Interaction Effects of Predominantly Linear and Branched Waxes on Yield Stress and Elastic Modulus of Waxy Oils

Abstract

To assess the relationship of wax chemical structure and viscoelastic properties of waxy gels, model oils composed of single and blended waxes were prepared at a fixed concentration of 7.5 wt % and then gelled. The investigation encompassed four different well-characterized commercial paraffin waxes, solubilized in a mineral oil matrix. Previous rheological measurements pointed out that these systems reproduce essential features of crude oil gels (e.g., gel-like mechanical response) when gelled. Among the employed waxes, two are predominantly linear, whereas the others are nonlinear branched molecules. The waxes were molecularly characterized to aid in the investigation by means of GC-FID, 13 C NMR, DSC, FT-IR, and XRD. Rheological properties were measured via a controlled-stress rheometer by oscillatory shear experiments. Polarized light microscopy was adopted for morphological characterization of precipitated crystals. It was found that yield stress and elastic modulus at linear viscoelastic region are highly correlated (R 2 = 0.94). For single wax systems, the increase in chain length resulted in a yield stress increase, although there is a competitive effect among chain length (positive effect) and branching content (negative effect). The results indicated that for blended systems the small-chain linear wax was able to interact favorably with the long-chain nonlinear wax, possibly due to its ability to accommodate within the later molecule, ensuring the highest yield stress value (630.2 Pa). The wax structural arrangement of 37 carbon atoms on average, including approximately three tertiary carbons, was effective for lowering the yield stress of particular blended systems. The lowest viscoelastic properties were measured for a blended system composed of nonlinear waxes, which was also characterized by the smallest and rounded crystals visible by optical microscopy.

Apparent wall slip effects on rheometric measurements of waxy gels

“…Previous rheological measurements confirmed that these model systems reproduce essential features of crude oil gels, exhibiting a low-temperature gel-like mechanical response to an imposed low-frequency oscillatory stress [30]. The physicochemical and structural properties of the wax and solvent used can be found in our previous work (L 29 wax) [30]. Seven different geometry configurations are employed, including combinations of smooth and grooved surfaces at oscillatory, creep, and steady-state rheological experiments.…”

“…A standard preparation protocol is followed to ensure reproducible rheological data [10,12]. Previous rheological measurements confirmed that these model systems reproduce essential features of crude oil gels, exhibiting a low-temperature gel-like mechanical response to an imposed low-frequency oscillatory stress [30]. The physicochemical and structural properties of the wax and solvent used can be found in our previous work (L 29 wax) [30].…”

“…Thus, model oils were freshly prepared under controlled conditions before each rheological test to avoid thermal and shear histories variation. Predominantly linear macrocrystalline wax with 29 carbons on average [30] and fusion range of 56-58 °C was purchased from Sigma-Aldrich. Wax weight fractions of 3.0 and 7.5 wt.…”

“…According to our previous measurements, this temperature is 22.7 °C higher than the wax precipitation temperature of the 7.5 wt. % system [30].…”

“…The occurrence of apparent wall slip is investigated by comparing the results for the yield stress provided by oscillatory and creep tests performed with different geometry assemblies. The discussion is extended to encompass the storage modulus G 0 , and it is shown that G 0 is strongly correlated to the yield stress for these systems [30]. Flow curves are also obtained and apparent wall slip is quantified as the relative difference between the areas under the curves obtained using geometries with smooth and grooved surfaces.…”