| Natural graphite is an important mineral substance with excellent physical and chemical properties.Expanded graphite is one of the natural graphite deep processing products,it is a porous and loose material,its low density,good heat resistance,radiation resistance,softness,anisotropy and excellent compression resilience,self-lubrication,impermeability and corrosion resistance and other physical and chemical properties,are widely used in the seal area.Flexible graphite gaskets made of expanded graphite are faced with the industrial problems of high leakage and high temperature oxidation,which restrict the application of flexible graphite gaskets in the sealing field.In order to enlarge the application range of graphite gaskets in harsh working conditions,the flexible graphite gaskets were prepared by pressing and compounding the expanded graphite,and then the flexible graphite gaskets were modified by impregnation and coating spraying respectively.Flexible graphite impregnated gasket was obtained by impregnation modification to improve its sealing performance.The flexible graphite coated gasket was modified by coating spraying to enhance its oxidation resistance at high temperature,and the leakage mechanism and oxidation resistance mechanism of modified graphite gasket were analyzed,which had important theoretical value and practical significance.Through the condition test,the compression resilience and tensile strength of flexible graphite sheet under different pressing processes were compared,and the appropriate pressing process scheme was determined:The compression rate,resilient rate and tensile strength of flexible graphite sheet were 40.43%,15.88%and 5.35 MPa,respectively,when the thickness,density and times of pressing were 0.8 mm,1.0 g/cm3and 3 times respectively.On the basis of the above pressing process,the flexible graphite sheet was further composite reinforced to prepare flexible graphite reinforced gasket.The performance of sprint gasket and composite gasket was compared.The flexible graphite sprint reinforced gasket was selected as impregnation modified material and coating modified material.The leakage rate was 10.70×10-4cm3/s,the compression rate was 33.91%,and the rebound rate was 21.34%.The flexible graphite sprint reinforced gasket was impregnated by impregnation method.The impregnation process of PTFE emulsion impregnation was determined as follows:impregnation vacuum of-0.095 MPa,vacuum pressure of 0.65 MPa,impregnation times of 2 times,impregnation time of 30 min,impregnation concentration of 60%,and impregnation temperature of 35℃.After impregnation,the gasket leakage rate was 2.03×10-4 cm3/s,weight gain rate was 4.39%,compression rate was 32.17%,rebound rate was 22.34%.The gasket leakage rate,weight gain rate,compression rate and rebound rate were 115.30×10-4 cm3/s,15.23%,22.15%and 8.91%respectively after phenolic resin impregnated flexible graphite reinforced gasket.Compared with the impregnation effect of two kinds of impregnation agents,PTFE emulsion impregnation agent had obvious advantages.The flexible graphite reinforced gasket was modified by coating spraying process.The basic process of coating spraying was determined as follows:the coating concentration was 25%,the mass ratio of boron nitride to aluminum dihydrogen phosphate was 2:1,and the number of spraying was 7.Compared with the gaskets without coating and with coating under the best spraying process at different temperatures and different times,the gaskets with coating flexible graphite enhanced oxidation weight loss rate was better than the gaskets without coating.The gaskets with coating flexible graphite enhanced oxidation weight loss rate was 0.65%and the leakage rate was 11.57×10-4cm3/s after roasting at 600℃for 4 h.Related tests（SEM,XRD,EDS,TG-DSC）were carried out on expanded graphite,low density flexible graphite sheet,flexible graphite sheet,flexible graphite sprint reinforced gasket and coating.It was found that the expanded graphite internal micro piece in the process of suppressing after bending,lap,meshing,overlapping process after the final formation of graphite gaskets lamella structure;Through impregnation,the PTFE emulsion can effectively fill up the gap between the graphite gaskets and permeate into the graphite gaskets along the interlayer to reduce the leakage of gaskets.The method of coating spraying can generate material with high melting point to cover the gasket surface,reduce the contact area between oxygen and gasket,and reduce the oxidation weight loss rate. |
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