The 5th ISFR (October 11-14, 2009, Chengdu, China)

MECHANOCHEMICAL RECYCLING AND PROCESSING
OF WASTE CROSSLINKED POLYMERS: WASTE TIRERUBBER AND WASTE XLPE FROM CABLE SCRAPS
Canhui Lu*, Xinxing Zhang, Mei Liang
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute,Sichuan University, Chengdu 610065, China
E-mail: canhuilu@scu.edu.cn
Abstract: The accumulation and disposal of polymer wastes including waste plastics, wasterubber, waste synthetic fibers and waste composites is a worldwide concern. Until now, thereare many technical barriers in recycling polymer wastes with crosslinked structure, such asvulcanized rubber, crosslinked polyolefine and other thermoset materials. In this study, amechanochemical approach was used to conduct the devulcanization of waste tire rubbers(WTR) and de-crosslinking of waste crosslinked polyethylene (XLPE) from cable scraps, forturning WTR and waste XLPE to re-formable materials. The stress-induced mechanochemical
devulcanization of waste tire rubber and decrosslinking of XLPE at ambient temperature canbe achieved through self-designed solid-state mechanochemical reactor. Several high-valuedcomposites, such as natural rubber/WTR, nitrile rubber/WTR, HDPE/XLPE, thermoplastic
vulcanizates (TPVs) from partly de-crosslinked XLPE scrap and partly devulcanisized WTR,
with improved mechanical properites were obtained through mechanochemical processing.
The objective of the present research is to develop a feasible technique for reusing
unsegregated waste crosslinked polymer materials.
1. Introduction
The accumulation and disposal of polymer wastes including waste plastics, waste rubber,waste synthetic fibers and waste composites is a worldwide concern. Until now, there aremany technical barriers in recycling polymer wastes with crosslinked structure, such as
vulcanized rubber, crosslinked polyolefine and other thermoset materials. The reuse orrecycling of waste crosslinked polymer has become an important topic and attracted many
researchers’ interests in recent years. Nowadays, most waste crosslinked polymeric materials
have been subjected to incineration or landfilling, and only a small percentage of them havegone into a variety of reuse/recycle options. However, incineration and landfilling produce
severe environmental problems such as air pollution and soil contamination. The reuse ofcrosslinked polymer scraps not only protects the environment but also saves limited
petroleum resource from which the raw material is originated.
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The previous work carried out in our laboratory demonstrated that scrap rubber
vulcanizates could be partially devulcanized through solid state mechanochemical millingwith a pan-mill type mechanochemical reactor, designed on the basis of a traditional Chinesestone-mill. Because of its unique structure, the pan-mill equipment acts as pairs of
three-dimensional scissors during milling, exerting very strong shear forces on milledmaterials, and showing multi-function such as pulverizing, dispersion, mixing as well asactivation.
In this study, a mechanochemical approach was used to conduct the devulcanization ofground tire rubbers (GTR) and de-crosslinking of waste crosslinked polyethylene (XLPE)from cable scraps, for turning GTR and waste XLPE to re-formable materials. Thermoplastic
vulcanizates (TPVs) from partly de-crosslinked XLPE scrap and partly devulcanisized GTR,
with improved mechanical properites were obtained through mechanochemical processing.
The objective of the present research is to develop a feasible technique for reusing
unsegregated waste crosslinked polymer materials.
2. Experimental
2.1 Materials
The crosslinked polyethylene used in this study is widely applied in insulated electric
wires and cables, in which the molecule chains of low density polyethylene have beencrosslinked by organic peroxide compounds, with a crosslinking degree (gel content) ofapproximately 80%. The raw XLPE were cut into chips of about 1 cm3 with a rotary blade
chipping machine.
Ground tire rubber, with an average particle size of 340 µm, was originated from groundtread layers of truck tires. Other vulcanizing ingredients such as zinc oxide, stearic acid,
2-mercaptobenzothiazole (MBT), tetramethyl thiuram disulfide (TMTD), sulfur and dicumyl
peroxide (DCP) were obtained commercially.
2.2 Mechanochemical treatment of crosslinked polymer scraps
The partial degradation of XLPE and GTR was carried out in the pan-mill typemechanochemical reactor at ambient temperature. XLPE/GTR mixture was fed into themechanochemical reactor through the hopper in the center of the milling pan at a rotating
speed of 30 rev min-1. The average retention time of the composite powders during milling is25~40 seconds per cycle, and the heat generated during milling was removed by watercirculation. A chain transmission system and a screw pressure system are set to regulate therotation speed of moving pan and imposed load respectively, which can strictly control two
major dynamic parameters during milling, velocity and force. Milling process of solid mass inthe equipment was as follows, the materials are fed to the center of the pan from the inlet,
driven by shear force, moving along a spiral route towards edge of the pan till come out from- 149 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
the outlet, thus one cycle of milling is finished. The decomposed samples were collected forthe measurements of gel fraction, rheological behaviors and mechanical properties.
2.3 Preparation of TPEs through dynamic vulcanization
Blends were prepared in a Brabender Plasticorder PLE-330 by melt-mixing thecomponents at a temperature of 160 oC and a rotor speed of 80 rev min-1 for 10 min. The
dynamically vulcanized blends were prepared from the formulations shown in Tab.1. Then,the blends were left to cool down at ambient temperature prior to molding. Samples of theblends were compression molded in an electrically heated hydraulic press. Hot-press
procedures involved preheating at 180 oC for 5 min, followed by compression for 8 min at thesame temperature and subsequently cooling under pressure for 5 min.
Tab. 1 Compounding formulations used to prepare
the TPEs with various vulcanization systems
Amount (phr)Ingredients
Sulfur system

Peroxide system

XLPE/GTR (50/50) blends

100100ZnO

2.5—

Stearic acid

1—

MBT

0.375

—

TMTD

0.75

—

Sulfur

1.5—

DCP

—

0.5

2.4 Gel fraction measurements
Gel fractions of the decomposed and dynamically vulcanized samples were measured by
Soxhlet extraction. The samples were reflux in toluene for 24 hours and then dried in avacuum oven at 60 oC for 6 hours. The weights of samples were recorded before and after thetreatments.
2.5 Rheological measurements
The rheological measurements were done in a Gottfert capillary rheometer (Rheograph2002) at different plunger speeds. The plunger speed was varied from 0.06 to 20 cm min-1.
The melt was extruded through the capillary at predetermined plunger speeds after a warm-up
period of 4 min. The measurements were done at a temperature of 180 oC.
2.6 Mechanical properties
The stress-strain properties of the samples were determined in accordance with ASTM
D412 using an Instron 5567 Universal Testing Machine at a crosshead speed of 100 mm/min.
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3. Results and discussion
3.1 Gel fraction reduction of XLPE/GTR blends during mechanochemical milling
The realization of partial degradation of XLPE and GTR was confirmed by gel fraction
measurements. Fig.1 shows the effect of mechanochemical milling on the gel fraction ofXLPE/GTR (50/50) blends. The gel fraction which correlated to the crosslink density of theblends was decreased significantly after solid state mechanochemical milling. In order toinvestigate whether both of the XLPE and GTR were partially decomposed or only GTR
component was devulcanized during mechanochemical milling, XLPE and GTR wereseparately milled 8 cycles and the variation of their gel fractions after milling was measured.
Tab.2 indicates that both of the XLPE and GTR could be partially de-crosslinked throughmechanochemical milling.
Tab. 2 Gel fraction of XLPE and GTR before and after separately mechanochemical milling
Samples

Without milling

After 8 cycles of milling

XLPE

80.6%

64.5%

GTR

91.3%

72.4%

Fig. 1 The effect of mechanochemical
milling on the gel fraction of XLPE/GTR(50/50) blends

Fig. 2 The variation of the capillary
viscosity as a function of the shear rate for
XLPE/GTR (50/50) blends with different
cycles of milling

3.2 Rheological studies
The flow properties of the materials studied in the molten state were analyzed byrheology. The variation of the capillary viscosity as a function of the shear rate for
XLPE/GTR (50/50) blends with different cycles of mechanochemical milling is presented inFig. 2. The apparent viscosities of the mechanochemically treated mixtures decrease withincreasing of shear rate, indicating pseudoplastic flow behavior. Furthermore, as the cycles of- 151 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
mechanochemical milling increases, the apparent viscosity decreases. Such a decrease inviscosity is in accordance with the gel fraction test. The partial de-crosslinking or
devulcanization of XLPE and GTR during mechanochemical milling may responsible for thedecrease in apparent viscosity.
3.3 Effect of different curing system on the mechanical properties of the blends
One of the key properties of thermoplastic vulcanizates is their mechanical properties,
which also yields the first information on the compatibility of blends. Tab.3 shows themechanical properties of the XLPE/GTR (50/50) composites after 8 cycles ofmechanochemical milling with and without dynamically vulcanizing, respectively. Twodifferent types of curing agent: sulfur and peroxide system have been investigated. Comparedwith the sulfur-cured system, the DCP-cured system showed deteriorated mechanicalproperties. DCP may cause crosslinking of polyethylene, therefore it is not strange that theDCP-cured system lead to poor processability. On the contrast, the mechanical performanceof the sulfur-cured system was significantly improved. The tensile strength increased by about
42% from 5.7 to 8.1 MPa. Meanwhile, the elongation at break rose by about 24% from
152.8% to 189.1%.
Tab.3 Mechanical properties of the XLPE/GTR (50/50) blends after 8 cycles ofmechanochemical milling with and without dynamic vulcanization
SamplesTensile strength (MPa)Elongation at break (%)

Without vulcanization

5.7

152.8

DCP-cured system

3.7

40.2

Sulfur-cured system

8.1

189.1

3.4 Effect of mechanochemical treatment on the mechanical properties of the blends
The sample of raw XLPE/GTR blends could not be obtained due to the fact that raw
blends could not be plasticized and thermoformed. However, the partially de-crosslinked
blends could be plasticized and thus be hot-pressed to a continuous sheets, indicating thecharacteristic of thermoforming. The effects of mechanochemical de-crosslinking on thetensile strength and elongation at break of the composites are shown in Fig.3 and Fig.4,respectively. The results showed that the tensile strength and elongation at break of thecomposites increase with the cycles of mechanochemical milling. After 20 cycles ofmechanochemical milling, the composites reached the maximum tensile strength (9.1 MPa)and elongation at break (201.2%). The longer time of mechanochemical milling treatment is
considered to induce higher degradation degree of XLPE/GTR blends and thus improved the- 152 -PARTⅢ MECHANICAL RECYCLING
compatibility of the XLPE/GTR blends due to improved molecule entanglement. Moreover,mechanochemical co-milling of XLPE and GTR gave rise to a better dispersion between bothpolymeric phases, and thus, a more homogeneous and continuous structure was obtained.

Fig. 3 The effect of mechanochemicaldegradation on the tensile strength ofXLPE/GTR (50/50) composites

Fig. 4 The effect of mechanochemicaldegradation on the elongation at breakof XLPE/GTR (50/50) composites

3.5 Morphological studies
Fig. 5 (a) and (b) show the cryogenically fractured surface of the XLPE/GTR (50/50)
composites after 8 cycles of milling without and with dynamic vulcanization, respectively. As
can be seen, the interfacial adhesion between XLPE and GTR was considerably improved
through dynamic vulcanization. The improved interfacial adhesion was ascribed to theultimately fine dispersion of rubber particles in the partially decomposed polyethylene phase
after dynamic vulcanization.

(a)

(b)

Fig. 5 SEM fractographs of XLPE/GTR (50/50) composites after 8 cycles of milling(a) withoutand(b) with dynamic vulcanization
3.6 Reprocessability studies
The reprocessability of the dynamically vulcanized XLPE/GTR (50/50) composites wasstudied by remixing the sample in a Brabender plasticorder and remolding the material. The
process was repeated for three times and the changes in tensile strength and elongation atbreak of the blend vulcanizates are shown in Fig.6. It is clearly observed from the figure that- 153 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
the tensile strength and elongation at break are kept almost the same even after three cycles ofreprocesses, which indicates a thermoplastic nature of the blend.

Fig. 6 The effects of reprocessing cycles on the tensile strength andelongation at break of XLPE/GTR (50/50) composites

4. Conclusions
The mechanochemical recycling of XLPE and GTR was realized using our self-designed
solid state mechanochemical reactor. The significant decrease in the gel fraction of
XLPE/GTR blends during mechanochemical milling confirmed the partial degradation ofthese polymer scraps. The apparent viscosity of the blends decreased with increasing themechanochemical milling time, which confirmed the partial mechanochemical degradation.
TPVs could be prepared from the partially decomposed XLPE/GTR blends through dynamic
vulcanization. However, the blends of raw XLPE and GTR could not be plasticized due totheir crosslinking structures. The composites obtained by dynamic vulcanization with thesulfur-accelerator system exhibit better mechanical properties than that of samples
dynamically vulcanized with DCP. The mechanical properties of XLPE/GTR composites canbe largely improved by mechanochemical treatment and dynamic vulcanization. The blend
vulcanizates maintain the reprocessability, indicating the thermoplastic elastomeric nature of
the blend. In comparison with the existing recycling methods, the recycling approachdescribed in this paper is cost-effective and easily to industrialization.
Acknowledgments
The authors would like to thank National High Technology Research and Development
Program (863 Program, 2008AA06Z343) for financial support.

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