Thermoplastic charring

A series of compounded flame retardants, based on finely divided insoluble ammonium polyphosphate together with char-forming nitrogenous resins, has been developed for thermoplastics (52—58). These compounds are particularly useful as iatumescent flame-retardant additives for polyolefins, ethylene—vinyl acetate, and urethane elastomers (qv). The char-forming resin can be, for example, an ethyleneurea—formaldehyde condensation polymer, a hydroxyethylisocyanurate, or a piperazine—triazine resin. 

It is well known that char formation during polymer burning is an important mechanism by which polymers resist burning. For example Van Krevelen (1-2) has shown that a correlation exists between the oxygen index (OI) of a polymer containing no heteroelements and the amount of char it forms when pyrolyzed in the absence of air. As shown in Table I, a similar correlation is seen to hold for a number of common engineering thermoplastics. 

This study shows that limited oxidation at 373 K or weathering (ambient) of coal feedstock reduces the thermoplastic properties of a coal This is manifest as a transformation in char type, from cenospheres to inertoids, at high rates of heating (10 -10 K s" ) in an Entrained Flow Reactor at 1273 K. The specific types of char are related to the chemical structure of the coal and an inverse relationship exists between the occurrence of cenospheric chars and the atomic 0/C ratio of the oxidised or weathered coal from which they are derived... 

The reduction in thermoplasticity of the metaplast is possibly the result of the promotion of aromadsation/char condensation reaaions at die expense of production of stable volatile species during coal pyrolysis. In this way, the generation of specific char types at 1273 K in the EFR can be related to the proportion of elemental oxygen within the original coal stmcture of the oxidised or weathered coal particle below an atomic 0/C ratio of =0.15 the predominant char is that of the cenosphere variety due to the maintenance of nunaplast fluidity during pyrolysis. 

Thermoplastic Point, Class 1 material shall fuse without charring and be capable of being drawn into a thread at a temp not lower than 145°C... 

Plastics can be put into one of two categories. If they melt or soften when heated (like poly(ethene), PVC and polystyrene) then they are called thermoplastics or thermosoftening plastics. If they do not soften on heating but only char and decompose on further heating, they are known as thermosetting plastics. 

The predominant mode of action of phosphorus-containing flame retardants (both additives and reactives), when present in thermoplastics or thermosets, is considered to be in the condensed phase. Generally, as with cellulose, flammable gas generation is reduced and char formation is promoted. In some cases, the char cohesiveness is also enhanced. The retention of phosphorus in the chars in... 

Martel, B., Charring process in thermoplastic polymers Effect of condensed phase oxidation on the formation of chars in pure polymers, J. Appl. Polym. Sci., 1988, 35, 1213-1226. 

The use of polyols such as pentaerythritol, mannitol, or sorbitol as classical char formers in intumescent formulations for thermoplastics is associated with migration and water solubility problems. Moreover, these additives are often not compatible with the polymeric matrix and the mechanical properties of the formulations are then very poor. Those problems can be solved (at least partially) by the synthesis of additives that concentrate the three intumescent FR elements in one material, as suggested by the pioneering work of Halpern.29 b-MAP (4) (melamine salt of 3,9-dihydroxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]-undecane-3,9-dioxide) and Melabis (5) (melamine salt of bis(l-oxo-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octan-4-ylmethanol)phosphate) were synthesized from pentaerythritol (2), melamine (3), and phosphoryl trichloride (1) (Figure 6.4). They were found to be more effective to fire retard PP than standard halogen-antimony FR. 

To further explore the influence of silica material properties (morphology, surface area, silanol concentration, and surface treatment) on the silica flame-retardant properties, various types of silicas (silica gel, fumed silicas, and fused silica) were investigated.50 51 Material properties of the various silicas are summarized in Table 8.6. These different types of silicas were added to polypropylene and polyethylene oxide to determine their flame-retardant effectiveness and mechanisms. Polypropylene was chosen as a non-char-forming thermoplastic, and polyethylene oxide was chosen as a polar slightly char-forming thermoplastic. Flammability properties were measured in the cone calorimeter and the mass loss rate was measured in the radiative gasification device in nitrogen to exclude any gas phase oxidation reactions. 

Myers et al. reported that partially dehydrated APB is an effective intumescent flame retardant in thermoplastic polyurethane.77 APB at 5-10 phr loading in TPU can provide 7- to 10-fold improvement in burn-through test. It is believed that in the temperature range of 230°C-450°C, the dehydrated APB and its released boric oxide/boric acid may react with the diol and/or isocyanate, the decomposed fragments from TPU, to produce a highly cross-linked borate ester and possibly boron-nitrogen polymer that can reduce the rate of formation of flammable volatiles and result in intumescent char. 

Melamine diborate (MB), known in the fire-retardant trade as melamine borate, is a white powder, which can be prepared readily from melamine and boric acid. It is partly soluble in water and acts as an afterglow suppressant and a char promoter in cellulosic materials. Budenheim Iberica79 claims that, in a 1 1 combination with APP, MB (10%-15%) can be used for phenolic bound nonwoven cotton fibers. In general, melamine borate can be used as a char promoter in intumescent systems for various polymers including polyolefins or elastomers. However, its low dehydration temperature (about 130°C) limits its application in thermoplastics that are processed at above 130°C. Melamine borate is also reported to suppress afterglow combustion in flame-proofing textiles with APP or monoammonium phosphate to meet the German DIN 53,459 and Nordtest NT-Fire 002.80... 

The FR properties of polymer-layered silicate nanocomposites have been studied for a wide range of polymers, especially for organomodified montmorillonites (OMMT) in thermoplastics. Depending on the nature of the polymer, the decomposition pathway and decomposition products may change.8 A major consequence of the introduction of modified clays is the formation or the enhancement of charred structure, caused by cross-linking processes possibly catalyzed by the nanoparticles. 

Table 11.4 shows the product yield of gas, oil/wax and char from the pyrolysis of other single plastics, including thermoplastics and thermoset plastics. Pyrolysis of polystyrene under moderate temperatures of between 500 and 600°C produces high levels of oil. Even at higher temperamres above 700°C, there is a high conversion of the polymer to oil. In fact the oil mainly consists mainly of the monomer styrene [8, 9, 23, 24]. 

Research on the pyrolysis of thermoset plastics is less common than thermoplastic pyrolysis research. Thermosets are most often used in composite materials which contain many different components, mainly fibre reinforcement, fillers and the thermoset or polymer, which is the matrix or continuous phase. There has been interest in the application of the technology of pyrolysis to recycle composite plastics [25, 26]. Product yields of gas, oil/wax and char are complicated and misleading because of the wide variety of formulations used in the production of the composite. For example, a high amount of filler and fibre reinforcement results in a high solid residue and inevitably a reduced gas and oiFwax yield. Similarly, in many cases, the polymeric resin is a mixture of different thermosets and thermoplastics and for real-world samples, the formulation is proprietary information. Table 11.4 shows the product yield for the pyrolysis of polyurethane, polyester, polyamide and polycarbonate in a fluidized-bed pyrolysis reactor [9]. 

Thermosetting foams can be defined as foams having no thermoplastic properties. Accordingly, thermosetting foams include not only cross-linked polymer foams, but also some linear polymeric foams having no thermoplastic properties, e.g., carbodiimide foams and polyimide foams. These foams do not melt and turn to char by heating. 

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