**4. Materials**

lower service temperatures. The binders produced at refineries without modification have a UTR of not more than 86°C, whereas the modified binder often have a UTR of more than 92°C. Therefore, UTR can be used as an indicator showing the degree of required modification and the cost needed for modification. As UTR increases, this cost increases accordingly [5].

The binder characteristics strongly influence the mechanical properties of asphalt mixture. To this point, binder should have a certain mechanical and rheological requirements as follows

• For homogeneous coating of aggregates, the bitumen should be fluid enough at mixing and

• To resist permanent deformation, the bitumen should be stiff enough at high temperatures

• To resist the cracking, the bitumen should be soft enough at lower temperature that pavement experiences (approximately down to −20°C depending on the local climate).

Accordingly, it can be concluded that obtaining bitumen to work well under all aforementioned conditions can be difficult. To surmount this problem, many researchers have tried to develop the asphalt pavement performance by improving the asphalt binder behavior using different modifiers. There are a large varieties of materials, which are often used for modifying the binder, of which polymers are widely known to be easy to use and cost effective. Referring to available literature (for example, see [6–9]), polymer addition may result in both a more flexible binder at low in-service temperature and enhanced properties at high in-service temperature, which significantly prevent the pavement from being deformed. They also improve the adhesive bonding to aggregate particles [10]. Polymers can exist in two dif-

• Amorphous, in which molecules are randomly oriented within the polymer when the material is cooled in a relaxed state. The cooled state of amorphous materials is highly similar to their molten state. The only difference between these two states is the molecules' distance. These polymers can easily be altered in shape and generally exist in a rubbery state.

• Semicrystalline is an arrangement of ordered molecules with some amorphous regions. As the semicrystalline polymer cools, a portion of the molecular chains forms crystals by folding up into densely packed regions. The polymer is classified as semicrystalline, if more than 35% of the polymer chain forms these crystals. These polymers are stiff and exist in a glassy state. The degree of crystallinity in a polymer is affected by different factors such as polymer type, additives, and cooling rate. The morphology and degree of crystallinity significantly influence the polymers' properties. Polymers with high degree of crystallinity have a higher glass transition temperature and higher modulus, toughness, stiffness, tensile strength, and hardness. In addition, they have more resistance to solvents but are less resistant to impact strength [11].

**3. Polymer modifiers**

112 Modified Asphalt

in order to fulfill the pavement criteria:

construction temperatures of about 160°C.

(about 60°C depending on the local climate)

ferent morphologies while in a solid phase:

#### **4.1. Bitumen**

Bitumen of C320 was used as a base material for this research, which was kindly supplied by Boral Ltd. This bitumen corresponds to the most common bitumen used in Australia. C320 is classified and manufactured in accordance with AS 2008 (2013) and is suitable for medium to heavy asphalt applications as well as for heavy duty and hot climate seals. The typical characteristics of bitumen C320 are presented in **Table 1**.

As emphasized earlier and shown in **Figure 3**, the complex structure of bitumen is composed of unsaturated structures divided into two main groups of asphaltenes (which are insoluble in n-heptanes) and maltenes. The maltenes are further split into saturates, aromatics, and resins. The proportion of bitumen fractions and the molecular weight of each fraction is presented in **Table 2**.


**4.2. HDPE**

Among plastics, polyethylene (PE) forms the largest portion followed by polyethylene terephthalate (PET). To this point, this study focused on polyethylene and particularly on highdensity polyethylene (HDPE). The HDPE used in this research were obtained from plastic recycling plant. As shown in **Figure 4(a)**, the HDPE is in the granular form with the particle size of 2.36 mm. HDPE like other plastics is a polymer consisting of very large molecules made up of smaller units called monomer, which are joined together in a chain by a process called polymerization. Polyethylene is semicrystalline material with a wide range of properties and appropriate resistance to chemicals and fatigue. A molecule of polyethylene has a very simple structure and is composed of a long chain of carbon atoms with two hydrogen atoms attached to them, as shown in **Figure 4(b)**. Sometimes other elements such as oxygen, nitrogen, chlorine, or fluorine are attached to these polymer molecules. These are lightweight molecules with low moisture absorption rates and good resistance to organic solvents.

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HDPE is one type of thermoplastics. As most of the thermoplastics can soften at temperature ranging from 130 to 140°C with no gas emission in the temperature range of 130–180°C, they can be a potential option for blending with bitumen in asphalt production because the heating temperature for bitumen ranges from 155 to 165°C in the whole processes for asphalt pavement construction [23]. **Table 3** presents the information regarding the thermal behavior of

In order to find the relevant properties of HDPE, some tests were conducted on this material

Crumb rubber used in this research was obtained from tyre recycling plant that processes the car tyres into crumb rubber through the ambient grinding method. The crumb rubber was

The particle distribution test was performed on crumb rubber. The result of sieve analysis is presented in **Table 5**. The ground tyre rubber has a particle size averaged between 8 and 50

polyethylene, which emphasizes its suitability as a binder modifier.

and the result of these test are presented in **Table 4**.

provided in granule form, as shown in **Figure 5(a)**.

**Figure 4.** Analyzed material (a) HDPE and (b) chemical structure of HDPE.

**4.3. Crumb rubber**

mesh (2.36 and 0.300 mm).

**Table 1.** Characteristics of the original bitumen.

**Figure 3.** Main compounds in representative structures of the four bitumen fractions [15].


**Table 2.** Proportion and molecular weight of bitumen chemical fractions [3, 16–19].

The rheological properties of bitumen are highly affected by the asphaltene content due to its physical parameters such as glass transition and bitumen viscosity [20, 21]. An increase in the asphaltenes content will generally result in harder bitumen with a lower penetration, higher softening point, and higher viscosity [22].

### **4.2. HDPE**

Among plastics, polyethylene (PE) forms the largest portion followed by polyethylene terephthalate (PET). To this point, this study focused on polyethylene and particularly on highdensity polyethylene (HDPE). The HDPE used in this research were obtained from plastic recycling plant. As shown in **Figure 4(a)**, the HDPE is in the granular form with the particle size of 2.36 mm. HDPE like other plastics is a polymer consisting of very large molecules made up of smaller units called monomer, which are joined together in a chain by a process called polymerization. Polyethylene is semicrystalline material with a wide range of properties and appropriate resistance to chemicals and fatigue. A molecule of polyethylene has a very simple structure and is composed of a long chain of carbon atoms with two hydrogen atoms attached to them, as shown in **Figure 4(b)**. Sometimes other elements such as oxygen, nitrogen, chlorine, or fluorine are attached to these polymer molecules. These are lightweight molecules with low moisture absorption rates and good resistance to organic solvents.

HDPE is one type of thermoplastics. As most of the thermoplastics can soften at temperature ranging from 130 to 140°C with no gas emission in the temperature range of 130–180°C, they can be a potential option for blending with bitumen in asphalt production because the heating temperature for bitumen ranges from 155 to 165°C in the whole processes for asphalt pavement construction [23]. **Table 3** presents the information regarding the thermal behavior of polyethylene, which emphasizes its suitability as a binder modifier.

In order to find the relevant properties of HDPE, some tests were conducted on this material and the result of these test are presented in **Table 4**.

#### **4.3. Crumb rubber**

The rheological properties of bitumen are highly affected by the asphaltene content due to its physical parameters such as glass transition and bitumen viscosity [20, 21]. An increase in the asphaltenes content will generally result in harder bitumen with a lower penetration, higher

**Description**

bitumen

asphaltenes determines the structural character of the

**Figure 3.** Main compounds in representative structures of the four bitumen fractions [15].

**Molecular weight**

**Characteristics Unit Methods Value** Softening point °C AS 2341.18 52 Penetration at 25°C dmm As 2341.12 Min 40 Flashpoint °C AS 2341.14 Min 250 Viscosity at 60°C Pa·s AS 2341.2 320 Viscosity at 135°C Pa·s As 2341.2 0.5 Specific Gravity Kg/m<sup>3</sup> AS 2341.7 1.03

Aromatics 40–65% 300–800 Major dispersion medium for asphaltenes

Saturates 5–20% 300–600 Non-polar viscous oil

**Table 2.** Proportion and molecular weight of bitumen chemical fractions [3, 16–19].

Asphaltenes 5–25% 600–3000 Substantial effect on bitumen rheological properties Resins 15–25% 500–1300 Dispersing agent for asphaltenes; their proportion to

softening point, and higher viscosity [22].

**Fraction Proportion of the** 

**overall bitumen**

**Table 1.** Characteristics of the original bitumen.

114 Modified Asphalt

Crumb rubber used in this research was obtained from tyre recycling plant that processes the car tyres into crumb rubber through the ambient grinding method. The crumb rubber was provided in granule form, as shown in **Figure 5(a)**.

The particle distribution test was performed on crumb rubber. The result of sieve analysis is presented in **Table 5**. The ground tyre rubber has a particle size averaged between 8 and 50 mesh (2.36 and 0.300 mm).

**Figure 4.** Analyzed material (a) HDPE and (b) chemical structure of HDPE.

#### 116 Modified Asphalt


**Table 3.** Thermal characteristics of polyethylene [23].


The properties of crumb rubber are presented in **Table 6**, which are obtained from conducting relevant tests on crumb rubber. It should be noted that tyre rubber is typically a composition of three polymers including polyisoprene (natural rubber), polybutadiene, and polystyrene-

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117

In this research, different polymers (i.e., bitumen, HDPE, and crumb rubber) were analyzed based on their calorimetric curve, thermal transition, and their overall quality. The analysis of the materials was performed by means of thermogravimetric analysis (TGA), differential

For performing analysis on polymers, the samples were prepared based on the requirements of equipment. In order to study the thermal behavior of individual polymers, in all cases, a small amount of material (5 to 10 mg) was placed in a measuring pan. To prevent pressure buildup during the test, it was advised to have a small opening in the small pan. After this

It is expected that the addition of polymers influences the microstructure of binder. In theory, the addition of polymers containing hard segments provides higher strength, whereas the soft segment polymers improve toughness and low-temperature cracking. Since the binder modification depends on the compatibility of bitumen and polymer as modifier, this chapter covers the study of the individual polymers to identify some of their physical and chemical properties, their thermal behavior, and their microstructure. The tests to characterize the analyzed materials were performed in the Advanced Materials Characterization Facility (AMCF) at Western Sydney University, Australia. These tests included thermal analysis, structural characterization, and microstructure analysis, and the main points of testing procedure are represented in the following sections.

Thermal analysis corresponds to a group of techniques used to measure the physical and chemical properties of materials as a function of temperature. The measurements can be performed

butadiene [24]. The main compounds in rubber are shown in **Figure 5(b)**.

**Characteristics Unit Methods Value** Density g/cm<sup>3</sup> AS 1141.5 0.982 Size mm AS 1141.11.1 1.18–2.36 Water absorption % AS 1141.5 0.1

scanning calorimetry (DSC), and scanning electron microscopy (SEM).

preparation, the samples were placed in DSC or TGA equipment.

**5. Methodology**

**Table 6.** Characteristics of crumb rubber.

**5.1. Sample preparation**

**5.2. Analysis methodology**

*5.2.1. Thermal analysis*

**Table 4.** Characteristics of high-density polyethylene (HDPE).

**Figure 5.** Analyzed material (a) crumb rubber and (b) chemical structure of rubber.


**Table 5.** Particle size distribution of crumb rubber.


**Table 6.** Characteristics of crumb rubber.

The properties of crumb rubber are presented in **Table 6**, which are obtained from conducting relevant tests on crumb rubber. It should be noted that tyre rubber is typically a composition of three polymers including polyisoprene (natural rubber), polybutadiene, and polystyrenebutadiene [24]. The main compounds in rubber are shown in **Figure 5(b)**.

## **5. Methodology**

**Characteristics Unit Methods Value** Density g/cm<sup>3</sup> AS 1141.5 0.963 Size mm AS 1141.11.1 2.36 Water absorption % AS 1141.5 0.0

**Characteristics Unit Methods Reported products**

, C<sup>2</sup> H6

Solubility in water — Nil — Softening temperature °C 100–120 No gas Decomposition temperature °C 270–350 CH<sup>4</sup>

Ignition temperature range °C > 700 CO, CO<sup>2</sup>

**Table 4.** Characteristics of high-density polyethylene (HDPE).

**Table 3.** Thermal characteristics of polyethylene [23].

116 Modified Asphalt

**Figure 5.** Analyzed material (a) crumb rubber and (b) chemical structure of rubber.

4 4.75 0.0 2.36 25.7 1.18 67.7 0.600 6.2 0.300 0.4 0.150 0.0 0.075 0.0

**Table 5.** Particle size distribution of crumb rubber.

**Sieve no. Sieve size (mm) Mass retained (%)**

In this research, different polymers (i.e., bitumen, HDPE, and crumb rubber) were analyzed based on their calorimetric curve, thermal transition, and their overall quality. The analysis of the materials was performed by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM).

#### **5.1. Sample preparation**

For performing analysis on polymers, the samples were prepared based on the requirements of equipment. In order to study the thermal behavior of individual polymers, in all cases, a small amount of material (5 to 10 mg) was placed in a measuring pan. To prevent pressure buildup during the test, it was advised to have a small opening in the small pan. After this preparation, the samples were placed in DSC or TGA equipment.

#### **5.2. Analysis methodology**

It is expected that the addition of polymers influences the microstructure of binder. In theory, the addition of polymers containing hard segments provides higher strength, whereas the soft segment polymers improve toughness and low-temperature cracking. Since the binder modification depends on the compatibility of bitumen and polymer as modifier, this chapter covers the study of the individual polymers to identify some of their physical and chemical properties, their thermal behavior, and their microstructure. The tests to characterize the analyzed materials were performed in the Advanced Materials Characterization Facility (AMCF) at Western Sydney University, Australia. These tests included thermal analysis, structural characterization, and microstructure analysis, and the main points of testing procedure are represented in the following sections.

#### *5.2.1. Thermal analysis*

Thermal analysis corresponds to a group of techniques used to measure the physical and chemical properties of materials as a function of temperature. The measurements can be performed in different atmospheres including inert atmosphere (nitrogen, argon, helium) or in an oxidative atmosphere (air, oxygen). The gas pressure can also selectively vary in thermal analysis. In this research, the thermal behavior of materials was investigated through DSC and TGA.

decomposition was verified in 5 mg samples in an aluminum crucible under air flow (100 mL/min) heated from 30 to 600°C at a heating rate of 10°C·min−1. The TGA curves and its differential (DTG) were carried out in an NETZSCH STA-449C thermogravimetric analyzer. The onset

Evaluation of Structural and Thermal Properties of Rubber and HDPE for Utilization as Binder Modifier

The microstructure of the materials was investigated under scanning electron microscope (SEM). Scanning electron microscopic analysis was done in 6510LV SEM employing between 10 and 20 kW. The specimens in this study were examined with magnifications of 100–1000, and the results of this study with best magnification are presented in the following sections.

) were

119

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), crys-

temperature of the mass loss effect (TO) and temperature of peak rate of mass loss (Tp

Heating the polymers results in a number of phase changes such as the glass transition (*T<sup>g</sup>*

location of these thermal parameters. In the DSC curves, the sharp peaks are related to the polymer melting and the areas under these peaks provide the heat of fusion (∆*H*). Furthermore, the smaller inconsistencies at the lower temperature are most likely related to the glass transition. In this research, DSC technique is used to investigate the transition temperatures and the crystallization degree of different polymers. Accordingly, the DSC curves were examined to evaluate the physical characteristics of individual materials. It should be noted that for DSC runs, the complete set of heating-cooling process were repeated three times for each polymer, where the first run is usually carried out to remove any impurities and moisture from the sample. In addition, in order to evaluate the transitions accurately, a temperature scan over a

In DSC analysis, the thermal parameters for bitumen depend on the refined petroleum source as well as the petroleum refining process. **Figure 6** shows the DSC thermograph of neat bitumen and its corresponding first-derivative curve. The effects detected in the thermograph, as

An increase in the heat capacity for neat bitumen can be observed in the DSC curve by an abrupt change in the slope of the curve placed in the low-temperature region (around −30°C)

frozen; therefore, the material becomes rigid and brittle at or below this temperature. The glass transition temperature of polymers is one of the most important parameters as it is related to the average molecular weight of polymers and hence provides information about their composition. Moreover, it demonstrates the viscoelastic behavior of polymers at low temperatures [31]. Therefore, the glass transition temperature of neat bitumen is believed to be closely related to the low-temperature performance of asphalts. As shown in **Figure 6**, the middle point of the temperature range where the transition occurs is considered as the

) is a material's temperature at which all molecular transitional motion is

), and melting point (*Tm*). DSC analysis is a useful technique to identify the

) of the bitumen. The glass transition

determined from TGA thermographs.

*5.2.1.3. Microstructure characterization*

**6. Results and discussion**

**6.1. Thermal analysis by DSC**

wide range temperature is considered for DSC analysis.

corresponding to the glass transition temperature (*T<sup>g</sup>*

assigned previously (for example, see [28–30]), are described below.

tallization transition (*T<sup>c</sup>*

temperature (*T<sup>g</sup>*

#### *5.2.1.1. Differential scanning calorimetry (DSC)*

Parameters such as glass transition temperature (*T<sup>g</sup>* ), melting point, and the degree of crystallization were monitored by DSC. It should be noted that the glass transition temperature is more important in polymer applications compared to the melting point, because it corresponds to the polymer behavior under ambient conditions.

In this research, DSC analysis was performed according to ASTM E473-85 in an NETZCH DSC 204 F1 to obtain the thermal critical points of materials. The test specimens weighing about 5 mg were heated up at different temperature ranges, depending on polymer type, in an aluminum crucible under an air flow (100 mL/min) at a rate of 10°C·min−1.

For bitumen, DSC experiments were carried out on about 5 mg samples in aluminum crucibles with perforated covers. Before conducting DSC, the bitumen sample was homogenized at 130°C for about 1 hour and then placed in the DSC equipment. To conduct DSC, first, the samples were cooled from room temperature to −100°C at a heating rate of −10°C/min. The samples were maintained at the low temperature for about 15 minutes to ensure a stabilized initial reading. Then, they were heated up to 200°C at a heating rate of 10°C/min. The DSC thermograph recorded during this heating scan is considered as the first scan. On completing the first scan, the sample was maintained at 200°C for 5 min to remove thermal history and then quickly cooled from 200°C to its starting temperature (−100°C) at a cooling rate of −10°C/ min and again held for about 15 min before being reheated to 200°C at a heating rate of 10°C/ min. The DSC thermograph recorded during this second heating scan is considered as the second scan. The same procedure was repeated to provide the third heating scan.

For rubber, similar to bitumen, three cycles of cooling and heating were considered as the method of the experiment with the same heating rate of 10°C/min, cooling rate of −50°C/min, and temperature range of −100–200°C. For HDPE, the DSC procedure was the same as bitumen and rubber with an exception of the temperature range, which was considered from −160 to 200°C.

Glass transition and melting point were measured from DSC curves. The percentage of crystallized fraction (CF) was determined from the following equation through dividing the observed melting enthalpy (∆*Hobs*) by the melting enthalpy of 100% crystalline material (∆*H<sup>o</sup>* ).

$$CF = \frac{\left\{\Delta H\_{sh} \times 100\right\}}{\Delta H\_{\circ}} \tag{1}$$

The values of ∆Ho depend on the material type and can be found in literature. For example, a value of 200 J/g was used by Lesueur et al. [4] and Claudy et al. [25]. Values of 180 and 121 J/g were used by Michon et al. [26] and Lu and Redelius [27], respectively.

#### *5.2.1.2. Thermogravimetric analysis (TGA)*

Thermogravimetric analysis (TGA) was performed to study the kinetics and to investigate the degradation process of materials at a higher temperature. In this study, the thermal decomposition was verified in 5 mg samples in an aluminum crucible under air flow (100 mL/min) heated from 30 to 600°C at a heating rate of 10°C·min−1. The TGA curves and its differential (DTG) were carried out in an NETZSCH STA-449C thermogravimetric analyzer. The onset temperature of the mass loss effect (TO) and temperature of peak rate of mass loss (Tp ) were determined from TGA thermographs.

#### *5.2.1.3. Microstructure characterization*

in different atmospheres including inert atmosphere (nitrogen, argon, helium) or in an oxidative atmosphere (air, oxygen). The gas pressure can also selectively vary in thermal analysis. In this research, the thermal behavior of materials was investigated through DSC and TGA.

lization were monitored by DSC. It should be noted that the glass transition temperature is more important in polymer applications compared to the melting point, because it cor-

In this research, DSC analysis was performed according to ASTM E473-85 in an NETZCH DSC 204 F1 to obtain the thermal critical points of materials. The test specimens weighing about 5 mg were heated up at different temperature ranges, depending on polymer type, in

For bitumen, DSC experiments were carried out on about 5 mg samples in aluminum crucibles with perforated covers. Before conducting DSC, the bitumen sample was homogenized at 130°C for about 1 hour and then placed in the DSC equipment. To conduct DSC, first, the samples were cooled from room temperature to −100°C at a heating rate of −10°C/min. The samples were maintained at the low temperature for about 15 minutes to ensure a stabilized initial reading. Then, they were heated up to 200°C at a heating rate of 10°C/min. The DSC thermograph recorded during this heating scan is considered as the first scan. On completing the first scan, the sample was maintained at 200°C for 5 min to remove thermal history and then quickly cooled from 200°C to its starting temperature (−100°C) at a cooling rate of −10°C/ min and again held for about 15 min before being reheated to 200°C at a heating rate of 10°C/ min. The DSC thermograph recorded during this second heating scan is considered as the

an aluminum crucible under an air flow (100 mL/min) at a rate of 10°C·min−1.

second scan. The same procedure was repeated to provide the third heating scan.

*CF* <sup>=</sup> (∆*Hobs* <sup>×</sup> <sup>100</sup>) \_\_\_\_\_\_\_\_\_\_

*5.2.1.2. Thermogravimetric analysis (TGA)*

were used by Michon et al. [26] and Lu and Redelius [27], respectively.

The values of ∆Ho

For rubber, similar to bitumen, three cycles of cooling and heating were considered as the method of the experiment with the same heating rate of 10°C/min, cooling rate of −50°C/min, and temperature range of −100–200°C. For HDPE, the DSC procedure was the same as bitumen and rubber with an exception of the temperature range, which was considered from −160 to 200°C. Glass transition and melting point were measured from DSC curves. The percentage of crystallized fraction (CF) was determined from the following equation through dividing the observed melting enthalpy (∆*Hobs*) by the melting enthalpy of 100% crystalline material (∆*H<sup>o</sup>*

∆*H<sup>o</sup>*

value of 200 J/g was used by Lesueur et al. [4] and Claudy et al. [25]. Values of 180 and 121 J/g

Thermogravimetric analysis (TGA) was performed to study the kinetics and to investigate the degradation process of materials at a higher temperature. In this study, the thermal

depend on the material type and can be found in literature. For example, a

), melting point, and the degree of crystal-

).

(1)

*5.2.1.1. Differential scanning calorimetry (DSC)*

118 Modified Asphalt

Parameters such as glass transition temperature (*T<sup>g</sup>*

responds to the polymer behavior under ambient conditions.

The microstructure of the materials was investigated under scanning electron microscope (SEM). Scanning electron microscopic analysis was done in 6510LV SEM employing between 10 and 20 kW. The specimens in this study were examined with magnifications of 100–1000, and the results of this study with best magnification are presented in the following sections.
