The present invention relates to a coating composition for coating food containers .

The packaging of food containers (food and beverages) is still today largely carried out using metal food containers.

Typically, the metal containers are made of aluminium, steel or tinplate (more commonly known as "tin"). Due to the effect of air oxidation and the corrosive action of food products (food and beverages), the metal containers are subject to phenomena of corrosion which can contaminate the product contained in them, altering the organoleptic qualities, or even rendering it unsafe for the customer.

In order to avoid these contaminations, the surfaces of the metal containers which come in contact with the food products are generally coated with a film of protective paint which delays its corrosion, and also improves its appearance.

In the specific case of containers for liquid products (for example, beverages or oil) in tins the coating of the internal surface is made preferably by utilizing coating compositions in liquid form, as such compositions permit to obtain coatings of higher quality and with less costs with respect to the coating compositions in powder form, applicable by electrophoretic or electrostatic coating. In order to guarantee an homogeneous coating of the application surfaces by utilizing coating compositions in powder form, it is in fact necessary to lay coating films definitely thicker than the thickness of the corresponding coatings obtainable by the use of coating compositions in liquid form. This entails, therefore, a greater consumption of coating product and, consequently, higher application costs.

The application of coatings on the internal surface of hollow containers, like the cylindrical tins for beverages, presents also technical difficulties which can prevent in fact the possibility of using coating compositions in the powder form in an economically convenient way. The application by means of an electrophoretic or electrostatic painting, in fact, is based on the creation of an electric field between the object to be coated and the device for the distribution of the coating composition. In the case of hollow containers like the tins, during the deposition electrostatic fields are created, able to prevent an appropriate application of the coating on the lower region of the tins, particularly on the bottom. This effect is the more pronounced, the smaller the volume of the containers is.

A further disadvantage of the application of coating compositions in powder form consists in that it does not guarantee the high painting speeds required at industrial level (typically over 1500 tins per minute) . - In the state of the art, the coating composition used for the coating of the metal surfaces contacting foods are compositions comprising bonding resins of the epoxy, phenolic or melamine type.

These coating compositions, thus avoiding the contamination of the products by the metal surface of the container, have the disadvantage that they are subject themselves to the release of substances, like bisphenol, phenol and formaldeyd.

Considering the always more stringent National and International Standards in matter of food safety, the transfer of these substances to food products is highly undesired, due to their toxicological characteristics.

The reason for which the transfer of undesired substances to the food products packed in metal containers occurs, are to be searched mainly in the specific chemical characteristics of the components of the paint and in the imperfect adhesion of the coating film applied on the metal surface.

In the technical field of the coatings for food metal containers, is therefore highly felt the need of identifying coating compositions alternative to those containing epoxy, phenolic or melamine resins, which permit to overcome the problems outlined above.

The American Patent US 5,266,628 describes aqueous-based coating compositions for the coating of metal food containers, comprising a bonding resin containing an acrylic polymer and a cross-linking agent containing ?-hydroxy-alkylamides . The coating films obtained with the coating compositions of the aforesaid patent have a high water resistance at high temperature, which makes them suitable for coating the inside of metal containers which must be subjected to sterilization and pasteurization processes. According to US 5,266,628, in order to obtain a high water stability at high temperature, the quantity of hydroxy- alkylamides in the composition must be chosen in a specific ratio with respect to the quantity of acrylic polymer. More precisely, the ratio between the quantity of hydroxy groups of the hydroxy-alkylamide and the quantity of carboxylic groups in the polymer must be lower than 0,5:1, preferably lower than 0,4:1, more preferably lower than 0,35:1. When such ratio is greater than 0,5:1 (up to approx. 1:1), the adhesion of the coating film to the metal surface decreases dramatically and the surface becomes easily prone to corrosion .

The aim of the present invention is to overcome the problems underlined from the state of the art.

A first object of the present invention is a coating composition for coating the surface of a metal container for foods, comprising

a) a bonding resin comprising an acrylic polymer having carboxylic groups and

b) a cross-linking agent comprising a β-hydroxy- alkylamide ,

characterized in that the ratio between the hydroxy groups of the β-hydroxy-alkylamide and the carboxylic groups of the bonding resin is comprised between 0,7:1 and 1,2:1.

A second object of the present invention is the use of the aforementioned composition for coating a surface of a metal food container.

A third object of the present invention is a metal food container having a surface coated with the aforesaid composition.

A further object of the present invention is a coating film applied on a surface of a metal food container.

The Applicant has surprisingly found that, by utilizing in suitable stoichiometric ratios a cross- linking agent comprising ?-hydroxy-alkylamides and a bonding resin comprising an acrylic polymer, it is possible to realize coating films characterized by a high adhesiveness and a surprisingly low level of transferring of undesired substances. Therefore, the coating compositions being the object of the present invention are particularly suitable for coating metal surfaces of containers destined to come in contact with food products, like foods and beverages.

For the aims of the present invention, metal food containers means containers of any kind and dimensions having metal surfaces destined to come in contact with liquid and solid food containers (for example, tins for beverages, containers for canned food, oil tanks, etc . ) .

The coating composition which is the object of the present invention is particularly suitable to cover the surface of a metal container of hollow shape, preferably a cylindrical container, more preferably a cylindrical container having a circular or elliptic cross-section. Preferably, the container has a volume equal or lower than 2 liters, more preferably equal or lower than 1 liter, still more preferably equal or lower than 0,5 1, still more preferably lower or equal to 0,35 1.

The cross-linking agents based on ?-hydroxy- alkylamides are products known in the state of the art and are commercially available (EMS-CHEMIE AG CH-7013 Domat/Ems ) .

The ?-hydroxy-alkylamides are represented by the

HO(R ₃ ) ₂ C(R ₂ ) ₂ C-N-C ^A-kC-N-C(R ₂ ) ₂ C(R ₃ ) ₂ OH following general formula (I)

(I)

in which:

A is chosen among

- a hydrogen bond,

a monovalent or polyvalent organic radical derived from a saturated or unsaturated alkylic radical containing from 1 to 60 carbon atoms (for example metyl, etyl, propyl, butyl, pentyl, exyl, eptyl, octyl, nonyl, decyl, eicosyl, triacontyl, tetracontyl, pentacontyl, exacontyl);

- an arylic radical with one or two aromatic rings (for example, phenyl or naphtyl) ;

- a tri-alkylene-aminic radical (for example, tri- metylene-amine or tri-etylene-amine ) ;

- an unsaturated radical containing one or more ethylenic groups (for example, etenyl, 1-metil-etenyl, 3-butenil-l, 3-diyl, 2-propenil-l , 2-diyl) ;

- a carboxy-alkenylic radical (for example, 3- carboxy-2-propenil ) or alcoxy-carbonil-alchenyl (for example, 3-metoxy-carbonyl-2-propenyl ) ;

Ri is hydrogen, an alkyl containing from 1 to 5 carbon atoms (for example, metyl, etyl, n-propyl, n- butyl, sec-butyl, tert-butyl, pentyl) or a hydroxy- alkyl containing from 1 to 5 carbon atoms (for example, hydroxy-etyl, 3-hydroxy-propyl , 2-hydroxy-propyl , 4- hydroxy-butyl, 3-hydroxy-butyl , 2-hydroxy-2-propyl- metyl, 5-hydroxy-pentyl, 4-hydroxy-pentyl , 3-hydroxy- pentyl, 2-hydroxy-pentyl and the pentyl isomers);

R ₂ e R ₃ , equal or different, are radicals selected from hydrogen, lower alkyls, linear or branched, containing from 1 to 5 carbon atoms, or one of the radicals R2 and one of the radicals R3 can be combined in order to create, together with the carbon atoms to which they adhere, a cyclo-alkyl (for example, cyclo- pentyl or cyclo-hexyl ) ;

n is an integer equal to 1 or 2 and n' is an integer comprised between zero and 2, n being equal to 2 when n' equals zero.

For the aims of the present invention, preferred /?-hydroxy-alkyl-amides among those of the general aforementioned · formula (I) are the following:

bis [N, N-di ( ?-hydroxy-ethyl ) ] adipamide,

bis [N, N-di ( ?-hydroxy-propyl ) ] succinamide,

bis [N, N-di ( ?-hydroxy-ethyl ) ] azelamide,

bis [N, N-di ( β-hydroxy-propyl ) ] adipamide,

bis [N-metil-N- ( /?-hydroxy-ethyl ) ] oxamide

and their mixtures.

The acrylic polymer comprising carboxylic groups can be prepared for radical polimerization of ethylenically unsaturated monomers. A broad range of acrylic monomers known in the art can be used for the polymerization, among which acrylic acid, methacrylic acid and their esters.

Typical acrylic esters are the lower alkyl-esters , among which metyl- (met ) acrylate , etyl (met ) -acrylate, n- butyl- (met ) acrylate and isobutyl- (met ) acrylate. Other acrylic esters include 2-ethyl-exyl- (met ) acrylate and dodecyl-acrylate . As far as possible, the use of esters of the methacrylic acid is to prefer for their greater resistance to hydrolysis.

For the same reason, the esters of branched alcohols are to prefer.

Other monomers can also be used, containing carboxylic groups and having unsaturations, among which crotonic acid, itaconic acid, fumaric acid, maleic acid and citraconic acid. The presence in the polymer of monomers containing acid groups guarantees the compatibility of the coating composition with water, after the neutralization of the polymer with a base (for example, amine). The acid groups are further necessary also for the reactions in which the β- hydroxy-alkyl-amides are utilized as cross-linking agents .

In a preferred embodiment of the present invention, the acrylic polymer, in addition to the carboxylic groups, can also contain carbonyl .

In order to efficiently perform both aforesaid functions, preferably at least 6% in weight on solid base of the used monomers for obtaining the acrylic polymer must contain acid groups, more preferably from 10% to 30%.

Expressed in terms of acid number (N.A.), the quantity of acid groups of the acrylic polymers varies from 80 to 180 mg KOH per gram of solid polymer (measured according to the Standard method DIN EN ISO 3682) . Preferably, N.A. varies from 90 to 150 mg KOH per gram of solid polymer.

As an alternative to monomers comprising carboxylic groups, anydrides can also be utilized, for example maleic anydride. In this case, each anydride molecule contributes to the acid number with two acid groups . The acrylic polymer can be obtained through polymerization only of acrylic polymers. Preferably, the acrylic polymer comprises also vinyl monomers (for example, styrene or vinyl-toluene) co-polymerized with the acrylic monomers.

The use of these monomers gives a high resistance to hydrolysis.

In the polymer composition monomers containing hydroxy groups can be introduced, as for example hydroxy-ethyl (met ) acrylate, hydroxy-propyl

(met) acrylate, a reaction product between (met) acrylic acid and glycidyl ester of the versatic acids. Smaller proportions (lower than 10% in weight, on solid base, with reference to the total weight of the polymer) of other monomers, as for example (met ) acrylamide, diacetone acrylamide (DAAM) or promoting adhesion monomers containing phosphoric groups, can be used in the polymer composition.

As known to the skilled in the art, the polymerization reaction of the monomers for producing the acrylic polymers requires, generally, the presence of a radical initiator. Examples of initiators of free radicals are azoic compounds like 2 , 2 ¹ -azobis- ( 2-metyl- iso-butyrro-nitryl ) ; peroxy-esters , like t-butyl- peroxy-acetate, t-butyl-peroctoate, amyl-peroctoate, amyl-t-peracetate, and t-butyl-perbenzoate ; alkyl- hydro-peroxides , like t-butyl-hydro-peroxyd; diacil- peroxydes, like dibenzoyl-peroxyd, and dialkyl- peroxyds, like di-t-butyl-peroxyd and dicumyl-peroxyd.

The synthesis of the acrylic polymers can be made in any organic solvent in which the monomers and the polymers are soluble. Among the different solvents which can be utilized there are ethers, like 2-butoxy- ethanol, dipropylene-glycol mono-etyl-ether , 1-metoxy- 2-propanol, and alcohols, like butanol, isopropylic alcohol and/or their mixtures.

The polymerization can also be made in an emulsion or in suspension in the absence of solvents according to known techniques.

At the end of the polymerization, the acid groups present on the polymer can be neutralized with a base, for example amine, in order to make the polymer water- soluble. Preferably the base is chosen among compounds which are volatile at the temperature at which the cross-linking is made, in order that the base does not remain on the cross-linked film.

Subsequently, the polymer can be reduced with water and mixed with the ?-hydroxy-alkyl-amide and, possibly, with additives (for example, anti-foam agents, slipping agents, polyvalent metal complexes like for example zinc or zirconium, etc.) useful, according to the known art, for the improvement of some chemical-physic and applicability characteristics. The compositions can contain also pigments in function of the specific application expected for the composition.

In particular, the coating composition which is the object of the present invention can also comprise blocked compounds of the isocyanate type or adipic di- hydrazide acid (DAH) able to improve the mechanical characteristics of the paint film.

When the acrylic polymer comprises carboxylic and carbonylic groups, the coating composition comprises preferably also the compound DAH. The relative quantities of ?-hydroxy-alkyl-amide and of acrylic polymers in the coating composition are determined by the ratio (R) between the quantity of the hydroxy groups present in the cross-linking agent and the quantity of carboxylic groups present in the acrylic polymer.

In order to obtain films with a high adhesion and a low level of transferring of undesired substances, it has surprisingly been found that R must be comprised between 0,7:1 and 1,2:1, i.e. in the coating composition from 0,7 to 1,2 hydroxy groups of the cross-linking agent must be present for every carboxylic group of the polymer.

Preferably, R is comprised between 0,8:1 and 1:1, as the coating films obtained with these compositions have the highest degree of cross-linking with consequent advantages on the resistance characteristics towards the aggression of water and other substances present in food products like acids, alcohols, salts.

The reduced permeability of the paint film, due to the high degree of cross-linking, reduces also the probability of transfer of substances from the coating film to the food products.

The coating compositions which are the object of the present invention are preferably formulated in a liquid form, typically as an aqueous dispersion, and can be applied on the metal surfaces of the food containers with the methods and the apparatuses conventionally utilized for the application of the water coating products containing epoxy-acrylic- phenolic resins. In particular, the application of protective films on the metal surfaces of food containers is generally made by spraying, rolling or by means of electrophoresys . Preferably, in the case of containers of hollow shape having a volume equal or lower than 2 liters, more preferably equal or lower than 1 liter, still more preferably equal or lower than 0,5 1, further more preferably lower or equal to 0,35 1, the application is made by spraying in order to avoid the application problems typically encountered in the state of the art for the coating compositions in powder form.

The protective film applied can have any desired thickness. Preferably, the surfaces of the metal food containers are coated with films having a thickness lower than 0, 025 mm, more preferably lower than 0,013 mm, still more preferably lower than 0,005 mm.

The cross-linking of the coating compositions which are the object of the present invention is mainly obtained by submitting the film applied to the surface to heating at a temperature comprised between 125°C and 400°C, for a duration variable from 30 minutes to 2 seconds, preferably between 150°C and 250°C for durations between 30 minutes and 1 minute.

The protective films realized on the metal surfaces with the coating compositions which are the object of the present invention, besides having a high degree of chemical resistance and a low level of transfer of undesired substances, are characterized also by a good flexibility.

The following examples of embodiments are given for a mere illustrative scope of the present invention and must not be understood in a limitative way within the protection range defined by the annexed claims. EXAMPLE 1

Synthesis of a bonding resin based on a acrylic carboxylated polymer

In a glass reactor provided with a stirrer, fallouts and nitrogen entrance, are introduced:

- glicol ethylene monobutyl ether 392 g

- di-propylene glicol monometyl ether 200 g Heating is made at 145°C at the same time providing to charge, within 3 hours, all the mixture 1) and 85% of the mixture 2) :

Mixture 1)

- styrene 332 g

- 2-etyl-esyl-acrylate 515 g - metacrylic acid 250 g

- idroxy-propyl-metacrylate 60 g; Mixture 2)

- t-amil peroxy-3, 5, 5-trimetyl exanoate 46 g

- dipropylene glicol monometyl ether 169 g. At the end, within 1 hour, the remaining 15% of the mixture 2) is fed, in order to complete the conversion of the monomers.

The resin obtained has a solid residue of 60% in weight, an acidity, expressed as acid number, of 136 mg KOH/g of solid and a viscosity of 14000 mPa-s. The glass transition temperature (Tg) calculated is 10°C.

EXAMPLE 2

Neutralization and dispersion in water of the resin .

In a reactor having a rapid stirring, is charged:

- resin of the example 1 : 243 g

- xylene : 50 g - butanol 70 g

- dimetyl-ethanol-amine 28 g and after homogenization

- distilled water 369 g.

EXAMPLE 3

Coating compositions

A first series of coating composition was prepared (series 1) containing the dispersion in water of the neutralized acrylic resin of the example 2 and a cross- linking agent made by bis [N,N-di (β-hydroxy- ethyl) ] adipamide (PRIMID® XL-552 of the Company EMS) . The cross-linking agent was dissolved in water (25% in weight) before being mixed with the resin. The samples have been prepared by mixing the acrylic resin and the cross-linking agent in such relative quantities in order to have a ratio R among hydroxy groups of the β- hydroxy-alkyl-amide and carboxylic groups of the acrylic resin variable from 0,2 to 2 (Table 1) . Table 1. Coating compositions (series 1)

Quantities expressed in grams. A second series of coating compositions (series 2) has been prepared by mixing the same dispersion of acrylic resin of the first series with a cross-linking agent made by bis [N, -di (β-hydroxy-propyl- adipamide) (PRIMID® QM-1260) . The cross-linking agent has been dissolved in water (27,54% in weight) before being mixed with the resin. Also the samples of this series have been prepared by mixing the acrylic resin and the cross-linking agent in such relative quantities to have a ratio R among hydroxyl groups of the β- hydroxy-alkyl-amide and carboxylic groups of the acrylic resin variable from 0,2 to 2 (Table 2) .

Table 2. Coating compositions (series

Quantities expressed in grams. The two series of coating compositions have been applied with a film-stretcher on a metal support made of a plane panel of phosphor-chromed aluminium or of tinplate .

The support coated with the coating support has been made cross-linked through heating at 204°C for 3 minutes. At the end of the cross-linking, the applied film had a thickness variable from 0,0023 to 0,0027 mm.

On each coated support a test was made of the cross-linking degree of the coating film (rub-test) according to the ASTM D-5402 method by using as solvent methyl ethyl ketone. The cross-linking degree was expressed by using a range of values variable from 0 (low resistance to methyl ethyl ketone) to 5 (good resistance to methyl ethyl ketone) . The evaluation was made after 100, 200 and 300 double hits. The results of the test are reported in the subsequent Tables 3 and 4. Table 3 shows the data regarding the coating compositions of the series 1 (PRIMID® XL-552), whereas Table 4 those regarding the coating compositions of the series 2 (PRIMID® QM-1260) .

Table 3. Rub-test made on the coating compositions the series 1 (aluminium phosphor-chromed panel)

^C: double hits.

Table 4. Rub-test made con the coating compositions the series 2 (aluminium phosphor-chromed panel)

^C: double hits.

The results show that, for both series of coating compositions, the cross-linking of the film is greatest when the ratio R between quantity of hydroxy groups in the /?-hydroxy-alkyl-amide and quantities of carboxylic groups in the acrylic resin is comprised from 0,7 to The two series of coating compositions listed in Tables 1 and 2, have been subjected to a sterilization test which provides the partial immersion of coated panels in water maintained at the temperature of 121°C for a period of 90 minutes. The degree of integrity of the film is evaluated (presence of stains, bubbles, loss of adhesion) . The degree of resistance to the sterilization of each film was expressed by utilizing a range of values variable from 0 (low resistance) to 5 (good resistance) .

The results of the sterilization test are reported in the following Tables 5 and 6. Table 5 shows the data regarding the coating compositions of the series 1 (PRIMID® XL-552), whereas Table 6 those regarding the coating compositions of the series 2 (PRIMID® QM-1260) . Both series of tests are referred to applications on plane panels of phosphor-chromed aluminium.

Table 5. Sterilization test in water made on the coating compositions of the series 1

Table 6. Sterilization test in water made on the coating compositions of the series 2

The results of the test show that the films obtained with both series of compositions have a good resistance to the sterilization when the ratio R among quantities of hydroxy groups in the /?-hydroxy-alkyl- amide and quantities of carboxylic groups in the acrylic resin is comprised between 0,7 and 1,2.

Further tests were also made destined to simulate the aggression on the paint film by food products.

The tests are made with the same methods of the sterilization test, by substituting the distilled water with solutions simulating the food products. In particular the solutions having the following compositions were tested:

1) "LAS solution" lactic acid 0,5 g

acetic acid 0,5 g

sodium chloride 0,5 g

distilled water 98, 5 g

2) "Citric acid solution

- citric acid 3 g

- deionized water 9 7 g

The evaluation criterion of the resistance to the chemical aggression is the same used for the sterilization tests in water.

The coating compositions tested are those of the series 2. The results of the tests are reported in Tables 7 and 8. Table 7 . Sterilization test in "LAS solution" made on the coating compositions of the series 2

RATIO OH/COOH (R)

Panel 0,2 0,4 0, 6 0, 8 1,0 1,2 1,4 1,6 1,8 2,0

Tin plate 4 5 5 5 5 4 3 3 3 3

Phosphor- chromed 5 6 6 6 6 6 6 6 6 5 Aluminium

Table 8. Sterilization test in "citric acid solution" made on the coating compositions of the series 2

A further sterilization test was made in a solution called "L-85 solution", the composition of which is reported as follows. The test was made by partially immersing the panel coated with the compositions of the series 2 in the L-85 solution at the temperature of 50°C for a duration of 12 days. The evaluation is made with the same criterion (0 = very bad; 10 = very good) . The results of the test are reported in Table 9.

The "L-85 solution" has the following composition:

- citric acid 9,2 g

- sodium chloride 0,71 g

phosphoric acid (85%) 3,33 g

distilled water 86, 76 g.

Table 9. Sterilization test in "L-85 solution" made on the coating compositions of the series 2

RATIO OH/COOH (R)

Panel 0, 2 0,4 0,6 0, 8 1,0 1,2 1, 1,6 1,8 2,0

Phosphor- chromed 7 7 7 8 9 9 9 8 5 1 Aluminium