UV paint for metal applications

Abstract

A UV-curable paint has been developed which exhibits improved adhesion to metal substrates. The paint composition comprises a) a mixture of different epoxy resins, b) an acrylate compound, c) a photoinitiator, d) a polycaprolactone triol, and e) cellulose acetate butyrate.

Description

[0001] The present invention relates to a UV-curable lacquer composition which is particularly suitable for use on plasma treated substrates such as tin-free steel, tin-plated steel or aluminium. These substrates can be used as industrial, food or beverage packaging material.

[0002] For many years, metal packaging material has been coated with thermally cured coating compositions. Such coatings contain volatile organic solvents and consume considerable amounts of energy. In order to reduce or even eliminate the amount of such organic solvents and to reduce the energy consumption required for complete curing of the coating, the applicant has developed various types of UV-curable coating systems. By using these UV-curable coatings, the amount of solvent released during curing can be reduced by more than 40% compared to thermally cured systems. Since UV-curing is a fast process (complete curing in less than 20 seconds), the use of UV-curable coatings leads to a faster production process at lower costs.

[0003] It was found that the use of these UV-curable lacquers on tin-plated sheet or tin-free steel exhibits some inconsistent properties, partly due to the adhesion properties of these coatings. It is known in the art that the above mentioned substrates need to be treated before a protective coating can be applied, since the surface of these substrates is often contaminated. This contamination reduces the adhesion of the lacquer applied to such surface.

[0004] It was found that by using a plasma pre-treatment, the surface of these substrates exhibits an improved adhesion of the applied lacquer compared to untreated surfaces. However, it was noticed that for the available UV-curable lacquers, defects leading to poor adhesion were still observed, which makes these coated substrates less suitable for use as industrial, food or beverage packaging material.

[0005] The present invention provides a UV-lacquer composition which exhibits excellent adhesion to substrates such as tin-free steel, tin-plated steel or aluminium which are commonly used as food or beverage packaging material.

[0006] The present invention relates to a UV-curable lacquer composition comprising

[0007] a) a mixture of different epoxy resins,

[0008] b) an acrylate compound,

[0009] c) a photoinitiator,

[0010] d) a polycaprolactone triol, and

[0011] e) cellulose acetate butyrate.

[0012] Tin free steel (TFS) is widely used as packaging material for food and beverages, for example as food container. TFS consists of a thin layer of chromium and a layer of chromium oxide deposited on a steel substrate. This material has a metallic gloss and good corrosion resistance.

[0013] Tinplate or tin plated steel consists of a thin layer of tin-iron alloy, tin, tin oxide and a passivation layer deposited on a steel substrate. This material has good corrosion resistance, good formability and is easy to weld.

[0014] Coating composition

[0015] The coating composition according to the present application comprises the following ingredients:

[0016] Epoxy resin

[0017] The composition according to the present application comprises a mixture of different epoxy resins. In one embodiment, this mixture comprises an aliphatic epoxy resin and an aromatic epoxy resin. In a further embodiment, the mixture of different epoxy resins comprises at least 85 wt% of one or more aliphatic epoxy resins, the wt% being calculated on the total weight of the epoxy resins present in the paint composition.

[0018] Suitable aliphatic epoxy resins which can be used in the composition according to the present application include the epoxy resins which are conventional in the art of epoxy resins. Examples of such epoxy resins include the following:

[0019] I) polyglycidyl ethers or poly-(p-methylglycidyl) ethers, which are obtainable by reacting a compound having at least one, preferably two, free alcoholic hydroxyl groups with epichlorohydrin or β-methyl epichlorohydrin under basic conditions or in the presence of an acidic catalyst and subsequent base treatment. Glycidyl ethers of this type are derived, for example, from acyclic alcohols, such as ethylene glycol, diethylene glycol or higher poly(oxyethylene) glycols, propylene-1,2-diol or poly(oxypropylene) glycols, propylene-1,3-diol, butane-1,4-diol, poly(oxytetramethylene) glycol, pentane-1,5-diol, hexane-1,6-diol, hexane-2,4,6-triol, glycerol, C12-14 OH (Araldit® DY-E) 1,1,1 -trimethylolpropane, pentaerythritol, sorbitol, and also from polyepichlorohydrins. Further glycidyl ethers of this type are derived from cycloaliphatic alcohols, such as 1,4-cyclohexane dimethanol, bis(4-hydroxycyclohexyl)methane, 2,2-bis(4-hydroxycyclohexyl)-propane or tricyclodecane dimethanol.

[0020] II) Cyclic aliphatic epoxy resins, 3,4-epoxycyclohexylmethyl-3,4- epoxycyclohexylcarboxylate is a cyclic aliphatic epoxy resin that can be synthesized by the reaction of 3-cyclohexene carboxylic acid 3-cyclohexenylmethyl ester with peracetic acid. These aliphatic resins exhibit many useful properties in the compositions according to the present application, such as thermal stability.

[0021] Suitable aromatic epoxy resins useful in the compositions according to the present application include glycidyl ethers prepared by the reaction of epichlorohydrin with aromatic compounds containing at least one hydroxyl group under basic reaction conditions. Other suitable epoxy resins can be prepared by the reaction of epichlorohydrin with mononuclear dihydroxy and trihydroxy phenolic compounds, such as resorcinol and phloroglucinol, selected polynuclear polyhydroxy phenolic compounds, such as bis(p-hydroxyphenyl)methane and 4,4-dihydroxydiphenyl. Epoxy resins suitable for use in the compositions of the present application have molecular weights generally in the range of 86 to 10,000, preferably 200 to 1500. A commercially available epoxy resin, 2,2-bis(4-hydroxyphenylpropane) (bisphenol-A) having a molecular weight of 400, an epoxy equivalent weight (ASTM D-1652) of 185-192 and an n value (from the formula above) of 0.2 is currently the preferred epoxy resin due to its low viscosity and commercial availability.

[0022] Generally, the compositions of the present application comprise 30 to 60 weight percent of epoxy resin, based on the total weight of the composition.

[0023] Acrylate compound

[0024] The compositions according to the present application comprise an acrylate compound. This acrylate compound should have at least two unsaturated carbon-carbon bonds. Such unsaturated bonds can be opened under the influence of ultraviolet light in the presence of a photoinitiator.

[0025] Suitable acrylate compounds useful in the compositions according to the present application include ethoxylated (3) trimethylolpropane triacrylate, glyceryl propoxy triacrylate or trimethylolpropane triacrylate.

[0026] Generally, the compositions of the present application comprise 25 to 35 weight percent of acrylate compound, based on the total weight of the composition.

[0027] Photoinitiator

[0028] The composition according to the present application comprises an effective amount of a photoinitiator or a mixture of different photoinitiators to achieve photocuring of the composition by exposure to ultraviolet light. Typically, this amount is from about 0.01 wt% to about 10 wt% based on the total weight of the composition. Examples of photoinitiators that can be used in the composition according to the present application include hydroxycyclohexyl phenyl ketone, benzophenone, triaryl sulfonium hexafluorophosphate in propylene carbonate, and the like.

[0029] Polycaprolactone triol

[0030] Polycaprolactone triol is present in the composition according to the present application and greatly enhances the strength of the coating film after curing. Typically, the composition comprises 5-10 wt% of polycaprolactone triol based on the total weight of the composition.

[0031] Cellulose acetate butyrate

[0032] Cellulose acetate butyrate is present in the composition according to the present application to improve the adhesion of the composition to various (metal) substrates. Typically, the composition comprises 10 wt% - 15 wt% of cellulose acetate butyrate based on the total weight of the composition.

[0033] Application method

[0034] The present application also relates to a method of applying the coating composition described above. This method comprises the following steps:

[0035] • prior to applying the coating composition, plasma treatment of the surface of the substrate,

[0036] • after the plasma treatment, applying the coating composition onto the plasma treated surface of the substrate, and

[0037] • curing the coating composition by exposing the substrate obtained in step ii) to ultraviolet radiation.

[0038] In this method, it is preferred to use tin-plated sheet, tin-free steel or aluminum as the substrate.

[0039] The plasma treatment of the metal substrate can be carried out, for example, by using the Openair-Plasma® process developed by the company Plasmatreat.

[0040] Examples

[0041] Different types of lacquer were applied to plasma-treated and non-plasma-treated substrates. Thereafter different tests were carried out and the adhesion of the lacquer to the substrate was evaluated as follows:

[0042]

[0043] The following paint was used:

[0044]

[0045] The following substrate was used:

[0046]

[0047] The results of the evaluation are listed in Tables 1 and 2 below.

[0048] Table 1 : Results of adhesion test

[0049]

[0050] Example 1

Claims

1. A UV-curable paint composition comprising... a) A mixture of different epoxy resins. b) Acrylic ester compounds, c) Photoinitiators, d) Polycaprolactone triol, and e) Cellulose acetate butyrate.

2. The UV-curable paint composition according to claim 1, wherein the mixture of different epoxy resins comprises aliphatic epoxy resin and aromatic epoxy resin.

3. The UV-curable paint composition according to claim 1 or 2, wherein the mixture of different epoxy resins comprises at least 85% by weight of one or more aliphatic epoxy resins, the percentage by weight being calculated based on the total weight of the epoxy resins present in the paint composition.

4. The UV-curable paint composition according to claim 2 or 3, wherein the epoxy resin mixture comprises bisphenol A type epoxy resin.

5. The UV-curable paint composition according to claim 1, comprising... a) A mixture of 30-60% by weight of different epoxy resins. b) 25-35% by weight of an acrylate compound or a mixture of different acrylate compounds. c) 0.1-10% by weight of photoinitiator or a mixture of different photoinitiators, d) 5-10% by weight of polycaprolactone triol or a mixture of different polycaprolactone triols, and e) 10-15% by weight of cellulose acetate butyrate, The weight percentage of the components is based on the total weight of the composition.

6. A method for applying a coating to a metal substrate, comprising the following steps: i. Before applying the coating composition, perform plasma treatment on the substrate surface. ii. After plasma treatment, the coating composition is applied to the plasma-treated surface of the substrate, and iii. The coating composition is cured by exposing the substrate obtained in step ii) to ultraviolet radiation. The coating composition applied in step ii) is an ultraviolet-curable paint composition according to any one of claims 1 to 5.

7. The method according to claim 6, characterized in that... The metal substrate is selected from tin-plated steel, tin-free steel, and aluminum.

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