Process for making metallic platelets

Abstract

A method is provided for producing metallic platelets. By determining the ratio of the diameter of the metal source to the thickness of the metallic platelet and the workability of the metal source, a high quality metallic platelet can be developed. According to this method, the platelets that are produced can be used in such applications as building and construction materials for improved weather-resistance, heat-reflection/management, anti-fouling resistance and architectural appearance.

Description

FIELD OF INVENTION

[0001] The present invention relates to the production of metallic platelets.

BACKGROUND OF THE INVENTION

[0002] The combination of increasingly stringent environment laws and the undesirably high costs associated with processing natural resources has caused a movement to develop homes that are both more environmentally friendly and more economical to run. Among the improvements that are being sought are means by which to reduce reliance on natural resources for cooling. This is particularly desirable for homes that are located in warmer climates, such as in Southern California or Florida.

[0003] One method for making homes less reliant on natural resources for cooling is the development of roofs that reflect rather than absorb heat from sunlight. For example, construction of roofs out of metals such as copper is one known option. However, roofs made entirely out of metals are prohibitively expensive.

[0004] One economical way to make use of the benefits of using metals in building and construction materials is to use materials that are only partially constructed of metal materials. For the case of roofs, small pieces of metal or metallic platelets may be incorporated into the roofing material to produce a reflective surface. Roofs that comprise metal pieces would be desirable if one were able to develop high quality platelets. Preferably, one would produce metallic platelets that are uniform in size, with minimal to no cracks.

[0005] Methods for processing metals and generating small pieces of metal are well known to persons skilled in the art and include, but are not limited to, technologies that employ splat automation, stamping and perforating of filled metal. However, these methods are not practical because they are difficult to control and cannot economically produce flat metal platelets with the appropriate dimensions and quality e.g. no cracks or splits. Thus, other methods need to be developed.

[0006] One potential process for producing high quality platelets is rolling, for example cold rolling. Traditionally, cold rolling has been used to reduce the thickness of a metal plate or slab into a sheet, strip or foil. Sheets and strips are referred to as rolled products with thicknesses of less than 0.25″. Foil generally has a thickness on the order of 0.010″ or less. The cold rolling process typically uses a series of rollers or stands, to reduce the thicknesses of the work piece consecutively to a desired level. The amount of reduction to a work piece during a single pass is determined by the type and/or size of the mill, and the properties of the metal work piece such as yield strength, hardness, chemistry, and rate of strain hardening.

[0007] Unfortunately, one side effect of using rolling technologies is crack formation, which is the result of excessive metal deformation during the rolling process. During rolling, there is a tendency for the metal to expand laterally as the thickness is reduced. The majority of the lateral expansion occurs in the outer edges of the work piece. The material in the center of the work piece is confined and mainly contributes to the length increase of the work piece during reduction. This inhomogeneous strain of distribution produces a residual stress that can lead to edge cracking. If the amount of deformation is excessive, then the residual stresses will be large enough to form a center split in the work piece. In practice, when rolling metal sheets, edge cracking may be minimized by running vertical rollers along the edges of the work piece. This process keeps the edges straight and minimizes the buildup of residual tensile stresses. This approach is impractical in the production of metal platelets.

[0008] Typically, cold rolling techniques are applied by passing metals through a plurality of rollers. However, with each additional pass through a pair of rollers, there is a greater likelihood of cracking caused by residual stresses. To avoid this, a common practice in cold rolling is to anneal the work piece between successive rolling passes so that the residual stresses are relieved, enabling further reduction. This annealing step increases the total cost of the rolled material. Thus, cold rolling could be a practical and economical method for forming metallic platelets if a method that uses only one pass through a set of rollers could be developed. The present invention addresses this problem by providing methods and processes for producing platelets that may be particularly useful in roofing applications or building and construction materials.

SUMMARY OF THE INVENTION

[0009] The present invention provides a process for making platelets through the use of rolling technologies. This process is an economical method for producing metallic platelets from metal shot, coarse powder, chops, cut-wire and other discrete pieces of metal in one step and permits the production of metallic platelets that are flat and free from visible cracks, splits, or any other undesirable defects associated with excessive deformation. According to one embodiment, the process comprises rolling a metal piece in one-step to form a platelet, wherein the ratio of the diameter of the metal piece to the thickness of the platelet is defined by the formula:

1≦D/t≦15;

[0010] wherein

[0011] D=diameter of the metal piece, and

[0012] t=thickness of the platelet.

[0013] The preferable D/t ratio for a particular type and/or source of metal will typically be within this range. However, for each particular metal, more preferred ranges may exist. These ranges are in part based on the workability and purity of the metal source. For example, for most easily accessible sources of copper, the ratio is preferably between 2 and 7. The term “workability,” which is also referred to as “formability” refers to the degree to which a material can be deformed before fracturing. More specifically, it is a function of the basic ductility of the metal and the type of stress and strain that may be imposed on the material. The concept of workability is well known to persons skilled in the art.

[0014] The present invention also provides a method for making a metallic platelet comprising:

[0015] a. determining a desired thickness of a metallic platelet;

[0016] b. selecting a metal piece based on said desired thickness of said metallic platelet; and

[0017] c. forming said metallic platelet from said metal piece.

[0018] The platelets produced according to the present invention may be subject to a secondary operation in order to impart desired attributes. For example, the platelets may be subjected to electroplating or other coating processes that improve the performance e.g. oxidation resistance) or visual appearance of the metallic platelet.

BRIEF DESCRIPTION OF THE FIGURES

[0019] FIGS. 1a and 1b are representations of optical micrographs of two rolled metallic platelets from Example 1 made with different D/t ratios.

[0020] FIG. 2 is a representation of an optical micrograph of rolled metallic platelets in Example 2. Samples J-M were rolled with different D/t ratios.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention provides methods for producing metallic platelets. The platelets produced according to the present invention may be produced quickly, efficiently and economically, and are substantially devoid of cracks or splits. In addition, the platelets are flat with a circular shape having an aspect ratio of <2. Typically, the aspect ratio is defined by dividing the length of the platelet by the width of the platelet. For example, if the platelet is round, the length and width of the platelet has equal dimensions so the aspect ratio is equal to 1. The round platelet with an aspect ratio of 1 are particularly beneficial for roofing applications or other building and construction materials because of its efficient geometry. Under certain circumstances, it is possible that if a material has a very high workability and the starting diameter is quite large (i.e. high D/t ratio), the rolled platelet will not be circular but will have an elongated oval or elliptical shape (aspect ratio>2). For example, when the platelet has an oval shape, the length is the longest dimension and the width is the shortest measurement that is often taken at ninety degrees to the length measurement. The elongated oval shape platelet is undesirable particularly when the maximum dimension is 0.25″ or so. This is because the platelet has more of a ribbon-like structure that requires more platelets to be used to cover the base material. The phrase “metallic platelets” refers to small pieces of one or more metals or alloys of metals.

[0022] The preferred embodiments of the present invention will now be described. These embodiments are presented to aid in an understanding of the present invention and are not intended, and should not be construed, to limit the invention in any way. All alternatives, modifications and equivalents that may become obvious to those of ordinary skill in the art upon reading this disclosure are included within the spirit and scope of the present invention. Further, this disclosure is not intended to be a treatise on rolling technologies. Readers are referred to appropriate available texts for additional information as necessary.

[0023] The present invention provides a process in which the appropriate selection of the size of the raw material in relation to the desired final thickness of the rolled platelet provides a useful product. According to this process, it is possible to generate high quality metallic platelets by controlling the ratio of the diameter of the starting metal to the thickness of desired platelet, which may be referred to herein as the “D/t ratio” where “D” is the diameter of the starting metal piece and “t” is the final thickness of the platelets.

[0024] If the D/t ratio is within a proper range, high quality metallic platelets can be formed such that they are regular in shape and contain minimal or no splits or cracks. If the D/t ratio is too large, the metal is subjected to excessive deformation that leads to splitting or cracking. If the D/t ratio is too small, especially as it approaches 1 (i.e. the thickness of the platelet is approaching the dimensions of the raw material), the metal is not deformed enough to produce a rolled material with a platelet structure. The optimal range of a D/t ratio of a particular metal source is in part dependent on the purity and chemical composition of that metal source. In general, the preferred D/t ratio is between about 1 and about 15. The D/t ratio range is important because the rolled platelet can be distributed onto a substrate and the platelet will lay flat. Where the D/t ratios are less than about 1.5, the percentage of non-flat platelets will increase substantially.

[0025] Under one embodiment, the present invention provides a process for a low cost, economical method for continuous production of metallic platelets using a one-step rolling process. According to this process, one may begin with a metal piece. Preferably, the metal piece is selected from the group of substances consisting of metal chops, shot, cut-wire, shavings, powders, granules and mixtures thereof. Further, the metal piece is preferably produced from a variety of metals and alloys including but not limited to: copper, bronze, brass, tin, aluminum, stainless steel, iron, lead, gold, silver, platinum, nickel and cobalt.

[0026] The precise acceptable range of sizes for a particular metal piece or metal source will in part be dependent on the “workability” of the metal source and in part on the desired size and shape of the metallic platelet. The workability of a metal source is dependent in part on the metal or metallic compound(s) selected, and in part on the purity of the metal source. Other factors that affect the workability of a compound include: (a) deformation conditions, which refer to a combination of factors including amount of strain, amount of stress, strain rate, temperature, geometry of workpiece, friction between deforming workpiece and tool (roller, die, etc.), surface finish of rollers, dies, etc.; and (b) the material properties, which refer to a combination of factors, including ductility (amount of plastic strain at failure) yield stress, grain size and structure, chemistry, content of second phase particulates, residual stress, annealing history, and degree of strain hardening. The factors that affect workability are well known to persons skilled in the art. With the knowledge of the workability of a particular substance or that ability to assess it, upon reading this disclosure, one skilled in the art would be able to determine the range of potential thickness of high quality metallic platelets that may be produced from metal pieces of a certain diameters.

[0027] For a metal with a higher workability, the upper limit of the critical D/t ratio for rolling acceptable metallic platelets will be higher. Table 1 on page 406 of the Chapter entitled Forming of Copper and Copper Alloys in the book entitled Forming, Metal Handbooks 8th Edition gives a relative ranking of the formability or workability of several different metal alloys, and is incorporated by reference herein. Using their ranking of excellent, good, fair and poor, the acceptable range of the D/t ratio should be highest for the materials with an excellent ranking and lowest for the poor ranking. Principles of workability and examples of workability of various substances are well known to persons skilled in the art and are discussed more fully in Dieter, Bulk Workability Testing, pp. 571-597 (1985), and ASM, Forming of Copper and Copper Alloys, pp. 405-408 (1969), both of which are incorporated by reference herein. The more workable a metal source is, the larger the potential ratio of D/t according to the present invention. For example, many easily accessible sources of copper are impure, and thus according to the present invention the D/t ratio is preferably greater than or equal to 2 and less than or equal to 7.

[0028] The workability of the metal source or metal piece will determine the range of ratios of D/t that are permissible according to the present invention. However, for any one particular application, one will also need to consider the range of desired thicknesses that are acceptable for a particular application. Based on these two parameters: (i) workability of the metal source; and (ii) range of desired thicknesses of the metallic platelets, one may determine an acceptable range of sizes of the metal source. According to the methods of the present invention, the starting metal pieces will preferably have diameters between about 0.01 inches and about 0.50 inches; more preferably between about 0.023 inches and about 0.100 inches.

[0029] The metallic platelets that are produced according to the processes of the present invention will have a distribution of sizes that are in part dependent on the size of the metal source, and in part on the operating conditions in which the rolling process takes place. Preferably, as many of the metallic platelets as possible will have a thickness between approximately 0.005 inches and 0.020 inches, more preferably between about 0.008 and about 0.012 inches. As a matter of practicality, some particles may be outside of a desired range, and it will not be economical to ensure that all particles are within this range. Thus, under normal operating conditions, preferably between about 80% and about 90% of the platelets have the desired size. Additionally, the metallic platelets will preferably have a diameter of between about 0.04 inches and about 0.25 inches; more preferably between about 0.08 and 0.18. If the platelets are too small they will be difficult to incorporate into roofing applications while maintaining the ability to reflect sunlight. Included in the problems associated with platelets that are too small is that they may become completely engulfed in the base roofing material. If the platelets are too large, they may be difficult to incorporate into the roofing material and may give an undesirable appearance.

[0030] According to the present invention, once a metal source of metal piece is selected, it is subjected to a rolling process to produce platelets. Because of the selection criteria, the desired size for the platelets may be produced in one-step. The phrase “one-step” as used herein refers to a rolling process in which each piece of metal passes through metal rollers only one time. Although the products of this one-step rolling could be subject to further rolling, the economics of the present invention are maximized when they are subject to only one-step rolling processes. Processes for rolling metals are well known to persons skilled in the art. Some of these processes are described in Chapter 17 of Mechanical Metallurgy, Dieter, (1976) which is incorporated by reference herein. According to the present invention, preferably, the metal pieces are passed through two rollers that are of the same diameter and operate at the same speed. The speed of the roller will need to be adjusted in accordance with the feed rate. Higher feed rates generally need higher rolling speeds to avoid metal pieces from being rolled together. The space between the rollers is set to produce a platelet with the desired thickness. Methods for determining feed rates and spacing of rollers is well known to persons skilled in the art.

[0031] In theory, one could provide unique or distinctive textures to the platelets produced according to the processes of the present invention if the rollers themselves were designed with textures. For example, if the rollers contain elevated lines or patterns on their surfaces, these lines or patterns would appear as depressions on the platelets. Conversely, depressions in the rollers would appear as elevations on the platelets. These textures could be included for aesthetic reasons. Alternatively, depending on into what environment the platelets are to be incorporated, textures may facilitate binding by for example, increasing surface area. This may be particularly beneficial when asphalt is used as a matrix for these platelets. Other matrices, which can be used with the metal platelets, are concentrate, fiberglass, epoxies, eurethanes, composite woods and other organic materials.

[0032] The metallic platelets that are produced according to the present invention may be used in applications that are now known or that come to be known to persons skilled in the art for the use of metallic platelets. For example, they may be used in building and construction materials such as in roofing applications, and impart improved weather resistance, heat-reflection/management, and architectural appearance. Methods for incorporating metallic platelets into roofing materials include combining the metallic platelets with asphalt by methods that are now known or come to be known by persons skilled in art of shingle manufacturing.

[0033] The metallic platelets that are produced according to the present invention may also be subjected to secondary operations in order to tailor them to specific applications, such as for aesthetic reasons or to prevent oxidation or rusting. For example, they may be subjected to an acidic or basic solution in order to clean up the metal or to remove tarnish. Another option is to subject the platelets to post-heat treatment under an atmosphere containing one or more substances selected from the group of substances consisting of air, hydrogen and inert gases in order to change the tint of the metal. They may also be electroplated or coated with an organic compound, as described in U.S. Pat. No. 5,723,516, which is incorporated by reference herein. All of these techniques are well known to persons skilled in the art. Electroplating is used primarily for cosmetic reasons. Coating with an organic compound prevents oxidization or improve the adherence to the matrix.

[0034] Having described the invention with a degree of particularity, examples will now be provided. These examples are not intended to and should not be construed to limit the scope of patent in any way.

EXAMPLES

Example 1

[0035] Copper shot ˜99.8% Cu of different sizes were deformed into platelets by passing the material through a roller mill. The roller mill in this example was manufactured by Stanat (Model TAH400). The rolling mill consisted of two equal diameter rolls that operated at a linear speed of 750 in/min. The thickness of the metallic platelets was varied by changing the opening or gap between the two rollers. Table 1 summarizes the starting diameter of the copper shot (D), and the thickness (t) and diameter of the metallic platelet. In addition, the calculated D/t ratio and comments about the quality of the metallic platelets formed are also shown in Table 1. 1 TABLE 1 Dimensions and properties of rolled copper shot produced in Example 1. Approximate Thickness of Diameter of Diameter of Metallic Platelet Calculated Metallic Platelet Sample Copper Shot (in) (in) D/t Ratio (in) Comments A 0.100 0.020 5.0 0.180 Good B 0.100 0.015 6.7 0.220 Good C 0.100 0.008 12.5 n/a Split D 0.06 0.017 3.5 0.093 Good E 0.06 0.011 5.5 0.113 Good F 0.06 0.006 10.0 n/a Spilt G 0.03 0.014 2.1 0.042 Good H 0.03 0.010 3.0 0.063 Good I 0.03 0.004 7.5 n/a Split

[0036] The results in Table 1 indicate that for this particular metal source, which has a moderate degree of workability, crack free metallic platelets can be formed when the D/t ratio is less than approximately 7. When the D/t ratio was greater than ˜7, the rolled platelets contained large edge cracks or splits. Examples of a metallic platelet produced with a D/t ratio of 6.7 (Sample B) and 12.5 (Sample C) are shown in FIGS. 1a and 1b, respectively. The metallic platelet in FIG. 1b is found to have a large center split.

Example 2

[0037] Copper shot with a size of 0.063″ was passed through the same roller mill used in Example 1. For this set of experiments, the opening between the rollers was varied to produce rolled material with a range of thickness. Table 2 summarizes the thickness of the rolled shot and the calculated D/t ratio. FIG. 2 is a stereo-micrograph showing the morphology of the rolled samples. Samples K, L and M have a well-defined platelet structure. For Sample J (D/t ratio=1.6), a platelet structure was not formed due to limited deformation of the shot (i.e. the gap between the rollers was too large). Based upon these results, it is desirable that the D/t ratio be greater than ˜2 in order to obtain a well-defined platelet structure during the rolling operation. 2 TABLE 2 Dimensions and properties of rolled copper shot produced in Example 2. Approximate Thickness of Platelet Diameter of Metallic Platelet Calculated Structure Sample Copper Shot (in) (in) D/t Ratio (yes/no) J ˜0.063 0.040 1.6 No K ˜0.063 0.028 2.2 Yes L ˜0.063 0.019 3.3 Yes M ˜0.063 0.011 5.7 Yes

Example 3

[0038] Stainless steel cut-wire shot was used to form a stainless steel platelet. The starting dimension of the cut-wire shot was ˜0.040″. The cut-wire shot was rolled into platelets approximately 0.010″ thick using a process similar to Example #1. The calculated D/t ratio for this material was 4. The resulting stainless steel platelet was free of any cracks or splits.

Example 4

[0039] Copper chops, produced from recycled wire, were rolled into platelets using a process similar to Example #1 The size distribution of the starting copper chops was approximately −18/+20 mesh fraction (0.033-0.39″). The copper chops were rolled into platelets with a thickness of ˜0.008″. The calculated D/t ratio for this sample ranged from ˜4 to 5. The rolled copper platelets were high quality, free from splits or cracks.

Example 5

[0040] Tin-coated copper chops, produced from recycled wire, were rolled into platelets using a process similar to Example #4. The tin-coated copper chops were rolled into platelets with a thickness of ˜0.008″. The rolled platelets had a uniform tin coating on the outer surface of the copper platelets. The tin coating gave the platelet a silvery/gray color and a highly reflective appearance.

Claims

1. A process for making a platelet, said process comprising rolling a metal piece in one-step to form the platelet, wherein:

D=diameter of the metal piece

t=thickness of the platelet

and 1≦D/t≦15.

2. A process for making a platelet according to claim 1, wherein said metal piece is selected from the group consisting of metal chops, shot, cut-wire, shavings, powders, granules and mixtures thereof.

3. A process for making a platelet according to claim 1, wherein said metal piece comprises a substance selected from the group consisting of copper, bronze, brass, tin, aluminum, stainless steel, iron, lead, gold, silver, platinum, nickel, cobalt and alloys thereof.

4. A process for making a platelet according to claim 1, wherein said platelet is flat and free from visible cracks or splits and an aspect ratio <2.

5. A process for making a platelet according to claim 1, wherein substantially all of said metal pieces have a diameter of from about 0.01 inches to about 0.50 inches.

6. A process for making a platelet according to claim 1, wherein substantially all of said platelets have a thickness of from about 0.005 inches to about 0.020 inches.

7. A process for making a platelet according to claim 1, wherein the D/t<7.

8 A process for making a platelet comprising the process of claim 1 and further comprising using a secondary operation to alter the appearance of the platelet.

9. A process according to claim 8, wherein said secondary operation comprises using an acidic, a basic or a solvent solution.

10. A process according to claim 8, wherein said secondary operation comprises using a post-heat treatment under an atmosphere containing one or more substances selected from the group consisting of air, hydrogen, nitrogen and inert gases.

11. A process according to claim 8, wherein said secondary operation comprises electroplating another metal onto the surface of the platelet.

12. A process according claim 8, wherein said secondary operation comprises coating the metallic platelet with an organic or organic coating.

13. A method for making a metallic platelet comprising:

a. determining a desired thickness of a metallic platelet;

b. selecting a metal piece based on said desired thickness of said metallic platelet; and

c. forming said metallic platelet from said metal piece.

14. A method for making a metallic platelet according to the method of claim 13, wherein step (b) comprises selecting said metal piece such that it has a diameter that is less than 15 times the desired thickness of said metallic platelet.

15. A method for making a metallic platelet according to claim 13, wherein said a diameter that is less than 7 times the desired thickness of said metallic platelet.

16. A method for making a metallic platelet according to claim 13, further comprising rolling said metal piece.

17. A method for making a metallic platelet of claim 14 for use in roofing, building and construction materials.

18. A method for making a roof comprising combining said metallic platelet of claim 11 with a roofing material.

19. A method for making a roof according to claim 18, wherein said roofing material is asphalt.