Nano-clays exfoliates

Filled polymer systems of industrial importance, e.g., filled rubber compounds, filled thermoplastics are thus meso or microcomposites, possibly with a structuration (of the dispersed phase) at the nano or meso scale. Whilst no sizeable commercial application yet exist for nanocomposites rubbers or thermoplastics (to the author s knowledge), considerable research has been made since 1984 with so-called ex-foliated layered silicate "nano-clays." Exfoliation means that individual clay sheets, of around 1 nm thickness, have been separated and adequately dispersed in the matrix. Some reinforcement has indeed been demonstrated with such exfoliated nanoparticles but, generally with very specific rubber systems and/or at a cost of preparation that is hardly compatible with reasonable chances of commercialization. 

Lan and Pinnavaia [6] showed that hybrid organic-inorganic composites exhibited mechanical properties superior to those for their separate composites. Dependence of tensile strength and modulus of epoxy nano-clay composites on the chain length of the clay-intercalated alkylammonium ions is shown in Fig. 28. The presence of the organoclay substantially increased both the tensile strength and modulus relative to the pristine polymer. The mechanical properties increased with any exfoliation in the order ... 

Nano-clay particles, 476 Exfoliated, 476 Intercalated, 476 Nanoclay, 190... 

Figure 15.1. Transmission electron microscopy images of polypropylene filled with nano-clay to 5 wt%. (a) The combination of exfoliated and intercalated clay platelets is evident, (b) After stretching the platelets appear to be less curved and ai e aligned in the stretch-direction...

Zheng, W. G., Y. H. Lee, and C. B. Park. 2006. The effects of exfoliated nano-clay on the extrusion microcellular foaming of amorphous and ciystaUine nylon. J Cell Plast 42 271-88. 

Keywords Montoraorillonite polyethylene nano composites, CPN, clay exfoliation, nanocomposites formulation, thermal stabihty, barrier propoerties, mechanical behaviour... 

The protein/glycerol/water/Cloisite Na (treated) sample also showed a single Tg at 118.0 °C, 5.5 °C higher than observed in the untreated nanocomposite, due to the further restriction in molecular motion of the protein by the predominantly exfoliated dispersion of the nano-clay. The secondary (or (3) relaxation in the neat protein/glycerol/water/Cloisite Na (untreated) sample was situated at... 

The study showed that shape recovery stress of shape memory polyurethane can be increased significantly with the inclusion of reactive nano-clay and subsequent exfoliation. Fully exfoliated 1 wt% PU/clay nanocomposite showed highest recovery stress, excellent shape recovery ratio and large enhancement in mechanical properties. A faster stress relaxation in the case of composite with 5 wt% limited the maximum shape recovery stress, although largest improvement in tensile properties was seen in this case. 

Over the past decade extensive work has been done to develop a novel extrusion process with the aid of high power ultrasound [18-22], A number of studies on the effect of ultrasound on polymers have been published and reported in various review articles and books. It was shown that ultrasonic oscillations can breakdown the 3-D network in vulcanized rubber within seconds. Ultrasound was found to improve the compatibilization of immiscible plastic blends, plastics/rubber and rubber/rubber blends during extrusion process [23]. In recent years, use of ultrasound to disperse nanofdler in a polymer matrix is gaining attention. Ultrasound helps in rapid exfoliation and intercalation of nano-clay in a polymer matrix [24]. 

The foam density variations of the foamed WPCs with different contents of wood fiber as a function of die temperature are shown in Figure 4. With no gas injection, the density reduction may be due to the volatiles (e.g., moisture or extractives) generated from WF during processing. The addition of 1% exfoliated nano-clay particles decreased slightly the foam density of the composites. This may be because nano-clay particles increase the melt strength and thereby the possibility of gas escape from the extrudate skin decreases. 

Micro-composites are formed when the polymer chain is unable to intercalate into the silicate layer and therefore phase separated polymer/clay composites are formed. Their properties remain the same as the conventional micro-composites as shown in Figure 2(a). Intercalated nano-composite is obtained when the polymer chain is inserted between clay layers such that the interlayer spacing is expanded, but the layers still bear a well-defined spatial relationship to each other as shown in Figure 2(b). Exfoliated nano-composites are formed when the layers of the day have been completely separated and the individual layers are distributed throughout the organic matrix as shown in Figure 2(c). 

Layered materials are of special interest for bio-immobilization due to the accessibility of large internal and external surface areas, potential to confine biomolecules within regularly organized interlayer spaces, and processing of colloidal dispersions for the fabrication of protein-clay films for electrochemical catalysis [83-90], These studies indicate that layered materials can serve as efficient support matrices to maintain the native structure and function of the immobilized biomolecules. Current trends in the synthesis of functional biopolymer nano composites based on layered materials (specifically layered double hydroxides) have been discussed in excellent reviews by Ruiz-Hitzky [5] and Duan [6] herein we focus specifically on the fabrication of bio-inorganic lamellar nanocomposites based on the exfoliation and ordered restacking of aminopropyl-functionalized magnesium phyllosilicate (AMP) in the presence of various biomolecules [91]. 

Examples of the synthesis of polysiloxane nanocomposites reported in the literature include Work by Ma et al (6) who modified montmorilIonite with short segments of PDMS and blended this into a polymer melt/solution to yield examples of fully exfoliated or intercalated PDMS/clay nanocomposites. Pan, Mark et al (7) synthesized well defined nano-fillers by reacting groups of four vinyl terminated POSS cages with a central siloxane core. These materials were subsequently chemically bonded into a PDMS network yielding a significant improvement in the mechanical properties of the polymer. 

In another interesting development, Yei et al. [124] prepared POSS-polystyrene/clay nanocomposites using an emulsion polymerization technique. The emulsion polymerization for both the virgin polystyrene and the nano composite started with stirring a suspension of clay in deionized water for 4h at room temperature. A solution of surfactant ammonium salt of cetylpyridinium chloride or POSS was added and the mixture was stirred for another 4 h. Potassium hydroxide and sodium dodecyl sulphate were added into the solution and the temperature was then raised to 50 °C. Styrene monomer and potassium persulfate were later on added slowly to the flask. Polymerization was performed at 50 °C for 8 h. After cooling, 2.5% aqueous aluminium sulphate was added to the polymerized emulsion, followed by dilute hydrochloric acid, with stirring. Finally, acetone was added to break down the emulsion completely. The polymer was washed several times with methanol and distilled water and then dried overnight in a vacuum oven at 80 °C. The obtained nanocomposite was reported to be exfoliated at up to a 3 wt % content of pristine clay relative to the amount of polystyrene. 

In practice, the techniques of blending, compositing and nano-reinforcement are often used together. Thermoplastic starch/poly(vinyl alcohol) (PVOH)/clay nanocomposites exhibited the intercalated and exfoliated structures [260]. Mont-morillonite (MMT) with three types of cation or modifier (Na", alkyl ammonium ion, and citric acid) was examined. The prepared nanocomposites with modified montmorillonite indicated a mechanical improvement in the properties, in comparison with pristine MMT. 

Diaminodecane and 1,10-decanedicarboxylic acid were polyconden-sated in the presence of an organophihc clay to polymerize a nylon 1012 clay nano composite [27]. X-ray diffraction and TEM observations revealed that the clay layers were exfoliated and uniformly dispersed in nylon 1012. The speed of crystallization of the nanocomposite increased compared with nylon 1012. Furthermore, the tensile strength and the elastic modulus in tension were improved, and the amount of absorbed water was decreased through the improvement of the barrier characteristics. 

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. 

The dispersion of clay platelets (exfoliation and intercalation level of the silicate layers) and surface area of silicate platelets have the potential to alter the rheological behavior of the nanocomposites. In-situ polymerized nano composites exhibit more exfoliated structure than the composites prepared by the melt blending technique. Irrespective of the processing parameter, the nanocomposites show shear thinning behavior at high shear rate (Figure 9.14), whereas the pristine polyamide exhibits Newtonian behavior (i.e., the viscosity remains almost the same). It has also been reported that the polymer nanocomposite possesses higher steady shear viscosity than pristine polyamide at low shear rates. 

The achievement of a high degree of exfoliation of layered clay minerals in nonpolar rubber matrices, such as NR, is still a major issue. This chapter presents a brief overview of studies of clay reinforcement in NR both in micro and nano scale. Although quite a lot of studies have been reported in the field of NR reinforcement with simple organoclay (OMt), plenty of scope still exists to improve the dispersion of MMT followed by property enhancement. Better dispersion and improvement in dilferent properties was observed in the case of EOMt-filled NR nanocomposites. 

The concept of toughening on the nano scale is the subject for much current research where particles of, for example, clays, carbon fibre and carbon nano-tubes, are being evaluated as tougheners for brittle, often high-temperature epoxy matrices. In the same way as for polymeric additives, the main difficulties being experienced are those of particle exfoliation, in the case of the clays, and dispersion in the case of the carbon particles. 

Flammability resistance. An increased flammability resistance has been noticed as an important property enhancement involving nano-platelets incorporated into polymers involving exfoliated clay this involves the formation of a stable carbon/nano-platelet or nanofibre surface. This surface exhibits analogous characteristics to intumescent coatings, whereby the resultant char provides protection to the interior of the specimen by preventing continual surface regeneration of available fuel to continue the combustion process. The primary advantage noted with nano-filler incorporation is the reduction in the maximum heat release rate. 

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