Microscopic observation test
The polylactic acid fibers were observed by microscope. The cross section of polylactic acid fibers was approximately circular, and the longitudinal section fibers were smooth and spotted.
Polylactic acid fibre cross section
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Combustion test
Fibers melt and shrink when they are near the flame; Fibers melt and burn when they contact the flame; When leaving the flame, the fiber melts and burns, and the melt falls. The fiber burns with a slight special sweet taste; The residue is a pale gray jelly. According to the combustion test method, the characteristics of polylactic acid fiber are similar to those of synthetic fiber, and there is no special characteristic phenomenon. However, polylactic acid fiber burns with a unique transparent blue flame when it leaves the flame, and its residue is a very small amount of yellowish gum, which is different from polyester fiber. However, this characterization phenomenon is not so obvious that it cannot be confirmed as polylactic acid fiber by this method alone.
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Melting point test
The polylactic acid fiber is melted at about 150℃ and completely melted at 165℃~170℃. In the melting point method, the melting point of polylactic acid fiber is between 165℃~170℃, the melting point of polyester fiber is generally about 252℃, nylon 6 fiber is about 220℃, nylon 66 is about 260℃, polypropylene fiber is about 180℃, ethylene fiber is about 160℃, although it can be distinguished from polyester and nylon, but easy to confuse with ethylene fiber, With the development of many low melting point polyester, it is not easy to use melting point method for qualitative identification.
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Infrared spectrum
The infrared absorption spectrum of polylactic acid was analyzed
It can be seen from the infrared absorption spectrum that there is a strong absorption peak of double bond stretching vibration at about 1747cm-1, C-O stretching vibration at about 1076cm-1, and there are two strongest absorption peaks at about 1179cm-1, indicating that the fiber has ester characteristics of absorption peaks, belonging to the polyester family. But C=O stretching vibration frequency position corresponding to benzene ring conjugated polyester class to be high, strength to be weak.
In the infrared absorption spectrum test, the spectral absorption map of polylactic acid fiber and polyester fiber has great similarities, but the characteristic peak of polylactic acid: 1076cm-1, 1179cm-1, 1747cm-1, indicating that the fiber has ester characteristics of the absorption peak, belonging to the polyester family. According to the conjugation of C=O group and benzene ring, the stretching vibration frequency of C=O group is higher than the above position, and the strength is strong or weak, polylactic acid fiber and polyester fiber can be distinguished.
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Chemical dissolution
The chemical dissolution test of polylactic acid fiber was carried out to observe the dissolution state of polylactic acid fiber in certain acid, alkali, salt and organic solvent under certain conditions of temperature and dissolution time. For details, see Table 1.
Note: S0 -- immediately dissolved, S -- dissolved, P -- partially dissolved, I -- insoluble, △ -- swollen.
In the dissolution test method, it can be seen from the above table that polylactic acid fiber can be dissolved in dichloromethane and boiled dimethylformamide at normal temperature, which can be distinguished from polyester, which is the key point to identify polylactic acid fiber and polyester fiber, polypropylene fiber and ethylene fiber. The chemical solubility of polylactic acid fiber is the same as that of polyester fiber, but polylactic acid fiber belongs to linear fatty family polyester fiber, while polyester belongs to aromatic family polyester fiber. The differences in molecular structure of these two fibers lead to obvious differences in chemical solubility.