Thermal stability fillers

Fillers can also be used to promote or enhance the thermal stability of the silicone adhesive. Normal silicone systems can withstand exposure to temperatures of 200 C for long hours without degradation. However, in some applications the silicone must withstand exposure to temperatures of 280 C. This can be achieved by adding thermal stabilizers to the adhesive formulations. These are mainly composed of metal oxides such as iron oxide and cerium oxide, copper organic complexes, or carbon black. The mechanisms by which the thermal stabilization occurs are discussed in terms of radical chemistry. 

Thermal stability is a crucial factor when polysaccharides are used as reinforcing agents because they suffer from inferior thermal properties compared to inorganic fillers. However, thermogravimetric analysis (TGA) of biocomposites suggested that the degradation temperatures of biocomposites are in close proximity with those of carbon black composites (Table-1). 

Diallyl phthalate when heated to 100°C with an initiator like t-butyl per benzoate yields a linear polymer linked through one allyl unit per monomer. This polymer having mol. wt. 10000-25000 is then compounded with fillers to get thermosetting moulding powder. Mouldings have thermal stability upto... 

For applications where only mechanical properties are relevant, it is often sufficient to use resins for the filling and we end up with carbon-reinforced polymer structures. Such materials [23] can be soft, like the family of poly-butadiene materials leading to rubber or tires. The transport properties of the carbon fibers lead to some limited improvement of the transport properties of the polymer. If carbon nanotubes with their extensive propensity of percolation are used [24], then a compromise between mechanical reinforcement and improvement of electrical and thermal stability is possible provided one solves the severe challenge of homogeneous mixing of binder and filler phases. For the macroscopic carbon fibers this is less of a problem, in particular when advanced techniques of vacuum infiltration of the fluid resin precursor and suitable chemical functionalization of the carbon fiber are applied. 

By the addition of glass fibers, textile fibers, or chopped fabrics to crosslinkable polymers molding materials are produced with increased tensile strength, stiffness, and thermal stability compared to the filler-free polymers. The so-called reinforcing fillers, like carbon black, have good adhesion to the matrix due to their nonpolar structure and their characteristic geometry. 

The tensile strength of polydimethylsiloxane irradiated by a dose of 60 kGy (6 Mrad) is 15% lower than that of PDMS cross-linked by peroxide, but, when 0.14 mol percent of vinyl unsaturation is substituted for the methyl groups, the tensile strength is 30% higher. Irradiation of PDMS containing 55 phr silica filler by a dose of 40 kGy exhibited an optimum tensile strength of 8 MPa (1160 psi) and excellent thermal stability at 523 (482°F).209... 

In rubber-plastic blends, clay reportedly disrupted the ordered crystallization of isotactic polypropylene (iPP) and had a key role in shaping the distribution of iPP and ethylene propylene rubber (EPR) phases larger filler contents brought about smaller, less coalesced and more homogeneous rubber domains [22]. Clays, by virtue of their selective residence in the continuous phase and not in the rubber domains, exhibited a significant effect on mechanical properties by controlling the size of rubber domains in the heterophasic matrix. This resulted in nanocomposites with increased stiffness, impact strength, and thermal stability. 

Compatibility of NA with organic polymer is much inferior to the compatibility of , 15A, and SP. Thus, Sl-NA-4 has inferior thermal stability as compared to the other three clays. Moreover, the intergallery spacing of NA is very small (only 1.22 nm, obtained from XRD results, Fig. 26a). Only a few chains of HNBR (with 34% acrylonitrile content), being bulky in nature, can find their way into such a small gallery space, which results in poor polymer-filler interaction. This is confirmed by both XRD and (Fig. 26a, b). 

The description of the physical properties of fluoroelastomers is necessarily less precise than that of fluoroplastics because of the major effect of adding curatives and fillers to achieve useful cross-linked materials of a given hardness and specific mechanical properties Generally, two parameters are varied increasing cross-link density increases modulus and decreases elongation, and raising filler levels increases hardness and decreases solvent swell because of the decreased volume fraction of the elastomer In addition to these two major vanables, the major determinants of vulcanizate behavior are the chemical and thermal stabilities of its cross-links The selection of elastomer, of course, places limits on the overall resistance to fluids and chemicals and on its service temperature range... 

The particular absorber to be used in a given application depends on several factors. One important criterion is whether the absorber will strongly absorb that portion of the ultraviolet spectrum responsible for degradation of the plastic under consideration. Compatibility, volatility, thermal stability, and interactions with other additives and fillers are other items that must be considered. When used in food wrappings, Food and Drug Administration approval must be obtained. While one or more of these considerations may rule out a given stabilizer or influence llie choice of one class over another, the final selection must await the results of extensive accelerated and long-term tests. 

These in situ generated silica fillers also give increased resistance to creep or compression set in cyclic deformations, as is illustrated in Figure 8.6.43 The in situ filled PDMS samples are seen to show very little compression set. They can also provide increased thermal stability, as demonstrated by the results presented in Figure 8.7. 44... 

By analogy with the works which dealt with cellulose micro crystal-reinforced nanocomposite materials, microcrystals of starch [95] or chitin [96, 97] were used as a reinforcing phase in a polymer matrix. Poly(styrene-co-butyl acrylate) [95,96], poly(e-caprolactone) [96], and natural rubber [97] were reinforced, and again the formation of aggregates or clustering of the fillers within the matrices was considered to account for the improvement in the mechanical properties and thermal stability of the respective composites processed from suspensions in water or suitable organic solvents. 

The quick overview of the mechanisms of action reveals that the formation of an expanded charred insulative layer acting as thermal shield is involved. The mechanism of action is not completely elucidated, especially the role of the synergist. Reaction may take place between the nano-filler and some ingredients of the intumescent formulation (e.g., the phosphate) in order to thermally stabilize the charred structure. Only physical interactions are observed (e.g., action of POSS with phosphinate), and these interactions permit the reinforcement of the char strength and avoid the formation of cracks. The development rate and the quality of this layer are therefore of the primary importance and research work should be focused on this. 

Furthermore, the effect of hydrated fillers on polymer fire retardancy will depend not only on the nature of the filler, including its particle characteristics (size, shape, and purity) and decomposition behavior, but also on the degradation mechanism of the polymer, together with any filler/ polymer interactions that might occur, influencing thermal stability of the polymer and possible char formation. 

Dando, N.R., Clever, T.R., Pearson, A., Stinson, J.M., Kolok, P.L., and Martin, E.S., Aluminum trihydroxide (ATH) as a filler for polymer composites Improvements in thermal stability by controlled precipitation, Proceedings from 50th Annual Technical Conference, Composite Institute, Society of Plastics Industry Inc., Washington D.C., Session 1-D, 1995, pp. 1—4. 

---