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.
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.
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.
|Particle Size (nm)
|Silica Content (%)
|Silica Surface Area (m²/g)
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.
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.