COALESCENCE OPTIMIZATION ON WATERBORNE METAL COATINGS

Mayara Cristina Correa, Luis Gustavo Morelli dos Santos, Thaís Ramos Fulgeri, Thiago Luiz

Theodoro Faria, Rodolfo Di Nápoli Nogueira

Dow Brazil

INTRODUCTION

Recently, environmental awareness and sustainability discussions for new regulations have motivated the coatings industry to develop more waterborne formulations. The absence of solvents is a challenge for film formation, so that for this role it is needed a coalescent agent.

The coalescent is in charge of providing disappearance of the boundary between two particles in contact followed by changes of shape, what leads to a reduction of the total surface area. Its application reduces the Minimum Film Formation Temperature, being a point of energy saving as well. The coalescent is essential in the application of emulsion polymers in coating formulation, in order to irreversibly bound matrix structures, so that the paint will not breakdown into particles after applied. This class of substance must have more affinity with the resin than with the water, in order to remain in the film until the coalescence is granted, instead of being dragged by the water evaporation.

The film formation is the main factor responsible for barrier properties, because independently on how good the polymer is, if the film is not well formed, the substrate will be exposed. The paints for metal coatings require high quality of film formation in order to prevent penetration of liquids and gases that bring on corrosion to the substrate.

This study intends to reveal how coalescents interfere on the aspects of quality, durability, and sustainability of acrylic water-based metal coatings that are applied to light to moderate industrial use, or decorative use. The aspects compared and its parameters analyzed are:

- Productivity, through MFFT (minimum film forming temperature); - Corrosion resistance, through salt spray; - Appearance, by gloss; - Water resistance, by blistering; - Adhesion, by tape removal.

Two styrene acrylic resins and 5 solvents from Dow Industrial Solutions portfolio were tested as coalescent agents. The acrylic resins are named in this work as Resin A (50.6% solids), and Resin B (44.9%solids), chosen due to being recommended for metal coating.

The coalescents for this project were selected among Glycol Ethers, a chemical class known in the coatings industry as good solvents and coalescence function. A good coalescent must evaporate slow enough to guarantee the coalescence of particles, but at the same time, fast enough to not to extend the drying time. The table I summarizes the technical properties of the coalescents tested, being Butyl CELLOSOLVE™ Solvent largely used currently in Brazilian waterborne metal coatings.

Table I: Glycol Ethers tested as coalescent agents

Trade Name Chemical Description Structural formula

Evaporation rate (BuAc=1)

Boiling point (⁰C)

Butyl CELLOSOLVE™ Solvent

Ethylene Glycol n-Butyl Ether

0.079 171

DOWANOL™ PnB

Propylene Glycol n-Butyl Ether

0.093 171

DOWANOL™ DPnP

Dipropylene Glycol n-Propyl Ether

0.014 213

DOWANOL™ PPh

Propylene Glycol Phenyl Ether

0.002 243

DOWANOL™ DPnB

Dipropilene Glycol n-Butyl Ether

0.006 230

Formulation

The formulation used as base is a suggestive formula for styrene-acrylic paints applied to metal substrate. The quantity of coalescent was adjusted on each paints prepared, according to the percentage of coalescent over resin solids desired for upcoming tests. An adjustment was done as well with the resin with different solids percentage.

Grind % Weight Water 10,72 Dispersant 1,23 Surfactant 0,14 Titanium Oxide 18,25 Defoamer 0,12 LetDown RESIN (50%solids) 57,24 Ammonia (15%) 0,38 Water 1,61 COALESCENT X Sodium Nitrite (15%) 3,14 Water 4,85 Rheology modifier 0,43

O

HO

EXPERIMENTAL PROCEDURES

Substrate and Coating Application

The substrate for coating application (except MFFT) were cold rolled panels of AISI 1020, with dimensions 20 x 10 x 0.15 cm. Before application, the panels were cleaned with acetone.

The paint was applied with brush, until achieving a wet thickness of 200 µm, measured with wet film gauge. After 7 drying days, adhesive tapes were placed along the edges of the panels that were going to be submitted to salt spray and water immersion, in order to protect exposed metal. The figure 1 illustrates these coated panels.

Figure 1: Example of prepared panels

Minimum Film Forming Temperature (MFFT)

The method used for this procedure is described on ASTM D2354 – 10 Standard Test Method for Minimum Film Formation Temperature (MFFT) of Emulsion Vehicles, which consists of observation of cracking or whitening of films that have dried over a substrate having controlled temperature gradient. The tests were done in triplicate.

Gloss measurement

The panels were submitted to the gloss meter equipment according to ASTM D523-14 Standard Test Method for Specular Gloss, under angles of light incidence of 60°, 20°, and 85°.

Adhesion

Adhesion tests were performed according to ASTM D3359 – 09 Standard Test Methods for Measuring Adhesion by Tape Test, following test method A, in which an X-cut is made through the film to the substrate, and a pressure-sensitive tape is applied over the cut and then removed. The adhesion is assessed qualitatively on the 0 to 5 scale, where a 5 represents no coating removal.

Corrosion resistance

The corrosion resistance was measured by salt spray according to ASTM D1654-08 Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments, in which a cut is done on the panels in order to expose the metal substrate and then submitting them to a chamber that simulates corrosive atmosphere. A

cycle of 218 hours was applied for these styrene acrylic paints. Afterwards, the coating was removed so that it is possible to measure the corrosion caused around the cut.

Water resistance

The panels were placed into a capped plastic box with deionized water. The results were evaluated by comparison with photographic reference standards described on ASTM D714 − 02 Standard Test Method for Evaluating Degree of Blistering of Paints.

RESULTS AND DISCUSSION

Coalescent quantification by MFFT results

It is known that some coalescents need less quantity to achieve a desired MFFT than others. For this reason, an initial MFFT test was performed to establish the percentage that will be considered for each coalescent, looking for a narrow range that appears in common among them.

The graph 1 and 2 show the results of MFFT tests for Resin A and Resin B, respectively.

Graph 1: MFFT reduction for Resin A. Its MFFT with no coalescent is 20,6⁰C.

The quantity of 5% of coalescent was tested commonly with all solvents. By observing this quantity and its results on the graph, it is easily identified the advantage of DOWANOL™ PPh and DOWANOL™ DPnB on reducing MFFT.

This graph also shows an MFFT in common around 8⁰C, on the points surrounded by the circle. These quantities were selected for further comparative tests instead of defining the same percentage for all coalescent, what would generate a wide disparity of results. For this reason, the coalescent concentrations for formulating paints at an MFFT of 8 °C with Resin A were: 2.5% of DOWANOL™ PPh, 2.5% of DOWANOL™ DPnB, 5% of DOWANOL™ PnB, 5% of Butyl CELLOSOLVE™ Solvent, and 15% of DOWANOL™ DPnP.

0

2

4

6

8

10

12

0% 5% 10% 15%

MF

FT

(⁰C

)

% of coalescent over resin solids

Resin AButyl CELLOSOLVE™

Solvent

DOWANOL™ PnB

DOWANOL™ DPNP

DOWANOL™ PPh

DOWANOL™ DPnB

Graph 2: MFFT reduction for Resin B.

By observing the MFFT results at 5% coalescent (in common for all solvents), it was possible to confirm the advantage of DOWANOL™ PPh and DOWANOL™ DPnB on reducing MFFT with lowest quantity.

The MFFT in common was around 5⁰C, so that the quantity of each solvent chosen to further comparative tests with formulated paints were: 5% of DOWANOL™ PPh, 5% of DOWANOL™ DPnB, 10% of DOWANOL™ PnB, 10% of Butyl CELLOSOLVE™ Solvent, and 10% of DOWANOL™ DPnP.

By analyzing these results in general, it is possible to affirm that DOWANOL PPh and DOWANOL DPnB are more efficient in reducing the MFFT of the styrene-acrylic resins. It would require 50% more to achieve similar results with other products.

When considering strictly MFFT, the ranking of coalescent according to the lowest quantity needed to achieve the same MFFT is shown on the table II.

Table II: Ranking of coalescents for MFFT.

Resin A Resin B

Butyl CELLOSOLVE™ Solvent 4 3 DOWANOL™ PnB 3 4 DOWANOL™ DPnP 5 5 DOWANOL™ PPh 1 2 DOWANOL™ DPnB 2 1

The list below summarizes the combination of resins and quantity of each coalescent that were taken to formulate paints to apply on metal panels for further tests. These quantities put all the coalescents on the same level of MFFT, what makes the further comparison more accurate than having the same percentage of all coalescent and getting large disparity of results.

0

5

10

15

0% 5% 10%

MF

FT

(⁰C

)

% of coalescent over resin solids

Resin BButyl CELLOSOLVE™

Solvent

DOWANOL™ PnB

DOWANOL™ DPNP

DOWANOL™ PPh

DOWANOL™ DPnB

Resin A 2.5% of DOWANOL™ PPh 2.5% of DOWANOL™ DPnB 5% of DOWANOL™ PnB 5% of Butyl CELLOSOLVE™ Solvent 15% of DOWANOL™ DPnP

Resin B 5% of DOWANOL™ PPh 5% of DOWANOL™ DPnB 10% of DOWANOL™ PnB 10% of Butyl CELLOSOLVE™ Solvent 10% of DOWANOL™ DPnP

Gloss Measurement

The coated panels were submitted to the gloss meter 7 days after the application. The average of measures is presented in the table III.

Table III: Gloss meter results for coated panels.

Resin A Resin B

Light incidence angle 20° 60° 85° 20° 60° 85° Butyl CELLOSOLVE™ Solv. 27.8 70.1 78.9 37.9 71.6 83.6

DOWANOL™ PnB 25.4 61.3 68.6 19.7 57.2 67.7 DOWANOL™ DPnP 13.2 44.7 54.0 19.7 76.2 76.2 DOWANOL™ PPh 19.7 54.3 54.0 25.8 67.4 71.7

DOWANOL™ DPnB 28.4 66.6 72.1 40.8 75.7 80.2

DOWANOL™ DPnB and Butyl CELLOSOLVE Solvent yielded the highest gloss. The gloss contributes to the aesthetic aspect, and might be considered in the ensemble of properties to develop paints, mainly the ones to be used over interior and exterior metal substrates for architectural and commercial applications.

Analyzing strictly the gloss, and ranking the coalescent from the highest gloss to the lowest, we get the table IV below.

Table IV: Ranking of coalescents for gloss.

Resin A Resin B

Butyl CELLOSOLVE™ Solvent 1 2 DOWANOL™ PnB 3 5 DOWANOL™ DPnP 5 4 DOWANOL™ PPh 4 3 DOWANOL™ DPnB 2 1

Adhesion

In general, the paint affected after tape removal was very little in all panels. The range of judgment varies from 0A (total removal) to 5A (no removal), according to ASTM D3359. Three panels were tested for each formulation, and three removals were done by panel, resulting in 9 tested areas by formulation. An example of 4A and 5A is found on Figure 2, and the average result is on table V.

Figure 2: Example of tested panel coated with formulation containing Resin A + 2,5% DOWANOL™ PPh.

Table V: Adhesion qualitative result that most appeared on the 9 X-tests, being 5A the best and 0A would be the worst.

Resin A Resin B

Butyl CELLOSOLVE™ Solvent 5A 4A DOWANOL™ PnB 5A 5A DOWANOL™ DPnP 5A 4A DOWANOL™ PPh 5A 4A DOWANOL™ DPnB 4A 5A

This difference among results was too subtle, and a ranking would not be representative for comparing the coalescents on adhesion, because other factors might interfere slightly, for instance, the thickness variation due to manual coating.

Corrosion resistance

The corrosion resistance defines the durability and maintenance cycle of metal substrate. The acrylic resins offer a balance of properties and cost benefits for low to moderate protective use, where the conditions are not severe. The salt spray test was applied in order to compare how the coalescence of each solvent tested performs as barrier to corrosion.

The panels had the coating removed in order to measure the corrosion advance. Some panels were corroded in all their areas, so that the quantification method could not be applied. For those that were measured, the results were in a range of 0.2 to 0.7 cm. The variation due to manual coating application might be remembered. The tables VI and VII on the next pages show the visual aspect of the panels immediately after 218 hours of salt spray, and the corrosion advance after removing the coating.

Table VI: Panels coated with formulation containing Resin A and each coalescent.

Coalescent Visual aspect Corrosion advance (cm)

Butyl CELLOSOLVE™ Solv.

Completely corroded

DOWANOL™ PnB

Completely corroded

DOWANOL™ DPnP

0,27

DOWANOL™ PPh

Completely corroded

DOWANOL™ DPnB

Completely corroded

Table VII: Panels coated with formulation containing Resin B and each coalescent.

Coalescent Visual aspect Corrosion advance (cm)

Butyl CELLOSOLVE™ Solv.

0.58

DOWANOL™ PnB

0.31

DOWANOL™ DPnP

0.7

DOWANOL™ PPh

0.37

DOWANOL™ DPnB

0.67

By analyzing strictly salt spray visual results, the ranking of the coalescent from the less corroded to the most corroded is expressed on the table VIII below.

Table VIII: Ranking of coalescents for corrosion resistance.

Resin A Resin B

Butyl CELLOSOLVE™ Solvent 3 1 DOWANOL™ PnB 5 3 DOWANOL™ DPnP 1 4 DOWANOL™ PPh 4 5 DOWANOL™ DPnB 2 2

Water resistance

The panels were submitted to water immersion for 218 hours, and had the blistering evaluated. The effect on most panels were yellowing, and blistering, but some had cracks. The table X present the qualitative result that most repeated on the 3 panels tested for each formulation. The result is expressed by a number followed by letter, being the number a reference of blister size (8 is the smallest) while the letter indicates frequency (F-Few, M-Medium, MD-Medium Dense, and D-Dense).

Table X: Blistering results by coalescent.

Resin A Resin B

Butyl CELLOSOLVE™ Solvent None 6M DOWANOL™ PnB None 2MD DOWANOL™ DPnP 6MD 8M DOWANOL™ PPh 4M 6D DOWANOL™ DPnB 2F 8M

Due to this double and independent aspects of blistering, no ranking was considered. Examples of panels after immersion are presented on figures 3 and 4, bellow.

Figure 3: Panels after water immersion - formulation containing Resin B and DOWANOL™ PnB, example of level 2MD of blistering.

Figure 4: Panels after water immersion - formulation containing Resin A and DOWANOL™ DPnB, example of level 2F of blistering.

DRYING

The evaporation profile of volatile portion in the formulations, i.e. the water content and respective percentage of each coalescent, was simulated by chemical software for the Resin A. Its result is presented on Graph 3, where all drying times are close, except DOWANOL™ DPnP that has faster evaporation. The evaporation profile confirms that the replacement brings no issue on drying time, among the tested quantities of Butyl CELLOSOLVE™ Solvent, DOWANOL™ DPnB, DOWANOL™ PPh and DOWANOL™ PnB.

Graph 3: Evaporation profile of volatile portion in paints for Resin A. Simulated for 25⁰C, 60% of Relative Humidity.

0

10

20

30

40

50

60

70

80

1 11 21 31 41 51 61 71 81 91 101

Tim

e (m

in)

DOWANOL™ DPnB

DOWANOL™ PPh

DOWANOL™ DPnP

DOWANOL™ PnB

Butyl CELLOSOLVE™ Solvent

% Evaporated

CONCLUSIONS

Various tests were performed varying only the coalescent agent, and the results were different, which confirms that these products play important role in formulation, impacting key paint properties by small changes in the molecule and its percentage added.

Although the ranking of best coalescent had changes when compared MFFT, Adhesion, Water resistance and Corrosion resistance, the coalescent DOWANOL™ DPnB had the best balance of results, being on the top of ranking on most tests performed, what makes DOWANOL™ DPnB the best suggestion for Styrene Acrylic waterborne metal coating. Besides that fact, it is important to highlight that the effective percentage of DOWANOL™ DPnB was considerably lower than other coalescents, over 50% less, with very close drying times.

These results also confirm that the largely used coalescent for waterborne metal coatings in Brazil, Butyl CELLOSOLVE™ Solvent, is not the coalescent that optimizes the performance of this kind of formulation.

DOWANOL™ PPh and DOWANOL™ DPnB are better to reduce MFFT of styrene acrylic resins, as its results were similarly effective on this property, reducing MFFT with smaller quantity needed. DOWANOL™ PPh has the slowest evaporation rate, demanding higher drying time, and this fact could have influenced some of the results found.

ACKNOWLEDGMENTS

I would like to special thank the laboratory technicians who supported the experiments performance: Thais Fulgeri, Thiago Faria, Rodolfo Nogueira and Ana Carolina da Silva.

I thank Luis Gustavo dos Santos for his support in sharing previous experience, as well as Francisco Serrano, Juliana Francisco, Leo Procopio, Brian Jazdzewski and Felipe Donate.

REFERENCES

Natalense, J.C.; Graziani, M.; Juriyama, R. Solventes. Tintas Ciência e Tecnologia; 4th edition, Fazenda J.M.R/Blucher: São Paulo, 2009; pp 558-563.

Gnecco, C. Curso Tecnologia de Tintas. ABRAFATI. Personal communication, 2015.

Dow Coatings Materials Home Page. http://coatings.dow.com/en/products/AVANSE (accessed May 9, 2015).

American Society for Testing and Materials. ASTM D2354 – 10 Standard Test Method for Minimum Film Formation Temperature (MFFT) of Emulsion Vehicles.

American Society for Testing and Materials. ASTM D523-14 Standard Test Method for Specular Gloss.

American Society for Testing and Materials. ASTM D3359 – 09 Standard Test Methods for Measuring Adhesion by Tape Test.

American Society for Testing and Materials. ASTM D1654-08 Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments.

American Society for Testing and Materials. ASTM D714 − 02 Standard Test Method for Evaluating Degree of Blistering of Paints.