UV-cured gaskets and formed-in-place seals that shrink significantly after cure create sealing problems — the gasket pulls away from sealing surfaces, compressive preload is lost, and the assembly leaks. Understanding why UV gasket materials shrink and how to minimize it allows engineers to select the right material and process for reliable sealing.
Why UV Polymerization Causes Shrinkage
All UV-curable materials shrink to some degree during polymerization. Shrinkage occurs because the distance between monomer molecules in the liquid adhesive is greater (determined by van der Waals forces between individual molecules) than the bond length in the polymer chain connecting those same molecules. When monomers polymerize into a chain, the effective volume occupied per monomer unit decreases — the material contracts.
The magnitude of shrinkage depends on the monomer molecular weight and functionality: low-molecular-weight, high-functionality monomers (such as small acrylate monomers with multiple reactive groups) shrink more during cure than high-molecular-weight oligomers with fewer functional groups per unit volume. A typical UV acrylate adhesive or sealant undergoes 2–8% volumetric shrinkage during cure.
For a formed-in-place UV gasket, this shrinkage manifests as dimensional change in the cured gasket — the gasket bead becomes slightly smaller in cross-section after cure than it was when dispensed and before UV exposure. If the gasket is intended to compress between two mating surfaces and provide a seal through the compressive recovery force, this size reduction reduces the available compressive contact force.
Material Shrinkage Varies Significantly by Formulation
Not all UV gasket materials shrink equally. The degree of shrinkage is a function of the formulation:
High-shrinkage materials: Low-molecular-weight monomer-rich formulations, UV-curable epoxy acrylates, and thin UV adhesives with high functional group density shrink 5–10% volumetrically. These materials are appropriate for rigid bond joints where dimensional change is accommodated, but problematic for gasket applications where sealing geometry is critical.
Low-shrinkage materials: High-molecular-weight urethane acrylate oligomers, silicone UV-curable formulations, and UV-curable materials with ring-opening cure mechanisms (cationic UV systems using epoxide or vinyl ether chemistry) shrink much less — 1–3% volumetrically. Low-shrinkage formulations are the appropriate choice for UV gasket applications.
Cationic UV cure. UV-curable materials that use cationic ring-opening polymerization (epoxide opening, oxetane opening) can exhibit near-zero or even positive volumetric change (slight expansion) during cure, because ring-opening chemistry does not have the same volumetric contraction as chain-addition polymerization. For sealing applications where dimensional stability after cure is critical, cationic UV gasket materials may offer better performance than acrylate alternatives.
If you need help selecting a low-shrinkage UV gasket material for your sealing application, Email Us and an Incure applications engineer can review the requirements and recommend appropriate formulations.
Overcure Increases Shrinkage
Additional UV dose beyond the minimum for complete cure drives additional crosslinking reactions. Each additional crosslink pulls polymer chains slightly closer together, incrementally increasing the total volumetric shrinkage beyond what occurs at the minimum cure dose. For gasket applications with tight dimensional tolerance requirements, operating at the minimum dose for full cure (not substantially above it) minimizes cure-related shrinkage.
Establish the minimum dose that achieves full mechanical properties and seal integrity, and use that as the production dose target rather than operating at maximum lamp output for maximum cure speed.
Post-Cure Relaxation
Some UV-cured materials undergo additional dimensional change after the UV exposure is complete — continuing to shrink slowly at room temperature as residual polymerization reactions proceed (particularly if the material was not fully converted to maximum degree during UV exposure), or as internal stresses relax.
If significant dimensional change is occurring in the hours or days after UV cure, post-cure relaxation is the cause. This can be accelerated by a brief thermal post-cure (60–80°C for 30–60 minutes), which drives the remaining conversion and allows stress relaxation to occur before the assembly is sealed and the gasket is placed under load.
Constraint During Cure Changes Apparent Shrinkage
A gasket cured under constraint — with both mating surfaces clamped against it during UV exposure — will shrink differently than a gasket cured in free space. Constrained cure forces the shrinkage to manifest as internal stress in the gasket material rather than as dimensional change, because the substrates resist the contraction.
When the clamp is released, the internal stress causes the gasket to relax and its dimensions to change. In some cases, constrained cure during UV exposure followed by pressure application results in better sealing performance than free cure because the gasket dimensions at room temperature are closer to the as-assembled geometry.
For UV gasket applications, evaluate whether curing with the mating surfaces partially or fully engaged (producing constrained cure) improves dimensional consistency and seal integrity compared to free-air cure of the dispensed bead before assembly.
Thermal Expansion and Contraction in Service
Even a gasket with minimal cure shrinkage may change dimensions in service due to thermal expansion and contraction. UV-cured silicone gaskets have relatively high CTE compared to metal hardware; urethane acrylate gaskets have intermediate CTE. If the gasket CTE is very different from the substrate CTE, thermal cycling causes relative dimensional change at the sealing surfaces.
Evaluate whether service temperature range is large enough to make CTE mismatch a problem. Select gasket materials whose CTE is reasonably matched to the substrate, or ensure the sealing geometry provides enough compression that the gasket maintains positive contact through the full thermal range.
Measuring Gasket Shrinkage
To characterize gasket shrinkage from a specific material and cure process:
- Dispense a free-standing bead of the gasket material onto a release-coated substrate
- Measure bead cross-section dimensions (width, height) before cure
- Cure with the production UV process
- Measure bead dimensions after cure
- Calculate volumetric shrinkage from the dimensional change
Compare this measured shrinkage to the supplier’s specification for the material. If measured shrinkage exceeds the specification, investigate whether overcure or process conditions are contributing.
Contact Our Team to discuss UV gasket material selection and cure process optimization for your sealing application.
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