Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2004-02-23
2004-11-16
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S50200C, C528S503000, C264S347000
Reexamination Certificate
active
06818737
ABSTRACT:
BACKGROUND OF THE INVENTION
The process of the present invention relates to methods of producing articles from compositions of syndiotactic vinyl aromatic polymers, particularly articles for use in food applications.
Food applications typically require a very low level of residual monomer contaminants within the polymers and articles produced therefrom. Many methods have been set forth to reduce levels of residual vinyl aromatic monomer in vinyl aromatic polymers and syndiotactic vinyl aromatic polymers. U.S. Pat. No. 5,877,271 discloses a drying process utilizing heat treatment of syndiotactic polystyrene (SPS) pellets to avoid free-radical polymerization of styrene, allowing a faster evaporation of styrene and reducing the residence time of the polymer at higher temperatures by very rapid heating.
EP 325,125 and U.S. Pat. No. 5,270,442 disclose a method of producing syndiotactic polystyrene moldings having crystallinity of at least 20% comprising subjection the moldings to heat treatment at a temperature of 120 to 270° C. in order for such crystallinity to be obtained. These crystalline polymers are extracted as shown in the examples, to remove unwanted atactic polystyrene as well as styrene monomer prior to molding and heat treatment. However, the extraction is an added step which adds time and cost to the process of producing such articles.
Therefore, there remains a need for an efficient and cost effective process of producing molded articles of syndiotactic vinyl aromatic polymers, particularly for articles which will be utilized in food applications, such that the level of residual monomer in the articles is less than 0.05 parts per hundred parts of syndiotactic vinyl aromatic polymer.
SUMMARY OF THE INVENTION
The present invention is a process of producing a molded article from a composition comprising a syndiotactic vinyl aromatic polymer, comprising:
I) molding the composition comprising a syndiotactic vinyl aromatic polymer, herein the syndiotactic vinyl aromatic polymer has a residual vinyl aromatic monomer content of less than 0.3 parts per 100 parts syndiotactic vinyl aromatic polymer, at a temperature of from 265° C. to 305° C. to produce a molded article; and
II) heat treating the molded article at a temperature of 210° C. to 230° C. for at least 2 minutes to thermally initiate free radical polymerization of the residual monomer such that the molded article has a residual vinyl aromatic monomer content of less than 0.05 per 100 parts syndiotactic vinyl aromatic polymer.
This process produces articles acceptable for food applications by utilizing a polymer having low residual monomer in the molding step and further heat treating the formed article to polymerize unreacted residual monomer within the molded article. Food application examples include food packaging and food handling equipment as well as direct contact applications such as baking trays and food storage and/or reheating containers.
DETAILED DESCRIPTION OF THE INVENTION
All reference to the Periodic Table of the Elements herein shall refer to the Periodic Table of the Elements, published and copyrighted by CRC Press, Inc., 2001. Also, any reference to a Group or Groups shall be to the Group or Groups as reflected in this Periodic Table of the Elements using the IUPAC system for numbering groups. For purposes of United States patent practice, the contents of any patent, patent application or publication referenced herein is hereby incorporated by reference in its entirety, especially with respect to the disclosure of analytical or synthetic techniques and general knowledge in the art.
The term “comprising” and derivatives thereof is not intended to exclude the presence of any additional component, step or procedure, whether or not the same is disclosed herein. In order to avoid any doubt, all compositions claimed herein through use of the term “comprising” may include any additional additive, adjuvant, or compound whether polymeric or otherwise, unless stated to the contrary. In contrast, the term, “consisting essentially of” excludes from the scope of any succeeding recitation any other component, step or procedure, excepting those that are not essential to operability. The term “consisting of” excludes any component, step or procedure not specifically delineated or listed. The term “or”, unless stated otherwise, refers to the listed members individually as well as in any combination.
The term “polymer”, as used herein, includes both homopolymers, that is, polymers prepared from a single reactive compound, and copolymers, that is, polymers prepared by reaction of at least two polymer forming reactive, monomeric compounds. The term “crystalline” refers to a polymer that exhibits an X-ray diffraction pattern at 25° C. and possesses a first order transition or crystalline melting point (Tm). The term may be used interchangeably with the term “semicrystalline”. The term “syndiotactic” refers to polymers having a stereoregular structure of greater than 90 percent syndiotactic, preferably greater than 95 percent syndiotactic, of a racemic triad as determined by
13
C nuclear magnetic resonance spectroscopy.
Syndiotactic vinyl aromatic polymers are homopolymers and copolymers of vinyl aromatic monomers, that is, monomers whose chemical structure possess both an unsaturated moiety and an aromatic moiety. The preferred vinyl aromatic monomers have the formula:
H
2
C═CR—Ar;
wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms, and Ar is an aromatic radical of from 6 to 10 carbon atoms, including alkyl or halo ring substituted aromatic radicals. Examples of such vinyl aromatic monomers are styrene, alpha-methylstyrene, ortho-methylstyrene, meta-methylstyrene, para-methylstyrene, vinyl toluene, para-t-butylstyrene, vinyl naphthalene, divinylbenzene, chlorostyrene, bromostyrene, and the like. Syndiotactic polystyrene is the currently preferred syndiotactic vinyl aromatic polymer. Typical polymerization processes and coordination catalyst systems for producing syndiotactic vinyl aromatic polymers are well known in the art and are described in U.S. Pat. Nos. 4,680,353, 5,066,741, 5,206,197 and 5,294,685, and elsewhere.
During polymerization of the vinyl aromatic monomer, the polymerization reaction is not typically carried to completion and a mixture of syndiotactic vinyl aromatic polymer and volatiles, such as residual monomers and process solvents, is produced. This mixture typically contains from about 2 to about 99 percent solid, non-volatile, high molecular weight polymer, preferably from about 30 to about 95 percent, more preferably from about 40 to about 95 percent, and most preferably from about 70 to about 90 percent by weight based on the total weight of the mixture. The bulk density of the feed is typically less than 400 kg/m
3
, preferably less than 350 kg/m
3
. The average particle size (dp
50
) is generally less than 500 &mgr;m, preferably less than 400 &mgr;m. The polymer can then be recovered from this mixture using a finishing process such as devolatilization to produce resins which are useful for forming injection molded articles, films, fibers, etc.
In the production of syndiotactic vinyl aromatic polymers such as syndiotactic polystyrene (SPS), a devolatilization step is typically used to remove residual monomers, process solvents, and other volatile components from the SPS polymer. The feed mixture is typically discharged from a polymerization reactor or polymer recovery system at a temperature below 100° C., typically from about 10 to about 90° C. This mixture is then devolatilized in the presence of steam and optionally one or more other catalyst deactivating agents at a temperature between the glass transition temperature (typically around 100° C.) and the melting point of the devolatilized syndiotactic vinyl aromatic polymer (typically from 200 to 320° C.). To reduce the time necessary to achieve the desired level of volatiles removal, the feed mixture may be heated to a temperature of at least 110° C., more preferably to at least 125° C. prior to or simultaneously
Hiel Clement
Jackson Peter
Schellenberg Juergen
Veraart Rudi
Vossen Roel
Dow Global Technologies Inc.
Teskin Fred
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