The Research On Char Forming Mechanism And Rheological Behavior Of Polycarbonate/ Polyhedral Oligomeric Silsesquioxane Composites

Polyhedral oligomeric silsesquioxanes?POSS?is a perfect and novel halogen-free flame-retardant.POSS has a good dispersion in matrix,and the organic groups which connect with Si have a good compatibility with polymer.The POSS particle size can reach a nano scacle,which can be mixed with polymer as a nanocomposite.The octaphenyl polyhedral oligomeric silsesquioxanes?OPS?,polyphenyl silsesquioxane?PPSQ?of ladder structure and polyhedral oligomeric silsesquioxanes of the caged structures with the DOPO groups?DOPO-POSS?are synthesized by our lab,and they have an excellent flame retardant property with a little content.6 wt%OPS can make PC reach UL-94 1.6 mm V-0rating;6 wt%PPSQ can make PC reach UL-94 1.6 mm V-0 rating;4 wt%DOPO-POSS can make PC reach UL-94 3.2 mm V-0 rating,but cannot reach 1.6 mm V-0 rating whatever DOPO-POSS content is.The paper uses the rheology analysis method to study the changing of composite melt interior structure;Thermal gravimetric-Fourier transform infrared spectrometer?TG-FTIR?and Pyrolysis-gas chromatography/mass spectrometry?Py–GC/MS?are used to analyze the gaseous phase products on charring and flame-retardant during the thermal degradation and pyrolysis process;the cone calorimeter and Scanning electron microscopy-X-ray energy spectrometer?SEM-EDS?are used to analyze the structure composition and char strength of the condensed phase,and study the effect of the condensed phase to the composite flame retardant.The specific research content and achievements as follow.1.The rheological behavior of PC/OPS and PC/PPSQ composites are studied.In the Oscillatory rheological test,when angle frequency??0.1 rad/s,the storage modulus of C/OPS and PC/PPSQ composites are higher than PC and the storage modulus is increased with OPS and PPSQ content increased.When the content of OPS and PPSQ exceed 6 wt%,a storage modulus platform is appeared which indicates that a three dimentional network is forming in the composite and this is benefical to the char forming during the combustion process.The complex viscosity of PC shows a Newton fluid behavior during the stress rheology test,and the PC/OPS composites exhibit a typical shear-thinning behavior of polymer melts.At low angle frequency,the complex viscosity of PC/OPS composies exhibit decreased first?OPS?3 wt%?and then increased?OPS?6 wt%?which indicates a solid-like behavior appeared in the composite melt.PC/PPSQ composites show a typical shear thining behavior.When PPSQ content increased,the complex viscosity is increased gradually and the fluid shear thining behavior phenomenon becomes weak.After PPSQ content exceed 10 wt%,the complex viscosity of PC/PPSQ composite is higher than PC and exhibit a approximate Newton fluid which indicates the polymer melt from a liquid-like behavior transfer to a solid-like behavior.The stress rheology analysis indicates that the appearance of three dimentional network,solid like behavior and the improment of complex viscosity can improve the resistant thermal degradation of PC/PPSQ composite and beneficial to the char crosslink during the combustion process.2.In the study of the gaseous phase products analysis,an obvious carbon dioxide?CO₂?release peak can be found during the thermal degradation for PC/OPS and PC/DOPO-POSS;the absorbance peak of CO₂ in PC/PPSQ can be found at 60?which indicates the thermal stability is better than PC.A symmetrical stretch vibration peak of Si-O-Si bond is detected by FTIR which indicates the silicious products appeared.During the pyrolysis process of OPS and PPSQ,a mount of benzene ring and fumed silica are released;the pyrolysis process of DOPO-POSS is more complicated than OPS and PPSQ,DOPO-POSS released lots of gaseous products fragments,including the DOPO and DOPO derivative,and the content of DOPO is the highest than others which indicates that DOPO is the main action in the gas phase flame retardant for PC/DOPO-POSS composite.From the total mass spectrum,it can be indicated that the addition of OPS,PPSQ and DOPO-POSS donot change the pyrolysis process of PC and this is accordance with TG-FTIR analysis results,but they inprove the gas concentration of small molecule compound which is beneficial to the swell of the char during the combustion.3.From the combustion performance,PC/PPSQ composite can greatly decrease the heat release rate peak?PHRR?,total heat release?THR?and total somke release?TSR?,and has the best flame retardant effect in the three PC/POSS composite,the effect of flame retardant of PC/DOPO-POSS composite is weaker than PC/OPS and PC/PPSQ composite.Both the outside of char residues of PC/OPS and PC/PPSQ composites have white silicon dioxide powder covered.Combine the gas phase analysis and the char residue FTIR curve of PC/PPSQ composite,it can be indicated that the fumed silica that generated by PPSQ during the pyrolysis process attach to the internal surface of char residue,and white SiO₂ is exposed to the top surface of char residue when top char residues is broken by cone heat radiation,and the SiO₂ can effectively have a heat and oxygen insulation effect.In the microstructure of the char residue,the interior char of PC/OPS and PC/PPSQ composie is continuous,smooth and have a compact char laminate structure which show ordered arrangement,all those features can make the composite effectively provent the heat transmission and restrain the release of the combustible gas and gas volatile matter during the combustion;PC/DOPO-POSS composite form a stable and high swelled char residue after a complete combustion,the interior of char of PC/DOPO-POSS composite is mooth and have much bubbles and pore attatched to he char.The char show a cone shape which is can support the char forming during the combustion process,and also can stable the char height.In the condensed phase of the char residue,in FTIR analysis,the exterior char structure of PC/OPS and PC/PPSQ composies are composed by-O-Si-O-bond and Si-O-C bond;the the exterior char structure of PC/DOPO-POSS composite is composed by aromatic hydrocarbon,-P?=O?-O-Si-bond and Si-O bond.The char strength analysis results show that the interior char structure of PC/PPSQ compact is more compact than other two composites and have a distribution even,and show a laminate stacked structure,and no bubbles and pores are found;the interior char structure of PC/OPS composite is compact,and have little bubbles and proe attach to he char;the char strength curve of PC/DOPO-POSS is more twists and tums which indicate there are lots of bubbles and pores exist in the char,but the large fluctuates also explain the interior char framework strength is much high which is beneficial to support the swelled the high char residue and guarantee the char cannot be collapsed during the combusition process.