Studies On The Novel Polyaspartic Ester Based Polyurea Coatings For Marine Concrete Protection

Concrete structures are widely fabricated in the harbours, submarine tunnels, and over sea bridges. Numbers of marine structures are constructed with the exploitation of the marine resources and utilize of the marine spaces. However, the marine structure corrosion occurs gravely due to environmental factors, which diminish the safety and durability of marine concrete structure fearfully. Thus, the preparation of new protective coating materials and the investigation of high performance coating techniques are significant since coating protection becomes the primarily and available method for the protection of marine concrete. Polyurea is a class of high performance polyurethane elastomer, and aromatic and normal aliphatic polyureas are there into generally applied in the preparation of the high weathering durability and anti-corrosion protective coatings. In the present study, new aliphatic polyurea- polyaspartic ester (PAE) based ployureas were studied for the first time as marine concrete structure protective coatings. And thereafter, FTIR, DSC, DMA, AFM and GPC methods were employed to analysis their structure, morphology, properties and the anti-corrosion behaviours in marine environment as well as protective properties of marine concrete structures. The main results are as following:A unique secondary trianime chain extender PAE-t was prepared from amine terminated polyoxypropylene (Jeffamine T403) and dialkyl maleates via Michael Addition Reaction. A series of novel PAE-t based ployureas were then synthesized from PAE-t chain extender and aliphatic polyisocyanates hexamethylene diisocyanate (HDI) trimer/HDI trimer-Jeffamine D2000 prepolymer (N prepolymer) and Jeffamine D2000 via one-step method or prepolymerization method.Two types of new secondary dianime chain extenders PAE-b and PAE-c were prepared by two-steps of addition reaction: (1) Michael Addition Reaction between 4,4'-methylenebis(cyclohexyl amine) (H12MDA) or 4,4'-methylenebis(2- methyl cyclohexyl amine)(Laromin C260) and dialkyl maleates; (2)The addition reaction between the residual anime of reaction (1) and E-51 epoxy resin. A series of new PAE-b or PAE-c based polyureas were then synthesized by the reaction of PAE-b or PAE-c chain extender and aliphatic polyisocyanates 4,4'-diisocyanato dicyclohexylmethane(H12MDI)-Jeffamine D2000 prepolymer (H prepolymer) via prepolymerization method. A unique flexibilized secondary dianime chain extender PAE-f was prepared from amine terminated polyoxypropylene (Jeffamine D230), Laromin C260 and dialkyl maleates via Michael Addition Reaction. A series of new PAE-f based polyureas were then synthesized by the reaction of PAE-f chain extender and H prepolymer via prepolymerization method.Reaction activity and mechanical properties results indicate that the synthesized PAE based polyureas exhibit slower reaction activity with polyisocyanates (gel time are about 8-86min)than aromatic and normal aliphatic polyureas, and the reaction activity can be adjusted through the variation of PAE and the variation of the polyureas composition, so they are more useful in practical engineering. These polyureas are elastomer coating materials with high strength, modulus and hardness. FTIR,DSC,DMA and atomic force microscope(AFM) results indicate that the hydrogen bonding degree of amidogen groups in hard segments of the polyureas are high, and the length are in a range of 0.303nm～0.309nm; the hydrogen bonding degree for urea carbonyl groups are 67.3%～82.7%. The hydrogen bonding degree of amidogen and urea carbonyl decrease with the decreasing of hard segment content. The segmented polyureas behave microphase separated morphology. It reduces the possibility of the microphase separated morphology creation when the hard segment content reduces and/or chemical crosslindk occurs. AFM topography images fractal dimension Dt and phase images fractal dimension Dp of the PAE based polyureas change regularly with the change of hard segment content, so they can be used to characterize the surface topography of the polyureas.The experimental results from the influence of marine corrosion to those PAE based polyurea coatings show that they exhibit excellence durability of outdoor insolation (after 350 days outdoor insolation tarnish percentage are 12.7%-18.5%, tensile strength change percentage are 5.2%-10.2% and elongation change percentage are 4.3%-6.8%). The behaviours of anti-salt fog exposure of these coatings are even better. The coatings have better aging resistance with more hard segment content, better ordered structure, higher curing temperature and larger thickness. After exposing to salt fog for 200 days, the surface of T3(PAE-t-HDI prepolymer-D2000H65)and the interface between T3 and the concrete substrate are all in good condition, and the tarnish percentage is 1.7%, tensile strength change percentage is 1.4%. Whereas, UV/ salt-fog alternating exposure destroys the molecular structure, surface morphology and the properties of the T3,B2 ( PAE-b-H12MDI prepolymerH66 ) and F2(PAE-f-H12MDI prepolymerH62)coatings more severely. FTIR,SEM and AFM results show that the molecular chains are broken in both soft segment and hard segment, pinholes and cracks are development and coatings are partially pull off from the concrete which lead to the inability to protect the substrate. The current researches indicate that PAE based polyurea coatings have comparative better anti-aging properties than normal anticorrosion coatings, such as polyacrylate and polyurethane, and aromatic and normal aliphatic polyurea. Therefore, PAE based polyurea coatings maintain better resistance of marine corrosion than normal anticorrosive coatings.The farther investigations on the influence of the salt fog and NaCl immersion exposure to the T3,B2,F2 and TM(T3 finishing coat/ MDI emulsion primer)coated concrete indicate that the coated specimens show favourable frost resistance, wet-adhesion, and resistance of Cl- diffusivity. In all, coating system is a significant influencing parameter to the protective properties. The TM coating with compact structure, tough bond between giant molecule chains, high coating/concrete interfacial adhesion, exhibit excellent resistance of salt fog exposure and 10% NaCl solution immersion exposure. It is reported by experiments that there are none structural defect and Cl- diffusion behaviour satisfy second Fick's law when the coatings are prepared under the condition of the temperature no less than 10oC, curing time no less than 72 hours, and more than 40±2um thickness.Compare with single NaCl solution immersion exposure, the coaction of load-NaCl solution immersion double factors and freezing-thawing cycles-NaCl solution immersion double factors have more influence to the properties of the coated concrete. Under the coaction of these double factors, the diffusion coefficient of chloride of T3,B2 and TM coated concrete are 10-14-10-15m2/s, adhesion of TM is about 2.5N/mm, indicating favourable anti-corrosion characteristics.Furthermore, equations to models the chloride penetration of T3,B2 and TM coated concrete and concrete substrate are set up according to diffusion theories under the conditions of NaCl immersion exposure, coaction of load-NaCl solution immersion and coaction of freezing-thawing cycles-NaCl solution immersion. The forecast of the models consist with experimental results. It note that the research work establish foundation for the research of durability protection of marine concrete.