Introduction
As a material that can quickly cross-link under specific wavelength of light, the curing process of light-curing resin directly determines the physicochemical properties of the final product. Traditionally, researchers have mostly used infrared spectroscopy or differential scanning calorimetry to monitor the degree of curing, but these methods are difficult to capture the dynamic evolution of the rheological properties of materials under light in real time and in situ. The integration of rheometer with UV light source provides a powerful technical means for in-situ study of the curing behavior of light-curing resins. The combined technology can simultaneously apply UV irradiation under controlled shear or oscillation conditions, and monitor the changes in rheological parameters such as modulus and viscosity of the resin from the liquid precursor to the solid-state network in real time, so as to accurately analyze the curing kinetics, gel spots, and final network structure formation process.
Technical principle
At the heart of the rheometer-UV combined system is the integration of the UV light source module with the measuring geometry head of the rheometer, such as a parallel plate. The light source typically employs a light-emitting diode or mercury lamp and directs UV light at a specific wavelength (such as 365 nm or 405 nm) evenly to the sample measurement area through a fiber optic or lens system. The system needs to ensure that the light intensity is evenly distributed and precisely controlled (often in mW/cm²) across the sample area, which is usually achieved with a calibrated light intensity meter.
During the measurement, the rheometer applies a small oscillatory strain (or stress) to the sample and measures its response, resulting in the calculation of the complex modulus (G*), energy storage modulus (G'), and loss modulus (G''). Its basic relationship is:
G* = G' + iG''
where i is an imaginary unit. When the UV light is turned on, the photoinitiator in the resin absorbs photons to produce free radicals or cations, triggering a chain polymerization reaction of monomers or oligomers, increasing the molecular weight and forming a three-dimensional network. This process is directly reflected in the change in rheological parameters: G' and G'' rise with curing time, and at the gel point, G' intersects with G (tanδ = G'/G' = 1), marking the beginning of the material's transition from a viscous liquid to an elastic solid.
Analysis methodology
This combination technique provides a range of key parameters to characterize curing behavior:
| Gel time | The time corresponding to the intersection of the energy storage modulus and the loss modulus |
| Curing rate | The slope of the energy storage modulus over time |
| final modulus | The platform modulus value reached after the curing reaction is basically completed |
| Light intensity threshold | The minimum light intensity required to initiate measurable gelation |
In addition, the kinetic parameters such as apparent activation energy of the curing reaction can be further obtained by fitting the modulus growth curve based on the principle of time-temperature-conversion rate superposition or based on kinetic models such as the Kamal model. These analyses are instructive for optimizing light conditions (light intensity, wavelength, exposure time) and resin formulation (photoinitiator concentration, monomer type).
Application examples
This method is widely used in the research of coatings, inks, 3D printing photosensitive resins, electronic encapsulants and other fields. For example, when studying acrylate resins for stereolithography, the effects on gel time and final network modulus can be systematically studied by varying the UV light intensity. Typically, increasing the light intensity significantly reduces the gel time and may increase the final modulus, but too high a light intensity can lead to local overheating or uneven reactions.
Key factors that influence measurement results include:
| Light intensity uniformity | Ensure consistent lighting throughout the sample being tested |
| Temperature control | Polymerization reaction exothermic requires precise temperature control to distinguish thermal and light effects |
| Oxygen inhibition | Surface curing may be blocked by oxygen in the air, and inert atmosphere protection needs to be considered |
| Sample thickness | Affects the penetration depth of UV light and the overall curing uniformity |
Therefore, the experimental design needs to take these factors into account and standardize the measurement geometry and sample preparation.
Conclusion
The combination of rheometer and UV light source provides a dynamic, in-situ, and informative characterization platform for in-depth understanding of the curing behavior of light-curing resins. It can directly correlate the changes in macroscopic mechanical properties with microscopic polymerization reactions, making up for the shortcomings of traditional chemical analysis methods. In the future, with the further development of light source technology (e.g., multi-wavelength switching, patterned irradiation) and rheological measurement technology (higher temporal resolution), this combined technology is expected to play a greater role in more complex curing scenarios (e.g., gradient curing, secondary curing) and the development of a wider range of advanced photoresist materials.
References
1. A review of the rheological characterization of photopolymerization in the journal Polymer Science and Engineering.
2. Rheological study of UV curing kinetics, ASTM related standard guidelines.
3. Application of Oscillatory Rheology in Crosslinking Systems, Polymer Materials Testing Handbook.