elongational viscosity as a tool to predict the foamability of polyolefins

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http://cel.sagepub.com/ Journal of Cellular Plastics http://cel.sagepub.com/content/42/3/207 The online version of this article can be found at: DOI: 10.1177/0021955X06063510 2006 42: 207 Journal of Cellular Plastics Henk Ruinaard Elongational Viscosity as a Tool to Predict the Foamability of Polyolefins Published by: http://www.sagepublications.com can be found at: Journal of Cellular Plastics Additional services and information for http://cel.sagepub.com/cgi/alerts Email Alerts: http://cel.sagepub.com/subscriptions Subscriptions: http://www.sagepub.com/journalsReprints.nav Reprints: http://www.sagepub.com/journalsPermissions.nav Permissions: http://cel.sagepub.com/content/42/3/207.refs.html Citations: What is This? - Apr 27, 2006 Version of Record >> at Faculty of Political Science on July 19, 2012 cel.sagepub.com Downloaded from

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http://cel.sagepub.com/Journal of Cellular Plastics

http://cel.sagepub.com/content/42/3/207The online version of this article can be found at:

 DOI: 10.1177/0021955X06063510

2006 42: 207Journal of Cellular PlasticsHenk Ruinaard

Elongational Viscosity as a Tool to Predict the Foamability of Polyolefins  

Published by:

http://www.sagepublications.com

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What is This? 

- Apr 27, 2006Version of Record >>

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Elongational Viscosity as a Toolto Predict the Foamability

of Polyolefins*

HENK RUINAARDy

SABIC EuroPetrochemicals

PO Box 475, 6160 Geleen

The Netherlands

ABSTRACT: The foaming of polyolefins (POs) by direct gas injection is adelicate balance between the melt strength of the expanding polymer and thepressure of the blowing gas in the growing cells.Important parameters in this process are: melting and crystallization

temperature of the polyolefin grade, viscosity in the crystallization temperaturerange and elongational viscosity in the crystallization temperature range.In particular, the increase in elongational viscosity or so-called strain hardeningis vital for a successful foam structure.This study shows how the elongational viscosity is measured according to

the rheological melt extension (RME) method. The experimental conditionsfor LDPE, LLDPE, HDPE, mPE plastomers, and PP are defined and used fora series of 25 PO grades.These experiments show that the strain hardening ratio (SHR) is influenced

by the average molecular length, molecular weight distribution (MWD) and thelong chain branching (LCB).

KEY WORDS: strain hardening, elongational melt extension, elongationalviscosity, SEC-MALLS, gyration parameter.

*This was presented at ANTEC 2005, May 1–5, 2005, Boston, Massachusetts.The copyright is held by Society of Plastics Engineers.yE-mail: [email protected] 1–6 appear in color online: http://cel.sagepub.com

JOURNAL OF CELLULAR PLASTICS Volume 42 — May 2006 207

0021-955X/06/03 0207–14 $10.00/0 DOI: 10.1177/0021955X06063510� 2006 SAGE Publications

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INTRODUCTION

Extrusion of PO foams by direct gas injection or so-called physicalfoaming can be separated into five distinctive steps, i.e., melting,

gas injection and gas mixing, melt cooling, melt shaping, and gasexpansion and foam cooling.

The gas expansion is the crucial step in the foam process. In orderto obtain a good foam quality, the gas-laden melt needs to be cooleddown to a temperature close to the crystallization temperature ofthe semi-crystalline polyolefin to increase the melt viscosity and reducethe time needed for the transition from melt to solid phase. In practice,this means that for semi-crystalline POs the temperature of the meltat the die exit is always a few degrees centigrade higher than thecrystallization temperature Tc (with the exception of the very lowcrystalline mPE plastomers). This condition is the starting point of thecell growth process, in which the molten PO is elongated biaxially bythe expanding gas and cooling down at the same time by endothermicheat loss of the expansion and heat loss to the environment. Themelt elongation will be stopped by the transition into the solid phasewhen the crystallization temperature is reached.

Experiences from the past have shown that, apart from a very goodnucleation, a high elongational viscosity and an increase of theelongational viscosity during this expansion (so- called strain hardening)are required to get a good foam quality (i.e., regular fine cell size andhigh closed cells content).

THEORY AND DEFINITIONS

Strain hardening can only be measured accurately by the rheologicalmelt extension (RME) method as developed by Meissner [1] in 1971.The elongational viscosity �el is measured by elongating a well-definedcompression moulded sample in a hot nitrogen filled oven at a well-defined temperature and strain rate. The �el is calculated from themeasured force and strain as a function of time [2].

The work of Meissner et al. [3], shows that the elongational viscosityat a given temperature, strain rate and strain equals 3 times theshear viscosity �sh at the same conditions as shown in the followingformula:

�þel þ ðtÞ ¼ 3�þshðtÞ

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However, �el will increase more than proportional with increasing timedue to disentanglements of the long chain branching (LCB) withincreasing strain, whereas �sh will only increase slowly and less thanproportional with time. The difference between �el and 3 �sh is calledstrain hardening.

This strain hardening is known to be of a different magnitude fordifferent PO grades [3], depending on their molecular weight distribu-tion (MWD) and LCB. It is also known that some PO grades show a verygood foamability while others are not foamable at all with the physicalfoaming process.

For the purpose of comparing the strain hardening of different POgrades, the so-called strain hardening ratio (SHR) was calculated asthe ratio between the maximum �el of the RME curve (at time tmax) and3 �sh of the dynamic mechanical spectrometry (DMS) frequency sweepcurve at time tmax as �el max i.e.,

SHR ¼�el max

3�sh at tmax

The strain hardening of a PO grade depends strongly on its MWD.A good way to measure the MWD of long chain branched POs is sizeexclusion chromatography (SEC) coupled to both a refractive index(RI) and a multiple angle laser light scattering (MALLS) detector. Thistechnique will further be called SEC-MALLS.

From the MWD, the following averages can be calculated: the numberaverage molecular mass Mn (most influenced by the shorter molecules),the weight average molecular mass Mw, and the z-average molecularmass Mz (most influenced by the longer and strongly branchedmolecules). The width or polydispersity of the MWD can be expressedin the ratios Mw/Mn and Mz/Mw. The shape of the MWD curve and asa consequence the polydispersity ratios depend strongly on the degreeof LCB of the PO grade. The non-LCB grades in this study showtypically an MWD with only one peak and rather low Mw/Mn and Mz/Mw

ratios. Polymers with high LCB show a broad MWD with anextra peak (shoulder) at the high molecular mass side of the MWD.The Mw/Mn and Mz/Mw ratios are dependant on the polymer typeand reaction process.

Using the SEC-MALLS technique, the branching degree can bedetermined by the gyration parameter (g-parameter), which is theratio of the radius of gyration of the branched versus the linear polymerof a certain molecular mass. By definition, non-branched polymershave a g-value of 1. The higher the branching degree, the lower theg-parameter, so values vary between 1 and 0.

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EXPERIMENTAL

The purpose of the following experimental program is to relate theelongational strain hardening (as expressed in SHR) and molecularcharacteristics (MWD and LCB as expressed in g-parameter) with theknown foamability behavior, for a set of 27 PO grades (Table 1).

PO grades from different polymerization processes (high pressuretubular, high pressure autoclave, low pressure slurry, low pressure gasphase, low pressure solution), catalysts (Ziegler-Natta, chromium,metallocene) and co-monomers (propene, butene, hexene, octene),were chosen for this comparison.

The temperature conditions for the RME experiments were basedon the Tc of the specific PO grade (Table 2) and were chosen close to

Table 1. Tested PO grades and experimental data.

SEC-MALLS

POgrade

PO type/process

MFI(dg/min)

Density(kg/m3) SHR

Mn

�103Mw

�103Mz

�103Mw/Mn

–Mz/Mw

LDPE 1 LDPE tubular 0.7 921 7.1 18 280 3000 15.5 10.7LDPE 2 LDPE tubular 0.9 921 6.5 24 320 4700 13.3 14.7LDPE 3 LDPE tubular 0.9 925 7.9 21 275 4800 13.2 17.5LDPE 4 LDPE tubular 1.5 926 4.1 17 225 4100 12.8 18.4LDPE 5 LDPE tubular 1.9 921 7.3 20 240 3000 11.8 12.8LDPE 6 LDPE autoclave 2.0 921 8.2 31 880 5400 28.4 6.1LDPE 7 LDPE tubular 2.5 921 6.5 18 235 3000 13.1 12.7LDPE 8 LDPE tubular 2.5 924 6.4 17 235 5400 13.8 23.1LDPE 9 LDPE tubular 4.7 921 6.3 19 228 3480 11.8 15.3LDPE 10 LDPE tubular 4.7 924 6.3 19 225 4200 12.2 18.7LDPE 11 LDPE autoclave 4.2 924 8.0 15 365 2450 24.6 6.7LDPE 12 LDPE tubular 5.0 924 7.0 15 200 2800 13.0 14.2LDPE 13 LDPE tubular 5.0 924 4.3 15 190 2250 12.7 11.8LLDPE 1 C4-gas phase 0.8 921 1.3 34 145 800 4.2 5.5LLDPE 2 C4-gas phase 1.0 918 1.3 36 170 920 4.7 5.4LLDPE 3 C4-gas phase 2.8 918 1.3 23 130 970 5.7 7.4LLDPE 4 C4-gas phase 7.0 932 1.5 18 86 620 4.8 7.2LLDPE 5 C8-solution 1.0 919 1.3 30 115 370 3.8 3.2LLDPE 6 C8-solution 4.4 919 2.4 24 93 310 3.9 3.4mPE 1 C8-solution 1.0 902 1.8 39 94 200 2.4 2.1mPE 2 C8-solution 3.0 902 1.8 32 81 170 2.5 2.1HDPE 1 Slurry 3.5 953 1.4 21 130 660 6.4 5.0HDPE 2 Slurry 0.5 962 12.1 26 155 1500 5.9 9.4PP 1 RCPP slurry 0.6 910 1.2 170 600 1700 3.5 10.7PP 2 HMS PP 2.8 912 49.0 83 710 3200 8.5 4.5PP 3 HMS PP 2.5 912 70.0 94 630 3100 6.7 4.9PP 4 HMS PP 2.4 912 28.2 99 550 2600 5.6 4.6

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the practical foaming conditions, i.e., approximately 10�C above the Tc ofthe PO types to prevent variations in the measurements dueto premature crystallization. All RME experiments were performed atthe same strain rate setting of 1/sec, which is expected to show thebest differentiation and is also close to the practical conditions(cell growth speed).

RESULTS AND DISCUSSION

The typical data of the PO samples and the results of RME andSEC-MALLS experiments are presented in Table 1.

A typical example of an RME curve is shown in Figure 1 for a tubularLDPE grade, an autoclave LDPE grade, a gas phase LLDPE grade,and an mPE plastomer grade. The SEC-MALLS curves (includingg-parameter vs. molecular mass) of these same grades are presentedin Figure 7.

Tubular and autoclave LDPE grades show strain hardening as theyall have a relatively broad MWD and a relatively high degree of LCB.The difference between tubular and autoclave LDPE is demonstratedby the steeper viscosity upswing, the slightly higher maximum �el andthe lower elongation at break for the autoclave grade due to its higherLCB content.

LLDPE is a linear molecule without LCB and consequently doesnot show strain hardening. However, linear molecules like LLDPE areknown to have a high elongation at break both in the solid phase as wellas in the molten phase as is demonstrated by LLDPE 3 in Figure 1.mPE plastomers are also linear molecules with a very narrow MWD,which is proved by the narrow peak in Figure 7, but the short C6 sidebranches of the octene co-monomer and the low content of LCB at highmolecular mass positively contribute to the melt strength and slightly tothe strain hardening.

Table 2. RME standard conditions.

PO typeTc-onset temperature

range in �CRME experimentaltemperature in �C

mPE plastomers 85–95 105LDPE 95–105 110LLDPE 105–115 125HDPE 110–120 130PP 120–140 160

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For tubular LDPE grades, the strain hardening depends only verylittle on the melt flow index (MFI) but more on the MWD as is shown inFigure 2. As expected, the �el is increasing with decreasing MFI due tothe increasing molecular mass, but this is not necessarily linked to an

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

0.1 1 10Time (s)

Visc

osity

(Pa

s)2 = LDPE 7; Tubular LDPE, MFI = 2.5 dg/min1 = LDPE 6; Autoclave LDPE, MFI = 2.0 dg/min3 = LLDPE 3; Gasphase LLDPE, MFI = 2.8 dg/min4 = mPE 2; Solution LLDPE, MFI = 3.0 dg/min 1

2

3

4

Figure 1. RME curves of a tubular LDPE, autoclave LDPE, gas phase LLDPE grade, and

mPE grade.

0.1 1 10Time (s)

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+071 = LDPE 1 2 = LDPE 23 = LDPE 4 4 = LDPE 75 = LDPE 9

12

34

5

Visc

osity

(Pa

s)

Figure 2. RME curves of tubular LDPE grades.

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increase in SHR. The SHR ranges from 4.1 for an MFI of 1.5 dg/minto 7.9 for an MFI of 0.9 dg/min.

Tubular LDPE grades have a wide MWD (Figure 8), which is shown inthe high Mw/Mn ratio (between 11.8 and 15.5) a high to very high Mz/Mw

ratio (between 10.7 and 23.1). In contradiction to most autoclave grades,tubular grades do not have a second peak in their MWD or bimodaldistribution, but more molecules with a higher molar mass (between106.5 and 107.5) than autoclave grades. The g-parameter shows that inparticular these long molecules are long chain branched. For tubularLDPE, these molecules are probably the ones that contribute most tothe SHR.

Autoclave LDPE grades are known for their high melt strength whichmakes them perfectly fit for the extrusion coating process. This highmelt strength is demonstrated in their high SHR ratios (between 8.0 and8.2; Figure 3). The superior strain hardening of these grades is ascribedto their increased proportion of LCB, which is shown as a second peakat higher MWD in the SEC-MALLS curves (Figure 9). For autoclaveLDPE grades, the SHR seems not to depend at all on the MFI,but completely on the amount and length of the LCB. The broadMWD results in a high Mw/Mn ratio (between 24.6 and 28.4) butresults surprisingly in a low Mz/Mw ratio (between 6.1 and 6.7). Theg-parameter shows that the LCB of autoclave grades is present ata lower molecular mass range (105–106) than for tubular grades.Although it is hard to put this down in one number, it is clear that this

0.1 1 10Time (s)

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+072 = LDPE 7; Tubular MFI = 2.5 dg/min3 = LDPE 11; Autoclave MFI = 4.2 dg/min4 = LDPE 10; Tubular MFI = 4.7 dg/min1 = LDPE 6; Autoclave MFI = 2.0 dg/min

1 234

Visc

osity

(Pa

s)

Figure 3. RME curves of tubular vs autoclave LDPE grades.

Elongational Viscosity to Predict the Foamability of POs 213

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high fraction of LCB in the 105–106 molecular mass range is responsiblefor the high SHR in the RME curves.

The tested LLDPE grades show a wide range in �el (e.g., at0.4 s between 104Pa s and almost 105Pa s) due to the MFI range of0.8–7.0 dg/min, however, this does not show in the SHR(Figure 4). LLDPE grades in general do not show strain hardeningas they typically have a very narrow MWD (Figure 10). This isdemonstrated by the low Mw/Mn ratio values of C4- and C6-LLDPEgrades (between 4.2 and 5.7 and the low Mz/Mw ratio (between 5.4and 7.2). The SEC-MALLS method shows that LLDPEs can have a verylow content of LCB (g-parameter of LLDPE 4 in Figure 10) but thecontribution to the strain hardening is very small. Consequentlythe SHR values of C4- and C6-LLDPE grades are very low (between1.3 and 1.5).

C8-LLDPE solution grades also have a low Mw/Mn ratio(between 3.8 and 3.9) and a very low Mz/Mw ratio (between 3.2and 3.4). Due to the broader MWD and a very small content of LCBof the longer molecules, C8-LLDPE solution grades show a betterSHR than the comparable C4-LLDPE gas phase grades e.g., LLDPE 6;MFI¼ 4.4 dg/min; SHR¼ 2.4 as compared to LLDPE 3; MFI¼ 2.8;SHR¼ 1.3.

mPE plastomers typically have a very narrow MWD (Figure 10),which is demonstrated by their very low Mw/Mn ratio (between 2.4and 2.5) and their very low Mz/Mw ratio (2.1). However, the shape of

0.1 1 10Time (s)

1.0E+03

1.0E+04

1.0E+05

1.0E+062 = LLDPE 2 3 = LLDPE 3 1 = LLDPE 14 = LLDPE 45 = LLDPE 56 = LLDPE 67 = mPE 18 = mPE 2

7

81

25

3

6

4

Visc

osity

(Pa

s)

Figure 4. RME curves of LLDPE grades.

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the RME curve (mPE 1 and mPE 2 in Figure 4) and the SHR value of1.8 suggests that these grades have some LCB. This is confirmed bythe g-parameter curves (mPE 1 in Figure 10), but possibly also thehigh octene content (approx. 20%), which introduces a high level ofshort chain branching, is responsible for this behavior.

HDPE grades in general have a relatively narrow MWD, which isdemonstrated by their relatively low Mw/Mn ratio (between 5.9 and 6.4)and relatively lowMz/Mw ratio (between 5.0 and 9.4). Surprisingly it wasfound that some slurry grades can have a very high SHR as is shownby HDPE 2 (MFI¼ 0.5 dg/min; Figure 5). The SEC-MALLS curve ofHDPE 2 (Figure 11) shows some LCB at high molecular mass (106–107),however, with a very steep g-parameter curve whereas HDPE 1(MFI¼ 3.5 dg/min) shows no LCB at all. The explanation for this veryhigh SHR of HDPE 2 is possibly also the strain-induced crystallizationthat was observed during this experiment. (Remark: This effectdisappeared when the experiment was repeated at 190�C. More workhas to be carried out to study this effect.)

PP grades (homo copolymers, block copolymers as well as randomcopolymers) are characterized by a very narrow MWD (Mw/Mn ratio 3.5),no LCB and consequently a very low melt strength and an absence ofstrain hardening (SHR¼ 1.2). However, the high melt strength (HMS)PP grades, which have obtained LCB by a special grafting process, showa much wider MWD (Mw/Mn ratio between 5.6 and 8.5; Figure 12) andthe expected high melt strength (Figure 6). Like autoclave LDPE grades,HMSPP grades have a relatively low Mz/Mw ratio (between 4.5 and 4.9)

0.1 1 10Time (s)

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+072 = HDPE 21 = HDPE 1

2

1

Visc

osity

(Pa

s)

Figure 5. RME curves of HDPE grades.

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and a slightly bimodal distribution. In spite of this, the Mw/Mn ratiois rather low (between 5.6 and 8.5). Comparison of the LCB of thesegrades with the linear molecule of the Randol Copol Poly Propylene(PP 1) reveals that most of the LCB is present in the mol mass range of106–107 in the second peak of the MWD. This is more or less comparablewith tubular LDPE, however, the SHR for these grades is so high

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

1.2

LDPE 10LDPE 11LLDPE 2mPE 1

d W

(M) /

d (l

og M

)

log M

g - p

aram

eter

LDPE 10LDPE 11LLDPE 2mPE 1

Figure 7. SEC–MALLS curves of a tubular LDPE grade, autoclave LDPE grade, gas

phase LLDPE grade, and mPE grades.

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

0.1 1 10

1 = PP 12 = PP 24 = PP 43 = PP 3

1

2

3

4

Time (s)

Visc

osity

(Pa

s)

Figure 6. RME curves of PP grades.

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(between 28 and 70) that it is probably partly caused by strain inducedcrystallization.

Due to the absence of a reference ‘linear high molecular PP grade,’the g-parameter could not be calculated. This will be subject forfuture work.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

1.2

d W

(M) /

d (l

og M

)

log M

g - p

aram

eter

LDPE 5LDPE 6LDPE 10LDPE 11

LDPE 6LDPE 10LDPE 11

Figure 9. SEC–MALLS curves of tubular and autoclave LDPE grades.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

1.2

d W

(M) /

d (l

og M

)

log M

g - p

aram

eter

LDPE 1LDPE 2LDPE 4LDPE 5LDPE 8

LDPE 1LDPE 2LDPE 4

Figure 8. SEC–MALLS curves of tubular LDPE grades.

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CONCLUSIONS

The excellent physical foamability of autoclave LDPE grades isconfirmed by their high SHR, their high Mw/Mn ratio, but morespecifically by the high content of LCB at medium molecular mass.This is exhibited by a relatively flat g-parameter curve.

0.0

0.2

0.4

0.6

0.8

2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

1.2d

W(M

) / d

(log

M)

log M

g - p

aram

eter

LLDPE 2LLDPE 4LLDPE 6mPE 1

LLDPE 2LLDPE 4LLDPE 6mPE 1

Figure 10. SEC–MALLS curves of LLDPE grades.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

1.2

d W

(M) /

d (l

og M

)

log M

g - p

aram

eter

HDPE 1HDPE 2

HDPE 2

Figure 11. SEC–MALLS curves of HDPE grades.

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Tubular LDPE grades show almost the same SHR as autoclave LDPEgrades, but they owe this to a high LCB content at higher molecularmass as exhibited in a rather steep g-parameter curve.

The total lack of physical foamability of C4�, C6�, and C8�LLDPEgrades is confirmed by their very low SHR and the lack of LCB.

mPE Plastomers exhibit a low SHR and only a very small LCBcontent, however, the physical foamability is rather good when theyare expanded at temperatures slightly below the crystallizationtemperature.

HDPE grades in general show a very bad or no physical foamabilitydue to the lack of LCB and consequently very low SHR, however, someslurry grades exhibit a very high SHR, which is possibly also causedpartly by strain-induced crystallization.

PP grades in general cannot be used for physical foaming due to theirlack of LCB and consequently their very low SHR. The HMSPP gradesexhibit a very high SHR due to the grafted LCB and possibly also partlydue to strain-induced crystallization.

ACKNOWLEDGMENTS

The author likes to thank J. Palmen, T. Sleypen, and H. Wallink of theRheological Department of DSM Research in Geleen, The Netherlandsfor their excellent rheological work over the years and in particularH. Wallink for the RME experiments executed for this article.

2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

1.2

d W

(M) /

d (l

og M

)

log M

PP 1PP 2PP 3PP 4

Figure 12. SEC–MALLS curves of PP grades.

Elongational Viscosity to Predict the Foamability of POs 219

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The authors also thank E. Gelade and S. Jacobs of the molecularcharacterization department of DSM Research in Geleen, TheNetherlands for their excellent molecular characterization work overthe years and in particular, S. Jacobs for the SEC-MALLS experimentsexecuted for this article and finally J. Krist and P. Neuteboom for theirhelp with the illustrations of this article.

REFERENCES

1. Meissner, J. (1971). Dehnungsverhalten von Polyathylenschmelzen, RheolActa, 10: 230–240.

2. Meissner, J. and Hostettler, J. (1994). A New Elongational Rheometer forPolymer Melts and other Highly Viscoelastic Liquids, Rheol Acta, 33: 1–21.

3. Wagner, M.H., Bastian, H., Hachmann, P., Meissner, J., Kurzbeck, S.,Munstedt, H. and Langouche, F. (2000). The Strain-hardening Behavior ofLinear and Long-chain-branched Polyethylene Melts in Extensional Flows,Rheol Acta, 39: 97–109.

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