A novel methodology for controlled migration of antifog from thin polyolefin films.
Transparent plastic films are widely used in agriculture as covering of greenhouses and in the food industrial packaging films. The films are usually prepared using low polar hydrophobic thermoplastics such as polyethylene (PE). Within a greenhouse, the temperature and humidity are usually higher than at the outside and fog may appear on the inside of the PE film [l]. The same phenomenon is observed when food, in plastic film packaging, is stored at low temperatures. The term "fogging" is used to describe the condensation of water vapor on a plastic film's surface in the form of small, discrete droplets causing diffuse reflection of the light, which damages the films transparency. Additional damage can occur by drizzling droplets which result in plants and food spoiling. Antifogging additives have been developed to overcome these problems , Antifogging (AF) additives are tensoactive materials, mainly nonionic surfactants, which increase the interfacial tension solid-vapor, thus creating sequential thin film of water rather than discrete droplets. The AF additives consist of two parts: a hydrophilic head and a lipophilic tail. When the surface tension of both the droplets and the film becomes equal, the droplets spread and a continuous film of water is formed, described as the "antifog" effect. When added to a polyethylene film, the AF additives migrate to the films' surface; simultaneously, parts of the AF molecules dissolve in the water droplets. As the water film is rinsed, the AF molecules surface concentration decreases. This generates a concentration gradient between the surface and the bulk which results in migration of AF molecules from the bulk to the surface. The loss of the AF effect is related to the decrease of the AF molecules concentration in the polymer skin.
The rate of migration of small AF molecules in polymeric matrix is controlled by its solubility and the diffusion coefficients. Additives of low solubility and high diffusion rate migrate fast to the polymer surface changing its wettability. The process of migration involves cooperative motion of the AF molecule and the polymer segments. Thus, the migration rate depends on the size and shape of the migrating additive, as well as, on the magnitude and distribution of the free-volume in the polymer. In polyethylene, the migration rate of AF additives decreases when the additive size increases [3, 4],
Efficient methods for extended AF performance have been sought in recent years. Researchers have focused on methods of grafting the AF molecules to polymer chains or coating substances, physical compounding of AF in the presence of nano-particles such as sol-gel/silica/alumina/ titania, or grafting the AF molecules to surfaces such as glass using corona treatment. To overcome additive migration, various methods for additive affixing have been suggested. For example, silica nanoparticles were suspended in methanol and grafted with methoxy silane and poly ethylene glygol (PEG) methoxy silane to create PEG grafted particles. The PEG grafted particles were used for coating, Meijers et al. . Nakayama et al.  suggested fixing inorganic particles with an unsaturated bond to a resin substrate surface, the resin coating the substrate surface is chemically bonded to the particles by graft polymerization. In another work, Lee et al. , low density polyethylene was grafted with trifluoroacetic acid allyl ester using [gamma]-ray irradiation.
In the present work, a new simple method of controlled migration of AF is described, by grafting the AF molecules to submicron silica particles by a radical reaction; the treated particles are then melt-mixed with the polymeric matrix. The silica particles are used as AF molecules physical migration inhibitors. Silica particles were chosen due to their small size, which does not scatter light significantly. It is thus possible to make composites that retain their optical clarity. Furthermore, the small size of the particles leads to large interfacial area for grafting . Glycerol monooleate (GMO) is used as an AF. During the grafting reaction two phase fractions are formed: A GMO fraction attached to the particles surface, which cannot be detached by hot ethanol extraction, and an unattached GMO fraction which can be dissolved by ethanol. Solvent extraction and TGA measurements have confirmed the existence of grafted GMO molecules to the silica surfaces. FTIR measurements and iodine number titration have indicated the disappearance of double bonds as a result of the radical reaction.
Commercial polyethylene (LLDPE) pellets were used as received and after grinding in a Thomas-Wiley laboratory mill, model 4275, 2-mm mesh (Nova Chemicals, Canada). Commercial GMO, shown in Fig. 1, was used without further purification. Benzoyl peroxide (BP) was used after recrystallization  (Merck-Schuhardt, Germany). LuperoxlOl peroxide was used as received (Arkema, France). Commercial modified fumed silica, treated with methacrylsilane, Aerosil R-711 was used as received (4.5-6.5% carbon content, 12 nm in size, specific surface area of 150 [m.sup.2] [g.sup.-l], Degussa, Germany). Ethanol 96% (Gadot Chemicals, Israel) was used as a solvent. Wijs' solution was used as a reagent for iodine number titration (Merck-Schuhardt, Germany). Sodium thiosulfate was used as a titration solution in the iodine number tests (Merck-Schuhardt, Germany). Chloroform was used as solvent in the iodine number titration (Biolab Chemicals, Israel). Mercury acetate dissolved in acetic acid was used as catalyst in the iodine number titration (Sigma-Aldrich, USA). Potassium iodide (KI) was used for the titration reaction (Merck-Schuhardt, Germany). Starch dissolved in distilled water was used as an indicator (Sigma-Aldrich, Germany).
Preparation of Modified AF
Modified AF particles were prepared as schematically shown in Fig. 2: A certain amount of GMO and different BP concentrations were dissolved in ethanol and mixed with the silica particles. Reactants were introduced according to the amounts depicted in Table 1. The reaction took place under nitrogen atmosphere, in an oven at 70[degrees]C, for 20 h. Following the reaction, the non-grafted unattached GMO fraction was extracted with ethanol at 60[degrees]C for 24 h, resulting only the GMO grafted inorganic particles. The product was vacuum dried at 60[degrees]C for 3 h. . The suggested reaction and its products are shown in Fig. 3.
Preparation of Modified AF Compounded PE Sheets
Modified AF molecules were mixed with PE pellets and ground PE prior to blending using a batch Brabender plastograph, equipped with a 50 [cm.sup.3] cell, 220[degrees]C, 50 rpm, for 15 min. Afterward, the well-mixed product was processed into a 300-pm film using a 40/50 ton hand operated hydraulic laboratory press (George E. Moore & sons, Birmingham) at 180[degrees]C, for 3 min.
The modified AF particles were analyzed using a TA 2050 (TA Instruments, United States) thermal gravimetric analyzer (TGA). Samples were heated under air, at a heating rate of 20[degrees]C [min.sup.-1], monitoring their weight loss as function of temperature. Grafted GMO on silica samples were assessed by TGA thermograms and decomposition temperatures were assessed by TGA thermogram derivatives (DTG).
Fourier Transform Infrared (FTIR) measurements were carried out using a Nicolet 6700 spectrophotometer (Thermo Scientific, United States). Measurements were made both in the ATR and transmittance modes. Each spectrum was recorded at a resolution level of 4 [cm.sup.-1] and 128 scans were used. The ATR mode is used for tracking the changes in the linking area of the molecules. The transmittance mode is used for quantitative determination of the total (bulk) additive concentration, a method used to perform migration tests . To perform quantitative analysis using FTIR spectra, several films containing different AF and silica concentrations were prepared. A calibration curve was calculated using the partial least square (PLS) method. The calibration curves yielded a good regression factor. Accelerated migration tests were performed under the following conditions: 300 fim thick films of polyethylene+ AF in a 75[degrees]C ethanol. The tested PE films were immersed in hot ethanol for a short time periods, followed by drying and FTIR analysis, in the transmittance mode. The AF concentration was determined using the calibration curve.
Nuclear magnetic resonance ([sup.1]H, [sup.13]C NMR) measurements were recorded on Bruker DPX 200 spectrometer (Bruker, United States) in deuterated chloroform.
The GMO double bond is located at the center of the molecules' lipophilic tail. It was essential to examine if it is reactive in the presence of peroxide. Reduction of double bonds content on the AF grafted particles was observed using iodine number titration. The volumetric titration is based on a redox reaction, as follows:
C[H.sub.2] = C[H.sub.2] + ICl [right arrow] CHI - CHCl
ICl + KI [right arrow] KCl + [I.sub.2]
[I.sub.2] + 2[Na.sub.2][S.sub.2][O.sub.3] [right arrow] 2NaI + [Na.sub.2][S.sub.2][O.sub.4]
A sample (0.2 g) containing neat GMO after reaction with different amounts of silica and peroxide (as shown in Table 1) was dispersed in chloroform (15 [cm.sup.3]); 25 cm3 of Wijs' reagent was added with 10 [cm.sup.3] mercury acetate as a catalyst under stirring, for 5 min. After 5-min reaction period, KI 15 wt% solution (15 [cm.sup.3]) was added and the sample was titrated with sodium thiosulfate until the color changes from brown to pale yellow. When the solution turned pale yellow; starch indicator was added as a reaction indicator. The iodine number was calculated according to Eq. 1:
IN = [12.69 x N([V.sub.0] - [V.sub.s])/w] (1)
where w is the weight in grams of the sample, N is the normal concentration of the sodium thiosulfate solution used (in mol [L.sup.-1]), [V.sub.0] is the volume in milliliters of the sodium thiosulfate used to titrate the blank and [V.sub.s] is the volume in milliliters of the sodium thiosulfate solution used to titrate the sample .
RESULTS AND DISCUSSION
Figure 4 depicts TGA thermograms in air of methacrylic silica, GMO/Silica grafted in ethanol with 1% peroxide concentration, at 70[degrees]C, before and after ethanol extraction.
Methacrylic silica has shown a weight loss of 6% attributed to the methacrylic groups of the silica used. Before extraction, the sample contains silica particles, attached and unattached GMO fractions. After extraction only the attached GMO and methacrylic phase remain (~9%), thus, it is suggested that the 3 wt% excess is the weight loss of the attached GMO , The DTG curves shown in Fig. 5a of the neat GMO reference sample exhibit three decomposition temperatures at 324, 428, and 495[degrees]C, while the unextracted sample shows only two decomposition temperatures at 202 and 350[degrees]C. It is suggested that the 202[degrees]C peak represents the attached silica particles fraction, and the 350[degrees]C represents elimination of residual GMO (boiling point of ~320[degrees]C). The DTG curves in Fig. 5b of the neat silica reference sample exhibit a single transition at 311[degrees]C. The extracted sample, however, shows two peaks at 288 and 317[degrees]C (Fig. 5b), which represent degradation of the attached GMO fractions.
Figure 6 depicts further characterization of the silicaGMO chemical grafting using FTIR spectra. The grafted sample is compared to both the methacrylic silica and to the neat GMO. Several changes in the spectra, comparing the three samples, are observed: (1) formation of peaks in the range 3000-2800 [cm.sup.-1] of the unextracted sample in comparison with the neat silica; (2) disappearance of a peak at ~ 1639 [cm.sup.-1] of the unextracted sample in comparison with the silica and (3) disappearance of a peak at ~3004 [cm.sup.-1] of the unextracted sample in comparison with GMO. The formed peaks between 3000 and 2800 cm"1 are attributed to the presence of GMO in the sample. The peak at ~ 1639 [cm.sup.-1] is the characteristic C=C stretching vibration of the methacrylic group. It is suggested that the disappearance of this peak is due to the reaction of the methacrylic groups with the AF, meaning, a grafting reaction (left type in Fig. 3) has taken place. The peak at ~3004 [cm.sup.-1] in the GMO spectra is related to the double bond . The disappearance of the double-bonds peaks, both in the GMO molecules and the methacrylic groups of the silica, further supports the suggested reaction of GMO with silica particles.
To investigate the reactivity of the double bond in the AF material and the possibility of interaction with the peroxide molecules C-NMR analysis was performed. The =CH signal (centered ~130 ppm) was monitored in order to evaluate its conversion. Figure 7a shows the C NMR results of GMO after reaction with luperox101 peroxide and GMO after reaction with BP in comparison with the neat GMO. The analysis exhibits a shift of double-bonds after peroxide treatment. In addition, a new signal has appeared in the region of 30-28 ppm, as shown in Fig. 7b. This region is attributed to the formation of -C[H.sub.2] groups. It is suggested that the peroxide molecules have successfully opened up the double bond and the radicals of the hydrocarbon chains have reacted with each other. This further supports our suggested reaction (left type in Fig. 3).
Iodine number tests, shown in Table 1, have shown a decrease in the number of double-bonds due to the radical reaction. Both iodine titration and the [sup.13]C NMR techniques strongly support the suggested reaction where the GMO double-bond is opened via radical reaction with peroxides.
To observe the migration rate of AF in thin polyolefin film, accelerated migration tests were performed. Figure 8 describes an accelerated migration test of neat GMO in polyethylene film and GMO grafted methacrylic silica (only the unattached AF fraction, modified AF, can migrate) in polyethylene film. A linear ratio between GMO concentration and square root of time is shown, meaning the migration rates follow the Fickian diffusion mechanism. Furthermore, and most important, the modified GMO migrates slower than the neat GMO from the polyethylene film.
In summary, this article describes a novel method for controlled AF migration through grafting the AF molecules onto silica particles' surfaces by three simple steps: mixing the reagents, heating and drying. This work has shown the feasibility of grafting GMO molecules on the surface of organo-modified silica particles via a peroxide treatment. Two GMO fractions present in the reaction products: free and grafted GMO, proven by solvent extraction and TGA tests. FTIR and NMR measurements have shown the disappearance of double-bonds, further supporting the suggested reaction. Consequent upon the success of the described reaction, a similar process is being implemented on other types of AFs.
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Jasmine Rosen-Kligvasser, (1,2) Ran Y. Suckeveriene, (1 3) Roza Tchoudakov, (1) Moshe Narkis1
(1) Department of Chemical Engineering, Technion-IIT, Israel
(2) Interdepartmental Program in Polymer Engineering, Technion-IIT, Israel
(3) Department of Water Industries Engineering, Kinneret College on the Sea of Galilee, Israel
Correspondence to: Moshe Narkis; e-mail: firstname.lastname@example.org Contract grant sponsor: Irwin and Joan Jacobs Fellowship Fund; Russell Berrie Nanotechnology Institute (RBNI), Technion; contract grant sponsor: Magnet Program; Israel Ministry of Trade and Industry.
Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Reaction parameters. Experiment AF type Si[0.sub.2]: Peroxide no. AF type 0 GMO -- -- 1 GMO 01:01 B.P 2 GMO 01:01 B.P 3 GMO 01:01 B.P 4 GMO 01:02 B.P 5 GMO 01:02 B.P 6 GMO 01:02 B.P 7 GMO 01:03 B.P 8 GMO 01:03 B.P 9 GMO 01:03 B.P Experiment %Peroxide Solvent Iodine no. (out of AF) number 0 -- -- 81.28 1 1 Ethanol 28.56 2 10 Ethanol 22.58 3 20 Ethanol 12.08 4 1 Ethanol 39 5 10 Ethanol 18.3 6 20 Ethanol 26.32 7 1 Ethanol 22.42 8 10 Ethanol 24.4 9 20 Ethanol 20.18
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|Author:||Rosen-Kligvasser, Jasmine; Suckeveriene, Ran Y.; Tchoudakov, Roza; Narkis, Moshe|
|Publication:||Polymer Engineering and Science|
|Date:||Sep 1, 2014|
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