Polymer-clay systems

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. 

This chapter is organized in the following way. First, we present some common techniques for characterizing the dispersion of nanoclays in polymer blends. The dispersion level has been shown to have a fundamental effect on the fire performance of polymer-clay nanocomposites (PCNs), as an exfoliated or intercalated polymer-clay system seems to enjoy reduced flammability. Second, the effects of nanoclays on the viscosity of polymer blends are discussed. With increased temperature in the condensed phase during combustion, most polymers (and hence polymer blends) have sufficiently low viscosity to flow under their own weight. This is highly undesirable, especially when the final products will be used in vertical orientation, because the melt can drip, having the potential to form a pool fire, which can increase fire spread. The results on thermal stability are presented next, followed by those for the cone calorimeter. The quantitative effects of nanoclays on the... 

Nanocomposites based on polymer-clay systems are of considerable interest for the development of new stmctural and functional materials. Recently, there has been much research into polymer/day nanocomposites such as epoxy, acrylic,polystyrene, and polyamide-6, owing to their unique and improved properties. For instance, compared to polyamide-6, polyamide-6/clay nanocomposites at 5wt.% day loading levd had the heat distortion temperature 87 °C higher. Also the tensile strength and tensile modulus were 49% and 68% higher, while the impact strength was almost unchanged. ... 

Up until this point, the discussion has centered on the role of thermodynamics in the formation of intercalated and exfoliated polymer clay systems, ignoring any kinetic effects. As can well be imagined, there are a number of activation barriers that must be overcome to allow... 

One may conjecture that the SS absorbs and re-irradiates the heat to cause rapid charring of the polymer, which gives rise to the rapid rise seen in the HRR curve. This charred polymer then undergoes thermal degradation but more slowly than does the virgin polymer since the SS now functions as a barrier to mass transport and partly insulates the underlying polymer from the heat source. Since a barrier process, similar to that seen for polymer-clay systems, is implied by the cone data, it is important to determine if a barrier does form and its composition. 

Recommended model particle systems are enzymes immobilised on carriers ([27,44,45,47,49]), oil/water/surfactant or solvent/water/surfactant emulsions ([27, 44, 45] or [71, 72]) and a certain clay/polymer floccular system ([27, 42-52]), which have proved suitable in numerous tests. The enzyme resin described in [27,44,47] (acylase immobilised on an ion-exchanger) is used on an industrial scale for the cleavage of Penicillin G and is therefore also a biological material system. In Table 3 are given some data to model particle systems. 

It is a common phenomenon that the intercalated-exfoliated clay coexists in the bulk and in the interface of a blend. Previous studies of polymer blend-clay systems usually show that the clay resides either at the interface [81] or in the bulk [82]. The simultaneous existence of clay layers in the interface and bulk allows two functions to be attributed to the nanoclay particles one as a compatibilizer because the clays are being accumulated at the interface, and the other as a nanofiller that can reinforce the rubber polymer and subsequently improve the mechanical properties of the compound. The firm existence of the exfoliated clay layers and an interconnected chain-like structure at the interface of CR and EPDM (as evident from Fig. 42a, b) surely affects the interfacial energy between CR and EPDM, and these arrangements seem to enhance the compatibility between the two rubbers. 

Clay size layer silicates also have the ability to catalyze the polycondensation of phenolic compounds and amino acids. Wang et al. (1985) examined the catalytic effect of Ca-illite on the formation of N-containing humic polymers in systems containing various phenolic compounds and amino acids. The yields and N contents... 

Schematic illustration of clay and CNTs morphology in chitosan nanocomposites is shown in Figure 4.8. In the composites based on chitosan/CNTs containing 0.4 wt % CNTs, nanotubes can be well dispersed in chitosan, but no filler network could be formed due to its low concentration (Figure 4.8a). In the composites based on chitosan/clay containing 3 wt % clay, formation of 2D clay platelets network is possible (Figure 4.8b). In chitosan/clay-CNTs ternary nanocomposites, ID CNTs are confined in 2D clay platelets network, which results in a much jammed and conjugated 3D clay-CNTs network (Figure 4.8c). The interactions and networks in the system can be divided into (1) clay-clay network, (2) clay-CNTs network, (3) CNTs-polymer-clay bridging, (4) polymer-polymer network. The formation of different networks and interactions could be the main reason for the observed synergistic reinforcement of CNT and clay...

Figure 4.8. Schematic illustration of morphology of clay and CNTs in chitosan nanocomposites (a) chitosan/0.4% CNTs (b) chitosan/3% clay (c) chitosan/3% clay/0.4% CNTs. The interaction and networks in the system could include (1) clay-clay network (2) clay-CNTs network (3) CNTs-polymer-clay bridging (4) polymer-polymer network. Reprinted with permission from ref (42).

The addition of polymer has no effect on the phase transition temperature between the gel and tactoid phases of the clay system. 

Thermogravimetric analyses have confirmed the general enhancement in the stability of the various polymer-clay nanocomposites relative to the base polymers and also provided useful information on the extent of the polymer loading in such composites. The thermogravimetric characteristics of several typical systems are highlighted below. 

Nabzar L, Pefferkorn E, Varoqui R. Stability of polymer-clay suspensions. The polyacrylamide-sodium kaolinite system. Colloids Surfaces 1988 30 345-353. 

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