Supercritical CO2 as an efficient medium for layered silicate organomodification: Preparation of thermally stable organoclays and dispersion in polyamide 6

In this study, the preparation of organoclays via a new process using supercritical carbon dioxide is described. This method turns out to be very efficient with various surfactants, in particular nonwater-soluble alkylphosphonium salts. The influence of the surfactant as well as of the clay nature on the thermal stability of the organoclay is evaluated by thermogravimetric analysis. Phosphonium-based montmorillonites are up to 90 °C more stable than ammonium-based montmorillonites. Moreover, the use of hectorite adds another 40 °C of thermal stability to the phosphonium-modified clays. These organomodified clays have been melt-blended with polyamide 6 and morphology as well as fire properties of the nanocomposites are discussed, in terms of influence of the stability of organoclays. For the first time, comparison of nanocomposites based on clay organomodified by ammonium and phosphonium salts of the very same structure is reported.     

Polyamide 66 nanocomposites based on organoclays treated with thermally stable phosphonium salts

Purification of bentonite clays and their modification with two thermally stable (alkyl and aryl) phosphonium organic salts were investigated. The organoclays were subsequently melt compounded with Polyamide 66 (PA66), with and without the use of an elastomeric compatibilizer. The morphology, melt flow, thermal stability, and mechanical properties of the binary and ternary nanocomposites were studied. The bentonite clay was purified by sedimentation, resulting in higher cation exchange capacity and thermal stability in comparison with unpurified clay. These were then used in the synthesis of two thermally stable organoclays by replacing the interlayer sodium cations with two (alkyl and aryl) phosphonium surfactant cations to circumvent the problem of low temperature decomposition of quaternary ammonium organoclays usually used in polymer nanocomposites. The organoclay with aliphatic groups showed more compatibility with PA66 in comparison with the organoclay with aromatic groups. Thus, the use of organoclay with aliphatic groups resulted in nanocomposites with higher tensile strength, higher modulus, higher elongation at break, and higher impact strength in comparison with the nanocomposites produced from the organoclay with aromatic groups. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Reaction of acid activated montmorillonites with hexadecyl trimethylammonium bromide solution

Acid-activated montmorillonites (AMt) prepared at different acid montmorillonite ratios were reacted with hexadecyl trimethylammonium (C16TMA) bromide solution. The acid treated Mt incorporated smaller amounts of the surfactants than the parent Mt, due to the reduction of the cation exchange capacity after acid-treatment. The powder X-ray diffraction patterns exhibited a similar basal spacing of 3.80 nm but with less ordered structure at higher acid montmorillonite ratios. A certain degree of conformational heterogeneity was observed by ¹³C CP NMR spectroscopy due to the different local environment of C16TMA⁺ ions in the interlayer space. The in-situ PXRD patterns showed an increase of the basal spacing of organo acid-activated montmorillonites when heated at intermediate temperatures (100–200 °C), while the basal spacing was almost constant in this range of temperature for the organomontmorillonite. Generally, the stability of the surfactant decreased when intercalated into the montmorillonites compared to the pure C16TMABr. This fact implies that the interlayer space influences the decomposition steps.     

Thermally stable phosphonium-montmorillonite organoclays

Sodium montmorillonite (MMT) was modified with several organic phosphonium salts. Organoclays with water soluble surfactants were prepared by the traditional cation exchange reaction. An alternative procedure was used to prepare organoclays with water insoluble salts. The effect of chemical composition and molecular weight of the salts on the thermal stability and basal spacing were evaluated. The phosphonium montmorillonites exhibit higher thermal stability than conventional ammonium organoclays. The basal spacing is generally larger for the phosphonium montmorillonites. These properties provide a good potential for the use of phosphonium organoclays for the synthesis of polymer/clay nanocomposites by melt processing.     

Changes in the morphology of organoclays with HDTMA surfactant loading

The detailed understanding of the interlayer structure of organoclays is of importance in the design of organoclay based materials and their industrial applications. In this study, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to provide new insights into the interlayer structure and morphology of HDTMA⁺/montmorillonite organoclays. XRD patterns show that thermal treatment has an important effect on the stability of organoclays, reflected by significant changes in the basal spacing. TEM and SEM micrographs demonstrate that the organoclays with lower surfactant packing density are mainly composed of irregular layer stacking with a number of curved organoclay layers, while those with higher surfactant packing density are mainly composed of regularly intercalated and flat layers. Variations of the interlayer distances exist in all organoclays and are more pronounced in the organoclays with lower surfactant packing density. This study demonstrates that not only the arrangement model of surfactant but also the morphology of organoclay strongly depend on the surfactant packing density within the montmorillonite interlayer space.     

Preparation and characterization of thermally stable phosphonium organoclays and their use in poly(ethylene terephthalate) nanocomposites

Purification of montmorillonite rich bentonite followed by surface modification using organic salts was performed. The bentonite was purified by sedimentation and then surface modified by ion exchange using alkyl‐ and aryl‐based phosphonium salts. The thermal stability, morphology, melt flow, and mechanical properties of the poly(ethylene terephthalate) (PET) nanocomposites prepared with these organoclays were studied with and without using a reactive elastomeric compatibilizer. TEM results showed that the alkyl based organoclay exhibited better dispersion and thus, higher tensile strength and elongation at break in the PET/organoclay/elastomer ternary nanocomposites than the aryl‐based organoclay did. The notched Charpy impact strength of PET increased from 2.9 to 4.7 kJ m^?2 and 3.4 kJ m^?2 for alkyl and aryl phosphonium organoclay‐based ternary nanocomposites, respectively. Upon compounding PET with alkyl and aryl phosphonium organoclays, the onset decomposition temperature of PET increased from 413°C to 420°C and 424°C, respectively. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A modified method for processing and characterization of organoclay‐based poly(ethylene terephthalate) nanocomposite fibers

In the current study, in order to prepare poly(ethylene terephthalate) (PET)/organoclay nanocomposite fibers, a slurry‐compounding method (SCM) was applied and compared to conventional melt‐compounding method (CMM) in terms of the dispersion of organoclays and the performance of the spun and or drawn fibers. The organoclays were synthesized by using three different alkyl phosphonium salts and compared with commercially available alkyl ammonium‐modified organoclays in terms of thermal stability and basal spacing. It was found that the alkyl phosphonium salts exhibited higher thermal stability and basal spacing with respect to commercial alkyl ammonium organoclays. Among them, tributylhexadecylphosphonium bromide resulted in superior properties; therefore, it was used to prepare the nanocomposite PET fibers. The organoclay content of 0.1–1 wt% was taken as the material parameter. It was demonstrated that the SCM yielded better dispersion of organoclays with respect to CMM. The drawn nanocomposite fibers prepared via SCM exhibited improved tensile strength and modulus in comparison to the neat‐PET. The maximum tensile properties for fibers were obtained at 0.5% organoclay loading in SCM. The thermal properties and the percentage of crystallinity were investigated by differential scanning calorimetry analysis. In addition, Fourier transform infrared spectroscopy was utilized to obtain the percentage of crystallinity of the fibers. POLYM. COMPOS., 34:887–896, 2013. © 2013 Society of Plastics Engineers     

Effect of intercalating agents on clay dispersion and thermal properties in polyethylene/montmorillonite nanocomposites

Alkyl pyridinium, 1‐vinyl alkyl imidazolium, 1,3‐dialkyl imidazolium, and tetraalkyl phosphonium bromides were successfully used as intercalants for the preparation of highly thermally stable organophilic montmorillonites. Nanocomposites of linear low density polyethylene (LLDPE) and linear low density polyethylene grafted with maleic anhydride (LLDPE/LLDPE‐g‐MAH) were prepared from those organoclays. The micro‐ and nano‐dispersions were analyzed through X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM): intercalation and/or partial exfoliation were found to occur only for formulations based on organoclays having an initial basal distance higher than 20 Å, suggesting the existence of a critical interfoliar distance for the delamination of silicate layers in a noninteracting polymer matrix. The properties of the nanocomposites were analyzed through differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and oscillatory rheometry. The dynamic crystallization of LLDPE was not significantly affected by the presence of clay. TGA in oxidative atmosphere proved to be very sensitive to the dispersion state of the organoclay: the thermal stability was drastically enhanced for intercalated and partially exfoliated formulations. However, the inherent thermal stability of the organoclay did not appear to influence significantly the overall thermal stability of the composite in the range of temperatures investigated (160–230°C). POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Ternary nanocomposites: Curing,morphology, and mechanical properties of epoxy/thermoplastic/organoclay systems

The influence of two organically modified montmorillonites on the curing, morphology and mechanical properties of epoxy/poly(vinyl acetate)/organoclay ternary nanocomposites was studied. The organoclays and poly(vinyl acetate) (PVAc) provoked contrary effects on the epoxy curing reaction. Ternary nanocomposites developed different morphologies depending on the PVAc content, that were similar to those observed in the epoxy/PVAc binary blends. The organoclays were only located in the epoxy phase independently of the morphology. All nanocomposites showed intercalated structures with similar clay interlayer distances. Both PVAc and organoclays lowered the T_g of the epoxy phase, the presence of clays did not influence the T_g of the PVAc phase. The addition of the organoclays to the epoxy improved stiffness but lowered ductility while the adition of PVAc improved toughness although reduced stiffness of epoxy thermoset. Ternary nanocomposites exhibited optimal properties that combine the favourable effects of the clay and the thermoplastic. POLYM. COMPOS., 37:2184–2195, 2016. © 2015 Society of Plastics Engineers     

Modification of montmorillonites with thermally stable phosphonium cations and comparison with alkylammonium montmorillonites

Alkylammonium montmorillonites have onset temperature of degradation in the same temperature range used for compounding of thermoplastics, thus, requiring more thermally stable organic cations. In the current study, phosphonium, imidazolium and pyridinium ions differing in their chemical architecture (length of alkyl chains, number of phenyl groups etc.) were exchanged on the surface of montmorillonites. The montmorillonites with two different cation exchange capacities (CECs) were used. The thermal behavior of the modified montmorillonites was analyzed by thermogravimetric analysis. To achieve quantitative insights into the onset of degradation, temperatures to reach 5% and 10% mass loss and peak degradation temperatures were compared. Time to reach 1% mass loss was also calculated in the dynamic TGA. The surfactants used in the study were more thermally stable than the conventional alkylammonium cations due to their delayed onset as well as peak degradation temperatures. The time required to attain certain extent of degradation was also much higher in the case of phosphonium montmorillonites, thus, confirming their better thermal resistance. Though the degradation temperatures as well as profiles of the modified montmorillonites with different CECs were similar, the dynamic TGA revealed better thermal behavior of the higher CEC montmorillonite. The X-ray diffraction studies also concluded that optimal modification of the montmorillonite surface could be attained, thus, confirming the high potential of phosphonium montmorillonites for nanocomposites.     

XRD study of intercalation in statically annealed composites of ethylene copolymers and organically modified montmorillonites. 2. One-tailed organoclays

Ethylene copolymers with different polar comonomers, such as vinyl acetate, methyl acrylate, glycidyl methacrylate, and maleic anhydride, were used for the preparation of polymer/clay nanocomposites by statically annealing their mechanical mixtures with different commercial or home-made organically modified montmorillonites containing only one long alkyl tail. The nanostructure of the products was monitored by X-ray diffraction, and the dispersion of the silicate particles within the polymer matrix was qualitatively evaluated through microscopic analyses. The effect of the preparation conditions on the structure and the morphology of the composites was also addressed through the characterization of selected samples with similar composition prepared by melt compounding. In agreement with the findings reported in a previous paper for the composites filled with two-tailed organoclays, intercalation of the copolymer chains within the tighter galleries of the one-tailed clays occurs easily, independent of the application of a mechanical stress. However, the shear-driven break-up of the intercalated clay particles into smaller platelets (exfoliation) seems more hindered. A collapse of the organoclay interlayer spacing was only observed clearly for a commercial one-tailed organoclay – Cloisite^® 30B – whereas the same effect was almost negligible for a home-made organoclay with similar structure.     

Recycled PET‐organoclay nanocomposites with enhanced processing properties and thermal stability

Preparation of thermally stable recycled PET‐organoclay nanocomposites with improved processing and mechanical properties is a challenging task from the environmental as well as industrial and commercial point of view. In this work, both modification of sodium‐type montmorillonite with 1,2‐dimethyl‐3‐octadecyl‐1H‐imidazol‐3‐ium chloride and additional treatment with [3‐(glycidyloxy)propyl]trimethoxysilane was performed. Thermal stability of the organoclays and nanocomposites prepared by melt compounding was tested by thermogravimetric analysis, differential scanning calorimetry, and melt rheology. In comparison with the organoclays modified with quaternary ammonium compounds, the prepared clays showed substantial suppression of matrix degradation during melt mixing. The increase in interlayer distance of silicate platelets and homogeneity of dispersions in the recycled and virgin PET matrices have been evaluated by transmission electron microscopy and wide‐angle X‐ray scattering. The higher degree of delamination in the nanocomposites filled with imidazole organoclays was in a good agreement with improved rheological characteristics and led to significant enhancement in mechanical properties and thermal stability. A difference in structure (besides the level of delamination and homogeneity of silicate platelets) of recycled versus virgin PET nanocomposites was detected by X‐ray diffraction patterns. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Phosphonium‐based layered silicate—Poly(ethylene terephthalate) nanocomposites: Stability,thermal and mechanical properties

PET‐clay nanocomposites were prepared using alkyl quaternary ammonium and phosphonium modified clays by melt‐mixing at 280°C using a micro twin screw extruder. The latter clays were prepared by synthesizing phosphonium surfactants using a simple one‐step method followed by a cation exchange reaction. The onset temperature of decomposition (T_onset) for phosphonium clays (>300°C) was found to be significantly higher than that of ammonium clays (around 240°C). The clay modified with a lower concentration (0.8 meq) of phosphonium surfactant showed a higher T_onset as compared to the clay modified with a higher concentration (1.5 meq) of surfactants. Nanocomposites prepared with octadecyltriphenyl phosphonium (C18P) modified clay showed a higher extent of polymer intercalation as compared with benzyltriphenylphosphonium (BTP) and dodecyltriphenylphosphonium (C12P) modified clays. The nanocomposites prepared with ammonium clays showed a significant decrease in the molecular weight of PET during processing due to thermal degradation of ammonium surfactants. This resulted in a substantial decrease in the mechanical properties. The molecular weight of PET was not considerably reduced during processing upon addition of phosphonium clay. The nanocomposites prepared using phosphonium clays showed an improvement in thermal properties as compared with ammonium clay‐based nanocomposites. T_onset increased significantly in the phosphonium clay‐based nanocomposites and was higher for nanocomposites which contained clay modified with lower amount of surfactant. The tensile strength decreased slightly; however, the modulus showed a significant improvement upon addition of phosphonium clays, as compared with PET. Elongation at break decreased sharply with clay. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Functionalization of nanoclays with ionic liquids for polypropylene composites

Cationic nanoclays were treated by ion exchange with various ionic liquids (ILs) containing cations and anions of different structure and/or molecular weight in order to investigate the effects of the IL structure and cation chain length on extent of clay dispersion, intercalation, and thermal stability. The modified clays containing imidazolium‐, pyridinium‐, and phosphonium‐based cations were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and X‐ray diffraction. Although the thermal stability of pure ILs was mostly controlled by the type of the anion present, high temperature thermal stability of the modified clays, at the IL cation loadings achieved in this work, was not significantly dependent on type, structure, or size of the cation. The latter parameters, however, were of significant importance in controlling degree of dispersion of the nanoclays during melt compounding with polypropylene (PP). Basal spacing increased proportionally to the size and type of the intercalated cations and showed little change in the PP composites. Although commercial organoclays were shown to be less thermally stable than IL modified clays, they exhibited larger basal spacing and better dispersion characteristics in the polyolefin matrix; however, they increased to the same extent the thermal stability of the PP matrix as the phosphonium modified clays. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Organoclay degradation in melt processed polyethylene nanocomposites

Melt processed nanocomposites were formed from low-density polyethylene, LDPE, and organoclays over a wide range of processing temperatures. These composites show limited exfoliation, and hence, their X-ray analysis reveals a distinct peak corresponding to the interplatelet distances in the unexfoliated clay galleries. The degradation of the quaternary ammonium surfactant of the organoclay in these systems was characterized by examining the change in the position of these peaks as a function of the melt processing temperature. Upon degradation, the mass of the surfactant within the clay galleries decreases, which causes the platelets to collapse and shifts the WAXS peak to lower d-spacings. The results of the WAXS analysis suggest that a significant portion of the surfactant is lost from the organoclays when the melt processing temperature is increased from 180 to 200 °C or higher. The extent of surfactant degradation in these composites was determined to be independent of the organoclay content. Organoclay degradation appears to limit the extent of exfoliation or dispersion in LDPE as revealed by stress-strain analyses of nanocomposites processed at different temperatures. The amount of surfactant lost during thermogravimetric analysis of various organoclays indicates that surfactants with multiple alkyl tails have greater thermal stability than those with a single alkyl tail. A comparison of the mass of surfactant lost during melt processing of nanocomposites and during thermogravimetric analysis of organoclays (in the absence of polymer) indicated that at a given time, a larger surfactant loss from the clay galleries occurs during extrusion than during the TGA experiment. This is attributed to the greater ease with which the degradation products (predominantly α-olefins) are solubilized in polyethylene for the composites as opposed to evaporated from the organoclay during TGA.     

Polyimide/organo‐montmorillonite nanocomposites: A comparative study of the organoclays modified with aromatic diamines

Six organophilic clays have been obtained through cation‐exchange between sodium montmorillonite (Na^+‐Mt) and the hydrochloride salts of aromatic diamines (DA1–6). The results obtained by thermogravimetric analysis (TGA) showed that the organophilic clays start to decomposition within 150–340°C, which shows that they are thermally stable compared with conventional montmorillonite modified with aliphatic long‐chain quaternary alkyl ammonium salts. The highest thermal stability and interlayer basal spacing were observed for the organoclay obtained from 3,3′‐sulfonyl dianiline (DA2), and therefore it was chosen for preparing clay/polymer nanocomposite materials (CPN). Polyimide/clay nanocomposite materials consisting of benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride (BTDA) and 2‐(5‐(3,5‐diaminophenyl)‐1,3,4‐oxadiazole‐2‐yl)pyridine (POBD) were also obtained by an in situ polymerization reaction through a thermal imidization. DA2‐Mt was used as filler at different concentrations. Both the thermal stability and the glass transition temperature (T_g) are increased with respect to pure polyimide (PI) at low clay loadings. At high clay concentrations, the organoclay particles make aggregate and as results of this phenomena T_g and thermal stability are decreased. POLYM. COMPOS., 36:613–622, 2015. © 2014 Society of Plastics Engineers     

Biodegradable PLA based nanocomposites for packaging applications: The effects of organo-modified bentonite concentration

The effects of concentration and surface modification of two Brazilian bentonite clays on nanocomposites' properties based on polylactic acid – (PLA) were investigated. The samples were prepared by the extrusion/injection method to obtain biodegradable packaging plastics. The raw materials and their bionanocomposites were characterized by various techniques. Natural clay samples presented a size of around 2 μm while the modified ones' size was 5–6 μm, probably due to the presence of cetyltrimethylammonium bromide in the interlayer space. The particle size and the contact angle increased with the treatment and the clay's density decreased. The organoclays were homogeneously dispersed in PLA, which can be associated with the interactions between PLA chains' carbonyl groups and the organoclays. The bionanocomposites present modified clay particles axis aligned to the flow direction of the extruder/injector. Chocolate organoclay acts as a nucleating agent to PLA crystal growth, increasing the sample's crystallinity, while Bofe organoclay interferes with the amorphous chain's mobility and diminishes the sample's crystallization. The addition of both organoclays to PLA diminished the sample's elongation at break and strength, although the organoclays increased the sample's Young modulus, even though Bofe organoclay is more active in PLA amorphous phase and Chocolate organoclay on the crystalline one.     

Analysis of tetramethylammonium–montmorillonite and retention of toluene from aqueous solution

Non-ionic organic contaminants, such as toluene, can be effectively absorbed by organo-montmorillionite. The tetramethylammonium (TMA)–montmorillonites were prepared by ion-exchange and used to evaluate toluene uptake from water. The TMA–montmorillonites were characterized by total organic carbon determinations, swelling index, X-ray diffraction, thermal analyses, and infrared spectroscopy. Toluene adsorption was measured by UV–visible spectrophotometry. The thermal stability of the TMA–montmorillonites decreased as TMA addition increased; the total mass loss decreased with increasing temperature up to 500 °C. The amount of carbon retained by the TMA–montmorillonites was proportional to the area of the infrared band at 1488 cm^− 1 (asymmetric C–H bending vibration of –CH3 group). The retention of toluene by TMA–montmorillonites increased from 0.2 to 0.22 mg g^− 1 mainly to interlayer adsorption. The adsorption of toluene influenced the OH-stretching vibration.     

Effect of organoclay purity and degradation on nanocomposite performance, Part 1: Surfactant degradation

The alkylammonium surfactants used to form commercial organoclays are known to begin to degrade at temperatures below the typical melt processing temperatures of some polymers. In this study, the thermal stability and degradation of various surfactants and their corresponding organoclays were investigated. Several factors, such as surfactant type and excess surfactant in the organoclay, that affect the thermal stability of surfactants on organoclays are explored. Nuclear magnetic resonance (NMR) spectroscopy was used to analyze the decomposition products. Thermogravimetric analysis (TGA) was used as the primary method to characterize the thermal stability of these surfactants and organoclays; the neat surfactants lose mass more rapidly, at a given temperature, than the corresponding organoclay. Washing the organoclay with methanol proved to be an effective way to remove the excess surfactant from the clay galleries. Such purification generally improves the thermal stability of the as-received organoclays. Depending on the availability of residual halide anions in the organoclay, the organoclays decompose via either S_N2 nucleophilic substitution or Hoffmann elimination pathways.     

Study on ibuprofen/montmorillonite intercalation composites as drug release system

The present paper focused on the intercalation of ibuprofen (IBU) into montmorillonite as a sustained release drug carrier. The intercalation compounds were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FT-IR), and thermogravimetric analysis (TGA). The basal spacing of montmorillonite increased from 1.25 nm to 1.57 nm. The decomposition temperature of intercalated IBU was increased to 471 °C. The in vitro release experiments revealed that IBU was released from MMT steadily and pH dependent.