HITACHI CABLE REVIEW No.23 (AUGUST 2004)24

ABSTRACT: Silane cross-linked polyethylene (Si-XLPE) is widely used for insulationof wires and cables, which are required to be recycled. For material recycling of cross-linked polyethylene (XLPE), a thermoplastic properties is preferable. We success-fully obtained thermoplastic polyethylene (PE) from Si-XLPE via a chemical reactionin supercritical alcohol. Mechanical and electrical properties of recycled PE wereadequate as insulating materials for wires and cables.Furthermore, we indicate in the present report that the investigation of the continu-ous process for chemical reaction in supercritical alcohol supported by NEDO (NewEnergy and Industrial Technology Development Organization). We conclude that theextruder was a useful instrument for injecting XLPE into the reactor and reactoritself for supercritical fluid. This technique will be useful for recycling wire and cableinsulation materials.

[1] Introduction

Recently, much attention has focused on methods to recycleindustrial waste from the plastics and rubbers used as insulatingjackets for cables. Fig. 1 shows retrieved plastics from cablejackets in Japan1). Thermoplastics like polyethylene (PE) andpoly-vinyl chloride (PVC) are recycled as materials for wires,cables and so on. On the other hand, most cross-linked poly-mers such as XLPE and rubber are burned as fuel or disposed ofin landfills. This is because waste from cross-linked polymersare difficult to process into other useful material due to theirlow fluidity and poor moldability properties.

Therefore, a technique that alters the structure of cross-linked polymers to thermoplastic polymers is required to re-cycle cross-linked polymers. As a result, thermo- plasticizationof XLPE by shear stress was investigated2). However, the cross-linking element cannot be selectively decomposed by thismethod, meaning that PE, which has the same properties as thePE before cross-linking (Raw PE), cannot have been produced.

Reporting the present report we discuss a method usingsupercritical alcohol, which makes it possible to recover PE withalmost the same properties as raw PE.

Recycling of Silane Cross-linked Polyethylene for Insulationof Cables using Supercritical AlcoholToshiharu Goto*Takanori Yamazaki*

[2] Cross-linked PE

The chemical structures of cross-linked PE are shown inTable 1. Peroxide cross-linked PE and irradiated cross-linkedPE have C-C bonds as their cross-linking element which is thesame molecular chain structure as PE. On the other hand, silanecross-linked PE has siloxane bonds (-Si-O-Si-) as the cross-link-ing element.

For selective decomposition of the cross-linking element,the cross-linking element should preferably have a differentstructure from PE, as different reactivities can be used for se-lective decomposition. Therefore, we investigated the silanecross-linked PE in the present study.

[3] Thermoplasticization of the XLPE by supercriticalfluid5),6)

Siloxane bond reactions have been studied using silicagel7~9). In silica gel, siloxane bonds dissolve at 200°C in wateror alcohol. These investigations indicate that the same reactionmust take place for the cross-linking element if alcohol can beused as a solvent for silane cross-linked PE. However, this can-not be performed at standard temperature and pressure.

In the present study, a supercritical state was applied tomake water or methanol a solvent for Si-XLPE. Generally, theproperties of supercritical fluid as solvents is different from* Advanced Technology Laboratories, Hitachi Cable, Ltd.

Rubber

XLPE

PE

30,000

ton/year

PVC

2%

19%

30%

49%

Fig.1 − Retrieved plastics from cable jackets in Japan.XLPE in the cable accounts for 19% of the cable plastic waste.

Table. 1 Structures of Cross-linked PE.Two types of cross-linked PE are applied to the cable.

-CH2-CH2- C H-CH2-

Peroxide cross-linked PERadiated cross-linked PE

Silane cross-linked PE PE

Moldability

Structure-CH2-CH2-CH2-CH2-

-CH2-CH2-CH2-CH2-

-CH2-CH2- C H-CH2-

CH2

CH2

-OSi-O- Si - O-SiO-

O

Si

CH2

CH2

-CH2-CH2- C H-CH2-

-CH2-CH2- C H-CH2-

MolecularChain of PE

Molecular Chainof PE

Cross-linkingElement

HITACHI CABLE REVIEW No.23 (AUGUST 2004) 25

Fig. 3 − Temperature dependence of gel fraction and number averagemolecular weight of recycled PE::Water :MethanolRecycled PE in which the gel fraction is 0% and molecular weight isalmost same as raw PE obtained by using supercritical methanol.

F(%) = x 100........................................................... ( i )

High temperature gel permeation chromatography (GPC;TOHSOH HLC-8121GPC/HT) was utilized to determine mo-lecular weight. O-dichlorobenzene was used as the solvent andpolystyrene was the reference used against the molecularweight.

Fig. 3 shows the temperature dependence of gel fractionand the molecular weight of recycled PE against temperaturesfor sub or supercritical water and supercritical methanol. Thehigh temperatures which are needed to reduce the gel fractionto 0% by methanol and water was 300°C and 370°C, respec-tively as shown in Fig. 3(a).

The molecular weight was determined using high tempera-ture gel permeation chromatography. The results are shown inFig. 3(b). The molecular weight of the products decreased above340°C in water and methanol. This indicates that the PE mo-lecular chain must be shortened by heat. Therefore, if water isused as the supercritical fluid, the molecular weight should belower to decrease the gel fraction to 0%.

On the other hand, for methanol, molecular weight remainedalmost constant to that of raw PE, and the gel fraction decreasedto zero between 300°C to 340°C. It is suggested that a selectivereaction occurs here for cross-linking elements.

(2)Chemical StructureThe structure of the specimens below were compared by

using FTIR.① Silane grafted PE② Si-XLPE③ Recycled PE (Si-XLPE treated by supercritical methanol)

Si-XLPE( ② ) was formed from Silane grafted PE( ① ) thatcan be extruded as cable. Silane grafted PE( ① ) was cured in acontainer saturated with steam at 80°C for 24hrs in order for thecross-linking reactions to occur. The recycled PE was obtainedfrom Si-XLPE exposed to supercritical alcohol at 320°C,10MPa for 30 min.

The FTIR spectra obtained was analyzed for chemical struc-ture. The FTIR spectra of specimens ① - ③ are shown inFig.410~13). The chemical structure relating to specific absorp-

Material Methanol Water

Critical Temperature (˚C)Critical Pressure(MPa)

239.6 8.0

374.3 21.8

Table 2 Critical points of methanol and waterThe critical point of alcohol is lower than water.

Super Critical State

0

50

100

150

200

250

300

350

0 100 200 300

Tem

pera

ture

( ˚

C)

(10MPa)

Foamed(0.1MPa)

Initial(0.1MPa)

( ) : Methanol pressure

Time (min.)

Swollen

Fig. 2 − PE visual characteristics in alcohol.PE pellets foam after swelling in the supercritical alcohol.

(a) Gel Fraction

0

20

40

60

10

30

50

70

250 300 350 400

Gel

Fra

ctio

n(%

)N

umbe

r av

erag

eM

olec

ular

Wei

ght

0

10000

20000

30000

40000

50000

250 300 350 400

Temperature(˚C)

(b) Number average Molecular WeightTemperature(˚C)

standard conditions. The critical points of each of the materialsused are shown in Table 25),6).

Three subjects exist that require further research to actual-ize the process using supercritical fluid for Si-XLPE. These are:1) Solubility of Si-XLPE in the supercritical fluid.2) Selective decomposition of siloxane bonds in Si-XLPE3) Development of a continuous process using supercriticalfluid

In the present study, each of these three subjects were in-vestigated.

3.1 Solubility of supercritical fluid into Si-XLPEExperiments were performed using an autoclave with a

transparent window to observe the sample in its supercriticalstate. A pellet of Si-XLPE and methanol were charged into theautoclave, while air was purged from the inner space of the re-actor using argon gas. Then, the reactor was heated for metha-nol to reach a supercritical state.

The change in the appearance of the Si-XLPE pellets isshown in Fig. 2. The pellet was swollen in supercritical metha-nol at a temperature of 330°C and pressure of 10MPa with thedissolution of methanol. The pellet was then foamed whencooled to ambient conditions. The methanol was supersaturateddue to the decrease in temperature and pressure. These resultsindicate that methanol can be used as a solvent for Si-XLPE.

3.2 Selective decomposition of siloxane bonds in Si-XLPE(1) Gel fraction and molecular weight distribution

Products treated in a supercritical state were evaluated us-ing gel fraction and molecular weight. Gel fraction was used toindicate the degree of cross-linking. Ordinarily, molecularweight is very important for PE properties.Gel fraction experiments were carried out as follows:

The specimen was dissolved in xylene at 110°C for 24hours, and then dried at 80°C with vacuum aspiration. Gelfraction(F) was defined by the following equation from the driedweight (aw) and weight before dissolving in xylene (bw).:

awbw

HITACHI CABLE REVIEW No.23 (AUGUST 2004)26

tion is shown in Table 3. The absorption peaks c (800cm-1), anda (1090cm-1) represent methoxysilane (-Si-OCH3) orhydroxysilane (-Si-OH) that originated from the breaking of thesiloxane bonds. Peak b at 1030cm-1 observed as the shoulder ofthe peak is related to the siloxane bond. Peak b observed inspecimen ① was weaker than that for specimen ②. Otherwise,peak c observed in specimen ② was weaker than that in speci-men ①. From the spectra of specimen ③, a small peak b, and strongpeak c, were observed. This indicates that cross-linking elementbonds were broken as a result of siloxane bonds changing tomethoxysilane.

Fig. 5 represents the chemical reaction revealed by the FTIRspectra analysis. A chemical reaction took place (from left toright in chemical equation) at cross-linking. A reverse reactiondominated in the supercritical alcohol. Consequently, the cross-linking element bonds were selectively broken 4).

(3)Electrical and mechanical properties of the recycled PETable 4 shows the tensile strength Tb(MPa), Elongation

Eb(%), dielectric loss tangent(Tan δ) and volume resistivity (ρ)measured using sheet samples in a comparative analysis withraw PE. These properties satisfy the standard of the insulationof 600V XLPE cable. Furthermore, ρ of recycled PE wassmaller than that observed in raw PE indicating that recycledPE is acceptable as insulation material for wire and cable.

3.3 Development of continuous process for supercritical fluidTo establish a practical process, a new efficient method

which can continuously decompose the plastics usingsupercritical alcohol needs to be developed. This is a commonproblem in the practical application of supercritical fluids forpolymer modifications or processing. Here, we investigated thecontinuous process shown in Fig. 6 that used an extruder as thereactor in recycling cross-linked polymer using supercriticalalcohol.

(1) Continuous process using extruderA schematic diagram of the apparatus is shown in Fig. 6. A

pipe with heater and pump was connected to a twin screw ex-truder to feed the supercritical alcohol. Si-XLPE was injectedinto a twin-screw extruder. The conditions of the extruder weremaintained at approximately 320°C and 10 MPa. Fig. 7 showsthe recycled PE extruded from the die and used as the reactorfor supercritical alcohol. These results indicate that we can suc-cessfully extrude recycled PE continuously

(2) Properties of recycled PE from continuous processRecycled PE extruded via the continuous process described

above was evaluated using gel fraction and molecular weight.

Peak Chemical Structure Comment

a 1090 -Si-OCH3 or -Si-OH Bond breaking ofCross-linked Element b 1030 -Si-O-Si- Cross-linking Element

c 800 -Si-OCH3 or -Si-OH Bond breaking of Cross-linked Element

Wave Number (cm-1)

Table 3 The chemical structure related to specific absorp-tion,13).Peaks a and c indicate the bond breakdown of the cross-linking ele-ment

2000 600Wave Number [cm-1]

100014001800

Silane Grafted PE

Silane Cross-linked PE

Recycled PE

a b c

%T

(T

rans

mitt

ance

)

Fig. 4 − FTIR spectra of silane grafted PE, Si-XLPE and the recycledPE. Si-XLPE turned into the recycled PE whose structure was close tothe silane grafted PE by supercritical alcohol

Fig. 5 − Reaction of cross-linking element of silane cross-linked PE.Cross-linked bonds breakdown as a result of the alcoholysis reaction insupercritical alcohol.

HO Si+ + ROHSi OR Si O Si

Cross-linking Element(Siloxane bond) Alkoxysilane Hydroxysilane

R: Alkyl group Alcoholysis

RecycledPE Raw PE

Tensile Strength(MPa) 16.7 15.3MechanicalProperties Elongation (%) 580

tan δat 50Hz,1kV/mm 0.00018 0.00022Electrical

Properties ρ(Ω . cm)at DC1kV/mm 1.2×1017 2.7×1018

540

Table 4 Properties of recycled PEProperties of recycled PE are almost same as raw PE.

Fig. 6 − Continuous process for supercritical alcohol using an extruder. Supercritical alcohol was fed to the extruder used as a reactor.

Silane cross-linked PE

Recycled PE

Supercritical Alcohol

Pump Alcohol Tank Heater

HeaterPipe

Fig. 7 − Appearance of continuously extruded recycled PERecycled PE can be produced continuously by the extruder.

Extruder dieRecycled PEstrand

HITACHI CABLE REVIEW No.23 (AUGUST 2004) 27

Fig. 8 − Molecular weight distribution comparison. Recycled PE fromcontinuous and batch processes respectively have similar molecularweight distributions as raw PE.

Raw PERecycled PE by continuousprocess (320˚C 10MPa) Recycled PE by batch process (320˚C 10MPa)

Frac

tion

(%)

0

20

40

60

80

100

Log Mn2 3 4 5 6 7

Mn: Number Average molecular weight

Properties JISStandard

MeasurementValue

Tensile Strength (MPa) 18.3 MechanicalProperties Elongation (%) 660

ElectricalProperties

Withstand voltage test at AC1500V/1min.

Insulation resistance (MΩ km)

Good

78,000

10

350

Good

2,500

JISC3605

Table 5 Properties of recycled wire (Cross section of con-ductor: 2 mm2, thickness of insulation: 0.8 mm)These properties satisfy the JIS standard of 600V XLPE cable*

and electrical properties of the wire prepared from recycled PEwere found to be acceptable as shown in Table 5.

[4]Conclusion

Recycling Si-XLPE to use as wire and cable insulators wasinvestigated. The structural analyses indicated that only thecross-linking bonds were broken using supercritical alcohol.This appears to be an ideal method for material recycling, as therecycled PE can be used as raw PE. To verify this, mechanicaland electrical properties, as well as processability were tested.All results indicated that recycled PE can be used as insulationmaterial for wire and cable.

As a consequence of the study of the continuous process,we have indicated that the an extruder is a useful piece of equip-ment to extrude the Si-XLPE into a supercritical alcohol solu-tion.

This investigation was supported by Key Technology Re-search Promotion Project of NEDO (New Energy and IndustrialTechnology Development Organization).

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Zairyo,48,25(2000)3) Goto,T., Yamazaki,T., Okajima,I., Sugeta,T., Miyoshi,T.,

Hayashi,S., Sako,T.: Koubunshi Ronbunnsyu,58,12(2001)4) Goto,T., Yamazaki,T.: Hitachi Densenshi, 23, 27 (2004)5) Saito,S.: Chourinnkai Ryuutai no Kagaku to Gizyutu,

Sankyo Business (1996)6) Sako,T., Sugeta,T., Okajima,I.: Shigenkankyo Taisaku,

34,31(1998)7) Kitahara,S.: Nihon Kagakkaishi,90,237 (1969)8) Kitahara,S., Asano,T.: Nihon Kagakukaishi, 91,109 (1970)9) Kitahara,S., Asano,T., Hirowata,T.: Nihon Kagakukaishi,

92,377(1971)10) Hjertberg, T., Palmlof, M. and Sultan, B.A.: J.Appl. Polym.

Sci., 42,1185 (1991).11) Hjertberg, T., Palmlof, M. and Sultan, B.A.: J.Appl. Polym.

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25,857(1985).

Fig. 9 − Appearance of the recycled wire. (Cross section of conductor:2mm2, Thickness of insulation:0.8 mm) Recycled PE can be extruded aswire.

The gel fraction of recycled PE was 0%, indicating that thecross-linking bonds were successfully broken. The molecularweight of the products are shown in Fig. 8 and compared withraw PE. Recycled PE from the batch-made process at 320°Cand from the continuous process both had similar molecularweight distributions to raw PE. These results suggest that themolecular chains of Si-XLPE were not broken in the continuousprocess. This data indicates that an extruder can be used as areactor using supercritical fluid.

During the next step, the recycled PE was extruded intowire as shown in Fig. 9. The wire surface appeared sufficientlysmooth to be used as an insulator. Moreover, the mechanical

Toshiharu GotoElectric Wires & Cables Technology Research Center,Advanced Technology LaboratoriesJoined the company in 1997.Currently engaged in development on dielectric material.Member of the Society of Polymer Science, Japan andthe Japan Society of Polymer Processing

Takanori YamazakiElectric Wires & Cables Technology Research Center,Advanced Technology LaboratoriesJoined the company in 1985.Currently engaged in development on dielectric material. Member of the Society of Polymer Science, Japan andthe Institute of Electrical Engineers of Japan and theSociety of Rubber Industry, JapanPh. D. in Engineering