13 October 2013
Year: 2013Price: 10.00
Double Bond Conversion (DBC) is an important indication of degree of cure (Photo-polymerization), which has a direct relationship with most of the physical properties of cured films, such as surface scratch resistance, surface tacky, double bond residue, and adhesion. DBC has widely been used as a tool of quality control in the UV curing industry [1].
Acrylate C=C of photo-polymerization have been characterized by using a variety of instruments, such as Fourier transforms infrared (FTIR), real-time Fourier transforms infrared (RT-FTIR) and photo-differential scanning calorimeter (photo DSC), people usually study an average double DBC throughout the entire film [2, 3]. In previous studies, author used a new approach [4-7], which is a combination of a traditional FTIR and statistical calculation, to analyze depth profile of DBC by each 5 m depth for a module formulation with Irg.819 or Irg.184 as photo-initiator. Overall, this approach is still an average DBC analysis, which includes tedious film thickness control and calculation in analytical performance.
There are two Major challenges in the UV curing industry: oxygen inhibition at the top layer of the cured films near the air and the inner filter effect at the bottom layer of the cured film, near the substrate. Most of the time, people are very interested in knowing the DBC near the air (to determine surface tacky /scratch resistance) and the DBC near the substrate (to determine adhesion/degree of cure) .The real-time Fourier transforms infrared attenuated total reflectance [FTIR-ATR] has been used to study the DBC at last 2 microns near substrates[8,9] in order to study inner filter effect; A con-focus RAMAN method has been used to study the DBC in 1-2 microns depth from the air in order to study Oxygen inhibition[10, 11 ].
Reducing oxygen inhibition is a hot topic in Photo-polymerization [12, 13]. For a Clear 25 Microns coating with Irg.184, UV curing first happened in the middle depth of cured film due to less oxygen inhibition and less inner filter effect[14,15]; The last part of cured film to be cured(to have a DBC higher than 90-95%) is at the top of cured film, due to oxygen inhibition.
It is important to have UVC output to cure the surface of cured films. In the UV curing industry, people used to use following methods to overcome Oxygen inhibition:
a. To use BPh and organic tertiary amines or their mixtures
b. To add more photo-initiators
c. To make UV curing in inert atmosphere (Nitrogen or Carbon Dioxide) or between laminate films, such as PET, PP, PE film
d. To increase UV Dose (mJ/cm2)
e. To increase UV light intensity (mW/cm2), it is effectively, but not completely.
f. To add Tribenzen Phosphine or Thio compounds
Obviously, in all above mentioned methods, there are some their own limitations in real industry processes. In this research, author will report the DBC difference of top 1 micron depth from the air using two different UV lamps: Microwave Power UV H Lamp (M.P. H Lamp) and electrode Power UV H Lamp (E.P. H Lamp) by using con-focus RAMAN (Fig.1) analytical method for a hard coating formulation containing Irg.184. An influence of enhanced UVC output UV Lamp on increasing the conversion at the stop 1 micron depth from the air of the cured film with short wavelength photo-initiator, Irg.184 for model formulations has been studied.
The RAMN scattering technique is based upon changes in the wavelength(1636 cm-1) of incident light interaction with the rotation and vibrational energy levels in double bond (C=C) of acrylate molecules and is perfectly well suited for the detection of C=C chemical bonds and their changes before and after UV curing. Con-focus RAMAN method is a combination of microscopy and Raman Spectroscopy to provide spatial resolution and the depth resolution. Thus, an optical slice of sample can be obtained without physical touching the sample. The finished coating was about 200μm. A 1000X objective with numerical aperture equal to 1 mm working distance was used. This con-focus setup provides a sport size of 1 um and depth resolution of 1 um. RAMAN spectra of the coatings were taken from the sample surface to 1 um depth before and after UV curing.
