CA1209314A - Process for producing articles from polytetrafluoroethylene - Google Patents
Process for producing articles from polytetrafluoroethyleneInfo
- Publication number
- CA1209314A CA1209314A CA000435981A CA435981A CA1209314A CA 1209314 A CA1209314 A CA 1209314A CA 000435981 A CA000435981 A CA 000435981A CA 435981 A CA435981 A CA 435981A CA 1209314 A CA1209314 A CA 1209314A
- Authority
- CA
- Canada
- Prior art keywords
- process according
- temperature
- article
- forming
- granules
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 239000004810 polytetrafluoroethylene Substances 0.000 title claims abstract description 43
- 238000000034 method Methods 0.000 title claims abstract description 35
- -1 polytetrafluoroethylene Polymers 0.000 title claims abstract description 13
- 229920001343 polytetrafluoroethylene Polymers 0.000 title claims description 38
- 238000010438 heat treatment Methods 0.000 claims abstract description 18
- 229940058401 polytetrafluoroethylene Drugs 0.000 claims abstract description 15
- 239000008187 granular material Substances 0.000 claims abstract description 13
- 238000002156 mixing Methods 0.000 claims abstract description 13
- 238000001816 cooling Methods 0.000 claims abstract description 11
- 238000007599 discharging Methods 0.000 claims abstract description 6
- 238000004519 manufacturing process Methods 0.000 claims abstract description 6
- 239000011347 resin Substances 0.000 claims description 25
- 229920005989 resin Polymers 0.000 claims description 25
- 238000012545 processing Methods 0.000 claims description 11
- 238000003825 pressing Methods 0.000 claims description 9
- 239000000654 additive Substances 0.000 claims description 4
- 238000000748 compression moulding Methods 0.000 claims description 4
- 238000005242 forging Methods 0.000 claims description 4
- 239000000758 substrate Substances 0.000 claims description 3
- 239000011247 coating layer Substances 0.000 claims description 2
- 239000000049 pigment Substances 0.000 claims description 2
- 239000004033 plastic Substances 0.000 abstract description 4
- 229920003023 plastic Polymers 0.000 abstract description 4
- 238000012360 testing method Methods 0.000 description 15
- 238000002844 melting Methods 0.000 description 8
- 230000008018 melting Effects 0.000 description 8
- 239000000463 material Substances 0.000 description 6
- 230000000704 physical effect Effects 0.000 description 5
- 238000007796 conventional method Methods 0.000 description 3
- 238000012544 monitoring process Methods 0.000 description 3
- 229920000642 polymer Polymers 0.000 description 3
- 239000000047 product Substances 0.000 description 3
- 230000005855 radiation Effects 0.000 description 3
- 239000000523 sample Substances 0.000 description 3
- 239000006057 Non-nutritive feed additive Substances 0.000 description 2
- 238000000576 coating method Methods 0.000 description 2
- 230000005484 gravity Effects 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 241000467686 Eschscholzia lobbii Species 0.000 description 1
- 239000004698 Polyethylene Substances 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 238000009529 body temperature measurement Methods 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 239000000919 ceramic Substances 0.000 description 1
- 238000001311 chemical methods and process Methods 0.000 description 1
- 238000002485 combustion reaction Methods 0.000 description 1
- 229920001577 copolymer Polymers 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 238000001125 extrusion Methods 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 239000003517 fume Substances 0.000 description 1
- 238000000691 measurement method Methods 0.000 description 1
- 229910001092 metal group alloy Inorganic materials 0.000 description 1
- 238000004377 microelectronic Methods 0.000 description 1
- 238000005065 mining Methods 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 239000013307 optical fiber Substances 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229920000573 polyethylene Polymers 0.000 description 1
- 238000004663 powder metallurgy Methods 0.000 description 1
- 230000000171 quenching effect Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 238000010008 shearing Methods 0.000 description 1
- 238000005245 sintering Methods 0.000 description 1
- 239000002904 solvent Substances 0.000 description 1
- 238000000807 solvent casting Methods 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 229920001169 thermoplastic Polymers 0.000 description 1
- 239000004416 thermosoftening plastic Substances 0.000 description 1
- 238000001721 transfer moulding Methods 0.000 description 1
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
- B29C48/25—Component parts, details or accessories; Auxiliary operations
- B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
- B29C48/475—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die using pistons, accumulators or press rams
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Processing And Handling Of Plastics And Other Materials For Molding In General (AREA)
Abstract
A B S T R A C T
A process for producing articles of polytetra-fluoroethylene (PTFE) is disclosed. The process allows some of the conventional plastic fabrication techniques to be used with PTFE which until now have not been possible. The process comprises the steps of intensively mixing and thermokinetically heating PTFE
granules in an enclosed container with a plurality of blades attached to arms rotating about an axis, with a blade tip speed of at least 30 meters/second, to a pre-determined discharge temperature of above 370°C, discharging the granules at the discharge temperature, forming the discharged, heated granules into an article of desired shape at or below the discharge temperature, and cooling the shaped article.
A process for producing articles of polytetra-fluoroethylene (PTFE) is disclosed. The process allows some of the conventional plastic fabrication techniques to be used with PTFE which until now have not been possible. The process comprises the steps of intensively mixing and thermokinetically heating PTFE
granules in an enclosed container with a plurality of blades attached to arms rotating about an axis, with a blade tip speed of at least 30 meters/second, to a pre-determined discharge temperature of above 370°C, discharging the granules at the discharge temperature, forming the discharged, heated granules into an article of desired shape at or below the discharge temperature, and cooling the shaped article.
Description
12(~5~31~
PROCESS FOR PRODUCING ARTICLES FROM POLYTETRAFLUOROET~YLENE
This invention relates to polytetrafluoroethylene ~PTFE). More specifically, the invention relates to the production of articles made from PTFE in shorter times and by simpler methods than has been previously possible.
Because of its high and low temperature properties PTFE has many commercial uses. PTFE has a usefuI tempera-ture range from -200C or lower, up to a maximum of 260C.
Production of articles is expensive as it has not been possible to use conventional plastic processing techniques.
At the present time powder metallurgy processing techniques such as those used with metal alloys and ceramics are applied to PTFE to produce molded products. In one embodi-ment PTFE granuIes are molded at pressures in the range of 13.8 to 68.9 megapascals followed by sintering at tempera-tures of about 380C. PTFE products can be ram extrudedat slow rates, but a processing aid, generally naphtha must be used and it is then necessary to remove this pro-cessing aid by heating after the molding step. Most exist-ing techniques produce a final product which is sintered and opaque, rather than fused, since the very high melting point of PTFE is close to the decomposition temperature.
Furthermore, since there is no known solvent for this very stable polymer, it has not been possible to fabricate sheets, films, fryingpan coatings, etc. ~y solvent casting tech-niques. There are some exotic techniques such as photo-chemical processes that produce fusible or transparentforms of PTFE, but they are most suitable for specialized applications such as micro electronic parts. Processible perfluorinated polymers, such as fluorinated ethylene, propylene copolymer, have been introduced to the market place, but the useful range of temperatures is then reduced to about 200C.
lZ~93 1~
It has now been found that PTFE reslns which are commercially available ma~ be directly formed into articles of a desired shape by initially carrying out a high intensity mixing and heating process wherein the resin is intensively mixed and thermokinetically heated in a manner which controls thermal degradation and without shearing action occurring. One type of high intensity mixer is shown by Goeser et al in U.S. patent 3,266,738, published August 16, 1966. This patent describes a high intensity mixer avail-able on the market today, under the trade mark Gelimat,made by Draiswerke GmbH. The mixer includes a plurality of blades which rotate about an axis within an enclosed con-tainer. In the past tip speeds have been in the order of up to 25 meters per second. Higher tip speeds have not generally been feasible because of problems controlling the temperature of the batch at the discharge. Tip speeds above 25 meters/second have recently been usea in accordance with a control system as disclosed in U.S. patent 4,230,615, issued October 28, 1980. According to this patent a system is provided for monitoring the batch temperature in the mixer separately from the mixer temperature, and then discharging the batch from the mixer when the batch temperature reaches a final predetermined level. It has surprisingly been found that PTFE resins may be first processed in a high intensity mixer having blade tip speeds of at least 30 meters/second and preferably 40 meters/second or higher. In this process the resin is intensively mixed and thermokinetically heated to the desired forming temperature which is above 370C and preferably 450C or higher. To reach this temperature range the mi~ing step requires a period of time in the order of 1 - 2 minutes.
After the heated unstabilized resin is dis-charged from the high intensity mixer it must be processed or formed into an article before the material cools. In the form it exits from the high intensity mixer it may be 3i~fl processed directly by some conventional or modified plastic fabrication techniques, such as compression molding, transfer molding, forging, stamping, ram extrusion or press-ing into the desired shape and thickness.
Fabricated articles produced by the process of the present invention have physical properties at least as good as those articles produced by conventional methods.
The present invention provides, a process for pro-duction of a shaped polytetrafluoroethylene article comprising, introducing polytetrafluoroethylene granules into a high intensity mixer comprising an enclosed container having a plurality of blades rotatable on arms about a central axis; rotating said blades at a blade tip speed of at least 30 meters per second whereby the polytetrafluoro-ethylene granules are subjected to an intense mixing andthermokinetic heating action of the rotating blades until the granules become heated to a predetermined discharge temperature of above 370C; discharging the polytetrafluoro-ethylene granules at said discharge temperature; forming the discharged, heated, granules into an article of desired shape at or below said discharge temperature; and cooling the shaped article.
In preferred embodiments, the blade tip speed is at least about 40 meters/second and the predetermined discharge temperature is at least about 450C. With a blade tip speed of about 35 meters/second and a discharge temperature of about 450C the mixing and heating step is in the range of about 80 - 120 seconds. When the blade tip speed is about 40 meters/second and the discharge tem-3Q perature in the range of about 475 - 480C the mixing and heating step is in the range of about 60 - 80 seconds.
In a further embodiment the article comprises a sheet of polytetrafluoroeth~lene and the forming step com-prises pressing the discharged resin between platens to form the sheet. The pressing occurs between hot plates at a temperature of at least about 150C and a pressure of at least about 2200 kilopascals, the pressing step lasts for about 3 minutes, followed by a cooling step for about 2 minutes. In another emhodiment the total processing time for making the article is in the range of about 6 - 7 minutes.
Commercial grades of PTFE resin, generally in granular form, may be processed without additives, or may include additives for stabilization. Additives such as pigments may be added for colour as desired.
The forming may take place in a press and in one embodiment is a coating layer on a substrate. In another embodiment the desired shape is a billet which is cooled to a temperature suitable for forming into a finished article by direct stamping or forging. The resin is preferably discharged from the intensive mixing step directly into a compression molding step. In yet a further embodiment the forming comprises extruding the discharged resin in a ram extruder. The article formed into a desired shape is prefer-~ ably held at an elevated temperature above a predetermined forming temperature, i.e. at least 150C or in some cases at least 170C, prior to cooling the shaped article.
High intensity mixers are known for processing thermo-plastic and thermo-setting materials. Reference is made to ~.S. patent 4,230,615 which discloses a control system utilizing a high intensity mixer. Ultra high molecuIar weight polyethylene articles may be processed in a high intensity mixer, as disclosed in U.S. patent 4,272,474.
The Gelimat, one example of a high intensity mixer, is satisfactory for operating the present process. Such a mixer has a plurality of blades attached at the end of arms radiating from a central axial shaft which rotates within an enclosed container. Variation in the speed of the rotating shaft changes the blade tip speed and in the present invention it is found that blade tip speeds of 30 lZ~5~3~
meters/second and higher are satisfactory. The Gelimat high intensity mixer may be of the screw feed design having a feed screw at one end which feeds resin into the mixer, or may be of the stop-start type which includes an input hatch at the top of the container, which is opened to feed material into the mixer. In both cases, in order to empty the mixer there is an exit flap which opens to permit the batch of material processed in the mixer to be dropped out and removed for further processing.
As disclosed in U.S. patent 4,230,615, the mixer is preferably fitted with a system for monitoring the infra-red radiation from the batch and hence temperature of the batch in the mixer separately from the temperature of the mixer walls and blades. The batch is discharged from the mixer when the batch temperature reaches a predetermined level. The measurement of the temperature of the batch by infrared radiation is instantaneous and is quite unrelated to the temperature of the mixer walls or blades. When the batch reaches the predetermined discharge temperature, the exit flap of the mixer opens and the batch is discharged for further processing. The energy from the high speed rotating blades in the mixer body intensively mix and thermo-kinetically heat the PTFE resin batch. As the control of the mixing and heating step is by temperature, it is found that the time of each batch can vary slightly due to a number of reasons, however, the state of each batch and its discharge temperature are constant.
In a series of tests, batches of PTFE resin ranging from 200 - 300 grams were processed in a Gelimat high intensity mixer having a capacity of 1.4 litres. The temperature of each batch was measured using an infrared monitoring system which determined the infrared radiation of the resin in the mixer through an optical fiber system. The system was capable of accurate temperature measurement in the range from 160 - 600~C. The temperature of the batch 1~93~
discharging from the mixer was also measured with a pocket probe digital pyrometer fitted with a needle-nose probe~
The time in seconds from introduction of the resin to the mixer until discharge, was measured with a stopwatch. Tests were carried out with the mixer having blade tip speeds of approximately 35 meters/second and 40.5 meters/second.
The batches of PTFE hot resin discharging from the Gelimat high intensity mixer were immediately pressed into sheets having a thickness of approximately 3 mm. The press plates were steam heated to temperatures of 170C
with the frame preheated to temperatures of up to 200C to avoid too great a thermal quenching effect. Thus, the temperature of the PTFE batch in the press was below the discharge temperature from the mixer. Pressures in the press were as low as 2200 kilopascals. The pressing cycle lasted for approximately 3 minutes at the raised tempera-ture, followed by an approximately 2 minute cooling phase, during which time water was circulated through the press to cool the plates. Whereas the tests carried out were for substantially thin sheets, the limitations on the press were limitations of existing equipment, larger sheets and articles couId be more easily formed in presses or molds heated to temperatures closer to the discharge temperature of the resin.
The properties of the pressed sheets from PTFE
were tested by standard measurement techniques. The specific gravity was measured by water displacement, hardness tests and tests for dielectric response were carried out r and a differential scanning calorimeter (DSC~ was used for deter-mining melting points at both heating and cooling rates of 20C per minute. In the DSC tests, after the first heatingcycle, the sample was maintained above the melting point for approximately 5 minutes before the cooling cycle com-menced and then held at approximately 45~C for 5 - lO
minutes before the second heating cycle was begun.
93~
Typical processing details for tests of PTFE
batches utilizing a screw feed 1.4 litre Gelimat high intensit~ mixer with a tip speed of 40.5 meters/second and at a ~atch weight of approximately 300 grams, are given below.
TABLE I
Test 1 2 Mixer temperature before batch added 257C 329C
Time to reach predetermined temperature 10 in mixer 62 sec. 77 sec.
Temperature of batch after discharge measured by pyrometer 475C 480C
Batches were pressed into sheets immediately after discharge from the mixer. The total time from the intro-duction of the resin batch into the mixer to the end of the cooling cycle for the sheet, was in the order of 6 - 7 minutes. Samples produced in these tests were analyzed to determine their physical properties and compare these physical properties against PTFE articles made by conven-tional methods.
Table II sets out the results of the physicalproperty analysis.
TABLE II
Conventional Test Test Product No: 1 No: 2 Values Specific Gravity 2.14 - 2.20 2.20 2.17 Hardness (D-Scale) 50 55 ~ 50 ~,50 Dielectric Content 2.1 n~2 ~ 2 Dielectric Loss Factor Low Low Low (Tan ) (Not Measurable) Limiting Oxygen Index (LOI) High >80 >80 Melting Point (C) 3~7 - 330 328.5 333.6 12~5~31'~
The measurement of the melting points was carried out on a Perkin Elmer Model 2C differential scanning calorimeter, the readings are taken from the second heating cycle. The melting points increased from the first heating cycle to the second heating cycle by 1 and 2.2C for the first and second tests respectively. When the melting points of a commercially available sheet of PTFE was measured on the DSC it was found that the melting point for the second heating cycle decreased by approximately 0.5C. This wouId indicate that the mixing and heating step causes the PTFE resin to pass through an effective melt stage so that the crystalline state of the processed resin is closer to the equilibrium condition of the material.
As indicated by the tests, the physical properties of the PTFE articles produced by the process of the present invention, are as good if not better than sheets of PTFE
produced by conventional methods.
Tests run at discharge temperatures of less than 370C could be pressed following discharge from the mixer, but had little integr~ty and retained a very sintered whitish appearance. Tests carried out with blade tip speeds in the mixer of less than about 30 meters/second produced batches that couId be pressed into sheets, however, the sheets were hand breakable. When precise temperature measuring capability above 450C was not available and longer processing times in the mixer were run, then a free-flowing melt was produced that in some cases burst spon-taneously into flames and gave off obnoxious fumes despite the polymer's known resistance to combustion in air.
Whereas pressing into sheets is the only further processing step in the forming of articles of PTFE resin disclosed herein, it will be obvious to those skilled in the art that some other types of conventional processing of plastic materials may ~e carried out with the output from the high intensity mixer, provided it is processed ~Z~931~
immediately after discharge from the mixer.
In one instance the PTFE resin discharged from the mixer i5 formed into a billet which is cooled to a temperature suitable for forming into a finished article by direct stamping or forging. In another embodiment the resin is discharged from the mixer directly into a com-pression molding step. The forming may also occur in a ram extruder.
PTFE applications include coatings on a substrate.
This application is particuIarly usefuI because of the chemical resistance properties and heat properties of PTFE.
Various amendments may be made to the process defined herein without departing from the scope of the present invention, which is limited only by the scope of the claims.
PROCESS FOR PRODUCING ARTICLES FROM POLYTETRAFLUOROET~YLENE
This invention relates to polytetrafluoroethylene ~PTFE). More specifically, the invention relates to the production of articles made from PTFE in shorter times and by simpler methods than has been previously possible.
Because of its high and low temperature properties PTFE has many commercial uses. PTFE has a usefuI tempera-ture range from -200C or lower, up to a maximum of 260C.
Production of articles is expensive as it has not been possible to use conventional plastic processing techniques.
At the present time powder metallurgy processing techniques such as those used with metal alloys and ceramics are applied to PTFE to produce molded products. In one embodi-ment PTFE granuIes are molded at pressures in the range of 13.8 to 68.9 megapascals followed by sintering at tempera-tures of about 380C. PTFE products can be ram extrudedat slow rates, but a processing aid, generally naphtha must be used and it is then necessary to remove this pro-cessing aid by heating after the molding step. Most exist-ing techniques produce a final product which is sintered and opaque, rather than fused, since the very high melting point of PTFE is close to the decomposition temperature.
Furthermore, since there is no known solvent for this very stable polymer, it has not been possible to fabricate sheets, films, fryingpan coatings, etc. ~y solvent casting tech-niques. There are some exotic techniques such as photo-chemical processes that produce fusible or transparentforms of PTFE, but they are most suitable for specialized applications such as micro electronic parts. Processible perfluorinated polymers, such as fluorinated ethylene, propylene copolymer, have been introduced to the market place, but the useful range of temperatures is then reduced to about 200C.
lZ~93 1~
It has now been found that PTFE reslns which are commercially available ma~ be directly formed into articles of a desired shape by initially carrying out a high intensity mixing and heating process wherein the resin is intensively mixed and thermokinetically heated in a manner which controls thermal degradation and without shearing action occurring. One type of high intensity mixer is shown by Goeser et al in U.S. patent 3,266,738, published August 16, 1966. This patent describes a high intensity mixer avail-able on the market today, under the trade mark Gelimat,made by Draiswerke GmbH. The mixer includes a plurality of blades which rotate about an axis within an enclosed con-tainer. In the past tip speeds have been in the order of up to 25 meters per second. Higher tip speeds have not generally been feasible because of problems controlling the temperature of the batch at the discharge. Tip speeds above 25 meters/second have recently been usea in accordance with a control system as disclosed in U.S. patent 4,230,615, issued October 28, 1980. According to this patent a system is provided for monitoring the batch temperature in the mixer separately from the mixer temperature, and then discharging the batch from the mixer when the batch temperature reaches a final predetermined level. It has surprisingly been found that PTFE resins may be first processed in a high intensity mixer having blade tip speeds of at least 30 meters/second and preferably 40 meters/second or higher. In this process the resin is intensively mixed and thermokinetically heated to the desired forming temperature which is above 370C and preferably 450C or higher. To reach this temperature range the mi~ing step requires a period of time in the order of 1 - 2 minutes.
After the heated unstabilized resin is dis-charged from the high intensity mixer it must be processed or formed into an article before the material cools. In the form it exits from the high intensity mixer it may be 3i~fl processed directly by some conventional or modified plastic fabrication techniques, such as compression molding, transfer molding, forging, stamping, ram extrusion or press-ing into the desired shape and thickness.
Fabricated articles produced by the process of the present invention have physical properties at least as good as those articles produced by conventional methods.
The present invention provides, a process for pro-duction of a shaped polytetrafluoroethylene article comprising, introducing polytetrafluoroethylene granules into a high intensity mixer comprising an enclosed container having a plurality of blades rotatable on arms about a central axis; rotating said blades at a blade tip speed of at least 30 meters per second whereby the polytetrafluoro-ethylene granules are subjected to an intense mixing andthermokinetic heating action of the rotating blades until the granules become heated to a predetermined discharge temperature of above 370C; discharging the polytetrafluoro-ethylene granules at said discharge temperature; forming the discharged, heated, granules into an article of desired shape at or below said discharge temperature; and cooling the shaped article.
In preferred embodiments, the blade tip speed is at least about 40 meters/second and the predetermined discharge temperature is at least about 450C. With a blade tip speed of about 35 meters/second and a discharge temperature of about 450C the mixing and heating step is in the range of about 80 - 120 seconds. When the blade tip speed is about 40 meters/second and the discharge tem-3Q perature in the range of about 475 - 480C the mixing and heating step is in the range of about 60 - 80 seconds.
In a further embodiment the article comprises a sheet of polytetrafluoroeth~lene and the forming step com-prises pressing the discharged resin between platens to form the sheet. The pressing occurs between hot plates at a temperature of at least about 150C and a pressure of at least about 2200 kilopascals, the pressing step lasts for about 3 minutes, followed by a cooling step for about 2 minutes. In another emhodiment the total processing time for making the article is in the range of about 6 - 7 minutes.
Commercial grades of PTFE resin, generally in granular form, may be processed without additives, or may include additives for stabilization. Additives such as pigments may be added for colour as desired.
The forming may take place in a press and in one embodiment is a coating layer on a substrate. In another embodiment the desired shape is a billet which is cooled to a temperature suitable for forming into a finished article by direct stamping or forging. The resin is preferably discharged from the intensive mixing step directly into a compression molding step. In yet a further embodiment the forming comprises extruding the discharged resin in a ram extruder. The article formed into a desired shape is prefer-~ ably held at an elevated temperature above a predetermined forming temperature, i.e. at least 150C or in some cases at least 170C, prior to cooling the shaped article.
High intensity mixers are known for processing thermo-plastic and thermo-setting materials. Reference is made to ~.S. patent 4,230,615 which discloses a control system utilizing a high intensity mixer. Ultra high molecuIar weight polyethylene articles may be processed in a high intensity mixer, as disclosed in U.S. patent 4,272,474.
The Gelimat, one example of a high intensity mixer, is satisfactory for operating the present process. Such a mixer has a plurality of blades attached at the end of arms radiating from a central axial shaft which rotates within an enclosed container. Variation in the speed of the rotating shaft changes the blade tip speed and in the present invention it is found that blade tip speeds of 30 lZ~5~3~
meters/second and higher are satisfactory. The Gelimat high intensity mixer may be of the screw feed design having a feed screw at one end which feeds resin into the mixer, or may be of the stop-start type which includes an input hatch at the top of the container, which is opened to feed material into the mixer. In both cases, in order to empty the mixer there is an exit flap which opens to permit the batch of material processed in the mixer to be dropped out and removed for further processing.
As disclosed in U.S. patent 4,230,615, the mixer is preferably fitted with a system for monitoring the infra-red radiation from the batch and hence temperature of the batch in the mixer separately from the temperature of the mixer walls and blades. The batch is discharged from the mixer when the batch temperature reaches a predetermined level. The measurement of the temperature of the batch by infrared radiation is instantaneous and is quite unrelated to the temperature of the mixer walls or blades. When the batch reaches the predetermined discharge temperature, the exit flap of the mixer opens and the batch is discharged for further processing. The energy from the high speed rotating blades in the mixer body intensively mix and thermo-kinetically heat the PTFE resin batch. As the control of the mixing and heating step is by temperature, it is found that the time of each batch can vary slightly due to a number of reasons, however, the state of each batch and its discharge temperature are constant.
In a series of tests, batches of PTFE resin ranging from 200 - 300 grams were processed in a Gelimat high intensity mixer having a capacity of 1.4 litres. The temperature of each batch was measured using an infrared monitoring system which determined the infrared radiation of the resin in the mixer through an optical fiber system. The system was capable of accurate temperature measurement in the range from 160 - 600~C. The temperature of the batch 1~93~
discharging from the mixer was also measured with a pocket probe digital pyrometer fitted with a needle-nose probe~
The time in seconds from introduction of the resin to the mixer until discharge, was measured with a stopwatch. Tests were carried out with the mixer having blade tip speeds of approximately 35 meters/second and 40.5 meters/second.
The batches of PTFE hot resin discharging from the Gelimat high intensity mixer were immediately pressed into sheets having a thickness of approximately 3 mm. The press plates were steam heated to temperatures of 170C
with the frame preheated to temperatures of up to 200C to avoid too great a thermal quenching effect. Thus, the temperature of the PTFE batch in the press was below the discharge temperature from the mixer. Pressures in the press were as low as 2200 kilopascals. The pressing cycle lasted for approximately 3 minutes at the raised tempera-ture, followed by an approximately 2 minute cooling phase, during which time water was circulated through the press to cool the plates. Whereas the tests carried out were for substantially thin sheets, the limitations on the press were limitations of existing equipment, larger sheets and articles couId be more easily formed in presses or molds heated to temperatures closer to the discharge temperature of the resin.
The properties of the pressed sheets from PTFE
were tested by standard measurement techniques. The specific gravity was measured by water displacement, hardness tests and tests for dielectric response were carried out r and a differential scanning calorimeter (DSC~ was used for deter-mining melting points at both heating and cooling rates of 20C per minute. In the DSC tests, after the first heatingcycle, the sample was maintained above the melting point for approximately 5 minutes before the cooling cycle com-menced and then held at approximately 45~C for 5 - lO
minutes before the second heating cycle was begun.
93~
Typical processing details for tests of PTFE
batches utilizing a screw feed 1.4 litre Gelimat high intensit~ mixer with a tip speed of 40.5 meters/second and at a ~atch weight of approximately 300 grams, are given below.
TABLE I
Test 1 2 Mixer temperature before batch added 257C 329C
Time to reach predetermined temperature 10 in mixer 62 sec. 77 sec.
Temperature of batch after discharge measured by pyrometer 475C 480C
Batches were pressed into sheets immediately after discharge from the mixer. The total time from the intro-duction of the resin batch into the mixer to the end of the cooling cycle for the sheet, was in the order of 6 - 7 minutes. Samples produced in these tests were analyzed to determine their physical properties and compare these physical properties against PTFE articles made by conven-tional methods.
Table II sets out the results of the physicalproperty analysis.
TABLE II
Conventional Test Test Product No: 1 No: 2 Values Specific Gravity 2.14 - 2.20 2.20 2.17 Hardness (D-Scale) 50 55 ~ 50 ~,50 Dielectric Content 2.1 n~2 ~ 2 Dielectric Loss Factor Low Low Low (Tan ) (Not Measurable) Limiting Oxygen Index (LOI) High >80 >80 Melting Point (C) 3~7 - 330 328.5 333.6 12~5~31'~
The measurement of the melting points was carried out on a Perkin Elmer Model 2C differential scanning calorimeter, the readings are taken from the second heating cycle. The melting points increased from the first heating cycle to the second heating cycle by 1 and 2.2C for the first and second tests respectively. When the melting points of a commercially available sheet of PTFE was measured on the DSC it was found that the melting point for the second heating cycle decreased by approximately 0.5C. This wouId indicate that the mixing and heating step causes the PTFE resin to pass through an effective melt stage so that the crystalline state of the processed resin is closer to the equilibrium condition of the material.
As indicated by the tests, the physical properties of the PTFE articles produced by the process of the present invention, are as good if not better than sheets of PTFE
produced by conventional methods.
Tests run at discharge temperatures of less than 370C could be pressed following discharge from the mixer, but had little integr~ty and retained a very sintered whitish appearance. Tests carried out with blade tip speeds in the mixer of less than about 30 meters/second produced batches that couId be pressed into sheets, however, the sheets were hand breakable. When precise temperature measuring capability above 450C was not available and longer processing times in the mixer were run, then a free-flowing melt was produced that in some cases burst spon-taneously into flames and gave off obnoxious fumes despite the polymer's known resistance to combustion in air.
Whereas pressing into sheets is the only further processing step in the forming of articles of PTFE resin disclosed herein, it will be obvious to those skilled in the art that some other types of conventional processing of plastic materials may ~e carried out with the output from the high intensity mixer, provided it is processed ~Z~931~
immediately after discharge from the mixer.
In one instance the PTFE resin discharged from the mixer i5 formed into a billet which is cooled to a temperature suitable for forming into a finished article by direct stamping or forging. In another embodiment the resin is discharged from the mixer directly into a com-pression molding step. The forming may also occur in a ram extruder.
PTFE applications include coatings on a substrate.
This application is particuIarly usefuI because of the chemical resistance properties and heat properties of PTFE.
Various amendments may be made to the process defined herein without departing from the scope of the present invention, which is limited only by the scope of the claims.
Claims (16)
1. A process for production of a shaped poly-tetrafluoroethylene article comprising:
introducing polytetrafluoroethylene granules into a high intensity mixer comprising an enclosed container having a plurality of blades rotatable on arms about a central axis;
rotating said blades at a blade tip speed of at least 30 meters per second whereby the polytetrafluoro-ethylene granules are subjected to an intense mixing and thermokinetic heating action of the rotating blades until the granules become heated to a predetermined discharge temperature of above 370°C;
discharging the polytetrafluoroethylene granules at said discharge temperature;
forming the discharged, heated, granules into an article of desired shape at or below said discharge temperature; and cooling the shaped article.
introducing polytetrafluoroethylene granules into a high intensity mixer comprising an enclosed container having a plurality of blades rotatable on arms about a central axis;
rotating said blades at a blade tip speed of at least 30 meters per second whereby the polytetrafluoro-ethylene granules are subjected to an intense mixing and thermokinetic heating action of the rotating blades until the granules become heated to a predetermined discharge temperature of above 370°C;
discharging the polytetrafluoroethylene granules at said discharge temperature;
forming the discharged, heated, granules into an article of desired shape at or below said discharge temperature; and cooling the shaped article.
2. The process according to claim 1 wherein the blade tip speed is at least about 40 meters/second.
3. The process according to claim 1 wherein the predetermined discharge temperature is at least about 450°C.
4. The process according to claim 3 wherein the blade tip speed is about 35 meters/second, the predetermined discharge temperature is about 450°C and the time period for the mixing and heating step is in the range of about 80 - 120 seconds.
5. The process according to claim 3 wherein the blade tip speed is about 40 meters/second, the predetermined discharge temperature is in the range of about 475 - 480°C
and the time period for the mixing and heating step is in the range of about 60 - 80 seconds.
and the time period for the mixing and heating step is in the range of about 60 - 80 seconds.
6. The process according to claim 1 wherein the article comprises a sheet of polytetrafluoroethylene and the forming step comprises pressing the discharged resin between platens to form said sheet.
7. The process according to claim 6 wherein the pressing step occurs between hot plates at a temperature of at least about 150°C at a pressure of at least about 2200 kilopascals, the pressing step occurring for about 3 minutes followed by the cooling step occurring for about 2 minutes.
8. The process according to any of claims 1, 6 or 7 wherein the total processing time for making the article is in the range of about 6 - 7 minutes.
9. The process according to claim 1, including the addition of pigments and/or other additives to the polytetrafluoroethylene resin for the mixing and heating step.
10. The process according to claim 1, wherein the forming takes place in a press and the desired shape is a coating layer on a substrate.
11. The process according to claim 1, wherein the desired shape is a billet which is cooled to a tem-perature suitable for forming into a finished article by direct stamping or forging.
12. The process according to claim 1 wherein the resin is discharged from the intensive mixing step directly into a compression molding step.
13. The process according to claim 1, wherein forming comprises extruding the discharged resin in a ram extruder.
14. The process according to claim 1 wherein the article formed into desired shape is held at an elevated temperature above a predetermined forming temperature prior to cooling the shaped article.
15. The process according to claim 14 wherein said predetermined forming temperature is at least 150°C.
16. The process according to claim 14 wherein said predetermined forming temperature is at least 170°C.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000435981A CA1209314A (en) | 1983-09-02 | 1983-09-02 | Process for producing articles from polytetrafluoroethylene |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000435981A CA1209314A (en) | 1983-09-02 | 1983-09-02 | Process for producing articles from polytetrafluoroethylene |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1209314A true CA1209314A (en) | 1986-08-12 |
Family
ID=4126008
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000435981A Expired CA1209314A (en) | 1983-09-02 | 1983-09-02 | Process for producing articles from polytetrafluoroethylene |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1209314A (en) |
-
1983
- 1983-09-02 CA CA000435981A patent/CA1209314A/en not_active Expired
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