METAL CONTAINER COATING COMPOSTIONS COMPRISING STABLE EMULSIONS OF WATER RESISTANT VINYL CHLORIDE-ETHYLENE COPOLYMERS

Abstract: A metal container coating composition comprising a crosslinking resin and a polyvinyl alcohol vinyl chloride-­ ethylene copolymer emulsion, the copolymer having a T g from 0 to 50°C, about 65 to 90 wt% vinyl chloride, about 5 to 35 wt% ethylene, 0 to 10 wt% olefinically unsaturated copolymerizable monomer and prepared by emulsion polymerization of vinyl chloride, ethylene and, optionally, an olefinically unsaturated comonomer in the presence of about 3 to 15 wt% polyvinyl alcohol as the dispersing agent.

Description: METAL CONTAINER COATING COMPOSITIONS COMPRISING STABLE EMULSIONS OF WATER RESISTANT VINYL CHLORIDE-ETHYLENE COPOLYMERS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of pending application Serial No. 06/572,726 filed 20 January 1984 entitled stable Emulsions of Vater Resistant Vinyl Chloride-Ethylene Copolymers", which is incorporated by reference. TECHNICAL FIELD The present invention relates to vinyl chloride-ethylene copolymer emulsions and metal container coating compositions containing such emulsions. BACKGROUND OF THE INVENTION Emulsion polymerization processes employing polyvinyl alcohol or celluloses such as hydroxyethyl cellulose as the emulsifier, or protective colloid, yield emulsion polymer products widely used as adhesives, binders and coatings. Such products have historically been known to possess poor resistance to water, generally manifested as a loss of adhesive strength to the substrate on exposure to water or as an unattractive whitening characteristically known as "blushing". This poor water resistance property has inhibited the use of such polymers prepared in the presence of polyvinyl alcohol or celluloses as coatings coming in contact with water. for example, as interior can coatings. To overcome this water sensitivity, crosslinking agents have been added to react with the polyvinyl alcohol of the polymer coating to render lt water insoluble. However, such coatings still demonstrate the unattractive blushing on exposure to water. U.S. 2,843,562 attempts to overcome the water sensitivity of graft copolymers of polyvinyl alcohol and vinyl chloride by incorporating a small amount of a cross-linking monomer containing it least two olefinic unswaturations. The inventor is aware that polymer emulsions which can deposit a polymer coating that has enhanced water resistance can be prepared by the aqueous emulsion polymerization of at least one ethylenically unsaturated monomer in a salt-free aqueous edium. The salt-free aqueous edits comprises a polyvinyl alcohol as the protective colloid and a redox system comprising hydrogen peroxide or an organic peroxy compound as the oxidant and an organic reductant. ihen this technology was applied to the preparation of vinyl chloride-ethylene copolymer emulsions using typical polymerization techniques including polyvinyl alcohol levels of 3 to 10 wt%, stable copolymer emulsions were not readily obtained. Another difficulty encountered is related to the solubility of the ethylene in that the ethylene pressure may rise uncontrollably during the polyEerization reaction. U.S. 3.399.157 teaches improving the stability of ethylene/vinyl chloride latexes by adding only a portion of the desired amount of emulsifying agent to the reaction mixture prior to the initiation of polymerization and adding a second portion to the reaction fixture after the polymerization is cosple$ed. U.S. 3.501.440 discloses in Example 5 and reference Exasples 5-6 the use of polyvinyl alcohol as the protective colloid in the copolyerization of ethylene with vinyl chloride at a reaction temperature between the critical temperature of ethylene and 60 C and at an ethylene pressure which is maintained substantially constant by withdrawing excess ethylene out of the reaction vessel during the polymerization. U.S. 3,642.740 discloses that vinyl chloride homo- and copolymers can be prepared in aqueous emulsion using as the emulsifier system an alkali metal salt of a sulfated C8-C18 fatty alcohol, a tallow fatty alcohol or an epoxidized unsaturated fatty acid oil, and a complex organic phosphate ester or salt derivative. Example 1 shows that the emulsion used in preparing the Molding paste resin product involves the preparation of a polymer seed latex. U.S. 3,689,447 discloses that heat resistant copolymers of ethylene and vinyl chloride can be prepared by the use of a seed latex in the aqueous composition for emulsion polyerization, together with heat-activated initiation at between about 50 and 85 C by water-soluble persulfates or peroxydlphosphates. The seed latex can be prepared by the emulsion polymerlzation of any polymerizable ethylenically unsaturated compound. The seed latex can be prepared beforehand in a separate vessel - and a desired aliquot can then bye introduced into the aqueous composition. Alternatively, the seed latex can be made in situ in all or part of the aqueous composition before the reactor is pressurized with ethylene. U.S. 3,725,367 describes a process for the preparation of polymers and copolymers having a vinyl base which comprises dispersing into the organic vinyl based monomeric system a seeding latex containing an excess amount of organo-soluble catalyst such that the seeding latex forms the dispersed phase and the organic monomeric system the continuous phase. U.S. 3,736,303 discloses a latex composition comprising a copolymer of vinylidene chloride. an ethylenically unsaturated sulfur acid having sulfur in a valent state of 6. optionally an ethylenically unsaturated carboxylic acid and another ethylenically unsaturated monomer prepared using a seed latex. an ascorbic acid-hydrogen peroxide redox system, and ionic buffers and surfactants. U.S. 3,875,130 discloses the preparation of homo- and copolymers of vinyl chloride in which the polymerization of the monomer composition is carried out in the presence of a seeding product prepared by the polymerization in emulsion or fine suspension. U.S. 4,150.210 discloses a one-step process for the emulsion polymerization of vinyl chloride and, optionally, comonomers using a water-soluble initiator or initiator system and a mixed emulsifier of (1) a C12-C18 straight chain alkyl or alkenyl surfactant; (2) a C14-C20 straight chain alkyl or alkenyl alcohol; and (3) a C5 straight alkyl chain sulfosuccinate emulsifier. The examples show the use of a hydrogen peroxide-ascorbic acid redox system. U.S. 4,189,415 discloses aqueous vinyl chloride-vinyl acetate-ethylene copolymer dispersions containing only polyvinyl alcohol as the protective colloid. All of the polyvinyl alcohol or only part of it can be introduced at the beginning. the ethylene pressure applied is kept constant and the polymerization temperature is 10-85 C. preferably 20-5ff C. U.S. 4,331,577 discloses a method for preparing ethylene-containing copolymer emulsions by the selective addition of the monomers mixture to the reactor in response to pressure variation and the saintenance of a monomer unsaturated condition in the reactor. SUbbNRY OF THE INVENTION The present invention provides stable vinyl chloride-ethylene copolymer emulsions which can deposit a polyvinyl alcohol-containing polyneric coating on a substrate that demonstrates enhanced water resistance and adhesion to the substrate. The stable resin emulsion comprises from 20 to 70 wtZ of a copolymer colloidally dispersed in an aqueous medium, the copolymer comprising about 65 to 90 wtZ vinyl chloride. about 5 to 35 wt% ethylene, and 0 to about 10 wtZ olefinically unsaturated copolynierizable monomer and having a Tg from about 0 to 500C. The e copolymer is prepared by emulsion polymerization, preferably in a substantially salt-free aqueous medium, in the presence of about 3 to 15 wt% polyvinyl alcohol as the dispersing, or emulsifying agent. The e wt% values are based on monomers incorporated into the copolymer. Vith regard to the invention, "salt-free" Deans the substantial absence of ionic materials, that is to say the presence of ionic materials at less than about 0.1 wt% based on solids. Accordingly, the free radical generating source for initiating and sustaining the polymerization process would be a redox system comprising hydrogen peroxide or an organic peroxy compound as the oxidant and an organic reductant. According to the invention the polymerization process comprises (a) forming an aqueous emulsion reaction Mixture containing substantially all the polyvinyl alcohol and a portion, preferably at least 5%, of the total vinyl chloride monomer, (b) pressurizing the reaction mixture with an ethylene pressure sufficient to provide the copolymer with about 5 to 35 stZ ethylene content, (c) initiating the reaction mixture by the addition of a free radical forming source and continuing polyterization until the rate of polymerization begins to decrease, (d) adding the remaining vinyl chloride, preferably at a substantially uniform rate over a period of time, while continuing polymerization until the reaction is no longer self sustaining, and (e) removing the unreacted ethylene and reducing the vinyl chloride free monomer content, preferably to less than 10 ppm. Particular embodiments of the invention, in addition to the resin emulsions, are a metal container, such as a metal can, coated with a polymeric film deposited from the vinyl chloride-ethylene copolymer emulsions and metal container coating compositions containing such emulsions and crosslinking agents or resins. As an advantage of the invention, the emulsion polymerization process provides vinyl chloride-ethylene copolymer emulsions possessing enhanced stability compared to copolymers prepared using conventional copolymerization techniques. As another advantage the vinyl chloride-ethylene copolymer emulsions deposit polymeric films of dramatically improved water resistance, especially if prepared in a salt-free medium. It is not necessary to use crosslinking agents to provide a high degree of water resistance although such agents can be used. In addition, compared to standard vinyl chloride-ethylene products the copolymers of this invention show improved dry film tensile hardness and improved adhesion to various substrates including wood, glass and metals such as steel and aluminum. DETAILED DESCRIPTION OF THE INVENTION The vinyl chloride-ethylene copolymers of the stable emulsions according to the invention contain about 65 to 90 wt% vinyl chloride. preferably about 75 to 80 wt%. The copolymerization reaction is performed under an ethylene pressure which is sufficient to provide the copolymer with about 5 to 35 wtz ethylene content, preferably about 20 to 25 wt%. Pressures of about 50 to 100 atm are generally used to afford such ethylene content. when the vinyl chloride content is less than about 65 wt%, the requisite ethylene pressures are difficult to handle and at greater than about 90 wt% vlnyl chloride stability becomes a problem. The e vinyl chloride-ethylene copolymers may also contain up to about 10 wt% of nether olefinically unsaturated monomers copolyermizable with vinyl chloride and ethylene. By way of illustration, other suitable olefinically unsaturated co-monomers for making the stable copolymer emulsions and water resistant copolymers of the invention include vinyl esters of Cl-C12 alkanoic acids. such as vinyl formate, vinyl propionate. and especially vinyl acetate; ; C3-C10 alkenoic acids, such as acrylic acid and methacrylic acid. and-alpha.beta-unsaturated C4-Clo alkanoic acids such as crotonic acid, isocrotonic acid, and their esters with Cl-C18 alkanols such as ethanol, ethanol. propanol and butanol. as well as 2-ethylhexyl alcohol, cyclohexyl alcohol and lauryl alcohol; vinylidene halides. such as vinylidene chloride; styrene; alpha,beta-unsaturated C4-C10 alkenedioic acids such as maleic acid. fumaric acid and itaconic acid and their monoesters and diesters of C1-C18 alkalnols; and nitrogen containing monoolefinically unsaturated monomers. particularly nitrides, amides. N-methylol amides. lower alkanoic acid esters of N-sethylol amides, lower alkyl ethers of -nethylol asides and allyl carbamates, such as acrylonitrile. acrylamide, methacrylamide, N-methylolacrylamide, N-methylol methacrylamide. N-methylol allyl carbamate, N-methylol lower alkyl ethers or N-methylol lower alkanoic acid esters of N-methylolacrylamide, N-methylol methacrylamide and N-methylol allyl carbamate, such as N-isobutoxymethylacrylamide. The preferred comonomers for the vinyl chloride-ethylene polymers are the C3-C10 alkenoic acids and nitrogen-containing monomers. such as acrylic acid, acrylamide, N-methylolacrylamide and N-isobutoxymethylacrylamide, preferably in about 1 to 5 wt%. The dispersing agent. or protective colloid, used in preparing the stable emulsions is at least one polyvinyl alcohol. A single polyvinyl alcohol may be used alone or mixtures of different polyvinyl alcohols can be used. Substantially the entire amount of polyvinyl alcohol is added to the aqueous medium prior to initiation. The polyvinyl alcohols which are suitable for use in the invention are, in general. 30 to 99 mole% hydrolyzed, preferably 87 to 89 mole% and have a degree of polymerization ranging from 100 to 10.000, preferably 200 to 2.500. The e amount of polyvinyl alcohol used in the polymerization reaction is about 9 to 15 wit8. preferably 4 to 10 wtb, based on monomers. substantially all of which is added initially to the aqueous medium, i.e. prior to initiation of polymerization. Less than about 3 wt% polyvinyl alcohol is unsuitable for providing stable copolymer emulsions because of emulsion coagulum while greater than 15% polyvinyl alcohol is generally unsuitable because of high viscosity at commercially acceptable solids. Additional amounts of polyvinyl alcohol can be added to the reaction mixture during polymerization provided that at least about 3 wt%, preferably at least about 4 wt, polyvinyl alcohol is present in the reaction mixture upon initiation. Such delay addition of polyvinyl alcohol does not enhance stability and may possibly adversely affect water resistance of the copolymer. For polymerization recipes containing up to about 8 wt% polyvinyl alcohol. a polyvinyl alcohol resin having a degree of polymerization of less than about 2500 should be used. and at about 8 to 15 wt% a polyvinyl alcohol of less than about 1000 degree of polymerization should be used. Other protective colloids, such as the celluloses or hydroxyalkyl celluloses. or typical emulsifying agents such as ionic or nonionic surfactants in combination with the polyvinyl alcohol may be used in amounts up to about equal proportions, preferably less than 50%. based on weight of polyvinyl alcohol, although water resistance may be impaired. Free radical sources. for example redox systems, used in the practice of this invention are conventional and used in conventional amounts. The polymerization is generally performed with quantities of redox system ranging from 0.03 to 3 wt% based on monomers. Typically, the entire quantity of either the oxidant or reductant component of the redox system. or a substantial portion. is introduced at the beginning and polymerization is initiated and controlled by metering in the other component. Obviously. the polymerization may be controlled by the simultaneous metering in of both components. Examples of the oxidizing component are ammonium persulfate, potassium persulfate. hydrogen peroxide and t-butyl hydroperoxide. Examples of the reducing component are sodium sulfite. sodium metabisulfite and zinc or sodium formaldehyde -. sulfoxylate, Although the use of such conventional redox systems affords stable emulsions and vinyl chloride-ethylene copolymer coatings having improved water resistance, adhesion and tensile hardness, the resistance to water can be further enhanced by polymerization in a salt-free aqueous environment. Accordingly. the redox system must be salt-free, that is to say, nonionic as far as the oxidant and reductant are concerned, For the salt-free system suitable oxidizing agents, or initiators, include hydrogen peroxide and organic peroxy compounds. Illustrative of the organic peroxides that can be used are alkylhydroperoxides such as t-butyl hydroperoxide, dialkyl peroxides such as di-t-butyl peroxide. peroxy acids such as peracetic acid and perbenzoic acid, diacyl peroxides such as diacetyl peroxide and dilauroyl peroxide. and peroxy esters such as t-butyl peracetate and t-butyl perbenzoate. The e preferred oxidant for use in the invention is hydrogen peroxide. The reductant component of the redox system used in the practice of the invention is a nonionic organic material such as a reducing sugar or other easily oxidizable polyhydroxy compound. Compounds frequently employed in this capacity are glucose. levulose, sorbose, invert sugar, ascorbic acid and its enantiomer erythorbic acid which are the preferred reductants, citric acid and the like. Other useful organic reductants include hydroxyl amines, thiols, pentamines and tartaric acid. The e preferred redox catalyst system for making stable vinyl chloride-ethylene copolymer emulsions comprises hydrogen peroxide and ascorbic acid or erythorbic acid. The oxidizing agent is generally employed in an amount from about 0.01 to 1.08. preferably 0.05 to 0.5%, based on the weight of monomers introduced into the polymerization system. The reductant is ordinarily added in an aqueous solution in the necessary equivalent amount. It is important that a reductant be present in the polymerization recipe because the oxidant. in most cases, does not furnish free radicals rapidly enough at temperatures below-about 80"C to expeditiously effectpolymerization of the monomers. The salt-free redox catalyst system may contain promoters such as ferrous sulfate in typically minor amounts. Such minor amounts of ionic materials would not adversely affect the water resistance of the emulsion ¯ polymers. Needless to say, the requirement of a salt-free aqueous medium rules out the use of ionic buffering agents as is so often used in many polymerization recipes for maintaining a particular pH range. However, it has been found that buffers are not required to make the emulsions whether or not salt-free conditions are used. Buffers may thus be used if desired. The e key to providing the stable vinyl chloride-ethylene copolymer emulsions of the invention is the particular emulsion polymerization procedure employed. The reaction temperature of the polymerization can be controlled by the rate of free radical source addition and by the rate of the heat removal. Generally, it is advantageous to maintain a mean temperature of about 550c during the polymerization of the monomers and to avoid temperatures much in excess of 800C. While temperatures as low as oe can be used, economically the lower temperature limit is about 300C. The e reaction time will vary depending upon other variables such as the temperature, free radical source, and the desired rate of the polymerization and reactor heat removal capacity. It is generally desirable to continue the reaction until the heat generation due to polymerization has subsided, i.e. the polymerization reaction is no longer self-sustaining. In carrying out the polymerization, substantially all the polyvinyl alcohol and a portion of the vinyl chloride is initially charged to the polymerization vessel which is then pressurized with ethylene. Most advantageously, at least about 5% of the total vinyl chloride to be polymerized is initially charged, preferably at least about 15%. The remainder of the vinyl chloride is added, desirably at a substantially uniform rate, during the course of the polymerization after the initially charged vinyl chloride monomer content has been substanitally reduced, as evidenced by a decrease in the rate of polymerization, to avoid overpressurization of the reactor. No more than about 60% of the vinyl chloride should be charged initially since it is necessary to generate a prepolymer in situ in order to obtain the stable emulsions. The quantity of ethylene entering into the copolymer is influenced by the pressure, the mixing, and the addition rate and amount of free radical generating source. Thus, to increase the ethylene content of the copolymer higher pressures. greater mixing and higher free radical source rate and amount are employed. By way of example. at a polymerization temperature of about 550C an ethylene pressure in the range of 750 psig to 1000 psig is required to provide a copolymer with about 20-25 wtS ethylene. Above about 1000 psig an undesirable pressure rise occurs while an ethylene pressure below 750 psig yields an unstable emulsion product. It appears that certain amounts of ethylene are needed to be polymerized with the initial vinyl chloride charge to yield a stable emulsion. Needless to say, the 750 to 1000 psig ethylene pressure range for 550C polymerization would vary with the reaction temperature employed. The e process of forming the vinyl chloride-ethylene copolymer emulsions generally comprises the preparation of an aqueous solution containing substantially all the polyvinyl alcohol dispersing agent. This aqueous solution and the initial charge of vinyl chloride are added to the polymerization vessel and ethylene pressure is applied to the desired value. As previously mentioned, the mixture is thoroughly mixed to dissolve ethylene in the vinyl chloride and in the water phase. Conveniently, the charge is brought to polymerization temperature during this mixing period. Mixing can be effected by shaking, by means of an agitator or other known mechanism. The polymerization is then initiated by introducing initial amounts of a free radical generating source. For example, either the oxidant or reductant component of a redox system could be initially added to the aqueous medium with the polyvinyl alcohol and vinyl chloride with the other redox component subsequently added to initiate the reaction. With the commencement of initiation. the addition of any third monomer. i.e. olefinically unsaturated copolymerizable monomer, is also-begun incrementally. After polymerization has started, delay addition of the free radical generating source is used to continue polymerization until the prepolymer reaction is essentially completed as evidenced by a reduction in the rate of the polymerization. As is well known in the art the rate of polymerization can be followed by plotting the temperature : difference (bT) between the reaction mixture and the reaction vessel jacket. The point at which aT begins to decrease corresponds to a reduction in the rate of polymerization. At this point, the remaining vinyl chloride is incrementally added along with additional free radical generating source and the remaining olefinically unsaturated monomer as delays to continue the polymerization. By "delay" addition is meant the addition of a component in a continuous or intermittent and, preferably, a substantially uniform rate. Vhen preparing a vinyl chloride copolymer having a Tg of about 20 to 50"C, the ethylene pressure during the polymerization reaction is neither decreased by venting nor maintained at a substantially steady pressure by the addition of make-up ethylene for that consumed, i.e. ethylene being neither withdrawn nor added during polymerization. Rather the ethylene pressure is permitted to increase, decrease or remain constant, i.e. float, and eventually to reduce gradually as ethylene in the sealed polymerization vessel is copolymerized. Once the requisite ethylene pressure is set in the reactor, it will rise for a short period of time with initiation of polymerization as the vinyl chloride in which it is soluble is reacted to give the polymer in which it is less soluble. After the initially charged vinyl chloride has been reacted and the vinyl chloride delay begun, the pressure essentially stabilizes over the remaining polymerization period and eventually decays. This procedure avoids uncontrollable ethylene pressure rises. For preparing copolymers having a Tg of about O to 200C, make-up ethylene will be required in suitable pressure reactors. Make-up ethylene is usually that amount needed to maintain the initial pressure. As mentioned, the reaction is generally continued until the polymerization reaction is no longer self-sustaining, desirably until the residual vinyl chloride content is below 0.5%. The completed reaction product is removed from the presence of ethylene and then maintained at a temperature above the Tg of copolymer while sealed from the atmosphere. The reaction mixture can also be transferred to a degasser with removal of ethylene. The unreacted vinyl chloride monomer content is reduced by its reaction with a vinyl acetate addition. Another method for producing the vinyl chloride-ethylene copolymers comprises first forming an aqueous emulsion of vinyl chloride and the polyvinyl alcohol stabilizing agent. The reactor is pressurized with ethylene and the resulting reaction mixture is adjusted to a temperature frogs about 10 to 300c. Polymerization is initiated by the addition of a free radical source at a rate such that the reaction mixture is brought to a temperature from 45 to 850C. preferably 50 to 60eC, within a period of 1 hour or less, preferably 30 minutes. The e polymerization is continued until the rate of the polymerization begins to reduce. The major portion of the vinyl chloride is then added to the reaction vessel as a delay. Although the water resistance of the vinyl chloride-ethylene copolymer emulsions of this invention is surprisingly enhanced, the water resistance can be further increased by cross linking the hydroxyl sites on the polymer polyvinyl alcohol, which is believed to be incorporated into the copolymer, with crosslinking agents. There are many known agents for crosslinking polyvinyl alcohol and these include formaldehyde and other aldehydes. in particular dialdehydes such as glutaraldehyde and glyoxal; dimethylol urea, tetrabutyl titanate, bis-3-methoxyl propylidene, pentaerythritol; diazonium and tetrazonium salts, boric acid. Polyvinyl alcohol may also be crosslinked by radiation. Other agents which might be used are those known to crosslink cellulose, for example N-methylol and N-methylol ether derivatives of amines, amides and ureas, such as dimethylol dihydroxy ethylene urea and ethyl-N,-dimethylol carbamate; diepoxides such as diglycidyl ether; ethyleneamine derivatives; divinyl sulphone and bis- (2-hydroxyethyl)sulphone; epichlorohydrin; phosgene and diacid-dichlorides; and 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone. Such crosslinking agents typically are added to the polymer emulsion just prior to the coating step in a 1 to 10 wt% range, based on emulsion solids. Acid catalysts such as phcsphoric acid, hydrochloric acid, p-toluenesulfonic acid, and oxalic acid are often added to catalyze the cross linking reaction upon heating of the polymer films or coatings. The vinyl chloride-ethylene copolymer emulsions may be used to prepare container coating formulations which, when applied to a metal substrate and cured, provide a polymer coating possessing enhanced water resistance. Such container coating formulations generally involve admixing the copolymer emulsions with crosslinking resins, coalescing agents and acid components well known in the art such as melamine formaldehydes. epoxies, dialdehydes. amines, diols, acid catalysts and organic co-solvents. A general can coating formulation would comprise (a) about 45 to 95 wt% copolymer emulsion, (b) about 5 to 40 wtZ crosslinking resin, based on solids, (c) up to 10 wt% organic cosolvent (coalescing agent) and, optionally, (d) up to 5 wtb acid catalyst. An illustrative container coating formulation would be a vinyl chloride-ethylene copolymer emulsion according to the invention and Cymel 303 melamine formaldehyde, 90/10 wt%, 40 wt% solids in water/diethylene glycol monoethylether acetate (90/10 wt%). These formulations are then applied to metal substrates such as steel or aluminum by spraying, dipping, roll coating or other application methods well known in the can coating art and cured by heating. The following examples are provided to illustrate the invention and are not intended to restrict the scope thereof. Test procedures used in evaluating the vinyl chloride copolymer resins for can coatings: Blushing: Blushing (film whitening) is observed immediately following removal of coated aluminum panel from test solution and rated using the following scale: Rating scale 10-0 (10 excellent or no change) 10 - No Blush 8 = Faint Blush 6 = Moderate Blush 4 = Definite 2 = Very White O - Terrible, Film Lifts Inpact Resistance: JISTM D 2794-69 Pencil Hardness: ASTM D 3363-74 Metal Adhesion: ASTM D 3359-78 using Ericsen Type 295 Adhesion tester MEK Resistance: A cotton swab saturated with MEK is rubbed back and forth across coated panel until coating is removed or until 100 rubs is reached. One back and forward movement across test panel constitutes one rub. EXAMPLE 1 The polymerization of various vinyl chloride-ethylene copolymer emulsions of Runs 1-26 was carried out in a pressure vessel equipped with a jacket and an agitation system involving turbine blades. In preparing the copolymer emulsion of Run 1 the following initial charge was introduced into the reaction vessel: INITIAL CHARGE Distilled water 555g Ferrous ammonium sulfate O.9g Sequestrine 30Aa 2.7g Vinole 205b PVOH (12% solution) 854g a Ethylenediamine tetraacetic acid sodium salt marketed by Ciba-Geigy as a 30% aqueous solution. b An 87 to 89 mole% hydrolyzed PVOH marketed by Air Products and Chemicals, Inc.. The e pH of the above charge was adjusted between 4.0 and 4.5 with acetic acid. The e vessel contents were agitated at 200 rpm and purged three times with ethylene (25 psig). Vinyl chloride monomer (240g) was then added and the reactor was heated to 550C and pressurized with ethylene (875 psig). The agitation was¯increased to 900 rpm and 7 ml of 10% aqueous solution of erythorbic acid (pH 4.5) was pumped into the reactor. After the temperature and pressure had been equilibrated, the polymerization was initiated with a 1% aqueous hydrogen peroxide solution. After the rate of polymerization began to decrease (substantially all the vinyl chloride had polymerized with ethylene), the remaining vinyl chloride monomer (1,415g) was added over a 4 hr period maintaining the polymerization temperatureof 55"C using approximately 1.2g hydrogen peroxide as a 1% solution and 2.7g erythorbic acid as the activator. Additional oxidant and reductant were used after the vinyl chloride monomer had been added to complete polymerization. A total of 1.67g hydrogen peroxide as a 1% solution and 5.0g erythorbic acid were used for the entire polymerization. The ethylene pressure was allowed to afloat during the polymerization without makeup or withdrawal. The emulsion was transferred to a degasser and the unreacted vinyl chloride monomer reduced to less than 10 ppm by the addition of vinyl acetate (15g) followed by t-butyl hydroperoxide (4g) and erythorbic acid (3g), ferrous ammonium sulfate (0.2g) and sequestrine 30A (0.8g) in water (50g). The vinyl chloride-ethylene copolymer of Run 1 was 83 wtZ vinyl chloride, 17 wt% ethylene and had a Tg of about 30"C. Emulsion solids were about 55%. Table 1 shows the amounts of the various monomers and other polymerization components for Runs 2-26 as well as physical data for the resulting emulsions. In general, for runs 2-26 the above procedure was modified as follows: The polymerization was initiated at 520C and the desired ethylene pressure, usually 875 psig to afford a copolymer having a Tg of about 20 to 35"C and suitable for can coating formulations. Upon initiation by the addition of the oxidant (H202) at a fixed rate, the temperature of the polymerization reaction mixture was allowed to rise to 550C where it was maintained automatically by the heat exchange function of the reaction vessel jacket as is well known in the art. Concomitantly, the pressure would rise, for example from 875 psig to almost 950 psig. The jacket temperature began to drop after initiation and, usually, after about one. half hour would reach a minimum. namely about 4000. In approximately 5 to 10 minutes the jacket temperature usually began to rise corresponding to the time when the rate of polymerization began to decrease. When the jacket temperature approached 45 to 500C, the amount of oxidant consumed was recorded and the vinyl chloride delay commenced. Again, this initial. or prepolymerization, step avoids the build up of excess unreacted vinyl chloride monomer in the sealed reaction vessel and reduces the possibility of the ethylene vessel pressure approaching the upper pressure limitation of the vessel, If the vinyl chloride monomer delay were commenced at polymerization initiation. an amount of cooling ability, which may be in excess of that of the reactor, would be required to permit reaction of sufficient vinyl chloride and ethylene monomer so as to prevent excessive pressure rise which could exceed the limits of the vessel. Typical plant reactors are not designed to withstand such conditions. In those Runs in which a terpolymer was made, the olefinically unsaturated canonomer was added in the delay mode upon initiation of the polymerization reaction. RUN 1 2 3 4 5 6 7 8 9 10 11 12 13 VINYL CHLORIDE 83 83 83 81 78 80 85 81 83 78 81 93 85 ETHYLENE 17 17 17 19 21 20 15 19 17 22 18 6 15 COMONOMER(%) -- 3% AAm -- -- -- -- -- -- -- -- 1% AA 1% AA 1% AA POLYVINYL ALCOHOL (%) 5 SLS 5 5 6 6 6 10 10A 10A 5 5 5 H2O2 NH4S2O4 H2O2 REDOX SYSTEM EA SFS SFS EA..............................(SAME FOR RUNS 4 - 13).................................... ACCELERATED SEDIMENTATION (%) --- --- --- 8 6 7 4 1 2 1 6 3.5 15 Tg ( C) 30 30 27 21.5 22.5 29 34 34 33.5 25 26.5 25 30 SOLIDS (wt%) --- 50 55 46 51 52 50 51.5 49 52 52 49 DRY FILM PENCIL HARDNESS --- 2H 5H 4H 5H 5H 5H 4H 4H 5H 4H 4H 5H MEK RUBS --- 6 3 3 1 2 4 3 3 1 3 3 2 BOILING WATER (2 min) BLUSH RESISTANCE --- 7 6 8+ 9 9 7 9+ 9 7+ 9+ 10 9 AL ADHESION --- 0 9 10 10 10 10 10 10 10 10 10 10 PENCIL HARDNESS --- 5B 4B H 2B -- B H H H 2B HB F WATER SOAK BLUSH RESISTANCE --- 10 10 10 10 10 10 10 10 10 10 10 10 PENCIL HARDNESS --- B HB 4H 2H 4H F HB 5H 3H 3H -- 5H ^ = Polyvinyl alcohol; DPn about 100 AA = acrylic acid AAm = acrylamide N - i -BMA = N-i-butoxymethylacrylamide EA = erythorbic acid SFS = sodium formaldehyde sulfoxylate SLS = sodium lauryl sulfate TABLE I (Continued) RUN 14 15 16 17 18 19 20 21 22 23 24 25 26 VINYL CHLORIDE 80 78 78 77 --- 79 81 --- --- --- --- 74 -- ETHYLENE 18 19 21 22 --- 20 18 --- (750 (700 (650 26 (1200 psig psig) psig) psig) (1100 & make-up) psig) COMONOMER(%) 2% AA 3% AA 1% AA 3% AA 3% AA 1% AA 1% N- i-BMA --- --- --- --- --- -- POLYVINYL ALCOHOL (%) 5 5 6 6 6 5 5 4.4 4 4 4 5 partial delay REDOX SYSTEM H2O2 EA.......................................................(SAME FOR RUNS 14 - 26)..................................................... ACCELERATED SEDIMENTATION (%) 10 20 1 1 --- 9 7 Extremely 13 20 30 4 Aborted gritty Tg ( C) 23 27.5 25.5 34 --- 27.5 38.5 discard 26 29 --- 6 Pressure Rose --- --- --- 48 > 1400 psig SOLIDS (wt%) 53.7 53 49. --- 53 45.6 DRY FILM PENCIL HARDNESS 4H 4H 4H 4H 5H 4H 5H MEK RUBS 3 3 3 3 3 3 20 BOILING WATER (2 min) BLUSH RESISTANCE 8+ 7+ 10 10 8 9+ 10 AL ADHESION 10 8 10 10 10 10 10 PENCIL HARDNESS 2B 2B HB H H 2B 5H WATER SOAK BLUSH RESISTANCE 9+ 10 10 10 10 10 10 PENCIL HARDNESS H 3H -- -- F 3H 5H The comparison of Run 2 with Run 3 shows the effect of polyvinyl alcohol in place of an ionic surfactant in the ethylene-vinyl chloride polymerization recipe. The dry film tensile hardness is improved and the adhesion of the film to aluminum is improved significantly as shown by its ability to withstand boiling water. When Run 3 was repeated using a salt-free system in Run 4 (erythorbic acid instead of sodium formaldehyde sulfoxylate as reductant). the blush resistance was improved and the hardness retention of the film after exposure to water was significantly better. Buns 5-7 demonstrate the effect of increasing the polyvinyl alcohol content to about 6 wt%. An improvement in blush resistance was attained over Run 3. A slight improvement in blush resistance was obtained in Run 8 by the addition of 4 wt% more polyvinyl alcohol to a 10 wt% total polyvinyl alcohol level. Runs 11-15 show the effect of adding one. two and three percent acrylic acid to a five percent polyvinyl alcohol containing ethylene-vinyl chloride polymerization recipe. Here the blush resistance and the hardness of the film has been improved. Runs 16-18 show the effect of increasing the polyvinyl alcohol content to 6 wtt at one and three percent acrylic acid levels. Outstanding blush resistance and improvement in boiling water film hardness were demonstrated. Runs 19 and 20 show the five percent polyvinyl alcohol stabilized vinyl chloride-ethylene copolymers also comprising a nitrogen-containing monomer, namely acrylamide and N-isobutoxymethylacrylamide, respectively. Run 19 showed good blush resistance and Run 20 showed excellent blush resistance and hardness. In addition, a substantial improvement in NEK rubs was obtained in Run 20 due to the selfcrosslinking nature of N-isobutoxymethylacrylamide. Run 21 used a partial delay of polyvinyl alcohol to the polymerization recipe. Initially 15% (0.66g) of the total polyvinyl alcohol (4.4g) was added prior to initiation and the remainder added by delay addition to the reaction mixture with the vinly chloride monomer. A suspensiontype polymer was obtained (extremely gritty). This Run demonstrates the need for at least 3 wt%, preferably 4 wit%, polyvinyl alcohol in the reaction medium when forming the prepolymer. Runs 22-24 were performed at ethylene pressure of 750, 700 and 650 psig. respectively, without ethylene makeup and yielded product which, although low in grits. showed unacceptably high unaccelerated sedimentation values. The maximum acceptable accelerated sedimentation was set as 10%. These Runs show that with no ethylene make up, ethylene pressures of greater than about 750 psig are required to provide the stable vinyl chloride-ethylene copolymer emulsion according to the procedure set forth for Run 2. Runs 25 and 26 showed that 1100 psig and no ethylene make up provided an acceptable emulsion even though a large amount of redox system was used. while 1200 psig and make up ethylene resulted in aborting the run due to uncontrollable pressure rise. Example 2 In this example several vinyl chloride-ethylene copolymer emulsions from Example 1 were tested with and without external cross-linkers as films deposited by a &num;8 wire rod onto aluminum panels and cured for 10 minutes at 380n e. The resultant data are set forth in Table 2. It is apparent from the data that conebining the polyvinyl alcohol/vinyl chloride-ethylene copolymers of the invention with cross-linking agents provide a container coating formulation which can deposit a resin film having good water resistance and hardness. TABLE 2 EMULSION RUN 2 2 3 3 9 9 11 11 11 11 11 12 CROSS-LINKER 10% 10% (% solids/solids) 10% 10% 10% --- 10% 10% CYMEL303 RESIMENE -- --- CYMEL303 --- CYMEL303 --- CYMEL303 DER32 CYMEL303 MC 730 pH --- 4.3 --- 4.6 --- 8.0 5.3 --- --- --- -- DRY FILM PENCIL HARDNESS 2H 5H 5H 5H 4H 5H 4H 4H 5H 5H 5H 4H MEK DBL RUB 6 10 3 4 3 17 3 3 3 10 3 3 BOILING WATER BLUSH RESISTANCE ( 2 min) 7 --- 6 --- 9 10 9+ 8 --- --- --- 10 BLUSH RESISTANCE (30 min) --- 1 0 0 8 10 9 --- 7 9 6 10 AL ADHESION ( 2 min) 0 --- 9 --- 10 --- 10 9+ ---- --- --- -- AL ADHESION (30 min) --- 10 10 0 10 10 10 --- 4 10 7 10 PENCIL HARDNESS ( 2 min) 5B ---4B --- 5H --- 2B 4B --- --- --- -- PENCIL HARDNESS (30 min) --- 4B 6B 5B H 3H F --- 3B 2B 6B H 24 HOUR SOAK(ACID/WATER) BLUSH RESISTANCE (ACID) --- 10 --- 5 10 10 10 --- 10 10 --- 10 BLUSH RESISTANCE (WATER) 10 10 10 10 10 10 10 10 --- --- --- 10 AL ADHESION (ACID) --- 7 --- 10 6 10 10 --- 10 10 --- 10 AL ADHESION (WATER) --- 4 --- 10 10 10 10 9 --- --- --- 10 PENCIL HARDNESS (ACID) --- B ---F 3H 4H H --- 2H 4H --- H PENCIL HARDNESS (WATER) B B HB H 5H 4H 3H HB --- --- --- 4H DER 732 is a water based epoxy resin marketed by Dow Chemical. CYMEL 303 is a melamine formaldehyde marketed by American Cyanamid. RESIMENE 730 is a melamine formaldehyde marketed by Monsanto. MC is methyl cellosolve. TABLE 2 (Continued) EMULSION RUN 12 15 15 15 15 15 16 17 17 19 19 CROSS-LINKER 10% 10% (% solids/solids) 10% --- 10% --- RESIMENE 5% --- --- CYMEL 303 --- 10% CYMEL303 CYMEL303 730 DER732 MC CYMEL 303 pH --- 5,8 --- 5.8 5.7 4.7 --- --- --- --- 4.6 DRY FILM PENCIL HARDNESS 4H 4H 5H 5H 5H 5H 4H 4H 5H 4H 5H MEK DBL RUB 4 3 20 3 12 2 3 3 18 3 3 BOILING WATER BLUSH RESISTANCE ( 2 min) --- 7+ --- 8 6 10 10 10 10 9+ -- BLUSH RESISTANCE (30 min) 9 6 10 --- --- --- 10 10 10 --- 4 AL ADHESION (2 min) --- 8 --- 10 6 10 --- 10 --- 10 -- AL ADHESION (30 min) 10 10 10 --- --- --- 10 10 10 --- 10 PENCIL HARDNESS (2 min) --- 2B --- HB 5B 4B --- H --- 2B -- PENCIL HARDNESS (30 min) F 4B H --- --- --- HB H H --- H 24 HOUR SOAK (ACID/WATER) BLUSH RESISTANCE (ACID) 10 9 10 --- --- --- 10 10 10 --- 10 BLUSH RESISTANCE (WATER) 10 10 10 10 10 10 10 10 10 10 10 AL ADHESION (ACID) 10 5 10 --- --- --- 10 10 10 --- 10 AL ADHESION (WATER) 10 10 10 10 7 10 10 10 10 10 10 PENCIL HARDNESS (ACID) H 5B H --- --- H --- 4H --- 2H PENCIL HARDNESS (WATER) 3H 3H 4H H 3B H 4H 3H 5H 3H 4H STATEMENT OF INDUSTRIAL APPLICATION The stable polyvinyl alcohol/vinyl chloride-ethylene copolymer emulsions according to the invention are useful in container coating compositions in that they provide a polymer coating on a metal surface possessing enhanced water resistance. improved dry coating tensile hardness and improved adhesion of the coating to the surface even after exposure to water.

Claims: CLAIM3 1. A metal container coating composition comprising (a) about 45 to 95 wt% of a stable aqueous vinyl chloride-ethylene copolymer emulsion, the copolymer having a Tg from 0 to 500C, consisting essentially of about 65 to 90 wt% vinyl chloride, about 5 to 35 wt% ethylene. and up to about 10 wt% C3-C10 alkenoic acid or nitrogen-containing comonomer, and prepared by the emulsion polymerization of the monomers in the presence of an emulsifying system consisting essentially of about 3 to 15 wt% polyvinyl alcohol, (b) about 5 to 40 wt% of a crosslinking resin. based on emulsion solids, (c) up to about 10 wt% organic cosolvent and, optionally. (d) up to 5 wt% acid catalyst. 2. The metal container coating composition of Claim 1 in which the copolymer is about 1 to 5 wt% alkenoic acid comonomer. 3. The metal container coating composition of Claim 2 in which the copolymer is prepared by emulsion polymerization in the presence of about 4 to 10 wt% polyvinyl alcohol. 4. The metal container coating cooposition of Claim 3 in which the copolymer is about 75 to 80 wt% vinyl chloride and has a Tg of about 20 to 35 C. 5. The metal container coating ccc8Esition of Claim 4 in which the alkenoic acid is acrylic acid. 6. The metal container coating cooposition of Claim 1 in which the copolymer is about 1 to 5 wt% nitrogen-containing comonomer. 7. The metal container coating composition of Claim 6 in which the copolymer is prepared by emulsion polymerization in the presence of about 4 to 10 wt% polyvinyl alcohol. 8. The metal container coating composition of Claim 7 in which th± copolymer is about 75 to 80 wt% vinyl chloride and has a Tg of about 20 to 35CC. 9. The metal container coating composition of Claim 8 in which the, nitrogen-containing monomer is acrylamide or N-isobutoxymethyl acrylamide. 10. The metal container coating composition of Claim 1 in which the polymerization is performed using a free radical generating source comprising hydrogen peroxide and ascorbic acid or erythorbic acid as the redox system. 11. The metal container coating composition of Claim 1 in which the emulsion polymerization process comprises forming an aqueous emulsion reaction mixture in a reaction vessel containing substantially all the polyvinyl alcohol and a portion of the total vinyl chloride monomer1 pressurizing the reaction mixture with an ethylene pressure sufficient to provide the copolymer with a 5 to 35 wt% ethylene content, initiating the reaction mixture by the addition of a free radical generating source and continuing polymerization until the rate of polymerization begins to decrease, adding the remaining vinyl chloride over a period of time while continuing polymerization until the reaction is no longer self-sustaining. and removing the unreacted ethylene and reducing the vinyl chloride free monomer content of the emulsion. 12. A metal container coating composition comprising (a) about 45 to 95 wt% of a stable aqueous vinyl chloride-ethylene copolymer emulsion, the copolymer having a Tg from 0 to 50"C, consisting of (i) about 65 to 90 wt%vinyl cMrof-ide and (-li-) ethylene,-and-prepared by emulsion polymerization of vinyl chloride monomer and ethylene in the presence of an emulsifying system consisting essentially of about 3 to 15 wt% polyvinyl alcohol. (b) about 5 to 40 wt8, based on emulsion solids, of a crosslinking resin, (c) up to about 10 wt% organic cosolvent and, optionally, (d) up to 5 wt% acid catalyst. 13. The metal container coating composition of Claim 12 in which the emulsion polymerization is performed in the presence of about 4 to 10 wt% polyvinyl alcohol. 14. The metal container coating composition of Claim 13 in which the copolymer is about 75 to 80 wt% vinyl chloride and 20 to 25 wt% ethylene and has a Tg from 20 to 35"C. 15. The metal container coating composition of Claim 12 in which the emulsion polymerization is performed using a redox system comprising hydrogen peroxide and ascorbic acid or erythorbic acid. 16. The metal container coating composition of Claim 12 in which the emulsion polymerization process comprises forming an aqueous emulsion reaction mixture in a reaction vessel containing substantially all the polyvinyl alcohol and at least 58 to no more than about 60% of the total vinyl chloride monomer, pressurizing the reaction mixture with an ethylene pressure sufficient to provide the copolymer with the requisite ethylene content, initiating the reaction mixture by the addition of a free radical generating source and continuing polymerization until the rate of polymerization begins to decrease, adding the remaining vinyl chloride monomer over a period of time while continuing polymerization until the reaction is no longer self-sustaining, and removing the unreacted ethylene and reducing the vinyl chloride free monomer content of the emulsion. 17. A metal container coating composition comprising (a) about 45 to 95 wt% of a stable aqueous vinyl chloride-ethylene copolymer emulsion, the copolymer having a Tq from 0 to 50"C, about 65 to 90 wt% vinyl chloride, about 5 to 35 wt% ethylene, 0 to 10 wt% olefinically unsaturated copolymerizable monomer and prepared by emulsion polymerization of vinyl chloride monomer, ethylene and, optionally, an olefinically unsaturated comonomer in the presence of an emulsifying system consisting essentially of about 3 to 15 wt% polyvinyl alcohol, the emulsion polymerization process comprising forming an aqueous emulsion reaction mixture in a reaction vessel containing substantially all the polyvinyl alcohol and a portion of the total vinyl chloride monomer. pressurizing the reaction mixture with an ethylene pressure sufficient to provide the copolymer with a 5 to 35 wt% ethylene content. initiating the reaction mixture by the addition of a free radical generating source and continuing polymerization until the rate of polymerization begins to decrease, adding the remaining vinyl chloride over a period of time while continuing polymerization until the reaction is no longer self-sustaining, and removing the unreacted ethylene and reducing the vinyl chloride free monomer content of the emulsion, (b) about 5 to 40 wt%, based on emulsion solids, of a crosslinking resin, (c) up to about 10 wtj% organic cosolvent. and, optionally, (d) up to 5 wt% acid catalyst.

Inventors: IACOVIELLO, JOHN GENEROSO
Applicants: AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1985
Main classification: C09D3
IPC codes: C09D5, C08L27, C08F214, C09D127, C08F2, C08L1