Process for preparing a coating composition having improved color retention

Abstract

The present invention relates to a process for preparing a blend comprising an aqueous dispersion of polymer particles, PMHS, an opacifying pigment and an extender. The blend of the present invention is useful in exterior paint applications, especially where color retention in the painted substrate is important.

Description

Background Art

[0001] The present invention relates to a process for preparing a coating composition that exhibits improved color retention when applied to a substrate.

[0002] Color retention, defined by the loss of color (or fading) caused by light degradation of the organic coating on the substrate, is highly desirable for exterior coatings. Premature and excessive fading typically occurs on surfaces overexposed to sunlight, which can also exacerbate chalking of the coating. Color loss is mechanically caused by UV degradation, which accelerates chemical changes in the colorant by generating hydroxyl and peroxyl radicals, which in turn lead to photo-oxidation (or photochemical degradation) in most polymers.

[0003] Color retention in pigmented coating formulations is particularly challenging. TiO is always present in paints. 2 The pigment particles strongly absorb UV radiation while providing excellent hiding power, which leads to the formation of oxidative hydroxyl radicals. 2 The adverse effects of the technology include the use of SiO 2 、ZrO 2 or both pre-treat the surface in a way that minimizes but does not eliminate the formation of hydroxyl radicals.

[0004] Mitigation of UV light and TiO 2 Attempts to influence color loss in pigmented coatings have included incorporating UV absorbers and antioxidants into paint formulations. These additives tend to have limited effectiveness over time; moreover, they tend to be more effective at reducing yellowing than at reducing fading. It would therefore be advantageous in the exterior coatings art to find a simple, effective, and cost-effective way to reduce fading in pigmented coatings. Summary of the invention

[0005] In one aspect, the present invention solves the need in the art by providing a method for forming a blend comprising an aqueous dispersion of polymer particles, polymethylhydrogensiloxane (PMHS), an opacifying pigment, and an extender; wherein the order of addition comprises any of the following steps:

[0006] a) mixing the polymethylhydrogensiloxane with an aqueous dispersion of the polymer particles to form a polymethylhydrogensiloxane / polymer particle blend; and then mixing the polymethylhydrogensiloxane / polymer particle blend with the opacifying pigment and the extender in any order; or

[0007] b) mixing the opacifying pigment with an aqueous dispersion of the polymer particles to form an opacifying pigment / polymer particle blend; and then mixing the opacifying pigment / polymer particle blend with the polymethylhydrogensiloxane and the extender in any order; or

[0008] c) mixing the extender with the aqueous dispersion of polymer particles to form an extender / polymer particle blend; and then mixing the extender / polymer particle blend with the polymethylhydrogensiloxane and the opacifying pigment in any order; or

[0009] d) adding the polymethylhydrogensiloxane to a blend comprising the aqueous dispersion of the polymer particles, the pigment and the extender;

[0010] The polymethylhydrogensiloxane is represented by the following formula:

[0011]

[0012] Each R 1 independently H or SiMe 3 ; R 2 CH 3 or phenyl; and R 3 C 1 -C 18 -alkyl, C 1 -C 18 - alkoxy or phenyl; m is 0 to 199 and n is 1 to 200.

[0013] The process of the present invention can be used to prepare pigmented paint formulations that exhibit excellent color retention when applied to a substrate. DETAILED DESCRIPTION

[0014] The present invention is a method for forming a blend comprising an aqueous dispersion of polymer particles, PMHS, an opaque pigment and an extender; wherein the order of addition comprises any of the following steps:

[0015] a) mixing the PMHS with an aqueous dispersion of the polymer particles to form a polymethylhydrogensiloxane / polymer particle blend; and then mixing the polymethylhydrogensiloxane / polymer particle blend with the opacifying pigment and the extender in any order; or

[0016] b) mixing the opacifying pigment with the aqueous dispersion of the polymer particles to form an opacifying pigment / polymer particle blend; and then mixing the opacifying pigment / polymer particle blend with the PMHS and the extender in any order; or

[0017] c) mixing the extender with the aqueous dispersion of polymer particles to form an extender / polymer particle blend; and then mixing the extender / polymer particle blend with the PMHS and the opacifying pigment in any order; or

[0018] d) adding the polymethylhydrogensiloxane to a blend comprising the aqueous dispersion of the polymer particles, the pigment and the extender;

[0019] The PMHS is represented by the following formula:

[0020]

[0021] Each R 1 independently H or SiMe 3 ; R 2 CH 3 or phenyl; and R 3 C 1 -C 18 -alkyl, C 1 -C 18 - alkoxy or phenyl; m is 0 to 199 and n is 1 to 200.

[0022] PMHS is preferably a homopolymer (m=0) or a copolymer, where m>0 and R 2 and R 3 Each independently is CH 3 ; In another embodiment, m is 0 to 99 and n is 1 to 100. In a first aspect, in a first step, PMHS is blended with an aqueous dispersion of polymer particles (latex), examples of which include polyacrylic acid, poly(styrene-acrylic acid), polyvinyl acetate, and polyurethane-based latex. In a second step, the PMHS / polymer particle blend is mixed with a pigment and an extender together or separately and in any order. As used herein, "opaque pigment" refers to a metal oxide having a refractive index greater than 2.0. Suitable opaque pigments include TiO 2 and ZnO. Suitable extenders include carbonates, silica, silicates, aluminosilicates, phosphates, and non-hollow organic microspheres. More specific examples of extenders include talc, clay, mica, sericite, CaCO 3 , nepheline syenite, feldspar, wollastonite, kaolinite, dicalcium phosphate and diatomaceous earth.

[0023] The concentration of PMHS is preferably in the range of 0.1 wt% to 5 wt% based on the weight of the blend (composition). The weight-weight ratio of PMHS to the total amount of opaque pigment and extender is preferably in the range of from 0.5:1, more preferably from 0.2:1 and most preferably from 0.1:1 to 0.001:1, more preferably to 0.005:1 and most preferably to 0.01:1. The preferred ratio of PMHS to opaque pigment is in the range of from 0.8:1, more preferably from 0.4:1 and most preferably from 0.2:1 to 0.002:1, more preferably to 0.01:1 and most preferably to 0.05:1. TiO 2 It is the preferred opaque pigment.

[0024] In a second aspect, an opacifying pigment is mixed with an aqueous dispersion of polymer particles to form an opacifying pigment / polymer particle blend, and the opacifying pigment / polymer particle blend is then mixed with the PMHS and extender together or separately and in any order.

[0025] In a third aspect, an extender is mixed with an aqueous dispersion of polymer particles to form an extender / polymer particle blend; the extender / polymer particle blend is then mixed with the PMHS and opacifying pigment together or separately and in any order.

[0026] In a fourth aspect, PMHS is added to a blend comprising an aqueous dispersion of polymer particles, a pigment, and an extender. Notably, the method of the present invention does not include contacting PMHS with an extender or pigment in the absence of an aqueous dispersion of polymer particles.

[0027] In another aspect, the total amount of pigment and extender used in the process of the present invention is such that the composition has a PVC above the critical pigment volume concentration (PVC). The term "critical pigment volume concentration" (CPVC) refers to the minimum concentration of polymer required to wet the surface of the pigment particles. Above the CPVC, the volume of the adhesive is insufficient to coat all of the pigment and extender. In another aspect, the process of the present invention forms a composition having a PVC of at least 60%, or at least 65%, and in one aspect no greater than 90%.

[0028] In yet another aspect, the total amount of pigment and extender used in the process of the present invention is such that the composition has a PVC below the critical pigment volume concentration (PVC). "Critical pigment volume concentration" (CPVC) refers to the minimum concentration of polymer required to wet the surface of the pigment particles. In this aspect, the process of the present invention forms a composition having a PVC of no greater than 60%, or no greater than 50%, and in one aspect at least 10%, or at least 25%, or at least 35%.

[0029] The CPVC of a coating can be conveniently determined using reflectance (integrating sphere, including spectral reflectance, 10 degree observer / D65) as follows: compare the difference in reflectance of a coated film in the dry state to the same film that has been rewetted with a penetrating solvent having a similar refractive index to the polymer that fills the air voids of the dried coating (such as Isopar L solvent). When the coating is above the CPVC, the gamma reflectance of the rewetted coating will be reduced by at least 2% from the initial gamma reflectance value of the dried coating. The PVC is defined by the following formula:

[0030]

[0031] Here, binder solids refers to the polymer contribution from the aqueous dispersion of polymer particles that binds the pigment and chain extender particles together.

[0032] The method of the present invention preferably also includes adding any or all solvents, rheology modifiers, defoamers, dispersants, preservatives, coalescing agents, neutralizers, colorants and additional water. The preferred order of addition of these materials is easily determined by those of ordinary skill in the art without undue experimentation.

[0033] It has surprisingly been found that exterior paints prepared using the process of the present invention provide coatings having excellent color retention when subjected to accelerated UV exposure.

[0034] Example

[0035] The above critical paint formulations were prepared according to Table 1. In the table, PRIMAL TM WDV-2001 refers to PRIMAL TM WDV-2001 acrylic copolymer (46.5% solids); METHOCEL TM J75MS refers to METHOCEL TM J75MS cellulose ether; Nopco NXZ refers to Nopco NXZ defoamer; Calgon N refers to Calgon N (10%) dispersant; TAMOL TM 165A refers to TAMOL TM 165A dispersant (21%); Ti-Pure R-746 refers to Ti-Pure R-746TiO 2 ; Attagel 50 refers to Attagel 50 attapulgite; Minex 4 refers to Minex 4 nepheline syenite extender; Durcal (Omyacarb 2) refers to Durcal (Omyacarb 2) CaCO 3Durcal(Omyacarb 40) refers to Durcal(Omyacarb 40)CaCO 3 ; Temisca#15 refers to Temisca#15 sand; Unimin 50-30 refers to Unimin 50-30 sand; Rozone 2000 refers to Rozone 2000 preservative; DOWANOL TM DOWANOL TM DPnB coalescing agent; PMHS / PDMS refers to the polymethylhydrogensiloxane / polydimethylsiloxane polymers shown in Table 2. PRIMAL, METHOCEL, TAMOL and DOWANOL are trademarks of The Dow Chemical Company or its affiliates.

[0036] Table 1 - Paint formulations above critical

[0037]

[0038]

[0039] a See Table 2

[0040] Five PMHS and one PDMS siloxane polymers were added to the paint formulation at 0.5 wt% based on the formulation weight. The polymers are summarized in Table 2:

[0041] Table 2 - Polysiloxanes

[0042] PMHS1 – XIAMETER TM MHX-1107 fluid (100% MeSiH)

[0043] PMHS2–Gelest HMS 501 Dimethylsiloxane Copolymer (45%-55% Methyl Hydrogen Siloxane)

[0044] PMHS 3–Gelest HMS 301 Dimethylsiloxane Copolymer (25%-35% Methyl Hydrogen Siloxane)

[0045] PMHS 4 – Gelest HMS-151 Dimethylsiloxane Copolymer (15%-18% Methylhydrogensiloxane)

[0046] PMHS 5 – Gelest HMS-071 Dimethylsiloxane Copolymer (6%-7% Methylhydrogensiloxane)

[0047] PDMS–Dow Corning 1Q-3563PDMS

[0048] XIAMETER is a trademark of The Dow Chemical Company or one of its affiliated companies.

[0049] The paint formulation was tinted with Colortrend Blue 888-7214 dye (1 wt%) and applied to aluminum panels (3.2 mm thick). The coated panels were dried at 25° C. and 50% relative humidity and then subjected to 5000 h of accelerated UV exposure (102 min sunlight followed by 18 min water spray on the coated surface) using a Ci 5000 Xenon Arc Weathering Instrument (Atlas Corp.) set to ASTM G-155 Cycle 1. The emissivity was set to 0.35 W / m 2 .

[0050] ΔE was measured using a Byk-Gardner Spectro-Guide colorimeter according to ASTM D-1925. The larger the ΔE, the worse the color retention. Typically, the human eye can detect a ΔE greater than 1. Table 3 shows the color retention (ΔE) of coatings formed from a paint formulation (PVC=82.7) above the critical without polysiloxane additives, PMHS additives 1-5, and PDMS additives at 5000 hours.

[0051] Table 3 - Color retention of above critical paint formulations with and without polysiloxane additives

[0052] Paint Examples Polysiloxane Additives ΔE Comparative Example 1 none 15.1 Comparative Example 2 PDMS 10.3 Example 1 PMHS1 4.8 Example 2 PMHS2 5.2 Example 3 PMHS 3 5.0 Example 4 PMHS 4 6.3 Example 5 PMHS 5 6.6

[0053] Table 4 shows the components and amounts of the subcritical paint formulations. In the following table, Rocima 63 refers to Rocima 63 microbicide; TERGITOL TM 15-S-9 refers to TERGITOL TM 15-s-9 secondary alcohol ethoxylate; Bioban IPBC40-LE refers to Bioban IPBC 40-LE antimicrobial agent; Foamstar A-34 refers to Foamstar A-34 defoamer; RHOPLEX TM VSR-50 refers to RHOPLEX TM VSR-50 acrylic latex; ACRYSOL TM RM-8W and ACRYSOL TM RM-2020NPR refers to ACRYSOL TM RM-8W and ACRYSOL TM RM-2020NPR Rheology Modifier. TERGITOL, RHOPLEX and ACRYSOL are trademarks of The Dow Chemical Company or one of its affiliates.

[0054] The paint formulation was tinted with Colortrend Blue 808-7214 dye (3 wt%) and applied to aluminum panels (0.25 mm thick). Accelerated UV testing was performed as described above.

[0055] Table 4 - Subcritical Paint Formulations without PMHS Additives

[0056] Grinding (g) Ti-Pure R-746 272.26 <![CDATA[ROCIMA TM 63]]> 10.00 Bioban IPBC 40-LE 6.00 <![CDATA[TERGITOL TM 15-S-9]]> 2.00 <![CDATA[TAMOL TM 165A]]> 19.50 Foamstar A-34 1.00 Omyacarb 8 144.37 Omyacarb 2 66.52 <![CDATA[RHOPLEX TM VSR-50]]> 492.07 Texanol 13.38 Foamstar A-34 0.50 <![CDATA[ACRYSOL TM RM-8W]]> 2.40 <![CDATA[ACRYSOL TM RM-2020 NPR]]> 10.00 water 113.44 total 1153.45 PVC 40.00

[0057] Table 5 shows the color retention (ΔE) of coatings formed from subcritical paint formulations (PVC=40) after 2000 h of accelerated UV exposure, these formulations do not contain polysiloxane additives (Comparative Example 3) and contain 0.5 wt% PMHS1 based on the formulation weight of Table 4 (Example 6).

[0058] Table 5 - Color retention of subcritical paint formulations with and without polysiloxane additives

[0059] Paint Examples Polysiloxane Additives ΔE Comparative Example 3 none 7.0 Example 6 PMHS1 1.9

[0060] The color retention data supports the functional importance of methylhydrogensiloxane in the silicone additive. While not being bound by theory, it is believed that unlike methylsilane groups, hydrogensilane groups can chemically bond to extender and pigment particles, thereby limiting the migration of PMHS, which serves to mitigate the effects of UV degradation during accelerated weathering testing.

Claims

1. A method for forming a blend comprising an aqueous dispersion of polymer particles, polymethylhydrogensiloxane, an opaque pigment, and an extender; wherein the order of addition comprises any of the following steps: a) mixing the polymethylhydrogensiloxane with an aqueous dispersion of the polymer particles to form a polymethylhydrogensiloxane / polymer particle blend; and then mixing the polymethylhydrogensiloxane / polymer particle blend with the opacifying pigment and the extender in any order; or b) mixing the opacifying pigment with an aqueous dispersion of the polymer particles to form an opacifying pigment / polymer particle blend; and then mixing the opacifying pigment / polymer particle blend with the polymethylhydrogensiloxane and the extender in any order; or c) mixing the extender with the aqueous dispersion of polymer particles to form an extender / polymer particle blend; and then mixing the extender / polymer particle blend with the polymethylhydrogensiloxane and the opacifying pigment in any order; or d) adding the polymethylhydrogensiloxane to a blend comprising the aqueous dispersion of the polymer particles, the pigment and the extender; The polymethylhydrogensiloxane is represented by the following formula: Each R 1 are independently H or SiMe3; R 2 is CH3 or phenyl; and R 3 C1-C 18 Alkyl, C1-C 18 Alkoxy or phenyl; m is 0 to 199 and n is 1 to 200; wherein the pigment is TiO2 or ZnO; the extender is carbonate, silica, silicate, phosphate or non-hollow organic microspheres; the aqueous dispersion of polymer particles is an aqueous dispersion of polyacrylic acid or poly(styrene-acrylic acid) polymer particles; the concentration of the polymethylhydrogensiloxane is in the range of 0.1 wt % to 5 wt % based on the weight of the blend; wherein The weight-to-weight ratio of the polymethylhydrogensiloxane to the total amount of the opaque pigment and the extender is in the range of 0.5:1 to 0.001:

1.

2. The method according to claim 1, wherein each R 1 is trimethylsilyl; R 2 and R 3 each is a methyl group; m is 0 to 99; and n is 1 to 100.

3. The method according to claim 1, wherein the pigment is TiO2; and the extender is carbonate, silica, aluminosilicate, phosphate or non-hollow organic microspheres.

4. The method according to claim 1, wherein the extender is talc, clay, mica, sericite, CaCO3, nepheline, feldspar, wollastonite, kaolinite, dicalcium phosphate or diatomaceous earth.

5. The method of claim 4, wherein the weight-to-weight ratio of the polymethylhydrogensiloxane to the total amount of the opacifying pigment and the extender is in the range of 0.2:1 to 0.005:

1.

6. The method of claim 5, wherein the weight-to-weight ratio of the polymethylhydrogensiloxane to the total amount of the opaque pigment and the extender is in the range of 0.1:1 to 0.01:1; and the ratio of polymethylhydrogensiloxane to opaque pigment is in the range of 0.2:1 to 0.05:1; wherein the total amount of pigment and extender is such that the blend has a pigment volume concentration above the critical pigment volume concentration.

7. The method of claim 5, further comprising adding at least one of the following components: a solvent, a rheology modifier, a defoamer, a dispersant, a preservative, a coalescing agent, a neutralizer, or a colorant.

8. The method of claim 1, comprising mixing the polymethylhydrogensiloxane with an aqueous dispersion of the polymer particles to form a polymethylhydrogensiloxane / polymer particle blend; and then mixing the polymethylhydrogensiloxane / polymer particle blend with the opacifying pigment and the extender in any order.

9. The method of claim 1, comprising mixing the opacifying pigment with an aqueous dispersion of the polymer particles to form an opacifying pigment / polymer particle blend; and then mixing the opacifying pigment / polymer particle blend with the polymethylhydrogensiloxane and the extender in any order.

10. The method of claim 1, comprising mixing the extender with an aqueous dispersion of the polymer particles to form an extender / polymer particle blend; and then mixing the extender / polymer particle blend with the polymethylhydrogensiloxane and the opacifying pigment in any order.

11. The method of claim 1 , wherein the polymethylhydrogensiloxane is added to the blend comprising the aqueous dispersion of the polymer particles, the pigment, and the extender, wherein at least one of the following components is combined with the blend prior to adding the polymethylhydrogensiloxane: a solvent, a rheology modifier, a defoamer, a dispersant, a preservative, a coalescing agent, a neutralizer, or a colorant.

12. The method of claim 1, wherein the total amount of pigment and extender is such that the blend has a pigment volume concentration below a critical pigment volume concentration.

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