Repozytorium Eny Politechnika Wrocławska

Physico-chemical interactions between the impregnating oil and the polypropylene foil (PP) are of great concern. Such phenomena like swelling and dissolution of polymer films into the oil can detonate the electrical properties of the oil - polymer system. It was found that the swelling control could be attained by the application of the effective diffusion barrier at the PP surface. Formation of thin polymer coatings in radio-frequency discharges was used for this purpose. The effect of surface modification of the PP film on the polymer-oil interaction phenomena were investigated

Pospieszna, J

Tyman, A

Juchniewicz, J

NINTH INTERNATIONAL SYMPOSIUM 
ON HIGH VOLTAGE ENGINEERING 
August 28 - September 1,1995 
GRAZ CONVENTION CENTER 
th 
AUSTRIA, EUROPE 
I S H  
EFFECTS OF SURFACE MODIFICATION OF POLYPROPYLENE FOIL 
ON POLYMER - OIL INTERACTION 
J. Pospiesna, A. Tymh, J. Juchniewicz 
Institute of Electrotechnics and Electrotechnology, 
Technical University of Wroclaw, 
POLAND 
Abstract 
Physico-chemical interactions between the impregnating oil 
and the polypropylene foil (PP) are of great concern. Such 
phenomena like swelling and dissolution of polymer films into 
the oil can detonate the electrical properties of the oil - polymer 
system. 
It was found that the swelling control could be attained by 
the application of the effective diffusion barrier at the PP 
surface. Formation of thin polymer coatings in radio-frequency 
discharges was used for this purpose. The effect of surface 
modification of the PP film on the polymer-oil interaction 
phenomena were investigated. 
1 .  Introduction 
The growing need for higher energy density capacitop of 
reduced size has led to replacement of the paper by polymer 
film. Actually, biaxially oriented polypropylene (PP) films are 
extensively used due to their good electrical and mechanical 
properties as compared to unoriented PP films. 
For oil-impregnated all polypropylene film system the oil- 
polymer interaction is of great concern. When the foils are 
impregnated with insulating oils, absorption of the oil by the 
6lm and dissolution of the atactic fractions of the polymer into 
the liquid occur simultaneously, resulting in swelling of the film. 
The impurities and additives dissolved from the PP films into 
the oil have been pointed out to play important role in the 
electrical properti& of oil-impregnated PP &stems. Generally, 
dielectric behaviour of these svstems have been emlained as an 
interfacial phenomena in terms of trapping or absohtion effects 
and the movement of ion impurities in the solid-liquid system 
[1,21. 
Studies by G. J. Fitz Patrik, J. Laghari at all [3] have shown 
that the swelling process definitely involves the surfaces of the 
film while the electrical and mechanical properties are all 
related to the bulk of the film. 
We pointed out, that the swelling control could be attained 
by application of an effective diffision barrier at the PP 
interface [4]. 
The plasma-polymerized styrene 6lms have been prepared by 
high frequency glow discharge. Plasma polymerization, due to 
its unique mechanism of film deposition from the vapour phase, 
gives rise to low pin-hole density and highly cross-linked 
polymers [5]. Hence, the plasma polymerization technique was 
chosen for depositing the barrier films. 
In the present study the effect of surface modification of the 
PP foil on the PP-oil interactions was studied. The swelling of 
the PP and dielectric properties of the polymer foil as well of 
the layer oil-impregnated PP system were measured. 
2. Experimental procedure 
The investigations were carried out on biaxially-oriented 
bolypropylene films 12 pm thick made in Finland by 
Tervakoski. The samples of PP were freely immersed in 
previously purified alkylke lbenzine oil (AKB) made in 7 Poland. They were kept at 60 C for definite time. 
The degree of the swelling of PP films was estimated from the 
change in weight of polymer films after the oil-impregnation. 
The oil adhering on surface of specimen was wiped away by 
gauze. Then the specimen was ultrasonic cleaned. 
Scanning Electron Microscopy (SEM) was used to examine the 
microstructure of studied samples. 
The modified samples of PP were obtained by two-sided 
deposition of monomer in a radio frequency glow discharge (27 
MHz) using a pair of circular parallel electrode plates mounted 
in a vacuum vessel. 
The dielectric spectroscopy was used to the investigation of 
the dielectric properties of studied samples. The dielectric 
' response over wide frequency range, from lo4 to lo4 Hz at 
very low electric field, were measured by means a Solartron- 
Model 1172 Frequency Response Analyzer with special I-U 
converter. 
The layer oil-impregnated PP systems with the unmodified and 
modified swollen foils were studied. Furthermore, the dielectric 
properties of specimens of the pure oil to be used for 
investigation and oil extracted from the swollen PP foil as well 
samples of swollen PP foils were measured. 
3. Results and discussion 
Figure 1 shows the primary process of the swelling of the 
unmodified and modified samples of PP foil, respectively. We 
see, that the degree of swelling is strongly reduced owing to. 
surface modification of PP ( J 60%). In both cases, the swelling 
takes place comparatively rapidly and saturates afterwards. 
0 2 4 6 8 1 0 1 2  
t [hours] 
Fig. 1. Swelling of unmodified (a) and modified (0) samples of 
PP foil in AKB oil at 60°C. 
The SEM photographs (fig. 2) show that some surface 
roughness of unmodified PP foil appears after staying in the oil 
and a disorientation occurred. These imply that dissolution of 
amorphous phase into the oil takes place here and the 
crystalline phase is exposed. The surface of modified PP 
remained smooth and there no change in the structure of 
polymer. 
Fig. 2. Scanning electron micrographs of a cross section (1) and 
a surface (2) of PP samples: a) virgin. b) unmodified 
swollen, c) modified swollen. 
Fig. 3 shows the changes of relationships between a 
frequency and dissipation factor of the oil-impregnated PP 
system with the time for which the unmodified film was 
immersed in oil at 60°C. 
log f [Hz] 
Fig. 3 .  Relationships between dissipation factor and frequency 
of oil-impregnated PP system (a), oil (0) and PP foil (V) 
a) after making, b) placed for 100 days at 60°C. 
It is clearly seen that the interaction between the PP film and 
the oil lead to a increase of the dissipation factor of the oil. As 
pointed out previously it can be related to dissolution of the 
impurities contained in the PP film into the oil. 
The typical polarization spectrum due to the swelling is seen in 
the low frequency region, which is related to the interface / 
space charge polarization phenomena of long time constant. 
Fig. 4 shows the frequency dependence of the dissipation 
factor due to the swelling of the modified PP films immersed in 
oil at 60°C for 100 hours. 
- 4  -3 -2 -1 0  1  2  3 4 
log f [Hz] 
Fig. 4. Relationships between dissipation factor and frequency 
due to the swelling of modified PP foils for : oil-PP 
system (a), PP foil (0) and virgin modified PP foil (V). 
As it is shown, the frequency spectrum of dissipation factor 
of swollen oil-modified polymer system is closely connected 
with the oil absorption of the film immersed in the fluid. 
Probably, the swelling of the modified foil takes place without 
the extraction of impurities dissolved into the oil from 
amorphous phase of PP foil. By this means modified foils 
impregnated with oil are plasticized by oil. 
In addition, cross-linkings near a PP surface of PP can act as 
carrier traps and reduce the dissipation factor of an oil-polymer 
system [6] 
4. Conclusions 
The results of our investigations confirm that 
the swelling control could be attained by application of the 
thin polymer coating on the foil. 
The plasma polimerization is very effective method to 
generate the diffusion banier, 
0 such surface modification can improve the electrical 
properties of the PP-oil system. 
Acknowledgements 
Thanks are due to the Polish Committee of Research 
for fundation the research - KBN No 3 P40103 1 06. 
References 
Ill D. Shaw, "AC conduction in impregnated polymer 
insulating systems", IEEE Trans. on Electrical Insulation, 
~01.-21, NO., pp.62-68, 1975. 
121 T. Umemura, K. Akiyama, T. Kashiwazaki , "Dielectric 
behaviour of solidniquid insulation system", IEEE Trans. 
on Electrical Insulation, Vol.EI-17, No. 3, pp.276-280, 
1982. 
131 G. J. Fitz Patric, J. Laghari, E. 0 .  Forster, "The effect of 
imprepants on the morphology and dielectric properties of 
polypropylene films", Proc. 2nd Inter. Conf. on 
Conduction and Breakdown in Solid Dielectrics, Erlagen, 
pp.211-216, 1986. 
141 S. Gtuchowski, J. Pospieszna, J. Juchniewicz, "Attempt to 
the control of the properties of polypropylene-oil system", 
Proc. 3rd Inter. Conf. on Properties and Applications of 
Dielectric Materials, Tokyo, pp.871-874, 1991. 
I51 R. K. Sadhir, H. E. Saunders, "Plasma processes for 
electrical and electronics applications", IEEE Electrical 
Insulation Magazine, Vol. 2, No. 6, pp.8-14, 1986. 
161 S. Ochiai, K. Kojima, A. Ohashi, M. Ieda, T. Mizutani, 
"Conduction current and loss tangent of oillpolymer 
composite system with polypropylene films sputtered in 
oxygen", 8th Inter. Symp. on High Voltage Engineering, 
Yokohama, pp. 103-106, 1993. 
Address of authors: 
Dr J. Posuieszna 
Dr A. Tymh 
Prof. J. Juchniewicz 
Technical University of Wroclaw (I-7), 
Wybrzeie Wyspiahskiego 27, 
POLAND 


Wrocław University of Science and Technology

Not a member yet

Effects of Surface Modification of Polypropylene Foil on Polymer - Oil Interaction

332

431