Surface modified nanoclay

Kamal, M. R., Galderon, J. U., and Lennox, R. B. 2009. Surface energy of modified nanoclays and its effect on polymer/clay nanocomposites. 

The U.S. military and NASA, in conjunction with Triton Systems, Inc., Chelmsford, Mass., were looking into nanoclay as a barrier enhancer for EVOH in long-shelf-life packaging. The increasing interest in clay nanocomposites has resulted in many companies coming up with surface modified or nanostructured clay in market. The market for nanoclay is dominated by companies such as southern clay products Inc (Cloisite series). Nanocor (Nanomer series), Elementis specialities (Bentone series), Sud Chemie Inc. 

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]. 

Similar to the studies performed on the SFRSF, the amount of microspheres, for each nanoclay-reinforced syntactic foam, was fixed at 30 vol%. The nanoclay was surface-modified montmorillonite mineral. Figure 2.15 shows the general structure of NCRSF. 

E - clay surface modified with 25-30 wt% octadecylamine (Nanoclay, USA),... 

The above results clearly showed that compatibility and optimum interactions between starch matrix, organic modifiers (if any) and the silicate l er surface were crucial to the formation of intercalated or exfoliated starch-layered silicate nanocomposites. Nanomer BOE is an onium ion surface modified montmorillonite mineral. Compared with natural MMT, BOE is more sinface hydrophobic, and therefore is not very miscible in the hydrophilic starch matrix. On the other hand, in the case of natural MMT, due to the strong interactions between small amounts of polar hydroxyl groups of starch and glycerol, and the silicate layers of the nanoclay (inorganic MMT Na+), the starch chains and glycerol molecules can intercalate into the interlayers of the nanoclay. 

Polymer nanocomposites represent a rapidly expanding research area. Nanocomposites refer to a class of reinforced polymer with a low percentage of well dispersed nanopartieles. These materials often demonstrate notable improvement in properties such as mechanic characteristics, tensile strength, heat and chemical resistance. Nanoparticles can be classified based on how many dimensions are on the nanoscale. The first type is plate-like (e.g. nanoclays) that has a thickness in the nanometer range and lateral dimensions in the sub-micron or micron range. Two types of nanoclay, 20A (Southern Clay) and MHABS (a surface modified clay 20A), are used in our research. The second type has two dimensions in the nanometer range, such as carbon nanofibers (CNF) or carbon nanotubes. The third type has three dimensions in the range of nanometer, such as spherical silica particles. The latter type of nanoparticle is not used in our study. 

The lowering of die swell values has a direct consequence on the improvement of processability. It is apparent that the processability improves with the incorporation of the unmodified and the modified nanofillers. Figure lOa-c show the SEM micrographs of the surface of the extrudates at a particular shear rate of 61.2 s 1 for the unfilled and the nanoclay-filled 23SBR systems. The surface smoothness increases on addition of the unmodified filler, and further improves with the incorporation of the modified filler. This has been again attributed to the improved rubber-clay interaction in the exfoliated nanocomposites. 

In-situ polymerization consists of three steps. In the first step, the clay is converted to organophilic. In this process, the preparation of the organically modified clay is synthesized from the normally available hydrophilic clay mineral and the surface is modified using surfactants. The second step consists of intercalation of the monomer, assisted by the presence of surfactant, which is followed by the polymerization of the monomer to prepare long-chain nanoclay. In other words, in this process a polymer precursor or a monomer is subjected to get embedded in between clay layers followed by the expression of the silicate platelet layers into the clay matrix undergoing polymerization. 

Sodium montmorillonite (Na-MMT) was originally modified with pro-tonated amino acids with different numbers of carbon atoms and subsequently swollen with e-caprolactam. Then it underwent polymerization to produce nylon-6 polymer-clay nanocomposite [18]. Later, this technique was also extended to manufacture other thermoplactics. One advantage of this in-situ polymerization technique is the tethering effect, which enables the organic chemical such as 12-aminododecanoic acid (ADA) situated at the surface of the nanoclays to link with nylon-6 polymer chains during polymerization. 

However, the simple melt mixing of polyolefins with natural clays, does not guarantee a sufficient level of dispersion of the nanoparticles, which are often present in the form of micron-size agglomerates. In order to overcome this problem, two main strategies have been followed surface functionalization of needle-like clays (usually by alkyl-silanes) or addition of a third polymeric phase (usually maleic anhydrite modified PP PP-g-MA), which acts as a compatibilizer between the matrix and nanofiller. Both methods tend to modify the surface energies of the nanocomposite system, in order to reduce the interparticle interaction and improve the dispersion. In the case of a reactive surface treatment only, the polymer-day interaction is expected to be enhanced, along with better nanoclay dispersion, which is very important for the final mechanical properties. 

Fire retardancy behavior of PP/PA66 blends compatibilized with PP-g-MAH and modified with untreated and treated nanoclays was studied (Kouini and Serier 2012). It was found that the intercalation, exfoliation of nanoclays of nanocomposites, and the flame retardancy properties were improved significantly. In addition a good balance of impact strength and flame retardancy was obtained for PP/PA66 nanocomposites in the presence of PP-g-MA compatibilizer. The presence of the clay led to an increase in the flammability time. In addition, the treatment made a more pronounced effect. A 23 % increase was observed only when 4 wt% nanoclay was added and a longer flammabiUty time was noticed with treated clay. This was attributed to the stacking of nanoclay which created a physical protective barrier on the surface of the material. Similar behavior has been reported by earlier workers (Kocsis and Apostolov 2004). 

Nanoclay is the term generally used when referring to a clay mineral with a phyllosilicate or sheet structure with dimensions of the order of 1 nm thick and surfaces of perhaps 50-150 nm. The mineral base can be natural or synthetic and is hydrophilic. The clay surfaces can be modified with specific chemistries to render them organophilic and therefore compatible with organic polymers. Surface areas of nanoclays are very large, about 750 m /g. When small quantities are added to a host polymer, the resulting product is called a nanocomposite. 

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