Page 18 - 201902 SCA February 2019 Volume 56 Number 1

Technical ARTICLE An easy, tried-and-true method to ensure quality
in coatings
stability and open time, but an overreliance on Uniform suspensions
cellulosic thickeners can result in a ropey, snotty or High-quality, U.S. gel grade attapulgite products
“livering” appearance and poor workability. Unlike provide long-term stability to liquid systems. Their
attapulgite, cellulose products are expensive, are particles remain suspended indefinitely given their
subject to microbial attack and fade after repeated small size and interaction with themselves and other
shear-thinning. Specifically, Min-U-Gel® 400 pigments. The gel formation can suspend relatively
attapulgite is commercially successful in reducing large or dense pigments or extenders. If any
HEC levels without reducing paint properties… particles do settle, they form a soft sediment that
except one, cost… and improves the paint’s overall is easily re-suspended. The amount of attapulgite
appearance and workability. Compared to HEC, needed is reduced at higher PVC values or, in other
Min-U-Gel 400 builds a higher initial yield point and applications, as solids rise. Mixtures of typical
provides more consistent shear stress values when pigments and extenders may need less than 0.5%
under shear (Figure 3). 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
Figure 3: Comparison of shear stress and viscosity at various shear rates for motion its shear-thinning thixotropic thickening
Min-U-Gel 400 attapulgite and HEC in water. The loading levels of both materials
have been exaggerated to produce comparable rheological values. In coatings, properties improve flow and leveling, workability
where the attapulgite network interacts with particulate solids, the dosage level (pumping, spraying, spreading, feathering, etc.).
of Min-U-Gel 400 is similar to HEC. As attapulgite’s lattice structure quickly reforms
it reduces spatter, sag, and overspray and
Min-U-Gel 400 is also useful as a co-thickener with allows higher film build. As the most economical
associative thickeners. These rheology modifiers thickeners in their class, high-quality U.S. gel
provide favorable leveling and gloss and maintain grade attapulgite products can reduce cost by
high viscosity under high shear for reduced spatter. partially or wholly replacing other thickeners. n
However, they can cause problems with syneresis,
pigment settling and floatation, color development Min-U-Gel is a registered trademark of Active
and sag resistance. Manufacturers often use Min-U- Minerals International LLC.
Gel 400 attapulgite to overcome these deficiencies
at a rate of 3 to 5 lb/100 gal (4 to 6 g/L) in semi-gloss References
paints and 3 to 7 lb/100 gal (4 to 8.5 g/L) in interior flat 1. Haden, Jr., W.L. Attapulgite: Properties and Uses, pp.284-290.
paints. Min-U-Gel 400 is also used as a co-thickening Paper presented at The Tenth National Conference on Clays
thixotrope with alkali-swellable and hydrophobically- and Clay Materials, Austin, Texas, October 14 – 18, 1961.
modified HEC thickeners to remedy sag resistance, 2. International Agency for Research on Cancer (IARC)
pigment settling and dripping problems. Typical Monograph, Volume 68, pg 247, Table 2. Samples labelled
addition levels is 3 to 5 lb/100 gal (4 to 6 g/L)., which NIOSH A and NIOSH B are from U.S. deposits of Floridin
Company and Engelhard, now operated by Active Minerals
adds stabilization to reduce syneresis, pigment International and BASF, respectively.
settling, and flotation while improving hide, color 3. International Agency for Research on Cancer (IARC)
development and sag resistance. Monograph, Volume 68, pg 262, section 5.5.

16 Journal of Surface Coatings Australia • February 2019