How Can You Get Accurate Results When Measuring Bulk Polymer Pellets?

For heterogeneous materials such as polymer pellets, a small sample may not be representative of the bulk material. Each pellet or group of pellets may have a slightly different composition than the next, so it is critical to obtain a measurement that is representative of the bulk sample rather than a small fraction of the material. This is often a significant challenge when using traditional Near-Infrared spectroscopy sampling methods.

Fourier Transform Near-Infrared spectroscopy (FT-NIR) is an ideal tool for at-line or near-line quality control analysis of polymer pellets.  NIR is a proven technology that is very useful for probing bulk material with little or no sample preparation. This spectroscopic method uses the near-infrared region of the electromagnetic spectrum, and is based on overtones and combinations of bond vibrations in molecules.  It is a common tool used for the analysis of industrial materials because it can typically penetrate much further into a sample than FTIR, and unlike Raman, is not affected by fluorescence. Although NIR spectroscopy is not as chemically specific as Raman or FTIR, accurate results can be achieved when a representative sampling method, like a sample cup spinner, is used.

A sampling cup with a quartz window used in combination with an FT-NIR analyzer provides a way to analyze greater amounts of material without having to empty the first sample and replace it with a new sample from the same batch. Once the sample is placed in the cup, it can be analyzed by two methods.

We conducted a study comparing two diffuse reflectance-sampling methods to determine the most efficient and accurate method for sampling polystyrene pellets. 

According to the American Chemistry Council, polystyrene (Styrofoam™) is a type of plastic that can be foamed and molded into very intricate parts:

“Polystyrene (PS) is a versatile plastic that can be rigid or foamed. General purpose polystyrene is clear, hard and brittle. Its clarity allows it to be used when transparency is important, as in medical and food packaging, in laboratory ware, and in certain electronic uses. Expandable Polystyrene (EPS) is commonly extruded into sheet for thermoforming into trays for meats, fish and cheeses and into containers such as egg crates. EPS is also directly formed into cups and tubs for dry foods such as dehydrated soups. Both foamed sheet and molded tubs are used extensively in take-out restaurants for their lightweight, stiffness and excellent thermal insulation.”

Because it can be used in health-related industries, ensuring the chemical make-up of the whole bulk batch matches customer requirements is of the utmost importance. With the high production rates in the polymer industry, it is essential that a quick, accurate, and easy-to-use analytical technique is available to monitor the quality of the material produced.

Traditional methods, such as titration or extraction followed by Gas Chromatography (GC) require sample preparation by a trained technician and often deliver results to the production personnel after a significant time lapse. This time lag between sampling and the completion of the analysis can produce out-of-specification material, resulting in manufacturing inefficiency, high scrap levels, and the need to rework product that does not meet quality standards.

Two Analysis Methods

A set of 17 polystyrene pellet samples were obtained from a proprietary source. The concentration of a UV-stabilizing additive ranged from 42% to 58% by weight. The pellet shapes and sizes varied slightly from sample to sample. The samples were placed into the open powder sampling cup, which has a 47.8 mm quartz window, and analyzed by diffuse reflectance.

A background was collected for each sample using the internal gold reference of the integrating sphere. The internal reference allows the background to be collected even if the sample cup is in place. All spectra were acquired at 8 cm-1 resolution and 16 scans with a collection time of less than 10 seconds. Spectra used to develop the method were obtained using a sample cup spinner accessory. The sample cup spinner was adjusted so that the largest amount of sample possible passed through the NIR beam in one complete revolution. Thirteen of the samples were used to develop the FT-NIR model and four samples were used to validate the performance of the model using the two sampling methods

Using the sample cup spinner accessory, the sample was rotated, constantly exposing new sample to the incident beam during data collection. A single spectrum was obtained that was representative of the material in the cup. The sample cup spinner allowed the largest volume of material to be analyzed in a single measurement.

Alternatively, multiple single point measurements were collected at different sample locations within the cup. Multiple spectra were produced for each sample and the results were averaged to obtain a representative answer. In order to obtain a representative answer, the user must manually rotate the cup then collect a spectrum. This process had to be repeated several times to ensure that the results would be indicative of the entire batch.

So what were the results of these two diffuse reflectance-sampling methods? Which method was determined to be the most efficient and accurate way to measure the amount of a UV-stabilizer additive in the polystyrene pellets?  We will discuss the results in our next article, but if you can’t wait, read the application note: Sampling Considerations for the Measurement of a UV Stabilizer in Polymer Pellets Using FT-NIR Spectroscopy.