PSA Property Enhancer

September 1, 2008
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AEROSIL® 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® 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.

Adhesion-in-peel was tested on Type 304 stainless steel. The results of both types showed a decrease in value with increased loading (see Figure 2).

Figure 3.

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).

Figure 4.

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).

Figure 5.

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).

Figure 6.

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).

Figure 7.

Viscosity 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).

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.

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