Photopolymers are normally made up of oligomers, monomers and photo-initiators that cross-link to create a network structure on exposure to light, normally in the visible or the ultraviolet region of the electromagnetic spectrum (Figure 1).
Figure 1. Schematic view of cross-linking mechanism of photopolymers when exposed to UV light.
Photocuring is a considerably quick process in comparison to thermal curing, and hence, the process can be deployed for selective curing with high-energy light sources, making the process ideal for printing circuit boards and fabricating microchips.
Photopolymers are extensively used in 3D-printing, medical, adhesive and photo-resist technologies. Rheological measurements are normally used for characterization of the progression of viscoelastic properties of photopolymers at the time of photo-curing. By determining the variation in complex modulus (G*), the cross-linking rate can be estimated.
Furthermore, photopolymers show a considerable post-cure shrinkage based on the monomer concentration. The rheometer has a normal force control feature that enables the measurement of vertical shrinkage, at the time of curing, from the gap change, when subjected to a constant applied force. This can also be used for determining the post-cure shrinkage percentage.
Photopolymer’s cross-linking kinetics has a high dependency on UV light intensity and exposure time length. It is also important to note that the light beam intensity decreases with the distance from the irradiative surface.
The cross-linking rate and the post-cure shrinkage rate of two distinct UV-curable adhesives were studied and compared under suggested process conditions. The Kinexus rotational rheometer was used with a UV accessory fitted to the cylinder cartridge to make measurements. Liquid adhesive was dispensed to a glass plate, through which irradiation of UV light was performed (Figure 2). For rheology measurements, a disposable parallel plate measuring system was used.
Figure 2. Cut-away view of UV cell with solvent trap cover.
Under a control strain of 0.1% at 1 Hz, a single frequency oscillatory measurement was performed on the sample with a thickness of 0.65 mm. To make sure that both samples were subjected to a reliable and a controllable loading protocol, a standard loading sequence was used. OmniCure® Series 2000 UV/Visible Spot Curing Unit was deployed, with an 8 mm diameter OmniCure® liquid light guide for sample illumination with UV light.
The light source used had a wavelength range of 320 - 500 nm. The curing unit was deployed in calibration mode, and an OmniCure R2000 radiometer was used for UV output density calibration. Configuration of the rSpace software was carried out to enable it to communicate with the OmniCure S2000 Curing Unit via RS232 connection. Control of the output intensity was possible by performing a standard pre-configured sequence in the software. For curing tests, a UV intensity of 0.5 W/cm2 was used.
The rheological measurements were all carried out at 25 °C, and the distance between the glass plate and the light guide end was kept constant. By controlling a constant 0 N normal force on the sample, free movement was possible along its verticle axis sue to the sample shrinking as the cross-linking progressed. The rSpace software was used to control the OmniCure® S2000 in order to record the rheological parameters of interest, together with UV intensity profiles.
Results and Discussion
A qualitative comparison of cross-linking kinetics of two distinct UV adhesives deployed in optical applications is shown in Figure 3. There is a rapid increase in the complex shear modulus (G*) after opening the UV shutter because of the UV reaction speed.
Figure 3. Comparison of rate of cross-linking and extent of cure of two typical UV adhesives.
The rate of cross-linking is quite different, even though the complex modulus of pre-cured adhesives is similar. When compared to Adhesive A, Adhesive B shows a lower modulus in the plateau region, revealing that the cross-linking density and related stiffness obtainable at the end of cure, with a specified irradiation level of 0.5 W/cm2, are less than that of Adhesive A.
For several UV adhesives, shrinkage caused by cross-linking is a major parameter in deciding the performance acceptance for end-use applications. Shrinkage data determined for Adhesive A under a constant normal force is shown in Figure 4.
Figure 4. Shrinkage data for a typical UV adhesive used in optical applications.
rSpace software has been developed for handling this shrinkage by enabling the user to select the gap in auto-tension mode at a predefined normal force. For loading of the sample, gap setting mode was used, but at the time of the curing test, a zero normal force was applied to allow the plate to be moved freely during contraction of the sample. Figure 4 results show that adhesive A exhibits 8% shrinkage towards the end of cure.
The article has discussed in-situ characterization of rheological properties of UV curable materials using a Kinexus rotational rheometer with UV curing accessory. The cross-linking rate and post-cure shrinkage rate can be determined from these measurements.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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