investigation of the plastification behavior of polymers in high-speed twin screw extruders

5/23/2018 INVESTIGATION OF THE PLASTIFICATION BEHAVIOR OF POLYMERS IN HIGH-SPEED TWIN SCREW E...

http:///reader/full/investigation-of-the-plastification-behavior-of-polymers-inProceedings o f the Polymer Processing Society 28th Annual Meeting ~ PPS-28 ~ December 11-15, 2012, Pattaya (Thailand)

INVESTIGATION OF THE PLASTIFICATION BEHAVIOR OF POLYMERS

IN HIGH-SPEED TWIN SCREW EXTRUDERS

G. Spohr1, A. Knieper

2, C. Beinert

3*

1Mechanical and Process Engineering, Technical University Darmstadt, Germany

[email protected] Polymer Institute (DPI), PO Box 902, 5600 AX Eindhoven, the Netherlands

[email protected] Processing, Div. Plastics, Fraunhofer-Institute LBF, Germany [email protected];

Abstract - A test stand to observe movement and deformation of thermoplastic polymers in a cross section of the

plastification zone of a co-rotating twin screw extruder is presented. Multiple experiments to examine the initial melting

processes in dependence of different process parameters such as rotational speed, temperature, kneading diskconfiguration, polymeric material, granule size and filling degree were conducted. The focus thereby was the

visualization of underlying effects by high-speed imaging. This allowed gaining insights into the basic energy

dissipation mechanisms and the most significant process parameters for the initial melting process.

Keywords: Twin-screw extruder, plastic energy dissipation, initial melting, visualization

Introduction

Research Objective

Since up to 80% of the energy input in a co-TSE

extruder takes place in the plastification zone [1], the

optimization of the plastification process offers thehighest potential for an increase of the economic

efficiency by energy savings, improvement of material

properties and increase of throughput.

The objective of this work is therefore studying the

process of initial melting by experimental

investigations in the plastification zone of a high-speedco-TSE. Here the responsible mechanisms for energy

input into the polymer are friction, heat transfer, plastic

and viscous deformation [1]. The mechanisms are

thereby coupled and change throughout the

plastification zone. They are also dependent onto

multiple parameters such as rotational speed,

temperature, kneading disk configuration, material

properties, granule geometry, filling degree, local

pressure, residence time, etc. The differentiation of the

four heating mechanisms is therefore difficult.

In order to have a basis for the development of newmelting models which are suitable for high-speed co-

TSE a visualization of the plastification zone wasconducted. Because the effects especially in a cross

section of the plastification zone have never been

observed, a test-stand that allowed looking at the side

faces of two kneading blocks was designed and built.High-speed imaging of deformation, movement and

melting of the granules in the cross section of the

plastification zone led to further insights into

underlying effects.

State of the art

First models to describe plastification in extruders werebased on models for single screw extruders [2], [3], [4].

The models are based on the assumption of continuous

solid melting (CSM) which postulates a solid bed ofgranules surrounded by a pool of melt. These models

are based on a continuous channel for material flow at

low rpm. Thus they do not describe the melting

processes adequately in twin screw extruders.

Therefore dispersed solid melting models (DSM) were

developed [5], [6], [7]. They assume that granules are

dispersed in melt. The melting of solids is therebyrelated to the dissipation of energy in the melt and

conduction of energy from melt to solids. These

models describe the melting process in twin screw

extruders much better and are nowadays mainly used in

commercial extruder software.

The main problem with the DSM models is that theinitial melting is neglected in all these models. They

are only valid for granules surrounded by a polymer

melt matrix. Since at least 25% (for spherical

geometries) of the material must be melted for granules

to be embedded in melt, all energy dissipation

mechanisms up to this point are neglected. But because

plastic energy dissipation is the primary energy

dissipation mechanism for initial melting it plays a

significant role. It cannot be neglected for complete

modeling of the melting process. Thought examination

of plastic energy dissipation in polymers was alreadyconducted by [8], [9], [10], [11], [12], [13], melting

models still lack accuracy. In order to expand theknowledge of the underlying mechanisms and process

parameters, experimental investigations on co-TSE

extruders are necessary. This will allow generating

melting models that integrate the initial melting.

Experimental

Experimental Setup

For the experiments a Coperion ZSK32 high-speed co-

TSE was used. The overall length of the extruder was

reduced to an L/D ratio of 21. The shafts protruded theextruder for approximately 70mm. The partial length of

shaft inside the extruder was equipped with conveying

elements; onto the screw tip kneading disks weremounted. This allowed positioning the first

O-05-166



compression zone and thus the plastification zone in

our test stand as seen in Fig. 1 and Fig. 3.

Figure 1Screw and extruder configuration

The test stand allowed positioning of a window 1/10th

of a millimeter away from the side faces of the lastkneading disks and thus allowed observation of the

processes in the plastification zone.

Design of the Test Stand

The test stand (Fig. 2) consists out of three different

parts:

The first part is the "granule removal plate" (green). Itis directly mounted onto the endplate (blue) of the

extruder. It is necessary for running the extruder

continuously, without filling of the "plastification

zone". An opening allows exiting of granules out of theextruder. By inserting the slider (grey) into the test

stand the experiment is started.The second part, the "plastification zone plate"

(orange), allows positioning the last kneading disk pair

in plane with the raise (in the middle). Also sliding

rails (red) were welded onto the "plastification zone"

The third part is the "sledge", in which a window is

mounted. The sledge (and thus the window) slides on

the rails (red). With screws (blue) the pressure of the

window onto the raise on the "plastification zone plate"

can be adjusted. This is done by tightening or

loosening springs (not shown) which are mounted onto

the screws (blue). At a predefined pressure inside theplastification zone the sledge thus moves away from

the "plastification zone plate", allowing granules to exit

the extruder at the end. This prevents brakeage of the

window. For window material a 10mm polycarbonate

plate was used. It was cut to fit into the sledge. Two

holes were milled into the windows to allow the shaftsto protrude the window. The windows were designed

as wear and tear parts.

Figure 2Cad model of test stand

This setup made it possible to observe movement,

deformation and melting of granules in a cross section

of the plastification zone. The experiments were

recorded by high-speed imaging with 3000 fps. This

allowed recording of around 5 seconds. In this

timespan the observation of the kneading disks fromcompletely empty to completely filled and partly

melted was possible. A window thereby inhibited any

movement of granules out of the extruder until apredefined pressure was exceeded. It therefore acted as

"back"-conveying elements.

Figure 3Test setup (left) and schematic of window

position at the screw tip (right)A parametric study of the initial melting with variation

of process parameters such as rotational speed,

temperature, kneading disk configuration, polymeric

material, granule size and filling degree inside the

melting zone was conducted.

Results and Discussion

Plastic Energy Dissipation in the Intermeshing Zone

It was shown that the energy required for melting is

primarily dissipated during one heavy, plastic

deformation in the intermeshing zone. Granules get

trapped between the active flanks and the upper or

lower wedge, the intermeshing zone. The granules are

thereby compressed into the void volume (~ 40% of

bulk volume) and the compressed bulk is deformed

(Fig. 4).

Both mechanisms lead to plastic deformation and thusplastic energy dissipation in approximately 0.005

seconds (@ 1200rpm). High strain rates of the material

occur. Furthermore a hydrostatic stress is existent

because granules are enclosed on all sides while

compression and deformation take place.

Figure 4Compression of polypropylene granules in

the intermeshing zone

Compression in Front of Active Flank

Compression in front of the active flank (Fig. 5) can be

observed during the experiments. Friction of granules

on the cylinder wall compresses material in front of theactive flank. Energy dissipation due to friction on the

extruder wall was thereby not observed to heat material

significantly. Heating only occurs due to "compression

into the void volume".

Figure 5Compression of polyamide granules in front

of the active flank

Influence of Granule Size

Granule size has the biggest influence onto heating andmelting of polymer material. Small granules thereby

heat up slower than large granules. Reason for this is



that small granules show an increased tendency to

evade compression and deformation by flowing out ofthe intermeshing zone (Fig. 6).

Figure 6

Small granules at low filling degreesLarge granules by contrast tend to get caught betweenthe kneading disks and the wedge even at low filling

degrees (Fig. 7). Granules that get caught thereby stop

any further movement of other granules through theintermeshing zone. The amount of material that is

caught in the intermeshing zone is thus higher.

Therefore the material has to undergo heavier

deformation and heats up much faster.

Figure 7Large granules at low filling degrees

This dependence of plastic energy dissipation ontogranule geometry is a significant scale-up problem for

initial melting. A melting process that is optimized for

one extruder size, may not work for another.

Influence of Filling Degree / Pressure

The second important influence onto melting behavior

of granules is the filling degree inside the plastification

zone. It is observable that flow of (small) granules

through the intermeshing zone decreases withincreasing filling degree: Because all space is already

filled by material, granules cannot evade compression

and deformation. Additionally the melting degree

increase with increasing amount of material used for

experimentation (Fig. 8). While at low filling degreesand small granule sizes no plastic energy dissipation is

present, it increases rapidly at highest filling degrees.

Figure 8Small granules at highest filling degrees

Only 10 to 14 compression and deformation cycles in a

fully filled plastification zone are then needed to

dissipate enough energy to partially melt all granules.This equals 5 to 7 rotations.

With increasing amount of material used during

experimentation the pressure inside the plastification

zone as well as the melting degree increased. Ittherefore seems reasonable that the hydrostatic stress,

which increases the compressive strength, is relevant

for increasing the amount of dissipated energy.

Influence of Rotational Speed on Granules

Experiments with two different rotational speeds (120

rpm & 1200rpm) were conducted. No dependence of

plastic and frictional energy dissipation onto therotational speed could be observed for granules. So

although the compressive strength increases at high

strain rates, no significant change of the amount of

energy that was dissipated per rotation resulted.

The experiments were also conducted with white PA12

powder (diameter: 80-120m) mixed with sootcoagulates. Here a significant change of effects in

dependence of the rotational speeds took place:

At lower rpm (120rpm) the soot coagulates were not

destroyed. No significant amount of plastic or frictionalenergy was dissipated (Fig. 9).

Figure 9Polyamide Powder mixed with soot

coagulates at 120rpm

At 1200rpm however some coagulates were already

ground in the conveying section (Fig. 10). High

relative speeds of the screws lead to frictional

destruction. In the plastification zone itself the powder

turned black within only few rotations and the powder

melted together. Plastic energy dissipation was not

observed and is known to decrease with decreasing

particle size. Therefore only friction remains as main

energy dissipation mechanism.

Figure 10Polyamide Powder mixed with soot

coagulates at 1200rpm

Melting

It can be observed that the bulk of granules does not

heat up evenly; but that a heated (transparent) partialquantity of granules is transported in front of the active

flank of the kneading disk (similar to the continuous

melting model of [4]). The energy is only dissipated

during the compression and deformation cycles in front

of the active flanks. The partial quantity of heated

granules grows with each rotation until all material is

transparent (Fig. 11).

Figure 11Melting inside a fully filled kneading zonewith small PP granules

Melting of the bulk of granules starts in front of the

active flanks (highlighted in Fig. 11 on the right).

During around 10 to 14 compression / deformationcycles (around 5-7 rotations) enough energy is

dissipated for melting.



Furthermore it is observable that, the granules are

embedded in a molten polymer matrix. In contrast tothe side faces of the kneading disks one can

distinctively see granules and melt (Fig. 12).

Figure 12Melting behavior of small PP granules at1200rpm and heated extruder wall (120C)

The partially melted mass thereby behaves at first like

granular material. The bulk of granules is pushed in

front of the active flank, it does not flow around the

kneading disks.

After few further rotations the material starts to flowaround the kneading disks.

Due to plastic and viscous energy dissipation the

melting degree increases: The viscosity of the melt-

granules mixture decreases and the mixture starts to

behave like a liquid. The bulk (granular material) does

not exist anymore. From this point on the meltingbehavior stands in very good comparison with the

dispersed melting models (DSM).

Because material can flow around the kneading disks,

material can also flow out of the intermeshing zoneduring the compression cycles. This results in receding

plastic energy dissipation. Therefore all further energy

can only be dissipated by viscous energy dissipation in

the melt or by heat transfer from heated extruder wall.

Conclusions

It was shown that most energy is dissipated by plastic

energy dissipation during one heavy deformation in the

intermeshing zone. Here granules get trapped between

the wedge and the two kneading disks and have to

undergo compression as well as deformation. It was

confirmed that plastic energy dissipation is the

dominant mechanism for initial melting. The otherenergy dissipation mechanisms (friction, heat transfer)

play only a secondary role for initial melting; viscous

energy dissipation can only occur when melt is present.

The rate of energy dissipation was thereby primarily

dependent from the granule size. Large granules were

compressed and deformed heavily and thus heated

quickly, whereas small granules evaded compression

and deformation by flowing out of the intermeshingzone. Significant heating of small granules occurred

thereby only at highest filling degrees, which lead to

higher hydrostatic stresses.

The rotational speed of the extruder did not have an

observable effect onto the amount of plastic energydissipation per rotation. Frictional energy dissipationincreased due to higher relative speeds, but was

thereby only significant for very small particle sizes

(powder) and at high rpm.

The experiments allowed observation of filling of the

plastification zone. The following plastic energy

dissipation led to the initial melting. Observation of

change from granular behavior to liquid behavior was

possible. Therefore the complete initial melting process

was visualized up to the point where the DSM models

describe the further melting process.

AcknowledgementsThis work is part of the Research Program of the Dutch

Polymer Institute (DPI), Eindhoven, the Netherlands,

project nr. #671.

The support from the Dutch Polymer Institute, Bayer

Material Science, Bayer Technology Services andDSM is gratefully acknowledged.

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investigation of the plastification behavior of polymers in high-speed twin screw extruders

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