Identification methods for synthetic fibers- Jiaxing Fuda Chemical Fiber Factory

Ordinary synthetic fibers mainly refer to the six traditional fibers, namely polyester, nylon, acrylic, polypropylene, vinylon, and chloroprene. Spandex, as a commonly used elastic filament, is also widely used.

1. The morphology and structure of various synthetic fibers

Due to the different chemical compositions of various synthetic fibers, their spinning and forming methods also vary. The spinning and forming methods have a significant impact on the morphology and structure of the fibers.

Melt spinning is used for polyester, nylon, and polypropylene fibers; Most acrylic fibers, vinylon staple fibers, and chlorinated fibers are wet spun; Spandex, some vinylon, and acrylic fibers are spun dry. During melt spinning, the molten polymer is pressed out through the spinneret hole and cooled and solidified in air. The fiber cross-section is related to the shape of the spinneret hole, and the conventional cross-section is circular. The wet spun silk strips solidify in solution due to solvent precipitation, with mostly non circular cross-section and obvious skin core structure. The longitudinal and cross-sectional morphological characteristics of common synthetic fibers are shown in Table 6-1 and Figures 6-1-4.

2. Combustion characteristics of various synthetic fibers

When using the combustion method to identify fibers, it is important to focus on observing the state of the fibers when they approach, come into contact with, and leave the flame, and pay attention to the odor generated during combustion and the characteristics of the residue after combustion. The combustion characteristics of commonly used synthetic fibers are shown in Table 6-2.

3. Chemical solubility performance of various synthetic fibers

The stability of various fiber materials to chemical reagents such as acids, alkalis, and organic solvents varies. The solubility of commonly used synthetic fibers is shown in Table 6-3.

4. Melting point of various synthetic fibers

The temperature at which the crystals in a polymer completely disappear, i.e. the temperature at which the crystals melt, is called the melting point. Synthetic fibers undergo changes in the intermolecular linkage structure under high temperature, softening first and then melting. Most synthetic fibers do not have exact melting points like pure crystals, and the same fiber may have different melting points due to different manufacturers or batch numbers. However, the melting points of the same type of fiber are fixed within a relatively narrow range, which can determine the type of fiber. Natural cellulose fibers, regenerated cellulose fibers, and protein fibers, due to their melting point higher than the decomposition point, do not melt under high temperature and decompose or carbonize.

The melting point method is generally applicable to identifying synthetic fibers with obvious melting point characteristics, but not to natural cellulose fibers, regenerated cellulose fibers, and protein fibers. Generally, it is not used as a separate means of qualitative identification, but can be used as a supplementary method for confirmation on the basis of other methods of identification.

Observing the temperature of fiber extinction under a melting point meter or a polarizing microscope with heating and temperature measurement devices to determine the melting point of fibers, in order to identify the type of fiber. Especially for synthetic fibers such as polyester, nylon, and polypropylene, their longitudinal and cross-sectional morphological characteristics and combustion performance are very similar, and using the melting point method for identification has significant advantages. The melting points of the main synthetic fibers are shown in Table 6-4.

5. Infrared spectra of common fibers

The study of infrared spectroscopy (IR) began in the early 20th century, when scientists had published infrared spectra of over 100 organic compounds, providing a powerful means of identifying unknown compounds. After the 1970s, on the basis of the development of electronic computer technology, Fourier transform infrared spectroscopy (FTIR) experimental technology entered the laboratory of modern chemists and became an important tool for structural analysis.

(1) The basic principles of infrared spectroscopy

When a beam of infrared light with continuous wavelengths is irradiated on the tested sample, and the vibration or rotation frequency of a certain group in the substance molecule is the same as the frequency of infrared light, the absorbed energy of the molecule transitions from the original ground state vibration (rotation) kinetic energy level to the higher energy vibration (rotation) kinetic energy level. After the molecule absorbs the radiation energy of infrared light, the vibration and rotation energy levels transition occurs, and the light at that wavelength is absorbed by the substance. Record the absorption of infrared light by molecules using an instrument to obtain an infrared spectrogram. Therefore, infrared spectroscopy is the analysis of fiber structure using the absorption characteristics of substances to infrared light. Each characteristic absorption band in the spectrum contains information about sample molecular groups and bonds, and different substances have different infrared absorption spectra.

Infrared spectrograms typically use wavelength（ λ) Or wavenumber（ σ) The horizontal axis represents the position of the absorption peak, and the vertical axis represents the absorption intensity using transmittance (T%) or absorbance (A).

(2) Zoning of infrared spectroscopy

The wavelength range of the infrared spectrum is approximately 0.75 to 1000 μ M. The infrared spectrum is usually divided into three regions: near infrared region, mid infrared region, and far infrared region. The relationship between wavelength and wavenumber is shown in Table 6-5.

Rotational spectra and vibrational spectra of certain groups. Due to the fact that the fundamental absorption bands of most organic and inorganic substances occur in the mid infrared region, the mid infrared region is the most studied and applied region, and the commonly referred to as infrared spectroscopy refers to the mid infrared spectrum.

According to the source of the absorption peak, the mid infrared spectrum can be roughly divided into two regions: the characteristic frequency region and the fingerprint region.

① Characteristic frequency range (4000-1300cm-1): The absorption peaks in the characteristic frequency range are mainly generated by the stretching vibration of the functional groups, with a small number but strong characteristic. Therefore, they are very valuable in group identification work and are mainly used for identifying functional groups. For example, carbonyl groups, whether in compounds such as ketones, acids, esters, or amides, always exhibit a stretching vibration of 5.9 μ A strong absorption peak appears around m, as shown in Figure 5.9 of the spectrum μ If there is a strong absorption peak around m, it can be roughly determined that there are carbonyl groups in the molecule.

② Fingerprint area (1300-400cm2): Multiple and complex peaks without strong characteristics, mainly generated by the vibration of some single bond C-O, C-N, C-H, O-H and other functional groups. When the molecular structure is slightly different, there are subtle differences in absorption in this region. This situation is like everyone has different fingerprints, hence it is called the fingerprint area. The fingerprint area is very helpful for distinguishing compounds with similar structures.

(3) Infrared spectra of common fibers

The characteristic groups of polymer substances have three main characteristics on the infrared spectrum: the position and intensity of the absorption peak, as well as the shape of the absorption peak. There are significant differences in the infrared spectra of different substances, and this principle can be used to identify textile fibers.

When using infrared spectroscopy to identify textile fibers, there are two main processes: "spectrum measurement" and "spectrum reading". Spectral measurement involves measuring the infrared spectrum of unknown fibers. Reading the spectrum involves comparing the infrared spectrum of unknown fibers with the standard spectrum of known fibers, thereby determining the category of unknown fibers. The qualitative analysis of textile fibers using infrared spectroscopy has the characteristics of being fast and convenient, and has been widely used.

The infrared spectra of common textile fibers are shown in Figures 6-5 to 6-12, and the main characteristic absorption spectra of various fibers can be seen in other reference materials.