Radiation Effects on Polymers - ACS Publications - American

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Chapter 31

Influence of Ionizing Radiation on Thermoset Plastics 1

H. Wilski

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Hoechst AG, 6230 Frankfurt 80, Germany

Thermoset plastics were irradiated under two very different conditions. One set of samples was irradiated with electrons up to 10 MGy under the exclusion of oxygen, using a high dose rate. The second set was irradiated with gamma-rays up to 1 MGy, in air, using a dose rate of only 13.8 Gy/h. The latter experiment lasted longer than 10 years. Theflexuralstrengths of the inorganic filled thermosets (epoxy, melamine-formaldehyde, unsaturated polyester and phenol-formaldehyde) remained nearly unchanged under these conditions though an influence of the dose rate was clearly seen in both the latter cases. Organicfilledthermosets were deteriorated, the influence of the dose rate being stronger. The deflection temperature was changed to the worse in all cases (independent of the dose rate) thus indicating chain scission.

Irradiation deteriorates all organic plastics if the dose is sufficiently high. For thermoplastics it is known that if the irradiation takes place in air the degradation is the more severe the lower the dose rate is. The interrelation between the dose rate and the degree of damage was thoroughly investigated for many thermoplastics in the past. Compilations of the papers published can be found elsewhere (1 - 4). This dependence of the deterioration on the dose rate was not understood for a long time. Today it is known that the oxygen concentration in the interior of the samples is the reason. The dissolved oxygen reacts with the radiation induced radicals and builds peroxides. These peroxides are not stable; they decay slowly under chain scission. The longer the irradiation time is, the more complete is the break down of the peroxides and the damage to the material. Gillen and Clough were thefirstto explain this reaction (5).

Current address: Am Sportplatz, 27 D-6231, Stulzbach, Germany 0097-6156/91/0475-0500S06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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The radiation resistance of thermoset plastics was also investigated several times. These experiments were always carried out with high dose rates. The thermoset plastics proved themselves to be very stable in comparison with most of the usual thermoplastics (6-9). But the influence of the dose rate was neglected so far in nearly all of these experiments. Consequently it is not yet known whether the dose rate has any influence on the result if thermosets are irradiated in air. An answer to this question is important if thermoset plastics are to be used for a long period of time in a radiation environment. Only if the influence of the dose rate is known reliably, one can estimate the prospective lifetime of components made from these materials. To learn more about the influence of the dose rate a number of thermoset plastics of different chemical compositions was irradiated with two very different dose rates. In this paper the influence of the irradiation on the flexural strength and the deflection temperature will be discussed.

Experimental The thermoset plastics used in this work were made from resin moulding compounds which are defined in the German standard DIN 7708 (1968). Detailed descriptions of these plastics can be found in the literature (10). The moulding compounds which we used were products of Hoechst AG, Frankfurt, Germany, with the trade name "Hostaset". Similar products are manufactured in many countries of the western world. The chemical composition of the resin moulding compounds used is given in the following Table I (allfiguresin percent; the rest mounting up to 100 % contains additives not shown here). From these compounds bars of thermoset plastics were made by compression moulding. The bars had a rectangular cross-section, 120 mm χ 15 mm χ 10 mm. The short term irradiation under (nearly) complete exclusion of oxygen was done with a 3 MeV electron beam accelerator (irradiation from both sides). The long term irradiation experiments were performed with cobalt60 sources with an average dose rate of only 13.8 Gy/h. These experiments lasted over a period of 10.51 years, ending with a dose of 1.27 MGy. The dose rate could not be held accurately constant during this long period of time. In fact it changed between 21 and 9.3 Gy/h. A few bars remained in the irradiation chamber after the end of the main part of the experiment. After 16.01 years of irradiation these bars had received a dose of 1.79 MGy. Theflexuralstrength was tested according to DIN 53 452 (ISO 178) with five bars at 23 °C., the deflection temperature was tested according to DIN 53 458 and 53 462 ("Martens method") with two bars only. In this test a bar is held at the top with a 240 mm long lever at a bending tension of 5 N/mm and heated at 50 K/h. The temperature, at which the end of the lever has sunk by 6 mm, is called the "deflection temperature". 2

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Table I. Chemical Composition of the Thermosets

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Inorganic Filled Thermosets Epoxy plastic (laboratory product) ΈΡ" 28.2 EP-resin (made of epichlorohydrin and diphenylolpropane), 3.0 mphenylene diamine, 1.3 m-toluylene diamine, 12.0 lime stone, 27.0 China clay, 26.0 glass fibers. Unsaturated polyester "L 1405" 24.0 Unsaturated polyester resin (made of terephthalic acid, fumaric acid and butandiol-1.4), 2.0 diallyl phthalate, 46.5 lime stone, 10.0 China clay, 15.0 glass fibers. Melamine-formaldehyde MF 'Typ 158" 50.0 Melamine-formaldehyde resin, 46.0 asbestos fibers, 3.0 glass fibers. Phenol-formaldehyde PF 'Typ 15" 30.3 Novolak, 4.1 hexamethylene tetramine, 55.0 asbestosfibers,6.3 cotton threads, 2.7 wood flour. Organic Filled Thermosets Melamine-formaldehyde MF 'Typ 150" 39.0 Melamine-formaldehyde resin, 8.0 urea-formaldehyde resin, 15.0 dust from Typ 150, 33.2 woodflour,3.1 chalk. Phenol-formaldehyde PF 'Typ 31 " 38.6 Novolak, 5.5 hexamethylene tetramine, 47.2 woodflour,3.6 chalk.

Results The test specimens as well as all other thermoset plastics were moulded from compounds containing very coarse components, which are never ideally homogenized. A relatively large scatter of the properties is the result. To minimize this scatter, the standards for the thermosets prescribe relatively thick test specimens. These thick pieces are, of course, not very suitable for the investigation of the influence of oxygen during a long term experiment. But there was no other choice for the test specimens since otherwise the reproducibility of the measurements could by no means be guaranteed. The flexural strength of the inorganic filled thermoset plastics as a function of the dose is shown in Figure 1. In thisfigurethe points drawn in the ordinate and marked with Έ" (Beginning) show theflexuralstrengths of the non-irradiated materials prior to the beginning of the experiments. The measured points "E" (End) stand for theflexuralstrengths of the non-irradiated materials measured after a storage time of more than 16 years. The two

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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125

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Figure 1. Flexural strength of thermoset plastics with inorganicfilleras a function of dose. · Irradiation with high dose rate under exclusion of air. Ο Irradiation with extremely low dose rate in air. EP = epoxy plastic, MF = melamine-formaldehyde Typ 158, UP = unsaturated polyester L1405, PF = phenol-formaldehyde Typ 15.

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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corresponding values are always the same within the limits of the experimental error. An influence of the storage time (with no irradiation) therefore can be excluded for all thermosets investigated. In case of the epoxy thermoset plastic there is also no influence of the irradiation with high dose rate up to a dose of 10 MGy. This was indeed expectedfromthe many experiments published by Schônbacher et al. (11). The long term irradiation as well has no influence on the flexural strength in this case. The same is true for melamine-formaldehyde plastic. The picture changes in case of the unsaturated polyester plastic. There is a small improvement of the flexural strength at high doses, but the material becomes deteriorated above 0.5 MGy if it is irradiated with the very low dose rate of only 13.8 Gy/h in air. The same seems to happen to the phenol-formaldehyde plastic. But in this case it has to be realized that the thermoset not only contains inorganicfillersbut also 9 % of organic components. Unfortunately this thermoset plastic was not available with only inorganicfiller.Therefore the observed changes to a certain extent are not the consequence of radiation damage of the phenolic plastic itself, but originate from the damage of the relatively small amount of the organic filler material. Evidence is brought for this in the Figures 2 and 3, which are shown on the same scale as Figure 1. The melamine-formaldehyde plastic and the phenol-formaldehyde plastic, bothfilledwith considerable amounts of wood flour, are readily deteriorated by irradiation. In both cases an influence of the dose rate is also seen clearly (the fat symbols denote high dose rate, the empty circles denote low dose rate). Because there was not almost seen a change in theflexuralstrength of the (inorganic filled) thermoset plastics, there is the question if there is no radiation induced change in the molecular structure of the plastic material itself. Since all the thermoset plastics are cross-linked with a high network density they are completely insoluble and consequently cannot be easily investigated with usual methods. But the heat deflection temperature may give some information about changes in the network density. As a matter of fact it is well known, that after-curing reactions, induced by annealing, can lead to a considerable improvement of the deflection temperature. For example, 15 hours annealing at 120 °C raises the deflection temperature for the phenol-formaldehyde plastic Typ 31from128 °C to 142 °C., in other words by 14 °C (12). This means it has to be expected that radiation induced cross-linking will lead to an increase in the deflection temperature. Figure 4 shows the result of the deflection temperature measurements. (The data points of the MF Typ 158 are not shown since they coincide with those of PF T^p 15.) A remarkable decrease of the deflection temperature is seen for all thermosets. This only can so far be explained by chain scission. It is also seen that the decrease of the deflection temperature does not depend on the dose rate. This may be explained by the extremely small diffusion coefficient for oxygen found for all of these thermosets (Pauly, S. Polymer, in press). During the long term irradiation only a very thin outer layer may be penetrated by oxygen and thus radiation chemical oxidized. The flexural strength measured at room temperature in the glassy state of the polymers may

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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31. WILSKI

Figure 3. Flexural strength of phenol-formaldehyde plastic Typ 31 with organicfiller(47.2 % woodflour)as a function of dose.

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Figure 5. Deflection temperature of thermoset plastics with organicfilleras a function of dose.

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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be affected by the brittle outer layer which was built in this way. On the other hand the deflection temperature lies well above the respective glass transition temperatures of the thermosets themselves and their oxidized outer layers. In this temperature region there exists no more brittleness and therefore no more any difference between high and low dose rate experiments. - A decrease of the heat distortion temperature of epoxy plastics was already observed by Colichman et al. (13). Figure 5 shows the heat deflection temperature of the organic filled thermosets MF Typ 150 and PF Typ 31. In these cases there was also no influence of the dose rate observed. The reason is the same as before. The decrease in deflection temperature of the (mostly) inorganicfilledPF Typ 15 at 10 MGy is 56 °C., the decrease of the organicfilledPF Typ 31 at the same dose is 55 °C. This is within the limits of error the same value, and that would mean the deflection temperature is really a property of the thermosets themselves and not of thefillermaterial. This fact provides support for the assumption that the phenolic thermosets do not cross-link by irradiation but in fact degrade. From a practical point of view it seems to be important that irradiation makes the thermosets more sensitive to the influence of heat - an aspect neglected so far. Concluding Remarks The experimental results described in this paper show that the dose rate is of importance in some cases, in others it is insignificant. But it shall be mentioned that other properties, e.g. the surface resistance and the tracking resistance are strongly influenced by the dose rate as a consequence of the surface chemistry of the thermosets. A detailed discussion of these results as well as a precise description of the experimental conditions belonging to this paper will be published in the near future (Gilfrich, H.P.; Rôsinger, S.; Wilski, H.). Acknowledgments Thanks are due to Dipl.-Ing. R. Bôrst, Hoechst AG, for making the computer drawings. Literature Cited 1 2 3 4 5 6

Clough, R.L.; Gillen, K.T.; Campan, J.-L.; Gaussens, G.; Schönbacher, H.; Seguchi, T.; Wilski, H.; Machi, S. Nuclear Safety 1984, 25, 238. Wündrich, K.Radiat.Phys. Chem. 1985, 24, 503. Wilski, H.Radiat.Phys. Chem. 1987, 29, 1. Wündrich, Κ. in Polymer Handbook; Brandrup, J. and Immergut, E.H., Ed., John Wiley, New York, N.Y., 1989 VI/463. Gillen, K.T.; Clough, R.L.J.Polym.Sci.Polym. Chem. Ed. 1985, 23, 2683. Collins, C.G.; Calkins, V.P. Radiation damage to elastomers, plastics, and

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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organic liquids; APEX 261, Techn. Publications GE-ANPD, Evendale, Ohio 1956. Rauhut, K.; Rösinger, S.; Wilski, H. Kunststoffe 1980, 70, 89. Schönbacher, H.; Stolarz-Izycka, A. CERN 79-08 1979. Beynel, P.; Maier, P.; Schönbacher, H. CERN 82-10 1982. Kunststoff Handbuch Duroplaste; Becker, G.W.; Braun, D.; Woebcken, W., Eds.; Hanser, München, Germany 1988, Vol. 10. Schönbacher, H.; Schreiber, B.; Stierli, R. Kunststoffe 1986, 76, 759. Loc. cit. 10, p. 252. Colichman, E.L.; Strong, J.D. Mod. Plastics 1957, 35 (October), 180.

R E C E I V E D May 29, 1991

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.