COMPOUNDING Definition The Process by which ingredients are

COMPOUNDING

Definition The Process by which ingredients are intimately mixed together into as nearly a homogeneous mass is known as compounding.

CRITERIA FOR COMPOUNDING Selection of polymer. • Selection of compounding ingredients. •

SELECTION OF POLYMER n The performance requirement of the intended end use. n The economic factors. n The availability of suitable and processing equipment.

Selection of compounding ingredients REQUIREMENT: • Their degree of effectiveness. • The efficiency of processing ingredients. • The condition of processing • The requirement of the end use • The environmental exposure to which the product will be subjected.

FACTORS THAT INFLUENCE COMPOUNDING Physical form & melting characteristics of the polymer considered. • Degree of dispersion or solubilisation of the additive in the final mixture. • Physical form of the plastics raw material or compound to be produced. •

METHODS OF INCOPORATING ADDITIVES Added to the monomer ie before or during the production of the polymer. • Added during the working up of the polymer. • Added in a separate manufacturing step. • Incorporated directly before or during molding of a plastic to a semifinished product or finished article. •

Addition of additives before or during a polymerisation process • Additives should be inert. • Used by the producer of the base resins. Examples. Stabilization of impact resistant PS and pre stabilasation of PVC produced by suspention polymerisation. (in both cases antioxidants are added to the monomers)

Addition of additives during the working up of the polymer • Used by polymer manufacturer. • Additives used must satisfy specific requirements determined by the process such as emulsifiability , solubility in certain solvents , low volatility or stability to hydrolysis. • Polymer must be in dispersed form. Example- In modification of styrene polymer, such as ABS.

Methods of Compounding n. There are four basic methods – Dry mixing, – Batch mixing, – Continuous mixing, – Compounding by Screw extruder. The selection of the method determined by –Condition of the material, –The volume of end product required, –The sensitivity of the material to breakdown during shear.

Dry Mixing Dry mixing the plastic with its additives in a 500 pound capacity paddle mixer. n The paddles are turned at 30 to 90 seconds or until the mix is homogeneous. n If the mix is too long, it will develop lumps of glass, while too short a mix will produce an unequal dispersion of glass fibres. n Dry mixer are preferred for powders, plastic pellets with fibers and also for mixing plasticizers with PVC. n

Types of Dry mixers n There are four types of dry mixers: – High speed impellers, – Ribbon mixers, – Paddle mixers and – Drum tumblers.

Dry mixer n n The mixer consists of an enclosed container with a higher speed impeller mounted at the bottom. An impeller is similar to the rotating blades of a helicopter. The impeller is capable of 80 to 3600 revolutions per minute (RPM). Mixing is fast with materials blended within 2 -4 minute cycles. Heat generated during the mixing process must be drawn off through a cooling jacket to retard decomposition of the material and to block the development of lumps.

drum tumbler n n n Drum tumblers are popular with molding operation for the dispersion of powdered colorants with the plastic pellets. In a drum tumbler, plastic pellets (with approximately 1 to 2% colorant) are placed in a 250 gallon drum and tumbled dry for approximately 30 minutes. Drum tumblers are used when the additives disperse easily throughout the mixture without tending to lump or cluster together.

High Speed Mixture

Continue…… n n n High speed mixture is widely used for PVC dry blending and for the dispersion of colorants, pigments etc…. And also it is using for mixing polymer powder with pellets. HSM is made of impellers mounted in the bottom of stationary vessel. Rotational speed of impellers is upto-1000 rpm. The motors are using for rotating impellers.

Two Roll Mill

Continue…… n n n Two roll mills are used mostly for mixing materials with coarse particles. In roll mills, the size reduction takes place between two rolls can have either a smooth or a profiled surface. It is reducing the particles size from 1 mm˃ to ˃ 0. 010 mm. Rolls are rotating as counter-rotating only. Toll roll mill are mainly using in calendering process of PVC sheet.

Banbury mixers :

Banbury mixers : q. The Banbury mixer is a brand of internal batch mixer, named for inventor Fernley H. Banbury. q. Banbury batch mixer consists of cylindrical chamber or shell with in which materials to be mixed are deformed by rotating blades or rotors. q. The blade motion causes the mixture to be shear between blade and tip and shell and creating shuffling of material Cycle time 2 -4 min Size 50 -75 kg Producing large shapeless lumps. q. Internal batch mixers such as the Banbury mixer are used for mixing or compounding rubber and plastics. Its invention resulted in major labor and capital savings in the tire industry. q. It is also used for reinforcing fillers in a resin system.

Ribbon Blender

Continue…… n n n Ribbon Blender is useful for convective mixing. In most of convective mixers, a mixing rotor within a static housing, moving groups of particles from one region to another. Ribbon mixtures are available both as batch mixture as well as continuous mixture. The rotational speed in ribbon blender is typically in the range of 10 to 60 rpm. Many different ribbon geometries are available.

Planetary Mixture

Planetary Mixture n n The mixing blade is set off center and is carried on a rotating arm. It therefore travels round the circumference of the mixing bowl while simultaneously rotating around its own axis. This is therefore a double rotation similar to that of a spinning planet rotating around the sun, hence the name. Planetary Mixers are used for dry mixing and dough mixing. The homogenous mixing if various lngredients of different densities in different proportions for solid/solid, liquid/solid, and liquld/Iiquid is achieved effectively by the use of Planetary Mixer.

Continue… n n Planetary Mixers for the preparation of ointment, toothpaste, creams. The blades each rotate on their own axes, and at the same time on a common axis, thereby providing complete mixing in a very short timeframe.

Extruders n n Screw extruders are the process tool used in compounding, injection molding, injection blow molding, extrusion, and extrusion blow molding. Screw extrusion technology is used as the plastic mixing and melting device. GF are efficiently compounded with plastic using screw extruders. Two Types – Single Screw Extruder – Twin Screw Extruder

A single screw extruders n n It classified by the ratio of the length of the screw to the inside diameter of the screw. Standard extruders have 30 to 1 ratios. The size of extruders range from small laboratory models with 1/2 inch diameters to large production extruders with 8" diameter barrels. n Consist of long steel screw which rotates steel tube called barrel n Extruder outputs range from a few pounds an hour up to 1, 000 pounds per hour.

Continue… Barrel, is wrapped with heating elements to maintain the barrel at the melt temperature of the plastic. n The plastic is pumped from one end of the screw to the other by rotating the screw. n The rate at which the plastic moves from one end of the extruder to the other is controlled by the speed of the screw. n

Zones of Extruder & its Functions: Feed Zone - Transport the material from hopper to compression zone. Compression Zone - Compacts, eliminates air gap - Transport the material from compression to metering zone. Softens the material Metering zone - Melts, material Mixes, the pressurizes and pumps the melt.

Twin Screw Extruder n n n The efficiency of the mixing action of the single screw extruder compounder depends on maintaining higher friction between the wall of the barrel and the material than between the materials and the screw surface. If the friction is higher on the screw surface, plastic will stick to the screw surface, decompose and clog the mixing process. In a single screw, extruder's plastic pastes, flakes. Sometimes low bulk density materials tend to stick to the screw surface. To overcome these restrictions, the twin screw extruder was developed. The twin screw extruder has two screws that are arranged side by side and intermesh. The intermeshing action of the two screws constantly self-wipes the screw flights.

Continue n It is impossible for plastics to stick to the screw surface. The positive pumping action of the intermeshing screws allows for the compounding of all forms of plastic materials not possible with a single two stage extruder.

Continue… There are two type of rotating 1. Co-rotating. 2. Counter –rotating. Co-rotating: When two screws rotates along same side in a barrel then the phenomena is called co-rotating

Continue… Counter-Rotating: When the two screws rotates the different side in a barrel, then the phenomena is called counter-rotating.

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