UV curing is commonly used for printing inks, adhesives, and coatings in all industries to reduce manufacturing times and remove the need for hazardous solvents. The curing start time can be accurately defined, and the reaction does not lead to major temperature or volume changes. This makes it suitable for many substrates such as wood, metal, glass, and polymers.
Rotational rheology combined with a UV light curing system
General aspects
Radiation curing is a process where high energy radiation (UV and visible light between 100 nm and 500 nm) is used to transform a liquid polymer solution to a solid polymer in order to coat or bond a product. The main ingredients of such polymer solutions are monomers, oligomers, photoinitiators and additives. When these liquids are exposed to UV light, the photoinitiators start to release free radicals, which trigger a polymerization and lead to a cross-linked polymer network (see figure 1).
The curing reaction only takes from seconds to a few minutes and therefore leads to high manufacturing throughput. The exact curing time can be determined by combining a UV light curing system with a rotational rheometer. The rheometer displays the mechanical transition of the curing process and allows determination of the exact point of solidification.
Figure 1: Schematic visualization of a UV curing reaction
Applications and benefits
Measurement examples
UV printing ink – influence of UV light intensity
The following figure shows the influence of UV light intensity on the curing speed of a UV printing ink. It can be seen that with increasing intensity the curing speed (G’ ~ sample stiffness) also increases. At 1 % intensity, it takes 140 seconds for the sample to cure, at 50 %, on the other hand, it takes only 10 seconds. This informs the ink manufacturer about the processing times of the product and allows for evaluation of the cost-benefit ratio of higher light intensities.
Figure 2 The curing time strongly depends on the intensity of the UV light. The higher the intensity (here values set up to 50 % of the maximum intensity of the UV source used), the faster the curing.
Typical test procedures
- Time tests in oscillation with permanent exposure
- Time tests in oscillation with partial exposure
- High sampling rate for fast curing processes
Measurement equipment
Typical rheo-UV systems consist of a temperature-controlled glass plate and a UV light source with an attached light guide. The light guide is usually fixated below the plate so that the sample can be radiated from below. Either high pressure mercury bulbs or LED lamps are used as light sources.
To ensure homogeneous illumination, measuring systems with diameters up to 20 mm are used. Depending on the application, the sample is illuminated either for a few seconds (partial illumination) or till the end of the measurement (permanent illumination).
More information about this setup can be found here.
Figure 3 Schematic sketch of the rheo-UV setup
Conclusions
Rheometers can be combined with UV light sources to directly measure the phase transition of UV-initiated curing reactions. As shown above, the curing process is strongly influenced not only by UV light intensity but also by humidity conditions.
The curing process is considered complete once no further change can be rheologically monitored, i.e. when the storage and loss moduli reach a plateau. Nevertheless, for some samples, some chemical reactions still occur. Besides the mechanical feedback from the rheometer, additional microscopic information can be gained by combining additional optical methods such as Raman or IR.