GB/T 6509-2025Determination of caprolactam and oligomers content in polycaprolactam (PA6) chip and fibre (English PDF)

3.1 Polycaprolactam

PA6 Molecular formula. (C6H11NO)n, a polyamide formed by ring-opening polymerization of caprolactam.

Note. Its resin chips are scientifically known as polycaprolactam chips; if spun into fibers, they are polycaprolactam fibers, also known as nylon 6.

3.2 caprolactam

Monomers used for the polymerization of polycaprolactam (PA6).

Note. Residue found in polycaprolactam (PA6) chips or fibers.

3.3 oligomer

Oligomers with a degree of polymerization of less than 10 that exist in polycaprolactam (PA6) chips or fibers.

4.1 Principle

Polycaprolactam (PA6) slices or fiber samples were dissolved in hexafluoroisopropanol solvent, and then methanol was used as a precipitant to remove impurities from the sample solution.

Macromolecules are reprecipitated and separated in the sample solution using ultra-high performance liquid chromatography (UHPLC) or high performance liquid chromatography (HPLC).

Oligomers (see Appendix A) were qualitatively identified by retention time and UV spectroscopy, and quantitatively identified by external standard method.

4.2 Reagents or Materials

Unless otherwise specified, use only chromatographic grade reagents.

4.2.1 Water. GB/T 6682, Grade II.

4.2.2 Hexafluoroisopropanol. CAS No. 920-66-1, purity >=99.5%.

4.2.3 Methanol. CAS No. 67-56-1.

4.2.4 Acetonitrile. CAS No. 75-05-8.

4.2.5 Caprolactam standard. CAS No. 105-60-2, purity >=99.5%.

4.2.6 Caprolactam cyclic oligomer standard or reference standard. purity >=95%. The preparation method for the caprolactam cyclic oligomer reference standard is attached. Record B. 4.2.7 Sealing film.

4.3 Instruments and Equipment

4.3.1 Ultra-high performance liquid chromatograph or high performance liquid chromatograph. equipped with a diode array detector or ultraviolet detector.

4.3.2 Volumetric flasks. 25mL, 250mL.

4.3.3 Polytetrafluoroethylene needle filter. hydrophobic type, pore size <=0.22µm.

4.3.4 Electronic balance. graduation value 0.1mg.

4.3.5 Constant temperature oscillating water bath. temperature control accuracy ±2°C.

4.4 Test Procedure

4.4.1 Chromatographic Reference Conditions

4.4.1.1 Reference conditions for ultra-high performance liquid chromatography

The reference chromatographic conditions for ultra-high performance liquid chromatography are as follows.

— Column. T3 column, 100mm x 2.1mm (inner diameter) x 1.7µm, or equivalent;

— Column temperature. 30°C;

— Flow rate. 0.3 mL/min;

— Detection wavelength..200nm;

— Injection volume. 2 µL;

— Mobile phase. Mobile phase A is water, and mobile phase B is acetonitrile (4.2.4);

— Eluting procedure. see Table 1.

Table 1.Elution Procedures for Ultra-High Performance Liquid Chromatography time min Mobile phase A Mobile phase B 0 95% 5% 7.0 60% 40% 7.5 10% 90% 8.5 4% 96% Table 1.Elution Procedures for Ultra-High Performance Liquid Chromatography (Continued) time min Mobile phase A Mobile phase B 10.0 95% 5% 12.0 95% 5%

4.4.1.2 Reference conditions for high performance liquid chromatography

The reference chromatographic conditions for high performance liquid chromatography are as follows.

— Chromatographic column. C18 column, 250mm x 4.6mm (inner diameter) x 5µm or equivalent;

— Column temperature. 30°C;

— Flow rate. 1.0 mL/min;

— Detection wavelength..200nm;

— Injection volume. 20 µL;

— Mobile phase. Mobile phase A is water, and mobile phase B is acetonitrile (4.2.4);

— Eluting procedure. see Table 2.

Table 2 High Performance Liquid Chromatography Elution Procedures time min Mobile phase A Mobile phase B 0 95% 5% 12.0 70% 30% 12.5 0% 100% 15.5 0% 100% 15.6 95% 5% 20.0 95% 5%

4.4.2 Plotting Standard Working Curves

Weigh caprolactam (C1) standard (4.2.5) and caprolactam cyclic oligomer (C2~C9) standard separately using an electronic balance (4.3.4).

Or, the reference standard (4.2.6) is placed in a 250 mL volumetric flask (4.3.2), with C1~C9 having masses of approximately 14 mg, 4 mg, 14 mg, 14 mg, and respectively.

14 mg, 14 mg, 14 mg, 8 mg, and 4 mg were dissolved in methanol (4.2.3) and diluted to volume to obtain a mixed standard stock solution. The concentration was determined by the actual weight.

The value was calculated. Then, the solution was serially diluted 2, 4, 8, 16, and 32 times to prepare a mixed standard working solution. Store in the dark at 4°C~8°C.

The mixed standard working solution was determined using a chromatogram (4.3.1) under the chromatographic conditions described in 4.4.1.The concentrations of caprolactam and its cyclic oligomers (C1-C9) were determined.

Chromatograms and UV spectra are shown in Figures C.1 to C.3 of Appendix C. The peak areas of C1 to C9 are plotted on the ordinate, and the concentrations of the corresponding components of C1 to C9 are plotted on the ordinate.

Using the x-axis as the horizontal axis, plot the standard working curves for C1 to C9 respectively. The linear correlation coefficient should not be less than 0.999; otherwise, the standard working curves should be re-plotted. Draw a curve.

4.4.3 Sample Preparation

Weigh 50 mg of the sample into a 25 mL volumetric flask (4.3.2) using an electronic balance (4.3.4), add 3 mL of hexafluoroisopropanol (4.2.2), and stopper the flask.