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®fumed-silica dispersions. 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 viscosity.
SIPERNAT®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®organofunctional silanes, which are used to improve adhesion and offered in a variety of functionalities.
Figure 1.
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).
Figure 2.
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).
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Conclusion
Evonik 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 lower viscosity.For more information, visit www.degussa.com.
