Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-03-12
2003-02-18
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S356000
Reexamination Certificate
active
06521714
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to novel, improved high quality brominated styrenic polymers eminently well suited for use as flame retardants in thermoplastic polymer compositions.
Brominated polystyrenes are well established as flame retardants for use in thermoplastics, e.g., polybutylene terephthalate, polyethylene terephthalate and nylon (a.k.a. polyamides). Recently, interest has been shown for expanding their use to syndiotactic polystyrene and polycyclohexylene dimethylene terephthalate. Generally, brominated polystyrenes are produced by a reaction between polystyrene and a brominating agent (e.g., bromine or bromine chloride) in the presence of a solvent (e.g., dichloroethane) and a Lewis acid catalyst. Heretofore the art has proffered many processes which are claimed to produce a superior brominated polystyrene. See, for example, U.S. Pat. Nos. 4,200,703; 4,352,909; 4,975,496 and 5,532,322.
Despite these efforts, previously-known brominated polystyrene flame retardants remain deficient in certain properties which translate into deficient performance of thermoplastic polymer blends in which they are used when the blends are subjected to polymer processing conditions.
To better understand some of the reasons for these deficiencies, it is helpful to consider some of the structural characteristics of previously known brominated polystyrenes. To begin with, the bromine content of a brominated polystyrene is the sum of (1) the bromine which is substituted onto the aromatic portions of the polymer, (2) the bromine which is substituted onto the aliphatic portion(s) of the polymer, e.g., the polymer backbone or alkyl substitution which is present due to alkylation of the aromatic portion of the polymer, and (3) any ionic bromine present, e.g., sodium bromide. The alkylation of aromatic rings in the brominated polystyrene is catalyzed by the Lewis acid catalyst used in producing the brominated styrenic polymer, and the reaction solvent (usually a 1-3 carbon atom dihaloalkane) serves as the alkylating agent. The bromine for (1) is referred to herein as aromatic bromide, while the bromine for (2) is referred to as aliphatic bromide. Even though ionic bromine can contribute to the total bromine content, its contribution to the total bromine content is small. Nevertheless, as pointed out in U.S. Pat. No. 5,328,983, ionic impurities in brominated polystyrene may degrade polymer formulations in respect to their ultimate electrical properties, and also may result in corrosion of processing equipment or in the corrosion of metallic parts in their end-use applications.
Aliphatic halide content of brominated polystyrenes is not desirable as aliphatic halide is not as thermally stable as aromatic halide and, thus, aliphatic halide can be easily converted to hydrogen halide, e.g., HBr or HCl, under normal end-use processing conditions. Aliphatic bromide and chloride are generally referred to by the art and quantified, respectively, as hydrolyzable bromide and hydrolyzable chloride since such halides are easily hydrolyzed as compared to aromatic halides.
Corrosion of metal processing equipment such as melt blenders, extruders, and molding machines, attributable to the release of hydrogen halide from prior brominated polystyrene flame retardants when molding thermoplastic polymers containing such flame retardants under thermal processing conditions is thus one deficiency of such flame retardants. In the presence of moisture, hydrogen chloride and hydrogen bromide released from the brominated polystyrene in the blend during exposure to the elevated polymer processing temperatures can result in acid formation and consequent metal corrosion.
To evaluate brominated styrenic polymers for their tendencies to release hydrogen halide under thermal processing conditions, use is made of the method described in U.S. Pat. No. 5,726,252 and referred to therein as the Thermal Stability Test Procedure. In essence, this method indicates the content of halogen atoms in the brominated polystyrene that is not bonded directly to the aromatic rings and thus is more readily released from the polymer when at elevated temperature. The Thermal Stability Test is described in greater detail hereinafter.
Prior brominated polystyrenes have oftentimes been deficient in their color characteristics. The manufacturer of molded thermoplastic articles generally finds it of advantage to have available a flame retardant which will not contribute excessive color to the molded products or otherwise interfere with color matching specifications applicable to a given product. Thus in general, the lower the color of the brominated styrenic polymer (i.e., the whiter the flame retardant), the better.
For many applications it is desirable to have a flame retardant which produces polymer blends having good flow characteristics. Good flow characteristics can significantly reduce processing time in the manufacture of molded thermoplastic products. In addition, good molten polymer flow tends to ensure that the molds are completely filled, thereby resulting in products with better surface characteristics and appearance.
The chlorine content of brominated polystyrenes is credited to chlorine which, like the bromine, is chiefly part of the polymer structure as an aromatic and/or an alkyl chloride. The use of bromine chloride as the brominating agent is the largest contributor to the chlorine content. However, chlorinated solvents and/or chlorine-containing catalysts used in the production of the brominated polystyrene may also contribute to the chlorine content of the brominated polystyrene. Apart from whether the halide is present as an aromatic or aliphatic halide, it is also desirable to minimize the total chlorine content of the brominated polystyrene as chlorine is not as efficacious or as stable a flame retardant constituent as is bromine.
Prior art brominated polystyrene polymers that have been evaluated for thermal stability have exhibited a 1% weight loss at temperatures less than 336° C. when submitted to Thermogravimetric Analysis (TGA) and, indeed, most have exhibited a 1% weight loss at temperatures around 300° C. Such low TGA 1% weight loss thermal stability temperatures, while acceptable in actual practice, are much less desirable than TGA 1% weight loss thermal stability temperatures of 340° C. or above, especially under the high temperatures to which flame retarded thermoplastics formulated with such brominated polystyrene polymers are exposed during processing.
It would be especially desirable if most if not all of the above-mentioned disadvantages of brominated polystyrenes could be avoided or at least minimized. For example, it would be of considerable advantage if a more thermally stable brominated styrenic polymer, e.g., brominated polystyrene, could be provided that releases minimal amounts of hydrogen halide under thermal processing conditions, thereby greatly reducing the opportunity for corrosion of processing equipment to occur. It would be especially desirable and of considerable advantage, if a brominated styrenic polymer, e.g., brominated polystyrene, could be provided that has both improved melt flow characteristics and improved thermal stability. It would also be of great advantage if these improvements could be achieved without material sacrifice of other important properties, and if possible with concomitant provision of other desirable properties such as low ionic bromide content, minimal (if any) chlorine content, and/or less odor properties due to solvent residues. It would also be very desirable and advantageous if, in addition to high thermal stability in the Thermal Stability test, a brominated styrenic polymer, e.g., brominated polystyrene, could exhibit low color-forming tendencies, and improved melt flow properties. It would also be of considerable advantage if a brominated styrenic polymer, e.g., brominated polystyrene, could be provided that, even though it has a total bromine content of 60 wt % or more, and indeed of 67 wt % or more, possesses the combination of improved melt f
Ao Meng-Sheng
Balhoff Donald E.
Dadgar Billie B.
Kolich Charles H.
Lin Homer C.
Albemarle Corporation
Lipman Bernard
Pippenger Philip M.
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