Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2002-05-15
2004-03-02
Boykin, Terressa (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S316000, C528S323000, C528S481000, C528S499000
Reexamination Certificate
active
06699962
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides a process and a device for removing low molecular weight constituents from polyamide 6 or from copolyamides based on caprolactam (herein (co)polyamides) by extraction.
The present invention also provides a process and a device for preparing (co)polyamides).
SUMMARY OF THE INVENTION
A process for removing low molecular weight constituents from polyamide 6 or from copolyamides based on caprolactam (herein (co)polyamide) is disclosed. The process comprise subjecting the (co)polyamide to liquid-extraction at a temperature which is above the boiling point of the liquid at atmospheric pressure said liquid containing water in an amount of at least 80 percent relative to its weight.
BACKGROUND OF THE INVENTION
Polyamides, their preparation and their use as industrial plastics are known. They are described, for example, in “Kunststoff-Handbuch” edited by G. W. Becker and D. Braun, published by Hanser-Verlag in Munich and Vienna 1998, vol. 3 “Technische Thermoplaste”, section 4 “Polyamide”.
The present invention provides polyamide 6 and copolyamides based on caprolactam. Polyamide 6 is the homopolymer of caprolactam. Copolyamides based on caprolactam, in the context of the present invention, are copolyamides which contain at least 50 wt. % of monomer units derived from caprolactam. In addition, these copolyamides based on caprolactam contain other repeating units linked via amide groups. For example, these repeating units may be those which are derived from adipic acid and hexamethylene diamine, that is polyamide 6,6 units. They may also be other repeating units which are derived from other dicarboxylic acids and other diamines. Or they may be repeating units which are derived from aminocarboxylic acids, for example the repeating units in polyamide 11 or polyamide 12.
(co)polyamides may be prepared by a number of processes. For example, they may be prepared by hydrolytic polymerisation of caprolactam. This is the most important industrial-scale process. It is also the process which is preferred according to the present invention. Furthermore, they may be prepared by alkaline lactam polymerisation. The processes for preparing (co)polyamides are described, for example, in the manual mentioned above, Kunststoff-Handbuch, vol. 3 “Technische Thermoplaste”, section 4 “Polyamide”, on pages 22 to 75.
(co)polyamides are generally prepared by the hydrolytic polymerisation of caprolactam. If copolyamides based on caprolactam are intended to be prepared, the comonomers required are also added. The comonomers are preferably either a mixture of dicarboxylic acids and diamines or aminocarboxylic acids or lactams.
Hydrolytic polymerisation, as is described, for example, in the manual mentioned above, Kunststoff-Handbuch, vol. 3 “Technische Thermoplaste”, section 4 “Polyamide”, in sub-section 2.2.2.2.4, is generally performed as a polymerisation under atmospheric pressure by adding usually 1 to 5 wt. % of water at temperatures of usually 240 to 270° C. with the exclusion of atmospheric oxygen. Reaction may be started with water-eliminating compounds such as aminocarboxylic acids or AH salt (salt of adipic acid and hexamethylene diamine). It is usually started with water. The molecular weight of the polyamide is predetermined by the water content, but in practice is normally stabilised by regulators, for example by carboxylic acids or amines.
(co)polyamides are usually prepared continuously. This continuous preparation usually takes place in vertical reactors, so-called VK tubes (VK=simplified continuous). The following procedure is normally used: The VK tube is filled from the top with aqueous caprolactam (optionally together with comonomers and optionally together with further auxiliary substances such as e.g. molecular weight regulators). Excess water is distilled off here in order to achieve a sufficiently high molecular weight. The molten material then flows through the tube in usually 15 to 30 hours at atmospheric pressure, wherein it is maintained at a temperature between usually 240 and 270° C. At the end of the VK tube, the molten polyamide is forced into a water-bath through round-section dies as threads, for example using a gear pump, cooled and then granulated.
Low molecular weight compounds are produced as secondary products during the preparation of (co)polyamides. These are in particular oligomeric compounds which are produced from caprolactam and optionally also comonomers. These low molecular weight compounds are detrimental to the general properties of (co)polyamides and are therefore usually removed. Cyclic dimers of caprolactam in particular exert a detrimental effect on the general properties of (co)polyamides.
The low molecular weight fractions lead to products which are prepared from polyamides, such as e.g. injection moulded items or films and the like, being of poor quality because the low molecular weight fractions diffuse to the surface and form a greasy film there. In addition, low molecular weight fractions which diffuse to the surface lead to a deterioration in the surface quality of products made of polyamides. The gloss is reduced and the colour itself is impaired.
Polyamides which are intended to be processed to form films require an especially low concentration of low molecular weight compounds. Immediately after polymerisation, polyamide 6 and copolyamides based on caprolactam typically contain a proportion of low molecular weight compounds (caprolactam and its oligomers and optionally oligomers of comonomers) of more than 5 wt. %. Polyamides which are intended to be processed to form injection moulded goods should usually have a concentration of low molecular weight compounds of less than 1 wt. %, preferably less than 0.5 wt. %. Even higher requirements are usually placed on polyamides which are intended to be processed to form films. In this case, a concentration of low molecular weight compounds of 0.1 wt. %, preferably 0.05 wt. %, should not be exceeded.
The low molecular weight compounds may be removed, for example, by extraction. Extraction is usually performed using water or liquids which contain mainly water.
After polymerisation and subsequent extraction, the polyamide 6 or copolyamide based on caprolactam obtained is usually dried. This takes place, for example, at temperatures between 90 and 130° C. in a stream of an inert gas. Nitrogen may be used, for example. The molecular weight of the polyamide may be increased for special applications, by so-called post-condensation, by increasing the drying temperature to 180 to 190° C.
Instead of the one-step VK tube mentioned above, plants with multi-step VK tubes or plants incorporating an upstream pressure-less prepolymerisation step may be used.
According to the prior art, extraction of (co)polyamides is performed batchwise or continuously with water at temperatures which are below the boiling point of water at atmospheric pressure (i.e. below 100° C.).
The processes disclosed in the prior art for preparing polyamide 6 and copolyamides based on caprolactam which include extraction of polyamide 6 or of copolyamides based on caprolactam, have disadvantages. One disadvantage of the known processes is in particular that a high residence time is required in the extraction devices in order to undershoot a certain concentration of the low molecular weight fraction in the extracted polyamide. That means that the space-time yield for the extraction process is low. In addition, the results of extraction by the known industrial processes are frequently not good enough. In particular, the proportion of low molecular weight substances left in the polyamide is too high. In particular, the proportion of cyclic dimers of caprolactam left in the polyamide is too high.
In addition, processes known from the prior art for extracting have the disadvantage that they require too large an amount of water per unit weight of the polyamide being extracted. Smaller amounts of water for extraction are desirable because this would simplify the process, because it would enable the eq
Gittinger Andreas
Göbbels Dieter
Haupt Heinrich
Triebeneck Konrad
Boykin Terressa
Matz Gary
Preis Aron
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