Jeffrey B. Carr, Director of Global Business Development, ActiveMinerals International, LLC
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 ben-
efits 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 develop-
ments 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
An Easy, Tried-and-True Method
to Ensure Quality in Coatings
Figure 1. Crude attapulgite mined by Active Minerals International LLC.
