Thermoplasticizing Technology
for the Recycling of Crosslinked Polyethylene
by Shigeru Tokuda *, Sanae Horikawa *, Kunio Negishi *,Kenji Uesugi *2 and Hiroshi Hirukawa *3

Because of its outstanding dielectric properties and heat resistance, crosslinked
polyethylene (XLPE) is widely used as insulation materials for electric wires andcables. However the three-dimensional lattice structure produced by crosslinking makes it impossible to melt it down again for molding. Thus almost all waste XLPE is currently burnt (as a fuel)or disposed of in landfills. In this work the authors have developed technology in which the application of suitable heating and shearing to XLPE decreases its molecular weight, producing a
recycled material that is sufficiently thermoplastic to allow it to be molded, and have evaluatedthe physical properties of the recycled material obtained. The crosslinked content remaining in
the recycled material is from about 1 to 40 %; the melt flow rate (MFR) is in the range of 0.1 to 30
g/10 min, and the chemical structure is substantially the same as the original material, making it
possible to recycle it to various applications using ordinary molding equipment.

ABSTRACT1.

INTRODUCTION

As part of Furukawa Electric’s wide-ranging environment
protection activities, we are actively participating in material recycling programs aimed at the reduction of industrial
waste and the reuse of waste materials, to develop comprehensive recycling technologies and systems, andachieve an environment-friendly society. Among
Furukawa Electric’s many product lines, electric wire andcable is one in which recycling systems are comparatively
well established, but while recycling rates for the copperand aluminum used as conductors are something like 99%, the amount of sheathing material that is recycled isvery low.
It is true that for some sheathing materials-- the PVC
widely used for electric power cable sheathing andtelecommunications cables as well as the polyethylene
used for telecommunications cables--a considerable proportion is made into pellets and reused. For other sheathing materials, most particularly XLPE, on the other hand,almost all but that portion that is thermally recycled by use
as auxiliary fuel is disposed of as industrial waste in landfills. It is estimated that the amount involved in Japan isabout 10,000 tonnes per year. This is because there is noeffective material recycling technology to handle XLPE,
and no hope that effective industrial-scale recycling canbe implemented. In the present work an attempt has been*

Ecology & Energy Lab., R&D Div.
Planning Dept., R&D Div.
*3 Zaiko Corp.
*2

made to use thermoplasticizing technology to recycle theperoxide crosslinked and silane crosslinked polyethylenescommonly used as the insulation of electric wires andcables, and a report is made on the various properties of
the recycled materials obtained and the results of analysis.

2.

RECYCLING OF XLPE

Crosslinked polyethylene, which is in wide use mainly as
insulation for electric wires and cables, is a material inwhich the polyethylene molecule chains have beencrosslinked by such means as organic peroxides, ionizing
radiation or silane compounds. Because of the formationof a gigantic three-dimensional lattice structure it becomesheat resistant, holding its shape even when heated aboveits melting point. It is thus an extremely useful material
that has superior dielectric properties, but unlike ordinary
thermoplastic materials it has the disadvantage that it cannot be melted down and molded for recycling. Thus likerubber and thermosetting plastics, it is virtually impossible
to reuse XLPE in the same application.
Techniques that have been used in the past to recycle
XLPE include conversion by pyrolysis into oils or waxes,
and pulverization for use as fuel or filling material 1) .
Recently much work has been done on developing recycling technology using supercritical fluids and it has beenreported that thermoplasticizing through selective crackingof the crosslinked structure has yielded high-quality recycled materials 2), 3), but it seems that problems persist, relating to the cost of equipment and the feasibility of continu-

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Thermoplasticizing Technology for the Recycling of Crosslinked Polyethylene

ous processing. There have also been reports of technology for adding thermoplastics to the waste XLPE and melting the mixture to obtain recycled material that is thermoplasticized 4), 5).
The work reported here has been supported by theEnterprise for Assisting in the Development and
Implementation of Industrial Technologies of NEDO, the
New Energy and Industrial Technology Development
Organization, and constitutes a technology for obtaining athermoplastic recycled material by breaking down the
crosslinked structure of XLPE through the use of appropriate heat and shearing. Since the recycled material can bemelted down and molded in the same way as ordinary
thermoplastic materials it has the advantage that it can bereused without additional material.

3.

RECYCLING BY THERMOPLASTICIZINGThe process for obtaining a thermoplastic recycled material by breaking down the crosslinked structure of XLPE
and reducing the molecular weight is referred to as thermoplasticizing. Figure 1 diagrams the process.
Since the recycled material obtained does not have thethree-dimensional lattice structure of XLPE, it is transformed into a thermoplastic material that can be melted
when heated above the melting point that polyethylene
had before it was crosslinked.
Figure 2 is a flow chart of the thermoplasticizing process
for obtaining recycled material using the XLPE waste fromelectric wires and cables as the raw material. XLPE waste
is removed from electric wires and cables and processed
into nuggets preparatory to be discharged. It may then, if
required, be separated or sorted, and be cleaned or haveforeign matter removed. It is then broken down into particles of a size suitable for feeding into the processing[Crosslinked polyethylene]

equipment. The XLPE chips are then loaded into the feeder unit, and supplied continuously in measured amounts tothe thermoplasticizing equipment. By making appropriate
adjustments to the structure of the thermoplasticizing
equipment and the processing conditions it is possible to
make from the XLPE a material of lower molecular weight.
The thermoplastic material discharged from the equipment
is then cooled and made into pellets.

4.

EXPERIMENTS

4.1 Raw Material
It would be desirable to use the XLPE waste recoveredfrom wires and cables in actual service, but since it is difficult to determine the history of the materials arising fromdifferences in usage environments, two XLPEs equivalent
to those used in wires and cables were manufactured foruse as the raw materials. These were:• XLPE-a (peroxide crosslinked polyethylene):a low-density polyethylene made by crosslinking
using organic peroxide compounds, with a degree ofcrosslinking (gel content) of approximately 80 %;• XLPE-b (silane crosslinked polyethylene):a linear low-density polyethylene made by crosslinking using silane, with a degree of crosslinking (gelcontent) of approximately 60 %.
4.2 Pre-processing and Thermoplasticizing
The raw materials were processed in a single-axis rotary
blade chipping machine into chips approximately 5 to 10
mm square. Photo 1 shows the material before processing
and the chips. No washing or cleaning, or processing to
remove foreign matter was performed.
The XLPE chips were then fed to the thermoplasticizing
equipment, where they were converted under differing
process conditions into pellets of recycled material measuring 2 to 3 mm in diameter by about 3 mm.

• Three-dimensional lattice
structure gives outstanding heat
resistance• Melting and molding is notpossible making material
recycling difficult

XLPE waste (from wires, cables, etc.)

Thermoplasticizing process(Sorting) Chipping

Measurement and feeding of materials[Recycled material]• Molecular weight is less andcrosslinked structure virtuallydisappears• Thermoplasticizing enables
ordinary recycling

Thermoplasticizing

Cooling and pelletization

Recycled XLPE

Figure 1

Outline of the process of thermoplasticizing
crosslinked polyethylene.

Figure 2

Flow chart of thermoplasticizing process for recycling of XLPE waste from electric wires and cables.

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Thermoplasticizing Technology for the Recycling of Crosslinked Polyethylene

160°C and a die temperature of 180°C. The extruded
sheets were taken up onto a cold roll yielding sheets
approximately 2.0 mm in thickness.
4.5 Analysis
4.5.1 Sample Materials
We used representative samples of recycled materials
and LDPE as a control.

Photo 1

4.5.2 Infrared Absorption Spectrum
The infrared absorption spectra of samples made by the
hot pressed film method were measured using FT-IR.

Raw XLPE and chips.

4.3 Evaluating Properties of Recycled Material
4.3.1 Sample Materials
Two recycled materials--XLPE-a and XLPE-b which had
been thermoplasticized from two types of XLPE wereused, together with low-density polyethylene (LDPE), having a density of 0.92 g/cm3 and an MFR of 1.0 g/10 min,as a control.
4.3.2 Observation
Surface condition and coloration, transparency, etc. wereobserved visually.
4.3.3 Melt Flow Rate
In accordance with JIS K7210, the MFR was measured at
a test temperature of 190°C and a test load of 21.8 N.
4.3.4 Degree of Crosslinking (gel content)
A sample of approximately 0.1 g (w1) was wrapped in 400mesh stainless steel mesh of known mass (w 2 ) and
exposed in 100 ml of xylene at 120°C for 24 hr. The stainless steel mesh was then removed and the mass measured after vacuum drying at 80°C for 16 hr (w3). The
degree of crosslinking was measured in terms of the percent gel content, using the equation
gel content (%) ={(w3 – w2)/w1}×100(1)4.3.5 Melting Point
Measurements of the melting point were made by differential scanning calorimetry (DSC) at a temperature rise rateof 10°C/min and of the quantity of heat to melting. The
peak top temperature of the heat absorption peak was
taken as the melting temperature.

4.5.3 Molecular Weight Distribution
The molecular weight distribution in the samples wasmeasured by gel permeation chromatography (GPC). We
used ortho-dichlorobenzene as the solvent, a Shodex AD806MS GPC system, a measuring temperature of 140°C,
and an infrared spectrophotometer as the detector.
Molecular weight was calculated by universal calibration
using a standard PS.
4.5.4 Short-Chain Branches
Short-chain branches were analyzed by measuring the
13C-NMR (nuclear magnetic resonance) spectrum. The
chip samples were first processed in a Soxhlet extractor
using acetone and then dissolved by heating to 120°C in amixed solvent of ortho-dichlorobenzene and deuterated
benzene.

5.

EXPERIMENTAL RESULTS AND DISCUSSION

5.1 Evaluation of Properties
Photo 2 shows pellets of recycled XLPE-a, and Table 1
shows the results of evaluating the materials tested.
The pellets of recycled material were pale brown to pale
gray, and both the recycled materials were more discolored than, and somewhat inferior in transparency to, the
LDPE used as a control. With respect to the surface, the
XLPE-a was smooth, but the XLPE-b was somewhat
rough.

4.4 Evaluating Molding Properties
4.4.1 Press Molding
After being mixed by an open roll, the recycled material
was formed on a hot press into sheets 1 mm in thickness,and its flow properties and the condition of the sheet surface were checked.
4.4.2 Extrusion Molding
Sheet extrusion was carried out using a 40 mm diameter
single screw extruder with a cylinder temperature of

Photo 2Pellets of recycled XLPE-a.

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Thermoplasticizing Technology for the Recycling of Crosslinked Polyethylene

Typical properties of materials tested.
XLPE-b(recycled)

LDPE(control)Good

Slightly roughGood

Pale brown

Pale gray

NaturalMFR (g/10 min)0.1~300.1~301.0

Gel content (%)

<1~10

20~40

0

Melting point (°C)

108

114

112

Heat of melting (mJ/mg)

121

103

126

Appearance
Surface
Color

There was a broad range of MFR for the recycled materials produced under various conditions, from 0.1 to 30
g/min. The gel content, which indicates the degree ofcrosslinking, dropped to about 1 % in most of the recycled
XLPE-a material, and the reduction in molecular weightproceeded until there was virtually no gel content. In
XLPE-b, by contrast, there was a residual gel content of
about 30 %, but most of the unmelted gel in the XLPE-b
was in the form of particles smaller than 1 mm, not a typeof gel content that would retain the configuration of thesample. Measurements of MFR for the recycled XLPE-balso showed adequate flow properties, so that, despite thefact that the macro crosslinked structures were largely
broken down, it seems not to have reached the point ofcomplete reduction in molecular weight.
Figure 3 is a chart showing the results of DSC for therecycled XLPE-a and XLPE-b, and the LDPE control
material. Both of the recycled materials were seen to
exhibit a clearly defined melting point, and it was confirmed that the quantity of heat absorbed was not significantly different from the ordinary LDPE.
5.2 Molding Properties
Both in press molding and extrusion molding the recycledmaterials exhibited exactly the same molding properties
as the LDPE used as a control. In extrusion molding, there
were some difference in flow properties due to the different MFRs but there was no increase or fluctuation in
extrusion torque, and the surface properties were satisfactory for both materials. These results confirmed that therecycled materials have satisfactory molding properties.

XLPE-b (recycled)

XLPE-a (recycled)

DSC (mW)

XLPE-a(recycled)

LDPE (control)

0

60

120

Figure 3

DSC chart.

XLPE-b (recycled)

XLPE-a (recycled)

LDPE (control)

3000

2000

1500

1000

800

600

Wavenumber (cm-1)

Figure 4
Table 2

FT-IR absorption spectra.
Molecular weight distribution.
XLPE-aXLPE-b(recycled) (recycled)

LDPE(control)Number-average molecular weight (Mn) 1.34 × 104 6.72 × 103 1.05 × 104
Weight-average molecular weight (Mw)

5.3 Results of Analysis
Figure 4 shows the FT-IR absorption spectra of the recycled XLPE materials and the LDPE used as a control.
XLPE-a showed virtually identical results to the LDPE, but
XLPE-b exhibited absorption due to silane crosslinking.
This was virtually the same as the absorption spectra of
the XLPE raw material, showing that there were virtually
no chemical changes, with the exception of the reduction
in molecular weight due to the thermoplasticizing processing. Among the points of difference was a slight increasein absorption at about 900 cm-1 for the recycled materials
due to the vinyl group and the vinylidene group, suggesting that there was a slight increase in the C=C doublebond due to thermoplasticizing.

180

Temperature (°C)

Light absorbency

Table 1

1.1 × 105

2.26 × 104 3.37 × 104

Polydispersity (Mn/Mw)

8.19

3.36

3.21

Peak top molecular weight

24900

10900

29400

Table 2 and Figure 5 show the results of measurementsof the molecular weights of the two recycled XLPE compounds and the LDPE used as a control. It can be seenthat while the weight-average molecular weight was about
30,000 for the LDPE versus about 100,000 for XLPE-a,the peak top molecular weight was, in contrast, lower forthe recycled materials. This is due to the comparatively
large amount of constituents having molecular weights
greater than 100,000 that remained in the recycled materials.

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Thermoplasticizing Technology for the Recycling of Crosslinked Polyethylene

1.0
XLPE-b (recycled)

dw/d (logM)

0.8

LDPE (control)

0.6

0.4
XLPE-a (recycled)
0.2

0
2

3

4

5

6

7

LogMW

Figure 5

Molecular weight distribution.

There was, on the other hand, a major difference
between recycled XLPE-a and recycled XLPE-b in termsof the molecular weight distribution, which was comparatively broad in the case of XLPE-a, whereas in XLPE-b the
distribution limits were comparatively sharp. This difference in distribution was apparent even in recycled materials having virtually the same MFR, showing that a difference in molecular weight reduction during thermoplasticizing appeared as a result of differences in the methods ofcrosslinking. It is necessary to note, however, that forrecycled XLPE-b, the molecular weight of the approximately 30 % gel content remaining was not included in themeasurement results.
Analysis of short-chain branch structures by 13C-NMR
showed that they were virtually the same in the LDPE
control and XLPE-a recycled material, whereas the XLPEb recycled material had a structure with relatively more
ethyl branches. This is primarily due to the polyethylenematerial prior to crosslinking, making it possible to confirm
that there was virtually no change in short-chain branch
structure due to thermoplasticizing. Also, C=C double
bonds were detected, though in miniscule quantities, fromthe recycled material, results that were congruent with theinfrared absorption spectra.
5.4 Mechanism of Thermoplasticizing Process
The results of our evaluations and analyses told us thatthe recycled materials made from XLPE by a process ofthermoplasticizing showed differences from virgin polyethylene in that they had a certain amount of minutecrosslinking and residual constituents of somewhat highmolecular weight, and that double bonds formed with thereactions reducing molecular weight, to which discoloration may be attributed. From the chemical standpoint,
however, it can still be called polyethylene.
The technology developed here is characterized in that,by optimizing the configuration and operating conditions ofvarious equipment, the level of molecular weight reductionof XLPE can be controlled to a level approaching that ofpolyethylene before crosslinking. In the thermoplasticizing
equipment, a molecular weight reducing reaction proceeds as the XLPE raw material is subjected to appropriate heating and shearing, and it is assumed that themechanism of this reaction is similar to the random

decomposition that occurs in the thermal cracking of ordinary XLPE.
Furthermore it has been found that polyethylene
crosslinked by organic peroxides yields a satisfactory
recycled material of good appearance and little residual
gel content. In contrast, when the raw material was polyethylene crosslinked by silane, the recycled material had asomewhat rough surface and even when MFR was comparatively high, a crosslinked component remained. It isthought that this is because when silane XLPE is subjected to molecular weight reduction, the polyethylene chainsare severed preferentially, while the siloxane bonds of thecrosslinks are not severed.
5.5 Suitability of Materials to Recycling
By a thermoplasticizing process of XLPE, it is possible toobtain a recycled material having virtually the same molding properties and molecular structure as polyethylene.
With the exception of the preparation and chipping of the
raw material, this process is simple and continuous, with ayield approaching 100 %. Productivity is comparatively
high, and depending on the scale of the equipment, outputs of several tonnes of recycled material per day are
possible.
Although the results of this work are based on studies of
the use of virgin XLPE, it was possible to use the recycledmaterial by itself in molding. Thus it can be used in a wide
range of material recycling applications, including reuseas sheathing for electric wires and cables.
When, however, it is desired to recycle XLPE waste salvaged from wires and cables that have been in use, it isnecessary to consider the deterioration caused by theconditions of use, the adherence of foreign matter, andthe admixture of materials other than XLPE during thestripping process. Since sheathing materials for electric
wires and cables must be superior in appearance,mechanical properties, electrical properties and durability,
we feel that in establishing closed-loop material recycling,
consideration must be given to salvage systems and technologies for reuse.

6.

CONCLUSION

It has been confirmed that through the technology of thermoplasticizing for recycling that has been developed inthis work, it has been possible to obtain from crosslinked
polyethylene a recycled material, the molecular weight ofwhich is sufficiently reduced to allow it to be reused. The
following conclusions have been reached:(1) Both peroxide-crosslinked and silane-crosslinked
polyethylenes can be thermoplasticized, and the recycled materials have gel contents of about 1~40 % andMFRs in the range of 0.1~30 g/10 min. The recycledmaterial from peroxide-crosslinked polyethylene issuperior to that from silane-crosslinked polyethylene
in terms of better appearance and lower residual gel
content.
(2) The recycled materials can be press-molded andFurukawa Review, No. 23

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Thermoplasticizing Technology for the Recycling of Crosslinked Polyethylene

extrusion molded without additional material.
(3) The chemical composition of the recycled materials
is substantially the same as that of polyethylene, withsome slight increase in double-bond component.
There are also residual components with molecularweights greater than 100,000.
These results confirm that thermoplasticizing recycling
technology enables crosslinked polyethylene to bereused. The technology presented here is highly feasible
from the standpoints of productivity and cost, and seems
to be an effective method of reducing environmental
impact by reusing the crosslinked polyethylene waste that
has in the past been used as fuel or disposed of as industrial waste.

REFERENCES
1) T. Uematsu, H. Ogiwara: Recycling Technology Revolution forElectric Power Cables (Sequel), Shi-Denki, Jun. 2000, 27 (inJapanese)2) K. Furumura et al.: Development of Recycling Technologies for
Crosslinked Polyethylene --Study on the Application Technology
of Supercritical Water--, Proceeding, National Convention of IEEJ,(2001), 630 (in Japanese)
3) T. Goto et al.: Recycling of Crosslinked Polyethylene Using
Supercritical Alcohol, Collected Papers on Polymers, Vol.58,
No.12 (2001), 703 (in Japanese)
4) K. Naka et al.: Link-Severing Technologies for Crosslinked
Polyethylene --Thermoplasticizing of Crosslinked Polyethylene--,Meeting for Reading Research Papers in Material Life Institute,Vol.11 (2000), 76 (in Japanese)5) K. Inoue et al.: Material Recycling of Crosslinked Polyethylene by
Thermoplasticizing, Kogyo Zairyo, Vol.48, No.3 (2000), 25 (inJapanese)

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