Fourier transform near-infrared spectroscopy (FT-NIR) has many advantages in comparison with traditional analytical methods. Noninvasive and nondestructive analyses can be performed. Reagents are not typically required and samples can be analyzed in situ through glass or polymer containers. Minimal sample preparation is generally needed. Simultaneous multi-component analysis is possible in a very short time. In most cases, the analysis time is typically less than one minute. This allows high sample throughput and quick feedback of critical analytical information to production personnel.
Since FT-NIR measurements are simple and rapid, the technique is an ideal tool for quality control. It is also a convenient tool for at-line or near-line process control analyses because of the rapidity in which results are obtained.
The near-infrared (NIR) region is usually considered to include wavenumbers between 14400 and 4000 cm-1. Absorption bands originate from overtones and combinations of the fundamental (mid-IR) bands [mostly from carbon-hydrogen (C–H), nitrogen-hydrogen (N-H) and oxygen-hydrogen (O-H) bonds]. Broad, overlapping and low intensity peaks usually dominate the NIR spectrum.
An experiment was conducted to develop an FT-NIR method for the determination of the plasticizer content in polyvinyl chloride (PVC). PVC is used for many products such as toys, cables and foils. The plasticizer content differs depending on the commercial use of the material since the level of plasticizer determines the physical properties of the PVC. The plasticizer used for the material in this study was dioctyl phthalate (DOP).
The amount of DOP present in PVC is typically determined by gravimetry after Soxhlet extraction. This technique is time-consuming, tedious and requires the use of organic solvents. Furthermore, the extract contains small amounts of antioxidants that can interfere with the gravimetric determination. Replacement of this procedure with a faster, easier and more specific FT-NIR method would clearly be advantageous. Mid-IR could also be used but it requires sample preparation making FT- NIR more attractive for routine analyses.
Two sets of samples were obtained for this work. The first sample set contained 22 samples of PVC plates with known thicknesses. These plates contained different levels of DOP in the range of 5-50%. The sample thicknesses were confirmed using a digital micrometer. The PVC contents of these samples were calculated based on the weight percentage of DOP added to each. Ten translucent PVC (Neralit™ N702) plates with the stabilizer Naftomix™ MCS 20 (2.8%) and twelve (12) transparent PVC plates with the stabilizer Interlite™ ZP 7,003 (1.4%) were produced for this study. The plates were kept separate from each other due to the known tendency of DOP to migrate between samples.
The second sample set was comprised of 11 PVC transparent films with different contents of DOP and with known thicknesses. The thicknesses of all of the samples were confirmed using a digital micrometer. As with the previous samples, the DOP contents were calculated from the amount used to prepare the formulations. The PVC (Neralit 652) films contained the stabilizer Landromark™ LZB 968.
Spectra were collected on an FT-NIR analyzer, equipped with a CaF2 beamsplitter and an InGaAs detector. Ninety scans were collected at 4 cm-1 resolution. The plates were analyzed as is without sample preparation. . The total spectral analysis time was typically shorter than one minute per sample.
The data presented proved that FT-NIR can be used to predict polymer composition quickly and with minimal or no sample preparation.
For more specifics, including the instruments used, the calibrations, techniques, model parameters, plots, spectra, and results, see the Determination of Plasticizer Content in PVC by FT-NIR Spectroscopy application note.