Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2001-07-11
2003-07-08
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S312000, C528S320000, C528S321000, C528S323000, C528S332000, C528S335000, C528S336000, C528S480000, C528S489000, C526S065000
Reexamination Certificate
active
06590064
ABSTRACT:
This invention relates to the use of metal oxides as heterogeneous catalysts in a process for producing polyamides by polymerization of lactams.
Polyamides based on &egr;-caprolactam are used for producing fiber, film and moldings. However, the hydrolytic polymerization of &egr;-caprolactam, which is one of the most important processes for producing nylon-6, does not go to completion. The crude polymer formed from caprolactam therefore includes a high level of &egr;-caprolactam and of low molecular weight reaction products (oligomer). Since both oligomer and caprolactam monomer are soluble and extractable in water, the level of low molecular weight constituents in the polymer is also known as its extractables content.
To prevent any impairment of product quality and of processing properties, for example during injection or extrusion molding or during spinning, the extractables content has to be lowered.
The requisite extraction is usually carried out with water at elevated temperatures, as described in DE-A 2 501 348, for example.
To increase the yield of the polymerization and to avoid any adverse impact on the environment, aqueous extracts are not disposed of as waste, but are recycled. To this end, for example, they are concentrated or extracted to be able to return their caprolactam monomer content into the polymerization.
The lower solubility of oligomer and especially dimer in the chips is not the least reason why complicated and energy-intensive processing steps have to be used for the extraction in order that polyamides of satisfactory quality may be obtained. In existing processes, caprolactam monomer is used as a solubilizer for lactam oligomer in the extraction of nylon-6. DE-A-43 24 616 therefore proposes adding caprblactam monomer to the water at the start of the extraction.
Processes are also known in which the extractables content is lowered by vaporizing the monomer and oligomer out of the polymer. DE-A-29 48 865 discloses a demonomerization process wherein the polymer is subjected to a vacuum in the molten state and in the form of thin films.
All the processes mentioned have the disadvantage of requiring in some instances multistage, costly and energy-intensive steps to demonomerize the polymer and to work up the aqueousextract.
It is therefore advantageous to produce polyamides which have a reduced extractables content and especially dimer content even in the unextracted state, so that the technical and economic effort needed to extract the low molecular weight constituents and to work up the aqueous extract can be reduced.
It is an object of the present invention to provide a process for producing polyamide where the formation of dimer and higher oligomer and the presence of monomer in the polymer is restricted and the polyamides produced thus have a low extractables content even before any extraction.
We have found that this object is achieved according to the invention by the use of metal oxides as heterogeneous catalysts in a process for producing polyamides by polymerization of lactams and optionally further polyamide-forming monomers wherein the metal oxides are used in a form which permits mechanical removal from the reaction mixture and are removed from the reaction mixture during or after the polymerization, to reduce the extractables content of the polyamide obtained.
A similar process is described in DE-A-198 08190, which has an earlier priority date than the present invention but was unpublished at the priority date of the present invention.
To produce low-extractables polyamides in the process of the invention, &egr;-caprolactam and optionally other polyamide-forming monomers are reacted, preferably continuously, in the presence of metal oxides which do not remain in the reaction mixture. Surprisingly, the process provides for a marked reduction in the level of dimer, the extraction of which is particularly difficult because of its solubility behavior in water.
The advantage of the present invention is that the total amount of low molecular weight constituents and especially dimers which have to be removed from the polymer in the subsequent extraction operation is reduced. This not only shortens the residence time in the extractor and thus raises the space-time yield, but also reduces the amount of extract to be worked up or disposed of. More particularly, the yield of dimers as well as of trimers and tetramers, which are more critical in removal and reuse than monomers can be reduced. Moreover, the reaction times may be shortened.
According to the invention, an aqueous lactam solution which optionally additionally contains other polyamide-forming monomers and has a water content of from 0.05 to 20% by weight, preferably of from 1 to 10% by weight, particularly preferably from 2 to 6% by weight, is reacted batchwise and preferably continuously in the melt phase at from 180 to 290° C. If desired, before being introduced into subsequent reaction stages, the monomers are melted and mixed with water in a makeup tank.
The metal oxide used can be any known metal oxide such as zirconium oxide, aluminum oxide, magnesium oxide, cerium oxide, lanthanum oxide and preferably titanium oxide as well as beta-zeolite and sheet-silicate. It was found that even silica gel and doped metal oxides, doped with ruthenium, copper or fluoride, for example, may distinctly reduce the extractables content. Preference is given to using metal oxides which are Broensted acidic. Particular preference is given to titanium dioxide in the anatase form. The anatase fraction is preferably at least 70%, particularly preferably at least 90%, especially 100%. According to the invention, the metal oxide bodies have a shape and size which permits mechanical separation of the polymer melt from the catalyst, for example by means of sieves and filters. The proposal is for the catalyst to be used in extrudate chip form or as coating on packing elements.
In another embodiment, the lactam is reacted with homogeneously dissolved acidic cocatalysts or a mixture of various catalytically active compounds in the presence of the abovementioned heterogeneous catalysts. The cocatalysts used here are preferably acidic catalysts selected from organic mono- and dicarboxylic acids, inorganic acids, their salts or mixtures thereof, such as the abovementioned carboxylic acids, terephthalic acid, adipic acid, propionic acid and isophthalic acid or oxygen-containing phosphorus compounds, especially phosphoric acid, phosphorous acid, hypophosphorous acid, the alkali metal and alkaline earth metal salts and ammonium salts, or oxygen-containing sulfur compounds, especially sulfuric acid and sulfurous acid.
ONE-STAGE PROCESS
If the mixture of starting materials is used in one reaction stage, this reaction stage preferably contains, the fixed or dumped bed of metal oxide bodies within the confines of the reactor in which the reaction mixture is present as a single liquid phase. In order that a high molecular weight may be achieved, the reactor should allow for the removal of water and an adequate residence time. Preferably, the polymerization in the single-stage embodiment is for that reason carried out in an upright downward-flow tube, a VK tube, which has a gas phase in the upper part. In this embodiment, the melt phase flows through the dumped metal oxide bodies. The reaction temperatures are set within the range from 180° C. to 270° C., preferably within the range from 190° C. to 250° C., particularly preferably within the range from 200 to 230° C. In a particularly preferred embodiment, the VK tube has a continuous or discontinuous temperature gradient whereby the temperature preferably rises from 180° C.-250° C. in the upper part of the flow tube to 240° C.-280° C. upstream of the outlet in the lower part of the flow tube.
TWO-STAGE PROCESS
The polymerization is preferably carried out in at least two reaction stages, the first stage being carried out under an elevated pressure and the second stage effecting the removal of a gas phase which contains condensation products such as water. The metal oxide bodies
Hildebrandt Volker
Mohrschladt Ralf
BASF - Aktiengesellschaft
Hampton-Hightower P.
Keil & Weinkauf
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