The problem:
The design and manufacture of a twin screw extruder for melt blending and compounding a mixture of clay and polymer substrate to give a nanocomposite. We have a specified L/D ratio, diameter of the screw and throughput. The other details like the helix angles total length other diameters have to be calculated.
Nanocomposites
This is basically a base polymer into which is added filler material just to achieve some properties. The property advantages that nanomaterial additives can provide in comparison to both their conventional filler counterparts and base polymer include:
•Mechanical properties e.g. strength, modulus and dimensional stability
•Decreased permeability to gases, water and hydrocarbons
•Thermal stability and heat distortion temperature
•Flame retardancy and reduced smoke emissions
•Chemical resistance
• Surface appearance
• Electrical conductivity
•Optical clarity in comparison to conventionally filled polymers
However it has been observed that the nanocomposites develop lesser toughness and weakened impact performance as compared to their components.
In our case the mixing of polymers with clay to form clay based nanocomposites is targeted. Clays are basically chains of alumino silicates with a sheet like layered structure, more specifically they are silica SiO4 tetrahedra bonded to alumina AlO6 octahedra in a variety of ways. The thickness of the layers (platelets) is of the order of 1 nm.
Synthetic Processing of Clay-Based Nanocomposites
The synthetic route of choice for making a nanocomposite depends on whether the final material is required in the form of an intercalated or exfoliated hybrid. In the case of an intercalate, the organic component is inserted between the layers of the clay such that the inter-layer spacing is expanded, but the layers still bear a well-defined spatial relationship to each other. In an exfoliated structure, the layers of the clay have been completely separated and the individual layers are distributed throughout the organic matrix. A third alternative is dispersion of complete clay particles (tactoids) within the polymer matrix, but this simply represents use of the clay as a conventional filler.
Polymers can be introduced either by melt blending, for example extrusion, or by solution blending. Melt blending (compounding) depends on shear to help delaminate the clay. Both thermosets and thermoplastics have been incorporated into nanocomposites, for e.g.
• Nylons
• Polyolefins, e.g. polypropylene
• Polystyrene
• Ethylene-vinyl acetate (EVA) copolymer
What is extrusion?
As has been already mentioned the melt blending can be done by extrusion of a mix of the clay and polymer. The extrusion can be performed by the either a single screw extruder or a twin screw extruder.
The single screw consists of a screw in a barrel .The single-screw extruder geometry relies on “drag flow.” Drag flow is the action of a rotating screw flight as it “drags” material that wants to stick or adhere to the barrel wall down the length of the extruder. If the material wants to stick to the screw more than the barrel wall, it will just remain at the same location on the screw. In this situation, there will be no material movement down the extruder
Twin screws can be :
1. Co-rotating and counter-rotating designs
2. Fully intermeshing and non-intermeshing designs
3. Parallel shaft and conical shaft designs.
The twin screw extruder is advantageous as it provides better mixing due to the fact there are 2 screws .Also in a single screw if the process material sticks to the screw and slips against the wall the objective is unfulfilled. This is a basic flaw of the drag flow mechanism that is employed in single screw design. This situation can be imagined as in the example of a nut and screw model shown in the diagram.
If both the nut and bolt are allowed to freely rotate, then the nut will remain at the same location on the bolt. However it the nut is fixed to a surface, it will move along the bolt as material would move down the extruder channel.
Another difference in the two can be due to the wiping effect.
The twin screw extrusion to be designed and fabricated will have the three basic sections:
1. drive section
2. process section
3. die section
