Jeffrey W. Gilman

Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites

In the pursuit of improved approaches to fire retarding polymers, a wide variety of concerns must be addressed in addition to the flammability issues. For commodity polymers, their low cost requires that the fire retardant (FR) approach also be of low cost. This limits solutions primarily to additive type approaches. These additives must be inexpensive and easily processed with the polymer. In addition, the additive must not excessively degrade the other performance properties of the polymer, and it must not create environmental problems when recycling or at the time of its final disposal. We have recently found that polymer layered-silicate (clay) nanocomposites have the unique combination of reduced flammability and improved physical properties. This paper is intended as an overview of the research to date, by our group and others, on the use of clays, dispersed at the nanometer level, in polymers for improving thermal stability and flammability.

Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites

Abstract Increasing the thermal stability of organically-modified layered silicates is one of the key points in the successful technical application of polymer-layered silicate nanocomposites on the industrial scale. To circumvent the detrimental effect of the lower thermal stability of alkyl ammonium-treated montmorillonite, a series of alkyl-imidazolium molten salts were prepared and characterized by elemental analysis, thermogravimetry (TGA) and thermal desorption mass spectroscopy (TDMS). The effect of counter ion, alkyl chain length and structural isomerism on the thermal stability of the imidazolium salts was investigated. Alkyl-imidazolium-treated montmorillonite clays were prepared by ion exchange of the imidazolium salts with Na-montmorillonite. These organically-modified clays were characterized by X-ray diffraction (XRD), TDMS and thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR), and compared to the conventional quaternary alkyl ammonium montmorillonite. Results indicate that the counter ion has an effect on the thermal stability of the imidazolium salts, and that imidazolium salts with PF6−, N(SO2CF3)2− and BF4− anions are thermally more stable than the halide salts. A relationship was observed between the chain length of the alkyl group and the thermo-oxidative stability; as the chain length increased from propyl, butyl, decyl, hexadecyl, octadecyl to eicosyl, the stability decreased. The results also show that the imidazolium-treated montmorillonite has greater thermal stability compared to the imidazolium halide. Analysis of the decomposition products by FTIR provides an insight about the decomposition products which are water, carbon dioxide and hydrocarbons.

PA-6 clay nanocomposite hybrid as char forming agent in intumescent formulations

This work deals with new flame retardant (FR) intumescent formulations for ethylene-vinyl acetate copolymers (EVA) using charring polymers polyamide-6 (PA-6) and polyamide-6 clay nanocomposite hybrid (PA-6-nano) as carbonization agents. Use of PA-6 nano improved both the mechanical and fire properties of FR EVA-based materials. The part played by the clay in the improvement of the FR performance was studied using FTIR and solid state NMR. It is shown that the clay allowed the thermal stabilization of a phosphorocarbonaceous structure in the intumescent char which increased the efficiency of the shield and, in addition, the formation of a ‘ceramic’ which can act as a protective barrier. Copyright

Flammability, thermal stability, and phase change characteristics of several trialkylimidazolium salts

Room temperature ionic liquids (RTILs) have emerged as tunable and potentially “greener” solvents for a multitude of applications. To investigate the solvent properties and potential use as a thermal fluid, a study was initiated to determine the effects of anion type, C-2 hydrogen substitution, and alkyl chain length on the flammability, thermal stability, and phase change characteristics of 1,2,3-trialkylimidazolium room temperature ionic liquids. A Setaflash flashpoint apparatus was used to determine the flammabilities of the RTILs. No flashpoints were detected for any of the imidazolium based RTILs below 200 °C, the maximum temperature of the instrument. The thermal stabilities of the RTILs were measured using the technique of thermogravimetric analysis. The 1,2,3-trialkylimidazolium compounds exhibit slightly higher thermal stabilities than the comparable 1,3-dialkylimidazolium compounds; RTILs with nucleophilic anions decompose about 150 °C lower than RTILs with bulky fluoride containing anions; the alkyl chain length does not have a large effect on the thermal stability of the RTILs; and the pyrolysis decomposition exhibits higher thermal stabilities via a different mechanism than the oxidative decomposition. In addition, it was found that although the calculated onset temperatures were above 350 °C, significant decomposition does occur 100 °C or more below these temperatures. The phase change behaviors of several imidazolium based RTILs were characterized by differential scanning calorimetry. The melting points of the RTILs increased with increasing alkyl chain length. Most of the salts studied exhibited significant undercooling, which decreased as the length of the alkyl chain was increased. The hexafluorophosphate and bromide RTILs exhibited polymorphic and liquid crystalline behaviors as the alkyl chain length was increased above C10. The clearing point temperatures increased more rapidly with alkyl chain length than the melting point temperatures.

Flame retardant mechanism of silica gel/silica

Various types of silica, silica gel, fumed silicas and fused silica were added to polypropylene and polyethylene oxide to determine their flame retardant effectiveness and mechanisms. Polypropylene was chosen as a non-char-forming thermoplastic and polyethylene oxide was chosen as a polar char-forming (slight) thermoplastic. Flammability properties were measured in the cone calorimeter and the mass loss rate was measured in our radiative gasification device in nitrogen to exclude any gas phase oxidation reactions. The addition of low density, large surface area silicas, such as fumed silicas and silica gel to polypropylene and polyethylene oxide significantly reduced the heat release rate and mass loss rate. However, the addition of fused silica did not reduce the flammability properties as much as other silicas. The mechanism of reduction in flammability properties is based on the physical processes in the condensed phase instead of chemical reactions. The balance between the density and the surface area of the additive and polymer melt viscosity determines whether the additive accumulates near the sample surface or sinks through the polymer melt layer. Fumed silicas and silica gel used in this study accumulated near the surface to act as a thermal insulation layer and also to reduce the polymer concentration near the surface. However, fused silica used in this study mainly sank through the polymer melt layer and did not accumulate near the surface. The heat release and the mass loss rate of polypropylene decreased nearly proportionally with an increase in mass loading level of silica gel up to 20% mass fraction. Polyethylene oxide samples with fumed silicas and silica gel formed physically strong char/silica surface layers. This layer acted not only as thermal insulation to protect virgin polymer but also acted as a barrier against the migration of the thermal degradation products to the surface.

Improved Thermal Stability of Organically Modified Layered Silicates

Bromide-containing impurities were found to decrease the thermal stability of quaternary alkyl ammonium-modified layered silicates. Improved purification procedures completely removed bromide and led to a 20°C to >100°C increase in organic modified layered silicate thermal stability. Using mass spectrometry and thermal and electrochemical analysis, N,N-dimethyl-N,N-dioctadecyl quaternary ammonium-modified montmorillonite and fluorinated synthetic mica were found to degrade primarily through elimination and nucleophilic attack by these anions. The nature of residual bromides was identified and quantified, and the efficiency of removing these anions was found to be solvent dependent; sequential extraction, first ethanol then tetrahydrofuran, gave the best results. This exhaustive extraction method represents a viable alternative to the use of expensive, more thermally stable oniumion treatments for layered silicates.

Fire-retardant additives for polymeric materials. I. Char formation from silica gel-potassium carbonate

Silica gel combined with potassium carbonate is an effective fire retardant for a wide variety of common polymers (at mass fraction of only 10% total additive) such as polypropylene, nylon, polymethylmethacrylate, poly(vinyl alcohol), cellulose, and to a lesser extent polystyrene and styrene-acrylonitrile. The peak heat release rate is reduced by up to 68% without significantly increasing the smoke or carbon monoxide levels during the combustion.

Synthesis and characterization of isosorbide-based polyphosphonates as biobased flame-retardants

A new isosorbide-based polyphosphate was synthesized and applied as a flame-retardant for polylactic acid (PLA). The storage modulus and glass transition temperature of PLA/polyphosphonate blends was unaffected by the inclusion of polyphosphonate, but moderate depressions of PLAs tensile strength (16%, 28%, and 45% reduction from PLA at a polyphosphonate mass percentage of 5%, 10%, and 15%, respectively) and strain-at-break (0%, 17%, and 30% reduction from PLA at a polyphosphonate mass percentage of 5%, 10%, and 15%, respectively) were observed. Modified UL-94 flammability testing indicated that isosorbide-based polyphosphonates are effective flame retardants for PLA and are able to self-extinguish flames in less than 2 s to achieve V2 and V0 ratings at polyphosphonate mass percentage of 5% and 15%, respectively. Fire test data indicates a gas phase mechanism that can quench the flame when no external radiant heat flux is present (e.g., in modified UL-94 testing) but does not affect the materials heat release rate in forced combustion (e.g., in cone calorimetry). Use of the biobased flame retardants described herein yields flame retardant PLA containing up to 97% by mass of bio-derived content.

Revealing the interface in polymer nanocomposites.

The morphological characterization of polymer nanocomposites over multiple length scales is a fundamental challenge. Here, we report a technique for high-throughput monitoring of interface and dispersion in polymer nanocomposites based on Förster resonance energy transfer (FRET). Nanofibrillated cellulose (NFC), fluorescently labeled with 5-(4,6-dichlorotriazinyl)-aminofluorescein (FL) and dispersed into polyethylene (PE) doped with Coumarin 30 (C30), is used as a model system to assess the ability of FRET to evaluate the effect of processing on NFC dispersion in PE. The level of energy transfer and its standard deviation, measured by fluorescence spectroscopy and laser scanning confocal microscopy (LSCM), are exploited to monitor the extent of interface formation and composite homogeneity, respectively. FRET algorithms are used to generate color-coded images for a real-space observation of energy transfer efficiency. These images reveal interface formation at a nanoscale while probing a macroscale area that is large enough to be representative of the entire sample. The unique ability of this technique to simultaneously provide orientation/spatial information at a macroscale and nanoscale features, encoded in the FRET signal, provides a new powerful tool for structure-property-processing investigation in polymer nanocomposites.

Molecular Dynamics Simulations of the Thermal Degradation of Nano-Confined Polypropylene

Molecular dynamics simulations of the thermal degradation of polymer nanocomposites were performed in an attempt to explain the reduction in the flammability of these materials, as compared to the pure polymer, that has been observed in experimental measurements. The relative thermal stabilities of a series of polypropylene/graphite layered nanocomposites were assessed by comparing the rates of mass loss from model polymers as a function of the distance of separation between the graphite sheets. The effect of the interactions with the graphite was separated from the effect of nano-confinement of the polymer by comparing the results obtained from simulations where the nonbonding interactions between the polymer and the graphite were turned on to the corresponding values obtained when these interactions were turned off.