Part 1 of this post explains how rheological studies can be greatly enhanced with a complimentary technology such as FTIR spectroscopy. A rheometer can only tell us what happens during the curing process; FTIR spectroscopy is able to identify and quantify chemical changes inside the sample.
In this post, we’ll describe the test method for monitoring the curing of the acrylate glue with rheology and simultaneous FTIR using a specially designed module that features a unique combination of a temperature control module and an Attenuated Total Reflection (ATR) cell with its own IR detector. Using this module, a rheometer can be combined with an FTIR spectrometer in one analytical setup to record the mechanical changes of the curing glue while at the same time and, even more importantly, on the same sample, IR spectra can be collected.
A consumer-grade two-component acrylate glue was prepared by mixing both components outside the rheometer according to its technical leaflet. Part of this mixture was transferred into the rheometer. When designing the test method, two important facts about curing materials were considered:
- The curing reaction starts outside the rheometer. To be able to compare different datasets, the test method contained an element to reset the internal time the moment the two components mix. Otherwise, any deviation in the loading procedure would lead to an undefined offset on the time axis.
- The biggest changes happen during the first moments of the curing process. The test method was optimized to start the test as quickly as possible after the sample was put onto the lower plate. The upper geometry was lowered to 10 mm before loading the sample to shorten the time to reach the measuring gap. The test itself started immediately after the measuring gap was reached without any time for thermal or mechanical equilibration.
The rheological part of the test method was an oscillation time curve, i.e. the oscillation parameters were kept constant to detect only changes in the sample due to the curing. Since drastic changes of the moduli were expected during the test, the rheometer’s CD-mode was used to ensure optimum signal quality throughout the whole test. A small amplitude from the uncured sample’s linear viscoelastic range (LVR) was selected, which yielded data with a good signal-to-noise-ratio from the uncured glue.
The rheological results can answer questions about the dosing and application properties of the liquid glue and the toughness of glue bond. The evaluation of the changing rheological properties gives the characteristic time spans like pot life, curing speed and the time to reach maximum strength of the bond.
With the information gained using this methodology, it is possible to understand why the curing process runs the way it does. Subsequently, a targeted approach to optimize a glue or to design a completely new formulation is now possible, knowing for example if it would be better to add more monomer or to increase the temperature to increase the mobility of the existing monomer.
Read Curing of an Acrylate Glue – Rheology with Simultaneous FTIR Spectroscopy for a more in-depth description of the test including the resulting figures and spectra.