Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Autogenously or by activation of dry coated particles
Reexamination Certificate
2002-08-13
2003-12-30
Lechert, Jr., Stephen J. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Autogenously or by activation of dry coated particles
C264S122000
Reexamination Certificate
active
06669881
ABSTRACT:
The invention relates to porous sintered moulded bodies made of thermoplastic polymers, a process for their manufacture and their use.
Porous moulded bodies made of thermoplastic polymers are frequently produced from solutions. Thus it is, for example, possible as described in DE-C2-3 205 289 to manufacture, in particular, porous membranes in that a polymer is heated in a special solvent mixture above its upper critical demixing temperature, the solution is cooled and the solvent is extracted.
The process there described which constitutes a further development of the so-called Accurel process disclosed in DE-OS 2 737 745, is rather unsuitable for the manufacture of more compact solid bodies. In particular, moulded bodies such as porous pipes can only be manufactured continuously with great difficulty. Likewise, the process for the manufacture of porous polyolefin membranes disclosed in DEC-2 4 226 205 is carried out with a molten phase, the phase separation being thermally induced.
Attempts have also already been made to process polymer powders of thermoplastic material by sintering. Thus, in U.S. Pat. No. 4,879,081 a process is described in which molecularly orientated thermoplastic material is comminuted; the particles are then subjected to a sintering process in order to obtain a material in which discrete particles having anisotropic properties are distributed. However, in the technology disclosed in that US patent specification, no porous moulded bodies are formed.
It is also known to sinter, e.g. under pressure, powders of thermoplastic polymers in a mould and to remove the moulded body after cooling form the mould. It is a drawback of this process that it can only be performed continuously with great difficulty, that only sinter elements of limited dimensions can be produced, e.g. having a length of approximately 1 m and that the pore size as well can only be controlled with great effort so that the reproducibility of the process leaves much to be desired.
In GB-PS 1 549 949 a process is described for the manufacture of open pore polymeric material, in which thermoplastic polymer in powder form is pressure moulded under pressure in the presence of an organic polar liquid having a boiling point of 5 to 20° C. above the softening temperature of the polymer; this structure is then heated at a heating rate of 20 to 190° C. per minute causing the liquid to escape in gas form and the formation of a directional pore structure having i.a. channel shaped pores. The formation of the directional pore structure may be further assisted by the presence of potassium bicarbonate which decomposes under such process conditions.
In DE-AS 2 657 943 a process for the manufacture of open-pore materials is described in which initially pulverulent polymer is pressure-moulded at a pressure of 10 to 250 kp/cm
2
in the presence of an organic liquid. The pressed body is thereafter heated causing the liquid to escape under boiling conditions and forms directional elongate pores. It is a drawback of this process that it is performed in two stages and relatively slowly, cannot be performed continuously and forms directional pores.
Although numerous processes are already known for the manufacture of porous moulded bodies, a demand nevertheless still exists for improved manufacturing processes which can provide economically sintered moulded bodies having good or improved properties and which can be adapted to a variety of applications.
A need has been recognised to make available a process which operates in a simple and reliable manner, by means of which porous moulded bodies of thermoplastic polymers can be provided in a reproducible manner with adjustable porosity and pore size, which can be performed continuously and which provides moulded bodies having valuable properties which can be applied in numerous manners, that, moreover, proceeds rapidly and essentially creates an isotropic structure.
The present invention provides a process for the manufacture of porous moulded bodies from thermoplastic polymers, wherein a mixture of pulverulent polymer and a liquid having a boiling temperature at least as high or higher than the sintering temperature, is introduced into a moulding apparatus, the mixture is sintered therein, optionally cooled, and the liquid is separated off.
The invention further provides moulded bodies, produced by the process having a porosity of 20 to 60% and a mean pore size of 0.2 to 50 micrometers and preferably having an outer diameter of 0.05 to 2.5 cm and an inner diameter of 0.03 to 2 cm, preferably composed of polyethylene.
The invention furthermore provides the use of the moulded bodies in the form of panels as support panels for filter cloths and filter membranes, as filter elements, a coalescing elements, as suction elements for free oils, as gas distribution or air distribution elements, as supports for the picking up or releasing of active substances, e.g. by slow release, e.g. for the taking up and release of insecticides, scent and fertilisers.
Pulverulent thermoplastic polymers are used for performing the process according to the invention. These are commercially available and may be obtained in a known manner, e.g. by precipitation from polymer solutions, by spraying or even by comminution of polymer material. The powders employed may be composed of a single polymer or even of polymer mixtures. The mean size of the powder particles may be within wide limits, e.g. may amount 2 to 300 micrometers, in particular, 2 to 80 micrometers. Preferably, powders having a mean particle size of 2 to 80 micrometers are used.
The polymer which, in particular, is present in a particle size of 2 to 65 micrometers is then mixed with the provided liquid or liquids. This may result in the formation of paste-formed or suspension-like mixtures depending on the selected mass ratios.
Suitable polymers include conventional thermoplastic, in particular synthetic polymers such as polyolefins, e.g. polyethylene, polypropylene, polymethylpentene etc. as well as corresponding copolymers, polycondensation polymers such as polyesters or polyamide 66 but also polyamide 6. Further thermoplastic polymers are listed in German published specification no. 2 737 745 which is hereby expressly cross-referred to. High molecular weight polyethylene is particularly suitable.
The liquid used or the liquid mixture used must not dissolve or practically not dissolve the polymer during mixing and during sintering. The boiling point of the liquid or of the liquid mixture is at least as high as the selected sintering temperature but is preferably higher than the sintering temperature, preferably at least 10°, in particular, 30° C. above the sintering temperature. The use of a liquid within the meaning of the invention is not counterindicated, even if the polymer/liquid mixture employed may be present at temperatures higher than the sintering temperature as a single or multiple phase liquid solution.
Besides organic liquids which under the sintering conditions are inert in relation to the polymer employed, i.e. do not attack it chemically or otherwise, such as e.g. natural oils, e.g. soya oil, castor oil or glycerine, polyglycols and the like, it is also possible to employ inorganic liquids. Thus, it is even possible to employ inorganic acids such as sulphuric acid according to the invention provided this does not attack the polymer under the sintering conditions.
The ratio of polymer/liquid can be set within wide limits and amounts preferably to 1:4 to 3:1. Preferably, the liquid content amounts to at least 30 percent by weight. It is important that sufficient liquid is present so as to embed or code the powder particles in order to ensure an optimal and uniform heating up of the sintering mixture. In a continuous manufacture the liquid in addition serves as a transporting aid.
By varying the ratio of polymer/liquid, the porosity and the pore size can be influenced. Thus, as the liquid content increases, the porosity increases. The pore size may be controlled by the selection of the particle size of the powder. The me
Lechert Jr. Stephen J.
Marshall & Gerstein & Borun LLP
Microdyn Modulbau GmbH & Co. KG
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