Ignition, polymer flammability

Factors asjogigtgd with polymer flammability have been previously reviewed. Stages in the polymer flammability process can be simply described as a) preheating, b) decomposition to give volatile compounds and residuals, 3) ignition of the volatiles in the presence of oxygen, and d) combustion. This may be considered as a sustained cyclic process since the combustion process itself is exothermic. Mechanisms to decrease the flammability of polymer have involved the interruption of the cyclic process at any stage. 

Flame retardants are added not to aid in processing, but rather to inhibit ignition or flammability of the end-use product. They are added so that the polymer compound is able to meet flammability standards established by either mandatory government regulations or voluntary specifications. 

Substances applied to or incorporated in a combustible material (e.g. organic polymers, nylon, vinyl and rubber, etc.) to reduce flammability. Act by retarding ignition, control/douse burning, reduce smoke evolution. Slow down or interrupt the self-sustained combustion cycle when the heat-flux is limited. Flame retardants (FRs) improve the combustion behaviour and alter the combustion process (cool, shield, dilute, react) so that decomposition products will differ from nonflame retarded articles. FRs are usually divided into three classes ... 

The fire properties of PVC have been put into perspective recently [4, 5]. They show that PVC is a polymer with a high ignition temperature and low flammability. Furthermore, PVC products are associated with a low rate of heat release as well as little total heat released [4-9]. This will depend, clearly, on the type of product, since plasticised PVC products are obviously more flammable than rigid ones. 

One of the most useful laboratory flammability tests is the oxygen index (OI) test (ASTM-D2043). In this test, the specimen is burned as a candle in controlled mixtures of oxygen and nitrogen. The minimum oxygen concentration which produces downward flame propagation is considered the OI of ignitability for the polymer. 

Dust explosions ignited hy static discharge are a recognized hazard encountered in the handling of polytmethyl methacrylate) powders or in the fabrication of polytmethyl methacrylatel plastic sheet. Mclhacrylic solution polymers are treated as flammable mixtures latex polymers arc nonflammable. 

The red allotropic form of phosphorus is relatively nontoxic and, unlike white phosphorus, is not spontaneously flammable. Red phosphorus is, however, easily ignited. It is a polymeric form of phosphorus, thermally stable up to ca. 450°C. In its finally divided form, it has proved to be a powerful flame-retardant additive.18 Elemental red phosphorus is a highly efficient flame retardant, especially for oxygen-containing polymers such as polycarbonates and polyethylene terephthalate). Red phosphorus is particularly useful in glass-filled polyamide 6,6, where high processing temperature (about 280°C) excludes the use of most phosphorus compounds.19 In addition, coated red phosphorus is used to flame retard nylon electrical parts, mainly in Europe and Asia.20... 

However, a common feature of the PNs is that, based on the flammability of various polymers that have been studied with a single nanoadditive, like clay, they can only show a retarded combustion process (i.e., the peak HRR and the average HRR can be lowered), while almost all the carbon sources will eventually burn out in most cases. The total heat released (THR) in CCA is not changed compared with that of the same amount of base polymer. Furthermore, like many flame-retardant composites, an earlier time to ignition (and perhaps also the time to peak HRR) for most PNs than the base polymer is also seen in the CCA experiment.117... 

---