Method of continuously producing polyamide 6 using recycled lact

Details Method of continuously producing polyamide 6 using recycled lact Method of continuously producing polyamide 6 using recycled lact

1998-02-27

1999-10-05

Yoon, Tae

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Process of treating scrap or waste product containing solid...

523343, 528323, 528499, 210808, C08J 1104, C08G 6914

Patent

active

059625384

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a multistage method of continuously producing a polyamide 6 using recycled (returned) lactam which is obtained in extraction stages and melt demonomerization stages for reducing the low molecular mass fraction in polyamide 6.
It is known that the polycondensation of .epsilon.-capro-lactam to polyamide 6 is accompanied by the establishment of a chemical equilibrium which is dependent on temperature. At a customary plant reaction temperature of 280.degree. C., a water-soluble fraction of approximately 13% becomes established.
For the further processing of the polymer, it is necessary to remove the unreacted fraction. Various techniques are known for this purpose. For example, granules are produced from the melt at equilibrium, and these granules are extracted with water. This removes .epsilon.-caprolactam, oligomers and dimers. This technique necessitates subsequent drying of the granules and working up of the aqueous extraction waters by evaporative concentration.
Also known are techniques where unreacted .epsilon.-caprolactam, dimers and oligomers are driven off from melt films by means of vacuum or inert gas.
For economic reasons, these recovered products should be returned to the polymerization process.
There are techniques in which PA 6 extract water concentrates with high or low solids contents are added in lesser or greater proportions to the fresh lactam so that they too can be converted to polyamide 6. In accordance with DE-B 25 01 348 and DE-A 27 32 328, polyamide 6 extract waters concentrated to a solids content of more than 90% and, respectively, to a solids content of 60-70% are polymerized, together with more than the equal amount of fresh lactam based on the solids content, with propionic and, respectively, benzoic acid in a reactor which is operated essentially at atmospheric pressure.
A disadvantage of this technique is that when polymerizing polyamide 6 extract water concentrates with monocarboxylic acids, the polymerization times required are longer than when polymerizing fresh lactam with monocarboxylic acid. The conversions of .epsilon.-caprolactam and of cyclic oligomers obtained, moreover, are approximately 2 to 3% below those of fresh lactam polymers, since the cyclic oligomers play essentially no part in the reaction mechanism. The high water content in the reaction mixture, in conjunction with the unpressurized operation of the polymerization reactor, leads to an unfavorable reaction course. The water is unable to act in the reaction since it evaporates.
It is additionally known that melt vacuum demonomerization processes are operated such that the demonomerization products drawn off in gas form, i.e. .epsilon.-caprolactam and cyclic oligomers, are condensed in fresh lactam and so also subjected to the polymerization (DE-A 28 21 886 and DE-A 29 48 865). In the case of similar processes as well, the cyclic oligomers, and especially the cyclic dimers, are not transformed into reactive compounds.
Also known is the strong catalytic effect of o-phosphoric acid on the "Wirkungsmechanismus verschiedener Katalysatoren bei der the polymerization of caprolactam! in Faserforschung und Textiltechnik 13 (1962)6, 282-283! in relation to the polymerization of cyclic oligomers (Schade, H. and Beckhaus, F. W.: Verfahren zum Polymerisieren von of .epsilon.-caprolactam!--DD Patent 24 307). Disadvantages of such a polymerization technology are, inter alia: reproducibility of the desired degree of polymerization, since the o-phosphoric acid acts essentially as catalyst and less as chain regulator granules of this type.
In order to circumvent these above-mentioned problems, it is common when reprocessing the aqueous extraction waters to separate off the oligomers by distillation. The oligomers separated off are landfilled, which pollutes the environment, or else subjected to energy-intensive depolymerization to give lactam, by processing with phosphoric acid, and the lactam is subsequently passed to a purification stage. The residues are landfilled.
For technical reasons and on g

REFERENCES:
patent: 4053457 (1977-10-01), Cordes et al.
patent: 4327208 (1982-04-01), Lehr et al.
patent: 4376680 (1983-03-01), Horak et al.
patent: 5646191 (1997-07-01), Wiltzer et al.

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