Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2002-07-26
2004-09-14
Hu, Henry S. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S481000, C528S50200C, C528S50200C, C524S545000, C526S250000
Reexamination Certificate
active
06790932
ABSTRACT:
The present invention relates to a non thermoprocessable fine powder of homopolymer or modified PTFE as defined below, and to the process thereof, via powder to be used in the lubricated extrusion wherein also low extrusion pressures can be used. The manufactured articles obtainable from said fine powders have a high purity degree and are surface defect free, such for example cracks, roughness.
It is known that the polytetrafluoroethylene manufactured articles are obtained in various forms, such for example, pipes, cables, wires or tapes, subjecting the PTFE fine powders to a lubricated extrusion process. Generally, a higher extrusion pressure leads to a defect increase of the extruded manufactured article, whereby it is always preferable to work I at a low extrusion pressure. Before extrusion it is necessary to subject the fine powders to drying. The extrusion pressure results depend on the temperature at which drying is carried out. The higher the drying temperature, the higher the polymer particle coalescence degree whereby it will be necessary a higher extrusion pressure. Therefore it is extremely advantageous to be able to dry the fine powder at a lower temperature, to work at low extrusion pressures. The PTFE fine powder obtained at lower drying temperatures results suitable to be extruded with a sufficiently low pressure, obtaining extruded manufactured articles surface defect free, such for example cracks, roughness.
It is known that the PTFE fine powders are obtained by the polymerization in dispersion (emulsion). In said process a sufficiently high amount of surfactant is used so to be able to stabilize the PTFE colloidal particles and a mild stirring is applied so to avoid the polymer coagulation. In the dispersion polymerization process stabilizers, initiators and other additives (for example nucleants such as ZnCl
2
) are added to control the polymer particle diameter. Then the latex obtained from said process is coagulated, and the obtained powder is called “fine powder”. The known conventional coagulation process comprises the following steps:
latex dilution with water and optional addition of a destabilizing electrolyte to coagulate the latex;
latex mechanical stirring which causes the aggregation of the colloidal particles and leads first to gelification then to granulation and lastly to flotation;
separation of the wet fine powder from the coagulum water;
optional polymer washing, under powder form, for several times with water to reduce the electrolyte concentration in the powder;
optionally drying of the fine powder.
In the dispersion polymerization process of PTFE, surfactants and polymerization initiators are commonly used, for example persulphates, which after the polymerization process remain in the latex. The polymer obtained from this conventional process is therefore contaminated by the initiator and surfactant. For said reasons the PTFE fine powders cannot be used for the porous manufactured article production, for example defect free thin membranes. Furthermore said fine powders cannot be used in the production of manufactured articles for the semicon industry. In fact, as well known, in the applications concerning the semicon industry, a high purity degree of the polymer is required, in particular having a content of residual cations <1 ppm, and of anions (in particular sulphates <1 ppm).
In U.S. Pat. No. 3,046,263 a continuous coagulation process of the PTFE latexes is described, comprising
a phase of strong mechanical stirring with a specific power of 1-100 (CV×sec)/gallon (196 kJ/m
3
-19,600 kJ/m
3
), preferably using a centrifugal pump with average residence time of the latex in the pump of 2 seconds;
passing through a capillary tube with hydraulic resistance of 0.5-20 p.s.i. (3.4-136 kPa);
granulation in the presence of air by mechanical stirring with specific power of 0.25-50 (CV×sec)/gallon (49 kJ/m
3
-9,800 kJ/m
3
) with subsequent separation of the fine powder from water.
The use of the mechanical stirring with so high specific powers (~98 kW/m
3
-9,800 kW/m
3
) by a centrifugal pump, which has a wide distribution of the residence times, causes a too compact structure of the fine powder, which does not mixes well with the lubricant and does not allow to work at low pressure in the lubricated extrusion process as described hereafter.
In U.S. Pat. No. 5,391,709 a purification process of the PTFE fine powders from contaminants is described, wherein a thin layer of fine coagulated powder is put on the surface of a polymer tissue and subsequently exposed to a hot air flow, which passes through the fine powder layer flowing from the upper part to the lower part. In said process the drying temperature is in the range 110°-200° C., preferably 160°-200° C. Tests carried out by the Applicant (see comparative Examples) show that by operating under the conditions described in this patent, a product free from initiator salts is not obtained; in fact the residual cation content is higher than 1 ppm, the residual anion content (sulphate) is higher than 1 ppm. Therefore the purification process described in said patent is not suitable to obtain manufactured articles usable in the semicon industry, which requires a residual cation content <1 ppm and a residual anion content (sulphate) <1 ppm. Besides, said patent refers to PTFE fine powders coagulated with the conventional coagulation process, as above described. Tests carried out by the Applicant (see comparative Examples) show that by subjecting said PTFE fine powder to the purification process indicated in said patent wherein the drying temperature is lower than 160° C., a product still containing some surfactant is obtained. Therefore the manufactured articles obtainable under said conditions have not the purity requirements to be used in the semicon industry. In fact it is necessary to work at a drying temperature of 190° C. to obtain a product purified from the surfactant. However in the extrusion phase the purified product according to said patent requires high extrusion pressures since the drying temperature is high. Therefore the purification process described in said patent does not allow to obtain a fine powder which can be subjected to low extrusion pressures.
The need was felt to have available non thermoprocessable fine powders of homopolymer or modified PTFE as defined below, which can be subjected to subsequent extrusion using low extrusion pressures. The obtainable manufactured articles have a high purity degree, are surface defect free, such for example cracks and roughness, and are used for the application in the semicon industry.
The Applicant has surprisingly found a purification process of homopolymer or modified PTFE which allows to obtain PTFE fine powders with the above advantages.
It is therefore an object of the present invention a process to obtain non thremoprocessable fine powders of homopolymer or modified PTFE, comprising the following steps:
A) obtaining of the polymer latex under the gel form;
B) washing of the polymer gel with acid aqueous solutions or neutral aqueous solutions, having a pH from 0.5 to 7, preferably from 1 to 4;
C) granulation of the washed gel by mechanical stirring, at a specific power in the range 1.5-10 kW/m
3
, until flotation of the PTFE fine powder, and separation of the floated fine powder;
D) drying in an aerated oven of the floated fine powder at a drying temperature in the range 90° C.-160° C., preferably 105° C.-150° C.
The process of the present invention allows to obtain PTFE fine PTFE powders suitable for low pressure extrusion. The powders ar substantially free from inorganic cations and from surfactants.
The fine powders of PTFE or PTFE modified after step D), as said, are substantially free from inorganic cations (residual amount <1 ppm), substantially free from inorganic sulphates (residual amount <1 ppm), and contain polymerization surfactants in an amount lower than the analytical detectable limits (<10 ppm, determined as indicated in the characterization methods).
Unexpectedly it has been found that
Kapeliouchko Valery
Palamone Giovanna
Wu Hua
Arent Fox PLLC.
Ausimont S.p.A.
Hu Henry S.
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