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Polypropylene: Morphology, defects and electrical breakdown
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Crystal structure, morphology and crystallization kinetics of melt-crystallized polypropylene and poly(propylene-stat-ethylene) fractions with 2.7 to 11.0 mol% of ethylene were studied by differential scanning calorimeter, wide- and small-angle X-ray scattering, polarized light microscopy, transmission electron microscopy and infrared spectroscopy. With increasing ethylene content the poly(propylene-stat-ethylene) fractions showed unchanged crystallinity, increased unit cell volume and constant crystal thickness in combination with a shortened helix length. This indicated that a fraction of ethylene defects were incorporated into the crystal structure. During the isothermal crystallization both α- and γ-crystals could be formed. The γ-crystal fraction increased with increasing ethylene content and increasing crystallization temperature. For samples with α- and γ-crystal contents, multimodal melting was observed and a noticeable γ- to α-crystal conversion was observed on slow heating. The spherulitic structure of the copolymers was coarser than that for the homopolymer.

The crystalline lamellae in copolymers exhibited profound curvature in contrast to the straighter cross-hatched α-crystals typical to the homopolymer. Area dependence of electrical breakdown strength was studied for thin polypropylene homopolymer films. The measurements were performed with an automatic measurement system equipped with a scanning electrode arm. Five different electrodes having areas between 0.045 cm2 and 9.3 cm2 were used and typically 40-80 breakdowns per sample and electrode area were collected. All measurements were performed on dry samples in air at room temperature. The data was analyzed statistically and the Weibull function parameters α and β, the first one related to 63% probability for the sample to break down and the second one to the width of the distribution were fitted to the obtained data. Different features concerning the measurement system and conditions, e.g. criteria for the automatic detection of the breakdowns, effect of electrode edge design, partial discharges, DC ramp speed and humidity were critically analyzed. It was concluded that the obtained α-parameter values were stable and repeatable over several years of time. The β-parameter values, however, varied ± 10-30%, more for the large than the small electrodes, and were also sensitive to the changes both in the sample itself and in the measurement conditions.

Breakdown strengths of over 50 capacitor grade polypropylene films were analyzed. The obtained α-parameter values were between 450 and 850 V/μm, depending on the film grade and electrode area. In addition to the high breakdown strengths, reflected by the obtained α-values, another, sparse distribution consisting of low breakdown strengths was revealed when the amount of measurement points was high enough. This means that more than one Weibull distribution could be needed to describe the breakdown strength behavior of a polypropylene film. Breakdown values showed decreasing area dependence with decreasing electrode area. Breakdown strengths for larger sample areas were predicted from the small area data by area- and Weibull extrapolation. The area extrapolation led to predicted α-values 50% higher than measured at 4 m2 whereas the Weibull extrapolation showed an accuracy of ±15 % when predicted and measured values were compared.

Breakdown strengths were also extrapolated for film areas similar to those in impregnated power capacitors. It turned out that the power capacitors, tested at the factory, performed much better than predicted by the extrapolation. However, a few weak spots with very low breakdown values were also found. For the poly(ethyelene terephtalate) dielectric, which is not swelled by the impregnation liquid, the large area breakdown strength was predictable. This indicates that for polypropylene film processing and impregnation led, in addition to the improved large area breakdown performance, also to sparse weak spots with low breakdown probabilities. Different Weibull distributions were responsible for the breakdown strengths for the processed and impregnated polypropylene than for the dry film samples.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , [71] p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2005:19
Keyword [en]
Chemical engineering, polypropylene, poly(propylene-stat-ethylene), crystal structure, morphology, crystallinity, crystallization kinetics, lamellae, spherulite, defect
Keyword [sv]
Kemiteknik
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-268ISBN: 91-7178-091-2 (print)OAI: oai:DiVA.org:kth-268DiVA: diva2:8366
Public defence
2005-06-17, Sal E2, Lindstedtsvägen 3, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2010-10-27Bibliographically approved
List of papers
1. Crystal structure and morphology of melt-crystallized poly(propylene-stat-ethylene) fractions.
Open this publication in new window or tab >>Crystal structure and morphology of melt-crystallized poly(propylene-stat-ethylene) fractions.
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1997 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 38, no 2, 371-377 p.Article in journal (Refereed) Published
Abstract [en]

Crystallinity, crystal structure and lamellar thickness in melt-crystallized samples of poly(propylene-stat-ethylene) fractions with 2.7–11.0 mol% ethylene comonomer and of approximately constant tacticity were assessed by wide- and small-angle X-ray scattering, differential scanning calorimetry and infra-red spectroscopy. Most of the samples were crystallized under isothermal conditions at 373 K. In comparison with an isotactic homopolymer of polypropylene, the copolymers showed lower crystallinity, melting enthalpy and average length of 3/1 helices, a slightly larger unit cell, a longer long period and an invariant lamellar thickness. The X-ray crystallinity of the copolymers remained approximately constant with increasing ethylene content, whereas the γ-crystallinity increased and the heat of fusion decreased moderately. It is suggested that the ethylene units are partially included in the crystals, and that this causes the invariance in crystallinity and crystal thickness. The observed gradual decrease in average 3/1 helix length with increasing ethylene content as assessed by infra-red spectroscopy is in accordance with this suggestion.

Keyword
poly(propylene-stat-ethylene), X-ray scattering, thermal analysis
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5300 (URN)10.1016/S0032-3861(96)00503-4 (DOI)
Note
QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2017-12-04Bibliographically approved
2. Crystallization kinetics and morphology of poly(propylene-stat-ethylene) fractions.
Open this publication in new window or tab >>Crystallization kinetics and morphology of poly(propylene-stat-ethylene) fractions.
1997 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 38, no 2, 361-369 p.Article in journal (Refereed) Published
Abstract [en]

The crystallization and melting behaviour and the morphology of fractions of poly(propylene-stat-ethylene) with 2.7–11.0 mol% ethylene were studied by differential scanning calorimetry, wide-angle X-ray scattering, polarized light microscopy and transmission electron microscopy, after etching with permanganic acid. The inclusion of ethylene co-repeating units in isotactic polypropylene (0–11.0 mol% ethylene) caused approximately linear decreases in kinetic and equilibrium melting temperatures and in the glass transition temperature with increasing ethylene content. X-ray scattering showed that the content of the γ form increased with increasing ethylene content, increasing crystallization temperature and decreasing cooling rate from the molten state. It was shown for one of the copolymers (8.7 mol% ethylene) that during heating approximately 50% of the γ form was converted to the α form before the final melting of the sample. The rest of the γ crystals melted without transformation to the α form. The multimodality of the crystal melting above the crystallization temperature in the polymers with a more uniform crystal structure was caused by recrystallization during heating, whereas polymers with appreciable contents of both α and γ forms exhibited multimodal melting at all the heating rates adopted. The size of the low temperature melting peak as assessed at a heating rate of 40 K min−1 was approximately proportional to the initial content of the γ form. By comparison with the spherulitic structure of homopolymers, that of the copolymers was coarser with internal and peripheral pockets of molten material during spherulite growth. The crystal lamellae exhibited more curvature in the copolymer samples than in the homopolymer.

Keyword
poly(propylene-stat-ethylene), fractions, melting
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5301 (URN)10.1016/S0032-3861(96)00502-2 (DOI)A1997WD22400013 ()
Note
QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2017-12-04Bibliographically approved
3. Area Dependence of Breakdown Strength of Polymer Films: Automatic Measurement Method
Open this publication in new window or tab >>Area Dependence of Breakdown Strength of Polymer Films: Automatic Measurement Method
Show others...
2007 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, no 2, 263-274 p.Article in journal (Refereed) Published
Abstract [en]

An automatic method for determining the breakdown strength of thin polymer films was evaluated. The equipment consists of a scanning electrode arm with which the breakdown voltage over the film sample is measured. The measurement electrodes were of solid brass and the ground electrode was of thin aluminum foil. five different electrode areas between 0.045 and 9.3 cm(2) were used and 40-100 individual measurements per electrode area per sample were performed. All measurements were performed in air. The Weibull function parameters alpha and beta were fitted to the obtained data. Different features concerning the measurement system and conditions, e.g. the criteria for the automatic detection of the breakdowns, the effect of the electrode edge design, the DC ramp speed, partial discharges and humidity were analyzed. In order to control the reproducibility of the results, also another type of measurement setup was tested. It was concluded that the measured alpha-parameter values were stable and repeatable over a period of several years. The average beta-parameter values varied +/- 10-30%, more for the large than the small electrodes, and were also sensitive to changes in the sample, in the measurement conditions and the choice of the electrode system.

Keyword
breakdown strength, polymer film, polypropylene film, dielectric film, capacitor insulation, measurement method, area dependence, Weibull distribution function
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-5302 (URN)10.1109/TDEI.2007.344603 (DOI)000245598900001 ()2-s2.0-34147175518 (Scopus ID)
Note
QC 20100831 Uppdaterad från submitted till published (20100831)Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2017-12-04Bibliographically approved
4. Influence of electrode area on dielectric breakdown strength of thin poly(ethylene terephthalate) films
Open this publication in new window or tab >>Influence of electrode area on dielectric breakdown strength of thin poly(ethylene terephthalate) films
2004 (English)In: Electrical Insulation and Dielectric Phenomena, 2004. CEIDP '04. 2004 Annual Report Conference on, 2004, 563-567 p.Conference paper, Published paper (Refereed)
Abstract [en]

DC breakdown voltages of 8-20 p thickpoly(ethy1ene terephthalate) (PET) films were measuredas a function of electrode area. The areas of the measurement electrodes were between 0.045-9.3 cm2.Also 13-15 m2 capacitor elements were tested. Weibull distribution parameters, scale parameter a and shapeparameter p, were fitted to the obtained breakdownstrength data. Based on the small area results, short termbreakdown strength of large area capacitor elementswas predicted. Area extrapolation based on the scaleparameter a was compared with Weibull extrapolation,in which also the shape of the distribution is taken into account. It was concluded that the Weibull extrapolation better represented the large area breakdown strength than the area extrapolation.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5303 (URN)10.1109/CEIDP.2004.1364312 (DOI)2-s2.0-17744389267 (Scopus ID)
Conference
2004 Annual Report Conference on Electrical Insulation and Dielectric Phenomena,17-20 Oct. 2004
Note
QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2010-10-27Bibliographically approved
5. DC Breakdown Strength of Polypropylene Films: Area Dependence and Statistical Analysis
Open this publication in new window or tab >>DC Breakdown Strength of Polypropylene Films: Area Dependence and Statistical Analysis
2007 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, no 2, 275-286 p.Article in journal (Refereed) Published
Abstract [en]

The dc breakdown strengths of 4-15 μm thick capacitor grade polypropylene (PP) films were determined. The measurements were performed with an automatic measurement system using five electrodes with areas between 0.045 and 9.3 cm2. Some samples werestudied with larger electrodes. Samples were not aged but were measured as received.The measurements were performed in air at room temperature and typically 40-80 breakdowns per sample per electrode area were collected. The Weibull distribution parameters α and β were fitted to the data. Depending on the electrode area and the film grade, the α-parameter values obtained were between 450 and 850 V/μm. The breakdown strength values showed decreasing area dependence with decreasing electrode area and the Weibull scaling law was not valid for the smallest electrodeareas. The α-values for the large sample areas were extrapolated from the small areadata by area and Weibull extrapolation. The area extrapolation overestimated the breakdown strength at 4 m2 by 40-50% whereas the Weibull extrapolation gave anaccuracy of ±15%. The extrapolated breakdown strengths for the full-scale powercapacitors deviated from the capacitor manufacturers test data. This indicated that different defect distributions were responsible for the breakdown strengths in the oil impregnated power capacitors than in the small dry film samples.

Keyword
polypropylene (PP), biaxially oriented PP (BOPP), breakdown strength, Weibull distribution, area dependence, extrapolation, capacitor, capacitor film, capacitor dielectric
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5304 (URN)10.1109/TDEI.2007.344604 (DOI)000245598900002 ()2-s2.0-34147181678 (Scopus ID)
Note
Uppdaterad från submitted till published: 20101027. QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2017-12-04Bibliographically approved

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