Through the adoption of increasingly more stringent environmental regulations, the coatings industry has been moving away from traditional oil-based paint systems and toward water-based systems that contain lower levels of VOCs. From an applications perspective, traditional oil-based alkyd systems produce excellent brush properties primarily due to well-developed high-shear viscosity, which in turn results in high film build, which allows for improved coverage, flow and leveling. Alkyd resins commonly used in these systems mimic, to some degree, the rheological properties of common oils, such as linseed, and many silicone fluids. Most of the resulting paint systems produced using these resins inherently produce high high-shear viscosities with near-Newtonian rheological profiles.
BackgroundInitial conversion to early latex systems was a challenge due in part to the types of thickeners/rheology modifiers used as well as the performance of the latex emulsions themselves. If one were to take a closer look at only the rheological properties associated with these early systems, obvious deficiencies would be evident, most noticeably the application properties, which were severely lacking when compared to their alkyd counterparts.
Alkyd systems are renowned for their creamy brush feel and superior flow and leveling, while the early latex systems suffered from poor application properties, poor coverage, and poor flow and leveling. Many of these shortcomings can be directly linked to poor high shear, (ICI), viscosity development. With HEC as the predominate thickener the ICI viscosities of these early systems were often well below 1.0 poise with rheological profiles that were closer to near pseudoplastic than to near Newtonian. Shear-thinning rheology is desirable for most systems, but the negatives of poor brush/roller feel, poor coverage, and poor flow and leveling were difficult to overcome.
As early as 1980 nonionic associative thickeners were being used to augment the rheological profile of latex paints with the goal being to improve on high-shear viscosity development or as many in the industry refer to as ICI viscosity. Improvement in latex technology and formulation techniques has resulted in latex paint application performance starting to approach that of the traditional alkyd oil-based systems.
Figures 1 and 2 compare the rheological properties of a traditional oil-based alkyd system to an HEC-thickened acrylic water-based system and a nonionic associative thickener-modified system. Both examples show that the traditional oil-based alkyd system yields a more Newtonian viscosity profile or less shear thinning behavior with a high-shear, (ICI), viscosity of 4.6 poise at 10,000 s-1. Also, the oil-based alkyd system yields acceptable sag resistance of ~300 µm with excellent flow and leveling. The HEC-thickened latex water-based system shows considerable shear thinning properties with nearly double the low-shear viscosity, resulting in excessive sag resistance with minimal high-shear viscosity, measured at 0.7 poise at 10,000 s-1. The third system based on a nonionic associative thickener yields a rheological profile more similar to that of the oil-based alkyd but with less low-shear viscosity, an acceptable sag resistance of 350 µm, and a much higher ICI viscosity of 2.2 poise at 10,000 s-1. The ICI viscosity of this acrylic paint system was further increased through further additions of nonionic associative thickener to as high as 4.5 poise, which was very close to the ICI viscosity of the alkyd test system. We would expect the paints with very similar ICI viscosities to behave the same upon application, but the acrylic system developed unusual application properties at ICI viscosities ≥ approaching 3.0 poise. Most notably among these was the sticky and unpleasant brush drag compared to the traditional oil-based alkyd system yielding a very creamy brush feel.
Dynamic Rheological MeasurementsFurther investigation into why the brush feel was so drastically different between the various paint systems was conducted using advanced dynamic rheological measurements, which measures the viscous and elastic components of viscosity using oscillatory measurements that separates viscosity into two components G’ and G”. G’, or the elastic component of a fluid, is determined based on how in-phase the fluid is with the oscillatory measurement and describes the entanglement or networking within the fluid; G”, or the viscous component, is sometimes referred to as the loss modulus. The G’ or elastic modulus of three paints (traditional alkyd at 4.6 poise ICI, the acrylic latex at 2.2 poise ICI, and the acrylic at 4.0 poise ICI) is shown in Figure 3 and reveals that the G’ response of the nonionic thickened latex system at an ICI of 2.2 poise is very similar to that of the oil-based alkyd. The latex paint with an ICI of 4.5 poise shows extremely strong G’ response.
This study suggests that the mechanism of developing ICI viscosity for the acrylic latex system is substantially different than the alkyd oil-based system and, therefore, the stickiness observed during the brush application can be attributed to the excessive elastic component of the higher ICI latex system. From this, most architectural coating formulations target ICI viscosity in the range of 1.0 to 1.8 poise with higher ICI viscosities often resulting in application difficulties in the field.
One theory into the mechanism of how nonionic associative thickeners develop high-shear viscosity consists of maintaining a higher hydrodynamic volume during the high-shear operation, which is obtained through the nonionic associative thickener interacting or associating with the latex particles.1 Proper selection of molecular weight, hydrophobe type and the correct three-dimensional structure of the resulting nonionic associative thickener dictate the high-shear response of the thickener, which has resulted in several commercially available systems for developing adequate ICI response. Two factors commonly encountered when formulating with ICI generators are: 1) syneresis control or separation as a result of bridge flocculation of the latex particle and 2) use level in formulation required to produce adequate ICI viscosity. Typical use levels for architectural coatings can range from 0.5 – 3% based on formula weight, and some formulations can exceed 5%. The use of nonionic associative thickeners at such high levels can negatively impact film properties such as adhesion, block, dirt pick-up, etc.
Through proper polymer design the issue of bridge flocculation can be improved upon or prevented, as demonstrated in Figure 4. The chart clearly shows that the G’ of the syneresis-free and stable system, (2), displays a more flat, linear response through the low-shear stress test region. The G’ of system 1 began to degrade very quickly with increasing shear stress. We have observed this phenomenon on many occasions and have been able to establish a loose correlation between the results and the occurrence of syneresis. The exact correlation however has proven to be formulation dependent.
Concerning the issue of use level, recent research has been conducted to the development of a novel system for generating a more efficient ICI generator or what may be called a “Hyper Efficient ICI Driver,” which is designed to yield the same rheological profile as current ICI generators but at substantially lower use levels.
ICI viscosity development and syneresis result from bridge flocculation and are to some degree concentration dependent; therefore at higher formulation concentrations we would expect to see improved ICI viscosity performance along with a reduction in syneresis as a result of bridge flocculation. The challenge that faces these hyper ICI drivers is that they are designed to be used at lower concentrations in formulation and, as a result, we can expect to see an increase in the tendency of these new polymers to develop severe bridge-flocculation related syneresis.
Figure 5 shows four systems with H1 being a conventional ICI driver and three recently developed “Hyper Efficient ICI Drivers”: U1, U2, and U29. This graph plots the low-shear viscosity of these four systems versus the ICI response with increasing use level. The H1 system exhibits an increase in ICI response and also a slight increase in the low-shear viscosity, while system U1 and U2 demonstrate more-efficient ICI viscosity development but as the use level is increased the low-shear viscosity decreases dramatically, which is an indication of poor stability or that bridge flocculation has occurred. The U29 system also shows increased ICI efficiency and also shows a slight increase in low-shear viscosity with no indication of bridge flocculation.
Figure 6 shows the heat-aged stability of the four low-VOC systems, with system H1 (conventional ICI driver) producing acceptable package stability and no syneresis. Systems U1 and U2 at low use levels display severe syneresis, which becomes less pronounced as thickener concentration is increased. This phenomenon is typical of bridge flocculation where low levels of an ICI driver will bridge latex particles together and result in an increase in low-shear viscosity. With the addition of more thickener the flocculation is resolved or dispersed, and the low-shear viscosity will actually decrease at higher formulated use levels. The U29 system shows, as predicted by the low-shear viscosity versus ICI plot, that the system is stable with no indication of bridge flocculation or syneresis found even at low polymer use levels when compared to U1 and U2.
Figure 7 shows the comparison of H1 (conventional ICI driver) with the U29 in an all-acrylic, high-gloss formulation, which shows that the use level required to achieve the desired rheology is substantially less for the U29 (~33% less) and still generating a strong ICI response. Additional comparisons indicate that U29 generates the “hyper” efficiency in various latex systems with minimal generation of syneresis.
ConclusionsOver the past 25 years or more, nonionic associative thickeners have been used to improve the application properties of latex paint with substantial progress made toward matching the rheology of traditional oil-based alkyd systems. Recent developments show that through proper polymer design (3-dimensional structure, proper hydrophobe selection and level) a “hyper” efficient ICI driver can be developed, which can allow a formulator to generate acceptable ICI response but with up to 30% less thickener demand. The polymer structure is also critical to other properties such as stability with improved syneresis resistance.
This paper was presented at The Waterborne Symposium, sponsored by The University of Southern Mississippi School of Polymers and High Performance Materials, and The Southern Society for Coatings Technology, 2008, New Orleans, LA.
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