Low temperature dielectric loss due to antioxidants in polyethylene

Certain ant/oxidants added to polyethylene are known to cause increased dielectric loss in the audio-frequency range at liquid helium temperature. Although the peak loss factor occurs at several kilohertz, the increase at power transmission frequency is sufficient to affect the performance o f an ac superconducting cable insulated with polyethylene tape. Contrary to recent reports, we now have reason to believe that the effect is an exclusive feature of antioxidant molecules which are 3, 5-ditert-butyl-4-hydroxy-phenyl compounds. It is attributable to tunnelling o f the hydroxy hydrogen atom. Other substituted phenols and non-phenol ant/oxidants can all effectively protect the polyethylene without increasing the dielectric loss, 4.2 K dielectric loss measurement may be used as a rapid, sensitive and specific test for the presence o f 3, 5-ditert-bu tyl-4-hydroxy-phenyl compounds in polyethylene,

Low temperature dielectric loss due to antioxidants in polyethylene J. le G. Gilchrist In 1975, Thomas and King 1 reported that the dielectric loss factor of polyethylene at 4.2 K often exhibits a broadened relaxation peak at several kilohertz. This appeared to be associated with the presence of certain antioxidant additives. Ethyl 330 (see Fig. 1) was cited as an example of an antioxidant having this property and BHT (3, 5-ditert-butyl-4hydroxy-toluene) as one that did not apparently cause increased dielectric loss. Yano et al2 confirmed these results and studied the temperature variations between 1.5 K and 4.2 K of the loss peak and its frequency. The peak loss factors reported by Thomas and King were around 1 - 2 x 10 -4 but Yano et al apparently found loss peaks of 2 - 3 x 10 -3. More recently we examined a series of low density polyethylene samples with various antioxidants, a'4 We classified the antioxidants into those containing the 3, 5-ditert-butyl-4hydroxy-phenyl group (alternatively the '2, 6-ditert-butylphenols'), other phenols and non-phenol antioxidants. How-~ ever, we failed to find a connection between the molecular structure of the antioxidant and the occurrence of the loss peak, other than a possible correlation with the molecular weight, and were unable to draw a firm conclusion.

New results We now report the study of a new series of low density polyethylene samples which were kindly supplied by A.R. Blythe who is with ICI Plastics Division, Welwyn Garden City. The loss peak values are given in Table 1 and they suggest that the increased loss is a specific feature of the antioxidants containing the 3, 5-ditert-butyl-4-hydroxy-phenyl group. Further investigations have given us reasons for supposing that previously reported results which disagreed with this conclusion were erroneous.

Ant/oxidant volatility Firstly, let us take the case of BHT. Of the various antioxidant molecules under consideration this is the lightest, with The author is at Centre de Recherches sue les TrSs Basses Temperatures, CNRS, BP 166 X, 38042 Grenoble C~dex, France. Received 9 January 1979.

CRYOGENICS. MAY 1979

molecular weight 220, and it is also the most diffusive and volatile. Following advice from Y. Moisan of CNET we suggested 3'4 that the absence of peak reported with this antioxidant 1'2 may have been due to its inadvertent loss by diffusion and evaporation. The present study lends support to this view. As shown in Fig. 2a, we found that vacuum exposure of the sample film (measuring 22 mm x 22 mm × 130pro) for several hours at 110°C caused the effect to disappear completely. We asked whether this was due to evaporation of the BHT or simply to its migration and possible segregation within the volume of the sample. If the latter were true it might still be effective as an antioxidant. We therefore annealed another film sample, which was pressed between glass plates at the same temperature. The peak diminished by less than 25% this time, presumably because the BHT was not able to evaporate except for a little bit around the edges of the film. We also exposed 10 g of the sample, as granules, and then took the ultraviolet spectrum of the ethanolic extract (Fig. 2b). The two absorption peaks which characterise the substituted phenol antioxidants s were diminished as compared with an extract of the as-received sample, and by comparing the absorptions at 283 nm wavelength we estimate that only 30% of the antioxidant remained after the exposure. Bearing in mind t[aat the surface to volume ratio of the 10 g sample was about 30 times less than of the Film, it is clear that virtually all the BHT can be expected to have evaporated from the latter during the 10 hour exposure. This finding is significant since it may be supposed that what happened in a matter of hours at 110°C may take place over a period of months at room temperature or of minutes at (for example) 160°C. The BHT loss peak was perhaps not observed earlier 1'2 because the samples had been stored for too long, or perhaps had been prepared using a moulding technique involving heating to 160°C or more for some time. The moulding technique used by us involved heating to 140°C for a few minutes only. The heat was switched off as soon as the polymer was found by visual inspection, to have flowed (the moulding plates were of glass and the vacuum vessel also, being a dessicator adapted for that use). One

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281

Table 1. Peak loss factors determined with a.capacitance bridge of a series of low density polyethylene samples at 4.2 K clamped between indium electrodes Antioxidant Polymer

1056max

Trade names

Amount ppm ICI Alkathene 11

Chemical name or formula

nil

<1

ICI Alkathene 11/01

220

topanol O BHT ionol CP

CH3X

ICI Alkathene Q4177

600

ionox 330 ethyl 330

C 6 ( C H3)3 (C H2X) 3

ICI Alkathene 74/01

800

topanol CA

1, 1, 3-tris (5-tert-butyl-4-hydroxy-2-methyl-phenyl) butane

< 1

ICI Alkathene 50/04

3300

nonox WSP permanax

bis [2-hydroxy-5 methyl-3 (1-methyl-cyclohexyl) phenyl] methane

< 1

6

13

specimen was deliberately left in its mould for 1 h at 140°C and its loss peak was then 3 x 10 -s, about half the value of normal specimens.

I I I

We were also warned that some of the other antioxidants might have evaporated. However, when we exposed a Film of polyethylene with ionox 330 (molecular weight 774) together CH3~ C with the BHT sample to the vacuum for 10h at 110°C, we C found that its loss peak was not significantly modified. Amongst the other antioxidants, there are some with relatively C CH 3 ~ C H 3 modest molecular weights - in particular topanol CA (544), nonox WSP (420), NiDEC (355) and DPPD (260). The last two molecules are nickel bis-(N, N-diethyl-dithiocarbamate) C and diphenyl-N, N'-paraphenylene-diamine. We verified that all these four are in fact effective as antioxidants in low H" density polyethylene while not adding significantly to its audio-frequency dielectric loss at 4.2 K. As an example we have shown in Fig. 3 the carbonyl stretch absorption band R in the infra-red spectrum of a low density polyethylene sample containing DPPD, kindly supplied by C. Huraux, who is with the Laboratoire de G~nie Electrique, Toulouse, France, following a thermal oxidation treatment. An absorption peak near 5.8/am wavelength is a fairly sensitive indicator of oxidation in a polyethylene sample, and it is clear that if CH3 CH3X x~ I ...x some antioxidant was lost, at least enough remained to be CH2 ~ C . CH2 effective after more than 100 h at 110 ° C. Neither before the CH2X 2 .c~ " ~c / oxidation treatment nor afterwards did there appear at 4.2 K (x--CH~--CH~'--i--O--CHz~--C a dielectric loss peak, the loss factor in both cases being below 10 -s throughout the audio frequency range. We 0 cH~C~c/C~cH3 similarly demonstrated the effectiveness of the other three I antioxidants, topanol CA and nonox WSP as in Table 1 and CH ( x - - CH2-~ CH2"~ ~ --0 --CH2)2--S NiDEC at 1000 ppm (sample kindly supplied by G. Scott who I O X is with the Chemistry Department, University of Aston in b Birmingham), while confirming that none of these caused Fig. 1 a -- The generic molecule 3, 5-ditert-butyl-4-hydroxy-phenyI-R. increased loss.

CH 3

0

CH3

J

J

L

I

II

Note that as the phenol hydrogen rotates about the C-O axis the symmetry is such that it must have two equivalent positions of potential minimum; b -- some commonly used antioxidants of the type illustrated in a -- which has been abbreviated to RX, X being the 3, 5-ditert-butyl-4-hydroxy-phenyl radical. The substituted benzene molecule shown here has trade names 'ionox 330' and 'ethyl 330'

282

Other phenols

The BHT question now being clarified, there remain our reports 3'4 of three phenol antioxidants other than 3, 5-ditert-

CRYOGENICS. MAY 1979

ultraviolet (cf Fig. 2b). The data obtained with that particular series of samples are therefore to be rejected and any conclusions based on them to be withdrawn. Nevertheless, it is perhaps still just worth recalling that the two to which 3, 5-ditert-butyl-4-hydroxy-phenyl compounds had been knowingly added (irganox l 0l 0 and irganox 1035 - two molecules shown in Fig. l) had higher loss peaks than the other three 3

f+.-----.+~ 6

m

Suggested

4 --

o

I

8

8

o

I

l

IK

3K

o

mechanism

We have seen that the experimental evidence favours the hypothesis that all anti-oxidant additives which are 3, 5-ditertbutyl-4-hydroxy-phenyl compounds give rise to the dielectric relaxation peak in polytheylene, (high density polyethylene1'2 as well as low) but that others do not. Thomas and King ~ suggested that the explanation lay in the quantum mechanical tunnelling of a polar chemical group, and Yano et al, 2 referring to the case of ionox 330, suggested that it is the 4-hydroxy hydrogen of the 3, 5-ditert-butyl-4-hydroxyphenyl group that tunnels. We believe that the present

/

o 300

loss

~ - I 10K

f Hz

A

k .5

o 220

I

I

I

240

260

300

b

340

x,.~

k

.5

-

Fig. 2 a -- Dielectric loss factor of low density polyethylene sample supplied as granules with 220 ppm of the a n t i o x i d a n t BHT. Data obtained at 4.2 K w i t h films f o r m e d by moulding between glass plates, + _ _ sample as moulded; • _ _ film exposed to vacuum for 10 hours at 100°C with a free surface; X - film annealed for 14½ hours at 110°C while pressed between glass plates; o _ _ after removal of the antioxidant by Soxhlet methanol extraction, b -- Transmission spectrum of ethanolic extract of low density polyethylene sample with BHT. l O g of polymer was refluxed with 5 0 m l of ethanol for 24 hours. A f t e r the solution had stood f o r a further 24 hours it was filtered and examined in a 1 0 m m cell with a Beckman D K 2 A spectrometer: spectrum A _ extract of granules as received; spectrum B _ _ after vacuum exposure for 10 hours at 110°C. The granules coalesced and the sample was afterwards cut into slices for the ethanol extraction

butyl-4-hydroxy-phenyl compounds, which apparently all caused the loss peak, though with values below 10 -4. We now believe that these samples also contained a small amount of some other unknown phenol antioxidant in addition to,the antioxidants stated. We examined the 'antioxidant-free starting material' from which these samples had been prepared and found that it too exhibited a characteristic relaxation peak at 4.2 K. This disappeared after methanol extraction and the sample then became more vulnerable to oxidative attack (cf Fig. 3). Moreover, the ethanolic extract of this material showed two characteristic absorption peaks in the

CRYOGENICS.

MAY

1979

C

0

I 6

__

I 7

X ,pro Fig. 3 Transmission spectrum of films ( 1 3 0 # m ) of low density polyethylene sample supplied with 700 ppm of the antioxidant DPPD spectrum A _ _ after exposure to air in darkness at 110°C for 167 hours (this is indistinguishable f r o m the spectrum obtained before exposure); spectrum B _ _ after removal of the antioxidant by Soxhlet extraction with azeotropic ethanol-benzene then with methanol followed by exposure to air in darkness at 110°C for 80 hours; spectrum C _ _ as B but after 167 hours exposure

283

findings lend support to both these suggestions since the two tert-butyl groups in the ortho positions, with respect to the phenol, provide the necessary symmetry and protection for such an effect to occur. Taking the group in isolation (Fig. 1a), the line of the C-0 bond is a two-fold symmetry axis. There may also be symmetry planes. In any case, as the hydrogen atom rotates about the axis defined by the C-0 bond it must find two equivalent positions of potential minimum. In classical physics we would say there were two equilibrium positions with the possibility of thermally activated transitions between them, the occupation probabilities being influenced by an applied electric field. Since by analogy with a wide variety of other low temperature relaxation phenomena, a quantum mechanical description is undoubtedly more appropriate we prefer to say that there are two states with transition probabilities between them determined by coupling to phonons: a ground state and a low energy excited state. The phonons are characteristic of the surrounding polyethylene at a given temperature. The occurrence of a transition does not require the acquisition of sufficient energy to overcome the potential barrier separating the two minima. Of course it is difficult to imagine exactly how the antioxidant molecule is embedded amongst the polyethylene chains but it seems unlikely that the latter will perfectly preserve the symmetry. A slight dissymmetry is not only inevitable, in fact, but even necessary for an explanation of the effect. We would, however, guess that in the case of a bare 4-hydroxy-phenyl compound the symmetry would be so completely destroyed by the surrounding polyethylene that no relaxation would be discernable. In the antioxidant molecules the ortho-tertbutyl groups enclose the - OH in a 'cage' or shield which presumably protects it from an excessive dissymmetry of its outer environment. Fortunately, although this shielding, or strong steric hindrance is necessary to the functioning of the antioxidant, the symmetry of the cage is not. Such molecules as nonox WSP and topanol CA which lack the symmetry, function as antioxidants like the others without inducing a dielectric relaxation. Other polymers If the ideas outlined here are correct we should expect to find that the antioxidants have a similar effect when added to other non-polar polymers, in particular other hydrocarbons. We already noted 6 in fact that a similar peak exhibited by polypropylene disappeared when the sample was Soxhlet methanol extracted, but we are unable to ascertain which antioxidant it contained except that it was a phenol type (cf Fig. 2b). A different antioxidant-free polypropylene sample failed to exhibit a peak. Exactly the same remarks apply to another commercially produced polyolet'm we have studied more recently - poly (4-methyl-pentene - 1) or TPX. Yano et al.2 studied polystyrene with 2000 ppm of ionox 330 and reported that its loss was 'very low'. Their figure records a loss factor of ~ 3 x 10 -4 however, which is already larger than any of the loss peaks reported here and

284

which does not exclude the possibility that ionox 330 has a similar effect in polystyrene as found in polyethylene by Thomas and King 1 and by the present author. Practical conclusions We have drawn two practical conclusions from this study. First, polyethylene tape to be used for the insulation of an ac superconducting power line should preferably be protected with any antioxidant other than a 3, 5-ditertbutyl-4-hydroxy-phenyl compound. Second, 4.2 K dielectric loss measurements provide a rapid, sensitive and specific test for the presence of these compounds in polyethylene requiring a sample of only 10-100 mg. N o t e added in p r o o f

The following additional findings confirm all that we have reported here. A polyethylene effectively protected with with santonox R (an unsymmetrically substituted phenol), kindly supplied by Mr J. Nury, ATO Chimie, had no discernable loss enhancement ( 6 m a x < lO-S). On the other hand two polypropylene samples, kindly supplied by Mr M. Cloarec, Papeteries Bollord, both exhibited loss peaks. One was stabilised with 800 ppm of ionox 330 (8 max = 33 x 10-s), the other with 400-500 ppm each of BHT and irganox 1010 ( 6 m a x = 24 x lO-S). We have also produced dielectric relaxations, generally very strong, by adding 3, 5 ditertbutyl-4-hydroxy benzene to high density polyethylene, low density polyethylene, polypropylene, poly-l-butene, poly-4methyl-l-pentene, polytetrafluorethylene and viscous pharmaceutical-grade paraffin; also by dissolving BHT in the paraffin, and (weaker relaxations) in heptane, carbon tetrachloride and a viscous potystyrene-paraxylene solution; by dissolving 3, 5-ditert-butyl-4-hydroxy benzoic acid and its n-butyl ester in the paraffin. The relaxation of BHT in the paraffin was strongly inhibited by the addition of other molecules having permanent dipole moments (esters) and weakly inhibited by the addition of other aromatic molecules. When the phenol OH of BHT was replaced by OD (by recrystallising the substance from a methanol CH3OD solution) the 4.2 K loss peak of its paraffin solution was shifted from 11 kHz to somewhere near 1 Hz. Further details will be published in due course.

References 1 2 3

4 5 6

Thomas,R.A., King, C.N. Appl Phys Lett 26 (1975) 406-408 Yano, O., Kamoshida, T., Sekiyama, S., Wada, Y. J Polymer Sci: Polym Phys Ed 16 (1978) 679-688 Gilchrist,J.leG. Dielectric Loss Spectra of Polyethylenes, Nonmetallic Materials and Composites at Low Temperatures, Clark, A.F., Reed, R.P., Hartwig, G. (eds) Plenum Press, New York (1979) 103-112 Gilchrist,J. le G. J Polymer Sci: Polym Phys Ed 16 (I 978) 1173-1787 Haslam,J., Willis, H.A., Squirrell, D.C.M. Identification and analysis of plastics 2nd edition, Iliffe, London (1972) Gilchrist,J. le G. Proceedingsof the Sixth International Cryogenic EngineeringConference, IPC Press, Guildford (1976) 372-75

CRYOGENICS. MAY 1979