Synthesis Of Heat-resistant And Anticorrosion Urethane Modified Siloxane Resins And Their Application In Coatings

Siloxanes have been used as coating materials since their industrial production due to their high thermal stability, low glass transition temperature and low surface energy. However, they are characterized by poor mechanical properties, and require high temperature of curing, which limits their use as coatings for substrates that are sensible to the high temperature. To overcome these drawbacks, several other polymers were introduced in the system of siloxane. They are modified by different polymers or copolymers such as epoxy resin, urethane, acrylates, etc. Thus, mechanical, adhesion, weathering and protective and anticorrosion properties were improved.In this thesis, new heat-resistant and anticorrosion modified crosslinked siloxane materials with low curing temperature and improved properties. Polyurethane is well-known for its advanced mechanic performance, and can be introduced into siloxane backbones with the aim of improving significantly the properties of siloxane binder. To achieve this goal, is has been proceeded as follows:(1) Firstly, intermediate products such as functionalized silicone resin (SR) and specially organoalkoxysilane (USCop) have been synthesized. TD (Trifunctional and Difunctional) SR has been synthesized through sol-gel process of difunctional and trifunctional alkoxysilanes as recommended in the synthesis of coating material. The ratios R/Si=1.4, Ph/R=0.5, Si/H2O=1.5and C2H5OH/Si=1.6have been chosen in order to synthesize SR with good flexibility and adhesion properties at180℃for10hours of curing. Such coating showed relatively good mechanical and adhesion properties. For the synthesis of the second intermediate product, i.e. urethane/urea copolymer,4,4’-diphenylmethylene diisocyanate (MDI) and Polybutylene Glycol Adipate (PBGA) were copolymerized according to NCO/OH=2:1, and then end-capped with3-aminopropyltriethoxysilane with the aim for using its alkoxysilane group in further reaction. Obtained intermediate product was referred to as USCop. The structure of USCop was characterized by Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (’H-NMR). Intermediate products have been used for the synthesis of different urethane siloxane hybrid coatings.(2) In order to synthesize heat resistant coating materials, SR and USCop was crosslinked prior to the presence of functional groups such as silanol and siloxane in their structure. Different proportions of SR and USCop were used with the aim to synthesize coatings with different siloxane contents. The ratio R/Si=1.4, Ph/R=0.5, Si/H2O=1.5and C2H5OH/S1=1.6was respected. Gel-permeation chromatography (GPC) of SRU coatings confirmed the presence of long and short macromolecular chains. FT-IR study of SRU coatings showed the presence of hydrogen bonds in their structures. Moreover, hydrogen bonding index increases with the increase of urethane content. Mechanical and adhesion properties of SRU coatings from180℃to350℃for several hours of exposition improved with the increase of urethane content. SRU coatings with low content of urethane showed good flexibility at short time of exposure. SRU coatings with relatively high content of urethane showed good adhesion properties. Scanning electron microscopic (SEM) study of SRU coating showed that the coating with70%of siloxane has relatively large molecule size and good particle size distribution due to hydrogen bonding. SRU coatings also exhibited good chemical properties when immersed in different chemicals for seven days. Thermogravimetric analysis (TGA) of SRU coatings confirmed their relatively high heat-resistance. Evaluation of protective and anticorrosion properties of SRU coatings by electrochemical impedance (EIS) measurement for21days of immersion confirmed the high protective capacity of SRU coatings.(3) Finally, through sol-gel processing of USCop, difunctional (dimethyl diethoxysilane and diphenyl dimethoxysilane) and trifunctional (methyl triethoxysilane and phenyl trimethoxysilane) alkoxysilanes, other urethane siloxane coatings were synthesized and characterized. These polyurethane/siloxane coatings were called SPUU. Ratios R/Si=1.4, Ph/R=0.5, Si/H2O=1.6and Si/C2H5OH=1.5were also respected. FT-IR study of SPUU coatings showed that the increase the urethane content, the increase the hydrogen bonding index. GPC study of SPUU coatings showed relatively high Mw and Mn as well as high polydispersity. SPUU coatings with90%of siloxane content in starting material, Mn=1.5×103, Mw=1.4×104and polydispersity d=10showed excellent impact resistance, adhesion and flexibility at different temperatures and times of exposition. However it cracked at350℃for four days of exposure. This coating also showed relatively large paricle size and good distribution at low and high temperatures. This SPUU coating showed excellent chemical resistance to inorganic and organic acids as well as salt and alkaline solutions. TGA analyses of SPUU coatings showed that they have excellent thermal stability and resistance. SPUU coatings with high content of siloxane showed the best thermal resistance and the lowest weight loss in inert environment and oxygen. EIS measurement of SPUU coating for21days of immersion showed that they are excellent providers of protection.