使用拉曼光谱识别聚合物母料

Masterbatches play an essential role in polymer manufacturing. Some widespread additive masterbatches reinforce plastics or make them resistant to flame or UV radiation, for example. Masterbatches are not only added to change polymers’ physical and chemical properties, they can also be used to color the polymer during the manufacturing process.

Measurement of masterbatches with handheld Raman spectroscopy requires no sample preparation and provides immediate results that easily distinguish between a polymer with different additives. Unique to Metrohm, the XTR® algorithm mitigates the inherent fluorescence of plastics and the background contribution from dyes; fluorescence mitigation is crucial for accurate library matching.

785 nm Raman excitation is considered the ideal wavelength for a high spectral signal-to-noise ratio. However, approximately 10% of Raman-active materials fluoresce under interrogation with 785 nm Raman [1]. Fluorescence overwhelms the Raman signal and can prevent positive identification of the target substance. Even the Raman spectra of uncolored polymers exhibit some inherent fluorescence, as do many hydrocarbon-based materials. Nearly 100% of deeply colored materials—tablets, foodstuffs, art, and plastics—can be problematic for traditional Raman analysis. The ability of XTR to remove the fluorescent background and reveal a high-resolution spectrum of colored plastics is particularly impressive.

Polypropylene (PP) is widely used in manufacturing, with many different types including homopolymer and copolymer. It also comes in specialized variants like flame-retardant and reinforced PP. Homopolymer PP provides high strength, stiffness, and chemical resistance, while copolymer PP offers flexibility and impact resistance.

Raman spectroscopy ensures fast, precise on-site confirmation of PP types. Figure 1 demonstrates the high specificity of Raman for very similar materials.

A series of strongly colored plastic marker casings were directly tested at the surface with a 785 nm handheld Raman spectrometer equipped with the XTR algorithm. Similarly to Sequentially Shifted Excitation (SSE), XTR uses multiple shifted spectra that are generated by internal algorithms during the experiment to distinguish the Raman shift from fixed fluorescence, permitting the fluorescence component to be isolated and extracted. The Raman data is optimized through an iterative process in a secondary automated process in real time. After identification and elimination of the fluorescence component, only a pure, unobstructed Raman spectrum remains.

The ability of XTR to return baselined spectra containing the signature Raman fingerprint peaks for a given substance is demonstrated across a spectrum of colors in Figure 2.

Only very saturated blue colors containing a cyan pigment showed strong spectral contribution from the dye (Figure 2). Indeed, most strongly blue-colored polymers are dyed with masterbatches featuring phthalocyanine chemistry [2].

Interestingly, the cyan signal was the main spectral contributor for only the very dark blue polypropylene (Figure 3). The resulting spectrum is an obvious blend of both polymer and dye masterbatch [3].

Despite significant contribution from both the dye and high levels of fluorescence, XTR permitted identification of both material and colorant (Figure 4). Note the very high Hit Quality Index (HQI = 0.99) values, indicating a high correlation between the sample and reference spectra.