Investigating UV-Induced Curing Reactions

The use of UV curing materials for the application or processing of inks, paints, coatings, adhesives, etc. is critically important in many industrial segments. This technology has a combination of economic and environmental advantages as well as improved product capabilities.

Since the build-up of a network in the sample is directly related to the change of the viscoelastic properties (G’, G”, etc.) of that sample, a curing process can be followed by dynamic rheological measurements.

The combination of oscillatory rheological measurements and a second analytical technique results in an even greater insight into the possible characteristics of curing processes. An example of such a supplementary tool is FTIR spectroscopy.

In general, a rheometer can analyze the time-dependent change of the viscoelastic properties of a material for a curing process or phase transition. However, the viscoelastic properties of a material depend on its structure and, most importantly, the structural changes it undergoes during a curing process. Infrared spectroscopy is a particularly useful tool for identifying structural changes on a molecular level.

This article will discuss the technical details of a new UV-curing setup for the Thermo Scientific^TM HAAKE^TM MARS^TM Rheometer. It will also demonstrate the experimental results of the investigation of a UV-curing process for an acrylate-based coating for optical fibers, presenting both spectroscopic and rheological data.

Measurement and Setup

Figure 1 shows how, with the patented* Thermo Scientific^TM Rheonaut module, a standard FTIR spectrometer with side port and the HAAKE MARS Rheometer are combined to create one measuring unit.

Figure 1. Left: HAAKE MARS rheometer with Rheonaut module and FTR spectrometer; Right: HAAKE MARS rheometer configuration for UV curing measurements incl. temperature control.

A new fixture for the HAAKE MARS Rheometer platform has been developed for investigating UV curing materials with the Rheonaut module. This module is comprised of an upper shaft with an integrated mirror and an exchangeable quartz glass plate (Fig. 2), and also a holder for a collimator plus light guide that is mounted to the measuring head (Fig. 2).

The UV light beam of a light source that is commercially available, initially bundled by the collimator and then reflected by the mirror, is directed from above into the sample through the quartz glass plate (Fig. 2).

Figure 2. Left: measuring geometry and adapter for mounting and adjusting the collimator and light guide; middle: upper shaft with integrated mirror; Right: holder for collimator plus closed hood.

The quartz glass plate is the upper plate of a plate/plate measuring geometry, but the lower plate is either the Rheonaut module for simultaneous FTIR spectroscopy and measurements of rheological properties or a standard temperature control module.

For measurements above ambient conditions, use of the optional sample hood is recommended. The hood can be used for temperatures up to 240^oC and is made of Teflon^TM. This new setup means that the user can expose a material to UV radiation, and, at the same time, collect spectroscopic and rheological data.

Materials

Typically, the single glass fibers in an optical fiber cable are coated with a polymetric material to protect them from physical damage and moisture. Usually, the coatings are UV-cured urethan acrylate composite materials applied during the drawing process to the outside of the fiber. In current practices, a dual layer coating system is used. Layers are applied at speeds of up to 1000 m/min. A comprehensive understanding of the curing process is essential to optimize the final coating behavior and reduce energy consumption during production. The following paragraph describes the spectroscopic and rheological investigation of the curing of an acrylate-based coating formulation.

Figure 3. Uncoated optical fibers.

Results

The following measurements were performed using the new UV curing cell in the HAAKE MARS Rheometer at 25^oC, and the Rheonaut module was used to collect the spectroscopic data. Figure 4 demonstrates the rheological data of the measurement of a UV-curing method for an acrylate-based glass fiber coating. The set strain value was 1%. After initially triggering the UV light source at 30 s, G’ and G” increased over several orders of magnitude.

Figure 4. Oscillation time experiment (CD-Mode) of a UV curing method for an acrylate based glass fiber coating (f=5 Hz, plates 20 mm, gap 100 ìm).

The sample was exposed to UV light every two seconds for a one second period during the curing process. After 100 s, G’ reaches its maximum and then remains constant for the remainder of the experiment. After the initial increase, G” runs through a maximum and then decreases by almost one order of magnitude, before reaching a plateau at 240 s.

This behavior indicates a two-step curing process. The second step influences the brittleness of the material but not the evolution of G’ or the overall stiffness of the material. The curing reaction is completed after 240 s and there are no other observable rheological parameter changes.

During the experiment, IR spectra were collected in addition to the rheological data. Figure 5 shows the results. The high spectrum acquisition rate is one of the advantages of FTIR spectroscopy. The addition of the Fast Oscillation function of the Thermo Scientific^TM HAAKE^TM RheoWin^TM Measuring and Evaluation Software means that it is possible to monitor even very rapid structural changes in a spectroscopic and rheological way.

Three representative spectra, taken before the uncured sample was exposed to light during the steep increase of the moduli and after G’ and G” reached their plateau values, can be seen in Figure 5. Some characteristic peaks are highlighted: decreasing peaks are observed at 1719 cm^-1 and at 1179 cm^-1.

Figure 5. IR spectroscopic data of acrylate based coating formulation before, during and after being exposed to UV light.

These are characteristic wavelength numbers for the stretching and vibration of carbonyl groups. As a result, it can be concluded that this functional group is actively involved in the curing process and that the amount of free carbonyl groups decreases over time. Another standout peak in the presented spectra is at 808 cm^-1. At this value, =C-H groups transform absorbed energy into bending vibration.

The examples discussed show how the structural changes within a curing sample can be monitored and evaluated on a molecular level. As well as rheological information from the oscillatory experiment, the combined measuring technique offers a comprehensive insight into complex processes. Therefore, it is an ideal tool for optimizing industrial curing processes in terms of energy efficiency and sample performance.

Acknowledgments

Produced from materials originally authored by Jan Philip Plog from Thermo Fisher Scientific, Karlsruhe, Germany.

*Resultec Analytic Equipment: DE 10140711, EP 02762251, US 6988393, JP 4028484

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.