Composition of Fibrous Filters


Small filters from a competitor’s product were reported to be comprised of fibers and particles. Previous analyses using Fourier transform infrared spectroscopy (FTIR) suggested two polymers were present in the filter material. MVA Scientific Consultants was asked to confirm the fiber/particle composition and determine how the two polymers were distributed within the filter.


An examination of the filter material by scanning electron microscopy (SEM) revealed the filter to be comprised solely of polymer fibers. It was noted that areas appeared to have peeled away or were missing from the fiber surface, suggesting that a coating may have been applied to the fibers (Figures 1 & 2). The confocal Raman microscope (CRM), with its ability to scan below a sample’s surface and provide full chemical information on the components present in a sample, was employed to confirm the SEM findings and determine the distribution of the two polymers.

Distribution of polymer fibers using SEM analytical testing

Fiber composition and distribution within filter by analytical testing


    A CRM depth profile, which produced an image of a cross-section of a representative fiber confirmed the presence of a coating. A color-coded composite image (Figure 3) was generated to show the distribution of the two components. The fiber (green) is surrounded by a second material (red), and yellow is the interface of the two materials. Comparison of the Raman spectra for each component with reference libraries revealed that the fiber was polypropylene coated with polyethylene terephthalate (Figure 4).

    Fiber coating analysis using depth profile imaging

    Coated polypropylene fiber analysis using confocal Raman spectroscopy