WO1994004581A1

WO1994004581A1 - Waterborne latices adapted for spray application

Info

Publication number: WO1994004581A1
Application number: PCT/US1993/006550
Authority: WO
Inventors: Kurt G. Olson; Mary Ellen Rosenberger; Giovanni Bianchini; Suryya K. Das; Barbara G. Piccirilli
Original assignee: Ppg Industries, Inc.
Priority date: 1992-08-13
Filing date: 1993-07-13
Publication date: 1994-03-03

Abstract

Spray applied, waterborne acrylic latex containing, coating compositions having impact resistance, sag resistance and solvent popping resistance include a latex comprising core/shell particles resulting from a two stage emulsion polymerization. The latex particles have a hydrophobic polymeric core and a hydrophilic polymeric shell, the glass transition temperature of the core being lower than that of the shell.

Description

WATERBORNE LATICES ADAPTED FOR SPRAY APPLICATION

Background of The Invention The present invention relates to spray applied, waterborne, coating compositions containing acrylic latex particles. Spray application of waterborne coating compositions is generally accompanied by problems with obtaining adequate flow, sag resistance, smoothness and solvent popping resistance. Poor flow and smoothness are characterized by rough or non-continuous coating film surfaces. Sag resistance is important for coating vertical surfaces, wherein the applied coating may have a tendency to flow down the vertical surface non-uniformly during the cure process, resulting in a phenomenon referred to as sag. Solvent-popping can result from the release of solvents or crosslinker blocking agents from within the film during the cure process after the surface of the coating film has begun to form a skin. Solvent popping can result in small holes in the final cured coating which degrade performance and appearance of the coating. The problem of solvent popping generally becomes more serious as film thicknesses are increased, and can become such a significant problem as to preclude spray application of some aqueous coating compositions when relatively high film builds are required (e.g., above 25 microns, particularly at about 75 microns or greater). Approaches to reducing solvent popping and improving film smoothness in spray applied waterborne latex coating compositions have involved the use of solvent additions and/or defoamers. However, such solvent and defoamer additions are accompanied by increases in the volatile organic component content ("V.O.C") of the final coating formulation. From the standpoint of V.O.C. compliance, an increase in V.O.C. can be counterproductive. Improving the sag control in spray-applied waterborne latex coatings compositions has been typically approached through the use of thickeners, which alter the rheological behavior of the formulation. The use of rheological control additives are known to adversely effect the atomization process, thus resulting in reduced efficiency in the spray application process. Various combinations of core and shell polymers for latices have been proposed by the prior art. Morgan et al., in Journal of Applied Polymer Science, Vol. 27, pages 2033-2042 (1982), discussed the effects of monomer feed sequence on minimum film forming temperature and concluded that the sequence did not play a significant role in the properties of the resultant latex. A core formed from relatively "soft" polymer combined with a shell formed from relatively "hard" polymers is disclosed by Morgan et al. as well as the combination of hard core with soft shell. The combination of a core that is soft and hydrophobic with a shell that is hard and hydrophilic is not disclosed by Morgan et al. Both the core and the shell in Morgan et al. appear to be relatively hydrophilic. Furthermore, no crosslinking functionality is provided in the Morgan et al. polymers for practical curing for coatings made from the latices.

A similar study of hard and soft core/shell combinations was conducted by Devon et al. as reported in the Journal of-Applied Polymer Science, Vol. 39, pages 2119-2128 (1990). Hardness was varied by changing the relative amounts of butyl acrylate and methyl methacrylate in the monomer mixtures. Although hydrophobicity and hydrophilicity of the polymers are discussed, Devon et al. do not provide particular level of hydrophilicity apart from hardness. No crosslinking functionality is provided.

U.S. Patent No. 4,107,120 (Plamondon et al.) discloses latices in which a shell is polymerized on a core of different composition, with the shell having a higher - than the core. The compositions are intended for treating fabrics.

U.S. Patent No. 4,325,856 (Ishikawa et al.) discloses latices in which a hydrophobic core is polymerized within a hydrophillic shell. The order of polymerization can significantly affect the properties of the latex product.

U.S. Patent No. 4,403,003 (Backhouse) discloses core/shell latices in which both the core and the shell have relatively low T„. Spray applied, waterborne, acrylic latex containing, coating compositions possessing solvent popping resistance, sag resistance and impact resistance, plus the possible combination of low V.O.C, have thus far not been adequately achieved through the above mentioned prior art approaches.

Summary of The Invention In accordance with the present invention, it has been found that waterborne coating compositions that lend themselves to spray application, i.e., having good solvent popping and sag resistance, while at the same time maintaining good impact resistance, can be obtained by using as the principal binder resin an acrylic "core/shell" latex formed by a two stage polymerization. The core of the latex particles (i.e., the polymeric product of the first stage of the polymerization) is characterized as being hydrophobic and "soft" (i.e., having a lower glass transition temperature or T_e) relative to the shell (the polymeric product of the second stage of the polymerization). Thus, the shell is relatively hydrophillic and "hard." Although the invention should not be considered limited to any particular theory, it is believed that the particular combination of core and shell properties described above yields a delayed coalescence of the latex particles during drying, thereby permitting solvent to readily escape from the film during the early stages of drying, thereby substantially reducing the occurrence of solvent popping.

The polymeric core may be formed by a first emulsion polymerization of an initial monomer composition comprising from 50 to 90 percent by weight acrylic or methacrylic monomers selected such that they produce a polymer having T below 0°C. In addition, 0 to 5 percent by weight of carboxyl group containing monomers, and between 1 to 20 percent by weight of hydroxyl group containing monomers, are also included in the reactants forming the core polymer. The polymeric shell will also be formed by a second emulsion polymerization carried out in the presence of the polymeric product of the first polymerization. The monomers employed to form the shell differ from those used to form the core. The monomers forming the shell will be chosen such that 40 to 90 percent by weight are acrylic or methacrylic monomers and will produce a polymer having its T„ above 20°C. In addition, 5 to 20 percent by weight of acid group containing monomer, and between 1 to 20 percent by weight of hydroxyl group containing monomer, also are included in the reactants used to form the shell. Additionally, 0 to 40 percent by weight of another monomer or oligomer having alpha/beta ethylenic unsaturation may be included in the second stage reactants to form the shell.

The core/shell latex, as described above, can be further characterized in terms of the resulting T difference between the core and the shell. The core has a T„ at least 50°C, preferably 75°C, and most preferably 100°C below that of the shell.

Detailed Description The expressions "core" and "shell" are used herein based on the theory that in forming the latex particles of the present invention, the first stage of polymerization results in the formation of a core region of the final particle, and the second stage polymerization results in the formation of a shell on the outside of the core. Although there is evidence that this "core/shell" morphology does in fact exist, its existence is not essential to the functioning of the present invention. For the purposes of this description, the polymer portion termed the "core" is intended to indicate that which is polymerized first.

The major monomer component used in the first stage polymerization to form the core of the latex particles is selected to yield a polymer having a below 0°C and at least 20°C lower than the T of the shell. Preferably this may be achieved by selecting acrylic monomers having at least four carbons in the side chain, for example, butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, n-hexyl acrylate, lauryl acrylate, tridecyl acrylate, isobornyl acrylate, stearyl acrylate, n-decyl methacrylate, benzyl acrylate, isobutyl acrylate, dicyclopentyl acrylate, isodecyl acrylate, tertiary butyl acrylate, palmitic acrylate, ethoxy ethyl acrylate, methoxy butyl acrylate, 2-(2-ethoxy ethoxy) ethyl acrylate,

2-phenoxyethyl acrylate, tetrahydrofurfuryl acrylate, methoxylated tripropylene glycol monoacrylate, 1H, 1H, 5H-octafluoropentyl acrylate, tri ethylsiloxyethyl acrylate, or the corresponding methacrylates of the foregoing. Monomers for the first stage also include a hydroxy group containing monomer, which is useful for providing crosslinking functionality to the polymeric product. Preferably, the hydroxy group containing monomer is a hydroxy functional acrylate such as hydroxyethyl acrylate, hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, acrylate esters of polyethylene glycols, acrylate esters of polypropylene glycols, glycerol monoacrylate, and the like, and the corresponding methacrylates of the foregoing.

No acid group containing monomer need be included in the first stage since the core is to be relatively hydrophobic. However, a small amount of acid functionality may be present, provided that the amount is insufficient to alter the hydrophobicity of the core relative to the shell. A small amount of acid in the core can be advantageous in carrying out the polymerization process. • Preferably the acid group containing monomer is acrylic acid, methacrylic acid, 2-sulfoethyl methacrylate, 2-acrylamido-2-methylpropane sulfonic acid, 2-acryloxymethoxy-0-phthalic acid, 2-acryloxy-l-methylethoxy-0- hexahydrophthalic acid, and the like.

The major monomer component used in the second stage polymerization to form the shell of the latex particles is selected to yield a polymer having a T above 20°C and a least 20°C higher than the T_ of the core. Thus this monomer component typically includes acrylates such as methyl methacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, or propyl methacrylate. It is also possible to use as comonomers in the second stage polymerization any of the acrylate or methacrylate monomers listed above in connection with the first stage polymerization, provided that the amounts are selected so as to yield the T properties required for the shell as defined herein.

As in the first stage monomer mixture, hydroxyl group containing monomers are included in the second stage to provide crosslinking functionality, and these may be selected from the same hydroxyl containing monomers disclosed above.

The reactants producing the shell include acid group containing monomers, generally to a greater extent than in the core. Sufficient acid group functionality is provided in the shell to render it hydrophillic relative to the core. Among the acid group containing monomers that may be employed are virtually any acid containing monomers that are copolymerizable with the other monomers, for example, methacrylic acid, or any of the acid group-containing monomers disclosed in connection with the first stage polymerization. The first and/or second stage polymerization may include an additional monomer having alpha-beta unsaturation and differing from the other specified monomers. These additional monomers may serve as diluents to reduce the cost of the latex, or as modifiers to refine the properties of the polymers. Examples include styrene, methylstyrene, vinyl esters, vinyl chloride, vinylidene chloride, 1,4-butadiene, and the like.

Optionally, the core, the shell, or both can be crosslinked. This crosslinking can be achieved by including crosslinking monomers such as allyl methacrylate, ethylene glycol dimethacrylate, trimethylolpropane triacrylate, and the like to the monomers feeds. Alternatively, monomers with coreactive functionalities can be include in the core, the shell, or both. For example, glycidyl methacrylate can be a component of the shell while acrylic acid is included in the core. Coreaction of the acid and epoxy groups results in a crosslinked (e.g., insoluble) particle which is within the scope of the present invention. The T_g of the core can be altered by the addition of plasticizer molecules. These molecules have the effect of lowering the T of the core polymer and thereby improving the flow and leveling characteristics of the composite system. Any of the plasticizers known to those of skill in the art can be used, preferably those which are water insoluble, such as tributyl phosphate or other phosphate esters, diesters of phthalic acid such as di-2-ethylhexyl phthalate.

It should be understood that the monomer combinations used to make the core and the shell may include mixtures of monomers from each of the groups disclosed above, as well as additional comonomers that may not themselves meet the criteria as to T or hydrophobicity for that portion of the latex, provided that the comonomers are present in relatively minor amounts such that the resulting polymer forming the core or the shell has the required properties.

Glass transition temperatures as used herein are calculated from the Fox Equation:

(i/τ_g) = ( _a/τ_ga) + ( _b/τ_gb) + ... + ( i/τ_gi) where:

W = Weight percent of monomer "a."

T = T_e (glass transition temp, of a homopolymer of monomer "a." Wr= Weight percent of monomer "b." ^■eh~~~ ^e (^c^^ass transition temp^, of a homopolymer of monomer "b." T = Calculated theoretical T of the polymer formed from monomers "a, b,....i." LATEX POLYMERIZATION EXAMPLES

EXAMPLE A This example describes the preparation of a preferred embodiment of the waterborne acrylic core/shell latex. The waterborne acrylic core/shell latex was prepared from the following mixture of ingredients:

Ingredients P rts y Weight (grams)

Deionized Water 2043.5 Sodium Bicarbonate 2.5 Ammonium Persulfate 7.5 Alipal® C0-436 ¹ 52.5 Triton® N-101 ² 3.0 Ammonium Persulfate 7.5 Butyl Acrylate 593.4 Tributyl Phosphate 165. Hydroxyethyl Acrylate 212.1 Sulfoethyl Methacrylate 8.2 Methacrylic Acid 58. Methylmethacrylate 493. Styrene 123.

Tridecyl Alcohol 5. Foamkill® 649 ³ 0.2 Dimethylethanolamine 10.0

TOTAL 3784.4

Anionic surfactant available from Rhone-Poulenc. Nonionic surfactant available from Rohm and Haas. ^J Defoamer available from Crucible Chemical.

A suitable reaction vessel, equipped with an appropriate agitator and a nitrogen gas hook up, was initially charged, at room temperature, with 1040.5 grams of deionized water, 2.5 grams of Alipal® C0-436 surfactant, 3.0 grams of Triton® N-101 surfactant, and 2.5 grams of sodium bicarbonate. The reaction vessel was then heated to 80°C. After the reaction vessel had reached 80°C, it was charged with 30.1 grams of butyl acrylate, 8.4 grams of tributyl phosphate, 4.2 grams of hydroxyethyl acrylate, 15.9 grams of deionized water, 1.3 grams of Alipal® CO-436 surfactant, and 0.2 grams of sulfoethyl methacrylate, this charge was then held for five minutes. At this point an additional 50.1 grams of deionized water plus 7.5 grams of ammonium persulfate were added, followed by a 30 minute hold. Next, over a period of 60 minutes, the following feed was added: 563.3 grams of butyl acrylate, 156.6 grams of tributyl phosphate, 78.9 grams of hydroxyethyl acrylate, 296.8 grams of deionized water, 23.7 grams of Alipal® C0-436 surfactant, and 3.0 grams of sulfoethyl methacrylate. After the completion of this feed, the reaction mixture was held at 80°C for 45 minutes. The following feed was then added over a period of 90 minutes: 58.0 grams of methacrylic acid, 493 grams of methylmethacrylate, 123 grams of styrene, 129 grams of hydroxyethyl acrylate, 5 grams of tridecyl alcohol, 530.2 grams of deionized water, 25 grams of Alipal® C0-436 surfactant, and 5.0 grams of sulfoethyl methacrylate. At the completion of this feed, the reaction mixture was held at 80°C for one hour. The reaction mixture was next cooled to room temperature followed by the addition, over 15 minutes, of 110 grams of deionized water and 10 grams of dimethylethanolamine. Upon the completion of this addition, 0.2 grams of Foamkill® 649 defoamer were added and the reaction mixture was transferred, through a filter, into an appropriate container.

The monomer feed compositions for Example A can be described as follows:

% Weight

TOTAL 100.0 100.0 100.0 EXAMPLE B This example describes the composition of a phosphatized epoxy used in the formulation of spray applied water-borne latex acrylic coatings. This resin is the reaction product of Epon® 828 epoxy monomer and phosphoric acid.

Ingredients W ht

Epon® 828 1 Phosphoric Acid

Butyl Cellosolve® Deionized Water Xylene

TOTAL 100.0

Bisphenol based epoxy monomer available from Shell Chemical Inc.

The following examples describe the synthesis of various waterborne latex acrylic resins which are subsequently_ysed in comparative tests of pigmented coating formulations.

COMPARATIVE RESIN EXAMPLE A This example describes the synthesis of an acrylic latex that is not core/shell. The water-borne latex was prepared from the following mixture of ingredients:

Ingredients Parts bv Weight (grams)

Deionized Water 971.6

Alipal® C0-436 26.3

Triton® N-101 1.5

Sodium Bicarbonate 1.3 Ammonium Persulfate 3.6

Methacrylic Acid 29.

Methyl Methacrylate 246.5

Styrene 61.5

Butyl Acrylate 296.7 Tributyl Phosphate 82.5

Hydroxyethyl Acrylate 106.1 Tridecyl Alcohol 2.5

Dimethyl Ethanol Amine 5.0

Foamkill® 649 0.1

Sulfoethyl Methacrylate 4.1

TOTAL 1838.3

A suitable reaction vessel, equipped with an appropriate agitator and a nitrogen blanket was initially charged, at room temperature, with 520.3 grams of deionized water, 1.3 grams of Alipal® CO-436 surfactant, 1.5 grams of Triton® N-101 surfactant, and 1.3 grams of sodium bicarbonate. The reaction vessel was then heated to 80°C. After the reaction vessel had reached 80°C, the following charge was added followed by a five minute hold: 1.1 grams of methacrylic acid, 9.0 grams of methyl methacrylate, 2.2 grams of styrene, 10.8 grams of butyl acrylate, 3.0 grams of tributyl phosphate, 3.9 grams of hydroxyethyl acrylate, 0.1 grams of tridecyl alcohol, 15.3 grams of deionized water, 0.9 grams of Alipal® C0-436 surfactant, and 0.15 grams of sulfoethyl methacrylate. At this point an additional 25.0 grams of deionized water plus 3.6 grams of ammonium persulfate were added, followed by a 30 minute hold. Next, over a period of 2.5 hours, the following feed was added: 27.9 grams of methacrylic acid, 237.5 grams of methyl methacrylate, 59.3 grams of styrene, 285.9 grams of butyl acrylate, 79.5 grams of tridecyl phosphate, 102.2 grams of hydroxyethyl acrylate, 2.4 grams of tridecyl alcohol, 406 of deionized water, 24.1 grams of Alipal® CO-436 surfactant, and 3.95 grams of sulfoethyl methacrylate. At the completion of this feed the reaction mixture was held at 80°C for one hour. The reaction mixture was then cooled to room temperature followed by the addition, over 15 minutes, of 5 grams of dimethyl ethanol amine and 55 grams of deionized water. Upon the completion of this addition, 0.1 grams of Foamkill 649 defoamer were added and the reaction mixture was transferred, through a filter, into an appropriate container.

The monomer feed compositions for Comparative Example A can be described as follows: % ht

Monomers

Butyl Acrylate Tributyl Phosphate Hydroxyethyl Acrylate Methacrylic Acid Methyl Methacrylate Styrene Tridecyl Alcohol

TOTAL 100.0

COMPARATIVE RESIN EXAMPLE B This example describes the synthesis of a water-borne core/shell latex resin where the core and shell compositions are reversed, that is, the shell was made from a monomer composition that would result in a Fox Equation derived T lower than that of the core. The "reversed" core/shell water-borne latex resin was prepared from the following mixture of ingredients:

Ingredients Parts bv Weight (grams)

Deionized Water 1015.3 Alipal® CO-436 13.8 Triton® N-101 1.5 Sodium Bicarbonate 1.3 Ammonium Persulfate 3.6 Methacrylic Acid 29. Methyl Methacrylate 246.5 Styrene 61.5 Hydroxyethyl Acrylate 107.6 Tridecyl Alcohol 2.5 Butyl Acrylate 296.7 Tributyl Phosphate 82.5 Sulfoethyl Methacrylate 4.1 Dimethyl Ethanol Amine 5.0 Foamkill® 649 0.1

TOTAL 1871.0 A suitable reaction vessel, equipped with an appropriate agitator and a nitrogen blanket was initially charged, at room temperature, with 520.3 grams of deionized water, 1.3 grams of Alipal® C0-436 surfactant, 1.5 grams of Triton® N-101 surfactant, and 5 1.3 grams of sodium bicarbonate. The reaction vessel was then heated to 80°C. After the reaction vessel had reached 80°C, the following charge was added followed by a five minute hold: 1.3 grams of methacrylic acid, 10.8 grams of methyl methacrylate, 2.7 grams of styrene, 2.8 grams of hydroxyethyl methacrylate, 0.1 grams of

10 tridecyl alcohol, 11.7 grams of deionized water, 0.6 grams of Alipal® CO-436 surfactant, and 0.1 grams of sulfoethyl methacrylate. At this point an additional 25 grams of deionized water plus 3.6 grams of ammonium persulfate were added, followed by a 30 minute hold. Next, over a period of 60 minutes, the following feed was added: 27.5

15 grams of methacrylic acid, 235.7 grams of methyl methacrylate, 58.8 grams of styrene, 61.7 grams of hydroxyethyl acrylate, 2.4 grams of tridecyl alcohol, 253.3 grams of deionized water, 11.9 grams of Alipal® C0-436 surfactant, and 2.4 grams of sulfoethyl methacrylate. After the completion of this feed, the reaction mixture was held at

20 80°C for 45 minutes. The following feed was then added over a period of 90 minutes: 296.7 grams of butyl acrylate, 82.5 grams of tributyl phosphate, 41.6 grams of hydroxyethyl acrylate, 156.3 grams of deionized water, 12.5 grams of Alipal® C0-436 surfactant, and 1.6 grams of sulfoethyl methacrylate. At the completion of this feed,

25 the reaction mixture was held at 80^βC for one hour. The reaction mixture was then cooled to room temperature followed by the addition, over 15 minutes, of 5 grams of dimethyl ethanol amine and 55 grams of deionized water. Upon the completion of this addition, 0.1 grams of Foamkill® 649 defoamer were added and the reaction mixture was

30 transferred, through a filter, into an appropriate container.

The monomer feed compositions for Comparative Example B can be described as follows: % i ht

TOTAL 100.0 100.0 100.0

COMPARATIVE RESIN EXAMPLE C This example describes the synthesis of a water-borne acrylic latex which is not core/shell. In this example, a water-borne latex is synthesized from a monomer composition similar to that used to make the core of the present invention. No shell type composition is present in this example. This "core monomer ingredients only" water-borne latex resin was prepared from the following mixture of ingredients:

Ingredients Parts bv Weight (grams)

Deionized Water 496.5 Alipal® C0-436 13.2 Triton® N-101 0.7 Sodium Bicarbonate 0.7 Ammonium Persulfate 1.8 Butyl Acrylate 296.7 Tributyl Phosphate 82.5 Hydroxyethyl Acrylate 41.6 Sulfoethyl Methacrylate 1.6 Foamkill® 649 0.1

TOTAL 935.4 A suitable reaction vessel, equipped with an appropriate agitator and a nitrogen blanket was initially charged, at room temperature, with 260.2 grams of deionized water, 0.7 grams of Alipal® C0-436 surfactant, 0.7 grams of Triton® N-101 surfactant, and 0.7 grams of sodium bicarbonate. The reaction vessel was then heated to 80°C. After the reaction vessel had reached 80°C, the following charge was added followed by a five minute hold: 13.8 grams of butyl acrylate, 3.8 grams of tributyl phosphate, 1.9 grams of hydroxyethyl acrylate, 9.8 grams of deionized water, 0.6 grams of Alipal® C0-436 surfactant, and 0.1 grams of sulfoethyl methacrylate. At this point an additional 25 grams of deionized water plus 1.8 grams of ammonium persulfate were added, followed by a 30 minute hold. Next, over a period of 2.5 hours, the following feed was added: 282.9 grams of butyl acrylate, 78.7 grams of tributyl phosphate, 39.7 grams of hydroxyethyl acrylate, 201.5 grams of deionized water, 11.9 grams of Alipal® CO-436 surfactant, and 1.5 grams of sulfoethyl methacrylate. After the completion of this feed, the reaction mixture was held at 80°C for 30 minutes. The reaction mixture was then cooled to room temperature followed by the addition of 0.1 grams of Foamkill® 649 defoamer. The reaction mixture was then transferred, through a filter, into an appropriate container.

The monomer feed compositions for Comparative Example C can be described as follows:

% Wei ht

Monomers Butyl Acrylate

Tributyl Phosphate

Hydroxyethyl Acrylate

Methacrylic Acid

Methyl Methacrylate Styrene

Tridecyl Alcohol

TOTAL 100.0 0 100.0 COMPARATIVE RESIN EXAMPLE D This example describes the synthesis of a water-borne acrylic latex which is not core/shell. In this example, a water-borne latex is synthesized from a monomer composition similar to that from which the shell of the present invention is made. No core type composition is present in this example. This "shell monomer ingredients only" water-borne latex resin was prepared from the following mixture of ingredients:

Ingredients Parts bv Weight (grams)

Deionized Water 551.5

Alipal® C0-436 13.2

Triton® N-101 0.7 Sodium Bicarbonate 0.7

Ammonium Persulfate 1.8

Methacrylic Acid 29.0

Methyl Methacrylate 246.5

Styrene 61.5 Hydroxyethyl Acrylate 64.5

Tridecyl Alcohol 2.5

Dimethyl Ethanol Amine 5.0

Sulfoethyl Methacrylate 1.6

Foamkill® 649 0.1

TOTAL 978.3

A suitable reaction vessel, equipped with an appropriate agitator and a nitrogen blanket was initially charged, at room temperature, with 260.2 grams of deionized water, 0.7 grams of Alipal® C0-436 surfactant, 0.7 grams of Triton® N-101 surfactant, and 0.7 grams of sodium bicarbonate. The reaction vessel was then heated to 80°C- After the reaction vessel had reached 80°C, the following charge was added followed by a five minute hold: 1.4 grams of methacrylic acid, 11.7 grams of methyl methacrylate, 2.9 grams of styrene, 3.1 grams of hydroxyethyl acrylate, 0.1 grams of tridecyl alcohol, 10.1 grams of deionized water, 0.6 grams of Alipal® C0-436 surfactant, and 0.1 grams of sulfoethyl methacrylate. At this point an additional 25 grams of deionized water plus 1.8 grams of ammonium persulfate were added, followed by a 30 minute hold. Next, over a period of 2.5 hours, the following feed was added: 27.6 grams of methacrylic acid, 234.8 grams of methyl methacrylate, 58.6 grams of styrene, 61.4 grams of hydroxyethyl acrylate, 2.4 grams of tridecyl alcohol, 201.2 grams of deionized water, 11.9 grams of Alipal® C0-436 surfactant, and 1.5 grams of sulfoethyl methacrylate. After the completion of this feed, the reaction mixture was held at 80°C for 30 minutes. The reaction mixture was then cooled to room temperature followed by the addition, over 15 minutes, of 5 grams of dimethyl ethanol amine and 55 grams of deionized water. Upon the completion of this addition, 0.1 grams of Foamkill® 649 defoamer was added and the reaction mixture was then transferred, through a filter, into an appropriate container.

The monomer feed compositions for Comparative Example D can be described as follows:

% Weight

Monomers

TOTAL 0 100.0 100.0

COATING FORMULA EXAMPLES Examples 1 through 4 describe the formulation of spray applied pigmented water-borne acrylic latex based coatings formulations which make use of the various latex resins as described in Example A and comparative Examples A through D. EXAMPLE 1 A pigmented water-borne acrylic latex based coating formulation in accordance with the present invention was made from the following ingredients:

Ingredients t t r

Resin from Example A 50% Dimethylethanolamine in Water White Tint Paste ¹ Cymel® 303 ² Phosphatized Epoxy Ammonium Benzoate (10% in water) Deionized Water

TOTAL 374.3

The white tint base is available from Cardinal Color Inc. as VA-AQI-1076 W/R White and has a pigment to binder ratio of 11.2

Hexamethoxy methylmelamine crosslinking agent available from American Cyanamid, Inc. The phosphatized epoxy is described in Example B.

EXAMPLE 2 This example shows the formulation of a pigmented water-borne acrylic latex based coatings formulation which makes use of the water-borne acrylic latex resin described in Comparative Example A. As described in Comparative Example A this latex resin was not core/shell in nature. This coating formulation was made from the following ingredients:

Ingredients Parts bv Weight (grams)

Resin from Comparative Example A 165.8

50% Dimethylethanolamine in Water 4.1 White Tint Paste ¹ 149.3

Cymel® 303 ² 20.0

Phosphatized Epoxy 6.5

Ammonium Benzoate (10% in water) 10.0

Deionized Water 50.0

TOTAL 405.7 The white tint base is available from Cardinal Color Inc. as VA-AQI-1076 W/R White and has a pigment to binder ratio of 11.2 o

Hexamethoxy methylmelamine crosslinking agent available from American Cyanamid, Inc.

° The phosphatized epoxy is described in Example B.

EXAMPLE 3

This example shows the formulation of a pigmented water-borne acrylic latex based coatings formulation which makes use of the water-borne acrylic latex resin described in Comparative Example B. As described in Comparative Example B this latex resin is core/shell in nature but was made with the core and shell compositions switched, that is the shell would have a Fox Equation derived T_ lower than that of the core. This coatings formulation was made from the following ingredients:

Ingredients Parts bv Weight (grams)

Resin from Comparative Example B 156.0 50% Dimethylethanolamine in Water 4.0 White Tint Paste *^■ 149.3 Cymel® 303 ² 20.0

Phosphatized Epoxy ³ 6.5

Ammonium Benzoate (10% in water) 10.0

Deionized Water 55.2 TOTAL 411.7

The white tint base is available from Cardinal Color Inc. as VA-AQI-1076 W/R White and has a pigment to binder ratio of 11.2.

Hexamethoxy methylmelamine crosslinking agent available from American Cyanamid Inc.

³ The Phosphatized Epoxy is described in Example B. EXAMPLE 4 This example shows the formulation of a pigmented water-borne acrylic latex based coatings formulation which makes use of the water-borne acrylic latex resins described in Comparative Examples C and D. These latex resins are not core/shell in nature, but each separately represents a core or a shell composition. They were formulated such as to provide the same core to shell weight ratio as would be present in a core/shell waterborne acrylic latex resin. This coatings formulation was made from the following ingredients:

Ingredients Parts bv Weight (grams)

Resin from Comparative Example C 77.0

Resin from Comparative Example D 84.9 50% Dimethylethanolamine in Water 2.9

White Tint Paste ¹ 149.3

Cymel® 303 ² 20.0

Phosphatized Epoxy 6.5

Ammonium Benzoate (10% in water) 10.0 Deionized Water 26.0

TOTAL 376.0

The white tint base is available from Cardinal Color Inc. as

VA-AQI-1076 W/R White and has a pigment to binder ratio of 11.2

₉

H Heexxaammeetthhooxxyy mmeetthhylmelamine crosslinking agent available from American Cyanamid Inc. 3 ^J The Phosphatized Epoxy is described in Example B.

The water-borne acrylic latex based formulations of the Examples described above were each spray applied over 10.2 centimeter by 30.5 centimeter Bonderite® 1000 panels available from ACT, Inc., the substrate panels being cold rolled steel pretreated with iron phosphate and a chrome rinse. The spray application involved first adjusting the spray viscosity to 21 seconds as measured using a number 2 Zahn cup, by adding additional deionized water. All of the coatings formulations were spray applied, tested and evaluated at the same time and under the same conditions. The relative humidity at which the water-borne coatings formulations were applied was 60%. The water-borne coating formulations had the following physical values prior to spray application:

Total solids refers to: 100 times (weight of resinous components + pigments + additives) divided by total formula weight.

Spray Viscosity is given in units of seconds and was determined using a number 2 Zahn Cup.

These formulations were first spray applied over Bonderite® 1000 steel panels at 60% relative humidity. The coatings formulations were spray applied using a liquid spray gun, (available from Binks Inc.). After allowing the coatings to flash for 7.0 minutes, they were cured in an electric oven set at 300°F, (149°C), for 15 minutes. The following table covers test data generated in the evaluation of the water-borne spray-applied coatings formulations.

60° Gloss ^L

Pencil Hardness Direct Impact (Pounds) Reverse Impact (Pounds)

C.F.T. ³ ₀f Minimum Sag (microns) 40.6 C.F.T. ³ of Minimum

Popping (microns) 45.7

60° Gloss was determined using a Pacific Scientific gloss meter. Pencil Hardness was determined as described in ASTM-D3363-

3 "C.F.T." is Cured Film Thickness. Direct and reverse impact were determined for final cured film thicknesses of between 1.0 and 1.3 mils using a Gardner Impact Tester as described in ASTM-D2749-90. Minimum sag was determined by applying the coatings formulation over a substrate in such a way as to produce increasing film thickness from the top of the substrate to its base. Prior to stoving the coated panels, lines are drawn horizontally through the wet coating. The coated panels were then cured in an electric oven, while hanging in a vertical position. Upon removing the coated panels from the oven, the cured film thickness at which the coating above the scribed line is seen to have flowed, (or sagged), down over the scribed line is designated as the "cured film thickness of minimum sag." Minimum popping was determined by applying the coating formulation over the substrate in such a way as to produce increasing film thickness from the top of the substrate to its base. The coated substrate is then flashed in an electric oven for 7.0 minutes. The coated substrate is then removed from the oven and the dry film thickness at whichrsolvent is observed to have occurred is designated as the "cured film thickness of minimum popping." The invention has been described in connection with specific embodiments, but it should be understood that the scope of the invention, as defined by the claims that follow, is not limited thereto, but includes variations and modifications as would be known to those of skill in the art.

Claims

Claim 1. A waterborne acrylic latex based coating composition having impact resistance and solvent popping resistance, the latex comprising particles having a hydrophobic polymeric core formed by initial emulsion polymerization of a first monomer composition comprising:

50-99 weight percent acrylic or methacrylic monomers selected to yield a polymer having T_g below 0°C, 0-5 weight percent acid group containing monomers, and

1-20 weight percent hydroxyl group containing monomers; the particles further having a hydrophilic polymeric shell formed by emulsion polymerization of a second monomer composition in the presence of the polymerization product of the first monomer composition, the second monomer composition comprising:

40-90 weight percent acrylic or methacrylic monomers selected to yield a polymer having T above 20°C,

5-20 weight percent acid group containing monomer, and

1-20 weight percent of hydroxyl group containing monomer.

Claim 2. The composition of claim 1 wherein the !_ of the polymerized product of the first monomer composition is at least 50°C lloowweerr tthhaann tthe T of the polymerized product of the second monomer composition.

Claim 3. The composition of claim 1 wherein the of the polymerized product of the first monomer composition is at least 75°C lloowweerr tthhaann 1the T of the polymerized product of the second monomer composition.

Claim 4. The composition of claim 1 wherein the of the polymerized product of the first monomer composition is at least 1 10000°°CC lloowweerr tthhaann tthhee T of the polymerized product of the second monomer composition.

Claim 5. The composition of claim 1 wherein the core is substantially free of acid groups.

Claim 6. The composition of claim 1 wherein the non-hydroxyl group containing and non-acid group containing acrylic monomers comprise at least 70 weight percent of the first monomer composition.

Claim 7. The composition of claim 6 wherein the non-hydroxyl group containing and non-acid group containing acrylic monomers comprise at least 60 weight percent of the second monomer composition.

Claim 8. The composition of claim 1 wherein the first and second compositions together include a sum of at least 10 weight percent of hydroxyl group containing monomers.

Claim 9. The composition of claim 1 wherein at least one of the first and second monomer compositions includes a crosslinking agent.

Claim 10. The composition of claim 1 wherein the first and second monomer compositions each include a crosslinking agent.

Claim 11. The composition of claim 1 wherein the acrylic or methacrylic monomers in the first monomer composition include compounds having the following structure:

||

CH₂

1 9 wherein R^x is H or CH3, and R contains at least four carbon atoms.

Claim 12. The composition of claim 1 wherein either or both monomer compositions further include 0 to 20 weight percent plasticizer.

Claim 13. The composition of claim 1 wherein the weight percentage of the first monomer composition is from 20 to 80 on the total weight of the sum of the first monomer composition and the second monomer composition.

Claim 14. The composition of claim 1 wherein the second monomer composition includes 0.5 to 40 weight percent monomer or oligomer having alpha, beta ethylenic unsaturation in addition to the previously recited monomers.

Claim 15. The composition of claim 1 wherein the first monomer composition include 0.5 to 40 weight percent monomer or oligomer having alpha, beta ethylenic unsaturation in addition to the previously recited monomers.

Claim 16. A waterborne acrylic latex based coating composition having impact resistance and solvent popping resistance, the latex comprising particles having a hydrophobic polymeric core formed by initial emulsion polymerization of a first monomer composition comprising: 50-99 weight percent acrylic or methacrylic monomers selected to yield a polymer having T below 0°C,

0-5 weight percent acid group containing monomers, and 1-20 weight percent hydroxyl group containing monomers; the particles further having a hydrophilic polymeric shell formed by emulsion polymerization of a second monomer composition in the presence of the polymerization product of the first monomer composition, the second monomer composition comprising: 40-90 weight percent acrylic or methacrylic monomers selected to yield a polymer having T_g above 20°C,

1-20 weight percent acid group containing monomer, the acid group content of the shell being greater than or equal to that of the core, and

1-20 weight percent of hydroxyl group containing monomer.

Claim 17. The composition of claim 16 wherein the acid group content of the shell is greater than that of the core.