Flame resistance, polymer flammability

Poly(vinyl chloride). PVC is a hard, brittle polymer that is self-extinguishing. In order to make PVC useful and more pHable, plasticizers (qv) are added. More often than not the plasticizers are flammable and make the formulation less flame resistant. Flammability increases as the plasticizer is increased and the relative amount of chlorine decreased, as shown in Table 7. The flame resistance of the poly(vinyl chloride) can be increased by the addition of an inorganic flame-retardant synergist. 

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

Mobility control, issues in, 18 626 Mobility control agents polyacrylamides as, 18 625 in polymer flooding, 18 622 Mobility control surfactants, in enhanced oil recovery, 18 625-628 Mobilizable vectors, for genetic manipulation, 12 471 Mobilization, of ascorbic acid, 25 771 Modacryhc fibers, 9 192 11 188, 189, 190 dyesite content of, 11 195 flame resistance of, 11 214 flammability of, 11 194 pigmented, 11 213 U.S. production of, 11 220t Mode conversion phenomenon, 17 422 Model agreements, 24 373-374 Model-based methods, for reliability, 26 1044... 

Flammability. Presence of chlorine, fluorine, bromine, or phosphorous in a polymer reduces flammability. Thermosets are more flame-resistant than thermoplastics. 

Since many polymeric materials are used as clothing, household items, components of automobiles and aircraft, etc. flammability is an important consideration. Some polymers such as polytetrafluoroethylene and PVC are naturally flame-resistant, but most common polymers such as PE and PP are not. Small-scale horizontal flame tests have been used to estimate the flammability of solid (ASTM D-635), cellular (ASTM D-1692-74), and foamed (ASTM D-1992) polymers, but these tests are useful for comparative purposes only. Large-scale tunnel tests (ASTM E-84) are more accurate, but they are also more expensive to run than ordinary laboratory tests cited before. 

The monomer that we have used as a backbone for our work toward flame retardant polymers is commonly called bisphenol C (BPC) or l,l-dichloro-2,2-bis(4-hydroxyphenyl) ethylene. As has been shown by many research groups, BPC can be used as a blendable additive in a commercial plastic or as part of a polymer backbone to effectively impart flame resistance to certain polymeric materials.When thermally decomposed, BPC exothermically produces volatile products such as HCl and CO2, and the unique structure formed upon thermal degradation leads to a very stable carbon structure (char). It is this pyrolysis byproduct and the high char forming nature of BPC that give inherently low flammability and flame retardancy (Fig. 3) in these polymers and blends. 

The flammability of a particular polymer depends mainly upon its structure. The amounts of char and incombustible gases formed upon thermal decomposition determine to a great extent the flame resistance of a polymeric material. A material with LOI < 26 is considered flammable ( ). Polymeric materials, on the basis of structure and LOI, can be broadly classified into three categories. 

Specifically, PVC blends with polyethylene, polypropylene, or polystyrene could offer significant potential. PVC offers rigidity combined with flammability resistance. In essence, PVC offers the promise to be the lowest cost method to flame retard these polymers. The processing temperatures for the polyolefins and polystyrene are within the critical range for PVC. In fact, addition of the polyolefins to PVC should enhance its ability to be extruded and injected molded. PVC has been utilized in blends with functional styrenics (ABS and styrene-maleic anhydride co-and terpolymers) as well as PMMA offering the key advantage of improved flame resistance. Reactive extrusion concepts applied to PVC blends with polyolefins and polystyrene appear to be a facile method for compatibilization should the proper chemical modifications be found. He et al. [1997] noted the use of solid-state chlorinated polyethylene as a compatibilizer for PVC/LLDPE blends with a significant improvement in mechanical properties. A recent treatise [Datta and Lohse,... 

Another crucial factor consists of flame resistance. Except for outstanding polymers like fluoro compoimds, Noryl, or aromatic polyamides, it is well known that most polymers based on organic compoimds bum well. (There have been many attempts to synthesize non-flammable inorganic polymers, but with minor commercial success.) However, there are polymers that are considered to be "self extinguishing" (more or less) such as PVC and others. 

It is clearly a truism that for reducing the fire risk in the applications of plastics, their flammability should be diminished. This is achieved either by reactive flame-retardants incorporated during the preparation (polymerization, polyaddition, polycondensation) of the polymer or by additive flame-retardants admixed in the course of plastics processing. The flammability of plastics is sometimes reduced by surface protection. The most recent methods of reducing flammability are the modification of the macromolecular structure and the development of thermally resistant polymers (high-temperature plastics). 

Il-polyacrylate (compared with BPA-polyacrylate) is mainly due to the low mass loss rate and the lesser amount of fuels generated. While, in the case of BPC Il-polyacrylate, release of less flammable decomposition products is another important factor for its low flammability. So, compared with some other commercial polymers, Chakon II-and BPC Il-polyacrylates are more heat and flame resistant, and BPC Il-polyacrylate has an especially low flammability. 

Flame resistance has become a legal requirement for commercial utilization of polymers and their blends in many applications. Innumerable test methods for flammability have been developed in different countries, and several books and handbooks are exclusively dealing with this subject. Discussion of the test methods that are en vogue in various countries is beyond the scope of this chapter thus only the most popular test methods are discussed. The fire-retardant chemicals and their suppliers are tabulated in Table 10.37. 

In the USA the National Institute of Science and Technology has discovered that the dispersion of a very small quantity of carbon nanotubes into PP greatly reduces the flammability of the polymer. An alternative approach to flame resistance is offered by the US PolyOne Corporation which supplies a range of nanoclay-based additives. These Nanoblend additives are claimed to be able to boost the fire resistance of flame-retardant PE and PP. 

Some polymers have some unique chemistry that allows the use of relatively low amounts of additives that catalyze a molecular rearrangement that provide some flame resistance. PC is an example. It is a polymer based on para orientation of chemical bonds. These para bonds can be changed to meta bonds by traces of sulfonic acids. Meta linkages are less flammable than the original para ones. Of course, some other properties are changed as well. Other aromatic-based polymers take advantage of this chemistry. Nomex is a/neta-bonded form of the better known Kevlar. Kevlar with para bonds has better properties but Nomex is more flame resistant and is used by firefighters worldwide. 

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