US3677471A

US3677471A - Apparatus and process thereof for coating with polytetrafluoroethylene and other materials

Info

Publication number: US3677471A
Application number: US119850A
Authority: US
Inventors: Stanley Thomas Deakin
Original assignee: Sealectro Corp
Current assignee: Sealectro Corp
Priority date: 1971-03-01
Filing date: 1971-03-01
Publication date: 1972-07-18
Anticipated expiration: 1989-07-18

Abstract

An apparatus and method thereof are described for coating surfaces with a continuous unitary film of polytetrafluoroethylene (PTFE) by spraying a heated stream of particles from a spray gun. The gun includes a heating means such as an electrical resistance unit, a mixing chamber where gas and preferably inert gas is mixed with the particles, and means for directing a stream of gas through the gun to force the heated particles onto the surface to be coated. Obviously, other plastic materials may be used at their appropriate melting temperatures.

Description

United States Patent Deakin [451 July 18, 1972 [54] APPARATUS AND PROCESS THEREOF FOR COATING WITH POLYTETRAFLUOROETI'IYLENE AND OTHER MATERIALS [72] Inventor: Stanley Thorns Deakin, Portsmouth, En-

gland [73] Assignee: Sealectro Corporation, Mamaroneck, NY.

[22] Filed: March 1, 1971 [21] Appl.No.: 119,850

Related US. Application Data [62] Division of Ser. No. 507,504, Nov. 12, 1965, aban FOREIGN PATENTS OR APPLICATIONS 1,054,610 10/1953 France ..239/85 Schleer et al. ..239/ l 35 Primary Examiner-Lloyd L. King Attorney-James M. Heilman ABSTRACT An apparatus and method thereof are described for coating continuous unitary film of polytetrafluoroethylene (PTFE) by spraying a heated stream of particles from a spray gun. The gun includes a heating means such as an electrical resistance unit, a mixing chamber where gas and preferably inert gas is mixed with the particles, and means for directing a stream of gas through the gun to force the heated particles onto the surface to be coated. Obviously, other plastic materials may be used at their appropriate 5 Clains, 7 Drawing Figures doned' surfaces with a 521 u.s.c|. ..239/85,239/8l,239/l33, 239/135 51 Int." B44d3/60 5s FieldoiSeareh ..239/142,144,135,133,79-s5 [56] References Cited UNITED STATES PATENTS meltinstemperawres- 3,415,450 12/1968 Hawk, Sr. "239/85 LLMBJMXH [U 7 if A 1 g "W LL w X224 "22 J22 Patented July 18, 1972 2 Sheets-Sheet l V mm W ew x Patented July 18, 1972 3,677,471

2 Sheets-Sheet 2 APPARATUS AND PROCESS THERmF FOR COATING WITH POLY'IEI'RAFLUOROEIHYLENE AND (Yl'HER MATERIAIS RELATED APPLICATIONS This application is a divisional application divided from Deakins copending application Ser. No. 507,504now abandoned, filed Nov. 12, 1965, in which claimswere elected to the method. This parent application isbeing abandoned herewith. This parent application was assignedtoS TRO LIMITED, a wholly owned subsidiary of the present assignee, and the assignment was recorded in Reel 1768, Frame 750.

This application is related to stand application Ser. No. 109,369, filed Jan. 1971, which is a continuation-in-part of application Ser. No. 834,292, abandoned on the same date. This application is also related to Stand U.S. Pat. application Serial No. 835,876, filed June 4, 1969, entitled Method of Depositing Heat Fusible Material and Apparatus Therefor, issuing into U.S. Pat..No. 3,591,759, granted July 6, 197l, and to Stand U.S. application Ser. No. 834,293, filed June [8, 1969, titled Spray Nozzle for Plasma Guns, issuing into U.S. Pat. No. 3,627,204, granted Dec. I4, 1971, and also to Stand and Streicher U.S. application Ser. No. 868,487, filed Oct. 22, 1969, titled Powder Feeder, issuing into U.S. Pat. No. 3,606,48l, granted Sept. 20, 1971. All the foregoing applications are incorporated by reference herein in their entirety, and these applications and the present application are assigned to the same arsignee, i.e., SEALECI'RO CORPORA- TION.

This invention relates to the apparatus for coating surfaces (e.g. the surfaces of metallic articles such as electrical components, bearings and cooking utensils and surfaces of other materials such as wood) with polytetrafluoroethylene (PTFE), or other plastics.

It is known to apply coatings of polytetrafluoroethylene in suspension in a carrier liquid to surfaces and, after drying, to heat the coated surfaces to sinter the polytetrafluoroethylene particles to produce a continuous adherent layer.- The temperature necessary to effect sintering is, however, high (approximately 300 350 C., for example, 325- 327 C. or above) and this may have deleterious effects (e.g. softening or distortion) on the article and is also a costly and inconvenient step. The present invention provides a coating method which avoids the need to heat the article and also apparatus for use in carrying out the method.

The inventionprovides an apparatus and coating of a surface with polytetrafluoroethylene in which the method comprises the step of projecting polytetrafluoroethylene particles through a heating zone, wherein the particles reach sintering temperature, and onto the surface to be coated and on which the hot particles coalesce.

Preferably the particles are projected by blowing a stream of gas (e.g. nitrogen, argon or air) through a bed or mass of the particles.

The heating zone may comprise a tubular heating element.

Preferably the heating zone is heated by a source of infrared radiation desirably within the wavelength range of from 7.5 to 9.0 microns or above 12.0

In accordance with one aspect of the present invention there is used in the apparatus and method, particles which have been prepared by compressing polytetrafluoroethylene (P'I'FE) (e.g. produced by flocculation or coagulation from a suspension produced by suspension polymerization), sintering the compressed particles to form a coherent body and them grinding the body to form fresh particles. The particles so produced and used in the method may be within the range of sizes from 10 microns through 100 microns to 1,000 microns. They may be of about the same size as the particles in the original suspension.

The particles may be produced by grinding (i.e., abrading) from the body or by crushing and grinding (e.g. in a ball mill). The grinding may be carried out wet and the particles may be dried before use.

The polytetrafluoroethylene particles may be presintered before being fed to the heating zone. Presintering has the effect of reducing or eliminating air spaces in the particles. The particles may cake together but the cakes can readily be brokenup before feeding the particles to the heating zone.

While for convenience and simplicity P'I'FE plastic coating isreferred to throughout, it is understood that any plastic or plastic-like coating material may be used just as satisfactorily, but that the herein stated temperatures would have to be adjusted according to the melting temperatures of the particular plastic, plasticqlike or combinations of plastics used. Alternatively powdered polytetrafluoroethylene may be used as supplied by the manufacturers, if it is subjected to pre-sintering, e.g. for 7 hours at 500 F. Particles of various sizes may be used in the methods of the invention but it has been found desirable to employ particles of substantially uniform size in any one coating operation.

Pigmented polytetrafluoroethylene may be employed in the method of the invention but the pigment must have suflicient thermal stability to withstand the temperature of the heating zone.

All kinds of surfaces may be coated by the method of the invention including surfaces, such as wood, which would normally char at the sintering temperature of polytetrafluoroethylene. Surfaces which can be coated include copper, brass, aluminum and steel. The method may be used to coat skis and sledge runners, tanks and vats used in chemical processes, metal foil and tempered steel parts. Usually surfaces to be coated are first degreased and where appropriate the surface may be etched or sand-blasted to improve adhesion of the polytetrafluoroethylene.

The particles of polytetrafluoroethylene may be projected at high speed even up to the speed of sound.

The coating of polytetrafluoroethylene may for example have a thickness of from 0.001 to 0.100 inch. It is possible to build up a thick coating gradually by application of two or more coatings one on top of another. In the method of the invention the time during which the particles are held at sintering temperatures is short and thus less degradation of the polytetrafluoroethylene will occur leading to better adhesion of the coating to the surface compared to the normal method of applying a layer of unsintered polytetrafluoroethylene and sintering the layer on the article. The coating produced by the method of the invention will usually have less porosity than a coating produced by conventional methods such as the one just mentioned.

Various methods of feeding the particles of polytetrafluoroethylene to the heating zone may be used in the present invention. According to a feature of the present invention, the particles of polytetrafluoroethylene are fed by mechanical means into a stream of gas passing to the heating zone.

The mechanical means may comprise a screw feed. Alternatively the mechanical means may comprise a rotatable cylindrical member provided on its cylindrical surface with a number of grooves to which powdered polytetrafluoroethylene is fed and a duct for gas which crosses the surface of the cylinder and is arranged so that a portion of the cylindrical member enters the duct and gas passing through the duct can pick up particles lodged in the grooves.

The invention also includes a feeding arrangement for polytetrafluoroethylene particles, for use in the methods of the present invention, which arrangement comprises a rotatable cylindrical member provided on its cylindrical surface with a number of grooves, means for feeding polytetrafluoroethylene powder to the surface of the cylindrical member so that the grooves pick up powder when the cylindrical member rotates, and a duct for conveying gas to the heating zone, which duct crosses the cylindrical surface of the cylindrical member and is open to part of that surface so that gas passing through the duct can pick up powder from any groove which cromes its path when the cylindrical member rotates.

The invention also includes apparatus for use in spraying particles of polytetrafluoroethylene at or above sintering temperature in accordance with the methods of the invention which apparatus comprises a tubular heating element, means at one end of the element for supporting a bed of polytetrafluoroethylene powder, means for directing a stream of gas through the tubular heating element and means for feeding polytetrafluoroethylene powder into said stream of gas so that polytetrafluoroethylene particles are carried through the tubular heating element and out of the other end thereof when the apparatus is in use.

The invention further includes apparatus for use in spraying particles of polytetrafluoroethylene at or above sintering temperature in accordance with the methods of the invention which apparatus comprises a tubular heating element, means at one end of the element for supporting a bed of polytetrafluoroethylene powder and means for directing a stream of gas (e.g. air) on to or through the bed to pick up particles and to carry them through the tubular element and out of the other end thereof.

Preferably the tubular heating element comprises a source of infra-red radiation.

The invention still further includes apparatus for use in spraying particles of polytetrafluoroethylene at or above sintering temperature in accordance with the methods of the invention which apparatus comprises an elongated tube transparent to infra-red radiation, an infra-red heating element surrounding said tube along a substantial proportion of its length, means for passing polytetrafluoroethylene particles through said elongated tube, whereby polytetrafluoroethylene particles may be brought to sintering temperature within said tube and projected on to a surface to be coated.

The apparatus may comprise means for passing a stream of gas through said elongated tube and mechanical means for feeding polytetrafluoroethylene into said gas stream.

The mechanical means may comprise a rotatably cylindrical member provided on its cylindrical surface with a number of grooves, means for feeding polytetrafluoroethylene powder to the surface of the cylindrical member so that the grooves can pick up powder when the cylindrical member rotates, and a duct for conveying gas to said elongated tube, which duct crosses the cylindrical surface of the cylindrical member and is open to part of that surface to that gas passing through the duct can pick up powder form any groove which crosses its path when the cylindrical member rotates.

The accompanying diagrammatic drawings illustrate various embodiments of the invention. In the drawings:

FIG. 1 is a sectional view of a spray gun in accordance with the invention;

FIG. 2 is a sectional view of another spray gun together with a feeding arrangement therefor;

FIG. 3 is a perspective view of a feeding arrangement for use in the arrangement of FIG. 2;

FIG. 4 is a sectional view of another arrangement for feeding polytetrafluoroethylene particles in accordance with the invention;

FIG. 5 is a sectional view of a valve for use in an arrangement of the kind shown in FIG. 4;

FIG. 6 is a sectional view of a further feeding arrangement in accordance with the invention; and

FIG. 7 is a perspective view of a stirrer used in the arrangement shown in FIG. 6.

Referring to FIG. 1, the gun comprises a cylindrical casing l with a platform 2 of 100 mesh gauze on which a bed 3 of dry polytetrafluoroethylene power may be supported. Beneath the platform there is an inlet 4 for air by which the bed may be fluidized and some of the powder particles incorporated into the air stream. On the axis of the casing, above the bed, there is a quartz outlet element such as a cylinder or tube 6 for the particle-carrying air, and a bafile 7, which may be adjustable towards and away from the mouth of the tube outlet element, to control the air flow. The cylinder or tube 6 passes through an electrical heating element 8 and terminates in a noale outlet l0.

The heating element 8 may be of any suitable material. One of the itens found to be effective is a silicon carbide element, cylinder or tube, preferably in one piece, in which the hot zone is in the form of a helix or some other equivalent shape. Obviously, many kinds and types of elements of various shapes and sizes may be used One of the elements is the kind sold by The Morgan-Crucible 00., Ltd. under their trade mar CRUSILITE."

There is a lead-in cylinder tube 12 around the inlet end of the element 8 with electrical connections to the element and surrounding the whole length of the heating element there is a refractory cylinder or tube 14 which, in turn, is surrounded by diatomaceous or other heat insulating material 15 in the space between the tube 14 and the casing. The ends of this space are closed by annuli 16, 17 of heat resisting material and there is also an annular partition 18 supporting the platform 2 and the tube 6.

In use air is admitted through the inlet in an amount sufficient to fluidize the polytetrafluoroethylene powder and to entrain some of the particles. The air passes out through the tube 6 and during its passage along the tube the polytetrafluoroethylene particles are heated to a little above their sintering temperature. The stream issuing from the nozzle 10 is directed on to a surface to be coated with polytetrafluoroethylene, and as the hot particles strike the surface, they combine to form a substantially continuous film. shown).

A further embodiment of the invention will now be described with reference to FIG. 2 and 3. This apparatus comprises a quartz tube 20 concentric with a second quartz tube 22 round which is helically wrapped an infrared heating element 23 connected to terminals 24, 25. A third quartz tube 26 surrounds the heating element and is provided on its inner surface with a reflecting film of gold or rhodium. Cooling air or other gas enters the annular space between

tubes

20 and 22 from inlet 40 and leaves by an outlet (not shown). Tube 20 is connected to a feeding device which comprises a closed hopper 27 containing powdered polytetrafluoroethylene 28. At the base of the hopper 27 there is a rotatable cylinder 29 mounted on a shaft 30 driven by a motor 31. The cylinder 29 has on its surface a plurality of helical grooves 32 which pick up polytetrafluoroethylene powder from the hopper 27. A duct 33 passes through the feeding device to the tube 20 and is open to part of the surface of the cylinder 29. Air or other gas blown through the duct 33 picks up polytetrafluoroethylene powder from any groove which crosses the path of duct 33 and conveys it to the tube 20.

In use the apparatus is placed with the nonle 21 of tube 20 directed at the surface 34 which is to be coated. The hopper 27 is filled with polytetrafluoroethylene powder and the motor 32 switched on to drive the shaft 30 and cylinder 29. The infra-red heating element 23 is switched on previously to allow it to warm up. Nitrogen is blown through the duct 33 and picks up polytetrafluoroethylene powder from the grooves 32 as portions of them cross the path of the nitrogen passing through the duct. The polytetrafluoroethylene powder then passes through tube 20 where it is heated to sintering temperature or above by the infrared heating element. The hot particles are sprayed on to the surface 34 where they coalesce to coat the surface. The infra-red radiation heats up the polytetrafluoroethylene to a much greater extent than the carrier gas.

The polytetrafluoroethylene particles may be preheated, e.g. to 250 C. in the feeding device.

Referring to FIG. 4, the arrangement comprises a container 50 having a porous plate 51 situated some way above its bottom. The container has a screw top 52 through which two

tubes

53 and 54 pass into the container from a duct 55. Tube 53 passes through the porous plate 51 and is open to the space below it, whereas tube 54 opens to the space above the plate 51. A rod 56 passes through tube 53 and is connected to a valve plate 57 and screw 58. The screw 58 may be adjusted to vary the distance between the end of tube 53 and valve plate 57 and thus the amount of gas passing into the container 50.

In use the gas for conveying polytetrafluoroethylene powder to the heating zone is passed through duct 55 and owing to a restriction comprising a throat 59in the duct some gas passes through tube 53 and enters the space below the porous plate 51. The gas passes through the porous plate 51 and fluidizes polytetrafluoroethylene particles stored in the container 50 above the plate. Some of these fluidized particles pass through tube 54 into duct 55 where they are conveyed to the heating zone which may be in a tubular heating element as shown in FIGS. 1 or 2, or a plasma torch or gun of known kind.

FIG. 5 shows an alternative valve arrangement for adjusting the amount of gas flowing into tube 53 of the arrangement shown in FIG. 4. The tube 53 is modified to provide a seating 60 for valve member 61 which can be screwed down into tube 53 to adjust the amount of gas flowing through it into container 50 for fluidizing the particles.

FIG. 6 shows a container 70 through which a duct 71 passes. A tube 72 which leads into duct 71 contains a screw feed 73 on the end of a rotatable rod 74 passing through the wall of the container 70. An opening 75 is provided in tube 72 to admit powdered polytetrafluoroethylene which is picked up by the screw feed 73 and conveyed to the duct 71. A stirrer 76 (FIG. 7) comprising a plate 77 having a hole 79 through which rod 74 passes and four prongs 78 is situated in the container 70 to operate around the bottom of tube 72.

in use gas passing through duct 71 picks up powder introduced into the duct by screw feed 73 and conveys it to the heating zone.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for coating a surface with a film of polytetrafluoroethylene comprising; a conduit made of a material transparent to infrared radiation for delivering a mixture of polytetrafluoroethylene particles to a surface to be coated through an exit noule, a cylindrical tube mounted around the conduit and made of a material transparent to infrared rays, an electrical resistance heater positioned around said tube for heating the particles by radiation to a temperature above the melting point of the particles, a reservoir for holding a quantity of said particles, said conduit connected to the reservoir at a position adjacent to the lower level of the particles, a pipe having an exit port connected to the reservoir for forcing gas through the particles, and means in the reservoir for agitating the particles.

2. Apparatus as set forth in claim 1 wherein the particles are heated above 327' C.

3. Apparatus as claimed in claim 1 wherein a rotatable cylindrical member is mounted within said reservoir, said cylindrical member formed with a helical groove on its outer surface, and a motor for turning the cylindrical member to agitate the particles.

4. Apparatus as claimed in claim 1 wherein said cylindrical tube is made of fused quartz.

5. Apparatus as claimed in claim 1 wherein said constriction is placed in the pipe connected between the reservoir and the conduit and a branch pipe is connected to the conduit, said branch pipe extending into the reservoir and having an entrance port adjacent to the upper surface of the particles.

Claims

2. Apparatus as set forth in claim 1 wherein the particles are heated above 327* C.
3. Apparatus as claimed in claim 1 wherein a rotatable cylindrical member is mounted within said reservoir, said cylindrical member formed with a helical groove on its outer surface, and a motor for turning the cylindrical member to agitate the particles.
4. Apparatus as claimed in claim 1 wherein said cylindrical tube is made of fused quartz.
5. Apparatus as claimed in claim 1 wherein said constriction is placed in the pipe connected between the reservoir and the conduit and a branch pipe is connected to the conduit, said branch pipe extending into the reservoir and having an entrance port adjacent to the upper surface of the particles.