Clay Slurry

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Materials and Interfaces

High Oxygen Barrier Thin Film from Aqueous Polymer/Clay Slurry Yixuan Song, Joseph Gerringer, Shuang Qin, and Jaime C. Grunlan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01077 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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Industrial & Engineering Chemistry Research

To be submitted to Industrial & Engineering Chemistry Research

High Oxygen Barrier Thin Film from Aqueous Polymer/Clay Slurry Yixuan Song,† Joseph Gerringer, § Shuang Qin,† Jaime C. Grunlan†,‡,§,* †

Department of Materials Science and Engineering, Texas A&M University, College Station,

Texas 77843, United States §

Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States

‡

Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843,

United States

KEYWORDS Solution casting, polymer clay composites, transmission electron microscope, oxygen gas barrier

ABSTRACT A thin film coating with tailorable thickness and clay concentration was prepared by solution casting an aqueous slurry containing polyvinyl alcohol (PVOH) and montmorillonite (MMT) clay. The anisotropic clay platelets have excellent alignment due to self-orientation during drying, which results in good transparency and oxygen barrier. A 50 wt% clay coating, with a thickness around 4 µm and visible light transmission of 58%, improves the oxygen barrier of a 179 µm poly(ethylene terephthalate) (PET) substrate by more than three orders of magnitude. This 1 ACS Paragon Plus Environment

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PVOH/MMT composite thin film also has good thermal stability and mechanical properties. This simple coating procedure could be used for a variety of packaging applications that use plastic film (e.g., food, pharmaceutical, and electronics).

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INTRODUCTION Flexible gas barrier films are of great interest for food and electronics packaging.1-2 In order to reduce food waste and prolong the life-span of sensitive electronics (e.g., LED displays), an oxygen transmission rate (OTR) below 10-2 cm3/(m2 day atm) is required.3 The most widely used barrier coating for plastic is metal (1.7 million tons used in the United States in 2005),4 which provides significant barrier improvement at low cost. Despite having good barrier, thin metal coatings often have poor transparency and/or lack of flexibility. These films also have environmental and health concerns due to significant energy consumption and a difficult recycling process.5 Polymer-clay composites, with good mechanical strength and adhesion, improve the gas barrier by providing a tortuous path that forces gas molecules to wiggle around impermeable platelets during permeation.6 Improving the alignment and dispersion of the clay platelets remains a challenge for these composite barriers.7 One strategy to tackle this challenge is layer-by-layer (LbL) assembly, which has been used to prepare flexible and transparent ultrahigh oxygen barriers.8-10 Although coating procedures and recipes have been optimized to reduce the processing time and steps,11-13 the biggest disadvantage of this technique is the complexity (i.e. multiple dipping, rinsing, and drying steps) required to achieve high barrier performance. Achieving a similar structure in a single processing step would be a significant achievement. In the present study, a one-step casting method was used to prepare gas barrier coatings with polyvinyl alcohol (PVOH) and montmorillonite (MMT) clay (Fig. 1). During mixing in water, PVOH intercalates and exfoliates MMT platelets, creating a homogenous paste that is stable and easy to process. As opposed to aligning clay by depositing one layer at a time, as is done with multilayer assemblies,14 MMT platelets self-orient during the casting and drying of the 3 ACS Paragon Plus Environment



slurry coating.15-16 This clay self-orientation can only be realized by satisfying several requirements: low viscosity of the polymer-clay dispersion, relatively low clay concentration and slow drying.17 Alignment improves as clay concentration increases, which is the result of the confinement between platelets that forces them to rotate and align. The degree of orientation ultimately decreases at very high clay loading due to hindered rotation in the high viscosity dispersion.16, 18 Alcohol containing polymer films are often used to reduce oxygen permeability due to the large number of hydrophilic –OH functional groups.19-20 The oxygen barrier is further improved because of the tortuosity created by added clay particles. Concentration, orientation, and aspect ratio of these nanoplatelets strongly influence the gas permeability of the composites.21 A 4 µm coating, containing 50 wt% clay, is able to reduce the oxygen transmission rate of a thick polyester substrate (>150 µm) by three orders of magnitude. Despite lacking nanoscale control over the coating structure, the single deposition step and excellent oxygen barrier make this technique very industrially feasible.

Figure 1. Schematic procedure for the preparation of composite coatings of poly(vinyl alcohol) and montmorillonite. Clay alignment improves in the drop casting and drying steps.


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EXPERIMENTAL SECTION Materials and Substrates Polyvinyl alcohol (PVOH) (Mw 146,000-186,000 g/mol, 87-89% hydrolyzed) was purchased from Sigma-Aldrich (Milwaukee, WI). A 2 wt% PVOH aqueous solution was prepared by mixing the polymer powder with deionized (DI) water and heating the mixture at 90 °C for 30 min while stirring. Natural sodium montmorillonite clay (MMT) (Cloisite NA+) was supplied by BYK Additives Inc. (Gonzales, TX). A 2 wt% MMT suspension was prepared in deionized (DI) water after rolling overnight to achieve homogeneity. 179 µm thick poly(ethylene terephthalate) (PET) film (ST505, Dupont-Teijin) was purchased from Tekra (New Berlin, WI) and used as the substrate for oxygen transmission rate (OTR) testing and transmission electron microscope (TEM) imaging. Prior to coating, PET substrates were rinsed with DI water, methanol, and DI water again. Corona treatment was performed on the PET substrates with a BD-20C Corona Treater (Electro-Technic Products, Inc., Chicago, IL) to improve coating adhesion. Coating Procedure As shown in Figure 1, the liquid slurries (i.e., PVOH/MMT complex) with various clay concentration were prepared by mixing the PVOH solution and MMT suspension, followed by stirring for 5 min until a homogenous paste is formed. The resulting polymer-clay complex was then poured onto a PET substrate sitting within an acrylic box, with a size of 10.2×15.2 cm, and dried at room temperature (~25 °C) for 6 hours.




Characterization Methods Coating thickness was measured with an Absolute Micrometer (Mitutoyo American Co., Aurora, IL), which is accurate to 0.1 µm. Each reported thickness value was the average of at least 30 measurements. Scanning electron microscope (SEM) imaging was done on cross-sectional samples with a JEOL JSM-7500F ﬁeld emission SEM (JEOL Ltd, Tokyo, Japan). Transmission electron microscope (TEM) images were taken using a Tecnai G2 F20 TEM (FEI, Hillsboro, OR), with an accelerating voltage of 200 kV. A small piece of coated PET (1 mm × 5 mm) was embedded into Epofix resin (EMS, Hatfield, PA) for TEM sample preparation, followed by curing overnight in a silicone mold. The embedded epoxy block was then trimmed and cut into 90 nm thick cross-sections using an Ultra 45° diamond knife (Diatome, Hatfield, PA). Sections were collected with a Perfect Loop (EMS, Hatfield, PA), then transferred and imaged on 300 mesh copper grids (EMS, Hatfield, PA). Mechanical properties (elastic modulus, tensile strength, and ultimate strain) were measured on free standing PVOH or PVOH/MMT films with a Q-800 dynamic mechanical analyzer (DMA) (TA Instruments, New Castle, DE). A strain ramp method was used at ambient conditions, with a strain rate of 0.1%/min until 3% strain was reached. Thermogravimetric analysis was performed with a Q-50 thermogravimetric analyzer (TA Instruments), using a heating rate of 10 °C/min from 25 to 800 °C. Visible light transmission was measured and averaged between 390 and 700 nm with a USB2000 ultraviolet-visible light (UVvis) spectrometer (Ocean Optics, Dunedin, FL). Oxygen transmission rate (OTR) testing was performed by MOCON (Minneapolis, MN) with an Oxtran 2/21 instrument, in accordance with ASTM D-3985 (at 23 °C and 0% relative humidity (RH)).


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RESULTS AND DISCUSSION

Coating Thickness and Optical Transparency Although polyvinyl alcohol is uncharged in water, several weak interactions exist between this polymer and montmorillonite platelets: hydrogen bonding, van der Waals interaction,16 and a sixmembered ring structure between PVOH and MMT.22 The stirring after combining the polymer and clay solutions promotes interaction and intercalation of the silicate layers,23 forming a homogenous paste. Coating thickness was measured as a function of PVOH/MMT complex weight cast on PET (10.2×15.2 cm), as shown in Figure 2(a). Greater film density with increasing clay concentration results in thinner cast films. The water evaporation during complex spreading and drying helps MMT platelets rotate and align, which transforms the yellowish liquid paste into a semitransparent coating. Figure 2(b) shows the optical clarity of a 7g (liquid complex weight) coating containing 50 wt% clay (~4 µm thick), which exhibits comparable contact transparency to the uncoated PET. The 25 wt% and 50 wt% clay films, prepared with 7g of aqueous paste, have average visible light transmission of 71% and 58%, respectively (Fig. S1).




Figure 2. (a) Thickness of PVOH/MMT coatings as a function of liquid complex weight cast on PET, the weight percent indicates the clay concentration in the dry coating. (b) Image of the bare PET substrate (left) and with a 7g coating of 50 wt% MMT (right).

Thermogravimetric Analysis Clay concentration in the final dry coating is controlled by the ratio of PVOH and MMT solutions used to prepare the aqueous coating paste, which is confirmed by the residue in TGA analysis (Fig. 3(a)). After the initial loss of absorbed moisture (3-5 wt%) at ~100 °C, PVOH starts to degrade at 275 °C in all four samples. Two main degradation regions are observed from the first derivative curve of weight loss (Fig. S2). The weight loss from 275 to 390 °C is due to the elimination of–OH sidegroups, while C-C cleavage and polymer chain rupture are responsible for the PVOH degradation from 390 to 500 °C.24-25 Assuming the weight loss is solely related to the PVOH degradation, the remaining PVOH weight percent as a function of temperature was calculated (Fig. 3(b)). The slightly increased PVOH degradation in the first region (275-390 °C) is possibly related to the presence of clay platelets, which are good thermal conductors. Although PVOH starts to degrade at nearly the same temperature (275 °C) for all four samples, it’s clear that montmorillonite helps to preserve PVOH and inhibits the polymer degradation at high temperature (Fig. 3(b)). One possible reason to explain this phenomenon is the reduced PVOH chain mobility and improved thermal stability through the interaction with clay, which ultimately slows down the C-C cleavage.26-28 It is for this reason that these coatings should provide flame-retardant and thermal barrier to the underlying substrate.


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Figure 3. (a) Total and (b) PVOH weight loss as a function of temperature for neat PVOH and the PVOH/MMT coatings.

Coating Structure Cross-sectional SEM images show a well-defined, nacre-like layered structure irrespective of clay concentration (Fig. 4(a-c)), which is due to the flow-induced self-orientation of anisotropic nanofillers (i.e., MMT platelets) during the spreading and drying process.17, 29 A more stratified structure is observed as the clay concentration increases. As shown in Figure 4(d-f) and Figure S3(a-c), TEM reveals the polymer-clay structure with higher resolution. It’s clear that most of the clay platelets are aligned with the orientation of the substrate (yellow double headed arrows), which leads to good optical clarity and gas barrier.




Figure 4. (a-c) Scanning electron microscope (SEM) and (d-f) transmission electron microscope (TEM) cross-sectional images of 7g PVOH/MMT coatings containing (a, d) 25, (b, e) 50, and (c, f) 75 wt% clay. The yellow double headed arrows represent the orientation of the substrates. The red circle highlights one example of clay misalignment.

It is observed that the 50 wt% MMT coating has a better clay alignment than the 25 wt% clay film due to a higher degree of freedom in the liquid complex, which prefers a more random orientation in the final dry coating at low clay concentration (low platelets/polymer ratio) (Fig. 4(d) and 5). Clay platelets are subject to more confinement as the MMT concentration increases in the complex, resulting in better clay packing and alignment (Fig. 4(e) and 5). As the clay concentration continues to increase beyond 70 wt%, platelets stop favoring the ordered structure due to hindered rotation in the nanofiller crowded complex during drying (Fig. 5).16-17 Figure 4(f) shows the resulting clay misalignment. Although the clay alignment is good in the ordered region for all three complex samples (Fig. S3(d-f)), defect-free and perfectly ordered polymer-


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clay thin films with high clay loading (~80 wt%) can only be prepared via layer-by-layer assembly.11-13 Thermal conductivity (k) of polymer composites is strongly influenced by nanofiller orientation.30 The 25 wt% clay film likely has low thermal conductivity in both transverse and longitudinal directions due to the lack of contact between clay platelets, as shown schematically in Figure 5. The percolation threshold in the longitudinal direction is reached at 50 wt% clay concentration, which results in high k in this direction, while the contact between clay particles (and therefore thermal conductivity) in the transverse direction remains low. The composite film becomes isotropic at 75 wt% clay loading due to the formation of a network, with high k (low thermal barrier) in both directions. With an almost perfectly ordered “brick and mortar” structure (Fig. 5), multilayer assembled composite films should have low k in the transverse direction, while maintaining high thermal stability (due to the high clay loading), which leads to excellent thermal insulation and flame retardant behavior.31-33

Figure 5. Schematics of the cross-sectional structure of PVOH/MMT films with 25, 50, and 75 wt% clay (schematics are based on the TEM images in Figure S3), and LbL assembly.

Mechanical Properties The mechanical properties of these PVOH/MMT slurry-cast films were measured with a dynamic mechanical analyzer (DMA) and are summarized in Table S1, with the representative stress-strain curves shown in Figure 6. Both tensile strength and elastic modulus are markedly 11 ACS Paragon Plus Environment



improved compared to neat PVOH due to the presence of well-dispersed clay platelets, which is well-established in the literature.34-35 As the clay concentration increases, the elastic modulus increases, while the ability to elongate decreases. The elastic modulus of the 75 wt% clay film (~36 GPa) is similar to that found with PVOH/MMT or other polymer-clay multilayer assemblies, but the ultimate strain and tensile strength are smaller.22, 36 This could be ascribed to clay misalignment at high loading in the slurry coating, which act as defects that promote crack formation.

Figure 6. Representative stress as a function of applied strain for neat PVOH and PVOH/MMT complex films with varying clay concentration. The measurements were performed until sample failed or 3% strain was reached.

Oxygen Barrier Oxygen transmission rate was tested for PVOH and PVOH/MMT complex coatings at 0% RH, as shown in Figure 7. An ~4 µm thick neat PVOH coating improves the oxygen barrier of a 179 um PET substrate from 8.6 to 1.04 cm3/(m2 day atm), due to the presence of highly polar –OH 12 ACS Paragon Plus Environment

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functional groups and the resulting high cohesive energy.37 The oxygen barrier is further improved by clay, which forces the gas molecules to follow a tortuous path during permeation. Greater clay concentration and better clay alignment increases the tortuosity of the 50 wt% clay film compared to 25 wt%, which results in better oxygen barrier. A similar thickness 50 wt% clay complex coating exhibits an OTR of 0.007 cm3/(m2 day atm), which is more than 1000 times lower than the thick PET substrate. As expected, the 75 wt% clay film has an increased OTR (i.e., worse oxygen barrier), due to clay misalignment that facilitates oxygen permeation. This oxygen barrier trend is consistent with mechanical strength and the microstructure observed in TEM images (Fig. 4). It is very likely that the oxygen barrier will remain the same or even improve if the coating thickness is reduced to hundreds of nanometers. Several existing coating techniques (e.g., spray coating, spin coating, and doctor blade coating) and other clay species (vermiculite, laponite, mica, etc.) can be used to make thinner films. 17, 38-40

Figure 7. Oxygen transmission rate of 7g PVOH and PVOH/MMT complex coatings (~ 4µm thick). The dashed line represents the detection limit of the testing apparatus (0.005 cm3/(m2 day atm)). 13 ACS Paragon Plus Environment



CONCLUSIONS A polymer-clay complex oxygen barrier coating was prepared with polyvinyl alcohol (PVOH) and montmorillonite clay (MMT), through a mixing and slurry casting method. This composite coating can be deposited in a single step and exhibits very low oxygen transmission. After flowinduced self-orientation during the complex spreading and drying, the clay platelets are highly aligned in the coating. The clay orientation improves until hindered platelet rotation occurs at high concentration. The mechanical properties and oxygen barrier are significantly improved as clay concentration increases up to ~50 wt%. A 4 µm thick coating produced from a 50 wt% clay slurry, on a 179 µm thick PET substrate exhibits an OTR of 0.007 cm3/(m2 day atm) (the uncoated PET has an OTR of 8.6 cm3/(m2 day atm)). At high clay loading (75 wt%), decreased tensile strength and oxygen barrier were observed, due to clay misalignment. With similar oxygen barrier improvement, this PVOH/MMT complex coating could be used as an alternative for multilayer gas barrier nanocoatings, especially when optical transparency is not required. Other coating techniques, including spraying and spinning, could potentially be used to produce thinner and more transparent coatings without sacrificing barrier.


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SUPPORT INFORMATION Additional visible light transmission, thermogravimetric analysis data, TEM images, and mechanical properties. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHER INFORMATION Correspondent Author * J. C. Grunlan. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors acknowledge the Texas A&M Engineering Experiment Station (TEES) for infrastructural support. The Microscopy and Imaging Center (MIC) at Texas A&M University is also acknowledged for assistant with transmission electron microscope imaging.




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To be submitted to Industrial & Engineering Chemistry Research 28. Leszczyńska, A.; Njuguna, J.; Pielichowski, K.; Banerjee, J. R., Polymer/montmorillonite nanocomposites with improved thermal properties: Part I. Factors influencing thermal stability and mechanisms of thermal stability improvement. Thermochim. Acta 2007, 453 (2), 75-96. 29. Ebina, T.; Mizukami, F., Flexible Transparent Clay Films with Heat-Resistant and High GasBarrier Properties. Adv. Mater. 2007, 19 (18), 2450-2453. 30. Gresil, M.; Wang, Z.; Poutrel, Q.-A.; Soutis, C., Thermal Diffusivity Mapping of Graphene Based Polymer Nanocomposites. Sci. Rep. 2017, 7 (1), 5536. 31. Jung, H.; Yu, S.; Bae, N.-S.; Cho, S. M.; Kim, R. H.; Cho, S. H.; Hwang, I.; Jeong, B.; Ryu, J. S.; Hwang, J.; Hong, S. M.; Koo, C. M.; Park, C., High Through-Plane Thermal Conduction of Graphene Nanoflake Filled Polymer Composites Melt-Processed in an L-Shape Kinked Tube. ACS Appl. Mater. Interfaces 2015, 7 (28), 15256-15262. 32. Han, Z.; Fina, A., Thermal conductivity of carbon nanotubes and their polymer nanocomposites: a review. Prog. Polym. Sci. 2011, 36 (7), 914-944. 33. Carosio, F.; Kochumalayil, J.; Cuttica, F.; Camino, G.; Berglund, L., Oriented Clay Nanopaper from Biobased Components—Mechanisms for Superior Fire Protection Properties. ACS Appl. Mater. Interfaces 2015, 7 (10), 5847-5856. 34. Chen, H.-B.; Zhao, H.-B.; Huang, W.; Shen, P., Effects of Gamma Irradiation on Clay Membrane with Poly (vinyl alcohol) for Fire Retardancy. Ind. Eng. Chem. Res. 2015, 54 (43), 10740-10746. 35. Liff, S. M.; Kumar, N.; McKinley, G. H., High-performance elastomeric nanocomposites via solvent-exchange processing. Nat. Mater. 2006, 6, 76. 36. Humood, M.; Chowdhury, S.; Song, Y.; Tzeng, P.; Grunlan, J. C.; Polycarpou, A. A., Nanomechanical Behavior of High Gas Barrier Multilayer Thin Films. ACS Appl. Mater. Interfaces 2016, 8 (17), 11128-11138. 37. Grunlan, J. C.; Grigorian, A.; Hamilton, C. B.; Mehrabi, A. R., Effect of clay concentration on the oxygen permeability and optical properties of a modified poly(vinyl alcohol). J. Appl. Polym. Sci. 2004, 93 (3), 1102-1109. 38. Tsurko, E. S.; Feicht, P.; Nehm, F.; Ament, K.; Rosenfeldt, S.; Pietsch, I.; Roschmann, K.; Kalo, H.; Breu, J., Large Scale Self-Assembly of Smectic Nanocomposite Films by Doctor Blading versus Spray Coating: Impact of Crystal Quality on Barrier Properties. Macromolecules 2017, 50 (11), 43444350. 39. Zhou, C.-H.; Shen, Z.-F.; Liu, L.-H.; Liu, S.-M., Preparation and functionality of clay-containing films. J. Mater. Chem. 2011, 21 (39), 15132-15153. 40. Das, P.; Malho, J.-M.; Rahimi, K.; Schacher, F. H.; Wang, B.; Demco, D. E.; Walther, D., NacreMimetics with Synthetic Nanoclays up to Ultrahigh Aspect Ratios. Nat. Commun. 2014, 6. 5967.


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TOC Graphic



Figure 1. Schematic procedure for the preparation of composite coatings of poly(vinyl alcohol) and montmorillonite. Clay alignment improves in the drop casting and drying steps.

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ACS Paragon Plus Environment

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Figure 2. (a) Thickness of PVOH/MMT coatings as a function of liquid complex weight cast on PET, the weight percent indicates the clay concentration in the dry coating. (b) Image of the bare PET substrate (left) and with a 7g coating of 50 wt% MMT (right).

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Figure 3. (a) Total and (b) PVOH weight loss as a function of temperature for neat PVOH and the PVOH/MMT coatings.

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Figure 4.(a-c) Scanning electron microscope (SEM) and (d-f) transmission electron microscope (TEM) crosssectional images of 7g PVOH/MMT coatings containing (a, d) 25, (b, e) 50, and (c, f) 75 wt% clay. The yellow double headed arrows represent the orientation of the substrates. The red circle highlights one example of clay misalignment. 403x234mm (300 x 300 DPI)



Figure 5. Schematics of the cross-sectional structure of PVOH/MMT films with 25, 50, and 75 wt% clay (schematics are based on the TEM images in Figure S3), and LbL assembly.

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Figure 6. Representative stress as a function of applied strain for neat PVOH and PVOH/MMT complex films with varying clay concentration. The measurements were performed until sample failed or 3% strain was reached.

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Figure 7. Oxygen transmission rate of 7g PVOH and PVOH/MMT complex coatings (~ 4m thick). The dashed line represents the detection limit of the testing apparatus (0.005 cm3/(m2 day atm)). 161x133mm (300 x 300 DPI)


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Figure S1. Visible light transmission of 7g PVOH/MMT complex coatings with varying clay concentration on a 179 µm PET substrate.

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Figure S2. First derivative of weight loss as a function of temperature for neat PVOH and PVOH/MMT films with varying clay concentration.

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Figure S3. Transmission electron microscope (TEM) images of 7g PVOH/MMT coating with (a, d) 25%, (b, c) 50%, and (c, d) 75% clay concentration. The yellow double headed arrows represent the orientation of the substrates.

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TOC graphic 283x180mm (72 x 72 DPI)


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Clay Slurry

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