Abstract : Through the analysis of the physical and chemical characteristics of polylactic acid fiber, the polylactic acid fiber was researched and certified by physical, chemical, microscopic and infrared absorption spectroscopy, and the polylactic acid fiber was qualitatively identified.

Polylactic acid fiber is made from fermented corn, cassava and other starch raw materials, fermented to obtain lactic acid, then polymerized from lactic acid into resin, and finally made by spinning. Therefore, polylactic acid fiber is also called corn fiber. It is a fully biodegradable synthetic fiber that can be obtained from cereals. After the product is discarded, it can be decomposed into carbon dioxide and water through the action of microorganisms in soil or seawater. When burning, it will not emit toxic gas and will not cause pollution. It is an ecological fiber. Polylactic acid fiber products can be widely used in the clothing market, household and decorative, non-woven fabrics, sanitary and medical fields. Knitting, weaving and weaving manufacturing processes can be used, and they are widely used in sportswear, underwear, and infants' close-fitting clothing [1~2].

Spinning of polylactic acid fibers can be achieved by solution spinning and melt spinning processes. Due to the complicated process of the solution spinning method, the solvent used has certain toxicity, the solvent recovery is difficult, the spinning environment is bad, and the finished product cost is relatively high; therefore, the melt spinning method currently used at home and abroad is basically. The process of melt spinning is: polylactic acid slicing → drying → screw extruder melting → filtration → metering → spinning → stretching → winding [3].

The research of this subject is based on the existing basic methods of fiber analysis, using the combustion test method, microscopic observation method, melting point test method, infrared spectrum absorption method and dissolution method to burn the polylactic acid fiber to the polylactic acid fiber. The melting properties, the absorption properties of the infrared spectrum and the solubility properties of the fibers were studied for the qualitative identification of polylactic acid fibers.

1 test materials, reagents and instruments

1.1 Test materials

Polylactic acid fiber: Aliphatic polyester fiber, commonly known as corn fiber, referred to as PLA fiber, sample supplied by Cargill Dow LLC.

1.2 reagent

Sulfuric acid, hydrochloric acid, formic acid, glacial acetic acid, sodium hydroxide, sodium hypochlorite, dimethylformamide, acetone, dichloromethane, and the like.

1.3 Instruments

Hastelloy slicer, biological microscope, thermal difference analyzer, electron microscope, electrothermal constant temperature water bath, infrared spectrum analyzer, etc.

2 identification method

The physical and chemical properties of polylactic acid fibers were studied by combustion test method, microscopic method, melting point test method, infrared absorption spectroscopy and chemical dissolution test method.

2.1 Combustion test

When the fiber is close to the flame, the fiber melts; when it contacts the flame, the fiber melts and burns; when it leaves the flame, the fiber melts and burns, and the melt falls; the fiber burns with a slight special sweet taste; the residue is a light gray gum. According to the combustion test method, the characteristics of polylactic acid fiber and synthetic fiber are similar, and there is no special characterization phenomenon.

2.2 Microscope observation test

The polylactic acid fiber was observed under a microscope, and its cross section was approximately circular, and the longitudinal section fiber was smooth and had obvious spots. The horizontal and longitudinal cross-sectional figures are shown in Fig. 1 and Fig. 2.

2.3 melting point test

The melting point test was carried out on the polylactic acid, and the test data is shown in Table 1.

Table 1 Melt performance of polylactic acid fiber

The polylactic acid fiber is initially melted at about 150 ° C and completely melted at 165 ° C to 170 ° C.

2.4 Infrared spectroscopy

The infrared absorption spectrum analysis of polylactic acid was carried out, and the infrared absorption spectrum thereof is shown in Fig. 3.

Figure 3 Infrared absorption spectrum of polylactic acid fiber

From the infrared absorption spectrum, it can be seen that there is a strong absorption peak of double bond stretching vibration around 1747 cm-1. The stretching vibration of CO is about 1076 cm-1, and there are two strongest absorption peaks around 1179 cm-1, indicating that the fiber has esters. The characteristic absorption peak belongs to the polyester family. However, the C=O stretching vibration frequency position is higher than that of the benzene ring conjugated polyester, and the strength is weak.

2.5 chemical dissolution test

The chemical dissolution test was carried out on the polylactic acid fiber, and the dissolution state of the polylactic acid fiber in a certain acid, alkali, salt and organic solvent under a certain temperature and dissolution time was observed, as shown in Table 2.

Table 2 Polylactic acid fiber dissolution performance table

Note: S - dissolve immediately, S - dissolve, P - partially dissolved, I - do not dissolve, â–³ - swell.

3 test results and analysis

3.1 Microscopy test

Compared with ordinary synthetic fibers, polylactic acid fibers are many like synthetic fibers in the identification. In the microscopic test method, the characteristics of the transverse cross section and the longitudinal cross section are easily confused with the synthetic fibers, and the distinction cannot be made clearly.

3.2 Combustion test method

In the combustion test method, the burning performance and state are similar to those of synthetic fibers. The difference is that the color of the residue is light, but the characterization phenomenon is not very obvious. It is not possible to confirm whether it is polylactic acid fiber by this method alone. .

3.3 melting point method

In the melting point method, the melting point of the polylactic acid fiber is between 165 ° C and 170 ° C, the melting point of the polyester fiber is generally around 252 ° C, the nylon 6 fiber is around 220 ° C, the nylon 66 is around 260 ° C, and the polypropylene fiber is at 180 ° C. Although the fiber is about 160 °C, it can be distinguished from polyester and nylon, but it is easy to be confused with ethylene. In addition, the development of many low-melting polyesters makes it difficult to qualitatively identify the melting point method.

3.4 Infrared absorption spectrum test method

In the infrared absorption spectrum test, the spectral absorption diagrams of polylactic acid fiber and polyester fiber have great similarities, but can be indicated by the characteristic peaks of polylactic acid: 1076 cm-1, 1179 cm-1, and 1747 cm-1. An absorption peak having an ester characteristic belongs to the polyester family. According to the conjugate of the C=O group and the benzene ring, the stretching vibration frequency of the C=O group is higher than the above position, and the strength is strong and weak, and the polylactic acid fiber and the polyester fiber are distinguished.

3.5 dissolution test method

In the dissolution test method, it can be seen from Table 1 that the polylactic acid fiber can be dissolved in methylene chloride at room temperature and boiled dimethylformamide, which can be distinguished from polyester, which is to identify polylactic acid fiber. And the key points of polyester fiber and polypropylene fiber and polyester fiber.