Subject flame retardancy

Automated soldering operations can subject the mol ding to considerable heating, and adequate heat deflection characteristics ate an important property of the plastics that ate used. Flame retardants (qv) also ate often incorporated as additives. When service is to be in a humid environment, it is important that plastics having low moisture absorbance be used. Mol ding precision and dimensional stabiUty, which requites low linear coefficients of thermal expansion and high modulus values, ate key parameters in high density fine-pitch interconnect devices. 

Many methods have been evolved in recent years for assessing flame retardants and the combustion characteristics of plastics and these have been the subject of comprehensive reviews. " ... 

A manufacturer considering using a thermoplastic elastomer would probably first consider one of the thermoplastic polyolefin rubbers or TPOs, since these tend to have the lowest raw polymer price. These are mainly based on blends of polypropylene and an ethylene-propylene rubber (either EPM or EPDM) although some of the polypropylene may be replaeed by polyethylene. A wide range of blends are possible which may also contain some filler, oil and flame retardant in addition to the polymers. The blends are usually subject to dynamic vulcanisation as described in Section 11.9.1. 

Fire retardancy is an often occurring theme in phosphazene chemistry and numerous reviews have focused on this subject over the years [ 10,44,387,393,396, 582]. In this article we will treat only aspects related to the flame-retardant properties of aryloxyphosphazene copolymers, which are the subject of the greatest number of applications. 

The antimony oxide/organohalogen synergism in flame retardant additives has been the subject of considerable research and discussion over the past twenty-five years (1-17). In addition to antimony oxide, a variety of bismuth compounds and molybdenum oxide have been the subject of similar studies (18-20). Despite this intensive investigation, relatively little has been conclusively established about the solid state chemical mechanisms of the metal component volatilization, except in those cases where the organohalogen component is capable of undergoing extensive intramolecular dehydrohalogenation. 

Once ignited they produced considerable amounts of heat and smoke. Flame retarded flexible PU foams became available in 1954-55, i.e. within a few years of flexible PU foams becoming available in commercial quantities(22). These FR PU foams contained trichloroethyl phosphate or brominated phosphate esters and resisted ignition from small flame sources. Unfortunately they may burn when subjected to a larger ignition source or when covered by a flammable fabric and may then produce as much heat and more smoke than the standard grade of PU foam(3). This was identified by UK room tests in the early 1970 s and has been confirmed more recently by furniture calorimeter tests at the NBS(21). 

Various types of additives are essential to give plastics new or enhanced properties. However, the safety of flame retardants, plasticisers and stabilisers, as regards potential health hazards, is attracting attention. Brome-based flame retardants have been the subject of safety disputes for several years and several substitutes have been developed. The shift to lead-free stabilisers is making further progress. 

Safety and Hazards. The linear carbonate solvents are highly flammable with flash points usually below 30 °C. When the lithium ion cell is subject to various abuses, thermal runaway occurs and causes safety hazards. Although electrode materials and their state-of-charge play a more important role in deciding the consequences of the hazard, the flammable electrolyte solvents are most certainly responsible for the fire when a lithium ion cell vents. The seriousness of the hazard is proportional to the size of the cell, so flame-retarded or nonflammable lithium ion electrolytes are of special interest for vehicle traction batteries. 

Decabromodiphenyl ether (BDE-209) is a major industrial product from the polybrominated diphenyl ethers used as flame retardants derivatives of this product have been detected in the environment. After exposure to the land surface, these contaminants adsorb on soil materials and may reach the atmosphere as particulate matter these particulates are subsequently subject to photolytic reactions. In this context, Ahn et al. (2006) studied photolysis of BDE-209 adsorbed on clay minerals, metal oxides, and sediments, under sunhght and UV dark irradiation. Dark and light control treatments during UV and sunlight irradiation showed no disappearance of BDE-209 during the experiments. Data on half-lives and rate constants of BDE-209 adsorbed on subsurface minerals and sediments, as determined by Ahn et al. (2006) and extracted from the literature, are shown in Table 16.6. 

Since combustion is subject to many variables, tests for flame retardancy may not correctly predict flame resistance under unusual conditions. Thus, a disclaimer stating that flame retardancy tests do not predict performance in an actual fire must accompany all flame-retardant products. Flame retardants, like many organic compounds, may be toxic or may produce toxic gases when burned. Hence, care must be exercised when using fabrics or other polymers treated with flame retardants. 

It has been reported that the effectiveness of copolymerized DOPO-type monomers can be further improved if the alcohol-amine derivatives of DOPO, for example, Structure 5.11, are used rather than similar structures not containing nitrogen.30 Of the FR fibers based on P-containing comonomers, it has been found that those based on Structure 5.10 are more hydrolytically stable, presumably because the P-containing group is in a cyclic structure and also should the hydrolysis of the P-0 bond occur, it will not lead automatically to a marked reduction in molecular weight.31 All the P-modified PETs appear to be subject to both the vapor-and condensed-phase mechanisms of flame retardance, with the former predominating.32 33... 

It has extremely low water vapor transmission, even with extruded thin transparent film. Chemical resistance is generally good, although not comparable to PTFE and other fluoroplastics. Chlorinated solvents, in particular, can affect PCTFE, and it can be subjected to stress cracking at elevated temperatures. The presence of chlorine atoms, used to enhance flame retardancy, keeps PCTFE in the non burning category with PTFE. It causes, however, a serious increase in dielectric losses, particularly at high frequencies. 

The UK therefore deemed it necessary to balance the risks to health from the brominated flame-retardants with the prevention of fires, even though alternative technologies or flame-retardants were available. Today, penta- and octa-BDE are banned in the EU, although the flame-retardant deca-BDE is still subject to ongoing investigation [311]. 

Trace analysis of organic compounds is primarily used in the detection and determination of harmful substances of natural origin (e.g., mycotoxins) as well as those that are the undesirable result of human activity, especially industrial and agricultural. Of the latter, the subject of interest could be either intentionally produced compounds (pesticides, flame retardants, chemical weapons, etc.) or unwanted impurities released in an uncontrolled manner in technological processes or from improper combustion of fuels and waste materials [1,2]. 

As mentioned previously, when polystyrene is subjected to the temperatures of a flame it pyrolyzes by a depolymerization mechanism to give monomer and oligomers [14]. The combustion of these volatile products in the vapor phase above the sample supplies heat back to the solid sample (Figure 29.3). If the energy supplied by combustion is sufficient to maintain the pyrolysis process, the flame is self-sustaining even after the test flame has been removed. In order to make polystyrene more flame retardant, the cycle of pyrolysis and combustion must be broken. Flame retardants may act in either the vapor or solid (condensed) phase. 

This study resulted in a series of reports on a comprehensive evaluation of fire-retardant treatments for wood (2-6). One hundred and thirty single chemicals or combinations of chemicals in the form of various salts were evaluated for flame-spread reduction, smoke, and corrosivity. Diammonium phosphate ranked first in reducing flame spread, followed by monoammonium phosphate, ammonium chloride, ammonium sulfate, borax, and zinc chloride. Zinc chloride, although excellent as a flame retardant, promoted smoke and glowing. Ammonium sulfate was the least expensive, but under certain environmental conditions it was corrosive to metals. None of the 130 compositions tested was considered ideal because of the adverse effects on some of the properties of wood. Several reviews of the subject are available and provide additional background material (J, 7-JO). 

Early studies involved treatment of wood specimens with fire-retardant chemicals, then subjecting the treated specimens to thermal analysis by TG. Browne and Tang (41) tested eight compounds, some of which were known to be effective fire retardants, and some of which were not. The TG results (Figures 8-10) indicate that all compounds increased the residual char weight of the material. Except for sodium tetraborate, the more effective the salt as a flame retardant the lower the temperature of active pyrolysis and the greater the amount of char. These results were confirmed through repeated experiments (45, 46). 

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