Determination of Partition Coefficients of Selected Model Migrants between Polyethylene and Polypropylene and Nanocomposite Polypropylene

Research Article Determination of Partition Coefficients of Selected Model Migrants between Polyethylene and Polypropylene and Nanocomposite Polypropylene Pablo Otero-Pazos,1David A. Pereira de Abreu,1 Raquel Sendon,1Ana Rodriguez Bernaldo de Quiros,1Inmaculada Angulo,2 Jose M. Cruz,3and Perfecto Paseiro-Losada1 1Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Pharmacy, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain 2Department of Plastic and Composites, GAIKER Research Centre, Zamudio, 48170 Bilbao, Spain 3Department of Chemical Engineering, ETSEI, University of Vigo, Vigo, 36200 Pontevedra, Spain Correspondence should be addressed to Raquel Sendon; [email protected] Received 8 November 2015; Accepted 24 December 2015 Academic Editor: Carola Esposito Corcione Copyright © 2016 Pablo Otero-Pazos 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. Studies on nanoparticles have focused the attention of the researchers because they can produce nanocomposites that exhibit unexpected hybrid properties. Polymeric materials are commonly used in food packaging, but from the standpoint of food safety, one of the main concerns on the use of these materials is the potential migration of low molecular substances from the packaging into the food. The key parameters of this phenomenon are the diffusion and partition coefficients. Studies on migration from food packaging with nanomaterials are very scarce. This study is focused on the determination of partition coefficients of different model migrants between the low-density polyethylene (LDPE) and polypropylene (PP) and between LDPE and nanocomposite polypropylene (naPP). The results show that the incorporation of nanoparticles in polypropylene increases the mass transport of model migrants from LDPE to naPP. This quantity of migrants absorbed into PP and naPP depends partially on the nature of the polymer and slightly on the chemical features of the migrant. Relation (𝑅PP/naPP) between partition coefficient𝐾LDPE/PP and partition coefficient 𝐾LDPE/naPP at 60∘Cand80 ∘CshowsthatonlyBHTat60 ∘Chasa𝑅PP/naPP less than 1. On the other hand, bisphenol A has the highest 𝑅PP/naPP with approximately 50 times more. 1. Introduction At the present time, multilayer systems are commonly used as food packaging. Plastic food packages contain additives (antioxidants, dyes, pigments, antifogging agents, stabilizers, and plasticizers) used to minimize degradation during processing, to facilitate processing and to increase stability during storage [1, 2]. The contamination of foodstuffs with substances from the packaging material that can arise either from the packaging process itself (e.g., via overprinting labels) or from migration processes [3]. From the food safety point of view, it is important to remark that some substances (volatiles, additives, monomers, and oligomers) are able to migrate from packaging to foods. Migration includes two phenomena (partition and diffusion) that are important in determining the concentration of contaminants in a food system at any time. In the migration process, the kinetic (diffusion) and thermodynamic equilibrium (partition) can be defined as an exchange of mass and energy between the packaging and food [4, 5]. Moreover, if food packaging is made up of various layers then it is important to know the capacity of substances to pass through them. Measurement of the rate at which the diffusion process occurs is expressed in terms of diffusion coefficient (cm2s−1). Theclassicaltheoryusedtomodelthediffusionprocessis Hindawi Publishing Corporation Journal of Chemistry Volume 2016, Article ID 3952631, 10 pages http://dx.doi.org/10.1155/2016/3952631 2Journal of Chemistry based on Fick’s laws. Diffusion (𝐷) is based on the fact that the rate of transfer (𝑅) of a migrant passing perpendicularly through unit area of a section is proportional to the concentration gradient between the two sides of the packaging: 𝑅 = 𝜕𝐶/𝜕𝑡 = −𝐷(𝐶)(d𝐶/d𝑋). In general, 𝐷is a function of the local diffusant concentration, 𝐶(mol m−3), 𝑡is the time (s), and 𝑋is the thickness of the material (m) [5, 6]. Amongst plastics, the highest diffusion rates occur in polyolefins, especially low-density polyethylene (LDPE) films. LDPE film was therefore chosen for the migration tests because it is widely used and provides a picture of the “worst case scenarios” [7]. Polypropylene(PP)isoneofthemostpolyolefinsusedinfood because it provides mechanical properties at low cost. Other vantages are their resistance to water vapor, acids, bases, grease, and solvents. The amount of package components that may migrate between layers of multilayer packaging material depends on the chemical and physical properties of them and the material itself. Different factors like partition coefficient, migrant concentration, time, temperature, packaging composition, molecular weight, solubility, diffusivity, and structures are the main controlling factors in migration [5, 8, 9]. In a multilayer packaging system, migrant transfer occurs to reach thermodynamic equilibrium. A partition coefficient (𝐾)is defined as the ratio of migrant concentration in the packaging material 𝐶𝑝1 and its concentration in another packaging material phase, 𝐶𝑝2 at equilibrium. 𝐾is defined as 𝐾= 𝐶𝑝1/𝐶𝑝2.When𝐾=1, the migrant concentration in each packaging material is equal, at equilibrium. 𝐾is higher when more migrant quantity is absorbed into packaging material 1 than in packaging material 2. Molecular size and structure of migrant and structure of polymeric matrix are the main factors that affect the partition coefficient [5]. The approach of the European Commission (EC) in controlling the safety of food packaging and other food-contact materials has been to assign specific migration limits (SML) to those substances with potential adverse toxicological properties. The SML restricts the quantity of the substance allowed tomigrate intofoods or foodsimulants. For instance, theSML established for some of the model substances evaluated in this study are 0.6 mg/kg for bisphenol A and Uvitex and 3 mg/kg for butylated hydroxyl toluene. Migration limits are based on the acceptable daily intake/tolerable daily intake established by the European Food Safety Authority for the substance in question and on the assumption that a person weighing 60 kg eats 1 kg of food each day, which has been packaged in the same material. One important consideration in the migration test condition is the ratio of the volume of the simulant to the area of the sample. The conventional value considered by the EC, 6 dm2/kg, is used in most cases. In some instances, where the additive is readily soluble in the simulant, it is considered acceptable to increase this ratio to improve the sensitivity of the analysis. Experimental studies of all these migrations are time consuming and highly expensive [10–13]. Through the mathematical model, experimental migration tests only will be carried out when the predicted migration values are higher than those established by legislation. For those reasons, it is interesting to get a mathematical model that allows predicting theses distribution processes. Moreover, nanotechnology is driving a long list of developments offering many alternatives. Organic-inorganic nanocomposites are compounds that exhibit hybrid properties derived from synergistic reactions between the two components. Nanocomposites based on organic polymers and inorganic clay minerals consisting of silicate layers are one of the most promising composite systems. Inside this system, a nanocompound can be intercalated or exfoliated, depending on the degree of dispersal of the nanoparticle between polymeric chains; however, a mixture of the two morphologies is often found. If the nanoparticles are completely dispersed between the polymeric chains, the nanocompound is exfoliated [14–17]; if they are not it is intercalated. Packages with exfoliated nanoparticles show better barrier properties than packages with intercalated nanoparticles. In general, thenanocompositeshavebeenfoundtoprovideeffective barriers to gaseous diffusion [16, 17]. Nevertheless, for its use in the field of food-contact materials in the European Union they should be specifically authorized, and at this moment only titanium nitride nanoparticles are approved for use as additives in polyethylene terephthalate (PET) intended to come into contact with foodstuffs (Commission Regulation (EU) number 10/2011 of 14 January 2011) [18]. The present study is focused on the determination of partition coefficients of trans,trans-1,4-diphenyl-1,3-butadiene (DPBD), 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan), Benzophenone, 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (Uvitex OB®), diphenyl phthalate (DPP), isopropyl- 9H-thioxanthen-9-one (ITX), butylated hydroxyl toluene (BHT), and bisphenol A (BPA) between the LDPE and polypropylene (PP) and between LDPE and nanocomposite polypropylene (naPP). Benzophenone and ITX are photoinitiators in inks, employed to print food packaging surfaces; DPBD is a fluorescent whitening agent; triclosan is a broadspectrum antimicrobial compound; DPP is a plasticizer that improves the performance of polyvinylpyrrolidone; BPA is used in the production of polycarbonate and epoxy resins, flame retardants, and other products; BHT is a small phenolic antioxidant, used in plastics at levels between hundreds of and afewthousandmgperkgasplasticstabilizer,andUvitexOB is an optical brightening agent [3, 7, 13, 19–22]. Selection of these substances was made on the basis of their representativeness as potential migrants from food-contact materials so that the results can be extrapolated to other substances of similar characteristics. The results were discussed in terms of the parameters that may have the greatest effect on migration. 2. Materials and Methods 2.1. Materials. The nanoclay used is natural montmorillonites, modified with a quaternary ammonium salt to facilitate dispersal in the polymeric matrix. Cloisite 15 A nanoparticles were used (provided by Southern Clay Product, Texas, USA; http://www.byk.com). Cloisite 15 A has an average particle size of 6 𝜇m and cationic exchange capacity of 125 meq/100 g of clay. Montmorillonite is suitable for incorporation in an apolar polymeric matrix such as polypropylene. The polypropylene selected was a copolymer of Journal of Chemistry 3 intermediate-high fluidity (ISPLEN PB 171 H1M), with MFI: 9.5 g/10 min, ISO 1133 (230∘C; 2.16 kg). 2.2. Reagents and Standard Solutions. All reagents were of analytical grade. Ethanol, acetonitrile, and tetrahydrofuran were purchased from Merck (Darmstadt, Germany). Water used for all solutions was obtained from a Milli-Q water purification system (Millipore; Bedford, MA). Model migrants standards were obtained as follows: Benzophenone and DPBD (98%) were from Sigma-Aldrich (Steinheim, Germany); triclosan (97%), Uvitex OB (≥99.0%), and BHT (≥99.0%) were from Fluka (Steinheim, Germany); DPP (≥99.0%), BPA (≥99.0%) and ITX (97%) were from Aldrich-Chemie (Steinheim, Germany). Stock standard solutions were prepared in ethanol and stored in amber flasks at 4∘C. 2.3. Film Samples. FilmsofLDPEwerespikedwiththe migrants by extrusion process: (i) LDPE film (thickness: 397.7±13.9𝜇m; 𝐶Benzophenone = 417.7 ± 67.4𝜇g⋅g−1;𝐶DPBD = 755.0 ± 31.6𝜇g⋅g−1; 𝐶Uvitex = 900.3±55.2𝜇g⋅g−1)wasspikedwithBenzophenone, DPBD, and Uvitex OB. (ii) LDPE film (thickness: 410.6±16.7𝜇m; 𝐶DPP = 457.3± 113.2𝜇g⋅g−1;𝐶ITX = 1138.0 ± 206.9𝜇g⋅g−1;𝐶BHT = 839.8 ± 170.3𝜇g⋅g−1)wasspikedwithDPP,ITX,and BHT. (iii) LDPE film (thickness: 394.8±17.6𝜇m; 𝐶BPA = 933.7± 57.5𝜇g⋅g−1;𝐶Triclosan = 1233.1 ± 85.1𝜇g⋅g−1)was spiked with BPA and triclosan. The obtaining of the exfoliated film was previously described by Pereira de Abreu et al. [16]. A polypropylene masterbatch was used to elaborate that the nanocomposite polypropylene (naPP) film contained equal percentages of polypropylene and polypropylene modified with maleic anhydride and Cloisite 15 A (C15A). This masterbatch was prepared in the rollers at temperature of 160∘C. The resulting mixture was then crushed and diluted in the PP matrix by compounding in a double screw extruder (MIKE 27 GL-36 D LEISTRITZ) (screw diameter: 27 mm, screw length: 36×𝐷, and screw speed range: 10–400 rpm). For both the PP and its nanocompound, the following conditions were used in the extruder: barrel was set to 160– 170∘C; Zones I and II were at 160∘C, Zones III, IV, and V were at 163∘C, Zones VI, VII, and VIII were at 165∘C, and Zone IX was at 170∘C. The inorganic content of the samples was determined by measuring the weight before and after burning the organic component. Screw speed was set to 300 rpm with a maximum temperature of 170∘C for the PP. The obtained pellet was passed through the double screw extruder twice to homogenize the mixture thoroughly. 2.4. Analytical Procedure. To carry out partition coefficients calculation,LDPE,PP,andnaPPwerecutinpiecesof4× 4cm2.Thethicknessalongthefilmwasmeasuredtoobtainan average value. LDPE film was put into contact with one face Table 1: High performance liquid chromatography parameters employed in the analysis of Benzophenone, diphenylbutadiene (DPBD), and Uvitex. Injection volume 20 𝜇L Eluents A: Milli-Q water B: THF 30% methanolic solution (v/v) Time (min) % Eluent A % Eluent B Eluent gradient 03070 43070 10 0 100 17 0 100 Posttime: 3 minutes Substance UV (nm) Detection Benzophenone 256 DPBD 330 Uvitex OB 372 of PP or naPP. Samples were then wrapped with aluminum foil to protect them from the light and were placed inside a plasticbagandincubatedintheovenat60and80 ∘C. The incubation times used at 60∘Cwere40,55,and70daysandat 80∘Cwere25,40,and60days,respectively.Then,atselected times,thesetwaswithdrawnoftheovenandthespikedLDPE film and the other plastic (PP or naPP) were separated. After this, an extraction of the selected migrants from spiked LDPE, PP, and naPP films was made. The films were extracted in a 60mLamberflaskwith50mLofethanolfor6hoursat70 ∘C. After this time, the flask was removed from the oven and the filmwasseparatedfromtheliquidphase.Thenanaliquotof this liquid phase was filtered by a 0.50 𝜇mfilterandanalyzed by HPLC. 2.5. High-Performance Liquid Chromatography Analysis. The HPLC-DAD-FLD system (Hewlett-Packard, Waldbronn, Germany) was fitted with a HP1100 quaternary pump, a degassing device, an autosampler, a column thermostatting system, a diode array UV detector, and a fluorescence array detector. HP ChemStation chromatographic software was used for data acquisition. Chromatographic separation was performed on a Kromasil ODS C18 column (25 ×0.32 cm ID, 5𝜇mparticlesize)at30 ∘Candaflowrateof0.5mL⋅min−1; detailed analytical conditions are shown in Tables 1, 2, and 3. 3. Results and Discussion Model migrants were selected based on their structure and chemical properties, trying to cover the maximum area in terms of molecular size and polarity. Ranges of temperature and time exposition were selected seeking a compromise between the speed of the assay and the stability of the molecules. Thus, 60∘Cand80 ∘Cwerechosen, withexposuretimesof40,55,and70daysand25,40,and60 days, respectively. Effectiveness of extraction procedure was tested in films spiked with the selected substances. These films were 4Journal of Chemistry Table 2: High performance liquid chromatography parameters employed in the analysis of diphenyl phthalate (DPP), 2-isopropyl- thioxanthone (ITX), and butylated hydroxyl toluene (BHT). Injection volume 10 𝜇L Eluents A: Milli-Q water B: Acetonitrile Time (min) % Eluent A % Eluent B Eluent gradient 040 60 140 60 17 0 100 20 0 100 Posttime: 5 minutes Substance DAD (nm) FLD (nm) Detection DPP 205 — ITX — Ex 250; Em 410 BHT 205 — Table 3: High performance liquid chromatography parameters employed in the analysis of bisphenol A (BPA) and triclosan. Injection volume 10 𝜇L Eluents A: Milli-Q water B: Acetonitrile Time (min) % Eluent A % Eluent B Eluent gradient 040 60 340 60 90 100 17 0 100 Posttime: 5 minutes Substance DAD (nm) FLD (nm) Detection Triclosan 205 — BPA — Ex 225; Em 305 Table 4: Calibration curves and retention times of analyzed substances. Substance 𝑅𝑡(min) 𝑅2Equation Benzophenone 5.3 0.99998 𝑦 = 216.28𝑥 −5.6776 DPBD 12.7 0.99988 𝑦 = 567.67𝑥 −22.856 Uvitex 14.8 0.99998 𝑦 = 260.44𝑥 +5.4778 DPP 11.7 0.99999 𝑦 = 174.88𝑥+ 0.5316 ITX 16.8 0.99997 𝑦 = 262.77𝑥+ 1.5187 BHT 17.8 0.99950 𝑦 = 161.96𝑥 +74.026 Bisphenol A 4.1 0.99999 𝑦 = 662.48𝑥− 1.731 Triclosan 11.0 0.99999 𝑦 = 239.82𝑥 +2.327 extracted several times. Subsequent extractions of the film resulted in quantities of substances that were undetectable. Quantification was obtained using a series of standards of selected substances solutions of known concentration. Calibration curves were constructed using five concentration levels and they were fitted to linear equations within the 0.25– 5𝜇gmL −1 range. The equations are shown in Table 4. Partition coefficient LDPE/PP or naPP for each model substance was calculated using the following equation: 𝐾LDPE/PP =𝐶LDPEeq 𝐶PPeq , 𝐾LDPE/naPP =𝐶LDPEeq 𝐶naPPeq , (1) where 𝐾LDPE/PP is the partition coefficient between LDPE and PP; 𝐾LDPE/naPP is the partition coefficient between LDPE and naPP; 𝐶PPeq is the concentration of substance in PP at equilibrium, in 𝜇gg −1;𝐶naPPeq is the concentration of substance in naPP at equilibrium, in 𝜇gg −1;𝐶LDPEeq is the concentration ofsubstanceintheLDPEatequilibrium,in𝜇gg −1. The partition coefficient of low-density polyethylene/polypropylene (𝐾LDPE/PP) and low-density polyethylene/nanocomposite polypropylene (𝐾LDPE/naPP), that is, the relative solubility of the migrants at equilibrium between the polyethylene and the polypropylene or nanocomposite polypropylene,isshowninTables5and6,respectively. The partition coefficient values correspond to the relative solubility of the migrant at equilibrium between the polyethylene and the polypropylene or nanocomposite polypropylene [23]. Benzophenone, DPP, DPBD, BHT, ITX, and triclosan show a 𝐾LDPE/PP near to 1. Because of the composition of polyethylene -[-CH2-CH2-]n- and polypropylene -[-CH2- CH(CH2)-]n-, with a similar structural base, these results were expected, whereas in the cases of Uvitex and bisphenol A the equilibrium was not reached at the end of the assay (i.e., at 60∘C, 70 days or at 80∘C, 60 days). The molecular size, chemical structure, and physicochemical properties (polarity, solubility, and hydrogen bonding) of migrants and type of packaging are important parameters affecting partition coefficient. Due to the large sizes of the polymer molecules, the secondary bond forces (van der Waals forces and hydrogen bonding) assume much greater roles in influencing physical properties than they do in small organic molecules [24]. Smaller molecules are absorbed more rapidly because they have a greater diffusivity than larger molecules. In fact, the partition coefficient of a series of compounds with the same functional group (i.e., esters or aldehydes) increased with increasing carbon chain length [5, 25–30]. The alcohols in aqueous solutions increase its partition coefficients to increase its molecular weight. The differences in polarity of migrant and polymer are another important factor in the absorption process [5]. Alcohols and short-chained esters (polar substances) have higher partition coefficients in the oil/polymer system than in the water/polymer system, while aldehydes (long carbon nonpolar chains) have lower partition coefficients in the oil/polymer than in the water/polymer [29]. In mixtures, some compounds exhibit a lower absorption rate than systems containing the individual compounds. This effect could be due to the fact that the absorbed components change the properties of polymer and/or the compounds compete for a limited number of active sorption sites [29, 30]. When more than one type of solids is present, alcohols preferentially are adsorbed to Journal of Chemistry 5 Table 5: Partition coefficient of polyethylene/polypropylene (𝐾LDPE/PP)at60 ∘Cand80 ∘C for all substances studied. Model migrant Temperature Time 𝐾LDPE/PP Model migrant Temperature Time 𝐾LDPE/PP Benzophenone 40 days 0.86 DPP 40 days 0.86 60∘C 55 days 0.84 60∘C55days1.02 70 days 0.86 70 days 0.89 25 days 1.08 25 days 1.21 80∘C40days1.04 80 ∘C40days1.04 60 days 1.03 60 days 1.16 DPBD 40 days 1.16 BHT 40 days 0.62 60∘C55days1.15 60 ∘C 55 days 0.66 70 days 1.09 70 days 0.56 25 days 1.36 25 days 0.90 80∘C40days1.31 80 ∘C 40 days 0.67 60 days 1.36 60 days 0.86 Uvitex OB 40 days 41.35 Bisphenol A 40 days 14.23 60∘C 55 days 16.43 60∘C 55 days 14.13 70 days 9.42 70 days 12.05 25 days 6.26 25 days 8.19 80∘C 40 days 4.50 80∘C40days7.17 60 days 3.61 60 days 5.70 ITX 40 days 0.99 Triclosan 40 days 0.88 60∘C 55 days 1.05 60∘C 55 days 0.93 70 days 0.99 70 days 0.91 25 days 1.26 25 days 1.13 80∘C 40 days 1.07 80∘C40days1.12 60 days 1.23 60 days 1.15 the solid with higher affinity for alcohols, until all binding sitesonthesolidhavebeenoccupied.Silicaandsoyprotein increase solubility of alcohols in water by adsorbing them through hydrophobic association and/or hydrogen bonds [31]. Some of these factors could be the reasons because the diffusion for Uvitex and bisphenol A was lower than the rest of compounds and the equilibrium was not achieved. However, the 𝐾LDPE/PP should be close to 1 when the equilibrium will be reached. PPhasnopolargroupsinitsbackboneandthehomogeneous dispersion of the silicate layers in PP is problematic without the utilization of a functional oligomer (maleic anhydride (MA) modified PP oligomer). There are two important factors in terms of the structures of the oligomers in order to obtain the complete homogeneous dispersion of the silicate layers. The oligomers should include a certain amount of polar groups to be intercalated between silicate layers through hydrogen bonding and the oligomers should be well miscible with PP. Functional oligomer is intercalated into the silicate layers at the first stage of the mixing process because the driving force of the intercalation originated from the strong hydrogen bonding between the maleic anhydride group (or COOH group generated from the hydrolysis of the maleic anhydride group) and the oxygen groups of the silicates. The interlayer spacing of the clay is increased and theinteractionsofthelayersaredecreased.Then,intercalated clays with the oligomers can enter in contact with PP under a strong shear field. If the miscibility of PP-MA with PP is good enough to disperse at the molecular level, the exfoliation oftheintercalatedclayshouldtakeplacesmoothly[32,33]. Polymer/silicate nanocomposites have better properties that include high barrier properties, improved tensile characteristics, higher heat deflection temperature, better scratch resistance,andincreasedflameretardancy[13,16,17,34]. Observing results obtained, only in the cases of the Uvitex and bisphenol A at 60∘C(70days)itwasnotpossibletoreach equilibrium in the LDPE/naPP system. Triclosan, bisphenol (80∘C), and DPP show the smallest partition coefficient of low-density polyethylene/nanocomposite polypropylene (𝐾LDPE/naPP), while Uvitex (80∘C) has the biggest 𝐾LDPE/naPP. Table 7 shows the properties of the substances used in this study. The molecular size, chemical structure, and physicochemical properties of migrants and naPP characteristics could explain these results. These substances have the capacity to form secondary bond forces, only DPBD has no polar surface area. The polar surface area of a molecule is defined as the area of its van der Waals surface that arises from oxygen or nitrogen atoms or hydrogen atoms attached to oxygen or nitrogenatoms.Assuch,itisclearlyrelatedtothecapacity of a compound to form hydrogen bonds [35]. But DPBD can form secondary bond forces (van der Waals forces). naPP has polar zones where the exfoliated nanoparticles and maleic anhydridegrouparestaying.Maleicanhydridegroupcanalso 6Journal of Chemistry Table 6: Partition coefficient of polyethylene/nanocomposite polypropylene (𝐾LDPE/naPP)at60 ∘Cand80 ∘C for all substances studied. Model migrant Temperature Time 𝐾LDPE/naPP Model migrant Temperature Time 𝐾LDPE/naPP Benzophenone 40 days 0.35 DPP 40 days 0.13 60∘C 55 days 0.33 60∘C 55 days 0.07 70 days 0.34 70 days 0.10 25 days — 25 days 0.10 80∘C 40 days 0.48 80∘C 40 days 0.09 60 days 0.45 60 days 0.12 DPBD 40 days 0.20 BHT 40 days 0.67 60∘C 55 days 0.18 60∘C 55 days 0.55 70 days 0.20 70 days 0.55 25 days — 25 days 0.84 80∘C 40 days 0.27 80∘C 40 days 0.79 60 days 0.32 60 days 0.97 Uvitex OB 40 days 11.57 Bisphenol A 40 days 0.87 60∘C55days12.58 60 ∘C 55 days 0.91 70 days 4.67 70 days 0.60 25 days — 25 days — 80∘C40days1.35 80 ∘C 40 days 0.15 60 days 1.73 60 days 0.11 ITX 40 days 0.41 Triclosan 40 days 0.05 60∘C 55 days 0.37 60∘C 55 days 0.05 70 days 0.41 70 days 0.04 25 days 0.57 25 days — 80∘C 40 days 0.60 80∘C 40 days 0.06 60 days 0.60 60 days 0.06 be found distributed in the PP without being attached to the nanoparticles. These areas could attract and form hydrogen bondsandvanderWaalsforcesbondswithmigrantsfrom the polyethylene. This ability to form secondary bond forces couldexplainthedecreaseinthevaluesof𝐾LDPE/naPP found for all these molecules. 𝑅PP/naPP is a measure of the relative solubility of a substance in PP compared to its solubility in naPP. Relation (𝑅PP/naPP) between 𝐾LDPE/PP and 𝐾LDPE/naPP at 60∘Cand80 ∘C for all substances studied is shown in Table 8. 𝑅PP/naPP is defined as 𝑅PP/naPP =𝐾LDPE/PP 𝐾LDPE/naPP .(2) When 𝑅=1, the migrant concentration in naPP at equilibrium equals the concentration in PP at equilibrium. 𝑅 is higher than 1 when more migrant is present into the naPP than in the PP. Only BHT at 60∘Chasan𝑅PP/naPP less than 1 but is very near to one. On the other hand, BPA has the highest 𝑅PP/naPP (approximately 50 times more). These results showthatallsubstancesemployedinthisstudyhavemore preference for naPP than for PP. At 60∘C(70days)theaffinity in decreasing order for naPP is triclosan >BPA >DPP > DPBD >Benzophenone >ITX >Uvitex >BHT. At 80∘C(60 days) the preference in decreasing order for naPP is BPA > triclosan >DPP >DPBD >Benzophenone >Uvitex >ITX > BHT. Generally, the incorporation of Cloisite to PP increased the affinity of the selected substances compared to PP and LDPE. These substances used in the preparation of food packaging could migrate into food. It is interesting to remark that the solubility of these substances has increased in the material with nanoparticles respect with the polypropylene alone. With this information, we can speculate about the behaviour of these kinds of plastics in contact with another element, for example, foodstuff. In this case, the increased solubility in nanoparticle polymer could represent a less quantity of substances that would pass away across the naPP and could beagoodoptiontoincreasethesafetyandsecurityinfood packaging. More investigation about this approach must be carried out. 4. Conclusions Partition coefficient of eight model substances between LDPE and PP and naPP was determined from their concentration in each polymer analysed. These concentrations were determined by using High-Performance Liquid Chromatography. The additives were of different chemical types, molecular weights and used for different functions. The obtaining of partition coefficients lower than 1 indicates that affinity of additives is greater for naPP than PP, even when substances with less tendency to migrate are present. Relation (𝑅PP/naPP) between 𝐾LDPE/PP and 𝐾LDPE/naPP Journal of Chemistry 7 Table 7: Chemical information of selected migrants. Migrant Chemical name CAS number MW Polar surface area ( ˚ A2) Molar volume (cm3⋅mol−1) Hbond donor Hbond acceptor Structure Benzophenone Methanone diphenyl 119-61-9 182.22 17.07 167.50 0 1 O C13H10O Bisphenol A 4,4󸀠-Dihydroxy-2,2- diphenylpropane 80-05-7 228.29 40.46 199.56 2 2 HO OH C15H16O2 Triclosan 5-Chloro-2-(2,4- dichlorophenoxy)phenol 3380-34-5 289.54 29.46 194.30 1 2 O Cl Cl OH Cl C12H7Cl3O2 DPBD trans,trans-1,4-Diphenyl-1,3- butadiene 538-81-8 206.28 0.00 199.20 0 0 C16H14 Uvitex 2,5-Bis(5-tert-butyl-2- benzoxazolyl) thiophene 7128-64-5 430.56 80.30 363.10 0 4 N O S O N C26H26N2O2S DPP Diphenyl phthalate 84-62-8 318.32 52.60 256.20 0 4 O O O O C20H14O4 8Journal of Chemistry Table 7: Continued. Migrant Chemical name CAS number MW Polar surface area ( ˚ A2) Molar volume (cm3⋅mol−1) Hbond donor Hbond acceptor Structure ITX Isopropyl-9H-thioxanthen-9-one 75081-21-9 254.35 42.37 212.12 1 1 S O C16H14OS BHT Butylatedhydroxyltoluene 128-37-0 220.35 20.23 237.50 1 1 OH C15H24O Journal of Chemistry 9 Table 8: Relation (𝑅PP/naPP)𝐾LDPE/PP/𝐾LDPE/naPP at 60∘Cand80 ∘C for all substances studied. Model migrant Temperature Time 𝑅PP/naPP Model migrant Temperature Time 𝑅PP/naPP Benzophenone 40 days 2.47 DPP 40 days 6.70 60∘C55days2.54 60 ∘C55days13.84 70 days 2.55 70 days 8.53 25 days — 25 days 11.92 80∘C40days2.15 80 ∘C 40 days 11.35 60 days 2.30 60 days 10.02 DPBD 40 days 5.69 BHT 40 days 0.93 60∘C 55 days 6.33 60∘C55days1.20 70 days 5.50 70 days 1.02 25 days — 25 days 1.07 80∘C 40 days 4.82 80∘C 40 days 0.85 60 days 4.27 60 days 0.89 Uvitex OB 40 days 3.57 Bisphenol A 40 days 16.30 60∘C55days1.31 60 ∘C55days15.54 70 days 2.02 70 days 20.19 25 days — 25 days — 80∘C40days3.34 80 ∘C 40 days 48.41 60 days 2.09 60 days 54.19 ITX 40 days 2.42 Triclosan 40 days 16.41 60∘C55days2.84 60 ∘C 55 days 16.98 70 days 2.44 70 days 23.01 25 days 2.22 25 days — 80∘C40days1.77 80 ∘C 40 days 18.28 60 days 2.03 60 days 18.60 at 60∘Cand80 ∘CshowsthatonlyBHTat60 ∘Chasa 𝑅PP/naPP lowerthan1butisverynearto1.Ontheother hand, BPA has the highest 𝑅PP/naPP (approximately 50 times more). The amount of migrants absorbed into PP and naPP depends partially on the nature of the polymer and slightly on the chemical features of the model migrant. Factors that affected absorption include molecular size of migrants, polarity, and solubility properties of both the polymers and themigrants.Theresultsobtainedcouldprovethepresence of interactions between the model migrants and the naPP. As far as we know there are no published studies over the partition coefficient of model migrants into nanocomposite polypropylene films. For that reason, the results obtained in the present study contribute to a better understanding of mass transport processes of migrants to nanocomposite film. Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgments The study was financially supported by the Xunta de Galicia (Project no. INCITE08PXIB203096). R. 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