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Achieve Desired Pigment Dispersion in WB Coatings with High Performance Polymeric Dispersants

SpecialChem – Jul 13, 2021

TAGS:   Architectural Coatings      Industrial Coatings      Automotive Coatings    

Overcoming Pigment Dispersion Challenges in Waterborne Coatings with High Performance Polymeric DispersantsDispersing agents are additives used in paint and coating formulations to enable the dispersion of pigments during manufacture, storage, and application.

To provide optimal performance, pigment particles must act independently of each other in the coating film and remain well dispersed throughout manufacture, storage, application, and film formation. A suitable dispersing agent can significantly reduce the energy and time required for pigment dispersion, as well as improve the final film’s performance properties.

Achieving the desired pigment distribution within a waterborne coating system is not an easy task considering the challenges of environmental regulations, demand for premium color coatings, and cost considerations. Fortunately, novel high-performance polymeric (HPP) dispersants have been developed to address these issues.

In this article, we will help you understand the current challenges faced during waterborne coatings formulation to achieve the desired pigment dispersion. Going further, you will be able to learn about the working mechanism of high-performance polymeric (HPP) dispersants and how they one HPP dispersant (STEPSPERSE® 61 from Stepan Company) provides overall value while easing environmental and cost burdens.

Let's begin by revisiting pigment dispersion process.


Three Stages of Pigment Dispersion in Paints and Coatings Formulations


Complete dispersion of pigment in the liquid medium is essential for optimal paint manufacturing properties. Moreover, the quality of the dry paint film such as long-term stabilization, color development, and gloss is very much related to the uniformity of the distribution of the pigment. Pigments must be dispersed and stabilized at their primary particle size (i.e., the “fundamental” non-aggregated particles) to provide maximum hiding and film properties.

There are three distinct stages to the dispersion process as shown in the Figure 1 below.
  1. Wetting
  2. Separation / Grinding
  3. Stabilization

Stages of the pigment dispersion process in paints and coatings
Figure 1: Stages of the dispersion process

Pigments are usually supplied in the form of dry powders. These dry powders consist of agglomerates that are much larger than the primary particles.

  1. In the first stage, the liquid replaces air on the surface of the pigment particles by decreasing the interfacial tension and wetting the pigment. All the air and moisture must be displaced from the surface of the pigment particles and between aggregates and replaced by the dispersant solution.

  2. The second stage involves pigment particle size minimization through mechanical energy such as grinding or milling. The pigment agglomerates are broken up, disrupted into smaller units, and uniformly distributed. The grinding process results in the so-called “millbase” that is finalized into a finished paint, in a let-down tank equipped with a mixer.

  3. In the final stage, the dispersant prevents the pigment from re-agglomeration through either electrostatic, steric, or a combination of the two mechanisms. The resultant suspension is stabilized due to the adsorption of dispersant and binder species at the pigment surface.


Pigment Dispersion Challenges - Waterborne Vs. Solventborne


Waterborne coatings present special challenges in providing optimal pigment dispersion. The selection of a dispersing agent is not a simple task. The wrong dispersant could:

  • Increase processing costs,
  • Jeopardize the coating’s appearance and,
  • At worst, degrade the mechanical properties of the coating.

Water miscible elements (e.g., impurities, electrolytes, neutralizing agents) in the waterborne formulation impact how dispersants affect performance properties. Consequently, it can be difficult to observe significant differentiation due to dispersion selection when it comes to performance compared to dispersants in solventborne coatings. With waterborne formulations, the dispersant chemistry must be exceptional to see a leap in performance.

In many solvent-based coatings, the solvent or the film-forming polymer may provide the wetting and dispersion function, but in waterborne, systems separate dispersants are required. The wetting step in solventborne systems is generally quite easy because of the low surface tension of organic solvents. Due to the significantly higher surface tension of water, dispersants are required to lower the surface tension and enable sufficient pigment wetting.

Furthermore, dispersants used in waterborne coatings require special functions that are necessitated by the aqueous medium and the type of pigments currently used in high performance coatings. These provide formulation and manufacturing challenges that can generally be solved by HPP dispersants.


High Performance Polymeric (HPP) Dispersants That Work for WB Systems


What are High Performance Polymeric (HPP) Dispersants?


HPP dispersants are comprised of a high-molecular-weight copolymer containing multiple functional pigment anchoring groups and long solvation chains to create steric hindrance around the pigment. These groups are connected to the dispersant’s backbone. As a result, the detachment of anchoring groups is prevented, and long-term stabilization is achieved. A key factor for a successful dispersion is the compatibility of the polymeric chains with water. They need to have a high degree of solubility to uphold steric stability.

Stepan Company has recently developed patent pending HPP technology that is very effective in the creation of pigment dispersions for waterborne coatings. A generalized structure of these dispersants is shown in the Figure 2 below. The dispersants are composed of three discreet components:

  • A linker, which can vary in length, functionality, flexibility, and number of appendages,
  • Affinity domains, which are designed to interact with the pigment surface, and can vary by the type, number, and arrangement of anchoring groups,
  • Stabilizing segments, which can differ by length, hydrophobicity, and functionality.

Generic structure of Stepan Company’s HPP dispersants
Figure 2: Generic structure of Stepan Company’s HPP dispersants

Each component of the generic structure can be varied independently. In addition to dispersion viscosity and paint compatibility, several additional criteria can be controlled by this molecular architecture, including dispersion stability, dispersant demand, pigment versatility, milling efficiency, paint color development, and product form.

Enhanced Value Provided by STEPSPERSE® 61 HPP Dispersant for Waterborne Coatings


One product, STEPSPERSE® 61, has emerged from Stepan Company’s technology efforts as the best overall performer, having broad dispersion capabilities.

STEPSPERSE® 61 is designed to be an alkyl phenol ethoxylates (APE)-free and VOC-free wetting agent and dispersant for a wide variety of organic pigments and carbon black in waterborne paint applications. This dispersant provides enhanced value to both the paint formulator and the end-user. Many of these qualities have a direct influence on overall cost. (As listed in table below and discussed more fully in the next section)

Formulation and Manufacture Performance Quality
  • Reduced dispersant concentration
  • Minimal impact on paint viscosity (KU drop)
  • Compatibility in multiple types of coatings
  • Multi-functional anchoring groups (to carbon black and organic pigments)
  • Faster and more efficient milling processes
  • Higher surface area / particle size reduction
  • Reduction in formulation components / inventory
  • Sustainable (VOC- and APE-free)
  • Improved long-term dispersion storage stability
    • No settling
    • No flocculation
    • No phase separation
  • Higher color strength development
  • Higher gloss
  • Greater film build
  • Better surface appearance
  • Improved dry film properties (primarily through lower dispersant loading and superior resin compatibility)
Enhanced value provided by STEPSPERSE® 61 HPP Dispersant for waterborne coatings


STEPSPERSE® 61 minimizes many of the challenges noted above for dispersing pigments in waterborne coatings. In doing so, it also provides an avenue to reduce formulation cost via:

  • Lower dispersing agent loading
  • Reduced pigment loading for equivalent color strength
  • Reduction in dispersing energy and time
  • Potential decrease in the number of formulation components
  • Broad pigment and resin compatibility

Reducing Coating Formulation Challenges Thanks to Broad Compatibility


STEPSPERSE® 61 is a high molecular weight, non-ionic-type dispersant that stabilizes by steric hindrance and provides a wetting effect. STEPSPERSE® 61 adsorbs onto the pigment surface by multiple anchoring groups and stabilizes the dispersion by solvation chains. Dispersants with smaller molecules or weaker single anchoring interaction could lead to desorption and flocculation. Stabilized pigments in water are often disturbed by impurities, such as other ions. STEPSPERSE® 61 is a non-ionic-type dispersant that is less affected by ionic impurities in the paint system.

STEPSPERSE® 61 can achieve excellent results in a wide variety of waterborne paint systems. This provides for a dependable and flexible solution to formulate coating systems with superior color strength. With carbon black pigments this is called “jetness” or “blackness.”

Table below provides an indication of the broad compatibility of STEPSPERSE® 61 compared to other commercial HPP dispersants. Resin compatibility indicates that this dispersant can be used in various resin systems and for different applications (architectural, automotive, industrial, etc.). Excellent resin compatibility also suggests its use in the formulation of universal masterbatch colorants for custom coloring.

STEPSPERSE® 61 Compatibility compared to other HPP dispersants
Industrial black tint: 15% Raven 5000 Ultra II (Birla Carbon) at 100% dispersant SOP. Tinting of paint at a 1:9 ratio

STEPSPERSE® 61 Compatibility compared to other HPP dispersants


The proprietary design of STEPSPERSE® 61’s chemistry facilitates the use of lower dispersant loadings when compared to other HPP dispersants. It was found that the optimal dosing to be 20% less (10% vs. 12.5% solids on pigment, SOP) than that of another commercial dispersant.

Stricter environmental legislation and regulation in waterborne coatings have also consistently challenged paint formulators and manufacturers in the choice and use of sustainable dispersants. The desire for VOC-free formulations, together with APE surfactant restrictions, represents new challenges for paint formulators. As a result, the demand for high-performance dispersants has intensified significantly.

Reducing Manufacturing Challenges with Multifunctional Wetting and Dispersing Ability


Once formulated, the paint system needs to be manufactured. Proper dispersant choice can impact grinding / milling time, rheology behavior, in-process stability, and particle size. This all impacts the time and cost to produce the paint system as well as the quality of the dry paint film.

Waterborne coatings tend to require longer milling times and greater control of rheological and dispersing properties. The grinding stage requires both wetting and dispersion. This is the most time and energy consuming process during the manufacture of paint systems.

Formulators sometimes use surfactants as milling agents and add a separate dispersant to provide particle separation and stability. By using high performance polymeric dispersants, both functions are combined into one product. This reduces the number of required components and decreases the production time and cost for manufacturing the paint system.

Reducing Dispersion Challenges for Organic Pigments and Carbon Black in Waterborne Paints


Pigments are usually the most expensive raw material in paint systems and can only show their full color strength if optimally dispersed. High performance dispersants can provide the required color quality with the minimum amount of pigment and, thus, help to minimize raw material costs. STEPSPERSE® 61 is recommended for use in dispersing organic pigments as well as carbon black pigments in waterborne paint.

The organic pigments market is projected to witness high growth due to the increasing demand for automotive and industrial coatings. Organic pigments are high in tint strength and brightness, but they are very difficult to disperse and stabilize because of the factors listed below.

  • Small primary particle size, which provides a larger total surface to be wetted
  • Non-uniform surface structure (e.g., both high and low polarities on a single crystal)
  • Due to the small particle size, Brownian motion causes increased flocculation
  • Low surface energy which inhibits wetting and dispersant adsorption

The particle surface area affects the level of dispersant required. An efficient and stabilized dispersion can only be reached if almost all particle surfaces are wetted and covered by the dispersant. In general, the recommended dosages of STEPSPERSE® 61 dispersant are as indicated below.

  • Solids on organic pigments: 4 – 40%
  • Solids on carbon black: 5 – 100%

Carbon black pigments are ideal for use in automotive, architectural, industrial, and protective and marine coating applications where they provide excellent color and hiding power and can ultimately improve coating performance. They have a wide range of surface properties and are difficult to disperse and stabilize, primarily due to their notoriously low surface charge and poor wetting characteristics.

The use of primary carbon black particles yields more intense black color as compared to larger particles. However, carbon black is generally considered to be the most time consuming and difficult pigment to disperse. This is especially true for waterborne systems because water is very polar, has high surface tension, and there is little interaction between the binder and the pigment. These properties require the use of a highly efficient wetting and dispersing agent.

STEPSPERSE® 61 HPP dispersant provides superior milling of industrial grade carbon black, achieving finer particles more rapidly when compared to other commercial HPP dispersants (Figure 3 below). Furthermore, the use of STEPSPERSE® 61 ensures excellent millbase stability maintaining initial viscosity after four weeks at 50°C.

Carbon black milling efficiency of STEPSPERSE® 61
Industrial black tint: 15% Raven 5000 Ultra II (Birla Carbon) at 100% dispersant SOP

Figure 3: Carbon Black Milling Efficiency of STEPSPERSE® 61


Summary of HPP Dispersant Benefits for Waterborne Paints and Coatings


STEPSPERSE® 61 is a new HPP dispersant consisting of a high-molecular-weight polymer containing multiple anchoring and solvating groups. The dispersant enables formulators to develop higher-quality pigmented paint systems as well as color concentrates. With this dispersant both wetting and dispersing functions are combined into a single product, which simplifies both the formulation and manufacturing processes.

STEPSPERSE® 61 is a VOC-free and APE-free dispersant intended for organic and carbon black pigments in waterborne paints and coatings. Application areas for STEPSPERSE® 61 dispersant includes automotive coatings, industrial coatings, and architectural coatings.

STEPSPERSE® 61 has shown multiple advantages for both the formulator and end-user. When compared to other HPPs, STEPSPERSE® 61 shows excellent color intensity, high gloss, and long-term stability. The new dispersant also shows very good compatibility and storage behavior.

Cost savings are possible when utilizing STEPSPERSE® 61 as it reduces the concentrations of dispersant or pigment required, reduces milling time and energy usage, and reduces the number of components required in the paint formulation thanks to its multifunctionality.

Access additional technical data on STEPSPERSE 61 polymeric dispersant



References


Vargas, C. et.al., “High Performance Dispersants for Carbon Black in Waterborne Systems” presented at the American Coatings Show, 2020.

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