Polymer coated substrate for packaging applications and a method for producing said coated substrate

Abstract

This relates to a coated substrate for packaging applications and a method for producing the coated substrate.

Claims

1. A process for manufacturing a polymer coated steel substrate for packaging applications, comprising the steps of: providing a steel substrate selected from: a single-reduced steel substrate, or a double-reduced steel substrate which was subjected to recrystallisation-annealing between the first and second cold-rolling step; electrodepositing a tin layer on one or both sides of the single-reduced or double-reduced steel substrate to produce a tin-coated steel substrate; annealing the tin-coated steel substrate at a temperature T.sub.a of at least 513 C. for an annealing time t.sub.a to convert the tin layer into an iron-tin alloy layer which contains at least 80 weight percent (wt. %) of FeSn; cooling the iron-tin alloy coated substrate at a cooling rate of at least 100 C./s; applying a polymer coating layer on one or both sides of the iron-tin alloy coated substrate, wherein during the applying of the polymer coating the iron-tin alloy coated substrate is heated; subjecting the iron-tin alloy coated substrate to a stretching operation at any moment after the polymer coating process wherein the stretching operation is achieved by: a. passing the iron-tin alloy coated substrate with the polymer coating layer through a temper mill and applying a thickness reduction between 0.2 and 3%; or by b. passing the iron-tin alloy coated substrate with the polymer coating layer through a stretcher-leveller.

2. The process according to claim 1, wherein the iron-tin alloy layer contains at least 85 wt. % of FeSn.

3. The process according to claim 1, wherein the annealing is performed in a reducing gas atmosphere while keeping the coated substrate in a reducing or inert gas atmosphere prior to cooling using non-oxidising or oxidising cooling medium, to obtain a stable surface oxide.

4. The process according to claim 1, wherein the steel substrate is a strip, wherein the fast cooling is achieved by water-quenching, wherein the water used for quenching has a temperature between room temperature and 80 C., and wherein the quenching process is designed to create and maintain a homogeneous cooling rate over the strip width.

5. The process according to claim 1, wherein the steel substrate is a strip, wherein the process comprises at least one feature selected from the group consisting of: (a) wherein the annealing comprises heating using a heating unit able to generate a heating rate exceeding 300 C./s in a hydrogen containing atmosphere, and optionally the heating is followed by a heat soak kept at the temperature T.sub.a to homogenise temperature distribution across the width of the strip, (b) wherein the cooling is performed in a reducing gas atmosphere, and (c) wherein the cooling is performed by water quenching using submerged spraying nozzles, wherein the water used for the quenching has a temperature between room temperature and 60 C., while keeping the iron-tin alloy coated substrate shielded from oxygen by maintaining an inert or reducing gas atmosphere prior to quenching.

6. The process according to claim 1, wherein coating weight of the tin layer onto one side of the steel substrate, or tin layers respectively onto both sides of the steel substrate, is at most 1000 mg/m.sup.2 of the steel substrate surface.

7. The process according to claim 1, wherein the polymer coating layer is an organic coating consisting of a thermoplastic single- or multi-layer polymer coating.

8. The process according to claim 1, wherein an additional coating layer is applied onto the iron-tin alloy layer prior to the polymer coating process, to protect the iron-tin alloy coated substrate against pitting corrosion, while retaining adhesion to additionally applied organic coatings, wherein a tin layer is optionally deposited onto the iron-tin alloy layer prior to the application of the additional coating layer and wherein this tin layer is optionally subsequently reflowed prior to the application of the additional coating layer.

9. The process according to claim 8, wherein the additional coating consists of a CrCrOx coating layer, deposited onto the iron-tin alloy layer prior to applying the polymer coating.

10. The process according to claim 9, wherein the CrCrOx-layer is deposited in one plating step from a plating solution comprising a mixture of a trivalent chromium compound, a chelating agent, an optional conductivity enhancing salt, an optional depolarizer, and an optional surfactant, and to which an acid or base can be added to adjust the pH.

11. The process according to claim 10, wherein the chelating agent comprises a formic acid anion, the conductivity enhancing salt contains an alkali metal cation, and the depolarizer comprises a bromide containing salt.

12. The process for producing a coated substrate for packaging applications according to claim 1, wherein the stretching operation is achieved by the passing of the material through the temper mill and applying the thickness reduction of 0.2-3%.

13. The process according to claim 1, wherein the iron-tin alloy layer contains at least 90 wt. % FeSn.

14. The process according to claim 4, wherein the water used for quenching has a temperature between room temperature and 60 C.

15. The process according to claim 1, wherein the coating weight of the tin layer onto one side of the steel substrate, or tin layers respectively onto both sides of the steel substrate, is at least 100 and/or at most 600 mg/m.sup.2 of the steel substrate surface.

16. The process according to claim 7, wherein the thermoplastic polymer coating is a polymer coating system comprising at least one layer comprising thermoplastic resin selected from the group consisting of polyesters, polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene resins, ABS resins, chlorinated polyethers, ionomers, urethane resins, functionalised polymers thereof, copolymers thereof, and blends thereof.

17. The process according to claim 11, wherein the chelating agent comprises a potassium cation, the alkali metal cation of the conductivity enhancing salt comprises a potassium cation, and the depolarizer comprises a potassium cation.

18. The process according to claim 1, wherein the substrate is a strip, wherein the annealing comprises heating using a heating unit able to generate a heating rate exceeding 300 C./s in a hydrogen containing atmosphere.

19. The process according to claim 18, wherein the heating is followed by a heat soak kept at the temperature T.sub.a to homogenise temperature distribution across the width of the strip.

20. The process according to claim 1, wherein the cooling is performed in a reducing gas atmosphere.

21. The process according to claim 1, wherein the cooling is performed by water quenching using submerged spraying nozzles, wherein the water used for quenching has a temperature between room temperature and 60 C., while keeping the iron-tin coated substrate shielded from oxygen by maintaining an inert or reducing gas atmosphere prior to quenching.

Description

(1) FIG. 1 shows a stress-strain curve of PET coated standard steel substrate and

(2) FIG. 2 shows the same after subjecting the PET coated standard steel substrate to a temper rolling reduction of 1%.

(3) FIG. 3 shows a stress-strain curve of a steel substrate after being exposed to two sequential heat treatments simulating diffusion-annealing & thermal lamination and

(4) FIG. 4 shows the same after a temper rolling reduction of 1%.

(5) FIG. 1 shows that the application of a polymer coating on an already temper-rolled SR-CA material results in a yield point elongation ((YPE) i.e. an aged substrate), which YPE can be removed by a second temper-rolling (FIG. 2). FIG. 3 shows what happens if the diffusion annealed substrate is coated with a polymer coating and then subsequently temper rolled: no YPE. In other words: only the temper-rolling (or stretching) of the polymer coated product results in a YPE-free material. Temper rolling only prior to polymer coating does not result in a YPE-free material. This YPE-free substrate is not susceptible to environmental stress cracking, whereas the substrate that is not YPE-free is susceptible to environmental stress cracking

EXAMPLE 1

(6) A PET film was applied by thermal lamination to a standard packaging steel substrate (TH340, continuous annealed SR low carbon steel) provided with a standard ECCS metal coating. These flat sheet polymer-coated materials were subsequently deformed either by Erichsen cupping or putting the material through a Gardner falling dart impact test. Some of the sheets were fed to a laboratory temper mill, reducing the material thickness by 1%, prior to applying the aforementioned deformation.

(7) For the polymer-steel laminates that did not receive a temper mill reduction, after deformation no cracking of the coating was observed visually, even at fairly large deformations as in a 6 mm Erichsen cup. When these deformed samples were left exposed to air, a minor amount of stress cracking did develop over a period of days. When these samples were exposed to a lubricant or wax, stress cracks developed within minutes and continued to grow for several hours. When these samples were exposed to ethanol, extensive stress cracking was observed immediately which did not develop further in time. Thus, the observed behaviour was a true environmental stress cracking (ESC) phenomenon arising from a combination of mechanical stress and contact with chemicals, where certain chemicals are much more aggressive than others.

(8) During the experiments it was noted that deformation in an Erichsen cup is not homogeneous but shows Lders' lines, in particular in freely deforming areas not supported by the indenter. Stress cracking of the coating appears to develop predominantly in those areas.

(9) It was found that samples that had received a temper mill reduction of 1% prior to deformation did not develop Lders' lines during Erichsen cupping and showed no signs of environmental stress cracking after exposure to ethanol.

(10) The stress-strain curves of the PET coated steel sheets with and without the temper mill treatment are shown in FIGS. 1 and 2. These Figures clearly show that yield point elongation is effectively suppressed by this stretching operation, which underpins the observation that no formation of Lders' lines was found for the specimens that received the 1% reduction.

(11) These results demonstrate that ESC of PET coated steel can be suppressed and/or eliminated provided that the material is substantially free from yield point elongation.

(12) This first example focuses on counteracting the effects of material ageing due to a thermal treatment associated with applying a PET film by thermal lamination. However, the inventors found that it is also possible to counteract the material ageing effects of successive heat treatments to which the steel substrate can become exposed during the consecutive application of coating processes, as demonstrated in example 2.

EXAMPLE 2

(13) A standard packaging steel substrate (TH340, continuous annealed low carbon steel, C=0.045%) was exposed to two sequential heat treatments (to which the material would be exposed when manufacturing a thermoplastic coated steel material, in which the steel substrate is provided with a FeSn alloy coating and a CrCrOx coating layer prior to application of a thermoplastic coating). The CrCrOx coating was applied from the trivalent Chromium plating solution as described hereinabove.

(14) During the diffusion-annealing process the sample was heated to a temperature of 600 C., applying a heating rate of 100 C./s, kept at 600 C. for 2 seconds, cooled back to room temperature by blowing Nitrogen gas, applying a cooling rate of 100 C./s (i.e. T.sub.a 600 C., t.sub.a 2 s) followed by standard thermal lamination of a PET film, including pre-heating the steel to a temperature of 220 C. to achieve thermal sealing/bonding of the PET film, followed by post-heating the substrate to a temperature exceeding 250 C. (above the melting temperature of PET) to modify the properties of the film.

(15) Some of the sheets thus prepared were fed to a laboratory temper mill which reduced the material thickness by 1%. Stress-strain curves were obtained from samples with (FIG. 3) and without (FIG. 4) being exposed to this temper rolling treatment. These results clearly demonstrate that it is possible to successfully counteract the effects of material ageing caused by exposing the bulk steel substrate to the successive thermal treatments associated with diffusion-annealing and thermal lamination. The results in relation to ESC were similar to the samples of Example 1. For ELC and ULC steels which are susceptible to ageing similar results are to be expected.