Polym. Bull. DOI 10.1007/s00289-016-1782-4 ORIGINAL PAPER
Properties of UV protective films of poly(vinylchloride)/TiO2 nanocomposites for food packaging Ljerka Kratofil Krehula1 • Ana Papic´1 • Stjepko Krehula2 • Vanja Gilja1 • Lucija Foglar1 Zlata Hrnjak-Murgic´1
•
Received: 30 July 2015 / Revised: 7 July 2016 / Accepted: 10 August 2016 Springer-Verlag Berlin Heidelberg 2016
Abstract This work studies the UV protection properties of poly(vinyl-chloride) (PVC) nanocomposites. A functional property of UV protection is achieved by adding the active component (titanium dioxide (TiO2) or titanium dioxide modified with silver nitrate and copper nitrate) to the PVC matrix. PVC nanocomposites were prepared by extrusion and then pressed into films. Prepared PVC nanocomposites were characterized by thermogravimetric analysis, UV–Vis spectroscopy, X-ray diffraction and scanning electron microscopy. The mechanical properties and antimicrobial activity were also studied. The results show that PVC nanocomposites’ thermal stability is improved in relation to a pure PVC polymer. The thermal stability and antimicrobial efficiency increase when higher silver nitrate content is used. The sample prepared with silver and copper nitrate shows the best thermal stability due to a modified mechanism of thermal degradation. Samples where & Ljerka Kratofil Krehula
[email protected] & Zlata Hrnjak-Murgic´
[email protected] Ana Papic´
[email protected] Stjepko Krehula
[email protected] Vanja Gilja
[email protected] Lucija Foglar
[email protected] 1
Faculty of Chemical Engineering and Technology, University of Zagreb, Marulic´ev trg 19, P. O. Box 177, HR-10000 Zagreb, Croatia
2
Division of Materials Chemistry, Rud¯er Bosˇkovic´ Institute, P.O. Box 180, HR-10002 Zagreb, Croatia
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nanoparticles are homogeneously dispersed in the polymer matrix show good mechanical properties. The results also show that adding the active component TiO2 modified with silver ions contributes to the improved UV protection property of nanocomposite materials. Keywords Food packaging Nanocomposite Poly(vinyl-chloride) Titanium dioxide UV protection
Introduction Polymer food packaging materials are developed to protect packaged food from various influences (light, heat, oxygen…) to obtain improved food quality and safety [1, 2]. Some food products are particularly susceptible to degradation and evaporation of volatile substances and demand greater efficiency in food protection. Photodegradation usually occurs in specific food constituents (pigments, fats, proteins, and vitamins), resulting in discoloration, off-flavors, and vitamin loss [3, 4]. During processing, packaging, storage, shipping and marketing food is exposed to daylight and artificial light, which significantly shortens its shelf life and harms the quality. Food sensitivity to light depends on many factors including the light source strength and the light type that it emits the length of exposure, the optical properties of packaging materials, oxygen concentration in the food and temperature. Thus, to protect the light sensitive food, the development of advanced, functional polymeric materials is of great importance. For instance, to avoid the UV degradation of packaged food products, fillers, stabilizers, absorbers or blockers can be incorporated in plastics packaging. Fillers are usually used to improve some material properties or to dilute the matrix with something less expensive than the polymer itself. However, nanofillers are today particularly used to develop one or more functional properties of polymer nanocomposites. These nanosubstances change the properties of polymers and render them more adaptable and versatile. A combination of several types of additives, easily mixed in a polymer into synergistic ‘‘packages’’, is becoming popular. The choice of additives must also be dictated by health and safety considerations when materials are in direct contact with food [5]. UV light absorbers in plastics act as UV screeners or coatings to reduce or eliminate the transmission of UV light and to protect UV sensitive substances [6] by inhibiting the first step in the photodegradation process. Titanium dioxide (TiO2), which is often used as a filler and/or pigment, can also be effective as a typical absorber that selectively absorbs UV light and reemits it at a less harmful wavelength, mainly as heat [7]. The polymer type, the UV absorber type, the thickness of the plastic product and the concentration of the UV absorber need to be considered for the specific application to meet the reduced UV percentage transmission requirements [8]. For the stability of food packaging materials it is very important that UV light be blocked, but especially interesting and challenging is to block visible light since total sunlight is constituted of *5 % UV light, 50 % visible light and 45 % IR light.
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As it is known, titanium dioxide exists in three crystalline forms: anatase, rutile, and brookite [9]. Anatase has a high photochemical activity and can degrade organic compounds under UV radiation. Brookite has no practical significance since it is stable only at low temperature, while rutile shows some interesting characteristics [10]. Rutile indicates the absorption of UV light up to the proximity of visible light wavelengths and transparency at visible light wavelengths. Because of those polymer nanocomposites containing rutile may be of great interest for producing visually transparent UV films as well as for coating UV sensitive materials [11, 12]. Thus, the extension of absorbance capacity of rutile by depositing metal ions like silver and copper on TiO2 nanoparticles is of great interest for preparing packaging materials for food [13]. This study aims to develop efficiently modified TiO2 photoabsorber that is going to significantly improve UV/Vis blocking in PVC films for food packaging. For the prepared PVC composite samples, the morphology, mechanical and thermal properties were studied as well as their antimicrobial activity.
Experimental Materials Poly(vinyl-chloride) used was PVC ONGROVIL S-5070, produced by BorsodChem (containing plasticizer diisononylphthalate, DINP, the content of plasticizer DINP was 50 phr (parts per 100 parts of PVC), Hungary. Titanium dioxide, TiO2, was produced by Acros Organics, silver nitrate (AgNO3) and copper nitrate (Cu(NO3)2) by Kemika, Croatia. During PVC extrusion Irganox (BASF) was used as antioxidant and zinc stearate (Kemika) as lubricant. Impregnation of TiO2 by silver and copper The solution mixing method of TiO2 with AgNO3 and Cu(NO3)2 is used. Such impregnation of TiO2 nanoparticles was carried out in a reactor (with mechanical stirrer), at room temperature, the speed of stirring was 250 min-1. The mass of TiO2 used in each synthesis was 10 g. The given mass of AgNO3 and Cu(NO3)2 was added and dissolved in a 100 ml of distilled water. Then the given mass of TiO2 (10 g) was suspended in that solution to obtain the required molar ratio of components (Table 1). The suspension was stirred for 6 h, and then left for 24 h to settle down. After that the suspension was dried in an air oven for 12 h at 100 C. The dried solid was ground and calcined at 450 C for 3 h. Ag-H denotes higher content of Ag in composite where molar ratio AgNO3:TiO2 is 1:10. Ag-L denotes lower content of Ag in composite where molar ratio AgNO3:TiO2 is 1:100. In sample, TiO2/Ag/Cu molar ratio Cu(NO3)2:AgNO3:TiO2 is 0.3:1:100.
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Polym. Bull. Table 1 The composition of the modified TiO2 active substances and PVC nanocomposites Samples TiO2
mol. % in TiO2 AgNO3; Cu(NO3)2
Samples TiO2 Mass %
PVC
Samples PVC
TiO2
0; 0
4.8
95.2
PVC/Ti
TiO2/Ag-L
1; 0
4.8
95.2
PVC/Ti/Ag-L
TiO2/Ag-H
10; 0
4.8
95.2
PVC/Ti/Ag-H
TiO2/Ag/Cu
1; 0.3
4.8
95.2
PVC/Ti/Ag/Cu
Nanocomposites preparation PVC nanocomposites were prepared by the extrusion process carried out on the Rondol Bench Top 21 twin screw extruder. Nanocomposites were first homogenized before extrusion by dry mixing with the addition of thermal stabilizer Irganox 1076 (0.5 wt%) and Zn-stearate (1.0 wt%). The temperature profile of the extruder was set to 140/150/160/165/165/170 C (from hopper to die) and the screws rotation speed was set to 100 rpm. Prepared samples were pressed into films. The exact composition is given in Table 1. Film preparation Film specimens of PVC nanocomposites were prepared using a hand-operated hydraulic press (Dake Machine Tools, model 44-226). A pre-weighted amount (3 g) of the sample was pressed at 150 C under the pressure of 200 bars. Nanocomposite bead specimens were pressed between two Teflon sheets for 4 min. The average thickness of PVC films was *200 lm. Characterization methods X-ray powder diffraction (XRD) To characterize the crystal structure of pure and modified TiO2 powder samples, XRD measurements were performed using an X-ray powder diffractometer Ital Structures APD 2000 (CuKa radiation, graphite monochromator, NaI-Tl detector). UV spectroscopy UV–VIS spectra of all TiO2 powder samples as well as PVC/TiO2 nanocomposite films were recorded at 20 C using a Shimadzu UV–Vis-NIR spectrometer (UV3600 model) with an integrated sphere. Barium sulfate was used as reference material.
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Thermogravimetric analysis (TGA) The thermal degradation of TiO2 samples and PVC/TiO2 nanocomposites was monitored by thermogravimetric analysis (TGA) using a TA Instrument Q500. Samples of *10 mg were heated from room temperature to 600 C at a rate of 10 C/min in a nitrogen gas flow of 100 ml/min. Scanning electron microscopy (SEM) The examination of the surface morphology of PVC/TiO2 nanocomposite samples was carried out using a scanning electron microscope TESCAN VEGA 3 SEM at 10 kV. Before recording samples were applied to the adhesive tape on the sample holder, then their surfaces were coated with a very thin layer of gold. Mechanical properties The mechanical properties of PVC/TiO2 nanocomposites were tested using a Zwick Testing Machine (model 1445) at a constant temperature of 23 C and the humidity of 65 % RH. The gauge length of 50 mm, crosshead speed of 50 mm/min and film sample size (120 9 10 9 0.2) mm were used to determine tensile strength and elongation at break. The results of tensile strength and elongation at break are given as the average values of five sample measurements. Antimicrobial activity Using sterile technique, 1 mL of a mixed microorganism culture suspension was transferred to sterile Petri dishes containing a previously prepared nutrient agar. Then the disc samples of PVC/TiO2 nanocomposite films 1 cm dia. were applied to the substrate basis. Petri dishes were placed in an inverted position in the thermostat to incubate at 37 C for 48 h. The antimicrobial activity was measured as a width of the inhibition ring around the sample of material (the area without microbial growth).
Results and discussion X-ray powder diffraction As shown in the XRD patterns of pure TiO2, TiO2/Ag-L, TiO2/Ag-H and TiO2/Ag/ Cu samples calcined at 450 C, the diffraction peaks display a combination of anatase and rutile phases for all samples, Fig. 1. The peaks (101), (103), (004), (112), (200), (105), (211), (214), (106) can be assigned to the anatase phase, and the peaks (110), (101), (111), (211), (220) to another crystal type, rutile, suggesting the combination of two crystal phases with the dominant anatase. The crystallite size and the amount of each phase for different modified TiO2 absorbers are reported in Table 2. Silver was detected in TiO2/Ag-H and TiO2/Ag/Cu samples. The absence
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Fig. 1 X-ray powder diffraction analysis of calcined TiO2 and TiO2 modified with silver and copper ions Table 2 The amount of each crystalline phase and the crystallite size for different modified TiO2 absorbers Sample
Crystalline phase % anatase/rutile
Crystallite size nm
TiO2
A:88 R:12
DA: 26 DR: 39
TiO2/Ag-L
A:88 R:12
DA: 26 DR: 39
TiO2/Ag-H
A:88 R:12
DA: 26 DR: 39
TiO2/Ag/Cu
A:88 R:12
DA: 26 DR: 39
of characteristic peaks corresponding to Ag in sample TiO2/Ag-L and to Cu in sample TiO2/Ag/Cu can be due to low metal concentration. Other researchers have reported a XRD detection limit of 5 wt% for the impregnated content on TiO2 [14, 15]. There are no lines for AgNO3 because it is not stable at such a high temperature. It is stable up to 120 C, whereas at higher temperatures it decomposes to silver, NO2 and O2 and evaporates from the sample [16]. The diffraction lines for Cu(NO3)2 were not detected, due to its low content. In all studied samples a certain
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content of rutile phase was observed that is responsible for TiO2 UV light absorbing capabilities [17]. UV–Vis spectroscopic analysis The UV–Vis absorption spectra of TiO2 modified by depositing metal ions (Ag and Cu) were recorded and are displayed in Fig. 2. As expected, TiO2 has no absorption band in the visible range and shows the characteristic spectrum with its fundamental absorption of the Ti–O bond in the ultraviolet light range from 200 to 400 nm. From the spectrum of TiO2/Ag-L, TiO2/Ag-H and TiO2/Ag/Cu samples, it can be seen that some differences in their UV–Vis spectra are observed in comparison with the pure TiO2. Their UV–Vis absorption intensity is obviously different because they show absorption in ultraviolet light and also to some extent in the visible light region. Sample TiO2/Ag-H shows the highest absorption band in the region from 400 to 600 nm, which is attributed to the electron transition from the valence band to the conduction band. Namely, the rutile phase of TiO2 indicates the absorbance of visible light wavelengths. TiO2 modification with Ag and Cu ions enhances the absorbance capacity and allows the absorbance of visible light of higher wavelengths as can be observed from the results. Rutile TiO2 has a high recombination rate of photogenerated electrons and holes, so rutile could be used as an UV light protective material [17]. Namely, UV protective behavior occurs primarily due to strong UV light absorption, a high recombination rate of photogenerated electrons and holes and high light scattering capability provided by rutile TiO2 submicrospheres.
Fig. 2 UV–Vis spectra of the active substances of TiO2 and TiO2 modified with silver and copper nitrate
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Such absorbed light TiO2 reemits at a less harmful wavelength, mainly as heat or IR light. In addition to that, it is notable that the absorption band of TiO2/Ag-H is stronger and wider than that of TiO2/Ag/Cu and TiO2/Ag-L, which is due to the fact that a higher concentration of silver ions leads to lower band gap energy. This suggests that a concentration of deposited metal ions to obtain effective UV absorption needs to be optimized. The enhanced capacity of visible light absorbance by a modified TiO2 absorber has been explained in terms of a photoelectrochemical mechanism in which the electrons generated by UV irradiation on the TiO2 semiconductor transfer to loaded metal particles, while the holes remain in the semiconductor, resulting in a decrease in the electron hole recombination [18, 19]. To prepare PVC/TiO2 nanocomposite films for food packaging with UV protection properties, modified TiO2 absorbers were mixed with PVC polymer during the extrusion and their UV–Vis spectra are shown in Fig. 3. In comparison to the neat PVC films, it is evident that all PVC/TiO2 nanocomposites show a significant increase of absorbance in the whole UV region (200–400 nm). The absorbance of modified PVC has increased 100 % (from 0.6 for pure PVC to 1.2 for other samples). In the visible region, strong bands with a peak wavelength at 450 nm were observed and attributed to the electron transition in the TiO2 absorber in PVC/TiO2 nanocomposite samples. The intensity is highest for samples PVC/TiO2/Ag/Cu and PVC/TiO2/Ag-H. The interaction between the additives and TiO2 possibly changes the absorbance property of pure TiO2. Hence, the photoelectric behavior of a modified titanium dioxide system becomes more complicated. Due to differences between the additives in ionicity, electron or hole energy levels, etc., the properties of the interface between TiO2 and additives differ.
Fig. 3 UV–Vis spectra of PVC nanocomposite films
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TG analysis TG analysis enables a good insight into the thermal behavior of polymeric materials during their degradation, which is always the result of the composition and structure of the material. First, the samples of modified TiO2 absorber were characterized thermally and the results are shown in Fig. 4 as TG curves in the range of thermal decomposition from &30 to 600 C. As expected, the samples of TiO2 absorber were thermally very stable since only 1.6 wt% of the sample decomposed up to 600 C. The thermal stability of studied samples can be estimated on the basis of the T99 values (temperature at which 1 mas % of sample is decomposed) where modified TiO2 samples were more stable than pure TiO2, Table incorporated in Fig. 4. The highest thermal stability during the whole decomposition temperature range was manifested by the TiO2 sample modified with silver and copper nitrate (TiO2/Ag/Cu). Its T99 was 331 C, which is more than 100 C higher than for pure TiO2. The TG and dTG curves of PVC/TiO2 nanocomposite films are shown in Figs. 5 and 6. The obtained results show that the thermal decomposition of PVC/TiO2 nanocomposites was performed in two steps, first from 200 to 300 C, then from 430 to 480 C. From the dTG results, it is evident that in the first step strong decomposition is present for almost all nanocomposites. For samples with Ag ions (PVC/TiO2/Ag-H, PVC/TiO2/Ag-L) some acceleration of decomposition was observed, as shown by the degradation rate values (dm1) in Table 3. The degradation rate increased from 20 %/min for pure PVC to 31 %/min and 37 %/ min for PVC/TiO2/Ag-H, PVC/TiO2/Ag-L samples, respectively. This is explained by the degradation mechanisms of PVC where first dechlorination takes place and soon after that the evaporation of hydrogen chloride, which always accelerates decomposition. The presence of Ag ions indicates an even higher acceleration. Higher acceleration results in an almost complete decomposition of samples in the first degradation step as observed for the PVC/TiO2/Ag-H nanocomposite sample.
Fig. 4 TG curves of TiO2 and TiO2 modified with silver and copper ions
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Fig. 5 TG curves of PVC nanocomposite films
Fig. 6 dTG curves of PVC nanocomposite films
Table 3 Values of initial decomposition temperature of the samples (T95), temperature of maximum rate of decomposition (Tmax1, Tmax2), the char residue (mr) and the decomposition rate of the samples (dm1, dm2) Samples
T95/C
Tmax1/C
Tmax2/C
mr/%
dm1 %min-1
dm2 %min-1
PVC
226.7
266.9
451.9
43
20
1.0
PVC/TiO2
238.4
270.6
447.6
40
24
1.9
PVC/Ti/Ag-H
234.7
263.8
–
50
31
0.6
PVC/Ti/Ag-L
234.1
264.4
450.7
40
37
1.7
PVC/Ti/Ag/Cu
232.9
259.5
460.6
31
12
8.4
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Further degradation is followed by the formation of benzene due to an intramolecular cyclization of polymer radicals formed by the cleavage of polymer chains. Polymer radicals react with each other and are then decomposed to form alkyl aromatic hydrocarbons and char residue [20, 21]. For the studied PVC/TiO2 nanocomposite samples two different degradation mechanisms of PVC were observed. One is that almost all sample decomposes in the first step at a significantly high rate of degradation (dm1 = 31 %/min) with the remaining high content of residual char (mr = 50 wt%). The second degradation mechanism involves decomposition in two steps. In the first step, the degradation temperature (Tmax1 = 259.5 C) and rate (dm1 = 12 %/min) fall significantly, whereas in the second step, the decomposition temperature (Tmax2 = 460.6 C) and rate (dm2 = 8.4 %/min) rise. It can be concluded that nanoparticles in PVC/TiO2 nanocomposites contributed to a higher thermal stability at initial thermal degradation (T95) because temperatures were cca 10 C higher for PVC nanocomposites than for pure PVC. Otherwise, nanoparticles in studied samples did not significantly enhance thermal stability except for the PVC/TiO2/Ag/Cu sample where thermal decomposition typical of polymer nanocomposites was observed. It is known from the literature [22] that nanoparticles have an effect on thermal degradation, as a consequence of polymer composite nanostructure reflected in the slow degradation rate and a lower content of char residue. SEM analysis To obtain PVC/TiO2 nanocomposites with efficient UV protection properties, it is of utmost importance to study their morphology. SEM micrographs of TiO2 and TiO2 modified with Ag or Ag/Cu are presented in Fig. 7. For studied PVC/TiO2 nanocomposite films, SEM micrographs were recorded and presented in Fig. 8. The most uniform morphology was observed for PVC/TiO2/Ag-L sample, followed by PVC/TiO2/Ag-H sample, Fig. 8b, c. The micrographs of these samples show a very low aggregation of modified TiO2 nanoparticles, indicating that the presence of silver ions affected and improved the miscibility of TiO2 and PVC polymer in comparison with sample PVC/TiO2, which is presented in Fig. 8a. On the other hand, the presence of copper ions in PVC/TiO2/Ag/Cu nanocomposite, where TiO2 was modified with silver and copper nitrate, caused a stronger aggregation of TiO2, Fig. 8d. The aggregation was an outcome of strong secondary interactions between the TiO2 particles that enhanced incompatibility with the PVC polymer matrix. The dispersion of particles is of great importance because it has a major impact on the nanocomposite properties (thermal, mechanical, barrier), and affects the application of the final material. The consequence of larger particle size formation is microparticles which deteriorate the properties of the material [23]. If the particles are homogeneously dispersed in the polymer matrix on the nano-level, then materials with improved or completely new properties will emerge, indicating good adhesion at the interface of two phases in a nanocomposite [24]. The best dispersion of a modified TiO2 absorber was observed in sample PVC/TiO2/Ag-L which showed the best mechanical properties and vice versa: the worst mechanical properties were observed in sample PVC/Ti/Ag/Cu with the presence of the largest aggregates.
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Fig. 7 SEM micrographs of a TiO2, b TiO2/Ag-L, c TiO2/Ag-H, d TiO2/Ag/Cu
Mechanical properties For studied PVC/TiO2 nanocomposite films, mechanical properties were determined because they are of crucial significance for the food packaging material. The results are given in Figs. 9 and 10 as tensile strength (r) and elongation at break (e). All PVC/TiO2 nanocomposite samples have the same fraction of the TiO2 absorber but they differ from each other only by the type of modifier. The results show that PVC/ TiO2/Ag-L sample with a low concentration of AgNO3 has the highest value of tensile strength (13.11 N/mm2) and elongation at break (205.42 %) where tensile strength is increased by 22 % and elongation at break by 30 % in comparison with the neat PVC polymer. The obtained results indicate a good miscibility of the modified TiO2 absorber and PVC matrix due to the dispersion on nano-level. The well-known fact [25] is that in polymer composites, nanostructure simultaneously
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Fig. 8 SEM micrographs of PVC nanocomposites: a PVC/TiO2, b PVC/TiO2/Ag-L, c PVC/TiO2/Ag-H, d PVC/TiO2/Ag/Cu
Fig. 9 Tensile strength of the studied nanocomposites
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Fig. 10 Elongation at break of the studied nanocomposites
causes an increase in tensile strength and elongation at break compared with the neat polymer, while these properties are in fact inversely proportional. Namely, the addition of microparticles to the polymer matrix will increase tensile strength and decrease elongation at break of a composite. Thus, the values of the mechanical properties of PVC/TiO2/Ag/Cu sample indicate a significant deterioration. It is evident that this is the result of the aggregation of TiO2 modified by silver and copper ions, which is confirmed by SEM micrographs. Furthermore, samples PVC/ TiO2 and PVC/TiO2/Ag-H that also contain nanoparticles show an increase in elongation at break of 4 and 26 %, and a decrease in tensile strength of 17 and 20 %, respectively. The obtained results of mechanical properties show the nanoparticles homogeneously dispersed in the polymer matrix where they initiated the formation of composite nanostructure, to the effect of a significant change in mechanical properties. Regarding the mechanical properties it is also possible to predict the appropriateness for a specific application, such as for polymer packaging films because it is necessary that the material has characteristic suppleness and flexibility. PVC used for study in this work was of industrial food grade and contained plasticizer (diisononylphthalate, DINP), tailored for this specific purpose and allowed to be used in contact with food which makes it easily processable, flexible and tough. Antimicrobial activity Since packed food is usually in contact with packaging materials, it is of great advantage when such packaging has an additional active role like antimicrobial activity. This is particularly suitable when the packaged fresh food contains high moisture content which initiates the growth of microorganisms. Since the studied TiO2 absorber was modified by silver ions, it was of practical use to examine the antimicrobial activity of prepared PVC/TiO2 films. The antimicrobial activity of
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prepared PVC/TiO2 nanocomposite films was studied by the disc diffusion method, which is a procedure approved by the Food and Drug Administration (FDA). A disc of studied material containing the antibacterial substance is placed in a sterile Petri dish with a nutrient agar and a suspension of the mixed culture of microorganisms. After the required time of incubation (48 h), antibacterial activity is measured as a length of the inhibition ring (an area around the disc of studied material without bacterial growth). The obtained results are presented in Fig. 11, and show that the inhibition of bacterial growth is not present for the samples PVC and PVC/TiO2, Fig. 11a, b. On contrary, bacterial growth is notably lower for PVC/TiO2/Ag-L and PVC/TiO2/Ag/ Cu samples, Fig. 11c, d. The highest antibacterial effect is noticed for the sample PVC/TiO2/Ag-H (Fig. 11e) where broader inhibition ring is formed. The antimicrobial activity of silver is based on its effect on bacterial cells where silver causes their structural and functional defects. Silver ions interact with the cell wall, cell membrane, bacterial DNA and proteins as well as with ribosomes and cause bacterial defects. The bacterial cell wall is formed of peptidoglycan so silver ions change its permeability, penetrate inside the bacterial cell, inhibit bacterial function, and cause its degradation and finally bacterial death [26]. For that reason many studies are focused on the development of food packaging materials with antimicrobial properties. The antimicrobial substances used for this purpose include natural additives or other substances: TiO2 [27–29] titania nanotubes [30], silver particles [31–35], gold particles [36], zinc oxide [37] and their modifications.
Fig. 11 Antimicrobial activity of PVC films: a PVC, b PVC/TiO2, c PVC/TiO2/Ag-L, d PVC/TiO2/Ag/ Cu and e PVC/TiO2/Ag-H
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Despite the good aspects of active nanocomposite materials for packaging, there are certain limitations and we must be careful during their potential commercialization and use. Namely, the complete knowledge about the influence of nanoparticles from active polymer packaging to human health and safety (absorption to blood and organs), due to nanoparticles migration, is still limited because such types of materials are relatively new. The effects of nanoparticles migration from food packaging to food are continuously studied. The processes of nanoparticles’ migration depend on their chemical and physical properties (size, shape, hydrophilic or hydrophobic nature). Many characterization procedures must be carried out before using these materials. However, risk assessments are still not conclusive [38]. All possible toxic effects on human health or environment should be analyzed.
Conclusions PVC/TiO2 nanocomposites as a packaging material can successfully protect food products during their transport and shelf life. Our results show that PVC nanocomposites have improved thermal stability in comparison with the pure PVC polymer. Thermal stability increases with a higher silver nitrate content. The sample prepared with silver and copper nitrate showed the best thermal stability due to a modified mechanism of thermal degradation in the presence of Ag and Cu ions. The results show that the PVC/TiO2/Ag-L has the highest value of tensile strength and elongation at break due to a good dispersion of TiO2 in the PVC matrix. The results also show that the adding of active component TiO2 modified with silver ions improves the UV blocking property of the nanocomposite. Samples TiO2/Ag-H and TiO2/Ag/Cu manifest the extension of absorbance to the visible region (400–800 nm). PVC/TiO2/Ag-H sample demonstrates an increased antimicrobial activity due to the highest silver content. Acknowledgments This work was financially supported in 2014 by the University of Zagreb, Croatia, through the Research Project PVC Nanocomposites with UV Protective Properties (TP1.29).
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