Polymer blends for enhanced asphalt binders

Polymer Blends for Enhanced Asphalt Binders

ABDELLATIF AIT-KADI, BRAHIM BRAHIMI,' and M O S T 0 BOUSMINA

Chemical Engineering Department Centre de Recherche en Sciences et Znghierie des Macromolkules

(CERSZM) Lava1 University

Ste-Foy, Quebec, Canada, G1 K 7P4

Straight asphalt binders have been modified by addition of both high-density polyethylene (HDPE) and a blend of HDPE and ethylene-propylene-diene-monomer (EPDM). The blend composition was fmed to 90/ 10 HDPE/EPDM to illustrate the possibility of adapting the polymer to be added to the asphalt binder for specific end-use applications. Linear viscoelastic properties of unmodified and polymer modified asphalts at concentrations ranging from 1 to 5 wt% were studied before and after Thin-Film Oven Test (TFOT) aging. Temperatures ranging from -15°C to 60°C were considered. Standard tests such as Ring-and-Ball softening point, Fraass breaking point and TFOT aging were also performed on the whole set of samples. It was found that addition of rubber-modified polyethylene (HDPE/EPDM) to the straight asphalt results in materials with enhanced overall properties, and most important, dispersed phase much more stable than the equivalent HDPE modified asphalt, mainly before TFOT aging. Good results were obtained for 1% HDPE/EPDM samples. Optimum design is, however, required for the desired properties to be obtained.

INTRODUCTION

sphalt binders for road paving experience a variety A0 f thermomechanical demands ( 1). Owing to their non-Newtonian nature, mainly in the low temperature region, their response to such demands may depend in a nonobvious manner on the history of the incurred deformation. Among the parameters that influence the rheological response of such materials is their chemical composition. Asphalts are generally consti- tuted of an oily phase composed of saturated hydro- carbons (paraffinics and maltenes), cyclic products (naphthenes and aromatics) and resins (polar aromatics); and a non-oily phase composed of asphaltenes, carbenes, etc. The ratio of asphaltenes to the other constituents (saturates, aromatics, and polar components) has a significant effect on the viscoelastic properties of asphalts (2, 3) and hence on their performance as road paving binders. The effect of temperature on the intrinsic properties of straight asphalts also has important consequences on their service properties (4). Rutting at high temperatures, crack initiation, and propagation in the low temperature region and other forms of pavement defects are due not only to steadily increasing traffic and size of truckloads (4) but also to the capability of the asphalt

I Present address: IRDI. 649 Prospect Blvd.. P.O. Box 518. Midland ON.. Canada L4R 4L3.

concrete to sustain temperature changes. Addition of natural or synthetic polymers to straight asphalts is known to impart enhanced service properties over a wide range of temperatures in road paving applications (5, 61.

A large number of research works have been de- voted to the study of the effect of these additives on the properties of such modified asphalts and their road performance. Several studies have confirmed the beneficial effects of polymer modification on asphalt binders (2, 4, 7-1 1): decreased thermal susceptibility and permanent deformation (rutting), increased fatigue, and low temperature cracking resistance. However, the major restriction encountered in such polymer modifications of asphalts remains the incompatibility of the modifying polymer and the asphalt matrix. In general, incorporating a polymer in an asphalt matrix results in a multiphase material with a great tendency to phase separate (6). The mechanisms and the kinet- ics involved in polymer-asphalt phase separation and stabilization are discussed in refs. 6 and 12. Steric stabilization by copolymer is known to be effective in such operations. This is the approach advocated in this work. The base polymer is a high-density polyethylene (HDPE) and the copolymer is an ethylene-propylene-diene monomer (EPDM). Ethylene-based copolymers are found to improve several properties of asphalt binders, such as temperature susceptibility

1724 POLYMER ENGINEERING AND SCIENCE, JUNE 1996, Vol. 36, No. 12


and resistance to rutting (14). The use of EPDM in this study serves two purposes: first as an emulsion sta- bilizer as reported for several copolymers of this type [see, for example, Hesp and Woodhams (6) and the references cited therein]; and second as an additive to enhance some other properties that cannot be im- proved by the use of polyethylene alone. A similar approach has been used in the literature with diblock or triblock styrene-butadiene-styrene (SBS) copolymers to enhance both low temperature crack resistance and high temperature rutting resistance of asphalts (15-17). The idea behind this approach is to adapt, through physical blending of existing polymers, the material to the end-use application. Mate- rial performance is discussed on the basis of rheological properties and morphology before and after aging. The importance of a more precise rheological characterization of asphalt binders is well recognized in the literature (1, 2, 4, 10, 11, 18-24). For discussion purposes, conventional tests such as softening point, Fraass point, and Thin film oven test are also performed on the materials.

EXPERIMENTAL

Materials

A 150-200 penetration grade asphalt provided by Ultramar Company is used as base material for polymer modification. The polymer used to modify the asphalt was a high-density polyethylene (HDPE, 25055N) provided by Dow Chemical, Canada, Inc. The copolymer used was, as mentioned earlier, an amor- phous ethylene-propylene-diene monomer (EPDM, Buna Huls AP54 1 containing 5 double bonds per 1000 C and 44% of propylene). Some properties of these products are reported in Table 1.

Blending

The blend of HDPE with the EPDM was prepared in the molten state using a twin-screw extruder (Haake Buchler System 40) at a rotating speed of 80 rpm and a temperature profile of 155°C 165"C, 175°C. and 175°C from the feed zone to the die. In order to obtain a homogeneous material, the blend was processed three times under the same conditions. The blend composition was 90/ 10 (HDPE/EPDM). This composition was chosen here only to illustrate the effective-

Table 1. Material Specifications.

Material Property Values

HDPE (25055N) Density (g/cm3) Tensile strength at yield (MPa) Tensile strength at break (MPa) Elongation (%) Melt flow index (g/lO min)

EPDM (AP541) Density (g/cm3) Tensile strength (MPa) Elongation at break (%) Ethylene content (%)

0.955 26 15

510 25

0.86-0.87 214

2300 50

ness of the modification. A more detailed study to optimize the composition of the additives is needed.

Polymer-modified asphalts were prepared using a mixing device designed in our laboratory. It consists in a jacketed reactor maintained at a constant temperature by a circulating oil and equipped with a high shear mixing element. Asphalts modified with 1, 3, and 5 wt% HDPE and HDPE/EPDM were prepared at 170°C and then stored in small metallic containers at -4°C.

Sample Preparation

Samples for rheological testing were prepared by rapidly heating the material in the container until it reached the liquid state and then pouring it on 25- mm-diameter disposable aluminum plates fixed to a special holding device. The samples and the holding device were then stored at -4°C. Testing is performed within the following 24 hrs.

CHARACTERIZATION

DJrrpmic Mechanical Analysis

Dynamic mechanical analyses were performed on all compositions using a Rheometrics System IV Rhe- ometer equipped with a convection oven and a liquid nitrogen cooling system. Rate sweeps were performed at frequencies ranging from 0.01 to 100 rad/s. Test temperatures were -15 ,O. 15,30,45 and 60°C. Strain sweeps were performed at all test temperatures to determine the linear viscoelastic (LVE) zone for each sample. Time sweeps were also carried out prior to rate sweeps to ensure that no structural modifications occurred during the time required for each test.

Morphological Analpi0

The morphologies of polymer modified asphalts were observed through a Zeiss optical microscope equipped with a Mettler FP-80 hot stage capable of maintaining a constant temperature. A drop of asphalt was placed between microscope slides and pressed into a thin film, which could be viewed by transmitted light.

softening Point

The softening point for each sample composition was determined according to the ASTM D36-76 (25-a). Two disks of asphalt, cast in shouldered rings, were heated at a controlled rate in water. The softening point is defined as the mean of the temperatures at which the asphalt disks soften and sag downward a distance of 25 mm under the weight of a standard steel ball (25).

Fraass Breaking Point

Fraass breaking point tests were performed according to the IP-80 standard (25-b). The Fraass breaking point is defined as the temperature at which a break appears on a thin layer of asphalt sprayed on an iron

POLYMER ENGINEERING AND SCIENCE, JUNE 1- Vol. 36, No. 12 1725

Abdellatif Ait-Kadi, Brahim Brahimi, and Most0 Bousrnina

T d 5 L 3 4

- , . I

a -

7 -

6 -

5 -

4 -

3 -

2 -

1 - I . : 5 % I 0 -4 -2 0 2 6

Log aTw (rad/s)

6

5

4 - 2 5

2 * 6

3

4

2

1

Fig. 1. Master curves for storage modulus, G’, and complex viscosity, T*. of unmodified and HDPE modifiid asphalt binders before TFOT aging. Reference temperature: 30°C.

slide. This slide is subjected to successive flexions under determined cooling conditions.

Thin-Film Oven Test (TFOT)

The aging of asphalt samples was performed according to the ASTM D1754-78 test (26). The objective of the test is to evaluate the effect of heat and air on the asphalt and to quantify the amount of volatile components in the sample. A 3-mm-thick, 50-g film of asphalt was heated in an oven for 5 hrs at 163°C. The effects of heat and air were determined from mass changes measured before and after the heat treat- ment.

5

6

7

6 ? d % 5

2 & 4

4 3

2

1

0

. 16

. 5

4 3

v z 6

4

3 2

2 . . v

I I

Log aTo (rad/s)

Fig. 2. Master curves for storage modulus, G’. and complex viscosity, T*, of unmodifiid and HDPE/EPDM modtfied asphalt binders before TFWT aging. Reference temperature: 30°C.

-4 -2 0 2 4 6 a

Log arw (rad/s)

Fig. 3 . Master curves for storage modulus, G’, and complex viscosity, TJ*. of unmodtfied and HDPE modtfied asphalt binders after TFWT aging. Reference temperature: 30°C.

RESULTS AND DISCUSSION Dynamic mechanical results at different tempera-

tures are presented in Figs. 1 and 2 for the storage shear modulus, G’, and the complex shear viscosity, q*, as a function of the reduced frequency, a+, where a, is the shift factor and o the imposed frequency. No correction for thermal expansion (vertical shift) was needed to construct the master curves at the reference temperature of 30°C. The data represented on these Figures are for unaged samples. It can be seen that the time-temperature superposition principle holds for these materials over the entire range of temperatures and frequencies. Some discrepancies are, however,

I 17

6

5

4

3

I n -4 -2 0 2 4 6 8

Log a,o (rad/s)

Fig. 4. Master curves for storage modulus, G’, and complex viscosity, q*, of unmodifiid and HDPEIEPDM rnodtfied asphalt binders after TFOT aging. Reference temperature: 30°C.



0.0 2.0 4.0 6.0 8 0 1 1

qs (X Pa s)

0

Rg. 6. Cole-Cole plot for unmodified and HDPE rnodifiid asphalt binders before TFOT aging. Reference temperature: 30°C.

Fig. 5. Mass percent loss after TFOT aging for control and polymer rnodifiid asphalt binders. Lines are for easy reading only.

observed in the low frequency region, especially for 5% HDPE-modified asphalt.

The results indicate that both unmodified and polymer modified asphalts show a viscoelastic behavior. The storage modulus data in the experimental frequency window of Figs. 1 and 2 show that the behavior of the tested materials is intermediate between that observed in the terminal zone where the storage modulus is proportional to w2 and the rubbery plateau characteristic of viscoelastic crosslinked or linear polymers of high molecular weight (27). The viscosity data show a shear-thinning behavior at high frequencies and tend towards the Newtonian plateau in the

low frequency region. The Newtonian viscosity in the low frequency region is, however, not completely reached for all samples. The results reported in Figs. 1 and 2 also show the effect of polymer concentration on the rheological behavior of the asphalts. The presence of both HDPE and HDPEIEPDM results in increased low frequency viscosity and storage modulus. For 1 % and 3% concentrations, the HDPE/EPDM is found to be more effective than HDPE alone. In the high frequency region, the effect of polymer is less pronounced particularly for the HDPE-modified asphalts. For the HDPE/EPDM-modified ones, a slight decrease of the storage modulus for the 1% concentration is

Table 2. Activation Energies (in kJ/mol) for Control and Polymer Modified Asphalts.

Polymer Concentration (YO) 0 I 3 5

Polymer Type Temp. ("C) log a, AH loga, AH log a, AH log aT AH

Before Aging HDPE -15 4.54 141.7 4.37 154.5 4.57 157.8 4.47 155.9 0 3.68 3.52 3.73 2.99 15 1.76 1.45* 1 .a8 2.20 30 0 0 0 0 45 -1.09 - 1.42 -1.13 -0.96 60 -1.21 -2.21 -2.20 -2.71

HDPVEPDM -15 4.54 141.7 4.71 158.3 4.60 152.2 4.95 171.4 0 3.68 3.40 3.13 3.92 15 1.76 1.20 1.74 2.04 30 0 0 0 0 45 -1.09 -1.27 -1.20 -1.36 60 -1.21 -2.30 -2.13 -2.40

-15 5.18 159.7 5.33 162.4 4.80 149.9 4.88 156.3 0 3.01 3.20 2.88* 3.15 15 1.22 1.26 0.92 0.86 30 0 0 0 0 45 -1.28 -1.13 -1.13 -1.07 60 -2.09 -2.06 -2.08 -2.30

HDPUEPDM -15 5.18 159.7 5.29 161.4 5.223 159.3 5.06 162.6 0 3.01 2.93 3.13 3.13 15 1.22 1 .oa 0.87 0.87 30 0 0 0 0 45 -1.28 -1.13 -1.16 - 1.20 60 -2.09 -2.25 -2.07 -2.45

After Aging HDPE

Value extrapolatad from the linear regrwsion using the other data points.

POLYMER ENGINEERING AND SCIENCE, JUNE 1996, Vol. 36, No. 12 1727

Abdellatif Ait-Kadi, Brahim Brahimi, and Most0 Bousrnina

20.0

17.5

15.0

2 12.5

0 10.0

I

0 I

Q. 7. Cole-Cole plot for unmodified and HDPEIEPDM modified asphalt binders before TFOT aging. Reference temperature: 30°C.

observed. This concentration of 1 % HDPEIEPDM seems to give promising results since it possesses higher G' and q* in the low frequency (or high temperature) region and lower G' and ?* in the high frequency (or low temperature) region. These are some features that are required for road paving applications. Indeed, in the high temperature region, higher viscoelastic properties are needed for enhanced rutting resistance, and in the low temperature region, lower modulus is needed to prevent crack initiation and propagation.

The effect of polymer on the viscoelastic properties of asphalts may be attributed to the presence of the more viscous polymer phase in the mixture and also to the solubility at high temperature of low molecular weight fractions of polymer in the asphalt. Diffusion of aromatics and saturates in the polymer phase also contributes to the increase of asphaltenes in the matrix, which results in increased rheological properties (2, 3).

Figures 3 and 4 show the corresponding LVE data after TFOT aging. The overall observations on the shape of the viscosity and the storage modulus are similar to those reported for the unaged samples. The viscoelastic properties of aged specimens are in general higher than those of unaged ones (see Figs. 1-4). These results are in agreement with the physics of aging processes. Temperature aging favors volatiliza- tion losses of low molecular weight constituents of asphalt, thereby increasing higher molecular weight components in the residual bitumen (3). This is clearly shown in Q. 5, in which we have reported the percent loss of weight as a function of the polymer content in the asphalt. The highest percent loss is observed, as expected, for the straight asphalt. For both polymer modifications, the percent loss reaches a constant value of approximately 0.36 for all polymer concentrations except for the 5% HDPE-asphalt composition, for which the result is suspicious and will not be discussed here.

5.0, , , , , . ,

4.0

I

d 4 3.0

: 3% : 5%

L %

T- * 2.0 L

c 1.0

0.0 0 0 5 0 1 0 0 150 200

7' (X P a s)

Fig. 8. ColeCole plot for unmodified and HDPE modified asphalt binders after TFOT aging. Reference temperature: 30°C.

5 . 0 , I 1 I 1

4.0

7 0,

d 3.0

P I 0 h * 2.0 L

c 1.0

0.0

3% . 5%

0.0 5.0 10.0 15.0 20.0

7' (X P a s ) Fig. 9. ColeCole plot for unmodijled and HDPE/EPDM modified asphalt binders after TFOT aging. Reference temperature: 30°C.

The decrease of volatile losses for polymer modified asphalts is generally attributed to the swelling effect of the polymer by the aromatics and the saturates of the asphalt phase ( 121. The swelling of the polymer can be determined for example by centrifugation at high temperature (6). The effect of polymer concentration and temperature, in the case of an SBS copolymer modified asphalt, was found to be relatively small and independent of the compatibility of the polymer-asphalt pair (2). Our results for HDPE and HDPE/EPDM samples confirm this effect of polymer concentration. Other aging phenomena such as oxidation and poly- merization may contribute to the change of the viscoelastic properties of aged specimens (1, 3, 11, 18). This will not be discussed here since no independent analyses were performed to quantify these aging phenomena.

1728 POLYMER ENGINEERING AND SCIENCE, JUNE 1- Vol. 36, No. 12


Table 3. Cole-Cole Parameters for Control and Polymer Modified Asphalts.

Polymer Concentration (YO) 0 1 3

After Aging

1.349 x 0.061 0.642 1.349 x 0.061 0.642 6.685 X 0.868 0.674 6.685 X 0.868 0.674

1 o4

104

1 o4

104

2.43 x 104 0.103 0.660 5.831 X lo4 2.009 0.671 6.842 X lo4 0.849 0.668 1.562 X 1 O5 7.122 0.639

6.573 X lo4 0.476 0.679 8.974 X lo4 0.525 0.621

0.887 0.643 1.314 X lo5 1.301 0.666

9.544 x 104

The horizontal shift factor, a,, used to obtain the master curves of Figs. 1-4 depends on temperature. In general, and depending on the temperature range, the variations of a, as a function of temperature may be represented by either an Arrhenius and/or a WLF type equation (27). In our case, the horizontal shift factor is found to be well represented by an Arrhenius type equation given by the following expression (27):

where AH is known as the activation flow energy, R the universal gas constant, T the current temperature, and To the reference temperature. Similar results are reported in the literature for a variety of polymer modified asphalts (2 , 10, 28). Activation energies before and after aging together with the shift factors at each temperature are reported in Table 2. The presence of polymer in the asphalt mixture results in an increase of the activation energy. This result has been reported in the literature for several polymer modified asphalts (10, 28). However, no specific trend of the effect of polymer concentration on this parameter can be extracted from the obtained results. Even though the physical significance of this parameter is still not clearly understood, it has been found to depend on a number of molecular characteristics such as molecular weight, molecular weight distribution, degree of branching, and the nature of the monomer unit (29, 30). The slight increase of the activation energy observed for most polymer concentrations indicates that, within this range of temperatures, the modified asphalts will be more sensitive to temperature changes than the unmodified one. However, this increase is not important enough to overshadow the other beneficial effects of polymer modification on the overall viscoelastic properties ofFigs. 1-4. This can be inferred from the values of the shift factors and the activation energies reported in Table 2.

The effect of polymer addition on the rheological properties of asphalt binder can be even more high- lighted and discussed using the so-called Cole-Cole plots (31). Indeed, it is found that the representation of the viscoelastic data using the complex plane locus provides a more convenient basis for analysis and discussion of eventual departures of modified materi-

als from the control one (31). This approach was first applied by Cole and Cole (31) to analyze dispersion and absorption data in dielectrics. It consists of rep- resenting the components of the complex viscosity (q* = q‘ - i 7”) in the complex plane (q‘, 7”). The locus of the experimental data in this representation is found to be a circular arc [or a number of circular arcs (32)] having its center below the real 7’ axis. The experimental data of Figs. 1-4 are converted and represented using Cole-Cole plots (see Figs. 6 to 9). The complex viscosity expression corresponding to the circular arc is given by (31):

where qo and qm are the zero-shear and the high-shear rate viscosities, o the frequency at the reference temperature, r0 a characteristic relaxation time and a a fitting parameter that will be discussed later. The real and imaginary parts of the complex viscosity corresponding to Eq 2 are (3 1):

7‘ - q m

sinh(1 - alx

cash( 1 - a ) ~ + cos 1 /2 CYT

[ cash( 1 - a)x + sin 1 /2 an

-1 (31

-1 (4) cos 1/2 a7T

[ 1

= 5 ( q o - q m ) 1 -

1 2 ’I” = - (‘I0 - 77-1

where x = In ( 0 7 ~ ) . The cord sustained by the circular arc represents the value (qo - qJ with qo being the intercept with the real axis far from the origin and q, the intercept with the real axis near the origin. The results of Figs. 6-9 show that q, is equal to zero for all our samples. The zero shear viscosity can be easily extrapolated for both the unaged and the aged samples except for the 5% concentrations. The effect of polymer concentration on qo is clearly shown in this representation. As the concentration of the polymer increases, qo increases. The increase is much more pronounced for HDPE/EPDM modified samples. The numerical values of the zero-shear viscosity together with the other Cole-Cole parameters are reported in Table 3. The values for 5% polymer are not reported on this Table since the range of experimental data cannot

POLYMER ENGINEERING AND SCIENCE, JUNE 1996, Vol. 36, No. 12 1729

Abdellatif Ait-Kadi, Brahirn Brahirni, and Most0 Bousrnina

0 hour 3 hours 24 hours 28hours

0 hour 3 hours 24 hours

(B) Fig. 10. Optical micrographs for 3% HDPE (a) and HDPE/EPDM (b) rnodtfied asphalts at deerent times before TFQT aging.

perform reliable fits. In the case of aged HDPE/EPDM modified asphalts, the experimental data show the characteristic deviation from a single circular arc, in- dicating the existence of a second relaxation process in the low frequency region (32). Extrapolation of the experimental data to obtain the zero-shear viscosity in these cases should be considered with care. The existence of this relaxation process can be attributed to the oxidation and crosslinking of the unsaturated EPDM rubber during the aging process.

The other information that can be extracted from the Cole-Cole representation concerns the relaxation time T,, and the a parameter. The relaxation time T~ is equal to 1 /corn,, where cornax is the frequency at which 7’’ presents a maximum (see Eq 4). The results show that for unaged HDPE modified asphalts, this parameter increases monotonously. For unaged HDPE- EPDM modified ones, the relaxation time sharply increases for the 1% modified asphalt and then decreases for the 3% one. The aged HDPE modified

1730 POLYMER ENGINEERING AND SCIENCE, JUNE 1996, Vol. 3f3, No. 12


samples show almost no effect of polymer concentration (0%. 1%, and 3 % ) while the aged HDPE/EPDM ones show a trend similar to that observed for the unaged samples. The values of T~ are higher for aged samples for both HDPE and HDPE-EPDM modified asphalts. The a parameter varies from 0 to 1 . For (Y

equal zero, the rheological behavior is described by a Maxwell model with single relaxation time. Values greater than zero indicate the existence of a distribu-

0 hour 3 hours

0 hour 3 hours

tion of relaxation times ( 3 1 ) . The relaxation time T ~ , also called the terminal relaxation time, corresponds to the maximum of the distribution function. The values reported in Table 3 do not indicate any clear trend of the effect of both polymer concentration and aging on this parameter.

The use of EPDM copolymer in the asphalt is also aimed at stabilizing the polymer/asphalt mixtures. This is clearly shown on the optical micrographs of

24 hours Zhours

24 hours

03) Fig. 1 1 . Optical micrographs for 3% HDPE (a) and HDPE/EPDM (b) modijEd asphalts at dflerent times after TMlT aging.

POLYMER ENGINEERING AND SCIENCE, JUNE 1996, Vol. 3s, No. 12 1731

Abdellatif Ait-Kadi Brahim Brahimi, and Most0 Bousmina

80

70

60

50

5 4 0 .

2 30

-.\

.a

0

p 20 1 0 -

0 .

-10

-20

Figs. 10 and 11. Figure 10 represents the 3% HDPE and HDPE / EPDM modified samples before aging. These samples were maintained at a constant temperature of 160°C for a prolonged period of time. The polymer phase in the HDPE sample presents a gross morphology after only 28 hrs while the HDPE/EPDM specimen remains well dispersed after more than 48 hrs under the same temperature. Figure 11 shows the corresponding micrographs after TFOT aging. The presence of EPDM in the mixture results in a more stable emulsion. It should be noted, however, that phase separation problems are less pronounced for the aged HDPE sample compared with the unaged one (see Figs. 10a and 1 la). This can be attributed to the fact that aging, as mentioned earlier, results in increased viscoelastic properties, which may contribute to delay the phase separation phenomena. The fact that the HDPE mixture is still dispersed after TFOT aging indicates that the weight loss of volatile components happens rapidly in the aging process. We should mention here that these "pictures" should be used for purposes of comparison only. Other phenomena (such as asphalt components/glass slides interactions and/or polymer/glass slides interactions, and negligible creaming) may lead to different "pictures" for other aging experiments (e.g., paving or tank storage aging).

The Ring-and-Ball softening temperature and the Fraass breaking temperature of all the samples exam- ined in this study are reported in Figs. 12 and 13 for unaged and TFOT-aged specimens, respectively. For unaged specimens, addition of pure HDPE to the straight asphalt is found to monotonously increase both the Fraass breaking temperature (from -1 9°C for the control asphalt to -1 1 "C for the 5% HDPE modified one) and the softening temperature (from 38.2"C for the control asphalt to 6 1.1 "C for the 5% HDPE modified one) as can be seen in Fig. 12. The plasticity interval, defined as the difference between the softening temperature and the Fraass breaking temperature, is also found to steadily increase as the HDPE concentration increases (from 57.2"C for 0% to 72.1"C

'

.

.

.

.

'

659 : HDPE : HDPE/EPDM

70

- 1 0 7 2 3 4 5 6 Polymer Content ( w t x )

Fig. 12. Ringand-ball softening point and Fraass breaking point for unmodgid, HDPE, and HDPE/EPDM modlfed asphalt binders before TFQT aging.

901 . , , , . , . , . , . , . 1

- a: HDPE : HDPL?/EPDM

-- - 1 0 1 2 3 4 5 6

P o l y m e r Content (wt%)

Fig. 13. Ringand-ball softening point and Raass breaking point for unmodQled, HDPE. and HDPE/EPDM modtfied asphalt bfnders after TFOT aging.

for the 5% HDPE specimen). For the HDPE/EPDM samples, the Fraass temperature first decreases to -22°C for the 1 % concentration and then steadily increases for the 3% and the 5% samples but remains always below that of the corresponding HDPE modified samples. The softening temperature of HDPE/ EPDM samples also increases monotonously with increasing polymer concentration. The plasticity interval increases with an increase in the HDPE/ EPDM concentration. Its value is 6 to 7 degrees higher than that of the HDPE modified samples for the 1% and the 3% concentration. The 1% modified HDPE/ EPDM sample I s found to have an even larger plasticity interval than the 3% HDPE sample (66°C and 6 1.3% respectively). Again, this sample presents the most interesting results. The aging process is found to increase both the Fraass breaking temperature and the softening temperature for almost all the samples. The HDPE/EPDM modified samples surpass the corresponding HDPE modified ones for both the Fraass breaking temperature and the softening temperature. The plasticity interval is also larger for the samples containing EPDM (except for 5%).

The effect of the polymer additive on the softening point and the Fraass point has been reported in several studies on polymer modified asphalts. Serfass et al. (33) found that the softening point monotonously increases when the concentration of the added styrene-butadiene copolymer is increased. The Fraass point was also found to slightly decrease at the first 5% of added polymer and decrease more sharply at higher concentrations. However, no morphological study was reported in this work that confirms the state of the dispersion of the polymer in the asphalt mainly at high polymer concentrations. Jain et al. (34) also reported the same effect of polymer concentration on the softening point for a variety of polymer modifi- ers [SBS, ethylene-vinyl acetate (EVA), low-density polyethylene (LDPE) and hydroxy-terminated poly- butadiene (HTPB)] and three different types of asphalts. In most studies, the Fraass point is found to



decrease with increasing polymer concentration (33, 34), a result slightly different from what we have reported here mainly for HDPE. The interpretation of the effect of polymer on these two parameters is not obvi- ous. It may depend on several factors such as the nature and the composition of the asphalt itself, that of the polymer and the asphalt/polymer pair. More detailed studies are needed to understand such effects.

CONCLUSIONS

Tailoring the polymer to be used as an additive for asphalt binders may be considered an efficient route. Polyethylene (HDPE) blended with ethylene-propylene-diene monomer (EPDM) was found to impart enhanced overall properties to asphalt binders. For the level of modification (10% EPDM in HDPE) used here to illustrate this route, it is found that on the basis of linear viscoelastic properties, morphology, TFOT aging, Ring-and-Ball softening point, and Fraass breaking point, the 1 % HDPE/EPDM modified asphalt presents the most promising results. The presence of EPDM in the mixture is also found to stabilize the morphology before TFOT aging. Optimum design is needed to obtain a modified materials with desired end-use properties.

ACKNOWLEDGMENTS

The authors acknowledge the financial support provided by the Natural Sciences and Engineering Re- search Council of Canada (NSERC) and the Fonds pour la formation de chercheurs et 1’ aide a la recherche (FCAR) of the province of Quebec. The authors also acknowledge the technical support provided by Dr. J.C. Moreux and the Laboratoire des chausskes of the Ministhre des transports du QuCbec, where part of the standard tests has been carried out.

REFERENCES

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