Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
1999-10-25
2001-03-13
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S322000, C528S323000, C528S328000, C528S332000, C528S336000, C528S362000, C528S363000
Reexamination Certificate
active
06201096
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT “NOT APPLICABLE”
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for the preparation of polyamides. More specifically, the invention relates to a continuous process for preparing polyamides by reaction of omega-aminonitriles in a countercurrent multistage reactor swept with steam.
2. Description of the Prior Art
Polyamides are conventionally prepared by the condensation polymerization of a diacid, such as adipic acid, and a diamine such as hexamethylene diamine, or by the polymerization of lactams such as &egr;-caprolactam. Other processes are known which involve preparation of polyamides by reaction of omega-aminonitriles with water. For example, Greenwalt U.S. Pat. No. 2,245,129 discloses preparation of polyamides by heating omega-aminonitriles in the presence of water in a two step process. In the first step, the reaction mixture is heated in a closed reaction vessel to form a low molecular weight polyamide. In the second step, the liberated ammonia and excess water is removed at atmospheric or reduced pressure with simultaneous and/or subsequent heating of the polyamide to increase the molecular weight of the polyamide. Curatolo et al. U.S. Pat. No. 4,568,736 discloses using oxygen-containing phosphorus compounds as catalysts during the reaction of omega-aminonitriles to form polyamides. Marks U.S. Pat. No. 5,109,104 discloses the batch polymerization of omega-aminonitriles to produce high quality polyamides. In international patent application PCT/EP/97/0460. corresponding to international publication number WO 98/0889. a process for producing a polyamide by reacting an aminonitrile with water at sequential staged temperatures and staged pressures is disclosed. It is also known to produce polyamide within a continuous reactive distillation column involving significant rectification of an aqueous salt solution of a diamine and dicarboxylic acid as disclosed for example in U.S. Pat. No. 3,900,480. In a recent international patent application PCT/EP/98/08239, corresponding to international publication number WO 99/43732 published after the priority date of the present application, the reaction of an aminonitrile with water by reactive distillation is taught.
Most of the processes that have been disclosed in the art have been batch, with attendant potential disadvantages of high operating labor, within-batch final product non-uniformity, and batch-to-batch product non-uniformity. The current invention provides a continuous process for preparing polyamides from omega-aminonitriles which overcomes these problems.
BRIEF SUMMARY OF THE INVENTION
In view of the above, the present invention provides a continuous process for the manufacture of polyamide from omega-aminonitrile in a vertical counter-current multistage reactor wherein temperature control for individual stages is critical to achieve a usable product efficiently. More specifically, the polyamide capable of making useful product (i.e. polyamide prepolymer capable of ultimately producing a number average molecular weights in excess of 14,000) is characterized for purposes of this invention as containing unreacted nitrile end groups of less than 20 gram equivalents terminal nitrile per million grams of polyamide. Furthermore, the polymeric linearity of useful polyamide is characterized as containing less than 25 and preferably less than 10 gram equivalents of secondary amino branching per million grams of polyamide.
In order to achieve the production of commercially useful product efficiently in a continuous counter-current reactor operation, it has now been discovered that the temperature profile along the vertical column and the reactor pressure must be controlled. More specifically, the selection of reactor conditions (particularly of the top stages of the reactor) must assist in the absorption of volatile starting reactants and simultaneously minimize their loss in the exit steam of volatile by-products. Furthermore, the selection and control of the reactor conditions must assist in directing of the counter-current flow of volatile by-products to the upstream part of the process where they are less likely to chemically limit or reverse the polymerization reaction.
Both of these benefits are partially realized in the process of the present invention, for example in the case where a catalyst is added to the liquid feed stream, by virtue of the temperature of at least one top stage of the reactor being maintained between 190 to 220° C., the temperature of at least one bottom stage of the reactor being maintained between 260to 290° C. and the pressure being maintained between 100 and 300 psig. In the case where no catalyst is added to the liquid feed stream the temperature of at least one top stage of the reactor is to be maintained between 230 to 250° C., the temperature of at least one bottom stage of the reactor is maintained between 260 to 290° C. and the pressure is maintained between 400 and 800 psig. In furtherance of the above, the discrete stages of the present invention provide locations where heat can be readily supplied or removed in order to control stage temperature and composition. A preferred temperature profile along the column involves the top stage temperature being 2 to 10° C., most preferably 5° C., hotter than the temperature corresponding to a partial pressure at which the pressure of pure stream is numerically equal to the operating pressure of the reactor. Simultaneously, the lower stages of the reactor for this preferred temperatures profile involve maintaining a temperature between 260 and 290° C., preferably about 270° C.
Thus, the present invention provides a continuous process for the manufacture of polyamide from omega-aminonitrile comprising the steps of:
a) providing a vertical countercurrent multistage reactor, the reactor being equipped with internal perforated barrier means for establishing a plurality of stages and for effecting contact of countercurrently flowing omega-aminonitrile stream and steam vapor stream;
b) introducing an omega-aminonitrile reactant stream near the top of the vertical countercurrent multistage reactor;
c) introducing a steam vapor containing stream near the bottom of the vertical countercurrent multistage reactor;
d) maintaining the temperatures and the pressure within the vertical countercurrent multistage reactor sufficiently high to achieve hydrolysis of the omega-aminonitrile in the upper stages of the reactor without excessive volatilization and simultaneously sufficiently high to achieve polymerization in the lower stages of the reactor without excessive degradation;
e) withdrawing a steam and ammonia containing stream overhead of the vertical countercurrent multistage reactor; and
f) recovering a polyamide containing product stream from the bottom of the vertical countercurrent multistage reactor, wherein said polyamide is characterized as having unreacted nitrile end groups of less than 20 gram equivalents per million grams of polyamide.
Preferably, the linearity of the polyamide being recovered is further characterized as containing less than 25 gram equivalents of secondary amine branching per million grams of polyamide, and most preferably less than 10 gram equivalents of secondary amine branching per million grams of polyamide.
The present invention further provides one preferred embodiment of the process wherein the manufacture of polyamide from omega-aminonitrile is performed in the presence of catalyst and wherein the temperature of at least one top stage of the reactor is maintained between 190 to 220° C., the temperature of at least one bottom stage of the reactor is maintained between 260 to 290° C. and the pressure is maintained between 100 and 300 psig and wherein polymeric linearity of the polyamide being recovered is further characterized as containing less than 10 gram equivalents of secondary amine branching per million grams of polyamide.
The present invention provides another preferred embodiment of the process wherein the manufac
Marchildon Ernest Keith
Mutel Ahmut Turgut
E. I. Du Pont de Nemours and Company
Hampton-Hightower P.
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