Clay particle flame retardants

Fillers. Fillers are not commonly added to CR adhesives. Calcium carbonate or clay can be primarily added to reduce cost in high-solids CR mastics. Maximum bond strength is obtained using fillers with low particle size (lower than 5 [jim) and intermediate oil absorption (30 g/100 g filler). In general, fillers reduce the specific adhesion and cohesion strength of adhesive films. Although polychloroprene is inherently flame retardant, aluminium trihydrate, zinc borate, antimony trioxide or... 

The nature of the organomodifier plays a role in the existence of true nanocomposite structures (intercalated for 15A and 30B, exfoliated for 25A, microcomposite for 10A), cone calorimeter results associated with x-ray diffraction (XRD) suggest that increased flame retardancy are more dependent on physical and thermal cross-linking of clay particles and polymer chains than on formation of nanocomposite structure. However, it can be concluded that the role of clay is crucial since PHRR values are reduced up to 70% in the presence of clays. 

Fire retardants used in polystyrene (PS) include montmorillonite clay, polytetrafluoro-ethylene (PTFE) [8], bromine-based flame retardants such as brominated bisphenol A [9], brominated phenyl oxide or tetrabromophthalic anhydride, or magnesium hydroxide [10,11]. Sanchez-Olivares and co-workers [12], in their study of the effect of montmorillonite clay on the burning rate of PS and PS-polyethylene terephthalate blends, showed that increased combustion rate accompanied the incorporation of montmorillonite particles in high-impact polystyrene (FlIPS) formulations. 

The type and amount of filler have an effect on shrinkage and sink marks more filler reduces both, but at the same time increases the mixture viscosity, which is critical when working with moulding compounds. A combination of coarse and fine particles produces the optimum results. Fillers, such as clay, calcium carbonate and wollastonite, judiciously selected and in relatively high concentrations, can also impart flame retardancy and serve as a stress transfer medium, as well as reduce the total material cost. It should be pointed out that fillers strongly influence the flow characteristics of moulding compounds. 

Nano-composites are the subject of intense research for a number of properties such as improved barriers to gas, higher mechanical strength, and improved flame retardancy. Plate-like particles of special clays, one nanometre (one billionth of a metre) thick by 1000 nanometres in diameter, are being studied as FRs in plastics by the US National Institute for Standards and Technology (NIST), Gaithersburg, Maryland. Initial research showed that the addition of as little as 5% of nano-sized clay particles could produce a 63% reduction in the... 

Fillers. Fillers are used in compounds to increase viscosity, increase solids, and to lower cost. Most fillers used in latex do not exhibit the reinforcing effect that is characteristic of their use in dry-type polychloroprene. Water-washed whiting (calcium carbonate) can be added directly to the latex. Most clays are acids and must be neutralized and slurred before adding to the latex. When used at levels ranging from 10- to 20-phr, fine clays such as DIXIE Clay (R. T. Vanderbilt Co.) can add some degree of reinforcement. Hard clays have much smaller particle size than soft clays. Feldspar can be added directly to the latex but will tend to settle quickly. Hydrated alumina is used primarily to improve flame retardancy and improve water resistance. Large-particle-size hydrated alumina can be added directly to the latex. 

Platy nano-particles, such as nano-clays and micas, have some potentially useful flame retardant effects, and are currently receiving a lot of attention for this application. This topic has recently been reviewed [34, 35]. The other forms of nano-particle do not seem to have the same effectiveness. This subject, which was briefly treated in Chapter 6, is discussed in more detail here, but the earlier chapter should be referred to for details of fire retardant tests, especially the cone calorimeter which is widely used in studies involving nano-plate fillers. 

The lack of effect on total heat release, smoke and carbon monoxide, is different from halogenated flame retardants, and is taken to show that the effects are all in the condensed phase. Using gasification equipment, which duplicates the pyrolysis conditions in the cone calorimeter, without flaming taking place, Gilman and co-workers found that the melt that formed on the surface of unfilled polymer was quickly converted to a solid, black char when nano-clays were present [39]. It is postulated that this char is reinforced with nano-particles and slows down the combustion processes. It is,... 

MWCNT, rather than the clay platelets. These results indicate that the FR is preferentially adsorbed onto the MWCNT surfaces, and hence no longer competes with the polymer in the formation of the in situ grafts on the clay platelet surfaces. As a result, the efficiency of the platelets in compatibilizing the blend is restored, and both blends are flame retardant with half the concentration of clay. It is interesting to note that the s-MWCNT blend satisfies the UL-94 VO requirement, whereas the blend containing the /-MWCNT only satisfies UL-94 V2, which may be because of improved FR particle dispersion. In addition, close examination of the TEM images shows that the s-MWCNT are dispersed in both the PS and PMMA phases, whereas the /-MWCNT are observed mostly in the PS phase. 

Although the incorporation of microscale particles as fillers into polymers has been well explored scientifically, the decrease in size of particles to nanometers, and the simultaneous increase in interface area, results in extraordinary new material properties.In one such application, the flammability properties of polymers have been improved with the addition of nanoscale particles. These filled nanocomposites provide an attractive alternative to conventional flame retardants. At present, the most common approach to improving flammability is the use of layered silicates such as clays, as described in Chapter 3. However, there are many different shapes and types of nanoparticles. (Here, a nano scale particle is defined as having at least one dimension on the nanometer scale.) When all three dimensions are on the order of nanometers, we are dealing with true nanoparticles, such as spherical silica particles, having an aspect ratio of 1. Another type of nanoparticle has only one dimension on the nanometer scale. Such nanoscale... 

Since it is evident from discussions in previous chapters that nanodispersed, functionalized, largely inert particles such as clays and synthetic alternatives cannot promote sufficient flame retardant activity alone but only in the presence of more conventional flame retardants, their potential usefulness will be determined by their ease of processing and the manner in which they influence both process and end product. Essential issues to be considered and resolved are nanoparticle compatibility with the polymer matrix and other additives present, the ability to maintain a nanodispersion during all processing stages, their influence on rheology, and the possible compromise between effective concentrations levels and optimization of these. 

The flame retardancy observed with polymer-clay nanocomposites also seems to occur with other nanofillers, such as carbon nanotubes and nano-fibers, and to some extent with colloidal particles, and the mechanism for flame retardancy is very similar (Chapter 10). 

A trend that has already begun to arise is the use of multiple types of nanofillers in the same polymer to yield a multicomponent nanocomposite. Some workers have found that some types of nanofillers cannot bring all of the desired properties to the final material, so clays have been combined with multiwall carbon nanotubes to bring enhanced properties.The observation for most polymer additives is that they cannot be used for all applications in all polymers, and the same observation will surely be made about nanocomposites. A clay may be used to enhance the flammability performance, bnt it could also be combined with a conductive nanoflller to impart antistatic aspects or electrical conductivity in the final system. One potential way to look at the use of multiple nanoparticles is that each nanoparticle plays a complementary role in flammability reduction. For example, one could choose a clay for mass loss rate or fuel release reduction, but then use a colloidal particle to flu in the gaps between clay plates as the nanocomposite thermally decomposes. Perhaps even more useful, the colloidal particle could have catalytic or flame retardant properties that encourage... 

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