What are high modulus and low modulus in sealants

What are high modulus and low modulus in sealants

In the sealant industry, “modulus” refers to the elastic modulus, specifically the tensile modulus or secant modulus. It measures a sealant’s ability to resist deformation under tensile stress, or simply put, its degree of “softness” or “rigidity.”

High-modulus sealant:

Properties: High rigidity, high hardness, and high tensile and shear strength.

Stress Performance: It tends to resist deformation when the joint undergoes movement (such as thermal expansion and contraction, or movement caused by wind loads). This means it requires greater force to stretch or compress. Higher internal stresses can be generated with smaller displacements.

Advantages:

Excellent bond strength and structural support.

Typically offers improved tear and abrasion resistance.

Performs better in areas subject to heavy static loads or requiring structural support (for example, in some structural sealing assembly applications).

Disadvantages:

Poor adaptability to dynamic movement. If significant joint movement is expected (typically exceeding ±12.5% of the joint width), the internal stresses generated by high-modulus sealants may exceed their inherent adhesive or cohesive strength, leading to bond failure (debonding) or tearing of the sealant itself.

High stresses applied to delicate substrates (such as certain natural stone, old concrete, insulation boards, etc.) may cause the substrate edges to break (“peeling effect”).

Typical Applications: Where high bond strength is required and joint movement is relatively small, such as:

Securing non-load-bearing building components (such as certain precast concrete panels and metal panels).

Structural glazing (must comply with strict code requirements).

Industrial adhesive sealants subject to small movement.

Low-modulus sealants:

Properties: Highly flexible, low hardness, relatively softer and more elastic.

Stress Behavior: When the joint moves, it is more susceptible to tension or compression. For the same amount of movement, it generates much less internal stress than high-modulus sealants.

Advantages:

Excellent movement resistance. Able to accommodate large joint movement (typically up to ±25% of the joint width, or even ±50% or more), absorbing deformation stress.

Implementing less stress on the substrate edge makes it more suitable for use on fragile or delicate substrates.

Performs more reliably in dynamic joints and is less susceptible to failure due to stress accumulation.

Disadvantages:

Adhesion and shear strength are typically lower than those of high-modulus sealants (although sufficient for most non-structural sealing requirements).

Abrasion and tear resistance may be relatively low (but still meet standard requirements).

Typical Applications: Where large movement is expected or the substrate is delicate, such as:

Building expansion joints (e.g., between exterior wall panels, between concrete slabs).

Joints between door and window frames and walls.

Joints between curtain wall panels (especially those subject to large thermal movement).

Roof joints and pipe penetrations.

Sealing fragile substrates such as natural stone, insulation materials (EPS/XPS), and gypsum board.

Key Selection Considerations:

Expected Joint Movement: The greater the movement, the more desirable a low-modulus sealant.

Substrate Type: Low-modulus sealants are preferred for fragile substrates (such as stone, old concrete, and insulation boards).

Bond Strength Requirement: When high structural strength or high loads are required, a high modulus is essential (but sufficient movement resistance must also be evaluated).

Sealant Type: Silicone sealants are typically available in a range of modulus grades, from low to high. Polyurethane sealants also come in different modulus grades, while MS (modified silane) polyether sealants are generally low-modulus. Consult the specific product data sheet when making a selection.

Standards and Specifications: Certain applications, such as structural sealing assemblies, have strict modulus requirements and testing standards.