Jean-Marie Letoffe

A STRUCTURE-RELATED MODEL TO DESCRIBE ASPHALT LINEAR VISCOELASTICITY

Viscoelastic properties of paving grade asphalt cements (AC) were measured on a Rheometrics RDA II. Thermal properties were studied with a Mettler TA 2000 B differential scanning calorimeter. A review of the literature showed that most authors proposed that the time‐temperature superposition principle applies to asphalt over the whole temperature range. However, the results presented herein demonstrate that it indeed fails at high temperature, especially for high asphaltene content and high crystalline content materials. This paper focuses on the role of asphaltenes, which are, by definition, insoluble in n‐heptane. Asphaltenes are suspended in a colloidal manner within the AC matrix and are peptizided by the species called resins. Thus, a temperature dependent solid fraction exists within a liquid (or glassy) matrix. This paper shows that the viscoelastic properties of asphalt are bimodal: they are governed by both solid and continuous phases.Viscoelastic properties of paving grade asphalt cements (AC) were measured on a Rheometrics RDA II. Thermal properties were studied with a Mettler TA 2000 B differential scanning calorimeter. A review of the literature showed that most authors proposed that the time‐temperature superposition principle applies to asphalt over the whole temperature range. However, the results presented herein demonstrate that it indeed fails at high temperature, especially for high asphaltene content and high crystalline content materials. This paper focuses on the role of asphaltenes, which are, by definition, insoluble in n‐heptane. Asphaltenes are suspended in a colloidal manner within the AC matrix and are peptizided by the species called resins. Thus, a temperature dependent solid fraction exists within a liquid (or glassy) matrix. This paper shows that the viscoelastic properties of asphalt are bimodal: they are governed by both solid and continuous phases.

Crude oils: characterization of waxes precipitated on cooling by d.s.c. and thermomicroscopy

Mixtures of 2 and 4 wt% of pure paraffins in a crude oil matrix and 14 crude oils were characterized by differential scanning calorimetry (d.s.c.) and thermomicroscopy at a cooling rate of 2 K min−1 in the temperature range from +80 to −20°C. Pour points were also determined. In the temperature range used, a constant value of 200 J kg−1 for the enthalpy of precipitation was determined. For crude oils which contain > 10 wt% of n-paraffins, the pour point is reached when ∼ 2 wt% of waxes have precipitated. By thermomicroscopy, it was observed for mixtures of pure paraffins and crude oil matrix that the size of the crystals is small and depends on the length of paraffinic chains at the pour point. For crude oils, at the pour point the same final state is obtained (unlike diesel fuels). Even if the rates of precipitation are very different, the crystal size is always small (1–3 μm). This fact suggests that each crude contains nuclei which are responsible for heterogeneous germination.

Polymer modified asphalts as viscoelastic emulsions

Linear viscoelastic properties of polymer modified asphalts (PMAs) were studied at various temperatures and frequencies. The materials consisted of blends of paving grade asphalt cements (ACs) and diblock poly(styrene-b-butadiene) (SB) or triblock poly(styrene-b-butadiene-b-styrene) (SBS) copolymer up to 6 wtu2009% concentrations, which yielded heterogeneous PMAs with an emulsionlike morphology: a polymer-rich phase dispersed within an asphalt phase. In addition, the 6% SB modified binder was studied before and after dynamic vulcanization (i.e., in situ crosslinking of the polymer-rich inclusions to increase the PMA stability). The rheological response of the blends was calculated using the Palierne emulsion model, knowing the mechanical properties of each phase, the volume fraction of dispersed phase and the capillary number of the dispersed droplets. The interfacial tension then acted as an adjustable parameter and was estimated to be of order of 10−5u2002N/m. As a consequence of the colloidal nature of ACs (a ...

Comparison of wax appearance temperatures of crude oils by differential scanning calorimetry, thermomicroscopy and viscometry

This paper presents the results of an experimental study of the determination of wax appearance temperatures (WAT) of 15 crude oils by differential scanning calorimetry (d.s.c.), thermomicroscopy and viscometry. Good agreement is obtained among the three techniques for crude oils having a significant precipitation rate. Considering the great diversity of crude oils, it is concluded that d.s.c. and thermomicroscopy should be used together for a better understanding of the determination of WAT of crude oils.

Thermal behavior of asphalt cements

Asphalt cements are highly complex mixtures of hydrocarbon molecules whose thermal behavior is of prime importance for petroleum and road industry.rnrnFrom DSC, the determination of several thermal properties of asphalts is given, e.g. glass-transition temperature and crystallized fraction content.rnrnThe dissolution of a pure n-paraffin CnH2n+2 in an asphalt, as seen by DSC, should be a single peak. For 20<n<32 two peaks were observed. This means that their dissolution occurs in two liquids, and explains the usual shape of the DSC traces of AC, which shows a minimum at 35° to 40°C.rnrnAt room temperature, an asphalt cement is a mixture of two liquids containing some crystallized fractions. Above 80°C, it appears as a single liquid phase. At low temperature, the study of the glasses formed was done, using either constant or variable heating rate DSC. This technique, shows two glass transitions, for an asphalt containing a high quantity of crystallized fractions. The Tg of these glasses change with time and temperature. The formation of the crystallized phases is superposed to the enthalpic relaxation of the glasses, making a kinetic study very difficult.

Determination des constantes thermodynamiques des hydrures simples et complexes de l'aluminium. IV. Enthalpie de formation de LiAIH2 et Li3AIH6

Abstract Heats of formation of α and γ aluminium hydrides have been measured by means of a scanning Calvet microcalorimeter: Thermal dissociation of AIH 3 γ shows an exothermic effect associated with hydrogen evolution followed by the endothermic decomposition of AIH 3 α. On the basis of the quantities of gas evolved, a solvent-free aluminium alanate structure, AI (AIH 2 ) 3 , is proposed for AIH 3 γ.

Annealing of a glycerol glass: Enthalpy, fictive temperature and glass transition temperature change with annealing parameters

Abstract The enthalpy change (relaxation enthalpy) of the glycerol glass with annealing time was studied using DSC for different annealing temperatures. The investigation of the fictive temperature and glass transition temperature variations with annealing parameters was also carried out. In this study, a well-annealed glass was used as a reference for the determination of relaxation enthalpy. Two enthalpy contributions to relaxation enthalpy were observed. The results were discussed in terms of a multi-step thermodynamic transformation of molecular associations in the glass transition range. A linear relation was also found between relaxation enthalpy and fictive temperature. This well-known conclusion was established from experimental results. Finally, an empirical relation was proposed for the variation of the fictive temperature with annealing parameters.

Quasi-static study of the glass transition of glycerol by DSC

Abstract The glass transition of glycerol has been studied using a DSC apparatus in step heating mode, on cooling and warming. Using the electrical model of the calorimeter, the supposed thermal behavior of the sample is computed, and compared with the experimental results. The glass transition of glycerol appears to be reversible, with a slow endothermic effect (5 J g−1) and a change in thermal capacity.

DSC and Rheometry Investigations of Crude Oils

Thermal characteristics and rheological behaviour of eight crude oils covered a wide range of fluid composition and properties were studied by differential scanning calorimeter (DSC) and viscometry. Wax appearance temperatures (WAT) of crude oils were determined by DSC and viscometry. Good agreement is obtained between the results. The dynamic viscosity in the Newtonian temperature range of the crude oils (above 30°C) generally obeyed a simple first-order Arrhenius type of temperature dependence. Activation energies of flow in the non-Newtonian range are not uniquely defined.

Heat transfer in a disc-type DSC apparatus. II. Theoretical representation

Abstract Using an electrical representation, a model of a heat flux DSC apparatus is given. Numerical values of the resistors are computed using experiments done with a Mettler TA 2000 B heat-flow DSC. Theoretical traces of the melting of an Indium sample with inert or melting indium as reference have been computed and compared with experiments: a good accord is obtained thus supporting the validity of the model.