Radiation

Resistance

and

of

Plastics

Elastomers

Tadao Seguchi, Yosuke Morita Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute (JAERI), Takasaki, Gunma 370-12, Japan

A. Introduction 1. General Comments 2. Criterion for Radiation Resistance 3. Factors of Influence and their Consideration in Tables 3.1. Type of Polymer and Formulation 3.2. Type of Radiation and Dosimetry 3.3. Dose Rate and the Atmosphere 3.4. Temperature 3.5. Other Stresses B. List of Symbols Used C. Tables of Radiation Resistance Table 1. Thermoplastics Table 2. Elastomers Table 3. Aromatic Polymers Table 4. Organic Composite Materials D. References A.

INTRODUCTION

1.

General Comments

VI-583 VI-583 VI-583 VI-583 VI-583 VI-584 VI-584 VI-584 VI-584 VI-584 VI-585 VI-585 VI-586 VI-587 VI-588 VI-588

The data on radiation resistance have been compiled mainly for practical polymer materials that have been classified into four groups: thermoplastics, elastomers, all aromatic polymers, and composite materials. The values of radiation resistance were determined by the change in mechanical properties. Radiation-induced degradation of polymer materials depends greatly on the irradiation conditions, as indicated in the tables; hence, the reader is advised to use these data with caution, considering all the factors. 2. Criterion for Radiation Resistance Radiation resistance is characterized by the half-value-dose of significant changes in mechanical properties. For

elastomers and thermoplastics, with large ultimate elongation by tensile test, the resistance is characterized by 100%

elongation at break. The half-value-dose means the absorbed dose that reduces a property to 50% of its initial value under defined environments. The SI unit gray (Gy; 1 Gy = 1 J/kg = 100 rad) is used, and values in tables are in units of MGy (106 Gy) when there is no notation. In cases where the absorbed dose in the experiment was terminated before reaching the criterion, the dose is given with the sign The properties can be affected differently, therefore more than one relevant property should be considered. In most cases, the properties used to characterize radiation resistance are the elongation at break for elastomers and flexible plastics, and the flexural strength for rigid plastics and composite materials. However, the elongation at break sometimes increases with elastomers deterioration, but in that case, the strength decreases greatly. 3. Factors of Influence and their Consideration in Tables Radiation resistance depends on polymer formulation as well as on the conditions of radiation exposure, such as the environmental atmosphere, temperature, dose rate, mechanical stress, etc. The most effective factor is the oxidation induced by radiation. Radiation resistance is indicated in two categories, with and without oxidation, together with the irradiation conditions employed. 3.1. Type of Polymer and Formulation The polymer samples reported were generally formulated by mixing with chemical agents, such as of stabilizers and fillers. Elastomers were normally crosslinked. The formulation and/or crosslinking have many variations for a given type of polymer. The formulation is determined by the suppliers based on the application (which may need, improve the mechanical or thermal properties), and the contents are usually not disclosed. Radiation resistance is influenced by formulation and crosslinking. In general, radiation resistance can be increased with inorganic fillers, with good

adhesion between the polymer and reinforcing fillers, with

antioxidants, and with aromatic compounds. Organic fillers usually decrease radiation resistance. Some of the aromatic polymers reported may contain fillers of glass or carbon fibers. The first column of the tables give the polymer name, along with the formulation or cross-linking. The second column gives the form of the test sample: either sheets (formed by heat press) or films (formed by extrusion or inflation), and their thickness (in mm). 3.2. Type of Radiation and Dosimetry The overall radiation effect does not depend on the type of radiation (such as X rays, gamma rays, electron beams, nuclear reactor irradiation, or other accelerated particle radiation), but only on the absorbed dose. There are some uncertainties about the given dose, especially to different types of radiation. In the following tables, the accuracy of the dose is estimated to be ±10% for gamma-rays and ±20% for electron beams or accelerated particles. 3.3. Dose Rate and the Atmosphere In an inert gas atmosphere or under vacuum, the dependence of dose rate on polymeric degradation is negligible at ambient temperature (1). If the other stresses, such as thermal degradation or light exposure, are eliminated during irradiation, the degradation should depend on the accumulated dose. If oxygen is present, the induced oxidation will decrease radiation resistance. The overall oxidation is determined by the rate of oxygen consumption by radiation-induced oxidation in the polymer matrix, and by the rate of oxygen supply into the matrix. The thickness of the polymer oxidation layer (L0x) is a function of radiation dose rate (I), oxygen partial pressure (P 02 ) a t o n e atmosphere total pressure, oxygen permeability coefficient (Pc) into the polymer matrix, and oxygen consumption factor ($), and is

B.

given by the following equation. (Ref. 2,3). (i) When the test material is a thick sheet, the oxidation layer occurs only at the surface, and the inside is not oxidized. The oxidation layer increases with decreasing dose rate. For most polymers, the degradation is more progressive with oxidation than without it. The dose rate affects the ratio of oxidation layer to material thickness. The degradation with oxidation can be observed by irradiation of a thin film; here, oxidation proceeds throughout the film. Oxidation throughout a thick film requires irradiation with oxygen under pressure rather than decreasing the dose rate (4). In the following tables, radiation resistance with oxidation implies that oxidation takes place throughout the material. The other dose-rate dependent degradation, chainreaction oxidation, has been reported at low dose-rate irradiations over long time periods. We believe that it is difficult to distinguish these chain reactions from thermal oxidation. In the tables, the data for very low dose rate have been excluded.

3.4. Temperature The irradiation temperature is ambient temperature, i.e. about 2 0 - 3 0 0 C . The data for lowtemperature irradiation is listed for few materials. Several experiments performed at higher irradiation temperatures are also listed. As the resistance depends on irradiation temperature, it is supposed that chemical reactions increase greatly due to the increased molecular motions during irradiation.

3.5. Other Stresses The tables do not consider mechanical stress and deformation during irradiation. These factors can have a large influence on degradation.

LIST OF SYMBOLS USED

a a 1/2 e e i/2 £100% aF crFi/2 IL IL1/2 7

Tensile strength at break (ultimate strength) Half of tensile strength at break Elongation at break (ultimate elongation) Half of elongation at break 100% of elongation at break Flexural strength Half of flexural strength Inter Lamella Shear Strength (ILSS) Half of ILSS 60 Co- 7-rays

EB n H+ RT Vac Air He N2 O2

Electron beams, ( ) energy Neutron, ( ) energy Proton, ( ) energy Room temperature Vaccum (including seal after vacuum) Air atmosphere Helium-gas atmosphere Nitrogen-gas atmosphere Oxygen gas, ( ) indicates pressure

C.

TABLES OF RADIATION RESISTANCE

TABLE 1.

THERMOPLASTICS Radiation resistance Property

Polymer

Form

Initial value

Poly(ethylene) low-density (LDPE), non formulated low-density (LDPE), non formulted cross-linked (XLPE), formulated, cable insulator

Sheet, 1 mm

r r : 18MPa c : 500%

Film, 0.1 mm

UHMW-PE (ultra high molecular weight-PE) Poly(propylene) (PP), isotactic PP

Fiber, 0.011 mm (p

a: £: a: £: a: e: a:

Sheet, 1 mm Tube, 5 mm 4>

Sheet, 0.5 mm Film, 0.1 mm

Poly(vinyl chloride) (PVC), formulated Poly(vinyl chloride) (PVC), cable jacket Poly (12-aminolauric acid), Nylon-12 Poly (8-caprolactam), Nylon-6 Poly(ethyleneterephthalate) (PET)

a- : 32 MPa S : 210% ۥ: 1200%

S-l/2 £

ioo%

^

1/2

£

100%

£ ioo%, £

100%

Dose (MGy)

a: 28MPa e : 370% Tube, a : 20MPa OD 18 mm 0 e : 190% 1.8 mm t Sheet, 0.5 mm o-: 80MPa c : 430% Sheet, 0.5 mm CT: 130MPa c : 400% Film, 0.1 mm a: 220MPa £ : 75% Film, 0.075 mm Sheet, 3 mm

Poly(styrene) (PS), atactic PS Poly(styrene) (PS), syndiotactic PS Poly(tetrafluoroethylene) (PTFE)

Sheet, 0.5 mm

Cross-linked PTFE (XL-PTFE) Poly(vinylidene fluoride) (PVDF)

Sheet, 0.5 mm

a : 280 MPa £ : 30%