As a general purpose resin, polypropylene (PP) is widely used in industries such as electronic manufacturing and construction, and is used in packaging materials due to its insulating and processing properties. PP resins must be melted first in order to be formed into the intended shape, and therefore flow properties are important characteristics which affect the production process. One parameter that describes the flow characteristics is the melt flow rate (MFR). This is a measure of the mass of material that extrudes from the die over a given period of time (ASTM D1238). The standard procedure requires a significant amount of work with packing the sample, preheating, and cleaning. With no sample preparation or chemicals needed, Vis-NIR spectroscopy allows the analysis of MFR in less than a minute.
PP pellets were measured with a DS2500 Solid Analyzer in reflection mode over the full wavelength range (400–2500 nm). To minimize particle size effects, a rotating DS2500 Large Sample Cup was employed. This accessory enables an automated measurement at different sample locations for a reproducible spectrum acquisition. As displayed in Figure 1, samples were measured without any sample preparation. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.
|DS2500 Solid Analyzer||2.922.0010|
|DS2500 Large Sample Cup||6.7402.050|
|Vision Air 2.0 Complete||6.6072.208|
The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for quantification of the density content. The quality of the prediction models was evaluated using correlation diagrams, which display the correlation between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.
|Figures of merit||Value|
|Standard error of calibration||4.99 g / 10 min|
|Standard error of cross-validation||7.00 g / 10 min|
This application note demonstrates the feasibility of NIR spectroscopy for the analysis of MFR in polypropylene samples. In comparison to the standard method (ASTM D1238) (Table 3), the reduction of analysis time and workload is a major advantage of NIR spectroscopy.
|Parameter||Method||Time to result and workflow|
|Melt flow rate||Extrusion ASTM D1238||∼20 minutes; packing material, preheating, measuring, cleaning|