Curing an Acrylate with FTIR Spectroscopy

Those who have worked with glues know that timing is a crucial issue. As a result, leaflets for glues sometimes look like timetables. Various terms are used to describe the properties of glues and to guide successful applications; these include curing time or time to reach maximum bonding strength, pot life, time for minor adjustments, and open time.

These times have to fit the product application in order for newly developed glues to be acceptable to the target market. For example, depending on the method of glue application, the open time needs to be adjusted in order to avoid curing before the parts have been joined.

A rheometer is essential for the characterization of the uncured glue as well as critical for the curing process itself. The Thermo Scientific^TM HAAKE^TM MARS^TM Rheometer and its wide range of accessories are the ideal tools for characterizing curing behaviors, whether for a drying glue, a 2-component system, a UV curing glue or a thermally curing glue.

However, a rheometer can only report what happens during the curing process, it cannot identify why. It is important to understand why a batch of glue shows unexpected properties, but the “why” is also important when development of a glue for a new application is indicated. In order to overcome this limitation, rheological data must be combined with data from a different analytical method in order to detect changes on a molecular level.

This molecular data provides the necessary “why” information to the rheological measurements. FTIR spectroscopy is a perfect match. This is a technique that can identify and quantify different chemical groups in a substance or mixture of substances.

Spectrometers and rheometers are two different types of analysis and often have their own, separate instruments. The main disadvantage of running tests on two separate instruments is that it takes extra effort to prepare two different samples as they each require different preparation procedures. Consequently, it is virtually impossible to collect both sets of data on two identical samples under the exact same conditions.

The Thermo Scientific^TM Rheonaut module has been developed to combine rheological tests with FTIR spectroscopy without the disadvantages previously mentioned. The module is a unique combination of an attenuated total reflection (ATR) cell with its own IR detector and a temperature control module.

Figure 1. Setup with the HAAKE MARS Rheometer, Rheonaut module* and FTIR spectrometer.

Figure 1 shows how the Rheonaut module can be used to combine the HAAKE MARS Rheometer with an FTIR spectrometer into one analytical setup. Using this unique combination is the only way to simultaneously record the mechanical changes of the curing glue whilst collecting IR spectra on the same sample in order to track the chemical changes in the sample.  

Experimental

A consumer-grade-2-component acrylate glue was prepared according to its technical leaflet by mixing both components outside the rheometer. Some of this mixture was transferred into the rheometer.

Two important facts about curing materials have to be considered when designing the test method:

1. The curing reaction starts outside the rheometer. In order to compare different datasets, the test method contains an element to reset the internal time when the 2 components start to mix (Fig. 2, steps 3 and 4). Without this, deviation in the loading procedure would lead to an undefined offset on the time axis.
2. The largest changes happen during the initial moments of the curing process. The test method has been optimized so that the test is started as promptly as possible after the sample is put onto the lower plate. In order to shorten the time to reach the measuring gap, the upper geometry is lowered to 10 mm before loading the sample (Fig. 2, step 6). The test starts immediately after the measuring gap has been reached, with no time for mechanical or thermal equilibration.

Figure 2. Test method for 2-component glues shown in Thermo Scientific^TM HAAKE^TM RheoWin^TM Measuring and Evaluation Software. In steps 3 and 4 the time is reset when the 2 components mix outside the rheometer. In step 5 the upper geometry moves to a 10 mm gap to minimize lift travel after the sample is put onto the lower plate. Step 8 moves the upper geometry to the measuring gap, and step 10 starts the test without waiting for temperature equilibration.

Figure 3. Curing of an acrylate glue; development of the moduli G’ and G“, the complex viscosity |h*| and the phase angle δ over time.

Figure 4. Radical polymerization of Methylmethacrylate (MMA) to Polymethylmethacrylate (PMMA). Marked in blue: C=C-bond of the monomer. Marked in red: ester bond in the polymer.

The rheological part of the test method is an oscillation time curve, shown in Figure 2, step 10, where the oscillation parameters are kept constant so that the only changes in the sample detected are due to curing. Drastic changes of the moduli are expected during the test, so the controlled deformation (CD) mode of the rheometer is used to ensure optimum signal quality throughout the entire test. A small amplitude within the linear viscoelastic range (LVR) is selected, which still yields data with a good signal-to-noise-ratio from the uncured glue. The evaluation can be based on the loss modulus G” representing the viscous part of the viscoelastic properties and the storage modulus G’ representing the elastic part (Fig. 3).

The freshly prepared glue is mainly viscous with G” dominating over G’ with phase angle (δ) values approximately 70^o (purely elastic: δ = 0^o; purely viscous: δ = 90^o). The curing reaction happens rapidly and after 3.2 min the crossover point, where G” = G’ or δ = 45^o, is reached. After this so-called gel time, the glue mainly behaves as elastic due to the wide-meshed network that has developed throughout the sample. Otherwise, any movement in the glue is either impossible or would decrease the final bonding strength. After 10 min, δ is reduced to 3^o and G’ reaches an almost constant value as the glue reaches its final strength. Acrylate glues however, do continue to cure at a slow rate and their final strength is reached after 12 – 24 hours.

FTIR spectra have been collected simultaneously alongside the rheological data at about every 13 s. This gives a yield of 115 IR spectra during the 25 min duration of the rheological test. Several characteristic signals are seen and these can be correlated with the chemical reaction as it progresses (Fig. 4). For example, the signal at 1637 cm^-1 is characteristic of the C=C-bond of the acrylate monomer. The consummation of the monomer during the curing reaction is demonstrated as the signal decreases over time.

On the other hand, the signal at 1241 cm^-1 is, among others, characteristic of the O=C-O-C ester bond in the polymeric acrylate that is formed during the glue curing (Fig. 5).

Figure 5. First IR spectrum (blue) and last IR spectrum after 25 min (red) collected during the curing of an acrylate glue at 23 °C. The signal at 1637 cm^-1 decreases over time while the signal at 1241 cm^-1 increases.

The Thermo Scientific^TM OMNIC^TM Spectroscopy Software with the optional OMNIC Series add-on enables the user to line up the spectra in chronological order in a 3D-graph so that the characteristic signal changes can be evaluated at the time of the test (Fig. 6).

Figure 6. 3D profile illustrating the time-dependent change of the IR spectra collected during the curing of the sample in the rheometer, created with the OMNIC Series software add-on.

Absorbance profiles are created cutting through the data set along the characteristic wave numbers which show how the corresponding chemical groups in the sample change.

Combining the spectroscopic profiles with the rheological data demonstrates that the initial increase of the moduli corresponds with the reduction in the amount of monomer (Fig. 7).

Figure 7. Curing of the acrylate glue monitored with rheology and simultaneous FTIR. The increase of the sample’s moduli (red and blue) corresponds with the decreasing signal of the monomer (green) and the increasing signal of the polymer’s ester bond (black).

The decrease of the monomer slows down significantly when G’ reaches its plateau value after 10 min. This is due to the monomer in the solidifying glue having reduced mobility. The increase of the ester bond in the polymer also decreases, but it continues with twice the speed of the decrease of the monomer. This suggests that the intramolecular processes are more important for the final curing stage than the free monomer, although this dominated the initial part of the curing.

This information facilitates understanding of why the curing process runs the way it does. As a result, it is now possible to have a targeted approach to designing a completely new formulation or to optimizing a current glue. For example, it is known whether it would be better to increase the temperature to increase the mobility of the existing monomer or to add more monomer.

Summary

An oscillation time sweep is a well-established characterization method for the curing of glues and similar curing materials. It demonstrates the change from the liquid to the solid-state base in the glue’s mechanical properties. Questions about the dosing and application properties of the liquid glue and the toughness of the bond can be answered by rheological results. Evaluating the changing rheological properties provides characteristic time spans, such as the curing speed, time to reach maximum strength of the bond and the pot life.

The HAAKE MARS Rheometer can be combined with an FTIR spectrometer using the Rheonaut module to simultaneously record on the same sample what happens during the curing process and also why it happens on a molecular level. This reduces sample preparation and analysis time significantly. It also excludes uncertainties, such as different sample composition or sample treatment that could be a result when running the analyses separately.

This unique combination of spectroscopic and rheological methods increases the quality of data collection as well as the time and cost efficiency of an analysis such as the one described in this report.

Acknowledgments

Produced from materials originally authored by Klaus Oldörp from Thermo Fisher Scientific, Karlsruhe, Germany.

*Resultec developed the Rheonaut module for exclusive resale by Thermo Fisher Scientific.

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.