Polyurethane, thermoplastic retardant

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. 

Bourbigot, S., Turf, T., Bellayer, S., and Duquesne, S., 2009. Polyhedral oligomeric silsesquioxane as flame retardant for thermoplastic polyurethane. Polym. Deg. Stab. 94 1230-1237. 

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. 

Myers, R.E., Dicksons, E.D., Licursi, E., and Evans, R. 1985. Ammonium pentaborate An intumescent flame retardant for thermoplastic polyurethanes, J. Fire Sci., November/December, 3(6), 432-449. 

The most common polymers used in FR wire and cable applications are PVC, polyolefins, fluoropolymers, and silicone polymers. Thermoplastic polyurethanes (TPUs) and other specialty polymers such as chlorosulfonated polyethylene also serve niche applications in wire and cable. The approaches to achieve flame retardancy in each of these polymer systems along with issues unique to wire and cable application are discussed in the following sections. 

Polybrominated biphenyls (PBBs) are inert, stable chemicals used primarily as additive flame retardants to suppress or delay combustion. In their use as flame retardants, PBBs were added to polymer materials, but were not chemically incorporated into the polymer matrix and therefore could migrate out of the polymer matrix with time. Hexabromobiphenyl was used as a Are retardant mainly in thermoplastics in electronic equipment housings. Smaller amounts were used as a Are retardant in coating and lacquers, and in polyurethane foam for auto upholstery. After the voluntary ban of hexabromobiphenyl in the late 1970s, polybrominated diphenyl ethers (PBDEs) and other flame retardants were used as replacements. 

Flame retardant intumescent formulations have been developed using charring polymers PA6, thermoplastic polyurethanes (TPUs), and hybrid clay-PA6 nanocomposites as carbonisation agents. The advantage of the eoncept is to obtain FR polymers with improved mechanical properties and to avoid the problem of migration and solubility of the additives. 

APP and APP-based systems are very effident halogen-free flame retardants mainly used in polyolefins (PE, PP), epoxies, polyurethanes, unsaturated polyesters, phenolic resins, and others. APP is a nontoxic, environment friendly material and it does not generate additional quantities of smoke due to intumescence. Compared to other halogen-free systems, APP requires lower loadings. In thermoplastic formulations, APP exhibits good processability, retention of good mechanical properties. 

Dibromo-2-butene-1,4-diol n. (dibromo-butenediol). HOCH2C(Br)=C(BR)CH2 OH. A low-molecular weight, chemically reactive, brominated primary glycol. It is used as a building block for condensation polymers that can be incorporated into a wide variety of polymers including esters, urethanes, and ethers. It is also used as a flame-retardant monomer for polyurethanes and thermoplastics, and as a substitute for methylene-bis-o-chloroniline (MOCA) in urethane foams. 

Clariant is the world s largest manufacturer of ammonium polyphosphate, which is the basis of its Exolit AP halogen-free intumescent flame retardants. They cut down smoke emission, evolve no corrosive gases and are recommended for both thermoplastic (including polyolefin and polyurethane) and thermosetting polymers. Exolit AP 750 is used to make injection moulded PP electrical parts and weatherable items that contain a HALS stabiliser. Ammonium polyphosphate is recommended for certain types of wood-plastic composite. 

Alloying different plastics with different flame retardant characteristics can also improve flame retardancy. A typical compound would be ABS-PVC alloys. Recently, PVC with antimony oxide has also been investigated for alloying with thermoplastic polyurethane. 

AMGARD 1045 can advantageously be added to polyethylene terephthalate melt before spinning. These products are also useful flame retardants for other thermoplastics, thermoset polyester resins, polyurethanes, polycarbonates, nylon 6, and textile backcoating. 

The unnotched Izod impact strengths of thermoplastic resins are generally reduced by the addition of glass fibers. Also, the addition of flame-retardants usually results in decreased impact strength. The glass-reinforced nylon, polycarbonate, and polyurethane resins are regarded as the toughest of the reinforced thermoplastic resins. 

Melamine is most commonly used in flexible polyurethane foams in combination with chloroalkyl phosphates and in intumescent coatings in combination with ammonium polyphosphate and pentaerythritol. Nevertheless, there is extensive patent literature on the use of melamine in thermoplastics and elastomers, which was reviewed by Weil and Choudhary. The review gives good insight into the mechanism of flame retardant action of melamine. It is known that melamine does not melt, but sublimes at about 350°C (actual volatilization starts at a lower temperature). Upon sublimation, significant energy is absorbed, which decreases the surface temperature of the polymer. This is especially important for polyurethane foams having very low thermal inertia. In a hot flame, melamine may decompose further, with creation of cyanamid, which is also a very endothermic process. 

The char formers commonly used in intumescent formulations for thermoplastics are polyols such as pentaerythritol, mannitol, and sorbitol. However, exudation and water solubility are problems associated with these additives. Moreover, these additives are often not compatible with the polymeric matrix, and the mechanical properties of the formulations are then very poor. We have developed intumescent polyolefin-based formulations using charring polymers [thermoplastic polyurethane (TPU) and polyamide-6 (PA6)] as carbonization agents. " These formulated blends have improved mechanical properties compared with polymers loaded with classical flame retardants, and they avoid the problems... 

Bugajny, M. Le Bras, M. Bourbigot, S. Poutch, F. Lefebvre, J.-M. The use of thermoplastic polyurethanes as carbonization agents in intumescent blends, 1 Fire retardancy of polypropylene/thermoplastic polyurethane/ammonium polyphosphate blends. J. Fire Sci. 1999, 17(11-12), 494-513. 

Yellowing in thermoplastic polyurethanes can be retarded by adding UV-stabilizers, antioxidants and/or reduction agents. Proven stabilizers are the combinations of a phenolic antioxidant with an absorber of the hydroxybenzophenone or the hydroxy-benzotriazole type, and the three-fold combination phenolic antioxidanf/UV absorber/sterically hindered amine. This has allowed significant improvements in the light resistance of thin cast films made from aromatic polyester urethane [86]. 

As the substrate sheet, a thin plastic film or tightly packed nonwoven cloth, such as polyester spanbond nonwoven cloth of approximately 0.15 mm, is used. A synthetic rubber, such as styrene-butadiene rubber or thermoplastic polyurethane elastomer, is used as a binder. In addition, a surfactant for hydrophilicity, an antioxidant for prevention of thermodeformation, and a silica-type inorganic filler for prevention of tackiness are used. For the superabsorbent polymer particles, various synthetic polymers, for example, polyacrylate and polyvinyl-type superabsorbent polymers, can be used. For this apphcation, the particle sizes are an important parameter, because they polymer is required to be within the coating layer, and as the absorption rate is no retarded, quickly protrude from the layer when swelling. 

Blending two or more polymers offers yet another method of tailoring resins to a specific application. Because blends are only physical mixtures, the resulting polymer usually has physical and mechanical properties that lie somewhere between the values of its constituent materials. For instance, an automotive bumper made from a blend of polycarbonate resin and thermoplastic polyurethane elastomer gains rigidity from the polycarbonate resin and retains most of the flexibility and paintability of the polyurethane elastomer. For business machine housings, a blend of polycarbonate and acrylonitrile-butadiene-styrene (ABS) copolymer resins offers the enhanced performance of polycarbonate flame retardance and ultraviolet (UV) stability at a lower cost. 

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