Initial Modulus and Stiffness Control
topic
Initial modulus of PET fibre (tangent modulus at origin of stress–strain curve): 50–80 cN/dtex (commodity FDY); 100–120 cN/dtex (high-modulus industrial yarn). Modulus controlled by: crystallinity (higher Xc → higher modulus), orientation (higher fc → higher modulus), temperature (modulus decreases above Tg). Aggregate model: 1/E = cos⁴θ/Ec + sin⁴θ/Ea (predicts modulus from crystalline/amorphous moduli and orientation angle θ). PTT fibre has lower modulus (~20 cN/dtex) than PET, enabling elastic stretch.
Role
Initial modulus determines fabric drape, handle, and structural rigidity. Control of polyester modulus through processing is key to differentiating apparel (soft, drapeable) from industrial (stiff, dimensionally stable) applications.
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