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Presence of Isoalkanes in Waxes and Their Influence on Their Physical Properties† D. Petitjean,*,‡ J. F. Schmitt,§ V. Laine,‡ M. Bouroukba,‡ C. Cunat,§ and M. Dirand‡ Laboratoire de Thermodynamique des Milieux Polyphasés, Ecole Nationale Supérieure des Industries Chimiques, Institut National Polytechnique de Lorraine, Nancy UniVersity, 1 rue GrandVille, 54001 Nancy Cedex, France, and Laboratoire d’Energétique et de Mécanique Théorique et Appliquée, Ecole Nationale Supérieure d’Electricité et de Mécanique, Institut National Polytechnique de Lorraine, Nancy UniVersity, 2 aVenue de la Forêt de Haye, 54504 VandoeuVre les Nancy, France ReceiVed July 21, 2007. ReVised Manuscript ReceiVed October 9, 2007
The purpose of this paper is to display the influence of isoalkanes on the structure and mechanical properties of waxes. This work was carried out by means of X-ray diffraction, differential thermal analyses, and dynamic mechanical analyses. Isoalkanes coming from an industrial wax were added in different quantities to a synthetic mixture of linear alkanes. The disorder generated by this addition was estimated by X-ray diffraction; it modifies the structural transformations versus temperature and therefore the mechanical properties. The branched chains of isoalkanes hinder the molecules motions, especially the rotation along the long axis. Thus, some phases usually formed at high temperature are not observed when increasing the temperature. With regard to the mechanical properties, the effect of the branched alkanes was studied using two industrial waxes with Gaussian distribution composition. The dynamic mechanical analyses have shown that the storage modulus (E′) is also affected by the isoalkane quantity.
Introduction It has become very important to improve the knowledge of the relatively high-molecular-weight hydrocarbons present at petroleum cuts. These compounds constitute a homogeneous series of molecular alloys, presenting many industrial applications when they are mixed. These materials have several properties sensitive to temperature. These last ones are tied to the structural state. Thus, the studies relative to alkanes and their mixtures must be correlated to structural characterization. The first structural characterization was made by Muller and Saville in 1925.1 Since then, many works have been published on this subject.2–5 Some of them were devoted to the establishment of binary-6–10 or ternary-phase diagrams.11,12 These representations allow us to deduce numerous information on the binary or ternary mixtures. The case of numerous normal alkane mixtures has been already investigated by structural analyses. In the literature, two cases are distinguished according to the general shape of the alkane molar distribution. Thus, mixtures with a “normal logarithmic distribution” type of linear alkanes crystallize into a single solid solution with an orthorhombic structure at room temperature.13,14 On the contrary, mixtures with an exponential decreasing distribution crystallize † Presented at the 8th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. Fax: 03-83-17-53-95. E-mail:
[email protected]. ‡ Ecole Nationale Supérieure des Industries Chimiques. § Ecole Nationale Supérieure d’Electricité et de Mécanique. (1) Muller, A.; Saville, W. B. J. Chem. Soc. 1925, 127, 599. (2) Craig, S. R.; Hastie, G. P.; Roberts, K.; Sherwood, J. N. J. Mater Chem. 1994, 4 (6), 977. (3) Gerson, A. R.; Roberts, K. J.; Sherwood, J. N. Acta Crystallogr., Sect. B: Struct. Sci. 1984, 47, 280. (4) Heyding, R. D.; Russel, K. E.; Varty, T. L. Powder Diffr. 1990, 2, 93. (5) Turner, W. R. Ind. Eng. Res. DeV. 1971, 10 (3), 238.
into more than one solid solution with an orthorhombic structure.15 These orthorhombic phases generally noted β′ are isostructural to the intermediate phases previously identified in the binary or ternary mixtures.9,11,12 These solid solutions evolve via solid–solid transitions when the temperature is increased. A description of high-temperature phases can be obtained in the following references.6,7 Scarce studies in the literature exist related to the effect of the composition and structure on the degree of crystallinity of waxes. There is also a certain degree of amorphousness in some waxes reported in the literature.16 This results from the presence of long “tie” molecules in natural waxes, which lead to a conformational disorder near the chain ends.17 In the case of Fischer–Tropsch or petroleum-refined waxes, oil content, isoparaffins, and branched chains may play a significant role in (6) Lüth, H.; Nyburg, S. C.; Robinson, P. M.; Scott, H. G. Mol. Cryst. Liq. Cryst. 1974, 27, 337. (7) Achour-Boujema, Z.; Bourdet, J. B.; Petitjean, D.; Dirand, M. J. Mol. Struct. 1995, 354, 197. (8) Denicolò, I.; Craievich, A. F.; Doucet, J. J. Chem. Phys. 1984, 80, 6200. (9) Dirand, M.; Achour, Z.; Jouti, B.; Sabour, A.; Gachon, J. C. Mol. Cryst. Liq. Cryst. 1996, 275, 293. (10) Métivaud, V.; Rajabalee, F.; Mondieig, D.; Haget, Y. Chem. Mater. 1999, 11, 117. (11) Nouar, H.; Petitjean, D.; Bouroukba, M.; Dirand, M. ReV. Inst. Fr. Pet. 1998, 53, 21. (12) Nouar, H.; Petitjean, D.; Bouroukba, M.; Dirand, M. Mol. Cryst. Liq. Cryst. 1999, 326, 381. (13) Chevallier, V.; Provost, E.; Bourdet, J. B.; Bouroukba, M.; Petitjean, D.; Dirand, M. Polymer 1999, 40, 2121. (14) Chevallier, V.; Petitjean, D.; Ruffier-Meray, V.; Dirand, M. Polymer 1999, 40, 5953. (15) Dirand, M.; Bouroukba, M.; Chevallier, V.; Petitjean, D. J. Chem. Eng. Data 2000, 47, 115. (16) Dirand, M.; Chevallier, V.; Provost, E.; Bouroukba, M.; Petitjean, D. Fuel 1998, 77, 1253. (17) Dorset, D. L. J. Phys. D: Appl. Phys. 1997, 30, 451.
10.1021/ef700423g CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2007
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Table 1. Characteristics of the Molecular Alloysa sample
origin
Wax 1 Synth S3 Wax 3
commercial synthetic synthetic industrial
composition in normal alkanes C20H42 C20H42 C19H40 C19H40
to to to to
C44H90 C44H90 C44H90 C44H90
percent in normal alkanes
nc
σ
84.8 100 100 67.7
25.9 25.9 29.6 29.6
2.4 2.4 3 3.4
an c represents the average chain length, and σ represents the root-mean-square deviation.
this disorder.18–21 Among these parameters are the width of the alkane distribution (i.e., root-mean-square deviation from the mean of the Gaussian distribution) and its role in the disorder appearance, which have already been investigated by X-ray diffraction and differential thermal analyses.22 These last ones allow us, on one hand, to study the thermal behavior of the sample and, on the other hand, to determine the range of temperature for which each phase is stabilized. Such results are useful for the X-ray characterization of the high-temperature phases and thus for the determination of the solid–solid transitions. The composition, especially the isoalkane amount in the wax, constitutes an important parameter. The presence of nonlinear alkanes induces important modifications of the structural state and mechanical properties. In this paper, we present original results about the influence of isoalkanes on the structure and mechanical properties of waxes. This work was carried out by means of X-ray diffraction and by measurement of the storage modulus (dynamic mechanical analysis) according to the temperature. Experimental Section The molecular alloys used in this study can be classified in two types according to their origin: synthetic (“Synth” or “S3”) or commercial/industrial (“Wax 1” or “Wax 3”). The characteristics of these waxes are presented in Table 1 and Figure 1. The first alloy (called “Wax 1”) was purchased from the Aldrich Company, and the second one (called “Synth”) was prepared by weighting pure components, mixing and melting them together. The pure components used were also purchased from the Aldrich Company. Purities were determined to be greater than 99% by gas chromatography. The Gaussian distribution of n-alkane molar fractions of the synthetic mixture, the average chain length (nc), and the rootmean-square deviation from the mean (σ) are identical to the ones of “Wax 1” (Table 1). Isoalkanes were added in different amounts ranging from 0 to 50% in mass to the synthetic wax; they come from the industrial wax “Wax 3” (Table 1) and were produced by successive crystallizations. Therefore, isoalkanes used in this study correspond to the liquid phase, resulting in the cooling (at 268 K) of “Wax 3”. The structural state at room temperature was established by means of X-ray diffraction. The X-ray diffractometer uses the copper radiation λKR Cu and is equipped with a curved detector CPS 120°. The apparatus allows the recording of X-ray beam diffracted in the angle range (0°, 120°). Upon an increasing temperature, the structural modifications were identified using the diffractometer equipped with a heating sample holder based on the Peltier effect. This allows the heating of the sample from room temperature to near 373 K. Two sample preparations have been used according to information expected. First, to identify and characterize the structural state, the sample was heated to the melting point and cooled slowly at 1 K min-1 on a copper sheet until reaching room temperature. With (18) Le Roux, J. J. Appl. Chem. 1969, 19, 39. (19) Retief, J. J.; Le Roux, J. H. S. Afr. J. Sci. 1983, 79, 234. (20) Basson, I.; Reynhardt, E. C. Chem. Phys. Lett. 1992, 198, 367. (21) Le Roux, J. J. Appl. Chem. 1969, 19, 203. (22) Laine, V. Ph.D. Thesis, Institut National Polytechnique de Lorraine (INPL), Nancy, France, 2005.
Figure 1. Molar distributions in linear alkanes (gray bars) and isoalkanes (black bars) in industrial and synthetic waxes.
this preparation method, preferential crystallographic orientations are obtained and, therefore, the relative intensities of (00l) reflections
Presence of Isoalkanes in Waxes
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Table 2. Characteristics of the DMA Experiments applied solicitation
tension/compression
imposed dynamic displacement sample shape and dimensions
3 µm cylinder length, l ) 8 mm; diameter, φ ) 4 mm M-BOND 200 adhesive 10 Hz parallel plates from room temperature to fusion temperature 0.5 K mn-1
glue frequency specimen holders range of explored temperature heating speed
are increased artificially. In such conditions, the determination of the number of phases and the calculation of the c crystallographic parameter with a good accuracy are easy. The second procedure consists of the examination of the sample as powder at room temperature. Sample examination according to a transmission method allows us to evaluate the crystallographic parameters a and b of the orthorhombic solid solution but also to determine the proportion of the diffusion halo, characteristic of the presence of an amorphous phase. Dynamic mechanical analyses (DMA) were carried out to study the thermomechanical behavior of the molecular alloys. A METRAVIB viscoanalyser (type VA 2000) was used for these experiments; the apparatus and the experimental protocol has been described in detail in a previous paper,23 and the main characteristics of the experiments are only mentioned here. Only the tension/ compression test mode was used. The parallel plate specimen holder was chosen because it is the most suitable to this kind of material. The samples (small cylinders) are stuck between parallel plates. An electromechanical shaker imposes a well-controlled sinusoidal displacement at the superior extremity of the sample, and an output transducer force measures the transmitted force, also in terms of phase and peak amplitude. The complex stiffness K* is obtained from the ratio of force/displacement. Calculations make it possible to evaluate the complex modulus E* of the material using a shape factor (this shape factor takes into account the geometry of the sample and the mechanical solicitation).24 We remind the reader that E* can be written as E* ) E′ + jE′′, where E′ is the storage modulus and E′′ is the loss modulus. The ratio of E′′/E′ ) tan δ is the loss factor; it is proportional to the ratio of energy lost to energy stored in one cycle. The characteristics of the DMA experiments are presented in Table 2.
Table 3. Structure and Crystallographic Parameters of the Molecular Alloys
Wax 1 Synth
percent in alkane
phase
a (nm)
b (nm)
c (nm)
nRX
nc
84.8 100
β′ β′
0.492 0.492
0.747 0.748
7.57 7.208
28.3 26.9
25.9 25.9
Table 4. Percentage of the Amorphous Phase According to the Mass of Isoalkane Added to the Synthetic Alloy percent (in mass) of isoalkanes in the synthetic alloy
percent of amorphous phase
0 12 25 50
19 22 59 69
“Wax 1” and “Synth” were examined by X-ray diffraction at room temperature according to a transmission method. The patterns indicate, on one hand, that the mixtures form one solid solution with an orthorhombic structure characterized by the reflections (110/111) and (200) and, on the other hand, the diffusion halo characteristic of the amorphous phase. The solid solution is isostructural to the ordered intermediate phase (noted β′) previously identified and characterized with binary6–10 or ternary11,12 alloys composed of linear alkanes. The crystallographic parameters a and b of the solid–solid solution of orthorhombic structure can be calculated from the hkl interplanar distances identified on the diffractogram (Table 3). The results are in good agreement with those previously reported in the literature.2 The crystallographic parameter c can be correlated to a length of a hypothetic linear alkane representative to the solid solution and, therefore, to an average carbon atom number. Thus, the nRX number reported in Table 3 is determined from the (00l) reflections and more precisely from the interplanar distances
d00l calculated using Bragg’s law. It was calculated with the help of the following relation:13–15 nRX ) (c (nm) - 0.37504)/ 0.25448, established using the parameter c of numerous linear alkanes reported in the literature.2 This carbon atom number can be compared to the mean carbon atom number calculated from the molar fraction xn of each mixture according to the relation nc ) ∑44 n)20xnn. The difference ∆n ) nRX - nc indicates an excess value of the carbon atom number attributed to the conformational disorder.25 This difference is weak for the synthetic mixture because it contains only normal alkanes. Furthermore, the analyses reveal that the disorder increases with the amount of isoalkanes added to the synthetic alloy. The ratio of the amorphous phase contained was evaluated by using the deconvolution procedure of informatics. It corresponds to the ratio between the area of the Gaussian function characteristic of the diffusion halo and the sum of the halos corresponding to the crystalline fraction and the amorphous phase. The ratio can be written as follows: percent amorphous ) Ahalo/(Ahalo + Apeaks), with Ahalo being the area of the Gaussian function characteristic of the amorphous fraction and Apeaks being the area of the Gaussian function characteristic of the crystalline function. The results of the estimation are collected in Table 4. Examination of the data indicates that the amount of the amorphous phase increases with the mass of isoalkanes added to the synthetic alloy, but the effect of the isoalkanes is not important until 12%. In fact, the difference of the amorphous phase levels is only 3%. Indeed, branched alkanes in a small proportion can occupy voids or interlamellar spaces of the lattice. This result is in agreement with those reported in the literature.26 For higher quantities, the disorder increases sharply (Table 4). As described before,7,13,14 increasing the temperature induces important modifications of the structure. The literature indicates that the orthorhombic solid solution evolves via several solid– solid transitions before melting. The first one transforms the “low-temperature phase” (β′) into a disordered orthorhombic phase, called β(Fmmm). The increase of the temperature provokes the transformation of this phase into a “hightemperature phase”, called the rotator phase (R-RII).6 This state is characterized by a free rotation of the molecule along the c axis.6 The diffractograms characteristic of these transformations can be seen in the following references.6,7,11–14 These solid–solid transitions are observed with pure odd alkanes and solid solution of the orthorhombic structure. These transformations are characterized by an onset temperature and a final temperature for the alloys. These last ones can be measured with accuracy
(23) Petitjean, D.; Schmitt, J. F.; Fiorani, J. M.; Laine, V.; Bouroukba, M.; Dirand, M.; Cunat, C. Fuel 2006, 85, 1323. (24) Greif, R.; Johnson, M. S. J. Eng. Mater. Technol. 1992, 117, 77.
(25) Clavell-Grunbaum, D.; Strauss, H. L.; Snyder, R. G. J. Phys. Chem. B 1997, 101, 335. (26) Dorset, D. L. Energy Fuels 2000, 14, 685.
Results and Discussion
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Figure 2. Evolution of the storage modulus (E′) versus temperature in the case of the pentacosane and the mixture called “Synth”.
on the thermograms. The structural changes associated with the transition are identified by means of X-ray diffraction. With regard to the studied alloys, these analyses revealed that the solid–solid transitions are similar in the case of “Wax 1” and the synthetic alloy. One finds the “low-temperature phase” ordered (β′), the disordered orthorhombic phase, called β(Fmmm), and finally, the rotator phase (R-RII). The addition of isoalkanes modifies the structural evolution versus temperature, especially from a 25% amount of nonlinear alkanes. The X-ray analyses have shown only one solid–solid transition. The disordered orthorhombic phase β(Fmmm) is not observed. Thus, versus temperature, the orthorhombic phase β′ evolves into the rotator phase (R-RII) before melting. Therefore, the branched alkanes hinder the transformation of β′ into β(Fmmm). We particularly studied the influence of isoalkanes on the thermomechanical properties of mixtures by a comparison of the results obtained for a real mixture (commercial or industrial origin) and for the corresponding synthetic mixture. We remind the reader that statistical characteristics of the mixtures analyzed by DMA are quasi-identical but the real mixture contains nonlinear alkanes (mainly isoalkanes). For that purpose, we compare the results obtained at first with the mixtures “Synth” and “Wax 1” (Table 1). This approach allows a correlation of the mechanical behavior with a small quantity of isoalkanes (15.2%). The second system “S3”/“Wax 3” allows us to correlate the mechanical behavior when their quantity is greater (32.3%) (Table 1). For a comparison of these products, we only present the evolutions of the storage modulus (E′) to avoid overloading figures. Let us compare at first the thermomechanical behavior of the pentocosane C25H52 and that of the synthetic mixture “Synth” (Figure 2). These results allow us to remind the reader of the typical behavior of an odd pure alkane23 but also to show the mixture effect. Indeed, as stated before, it is possible to assimilate the mixture “Synth” to a hypothetical n-alkane of formula C25.9H51.8.22,23 Furthermore, both C25H52 and “Synth” crystallize at room temperature in an orthorhombic structure. In these conditions, it is thus judicious to compare the behavior of this mixture with that of the pure odd alkane (C25H52) with the average carbon atom number near the one of the alloy. The evolution of E′ versus temperature shows several steps that are correlated to the solid–solid transitions determined by X-ray diffraction and differential thermal analyses. The comparison between the mixtures “Synth” and “Wax 1” is presented in Figure 3. The curves present domains correlated to the structural evolution of the mixtures versus temperature. Note that, in the case of “Wax 1”, local maxima associated with the solid–solid transitions observed by calorimetry are also detected on tan δ; it corresponds to an increase of the energy dissipated in the material induced by a consumption of enthalpy.
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Figure 3. Evolution of E′ versus temperature in the case of the mixtures called “Wax 1” and “Synth”, respectively.
Figure 4. Evolution of E′ versus temperature for the synthetic mixture “S3” and the industrial mixture “Wax 3”.
These data are recorded during the experiments but are not presented in this paper to avoid overloading figures. A small decrease of E′ is observed from ambient temperature because of the disorder within the mixtures; we remind the reader that the origin of the disorder is statistical and compositional for “Wax 1” and only statistical for “Synth”. The curves are almost merged, but a maximum is detected toward 322 K for the mixture “Synth” in the phase R-RII; it corresponds to a partial recovery of the initial mechanical properties of the lowtemperature phase between 315 and 322 K but is interrupted by the fusion. This phenomenon connected to the rotator phase is less visible in the case of the commercial mixture; it is likely that the presence of nonlinear compounds partially prevents this recovery from occurring. In Figure 4, we compare the mechanical behaviors of the mixtures “Wax 3” and “S3”. These mixtures present similar statistical characteristics, but their composition is different; indeed, only the mixture “Wax 3” contains 32.3% of alkanes (Table 1). The evolutions of the storage modulus are quite different. On one hand, a regular decrease of E′ is recorded from ambient temperature and until the temperature of the beginning of the fusion (321 K) for the mixture “Wax 3”. From this temperature, the weak detected change of hillside can be correlated with the viscous character of the material because of the beginning of the fusion. On the other hand, E′ evolves by stages in the case of the synthetic mixture “S3”, even if a weak decrease is recorded from ambient temperature. In particular, the existence domain of phase β′ is well-identifiable (until 320 K). The values of E′ are quasi-identical for the R-RII phase of both mixtures. Summary and Conclusion The aim of this study is first to determine the effect of the branched alkanes on the structural state and mechanical proper-
Presence of Isoalkanes in Waxes
ties of a molecular alloy. A commercial wax called “Wax 1” and an artificial molecular alloy were studied. The latter was prepared by weighting pure components one after the other and then mixing and melting them together. The distribution in normal alkane is identical to the one of “Wax 1”. Both samples were characterized by means of X-ray diffraction and differential thermal analyses (DTA). The solid–solid transitions identified by means of DTA were characterized by X-ray diffraction. Branched alkanes were added to synthetic alloy. The characterizations of the obtained alloy have shown an increase of the disorder at room temperature. The latter was determined with the help of the deconvolution of the X-ray patterns of informatics. The effect of branched alkanes on the structural modifications induced by an increasing temperature was also
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evaluated. The differential thermal analyses and the X-ray experiments have shown that branched alkanes hinder the molecular motions; the disordered orthorhombic phase called β(Fmmm) was never observed on the system synthetic plus branched alkanes (25%). Finally, the incidence of the presence of isoalkanes on the mechanical behavior was evaluated by dynamic mechanical analyses (DMA). Results reveal that the isoalkanes modify the storage modulus E′. In a small quantity, isoalkanes tend to prevent the partial recovery of the initial rigidity of the low-temperature phase from occurring. When their quantity is more important, the decrease of the storage modulus by stages tends to disappear and become continuous. EF700423G
