976
ANALYTICAL CHEMISTRY, VOL. 50,
NO. 7,
JUNE 1978
Effect of Molecular Weight and Composition on the Glass Transition Temperatures of Asphalts Hon Kiet Huynh, Trung Dong Khong, Shadi La1 Malhotra, and Louis-Philippe Blanchard" D6partement de G6nie Chimique, Facult6 des Sciences et de GGnie, Universit6 Laval, Qugbec, QuB., Canada G1K 7P4
Glass transition temperatures ( T , ' s ) of an asphalt and its fractions obtained by prep-gel permeation chromatography were determined by DSC-2. Values decreased drastically from 17.5 to -63.3 OC as the molecular weight of the fractions increased from 500 to 800, then increased regularly to -53 OC as the molecular weight increased further to 3000. Composition rather than molecular weight of the fractions is believed to be responsible for the control of the T, values. The Van Krevelen theory relating T, to composition of polymers is applicable within limits to asphalt fractions as is the Gordon-Taylor theory for the calculation of copolymer T,'s which, with the asphalt fraction data gave a calculated value of -47 f 2 OC for the parent material while the experimental value was established at -38 f 4 OC.
The glass transition temperatures of asphalts have been the object of several studies (1-11). Schmidt et al. (1-3) were among the first to report on asphalt transitions. They used a modified differential thermal analyzer (DTA) capable of recording precise thermal expansions related directly to glass softening points ( Tgsp)and glass transition temperature ( T,). Breen and Stephens (6) considered asphalts as pseudopolymers and developed an empirical model to predict their T , values. The idea that asphalts might be treated like oligomers or low molecular weight polymers originates from viscoelastic studies (12-16) where both asphalts and oligomers show similar behavior patterns. The relations between the composition of asphalts and their T gvalues were reported on by Connor and Spiro (5),Noel and Corbett (3, Giavarini and Pochetti (8),Savu et al. (9, I O ) , and Quedeville ( I I ) , who showed that the T , values of asphalts increase with increasing percentage of asphaltenes and decreasing percentage of paraffinic carbon atoms. The mean value of the glass transition temperature of the saturated asphalt fractions (paraffins) was found to be -70 "C while the corresponding value for the naphthenic and aromatic fractions was about -40 "C (11). The effect of T gon the rheological properties of asphalts was studied by Stearns et al. (17),Majidzadeh and Schweyer ( I @ , Schweyer ( I 9 ) ,and Pechenyi et al. (20), who showed that, using the Williams-Landel-Ferry equation (21), absolute viscositites of asphalts could be computed from their respective glass transition temperatures. From this literature survey, one notes that, although glass transition temperatures have been related empirically to the molecular weights of asphalts, the T , values were only hypothetical (6). Furthermore, in other studies where T,'s were related to the composition of the asphalts, the Tg'swere measured by either differential thermal analysis (DTA) (5, 11) or by differential scanning calorimetry (DSC-lb) (7, I O ) , neither of which allows the measurement of precise values. In the present study, a more sophisticated instrument, the Perkin-Elmer DSC-2 was used to study the T, values of asphalt fractions, fractionation having been achieved (22) with a preparative-type gel permeation chromatograph (Prep-GPC). This instrument separates both low and high molecular weight products on the basis of their hydrodynamic volumes. The principal results related to the 0003-2700/78/0350-0976$01 .OO/O
Tgof asphalts are given in the following paragraphs. EXPERIMENTAL The data on the origin of the asphalt, its separation into various fractions by Prep-GPC and their characterization by analytical GPC, vapor pressure osmometry (VPO), nuclear magnetic resonance (NMR),infrared spectroscopy (IR) and element analyses has been reported elsewhere (22, 23). T,Measurements. Tg'swere measured with a Perkin-Elmer DSC-2 equipped with an autoscanning zero device which enabled perfect baselines to be obtained. Asphalt samples weighing 10 mg each were placed in standard DSC aluminum sample pans. For each Tgmeasurement, an empty reference pan was also used. The pans (both sample and reference) were closed with aluminum covers and sealed with a special sample pan press. The pans were then placed in the cell compartment of the DSC instrument, where a continuous stream of pure dry helium is made to flow at a rate of 20 cm3/min. The asphalt sample was then heated to a temperature of 30 to 40 "C for a few minutes to bring it to a homogeneous state. Following this the sample was cooled to -120 "C at a rate of 320 "C/min. Once cooled, the Tgof the sample was determined by heating the sample successively at several different heating rates. RESULTS AND DISCUSSION In Figure 1 are shown three typical DSC thermograms obtained with asphalt fractions 5,9, and 13. For the purpose of comparison, the T i s of asphalts were measured two different ways: (a) a t the half height of the observed ACp (the change in heat capacity a t the glass transition) (24-26) and (b) at the point of intersection of the baseline with the tangent to the endothermic curve as suggested by Griffiths and Maisey (27) (see Figure 1). One notes in this figure that the higher molecular weight asphalt fractions ( 5 and 9) show two other thermal zones, one exothermic representing crystallizations, the other endothermic characteristic of fusions. Asphalt fraction 13, the lowest molecular weight fraction shown yielded but a single endothermic peak related to fusion. According to Noel and Corbett ( 7 ) , the phenomena of crystallization and fusion are related to the presence in the asphalt of paraffins which are susceptible to cold crystallization. On heating to temperatures above T g ,molecular mobility increases sufficiently for the metastable amorphous state to revert to the crystalline state, which, in turn, subsequently melts a t yet higher temperatures. The absence of the crystallization peak in asphalt fractions of low molecular weight can best be explained by the presence of a smaller proportion of paraffins in these fractions. These observations agree well with those based on the structure parameter analyses reported earlier (23). In Table I are summarized the principal experimental Tg results obtained in this study. In Table I1 are also presented, for comparison purposes, some of the T, data available in the literature. The T , values were found to increase with decreasing molecular weight of the asphalt fractions (Table I, Figure 2). This is contrary to what was observed in the case of polystyrene (24) where Tgwas found to decrease with decreasing molecular weight; however, before attempting to draw any conclusions concerning a parallel between asphalts and polymers, one must keep in mind that, although the molecular weights of the latter can differ significantly, these 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 Table I. Influence of Heating Rate
q
977
("C/min) and Molecular Weight on T g Tg, " C
ii?,
Fraction
(VPO)
Mw
(GPC)
No.
(22)
(22)
5 6
2220 1520 1090 900
2950 1930 1290 1030 8 60
Baseline and tangent intersection method ( 2 7 ) q=80 q=40 q = 2 0 q=1"
-36.8 -42.8 -45.3 -41.3 -48.8 -41.3 -22.8 -9.8 +4.7 t21.2 +29.2
-41.5 -45.0 -47.5 -48.5 -51.0 -43.0 -25.0 -11.5 +3.0 +19.5 t27.5
-51.8 -54.7 -57.0 -57.5 -61.0 -53.6 -34.0 -19.0 -5.6 +8.5 +17.5
-46.0 -49.5 -50.0 -52.5 -53.5 -48.5 -36.5 -23.8 -12.5 +3.1 +15.5
-41.0 -51.0 -53.0 - 54.0 -55.0 -51.0 - 38.0 -25.7 -14.0 t2.1 + 14.1
- 50.5
t 2.0 t14.0
- 52.7 -55.6 - 63.3 -59.3 - 62.5 -57.9 -41.0 -26.7 -15.0 + 2.0 +13.0
-20.8
-24.0
-34.3
-30.8
-33.0
-34.8
-42.0
720 550 550 490 580 510
590 590 650 650
34.0 40.5 -42.5 - 44.0 - 45.5 - 38.5 -20.5 -1.5 + 8.0 .t 23.5 -t 32.0
750 (GPC)
1170
-18.5
I 8 9
170
10 11
12 13 14 15 Parent asphalt
Values a t ACp/2 ( 2 4 - 2 6 ) q=80 q=40 q=20 4'1"
740 65 0
" Extrapolated value using equation:
-
-
log q
=a-
-51.5 - 55.0 - 54.8 - 56.5 -52.0 - 38.5 - 25.8 - 14.0
b / T , ( 2 4 j.
Table 11. Glass Transition Temperatures of Different Asphalts
I 0
Asphalt origin
Technique of measurement used
California Venezuelan Kuwait Saskatchewan Venezuelan Alberta California Montreal
dilatometry dilatometry dilatometry DTA DTA DTA DTA
, , ,
1
1
, , , ,
DSC-2
,
, ,
, ,
,
Ref.
Tg ( " C ) - 13.0 to - 23.0
(3) (11) (11) ( 5 , 7) ( 1 , 7, 8 ) (7) ( 1j
-13.0 t o - 35.0 - 34.0 t o -42.0 26.0 to- 33.0 - 21.0 t o - 33.0 - 22.0 to - 32.0 -4.0 t o - 14.0 - 34.0 to - 42.0 ~
Present study
, , , 05
2
I
MOLECULAR WEIGHT
3 ( Xld')
Figure 2. T, (at 9 = 1) as a function of molecular weight. ( 0 )Mw and (0)M, (27); (A)M, and (A)M, (24-26)
Table 111. Structural Parameters for Various Asphalt Fractions ( 2 2 ) Fraction No.
5 6
I 8 9 11
12 13 15
,c,
%
54.7 55.6 61.7 65.3 58.2 50.9 43.9 37.7 39.8
cam %
23.0 20.0 15.0
13.0 19.0
22.0 29.0 33.0 32.0
Cnaph %
22.3 24.4 23.3 21.7 22.8 27.1 27.1 29.3 28.2
considerable interest t o study the changes in the Tgvalues of asphalt fractions with the percentage of paraffinic, naphthenic, and aromatic carbons determined experimentally and presented in Table 111.
978
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 t 20
,
I
,
I
-!
-
2o
0 -
E
-20
e
-
t
o t
I
-20
m
c
-40
-60
-- 4400
~
50 %
60
I5
c par
Figure 3. T, (at 4 = 1) as a function of % C.,
Cyclihexane Benzene Naphthalene
(0)(27);(A) (24-26)
/
i
O
Molecular weight
Mi 14 84 I8 128
25
35
Figure 4. T, (at 9 = 1) as a funcion of
YO C.,
(0)(27);(A) (24-26)
For an asphalt sample having a number of different functional groups, the overall T , must be computed from Equation 2 : FYgi
ygi
Tg = M
(Eq l )
(K.g/mol) Tgi, K Tgi (" C) 2700 25O0Oa 31 000 26O0Oa 32000 58000
193 369
-80 96
410 453
137 180
Values used in the present study after accounting for various possible interactions (29). In Figures 3 and 4 are shown, respectively, the variations of T gwith the percentage of paraffinic and aromatic carbon. One notes that Tg decreases as the % Cpar increases but increases with increasing % C,,,. It would appear that T g reaches a minimum value (-60 "C) when the asphalt fraction contains 58% Cpar, 19% C,,,, and 23% Cnaph. Variation of T gwith % Cnaphwas also carried out; however, no conclusion could be drawn from the data obtained. From this study one can relate the change of T , to the experimentally obtained molecular weight and also to the composition of the asphalt fractions. Calculation of T , Using Molar Glass Transition Functions for the Different Groups Present in Asphalts. Van Krevelen and Hoftyzer (28) have shown that the glass transition temperatures of polymers or asphalts can be calculated if the proportions of each of the different functional groups present in these substances are known. According to these authors (28, 29), the molar glass transition values Yg (K.g/mol) of the different functional groups e.g. methylene, benzene, etc., are related to the T g (glass transition temperatures) of these functional groups via their molecular weights. Thus
Yx . =
T
/
%Corm
Table IV. Theoretical T Values for Different Constituent Groups Base8 on Molar Glass Transition Function Values (28, 2 9 )
Me thvlene
;o
40
Functional group
t
Mi where i represents the given functional group. gr
(1)
(2)
where M is the weight average molecular weight of the asphalt fraction. Tgrvalues from the literature (29) of functional groups in asphalts are shown in Table IV. Using Equations 1 and 2 , other T , values were calculated and the results are likewise listed in Table IV where the order of the functional groups follows that of increasing values of T,. It may be seen in Table IV that the methylene groups (paraffins) have very low (-80 "C) Tg's when compared to those of aromatics or naphthenics. The low values of T gfor asphalt fractions would suggest that the contribution of paraffins towards T, is very important and it is probable that these are the principal cause for such transitions. In an earlier publication (23), numerical values for the different types of carbon atoms viz CBCH2, C, and Cmphwere computed. From the C,,, and Cnaphvalues, the number of aromatic and naphthenic rings were then calculated (30). These are presented in Table V. Using these in combination with the Ypi values in Table IV and &fw values in Table I, the Tg'sof a few fractions were computed and compared with those obtained experimentally (see Table V). The calculated and experimental T gvalues for all of the fractions show a substantial discrepancy which nevertheless falls within the predicted limits (29). The reason for these differences lies in the limited availability of data on Ygr(28, 29) to compute T,. Moreover, the calculations of the number of aromatic and naphthenic rings pertaining to this study also introduce errors. The results however do show that with increasing paraffinic in Table V) even the calculated Tgvalues chain length (CBCH2 increase, an observation already pointed out in the present study from the experimental data (see Figure 3 ) . T , Calculations for Parent Asphalt Based on the T, Values of Its Fractions. According to the theory of Gordon and Taylor (31, 32), the T , of a copolymer built up of two different monomers may be calculated from the individual T g of their homopolymers. Making use of the equations developed by these authors, one may also write a parallel
Table V. Comparison of Experimental and Theoretical T g Values Calculated from the Number of Paraffinic (CBCH,) and Naphthenic (Cnaph) Rings Carbon Atoms and Aromatic (C,) Aromatics Naphthenics Tg ("C), expl. Paraffinics, No. of No. of Tg ("C) No. CBCH, Cama rings (30) Cnaphb rings (30) calcd (24-26) (27) 7 9
10 11 13 a
cam = C R I
24 18
11 9 1
CRPC
19 18 18 15 19 C R P H (23).
4.0 3.5 3.5 3.0 4.0
22 14 13 12 13
Cnaph= C A N C H , 4- CANCH,
5.0 2.5 2.5 2.5 2.5
+
CBNCH,
- 45 - 38 - 25 -20 +13
+
CBNCH,
-57.0 - 61.0 -53.6 - 34.0 - 5.6 (23).
-63.3 - 62.5 -57.9 - 41 .O - 15.0
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
expression for asphalts as is shown in Equation 3:
C Q , W , ( T , - T g r ) =0
(3)
where T , is the glass transition of the parent asphalt, T,, that of one of its fractions i, W , the weight fraction of fraction i and Q, a characteristic constant of fraction i. Quedeville (11) has shown that the constant Q, has a value which is of the same order as that of the parent asphalt. Equation 3 thus may be rewritten as
c W , ( T , - T g r =) 0 1
(4)
With the help of Equation 4 and T , data of fractions 5 to 15 in Table I a value of -47 f 2.0 "C was calculated for the parent asphalt. T h e T , data for the high molecular weight fractions (1 to 4) and the low molecular weight fractions (16 to 23) was difficult to obtain and was not available for the above computations. It is assumed however that because of their identical weight proportions in the parent asphalt and their opposing T gvalues, the contribution due to these high and low molecular weight fractions would cancel out. The experimental values of -34.3 "C obtained by the ACJ2 method (24-26) and -42 "C using the method suggested by Griffiths and Maisey (27) suggest that because of the high aromatic and low paraffinic content in the lower molecular weight fractions, their contribution toward T gof the parent asphalt is more significant than that due to the higher molecular weight asphalt fractions. These results agree with those reported on the T g contributions of various substituents reported in the literature (28, 29).
CONCLUSION The main conclusions to be drawn from this study may be summed up as follows. (1)The T , values obtained with the DSC-2 instrument are more precise than those found with the DSC-1 instrument or other DTA techniques. (2) It is the composition of a given asphalt fraction which controls its TEand not the molecular weight; furthermore, the longer the paraffin chain in an asphalt fraction, the lower will be its T,. (3) Approximate T,'s of asphalt fractions may be calculated from the individual molar glass transition values of their various functional groups. as suggested by Van Krevelen (28, 29). (4) The Gordon-Taylor (31, 32) theory of T , calculations for copolymers, based on the T, values of the homopolymers,
979
may also be extended t o asphalts. Further studies on the thermal and rheological behavior of asphalts are underway. Results on this work will be reported in due course.
LITERATURE CITED (1) R. J. Schmidt and E. M. Barraii, J . Inst. Pet., 51, 162 (1965). (2) R. J. Schmidt and L. E. Santucci, Proc. AAPT., 35, 91 (1966). (3) R. J. Schmidt, R. E. Boynton, and L. E. Santucci, Am. Chem. SOC.,Div. Pet. Chem. Prepr., 11, 17 (1966). (4) P. du Bois, Bitumen, Teere. Asphalte, Peche, 17, 254 (1966). (5) H. J. Connor and J. G. Spiro, J . Inst. Pet., 54, 137 (1968). (6) J. J. Breen and J. E. Stephens, Proc. AAPT., 38, 706 (1969). (7) F. Noel and L. W. Corbett, J . Inst. Pet., 5 8 , 261 (1970) (6) C. Giavarini and F. Pochetti, Riv. Combust., 25, 256 (1971). (9) C. Savu, C. Giavarini, and I.Zirna, Pet. Gaze, 23, 153 (1972). (10) C. Savu, C. Giavarini, and I . Zirna, Pet. Gaze, 23, 418 (1972). (11) A. Quedeviiie, Bull. Liaison Lab., 81, 125 (1972). (12) J. G. Brodnyan, Nati. Res. Counci/(U.S.)Highway Res. Bd. Bull., 192 119581 \
- - - I
(13) F. H. Gaskins, J. G. Brodnyan, W. Phiiippoff, and E. Thelen, Trans SOC. Rheol.. 4. 265 (1960). (14) J. G. Brodnvan, F. H. &skins, W. PhiliDDOff, and E. Theien, Trans. Soc. .. Rheol., 4, 279 (1960). (15) Y . Wada and H. Hirose, J . Phys. SOC.Jpn., 15, 1885 (1960). (16) Y. Wada and H. Hirose, J . Appi. Phys. Jpn., 30, 40 (1961). (17) , . R. S. Stearns. I. N. Duiino. and R. H. Johnson. Ind. €no. Chem.. Prod Res. Dev., 5; 306 (1966). (18) K. Majidzadeh and H. E. Schweyer, Proc. AAPT., 38, 80 (1967). (19) H. E. Schweyer, Highway Res. Rec., 488 (1973). (20) B. G. Pechenyi, E. P. Zhelezko, and A. A . Akhmetova, Khim. Technoi. Top/. Masel, 2, 59 (1976). (21) M. L. Williams, R. F. Landei, and J. D. Ferry, J . Am. Chem. SOC.,77, 3701 (1955). (22) H. H. Kiet, L. P. Bhnchard, and S. L. Malhotra, Sep. Sci., 12, 607 (1977). (23) H. K. Huynh, DSc. Thesis, Lavai University, Quebec, (1978). (24) L. P. Blanchard, J. Hesse, and S. L. Maihotra, Can. J . Chem., 52, 3170 (1974). (25) Instruction Manual, Differential Scanning Calorimeter, Model DSC-2, Perkin-Elmer, Norwalk, Conn. 1974. (26) W. P. Brennan, Thermochim. Acta, 17, 285 (1976). (27) M. D. Griffiths and L. J. Maisey, Polymer, 17, 869 (1976). (28) H. G. Weyland, P. J. Hoftyzer, and D. W. Van Krevelen, Polymer, 11, 79 (1970). (29) D. W. Van Kreveien, "Properties of Potyrners", Elsevier, Amsterdam, 1976. (30) G. A . Haiey, Anal. Chem., 43, 371 (1971). (31) M. Gordon and J. S. Taylor, J . Appl. C'lem., 2, 493 (1952). (32) A. D. Jenkins, "Potymer Science", Voi. I, N M b Holhnd Publishing Company, Amsterdam, 1972.
-
RECER-EDfor review December 22, 1977. Accepted March 17, 1978. The authors gratefully acknowledge the financial assistance received from the National Research Council of Canada, the Department of Education of the Government of Quebec, and B.P. Canada Limited. The work described in this paper forms part of the general research program undertaken by the "Groupe de Recherches en Sciences MacromolBculaires" a t Lava1 University.
