ydrocarbon resins(HCR) can be extrud-ed with polypropyleneresin (PP) directlyfrom a dry blend tosave tolling costs in amasterbatchingprocess. A glassy, low-molecular-weight poly-mer, HCR is compati-ble with a wide range

of polymers. When used in film, itsignificantly improves barrier proper-ties, clarity, stiffness, and many otherattributes. Because of its low soften-ing temperature and low viscosity,most applications use HCR master-batch to avoid extrusion instabilityand poor mixing. In most applica-tions, up to 15% to 20% of HCR areused in order to optimize the finalproperties. If an extrusion system canbe developed to directly feed theHCR to the extruder, a significantcost saving can be achieved.

Polymer Properties HCRHCR is used mostly as a modifier inadhesives to improve tack, lower vis-cosity, and provide other properties.It is a glass-like, brittle, and stickypolymer produced by polymerization

of byproduct streams resulting mostlyfrom the cracking of petroleumhydrocarbons for ethylene produc-tion. The three principal raw-materialstreams to produce HCR are linearC5s such as piperylene and amylene,cyclic C5s such as cyclopentadiene,and aromatic C9 and C10 fractions,heavy in reactive monomers such asstyrene. All hydrocarbon resins can behydrogenated to improve color, odor,and stability, as well as modifyingcompatibility.

PP modified with HCRThe most popular application of PP-HCR blends is in polypropylenehomopolymer film, especially biaxial-ly oriented film. Figure 1 illustratesthe advantages ofadding HCR in biaxi-ally oriented PP film.Many other applica-tions also take advan-tage of HCR’s uniquecharacteristics.

Experimental Material handling Handling of HCRneeds special caution.The brittle pellets tendto generate fines,

which can be dusty and sticky. Awell-designed conveying system thatcan handle the fines is strongly rec-ommended.For this study, a multicomponentloss-in-weight feeding system wasused to prepare the HCR and PPblend in line. The feed streamsentered an in-line blender beforedropping into the extruder hopper.The ratio of PP to HCR was adjustedby the individual feeding rate. Theproportion of HCR in the feedstream ranged from 0% to 20% dur-ing the trial.

ExtruderA 2.5-inch, 30:1 L/D well-instru-mented extruder was used for the

28 | PLASTICS ENGINEERING | JUNE 2011 | www.4spe.org

Jay Keung, Yann Devorest, Marty Levine, and C.Y. Cheng (consultant)ExxonMobil Chemical, Baytown, Texas, USA

Improve moisture & gas barrier by 50-70%

Lower seal initiation temperature

BARRIER

OPTICS

SHRINKAGE SEALING

Higher stiffness Excellent dead-fold properties

PROCESSING

STIFFNESS

Higher Shrinkage

- Lower melt viscosity - Reduced power to stretch film - Lower processing temperatures

Lower haze and higher gloss in clear PP film

HFig. 1. Advantages of HCR in OPP film.

trial. The extruder had a 75-HP DC drive with maximumscrew speed of 140 rpm. Five pressure transducers werelocated along the extruder, as shown in Figure 2, to meas-ure the internal pressure developed by the screw. Animmersion melt thermocouple was used to measure thetrue melt temperature.To simulate the back pressure from a die, a restrictorvalve was used to adjust the head pressure normallyencountered in a commercial process. Samples of extru-date were taken at the valve discharge to analyze the dis-persion of the HCR in the PP phase.

Screw A was designed for polypropylene resin. Thescrew geometries shown in Figure 3 are: F: Feed section length, 9 DHf: Feed section depth 0.540 inch, square pitchT/B: Transition/barrier section, 12 D Pm: Major flight pitch 3.50 inches (constant)Pb: Barrier flight pitch 3.50 inches (constant) Melt channel depth: 0.115 inch–0.318 inchSolid channel depth: 0.550 inch–0.130 inchBarrier flight undercut: 0.065 inchMetering section length: 10 DMetering section depth: 0.310 inchMixing section length: 6 inches

Screw B was designed to maximize the output rate ofpolyolefin resin in general. The screw geometries shownin Figure 3 are: F: Feed section length, 6 D Hf: Feed section depth 0.480 inch, square pitch T/B: Transition/barrier section, 16 DPm: Major flight pitch 3.50 inchesPb: Barrier flight pitch 3.75 inchesMelt channel depth: 0.250 inch (constant)Solid channel depth: 0.480 inch–0.135 inchBarrier flight undercut: 0.045 inchMetering section length: 9 DMetering section depth: 0.250 inchMixing section length: 7 inches

Screw C was designed specifically for blown-film-gradelow-density polyethylene. The screw geometries shown inFigure 3 are: F: Feed section length, 5 D Hf: Feed section depth 0.495 inch, square pitchT/B: Transition/barrier section, 17 D Pm: Major flight pitch 3.50 inches Pb: Barrier flight pitch 3.75 inches Melt channel depth: 0.220 inch–0.415 inchSolid channel depth: 0.480 inch–0.135 inchBarrier flight undercut: 0.080 inch–0.050 inchMetering section length: 9 DMetering section depth: 0.415 inch Mixing section length: 7 inchesResinsPP4712E1 polypropylene from ExxonMobil ChemicalCo. was used as the PP component. This grade is ahomopolymer used mainly for film applications, such asBOPP film. It has a nominal MFR of 2.8 g/10 min andtypical density of 0.90 g/cm3. The peak melting point is162°C.The Oppera PR100A HCR, from ExxonMobilChemical Co., used in this experiment has a ring and ballsoftening point of 135°C–140°C and Tg of 85°C. Its typ-ical molecular weight of 675–986 is very low compared toPP. Since it is glassy and brittle, the pellet size is some-what nonuniform and some fines occur.

ProceduresThe percentage of HCR in the PP blend ranged from0%–20% in 5% increments. For each blend ratio, extru-sion performance at 40 rpm and up to 120 rpm was eval-uated. The output rate, pressure profile along the barrel,and melt temperature were recorded for each screw speedafter the extrusion condition reached a steady state.

Results and Discussion Overall observation To avoid premature softening and melting of HCR at thethroat of the extruder, it was necessary to have extruder

www.4spe.org | JUNE 2011 | PLASTICS ENGINEERING | 29

Fig. 3. Screw designs A, B, & C.

9 D 12 D 10 D

0.540” 0.310” 3.5” 3.5” 0.310”

6 D 16 D 9 D

0.350” 0.375” 0.480” 0.250”

0.250”

5 D 17 D 9 D

0.350” 0.375” 0.495” 0.415” 0.415”

0

D9

540”0.

D6

0 480”0.

D5

D12

540” 5”3. 5”3. 0.

D16

350”0. 375”0. 250”0.

D17 9

D10

310”0.310”

D9

2500. ”

D

Sc3.g.Fi

0 495”0.

C&,B,AgnsisdewerSc

350”0. 375”0. 415”0.

.C

00.41415”

Fig. 2. Extruder setup.

Fig. 3. Screw designs, A, B, & C.

30 | PLASTICS ENGINEERING | JUNE 2011 | www.4spe.org

throat cooling. Without it, the fines of the HCR tend tosoften and stick to the hopper base and hinder smoothfeeding of the blend.The irregular shape of the HCR pellets did not seem toaffect the homogeneity of the blend or the free flowing ofthe blend in the extruder hopper. The transition from PPto PP/HCR blend during extrusion was smooth, and itreached a steady state in 20 to 30 minutes.

Performance of screw C The screw design has a strong influence on the extrusionperformance of the blend. Screw C, designed specificallyfor LDPE, has a shorter feed section and low compressionratio. It was not possible to achieve a steady extrusionstate with the PP/HCR blend. Surging occurred undermost extrusion conditions. It is believed that the shortfeed section and low compression ratio created a solidplug at the beginning of the barrier section, where signifi-cant volume compression occurred.Because the blend has a higher melting point and heatof crystallization than polyethylene, the screw design doesnot allow sufficient residence time to convey the polymersbefore melting occurs and the compression of the solidbed can be applied. Further, the highly crystallinepolypropylene homopolymer is not as compressible asLDPE. Hence, the solid-bed plugging at the early stage ofthe screw causes the melting to become unstable. Becauseof the deep metering depth, the low melting rate at thebarrier section was not able to match the conveying rateof the metering section, further causing the screw tosurge.

Performance of screw BThis screw has a longer feed section, square pitch, andshallower metering section than screw C. It was possibleto obtain a steady extrusion condition for all blend ratios.

Figure 4 compares the output rate for different percent-ages of HCR in the blend. It is interesting to note thatthe specific output rate is very high up to 40 rpm. Thenthe output rate tapers off and remains almost the same upto 120 rpm at all blend ratios. In fact, there is a slightdecrease in output around 80 rpm.In this screw design, the flight pitch increases from 2.5inches at the feed section to 3.5 inches at the barrier sec-tion, and the channel depth changes from 0.480 to 0.135inch over 16.5 D. Therefore, the solid-bed compression isvery gradual and the polymer has a longer residence timeto preheat and initiate melting, resulting in high outputrate at low screw speed.

Figure 5 compares the internal pressure profiles along

the extruder at 40 and 80 rpm for PP and 20% HCRblend. Note that at 80 rpm, the screw is basically starveduntil the last 20% of the screw length. There must havebeen a blockage at the early stage of the screw, where itwas not capable of developing sufficient pressure at thatscrew speed.

Performance of screw AThis screw was designed for polypropylene. It has a longfeed section (9 D) and constant melt and solid channelwidths. Its longer and deeper feed section provides alonger residence time for the polymer blend to reach ahigher temperature with the solid-bed compression at theonset of the barrier flight.The output rates for different blend ratios are shown in

Figure 6. As is typical in screw performance, the outputrate increases monotonically with increasing screw speed.

Extrusion of PP and HCR Blends CONTINUED

Fig. 4. Screw B performance.

Fig. 5. Screw B internal pressure profile.

The difference in output rate between the different blendratios is also much smaller than with screw B.

Effects of barrel temperatureFigure 7 shows the output rate for 15% HCR at 80 rpmusing screw A. It shows that the barrel temperature affectsthe output significantly. Since PP is a major componentof the blend, a higher barrel temperature setting, especial-ly at the melting section, significantly improves the out-put rate of the blend. This indicates the melting ratecould be the rate-limiting factor.Because of the low softening point of the HCR, it isnot clear how the higher feed-section temperatures affect

solid-bed conveying. It is possible that a low feed-sectiontemperature setting improves solid conveying by delayingthe melting of the HCR, which may act as a lubricantand hinder solid conveying. However, because of the verylow viscosity of HCR, the high feed-section temperaturemay not cause significant slippage in the solid conveyingzone. The last condition on the right-hand side of Figure7 seems to confirm this argument.

Internal pressure profileFigure 8 shows pressure profiles along the extruder underdifferent temperature profiles. Note that for all conditionsexcept condition F (the highest temperature on all zones),

the early part of the screw exhibits no or very low pres-sure, especially in conditions A, B, and E.The low pressure indicates that the shear stress betweenthe solid bed and the barrel is low and therefore viscousshear heating would be less than when there is high inter-nal pressure.At the highest-output condition (F), high pressure isobserved at the 6 L/D location and remains at a higherlevel compared to the lower-output conditions. Based onthis observation, it is possible to modify the barrier flightdesign to relieve this high pressure early, and raise thepressures downstream, which would increase the outputrate.

Specific energyOnce melted, HCR has a very low melt viscosity becauseof the low molecular weight of the polymer. Therefore,the addition of HCR significantly decreases the overallmelt viscosity of the blend, resulting in lower extrudertorque, horsepower consumption, and melt temperature.The specific energy, expressed in hp-hr/lb, can be usedto measure the energy consumption. The lower the specif-ic energy, the lower the viscous shear heat dissipation, andtherefore a lower melt temperature can be expected.Figure 9 compares the specific energy for all blends underthe same temperature profile for screw A.

Dispersion of HCR in PP phase In order to produce a consistent product with attributesincorporating HCR, it is necessary that HCR be uniform-ly dispersed in the PP phase. With a barrier flight and aMaddock-type mixing section to enhance shear mixing,good dispersion of the HCR in PP is expected.To verify if the polymer blends were well dispersed,samples with various blend ratios and output rates usingextrudate from screw A were analyzed by extraction andscanning electron microscopy (SEM).In an extraction test using toluene, no HCR was detect-ed in the extracted solvent, indicating that HCR is well-dispersed in the PP matrix. At low SEM image magnifica-tion, we did not observe vacuoles or the presence of a sec-

www.4spe.org | JUNE 2011 | PLASTICS ENGINEERING | 31

Fig. 6. Screw A performance.

Fig. 7. Effect of barrel temperature profile (°C).

Fig. 8. Screw A pressure profile comparison.

32 | PLASTICS ENGINEERING | JUNE 2011 | www.4spe.org

ond phase formed by the undispersed HCR. At highmagnification, it appears that HCR is well-dispersed inthe amorphous regions such as the boundary of thespherulites and between the lamellae inside the

spherulites. Figure 10 compares the images at 100X and1000X for the 20% HCR blend extrudate at 100 rpm.

Conclusions The output rate of PP and PP/HCR blends under thesame condition is shown in Figure 11. Also shown are twooptimized process conditions for the blend. It clearly indi-cates that PP/HCR blends can be extruded at a rate nearthe neat PP output rate. Similar to polypropylene extru-sion, a high barrel temperature profile is desirable toachieve a higher output rate.With a non-pneumatic raw-material–handling systemand a well-designed screw, neat HCR could be dry blend-ed and extruded directly with PP in a single-screw extrud-er to enhance the properties of PP. This approach elimi-nates the need to use a higher-cost HCR masterbatch. Asteady extrusion with well-dispersed extrudate can beachieved to produce the desired product.

References 1. US Patent 5,213,774 (1993). 2. C.Y. Cheng, “Influence of Polypropylene Melt Rheology on ExtrusionBehaviors,” AIChE Annual Meeting, November 2000.

3. C.Y. Cheng, “Effects of Polypropylene Crystallinity on Extrusion,” SPEANTEC 1995.

4. H.F. Giles, J.R. Wagner, and E.M. Mount, Extrusion: the Definitive ProcessingGuide and Handbook.

5. 2010.1822 Internal to EMCC Traceability.

*****

Note: The authors presented a version of this paper atANTEC® 2011 in May in Boston, Massachusetts, USA.

Extrusion of PP and HCR Blends CONTINUED

Fig. 9. Specific energy comparison.

Fig. 10. SEM images of 20% blend, 100X & 1000X.

Fig. 11. Comparison of output for PP & blends.