Claims
- 1. A method for processing flowable solid polytetrafluoroethylene material by radiation, to degrade said material to lower its molecular weight and render it grindable into a powder, comprising
- (a) supplying said material to a processing vessel,
- (b) supplying radiation to a selected region of said processing vessel,
- (c) agitating said material in said processing vessel during said processing thereby to repeatedly move said material into and out of said selected region whereby said material is uniformly irradiated, and,
- (d) cooling said material to maintain a temperature below 500.degree. F. during said processing.
- 2. A method as specified in claim 1 wherein air is provided to said material to cool said material, fluidize said material, and provide oxygen to said material during said processing.
- 3. A method as specified in claim 1 or 2 wherein additional material is supplied to said vessel during said irradiation.
- 4. A method as specified in claim 1 or 2 wherein air is supplied to said material during irradiation, and wherein said vessel is vented by negative pressure to remove airborne particles and gases.
- 5. A method as specified in claim 4 wherein said removed material is recovered.
- 6. A method for processing flowable solid polytetrafluoroethylene material by radiation, to degrade said material to lower its molecular weight and render it grindable into a powder, comprising
- (a) supplying said material to a processing vessel,
- (b) supplying radiation to a selected region of said processing vessel,
- (c) agitating said material in said processing vessel during said processing thereby to repeatedly move said material into and out of said selected region whereby said material is uniformly irradiated, and
- (d) supplying water to said material to maintain it below a selected temperature during said processing.
- 7. A method as specified in claim 6 wherein the material processed is virgin polytetrafluoroethylene and the selected temperature is about 260.degree. F.
- 8. A method as specified in claim 7 wherein the water supply is diminished or interrupted when the temperature of the material is lowered to about 245.degree. F.
- 9. A method as specified in claim 6 wherein the material processed is sintered scrap polytetrafluoroethylene which has had a heat treatment history and the selected temperature is below aoout 390.degree. F.
- 10. A method as specified in claim 9 wherein the water supply is diminished or interrupted when the temperature of the material is lowered to about 375.degree. F.
- 11. A method as specified in claim 6 wherein the material processed is non-virgin polytetrafluoroethylene and the selected temperature is about 265.degree. F.
- 12. A method as specified in claim 11 wherein the water supply is diminished or interrupted when the temperature of the material is lowered to about 250.degree. F.
- 13. A method as specified in claim 6 wherein the material is granular polytetrafluoroethylene chips and the selected temperature is about 265.degree. F.
- 14. A method as specified in claim 13 wherein the water supply is diminished or interrupted when the temperature of the material is lowered to about 250.degree. F.
- 15. A method as specified in claim 6 wherein the material processed is scrapped teflon tape and the selected temperature is about 265.degree. F.
- 16. A method as specified in claim 15 wherein the water supply is diminished or interrupted when the temperature is lowered to about 250.degree. F.
- 17. A method as specified in any one of claims 6 through 16 wherein additional material is supplied to said vessel during said irradiation.
- 18. A method as specified in any one of claims 6 through 17 wherein said vessel is vented by negative pressure to remove airborne particles and gases.
- 19. A method as specified in claim 18 wherein said removed material is recovered.
BACKGROUND OF THE INVENTION
This application is a continuation-in-part of application Ser. No. 374,015, filed May 3, 1982, for "Apparatus and Method for Radiation Processing of Materials."
This invention relates to radiation processing for the degradation of materials and specifically to radiation degradation of polytetrafluoroethylene (PTFE).
U.S. Pat. No. 3,766,031 to Dillon discloses a method for radiation processing of polytetrafluoroetylene, wherein the polytetrafluoroethylene is exposed to radiation and thereafter subjected to comminution to reduce the polytetrafluoroethylene to a fine particle powder. This powder is useful as a dry lubricant, for example, in paints and inks. This and other methods for carrying out this technique generally made use of electron beam or cobalt sources for irradiating the polytetrafluoroethylene. The material being irradiated was arranged in trays and exposed to multiple doses of radiation, approximately 2 to 15 MR per pass, so that the temperature of the material does not rise excessively, thereby to avoid discoloration of the material and the possible generation of noxious gases. The total dose is approximately 35 to 150 MR.
The tray irradiation technique has a radiation utilization efficiency of only approximately 35%. Radiation inefficiencies arise from the three factors, which are (1) overscan of trays, (2) gaps between trays and (3) depth-dose characteristics for an electron beam.
The need to assure complete and uniform radiation of a tray requires some overscan by the radiation and an efficiency loss of 5 to 15%. Likewise there is usually some space between trays of material which causes a further 10 to 15% loss of efficiency.
The largest efficiency loss arises out of the fact that the dose received by the material varies with material depth. This variation is illustrated in the graph of FIG. 6. Typically the dose at the surface, designated R.sub.o, is taken as the nominal dose for the material. Beam energy and/or material depth is adjusted so that an equal dose at the opposite surface of the material (Depth D). Radiation which passes entirely through the product (Area C) is not utilized. Radiation in Area B in excess of the nominal dose R.sub.o is likewise not used. This causes further inefficiency, and in some instances may result in undesired properties of the resultant product. Overall this depth-dose characteristic can cause processing inefficiency of up to 50%.
The prior art technique of slow irradiation of material by subsequent exposures to doses of radiation, occasionally coupled with stirring the material between exposures, tends to cause the production of radiation degraded polytetrafluoroethylene powder to be an expensive, inefficient and time consuming operation.
It is an object of the present invention to provide an apparatus and method for more efficiently producing radiation degraded polytetrafluoroethylene.
In accordance with the present invention there is provided an apparatus forradiation processing of chopped flowable solid materials having a high molecular weight to degrade the material to a lower molecular weight and render it grindable into a powder. The apparatus includes a processing vessel for holding the material, a radiation source for supplying radiation to the processing vessel, means for agitating the material during the processing, and cooling means for maintaining the vessel and the material below a selected temperature.
In a preferred embodiment the apparatus includes a dust cover over the vessel and dust collecting means for gathering airborne material particles which accumulate in may be supplied to the material in order to fluidize the material, and to additionally cool the material by the flow of air or the addition of cooling water.
The processing vessel is preferably a ribbon blender which includes a vessel and motor driven stirrer for agitating the contents of the vessel. A preferred radiation source is a source of high energy electrons. The processing vessel can be cooled by a water flow channel surrounding the vessel. Cooling can also be accomplished, alternatively in part, by the addition of water to the processing vessel.
In accordance with the invention there is provided a method for processing flowable solid polytetrafluoroethylene material by radiation degradation to reduce the molecular weight of the material and render it grindable into a powder. The method includes exposing the material to radiation, agitating the material during exposure, and maintaining the material at a temperature below 500.degree. F. during exposure.
In preferred embodiments the material is fluidized by an air flow during exposure and maintained below 250.degree. F. or, alternatively, water is added to the material during exposure and the mixture maintained within an appropriate temperature range.
For better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.
US Referenced Citations (15)
Foreign Referenced Citations (4)
| Number |
Date |
Country |
| 17349 |
Oct 1980 |
EPX |
| 1019440 |
Feb 1966 |
GBX |
| 1505284 |
Mar 1978 |
GBX |
| 1516648 |
Jul 1978 |
GBX |
Non-Patent Literature Citations (1)
| Entry |
| Chemical Engineer's Handbook, p. 1210, (3rd Edition, J. Perry Editor, 1950). |
Continuation in Parts (1)
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Number |
Date |
Country |
| Parent |
374015 |
May 1982 |
|
Patent Grant
4777192
