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Biofilms on silicone rubber for outdoor high voltage insulation
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Silicone rubber high voltage insulators are sometimes colonised by microorganisms which form a biofilm on the surface of the infected unit. In this work insulators exposed to the outdoor environment in Sweden, Sri Lanka and Tanzania respectively have been studied. The biofilms colonising the insulators were shown to be of roughly the same composition regardless of their origin. Algae in association with bacteria dominated the biofilms and provided nutrition to mold growth. The isolated microorganisms were further used to study the effect of a biofilm on different silicone rubber materials. New tools for diagnosing biological growth on polymeric materials were developed and used to analyse the silicone rubber samples.

No evidence of biodegradation of the polydimethylsiloxane (PDMS) molecule has been found in this work. However, this does not mean that PDMS rubbers used in high voltage insulators can be called bioresistant. Silicone insulating materials always contain additives and these may promote or hinder growth. For this reason, an extensive test program was developed, in order to evaluate the effect of different additives on the degree of biological growth. The program spanned from fast and easy methods, useful for screening large amount of samples, to the construction of specially designed microenvironment chambers in which mixed biofilms, similar to those formed on the surface of silicone rubber insulators in the field, were successfully grown.

The test program showed that the flame retardant zinc borate protected the materials, whereas alumina trihydrate (ATH) did not hinder biological growth. On the contrary, environmental scanning microscopy (ESEM) in combination with X-ray energy dispersive spectroscopy (EDS) showed that the surface roughening caused by the addition of ATH to the silicone rubber matrix made the materials more difficult to clean.

Furthermore when the hydrophobic surface of a silicone rubber insulator is covered by a hydrophilic biofilm this leads to a reduction of the surface hydrophobicity of the material. This may alter the electrical properties of the insulator. It is therefore important to develop methods to identify biofouled units. In this work, laser-induced fluorescence (LIF) spectroscopy was explored as a tool for the detection of biofilms on silicone rubbers. The experiments revealed that weak traces of algae or fungal growth, even those not visible to the naked eye, could be detected by this technique. In addition, it was shown that photography and subsequent digital image analysis could be utilised to estimate the area covered by biofilm growth. The results obtained indicate that LIF spectroscopy in combination with image analysis could be used for field diagnostics of biological growth on insulators in service.

Place, publisher, year, edition, pages
2005.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2005:13
Keyword [en]
Chemical engineering, polymer technology
Keyword [sv]
Kemiteknik
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-171ISBN: 91-7283-999-6 (print)OAI: oai:DiVA.org:kth-171DiVA: diva2:7667
Public defence
2005-04-21, Kollegiesalen, KTH, Valhallavägen 79, Stockholm, 13:00
Opponent
Supervisors
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2012-03-21Bibliographically approved
List of papers
1. Development and comparison of test methods for evaluating formation of biofilms on silicones
Open this publication in new window or tab >>Development and comparison of test methods for evaluating formation of biofilms on silicones
2002 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 78, no 2, 257-262 p.Article in journal (Refereed) Published
Abstract [en]

Silicone rubber formulations used for outdoor high voltage insulation are sometimes reported to be colonized by microorganisms. Different formulations show different sensitivity towards biological growth. In this study five rubbers were tested. Two standard practices were used, ASTM G21-90 and IEC 68-2-10. In addition a new method was developed to measure the rate of colonisation of a sample by biological growth. Results showed that the tested rubbers supported fungal growth when an external carbon source was available. Thus none of the silicones can be called bio-resistant. However the materials are not biodegradable either. This was clearly shown when mould spores were added to the samples in clean water, when none of the rubbers was contaminated. Some of the materials did, however, support growth when only nutrient salts, no carbon source, was added. The most bioresistant formulations contained the flame retardant zinc borate, indicating that this additive suppresses fungal growth. Further investigation with the new method supported that theory.

Keyword
Biofilm, Biological growth, Flame retardants, Fungus, High voltage insulation, Silicone rubber
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5032 (URN)10.1016/S0141-3910(02)00140-4 (DOI)000178534800007 ()
Note
QC 20100908. Uppdaterat från In press till Published (20100908)Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
2. Biofilms on silicone rubber insulators; microbial composition and diagnostics of removal by use of ESEM/EDS - Composition of biofilms infecting silicone rubber insulators
Open this publication in new window or tab >>Biofilms on silicone rubber insulators; microbial composition and diagnostics of removal by use of ESEM/EDS - Composition of biofilms infecting silicone rubber insulators
2004 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 85, no 2, 841-846 p.Article in journal (Refereed) Published
Abstract [en]

Silicone rubber high voltage insulators are sometimes reported to be colonised by microorganisms. When the hydrophobic polymeric surface is covered by a hydrophilic biofilm, the isolating properties of the insulator are reduced. However, cleaning the surface can restore the function of the insulator. In this study biofilms colonising insulators from Tanzania, Sri Lanka and Sweden were investigated. Results showed that the examined biofilms shared several properties. Small unicellular green algae associated with bacteria and filamentous fungi dominated all samples. Environmental scanning electron microscopy (ESEM) with X-ray energy disperse spectroscopy (EDS) is proposed as a new method to determine if an infected surface can be cleaned.

Keyword
silicone rubber, high voltage insulation, biofilm, algae, fungi, ESEM/EDS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-5033 (URN)10.1016/j.polymdegradstab.2004.02.014 (DOI)000223254300012 ()2-s2.0-3042647452 (Scopus ID)
Note
QC 20100901 Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
3. Microbiological growth testing of polymeric materials: an evaluation of new methods
Open this publication in new window or tab >>Microbiological growth testing of polymeric materials: an evaluation of new methods
2005 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 24, no 5, 557-563 p.Article in journal (Refereed) Published
Abstract [en]

Biofilms growing on high voltage insulators made of silicone rubber cause changes in appearance and properties of the silicone material. This study presents the design and building of microenvironment chambers simulating outdoor environment and the use of these for long-term studies of the development of mixed biofilms on silicone rubber materials. Results from the microenvironment chambers are compared to standard test procedures used in combination with new methods to evaluate the effect of two common flame retardants, ATH and zinc borate, on the development of a biofilm. Algae, bacteria and fungi isolated from silicone rubber insulators collected from Tanzania, Sri Lanka and Sweden, respectively, were used in the tests performed. Results show that zinc borate has a protective effect against all the microorganisms tested.

Keyword
Algae; Biofilm; Fungi; Insulators; Silicone rubber; Zinc borate
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-5034 (URN)10.1016/j.polymertesting.2005.02.005 (DOI)000229979000004 ()2-s2.0-19144366150 (Scopus ID)
Note
QC 20100811Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
4. Image analysis and laser induced fluorescence combined to determine biological growth on silicone rubber insulators
Open this publication in new window or tab >>Image analysis and laser induced fluorescence combined to determine biological growth on silicone rubber insulators
2005 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 88, no 3, 394-400 p.Article in journal (Refereed) Published
Abstract [en]

High-voltage outdoor insulators made from silicone rubber are sometimes reported to be colonised by microorganisms. When the hydrophobic polymeric surface is covered by a hydrophilic biofilm, the electrical properties of the insulator are altered. In this work, mixed biofilms, similar to those formed on the surfaces of polymeric insulators in the field, were successfully grown on five types of silicone rubber substrates in the laboratory, using specially designed microenvironment chambers. Photography and digital image analysis were utilized to estimate the areas covered by the growth. It was found that direct UV-light exposure hindered growth of the biofilms. Further, growth was also hindered on samples where zinc borate had been added as flame retardant. In contrast, addition of ATH did not influence the growth. In parallel, LIF spectroscopy was explored as a tool for detection of biofilms on silicon rubber samples. Experiments revealed that even weak traces of growth, not visible to the naked eye, could be detected. Finally, it is believed that LIF spectroscopy in combination with image analysis can be used for field diagnostics of biological growth on insulators in service.

Keyword
silicone rubber, biological growth, biofilm, image analysis, LIF, insulators
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5035 (URN)10.1016/j.polymdegradstab.2004.12.009 (DOI)000228789700005 ()2-s2.0-14844286478 (Scopus ID)
Note
QC 20100917Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
5. Effects of microbiological growth on PDMS materials
Open this publication in new window or tab >>Effects of microbiological growth on PDMS materials
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-5036 (URN)
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2011-09-20Bibliographically approved
6. Fungus covered insulator materials studied with laser-induced fluorescence and principal component analysis
Open this publication in new window or tab >>Fungus covered insulator materials studied with laser-induced fluorescence and principal component analysis
Show others...
2005 (English)In: Applied Spectroscopy, ISSN 0003-7028, E-ISSN 1943-3530, Vol. 59, no 8, 1037-1041 p.Article in journal (Refereed) Published
Abstract [en]

A method combining laser-induced fluorescence and principal component analysis to detect and discriminate between algal and fungal growth on insulator materials has been studied. Eight fungal cultures and four insulator materials have been analyzed. Multivariate classifications were utilized to characterize the insulator material, and fungal growth could readily be distinguished from a clean surface. The results of the principal component analyses make it possible to distinguish between algae infected, fungi infected, and clean silicone rubber materials. The experiments were performed in the laboratory using a fiber-optic fluorosensor that consisted of a nitrogen laser and an optical multi-channel analyzer system.

Keyword
fluorescence, polymeric insulators, fungal growth, lidar, remote sensing
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5037 (URN)10.1366/0003702054615214 (DOI)000231134300011 ()16105213 (PubMedID)2-s2.0-23844473759 (Scopus ID)
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
7. Fluorescence lidar imaging of fungal growth on high-voltage outdoor composite insulators
Open this publication in new window or tab >>Fluorescence lidar imaging of fungal growth on high-voltage outdoor composite insulators
2005 (English)In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 43, no 6, 624-632 p.Article in journal (Refereed) Published
Abstract [en]

Remote fluorescence imaging of fungal growth on polymeric high-voltage insulators was performed using a mobile lidar system with a laser wavelength of 355 nm. Insulator areas contaminated by fungal growth could be distinguished from clean surfaces and readily be imaged. The experiments were supported by detailed spectral studies performed in laboratory using a fibre-optic fluorosensor incorporating an optical multi-channel analyser system (OMA) and a nitrogen laser emitting radiation at 33 7 nm.

Keyword
lidar, fungal growth, polymeric insulators, fluorescence, remote sensing
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5038 (URN)10.1016/j.optlaseng.2004.09.019 (DOI)000227961800002 ()2-s2.0-13844299424 (Scopus ID)
Note
QC 20100915Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved

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