Method for preparing a microbe resistant latex

Abstract

The present invention relates to a comprising the steps of a) stripping an acrylic or styrene-acrylic latex containing greater than 1000 ppm of one or more acrylic and / or styrenic monomers to achieve a concentration of the monomers of less than 1000 ppm, then b) adding from 150 ppm to 1000 ppm of a C4-C10-alkyl hydroperoxide to the steam-stripped latex. The method of the present invention provides a biocide-free approach for preserving latexes.

Description

[0001] Method for Preparing a Microbe Resistant Latex

[0002] Background of the Invention

[0003] The present invention relates to a method for preparing a latex that is resistant to microbe formation without the use of a biocide.

[0004] Aqueous dispersions of polymer particles (i.e., latexes) used in the coatings industry are preserved with antimicrobial agents to inhibit the formation and growth of biological organisms such as bacteria, yeast, and mold while in storage. Inhibition of these organisms prevents product degradation and spoilage, as well as off-gassing of volatile products and consequent pressure build-up in closed containment. Preservation is therefore essential for reasons of health, safety, and performance.

[0005] In-can preservatives such as isothiazolinones are facing intense regulatory scrutiny for their real or perceived adverse impact on health, safety, and the environment; in fact, an outright ban of these biocides in many parts of the world appears in the offing. Inasmuch as the development of new biocides is unlikely for reasons of cost and a widespread perception, justified or not, of their inherent dangers, a need exists to supplant biocides with alternative non-biocidal preservatives that are safer and more sustainable.

[0006] A recent example of a non-biocidal approach for preserving paints against microbial contamination can be found in EP 3 456 787 Bl, which discloses a water-borne coating formulation adjusted to a pH in the range of 10 to 12.5. While ostensibly effective, these very high pH formulations create additional safety and health concerns that render this approach impractical. Other non-traditional approaches such as the addition of silver or zinc ions may adversely affect the properties of the paint and face regulatory scrutiny as well. For these reasons, other safer and more sustainable approaches for preserving paints, and materials that are used in paints, are needed.

[0007] Summary of the Invention

[0008] The present invention addresses a need in the art by providing a method comprising the steps of a) stripping an acrylic or styrene-acrylic latex containing greater than 1000 ppm of one or more acrylic and / or styrenic monomers to achieve a concentration of the monomers of less than 1000 ppm, then b) adding from 150 ppm to 1000 ppm of a C4-Cio-alkyl hydroperoxide to the steam-stripped latex. The process of the present invention provides an acrylic or styrene- acrylic latex that exhibits resistance to microbial growth.

[0009] Brief Description of Drawings

[0010] FIG. 1 is a schematic of an apparatus used to steam strip the latex.

[0011] Detailed Description of the Invention

[0012] The present invention is a method comprising the steps of a) stripping an acrylic or styrene- acrylic latex containing greater than 1000 ppm of one or more acrylic monomers and optionally one or more styrenic monomers to achieve a concentration of the monomers of less than 1000 ppm, then b) adding from 150 ppm to 1000 ppm of a C4-Cw-alkyl hydroperoxide to the steam- stripped latex.

[0013] Stripping is a distillation process used to reduce the concentration of the monomers as well as other volatile organics from the latex; the term refers to removal of the monomers by steamstripping or non-condensable gas-stripping using, for example, air or nitrogen. The removal of the volatile organics is advantageously carried out at advanced temperatures and in vacuo. As used herein, the term “acrylic latex” refers to an aqueous dispersion of polymer particles that comprise at least 50, or at least 70, or at least 80, and or at least 90 weight percent structural units of acrylate, methacrylate, and acid monomers. The term “structural units” of the recited monomer refers to the remnant of the monomer after polymerization. For example, a structural unit of methyl methacrylate is as follows: structural unit of methyl methacrylate where the dashed lines represent the points of attachment to the polymer backbone.

[0014] The latex is advantageously prepared by emulsion polymerization, which invariably leaves a remnant of residual unreacted monomer that must be removed to regulatorily acceptable levels, generally on the order of less than 1000 ppm or less than 500 ppm or less than 100 ppm. The concentration of such monomers can be removed by chasing, that is, contacting the latex containing excess monomer with a redox coupler, which is a combination of an oxidizing agent such as / -amyl hydroperoxide (t-AHP) or t-butyl hydroperoxide (f-BHP) and a reducing agent such as isoascorbic acid. In the present invention, however, the concentration of excess monomer is reduced by stripping. In a second step, microbial resistance is achieved without the use of a biocide by addition of from 150 ppm or from 200 ppm or from 220 ppm to 1000 ppm or to 700 ppm or to 500 ppm of a C4-Cio-alkyl hydroperoxide, preferably t-AHP or t-BHP.

[0015] Examples of monomers suitable for the preparation of acrylic latexes include ethyl acrylate, / / -butyl acrylate, t-butyl acrylate, / / -hexyl acrylate, 2-ethylhexyl acrylate, 2-octyl acrylate, 2-propylheptyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, ureido methacrylate, acrylic acid, methacrylic acid, and itaconic acid. One or more of these monomers and a styrene monomer such as styrene and a-methyl styrene are suitable for the preparation of styrene-acrylic latexes.

[0016] After the latex is subjected to steam stripping under conditions to reduce the concentration of monomers in the latex to the desired level, the C4-C -alkyl hydroperoxide is added to provide resistance to microbial contamination.

[0017] Examples

[0018] Example 1 - Preparation of an Acrylic Latex

[0019] A first monomer emulsion (MEI) was prepared by mixing deionized water (160 g), sodium dodecylbenzene sulfonate (36.96 g, 23% active), a blend of Ci-Cs-acrylate monomers (374 g), methyl methacrylate (303.28 g), and methacrylic acid (2.72 g).

[0020] A second monomer emulsion (ME2) was prepared by mixing deionized water (270 g), Disponil FES 993 surfactant (23.8 g, 30% active), butyl acrylate (255 g), ethyl acrylate (306 g), methyl methacrylate (383.52 g), acetoacetoxy ethyl methacrylate (34 g, 95% active), phosphoethyl methacrylate (30.6 g, 60% active), sodium 4-vinylbenzenesulfonate (11.33 g, 90% active), and methacrylic acid (2.38 g).

[0021] Deionized water (1000 g) and sodium dodecylbenzene sulfonate (14.79 g, 23% active) were added to a 5-L, four necked round bottom flask equipped with a paddle stirrer, a thermometer, N2 inlet, and a reflux condenser. The contents of the flask were heated to 85 °C under N2 and stirring was initiated. A portion of MEI (105 g) was then added, quickly followed by addition of an aqueous solution of ammonium persulfate (5.1 g) dissolved in deionized water (25 g) followed by a rinse of deionized water (5 g). After stirring for 10 min, the remainder of MEI and a solution containing ammonium persulfate (0.67 g) dissolved in deionized water (30 g), were each added linearly and separately to the flask over a total period of 45 min. The contents of the flask were maintained at a temperature of 85 °C during the addition of MEI. When all additions were complete, the flask containing MEI was rinsed with deionized water (25 g), which was then added to the flask, and the contents of the flask were maintained at 85 °C for 15 min.

[0022] After the 15-min hold, ME2 and a solution containing ammonium persulfate (1.03 g) dissolved in deionized water (50 g), were each added linearly and separately to the flask over a total period of 70 min. The contents of the flask were maintained at a temperature of 85 °C during the addition of ME2. When all additions were complete, the flask containing ME2 was rinsed with deionized water (25 g), which was then added to the flask, and the contents of the flask were maintained at 85 °C for 10 min. After the 10-min hold, a solution containing ammonium hydroxide (10 g, 29% active) and deionized water (20 g) was added to the flask over 5 min.

[0023] The contents of the flask were cooled to room temperature. The resultant dispersion of polymer particles was neutralized to pH ~ 9.2 with an aqueous solution of ammonium hydroxide (30 g, 29% active) and Tergitol 15-S-40 surfactant (12.15 g, 70% active) in deionized water (40 g). An aqueous solution of Dequest 2016 (3.35 g, 33% active) in deionized water (10 g) was then added to the dispersion. The z-average particle size was found to be 113 nm using a Malvern Particle Size Analyzer; the measured solids was 47.9%.

[0024] Example 2 - Preparation of a Styrene-Acrylic Latex

[0025] A monomer emulsion (ME) was prepared by mixing deionized water (445 g), sodium lauryl sulfate (42.25 g, 28% active), Disponil FES-993 surfactant (12.09 g, 30% active), butyl acrylate (1143.18 g), styrene (782.43 g), vinyltrimethoxysilane (5.92 g), sodium 4-vinylbenzenesulfonate (6.57 g, 90% active), and acrylic acid (33.52 g).

[0026] Deionized water (770 g) and sodium lauryl sulfate (1.5 g, 28% active) were charged to a 5-L, four-necked round bottom flask equipped with a paddle stirrer, a thermometer, N inlet, and a reflux condenser. The contents of the flask were heated to 87 °C under N2 and stirring was initiated.

[0027] A portion of the ME (86 g) was then added, quickly followed by an aqueous solution of sodium persulfate (2.35 g) dissolved in deionized water (47 g) followed by a rinse of deionized water (5 g). After stirring for 5 min, the remainder of the ME and a solution containing sodium persulfate (7.05 g) and sodium hydroxide (3 g, 50% active) dissolved in deionized water (197 g) were each added separately to the flask over a total period of 150 min. The contents of the flask were maintained at 87 °C during the addition of ME. When all additions were complete, the vessel containing the monomer emulsion was rinsed with deionized water (25 g), which was then added to the flask.

[0028] The contents of the flask were cooled to room temperature. After cooling, the dispersion of polymer particles was neutralized to pH ~ 9.0 with an aqueous solution of sodium hydroxide (25 g, 50% active) and Tergitol 15-S-40 surfactant (14.25 g, 70% active) dissolved in deionized water (240 g). The z-average particle size was found to be 147 nm using a Malvern Particle Size Analyzer; the measured solids was 50.3%.

[0029] Method for Steam Stripping Volatiles from the Latex

[0030] FIG. 1 illustrates an example of a schematic of the equipment used to strip volatiles from latex, specifically by steam stripping. In an example of a method for removing VOCs from the aqueous dispersion of polymer particles (the latex), the latex is fed at a rate of 390 g / min from a heated (60 °C) stainless-steel jacketed vessel (1), via pump (2) to the process. The steam (flow rate = 55 to 110 g / min) enters through steam separator (9), needle valve (10), steam flowmeter (11), then contacts the feed material through a Tee at the entry to steam stripper (3). The mixed stream flows concurrently through steam stripper (3), which is a 1 -inch jacketed glass column with a jacket temperature of 60 °C, then into vertically oriented glass separator unit (4), where the gas liquid separation occurs. The liquid stream falls into the bottom of separator unit (4), then transferred continuously into a product collection bucket by way of pump (5). The gas stream, which contains steam and the volatile organic compounds (VOCs), exits separator unit (4) from the top and then is condensed by shell and tube type condenser (6). The condensate then falls into glass condensate collection pot (7) and finally transferred to a waste disposal site via pump (8). Vacuum pump (13) creates and controls the pressure to 90 mm Hg.

[0031] Preparation of Samples for Microbial Resistance

[0032] Samples were tested for microbial resistance “as-is” (not heat-aged) as well as after being subjected to 50 °C for four-weeks (heat-aged). A 10-g aliquot was taken from each sample and inoculated three times at 7-d intervals with 106-107colony forming units per milliliter of sample (CFU / mL) of a standard pool of bacteria, yeasts, and molds obtained from American Type Culture Collection (ATCC) that are common contaminants in coatings. Once inoculated, the samples were stored in 25 °C incubators. Test samples were monitored for microbial contamination by agar plating using a standard streak plate method. Samples were plated 1 day and 7 days after each microbial challenge onto trypticase soy agar (TSA) and potato dextrose agar (PDA) plates. All agar plates were checked daily up to 7 days after plating to determine the number of microorganisms surviving in the test samples. Between checks, the agar plates were stored in incubators at 30 °C for TSA plates and at 25 °C for PDA plates. The extent of microbial contamination was established by counting the colonies, where the rating score was determined from the number of microbial colonies observed on the agar plates. Reported results come from day 7 readings, and are summarized for both the “as-is” and heat-aged samples. Results are described by the rating score for each type of microorganism: B = bacteria, Y = yeast, and M - mold. For example, a 3B describes a plate with 3 rating score for bacteria, or a Tr Y(l) describes a plate with trace yeast (1 colony on plate). Table 1 illustrates the rating system used to estimate the level of microbial contamination on streak plates. Colonies refers to the number of colonies on the plate.

[0033] Table 1 - Rating system for Estimating Microbial Contamination

[0034] In Table 1, “Pass” means fewer than ten colonies were detected on plates on the specified day (Day 1 (DI) or Day 7 (D7)) after inoculation. “Fail means that ten or more distinct colonies were detected on plates on the specified day after inoculation. Preparation of a t-AHP-Containing Latex

[0035] After the latexes were steam-stripped, t-AHP was added to the samples at amounts of 250 ppm and 500 ppm. Table 2 illustrates the impact of t-AHP post-addition to resistance to microbial growth.

[0036] The results show that all steam-stripped latexes subjected to post-addition of 250 ppm or 500 ppm of t-AHP passed two challenge tests, while the latex with no post-added t-AHP failed both challenge tests.

Claims

Claims:

1. A method comprising the steps of a) stripping an acrylic or styrene- acrylic latex containing greater than 1000 ppm of one or more acrylic and / or styrenic monomers to achieve a concentration of the monomers of less than 1000 ppm, then b) adding from 150 ppm to 1000 ppm of a C4-Cio-alkyl hydroperoxide to the steam-stripped latex.

2. The method of Claim 1 where the stripping step achieves a concentration of the monomers of less than 500 ppm, wherein the stripping is carried out by way of steam stripping.

3. The method of Claim 1 where the steam stripping step achieves a concentration of the monomers of less than 100 ppm.

4. The method of Claim 1 wherein the amount of the C4-Cio-alkyl hydroperoxide added to the steam stripped latex is in the range of from 200 ppm to 500 ppm.

5. The method of Claim 4 wherein the C4-C -alkyl hydroperoxide is / -butyl hydroperoxide or / -amyl hydroperoxide.

6. The method of Claim 4 wherein the C4-Cio-alkyl hydroperoxide is / -amyl hydroperoxide, wherein the amount of / -amyl hydroperoxide added to the steam stripped latex is in the range of from 220 ppm to 500 ppm.

7. The method of Claim 4 wherein the C4-C -alkyl hydroperoxide is / -butyl hydroperoxide, wherein the amount of / -butyl hydroperoxide added to the steam stripped latex is in the range of from 220 ppm to 500 ppm.

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