FIELD OF THE INVENTION The invention relates to waterborne coatings and a way to reduce or eliminate blocking problems with the cured coatings.

BACKGROUND OF THE INVENTION

Waterborne coatings desirably have low contents of volatile organic compounds (VOC), but the resulting paints and stains can suffer from poor wetting, color issues, and surface defects when compared with their solvent-borne counterparts. The primary causes for these problems are the high surface tension of water and difficulties in forming good films with polymeric binders in aqueous media.

The polymeric binders in waterborne systems are acrylic latex emulsions. After the paint is applied, water evaporates, and the polymer latex droplets coalesce, ideally to give a uniform, solid film. Relatively soft latex droplets facilitate coalescence to produce continuous films, which are essential for good performance. To help with this process, waterborne coating formulations often include low-T _g polymeric binders or low-VOC coalescing agents. However, these components can cause sticky coatings with blocking problems even after curing.

Blocking is undesirable adhesion between two painted surfaces that stick together when pressed against each other. The paint sticks to itself when a window or door is opened and usually leaves behind bare patches of substrate. Blocking problems can sometimes be avoided with additives or by increasing the solids level of the paint. Polyoxyalkylene siloxanes and fluorochemicals have been proposed as anti blocking additives (see, e.g., U.S. Publ. No. 2008/0145552 and EP 1961797). However, fluorochemical-containing coatings are difficult to recoat, and the additives can negatively impact the environment and human health.

U.S. Pat. No. 8,822,580 describes a point-of-sale tinting system that contains a polyalkylene glycol humectant or ethoxylated surfactants that help to improve the blocking resistance of the paint. In this case, the humectant helps the paint stay wet for longer to allow latex particles more time to coalesce.

Z. Dou et al. (Polym. Paint Colour J. 198 (2008) 22) describe ethoxylated phosphate esters as APE-free emulsifiers for latex emulsion polymerization and as wetting agents for coatings with benefits that include blocking resistance. However, no benefit of non-ethoxylated alkyl phosphates is discussed.

Alkali metal salts of certain non-ethoxylated phosphate esters are known as wetting agents that impart good anti-blocking properties to waterborne coatings. Unfortunately, acidic phosphate esters and their corresponding alkali metal salts (and ammonium salts) suffer from relatively poor water solubility and give hazy or phase- separated mixtures even at low concentration. In addition, the ability of these materials to rapidly reduce surface tension is limited.

The coatings industry would benefit from the availability of paint additives that effectively deal with blocking problems of waterborne coatings. Ideally, the additives would be cost-effective, would be easy to introduce without causing phase separation, would assist in rapid film formation, and would contribute to a good balance of coating properties.

SUMMARY OF THE INVENTION In one aspect, the invention relates to a method for boosting the high-temperature blocking resistance of a cured coating made from a waterborne coating formulation. Blocking resistance is boosted by incorporating into the coating formulation an effective amount within the range of 0.010 to 1.0 weight percent, based on the amount of the coating formulation, of an anti-blocking additive. The anti-blocking additive comprises an organoamine salt of a non-ethoxylated C4-C15 alkyl phosphate. The resulting cured coating has a blocking resistance rating within the range 5 to 10. In some aspects, the blocking resistance rating is improved by at least 2 units when compared with the same cured coating produced in the absence of the anti-blocking additive.

In other aspects, the invention includes a waterborne coating formulation and a cured coating produced from the formulation. The coating formulation comprises an acrylic latex, water, a dispersant, a pigment, and from 0.010 to 1.0 wt.%, based on the amount of the coating formulation, of an anti-blocking additive comprising an organoamine salt of a non-ethoxylated C4-C15 alkyl phosphate.

We found that organoamine salts of non-ethoxylated C4-C15 alkyl phosphates provide a valuable improvement in blocking resistance of waterborne coatings. Surprisingly, the organoamine salts demonstrate good wetting properties as well as improved high-temperature blocking resistance, much better water solubility, and easier handling when compared with the corresponding alkali metal or ammonium salts. The additives are cost-effective and offer environmental and recoating advantages when compared with the fluorochemicals currently available as anti-blocking additives.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a plot of dynamic surface tension versus surface age for a series of aqueous mixtures containing 0.3 wt.% of various non-ethoxylated Cs-C-io alkyl phosphate salts.

DETAILED DESCRIPTION OF THE INVENTION Cured coatings with high-temperature blocking resistance are prepared from waterborne coating formulations. Blocking resistance is boosted by incorporating into the coating formulation an effective amount of an anti-blocking additive.

Anti-blocking additive

In some aspects, the anti-blocking additive comprises an organoamine salt of a non-ethoxylated C4-C15 alkyl phosphate and may include one or more other components helpful for improving solubility, compatibility, or other properties of the waterborne coating formulation. In other aspects, the anti-blocking additive consists essentially of the organoamine salt of the non-ethoxylated C4-C15 alkyl phosphate.

By “effective amount,” we mean an amount of anti-blocking additive sufficient to impart improved blocking resistance of the cured waterborne coating when compared with that of the same cured coating prepared without the anti-blocking additive. The anti-blocking additive is included in the waterborne coating formulation in an amount within the range of 0.010 to 1.0 wt.% based on the amount of the coating formulation. In other aspects, the anti-blocking additive is used in an amount within the range of 0.050 to 0.5 wt.%, or from 0.080 wt.% to 0.3 wt.%, based on the amount of the coating formulation.

“Waterborne” coating formulations can (and often do) include a minor proportion of an organic solvent, which is typically included as a coalescing agent or to modify film forming properties. Generally, waterborne coating formulations will comprise at least about 25 to 40 wt.% of water in addition to an acrylic latex and other components.

By “coating formulation,” we mean formulations suitable for use as water-based paints, inks, varnishes, architectural coatings, industrial coatings, OEM coatings, special- purpose coatings, enamels, caulks, sealants, and other polymeric coatings for which improved blocking resistance is desirable.

In some aspects, the anti-blocking additive comprises an organoamine salt of a non-ethoxylated C4-C15 alkyl phosphate. The salts are generated by neutralizing the corresponding acidic alkyl phosphates with an organic amine. Although ethoxylated alkyl phosphates are used in the coatings field as hydrophilic surfactants, we found that ethoxylation can detract from good blocking resistance. Additives used in the inventive method are not ethoxylated, i.e., no oxyethylene units are introduced between the parent C4-C15 alcohol and the phosphate ester groups.

The C4-C15 alkyl phosphates (also described herein as “acidic phosphate esters” or “phosphate esters”) are made by known methods from the corresponding C4-C15 alcohols and a phosphating agent. Suitable phosphating agents include, for example, combinations of phosphorus pentoxide with hypophosphorous acid, polyphosphoric acid, or the like. Examples of suitable phosphating procedures are provided below. The phosphate esters can comprise monoesters, diesters, or combinations of these. A minor proportion of phosphate triester can also be present.

Suitable C4-C15 alcohols for making the alkyl phosphate esters are linear, branched, or cycloaliphatic. The alcohols can be pure compounds or mixtures. In some aspects, the alcohols are C6-C14 alcohols or Cs-C-io alcohols, particularly linear C6-C or C8-C10 alcohols.

The C4-C15 alkyl phosphates are neutralized with organoamines to give the desired organoamine salts. Suitable organoamines include C1-C20 primary, secondary, and tertiary amines or alkanolamines. Examples include methylamine, ethylamine, isopropylamine, n-butylamine, n-hexylamine, n-octylamine, 2-ethylhexylamine, diethylamine, di-n-butylamine, diisopropylamine, triethylamine, tri-n-butylamine, benzylamine, 2-phenyl-ethylamine, 2-amino-2-methyl-1 -propanol, ethanolamine, diethanolamine, triethanolamine, isopropanolamine, N-methylethanolamine, N- methyldiethanolamine, N,N-dimethylethanolamine, and the like, and mixtures thereof.

Neutralization of the C4-C15 alkyl phosphates with the organoamines is generally straightforward. The acidic phosphate esters are converted to the corresponding organoamine salts by adding stoichiometric amounts of the organoamines either neat or in aqueous solution to the acidic phosphate esters. When the phosphate esters are neutralized in neat form (i.e., not in aqueous solution), the pH of a 5 wt.% aqueous solution of the final product is ~7 to ~9. The phosphate ester can also be diluted in water first and subsequently neutralized with the amine to a solution pH of ~7 to ~9. Depending on the nature of the organoamine and phosphate ester, a viscous stage may occur during neutralization, for instance, when the total solids is greater than about 30 wt.%. Sufficient mixing ensures homogeneity during this process. Examples of how to make the organoamine salts with diethanolamine or tri-n-butylamine appear below.

In some aspects, the organoamine salt is an alkanolamine salt of a non- ethoxylated OQ-OM alkyl phosphate. In other aspects, the organoamine salt is an alkanolamine salt of a non-ethoxylated OQ-OM alkyl phosphate, especially a diethanolamine salt of a non-ethoxylated Cs-C-io alkyl phosphate.

The organoamine salts are included in the waterborne coating formulations in an amount effective to boost the high-temperature blocking resistance of a cured coating made from the formulation. High-temperature blocking resistance is measured by ASTM D4946-89 at 50°C, and results are evaluated on a scale of 0-10, with 0 signifying very poor blocking resistance (75-100% sealing of the painted layers) and 10 signifying perfect (or near perfect) blocking resistance (i.e., no tackiness detected). The applicable sliding scale for evaluation appears below in Table 3.

Generally, the organoamine salts can boost the blocking resistance rating of the cured waterborne coating to a value within the range of 5 to 10, or in some aspects, to 6.0 to 9.5 or from 7.0 to 9.0. In other aspects, the improvement is at least 2, at least 4, at least 6, or at least 8 units higher than the blocking resistance of the same coating produced in the absence of the organoamine salt.

In other aspects, the invention relates to a waterborne coating formulation. The formulation comprises an acrylic latex, water, a dispersant, a pigment, and from 0.010 to 1.0 wt.%, based on the amount of the coating formulation, of an anti-blocking additive comprising an organoamine salt of a non-ethoxylated C4-C15 alkyl phosphate. In other aspects, the waterborne coating formulation may include one or more additional components selected from defoamers, rheology modifiers, solvents, biocides, neutralizing agents, preservatives, fillers, pigment extenders, and the like. Suitable acrylic latexes can be made by emulsion polymerization of acrylic monomers and other components according to well-known methods (see, e.g., WO 2020/185513 at pp. 26-27 and WO 2019/161323 at pp. 10-12). Suitable pigments (or pigment dispersions) used for making coatings are well known and readily available. Examples of suitable pigments appear in U.S. Pat. No. 7,442,724, the teachings of which are incorporated herein by reference. Suitable dispersants, defoamers, biocides, solvents, neutralizing agents, rheology modifiers, and other components used to formulate the waterborne coatings are also well known and are considered conventional.

In some aspects, the waterborne coating formulation comprises 35 to 55 wt.% of the acrylic latex, 25 to 45 wt.% of the pigment, and 5 to 40 wt.% of water. In other aspects, the waterborne coating formulation comprises 40 to 50 wt.% of the acrylic latex, 30 to 40 wt.% of the pigment, and 10 to 30 wt.% of water. In these aspects, conventional fillers and/or pigment extenders (e.g., barium sulfate, aluminum trihydrate, bentonite, calcium carbonate, aluminum silicate, mica, silicas, silica-aluminas, magnesium silicate, or the like) can be part of the 25 to 45 wt.% or 30 to 40 wt.% of the pigment component.

The following examples illustrate the invention. Those skilled in the art will recognize many other variations that are within the scope of the claims.

N-Octyl phosphate ester (A1-A7 precursor)

A round-bottom flask equipped with an agitator, heating mantle, and nitrogen sparge is charged with dry n-octyl alcohol (825 g, 6.35 mol) and hypophosphorous acid (50% aq. solution, 0.90 g). Phosphorus pentoxide (301 g, 2.1 mol) is slowly added over 4 h with mixing and cooling to control the exotherm below 75°C. The reaction mixture is then held at 75°C for 2 h and at 90°C for 3 h. The product is a colorless liquid. Measured acid values corresponding to endpoints 1 , 2, and 3 are 214, 324, and 325 mg KOH/g, respectively, indicating little or no free phosphoric acid as a by-product. lso-Cii-Ci4 alkyl phosphate ester (A10 precursor)

A round-bottom flask equipped as described earlier is charged with EXXAL™ 13 branched tridecyl alcohol (248 g, 1.3 mol, product of ExxonMobil). Polyphosphoric acid (115% H3PO4 basis, 74 g) is added over 5 minutes with mixing, and the reaction mixture exotherms from 22°C to 52°C. The mixture is heated to 68°C and is held at 68°C for 1 .5 h. Phosphorus pentoxide (37 g) is added over 25 minutes. The mixture is heated at 68°C for 40 minutes followed by heating at 90°C for 3 h. Water (5.3 g) is added, and the mixture is kept at 90°C for 3 h. ³¹ P NMR indicates that the product contains an 85:15 molar mixture of mono- and diphosphate esters and is free of pyrophosphates. Measured acid values corresponding to endpoints 1 , 2, and 3 are 216, 411 , and 450 mg KOH/g, respectively, indicating 6.7 wt.% of phosphoric acid by-product.

Phosphate ester precursors to additives A8 and A9 are similarly prepared from (respectively) the corresponding Ce or Cs-C-io alcohols. The phosphate esters are diluted with water and neutralized to pH ~7 to ~9.

Orqanoamine salts of the phosphate esters

Acidic phosphate esters are converted to the corresponding organoamine salts by adding stoichiometric amounts of organoamines (alkylamines, alkanolamines) either neat or in aqueous solution to the acidic phosphate esters. When the phosphate esters are neutralized in neat form (i.e., not in aqueous solution), the pH of a 5 wt.% aqueous solution of the final product is ~7 to ~9. The phosphate ester can also be diluted in water first and subsequently neutralized with the amine to a solution pH of ~7 to ~9. Depending on the nature of the amine and phosphate ester, a viscous stage may occur during neutralization, for instance, when the total solids is greater than about 30 wt.%. Sufficient mixing ensures homogeneity during this process. The water solubility of these additives is evaluated visually under ambient conditions. The clarity of the sample is also recorded. Table 1A, below, lists organoamine salts prepared for testing in waterborne coating formulations.

Tri-n-butylamine-neutralized phosphate ester A6 n-Octyl phosphoester (5.61 g) is charged to a glass jar equipped with a mixer. Deionized water (12.7 g) is added with mixing. T ri-n-butylamine (5.5 g) is added dropwise with mixing. The pH of the solution is monitored during neutralization until the solution becomes homogeneous and its pH is stable. The product is a clear liquid (46.3 wt.% solids; pH: 7.3).

Diethanolamine-neutralized phosphate ester A7 n-Octyl phosphoester (455 g, 2.63 mol total acid) is charged to a beaker with agitation. Diethanolamine (201 g, 1.91 mol) is slowly added over 10 minutes. The pH of a 5 wt.% solution in isopropanol/water (50/50) is 7.1. The warm liquid becomes a paste after a few hours. Warming the DEA salt to 80°C and diluting it with warm deionized water to 30 wt.% solids provides a clear, free-flowing liquid. Alkali metal or ammonium salts of the phosphate esters (comparative examples)

For comparative purposes, alkali metal or ammonium salts of n-octyl phosphate esters are prepared. Thus, sodium hydroxide, potassium hydroxide, or ammonium hydroxide solutions are combined with aqueous solutions of the acidic phosphate ester to achieve a pH within the range of 7.0 to 9.0. An acidic phosphate ester is also used “as is” for comparison. Each of the resulting comparative additives (see Table 1B) is mixed well prior to use.

Paint formulations

The control paint formulation (see Table 2) is a low-VOC, semi-gloss latex paint prepared with pre-dispersed titanium oxide (pigment concentration: 25.4 vol.%). The acrylic latex is made by emulsion polymerization of n-butyl acrylate (52 wt.%), methyl methacrylate (46 wt.%), and methacrylic acid (2 wt.%) to a targeted T _g for the latex of about 0°C. The components are added slowly to a vessel under proper shear using a Cowles mixing blade. The final viscosity is adjusted to 90 to 100 KU, and pH is adjusted to about 9. The control formulation contains no blocking-resistance additive. See Table 4 for a summary of the paint formulations.

Test formulations containing the control paint formulation of Table 2 and 0.1 to 1.0 wt.% of additives A1-A10 are prepared as identified in paint Examples 1-16 and Comparative Examples 18-22. In each case, the phosphate ester is added to the otherwise-complete paint formulation slowly with proper mixing, and mixing continues for 0.5 h to ensure a homogeneous mixture. Ammonium hydroxide solution is used to adjust the pH to about 9. Comparative Example 17 is a control example with no anti-blocking additive. High-temperature blocking resistance

High-temperature (50°C) blocking resistance is measured by ASTM D4946-89. Paint samples are cast on a sealed Leneta WB chart to a uniform 6-mil wet film thickness. The film dries in a horizontal position under ambient conditions for 7 days. The film is cut into 1.5” by 1.5” squares, and the blocking resistance is tested by placing the squares face-to-face with a 1 -kg weight on top for 0.5 h in a 50°C oven. Blocking resistance is rated visually on a scale of 0 to 10 (see Table 3) after the sample cools for 0.5 h. A higher rating indicates better high-temperature blocking resistance. As shown in Table 4, organoamine salts of alkyl phosphate esters significantly boost the blocking resistance of waterborne coatings (Examples 1-16) when compared with a control example with no additive (Comparative Example 17), even when used at very low concentration (see Example 7). The performance in anti-blocking properties rivals that available from a commercial fluorosurfactant (Comparative Example 22). When compared at the same use level (0.30 wt.%) with the free acid or ammonium or alkali metal salts of the phosphate esters, the organoamine salts are much better at imparting anti-blocking character to the cured waterborne coatings (see Example 10 versus Comparative Examples C18-C21).

Water solubility

Surprisingly, the phosphate ester salts from organoamines are far more water soluble than their ammonium or alkali metal counterparts. Better water solubility significantly improves handling and convenience of use of the salts. Table 5 compares the appearance of various salts of n-octyl phosphate esters as a function of actives content (in wt.%). As shown in the table, the diethanolamine salt provides a clear liquid at all tested actives levels from 0.30 wt.% to 77 wt.%. In contrast, the unneutralized n- octyl phosphate ester and the ammonium or alkali metal salts phase separate at 10 wt.% actives and are opaque or hazy liquids at much lower actives levels. Surface free energy

Surface free energy reflects the interaction between the atoms and molecules in a substance. The surface free energy encompasses two components: dispersive energy and polar energy. Dispersive energy is caused by interactions between temporary fluctuations of the charge distribution in the atoms/molecules (van der Waals interaction). Polar energy is caused by Coulomb interactions between permanent dipoles and between permanent and induced dipoles (e.g., hydrogen bonds). The surface free energy si is made up of dispersive energy si ^ά and polar energy s according to: s _i = s† + af Comparing the ratio between the dispersive and polar parts of the surface energy for two phases enables a prediction of the adhesion and compatibility between these two phases. The closer the ratios, the more phase interactions possible, and the higher the expected adhesion and compatibility.

Surface free energies, dispersive energies, polar energies, and water contact angles of paint films are measured using a mobile surface analyzer (from Kruss) under ambient conditions. Paint films are prepared according to ASTM D4946-89. After the films dry for 7 days, analysis is performed on five different locations on the film. An average of 5 measurements is recorded as the surface free energy value in Table 6. Surprisingly, the water-soluble organoamine salts impart excellent water repellency to the paints. Table 6 summarizes water contact angles of the tested paint films. A higher water contact angle indicates higher water resistance. A common problem for waterborne coatings is their sensitivity to water when compared with their solvent- borne counterparts. This can cause color inconsistency and “snail trails” in high-humidity environments. At only 0.1 wt.%, the organoamine salts can boost the water contact angle of a paint film more than 20 degrees. In contrast, the commercial fluorosurfactant (Comparative Example 22) also reduces surface energy of the paint, but it cannot match the increase in water repellency attributable to the organoamine salts. Table 6 shows that the organoamine salts can reduce the surface energy of paint films (Examples 1 -6, 8, and 15) significantly when compared with that of films that contain no additive (Comparative Example 17). The reduction in surface energy is like that of a film made using a commercial fluorosurfactant additive (Comparative Example 22). The reduced surface energy suggests that the paint has improved ability to wet hydrophobic substrates, reduce surface defects, and improve leveling and flow.

In some aspects, the anti-blocking additive comprising the organoamine salt is incorporated into the cured coating in an amount effective to reduce its total surface energy. Thus, the total surface energy of the cured coating is preferably at least 10%, at least 20%, or at least 30% less than that of the same cured coating made without the anti- blocking additive.

Surface energy has two components: dispersive energy and polar energy. As shown in Table 6, the paint made with the fluorosurfactant has higher polar energy, while paints made with the organoamine salts have higher dispersive energy. The higher dispersive energy and lower polar energy for the paints containing the organoamine salts make them more compatible with resins and more water-resistant. Although the fluorosurfactant gives the paint films a lower total surface energy, the higher polar energy and lower dispersive energy make them less compatible with resins and more water- sensitive. Consequently, paints made using the organoamine salts can avoid the surface defects and poor recoatability of paints that incorporate a fluorosurfactant. Dynamic surface tension

Dynamic surface tension measurements are conducted with a bubble pressure tensiometer BP-100 (from Kruss) under ambient conditions. The additives are combined at 0.3 wt.% with deionized water. The surface age of the bubble is controlled from 10 to 50,000 milliseconds.

Fig. 1 compares the results of measuring dynamic surface tension as a function of surface age for aqueous mixtures that contain 0.3 wt.% of various anti-blocking additives. The C8-C10 alkyl phosphate diethanolamine salt significantly improves dynamic surface behavior when compared with the free Cs-C-io alkyl acidic phosphate ester or the Cs-C-io alkyl phosphate ammonium or alkali metal salts.

Dynamic surface tension reflects the speed of an additive’s movement toward the interface as well as its ability to reduce surface energy. Fig. 1 shows that the diethanolamine salt migrates to the interface much faster than the other salts or the free acid, a clear advantage for paint films that dry quickly.

Experiments are performed to show the impact of ethoxylation on an additive’s ability to resist blocking. For this purpose, a series of ethoxylated tridecyl phosphate ammonium salts is prepared (ethoxylation with 3, 6, or 12 moles of EO per mole of tridecyl phosphate) and tested at 0.3 wt.% in the control paint formulation described earlier. In general, the blocking resistance ratings for these films is less than or equal to 2, i.e., poor in comparison to the non-ethoxylated phosphate ester ammonium salts.

The preceding examples are only illustrations. The following claims define the scope of the invention.