KR19990044235A - Powder coatings and their use for internal coating of packing containers - Google Patents

Abstract

The present invention

1) at least one epoxy resin having an epoxide equivalent of 300 to 5500, at least one curing agent having a hydroxyl equivalent of 100 to 500 to one or more phenolic hydroxyl groups and phenolic OH groups per molecule, and

2) preferably comprises up to 2% by weight of lubricant;

a) at least 90% by weight of the powder coating particles have a particle size of 1 to 60 μm,

b) the maximum particle size of the powder coater particles is 100 μm or less relative to at least 99% by weight of the particles,

c) the average particle size of the powder coating particles is 5-20 탆,

d) having a particle size distribution such that the slope of the particle distribution curve at the bending point is at least 100,

A powder coating based on epoxy resins and phenolic curing agents for the internal coating of packing containers.

Description

Powder coatings and their use for internal coating of packing containers

Packing containers, such as cans for canning, two- and three-component beverage cans, etc., on the one hand prevent deterioration of the contents due to elution of the sheet metal components, and on the other hand, corrosion of the sheet metal by corrosive contents. In order to avoid the coating layer is provided therein.

In practice, the packing containers are coated in this way mainly with organically dissolved coatings. However, this increases the environmental pollution by the solvent during the drying period of the coating film. Thus, more and more attempts have been made to replace these coatings with low-solvent or solvent-free coatings. For example, thermoplastic powder coatings are often already used to cover the weld seam of cans. These products are made from the corresponding thermoplastics by costly low temperature grinding.

Thermosetting powder coatings for coating weld seams of metal containers used for holding food or beverages are known from EP-B-119164. These thermosetting powder coatings include a mixture of aromatic epoxy resins having an average of two or less epoxide groups per molecule and binders having an average of two or more epoxide groups per molecule as a binder. As the curing agent, condensation products of bisphenol A or bisphenol A or acid polyesters having an equivalent of 220 to 280 based on phenol hydroxyl groups or mixtures thereof are used.

These powder coatings are only used to coat the weld seam of the packing vessel. EP-B-119164 does not disclose the use of these powder coatings for internal coating of packing containers or to modify these powder coatings for use as internal protective coatings. In particular, EP-B-119164 does not disclose any information on the particle size and particle size distribution of powder coatings. However, the use of the powder coating of EP-B-119164, which has a typical particle size distribution in powder coatings, results in coatings having too large porosities at small coating thicknesses of 15 μm or less, which is typical for internal coating finishing.

Epoxy resins having at least one 1,2-epoxide group per molecule and polyols having at least one phenol OH group per molecule, as well as other conventional auxiliaries and additives, for the coating of bodies, machines, plants and containers of motor vehicles. Thermosetting solvent-containing and solvent-free coatings are also known from German Patent No. 23 12 409.

For the production of automotive topcoats, powder coatings having a particle size of 0.044 mm or less are applied in a dry-film thickness of 25 μm according to German Patent No. 23 12 409. No particle size or particle size distribution of powder coated particles has been reported for powder coatings suitable for the preparation of the inner coating of a packing container. German patent 23 12 409 also does not disclose that the particle size and particle size distribution must be specifically adjusted according to the desired use of the powder coating. However, powder coatings having a maximum particle size of 44 μm and a typical particle size distribution are not suitable for forming the inner coating layer of the packing container with a typically small coating thickness of 15 μm or less because of their porosity.

Powder coatings for the inner coating of cans, including epoxy resins and curing agents, are also disclosed in US Pat. No. 3,962,486. U.S. Patent No. 3,962,486 refers to catalyst curing agents, aromatic amines, epoxy-amine adducts and acid anhydrides as typical curing agents, while phenolic curing agents are not mentioned. Coatings that meet the requirements typically imposed on the inner coatings of food packing containers, even with small coating thicknesses of less than 13 μm, can be prepared by the plasma spray coating method. In order to allow application by the plasma spray method, only powder coatings having a maximum particle size of 100 μm or less and sufficiently low melt viscosity can be used. However, the particle size distribution of the powder coatings used is not characterized in more detail in US Pat. No. 3,962,486.

A disadvantage of the powder coatings disclosed in US Pat. No. 3,962,486 is the inadequate disinfection of the resulting coatings resulting from the use of amine based curing agents. It is also a disadvantage that epoxy resins cured with amines tend to embrittle and have very poor elasticity. Acid anhydride curing agents have the disadvantage that they are very irritating, requiring special safety precautions during the manufacture of powder coatings.

Powder coatings for the internal coating of cans, likewise comprising epoxy resins and amine curing agents, are also known from US Pat. No. 4,188,974. These powder coatings have an average particle size of 1 to 100 μm, preferably 1 to 10 μm. The resulting coating already exhibits the low porosity required at a coating thickness of 13 μm or less, but this time an improvement in sterilization resistance is required. It is also a disadvantage that the epoxy resin cured by the amine tends to embrittle and have very poor elasticity.

Moreover, powder coatings which are based on epoxy resins and which are suitable for both the inner coating of the packing vessel and the coating of the weld seam after appropriately adjusting the particle size distribution of the powder coating particles are known from German patent application P 40 38 681.3. These powder coatings contain polyesters containing carboxyl groups as curing agents. The use of phenolic curing agents is not disclosed in this patent application.

Finally, powder coatings based on epoxy resins and phenolic curing agents are known. Such coatings are suitable for coating packing containers. However, the properties are still not considered satisfactory, in particular in terms of flexibility, transparency of the coating layer and processing speed.

The present invention relates to powder coatings for the internal coating of packing containers, based on powder coatings, in particular epoxy resins and phenolic curing agents, and methods for their preparation.

The invention also relates to a method of internal coating of packing containers and to the use of powder coatings.

Accordingly, the present invention is based on the object of providing a powder coating that, when used for the inner coating of a packing container, meets the requirements normally imposed on the inner coating of the can, even when applied with a small coating thickness of 15 μm or less. . In particular, these internal coatings should not be porous (measured using so-called enamel evaluation tests), should exhibit good adhesion to the substrate, have high elasticity, and be stable under conventional pasteurization and sterilization conditions. do. In this case the powder coating should typically be curable over a short drying time in the coating of the can.

This object is surprisingly achieved by powder coatings based on epoxy resins and phenolic curing agents, in particular for the inner coating of the packing container, wherein the powder coating agent

1) at least one epoxy resin having an epoxide equivalent of 300 to 5500, at least one curing agent having a hydroxyl equivalent of 100 to 500, based on at least one phenolic hydroxyl group and phenolic OH group per molecule, and

2) preferably comprises up to 2% by weight of lubricant;

a) at least 90% by weight of the powder coating particles have a particle size of 1 to 60 μm,

b) the maximum particle size of the powder coating particles is 100 μm or less relative to 99% by weight of the particles,

c) the average particle size of the powder coating particles is 5-20 탆,

d) has a particle size distribution such that the slope of the particle distribution curve at the bending point is at least 100.

The invention also relates to a method of internal coating of a packing container, applying such a powder coating.

Finally, the present invention also relates to the use of powder coatings for internal coating of packing containers.

It is surprising and unpredictable that the property profiles of epoxy resins and phenolic curing agents, and lubricant coatings based on lubricants, and thus their desired use, can be specifically controlled by defining specific particle size distributions. Powder coatings according to the invention can also be cured quickly, are easy to handle and apply, have good processing speed, good flexibility and transparency after coating.

Moreover, the powder coating according to the invention is characterized by the fact that a coating layer with a very small coating thickness of 15 μm or less has the properties that can manufacturers require for internal coating. In particular, these coatings have the desired small porosity even at small coating thicknesses of 15 μm or less. Moreover, these coatings are characterized by good adhesion, high flexibility, and good pasteurization and sterilization resistance.

Now, the individual components of the powder coating according to the invention will be explained in more detail below.

The epoxy resin used in the powder coating of the present invention is a solid epoxy resin having an epoxide equivalent of 300 to 5500. Aromatic, aliphatic and / or cycloaliphatic epoxy resins are suitable. Preference is given to using aromatic epoxy resins based on bisphenol A and / or bisphenol F and / or epoxy resins of the novolak type. Particularly preferably used bisphenol A or bisphenol F-based epoxy resins have an epoxide equivalent of 500 to 2000. Particularly preferably used novolak type epoxy resins have an epoxide equivalent of 500 to 1000. In this regard, bisphenol A or bisphenol F-based epoxy resins generally have up to two functional groups, and novolak type epoxy resins generally have two or more functional groups. However, bisphenol A or bisphenol F-based epoxy resins may also have two or more functional groups by branching, for example using trimethylolpropane, glycerol, pentaerythritol or other branching agents.

Other epoxy resins, such as alkylene glycol diglycidyl ether or branched secondary products thereof, bisphenol A or bisphenol F-based epoxy resins made flexible by alkylene glycol, and the like, can of course also be used. Mixtures of the various epoxy resins mentioned above are also suitable.

Examples of suitable epoxy resins include those sold under the following trade names: Epikote® 154, 1001, 1002, 1055, 1004, 1007, 1009 and 3003-4F-10 (manufactured by Shell-Chemie), XZ 86 795 and DER® 664, 667, 669, 662, 642U and 672U (manufactured by DOW), and Araldit®, GT 6064, GT 7072, GT 7203, GT 7004, GT 7304, GT 7097 and GT 7220 (Manufactured by Ciba Geigy).

FDA-approved epoxy resins are preferably used in the present invention.

Suitable curing agent components are all one, preferably 1.8 to 4, particularly preferably up to 3 phenol OH groups per molecule, particularly preferably from 1.8 to 2.2 phenol OH groups per molecule, and from 100 to based on said phenol OH groups 500, preferably a solid compound having a hydroxyl equivalent of 200 to 300.

Bisphenol A and / or bisphenol F-based curing agents are preferably used as the curing agent. Particular preference is given to condensation products of diglycidyl ethers of bisphenol A or bisphenol F with bisphenol A or bisphenol F, in particular condensation products having an equivalent of 220 to 280 based on phenol hydroxyl groups. These condensation products are usually prepared by reacting an excess of bisphenol generally with bisphenol diglycidyl ether in the presence of a suitable catalyst. Preferably, the condensation product is prepared by reacting diglycidyl ether with bisphenol in a weight ratio of 0.5 to 2. These curing agents based on the condensation product of the bisphenol diglycidyl ether and bisphenol generally have up to two functional groups and may have more functional groups by branching agents.

Furthermore, reaction products of bisphenols with novolac type epoxy resins are also suitable as curing agents. These curing agents are preferably prepared by reacting an epoxy resin in the presence of a suitable catalyst at a weight ratio of 0.5 to 2 with bisphenol.

For example, phenolic curing agents disclosed in DE-PS-23 12 409, column 5, line 2 to column 6, line 55 are suitable. These polyphenols correspond to the formulas

In the above formulas,

A is a divalent hydrocarbon radical or radical having 1 to 6 carbon atoms

ego,

X is hydrogen or alkyl of 1 to 4 carbon atoms,

n is on average 1 to 9, preferably 2 to 7,

y is 0 or 1.

Furthermore, the phenolic curing agents disclosed in DE-A-30 27 140 can also be used.

Curing agents modified with branching agents and / or flexible curing agents are of course also suitable. Mixtures of the various curing agents mentioned above may also be used. FDA-approved hardeners are preferably used in the present invention.

The epoxy resin component is usually used in the powder coating of the present invention in an amount of 29 to 80% by weight, preferably 39 to 60% by weight, based on the total weight of the powder coating. The curing agent component is usually used in the powder coating of the present invention in an amount of 10 to 50% by weight, preferably 15 to 40% by weight, based on the total weight of the powder coating.

The powder coating of the present invention comprises as an additional component one or more curing catalysts in an amount usually of from 0.01 to 5.0% by weight, preferably from 0.05 to 2.0% by weight, based on the total weight of the powder coating.

The catalyst is imidazole, 2-methylimidazole, ethyltriphenylphosphonium chloride or another salt thereof, for example a quinoline derivative as disclosed in EP-B-10805, primary, secondary or tertiary aminophenols. , Aluminum acetylacetonate or toluenesulfonic acid salts, or mixtures of various these catalysts are advantageous.

Commercially available curing agents containing hydroxyl groups usually already already contain a curing catalyst.

Examples of such commercially available hydroxyl group-containing curatives which are preferably used are commercially available products having the following trade names: DEH® 81, DEH® 82 and DEH® 84 (Manufactured by DOW), curing agent XB 3082 (manufactured by Ciba Geigy), and Epikure® 169 and 171 (manufactured by Shell-Chemie).

Furthermore, the powder coating of the present invention may comprise 0 to 55% by weight, preferably 15 to 25% by weight of filler. FDA-approved fillers are preferably used.

Inorganic fillers such as titanium dioxide such as Kronos 2160 (Kronos Titan), Rutil R 902 (Du Pont) and RC 566 (Sachtleben), barium sulfate, and silicate based fillers such as talc, kaolin, magnesium silicate Aluminum, mica, etc. are usually used. Titanium dioxide and fillers of quartz sand type are preferably used.

Further, the powder coating of the present invention optionally comprises from 0.01 to 10% by weight, preferably 0.1 to 2% by weight, of other auxiliaries and additives, based on the total weight of the powder coating. Examples of these include leveling agents, flow aids, air inhibitors such as benzoin, pigments and the like.

For use in the inner coating of the packing vessel, the particle size distribution is adjusted such that at least 90% by weight of the powder coater particles have a particle size of 1 to 60 μm, ie d 90 = 1 to 60 μm. Preferably 90% by weight of the powder coating particles have a particle size of 1-40 μm (d 90 = 1-40 μm), particularly preferably 5-25 μm (d 90 = 5-25 μm). The maximum size of the powder coater particles is 100 μm or less, preferably 60 μm or less, particularly preferably 40 μm or less, relative to 99% by weight of the particles. The average particle size of the powder coating particles is 5-20 μm, particularly preferably 5-12 μm. Furthermore, when the powder coating agent is used for the inner coating of the packing container, it is preferable to adjust the particle size distribution such that the slope S of the particle distribution curve at the bending point is 100 or more, preferably 150 or more, particularly preferably 200 or more. Is essential. In order to obtain a coating with particularly good properties, powder coatings with a slope S of a particle size distribution curve at the bending point of 300 or more are particularly preferably used.

In the present invention, the slope S is a value defining f (x ₂ ) -f (x ₁ ) at the bending point of the particle distribution curve is (f (x ₂ ) -f (x ₁ )) / log ((x ₂ / x ₁ )) = defined as tending to zero. In the present invention, the particle distribution curve is a graph of accumulation weight% (f (x)) relative to absolute particle diameter (x), where the particle diameter is plotted on an algebraic scale, and the accumulation weight% is plotted on a primary scale. It is. For use as an inner coating on a packing container, it not only has a small proportion of very fine particles (particle size <5 μm) but also a very small proportion of coarse powder coating particles (particle size> 25 μm), ie as narrow as possible. Particularly suitable are powder coatings having a particle size distribution.

Specific particle size distribution of powder coatings, using suitable grinding units, optionally with suitable shifting and sifting devices, for example fluid bed countercurrent grinder with turboflex extra-fine shifter (Alpine, Augsburg) Adjust using (AFG, Alpine, Augsburg).

Also essential to the invention is the addition of lubricants. This lubricant is added in an amount of less than 2% by weight according to the invention. Preference is given to additions of less than 1% by weight. Optimum proven range is 0.1 to 0.6% by weight. Lubricants used are propylene waxes, modified polyethylene waxes and acid amides.

According to the invention, preference is given to using propylene wax in finely divided form. Such waxes are especially described in Hoechst-Wachs pp. 230, Hoechst-Wachs C, Hoechst-Wachs C Mikropulver PM and Hoechst-Wachs R 31 are commercially available.

Polyethylene waxes are generally polytetrafluoroethylene-modified waxes. These are in particular commercially available as Hoechst-Wachs PED 121, 191, 136, 153, 261, 521, 522, and also Hostalup H 12 and Hostalup H 22.

Acid amides which can be used according to the invention are especially erucamides and oleamides.

Surprising properties are achieved with the addition of lubricants according to the invention, in particular when powder coatings are used for the inner coating of the packing container. The coatings obtained have especially high flexibility and surface scratch resistance. Corrosion resistance is also improved. Finally, the material according to the invention shows excellent transparency as a coating. Furthermore, it is also possible to crosslink within 10 seconds to several minutes at a temperature of 21 to 350 ° C.

Powder coatings are prepared by known methods, for example by homogenization and dispersion using extruders, screw kneaders and the like (for example compare with product information from BASF Lacke & Farben AG, "Pulverlacke", 1990). . After the preparation of the powder coatings, it is essential to the present invention to adjust them to a particle size distribution suitable for the intended use by grinding and optionally shifting and sieveing.

According to the invention, the lubricant is preferably extruded and ground together with the powder coating material. This process makes it possible to obtain particularly good property profiles according to the invention. On the other hand, if desired, grinding may be followed by shifting and application by corona unit or tribo-electric charging. The production and application of powder coatings in the manner described yields surprisingly high levels of matting, surface hardness, flexibility and bonding enhancement.

The packing container coated with the powder coating according to the present invention may be made of a wide variety of materials, may have a wide variety of sizes and shapes, and may be produced by various processes. However, metal containers are especially coated with the powder coating according to the invention. Such metal containers may be made by first rolling the sheet metal and then folding the rim back to join. The final pieces can then be attached to the cylinder formed as above. Powder coatings according to the invention are generally used for the internal coating of can bodies which already have a base. Deeply drawn metal containers may also be internally coated with the powder coating according to the invention. However, of course, the powder coating is also suitable for coating can lids and can bases.

Packing containers can be made of a wide variety of materials, such as aluminum, black sheets, tin sheets and various iron alloys provided with passivating coatings based on nickel, chromium and tin compounds as the case may be.

Containers of this type are usually used not only as containers for food and beverages, such as beer, juice, soda, soups, vegetables, meat, fish and vegetables, but also as containers for animal feed, for example.

Application of the coating is carried out by known methods, for example by the method disclosed in US Pat. No. 4,183,974. Electrostatic filling of powder coated particles is carried out by friction (triboelectric) in the present invention. Powder coated particles are applied using special spray heads known to those skilled in the art. It is of course also possible that the powder coatings according to the invention can also be applied by known electrostatic support.

For the inner coating of the packing container, powder particles are usually applied at a coating thickness of 15 μm or less, preferably 10 to 14 μm.

Even at such small coating thicknesses, these coatings meet the requirements imposed on such films. However, it is of course also possible to apply powder coatings to thicker coating thicknesses.

Subsequently, a heat treatment for curing the powder coating is applied to a packing container provided with the powder coating according to the present invention. Such heat treatment can be performed in a variety of ways. In practice, the containers are often moved through tunnel ovens for this purpose. During this process, the powder coating typically cures completely at a target temperature of 230 to 350 ° C. for 5 to 30 seconds. Tunnel ovens in the present invention may be operated at constant temperature or have a temperature profile that is adjusted to suit a particular situation.

The invention will now be described in more detail by way of examples. All parts and percentage data in the examples are by weight unless otherwise indicated. The powder coating is metered in each case to all the ingredients in the vat and premixed in a premixer, the mixture is homogenized at 60 to 80 ° C. using an extruder and cooled as quickly as possible, which is then Prepared by adjusting to the desired particle size distribution.

Example 1

The following components were processed to yield powder coating 1:

580 parts of an epoxidized novolak resin (commercially available from Dow D.E.R® 642U) having a commercially available EEW 500,

270 parts of a commercially available curing agent, based on bisphenol A having a hydroxyl equivalent of 250, based on OH group-containing curing agent (commercially available from Dow, D.E.H (R) 82),

5 parts of commercially available equalizers based on oligomeric acrylates,

143 parts of finely divided silicate filler of quartz sand type, and

2 parts of fluidization aids based on pyrogenic silica or aluminum oxide.

This powder mixture is extruded and ground with erucamide. In two experiments, amounts were added in amounts of 0.2% and 0.5% by weight.

Using a grinding unit, the particle size distribution was adjusted so that at least 90% by weight of the powder coater particles had a particle size (d 90 = 1-25 μm) of 1-25 μm. The maximum particle size is 100 μm or less for at least 99% by weight of the particles, with an average particle size of 9 μm. The slope S is 250 at the bending point of the particle distribution curve.

Ten tin panels were coated by corona filling with a) a powder coating containing 0.2% erucamide and b) a powder coating containing 0.5% erucamide, and the coating layer was crosslinked in an oven at 220 ° C. for 3 minutes. Combined. Club cans, ie fish preservation cans, were stamped from the panel and sterilized for 30 minutes at 128 ° C. The media tested were water, 2% concentration of lactic acid solution, and 3% concentration of acetic acid solution. Test results showed that the powder coating without erucamide cracked (fragile) upon punching and gave a milky appearance. The can also exhibited corrosion. Powder coatings with 0.2% lubricant are highly flexible and suitable for punching, but have a milky appearance due to the 2% concentration of lactic acid solution. In the case of 0.5% of lubricant, there was no problem with this property.

The results are shown in Table 1 below.

Sterilization: 30 minutes at 128 ℃ No grease + 0.2% erucamide + 0.5% erucamide Out of H ₂ O ++ ++ ++ Lactic acid at 2% concentration + + ++ 5% concentration of lactic acid + ++ ++

Claims (14)

1) at least one epoxy resin having an epoxide equivalent of 300 to 5500, at least one curing agent having a hydroxyl equivalent of 100 to 500, based on at least one phenolic hydroxyl group and phenolic OH group per molecule, and 2) preferably comprises up to 2% by weight of lubricant; a) at least 90% by weight of the powder coating particles have a particle size of 1 to 60 μm, b) the maximum particle size of the powder coater particles is 100 μm or less relative to at least 99% by weight of the particles, c) the average particle size of the powder coating particles is 5-20 탆, d) having a particle size distribution such that the slope of the particle distribution curve at the bending point is at least 100, Powder coating based on epoxy resin and phenolic curing agent for internal coating of packing containers. The method of claim 1, a) at least 90% by weight of the powder coating particles have a particle size of 1 to 40 μm, b) the maximum particle size of the powder coater particles is 60 μm or less relative to 99% by weight or more of the particles, c) the average particle size of the powder coating particles is 5-12 μm, d) a powder coating having a particle size distribution such that the slope of the particle distribution curve at the bending point is at least 150. The method of claim 1, a) at least 90% by weight of the powder coating particles have a particle size of 5 to 25 μm, b) the maximum particle size of the powder coating particles is 40 μm or less relative to 99% by weight or more of the particles, c) the average particle size of the powder coating particles is 5-12 μm, d) a powder coating having a particle size distribution such that the slope of the particle distribution curve at the bending point is at least 200. The method according to any one of claims 1 to 3, A component coating as component A comprising an epoxy resin based on bisphenol A and / or bisphenol F and / or an epoxidized novolak resin. The method according to any one of claims 1 to 4, Component A, characterized in that it comprises an epoxy resin based on bisphenol A and / or bisphenol F having an epoxide equivalent of 500 to 2000 and / or a novolak type epoxy resin having an epoxide equivalent of 500 to 1000. Powder coating made. The method according to any one of claims 1 to 5, Component B, comprising a curing agent having a hydroxyl equivalent of 200 to 300 based on phenol OH groups. The method according to any one of claims 1 to 6, Component B, comprising a curing agent having from 1.8 to 4, preferably up to 3, phenolic hydroxyl groups per molecule. The method according to any one of claims 1 to 7, A component coating, comprising component B, a curing agent based on bisphenol A and / or bisphenol F. The method according to any one of claims 1 to 8, A) 29 to 80% by weight of epoxy resin component A, based on the total weight of the powder coating, and B) 10 to 50% by weight of curing agent component B, based on the total weight of the powder coating. . The method according to any one of claims 1 to 9, Powder coating, characterized in that the proportion of the lubricant is 1% by weight or less, preferably 0.1 to 0.6% by weight. The method according to any one of claims 1 to 10, Powder coating, characterized in that the lubricant used is acid amide, polypropylene wax, modified polyethylene wax. 1) at least one epoxy resin having an epoxide equivalent of 300 to 5500, at least one curing agent having a hydroxyl equivalent of 100 to 500, based on at least one phenolic hydroxyl group and phenolic OH group per molecule, and 2) preferably comprises up to 2% by weight of lubricant; a) at least 90% by weight of the powder coating particles have a particle size of 1 to 60 μm, b) the maximum particle size of the powder coater particles is 100 μm or less relative to at least 99% by weight of the particles, c) the average particle size of the powder coating particles is 5-20 탆, d) extruding and pulverizing the powder coating having a particle size distribution such that the slope of the particle distribution curve at the bending point is equal to or greater than 100, A process for the preparation of powder coatings based on epoxy resins and phenolic curing agents for the internal coating of packing containers. Method for coating the interior of a packing container, characterized in that the powder coating according to any one of claims 1 to 9 is applied at a coating thickness of 15 μm or less. Use of a powder coating according to any one of claims 1 to 9 for the inner coating of a packing container.