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
1999-08-02
2001-01-23
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S068000, C525S132000, C525S149000
Reexamination Certificate
active
06177512
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a polymerization process for producing polystyrene having, compared to conventional high impact polystyrene, improved properties, such as heat resistance, impact strength at low temperature, environmental stress crack resistance and colorability. More particularly, the present invention relates to a bulk process for producing engineering polystyrene resins containing rubber components, such as butadiene rubber, and also containing polyphenylene ether.
BACKGROUND OF THE INVENTION
Polystyrene is a high molecular weight linear polymer produced by polymerizing styrene. When made from only the styrene monomer, the resulting polymer has a glass transition temperature of about 100° C. and is brittle, showing very poor strength (impact strength, elongation to rupture and dart-drop impact strength) characteristics. The strength characteristics can be improved by incorporating rubber modifiers, such as butadiene rubber. When a rubber is incorporated, the product is called “HIPS” for High Impact Polystyrene.
HIPS usually contains about 5 to 15 wt. % polybutadiene or styrene-butadiene copolymer rubber. However, the presence of polystyrene occlusions within the rubber phase typically results in about 10-40% volume fraction of the reinforcing rubber phase. When polymerizing styrene in the presence of polybutadiene, polybutadiene grafts with polystyrene. There is a phase inversion period as the polymerization progresses. The polybutadiene-styrene phase is the continuous phase during the earlier part of the polymerization, with polystyrene dispersed in the polybutadiene-styrene phase. After progressively more polystyrene is formed, there is a phase inversion period and styrene-polystyrene eventually becomes the continuous phase, with polybutadiene dispersed therein, as explained by Freeguard,
Br. Polym. J
., vol. 6, page 205, 1974.
Whereas various rubbers, such as polybutadiene, have been used to modify polystyrene to improve the strength of polystyrene, polyphenylene oxide has been used to raise the softening point of polystyrene.
Polyphenylene oxide and the like are sometimes referred to as polyphenylene ethers, abbreviated as PPE, and are described in numerous publications, including an article by D. M. White titled “Poly(phenylene oxide)s” at chapter 28, pages 473-481 of
Comprehensive Polymer Science
, vol. 5, Pergamon Press, 1989, and including U.S. Pat. Nos. 3,306,874; 3,306,875; 3,257,357 and 3,257,358. PPE can be made to grades of sufficiently high molecular weights. Furthermore, high temperatures are necessary for processing PPE resins because of a high softening point in a range of about 210° C. to 280° C.
The incorporation of PPE in polystyrene is typically done by blending, such as blending PPE with polystyrene in an extruder at high temperatures to form PPE and polystyrene blends. For instance, European Patent 121,974 to Van der Meer et al. discloses in Example II the blending of PPE with polystyrene (PS) by heating blends of PPE with PS followed by extrusion. The PS used included rubber-modified polystyrene, and/or styrene-butadiene block copolymer (unsaturated), and/or styrene-butadiene block copolymer (saturated), and various additives. European Patent 168,566 and U.S. Pat. No. 4,423,189 also disclose blends of PPE and PS.
U.S. Pat. No. 3,664,977 to Nakanishi et al. discloses a combination of PPE and HIPS where the PPE is added in situ, that is, during the polymerization reaction to form HIPS type polystyrene from styrene monomer and rubber, as opposed to blending PPE with preformed polystyrene in an extrusion operation.
According to the Nakanishi patent, the PPE is added after phase inversion, in particular, between the time immediately after phase inversion up to the time the total solids level reaches 40%, and preferably when the solids level is 15-35%. Also, the Nakanishi patent states at col. 2, line 38, that the PPE was added in the form of a styrene solution of PPE. Further, at col. 3, line 7, the Nakanishi patent states that, in their process, a considerable amount of PPE is incorporated into the rubber particle portion of the HIPS product.
U.S. Pat. No. 5,660,776 to Wooden et al. is another reference where PPE is added during the polymerization process in the form of a solution of PPE in styrene. The Wooden et al. patent is directed to so-called GPPS (general purpose polystyrene), or crystal polystyrene, as opposed to HIPS. In the Wooden et al. patent, the feed PPE for the process is a solution of PPE in styrene. The solution is obtained by dissolving PPE in styrene. Thus, Wooden states that he provides a process to produce a solution of a styrenic polymer containing up to 15 weight % of polyphenylene ether comprising:
(a) dissolving a polyphenylene ether polymer in one or more of the monomers for said styrene polymer;
(b) feeding the solution obtained in (a) above together with up to 0.05 weight % of one or more organic free radical initiators and from 0 to 10 weight % of an organic diluent and additional monomer to produce a concentration of polyphenylene ether in monomer of up to 15 weight % to one or more reactors;
(c) subjecting said solution to heat for a time sufficient to polymerize at least 70 weight % of the monomers;
(d) subjecting said polymerized solution of styrenic polymer containing polyphenylene ether to temperature and pressure conditions to devolatilize unpolymerized monomer and diluent; and
(e) extruding said solution of styrenic polymer containing polyphenylene ether as strands and cooling said strands and chopping said strands into pellets.
In referring to the Nakanishi patent, Wooden states that the Nakanishi U.S. Pat. No. 3,664,977 reference teaches blending polyphenylene ether into the bulk polymerization of high impact polystyrene (HIPS) between a point after rubber phase inversion (e.g., after the point when the phase volume of rubber in styrene is about equal to the phase volume of polystyrene in styrene) to a point when the total polymer concentration has become 40%.
SUMMARY OF THE INVENTION
According to the present invention, a process is provided for preparing polystyrene containing polyphenylene ether (PPE), which comprises:
(a) polymerizing styrene in the presence of a rubber to a point past phase inversion, preferably far beyond the phase inversion point, in a first reaction zone to obtain a first reaction mix;
(b) passing the first reaction mix to a second reaction zone wherein the polymerization of styrene is continued;
(c) adding a slurry of PPE in styrene, containing at least 15 wt. % PPE, to the first reaction mix in the second reaction zone; and
(d) further polymerizing styrene in the slurry of PPE in styrene with the first reaction mix in the second reaction zone.
In a broad embodiment of the present invention, the slurry contains at least 15 wt. % PPE. However, preferably, the slurry contains at least 20 wt. % PPE, and more preferably at least 22 wt. % PPE, and most preferably at least 25 wt. % PPE. Preferably, the upper range of PPE in the slurry is below 50 wt. %, more preferably below 40 wt. %.
In the present invention, the slurry of PPE in styrene is added to the process in situ, that is, during the course of styrene polymerization rather than adding PPE by blending. Adding PPE by blending as part of the extrusion step is the prevalent method of adding PPE in accordance with the prior art. In the present invention, the PPE is introduced in situ, and beyond phase inversion point. The slurry of PPE in styrene preferably is introduced to the process of the present invention after the total polymer solids level in the reaction mixture is advanced to greater than 25 wt. %, more preferably greater than 30%, and most preferably greater than 40%. The total polymer solids include polystyrene and rubber.
The first reaction zone in the present invention may comprise one or more reactors, and the second reaction zone may comprise one or more reactors. The reactors preferably are CSTR (continuous-flow stirred-tank reactors) type reactors. However, many other types of reactors c
Dotson Seldon L.
Gibbons Loren K.
Hanner Michael J.
Miller Jason J.
Silloway Jeffrey
Chevron Chemical Company LLC
DeJonghe Thomas G.
Short Patricia A.
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