An Easy, Tried-and-True Method to Ensure Quality in Coatings
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.
Figure 2. Attapulgite lattice network when dispersed in a liquid.
