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SC:2889-01

Design of VeoVa™ Based Latices with Low Odor/VOC for Flat Interior Wall Paints

By 

M ichae ichaell J . Watk Watkins ins Françoise Fran çoise Decocq Francis Defoor Hélène Petit Claude Nootens David Vanaken

EPIKOTE

™

EPON

™

EPI-CURE

™

EPI-REZ

™

HELOXY

™

CARDURA

™

VEOVA

™

SC:2889-01 / Design of VeoVa™ Based Latices with Low Odor/VOC for Flat Interior Wall Paints

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Abstract

 The demand for interior paints with low odor and low VOC continues to increase. Formulating such paints over a broad PVC-range, while maintaining high levels of performance (good drying time, with resistance to scrubbing and mudcracking) is a challenge. This paper describes approaches to formulate these paints with latices based on vinyl acetate, tertiary branched vinyl esters (notably VeoVa 10 Monomer), and selected acrylates. Formulations for both colloid-free and colloid-stabilized latices are discussed. These latices are prepared with extremely low free monomer levels, without solvent coalescing aids, and with environmentally friendly surfactant types. High performance flat paint formulations using these latices are discussed.

Introduction

 The reduction of volatile organic compounds (VOC) is an issue common to all coating types. Significant research and development efforts have been expended to accomplish this for such diverse coating types as architectural paints, automotive coatings, and can coatings. Formulators in each area are faced with the problem of maintaining low cost and high levels of performance, while making significant formulation changes to reduce VOC. Many architectural coatings used today are waterborne, and are based on synthetic latices. As such, they are relatively low in VOC content. However, there are two major drivers to reduce VOC further, particularly in interior architectural coatings. The first is simply to reduce the release of organic compounds that are viewed as sources of photochemical smog and pollution. The second driver is that these interior paints are often used in confined spaces, where the odor of volatile organic compounds may be perceived by the user to be unpleasant, or even harmful. In most cases, the user would prefer an odor-free product.

 Three primary sources of VOC and odor in architectural latex coatings are: 1. The use of organic solvents as coalescing aids 2. Residual (unreacted) free monomers 3. Solvents or other volatile organic compounds in paint additives  The addition of organic solvents, which act as coalescing aids during film formation, is a primary source of VOC in architectural latex paints.1 The latex particles are generally high molecular weight polymers, in order to achieve high levels of performance. Solvents act to plasticize the polymer, reducing the modulus of the latex particles. This lowers the minimum film forming temperature (MFFT) of the latex to a desirable range (e.g. 0-5°C), enhances the ability of the particles to deform during film formation, and also promotes diffusion of polymer segments across particle boundaries. This low MFFT facilitates the application of paints ina wide temperature range without affecting film performance properties. Good film formation at normal application temperatures, in the absence of coalescing aids, is only possible if  the latex has an MFFT around 0-5 °C. On the other hand, the polymer film should not be too soft if high performance is required. Therefore, eliminating solvent coalescing aids from architectural latex coatings is a significant challenge.

Latex film formation is a very complex process, and a complete description is beyond the scope of this paper. References 1-7 are recommended for more complete discussions.

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SC:2889-01 / Design of VeoVa™ Based Latices with Low Odor/VOC for Flat Interior Wall Paints

Introduction (cont.)

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Broek8 published an article, “Environment-friendly paints: their technical (im)possibilities.” He repeatedly used the term “(im)possibilities” throughout the paper to highlight the challenge of reformulating high performance architectural coatings. In this work, we have demonstrated that it is not an “impossibility” to formulate latices with vinyl acetate and branched vinyl ester co-monomers which can subsequently be use to formulate high performance interior flat wall paints with low VOC/odor. Vinyl acetate copolymer latices comprise one of the most important binders for architectural coatings worldwide.9 Branched vinyl esters, notably the vinyl esters of VERSATIC Acid™, comprise an important class of co-monomers for vinyl acetate, and are extensively being used in a wide variety of architectural paints. Binders (based on branched vinyl esters), suitable for low odor/low VOC paints, have been specifically developed and are presented in this article. In order to achieve a sufficiently low MFFT, soft monomers such as VeoVa 10 Monomer, VeoVa 11 Monomer, 2-ethyl hexyl acrylate, or butyl acrylate were incorporated as co-monomers. All of these latices can be formulated into paints without coalescing agent, and with only environmentally acceptable additives and surfactants. Their residual monomer content can also be reduced to extremely low levels using a post treatment based on a redox initiator system. These latices are therefore especially suitable for formulation into low-odor and odor-free interior emulsion paints with virtually no emission of volatile organic compounds. As a result of the significant changes made in the latex formulations compared with those used in traditional paints, it was also necessary to adjust the emulsion paint formulations in order to maintain good performance with respect to properties such as mudcracking and wet scrub resistance. Two interior solvent-free paint formulations (60% and 72% PVC) were adapted accordingly.

VeoVa™ Monomers in Conventional Emulsion Paints

VeoVa™ monomers are the vinyl esters of Versatic Acids (saturated, tertiary, branched, carboxylic acids). The VeoVa monomers are designated with a number that indicates the number of  carbon atoms in the acid. For example, VeoVa 10 Monomer is made from Versatic 10 Acid, and contains ten carbon atoms in the acid (plus two in the vinyl functionality). The general structure for the branched vinyl ester monomers is provided in Figure 1.

SC:2889-01 / Design of VeoVa™ Based Latices with Low Odor/VOC for Flat Interior Wall Paints

VeoVa™ Monomers in Conventional Emulsion Paints (cont.)

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Figure 1/Chemical Structure of VeoVa Branched Vinyl Esters (R1 and R2 = Alkyl Groups)

Vinyl acetate VeoVa latices can be stabilized by surfactants alone (colloid-free system), or by a combination of surfactants and protective colloid (colloid-stabilized system). The resulting latices have a relatively high solids content in combination with a very good colloidal stability. VeoVa Monomers and vinyl acetate have virtually identical reactivity in radical polymerization, which ensures easy preparation over a wide temperature range and results in randomly distributed monomer units along the polymer chain. This is important, because it allows the key performance properties of the branched hydrophobic structure to be exploited most efficiently. The incorporation of VeoVa Monomers upgrades the polymer in areas where vinyl acetate is weak, notably hydrophobicity and alkali resistance. The unique, highly branched, carbon-rich structure of VeoVa 10 Monomer sterically protects its own ester group from being hydrolyzed. More importantly it also protects several neighboring acetate groups 10 (sometimes referred to as an “umbrella effect”), thus improving the hydrolytic stability of  the polymer. This enables such polymers to be successfully used as paint binders on alkaline substrates, such as exterior concrete.11

 This “umbrella effect” is demonstrated in Figure 2. The hydrolysis of ester groups has been measured for a range of colloid-stabilized latices with different vinyl acetate/VeoVa 10 Monomer ratios. They were exposed to 2% aqueous sodium hydroxide solution for four days.  The alkali resistance of the polymer is expressed in terms of the percentage of the total number of ester groups hydrolyzed. From the results, it is easily concluded that VeoVa 10 Monomer reduces the ester group hydrolysis much more than proportionally to its concentration. Figure 2 also shows a data point indicating the alkali resistance of a vinyl acetate/ butyl acrylate (75/25) polymer. This demonstrates that VeoVa 10 Monomer clearly outperforms butyl acrylate in this regard.

SC:2889-01 / Design of VeoVa™ Based Latices with Low Odor/VOC for Flat Interior Wall Paints

VeoVa™ Monomers in Conventional Emulsion Paints (cont.)

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Figure 2/Ester Bond Hydrolysis as Function of the Hydrophobic Monomer Content in the Polymer

Vinyl acetateVeoVa latices are easily formulated into emulsion paints because of their good shear stability and compatibility with other raw materials. The shelf life of VeoVa based emulsion paints is very good, because of good hydrolytic stability and hence, low pH- and viscosity drift. The latices are versatile and can be used to formulate a variety of flat, semigloss, and gloss paints over a wide PVC range. VeoVa based emulsion paints have very good rheological properties, resulting in easy application and good film build. The unique branched structure and the hydrophobic nature of VeoVa 10 Monomer makes it an excellent co-monomer for vinyl acetate. The scrub resistance improves with increasing VeoVa 10 Monomer content of the binder, as shown in Figure 3. It is quite remarkable as this coincides with simultaneous improvements in flexibility and minimum film forming temperature (MFFT).  The choice between using a colloid-free and a colloid-stabilized latex affects the paint properties in various ways. Colloid-free latices with their small average particle size tend to perform better with respect to pigment binding power. This is demonstrated in Figure 3, where the use of colloid-free latices leads to better wet scrub resistance compared to colloid-stabilized latices

SC:2889-01 / Design of VeoVa™ Based Latices with Low Odor/VOC for Flat Interior Wall Paints

VeoVa™ Monomers in Conventional Emulsion Paints (cont.)

Design of LowMFFT VeoVa™ Ester Latics

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Figure 3/ Wet Scrub Results of 60% PVC Conventional Flat Paints Based on VeoVa Latices (ASTM D 2486-89)

Vinyl acetate latices modified with the vinyl ester of Versatic 10 have historically been used successfully to formulate traditional interior latex paints. But the combination of these two monomers alone does not permit the formulation of latex with a sufficiently low MFFT to produce solvent-free paints. The reason for this is that the homopolymer Tg for the vinyl ester of Versatic 10 is -3 °C, so the target Tg range of 0-5 °C cannot be achieved for copolymers with moderate levels of the vinyl ester of VeoVa 10. However, the vinyl ester of VeoVa 11 has a homopolymer T g of -40 ° C. Therefore, it affords the opportunity to formulate vinyl acetate copolymers that do have a sufficiently low Tg (and MFFT) at modest levels of modification. Another possibility is to use other soft acrylic monomers (such as 2-ethylhexyl acrylate, with a homopolymer Tg of -50 °C, or butyl acrylate with a homopolymer Tg of -40°C) in conjunction with the VeoVa 10 and vinyl acetate. Examples of such formulations were evaluated in this study. Latices evaluated include:

•Latex 1 is a colloid-stabilized, vinyl acetate and VeoVa 11 copolymer (50/50 ratio). The latex has an MFFT of 0 °C and is prepared in a very similar way to the VeoVa 10 based latices. See Table 1 for the formulation. •Latex 2 is a colloid-stabilized, vinyl acetate latex modified with a combination of the VeoVa 10 and 2-ethylhexyl acrylate (55/20/25 ratio). This latex has an M FFT of 0 °C and imparts good application properties, notably easy painting and good flow. See Table 1 for the formulation.

SC:2889-01 / Design of VeoVa™ Based Latices with Low Odor/VOC for Flat Interior Wall Paints

Design of LowMFFT VeoVa™ Ester Latics (cont.)

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 Table 1/Colloid Stabilized Formulations for Latices 1 and 2 Latex

1

2

parts weight 313.5 1.5 – 1.0 20.0

parts weight 357.0 1.5 2.0 1.0 5.0

Monomer pre-emulsion Demineralized water Sodium bicarbonate Sodium dodecylbenzene sulfonate Non-ionic surfactant (linear alcohol ethoxylate type) VeoVa 11 Monomer VeoVa 10 Monomer 2-ethylhexyl acrylate Vinyl acetate

448.0 4.0 1.5 20.0 500.0 – – 500.0

400.0 2.0 2.0 20.0 – 200.0 250.0 550.0

Initiator solution Demineralized water Potassium persulphate

120.0 2.3

120.0 2.3

Initial reactor charge Demineralized water Sodium dodecylbenzene sulfonate Sodium bicarbonate Potassium persulphate Hydroxyethyl cellulose

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Design of LowMFFT VeoVa™ Ester Latics (cont.)

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• Latex 3 is a colloid-free, vinyl acetate latex modified with a combination of the VeoVa 10 and butyl acrylate (ratio 60/20/20). The latex is stabilized with a small amount of acrylic acid and has an MFFT of 5ºC. The latex has a low particle size and hence very good pigment binding power. See Table 2 for the formulation.  Table 2/Colloid-free Formulation for Latex 3 Initial reactor charge Demineralized water Sodium dodecylbenzene sulfonate Potassium persulphate

parts weight 439.5 0.5 1.0

Monomer pre-emulsion Demineralized water Sodium dodecylbenzene sulfonate Non-ionic surfactant (linear alcohol ethoxylate type) Sodium carbonate VeoVa 10 Monomer Butyl acrylate Acrylic acid Vinyl acetate

435.0 9.5 20.0 1.0 200.0 200.0 0.5 600.0

Initiator solution Demineralized water Potassium persulphate

120.0 1.0

A number of other factors were considered in the design of the latices in this study to reduce the VOC or odor:

• No solvents were used for coalescence or freeze-thaw resistance • Additives were chosen to be VOC-free • Buffer systems containing acetic acid were not used • Additives (in particular, surfactants) chosen were free of alkyl phenol derivatives • Amines or ammonia were not used for the neutralization of colloid-free latices or dispersing agents Preparation of Latices for Cosolvent-free Paints

A stable monomer pre-emulsion was prepared by adding the monomers to the vigorously stirred aqueous phase, according to the recipe. The initial reactor charge was heated to 60 °C under a nitrogen purge. When the temperature of 60 °C was reached, the nitrogen stream was stopped and a portion (10% for Latices 1 and 2, and 2.5% for Latex 3) of the monomer pre-emulsion was added to the reactor to form the seed polymer. When this charge had polymerized, reflux of vinyl acetate stopped, and the temperature was raised to the desired reaction temperature (76 °C for Latex 1 and 80 ºC for Latices 2 and 3). Then, the remainder

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Preparation of Latices for Cosolvent-free Paints (cont.)

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of the monomer pre-emulsion was gradually added over a period of 3 hours, while maintaining the desired reaction temperature. The initiator solution was added simultaneously with a separate line. The addition time for the initiator solution was fifteen minutes longer to improve monomer conversion. After monomer addition, a post-reaction treatment of 2 hours at 80 °C was applied to further improve monomer conversion. Finally, the latex was cooled to 30 °C, and filtered. Latex 3 is colloid free, and it was neutralized with a 7% solution of sodium hydroxide. Neutralization was done slowly and in 2-3 steps so that the pH did not exceed pH 9 and the final pH did not drop below pH 7 after a few hours.

Experimental and Results

Evaluation of Latices for Cosolvent-free Paints  Table 3 summarizes the properties of both the latices, and the clear films cast fromthese latices. The three design routes allow the preparation of good performing latices within the target MFFT window of 0-5°C. This illustrates the design versatility of VeoVa Monomers to produce colloid-stabilized or colloid-free binders with vinyl acetate and optionally acrylates. In general, these latices show good stability at high solids content and good clear film performance. The particle size and pH depend on the stabilization system.

 Table 3/Latex and Clear Film Properties Latex VeoVa Monomers used Other Flexible monomers used Latex properties Solids content (% m/m)1 pH

Viscosity (mPa•s)2 MFFT (°C)3 Particle size (nm)4

1 11

2 10 2-EHA

3 10 BA

53 ±1 4.5

53 ±1 4.5

315 ±50 95

>95

liquid >60 passes

liquid >60 passes

liquid