Natural and synthetic clay-filled coatings for insulation barrier applications.
Keywords Clays, Coatings, Oxygen barrier, Insulating foam
Functional gas barriers effectively restrict the transport of penetrants across a boundary, with industrial applicability in food and beverage packaging, as well as other areas. (1-3) Common approaches range from homogeneous layers of thin film materials with inherently high barrier characteristics such as glass, aluminum, or metal oxides, to laminated high barrier polymers or polymers loaded with inorganic fillers. Film-forming combinations of the polymer/filler blends can be attractive alternatives in settings where specific mechanical flexibility, recycling, optical, or other esthetic characteristics are required.
Gas transport through solid barriers is typically described through a solution-diffusion mechanism where the transmission rate is primarily dependent upon the solubility of the gas within, and its diffusion through, the barrier material. As the specific gas is absorbed into the material, a higher partial pressure on one side of the barrier drives its diffusion through the film, where it then desorbs into the lower partial pressure environment. The diffusion stage kinetics are the overall rate-limiting aspect, and are impacted by both the permeant molecule size and the general characteristics of the barrier polymer itself, including degree of crystallinity, orientation, and chemical structure of the polymer chains, as well as environmental considerations (temperature and humidity).
Effective gas (oxygen, carbon dioxide, and nitrogen) permeability of polymeric membranes containing dispersed inorganic particles exhibits strong dependency on particle shape/aspect ratios, geometric arrangement, and concentration of the filler particles themselves. These characteristics couple with mechanical defects and physical disruptions in the film to determine the observed permeability. Models (4,5) facilitate permeability predictions based on the prevailing structure and integrity of films. Lamellar plate-like flakes such as exfoliated mica and vermiculite natural clays aligned within the plane of a thin coating (6-8) are of particular interest due to their demonstrated performance against oxygen intrusion in a variety of application areas. Extension of this concept to synthetically derived clays (9) enables additional attributes to be attained which are particularly attractive to insulation foams. Foamed boards are commonly extruded on a continuous basis and comprised of a network of closed cells containing a mixture of gases (such as R-142b and carbon dioxide) which enhance resistance to heat transport. (10) Counter diffusion between these gases and those in the ambient environment over time can result in varying cell gas pressure as shown in Fig. 1; as oxygen and nitrogen in air diffuse inward, the overall thermal resistance of the foam is reduced.
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The rate of reduction can be considerably delayed through application of a gas barrier applied to the foam surface, which can comprise a multi-layer film containing an encapsulated barrier polymer (11) such as an ethylene vinyl alcohol copolymer (EVOH), or a coating of aqueous dispersion containing dispersed and exfoliated clay platelets. (9) Key requirements for such a coating include adhesion to the substrate, uniform wetting, and desirable optical characteristics. Figure 2 illustrates a magnified coated foam surface substrate topology via SEM imaging; the tortuous path provided by successive layers of overlapping impermeable platelets within the coating provides the basis for barrier enhancement as illustrated within the inset.
Clay types vary widely, and the aspect ratio is typically less than 200. Coating layer thicknesses for this application are in the ~0.005-0.025 mm (0.2-1 mil) range. Approaches to optimize barrier performance include maximizing platelet aspect ratio and filler loading fractions, in conjunction with film integrity in the final dried state. Figure 3 illustrates idealized oxygen transmission rate (OTR) tradeoffs associated with these variables; overall OTR can be successively reduced with increasing thickness, though cost can become limiting.
Clays for barrier coatings can be derived from both natural mineral sources and synthetic routes. Barrier-effective natural smectite clays such as montmorillonite require considerable purification and modification during refinement from raw bentonite ore, resulting in the generation of large amounts of waste. Natural clay consistencies also vary between the specific deposits from which they are mined. Further compatibilizing surface treatments and modification via ion exchange processes (12,13) can enable chemical functionality by attaching organophilic species to generate organoclays, facilitating platelet exfoliation within a polymer matrix. Conversely, inexpensive oxides and fluorides such as [Na.sub.2]Si[F.sub.6], MgO, and Si[O.sub.2] can be combined to form sodium fluoromicas (Na[Mg.sub.2.5] [Si.sub.4] [O.sub.10] [F.sub.2]) via synthetic routes to achieve >90% purity with relatively low potential costs. Additional treatments applied to reduce agglomeration and improve exfoliation lead to considerable enhancement in performance over natural sourced clays.
In this work clay types and coating formulation variables are evaluated with respect to their ability to meet the performance and barrier demands of foam applications.
Both natural and synthetic clays were selected as shown in Table 1; individual particle/platelet sizes most suitable for these applications are typically under 0.5 pm with thicknesses ~10 nm or smaller. The natural clays include Hydrafine 90, which is a delaminated kaolin with an aspect ratio of ~<15 and is widely used in paper coating. Bentone EW is a naturally occurring hectorite with an aspect ratio of ~25. Sud-Chemie Bentonite is a partially refined clay with montmorillonite as the major fraction and an aspect ratio of ~100. Similarly, Cloisite Na is a more refined montmorillonite. Microlite 963++ is supplied as an aqueous dispersion of vermiculite and was used asreceived. The synthetic Somasif ME-100 and Topy grades are sodium fluoromicas, where Topy D is a variant of DMA-350, with an aspect ratio greater than several hundred.
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Clays were dispersed in water to the concentrations shown in Table 1 and then blended with polymer latexes (styrene-butadiene copolymer) DL460NA, (14) or polyurethane dispersions (15) to form stable, aqueous blends capable of forming films and being deposited via typical coating techniques. Wetting on relatively low surface energy polymer substrates is necessary to ensure that no small breaks within the film barrier persist; behavior can be modified via surface tensionlowering wetting agents and surfactants, though no additional agents were added to the as-received latex dispersions in this work. A tetrasodium pyrophosphate deflocculant was used to enhance clay particle dispersion within the slurries generated. Physical mixing of latexes and clay aqueous dispersions was performed with a Caframo BDC3030 mixer using a Cowles blade. For experimental purposes, drawdown via a Mayer rod or doctor blade onto a 0.032 mm (1.25 mil) polystyrene film substrate was performed, followed by convection-assisted drying with resulting film thicknesses on the order of 0.005-0.038 mm (0.2-1.5 mil).
OTR of the latex/clay-coated polystyrene films was measured by a Mocon Ox-Tran 2/20 unit (16) fitted with a coulometric sensor; measurements were performed according to ASTM D3985-05 (17) at a temperature of 23[degrees]C and 60-80% relative humidity conditions. The OTR response of the coating layer itself can be differentiated from that of the substrate using the lamination equation (2):
[OTR.sub.coating] = ([OTR.sub.uncoated] x [OTR.sub.coaled]) / ([OTR.sub.uncoated] - [OTR.sub.coated]).
Coatings were thin-sectioned at -95[degrees]C using a Leica UC6 microtome with a cryo chamber to preserve the section integrity, and examined with a JEOL JEM-1230 Transmission Electron Microscope. (18) Film Yellowness Index was determined according to ASTM E313-00 (19) with a Macbeth Color-Eye Model 2020PL. (20)
Key elements in the development of an effective barrier include clay type, its loading within the coating layer, the preparation and refinement techniques applied to both the clay and its aqueous dispersion, and the carrier polymer characteristics themselves. For many applications, a 0.025 mm (1 mil) barrier which exhibits a permeability of <0.79 [cm.sup.3] mm/[m.sup.2] day-atm (2 cc-mil/[100in.sup.2] day-atm) would be desirable; variables around achieving levels within this range are described in turn.
[FIGURE 4 OMITTED]
Theoretically, large clay platelets with high aspect ratios, high clay loading, and good exfoliation enable platelet overlap with subsequent formation of tortuous pathways to slow gas diffusion as illustrated in Fig. 2. Figure 4 contrasts several natural and synthetic clays with clear differences in achievable barrier; all were prepared with the same DL460NA latex. While, as expected, OTR is reduced with increasing coating thickness, the figure also suggests that the OTR improves (higher to lower) across the series Flydrafine 90 < Bentone EW < Sud-Chemie Bentonite < Cloisite Na < Somasif ME100, which is consistent with the trend in the estimated aspect ratio.
Select natural clays such as vermiculite can generate excellent barrier performance at appropriate loadings and with appropriate preparation routes. Microlite 963++ was blended with a polyurethane dispersion at various clay loadings; Fig. 5 demonstrates associated low OTRs, but also illustrates that resultant coating characteristics such as Yellowness Index can increase considerably. This is to be anticipated, given that natural clays can contain residual organic materials (21) such as cellulose or lignins, or iron/silicon/calcium compounds. In contrast, the higher purity synthetically derived clays (like Somasif ME 100) retain relative low color and good transparency as shown in Fig. 6, allowing the substrate appearance to remain intact, minimizing esthetic impact.
The synthetic fluoromica advantages thus warrant particular focus. Figure 7 contrasts achievable barrier levels of several synthetic fluoromicas at 30% clay loading. Topy E3 contains large, non-swelling particles, and jet milling can dramatically decrease the size of the smectite agglomerates. Topy D is a highly swellable smectite. All the coatings achieved OTR less than 31 [cm.sup.3]/[m.sup.2] day-atm at a thickness less than 0.025 mm. Topy D showed the best barrier performance, followed by Somasif ME-100 and then jet milled Topy E3.
Transmission electron micrographs in Fig. 8 illustrate the clay platelet orientation and degree of overlapping network interconnection, which is the source of barrier effectiveness with this approach. Among the characteristics in the formed coating, the effective aspect ratio of the clay is perhaps the most critical, which is highly dependent on the degree of clay exfoliation in the coating. (22) TEM pictures show good clay exfoliation is achieved, with clay stacks of less than 10-20 platelets. This dramatically increases aspect ratio, though its determination via optical means is difficult due to the deformation of clay platelets within the polymer matrix.
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Estimations of effective aspect ratio are possible with the aid of model predictions. Equation (2) describes one such approach4 assuming a distribution of random flakes aligned within a layer, where Vf is the volume fraction of clay within the dried coating layer. Superimposition of experimental data generated by determining permeability of coatings formed from different clays at different loading levels, with predicted permeability reduction as a function of aspect ratio enables approximation of an effective aspect ratio achieved in the different systems (Fig. 9). Based upon this comparison, clay in films from Topy D has the largest aspect ratio (~400), followed by Somasif ME100 (~200), Topy E3 jet milled (100-150), and Cloisite Na (50-75).
[Perm.sub.Unfiiied]/[Perm.sub.Filliedcoating] = 1 + [(Aspect ratio).sup.2] [([V.sub.f]).sup.2]/(1- [V.sub.f]]. (2)
[FIGURE 7 OMITTED]
As previously suggested, volumetric loading is a further critical requirement for clay-based barrier generation, and can be balanced against aspect ratio and refinement cost. Figure 10 compares transmission performance of various clay types vs loading levels in films derived from DL460NA latex. All films demonstrate OTR reduction as clay loading increases, though gains are typically diminished beyond a given filler fraction. This is likely due to achievement of a critical threshold where an interconnected inorganic network is formed; further loading of clay then does not result in significant enhancement of the diffusion path or tortuosity, and therefore, only modest improvements in OTR result. (23) The optimum use level is specific to clay type, which has economic implications. A similar relationship is observed with respect to coating thickness.
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[FIGURE 9 OMITTED]
Increases in clay efficiency can accompany the specific refinement route used on the clay. Figure 11 illustrates the extent of variation in efficiency as a function of refinement for Topy E3. Four Topy E3 dispersions were prepared through differing preliminary treatment techniques, followed by admixing with DL460NA latex such that resultant clay loadings were identical at 30 wt% in the final films. The refinement techniques consisted of (i) as-received clay powder which was directly dispersed into water at 5 wt% concentration and subjected to sonication; though a significant amount of sedimentation occurred, the dispersion was agitated and used 'as is'; (ii) the 5 wt% sonicated dispersion was filtered through a 200 pm filter bag to remove large particles, reducing the final clay content in the dispersion to ~4.1 wt%; (iii) the 5 wt% sonicated dispersion was treated with a 2 wt% sodium pyrophosphate solution, re-sonicated for a duration of 15 min, followed by centrifuging at 2500 rpm for 10 min, and filtering through a #3 Whatman filter paper (where the supernatant was retained, with a final clay content of ~1.8 wt%); and (iv) the clay powder was jet milled and then dispersed into water at a 6 wt% concentration. Figure 11 compares the effectiveness of the different refinement routes (i-iv). The high yielding process of jet milling yields a final performance approximately equivalent to that of the more involved and wasteful centrifugation and filtration route.
The major driver for barrier improvement was removal/attrition of non-clay impurities, and large, non-swellable clay particles as evidenced by Fig. 12, which compares the coating topology resulting from clay refinement methods (i) and (iii). Particles as large as 50 pm are evident without application of rigorous refinement. Conversely, with these treatment steps applied, the remaining clay is more readily exfoliated within the coating formulation, and thus better able to maintain the exfoliated morphology when ultimately dried. Rigorous refinement enables good barrier performance at low clay loading or film thickness. Dispersion stability is a further practical consideration for these types of particle suspensions, and is impacted by the clay refinement protocol.
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Polymeric coating carrier
Barrier characteristics of the coating polymers themselves contribute to overall transport restriction, though unless large inherent differences prevail, their contribution is minor relative to that of the clay content. For the DL460NA S/B latex and polyurethane polymers (PU) used, measured permeability was 112 and 15 [cm.sup.3] mm/[m.sup.2] day-atm (285 and 40 [cm.sup.3]-mil/ 100 [m.sup.2] day-atm), respectively. As a result, PU/Somasif ME-100 coatings could achieve an OTR of 31 [cm.sup.3]/ [m.sup.2] day-atm at 15 wt% clay loading, while fully 30% clay was required to reach this level of performance for a corresponding DL460NA/Somasif ME-100 coating.
The effects of ultra violet (UV) exposure on the polymeric carrier (24) and its adhesion to the substrate are also relevant when considering use within the present application, though exposure duration is anticipated to be relatively brief since outer siding/claddings are typically applied throughout the structure lifetimes. UV bulbs with emissions peaking at 340 nm (~315-400 nm wavelength) were selected to simulate sunlight. Coated samples were subjected to a repetitive cycle of UV exposure for 8 h at 60[degrees]C, followed by 4 h at 50[degrees]C with no UV exposure in the presence of moisture to simulate short term behavior. This latter stage generated sustained standing condensation on the surfaces, followed by evaporation as the sequence restarted. While minimal change in barrier between pre- and post-exposure after 118-168 h duration (10-14 cycles) for a range of clay loadings was experienced, exposure to UV energy does, however, weaken the bond between the coating and the substrate via qualitative peel testing, though no delamination occurred spontaneously. UV absorber additives typically impact energy in the 300-370 nm range, and would be anticipated to minimize the photodegradation and dissociation of polymer molecules both binding the clay and providing adhesion to the substrate.
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Inorganic clay fillers provide an effective barrier against gas transmission when employed within polymer-based coatings. The performance of a given clay depends upon purity, shape characteristics, and preparation/refinement route. Aqueous based barrier coating formulations offer advantages that are attractive, and, unlike sheet lamination, can be applied to irregular shapes.
Elimination of agglomerates and improvements to the overall extent of exfoliation within coatings are the most critical elements in maximizing effective aspect ratio of the clay and, in turn, enabling optimum barrier performance. In general, synthetic clay is preferred over purified natural clay because greater consistency of the lamellar platelet structure is achievable which, in turn, assures a high-quality barrier in coating films. Good barriers to oxygen were observed with films containing 5-30 wt% of clay with effective aspect ratios of ~100-400. When coatings exhibiting these characteristics are applied to foam board insulation, high thermal resistance can be maintained for extended periods of time.
R. T. Fox ([mail]). H. Zhang, M. A. Barger, C. Han
The Dow Chemical Company, Midland, MI 48674, USA
BNI LLC, Midland. MI 48642, USA
Acknowledgments The authors thank Joe Harris of The Dow Chemical Company for providing the transmission electron microscopy.
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Table 1: Clay types and sources used Tradename Clay type Hydrafine 90 Kaolin Bentone EW Hectorite Sud Chemie Bentonite Bentonite/montmorillonite Cloisite Na (MMT) Montmorillonite Somasif ME-100 Synthetic fluoromica Topy E3 Synthetic fluoromica Topy D Synthetic fluoromica Microlite 963++ Vermiculite Tradename Source Solids (wt%) in final dispersion Hydrafine 90 Huber engineered materials 50 Bentone EW Elementis Specialities Inc. 3 Sud Chemie Bentonite Sud Chemie 3 Cloisite Na (MMT) Southern clay products 3 Somasif ME-100 Co-op Chemical Co. Ltd. 3-10 Topy E3 Topy industry 5 Topy D Topy industry 2.85-3 Microlite 963++ W. R. Grace 4
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|Author:||Fox, R.T.; Zhang, H.; Barger, M.A.; Han, C.; Paquette, M.|
|Publication:||Journal of Coatings Technology and Research|
|Date:||Jan 1, 2016|
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