Synthesis Of Phosphorus Silicon Containing Polyurethane And Its Effects On Structures And Properties Of Intumescent Flame Retardant EPDM/PP Thermoplastic Elastomers

EPDM/PP thermoplastic elastomer prepared by dynamic vulcanization has high elasticity,mechanical properties and good processing fluidity,and is widely used in automotive,electronics,construction and other fields.As EPDM/PP is a flammable material,its oxygen index is only about 18%,which is prone to fire and release toxic gas,causing casualties and environmental pollution,which greatly limits the application and development of EPDM/PP.Therefore,the design and development of flame retardants suitable for EPDM/PP with high efficiency is the key.In this thesis,six thermoplastic polyurethanes（TPUs）containing phosphorus,silicon,phosphorus silicon or non-phosphorus silicon were synthesized with 4,4’-diisocyanate diphenylmethane（MDI）and 1,4-butanediol（BDO）as hard segments and polycaprolactone diol alone or in combination i.e.（polyphosphate diol OP550,polyphosphate diol BY3009T and polysiloxane diol MY8815）,as soft segments.TPUs synthezied were compounded with ammonium polyphosphate（APP）as carbon forming agents to swell flame retardant EPDM/PP.The microencapsulated expandable graphite（MEG）was obtained by modifying expandable graphite（EG）by in situ polymerization of tris（hydroxymethyl）phosphine oxide（THPO）and toluene-2,4-diisocyanate（TDI-100）,and it was compounded with the preferred APP/phosphorus silicon containing TPU for flame retardant EPDM/PP.Thermogravimetric（TG）,oxygen index instrument（LOI）,vertical combustion instrument（UL-94）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy and rotational rheometer were used to study the effects of the flame retardant system on the thermal stability,flame retardant properties,mechanical properties and processing rheology of EPDM/PP composites.The results of infrared spectroscopy（FTIR）showed that the absence of-OH absorption peak near 3400 cm^-1and-NCO absorption peak at 2270 cm^-1,as well as the presence of-NH stretching absorption peak near 3330 cm^-1,the result of ~1H-NMR showed that the peak value at chemical shift 9.50 ppm was H atom on-NHCOO-,indicate the successful synthesis of thermoplastic polyurethane.The results of elemental analysis showed that phosphorus and silicon elements had been introduced into the corresponding TPUs,in which the phosphorus-silicon containing thermoplastic polyurethane TPUOM（soft segments of OP550 and MY8815）and TPUBM（soft segments of BY3009T and MY8815）contained 17.2%and 8.3%of phosphorus-silicon,respectively.The higher the phosphorus content,the higher the residual carbon rate of TPU at 700°C.The introduction of silicon element further increased the residual carbon rate of TPUs,with TPUOM having the highest residual carbon rate at700℃of 33.1%.When the total content of flame retardant was 40%,the residual carbon rate at 700℃of EPDM/PP/APP was 17.7%,and the LOI value was 25%,which did not pass the vertical combustion grade;after the addition of pentaerythritol（PER）,the flame retardant of EPDM/PP/APP/PER improved,but its residual carbon rate at 700℃was only 17.1%;and the thermoplastic polyurethane compounded APP had better flame retardant effect on EPDM/PP,in which the APP/TPUOM（mass ratio of 3:1）flame retardant composite showed21.5%residual carbon rate at 700℃,26.5%the LOI value,and V-0 grade the vertical combustion,as well as better mechanical properties with tensile strength 42%higher than that of EPDM/PP/APP/PER system.The addition of flame retardant hindered the crystallization of EPDM/PP matrix and reduced the crystallinity.The effect of adding APP/TPU on the reduction of the crystallinity of the composites is smaller than that of adding APP or APP/PER,and the crystallinity of EPDM/PP/APP/TPUOM is 4.1%higher than that of EPDM/PP/APP/PER.Compared with the addition of APP/PER,the composites with APP/TPUOM have lower complex viscosity,indicating better melt flow and better processing properties.The IR spectrum of MEG showed a C=O absorption peak in-NHCOO-at 1730 cm^-1,a-NH bending vibration peak at 1535 cm^-1 and P=O absorption peaks at 1220 cm^-1 and 1065cm^-1,scanning electron microscopy-energy spectroscopy（SEM-EDS）results showed the successful preparation of MEG.TG results showed that MEG has better thermal stability,with the residual carbon rate at 700℃of 74.5%.After the expansion test at 350°C,the expansion multiplicity of MEG was increased by nearly 25.7%compared with that of EG.When adding equal mass fraction of MEG or EG,the EPDM/PP/MEG system has higher LOI value and vertical burning grade than the EPDM/PP/EG system.When the total addition of flame retardant was 30%,the LOI value of MEG flame retardant composites was 25%and the vertical burning was V-0 grade;when MEG was compounded with APP/TPUOM flame retardant（mass ratio of 2:1）,the LOI value of the flame retardant composite reached 27.8%and the vertical burning was V-0 grade,which showed the synergistic flame retardant effect of MEG and APP/TPUOM.Better tensile strength was observed for EPDM/PP/MEG/APP/TPUOM than EPDM/PP/MEG system.The rheological results showed that the EPDM/PP/MEG composites had lower complex viscosity than EPDM/PP/EG composites under the conditions of equal flame retardant dosage.After partial replacement of MEG with APP/TPUOM,the complex viscosity of the composites was further reduced,indicating the increased melt flowability,which was conducive to improving the processing performance.