WO2018055099A1

WO2018055099A1 - Tab for container ends, process to produce it and use thereof

Info

Publication number: WO2018055099A1
Application number: PCT/EP2017/074077
Authority: WO
Inventors: Arnoud Cornelis Adriaan DE VOOYS; Dammes Hans Van Der Weijde
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2016-09-22
Filing date: 2017-09-22
Publication date: 2018-03-29
Anticipated expiration: 2019-03-22

Abstract

Tab for container ends produced from unplated steel strip or sheet tabstock or from plated steel strip or sheet tabstock, wherein the is provided with a layer of a zinc-rich primer, and process for producing the tab, and use thereof.

Description

TAB FOR CONTAINER ENDS, PROCESS TO PRODUCE IT AND USE THEREOF

The present invention relates to an opening tab for container ends produced from tabstock provided with a coating layer. The invention particularly relates to opening tabs for retained tab beer and beverage container ends and for easy open ends for foodstuffs. Foodstuffs may be for consumption by humans or animals (e.g. pet food).

Pull tabs for container ends are used on various types of containers, including beer and beverage containers, food containers, and many other types of containers for household or industrial use. The most prevalent type of pull-tab container ends is the retained-tab end, which have a tab designed to remain on the container end after the container is opened, such as with beer and beverage metal containers. See figure 3 for a specific example of the use of a tab on a beverage container.

The typical non-detachable tabs used in the beverage container industry all are of generally the same design (see e.g. W09926741-A1). These tabs have a nose portion, a lift end portion, separated by a central body portion that has an aperture provided for securing the tab to the end by a rivet. The tab is manufactured separately from the end, and is secured to the end such that the nose is positioned over the score line of the end . When the tab is lifted at the lift end by the user, the nose of the tab is pushed down on the end panel to fracture the score and open the container. The rivet, therefore, acts as a fulcrum for the rotation of the tab, and a central hinge area of the tab bends adjacent the rivet across a hinge line of the tab.

Containers are designed to provide liquids or food to consumers without spoilage, which requires any bacteria or other microbiological species to be eliminated. The latter is done by a heat treatment of the containers after the lid, including the tab, has been placed in an airtight manner. The heat treatment is referred to as sterilising or pasteurising, depending on the time-temperature cycle used, and is usually done in hot water and/or steam. One of the prime criteria of tabstock is therefore the ability to withstand the heat treatment and subsequent storage without any visual or other undesirable change, like the formation of red rust.

The material from which the tab is produced (aka tabstock) is generally a steel strip or a steel sheet. Steel has a tendency to corrode when it is not protected, and therefore tabstock is usually provided with a tin layer or an electrolytically deposited zinc (electro-zinc (EZ)) layer. In addition, the tabstock may be provided with a lacquer. After the forming of the score line the coating may be locally damaged, and a score repair treatment, usually involving the application of a lacquer, may be performed .

However, tin does not provide galvanic protection in many applications, leading to undesirable red rust formation at the cutting edges and at other positions where the steel substrate is exposed. Electrolytically depositing zinc layers do provide galvanic corrosion, and therefore suppress the formation of red rust, but the visual appearance is of a lower quality (it is darker and more dull) and is an expensive process requiring a dedicated EZ-line.

It is an object of the invention to provide a tab made from a steel strip or sheet having improved properties.

It is another object of the invention to provide a tab made from a steel strip or sheet that is cheaper to produce than the known steel strip or sheet.

According to the invention, one or more of these objects is reached with a tab for container ends produced from unplated steel strip or sheet tabstock or from plated steel strip or sheet tabstock, characterized in that the tabstock is coated with a layer of a zinc-rich primer.

This embodiment provides the steel with the necessary protection against corrosion during its most critical process step (the heat treatment of the canned product) and thereafter during its shelf-life.

The use of zinc-rich primer in heavy duty applications such as automotive applications, construction and industrial applications is known.

The protection offered by the zinc-rich primer to a steel substrate is cathodic protection. The zinc is anodic, and the steel is cathodic. The polarity of a zinc- steel galvanic couple may reverse under certain conditions, which means that the steel becomes the anode, and the zinc becomes the cathode. The change in the zinc electrode potential is chiefly responsible for the reversal of polarity since the potential of the steel remains relatively unchanged . Figure 4 shows that polarity reversal does not occur in distilled water between 50 and 60 °C (the solution contains 70-80 ppm HC0₃ ^", 10 ppm S0₄ ^2" and 10 ppm N0₃ ^" (Figure 7.15 from Corrosion and Electrochemistry of Zinc, Zhang, Plenum Press, New York 1996)). Many other factors, besides temperature, such as dissolved ions, pH, and time of immersion, are found to affect the polarity of a zinc/steel couple. In normal circumstances, such as in cold water, the zinc is anodic to the steel and thus provides sacrificial protection for the steel. When polarity reversal occurs for a galvanized steel, the steel is not protected cathodically by the zinc coating, and the corrosion of the steel underneath the zinc coating occurs when the coating is completely penetrated by localized corrosion such as pilling, or at a cut edge. On the other hand, the corrosion of zinc is reduced because the zinc coating is in a passive state. The zinc, instead of being galvanically corroded, is actually cathodically protected by the steel. Temperature is a critical factor in the reversal of polarity. The critical temperature, i.e., that at which polarity reversal occurs, is influenced by the solution composition. At a given composition, the tendency for potential ennoblement of the zinc electrode over time increases with increasing solution temperature as shown in Figure 4 which shows that polarity reversal in hard hot water occurs when the solution temperature is over 60°C and disappears if the previously heated sample is cooled to below 60°C.

The phenomenon of polarity reversal does not occur in the known applications of zinc-rich primer such as automotive applications, construction and industrial applications, because the temperature does not generally exceed the threshold temperature while being immersed in a solution.

Contrary to that, the tabs for container ends are generally being subjected to a high temperature treatment for food conservation, such as sterilisation in water in an autoclave. The food product in the container must then reach a temperature of 110 to 121 °C, depending on the food product, and the processing times to reach these maximum temperatures in the product are in the order of hours, depending on the container size and shape. To achieve this high temperature the sterilisation is performed in an autoclave in water. The high pressure in the autoclave allows the water to be heated above 100 °C. The temperature of the tab is therefore far above the temperature for polarity reversal, and normally corrosion of the cut edge or at a scratch would occur. So from this point of view the skilled person would not consider zinc-rich primer coatings as suitable for application as tabstock.

However, the inventors surprisingly found that no such corrosion occurs, indicating that the polarity reversal does not occur, or that the adverse effect of polarity reversal on the corrosion is not detectable. Particularly at the cut edge such corrosion would potentially be prominent, but was not observed during normal use.

FR2249141 discloses a zinc powder primer system for use on a metal which has been preliminarily treated by phosphating or chromating. The zinc powder primer system comprises a two layer system consisting of a lower layer (a) and an upper layer (b). The lower layer (a) consists of a zinc powder primer having a zinc content of 80 to 95 wt.% zinc and a thickness of 10 to 20 μιτι, and an upper layer (b) of 2 to 5 μιτι consisting of a 30-50 wt.% of an organic binder and 70-50% of a lubricant selected from the group consisting of oil, silicone oil, wax, fat, graphite, molybdenum sulphide and other heavy metal sulphides. The use of the system without layer (b) is not possible because this led to flaking off during forming of the steel substrate and contamination of the forming tools. The process to apply this system involves phosphating or chromating the substrate, applying layer (a) and drying at 240 to 260 °C, cooling to 20-25 °C, applying layer (b) and drying at 240 °C. The use of the lubricant would also render the product unsuitable in view of food safety.

The absence of corrosion at the cut edge means that the zinc-rich primer coating layer effectively protects the cut edge during the sterilisation treatment, and that no additional lacquering treatment is needed.

It is noted that the term primer is used in the context of this invention to denote a coating layer, but it should be stressed that it may, or may not be, the first layer on a steel strip or sheet. The term primer is also used if the layer is coated onto a steel strip or sheet that is already provided with a plating layer of a different kind, such as a tin layer. It should be noted that the plated steel, prior to and/or after plating, or the blackplate may have been pre-treated, prior to coating with the zinc-rich primer. This pre-treatment may form instance be a passivation treatment in case of tinplate.

It should be noted that the steel strip can be cut into sheets, and that tabs may be produced from steel strip, or from steel sheet, depending on the equipment available at the tab-makers. The zinc-rich primer may be applied to the steel strip before cutting the strip into sheets, or after cutting the strip into sheets.

In an embodiment of the invention the layer of a zinc-rich primer is further coated by a lacquer. Although the inventors found that the zinc-rich primer coating layer already effectively protects the cut edge, this lacquer layer provides additional protection to the layer of zinc-rich primer, and to the sides of the tab which may not be provided with the zinc-rich primer. The lacquer may be applied to the steel strip before cutting the strip into sheets, or after cutting the strip into sheets, or even after forming the tabs. In an embodiment of the invention the layer of a zinc-rich primer comprises between 4 and 1350 mg/dm² of zinc. The protection by the layer takes effect already at 4 mg/dm² of zinc. At a lower level than 4 mg/dm² the protection was insufficient. Values of over 1350 mg/dm² do not contribute further to the protection. A suitable minimum value is 10 mg/dm² of zinc. A suitable maximum is 1200 mg/dm² of zinc, a more suitable maximum is 1000 mg/dm² of zinc and an even more suitable maximum is 800 mg/dm² of zinc. There is a balance between the use of zinc, the protection offered and required and the cost of the tabs.

In an embodiment of the invention the unplated tabstock, i.e. the steel strip or sheet, is blackplate. A blackplate strip or sheet is still uncoated and is therefore unprotected against corrosion. The application of the layer of a zinc-rich primer provides the steel with the required protection. Preferably, certainly in the case of blackplate as tabstock, the tabstock is coated with a layer of a zinc-rich primer on both sides of the blackplate sheet or strip. It should be noted that the coating thickness and/or the amount of zinc does not necessarily have to be the same on both sides, and can be optimised from a cost and/or corrosion protection perspective.

In an embodiment of the invention the tabstock is plated before providing it with a layer of zinc-rich primer, and preferably the plated tabstock is tinplate, reflown tinplate, ECCS, or blackplate with a chrome/chrome oxide layer.

Tinplate is a thin sheet or strip of soft steel, coated electrolytically on both sides with a tin-layer. The tin layer may be different in thickness on both sides. In some cases the tin layer is melted after application and after solidification this is called reflown tinplate. Chromium coated steel is a sheet or strip of steel electrolytically coated with a layer of chrome, usually thinner than a micron. This was originally called TFS (Tin Free Steel), but it is better known by the acronym ECCS (Electrolytic Chromium Coated Steel). ECCS is produced using hexavalent chromium plating baths, which are considered harmful and will eventually be banned. Recent developments have been made to avoid these baths and it is now possible to provide blackplate (or tinplate) with a chrome/chrome oxide layer from harmless trivalent chromium plating baths as described in WO2014202316.

In the invention the layer of a zinc-rich primer is between 0.2 and 20 μιτι in thickness. Depending on the loading of the primer with zinc particles, the minimum thickness of the layer is 0.2 μιτι, and the maximum is 20 μιτι . A thinner layer is unlikely to provide the desired full protection, whereas a thicker layer adds additional costs without adding additional benefits. A suitable maximum value is 15 μιτι and a more suitable maximum is 10 μιτι. An even more suitable maximum value is 7.5 μιτι. A suitable minimum value is 0.25 μιτι, a more suitable minimum is 0.3 μιτι, and an even more suitable value is 0.35 μιτι.

The amount of zinc in the zinc-rich primer determines the degree of galvanic protection offered by the layer of zinc-rich primer. The inventors found that a limited degree of protection is already provided at by a layer of zinc-rich primer with a zinc content of 5 vol.%, but that there is a strong increase in protection when the layer of zinc-rich primer contains at least 30 vol.% of zinc. At this moment the strength of the galvanic protection is increased considerably as a result of the connectivity of the zinc particles with each other and the steel strip or sheet tabstock. In an embodiment of the invention the layer of a zinc-rich primer comprises 30 to 95 vol.% of zinc. After curing the zinc-rich primer consists of zinc particles embedded in a matrix. It is important that the zinc is in electrical contact with the substrate. So, even though a lower content, starting at around 5 vol.% of zinc, but preferably of 20 or 25 vol.% already offers some protection, the inventors found that the minimum zinc content of the zinc rich primer where the performance of the tabs in use starts to excel is the value at which the zinc-rich primer starts to short-circuit all local cell activity on steel. The inventors found that a value of 30 vol.% of zinc is sufficient to ensure this.

According to a second aspect, a process is provided for producing the tabstock. In this embodiment tabstock is produced by producing a cold-rolled and recrystallisation annealed steel strip or sheet, or a recovery annealed steel strip or sheet, or a double reduced steel strip, or a recovery annealed double reduced steel strip or sheet, said strip or sheet having a thickness between 0.1 and 0.5 mm, wherein :

a) a layer of a zinc-rich primer is applied onto at least one side of the strip or sheet, eventually followed by forming the tab from the tabstock, or wherein

b) a layer of a zinc-rich primer is applied onto at least one side of the plated strip or plated sheet, eventually followed by forming the tab from the tabstock.

With this process it is possible to produce the steel strip or sheet as discussed above, using conventional equipment. The steels used for producing tabs are usually of the high strength variety, or have received additional strengthening by using it in its recovered state rather than in a recrystallised state. However, the method according to the invention can be used on any steel that are suitable to produce tabs from. The zinc-rich primer coating layer is preferably be applied at temperatures below 50 °C, and more preferably at room temperature (i.e. the temperature of the surrounding atmosphere = ambient temperature). Curing or drying in an oven may be used if needed to accelerate or promote the drying or curing process. The drying time or curing time depends a.o. on whether the zinc-rich primer is based on a one component, two- or multicomponent system, on the ambient temperature, the humidity of the surrounding atmosphere and the thickness of the applied layer.

The zinc-rich primer coating layer may be applied in a conventional coil coating line wherein the zinc-rich primer is applied, preferably at temperatures below 50 °C, and more preferably at room temperature, and wherein a curing or drying oven may be used if necessary to accelerate or promote the drying or curing process.

In an embodiment of the invention the layer of a zinc-rich primer is applied from a one component system. This is a relatively simple solution resulting but the functionality of the layer can be improved by using a two- or multi-component system. The processing of these latter types is usually more complicated and required good process control in the preparation of the primer.

In an embodiment of the invention the layer of a zinc-rich primer is applied by means of one of the following application methods: spraying, coil-coating or roll-coating. These methods are suitable for the application of the layer. Preferably the layer of a zinc-rich primer is applied in-line with the plating of the tabstock, preferably immediately following the plating.

In an embodiment of the invention wherein the steel strip or sheet is provided with the layer of a zinc-rich primer at a running speed of the strip of higher than 9 metres per minute, preferably a speed higher than 15 metres per minute, more preferably having a speed higher than 30 metres per minute.

According to a third aspect, the invention is also embodied in the use of a tab according to the invention from the steel strip provided with a layer of a zinc- rich primer according to the invention for opening tabs for container ends for beer and beverage container ends or for easy open ends for foodstuffs, or for an opening tab for container ends for beer and beverage container ends or for easy open ends for foodstuffs, which will be pasteurised or sterilised during its lifecycle. In an embodiment of the invention the strip or sheet having a thickness between 0.11 and 0.29 mm.

The invention will be elucidated in the Examples hereinafter.

The inventors found that the method according to the invention can be used on any steel that are suitable to produce tabs from, and is thus not limited to the steels used in the examples. The steel substrate used in the experiments is a high-nitrogen low carbon steel. The steel substrate is produced in a conventional way by casting a slab or strip with a suitable composition, hot-rolling and cold- rolling the material to a cold-rolled thickness, followed by annealing to recrystallise the steel strip. After annealing the strip was temper rolled with a deformation of between 5 to 10%. This steel would be referred to as double reduced steel. The temper rolling is the second reduction, the first reduction is the reduction in the cold-rolling mill of the hot rolled strip thickness to a cold rolled thickness of around 0.20-0.50 mm. The temper rolling reduction may be considerably higher than 10%, in which case it is usually not referred to anymore as temper rolling, but rather a second cold-rolling reduction, although in relation to this invention there is no principal difference between the two beside the degree of reduction.

The examples were prepared using uncoated material (referred to as blackplate), tinplate or reflown tinplate (in which case a tin layer is applied to produce tinplate, which layer is then melted (reflown) to change its appearance and properties). The blackplate can also be provided with a thin layer of chromium provided with an even thinner layer of chromium oxide (ECCS) deposited from a hexavalent chromium bath, or with a layer of chromium-chromium oxide- chromium carbide deposited from a trivalent chromium bath (see WO2014202316).

The thickness of the strip was 0.2 mm in all cases. From this substrate the following products were produced and used in the experiments:

- reflown tinplate (2.8 g/m²)

- non-reflown tinplate (2.8 g/m²)

- ECCS (100 mg/m² chromium metal, 10 mg/m² oxide)

- Cr-CrOx (130 mg/m² as in WO2014202316)

- uncoated steel ('blackplate')

- electroplated zinc (2 μιτι)

- hot-dipped zinc (35 g/m², 1.6 % AI/1.6 % Mg/rest Zn) These substrates were subsequently coated on one side with 30 mg/dm² of a zinc-rich primer. The application was done by spraying an aerosol and drying in air at room temperature. After evaporation of the solvent a zinc-rich primer remained behind on the substrates comprising of a binder and metallic zinc particles. In the example the binder was an epoxy-ester and the primer contained 90 vol.% zinc. The thickness of the zinc-rich primer layer was just under 5 μιτι. The reference samples were not provided with the zinc-rich primer.

For the purpose of the experiment the other side of the substrate was coated with an epoxyphenolic gold lacquer (Vitalure 345, AkzoNobel), a widely accepted lacquer in packaging industry.

The samples were subsequently formed in a proprietary Wedge bend test as described in § 9.3 of "Canmaking. The technology of metal protection and decoration" by T.A. Turner, Blackie Academic & Professional (1^st ed. 1998). The sample of about 14 x 5 cm to be tested is bent along the long centreline with T = 0 (i.e. folded with a sharp edge) at the bottom, and the remainder of the sample has a larger radius (see figure 2). 10 samples were made per variant. The surface was not prepared in any way.

The samples were placed in three beakers containing either demineralised water, tap water or 3.6 % sodium chloride in demineralised water so that half the sample was immersed . 5 samples were placed with the T = 0 side immersed, 5 with the T = 0 side above the water level.

The beakers were placed in a Tuttnauer 3850 ELV autoclave and sterilised for 60 minutes at 121 °C. The lid was opened and the samples were left in the beakers until the water cooled to 40 °C, which took longer than half an hour. This procedure causes residual steam to condense on the parts above the water level, and the cooling rate was relatively low. The samples were taken from the water and left to dry. Once dry, pictures were taken, and the cut edge and T = 0 bend was evaluated for red rust. The results are summarised in Tables 1-3. All substrates were tested with and without a layer of the zinc-rich primer (ZRP).

Table 1: Sterilisation corrosion in demineralised water

Sample appearance of non- cut-edges, % red rust lacquered surface, %

red rust

Reflown tinplate + ZRP 0 20 Reflown tinplate 0 50

Non-reflown tinplate + ZRP 0 20

Non-reflown tinplate 20 50

ECCS + ZRP 10 30

ECCS 20 50

Cr-CrOx + ZRP 0 10

Cr-CrOx 50 60

Blackplate + ZRP 5 20

Blackplate 50 20

Table 2: Sterilisation corrosion in ta water

The mechanism of the protection is schematically presented in Figure 1. The presence of strongly electronegative zinc-rich primer short-circuits all local cell activity on steel or the plating layer. The steel becomes totally cathodic to the zinc anode. The zinc corrodes, but the steel will not corrode even at bare spots such as the cut-edge.

The galvanic effect of zinc is demonstrated by the electroplated- and hot- dipped zinc-containing samples. These samples did not show any red rust formation, but were otherwise of an unacceptably poor surface quality after sterilisation.

It is necessary that the zinc coating is in electrical contact with the substrate. So the minimum zinc content of the zinc rich primer is the value at which the zinc-rich primer starts to short-circuit all local cell activity on steel. The thicker the layer of zinc-rich primer, the longer the duration of protection, and the longer the shelf life. However, the most corrosive conditions are experienced for only a limited time (the sterilisation period), so a limited layer thickness will provide adequate shelf life under most circumstances. For tabs the thickness of the layer of 0.2 to 20 μιτι was found to be sufficient.

It will be appreciated that the coatings and the coating method can also be used for strip having a composition different from that used for the above experiments.

Claims

Tab for container ends produced from unplated steel strip or sheet tabstock or from plated steel strip or sheet tabstock, characterized in that the tabstock is coated with a layer of a zinc-rich primer.

Tab according to claim 1, in which the layer of a zinc-rich primer is further coated by a lacquer.

Tab according to claim 1, in which the layer of a zinc-rich primer comprises between 4 and 1350 mg/dm2 of zinc.

Tab according to claim 1, wherein the unplated tabstock is blackplate.

Tab according to claim 1, wherein the plated tabstock is tinplate, reflown tinplate, ECCS or blackplate with a chrome/chrome oxide layer deposited from a trivalent chrome plating bath, preferably blackplate with a chrome/chrome oxide layer deposited from a trivalent chrome plating bath in accordance with WO2014/202316.

Tab according to claim 1, wherein the layer of a zinc-rich primer is between 0.2 and 20 μιτι in thickness.

Tab according to claim 1, wherein the layer of a zinc-rich primer comprises 5 to 95 vol.% of zinc, preferably at least 30 vol.%.

Process for producing a tab according to any one of claim 1 to 7 wherein tabstock is produced by producing a cold-rolled and recrystallisation annealed steel strip or sheet, or a recovery annealed steel strip or sheet, or a double reduced steel strip, or a recovery annealed double reduced steel strip or sheet, said strip or sheet having a thickness between 0.1 and 0.5 mm, wherein :

a) a layer of a zinc-rich primer is applied onto at least one side of the strip or sheet, eventually followed by forming the tab from the tabstock, or wherein b) a layer of a zinc-rich primer is applied onto at least one side of the plated strip or plated sheet, eventually followed by forming the tab from the tabstock.

9. Process according to claim 8 wherein the layer of a zinc-rich primer is applied from a one component system.

10. Process according to claim 8 wherein the layer of a zinc-rich primer is applied from a two- or multi-component system.

Process according to claim 8 wherein the layer of a zinc-rich primer i applied by means of one of the following application methods: spraying, coil coating or roll-coating.

Process according to claim 8 wherein the layer of a zinc-rich primer is applied in-line with the plating of the tabstock, preferably immediately following the plating.

Process according to any one of the claims 8, wherein the steel strip is provided with the layer of a zinc-rich primer at a running speed of the strip of higher than 9 metres per minute, preferably a speed higher than 15 metres per minute, more preferably having a speed higher than 30 metres per minute.

Use of the tab according to claim 1 to 7 as an opening tab for container ends for beer and beverage container ends or for easy open ends for foodstuffs.

15. Use of the tab according to claim 1 to 7 as an opening tab for container ends for beer and beverage container ends or for easy open ends for foodstuffs, which will be pasteurised or sterilised during its lifecycle.