Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2002-09-26
2004-03-23
Boykin, Terressa (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C422S131000, C422S135000, C524S451000, C264S176100, C264S219000
Reexamination Certificate
active
06710159
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for producing a polyamide or copolyamide freed from volatile constituents by removing the volatile constituents, in particular the monomers and dimers, in at least two stages from the polymer melt using an, in particular, gaseous entrainer. The invention also relates to polyamide compositions made of polyamide 6 or a copolyamide with a reduced content of volatile constituents, as well as moulding compositions made of the above-mentioned polyamides, and moulded articles and fibres which are produced from these moulding compositions.
BACKGROUND OF THE INVENTION
When producing polyamide 6 (PA 6) and using copolyamides with PA 6 as the main constituent, the conversion in the reaction is limited by a chemical equilibrium. As a result, the mixture leaving the reaction at 270° C. still contains residual monomers (&egr;-caprolactam) and oligomers (cyclic dimers, cyclic trimers, cyclic tetramers etc., which are called dimers, trimers or tetramers hereinafter) in a total quantity of approximately 10%.
Polyamide is distinguished from other polymers, such as polycarbonate, SAN resin or polystyrene, in that the volatile components (in particular the oligomers) have an extremely low vapour pressure in the reaction mixture during production (for example the vapour pressure of the dimer of &egr;-caprolactam at 300° C. is only 100 mbar). To process the product (for example by the injection moulding process) it is necessary to reduce the dimers as well as the monomers to a very low residual content as undesirable effects, such as depositions in the injection moulding tool or other parts of the machinery, otherwise occur during processing and a thick covering forms in the case of dark coloured conditioned polyamide moulded articles.
Owing to the low vapour pressure of the dimer, the reduction of monomers and dimers by an evaporation process requires a high processing temperature (typically 250 to 320° C.) and evaporation at very low pressure. The high temperature is also required to keep the product flowable and pumpable. The high temperature in an evaporation process is generally detrimental to the quality of the product (the product tends to yellow and chain breakages occur in the polymer). Therefore, short residence times are required when carrying out the reaction.
Initially, effective heat transfer from the heating medium to the product is required in an evaporation process in order to obtain the evaporation enthalpy necessary for evaporating the volatile constituents at the lowest possible heating temperature. At the end of an evaporation process when only small monomer or oligomer contents are still present, mass transfer by diffusion is crucial to the outcome of degassing. This necessitates a high driving force for the diffusion process and a low partial pressure of the components in the gaseous phase, large material exchange surfaces and the longest possible residence times under vacuum.
Two fundamentally different procedures are known for reducing the volatile components in polyamide 6 and copolyamides with PA 6 as the main constituent.
In the extraction processes described in Offenlegungsschriften DE 43 24 616 A1 and DE 19 505 150 A1, the highly viscous reaction mixture is cooled and granulated after the reaction. The monomers and oligomers contained in the granules are then extracted with water or a caprolactam/water mixture formed or pure caprolactam by counter-current extraction lasting approximately 24 hours. The extracted granules are dried in a drying stage which also lasts 24 hours. The aqueous solution from extraction, which now contains the monomers and oligomers, is concentrated in an evaporation stage. The so-called concentrated liquor produced in the process is supplied to the reaction, the evaporated water to the extraction. The granules obtained by this process can have very small volatile constituent contents (content of &egr;-caprolactam ≦0.35%, dimer ≦0.15%, trimer ≦0.25% and tetramer ≦0.3%). Furthermore, the exposure to heat is comparatively low.
The major disadvantage of this process is the very long residence time in the extraction and drying phases and the associated very bulky apparatus for large throughputs of product. This means that a high level of expenditure is required to carry out such a process. A further disadvantage is the large amount of energy required for drying and evaporating the extracted water.
Evaporation processes, in particular mechanical processes, are also known. Machines such as extruders (single- or multi-screw extruders) or thin film evaporators are often used for evaporating polymers (cf. DE 3 310 676.A1, DE 4 328 013.A1 or U.S. Pat. No. 3,630,689). The disadvantages of mechanical processes are the expensive evaporation extruders or thin film extruders, the wear of rotating parts and the difficulties in handling the very heavy machine parts required. In particular with a high throughput of product, there is also a high mechanical load on the product which leads to a very high product temperature and subsequent damage to the product. In addition, these processes do not generally satisfy the high demands placed on industrial polyamide raw materials with the low oligomer content, in particular the dimer content, required for processing and the properties of the end product.
Evaporation processes using devices without moving mechanical parts are also known. The Offenlegungsschriften DE 3 409 066 and EP 200 368 A1 describe a two- and three-stage process using two and three shell-and-tube heat exchangers with separators immediately following the bank of tubes which are operated at various pressures below atmospheric pressure (during the second and third stage in the three-stage process or both stages in the two-stage process). The processes are used for removing volatile constituents from styrene polymers and styrene copolymers or rubber-modified styrene polymers. In this process liquid entrainers, such as water or alcohols, are used prior to the last stage of evaporation. Liquid entrainers are undesirable because sudden evaporation, which is associated with boiling of the film, occurs when the entrainer contacts the hot polymer melt, owing to the high temperature differences between melt and entrainer. This causes pulsation in the reaction chamber and adversely affects dispersion of the entrainer. When using organic entrainers with polyamide 6 and copolyamides, additional safety precautions are required as the danger of explosion has to be taken into account, and this entails high expenditure on safety.
A further method of isolating polymer solutions with simple apparatus is a process using a combination of a coiled tube evaporator and extrusion evaporator as described in the publication WO 99/04023. This process is not suitable for polyamides because the residence time in the extrusion evaporator is not sufficient for the residual contents required in this case, in particular as it is not possible to use an entrainer with the extrusion evaporator without the strands breaking.
DETAILED DESCRIPTION OF THE INVENTION
The object of the invention is to reduce the monomer (&egr;-caprolactam) and cyclic dimer contents in the polyamide 6 by an evaporation process to a monomer content of at most 0.7%, in particular at most 0.35% and preferably at most 0.2% and to a dimer content of at most 0.3%, in particular at most 0.2% and preferably 0.15% (measured by HPLC analysis as described below). A simple method which can be carried out using inexpensive devices is to be employed. The distinction from known evaporation processes is that, on the one hand, the monomers and dimers have a very low vapour pressure and, on the other hand, the very low diffusion coefficient of the dimers (of about 10
−11
m
2
/s) makes exchange of material very difficult.
The object is achieved according to the invention by two-stage evaporation of the polymer melt using a shell-and-tube heat exchanger and a loop-type evaporator or two shell-and-tube heat exchangers connected in series. A special feature o
Göbbels Dieter
Kohlgrüber Klemens
Morhenn Heinrich
Scherer Cliff
Triebeneck Konrad
Bayer Aktiengesellschaft
Boykin Terressa
Gil Joseph C.
Mrozinski, Jr. John E.
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