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Measuring Hydroxyl (OH) Values OF Polyols using FT-NIR Spectroscopy

Measuring the hydroxyl content of polyol starting materials is the key to getting consistent output from your production process.  If you are currently using wet chemistry methods like ASTM D4274-11 to measure the hydroxyl content of the polyols in your polyurethane process, let Galaxy Scientific help you get more efficient and free up valuable lab time.  Wet chemical methods are overly time consuming, requiring the polyol to be dissolved in acid and then titrated with potassium hydroxide, making it expensive and prone to error.  If you're using the ASTM method it can take 12-15 minutes per sample to determine the hydroxyl content.  Now there's a better way!

Recently, FT-NIR spectroscopy has been accepted as a rapid and accurate technique for determination of hydroxyl value of polyols.  ASTM D6342-12 defines a method for measuring hydroxyl content of several materials.  Using Galaxy Scientific's QuasIR series FT-NIR spectrometers, hydroxyl determination takes typically less than 1 minute per sample and requires no solvents, no reagents, and no columns or other consumables.  The analysis does not even need to be performed in a lab or by a chemist, freeing them up for more important duties.

QuasIR 4000 FT-NIR with cuvette

QuasIR 4000 FT-NIR


FT-NIR spectroscopy works by measuring the absorbance of light through your sample.  The light is produced by a halogen bulb which emits wavelengths of light in the near-infrared spectrum (approximately 800nm through 2500nm).  The light is then sent through an optical interferometer that contains a fixed arm and a variable arm.  The variable arm of the interferometer automatically and precisely scans through a pathlength while high speed electronics measure the signal on the InGaAs detector.  The resulting spectral data have peaks where light is highly absorbed due the molecular construction of the sample.  For example, in the spectra shown below, the absorbance peak at 5150cm-1 is from the CO stretch double bond in the molecule.  More importantly, the peak around 7000cm-1 is due to the O-H stretch of the present hydroxyl groups.  

FT-NIR Spectra of Polyols

Using this information you can imagine building a calibration that measures the height of the O-H peak to determine the hydroxyl content of the polyol. However, the algorithms contained in Galaxy Scientific's Spectral Sage PLS software can do even better by applying a partial least squares algorithm along with pre-processing to improve the accuracy of the results.  The calibration shown below was developed using 55 samples where the hydroxyl values were grouped around two levels, low at around 38 mg KOH/g and high around 56 mg KOH/g.  To avoid any temperature influence on the OH bands all sample spectra were collected using the QuasIR 4000's precision peltier vial heater set to 70 °C.

FT-NIR Polyol PLS plot

Even using this limited sample set we could accurately measure the hydroxyl content of these samples within an RMS error of 0.28 mg KOH/g.  Note that the output units for the calibration are based on the units provided by the reference values.  So, even though we're not doing a KOH titration, we still get the output of the near infrared analysis in milligrams of KOH per gram of sample!  So the results of this analysis are very familiar and you don't have to worry about downstream processes needing to be re-calibrated or changed to accommodate the new measurements.


Right now maybe you just want to get more efficient in the lab but, down the road, implementing near-infrared could be a game changer for your company or product line.  FT-NIR can be setup next to your process and run by non-technical operators who can use the rapid answers to quickly dial in their process. Even better, FT-NIR can be brought entirely online through the use of fiber optic probes and flow cells to create a fully automated process.  An FT-NIR isn't just for measuring OH number, it can also be used for many other needs such as measuring the isocyanate number during a polyurethane reaction or determining the styrene concentration in SBC pellets.  If you want to learn more about these other possibilities be sure to check out our whitepaper, FT-NIR as a Real Time QC Tool for Polymer Manufacture.


We're not the only near-infrared company and we're certainly not the biggest but when you work with Galaxy you're partnering with the leading innovator in FT-NIR.  We're pushing the technology to new heights by making our spectrometers smaller, more reliable, and less expensive than any other comparable product out there.  

What makes Galaxy Scientific's QuasIR series of spectrometers better?

  • Compact and portable - if you're short on space or think you may want to move the instrument around, Galaxy's QuasIR series is built for your needs.  It's lightweight yet rugged enough to get knocked around if you want to place it near your manufacturing process.
  • Robust - Interferometers are highly precise optical pieces that require the internal mirrors to remain parallel down to arc-seconds of tilt.  Our unique Permalign optics have made misalignment nearly impossible.  Solid quartz plates hold the optics in perfect alignment even when temperatures change or vibrations are present.
  • Affordable - We work hard to keep our products affordable but don't just look at the sticker price.  We won't gouge you for ongoing support contracts, software upgrades, or exorbitant service calls.  If something goes wrong, we'll do our best to support you remotely and, if you need a repair, zip the instrument back to our factory and we'll take care of it within 24 hours.  No waiting for service calls - no high priced repairs.
  • Easy-to-use - Our software, Spectral Sage, is purposely built to keep things as simple as possible without sacrificing the features needed to get the best results.  We provide everything you need in a clean, consistent user interface.  Our PLS software doesn't require advanced chemometric knowledge to put it to use.

When you partner with Galaxy Scientific, you're not just a number to a mega-corporation.  We're here to help you be successful; before, during, and after your purchase.  Our applications scientists have combined more than 60 years of experience in helping customers implement spectroscopy to make their application work.  We're sure our application knowledge and personal support sets us apart from everyone else out there.  



A quick phone call will help us understand your needs and how we can best serve you.  Even if you just want a budgetary quote, we'd love to hear about your plans and how we might be able to help.

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