Coatings are complex composites designed to have a stable shelf-life, apply easily to a substrate, and provide lasting durability in varied environments. These performance criteria require disparate materials to be finely dispersed and work cohesively, which presents a major challenge as composites can be unstable and ultimately fail at points of inconsistency. To address this, the first thing a new paint chemist learns is that wetting agents are mandatory in allowing mineral fillers to disperse properly. Without them, these inorganic particles generally reject their organic media, agglomerate and settle out. It would be imprudent to forgo wetting agents.

Another well-known, tried-and-true technique to stabilize composites and improve their uniformity is to use high-quality, gel-grade attapulgite mineral products. They have been common in paint applications for several decades as thixotropic thickeners for low-shear flow and leveling, sag resistance and other rheology benefits. However, a seemingly forgotten higher value is their function as excellent stabilizers due to their unique colloidal lathe-shaped particles forming a lattice structure. This keeps liquids and particles more evenly dispersed and suspended, generating important benefits well beyond rheological properties.

This article gives a brief history of attapulgite, a description of its mechanism that sets it apart from other thixotropic thickeners and stabilizers, and discusses its performance benefits in coatings.

Chemistry and Morphology

Attapulgite is a naturally occurring hydrous magnesium aluminosilicate clay mineral with the formula (Mg,Al)2Si4O10(OH)•4(H2O). The composition and structure of attapulgite were determined in the early 1940s and 1950s.1  It has a lathe or bristle morphology that is non-swelling and non-fibrous (Figure 1) and is inert and stable under a wide pH, temperature and chemical range. Attapulgite particles are naturally colloidal with weak electrical charges. They have a high aspect ratio of 100:1 or higher and are in tightly compressed large bundles that must be dried and separated.

Commercial development soon followed to capture the benefits of attapulgite’s unique ability to form an electro-mechanical lattice structure that interacts with particulate solids, when it’s well-dispersed in a liquid. High-quality gel-grade attapulgite products, produced from deposits in the United States, proved to be ideal low-shear rheology modifiers, syneresis control agents and suspension stabilizers for a wide variety of water-based and organic liquid systems. They are especially effective in mixtures containing high solids. Drilling muds, coatings, and tape joint compounds were among the first applications. Later developments included suspension fertilizers, animal feed suspensions, adhesives, sealants, inks, color concentrates and asphalt coatings.

Although this sedimentary clay is found on nearly every continent the quality varies greatly with two primary distinctions. First, the majority of deposits and the quality within the deposits are not gel grade, making them only suitable for sorbent applications, like filtration or floor absorbents. The other variance is particle length. United States gel grade attapulgite products have a consistent particle size of 2.5 microns long and 25 nanometers wide. Other deposits have much longer and varied lengths2, which makes their primary value of forming a lattice structure inconsistent. This non-uniform bristle length causes some variability in performance and in a composite’s shelf-life. In addition, the International Agency for Research on Cancer (IARC) found that particle lengths greater than 5 microns have evidence of carcinogenicity.³

How Attapulgite Works

Attapulgite bristles have weak electrical charges, positive on the ends and negative along the shaft. When well dispersed in liquids, the charged particles “at rest” are in close proximity and form weak electrical bonds, end-to-face, creating a three-dimensional lattice network. This structure entraps liquids and keeps insoluble particles both separated and suspended. Under shear the weak lattice structure dissociates, resulting in lower viscosity and markedly improved mixing, pumping and workability. When the system returns to “at rest”, the rate of ‘thixotropic recovery’ – or rebuilding of the lattice structure – is exceptionally fast with an immediate return to a stable structure (Figure 2). Since attapulgite mineral does not swell, there is no post-thickening or creep viscosity. It is not a dispersant but further aids the separation of liquids and particles and keeps them suspended.

Unlike most other thixotropes, this shear-thinning can be repeated endlessly without loss of performance.

Particle Network and Performance

The thixotropic effect, coupled with suspension and rapid recovery, provides benefits beyond just rheology modification.

The lattice network “at rest” provides a uniform and stable mix with all components well dispersed. This better ensures the mixture can perform optimally as well as maintain consistent quality throughout shelf-life. Benefits are:
• Syneresis Control;
• Freeze-Thaw Stability; and
• Pigment Suspension (reduced pigment float & flooding and improved tint strength).

Under motion, the attapulgite bristles align with the direction of flow and stay dispersed among the particulate solids, keeping them separated. This optimal dispersion provides lower viscosity and markedly improved flow, pumping, spraying and workability.

• Low shear rates maintain high viscosity, which minimizes sag and flow after application, controls the penetration rate of adhesives and stains, and prevents adhesive squeeze-out during high pressure laminating.
• High shear rates produce low viscosity that permits faster mixing prior to application, improves leveling during the application of high-viscosity mixtures, and improves workability (pumping, spraying, spreading, feathering, etc.).

Returning to “at rest”, or rebuilding of the lattice structure, is very fast. Mixtures quickly return to their original higher viscosity after mixing or after being applied, resulting in:
• Improved flow and leveling of paints, adhesives, sealants, etc.;
• Reduced settling and sag;
• Reduced overspray and spattering; and
• Reduced permeability and shrinkage.

It is important to reiterate that this cycle of lattice formation and reformation can be repeated indefinitely without fade. This is the behavioral attribute of high-quality, U.S.  gel grade, attapulgite products that keeps coatings and other composites stable and allows consistent, optimum performance over their intended shelf-life.

Features and Performance

The key features in selecting an attapulgite product are gel quality and fineness of residue. Products suitable to coatings, adhesives, sealants and many other fine applications require a very low +325 mesh residue, typically less than 1%. This is not only critical for the end-use applications but also reflects the degree of separation of attapulgite’s naturally compressed particles by the manufacturing process. With greater separation, the attapulgite particles disperse easier and more completely permitting optimal performance.

Exceptional Performance and Economies

High-quality U.S. gel grade attapulgite products are the most economical low-shear rheology modifiers in their class and provide consistent performance over diverse liquid systems. Their dosage in most applications is similar to not-in-kind products, about 0.1 to 2.0% by weight, and yet their cost per pound is usually far less than half.

Most coatings require a mix of different rheology modifiers to provide a fuller rheological profile to meet the demands of in-can stability (Stormer viscosity) and paint application (ICI viscosity). Paint manufacturers have found that U.S. attapulgite products add key rheology benefits while wholly or partially replacing more expensive additives, such as associative thickeners (alkali-swellable acrylics), urethane-based rheology modifiers, and cellulosic thickeners.

Attapulgite products have distinct advantages over other commonly used thickening and suspending agents. They do not fade under repeated shear and are chemically inert and non-swelling. Additionally, attapulgite products are easier to use as they do not require a higher pH, special solvent or modifier to activate them. They are ideal for low-pH paints. In solvent-based systems, with less than 10% water, a surfactant is often needed.

Hydroxyethyl cellulose (HEC) is a popular co-thickener for emulsion coating systems. Key benefits of HEC include syneresis control, freeze-thaw stability and open time, but an overreliance on cellulosic thickeners can result in a ropey, snotty or “livering” appearance and poor workability. Unlike attapulgite, cellulose products are expensive, are subject to microbial attack and fade after repeated shear-thinning.  Specifically, Min-U-Gel® 400 attapulgite  is commercially successful in reducing HEC levels without reducing paint properties…except one, cost…and improves the paint’s overall appearance and workability. Compared to HEC, Min-U-Gel 400 builds a higher initial yield point and provides more consistent shear stress values when under shear (Figure 3).

Min-U-Gel 400 attapulgite is also useful as a co-thickener with associative thickeners. These rheology modifiers provide favorable leveling and gloss and maintain high viscosity under high shear for reduced spatter. However, they can cause problems with syneresis, pigment settling and floatation, color development and sag resistance.  Manufacturers often use Min-U-Gel 400 to overcome these deficiencies at a rate of 3 to 5 lb/100 gal (4 to 6 g/L) in semi-gloss paints and 3 to 7 lb/100 gal (4 to 8.5 g/L) in interior flat paints. Min-U-Gel 400 is also used as a co-thickening thixotrope with alkali-swellable and hydrophobically-modified HEC thickeners to remedy sag resistance, pigment settling and dripping problems. Typical addition levels is 3 to 5 lb/100 gal (4 to 6 g/L)., which adds stabilization to reduce syneresis, pigment settling, pigment settling and flotation while improving hide, color development and sag resistance.

Uniform Suspensions

High-quality, U.S. gel grade attapulgite products provide long-term stability to liquid systems. Their particles remain suspended indefinitely given their small size and interaction with themselves and other pigments. The gel formation can suspend relatively large or dense pigments or extenders. If any particles do settle they form a soft sediment that is easily re-suspended. The amount of attapulgite needed is reduced at higher PVC values or, in other applications, as solids rise. Mixtures of typical pigments and extenders may need less than 0.5% by weight for suspension or soft settle and those with large, denser particles may need up to 2.0%.

In-Can Stability

U.S. gel grade attapulgite products maintain the quality of the finished product and reduce pre-application mixing time. Their stable gel strength prevents the settling of solids and phase separation or syneresis during storage. The gels are unaffected by pH, temperature fluctuations, soluble ions, microbes or almost any additive. They also do not swell or increase aged viscosity.

Conclusions

Attapulgite serves as the undercarriage of many high solids mixtures, ensuring product quality throughout their life cycles as well as providing valuable thixotropic properties.  When well dispersed in a liquid, high-quality U.S. gel grade attapulgite products improve the stability of complex composites by entrapping liquids and particles when the mixture is at rest.

Maintaining this dispersed state is reflected in improved stability and components being able to work to a level of optimal performance. In coatings this is demonstrated by reduced syneresis, pigment flooding/float, and improved freeze-thaw properties. In addition, when the mixture is in motion its shear-thinning thixotropic thickening properties improve flow and leveling, workability (pumping, spraying, spreading, feathering, etc.). As attapulgite’s lattice structure quickly reforms it reduces spatter, sag, and overspray and allows higher film build. As the most economical thickeners in their class, high-quality U.S. gel grade attapulgite proudcts can reduce cost by partially or wholly replacing other thickeners. 

Min-U-Gel is a registered trademark of Active Minerals International LLC.

References

1. Haden, Jr., W.L.  Attapulgite: Properties and Uses, pp.284-290. Paper presented at The Tenth National Conference on Clays and Clay Materials, Austin, Texas, October 14 - 18, 1961.
2. International Agency for Research on Cancer (IARC) Monograph, Volume 68, pg 247, Table 2. Samples labelled NIOSH A and NIOSH B are from U.S. deposits of Floridin Company and Engelhard, now operated by Active Minerals International and BASF, respectively.
3. International Agency for Research on Cancer (IARC) Monograph, Volume 68, pg 262, section 5.5.

For nearly two decades, Jeff Carr has been a leader in sales, marketing and business management for the two largest global attapulgite producers. He has developed new products and applications worldwide as well as led a turnaround for one producer. He currently serves at Active Minerals International and can be reached at j.carr@activemninerals.com.