Compounding Processes

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Blending

Stirring together/blending of a number of solids, e.g.polypropylene

powder, pigment, antioxidant, etc.

The results is a mixture of powders; the indiv idual powder remain and

can be separated (in principle)

Compounding

Involves more intimate dispersion of the additives into the polymeric

matrix

It requires;

A physical change in the component High shear force to bring about the change

The polymer to be in the molted or rubbery state during mixing

Some Processes and Machine (Blending)

Vary from the simplest to sophisticated high speed

machine

The simplest- is to tumble together dry ingredients, e.g.

using a twin-drum tumbler

Twin-drum tumbler

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Ribbon-blender

A tumbling action takes place

The chamber is stationary and the ribbons rotate

constantly scooping the material from the outside to

the centre

Some Processes and Machine (Blending)

Ribbon blender

High speed mixer

More sophisticated & rapid machine for blending

Widely used for PVC dry blends, drying, incorporated pigments,

antioxidant, etc.

Run at several thousand rpm, and form a circulating powders

which becomes heated by friction (150-200C)

Mixing tank can be single wall or jacketted for temp. control

Some Processes and Machine (Blending)

High Speed MixerMixing tank

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Dough Moulding Compound

Dough-like blend, e.g. DMC are made in Z-blade (The name

comes from the shape of the mixing blade)

The mixer itself comprises twin stainless steel bowls in each of

which is a mixing blade. The blades rotate in opposite directions

(and rarely at different speeds).

Some Processes and Machine (Blending)

Ball Mill Comprises of cylindrical vessel containing large number of steel or

ceramic balls

It rotates, the balls tumble inside together with the powder

Agglomerates of powder are broken down by the grinding action of

the tumbling balls

Some Processes and Machine (Blending)

Ball Mill

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Aim of Mixing

Reduce composition non-uniformity

Increase the probability of finding volume element of the minor at any

point in the blend (acceptable probability distribution)

Decrease the size of the domain

Break agglomerated particles

The mixing of two or more fluids or solid particles in a fluid consist of two

process, dispersive (intensive) mixing and distributive mixing.

Mixing and Composites Chee 490 18.10

Mixing

In almost all cases, good

distribution AND good

dispersion are required.

Distributive mixing can be

achieved by providing

convoluted flow paths that split

and reorient the flow repeatedly

Dispersive mixing can be

achieved by passing the

mixture through small regions

of intense deformation.

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Polimer mixing

Dispersive (Intensive) Mixing

Process whereby the solid particles or the minor

fluid phase are broken into smaller sizes.

Distributive Mixing

Process wheres the broken up domains distributed

into space and homogenized the the mixture.

mixing

increases only the

degree of

of the dispersed phase

mixing

increases the degree of

and

decreases of the

dispersed phase by

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Mixing and Composites Chee 490 18.14

Dispersive MixingDispersive mixing involves generating high stresses (shear andelongational) in the material to break down dispersed particles.These particles may be insoluble fillers (composites) or a second

polymer melt (blend).

Dispersive mixers force the material to flow over barriers that

form narrow clearances between mixing elements (i.e. between

extruder wall and a screw element).

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Dispersive Mixing

Concern in this process:

Droplet breakup

Happen by Interfacial tension, which keeps droplets whole.

Solid agglomerates breakup

Happen by Internal force, which makes solid particles form an

aggregates.

Particulates are used as fillers, color additives, modifiers and reinforcing

agent in polymers.

Because of electrostatic force etc. between primary particles of the

particulates, they form an agglomerates.

To get homogeneous mixture, its necessary to get the agglomerates be

broken, to their primary particles if possible

Dispersive Mixing

Break these agglomerates by using stresses generated by the flowing

fluid component.

Consider that force used to break the agglomerates = result of the

flow of the surrounding f luid.

Assume that the particles will break away as soon as the force

exceed the internal force (electrostatic, etc.)

Then assuming that the particle do not disturb the fluid flow field, the

flow field is homogeneous everywhere , making the deformation is the

same everywhere.

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Forces in Mixing

How the force transmitted, to break down agglomerates of additive

particles? By fluid mechanical stress in the mixer

Less energy is needed

Under high viscosity conditi

To achieve good dispersion

WHY?

Routes for Mixing

Route 1: with well distributed

but poorly dispersed additive,

will entail lower viscosity than

route 2.

Previous equation suggest

that Route 1 will require

more energy than Route 2

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Why distributive mixing is difficult to achieve in melts?

For example low viscosity system exhibits turbulent mixing

The boundry between laminar flow and turbulent flow is described by

Reynolds equation

V= velocity of the fluid, = density, = viscosity, D = diameter of circular

channel

Renolds Number must exceed 2000 for turbulence flow

Based on Reynolds equation, three viscosity regimes are seen to be;

At low viscosity, turbulence results in efficient distribution

At high viscosity (as found in polymer melts), turbulence cannot occur &dispersion is poor

At very high viscosity (as in rubber), there is sufficient shear to break downagglomerates & efficient distribution and dispersion can occur.

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Consider a channel where,

D = 0.5 cm = 0.005 m

= 150 Pas

= 1000 kg m-3

Q = 250 cm3 s-1 (Q is the amount of materials put

through a process/volume throughput)

Find velocity from the volume throughput

Low value of Re indicates that turbulent flow

cannot occur in polymer melts.

Distributive Mixing

Not concerned with interfacial energy, Particle breakup or domain

size reduction.

Not concerned with level of stress.

Concerned with rapidity and uniformity.

Application:

Miscible system (no interfacial tension)

Immiscible systems during initial stages (large domains) or afterdomain breakup

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Distributive Mixing

Distributive mixing involves stretching, dividing and

reorientating the flow of the polymer melt compound in order toeliminate local variations in material distribution and produce a

more homogeneous mixture.

Distributive mixers must impose high strain on the materialtogether with splitting and reorienting the flow.

Achieved by introducing barriers with large clearances thatgenerate a long, convoluted flow path.

Mixing and Composites Chee 490 18.24

Distributive MixingDistributive mixing involves stretching, dividing and reorienting

the flow of the polymer melt compound in order to eliminate local

variations in material distribution and produce a more

homogeneous mixture.

Distributive mixers must impose high strain on the materialtogether with splitting and reorienting the flow.

Achieved by introducing barriers with large clearances that

generate a long, convoluted flow path.

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Mixing and Composites Chee 490 18.25

Mixing Devices

Mixers can be classified into two categories:

Batch mixers: They are the oldest type of mixing equipment and are

still widely used. They are versatile units, because operating

conditions and the time at which the additives are incorporated can

be varied during a cycle to achieve optimum mixing.

Continuous mixers: They include single and twin-screw extruders,

and modified variants. Continuous mixing offers higher throughputs,

reduced manpower requirements and greater product uniformity,

leading to easier quality control. Disadvantages include less

processing flexibility, requirement for expensive feeding equipmentand generally lower dispersive mixing quality

Mixing and Composites Chee 490 18.26

A two-roll mill consists of a parallel pair of counter-rotating, heated metalrolls that turn at a slightly different rate (roll ratio), and provide an

adjustable gap (or nip) between them.

The shear stresses generated in

the gap are substantial, and provide

further compound mixing while shaping

the compound into a sheet.

Temperature control is more easily affected in a mill, when compared to a

Banbury-type mixer that completely encloses the compound.

Batch Mixing Devices

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Mixing and Composites Chee 490 18.27

Batch Mixing Devices

Internal mixers are high intensity mixers that work especially well in the

dispersion of solid particle agglomerates.

The dispersion of agglomerates

depends strongly on mixing time,

rotor speed, temperature and rotor

blade geometry.

The tangential rotor

design, or Banbury is

shown here, but several

variations exist, includingthe interlocking rotor

design known as

the Intermix type.

Mixing and Composites Chee 490 18.28

Continuous Mixing DevicesSingle Screw Extruders:

The SSE is a relatively poor mixing

device, and specialized screw

sections are needed to improve the

mixing efficiency of this apparatus.

Distributive mixing can be improved using static mixers or mixing heads

such as the pineapple configuration shown here.

Pineapple Mixing SectionKenics Static Mixer

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Mixing and Composites Chee 490 18.29

Continuous Mixing Devices

Dispersive mixing is more difficult, as it requires that high shear

stresses be generated. There are numerous designs for mixingsections, which can improve dispersive mixing (Maddock or Union

Carbide mixing section, Egan section).

These devices force the melt to flow over barriers, which form narrow

clearances between the screw and the barrel, thus improving

dispersive mixing.

Extrusion :Reciprocating

Control suhu mixingyang akurat

Shear stress secararelatif lebih rendah dariTwin Screw

Head pressure lebihrendah dibanding TwinScrew

Untuk CompoundFoam.

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Mixing and Composites Chee 490 18.31

Continuous Mixing Devices

Twin Screw Extruders:

TSEs have two screws within the barrel, and can be co-rotating orcounter-rotating, intermeshing or non-intermeshing. They are favoured for

Mixing and compounding

Chemical reactions / reactive extrusion

Devolatilization

Twin Screw Extruder

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Komponen screw

Extrusion: Continous Mixer

High Ouput

High Shear

Untuk mendispersikan

filer dengan ukuran

partikel mikroskopis

dan low aspek rasio

seperti carbon black,

pigment TiO2

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Mixing and Composites Chee 490 18.35

Continuous Mixing Devices

Mixing and Composites Chee 490 18.36

Continuous Mixing DevicesTwin-screw extruders use modular screws that are assembled from

screw segments fitted onto a common shaft. Conveying segments can

provide pumping, mixing and devolatilization, while non-conveying

elements such as discs and rotors are designed for kneading and mixing.

Thus, screws are easily optimized for a wide range of applications,

making twin-screw extruders extremely versatile.

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Dough Moulding Compound

The powders are charged first andblended for about a minute.

The resin (containing catalyst) andshrinkage control additive areadded and mixing is continued forbetween 15 and 30 minutes.

During this time the mixer isstopped occasionally and thewalls of the bowl scraped clean.

Once the slurry is homogeneousthe reinforcing fibres are added

gradually. These are blended intothe slurry until they are justdistributed throughout the mix - atime of about 5 minutes, typically.

Compone

nt

PHR Note

Res in 100 High reactivity resin having a maleic : phthalic

ratio around 2 : 1 and an alkyd : styrene ratio ofabout 70 : 30 by weight. The resin viscosity is inthe order of 10Pas.

Shrinkage

ControlAdditive

40-65 Also called a low profile additive. The material is

a solution of a thermoplastic polymer such aspolystyrene, polymethyl methacrylate, polyvinyl

acetate or polycaprolactone in styrene. Thesolution contains about 30% polymer by weight.

Used at ratios of 40:60 or 30:70 by weight on thehost resin.

Fillers 150-25

0

General purpose fillers include calcites and

dolomites. Eff ects fillers include aluminatrihydrate. The mean particle size of all fillersused in DMC is in the range 5mm-20mm with anoil absorption between 20-30. Exceptionally

larger fillers or fillers having higher oil absorptions

(up to 45) are used but at relatively lower levels.

Lubricant 3-5 Zinc Stearate, Calc ium Stearate

Catalyst 1-2 A relatively stable peroxide having a half-li fe of a

few minutes in the temperature range 100oC -130oC is used. Commonest materials are t-butylperbenzoate, peroctoate or dicumyl peroxide.Combinations of peroxides are used to fine-tune

the rate of crosslinking.

Pigment 5-10

Mixing and Composites Chee 490 18.38

Polyethylene-Nanosilica Composites

Transmission electron microscopy (TEM) images of

13 wt% SiO2/PE-g-PA 5 wt% SiO2/HDPE

(scale bars correspond to 200 nanometres).

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Mixing and Composites Chee 490 18.39

Polyethylene-Nanoclay Composites

Transmission electron microscopy (TEM) images of

40 wt% NR4+-MM/PE-g-PA 5 wt% NR4+-MM/HDPE

(scale bars correspond to 200 nanometres).

Mixing and Composites Chee 490 18.40

Polyolefin Blends - Nanocomposites

Idea to locate fillers preferentially in

one phase, to selectively reinforce it.

This can be accomplished by adding

functional groups having better

affinity with the filler, so that it will

migrate and exfoliate to thepreferred phase.

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Involve high shear process & much more powerful machinery

The simplest technique is two-roll mill

Some Processes and Machine (Compounding)

Two-roll millPair of rollers with a vertical nips between them

The polymer and additives are subjected to high shear in

the nip as the rolls rotate in opposite directions

Two-roll mill mixing started with rubber processing, now

exist for various function

Mixing on two-roll mill is time consuming, 2 h for a 200 kg

mix on a 84 wide mill, and depends on the skill of mill

operator

Schematic illustration of two-roll mill

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Banbury Mixer

2 rotors-counter-rotating within a chamber

Each has two or four blades which mix by smearing the

materials against the chamber wall

A weighted ram keeps the mix in place inside the chamber

Banbury Mixer vs. Two-roll Mill

The rate of output (200 kg batch of rubber

compound would take 2 h- two-roll mill. A

number 11 Banbury mixer produce 350 kg in

15 min or less)