plastic energy dissipation and its role on heating/melting of single-component polymers and...

Plastic Energy Dissipationand Its Role on Heating/Meltingof Single-Component Polymersand Multi-Component PolymerBlends

BAINIAN QIAN, DAVID B. TODD, COSTAS G. GOGOSDepartment of Chemical Engineering, NJ Institute of Technology, Newark, New Jersey 07102Polymer Processing Institute, NJ Institute of Technology, Newark, New Jersey 07102

Received: November 4, 2002Accepted: January 9, 2003

ABSTRACT: Plastic energy dissipation (PED) of polymer particulates is,essentially, the energy dissipated during large and repeated plastic deformationsof compacted polymer particulates while still in the solid state. PED is higher ormuch higher than VED, the viscous energy dissipation source of polymeric melts,because the stresses necessary to plastically deform viscoelastic polymer solidsare orders of magnitude higher than the stresses needed to support viscous flow.In the last few years our group has demonstrated experimentally the dominantrole which PED plays in the heating/melting of solid polymer (compacted)particulate beds in compounding processing equipment, such as twin-screwextruders and counterrotating continuous mixers/melters, in which thedeformation of solid polymers is mandatory. We have also developed simpleempirical methods of predicting the total axial distance needed for melting agiven polymer in specific processing/compounding machines and processing

Correspondence to: Bainian Qian; e-mail: [email protected].

Contract grant sponsor: Members of Polymer Mixing Study atPPI.

Advances in Polymer Technology, Vol. 22, No. 2, 85–95 (2003)C© 2003 Wiley Periodicals, Inc.

PLASTIC ENERGY DISSIPATION ON POLYMER MELTING

conditions, as well as the melting rates, all based on the mechanical energydissipated during solid particulate compression. This work explores the morecomplex issue of how the PED behavior of single-component polymers may affectthe PED (and the heating/melting) behavior of multi-component polymer blends.C© 2003 Wiley Periodicals, Inc. Adv Polym Techn 22:85–95, 2003; Published onlinein Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10039

KEY WORDS: Plastic energy dissipation, Melting mechanism, Polymerblends, Twin-screw extrusion

Introduction

M elting as an important elementary step1 inmost polymer processing applications has

been studied extensively in single-screw2,3 and morerecently in twin-screw extruders.4–12 Poor meltingwill have profound consequences on the entire pro-cess, such as discharge surging, nonuniform prod-uct, high machine wear, and coarse blend morpho-logy. A uniform and rapid melting stage is essentialin most polymer processing applications, especiallyin reactive polymer modification where the un-melted “escapees” will stay unchanged and not un-dergo the intended chemical reactions, leading to aproduct with nonuniform structure and properties.

All existing studies show that melting in twin-screw extruders occurs over very short axial lengthsand is totally different from what is observed insingle-screw extruders.4–11 Instead of the existence ofclear boundaries between melt pool/film and solidsbed as in the melting section of single-screw extrud-ers, polymer solids are intimately mixed with poly-mer melt in the melting section of their twin-screwcounterparts. The theoretical models Bawiskar andWhite5 and Potente and Melisch6 describing melt-ing in twin-screw extruders have adopted the sameapproaches as those for single-screw extruders, withprogressive development of a molten layer from thebarrel towards the screws, which is barely observedin the experiments. The “composite” or “equivalent”viscosity to account for the presence of high frac-tion of solids in the melt matrix was used by Todd4

and Vergnes et al.9 to model the melting of solidparticulates in the solids-rich and melt-rich suspen-sions in the later stage of melting when a suspen-sion of solid particulates in a molten matrix has beenformed. Obviously, it is not possible for this “com-posite” or “equivalent” viscosity approach to de-pict the early stage of the melting process when nosuch polymer solids/melt suspension exists. Gogos

et al.7,8,10 showed the important roles of pellet plas-tic deformation, for which he coined the term “plas-tic energy dissipation (PED),” on the initial heat-ing/melting of polymer solids in any two-rotor-typecompounder or melter. For most polymer solids-fedmelt-processing equipment, melting is an energy-intensive process that consumes about 70–80% ofthe total energy. This amount of energy required formelting is supplied either by heat transfer from theelectric heaters/heated medium for the barrels or bydissipation of the mechanical energy supplied by theextruder drive. The mechanical energy is mainly dis-sipated in three different ways7,8,10,11: frictional en-ergy dissipation (FED) from the frictional movementof polymer solid particles, plastic energy dissipation(PED) from the irreversible deformation of solid par-ticulates, and viscous energy dissipation (VED) fromthe irreversible deformation, i.e., flow, of polymermelt.

In the last few years our group has demonstratedexperimentally the dominant role which PED playsin the heating/melting of compacted polymer par-ticulates in compounding processing equipment,such as twin-screw extruders and counterrotatingcontinuous mixers/melters, in which the deforma-tion of solid polymer particulates is mandatory.7,8,10

The mandatory deformation of consolidated partic-ulates (“solids plug” with relatively weak bound-ary between individual particulates) provides a veryeffective, rapid, and homogeneous way of heat-ing and melting in corotating twin-screw extrud-ers. Experimental evidences show that PED of in-dividual pellets in the partially filled kneadingsection can raise the initial temperature of poly-mer pellets significantly.8 This deformation energyalone can provide enough energy to heat up amor-phous polystyrene (PS) pellets to above their glass-transition temperature and, therefore, PS pellets mayget melted in the partially filled kneading section. Al-though semicrystalline polypropylene (PP) pelletscannot get melted in the partially filled kneading

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section, the significant temperature rise of feedstockin this region caused by the PED of individual PPpellets contributes greatly to the subsequent melt-ing process.

The larger PED of amorphous PS, compared tosemicrystalline PP or PE (polyethylene), evaluatedfrom simple unconfined uniaxial compression ofmolded cylindrical polymer samples, was attributedqualitatively to the faster heating/melting rate ofsingle-component PS pellets than that of PP or PEpellets in the twin-screw melting section.10 In thepresent work, we studied the heating/melting be-havior of polymer blends in twin-screw extrudersthrough the PED behavior of individual polymeringredients as well as their blends. Carcass exper-iments of the melting of polymers and blends werealso carried out under different processing condi-tions. The results from the PED response of polymeringredients and blends were correlated to their melt-ing behavior from carcass experiments.

Experimental

MATERIALS

An amorphous polystyrene (STYRONTM 685 ofDow Chemical, with a melt-flow rate of 2.4 g/10 minand a density of 1.05 g/cm3) and a semicrystallinemetallocene-catalyzed PE (EXCEEDTM 350D60 ofExxonMobil Chemical Company, with a melt-flowrate of 1.0 g/10 min and a density of 0.917 g/cm3)were used to study the PED behavior of individualpolymers as well as their blends. Both resins wereprovided in pellet form, with a diameter of 3–4 mm.The specific heats of the PS and PE were measuredusing Perkin-Elmer differential scanning calorime-try (DSC) at a heating rate of 20◦C/min. This is todetermine the amount of energy required for melt-ing of each polymer. An Instron capillary rheometerwas used to measure their melt viscosity. The cap-illary die used was 0.7214 mm in diameter with anL/D of 50. The capillary measurements were con-ducted at 200◦C.

UNIAXIAL COMPRESSION

The test samples were molded using a hot pressat the melt processing temperature of each polymer.Two identical molds were used. Each mold couldmake “thin” cylindrical polymer samples of about

10 mm in diameter and 3.3 mm in thickness. Whenstacked together through two guiding pins, the twomolds could make “thick” samples of the same diam-eter with twice the thickness (�10 × 6.6 mm, 10 mmin diameter and 6.6 mm thick). Only bubble-freesamples were selected for compression testing.

The molded cylindrical polymer samples wereuniaxially compressed in an unconfined manner,using a Tinius-Olsen Universal Tester at differenttemperatures. Samples were conditioned in an en-vironmental chamber for at least 30 min at thetest temperature before being compressed. Bothcompression force and displacement were recordedusing a data-acquisition system. This procedure wassimilar to that reported in our previous work.7,8,10

CARCASS EXPERIMENTS

The carcass experiments for melting were con-ducted in the Twin Screw Mixing/Melting ElementEvaluator (TSMEE©R). The TSMEE©R has the samescrew geometry as a ZSK-30, but has a split barreldesign and quick-quench features. After the processbecame steady, normally after 30 min of running un-der set conditions, the extruder was “dead-stopped.”All barrel sections were then quenched with circu-lating cold water to preserve the melting and mix-ing states inside the extruder. Images of the quicklyfrozen carcass—revealing the evolution of meltingalong the screw axis—were captured via a digitalcamera.

In order to visually determine the melting/mixingstate from the solidified carcass, some of the feedpellets used were precompounded into colored pel-lets that contain a very small amount of pigment.We assume that the presence of this small amountof pigment does not affect the melting character-istics of the polymer. Basically, two types of melt-ing screw configurations were examined. The firstone was comprised of 45◦ staggering angle for-ward kneading blocks, each kneading block con-taining five kneading paddles with a total length of28 mm, followed by a half-lead reverse screw ele-ment 10 mm long. The other melting screw designwas just four neutral kneading blocks (each withfive kneading paddles and the same total length of28 mm) without any reverse elements. The objectiveof carcass experiments was to examine whether thehigher PED amorphous PS would also melt fasterthan the lower PED semicrystalline PE in PS/PEblends, the same phenomena observed as the melt-ing of each individual component in twin-screwextruders.

ADVANCES IN POLYMER TECHNOLOGY 87


Results and Discussion

MATERIAL CHARACTERIZATION

The specific heat curves of the LLDPE and thePS obtained from DSC are shown in Fig. 1. LLDPEhas a higher specific heat than the amorphous PS atthe temperature range tested. The melting temper-ature of the LLDPE detected was a little bit lowerthan 120◦C. The glass-transition temperature of thePS was found to be slightly higher than 100◦C. By in-tegrating the specific heat curves shown in Fig. 1, thespecific enthalpy curves for the polymers were ob-tained (shown in Fig. 2). From the specific enthalpycurve of each polymer we know that the amountof energy required to heat up LLDPE to its meltingtemperature is about 350 J/g, while less than half ofthat much, about 150 J/g, is needed to heat up PS tothe same temperature. As we described before, thisamount of energy has to be provided in the melt-ing process through either heat transfer and/or anyof the heat dissipation mechanisms (FED, PED, andVED).

The Instron capillary melt viscosities of the twopolymers measured at 200◦C are shown in Fig. 3.The LLDPE shows a higher viscosity than the PS overthe shear rate range tested. Melt-flow instabilitieswere observed for the LLDPE at higher shear rates.The viscous heating (VED) will be more effective ifthe more viscous LLDPE forms the continuous phaseof melt in the melting process.

Temperature (oC)

20 40 60 80 100 120 140 160 180 200

Spe

cific

Hea

t (J/

g*o C

)

0

2

4

6

8

10

LLDPE

PS

FIGURE 1. Specific heat curves of LLDPE and PSobtained by DSC at a heating rate of 20◦C/min.

Temperature (oC)

0 20 40 60 80 100 120 140 160 180 200

Spe

cific

Ent

halp

y (J

/g)

0

100

200

300

400

500

600

700

LLDPE

PS

FIGURE 2. Specific enthalpy curves of LLDPE and PSobtained through the integration of their respectivespecific heat curve shown in Fig. 1. Much more energy isrequired to heat up LLDPE than PS to the sametemperature.

PED BEHAVIOR OF PS, LLDPE,AND THEIR BLEND

Uniaxial compression experiments were con-ducted at different temperatures for the moldedcylindrical PS and LLDPE samples with a crossheadspeed of 25.4 mm/min. The molded samples wereweighed and their dimensions were measured priorto the test. Figures 4 and 5 show, as examples,the results of compressive force vs. displacementat 25◦C for the LLDPE and the PS “thick” sam-ples (∼�10 × 6.6 mm), respectively. The results are

Shear Rate (s-1)

100 101 102 103 104 105

Vis

cosi

ty (

Pa*

s)

101

102

103

104

PSLLDPE

FIGURE 3. Polymer melt viscosity curves of PS andLLDPE measured at 200◦C.

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Displacement (mm)

0 1 2 3 4 5 6

Com

pres

sion

For

ce (

N)

0

5000

10000

15000

20000

25000

30000

35000

sample 1 (0.425 g)sample 2 (0.425 g)sample 3 (0.429 g)

FIGURE 4. Compression force vs. displacement duringuniaxial compression of molded LLDPE samples at 25◦C(crosshead speed: 25.4 mm/min; samples:∼φ10 × 6.2 mm).

quite reproducible. Amorphous PS and semicrys-talline LLDPE behaved very differently under com-pression. PS was much tougher and showed obviousyielding (decreasing in compressive stress, as shownmore clearly later on in Fig. 11) during the compres-sion. Figure 11 also indicates that there was no ob-vious stress-softening for the semicrystalline LLDPEunder compression.

Figures 6 and 7 show, respectively, the tem-perature dependence of compression force vs.displacement curves of LLDPE and PS “thick”samples during compression. For both materials, the

Displacement (mm)

0 1 2 3 4 5 6

For

ce (

N)

0

5000

10000

15000

20000

25000

30000

35000

sample 1 (0.546 g)sample 2 (0.562 g)

FIGURE 5. Compression force vs. displacement duringuniaxial compression of molded PS samples at 25◦C(crosshead speed: 25.4 mm/min; samples:∼φ10 × 6.7 mm).

Displacement (mm)

0 1 2 3 4 5

For

ce (

N)

0

2000

4000

6000

8000

10000

25C50C75C100C

FIGURE 6. Typical force vs. displacement curves duringcompression of molded LLDPE samples at differenttemperatures (crosshead speed: 25.4 mm/min; sampledimension: ∼φ10 × 6.2 mm; average sample weight:∼0.43 g).

compression forces decreased dramatically with in-creasing temperature; the higher the sample temper-ature, the less energy input during compressive de-formation. In other words, PED decreases drasticallywith increasing temperature for both materials.

The area under each force vs. displacement curvegives the total mechanical energy input during thecompression. By integrating the compression forcevs. displacement curve, we can calculate the totalmechanical energy input in the compression pro-cess. Specific mechanical energy (SME) input was ob-tained by dividing this mechanical energy input by

Displacement (mm)

0 1 2 3 4 5

For

ce (

N)

0

5000

10000

15000

20000

25000

25C-1 (0.546g)25C-2 (0.562g)75C-1 (0.553g)75C-2 (0.562g)100C-1 (0.551g)100C-2 (0.560g)

FIGURE 7. Force vs. displacement curves duringcompression of molded PS samples at differenttemperatures (crosshead speed: 25.4 mm/min; sampledimension: ∼φ10 × 6.7 mm; average sample weight:∼0.56 g).



the preweighed sample weight, noting that the dif-ferences in sample weight were very small. Figure 8shows the results for the SME during the compres-sion plotted against the engineering strain, whichwas defined as the ratio of compressive deformationto the original sample thickness. It is obvious fromFig. 8 that much more mechanical energy (about threetimes as much) was required to deform amorphous PSthan to deform semicrystalline LLDPE solid samples atthe same initial sample temperature. Coincidentally, theSME of the LLDPE at 25◦C was found to be similarto that of the PS at 100◦C. From the specific enthalpycurves of those polymers in Fig. 2, we can estimatethe temperature rise of the samples during the com-pression from their respective SME values, assum-ing, as a first approximation, that all mechanical en-ergy input during compression was dissipated intoheat. For a compressive deformation to an engineer-ing strain of 0.8, the SME input during the compres-sion and the resultant temperature rise this amountof energy can provide for the PS and the LLDPE sam-ples are shown in Table I.

In Table I, SME refers to the specific mechanicalenergy input during uniaxial compressive defor-mation of molded polymer cylindrical samples toan engineering strain of 0.8 at crosshead speed of25.4 mm/min. As a first approximation, we assumethat all mechanical energy input during the compres-sion is converted into heat, neglecting the elastic partof energy stored in the sample. The predicted finalsample temperature is the theoretical temperaturerise derived from the total mechanical energy inputduring the compression and the specific enthalpycurves shown in Fig. 2. The results in Table I clearly

Engineering Strain

0.0 0.2 0.4 0.6 0.8

Spe

cific

Mec

hani

cal E

nerg

y In

put (

J/g)

0

20

40

60

80

100

120PE-25-1PE-25-2PE-25-3PE-75-1PE-75-2PE-75-3PE-100-1PE-100-2PE-100-3PS-25-1PS-25-2PS-75-1PS-75-2PS-100-1PS-100-2

PS at 25oC

PS at 75oC

PS at 100oCPE at 25oC

PE at 75oCPE at 100oC

FIGURE 8. Specific mechanical energy (SME) input asa function of compression strain during uniaxialcompression of molded PS and LLDPE samples atdifferent initial sample temperatures (crosshead speed:25.4 mm/min).

TABLE IPED Behavior of PS and LLDPE at Different InitialTemperatures

Initial Test Temperature

25◦C 75◦C 100◦C

PSSME (J/g) 98 66 34Predicted final sample 93 113 117

temperature (◦C)Predicted sample 68 38 17

temperature rise (◦C)LLDPE

SME (J/g) 33 19 11Predicted final sample 40 81 102

temperature (◦C)Predicted sample 15 6 2

temperature rise (◦C)

Crosshead speed: 25.4 mm/min; engineering strain: 0.8.

indicate that one compressive deformation to anengineering strain of 0.8 can provide enough energyto heat up the PS from room temperature (25◦C) to93◦C, very close to the glass-transition temperatureof the PS. The same deformation can only heat up theLLDPE from room temperature to 40◦C. This totallydifferent PED behavior of PS and LLDPE shouldrender different initial heating/melting phenomenain PED-dominant melting devices like twin-screwextruders. We have attributed this different PEDbehavior to the differences in the heating/meltingrate of single-component polymers in twin-screwextruders. The higher PED of the amorphous PSduring compressive deformation results in its highermelting rate as a single-component in twin-screwextruders.8

We next turn our attention to LLDPE/PS blendto examine the blend melting behavior, since the in-dividual blend components have a drastically andquantitatively different PED behavior. To under-stand that, we started with the study of the compres-sive deformation behavior of two stacked polymersamples.

The same LLDPE was compounded with a smallamount of blue pigment to color the pellets blue.The blue LLDPE and original white LLDPE pel-lets were molded into both “thin” (∼3.3 mm thick-ness) and “thick” (∼6.6 mm thickness) samples.Two thin samples of different colors were moldedagain into one thick sample with half blue and halfwhite. The LLDPE samples prepared in different

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ways (molded or stacked) were compressed underthe same conditions. Compression force vs. displace-ment curves were integrated to obtain the mechani-cal energy input. Figure 9 shows the results of SMEinput vs. engineering strain at 25◦C for LLDPE sam-ples prepared in different ways. The molded sam-ples of the same color and of different colors gavesimilar results as the ones of physically stacking twothin samples together. Similar results were obtainedfor the PS. In other words, for samples molded fromthe same material under unconfined uniaxial com-pression, physically stacking two thin samples to-gether with no interfacial adhesion behaved simi-larly to the molded thick sample with no such weakinterface. Furthermore, since the colored LLDPE andPS samples were precompounded, i.e., with one ad-ditional thermal history, we can state that one-passcompounding with a small amount of pigment hasnot affected their PED behaviors.

We were unsuccessful to mold a thick sample fromtwo thin samples of immiscible PS and LLDPE, be-cause of the lack of sufficient molecular diffusionat the interface even after 2 h at their melt process-ing temperature. The compression of PS and LLDPEblend samples can currently be carried out only byphysically stacking two thin samples together with-out any adhesion in the interface. The compressionforce vs. displacement curves for the stacked PSand LLDPE samples at 25 and 75◦C are shown inFigs. 10a and 10b respectively. The results for molded

Engineering Strain

0 1

Spe

cific

Mec

hani

cal E

nerg

y In

put (

J/g)

0

20

40

60

80

100

molded PE (blue)stacked PE halvesmolded halves (white/blue)molded halves (blue/white)molded PE

FIGURE 9. SME input vs. engineering strain for LLDPEat 25◦C prepared in different ways (crosshead speed:25.4 mm/min): stacking two molded “thin” LLDPEsamples together behaved almost the same incompression as one “thick” molded sample of equalthickness as those two “thin” samples.

Displacement (mm)

0 1 2 3 4 5

Com

pres

sion

For

ce (

N)

0

5000

10000

15000

20000

25000

molded PS at 25oC

stacked PS and PE at 25oC

molded PE at 25oC

(a)

(b)

Displacement (mm)

0 1 2 3 4 5

Com

pres

sion

For

ce (

N)

0

5000

10000

15000

20000

molded PE at 75oC

molded PS at 75oC

stacked PS and PE at 75oC

FIGURE 10. Compression force vs. displacementcurves of stacked PS and LLDPE “thin” samplescompared with molded PS and LLDPE “thick” samples(crosshead speed: 25.4 mm/min). Initiation sampletemperature: (a) at 25◦C; (b) at 75◦C. Initially the stackedsamples of PS and PE behaved quite like molded PEsamples. After the soft half of PE was nearly squeezedout, the compression force started to increase.

PS and molded LLDPE samples are also shown forcomparison. Initially, the stacked samples of PS andLLDPE behaved quite like molded LLDPE samples,the softer part of the two materials. After the softhalf of LLDPE was nearly squeezed out (at a dis-placement close to the thickness of one “thin” sam-ple: ∼3.3 mm), the compression force started to in-crease, and approached the compression behavior ofthe tougher PS samples. The lower value in compres-sion forces after relatively soft LLDPE was squeezedout is mainly due to the difference in the cross-sectionarea of the PS thin and thick samples at those nomi-nal displacements. The cross-section area of “thick”



samples should be nearly doubled at displacement of∼3.3 mm while the cross-section area of the “thin”samples was barely changed at this displacement,since most of the deformation was due to the squeez-ing out of the softer LLDPE part of stacked samples.

The compressive deformation behaviors ofstacked samples of PS and LLDPE can be explainedfrom the stress–strain relationship of each compo-nent, as shown in Fig. 11. For example, at the samecompressive stress level of 20 MPa, the engineeringstrain (deformation) was only about 0.03 for the PS,but was nearly 20 times higher for the LLDPE. Inother words, most of the initial compressive deformation(nearly 95%) was accommodated by that of the relativelysofter LLDPE component. This explains why the com-pressive behavior of stacked PS and PE behaved ini-tially much like the soft PE part (see Fig. 10). Whenthe compressive stress was larger than ∼28 MPa (seeFig. 11), the deformation of soft PE was close to engi-neering strain of 1, i.e., it was squeezed out. PS thenbecame the sole force-bearing component during thecompression.

The areas under the stress–strain curves (Fig. 11)are the total energy input during the compression. At20 MPa, the energy input integrated from the curvesin Fig. 11 is 0.33 for the hard PS and 10.12 for the softLLDPE. In other words, more than 95% of mechanicalenergy was absorbed by the soft LLDPE component duringthe compression of stacked, i.e., placed “in series,” PS andLLDPE samples.

The large PED of the strong and tough amorphousPS resulted in fast melting as single-component in

Engineering Strain

0.0 0.2 0.4 0.6 0.8

Com

pres

sive

Str

ess

(MP

a)

0

20

40

60

80

100PS-1PS-2PE-1PE-2

FIGURE 11. Compressive stress–strain behavior of PSand LLDPE at 25◦C (crosshead speed: 25.4 mm/min); atcompression stress of 20 MPa, the deformation of softLLDPE was nearly 20 times larger than the tough PS part.

the PED-dominant melting section of twin-screwextruders.7,8,10 The mandatory deformation, broughtabout by the interaction of two rotors, will forcethe particulates of compacted polymer blends to de-form. This deformation is similar to the compres-sions of stacked PS and LLDPE samples, thoughin a more confined manner in extruders. The PEDbehaviors of polymer blends should have direct ef-fects on the heating/melting behavior of individualcomponents in the PED-dominant melting devicessuch as twin-screw extruders. Thus, the different re-sponse to plastic deformation of various polymerswhen blended together may result in a totally dif-ferent heating/melting behavior for the polymer blendsas compared to the melting of individual components inPED-dominant melting devices. For example, single-component PS normally has a much shorter meltinglength than PE in twin-screw extruders partly be-cause, as indicated in Fig. 8, the PS has a larger, thus,more effective PED than PE. The heating/meltingof PS/PE blend is much more complex and by nomeans follows the trend of the melting of each in-dividual component. PS may not necessarily meltfaster than PE when the two are blended together. Ifthe forces causing the “hard” PS to deform are onlythose from “soft” PE neighbors, the PED of “hard”PS might be trivial because of the resultant verysmall deformation for the material of higher mod-ulus and toughness, though the same reason alsocauses its large PED and resultant fast melting as asingle-component polymer. The relation between thecompressive deformation PED behavior of polymerblends and their heating/melting behavior in twin-screw extruders are examined and discussed in thenext section through carcass experiments.

MELTING BEHAVIOR

Melting of single-component LLDPE was carriedout using melting screw design with 45◦ staggeringangle forward kneading blocks followed by a reversescrew element. Typical results are shown in Fig. 12a.All three barrel-sections were set at 200◦C. The screwspeed was 200 rpm, and the flow rate 16 kg/h. Fromthe carcass image, it was found that melting com-menced at the fully consolidated region. Withoutconsolidation, the forward kneading blocks behavedsomewhat like conveying screw elements, as shownin Fig. 12a. Although a few pellets may be caughtand get deformed, the majority of pellet feed wasconveyed with little change in the partially filledsection. Melting was very effective once polymerparticulates were consolidated because of the barrier

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FIGURE 12. The melting carcasses of LLDPE/PS blends in a Co-TSE. (a) LLDPE; 4 × KB45/5/28+20/10(R); 200 rpm,16 kg/h, 200◦C; Bulk melting commenced only in the consolidated section; (b) LLDPE/PS blend (90/10 wt);3 × KB45/5/28+20/10(R); 100 rpm, 12 kg/h, 200◦C; 10% of LLDPE pellets colored in light blue; (c) LLDPE/PS blend(90/10); 4 × KB90/5/28; 100 rpm, 8 kg/h, 200◦C/200◦C/200◦C; 10% of LLDPE pellets colored in light blue; PS pelletscolored in red; (d) LLDPE/PS blend (90/10); 4 × KB90/5/28; 100 rpm, 24 kg/h, 200◦C/200◦C/200◦C; 10% of LLDPEpellets colored in light blue; PS pellets colored in red; (e) LLDPE/PS blend (90/10); 4 × KB90/5/28; 100 rpm, 8 kg/h,100◦C/130◦C/123◦C; 10% of LLDPE pellets colored in light blue; PS pellets colored in red; (f) LLDPE/PS blend (50/50);4 × KB90/5/28; 100 rpm, 8 kg/h, 100◦C/129◦C/119◦C; 10% of LLDPE pellets colored in light blue; 10% of PS pelletscolored in red.

(reverse) screw element downstream, and very shortmelting length (1–2 L/D) was observed. Comparedwith the melting of single-component amorphousPS,8 semicrystalline PE has a much longer melting

length. Mixing or blending morphology evolves con-currently with the melting process.

The typical melting carcass of two-componentpolymer blend (PE/PS: 90/10 wt) is shown in



Fig. 12b. The blend consisted of 80% of “virgin”PE, 10% of the colored PE in light blue, and 10%of PS, all in pellet form. Again, all three barrels wereset at 200◦C. The feeding rate was 12 kg/h and thescrews were running at 100 rpm. The total meltinglength for the blends was comparable to that of asingle-component PE. More unmelted PS “escapees”(detected as more transparent spots in the meltingcarcass while it was held against the light) were ob-served than PE “escapees” (seen from the light bluespots in the carcass) just before the reverse screw el-ement. Amorphous PS, while it melts much faster as asingle-component polymer, did not show the same highrate of melting as a participant in polymer blend meltingwith softer PE, which can be readily explained in thePED behavior of the blend.

As a single component, amorphous PS withhigher level of PED exhibited a shorter meltinglength in twin-screw extruders than the relativelysofter semicrystalline PE.8,10 The melting behaviorof their blends, however, is more complicated. Thetougher PS, which has a much shorter melting lengthwhen melted alone, took much longer to get meltedas a blend component with the softer PE. The sameblend melting behavior was observed in a meltingscrew configuration composed of all neutral knead-ing blocks (see Figs. 12c–12e) for the same LLDPE/PSblends of weight ratio of 90/10. The only processvariable difference between the samples shown inFig. 12c and 12d was the flow rate, which was muchhigher in the latter. Figures 12c and 12e differ inthat the latter had much lower barrel temperature.Figure 12f also shows low barrel temperature melt-ing, but with blend of LLDPE and PS in a 50/50weight ratio.

From all above carcass melting experiments ofLLDPE/PS blends under various conditions andcompositions, the softer LLDPE part always meltedfaster than the harder PS component. This can also beexplained from their different PED behaviors whencompressed together in series. Although PS has ahigher PED, and thus melts faster as an individ-ual component than LLDPE, the blend PED behav-ior showed that the majority (∼95%) of the plas-tic energy was dissipated inside the softer LLDPEpart. The much larger TSE-generated PED of LLDPEthan PS in their blends resulted in a faster heat-ing/melting of LLDPE in twin-screw extruders, eventhough LLDPE is the softer component. After thesoft LLDPE melts and forms a continuous phase, themelting of the remaining PS pellets and LLDPE “es-capees” is via heat conduction from the surroundingmelt, which is heated mainly by VED. A physically

sound melting model should be able to account forthe initial heating/melting of polymers or polymerblends because of their respective PED behaviorsand subsequent melting of “escapees” owing to heattransfer and VED of the melt. Although not specifi-cally studied, it is possible that in high melt viscosityblends, the “escapees,” remaining as solid particu-lates, may undergo deformation and thus generatePED.

Conclusions

PED is a very effective melting mechanism. Onecompressive deformation to an engineering strainof 0.8 could provide enough energy to heat up PSfrom room temperature to 93◦C, close to its glass-transition temperature. The same deformation couldheat up LLDPE from room temperature to 40◦C. Thelarge PED results in a very fast melting of single-component PS in PED-dominant melting devices liketwin-screw extruders.

The melting of multi-component polymer blendsis more complicated, totally different, but physicallyentirely reasonable based on the melting of the in-dividual blend components. The tougher polymer,which normally has a much shorter melting lengthwhen melted alone, is not necessarily the fastermelting component in blends. This is because inblends, not unlike stacked cylinders of the blendcomponents undergoing unconfined compressivedeformation, the softer component “absorbs” mostof the plastic deformation energy. Thus, relativerates of initial heating/melting of the blend compo-nents depend inversely on their individual modulusand mechanical strength at high deformationlevels.

References

1. Tadmor, Z.; Gogos, C. G. Principles of Polymer Processing;John Wiley & Sons: New York, 1979.

2. Tadmor, Z. Polym Eng Sci 1966, 5, 185.3. Tadmor, Z.; Klein, I. Engineering Principles of Plasticating

Extrusion; Reinhold Book Co.: New York, 1970.4. Todd, D. B. SPE ANTEC 1992, 39, 2528.5. Bawiskar, S.; White, J. L. Polym Eng Sci 1998, 38(5), 727.6. Potente, H.; Melisch, U. Int Polym Proc 1996, 11, 101.

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7. Gogos, C. G.; Tadmor, Z.; Kim, M. H. Adv Polym Tech 1998,17, 285.

8. Qian, B.; Gogos, C. G. Adv Polym Tech 2000, 19(4),287.

9. Vergnes, B.; Souveton, G.; Delacour, M. L.; Ainser, A. IntPolym Proc 2001, 16, 351.

10. Gogos, C. G.; Qian, B. Adv Polym Tech 2002, 21(4),287.

11. Kim, M. H. Ph.D. dissertation, Stevens Institute of Technol-ogy, Hoboken, NJ, 1999.

12. Esseghir, M.; Yu, D.; Gogos, C. G.; Todd, D. B; Tadmor, Z. SPEANTEC, 1997, 43, 3684.


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