Study On The Resistance To Water,Alkali And Salt Solutions Of Pultruded Polyurethane-based CFRP Plates

Fiber reinforced polymer?FRP?materials with light weight and high strength,corrosion resistance and convenient construction,etc.,have been widely used in the reinforcement,repair and strengthening of civil engineering structures.Since FRP materials used in civil engineering will suffer from vaious harsh environments during the service and FRP materials are of great variety and vary in performance,the service performance of FRP materials under different environments has not been clear so far,which has become one of the key factors affecting its engineering application.In recent years,the two-component thermosetting polyurethane?PU?with excellent mechanical properties and processing performance is gradually used in FRP materials.In this article,the degradation in the performance of PU resin and its carbon fiber reinforced polymer?CFRP?plates in distilled water,alkaline solution and seawater?salt?was studied systematically using the accelerated aging method,and then the long-term performance of PU-based CFRP plates subjected to the combined effects of immersion and load was also explored,and finally the research results were compared to the common epoxy resin and its CFRP plates.The main research contents and achievements include:?1?The water absorption and diffusion behavior of PU resin immersed in distilled water/alkaline solution/seawater at 20?,40? and 60? was studied;the mechanisms in the water absorption and diffusion of PU resin were explored through the water desorption test and infrared spectroscopy testing methods.The results show:the absorption and diffusion behavior of water molecules in the PU resin follows the Fick's law and the difference in the effects of different environments is small;the equilibrium water uptake of PU resin decreased with increasing immersion temperature;the unreacted-N=C=O group in resin reacted with water molecules,resulting in the released of CO₂ and further a certain mass loss.Unlike the epoxy resin from the cited references,immersion did not cause significant hydrolysis of PU.?2?The effects of the distilled water/alkaline solution/seawater immersion on the viscoelastic and mechanical properties of PU resin were studied using the accelerated aging method by heating.The results found:unlike the epoxy resin from the cited references,the evolutions in the viscoelastic and mechanical properties of PU resin is independent on the immersion environment;both the reductions of the glass transition temperature?T_g?and tensile properties caused by the water absorption plasticization effect recovered after drying,and even their values exceeded the initial ones due to the improvement of the cross-linking degree of resin.?3?The water absorption and diffusion behavior of PU-based CFRP plates subjected to distilled water,alkaline solution and seawater immersion at 20?,40? and 60? was studied;the mechanisms in the absorption and diffusion of water molecules in the CFRP plates were explored based on the PU-carbon fiber interfacial bond performance testing,water desorption test and infrared spectroscopy testing.The results found:the water absorption and diffusion behavior of PU-based CFRP plates follows the two-stage model and the difference in the effects of different environments is small;as similar with the PU resin,the equilibrium water uptake of PU-based CFRP plates decreased with increasing immersion temperature and mass loss occurred during immersion;the formation of the PU-carbon fiber interfacial micro-cracks promoted the diffusion of water molecules along the fiber direction.?4?The law and mechanism in the effects of the distilled water/alkaline solution/seawater immersion on the viscoelastic and mechanical properties of PU-based CFRP plates were studied using the accelerated aging method by heating.The results found:the degradation law in viscoelastic and mechanical properties of PU-based CFRP plates is independent on the immersion environment;as similar with the PU resin,immersion leaded to a decrease in T_g,and T_g regained and even exceeded its initial value after drying;the effects of immersion on the flexural properties are small,and a little degradation in the flexural strength occurred due to the formation of the interfacial micro-cracks after drying;the effects of immersion on the tensile properties are small,and even the high-temperature immersion resulted in the increase of the tensile strength.?5?The effects of the coupling of distilled water/seawater immersion and sustained bending strain?0%,30%and 58%of ultimate tensile strain?on the long-term performance of PU-and epoxy-based CFRP plates were studied comparatively.The research achievements include:the increase of sustained bending strain leaded to the decrease of the water uptake?immersion in the late stage?and mechanical properties of CFRP plates;an influence mechanism model of sustained bending strain on the water uptake of CFRP plates was proposed and the degradation rate of the tensile strength of CFRP plates under the coupling effect was created;in contrast to the epoxy-based CFRP plates,the long-term service performance of the PU-based CFRP plates is better.In this article,the evolution law and mechanism in the long-term performance of PU resin and PU-based CFRP plates immersed in water,alkaline solution and seawater were obtained,and the research results will provide a reliable durability data for the application of PU-based CFRP plates in civil engineering.