Degree clay dispersion

Clays have long been used as fillers in polymer systems because of low cost and the improved mechanical properties of the resulting polymer composites. If all other parameters are equal, the efficiency of a filler to improve the physical and mechanical properties of a polymer system is sensitive to its degree of dispersion in the polymer matrix (Krishnamoorti et ah, 1996). In the early 1990s, Toyota researchers (Okada et ah, 1990) discovered that treatment of montmorillonite (MMT) with amino acids allowed dispersion of the individual 1 nm thick silicate layers of the clay scale in polyamide on a molecular. Their hybrid material showed major improvements in physical and mechanical properties even at very low clay content (1.6 vol %). Since then, many researchers have performed investigations in the new field of polymer nano-composites. This has lead to further developments in the range of materials and synthesizing methods available. 

Recently a lot of attention is being given to the field of latex-based nanocomposites. Various organoclays as well as pristine clays have been intercalated in aqueous medium with NR latex, SBR latex, NBR latex, as well as carboxylated nitrile mbber (XNBR) latex [184—187], to achieve a good degree of dispersion. 

Dispersion The degree of dispersion of the nanoplatelets is determined by the degree of delamination of the clay. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite. This means that the clay particles that grow as aggregates or books of sheets must be broken up or exfoliated into individual sheets that have a thickness of the order of 1 nm, with lengths and widths of the order of 500 nm. 

In the ceramic industry the rheology of clay suspensions plays a major role. Their colloidal properties are of especial significance in the process of slip-casting, in which a clay slurry is put in contact with a porous plaster mould, which sucks up the water to leave a solid form ready for firing, Here the suction provided by the porous mould in relation to the rate of flow of water through the clay matrix is important. The structure of the clay slurry, as determined by its degree of dispersion, has also to be controlled by suitable additives. 

Humic acids are soluble in weak alkaline solutions and are essentially insoluble in water and mineral acids. They may be precipitated from solution by the action of mineral acids and bivalent or trivalent cations, however, they are fairly resistant to the acid hydrolysis. They are dark spherocol-loids with a cross-linked structure which plays a part in their high sorption capacity. They exhibit different degrees of a tendency to aggregation and very different degrees of dispersion. In comparison with other types of natural organic substances, the humic acids are characterized by their extraordinary stability in the soil. This stability is due to their ability to form organomineraJ complexes, particularly with clay minerals and with aluminium and iron hydroxides. 

Adsorption complexes — these are formed by a reaction of humic acids and their salts with the surface of mineral particles, thus yielding complex microaggregates of different degrees of dispersion. They are most frequently formed by coagulation, involving precipitation of humus substances on the surface of clay minerals by means of exchangeable cations ... 

The CPNC performance depends on the degree of platelets dispersion, with some properties more affected by it (e.g., mechanical, barrier) than others. The degree of dispersion is controlled by the thermodynamic miscibility between polymer and organoclay, solidification of organics on the clay platelets, and their concentration [Utracki, 2004, 2008a]. The PVT measurements and interpretation of the CPNC behavior provide a direct means of extracting information about matrix-clay... 

The degree of clay dispersion in a PS matrix was characterized by x-ray diffraction analysis (XRD), and electron microscopy that is, scanning [with field emission gun scanning electron microscopy (FEGSEM)] and transmission (TEM) the results are listed in Table 14.1. The XRD scans were obtained at a scan rate 0.3°/min. The specimens were prepared by compression molding at T= 200°C and a compressive... 

There is a growing tendency to incorporate nanofillers into polymer blends. When the two polymers differ significantly in rigidity, their behavior resembles that of TPE. For example, a blend of PA-6 with PP (PA-6/PP = 70/30) compatibilized with EPR-MA was melt-compounded with 4 phr of MMT-ODA [Chow et al., 2005]. The CPNC had a high degree of clay dispersion and distribution. The dynamic mechanical thermal analyzer (DMTA) data (at 10 Hz) showed a tendency opposite to that observed for TPE The largest enhancement of E was obtained for non-compatibilized CPNC at the lowest temperature of -100°C (by about 25%) the addition of EPR-MA reduced this effect by one-half, up to - -100°C. However, for these systems the tensile moduli measured in steady state and dynamic mode at 23°C were comparable (i.e.. 

FIGURE 16.31 Relative storage vs. relative tensile modulus for PA, PP and their melt compounded CPNC at r = 25°C with different degree of clay dispersion. (From Sepehr and Utracki [2006].)... 

Rocha e Silva et al. [52] have prepared layered PHB nanostructured materials by adding different amounts of commercial clays on PHB matrix. They have demonstrated that the obtained nanostructured materials exhibited an excellent degree of dispersion (intercalation/exfoliation) and that the clay was able to promote changes in the structure of neat polymer by reducing its crystallinity degree. 

Maiti and co-workers [43] investigated the effect of organic modifiers of various chain lengths in different types of clays, smectite, MMT, and mica on the degree of dispersion of clay in PLA. They observed that the nanocomposites of MMT and mica are intercalated and well-ordered compared to smectite. The... 

Layered nanoparticles, like the aggregates of silicates talc and mica, form close proximity sheets of polymer—clay hybrids due to the immiscibility of clay in polymer. The degree of dispersion in these composites is normally referred to as the following ... 

Depending on the degree of modification, intercalated or exfoliated nanocomposites can form. The degree of dispersion of clay platelets in thermoplastic polypropylene matrix also depends on the type of the com-patibilizer used. The compatibilizer as such may be compatible with the clay surfaces and hence easily intercalates the interlayer spacing of the clay... 

The TEM studies carried out on clay-PP nanocomposites explained the degree of dispersion of clay platelets in the PP matrix [15-16,26-27,58,78]. Figure 9.20 shows the dispersion of (dimethyl, dihydrogenated tallow quaternary ammonium) modified clay (Closite 20A) with and without the assistance of compatibilizer. 

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