Mineral fillers raise processing performance in cable compounding

Plastics Additives & Compounding November/December 2005

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Mineral fillers raise processing performance incable compounding

Polymer compounds highly loaded with particulated fillers are widely used inextrusion processes.The mineral filler plays a crucial role in achieving certainphysical or functional properties.Low Smoke,Free of Halogen (LSFOH)compounds used for fire retardant cables represent a particularly important classof materials within this field.The development of LSFOH compounds has seenmany improvements in extrusion behaviour,but there are still some problemsregarding restrictions in extrusion performance and therefore the economics ofLSFOH cable extrusion.New processing aids specifically designed to solve theextrusion behaviour of highly filled compounds are now permitting furtherimprovements.Dr.Reiner Sauerwein of Nabaltec GmbH describes a newmineral-based processing aid concept.

Originally used to reduce the overall costs ofpolymer compounds, the current focus ofmineral loading is functionality.Unfortunately, achieving certain propertiesthrough the addition of specific purposefillers is often accompanied by thedeterioration of other properties. Reducedmaterial performance compared to thevirgin resin, for example in decreasedelongation at break, can be adjusted duringcompound development to be acceptable

and within required specifications.However, significant differences inextrusion behaviour of filled and non-filledcompounds are often not accepted by theend-user, because these converters normallyprocess both kind of materials on the sameequipment.Cable drawing is one the most importantextrusion processes and uses non-filled aswell as filled polymer compounds. Flameretardant (FR) cables consist of many

different polymer materials and electricalcables with FR performance can have quitecomplex constructions including insulationcompounds, filling masses and sheathingcompounds. In all of these compoundsaluminium trihydrate (ATH), magnesium(di)hydrate (MDH) and calcium carbonatecan be present. In Europe, the base resin forsuch flame retardant compounds is mainlya blend of polyolefins making the product aLow Smoke, Free of Halogen (LSFOH)

ISSN1464-391X/05 © 2005 Elsevier Ltd.All rights reserved.

Figure 1:Die drool. Figure 2:MVR as function of temperature (21.6kg); the darkorange curve belongs to the product showing poor physicalproperties (Car2_SB1).

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compound. Filler loadings for LSFOHcompounds are in the 60-65 wt.% range.Flame retardant compounds based on PVCare still the preferred solution in NorthAmerica, Eastern Europe and parts of Asia.They normally contain 20-40 wt.% ATHor MDH as smoke suppressants. However,PVC cables for special surroundings (forexample plenum cables) are made ofcomplex filler mixes at much higher totalloadings.

Extrusion problemsThe range of filled materials described foruse in cable drawing show that extrusionproblems caused by high mineral loadingsare not restricted to LSFOH compounds.However, the development workundertaken relates to ATH-filled LSFOHcompounds - a particularly important classof polymer material in this field.The problems and limitations found inLSFOH compound extrusion are not onlyrelated to high mineral loadings, but arepartly determined by physical and chemicalparameters of the ATH filler. While theendothermic decomposition reaction ofATH is essential for its FR activity, the earlyonset of this process - starting around200°C - is a challenge for compoundingand extrusion. Therefore, the most obviousdrawback of the mineral load - the highviscosity of the compound - cannot be easilyovercome by employing a higher extrusiontemperature. Although the use of MDHcould offer this option because it has ahigher decomposition temperature, higher

extrusion temperatures are not consideredan acceptable solution to the problem. Aswell as a higher price level for MDH,polymer stability particularly for commonlyused EVA, is the key limitation. Lowextrusion speeds used to be the majordisadvantage of LSFOH compounds, butthis has become of lesser importance overthe years. This is because of performanceimprovements in the resin and ATH inparticular. This has enabled thedevelopment of compounds that showsignificantly better extrusion rates [1].Two other problems are now the main focusof current development work. One is thesurface finish of the extruded insulated wireor cable. Some LSFOH compounds giverough surfaces. A reason for this could bebad dispersion of the mineral filler thatleads to agglomerates forming on thesurface of the extrudate. This is primarily acompounding problem. Shark skinning is atypical extrusion problem, whichsometimes can be overcome by adjustingextrusion parameters, such as providing alower drawing speed combined with a

higher nozzle temperature. However,obviously this kind of remedial action is oflimited value and not economical. Thesecond problem can become a severe onewith respect to LSFOH compounds: diedrool or die-lip-build-up. Die drool isdefined as the leakage of resin from a nozzleor around the nozzle area of a steel blockthrough which plastic is extruded [2].Figure 1 illustrates this phenomena. Thepotential causes of this problem areplentiful, but the fact that die drool is oftenfound with ATH filled compounds initiallyfavours degradation of materials throughoverheating. On the other hand, the diedrool itself mainly shows an enrichment ofpolymer and contains much less filler thanthe bulk compound itself. This canprobably be considered the result of acombination of different factors, one ofwhich is certainly the platy structure of thefiller particles and their polarity. This leadsto a certain adhesion of the melt onto thedie, retarding the release of the compound.When the die drool has achieved a certainsize and mass it is picked up by the insulated

Table 1: Principle composition of investigated process aids

Figure 3: Line speed and amperage during extrusion at40rpm extruder speed.

Figure 4: Line speed and motor load at 60rpm revolution ofthe extruder.


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wire and often causes failure within theinsulation - recognized by an electrical shortcut counting down the drawing line.Extruder shut downs are the unavoidableconsequence.

Extrusion processing aidProcessing aids or lubricants arecommonly used in moulding processes,especially as release agents. Fatty acids,salts of fatty acids and hydrocarbons areprobably the most commonly usedproducts, while custom or applicationformulated lubricants and processing aidsare becoming increasingly important.Custom-made lubricants can demonstratea good balance between internal andexternal lubrication. The differencesbetween these mechanisms has beendescribed previously in Plastics Additives &Compounding [3]. Internal lubricants arecompatible with the polymer matrix andare therefore soluble. However, externallubricants are not. Process aids with aninternal lubrication effect reduce theviscosity of the melt while external processadditives primarily do not. The drawbackof internal lubrication can be alteredphysical properties. Process aids used inextrusion processes are usually of theexternal lubricant type, working in theinterface between the polymer melt andthe steel die. Fluorinated polymers andsilicones are the most established products.However, as well as being relativelyexpensive, fluorinated polymers are notthe preferred choice of additive for usewith halogen-free compounds. Siliconesalso have some drawbacks. Fogging, therelease and condensation of volatiles, is themost important one. Recently somesilicon suppliers have developed additivesthat are based on a polymer carrier.Although this polymer carrier conceptmay simplify the incorporation of siliconduring compounding, it does not changethe ‘release mechanism’ of the surfaceactive silicone.This article describes a modified releasemechanism for silicones. The aim of theproject was to find a mineral carriermaterial, which may also be a blend ofminerals, and a blend of silicone polymers,which would allow the following properties.

• Easy incorporation during LSFOHcompounding

• No or only minimal impact on physicalproperties,

• Good surface properties• High extrusion speeds• No die drool• No negative impact on flame retardancyA screening process for a typical LSFOHcompound formulation was performed.The base compound consisted of a PE/EVAblend, 60 wt.% loading of ATH (Apyral®

40CD) and a vinylsilane peroxide coupling.Table 1 shows the principle composition ofthe six process aids investigated. They weremade of three different minerals or mineralblends (Car1 - Car3) and two types ofsilicone blends. One silicon blend consistedmainly of low to medium molecular weightsilicones (SB1), while the other was made ofsilicones resulting in a higher molecularweight distribution (SB2). As well asdifferences in their chemical composition,the mineral carriers used can bedifferentiated by their specific surface area,according to BET and pore volumes. The investigation was completed with twocommercial process aids - a low molecularliquid silicone with vinylsilane functionality(Com1) and a silicone masterbatch basedon a polyolefin carrier (Com2). The dosageof the trial products was adjusted accordingto the active silicone contents keeping theactive silicone matter in all compoundsconstant (1.4%). The commercial productswere used according to suppliersrecommendations.

CompoundingCompounding was done on a Buss-co-kneader. Split feed set up was used and theprocess aids were added through the

second port. This process was modified forthe liquid silicone, which was fed togetherwith the polymer granules as pre-blendupstream of the extruder. Feeding of themineral carrier type products was easy,while the commercial polymermasterbatch, made of relatively largepellets, was not easy to dose.

Physical propertiesMechanical properties were tested onextruded tapes. Only small variations inmechanical properties were observed for thecommercial products and nearly all testproducts. The only trial product thatresulted in significantly lower tensilestrength compared to the reference (noprocess aid, No PA), was the one made ofthe mineral carrier with the lowest BET-specific surface area and the silicone blendhaving the lower molecular weight(Car2_SB1).It is assumed that this compound shows amuch higher silicone concentration in thepolymer bulk, which is a result of thecarrier's lower absorption capacity due tothe lowest BET-specific surface area.Additionally, the molecular weight of thesilicon blend has an influence on its release.

Figure 5:Mineral carrier forced to theinterface between compound andmetal tool by partial release of activesilicones from particle surface inducedby back pressure.


Figure 6:Die drooling behaviour.No drooling at 40rpm (left) and ‘drifting’ die drool at60rpm extruder speed (compound based on Car3_SB2 = Actilox® PA-14).

40rpm 60rpm;3min 60rpm;6min 60rpm;9min



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The trial product made of the same mineralcarrier, but using the higher molecularsilicone blend (Car2_SB2) has goodphysical properties. Figure 2 displays the melt volume ratevalues for all compounds as a function ofmeasuring temperature (at 21.6kg weight).The curve representing by far the highestMVR-values belongs to the productshowing poor physical properties(Car2_SB1). Therefore, this confirms thehigher solubility of the silicone, or in otherwords the internal lubrication mechanismfor this specific product. All other curves areadjacent to the reference compound (NoPA). Most trial products have MVR-valuesinbetween the reference and the twocommercial products (Com1 and Com2),which show reduced MVRs compared tothe reference material.Although almost all trial products gave noimprovement in melt flow. Nevertheless,when investigated in a laboratory kneader asignificant reduction in peak torque can beobserved. This difference between in-staticmelt performance and behaviour undershear is of course essential for the ability ofprocess aids to improve extrudability.

Water uptakeElectrical performance after waterimmersion is an important criteria formaterial approval in the cable industry.Therefore, the water uptake of thecompounds was tested. Four test productsshowed significantly reduced wateruptake, while the two process aids basedon the mineral carrier having the highestBET-specific surface area (Car1) did

perform worse. Combined with the highermolecular weight silicon blend(Car1_SB2), the water uptake iscomparable to the reference material.However, when the lower molecularweight silicone mixture was used, wateruptake increased (Car1_SB1). Thecomposition of carriers used in thisscreening is proprietary, but it is essentialto state that the observed differences inwater absorption are not only based ondifference in BET-specific surface area.

Extrusion speedExtrusion trials were performed on aFrancis Shaw extruder equipped with astandard BM screw and pressure tooling forthe drawing. A single round copperconductor (1.5mm2) was insulated to atotal diameter of 3.0mm. The first trialseries was done with an extruder revolutionset at 40rpm. Line speed and amperage(currency uptake or motor load) wasrecorded.Figure 3 displays the results in form of a bardiagram. The broad bars represent the linespeeds achieved (left axis) and the thin redbars show the motor load. At first glance theperformance of the six trial products basedon the mineral carrier concept isdisappointing. While the two commercialprocess aids give at least comparable(Com1) or slightly enhanced (Com2) linespeeds, the development products result inreduced drawing speeds. However, whenlooking at motor load, a significantreduction can be observed for the six trialproducts. This reduction enables anincrease in extruder revolution speed.

Figure 4 plots the results for a trial seriesdone at 60rpm. The reference compoundwithout process aid (No PA) and the twocompounds based on commercial productscould not been run properly at this extruderspeed because the load on the motor wastoo high - the motor load or amperage ofthe pilot extruder used was limited to 80A.With the mineral carrier based process aidsline speed can be increased by up to 30%(Car2_SB1 and Car3_SB2). This is asignificant increase with potential economicimportance for cable manufacturers.The influence of the extruder speed can beexplained by the increase of friction energyand die pressure build-up when running ata higher revolution. The release of thesilicones from the mineral surface is afunction of die pressure. When sufficientback pressure builds up the silicones areforced to migrate from the mineral carrierand can become active at the interfacebetween compound melt and die (seeFigure 5). As long as they are not needed(ie: under low shear and pressure) thesilicones stay at the carrier surface andtheir concentration in the polymer matrixis very low.

Die drooling and surfacefinish

The reference compound (No PA) showeda conical die drool at 40rpm revolutionspeed and a line speed of 126m/min. Mostof the development products screened inthis study did not show any die droll at40rpm extruder speed. This phenomenacannot be explained by reduced die head


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pressure. In fact die head pressure of allcompounds containing process aids(development and commercial products)was increased against the reference. Whengoing up to 60rpm screw revolution andhigher line speeds performance differenceswere observed. None of the process aidsscreened could fully avoid the formation ofdie drool at these conditions. However,two products (Car3_SB2 and Car2_SB2)did perform in a very positive manner, asillustrated in Figure 6. The die droolformed by these compounds is notconically closed around the nozzle, butforms on one side of the die only. Whengrowing, it slowly drifts away from thenozzle and finally drops. A potential pickup by the insulated wire is avoided.Surface finish is at least partly related to diedrool. In this investigation we observed thatthose products giving best die droolperformance also resulted in the best surfacefinish qualities.Figure 7 summarizes the performance of alltrial products according to the most imp-ortant parameters. The product presentedin light green is the best performingproduct. This product, Car3_SB2, has nowbeen commercialized as Actilox® PA-14.

Flame retardant propertiesSome process aids, especially when basedon hydrocarbons or also some fatty acids,can deteriorate flame retardantperformance properties. Figure 8 displays

the heat release rate measured by ConeCalorimeter at a heat flux of 50KW/m2.The measurement was done on insulatedwires cut to 10cm length and set togetherone next to the other to form specimensof 100cm2.No negative impact on peak heat releaserate is detected. Indeed the wiresinsulated with Actilox® PA-14 containingcompound show a decrease of peak heat release rate. This positive effect onflame retardant properties is assumed toresult from a good char formingbehaviour of the mineral carrier siliconecombination.

Conclusion and outlookProcess aids based on a mineral carrierconcept can efficiently improve theextrusion process performance. In orderto avoid any negative impacts on otherproperties, a proper selection of the rawmaterials is important.The chemical composition of the carriermineral, its specific surface area and porevolume as well as the molecular weightdistribution of the silicones used as activecomponents are the most importantproduct parameters.The commercialized product Actilox®

PA-14 is made of a mineral carrier with medium specific surface areaaccording to BET and a silicone blendconsisting of medium to higher molecularpolymers.

Actilox® PA-14 has demonstrated goodperformance in an exemplary LSFOHcompound formulation. Further work iscurrently performed on mineral filledpipes and conduits. In each individualapplication and compound formulation athorough study on dosing level is crucialfor an optimum result.

References:1. R. Sauerwein, "New ATH developments

drive flame retardant cablecompounding", Plastics Additives &Compounding, December 2002, pp22-29

2. M. del Pilar Noriega E. and C.Rauwendaal, “Troubleshooting theExtrusion Process", Hanser Publishers,Cincinnati, OH (2001).

3. N. Teufel, "Internal lubricants: processadditives yield profits and productivity",Plastics Additives & Compounding,January 2001, pp26-29.

Contact:Dr. Reiner SauerweinManager Technical Services, BU FillersNabaltec GmbHAlustr. 50-52DE-92421 SchwandorfGermanyTel. +49 9431 53526Fax. +49 9431 61557E-mail: [email protected]: www.nabaltec.de


Figure 8:Rate of Heat Release (RHR) curve for insulated wiresmade of compound without process aid (No PA) and withActilox® PA-14.A reduction of Peak Heat Release Rate isdetected for the wires containing process aid.

Figure 7: Performance diagram for all screened process aidsbased on the mineral carrier concept.

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Mineral fillers raise processing performance in cable compounding