a simple way to suppress surface defects in the processing of polyethylene

J. Non-Newtonian Fluid Mech. 124 (2004) 103–114

A simple way to suppress surface defects in theprocessing of polyethylene

Oleg Kulikov, Klaus Hornung∗

Institut fur Stroemungsmechanik, Universitaet der Bundeswehr Muenchen, LRT-7, Neubiberg 85577, Germany

Abstract

During extrusion of linear low density polyethylene (LLDPE) the product leaving the die is subjected to a variety of surface defects –“melt-fracture”. The present paper proposes the use of rubber coatings for the die land area to stabilise such flows. Defect-free extrusion isdemonstrated for flow rates about 10 times higher as compared to the case of dies from a Teflon tube. The improvements were verified fortubular as well as for annular dies. No accumulation of polyethylene crumbs at the external die surface (“die drool”) was detected becauseof the very low adhesiveness between the LLDPE and the rubberised surface. The results may have an important impact on the improvementof the processing of those thermoplastic materials, which have a narrow molecular weight distribution. The use of rubber coatings moreoverallows to reduce the processing temperature. This is especially valuable in extrusion of raw elastomers, which are sensitive to heating.©

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2004 Elsevier B.V. All rights reserved.

eywords:Blown film; Die drool; Extrusion; Melt-fracture; Polyethylene; Rubber; Sharkskin

. Introduction

Modern polyethylene resins and several other thermoplas-ic materials including polyvinyl chloride, acrylic polymersre characterised by a narrow molecular weight distributionnd advantageous mechanical properties, but are subjected

o surface deterioration or “melt-fracture” during processingy extrusion. At low velocities the product has a smooth sur-

ace but above some critical velocity the extrudate exhibits aariety of surface defects, which increase in severity as theroduct velocity is increased. About 50% of all polyethy-

ene resins are processed into films by a blowing processnd the “melt-fracture” results in commercially unacceptableroducts. The investigations devoted to the problem have a

ong history (about 50 years) and an extensive list of publi-ations. We hesitate to review the literature of the topic andnstead would refer a recent article[1], which contains thetatus of understanding and control of the “melt-fracture”henomenon.

The influence of die surface properties on the “melt-racture” as well as the influence of coatings are well known,

e.g.[2–10]. The use of fluorinated dies[8,11,12]could sig-nificantly delay the appearance of such defects. Also thof processing additives[13–17]may substantially postpontheir onset. Such additives may migrate to the polymewall interface and enhance wall slip. Lubrication of theland area has also been used, especially in connectionextrusion of raw elastomers (rubber)[18]. Among the varietof die coating materials used to control “melt-fracture”did not find any note of rubber coatings. It seems very strto us because already a classic work[19] was dedicated tthe extrusion of rubber.

The present work investigates the use of die land cings, made from elastomers to suppress melt-fracture.elastomers (or rubbers) have very low values of Young’s mulus (1–10 MPa) opposite to plastics (1.7–3.5 GPa) and m(70–350 GPa). Moreover they have very specific molecstructure. Long molecular chains of rubber can be eaged as relatively compact coils randomly joined by scross-linking segments in a three-dimensional net. Theber material has inner voids in nanometer scale so thalow molecular weight hydrocarbons like oil and fuel co

∗ Corresponding author. Fax: +49 89 6004 4092.E-mail address:[email protected] (K. Hornung).

penetrate and “weep” through the rubber[20]. Among mod-ern synthetic elastomers the following groups, including hy-drogenated nitril rubber, fluorinated hydrocarbon elastomers,

d.
377-0257/$ – see front matter © 2004 Elsevier B.V. All rights reserveoi:10.1016/j.jnnfm.2004.07.009

104 O. Kulikov, K. Hornung / J. Non-Newtonian Fluid Mech. 124 (2004) 103–114

perfluorinated elastomer, silicone elastomers and fluorosili-cone elastomers have outstanding heat stability and chemicalresistance. The elastic properties of silicone elastomers andof fluorosilicone elastomers are retained up to temperatureslarger than the processing temperature of polyethylene andpolypropylene (250◦C and above). The silicone elastomershave a structure of organically modified silica and adhere wellto a clean surface of glass and metal oxides. Coefficients ofstatic friction and tactile impressions of a rubberised surfacediffer much from those of “dry lubricants” like Teflon and tal-cum. Nevertheless, the silicone elastomers have a pronouncedrelease effect towards organic materials, such as polyester,epoxide, polystyrene, PVC, etc. This effect is exploited inthe use of silicone rubber compounds to make moulded partsand reproductions[21].

2. Experimental set-up

2.1. Material

We used LL1201 XV – a commercial LLDPE from Exxon-Mobil Chemical[22], which is specially recommended forblown film production. It has thermal stabilisers but no pro-cessing aids. Its density is 0.926 g/cm3, its melting point is1 ◦ llyii tures( e ino elyl

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a pressure transducer from WINTEC[25], which has a lin-earity of 0.5% in the range from 0 to 100 bar. The position ofthe piston was measured with a transducer from BALLUFF[26], which has a precision of about 5�m. The analog signalof the position transducer was calibrated to the mean flowvelocity by weighing the extruded product. An analog out-put from the board PCI6023E from NATIONAL INSTRU-MENTS [27] was used to control the flow rate through thedie in the range from 0.5 to 400 mm/s.

2.3. Flow visualisation

The extrusion process was tape-recorded with 25 framesper second using a SONY D8 camcorder[28]. The prod-uct was illuminated with a special stroboscope, which wassynchronised with the tape-recorder in order to get sharp pic-tures at high velocities of the product. The tape-recording andthe other measurements were triggered by the same pulse totime-correlate the product outlook and the measured valuesof pressure and extrusion rate. For microscopic observationof the flow we have used a three-dimensional translation stagewith about 5�m precision to scan the camera with an attachedmicro-objective lens along and across the flow.

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23 C and the melt index is 0.7 g/10 min. If it is not speciandicated the working temperature was about 158◦C whichs below the range of recommended processing temperafrom 180 to 220◦C). We choose such a low temperaturrder to investigate the onset of “melt-fracture” at relativ

ow velocities.

.2. Extruder

For extrusion at controlled flow rate we used a hydraally driven press extruder from LOOMIS PRODUCTS[23]ith a barrel of 60 mm in diameter and 200 mm in lenhe press extruder is the same as in[24]. Maximum pressur

n the barrel could be up to 400 bars. The maximum terature of the barrel could be up to 220◦C. We used tubulaies of 6 mm inner diameter and annular dies of about 20uter diameter. The tubular dies were fixed to the barrelholder of an inner diameter 28 mm. The entrance o

ubular dies was plane. The procedure to load the barres follows: portions of LLDPE granules were loaded toeated barrel and were molten after evacuation of the bhis procedure was repeated until the barrel was complled with LLDPE melt. To equalise the temperature disution in the barrel the time interval between loadingeasurements was at least 8 h.The temperature of the barrel was controlled by he

nd measured by a thermocouple. The melt temperatureasured also inside the barrel by a contact thermocohe die temperature and the temperature of the extrudedct were measured by a non-contact infrared pyrometerressure was measured in the bottom part of the barre

. Investigation of “melt-fracture” for known dieaterials and configurations

First we performed an experimental study of the “mracture” using dies of various materials, various lengthiameter ratios and various temperatures in order to comith existing published data. These measurements gabetter insight into the problem and into the performa

f our setup. Nevertheless we consider it is not worthwo present all resulting data, since they are close to whublished elsewhere. We confirm observations in[1] that nei-

her shear stress nor shear rate is the parameter that cohe onset of the “melt-fracture”. Therefore we used the mow velocity in the die and the pressure at the position oie entrance as the parameters to characterise the plastihe values of apparent shear rate and apparent shearan be easily calculated from these values.

.1. Glass dies

A set of experiments was performed with a die madglass tube with 6 mm inner diameter, 8 mm outer diamnd 32 mm length. By this we could observe the interio

he die as well as the movement of scattering particlesoids above a few micrometers in size. A typical flow cuor a temperature of extrusion of 158◦C is presented inFig. 1y a solid line. The corresponding appearance of the pro

s shown inFig. 2. At very low extrusion rate the product hglossy surface (seeFig. 2A) and then small scale defec

Fig. 2B). With rising extrusion rate the surface structureuch more pronounced (Fig. 2C–E). Thereafter the pro

O. Kulikov, K. Hornung / J. Non-Newtonian Fluid Mech. 124 (2004) 103–114 105

Fig. 1. Flow curves in extrusion of polyethylene for tubular dies withD = 6 mm andL = 32 mm from glass (solid line), Teflon (dashed line) and silicone rubbercoated glass die (dotted line) at 158◦C. Letters correspond to those ofFigs. 2, 4 and 6.

uct shows periodic alterations of defect and smooth surface(Fig. 2F) and on the flow curve oscillations of the pressure anda drop in the mean pressure appears. At a certain velocity theextrusion gives a product with only small scale surface rough-ness and the pressure drops further (“spurt flow”,Fig. 2G).

There is common agreement[29] that such pressure dropsand oscillations are caused by a transition from stick to slipboundary conditions at the die wall.

At higher velocities the product gets some irregulargrooves and small scale craters on its surface and crumbs of

Fig. 2. Product appearance for a glass die withD = 6 mm andL
= 32 mm. Frames correspond to points A–H inFig. 1.


Fig. 3. Microscope images of the flow of polyethylene inside a glass die at 158◦C.

material accumulate on the exit face of the die (“die drool”,Fig. 2H). The number of these grooves grows until the wholesurface gets disturbed by many crater-like cavities (not pre-sented). At velocities of the “spurt flow” (Fig. 2G) we de-tected voids inside the die, which indicate cavitation withinthe melt. By analysing a sequence of video frames we couldrecognise that these voids are transported outside and de-velop into irregular surface craters. In this case we couldalso observe detachment, i.e. areas with some gap betweenthe die wall and the polymer flow. Such wall detachmentshave been observed earlier for the case of clay mixtures[24].Microscope images of the die near its exit are presented inFig. 3 to illustrate “melt-fracture” (Fig. 3A), the appearanceof small and large scale cavitation voids in the flow (Fig. 3Band C), areas of product detachment and crumbs (“die drool”)at the die exit face (Fig. 3D) [30] originating from voidsand from areas of detachment when they are dragged out-side the die. Horizontal lines inFig. 3A and B indicate thedie exit position. We see inFig. 3A that fracture occurs atthe die exit and it propagates to the axis of the die and up-stream.

We also made records of extrusion between crossed po-larisers to detect double refraction of the melt, which wouldindicate areas of polymer chain orientation. Because of theround shape of the transparent die we could perform onlya theya ure”o dou-b neart ruc-t eamd

3

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are smaller for the Teflon die. Also the appearance of ‘melt-fracture“is delayed to higher velocities. The points on the flowcurve AT and BT (Fig. 1) have to be shifted to the right sideof the plot in order to represent about the same appearanceas the pints A and B of the glass die. The low values of thepressure drop in the Teflon die at velocities below 10 mm/sindicate a slip of the product along the Teflon surface butat higher velocities the pressure is growing to the values forthe uncoated glass die. So, the pressure drop in points C, D,E, F of the flow curve as well as the product appearance inFig. 4(frames C, D, E, F) are very close to the correspondingvalues for the glass die and to the corresponding frames inFig. 2.

In Fig. 5 the values of the specific period for the surfacestructure, that is the ratio of the length scale of the observedsurface periodicity to the mean flow velocity, are presentedversus the mean flow velocity. The length scale varies in arather large range within orders of magnitude, but after com-bination with the mean flow velocity the resulting time scaleshows only small variations between 15 and 33 ms for theglass die and between 7 and 24 ms for the Teflon die. Thismight lead us to the suggestion that all these defects mighthave a common cause. This means that micro-roughness,“shark-skin” and “bamboo-like” stick-slip variations maybe a manifestation of the same phenomenon, called “melt-f uresa -c e fortr elt-f

ergyc rt -t BN).T brassd caseo n re-s BNp g thedp om-

qualitative evaluation of the pictures and thereforere not presented here. At the moment of “melt-fractnset a small (less then 0.5 mm in length) area ofle refraction appeared inside and outside the die just

he exit. With further development of the surface sture this chain orientation area was growing in upstrirection.

.2. Teflon and other dies with low surface energy

A set of experiments was performed with a die consisf a Teflon tube with 6 mm inner diameter and 32 mm lentypical flow curve for an extrusion temperature of 158◦C

s presented inFig. 1 by a dashed line. The correspondppearance of the product is shown inFig. 4 for about theame values of the mean flow velocity as in the case olass die.Fig. 1 shows that at low velocities friction loss

racture”. For example, the generation of periodic fractt the die exit (seeFig. 3A) could be a manifestation of loal stick-slip transitions, as it is assumed to be the cashe more pronounced “bamboo-like” structures[29]. In theemainder of this paper we will therefore use the term “mracture” for surface roughness in general.

Besides Teflon, other materials with low surface enould be used.Table 1contains critical velocity values foubular dies (L = 12 mm,D = 6 mm) for the following maerials: stainless steel, brass, Teflon, and boron nitride (he table shows that the surface deterioration for theie is delayed to higher velocities as compared to thef the stainless steel die. The use of the die from Tefloults in a further delay of surface defects. The die fromrovides the highest defect-free rate of extrusion amonies investigated. Also as it is presented inFig. 4the specificeriod of the surface structure is lower for Teflon as c


Fig. 4. Product appearance for a Teflon die withD = 6 mm andL = 32 mm. The frames A and B correspond to points AT and BT in Fig. 1dashed line. Otherframes correspond to those points inFig. 1.

pared to glass. This observation shows a connection betweensurface properties of the die wall and “melt-fracture” outsidethe die.

These measurements with various die materials brought usto the idea to try “elastic” dies, because we expected that anelastic material could absorb the sudden pressure changesin “stick-slip” transitions. The idea turned out to be suc-cessful immediately when we first tried it. However, fromfurther experiments we obtained evidence that it is not themacroscopic elasticity of the die coating, which producesthe effect, but some more complicated interfacial interac-

velocit

tion between the elastic material and the molten polyethy-lene.

4. New die design to suppress melt-fracture: use ofrubber coatings in the die land area

4.1. Material and coating procedure

The die land area was coated with a silicone rubber bydipping it into a fluid silicone compound diluted by xy-

Fig. 5. The specific period of the surface structure vs. the mean flow
y for the glass and the Teflon dies at 158◦C used to get the flow curves inFig. 1.


Table 1Critical mean velocities for the onset of surface defects (“shark-skin” for therigid die materials and first defect occurring for the coated dies)

Die material, length, temperature Critical meanvelocity (mm/s)

Common tubular rigid diesGlass, 6 mm× 32 mm, 158◦C 5.9Stainless steel, 6 mm× 12 mm, 145◦C 3.6Brass, 6 mm× 12 mm, 145◦C 4.3Teflon, 6 mm× 12 mm, 145◦C 4.5Teflon, 6 mm× 32 mm, 158◦C 7.0BN, 6 mm× 12 mm, 145◦C 5.0

Tubular dies with rubber coatingsSilicon rubber “Elastosil”, 6 mm× 32 mm, 158◦C 180.0Silicone rubber “Ceresit”, 6 mm× 12 mm, 145◦C 126.4Silicone rubber “RTV-ME”, 6 mm × 24 mm,

145◦C130.0

Tubular dies with rubber insertsSilicone rubber cylinder, 6 mm× 12 mm, 146◦C 124.6Silicone rubber cylinder, 6 mm× 6 mm, 146◦C 50.0Silicone rubber ring, 6 mm× 1.5 mm, 146◦C 30.3Viton rubber ring, 6 mm× 1.5 mm, 148◦C 32.0

Metal annular dieStainless steel, 20 mm× 1 mm× 5 mm, 158◦C 3.9

Annular die with rubber coatingsSilicone rubber “Elastosil”, 20 mm× 0.7 mm×

5 mm, 158◦C89.0

lene, vulcanisation at a temperature of 90◦C in the pres-ence of water vapour, and finally fixation at a temperatureof 200◦C. We used the silicone rubber Elastosil N10[21].The vulcanised rubber is transparent, has an average density

m coated inside by silicone rubber. Frames correspond to those points inFig. 1.

of about 1.07 g/cm3, tensile strength of about 1.6 N/mm2,shear strength of about 2 N/mm2 and an elongation at breakof about 400%. The thickness of the coating was varied bychanging the degree of dilution by xylene.

4.2. Rubberised tubular dies

We used glass tubes of 6 mm inner diameter, 8 mm outerdiameter and 32 mm length. They were coated inside alongthe whole length of 32 mm by silicone rubber with a thick-ness of about 0.05 mm as described above. The flow curvemeasured at a temperature of 158◦C is presented inFig. 1bya dotted line. The corresponding appearance of the product isshown inFig. 6at about the same values of the mean veloc-ity as above for comparison. The changes in the flow curveand in the product appearance are striking. In the case of therubberised die the flow curve in log–log coordinates is muchcloser to “linear” and the product has no visible roughnessat the surface up to velocities of about 175 mm/s. At veloc-ities above 300 mm/s we see cavitation voids coming fromthe entrance area and large scale roughness of the productsurface.

4.3. Influence of coating length

tedo lueso , 18a owc d

Fig. 6. Product appearance for a glass die withD = 6 mm andL = 32 m

To find out the influence of coating length, we first coanly at the die exit face and then we used the following vaf length from the die exit in upstream direction: 0.5, 1.5, 6nd finally again the whole length of 32 mm. Resulting flurves as measured at a temperature of 158◦C are presente


Fig. 7. Flow curves for glass dies withD = 6 mm andL = 32 mm and with silicone rubber coatings of various lengths at 158◦C (solid: only die face coated,dashed: 6 mm, dotted: 18 mm, dash-dotted: 32 mm). The onset of melt-fracture is marked by a horizontal cross for those lengths for which flow curves areshown, and by a cross under 45◦ for the lengths for which no flow curves are plotted.

in Fig. 7for the cases: only die face coated, 6, 18 and 32 mm.The positions of “melt-fracture” onset are marked by crossesin circles (a horizontal cross for the lengths where flow curvesare shown and under 45◦ else). The corresponding pictures ofthe product outlook are presented inFig. 8. The flow curvesare getting more “linear” and the pressure values becomesmaller for the same extrusion rate when proceeding to longercoatings. For comparison the onset of the “melt-fracture” inthe case of a clean glass die is also presented inFig. 7. Thedata clearly show that the longer the rubber coating the furtherthe “melt-fracture” onset is delayed.

4.4. Influence of coating thickness

We tried to vary the thickness of the coating and to deter-mine its influence on the “melt-fracture” onset. We have onlyfew observations to discuss because of difficulties to mea-sure and to produce coatings with thickness below 0.05 mm.The use of the highly diluted and low viscosity mixtures ofraw rubber with xylene produced beads of larger thicknessand simultaneously some areas were coated with a very thin

coating

layer. In the range of thickness from about 0.05 to 0.5 mmwe did not observe any change in the “melt-fracture” onset.A treatment of the glass die by 1% solution of raw rubber inxylene produces some coating which could be detected onlyby a small change in light scattering. Already this very thinlayer of rubber delays the “melt-fracture” onset to the sameextrusion rate as do much thicker coatings. This fact leads usto the tentative conclusion that not the macroscopic elastic-ity of the rubber but some unique feature of its adhesion tomolten polymers is the reason for the delay of the defect’sonset.

4.5. Velocity profiles in tubular dies

Velocity profiles were measured inside transparent tubu-lar dies 32 mm in length and 6 mm in inner diameter at aposition of about 0.5 mm upstream of the die exit. We useda camcorder equipped with an additional objective lens of20 mm focal length. We focussed to inside the die and mea-sured the displacement of tracing particles of boron nitride(BN) between subsequent frames. The maximum possible

Fig. 8. Product appearance for glass dies with silicone rubber
s of various lengths at 158◦C. Frames correspond to the points inFig. 7.


Fig. 9. Velocity profiles inside a glass die and a glass die coated by silicone rubber withD = 6 mm andL = 32 mm at a cross-section about 0.5 mm from itsexit as derived from the motion of tracing particles. The velocity values are normalised to the mean flow velocity. The radial position is normalised tothe dieradius. A fit curve for the glass die isV=V0 × 1.606(1− X3.1), whereV0 = 2.9 mm/s; a fit curve for the rubberised die isV=V0× 1.23(1− X2.8) + 0.28, whereV0 = 11.1 mm/s.

resolution of the pictures was about 1.5�m per pixel but inthe case of the rubberised dies the image of tracing particleswas disturbed by the thickness non-uniformity of the rubberlayer. The error in the radial coordinate was about 0.2 mm,caused by the finite depth of the image field. The relativeerror in the velocity measurements was about 20% causedby mechanical vibrations of the die relative to the camcorderand by fluctuations in the period of the stroboscope flashes.The resulting velocity profiles are presented inFig. 9by opencircles for the uncoated glass die and by solid circles for therubberised die. The velocity values are normalised in bothcases to the mean velocity of the flow and the radial distancevalues are normalised to the inner radius of the tubes. Thedata ofFig. 9are shown along with fits to power laws. Bothcurves were measured at velocities below “melt-fracture” on-set. There is a marked change in the flow profile in the caseof the rubberised die. The velocity does not go to zero at thewall, as it is the case for clean glass dies. This could meanthat the LLDPE is slipping at its interface with the rubbercoating.

4.6. Impact of extrusion temperature

The ratio of slip velocity to mean velocity was mea-sured at various extrusion temperatures and the results arep -c ue ofa de yE allera

Corresponding flow curves, measured with the same tubu-lar die 32 mm in length and 6 mm in inner diameter are pre-sented inFig. 11 for temperatures of 158 and 207◦C. Thepositions of “melt-fracture” onset are marked by crosses onthe curves. The picture shows that opposite to the case ofuncoated glass dies the “melt-fracture” onset could occurearlier for higher temperatures in the case of a rubberiseddie.

4.7. Extrusion with an annular die

We present results, which were done with an annular diefrom stainless steel to obtain data relevant to industrial ap-plication. The die has a “spider-like” suspension of the core.The part of the core, which is adjacent to the die exit of thedie consists of a cylinder of 18 mm in diameter and 16 mmin length. The collar of the die consists of a disk of 5 mm inthickness with a cylindrical hole of 20 mm in diameter. Thedie collar could be adjusted to a certain position in cross-axisdirection in order to get an even thickness of the extrudedtube. The result of extrusion at temperature 158◦C with agap of the rigid die of 1.0 mm, a die land length of 5 mm, anda product outer diameter of 20 mm is presented inFig. 12(left frame). The die was not coated in this case.

In a second variant the die collar was coated with sili-c bettera oxi-d bberc f thea ands

resented inFig. 10. The ratio is quickly growing with inreasing mean velocity and goes asymptotically to a valbout 37% for temperatures 144 and 158◦C. At more elevatextrusion temperatures (187◦C), which is recommended bxxon[22], we have measured a smaller growth and a smsymptotic value (32%).

one rubber, whereas the core remained uncoated. Fordhesion, the metal was treated by an open flame toise the surface before potting. The thickness of the ruoating was about 0.15 mm that is the resulting gap onnular die was about 0.85 mm. It is visible on the right-hide of the central frame ofFig. 12 that “melt-fracture” is


Fig. 10. Ratio of wall slip velocity to mean flow velocity vs. mean flow velocity at temperatures of 144, 160 and 187◦C.

suppressed only at the exterior of the product. The rough-ness which shows comes from the inner side of the extrudedcylinder only. The produced cylinder of polyethylene looksdisturbed in shape, which could be explained by the differ-ence in friction at the inner (uncoated) and outer (coated)surfaces. The pressure drop measured for a product velocityof about 50 mm/s is less for the case of the rubberised collaras compared to the case of the uncoated one.

In a third variant the die core was also coated. The result-ing gap of the annular die was about 0.7 mm. The right frameof Fig. 12shows the resulting product outlook. No surfacedeterioration is visible up to an average product velocity ofabout 50 mm/s and the pressure drop on the die for a productvelocity of about 30 mm/s is smaller than for the case of the

a tubu

one-sided coating. Some swell in the product diameter couldbe explained by the profile of the rubber coating at the diecollar, which became thinner towards the edge of the collar,i.e. near the die exit the collar surface could be consideredas a divergent cone. The values of the critical velocity of the“melt-fracture” onset for the case of annular dies are pre-sented inTable 1. The data clearly show that rubber coatingscould delay the onset of surface defects to an extent whichis far beyond what has been achieved up to now with anyrigid die at the same temperature. The silicone rubber coat-ings were mechanically stable in the investigated range ofextrusion rates and gave similar results in the delay of “melt-fracture” after three days of contact with molten polyethyleneand after several successive extrusions. The question of long

Fig. 11. Flow curves for extrusion temperatures of 158 and 207◦C for
lar die withD = 6 mm andL = 32 mm coated inside by silicone rubber.


Fig. 12. Product appearance for annular dies. The die orifice is of 20 mm outer diameter and 18 mm inner diameter. Frame A – uncoated die. Frame B – onlythe die collar is coated with thickness of about 0.15 mm. The “melt-fracture” is suppressed only at the outer surface of the extrudate. Frame C – both the diecollar and the die core are coated with thickness of about 0.15 mm.

term stability of the coatings to wear and deterioration, how-ever, needs further investigation.

4.8. Use of other sorts of rubber coatings

We have also tried extrusion with inserts from rubber ringsof various lengths. They were cut from rubber tubes withan inner diameter of 6 mm and a wall thickness of 1.5 mm(silicone rubber and fluorinated rubber: Viton[31]). Short(1.5 mm) rings were pressed to the exit of a brass die withD= 6 mm,L = 24 mm. Longer inserts were glued inside the diebase between two supporting diaphragms of diameter 7 mm.Resulting values for the critical values of mean flow veloc-ity are summarised inTable 1. For the long rubber insertsshark-skin is completely absent. Therefore the critical meanvelocities, contained in the table are related to such insta-bilities, which first occur after defect-free extrusion. We alsoused the silicone rubber “Ceresit” and “RTV-ME”, both avail-able from industry[21,32] to coat the metal frame of the diewith good results in the delay of surface deterioration.

5. Brief discussion of the results

The physical cause of the effect found is unclear at present.T n ef-f ibleS –Sil al neto a bigd faceo casew bers facea ther ings ther

increased. In the case where the molten polyethylene is in acontact with a pure glass surface, the polymer sticks to thesurface up to some threshold value of the mean velocity, be-yond which it undergoes a very sharp “stick-slip” transition.

The reason of the much lower threshold for transition topartial slip along a rubber surface is not yet understood. Onetentative explanation could consist in some flexibility of therubber surface at a micro-molecular level. We could imaginethat the polymer chain has some branch, which gets hookedinside a “micro-cave” in the rubber surface or inside a voidthat is open to the rubber surface. The polymer flow dragsthe polymer chain and deforms the rubber molecule networklocally so that the cave gets shallow or disappears. Thereforethe polymer branch will escape from the void and migrateto the next one which is not yet deformed. In other wordsthe adhesion of the polyethylene chains to the rubber surfacemay drop if the rubber is stretched in comparison with therelaxed state of the rubber. If this would be a meaningful rea-soning, then it would be energetically advantageous for thepolymer chain to migrate along the surface to a place withhigher adhesion. The higher the shear rate in the polymer flowthe more frequently such migration processes would happen.A low adhesion value also would make the migration to hap-pen more often. The use of fluorinated rubber (perfluorinatedelastomer), especially fluorinated silicone rubber (fluorosili-c easet sur-f tureso is nos therei longt rigids ll in-t ers as sur-f also,b mall

his fact meant a challenge to us to at least undertake aort in a qualitative way. Silicone rubber consists of flexi–O–Si chains joined in a three-dimensional net by Si–O

inks. Fused silica or glass also has a three-dimensionf Si–O–Si chemical bonds. It is amazing to see suchifference in adhesion of the molten polymer to the surf a silica based glass and of a silica based rubber. In thehere the molten polyethylene is in contact with the ruburface, the polymer starts to slip partially along the surlready above 0.5 mm/s of the mean velocity value. Andatio of the slip velocity to the mean velocity is increasmoothly as the mean velocity in the tubular die is fur

one elastomer) with its low adhesion value would incrhis partial slip of the molten polymer along the rubberace and suppress “melt-fracture” at elevated temperaf processing. In the case of a rigid die surface thereuch deformation of the surface structure and therefores no reason for micro-migration of the polymer chains ahe surface. The polymer chains stay connected to theurface until a shear fracture comes along the flow–waerface with an abrupt pressure discharge and this trigghift of the material to the exit. In the case of the rubberace, on the other hand, these fractures would happenut much more often and at much less amplitude, too s


to trigger fracture outside the die. The whole picture wouldthen imply a stabilisation of the flow by continuous energydissipation during the release of stress in many small steps.Such a way of reasoning is known from the consideration ofearthquakes, where stress release may occur in many smallor in fewer big events. The macroscopic result would thenbe “partial slip” produced in the above suggested way on amolecular level.

The potential of industrial use of rubberised dies has yetto be verified, wear being one of the main issues. Whereas weexpect the problem to be of less severity for clean LLDPE,there is a need for investigation of the case of abrasive wearby particles of fillers in cable coating, film blowing of filledpolymers, and in extrusion of profiles from raw elastomers.However, already a coating of the die exit face will stop thenasty accumulation of material crumbs (so called “die drool”)without any danger of abrasion wear. Also from our experi-ence we could propose to coat by silicone rubber all metalparts which are in contact with the molten polymer, like thefollowing: the barrel, the piston, the die entrance, etc. Wealways could see a decomposition of the polymer materialwhen it is in contact with a pure metal wall. The coating ofthis metal surface by silicone rubber makes the material toslip along the surface and therefore reduces the exposure ofthe polymer to elevated temperatures and therefore its de-c uchl Theo ce tot ando ary.S arei bera ucet ingo

6

itiesf flon,a dataf sertsw iono f ther e ex-t up tov se ofT e firstr tionsa PE,a mento rrowm andi e thep ty of

the blown bubbles and reduce thermal decomposition of thematerial in dead corners of the press extruder. The resultsmay have an impact on the improvement of film blowing,fibre spinning, extrusion of heat sensitive raw elastomers aswell as on measurements of viscosity in capillary rheometers.

Acknowledgements

We would like to thank Mr. P. Sprau and Dr. J. Kissel forsupport of the research. We gratefully acknowledge the assis-tance of Mr. D. Zhelondz in programming, Mr. L. Kasseckerand Mr. H. Meier in manufacturing hardware components andMrs. S. Becker for an advise for the coating technique. Wealso thank Prof. H. Wengle and Mr. H. Hoefner for stimulat-ing discussions of the results, and Mr. E. Nelson for valuableassistance in manuscript preparation.

References

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rials

faceatent

sion2.ure-

lene,

ur-Non-

ity995)

opy-Con-(En-

[ atent

[ of997)

[ om-n-

[ nd a

[ net,

[ D.L.the

. 40

omposition. Additionally a silica based rubber has mess catalytic activity in comparison with metals alloys.utstanding release properties of the rubberised surfa

he polymer makes it much easier to clean the barrelther metal parts from polymer if this would be necesso, we think that the coating of any metal parts which

n contact with the molten polymer by tight silicone rubnd especially by fluorinated silicone rubber would red

he decomposition of the polymer and simplify the servicf the machinery.

. Summary

We first investigated flow curves and extrusion instabilor conventional tubular dies from steel, brass, glass, Tend BN in order to be able to compare to published

or our experimental setup. Then rubber coatings and inere used to delay the onset of “melt-fracture” in extrusf polyethylene. The influence of length and thickness oubber coatings and inserts was investigated. Defect-frerusion is demonstrated for the case of rubber coated dieselocities of at least 10 times higher as compared to the caeflon dies of the same diameter and length. Our data aresults and further experimental and analytical investigare required. The experiments were performed with LLDnd the results therefore may contribute to an improvef extrusion of thermoplastic materials, which have a naolecular weight distribution. The use of rubber coatings

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a simple way to suppress surface defects in the processing of polyethylene

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