Two step porous boiling surface formation

[57] ABSTRACT

A method of coating metal substrates with a porous metal layer, comprising the steps of:

a. providing a metal substrate to be porous metal coated;

b. applying a liquid binder solution consisting essentially of at least one high volatility solvent component and at least one low volatility component in a layer from 0.002 to 0.040 inch thick onto said metal substrate;

c. applying a 0.005 to 0.050 inch thick layer of metal powder 30-500 mesh onto the binder layer so that the binder layer wets the metal powder coating and is saturated thereby and retains substantially all of said powder against said metal substrate;

d. evaporating substantially all of the high volatility component of said liquid binder solution at about ambient temperature;

e. heating the metal powder coated metal substrate of (d) to temperature sufficient to evaporate substantially all of the binder low volatility component;

f. further heating the metal powder coated metal substrate to temperature sufficient to bond the metal powder to itself and to the metal substrate so as to form a porous metal layer having interconnected surface and subsurface cavities; and

g. cooling the porous metal layer bonded metal substrate.

Claims, No Drawings

3,753,757

TWO STEP POROUS BOILING SURFACE FORMATION

This invention relates to coating metal substrates such as the inner and outer surfaces of heat exchanger tubes with a porous metal layer to enhance the liquid boiling properties of such metal substrates.

The usefulness of porous boiling surfaces is well established in the heat exchange art, see U.S. Pat. No. 3,384,154. The problem which this invention deals with is that of forming porous boiling surfaces in a fast and efficient way. The prior art teaches the formation of such porous surfaces by providing a slurry of metal powder and a liquid plastic binder material and applying the slurry in a coating to a base metal by dipping or spraying. The resulting coating is then air dried and the bulk of the solvent removed by evaporation, leaving a self-supporting layer of metal powder which is held in place by the remaining binder. The resulting substrate and powder metal coating are then heated for a sufficient time to sinter the metal powder particles together and to the substrate. However, various difficulties are experienced with this type of coating procedure when used in high rate production. The coating is inconvenient and difficult to apply, requires long drying time to remove the excess solvent from coated surfaces, is wasteful of materials, and results in non-uniform thickness coatings. In many instances the slurry method requires the use of centrifual force or gravity to retain the coating against the substrate until substantially all the solvent has evaporated.

An object of this invention is to provide a method for applying metal powder to a metal substrate for the purpose of forming a porous metal layer thereon, which method is suited to mass production techniques, is efficient in use of materials, and results in uniform thickness coatings.

Another object of this invention is to provide highly efficient apparatus for applying porous metal surfaces to tubular substrates, as for example by the method of this invention.

Other objects shall be apparent from the disclosure and appended claims.

SUMMARY OF INVENTION

An object of this invention is accomplished by a method of coating metal surfaces with a porous metal layer, comprising the steps of:

a. providing a metal substrate to be porous metal coated;

b. applying a liquid binder solution comprising at least one high volatility solvent component and at least one low volatility component in a layer from 0.040 to 0.040 inch thick onto said metal substrate;

c. applying a .050 to 0.050 inch thick layer of metal powder 30-500 mesh size onto the binder layer so that the binder layer wets the metal powder coating and retains it against said metal substrate;

d. evaporating substantially all of the high volatility component of said liquid binder solution;

e. heating the metal powder coated metal substrate of (d) to temperature sufficient to evaporate substantially all of the binder low volatility component;

f. further heating the metal powder coated metal substrate to temperature sufficient to bond the metal powder to itself and to the metal substrate so as to form a porous metal layer having interconnected surface and subsurface cavities; and

g. cooling the porous metal layer bonded metal substrate.

Another object of this invention is accomplished by an apparatus for externally coating metal tubes with a porous metal layer comprising:

a. conveyor means for supporting the metal tubes and means for rotating same about their longitudinal axis;

b. liquid binder spray means arranged to longitudinally traverse the metal tubes while simultaneously spraying a layer of liquid binder thereon;

c. metal powder spray means arranged to longitudinally traverse the metal tubes which have been sprayed with liquid binder while simultaneously spraying a layer of metal powder thereon.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a vertically arranged metal tube coating station representing an apparatus embodiment of the invention for consecutively spraying liquid binder and metal powder onto metal tubes.

FIG. 2 is a perspective view of an alternative means of applying liquid binder and metal powder to the outside of metal tubes by dipping said tubes in liquid binder, draining said dipped tubes and rolling same below a shaker box for externally coating said tubes with metal powder.

DETAILED DESCRIPTION

In essence, the coating method of this invention for producing porous boiling surfaces on metal surfaces comprises first coating the metal substrate with a liquid binder solution comprising a low volatility polymer diluted with a high volatility solvent material. The maximum coating thickness for a particular binder solution is that which will not cause the liquid binder to sag or run thereby ensuring a coating of uniform thickness. Once applied the binder coating surface is wet and tacky. Following application of such binder layer, sufficient metal powder is uniformly applied to the binder layer such that substantially all the metal powder is wetted and retained against the metal substrate by said liquid binder. Following these separate surface coatings, substantially all the high volatility solvent component of the liquid binder is allowed to evaporate. Thereafter the powder coated product is furance heated preferably in a mildly reducing atmosphere to sufficient temperature and for sufficient time to evaporate the low volatility binder component without causing blistering or other disruption of the metal powder coating. After evaporation of the binder low volatility component, the product is further heated to temperature sufficient to cause metal bonding of the particles of metal powder to each other and to the metal substrate to produce a desired strong porous metal coating having small surface and subsurface interconnected cavities.

Such porous metal coatings may be applied to either flat or curved surfaces oriented horizontally or vertically. When such coatings are applied to tubes the tubes are preferably rotated so as to provide for uniform coating of binder and metal powder.

Important advantages of the coating method of this invention are that more uniform coatings of desired thickness are reliably attainable as compared with dipping or spraying an article to be coated with a slurry of

3,753,757 metal powder and liquid binder. In addition, the coating process of this invention is more flexible in terms of selecting desired coating thicknesses and is more easily mechanized than the previous slurry coating methods thereby resulting in lower mass production coating costs. Another advantage of the method of this invention is that unwetted metal powder may be reclaimed and recirculated to the powder coating step and less binder is required for the application of a porous metal layer of a given thickness.

The liquid binder solutions of this invention comprise at least one high volatility component and at least one low volatility, high molecular weight component. One reason for this multi-component binder solution is that once the binder is applied to a metal substrate, the high volatility component keeps the low volatility component flexible until the metal powder layer is applied. When the metal powder contacts the binder layer the binder wets the metal powder and as the high volatility component evaporates for example at about ambient temperature the low volatility component retains the metal powder in a firm coating against the relatively smooth metal substrate to which the coating is being applied. In general, the low volatility component can be characterized by:

a. being soluble in a high volatility component liquid, b. the resulting liquid binder solution when applied to a surface in a thin layer remains tacky at about ambient temperature for at least about two minutes, and

c. the low volatility component of the binder liquid solution must evaporate sufficiently at below the metal powder bonding temperature so as not to interfere with such metal bonding.

The high volatility component is characterized by a boiling point at atmospheric pressure of not more than about 400° F.

Examples of such low volatility binder component materials useful in this invention comprise high molecular weight hydrocarbons such as isobutylene polymer having a molecular weight of at least about 90,000 such as commercially available “Vistanex” (trade name), and styrene polymers having a molecular weight at least about 35,000. Other suitable low volatility binder components would be acrylics having the characteristic of providing a thin tacky coating at about ambient temperature. Some high volatility binder solvent components useful in this invention are kerosene, mineral spirits and toluene. Choice of a solvent depends on the method of application of the binder and the powder coating thickness desired.

For general coating of metal surfaces with liquid binder wherein the aforementioned isobutylene polymer is the low volatility component, it should be diluted with a solvent of selected volatility and mixed in such proportion that the coated surface will remain tacky for a reasonable period of time after coating, for example, up to about ten minutes. In addition, the binder solution should be able to absorb considerable metal power reasonably quickly by the mechanism of the liquid flowing around the powder by capillary action. It is desired that the metal powder be drawn into the binder liquid so as to contact the base metal surface and not just have the powder retained on the surface of the liquid binder. For general application a useful solvent such as mineral spirits or kerosene can be mixed with equal parts of Vistanex by weight. An important con4 sideration in determining liquid binder composition and coating thickness is that the liquid binder coating should not flow or slump downwardly appreciably on a vertically suspended surface. An important advantage of this invention is that by choosing a proper binder neither the binder coating nor the metal powder saturated binder coating will slump or otherwise translate with respect to the surface being coated. The binder coating thickness generally influences the quantity or thickness of metal powder which can be retained by it prior to the heating and metal bonding step. A sprayed binder coating of 2-40 mills thickness in general should be used followed by applying all the metal powder that the binder will absorb and retain. If an exceptionally thick porous metal surface is desired then a metal surface which has been coated with binder and metal powder can be recoated with binder and metal powder to increase the metal powder coating thickness prior to furnace heating.

While the preferred liquid binder high molecular weight, low volatility component is isobutylene polymer and the high volatility solvent component is kerosene with the two components mixed in equal parts by weight, other dilution ratios and other solvents such as acetone, alcohol or toluene are suitable. In considering various applications of binder and metal powder to metal substrates, two of the most common applications are on the inside and outside of metal tubing. For both the external and internal coating of metal tubing it is desirable to evaporate the high volatility solvent from the coating slowly and preferably at ambient temperature. If the solvent is evaporated too quickly the coated surface tends to blister and lift off the base metal. It is an advantage of the method of this invention that considerably less binder material is needed than for a slurry application method, and that the high volatility solvent portion can be evaporated at room temperature without the need for additional heat prior to furnace bonding of the porous surface to the base metal. It is also important to note that the low volatility binder component which is evaporated during the initial furnace heating step requires that such initial furnace heating take place at a rate such that the low volatility component does not cause blistering of the metal powder layer during evaporation. An example of a heating rate is less than about 3,000° F per hour for isobutylene polymer.

Metal powders of various kinds such as copper, cupro-nickel, iron, steel and stainless steel may be used in this invention. Powder particle sizes ranging from about 30 to 500 mesh are generally useful. Coarser powder cannot be readily fluidized and transported for application and finer powders than 500 mesh and are not very useful for producing porous boiling surfaces by spraying. In producing porous boiling coatings for a particular boiling application, the first consideration is to select an appropriate metal powder mesh size range, inasmuch as the powder particle size range is dependent upon the physical properties of the fluid to be boiled. For example, high surface tension fluids such as water boil most efficiently with porous surfaces having relatively large pore sizes as provided by large metal particles, for example, 30-100 mesh, whereas low surface tension fluids such as cryogenic liquids or fluorinated hydrocarbon refrigerants boil most efficiently with porous surfaces having relatively small pore sizes

3,753,757 as provided by metal particles ranging from 200-400 mesh in size.

After selecting an appropriate metal powder particle size range, the next consideration is the desired thickness of the bonded porous metal coating. Inasmuch as 5 the final bonded porous coating thickness shrinks very little during the heating and bonding step the final po6 determined which would be preferable for particular applications, such as for coating flat surfaces or coating tubes either externally or internally. The metal coating thickness may range from about 0.005 to about 0.050 inch. Table I shows coating examples for various surface orientations, metal powder particle sizes and binder layer thicknesses.

TABLE I.—TYPICAL COATING PARAMETERS

Horizontal Vertical Internal external surfaces external surfaces tube surfaces

<td> Desired mesh size range of metal powder..........</td><td> 100-200</td><td> 200-400</td><td> 100-270</td><td> 270+00</td><td> 100-270</td><td> 270400</td>

<td> Desired final thickness of powder, in..............</td><td> .0.020</td><td> 0.015</td><td> 0.020</td><td> 0.015</td><td> 0.023</td><td> 0.019</td>

<td> Percent void of powder___________________________</td><td> 70</td><td></td><td> 60</td><td> 50</td><td> 70</td><td> 60</td>

<td> Required thickness of liquid binder, in...........</td><td> 0.015</td><td> 0.010</td><td> 0.013</td><td> 0.008</td><td> 0.017</td><td> 0.012</td>

<td> Viscosity of binder liquid, seconds*............... Components of binder liquid:</td><td> 30</td><td> 30</td><td> 32</td><td> 32</td><td> 48</td><td> 48</td>

<td> Isobutylene polymer, wt. percent_____________</td><td> 16</td><td> 15</td><td> 18</td><td> 17</td><td> 36</td><td> 34</td>

<td> Kerosene, wt. percent.........................</td><td> 84</td><td> 85</td><td> 82</td><td> 83</td><td> 0</td><td> 0</td>

<td> Toluene, wt. percent..........................</td><td> 0</td><td> 0</td><td> 0</td><td> 0</td><td> 64</td><td> 66</td>

<td> Thickness ratio of liquid binder/metal powder....</td><td> 0.75</td><td> 0.67</td><td> 0.65</td><td> 0.53</td><td> 0.74</td><td> 0.63</td>

•Binder viscosity as determined by a paint viscosity gage having H in. diameter orifice in bottom of 2 in. deep cup.

rous coating thickness will be only slightly less than the powder coating thickness existing before the bonding step. After determining the desired thickness of the porous coating it is necessary to determine the binder coating thickness required to wet and retain the desired thickness of metal powder. Such binder coating thickness is influenced mainly by the percentage of void in the metal powder mixture and the viscosity of the binder solution used. Powder mixtures having large percentage voids, i.e., relatively large particle sizes, will require somewhat more binder solution to wet and retain them than will powder mixtures having low void space. Binder solutions having low viscosity must be applied in thinner coatings to maintain uniform thickness and avoid running as compared with binder solutions having high viscosity. Viscosity of the binder solution is in turn determined by the quantity and characteristics of the solvent or solvents used with the low volatility binder component. Viscosity of the binder solution influences the rate at which the metal powder coating is wetted and its volatility influences the rate of evaporation of surplus solvent from the combined binder-metal powder coating. For example, externally coated surfaces preferably use a relatively slowly evaporating high volatility component such as kerosene to keep the binder coating tacky for up to about ten minutes until it can be coated with metal powder. However, internally coated surfaces, such as tube passages, preferably would employ faster evaporating high volatility components such as toluene so that the solvents can be removed quickly from such inner passages without the need for special ventilation after the metal powder coating is applied.

A reasonably constant ratio exits between the thickness of the binder liquid coating applied to a metal substrate and the thickness of metal powder applied to the binder coated metal substrate, such that the liquid will be saturated by the powder and retain substantially all the powder applied. The use of excess liquid binder as previously described compared to the quantity of metal powder applied causes the combined coating to be too wet and to run or slump away from vertically oriented surfaces during subsequent handling prior to furnace heating and porous surface bonding. Such a liquid binder powder thickness ratio can be experimentally

A typical metal powder particle size distribution for an externally porous metal coated heat transfer tube is shown in Table II.

TABLE II

TYPICAL METAL POWDER PARTICLE SIZE DISTRIBUTION

Standard Screen Mesh Percentage Ranges +30 10% Maximum

-30 +60 20-50%

-60 +100 30-50%

-100 +200 15-35% —200 +325 5% Maximum + means the particles are larger than the screen mesh size listed. — means particles are smaller than the screen mesh size.

An example of typical binder solutions found useful for external spray coating of metal tubes are shown in ₄₀ Table III.

TABLE III

Material Weight Percent

Isobutylene Polymer15-25

Toluene40-60

Kerosene25-40

After the liquid binder solution and metal powder have been applied to a metal surface and after sufficient time, e g., two hours, has passed to evaporate sub50 stantially all of the high volatility component of the liquid binder, furnace heating will serve to remove the remaining low volatility liquid binder component and to sinter, braze or otherwise fuse the metal powder particles together and to the metal surface.

₅₅ The application of liquid binder solution to a substrate can be by dipping the substrate into the binder solution and letting excess binder solution drip free, or by pouring liquid binder solution over the substrate to be coated and again letting excess binder solution drip ₆₀ free, by brushing or by spraying liquid binder solution onto the substrate to be coated. Of these methods, spraying is preferred. To improve spraying efficiency electrostatic spray guns may be used and spraying can take place with the substrate to be coated in any orien₆₅ tation.

Applying metal powder to the binder coated metal substrate can be accomplished by sprinkling the powdered metal over a binder solution coated metal sur3,753,757 face, by dipping a binder solution coated metal surface into a fluidized bed of metal powder or by spraying metal powder onto a binder solution coated metal substrate. Spraying is a preferred method of applying metal powder and electrostatic spraying has been found to give extremely uniform powder application especially in the case of spraying irregular surfaces. The voltage level of electrostatic spraying in general depends upon the characteristics of the material being sprayed, its density, electrical properties, etc. Because metal powders are electrically conductive and are heavy they require high voltages, that is, voltages ranging from about 42 to about 100 kv. D.C. In general, the advantage of electrostatic spray coating over other spray coating methods is the greater speed and uniformity of coating an article and more efficient use of coating materials because there is much less overspray and consequently smaller coating materials loss. Even with the advantages of electrostic spraying, a portion of sprayed metal powder still does not adhere to the article being sprayed and falls to the bottom of the powder spray station. Because this metal powder is valuable, collection means are preferably provided at the bottom of such spray station so that the powder can be recirculated to the spray gun.

It should be noted that for spray coating of metal surfaces with liquid binder solution it is preferred to use isobutylene polymer diluted with a high volatility solvent and a slightly lower volatile solvent, for example, toluene, having a boiling point of 230° F, which facilitates atomizing the liquid binder for spraying and, mineral spirits, having a boiling point of about 325° F which helps the liquid binder coating remain tacky after spraying until it is contacted with the metal powder. Such use of two different solvents is especially desirable for the external spray coating of heat exchange tubes. For spraying of liquid binder on the internal surface of heat exchange tubes it is desirable to use a highly volatile solvent in order to promote rapid evaporation following the powder coating step.

Heat exchanger tubes can be internally coated using the method of this invention by pouring liquid binder solution into the upper end of an inclined tube which rotates slowly, e.g., about 20RPM, to coat the binder reasonably evenly onto the inner wall and allowing the surplus binder liquid to drain out the lower end of such tube; thereafter rotating the tube slowly while introducing metal powder into the upper end and flowing the metal powder downwardly through the rotating tube so that the powder coats the binder uniformly and any surplus powder flows out the lower end of such tube. If such method is used a liquid binder comprising 35 percent isobutylene polymer and 65 percent toluene by weight should be suitable for most coating applications.

Referring to the drawings:

FIG. 1 shows a spraying station 1 with a liquid binder spray booth 2 and a metal powder spray booth 3 having a conveyor 4 which transports vertically hung heat exchange tubes 5 consecutively past spray booth 2 and spray booth 3 within spray station 1. A sample transport speed would be about 1.5 ft/min. Liquid binder is stored in container 6 and pumped to spray gun 7 by means of pump 8. Metal powder is stored in container 9 and by means of compressed gas supplied through line 10 such metal powder is fluidized and is delivered to spray gun 11. Both spray guns 7 and 11 move vertically, e.g., at about 4 ft./sec., to rapidly longitudinally traverse the tubes 5 which tubes are rotated, e.g., at about 1-6 RPM, as they pass through station 1 by rotation means 12 as for example wheels which rotate as they engage track 13. The tubes should rotate at least about one half turn for each vertical traverse of the spray gun nozzles.

A portion of the sprayed metal powder does not adhere to the tubes but falls to the bottom of booth 3. Because this metal powder is valuable, collection means 15 is provided and is attached to metal powder container 9 by reclaimed powder conduit 16. If desired, electrostatic spray equipment may be used in conjunction with spray guns 7 and 11 to establish a strong electrostatic field between the spray guns and tubes 5. Such electrostatic field increases the percentage of powder contacting the tubes so that a more uniform powder coating is obtained especially when tubes having irregular surfaces are being coated and less surplus powder falls to the bottom of booth 3.

Spray nozzles 7 and 11 are moved vertically along guides tracks 17 by chains 18 moved by sprockets 19 driven by electric motors 14. Alternatively, pneumatically or hydraulically operated devices could be used to move the spray guns vertically as required.

Although FIG. 1 illustrates apparatus for spraying binder solution and metal powder onto a vertical surface, such spraying can with equal effect be done on horizontal surfaces. In the case of a horizontal tube arrangement the spray nozzle could be located above the tubes which are rotated slowly while being conveyed past the spray nozzles. In addition, such physical arrangement would permit powder coating of the tubes by gravity if such method were desired or possibly by conveying the binder coated tubes through fluidized bed of metal powder.

For external spray coating of tubes, it is preferred to apply a second layer of metal powder after the first metal powder boating has been substantially wetted by the liquid binder coating. Accordingly, a second spray gun for spraying metal powder can be provided adjacent spray gun 11 such that after tubes 5 are conveyed past gun 11 they move on past the second gun (not shown) and receive a second layer of metal powder. Such use of a second powder spray gun ensures that the liquid binder layer is saturated with metal powder.

When externally spray coating articles having irregular shaped surfaces such as corrugated tubes, it is preferred to apply a high DC voltage, e.g. 50,000 volts, between the spray guns and the articles being coated. Such voltage produces a strong electrostatic field which provides for more uniform coating of the irregular shaped surface without bridging over indentations.

FIG. 2 shows another means by which liquid binder and metal powder can be applied to a metal surface prior to heat treatment to form porous boiling surfaces. In the FIG. 2 embodiment heat exchange tubes 20 are conveyed by conveyor means (not shown) through a liquid binder bath 21 for dipping therein. Preferably the tubes would have plugged ends so that liquid binder coating is restricted to the external surface of the tubing. After passing through the liquid binder bath, tubes 20 are elevated from the bath and allowed to drip excess binder back into the bath prior to continuing. Such draining takes place in region 22. After tubes 20 are dipped in liquid binder and drained, they are passed under a powder metal shaker box 23 which deposits a layer of metal powder on the liquid binder coated

3,753,757 tubes. To insure that the metal powder coating is uniform around the tubing it is desirable to rotate tubes 20 as they pass under shaker box 23. Tubes emerging from under shaker box 23 are thereafter allowed to evaporate the high volatility binder component and thereafter are heat treated as described in detail heretofore. The shaker box can be replaced by powder spray gun means if so desired.