Manufactured Fibre Technology_removed (4)

9.1 INTRODUCTION

  • Synthetic fibers such as nylon and polyester are extruded as continuous filaments.

  • These filaments are subsequently drawn to achieve desired strength and stability, resulting in semi-crystalline and oriented structures.

  • Thermal Shrinkage: Occurs when heated above glass transition temperature and below crystalline melting point, mainly due to molecular chain relaxation.

  • Shrinkage magnitude is influenced by:

    • Structural parameters: orientation and crystallinity.

    • External variables: temperature, tension, and time.

  • Dimensional Stability Issue: Drawn but unset fibers, despite having good tenacity and elasticity, lack dimensional stability, making them unsuitable for textile and technical applications.

  • Twisted or doubled filaments tend to curl, negatively impacting further processing.

  • Unset continuous filaments in fabrics show poor creasing behavior, requiring a heat-setting process.

  • Heat-setting Methods: Can involve hot water, saturated vapor, or dry heat treatment with or without tension.

  • Benefits of Setting: Relieves strains from manufacturing, enhancing resistance to shrinkage, dimensional changes, curling, and creasing.

9.2 NATURE OF SET

  • Definition: Setting refers to stabilizing a structure into a specific form, reaching equilibrium in textile materials.

  • Types of Set:

    • Temporary Set: Loses stability with mild conditions like light or warmth.

    • Semi-permanent Set: More stable but can be reversed by harsh treatment.

    • Permanent Set: Involves structural changes and cannot be fully reversed without damage.

9.2.1 MECHANISM OF TEMPORARY SET

  • Temporary sets can be induced in fibers like polyester and nylon through heating above specific temperatures, followed by cooling.

  • Intermolecular forces (van der Waals or hydrogen bonds) play a crucial role, allowing for temporary configuration changes that can be reversed.

9.2.2 MECHANISM OF PERMANENT SET

  • Permanent setting involves changes in crystalline morphology through partial melting and recrystallization:

    1. Weakening of intermolecular bonds at elevated temperatures.

    2. Thermal relaxation of the polymer structure.

    3. Stabilization and reformation of bonds upon cooling.

  • Heat causes molecular chains to relax, reducing crystalline structure and increasing amorphous content.

  • Reorganization of chains leads to a lower energy configuration.

9.3 HEAT-SETTING BEHAVIOR OF POLYAMIDE AND POLYESTER FIBRES

  • Heat-setting affects numerous physical properties:

    • Tenacity

    • Elongation

    • Modulus

    • Work recovery

    • Shrinkage and swelling properties

  • Dismore and Statton’s study on nylon 66:

    • High-temperature annealing drastically reduces tensile strength.

    • Shrinkage trends indicate complex interaction with elongation, showing that shrinkage is a disorienting process followed by folding into a stable structure.

9.3.1 CHANGES IN STRUCTURE AND PROPERTIES ON HEAT-SETTING

  • Heat-setting can be significantly different between free and tensioned conditions. Generally, free annealing reduces tenacity but increases diameter.

  • Tension annealing generally increases tenacity without altering diameter significantly.

  • Studies indicate that changes in crystallinity and internal structure from annealing influence mechanical properties and dye uptake behavior.

9.3.3 THERMAL HEALING

  • Thermal treatment can heal surface cracks and internal flaws, resulting in an even strength distribution across fibers post-heat-setting.

  • Chemical healing plays a notable role alongside physical healing in restoring strength.

9.3.4 SETTING UNDER DRY AND WET HEAT

  • Steam-setting of nylon enhances density more than dry heat due to better mobility in the presence of moisture.

  • Polyester shows improved density under heat but not as pronounced as nylon.

9.4 HEAT-SETTING OF CELLULOSE TRIACETATE FIBRES

  • Cellulose triacetate fibers are thermoplastic and can undergo significant changes upon heat-setting.

  • Studies show that crystallinity increases but tenacity decreases upon heat-setting.

  • The behavior of dyadic structures such as Arnel 60™ varies with heat treatment compared to Arnel™, affecting tensile properties.

9.5 SETTABILITY AND THE MEASUREMENT OF THE DEGREE OF SET

  • The degree of set can be quantified using two main methods:

    • Critical Dissolution Time (COT): Measures time taken for a filament to dissolve in a solvent, providing insights into crystallinity and molecular-ordering.

    • Recovery from Bending Strains: Assesses the ability of fibers to recover from deformation, significant for applications in apparel fabrics.

  • Results show that polyester retains its set better than nylon over time, particularly under varying temperatures.