Clean up your QC! Monitoring Soap with NIR

Did you celebrate National Shower with a Friend Day last Month?

Before you start to think that this blog has gotten NSFW (not safe for work), let us explain.

The cheeky National Shower with a Friend Day was born out of a marketing strategy by a water filtration company in 2014. The goal: educate people on the dangers of bathing with chlorinated water while encouraging them to shower with filtered, chlorine-free water.

To honor this cheeky holiday, we’ll present some recent work in the lab using NIR spectroscopy to monitor important properties of soap.

Don’t let your QC slip!

Have you ever wondered how your favorite bar of soap always smells, foams, lathers, and feels the same? This isn’t a happy accident—a lot of work goes into making sure it does! Monitoring the quality of raw ingredients and the consistency of the soap-making process are two major factors to ensure uniform, high-quality finished products. Three key properties routinely checked in soap-making are the %-active ingredient, %-free fatty acids (FFA), and %-moisture.

A QC monitoring program ensures that a batch of soap leaving the plant passes product specifications and meets customer expectations. Out-of-spec batches can’t be released, and instead, must be reworked or repurposed.

Wet chemistry methods can be used to test for out-of-spec products, but they are labor-intensive and time-consuming, often leading to production delays. Alternatively, NIR offers a fast, accurate measurement that can speed up QC and inform decision-making for process control.


Here’s a great example from our NIR Specialists.

In this work, two BUCHI NIR systems were used to monitor the finished product quality of two soap formulations (Formulation A and Formulation B). The formulations varied in both the amount and type of active ingredient.

Before product release, random samples from each formulation batch were tested for conformance on both a BUCHI NIRFlex N-500 FT-NIR system and ProxiMateTM at-line NIR system (Figure 1). Products that were within specifications were released for shipping. Out-of-spec products were held until the results were confirmed by primary wet chemistry techniques.

Figure 1. Soap samples undergoing quality control testing on the BUCHI NIRFlex N-500 FT-NIR spectrometer (left) and ProxiMateTM NIR at-line spectrometer (right).

Samples spanning a four-hour production run were analyzed by NIR then subsequently compared to wet chemistry reference data. The accuracy of the ProxiMateTM NIR predictions for all three variables: active ingredient (%), FFA (%), and moisture (%), is illustrated for both Formulations A (Figure 2) and B (Figure 3). The minimal difference between reference and predicted values illustrates the prediction accuracy for all three calibration models.

Figure 2. Reference (blue) vs. NIR (orange) measurements, and difference (gray) for Active (%), FFA (%), and Moisture (%) in Formulation A samples.
Figure 3. Reference (blue) vs. NIR (orange) measurements, and difference (gray) for Active (%), FFA (%), and Moisture (%) in Formulation B samples.

Circling back to the “Shower with a Friend” theme.

In this work, calibrations employed on the ProxiMateTM NIR were transferred directly from the NIRFlex N-500 FT-NIR using Transpec, the BUCHI calibration transfer tool. Transpec enables calibrations developed on the FT-NIR system in the lab to easily transfer to the more rugged and at-line ready diode-array-based ProxiMateTM NIR. For more details on calibration transfer, check out our last post Transcend your calibration transfer woes in 10 steps (with Transpec).

Before you towel dry…

In addition to monitoring finished products, BUCHI NIR-OnlineTM NIR sensors can be used for real-time monitoring of soap formulations on chill-rolls during the production process (Figure 4). Process monitoring enables operators to adjust ingredient proportions and processing variables as soon as deviations are observed. For a high-throughput, full-time soap plant, the benefits of in-process monitoring with NIR cannot be overstated.

Figure 4. NIR-Online process sensor installed on a chill-roll to measure soap formulation parameters in real-time.

Now your QC is squeaky clean!

If your lab is looking for some help getting started with applications like these, let us know!

We look forward to bringing you another exciting dispatch from the NIR world!

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