Curing Reactions of Polyurethane Resins

Detailed analysis of curing reactions of polyurethane resins using technology for simultaneous rheometry and FTIR

In our last article we suggested that for a comprehensive understanding of a material, usually more than just one analytical technique is required. We advised that simultaneous rheometry and Fourier Transform Infrared (FTIR) is achievable and is the preferred method for obtaining a detailed analysis of curing reactions of polyurethane resins. Here is an outline of the experiment we conducted utilizing the combination technology.

Instrumentation

As mentioned in our previous article, we used instrumentation that included a standard FTIR spectrometer with side port and a rheometer coupled to form one measuring unit. The stationary plate of the rheometer featured a monolithic diamond element that served as the ATR (attenuated total reflection) sensor, offering a single internal reflection. A motor-driven horizontal movement of the lower plate enabled the ATR sensor to be positioned at different distances between the center and a maximum of 45 mm. Instrument software enabled temperature control as well as the horizontal positioning of the lower plate and the communication with the spectrometer software for simultaneous acquisition of the rheological data and spectra.

Materials and Sample Preparation

The investigated PU resin was composed of two reactants. One reactant contained a Diisocyanate prepolymer, the second reactant contained two different ester compounds each endcapped with hydroxyl groups. The PU resin formulation was prepared by mixing the starting materials with equal stoichiometric ratio between isocyanate and hydroxyl groups ([NCO]/[OH] = 1). After 5 minutes of mixing and homogenizing, the reactive sample was transferred into the rheometer. The basic chemical reaction is as follows:

O=C=N-R*-N=C=O +
HO-R’-COO-R°-OH
-> O=C=N-[-R*-NH-COO-R’-COO-R°-]-OH O=C=N-R*-N=C=O +
HO-R’’-COO-R°°-OH
-> O=C=N-[-R*-NH-COO-R’’-COO-R°°-]-OH

Methods

The curing of the investigated PU resin was monitored in small amplitude oscillatory shear (SAOS). Experiments were carried out using plate/plate geometries with 20 mm diameter and 1 mm gap. At a frequency of 1 Hz the samples were both tested in CS (controlled stress) at a shear stress of 50 Pa and in CD (controlled deformation) at a strain of 0.01. Experiment duration was 180 minutes with data sampling every 60 seconds. The infrared spectral range was 400 cm-1 to 4000 cm-1 with a spectral resolution of 4 cm-1. Each spectrum consisted of 8 co-added scans. In total, 180 data sets, each containing rheological and spectral data, were collected.

Results

The storage modulus G’ and loss modulus G’’ of the PU resin was shown as a function of time. The curing reaction was monitored under different deformation conditions as described earlier. The viscous behavior dominated the initial part of both experiments (G’’ > G’) due to free chain mobility. Beyond the first inflection point in G’, the curing speed increased and finally a crossover was monitored. At the end of both experiments the elastic behavior dominated (G’ > G’’), because the formation of a 3-dimensional network enabled the sample to store more energy elastically than was dissipated via viscous flow.

The cross-over shifted to a later time when working in controlled deformation (CD) mode. Under this condition, the curing was slower which coincided with the Isocyanate absorbance band development at 2266 cm-1. The Isocyanate consumption that correlated with the curing rate in the infrared spectrum was higher in CS mode compared to the controlled deformation experiment.

One way to quantify the gel point of a thermosetting resin is to determine the inflection point of G’ during its rapid increase. Another method is the determination of the cross-over between G’ and G’’. At this specific point the viscous and elastic properties are even and thus a correlation with a working life timescale is argumentative.

Curve sketching of G’ yields the parameters summarized in the table below. The determined parameters allow for the fragmentation of G’ into several specific sections. To assist the understanding of the chemical aspects in each section, an examination of the simultaneously acquired FTIR spectra is necessary.

An assortment of IR absorbance spectra in the Carbonyl region and their origin was assigned. The absorbance band development of associated Urethane at 1687 cm-1 was seen in spectra. Furthermore, the graph depicted the discontinuity at 11 minutes and the first inflection points at 47 minutes and 72 minutes respectively, after which both absorbance band development curves showed an equivalent run. In contrast, the absorbance band development of free Urethane at 1721 cm-1 runs congruent for both CS and CD over the entire period of time.

From the chemical point of view, the absorbance band development reflects the cross-linking process of free Urethane blocks by formation of Hydrogen bonds. Applying stress or deformation during the curing process obviously affects the kinetics of Hydrogen bond formation in the early curing stage. In particular, a better mixing due to higher deformations obtained in CS mode is achieved, thus increasing reactant mobility, and finally being responsible for the cross-over shifting to earlier times.

An assortment of IR absorbance spectra containing the Amide II band at 1522 cm-1 was displayed. The absorbance band development of the Amide II band for both methods applied was seen. The absorbance band at 1522 cm-1 is assigned to the stretching vibration of the C-N bond and the deformation vibration of the C-N-H bonds. These bonds are thus affected by applying stress or deformation during the curing process beyond the first inflection point.

It was illustrated that the absorbance band development of the Amide III band at 1242 cm-1 for both methods applied. This absorbance band is assigned to the deformation vibration of the N-H bond and the deformation vibration of the O-C-N bonds. These bonds are thus affected by applying stress or deformation during the curing process beyond the intersection point at which G’ equals G’’. Up to that point, both curves run congruent and the assigned Amide bonds are affected by neither of the applied methods.

Conclusion

The gel point is one of the basic parameters characterizing the processability of thermosetting resins and oscillatory shear is the method of choice to gather information about it.The specific combination instrumentation enables the simultaneous acquisition of FTIR spectra to track the evolving chemical constitution in all details and can thus reveal the relation between the visco-elastic properties and individual reactive groups of a complex polymer. Further strengths include kinetic analysis depending on mechanical impacts during the curing process; discovering the root cause of artifacts, and much more. To summarize, the combination instrumentation is valuable technology in obtaining real-time conversion information for studying of cross-linking reactions at which a transition from the liquid to the solid phase takes place.

You can see all charts, spectra, and additional details in the application note: Detailed analysis of curing reactions of polyurethane resins using the Rheonaut technology for simultaneous rheometry and FTIR.