Plastic and nonmetallic article shaping or treating: processes – With twining – plying – braiding – or textile fabric formation
Reexamination Certificate
1999-04-12
2001-05-29
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With twining, plying, braiding, or textile fabric formation
C057S295000, C057S310000, C264S127000, C264S129000, C264S171100, C264S175000, C264S288400, C264S290500
Reexamination Certificate
active
06238605
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the production of a low-fibrillation moulded body containing polytetrafluorethylene, low-fibrillation moulded bodies, which can be obtained as a result of the process in accordance with the invention and the use of these as dental floss.
Polytetrafluorethylene (PTFE) is a prized material due to its thermal stability and chemical inertness. Monoaxially stretched films made from sintered PTFE are known from AT-B 370.673, the strength of which has values of between 50 N/mm
2
and 140 N/mm
2
in the stretching direction. Said films are produced when PTFE powder is first of all pressed to a cylindrical moulded body. The moulded body is then sintered, whereupon films are peeled off, heated to temperatures of at least 327 C and stretched.
GB-A 2 025 835 describes the production of porous PTFE moulded bodies in accordance with the paste extrusion process whereby a paste-like mass, which basically comprises PTFE powder and a lubricant (hydrocarbon), is pressed through dies whereby the lubricant is removed by drying. The moulded body is then heated to above the crystallite melting point of the PTFE (327° C.) and stretched during heating.
EP-A 0 391 887 describes a process for the production of a monoxially stretched moulded body of PTFE, in which a paste-like PTFE mass is continuously processed to a moulded body which is led over a number of rollers or cylinders, heated and stretched, the moulded body being heated to a temperature between 327° C. and 450° C. before applying the stretching and sintered. This process allows the production of a monoaxially stretched moulded bodies of PTFE with tenacity values in the stretching direction of at least 22 cN/tex (500 N/mm
2
).
Furthermore, it is known that moulded bodies of PTFE can be made available with different filling substances. Moulded bodies of this kind containing filling substances are for example known from AT-B399 882 and contain talc and/or mica and/or a high temperature resistant polyimide of between 20 and 30 wt. % as a filling material. The PTFE moulded bodies described in AT-B 399 882 containing filling materials are for example suitable for use as dental floss.
Dental flosses of PTFE tapes are moreover described in EP-A 0 335 466, U.S. Pat. No. 5,209,251 and U.S. Pat. No. 5,220,932. The PTFE-tapes can have different densities and can be in a folded or unfolded state.
Compared to conventional dental flosses e.g. of polyamide, the PTFE material is softer which means greater suppleness and softness which results in a lower risk of hurting the flesh of the gums when in use.
Since they are in a stretched or expanded state, PTFE tapes tend to fibrillate which means that individual fibres split. This leads to problems either when unreeling the dental floss from the dental floss dispenser respectively when the dental floss splits when in use leaving traces of dental floss between the teeth.
In PCT-WO 95/34252 a dental floss is suggested from an unfolded expanded PTFE fibre which has a regular thickness and width overall whereby the parallel placed edges of the dental floss are resistant to fibrillation. In this respect a thicker PTFE sheet in comparison to state of the art is cut into equally thick and broad strands. The strands are rolled up without prior folding.
SUMMARY OF THE INVENTION
The present invention sets itself the task of providing a moulded body containing PTFE, which is particularly well suited as a dental floss material, and a process for the production thereof whereby the moulded body is to be particularly resistant to fibrillation. The process for the production of the moulded body should be both simple and cheap.
The task of the present invention is solved by a process for the production of a low-fibrillation moulded body containing polytetrafluorethylene characterised in that a pre-moulded body of polytetrafluorethylene is twisted and then pressed flat.
Surprisingly it has been shown that as a result of the measure of twisting a pre-moulded body containing PTFE and then pressing the twisted pre-moulded body flat again, moulded bodies can be obtained which are particularly resistant to fibrillation. The process in accordance with the invention is characterised in that it can be easily performed. The process in accordance with the invention can also be performed continuously.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purpose of the present invention by “pre-moulded bodies” moulded bodies produced according to the known methods of state of the art, e.g. a fibre or a tape, are meant.
Compared to PTFE products folded according to state of the art, the pre-moulded body in the present invention is not folded but rather completely twisted respectively twined.
In this respect it has shown itself to be advantageous when the pre-moulded body is twisted with 30 twists per m (T/m) up to 400 twists per m (T/m).
The pre-moulded body can preferably be twisted around itself. In this case it is favourable when the pre-moulded body is twisted by 220 twists per m up to 270 twists per m, and preferably 245 twists per m up to 255 twists per m. In the case of a pre-moulded body mainly comprising PTFE this results in a moulded body mainly comprising PTFE.
One preferred embodiment of the process in accordance with the invention is characterised in that the pre-moulded body of polytetrafluorethylene is twisted around another material. This other material can preferably be for example a mono- or multifilament, preferably of polyamide, polyester or yet again polytetrafluorethylene. The other material can be a spun yarn, preferably of cotton, viscose, polyamide, polyester or acrylic.
With this embodiment of the process in accordance with the invention it becomes possible, to combine the properties of the twisted other material, e.g. a higher strength, with the advantageous properties of PTFE, particularly suppleness and the good gliding property.
The titre of the other material preferably lies in the range between 100 dtex and 1000 dtex, preferably between 300 dtex and 500 dtex.
When the pre-moulded body of PTFE is twisted around another material, it is advantageous that the pre formed body is twisted around the other material with 50 twists per m to 250 twists per m, preferably 100 twists per m up to 150 twists per m.
One other advantageous embodiment of the process in accordance with the invention is when the other material contains flavourings and/or substances which serve dental hygiene. It is known that PTFE moulded bodies, particularly when they are used as dental floss, contain flavourings and/or substances which serve dental hygiene. Materials which can be used for this are e.g. described in PCT-WO 95/34252.
It has proven itself favourable when additives of this kind are contained in the other material around which the pre-moulded bodies are twisted. These additives can for example serve as a depot.
The twisted pre formed body is preferably pressed flat as a result of calendering preferably between two rolls. In this mode of operation in particular it is possible to continuously perform the process in accordance with the invention.
The pre-moulded body used is preferably a monoaxially stretched moulded body of PTFE. The pre-moulded body can preferably be in the shape of a tape prior to twisting. The titre of the pre-moulded body can be between 400 dtex to 1800 dtex, and preferably 600 dtex to 1200 dtex.
Monoaxially stretched pre-moulded bodies are particularly well suited to the process in accordance with the invention which have a tenacity value in the stretching direction of at least 20 cN/tex, and preferably 25 cN/tex to 30 cN/tex, and a density of less than 2 g/cm
3
, preferably of less than 1.8 g/cm
3
, and in particular from 1.6 g/cm
3
to 1.8 g/cm
3
. Such pre-moulded bodies of this kind can for example be produced in a similar way to the processes described in EP 0 391 887 A1 or U.S. Pat. No. 3,953,566.
It appears to be favourable when the pre-moulded body contains flavourings and/or substances which serve dental hygiene.
For use as dental flos
Baker & Botts L.L.P.
Lenzing Aktiengesellschaft
Tentoni Leo B.
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