Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-08-03
2003-09-09
Wilson, Donald R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S486000, C528S487000, C528S491000
Reexamination Certificate
active
06617420
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for diminishing residual monomers contained in an N-vinyl compound polymer. The invention further relates to a process for producing an N-vinyl compound polymer solution or powder having a regulated pH.
DESCRIPTION OF THE RELATED ART
N-vinylpyrrolidone, N-vinylcaprolactam, N-vinyloxazolidone, N-vinylacetamide, N-vinylformamide, N-vinylimidazole and the like have advantages of biocompatibility, safety, hydrophilicity, etc., and therefore are utilized in various uses. In particular, N-vinylpyrrolidone makes it possible to synthesize polymers having a broad range of molecular weight of about 10 to 100 in terms of K value as determined by the Fikentscher's method, and such polymers are extensively used in various fields such as medicines, cosmetics, pressure-sensitive and other adhesives, coatings, dispersants, inks and electronic parts. Further, crosslinked vinylpyrrolidone polymers are useful as a water-absorbing resin in various applications where water absorption and water retention are required, e.g., paper diapers. By the way, there is the case that unpolymerized monomers may often remain in those N-vinyl compound polymer products. Amount of the monomer remained is generally in a level of 1% to several hundreds ppm, but even such a small amount of the residual monomers poses problems concerning, in particular, a fear of toxicity and odor. In recent years, there is a growing desire for diminishing the residual monomers contained in N-vinyl compound polymer products for use especially in medicines and cosmetics. Methods for removing a residual monomer from a vinylpyrrolidone polymer have been proposed. For example, JP-B-7-59606 (the term “JP-B” as used herein means an “examined Japanese patent publication”) proposes a method in which an aqueous vinylpyrrolidone polymer solution is treated with an adsorbent. Further, JP-W-7-503749 (the term “JP-W” as used herein means an “unexamined published PCT application”) proposes a method which comprises adding carbonic acid, formic acid, acetic acid, phosphoric acid or sulfuric acid to an aqueous vinyllactam polymer solution and heating the resulting mixture at 50 to 150° C. Use of an inorganic hydroxide as an example of a neutralizing agent to be used after the acid treatment is described therein.
The method disclosed in JP-B-7-59606 has a drawback that the aqueous solution to be treated by this method should be regulated so as to have a viscosity of 200 mPa·s or lower and, hence, the method is unsuitable for use in the treatment of polymers having a high viscosity. Further, for use of an adsorbent such as ion-exchanged resins, complicated procedures such as pre-washing or regeneration treatment of adsorbent must be conducted, resulting in increase of production costs.
The method disclosed in JP-W-7-503749 has a drawback that when carbonic acid, formic acid or acetic acid, which each are a volatile acid, is used at a reaction temperature of 80° C. or higher, then the acid volatilizes to escape from the system or move to the gaseous phase. As a result, it becomes impossible to maintain the pH of the solution at a desired value, making it difficult to speedily diminish the residual monomer. On the other hand, when the reaction temperature is reduced to 80° C. or lower, it becomes difficult to speedily diminish the residual monomer although acid volatilization is reduced. Furthermore, there have been cases where the acid volatilizes during the drying of the polymer solution, making it impossible to obtain a dried polymer having a desired pH value. A further problem of the proposed method is that use of an inorganic acid such as phosphoric acid or sulfuric acid results in an increased ash content in the resulting aqueous solution or in the dried solid. It has further been found the problem that where oxygen has entered in a reaction system, molecular weight of polymer reduces.
Moreover, there is a problem that in the case where an acid is added to an N-vinyl compound polymer to lower than pH of the system to below 7.0, the polymer deteriorates with the lapse of time, resulting in a decrease in molecular weight and, in the case of an aqueous solution, a decrease in solution viscosity. In addition, there has been a further problem that when an aqueous polymer solution having a pH lower than 7.0 is dried at a high temperature exceeding 100° C., the molecular weight of the polymer decreases. JP-W-7-503749 proposes a technique in which the polymer treated with an acid is neutralized with an inorganic hydroxide. However, use of an inorganic hydroxide poses a problem that the resultant aqueous solution or dried solid has an increased ash content. Furthermore, the present inventors have found that there is a problem that the presence of an inorganic hydroxide lowers the pH of the aqueous polymer solution or of the dried solid during drying or with the lapse of time and even deteriorates the polymer.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a method for diminishing residual monomers contained in N-vinyl compound polymers which can speedily diminish the amount of residual monomers to less than 100 ppm, preferably less than 10 ppm, and also is free from the drawbacks described above.
Another object of the invention is to provide a process for producing an N-vinyl compound polymer solution or powder having a regulated pH and free from the drawbacks described above.
As a result of intensive investigations to overcome the above drawbacks, it has been found that a method effective for diminishing a residual monomer comprises adding an organic acid having a boiling point of 140° C. or higher at ordinary pressure to an aqueous solution of an N-vinyl compound polymer. The organic acid having a boiling point of 140° C. or higher at ordinary pressure according to the invention does not volatilize even at high temperatures and functions to maintain the pH of the solution constant. Consequently, the residual monomer can be speedily diminished, whereby a polymer solution or a dried polymer each having a desired pH can be easily obtained. Furthermore, an ash-free aqueous polymer solution and an ash-free dried solid can be obtained.
A method for diminishing a residual N-vinyl compound monomer has further been found, which comprises adding at least one acid to an aqueous solution of an N-vinyl compound polymer, wherein the gaseous phase in the reaction vessel is regulated so as to have an oxygen concentration of 5.0% by volume or lower. When this method of the invention in which the gaseous phase in the reaction vessel is regulated so as to have an oxygen concentration of 5.0% by volume or lower is used, for example, vinyllactam polymer is prevented from suffering a decrease in K value (determined by the Fikentscher's method) even when treated with an acid at high temperatures. Thus, a vinyllactam polymer having a desired value of K can be obtained with satisfactory reproductivity.
As a result of further intensive investigations, there has been found a process for producing an aqueous N-vinyl compound polymer solution which comprises adding an organic base to an aqueous N-vinyl compound polymer solution containing an acid and having a pH lower than 7.0 to thereby neutralize the solution and regulate the pH thereof. By using an organic base according to this process of the invention, not only an ash-free aqueous solution and an ash-free dried solid can be obtained, but also the pH of the pH-regulated aqueous solution can be maintained constant over a prolonged period of time. Thus, an N-vinyl compound polymer which suffers no change in K value or pH during drying and during storage after drying and has excellent stability with the passage of time can be obtained.
The invention provides a method for diminishing a residual monomer which comprises adding an organic acid having a boiling point of 140° C. or higher at ordinary pressure to an aqueous solution of an N-vinyl compound polymer.
The invention further provides a method for diminishing a residual mon
Inui Yasuko
Kuriyama Toshiaki
Nippon Shokubai Co. , Ltd.
Wilson Donald R.
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