| dc.description.abstract | In contemporary composite science, polymer nanocomposites are the promising functional materials in the research field. Among them, the nano-sized filler, graphene, supplements the flaws of polymeric materials with their exclusive mechanical, electrical, and gas barrier properties. Recently, the exceptional corrosion resistance of polymer/graphene nanocomposites provides high efficiency for metal surface protection. In particular, the metal pipes for the transportation of oil and gas are very vulnerable to corrosion in the industries. The protection layer of polymer/graphene nanocomposites effectively mitigates the corrosion on the metal pipe due to the plate-like structure of graphene impeding the diffusion of corrosive agents through the polymer matrix.
In this thesis, polyurethane (PU) nanocomposites incorporated with graphene were considered as the protective coating layer of pipeliners in oil sands transportation. Pipeliner is an essential segment for oil sands transportation, and PU is generally used for the pipeliners to protect pipe inside against corrosive oil sands slurry. However, there has been a need for the improvement of the anti-corrosion performance and mechanical properties of PU. This is because these properties are correlated to the management cost in oil and gas industries. At this point, PU/graphene nanocomposites enable a longer lifetime of pipeliners and higher transport rates of oil sands due to the excellent physical properties of graphene. In addition, graphene nanoplatelets (GnP), which means multiple-layer graphene, were used as fillers for more practical research. GnP is commercialized in the market and provides various size for the polymer nanocomposites. Therefore, three phases were conducted in this thesis for the development of the protective coating layer based on PU/GnP nanocomposites: (I) size effect of GnP in the PU nanocomposites on mechanical and corrosion properties, (II) comparison of GnP and GO in the PU nanocomposites on mechanical and corrosion properties, and (III) effect of chemically functionalized graphene oxide (f-GO) in the PU nanocomposites on mechanical and corrosion properties.
In the first phase, the size effect of GnP influencing filler dispersion within the PU matrix was thoroughly investigated. 4 different sizes of GnP, specifically average diameter, were used for the first phase. They showed an average diameter from 100 μm to under 2 μm and were sufficient to compare their size. This study demonstrated that the GnP, with a small diameter significantly improved the anti-corrosion performance of the PU nanocomposites. Protection efficiency (PEF), which is a parameter to quantify anti-corrosion performance, was increased up to 99.6 % by the PU nanocomposites with the small GnP from 97.5 % of the neat PU. This is because the small GnP indicated a more uniform dispersion within the PU matrix than other GnP. The uniform dispersion of the small GnP formed a complicated pathway in the PU/GnP nanocomposites to suppress the diffusion of corrosive agents. However, the small GnP was not effective to improve mechanical properties of PU nanocomposites. The interfacial adhesion between GnP and the PU matrix was insufficient to distribute external stresses to the composite inside. On the other hand, large GnP indicated to the improvement of tensile modulus of the PU nanocomposites. The high aspect ratio of the large GnP contributed to the improvement of the tensile modulus, which means elastic deformation in a small range of strain. In consequence, the size of GnP was a critical factor influencing the dispersion of GnP and the anti-corrosion performance of PU/GnP nanocomposites. However, the interfacial adhesion between GnP and the PU matrix needs to be improved to enhance mechanical properties.
In the second phase, GO was synthesized from GnP of the phase 1 to improve the interfacial adhesion between GnP and the PU matrix. GO has various hydrophilic functional groups, such as hydroxyl (-OH), carbonyl (-C=O), and epoxy (-C-O-C-) groups. These functional groups could lead to the exfoliation (ie. delamination) of graphene layers and the formation of physical bonding with the PU matrix, such as hydrogen bonding. These effects contributed to the improvement of both mechanical properties and anti-corrosion in the PU/GO nanocomposites. However, GO was thermally more unstable than GnP due to the decomposition of the hydrophilic functional groups. Tensile modulus and hardness of PU/GO were increased up to 53 % and 40 % with the neat PU. In addition, PU/GO indicated 99.9 % of PEF while the neat PU indicated 98.9 % of PEF. In this study, large GO provided better mechanical properties and anti-corrosion in comparison to small GO in the PU nanocomposites. This result was attributed to formation of the hard-segment in the PU/GO nanocomposites. PU consists of the hard and soft segments and elasticity or hardness of PU depend on the size or amount of the hard segment. The hydrophilic functional groups on GO provides a nucleation site for the formation of the hard-segment. Thus, extremely high nucleation sites of the small GO resulted in a decrease of the hard-segment size. As a result, the hard segment size was not sufficient in the PU nanocomposites containing the small GO for appropriate mechanical properties Consequently, it was found that GO with large size eminently improved both mechanical and anti-corrosion properties of PU/GO. However, the thermal stability of GO needs to be improved for the pipeliner application because the oil sands slurry is hot (~80 °C).
In the third phase, f-GO was synthesized from GO of phase 2 using amine groups to improve the thermal stability of GO, maintaining high interfacial interaction among the PU matrix and fillers. Two different aliphatic amine groups (docecyl (DA) and tert-butyl amine groups(tBA)) and one aromatic group (2-naphthyl amine group (2NA)) were used to functionalize GO. DA and tBA groups were investigated for interaction with the polyol part of PU while the 2NA was for the one with the isocyanate part of PU. f-GO indicated higher thermal stability than GO because the amine functional groups have stronger bonding with GO than the hydrophilic functional groups on GO. Among the PU/f-GO nanocomposites, PU/2NA-GO indicated the most improved mechanical properties and anti-corrosion performance. PU/2NA-GO showed 123 % and 49 % improvements in tensile modulus and hardness, respectively than the neat PU. PEF of PU/2NA-GO was improved up to 99.9 % from 86.9 % of the neat PU. This improvement of PU/2NA-GO was attributed to π-π interaction between the PU matrix and 2NA, inducing overlapped aromatic groups, such as the 2NA and phenyl groups of isocyanate. It led to a synergetic effect on mechanical properties and anti-corrosion of the nanocomposite. In consequence, this study revealed that the aromatic component on GO led to π-π interaction with the PU matrix. This interaction contributed to both improvements in mechanical properties and anti-corrosion performance.
In conclusion, the results from the 3 phases have shown critical factors to design mechanically improved and highly anti-corrosive PU/GnP nanocomposites. GnP size was very important to decide the dispersion of the fillers. Functional groups on GO were crucial to improve the interfacial interaction between the PU matrix and graphene fillers. As a result, aromatic group, which is covalently bonded with f-GO, indicated the best performance in the PU nanocomposites due to π-π interaction. These studies in this thesis can be suitable for oil and gas industries. In particular, they can provide a practical insight into the development of PU pipeliners for the transportation of oil sand slurry. | en |
