Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-10-17
2003-12-09
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S827000, C526S173000, C526S264000
Reexamination Certificate
active
06660800
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for making crosslinked polyvinylpyrrolidone. In particular, the invention is a process for reducing the toxicity and volume of aqueous waste generated in a base-catalyzed process for making the polymer.
BACKGROUND OF THE INVENTION
Polyvinylpyrrolidones have diverse utility. They are used in polymer films, adhesives, hair and skin-care formulations, pharmaceutical tablet binders and disintegrants, and beverage clarifiers. Polyvinylpyrrolidones are normally produced by free-radical or base-catalyzed polymerization of N-vinylpyrrolidone (NVP).
Free-radical initiators, such as hydrogen peroxide or organic peroxides, polymerize NVP to give polymers having relatively low molecular weight and a low degree of crosslinking. These products (e.g., “PVP K30” and “PVP K90”) are soluble in water and alcohols, and they can be purified by treating their solutions with adsorbants or ion-exchange resins (see, e.g., U.S. Pat. No. 4,795,802).
In contrast, crosslinked polyvinylpyrrolidone (“crosslinked PVP” or “PVP-P”) has a high molecular weight and a high degree of crosslinking. It is produced by base-catalyzed polymerization of NVP. Crosslinked PVP is usually produced by one of two general methods. In one approach, NVP is polymerized in the presence of an added difunctional crosslinker. In another approach, the crosslinker is generated “in situ” in a two-stage process. In the first stage, an aqueous mixture containing N-vinylpyrrolidone (NVP) and about 0.4 to 0.8 wt. %, based on the amount of NVP, of an alkali metal hydroxide (usually NaOH) is heated to about 140° C. to generate divinyl crosslinkers. After several hours, the mixture is cooled to about 100° C., and polymerization begins.
Crosslinked PVP is not soluble in water or alcohols. Therefore, impurities cannot be removed by forming a solution and treating it with, for example, an ion-exchange resin or activated carbon. Instead, a typical workup for PVP-P starts with extensive water washing to remove residual alkali metal hydroxide residues. Usually, the polymer is washed until the pH of the washings is close to 7. This is followed by washing with aqueous acid to neutralize base and convert residual NVP to the less-toxic 2-pyrrolidone. A final water wash is then used to remove traces of acid from the PVP-P.
A large volume of water is needed in the three steps to purify the polymer, so a lot of wastewater is generated. Consequently, a PVP-P manufacturer has high disposal costs. Moreover, because the waste-water from the initial washing step normally contains a high concentration (100-1000 ppm) of NVP, the manufacturer must find an environmentally acceptable way to dispose of this relatively toxic waste stream.
U.S. Pat. No. 5,239,053 teaches a process for purifying vinyl lactam polymers, including crosslinked and linear (uncrosslinked) PVP. The reference does not deal with issues of waste volume or toxicity. Residual NVP is eliminated by treatment with an acid or carbon dioxide. In the examples that show how to treat crosslinked PVP (see Examples 1, 9, 10, and 12), the polymer samples are first washed several times with water and are then “reconstituted” with water to give an aqueous mixture having an approximately neutral pH. These washing steps, which are performed prior to any acid or carbon dioxide treatment, generate an aqueous waste stream that contains a substantial amount of NVP. Ideally, such a waste stream would be avoided.
One way to avoid using large volumes of water is to simply reduce the amount of water in the aqueous wash solutions. Another possible solution is to skip water washing and use only aqueous acid (to remove NVP) followed by aqueous NaOH (to neutralize acid). Unfortunately, these approaches usually give PVP-P that does not meet at least one of the important product specifications. Crosslinked PVP used in beverage clarification, for example, requires a neutral polymer having residual NVP<5 ppm and residual Na<250 ppm.
In sum, the industry would benefit from improved ways of making crosslinked PVP. In particular, a process for making PVP-P that generates a reduced amount of aqueous waste is desirable. A process that produces aqueous waste streams that contain little or no N-vinylpyrrolidone is especially needed. Ideally, the process would give crosslinked PVP that meets or exceeds important product specifications.
SUMMARY OF THE INVENTION
The invention is a four-step process for making crosslinked PVP. First, an aqueous mixture that contains N-vinylpyrrolidone and an alkali metal hydroxide is heated in a sealed reactor under added pressure to generate a crosslinker. The reactor temperature is then reduced to initiate polymerization and produce a mixture that contains crosslinked polyvinylpyrrolidone (PVP-P) and residual N-vinylpyrrolidone. Water is added, and the resulting aqueous PVP-P mixture is heated in the presence of a protic acid at pH<4 to eliminate NVP. Finally, the PVP-P is neutralized with aqueous alkali metal hydroxide.
By using the process described above, we reduced the volume of aqueous waste generated to less than about 20 L/kg of PVP-P produced. Moreover, the purified PVP-P has residual NVP less than 5 ppm and residual alkali metal content less than 250 ppm. Importantly, none of the aqueous waste generated has a residual NVP concentration greater than about 10 ppm.
DETAILED DESCRIPTION OF THE INVENTION
The process of the invention gives high-quality crosslinked polyvinylpyrrolidone (PVP-P) in four steps while generating a reduced quantity of aqueous waste having relatively low toxicity.
In step one, a crosslinker is generated in situ. An aqueous mixture that contains from about 70 to about 90 wt. %, preferably from about 75 to about 85 wt. %, of N-vinylpyrrolidone (NVP) is heated in the presence of an alkali metal hydroxide. Suitable alkali metal hydroxides include sodium hydroxide, potassium hydroxide, lithium hydroxide, and the like. Sodium hydroxide is particularly preferred. The amount of alkali metal hydroxide used in this step is preferably less than about 1.0 mole %, more preferably less than about 0.7 mole %, based on the amount of N-vinylpyrrolidone used. This amount is relatively low compared with the amount generally used, which is typically 1.5 to 2.5 mole %. For example, a typical amount of sodium hydroxide used in the industry is about 0.6 wt. % (about 2 mole %).
The ability to use a low concentration of alkali metal hydroxide in step one is an advantage of the invention because it facilitates the preparation of PVP-P that meets product specifications for residual NVP and residual alkali metal content. There is a “domino effect” here: the less alkali metal hydroxide used in step one, the less acid needed for step three, and consequently, the less base needed for neutralization step 4, and the less residual alkali metal in the PVP-P.
The aqueous NVP and alkali metal hydroxide are heated at a temperature within the range of about 130° C. to about 150° C. to generate the crosslinker. A more preferred range is from about 135° C. to about 145° C.; most preferred is about 140° C. As discussed above, the idea of generating a crosslinker in situ prior to polymerization of NVP is well known, but it is normally generated at higher base concentrations.
Step one is performed under added pressure, preferably at least about 40 psig, more preferably at least about 50 psig. This is conveniently done by sealing the reaction vessel (ideally an autoclave reactor or the like) and pressurizing to at least about 40 psig prior to heating. A similar approach is described in WO 94/20555, which teaches that elevating the initial reactor pressure to at least 2 bars (about 29 psig) reduces the “induction time,” i.e., the amount of time needed for polymerization to begin once the temperature is dropped to about 100° C.
Most of the NVP polymerizes in step two. The reaction mixture from step one is simply cooled (or allowed to cool) to a temperature within the range of about 95° C. to about 105° C., preferably from about 98° C. to about 102° C., to
Nandi Manish K.
Steinmetz Beth M.
Arco Chemical Technology L.P.
Pezzuto Helen L.
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