Gas and Vapor Transport in Clay Filled Copolyesteramides - American

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Gas and Vapor Transport in Clay Filled Copolyesteramides Scott Matteucci,* Leonardo López, and Sarada Namhata The Dow Chemical Company, 1776 Building, Michigan Operations, Midland, Michigan 48764, United States ABSTRACT: In recent years, macromolecular self-assembling (MSA) polymers have attracted a great deal of interest. These polymers have low molecular weight and self-assemble through secondary interactions such as hydrogen bonding. Polymer selfassembly causes the solid state polymer to take on properties similar to those of polymers with significantly higher molecular weights. These materials separate processability from solid state physical properties. Although the physical properties of MSA polymers are sufficient for many end uses, it is believed that further opportunities would be available if the gas and vapor transport properties of these materials could be enhanced. In an effort to improve barrier properties both organic functionalized and untreated montmorillonite clays were added to the MSA to change the gas and vapor transport properties of the composites from those of the neat polymer. The addition of clays increased melt viscosity for these materials with increasing clay loading. Both types of clays reduced O2 transport, but the untreated clay increased water vapor transport with increasing clay loading. This is in contrast to the behavior of the organic functionalized clay, which experienced a reduction of water vapor transport that was similar to the loss in O2 transport.

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INTRODUCTION

Most organic modifiers that facilitate clay dispersion and exfoliation are unstable at temperatures where many industrially important polymers are melt processed.10 The instability of the organic modifier has reduced the applications and polymer matrixes, in which the clays can be used. MSA polymers can generally be processed at a temperature range where the organic modifier is stable. This report documents the dispersion of both organic treated and untreated smectite clays in MSA copolyesteramides. This report examines the influence of particles and particle surface chemistry on the composite melt viscosity and permeation, and explores the gas and vapor transport in relation to literature models.

Recently, macromolecular self-assembling (MSA) polymers have been developed that disconnect melt viscosity from solid state physical properties.1,2 These materials are low molecular weight polymers, such as polyesteramides or polyurethanes, which contain sufficient chain mobility to self-assemble through hydrogen bonding. The hydrogen bonded segments cause the polymer solid state properties to mimic those of much higher molecular weight polymers.3 There are many applications where MSA melt viscosity behavior is very desirable, yet the neat-polymer barrier properties are not sufficient to meet application requirements. An example of this would be in coatings and packaging applications where the MSA melt viscosity and excellent adhesion characteristics would facilitate processing, but O2 and water vapor barrier properties are insufficient for the end use. Our proposed method for improving physical and/or barrier properties is to incorporate smectite clays into MSA polymers. Polymer−clay composites have elicited significant interest, since the discovery by Toyota that organoclays can be dispersed on the nanoscale in certain polymers.4 Such dispersions have been shown to improve certain composite properties far in excess of what would be expected for equivalent loadings of fillers that do not exhibit nanoscale dispersion. Various groups from industry and academia have reported a significant increase in the composite modulus,5,6 barrier properties,7 and flame resistance.8 One common drawback to clay−polymer composite physical properties has been an increase in melt viscosity with an increasing concentration of dispersed organoclays.9 An increase in melt viscosity can both limit the concentration of particles that can be dispersed in a given polymer, and reduce the applications where nanocomposites can be melt processed. Dispersing clays into MSA polymers presents a unique situation where the clays can enhance certain properties without causing an unacceptable increase in melt viscosity. © 2013 American Chemical Society

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BACKGROUND Macroscopically, smectite-type clays such as montmorillonite are composed of rather large particles with length scales on the order of micrometers; however, these large particles are composed of nanoscale inorganic lamella that have a thickness of around 1 to 2 nm. Generally, untreated clays do not disperse well in organic polymers, so the clays are often treated with organic surface modifiers in order to facilitate dispersion.4 Dispersed organoclays can be either fully exfoliated (i.e., when lamellae are individually dispersed within a matrix) or intercalated (i.e., when polymer diffused into the interlayer region causes an increase in interparticle or interlamellar spacing). The patent and academic literature does not make a consistent differentiation between these two forms of organoclay dispersion. However, it is generally agreed that there are only a few methods for dispersing organoclays. Initial dispersion of organoclays in polymeric materials occurred by exfoliating the clay particles in a medium such as monomers or Received: Revised: Accepted: Published: 10284

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Figure 1. Copolyesteramide structure.

in a solvent containing a polymer.6 The lamellae were then captured in the exfoliated state by polymerization or by removing the solvent.6 There has been a substantial drive to exfoliate organoclays in polymers during melt processing.7,10,11 This method for dispersing organoclays would eliminate problems with polymerization in the presence of particles, or the removal and disposal of solvents. During melt processing, particle stacks (tactoids) must be broken up either by shear or by polymer diffusion into the interlayer spacing.7 The shear depends upon the processing conditions as well as the polymer viscosity, whereas the polymer diffusion is strongly influenced by polymer−particle interactions.7 Melt processing has been carried out using relatively high viscosity polymers such as Nylon 6, Nylon 6,6 etc.7,10 In melt processing, these polymers induce a significant amount of shear on the organoclays, which acts to break up tactoids, and facilitate polymer diffusion into the interparticle spacing (galleries).7

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Table 1. Compression Molding Parameters for Copolyesteramide Based Composites step

time (min)

temperature (°C)

load (kg)

1 2 3 4

5 4 3 5

140 140 140 37.8

608 4536 18143 450

sample material that extended beyond the edge of the holders was removed. A dynamic strain sweep was conducted beginning at a strain of 0.1% at 170 °C and 1 Hz. Frequency sweeps were performed from 100 to 0.1 rad/s at 170 °C. O2 and Water Vapor Permeation. O2 transport measurements were conducted using a Mocon Ox-Tran 2/21 at 23 °C on the 4 in. × 4 in. × 0.02 in. plaques of the pure polymer and composites. Barometric pressure was about 750 mmHg, and relative humidity for both the permeant and carrier gas was near 50%. Samples from the O2 transport experiments were used for water vapor transport measurements. A Mocon Permatran-W 700 was used to characterize water vapor transport. Experiments were conducted at 760 mmHg barometric pressure, 38 °C, and 100% relative humidity. X-ray diffraction (XRD). For acquisition of X-ray diffraction patterns, a Bruker D-8 Advance θ−θ X-ray diffractometer was employed. The system was equipped with a copper sealed-tube source and scintillation counter detector. The incident beam was collimated via a 2.5° primary Soller slit. The diffracted beam was passed through a 0.5% Ni K-β filter and 2.5° secondary Soller slit. A 0.6 mm antiscattering slit and 0.2 mm detector slit were used. The tube was operated at 40 kV and 40 mA and the samples were illuminated with copper K-α radiation (K-α average wavelength = 1.5418 Å). Data were collected from 2θ = 1 to 50° with a step size of 0.02° and an acquisition time of 2 s per step. The samples were not rotated during data acquisition. Analysis of the resulting X-ray diffraction patterns was performed using JADE X-ray pattern analysis software V8.5 Transmission Electron Microscopy (TEM). Samples of approximately 0.5 mm thickness were cut and mounted for ultracryomicrotomy. The samples were cooled to −100 °C in the microtome and trimmed into a trapezoid. Approximately 80 nm thick sections were prepared and examined using a JEOL 1230 TEM at 120 KeV. Using a Gatan Multiscan 794 camera, microstructure images were recorded at multiple magnifications.

EXPERIMENTAL SECTION

Materials. A copolyesteramide, prepared at Dow Chemical Company as described elsewhere,1 was used for this study. The structure of the polymer is shown in Figure 1, where x is 1 and y is 1. The copolyesteramide has a Mn of 7500 g/mol as determine by 1H NMR. Smectite clays were used for this study due to the extensive research that has already been conducted both in industry4 and academia,6,7 which provide a robust technology base for exploration of the dispersion of clays. The two clays in this study are montomorillonites that were used as received from Southern Clay Products. Cloisite 30B is an organically modified clay, where the cation is methyl, tallow, bis-2-hydroxyethyl, qurternary ammonium. Cloisite Na+ is not organically modified and contains sodium cations. Composite Compounding. Polymer and particles were preweighed and stored separately. Samples were compounded using a Haake PolyLab Rheocord blender (20 cm3 bowl). All zones of the blender were set to 160 °C. An air cooling hose at the central zone maintained temperature control. First, the Haake bowl was loaded with all of the polymer, and the polymer was given sufficient time to melt. Next clay powders were added, and the plunger was lowered into the throat of the Haake mixer. The polymer melt and fillers were compounded at a rotor speed on 200 rpm for 2.5 min. Composite Molding. All polymer and composite samples were dried for at least 16 h at 65 °C under vacuum. Samples were compression molded into 4 in. × 4 in. × 0.02 in. plaques using a Tetrahedron MPT-14 press. The molding parameters for copolyesteramide based materials are listed in Table 1. Melt Viscosity. Circular samples of approximately 1 cm in diameter were die cut from the molded plaques. An Ares Rheometer from TA Instruments with a parallel plate geometry holder was heated to 170 °C and zeroed. Samples were loaded onto the holder. The top plate was lowered into contact with the sample so that the equipment registered some normal force on the sample. The sample was allowed to heat and melt. Any

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RESULTS AND DISCUSSION Melt viscosity. Melt viscosity increases with increasing particle loading for all filler systems. The extent of melt viscosity increase strongly depends on both the particle surface treatment and the particle loading. For instance, copolyesteramide filled with 2 wt % Cloisite 30B exhibits almost an order of magnitude increase in melt viscosity as compared to the unfilled polymer, and melt viscosity continues to increase by 10285

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loadings, the Cloisite Na+ filled composites exhibit melt viscosities that are lower than the melt viscosities of similar loadings of organic treated clays. The copolyesteramide containing 20 wt % untreated fillers has approximately the same order of magnitude melt viscosity as the copolyesteramide containing 2 wt % Cloisite 30B. In the composites containing untreated fillers, the polymer may be able to slip across the surface of the particle without any encumbrance to polymer chain mobility. Dispersion and Exfoliation. The extent of particle exfoliation is generally determined by using a combination of X-ray diffraction and microscopy. X-ray diffraction provides a statistically significant characterization of exfoliation. The Cloisite 30B particles, when dispersed in MSA polymers, exhibit higher interparticle spacings than the neat clay. The interlayer spacing for Cloisite 30B at each loading is presented in Figure 4. The interlayer spacing is nearly 3.6 nm at 2 wt %

orders of magnitude as the loading increases, as shown in Figure 2. The dramatic increase in melt viscosity may be

Figure 2. Melt viscosity behavior of copolyesteramide filled with various loadings of Cloisite 30B.1

attributed to entanglement that is believed to occur between the clay surface modification and the polymer. In such systems, the polymer chains are not free to move near the particle surface, and this impediment to chain mobility acts to increase the macroscopic melt viscosity. Similar changes in melt viscosity have been reported in the nanocomposites literature.10 For instance, Fornes et al. reported an increase in Nylon 6 shear viscosity from around 300 Pa·s to a range of 700 to 800 Pa·s by adding as little as 3 wt % organic modified montmorillonite.6 An important point, however, is that the melt viscosity of MSA polymers filled with up to 30 wt % Cloisite 30B is low enough to allow easy processing. The fillers that do not have surface treatments (i.e., Cloisite Na+) do not influence melt viscosity to the same degree as the surface treated fillers, as shown in Figure 3. However, the melt viscosity does increase with increasing particle loading, and at particle loadings above 40 wt % the composite melt viscosity can be near the same order of magnitude as the copolyesteramide filled with 30 wt % Cloisite 30B. At lower

Figure 4. Interparticle spacing of Cloisite 30B in copolyesteramide.

Cloisite 30B in copolyesteramide, and decreases to around 3.4 nm at high inorganic concentration (i.e., 40 and 50 wt % Cloisite 30B). The interlayer spacing for neat Cloisite 30B is 1.8 nm, which is consistent with literature values.13 The swelling of the interparticle spacing in Cloisite 30B is indicative of either exfoliation or intercalation of the organoclays. The limit to interparticle spacing may be due to the high particle concentrations that have been incorporated into the polymer matrix. For instance, at 50 wt % Cloisite 30B, the composite material is approximately 35 vol % organoclay, which leaves very little polymer to fill the interparticle space. Therefore, it is not unreasonable to see an asymptote in interparticle spacing at high particle loadings, and such an asymptote does not represent an inability of the polymer to exfoliate the organoclay, but rather a physical limit induced by polymer−particle volume ratios. Cloisite Na+ did blend readily in the copolyesteramide. However, XRD of unmodified nanoclays in copolyesteramide did not exhibit distinct peaks that could be used to assign dspacing. The images presented in Figure 5 exhibit copolyesteramide containing Cloisite 30B. These images demonstrate that at low particle loadings, i.e., 10 wt %, the tactoids are well dispersed, and that they contain between 2 to 4 lamella per tactoid (although the TEM results in this report are not meant to be

Figure 3. Melt viscosity behavior of copolyesteramide filled with various loadings of Cloisite Na+.2 10286

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Figure 6. TEM images of copolyesteramide filled with (a) 5 wt %, (b) 30 wt %, and (c) 50 wt % Cloisite Na+. Figure 5. TEM images of copolyesteramide filled with at (a) 10 wt %, (b) 30 wt %, and (c) 50 wt % Cloisite 30B organoclay.1

quantitative). There are numerous particles that are dispersed individually at low loadings as well. At higher particle loadings, i.e., greater than 30 wt %, the tactoids contain a large number of lamella. The increase in lamella per tactoid with increasing clay loading could be attributed directly to the reduction in polymer concentration as clay loading increases as stated above for the interparticle spacing; i.e., there is insufficient polymer in the composite to fully exfoliate or further disperse the layers at high clay loadings. In contrast to Cloisite 30B, Cloisite Na+ did not exfoliate in the copolyesteramide. This is readily apparent in the images shown in Figure 6. In these images, the magnification is 12.5x (which is a lower magniciation than the images shown in Figure 5). The agglomerates in these images show Cloisite Na+ to be multiple micrometers in size even at low loadings of particles. Interestingly, these particles appear to be better dispersed at high loadings, where individual lamella or small agglomerates are visible. The large particles, indicative of poor particle dispersion, may be the reason that blends containing Cloisite Na+ have much lower viscosity than compositions with Cloisite 30B at the same concentration levels. O2 and Water Vapor Transport Properties. Figure 7 presents the O2 permeation values for copolyesteramide filled with increasing concentrations of Cloisite 30B and Cloisite Na+ clay. For composites filled with either type of clay, O2 permeation decreases with increasing organoclay concentration up to the maximum loading tested. The behavior of the

Figure 7. O2 Permeation in copolyesteramide filled with Cloisite 30B and Cloisite Na+.1,2

composites is consistent with traditional gas transport properties in filled polymer matrices, where the organoclays increase the tortuosity of the O2 diffusion pathway and reduce the concentration of gas that can be sorbed in a given volume of composite.14 The lowest observed O2 permeation values are approximately 77% lower than the unfilled polymer. The minimum O2 permeation values are similar to those reported for unplasticized polyvinylchloride (PVC), and are reported for comparison with a select group of polymers in Table 2. The barrier 10287

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on eq 1 using a range of “a” values. ϕF is the volume fraction of filler in the composite which is estimated as follows:

Table 2. O2 and Water Vapor Transport for MSAComposites and Common Polymersa material

O2 permeation (cm3·mil) / 100 (in2·day·atm)

water vapor permeation (g·mil) / 100 (in2·day)

125 23

240 23

17 ± 13

3.0 ± 2.0

2.6 4.5 ± 1.5

19 ± 3 1.2

234 250 ± 150

10 0.5 ± 0.2

copolyesteramide copolyesteramide +30 wt % Cloisite 30B unplasticized polyvinylchloride (PVC) nylon 6 poly(ethylene terephthalate) (PET) polycarbonate (PC) polypropylene (PP)

ϕF =

MF/ρF + MP/ρP

(2)

where MF and MP are the weight of filler and polymer in the composite, respectively. ρF and ρP are the densities of the filler (i.e., 2 g/cm3) and polymer (i.e., 1.1 g/cm3), respectively. Interestingly, the experimental values of the permeability ratio for Cloisite 30B filled copolyesteramide do not follow predictions for any one “a” value. At 0.05 vol % Cloisite 30B, the O2 permeability is consistent with an “a” value greater than 10. However, as the Cloisite 30B loading increases, the O2 permeability crosses over the predicted permeability for “a” equals 10 and approaches that of “a” equals 5 at 27 vol % Cloisite 30B. The Cloisite 30B copolyesteramide composite permeability behavior is consistent with an increasing “a” value with increasing particle loading. This agrees with the increase in overall tactoid size that has been observed in TEM images of copolyesteramide Cloisite 30B composites, where the width of the tactoid is expected to remain relatively constant, whereas the tactoid thickness increases dramatically with an increasing number of lamellae per tactoid at high particle loadings.13 Water vapor transport in the MSA-Cloisite 30B nanocomposites appears to follow the same trends that were observed for O2 permeation, as shown in Figure 9. That is,

a

Permeation values for PVC, Nylon 6, PET, PC, and PP were obtained using ASTM D-1434 and ASTM E-96 testing methods for O2 permeation and water vapor permeation, respectively.14

properties of these films are not improvements over traditional materials used for barrier applications, especially PET. Although not part of this work, methods for further improving the barrier properties of Cloisite 30B filled copolyesteramides would include more rigorous attempts to orient the clay within the polymer, which might be as simple as melt extrusion of films rather than compression molding. The barrier properties of the composite films are consistent with theoretical composite behavior.15 With increasing particle loadings, the O2 permeability of the composite is expected to decrease. Such behavior is predicted from the following equation for permeation in a permeable matrix filled with infinitely long, impermeable ribbon like structures:15 1 − ϕF PX = P0 1 − ϕF + a 2ϕF2

MF/ρF

(1)

where PX and P0 are the O2 permeabilties of the composite and the unfilled polymer, respectively, and “a” is the ratio of the filler width to the filler thickness. Since “a” is not necessarily a known parameter, Figure 8 presents permeability ratios based

Figure 9. Water transmission in copolyesteramide filled with Cloisite 30B and Cloisite Na+.1,2

water vapor transport decreases with increasing particle loading. The water vapor permeation in copolyesteramide filled with 40 wt % Cloisite 30B is approximately 81% lower than water vapor permeation in unfilled copolyesteramide. As shown in Figure 8, these results tend to agree with the model from eq 1 although water does not exactly replicate the behavior of O2. As shown in Figure 9, the permeation of water vapor increases with increasing Cloisite Na+ concentration. Interestingly, the water vapor permeation values in copolyesteramide/ Cloisite Na+ composites do not behave in a manner predicted by traditional composite theories. In fact, water vapor permeation is not behaving in the same manner as O2 permeation in the same composites, where O2 permeation decreases with increasing particle loading due to an increase in the diffusion pathway tortuosity in the composites. It would be expected that the Cloisite Na+ particles influence water vapor

Figure 8. Relative permeability (Composite permeability/Unfilled polymer permeability) for O2 (●) and water vapor (■) in copolyesteramide/Cloisite 30B composites and values predicted by eq 1 for various values of “a”. 10288

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imaging, as well as Brian Pate who provided assistance with XRD measurements.

diffusion pathways in the same manner as O2. However, it is possible the water vapor either swells the clay particles in such a way as to reduce the diffusion pathway of the water vapor, or the water vapor has a favorable interaction with the Na+, which causes the concentration of water around the particles to be higher than in the unfilled polymer. Permeation of gases and vapors through polymer composites depends on both the diffusion coefficient and concentration of the penetrant in the substrate. In such a system, the increase in water concentration in the composite may be able to compensate for the increased diffusion pathway tortuosity that particles create, which could result in overall higher water vapor permeation. The divergence between water vapor transport and O2 permeation may not be useful for traditional barrier applications where all gases need to be impeded, however the copolyesteramide/Cloisite Na+ composites may prove useful in areas where barrier properties for one gas are needed in the same application where water vapor transport is required. Such applications would include food packaging, insulation, as well as other industrial applications.1,16

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CONCLUSIONS

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AUTHOR INFORMATION

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REFERENCES

(1) Lopez, L. C.; Matteucci, S. T.; Namhata, S. P. Polymer Organoclay Composites. US8440297, 2013. (2) Lopez, L. C.; Matteucci, S. T. Polymer Inorganic Clay Composites. US8268042, 2012. (3) Lips, P. A. M.; Broos, R.; Heeringen, P.; Dijkstra, J.; Feijen, J. Synthesis and Characterization of Poly(Ester Amides)s Containing Crystallizable Amide Segments. Polymer 2005, 46, 7823. (4) Okada, A., Fukushima, Y.; Kawasumi, M.; Ingaki, S.; Usuki, A.; Sugiyama, S.; Kurauch, T.; Kamigairo, O. Composite Material and Process for Manufacturing Same. US 4739007, 1988. (5) Cho, J. W.; Paul, D. R. Nylon 6 Nanacomposites by Melt Compounding. Polymer 2001, 42, 1083. (6) Fornes, T. D.; Paul, D. R. Nylon 6 Nanacomposites: The Effect of Matrix Molecular Weight. Polymer 2001, 42, 9929. (7) Krook, M.; Morgan, G.; Hedenqvist, M. S. Barrier and Mechanical Properties of Injection Molded Montmorillonite/ Polyesteramide Nanocomposites. Polym. Eng. Sci. 2004, 45, 135. (8) Ross, M.; Kaizerman, J. Organoclay/Polymer Compositions with Flame Retardant Properties. Eur. Pat. Appl. 1090954, 2001. (9) Ristolainen, N.; Vainio, U.; Paavola, S.; Torkkeli, M.; Serimaa, R.; Seppaelae, J. Polyprolyene/Organoclay Nanocomposites Compatibilizeed with Hydroxyl-Functional Polypropylenes. J. Polym. Sci. Part B: Polym. Phys. 2005, 43, 1892. (10) Chan, K. P.; White, J. M. Compositions and Methods for Polymer Composites. PCT. Int. Appl. 2008002869, 2008. (11) Vlasveld, D. P. N.; Vaidya, S. G.; Bersee, H. E. N.; Picken, S. J. A Comparison of the Temperature Dependence of the Modulus, Yield Stress, And Ductility of Nanocompsites Based on High and Low MW PA6 and PA66. Polymer 2005, 46, 3452. (12) Fornes, T. D.; Yoon, P. J.; Hunter, D. L.; Keskkula, H.; Paul, D. R. Effect of Organoclay Structure on Nylon 6 Nanocomposite Morphology and Properties. Polymer 2002, 43, 5915. (13) Chavarria, F.; Paul, D. R. Morphology and Properties of Thermoplastic Polyurethane Nanocomposites: Effect of Organoclay Structure. Polymer 2006, 47, 7760. (14) Tock, R. W. Permeabilities and Water Vapor Transmission Rates for Commercial Polymer Films. Adv. Polym. Technol. 1983, 3, 223. (15) Lape, N. K.; Nuxoll, E. E.; Cussler, E. L. Polydisperse Flakes in Barrier Films. J. Membr. Sci. 2004, 236, 29. (16) Matteucci, S. T.; Lopez, L. C.; Feist, S. D.; Nickias, P. N.; Khot, S. N. Self Assembling Polymer Membranes in Food Packaging Applications, U.S. Pat. Appl. 20130074451, 2013.

Organic treated and untreated montmorillonite clays have been demonstrated to increase melt viscosities of copolyesteramides. The extent of melt viscosity increase depends on the treatment on the clay as well as the clay loading, with organic treated clays increasing melt viscosity to a much greater extent than the equivalent loading of untreated clay. Oxygen and water vapor transport was studied for organic treated and untreated montmorillonite filled copolyesteramides. For both sets, clay filled composites O2 permeability decreased in a manner that is consistent with theory. This is also true of the water vapor transport in copolyesteramides filled with organic treated clays. Interestingly, untreated clay filled composites exhibited increasing water vapor permeation with increasing particle loading, which has been attributed to the Na+ or clay structure interacting with the water in a manner conducive to enhancing water permeation. This research demonstrates that macromolecular selfassembling polymers filled with high loadings of untreated montmorillonite exhibit a fundamental divergence in transport properties for water vapor compared to permanent gases such as O2. This divergence becomes more pronounced with increasing montmorillonite loadings. At the high montmorillonite loadings enabled by the inherent low melt viscosity of the macromolecular self-assembling polymers, water vapor/O2 flux ratios increase almost 10-fold. Such properties may prove useful in various industrial applications, such as food packaging and insulation.

Corresponding Author

*Phone +19896381748. Fax: +19896386225. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the following individuals for their help: Robert Cieslinski and Joseph Harris both of The Dow Chemical Company for their assistance with TEM 10289

dx.doi.org/10.1021/ie401532f | Ind. Eng. Chem. Res. 2013, 52, 10284−10289