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
1999-11-15
2001-12-25
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S332200, C525S332300, C526S293000, C526S326000, C526S334000, C526S340000, C526S347000, C524S127000, C524S139000
Reexamination Certificate
active
06333385
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a method of producing styrenic polymers having improved thermal stability and flame retardant styrenic polymers produced using the method.
BACKGROUND
Cross-linking is a general strategy which has been investigated for some time now as a process to enhance the thermal stability of various polymers, including polystyrene (PS). The cross-linking strategy attempts to drive the introduction of cross-linkages into an otherwise non-crosslinked polymer matrix at elevated temperatures only (i.e., under conditions where the polymer is challenged by fire and/or high heat).
To date, this approach has met with only limited success. For example, in PS, Grassie and Gilks (1973)
J. Polym. Sci.: Polym. Chem. Ed.
11:1985, used tin tetrachloride as a catalyst and p-di(chloromethyl)benzene in dichloroethylene as a cross-linking agent. A cross-linking reaction occurred under the conditions described in the paper, but the resultant cross-linked polymer has a lower thermal stability than the uncross-linked polymer itself. Similarly, Brauman (1979)
J. Polym. Sci.: Polym. Chem. Ed.
17:1129, describes using antimony chloride as a catalyst in conjunction with various alkylating and acylating agents, and Rabek and Lucki (1988)
J. Polym. Sci.: Part A: Polym. Chem.
26:2537, describe cross-linking PS at room temperature using aluminum trichloride as a catalyst. Both reactions, however, take place at a temperatures far too low to be useful to impart flame retardancy.
A primary concern in exploiting cross-linking as a flame retardant mechanism is that the cross-linking reaction should not occur under polymer processing conditions nor under the normal, operating environment in which the polymer is placed. Because the cross-linking reaction alters the physical properties of the polymer, to be practically useful the cross-linking reaction should have a sufficiently high barrier to activation so that the reaction does not auto-initiate under normal operational conditions. To be optimally useful as a flame-retardant mechanism, the cross-linking reaction should occur only when the finished polymer is exposed to open flame or heat which produces temperatures above the processing range of the polymer. Conversely, the activation barrier for the cross-linking reaction should not be higher than the degradation temperature of the polymer itself. Otherwise, the polymer degrades before the cross-linking reaction is initiated. In all of the efforts noted above, the cross-linking reaction described occurs at a temperature which is too low to be useful for thermal protection of the polymer.
The thermal degradation of PS homopolymer proceeds by two mechanisms: end-chain scission and random scission. The degradation products are styrene monomer, styrene oligomers, benzene, and toluene, all of which are highly flammable. The degradation commences at about 360° C. and is complete by about 450° C. Consequently, an optimal flame retardant/thermal stabilization system for styrenic polymers would remain inactive at temperatures at or below the processing temperature of PS (roughly about 200° C. to about 250° C.), and initiate cross-linking within the polymer matrix at temperatures greater than about 200° C. to 250° C. but not greater than about 360° C., the temperature at which pure PS begins to thermally degrade. Of course, the optimal temperature at which the cross-linking reaction is initiated should be tailored to the specific nature of the styrenic polymer at hand (e.g., PS homopolymer, PS-containing copolymer, etc.). For example, high-impact polystyrene (HIPS), a graft copolymer, begins to thermally degrade at about 300° C.
As reported by Li & Wilkie (1997)
Polym. Degrad. and Stability
57:293-299, Friedel-Crafts chemistry, using 1,4-benzyenedimethanol as a bifunctional alkylating agent and a zeolite catalyst, can yield cross-linkages within PS. However, the reactions leading to cross-linked products were performed in sealed vessels, where gaseous products are retained under high pressure and are available for further reaction. When the same reactions were conducted in a flowing nitrogen atmosphere, the diol was volatilized before it could react and cross-linking was not observed.
SUMMARY OF THE INVENTION
The present invention is a process to impart increased thermal stability to styrenic polymers and the resultant composition of matter. Specifically, the invention is directed to polymer compositions in which a bifunctional Friedel-Crafts-type alkylating or acylating agent is incorporated into a pre-formed styrenic polymer. In effect, the aromatic moieties of a pre-formed styrenic polymer are functionalized to contain a Friedel-Crafts-type alkylating or acylating agent. The functionalized styrenic polymer is then combined, either batch-wise or in continuous fashion, with a suitable Lewis or Brønsted-Lowry acid catalyst. The catalyst initiates (at a desired, elevated temperature, preferably above about 200° C.) the formation of inter- and intramolecular cross-linkages between the aromatic moieties present in the styrenic polymer, presumably via an electrophilic Friedel-Crafts-type alkylation/acylation mechanism, to yield a less volatile, more thermally stable, cross-linked, degraded polymer. Overall, the result is a styrenic polymer which, due to the introduction cross-linkages at elevated temperatures, is indistinguishable from conventional styrenic polymers during processing and at normally-encountered operational temperatures, but which degrades to a more thermostable, cross-linked polymer upon exposure to excessive heat.
One difficulty overcome by the present invention is that while Friedel-Crafts chemistry can be used to alkylate or acylate a styrene moiety under certain conditions, the reaction is quite capable of proceeding at only modest temperatures, so that with certain electrophiles, the reaction will occur under processing conditions. Another difficulty overcome by the invention is that because the alkylating/acylating agent is incorporated directly into the polymer itself, the agent cannot volatilize away from the bulk polymer before the cross-linking reaction can occur. Consequently, upon the exposure to sufficiently high heat, the alkylating/acylating agent is able to react, presumably via an electrophilic addition mechanism, to aromatic moieties present with the styrenic polymer.
The invention is drawn to a method of imparting increased thermal stability to styrenic polymers comprising reacting a pre-formed styrenic polymer with a bifunctional cross-linking reagent selected from the group consisting of:
X—R—Y
wherein X and Y are alkylating and/or acylating agents and R is an aliphatic or aromatic group which links X and Y together. Specifically, R can be any aliphatic or aromatic group, preferably phenyl, naphthyl, anthryl, etc.; and X and Y are independently selected from the group consisting of halo, hydroxy, nitro, alkyl, alkenyl, haloalkyl, hydroxyalkyl, alkyloxy, alkyloxyalkyl, alkylcarbonyl, alkylcarbonylalkyl, alkyloxycarbonyl, alkyloxycarbonylalkyl, alkylcarboxy, alkylcarboxyalkyl, C
1
to C
6
dicarboxylates, substituted or unsubstituted phenyl or naphthyl, benzoate, mononitrobenzoate, polynitrobenzoate, phosphate, alkylphosphate, phenylphosphate, diphenylphosphate, alkyl-diaryl-phosphate, carbonate, phenyl carbonate, inorganic acid esters, mesylate, and tosylate.
In the preferred embodiment, the bifunctional cross-linking reagent is selected from the group consisting of:
wherein X and Y (and, where appropriate, the methylene to which X and Y are attached) are independently selected from the group consisting of hydrogen, halo, hydroxy, nitro, alkyl, alkenyl, haloalkyl, hydroxyalkyl, alkyloxy, alkyloxyalkyl, alkylcarbonyl, alkylcarbonylalkyl, alkyloxycarbonyl, alkyloxycarbonylalkyl, alkylcarboxy, alkylcarboxyalkyl, C
1
to C
6
dicarboxylates, substituted or unsubstituted phenyl or naphthyl, benzoate, nitrobenzoate, phosphate, alkylphosphate, phenylphosphate, diphenylphosphate, alkyl-diaryl-phosphate, carbonate, phenyl carbonate, inorganic acid
McKinney Michael A.
Wilkie Charles A.
Yao Hongyang
Zhu Jin
Asinovsky Olga
DeWitt Ross & Stevens S.C.
Leone, Esq. Joseph T.
Marquette University
Seidleck James J.
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