INSIGHT & OUTLOOK: Compounding process

80 Modern Plastics & Polymers | February 2012

Ashish Kothavade

Polymer products are rarely

pure materials. Often these

are mixtures or compounds of

different polymers. Additives

are routinely added to modify or

improve polymer properties. Mixing (or

compounding) is a vital step in polymer

processing, as mechanical, physical

and chemical properties

& appearance of the final

product depend strongly

on attaining a uniform

composition. It is important to

review the steps involved and

useful fundamentals to avoid

problems in the compounding

process. A compounding

facility comprises preliminary

or up-stream processing,

compounding in twin-screw

extruder and downstream

processing, finishing and

packaging of products.

Pre-treatment Any anomaly in the

temperature, pressure and

moisture conditions in the extruder

can depolymerise the polymer during

processing. Drying is important while

processing certain polymers, especially

those produced by polycondensation.

Coating of fillers such as calcium carbonate

or titanium dioxide may be done by the

supplier or at the compounding facility.

The two filler materials and carbon black

agglomerate during mixing & coating

and help reduce this effect.

Pre-mixing Dry mixing with solid and liquid

ingredients with low-, medium- or high-

intensity mixers is called blending or

pre-mixing. Generally, the polymer does

not melt during this process, but wetting

or blinding agents such as waxes can melt.

The knowledge of sequence of adding

certain organic colourants is important

to avoid re-agglomeration. Pre-mixing of

selected ingredients with some quantity

of resin is always required.

Principle of extruder mixing Rotation of screw, friction at barrel wall

and forward movement of the screw

produce a three-way internal mixing of

pellets & additives. The mixing process

takes place as the friction between the

hot wall of the barrel and plastics melts

the outside of the plastic pellet, rotates

the screw rolls and tumbles the mixture

into the centre of the space between

screw flights. The leading edges of the

screw flight scrape the melted plastics

off the wall of the barrel and collect it

into an expanding pool. Mixing is of two

types – distributive and dispersive.

Distributive mixing: It involves

stretching, dividing and reorienting the

flow of the polymer melt compound

to eliminate local variations

in material distribution and

produce a more homogeneous

mixture. The mixer must

impose high strain on the

material, while splitting and

reorienting the flow.

Dispersive mixing: It

involves generating high

stresses in the material to

break down dispersed particles.

These particles may be

insoluble fillers (composites)

or a second polymer melt

(blend). These force the

material to flow over barriers

that form narrow clearances

between mixing elements.

Mixing equipmentBatch mixers: These are versatile units,

as operating conditions and the time

at which additives are incorporated

can be varied during a cycle to achieve

optimum mixing. High-intensity batch

mixers are used to process high-viscosity

plastics. Batch mixers are used to feed

calendering process. The calendering

process consists of large multiple rolls

flattened into sheets.

Imparting vigour to polymersPolymer blends and alloys, produced by mixing two or more polymers to generate a material with a broader range of properties, are finding increased applications today. Additives are incorporated into polymers before the final shaping operation. Here is a review of the fundamentals, the steps involved and equipment used for compounding of plastic materials to avoid problems in future.

Courtesy: OPTI

Courtesy: Madhu Plastic Industries

82 Modern Plastics & Polymers | February 2012

Two-roll mills: These have a parallel

pair of counter-rotating, heated metal

rolls that turn at a slightly different rate

(roll ratio) and provide an adjustable gap

between them. The shear stress generated

in the gap is substantial and facilitates

further compound mixing while shaping

the compound into a sheet.

Internal mixers: These high-intensity

mixers work well in dispersion of solid

particle agglomerates. This process

depends on the mixing time, rotor speed,

temperature and rotor blade geometry.

Continuous mixers: Here, the material

is continuously fed into a hopper end

while it is mixed and transported by an

intermeshing irregular screw device to

the opposite end. Mixing occurs between

the rotating screws and chamber wall, as

the material is kneaded between the two

screws and scraped off the wall surface.

The amount and quality of material

is controlled by adjusting the rotating

speed of screws and amount of material

leaving the chamber.

Devices used in continuous mixingSingle-screw extruder: It consists of a

long auger screw that fills the opening

of a steel tube. Plastics and additives are

placed at one end of the screw, mixed

& melted when the screw turns. The

size of extruders ranges from small

laboratory models (diameter, 0.5 inch) to

large production extruders with 8-inch

diameter barrels. With increase in the

size of extruders, the ability to produce

mixed plastic materials also increases.

The extruder used for compounding

is enclosed in a steel tube with two small

openings at both ends. One opening on

top allows entry of plastics and additives.

The second opening at the opposite end

shapes the melted and mixed plastics

into rods. A standard extruder screw

is divided into three sections – mixing,

compression & metering – which are

identified by changes in root diameter

of the screw.

a) Mixing section - The plastic material

and additives are introduced into

the mixing section at one end of the

screw. Here, the screw root diameter

remains constant. The root of a screw

is the centre rod that screw flights

are attached to and wrapped around.

The root diameter increases in size

from the mixing section towards

the metering section on an extruder

screw. As the screw rotates, plastics

and additives soften and get mixed.

The small constant root diameter in

the mixing section mixes plastics and

additives without compressing them.

b) Compression section - The screw

root diameter increases here. This

increase in screw root diameter and

the rotating screw pushes the plastic

pellets & additives against the wall

of the barrel & screw flights. This

intensifies the shearing and mixing

action of the screw and completes the

melting of the plastic material.

c) Metering section - Here, the root

diameter is the largest and remains

constant. The mixing action intensifies

and encourages homogenisation of

the plastic melt with its additives.

When the plastic material leaves the

extruder, it emerges with physical

and thermal homogeneity, ready for

shaping by a strand die.

Two-stage compounding extruders:

The high shear rates induced by the

extruder’s compressive section and

abrasive properties of certain additives

require that a compounding extruder

screw be constructed in two stages

with six sections. The two-stage screw

is similar to two single-stage screws

attached end to end.

The first stage allows for initial low-

intensity melting of the plastic material

with the additives. The heat from the

barrel and shearing action of the screw

converts moisture into steam. Between

the two stages, the root diameter of the

screw becomes narrow in the mixing

section. The mixing pressure is zero in the

narrow section between the two stages.

Moisture and gas molecules generated

in the first stage is drawn off by a

vacuum centre vent. The second stage

of the compounder extruder provides a

second opportunity for introduction of

shear-sensitive additives. At the end of

the final metering section, a one-inch-

thick steel screen, called breaker plate,

restricts the plastics flow and provides

back pressure on the mixing action. The

size of the screen determines the amount

of back pressure and contributes to the

intensity of the mixing action.

Twin-screw extruders: The efficiency

of the mixing action of the single-

screw extruder compounder depends

on high friction between the barrel

wall and material. If friction is high

on screw surface, plastic melt will stick

to it, decompose and clog the mixing

process. The twin-screw extruder was

developed to overcome this problem. It

has two screws arranged side by side and

intermesh. The intermeshing action of

the two screws constantly self-wipes the

screw flights. The cross-section of the

barrel has a shape of figure eight. The

two screws rotate forcing the materials to

form a figure eight pattern. The positive

pumping action of the intermeshing

screws allows compounding of all forms

of plastic materials.

Knowledge matters The compounding process results in the

chemical union of the ingredients. Thus,

adequate understanding of the mixing and

melting capabilities of screw extruders and

intermediate processes is crucial for better

grasp of the compounding technology.

Ashish Kothawade is

Senior Manager-Business

Development at

Plastiblends India Pvt Ltd.

Email: ashish.kothavade@

kolsitegroup.com

Mixing action of single-screw extruder compounder

Co-rotating Counter-rotating

Non-intermeshing

Counter-rotating

Intermeshing

Compounding process