Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Utility Patent
1999-03-31
2001-01-02
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S313000, C528S322000, C528S332000, C528S335000, C528S336000, C528S340000
Utility Patent
active
06169161
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing polyamides by polymerizing materials comprising essentially of caprolactam. More precisely, it relates to an efficient method for producing polyamides by polymerizing materials comprising essentially of caprolactam, in which the unreacted caprolactam content and the oligomer content are both reduced to a satisfactorily low level.
BACKGROUND OF THE INVENTION
As having good properties, polyamides produces from materials comprising essentially of caprolactam are used as fibers for clothing and industrial fibers, and are further widely used as injection moldings in various fields of automobiles, electric and electronic appliances and others, and as extruded films and stretched films essentially in the field of food wrapping and packaging.
Polyamides produced from materials comprising essentially of caprolactam are generally produced by heating caprolactam in the presence of a small amount of water. The production process is relatively simple and is widely employed in industrial fields. One typical method of caprolactam polymerization, which is described in “Polyamide Resin Handbook” (edited by Osamu Fukumoto, published by Nikkan Kogyo Shinbun-sha in 1988), pp. 63 to 65, is summarized below.
Caprolactam is melted in a melter, to which is added a polymerization degree regulator such as acetic acid or the like. The starting materials are fed into an atmospheric-pressure polymerization tower heated at about 260° C., and kept there for about 10 hours. The resulting polyamide is taken out from the bottom of the tower in the form of strands into water, and the strands are pelletized. The resulting polyamide (nylon 6) pellets contain a large amount of caprolactam monomer and oligomer resulting from polymerization equilibrium. Therefore, they are transferred into a hot water extraction tower, in which the pellets are countercurrent-wise contacted with hot water fed from the bottom of the tower whereby the monomer and others are extracted and removed from the pellets, and thereafter the pellets are taken out from the bottom of the tower. The extracted pellets contain a large amount of water, and are therefore dried in vacuum or in an inert gas atmosphere at about 100° C.
Thus, the polymerization of caprolactam is extremely simple, but must be indispensably followed by the hot water extraction step and the subsequent drying step for the purpose of removing the unreacted caprolactam and the oligomer from the polymerization product. The costs for the two post-treatment steps are high. Therefore, it is desired to develop a process not requiring the two steps.
Given that situation, there have been proposed a method of removing the unreacted caprolactam and the oligomer from the polymerization product, by processing the molten polyamide at a high temperature in a high vacuum; and a method of reducing the amount of the unreacted caprolactam and the oligomer formed during the polymerization.
For example, in U.S. Pat. No. 3,558,567, there has been proposed a method of processing a molten prepolymer in a flusher or finisher, to thereby remove the unreacted caprolactam and the oligomer from it under reduced pressure.
In U.S. Pat. No. 3,741,939, there has been proposed a high-pressure polymerization method for reducing the amount of the unreacted caprolactam and the oligomer formed during polymerization.
In JP-B 50-26594, there has been proposed a low-temperature polymerization method for reducing the amount of the oligomer.
The production method disclosed in U.S. Pat. No. 3,558,567 comprises polymerizing caprolactam with a water content of from 0.2 to 1.5%, at a temperature between 225 and 280° C. for about 1 hour to a conversion of lower than 45%, followed by heating it at a temperature between 250 and 300° C. under pressure. This method leads to a reaction product of which the unreacted caprolactam content is equal to its equilibrium concentration and the oligomer content is lower than 2% by weight. In Example 1 in this U.S. Patent, the unreacted lactam content and the oligomer content of the reaction product produced are 12% and 1.9%, respectively, and the reaction product is processed in a flusher at 50 mmHg for 10 minutes, and then further in a single-screw finisher at 280° C. and 5 mmHg for 120 minutes, thereby obtaining a polyamide having an unreacted caprolactam content and an oligomer content of lower than 0.1% and 1.7%, respectively. In Example 2 and Example 3 therein, the same process as in Example 1 is repeated under different conditions to obtain a polyamides having an oligomer content of 1.3%. In the proposed method using such a flusher and a finisher, the extraction step and the drying step that are indispensable in the prior art could be omitted. However, the process itself of the method is still complicated, and, in addition, the oligomer content accomplished by the method is not as yet satisfactory.
The production method disclosed in U.S. Pat. No. 3,741,939 comprises heating caprolactam at a temperature between 120 to 320° C. and at a pressure higher than 2000 atmospheres. However, the lowest content of extractables accomplished by the method is 2.4%. This value is still so high that the extraction step can not be omitted. In addition, the equipment cost and even the equipment maintenance cost for the ultra-high-pressure process of the method are high, and the process itself is dangerous. Therefore, the method can not be practically used.
The method disclosed in JP-B 50-26594 comprises polymerizing caprolactam with a water content of not higher than 0.5% by weight at a temperature between the melting point of the polyamide to be produced and a temperature of “melting point +20° C.” to produce a polyamide having a reduced cyclic oligomer content. The lowest cyclic oligomer content accomplished by the method is 0.9% by weight. In this example, however, the unreacted caprolactam content of the polymer is 15.76%, which is high. Therefore, this example faces the problem of low polymer yield. Further, in this example, the polymer produced contains 0.45% by weight of a linear oligomer, in addition to the cyclic oligomer. Consequently, the total oligomer content of the polymer produced is 1.35% by weight. In view of these data, it is in fact difficult to omit the extraction step in this method.
As in the above, an practicable method has heretofore been unknown for producing polyamides from materials comprising essentially of caprolactam, in which high-quality polyamides with a low content of unreacted caprolactam and oligomer can be obtained in high yield without using a hot water extraction step.
SUMMARY OF THE INVENTION
The object of the invention is essentially to provide an efficient method with high producibility for producing polyamides having a low content of unreacted caprolactam and oligomer, which does not require any hot water extraction step for removing the unreacted caprolactam and oligomer, and therefore, the energy cost is low.
Specifically, the invention is to provide a method for producing a polyamide having an unreacted caprolactam content of from 0 to 0.5% by weight and an oligomer content of from 0 to 1.25% by weight and having a relative viscosity of 2.2 to 8.0 in a 0.01 g/ml solution of sulfuric acid at 25° C., which comprises a first step of polymerizing a starting material comprising caprolactam, under atmospheric pressure to prepare a polymerization product having an unreacted caprolactam content of not larger than 25% by weight and an oligomer content between 0 and 1.8% by weight and a second step of heating the polymerization product as prepared in the first step, at a temperature T satisfying the following formula (1) and under reduced pressure to thereby remove the unreacted caprolactam and the oligomer from it:
Glass transition point of polyamide<T<melting point of polyamide+150° C. (1)
wherein the glass transition point and melting point of the polyamide are measured by differential scanning calorimetry (DSC) are using a sample prepared by rapidly cooling a molt
Okita Shigeru
Onishi Koji
Tachibana Yasuto
Hampton-Hightower P.
Miller Austin R.
Toray Industries Inc.
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