Dispersion barrier effects

Orientation Once the particles are dispersed in the polymer, they must be oriented so that the flat surface of the clay is parallel to the surface of the packaging material to maximize the barrier effect. Several models have been developed in order to describe the mass transfer within the nanocomposites. Most models assume that the platelets have a regular and uniform shape (rectangular, sanidic, or circular) and form a regular array in space. They are either parallel to each other or have a distribution of orientations, with the... 

The surfactants used for the preparation of disperse systems are seldom effective in maintaining the long-term physical stabihty (absence of flocculation and/or coalescence) of the formulation. This is due to their weak and reversible adsorption and lack of the presence of a high-energy barrier that prevents flocculation as a result of van der Waals attractions. For this reason, dispersants and emulsifiers of polymeric nature that are strongly and irreversibly adsorbed at the interface are required. In addition, these polymeric dispersant provide effective repulsive forces (referred to as steric repulsion) that overcomes the van der Waals attractions. The criteria for an effective dispersant are [1, 2] ... 

Figure 28 shows the relationships between the amount of inorganics in the clay and the gas permeability coefficient. The gas permeabiUty coefficient decreased as the amount of added clay increased. The gas barrier performance of PPCN-5 increased by 1.7 times. It has been reported that the barrier performance of the nylon-clay nanocomposites and polymer-clay nanocomposites was improved. This barrier effect is explained as being attributed to the geometrical detour effect of the dispersed nanosized silicates. The barrier effect of PPCN, however, was smaller than that of the nylon-clay nanocomposites. hi the case of the nylon-clay hybrid, the addition of 1.8 wt % of mont-morillonite caused its hydrogen permeability to decrease to 70%. In the case of PPCN, about 3 wt % of montmorillonite must be added to obtain the same... 

By adding a low ftaclion to the polymer matrix, the clay layers are well dispersed, the barrier effect is predominant, but with increasing loading, the promoter effect rapidly rises and becomes impressible, so that the thermal stabiUly of the nanocomposites decreases. This accelerating effect is mainly due to ... 

The improvement in thermal stability of the nanocomposites compared to the neat EVA/natural rubber is due to the barrier effect and insulating properties of organoclay. The well dispersed plate-like silicate layers form a tortuous path in the polymer matrix which gives a barrier effect and inhibits the diffusion of volatile degradation product from the inside of the polymer matrix. Moreover the well-dispersed silicate layers restrict the movement of polymeric chains, hence reducing the free volume for diffusion of volatile degradation products. Other researchers also confirm that organoclay tends to form a compact char-like residue on the surface of the nanocomposites when it is burnt. This char-like structure is incombustible and acts as an insulator which inhibits heat transfer to the inside of the nanocomposites. At 8 phr... 

At the same time, crosslinking induced by free radicals will also be reduced due to the barrier effect. However, as the irradiation dose increases, more free radicals will be formed. The large amount of free radicals can overcome the barrier effect caused by well dispersed clay. This has been proved by the gel fraction yield. At lower irradiation doses the gel fraction is significantly reduced for the nanocomposites, whereas at 200 kGy the gel fraction for all the nanocomposites is almost the same as that for the pristine blend. 

The slower thermal decomposition observed for the PA6/OMT/DB-AO nanocomposite has been ascribed to barrier effects arising from the nano-dispersed clay sheets. Synergetic effects between the OMT and DB-AO probably occur to augment PA6 flame retardancy [reactions Moreover,... 

These results make LDHs excellent candidates as fillers for polymer matrices, to be used as degradant additives, which permits a precise control of the lifetime of the plastic product, decreasing its negative impact on the environment. The improvement in the thermal properties of PE/LDH nanocomposites is usually explained assuming that the LDH layers act as a sort of barriers. Yue et al. [56] claim formation of nanostructures from the dispersed LDH particles, which decrease heat transfer, thus stabilizing the polymer chains. Dispersion of the LDH particles thus improves the barrier properties of the polymer nanocompounds because of their easy exfoliation. Fluid diffusion, especially gases, is strongly affected by this barrier effect. 

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