EP1095086A1

EP1095086A1 - Heat-curable can-coating lacquer

Info

Publication number: EP1095086A1
Application number: EP99927985A
Authority: EP
Inventors: Isabelle Frischinger; Michael Vogel; Jürgen Finter
Original assignee: Vantico GmbH
Current assignee: Huntsman Advanced Materials Switzerland GmbH
Priority date: 1998-06-26
Filing date: 1999-06-15
Publication date: 2001-05-02
Also published as: CN1307606A; JP2002519455A; WO2000000533A1; BR9911571A; KR20010053217A

Abstract

A heat-curable can-coating lacquer, which comprises an advanced epoxy resin which is the reaction product of one or several bisphenol diglycidyl ether(s), one or several bisphenol(s) and of an adduct of epoxidised soybean oil with a monocarboxylic acid, which has an epoxy equivalent weight of 350 to 600, and a crosslinking agent for epoxy resins.

Description

Heat-curable can-coating lacquer

The present invention relates to a heat-curable can-coating lacquer and to a process for the production of a coating for a food container by means of said can-coating lacquer.

One field of application for heat-curable epoxy compositions are can-coating lacquers. By means of heat-curable can-coating lacquers it is possible to coat materials which are resistant against the necessary curing temperatures and which come into contact with foodstuffs, and which materials, if not correspondingly coated, either have an adverse effect on the quality of these foodstuffs or are themselves attacked by the foodstuff's components. Can-coating lacquers are used in particular for coating the interior of metallic cans.

A widely used formulation component for standard can-coating lacquers are the so-called advanced epoxy resins which are obtainable by reacting one or several difunctional phe- nol(s) with one or several bisphenol diglycidyl ether(s) in the presence of a suitable catalyst, the bisphenol diglycidyl ethers needing to be present in stoichiometric excess over the difunctional phenols. The advancement proceeds, for example, according to the following known reaction scheme:

In this scheme,

O O — BLR- O

/ \ /

is a virtually monomeric diglycidyl ether based on bisphenol HO-[BPh]¹-OH, wherein BLR is a group of the following chemical formula:

wherein n is a number from 0 to 0.3 corresponding to the average number of the structural repeating units -[BPh]¹OCH₂CH(OH)CH₂O- in the diglycidyl ether molecules, so that such a diglycidyl ether consists to about 99.7 % or more of monomers of the formula:

O O - [BPh]¹ - O O / \ / \ / \ ι thus being (virtually) monomeric. In the above scheme, a is a number higher than 1 , for example 1.1 and higher. The upper limit of a is preferably 2. Both the quotient [1/(a-1)] and the index n are an average value for the sum of all molecules of the respective resin and can therefore also be fractional numbers if a polydisperse resin, i.e. a mixture of molecules of different chain lengths, is present. Such advanced epoxy resins are often produced e.g. from bisphenol A and liquid, virtually monomeric diglycidyl ether of bisphenol A.

However, in the case of the above advancement, a certain proportion of the monomeric bisphenol diglycidyl ether used is not reacted so that the advancement resin obtained as product, and therefore any can-coating lacquer formulated therefrom, still contains a relatively high proportion of unreacted monomeric bisphenol diglycidyl ethers, which is increasingly viewed as undesirable. Using specific techniques it is possible to reduce the content of bisphenol diglycidyl ether monomers in conventional advancement resins, these techniques, however, are generally elaborate and expensive and are therefore hardly suitable in connection with the production of a cheap mass product such as a tin.

This invention provides another solution for the above problem. The invention is based on the insight that the production of an advanced epoxy resin from one or several monomeric bisphenol diglycidyl ether(s) and from one or several bisphenol(s) gives an advanced epoxy resin containing a markedly reduced proportion of unreacted monomeric bisphenol diglycidyl ethers if, in addition to the bisphenol diglycidyl ether, an adduct of epoxidised soybean oil with a monocarboxylic acid takes part in the advancement reaction, which adduct has an epoxy equivalent weight of about 350 to 650. Epoxy resins advanced in this manner which are based on adducts of vernonia oil, a naturally occurring epoxidised vegetable oil, and of a monocarboxylic acid, and also heat-curable compositions based on these advanced epoxy resins and on certain crosslinking agents for epoxy resins, for example polyphenols, have been known for some time and are mentioned, inter alia, in EP-A-0555 589. The state of the art, however, does not describe the amount of unreacted bisphenol diglycidyl ether monomers in these advancement resins. The reduction of the percentage by weight of the undesirable monomeric bisphenol diglycidyl ethers in the advancement resins brought about by a modification of the advancement reaction using a certain portion of an adduct of epoxidised soybean oil and benzoic acid is, surprisingly, substantially higher than the reduction of the percentage by weight of monomeric bisphenol diglycidyl ethers which are used for the production of the modified advancement resin caused by that modification. Thus it was found, for example, that the advancement of monomeric diglycidyl ethers of bisphenol A with bisphenol A, using 20 % by weight of an adduct of epoxidised soybean oil and benzoic acid, based on the total amount of adduct, diglycidyl ether of bisphenol A and bisphenol A, results in an approximately 90 % reduction of the proportion of free diglycidyl ethers of bisphenol A monomers in the advanced resin (0.2 % by weight in the modified resin compared to about 1.6 % by weight in a conventional advancement resin) even though the proportion of diglycidyl ether of bisphenol A monomers, which are used for the production of the modified advancement resin, is lowered only by about 23 % by weight compared to that of the conventional resin (51.5 % by weight proportion of diglycidyl ether of bisphenol A monomers in the case of the modified resin compared to 66.7 % by weight in the case of the unmodified advancement resin).

Accordingly, this invention relates to a heat-curable can-coating lacquer, which comprises an advanced epoxy resin which is the reaction product of one or several bisphenol diglycidyl ether(s), one or several bisphenol(s) and of an adduct of epoxidised soybean oil with a monocarboxylic acid, which has an epoxy equivalent weight of 350 to 600, and a crosslinking agent for epoxy resins.

This invention also relates to a process for the production of a food container, one or several sides of which are coated entirely or partly with such a can-coating lacquer, which coating is heat-cured.

The novel can-coating lacquer preferably contains an advanced epoxy resin having an epoxy equivalent weight of 1000 (corresponding to 1 equivalent of epoxide per kilogramme of resin) to 4000 (about 0.25 equivalent of epoxide per kilogramme of resin), preferably of 1400 (about 0.7 equivalents of epoxide per kilogramme of resin) to 2000 (0.5 equivalents of epoxide per kilogramme of resin).

The advanced epoxy resin may be produced by reacting e.g. epoxidised soybean oil first with the monocarboxylic acid. Suitable monocarboxylic acids are, for example, monocarboxylic acids containing 1 to 20, preferably 7 to 20, carbon atoms. Particularly preferred is the use of aromatic and cycloaliphatic carboxylic acids, for example benzoic acid or abietic acid. Owing to its good availability, benzoic acid is very particularly preferred. The reaction of the epoxidised soybean oil with the monocarboxylic acid reduces the epoxy functionality of the epoxidised soybean oil, which is in practice at about 4, to a value from about 2 to 3, preferably from 2 to 2.5. This lower functionality prevents too strong a crosslinking during advancement and results in a lower melt viscosity of the advanced resin and therefore generally also of that of the can-coating lacquer based thereon. The epoxy equivalent weight of the epoxidised soybean oil, which is normally in the range from 230 to 250, is raised to the range from about 350 to 600 by the reaction with the monocarboxylic acid. The epoxy equivalent weight of the adduct is preferably in the range from about 450 to 600. In order to achieve the desired reduction of the functionality of the epoxidised soybean oil, the monocarboxylic acid must be used in an amount of about 1 to 2 mol per 4 epoxy equivalents of the epoxidised soybean oil. The reaction can, for example, take place at a temperature from 120 to 160°C and is usefully carried out in the presence of a suitable catalyst, for example a tertiary amine, such as tributylamine, or N,N-ethylmethylpiperidinium iodide, which is used in customary catalytic amounts, for example from 0.0001 to 10 % by weight, based on the epoxy resin. The reaction is usually carried out under protective gas (e.g. nitrogen) and/or under a slight vacuum, preferably in the range from 0.01 to 0.07 MPa (100 to 700 mbar). The completion of the reaction can be determined in simple manner, for example by checking the epoxy value of the reaction mixture.

The adduct obtained from the epoxidised soybean oil and the monocarboxylic acid can then be further reacted with one or several different bisphenol diglycidyl ether(s) and one or several different difunctional phenol(s) to give the finished advanced epoxy resin. This reaction is preferably carried out immediately after the preparation of the reaction product in the same reaction vessel. ln this application, bisphenol diglycidyl ethers will be taken to generally mean diglycidyl ethers of difunctional phenols. The difunctional phenols, on which the cited diglycidyl ethers are based, can, for example, be mononuclear, such as resorcinol or hydroquinone, but are preferably polynuclear, in particular binuclear, such as biphenol(4,4'-dihydroxybiphenyl), bisphenol F, bisphenol A ; 4,4'-dihydroxydiphenylsulfone or 4,4'-dihydroxybenzophenone or bis(4-hydroxyphenyl)ether. It is also possible to use mixtures of one or several diglycidyl ether(s), such as the ones mentioned above. The diglycidyl ethers used conveniently have an epoxy equivalent weight in the range from 110 to 250.

Diglycidyl ethers which are particularly preferred for use in this invention are diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F, mixed diglycidyl ethers of bisphenol A/bis- phenol F and mixtures of the cited diglycidyl ethers, most preferably diglycidyl ether of bisphenol A.

The bisphenols, which are preferably used for the production of the advanced epoxy resins, are likewise mononuclear or, preferably, polynuclear, particularly preferably binuclear. Examples of suitable bisphenols are, in particular, the bisphenols cited above in the description of the bisphenol diglycidyl ethers. In the case of the bisphenols it is also possible to use mixtures of two or more bisphenols. Particularly preferred bisphenols are bisphenol F and, most preferably, bisphenol A.

When reacting the adduct of epoxidised soybean oil and the monocarboxylic acid with the bisphenol and the bisphenol diglycidyl ether, the participating epoxy components must, as is usual in the case of advancements, be present in such an amount that epoxy groups are present in the reaction mixture in an excess over the hydroxyl groups of the bisphenol, as the epoxy resin of higher molecular weight is obtainable only then.

The proportion of the reaction mixture in the adduct of epoxidised soybean oil and monocarboxylic acid can vary within wide limits and is conveniently from at least 5 up to 50 % by weight, based on the total weight of the adduct and of the bisphenols and bisphenol diglycidyl ethers participating in the advancement. If the adduct proportion is too small, the resulting reduction of the content of free bisphenol diglycidyl ether monomers is relatively small, whereas a very high adduct proportion can lower the softening point of the resin and may, depending on the circumstances, already adversely affect the curability of a can-coating lac- quer produced with such advanced epoxy resins, which is crucial e.g. for the chemical resistance of the can-coating lacquer coating. Particularly preferably, the adduct proportion is in the range from 5 to 35 % by weight, more preferably from 10 to 35 % by weight, very particularly preferably from 15 to 25 % by weight. The adduct proportion should usefully be in the upper range if the desired advanced epoxy resin has a relatively low epoxy equivalent weight.

The advancement is usefully carried out in the presence of a customary advancing catalyst. Suitable catalysts are described, inter alia, in US patent (US-A-)5,095,050, which disclosure is explicitly referred to here. Preferred examples of catalysts are tertiary amines, such as tri- ethylamine, tripropylamine, tributylamine, 2-methylimidazole, 2-phenylimidazole, N-methyl- morpholine, N,N-ethylmethylpiperidinium iodide, quarternary ammonium compounds, tetra- alkylphosphonium compounds, such as tetrabutylphosphonium bromide, and alkali metal hydroxides. Combinations of different catalysts are also useful in some instances. The catalysts are used in customary catalytic amounts, for example in amounts from 0.0001 to 10 % by weight, based on the epoxy resin. Particularly preferred catalysts are tetrabutylphosphonium bromide, tributylamine and N,N-ethylmethylpiperidinium iodide and mixtures thereof.

The reaction temperatures during the advancement are preferably in the range from 80 to 250°C, more preferably from 130 to 210°C. The completion of the reaction can be determined in simple manner, for example by checking the epoxy value of the reaction mixture.

It is also possible to produce the advanced epoxy resin for the novel can-coating lacquers in a one-pot reaction, i.e. by reacting suitable amounts of epoxidised soybean oil, monocarboxylic acid, bisphenol diglycidyl ethers and bisphenol, without prior formation of the adduct from soybean oil and monocarboxylic acid, in the presence of a suitable advancing catalyst.

Crosslinking agents suitable for the epoxy resin material of this invention are, in principle, all crosslinking agents for epoxy resins suitable for food purposes. The hardeners are used in the customary amounts, which are known to the skilled person, preferably in an about stoi- chiometric ratio of the epoxy groups of the advanced epoxy to the epoxy-reactive groups (e.g. amino, hydroxyl, carboxyl, or α,β-dicarboxylic acid anhydride groups) of the crosslinking agent. Certain deviations from the approximate stoichiometry are also possible in the can- coating lacquers of this invention without causing any problems; the epoxy groups can, for example, also be present in up to 30 % stoichiometric excess over the epoxy-reactive groups, and vice versa.

Crosslinking agents preferably used for the novel can-coating lacquers are acid anhydrides, for example trimellitic anhydride, and polyphenols, preferably phenolformaldehyde condensates. Such epoxy hardeners are known to the skilled person and are commercially available in a multitude of embodiments.

Other crosslinking agents preferably used for the novel can-coating lacquers are those reaction products which can be obtained by heating trimellitic anhydride together with at least one polyol at a molar ratio of 2 to 1.25 to a temperature from 190 to 250°C for at least four hours at a pressure from 0.666 to 4 kPa. Crosslinking agents of this type are described, inter alia, in US patent (US-A-)4,226,755. The hardener is preferably used in an amount from 10 to 30 % by weight, based on the advanced epoxy resin. A very particularly preferred cross- linking agent is the corresponding reaction product of trimellitic anhydride with ethylene gly- col and glycerol, which reaction product has an acid number from 520 to 560 mg KOH per gramme of crosslinking agent and an anhydride content of about 2 to 3 equivalents per kilogramme of crosslinking agent.

Besides the cited components, the novel can-coating lacquer can contain other additives customarily used in this field of application in the respective customary amounts. Examples of such additives are light stabilisers, colourants, pigments, e.g. titanium dioxide, adhesives, thixotropic agents, flow control agents and suitable solvents, preferably methoxypropyl acetate, where appropriate in admixture with another solvent, such as cyclohexanone.

The novel can-coating lacquer is used for entirely or partly coating one or several sides of a food container, e.g. of a tin or a tube, in customary manner. The lacquer is dried after being applied to the substrate. The lacquer is preferably cured by heating to a temperature in the range from 160 to 240°C. The curing time is preferably from about 10 to 30 minutes. The novel can-coating lacquers are particularly suitable for coating the interior sides of food containers, in particular of metallic food containers. Example 1 (Production of an adduct of epoxidised soybean oil and benzoic acid and of an advanced epoxy resin based on 10 % by weight of the adduct:

In a 1.5 litre 5-necked reaction vessel, equipped with a mechanical steel agitator, digital thermometer and temperature-controlled heater, 82.8g (0.34 epoxy equivalents) of epoxidised soybean oil (lrgaplast^®39; 4.1 epoxy equivalents/kg) are heated to 120°C (measured externally) under a vacuum of 100 mbar, and then 17.2 g (0.14 carboxyl equivalents) of benzoic acid (8.2 carboxyl equivalents/kg) are added. Subsequently, 0.016g (0.063mmoles) of the catalyst N,N'-methylethylpiperidinium iodide is added in the form of a c. 20 % solution in ethanol and the temperature is increased to 143°C and kept constant for about half an hour. The resulting adduct of epoxidised soybean oil and benzoic acid has an epoxy value of about 2.5 equivalents/kg.

Subsequently, the reactor is charged with 591.4 g (3.16 epoxy equivalents) of a diglycidyl ether of bisphenol A (epoxy value 5.35 equivalents/kg), with 308.5 g (2.71 hydroxyl equivalents) of bisphenol A (8.77 hydroxyl equivalents/kg) and with 1.99g (1.56mmoles) of the catalyst N,N'-methylethylpiperidinium iodide (as a solution of c. 20 % by weight in ethanol). The temperature is continuously raised over 2 hours and 30 minutes to 205°C (measured externally) and is then kept constant for another 28 minutes. The pressure in the reaction vessel rises to about 300 mbar during the reaction. The advanced epoxy resin so obtained has an epoxy value of 0.67 equivalents/kg, a softening point of 112°C (measured by means of a Mettler thermosystem FP 800) and a viscosity according to Hόppler (40% in butylcarbitol according to DIN 53015) at 25°C of 1386 mPa-s. The proportion of unreacted monomeric diglycidyl ether of bisphenol A in the advanced resin is 1 % by weight (lOOOOppm).

Example 2 (Production of an adduct of epoxidised soybean oil and benzoic acid according to Example 1 and of an advanced epoxy resin based on 20 % by weight of the adduct:

In a 1.5 litre 5-necked reaction vessel, equipped with a mechanical steel agitator, digital thermometer and temperature-controlled heater, 165.6 g (0.68 epoxy equivalents) of epoxidised soybean oil (lrgaplast*39; 4.1 epoxy equivalents/kg) are heated to 120°C (measured externally) under a vacuum of 100 mbar, and then 34.4 g (0.28 carboxyl equivalents) of benzoic acid (8.2 carboxyl equivalents/kg) are added. Subsequently, 0.034g (0.133mmoles) of the catalyst N.N'-methylethylpiperidinium iodide in the form of a solution of c. 20 % by weight in ethanol is added and the temperature is raised to 143°C and kept constant for about half an hour. The resulting adduct of epoxidised soybean oil and benzoic acid has an epoxy value of about 2.4 equivalents/kg.

Subsequently, the reactor is charged with 515.4 g (2.75 epoxy equivalents) of a diglycidyl ether of bisphenol A (epoxy value 5.35 equivalents/kg), with 284.6 g (2.5 hydroxyl equivalents) of bisphenol A (8.77 hydroxyl equivalents/kg) and with 1.04g (4.08mmoles) of the catalyst N.N'-methylethylpiperidinium iodide (as a solution of c. 20 % by weight in ethanol). The temperature is continuously raised over 1 hour and 45 minutes to 205°C and is then kept constant for another 15 minutes. The pressure in the reaction vessel rises to about 200 mbar during the reaction. The advanced epoxy resin so obtained has an epoxy value of 0.60 equivalents/kg, a softening point of 106°C (measured by means of a Mettler thermosystem FP 800) and a viscosity according to Hόppler (40% in butylcarbitol according to DIN 53015) at 25°C of 1409 mPa-s. The proportion of unreacted monomeric diglycidyl ether of bisphenol A in the advanced resin is about 0.2 % by weight (2100 ppm).

Example 3 (Production of an adduct of epoxidised soybean oil and benzoic acid according to Example 1 and of an advanced epoxy resin based on 30 % by weight of the adduct:

In a 1.5 litre 5-necked reaction flask, equipped with a mechanical steel agitator, digital thermometer and temperature-controlled heater, 248.4 g (1.02 epoxy equivalents) of epoxidised soybean oil (lrgaplast^®39; 4.1 epoxy equivalents/kg) are heated to 120°C (measured externally) under a vacuum of 100 mbar, and then 51.6 g (0.42 carboxyl equivalents) of benzoic acid (8.2 carboxyl equivalents/kg) are added. Subsequently, 0.05g (0.196mmoles) of the catalyst N.N'-methylethylpiperidinium iodide in the form of a solution of c. 20 % by weight in ethanol is added and the temperature is raised to 143°C and kept constant for about half an hour. The resulting adduct of epoxidised soybean oil and benzoic acid has an epoxy value of about 2.2 equivalents/kg.

Subsequently, the reactor is charged with 438.9 g (2.35 epoxy equivalents) of diglycidyl ether of bisphenol A (epoxy value 5.35 equivalents/kg), with 261.1 g (2.3 hydroxyl equivalents) of bisphenol A (8.77 hydroxyl equivalents/kg) and with 1.28g (5.02mmoles) of the catalyst N.N'-methylethylpiperidinium iodide (as a solution of c. 20 % by weight in ethanol). The temperature is continuously raised over 1 hour and 45 minutes to 205°C and is then kept constant for another 15 minutes. The pressure in the reaction vessel rises to about 400 mbar during the reaction. The advanced epoxy resin so obtained has an epoxy value of 0.60 equivalents/kg, a softening point of 88°C (measured by means of a Mettler thermosystem FP 800) and a viscosity according to Hόppler (40% in butylcarbitol according to DIN 53015) at 25°C of 693 mPa-s. The proportion of unreacted monomeric diglycidyl ether of bisphenol A in the advanced resin is about 0.1 % by weight (1300 ppm).

The following Table compares the proportion of diglycidyl ether of bisphenol A in the reaction mixtures of Examples 1 to 3 with the proportion of unreacted monomeric diglycidyl ether of bisphenol A in the advanced resin obtained.

advancement resin obtained by reacting 667 g of monomeric diglycidyl ether of bisphenol A and 333 g of bisphenol A.

Example 4 (Production of an adduct of epoxidised soybean oil and benzoic acid and of an advanced epoxy resin based on 20 % by weight of the adduct:

In a 1.5 litre 5-necked reaction flask, equipped with a mechanical steel agitator, digital thermometer and temperature-controlled heater, 1000g (4.12 epoxy equivalents) of epoxidised soybean oil (lrgaplast^®39) are heated to 100°C with flushing with nitrogen, and then 207.58g (1.70 carboxyl equivalents) of benzoic acid are added. When a temperature of 108°C is reached, 0.200g (0.784 mmol) of the catalyst N.N'-methylethylpiperidinium iodide (about 1.0 ml of a solution of 20 % by weight of N.N'-methylethylpiperidinium iodide in ethanol) is added and the temperature is raised to 140°C and kept constant for half an hour. The resulting reaction product of epoxidised soybean oil and benzoic acid has an epoxy value of 2.4 equivalents/kg. In a 1.5 litre 5-necked reaction vessel, equipped with a mechanical steel agitator, digital thermometer, temperature-controlled heater, evacuating appliance and reflux condenser, 200g (0.478 epoxy equivalents) of the reaction product of Example 1 (epoxy equivalent weight 418) are heated to 100°C. Subsequently, the reactor is charged with 525.30 g (2.805 epoxy equivalents) of a diglycidyl ether of bisphenol A having an epoxy equivalent weight of 187, with 274.7g (2.409 hydroxyl equivalents) of bisphenol A and with 1.054g (4.133 mmol) of N.N'-methylethylpiperidinium iodide (solution of 20 % by weight of N,N'-methylethylpiperidi- nium iodide in ethanol). The temperature is continuously raised over 1 hour and 55 minutes to 205°C (external reactor temperature) and is then kept constant for another 22 minutes (internal reactor temperature about 199°C). During the reaction, a vacuum (pressure about 100 to 300 mbar) is maintained in the reactor. The resulting advanced epoxy resin has an epoxy equivalent weight of 1538, a softening point of 105°C (measured by means of a Mettler thermosystem FP 800) and a viscosity according to Hόppler (40% in butylcarbitol according to DIN 53015) at 25°C of 1586 mPa-s. The proportion of free monomeric diglycidyl ether of bisphenol A in the advanced resin is 2300 ppm, and the proportion of free bisphenol A is 202 ppm.

Example 5:

The can-coating lacquers listed in the following Table are produced by first preparing the solution of the crosslinking agent indicated in the Table and then mixing that with a solution of 50 % by weight of the respective advanced epoxy resin in methoxypropyl acetate (MPA) and adding the indicated amount of additional methoxypropyl acetate. Using a roll coater, the curable compositions are then applied to aluminium sheets (to determine the mechanical properties of the coatings) and to tin sheets (to carry out the sterilisation tests and to determine the resistance to acetic acid).

The impact strength; r( reverse side) is determined by dropping a 2 kg die, on the bottom side of which is a ball 20 mm in diameter, bottom first from a specific height from the reverse side on to the coated area. The value indicated is the product of the weight of the die in kg and of the test height in cm at which no damage of the coating is found yet, i.e. the coating remains uncracked.

The acetone rubbing test is carried out by drenching a wool cloth with acetone and rubbing it twenty times back and forth over the coating. The grading scale is as follows: 0=unchanged; 1=retarding, cannot be scratched with fingernail; 2= hard to scratch, cotton wool possibly coloured; 3= easy to scratch; 4= film starts to peel or dissolve; 5= film completely peeled off

Grading scale: 0= no cracks; 1= few cracks, only visible by means of a magnifying glass (9x); 2= many cracks, only visible by means of a magnifying glass (9x); 3= few cracks, visible to the eye; 4= many cracks, visible to the eye; 5= film flaked off or peeled off

Grading scale: T= cloudy films, B= bubble formation, St= rough surface, F= change of hue, G= gloss reduc - tion, Fl=s stain formation, H= reduction of adhesion, K= scratchability, Q= swelling, R= crack formation, W= softening of the films; the grade of the flaws is expressed by the preceding number, the numbers having the following meanings: 0=unchanged; 1= traces of change; 2= minor change; 3= medium change; 4= highly changed; 5= very highly changed.

Mixture of trimellitic anhydride and an ester of CAS-No. 43011-20-7 (acid number 500-540 mgKOH/g; anhydride content 3.00 - 4.00 equ./kg; softening point according to Mettler, DIN 51920 95 to 110°C)

Example 6:

The can-coating lacquer listed in the following Table is produced and applied according to the description of Example 5, the properties indicated in the Table being found.

55% solution of a phenolic polymer of CAS-No. 70750-15-1 in a solvent mixture consisting of butanol and methoxypropyl acetate.

7)

Grading scale as for 4)

Claims

What is claimed is:

1. A heat-curable can-coating lacquer, which comprises an advanced epoxy resin which is the reaction product of one or several bisphenol diglycidyl ether(s), one or several bisphenol(s) and of an adduct of epoxidised soybean oil with a monocarboxylic acid, which has an epoxy equivalent weight of 350 to 600, and also a crosslinking agent for epoxy resins.

2. A can-coating lacquer according to claim 1 , wherein the advanced epoxy resin has an epoxy equivalent weight of 1000 to 4000.

3. A can-coating lacquer according to claim 2, wherein the advanced epoxy resin has an epoxy equivalent weight of 1400 to 2000.

4. A can-coating lacquer according to any one of claims 1 to 3, wherein the adduct of epoxidised soybean oil with a monocarboxylic acid has an epoxy equivalent weight of 450 to 600.

5. A can-coating lacquer according to any one of claims 1 to 4, wherein the advanced epoxy resin is produced using 5 to 35 % by weight of the adduct of epoxidised soybean oil with a monocarboxylic acid, the cited percentages relating to the total weight of the adduct of epoxidised soybean oil and of monocarboxylic acid, bisphenol and bisphenol diglycidyl ether.

6. A can-coating lacquer according to any one of claims 1 to 5, wherein the one or several bisphenol diglycidyl ethers are selected from the group consisting of diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol A, mixed diglycidyl ethers of bisphenol A/bisphenol F diglycidyl and mixtures of the cited diglycidyl ethers.

7. A can-coating lacquer according to any one of claims 1 to 6, wherein the one or several bisphenols are selected from the group consisting of bisphenol A, bisphenol F and mixtures of these two bisphenols.

8. A can-coating lacquer according to any one of claims 1 to 7, wherein the monocarboxylic acid is benzoic acid.

9. A can-coating lacquer according to any one of claims 1 to 8, wherein the advanced epoxy resin is produced by advancement of the components in the presence of an advancing catalyst selected from the group consisting of the tetraalkylphosphonium salts, tributylamine and N.N'-ethylmethylpiperidinium iodide and mixtures of these catalysts.

10. A can-coating lacquer according to claim 9, wherein the advancing catalyst is a mixture consisting of tetrabutylphosphonium halide and tributylamine or N.N'-ethylmethylpiperi- dinium iodide.

11. A can-coating lacquer according to any one of claims 1 to 10, wherein the crosslinking agent is an acid anhydride or a polyphenol.

12. A can-coating lacquer according to any one of claims 1 to 10, wherein the crosslinking agent is a product which is obtainable by heating trimellitic anhydride together with at least one polyol at a molar ratio from 2 to 1.25 for at least four hours at a pressure of 0.666 to 4 kPa to a temperature in the range from 190 to 250┬░C.

13. A can-coating lacquer according to claim 12, wherein the polyol is ethylene glycol and glycerol, and the crosslinking agent has an acid number of 520 to 560 mg KOH per gramme and an anhydride content of about 2 to 3 equivalents per kilogramme of crosslinking agent.

14. A process for the production of a food container, which comprises coating one or several sides of the food container entirely or partly with a can-coating lacquer according to any one of claims 1 to 13 and heat-curing this coating.

15. A process according to claim 14, which comprises coating the interior sides of the food container with the can-coating lacquer.

16. A process according to claim 14, wherein the food container is metallic food container.