Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-02-14
2003-01-28
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S173000
Reexamination Certificate
active
06512066
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for making crosslinked polyvinylpyrrolidone. In particular, the invention is a productive way to make crosslinked polyvinylpyrrolidone that is easy to isolate and has a low swell volume.
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). 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 are commonly known as “PVP K30” and “PVP K90.”
In contrast, crosslinked polyvinylpyrrolidone (hereinafter called “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 such as divinylimidazolidone. In an alternative method, the crosslinker is generated “in situ” by heating the reaction mixture at 130° C. to 140° C. in an initial stage. The temperature is then reduced to about 100° C. After an “induction period,” during which little or no polymerization occurs, the reaction rate increases rapidly, usually accompanied by an exotherm, and polymerization proceeds to completion. Unfortunately, the induction period can last for hours, which hampers productivity. Thus, an important challenge in making crosslinked PVP involves reducing or eliminating the induction period.
In PCT Int. Appl. WO 94/20555, Tseng teaches a process for making highly crosslinked PVP having a swell volume less than about 65 mL H
2
O/10 g polymer using an in situ-generated crosslinker. By performing the polymerization under an initial gas pressure of at least 2 bars (about 30 psig), Tseng was able to practically eliminate the induction period. Under similar conditions (described in U.S. Pat. No. 3,277,066), but without the added gas pressure, an induction period of 2-3 hours is typically observed.
While eliminating the induction period is valuable, Tseng's process starts with a relatively high concentration of N-vinylpyrrolidone (75-85 wt. %) in the aqueous mixture. Consequently, the resulting crosslinked PVP mixture is highly concentrated, which makes the polymer difficult to isolate. Ideally, a more dilute solution of N-vinylpyrrolidone could be used to facilitate polymer isolation. Moreover, the polymer produced in this process usually has a swell volume of about 50-60 mL H
2
O/10 g polymer. For some applications, PVP-P having a somewhat lower swell volume (30-40 mL H
2
O/10 g polymer) is desirable.
U.S. Pat. No. 5,286,826 teaches a two-stage process for making highly crosslinked PVP having a low swell volume. A relatively concentrated aqueous NVP solution (80 wt. %) is heated in the presence of base under low pressure (0-3 psig) to generate an in situ crosslinker. Next, the solution is cooled to about 30° C., and is diluted with water to reduce the NVP concentration to 5-30 wt. %. The mixture is then reheated to 100° C., whereupon polymerization proceeds to give a product having a swell volume of less than 45 mL H
2
O/10 g polymer. The process is time-consuming because of the need for cooling, diluting with water, and reheating after generating the crosslinker.
Still needed is a productive process for making crosslinked PVP. Preferably, the induction period for making the PVP-P would be minimized or eliminated. A valuable process would operate at relatively low concentrations of N-vinylpyrrolidone to facilitate polymer isolation. Preferably, the process would avoid the need for a water dilution step. Ideally, the process would give crosslinked PVP having a low swell volume.
SUMMARY OF THE INVENTION
The invention is a process for making crosslinked polyvinylpyrrolidone. The process comprises two steps. In a first step, an aqueous mixture that contains from about 65 to about 70 wt. % of N-vinylpyrrolidone and from about 1.5 to about 6.0 mole % of an alkali metal hydroxide (based on the amount of N-vinylpyrrolidone) is heated under added inert gas pressure. This heating step, which generates a crosslinker in situ, occurs in a sealed reactor at a temperature within the range of about 130° C. to about 150° C. at an ultimate reactor pressure of at least about 55 psig. In a second step, the reactor temperature is reduced to a value within the range of about 95° C. to about 105° C. to initiate polymerization.
The two-step process is an easy and productive way to make crosslinked polyvinylpyrrolidone. Applying pressure in the first step effectively reduces or eliminates the induction period. Surprisingly, a relatively dilute aqueous solution of N-vinylpyrrolidone can be used, so the polymer product is exceptionally easy to isolate and purify. The resulting PVP-P has a swell volume less than about 40 mL H
2
O/10 g polymer, which makes it particularly valuable for beverage clarification.
DETAILED DESCRIPTION OF THE INVENTION
The invention is a two-step process for making crosslinked polyvinylpyrrolidone by base-catalyzed polymerization of N-vinylpyrrolidone. As used herein, “crosslinked polyvinylpyrrolidone,” “crosslinked PVP,” and “PVP-P” refer to polymers of N-vinylpyrrolidone that are highly crosslinked and insoluble in water. Preferably, N-vinylpyrrolidone is the only monomer used. However, minor amounts (up to about 10 mole %) of other ethylenic monomers, including other N-vinyl monomers, can be included.
Step One: Generation of an “In Situ” Crosslinker.
In the first step, an aqueous mixture that contains N-vinylpyrrolidone (NVP) and an alkali metal hydroxide is heated under added inert gas pressure. The aqueous mixture contains from about 65 to about 70 wt. % of NVP. If the NVP concentration is significantly less than about 65 wt. %, the induction period is too long (see Comparative Example 3, below) or polymerization does not occur (see Comparative Example 4). On the other hand, if the NVP concentration is much greater than about 70 wt. %, the polymer product is too concentrated and is more difficult to isolate and purify. Moreover, when the NVP concentration is as high as 80 wt. %, the swell volume of the polymer is usually significantly greater than 40 mL H
2
O/10 g polymer (see Comparative Examples 7 and 8).
The aqueous mixture contains an alkali metal hydroxide. Suitable alkali metal hydroxides include sodium hydroxide, lithium hydroxide, potassium hydroxide, cesium hydroxide, and the like. Potassium hydroxide and sodium hydroxide are economical and effective, and are therefore preferred. Sodium hydroxide is particularly preferred.
The amount of alkali metal hydroxide used is within the range of about 1.5 to about 6.0 mole % based on the amount of N-vinylpyrrolidone used. For example, 2.5 mole % of sodium hydroxide would be 0.025 moles of sodium hydroxide for every mole of N-vinylpyrrolidone used. A more preferred range is from about 2.0 to about 4.0 mole %. If the concentration of alkali metal hydroxide is too low, it is often difficult to achieve a swell volume that is less than 40 mL H
2
O/10 g polymer (see Comparative Example 5). Most preferably, about 2.8 mole % of sodium hydroxide is used.
The reaction temperature in the first step is within the range of about 130° C. to about 150° C., preferably from about 135° C. to about 145° C. Most preferably, the reaction temperature is about 140° C. The reaction mixture is held at this temperature for a time effective to generate “in situ” a difunctional crosslinker mixture, which typically includes mostly ethylidene vinylpyrrolidone (EVP) and a small fraction of ethylidene bis(vinylpyrrolidone) (EBVP). The in situ-generated crosslinkers act as starters for the polymerization, which occurs in step 2. The polymerization is sometimes called “popcorn” polymeri
Arrell Robert M.
Steinmetz Beth M.
Arco Chemical Technology L.P.
Pezzuto Helen L.
Schuchardt Jonathan L.
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