Self-retarding effect

It is self evident that mineral fillers need to be stable at the temperatures (up to 350 °C) experienced in processing thermoplastics. Most fillers are stable to much higher temperatures and so this is not usually an issue. However, it is a very important topic for flame retardant fillers which function by decomposing endothermically with the release of inert gasses. To be effective, this decomposition must occur near to the temperature at which the polymer begins to decompose and release flammable volatiles. This is usually not too much above the processing temperature in the case of thermoplastics and hence the exact temperature at which decomposition commences is of great importance. The size and position of the endotherm and the rate at which the inert gas is released are also of importance to the flame retardant effect itself [23]. 

Fig. 23 A possible instantaneous configuration of a row of stems perpendicular to the crystal surface schematically illustrating self-poisoning in a polydisperse polymer. The same picture could apply to a layer of stems depositing onto the crystal surface and parallel to it in that case it would illustrate the retarding effect on layer spreading rate v...

Silicone coated glass fabric has good flame-retardant properties, releases little smoke, leaves no toxic combustion products and is more weather resistant than PVC. However, its self-cleaning effect is inferior to that of PTFE. Silicone is more flexible and protects the glass fabric better than PTFE and has very good translucency, because the refraction indices of fibreglass and silicone are similar. 

These are often melamine and derivatives or polyphosphate compounds such as APP. The mode of action of melamine appears to involve endothermic sublimation, acting as a heat sink, vapour-phase dissociation and also self-condensation under suitable conditions. APP achieves its flame retardant effect by intumescence and char formation acting as a barrier to combustion reactions. 

Experimental results of thermal analysis and combustion tests (LOI self-ignition) of nylon-6,6/ SI compositions are presented in Table 5.4 and Fig. 5.2. LOI results clearly showed that incorporation of only SnClj + Si (2 3 %wt.) of the SI -composition in nylon-6,6 has substantial flame retardant effect in comparison with the other SI - systems [225]. 

This approximation neglects the retardation effects. Moreover, it is supposed, that the self-consistent field reacts instantaneously to the changes in the density p. The second functional derivative of the action... 

In the approximation of the first-order self-consistent perturbation theory Z(p) = 1 and (p) is given by the equation above. The quantities C and in Eq. (4.29) are functions of the four-momentum. This generality is required to treat retardation effects. 

The self-extinguishing characteristics of the chlorine-containing resins are improved by incorporation of antimony oxide but this approach is not possible where translucent sheet is required. As an alternative to chlorine-based systems a number of bromine-containing resins have been prepared and, whilst claimed to be more effective, are not currently widely used. It is probably true to say that fire-retarding additives are used more commonly than polymers containing halogen groupings. 

Houben [256] has compared the determination of flame-retardant elements Br, P, S, K, Cl and F in polycarbonate using commercial (X40 and UniQuant ) software. For the X40 method, a calibration line for each element in PC or PC/ABS blends was mapped for the conversion of intensities to concentrations. With the universal UniQuant method, sensitivity factors (ks) were calibrated with pure standards. The X40 method turned out to be more reliable than UniQuant for the determination of FRs in PC and PC/ABS blends, even in the case of calibration of k values with PC standards. Standard errors of 5 % were achieved for Br, P, S and K, and 20% for Cl and F the latter element could not be determined by means of UniQuant (Table 8.44). GFR PC cannot be quantified with these two methods, because of the heterogeneous nature of the composites. Other difficult matrices for XRF analysis are PBT, PS and PP compounds containing both BFRs and Sb203 (10-30wt %) due to self-absorption of Sb and interelement effects. 

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