed ﬁlms, it can be observed that the tear elongation of the the new one, a fact conﬁrmed by the EDXS results. The recycled ﬁlm is 50% higher than of the new ﬁlm. presence of the graphene network in the newly prepared All the other mechanical strength parameters of the ﬁlm affects its structure, in that, having a non-polar charac- recycled ﬁlm are superior to those of the new ﬁlm: the tear ter, the carbon network creates, together with the non-polar resistance is 58.33% higher, the burst resistance is 24% methyl groups of PLA, repulsions that make the matrix higher, and the bending resistance is 14.7% higher. The unable to compact. increase in mechanical resistance properties as a result of The WVP of the new and recovered ﬁlms is 2.2 and 2.5 the PLA’s additivation was also reported in literature, follow- times, respectively, higher than that of PLA’s (Table 4). This ing the addition of multiscale cellulosic biocomposites [21], is a negative aspect explained by the elemental composition chitosan [24], or halloysite nanotubes [25]. The mechanical and structure of the new prepared composite different from resistance of the recycled ﬁlm is higher than of the new and the recovered one. In the newly prepared composite, the unmodiﬁed PLA ﬁlm and is explained by the multitude of presence of graphene inserted in the PLA’s network provides 8 Advances in Polymer Technology added hydrophobic role to the composite matrix. Instead, in disadvantage is considerably reduced in the recycled ﬁlm (Figure 2b) due to the graphene’s very low concentration. the recycled ﬁlm, graphene was broken down during the recovery; thus, Ti, Ag, and traces of C remained only and This explains why the recycled ﬁlm has superior mechanical properties to the newly prepared ﬁlm. Moreover, the active the hydrophobic supply was considerably reduced. The antioxidant activity of the modiﬁed ﬁlms is lower carbonyls from the proximity of TiO and Ag active centers stimulate the electronic transfer and intensify the formation than that of the unmodiﬁed ﬁlms, and, moreover, that of the recycled ﬁlm is smaller than that of the newly prepared ﬁlm. of reactive oxygen anions, demonstrated by appearing of new peaks in the FTIR spectrum of the recovered ﬁlm. In general, the solubility in food simulants of the recycled ﬁlm is lower than that of the newly prepared ﬁlm, even that 4. Results of Curd Packaging in the New and the of the unmodiﬁed one, which is also explained by the com- Recycled Films posite network different from that of the newly prepared ﬁlm. So, it can be appreciated that the mechanical resistance, 4.1. Visual Appearance of the Curd Cheese. The curd cheese oxygen barrier, and food simulants properties of the recycled samples packed in each of the three types of packaging did ﬁlm are superior to the newly prepared ﬁlm, due to the fact not change from the organoleptical point of view during that another type of composite with different elemental com- 14 days of refrigeration (Tables 5 and 6). positions was obtained during the recovery procedure, that From day 14 to 21, signiﬁcant organoleptic changes have created superior qualitative and quantitative connections in occurred, especially in the case of the cheese kept in the neat the PLA matrix. This was also supported by DSC and FTIR PLA. Thus, after 14 days of storage, a positive inﬂuence of the results. modiﬁed PLA on the cheese’s organoleptical properties has The behavioral difference between the newly prepared been observed in comparison to that of the sample from the ﬁlm and the recycled ﬁlm lies in the following mechanism. unchanged PLA, assigned by the presence of the composite The chemical reactivity and kinetic stability of the compo- Ag-graphene-TiO . The photocatalytic activity of TiO [4] and 2 2 nent phases from the ﬁlm and the composite are explained the antimicrobial efﬁciency of Ag and graphene are already by the electron occupation of HOMO and LUMO molecular known [14, 19, 20, 24]. The cheese in PLA changed its color orbitals. HOMO is the most electron-occupied molecular from white to yellow, became crumbling and acquired, a orbital, and LUMO is the poorest in electrons. Reactivity is strong fermented smell. In contrast, the new ﬁlm sample due to the ability of electrons to be transferred from HOMO removed a little whey during storage and became yellowish. to LUMO and increases with the reduction of the energy The sample in the recycled ﬁlm suffered the fewest changes difference between HOMO and LUMO. In PLA, HOMO is after 21 days of storage. So, from the organoleptic point of located in the carbonyl groups C = O, which are the most view, PLA and the newly prepared ﬁlm ensure a shelf life of reactive and serve as electron donors [17]. In TiO , HOMO 14 days for cheese and the recycled one of 21 days, according to and LUMO are found in the titanium atoms and act as SR3664:2008. electron acceptors. In graphene, HOMO is in the carbon cycles and acts as electron donors [26]. In the new ﬁlm 4.2. Physical–Chemical Characterization of the Curd Cheese Ag-graphene-TiO -PLA (Figure 2a), as reported, there are 2 Wrapped in the New and Recycled Films. The variation of the reactive and polar areas (carbonyl groups) but also nonpolar physical–chemical parameters of the curd cheese kept in the areas (methyl groups). The carbonyl reactive areas, which act investigated ﬁlms is shown in Figure 3(a)–3(e). The moni- as electron donors, transfer the electrons to the Ti atoms tored parameters are standardized according to the Roma- (pathway A), by activating them, as the graphene network nian SR 3664 from 2008 in force, namely mass loss, titratable attached to the semiconductor (pathway B). The two path- acidity, dry mass, and fat and protein contents. ways of TiO activation are also added to photochemical When taking into account the values obtained at each activation under the action of UV radiation in the solar time interval, the one-way ANOVA analysis—Tukey’s test spectrum (pathway C) [27]. Ag nanoparticles attached to model showed that the variation of mass loss (Figure 3(a)), the TiO network stimulate the electronic transfer, reduce 2 dry matter (Figure 3(c)), fat (Figure 3(d)), and protein the recombination of charge carriers, and initiate the reduc- (Figure 3(e)) of the curd cheese, depending on the wrapping tion processes. The electrons activated by the nano-Ag can, material, is statistically insigniﬁcant. subsequently, be involved in two ways: (1) reduce the carbo- If only 21-day values were taken into account, differences cations in the PLA structure, formed due to the polarization in the mass loss of cheese samples packed in the three pack- of the carbonyl bond, generating oxygen anions that alter the age types were statistically signiﬁcant higher than 99%, while PLA’s surface state and affect its surface tension, as demon- those corresponding to the dry matter were statistically non- strated by the DSC results and (2) reduce species in the signiﬁcant. The differences in fat content are statistically environment surrounding the ﬁlm. Also, the connection of nonsigniﬁcant when compared to the sample wrapped in the PLA network with TiO is made by a considerably number the recycled ﬁlm and in the newly prepared ﬁlm, respectively, of hydrogen bonds [17, 28]. The presence of the graphene but that between the sample from newly prepared ﬁlm and network in the newly prepared ﬁlm has a disadvantage, that stored in the unchanged PLA is signiﬁcant (>99%). namely the fact that having a nonpolar character, the carbon When considering the protein variation as a function of network creates, next to the nonpolar methyl groups of PLA, the ﬁlm type, at 21 days, the difference between the sample in repulses that make the matrix unable to compact. This the recycled ﬁlm and that from the newly prepared ﬁlm is Advances in Polymer Technology 9 New prepared PLA-Ag-graphene-TiO film Graphene Nonpolar Repulsion Repulsion Repulsion Nonpolar CH CH CH CH CH CH 3 3 3 3 3 3 PLA chain OCH C O CH C O CH C O CH CO CH C OCH C OCH C O CH C OC CH OCH CO CH C O CH C O CH C e donor O CH O O CH O CH O O CH O O CH O O CH O O CH O 3 3 3 3 3 3 3 A transfer PLA’s surface state e transfer alteration (1) Ag Ag (2) − − − e e e − Ag CB e transfer transfer Species from the medium e donor surrounding the film UV e transfer TiO e acceptor VB + + + h h h ðaÞ Recycled film prepared from PLA with recovered composite CH CH CH CH CH CH 3 3 3 3 3 3 PLA chain OCH C O CH C O CH C O CH CO CH C OCH C OCH C O CH C OC CH OCH CO CH C O CH C O CH C e donor O CH O CH CH O O CH O O CH O O CH O O CH O O O 3 3 3 3 3 3 transfer A PLA’s surface state e transfer alteration 1) Ag Ag 2) − − − e e e − Ag CB e transfer transfer Species from the medium surrounding the film UV e transfer TiO e acceptor VB + + + h h h ðbÞ FIGURE 2: Bonds formation in the new prepared ﬁlm (a) and recycled ﬁlm (b). signiﬁcant (>95%), as well as that between the cheese from ﬁlm (17.2%). The highest capacity to preserve the protein recycled ﬁlm and neat PLA. content was provided by the recycled ﬁlm, as a result of The highest variation in the mass loss after 21 days of the ﬁlm structure and the most intense degree of compaction storage was registered in the curd cheese kept in the new ﬁlm (Figure 2(b)) compared to the newly prepared ﬁlm (Figure 2(a)). (11.6%) and the lowest in that kept in the recycled ﬁlm Thus, the OTR and solubility (Table 4) of the recycled ﬁlm (4.6%). The most accentuated decrease in fat content occurred signiﬁcantly lower in comparison to the newly prepared ﬁlm, in curd cheese kept in the neat PLA (10.23%), while the lowest reduced the oxygen penetration and the ﬁlm dissolution, in the recycled ﬁlm (1.63%). respectively, and implicitly the intensity of oxidative processes The same variation was determined for the protein con- in the intrapackaging space. tent, so the protein was the most severely degraded during When analyzing the curd cheese’s titratable acidity as a the storage in the neat PLA and in the new ﬁlm (25.35%) and function of the packaging material (Figure 3(b)), it is noticed the least affected during the cheese packaging in the recycled that variation of the acidity from the recycled ﬁlm is Oxidation Oxidation 10 Advances in Polymer Technology TABLE 5: Visual appearance of the curd cheese wrapped in the new and recycled ﬁlms and neat PLA under refrigeration. Period (day) New ﬁlm Recycled ﬁlm Neat PLA 14 Advances in Polymer Technology 11 TABLE 5: Continued. Period (day) New ﬁlm Recycled ﬁlm Neat PLA 21 12 Advances in Polymer Technology TABLE 6: Organoleptic properties of the curd cheese stored in the new and recycled ﬁlms and neat PLA under refrigeration, established based on the Romanian Quality Standard for fresh cheese SR3664:2008. Period (day) Characteristics New ﬁlm Recycled ﬁlm Neat PLA Appearance Homogeneous, clean paste without whey leakage Consistency Fine paste with a low grounty consistency 0 and 7 Color White Flavor Pleasant Taste Pleasant, taste of lactic fermentation, with no foreign tastes (acidic, bitter, of mold and yeast, smocked, etc.) Appearance Consistency 14 Color No changes when compared with those from day 0 Flavor Taste Appearance Homogeneous, with a very reduced whey leakage Homogeneous, with whey leakage Consistency Slightly crumbling Fine paste Crumbling Color White-yellowish White Yellowish Flavor A little bit pungent Pleasant, with no foreign tastes Acidic and of yeasts 14 230.0 210.0 190.0 170.0 150.0 6 130.0 110.0 90.0 70.0 50.0 0 510 15 20 0 5 10 15 20 Time (days) Time (days) Recycled film New film Recycled film New film Neat PLA Neat PLA ðaÞ ðbÞ 33.5 31.00 30.50 33.0 30.00 32.5 29.50 32.0 29.00 31.5 28.50 31.0 28.00 30.5 27.50 30.0 Minimum accepted limit according to SR 3664 : 2008 Minimum accepted limit according to SR 3664 : 2008 27.00 29.5 26.50 29.0 26.00 0 5 10 15 20 0 5 10 15 20 Time (days) Time (days) Recycled film New film Recycled film New film Neat PLA Neat PLA ðcÞ ðdÞ FIGURE 3: Continued. Dry mass (%) Mass loss (%) Fat content (%) Titratable acidity (Thorner degrees) Advances in Polymer Technology 13 Minimum accepted limit according to SR 3664 : 2008 0 510 15 20 Time (days) Recycled film New film Neat PLA ðeÞ FIGURE 3: Physical–chemical parameters of the curd stored in the recycled and new ﬁlms and in neat PLA: (a) mass loss, (b) titratable acidity, (c) dry mass, (d) fat, and (e) protein. signiﬁcant in comparison with that from the new ﬁlm during compared to day 0, but the nonmodiﬁed grapes were deeply 21 days of storage (p<0:05). However, the acidity evolution altered. of the cheese from new ﬁlm and the neat PLA is statistically Both the results of structural, thermal, and mechanical resistance analyses and the results obtained when storing nonsigniﬁcant. Thus, the acidity of the cheese kept in the recycled ﬁlm increased by 40.5% after 21 days of refrigera- cheese demonstrate that the recycled ﬁlm has a more con- servative role on the cheese than the newly prepared one, tion, while in the case of the new ﬁlm and neat PLA, it which further encourages the recycling. decreased by 28.8% and 59.1%, respectively. The rise of acid- ity during storage was also reported by Jafarzadeh et al. [29] 5. Conclusions who coated active edible ﬁlm based on whey protein modi- ﬁed with white tea extract on the fresh cottage cheese. This The study aims to assess the structural, morphological, behavior could be explained by the fact that the recycled mechanical resistance, physical–chemical and biochemical packaging ﬁlm did not affect the functionality of lactic bac- characteristics, as well as the preservative role on curd cheese teria, as the newly prepared ﬁlm or neat PLA. This is due to of a recycled PLA ﬁlm, obtained by recovering the nano-Ag- the different compositions of the recycled composition and graphene-TiO composite from the used PLA-based ﬁlm, the different structures of the recycled ﬁlm compared to the followed by its incorporation in the new PLA network. Visu- new one, demonstrated by DSC and FTIR analyses and illus- ally, the recycled ﬁlm looked just like the new one. The trated in mechanism, as shown in Figures 2(a) and 2(b). The elementary quantitative analysis of the newly prepared and maximum accepted limit of titratable acidity and the mini- the recovered Ag-graphene-TiO composite, used as active mum accepted limit of dry matter, fat, and protein according compound in the PLA ﬁlm, showed that the graphene was to SR 3664:2008 have not been reached in the case of any carbonized during the recovery procedure, so the newly pre- cheese sample, so the curd cheese can be stored safely for pared composite is compositionally different from the newly 21 days in all three types of studied packaging materials. prepared one. This induced variable DSC and FTIR results Paulsen et al. [30] who monitored the evolution of broc- was manifested by: (1) higher crystallization of the recycled coli under three different storage conditions (LLDPE pack- ﬁlm network than the newly prepared one, (2) recovered aging, biodegradable packaging containing PBAT/PLA, and composite reduced the surface tension of the PLA in a higher unpackaged) demonstrated that the smallest changes in organ- extend than the newly prepared composite, which showed oleptic characteristics, mass loss, oxygen and carbon dioxide that the recovered composite is better incorporated into the evolution, ﬁrmness, chlorophyll, and carotenoid content of PLA network, and (3) new bonds appear in the recycled ﬁlm, broccoli were recorded during its storage in LLDPE and in which also demonstrates that the recycled ﬁlm is more com- biodegradable ﬁlm, highlighting that the PBAT/PLA has pact than the new one. The mechanical and barrier resistance similar conservation characteristics to LLDPE and is also properties of the recycled ﬁlm superior to the newly prepared biodegradable. one are due to the new connections appeared in the matrix Zheng et al. [31] have successfully obtained active ﬁlms following recycling, ensuring a high compaction degree. The containing a cocktail of biodegradable materials (PBPA, PLA, curd cheese has been successfully stored in the recycled pack- PCL) modiﬁed with TiO and natamycin, with mechanical aging for 21 days, the organoleptic characteristics being and microbiological properties superior to the conventional superior to those of cheese kept in new ﬁlm or neat PLA. ﬁlms and showed their strong preservative effect on the The recycled packaging reduced the protein degradation of grapes. After 30 days of storage at 0.5°C, the grapes kept in cheese by 8% compared to the newly prepared one. 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Li, “The reuse of nano-TiO 2− visualization; Leonard Mihaly Cozmuta—statistical analysis; under different concentration of CO using coagulation Nicula Camelia—methodology and visualization; Goran Drazic, process and its photocatalytic ability in treatment of methyl Antonio Penas, and Stefania Silvi—supervision. orange,” Separation and Puriﬁcation Technology, vol. 282, Part B, Article ID 120152, 2022. Acknowledgments [10] A. Khodanazary and B. Mohammadzadeh, “The effects of polylactic acid-whey protein isolated bi-layer ﬁlm incorpo- This work was supported by the Technical University of rated with ZnO nanoparticles on the quality of common Cluj-Napoca (UEFISCDI contract no 72/2017), National carp Cyprinus carpio,” Journal of Food Measurement and Characterization, vol. 17, pp. 4684–4694, 2023. Institute of Chemistry, Ljubljana, Slovenia (MIZS 4126), [11] F. Izadi, M. Pajohi-Alamoti, A. Emamifar, and A. 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Publisher: Wiley
ISSN: 1098-2329
eISSN: 0730-6679
DOI: 10.1155/2023/9937270
Publisher site: See Article on Publisher Site

Abstract

Hindawi Advances in Polymer Technology Volume 2023, Article ID 9937270, 15 pages https://doi.org/10.1155/2023/9937270 Research Article Polylactic Acid-Based Film Modiﬁed with Nano-Ag-Graphene-TiO : New Film versus Recycled Film 1 2 1 2 Anca Peter , Camelia Nicula , Anca Mihaly Cozmuta , Goran Drazic , 3 4 1 Antonio Peñas , Stefania Silvi , and Leonard Mihaly Cozmuta Technical University of Cluj Napoca, Faculty of Sciences, Victoriei 76, 430072, Baia Mare, Romania National Institute of Chemistry, Hajdrihova 19 P.O. Box 660 SI-1001, Ljubljana, Slovenia Andaltec, Pol. Ind. Cañada de la Fuente Vílches s/n, 23600, Martos-Jaén, Spain University of Camerino, School of Bioscience and Veterinary Medicine, Gentile III da Varano, MC 62032, Camerino, Italy Correspondence should be addressed to Anca Peter; [email protected] Received 11 August 2023; Revised 26 October 2023; Accepted 30 October 2023; Published 25 November 2023 Academic Editor: Jun Ling Copyright © 2023 Anca Peter et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The increase in the polymer-based materials needs has induced along the waste accumulation, thus argued higher interest in recycling. The study aims to assess the structural, morphological, mechanical resistance, physical–chemical and biochemical characteristics, as well as the preservative role during the curd cheese storage of a recycled polylactic acid (PLA)-based ﬁlm modiﬁed with Ag-graphene-TiO nanostructured composite, obtained by recovering the composite from the used ﬁlm, followed by its incorporation in new PLA. The breaking load of the recycled ﬁlm was 24% lower than that of the new ﬁlm and 10% higher than of the neat PLA. Differential scanning calorimetry (DSC) showed changes of the recycled PLA’s surface tension and crystallization degree in a greater extent than in the newly prepared ﬁlm, revealing better incorporation of the recovered composite into the PLA matrix. Fourier transformed infrared spectroscopy showed the formation of C–O–Ti bridges between composite and PLA both in new and recycled ﬁlm. Oxygen transmission rate (OTR) of the new and recycled ﬁlm decreased by 33% and 45%, respectively, in comparison with reference PLA. The curd cheese was successfully stored in the recycled packag- ing; the organoleptic characteristics of cheese wrapped in recycled ﬁlm were superior in comparison with the new ﬁlm. The variation of fat and protein contents and mass loss was the lowest when the recycled ﬁlm was used as packaging material. The study successfully showed the possibility to recover and recycle the used PLA-based ﬁlms modiﬁed with inorganic nanocomposites. reduces its persistence in the environment and, consequently, 1. Introduction its impact. However, their effectiveness, as food packaging The sharp and constant increase of waste from nondegrad- materials, whether are not modiﬁed with active compounds, able plastics is a cruel reality, with negative effect over the is limited. For example, Ramos et al. [2] showed that the environmental components. Almost 448 million tons of non- presence of thymol and nano-Ag modiﬁes the thermal, opti- degradable plastics are discarded annually [1], which has cal, and barrier properties of polylactic acid (PLA) and prompted the need to ﬁnd new, efﬁcient, and fast solutions enhances its disintegration rate, in a higher extend, than in to reduce the plastic environmental impact. Thus, different the case of pristine PLA. Zabidi et al. [3] have obtained PLA approaches are under study at lab, pilot, and industrial scale, modiﬁed with cellulose and thymol/curry essential oil and which are mainly focused on two strategies: (1) using biode- they have demonstrated the antimicrobial and preservative gradable materials and (2) recycling. activity of the active ﬁlms during the tomato storage in com- Concerning biodegradable materials, their main advan- parison with the nonmodiﬁed PLA. Peter et al. [4] showed tage is the short degradability period (a few months), which that the best preservative role of PLA during the curd storage 2 Advances in Polymer Technology was achieved when activating with a content of the active (Ag- andeliminatedinthe form of CO and the composite materials graphene-TiO ) composite (in the range of 0.5–3wt%). More- based on TiO suffer reduced structural changes. Herein, we 2 2 over, the study reveals that the mechanical resistance was report a study aimed to recycle a modiﬁed PLA ﬁlm with Ag, enhanced by 30% by composite addition. Chi et al. [5] have graphene, and TiO composite and to establish the variations in demonstrated the preservative activity of PLA modiﬁed with structure, morphology, mechanical properties, and preservative essential oil, nano-Ag, and nano-TiO during the mango stor- activity during the curd cheese storage for a newly prepared and age in comparison with that of neat PLA by delaying the recycled ﬁlm. ﬁrmness loss, the color, total acidity, and vitamin C 2. Materials and Methods depreciation. The recycled materials are generating lower carbon foot- 2.1. Materials. TEOS, HNO , phenolphthalein, anhydrous potas- print than the pristine material and are providing unique sium sulfate (K SO ), selenium (Se), hydrochloric acid (HCl), 2 4 opportunity to obtain materials with high added value from and potassium bromide (KBr) were acquisitioned from Merck traditionally materials considered wastes, such as used food (Germany). Ninety-eight percent sulfuric acid (H SO )solution 2 4 packaging. Peter et al. [4] showed that the preservative role of was purchased from Lach-Ner, Czech Republic. Anhydrous the PLA-based ﬁlm for curd packaging, after the second use, ethanol (C H OH), AgNO , sodium hydroxide (NaOH), 35% 2 5 3 remained unchanged when the content of the active compos- hydrogen peroxide (H O ), boric acid (H BO ), and salicylic 2 2 3 3 ite into the PLA was 0.5 wt%. However, whether the percent of acid (C H O ) were purchased from Chemical Company 7 6 3 the active agent increased to 3 wt%, the active role of the (Romania). Ultrapure water was used for the gels preparation reused ﬁlm decreased to 85% from the maximum capacity and was prepared by using an equipment for ultrapure water of the new ﬁlm. Geetha et al. [6] showed that TiO –SO 2 4 Thermo Fisher Scientiﬁc (USA) device. Graphene powder was catalysts can be successfully used to piperidine synthesis GP500 Graphene Nanoplatelets from GrapheneTech (Spain). even ﬁve times, the process yield decreasing only by 2%. PLA NatureWorks 4043D (USA) has been used. The molecular Menon et al. [7] showed that the photocatalytic removal of weight of PLA 4043D from GPC is 154.378 g/mol [12]. estrogenic compounds over the TiO –ZnO nanocomposite is efﬁcient even after three cycles under visible irradiation with- 2.2. Preparation of the Nano-Ag-Graphene-TiO Composite. out loss in activity. The preparation procedure is described in our previous study Plavec et al. [1] showed that PLA-PHB blend can be [4]. The titanium dioxide sol was prepared by the sol–gel successfully recycled and the mechanical and thermal prop- method in acidic catalysis by TEOS, ultrapure water, anhy- erties were not negatively affected. Yan et al. [8] have dem- drous ethanol, and 63 wt% HNO . The molar ratio of the onstrated that CeO –TiO nanoarrays could be successfully reactants was TEOS:water:ethanol:HNO = 1 : 3 : 20 : 0.08. An 2 2 3 three times reused and also recycled in the water sterilization amount of 0.1 wt% graphene powder was, subsequently, process by keeping the same efﬁcacy. Liu et al. [9] reported added under continously magnetic stirring. The obtained black gels were immersed in 0.015 M AgNO solution for the recycling capacity of TiO -based ﬂocs modiﬁed with Al 2 3 and carbonate ions, having petal-like mesoporous structures 24 hr and were dried at 60°C and heat treated at 500°C for 2 hr. A black powder was obtained. and relatively high surface area and photoactivity. Khodanazary and Mohammadzadeh [10] and Izadi et al. 2.3. Preparation of the New Nano-Ag-Graphene-TiO -PLA [11] obtained active packaging based on PLA modiﬁed with Film. The nano-Ag-TiO -graphene composite was used to whey protein and ZnO nanoparticles [10] and with Thymus develop the Ag-graphene-TiO -PLA ﬁlm. A mixture of com- daenensis Celak, essential oil, beta-cyclodextrin, and nano-Ag, posite 0.5 wt% and PLA grains was prepared by using the respectively [11], that have been used successfully for ﬁsh and internal mixer equipment at 170°C. The homogenate was, beef meat storage. The modiﬁed PLA demonstrated a syner- subsequently, cooled down at room temperature. Then, the gistic effect in retarding the microbial growth and is preserv- mixture was extruded with a single screw extruder and the ing the organoleptical characteristics of the ﬁsh meat. obtained ﬁlament was chopped with a pelletizer in order to obtain Moreover, the efﬁcient entrapment of the essential oil in composite-PLA grains (∼2-mm diameter). Subsequently, the beta-cyclodextrin extended the shelf life of beef meat. composite-PLA ﬁlm was prepared by blow extrusion at 180°C. The novelty of this study lies in the fact that, at present, in the specialized literature, the information about the recyclability 2.4. Storage of the Curd Wrapped in the Nano-Ag-Graphene- of materials based on PLA is extremely low. The importance of TiO -PLA Film. The nano-Ag-graphene-TiO -PLA ﬁlm was 2 2 the inorganic composite recycling consists of the fact that the used for the curd storage according to the experimental con- precursors used for the Ag-graphene-TiO composite’sprepa- ditions described in our previous studies [4, 13]. The curd ration, namely titanium tetraisopropoxide (TEOS), silver nitrate cheese was homemade prepared according to the traditional (AgNO ), anhydrous ethanol, and nitric acid (HNO ), are very recipe used in the rural households from Northwestern 3 3 toxic. In this regard, the possibility of the inorganic composite’s Romania. An amount of 100 g curd cheese was wrapped in recycling will reduce the harmful effect of these substances both a 40 cm/40 cm sheet of nano-Ag-graphene-TiO -PLA ﬁlm over the researcher/worker that is preparing the composite and and the obtained packages were stored under UV-illuminated personal from the company that supplies these reagents. The refrigerator at 4°C. Samples were periodically analyzed in recycling procedure was applied due to the fact that it is known terms of organoleptic and physical–chemical changes (mass that by heat treatment at 500°C, the organic matter is charred loss, titratable acidity, dry mass, fat and protein contents). Advances in Polymer Technology 3 The organoleptic changes were monitored according was calculated by using the Kubelka–Munk equation. The to the Romanian Quality Standard for fresh cheese thermal stability of the composite was determined by thermo- SR3664:2008. Appearance, consistency, color, ﬂavor, gravimetric analysis (TGA)/differential scanning calorimetry and taste were evaluated and compared with the accepted (DSC). The photoactivity of the composite was carried out characteristics. during the photodegradation of salicylic acid under UV light illumination. The speciﬁcation of the abovementioned proce- Mass loss was determined by monitoring the difference dures is described in detail in our previous study [4]. of the package mass between day 0 (beginning) and 21 (ending) of the storage experiment. 2.8. Characterization of New and Recycled Nano-Ag-Graphene- Titratable acidity was determined by cheese titration TiO -PLA Film. The scanning electron microscopy (SEM) was with solution 0.1 N NaOH, in the presence of phenol- performed according to the method described in our previous phthalein 1%, until the pink color persisted at least 30 s. study [4]. The STEM measurements were performed using a The fat content was determined by using the solvent JEOL ARM 200CF STEM Cs-corrected system equipped with extractor VELP Scientiﬁca and anhydrous ethanol was a JEOL Centurio EDXS SDD spectrometer. The operating used as solvent. The working temperature was 210°C. voltage was 80 kV. A Gatan Quantum ER Dual EELS system The ethanolic solution obtained after extraction was was used to obtain the EELS spectra and to estimate the sam- dried in a desiccator until reaching the constant mass. ple thickness. Grammage, thickness, breaking length, tensile, tear, burst- The protein content was determined by Kjeldahl method ing, and folding resistance were performed according to our by using the VELP Scientiﬁca system. A mixture of 2 g previous study [14]. The grammage is the mass of the unit cheese, 7 g anhydrous potassium sulfate, 5 mg selenium, area of paper or paperboard determined by a speciﬁc test 7mL 98% H SO solution, and 5 mL 35% H O was 2 4 2 2 method. The breaking length was determined by using Elec- prepared for digestion, which took place at 420°C for tronic Testing System Instron 4411, according to Romanian 30 min. The distillation was performed in the presence ISO 1924-2:2009 standard. The bursting resistance was per- of 35% NaOH. The distillate was combined with 25 mL formed by using the Frank-Bursting Strength Tester 18530 4% H BO . The ﬁnal titration was performed with 0.2 N 3 3 F000 and according to Romanian SR EN ISO 2758:2015 stan- HCl in the presence of Tashiro indicator when a color dard. The folding endurance was determined by using the change from green into purple occurred. The protein Schopper type equipment according to the Romanian stan- content was calculated in mg of nitrogen in 1 g of cheese. dard SR ISO 5626:1996. DSC was performed according to our previous study 2.5. Recycling of the Nano-Ag-Graphene-TiO -PLA Film. The [14]. The degree of crystallinity (X ) was determined as fol- used nano-Ag-graphene-TiO -PLA packaging ﬁlm was cleaned lows: from the curd waste, washed three times with ultrapure water, rinsed for 1 min in anhydrous ethanol, and dried in air for 24 hr. ðÞ ΔH − ΔH m CC X ¼ × 100; ð1Þ The cleaned ﬁlm was calcinated in Carbolite furnance at 500°C ΔH° for 4 hr, the temperature increasing rate being 2°C/min. The removal of organic matter occurred during calcination and, where ΔH is melting enthalpy (J/g), ΔH is enthalpy of m CC ﬁnally, the inorganic composite (black powder) was obtained. cold crystallization (J/g), and ΔH° is enthalpy of crystalline Then, the recovered composite was included in new PLA fusion (i.e., 100% for PLA) (93 J/g) [15]. ﬁlm, according to the procedure presented in section “Prep- Oxygen transmission rate (OTR) was performed by cou- aration of the new nano-Ag-graphene-TiO -PLA ﬁlm.” lometric sensor method. The ﬁlm samples were cut using the template provided with the equipment. Before preparation, 2.6. Storage of Curd Wrapped in the Recycled Nano-Ag- the ﬁlm samples were conditioned in a desiccator containing Graphene-TiO -PLA Film. The recycled nano-Ag-graphene- anhydrous calcium chloride for a minimum of 48 hr. The TiO -PLA ﬁlm was used for the curd storage in identical ﬁlm sample was ﬁxed in the middle of test chamber to sepa- conditions with those presented in the section “Storage of rate the chamber into upper room and lower room. When the curd wrapped in the nano-Ag-graphene-TiO -PLA ﬁlm.” oxygen and nitrogen ﬂow in upper and lower rooms, respec- 2.7. Characterization of the Nano-Ag-Graphene-TiO Composite. tively, the oxygen molecules penetrate the ﬁlm sample into The nano-Ag-graphene-TiO composite was characterized as the lower room and the coulometric sensor system detects follows. Morphology was established by optical and electronic and analyzes the oxygen content and calculates the OTR. microscopy (scanning transmission electron microscopy When testing the container, oxygen is released outside and (STEM)-high-angle annular dark ﬁeld (HAADF)). Structure nitrogen inside of the container. The time required to reach was determined by performing Fourier transformed infrared steady state for OTR in sample was 64 hr. spectroscopy (FTIR) and X-ray diffraction (XRD) and by FTIR, ash content, pH, electrical conductivity, and anti- determining elemental composition by energy-dispersive oxidant activity were performed according to the procedure X-ray spectroscopy (EDXS). The optical analysis was evalu- detailed in a study by Peter et al. [14]. Water vapor perme- ated by performing UV–vis spectroscopy, and the gap energy ability (WVP) and grease permeability were determined 4 Advances in Polymer Technology according to the procedure described in a study by Peter observed at 60.2 Æ 2.2°C for neat PLA, at 58.1 Æ 3.3°C for et al. [4]. new ﬁlm, and at 59.7 Æ 4.8°C for the recycled ﬁlm. An exo- The barrier properties against UV–vis light was deter- thermic peak was formed at 121.9 Æ 9.2°C for the neat PLA, mined by using the PerkinElmer Lambda 35 Spectrophotom- at 117.6 Æ 8.1°C for the new ﬁlm, and at 95.2 Æ 3.4°C for the eter [9]. The experiment was performed in triplicate, and the recycled ﬁlm, due to the crystallization process of PLA, caus- opacity was determined by using the formula as follows: ing the reordering of the macromolecular chains from the PLA’s matrix and leading to mobility after thermal treatment Abs at 600 nm [16–18]. The endothermic peak corresponding to the melt- ð2Þ OpacityðÞ a:u: at 600 nm=mm¼ ; ing process is visible at 150.5 Æ 4.4°C for neat PLA, 148.1 Æ 7.9°C for the new ﬁlm, and at 178.6 Æ 10.2°C for the recycled ﬁlm. where Abs (absorbance) at 600 nm and x is ﬁlm thick- The incorporating Ag-graphene-TiO composite nano- ness (mm). particles slightly decreases the PLA’s T from 60.2 to 58.1°C in the new ﬁlm and to 59.7°C in the recycled ﬁlm when 2.9. Statistical Analysis. The characterization experiments, as compared to neat PLA (Table 2 and Figures S1–S3). This is well as those corresponding to the curd storage, were per- explained by the PLA chain shrinkage assigned by the inter- formed in triplicate and the values were reported as mean, by connectivity with the composite particles [19]. Similar results using the Excel program. The level of signiﬁcance was deter- were obtained by Deghiche et al. [17] and Nomai et al. [19]. mined by using the Statistica 7.0 software (StatSoft, Inc., The crystallization temperature (T ) of neat PLA and the CC Tulsa, USA) and one-way analysis of variance (ANOVA— new ﬁlm was similar (121.9 and 117.6°C, respectively), Tukey’s test). whereas that of the recycled ﬁlm decreased to 95.2°C. The crystallization degree increases in the new and recycled ﬁlms 3. Results and Discussion when compared to neat PLA because the composite nano- 3.1. Characterization of the New and Recycled Nano-Ag- particles act as nucleation centers [17]. It is a considerably Graphene-TiO -PLA Film 2 difference of X between the new and recycled ﬁlm, explained by the highly crystallized recovered composite, that induces a 3.1.1. Visual Appearance and Morphology of the Films. The more considerable crystallization in the recycled ﬁlm as in visual appearance of the new and recycled ﬁlms is shown in the new ﬁlm. There are differences in T and melting tem- cc Table 1. The image of the new ﬁlm clearly highlights the perature (T ), respectively, between the newly prepared ﬁlm presence of composite particles, having different dimensions, and the recycled ﬁlm. Moreover, the new ﬁlm has T and T cc m unevenly dispersed in the PLA matrix. The macroscopic values closer to the unmodiﬁed PLA than the recycled ﬁlm. image of the recycled ﬁlm is similar to that of the newly These differences are due to the elementary composition of prepared ﬁlm, that is, the composite particles distributed the newly prepared ﬁlm different than that of the recovered nonuniformly in the matrix and having different dimensions ﬁlm (Figures S1–S3). Also, differences between the compos- are observed. ite structure from new and recycled ﬁlm can be achieved in Table 1 also shows the SEM images of the new and the FTIR spectra (Figure S4). It was previously mentioned recycled samples. The observations are similar to those in that the recovered composite contains only traces of carbon, macroscopic analysis, meaning that composite particles with due to the heat treatment applied during the recovery, which dimensions of 10–20 nm in diameter are heterogeneously demonstrates that the graphene has been degraded. The distributed in the network. No difference in the morphology almost identical values of T determined for neat PLA and cc as a result of the recycling procedure could be observed. The new composite PLA demonstrate that the new Ag-graphene- STEM microscopy performed in our previous study [4] TiO composite did not induce signiﬁcant structural changes showed that the Ag-graphene-TiO composite contains tita- of the polymer matrix because of the strong PLA’ssurface nia and silver particles with approximately 20 and 5 nm in tension [20]. The signiﬁcant decrease in the T value (increase diameter, respectively, dispersed in the graphene matrix. cc in T ) achieved for recycled ﬁlm demonstrates that the com- The elemental analysis of the recycled and the new ﬁlm m position of the recovered material is structurally different from and of the neat PLA performed by EDXS analysis, as shown that of the new one and induced modiﬁcations in the PLA’s in Table 1, reveals that the difference of element content surface tension and crystallization in a greater extent than in between the new and recycled ﬁlm is statistically nonsigniﬁ- the newly prepared composite. Thus, it is inferred that the cant (one-way ANOVA—Tukey’s test). In contrast, the ele- recovered composite is better incorporated into the PLA mental composition of the newly prepared composite differs matrix than the new prepared one. signiﬁcantly from that of the recovered composite (Figure 1) in terms of carbon content. The explanation is the degrada- 3.1.3. FTIR Spectroscopy of the Films. There are characteristic tion occurred by carbon during the thermal treatment. signals corresponding to PLA, TiO , and graphene in the FTIR spectra of the new and recovered PLA-based ﬁlms, 3.1.2. Thermal Transitions of the Films (DSC). Figures S1–S3 show the DSC thermograms of the new and recycled PLA- showing the formation of the chemical connections between the PLA and the composite (Figure S4 and Table 3). The signals based and neat PLA ﬁlms in which the thermal transitions of −1 PLA are visible. The glass transition temperature (T ) was at 2,973, 1,750, 1,451, 1,182–1,190, 1,081, and 867–689 cm g Advances in Polymer Technology 5 Æ Æ Æ Æ Æ Æ TABLE 1: Images and elemental composition of the new and recycled nano-Ag-graphene-TiO -PLA ﬁlms. New Ag-graphene-TiO -PLA Recycled composite PLA Neat PLA Visual appearance SEM image a a a C(wt%)52 5 58 1 60 1 a a a O (wt%)47 4 42 2 40 1 a a Ti (wt%) 0.2 0.01 0.11 0.01 – a a Ag (wt%) 0.1 0.01 0.1 0.01 – In each line, mean values with superscript letter ( ) are statistical nonsigniﬁcant different at p<0:01 (one-way ANOVA—Tukey’s test). 6 Advances in Polymer Technology OTi C Ag New composite Recovered composite FIGURE 1: Elemental composition of the new and recovered Ag-graphene-TiO nanocomposite. TABLE 2: Thermal properties of the ﬁlms. Sample T (°C) T (°C) T (°C) ΔH (J/g) ΔH (J/g) Xc g CC m CC m a a a a a a Neat PLA 60.2 Æ 2.2 121.9 Æ 9.2 150.5 Æ 4.4 17.0 Æ 0.7 17.12 Æ 5.1 0.129 Æ 0.001 a b b b a b New ﬁlm 58.1 Æ 3.3 117.6 Æ 8.1 148.1 Æ 7.9 15.1 Æ 1.1 17.2 Æ 5.3 2.258 Æ 1.7 b c c c b c Recycled ﬁlm 59.7 Æ 4.8 95.2 Æ 3.4 178.6 Æ 10.2 21.2 Æ 1.9 32.2 Æ 3.9 11.827 Æ 4.8 T , glass transition temperature; T , crystallization temperature; T , melting temperature; ΔH , enthalpy of cold crystallization; ΔH , melting enthalpy. In g CC m CC m a,b,c each line, mean values with superscript letters ( ) are statistical nonsigniﬁcant different at p<0:01 (one-way ANOVA—Tukey’s test). TABLE 3: Speciﬁc signals and description from the FTIR spectra. −1 Peak (cm ) New ﬁlm Recycled ﬁlm Neat PLA Description 2,973 √√ √ Stretching vibration of aliphatic methyl groups from PLA [17] 1,750 √√ √ Stretching vibration of the C═O bond from the ester group from PLA [17, 21] 1,451 √√ √ 1,395 – √ – Vibration of ─Ti─O─C bonds between recovered composite and PLA 1,382 √ – √ ─O─C─O─ stretching from PLA [22] 1,368 – √ – Vibration of ─Ti─O─C bonds between recovered composite and PLA 1,303 √ (small) √ – Stretching of ─Ti─O─C included in the PLA matrix 1,182–1,190 √√ √ ─C─O─ stretching from PLA [17] 1,127 and 1,044 √ (shoulder) √ – Presence of composite into the PLA matrix 1,081 √√ √ Vibration of ─O─CH─CH from PLA [17] 952 √ (small) √√ (small) Presence of composite into the PLA matrix 867 √√ √ Vibration of C─C bond from the ─C─COO─ in PLA [21] Vibration of 751 √√ √ Ti─O─Ti and C─O─Ti [23] 689 √√ √ −1 found in all FTIR spectra are characteristic to the vibration of peaks at 1,395 and 1,368 cm found in the FTIR spectrum speciﬁc bonds in the PLA skeleton [17, 21]. of recovered ﬁlm. The peaks at 1,303, 1,127, 1,044, and −1 Speciﬁc signals assigned by the composite presence can 952 cm are small in the FTIR spectrum of the new ﬁlm −1 be found at 1,382, 1,303, 1,127, 1,044, and 952 cm and are but become stronger in that of the recovered ﬁlm. These assigned by the stretching of the Ti─O─C connections peaks are assigned to the interconnection of PLA with the [22, 23]. Moreover, the formation of these C─O─Ti bonds TiO network and the formation of C─O─Ti bridges [23] −1 is also demonstrated by the signals in the range of 600–900 cm and the different signal intensities demonstrate the different that overlaps with those of PLA. structures of the recovered composite in comparison with The FTIR spectra of the new ﬁlm and of the recycled ﬁlm the new one. are different in the regions at 1,368–1,395, 1,303, 1,127, These observations are supported by the DSC results, −1 −1 1,044, and 952 cm . Thus, the peak at 1,382 cm from which showed that the recycled composite, due to its differ- the FTIR spectra of new ﬁlm and of PLA is splitted in two ent compositions in comparison with the new one, altered Element content (%) Advances in Polymer Technology 7 TABLE 4: Characteristics of the new and recycled nano-Ag-graphene-TiO -PLA ﬁlms. Characteristic New ﬁlm Recycled ﬁlm Neat PLA 2 a a a Grammage (g/m 39.6 Æ 8.54 41.66 Æ 5.68 40.95 Æ 4.24 a a b Thickness (µm) 79 Æ 2.57 75 Æ 0.87 23 Æ 1.68 a b c Breaking length (%) according to SR EN ISO 1924-2:2009 4.11 Æ 0.45 6.19 Æ 0.42 15.28 Æ 2.45 a b a Tearing resistance (mN) according to SR EN ISO 1974:2012 120 Æ 5.46 190 Æ 4.23 100 Æ 3.61 a b c Slashing resistance (kPa) according to SR EN ISO 2758:2015 167 Æ 5.89 207 Æ 4.28 135 Æ 5.63 a b a Folding resistance (no.) according to SR ISO 5626:1996 3,484 Æ 21 3,998 Æ 35 3,452 Æ 42 3 2 a b c Oxygen transmission rate (OTR) (cm /m × day) 480 Æ 156 393 Æ 89 719 Æ 91 a a b Opacity (a.u. at 600 nm) 0.328 Æ 0.013 0.304 Æ 0.013 0.921 Æ 0.56 a a b Ash (%) 0.49 Æ 0.0045 0.4 Æ 0.088 0.03 Æ 0.001 a a a pH 5.86 Æ 0.04 5.72 Æ 0.089 5.91 Æ 0.09 a b c Electrical conductivity (µS/cm) 26.43 Æ 3.15 33.3 Æ 7.23 13.2 Æ 5.82 10 a b c Water vapor permeability (WVP) (g/s m Pa) × 10 6.903 Æ 0.85 7.98 Æ 1.05 3.13 Æ 0.85 Grease permeability (%)0 0 0 a b c Antioxidant activity (%) 13.3 Æ 0.24 10.15 Æ 0.37 14.57 Æ 0.26 a b c 20°C 5.54 Æ 0.54 0.4 Æ 0.01 3.52 Æ 0.57 Solubility in simulant A (distilled water) (%) a b c 4°C 8.6 Æ 0.79 2.4 Æ 0.08 3.99 Æ 0.42 a b c 20°C 5.54 Æ 0.42 12.1 Æ 1.47 3.36 Æ 0.42 Solubility in simulant B (acetic acid 3%)(%) a b c 4°C 7.81 Æ 1.23 6.2 Æ 0.21 1.91 Æ 0.21 a b c 20°C 3.8 Æ 0.22 1 Æ 0.04 3.61 Æ 0.47 Solubility in simulant C (ethanol 65%)(%) a b c 4°C 3.5 Æ 0.24 5.9 Æ 0.99 4.28 Æ 0.98 a,b,c In each line, mean values with different letters ( ) are signiﬁcantly different at p<0:01 (one-way ANOVA—Tukey’s test). the PLA’s surface tension and induced more efﬁcient chemi- new bonds arising between the PLA network and the com- cally connections with the PLA chain. posite particles, proven by the FTIR spectroscopy analysis, i.e., O─C─O─ and ─Ti─O─C─PLA bonds (Figure S4 and 3.1.4. Physical–Chemical and Biochemical Characteristics of Table 3), which physical hydrogen bonds are added to [25]. the Films. The characteristics of the new and recycled ﬁlm in All of these bonds restrict the mobility of PLA chains, comparison with neat PLA are shown in Table 4. The differ- increase its strength, and ensure the matrix’s rigidity. This ence between the two ﬁlm types and neat PLA in terms of explains why the elongation at break of the modiﬁed ﬁlm is grammage, pH, and fat permeability is statistically nonsig- much lower in comparison to that of the neat PLA. niﬁcant. However, the opacity and the ash content variation OTR of the modiﬁed ﬁlms has decreased considerably by are also statistically nonsigniﬁcant when comparing the new 33% (in the new ﬁlm) and 45% (in the recycled ﬁlm), respec- ﬁlm with the recycled ﬁlm. tively, compared to PLA (Table 4). Among the modiﬁed In contrast, the difference of mechanical characteristics, ﬁlms, OTR of the recycled ﬁlm is 18.1% lower than of the OTR, WVP, solubility in food simulants, electrical conduc- new ﬁlm. This is correlated with the mechanical resistance tivity, and antioxidant activity is statistically signiﬁcant in a characteristics, i.e., the recycled ﬁlm is least permeable for quantum higher than 99% (p<0:01, one-way ANOVA— oxygen and the most mechanically resistant due to the com- Tukey’s test) between new and recycled ﬁlms. paction of the composite PLA network in a greater extent The largest tear elongation was determined for neat PLA, than in the case of the new prepared ﬁlm (Table 4). Subse- 73% and 59% higher than for the newly prepared ﬁlm and for quently, this is a result of different chemical compositions the recycled ﬁlm, respectively. When comparing the modi- and structures of the recovered composite when compared to ﬁed ﬁlms, it can be observed that the tear elongation of the the new one, a fact conﬁrmed by the EDXS results. The recycled ﬁlm is 50% higher than of the new ﬁlm. presence of the graphene network in the newly prepared All the other mechanical strength parameters of the ﬁlm affects its structure, in that, having a non-polar charac- recycled ﬁlm are superior to those of the new ﬁlm: the tear ter, the carbon network creates, together with the non-polar resistance is 58.33% higher, the burst resistance is 24% methyl groups of PLA, repulsions that make the matrix higher, and the bending resistance is 14.7% higher. The unable to compact. increase in mechanical resistance properties as a result of The WVP of the new and recovered ﬁlms is 2.2 and 2.5 the PLA’s additivation was also reported in literature, follow- times, respectively, higher than that of PLA’s (Table 4). This ing the addition of multiscale cellulosic biocomposites [21], is a negative aspect explained by the elemental composition chitosan [24], or halloysite nanotubes [25]. The mechanical and structure of the new prepared composite different from resistance of the recycled ﬁlm is higher than of the new and the recovered one. In the newly prepared composite, the unmodiﬁed PLA ﬁlm and is explained by the multitude of presence of graphene inserted in the PLA’s network provides 8 Advances in Polymer Technology added hydrophobic role to the composite matrix. Instead, in disadvantage is considerably reduced in the recycled ﬁlm (Figure 2b) due to the graphene’s very low concentration. the recycled ﬁlm, graphene was broken down during the recovery; thus, Ti, Ag, and traces of C remained only and This explains why the recycled ﬁlm has superior mechanical properties to the newly prepared ﬁlm. Moreover, the active the hydrophobic supply was considerably reduced. The antioxidant activity of the modiﬁed ﬁlms is lower carbonyls from the proximity of TiO and Ag active centers stimulate the electronic transfer and intensify the formation than that of the unmodiﬁed ﬁlms, and, moreover, that of the recycled ﬁlm is smaller than that of the newly prepared ﬁlm. of reactive oxygen anions, demonstrated by appearing of new peaks in the FTIR spectrum of the recovered ﬁlm. In general, the solubility in food simulants of the recycled ﬁlm is lower than that of the newly prepared ﬁlm, even that 4. Results of Curd Packaging in the New and the of the unmodiﬁed one, which is also explained by the com- Recycled Films posite network different from that of the newly prepared ﬁlm. So, it can be appreciated that the mechanical resistance, 4.1. Visual Appearance of the Curd Cheese. The curd cheese oxygen barrier, and food simulants properties of the recycled samples packed in each of the three types of packaging did ﬁlm are superior to the newly prepared ﬁlm, due to the fact not change from the organoleptical point of view during that another type of composite with different elemental com- 14 days of refrigeration (Tables 5 and 6). positions was obtained during the recovery procedure, that From day 14 to 21, signiﬁcant organoleptic changes have created superior qualitative and quantitative connections in occurred, especially in the case of the cheese kept in the neat the PLA matrix. This was also supported by DSC and FTIR PLA. Thus, after 14 days of storage, a positive inﬂuence of the results. modiﬁed PLA on the cheese’s organoleptical properties has The behavioral difference between the newly prepared been observed in comparison to that of the sample from the ﬁlm and the recycled ﬁlm lies in the following mechanism. unchanged PLA, assigned by the presence of the composite The chemical reactivity and kinetic stability of the compo- Ag-graphene-TiO . The photocatalytic activity of TiO [4] and 2 2 nent phases from the ﬁlm and the composite are explained the antimicrobial efﬁciency of Ag and graphene are already by the electron occupation of HOMO and LUMO molecular known [14, 19, 20, 24]. The cheese in PLA changed its color orbitals. HOMO is the most electron-occupied molecular from white to yellow, became crumbling and acquired, a orbital, and LUMO is the poorest in electrons. Reactivity is strong fermented smell. In contrast, the new ﬁlm sample due to the ability of electrons to be transferred from HOMO removed a little whey during storage and became yellowish. to LUMO and increases with the reduction of the energy The sample in the recycled ﬁlm suffered the fewest changes difference between HOMO and LUMO. In PLA, HOMO is after 21 days of storage. So, from the organoleptic point of located in the carbonyl groups C = O, which are the most view, PLA and the newly prepared ﬁlm ensure a shelf life of reactive and serve as electron donors [17]. In TiO , HOMO 14 days for cheese and the recycled one of 21 days, according to and LUMO are found in the titanium atoms and act as SR3664:2008. electron acceptors. In graphene, HOMO is in the carbon cycles and acts as electron donors [26]. In the new ﬁlm 4.2. Physical–Chemical Characterization of the Curd Cheese Ag-graphene-TiO -PLA (Figure 2a), as reported, there are 2 Wrapped in the New and Recycled Films. The variation of the reactive and polar areas (carbonyl groups) but also nonpolar physical–chemical parameters of the curd cheese kept in the areas (methyl groups). The carbonyl reactive areas, which act investigated ﬁlms is shown in Figure 3(a)–3(e). The moni- as electron donors, transfer the electrons to the Ti atoms tored parameters are standardized according to the Roma- (pathway A), by activating them, as the graphene network nian SR 3664 from 2008 in force, namely mass loss, titratable attached to the semiconductor (pathway B). The two path- acidity, dry mass, and fat and protein contents. ways of TiO activation are also added to photochemical When taking into account the values obtained at each activation under the action of UV radiation in the solar time interval, the one-way ANOVA analysis—Tukey’s test spectrum (pathway C) [27]. Ag nanoparticles attached to model showed that the variation of mass loss (Figure 3(a)), the TiO network stimulate the electronic transfer, reduce 2 dry matter (Figure 3(c)), fat (Figure 3(d)), and protein the recombination of charge carriers, and initiate the reduc- (Figure 3(e)) of the curd cheese, depending on the wrapping tion processes. The electrons activated by the nano-Ag can, material, is statistically insigniﬁcant. subsequently, be involved in two ways: (1) reduce the carbo- If only 21-day values were taken into account, differences cations in the PLA structure, formed due to the polarization in the mass loss of cheese samples packed in the three pack- of the carbonyl bond, generating oxygen anions that alter the age types were statistically signiﬁcant higher than 99%, while PLA’s surface state and affect its surface tension, as demon- those corresponding to the dry matter were statistically non- strated by the DSC results and (2) reduce species in the signiﬁcant. The differences in fat content are statistically environment surrounding the ﬁlm. Also, the connection of nonsigniﬁcant when compared to the sample wrapped in the PLA network with TiO is made by a considerably number the recycled ﬁlm and in the newly prepared ﬁlm, respectively, of hydrogen bonds [17, 28]. The presence of the graphene but that between the sample from newly prepared ﬁlm and network in the newly prepared ﬁlm has a disadvantage, that stored in the unchanged PLA is signiﬁcant (>99%). namely the fact that having a nonpolar character, the carbon When considering the protein variation as a function of network creates, next to the nonpolar methyl groups of PLA, the ﬁlm type, at 21 days, the difference between the sample in repulses that make the matrix unable to compact. This the recycled ﬁlm and that from the newly prepared ﬁlm is Advances in Polymer Technology 9 New prepared PLA-Ag-graphene-TiO film Graphene Nonpolar Repulsion Repulsion Repulsion Nonpolar CH CH CH CH CH CH 3 3 3 3 3 3 PLA chain OCH C O CH C O CH C O CH CO CH C OCH C OCH C O CH C OC CH OCH CO CH C O CH C O CH C e donor O CH O O CH O CH O O CH O O CH O O CH O O CH O 3 3 3 3 3 3 3 A transfer PLA’s surface state e transfer alteration (1) Ag Ag (2) − − − e e e − Ag CB e transfer transfer Species from the medium e donor surrounding the film UV e transfer TiO e acceptor VB + + + h h h ðaÞ Recycled film prepared from PLA with recovered composite CH CH CH CH CH CH 3 3 3 3 3 3 PLA chain OCH C O CH C O CH C O CH CO CH C OCH C OCH C O CH C OC CH OCH CO CH C O CH C O CH C e donor O CH O CH CH O O CH O O CH O O CH O O CH O O O 3 3 3 3 3 3 transfer A PLA’s surface state e transfer alteration 1) Ag Ag 2) − − − e e e − Ag CB e transfer transfer Species from the medium surrounding the film UV e transfer TiO e acceptor VB + + + h h h ðbÞ FIGURE 2: Bonds formation in the new prepared ﬁlm (a) and recycled ﬁlm (b). signiﬁcant (>95%), as well as that between the cheese from ﬁlm (17.2%). The highest capacity to preserve the protein recycled ﬁlm and neat PLA. content was provided by the recycled ﬁlm, as a result of The highest variation in the mass loss after 21 days of the ﬁlm structure and the most intense degree of compaction storage was registered in the curd cheese kept in the new ﬁlm (Figure 2(b)) compared to the newly prepared ﬁlm (Figure 2(a)). (11.6%) and the lowest in that kept in the recycled ﬁlm Thus, the OTR and solubility (Table 4) of the recycled ﬁlm (4.6%). The most accentuated decrease in fat content occurred signiﬁcantly lower in comparison to the newly prepared ﬁlm, in curd cheese kept in the neat PLA (10.23%), while the lowest reduced the oxygen penetration and the ﬁlm dissolution, in the recycled ﬁlm (1.63%). respectively, and implicitly the intensity of oxidative processes The same variation was determined for the protein con- in the intrapackaging space. tent, so the protein was the most severely degraded during When analyzing the curd cheese’s titratable acidity as a the storage in the neat PLA and in the new ﬁlm (25.35%) and function of the packaging material (Figure 3(b)), it is noticed the least affected during the cheese packaging in the recycled that variation of the acidity from the recycled ﬁlm is Oxidation Oxidation 10 Advances in Polymer Technology TABLE 5: Visual appearance of the curd cheese wrapped in the new and recycled ﬁlms and neat PLA under refrigeration. Period (day) New ﬁlm Recycled ﬁlm Neat PLA 14 Advances in Polymer Technology 11 TABLE 5: Continued. Period (day) New ﬁlm Recycled ﬁlm Neat PLA 21 12 Advances in Polymer Technology TABLE 6: Organoleptic properties of the curd cheese stored in the new and recycled ﬁlms and neat PLA under refrigeration, established based on the Romanian Quality Standard for fresh cheese SR3664:2008. Period (day) Characteristics New ﬁlm Recycled ﬁlm Neat PLA Appearance Homogeneous, clean paste without whey leakage Consistency Fine paste with a low grounty consistency 0 and 7 Color White Flavor Pleasant Taste Pleasant, taste of lactic fermentation, with no foreign tastes (acidic, bitter, of mold and yeast, smocked, etc.) Appearance Consistency 14 Color No changes when compared with those from day 0 Flavor Taste Appearance Homogeneous, with a very reduced whey leakage Homogeneous, with whey leakage Consistency Slightly crumbling Fine paste Crumbling Color White-yellowish White Yellowish Flavor A little bit pungent Pleasant, with no foreign tastes Acidic and of yeasts 14 230.0 210.0 190.0 170.0 150.0 6 130.0 110.0 90.0 70.0 50.0 0 510 15 20 0 5 10 15 20 Time (days) Time (days) Recycled film New film Recycled film New film Neat PLA Neat PLA ðaÞ ðbÞ 33.5 31.00 30.50 33.0 30.00 32.5 29.50 32.0 29.00 31.5 28.50 31.0 28.00 30.5 27.50 30.0 Minimum accepted limit according to SR 3664 : 2008 Minimum accepted limit according to SR 3664 : 2008 27.00 29.5 26.50 29.0 26.00 0 5 10 15 20 0 5 10 15 20 Time (days) Time (days) Recycled film New film Recycled film New film Neat PLA Neat PLA ðcÞ ðdÞ FIGURE 3: Continued. Dry mass (%) Mass loss (%) Fat content (%) Titratable acidity (Thorner degrees) Advances in Polymer Technology 13 Minimum accepted limit according to SR 3664 : 2008 0 510 15 20 Time (days) Recycled film New film Neat PLA ðeÞ FIGURE 3: Physical–chemical parameters of the curd stored in the recycled and new ﬁlms and in neat PLA: (a) mass loss, (b) titratable acidity, (c) dry mass, (d) fat, and (e) protein. signiﬁcant in comparison with that from the new ﬁlm during compared to day 0, but the nonmodiﬁed grapes were deeply 21 days of storage (p<0:05). However, the acidity evolution altered. of the cheese from new ﬁlm and the neat PLA is statistically Both the results of structural, thermal, and mechanical resistance analyses and the results obtained when storing nonsigniﬁcant. Thus, the acidity of the cheese kept in the recycled ﬁlm increased by 40.5% after 21 days of refrigera- cheese demonstrate that the recycled ﬁlm has a more con- servative role on the cheese than the newly prepared one, tion, while in the case of the new ﬁlm and neat PLA, it which further encourages the recycling. decreased by 28.8% and 59.1%, respectively. The rise of acid- ity during storage was also reported by Jafarzadeh et al. [29] 5. Conclusions who coated active edible ﬁlm based on whey protein modi- ﬁed with white tea extract on the fresh cottage cheese. This The study aims to assess the structural, morphological, behavior could be explained by the fact that the recycled mechanical resistance, physical–chemical and biochemical packaging ﬁlm did not affect the functionality of lactic bac- characteristics, as well as the preservative role on curd cheese teria, as the newly prepared ﬁlm or neat PLA. This is due to of a recycled PLA ﬁlm, obtained by recovering the nano-Ag- the different compositions of the recycled composition and graphene-TiO composite from the used PLA-based ﬁlm, the different structures of the recycled ﬁlm compared to the followed by its incorporation in the new PLA network. Visu- new one, demonstrated by DSC and FTIR analyses and illus- ally, the recycled ﬁlm looked just like the new one. The trated in mechanism, as shown in Figures 2(a) and 2(b). The elementary quantitative analysis of the newly prepared and maximum accepted limit of titratable acidity and the mini- the recovered Ag-graphene-TiO composite, used as active mum accepted limit of dry matter, fat, and protein according compound in the PLA ﬁlm, showed that the graphene was to SR 3664:2008 have not been reached in the case of any carbonized during the recovery procedure, so the newly pre- cheese sample, so the curd cheese can be stored safely for pared composite is compositionally different from the newly 21 days in all three types of studied packaging materials. prepared one. This induced variable DSC and FTIR results Paulsen et al. [30] who monitored the evolution of broc- was manifested by: (1) higher crystallization of the recycled coli under three different storage conditions (LLDPE pack- ﬁlm network than the newly prepared one, (2) recovered aging, biodegradable packaging containing PBAT/PLA, and composite reduced the surface tension of the PLA in a higher unpackaged) demonstrated that the smallest changes in organ- extend than the newly prepared composite, which showed oleptic characteristics, mass loss, oxygen and carbon dioxide that the recovered composite is better incorporated into the evolution, ﬁrmness, chlorophyll, and carotenoid content of PLA network, and (3) new bonds appear in the recycled ﬁlm, broccoli were recorded during its storage in LLDPE and in which also demonstrates that the recycled ﬁlm is more com- biodegradable ﬁlm, highlighting that the PBAT/PLA has pact than the new one. The mechanical and barrier resistance similar conservation characteristics to LLDPE and is also properties of the recycled ﬁlm superior to the newly prepared biodegradable. one are due to the new connections appeared in the matrix Zheng et al. [31] have successfully obtained active ﬁlms following recycling, ensuring a high compaction degree. The containing a cocktail of biodegradable materials (PBPA, PLA, curd cheese has been successfully stored in the recycled pack- PCL) modiﬁed with TiO and natamycin, with mechanical aging for 21 days, the organoleptic characteristics being and microbiological properties superior to the conventional superior to those of cheese kept in new ﬁlm or neat PLA. ﬁlms and showed their strong preservative effect on the The recycled packaging reduced the protein degradation of grapes. After 30 days of storage at 0.5°C, the grapes kept in cheese by 8% compared to the newly prepared one. The active packaging did not change their organoleptic properties study shows that the recycling procedure was a success Protein (%) 14 Advances in Polymer Technology from two points of view: (1) packaging waste reduction and [5] H. Chi, S. Song, M. Luo et al., “Effect of PLA nanocomposite ﬁlms containing bergamot essential oil, TiO nanoparticles, (2) production of packaging material with a preservative and Ag nanoparticles on shelf life of mangoes,” Scientia action even more efﬁcient than the newly prepared ﬁlm. Horticulturae, vol. 249, pp. 192–198, 2019. [6] S. Geetha, A. Thangamani, R. Valliappan, S. Vedanayaki, and Data Availability A. Ganapathi, “Sulfated titania (TiO –SO )asan efﬁcient 2 4 ⁻ and reusable solid acid catalyst for the multi-component All the data that make the subject of the manuscript results synthesis of highly functionalized piperidines,” Chemical Data are available on request. Collections, vol. 30, Article ID 100565, 2020. [7] N. G. Menon, L. George, S. S. V. Tatiparti, and S. Mukherji, Conﬂicts of Interest “Efﬁcacy and reusability of mixed-phase TiO –ZnO nanocompo- sites for the removal of estrogenic effects of 17β-Estradiol and The authors declare that there are no conﬂicts of interest 17α-Ethinylestradiol from water,” Journal of Environmental regarding the publication of this paper. Management, vol. 288, Article ID 112340, 2021. [8] H. Yan, R. Wang, R. X. Liu et al., “Recyclable and reusable Authors’ Contributions direct Z-scheme heterojunction CeO /TiO nanotube arrays 2 2 for photocatalytic water disinfection,” Applied Catalysis B: Anca Peter—conception, experimental design, and writing; Environmental, vol. 291, Article ID 120096, 2021. Anca Mihaly Cozmuta—carrying out measurements and [9] Y. Liu, Y. Xiang, H. Xu, and H. Li, “The reuse of nano-TiO 2− visualization; Leonard Mihaly Cozmuta—statistical analysis; under different concentration of CO using coagulation Nicula Camelia—methodology and visualization; Goran Drazic, process and its photocatalytic ability in treatment of methyl Antonio Penas, and Stefania Silvi—supervision. orange,” Separation and Puriﬁcation Technology, vol. 282, Part B, Article ID 120152, 2022. Acknowledgments [10] A. Khodanazary and B. Mohammadzadeh, “The effects of polylactic acid-whey protein isolated bi-layer ﬁlm incorpo- This work was supported by the Technical University of rated with ZnO nanoparticles on the quality of common Cluj-Napoca (UEFISCDI contract no 72/2017), National carp Cyprinus carpio,” Journal of Food Measurement and Characterization, vol. 17, pp. 4684–4694, 2023. Institute of Chemistry, Ljubljana, Slovenia (MIZS 4126), [11] F. Izadi, M. Pajohi-Alamoti, A. Emamifar, and A. Nourian, University of Camerino and Synbiotec Srl, Italy (MIUR-con- “Fabrication and characterization of active poly(Lactic Acid) tract no 1-2895), and Andaltec, Spain (MINECO contract no ﬁlms containing Thymus daenensis essential oil/Beta- PCIN-2017-037) in the frame of the GRAFOOD project cyclodextrin inclusion complex and silver nanoparticles to (M-ERANET program). extend the shelf life of ground beef,” Food and Bioprocess Technology, 2023. Supplementary Materials [12] K. Poonsawat and M. Seadan, “Determination of molecular weight and molecular weight distribution of poly(lactic acid) Figure S1: differential scanning calorimetry (DSC) curves of by dynamic mechanical properties of polymer in melted state,” new prepared nano-Ag-graphene-TiO -PLA ﬁlms. Figure S2: poster accessed Aug 11 2023 http://www.dpst.sc.su.ac.th/asse differential scanning calorimetry (DSC) curves of recycled ts/news/poster-stcy11/Determination.pdf. nano-Ag-graphene-TiO -PLA ﬁlms. 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Polylactic Acid-Based Film Modified with Nano-Ag-Graphene-TiO<sub>2</sub>: New Film versus Recycled Film

Polylactic Acid-Based Film Modified with Nano-Ag-Graphene-TiO<sub>2</sub>: New Film versus Recycled Film

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Polylactic Acid-Based Film Modified with Nano-Ag-Graphene-TiO<sub>2</sub>: New Film versus Recycled Film

Polylactic Acid-Based Film Modified with Nano-Ag-Graphene-TiO<sub>2</sub>: New Film versus Recycled Film

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