Interfacial interaction between degraded ground tire rubber and polyethylene

Accepted Manuscript Interfacial interaction between degraded ground tire rubber and polyethylene Song Pan, Shuo Li, Shifeng Wang PII:

S0141-3910(17)30184-2

DOI:

10.1016/j.polymdegradstab.2017.06.020

Reference:

PDST 8273

To appear in:

Polymer Degradation and Stability

Received Date: 22 April 2017 Revised Date:

31 May 2017

Accepted Date: 19 June 2017

Please cite this article as: Pan S, Li S, Wang S, Interfacial interaction between degraded ground tire rubber and polyethylene, Polymer Degradation and Stability (2017), doi: 10.1016/ j.polymdegradstab.2017.06.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Interfacial interaction between degraded ground tire rubber and polyethylene Pan Song, ShuoLi, Shifeng Wang*

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Department of Polymer Science and Engineering, Shanghai Key Lab. of Electrical Insulation and Thermal Aging, Shanghai Jiao Tong University, Shanghai, 200240, China Abstract

The interfacial interaction between the linear low density polyethylene (LLDPE) and ground tire

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rubber (GTR) suffered from various degrees of degradation was investigated using the scanning electron microscopy (SEM), thermogravimetric analyses (TGA), differential scanning calorimetry

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(DSC) and mechanical testing. The effect of the maleic anhydride-grafted high density polyethylene (HDPE-g-MAH) on the interaction was also investigated. The results showed that the interfacial interaction was greatly influenced by the degradation degree of ground tire rubber and the presence of HDPE-g-MAH. During the melt process, a mechanochemical reaction between LLDPE and the degraded GTR was observed. The addition of the HDPE-g-MAH promoted a more homogeneous

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dispersion of degraded GTR in the LLDPE, and the thermal and mechanical performance of the composites was higher than those of without HDPA-g-MAH. The degraded molecular chains and the exposed carbon black reacted with the HDPA-g-MAH, which was confirmed by the Fourier

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transform infrared spectroscopy (FTIR).

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Keywords: polyethylene; ground tire rubber; interfacial interaction; light pyrolysis

*

Corresponding author. Tel: 86-21-54742671; Fax: 86-21-54741297 E-mail address: [email protected] 1

ACCEPTED MANUSCRIPT 1. Introduction Every year, 1.5 billion tires are manufactured and more than 500 million waste tires are discarded without any treatment [1-3]. How to economically, eco-friendly and properly dispose of these waste tires have presented a great challenge since the development of the auto industry [4-6].

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As a highly efficient and prosperous treatment, thermoplastic vulcanizates (TPVs) based on reclaiming tire rubber have been widely investigated due to the inherent crosslinking characteristics of GTR, low cost and attractive processing ability of TPV [7, 8].

The ideal TPVs must be required to meet the following requirements: 1) two components must

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possess good interfacial adhesion and the size of the dispersed phase domain must be of the micro size scale; 2) there must be no significant phase separation between the polymer components during

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processing and usage; and 3) the performance of the blends are different with each kind of polymers [9]. However, the presence of crosslinking prevents the GTR from finely dispersing at the micro-scale. Therefore, TPVs based on GTR and polyolefins have been frequently investigated with regards to compatibility, size reduction, phase structure, and mechanical performance. Compatibility of TPVs was first investigated based using original GTR. Zhang et al. [10] prepared

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asphalt/polypropylene/ground tire rubber composites processed by a screw extruder. They found that the presence of asphalt and compatibilizer (SEBS-g-MAH) improved the elongation at the break point, thermal stability and processability of the composites. Awang et al. [11] added N,

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N'-m-phenylenebismaleimide (HVA-2) and trans-polyoctylene rubber(TOR) to polypropylene/GTR composites, with the addition of these additives leading to the formation of chemical bonds at the interphase. Therefore, the tensile strength, solvent resistance and heat resistance of composite

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materials were greatly improved. Although the TPVs based on GTR and polyolefins have gained increased performance by addition of a compatibilizer, the adhesion between GTR and polyolefins remains weak because of the crosslinked characteristics of GTR. Many methods can be implemented to modify the GTR and solve this problem, such as surface coating [12, 13], surface treatment [14, 15] and devulcanization of the GTR. An efficient method for enhancing the interfacial interaction with polyolefins is to degrade it [16]. Twin-screw extrusion technology provides an economical, rapid and efficient process for manufacture of reclaimed rubber or liquid rubber [17-19], which can also be further blended with polyolefins for preparing the composite materials. Ahmad et al. [20] used liquid rubber as an additive 2

ACCEPTED MANUSCRIPT to increase the compatibility of natural rubber and LLDPE. Their results indicated that the mechanical properties of the blends were best with a natural rubber, liquid rubber and LLDPE ratio of 45/15/40. The liquid natural rubber obtained from the GTR after reclamation provided molecular chains with better flexibility, enhancing the interface binding force of the natural rubber and LLDPE

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[21]. In summary, the ability to reasonably crack the crosslinked structure and improve interfacial adhesion between the GTR and polyolefin plastic is a key factor in improving the performance of composite materials.

Polyolefins grafted with maleic anhydride (MAH) can be used as an effective compatibilizer for

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GTR and polyolefin plastic composites [22-24]. The grafted polyolefin is compatible with the plastic and the allyl group presenting on the side of a double bond in the natural rubber can react with the

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MAH via the action of radical initiation [25-27]. Therefore, using the polyolefin-grafted MAH as a compatibilizer can reduce the interfacial tension, improve the uniformity of the dispersed phase, decrease the size of the phase domain and maintain the stability of the blends [28]. Zhang et al. [29] added SEBS-g-MAH to the polypropylene/GTR composite materials and found that the butylene in the compatibilizer exhibited good compatibility with PP, and that the MAH could react with the

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hydroxyl groups of the GTR.

However, the mechanism of these interfacial reactions remains unclear because of the complicated composition and varied structural evolution of the GTR during processing. We have

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discovered the structure of degraded NR and GTR based on the preparation of light pyrolysis rubber at different temperatures. The structures of the sol fraction and gel fraction were characterized. Some functional groups are produced due to mechanical shearing and oxidation during the light pyrolysis

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process [30]. In addition, carbon black possessing a core-shell structure was separated from the light pyrolysis ground tire rubber at 300

, with the carbon black bearing many functional groups on its

surface [31, 32]. Therefore, the exploration of the interfacial bonding behavior between LPGTR and various polyolefins is relatively simple and high-performance composite materials can be prepared. In this work, the use of twin-screw extrusion technology led to the preparation of different degraded GTR at different reclamation temperatures. The light pyrolysis ground tire rubber was defined as LPGTR according to our previous study. Based on the decrosslinking mechanism of the GTR in the light pyrolysis conditions, LPGTR suffering from different degrees of degradation was blended with LLDPE and a compatibilizer (HDPE-g-MAH). The mechanism of the interfacial 3

ACCEPTED MANUSCRIPT interaction between the LPGTR and LLDPE was studied. Additionally, reactivity between HDPE-g-MAH and waste rubber or released carbon black was also researched.

2. Experimental

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2.1 Materials The ground tire rubber (GTR, 40 meshes) was obtained from whole truck tire, which was provided by Jiangsu Anqiang Rubber Co., Ltd. The composition of the GTR is presented in Table 1. Table 1 The composition of the ground tire rubber

Amount (%wt)

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Property natural rubber

40

carbon black inorganic filler soluble material

15

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synthetic rubber

30 8 7

Linear low density polyethylene (LLDPE) was supplied by China Petroleum & Chemical Co., Ltd. The density was 0.92 g.cm-3 and the melt flow index was 2 g.10 min-1.

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HDPE-g-MAH was provided by Shanghai Rizhisheng Material Co., Ltd. The grafting rate of the maleic anhydride was 1wt%.

2.2 Preparation

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2.2.1 Preparation of LPGTR

The GTR was extruded by an inter-meshed twin screw extruder (ZE25A, Berstorff GmbH,

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Germany). There were four heating zones in the twin screw extruder with an aspect ratio (L/D) of 41 and its diameter was 25 mm. The extrusion rate of the twin screw extruder was 5 Kg.h-1 and the screw rotation speed was set at 300 rpm. The GTR was extruded at 260, 280 and 300

.The

degraded GTR obtained at different temperatures was labeled as LPGTR-260, LPGTR-280 and LPGTR-300. The prepared samples were completely dried in a vacuum oven at a temperature of 50 for 2 h after extrusion. 2.2.2 Preparation of LPGTR/ LLDPE composites The LPGTR, LLDPE and HDPE-g-MAH were kneaded in a torque rheometer (HHAKE Polylab OS, Thermo Electron Gmbh, Germany) at a temperature of 150 4

. The Banbury rotor speed was 60

ACCEPTED MANUSCRIPT rpm and mixing time was 6 min. The samples were compressed in a vulcanizing machine (LP-S-50, Labtech Engineering Co., Ltd., Thailand) at a temperature of 150

for 5 min. Subsequently, the

samples were cold compressed for 4 min. In order to impart better elasticity to the composite, the rubber is often added in large quantities and the ratio of rubber to plastic is 2 to 1 [33]. Therefore,

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100 parts of LPGTR and 50 parts of LLDPE used to prepare composites and the LPGTR-260 and LLDPE composites were labeled as LPGTR-260/LLDPE. If different ratios of HDPE-g-MAH were added to the composites, it was marked as LPGTR-260/LLDPE/HDPE-g-MAH. For example, the LPGTR-260/LLDPE/7.5HDPE-g-MAH stood for 7.5 phr HDPE-g-MAH was added to the blends.

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The specific formulations are shown in Table 2.

Table 2 Formulations of LPGTR/LLDPE composites

LPGTR-260/LLDPE LPGTR-280/LLDPE LPGTR-300/LLDPE LPGTR-260/LLDPE/7.5HDPE-g-MAH LPGTR-280/LLDPE/7.5HDPE-g-MAH

2.3 Characterization

LPGTR-

LPGTR-

260

280

300

100

0

0

50

0

0

100

0

50

0

0

0

100

50

0

100

0

0

50

7.5

0

100

0

50

7.5

0

0

100

50

7.5

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LPGTR-300/LLDPE/7.5HDPE-g-MAH

LPGTR-

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Composites

LLDPE

HDPE-gMAH

2.3.1 Extraction of the blends

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Approximately 2 g of the composite materials were wrapped in filter paper, then placed in a Soxhlet extractor and extracted with acetone at 50

for 48 h to remove small polar molecules such

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as plasticizers and accelerators from the LPGTR. Subsequently, the sample was extracted with toluene at 110

for 72 h to eliminate nonpolar substances. The extracted samples were dried

completely in vacuum oven at 50

for 2 h. The sol fraction of LLDPE, LLDPE/7.5HDPE-g-MAH,

LPGTR-260, LPGTR-280 and LPGTR-300 could be obtained. According to the sol fractions of different components, the theoretical sol fraction of the LPGTR/LLDPE composites could be calculated and compared with the actual sol fraction of the composites in the extraction experiment. For example, the theoretical sol fraction of the LPGTR-260/LLDPE is calculated by the sol fraction of LPGTR-260 plus the sol fraction of LLDPE according to the respective weight ratio. The calculation of the actual sol fraction and theoretical sol fraction is shown in equation (1) and (2). 5

ACCEPTED MANUSCRIPT Actual sol fraction (wt%) = (m0– m1) / m0 *100

(1)

Theoretical sol fraction (wt%) =∑(siwi)

(2)

Where m0 is the mass of the sample before extraction and m1 is the mass of the dried samples after extraction. Where si is the sol fraction of the ith components and wi is the mass fraction of the ith

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components. 2.3.2 SEM measurements

Untreated samples: The section morphology of the sample was observed with scanning electron microscopy (SEM, S-2150, Hitachi High-Technologies Corp., Japan). The samples were cut into

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strips and placed in a liquid nitrogen container for 12 h, then then removed to simulate brittle fracture.

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Etched samples: The samples were cut into strips and placed in a liquid nitrogen container for 12 h, then fractured to a brittle state. After brittle fracture, the samples were extracted with the toluene for 24 h and were dried in a vacuum oven at 50 2.3.3 FTIR analysis

for 2 h.

Samples of the composite materials were characterized using FTIR (Spectrum 100, Perkin

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Elmer, Inc., USA), in the wavenumber range from 4000 to 250 cm-1. 2.3.4 TGA analysis

Thermogravimetric analysis (TGA, Q5000IR, TA Instruments, USA) of the composites was

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conducted under a nitrogen atmosphere. The samples were heated from room temperature to 800 °C at a heating rate of 10 °C.min-1.

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2.3.5 DSC analysis

DSC analysis (Pyris 1, Perkin Elmer, Inc., USA) was used to analyze the thermodynamic behavior and crystallization behavior of the composite materials. The samples, under nitrogen atmosphere, were heated from room temperature to 200 °C at a heating rate of 10 °C.min-1 and kept at this temperature for 10 min. Afterwards, the samples were cooled down to 25 °C and then the temperature was raised to 200 °C at 10 °C.min-1. 2.3.6 Tensile properties The dumbbell samples were measured on a universal electronic tensile machine (Instron 4465, Instron Corp., USA). The dumbbell shape samples of 75×4×1 mm3 was tested at 200 mm.min-1. 6

ACCEPTED MANUSCRIPT 3 Result and discussions 3.1 Interaction analysis of LLDPE/LPGTR composite by extraction The LPGTR/LLDPE composites were extracted with toluene to study the interfacial interaction of the composites, as shown in Table 3. It can be seen that the difference between the

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theoretical sol fraction and the actual sol fraction increased with increasing of extrusion temperature. The difference value signifies the bonding force between the different LPGTR and LLDPE. Compared with the difference value of LPGTR-260/LLDPE, the difference value of LPGTR-280/LLDPE increases by 4.7%, and LPGTR-300/LLDPE increases by 58.6%. This is due to

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the crosslinking network of LPGTR-260 being partially destroyed and most of the rubber being covered on the carbon black. However, the crosslinking network of LPGTR-280 and LPGTR-300

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was completely destroyed. Furthermore, the bound rubber on the surface of the carbon black was destroyed and the carbon black in LPR-300 was exposed [31]. A comparison of the difference value between LGTPR-260/LLDPE and LPGTR-280/LLDPE, shows that the degree of destruction of the cross-linked network has little impact on the binding force between LPGTR and LLDPE. However, the exposure of carbon black enhances the degree of bonding between LPTR-300 and LLDPE. Thus,

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this is an important role in the reaction process of LPGTR and LLDPE. The degree of binding between LPGTR and LLDPE is largely improved by the addition of 7.5 parts of HDPE-g-MAH. Compared to the blends with no compatibilizer, the actual sol fraction

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decreases largely and the difference value increases obviously. Because the HDPE and LLDPE have good compatibility and the grafted MAH reacts with LPGTR to a certain extent, the addition of

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HDPE-g-MAH significantly improves the compatibility between the LPGTR and LLDPE. Table 3 Extraction of LPGTR/LLDPE composites Theoretical sol

Actual sol

Difference value

fraction wt%

fraction wt%

wt%

LPGTR-260/LLDPE

45.3

24.3

21.0

LPGTR-280/LLDPE

55.7

33.7

22.0

LPGTR-300/LLDPE

66.7

33.4

33.3

LPGTR-260/LLDPE/7.5HDPE-g-MAH

40.8

14.0

26.8

LPGTR-280/LLDPE/7.5HDPE-g-MAH

50.5

18.9

31.6

LPGTR-300/LLDPE/7.5HDPE-g-MAH

67.5

36.2

31.3

Composites

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b

c

d

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a

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3.2 Interfacial analysis of the composite morphologies

f

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e

Fig. 1. SEM micrographs of the fracture surface of the LPGTR/LLDPE composites: (a) LPGTR-260/LLDPE (b)

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LPGTR-280/LLDPE (c) LPGTR-300/LLDPE (d) LPGTR-260/LLDPE/7.5HDPE-g-MAH (e) LPGTR-280/LLDPE/7.5HDPE-g-MAH (f) LPGTR-300/LLDPE/7.5HDPE-g-MAH

The fracture surface of the LPGTR/LLDPE composites was observed with SEM, as shown in the Fig. 1. It was found that the continuous matrix formed by the LLDPE and the LPGTR disperses in it. A comparison of the morphologies shown in Fig. 1(a), Fig. 1(b) and Fig. 1(c), suggests that a combination of LPGTR and LLDPE improve with the increasing of the extrusion temperature. As shown in Fig.1(a) and Fig. 1(b), the LPGTR/LLDPE composites presents a layered structure and the LPGTR particles is not seen clearly, which can be attributed to some rubber gel caused by the cross-linked network of the LPGTR is destroyed partially. Therefore, the weak interaction between 8

ACCEPTED MANUSCRIPT LPGTR particles and LLDPE causes LPGTR particles is embedded in LLDPE when the composites are cryogenically broken. Furthermore, as shown in Fig.1(c), the lamellar structure gradually disappears and a granular structure is gradually formed. This is a result of the rubber gel being converted to a sol fraction with increasing of extrusion temperature. In addition, the surface of the

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carbon black is exposed in the LPGTR, which possesses more active groups that produce certain interactions with the LLDPE.

As shown in the Fig. 1(d), Fig. 1(e) and Fig. 1(f), it can be seen that the dispersed phase becomes more uniform and the size of the rubber particles becomes smaller, which indicates that the addition

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of the HDPE-g-MAH significantly improves the compatibility between the LPGTR and LLDPE. This phenomenon can be explained by the HDPE and LLDPE having excellent compatibility and the

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grafted MAH reacting with the exposed carbon black. Because of increased interfacial interactions, the LPGTR covered with HDPE-g-MAH was exposed when the composite cryogenically broken as shown in Fig. 1(d), Fig. 1(e).

In order to further study the extent of the two-phase combination, the morphology of the brittle fracture samples etched with toluene were examined.

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It can be seen from the Fig. 2 that the sol fraction of the composite materials can be extracted in toluene and the cross-section shows a porous structure. It can be seen from Fig. 2(a) that the interaction between LPGTR-260 and LLDPE is limited to the material’s surface, which do not form a

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well dispersed two-phase structure. However, an interpenetrating network is present in the LPGTR-280/LLDPE (Fig. 2(b)) and LPGTR-300/LLDPE (Fig. 2(c)) composites. According to the magnified picture, the diameter of the porous structure is approximately 4-5 µm. The surface tends to

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be smooth after the addition of HDPE-g-MAH to the composites, which is due to the addition of the HDPE-g-MAH strengthening the interface binding force between the two phases.

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Fig.2. SEM micrographs of toluene-extracted samples of LPGTR/LLDPE composites: (a) LPGTR-260/LLDPE (b) LPGTR-280/LLDPE (c) LPGTR-300/LLDPE (d) LPGTR-260/LLDPE/7.5HDPE-g-MAH (e) LPGTR-280/LLDPE/7.5HDPE-g-MAH (f) LPGTR-300/LLDPE/7.5HDPE-g-MAH

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The crosslinking network is gradually destroyed following an increase in the extrusion temperature. The higher temperatures lead to an increase in the amount of sol fraction content and causes the exposure of carbon black which have a better combination with LLDPE. The addition of HDPE-g-MAH significantly enhances the compatibility of the composites. Additionally, the grafted maleic anhydride reacts with the devulcanized rubber chain and the hydroxyl groups generated in the reclamation process [34-36]. In particular, the carbon black in the rubber is exposed extensively [31, 32], and the hydroxyl groups on the surface of the carbon black can further react with the grafted maleic anhydride. In addition, the presence of a carboxyl group promotes the interaction between the LPGTR and the grafted maleic anhydride. 10

ACCEPTED MANUSCRIPT The reaction mechanism between the grafted maleic anhydride and LPGTR is shown in the Fig. 3. Firstly, during the degradation process, the molecular chains of the natural rubber and the synthetic rubber are oxidized to produce hydroxyl groups. These hydroxyl groups then react with the grafted maleic anhydride, as shown in Fig. 3(a) and Fig. 3(b). Carbon black is exposed in LPGTR-280 and

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LPGTR-300 during the extrusion process, and the hydroxyl groups on the carbon black surface react with the grafted maleic anhydride to produce new bonds. These reactions promote combination between LPGTR and LLDPE. The active groups present on the surface of the exposed carbon black play a critical role in the interaction of the composite components. H

C C CH2 H2C

H2 H2 C C C CH H

Oxidation

H3C

H

C C CH2 CH HO

Light Pyrolysis

C H2

Oxidation

H C

Light Pyrolysis

Ground Tire Rubber

H C C R2 H2

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R1

C

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b

O

O

C

+ R3

OH C H

CH

CH

Separation

Bound rubber O

OH

R3

O H H2 O C C C C OH R1

O

C C R2 H2 H

Fig.3. Schematic reaction between HDPE-g-MAH and LPGTR

11

H2 C

OH Carbon black

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C H2

H C

Light Pyrolysis

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H3C

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a

ACCEPTED MANUSCRIPT 3.3 FTIR spectra of the LPGTR/LLDPE composites

3500

3000

1712

1721

c d

1720

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1790

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a b

1720

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4000

LPGTR-260/LLDPE /7.5HDPE-g-MAH LPGTR-280/LLDPE /7.5HDPE-g-MAH LPGTR-300/LLDPE /7.5HDPE-g-MAH HDPE-g-MAH

Transmittance in arbitrary units

Transmittance in arbitrary units

915

1900

1800

2500

1700

Wavenumbers/cm-1

2000

1600

1500

1000

-1

Wavenumbers/cm

Fig. 4. FTIR spectra of composites added the HDPE-g-MAH

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To further confirm the reaction mechanism mentioned above, the interactions between HDPE-g-MAH and the LPGTR were studied using FTIR (Fig. 4). The spectrum in the range of 1500-1950 cm-1 was magnified to show the characteristic peaks. The absorption band at 1790 cm-1 corresponds to the carbonyl group of the maleic anhydride [37-39] and the band at 1712 cm-1 can be

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assigned to the carboxyl group of the hydrolyzed maleic anhydride [39]. The two absorption peaks can be seen clearly in the HDPE-g-MAH spectrum. However, the absorption peak at 1790 cm-1,

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corresponding to the carbonyl stretching vibrations of maleic anhydride in the HDPE-g-MAH, disappears. A new peak at 1720 cm-1 that corresponds to carbonyl stretching vibrations of the carboxylic acid or ester appears due to reaction between the LPGTR and HDPE-g-MAH. The reaction mechanism is shown in Fig. 3. In addition, the absorption peak at 914 cm-1 that corresponds to a -C-O-C- group [37] stretching vibration in the MAH also disappears, which indicates that a ring-opening reaction occurs in the MAH and further reacts with the LPGTR.

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ACCEPTED MANUSCRIPT 3.4 Thermogravimetric analysis of the composites 3.0 LPGTR-260/LLDPE

LPGTR-280/LLDPE

2.5

LPGTR-300/LLDPE

2

LPGTR-280/LLDPE/7.5HDPE-g-MAH

2.0

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LPGTR-300/LLDPE/7.5HDPE-g-MAH HDPE-g-MAH

1.5

LLDPE

460

1.0

0.5 360

370

380

390

200

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0.0 100

470

480

490

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Derive.Weight Change/in-1

LPGTR-260/LLDPE/7.5HDPE-g-MAH

300

400

500

600

700

Temperature/°C

Fig. 5. DTG curves of the LPGTR-260/LLDPE, LPGTR-280/LLDPE, LPGTR-300/LLDPE, LPGTR-260/LLDPE/7.5HDPE-g-MAH, LPGTR-280/LLDPE/7.5HDPE-g-MAH,

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LPGTR-300/LLDPE/7.5HDPE-g-MAH, HDPE-g-MAH and LLDPE.

Fig. 5 shows the derivative thermogravimetric curves that represent the thermal decomposition rates of the composites. Two decomposition peaks are present in the LPGTR/LLDPE composites.

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The temperatures at which the maximum rate of decomposition occurs (Tmax1 and Tmax2) are listed in Table 4. The first peak appears due to the decomposition of the LPR and the second peak is related to

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the decomposition of the LLDPE. It can be seen in Table 4 that Tmax1 of the LPGTR/LLDPE composites changes slightly with an increasing pyrolysis temperature for LPGTR. Similarly, when HDPE-g-MAH was added, the Tmax1 of the composites increased slightly. Tmax2 of the LPGTR/LLDPR composites is obviously higher than that of LLDPE, and it is further increased following the addition of HDPE-g-MAH. This phenomenon can be explained by the chemical interactions occurs between LPGTR and LLDPE with the addition of the HDPE-g-MAH, and the decomposition temperature of the HDPE is higher than LLDPE. It was also discovered that the Tmax2 of the composites decreases with increasing of the extrusion temperature, which can be ascribed to the decrosslinking low molecular chains and it has poor thermal properties. 13

ACCEPTED MANUSCRIPT Table 4 The decomposition temperatures of composites at different stages. Tmax1/ °C

Tmax2/ °C

HDPE-g-MAH

-

477.0

LLDPE

-

464.5

LPGTR-260/LLDPE

378.0

481.5

LPGTR-280/LLDPE

378.4

477.6

LPGTR-300/LLDPE

379.8

LPGTR-260/LLDPE/7.5HDPE-g-MAH

378.1

LPGTR-280/LLDPE/7.5HDPE-g-MAH

381.8

LPGTR-300/LLDPE/7.5HDPE-g-MAH

380.0

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Composites

474.6

481.7 480.7

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479.0

3.5 Characterization of the composites by DSC

HDPE-g-MAH LLDPE

Heat flow/W·g-1

Heat flow/W·g-1

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a

100

120 Temperature/

140

160

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80

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LPGTR-260/LLDPE LPGTR-280/LLDPE LPGTR-300/LLDPE LPGTR-260/LLDPE/7.5HDPE-g-MAH LPGTR-280/LLDPE/7.5HDPE-g-MAH LPGTR-300/LLDPE/7.5HDPE-g-MAH

80

100

120

Temperature/

14

140

160

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Heat flow/W·g-1

LPGTR-260/LLDPE LPGTR-280/LLDPE LPGTR-300/LLDPE LPGTR-260/LLDPE/7.5HDPE-g-MAH LPGTR-280/LLDPE/7.5HDPE-g-MAH LPGTR-300/LLDPE/7.5HDPE-g-MAH

HDPE-g-MAH LLDPE Heat flow/W·g-1

b

80

90

100 110 120 Temperature/

130

140

70

80

90

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70

100

110

120

130

140

150

Temperature/

Fig. 6. DSC thermograms of LPGTR /LLDPE composites (a, melting process; b, crystallization process)

The thermal properties of the composites were measured by DSC, as shown in Fig.6. The melting temperature (Tm), crystallization temperature (Tc) and the degree of crystallinity (χc) are

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shown in Table 5. It can be seen that, without addition of the compatibilizer, there is little change in the Tm and Tc of the composites as the extrusion temperature increases. However, the values of Tm and Tc increase with the addition of the HDPE-g-MAH. The result can be explained by the melting

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and crystallization temperatures of HDPE are higher than that of LLDPE. The degree of crystallinity (χc) of the composites decreases with increasing pyrolysis

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temperature of the LPGTR and is independent of the presence of the compatibilizer. This can be attributed to the ability of LPGTR and LLDPE to combine becoming greater at a higher extrusion temperature, which destroys the regularity of LLDPE’s molecular chains and reduces its crystallinity [40, 41]. The crystallinity of LPGTR-300/LLDPE most likely decreases because the extensive exposure of carbon black promotes the combination of the composites at 300

. The addition of

HDPE-g-MAH results in an increase in crystallinity due to the HDPE itself having a higher degree of crystallinity.

15

ACCEPTED MANUSCRIPT Table 5 DSC thermograms of LPGTR/LLDPE composites Tm/°C

Tc/°C

∆Hc/J g-1

χc/%

HDPE-g-MAH

131.3

118.5

198.0

73.3

LLDPE

121.0

105.9

100.3

34.2

LPGTR-260/LLDPE

121.4

106.6

10.8

11.1

LPGTR-280/LLDPE

121.3

107.7

10.0

10.2

LPGTR-300/LLDPE

122.6

106.2

LPGTR-260/LLDPE/7.5HDPE-g-MAH

124.6

112.6

LPGTR-280/LLDPE/7.5HDPE-g-MAH

124.9

111.6

LPGTR-300/LLDPE/7.5HDPE-g-MAH

123.5

113.8

9.0

14.8

15.2

13.1

13.4

12.5

12.8

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3.5

3.0

2.5

2.0

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1.5

1.0

0.5

0.0

10

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0

EP

Stress/MPa

8.8

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3.6 Mechanical properties of the composites

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Composites

a. b. c. d. e. f.

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LPGTR-260/LLDPE LPGTR-280/LLDPE LPGTR-300/LLDPE LPGTR-260/LLDPE/HDPE-g-MAH LPGTR-280/LLDPE/HDPE-g-MAH LPGTR-300/LLDPE/HDPE-g-MAH 30

40

50

60

Strain/% Fig. 7. Stress-strain curves of composites

In order to study the effects of the degree of devulcanization on the properties of the composites, the mechanical properties of the LPGTR/LLDPE composites were tested at different temperatures. The influence of the compatibilizer was further studied (Fig. 7), and the results are listed in Table 6. It can be seen that the tensile strength of the LPGTR-260/LLDPE composites are higher than that of the LPGTR-280/LLDPE and LPGTR-300/LLDPE composites. This feature can be explained by LPGTR-260 being stronger relative to LPGTR-280 and LPGTR-300. However, the presence of 16

ACCEPTED MANUSCRIPT abundant sol fractions in LPGTR-280 and LPGTR-300 facilitate a certain degree of recombination with the LLDPE, which reduce the rigidity of LLDPE and enhance the toughness of the composites. Therefore, the elongation of break point increases inversely. The tensile strength and elongation of the break point of the composites, compared to those with compatibilizer,

is

improved

following

the

addition

of

HDPE-g-MAH.

Composite

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no

LPGTR-300/LLDPE/7.5HDPE-g-MAH has a higher tensile strength and elongation break point, indicating that the bound-rubber on the surface of the carbon black is destroyed and so carbon black is exposed and the rubber is strengthened. Additionally, the carbon black itself reacts with the MAH

SC

and, to a certain extent, promotes the combination of LPGTR and LLDPE. Table 6 The mechanical properties of composites

LPGTR-260/LLDPE LPGTR-280/LLDPE LPGTR-300/LLDPE 260LPGTR/LLDPE/7.5HDPE-g-MAH 280LPGTR/LLDPE/7.5HDPE-g-MAH

4 Conclusions

Tensile Strength /MPa

25.5

1.6

26.6

1.5

33.4

1.3

40.5

3.2

40.0

2.7

52.9

2.8

TE D

300LPGTR/LLDPE/7.5HDPE-g-MAH

Elongation at break /%

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Compounds

The interfacial interaction between LLDPE and degraded GTR obtained at different pyrolysis

EP

temperatures was investigated for the purpose of understanding the mechanism of interaction between PE and GTR. The effect of HDPE-g-MAH on the LLDPE/LPGTR composites was also

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studied. The above studies indicate the following: 1) As the extrusion temperature increases, the degree of degradation gradually increases and carbon black is exposed to a greater extent in LPGTR. The extent of degradation of the GTR enhances the interfacial interaction between the LPGTR and LLDPE. According to a series of micrographs, the dispersion of the rubber particles becomes more uniform. The thermal performance, Tmax1, Tm and Tc, do not change significantly, whereas the Tmax2 and the tensile strength of the blends decreases with an increase in extrusion temperature. 2) Use of HDPE-g-MAH as a compatibilizer improved the compatibility of the LPGTR/LLDPE blends. The addition of HDPE-g-MAH to the blends allows for a more homogeneous dispersion of 17

ACCEPTED MANUSCRIPT the LPGTR in the LLDPE and its size becomes smaller. The decomposition temperature, melting temperature, crystallization temperature and crystallinity, increased slightly. In addition, elongation at the break point and tensile strength increased significantly because of better compatibility. The improved interfacial interaction can be ascribed to the hydroxyl groups present in the rubber chains

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or the surface of the exposed carbon black reacting with the HDPE-g-MAH. 3) The size of the LPGTR particles in the LLDPE/LPGTR/HDPE-g-MAH composites further decrease with an increase in pyrolysis temperature. HDPE-g-MAH has better compatibility with LPGTR at a higher degree of devulcanization. However, the thermal performance and mechanical

SC

performance become worsen as the pyrolysis temperature increases.

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Acknowledgments

The authors are grateful for the research foundation provided by the Jiangsu Science and Technology Project (BA2016002) and Guangzhou Science and Technology Project (201604020126). The authors

References

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are also deeply grateful for support from Prof. Isayev’s research group at the University of Akron.

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