Kinetic modeling of polyurethane pyrolysis using non-isothermal thermogravimetric analysis

International audienceThe pyrolysis of polyurethane was studied by dynamic thermogravimetry analysis (TGA). Thestudied polyurethane is used as organic binder in casting process to make sand cores and molds. Asemi-empirical model is presented that can be used to describe polyurethane pyrolysis occurringduring TGA experiments. This model assumes that the polyurethane is pyrolysed by severalparallel independent reactions. The kinetic parameters of polyurethane pyrolysis were evaluatedby fitting the model to the experimental data obtained by TGA over a wide variety of heatingrates. A nonlinear least-squares optimization method is employed in the fitting procedure. Ahybrid objectives based simultaneously on the mass (TG) and mass loss rate (DTG) curves hasbeen used in the least-squares method. The values of the activation energy obtained by the nonlinearfitting were then recalculated by the methods of Kissinger and Friedmand. Furthermore,the parameters obtained in the present paper were then compared with those reported in theliterature

Basal twinning of Greenland corundum

International audienceNatural corundum shows two types of twins: “basal twin”, by reflection on (0003) pinacoid, very rare, and “rhombohedral twin”, by reflection on (1011), more frequent. The analysis of the structural continuity across the composition plane does not show any reason for a large difference in occurrence frequency, which is likely related to the limited development of the (0003) plane in the characteristic morphology of corundum. “Basal twins” occur with unusually high frequency in samples from Greenland, which also present an atypical platy morphology, where the (0003) face is well developed. This observation seems to confirm a morphological control on the occurrence of the “basal twin”. All analysed twinned samples show macrosteps on their pinacoidal faces and this feature has been related to the high-temperature conditions and intense fluid-rock interactions of Greenland deposit. This clearly suggests a strong relationship between the “basal twin” occurrence, the development of basal faces, and the formation conditions. However, due to the complex geological context and the different features of samples (e.g. two individuals with almost the same size versus several lamellae stacked along c axis), it is not possible to establish with certitude if the “basal twins” observed in Greenland samples are growth or mechanical twins

Photodegradation of disposable polypropylene face masks: Physicochemical properties of debris and implications for the toxicity of mask-carried river biofilms

International audienceCOVID-19 outbreak led to a massive dissemination of protective polypropylene (PP) face masks in the environment, posing a new environmental risk amplified by mask photodegradation and fragmentation. Masks are made up of a several kilometres long-network of fibres with diameter from a few microns to around 20 µm. After photodegradation, these fibres disintegrate, producing water dispersible debris. Electrokinetics and particle stability observations support that photodegradation increases/decreases the charge/hydrophobicity of released colloidal fragments. This change in hydrophobicity is related to the production of UV-induced carbonyl and hydroxyl reactive groups detectable after a few days of exposure. Helical content, surface roughness and specific surface area of mask fibres are not significantly impacted by photodegradation. Fragmentation of fibres makes apparent, at the newly formed surfaces, otherwise-buried additives like TiO2 nanoparticles and various organic components. Mortality of gammarids is found to increase significantly over time when fed with 3 days-UV aged masks that carry biofilms grown in river, which is due to a decreased abundance of microphytes therein. In contrast, bacteria abundance and microbial community composition remain unchanged regardless of mask degradation. Overall, this work reports physicochemical properties of pristine and photodegraded masks, and ecosystemic functions and ecotoxicity of freshwater biofilms they can carr