Leveraging Structure-Property Relationships of Surfactants to Meet Emerging Needs in Paints

Summary

All surfactants are amphiphilic molecules oriented toward interfaces. However, the behavior of a surfactant at these interfaces is determined by its complete chemical structure. The behavior of a surfactant is well understood in diluted systems, but in the complex formulation of most paint applications, predicting the behavior of these materials becomes considerably more difficult. Guidelines developed for general surfactants in diluted detergent systems are insufficient for paints. Because for use in paints, understanding surfactants well is as important as knowing their interactions with other components. These interactions significantly affect a surfactant's behavior, leading to performance losses, unexpected effects in applied paints, and difficulties for chemists in optimizing formulations. This article will discuss some of these effects and review studies on new surfactant structure-property relationships in pigmented systems. These studies allow us to gain deeper insight into how chemical structural characteristics in different varieties determine the interactions of resulting surfactants with other components in paint. Additionally, how combinations of these structural elements create next-generation wetting agents will be explained.

Introduction

Surfactants are common in water-based paint formulations and impart various performance benefits. Some of their most common properties include wetting surfaces, providing good dispersion and emulsion, and increasing color acceptance and intensity. However, surfactants are also effective in many other areas. With continuous formulation changes made to meet increasingly stringent environmental, health and safety requirements, the effects of surfactants have become increasingly important. Frequently, suboptimal product selection has led to adverse formulation effects, higher usage rates, and increased workload. Much of current understanding of surfactant formulation is based on older theories such as HLB1,2 (Hydrophilic-Lipophilic Balance), which remain limited in complex systems like paints. Newer theories and approaches are needed to develop products that can be used optimally and to shorten formula development time. More detailed understanding of surfactant behavior in complex formulations is an extremely important tool in promoting innovation and is being applied to achieve good results in many areas. The chemistry and function of dispersing agents can be customized and even within complex structured paints can perform the function of creating latex particles; it can provide benefits in film coalescence and reducing film formation temperature. This performance provides options for reducing solvents, optimizing formulation, or obtaining paint behaviors with different properties. Chart 1 below highlights the effect of such a surfactant in a styrene-acrylic latex, as well as its use in a paint formulated as zero-VOC on this latex.

Chart 1: Effect of wetting agent on Minimum Film Formation Temperature (°C)

The left portion of Chart 1 shows MFFT (minimum film formation temperature measured in a manner similar to ASTM D2354-10 standard) results in styrene-acrylic latex. While the film formation temperature of the latex alone is 23°C, using 1% by weight of the recommended auxiliary solvent reduces MFFT to 20°C, while using a coalescing surfactant at the same rate provides a noticeably lower MFFT of 10.5°C. The right portion of Chart 1 examines results of a fully formulated semi-gloss paint. The resin supplier's recommended model formulation and the same formulation containing 0.4% by weight coalescing surfactant show MFFT values of 8.6°C and 5.5°C, respectively. Low-level coalescing surfactant use has a noticeable effect on MFFT and provides considerable contribution to wetting performance. This effect gives the formulator the opportunity to use less solvent or achieve a greater reduction in MFFT, depending on performance requirements. Chart 2 below shows results for a different surfactant designed to affect open time in low and zero VOC water-based paints. Open times were measured according to ASTM 7488-11, and are shown for a commercial wall paint formulated with and without a surfactant designed to extend open time. A significant increase in open time was measured at 1% by weight usage level. This provides the formulator with the potential to meet market demands.

Chart 2: Effect of open time-extending surfactant on working time (min.)

These performance properties depend on the distribution and behavior of the surfactant between the continuous phase, the particle surface, and other interfaces in the complex paint formulation. Optimizing this behavior is fundamental to maximizing desired performance while minimizing side effects in the final product. These examples highlight just two current approaches that add distinctive value to the formulator in surfactant technologies. More effects can be achieved through more comprehensive understanding of component interactions. What is more challenging and technically more compelling is the adaptation of surfactants to pigmented systems and their interaction with dispersing agents and resins. Work in this area will be examined in this article and the details of an experimental approach providing optimized surfactants for colorants and pigmented paints will be explained.

Interfacial Activity in Complex Systems

It is common to find surfactant performance measured and explained in pure water. Equilibrium and dynamic surface tension (DST) measurements and foam stabilization assessments are useful in understanding basic characteristics but behavior in pure water can differ significantly from even simple formulations. The addition of other components can fundamentally change a surfactant's behavior. Chart 3 below demonstrates this by comparing the surface tension reduction ability of a commercial ethoxylated alcohol surfactant in pure water (purple) and the same surfactant in the presence of a small amount of organic pigment (black).

Chart 3: Surface tension reduction in the presence of Red Pigment 22

By adding 0.05% by weight of active surfactant, this surfactant shows the lowest surface tension in pure water at CMC (critical micelle concentration): 0.1 dyne/cm (measured using a bubble tensiometer). In the presence of 2% pigment, significant surfactant depletion at the pigment-water interface is indicated, markedly reducing dynamic surface tension. With pigment addition, the surfactant in the system is depleted from the continuous phase to reduce surface tension at the gas-liquid interface. The lowest surface tension is not observed without at least five-fold surfactant loading. While the data may not fully elucidate the material's behavior, it suggests a new CMC at 0.3% by weight concentration.

The curve derived from the data is likely non-linear due to secondary horizontal progression at the solid-liquid interface due to adsorption.3

Adsorption behavior may be preferable when selected for a surfactant intended to assist in pigment stabilization, but using such a surfactant for surface wetting is definitely undesirable. A fully formulated paint contains many interfaces. Additionally, the situation becomes more complex with large surface areas causing depletion of each type of surface-active additive in the system. Schema 1 below explains this in a simplified diagram. Surfactants in a formulation will be distributed across all phases and interfaces present in a complex system.

Schema 1 - Location of surfactants in a complex formulation

Comprehensive understanding of each surfactant's location in a formulation could be ideal, but the system's complexity makes direct analysis difficult. For now, a stepwise investigation can provide insight and guide product development.

Surfactants in Pigmented Systems

In pigmented systems, the use of multiple surfactants is frequently observed. Dispersing agents, grinding resins or binders are used against agglomeration; wetting agents are mostly used for surface and pigment wetting. Additionally, alkoxylated surfactants are typically widely used and effective in stabilization and surface tension reduction across diverse formulations. This second category is of interest because the behavior of these products is likely the least understood and most prone to misuse, overdosing, and unexpected side effects. Schema 2 below outlines the general characteristics of these product groups.

Schema 2, Surfactants in a pigmented system

  On the left are wetting agents, providing optimum surface tension reduction for application and processing. Products achieve orientation by maximizing the surfactant concentration present in the continuous phase.

It is also optimized to maximize energy reduction with the formation of a new interface such as a surface. On the right are dispersing agents, optimized for stabilizing pigments or other solids against agglomeration and designed to show strong interaction at the solid-liquid interface.

However, stabilizing surfactants are typically exploited for many purposes. Alkylphenol ethoxylates, alcohol ethoxylates, and specialized chemical structures are used as processing aids and to provide benefits for grinding improvement and color enhancement, steric stabilization benefits, and color acceptance, while reducing costs in wetting. However, side effects are commonly seen, especially from incorrect product selection and overdosing. Effects on rheology, water sensitivity, settling, reduced stain resistance, and foaming are common results. Product optimization and proper use reduce formulation costs to the lowest dosage levels and minimize negative effects that reduce paint performance. Therefore, the ideal grinding surfactant will provide optimum process and performance effect at the lowest usage level without any negative side effects.

Interfacial Activity

Grinding surfactants create the impression that optimum performance requires balance across all properties. Reduction in surface tension is a known requirement for pigment grinding. 4 Adequate interfacial tension reduction ensures complete pigment wetting and reduces the energy required to grind the pigment. This is certainly a fundamental component, but if surface tension control were the only guide, formulators would be expected to prioritize wetting agents. However, this is not the case. Yet alkoxylated surfactants appear to play a greater role than wetting agents alone. Additionally, steric stabilization is an important factor in the final product's grinding, compatibility, and stability. This has been discussed extensively in the literature5, but the insufficiently elucidated point is the optimal combination of these two properties in a complex paint formulation.

The widespread concurrent use of multiple surfactants in paints typically suggests that achieving optimum performance requires a balance that is generally understood through trial-and-error and flawed testing. Predicting this balance is one of the important first steps leading to surfactant optimization.

One of the more common parameters in surfactant selection is the surfactant's HLB value. This is essentially an explanation of the relative size of the surfactant's steric stabilization component and is generally used to guide product selection. While HLB theory may be useful in dilute emulsions, observational data shows its utility for paints is limited. It is not uncommon to see surfactants with vastly different HLB values serve a system equally well. A study addressing the applicability of HLB in organic orange pigment is shown in Chart 4 below. This system involves selecting and substituting surfactants containing APE with surfactants not containing APE having similar HLB values to the product in question. All evaluated surfactants have an ethoxylated A-B structure but differ in hydrophobicity structure, shape, and size. No trend related to HLB value was observed, and large differences in performance were noted among surfactants with the same HLB value.

Chart 4 -Surfactant HLB and color improvement

This data shows that single factors such as HLB used to characterize a surfactant may be insufficient for more comprehensive product selection. Other actors affecting this situation exist, and factors such as hydrophobicity dimension, adhesion interactions, structure, and stabilization characteristics may also be potential areas of research.

Surfactant Depletion

To discover surfactant behavior in a complex system, an initial attempt was made to determine the structure of surfactant-pigment interaction. To successfully accomplish this determination, dispersions based on various surfactants for grinding were prepared. Color formation and stability were evaluated as critical performance criteria. Additionally, a centrifuge (10,000-25,000 rpm) was used to directly determine the amount of surfactant depletion between the pigment-water interface and the continuous phase. Completed dispersions were diluted with water at a 1:1 ratio prior to centrifugation to eliminate any rheological differences between them. Any deviation in the actual depletion of surfactant in the system from this condition is expected. However, it should allow for valid comparative analysis between different surfactant types. Additionally, dilution and centrifugation are expected to result in net increasing surfactant content in the supernatant phase and the residual value remaining with the pigment in the dispersion is expected to be proportionally minimal.

To increase the expected difference between surfactants, uncoated phthalocyanine pigment with greater surface area was used. This was combined with a strong pigment-seeking dispersing agent not expected to remain in the continuous phase.

Numerous preliminary assessments before comprehensive analysis confirmed that no dispersing agent was found in the collected liquid phase, and no other material was significantly present in the supernatant. Liquid chromatography was used for direct analysis of surfactant content in the stationary phase. These chromatographic analyses were extended with mass balance measurements of the separated phases. FTIR (Fourier Transform Infra-red) was used as a partial check to confirm general material structures and verify that other components did not separate. The alcohol ethoxylate used as a general reference was initially evaluated in subsequent investigation (also shown in Table 1 below) and provides some interesting insights.

Table 1: Amount of surfactant found in supernatant in relation to surfactant concentration in the dispersion

The impression is created that most of the reference surfactant remains on the pigment. This suggests that this surfactant would be more suitable for pigment stabilization and show less suitability for utilization at other interfaces or for wetting function. Even at high usage levels, the impression is created that the surfactant is depleted at the pigment interface and remains there even after dilution and centrifugation. The same approach was applied to a series of surfactant types to determine whether these technical differences were evidence. Chart 5 below shows a compilation of different surfactant results based on different hydrophobicities, A-B with A-B-C structures, and alkoxylation levels. This evaluation could be compared with the situation described above, and surfactants were consistently compared at 2% by weight usage level. Advanced studies were conducted on the selected examples and showed similar trends to the results previously shown in Table 1. Products showing adequate dispersion stability and rheology performance are only the surfactants shown in Charts 6, 7, and 8. Products showing insufficient stabilization or resulting in inadequate dispersion rheology were excluded to avoid potentially misleading results.

Chart 5: ΔE values as a function of surfactant amount found in supernatant

Results in Chart 5 indicate a positive relationship between color formation of uncoated phthalocyanine blue and the surfactant content found in the supernatant. Color formation was evaluated through projections in a tinted white-based paint and ΔE (CIELab) values were calculated in relation to a controlled sample prepared without surfactant in the grinding process. This performance may be related to reduction in surface tension, but contributions from other factors related to steric stabilization are also expected. Surfactant content found in supernatant was 0.02 to 0.1% by weight, well above CMC values for these surfactant types.

The data indicator in the lower left of Chart 5 is a result of an A-B type surfactant with strong hydrophobicity expected to show greater association tendency for the pigment surface, with hydrogen bonding capability.

This provides greater confidence in the suggestion that the results captured differences between surfactant depletion and that the situation was not an artifact of another condition. In the next step, 4 surfactants were selected based on the data, and analyses were repeated with changes to dispersing agents and pigment structure, extending the experiment. Chart 6 below repeats Chart 5 results for these 4 selected surfactants containing phthalocyanine pigment. Here the surfactant measured in the supernatant is shown as a bar, and color formation results are shown as a line. Chart 7 shows results for the same surfactants with a change in dispersing agent from styrene acrylic to polycarboxylate type. Chart 8 shows results for the same surfactants with a change to organic yellow pigment (PY83). The trend/behavior of these 4 surfactants is consistent across formulation structure modifications.

All four provided stable dispersions at comparable rheologies, but clear differences were noted in color formation. It appears that this situation follows the trend of increasing 'free' surfactant content in the supernatant.

Results from free surfactant analysis support the assumption that surfactant performance in a pigmented system is related to how surfactant depletion occurs between the pigment-liquid interface and the continuous phase. They also show that surfactant characteristics are a larger factor than system structure. This does not contradict current theories. It can also provide clues to answer why some surfactants succeed while others fail. A surfactant providing steric stabilization by depletion at the pigment surface while balancing interaction with new or altered interfaces in the continuous phase to provide wetting benefit is predicted to deliver optimum efficiency. Additionally, the suggestion that surfactant structure is more important than system structure is a significant advantage for additive designers. Because the effects from the formulation are small, greater flexibility in overcoming regulatory barriers during surfactant structure development is provided. Product volatility and indoor air emissions, higher molecular weights, fewer byproducts and unreacted alcohol can be considered. Products can be developed to avoid or reduce environmental, health and safety concerns that limit compliance with initiatives such as LEED or EcoLabel6 or create risks for formulators.

Product Development

By examining the experimental findings described above in depth, work was initiated to develop a product that provides optimal performance balance and prevents EH&S, VOC, and emissions effects restricting usability across all water-based paint types, particularly interior applications. Details of this work are not suitable for sharing due to the proprietary nature of material structures, but as explained in this article, focus was placed on depletion studies. Chart 9 below shows the result of this work, comparing a zero-VOC paint example formulated with a high-performance branched polymer dispersing agent known as prototype in the grinding stage using yellow PY74 pigment against a high-performance comb polymer dispersing agent. Increases in color and stabilization improvement are noticeably apparent compared to references.

Chart 9: Prototype performance in PY74 paste – ΔE increase compared to standard sample

Benefits from the identification and characterization of the surfactant and optimized performance results enable the paint to maintain an ideal balance of depleted surfactant and free surfactant for paint absorption. Similar trends were observed with different dispersing agents. The impression is obtained that this performance is also effective in fully formulated paints. Increasing complexity in paint structure restricts clear demonstration of differences between surfactants in a single property like color, but instead good results are achieved with subtle variations and some defects in paint performance. Coverage, rheology, water sensitivity, blocking, and stain resistance are general areas where surfactants can demonstrate notable and measurable changes in performance. Table 2 below shows results of a prototype surfactant in a zero-VOC, semi-gloss interior paint formulation based on styrene-acrylic latex. This system was taken from a published model formulation in which the typical use of a surfactant containing alcohol ethoxylate during grinding was used.

Table 2: Paint performance properties when prototype surfactant is used in zero-VOC, interior, semi-gloss formulation

Paint was prepared as specified and evaluated for typical paint performance properties. The control formulation showed minor defects related to antifoam agents along with signs of inadequate wetting on various surfaces. Extrapolating from results discussed in this article, it is likely that the alcohol ethoxylate is largely depleted in the continuous phase of the formula and is insufficient for surface wetting. The typical recommended solution for this is to use another surfactant, a wetting agent, to improve performance. This is shown in column 2 results by adding 0.1% by weight additional wetting agent. Inadequate wetting was resolved, but the small amount of surfactant resulted in a noticeable decrease in blocking and only minor improvement in extraction performance. This situation can be attributed to general properties declining due to surfactant content depleting at improper interfaces in the paint formulation. A substitution with prototype surfactant at the same usage level eliminates these deficiencies and improves both wetting and blocking performance of the control model formulation. The efficiency of the prototype surfactant even sustains its performance characteristics with 50% lower loading, allowing for even lower usage. Proper location of the surfactant in the complex paint system provides optimal performance at minimal usage level.

Product Development

The research and product development discussed in this article demonstrate that surfactant performance can change significantly in formulated systems. There are important indications that the diversity in change may be related to surfactant depletion at different interfaces in a formulation and the material structure itself. Whether as auxiliary dispersing agents for pigment wetting or as surfactants for combining latex particles, a range of products was customized for this work. This optimization approach is applicable to general surfactants and is particularly effective in water-based, pigmented formulations. A surfactant with appropriate balance in its characteristics maintains sufficient free surfactant content for wetting and process requirements while providing the stabilization performance, dispersion stability, and compatibility required for paint absorption. This balance reduces the risk of performance defects typically associated with surfactant overdosing, such as foaming, blocking problems, extraction, water resistance, or rheology issues, enabling the formulator to apply minimal surfactant loading. Optimally performing surfactants that can meet current and future regulations are a fundamental step in sustaining innovation and performance improvements in paint technology.

Acknowledgment

The authors acknowledge Timothy Smith, Michael Pauley, Renae Bennett, Jonathan Sefko, and Wilco Chaigneau for the research, analyses, and contributions to product development work referenced in this article.     Christine Louis Technical Marketing Manager / Automotive and Transportation Coatings Evonik         Mike Peck Senior Product Development Manager / Coating Additives Evonik           Duygu Özgün Customer Manager / Coating Additives Evonik Tic. Ltd. Şti.         Elif Küçükosman Customer Manager / Coating Additives Evonik Tic. Ltd. Şti.    

References 1 Griffin, William C. (1949), Classification of Surface-Active Agents by 'HLB', Journal of the Society of Cosmetic Chemists, 1 (5): 311-26 2 Davies JT (1957), A quantitative kinetic theory of emulsion type, I. Physical chemistry of the emulsifying agent, Gas/Liquid and Liquid/Liquid Interface, Proceedings of the International Congress of Surface Activity, pp. 426–38 3 Johnsson, Bo, et al(1998), Surfactants and Polymers in Aqueous Solution, John Wiley & Sons, pp 219-220 4 Winkler, Jochen (2012), Dispersing Pigments and Fillers, pp 59-96 5 Tadros, Tharwat F.(2005), Applied Surfactants: Principles and Applications, chapters 5, 6, 7 6 www.ecolabel.eu