Asphalt modified by thermoplastic elastomer based on recycled rubber

Construction and Building Materials 93 (2015) 678–684

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Asphalt modified by thermoplastic elastomer based on recycled rubber Shifeng Wang a,⇑, Qiang Wang b, Xiaoyu Wu a, Yong Zhang a a b

Research Institute of Polymer Materials, Shanghai Jiao Tong University, Shanghai 200240, PR China Department of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China

h i g h l i g h t s  Asphalt modified by TPE based on recycled rubber has been discussed.  Dynamic devulcanization and dynamic vulcanization improve the dispersion of GTR in the asphalt.  Asphalt modified by TPE has good processing property and high storage stability.

a r t i c l e

i n f o

Article history: Received 2 May 2015 Received in revised form 18 June 2015 Accepted 23 June 2015 Available online 26 June 2015 Keywords: Ground tire rubber Thermoplastic elastomer Dynamic vulcanization Modified asphalt Morphology

a b s t r a c t Thermoplastic elastomers (TPEs) based on ground tire rubbers (GTRs) and polyethylene were used for asphalt modification. TPEs were prepared by dynamic devulcanization (DD) in the presence of desulfurizer and dynamic vulcanization (DV) with sulfur, respectively. Structure and properties of TPEs and their modified asphalts were characterized by morphology, thermal, sol–gel content and mechanical testing. It was found that DD process decreased the physical properties, while the DV process strengthened that of TPE. The results also showed that the compatibility between PE and GTR was improved by DV, which further enhanced physical property and stabilized the TPE in the asphalt. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Recycling waste rubbers as asphalt modifiers has been considered as the most potential way from both economic and environmental points. However, the insoluble properties of recycled waste rubbers hindered their wide usage. The chemical crosslink structure of the recycled waste rubbers makes them act as elastic fillers, which causes the high initial cost and uncertain properties of asphalt modified by the rubbers. The uncertain properties arise from the degradation and settlement of ground tire rubber (GTR) during processing GTR modified asphalt. Thermoplastic elastomers (TPEs) are a class of copolymers or a physical mixture of polymers (usually a plastic and a rubber) which consist of materials with both thermoplastic and elastomeric properties. TPEs have attracts more attentions in industry since they combine the properties of vulcanized rubber with the ease processing of thermoplastics [1–6]. Styrene–butadiene–styrene copolymer (SBS) is a typical synthesized copolymer and the largest TPE being used as asphalt modifier because of its good ⇑ Corresponding author. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2015.06.047 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

compatibility with asphalt, workability and high performance. However, the poor oxidation resistance caused by the presence of double bonds in backbone of SBS and high cost are the main challenges before selecting it as asphalt modifier [7–11]. Therefore, more competitive TPEs are deserved to develop for asphalt modification. TPEs based on recycled tire rubbers have been widely investigated for the purpose of improving the performance and recycling tire rubbers. Karger-Kocsis et al. [12] have given a thorough review on processing GTR into TPEs. The biggest challenge of using GTR in TPEs is how to obtain favorable properties compared to those traditional TPEs. The use of GTR component leads to the deterioration of the mechanical properties of TPE because of the poor compatibility between the components. Devulcanization, surface activation, compatibilizer and dynamic vulcanization are considered as useful tools to improve the properties of GTR based composites [12–23]. Maleic anhydride and irradiation approach are also used to improve compatibility [12]. Magioli reported using dynamic vulcanization technology to enhance the properties of TPE [1]. Frequently, GTR was reclaimed before the process of dynamic vulcanization to further improve the homogeneous structure and physical properties of TPE.

S. Wang et al. / Construction and Building Materials 93 (2015) 678–684

The main purpose of above prepared TPEs based on GTR is to improve the physical properties. However, as far as we know, the TPEs based on GTR have not been widely used because of their low properties and odor. Therefore, it is quite necessary to try to use these TPEs as asphalt modifiers for the purpose of ease processing and balanced properties for asphalt pavement. The objective of this paper is to investigate the basic structure and properties of asphalt modified by TPE based on GTR. Dynamic devulcanization (DD) is firstly applied when GTR and LLDPE are blended in a twin-screw extruder. In this process, both thermo-shearing force and chemical desulfurizer (aryl disulfides) are used to partially de-vulcanize GTR and decrease the particle size of GTR. The blends are further dynamically vulcanized to improve the properties of TPE and its effect on the modified asphalt. 2. Experimental 2.1. Materials GTR (30 mesh) was obtained by grinding a whole waste truck tire. Linear low density polyethylene (LLDPE) with the melting index of 2.0 g/10 min (load of 2.16 kg, at 190 °C) was used. Desulfurizer was aryl disulfides. Processing oil was the engine oil with a viscosity of 46 centi-Poise (25 °C). The basic properties of asphalt were listed as following: softening point was 45.3 °C, penetration at 25 °C was 70.1 (0.1 mm). 2.2. Preparation of TPEs and their modified asphalts 2.2.1. Preparation of TPEs Dynamic devulcanization (DD) process: GTR was mixed with linear low density polyethylene (LLDPE) and devulcanized in a twin-screw extruder (L41/D1, ZE 25A). Blends were passed through the extruder at the screw speed of 200 rpm and 180 °C. Dynamic vulcanization (DV) process: The resulting DD blends were dynamically vulcanized on a two-roll mill at 150 °C for 5 min. Sulfur vulcanization system (S 1.5, ZnO 1.5, Stearic acid 0.3, and DM 0.5 phr) was used as well (see Table 1). 2.2.2. Preparation of TPEs modified asphalts Asphalt was heated till fluid in an iron container. Then upon reaching at about 180 °C, TPEs were added to the asphalts. The modified asphalts were blended by using a high shear mixer at 180 °C with a shearing speed of 4000 rpm for 30 min. 2.3. Characterization 2.3.1. Mechanical measurements Tensile and tear measurements were performed on a universal testing machine at room temperature with a crosshead speed of 500 mm/min, according to ASTM D412 and D624 respectively. Sheets of TPE were prepared by compression molding. The

Table 1 Recipes of different TPEs. Code

GTR (phr)

LLDPE (phr)

Oil (phr)

Desulfurizer (phr)

Sulfur (phr)

DD0 DD1 DV0 DV1

70 70 70 70

30 30 30 30

15 15 15 15

0 0.3 0 0.3

0 0 1.5 1.5

The dosage of desulfurizer was selected according to literature [21], phr means part per hundred rate.

679

dumbbell-shaped and un-nicked (90 °C) specimens were punched out. 2.3.2. Conventional properties and storage stability of the modified asphalts The conventional properties of modified asphalt like penetration, softening point and ductility were measured according to ASTM D5-06, D36-06, and D113-07, respectively. The high-temperature storage stability was evaluated through the following procedure. The samples of prepared blends were transferred into an aluminum toothpaste tubes (25.4 mm in diameter, 140 mm in height). The tubes were sealed and stored vertically in an oven at 163 °C for 48 h. After that, the samples were cooled down to room temperature. The aluminum tubes were then cut horizontally into three equal parts. The storage stability of the TPEs modified asphalt was evaluated by measuring the difference in softening points (DSP) between the top and bottom sections. If the value is less than 3.5 °C, the blend was considered as stable. 2.3.3. Structure analysis Sol–gel analysis was carried out by Soxhlet extraction of the samples with toluene at 110 °C. The samples were extracted for 24 h at atmospheric pressure. After that, the toluene was removed from the samples in an oven at 60 °C for 12 h. The gel fraction was calculated as

Gel-fraction ð%Þ ¼ W 1 =W 0  100%

ð1Þ

where W0 is the original weight of the sample before extraction, and W1 is the weight of dried sample after extraction. Swelling behavior was determined by immersing 0.1 g blends in 30 ml toluene for 72 h to reach swelling equilibrium. Every 12 h, the samples were taken out from toluene and the solvent was blotted from the surface of the sample then weighed immediately. Then, the samples were dried out at 60 °C after the constant weight was reached. The swelling ratio is calculated as

Swelling ratio ð%Þ ¼ ðW S  WÞ=W  100%

ð2Þ

where W is the original weight of the sample before swelling, and WS is the weight of sample containing toluene at the equilibrium. 2.3.4. Thermal analysis Differential scanning calorimetry (DSC) was performed on a differential scanning calorimeter under nitrogen flow. The tests were conducted as follows: the samples were heated from 40 to 200 °C, then cooled from 200 to 80 °C and reheated to 200 °C. The heating and cooling rate was 20 °C/min. To erase the differences in thermal history, the glass transition temperature was measured in the second and third steps. Thermo gravimetric analysis (TGA) was carried out using thermo gravimetric analyzer with nitrogen atmosphere. The temperature ranged from 40 to 600 °C at a heating rate 10 °C/min. Dynamic mechanical analysis (DMA) was implemented with a dynamic mechanical analyzer. The experiments were carried out using the rectangular samples and tension mode over the temperature ranging from 80 to 50 °C, at a heating rate of 3 °C/min. The samples were scanned at a frequency of 1 Hz, and a strain of 0.01% was applied. 2.3.5. Morphology Scanning electron microscopy (SEM) was conducted by observing the topography of the cryogenic fracture surface of the specimens, using a Scanning Electron Microscope. The samples were sputter coated with a fine layer of gold under vacuum for 60 s before scanning. Optical microscopy was used to observe the morphologies of the modified asphalts. A small drop of asphalt was placed between

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two heated microscopic glass slides and squeezed to form a thin film. The morphology of the asphalt ternary blends was observed under an optical microscope by using common light source with a magnification of 100 times. 3. Results and discussion 3.1. Mechanical properties

Fig. 1. Proposed reaction mechanism of devulcanization with desulfurizer, adapted from [21].

100 1# - DD0 2# - DD1

80

Swelling ratio (%)

TPEs unite the application properties typical of rubbers with the beneficial processing possibility of thermoplastics. TPEs are usually processed by dynamic vulcanization (DV). DV refers to a process of selectively vulcanizing an elastomer during its intimate melt mixing with a thermoplastic polymer leading to a two phase material in which particulate crosslinked elastomer phases are dispersed in a melt-processable plastic matrix. TPEs produced by DV require the rubber phase is finely dispersed in submicron scale in the thermoplastic matrix [12]. However, the size of GTR is in millimeter scale which is hard to disperse in thermoplastic matrix because of the cross-linked structure. Therefore, dynamic devulcanization (DD) is firstly attempted to improve the compatibility of the blends, then the DV is used to improve the physical properties of the TPEs. Table 2 shows the mechanical properties of GTR/LLDPE blends prepared by DD and DV. It can be seen that the desulfurizer makes worse the tensile strength of GTR/LLDPE blends during DD process carried out by screw extrusion. The desulfurizer is a radical scavenger which leads the crosslink point losing activity [21], as depicted in Fig. 1. Subsequently, the DV process is effective for improving the mechanical properties of blends. The permanent sets of the blends increase, which is due to the mechano-chemical interaction between GTR and LLDPE during processing. This is also proven in sol–gel analysis and thermal testing in the following parts. The possible mechanism of dynamically devulcanized blends with treatment of desulfurizer is shown in Fig. 1. Aryl disulfide reacts with the point of broken sulfur bond, which makes it lose the activity of recombination. It could also react with radicals generated by mechano-chemical effect [12]. Therefore, the efficiency of dynamic vulcanization for the improvement of compatibility is impaired.

3# - DV0 4# - DV1

60 40 20 0

1#

2# 3# Different TPEs

4#

Fig. 2. Swelling ratio of different TPEs prepared by DD and DV.

3.2. Structure analysis

70 1# - DD0

Table 2 Mechanical properties of TPEs prepared by DD and DV. Code

DD0 DD1 DV0 DV1

2# - DD1

Gel-fraction (%)

Swelling ratio is introduced to compare the evolution of network instead of cross-link density because of the complex compositions of GTR. The higher the swelling ratio is, the more network structure is damaged [12]. As shown in Fig. 2, the process of DV decreases the swelling ratio, which indicates that GTR is crosslinked again during the process of DV. The presence of desulfurizer had little influence on the swelling ratios of the blends after the processes of DD and DV. This is mainly caused by the formation of the continuous phase of LLDPE in TPE constrains the swelling of TPEs in toluene. The gel fraction of the blends is measured by Soxhlet extraction, which can be used to evaluate the degree of devulcanization and

3# - DV0

60 4# - DV1

50

40

1#

2# 3# Composites of the TPEs

4#

Fig. 3. Gel fractions of TPEs prepared by DD and DV.

Hardness (shore A)

Tensile strength (MPa)

Elongation (%)

Tear strength (kN/m)

Permanent set (%)

78 79 82 82

3.5 3.2 5.4 4.3

104.7 84.6 109.0 96.3

27.2 25.1 27.6 25.5

15 14 24 22

the mechano-chemical reaction in the DD process [12]. Fig. 3 shows the gel fractions of the blends prepared by DD and DV. The desulfurizer decreased the gel fraction of the devulcanized blends, which is due to the degradation and crosslinking of the main and side chains of the GTR. With the addition of desulfurizer, both the sulfur–sulfur bonds and the main chains of rubber were

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cut off. Consequently, the fragments of rubber can be dissolved into toluene. The gel fraction of TPEs increases after DV because of the crosslinking in the process of DV. The components of LLDPE and processing oil can be extracted by toluene. The original mass fraction of GTR is only 60.9%, which will become smaller if the soluble component (like aromatic oils) of GTR is considered. However, the gel fractions of the TPE after DV are over 62%. Therefore, there must be some LLDPE has reacted with GTR which is not extracted by toluene. This reaction is usually called mechano-chemical interaction between GTR and LLDPE, which is further proved by the following DSC analysis. 3.3. Thermal analysis Differential scanning calorimetry (DSC) is a testing method to detect the melting behavior of materials. LLDPE is a semi crystalline plastic with a melting peak around 120 °C, which can be considered as indicator represent whether LLDPE combine with GTR or not. DSC curves of different TPEs extracted by toluene are shown in Fig. 4. There are no melting peaks in extracted TPE blends which are processed by DD. On the contrary, there are melting peaks of LLDPE for the TPE blends processed by DV. It indicates that some LLDPE macromolecules have combined with the GTR because of the mechano-chemical reaction taking place in the process of DV. The decrease of melting peak of DV1 in the presence of desulfurizer means that the desulfurizer weakens the mechano-chemical effect. Actually, the mechano-chemical reactions also take place in the process of DD, while it is too weak to appear. There is a dynamic balance of building and breaking bonds between GTR

1#-DD 0-Extraction

and LLDPE during DD or DV. The chemical bonds are hard to be maintained in the blends during the process of DD. 3.4. Thermal gravimetric analysis TGA is a useful tool to clarify the decomposition behavior of the different materials, which reflects the interaction between the rubber and polyethylene in the TPEs blend. Fig. 5 shows the TGA curves of TPEs and the extracted TPEs, which reveals the interaction of GTR and LLDPE influences the degradation and carbonization of TPEs. It can be seen that the final residues increased slightly because of the DV process. It indicates that the process of DV makes the blend more facile to carbonization instead of degradation into volatile materials. As shown in Fig. 5(b), there is no apparent difference among the extracted blends. However, the presence of LLDPE in the extracted blends affects the mechanical properties and the compatibility between TPE and asphalt as shown in following section of improved mechanical properties and finely dispersed morphology. Therefore, this little proportion of LLDPE can be taken as compatibilizer for GTR/LLDPE system. 3.5. Dynamic mechanical analysis DMA test is introduced for the purpose of acquiring the dynamic mechanical properties of TPEs at different temperatures. Fig. 6 shows the storage modulus (E0 ) and the loss tangent (tan d) versus temperature plots for the blends. From Fig. 6(a), it can be seen that the addition of desulfurizer decreases the storage modulus of blends, while DV endows the TPE blends with higher storage modulus than those without DV. These results indicate that desulfurizer leads to an unrecoverable damage to the network structure of GTR, and DV bring structural integrity into the TPEs. The peaks of tan d move to higher temperatures after DV, as shown in Fig. 6(b). It implies that glass transition temperature of rubber increase, which proves the improvement of compatibility.

2#-DD 1-Extraction 3.6. Scanning electron microscopy

ENDO

3#-DV 0-Extraction

The morphology analysis is used to investigate the transformation of phase structure and the interaction between GTR and LLDPE during the process of DD and DV. Fig. 7 shows cryogenically fractured surface of different TPEs. It can be seen that all the blends had co-continuous phase structure. The fracturing surfaces of TPEs become rougher after DV. The DV blend without desulfurizer has rougher surface than that with desulfurizer, implying the mechano-chemical effect reinforces cohesive energy on the interface. The size of GTR particles reduce from 0.6 mm to micron scale, which makes the GTR evenly distributed in LLDPE matrix.

4#-DV 1-Extraction

0

50 100 o Temperature ( C)

150

Fig. 4. DSC curves of different TPEs extracted by toluene.

(a) 100

1# - DD0

(b) 100

Weight Percentage (%)

2# - DD1 3# - DV0

80

4# - DV1

60 40 20

0

100

200

300

400 o

Temperature ( C)

500

600

Weight Percentage (%)

-50

80

LLDPE GTR 1# - DD0 - Extraction

60

2# - DD1 - Extraction 40

3# - DV0 - Extraction 4# - DV1 - Extraction

20

0

100

200

300

400 o

Temperature ( C)

Fig. 5. TGA curves of TPEs and the extracted TPEs.

500

600

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(a)

(b) 1.00

1# - DD0

4000

2# - DD1

1# - DD0 2# - DD1

0.75

3# - DV0

3# - DV0 0.50

4# - DV1

Tan

E' (M Pa)

3000

2000

4# - DV1

0.25 0.00

1000

-0.25 0 -75

-50

-25

0

25

50

-0.50 -75

-50

o

-25

0

25

50

o

Temperature ( C)

Temperature ( C)

Fig. 6. Storage modulus and loss tangent of different TPEs.

(a) DD0

(b) DD1

(c) DV0

(d) DV1

Fig. 7. SEM microphotographs of cryogenic fracture of different TPEs.

3.7. Conventional properties of different TPEs modified asphalts The conventional properties of TPE modified asphalts are shown in Table 3. It can be seen that the softening points, penetration and ductility of different TPEs modified asphalts have a slight change. The DV process increases the softening points and decreases the penetration comparing with the DD process, indicating the improvement of high temperature resistance by DV. As shown in Table 3, the TPEs processed by DD exhibit high phase separation at high storage temperature, which are confirmed by DSP value of 12.0 °C. The DSP values obviously reduced to 2.0 °C after processed by DV. During DV process, the mechano-chemical reaction happens between GTR and LLDPE which binds them together. These interactions will help combine the polyethylene and GTR together, which prevents the TPE from phase separation. The density discrepancy between asphalt and TPE particles is also very important for the hot storage stability. The buoyancy of polyethylene and the settlement of GTR in the asphalt are cured

Table 3 Conventional properties and storage stability of different TPEs modified asphalts. Samples

Softening point (°C)

Penetration (25 °C) (0.1 mm)

Ductility (5 °C) (cm)

Viscosity (135 °C) (Pa s)

DSP (°C)

DD0 DD1 DV0 DV1

58.5 57.0 62.0 61.0

56.2 58.9 54.3 55.6

4.6 4.2 4.8 4.3

0.73 0.78 0.95 0.87

12.0 8.5 2.0 1.5

by the interaction between GTR and polyethylene which generates during DV. 3.8. Optical microscopy The morphologies of LLDPE and different TPEs modified asphalts are shown in Fig. 8. From Fig. 8(a), it can be clearly seen

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(a) 3% PE modified asphalt

top

bottom

(b) 11.5% DD0 (3PE+7GTR)

top

bottom

(c) 11.5% DD1 (3PE+7GTR)

top

bottom

(d) 11.5% DV0 (3PE+7GTR)

top

bottom

(e) 11.5% DV1 (3PE+7GTR)

top

bottom

Fig. 8. Optical microscopic images of the top and bottom sections of different TPEs modified asphalts.

that spherical LLDPE particles disperse in the yellow asphalt-rich phase. The LLDPE particles coalescence and flow to the top layer after hot storage, which indicates the immiscibility between PE and asphalt. After DD process, a different morphology is obtained

(Fig. 8(b)). The irregular shape of polyethylene and GTR are dispersed in the brown asphalt. After hot storage, most of polyethylene particles flow to the top layer, while the GTR particles go to the bottom. When the TPEs are processed by DD in the presence

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of desulfurizer (Fig. 8(c)), the GTR disperses finely in the asphalt, and the coalescence of polyethylene becomes to a less extent. Comparing with the hard dispersing ability of original GTR, the GTR of TPE has a fine dispersion in asphalt under 180 °C in 30 min, which indicates a good processing property of TPE. After process of DV, the spherical PE particles disperse in the brown bituminous phase and GTR particles also become smaller, indicating an improved compatibility by DV. The storage stability of the TPEs modified asphalts is significantly improved as shown in Table 2 and Fig. 8(d and e). These are more obvious in blends DV0 and DV1 with DSP value of 2 and 1.5 °C, respectively. In comparison of the morphology of Fig. 8(d and e), DV1 shows an interconnected network in Fig. 8(e). These morphological analyses support the assumption that DV process improves the hot storage stability in the modification system. 4. Conclusion Asphalt modified by TPEs based on recycled rubber and polyethylene was subsequently prepared by dynamic devulcanization and dynamic vulcanization. The stable and easy processing modified asphalt was realized by using the TPEs, and its physical and processing properties were totally different from that of the traditional rubberized asphalt. The original elasticity and strength of GTR could be effectively preserved to the TPEs owing to partial devulcanization. The addition of desulfurizer increased the degree of devulcanization, which decreased the mechanical strength and increased the dispersion of GTR in the blend. The DV process further enhanced the interaction between GTR and LLDPE, which obviously improved the hot storage stability of TPE modified asphalt. It was mainly due to the interpenetration and entanglement GTR and LLDPE during process. Acknowledgements The authors are grateful to the foundation from International Cooperation Project (2013DFR50550) and NSFC (51273110). References [1] M. Magioli, A.S. Sirqueiraand, B.G. Soares, The effect of dynamic vulcanization on the mechanical, dynamic mechanical and fatigue properties of TPE based on polypropylene and ground tire rubber, Polym. Testing 29 (7) (2010) 840–848. [2] Z.X. Zhang, S.L. Zhang, S.H. Lee, D.J. Kang, D. Bang, J.K. Kim, Microcellular foams of thermoplastic vulcanizates (TPEs) based on waste ground rubber tire powder, Mater. Lett. 62 (28) (2008) 4396–4399. [3] V. Sridhar, Z.Z. Xiu, D. Xu, S.H. Lee, J.K. Kim, D.J. Kang, et al., Fly ash reinforced thermoplastic vulcanizates obtained from waste tire powder, Waste Manage. 29 (3) (2009) 1058–1066.

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