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Crisp beverage bags and even dog food packaging are made of many polymer layers, which are glued together to optimize the barrier and mechanical properties of the packaging.
Conventional laminating adhesives used in multilayer flexible packaging are usually the product of a solvent-based or solvent-free two-pot system that has undergone the following reactions:
Polyol (-OH) isocyanate (-N=C=O) → polyurethane laminating adhesive (-NHCOO-)
Traditionally, the packaging industry used FTIR to screen and test the decay rate of isocyanate (NCO) in newly applied adhesives. This method is critical because unreacted aromatic NCO molecules can migrate into the food through laminated packaging. If this happens, these molecules may react with water molecules to produce carcinogenic primary aromatic amines, which are harmful to humans and pets.
At ~2270-2250 cm-1, the measured value of NCO absorbance peak height decreases with time, so that this attenuation can be used as an indicator of adhesive curing speed and food packaging safety. The spectrum shown below was taken over time from a single solvent-free adhesive on a single Arrow slide:
Figure 1. In a two-pot packaging adhesive with a standard mixing ratio, the NCO decays over time in absorbance mode.
You can also use multiple Arrow Slides to compare the curing speed of different adhesives at the same time. There are many variables in the curing speed of the adhesive, including the mixing ratio of OH and NCO, relative humidity, curing temperature, the crosslinking ability of the starting components, and the reactivity of the aromatic NCO component.
The curing process can sometimes take days or even weeks, so it is rarely prudent to occupy a single traditional crystal ATR disc for a long time. Large batch sampling using Arrow consumables ATR slides is a better option, saving time, speeding up work and ultimately making the organization more competitive.
Once the polyurethane product is formed on the crystals, it is difficult to remove. Of course, the adhesive is designed to stick, so this process risks damaging the relatively more expensive crystal plate.
Figure 2. The curing speed of the adhesive at different mixing ratios.
Overall, the use of Arrow consumables ATR slides in packaging adhesive analysis has many obvious advantages. They are ideal for dealing with irreversible reactions-once the polyurethane thermosetting material is cured, it will not return to its original composition and is difficult to remove.
No sample preparation is required. The traditional FTIR transmission method requires laminating by machine or by hand, which is time-consuming and wasteful of packaging substrates. Arrow refuses to use hazardous cleaning solvents because lamination is not required. It also allows monitoring of multiple samples in an inexpensive way.
This information is derived from materials provided by Specac Ltd. and has been reviewed and adapted.
For more information on this source, please visit Specac Ltd.
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