Chemical structure, polymer flammability

This paper is concerned with the flammability of polymer materials. It is an attempt to review the state of the art in the field of ignition and combustion of these materials. Attention is paid to the establishment of fundamental relationships between chemical structure and flammability of polymers. The existing theories for the action mechanisms of various flame retardants are critically analyzed. 

The degree of flammability of a polymeric material may be predicted from its chemical structure. One of the most valuable criteria in fire research, the so-called limiting oxygen index (LOI) may be estimated either from the specific heat of combustion or from the amount of char residue on pyrolysis. Since both quantities can be determined if the chemical structure is known, also the LOI can be estimated. An approximate assessment of the LOI value direct from the elementary composition of the polymer is also possible. 

The chemical structure of a polymer determines its flammability, or to be more specific, the chemical structure of the polymer degradation product dictates the... 

Small rubber particles embedded in the PS matrix provide high impact resistance [134, 135]. However, this polymer is highly flammable as it is exposed to fire, due to its chemical structure, where aliphatic groups bound to aromatic moieties provide hydrogen to the combustion reaction without forming any char layer that may prevent Are propagation [132, 136]. 

Thermal relationships between flammability and structure/composition of these polymers was explored. It is found that BPC Il-polyarylate is an extremely fire-resistant thermoplastic that can be used as an efficient flame-retardant agent to be blended with the other polymers. Chakon Il-polyarylate is of interest as a UV/visibk-sensitive polymer with a relatively low HRR and a high char yield. Pyrolysis combustion flow calorimetry (PCFC) results show that the total heat of combustion of the copolymers or blends changes linearly with the composition, but the change of maximum HRR and char yield depends greatly on the chemical structure of the components. 

Physical Properties. Physical properties include density, properties connected to their combustion tendency (flammability and oxygen index), optical properties (refractive index and yellow index), and the ability to absorb water. Density p, ie, the mass per unit volume, depends on the nature of atoms present in the chemical structure and the way molecules (chains) pack together. Polyoleflns, composed of C and H only, have densities in the range 0.85-1 organic polymers... 

Polymeric foamed materials are very specific in terms of ignitability and flame spread. It has been shown that differences in the surface area of foamed polymers and cell size have a larger effect on flammability than do density or differences in chemical structure. The chemical stmcture, of course, may dictate the surface area or porosity in the formation of foam. For example, flexible polyurethane foams can be ignited by a smoldering cigarette. A textile material normally used to enclose the foam, as is common in upholstered furniture and mattresses, actually helps ignition if snitable flame retardant-treated textiles are not used. 

Most of the basic commercial polymers are flammable [1]. Burning of plastics is a complicated phenomenon depending mainly on the chemical structure of polymers and on some physical factors [2]. The flammability of plastics is particularly severe when the basic polymers undergo depolymerization to form flammable monomers or active products. Such is the case for PS, PMMA, polyoxymethylene (POM), NR, etc. 

Throughout the text we will relate polymer structure to the properties of the polymer. Polymer properties are related not only to the chemical nature of the polymer, but also to such factors as extent and distribution of crystallinity, distribution of polymer chain lengths, and nature and amount of additives, such as fillers, reinforcing agents, and plasticizers, to mention a few. These factors influence essentially all the polymeric properties to some extent including hardness, flammability, weatherability, chemical stability, biological response, comfort, flex life, moisture retention, appearance, dyeability, softening point, and electrical properties. 

Acrylonitrile resembles VC, a carcinogen, in structure. It is a flammable, explosive liquid (b.p. 77 C, V.P. 80 mm at 20°C). AN is a component of acrylic and modacrylic fibers produced by copolymerization with other monomers, e.g., with methyl acrylate, Me-methacrylate, vinyl acetate, VC and VDC. Other major uses of AN include copolymerizations with butadiene and styrene to produce ABS polymers, and with styrene to yield SAN resins which are used in the manufacture of plastics. Nitrile elastomers and latexes are also made with AN, as are a number of other chemicals, e.g. acrylamide and adiponitrile. Acrylonitrile is also used as a fumigant. 

The low burning result and the self-extinguishing characteristic obtained were not surprising. I say this in hindsight, for the inherent flammability of polyurethane foam, in general, is almost entirely a function of the chemical composition of the solid polymer and one route used to produce flame retardant polyurethane foams has been to alter the structure of the ure-... 

Benrashid, R. and Nelson, G.L., Flammability improvement of polyurethanes by incorporation of a silicone moiety into the structure of block copolymers, in Fire and Polymers (ACS Symposium, Series 599), American Chemical Society, Washington D.C., 1995, p. 217. 

The general aspects of ignition, combustion and extinction in polymeric materials have been considered. There is a relationship between the chemical nature of the polymer and its flammability, which can be estimated from the influence of the polymer structure on the stoichiometry and the specific heat of the combustion reaction. 

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