先进的双螺杆挤出机Advances in Twin Screw Extruders

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Advances in Twin Screw Extruders for Powder Coating

applications.

Michael Dillon

Introduction

Powder coatings have become widely establisheddue to their ease of use without the need forsolvents, together with high physical strength and arange of surface finishes. Formulations based onpremixes of epoxy, polyester, acrylic andpolyurethane thermosetting resins are compoundedwith a variety of pigments, fillers, curing agents,flow modifiers etc. to suit specific end-userequirements. During the compounding process, theresin is melted and the ingredients are mixeduniformly to produce a homogenous melt which issubsequently cooled and flaked before grindinginto a fine powder and classifying.

The compounding process is well suited to theunique operating and mixing behaviour of the co-rotating twin screw extruder. This report outlinesthe main design, performance and operationalcharacteristics of this type of extruder withparticular emphasis on processing thermosettingPowder Coating materials. The major advances inthese extruders that have allowed for substantialoutput improvements are also discussed.

1

APV Baker

Industrial Extruder DivisionManor DrivePaston Parkway

Peterborough PE4 7APEngland

Tel: +44 (0) 283000Fax: +44 (0) 283001

The mixing process

It is useful to consider the overall mixing process ina solid/liquid system as consisting of three stageswhich are quite distinct in nature but overlap inpractice: namely; wetting of the powder,breakdown of agglomerates ( and aggregates ) andfinally stabilisation of the mixture by distributionof the solid particles 1.

The primary particle size of solid fillers andpigments is usually small enough for furtherbreakdown during compounding to be unnecessary.However, when dry, powders can often containsome aggregates and agglomerates or indeed thesemay be formed during premixing or conveying inthe solid state. The initial mixing process in theextruder involves melting the resin and wetting outthe solid particles. Once wetted, mechanical energyinput is required to bring about their completeseparation or dispersion. The finely dividedparticles and other formulation ingredients are thendistributed uniformly within the molten resin. Thedispersion is usually stabilised by the relativelyhigh viscosity of the molten resin and can be

assisted by surface active chemicals.

Factors influencing mixing

The surfacechemistry (polarity) of powdered additives is veryimportant in determining their tendency to formaggregates and agglomerates and converselydefines the mechanical forces necessary to breakthem down. In addition, the presence of moisturecan increase agglomerate strength. A number ofsurface treatments are used to assist the dispersionof powders, including silanes and stearates. Thesecan have the added benefit of lubricating themolten mass and reduce melt temperatures or allowhigher levels of filler to be used.

Other factors, such as the chemical reactivity of theresin system used can impose temperature andresidence time limits on the compounding process.All formulation ingredients areusually premixed for operational simplicity andlower installation costs. It is important that thepremix remains free-flowing and uniform inconsistency to ensure stability of the mixingprocess and in particular, the mechanical energyinput provided by the extruder. Both low speedribbon blenders and high speed turbine mixers areused but the high speed variant is more effective incontributing to the overall mixing process due tothe increased attrition and impact forces on the dryblend giving a more uniform particle size. Higher shear stresses foragglomerate breakdown are

imposed

with

increasing resin viscosity according to the formula:

2

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t = g.h

where:

t = shear stressh = viscosityg = shear rate

Conversely, filler particles will be wetted moreeasily by low viscosity resin melts. Viscosity isrelated to temperature and shear rate, and can beinfluenced by the interaction between the resin meltand additive.

The shear historyof the material is defined as the combined effect ofthe average shear rate imposed by the extruder (afunction of the screw speed) and the residence timewithin this shear rate regime (related to screwspeed and output). These two factors determine theprobability of high shear stresses being imposed ona given particle and hence the mixing efficiency.The breakdown of agglomerates will only occurafter a certain minimum stress, or yield stress, hasbeen exceeded. Therefore, lower shear mixers, e.g.single screw extruders, will be expected to needhigher residence times to achieve a comparableshear history to the high average shear rate twinscrew extruder. The minimum yield stress forefficient dispersion may never be achieved in lowshear mixers despite very long residence times ifthe average shear rate is too low.

Specific Energy

Melting and mixing are accomplished mosteffectively by the application of work energy as thethermal properties of polymers limit the amount ofheat transfer through surfaces. However, heattransfer can be important in controlling the wall

film properties such as friction coefficient,viscosity and wettability. All of these can influencethe amount of work energy applied by thecompounder 2.

The mechanical energy input to the extrusioncompounding process can be estimated byreference to the Specific Energy value (S.E.)defined as follows:-This parameter is the total mechanical energy usedduring material conveying, melting and mixing,with units of kW hr / kg, and is related to theenthalpy and viscosity characteristics of the rawmaterials as well as the intensity of the mixingprocess. Typical values for Powder Coatingmaterials are in the range 0.05 to 0.09 kW hr / kg(Epoxy to Polyester resins).

Design of the co-rotating twin screw extruderThe most important design parameter for twinscrew extruders is the ratio of the centrelinedistance between the two shafts (C) to the radius (r)- refer Figure 1a which illustrates a cross sectionalview through the twin screw barrel. In order tomaintain fully intermeshing screws with selfwiping and co-rotating operation, the crosssectional shape of the screw components is fixedby the C/r value 2,3,4.

Self wiping will occur for a screw form with nparallel helices (or screw lobes) when: -

C/r >= Ön

3

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Therefore, twin lobe designs, the most commonlyused commercially, will occur over a C/r rangefrom 1.41 to 2.00. Some of the most importantfactors, screw root diameter, which determines thetorque bearing capacity of the screw shafts, and thefree area within the extruder for processingmaterial, are illustrated schematically for C/r valuesof 1.55 and 1.70 in Figures 1b and 1c. For aconstant screw outside diameter, increasing thecentreline distance increases the screw rootdiameter but at the same time reduces the free areawithin the extruder - shown by the shaded areas ofFigures 1b and 1c.

The torque bearing capacity of the screws is related

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to the screw root diameter (or more precisely theshaft diameter) raised to the power of 3 andincreases correspondingly with increasing C/r. Freearea, on the other hand, decreases with increasingC/r up to a C/r value of 2 when the theoretical freearea is zero since at this value the screw rootdiameter equals the screw outside diameter.The effect of C/r on free area and maximum torquebearing capacity from C/r values from 1.41 to 2.00is shown in Figure 2. For convenience, free area onthis graph is normalised by dividing by thediameter squared. The relative maximum torquecapacity is also represented on the same y-axis bythe ratio of the screw root diameter at a given C/rvalue to the screw outside diameter, raised to thepower of 3. For the examples shown in Figure 1,increasing the C/r value from 1.55 to 1.70 allows

Screw shaft torque is used to apply mechanicalenergy (Specific Energy - refer above) to theprocess using mixing paddles. Therefore the C/rchosen by the extruder manufacturer represents abalance between the torque available for applyingmechanical energy to the process and the space inthe extruder to apply it. Modern twin screwextruders have C/r values from 1.55 to 1.83 but dueto the low bulk density and low specific energycharacteristics of Powder Coating materials, lowC/r ratio designs are preferred for theseapplications to maximise the output. The APV MPrange of extruders has been designed with a C/rvalue of 1.55 for Powder Coating applicationswhich gives the highest free volume available for

the maximum shaft torque to be doubled but this isaccompanied by a 33% reduction in free area.

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melting and mixing applications.Modular screw design

Screws are assembled from modular sections into5