Stress Cracking Behavior Of Polycarbonate In Sodium Hydroxide Solution And Surface Coating Protection

Polycarbonate(PC)has been gradually expanding into high-end field such as airspace and national defense,which proposes greater demands on the stability and safety of products.With the diversification of service environment,PC is prone to suffering stress cracking after contact with chemical liquid medium such as alkaline solution,causing premature failure of the product.And the failure mechanism is different from that of conventional organic solvents.Therefore,this paper aims to study the stress cracking behavior of PC exposed to alkaline solution and the influence mechanism,and to discover new method for improving stress cracking resistance of products.It has a scientific significance for deep understanding of the stress cracking failure behavior of PC on service under complex environment.In addition,it possesses great value from the point of engineering for the research,manufacture and service safety of high-performance transparent parts.This paper focused on the stress cracking behavior and mechanism of PC in sodium hydroxide solution of different concentrations.The effects of high concentration and low concentration of sodium hydroxide solution on the stress relaxation,crack generation and development,fracture surface morphology of PC were investigated,and the relationship between stress cracking behavior and temperature,initial load and other factors was also explored.On this basis,the protective mechanism of silicone coating on the stress cracking of PC transparent parts under sodium hydroxide solution was further discussed.The main tasks were as following:1.The influence and mechanism of sodium hydroxide solution of different concentration on the stress cracking behavior of PC were studied.It was found that PC showed distinct stress cracking behavior in low concentration(0.05mol/L~3mol/L)and high concentration(3.5mol/L~ 5mol/L)sodium hydroxide solution.Hydrolysis causes the breaking of PC molecular chains in the low concentration of sodium hydroxide solution,which in turn promotes the generation of cracks expanding into the interior of the sample.All of samples under this condition were broken.However,severe hydrolysis destroys the crazing area and blunts the crack tip in the high concentration sodium hydroxide solution,which delays or even prevents the generation and development of cracks.2.The effect of different ambient temperature and initial load on the stress cracking behavior of PC in sodium hydroxide solution was investigated.The results showed that the stress cracking behavior of PC could be accelerated by increasing the temperature and the initial load.This was because they can both accelerate the diffusion of the solution and reaction rate of hydrolysis of PC,which was prone to cracking.These would cause samples to break rapidly under low concentration solution and the cracks increase under high concentration solution.3.The protective mechanism of the silicone coating on the stress cracking behavior of PC was discussed.The research showed that coated samples have lower stress relaxation rate and longer crack initiation time,so stress cracking resistance of products was improved,but this protective property gradually decreased with the increase of sodium hydroxide concentration.This was mainly because the silicone coating blocked absorption of solution by PC and slowed down the occurrence of cracks.However,the sodium hydroxide solution could damage the chemical structure of the silicone coating,resulting in reducing protective performance of the coating.