The attenuated total reflectance technique is an alternate method for spectral collection, whereby a liquid or solid sample directly comes into contact with an ATR crystal material.
Internally reflected and measurable light from the sample crystal interface causes a spectrum, which is being collected for the sample from a particular penetration depth into the sample itself.
It is possible to use a monocrystalline type Ilia diamond ATR crystal to achieve a similar spectral range of 4000cm-1 to 400cm-1. The Specac Quest™ ATR accessory offers the choice of ZnSe, diamond or germanium ATR crystalpucks to be used on a dedicated optical unit with mirrored reflectance optical components only.
ATR measurement of the “curing” reaction of a 2 part epoxy glue from combination of the epoxy resin and the polyamine (hardener) component materials.
Equipment and Method
The Quest ATR Accessory P/N GS10801-B was used along with the single reflection diamond extended range ATR crystal puck option on the black surface colored optical unit for the ATR measurement of the epoxy resin curing reaction. This allowed for the collection of a full spectral range for the sample between 4000cm-1 and 400cm-1
A Thermo Nicolet IS5 at standard room temperature was used to collect the spectra. Specac’s Quest ATR Accessory detector system was set at a resolution of 4cm-1 for 16 scans. The selected sample was Araldite “Rapid” 2 part epoxy glue.
Epoxy resin part is 1 4-butanediol diglycidyl ether. Polyamine (hardener) part is n(3-dimethylaminopropyl)-1 3- propylene diamine.
To begin with individual spectra were taken for both the epoxy resin and polyamine (hardener) component materials of the 2 part reaction mixture. A 50:50 mixture of hardener and resin were mixed together and placed over the diamond crystal of the puck assembly. Spectra were obtained for this sample mixture from the beginning of mixing from when it was initially placed on the diamond crystal and at 5 minute intervals after that to monitor the reaction until the hardening of the epoxy resin was complete.
Figures 1 and 2 show the infra red spectra collected for the epoxy resin and polyamine hardener materials. Figures 3 and 4 show the spectra collected for the curing reaction mixture. Figure 3 shows the complete spectral range from 4000cm-1 to 400cm-1 comparison overlays for the reaction mixture after 1, 3, 6, 10 and 20 minutes after mixing and Figure 4 is a comprehensive comparison for the same spectra shown over the spectral range from 900cm-1 to 760cm-1
The hardener and resin samples are both mobile fluids. They were spotted into place onto the diamond crystal of the puck and no force for contact was needed by employment of the flat or pellet anvil from the arm assembly of the Quest ATR optical unit.
Figure 1. Araldite Rapid Resin on diamond puck of Quest ATR Accessory
Figure 2. Araldite Rapid Hardener on Diamond puck of Quest ATR Accessory
For the spectral acquisition of the start constituents (resin and hardener), the initial reaction mixture was in a mobile liquid-like state and of a plae yellow coloration so neither the pellet or flat anvil option was used for the sample to make good contact with the diamond ATR crystal.
Then an IR spectrum was collected which has been shown in Figures 3 and 4 for the blue-colored trace labeled as 1 minute after mixing. The reaction curing process continues and regular spectral acquisitions were taken to monitor any changes occurring during the curing time.
Figure 3. Araldite Rapid Mixture on diamond puck of Quest ATR Accessory
Figure 4. Araldite Rapid Mixture on diamond puck of Quest ATR Accessory
Observing the IR Spectra over this time period after 3, 6, 10 and 20 minutes shows a physical change of opacity and hardening manifesting itself in a gradual change to the spectral profile. This can be seen in the examples in figure 3.
This observation alone may indicate there is a change occurring in the sample state. On closer inspection, an absorbance feature at circa 860cm-1 is decreasing in peak intensity as the reaction time continues. Figure 4 shows this feature decreases for the first 10 minutes and has stabilised from 20 minutes onwards.
IR Spectra collected after 20 minutes showed no changes other than an overall increase to the background profile. After 60 minutes, the sample had become very hard making it difficult to make any indentation into its surface.
After the reaction process is completed, this cured hardened sample was relatively easily removable from the diamond crystal puck. This type of sample reaction monitoring could not be carried out using an ATR accessory with a ZnSe crystal as in the process of hardening and adherence of the sample to the ATR crystal. It would not be possible to remove the sample without permanent destruction and damage to the ZnSe crystal.
However, due to the diamond crystal hardness, the hardened cured resin sample was “shocked” from the puck by placing the sharp edge of a screwdriver blade to the side of the sample and knocked with a hammer. The resin sample came away from the diamond crystal completely, but there were some residual parts of the resin material stuck to the stainless steel puck around the diamond area. With subsequent cleaning, the puck was cleared of remaining sample and was ready for use with a new sample.
The has been shown that the Quest ATR accessory with a diamond puck Is suitable for use In applications whereby a sample such as a two part epoxy resin adhesive can be studied for its reaction of bonding and co-polymerisation. Using a diamond crystal as the ATR top plate allows for a wide spectral range In the IR from 4000cm-1 to 400cm-1 to be studied and recorded for the sample material, but most Importantly the hardness, structural and chemical Integrity of a diamond crystal allows for such samples to be studied using the ATR technique, as the sample can be easily removed from the crystal puck after a reaction curing process, with minimal.
This information has been sourced, reviewed and adapted from materials provided by Specac.
For more information on this source, please visit Specac.