Functional group and viscosity analysis (ASTM D789) of polyamides can be a lengthy and challenging process due to the sample’s limited solubility.
This application note demonstrates that the DS2500 Solid Analyzer operating in the visible and near-infrared spectral region (Vis-NIR) provides a cost-efficient and fast solution for a simultaneous determination of the relative viscosity as well as the amine, carboxylic, and moisture content in polyamides. With no sample preparation or chemicals needed, Vis-NIR spectroscopy allows for the analysis of polyamides in less than a minute.
Polyamide pellets were measured with a DS2500 Solid Analyzer in reflection mode over the full wavelength range (400–2500 nm). A rotating DS2500 Large Sample Cup was employed to overcome the distribution of the particle sizes and chemical components. This allowed for an automated measurement at different sample locations for a reproducible spectrum acquisition. As displayed in Figure 1, samples were measured without any preparation. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.
Equipment | Metrohm number |
---|---|
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 relative viscosity and amine end group, carboxylic end group, and moisture content in polyamides. The quality of the prediction models was evaluated using correlation diagrams, which display the relationship between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.
Result relative viscosity
Figures of merit | Value |
---|---|
R2 | 0.986 |
Standard error of calibration | 0.046 dl/g |
Standard error of cross-validation | 0.055 dl/g |
Result carboxyl end group content
Figures of merit | Value |
---|---|
R2 | 0.972 |
Standard error of calibration | 6.1 meq/kg |
Standard error of cross-validation | 11.1 meq/kg |
Result amine end group content
Figures of merit | Value |
---|---|
R2 | 0.981 |
Standard error of calibration | 2.5 meq/kg |
Standard error of cross-validation | 4.1 meq/kg |
Result moisture content
Figures of merit | Value |
---|---|
R2 | 0.991 |
Standard error of calibration | 0.041% |
Standard error of cross-validation | 0.067% |
This study demonstrates the feasibility of NIR spectroscopy for the analysis of some key quality parameters of polyamides. In comparison to wet chemical methods (Table 6), the time to result is a major advantage of NIR spectroscopy, since all parameters are determined in a single measurement in less than a minute.
Parameter | Procedure | Time to result |
---|---|---|
Relative viscosity | Viscometry | ∼90 min (preparation) + ∼1 min (viscometry) |
Carboxyl end group | Titration | ∼90 min (preparation) + ∼20 min (titration) |
Amine end group | Titration | ∼90 min (preparation) + ∼20 min (titration) |
Moisture | KF Titration | ∼25 min (preparation) + ∼5 min (Karl Fischer titration) |