US2337972A - Process of coating metals - Google Patents

Description

contact with, for example, foodstuffs.

Patented Dec. 1943 William Clayton and Robert Ian Johnson,

London, and Harry Kenneth Dean, Baildon, England, assignors to Crosse & Blackwell Limited, London, England, a British company No Drawing. Application June 27, 1939, Serial No. 281,490. In Great Britain June 29, 1938 This invention relates to the coating of metal Claims.

surfaces and to lacquers adapted for such purposes, and more particularly toimproved lacquers and methods suitable for protecting the interior of sheet metal containers, such as food containers.

An object of .the invention is to provide an improved and inexpensive coating material for metals.

A -iurther object of the invention is to provide a method of applying the coating material so as to obtain a'water resistant coating and more particularly a coating which is resistant to attack by water at temperatures at which foodstufis are normally sterilised.

These and other objects will appear from the following description. It has been found that drying at semi-drying oils in the raw state and containing a drier can be applied to a clean surface by mechanical means such as rolling, brushing or spraying and,

that on stoving (for example for 40 minutes at 350 F.) a film is obtainable which is mechanically hard but which is, nevertheless, unsuitable for coating the interior surface of a sheet metal,

for example tinplate, food container since the: v lacquer is attacked by water at the elevated temperature used in sterilizing the foodstufis,

and is thereby softened.

It has now been found that-the requisite water.

resistance of lacquers, of which .the.oil comporaw drying or semi-drying oils, may be achieved if the lacquer is first dispersed in an aqueous medium and then electro-deposited iromthis dispersion directly on to a metal surface, which functions as anode, in a suitable electric circuit. The resulting deposit, after washing free from adhering drops of the dispersion, drying and stoving, gives a film which .is quite suitable for The electrodeposited coating for the interior surface of metal containers may be formed at any-convenient stage in the fabrication of the said containers. Thus the flat sheets of tinplate -from which the containers are to be made, may be coated with the raw oil lacquer by electro-depo- The invention includes the use of those drying oils and semi-drying oils ordinarily used for paints and varnishes, and mixtures of these oils or mixtures of one or more of these oils with a partially oxidised oil, such as a blown oil or a partially polymerised oil, such as stand oil, maybe employed. Drying oils such as varnish linseed oil and tung "oil, and" semi-drying oils, such as pilchard oil, have been found to give satisfactory results. 1

Various dispersion media may be used but it is preferred to use an aqueous solution of sodium aluminate.

This method of utilising raw oils for coating compositions has been found to have very general application with uniformly excellent resultsif certain factors are brought under control.

Y example, where the aqueous system in which the oil is emulsified is such that little or no reinforcing-"rnaterial is co-deposited with the lacquer during the electro-deposition process, for example when the lacquer is emulsified in dilute aqueous solutions of ammonia, then it is desirable, par-- ticularly in those cases where the viscosity of the a nents consist of, or contain a large proportion of,

oilphase is naturally low, to increase the viscosity of the oil by the addition of such compounds as aluminium stearate, resins, stand oils, blown oils,

. et'c., as ,will be found to have the desired effect without impairing the properties of the film. If

sodium aluminate solution is usedas the aqueous" phase then the aluminous'compound codeposited with the lacquer is found to bind the film and make it unnecessary to increase the viscosity of the oil.

'A proportion of resin may be added when the lacquer is to be depositedupon containers for dry products. This has the advantage that inexpenisive resins may be employed so as further to reduce the total costs of the lacquer, which is already less expensive than the lacquers heretofore employed owing to the elimination of the cost of pre-treatment of the oils.

To achieve stoving of lacquer according to the invention, in a period equivalent to the stoving sltion, or the lacquer may be electrically deposited on to the tubular bodies and on to the ends separately. Alternatively raw oil lacquer may b d posited on to the interior, or on to the interior and exterior, of abuilt up container consisting -of the tubular body and one end applied thereto, or having both ends applied where oneend is provided with a dispensing orifice throughwhich an electrode may be inserted into the can.

time employed with the usual stoving lacquers, the driers for incorporation in the oils should be selected with care. I l

Driers jEo'r-raw oil lacquers Driers may beadded to the oils either as insoluble or as soluble salts. be heated with oils to high temperatures which are maintained until a clear solution is obtained, this results in a certain amount of darkening, oxidation and polymerisation and cannot be em- For Insoluble salts must.

by means of solid salts or their solutions in turpentine or white spirit, and these arethe types preferred. A further advantage arising from the use of soluble driers is that less heat treatment is required to effect solution of the drier.

Preferred methods of incorporation are as follows: either (a) the calculated amount of solid linoleate or naphthenate is added to.the oil at 150 C. and the temperature raised to 190-200 C., only until a clear solution is obtained, (usually about 5 minutes) and the oil then cooled; or, (b) the calculated quantity of drier is dissolved by heating or intensive stirring in its own weight of turpentine and this solution stirred into the cold oil. I We have found that raw oils need rather different combination of driers from bodied oils and some general features will therefore be outlined.

Drying oils Linseed oil (also perilla, mercuriales, lumbang, i. e. drying oils containing major proportions of linoleic and linolenic acids). Mixtures of cobalt and manganese are suitable for these oils and about 0.05-0.25% of metal having a ratio of cobalt to manganese of between 1:4 and 1:2 or manganese alone may be used but cobalt alone tends to produce rapid surface drying with the result that the film remains a little soft underneath.

Tung oil (also oiticica, neou, po-yok, i. e. drying oils whose component acids contain conjugated systems ofthree double bonds). For these oils we use a-drier containing equal amounts of cobalt .and manganese. (0.01-0.075%) in con- Junction with 0.1-0.3% zinc or lead in order to reduce the frosting ofthe film. Drlers containing zinc or lead should not be heated with the oilsbut should be added as in method (1)). In addition, certain commercial preparations such as Medium 109J (Albert products) can be add to minimise frosting.

Semi-amine oils Soya' oil (also sunflower, fish, saiiiower, maize, i. e., semi-drying oils containing major proportions of chic and linoleic acids). Here the ratio of cobalt to manganese must be increased to between 1:2 and 1:1 and the percentage of metal added may be 0.1 to 0.4%.

The emulsions of these oils are readily prepared by known methods but the'best results are obtained when the warm oil (50-100 C.) containing V t-1% added acid (e. g'. .oleic acid) is run into the agitated alkaline aqueous medium in which case the emulsifying agent (on oleate) emulsion and control is frequently possible by adjustment of the concentration of the emulsifying agent. f 'i The preferred method of carrying out the electo be coated than is necessary for other lacquers;

for example, when coating tinplate food containers, of which the interior surface area with one end seamed on is 36 sq. inches, the circuit resistance is adjusted to give an initial current of 4-5 ampsawith an E. M. F. of 36-40 volts, the stability of the dispersion being adjusted so that a satisfactory film is obtained in 2-4 seconds.

In order to illustrate the invention the follow-' ing examples are given of emulsions according to theinvention suitable for coating metal containers intended for the canning of. foodstuffs.

Example 1 Into 400 lbs. of varnish linseed oil is incorporated 8 lbs. of cobalt-manganese drier by heating to 200 C. When the oil has cooled to 110 C., 4 lbs. of oleic acid is added, and the mixture is emulsified in 60 gallons of softened water containing 1.5% sodium .aluminate, at

It has been found that varnish linseed oil does not emulsify as readily as, for example, tung oil and the ease with which an emulsion of the linseed oil may be obtained depends to some extent upon the emulsifier used. A high speed propeller unit designed to give a definite circular tion under the action of considerable disintegrating forces has been found to give a satisfactory emulsion. The addition of tung oil has also been found to facilitate the emulsification of the linseed oil.

Variations in the type of film obtained on electro-deposition maybe secured by (a) varying the concentration of sodium aluminate;- (b) varying the amount of oleic acid.

Example 2 trample 3 2 lbs. of medium 109.! (an anti-frosting agent) dissolved in 2=lbs. toluol, 2 lbs. zinc naphthenate 8% metal) and $6 lb. cobalt naphthenate (6% nhetal) is stirred into lbs. of cold raw tung o s i The mixture is heated to 60 C. and emulsified as in Example 1, oleic acid being added to the oil it necessary.

Example 4 as lbs. of pilchard oil and 12 lbs. raw tung oil is heated to 200 0., 4 lbs. of cobalt manganese drier (10% metal concentration) (C0:Mn=1:2) 2:51:80 added when, the temperature reaches The ofl is then-cooled to 0., ,5 lb. of oleic acid is added and the mixture emulsified as in Example 1.

The full importance of the invention-lies not only in the excellence of the films produced by this method but further inv the considerable economic and technical advantages resulting from the use of these coating compositions. For ex- 7 Y ample, no expensive oil blowing or polymerizing plant is required as in most cases, the raw'oil as supplied can (after incorporation of the suitable driers) be immediately emulsified in the appropriate aqueous system and used for electro-deposition. The conventional lacquer manuposited film very much easier than in the case of films of deposited lacquers of other types. The eleotrodeposition of raw oil lacquers may follow known pre-treatments of the metal surface which are designed to improve the resistance to discolouration by sulphur compounds in the pack, or to improve the adhesion. of the lacquer. For example, the surface of the tinplate to be-coated may be subjected to anodic oxidation, for example in the manner described in the specification of copending ,U. S.'application Serial No. 145,591. Patent No. 2,215,165, or a metallic oxide, such as aluminium oxide, may be deposited upon the surface by making the survide the oxide coating.

Thelacquer and the processes according to'the invention are applicable to the coating of any metallic surface, for example tinplate, black plate or aluminium.

Having thus described the invention, what we claim as new and desire to secure by Letters Patent is:

1. The process of forming a protective coating on a metal surface, saidcoating being capable of resisting attack by water at the temperature 4 at which foodstuffs are normall sterilised, which comprises preparing a dispersion of a raw drying oil and an oil-soluble drier in an'aqueous alkaline dispersion medium, electrolytically provoking a deposit from the dispersion uponthe metal surface as anode to provide anadherent film containing the oil and drier with the oil constituting the preponderant coating component, and then washing, drying and stoving the-depositedcoating film. v v

2. The process of forming: a protective coating on a metal surface, said coating being capable of resisting attack by water at the temperature at which foodstuffs are normally sterilised, which v comprises preparing a dispersion of a raw drying oil and an oil-soluble drierin an ammoniacal aqueous alkaline dispersion medium, electrolytically provoking a deposit from the dispersion upon the metal surface as anode to rovide an adherent film containing theoil and drier with the oil constituting the; preponderant coating component, and-then'washing, drying and stoving the deposited coating film. A o

3. The process of forming a protective coating on a metal surface, said coating being capable.

of resisting attack by water at the temperature at which foodstuffs are normally sterilised, which comprises preparing a-dispersion of a raw drying oil and a drier'in an aqueous alkaline sodium aluminate solution in the presence of a quantity of oleic acidcorresponding to substantially one-half to one per cent of the amount of oil, the drierbeing an oil-soluble mixture of cobalt and manganese salts having a metal concentration of 0.01 to 0.4 per cent based on the oil and with a cobalt:manganete ratio of essentially 1:4 to 1:1, electrolytically provoking a deposit from the dispersion upon the metal surface with said surface as anode and at a current density of substantially 4 to 5 amperes per 36 square inches to provide a film containing the oiland drier with the oil constituting the preponderant coating component, and then washing,- drying and stoving the deposited coating film.

4. The process of forming a, protective coating on a metal surface, said coating beingcapable of resisting attack by water at the temperature at which foodstuffs are normally sterilised, which comprises preparing a dispersion of a raw drying oil whose component acids contain conjugated systems of three double bonds in the-molecule,

together withan oil-soluble drier, in an aqueous alkaline sodium aluminate solution, the drier being an oil-soluble mixture of equal amounts of cobalt and manganese salts having a metal concentration of 0.01 to 0.075 percent of each based on thegoil, electrolytically provoking a deposit from tha dispersion upon the metal surface with coating component, and then washing, drying and stoving the deposited coating film.

5. The process of forming a protective coating on a metal surface, said coating being capable at which foodstuffs are normally sterilised, which comprises preparing a dispersion of a. raw drying oil of the kind having linoleic and linolenlc acids as the major acid component in the oil molecules,

together with a drier, in an aqueous alkaline,

sodium aluminate solution, the drier being an oilsoluble mixture of cobalt and manganese salts having a metal concentration of 0.01 to 0.25 per cent based on the oil and with a cobaltzmanganese ratio of essentially 1&4 to 1:2, electrolytically provoking a deposit from the dispersion upon the metal surface with said surface as anode and at a current density of substantially 4 to. 5

. amperes per 36 square inches to provide, a film containing the oil and drier with the oil constitu'tin'g the preponderant coating component,

HARRY, KENNETH DEAN. WILLIAM CLAYTONL ROBERT IAN JOHNSON.