(154x) Production of Polystyrene Microparticles and Microfibers from Waste Expanded Polystyrene Using Spinning Disk Technique

Expanded Polystyrene (EPS) is a non-toxic inert polymer which is used for versatile applications such as materials for packing, decoration purposes, manufacture of cups and trays and so on, due to its fascinating features of low cost, light weight, corrosion-resistance, and processability. Annually, it is estimated that 14.7 MMT of polystyrene is produced, among this 6.6 MMT is EPS. It is found that 6.63 tons/day of polystyrene waste in the form of waste expanded polystyrene (WPS) is produced around the world. WPS causes several environmental problems due to its non-biodegradability and demands considerable space due to its large volume to weight ratio (Chaukura, 2016). Hence disposing or recycling WPS is a challenge. Most of the WPS is disposed of by landfill or by incineration. Landfill causes soil and water pollution and also increases disposal sites. Incineration is an energy recycling method, but incineration of polystyrene produces hazardous chemicals. There are several mechanical recycling methods, but these are more expensive than producing virgin polystyrene. Moreover, these methods are mainly aimed at eliminating polystyrene rather than converting it into a valuable product. Hence, recycling of WPS into a valuable product in an environmentally friendly as well as economical manner is highly necessary (Mangalara, 2016).

Conversion of WPS into valuable products like nano or microparticles and nano or microfibers is a possible way of recycling WPS as they can be used for several purposes like biomedical applications, imaging, preparation of engineered colloid particles, oil adsorption and so on. Particularly, polystyrene microparticles can be used for lateral flow tests, latex agglutination tests, flow cytometry, fluorescence microscopy, and as calibration particles. Polystyrene microfibers with nano-porous structure are considered to be one of the best oil sorbents to absorb oil from the ocean in the event of an oil spill. Polystyrene fibers are also used as fiber membranes and as additives in the paint industry (Mangalara, 2016).

Polymer microparticles with narrow size distribution are preferred due to uniformity in properties, controllable behavior for specific applications and simplified processing. Several existing methods for bulk production of microparticles such as spray drying, fluidized bed drying, electro-spraying and polymerization have limitations especially in the control over particle size distribution and surface morphology. Also, these methods require challenging operating conditions such as high gas flow rates, high temperature for drying, pressures greater than atmospheric conditions and so on. Microfluidic generation of droplets is one method by which highly monodisperse particles can be produced but there are a number of drawbacks concerning lower production rates, handling viscous and impure solutions, separation of particles from continuous phase, wetting issues and so on. Thus, a technique that can overcome all these aforementioned disadvantages and can generate droplets with narrow size distribution is required.

Solvent-antisolvent precipitation can be one method for producing solid particles from microdroplets. In this method, particles are formed when microdroplets of a solution that contains a solvent and a solute come in contact with another solvent (antisolvent) which is miscible with the solvent but does not dissolve the solute (Joye, 2013). Microdroplets can be generated using several atomization techniques such as pressure atomization, air atomization and spinning disk atomization. Air atomization and pressure atomization techniques have similar drawbacks of microfluidic techniques mentioned earlier, as well as produce microdroplets with a broad size distribution. Spinning disk atomization exhibits certain advantages over the other atomization techniques. These include separation of droplets according to their size, no requirement of narrow nozzles, ease of operation and available control variables to control the droplet size from the disk. The droplets are generated when a liquid is introduced to the surface of a high speed rotating disk requiring only moderate pressure to transport the fluids (Ahmed, 2012). Hence, compared to any other atomization techniques, spinning disk atomization when combined with a suitable system for collecting the particles at regions located at different distances from the center of the disk, is found to be an efficient and simple method for producing droplets with a given average size and a narrow size distribution.

In this study, solvent-antisolvent precipitation method is combined with spinning disk atomization (SDA) technique for producing polystyrene microparticles with a narrow size distribution using custom-made SDA equipment. Here, tetrahydrofuran (THF) is chosen as the solvent because of its versatility of dissolving most of the compounds and its miscibility with most of the solvents (Rajeev, 2016). Unlike water which is used as an antisolvent in the production of polystyrene particles by nanoprecipitation, iso-butanol is chosen as an antisolvent. Polystyrene is denser than water, hence when the atomized droplets of polystyrene solution come in contact with water, instead of forming particles; they form circular sheets of solid polystyrene on the surface of water. Hence, iso-butanol, which is less dense than polystyrene, is used as an antisolvent.

In the experiment conducted using the custom-made SDA equipment, the polystyrene-THF solution is atomized using high speed spinning disk to form microdroplets and these droplets are collected in a tray containing iso-butanol located at a desired distance from the center of the disk to form microparticles with a narrow size distribution. The observations from this experiment indicate that the average size of the particles can be tuned by varying: (i) the concentration of EPS, (ii) the speed of the spinning disk, (iii) flow rate of the polystyrene solution, and (iv) location of the antisolvent tray used for precipitation.

In case of polystyrene microfibers production, current techniques like electrospinning and centrifugal spinning have several disadvantages. Electrospinning does not have a higher production rate, requires a high voltage external power source and may not be able to handle all kinds of hydrocarbon solvents and viscous solutions (Liu, 2015). Centrifugal spinning, an emerging method, has several demerits such as lower production rate, handling viscous solutions and clogging issues (Doan, 2019). Hence, an alternative method is required for the production of microfibers that addresses the aforementioned demerits. Spinning disk technique with drying can be used for the production of polystyrene microfibers. In this technique, polystyrene solution at a high concentration fed to the spinning disk with simultaneous gas drying forms polystyrene microfibers.

For the production of microfibers, the concentration of the polystyrene in THF solvent is increased to five times that required for particle production. An additional gas distributor fixed above the spinning disk inside the SDA equipment, supplies nitrogen for drying the fibers. The main parameters for microfiber production are polymer concentration, feed liquid flow rate, speed of the disk and gas flow rate.

There is no method reported to produce both microparticles and microfibers using single equipment. This can be achieved in this custom-made SDA equipment with two separate feed systems where one system contains a highly concentrated polystyrene-THF solution and the other one contains only THF. A static mixer at the downstream of these two feed systems mixes the two liquids before feeding the solution to the disk. By varying the flow rates of the liquids in these two systems and thereby altering the concentration of the feed liquid, microparticles or microfiber production can be achieved.

Analysis of the microscopy images of polystyrene microparticles collected at a particular distance from the spinning disk indicates a narrow size distribution with an average particle size 107 μm. This suggests that, using SDA technique, conversion of EPS to polystyrene microparticles of narrow size distribution can be achieved in a controlled and reproducible manner. The microfibers produced by spinning disk with drying technique have nano-porous structure at the surface with an average fiber size 4 μm. Using SDA equipment with two feed systems and by altering the concentration of the feed liquid, microparticles or microfibers production can be achieved by the same equipment. This equipment and techniques can be used for the conversion of other waste polymers into microparticles and microfibers and can find new applications in various fields such as production of drug particles and microencapsulation of food ingredients.

References:

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