Montmorillonite elastomers

Keywords layered fillers, layered silicates, montmorillonite, elastomers, nanocomposites, intercalation, exfoliation, ionic liquids. 

Stress whitening (indicating cavitation during stress) for the montmorillonite containing elastomer-toughened epoxy was much more extensive that the stress whitening with the elastomer-toughened epoxy without montmorillonite. This unusual behavior will be discussed in Chapter 7 with the review of montmorillonite-elastomer nanocomposites and an unusual amorphous polyester nanocomposite, terephthal-ate copolyester prepared with 2,2,4,4-tetramethyl-l,3-cyclobutanediol and propanediol. 

The characterization of the physical and chemical changes that occur in montmorillonite/PDMS nanocomposite elastomers as they are thermally aged is reported. Broadband Dielectric Spectroscopy (BDS) was used to track changes in the physical interaction between the polymer and clay associated with increases in non-oxidative thermal stability (as determined by TGA). The evolution of volatile siloxane species from the elastomers was characterized with Thermal Volatilization Analysis (TVA). Results suggest that the improved thermal stability and the increases in polymer/clay association are a result of significant re-structuring of the polymer network. 

A series of five nanocomposite elastomer systems were prepared for this study incorporating 0, 1, 2, 4 and 8% (on total resin mass) of the organically modified montmorillonite clay Cloisite 6A. The appropriate level of Cloisite was dispersed in a starting resin blend of OH terminated PDMS (M -77,000 and Mn -550 g mol in a 3 1 ratio) by a combination of mechanical mixing and ultrasonic processing to give a nano-dispersion of clay platelets. The blend was subsequently crosslinked with a stoichiometric level of tetrapropoxysilane (TPOS) in the presence of 5% diphenylmethylsilanol (DPMS) chain terminator and 5% tin(II) 2-ethylhexanoate catalyst, cured in an open mould at 65°C for twenty minutes, then removed from the mould and post cured for a further fifteen hours at 65°C to give an elastomeric mat. 

Substances that have been used in this context include glass fiber (occasionally glass beads), carbon fiber, carbon nanotubes, carbon black, graphite, fuUerenes, graphite chemically modified clays and montmorillonites, silica, and mineral alumina. Other additions have been included in polymer formulations, including calcium carbonate, barium sulfate, and various miscellaneous agents, such as aluminum metal, oak husks, cocoa shells, basalt fiber, silicone, rubbery elastomers, and polyamide powders. The effects of such additions of polymer properties are discussed next. 

Relative modulus versus talc clay-reinforced agent content for nanocomposites based on a thermoplastic polyolefin or a triphenylene oxide matrix polypropylene plus ethylene-based elastomer showed that relative to a particular filler content, an appreciably higher modulus content was obtained for the montmorillonite reinforcing agent than for talc [156]. Doubling the modulus of the phenylene oxide requires about four times more talc than montmorillonite, with the talc-reinforced polymer having an improved surface finish. In the case of the talc-reinforced polymer, exfoliation is appreciably better than with clay reinforcement. The talc-reinforced polymer has automotive applications. 

Microcellular foams can be produced by noncontinuous processes such as a batch process [2, 12, 15, 16, 31, 32, 34, 35], continuous processes such as extrusion and injection molding [24,33,36,37], orby asemicontinuousprocess [38]. Since the semicontinuous process is not extensively used in the scientific community or in the industry, it will not be described in this chapter. Readers are encouraged to refer Ref. 38 for detailed information on this process. To date, microcellular foams have been produced in amorphous polymers [12, 31, 32, 34], semicrystalline polymers [35], and in elastomers [16]. Recently, MCF structures have also been produced in plastics filled with inorganic nanoparticles (montmorillonite) [39-43], as well as organic cellulosic fiber filled plastic composites [12, 31, 32, 34]. 

The use of a commercial Cloisite 20A organoclay to prepare SBS-based nanocomposites by melt processing was recently reported [63]. In this case, the nanocomposite morphology was characterized by a combination of intercalated and partly exfoliated clay platelets, with occasional clay aggregates present at higher clay content. For this particular thermoplastic elastomer nanocomposite system, well-dispersed nanoclays lead to enhanced stiffness and ductility, suggesting promising improvements in nanocomposite creep performance. The use of stearic acid as a surface modifier of montmorillonite clay to effectively improve the clay dispersion in the SBS matrix and the mechanical properties of the SBS-clay nanocomposites was reported [64]. 

ATH and MH are used primarily in wire and cables in poly( vinyl chloride) (PVC), polyethylene, and various elastomers. There is also some limited application of MH in polyamide-6. To pass flame retardancy tests, 35 to 65 wt% of metal hydroxide is required. Decreasing the loading of metal hydroxides will result in a significant gain in physical properties, especially low-temperature flexibility therefore, combinations with red phosphorus, sUicones, boron compounds, nanoclays (treated montmorillonites), and charring agents have been explored. Surface treatment of metal hydroxides also helps to improve physical properties and sometimes improves flame retardancy, due to better dispersion. 

Figure 9.7 Comparison of talc reinforcement with nanoclay (montmorillonite) reinforcement, in terms of the ratio of composite modulus to matrix modulus. The matrix material is a blend of polypropylene and thermoplastic elastomer (TPO). (Reproduced from Lee H-s. et al. (2005) TPO based nanocomposites. Part 1. Morphology and mechanical properties Polymer, 46, 11673-11689. Copyright (2005) Elsevier Ltd.)...

Arroyo M, Lopez-Manchado M A and Herrero B (2003) Organo-montmorillonite as substitute of carbon black in natural rubber compounds. Polymer 44 2447-2453. Donnet J B (2003) Nano and microcomposites of polymer elastomers and their reinforcement, Compos Sci Technol 63 1085-1088. 

Chiu F-C, Fu S-W, Chuang W-T and Sheu H-S (2008) Fabrication and characterization of polyamide 6,6/organo-montmorillonite nanocoinposites with and without a maleated polyolefin elastomer as toughener, Po/ywer 49 1015-1026. 

The oil absorption parameter was calculated on the basis of the amount of dibutyl phthalate oil absorbed by the filler particles. This parameter can characterize the tendency of the filler to create own structure in elastomer matrix. The values of this parameter for modified fillers were, respectively 65,2 g/lOOg for MMT I.30E modified with octadecylamine, 78 g/lOOg for MMT 1.31 PS modified with octadecylamine and aminepropyltriethoxysilane, 72,1 g/lOOg for MMT 1.34 TCN. The montmorillonites modified with ionic liquids characterized the similar values of oil absorption parameters (MMT mod. ADDMA 75,2 g/lOOg, MMT mod. SDDMA 82,5 g/lOOg)... 

Epoxies have been toughened with elastomer-dispersed phases with a decrease in modulus. Work by Balakrishnan et al. [35] utilized organo-montmorillonite as a dispersed phase in elastomer-toughened epoxies to recover this lost modulus. The montmorillonite was modified with octadecyl ammonium ion. The elastomer-dispersed phase in the epoxy was prepared by free radical polymerization of acrylic monomer within the epoxy. The acrylic elastomer-dispersed phase had epoxy functionality provided through the utilization of glycidyl methacrylate as a comonomer. 

This preformed elastomer-reinforced epoxy is blended with additional epoxy and the organomontmorillonite to produce the final composite. The epoxy was DER 331 from Dow the curing agent was piperdine. The formulations were based on phr. The authors did convert the montmorillonite content to wt.% when illustrating the results. 

The difficulty in preparing aligned and exfoliated montmorillonite composites in epoxy was again apparent. This complication was further compromised with the preferred association at the epoxy-elastomer interface of the intercalated organomontmorillonite bundles. The result was a greater randomization of the montmorillonite dispersed phase in the epoxy. 

The composite that contained 16 phr of elastomer and 5.5 wt.% of organomontmorillonite provided unexpected results. The Young s modulus recovered modestly when the montmorillonite was in the formula. The unexpected result was the increase of percent elongation to failure from 10.0% for the elastomer toughened epoxy to 11.5% when the montmorillonite was present. 

This phenomenon of increased percent elongation of polymer composites as a function of increasing montmorillonite content will be addressed later in the chapter when the same behavior is observed with an unusual amorphous polyester and elastomers. This behavior is not observed with... 

When the ratio of PP-g-MA to Cloisite 20A is fixed at 1, the number-average particle size of the elastomer significantly decreases as a function of increasing montmorillonite content (0.86 pm for 0% montmorillonite content to 0.55 pm for 7% montmorillonite content). The number-average of the aspect ratios increased from 2.12 for 0% montmorillonite to 4.75 for 7% montmorillonite content in the TPO-montmorillonite composite with PP-g-MA as the compatibilizer. The tensile modulus for the 5 and 7% montmorillonite containing TPO composites increased rapidly up to a PP-g-MA/montmorillonite ratio of 1. The composite of TPO with and without PP-g-MA with no montmorillonite content had a tensile modulus of approximately 0.78 GPa. 

The morphology of the composites were carefully analyzed by WAXS and TEM for the montmorillonite and AFM for the elastomer-dispersed phase. The additional mechanical property that was measured in this work was the CTE with the same equipment, TMA 7, and protocol that was employed in the second article in this series (see above). 

As a function of the anisotropic orientation of aspect ratio for the montmorillonite and elastomer in the preferred direction of the flow of the injection molder, the expansion coefficient of the composite in the flow direction decreased from approximately 9.7 x 10 mm/mm°C for 0% montmorillonite content to approximately 5.3x10 mm/mm°C for 7% montmorillonite content when the ratio of compatibilizer to montmorillonite was 2. Similar values were observed for compatibilizer montmorillonite ratios of 0.5 and 1.0. 

As a measurement of the significance of the anisotropic orientation of the dispersed phases (elastomer and montmorillonite), the CTE in the direction orthogonal to the flow direction of the injection molder decreased from approximately 14.4x 10 mm/mm°C for 0% montmorillonite content to approximately 10.5x10 mm/mm°C for 7% montmorillonite content in the composite for compatibilizer/montmorillonite ratios of 0.5, 1.0, and 2.0. This perspective is consistent with the previous... 

Elastomers (rubber) have the advantage of lower Tg than polymers for engineered applications and equipment uniquely designed for the preparation of rubber compounds that allows for the incorporation of much larger amounts of montmorillonite without a detrimental sacrifice of the hardness-flexibility balance of the nanocomposites. The alignment of intercalated composites for the maximum benefit of mechanical properties is much easier. 

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