"Vinylite" Resins for Can and Container Coatings2 ARTHUR K. DOOLITTLE Carbido and Carbon Chamicets Corp., South Charleston, W. Va.
A
TRULY protective surface coating film must be composed of structural units that are mechanically strong and adequately resistant to the physical and chemical agencies encountered during exposure. Such structural units or molecules must, therefore, have slight chemical reactivity and relatively great size. These large molecules may either be developed after application by means of chemical reaction, or they may be fully developed during the process of manufacture of the resinous materials, hence prior to application. This is the essential difference that distinguishes paints and varnishes from lacquers. The previous paper3 discusses an outstanding resinous material of the first class in which the development of the giant molecules is accomplished after application by means of heat. Let us now consider an equally outstanding resinous material of the second class in which the resin macromolecules have achieved their ultimate growth prior to incorporation in the lacquer, and therefore take part in no further chemical reaction after application. The resin that will be discussed is one of several polymeric resins of the vinyl family, all of which are synthesized from Nature's abundant raw materials—gas, water, air, and salt. The particular member of the vinyl group of resins that is most widely used in surface coatings is a copolymer of vinyl chloride and vinyl acetate, sold under the trade name "Vinylite" resin VYHF. Subsequent reference to copolymer vinyl resin in these remarks will be understood to refer to this grade only.
moles of vinyl chloride to 1 mole of vinyl acetate in the simplest unit of the resin macromolccule. The growth of giant molecules from much smaller molecules, characteristic of the heat-hardening or thermosetting resins, advances in all directions by means of primary valence linkages. This results in a rigid three-dimensional structure which may be insoluble, infusible, hard, and often brittle. The |>olymerization reaction, on the other hand, generally advances m a single direction, giving long threadlike molecules from which surface coating films may be formed by deposition from solution. These films are generally soluble in solvents similar to those from which they were originally deposited, are softened by heat, and may be flexible or brittle depending on the nature and location of the so-called ''active" groups on the macromolecules. Copolymer vinyl chloride-acetate resin is an example of a polymeric resin that is hard but tough and flexible—a circumstance that is, no doubt, occasioned by the infrequency of the oxygen-bearing groups on the resin macromolecules.
Solution Characteristics Protective coatings of copolymer vinyl resin are applied from solutions of the
resin in volatile solvents. Suitable admixture of pigments, oils, plasticizers, or certain other resins may be made where required, but the essential constituent of the lacquer is the solution of the vinyl resin. The effectiveness of solvents for copolymer vinyl resin is judged by the viscosity behavior of solutions of the resin in mixtures of the solvent and a hydrocarbon diluent. Most liquid materials familiar to us in our everyday life exhibit but one viscosity phase, which we call "normal" or "viscous" flow. With solutions of this resin, however, three distinct viscosity effects are encountered which are termed the "viscous", "plastic", and "gel" phases. The transition from the viscous through the plastic to the gel phase can be made by increasing the solids content, by reducing the proportion of solvent in the volatile vehicle, or by reducing the temperature. With most solvents, the region in which plastic flow predominates is also thixotropic that is, the viscositv is changed when the solution is agitated but recovers its original value when the agitation ceases. Figure 1 illustrates the flow behavior of vinyl resin solutions in each of these three viscosity phases. The solutions are all of the same solids content (18 per cent) and differ from each other only in the proportion of methyl isobutyl ketone to toluene in the volatile vehicle.
Polymerization Polymerization is the result of repeated unions of simple chemical molecules with each other. Copolymerization implies that more than one chemical individual takes part in the polymerization reaction. It is usually possible to control the proportions in which the monomers unite to form the copolymer, and in the case under discussion conditions are maintained which produce a resin of approximately 87 per cent vinyl chloride and 13 per cent vinyl acetate. This corresponds to about 9 1
Trademark, registered. 2Presentedat the Protective Coatings for Industry meeting held by the Bakélite Corp. at The Franklin Institute. Philadelphia, Peana.. January 30.1940. > Weith. A. J.. News ED.. 18, 241 (1940).
Figure 1.
303
Different conditions of flow
Ν E W S E D ΙΤ ΙΟ Ν
304 The lx>t solvents an* those that dissolve a large quantity of the resin M o r e ab normal viscosity effects appear. Ac cording to this criterion, ketone* us a e l a » give the l*M |>crfornianee with copolymer vinyl resin. Xitroparallhis and o t e r s follow in the order named. Owing to diminishing solvent strength with increasing molecular weight, it has long l>een a problem to produce a highboiling solvent for eo|>olymcr vinyl resin that would |iennit Mic formulation of rollcoating finisher, graining inks, and stencil pastes. This problem hiis recently been solved in a most gratifying manner by the synthesis of isophoronc, a complex ketone with multiple active or solvating grout*. The eomjMirison of the viscosity phase diagram (/) of i>ophorone-xylene with that of methyl isobutyl kctone-toluene, Figure 2, is significant. Isophoronc, which eva|iorates more slowly than Tetrulin, gives solutions of normal viscosity liehavior over practicallv the same range of solids content as is indicated for the simpler ketone of much faster e v a l u a t i o n rate. Furthermore, the tendency toward thixotropism in the plastic region is very much less with isophoronc, ami higher solids content solutions can l>e made without gelation. Complete viscosity studies have been made of most commercially available vinyl resin solvents. These cover a wide range of eva|*>ration rates, making it possiUc to formulate most classes of in dustrial finishes. Vinyl resin la< mers may l>c applied by spraying or roll coating, as they have no tendency to string or cobweb.
Resin Behavior on Metals The behavior of cojiolymer vinyl re>in films over various metals is worthy of mention, since this liehavior is intimately related to the protective value of the coating. It is usually desirable to bake the coatings in order to improve the ad herence and expel residual solvent. Many surfaces are inert or "neutral" to the resin, but certain metals appear to accelerate its thermal decomposition. Zinc, tin plate, and iron are the worst offenders, and vinyl finishes applied over these metal surfaces must be stabilized or else separated from the metal by means of a neutral undercoat when high baking temperatures are em ployed. Figure 3 compares the effect of various baking times and temperatures on an unstabilized vinyl resin finish over glass, aluminum, steel, and tin plate. Very little darkening appears on glass or alumi num after 2 hours at 350° F. The same finish on steel shows a slight dark ening after 1 hour, whereas on tin plate it is discolored in 20 minutes at this tem perature. The resin can be ready stabi lized against thermal decomposition by the use of certain lead compounds. Non toxic stabilizers are also available where lead is undesirable. Another example of the influence of the
TViCAi VISCOSITY »HASI Ο*Α·ΡΑΜ comxm» ν«νι Λ*Η.ΜΛΛΛΛΙ MOL wt looo-moo
ηττη mf tnirr rnnir ΠΜΕΒΑΜ frTIWH W1\ **»'*£**.
MOL.WT. ΛΟΟΟΙΛΟΟΟ
Figure 2 metal surface on the diameter of the vinyl resin coating is found in the thickness of the film that must be applied to give a continuous coating. This subject has been extensively studied by Young and his associates (4), who have demonstrated that tl. nature of the metal surface con trols the minimum thickness of vinyl resin coatings of equivalent protective value. An electrical method was used in which the coated panel was made one panel of a galvanic cell, while the vinyl resin coating constituted the principal internal resistance. A very sensitive ballistic galvanometer receives a momentary im pulse when the key is touched. The de flections observed when the cell is dis charged through each of two known re sistances are used to compute the internal resistance of the cell. A film of such thickness that it gave an internal resistance infinite with respect to the range of the instruments was considered to be con tinuous. Young's results show that the minimum coating weights for continuous films over iron, aluminum, and tin plate are in the order of 10, 6, and 3 mg. per square inch.
Applications With this background of vinyl resin technology, let us now consider some applications of these films as protective coatings in the container field. The vinyl coating was found to be ideal for use in food containers (5) because it is non toxic, odorless, tasteless, and resistant to grease, alcohol, and water, and is im-
Vol. 18, No. 7 mune to Imcterial and enzymatic attack. It is furthermore tough and flexible and its thermoplastic character gives it heatsealing properties. The first large-scale application of copolymer vinyl resin in the container field was for the lining of beer barrels and later beer cans. Beer in cans made its a p p e a r r a n c e in1935andwas favorably re ceived by a large proportion of the con suming public. In 1937, according to the U. S. Census of Manufactures, 631,000,000 beer cans were produced. Copolymer vinyl resin, which has been used in con nection with beer-can linings from the beginning, is still unsurpassed in this field and has faithfully played its part in making canned beer popular. There are several reasons for the unique adaptability of this resin to the lining of beer cans. 1. The resin is tasteless. It introduces no taste to the beer and it likewise absorbs or removes none. 2. It has good resistance to water (and beer at the temperatures used in processing beer. Many materials have been evaluated as beer-can linings but few have equaled copolymer vinyl resin in this respect. 3. Very thin films of this resin that arc free from pinhole are impermeable to metal ions. Dialysis esperiments have been made to .study the diffusion of ferric ions through films of the order of 3 to 5 mg. of resin per square inch The teste were conducted by placing a saturated ferric chloride solution on one side of the mem brane and water at pi I 4.2 on the other. The water was tested |>eriodt tally, but no trace of iron could IK» detected after nearly three weeks, when the e&|ieriiiieiits were terminated 4. Copolymer vinyl chloride-acetate resin is absolutely unaffected by alcohol. Kxtraetion tests made on the dry powdered resin with 190 proof ethyl alcohol showed no dissolved matter after shaking for IS hours at room temperature. 5. And perhaps most important of all, the film is tough and flexible and will take a draw without rupture. By virtue of this profierty of copolymer vinyl resin, it is |K>s*ihlc to coat the end stock in the flat and stamp the ends out subsequently. A fur ther draw is given the ends when they are seamed to the can bodies and the need for a tough and flexible coating to withstand these operations is quite obvious. The o|)eration of lining the cans is interesting, inasmuch as the vinyl resin lining is applied after the can body is formed. The steps in the fabrication of beer-can bodies are: First, the sheets of tin plate are primed on one side. An unprimed strip is left where the solder must Cow when the side seam is closed. The lithographed decorations are next applied on the reverse side of the sheet. The body blanks are stamped out and formed into the cylindrical shell of the can, which is both crimped and lapped to make an especially strong seam. The can bodies are then ready to be lined with the v»nyl resin film. This is a spraying operation, w hereby a continuous film is applied which completely covers the inside of the can. After the vinyl resin lining has been prop erly baked to remove any trace of retained solvent, the bottom ends are seamed and
NEWS E D ΙΤ ΙΟΝ
April 10, 1940 the cans are ready to 1M» ship|»ed to the brewers. In addition to beer, Mime wine, gra|»e juice, and apple juice an* also packed in cans lined with copolymer vinyl resin. There is, no doubt, a |x>tential outlet for vinyl resins in lining cans for carbonated beverages and citrus fruit juices where high processing temperatures are not re quired. Work is in progress *ith these objectives in view. Other applications in the food container field are the lining of metal beer barrels, (mils for soft drink con centrates, and tank cars for shipping wme and essential oils. The resistance of copolymer vinyl resin to alkali and to grease and alcohol recom mends the use of these coatings for many applications in the container field l>eside> the packing of beverages. Consider the lining of pails for cold-water paints. In this instance a one-coat, lead-stabilize! vinyl resin lining is applied directly over the bare iron. The commercial use of these pails for casein paints is evidence that this lining is alkali-resistant. Stor age tests made tn our laltoratory add fur ther emphasis to this feature. I· one such test, iron |MiiU lined with a leadstabilized vinyl resin first coat followed by a clear vinyl resin finish coat wen» fill«»d with 25 per cent caustic MM la solution and stored from August, HWti, to June, 1939. At the end of this three-year period, the tests were terminated with no failures a»>parent on any of the test specimens. A field closely allied to the lining of containers is the protection of cajie and closures for food, cosmetic, and miHUcinal products. This work is generally finished in the flat, then punched, drawn, or spun into shape. Hen» again the hard, tough, and flexible vinyl resin film exhibits the ability to take an extreme draw without rupture. The general use of liquor bottle
305
«•-J** ami cosmetic container closures finished with co|w>lymer vinyl resin offers fi rther proof of the resistance of this resin to alcohol and grease. Closures used for pickle jars illustrate the resistance» of the resin to acid foods. The use of coated foil and coated pa|M»r for |>ackaging a thousand miscaper for closure liners. Most of the high-grade liner stock used today is vinyl resin calen der-coated |Mipcr. because this resin com bines in one product excellent resistance to a great variety of agents. A consider able saving is thus made in inventory by use of calender-coated paper. TN» study of the mo.e geirrul field of ovcrvarnishes for printed foil or paper ha* devclofied a nit her s|>ecial technique of application (J) that has ρ π χ 1 need some pleasing results. The st'»ck may IM» coated in the roll or by the sheet, but the drying o|M»ration is preferably carried out in two stages. In the first, a moderate tem|M»rature s e r v o to remove the bulk of the volatiles while, in the second stage, a brief e.\|>osure to a high tempenitun» (400° F.) raises the glosr, impmves the adhesion, and exjiels residual solvent from the film. Several commercial installations have been made of coating and baking equipment csjiecially designed to provide these features, and the results obtained with the correct application technique demonstrate the utility of this procedure.
The manufacturer of fabricated articles today no longer ex|iects to buy an allpur|M»c surface coating. In fuct, every problem of surface prot«-ction demands a unique (icrformancc of the surface coating, and the wise bu\er must inform himself regarding the various classes of coatings available. Our organization has accu mulated a fund of technical information relating to vinyl resin coatings and is pre pare! to offer a»istance in the solution of Mipfacc coating problems.
Literature Cited I» Donlittlr. lui. Ena.
