Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-01-12
2004-06-08
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S247000, C526S255000
Reexamination Certificate
active
06747108
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing a modified polytetrafluoroethylene fine powder having good molding properties and heat resistance, and a modified polytetrafluoroethylene fine powder produced by such a process.
PRIOR ART
Polymerization processes of polytetrafluoroethylene (PTFE) roughly include two types, that is, suspension polymerization and aqueous dispersion (emulsion) polymerization Granular PTFE obtained by the suspension polymerization is finely pulverized to form a powder (granular resin), and the powder as such or the powder, which is granulated to impart flowability to the powder, is processed mainly by compression molding or ram extrusion molding.
In the case of a latex obtained by the aqueous dispersion (emulsion) polymerization, the polymer fine particles are coagulated and dried to obtain fine powder, and the fine powder is processed by paste extrusion after compounding a liquid lubricant in the fine powder. Alternatively, a surfactant is added to the latex obtained by the polymerization to stabilize the latex, and then the latex is further concentrated and processed by coating or dipping.
Low molecular weight PTFE, which is called “waxes”, is often used to modify the properties of other thermoplastic resins, thermosetting resins, coatings, inks, oils, etc. with blending, by making use of the excellent properties of PTFE, for example, lubricity, although the low molecular weight PTFE does not have sufficient mechanical strength for practical applications.
As explained above, the PTFE fine powder is mainly used in a paste extrusion method in which a liquid lubricant is compounded in the PTFE fine powder and the compound is extruded with an extruder in the form of a relatively thin rod or a tube. The extruded articles is rolled to form a film which is used as a sealing material, or further stretched to form a porous film which is used as a filter. Alternatively, the PTFE fine powder is sintered to form a final product in the case of a tube or wire-coatings.
From the industrial point of view, the PTFE fine powder is required to have high productivity in the paste extrusion molding process. The most important factor is a reduction ratio (RR) in the course of extrusion.
The reduction ratio is expressed by a ratio of a cross sectional area (S) of a cylinder in which the powder to be extruded is filled to a cross sectional area (s) of a die outlet (S/s). It is preferable to increase the reduction ratio as much as possible so that a larger amount of the powder is charged in the cylinder and extruded in one extrusion cycle. However, when the reduction ratio exceeds the limit of the processing properties of the powder, a normal extrudate cannot be obtained since the extrudate tends to wave, the surface and/or inside of the extrudate tend to be cracked, or the extrudate tends to be broken in the course of extrusion.
Furthermore, it is important for an extrusion pressure not to increase excessively in the course of paste extrusion from the viewpoint of productivity. If the extrusion can be carried out at the high reduction ratio but the required extrusion pressure is very high, such a high pressure may exceed the capacity of the extruder and thus the it is not preferable in the industry. In addition, the reduction ratio and the extrusion pressure have a deep relationship with an extrusion speed. As the extrusion speed increases, the extrusion pressure increases while the possible reduction ratio decreases.
Under the above circumstances, various techniques have been proposed to improve the paste extrusion properties including the increase of the reduction ratio at which the extrusion is possible and the suppression of the increase of the extrusion pressure.
Among them, the earliest proposal is a polymerization process of tetrafluoroethylene (TFE) disclosed in JP-B-37-4643 (U.S. Pat. No. 3,142,665). This process is characterized in that a modifier is introduced in a polymerization reaction system prior to the consumption of 70% of TFE to be polymerized. Preferred examples of the modifier described are hexafluoropropylene (HFP) which is a comonomer, and methanol, etc.
The introduction of the modifier to PTFE decreases the crystallinity of the produced polymer and suppresses the fibrillation properties specific to PTFE. In general, the extrusion pressure increases as the reduction ratio is high. It is assumed that the extrusion under a high pressure may apply a high shear force on primary particles to accelerate the fibrillation of the particles. As a result, an abnormal flow condition is induced during the extrusion so that defects such as cracks are generated in the extrudate or the sintered extrudate. Thus, the suppression of the fibrillation properties with the introduction of the modifier functions to relieve the extrusion defects at the high reduction ratio caused by the excessive fibrillation.
JP-B-56-26242 discloses a method for improving the paste extrusion properties of PTFE with the introduction of chlorotrifluoroethylene in a reaction system at the start (initiation) of the reaction and when 70 to 90% of TFE is consumed.
JP-A-63-56532 discloses a method for improving the paste extrusion properties and heat resistance of a PTFE fine powder by copolymerizing a fluoroalkyl vinyl ether with TFE in the core part of the particle and copolymerizing chlorotrifluoroethylene (CTFE) in the shell part of the particle to obtain a PTFE fine powder.
Furthermore, JP-B-64-8022 proposes a primary particle having a three-layer structure in which PTFE in the second layer is not modified, while PTFE in the first and third layers is modified.
JP-B-3-69926 and JP-B-8-26102 use a perfluoroalkyl-ethylene as a modifier.
The introduction of a modifier in the PTFE fine powder is sometimes employed to improve properties other than paste extrusion properties. In particular, when a copolymerizable modifier is used, usually the crystallinity of the polymer decreases, and often a sintering rate increases and the transparency and mechanical properties of the polymer increase.
For example, JP-B-50-38159 describes that, when TFE is polymerized using a perfluoroalkyl vinyl ether as a modifier and a persulfate salt initiator, the mechanical properties, in particular, flexural life of the polymer prepared can be significantly improved.
In the meantime, the fine powder for high RR (HRR) extrusion molding is used to coat electric wires or processed in the form of thin tubes, and often used in the fields where high quality is increasingly required such as the fields of aircraft, automobiles, medical equipment, precision machines, etc.
From the viewpoint of processing, not only the HRR extrusion properties which influence the productivity and also the reduction of a sintering time are desired. Therefore, it is necessary to raise a sintering temperature, and the sintering temperature often exceeds 400° C. In general, as the sintering temperature rises, the sintering temperature shortens. However, the polymer tends to be thermally degraded to lead to the deterioration of the mechanical properties such as tensile strength of the final product of the polymer. Thus, the high sintering temperature is not preferable from the practical viewpoint. Therefore, the increase of the heat stability of the polymer is required.
In the applications of the final products, the improvement of long-term heat resistance is desired, since the temperature in the service environment increases, for example, in the case of coated electric wires used in engine rooms.
Hitherto, a thermal instability index (TII; ASTN D4895) is used as a measure of the heat resistance of PTFE.
TII is obtained from a difference of densities of molded articles which are molded with changing a sintering time. In general, the smaller TII means the smaller change of the crystallinity and less heat degradation of the polymer. Therefore, PTFE with the small TII is preferably used in the above applications requiring the heat resistance.
TII of the unmodified PTFE fine powder which is practically used does not exceed 5
Hosokawa Kazutaka
Nanba Yoshinori
Uchida Tatsuro
Yano Shinichi
Daikin Industries Ltd.
Lipman Bernard
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