| 1. |
- Atari Jabarzadeh, Sevil, 1981-
(författare)
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Prevention of Biofilm Formation on Silicone Rubber Materials for Outdoor High Voltage Insulators
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Microbial colonization on the surface of silicone rubber high voltage outdoor insulators often results in the formation of highly hydrated biofilm that influence the surface properties, such as surface hydrophobicity. The loss of hydrophobicity might lead to dry band formation, and, in the worst cases, flashover and failure of the insulator.In this work, the biocidal effects of various antimicrobial compounds in silicone rubber materials were determined. These materials were evaluated according to an ISO standard for the antimicrobial activity against the growth of aggressive fungal strains, and microorganisms that have been found colonizing the surfaces of outdoor insulators in several areas in the world. Several compounds suppressed microbial growth on the surfaces of the materials without compromising the material properties of the silicone rubber. A commercial biocide and thymol were very effective against fungal growth, and sodium benzoate could suppress the fungal growth to some extent. Thymol could also inhibit algal growth. However, methods for preservation of the antimicrobial agents in the bulk of the material need to be further developed to prevent the loss of the compounds during manufacturing. Biofilm formation affected the surface hydrophobicity and complete removal of the biofilm was not achieved through cleaning. Surface analysis confirmed that traces of microorganisms were still present after cleaning.Further, surface modification of the silicone rubber was carried out to study how the texture and roughness of the surface affect biofilm formation. Silicone rubber surfaces with regular geometrical patterns were evaluated to determine the influence of the surface texture on the extent of microbial growth in comparison with plane silicone rubber surfaces. Silicone rubber nanocomposite surfaces, prepared using a spray-deposition method that applied hydrophilic and hydrophobic nanoparticles to obtain hierarchical structures, were studied to determine the effects of the surface roughness and improved hydrophobicity on the microbial attachment. Microenvironment chambers were used for the determination of microbial growth on different modified surfaces under conditions that mimic those of the insulators in their outdoor environments. Different parts of the insulators were represented by placing the samples vertically and inclined. The microbial growth on the surfaces of the textured samples was evenly distributed throughout the surfaces because of the uniform distribution of the water between the gaps of the regular structures on the surfaces. Microbial growth was not observed on the inclined and vertical nanocomposite surfaces due to the higher surface roughness and improved surface hydrophobicity, whereas non-coated samples were colonized by microorganisms.
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| 2. |
- Kaali, Peter, 1984-
(författare)
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Antimicrobial Polymer Composites for Medical Applications
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The current study and discuss the long-term properties of biomedical polymers in vitro and invivo and presents means to design and manufacture antimicrobial composites. Antimicrobialcomposites with reduced tendency for biofilm formation should lead to lower risk for medicaldevice associated infection.The first part analyse in vivo degradation of invasive silicone rubber tracheostomy tubes andpresents degradation mechanism, degradation products and the estimated lifetime of thematerials.. It was found that silicone tubes undergo hydrolysis during the long-term exposurein vivo, which in turn results in decreased stability of the polymer due to surface alterationsand the formation of low molecular weight compounds.The second part of the study presents the manufacturing of composites with single, binary andternary ion-exchanged zeolites as an antimicrobial agent. The ion distribution and release ofthe zeolites and the antimicrobial efficiency of the different systems showed that single silverion-exchanged zeolite was superior to the other samples. Antimicrobial composites wereprepared by mixing the above-mentioned zeolites and pure zeolite (without any ion) withdifferent fractions into polyether (TPU), polyether (PEU) polyurethane and silicone rubber.The antimicrobial efficiency of binary and ternary ion-exchanged samples was similar whichis thought to be due to the ion distribution in the crystal structure.The changes in the mechanical and surface properties of the composites due to the zeolitecontent demonstrated that the increasing zeolite content reduced the mechanical propertieswhile the surface properties did not change significantly. The antimicrobial tests showed thatthe silver-containing composite was the most efficient among all the other samples. Thebinary and ternary ion-exchanged composites expressed similar antimicrobial efficiency as itwas seen previously for the different zeolite systems. Biocompatibility was studied byexposure to artificial body fluids to simulate the degradation of the composites in the humanbody. Significant changes were observed in the morphology, the surface properties and the chemical structure.
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| 3. |
- Kittikorn, Thorsak, 1974-
(författare)
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Tuning the long-term properties to control biodegradation by surface modifications of agricultural fibres in biocomposites
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sustainable polymeric materials put emphasis on mastering the whole life-cycle of polymeric materials. This includes the choice of raw materials, selection of synthesis and processing, environmental impact during long-term use followed by detailed knowledge about recycling and waste management. Within this large efforts are put in the design and development of new biocomposites using renewable fibres instead of inert ones. The thesis deals with surface modifications of agricultural fibres and the design of biocomposites with optimal long-term properties balancing the potential risk for biodegradation. The first part of this thesis involved surface modifications of oil palm fibres and production of biocomposites with PP as matrix. The chemical surface modifications of oil palm fibres explored propionylation, PPgMA grafting via solution modification and reactive blending and vinyltrimethoxy silanization as methods. All modified fibre/PP biocomposites showed improvements in the mechanical properties followed also by an improvement of water resistance. In comparison with unmodificed fibres/PP matrix the highest water resistance after the surface modifications of oil palm fibres were observed for silanization followed by PPgMA modified, PPgMA blending and propionylation.The second part aimed at producing fully biodegradable biocomposites and analysing the resulting properties with respect to potential risk for biodegradation. Sisal fibres were incorporated in PLA and PHBV and the resulting risk for biodegradation using a fungus, Aspergillus niger, monitored. Neat PLA and PHBV were compared with the corresponding biocomposites and already without fibres both polymers were notably biodegraded by Aspergillus niger. The degree of biodegradation of PLA and PHBV matrices was related to the extent of the growth on the material surfaces. Adding sisal fibres gave a substantial increase in the growth on the surfaces of the biocomposites.Correlating the type of surface modification of sisal fibres with degree of biodegradation, it was demonstrated that all chemically modified sisal/PLA biocomposites were less biodegraded than unmodified sisal biocomposites. Propionylated sisal/PLA demonstrated the best resistance to biodegradation of all biocomposites while sisal/CA/PLA demonstrated high level of biodegradation after severe invasion by Aspergillus niger.In general, the biodegradation correlated strongly with the degree of water absorption and surface modifications that increase the hydrophobicity is a route to improve the resistance to biodegradation.Designing new biocomposites using renewable fibres and non-renewable and renewable matrices involve the balancing of the increase in mechanical properties, after improved adhesion between fibres and the polymer matrix, with the potential risk for biodegradation.
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