Flame retardant phosphorus esters

Phosphorus-containing flame retardants, phosphate esters... 

Aluminum hydroxide, antimony oxide, flame retardants (phosphate esters, halogenated phosphorus compounds, chlorinated paraffins), zinc borate... 

Flame retardants phosphate esters, halogenated phosphorus compounds, zinc borates... 

With a recent push toward non-brominated flame retardants, phosphorus-based alternatives, such as phosphate esters, are used more frequently for various applications. Their use as plasticizers is also well known. However, their function as environmental stress crack agents of various thermoplastics is less well recognized. Two case studies, one - in which a triaryl phosphate was a component of the formulation, the other - in which it was migrating from an adjacent component illustrate some of the problems with their use. Fractographic analysis and various analytical techniques were used to determine a root cause of each of the two failures. 

The flame retardant mechanism for phosphorus compounds varies with the phosphorus compound, the polymer and the combustion conditions (5). For example, some phosphorus compounds decompose to phosphoric acids and polyphosphates. A viscous surface glass forms and shields the polymer from the flame. If the phosphoric acid reacts with the polymer, e.g., to form a phosphate ester with subsequent decomposition, a dense surface char may form. These coatings serve as a physical barrier to heat transfer from the flame to the polymer and to diffusion of gases in other words, fuel (the polymer) is isolated from heat and oxygen. 

Commercially available flame retardants include chlorine- and bromine-containing compounds, phosphate esters, and chloroalkyl phosphates. Recent entry into the market place is a blend of an aromatic bromine compound and a phosphate ester (DE-60F Special) for use in flexible polyurethane foam (8). This paper describes the use of a brominated aromatic phosphate ester, where the bromine and phosphorus are in the same molecule, in high temperature thermoplastic applications. 

Three flame retardants were compared in this study, namely, a brominated polycarbonate oligomer (58% bromine), a brominated polystyrene (68% bromine), and a brominated triaryl phosphate ester (60% bromine plus 4% phosphorus). These are described in Table I. Figures 1 and 2 compare the thermal stability of the brominated phosphate with commercial bromine-containing flame retardants by thermogravimetric analysis (TGA) and by differential scanning calorimetry (DSC). The brominated phosphate melts at 110°C and shows a 1% weight loss at 300°C. Brominated polycarbonate and brominated polystyrene are polymeric and are not as volatile at elevated temperatures as the monomeric flame retardants. 

The early patent disclosures have claimed the application of a wide spectrum of gas-evolving ingredients and phosphorus-based organic molecules as flame retarding additives in the electrolytes. Pyrocarbonates and phosphate esters were typical examples of such compounds. The former have a strong tendency to release CO2, which hopefully could serve as both flame suppressant and SEI formation additive, while the latter represent the major candidates that have been well-known to the polymer material and fireproofing industries.The electrochemical properties of these flame retardants in lithium ion environments were not described in these disclosures, but a close correlation was established between the low flammability and low reactivity toward metallic lithium electrodes for some of these compounds. Further research published later confirmed that any reduction of flammability almost always leads to an improvement in thermal stability on a graphitic anode or metal oxide cathode. 

Benzoylresorcinol based phosphate esters are obtained by reacting a benzoylresorcinol compound with a chlorophosphate, e.g., benzoylresorcinol with diphenyl chlorophosphate or phosphorus oxychloride. These esters can function both as flame retardants and UV stabilizers for PC/ABS and a series of other polymers (78). 

Aluminum bromide AlBr3 is used as a catalyst and parallels AICI3 in this role. Strontium and magnesium bromides are used to a limited extent m phamiacentical applications. Ammonium bromide is nsed as a flame retardant in some paper and textile applications potassium bromide is used in photography. Phosphorus tribromidc PBr3 and silicon tetrabromide SiBi4 are nsed as intermediates and catalysts, notably in the production of phosphite esters. 

Oligomeric Phosphorus Esters with Flame Retardant Utility... 

Pioneering work on phosphorus ester oligomers has been done by Monsanto in the U.S. (1), by Hoechst in Germany (2), and in the Soviet Union. These "itudies involved synthesis and flame retardant applications. The polycondensation of 2-chloroethyl phosphates as a route to oligomeric phosphorus esters (Equation 1) was first reported by Korshak al. (3). This Russian publication describes the polycondensation of tris(2-chloroethyl) phosphate at 240-280° under non-catalytic conditions. 

Phosphate esters (alkyl or aryl, or mixed) of phosphoric acid constitute an important family of organophosphorus flame retardants.25 Triethylphosphate, a colorless liquid boiling between 209°C and 218°C, and containing 17 wt % phosphorus, has been used commercially as an additive for polyester resins/laminates and in cellulosics. In polyester resins, it functions as a viscosity depressant as well as a flame retardant. Trioctylphosphate is employed as a speciality flame-retardant plasticizer for vinyl composites where low temperature flexibility is critical. It can be also included in blends, along with general purpose plasticizers, such as phthalate esters, to improve low temperature flexibility. 

Concerns exist in Japan over the use of phosphorus flame-retardants. The concern with the use of red phosphorus is the potential generation of phosphine gas during burning. There are also concerns with the use of phosphate esters. There is also dust concern over the use of antimony trioxide. 

Also phosphorus- and nitrogen-containing polyols are shown to be effective in flame retardancy of PU foams24 such as polyols based on phosphonic acid ester or obtained by partial or full substitution of methylol groups of tetrakis(hydroxymethyl)phosphonium chloride with amine several examples of such polyols were reported by Levchik and Weil.15 Rigid PU foam modified with these polyols showed improved oxygen index values moreover better results were achieved with higher functionality polyols. 

Brominated styrene grafted onto PET is capable of acting as a nonvolatile FR for PET fabrics [328]. Polyphosphate esters can be used as flame retardant plasticizers [204], phosphorus containing moieties act as color improvers and thermostabilizers in polyesters [198, 205]. Antistatic properties of copolymers containing moieties with sulfonic acids have been reported [107]. Some polymers, like poly(JV-vinylimidazole) impart anticorrosive and antirusting properties to protective coatings of metals [329]. 

The third most commonly used class of flame-retardants is phosphorus-containing compounds such as phosphoric acid esters, diphenyl cre-sylphosphate, dimethyl methylphosphonate, and dibutyl dihydroxyethyl diphosphate. They have good flame-retarding performance and are effective in small amounts. They are, however, expensive and have low hydrolysis stability. 

Applications. Phosphorus(V) oxychloride is mainly used in the manufacture of aliphatic and aromatic esters of phosphoric acid, which are used as flame retardants and plasticizers in plastics, as hydraulic fluids and as extraction agents. 

Bellstein Handbook Reference) AI3-08678 BRN 0878263 CCRIS 876 DImethoxymethyl-phosphIne oxide Dimethyl methanephosphonate Di-methyl methylphosphonate DMMP EINECS 212-062-3 Fyrol DMMP HSDB 2590 Methyl phosphonic acid, dimethyl ester NCI-C54762 NSC 62240 Phosphonic add, methyl-, dimethyl ester Pyrol dmmp. Flame retardant for applications where high phosphorus content, good solvency, and low viscosity are desired lowers viscosity of epoxy resins and unsaturated polyesters filled with hydrated alumina oxide. Liquid bp = 181 , bp20 = 79.6" d ° = 1.4099 Am = 217 nm (e = 13, EtOH) soluble in H2O, Et20, EtOH LDsO (rat orl) > 5000 mg/kg. Akzo Chemie. 

Organophosphorus compounds (OPs) are utilized on a large scale as flame retarding agents and plasticizers in a variety of products, such as plastic materials, rubbers, varnishes, lubricants, hydraulic fluids, and other industrial applications. This family of chemicals consists of alkylated and arylated phosphate or phosphonate esters and related compounds such as phosphites, phosphines, and related dimeric forms as well as ionic forms (Figure 31.2). " The low volatility of phosphoric acid and derivatives makes it the preferred choice of the phosphorus based FRs. These FRs are most effective in polymers that char readily. Also halogenated phosphate esters, such as tris(l-chloroisopropyl) phosphate (TCPP), and tris(2-chloroethyl) phosphate (TCEP), are widely used. These combine the properties of both the halogen and the phosphorus compounds. 

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