US 3676181 A
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United States Patent 3,676,181 ENT ELECTRICAL DISCHARGE TREATM OF TETRAFLUOROETHYLENE/HEXA- FLUOROPROPYLENE COPOLYMER IN US. Cl. 117-47 A ABSTRACT OF THE DISCLOSURE Treatment of tetrafluoroethylene/hexafluoropropylene copolymer surface by electrical discharge In an atmosphere containing 3 to 40% by volume of acetone to render the surface more adherable to another material.
This invention relates to a process for the treatment of a tetrafiuoroethylene/hexafluoropropylene surface to render it more adherable to another surface. More particularly this invention relates to the treatment of a fluorocarbon surface by electrical discharge in an atmosphere containing between 3 and 40% acetone.
It is known in the art that fluorocarbon surfaces are highly inert chemically. Because of this it is dlfficult to obtain good adhesion between a fluorocarbon surface and another material. One solution suggested by the prior art to improve the adherability of fluorocarbon is to treat it with a corona discharge. By means of such treatment it is possible to obtain a fluorocarbon surface that Wlll adhere to another material such as a polyimide layer. Such a process is fully disclosed in the Anderson et al. lat. 3,352,714.
It is also known to improve the adherabihty of a flllOI'O carbon surface by electrical discharge treatment of the surface in an atmosphere containing less than 5% by volume of an organic compound, such as glycidyl methacrylate. See US. Pats. 3,296,011 and 3,274,089.
The process of this invention for improving the adherability of a fluorocarbon surface to another matenal comprises treating the fluorocarbon surface with an electrical discharge in an atmosphere containing between 3 and 40% by volume of acetone. Preferably, the atmosphere contains between 15 and 30% by volume of the acetone, while the most preferred concentration is about 20% by volume. The atmosphere also contains a carrier gas that is substantially inert under the conditions of treatment. Such gases include nitrogen, helium, argon, carbon dioxide and the like as well as mixtures thereof which contain less than 600 p.p.m. by volume oxygen. The preferred oxygen content is less than 100 p.p.m. by volume.
A suitable apparatus for carrying out the process of the present invention is fully described in US. Pat. 3,274,089. A continuous self-supporting fluorocarbon film is passed continuously between a set of spaced electrodes consisting of a rotating metal roll which is connected electrically to ground, and one or more stationary hollow metal tubes disposed parallel to the longitudinal axis of the roll and spaced a distance of from 0.03 to 0.125 of an inch from the surface thereof. The tubes are each connected electrically to a suitable power source which supplies an alternating (or pulsating direct) current of the required intensity at the required voltage and frequency. The acetone containing atmosphere, i.e., acetone admixed with a suitable carrier gas, is fed continuously to the hollow interior of the electrode tubes through distributor ducts and issues from the tubes, through suitable openings therein, at the gap betwen each tube and the roll. The electrical discharge takes place in the atmosphere containing the acetone. The atmosphere may also be introduced into the reaction zone through one or more tubes separate from the electrode ice assembly. The assembly just described is suitably enclosed in a chamber, held at substantially atmospheric pressure and provided with the necessary openings to facilitate maintenance of the atmosphere of carrier gas and acetone therein and to permit controlled exhaust of the vapors therefrom. The treated film may be passed through a heating zone and/or a coating apparatus whereby to further condition the surface of the film to enhance the permanency of the effect of the treatment.
In carrying out the surface treatment of this invention the potential difference between the electrodes may vary from low voltages in the order of 1000 volts up to pulsating voltages of 100,000 and above. In general, however, it is preferred to maintain the voltage in excess of 2000- 3000 volts. Frequencies from 350 cycles per second up to 500,000 cycles per second and above can be used. Frequencies in the range of about 6,000 to 15,000 cycles are preferred in order to obtain effective treatment at commercially acceptable exposure times. While the current to the electrodes may range up to 360 R.M.S. (root mean square) milliamperes per square inch of electrode or more, for optimum results a range of from 20 R.M.S. milliamperes per square inch of electrode to 230 R.M.S. milliamperes per square inch of electrode is preferred. Power to the electrodes may range from 10 watts per lineal inch of the electrode length to watts per lineal inch of the electrode length.
The electrodes are preferably spaced from about .03 inch to about 0.1-25 inch. Useful results can be obtained when the electrode gap is as low as 0.015 inch to as much as 0.25 inch, provided suitable adjustments in such features as amount of current, electrode dimension and exposure time are made. Time of exposure to the electrical discharge treatment is not especially critical and effective treatments are realized at exposure times as short as 0.01 second and no adverse effects are noted at times as long as 60 seconds. Preferably the exposure time should not be less than 0.1 second. For economic reasons, exposure times as short as possible consistent with effective treatment would normally be employed.
In order to achieve the desired increase in adhesion, the tetrailuoroethylene/hexafluoropropylene surface should be subjected to between 0.15 and 2.5 watt hrs. per square foot of sheet surface treated.
Flow of the carrier gas/ acetone mixture to the electrodes may be as low as one-twelfth cubic foot per foot of electrode per minute up to 1.67 cubic feet per foot of electrode per minute. Higher flow rates can be used though economic considerations would dictate against use of amounts exceeding those required to produce the desired effects.
The carrier gas/acetone mixture may be obtained by bubbling the carrier gas through acetone. Carrier gas that is approximately saturated with acetone will be obtained using this method. In the case where nitrogen is used as the carrier gas, at room temperature (20 C.) the amount of acetone is about 20% by volume. The amount of acetone in the atmosphere can be regulated by regulating the temperature of the carrier gas, or the amount of acetone regulated by feeding into the carrier gas/acetone mixture additional carrier gas.
The tetrafiuoroethylene/hexafluoropropylene copolymer films may be treated either on one surface or on both surfaces, depending on the use to which the films are to be put after treatment.
Tetrafluoroethylene hexafluoropropylene copolymer films treated in accordance with the present invention form bonds that are more durable than those formed in previously used treatments, i.e., treatments in atmospheres containing glycidyl methacrylate, when exposed to moisture or when exposed to temperatures of about 200 C. Furthermore, the surface is more resistant to loss of adherability caused by abrasion or exposure to ultraviolet light.
This film has the further advantage that it does not adhere tightly to itself, and thus may be wound in rolls without an interleaf, or sheets of the film may be stacked without severe blocking.
EXAMPLE 1 A sample of mil thick tetrafiuoroethylene/hexafluoro propylene copolymer (wt. ratio 85/15) film of the type described by US. Pat. 2,946,763, was placed between metallic electrode plates 2 /2 inches in diameter and spaced 50 mils apart, positioned inside a gas-tight enclosure. After purging the enclosure with nitrogen until its oxygen concentration had dropped to 50 p.p.m. by volume, acetone vapor was introduced into the space between electrodes. This was done by passing a stream of nitrogen at about 20 C. through a gas washing bottle filled with liquid acetone, and then directing the resulting mixture of nitrogen/acetone to a flat distributor nozzle located just alongside the electrodes. An electrical discharge was then established across the electrodes by supplying 6,000 volts at 60 Hz. from an oscillator and power amplifier. A current flow of 165 a. was obtained. After 30 seconds the power was cut off and the treated sample removed.
Adherability of the treated surface was measured by laminating the film to an aluminum strip 6 inches long by 1 inch wide by 35 mils thick. The aluminum was first coated with Du Ponts Adhesive No. 6840 (an acrylic copolymer adhesive in which the copolymer is dissolved in a mixture of alcohol and aromatic hydrocarbon) and permitted to air dry overnight. A film sample cut to the same 1" wide x 6" long dimensions was then placed such that the treated surface of the film was in contact with the adhesive coated aluminum strip and then laminated at 20 p.s.i. in a Sentinel Heat Sealer, Model l2AS, for 60 seconds at 375 F. Only the last inch of length was pressed and cured together, leaving the remaining lengths unbonded so they could be gripped in the jaws of an Instron tensile tester and pulled apart in 180 peel to measure bond strength. In this test, the acetone treated film tore, indicating that bond strength exceeded film tensile strength.
The experiment was repeated with glycidyl methacrylate, approximately 0.05% by volume in nitrogen. Film peeled away from the test laminate at 6.7 lbs./ inch.
EXAMPLE 2 TABLE Bond strength Film in 180 thickness, peel, mils lbs/in.
20 10 8. 3 10 16. 2 Methyl alcohol. 5 11. 1 Methyl methacrylate 5 8. 5 Xylene. 5 2 10 1 Where film tore rather than delamlnated from the aluminum strip, bond strengths are shown as greater film tensile strength values.
2 Dropped to 5.6 lbs/in. after 2 weeks aging at room temperature.
EXAMPLE 3 A device was constructed which comprised a stationary bar electrode, A" wide and 6" long, positioned parallel to and above a rotatable, electrically grounded drum upon which film samples could be mounted. The whole assembly was enclosed in a gas-tight transparent case inside which '4 an essentially oxygen-free atmosphere could be maintained by purging with nitrogen. A perforated-tube gas distributor was provided for directing the organic vapor and nitrogen mixture into the gap between electrode and drum.
After first covering the drum with a dielectric bufier material to prevent any possible bum-through of the film being treated, an 8 inch x 14 inch sample of a tetrafluoroethylene/hexafluoropropylene copolymer (wt. ratio 15) film was wrapped tightly around the drum and clamped in position. With the enclosure replaced, nitrogen was purged through until an oxygen analyzer showed the interior to have only 50 p.p.m. by volume of 0 present. The drive motor was then started and the drum continually rotated at a surface speed of 8.3 feet per minute. Nitrogen, regulated at a fiow rate of 3,500 ml./min., was bubbled through liquid acetone to become essentially saturated with acetone vapor and this mixture was fed to the gas distributor and into the gap between electrode and drum. Power was supplied to the electrode to establish an electrical discharge, which was maintained for two complete revolutions of the drum and then shut off. The power source consisted of an oscillator which generated an alternating current at 10,000 Hz., coupled to a 400 VA power amplifier and a 30:1 step-up transformer. Voltage at the electrode was kept at 6,000 v.
The treated film was removed from the drum and cut into 1 inch x 6 inch strips for testing. Adherability was measured by the method described in Example 1. To determine resistance of the adherable surface to degradation by moisture, several of the samples were first immersed in boiling water for 16 hours and then tested.
Similar samples were prepared in the same manner except that glycidyl methacrylate was used in place of acetone.
Results are compared below:
Peel strength, lbs./in.
The objective of this experiment was to compare ability of the treated surfaces to withstand abrasion Without extensive loss of adherability. This property is important to manufacturers who use such film for making electrical printed circuits.
Samples of both acetone and glycidyl methacrylate treated film were prepared by the method described in Example 3. They were then abraded by a weighted bristle brush in a Gardner Straight-Line Wash Tester, with succesive samples receiving an increased number of brush strokes. Adherability of each sample was then measured by the peel test described in Example 1. Results are tabulated below:
Peel strength, lbs/in.
Additional samples were prepared in the same way and tested for resistance to ultraviolet light by exposing them in a Fadeometer for 200 hours. Adherability was measured after exposure. Results are:
Peel strength, lbs./iu.
GMA Acetone treated treated AS made 6. 64 l 20. After 200 hrs. in Fadeometer 4. 64 11. 16
1 Sample tore.
Peel strength, lbsJin.
GMA Acetone treated treated As made 1 9. 3 1 9. 5 After 8 hrs. at 225 C. in alr 8. 3 1 9. 0 After 1 day at 225 C. in alr- 3. 6 1 8. 7 After 2 days at 225 C. in air.-- 0 7. 4 After 4 days at 225 C. in air 0 3. 4 After 8 hrs. at 225 0.111 N2..-" 3. 0 1 9.0 After 1 day at 225 C. in N2.... 1.0 1 9. 0 After 2 days at 226 C. in N2- 0 l 8.7 Aiter4daysat 225 C.1I1Nz--- 0 8.7
4 Film tore.
What is claimed is:
1. In a process for the production of a tetrafiuoroethylene/hexafiuoropropylene copolymer film that has a surface that is adherable to another material which comprises subjecting said fluorocarbon film to an electric discharge between spaced electrodes in a gaseous atmosphere, the improvement which comprises employing as the atmosphere a mixture containing substantially inert carrier gas and between 15 and 30% by volume acetone.
2. The process of claim 1 in which the film is subjected to between 0.15 and 2.5 watt hours of electrical energy per square foot of sheeting surface treated.
3. The process of claim 1 in which the carrier gas is nitrogen.
4. The process of claim 1 in which the film is treated on both surfaces.
5. The process of claim 3 in which the nitrogen is at about 20 C.
References Cited UNITED STATES PATENTS 3,415,683 12/1968 Coifman et al 204-168 3,507,763 4/1970 McBride 204-169 3,255,099 6/1966 Wolinski 204----169 3,274,089 9/ 1966 Wolinski 204-169 3,274,090 9/1966 Amborski 204168 3,274,091 9/1966 Amborski 204-169 3,275,540 9/1966 McBride 204-169 3,296,011 1/ 1967 McBride 204-169 OTHER REFERENCES NASA Develops. Technique for Plating Metals on Metals or Fluorocarbon Polymers, Products Finishing, pp. 77-74, March 1966.
MURRAY KATZ, Primary Examiner W. R. TRENOR, Assistant Examiner U.S. Cl. X.R.
117-93.1 CD, 119, 138.8 UF; 204-168, 169