- The extrusion process for pharmaceutical has been designed for mixing drugs with carriers in several dosage forms. The major significant difference between thermoplastic and pharmaceuticals extruder is the equipment used and increase contact surface which must meet regulatory requirement. The equipment must fulfil standard validation to pharmaceutical industry.
HME consists of an extruder, downstream auxiliary equipment and some other tools which are used for evaluation of product quality and performance [Kruder GA. Extrusion. In: Encyclopedia of Polymer Science and Engineering Vol. 1, 2nd ed. John Wiley & Sons Inc., New York (1985) 571-631]. The extruder contains a feeding hopper, barrels, and the die, screw driving unit, single or twin screws and heating or cooling device. Downstream equipment is used to collect the powder for further process and temperature gauges, screw speed controller, pressure gauges and extrusion torque monitor include in monitoring device.[ Breitenbach J. Melt extrusion: from process to drug
delivery technology. Eur. J. Pharm. Biopharm. (2002) 54: 107-117]
For east theoretical understanding, Extrusion process divide into four parts-
Feeding material of extruder
Conveying and kneading part for material mixing and movement
Die for to formulate extrudates
Downstream equipment for cooling, pelletizing and collecting
Some additional equipments such as analytical technology, vacuum pumps, calendaring equipment and pelletize equipment required and parameters like zone and die temperature, pressure, feed rate, melt viscosity,, screw speed and power consumption are considered.
All zones of HME are preset before the process it started. A powder blend contain thermoplastic polymer or waxy material, drug substance and other processing materials which are fed into the barrel of extruder through hopper and transferred in the heated barrel by rotating screw.
Figure (A) Twin screw extruder schematic set up
The extruder contain barrels of single or twin screws which gives a particular shape and transport and force the melt through die. Barrels can be heated to desired temperature. The polymer of waxy material is plasticized and increases the viscosity due to external heat, shearing effect and begins to melt or soften as enters into the compression section of extruder. Temperature into different zones of the barrel are controlled by electrical bands and monitored by thermocouples.[Pharmaceutical Extrusion Technology By Isaac Ghebre-Selassie, Charles Martin, Ebooks Corporation, Marcel Dekker (Firma comercial),pp-166-170] Normal temperature of melting section is normally set at 30-60ÂÂ®C which is above the glass transition temperature of the amorphous polymer or melting of semi crystalline polymers. The extruder is fed up from one side and exit from other side which is continuous process. [Cuff G, Raouf F. A Preliminery evaluation of injection molding as a technology to produce tablets. Pharmceutical Technology, june, 2008, 22 (6), pp-97-106] The HME comprises into three steps- melting, mixing and shaping. (Figure-B)
Figure-(B) Principle of Hot-Melt Extrusion
The barrel mass pumped through the die which is attached to the end of barrel and further processing by auxiliary downstream devices. A good flow property of feedstock is required inside in the hopper for continuous process. To demonstrate a good flow, the angel between the sidewall of feeding hopper and horizontal line should be larger than the angle of repose of feeding material. In the case of very fine powder or cohesive material, a solid bridge is formed at the throat of hopper to maintain erratic powder flow with the use of force feeding material. Powder properties such as bulk density, particle size and shape and material compatibility may influence to flow of material in hopper of extruder. [Aitken-Nichol C, Zhang F, Mcginity JW, Hot-melt extrusion of acrylic films, Pharmaceutics Research, 1996,13, pp-804-808]
The function of screw is to transfer the material into the barrel and to mix it, compress and melt of materials and pump through the die so design of the extrusion screw has significant effect on the efficiency of Hot-Melt Extrusion process and can be designed to achieve either high or low shear. The length of screw in process is given in the term of length of screw divided by screw diameter (L/D). Some parameters are also defined to measure the feature of screw. [ Klucel Hydroxypropylecellulose, Physical and Chemical Properties. Technical Bulletin 250-2W. Wilmington: Hercules, Inc, 1997, Repka MA, Mcginity JW. Influence of Vitamin E TPGS on the Properties of hydrophilic films produced by hot-melt extrusion. International Journal of Pharmaceutics, 2000, pp-202-63-70]
Most of screw are made from stainless steel and steel in coated to friction and erosion or decay which may be done during extrusion process. Based on geometrical design and function of screw at the point, extruder is divided into three zones: feeding zones, melting or compression zones and metering zones.
Figure (C) Extruder illustrating various functional zones including hopper.
The starting material feed into feed section from hopper which has deeper flights. This geometry enables easily to fall material into screw along to barrel. A speed of rotation is depending on the pitch and helix angle. The materials transported as a solid plug into transition zone where it is mixed, compressed, melted and plasticized. Compression of material can be achieved by decreasing the thread pitch by maintaining a constant flight depth or decreasing flight depth by maintaining a constant thread pitch. [Johnson PS. Development in Extrusion Science and Technology, Polysar technical publication, Ontario, (1982)] as a result of these, pressure increases and material moves to barrel. Melt material moves by circulation in helical pathway by drag flow, pressure flow, and transverse flow. Space between screw diameter and width is normally 0.1 to 0.2mm. [Martin C. Guidelines for Operation of Leistritz Twinscrew Extruder, American Leistritz Corporation,Somerville (2001)] the material reaches to metering zone is homogeneous form and metering zone reduce pulsating flow and ensure a uniform delivery of material.[ Breitenbach J. Melt extrusion: from process to drug delivery technology. Eur. J. Pharm. Biopharm.(2002) 54: 107-117] downstream extrusion is used to shape and analyse of final material. Due to consideration of particle size, cooling and cutting has been performed after continuous strand and analytical technology may be used to check the homogeneity of active ingredient.
Single screw extruders were used in early days. Twin screw extruders are currently used for large quantities of material. In the twin screw, the screw can either rotate in same direction (co-rotating) or in opposite direction (counter-rotating).
Co-rotating extruders are most used in the industrial applications because it can be operated at high screw speed, high output and provide good mixing and conveying characteristic and good flexibility.
Advantages of twin screw over single screw extruders-
Major advantages is mixing abilities
Less residence time and high dispersing capacities
Easy feeding, less tendency and high kneading potential
Advantages of single screw over twin screw extruder-
Simple and cost effective