DE4011801A1 - METHOD FOR THERMALLY COATING SURFACES WITH A FLUOROPOLYMER - Google Patents

Abstract

The invention concerns a process for the thermal coating of surfaces with a fluoropolymer in the form of a powder, sheet, dispersion or suspension which is applied to the surface, at the same time heating the fluoropolymer and the substrate. In order to produce coatings from fluoropolymers wich are prepared from melts, by means of a simple and inexpensive process involving only a few operations, the invention proposes that the substrate be heated and, simultaneously, for the fluoropolymer to be melted using a laser beam, the temperature of the surface being determined over the coating zone and maintained by on-line control within the range between the melting point of the fluoropolymer and 100 DEG C above the melting point. At a film thickness of 100 mu , the fluoropolymer has an absorption coefficient which is lower than 80 % in the laser-beam wavelength range.

Description

Polymers that contain fluorine show a comparison to the analog fluorine-free polymers - essentially due to the high binding energy between Carbon and fluorine atom and the higher surfaces energy - a number of special properties. In addition to good mechanical and electrical properties are their salient features of wide temperature application range between -200 and + 260 ° C, excl excellent resistance to solvents, chemicals and oxidizing agents; they are water and oil repellent, flame retardant to non-flammable and weatherproof constantly and have a low coefficient of friction. They are therefore used for coating devices such as  e.g. B. heat exchangers, evaporators, containers that in Contact with corrosive liquids, vapors and gases operated, used.

According to the usual powder coating process total volume of the substrate to be coated a temperature higher than the melting point of the fluorothermoplastic preheated. During the on closing order of the fluorothermoplastic powder for example electrostatic spraying or vortex sinter melts on the heated substrate surface Part of the fluorothermoplastic powder. A complete ver melting is the result of subsequent heating in one Oven at temperatures that are also above the Melting point of the fluorothermoplastic used lie, reached. By repeated application and In this way, heating can achieve layer thicknesses that just below the discharge limit of this fluorotherm plastic lying.

When coating large equipment or large parts with high substrate volume must match the dimensions of this Apparatus or parts corresponding furnaces provided be due to the complete heating of the Volume be designed for a high energy output have to; a corresponding energy consumption takes place at the heating of the substrate instead. The control and Control of such large furnaces requires a high one Effort because a homogeneous temperature distribution for a uniform layer thickness is required that especially not achieved on edges or projections will.  

Because the permeability of such coatings in particular depending on the layer thickness, one tries to Apply layers as thick as possible to the substrate to protect against attack by aggressive media. One tries such thick layers by repeated Apply and fuse the fluorothermoplastic powder to achieve. However, this process cannot be arbitrary often repeated, especially when vertical in Coating surfaces arranged the melt of the furnace Fluorothermoplastics flow as a result of gravity starts and drains.

The application of the fluorothermoplastic powder to the pre warmed substrate leads in addition to thermal stress of the person making the coating also becomes a difficult handling in itself. In addition, at Fluorothermoplastics from heating above the cer toxic gases are generated, which lead to a Can lead to health risks. It is therefore he to provide considerable effort by means of a coating tion of one about the decomposition temperature of the fluorine thermoplastic heated substrate is prevented; at the same time, however, care must be taken for who that, especially with a small substrate volume, that a heat dissipation too quickly below the melting temperature the fluorothermoplastic is excluded.

The invention is therefore based on the object layers of fluoropolymers, which come from the melt are processable, by means of a simpler, cost cheaper and fewer process steps Manufacturing process. Among fluoropolymers preferably non-cross-linked fluorothermoplastics and PTFE (Polytetra-fluoroethylene) understood.  

This problem is solved by a method of a gangs mentioned type, which by the simultaneous Er heating of the substrate and melting of the fluorine polymerize with a laser beam during an investigation the surface temperature along the coating area and the on-line regulation of the surfaces temperature in a range between the melting temperature temperature and a maximum of 100 ° C above the melting temperature of the Fluoropolymers, which have an absorption less than 80% a layer thickness of 100 µm in the wavelength range which has laser beams, is marked.

Should heating to a maximum of 100 ° C above the Melting temperature of the fluoropolymer or even more done about it is on a very short-term Attention to temperature load. Preferably one is Heating up to 80 ° C above the melting temperature of the to provide respective fluoropolymers.

Further advantageous developments of the invention are in given the subordinate claims.

According to the invention, all known fluoropolymers can advantageously be used which have an absorption of less than 80% with a layer thickness of 100 μm of the laser beams in the wavelength range of 10.6 μm (CO ₂ laser) or 1.06 μm (solid-state laser). These are preferably fluorothermoplastics and PTFE. By absorption of less than 80%, the substrate is advantageously heated and the fluoropolymer is melted at the same time.

Fluoropolymers that can be processed from the melt and are used in the process according to the invention usually have a melt viscosity of <1 × 10 ⁶ Pa × s at the processing temperature. Such melt-processable fluoropolymers can e.g. B. be homopolymers, such as polyvinylidene fluoride, polyvinyl fluoride or preferably polychlorotrifluoroethylene. Likewise, these can be copolymers, preferably those which, in addition to copolymerized units of tetrafluoroethylene or chlorotrifluoroethylene, also contain at least one further ethylenically unsaturated comonomer in sufficient quantities in order to ensure processability from the melt. Such copolymers are selected in particular from the following groups:

• a) copolymers of tetrafluoroethylene with higher per fluoroolefins, especially with hexafluoropropylene; Copolymers of tetrafluoroethylene with perfluoroalkyl vinyl ethers of the formula CF ₂ = CF-OR _f where Rf is a perfluorinated alkyl radical, preferably with per fluoropropyl vinyl ether; Copolymers of tetrafluoroethylene which contain both hexafluoropropylene and a perfluoroalkyl vinyl ether, in particular perfluoropropyl vinyl ether.
• b) copolymers of tetrafluoroethylene with ethylene, where such copolymers preferably at least one other contain copolymerizable monomer, often also their two or more. Such comonomers are preferred wisely selected from the group of perfluorinated Olefins, with hexafluoropropylene being preferred; out the group of the perfluoroalkyl vinyl ethers of the above given formula, wherein perfluoropropyl vinyl ether be is preferred; from the group of fluorine-containing oils fine, preferably 3,3,3-trifluoro-2-trifluoromethylpro pylene; from the group of vinyl esters and from the Vinylidene fluoride and trifluorochloroethylene group.  Such copolymers of the tetrafluoroethylene / ethylene type, optionally with further monomers consisting of at most 60 mol% tetrafluoroethylene, 60 to 40 mol% Ethylene and 0 to 10 mol% of the proportion of the ge called third and optionally fourth monomers.
• c) copolymers of tetrafluoroethylene with vinylidene fluoride, such copolymers preferably still at least one other ethylenically unsaturated, preferably contain fluorine-containing comonomers; in particular, hexafluoropropylene or Perfluoroalkyl vinyl ether, if appropriate also the combination of the two; in thermoplastic Copolymers of this type is tetrafluoroethylene in Proportions from 50 to 80, in the case of the ter and quaters polymers from 50 to 65 mol%, the vinylidene fluoride contained in proportions of more than 20 mol%; a preferred combination is tetrafluoroethylene / viny lidenefluoride / hexafluoropropylene.
• d) copolymers of tetrafluoroethylene with chlorotrifluor ethylene, with both tetrafluoroethylene and Chlorotrifluoroethylene is the predominant ingredient can be.
• e) Copolymers of chlorotrifluoroethylene with ethylenic unsaturated fluorine-containing monomers, such as esp special hexafluoropropylene, tetrafluoroethylene and Vinylidene fluoride.
• f) copolymers of chlorotrifluoroethylene with ethylene, these copolymers also preferably at least another, often two or three more may contain ethylenically unsaturated comonomers,  which are selected from the same groups can, as described above for copolymers of the Tetra type fluoroethylene / ethylene is specified.

Regarding the production of copolymers of the above ge mentioned type is, for example, in the following US Patents referenced: 29 46 763, 31 32 123, 31 32 124, 40 29 868, 42 62 101, 36 24 250, 38 59 262, 38 17 951, 39 60 825, 38 47 881, 41 23 602, 24 68 054, 32 35 537, 25 13 312, 26 62 072, 30 53 818, 27 38 343, 27 52 332; also on the euro European patent specifications 2809 and 50 437 and the bel gische patent specification 8 44 965.

Within the scope of the method according to the invention, however also find such fluoropolymers because of their high melt viscosity does not come from the melt are processable. This term is used in primarily the homopolymer, namely the polytetra fluorethylene itself, but also the so-called "modified" tetrafluoroethylene polymers, i.e. H. such polymers of tetrafluoroethylene, the one fluorinated or preferably perfluorinated comonomer in such a small amount - mostly ≦ = 1% by weight - ent hold so that the character of the polymer as "from the melt cannot be processed "not decisive is changed. Examples of such comonomers are Chlorotrifluoroethylene. Hexafluoroisobutylene, 1-perfluoro butene, but especially perfluoroalkyl perfluorovinyl ether with a perfluoroalkyl radical of 1 to 5 carbon atoms and Hexafluoropropylene. As far as in this description Abbreviation PTFE is used, it refers to all of these types of polytetrafluoroethylene.  

The diagrams ( FIGS. 1 to 5) show the transmission as a function of the wavelength. In the preferred use of a CO ₂ laser operating in continuous operation with a wavelength of 10.6 μm (marked with a line in the diagrams), the fluoropolymers have polytetrafluoroethylene, the copolymers tetrafluoroethylene / hexafluoropropylene, tetra fluorethylene / perfluoropropyl vinyl ether, tetrafluoroethylene / hexafluoropropylene , Terpolymer of tetrafluoroethylene / vinylidene fluoride / hexafluoropropylene a transmission for the laser beams by 50% based on a layer thickness of 100 microns.

When the substrate is heated by means of a CO ₂ laser operating in continuous operation and having an output power greater than or equal to 100 watts and a wavelength of preferably 10.6 μm, the temperature profile introduced into the substrate is limited in level and depth . During the coating process, the surface temperature along the coating area is determined and kept constant by on-line control of the surface temperature in a range between the melting temperature and a maximum of 80 ° C. above the melting temperature of the fluoropolymer. This safely prevents heating above the decomposition temperature of the fluoropolymer. Due to the limited track widths, transversely overlapping tracks are drawn when coating with the fluoropolymers to process larger functional areas. Either the workpiece or the laser beam can be moved along the track. The small amounts of fluoropolymer powder melted in the process reduce the risk of heating above the decomposition temperature.

The coating point per can be particularly advantageous Process coupled (on-line) with a laser beam be warmed, thereby cooling the preferably before hitting the surface of the substrate Laser beam supplied fluoropolymer powder on the cold surface is prevented. Liability will through this coupled with the coating process Preheating improved.

According to a preferred execution method, Preheating the focal spot of the laser beams over a Optics widened (line or area beam focus), see above that an impact area of this focal spot arises, in the coating direction before the coating point le strikes the surface of the substrate.

Other options for online preheating are splitting the laser beam into two partial beams, the oscillation of the laser beam or in the ver use of a separate laser for preheating.

The layer thickness of such coatings can by initially mentioned procedures up to any Order of magnitude can be increased because of the coating tete surface of the substrate any number of more Layers of fluoropolymer powder applied and can be fused because only the binding surface the underlying fluoropolymer layer is heated and the fluoropolymer powder applied to them is melted. The permeability of the coating can thus be significantly reduced. Dangerous Corrosion as used in contact with  aggressive chemicals due to limited layer Thicknesses are avoided.

A reduction in the unfavorable impact of the Per Meation of aggressive substances can be caused by the arrangement an intermediate layer with greater permeability between between the layer and the substrate and / or between the layers can be reached if this intermediate layer is connected to the environment or ex gassing or degassing. Through this intermediate layer The removal of the permeate and thus a ge allows for a smaller layer thickness.

Fluoropolymers at a given wavelength the laser radiation of 10.6 µm, for example, is a very have high transmission, can by targeted administration of fillers in their transmission properties can be set. Both fillers are used Fluoropolymers with lower transmission than that of Matrix polymers, such as. B. PTFE powder as well Processing temperature range resistant organic and inorganic additives to the fluoropolymer powder mixed and an absorption between 20 and 80% pulled to a layer thickness of 100 microns.

The coating point with a surrounded by inert gas, causing oxidation of the be layering substrate surface is prevented.

According to the inventive method temperature stable surfaces are coated, d. H. the Substrate must be made of a material that the Fusion of the fluoropolymer layer and with it associated thermal load no adverse  Undergoes changes. The method is therefore suitable for the coating of metal, glass, graphite and Ceramic surfaces, but also for the coating of high temperature resistant plastics.

Before the actual coating process, the coating surface ent according to customary methods greases, for example by vapor phase degreasing, Be act in alkaline baths or possibly also by heating the object to be coated to approx. 400 to 450 ° C.

An improvement in the adhesion of the coating can by roughening the surface, for example by Sandblasting or etching can be achieved, if necessary also by applying ceramic or metalli Intermediate layers with a large surface roughness speed, for example by flame spraying or plasma coating. Are very high demands on the Adhesion of the coating provided, so before opening apply an adhesion promoter layer to the layer brought, usually from the same Fluoropolymers with the addition of adhesion-promoting sub punching exists. As an adhesion promoter for such fluorine polymers are suitable for high temperature resistant bandages resins such as epoxy resins, polyamides, polyamideimides, Polyimides, polytriketoimidazolidines, polyphenylene sul fide, polyether sulfides, polyether ketones, polyhydantoins or also inorganic substances, such as Alkali silicates, chromic anhydride, phosphoric acid or Aluminum chlorophosphates. The adhesion promoter layer is as powder according to the usual powder coating methods or with the laser beam or in the form of disper ions, suspensions or solutions by spraying,  Dip or brush applied.

Another option is to roughen the Substrate surface by melting with the Laser beam. After application, the adhesion promoter layer dried and baked if necessary. Be drying and baking is preferably carried out using the laser beam directed at the preheating point during of the coating process.

To carry out the method, a device 10 shown in FIG. 6 is advantageously used, which has a CO ₂ laser 11 working in continuous operation (cw operation) and an optical system arranged in the beam path 12 and a power unit 15 . The power section is assigned an on-line Regeleinrich device for controlling the surface temperature, the control device having a radiation pyrometer 16 connected to a measuring circuit, which detects changes in the surface temperature, which are compared with a setpoint value set on the setpoint generator 18 and the deviations thus determined be fed to the controller 17 in the surface temperature. Depending on this, this emits control signals to an actuator which regulates the power of the laser. The power of the laser is regulated in a period between 10 microseconds and one microsecond. The laser beam 20 is shaped via an optic 19 assigned to the laser nozzle 21 , a preheating region ( 22 , FIG. 7b) of the laser beams running locally in front of the coating location being generated. The focal spot has a higher intensity at the preheating point ( FIG. 7a), since, owing to the heat conduction of the substrate 23, a greater amount of energy has to be introduced than is required for the subsequent melting of the fluoropolymer powder. The intensity distribution depends on the substrate material.

The fluoropolymer powder is preferably fed in via the laser nozzle ( FIG. 6). The laser nozzle has at least one feed line 24 , 25 for the fluoropoly merpulver 26 , which is passed through a compressed gas, for example the inert gas, into the laser beam. The fluoropolymer powder is thereby advantageously brought into a plastic, liquid or preheated state before it emerges together with the laser beam from the outlet opening of the laser nozzle and strikes the processing point ( 27 , FIG. 7b). The feed line is aligned with the processing point ( 27 ) in the laser nozzle. As can be seen from FIGS . 7b and c, the feed line can also be arranged upright outside the laser nozzle.

The quantity control of the powder feed and the optics to expand the laser beam are known per se and therefore do not need to be described in more detail.

Claims (14)

1. A method for the thermal coating of surfaces with a fluoropolymer in the form of a powder, a film, dispersion or suspension, which are applied to the surface by heating a substrate and the fluoropolymer, characterized by the simultaneous heating of the substrate and the melting of the Fluoropolymers with a laser beam when determining the surface temperature along the coating area and the online control of the surface temperature in a range between the melting temperature and a maximum of 100 ° C above the melting temperature of the fluoropolymer, which has an absorption of less than 80% with a layer thickness of 100 microns has in the wavelength range of the laser beams. 2. The method according to claim 1, characterized, that the fluoropolymer used is one of the Melt processable fluorothermoplastic or PTFE is. 3. The method according to claim 1 or 2, characterized, that the laser beam has an output power greater / equal to 100 watts and a wavelength of 10.6 µm or 1.06 µm. 4. The method according to any one of claims 1 to 3, characterized, that the heating of the substrate in the plane and the Depth is limited.   5. The method according to any one of claims 1 to 4, characterized, that the coating point is process-linked (on line) is preheated with a laser beam and that Fluoropolymer powder on the processing site brought. 6. The method according to any one of claims 1 to 5, characterized, that the focal spot of the laser beams over a Optics preferably as a line or area beam focus is formed and an area of this focal stains locally in front of the coating point on the Surface of the substrate hits. 7. The method according to any one of claims 1 to 6, characterized, that the focal spot of the laser beam on the front hot spot has a higher intensity than at the processing point. 8. The method according to any one of claims 1 to 7, characterized, that an adhesion promoter on the coating site layer applied and coupled to the process Laser beam is dried or baked. 9. The method according to any one of claims 1 to 8, characterized, that on the coated surface of the substrate at least one further layer of a fluorine polymers while heating the binding surface of the first layer and the melting of the second Layer is applied with a laser beam.   10. The method according to any one of claims 1 to 9, characterized, that between the layer and the substrate and / or an intermediate layer with between the layers greater permeability is arranged and the Interlayer communicates with the environment or is externally gassed or degassed. 11. The method according to any one of claims 1 to 10, characterized, that the transmission property of the fluoropoly with respect to the wavelength of the laser beams through the targeted addition of fillers is posed. 12. The method according to any one of claims 1 to 11, characterized, that as fillers both fluoropolymers with ge lower transmission than that of the matrix polymer as well as in the processing temperature range organic and inorganic additives ver be used and a transmission between 20% and 80% is set. 13. The method according to any one of claims 1 to 12, characterized, that the coating point added an inert gas leads. 14. Device for carrying out the method one of claims 1 to 13 with a laser beam emitting laser as well as a beaming arranged optical system and a Leis part,  marked by an online rule assigned to the power section device for regulating the surface temperature, in a range between the melting temperature and the decomposition temperature of the fluoropolymer powder, the control device having a measuring circuit connected radiation pyrometer, the Ver changes in surface temperature recorded, which with a set on the setpoint generator Setpoint value compared and the Ab deviations in the surface temperature to the controller be fed in, which depending on signals to a regulating the power of the laser Outputs actuator and a laser nozzle with a Optics, the feeds for that via a compressed gas introduced fluoropolymer powder are assigned.