Mechanical properties exfoliated montmorillonite

Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (2), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-10). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. 

The addition of plasticizer used for the improvement of mechanical properties leads generally to an increase in the oxygen permeability coefficient due to the higher mobility of the polymer chain and higher free volume [102]. On the contrary, the dispersion of nanoclays in PLA makes it possible to divide the permeability coefficient by 2 or 3 depending upon the type of the nanoclays (e.g. organomodified montmorillonite, cloisite 25A or 30B, organomodified synthetic fluorine mica) and exfoliation [129-131]. 

Mechanical Properties Toyota Central Research Laboratories in Japan was the first to obtain significant mechanical improvement of a PA matrix by adding as little as about 2 wt% of montmorillonite (MMT) [Kojima et ah, 1993 Usuki et ah, 1993 Okada and Usuki, 2006], Improvement in the mechanical properties on the vitreous and rubbery plateau by layered silicate nanoparticles depends on several factors, including clay surface modification, polymer chemistry, processing method, level of exfoliation, and clay orientation. In this section we present an overview of the influence of these factors on the dynamic mechanical properties of PLSN. 

Botana et al. [50] have prepared polymer nanocomposites, based on a bacterial biodegradable thermoplastic polyester, PHB and two commercial montmorillonites [MMT], unmodified and modified by melt-blending technique at 165°C. PHB/Na and PHB/ C30B were characterized by differential scanning calorimetry [DSC], polarized optical microscopy [POM], X-ray diffraction [XRD], transmission electron microscopy [TEM], mechanical properties, and burning behavior. Intercalation/exfoliation observed by TEM and XRD was more pronounced for PHB30B than PHB/Na,... 

Choi and Chung [16] were the first to prepare phenolic resin/layered sihcate nanocomposites with intercalated or exfoliated nanostructures by melt interaction using linear novolac and examined their mechanical properties and thermal stability. Lee and Giannelis [10] reported a melt interaction method for phenolic resin/clay nanocomposites, too. Although PF resin is a widely used polymer, there are not many research reports on PF resin/montmorillonite nanocomposites, and most of the research investigations have concentrated on linear novolac resins. Up to now, only limited research studies on resole-type phenolic resin/layered silicate nanocomposites have been published [17-19] and there is still no report on the influence of nano-montmorillonite on phenolic resin as wood adhesive. Normally H-montmorillonite (HMMT) has been used as an acid catalyst for the preparation of novolac/layered silicate nanocomposites. Resole resins can be prepared by condensation reaction catalyzed by alkaline NaMMT, just as what HMMT has done for novolac resins. 

Bragaiifa et al. [97] showed the importance of the clay lamellar intercalation and exfoliation to define the material mechanical properties of nanocomposites. In this study, montmorillonite clay with different interlamellar cations (NaL Li+, K+, and Ca +) was mixed with styrene-acrylic latex and dried to produce nanocomposites. An ultra-thin cut of the sample was analyzed by ESI-TEM. The elemental maps of carbon, silicon, and calcium were used to identify the polymer, clay, and cation domains, as presented in Fig. 8.13. 

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]. 

The choice of Cloisite 30B has been demonstrated in the work above as an excellent choice for exfoliation into nylon 6. The choice of Cloisite 15A is not good, based on the work above. One must be cautious when comparing the results with Cloisite 15A and Cloisite 20A. Both organo-montmorillonites utilize the same quat treatment. However, Cloisite 15A has an excess of approximately 30 meq/lOOg of montmorillonite of quat past the exchange capacity of the montmorillonite. This excess quat could diffuse into the polymer and alter the mechanical properties of the composite. The counterion of this excess quat will be chloride. The montmorillonite content in the polymer was 3.7 wt.% for Cloisite 30B and 3.1 wt.% for Cloisite 15A. 

Epoxy ring opening is catalyzed by acids and bases. Evaluations of the role of ammonium ions as weak acids indicate that they do catalyze the epoxy cure. This can lead to a lowering of the Tg of the polymer. This compromises mechanical performance. TEM and mechanical properties indicate that the exfoliation and particle alignment of montmorillonite in the epoxy polymer in the above references are also significant variables. 

Intercalation, improved montmorillonite dispersion, and alignment of the montmorillonite in the direction of stress during testing were all positive factors in the significant improvement of mechanical properties. Greater hydrophobicity at the montmorillonite surface through the utilization of a long hydrocarbon chained carboxylic acid with the quat did not overcome the barrier to full exfoliation in the block copolymer. 

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