Deep-seawater Absorption Behavior And Anti-water Absorption Methods Of Polymer Matrix Composite Materials

The paper focuses on the water absorption behaviors and failure mechanism ofglass fiber reinforced epoxy resin composites (GF/CYD-128) in deep-seawaterenvironment. Methods to improve mechanical properties of the composites immersed inthe deep-seawater environment were investigated. The methods are based onmodification of the inter-phase of fiber/matrix by using coupling agents, enhancedproperties of the matrix by using nano-particles and isolating moisture from thecomposite structures by forming a superhydrophobic surface layer on it. The mainresearch work is as follows:Experimental investigation on water absorption of the glass fiber reinforced epoxymatrix composites immersed in the deep-seawater environment was conducted. The0-300meters deep-seawater environment was simulated by using a self-madecylindrical pressure vessel filled with seawater and0.1-3.0MPa high-pressure nitrogengas. The moisture contents and saturated swelling of specimens of the GF/CYD-128composites immersed in the deep-seawater environment were obtained. Results showthat the Langmuir-model can be used to describe the water absorption behavior of theGF/CYD-128composites, and the predicted data with Langmuir-model were agreedwith the experimental data of GF/CYD-128under pressures of0.1-3MPa.Investigation on effects of water absorption on mechanical properties of thecomposites was conducted. Flexural strength, inter-laminate shear strength (ILSS) andfatigue life of the GF/CYD-128specimens with different contents of absorbed seawaterwere test. Finite element models for ordinary and interphase precession waterabsorption of fiber reinforced polymer matrix composites were developed and stressdistribution in the composites were obtained. Results show that the interface debondingwas a dominating mechanism for the failure of the composites absorbed water underexternal loadings. Degradation of the mechanical properties of the composites absorbedwater was due to the internal stresses induced by swelling of the composites afterabsorbed water which results in the interface debonding and matrix cracking of thecomposites.In order to improve the interface bonding property of the composite materialsduring water absorbed process, silicic-alkyl coupling agents of RSiX3were used tomodify the glass fiber/epoxy matrix interphase. Effects of treatment method andmolecular structures of the coupling agents on the moisture absorption behavior of thecomposite material were investigated. Results show that the fiber pretreatment with thecoupling agents was better than the mixing of the coupling agents with epoxy resin toimprove the mechanical properties of the composites. Comparing to the original composites, the rate of moisture absorption of the GF/CYD-128composites with fiberpretreatment of KH550coupling agents decreased31.8%, while the saturated water inthe composites has no significant changes. With fiber pretreatment of KH550couplingagents, ILSS of the dry and wet composites immersed in water for79days increased16.9%and10.3%, respectively. And the same effect can be benefit from more reactiveR-group and less resistance of X-group in the RSiX3.To improve moisture resistance of resin matrix in the composites the nano-SiO2particles were added into the resin matrix of the GF/CYD-128composites. The moistureabsorption and mechanical properties of GF/CYD-128composites were tested. Resultsshow that the maximum swelling rate of the GF/CYD-128with nano-SiO2modifieddecreased61.5%. Compared with dry specimens, the flexure strength of the saturatedunmodified GF/CYD-128decreased15.3%, while the saturated modified GF/CYD-128increased5%. The ILSS of the saturated unmodified GF/CYD-128decreased31%,while that of the saturated modified GF/CYD-128decreased only12%. Fatigue life ofthe nano-SiO2modified composites with saturated moisture under tension-tensioncycling loading was increased142.8%than that of the saturated unmodifiedGF/CYD-128.A method for protecting the whole composite structures from moistureenvironment was proposed and tested. The idea for the method is try to form an outersuperhydrophobic surface on the composite structures by isolating the moistureenvironment from contacting with the composite structures. Through material design forthe superhydrophobic surface layer, chemical etching, surface modification andparameter optimization, a superhydrophobic surface on the GF/CYD-128compositeswas successful fabricated. It was showed that the superhydrophobic surface layer ofCaCO3/CYD-128and ZnO/CYD-128with contact angle of larger than150°weresuccessfully formed by chemical etching with50wt%acetic acid and surface modifiedwith1wt%stearic acid. The prepared superhydrophobic surfaces have long-termstability at room temperature and thermal stability in rang of10to80℃. In addition, theprepared superhydrophobic surface can effectively reduce the hygroscopic rate andsaturated moisture content of GF/CYD-128composites in the moisture environment.