Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-10-08
2003-08-19
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S228000, C526S346000, C521S056000, C521S060000, C521S146000
Reexamination Certificate
active
06608150
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to processes defined in the art of chemistry as free radical initiated polymerization processes, more specifically to the free radical initiated polymerization of styrene monomers. Still more specifically, it relates to such processes for the polymerization of expandable styrene polymers (“expandable polystyrene”). It also relates to compositions of matter employed in such processes and to the compositions and articles of manufacture produced thereby.
The established method to produce expandable styrene polymers, generically designated as EPS, is by aqueous suspension polymerization. It is typically a batch process where two or more monomer-soluble polymerization initiators are used with a rising stepwise, continuous, or combination temperature profile. Initiators for the process are selected on the basis of their half life temperatures to provide a measured supply of radicals at selected points along the temperature profile such that effective conversion occurs within an acceptable period of time. For styrene polymerization, it is convenient to describe initiator decomposition performance in terms of one hour half life temperature, defined as that temperature sufficient to cause decomposition of one half the starting concentration of initiator over a one hour time period.
Traditionally, suspension polymerization to prepare EPS is conducted in a process using two different temperature stages and two initiators with different half life temperatures, each appropriate for the particular temperature stage. Dibenzoyl peroxide (BPO) is often used as the first stage initiator at a reaction temperature of about 82° to 95° C. Other first stage initiators useful in this temperature range might include tertiary butyl peroxy-2-ethylhexanoate, tertiary amyl peroxy-2-ethylhexanoate and 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane. Initiators such as tertiary butyl peroxybenzoate (TBP) or dicumyl peroxide (DCP) are widely used for the higher temperature stage at 115° to 135° C. The second stage is usually a finishing step intended to minimize residual monomer in the EPS. In commercial processing, this stage is often held above 125° C. for prolonged intervals to reduce monomer content to acceptable levels.
EPS, as prepared in the suspension process, is in the form of essentially spherical beads with typical diameters of approximately 0.2 mm to 2.0 mm. In order to render the beads “expandable”, it is necessary to impregnate the polymer with a blowing agent, most often low molecular weight alkane hydrocarbons like butane, 2-methylbutane, pentane and cyclohexane. EPS can be prepared in a one-step process or a two-step process. The former allows blowing agent impregnation during the polymerization and has the obvious advantage of reduced polymer handling operations. The two-step process isolates the polymer beads and segregates by size prior to a separate impregnation operation. The advantage in this case is that of precise control of bead size, a more critical parameter in some polymer molding operations. Peroxide initiator concentrations used to expedite conversion in the present invention may readily be adjusted by one of ordinary skill in the art to accommodate either process.
Characteristic shortcomings of the traditional process are long reaction times necessary to obtain adequate conversion in the first stage and relatively high finishing temperatures required in the second stage. Initiators and their use conditions described in the present invention serve to address these deficiencies. Reduced conversion time offers obvious productivity benefits. Lower finishing temperatures may offer additional process advantages such as reduced oligomer content and reduced water content in the product polymer. Oligomers may contribute undesirable polymer properties and water incorporation may cause difficulties when fabricating EPS beads into molded articles. Reducing process temperature is also an increasingly important concern as energy costs rise.
There have been previous attempts to reduce reaction times for styrene suspension polymerizations. In U.S. Pat. No. 4,029,869, Ingram teaches a suspension polymerization process using difunctional monomer additives having asymmetrical reactivity to give more desirable distributions of molecular weight. Without such additives, higher temperature styrene suspension polymerization using tertiary butyl peroxybenzoate, while reducing conversion time, yields polymer with narrow molecular weight distribution (polydispersity index), unsuitable for polymer processing. The process of the present invention requires no such monomer additives to obtain suitable polydispersity.
U.S. Pat. No. 5,266,603 teaches use of the conventional two temperature stage process using at least two peroxide initiators having lower and higher initiation temperatures to obtain low residual benzene content employing particularly specified perketal and/or monoperoxycarbonate initiators as the higher temperature initiators. The particular peroxyketals specified are not “intermediate” half life temperature initiators such as are contemplated by the present invention and, in particular, the peroxy groups substituted on the aliphatic or cycloaliphatic chains of the perketals are limited to t-butyl peroxy groups. Such peroxy compounds and their close analogs are known to have a half life temperature outside the range for “intermediate” half life initiators as contemplated by the present invention.
In the Journal of Applied Polymer Science, Vol. 50, 327-343 (1993), Hamielec notes the generally recognized fact that styrene conversion rates for suspension polymerizations can be increased by elevating initiator concentration, but this causes unacceptably low molecular weight in the typical process. To overcome this circumstance, Hamielec resorts to somewhat higher temperatures and use of a symmetrical difunctional initiator with a one hour half life temperature of approximately 98° C. This improves conversion rate over BPO in a similar process and preserves much of the molecular weight. However, the process consumes a very substantial amount of the difunctional initiator and still appears to yield low polydispersity index. Also, no data is provided to assess effectiveness of the process to minimize residual monomer. The process of the present invention uses significantly lower concentrations of first stage initiators with one hour half life temperatures higher than that of BPO to more quickly obtain polymer of adequate molecular weight and relatively low residual monomer levels.
Glück et al. in U.S. Pat. Nos. 5,908,272 and 6,046,245, teaches the production of expandable styrene polymers using polymerization in aqueous suspension in the presence of two peroxides which decompose at different temperatures wherein the peroxide which decomposes at the higher temperature is multifunctional.
None of the above references teach or suggest the improvements provided by the present invention to the two temperature step process for the polymerization of styrene to produce expandable polystyrene.
OBJECTS OF THE INVENTION
An object of the present invention is to employ organic peroxide initiators included in a specific half life temperature range (optionally in combination with conventional peroxide initiators) to produce expandable styrene polymers at accelerated conversion rates.
Another object of the present invention is to use these specific organic peroxide initiators to obtain EPS resin with molecular weights suitable for typical EPS applications.
Another object of the present invention is to employ these specific organic peroxide initiators in a process with significantly reduced process finishing temperature while still obtaining relatively low (less than 1000 ppm) residual monomer levels in the final polymer.
SUMMARY OF THE INVENTION
The above objects have been realized by either partially or entirely replacing conventional peroxide initiators like BPO, TBP and DCP with peroxides whose one hour half life temperatures are higher than that of BPO
ATOFINA Chemicals, Inc.
Mitchell William D.
Teskin Fred
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