fumed silica has long been used in adhesives and sealants to provide thixotropy
and reinforcement in structural adhesives and construction sealants.
AEROSIL is an amorphous silica produced through the
flame-hydrolysis method, which, through its structure of aggregated particles,
reduces flow properties. This results in higher viscosity, reduced flow and
leveling, and reduced sag. By replacing more traditional thickeners (such as
cellulosics, associate thickeners and clays) viscosity can still be maintained,
but with the added benefit of reinforcement. The same structure that controls
viscosity response also sets up an internal network of inorganic particles that
improve the bulk properties of the dried film.
The apparent adhesion of a material is its combined cohesive and adhesive
properties; each must be balanced to obtain the final desired properties. Poor
adhesive or cohesive properties may result in early failure of the adhesive.
AEROSIL fumed silica enhances the cohesive properties of the film, thus
building a more integrated bond line.
Evonik offers AEROSIL fumed silica as a powder or water-based liquid
dispersion. Powders are available in hydrophilic and hydrophobic grades.
AEROSIL hydrophobic fumed silicas are offered in a range of hydphobicity.
Also available are AERODISP®
These products offer the formulator the ability to skip the dispersion phase
associated with fumed silicas. Replacing AEROSIL powder with AERODISP
dispersion still results in the combined properties of reinforcement and
precipitated silica is created through a wet
method to produce more discrete particles of a highly porous nature. While the
higher moisture content may make them more difficult to use in certain systems,
they are easily incorporated into waterborne adhesives.
Evonik also offers Dynasylan®
which are used to improve adhesion and offered in a variety of functionalities.
AEROSIL Fumed Silica in Waterborne PSAs
Fumed silica can be added to adjust strength and tack - key
elements of adhesive performance. These factors can also increase viscosity;
the converter may need to adjust properties to ensure proper film thickness.
The Pressure Sensitive Tape Council has developed a number of test methods to
determine adhesion-in-peel, shear strength and tack. These basic tests were
used to compare the performance of AEROSIL 200 and AERODISP W7520. UCAR®
Latex 9037 from Dow Chemical Co. was used as the base latex. A ladder series of
1%, 2%, 3%, 4% and 5% dry fumed silica on polymer solids was tested and
compared to a control with no silica. AEROSIL 200 (red) was compared to
AERODISP W7520 (blue).
Shear adhesion was tested using a ½ by ½ by 1000 g configuration. The AERODISP®
W7520 shows an increase in shear times with increased loading levels. The use
of an equivalent powder did not show the same improvements with loading levels
(see Figure 1).
Adhesion-in-peel was tested on Type 304 stainless steel. The
results of both types showed a decrease in value with increased loading (see
Loop tack showed a decrease
with both forms, but a marked decrease using the powder version (see Figure 3).
Table. Styrene-Block-Copolymer-Based Formulation
Organofunctional Silanes in Hot-Melt Pressure-Sensitive Adhesives
Dynasylan silanes also have a long history of use in
structural adhesives and construction sealants. These are used primarily for
adhesion promotion, crosslinking and as coupling agents.
The mechanism for silanes involves a two-step process of hydrolysis followed by
condensation. The hydrolysis step produces an alcohol related to the
functionality of the silane. In the case of methoxy-functional silane, it is
methanol; ethoxy produced ethanol.
For this study, an amino-and-epoxy-functional silane was chosen with ethoxy
groups to avoid the lower flash point methanol, should hydrolysis occur too
quickly. Dynasylan AMEO and GLYEO were used. Each was added at 1 and 2 phr and
tested for shear, adhesion-in-peel, and loop tack, as above.
The formulation is a simple styrene-block-copolymer-based system with resin,
oil and antioxidant added (see Table 1).
180° shear showed a dramatic increase when Dynasylan AMEO
was used. Formulations with Dynasylan GLYEO were still higher than the control,
with double the time using 2 phr. Aminofunctional silanes have the ability to
autocatalyze the hydrolysis step, and may provide a higher crosslink density in
a polymer with unsaturated bonds (see Figure 4).
Adhesion-in-peel showed increases in all cases, with the
best at 1 phr Dynasylan GLYEO. Any such crosslinking for the Dynasylan AMEO did
not translate to improvements in this parameter (see Figure 5).
Loop tack is increased for
all systems with higher values for Dynasylan GLYEO. This more aggressive tack
may be seen as an advantage for permanent labels (see Figure 6).
was also tested at 350°F, as this may influence the coat weight of the
adhesive. The Dynasylan GLYEO demonstrated a considerable drop in viscosity
without a decrease to the above properties (see Figure 7).
Degussa Corp. offers solutions for pressure-sensitive adhesive formulators to
improve various standard properties. AEROSIL fumed silicas can help control
tack and peel adhesion, while Dynasylan silanes can increase the strength and
For more information, visit www.degussa.com