2174
Energy & Fuels 2006, 20, 2174-2180
Characterization of Chemical Reactivity of Liquid Antistripping Additives Using Potentiometric Titration and FTIR Spectroscopy U. Bagampadde† and U. Isacsson*,‡ Faculty of Technology, Makerere UniVersity, P.O. Box 7062, Kampala, Uganda, and DiVision of Highway Engineering, Royal Institute of Technology (KTH), S-100 44, Stockholm, Sweden ReceiVed NoVember 28, 2005. ReVised Manuscript ReceiVed April 20, 2006
Chemical reactivity of two liquid antistripping additives mixed with two bitumens of diverse acid numbers was evaluated. Additives present in the blends were detected by use of potentiometric titration and infrared spectroscopy. Tests were done at dosages of 0, 0.5, 1.0, and 2.0%; storage temperatures of 25, 100, 140, and 150 °C; and storage times of 1, 24, and 72 h. At 0.5% dosage, close to typical field values, the more basic additive mixed with bitumen of high acid number almost ceased to be detected after 24 h of storage at 140 °C. The less basic additive could be detected beyond these conditions, irrespective of the bitumen used. At higher dosages, reactions with the bitumens were found to be more pronounced with the more basic additive. The reactions between the additives and bitumens studied seemed to be higher in the bitumen with higher acid number, irrespective of the dosage. Statistical analysis indicated that all the parameters studied significantly affected change in amount of additives detected in the blends. A correlation was established between potentiometric titration and infrared spectroscopy in detecting amine additives. This correlation notwithstanding, infrared spectroscopy was found to not be a good tool for measuring amines in the blends, especially at low concentrations.
Introduction Several types of liquid antistripping additives are extensively used to promote bonding between bitumen and aggregate surfaces in wet conditions. When liquid additives are utilized in paving technology, their effectiveness can be influenced by chemical reactivity with bitumen during mixing and storage at high temperature. Chemical reactivity of a liquid antistripping additive influences its ability to withstand high temperature and prolonged storage time without reacting with bitumen components, giving nonfunctional forms of reduced antistripping capability.1 Despite extensive efforts to determine conditions under which thermal degradation of liquid additives due to reactivity is negligible, there seems to be few scientific publications in this area. The most commonly used liquid antistripping additives are amine-based.2 It is reported by Dybalski,1 among others, that reactivity of liquid additives with bitumen is likely to be influenced by chemical composition of bitumen, chemical nature and concentration of additive, and hot storage conditions (storage temperature and time) before mixing with hot aggregate. There is much need to optimize these parameters in order to guarantee proper adhesion at the interface between bitumen and aggregate. The objective of the present work was to study the influence of additive concentration, temperature, and time of hot storage on chemical reactivity of two commercial amine-based additives, * Corresponding author. Phone: +46 8 7908700. Fax: +46 8 4118432. E-mail:
[email protected]. † Makerere University. ‡ Royal Institute of Technology (KTH). (1) Dybalski, J. N. Proc. Assoc. Asphalt PaVing Technol. 1982, 51, 293297. (2) Curtis, C. W.; Ensley, K.; Epps, J. Fundamental Properties of AsphaltAggregate Interactions Including Adhesion and Absorption. SHRP Report A-341; National Academy of Sciences: 2101 Constitution Avenue N.W., Washington, DC 20418, 1993.
when each additive is mixed with two bitumens of diverse acid numbers. In this study, chemical reactivity of the additives was characterized using potentiometric titration and Fourier transform infrared spectroscopy (FTIR) spectroscopy. Review of Literature Use of antistripping additives has been a subject of numerous research efforts. For example, the report by Curtis,3 who performed a comprehensive literature study on a large set of antistripping additives and their effectiveness, is a useful source of references. Curtis3 indicates that most of the liquid antistripping additives commonly used contain nitrogen and are in the form of amines, fatty amines, substituted amines, and polyamines. When added to bitumen, the additives enter into a bituminous body with a very large number of molecules of diverse chemical composition, polarity, and structure. Some of the additives tend to become less effective in promoting adhesion, after reacting with bitumen at high temperatures. Consequently, the effectiveness of such additives in hot bitumen may be influenced by composition of bitumen. Composition and structure of bitumen have been the subject of continuing controversy and research. A famous notion is the colloidal dispersion of asphaltenes in maltenes that was advanced by Nelensteyn in 1924 and Pfeifer and Saal.4 The 5-year Strategic Highway Research Program (SHRP) study that ended in 1992 proposed bitumen to be a homogeneous mix of complex molecules being either polar or nonpolar.5 Characteristic (3) Curtis, C. W. A Literature Review of Liquid Antistripping and Tests for Measuring Stripping. SHRP Report A/UIR - 90 016; National Academy of Sciences: 2101 Constitution Avenue, N.W., Washington, DC 20037; 1990. (4) Pfeiffer, J. P.; Saal, R. J. N. J. Phys. Chem. 1940, 44, 139-149. (5) Jones, D. R.; Kennedy, T. W. Report on SHRP A-001 Contract; Texas A & M University: College Station, TX, 1992.
10.1021/ef050397y CCC: $33.50 © 2006 American Chemical Society Published on Web 08/08/2006
Chemical ReactiVity of Liquid Antistripping AdditiVes
behavior of bitumen seems to be mainly influenced by functional groups resulting from heteroatoms. Amine antistripping additives are basic and would mainly react with bitumen acidic components. Ion exchange chromatography developed by SHRP indicated that functional groups in bitumen are either acidic, basic, or amphoteric.6 Pyridines are moderately strong bases, while the 2-quinolones, sulfoxides, and ketones are weak bases.7 Dutta and Holland consider pyrrolics and amides to be weak bases, although pyrrolics have an acidic hydrogen atom.8 Carboxylic acids and phenols are acidic, although the latter can also be basic depending on the nucleophility of the compound it reacts with. It is principally these functional groups and their locations on bitumen molecules that may primarily influence interaction with polar functionalities of the amine additives.9 Amine additives (like amines, diamines, amidoamines, or imidazolines) probably interact with both bitumen components and charged aggregate surfaces. However, from the literature reviewed, it is still unclear how exactly the additives interact with bitumen. Since nitrogen-containing polar groups in liquid antistripping additives are basic, the additives perhaps mainly interact with bitumen acidic functionalities. The remaining additives that reach the interface between bitumen and aggregate act as surfactants with their polar ends attracted to the oppositely charged aggregate surfaces, while the long organic chain remains anchored in the bitumen.10,11 Chemical reactivity of additives may be influenced by length or degree of saturation of the alkyl chain, number of nitrogencontaining groups and their positions on each molecule, and molecular shape, which determines level of steric hindrance.12 Amines are good Lewis bases and nucleophiles because of the lone electrons on the nitrogen. For an amine, the greater the availability of a lone pair of electrons on the nitrogen, the higher is the basic character and, hence, its propensity to react with bitumen acidic functionalities. Secondary amines are generally stronger bases than primary amines because of the induction effect of the two alkyl groups. However, tertiary amines exhibit decreased basic character due to steric hindrance, despite the presence of three alkyl groups.12 For amines with a ring structure (e.g., imidazoline), the heterocyclic nitrogen lone pair of electrons may not be available, since they are used to stabilize the ring structure. Liquid antistripping additives are typically applied in small concentrations, and the effects of reactivity with bitumen (if present) may be pronounced during mixing and storage. On the basis of the review made, it is prudent to assume that, if amines can be detected in an additive/bitumen blend, then the additive has not reacted with bitumen and is, therefore, still present in (6) Duvall, J. J.; Miyake, G.; Catalfomo, M. W.; Kim, S. S.; Colgin, D. C.; Branthaver, J. F. Size Exclusion Chromatography and Ion Exchange Chromatography Separations of Asphalt. SHRP Report A-663; National Academy of Sciences: 2101 Constitution Avenue, N.W., Washington, DC 20037, 1993. (7) Branthaver, J. F.; Petersen, J. C.; Robertson, R. E.; Duvall, J. J.; Kim, S. S.; Harnsberger, P. M.; Mill, T.; Ensley, E. K.; Barbour, F. A.; Schabron, J. F. Binder Characterization and Evaluation. Volume 2: Chemistry. SHRP Report A-368; National Academy of Sciences: 2101 Constitution Avenue, N.W., Washington, DC 20418, 1993. (8) Dutta, P. K.; Holland, R. J. Fuel 1984, 63, 197-210. (9) Petersen, J. C.; Plancher, H. J. Pet. Sci. Technol. 1998, 89-131. (10) Tarrer, A. R.; Wagh, V. The effect of the Physical and Chemical Characteristics of the Aggregate on Bonding. SHRP Report A/UIR-91-507; National Academy of Sciences: 2101 Constitution Avenue, N.W., Washington, DC 20418, 1991. (11) King, G.; Bishara, S. W.; Fager, G. Proc. Assoc. Asphalt PaVing Technol. 2002, 71, 147-175. (12) Matthews, P. Textbook of AdVanced Chemistry; Cambridge University Press: New York, 1996; pp 491-493; ISBN 0 521 56693 3.
Energy & Fuels, Vol. 20, No. 5, 2006 2175 Table 1. Properties of Bitumens Used in This Study property penetration at 25 °C (×0.1 mm) softening point (°C) ductility at 25 °C (cm) Brookfield viscosity at 135 °C (mPa‚s) total acid number (mg of KOH/g)
bitumen A
bitumen B
84 44.5 126 302
86 46.2 118 325
EN1426 EN1427 ASTM D113 ASTM D4402
3.59
0.28
ASTM D664
standard
the blend. There is no obvious method for detecting these amines, owing to complexity of the bitumen. However, some methods have been proposed for detection of bases in bitumen, such as vapor spectrophotometric analysis,13 mass spectrometry,14 thermogravimetry,15 UV spectroscopy,16 nonaqueous potentiometric titration,17 and others. Experimental Section Materials and Reagents. Two penetration-graded 70/100 bitumens were studied. They were designated A and B, respectively, and were supplied by Nynas, Sweden. Bitumen A was produced from Venezuela (Laguna) crude oil, while the source of bitumen B was not known. Selected properties of the bitumens, tested in triplicate, are summarized in Table 1. The two bitumens were chosen because they exhibited diverse total acid numbers (cf. Table 1). As a consequence, it was speculated that chemical reactivity of basic amine liquid additives with bitumen could probably differ for the two bitumens. Infrared spectroscopy done on these bitumens indicated that bitumen A exhibits a significantly higher concentration of carbonyls (stronger and broader peak at 1706 cm-1) than bitumen B. Two liquid commercial additives supplied by Akzo Nobel, Sweden, were selected and designated AM1 and AM2, respectively. Additive AM1 was white in color, while additive AM2 was brown. According to supplier information, the additives were multicomponent materials. AM1 was a fatty diamine with the main component being N-oleyl-1,3-diaminopropane, and AM2 was mainly a fatty imidazoline. The titration reagents used were all from Merck, Germany, and included (a) 0.1 N perchloric acid as the titrant, (b) glacial acetic acid (99.5%), and (c) chlorobenzene. A solvent mixture used to dissolve bitumen or additive/bitumen blend before titration was prepared in accordance with the Swedish Standard ISO 3771 by mixing one volume of acetic acid with two volumes of chlorobenzene. Preparation and Storage of Additive/Bitumen Blends. Typically, the required amount of liquid additive was carefully added to hot bitumen that had been preheated for 3 h at 100 °C in metallic cans and stirred continuously for 5 min using a warm spatula to ensure thorough mixing. Additive concentrations of 0, 0.5, 1.0, and 2.0% w/w, respectively, were used in this work to ensure an extended range of additive concentrations in testing, although real field concentrations are generally within 0.50.7% w/w. The cans containing the additive/bitumen blends were sealed and placed in an oven set at the storage temperature (13) Maupin, G. W., Jr. Test for the Presence of Asphalt Antistripping Additive. Trans. Res. Rec. 2005, 1929, 46-51. (14) Hilble, J. J. High capacity thermally regenerated supported amine sorbents for CO2 removal. ES CSTC Yr 3, Environmental Sciences Commercial Space Technology Center: Florida, 2004. (15) Donbavand, J. Thermochim. Acta 1984, 79, 161-169. (16) Tarrer, A. R. Stripping of asphalt concrete-chemical testing. Final report FHWA/AL-105B/88, Alabama, Highway Department: Alabama, 1987. (17) Tarrer, A. R.; Yoon, H. H.; Kiggundu, B. M.; Roberts, F. L.; Wagh, V. P. Trans. Res. Rec. 1989, 1228, 128-137.
2176 Energy & Fuels, Vol. 20, No. 5, 2006
Bagampadde and Isacsson
Figure 1. Direct MS analysis of the commercial amine additives.
for a particular time. Four storage temperatures (25, 100, 140, and 150 °C) and three storage times (1, 24, and 72 h) were used. Duplicate specimens were tested for each combination of bitumens, additives, dosages, storage temperatures, and times, giving a total of 384 samples tested. Analytical Procedures Selected. Mass Spectrometry. The two commercial additives used were qualitatively analyzed by mass spectrometry (MS). Information obtained from the supplier about these two additives indicated that the main components for AM1 and AM2 were N-oleyl-1,3-diaminopropane and alkyl imidazoline, respectively. These liquid additives were quite polar with long chain molecules and could not be analyzed successfully using GC-MS, as they would permanently adsorb onto chromatographic columns. Therefore, direct MS analysis was applied by heating a probe inlet to several hundred degrees. Separation of mixtures of compounds in the commercial additives was expected, owing to differences in boiling points. The analytes were solutions of the additives in methanol/ methylene chloride (50%:50%) prepared at 500 ppm (w/w). Aliquots of the analytes were put into a mass spectrometer (Finnigan SSQ 7000 model). The probe heating was programmed from 50 °C for 0.5 min, followed by a ramp to 400 °C at 200 °C/min, and finally held at 400 °C for 10 min. The resultant mass spectra for volatile compounds were noted. The MS scan range used was m/z 29-800 at 2 scans s-1. Potentiometric Titration. A sample of 2.000 ( 0.05 g of bitumen or additive/bitumen blend was weighed and put in a 250 mL beaker. The solvent mixture (100 mL) was poured into the beaker followed by magnetic stirring to ensure that bitumen completely dissolved in the solvent. Titration was carried out with an automatic Schott Titroline Easy titrator set to dispense 0.1 mL per 10 s of titrant into a magnetically stirred solution of the analyte until the electrode potential (Pe) reached a constant value. For each set of samples, a blank titration on 100 mL of the titration solvent mixture was made in increments of 0.01 mL per 10 s of titrant. Titration curves were generated by plotting Pe against volume of titrant (Vt). The end points were determined as the maximum points after plotting first derivatives (∆Pe/∆Vt) against Vt using Microsoft Excel. The volume of titrant required to reach the end point was, hence, determined. The total base number (TBN), expressed in equivalent milligrams of KOH per gram of a sample, gives its basicity, which
was calculated for all samples according to the following formula (Swedish ISO 3771),
TBN ) 56.1(V2 - V1)To/m
(1)
where V1 is the volume (mL) of the titrant required in the blank test to inflection point, V2 is the volume (mL) of the titrant required to reach the end point for the test analyte, To is the normality of the standard perchloric acid solution, and m is the mass, in grams, of the sample tested. In this study, TBN was assumed to increase with total concentration of basic aminebearing components in the examined sample. FTIR Spectroscopic Analysis. Chemical reactivity of the liquid additives with bitumen was also studied using infrared spectroscopy on the blends prepared as described earlier. Infrared spectroscopy was used to detect the presence of amines in the additive/bitumen blends. Absorbance spectra in the mid-IR spectral bandwidth (4000-700 cm-1) were obtained using an infinity 60AR Mattson spectrometer (resolution 0.125 cm-1 and 32 scans) with the iris set at 5% for both background and sample scans. Samples of additive/bitumen blend solutions (5% w/w) were prepared in carbon disulfide (CS2) as a spectral solvent. Background and sample scans were performed using sealed 1 mm circular cells of zinc selenide (ZnSe) windows. Owing to absorption of CS2 between 1600 and 1400 cm-1, the bands of N-H amine bend around 1450 cm-1 could not reliably be used in this study. Amines exhibit absorption bands around 3460 cm,-1 attributable to N-H stretch,18 and this band was used to detect the amount of amine remaining in the blends after storage. Absorbances for the band of interest were calculated using integrated peak areas between 3492 and 3425 cm-1 as the first and second integration points. Results and Discussion Qualitative Analysis of Additives by Mass Spectrometry. The chromatogram resulting from heating of AM1 is shown in Figure 1a. The total ion chromatogram (on the top) indicates that the analyte contains many substances, with the last peak (detected after 2.32 min) being more abundant than other peaks. (18) Visek, K. E. Kirk-Othmer Encyclopedia of Chemical Technology. Akzo Chem. Inc., McCook Research Center: McCook, IL, 1990.
Chemical ReactiVity of Liquid Antistripping AdditiVes
Energy & Fuels, Vol. 20, No. 5, 2006 2177
Figure 2. TBN data of bitumen A for different additives, storage temperatures, and times.
Since m/z 30 and 44 are two characteristic ions of aliphatic polyamines (especially of primary amines), selected ion chromatograms were reconstructed at m/z 30 (middle) and 44 (bottom), respectively. These chromatograms clearly show that the largest peak was that of aliphatic polyamines. This observation is in agreement with the product information from the supplier. Other series of ions from aliphatic amine-like chemicals were monitored, i.e., m/z 58, 72, 86, and 100, and their chromatograms confirmed the above conclusion. An ion was observed at m/z 55, indicating that the main component may contain a double bond. To identify the exact chemical structure of the main component was very difficult. First, the MS spectrum could possibly still be mixed by several compounds with similar structures and boiling points. Second, the standard MS spectrum of the target analyte was not available in the MS library. Third, the molecular ion of aliphatic polyamines is usually very weak or nonexistent, because the chemical structure of nitrogen-containing chains is easy to break during electron ionization. Nevertheless, the results obtained seem not to conflict with supplier information. The main components in AM2 were checked basing on the supplier’s chemical information. Ions of interest were checked, including those from the amine group (primary and secondary amine, respectively), using similar methodology as described for AM1. The total ion chromatogram (top part) and selected ion chromatograms at m/z 30 (middle) and 44 (bottom), respectively, were constructed as shown in Figure 1b. The two major peaks in the total ion chromatogram may be amine-like components, as suggested in selected ion chromatograms. Details
of MS spectrums were checked to ascertain if these two peaks contain imidazole or imidazoline ring (five-member ring) or piperazine ring (six-member ring). Some characteristic ions were selected to construct the ion chromatograms (m/z 58, 68, 70, 84, 85, and 96, respectively). Two overlapped main peaks were observed in all these selected ion chromatograms. In addition, the MS spectra at these two peaks were similar, indicating that these two components may have similar molecular structure. Since ions corresponding to m/z 44 were more dominant, and because the infrared spectrum of AM2 showed no carbonyl peak in the IR analysis, an imidazoline ring may probably be present other than a piperazine ring based on MS library information. Similar to AM1, the m/z 55 chromatogram for sample AM2 indicates that the aliphatic chain (R-) in the molecule contains double bonds. Potentiometric Titration Results. The results obtained from potentiometric titration are shown in Figures 2 and 3 for bitumen A and B, respectively. The TBN values were calculated using eq 1. In these figures, plots of TBN ((mg of KOH)/g) at different additive concentrations, storage temperatures, and times are shown. The data in each diagram indicate that, for each combination of bitumen type, additive type, and additive concentration (except samples without additives), the highest TBN was generally observed at a storage time of 1 h and storage temperature of 25 °C. For ease of analysis, the TBN values at these conditions were arbitrarily assigned values of 100%. The values at other conditions were normalized relative to this value. By way of example, Table 2 shows these data as normalized percentages at 0.5% additive concentration. This normalization
2178 Energy & Fuels, Vol. 20, No. 5, 2006
Bagampadde and Isacsson
Figure 3. TBN data of bitumen B for different additives, storage temperatures, and times. Table 2. Normalized TBN (as percentages) at 0.5% Additive Concentration bitumen A additive AM1
bitumen B additive AM2
additive AM1
additive AM2
temp
1h
24 h
72 h
1h
24 h
72 h
1h
24 h
72 h
1h
24 h
72 h
25 °C 100 °C 140 °C 150 °C
100 95 91 83
94 85 74 59
89 81 64 53
100 90 86 82
82 76 51 50
77 69 50 49
100 99 93 87
99 93 86 82
98 89 83 81
100 101 103 97
102 103 87 78
86 83 78 75
was also done at additive concentrations of 1.0 and 2.0%. For the two pure bitumens (cf. the two uppermost diagrams in each of Figures 2 and 3), the variations seem to be marginal. Effect of Storage Temperature. For the bitumens containing antistripping additives, it appears that TBN is sensitive to changes in storage temperature (cf. Figures 2 and 3). For example, the data in Table 2 show that, for blends of bitumen A and additive AM2 at additive concentration of 0.5% (close to values typically used in practice), an increase of storage temperature from 100 to 140 °C had corresponding decreases in TBN (expressed as normalized percentages) of 25 and 19%, respectively, at storage times of 24 and 72 h. For bitumen B, the corresponding reductions in TBN were 16 and 5%, respectively. A further increase in storage temperature from 140 to 150 °C at 0.5% additive concentration shows that the reductions in TBN at 24 and 72 h were much less pronounced for additive AM2 blended with bitumen A (1% in both cases) than those for additive AM1 with the same bitumen (15 and 11%, respectively). The reason for this difference between the two additives may be that, in principle, all AM2 had reacted with bitumen A at 140 °C after 24 h. This difference could
also be attributed to the depletion of the reactive bitumen components. However, by comparison with blends containing higher concentrations of additives, a continuous decrease in TBN was observed at 150 °C, which indicates that these reactive bitumen components were still present. On the basis of these observations, it was concluded that, at a 0.5% dosage, almost all of additive AM2 had reacted with bitumen A after storage at 140 °C for 24 h. Effect of Storage Time. As seen in Table 2, when the blends were stored at 25 °C, the TBN (normalized percentage) changed marginally in bitumen B when storage time was increased from 1 to 24 h. The change of TBN in bitumen A under the same conditions was higher. At storage temperatures of 140 and 150 °C, the TBN decreased more sharply for the same change in storage time (1-24 h), except in blends from bitumen B mixed with additive AM1. The TBN values above 100% in the blends of additive AM2 and bitumen B seem to be artifacts. A change of storage time from 24 to 72 h resulted in smaller decreases in TBN in blends of both additives and bitumens. Regarding the influence of storage time on the interaction between the additives and bitumen, increasing the storage time seems to cause greater
Chemical ReactiVity of Liquid Antistripping AdditiVes
Energy & Fuels, Vol. 20, No. 5, 2006 2179
Table 3. IR Absorbance of NH Stretch and Sulfoxides at 1.0 and 2.0 % Dosagesa 1 hb
24 h
100 °C BitA BitA + 1% AM1 BitA + 2% AM1 BitA + 1% AM2 BitA + 2% AM2 BitB BitB + 1% AM1 BitB + 2% AM1 BitB + 1% AM2 BitB + 2% AM2 a
72 h
100 °C
140 °C
100 °C
140 °C
NH
SdO
NH
SdO
NH
SdO
NH
SdO
NH
SdO
1.14 1.57 1.72 1.66 1.73 0.89 1.13 1.25 1.19 1.31
0.44 0.39 0.42 0.40 0.40 0.42 0.42 0.38 0.39 0.38
1.14 1.51 1.67 1.58 1.66 0.89 1.10 1.21 1.16 1.27
0.44 0.41 0.39 0.42 0.41 0.42 0.41 0.36 0.38 0.37
1.06 1.18 1.35 1.28 1.33 0.86 0.98 1.09 1.03 1.11
0.49 0.55 0.53 0.51 0.49 0.49 0.53 0.49 0.50 0.49
1.14 1.47 1.59 1.44 1.58 0.89 1.08 1.15 1.07 1.19
0.44 0.48 0.47 0.45 0.44 0.42 0.40 0.41 0.39 0.41
0.96 1.03 1.11 0.98 1.13 0.82 0.94 1.02 0.98 1.04
0.51 0.55 0.57 0.56 0.59 0.49 0.54 0.56 0.62 0.64
BitA ) bitumen A and BitB ) bitumen B. b No tests were performed after storage at 140 °C for 1 h.
reactions between both additives and bitumen A than between both additives and bitumen B, irrespective of temperature. Influence of Bitumen Acidity. As noted earlier, the two studied bitumens were selected because they exhibited diverse total acid numbers. On the whole, the data in Table 2 indicate that the changes in TBN (normalized percentage) with storage temperature and time seem to be higher in bitumen A than in bitumen B, irrespective of the additive used. For example, for additive AM2, after storage at 150 °C for 72 h, the TBNs for bitumen A and bitumen B are 49 and 75%, respectively. The corresponding values for additive AM1 and both bitumens are 53 and 81%, respectively. This observation was also made in the data at additive concentrations of 1.0 and 2.0%. Influence of AdditiVe Type and Concentration. Alkalinities for the additives were determined using potentiometric titration, and the values from triplicate runs were as follows: for AM1, 304, 354, and 328 (mg of KOH)/g, respectively, and for AM2, 459, 471, and 470 (mg of KOH)/g, respectively. These results indicate that additive AM2 might contain a higher concentration of base in comparison with additive AM1. The effect of this observation is apparent from the data of bitumen A (more acidic bitumen) in Table 2. It appears that, at 0.5% additive concentration, very little of amine AM2 is still remaining at 140 °C compared to AM1, and hence, there are smaller decreases in percentages for AM2 than for AM1, when temperature changes from 140 to 150 °C. Increasing concentration of additives seems to be followed by an increase in TBN, as indicated in Figures 2 and 3. This is probably true, since the concentration of bases in the additives is much greater than that in pure bitumens. Statistical Analysis. The statistical analysis presented in this section is concerned with identifying what factors and their interactions might contribute significantly to the measured total base number of the amine/bitumen blends studied. The factors considered were bitumen type (two levels), additive type (two levels), additive dosage (four levels), storage time (three levels), and storage temperature (four levels). Analysis of variance (ANOVA) was applied to the overall data generated from the tests at a 0.05 level of significance. F-tests were used to identify the contributions of the factors and their two-way interactions. In other words, after determining whether a factor or interaction of factors is/are significant (p < 0.05), ANOVA was used to rank the effects and their interactions in order of importance (the higher the F-statistic, the more important is the factor). The results of this analysis indicated that all the factors and their two-way interactions were significant, except the binder/ amine and amine/temperature interactions. The analysis ranked the factors in terms of importance toward contribution to change in TBN as follows: additive concentration (F ) 1319) > amine type (F ) 316) > bitumen type (F ) 286) > time of storage
(F ) 105) > temperature of storage (F ) 102). The most important interaction was between amine type and additive concentration (F ) 61), and the rest of the interactions showed relatively low F-values (
