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Colloidal Silica Additives For Pressure-Sensitive Adhesives

Grace.png Pressure-sensitive adhesives (PSAs) play a significant role in many materials used in everyday commercial and household settings. They line the tapes and labels that seal or identify packaged goods. They are also applied to films that protect digital display screens and help sticky notes adhere to walls and computer monitors.



Since there are many different applications for pressure-sensitive adhesives, certain properties must be optimized to perform for each specific application. Particularly, tack, peel adhesion, and shear strength must be adjusted per adhesive formulation to ensure each attribute functions as intended for its end use. These three main performance properties are defined as follows:

 

  • Tack: The speed at which an adhesive bonds two substrates, where higher-tack adhesives bond surfaces together more quickly and permit less time for repositioning.
  • Peel adhesion: The measure of required force to peel an adhesive away from a surface, which breaks its bonds and allows separation.
  • Shear strength: An adhesive’s ability to resist and withstand lateral forces applied in the same plane.

While many additives exist for PSA formulators to control the properties above, few are as simple, safe to use, and cost-effective as LUDOX® colloidal silica.

What Is LUDOX® Colloidal Silica?


LUDOX® colloidal silica consists of monodisperse, amorphous silica particles sold as an aqueous dispersion stabilized by counterions. The silica particles of LUDOX® are discretely spherical in shape, range in diameter between 5 and 40 nanometers, and feature surface areas between 75 and 435 m²/g that contain numerous hydroxyl groups. The combination of high surface area and hydroxyl groups makes LUDOX® highly reactive and a useful additive in pressure-sensitive adhesive applications. The images below illustrate the LUDOX® colloidal silica dispersion and one of its individual particles.
 

LUDOX® colloidal silica dispersion featuring monomodal, uniformly sized particles

LUDOX® silica particle displaying hydroxyl functional group content

How Does LUDOX® Colloidal Silica Influence Pressure-Sensitive Adhesive Properties?


When LUDOX® is added to a polymer latex in a PSA, the silica particles orient themselves on the surface of latex polymer spheres and form ionic or hydrogen bonds. As coalescence occurs during the drying of the adhesive, LUDOX® becomes an integral part of the polymer matrix. This mechanism of action is depicted below.

Shear Strength

When LUDOX® is introduced into the PSA’s polymer matrix, shear and cohesive strength are enhanced. This is because LUDOX® serves as a cross-linker between the polymers, increasing the adhesive film’s strength and resistance to both lateral and vertical stress. When an adhesive is subjected to these types of stress, it also helps to distribute the stress more evenly along the entire film. As a result, LUDOX® makes pressure-sensitive adhesives more durable.

Peel Adhesion

As seen in the graphic above, some LUDOX® particles will integrate themselves between the polymer film and the substrate it is applied to, leading to increased adhesion to the substrate upon curing. However, there will also be particles that cure on the opposite, exposed face of the adhesive that is meant to seal to a second material. These particles do not contribute to additional bond formation. Thus, the total surface area in which the adhesive can adhere to the secondary material is reduced. In general, if LUDOX® is added in high enough concentrations in certain PSAs, it can decrease peel adhesion for certain applications.

Tack

Noting the above, LUDOX® will decrease tack in PSAs. As the concentration of LUDOX® used in the pressure-sensitive adhesive formulation increases, there will also be less surface area available for the pressure-sensitive adhesive polymer to tack to the material that needs to be joined.

Use Cases for LUDOX® in Pressure-Sensitive Adhesives


The types of PSAs that will benefit the most from incorporating LUDOX® as an additive are those designed to be peeled and reused. Examples of materials that benefit from such adhesives include sticky notes, posters, certain tapes and labels, protective films, and more.

Since LUDOX® reduces tack and can reduce peel adhesion, substrate failure risk will be minimized, allowing the pressure-sensitive adhesive product to be pulled away intact. In addition, the increased shear and cohesive strength that LUDOX® provides ensure that the adhesive separates cleanly from the second material it is applied to, minimizing cohesive failure risk and separation due to shear stress.

LUDOX® Colloidal Silica Grades


LUDOX® colloidal silica is available in several grades varying in particle size, pH, and stabilizing counterions. The right grade for each application will depend on the following:

 

  • Requirement for surface area and desired particle size in the system
  • pH of the adhesive system
  • Compatibility of the adhesive system with the specific counterion present

A few LUDOX® grades are detailed below, along with information that can help with selection.
 
Grade Particle Size (nm) Silica Content (%) Stabilizing Counterion pH (25°C) Silica Surface Area (m²/g)
SM 7 30 Sodium 9.7–10.3 320–400
HS-40 12 40 Sodium 9.2–9.9 198–258
HS-30 12 30 Sodium 9.5–10.2 198–258
AS-30 12 30 Ammonium 9.1–9.7 198–258
AM 12 30 Sodium 8.6–9.3 198–258
TM-40 22 40 Sodium 8.5–9.5 110–150
CL-P 22 40 Chloride 3.5–4.5 110–150

How to Use LUDOX® Colloidal Silica in Adhesive Systems


The additive benefits of LUDOX® described previously are only achieved when the hydroxyl groups on the surface of LUDOX® begin to react with the PSA's polymer matrix in a sol–gel reaction. This reaction can be initiated by removing water, changing pH, or adding a salt or water-miscible organic solvent.
Generally, grades like LUDOX® SM, HS-40, HS-30, AS-30, and TM-40 are stable in polymer emulsions at pH 8 or above. If the pH of the system drops below pH 8, LUDOX® will begin to gel. For adhesive systems with lower pH, grades like LUDOX® AM are useful as they will be stable from pH 4 to 10. In general, silica must be compatible with the adhesive’s polymer latex, and this latex must be fluid and should not increase in viscosity during aging.

Summary


LUDOX® colloidal silica is a safe, cost-effective additive used in latex-based pressure-sensitive adhesive systems to achieve increased shear and cohesive strength, reduce or maintain peel adhesion, and decrease tack. It increases performance for pressure-sensitive adhesive formulations designed to be repositionable and resealable, such as those used in sticky notes, posters, temporary films applied to glass, and some tapes and labels. In general, LUDOX® is ideal for pressure-sensitive adhesives that require adhesive failure before other types of bond failure, such as cohesive or substrate failure. Click below for more information or to request a sample of LUDOX® colloidal silica today.

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