Scaling up thermoplastic starch (TPS) extrusion introduces challenges akin to those encountered in various industrial processes. Key issues include:
- Decreased Surface-to-Volume Ratio: Larger equipment results in a reduced surface-to-volume ratio, limiting heat transfer possibilities.
- Temperature Gradients in Larger Equipment: Even with a consistent temperature difference, larger extruders exhibit smaller temperature gradients and heat fluxes compared to smaller counterparts.
- Diffusion Limitations in Larger Extruders: Distributive mixing-related diffusion limitations become more significant in larger extruders, even with identical shear fields as in smaller equipment.
In scaling up single-screw extruders, considerations must be given to thermal factors. Adiabatic processes, where there is no heat loss or gain, maintain consistent energy input per material amount. Achieving absolute thermal similarity involves adjusting the screw rotation rate with the increasing screw diameter. Several similarities play a role in extruder scale-up:
- Geometric Similarity: Ratios between length parameters in large-scale equipment mirror those in the small-scale model.
- Hydrodynamic Similarity: Demands equal dimensionless flow profiles and, for twin-screw extruders, the same filled length.
- Residence Time Similarity: Striving for equal residence times in both small-scale and large-scale equipment.
- Thermal Similarity: While challenging, maintaining equal average end temperatures is a favorable scale-up rule.
Implementing these rules optimally reproduces laboratory-scale results. However, some may deviate at the laboratory scale, using this information as a starting point for pilot plant or industrial-level designs.
Starch Extrusion Process at a Glance:
Starch films, crucial for industrial purposes, are typically produced through thermoplastic processing. The extrusion process involves subjecting native starch to shear forces at temperatures of 90-180°C in the presence of a plasticizer like glycerol. A single-screw extruder (SSE) or a twin-screw extruder (TSE) may be utilized.
- Process Control: Parameters such as raw material feed rate, temperature, screw speed, and profile must be meticulously controlled to achieve desirable TPS properties.
- Granule Transformation: Achieving a homogenous plastic state can be challenging, particularly with high-amylose starches. Residual granules may require techniques like microscopy and X-ray diffraction for detection.
Innovations in Starch Modification:
- Reactive Extrusion (REX): Concurrent reaction and extrusion have been employed for chemical modifications, leading to improved properties and innovative materials like plasticized starch-silica-poly(vinyl alcohol) composite films.
- Studies on Modification: Maleic anhydride (MA), polyurethane microparticles, and poly(butylene adipate-co-terephthalate) (PBAT) have been used in REX to enhance TPS properties.
Designing an effective extrusion process for TPS materials requires iterative testing, considering factors like feed starch type, formulation, processing conditions, and subsequent environmental conditions. Continuous optimization ensures the creation of uniform TPS melts with desirable properties.