Method for selecting polyester film, method for producing multilayer body, method for producing package, and multilayer body

A method comprising:

• a first step including:
• determining, by performing FT-IR analysis by a reflection method on an amorphous polyethylene terephthalate film, a first absorbance at a wavenumber of 1,340 cm−1 (I11340), a first absorbance at a wavenumber of 1,370 cm−1 (I11370), and a first absorbance at a wavenumber of 1,409 cm−1 (I11409) and assigning I11340, I11370, and I11409 to:
• I11409=p1×I11340+p2×I11370 (Formula 1a);
• determining, by performing FT-IR analysis by the reflection method on a crystalline polyethylene terephthalate film that has a different crystallinity than the amorphous polyethylene terephthalate film, a second absorbance at a wavenumber of 1,340 cm−1 (I21340), a second absorbance at a wavenumber of 1,370 cm−1 (I21370), and a second absorbance at a wavenumber of 1,409 cm−1 (I21409) and assigning I21340, I21370, and I21409 to:
• I21409=p1×I21340+p2×I21370 (Formula 1b);
• a second step including:
• obtaining simultaneous equations of p1 and p2 from the Formula 1a and the Formula 1b and solving the simultaneous equations for p1 and p2;
• a third step including:
• determining, by performing FT-IR analysis by the reflection on a polyethylene terephthalate film of interest, a third absorbance of the polyethylene terephthalate film of interest at a wavenumber of 1,340 cm−1 (I31340) and a third absorbance of the polyethylene terephthalate film of interest at a wavenumber of 1,409 cm−1 (I31409);
• measuring a crystallinity of the polyester polyethylene terephthalate film of interest by:
• Crystallinity [%]=p1×(I31340/I31409)×100 (Formula 2)
• wherein, p1 is the obtained p1 from the second step;
• and
• a fourth step including:
• selecting the polyester polyethylene terephthalate film of interest as a first polyethylene terephthalate film of a sealant layer of a multilayer body when the measured crystallinity of the polyethylene terephthalate film of interest is from 2 to 15%, resulting in the multilayer body including a base material layer including crystalline polyester film, an adhesive layer, and the sealant layer in this order.