Cure Monitor Services

* Help Monitoring Degree of Cure of Coatings, Films or Adhesives

* Monitor Thickness

* Correlate to Coating Property including Solvent Rub and Hardness

* Fast - 20 Millisecond Measurement Possible

* Non-destructive Technique

* Adaptable to On-Line Monitoring

SGL offers consulting and cure monitoring services for your quality control and research needs. SGL has pioneered the commercial development of patented fluorescence technology for monitoring properties of coatings and other polymers. This fluorescence ratio technique is a reliable and simple method for monitoring degree of cure or other properties such as solvent rub or micro-hardness. It can also be used to monitor thickness or coat-weight. It is useful for quality control and assurance of both resins and finished products.

Cure Monitor Basics:

This technology is based on a simple principle - a fluorescent probe which is sensitive to its immediate environment. The probe response changes as the molecular environment of the resin - the micro-viscosity and micro-polarity changes with cure. A fluorescent material is added to a resin or any curable material in very small concentrations, typically 0.001 - 0.1 weight percent. When excited by UV light, the probe fluoresces. As the resin polymerizes, the fluorescence spectrum of these unique probes shifts to shorter wavelength.

Figure 2. Fluorescence spectra of DASD probe in an acrylate coating at several stages of cure. As the coating goes from the monomer state (0%) to 40% and then 72% double bond conversion, as determined by FTIR, the fluorescence emission spectrum shifts to the left (blue). The ratio of the intensity at two wavelengths, 456 nm and 558 nm is used to monitor the cure; the ratio increases with cure. The optimal wavelengths were selected based on the difference spectrum between the cured and uncured states.

 

This shift occurs because the probe can exist in energetically different conformations, the relative concentrations of which are dependent on the resin micro environment.

Correlation to Degree of Cure or Other Properties:

This shift in fluorescence emission can be directly related to degree of cure or other polymer properties. In practice the ratio of fluorescence intensity at two selected emission wavelengths is determined and correlated to an independent measure of cure. This ratio acts as an internal standard and is independent of other variables such as light intensity and coating thickness. Excellent correlation between probe response and cure is attained in UV, EB and thermal polymers using measures of cure including FTIR double bond conversion, or performance properties including micro-hardness and solvent resistance (methyl ethyl ketone double rubs).

Correlation between Cure Monitor and Other Analytical Techniques

Figure 3. Correlation curve for an acrylate coating between the fluorescent probe technique and C=C double bond conversion or degree of cure. The fluorescent probe, coumarin 1, at 0.02 w% was added to the liquid resin. The UV coating was cured with varying light doses then measurements taken by both the CM 1000 and by FTIR. The maximum cure was 88% double bond conversion for this coating. The fluorescence ratio of emission intensities at 400/444 nm, using excitation at 350 nm, was found to give optimal sensitivity. Note that a 1% difference in degree of cure was easily detectable in this system. The resolution was better than 0.5% degree of cure.

Figure 4. Correlation between the fluorescence intensity ratio and MEK double rubs for a white flexographic ink. The fluorescent probe DASB (5-dimethylamino napthalene-1-sulfonyl-n-butylamide) was incorporated into the ink at 0.1 w%. Thin layers (5-10 micron) were applied to metal plates and cured by exposure at one or more passes through a Hg lamp at 601 mJ/cm² (200-600 nm) per pass. The fluorescence intensity ratio at 442/534 nm was recorded and the number of MEK double rubs required to remove the ink from the surface was also determined. The results show a good correlation between the two techniques and indicate that the fluorescence ratio, which is a much simpler method to perform, could be used to determine cure of the ink.

Once a correlation curve is established, degree of cure becomes a simple optical measurement. The measurement can be as fast as 20 milliseconds.

Monitor Coat-weight or Thickness:

This fluorescence technology can also be used for monitoring coat-weight or coating thickness. In this case the coat-weight is directly related to the fluorescence intensity (or ratio) at one wavelength.

Coat-Weight or Thickness Measurement

Figure 5. Correlation between fluorescence intensity and coat-weight in an epoxysilicone coating. The fluorescence intensity at a single wavelength is linearly proportional to the coat-weight or thickness. In this case an anthracenyl derivative (0.1 w%) was incorporated into the UV curable epoxysilicone resin, UV100, supplied by GE Silicones. The resin was coated on biaxially oriented polypropylene film at different coat-weights, then cured. The fluorescence intensity was monitored using 350 nm excitation and 510 nm emission. The coat-weight was determined by standard techniques.

Again, once a correlation curve is established with an independent measure of coat-weight, thickness monitoring becomes a fast optical observation. The coat weight can be monitored simultaneously with degree of cure.

Real-Time Monitoring and Resin QC/QA:

The CM 1000 can be used to obtain real-time cure profiles of polymers. For photopolymers, the resin can be cured with the excitation source of the CM 1000 while simultaneously monitoring the ratio. When using carefully controlled procedures this technique can be useful for QC/QA of resins.

Cure Profile Analysis

Figure 6. Comparison of real time cure profiles for several coating formulations. The curing profiles of several monomers and one formulated resin are shown. In this case the fluorescent probe DASB was used. The excitation beam of the CM 1000 (350 nm) was used to cure a 100 micron layer of the resin, while simultaneously monitoring the intensity ratio at 465/556 nm to obtain the cure profile. When the measurement conditions are kept constant, the following parameters can be reliably obtained:

Initial ratio -	is indicative of the reproducibility of the resin composition and should remain stable unless a component is changed or the resin has aged.
Initial rate -	is dependent on the initiator concentration in the resin tested.
Induction time -	is indicative of the concentration of inhibitors of polymerization.

 

For additional information, please call us at (419) 837-9783 or e-mail us at info@sglinc.com

Spectra Group Limited, Inc. • 27800 Lemoyne Rd., Suite J

Millbury, OH 43447 • 1-419-837-9783

Updated 4/29/08