Properties And Ceramifying Mechanism Of Ceramifiable HNT/ Boron-containing Compounds/polysiloxane Composites

Ceramic polysiloxane composite is one kind of special fire retartant materials,which shows the similar processing and mechanical properties with common silicone rubber.The composites will be transformed to be self-surpported ceramics after being exposed at high temperature or in fire enviroment,thus the fire is hindered to spread into the inner of the composites.Ceramic polysiloxane composites show diverse application in aerospace and electric wire and cable industries.Ceramic fillers are the key parts of the ceramic composites,including mica,kaolin and montmorillonite etc.Halloysite nanotube?HNT?is one kind of natural multi-wall nanotube,mainly used in polymer nanocomposites at present.However,there have few reports about its applicaiton as a ceramic filler.In this work,ceramic polysiloxane composites were prepared by using HNTs as ceramic fillers and glass frit with low softening point and boron-containing compounds as fluxing agents.The mechanical and electrical insulated properties of the composites and the ceramic strength and microstructure of the ceramic residues were investigated.The ceramifying mechanism of the compositeswas revealed by SEM and XRD analysis.Firstly,ceramic polysiloxane composites were prepared by using HNTs as ceramic fillers and glass frits with low softening point as fluxing agents.The effect of the HNT content on the mechanical properties of the composites and ceramic residues was investigated.The results showed that as the concentration of HNTs increased,the mechanical and electrical insulated properties of the composites decreased,and the linear shrinkage and the weight loss of the composites decreased,and the flexural strength and impact strength of ceramic residue initially increased and then decreased.When the HNT content is 40 phr and the glass frit content is 15 phr,the ceramic strength of the ceramic residues formed at 1000oC was maximal with the flexural strength of 26.7 MPa and impact strength of 4.9 J·m^-1.The glass frits were melted to be liquid phase at 1000oC to form shrinked ceramic residues accompanied with the formation of cristobalite.Secondly,ceramic polysiloxane composites were prepared by using HNTs as ceramic fillers and sodium tetraborate,ammonium biborate or zinc borate as fluxing agents.The effect of the fluxing agents on the mechanical properties of the composites and ceramic residues was investigated.The tensile strength and elongation at break of the composites added with zinc borate are 5.4 MPa and 438%,respectively,and the tear strength is 14.8 kN·m^-1,and the volume and surface resistivity are 1.65×10¹⁴?·cm and 7.88×10¹⁴?,respectively,which are better than those of the composites with sodium tetraborate or ammonium biborate.Moreover,the ceramic residues obtained at 1000oC showed smooth and dense impact fratures with the flexural and impact strength of 28.7 MPa and 4.5 J·m^-1,respectively.Lastly,the effect of the zinc borate content on the mechanical properties of the composites and ceramic residues was investigated.The results showed that the mechanical and electrical insulated properties of the composites increased with the addition of zinc borate.When the content of zinc borate increased from 0 to 10phr,the flexural strength of the ceramic residue formed at 1000oC increased from 2.9 to 42.8 MPa,and the impact strength increased from 2.1 to 5 J·m^-1.After firing at 1000oC,zinc borate was decomposed to be zinc oxide and boron oxide,which reacted with amorphous silica and halloysites to precipitate cristobalite and gahnite after cooling of the residues.The new generated crysals will improve the strength of the ceramics.