ITTO20090872A1 - PAINTING COMPOSITION FOR THE COATING OF FOOD CONTAINERS - Google Patents

Description

PAINTING COMPOSITION FOR THE COATING OF

FOOD CONTAINERS

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

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

Typically, metal containers are made of aluminum, steel or tinplate (more commonly referred to as "tin"). Due to the oxidation of the air and the corrosive action of food products (food or drinks), metal containers are subject to corrosion phenomena that can contaminate the product they contain, altering its organoleptic qualities or even making it dangerous. for the consumer.

To avoid these contaminations, the surfaces of metal containers that come into contact with food are generally coated with a protective paint film that delays corrosion, as well as improving their appearance.

In the state of the art, the painting compositions used for the coating of metal surfaces in contact with food are compositions comprising binder resins of the epoxy, phenolic or melamine type.

These painting compositions, while avoiding the contamination of the products by the metal surface of the container, have the disadvantage that they themselves are subject to the release of substances, such as bisphenol, phenol and formaldehyde.

In consideration of the increasingly stringent national and international regulations on food safety, the transfer of these substances to food products is highly undesirable, due to their toxicological characteristics.

The reasons for which unwanted substances are transferred to food products packaged in metal containers are mainly to be found in the specific chemical characteristics of the paint components and in the imperfect adhesion of the coating film applied to the metal surface.

In the technical sector of coatings for food metal containers, there is therefore a strong need to identify alternative painting compositions to those containing epoxy, phenolic and melamine resins, which allow to overcome the drawbacks highlighted above.

The American patent US 5,266,628 describes aqueous-based painting compositions for the coating of metal food containers comprising a binder resin containing an acrylic polymer and a cross-linking agent containing β hydroxyalkylamides. The coating films obtained with the painting compositions of the aforementioned patent have a high resistance to water at high temperatures, which makes them suitable for coating the inside of metal containers that must be subjected to sterilization and pasteurization processes. According to US 5,266,628, to obtain high water stability at high temperature, the amount of hydroxyalkylamides in the composition must be chosen in a specific ratio with respect to the amount of acrylic polymer. More specifically, the ratio of the amount of hydroxyalkylamide hydroxyl groups to the amount of carboxyl groups in the polymer should be less than 0.5: 1, preferably less than 0.4: 1, more preferably less than 0.35: 1 . When this ratio is greater than 0.5: 1 (up to about 1: 1), the adhesion of the coating film to the metal surface drastically decreases and the surface becomes easily corrodible.

The object of the present invention is to overcome the drawbacks highlighted by the state of the art.

A first object of the present invention is a painting composition for coating the surface of a metal container for food comprising a) a binder resin comprising an acrylic polymer having carboxylic groups and

b) a cross-linking agent comprising a β hydroxyalkylamide,

characterized in that the ratio between the hydroxyl groups of the β-hydroxyalkylamide and the carboxyl groups of the binder resin is between 0.7: 1 and 1.2: 1.

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

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

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

The Applicant has surprisingly found that, by using in suitable stoichiometric ratios a cross-linking agent comprising β-hydroxyalkylamides and a binder resin comprising an acrylic polymer, it is possible to produce coating films characterized by high adhesiveness and a surprisingly low level of release of undesirable substances. . Therefore, the painting compositions object of the present invention are particularly suitable for coating metal surfaces of containers intended to come into contact with food products, such as food and drinks.

For the purposes of the present invention, metal containers for food are meant containers of all kinds and sizes having metal surfaces intended to come into contact with liquid and solid food products (for example, beverage cans, canned food containers, oil tanks , etc.)

Cross-linking agents based on β hydroxyalkylamides are products known in the state of the art and commercially available (EMS-CHEMIE AG CH-7013 Domat / Ems).

The β-hydroxyalkylamides are represented by the following general formula (I)

(THE)

in which:

A is chosen from

- a hydrogen bond,

- a monovalent or polyvalent organic radical derived from a saturated or unsaturated alkyl radical containing from 1 to 60 carbon atoms (for example methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, eicosyl, triacontyl, tetracontile, pentacontile, hexacontile);

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

- a trialkyleneamine radical (for example, trimethyleneamine or triethyleneamine);

- an unsaturated radical containing one or more ethylene groups (for example, ethenyl, 1-methylethenyl, 3-butenyl-1,3-diyl, 2-propenyl-1,2-diyl);

- a carboxyalkenyl radical (for example, 3-carboxy-2-propenyl) or alkoxy-carbonyl-alkenyl (for example, 3-methoxycarbonyl-2-propenyl);

R1 is hydrogen, an alkyl containing 1 to 5 carbon atoms (e.g., methyl, ethyl, npropyl, n-butyl, sec-butyl, tert-butyl, pentyl) or a hydroxyalkyl containing 1 to 5 carbon atoms (e.g. example, hydroxyethyl, 3-hydroxypropyl, 2-hydroxypropyl, 4-hydroxybutyl, 3-hydroxybutyl, 2-hydroxy-2-propylmethyl, 5-hydroxypentyl, 4-hydroxypentyl, 3-hydroxypentyl, 2-hydroxybutyl and the pentyl isomers);

R2 and R3, the same 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 joined to form, together with the carbon atoms a to which a cycloalkyl (for example, cyclopentyl and cyclohexyl) are attached;

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

For the purposes of the present invention, preferred β-hydroxyalkylamides among those of the above general formula (I) are the following:

bis [N, N-di (β-hydroxyethyl)] adipamide,

bis [N, N-di (β-hydroxypropyl)] succinamide, bis [N, N-di (β-hydroxyethyl)] azelamide,

bis [N, N-di (β-hydroxypropyl)] adipamide,

bis [N-methyl-N- (β-hydroxyethyl)] oxamide

and their blends.

The acrylic polymer comprising carboxyl groups can be prepared by radical polymerization of ethylenically unsaturated monomers. A wide range of acrylic monomers known in the art can be used for polymerization, including acrylic acid, methacrylic acid and their esters.

Typical acrylic esters are the lower alkyl esters, including methyl- (meth) acrylate, ethyl (meth) acrylate, n-butyl- (meth) acrylate and isobutyl- (meth) acrylate. Other acrylic esters include 2-ethylhexyl- (meth) acrylate and dodecyl acrylate. As far as possible, the use of methacrylic acid esters are to be preferred due to their greater resistance to hydrolysis.

For the same reason, esters of branched alcohols are preferred.

Other monomers containing carboxylic groups and having unsaturations can also be used, including 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 painting composition with water, after neutralization of the polymer with a base (for example, an amine). Acid groups are also required for reactions in which β-hydroxyalkylamides are used as cross-linking agents.

To effectively perform both the above functions, preferably at least 6% by weight on a solid basis of the monomers used to obtain the acrylic polymer must contain acid groups, more preferably between 10% and 30%.

Expressed in terms of acid number (N.A.), the quantity of acid groups of the acrylic polymer varies from 80 to 180 mg of 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 of KOH per gram of solid polymer.

As an alternative to monomers including carboxylic groups, anhydrides can also be used, for example, maleic anhydride. In this case, each anhydride molecule contributes to the acid number with two acid groups.

The acrylic polymer can be obtained by polymerization of only acrylic monomers. Preferably, the acrylic polymer also comprises vinyl monomers (for example, styrene or vinyltoluene) co-polymerized with the acrylic monomers.

The use of these monomers confers high resistance to hydrolysis.

Monomers containing hydroxyl groups, such as for example hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, reaction product between (meth) acrylic acid and glycidyl ester of versatic acids, can be inserted in the polymer composition. Smaller proportions (less than 10% by weight, on a solid basis, with respect to the total weight of the polymer) of other monomers, such as (meth) acrylamide or adhesion promoting monomers containing phosphoric groups, can be used in the composition of the polymer.

As known to the skilled in the art, the polymerization reaction of monomers to produce acrylic polymers generally requires the presence of a radical initiator. Examples of free radical initiators are azo compounds such as 2,2'-azobis- (2-methylisobutyronitrile); peroxyesters, such as t-butyl peroxyacetate, t-butyl peroctoate, amyl peroctoate, amyl t-peracetate, and t-butyl perbenzoate; alkyl hydroperoxides, such as t-butyl hydroperoxide; diacylperoxides, such as dibenzoyl peroxide, and dialkylperoxides, such as di-t-butyl peroxide and dicumyl peroxide.

The synthesis of acrylic polymers can be carried out in any organic solvent in which the monomers and polymers are soluble. Among the different solvents that can be used are ethers, such as 2-butoxyethanol, dipropylene glycol monoethyl ether, 1-methoxy-2-propanol, and alcohols, such as butanol, isopropyl alcohol and / or their mixtures.

The polymerization can also be carried out in emulsion or 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, such as an amine, to make the polymer soluble in water. Preferably the base is selected from compounds which are volatile at the temperature at which the crosslinking is made, so that the base does not remain on the crosslinked film.

Subsequently, the polymer can be reduced with water and mixed with the β-hydroxyalkylamide and, optionally, with additives (for example, antifoamers, sliding agents, polyvalent metal complexes such as for example zinc or zirconium, etc.) useful, according to the technique note, to the improvement of some chemical-physical characteristics and applicability. The compositions may also contain pigments depending on the specific application envisaged for the composition.

In particular, the painting composition object of the present invention can also comprise compounds of the blocked isocyanate type, capable of improving the mechanical characteristics of the paint film.

The relative quantities of β-hydroxyalkylamide and acrylic polymers in the painting composition are determined by the ratio (R) between the quantity of hydroxyl groups present in the cross-linking agent and the quantity of carboxyl groups present in the acrylic polymer.

In order to obtain films with high adhesiveness and a low level of release of unwanted substances, it was surprisingly found that the R must be between 0.7: 1 and 1.2: 1, i.e. in the painting composition there must be from 0.7 to 1 , 2 hydroxyl groups of the crosslinking agent for each carboxylic group of the polymer.

Preferably, R is between 0.8: 1 and 1: 1 since the coating films obtained with these compositions have the highest degree of crosslinking with consequent advantages on the characteristics of resistance to aggression by water and other substances present. in foods such as acids, alcohol, salts.

The reduced permeability of the paint film, resulting from the high degree of crosslinking, also reduces the likelihood of transfer of substances from the coating film to food.

The painting compositions object of the present invention can be applied on the metal surfaces of food containers with the methods and equipment conventionally used for the application of water-based paint products containing epoxy-acrylic-phenolic resins.

In particular, the application of protective films on the metal surfaces of food containers is generally carried out by spray, roller or electrophoresis.

The applied protective film can have any desired thickness. Preferably, the surfaces of the metal containers for food are coated with a film having a thickness of less than 0.025 mm, more preferably less than 0.013 mm, even more preferably less than 0.005 mm.

The crosslinking of the painting compositions object of the present invention is obtained by subjecting the film applied to the surface to heating at a temperature between 125 ° C and 400 ° C, for a period of time ranging from 30 minutes to 2 seconds, preferably between 150 ° C and 250 ° C for times from 30 minutes to 1 minute.

The protective films made on metal surfaces with the painting compositions object of the present invention, in addition to having a high degree of chemical resistance and a low level of release of unwanted substances, are also characterized by good flexibility.

The following embodiment examples are provided for illustrative purposes only of the present invention and should not be construed as limiting the scope of protection defined by the attached claims.

EXAMPLE 1

Synthesis of a binder resin based on a carboxylated acrylic polymer

In a glass reactor equipped with a stirrer, reflux and nitrogen inlet, are introduced:

- ethylene glycol monobutyl ether 392 g - dipropylene glycol monomethyl ether 200 g It is heated to 145 ° C and proceeds to load simultaneously, within 3 hours, all the mixture 1) and 85% of the mixture 2):

mixture 1)

- styrene 332 g - 2-ethylhexyl acrylate 515 g - methacrylic acid 250 g - hydroxypropyl methacrylate 60 g; mixture 2)

- t-amyl peroxy-3,5,5-trimethyl hexanoate 46 g - dipropylene glycol monomethyl ether 169 g. At the end, the remaining 15% of the mixture 2) is fed within 1 hour in order to complete the conversion of the monomers.

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

EXAMPLE 2

Neutralization and dispersion of the resin in water.

In a reactor equipped with fast stirring, the following is charged:

- resin of example 1: 243 g - xylene: 50 g - butanol: 70 g - dimethyl-ethanolamine: 28 g

and after homogenization

- distilled water: 369 g.

EXAMPLE 3

Paint compositions

A first series of painting compositions was prepared (series 1) containing the dispersion in water of the neutralized acrylic resin of Example 2 and a cross-linking agent consisting of bis [N, N-di (β-hydroxyethyl)] adipamide (PRIMID® XL-552 of the EMS company). The crosslinking agent was dissolved in water (25% by weight) before being mixed with the resin. The samples were prepared by mixing the acrylic resin and the crosslinking agent in relative quantities such as to have an R ratio between hydroxyl groups of the β-hydroxyalkylamide and carboxylic groups of the acrylic resin ranging from 0.2 to 2 (Table 1).

Table 1. Paint compositions (series 1)

<1> Quantity expressed in grams.

A second series of paint compositions (series 2) was prepared by mixing the same acrylic resin dispersion of the first series with a cross-linking agent consisting of bis [N, N-di (β-hydroxypropyl adipamide) (PRIMID® QM-1260). The crosslinking agent was dissolved in water (27.54% by weight) before being mixed with the resin. The samples of this series were also prepared by mixing the acrylic resin and the cross-linking agent in relative quantities such as to have an R ratio between the hydroxyl groups of the β-hydroxyalkylamide and the carboxylic groups of the acrylic resin ranging from 0.2 to 2 (Table 2 ).

Table 2. Paint compositions (series 2)

<1> Quantity expressed in grams.

The two series of paint compositions were applied with film spreaders on a metal support consisting of a flat panel of phosphochromatised aluminum or a tinplate.

The support coated with the paint product was cross-linked by heating at 204 ° C for 3 minutes. At the end of the crosslinking, the applied film had a thickness ranging from 0.0023 to 0.0027 mm.

On each coated support a cross-linking degree test of the coating film (rub-test) was carried out according to the ASTM D-5402 method using methyl ethyl ketone as solvent. The degree of crosslinking was expressed using a scale of values ranging from 0 (poor 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 test results are reported in the following Tables 3 and 4. Table 3 shows the data relating to the coating compositions of series 1 (PRIMID® XL-552), while Table 4 those relating to the coating compositions of series 2 (PRIMID® QM -1260).

Table 3. Rub-test performed on series 1 paint compositions (phosphochromatised aluminum panel)

<1> DC: double hits.

Table 4. Rub-test performed on series 2 paint compositions (phosphochromatised aluminum panel)

<1> DC: double hits.

The results show that, for both series of coating compositions, the crosslinking of the film is maximum when the ratio R between the quantity of hydroxyl groups in the β-hydroxyalkylamide and the quantity of carboxylic groups in the acrylic resin is between 0.7 and 1.2. .

The two series of painting compositions listed in Tables 1 and 2 were subjected to a sterilization test which involves the partial immersion of painted panels in water maintained at a 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 sterilization of each film was expressed using a scale of values ranging from 0 (poor resistance) to 5 (good resistance).

The results of the sterilization test are reported in the following Tables 5 and 6. Table 5 reports the data relating to the coating compositions of series 1 (PRIMID® XL-552), while Table 6 those relating to the coating compositions of series 2 (PRIMID ® QM-1260). Both series of tests refer to applications on phosphochromatised aluminum flat panels.

Table 5. Water sterilization test performed on series 1 paint compositions

Table 6. Water sterilization test performed on series 2 paint compositions

The results of the test show that the films obtained with both series of compositions have a good resistance to sterilization when the ratio R between the quantity of hydroxyl groups in the β-hydroxyalkylamide and the quantity of carboxylic groups in the acrylic resin is between 0.7 and 1 ,2.

Tests were also carried out to simulate the aggression on the paint film by food substances.

The tests are carried out with the same methodology as the sterilization test, replacing distilled water with food simulating solutions. 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 97 g

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

The paint compositions tested are those of series 2. The test results are reported in Tables 7 and 8.

Table 7. Sterilization test in "LAS solution" performed on the paint compositions of series 2

Table 8. Sterilization test in “citric acid solution” performed on the paint compositions of series 2

A further sterilization test was performed in a solution called “L-85 solution”, the composition of which is shown below. The test was carried out by partially immersing the panel painted with the compositions of series 2 in the L-85 solution at a temperature of 50 ° C for a duration of 12 days. The evaluation is carried out with the same criterion (0 = very poor; 10 = very good). The test results are shown 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 "solution L-85" performed on the paint compositions of series 2

Claims (12)

CLAIMS 1) Paint composition for coating the surface of a metal container for food comprising a) a binder resin comprising an acrylic polymer having carboxylic groups and b) a crosslinking agent comprising a β-hydroxyalkylamide, characterized in that the ratio between the hydroxyl groups of the β-hydroxyalkylamide and the carboxyl groups of the binder resin is between 0.7: 1 and 1.2: 1. 2) Composition according to claim 1, characterized in that the ratio between the hydroxyl groups of the β-hydroxyalkylamide and the carboxyl groups of the binder resin is between 0.8: 1 and 1: 1. 3) Composition according to claim 1 or 2, characterized in that the acrylic polymer has an acid number ranging from 80 to 180 mg of KOH per gram of solid polymer. 4) Composition according to one or more of the preceding claims, characterized in that the acrylic polymer has an acid number ranging from 90 to 150 mg KOH per gram of solid polymer. 5) Composition according to one or more of the preceding claims, characterized in that the acrylic polymer is the product of a polymerization reaction of monomers selected from the group consisting of acrylic acid, methacrylic acid and their esters. 6) Composition according to one or more of the preceding claims, characterized in that the acrylic polymer is the product of a copolymerization reaction of acrylic monomers with vinyl monomers. 7) Composition according to the preceding claim, characterized in that the vinyl monomers are a styrene monomer. 8) Composition according to one or more of the preceding claims, characterized in that the acrylic polymer has a glass transition temperature (Tg) ranging from 0 to 60 ° C. 9) Composition according to one or more of the preceding claims, characterized in that β-hydroxyalkylamide is selected from the group consisting of bis [N, N-di (β-hydroxyethyl)] adipamide, bis [N, N-di (β-hydroxypropyl)] succinamide , bis [N, N-di (β-hydroxyethyl)] azelamide, bis [N, N-di (β-hydroxypropyl)] adipamide, bis [N-methyl-N- (β-hydroxyethyl)] oxamide and their mixtures . 10) Use of a composition according to claim 1 to coat a surface of a metal container for food. 11) Metal container for food having a surface coated with a composition according to claim 1. 12) Coating film applied to a surface of a metal food container.