Research in Nondestructive Evaluation, 15: 55-63, 2004Copyright © American Society for Nondestructive TestingISSN: 0934-9847 print/1432-2110 on1ineDOI: 10.1080/09349840490443612

9Taylor & FrancisTaylor &. Francts Group

NONDESTRUCTIVE CONTROL OF POLYETHYLENE BLANKETINSULATION BY MEANS OF LOCK-IN THERMOGRAPHY

Carosena Meola," Giovanni Maria Carlomagno," Umberto Prisco," Antonio Vitiello''

lDipartimento di Energetica, Termofluidodinamica Applicata e CondizionamentiAmbientali (DETEC),Università di Napoli "Federico Il'', Napoli, Italy2Dipartimento di Ingegneria dei Materiali e della Produzione (DIMP),Università di Napoli "Federico Il'', Napoli, Italy

N...-<oN It is well known that cross-linking of polyethylene molecules into three-dimensional net-

works improves ma teriaIproperties; in particular, it enhances impact strength, thermal per-formance and chemical resistance. A product of good characteristics can be obtained if themanufacturing process is adequate and ali involved machines are working properly. In thiscontext, infrared thermography as a remote imaging technique may be a valuable tool tomonitor the polyethylene cross-Iinking processes. Experimental tests were carried out onspecimens without treatment and on specimens treated with either chemical processes orelectron beam irradiation. Our results prove that lock-in thermography can detect localnon-uniformity of material characteristics due to either the extrusion or cross-linkingprocesses, and material differences Iinked to the different compound.

Keywords: Nondestructive evaluation, lock-in thermography, cross-linked polyethylene

INTRODUCTION

Cross-linking of polyethylene (PE)molecules into three-dimensional net-works improves material properties such as impact strength, chemical resist-ance, and thermal characteristics, thereby allowing for application in variousfields such as: liquid transport (mains), alimentary (food packaging),medicine (orthopaedic prostheses).

Cross-linked polyethylene (PEX) is replacing traditional materials in theproduction of pipes for cold and hot water installations and has many advan-tages including [1] water tight systems, corrosion resistance, low encrus-tation tendency, noise dampening-elastic material, and long term pressureresistance.

Another important application is the production of thermo-shrinkingblanket insulation, made of low density polyethylene (LDPEX) for low andmedium voltage electrical cables. Cross-linking, in this case, may be doneusing chemical agents such as silane grafting, or by radiation. The main

The authors wish to thank the staff at Megarad for providing the materials and the related infor-mation.

Address correspondence to Or. Carosena Meola, Laboratorio di Gasdinamica-DETEC, Università diNapoli "Federico Il'', Via Claudio, 21 80125 Napoli, Italy. E-mail: carmeola@unina.it

55

56 C. MEOLA ET AL.

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potential problem may be a non-uniform treatment which can result in crackformation during the reshaping phase.

The beneficial changes observed in PEX comply with most of thepackaging material standards, which generally require that a material mustbe translucent, sealable, pinhole-resistant, and possess appropriate vapor,moisture, aroma and flavor barrier properties.

Cross-linked ultra high molecular weight polyethylene (UHMWPEX) isused for prostheses in the orthopaedic medicine especially for hips [2-4].Cross-linking for medicai applications cannot be accomplished withchemical agents because of potential toxic effects. The more efficientand generally accepted method is to use ionizing radiation which canbe gamma rays or high energy electron beams. Both methods may leadto prosthesis failure caused by oxidation [3] or hydrogen bubble formation[4]. Research is focused on manipulating various factors to improvecrossl inki ng.

In this context, a nondestructive technique that can detect the formationof voids in industriai products and provide information about the perform-ance of the cross-linking treatment is helpful. Active infrared thermographywhich is being used extensively in nondestructive testing basically can beapplied in the pulsed or modulated (lock-in) mode. Pulse thermographyconsists simply in the stimulation of an object by means of a pulse of energyand monitoring the variation of its surface temperature during the transientheating, or cooling, phase.

Modulated (lock-in) thermography first was described by Carlomagnoand Berardi [5] and later investigated further by other researchers [6-8]. Inthe lock-in mode, the thermographic system is coherently coupled to a ther-mal wave source which is operated in a way that results in sinusoidal tem-perature modulation. The thermal wave is strongly damped so that itpenetrates into the object only to a certain depth which depends on the wavecycle time (the slower the wave, the deeper the penetration) and theproperties of the material (heat conductivity, heat capacity and density).The modulated heat transport can be described in terms of a heavily dampedwave where real and imaginary parts of the complex wave number are givenby the inverse of the thermal diffusion length J1

(1 )

with thermal diffusivity o:= klpc, density p, specific heat capacity c, heatconductivity k, and modulation frequency f. The depth range is given byJ1 for the amplitude image whereas the maximum depth, which can beinspected with the phase image, is approximately 1.8 Il [9-12].

Photothermal radiometry (of which lock-in thermography is the multi-plexed version) was employed in the early 19805 in the inspection ofpolymers; in particular, Busse and Eyerer [13] investigated the effect of

NONDESTRUCTlVE CONTROl OF POl YETHYlENE 57

molecular orientation in stretched PE. The characteristics of ultra-highmolecular weight PE of a retrieved hip cup after 9 years implant, have beeninvestigated by the same authors [14], who related the local variation ofthephase angle to the local variation of material density.

The goal of the current study was to add information about the use of in-frared thermography in the production of thermo-shrinking LDPEX as a toolof either nondestructive control of end products, or validation of productionmethods. Materials tested mostly are used as blanket insulation for low andmedium voltage electrical cables, telephone cables, and anti-corrosion pro-tection for gas pipes.

MATERIAL DESCRIPTION

The material tested, as already specified, was thermo-shrinking LDPEX,which is produced by Megarad s.r.l. (Mignano Montelungo Caserta Italy).Specimens were made of pieces of commerciai blanket insulation or theywere specifically manufactured. Commerciai products were used in differentindustriai mains sectors (telephone, electricity, gas, etc.); thus they must havespecific characteristics which are adjusted either by the production processor by subsequent treatment.

The basic compound included: poly-ethylene, butyl-acrylate, methyl-acrylate, vinyl-acetate. Because of the specific material requirements, a cer-tain percentage of additives enhances stabilization and/or self-extiriguishingcapabilities. Dyes also were added to give the material a specific color,which may be linked to the material properties, or which may simply helpto manufacture electric, or electronic wiring.

Treatments included cross-linking, which were attained by either chemi-cal, or electron beam irradiation processes (more details are reported byMeola et al. [15]).

Specimens were prepared by cutting pieces from five different products,which entai I a different percentage of additi ves, and are referred to here asPl, P2, P3, P4, and P5. The material properties such as density p, operativetemperature Top, temperature of contracture Te, embrittlement temperatureTe, and dielectric strength T, are:

Pl p = 1.010 g/cm3, Top = -40 to 125°C, Te = 130°C,Te = -45°C, T> 20 kV /mm;

P2 p = 1.076 g/cm3, Top = -40 to 125°C, Te = 130°C,Te = -45°C, T> 18 kV /rnrn;

P3 p = 1.012 g/cm3, Top = -50 to 125°C, Te = 135°C,Te = -55°C, T > 16 kV /rnm:

58 C. MEOLA ET AL.

P4 p = 1.069 g/cm3, Top = -50 to 125°C, Te = 130°C,

Te = -55°C, T > 16 kV /mrn;

P5 p = 0.86 g/cm3, Top = -35 to 115°C, Te = 120°C,

Te = -40°C, T> 12 kV/mm.

These values were supplied together with the specimens by Megarad.The thermal behavior of materials generally was evaluated with DSC (differ-ential scanning calarimeter) tests, which allow far the measurement of thetemperature value where a certain material modification arises. From suchmeasurements one can obtain the melting temperature or the embrittlementtemperature [16].

The thickness of the samples ranged fram appraximately 0.5 mm to1.5 mm. In addition to compound and treatment, specimens were character-ized by geometrical differences. Specimens of type Pl were pieces of a blackstrip (manufactured far this investigation) 15mm diameter and 1.5 mm thick,irradiated at a dose (very low) of 38 kGy. Specimens of type P2 are yellowcircular pipes of 11 mm outer diameter and 1 mm wall thickness.This category of specimens was subdivided into three categories:

1. P2-A without any crass-linking treatment;2. P2-B chemically crass-linked via silane grafting and exposure of 90 hours

in hot water to reach the apprapriate (far commerciai electrical products)gel fraction of about 73%;

3. P2-C cross-linked by an electran beam irradiation pracess at a dose toreach the gel fraction of 73%.

Specimens of type P3, P4 and P5 were ali black circular pipes with 6 mmouter diameter and 0.5 mm thickness. They differed fram each other by theamount of stabilizing and/or self-extinguishing substances. In addition, aliwere cross-linked by electran beam irradiation, but by varying the dose.P3 included 30% of self-extinguishing powder and was irradiated at a doseof 74.8 kGy. P4 included-1 % of stabilizing substance and was irradiated at adose of 98.7 kGy. P5 included 20% of stabilizing substance and wasirradiated at a dose of 82.3 kGy. To complete the study, a black sheet ofthe same compound of specimens P5, 1 mm thick, was divided into threeparts:

1. Part A was untreated.2. Part B was irradiated with a dose of 80 kGy.3. Part C was subjected to an overdose till burning.

a specimen about 50mm wide and 1OOmmlong was obtained fram each part.

NONDESTRUCTIVE CONTROL OF POLYETHYLENE 59

EXPERIMENTAL PROCEDURE

N•.....•C>N

The measurements were performed with an Agema 900LW Thermovi-sion system (formerly produced by Agema, Sweden) containing the lock-inoption with modulation frequencies ranging from 3.75 Hz (267 ms) downto 0.0037 Hz (273 s) in 15 intervals. The maximum depth is achieved atthe minimum frequency and depends on the material diffusivity (Eq, 1).

The lamp was calibrated previously for each of the thermal wave fre-quencies to ensure that the temperature waveform is sinusoidal.

Generally, the following test procedure was employed: first, a highmodulation frequency (depending on the material diffusivity a (Eq.1)) waschosen to investigate the near-surface layer and a phase (or magnitude) im-age was acquired. Then a lower frequency value was used to investigate amore in-depth layer and the related image was acquired; this procedurecontinued down to the minimum frequency value (f = 0.0037 Hz), whenthe entire thickness was passed through by the thermal wave. Images (phaseandjor magnitude) were stored for successive analysis [17].

a)

b) close-up view

FIGURE 1. Phase image of Pl specimens; a) 1.2 pixelsl mm; b) 3.2 pixeisf mm.

60 C. MEOLA ET AL.

chemical

no cross-linking

electron beam irradiarion

FIGURE 2. Camparisan of different treatments far the P2 specimen.

DAl A ANAL YSISN•.....•<=>N Two phase images related to specimens of type Pl, for f = 0.23 Hz are

shown in Fig.l (a, b). The first image (Fig.1a), which was acquired by usinga 20° lens with a spatial resolution of approximately 1.7 pixels/mm. includesfive specimens next to each other. It is possible to note, for each specimen,two effects: the presence of a lighter centrai core which is not uniform overthe entire length. For a closer inspection, the zone in the rectangle wasviewed again by using a close-up lens, which enhanced spatial resolutionup to 3.2 pixels/rnm: the centrai core has a nodular appearance. This effect,which indicates local thickening, may be due to extrusion (perhaps intermit-tent) and cooling after extrusion.

The phase image in Fig.2 was obtained, on three specimens of type P2,for f = 0.12 Hz. Two observations were made:

1. There was a change of color (variation of phase phase angle) which indi-cates variation of material characteristics.

2. The phase angle was quite uniform for the material without treatmentwhile it displayed local variation after cross-linking.

The slightly uneven distribution of the phase angle probably is due to aninhomogeneous mixture of silane with the other constituents beforeextrusion, for the chemical method; otherwise, the irradiation method iscertainly affected by the winding speed and the accelerator specifications.

P3."",C .{,.2;:I\M '"~ ..",,::;,~%:1:;;.;%.';;.;,~{;;~;:;::,~.;:~:~~:~:,,:'.; '.;~:./ .::: Pcf

P5

FIGURE 3. Camparisan of specimens P3, P4, and P5.

NONDESTRUCTlVE CONTROl OF POl YETHYlENE 61

a)

N..--<oN-.,....•....o...<("')

..--<'Cl("')

("')

o.•...•(lÌ

~>o"d b)(lÌP-.;:a(lÌ.-:::Ul•....Il)

>'82>-,

...o"'d

Il)"d(lÌo

'"'a~oCl

c)FIGURE 4. Sheets at different irradiation levels; a) part A; b) part B; c) part C.

The phase image in Fig. 3, taken for f = 0.23 Hz, relates to the three spe-cimens P3, P4 and P5; the phase angle differs significantly for the three speci-mens which probably is due to differences in material composition. Inaddition, the color over each specimen is more uniform, indicating a moreuniform cross-linking distribution as compared with specimens Pl and P2shown in Figures 1 and 2, respectively. It may be inferred that the presenceof additives favors the uniformity of cross-linking.

62 C. MEOlA ET Al.

As for the blaek sheet, Fig. 4 (a-e) shows three phase images (taken at afrequeney of 0.12 Hz) that were obtained on samples eut from three differentparts of the sheet. The first image (untreated material) displays loeal phaseangle variation, perhaps due to manufaeturing proeesses (eompounding, ex-trusion, eutting). On the seeond image (part B) a slight variation of the aver-age phase angle is observed; local inhomogeneities may be due to eitherextrusion, or irradiation. To reach the desired cross-linking degree the ma-terial passes more times under the accelerator window. On the right sideof part C dark spots appeared which indicate material deterioration; thiszone was exposed to a too high dose which caused local burning.

CONCLUSIONS

The current investigation includes a nondestructive analysis of PEX.The results presented here indicate that lock-in thermography is able to

detect:

1. material inhomogeneities due to the extrusion process;2. modification of the material properties as a consequence of cross-linking.3. non-uniformity of the cross-linking degree;4. modification of the material properties attributable to the compounding

processo

Thus, this technique can be employed advantageously for quality morii-toring in the industriai production. Phase images may be quantitatively aria-Iyzed to obtain more detailed information.

The performance of end products generally is verified by mechanicaltests, which include measurement of gel content and modulus of elasticity.The 'phase angle, as seen, is sensitive to the variation of the irradiation dose,which in turn, entails variation of gel content and modulus of elasticity.Therefore, the phase angle is potentially a novel parameter to verify cross-linking of polyethylene.

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NONDESTRUCTIVE CONTROL OF POL YETHYLENE 63

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