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We have shown that the addition of nanoclays is an effective means for enhancing the flame retardant properties of polymer blends. Polymer blends are difficult to render flame retardant even with the addition of flame retardant agents due to dispersion and phase segregation during the heating process. We show that the addition of 5% functionalized Cloisite 20A clays in combination with 15% decabromodiphenyl ether and 4% antimony trioxide to a polystyrene/poly(methyl methacrylate) blend can render the compound flame resistant within the UL-94-V0 standard. Using a variety of micro-characterization methods, we show that the clays are concentrated at the interfaces between the polymers in this blend and completely suppress phase segregation. The flame retardant (FR) is absorbed onto the clay surfaces, and the exfoliation of the clays also distributes the FR agent uniformly within the matrix. TGA of the nanocomposite indicates that prior to the addition of clay, the dissociation times of the individual components varied by more than 20 °C, which complicated the gas-phase kinetics. Addition of the clays causes all the components to have a single dissociation temperature, which enhanced the efficacy of the FR formula in the gas phase. Cone calorimetry also indicated that the clays decreased the heat release rate (HRR) and the mass loss rate (MLR), due to the formation of a robust char. In contrast, minimal charring occurred in blends containing just the FR. SEM examination of the chars showed that the clay platelets were curved and in some cases tightly folded into nanotube-like structures. These features were only apparent in blends, indicating that they might be associated with thermal gradients across the polymer phase interface. SEM and SAXS examinations of the nanocomposites after partial exposure to the flame indicated that the clays aggregated into ribbon-like structures, approximately microns in length, after the surfactant thermally decomposed. Thermal modeling indicated that these ribbons might partially explain the synergy due to better distribution of the heat and improve the mechanical properties of the melt at high temperatures, in a manner similar to the one reported for carbon nanotubes.

We demonstrated that self-extinguishing polymer nanocomposites, which can pass the stringent UL 94 V0 standard, can be successfully prepared by combining modified organoclays with traditional flame retardant (FR) agents. Using secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM), we determined that the addition of modified clays, which can intercalate or exfoliate in the matrix, also improved the dispersion of the FR agents. Dynamic mechanical analysis (DMA) indicated that the clays increased the modulus of the polymer above Tg, which prevented dripping during burning. Cone calorimetry test showed that the nanocomposites with both FR and organoclay, had a lower peak heat release rate (PHRR) and average mass loss rate (MLR) than those with only clay or the FR agents. Extended X-ray absorption fine structure (EXAFS) data confirmed that no FR/clay interactions occurred in the solid phase, and that the synergistic effects were due to gas phase reactions. Since this mechanism is not specific, it opens the possibility of formulating self-extinguishing materials from a large class of polymers. 2006 Elsevier Ltd. All rights reserved.

PMMA nanocomposites using natural Cloisite 6A and synthetic Lucentite SPN clays were produced via melt blending. The degree of intercalation or exfoliation was measured for both composites using transmission electron microscopy (TEM) and the results were confirmed with small angle X-ray scattering (SAXS). The smaller dimensions of the synthetic SPN, as well as the complete absence of Fe ion impurities allowed for the production of a composite which was optically clear and completel colorless. TEM and SAXS measurements were performed on samples heated to high temperatures. The data showed that in both cases large ribbons composed of the individual platelets were formed. We postalated that these ribbons formed a barrier against spreading of the flame and hence the smaller clays, which are more mobile, had a higher efficiency.

We have studied the morphology of blends of PS/PMMA, PC/SAN24, and PMMA/EVA and compared the morphologies with and without modified organoclay Cloisite 20A or Cloisite 6A clays. In each case we found a large reduction in domains size and the localization of the clay platelets along the interfaces of the components. The increased miscibility was accompanied in some cases, with the reduction of the system from multiple values of the glass transition temperatures to one. In addition, the modulus of all the systems increased significantly. A model was proposed where it was proposed that in-situ grafts were forming on the clay surfaces during blending and the grafts then had to be localized at the interfaces. This blending mechanism reflects the composition of the blend and is fairly nonspecific. As a result, this may be a promising technology for use in processing recycled blends where the composition is often uncertain and price is of general concern.

Significant enhancement of the mechanical properties in PS/EVA blends is achieved via sc CO2 exposure in the pressure/temperature domain where the amplitude of the density fluctuations is maximal. Before exposure, bulk samples of the blends elongate an average of 134% under uniaxial tension, whereas the average elongation is 682% after exposure. SEM micrographs of the exposed blends indicate that foams with pores roughly 5−15 μm in diameter, separated by splines 1−2 μm wide, are formed at all pressures higher than 800 psi. We attribute the enhancement of the mechanical properties in PS/EVA to an increased degree of miscibilization of the immiscible polymers along the spline surfaces. When foaming occurs, the surface-to-volume ratio is increased, thereby increasing the regions where surface miscibilization can occur. Since tensile stresses in the material are sustained by the splines, reinforcement of the mechanical properties in this specific area results in an overall enhancement of the tensile propertie...

Flame-retardant Elvacite acrylic resin/Cloisite 6A nanocomposites were prepared via direct melt intercalation. Transmission electron microscopy micrographs showed that 75% of the clay platelets were completely exfoliated.This high degree of exfoliation resulted in a large improvement in thermal stability and UV absorption properties without sacrificing optical clarity. Cone calorimetry tests clearly showed that the heat release rate was far lower and more gradual in the nanocomposites than in pure resins. Thermal gravimetric analysis measurements showed that the thermal stability of nanocomposites was enhanced by almost 50°C (at 50 wt% loss) when the samples were thermally degraded under nitrogen. These results are consistent with a dramatic increase in the specific heat of the nanocomposites as verified by high precision differential scanning calorimetry measurements. Additionally, Fourier transform infrared spectroscopy results indicated that the introduction of clay did not change the chemical structure of acrylic resins.

Dynamic secondary ion mass spectrometry (DSIMS) measurements were performed to measure the effects of functionlized organosilicate clay (Cloisite 6A) on the tracer diffusion coefficient, D, in polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrices. The results indicate that D is unaffected by the addition of 5% (by volume) of PS and is reduced by a factor of 3 with the addition of the same volume fraction in a PMMA matrix. The same ratio was also obtained for the extrapolated zero shear rate viscosity of PS and PMMA melt-mixed composites with the same volume fraction of clay. DSIMS was also used to measure the mobility of the clay platelets, which was found to be much lower than the polymer chains in both matrices. Examination of the sample morphology with scanning force and transmission electron microscopy indicated that in contrast to melt mixing, spin casting produced a large degree of exfoliation in both PS and PMMA matrices. Hence the difference in the dynamics in the presence of clay was asc...

Morphology, thermal and rheological properties of polymer-organoclay composites prepared by melt-blending of polystyrene (PS), poly(methyl methacrylate) (PMMA), and PS/PMMA blends with Cloisite® organoclays were examined by transmission electron microscopy, small-angle X-ray scattering, secondary ion mass spectroscopy, differential scanning calorimetry, and rheological techniques. Organoclay particles were finely dispersed and predominantly delaminated in PMMA-clay composites, whereas organoclays formed micrometer-sized aggregates in PS-clay composites. In PS/PMMA blends, the majority of clay particles was concentrated in the PMMA phase and in the interfacial region between PS and PMMA. Although incompatible PS/PMMA blends remained phase-separated after being melt-blended with organoclays, the addition of organoclays resulted in a drastic reduction in the average microdomain sizes (from 1–1.5 μm to ca. 300–500 nm), indicating that organoclays partially compatibilized the immiscible PS/PMMA blends. The effect of surfactant (di-methyl di-octadecyl-ammonia chloride), used in the preparation of organoclays, on the PS/PMMA miscibility was also investigated. The free surfactant was more compatible with PMMA than with PS; the surfactant was concentrated in PMMA and in the interfacial region of the blends. The microdomain size reduction resulting from the addition of organoclays was definitely more significant than that caused by adding the same amount of free surfactant without clay. The effect of organoclays on the rheological properties was insignificant in all tested systems, suggesting weak interactions between the clay particles and the polymer matrix. In the PS system, PMMA, and organoclay the extent of clay exfoliation and the resultant properties are controlled by the compatibility between the polymer matrix and the surfactant rather than by interactions between the polymer and the clay surface. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 44–54, 2003

A series of nanocomposites prepared by melt-blending of cloisite-based organoclays with poly(ethylene-vinylacetate) (EVA) and neutralized poly (ethylene-methacrylic acid) (EMA) copolymers were investigated via DSC, small-angle X-ray scattering (SAXS), and rheological techniques. SAXS results indicated partial clay exfoliation in all samples. In both EMA and EVA systems, the nominal melting temperature T m and bulk crystallinity are not significantly affected by the presence of organoclays, suggesting that clay particles are predominantly confined in the amorphous phase. In rheological measurements (above T m ), the EVA-clay system demonstrated a solid-like rheological behavior under the small-strain oscillatory shear, yet it was able to yield and flow under a steady shear, which is the characteristic of physical crosslinking. In contrast, the EMA-clay system exhibited a melt-like rheological behavior, where the influence of organoclay on the thermorheological behavior of the EMA composite was quite minimal. We propose that the carbonyl groups of vinylacetate in EVA interact with the clay surface, resulting in a strong physically crosslinking like interaction in the melt. On the other hand, the interaction between EMA and clay is weak because of repulsion between carboxyl anions and negatively charged clay surface.

Thermal transitions and morphological changes in Cloisite organoclays were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and in situ simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) over the temperature range of 30-260 degrees C. On the basis of DSC and FTIR results, the surfactant component in organoclays was found to undergo a melting-like order-disorder transition between 35 and 50 degrees C. The transition temperatures of the DSC peaks (Ttr) in the organoclays varied slightly with the surfactant content; however, they were significantly lower than the melting temperature of the free surfactant (dimethyldihydrotallowammonium chloride; Tm = 70 degrees C). FTIR results indicated that within the vicinity of Ttr, the gauche content increased significantly in the conformation of surfactant molecules, while WAXD results did not show any change in three-dimensional ordering. Multiple scattering peaks were observed in SAXS profiles. In the SAXS data acquired below Ttr, the second scattering peak was found to occur at an angle lower than twice that of the first peak position (i.e., nonequidistant scattering maxima). In the data acquired above Ttr, the second peak was found to shift toward the equidistant position (the most drastic shift was seen in the system with the highest surfactant content). Using a novel SAXS modeling technique, we suggest that the appearance of nonequidistant SAXS maxima could result from a bimodal layer thickness distribution of the organic layers in organoclays. The occurrence of the equidistant scattering profile above Ttr could be explained by the conversion of the bimodal distribution to the unimodal distribution, indicating a redistribution of the surfactant that is nonbounded to the clay surface. At temperatures above 190 degrees C, the scattering maxima gradually broadened and became nonequidistant again but having the second peak shifted toward a scattering angle higher than twice the first peak position. The changes in SAXS patterns above 190 degrees C could be attributed to the collapse of organic layers due to desorption and/or degradation of surfactant component, which was supported by the TGA data.

Air plasma etching followed by scanning electron microscope (SEM) imaging technique for visualizing the inorganic nanofillers in polymeric composites is a promising technique for quick screening of exfoliation and dispersion of nanofillers in nanocomposites in manufacturing and materials development environments. The method was applied to study montmorillonite (MMT) clay with vinyl ester and epoxy nanocomposites samples produced using high shear mixing. Results were compared with transmission electron microscope data. The study details the versatility and the use of this technique for visualization of inorganic platelets. The etch rate for 2% MMT/vinyl ester and 1% MMT/epoxy nanocomposites varies from 1 to 2 nm/minute.

Self-extinguishing PMMA/Clay nanocomposites with conventional flame retardant agents, decabromodiphenyl oxide (DB) and antimony trioxide (AO) have been prepared by melt blending using Brabender. The combustion behavior was first investigated by subjecting slabs of the polymer to UL-94 V0 testing. We found that the polymers with only the retardant agents or the clay could not pass. On the other hand all polymers with both components were self-extinguishing. DMA results show that the introduction of 5 wt% clay improves PMMA bending modulus by 28 % and increases Tg by 8 °C. the addition of clay can effectively avoid dripping during burning test. Based on the cone calorimeter data, the general mechanism is proposed that the synergy between the char formation promoted by the clay in condense phase and free radical capture in gas phase due to the DB and AO makes the material become self-extinguishing.