US2407881A - Preparation of galvanized sheets to receive resinous coatings - Google Patents

Description

Patented Sept. 17, 1946 PREPARATION OF GALVANIZED SHEETS TO RECEIVE RESINOUS COATINGS George R. Hoover and Noble E. Hays, Middletown, Ohio, assignors to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio No Drawing. Application July 30, 1940, Serial No. 348,442

6 Claims.

Our invention relates to the preparation of ferrous sheets to receive resinous coatings, usually referred to as baked enamels. Enameled sheets and structures formed from sheets of this Without desiring to be bound by theory we believe that blistering is caused by moisture and hydrogen coming from the sheet surface and failing to escape through the enamel. Formerly character are today widely used in industry-as 5 most of the enamels which were applied to metal one example, in the making of the cabinets of parts and then heated to a drying temperature, household refrigerators. For many uses galvancontained, along with a suitable vehicle, thermoized sheets are desirable; and it has come to be plastic resins which softened at the elevated drythe practice to give such sheets a pretreatment ing or baking temperature but which increased to promote their acceptance of the enamel and the viscosity of the enamel vehicle sufliciently so the bonding of the enamel to the sheets. that the enamel would not run off and produce In the early days of the manufacture of baked too thin a coating. The elevated temperature enamel structures relatively little diificulty was had to be maintained until the vehicle polymerencountered with blistering, though a number of ized and hardened. There was thus a considercomplaints arose. The comparative unimporta e ak g t during Which the Viscosity was ance of blistering was due to the substantially not so high, as to prevent the escape of any bubuniversal use of enamel substances of continubles of entrained gas. In the newer types of ously thermoplastic character. More recently, enamel a thermo-setting resin is used which, however, there has been a shift to enamels which with the vehicle, not only makes the enamel more consist of resins of thermo-setting type, or conviscous as applied, thereby yielding a heavier tain large quantities of such resins; and blistercoating, but in itself undergoes a chemical ing difilculties have become serious. change so that it hardens and becomes perma- The principal objects of our invention are the nently infusible. This typ o enamel hae the solution of the blistering problem, and specifiadvantage of. setting much more quickly and at cally the provision of a treatment for the sheets taming the maximum hardness after a shorter which is effective to prevent blistering, baking operation. It has the disadvantages of It is an object of our invention to provide a hampering the escape of bubbles of gas forming treatment for the purpose which may be applied therein, of forming a film which tends to entrap by the manufacturer of the sheets, and which the gases, and of hardening so rapidly as to preproduces a permanent effect so far as the sheets vent the escap f t gasefi Resins 0f the therare concerned, so that blistering does not appear mo-setting variety which, along with suitable vein spite of normal handling, shipping and storhicles, constitute or are ingredients in the more age conditions. It is our object to provide a modern types of baking enamels comprise gentreatment of such cheapness as not to add sigerally resins of the urea-formaldehyde types, the nifically to the cost of the sheets for enameling. phenol-aldehyde types, the glycerol-polybasic These and other objects of our invention acid resins, the acetylene derivatives, and the which will be set forth hereinafter or will be appoly-olefin resins. Enamels containing any of parent to one skilled in the art upon reading these resins present a blistering problem which is these specifications, we accomplish by that cersolved by the treatment of this invention. tain procedure and treatment and in that cer- In the formation of our improved sheets, the tain article of which we shall now describe exferrous sheet metal is galvanized in any suitable emplary embodiments. way by being cleanedand treated with molten The blistering problem is by no means conzinc or shelter, though we prefer to produce our fined to galvanized sheets which have been pre galvanized sheets in accordance with the teachtreated to promote acceptance and adhesion of ings of Patent No. 2,110,893. 7 V enamels. Blistering upon ordinary galvanized The galvanized sheets are then passed through sheets which have not been given any such treata solution containing phosphoric acid, zinc phosment is also a problem and is equally amenable phate and an oxidizing agent such as sodium to the treatment which we hereinafter set forth. nitrate. This bath applies a coating of zinc A galvanized sheet which has been Bonderized" phosphate to the sheet surface. The sheets enor given an adhesion and acceptance promoting ter the bath preferably through a set of rubber treatment is, in our view, much more desirable rolls kept wet with water, and they leave the for enameling use; and in the exemplary embodbath through a set of rubber squeeze rolls which iment hereinafter taken up, we shall describe our remove most of the phosphate solution from the process in connection with such a sheet. sheet surface. Next they pass through a. cold a temperature approximately 180 F. Finally they are passed through a set of squeeze rolls to remove the excess chromic acid solution and finally through a blast of hot air to dry the surfaces.

The treatment above is a treatment preferred by us for giving to the galvanized sheets an acceptance and adhesion promoting surface.

After extended research we have found that we can prevent the blistering of baked enamels as applied either to plain galvanized sheets or sheets which have been Bonderized as described above by giving to the sheets a heat treatment. Curiously enough, the effect of this heat treatment i permanent and the sheets after having once been freed of their tendency to produce blistering in baked enamels never regain that tendency, in spite of long periods of normal handling, shipping and storage.

The heat treatment to which we refer is a specialized heat treatment of which the most commercially available temperature range is from 450 to 500? F., within which temperatures the treatment can be completed in from 3 to '7 minutes or thereabouts. In investigating the effects of temperature we havefound that heat treatments of temperatures as low as 250 F. will be effective if the time duration of the treatment is long enough, say 4 to 6 hours. limit of temperature is essentially a matter determined by manufacturing convenience and depending upon the lengths of time available for the heat treatment and the economics of relatively lengthy heat treatments. The upper limit of temperature is determined by the maximum temperature which will not adversely affect the adherence of the galvanized coating itself to the ferrous sh'eet. Different types of galvanized coatings will withstand different maximum temperatures. Certain galvanized coatings applied by the lead-zinc pot process have been known to begin to lose their adherence at below 500 F. Regular galvanized coatings can be heated to approximately 500 to 600 F. before adherence is damaged. Coatings made by the process of the patent referred to above can be heated much higher with entire safety, and can occasionally be heated as high as 900 F. for a short length of time before adherence is affected. Adherence tests for galvanized coatings are commonly used by manufacturers and in the light of these tests made at various temperatures the skilled Worker in the art can readily determine the maximum permissible. temperature for his particular product. Due to the shortness of the cycle (say 3 to 7 minutes) we recommend the temperature range of 450 to 550 F. for all galvanized products which will withstand these temperatures. If the skilled worker is working witha product which will not withstand these temperatures, he will be able in the light of the teachings above to select a time of treatment consonant with a temperature low enough not toafiect the adhesion of his galvanized coating.

' Our treatment therefore is effective in producing a galvanized sheet which as manufactured has no tendency to produce blistering with the Thus the lower 4 modern baked enamels and which, unobviously, is permanent in this respect.

The heat treatment may be carried out in a variety of ways because the sheets may be treated either singly or in packs. Where the time cycle is short enough to permit it, the heets may be passed singly or in groups through an open heat treating furnace equipped with a conveyor (or strip may be pulled through the furnace) or the sheets (or coils) may be stacked on a platform, covered with a box or not as desired and heated in any suitable heat treatment apparatus capable of maintaining the desired temperature for the desired length of time.

Modifications may be made in our invention Without departing from the spirit of it. Having thus described our invention, what we claim as new and desire to secure byLetters Patent is:

1. A process for the treatment of galvanized ferrous articles including steps to free them from a blistering tendency when coated with thermosetting enamels, which comprises heat treating the galvanized sheets at a temperature andfor a time suffieient to destroy the blistering tendency, but below a temperature and time cycle at which adherence of a galvanized coating will be adversely affected, and hence without altering the extent of alloy formation, said temperature being applied by passing the articles through a conan acceptance and adhesion-promoting treatment 7 involving the formation of an insoluble zinc compound on their surfaces, and thereafter heat treating the sheets but below a temperature and time cycle at which adherence of a galvanized coating will be adversely affected, and hence without altering the extent of alloy formation, by passing them through a continuous annealing furnace and subjecting them to a temperature of 450 to 550 F. for from 3 to 15 minutes, thetime duration of the heat treatment varying substantially inversely to the temperature thereof.

3. The process as claimed in claim 2 in which said acceptance and adhesion-promoting treatment comprises treating the galvanized sheets with a solution of phosphoric acid, zinc phosphate and an oxidizing agent, thereafter washing said sheets, treating said sheets with a chromic acid solution, and drying said sheets.

4. A process for the treatment of galvanized ferrous articles including a treatment to free them from a blistering tendency when coated with thermosetting enamels, which comprises heat treating the galvanized sheets at a temperature and for a time sufficient to destroy the blistering tendency but below a temperature and time cycle sufficient to impair adhesion of a galvanized coat ing and hence without substantially altering the extent of alloy formation, the temperature :of said heat treatment lying substantially within the range of 250 to 600 F. and the time lying substantially between 3 minutes and 6 hours, the time duration of the heat treatment varying substantially inversely to the temperature thereof,

and thereafter coating said articles with a thermosetting enamel.

5. A process of preparing galvanized iron and steel sheets for the reception of baked enamels containing thermosetting resins and of destroying the tendency of said sheets to produce blistering, which comprises galvanizing the sheets, giving the sheets an acceptance and adhesionpromoting treatment involving the formation of an insoluble zinc compound on their surfaces and thereafter heat treating the sheets in a temperature and time cycle below that at which adherence of the galvanized coating is impaired and hence without substantially altering the extent of said alloy formation, said heat treatment being at temperatures substantially within the range of 250 to 600 F. for from substantially 3 minutes to substantially 6 hours, the time of heat treatment varying inversely to the temperature thereof.

6. A process of preparing galvanized iron and steel sheets for the reception of baked enamels containing thermosetting resins, and of destroying the tendency of said sheets to produce blistering, which comprises galvanizing the sheets, giving the sheets an acceptance and adhesionpromoting treatment involving the formation of an insoluble zinc compound on their surfaces and thereafter heat treating the sheets in a temperature and time cycle below that at which adherence of the galvanized coating is impaired and hence without substantially altering the extent of alloy formation, said heat treatment being at a temperature of substantially 250 F. for substantially 6 hours, the said sheets being stacked on a platform and covered with. an annealing box.

GEORGE R. HOOVER.

NOBLE E. HAYS.