Study On Char Strengthening Of Expandable Fireproof Silicone Rubber

At present,the fire-resistant sealant and thermal expansion rubber sleeve used in the marine fireproof sealing device for cable penetration adopt the combination of expandable graphite（EG）and inorganic flame retardant.When it exposed to flame combustion,the expanded char layer formed was loose and porous,and has excellent flame retardancy and heat insulation performance.It can pass the A60 fire resistance test required for marine applications,but the poor mechanical strength of the char layer makes it difficult to be used in the sealing of plastic pipe penetration.The black appearance of EG also limits its application.Silicone rubber,which contains porcelainable components,can form a hard and dense char layer at high temperature.It could resist the further spread of flame for a long time.However,the densified structure needs to be strengthened in terms of heat isolation.Therefore,if the char layer of silicone rubber can take into account both the porous heat insulation of the expanded char layer and the sturdiness and durability of the porcelainized char layer,it’s of great significance to improve the fire resistance of silicone rubber.In this paper,char reinforcing materials which could melt,melt and foam,or decompose to form hard skeletons were used with expansion flame retardant NH₂-C,such as sodium silicate hydrate,BF550,Al（OH）₃.The effects of different char forming materials on the mechanical properties,fire-retardant and fireproof properties of silicone rubber,and on the compressive strength of char layer were analyzed.The influence of microstructure of char layer on strength,fireproof and heat insulation was discussed.Then,for the materials which can optimize the char layer,the influence of its dosage and the interaction with other components on the structure and performance was analyzed by orthogonal experiment.Finally,the flame propagation performance of silicone rubber was studied by surface flammability test.The silicone rubber was used along with fireproof tape to seal the PP,PE,PVC,epoxy plastic penetration pipes,and fireproof door.According to the actual application configuration and size,the components were made and their fire resistance were tested.Research indicates:（1）Silicone rubber containing the expansion flame retardant NH₂-C only has excellent flame retardant performance and oxygen index was up to 44.5.Its char layer has molten expansion deformation,and the internal porous structure was extremely uneven with the compression strength only 0.03MPa.The silicone rubber with sodium silicate monohydrate as the char reinforcing material has excellent fire resistance and shows obvious expansion after combustion with residual char compression strength of 0.57MPa.Its deep curing rate was fast because the crystal water was easy to react with keto oxime group.As a result,its storage life was short.The flame retardant and fireproof of silicone rubber with BF550 as char reinforcing material was slightly poor.BF550 greatly improves the firmness of the char layer and the compression strength was up to 9.88 MPa.The char layer has obvious shrinkage.Inner holes show small size and even distribution.The silicone rubber with Al（OH）₃has good flame retardant performance and oxygen index was 45.Its char layer had a certain degree of expansion,and the compression strength reaches1.08 MPa.By contrast,the laminated structure of EG has great damage to the mechanical property strength of silicone rubber,and the loose char layer formed after combustion has basically no mechanical property.Therefore,BF550 and Al（OH）₃had positive effects as reinforcing materials of expanded char layer.（2）Taking BF550,Al（OH）₃and NH₂-C as three factors,the three-factor three-level orthogonal experiment was designed.The range analysis shows that Al（OH）₃,NH₂-C and BF550 had the greatest influence on the mechanical properties,flame retardancy and fireproof,the volume change before and after calcining and the compression strength of char layer,separately.The weight analysis shows that the optimal ratio of silicone rubber contains Al（OH）₃18%,BF550 18%and NH₂-C 25%.Under this ratio,the silicone rubber had good mechanical properties,and the tensile strength,bond strength and elongation at break were 1.63 MPa,1.75 MPa and 240%,respectively.It had good flame retardant with oxygen index of 45.The char layer was significantly enhanced and the compression strength was up to 4.67 MPa.Under the burning of 1200^oC hydrocarbon flame,the fire resistance time of the 8mm thick sample was up to 18 min,which means the time needed for back surface temperature rise of 180^oC.（3）Phase analysis of the char layer revealed the presence of crystalline and amorphous Si O₂.Crystalline Si O₂originated from amorphous Si O₂dissolved in the molten phase of silicate BF550 and then cooled out.The diffraction peaks of Al PO₄and Ca₅（PO₄）₃（OH）indicated that the phosphate group interacted with the decomposition products of Al（OH）₃and Ca CO₃at high temperature to form new crystals.As the skeleton of porous structure,it greatly improved the strength of char layer.（4）Char enhanced silicone rubber showed low flame propagation under the dual action of hydrocarbon flame and thermal radiation field.This complied with the requirements of SOLAS chapter II-2.The flame spread distance,duration and total heat release were less than 370mm,800s and 0.7 MJ,respectively.There were no drips during the combustion.（5）The fire resistance of GRP/GRE glass steel tubes and PP,PE and PVC pipes with diameters of 90 and 160 were tested.At 60min,the maximum temperature rise of any temperature sensing point on the backside surface was less than 180°C,and the average temperature rise was less than 140°C.The fireproof sealing system reached grade A60 for marine pipe penetration.