GB2097812A

GB2097812A - Drawable coatings for forming metal containers and can-forming process

Info

Publication number: GB2097812A
Application number: GB8202576A
Authority: GB
Original assignee: American Can Co
Current assignee: Primerica Inc
Priority date: 1981-05-04
Filing date: 1982-01-29
Publication date: 1982-11-10
Also published as: GB2097802A; GB2097802B

Abstract

A drawable precoating composition for a metal substrate for forming containers, e.g. from precoated metal sheet by a multiple drawing process, comprises a basic coating material which will adhere to a metal substrate and 3-10% by wt. of an incompatible lubricant having a specific gravity lower than that of the uncured precoating so that upon curing of the coating it forms a concentration gradient having its greatest concentration near the surface of the coating and furthest from the metal substrate. The lubricant facilitates forming of the coated metal substrate without loss of adhesion of the coating.

Description

1

GB 2 097 812 A 1

SPECIFICATION

Improved coatings for multiple formed cans and can-forming process

The present invention relates to improved coatings for multiple formed cans and can-forming process.

5 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 10 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 15' 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 coils handles 110 — 3' (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 20 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 25 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 ±mg 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 30 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 35 necessary to overcome the extreme pressures during a metal forming operation whereby the precoating and the metal are protected from such stresses. Blooming is not necessary with coil coated plate as the lubricant squeezes from the polymer matrix at pressure points as the internally lubricated plate is formed between the punch and die.

It has been the practice to add a small amount of lubricant to the precoating in order to work with 40 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 subjects of our copending U.K. patent applications Nos. 8136915 and 45 8136916. Containers embodying the present application can take the form disclosed and claimed in our copending U.K. patent application No. 8136914.

The present disclosure relates to lubes included in the coating primarily destined to reside on the external surface of the finished, formed product. One particular example is the epoxy-phenolic coated surface of a drawn and redrawn food container.

50 According to the present invention, there is provided from one aspect a drawable precoating composition for a metal substrate for forming containers, including a basic coating material which will adhere to a metal substrate the coating material in its uncured form being mixed with an incompatible lubricant having a specific gravity lower than that of the uncured precoating and present at a weight percent level of 3 to 10.

55 From another aspect, the invention provides a method for forming a hollow cylindrical container open at one end and having an integral bottom at the other end from sheet metal comprising the following steps:

providing a thin sheet of tin free steel of the chrome type,

coating the steel surface with an epoxy phenolic based coating composition having 3 to 10% by 60 weight of lubricant which is incompatible with the epoxy phenolic matter,

curing the coating and lubricant combination with heat while the lubricant blooms to the surface of the coating so as to develop a concentration gradient of lubricant across the coating thickness, and forming the hollow container by a three-draw multiple forming operation at speeds of 100 cans per minute output without scuffing the coating and without adding topical lubrication to the coating.

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60

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GB 2 097 812 A 2

In practising this invention, it may be preferred to restrict the upper limit to 8%, i.e. the lubricant then comprises 3 to 8% by weight of the basic coating composition. It may be preferred to employ a minimum lubricant loading of 5% by weight.

The invention is particularly, but not exclusively, suitable for use in a triple-draw can-making 5 process which produces can bodies whose diameters are about 75% of their heights. 5

To overcome the oroblems of prior techniques of applying topical lubrication to precoated metal,

large quantities of compatible lubricating materials are now added to the precoating composition. Using the resulting precoating mix, subsequent topical lubrication can be reduced or eliminated.

While it has been known to mix lubricants into precoatings in levels of up to about 1.5% by weight 10 for various purposes, we here teach a means by which this technique can be used specifically for a high 10 stress multiple forming operation. When animal-type lubricants, such as lanolin, or naturally occurring lubricants, such as petrolatum, are used according to the invention, they can amount to 5 to 10% by weight of the exterior precoating. Such lubricant-containing coatings have been found to work successfully at coping with the stresses of multiple forming operations. Such high levels of lubricant 15 applied with the precoating are not only capable of blooming during the curing of such coatings (except 15 when coil coating) but ensures sufficient lubricant is available so that upon subjection to the pressures of a forming process the lubricant can be squeezed to the surface at the pressure points and is thus available for facilitating the forming operation. It has been found that the choice of lubricant is an essential consideration for successfully overcoming difficulties of non-adhesion during the forming of or 20 subsequent processing of the precoated container. Solvents present in the uncured coating material 20 per se or in the lubricant may facilitate the bloom movement of the lubricant to the coating surface during curing.

More specifically, a lubricant that has a high molecular weight and is insoluble in the precoating is believed essential to successful operation since the lube must act compatibly with respect to the 25 adhesion properties of the coating and should have an adequately high molecular weight for coping 25 with high stresses. The preferred outside precoating is applied at a rate of 9 to 12 mg per 4 square inches (103 sq. cm) and the lanolin lubricant is used at a rate up to 7-1/2% by weight of the basic precoating composition. The latter is an epoxy phenolic resin or organosol. The specific gravity of the lanolin lubricant is less than one and usually from 0.9 to 0.98, whereas the specific gravity of the 30 coating is greater than 1 and can be as high as 1.3. The relationship between the specific gravity of the 30 lubricant and the coating is thought critical to the blooming in that the lubricant is squeezed to the surface of the precoating composition as cross linking occurs. It has been found that lubricant that remains encapsulated in the cured precoating, i.e., that portion of the lubricant that does not bloom, also assists fabrication because the immediate pressure during the multiple forming operation appears to 35 cause said lubricant to ooze from the surface of the precoating just ahead of the forming pressure line. 35 This can be verified by washing the surface of the formed container with di-iso octane to dissolve the residual lubricant (made available during forming). Such testing has verified that about 50% additional lubricant is available on sheet coated stock due to forming pressure, while the coil coated stock virtually 100% is encapsulated.

40 The invention will now be disclosed in more detail by way of example only in the following 40

description.

This invention relates to the precoating of metal plate or sheet of thin gauge, and of various types, for subsequent use e.g. 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 45 of withstanding elevated internal pressures during processing at high temperature, and of withstanding 45 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 the outside of the container. The coating must be capable of withstanding the severe drawing and redrawing operations as well as bottom profiling operation which occur in the high speed conversion of 50 flat, thin-gauged precoated sheet into hollow cylindrical bottomed vessels. Normally coatings are 50

topically lubed to aid vessel fabrication. We have found incorporation of internal lubrication aids can fabrication even without topical lubrication.

Can container size in this disclosure uses the conventional can makers terminology (the can makers convention gives the diameter across the completed doubleseam in inches plus sixteenths of an 55 inch, then the height in inches plus sixteenths of an inch). Therefore, a container with a 4-4/16" 55

(107.9 mm) diameter by 3-7/16" (87.3 mm) height would be called a 404 by 307.

The material used in connection with forming containers as disclosed herein is 75 # 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" 60 by 20" (35.6 X 50.8 cm) or 31,360 square inches of plate, 20.2 sq. metres on one side. Currently, the 60 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.

65 The coating can be experimentally tested by means of a tape test before and/or after food 65

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GB 2 097 812 A 3

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 redrawn. The tape is pressed to the surface with sufficient pressure to make complete contact (removing the air bubbles therebetween). The tape test 5 requires that the tape be quickly peeled from the coating surface to which it is adhered in an effort to 5 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 provided freshly made, scored edges which provide sites for the initiation of any peeling that might occur.

10 A test which is representative of the planned use for such containers is called a "water pack test". 10

The formed containers are filled with distilled water almost to the top. A 1/4" (6 mm) headspace remains at the time the containers which are closed, hermetically sealed and a vacuum of 13" of mercury is applied. Such cans are then placed in a retort and steam processed for 60 minutes at 250°F (121 °C). Subsequent to the retort processing the cans are pressure cooled for 7 minutes. This 15 procedure subjects the containers to conditions similar to those which would be incurred in a 15

commercial use of the container for containing and packing 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 metal surface. Such oxidation indicates the degrees of 20 resistance the precoated container has to processing and the integrity of the outside coating. 20

EXAMPLE 1

This is the preferred combination wherein 3% petrolatum by weight was added to the resin solids of an epoxy phenolic coating MC 9372—006 made by Mobil Chemical. That combination was applied to 75 # per base box TFS—CT and ETP by roller coater at a rate of 12 mg per 4 square inches and 25 cured for 8 minutes at 400°F (204°C). No topical lubrication was used. The plate was formed into 211 25 by 400 cans (68.3 x 101.6 mm) in a three draw multiple forming operation at press speeds of about 100 strokes per minute. The resulting containers showed good forming properties and good coating adhesion on at least the TFS—CT but not on ETP. See Table 1 for relative ranking.

EXAMPLE 2

30 This experiment had 5% petrolatum by weight to the resin solids of an epoxy phenolic coating 30 MC 9372—006 made by Mobil Chemical. That combination was applied to TFS—CT and ETP both were 75 # plate by roller coater at a rate of 12 mg per 4 square inches and cured for 8 minutes at 400°F. No topical lubrication was used. The plate was formed into 211 by 400 cans in a three draw multiple forming operation at speeds of about 100 strokes per minute. The resulting containers showed 35 good forming properties and good coating adhesion on at least the TFS—CT, but the adhesion on the 35 ETP was worse than that found with the combination of Example 1. The reason was that the additional lubricant interferes with adhesion to the ETP to which bonding is more difficult. See Table 1 for relative ranking.

EXAMPLE 3

40 The test had 3% lanolin by weight added to the resin solids of an epoxy phenolic coating 40

MC 9372—006 made by Mobil Chemical. That combination was applied to 75 # TFS—CT and ETP by roller coater at a rate of 12 mg per 4 square inches and cured for 8minutes at 400°F. No topical lubrication was used. The plate was formed into 211 by 400 cans in a three draw multiple forming operation at speeds of 100 strokes per minute. The resulting containers showed good forming 45 properties and good coating adhesion on at least the TFS—CT but not ETP. See Table 1, for relative 45 ranking.

EXAMPLE 4

75 # TFS—CT plate was commercially coated using 0, 5 and 7-1/2% added petrolatum on a solids on solids percentage basis. Plate was baked for 8 minutes at 400°F and then the inside coating » 50 was applied and baked. The coated plate with added internal lubricant showed a thin film of lubricant 50 present. A control portion of the plate with no added lube was topically lubed. 303 x 406 triple drawn cans were made with the following results.

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GB 2 097 812 A 4

Added Petrolatum

Topical Lubrication

Can Fabrication Results

4-1.

0

Yes

Satisfactory

4-2.

0

No

Unsatisfactory

4-3.

5

No

Satisfactory

4-4.

7-1/2

No

Satisfactory

Example 4-1 was our commercial control and no fabrication problems occurred. Example 4-2 followed and can break up occurred after 6 cans were made. Example 4-3 was run next and startup occurred on dry dies. 1600 cans were made. Example 4-4 was next and 1600 cans were made. 10 Cans were autoclaved and no blushing or adhesion loss of either the outside or inside coating was 10 noted.

EXAMPLE 5

65 # TFS—CT was commercially coil coated using 5% added petrolatum on solids on solids basis. The coil was coated at 600 feet per minute (183 metres/minute) receiving a bake of 19 seconds at an 15 air temperature of 575°F (302°C). Lubricant squeeze-out from the film was not evident as with sheet 15 baked material. 303 by 406 cans (81 x 111 mm) were made and autoclaved with satisfactory results.

TABLE 1

Outside Coating Adhesion by Tape Test 311 x 400 Drawn Can

TFS—CT Tinplate

Ex. 1

3% Petrolatum

0

8

Ex.2

5%

0

10

Ex.3

3% lanolin

2

8

0 — No adhesion loss 10 — Complete adhesion loss

It is essential that the lubricant be incompatible with respect to the basic coating material and the specific gravity of the lubricant be less than that of the uncured coating. The reason is so as to allow the 20 lubricant to float to the surface of the coating during curing. More specifically, the lubricant is mixed into 20 the coating at a preferred solids-on-solids weight percent of 3 to 8. At these percentages, careful handling is required in order to keep the combination of lube and coating a mixture. The mixture is then applied to the metal and cured in a bake oven. During baking the metal being the best conductor of the three in the system (metal, lube, coating) becomes hot first, and tends to activate the coating such that 25 Brownian type movement of the lubricant occurs due to the kinetic energy imparted to the mixture by 25 the curing oven. More particularly, the lubricant forms a concentration gradient with more of it migrating to the surface of the coating during curing. It is this concentration gradient which operates to facilitate the successful multiple forming operations of a precoated metal and ensures that the coating adheres to the metal even though a high concentration of lubricant is included. It has been found that 30 having more than a 10% lubricant in the coating interferes with the clamping necessary to successfully 30 draw a container having a depth to diameter ratio greater than one.

While specific combinations of precoating and lubricant have been disclosed, the invention is broadly any precoating composition including lubricant at a weight percentage of 3 to 10 as included in the uncured coating. In particular, preferred are natural-type lubricants which are incompatible with the 35 coating so as to bloom to the surface of the coating and thus become available in quantities sufficient to 35 act as a lubricant between the precoated metal and the tools for forming same.

Claims

1. A drawable precoating composition for a metal substrate for forming containers, including a basic coating material which will adhere to a metal substrate the coating material in its uncured form

40 being mixed with an incompatible lubricant having a specific gravity lower than that of the uncured precoating and present at a weight percent level of 3 to 10.

2. A drawable precoating composition for a metal substrate for forming containers, including a basic coating material which will adhere to a metal substrate the coating material in its uncured form being mixed with an incompatible lubricant having a specific gravity lower than that of the uncured

GB 2 097 812 A

precoating and present at a weight percent level of 3 to 8.

3. The precoating composition according to claim 1 or claim 2 wherein solvents in either the precoating or the lubricant act to facilitate the bloom movement of the lubricant with respect to the coating material during curing.

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4. The precoating composition according to claim 1, 2 or 3, for use on a metal substrate of tin free 5

steel of the chrome type, wherein the coating material is an epoxy phenolic resin.

5. The precoating composition according to any of claims 1 to 4, wherein the lubricant is petrolatum.

6. The precoating composition according to any of claims 1 to 4, wherein the lubricant is lanolin.

0

7. The precoating composition according to claim 5, wherein the petrolatum is present at a weight 1 q percentage of about 7-1/2%.

8. A method for forming a hollow cylindrical container open at one end and having an integral bottom at the other end from sheet metal, comprising the following steps:

providing a thin sheet of tin free steel of the chrome type,

5 coating the steel surface with an epoxy phenolic based coating composition having 3 to 10% by 15

weight of lubricant which is incompatible with the epoxy phenolic matter,

curing the coating and lubricant combination with heat while the lubricant blooms to the surface of the coating so as to develop a concentration gradient of lubricant across the coating thickness, and forming the hollow container by a three-draw multiple forming operation at speeds of 100 cans per minute output without scuffing the coating and without adding topical lubrication to the coating. 20

9. A method for forming a hollow cylindrical container open at one end and having an integral bottom at the other end from sheet metal, comprising the following steps:

providing a thin sheet of tin free steel of the chrome type,

coating the steel surface with an epoxy phenolic based coating composition having 3 to 8% by weight of lubricant which is incompatible with the epoxy phenolic matter, 25

10. The method according to claim 8 or 9, wherein the lubricant is lanolin and the coating 30 composition is applied to the surface of the metal sheet destined to form the outside of the container.

11. The method according to claim 8 or 9, wherein the lubricant is petrolatum and the coating composition is applied to the surface of the metal sheet destined to form the outside of the container.

12. A container forming method according to claim 8, and substantially as herein described by way of example. 35

13. A container body made by the method claimed in any of claims 8 to 12.

14. A drawable precoating composition suitable for metal sheet destined to be used in a multiple forming container making process, substantially as herein described with reference to the Examples.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1982. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained