CHAPTER 2. LITERATURE REVIEW
2.8 Fibers Synthesis
• In pharmaceutical products, the release of the active ingredient is either obtained by biodegradation of the shell or by diffusion through the shell.
• For self-healing applications, the release mechanism is usually the mechanical rupture of the shell to release the active ingredient.
For asphalt applications, the use of self-healing microcapsules containing a rejuvenator is
promising, as it will allow the material to resist the initiation and propagation of cracking caused by vehicular and environmental loading. When cracking occurs due to the aging process of the binder, the shell of microcapsules containing rejuvenator will rupture, and the rejuvenator will flow through the binder, which consequently results in reversing the aging process.
Every fiber synthesis technique has 3 basic steps: (1) production of polymer solution, (2)
extrusion of the fibers, and (3) solidification of the fibers. The first step consists in dissolving the polymer into a solvent to form a stable solution. When a thermoplastic polymer is utilized, the polymer is usually melted to form the solution. The extrusion of the polymer solution is usually performed by utilizing a spinneret. A spinneret is an instrument with an appearance similar to a bathroom shower head with many holes ranging from one to several hundreds. Lastly, the solidification of the fibers can be performed by either wet-spinning, dry spinning, melt spinning, and gel spinning. The process of extrusion and solidification of the polymer solution is called spinning process [107].
2.8.1 Wet Spinning
This technique is usually utilized for polymers that are dissolved in a solvent. A wet spinning consists in extruding the polymer solution into a coagulation bath (i.e. non-solvent) to precipitate the polymer. Then, the extruded fibers are washed to remove the excess coagulant and
subsequently wound on a bobbin [108, 109]. Figure 2.22 shows a schematic diagram of a wet spinning procedure.
Figure 0.22. Schematic Diagram of Wet Spinning Procedure [110]
Calcium alginate fibers are commonly developed using a wet spinning process. Alginate is a natural polymer commonly used in the textile industry. The application of calcium alginate fibers for asphalt applications emerges with the idea to avoid negative effects of using hazardous materials (i.e. formaldehyde) for the development of self-healing mechanisms. Calcium alginate is not only a non-toxic and safe material for the environment and the user, but also has a low-cost and biodegradability property. Furthermore, alginate is a renewable resource with unlimited supply in nature [111, 112].
The wet spinning process to develop calcium alginate fibers starts by dissolving sodium alginate powder in water. Then, the sodium alginate solution is extruded through a spinneret into a calcium chloride coagulation bath. During the extrusion process, there is an exchange between the calcium and sodium ions resulting in a firm gel structure [113]. Based on previous studies, some factors that affects the production efficiency and the product performance are discussed in the next sections.
2.8.1.1 Molecular weight of the alginate powder. Studies have shown that the strength of the fiber will be enhanced with a higher molecular weight of the alginate powder [113]. However, the viscosity of the alginate solution increases with a higher molecular weight making it difficult to dissolve a high concentration of alginate. A study recommended that a particular 1% alginate solution should have a viscosity ranging from 40-100 mPa in order to have a balance between the production efficiency and the product performance [111].
2.8.1.2 Concentration of the spinning solution. As mentioned in the previous section, the viscosity of the alginate solution increases as the concentration increases. For wet spinning process, a concentration of 5%-6% of sodium alginate for the spinning solution is recommended [113].
2.8.1.3 Temperature of the solution. It is recommended to heat the sodium alginate solution during the dissolution process as the viscosity decreases as temperature increases, at a rate of about 2.5% per degree Celsius. However, a long exposing time to high-temperature such as 50
◦C can be depolymerized the alginate powder resulting in a loss of viscosity and molecular weight [113].
2.8.1.4 pH of the solution. The viscosity of the alginate solution is not affected over the range pH= 5-11. Below a pH= 5, a higher viscosity occurs as the free –COO− ions starts to protonate into -COOH. Furthermore, hydrogen bonds start to form as the electrostatic repulsion between the -COOH chains is reduced. The result of the formation of hydrogen bonds is the formation of a gel between pH= 3-4. For a wet spinning process, it is recommended to use deionized water with pH around 7 [113].
2.8.2 Dry Spinning
In an analogous way than wet spinning, the polymer is dissolved in a solvent and then is extruded. During the extrusion process, the solution emerges through the spinneret where the solvent is evaporated off with hot air. The solvent can be recollected and reuse it for another spinning process. Some common examples of fibers produced with this technique are acetate fiber, triacetate fiber, acrylic fiber, modacrylic fiber, and vinyon. Figure 2.23 shows a schematic diagram of a dry spinning procedure.
Figure 0.23. Schematic Diagram of Dry Spinning Procedure [107]
2.8.3 Melt Spinning
This technique is usually performed in polymers that can be melted. The melted polymer is extruded through the spinneret where a quench air cools the fibers as they emerge. For this particular technique, the winding speed is an important factor for the product performance as it influences the strength of the fibers. Figure 2.24 Schematic diagram of melt spinning procedure.
Some advantages of melt spinning process are the following: (1) it is cost effective, (2) no washing is required as no solvent is used; and (3) it is high speed, usually the winding speed is around 8000 m/minute. Melt spinning is usually performed with polymers that will not get thermal degradation at high-temperatures required to form the melt solution. Some examples of fibers produced with this technique are nylon, polyethylene, polypropylene, and polyester.
Figure 0.24. Schematic Diagram of Melt Spinning Procedure [107]