Film Formation in Waterborne Coatings - American Chemical Society

Chapter 22

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The Relationship Between Film Formation and Anticorrosive Properties of Latex Polymers R. Satguru, J. C. Padget, and P. J. Moreland Research and Technology Department, Zeneca Resins, P.O. Box 8, The Heath, Runcorn, Cheshire WA7 4QD, United Kingdom

Attainment of coherent and defect free film formation in latex polymers is essential for achieving good protective properties. Two case studies are reported to highlight this effect in terms of achieving good anticorrosive properties. In the first study, the positive influence of the addition of a non ionic surfactant to a chloro polymer latex in terms of enhanced particle coalescence leading to excellent anticorrosive properties is discussed. In the second case, the deleterious influence of particle pre-crosslinking on coalescence in a styrene-acrylic latex and its consequence on anticorrosive properties is highlighted.

The rate and extent of particle coalescence during latex film forming process has been a topic of interest for many years. Many theoretical and experimental papers have been published and the extent of activity has never been greater than at the present time, with the advent of powerful techniques for studying the process such as neutron scattering (/) atomic force microscopy (2) and fluorescence spectroscopy (5). This work has been driven in part because of the reasonable belief that the rate and extent of particle coalescence will have a profound effect on the properties of the ultimate coating. Nevertheless there has been remarkably little published work on the relationship between coalescence and anticorrosive properties. The anticorrosive properties of a coating are profoundly influenced by ingress of oxygen and water passing through the coating to the metal surface because corrosion processes depend fundamentally on the presence of oxygen and water. Preventing or at least restricting the ingress of water, oxygen, and/or electrolyte should be the primary consideration for designing an anticorrosive coating. Apart from selecting the optimum polymer type for this condition, coherent film formation is the key for achieving good protection. Attaining coherent film in solvent borne coatings is fairly straight forward. In water borne polymers however, it is more difficult as the film formation has to be derived from dispersed colloidal

0097-6156/96/0648-0349$15.00/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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FILM FORMATION IN WATERBORNE COATINGS

particles. In this paper we report two related case studies to highlight the importance of coherent film formation to obtain good anticorrosive properties. The first study deals with the influence of post added surfactant on film formation of a chloropolymer latex and the second study looks at the influence of crosslinking on film formation of a styrene-acrylic latex.


Experimental The chloropolymer and styrene-acrylic latices were synthesised using conventional emulsion polymerisation method. For water blushing, A C impedance and salt spray testing, films were cast on mild steel test panels using an applicator bar at the specified dry film thickness. Coated films were then dried for 7 days at 25°C and 50% relative humidity before subjecting to the respective testing method. A C impedance measurement was carried out using a Frequency Response Analyser, Solartron Model 1174. Atomic Force Microscopy (AFM) measurement was performed using a Nanoscope II instrument. Latex films were cast on acetate sheets and allowed to dry for 7 days at room temperature before subjecting to A F M examination. Moisture Vapour Transmission Rate (MVTR) values were obtained using the gravimetric weighed cup method at 25 °C and 75% relative humidity. Coatings were applied as l x 12um dry film thickness on to cascade board and dried at room temperature for 7 days prior to M V T R measurement. Results and Discussion Case Study 1 : Chlorine containing vinyl acrylic latex. Chlorine containing polymers exhibit excellent barrier to transmission of both water (liquid/vapour) and oxygen. For this reason solvent borne chloropolymers are well established as anticorrosive coatings. We have found that the attainment of similarly high level of corrosion protection from a waterborne chloropolymer latex is crucially dependent on achieving good particle coalescence. Investigations based on one particular chloropolymer latex (Haloflex* 202) led to some interesting findings. This latex was prepared at a very low anionic surfactant content with colloid stabilisation essentially derived from the initiator end groups (4,5). The average particle diameter of the latex was 230nm and the Minimum Film Forming Temperature (MFFT) was 15°C. It was found that clear films cast from the latex on to steel panels and room temperature dried for 7 days gave excellent performance in a conventional salt spray test. However formulation of the latex into a stable paint formulation (pH=4.5) required the addition of a non ionic surfactant in order to provide sufficient colloid stability. For this reason we studied the effect of surfactant addition on coalescence and coating properties. The chosen surfactant was Synperonic PE 39/70 (ICI), a polyethlene oxide - polypropylene oxide block copolymer surfactant of nominal composition (EO) (PO) (EO) . Latex samples containing different proportions of non ionic surfactant were cast on to mild steel panels at a dry film thickness of 60 um. After drying for 7 days 64

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In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

22. SATGURU ET AL.

Anticorrosive Properties of Latex Polymers

351

at 25 *C / 50% relative humidity the panels were immersed for 7 days in distilled water and then inspected. A qualitative assessment of film blushing at varying levels of surfactant addition is given in Table I.


Table I. Water blushing as a function of post added non ionic surfactant Extent of Blushing Wt% Surfactant 0

Severe

1

Moderate

2

Slight

3

None

4

Very Slight

The film containing no added surfactant exhibited worst blushing ie. a white haze developed. Progressively increasing the surfactant content in the range of 0 - 3 % (wt% on latex solids) gave rise to a progressive decrease in water blushing, with blushing being virtually absent at 3% concentration. A further increase to 4% caused reintroduction of blushing, but significantly less than for the surfactant free film. Evidence that the observed blushing was indeed due to the ingress of water was provided by measuring the capacitance increase of the coatings by an A C impedance technique (

Θ (2.0%) Figures total

i n parenthesis are

weight added surfactant

expressed as % of

latex

solid

weight

I

ι

1

ι

ι

ι

ι

ι

1

I

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 1-0 Concentration off surfactant in aqueous phase (WT.%)

Figure 2. Adsorption isotherm of non ionic surfactant on to chlorine containing vinyl acrylic latex. (Reproduced with permission from reference 6. Copyright 1983/ Coatings Tech.)


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for the polymer, a fact which presumably explains the observed ability of the surfactant to increase the rate of particle coalescence. A n additional possible role of the surfactant is to increases the inherent colloid stability of the parent latex, which in turn allows the particle coalescence to take place in a uniform manner by maintaining its particle integrity (ie. free from flocculation) through out the drying process, resulting in a defect free film. It is however important to note that the optimum protective properties of the film was observed at the monolayer coverage concentration of the surfactant ie. 3 wt% , and at this concentration the water vapour barrier properties of the latex cast film was comparable to that of the corresponding solvent cast film based on the same copolymer. Case Study 2 : Styrene acrylic latex. This study highlights the deleterious influence of particle pre-crosslinking on coalescence and its consequence on anti corrosive properties of the derived coatings. Two latices were chosen for this study: Latex AC1 - a styrene acrylic latex, where deliberate crosslinking was introduced during the polymerisation via the use of specific monomer choice. Latex AC2 - also a styrene acrylic latex composed of the same backbone as AC1 but no crosslinking monomer was employed ie. uncrosslinked. Particle diameter of both latices was 75 nm and MFFT of AC1 was 20* C while the MFFT of AC2 was 16 C. The study was based on assessing the respective anticorrosive performance of the coatings as a function of coalescent amount required to form coherent films. Dowanol DPnB and Dowanol PnB (Dow Chemical) combination at a ratio of 3:2 was used as the coalescing agent for this study. Latices containing 0 to 25 wt% (based on polymer solid content) of coalescent were prepared and 45 μπι d.f.t. films were cast on mild steel panels at room temperature (22 C) and allowed to dry for 7 days. The panels were then subjected to A S T M hot salt spray testing. The results showed (see Plates 1 and 2), that latex AC1 required a coalescent level of 20 to 25 wt% while latex AC2 required only 10 wt% for good performance up to lOOOh salt spray exposure. Considering the MFFT difference of the two latices is not very large, the above result was significant. In an attempt to obtain insight into the film formation process of latices AC1 and AC2, films containing different levels of coalescent were subjected to atomic force microscopy (AFM) and moisture vapour transmission measurement (MVTR) studies. A F M provides information on the topology of the film and hence the coalescence process, while M V T R values give information on the barrier to water vapour of the film and hence the coherency of the film. A F M results (see Plates 3 and 4), showed that a coalescent level of approximately 20 wt% was required for latex AC1 while 10 wt% coalescent was sufficient for Latex AC2 to form smooth films. This result was confirmed by measuring the root mean square roughness value from the respective micrographs. The M V T R values presented in Figure 3, also showed similar results indicating that lowest M V T R value for latices AC1 and AC2 were obtained at coalescent concentrations of approximately 15 -20 wt% and 10 wt% respectively. These results clearly supports the salt spray performance of the respective latex films, and confirms the necessity of higher level of coalescent for pre-crosslinked latex particles to achieve good anti corrosive performance. #

e




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Plate 2. Clear coatings of latex AC2 at varying coalescent levels after 1000 hour exposure in hot salt spray.


(SI

o



356

Plate 3. Atomic Force Micrographs of Latex AC1 films containing varying levels of coalescent.




22. SATGURU ET AL.

Plate 4. Atomic Force Micrographs of Latex AC2 films containing varying levels of coalescent.


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In addition, this study also showed the usefulness of A F M and M V T R measurements to assess the degree of coalescence in latex polymers in relation to its protective properties. * The word "Haloflex" is a registered trademark of Z E N E C A Ltd. Acknowledgments The authors wish to thank M r J Farrar Mrs Ε Jones and Mr Ν Ormesher of Zeneca Resins for their contributions to this work. Journal References: 1. 2. 3. 4. 5. 6. 7.

Yoo J N , Sperling L H, Glinka C J, Klein A ., Macromol. 1990, 23, 3962-3967. Juhue D, Lang J., Langmuir 9, 792, 1993. Winnik M A, Wang Y , Haley F., J. Coatings Tech., 1992, 64, 811, 51-61. Burgess A J, Caldwell D, Padget J C., J. Oil Colour Chemistry., 1981, 64, 175. Moreland Ρ J, Padget J C, Lim Yoo Keng., Proc. Corrosion Asia 1994, No 1042. Padget J C, Moreland Ρ J., J. Coatings Tech., 1983, 55, 698, 39. Brasher D M , Kingsbury A H., J. Appl. Chem., 1954, 4, 62.


Film Formation in Waterborne Coatings - American Chemical Society

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