Polytetrafluoroethylene fine powders and their use

Details Polytetrafluoroethylene fine powders and their use Polytetrafluoroethylene fine powders and their use

1998-04-23

2003-01-07

Seidleck, James J. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C524S546000, C524S805000, C526S250000

Reexamination Certificate

active

06503988

ABSTRACT:

TECHNICAL FIELD
The present invention relates to polytetrafluoroethylene (hereinafter referred to as PTFE in some cases) fine powder having superior powder flowability. This PTFE fine powder is suitably used as a molding material for paste extrusion molding and the like, alternatively, as a resin additive for reforming the properties of a variety of resins.
BACKGROUND ART
Conventional PTFE fine powders have the properties of being apt to coagulate and thus exhibit poor working efficiency when they are taken out of containers for being used in molding or the like. Poor working efficiency can be seen, for example, when PTFE fine powders are mixed with an extrusion aid in the step of extruding rod or tube-shaped premolded articles by paste extrusion method. In addition, there are the following problems. In cases where PTFE fine powders are added to plastic for imparting sliding properties or antidripping properties, it is necessary to premix the PTFE fine powder with a material resin. At that time, however, the powders coagulate in a Henschel mixer or an extruder's feeder to cause blocking, which considerably degrades working efficiency and productivity. in addition, since PTFE fine powders have poor dispersibility to material resins, silver lines caused by the segregated PTFE may occur on the surfaces of molded articles, depending: on the conditions of compounds. The term “silver line” is a silvery bar trace that generates on the surfaces of molded articles in the flow direction of resins.
Detailed description is given of the cases where PTFE fine powder are used as an antidripping agent.
As to the flame resistance regulations for plastics, the standards of various countries have been integrated in recent years. In Japan the regulations have been severer because of the tendency of product security. As the safety standards of electronic and electrical equipment, the regulations of UL (Underwriters's Laboratories) in the USA have the outstanding contents, which is not the domestic standards in Japan, but have a significant influence. In these circumstances, the demand for flame resistive resins is expected to increase for the future.
Since much of thermoplastic resins used in electrical appliances and office machinery are flammable, flame retardants are usually added for improving the incombustibility of resin molded articles. To meet the requirements for miniaturization and thin-wall of molded articles, the regulations of UL 94 required for flame resistive resins have become severer in these years. In the latest notebook PCs, for example, the wall thickness does not exceed 1 mm so as to obtain 300-g frames for compactness and lightness. Conventional flame resistive resins can make molded articles non-flammable, however, if used for such thin articles, it is liable that once they begin to burn, thermoplastic resins are in the liquid state and drip while burning, and then spread. On the other hand, it is known to prevent dripping during burning by adding and kneading PTFE fine powder into thermoplastic resins. This utilizes the characteristics that PTFE fine powders are easily fibrilized.
As to flame retardants, bromine-containing flame retardant DBDE (decabromodiphenyl ether), has often used for high impact polystyrene (HIPS), ABS, polycarbonate (PC), polybutyleneterephthalate (PBT), etc. However, replacement of DBDE has proceeded rapidly because of dioxin that is considered to generate in burning DBDE. As a substitute, brominated epoxy oligomer is presently the most effective from the viewpoints of flame resistance and weather resistance. However, the bromine content of brominated epoxy oligomer is about two- thirds of that of DBDE, and thus it is required to increase the amount of addition. Furthermore, the price of brominated epoxy oligomer is about 1.5 times that of DBDE, thereby increasing the cost of flame resistive resins. Therefore, a reduction of flame retardant is a big subject. In this regard, it is known that flame retardant can be reduced if an antidripping function is imparted by the addition of PTFE fine powder. However, as described earlier, PTFE fine powders are generally apt to coagulate. Hence, the powders mixed with a material resin may solidify in a Henschel mixer or an extruder's feeder during premixing, to cause blocking, which may often significantly deteriorate handling characteristics and productivity. Also, due two poor dispersibility, silver lines caused by the segregated PTFE may occur on the surfaces of molded articles, depending on the conditions of compounds.
Meanwhile, it is well known that fibrillating polytetrafluoroethylene is incorporated into powder having dusting characteristics to prevent dusting characteristics (JP-B-32877/1977, JP-B-24872/1993, JP-A-91993/1989 and JP-A-81882/1989).
In industrial manufacturing, economy, productivity and working efficiency must be taken into consideration. In order to uniformly disperse polytetrafluoroethylene in a desired concentration, it is desirable that polytetrafluoroethylene is supplied individually by an automatic weight or volumetric counting feeder.
However, due to poor powder flowability of conventional polytetrafluoroethylene, the jamming of powders has occurred in automatic weight or volumetric counting feeders, so that working efficiency is significantly reduced.
Accordingly, it is an object of the present invention to provide PTFE fine powders excellent in powder flowability which are used as molding materials, resin additive, or the like.
It is another object of the present invention to provide antidripping agents, excellent in handling characteristics and dispersibility while maintaining antidripping property, as well as flame resistive resin compositions containing the antidripping agent.
It is still another object of the present invention to provide compositions for inhibiting dusting characteristics that are excellent in handling characteristics and working efficiency.
DISCLOSURE OF THE INVENTION
The present invention relates to a polytetrafluoroethylene fine powder excellent in powder flowability which comprises fine particles (the primary particle) having a mean particle diameter of 0.05 to 1 &mgr;m, an apparent density of 0.52 to 0.70 g/ml, a standard specific gravity (SSG) of 2.14 to 2.23, and a secondary mean particle diameter of 100 to 1000 &mgr;m. The invention also relates to antidripping agents using the above polytetrafluoroethylene fine powder, flame resistive resin compositions containing this powder, and compositions, for inhibiting dust characteristics containing this powder.
The polytetrafluoroethylene fine powder of the invention is characterized in having specific mean particle diameters of the primary and secondary particles, specific apparent density and standard specific gravity, as defined above.
PTFE fine powder of the invention can be prepared by a method in which a polymer latex is obtained by known emulsion polymerization (JP-B-4643/1962, JP-B-14466/1971, JP-B-26242/1981, and U.S. Pat. No. 2,965,595), and a surfactant is then added into the latex when subjected to a coagulation.
It is not fully why the apparent density of PTFE fine powder increases by the addition of surfactant in coagulating the latex after the reaction, however, it is estimated that the primary particles within the secondary particles are closely filled when water evaporates in the step of drying. As a result, powders in which particles are highly packed with a high apparent density are obtained, improving powder flowability.
The fibrillating properties of PTFE fine powder are the characteristics that can be normally recognized in the PTFE whose molecular weight is too high to perform melt molding (i.e., the melt viscosity at 380° C. is more than 10
8
poise). Specifically, such PTFE have a standard specific gravity (ASTM D-1457) of not more than 2.23, preferably in the range of 2.14 to 2.23 (as the value of standard specific gravity decreases, the molecular weight increases). Over 2.23, i.e., in lower molecular weights, it is less susceptible to fibrillation. On the contrary, th

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