GB2097802A - Compositions for precoating metal substrates for conversion into containers - Google Patents

Abstract

Compositions for coating sheet metal for converting into can bodies by a multiple-forming, deep drawing and ironing process, comprise a dry film lubricant consisting of a polymeric or natural wax (e.g. Carnauba) in the coating composition, the dry film lubricant being one having a softening point higher than that of the basic coating composition with which it is amalgamated.

Description

SPECIFICATION Improvements relating to precoated metal substrates for conversion into containers The present invention relates to improvements relating to precoated metal substrates for conversion into containers.

This disclosure relates to the lubrication of precoated metal, such as tin free steel to be used in a multiple forming operation, the desire being for a finished product that has a continuous coating left on it. Lubrication is necessary in order to protect the coating material from being scuffed, torn or otherwise damaged during forming. It is common to use topically applied lubrication over the precoating.The lubrication is applied by means of rollers, sprays, brushes, electrostatic dispersion or spot application either immediately after the precoating process or just prior to the press operation wherein the product is made by a series of multiple forming operations; The most economical approach to applying lubrication over a precoating has been by means of a hot spray applied at speeds of 600 to 800 surface feet per minute (183 to 244 metres per minute) simultaneously to both sides of precoated coiled metal.

More specifically, the coil coater takes the precoated sheet after it cools from the baking oven where the precoating is cured and applies a thin film of lube by hot spray. Because of the high speed, there is no visible bloom of any internal lubricant on the surface of the precoated stock.

A slower process used for coating sheets as opposed to coiis handles 110 - (0.91 m) long sheets per minute on one side only. Thus, the speed of this arrangement is about one-fourth that of the coil coater which coats both sides simultaneously. Unlike the coil coater, there is a 10 minute bake to bloom the internal lube. Each sheet is held by a wicket in a wicketing rack as it passes through the oven.

Such an arrangement permits blooming with relative uniformity such that a consistent low level of lubricant (up to about 1.5% by weight) is visible, with the sheet coating operation. In addition, topically applied lube is more accurately applied at the slow sheet coating speed. In contrast, the coil coater has speed but little or no accuracy in terms of topical application rates or uniformity. The preferred amount of lube is 12 mg per square foot per side (1.29 mg/m2) for both a coil and sheet coater. The tolerance on both is +7 mg which is more difficult to achieve on the coil coater than with a sheet coater.

It is also critical that the right lube be selected. The lube should not affect the adhesion of the precoating to the metal substrate and should be compatible with future use requirements of articles formed by the multiple forming operation. The lubricant has, however, incompatibility with the precoating which does not affect the adhesion of the precoating during the multiple forming operation or the subsequent use of the formed article. The incompatibility works to make the lubricant float to the surface of the coating during curing; this is called blooming. Either condition would be acceptable.

Therefore, it becomes important to select a lubricant which has the required incompatibility but will mix with the precoating without separating before application and provides the lubricating properties necessary to overcome the extreme pressures during a metal forming operation whereby the precoating and the metal are protected from such stresses.

It has been the practice to add a small amount of lubricant to the precoating in order to work with a subsequent topical overlube. More particularly, less than 2% by weight of lubricant has been added to the precoatings and that in combination with a topical overspray has been found to provide sufficient lubrication when the topical overspray is applied uniformly at the prescribed rate.

Formed articles such as containers are preferably made by a multiple forming operation, and using tooling, respectively the subject of our copending U.K. patent applications Nos. 8136915 and 8136916. Containers embodying the present application can take the form disclosed and claimed in our copending U.K. patent application No. 8136914. In these applications, the metal is subjected to ironing so that the criticality of lubrication becomes most severe. Most coating lubricant combinations tend to break down due to the loads and stresses of such processing. It is, therefore, important to have specific lubricating properties which can withstand the heat and pressure occurring during an ironing operation without the need for any additional coolant or lubricant.

According to one aspect of the invention, there is provided a method of providing a metal substrate with a coating having increased lubricity and resistance to scuffing when both coating and metal are subjected to multiple forming operations, the method including the following steps: providing a coating material which retains adherence to a metal substrate when the latter is formed by stretching and forming operations, mixing a dry film lubricant into the coating material before application to the substrate, the dry film lubricant having a softening point higher than that of the basic coating material into which it is mixed, and applying the coating and lubricant material combination to the metal substrate in a thin film layer which is then cured to adhere it to the metal substrate.

Also according to the invention, there is provided a method of increasing the lubricity and resistance to scuffing of a curable metal coating composition, wherein to a metal coating composition, which retains adherence to a metal substrate after curing and which is capable of withstanding stretching and forming operations, is added a dry film lubricant having a softening point higher than that of the coating composition per se.

The invention further provides a precoating composition for a metal substrate adapted to be cured to form an adherent film thereon, comprising a coating material which adheres well to metal surfaces after curing and which is capable of being formed with the metal by stretching and drawing operations, and a dry film lubricant uniformly mixed with the coating material, the said dry film lubricant having a softening point higher than that of the said coating material per se.

During multiple forming at high levels of pressure, heat is generated. Lubricants must be able to work under both the heat and pressure in order to protect the precoating and the precoated metal from destruction. It has been found that dry film type lubricants can be dispersed in solvents and incorporated in the coating composition per se so that at the temperature resulting during forming operations, the lubricant is available at the surface as a hard, solid protective layer. It is essential that the melting point of the solid lubricant be adjusted to conform generally with the levels of heat existing during the multiple forming steps, so that the lubricant is the first entity to become available in a flowable form at the time when the temperature exceeds a predetermined level. In the preferred embodiment temperatures around 2800F (1 380C) have been found.To solve problems of scuffing and tearing on drawn and ironed containers by the processes disclosed in the copending U.K. applications noted above, containers have been precoated with epoxy phenolic or epoxy urea formaldehyde formulations containing a high melting lubricant applied upon the surface which is ultimately external. These high melting temperature waxes can be polyethylene or Carnauba; both can be dispersed in n-butanol. Any polymer hydrocarbon of Fisher-Tropsch type which can function as a slip agent is usable so long as the melting point is adjusted to work with the particular multiple forming conditions by including sufficient amounts of such waxes in the precoating composition.The need for additional topical lubrication is reduced or eliminated and the ability to produce a finished article formed from precoated metal can be maintained even in a high speed high stress operation such as combined drawing and ironing.

The invention will now be disclosed in more detail by way of example only in the following description.

This invention relates to the precoating of metal plate or sheet of thin gauge, and of various types, for subsequent use for instance for forming into containers having diameters usually less than their heights. Such containers are commonly used for packing processed foods and beverages, and must be capable of withstanding elevated internal pressures during processing at high temperature and of withstanding the external pressure due to the vacuum generated upon cooling. The precoating with which this particular disclosure is concerned is that coating which is on the surface that ultimately becomes either the inside or outside of the container.The coating must be capable of withstanding the severe drawing and redrawing operations as well as bottom profiling operations which occur in the high speed conversion of flat, thin-gauge precoated sheet metal into hollow cylindrical bottomed vesseis.

Normally coatings are topically lubed to aid vessel fabrication. We have found that incorporation of internal lubrication aids can fabrication and, if desired, even enables topical lubrication to be omitted.

Can container size in this disclosure uses the conventional can makers terminology. (The can makers convention gives the diameter across the completed double seam in inches plus sixteenths of an inch then the height in inches plus sixteenths of an inch.) Therefore, a container with a 4-4/16" (107.9 mm) diameter by 3-7/16" (87.3 mm) height would be called 404 by 307.

The material used in connection with forming containers as disclosed herein is 65# per base box.

This reference to the base box terminology for base weight is familiar to can makers. Such terminology originally referred to the amount of steel in a base box of tin plate consisting of 112 sheets of steel 14" by 20" (35.6 x 50.8 cm), or 31,360 square inches of plate, 20.2 sq metres on one side. Currently, the base box has related to base weight reference to the amount of steel in 31,360 square inches of steel whether in the form of coil or cut sheet. Tin free steel of the chrome type is commonly designated TFS-CT; electrolytically deposited tin on steel is designated ETP. The amount of tin is also designated in terms of so many pounds per base box.

The coating can be experimentally tested by means of a tape test before and/or after food sterilization wherein each sample of a container multiply formed from precoated stock can be tested with a pressure sensitive adhesive tape such as a 1" wide strip of 3M tape #610 applied to the cured coating before and after it is drawn and multiply drawn The tape is pressed to the surface with sufficient pressure to make complete contact (removing the air bubbles therebetween). The tape test requires that the tape be quickly peeled from the coating surface to which it is adhered in an effort to peel the coating, lifting any poorly adhering coating. In order to further test the coating adhesion, crosses are scribed in the coating with a sharp pointed instrument before the tape is applied.These crosses provide freshly made, scored edges which provide sites for the initiation of any peeling that might occur.

Once a container has been made from precoated stock by means of multiple forming operations it is important to be able to ascertain whether or not the container is adequately protected by the precoating. A rapid test to demonstrate whether or not the outside coating has fractured during the multiple forming operations includes the use of copper sulfate. Each container is immersed in a copper sulfate solution for two minutes. The solution is made by mixing 26 1/2 ozs (751.3 g) of copper sulfate crystals with 6 1/2 fluid ozs (185 ml) of concentrated hydrochloric acid diluted with one gallon (4.55 litres) of distilled water. After immersion in the solution the can is rinsed in water then examined visually for copper deposits. Any traces of copper on the surface indicate a lack of coating continuity and, more particularly, shows the exact nature of the discontinuity in terms of shape and location.

A further test which is more representative of the planned use for such containers is called, a "water pack test". Containers formed by drawing and redrawing are filled with distilled water almost to the top. A 1/4" (6 mm) headspace remains at the time the containers are closed and a vacuum of 13" (33 cm) of mercury is applied. Such cans are then placed in a retort and steam processed for 90 minutes at 2650F (1290C). Subsequent to the retort processing, the cans are pressure cooled for 7 minutes. This procedure subjects the container to conditions similar to those which would be incurred by the container for a pack of comestibles. The retorted containers are then evaluated and more specifically, the outside coating is examined after the containers have been allowed to air dry overnight.

These containers can also be stored in a high humidity chamber if necessary to encourage oxidation of any exposed exterior metal surface. Such oxidation indicates the degree of resistance the precoated container has to processing. Similarly, copper sulfate test can also be used. That is to say that, after the processing test they may be immersed in copper sulfate to specifically isolate areas of coating discontinuity.

EXAMPLE 1 A 20% dispersion of a high melting polyethylene (2800F or 138"C softening point) in n-butanol SL280 by Daniel Products was added as a lube to an epoxy-urea-formaldehyde formulation GL650C136, manufactured by SCM Glidden Coatings and Resins. The level of dry lubricant used ranged from one-half to 6% of the total non-volatile resin solids in the coating, and thereby allowed a two-fold function: (a) As internal lubricant only, at one-half to 2%, the polyethylene addition in the coating enabled multiple drawing with ironing of 65 lb base box weight TFS plate precoated with this composition at speeds up to 125 spm.

(b) As a complete lubricant at 2, 4 and 6% levels, the polyethylene permits a triple drawing and ironing operation (no topically applied petrolatum was necessary to form a commercially acceptable can without scuffing).

The adhesion and integrity of the coating with the polyethylene lubricant were excellent even after retort processing at 2650F (1 290C) for 90 minutes.

EXAMPLE 2 A 24% dispersion of polyethylene SL177 (2800F or 1 380C softening point) in xylene by Daniel Products was added in an epoxy-phenolic coating made by Mobil Chemical: MC 9372-006.

The levels of lubricant used were 2% and 4% by weight of the total resin solids in the coating.

Applied at 9 mg/4 sq in (9 mg over 103 sq cm) and baked 9 minutes at 400"F (2040C), the coating allows triple drawing with ironing of 65 Ib TFS plate.

EXAMPLE 3 A 24% dispersion of the same polyethylene Sly 77 in the preceding Example was used in combination with a 20% dispersion of a polymer wax SL425 by Daniel Products (2250F softening point) at a 4:1 ratio in the Mobil Chemical epoxy phenolic MC 9372-006. The low level of additional polymer wax improved the anti-scuff drawing and ironing properties of this epoxy phenolic on both 65 Ib TFS and 65 Ib No. 25 ETP plate.

EXAMPLE 4 A 20% dispersion of polyethylene SL 50 (2250F or 1 070C softening point) in n-butanol, by Daniel Products, was incorporated in three white pigmented inside enamels. The first was Glidden's phenolicmodified epoxy-urea formaldehyde GL588-92C. The others were Watson-Standard's WS28--419 and WS28-420, both vinyl organosols. At dry polyethylene content level of 2%, these precoats provided good fabrication with no loss of adhesion being observed. The SL50 is approved by the Food and Drug Administration for use with coatings in contact with food.

The following examples have lower melting point Carnauba waxes for lubrication and do not perform as well in a continuous operation thereby indicating the importance of the condition of the lube at the point where the forming stress and heat occurs.

EXAMPLES 5 AND 6 Two coatings were prepared containing Carnauba wax dispersed at 1.95% by weight on a solids on solids basis in a Mobil Chemical epoxy-phenolic formulation MC 9372-007 an an epoxy-ureaformaldehyde formulation MC 8406-020. These coatings were evaluated as outside precoatings for drawn and ironed cans. The cans were made on a pilot line by first cupping in one press then by multiple forming or redrawing in another press. On transferring to a press having consecutive forming sequences the precoat containing Carnauba wax failed due to the greater heat generated in forming. The lag time between cupping and forming stations on the former two-press arrangements was such that there was heat dissipation between stages.In the press with a continuous operating sequence, higher instantaneous temperatures occurred and Carnauba wax was not effective. Consequently, lubricants with higher softening points as in Examples 1 to 4 are necessary in a continuous high heat multiple forming sequence. Carnauba wax is a suitable dry film lubricant alternative to e.g. polyethylenes where ample time is available for heat dissipation between the individual forming steps.

EXAMPLES 7 AND 8 Dispersions of Carnauba at 1% and 2% by weight solids on solids in Glidden Coatings epoxy-ureaformaldehyde GL588-44 were prepared and tested as outside precoatings for high speed multiple forming by drawing with ironing. The 1% example had a trace of surface roughness after a water test.

The 2% example permitted good cans to be fabricated and they passed both the water pack and copper sulfate tests. While this wax was sufficient for triple drawing with ironing on the two press arrangement, the results on the press with a continuous operating sequence are consistent with those found in Examples 5 and 6.

The softening point is the temperature or range of temperature at which a substance softens. To determine this property, a thermomechanical analyzer, Perkin-Elmer TMS-1, has been used.

A sample of the coated plate is punched from a sheet and placed under a weighted probe. A predetermined, programmed range of temperatures is then scanned. When the coating starts to soften, the probe penetrates the coating, and a distinct break occurs on the trace of chart recorder of the analyzer. The softening point is taken as that temperature corresponding to the midpoint of a line drawn tangential to the break in the curve, between pre-transition and post-transition base lines.

The softening point is a factor influencing the drawing and ironing properties of a coating. If the internal lubricant is soft or liquid, the softening point of a coating is depressed to a larger extent. Thus, 0.5% lanolin in a Glidden formulation results in a softening point of 1 830F (840C) for the cured coating.

In contrast, when a hard, solid wax such as polyethylene with a melting point of 2800F (1380C) is incorporated in the same Glidden formulation the coating exhibits a softening point of 1 940F (90or).

The first example failed to draw and iron satisfactorily; the second example passed this fabrication test.

The following tabulation summarizes the foregoing Examples.

Coating Coating Internal Lube Wt. Softening Water CuSo4 Example Type Code (m pt) F % Point C.O.F. Fabrication Pack Test 1. epoxy-UF GL 588-67M 280 polyethylene 0.5% 194 F - no scuffing good passed epoxy-UF GL 588-67A 280 polyethylene 1.0 194 .09 no scuffing good passed *epoxy-UF GL 588-67B 280 polyethylene 2.0 192 .08 no scuffing good passed *epoxy-UF GL 588-67C 280 polyethylene 4.0 189 .08 no scuffing good** passed *epoxy-UF GL 588-67D 280 polyethylene 6.0 185 .08 no scuffing good passed 2. epoxy-phenolic MC9372-006 280 polyethylene 2.0 183 .05 no scuffing good passed 3. epoxy-phenolic MC9372-006 280 polyethylene 2.0 190 .05 no scuffing good passed with 225 polymer wax 0.5 4. epoxy-UF (mod.) GL 588-92C -225 polyethylene 2.0 - - no stripping - problem on punch vinyl WS 28-419 225 polyethylene 2.0 - - no stripping good problem on punch vinyl WS 28-420 225 polyethylene 2.0 - - no stripping good problem on punch 5. epoxy-phenolic MC9372-007 185 Carnauba 1.95 198 .06 no scuffing on good passed pilot line 6. epoxy-UF MC8406-020 185 Carnauba 1.95 180 - no scuffing on good passed pilot line 7. epoxy-UF GL 588-44A 185 Carnauba 1.0 - - no scuffing on trace passed pilot line roughness 8. epoxy-UF GL 588-44B 185 Carnauba 2.0 - - no scuffing on good passed pilot line N.B.* No topically applied lubricant on these samples.

** Best variable in Example 1.

Other examples given in the tabulation show that the softening point is inversely proportional to the concentration of dry film or wax as internal lube. These examples work in spite of the lower softening points because of partial migration of the lubricant to the surface of the coating. High melting waxes form a hard, protective layer over the coating film, while low melting waxes and liquid lubricants liquefy, wipe off, and leave a relatively unprotected coating film vulnerable to the fabrication stresses.

The combination of dry film lubricants with basic coating compositions yield precoating agents well able to survive the stresses experienced in multiple forming without serious damage occurring. The presence of the lubricant in the coating can facilitate and accelerate can manufacture and enough lubricant can be provided in this way to obviate the need for topically-applied lubrication.

While specific additions of various dry film lubricants such as polymer waxes and Carnauba wax have been disclosed and explained, the invention in its broadest context is the use of any dry film lubricant substance in a coating in sufficient quantity to effectively raise the softening point of the coating. That is to say that, the dry film lubricant must have a softening point as high as the coating or higher in order to be most useful in connection with protecting the coating during a multiple forming operation. Therefore, in the claims which follow any dry film material added to a precoating and the application of the combination to a metal substrate is sought to be covered.

Claims (12)

1. A method of providing a metal substrate with a coating having increased lubricity and resistance to scuffing when both coating and metal are subjected to multiple forming operations, the method including the following steps: providing a coating material which retains adherence to a metal substrate when the latter is formed by stretching and forming operations, mixing a dry film lubricant into the coating material before application to the substrate, the dry film lubricant having a softening point higher than that of the basic coating material into which it is mixed, and applying the coating and lubricant material combination to the metal substrate in a thin film layer which is then cured to adhere it to the metal substrate.

2. A method of increasing the lubricity and resistance to scuffing of a curable metal coating composition, wherein to a metal coating composition, which retains adherence to a metal substrate after curing and which is capable of withstanding stretching and forming operations, is added a dry film lubricant having a softening point higher than that of the coating composition per se.

3. A precoating composition for a metal substrate adapted to be cured to form an adherent film thereon, comprising a coating material which adheres well to metal surfaces after curing and which is capable of being formed with the metal by stretching and drawing operations, and a dry film lubricant uniformly mixed with the coating material, the said dry film lubricant having a softening point higher than that of the said coating material per se.

4. The invention as defined in any of claims 1 to 3, wherein the dry film lubricant is a polymeric wax.

5. The invention as defined in claim 4, wherein the polymer wax is added to the coating material in an amount up to 1% by weight.

6. The invention as defined in claim 5, wherein the polymeric wax is added to the precoating material in an amount of at a weight percentage half of 1% by weight.

7. The invention as defined in claim 6, wherein said dry film lubricant has a softening point in excess of 210"F (990C).

8. The invention as defined in any of claims 4 to 7 wherein the coating material is an epoxy phenolic resin material and said dry film lubricant is polyethylene wax.

9. The invention as defined in any of claims 1 to 3, wherein the lubricant is a naturally occurring wax with a softening point of at least 1 850F (850C).

10. A method according to claim 1 and substantially as herein described by way of example.

11. A method according to claim 2 and substantially as herein described by way of example.

12. Precoating compositions substantially as herein described with reference to any one of the Examples 1 to 8.