典型热塑性聚合物热解行为研究

利用热失重分析仪采用不同的加热速率对典型热塑性聚合物PP(聚丙烯)、PE(聚乙烯)和PMMA(聚甲基丙烯酸甲酯)在空气氛围中的热解行为进行了研究,通过热失重TG和微商热失重DTG分析了三种聚合物的热解过程及加热速率对热解过程的影响,通过Coats-Redfern方法建立了聚合物的热解动力学模型,并分析讨论了聚合物的热解机理和氧气对热解的影响,最后通过计算曲线和实验曲线的比较,验证了该文聚合物热解动力学模型的可靠性。     

Formation of 2,3,7,8‐tetrabromodibenzodioxin and ‐furan by thermolysis of polymers containing brominated flame retardants

Several plastic products which are used in areas exposed to fire hazards contain flame retardants. A group of important flame retardants are brominated aromatic compounds, and it is believed that they thermally generate polybrominated dibenzodioxins (PBrDD) and dibenzofurans (PBrDF).

The formation of 2,3,7,8‐Tetrabromodibenzodioxin (TBrDD) and ‐Tetrabromodibenzofuran (TBrDF) from various plastics with brominated flame retardants under different thermolysis conditions is presented. At 600 °C, 2,3,7,8‐TBrDD and ‐TBrDF in concentrations of 0.01–7 and 0.01–6 ppm, respectively, are formed from plastics containing decabromodiphenylether or polybromodiphenylether as flame retardant. With increasing temperature the concentration of these isomers decreases until they are no longer detectable above 800 °C (detection limit: 0.01 ppm).

The hypothetical reaction pathways for the formation of 2,3,7,8‐TBrDD and ‐TBrDF by thermolysis of decabromodiphenylether are proposed.

The solubility and photochemical properties of 2,3,7,8‐TBrDD in different solvents are given.     

Thermolysis of 2,4,6-trichlorophenol chemisorbed on aluminium oxides as example of fly ash mediated surface catalysis reaction in PCDD/PCDF formation

The influence of aluminium cation as a strong electrophilic centre on the thermolysis of chlorophenols chemisorbed on Al(OH)₃ surface was investigated. If thermolysis is carried out at 300 °C the spontaneous rupture of the bond between aluminium and oxygen of phenol takes place in the temperature range of 260–280 °C. The thermolysis of chlorophenoxy aluminium compounds occurs through homolytic and heterolytic bond cleavage. In the case of heterolytic cleavage the leaving chlorophenoxy anion causes a simultaneous formation of the aluminium cation, which is the driving force for the rearrangement of the unstable intermediate. By homolytic cleavage of the Al–O bond the chlorophenoxy radical is formed. The isolation of reaction products of the thermolysis of the system Al(OH)₃/2,4,6-trichlorophenol gave five isomers of dimeric compounds of resonance stabilised 2,4,6-trichlorophenoxy radical. The compounds are stable in nonaqueous, aprotic solution, but they are very sensitive to acid catalysis. They quickly transform into aromatic hydroxydiphenyl ethers. The process of dechlorination and aromatisation of cyclohexadienone dimers gives PCDD/PCDF.     

Investigation on the urea deposit formation and thermal decomposition characteristics in the SCR aftertreatment system of a diesel engine

Selective catalytic reduction is the most efficient and reliable equipment for NOx control in current diesel engines. However, the issue of urea crystallization becomes increasingly serious with the implement of the new emissions standards. In this paper, urea deposit samples collected from engine test bed and tube furnace were characterized by thermogravimetric analysis and Fourier transform-infrared analysis to aid the comprehension of urea deposit formation. Moreover, thermogravimetric tests were conducted to disclose the effects of catalyst on the thermal decomposition processes of urea deposit. The results indicated that less temperature resistant species are formed in the engine test bed than in the tube furnace at conditions with the same temperatures. The main compositions in the World Harmonized Transient Cycle (WHTC) urea deposits are urea, cyanuric acid (CYA) and ammelide, implying that accelerating the decomposition of these species could prevent the accumulation of urea deposit. CuWTi, Cuβ and CuZSM catalysts could lead to increased yield of CYA during pure urea thermolysis. Cuβ, CuWTi and VWTi catalysts tend to promote the thermolysis of CYA while VWTi has the most significant catalytic effects on the thermal decomposition of ammelide and ammeline.