In PSA industry, release liner is one of the key components. The main functions of release liner are to provide protection of the adhesive layer during storage and transportation, and ensure a residue-free removal from the protected adhesive layer without damaging or contaminating it.
Therefore, the silicone release agent must be even and consistently applied to a substrate and completely cross-linked, allowing no migration from the silicone into the adhesive layer. The degree of cross-linking can be determined by extraction method or IR spectroscopy method.
Johanna Hofmann from Indiana company adopted this two methods to determine the thermal cross-linking reaction or curing of PDMS via hydrosilylation reaction as schematically shown in below .
3.1 Extraction Measurement
The extraction measurement, used in the silicone coating industry, mostly consists of a combination of XRF measurement and atomic absorption spectroscopy (AAS). The coating thickness of the silicone is determined by XRF measurements and the extractable amount of silicone, or the uncured portion, is measured by AAS. With the ratio of the two results, the degree of silicone curing can be determined.
Extraction method
Procedures:
- Determine coating thickness or coating amount of the silicone release agent by XRF measurement, which is total amount of silicone
- Place the silicone sample with defined size and weight into a headspace vial and filled with isobotylmetylketone (MIBK)
- The vials were gas-tight sealed to prevent evaporation of the volatile MIBK
- Dissolve the non cross-linked silicone completely
- Leave the samples for at least 24 h and mixed shortly before the AAS measurement to determine the uncured silicone amount
- Divided the uncured amount by total amount to obtain the degree of cross-linking
FTIR method
The FTIR spectra are from Johanna Hofmann‘s publication in the internet. In theory, both Si-H and Si-C=C signal can be used to monitor the curing reaction. However, the vinyl stretching at 1600 cm-1 and 3070 cm-1 has a lower extinction coefficient and is present in lower concentrations, thus the change of Si-H absorption at 2168 cm-1 is adopted to measure the cross-linking reaction. Another signal is also needed to be adopted as an internal standard which needs no change with cross linking reaction. Study shows both CH3 signal at 2961 cm-1 and for siloxane at 1945 cm-1 are quite stable and good for the use as the internal standard.