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
1999-10-26
2001-06-26
Nutter, Nathan M. (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...
C525S193000, C525S210000, C525S221000, C525S222000, C525S227000, C525S232000, C525S233000, C525S234000, C525S235000, C525S238000, C525S239000, C525S241000
Reexamination Certificate
active
06251996
ABSTRACT:
BACKGROUND OF THE INVENTION
Regular treatment of the condensate water of power plants is necessary for preventing corrosion and maintaining efficient water flow rates. Condensate water commonly contains dissolved and suspended materials, especially oxidized iron commonly referred to as “crud”. If not maintained at low levels, crud can build up within the steam loop of the power plant causing pressure build up, reduced flow rates, reduced plant efficiency and expensive repairs. Proper crud removal is particularly important in boiling water nuclear reactors (BWRs) where crud can become radioactive as a result of passing through the “hot” side of the steam loop.
U.S. Pat. No. 4,975,201 (Re. 34,112), reissued Oct. 27, 1992 (incorporated herein by reference), discloses processes for treating power plant condensate water by contacting the water with a mixed bed ion exchange resin comprising cation and anion microporous copolymer beads. These beads include an interpenetrating network of polymer components made by way of known suspension “seeded” polymerization methodologies. As described, the copolymer beads may be made by a variety of seeded polymerization techniques including insitu-type single and second stage processes. These techniques typically include forming polymeric seed particles (e.g., a first polymer component), suspending the seeds in a suitable suspension medium and continuously adding (“con-add”) a polymerizable monomer (e.g., a second polymer component) thereto, thereby forming an interpenetrating polymer network. In one alternative approach, the seeds are imbibed with a monomer mixture (e.g., a third polymer component) which is substantially polymerized prior to the subsequent addition of the second polymer component noted above. As indicated, suspension polymerization methodologies are well known in the art, see for example U.S. Pat. No. 4,564,644, which is incorporated herein by reference. These resins have been found to have a high capacity for removing crud from condensate water in BWR nuclear power plants.
One drawback with current cation exchange resins is that they degrade with time to release a variety of sulfonated organic compounds. The mechanism for this resin degradation is believed to be due to an oxidative attack at the benzylic carbon groups of the resins. Subsequent chemical reactions cause the breakage of the copolymer chains. When two or more breakages occur between crosslinked groups, sulfonated organic compounds, i.e., “leachables”, are formed which can diffuse out of the resin and into the surrounding water. The molecular weight of these leachables is typically between 150 and 100,000 daltons. Under the high operating temperatures of the steam loop, these leachables can subsequently desulfonate and release highly corrosive inorganic sulfate into the water.
Anion exchange resins are effective at removing cation exchange resin leachables from solution so long as the total amount of leachables are low and so long as the molecular weight of the leachables is relatively low, i.e., below 10,000 (more preferably below about 5,000, and most preferably below about 1,000). Higher molecular weight leachables from the cation exchange resin are not effectively removed from solution by an anion exchange resin. As such, these leachables remain in the process water as an impurity. Additionally, high molecular weight leachables adsorbed by an anion exchange resin can lead to decreased kinetic performance of the anion exchange resin. Thus, it is desirable to reduce the average molecular weight of the cation exchange resin leachables while minimizing the total amount of leachables released into the process stream.
One way to reduce the total amount of leachables released by the cation exchange resin is to incorporate an antioxidant into the resin as described in U.S. Pat. No. 4,973,607; however, improved cation exchange resins are sought which release lower molecular weight leachables, and which release lower amounts of total leachables even without the addition of such antioxidants. It is further desired to provide cation exchange resins which provide improved crud removal properties.
Another approach to reducing leachables from cation exchange resins is to increase the extent of crosslinking of each polymer component used therein. Although the use of increased crosslinking does not reduce oxidative attack of benzylic carbon atoms, it does reduce the likelihood that any given cleavage of a benzylic bond will lead to a leachable species. Unfortunately, with conventional exchange resins the degree of crosslinking and capacity for crud removal are somewhat inversely related. Thus, as the extent of crosslinking is increased to reduce leachables, capacity for crud removal is compromised.
SUMMARY OF THE INVENTION
The present invention is a gel-type copolymer bead and method for making the same in which the subject copolymer beads have an interpenetrating polymer network of at least two polymer components including:
a first polymer component derived from a first monomer mixture comprising: a first monovinylidene monomer having a styrenic content less than about 50 molar percent;
a second polymer component derived from a second monomer mixture comprising: a second monovinylidene monomer having a styrenic content greater than about 50 molar percent and a second crosslinking agent; and
wherein the ratio of the molar percent crosslinking agent of the first polymer component to the second polymer component is less than about 0.7.
An object of the present invention is to provide gel-type copolymer beads which are useful as ion exchange resins and which have particular utility in the treatment of power plant condensate water. The subject beads provide ion exchange resins which are more effective at removing colloidal iron from condensate water, release fewer total leachables, and/or release leachables of a lower molecular weight.
A further object of the present invention is to provide methods for making the subject copolymer beads and methods for their use in treating aqueous solutions.
DETAILED DESCRIPTION OF THE INVENTION
As indicated previously, the mechanism for cation exchange resin degradation is believed to be due to oxidative attack at the benzylic carbon groups of the resins. Subsequent chemical reactions cause the breakage of the copolymer chains leading to “leachables” (i.e., cleaved portions of the crosslinked resin) which diffuse out of the resin and into the surrounding water. The present inventors have discovered that by selectively reducing the styrenic content of the polymer components most susceptible to oxidative attack, (i.e., those having relative lower molar percent crosslinking agent), one can significantly reduce the average molecular weight and/or the total quantity of leachables. For example, conventional seeded sulfonated resins useful in treating power plant condensate water comprise a seed component consisting of lightly crosslinked polystyrene, along with additional polymer components, (e.g., imbibed and/or continuously added) consisting of higher crosslinked polystyrene. By replacing at least 50 molar percent of the styrene content in the polymer component having the lowest degree of crosslinking (e.g., the seed component) with a non-styrenic monomer (e.g., acrylate, methacrylate, butadiene, ethylene, propylene, acrylonitrile, vinylidene chloride and vinyl chloride), one can produce a strong, high capacity resin having lower total leachables and/or leachables of a significantly reduced average molecular weight and/or leachables containing a lower sulfur content while simultaneously maintaining and even increasing the crud removal properties of the resin.
Pursuant to the present invention, the polymer component(s) that preferably have styrenic contents below about 50 molar percent (and more preferably below about 10 molar percent) are determined by the ratio between the molar percent crosslinking agent of any two polymer components. More specifically, if the ratio between the molar percent crosslinking agent of any two polymer components is less
Foster Kenneth L.
Stahlbush James R.
Black Edward W.
Nutter Nathan M.
The Dow Chemical Company
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