FR2499092A1 - COATING FOR METAL, EMBROIDABLE AND SULFUR RESISTANT - Google Patents

Abstract

A precoating material is described for a metallic blank which can be shaped by repeated drawing to give a container having the precoating material on the inside. This precoating material contains a phenolic resin, an epoxy resin, a vinyl chloride/vinyl acetate copolymer and a polyvinyl chloride dispersion resin in a solvent system containing saturated solvents, so that sulphides of low molecular weight, such as sulphur-containing compounds, cannot react to form additional products. The container provided with a precoating material of this type is corrosion-resistant and, if used for preserving foodstuffs, does not have an adverse effect on the taste of the foodstuff.

Description

 For many years, foods such as vegetables, fruits and soups are packaged in boxes of steel or tinplate. These boxes were made by the so-called three-piece manufacturing technique. In other words, they are made by shaping from sheet metal a cylindrical body whose abutting edges are joined by brazing or welding. A flange is then formed on the cylindrical body to which a lower bottom is fixed by the crimping technique. The other bottom is set by the boxing company after filling the box.

 The three-piece metal boxes are coated with protective enamels to prevent their corrosion, especially when it comes to steel tin-free (AEE). It is also possible to manufacture tin-plated steel boxes and these can be coated or not with protective enamels depending on the type of foodstuff to be packaged.

 Care protective enamels for preserves are carefully chosen so that they provide the required corrosion resistance and, furthermore, they do not tend to give a taste to the food product. Among the enamels commonly used are phenolic resins, epoxy resins, amine resins, oleoresinous materials such as Chinese wood oil and vinyl-type resins, such as polyvinyl chloride or vinyl chloride-vinyl acetate copolymers. These enamels are prepared in specific solvent mixtures such as ketones, esters, alcohols and aromatic or aliphatic hydrocarbons so that their application is easy and fast. They are applied by well known coating techniques, for example by roller or projection.

The coatings are usually of the thermosetting type and are reacted or cooked in conventional baking ovens.

 The coatings applied to three-piece boxes do not undergo any mechanical deformation during the box-making process. For this reason, it is possible to prepare the enamels of the type for three-piece boxes from very varied resin components such as to bake highly cross-linked structures by baking. This crosslinking makes it possible to obtain maximum protection against corrosion and resistance to chemical agents, which ensures the protection of the packaged food product in the boxes.

 For two-piece metal boxes manufactured by stamping several passes of flat metal blanks, it is necessary to take into account various criteria in the choice of enamels. For reasons of economy, it is preferable to pre-apply the enamel on the blank and then convert it into a bowl by forming in a suitable stamping machine. The forming subjects the enamel to severe mechanical stresses that test its adhesion to the metal and its extensibility or ability to flow without breaking.

For this reason, the formulation of enamels for stamped boxes is difficult to establish and it is necessary to carefully choose the mixtures of resins and solvents adopted, so that they form baking strongly adherent films with the appropriate degree of crosslinking for to provide good resistance to corrosion and chemical agents, but with a greater degree of flexibility than comparable three-piece can-enamels to undergo the forming operation without breaking.

 To properly adjust the degree of crosslinking of these enamels for stamped cups, the resin dose can be adjusted to cross-link or lower the cooking temperatures a little compared to those of three-piece box enamels. The result is to obtain enamel coatings a little more soluble in organic solvents, after firing, than the corresponding enamels for three-piece boxes. Otherwise, it can remain in the enamels for stamped boxes, after cooking, small amounts (parts per million) of some solvents. These solvent residues may be released after canning the product, giving the latter an unpleasant taste. Sometimes, the solvent is relatively harmless if it is present inside the filled bowl. A solvent can be in itself harmless, but likely to give by reaction with the food product a very unpleasant flavor and aroma.

 Metal cans manufactured by multi-pass stamping of pre-coated tin-free steel sheet can carry various types of thermosetting organic coatings containing phenolic resins, epoxy resins, acrylic resins, amine resins, polyvinyl chloride resins and the like. their copolymers, polyester resins. The inside and outside of each sheet metal or blank is coated, the inside and the outside being the faces which will be respectively inside and outside the box.

 In the manufacture of three-piece tin cans, the coating of the sheet of steel sheet, either tin-free or tin-plated, is carried out in a separate operation. Whereas the three-piece cans may have to be coated on both sides, the inside and the outside, it is usually necessary to provide several coating operations to ensure the finished box adequate protection against corrosion. For example, the outer coating is applied in one operation and the inner liner, which may include one or two layers, requires a separate layered operation. It may therefore take three passes through a coating device to make a three-piece box. The coated box can then be divided into body blanks by a slitting operation succeeding those of coating. The body blanks must be transferred to a body-making machine which places them in a cylindrical form and operates brazing or welding along the lateral seam. During a final operation, a box bottom is crimped on the bottom of the cylindrical body. These numerous operations considerably increase the time and cost of manufacturing three-piece boxes.

 The manufacture of cans by the more recent process of stamping in multiple passes or multiple stamping of two-piece can from pre-coated sheet metal allows a sharp reduction in the cost price because some of the operations of manufacturing boxes in three parts described above are either merged or deleted. The sheet to be converted into two-piece boxes by multiple stamping can be coated in the wound state. This method of coating makes it possible to coat both the inside and outside faces of the sheet in a single operation, which reduces manufacturing costs.

 Cans can be made by multiple stamping from either tin free steel or electrolytic tinplate. For reasons of economy, most tin cans are made from tin - free steel. The tin-free steel sheet may be of any thickness, but it is customary to use the American continuous-casting type with a filling of 34 kg per base box, T-4, or a basis weight of 29.5 kg. by "base-box", DR9.

"Base-box" means a unit of measurement, used by the
2 cans of tin cans, and representing about 20 m of area (on one side) for a given weight of metal.

T-4 and DR9 are designations of hardness used in steelmaking in the United States and from which one can deduce the rolling mode adopted to manufacture the sheet.

 To form multi-stamped boxes from pre-coated sheet metal, careful selection of the organic coating materials is required. Because the sheet is pre-coated, its coating undergoes during the forming of the box the same operations of drawing and shaping as the metal. As a result, the coating must adhere well to the metal substrate and have the desired mechanical properties to withstand drawing and forming without being broken or peeled off from the substrate. The multi-stamped shaped cups are distinguished from the three-piece ones. in that they comprise a bottom in one piece and no lateral seam. This results in a saving on the cost price by eliminating the soldering operation or welding of the side seam, that of bead coating of the side seam and that of crimping the bottom. In addition, the stamping process is implemented continuously. In other words, the pre-coated sheet arrives at a multiple stamping press. Finished stampings coming out of the press continuously reach machines to form the edges, to form the beads and to carry out the test with compressed air, then are deposited on pallets. The continuous implementation of the multi-drawing forming process removes a lot of labor during manufacturing, which provides additional savings.

 Given the rigorous mechanical forces imposed on the coating during the stamping in several passes, one might think that rubbery materials or very flexible polymers are more suitable for the pre-coating stamped boxes. In fact, while the rubbery materials are well suited for stretching when forming multi-stamped cans, their chemical resistance and adhesion properties are generally lower than those of thermosetting coatings. We must therefore aim at a medium term between the flexibility required by the multiple stamping and the chemical resistance necessary to protect the metal against the food packaged in the box. Another difficulty raised by the rubber is that the taste conferred by the agents Vulcanization is likely to be absorbed by food packaged in metal cans pre-coated with rubber. For this reason, such materials can not be used for coating metal boxes.

 Coatings with a good fluidity when they are applied to a metal surface are able to provide the metal with a continuous cover substantially free of frizz or blistering. This is particularly important for the packaging of foodstuffs, because any part not coated with the box offers a location of corrosion that can cause a perforation of the box at its level. The deterioration of the coating usually occurs in the 25 mm high area nearest the rim, because it is known that it is in the area of the rim that the metal undergoes the greatest efforts of axial stretching and peripheral compression. A frequently used method for testing the degree of metal coverage by the coating is the rapid enameling evaluation test. This test is carried out with a special test instrument and essentially consists of measuring the electrical conductivity of the coating. inside the box filled with an electrolyte.

 To perform a quick test, you need a particular instrument, namely a WACO Model 1071 Email Tester with 0 to 1 mA accessory. This instrument comprises a mobile electrode vertically along the axis of a bolte placed below it to enter the box and out. The electrode is placed about 25 mm from the bottom of the box. The box is held in place by a vice-like device which surrounds it around its bottom, holding it so that its open end is directed upwards towards the electrode. The box is filled with a 2% solution of sodium sulfate which is allowed to soak for at least 30 seconds before allowing the electrode to fall into the can. The temperature of the solution is to be maintained between 22 and 26 "C, and the box must be filled so that once the electrode is in the low test position, the solution arrives at about 3.2 mm below the rounded edge of the top rim of the box. Carefully avoid wetting the rim, which would lead to a false high result.The milliammeter of the tester is connected to the device of the kind that holds the bottom of the box.The electrode is connected to another milliammeter. adjust the zero of the instrument and the operator adjusts the milliammeter so that "T" is displayed on the scale, and soon after the zero adjustment, a light is illuminated and the result should be read immediately. we apply this method to b If properly pre-coated, the result should be between 0 and 8 mA, such a result indicating that the bolte is acceptable.

 It was therefore a question of a coating system for metal substrate capable of withstanding without damage the rigorous efforts of multiple stamping. The object of the invention is to provide a coating system which protects the metal substrate, prevents corrosion and is free of taste.

The invention aims
to propose a means of manufacturing by multiple stamping a metal box coated with an enamel which is substantially free of taste;
- To manufacture such a stamped box from pre-coated sheet, for example tin-free steel sheet (AEE) or tin-plated steel (AE);
to provide the most economical sheet metal coating method by using coatings of the desired composition to be applied by roller coating either directly or inverted;
providing an intermediate thermosetting coating formulation having the desired solvent release power to be applied to the sheet by either a moderate temperature sheet coating process or a higher temperature coil coating process; ;
- to describe a mixture of resinous precoat that provides good protection against corrosion of cans filled with a variety of different food products;
- To describe a mixture of resinous coating likely to withstand the mechanical stresses and strains generated by the multiple stamping without breaking or detaching;
to provide a solvent system vis-à-vis the e-mail, which has the desirable properties for applying the coating on the sheet, but does not contain chemicals conferring an undesirable taste to the food product, nor solvents capable of reacting with chemicals, such as hydrogen sulfide, released by the food product, such chemical reactions causing the formation in the food of unpleasant or harmful taste by-products; ;
finally, to describe a mixture of resinous enamel and of solvent having the desired properties for application to sheet metal and of being able to be rid of the quasi-solvent of the solvent during the cooking, so that, when the pre-coated metal undergoes passive reheating during the multi-drawing process, there is no release, from the enamel, residual traces of solvent, or other small molecules from catalyst residues or plasticizer, this which would give a strange taste to the packaged food product.

 In order to solve the problems posed by the prior art manufacturing of cans and in order to achieve the above-mentioned objects, the present invention proposes an original combination of materials making it possible to transform by multiple stamping the pre-coated sheet into an interesting finished box and of good quality.

 Due to the mechanical stretching and shaping forces exerted during the manufacture of multi-stamped cans, the coatings of the metal substrate must have particular characteristics. There is a need for an internal and external coating system that is suitable for multiple stamping.

An outer coating is applied by coating either to the roll or coils, and is baked. If it is to coat the sheets, the coating is fired at a temperature of 149 to 2040C for about 6 to 10 minutes. The coating film is usually applied to the metal substrate at a rate of about 0.31 to 0.58 mg, and preferably about 0.35 to 0.46 mg per square centimeter of sheet area. This coating can be of various chemical types, such as vinyl organosol, epoxy resin, amine resin, phenolic resin or suitable mixtures of these resins.

The coating rate is generally lower inside than inside, for reasons of economy and because there is less risk of corrosion on the outside.

 The liner is generally applied at a rate of about 0.58 to 1.36 mg per square centimeter of sheet area. It can be applied on sheets or on coils. The firing is generally carried out at a temperature of 149 to 2040C for 8 to 10 minutes for a coating of sheets. In addition, the sheet coatings can be applied in addition to one operation. For example, for a stamping box in a single pass, it is possible to first apply a base coating, for example of the epoxy type. The second application may be that of a coating of a different chemical type, such as an enamel oleoresin. This flexibility allows the box manufacturer to apply the best enamel system to the metal adhesion and corrosion resistance of the packaged food product.

 The liner can also be applied to the metal substrate by coil coating. In this case, the coatings both inside and outside are applied simultaneously. This technique saves costs, because the entire coating process is done in a single operation. For coil coating, the baking takes place at a cooking temperature of 260 to 3160C for a time of about 20 seconds, just sufficient to remove up to the last trace of solvent, since the inner or outer coating has its formulation 75% of solvent.

 In view of the severe mechanical deformations that they suffer, the coatings of boxes subjected to a multiple stamping must satisfy particular mechanical requirements as to their adhesion and their mechanical strength. The preferred interior coatings for these boxes are of the organosol type. These coatings contain mixtures of phenolic resin, epoxy resin, vinyl solution resins of the vinyl acetate-vinyl chloride copolymer type, and high molecular weight polyvinyl chloride dispersion resins. The molecular weight range of polyvinyl chloride has been found to be critical for obtaining good inner coatings for multiple stamping. If the resin does not exceed a certain molecular weight, the coating suffers a loss of adhesion or breaks down. level of the upper third of the box during the embossed, so that the behavior of the filled box is unsatisfactory. It has been found that the average molecular weight must be greater than 110,000, and preferably 150,000. The molecular weight is determined by measurements on a gel permeation chromatograph with known pore-size polystyrene gel liners. .

 The mechanical work undergone by the metal pre-coated in the dies of the press causes an increase in the temperature of the metal box being formed. The temperatures of the tools of the press, and therefore of the box, reach 660C at the post of the first stamping pass, but can reach 920C at the post of second stamping pass. The increased temperature softens the coating of the box which, as a result, becomes fluid again. The range of Tg (transformation point, or transition, vitreous) of the coatings is important. Tg should not be large enough to break brittle or weak enough for the coating to soften and cease to be continuous. There is, during the forming process, passive heating, all the more so as multiple stamping is accused. The beneficial effects of heating on the behavior of coatings have been previously described in US Pat. No. 3,206,848 . The fact that the coating becomes fluid again is evidenced by the increased gloss acquired by the outer coating during stamping. It is due to the increased temperatures and pressures.

 Lubrication applied to the coating is another critical aspect in the shaping of multi-stamped boxes. The lubricant allows the slipping necessary during the forming of the pre-coated blank between the tools of the press. In the absence of proper lubrication, the coatings are loosened by the press tools, resulting in defective boxes and risk of damaging the punches and dies of the press. Lubricants such as Boler wax, lanolin or petrolatum can be used. For the multiple stamping of boxes, the petrolatum is the most satisfactory as for the lubrication of the tools and the absence of taste. The lubricant is applied by spraying from standard spray guns or fog generated on the surface of the electrostatically coated coating sheet.

 It is important that the coatings be intact, not only from the point of view of corrosion resistance, but also that of taste. Therefore, the residues of solvents or small molecules present in the film should be kept to a minimum due to incomplete removal during baking of the coating, because they give the product an undesirable taste. This is particularly the case for a stamped box because the stamping expresses the residual solvent coating. The materials and solvents remaining in the coatings must be chosen on the one hand so as not to alter the taste, but in addition so as to have the desired degree of crosslinking and not to risk interacting with small molecules from the food product by side reactions with solvent residues, which would cause an unpleasant alteration of taste and aroma.

 A well-known feature of food products such as peas, but, beans, etc., is that they release after packaging small molecules such as cystine and hydrogen sulfide. These chemical compounds result from the dissociation of proteins from the food and can react with the residual and oligomeric solvents of the coating film to form reagents that impart undesirable taste and aroma characteristics to the product. More particularly, the taste and aroma characteristic resulting from the mesityl oxide reaction, impurity present in certain coating solvents such as diacetone alcohol, with hydrogen sulfide can give rise to a sneaky odor. 4-methyl-4-mercaptopentan-2-one has been identified as altering the flavor and aroma of some canned meat and vegetables. This mercapto-ketone is associated with a sneaky odor. Therefore, it is essential to remove the mesityl oxide, for example, pressed tin can coating systems so that these boxes remain free of flavor and aroma effects.

Reference will now be made, to describe the invention in detail, to the accompanying drawings in which
Figure 1 is a partial side view in cross section of a box shaped by stamping pre-coated sheet;
Figure 2 is a sectional view taken along line 2-2 of Figure 1;
FIG. 3 is a partial cross-sectional side view of the side wall and the crimped bottom of a box manufactured by winding a flat blank into a cylinder, the side seam of which is welded, then forming on the cylinder a rim on which crimped a flat circular bottom; and
FIG. 4 is a cross-sectional view along line 4-4 of FIG. 3, passing in particular through the lateral seam shown in FIG.

 Figures 1 to 4 illustrate the difference between the two-piece bofte shaped by multiple stamping pre-coated sheet metal and the three-piece box shaped by winding the sheet and side seam. We see that it takes less layers of coating for a box in two pieces than for a box in three pieces. In particular, in FIG. 1, the multi-pass stamped box 10 has inner 11 and outer 12 pre-coating layers which adhere to the support metal 13: tin-free steel or electrolytic tin-plate, despite the process. stamping required to put the pre-coated flat sheet in the form of stamped diameter usually lower than its height.

 On the other hand, the three-piece box 14 shown in FIG. 3 carries two inner layers 15 and 16 and an outer layer 17 of pre-coating. This box is made from sheet metal 18 cut into blanks that can be wound into the form of a cylinder having a lateral seam 19 made along its longitudinal edges joined by welding (case of FIGS. 3 and 4), adhering union or box seaming and soldering. Whatever the method of obtaining the side seam, it is necessary to refurbish, coat and bake the interior with a bead 20 of side seam coating material, in order to prevent the later packaged edibles to attack the side seam or the adjacent metal. It is necessary to provide a bottom 21 crimped at 22 on the cylindrical body and thus ensure with a joint composition 23 the hermeticity of the bottom. The additional operations of laying the side seam 20 and crimping the bottom 21 of a three-piece box represent for the manufacturer an additional charge compared to the stamped two-piece box. For many applications, it is also necessary to apply to the outside of the box, on the lateral seam 24, a protective coating and to cook the latter. It is conceivable that it is essential to appropriately coat the inside 11 and the outside 12 a two-piece stamped box in order to obtain a safe and inexpensive container that is not likely to corrode or affect the flavor and aroma of the packaged foods.

The preferred outer coating 12 is primarily comprised of a combination of an epoxy resin and a slight amount of lubricant applied as exposed at a level of 0.35 to 0.47 mg / cm 2. The particular epoxy coating is Mobil Chemical MC9372 and more than half the weight of material is epoxy resin. About 75% of epoxy resin has been used successfully in the combined coating. The preferred interior liner 11 is composed of polyvinyl chloride, epoxy or phenolic resin or chloride-vinyl acetate copolymers as the resin in solution, lanolin as a forming lubricant and carbon dioxide. titanium as a pigment. The above resins, pigment and lubricants are dispersed or dissolved in suitable solvents so as to obtain a viscosity expressed in a time of 160 seconds in a Ford nO 4 viscosity test cup. In practice, the solvents represent, by weight, a little more than half of the total. The composition of polyvinyl chloride, epoxide, phenolic resin, chloride-vinyl acetate, lanolin, solvent and titanium dioxide copolymers is sold by the Company.
Midland-Dexter under the designation MM478A. An analogous formulation of the dispersion and solution resins, lubricant and pigment may be the following
All resins except PVC: 30% by weight of body
non-volatile
Dispersion resin of the PVC type: 43% by weight of body
non-volatile
Pigment: 27% by weight of body
non-volatile.

 It has been found that the molecular weight of the polyvinyl chloride dispersion resin is particularly important for obtaining a satisfactory precoat for a box for multiple drawing.

If the molecular weight of this resin is below a certain critical value, defined by the intrinsic viscosity, the coating ceases to adhere or breaks near the top of the box due to the mechanical stretching and axial compression experienced at during multiple stamping passes accused.

The deterioration of the coating is reflected by the test result of the coating on the enamel tester. For organosol coatings having formulas having molecular weight polyvinyl chloride dispersion resins in three different ranges, applied at a rate of 1.43 mg / cm and baked at 193 ° C. for 10 minutes, the results of The enamel tester test, after forming by multi-pass stamping of a box 81 mm in diameter and 111 mm high, is as follows
Test results on enamel tester of pressed cups
with internal organosol coating
PVC Resin Viscosity Results obtained at
Intrinsic intrinsic enamel tester (mA)
A 1.02 50
B 1.00 45
C 1.40 2
As can be seen, it is the organosol coating with PVC resin C with the highest intrinsic viscosity that gives the enamel tester the weakest result, indicating that it is best suited for Shape with the metal substrate.

 Behavioral testing in the presence of content, or "sub-content" testing, is an important step in assessing the behavior of stamped can linings. Initial ranking tests are generally performed with three types of content. One of these is water, because many foods such as corn, peas or tomatoes have a high water content. The water causes rapid corrosion and the lining ceases to adhere if it shows discontinuities, fences or breaks. The chicken broth is used to try the boxes to contain most soups, because of its fat content. These fats have the effect of softening the coating.If a coating is insufficiently cooked, or if its formulation does not ensure the achievement of an adequate degree of crosslinking, chicken broth fat often soften it, causing losses of adhesion and swelling with subsequent rupture of the coating film. Apple juice is mainly used for flavor and flavor alteration tests and also as an agent to strongly stimulate corrosion at the discontinuities of the coating film. Apple juice is an important test agent for determining the effects on taste of a can liner. The table below indicates the adhesion behavior of the coating for organosol inner precoated stamped boxes subjected to tests "under content" with these three types of contents.

Results of tests "under contents" of stamped boxes
with internal organosol coating.

Nature of the content

<tb> Resin <SEP>: <SEP> Viscosity <SEP>: <SEP> Water <SEP> at <SEP> 1210C <SEP>: <SEP> Broth <SEP>: <SEP> juice <SEP> from
<tb><SEP> PVC <SEP>: <SEP> intrinsic: <SEP> 50 <SEP> min. <SEP>: <SEP> of <SEP> chicken,: <SEP> apple,
<tb><SEP>:<SEP>:<SEP>:<SEP> 3 <SEP> months <SEP>: <SEP> 3 <SEP> months
<tb><SEP> A <SEP>: <SEP> 1, or <SEP><SEP>:<SEP> bad <SEP>: <SEP> bad <SEP>: bad
<tb><SEP> B <SEP>: <SEP> 1.00 <SEP>: <SEP>. <September>
<Tb>

<SEP> C <SEP>: <SEP><SEP> 1.40 <SEP>: <SEP> Good <SEP>: <SEP> Good <SEP>: <SEP> Good
<Tb>
As can be seen, it is the organosol coating having the highest intrinsic viscosity that gives the best results in the "under content" test. These results were all obtained on plates coated with an organosol interior baked at a temperature of 1930C for 10 minutes. With pre-coated organosol cans baked at 2880C for 20 seconds, the results of enamel tester tests and sub-content tests were similar.

 The vitreous transformation point of the liner must be properly adjusted during formulation, so that the coating is neither scuffed nor brittle due to heat and forming pressure.

In a similar way, the composition of the resins is essential for obtaining a suitable coating also resistant to chemical attack by the packaged products. In other words, if the vitreous transformation point is too low, the coating undergoes softening and swelling under the effect of fats and hydrolysis in the presence of acidic products. Some products also leach out part of the coating, with which they combine to give added flavors and aromas. The glass transformation point must be adjusted so that the inner and outer coatings can not degrade due to the temperatures generated by the forming, but can undergo a passive return to the fluid state. The vitreous transformation point must not be high enough so that the coating ceases to be flexible and resume some fluidity during forming. Therefore, it is the formulation as stated having the appropriate proportions of materials, especially resins in solution, which gives the correct degree of crosslinking and therefore the appropriate glassy transformation point.

 Measurements of determination of the apparent vitreous transformation point of the inner and outer coatings give the following results.

Apparent vitreous transformation point of coatings
of stamped boots
Glass transformation point (C)
Internal coating 58
Exterior coating 87
In forming metal canisters by several-pass stamping, pre-coated metal is often used to avoid more expensive post-forming coating operations. Forming causes considerable stretching and extension of the coating. It also increases the temperature of the metal substrate and thus of the coating applied to it. If the coating film contains resins with a molecular weight below a critical value, the multi-pass molding opens the coating film and decreases its resistance to the migration of chemical species present in the packaged product to its breast and to the outside, solvents, resin oligomers, etc. present in the film.

 A well-known feature of food products such as peas, corn, beans, etc. is that they release small molecules such as cystine and hydrogen sulphide after packaging. These chemical compounds result from the dissociation of proteins from the food and can react with the residual solvents and oligomers of the coating film to form reactive products which impart undesirable foreign tastes to the food product.

 It has been found that certain solvents, added to organosol coatings, can lead to the appearance of foreign tastes in certain packaged foods, including foods known to release sulfides such as hydrogen sulfide. Examples of such foods are whole grain soybeans or soybeans. Frequently, a coating is prepared in a solvent system of esters and alcohols not containing per se compounds capable of interaction with sulfides. When the coatings are diluted in the manufacturing plant to give them the desired viscosity, certain solvents are used as diluents for reasons of economy. An example of such diluents is a mixture of xylene and diacetone alcohol or xylene and isophorone. Diacetone alcohol or isophorone are acetone condensation products often used as diluents in metal coatings, because they are not expensive or have an excellent solvent power with regard to many industrial coatings or canning.

 Although these solvents have technically interesting coating properties, both isophorone and diacetone alcohol usually contain traces of mesityl oxide. Mesityl oxide is a highly reactive unsaturated ketone compound that easily adds to sulfides such as hydrogen sulfide to create odors and harmful tastes in foods or meats. In the hydrogen sulfide release example in the presence of mesityl oxide, the reaction product was identified as 4-methyl-4-mercaptopentan-2-one. The table below gives comparative values of foreign taste index and residual solvent content for boxes stamped in several passes, of height greater than their diameter.

Taste test with whole grain corn.

<Tb>

NO: <SEP> Box <SEP>: Index <SEP> of <SEP> Index <SEP> of: Index <SEP>: Index <SEP> of <SEP> taste
<tb><SEP> solvent <SEP><SEP> Oxide diacetone <SEP> from <SEP> foreign <SEP> (scale)
<tb><SEP>'single<SEP> (TAI) <SEP> alcohol <SEP>'<SEP> mesityle <SEP><SEP> from <SEP> 1 <SEP> to <SEP> 9)
<tb> 1 <SEP>: <SEP> indicator:
<tb><SEP> .3 <SEP> pieces.
<Tb>

<SEP> (AE) <SEP> 213 <SE> O <SEP>
<Tb>
Taste test of whole grain corn (continued)

<tb> N: <SEP> Box <SEP>: <SEP> Index <SEP> of: <SEP> Index <SEP> of: <SEP> Index <SEP>: Index <SEP> of <SEP> taste
<tb><SEP> solvent <SEP> re- <SEP>: diacetone: oxide <SEP> from: foreign <SEP> (scale <SEP>
<tb><SEP> single <SEP> (TAI) <SEP> alcohol <SEP> mesityl <SEP> zonele <SEP> from <SEP> 1 <SEP> to <SEP> 9)
<tb> 2 <SEP>: Embossed:
<tb><SEP> Organo- <SEP> no <SEP>
<tb><SEP> soil <SEP> 1 <SEP> 34.0 <SEP>: <SEP> 2,6
<tb> 3 <SEP>: Embossed: <SEP>: <SEP> :: <SEP>
<tb><SEP> .Organo- <SEP>: <SEP>: <SEP>: <SEP>: <SEP>
<tb><SEP> soil <SEP> 2 <SEP> 38.0 <SEP> 2.6 <SEP> 4.8 <SEP> greater <SEP> to <SEP> 9
<Tb>
The test results show that the presence of small traces of a combinable molecule with sulfides, such as mesityl oxide or its parent, diacetone alcohol, gives the corn a flavor and aroma quite undesirable. With other food products such as beans, there are similar trends in flavor and aroma.

Flavor and aroma test with beans

<tb> N: <SEP> Box <SEP>: <SEP> Mix <SEP> of <SEP> solvents <SEP>: <SEP> Index <SEP> of <SEP> taste
<tb><SEP> diluent <SEP> the <SEP> coating: (<SEP> scale from <SEP> 1 <SEP> to <SEP> 9)
<tb> 1 <SEP>: <SEP> Control <SEP>: xylene <SEP> and <SEP>"butyl
<tb><SEP> 3 <SEP> pieces <SEP> Cellosolve "<SEP>: <SEP> 1.0
<tb> 2 <SEP>: <SEP> Organosol <SEP> 1 <SEP>: xylene <SEP> and <SEP>"butyl
<tb><SEP> Cellosolve "<SEP>: <SEP> 1.0
<tb> 3 <SEP>: <SEP> Organosol <SEP> 1 <SEP> xylene <SEP> and
<tb><SEP> diacetone <SEP> alcohol <SEP>: <SEP> 3,4
<Tb>
These results show that the coating diluted with the xylene-diacetone alcohol mixture, in which the diacetone alcohol was found to contain mesityl oxide at 1 to 2 parts per million, led to flavor and aroma indices. higher than those of diluted coatings with the mixture xylene and "butyl-Cellosolve".

 Similar results were obtained with organosol coatings diluted with the xylene-isophorone mixture.

 In general, the solvents must be such that low molecular weight sulfides, hence sulfur-containing compounds, can not react to form additional compounds. They may be chosen from saturated compounds, for example esters, ethers, alcohols or others. More specifically, it is possible to use a system of solvents comprising, by weight, 5 to 25% of aromatic hydrocarbons, 10 to 15% of ketones, and up to 60% of alcohols, ethers, esters, or others.

 The detailed description given of preferred coatings for the interior and exterior of a metal substrate to be converted into a box by stamping in several passes is devoid of any limiting character. The object of the invention is an internal coating formulation which, once applied, can be pressed in several passes without degradation, because the resins are sufficiently flexible because of their molecular weight and, on the other hand, is not likely to react badly with the food packaged in the box to give rise to foreign tastes or odors.

Claims (9)

 1. Pre-coating to be applied on the "inner" face (11) of a metal blank (13) to be converted into a box (10) by stamping in several passes, characterized in that it comprises a phenolic resin, an epoxy resin, a polyvinyl chloride-acetate copolymer and a polyvinyl chloride dispersion resin in a solvent system having saturated solvents such that low molecular weight sulfides, such as sulfur compounds, can not react to form additional products.  2. precoating system according to claim 1, characterized in that said polyvinyl chloride type dispersion resin is about 43% by weight and all other resins represents about 30% by weight of the total non-volatile bodies, the the rest being pigment.  3. Pre-coating system according to claim 2, characterized in that the pigment is titanium dioxide.  4. precoating system according to claim 2, characterized in that the saturated solvent is of the ester type, ether, aliphatic compound or aromatic compound.  5. pre-coating system according to claim 4, characterized in that metal blank comprises another pre-coating (12) for the outside of the box, the outer coating being mainly composed of an epoxy resin and a slight amount of lubricant .  6. Pre-coating system according to claim 5, characterized in that the epoxy resin is 75% by weight of the coating.  7. Pre-coating system according to claim 6, characterized in that the outer coating is applied in a film at a rate of about 0.35 to 0.46 mg / cm.  8. Pre-coating system according to claim 5, characterized in that the inner coating is applied in a film at a rate of 1.28 mg / cm 2 approximately.  9. Pre-coating system according to claim 4, characterized in that the solvent system comprises, by weight, 5 to 25% of aromatic hydrocarbons, 10 to 15% of ketones, and up to 60% of alcohols, ethers , esters, or others.