Energy & Fuels 2007, 21, 2311-2316
2311
Wettability under Imposed Flow as a Function of the Baking Temperatures of a DGEBA Epoxy Resin Used in the Crude Oil Industry Cristina M. Quintella,*,† Leila A. Friedrich,† Ana Paula S. Musse,† A ˆ ngelo M. V. Lima,† ‡ ‡ § Marcelo A. Maceˆdo, Ramires M. Silva, Iuri M. Pepe, Eduardo B. Silva,† Heitor M. Quintella,† and Luiz Carlos S. Soares, Jr.‡ Inst. Quı´mica, UniVersidade Federal da Bahia, Campus de Ondina, 40170-290, SalVador, BA, Brazil, Dep. Fı´sica, UniVersidade Federal de Sergipe, CEP: 40.100-000, Sa˜ o Cristo´ Va˜ o, SE, Brazil, and Inst. de Fı´sica, UniVersidade Federal da Bahia, Campus de Ondina, SalVador, BA, Brazil ReceiVed NoVember 4, 2006
The baking temperature (Tb) of an epoxy resin was optimized in order to decrease the dynamic interfacial tension, identifying the lowest wettability conditions for liquids flowing at a high rate. The dependence of dynamic interfacial tension on Tb was evaluated for the diglycidyl ether of bisphenol A. This resin was used to coat a sucking rod at the oil field of Bacia de Sergipe/Alagoas, Brazil in order to reduce blockage occurrences, maintenance stops, and the pumping capacity required. The samples were baked for 24 h between 100 and 180 °C. Their color and absorption spectra showed progressive dependence of Tb, indicating a migration from polymerization, through polymer network degradation, until carbonization. Spectrofluorimetry showed an initial increase in the energy gap between absorption and emission followed by a decrease that was attributed to changes of the chemical environment isotropy. Fourier transform infrared spectroscopy showed that the maximum polymerization occurred at 140 °C. Dynamic interfacial tension was evaluated by fluorescence depolarization of induced flowing liquids, using polarized laser-induced fluorescence within induced liquid flows and was clearly dependent on Tb. The lowest dynamic wettability was at 120 °C, just before full polymerization, which was attributed to two competing effects as Tb increased: polymerization and progressive yielding of compounds from the epoxy degradation. This points to the need to review the standard application procedures of these resins.
1. Introduction A recent trend in crude oil transport involves the use of ducts made of plastics, amorphous polymers, and fiber-reinforced plastic polymers. They may be used either as coatings or linings for metallic ducts or as massive ducts. Although they cannot stand high temperatures and their low mechanical resistance is a concern, they have the advantage of impeding the formation of paraffin deposits1-4 and a much lower corrosion rate than unlined metallic ducts. They are already being used5 to recuperate condemned metallic ducts by insertion of a plastic layer as inner lining, thus reducing the costs of welding and equipment maintenance or replacement. Nowadays, most of the ducts for production and transport are metallic. Nevertheless, in the middle to long term, most of the production tubing and tools, transportation ducts, and * Corresponding author. Tel. 55-71-88677876. Fax. 55-71-32355166. E-mail:
[email protected]. † Inst. Quı´mica, Universidade Federal da Bahia. ‡ Universidade Federal de Sergipe. § Inst. de Fı´sica, Universidade Federal da Bahia. (1) Slack, M. Mater. Perform. 1992, 31, 49. (2) Attou, A.; Benamar, A.; Inglebert, G. Mec. Indust. Mater. 1997, 50, 109. (3) Quintella, C. M.; Musse, A. P. S. M.; Castro, T. P. O.; Watanabe, Y. N. Energy Fuels 2006, 20, 620. (4) Quintella, C. M.; Lima, A. M. V.; Silva, E. B. J. Phys. Chem. B 2006, 110, 7587. (5) Refinarias Petrobras, Lubnor: Lubrificantes e Derivados de Petroleo do Nordeste. www2.petrobras.com (accessed May 2007).
pipelines will need to be customized to the operational conditions and to the types of fluids they transport in order to decrease financial losses and to preserve the environment. These tailormade ducts must reduce the chemical affinity with the fluids, that is, decrease the wetting efficiency that leads to degradation like corrosion and formation of asphaltenic and paraffinic deposits. At present, epoxy resins are among the most used materials for linings and coatings in the crude oil industry due to their availably in the international market, low price, and application versatility. They may also be used with fibers6 or as composites with other materials. Intrinsic luminescence of the epoxy resins has been studied before and related to cure degree. At room temperature, this is due mainly to7 fluorescence of the bisphenol from base resin and from the hardener, the aliphatic amines being well-known quenchers of the aromatic hydrocarbons. As a result of the cure process, the aliphatic amines are converted from primary to tertiary amines, increasing the quenching effects. On the basis of these, several methods were proposed to evaluate the cure degree and the oxidation process of the epoxy resin.7-9 (6) Holmes, G. A.; Feresenbet, E.; Raghavan, D. Compos. Interfaces 2003, 10, 515. (7) Gallot-lavalle, O.; Teyssedrea, G.; Laurenta, C.; Rowe, S. Polymer 2005, 46, 2722. (8) Rigail-Cedeno, A.; Sung, C. S. P. Polymer 2005, 46, 9378. (9) Younes, M.; Wartewig, S.; Lellinger, D.; Strechmel, B.; Strechmel, V. Polymer 1994, 35, 5269.
10.1021/ef060551w CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007
2312 Energy & Fuels, Vol. 21, No. 4, 2007
Epoxy resins can be produced with a wide variety of prepolymers and hardeners as well as catalyzers, yielding resins with different surface distributions of chemical groups. This should yield specific interfacial tension (ΓSL) and, consequently, different wetting efficiencies. The drag increases as the wetting between the pipeline walls and the fluids increases. Because the interfacial interactions take place at the boundary layers, where molecular effects overtake hydrodynamic ones, the flow rate should be a variable with high significance since high flow rates also increase the molecular effects over the fluid dynamic ones.10 ΓSL has been determined for many decades under static or slow flow conditions by sessile drop contact angle (θc) measurements.10 It is known that θc depends on surface roughness,10 on the chemical composition of both the solid surface and the flowing liquid,11-13 and on the relative orientation of the surface chemical groups.14 Extrand15 proposed a simple thermodynamic model for free energies of wetting obtained from the θc of small sessile drops spread on a solid surface and found reasonable agreement with bond strengths measured by other experimental methods. Recently, it was observed that θc could not discriminate between the interactions with polyethylene surfaces with different branching degrees.16 It was also shown3,4,17-20 that θc characterization cannot be applied straightforwardly to interactions between walls and liquids flowing at high rates as it becomes unreliable due to not taking into account flow effects within the liquid like molecular chains orientation. Dynamic interfacial tension (DIT) for liquids flowing at higher velocities has been recently observed by fluorescence depolarization using the technique of polarized laser-induced fluorescence within induced liquid flows (PLF-FI). This technique has the advantage of observing processes at the molecular level that take place at the solid-flowing liquid interface. Fluorescence depolarization of rhodamine in free-flowing films (FFFs) generated by a liquid lobed jet21 impinging on a nearly vertical solid surface has proved to be sensitive to different chemical constitutions of both the surface22 and the flowing liquid,23 as well as the orientation of surface hydroxyls.14 (10) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons Inc.: New York, 1997. (11) Suda, H.; Yamada, S. Langmuir 2003, 19, 529 (10.1021/la0264163). (12) Quintella, C. M.; Gonc¸ alves, C. C.; Castro, M. T. P. O.; Pepe, I.; Musse, A. P. S.; Lima, A. M. V. J. Phys. Chem. B 2003, 107, 8511. DOI: 10.1021/jp0274482. Available online, in English, at http://pubs.acs.org/cgibin/article.cgi/jpcbfk/2003/107/i33/pdf/jp0274482.pdf (accessed May 2007). (13) Extrand, C. W. J. Colloid Interface Sci. 2002, 248, 136. DOI: 10.1006/jcis.2001.8172. (14) Quintella, C. M.; Lima, A. M. V.; Gonc¸ alves, C. C.; Watanabe, Y. N.; Schreiner, M. A.; Mammana, A. P.; Pepe, I.; Pizzo, A. M. J. Colloid. Interface Sci. 2003, 262, 221. (15) Extrand, C. W. Langmuir 2003, 19, 646. DOI: 10.1021/la0259609. (16) Quintella, C. M.; Musse, A. P. S.; Castro, M. T. P. O.; Gonc¸ alves, C. C.; Watanabe, Y. N. J. Colloid Interface Sci. 2005, 281, 201. (17) Kenyon, A. J.; McCaffery, A. J.; Quintella, C. M.; Winkel, J. F. Mol. Phys. 1991, 74, 871. (18) Kenyon, A. J.; McCaffery, A. J.; Quintella, C. M. Mol. Phys. 1991, 72, 965. (19) Musse, A. P. S. 1° Place for Undergraduation in 5° Preˆmio Petrobra´s de Tecnologia de Ductos. http://www2.petrobras.com.br/tecnologia/portugues/programas_tecnologicos/premio02.stm (accessed May 2007). (20) Musse, A. P. S.; Castro, M. T. P. O.; Quintella, C. M. Bol. Te´ c. PETROBRAS 2004, 47, 1. (21) Quintella, C. M.; Musse, A. P. S.; Gonc¸ alves, C. C. J. Phys. Chem. B 2004, 108, 2751. (22) Quintella, C. M.; Gonc¸ alves, C. C.; Pepe, I.; Lima, A. M. V.; Musse, A. P. S. Braz. J. Chem. Soc. 2001, 12, 780. Available online, in English, at http://jbcs.sbq.org.br/jbcs/2001/vol12_n6/14.pdf (accessed May 2007). (23) Quintella, C. M.; Gonc¸ alves, C. C.; Pepe, I.; Lima, A. M. V.; Musse, A. P. S. J. Autom. Methods Manage. Chem. 2002, 24, 31. Available online, in English, at http://taylorandfrancis.metapress.com.
Quintella et al.
Recently,3,4,19,20 we confirmed by DIT the limitations of the θc technique when there is emphasis of the molecular effects over the flow effects due to the liquid being at a high flow rate. The study3,4 of the potential for paraffin deposition inhibition of polypropylene (PP), high-density polyethylene (HDPE), and vinyl acetate copolymer with a 28% oxygen content (EVA28) showed that the interfacial interactions have a strong dependence on the liquid being static or at high flow rate. Under flow, PP proved to be more suitable for the transportation of crude oil rich in paraffins with more than 36 carbon atoms, while HDPE was more suitable for those with smaller paraffinic chains. There are a wide variety of studies of the diglycidyl ether of bisphenol A (DGEBA) wetting of fibers, mostly in order to produce their composites. These studies are concerned with adhesion between DGEBA and the fibers. There are also studies24 of wetting by liquids at low velocities, using dynamic contact angle, that discuss the epoxy surface as a function of its acidity and basicity. Nevertheless, there are no studies of the epoxy wettability under fast liquid flow. There is also an absence on how dynamic wettability is affected by the different proportions of chemical groups present on the surface, yielded by the cure degree of the DGEBA epoxy resin. In this absence of data, it is usual practice to assume that the cure conditions that achieve minimum wettability correspond to those that yield maximum polymerization. The aim of this paper is to verify which baking temperature (Tb) of epoxy resin is more suitable for hindering wall depositions, that is, which has the lowest wettability, as well as relate the wettability with the surface chemical constitution yielded by Tb. For this purpose, we evaluate the dependence of DIT on Tb for the commercially available and most used epoxy resin, DGEBA. The resin here studied has been used as a coating on a sucking rod to pump crude oil at the Bacia de Sergipe/Alagoas, Brazil. The DIT was determined by fluorescence depolarization for a FFF on the surfaces. As far as we are aware, this is the first time that a wetting study is reported that considers the Tb of epoxy resin as a relevant variable to hinder depositions on walls coated with epoxy resins. Adding to this, it is also the first time, as far as the authors know, that fluorescence depolarization is used to determine the best nonwetting conditions of epoxy resins by liquids at a high flow rate. 2. Experimental Section 2.1. Epoxy Surfaces. The solid surfaces consisted of five slides of SAE 1006 stainless steel with one side covered by the epoxy layer. They measured 2 cm × 4 cm × 1 mm. The resin was a commercial formulation25 composed of the prepolymer of bisphenol A type epoxy resin, with a proportion of 80-92% bisphenol A epoxy resin and 10-18% aliphatic glycidyl ether. The hardeners were 30-42% isoforonadiamine, 30-42% benzyl alcohol, and 5-11% trimetyl hexametilenodiamina. With this composition, thermal stability was obtained up to 180 °C. The epoxy glass transition was below 70 °C. The samples were deposited on stainless steel and cured in the air for 24 h, for Tb’s of 100, 120, 140, 160, and 180 °C. The epoxy cured films had an average thickness of ∼0.5 mm. The surfaces had a similar roughness that was under 100 nm, as determined by perfilometry. (24) Page, S. A.; Berg, J. C.; Manson, J. A. J. Adhes. Sci. Technol. 2001, 15, 153. (25) Huntsman Home Page. www.huntsman.com (accessed May 2007).
Wettability under Imposed Flow
Energy & Fuels, Vol. 21, No. 4, 2007 2313
Figure 1. Scheme of the experimental setup to acquire absorption spectra. (1) Light source; (2) diffraction grate and step motor; (3) collimator; (4) chopper; (5) sample; (6) detector; (7) lock-in amplifier; (8) personal computer.
The samples were analyzed by differential scanning calorimetry with a Shimadzu DSC 50, under a nitrogen flow at 50 mL min-1, with heat flow as a function of the temperature from 100 to 200 °C. The curves were monotonic crescents without inflections and presented no vitreous transitions within the Tb range studied. The resin linings were characterized by absorption spectroscopy. As the resin was deposited on polished metal surfaces, the absorption spectra were not obtained by transmission but by reflection using eq 1: Iincident ) Iabsorbed + Ireflected
(1)
where Iincident, Iabsorbed and Ireflected are the intensities of the radiation respectively incident, absorbed, and reflected. The substrate polished metal was used as a reference. A home-made spectrometer developed at LAPO26,27 was used (Figure 1). The polychromatic light source consisted of an EHJ halogen lamp operating at 24 V and 250 W, with emission from 380 to 980 nm. The light passed through a monochromator with a 90 mm entrance collimator, used to generate a 2.54 cm circular light beam, and was initially diffracted by a plane diffraction grating attached to a step motor and then focused by a convergent lens, with a 190 mm focal length, into the 2 mm entrance slit of a second collimator. This collimator selected and generated a monochromatic circular light beam with a 1.27 cm diameter. The total optical path of these apparatuses is 950 mm, giving a final resolution of 0.602 nm. A chopper, at 25 Hz, modulated the monochromatic light to reduce the influence of the DC external light and of the electromagnetic noise. The light coming out from the sample surface was focused into a photodiode by a lens with a 10 mm focal length. The electrical signal generated by the diode was applied to a lockin analyzer that amplified and integrated the signal. The whole system (optics, electrical controls, and measurements) was remotely controlled by a PC and QBasic software. Assuming that no light was transmitted through the sample, one can derive the absorption from the reflected intensity using eq 1. The chemical groups of the cured samples were characterized by Fourier transform infrared spectroscopy (FTIR) using an ABB Bomem MB series spectrometer, from 300 to 4000 cm-1. A small part of each sample was powdered, and 5.0 mg was mixed with 100 mg of potassium bromide and pressed under 5 kgf cm-2 (4.8 atm) into a 1 mm diameter disk. The samples were characterized by spectrofluorimetry. Initially, the excitation wavelength (λexc) was fixed at 350 nm, and all the (26) da Silva, A. F.; Veissid, N.; Na, C. Y.; Pepe, I.; de Oliveira, N. B.; da Silva, A. V. B. Appl. Phys. Lett. 1996, 69, 1930. (27) Roman, L. S.; Valaski, R.; Canestraro, C. D.; Magalhaes, E. C. S.; Persson, C.; Ahuja, R.; da Silva, E. F.; Pepe, I.; da Silva, A. F. Appl. Surf. Sci. 2006, 252, 5361.
Figure 2. (A) Scheme of the FFF, generated by a free-liquid jet impinging on a solid surface, for PLF-FI [reprinted with permission from Journal of Colloid and Interface Science (doi:10.1016/j.jcis.2004.08.085), ref 16. Copyright 2005, Elsevier]. (B) Experimental setup for PLF-FI to detect fluorescence depolarization. CW, laser; M, mirrors; L1 and L2, lenses; P1 and P2, Glan-Thompson polarizer; PD1 and PD2, photodiodes; I, interface; PC, personal computer; FI, liquid flow; F, color filter; BS, beam splitter [reprinted with permission from Quı´mica NoVa, vol. 28, nο. 2, pp 227-339. Copyright 2005, Sociedade Brasileira de Quı´mica].
emission wavelengths (λem) were scanned from λexc plus 30800 nm, except 30 nm before and 30 nm after the harmonics of λexc. Then, λexc was increased by 50 nm, and the λem scan was repeated. This procedure was repeated until λexc was 650 nm. 2.2. Free Flowing Films (FFF). The apparatus employed to produce FFF (Figure 2, top) has been previously described.16 Briefly, the liquid jet exited a capillary tube with an inner diameter of 0.8 mm and impinged on the nearly vertical surface at 20° from a vertical offset and to one side by 15°, forming a parabolic-shaped liquid film on the surface. The liquid film flowing on the surface generated a middle valley where the flow was the thinnest. The fluid was recirculated at 3.5 m s-1 in a closed loop, and the temperature was maintained at 26.5 ( 0.5 °C. The flow had a Reynolds number below 3. Mono ethylene glycol (MEG) from Merck (99.5% purity) has been widely used as the flowing liquid for PLF-FI12,14,16-18,21,22,28-30 due to its two hydroxyl groups per molecule that increase the strength of the intermolecular network of the bulk. Additionally, it has high polarity and a low evaporation rate, and its high viscosity reduces the oxygen uptake and, consequently, the fluorescence quenching of the seeded fluorescent probe. Rhodamine B from Merck (99.9% purity) was used as the fluorescent probe31 seeded in MEG. A concentration of 1.9 × 10-3 mol L-1 was chosen in order to fulfill the requirements for the fluorescent probe and still obtain a good signal-to-noise ratio. Fluorescence emitted by dye adhering to the surface or present in a vicinal solvent layer may differ from that of dye molecules in the bulk. However, the contribution from such probes did not exceed (28) Quintella, C. M.; Lima, A. M. V.; Mammana, A. P.; Schreiner, M. A.; Pepe, I.; Watanabe, Y. N. J. Colloid. Interface Sci. 2004, 271, 201. (29) Quintella, C. M.; Watanabe, Y. N.; Lima, A. M. V.; Korn, M.; Pepe, I. M.; Embiruc¸ u, M.; Musse, A. P. S. Anal. Chim. Acta 2004, 523, 293. (30) Quintella, C. M.; Musse, A. P. S.; Gonc¸ alves, C. C.; McCaffery, A. J. Exp. Fluids 2003, 35 (1), 41. DOI: 10.1007/s00348-003-0620-2. Available online, in English, at www.springerlink.com/app/home/issue.asp (accessed may 2007). (31) Bojarski, P.; Jankowicz, A. J. Lumin. 1999, 81, 21.
2314 Energy & Fuels, Vol. 21, No. 4, 2007
Quintella et al.
3 × 10-3%. This estimate is based on an assumed thickness of the vicinal solvent layer of 25 Å and an minimum optical path length of 0.1 mm. Absorption spectra obtained before and after the experiment showed no evidence of residual rhodamine absorbed on any of the surfaces. 2.3. Depolarization of the Fluorescence Induced by Laser in Induced Liquid Flows (PLF-FI). Fluorescence depolarization consists of an evaluation of the molecular alignment and is obtained by polarization of the laser-induced fluorescence (PLF). It compares the polarization of the absorbed laser light to that of the fluorescence emission. It is not concerned with fluorescence intensity or the absorption or emission wavelengths. The polarization of the laser selects the molecular probe orientation within the liquid flow. The fluorescence polarization consists of the percentage of the emission with the same polarization as that of the absorbed light. For thin flows, the depolarization of the fluorescence, compared to the laser polarization, can be interpreted as a bidimensional phenomenon in terms of polarization (P)17,18,32 using eq 2: P)
I// - I⊥ I// + I⊥
(2)
where I// and I⊥ are respectively the fluorescence components parallel and perpendicular to the axis defined by the direction of the laser polarization. The flow velocity gradient induces an inner stress that changes the chemical environment; that is, the intermolecular interactions increase the local anisotropy, hindering the rotation of the fluorescent probe.32 This increases the probability of the probe still being aligned along the flow when it suffers fluorescent decay, thus increasing the net polarization of the fluorescence emission. When the interaction between the liquid flow and the solid surface increases, the liquid intermolecular network, which was previously aligned vertically by the vertical flow, has additional interactions along the horizontal that partially misalign the molecular domains and thus the fluorescent probes, decreasing I// and consequently P. PLF is a technique known for many years to be a sensitive probe of molecular alignment in liquid flows.33 It was applied17,18 to free liquid laminar jets at high velocity for the first time, and it was found that the intermolecular alignment along the flow is between 1 and 3 orders of magnitude greater than that achieved by conventional flow or electric field alignment techniques.32 Quintella et al.22 applied it for the first time to liquids flowing on solid surfaces and found its relation with DIT. For liquids flowing at a high flow rate through and out of a thin slit, it proved possible21,30 to visualize directly the boundary layer at a molecular level, to infer the macroscopic velocity gradient, to generate the velocity profiles throughout the flow by functional integration, and to discuss macroscopic fluid dynamic concepts in terms of interactions between the molecules that comprise the flowing liquid. The experimental setup for PLF-FI (Figure 2, bottom) has been described before.12 Briefly, a Coherent Inova60 argon laser (CW) in multimode, multiline, and at 60 mW was reflected by two mirrors (M) through a biconvex borosilicate lens (L1) of a 400 mm focal length that focused the laser to a 0.02 mm2 diameter spot on the sample (FI). In order to monitor the laser intensity fluctuations in real time, 10% of the laser beam was diverted by a beam splitter (BS) onto a BPW-21 RS-Electronics photodiode (PD1) with a 7.5 mm2 active area, operating as a current-to-voltage converter with seven optional sensitivity ranges. A vertical Glan-Air polarizer (P1) insured a 100% vertically polarized laser beam. Fluorescence was collected frontally within a 0.02 sr solid angle by a biconvex borosilicate lens (L2) of 50 mm focal length and focused onto a detector consisting of an OPT 202 Burr-Brown photodiode (PD2) with a 5.22 mm2 active area, operated as a (32) Bain, A. J.; Chandna, P.; Butcher, G. Chem. Phys. Lett. 1996, 260, 441. (33) Feofilov, P. P. The Physical Basis of Polarized Emission; Consultants Bureau: New York, 1961.
Figure 3. Absorption spectra of the epoxy resin as a function of Tb.
current-to-voltage converter with four optional sensitivity ranges. A 550 nm cutoff filter (F) blocked the laser radiation. The GlanThompson polarizer (P2) was manually rotated to select either the vertical (I//) or the horizontal (I⊥) fluorescence intensities. The outputs of both PD1 and PD2 were captured by an interface (I) connected to a computer (PC). The QBasic program controlled both data acquisition and sample positioning. The surface position was varied uniformly in relation to the horizontal and the vertical of the laser beam direction, using a twoaxis translation frame23 with 10 µm resolution and repeatability better than 0.1%. Each fluorescence intensity obtained by PD2 was divided by the laser intensity obtained in real time by PD1 in order to correct for laser intensity fluctuations. The polarization error margin was (0.5%.
3. Results and Discussion For each epoxy surface were determined absorption spectra, spectrofluorimetry maps, FTIR spectra, and DIT by PLF-FI. Visually, the epoxy coloration changed with Tb, from a transparent yellowish, through a light yellow that darkened progressively, becoming a dark brown. The absorption spectra (Figure 3) showed that the decay of the absorption band moved progressively to longer wavelengths. It was possible to observe that, at 100 and 120 °C, the absorption curve decreases after a sharp maximum, while for higher Tb’s, there was a second absorption band at longer wavelengths that was flattened. This was more evident by the crossing, at ∼570 nm, of the absorption spectra of the samples baked at 120 and 140 °C. For samples baked at 100 °C, the absorption occurred mainly at 350 nm and decreased for longer wavelengths. For the 120 °C baked samples, the absorption band increased until reaching 400 nm, decreasing afterward but not as much as the other curves. For the 140 °C baked samples, the absorption band started decaying only at about 470 nm. The sample baked at 160 °C had a band up to 580 nm. The 180 °C baked sample presented an absorption band that reached 650 nm. The increase of the absorption band toward the visible was in accordance with the visual inspection of color and opacity. The literature7-9 usually reports either excitation or emission curves. Here, all the emission curves of the resins, obtained by spectrofluorimetry, were juxtaposed, generating luminescence maps (Figure 4). No emission was detected from the sample baked at 180 °C, which is expected due to the increase of quenching groups as the cure proceeds.8 The other samples had a clear maximum that, as Tb increased, moved to higher excitation and emission wavelengths. As Tb increased, the
Wettability under Imposed Flow
Energy & Fuels, Vol. 21, No. 4, 2007 2315
Figure 4. Maps of the luminescence of the epoxy resins as a function of Tb, acquired by spectrofluorimetry, as a function of both the excitation and the emission wavelengths. Table 1. Maximum of the Luminescence of the Epoxy Resins as a Function of Tb, Acquired by Spectrofluorimetry Tb excitation (nm) emission (nm) energy gap (eV)
100 °C
120 °C
140 °C
160 °C
380 440 0.44
448 577 0.62
497 599 0.42
549 600 0.19
energy gap (Table 1) of the absorption between the fundamental state and the excited absorbing state decreased from 3.26 to 2.26 eV, while the gap between the fundamental state and the excited emission state decreased from 2.82 to 2.07 eV. At 120 °C, the energy gap of the excited states of absorption and emission is the largest (0.62 eV). This was attributed to the environment of the luminescent probes suffering a progressive change, as Tb increases, from disorganized incomplete polymerization, followed by maximum polymerization, polymer network destruction, and finally reaching partial carbonization. This progressive change corresponds to an initial decrease of the isotropy of the chemical environment with increasing symmetry (polymerization) followed by the progressive increase of isotropy, where the intermolecular interactions per mole diminish, generating a less energetic system. FTIR has been widely used to follow the cure of epoxy resins34-36 as it allows monitoring the presence of specific chemical groups. The FTIR analysis of the cured samples (Figure 5) showed high similarity between 4000 and 2000 cm-1, as well as below 750 cm-1. There was a high amount of peaks between 2000 and 750 cm-1. At 3400 cm-1, there was a high-intensity band that may be attributed to the bisphenol A hydroxyl stretch as well as the eventual water in the KBr disk.37 At 916 cm-1, there was a small band that may be attributed to the stretch of the terminal epoxy ring. It was clearly present only at 100 °C, pointing to an incomplete cure of the resin baked at 100 °C.38 From 2850 to 2960 cm-1 were observed bands that can be assigned to the vibration of the axial deformation of the epoxy resin CH2 group. The curve at 140 °C clearly had an absorption stronger than those for the other temperatures. The small band at 720 cm-1 was observed before, during epoxy thermal degradation studies,39 and may be attributed to the vibration of the angular deformation of the CH2 group. This peak was (34) Bartolomeu, P.; Chailan, J. F.; Vernet, J. L. Eur. Polym. J. 2001, 37, 659. (35) Roma˜o, B. M. V.; Diniz, M. F.; Azevedo, M. F. P.; Lourenc¸ o, V. L.; Pardini, L. C. R.; Dutra, C. L.; Burel, F. Polı´m.: Cieˆ ncia Tecnol. 2003, 13, 1428. (36) Jiawu, G.; Kui, S.; Mong, G. Z. Thermochim. Acta 2000, 352, 153. (37) Urbanski, J.; Czerwinski, W.; Janicka, K.; Majewska, F.; Zowall, H. Handbook of Analysis of Synthetic Polymers and Plastics; John Wiley: New York, 1977. (38) Roma˜o, B. M. V.; Diniz, M. F.; Pardini, L. C.; Dutra, R. C. L. Polı´m.: Cieˆ ncia Tecnol. 2004, 14, 142.
Figure 5. Infrared Fourier transform spectra of the epoxy resin as a function of Tb. Insets: (A) CO2 [667 cm-1]; (B) epoxy ring [916 cm-1]; (C) nitrites and nitrates [1650 cm-1]; (D) nitrites and nitrates [1720 cm-1].
maximum at 140 °C and decreased as Tb increased, almost vanishing at 180 °C, pointing to the maximum polarization occurring at 140 °C. The 1580 cm-1 region corresponds to the aromatic group attributed to the benzenic ring C-C bond present in the bisphenol A epoxy resin as well as in the benzylic alcohol hardener, and its band intensity remained almost constant for 100 and 120 °C, increased at 140 °C, and decreased progressively for higher Tb’s, which can be explained by the maximum polymerization at 140 °C followed by polymeric degradation. The peaks at 1460 and 1410 cm-1 may be attributed to alkyls and may be used to monitor the cross polymerization or degradation. They were constant until 160 °C and strongly reduced at 180 °C, pointing to the degradation occurring mainly after 160 °C. At 1610 cm-1, there was a band that may be attributed to the benzenic ring C-C bond present in the bisphenol A epoxy resin.37 It was almost constant until 160 °C and decreased for samples baked at 180 °C. The main absorption that can be attributed to the hardener agents is at 1513 cm-1 and corresponds to the amine N-H bond.34,40 It was almost similar except for 180 °C, where it clearly decreased, pointing to the degradation of the remaining amount of cure agent. The peaks at 1720 and 1650 cm-1 may be attributed to nitrates and nitrites. They were almost nonexistent at 100 °C, appeared (39) Bormashenko, E.; Pogreb, R.; Sheshnev, A.; Shulzinger, E.; Bormashenko, Y.; Sutovski, S.; Progreb, Z.; Katzir, A. Polym. Degrad. Stab. 2000, 72, 125. (40) Rong, M.; Zeng, H. Polymer 1998, 39, 2525.
2316 Energy & Fuels, Vol. 21, No. 4, 2007
Quintella et al.
Table 2. Polarization as a Function of Tb Tb (°C) polarization (%)
100 10.2
120 14.5
140 11.8
160 6.0
180 3.5
at 120 °C, and increased progressively with Tb, pointing to further degradation. The peaks at 2351 and 667 cm-1 may be attributed to carbon dioxide addition during sample preparation.41 These peaks decreased with the increase in Tb and almost vanished, pointing to the progressive liberation of this gas. The wettability was strongly sensitive to Tb. The polarization (Table 2) of the liquid flow on the samples increased up to 120 °C and then decreased substantially. As polarization decreased, the wettability increased. The minimum wettability at 120 °C corresponded to the Tb where the gap between absorption and emission wavelengths was at a maximum (Table 1); thus, it corresponded to the region where the resin was more anisotropic and the polymer was more structured. This optimum lower wettability of the baking at 120 °C may be related to the sample’s chemical constitution and its interaction at the interface between the epoxy and the liquid flow. The presence of nitrites and nitrates was more pronounced at 140 °C, when the wettability was strongly reduced. This was also in accordance with the new absorption band that appeared at longer wavelengths. Although polymerization was at a maximum between 140 and 160 °C, as determined by FTIR, the increasing presence of nitrites and nitrates became relevant to the wettability. At 120 °C, although the polymerization was not at a maximum, the degradation of the polymer network had already started yielding chemical groups originated from degradation of the polymer, hardener, and prepolymer and from carbonization. At 140 °C, these groups caused a pronounced interaction with the liquid, increasing the wettability and decreasing polarization. Thus, the wettability was related with two simultaneous phenomena: polymerization and degradation of the epoxy. The presence of degraded chemical groups strongly increases wettability. Thus, although polymerization was not at a maximum (41) Akinade, K. A.; CampbelL, R. M.; Campton, D. A. C. J. Mater. Sci. 1994, 29, 3802.
at 120 °C, the wettability was at a minimum because the amount of degraded chemical groups was still at a minimum and polymerization was already considerable. 4. Conclusion For the first time, the nondestructive test of DIT, obtained by fluorescence depolarization, was used to determine the best Tb to decrease the chemical interaction between the flowing liquid and a commercial DGEBA epoxy used as a coating on a sucking rod at the oil field of Bacia de Sergipe/Alagoas, Brazil. In this first step were selected the cure variables according to the maker recommendations: 24 h and a Tb ranging from 100 to 180 °C. All the experimental characterizations of the samples pointed to the increase of Tb, causing progressive resin polymerization, followed by degradation and posterior carbonization. From FTIR, it was possible to observe that the cure of the resin baked at 100 °C was incomplete. At 140 °C, the resin polymerization was at a maximum. The lower wettability corresponded to 120 °C, where the spectrofluorimetric analysis identified the highest energy gap and where there was already formation of residues from chemical components’ degradation. After 120 °C, the products of thermal degradation drastically increased the wettability, increasing the chemical interaction with the fluid and, thus, increasing the probability of wall depositions. This means that, for DGEBA epoxy resin, the more suitable Tb to inhibit wall deposits and blockages, as well as reduce the frequency of stops for interventions, is just before that of full polymerization as the latter also already has a small amount of chemical groups yielded by the beginning of thermal degradation. Acknowledgment. We acknowledge the CNPq and FINEP for partial grant support for this work. Prof. J. C. Scaiano from the Department of Chemistry, University of Ottawa, is acknowledged for the use of the spectrofluorimeter. A.P.S.M. and A.M.V.L. acknowledge CNPq and Preˆmio Petrobras de Tecnologia de Ductos for scholarships. A.P.S.M., A.M.V.L., and L.C.S.S., Jr. acknowledge CNPQ and FINEP for technological scholarships. C.M.Q. acknowledges a senior research scholarship from CNPq. EF060551W
