Thermally bonded yarn composite processes combine fibers or materials with distinct properties to achieve multifunctionality (e.g., high strength, elasticity, conductivity). Below are detailed implementation methods and key technical considerations:
I. Core-Sheath Yarn Production Process
Structural Features: A high-performance fiber core (e.g., aramid, glass fiber) is coated with a thermoplastic polymer sheath (e.g., TPU, PA).
Key Steps and Technologies:
Core Pre-Treatment:
Core Material Selection: High-modulus fibers (carbon fiber, steel wire) or functional fibers (conductive, antimicrobial).
Surface Modification: Plasma treatment or chemical coatings (e.g., silane coupling agents) to enhance core-sheath adhesion.
Sheath Melt Coating:
Co-Extrusion Die Design:
Concentric dual-channel nozzle with independent temperature control for core and sheath melts (temperature difference ≤10°C).
Tapered sheath melt channel to reduce interfacial shear stress.
Process Parameters:
Sheath melt viscosity (MFI) must be lower than the core material to prevent core displacement (e.g., TPU sheath MFI=15 g/10min; carbon fiber core preheated to 200°C).
Synchronized control of haul-off speed and extrusion rate (error <±0.5%).
In-Line Composite and Cooling:
Two-stage cooling system:
Primary Cooling: Air cooling (20–25°C) for rapid sheath surface solidification.
Secondary Cooling: Water bath (40–50°C) to regulate crystallinity and minimize internal stress.
Application Example:
Carbon fiber/TPU core-sheath yarn for smart textile strain sensors: TPU sheath provides elasticity, while the carbon fiber core enables conductivity.
II. Covered Yarn Production Process
Structural Features: An elastic filament core (e.g., spandex) is helically wrapped with thermoplastic short fibers or filaments (e.g., PET, PP).
Key Steps and Technologies:
Core Pre-Drafting:
Spandex core drafting ratio: 300–500%, stabilized via heated rollers (60–80°C) to ensure elastic recovery.
Outer Layer Covering Methods:
Air Covering:
High-pressure airflow (0.3–0.5 MPa) wraps short fibers onto the core, suitable for bulky yarns (e.g., thermal fabrics).
Mechanical Covering:
Filaments spiral-wrapped around the core via hollow spindle (helix angle: 30–45°), ideal for high-strength industrial yarns.
Thermal Bonding:
Infrared heating (wavelength 2–5 μm) partially melts the thermoplastic outer layer, enabling "spot welding" by penetrating core gaps.
Temperature control: Slightly above the melting point of the thermoplastic (e.g., PET melts at 260°C; heated to 265–270°C).
Application Example:
Spandex/PET covered yarn for sportswear: Spandex provides elasticity, while the PET outer layer enhances abrasion resistance and dyeability.
III. Technical Challenges and Solutions
| Challenge | Root Cause | Solution |
|---|---|---|
| Interfacial Delamination | Mismatched thermal expansion (e.g., PA6 vs. steel) | Add compatibilizers (e.g., maleic anhydride-grafted polymers). |
| Non-Uniform Coating | Tension fluctuations in air/mechanical covering | Dynamic tension sensors + servo motor closed-loop control (±0.1N precision). |
| Core Breakage | Molecular chain rupture in spandex under high drafting | Stepwise gradient drafting (e.g., 50% → 100% → 300%). |
| High Energy Consumption | Energy loss during melting/cooling | Heat pipe waste heat recovery (20–30% energy savings). |
IV. Advanced Composite Technologies
Multi-Component Electrospinning:
Coaxial electrospinning under high voltage (50–80 kV) to produce nanoscale composite fibers (diameter <500 nm).
3D-Printed Coating:
Fused Deposition Modeling (FDM) for layer-by-layer core coating, enabling customized structures (e.g., gradient-hardness sheaths).
Smart Process Monitoring:
Laser diffraction for real-time coating thickness analysis, with AI-driven parameter adjustments.
V. Quality Control Standards
Interfacial Bond Strength:
ASTM D1876 T-peel test: Minimum requirement ≥5 N/cm.
Coating Coverage:
Microscopic image analysis: ≥95% for textiles; ≥99% for industrial applications.
Elastic Recovery Rate:
Spandex core yarns after 5-cycle stretching (300% strain): Recovery rate ≥90%.
The success of thermally bonded yarn composites hinges on material compatibility, interfacial control precision, and energy efficiency. Innovations such as nano-coated anti-stick dies, dynamic tension systems, and intelligent temperature control enable stable, high-throughput production. Future trends include:
Bio-based thermally adhesive materials (e.g., PLA sheaths) to reduce carbon footprint;
Multifunctional composites (conductive/antibacterial/phase-change) for medical and aerospace applications.
