Novel Waterborne Soy Hybrid Dispersions and Soy Latex Emulsion for

Chapter 9

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Novel Waterborne Soy Hybrid Dispersions and Soy Latex Emulsion for Coatings Applications Madhukar Rao,* Gamini Samarnayake, James Marlow, and Richard Tomko The Sherwin-Williams Company, 601 Canal Road, Cleveland, Ohio 44113 *E-mail: [email protected]

We will be discussing two novel approaches taken at The Sherwin-Williams Company to develop soy based polymer technologies to meet unmet needs in the coatings market. The first approach is a novel dispersion of hydrophobic soy hybrid Polymer in water. This low VOC water based dispersion performs like a traditional solvent borne alkyd polymer with similar oxidative cure profile and application characteristics. But in contrast, water clean up is a value added feature in water borne alkyds. The second approach uses a more classic emulsion polymerization of soy based polymer with monomers to create novel soy hybrid latex. This latex performs like traditional latex with film formation by evaporation of water and particle coalescence, but with oxidative cure.

Background The United States paint market is approximately 1.2 billion gallons of which 850 million gallons is water-borne and 350 million gallons is solvent-borne (1). The architectural market segment, which is approximately 650 million gallons, is dominated by waterborne coatings, whereas the specialty purpose coating (industrial) and OEM is dominated by solvent borne technologies (1) (Figure 1).

© 2014 American Chemical Society In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 1. Shares and USA coating market by segments 2010: Segment percents based on gallons. Source: US Census Bureau

The recent record-high cost of petroleum and oil based raw materials, as well as uncertain availability due to increased demand overseas, has made complete dependency on synthetic raw materials questionable. There is a strong need to find different materials, preferably based on renewable resources. Furthermore the changing and tightening VOC regulations set forth by OTC (Ozone Transport Commission) and SCAQMD (South Coast Air Quality Management District) necessitates changes in architectural and industrial coatings. These regulations have prompted a search for alternate VOC compliant waterborne technologies, which perform like solvent-borne technologies. Today, latex emulsions dominate waterborne coatings; whereas the solvent borne coatings are dominated by alkyds. Each of these technologies has challenges in meeting the performance and application properties. To address this challenge, The Sherwin-Williams Company developed a novel a low VOC, waterborne soy hybrid technologies (2, 3) by utilizing the concepts of sustainability and green (naturally occurring resource materials). The waterborne technology was designed to meet key performance attributes of solvent borne alkyds, but at lower VOCs and with excellent hydrolytic stability similar to latex paints for industrial maintenance and architectural coatings applications

Waterborne Low VOC Soy Hybrid Dispersion Technology Coatings formulated from water-based soy dispersion technology (2, 3) perform like conventional solvent-based alkyd paints with high gloss, excellent adhesion, excellent moisture resistance and hydrolytic (shelf) stability. This “no surfactant “technology enables alkyd like properties with water clean up 194 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


and less odor than currently available conventional and high solids alkyd and latex technologies. In addition, because of the gloss enhancing and excellent adhesion characteristics, the hybrid dispersion technology could be used as a secondary binder or as a booster emulsion to improve film formation and improve performance properties of conventional latex paints. Innovation here was incorporating different value added functionalities through concepts of sustainability and renewable materials to create a hydrolytically stable value added waterborne soy dispersion with superior performance properties than the currently available latex, alkyd, sucrose ester and reactive diluents technologies. Recycled PET (polyester) and soybean oil were used along with commonly available raw materials to create a waterborne alkyd technology without any surfactants (4), and formulated into value added VOC compliant Industrial Maintenance and Architectural Coatings.

Soy-PET Polymer Dispersion PET (polyethylene terephthalate) polyester is the plastic commonly used in beverage bottles and typically ends up in landfills after its use. The chemistry to develop the novel coatings technology utilizes PET as a starting material primarily because it is a low cost and readily available raw material, and secondarily because it is less prone to hydrolysis due to the semi-crystalline polyester backbone and hydrophobic properties from terephthalic acid units. By controlled scission, it is possible to reduce the chain length of PET molecules. This is accomplished through acidolysis of the ester linkages and exchange of the terephthalic acid units of PET molecules with soya fatty acid. The exchange continues until a new equilibrium is established between PET, shorter chain PET, shorter chain length PET substituted with soy fatty acid, soya fatty acid and terephthalic acid. These can be reacted with polyols to form soy terminated PET containing liquid polyester. The Soy terminated PET containing polyester is then graft polymerized with suitable acrylic monomers, and finally dispersed into water to form an anionic aqueous dispersion. The following steps were carried out to make the hybrid dispersion

•

•

•

A controlled digestion of PET with Soya fatty acid with the resulting fatty acid terminated PET units converted into liquid soy functional PET Polyester by reacting with polyols. See Figure 2a and 2b. The Soya PET polyester is grafted with hydrophobic and hydrophilic acrylic monomers by graft polymerization in presence of Soybean oil instead of solvent that works as a reactive diluent to oxidatively cure into the final coating (Figure 3). The acidic pre-polymer was dispersed in water using an amine under high shear conditions. Under these conditions, the polymer inverts from water-in-oil to oil-in-water emulsion (Figure 4). 195 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 2. (a) Acidolysis and ester exchange step of making soy polyester; (b) Repolymerization into liquid PET soy polyester

Figure 3. Acrylic grafting of soy PET polyester

196 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 4. Dispersion of grafted polymer

The resulting novel polymer dispersion has hard PET segments (1-2 microns, Figure 5) that contribute to film hardness, the acrylic functionality for improving dry times & barrier properties, and the soy functionality to help in film formation, gloss, flexibility and cure.

Figure 5. TEM Micrograph of soy polyester/acrylic hybrid particles:. Courtesy of PCI Magazine (see Ref. (4))


Waterborne Soy Dispersion: Physical Properties NVM: 42 % Visc: < 2000 cps pH: 7.6-8.0 Dry Tack free: < 30 minutes Hydrolytic Stability: > 3 months at 120 ºC Morphology: Complex, MW: 30,000-40,000


Table I shows the targeted paint properties and the paint properties achieved with the soy hybrid dispersions.

Table I. PAINT PROPERTIES OF SOY POLYESTER ACRYLIC HYBRID Targeted Paint Properties

Specifications

Pigmented Prototype

90-100

98

9.60

9.60

SAG

12 min.

12

pH

8.5 - 9.5

9.0

>80

85

Dry Time

< 4 hrs

2 hrs

Adhesion (Tape)

Tape 5B

5B

Viscosity (KU) Weight per gallon

Gloss @ 60

Table II shows a comparison of the hybrid dispersion features compared to traditional solution alkyds, water-reducible (wr) alkyds and traditional latex emulsions. The traditional alkyds are high molecular weight fatty acid modified polyesters dissolved in solvents ranging from mineral spirits to ketones. The wr alkyds are made at high acid value and cut in water-reducible solvents and then neutralized with bases to dissolve in water. The traditional latex emulsions are made by free radical dispersion polymerization in water. The waterborne hybrid dispersion technology delivers the key performance attributes of conventional solvent borne alkyd technologies, while delivering on lower VOCs, exterior durability and less yellowing. Soy hybrid technology outperforms latex technology in gloss, application and adhesion properties.


Table II. PERFORMANCE COMPARISON OF TECHNOLOGIES: Solvent Borne vs. Waterborne


Performance Features

Solvent Alkyds

Latex

WR Alkyds

Hybrid Dispersion

Appearance

Solution

Solution

Dispersion

Dispersion

Low VOC Capability

-

-

++

++

High Gloss

+++

+++

-

+++

Hydrolytic Resistance

-

-

+++

+++

Moisture Resistance

+

+

+

+

Corrosion Resistance

+++

+++

++

+++

Dry Time

++

++

+++

+++

Shear Stability

++

++

+

++

Gloss Retention

-

-

+

++

Yellowing

+

+

++

+

Open Time

+++

++

-

-

Morphology

Solution

Solution

Particles

Particles

Molecular Weight

100000

100000

Millions

30000-40000

Film Formation

Oxidative Cure

Oxidative Cure

Coalescence

Coalescence/ Oxidative Cure

Soy hybrid technology was compared to some noteworthy VOC compliant technologies introduced recently in the market and compared in the spider graph (Figure 6) - and Table III. The waterborne soy hybrid technology outperforms solvent borne high solids sucrose ester technology in all of the performance properties. In addition, it has water clean-up attributes. The RC-Sun waterborne coalescent technology is a latex booster technology and fared similar to conventional latex technology in performance. The hybrid outperformed RC-Sun technology in commonly tested systems. The waterborne soy hybrid technology offers a balance of desired performance properties either as a sole binder or as a booster binder to enhance gloss, adhesion without compromising VOC of coating.



Figure 6. Spider graph comparing different polester dispersion technologies

Table III. COST COMPARISON OF TECHNOLOGIES Technology

Alkyd

VOC Compliance

NO Exceed and Fee to be paid Solvent borne

COST

HS Sucrose Ester

RC-SunCoalescent

Soy hybrid Technology

YES YES Supply Performance Chain challenges challenges

YES Supply chain challenges

YES Bio-based, Available materials

+ 10 %

High

+20%

Latex

High

The costs mentioned in Table III are comparative and normalized for disclosure. The lower costing alkyd and latex technologies lack performance properties as shown in the Table II. The VOC compliant and performance effective water based soy hybrid technology brings value to replace solvent borne alkyd coatings technology. Benefits of Soy Hybrid Dispersion Technology A novel green waterborne soy hybrid technology with the best performance attributes of both alkyd and latex technologies was developed. This hybrid dispersion has all the value added performance properties of alkyds with water 200 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

clean-up and outstanding hydrolytic shelf stability – a feature not seen in Polyester containing technologies. Because the soybean oil is the solvent, the technology has low odor and low VOC- a desired feature for consumers. This enabling technology was used in formulating a series of waterborne products to replace solvent borne Interior/ exterior products lines.


Waterborne Low VOC Soy Polymer Hybrid: Latex Emulsion Technology In previous section we described water borne soy polyester hybrid dispersions produced by free radical grafting of vinylic monomers on to soy polyester prepolymer and then dispersing that hybrid in water (2, 3). The method relied on the self-emulsifying ability of the hybrid having base reactive functional groups. Two other technologies: one, the direct emulsification of alkyds (5) and the other, emulsion polymerization of alkyds in the presence vinylic monomers (6, 7) have been successful in preparation of water borne alkyds. The following discussion illustrates the technology of preparing hybrid latex by emulsion polymerization of soy polyester with vinylic monomers. Coatings of these latexes performed similar to traditional solvent born alkyds, while carrying characteristics of vinylic latex.

Alkyd Emulsions and Latex Latexes used in coating applications typically have particle diameter in 50-500 nm range. To reach this particle size in emulsions of solid polymers or highly viscous liquids, as most alkyds are, a certain amount of co-solvent is needed to dissolve the polymer (6). However, solvents contribute to volatile organic content (VOC). Use of vinylic monomers in place of the solvent avoids this situation. Eventually, vinylic monomer/polymer emulsion can be polymerized to stable, water borne latex of proper particle size. Miniemulsion technology has been applied successfully in preparation of these type latexes (6, 7). Miniemulsion and conventional emulsion polymerization differ in their particle nucleation mechanisms. In ab-initio emulsion polymerization of vinylic monomers, the particle nucleation occurs in surfactant micelles of diameter 5-10 nm, which would grow into polymer particles of the size range of 50-500nm. In miniemulsion process, the monomer mixture is pre-emulsified to a fixed particle of size in the range 50-500nm (8); the nucleation occurs in droplets converting the whole droplet to a particle. Ideally, the particle size remains unchanged. Low molecular weight, low softening point, and good solubility in solvents, alkyds make good substrates for emulsions. Polymerization of the emulsion produces an alkyd/vinylic hybrid latex. It is conceivable that alkyds can participate in polymerization by rafting (9) through unsaturated fatty acid moieties. In nano-phase-domain the particles are heterogeneous in that the alkyd phase and vinylic phase has various degrees of interactions leading to different particle morphologies in water (10). Particle morphology influences film formation, a major considerations in paint films (11). 201 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Mechanics of Generating Nano Scale Particles


Several equipment are available for emulsification: Rotor stator, ultrasonic, high pressure homogenizer, and recently, static mixture. They all differ in there shear rates and power density (12). Two forces are involved in emulsion formation (13): viscous force that depends on the viscosity (η) of dispersed phase, which has to be overcome by the shearing mechanism and, the surface tension force that opposes the droplet breaking. All high shear homogenizers operate in turbulent flow regime that the droplet breakup is correlated to Weber Number (We) and viscosity ratio of continuous to dispersed phase.

Where, ρc = Continuous phase density; U = flow velocity; D = characteristic length for homogenizer; σ = interfacial tension. The relationships of these parameters to particle size are discussed in ref. (13). Surfactants are used to reduce surface tension whereas the dilution of the alkyd with monomer reduces the viscosity ratio, typically in the range 0.05 -5. Finally, the smallest particle size achievable is determined by the energy density of the homogenizer.

Stability of Mini Emulsion Particle size need to be less than 500 nm to have a stable miniemulsion, larger than that could result settling or creaming. However, smaller particles are prone to Oswald ripening: the growth of larger particles in the expense of smaller particles. This is correlated to the higher Laplace pressure (2σ/r, where r is droplet radius) in smaller droplets compare to that of larger ones. Use of a proper surfactant can minimize this effect.

Soy Hybrid Latex Process To a solution of soy polyester and monomer in a stirred reactor vessel, an aqueous phase containing, buffer and surfactant is added slowly and the mixture is until clear. The mixture is then homogenized at high pressure until droplet size distribution between the range of 50-500 nm. At an elevated temperature, an initiator is fed to the reactor for several hours, and the content was held until all the monomer is reacted. A step to scavenge residual monomers may be included. Typical properties of representative examples of the latex are given in Table IV. 202 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Table IV. PROPERTIES OF SOY POLYMER/ACRYLIC LATEX Latex

% Soy polyester

Tg, Latex

% Soy in dry latex

Particle size (nm)

pH

Solids%

1

50

23.6

8

178

7.2

48

2

50

23.6

8.2

172

7.7

49

3

50

23.6

9.4

176

7.7

50


Particle Morphology Equilibrium spreading coefficient of individual phases determines the morphology of a hybrid particle (10). Differential scanning calorimetry (14) or Dynamic mechanical analysis (15) can be used for semi-quantitative analysis of the compatibility of the two phases. The degree of separation of two glass transition temperatures indicates the degree of compatibility. Higher gloss of the coating is evident of better compatibility of the phases. The compatibility ensures better polymer diffusion in forming a continuous film free of micro voids. This is important not only in gloss, but also, in corrosion resistance on steel and on blister- formation. Figure 7 shows TEM of two soy hybrid latexes for two types of soy polyesters. In Figure 7A the polyester phase is shown in stained shell. The opposite configuration is shown in Figure 7B, where the polyester is in the shell, is the desired morphology that would protect the polyester from hydrolysis in aqueous medium. The latex examples listed in Table IV above have the core –shell morphology of Figure 7A.

Film Properties of Soy Polymer Hybrid Latex Coatings The hybrid soy polymer showed excellent adhesion, gloss and dry time compared to industrial alkyd coating (Table V).

Benefits of Soy Hybrid Latex Technology The process uses of 100% soy polyester as the starting material which has substantial bio renewable content form soy oils. Since the vinylic modification is done by emulsion process that does not require the use of solvents, it enables development of paint formulations less than 50 g/L for Industrial applications. The performance of the hybrid is comparable to typical solvent borne alkyd paints with additional improvements in tack free time, dry to touch time, characteristic to traditional latex polymers.



Figure 7. TEM micrographs of soy polymer hybrid latex, 7A soy polyester in shell and, 7B soy polyester in core


Table V. Film Properties of Soy Polyester/Acrylic Hybrid Latexa


a

Technology

% Alkyd

Tg, Latex

600 gloss on CRS

Pencil Hardness

DT/TF/DH hr

Corrosion resistance

Humidity resistance

Soy hybrid latex

50

8

81

2B

0.4/.08/1

+++

+++

Solvent borne Alkyd

0

N/A

87

3B

+++

+++

PS = particle diameter; CRS = Steel; DT = dry to touch; TF = tack free; DH = dry hard

Conclusions Both waterborne soy hybrid technologies presented here have the value added performance properties of alkyds with and outstanding hydrolytic shelf stability – a feature not seen in polyester containing technologies. • • •

•

Low VOC feature helps eliminating pollution and solvent clean up problems associated with typical oil based, solvent borne alkyd products Enables up-cycling of recycled PET in case of soy hybrid dispersion Utilize soybean oil – Helping The United States agro economy and soy bean farmers repacing crude oil from the solvents and oil based alkyd polymers it can replace Reduce VOCs going into environment and potentially contributing to ozone depletion

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3.

4. 5. 6. 7. 8. 9.

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Novel Waterborne Soy Hybrid Dispersions and Soy Latex Emulsion for

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