2008 gahleitner addcon pp_film nucleation

8/8/2019 2008 Gahleitner Addcon PP_film Nucleation

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OPTICAL AND MECHANICAL CONSEQUENCES OFNUCLEATION AND STERILISATION IN PP CAST FILMS

Markus Gahleitner*, Johannes Wolfschwenger & Christelle GreinBorealis Polyolefine GmbH, St. Peterstr. 25, 4021 Linz, Austria,

email: [email protected]

Rebecca Blell

Universit Louis Pasteur, Strasbourg, France

Thomas KochVienna University of Technology, Institute for Material Science and Technology,

Favoritenstrae 9-11, 1040 Vienna, Austria

BIOGRAPHICAL NOTE

Dr. Markus Gahleitner graduated from Johannes Kepler University Linz, Austria,with a PhD thesis in the field of polymer melt rheology. Since 1992 he works forBorealis Polyolefine GmbH (formerly PCD Polymere) in research and development,covering different projects from basic catalyst to application development. Presently

he holds the position of IPR Group Expert in the companys IPR department. He isauthor of more than 40 scientific papers in international refereed journals as well asseveral book contributions, and he is inventor of more than 15 patents andapplications. His special fields of interest are polymer crystallization, processing-related rheology problems and sustainability aspects of polymers.

ABSTRACT

An extensive investigation of two series of ethylene/propylene (EP) random copolymers was performed tounderstand the factors influencing haze increase in the steam sterilization of extrusion cast films from suchmaterials. Different analytical methods were employed to elucidate structural changes determining filmoptics, and next to the polymer parameters also nucleation and processing effects were studied. The findingsclearly show that a combination of efficient comonomer distribution and nucleation can partly inhibit lamellar

thickening in sterilization, thus preserving high transparency even after a heat treatment.

Introduction

Extrusion cast film, a major application area for polypropylene, requires an excellent combination ofmechanical and optical properties. Next to homopolymers, ethylene-propylene (EP) copolymers are the mostimportant materials for this segment, taking advantage of their remarkable see-through performance, a resultof their slower crystallization speed compared to standard PP [1]. As a growing fraction of this segment issubjected to pasteurization or sterilization processes (like in medical applications, see Fig. 1), themechanical and optical consequences of such a treatment must be considered in material design already [2].Numerous studies regarding controlled aging or annealing at different temperatures for various times havebeen performed on injection or compression molded thick samples. Mechanical tests along with observationof the microstructure at different scales are often performed to follow the induced changes and understand

their origin [3,4], but such results are limited in relevance to predict the behaviour of films, especially withrespect to an alteration of the optics after a defined heat treatment.

The present study was therefore directed at understanding both processing and sterilization effects on thecrystalline structures and the resulting film performance for both PP homopolymers and EP randomcopolymers, including versions with different nucleating agents frequently used for improving transparency[5]. Influence of processing parameters (chill-roll temperature. film thickness. co-extrusion line), polymerparameters (C2-content of EP-copolymers, catalyst systems) and nucleation agents will be discussed, andthe changes in optics will be correlated to crystallinity and morphology variations assessed by differentialscanning calorimetry (DSC), polarizing optical microscopy (POM), wide- and small-angle X-ray diffraction(WAXD/SAXD) and transmission electron microscopy (TEM).
mailto:[email protected]:[email protected]


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Figure 1 Flexible infusion pouch for medical application producedfrom a PP-based multilayer film; typical example for an advancedpackaging application requiring sterilization

Experimental work

Two series of PP materials produced on pilot facilities of Borealis were investigated:- In the first one, 6 polymers produced with a high yield 4 th generation Ziegler-Natta catalyst (C1) having

an ethylene content between 0 and 5 wt% were tested. All grades had a reactor melt flow rate (MFR230C / 2,16kg) of 1,5 g/10 min, were visbroken with peroxide to an MFR of 8 g/10min and equippedwith a standard additivation package.

- In the second one, EP-copolymers with a C2-content of about 3.5 wt% were produced using threedifferent catalyst systems from the same category. The grade made with catalyst C1 had an MFR ofabout 1.5 g/10min and was subsequently stabilized, additivated and visbroken to an MFR of 8 g/10min,while the grades made with catalysts C2 and C3 were neat reactor grades with an MFR of about 6g/10min. Different nucleating agents (A1 to A3 for the -modification and B for the -modification) topromote either the - or the -modification of the EP-copolymers were added during the compoundingstep, testing against non-nucleated references.

For all PP materials, extrusion cast films of 50 and in some cases 130 m thickness were produced on aPM30 type laboratory extruder with a coathanger slit die 200 mm wide and with a gap range of 0.55 to 0.6mm. The melt temperature was around 250C and the chill roll temperature was varied for the second series(20C. 55C and 90C).

After the extrusion, all fi lms of series 1 and 2 were optically characterized according to ASTM D 1003 using aBYK-Gardner Hazegard Plus Instrument. The haze was chosen as most relevant parameter to assessdifferences in optics before and after sterilization. The measurements were done at least 96h after filmproduction or sterilization.

Steam sterilization was performed in a Systec D series machine. The samples were heated up at a heat rateof 5 C/min starting from 23C. After having been kept for 30 min at 121C, there were removed immediatelyfrom the steam sterilizer and stored at room temperature till processed further.

Crystallinity and morphology of the films before and after sterilization were studied by DSC, POM,WAXD/SAXD and TEM, however not employing every method in each case. DSC measurements werecarried out on a power compensated DSC 2920-Co (TA instrument) on all samples before and aftersteril ization. Two heating scans and one cooling scan were done in between -10C to 210C at a rate of10C/min. The analyzed samples had a weight of about 5 mg. The first heating cycle was considered to bethe most interesting as it did not erase the history of thermal treated samples and was thus believed to givefairly reliable indications on the morphology of the investigated materials this is acceptable even though thetest is not as accurate as those provided by non destructive methods (e.g. WAXD). Melting temperature asthe maximum of the considered peak and melting enthalpies as the area under the relevant peak wereassessed.


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Figure 2 DSC thermograms recorded at10 K/min for EP random copolymer withMFR 6 and 3,5 wt% C2 processed at a

chill roll temperature of 55C; scansbefore and after sterilization

An example of the first heating cycle of a DSC thermogram of a neat and retorted sample is provided in Fig.2 for an EP-copolymer. The main challenge in this evaluation is to fix in a reproducible way the baseline ofthe DSC traces and the split between the two peaks appearing after retorting, which for simplicity was doneby taking the baseline as the line between the first minimum points to the left and the right of the peak andthe split between two peaks as a vertical section at the minimum between the two peaks.

Polarizing optical microscopy was performed on 10 m thin sections across the film thickness undertransmission mode using a Light Microscope from Olympus with crossed polarizers. The aim was to see thedifference in morphology - before and after sterilization - induced by and -nucleation compared to neatsamples and to visualize changes between the structures induced with the different catalyst systems (C1, C2and C3).

Some selected samples were submitted to WAXD investigations at Vienna University of Technology and toTEM investigations at the Center for Electron Microscopy Graz, Austria. Wide-angle X-ray was done on thefull films with a Philips XPert Pro instrument using CuK radiation in reflection mode. The peak integrationfor determining crystallinity was performed as reported before [6]. For TEM, ultrathin sections of samplespecimens contrasted with ruthenium tetroxide to allow differentiation between regions of high and low

crystallinity were prepared. Images were recorded on a Tecnai G 12 from FEI, equipped with a CCD camera(Gatan Bioscan) at 100 kV acceleration voltage.

Results and Discussion

The basic composition and melting points determined on cast films are summarized for both series in tables1 and 2 below. Two effects are obvious immediately from these tables: The rather linear melting pointreduction with increasing C2 content for series one (presented in Fig. 3), and the fact that both nucleationand catalyst type co-influence the quenching effect achieved by reducing the chill roll temperature in seriestwo. Here, the chill-roll temperature (T roll) has been varied between 20 and 90C to simulate cases of a:

(i) highly amorphous neat structure with moderate stiffness (T roll: 20C)(ii) a stiff resin at good starting optics (T roll: 55C)(iii) a maximum crystalline grade (T roll: 90C)

Samplenumber

EthyleneContent(wt %)

MFR(g/10 min) Tm (C)

1/1 0.0 8.6 162.81/2 1.6 7.4 155.91/3 2.4 7.6 151.91/4 3.3 8.5 148.01/5 4.3 7.4 143.01/6 5.0 7.2 140.2

Table 1 Basic properties of the polymers from the first series (melting point from DSC on cast film beforesterilization)

0 20 40 60 80 100 120 140 160 180

Temperature (C)

E n d o t

h e r m

i c H e a

t F l o w

( W / g ) Before Sterilization

After Sterilization

0 20 40 60 80 100 120 140 160 180

Temperature (C)

E n d o t

h e r m

i c H e a

t F l o w

( W / g )

E n d o t

h e r m

i c H e a

t F l o w

( W / g ) Before Sterilization

After Sterilization

Before SterilizationBefore Sterilization

After SterilizationAfter Sterilization


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Samplenumber

Ethylenecontent(wt%)

Catalyst MFR(g/10min)Reactor / CR Nucleation Tm(C) at chill roll temp.

20C 55C 90C2/1 3.5 C1 8.2 CR No 143.1 144.5 146.32/2 3.5 C2 5.7 RE No 146.3 144.5 150.02/3 3.5 C3 6.0 RE No 142.0 143.3 145.32/4 3.5 C1 7.8 CR A1 144.7 144.9 145.22/5 3.5 C2 5.8 RE A1 146.7 148.1 149.52/6 3.5 C3 6.4 RE A1 143.0 144.0 142.72/7 3.5 C1 8.1 CR A2 144.1 145.2 145.82/8 3.5 C2 5.6 RE A2 148.4 147.9 149.22/9 3.5 C3 6.1 RE A2 143.1 143.8 140.5

2/10 3.5 C1 7.9 CR A3 143.6 144.3 145.52/11 3.5 C2 5.7 RE A3 147.7 147.8 149.02/12 3.5 C3 6.2 RE A3 142.4 143.4 145.42/13 3.5 C1 8.6 CR B 143.2 144.1 145.12/14 3.5 C2 5.9 RE B 146.8 147.2 149.4

2/15 3.5 C3 6.1 RE B 142.1 143.4 145.3

Table 2 Basic properties of the polymers from the second series (visbroken grades indicated by CR forcontrolled rheology; melting point from DSC on cast film processed at different chill roll temperatures beforesterilization)

Figure 3 Changes in meltingtemperature and haze change insterilization at 121C dependingon the C2-content of a 50 mthick Cast Film for series 1

polymers

The comonomer influence in the first series is clearly reflected in the optical changes during crystallization.As Fig. 3 shows, the haze increase remains rather constant up to an ethylene content of about 2,5 wt% andthen rises significantly. For better understanding this phenomenon, the fraction of the melting enthalpy curvebelow the sterilization temperature of 121C was calculated in relation to the total melting enthalpy. Thediscontinuity at 2,5 wt% found in figure 3 could not be reproduced here, but the reason for this becomes

obvious when taking a closer look at the DSC curve (heat 1) before sterilization as recorded for material 1/2(see figure 4).

Three peaks could be distinguished here, corresponding to three transitions [7]:i. an endothermic peak (T1) with its maximum at around 60Cii. an exothermic peak (T2) with its maximum at around 100Ciii. a large endothermic peak (T3) with its maximum at around 160C in the case of the homopolymer

(resp. 156C for the polymer considered here)Peak T1 is assumed to be the superposition of peak T2 and T3. T1 is therefore assumed to be the beginningor onset of peak T3. Peak T2 is the result of increased mobility of the crystalline phase starting at 50C [8].

With this, further crystallization takes place. This is because thick lamellas attract thin ones as well ascrystallisable parts of the amorphous phase becoming mobile. Lamellar thickening takes place and the

y = -4,5715x + 162,95R2 = 0,9994

135

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165

0 1 2 3 4 5

C2 content / wt%

T m

( D S C ) / C

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D e l

t a H a z e

/ %


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amorphous region is rearranged thus a crystallization peak. Evaluation of the area of peak T2 showed alinear decrease of this area with the increase in ethylene content. This area evaluation is not exact as thesuperposition of the two peaks leads to high error but it is representative of the evolution. The increase inethylene content therefore improves the optical properties of the films before sterilization by reducing haze,increases the difference in the optical properties of the films with sterilization and encourages secondarycrystallization; this increased quenchability of PP grades with reduced regularity due to stereodefectsand/or comonomer incorporation has been postulated before [10].

Figure 4 DSC thermogram (heat 1) recorded at 10K/min for the 1.6 wt.% ethylene content film beforesterilization

After sterilization, the DSC analysis of the films reveals a double melting peak as already evident from figure1. Surprisingly, the first peak of these is always at the same temperature of ~ 131C, indicating a secondarycrystalline structure being generated in the sterilization step which is independent of the ethylene content.Figure 5 gives the relative areas for the primary (high T m , depending on C2 content) and secondary (low T m ,131C) crystallization fractions in the films. The qualitative correlation of the latter part to the haze increase infigure 3 is quite striking at the highest comonomer content the chain irregularities obviously even reducethe capacity for post-crystallization, in line with the overall crystallization speed reduction found for suchmaterials before [1].

Figure 5 Enthalpy area for primary andsecondary crystallization melting for thesterilized cast films from series 1

In the second series, where the focus was on processing and nucleation effects, the optical properties werefound to deteriorate with increasing the chill roll temperatures as a result of an increase in crystallinity asanticipated from literature [10]. When exemplified with material 2/1 (non nucleated sample), this featurecorresponds to an enhancement of the melt enthalpy from 64.0 J/g for a chill roll temperature of 20C, to76.1 J/g for a chill roll temperature of 55C, to 82.7 J/g for a chill roll temperature of 90C. Two factors play aparallel role in this effect for high chill roll temperatures: (i) the reduction of the rate of crystallization allowingthe crystallites to grow to large spherulites, in turn increasing the haze (see figure 6) and (ii) a reduction ofthe amount of mesomorphic phase in the system [11].

0 50 100 150 200

Temperature (C)

E n d o t

h e r m

i c H e a

t F l o w

( W / g )

T1

T2

T3

0 50 100 150 200

Temperature (C)

E n d o t

h e r m

i c H e a

t F l o w

( W / g )

0 50 100 150 200

Temperature (C)

E n d o t

h e r m

i c H e a

t F l o w

( W / g )

E n d o t

h e r m

i c H e a

t F l o w

( W / g )

T1

T2

T3

01.6

2.43.3

4.35

Secondary Crystallization

Primary Crystallization

0

10

20

30

40

50

60

70

80

90

100

A r e a

( m W

. C )

C2-content (%wt)

01.6

2.43.3

4.35

Secondary Crystallization

Primary Crystallization

0

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A r e a

( m W

. C )

C2-content (%wt)


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Figure 6 Haze change with chill colltemperature variation for samples 2/1, 2/4 and2/13 (see also table 1)

Even though the optical performance worsened with the chill roll temperature, this picture was notablyinfluenced by nucleation as highlighted in figure 6. While the haze increased steadily from 4.9% (T roll 20C) to31.3% (T roll 90C) for the non-nucleated samples, the recorded haze values stood roughly constant at around6% for the nucleating agent A1. This result confirmed with the other -nucleators - but at other haze levels- suggests that the morphology and thus the optical performance - of random copolymers can be controlled

by -nucleation independently of the conversion temperature. This feature can be ascribed to their highercrystallization rate promoted by heterogeneous nucleation [12].

Another way to visualize the influence of both processing conditions and additivation is provided in figure 7.There the difference in haze between different chill roll temperatures as a function of the nucleation isreported. On one hand the low sensitivity of -nucleated samples to cooling rates is evident as assessed bya relative change in optics of maximum 80% starting from a very low haze level of around 3 to 6 %. On theother hand, non-nucleated and -nucleated samples are influenced to a large extent (up to 670%) by theprocessing conditions as already mentioned earlier. The low difference in haze between a chill-rolltemperature of 55 and 90C suggests that the crystall ization speed of the studied polymer is not overruled byprocessing parameters, while the opposite holds for chill-roll temperatures of 20C independently of thematerial under investigation (C1, C2 or C3).

-50,0

50,0

150,0

250,0

350,0

450,0

550,0

650,0

C1 C2 C3 C1,A1 C2,A1 C3,A1 C1,A2 C2,A2 C3,A3 C1,A3 C2,A3 C3,A3 C1, C2, C3,

Film Sample Numbers

D e l

t a H a z e

( % )

Delta Haze between films Chill Rolled at 55C and 20 CDelta Haze between films Chill Rolled at 90C and 55 CDelta Haze between films Chill Rolled at 90C and 20 C

-50,0

50,0

150,0

250,0

350,0

450,0

550,0

650,0

C1 C2 C3 C1,A1 C2,A1 C3,A1 C1,A2 C2,A2 C3,A3 C1,A3 C2,A3 C3,A3 C1, C2, C3,

Film Sample Numbers

D e l

t a H a z e

( % )

Delta Haze between films Chill Rolled at 55C and 20 CDelta Haze between films Chill Rolled at 55C and 20 CDelta Haze between films Chill Rolled at 90C and 55 CDelta Haze between films Chill Rolled at 90C and 55 CDelta Haze between films Chill Rolled at 90C and 20 CDelta Haze between films Chill Rolled at 90C and 20 C

Figure 7 Difference in haze between the cast films extruded at different chill roll temperatures beforesterilization for the 15 samples of series two

Also in this series, steam sterilization induces changes in the optical performance of the studied grades.However, these variations are more or less pronounced depending on the initial crystalline state of thegrade. Quenched films obtained with a chill roll temperature of 20C as shown in figure 8 are more sensitiveto a heat treatment than more crystalline samples manufactured at a chill-roll temperature of 90C. They

C1C1,A1

C1,B

20

55

90

0

5

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35

Haze Values

(%)

Samples

Chill Roll

Temperature (C)C1

C1,A1C1,B

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90

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Haze Values

(%)

Samples

Chill Roll

Temperature (C)


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undergo the largest reorganization and lamellar thickening with sterilization, as a combination of annealingand post-crystallization effects as widely documented in the literature [3,4,7,8,9].

The occurrence of lamellar thickening in post-crystallization as a result of a reduction of the magnitude onthe interfacial free energy [4] could be confirmed by TEM (see figure 9) and in WAXD/SAXD analysis (seetable 3). On the example of a nucleated sample processed at a chill-roll temperature of 90C, it becomesobvious that a heat treatment at 121C for 30 min promotes the formation of thicker lamella and lead to a

denser network of crystalline structures. The X-ray data moreover show two very interesting phenomena:i. the crystallinity increase in sterilization is significant only for the non-nucleated sample, providing anadditional explanation for the reduced haze increase in nucleated materials (see again Fig. 8)

ii. in contrast to expectations from theory (which predicts an increase due to the higher crystallizationtemperature [13]), nucleation does not affect the lamellar thickness in cast-film processing

Haze Difference at 20C chill roll temperatureHaze Difference at 55C chill roll temperatureHaze Difference at 90C chill roll temperature

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1 2 3 4 5 6 7 8 9 1 11 12 13 14 15

Film Sample Numbers

D e l

t a h a z e

( % )

Haze Difference at 20C chill roll temperatureHaze Difference at 55C chill roll temperatureHaze Difference at 90C chill roll temperature

-50

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1 2 3 4 5 6 7 8 9 1 11 12 13 14 15

Film Sample Numbers

D e l

t a h a z e

( % )

Figure 8 Haze difference between sterilized and non-sterilized films for the 15 samples of series two at thethree different chill roll temperatures tested

Figure 9 TEM images of the lamellar morphology of material 2/6 (catalyst C3, MFR 6, 3.5 wt% C2) fromcast film extruded at chill roll temperature of 90C before (left) and after (right) sterilization at 121C; scalebar dimension 200 nm


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Table 3 Results of WAXD/SAXDanalysis of films from materials 2/3and 2/6 (catalyst C3, MFR 6, 3.5 wt%C2) before and after sterilization;overall crystallinity from WAXD andlong period L from SAXD given

Also for the second series, an attempt to correlate haze variations with crystallinity changes before and aftersterilization was made using the DSC traces of the first heat after sterilization. As explained before, a doublepeak was found with high reproducibility for all samples. In contrast to the results shown in Fig. 5, where aclear correlation of both primary and secondary peak with the comonomer content could be established, thesituation was found to be far more complex here.

In general, the relative amount (enthalpy) of the secondary crystallization was higher for the 20C chill rolltemperatures than for the 90C chill roll temperature, confirming the increased mobility in more stronglyquenched films observed for PP homopolymers before [9]. Another repeated pattern seen in Figure 10 is thelower area of secondary crystallization in the C2 polymerization method for the quenched films. Thesecondary crystallization of the C2 polymerized films showed the lowest area implying the least secondarycrystallization. This could be the effect of the higher melting point mentioned above for these films whichleads to less reorganization due to melting. Another effect could also be the decrease of lamellar thickeningwith the C2 polymerization as the lamellae are already thick enough and could not thicken further. Theinfluence of the nucleating agents on the secondary crystallization is however not visible in the DSC analysisas the areas of all the films for a given polymerization method are nearly constant and the area of the 15films are in the same range as seen in Figure 10. This is true for all three chill roll temperatures.

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50

No nucleation A1 A2 A3 Beta

Nucleation

A r e a ( m

W .

C )

C1 C2 C3

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50

No nucleation A1 A2 A3 Beta

Nucleation

A r e a ( m

W .

C )

C1 C2 C3C1 C2 C3

Figure 10 Area (enthalpy) of secondary crystallization after sterilization for selected materials from seriestwo extruded at T roll =20C

Sample state X cr / % L / nm2/3, non-nucl. extruded 90C 56 11,32/6, nucleated steri lized 121C 64 15,12/3, non-nucl. extruded 90C 61 11,52/6, nucleated steri lized 121C 63 15,2


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Summary and Outlook

We investigated the structure of EP random copolymer films before and after sterilization at 121C for 30minutes by DSC, TEM, POM and WAXD/SAXD to explain the changes in optical properties (haze) ofextrusion cast films with changing nucleating agents, extrusion conditions and ethylene content as well aspolymerization conditions. Two series of films were studied, varying the comonomer content in the first oneand catalyst, nucleation and processing conditions in the second one. The findings clearly show that a

combination of efficient comonomer distribution and nucleation can partly inhibit lamellar thickening insterilization, thus preserving high transparency even after a heat treatment.

In detail, attention has to be paid to the combined effects of physical ageing and post-crystallization, whichboth are affected by the chain regularity. The appearance of a rather constant secondary melting point inDSC at a temperature of 10 K above the sterilization temperature clearly points to the existence ofsecondary crystalline structures postulated before by several authors [4,7,9]. More extensive investigation,especially using the possibilities of X-ray diffraction, will be required to develop a more completeunderstanding.

Acknowlegements

The authors want to thank Dr. Elisabeth Ingolic from the Center of Electron Microscopy (ZFE) Graz, Austria,for preparing the excellent TEM images.

References

1 M. Gahleitner, P. Jskelinen, E. Ratajski, C. Paulik, J. Reussner, J. Wolfschwenger & W. Neil,PropyleneEthylene Random Copolymers: Comonomer Effects on Crystallinity and Application Properties .J.Appl.Polym.Sci, 95 (2005) 1073-812 K. Resch, G.M. Wallner, C. Teichert, G. Maier & M. Gahleitner, Optical Properties of Highly Transparent Polypropylene Cast Films: Influence of Material Structure, Additives, and Processing Conditions .Polym.Eng.Sci, 46 (2006) 520-313 S. Piccarolo, Ageing of isotactic polypropylene due to morphology evolution, experimental limitations of realtime density measurements with a gradient column , Polymer 47 (2006) 5610-56224 H. Marand, A. Alizadeh, S. Sohn, J. Xu, R. Farmer, V. Prabhu, S. Cronin, V. Velikov, A Model for the Physical Aging of Semicrystalline Polymers Above Tg: Secondary Crystallization Induced Constraining Effects . Proc. SPE ANTEC, 59 Vol. 2 (2001) 185618595 N. J. Macauley, E. M. A. Harkin-Jones, W. R. Murphy, The Influence of Nucleating Agents on the Extrusion and Thermoforming of Polypropylene , Polym.Eng.Sci. 38 (1998) 662-706 T. Koch, S, Seidler, E. Halwax, S. Bernstorff, Microhardness of quenched and annealed isotactic polypropylene , J.Mater.Sci. 42 (2007) 5318-267 N. Alberola, M. Fugier, D. Petit, B. Fillon, Microstructure of quenched and annealed films of isotactic polypropylene . Part I , J.Mater.Sci. 30 (1995) 1187-958 J.M.K. Agarwal, J.M. Schultz, The Physical Aging of Isotactic Polypropylene , Polym.Eng.Sci. 21 (1981)7767819 M. Gahleitner, J. Fiebig, J. Wolfschwenger, G. Dreiling & C. Paulik, Post-Crystallization and Physical Ageing of Polypropylene: Material and Processing Effects . J.Macromol.Sci.-Phys., B41 (2002) 833-4910 K. Resch, G.M. Wallner, C. Teichert & M. Gahleitner, Highly Transparent Polypropylene Cast Films: Relationships between Optical Properties, Additives and Surface Structure . Polym.Eng.Sci., 47 (2007) 1021-3211 A. Martorana, S. Piccarolo, F. Schichilone, The X-ray determination of the amounts of the phases in samples of isotactic poly(propylene) quenched from the melt at different cooling rates , Macromol.Chem.Phys. 198 (1997) 597-60412 N. J. Macauley, E. M. A. Harkin-Jones, W. R. Murphy, The influence of Nucleating Agents on the Extrusion and Thermoforming of Polypropylene , Polym.Engin.Scie. 38 (1998) 662-7313 B. Puknszky, I. Mudra, P. Staniek, Relation of crystalline structure and mechanical properties of nucleated polypropylene , J.Vinyl.Addit.Technol. 3 (1997) 53-7

2008 gahleitner addcon pp_film nucleation

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