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Electron beams in cable engineering
Radiat. Phys. Chem. Vol. 14, pp.911-918. Pergamon Press Ltd. 1979. Printed in Great Britain.
ELECTRON BEAMS IN CABLE ENGINEERING E.A. Abramyan I n s t i t u t e of High Temperatures, Academy of Sciences, USSR E.E. Finkel, All-Union S c i e n t i f i c Research I n s t i t u t e of Cable Industry
SUMMARY The paper covers methods of radiation modification of polymeric cable insulation~ i t s properties and service conditions. Typical electron accelerators and conveyer devices used in cable industry are described.
Nowdays much attention is given to new s c i e n t i f i c trends the r e a l i zation of which in industry is capable of accelerating s c i e n t i f i c and technical progress~ in p a r t i c u l a r , creation of new materials and effective methods of t h e i r processing. As is apparent from the research and development results accumulated by the present time in the USSR (1) and abroad (2), polymer radiation chemistry is one of the most promising trends. Modern engineering is characterized by progressing complication of equipment and apparatus operating conditions which finds natural reflection in more severe requirements for the materials used including polymers. The problem of creating polymeric material with a given set of service characteristic is far from being always solved by synthesis of a new polymer; in a number of cases the more effective and sometimes the only possible way is the modification of polymers already manufactured by the industry. I t is j u s t in this respect that polymer radiation chemistry opens wide prospects. The formation of three-dimenshional network in polymer due to the effect of ionizing radiation of high energy s i g n i f i c a n t l y i n f l u ences all i t s properties and with the appropriate selection of absorbed radiation dose and i r r a d i a t i o n conditions results in appreciable improvement of a number of performance characteristics (the a b i l i t y to flow at temperatures exceeding melting temperature is l o s t , the e l a s t i c modulus and the resistance to cracking in stressed state increase, e l e c t r i c a l and mechanical properties at elevated temperatures improve, etc. (3). With a l l variety of application of i r r a d i a t i o n modified polymers, products of e l e c t r i c a l purpose have come f i r s t for many years. 911
912
E.A.
Abramyan and E. E. Finkel
The possibility of using irradiation modified polymers at elevated temperatures puts in the forefront the problem of their thermos t a b i l i z a t i o n , i . e . oxidation protection in operation. This is espec i a l l y c r i t i c a l for p o l y o l e f i n s . Though i r r a d i a t i o n of polymers including polyolefins increases t h e i r heat-resistance, i t at the same time leads to the reduction of t h e i r thermal s t a b i l i t y due to the accumulation of radiation defects in the structure. As a results of t h i s f a c t , the process of i r r a d i a t e d polyethylene oxidation in the a i r at a temperature exceeding melting temperature proceeds more i n t e n s i v e l y than i t happens with non-irradiated polymer, the speed of oxidation reaction increasing with radiation dose. In connection with these p r i n c i p a l l y new methods of c r y s t a l l i zing polymers s t a b i l i z a t i o n were developed (4) which allowed to r e a l i z e in f u l l measure the p o t e n t i a l i t i e s of such material as i r r a diated polyethylene. Common commercial polyethylene i r r a d i a t e d up to optimum radiation dose retains i t s perfoKmance c h a r a c t e r i s t i c s in the a i r at 20~ C for some hours, at 150vC - for dozens of hours and at lO0-110 C - for some hundreds of hours. D u r a b i l i t y of operation in the indicated above conditions is l i m i t e d by severe decrease of e l a s t i c i t y and increase of material b r i t t l e n e s s due to the process of oxidation occuring rather quickly at elevated temperatures. High effeciency of the developed thermostabilizing systems as compared to the conventional standard antioxidants is aparent from data in Fig. I . Service l i f e of i r r a d i a t e d polyethylene (estimated by the time of elongation reduction in the process of thermal ageing up to I00%) containing thermostabilizing systems formulated on the basis of structural-and-chemical priRciple of s t a b i l i z a t i o n ("A" composition) exceedes 7000 hours at 150vC and 200 hours at 200°C (versus several dozens of hours and some hours, respectively, for i r r a d i a t e d polyethylene with a standard a n t i o x i d a n t ) . bp~ 500
~,~
60C
J;,o J. ,o,.,... 20C
--
ial (
Fig.l.
P
N
~00 5000 TimO, ,~2
$4
1
Elongation as a functiRn of ageing tim 8 in the a i r at 200~C (a) and 150 C (b): l - "A" composition; 2 - "B" composition; 3 - "C" composition
I t is aparent that the p o s s i b i l i t y of such thermostabilized polyethylene longterm operation in the a i r at elevated temperatures s i g n i f i c a n t l y enlarges perspectives of usage and f i e l d s of application of t h i s material.
913
Electron Beams in Cable Engineering
The value of absorbed radiation dose at which the required set of radiation-modified polymer properties is achived has great pract i c a l importance. The reduction of the appropriate dose increases proportionally the capacity of the radiation - processing plant with a given radiation source power and leads to the decrease of concentration of radiation defects accumulated in the material. The work that had been carried out resulted in the creation of polyolefin-based compositions having the optimum set of properties which may be achieved with lower absorbed doses as compared to "A" composition. Fig.2 shows the dependence of gel-fraction content on the absorbed radiation dose. I t may be seen that while 70% - gelfraction content of "A" composition (curve l ) assuring the required set of material properties achieved at lO0 Mrad, the technological dose of "B" composition (curve 2) is reduced to 50 Mrad and for "C" composition which is capable of self-extinguishing when removed from flame° evolves l i t t l e smoke when burning and causes no metal corrosion (curve 3) the dose is reduced to lO-15 mrad. The service l i f e of "B" and "C" compositions is approximately on the same level as that "A" composition.
~00
-
~. 80 ,~. 5o
I
30 2o
'oJi 50
~00
200
3OO
~00
5O0
.~gso~ged dose, ~ d
Fig. 2. The accumulation of gel-fraction in d i f f e r e n t compositions based on polyolefins: ],4 - t y p e "A"; 2 - type "B"; 3 - type "C"; 5 - type "D" Early developments of polymer compositions for subsequent radiation modification involved mainly polyolefins. Alongside with these com. positions, PVC-based cross-linkable compositions are of substantial m~rest for insulating materials and cable technology. By the present time a number of formulations of i r r a d i a t i o n cross-linked PVC compounds have been developed including "E" type of improved p r o c e s s i b i l i t y (admits high-speed extrusio R and thin layer application) for operating temperature up to +70vC and "F" type for operating temperature up to +]05 C. Fig. 3 shows gel-fraction content dependence on i r r a d i a t i o n doze for i r r a d i a t i o n cross-linked PVC compound of "E" t y p e (curve l ) and the dependence of equilibrium deformation value of this compound
R.P.C. 14/3~5--PP
914
E.A.
Abramyan and E. E. Finkel
90' 80'
~
60'
! ¢~a ..40
3o P
3 to r~
~0
.OStorStd do~e ,
Fig.
MpoO
3. The absorbed dose dependence of gelfraction content and equilibrium deformation under the load of 0,5 kg at 150vC for PVC compound: 1,2 type "E"; 3 - type "F"
specimen on i r r a d i a t i o n dose in thermomechanical tests (curve 2). I t may be seen that in 15-20 Mrad range of absorbed dose rather high content of gel-fraction is achieve~ and the deformation of specimen under the strain of 0,5 kg at 150 C does not exceed 40% of i t s original thickness. From this figure i t is aparent that optimum properties of i r r a d i a t i o n cross-linked compound of "F" t y p e (curve 3) are attained at s i g n i f i c a n t l y lower absorbed radiation dose - about 5 Mrad. At present radiation processing is successfully applied to fluoropolymers (polyvinylidenefluoride, ethylene-tetrafluoroethylene copolymer, e t c . ) . Wide use of irradiation-modified polymers of d i f f e r e n t type for wire and cable insulation and sheath allows to improve t h e i r operating characteristics, manufacturing technology and realize not only valuable technical but also appreciable economiC effect (5). I t should be outlined that final properties of i r r a d i a t i o n modified polymeric material are unambigyously determined by absorbed radiation dose and by attaining the required uniform d i s t r i b u t i o n throughout the whole material volume, but p r a c t i c a l l y they do not depend on the kind of radiation (and accordingly on the type of radiation source employed in the conditions eliminating the possibil i t y of some other side processes which could effect the final proper ties of the product). At the same time technical expediency and economic factors of manufacture with radiation source of one or another type are determined by p e c u l i a r i t i e s of a particular manufacturing process. Radiation modification of wire and cable polymeric insulation and sheath is preferably carried out with the use of electron beams. High dose rates created in large space by modern heavy-current electron accelerators enable continuous i r r a d i a t i o n process by means of transportation (rewinding) of cable product in the electron beam
Electron Beams in Cable Engineering
915
zone and besides the inevitable processes of radiation oxidation are less c r i t i c a l due to short-term presence of the product in the radiation f i e l d (5). In the USSR for the organization of the f i r s t commercial production of wire and cable with irradiation-modified insulation electron accelerators of transformer ELT type have been developed and since 1968 used (6, 7). In the last few years on the basis of longterm operating experience of ELT plants a series of modified plants of ELV type has been created. Accelerators of both types are based on transformer method of voltage step-up. The principle difference between these plants is in the method of voltage r e c t i f i c a t i o n : in ELT an accelerating tube is used as a r e c t i f y i n g device assuring constant accelerating voltage due to the modulation of beam current; in ELV plant each section of the transformer multisection secondary winding contains a r e c t i f y i n g device similary to ICT generators. A number of main units of ELT and ELV accelerators are unificated and primary and secondary windings of ELT transformer, magnetic c i r c u i t parts~ accelerating tube, etc. are used with some modifications in ELV plant. Electric and design c i r c u i t of both plants are shown in the report of this conference (8). Parameters and overall dimensions of the accelerators used in cable industry are l i s t e d in the table. Maximum accelerator parameters attained in tests are given in brackets. Accele- Electron Average Housing Housing Supply E f f i c i - Date of rator e n e r g y , power, diameter height voltage e n c y , developtype MeV kW m m % ment ELT-I.5
0.7(1.5)
1 5 ( 2 5 ) 1.2
2
ELT-2.5 ELV-I
1.7(2.2) l
1 5 ( 2 0 ) 1.8 20 1.2
4.5 2
ELV-2
1.5
20
2
1.2
227 V 50 Hz -"380 V 400 Hz -"-
90
1964
90
1967
70 70
1973 1974
I t is l i k e l y that in the nearest coming years new accelerators for higher voltages and power (Electronica-500, ILU-6 and others) which are being under development w i l l be used (8). The development of commercial accelerators for cable industry may proceed in two other directions: the creation of 3-5 MeV energy plants with electron energy conversion in gamma-ray for large products i r r a d i a t i o n and the development of portable low energy accelerators with I-5 kW beam power for the i r r a d i a t i o n of small diameter cable insulation. Despite the evident advantages of electron accelerators as radiation sources for the organization of industrial radiation-processing of long-length (extended) product modification, the use of highpower electron beams for this aim is connected with some d i f f i c u l t i e s . High dose rate along with low heat conduction of polymers makes the problem of heat-removal from the irradiated product even more d i f f i c u l t . Specific p e c u l i a r i t i e s of cable products (combination of polymer and metal in the construction, c y l i n d r i c a l configuration, long-length with r e l a t i v e l y small diameter, etc.) predetermine considerable d i f f i c u l t i e s in attaining appropriate d i s t r i b u t i o n of the absorbed radiation dose over the whole insulation (sheath) volume. According to the p e c u l i a r i t i e s of irradiated cable products, primarily according to the maximum insulation (sheath) thickness and product diameter and hence the required maximum electron energy,
916
E.A.
Abramyan and E. E. Finkel
there are two variants of radiation-processing equipment arrangement: the use of local shielding incorporating the accelerator o u t l e t scan horn and the adjecent processing equipment forming the i r r a d i a t i o n zone or the arrangement of the accelerator and processing equipment mentioned above in a special vault the walls of which function as biological shielding. I t is well known that the d i s t r i b u t i o n of the absorbed dose over the depth of the i r r a d i a t e d material with f l a t geometry of the object and the d i r e c t i o n of incident beam of monoenergetic electrons being perpendicular to i t s surface is not uniform. The maximum of d i s t r i bution is at a depth of approximately one t h i r d of the maximum electron path and at a depth equal to two thirds of this value the dose again becomes equal to the dose on the surface. This leads to some d i f f i c u l t i e s in a t t a i n i n g the required uniformity of the absorbed dose over the whole volume of even f l a t object. The i r r a d i a t i o n of wire and cable is s t i l l more d i f f i c u l t due to the v a r i a t i o n of insulation thickness around the diameter in the plane perpendicular to wire axis, in the d i r e c t i o n of incident electron beam: as well as due to the screening e f f e c t of the conductor. One of the ways of overcoming the mentioned d i f f i c u l t i e s is the use of processing equipment in the form of a m u l t i r o l l e r system (Fig. 4a). This system allows continuous passing of the cable product under i r r a d i a t i o n through the zone of electron beam, scanned in Awo mutually perpendicular d i r e c t i o n s , with a revolution through 180v a f t e r each pass to eliminate the screening e f f e c t of the conductor. Multiple successive passing of wire or cable through the i r r a d i a t i o n zone allows to gain the required absorbed radiation dose by separate cycles~ the r e l a t i o n between the i r r a d i a t i o n cycle duration and the interval between successive cyles may be varied by processing equipment parameters in such a way that insulation would not be overheated above the permissible temperature in natural or forced cooling conditions. Such system of " f r a c t i o n a l " i r r a d i a t i o n provides reasonable heat-removal from the i r r a d i a t e d cable product at high
do~e r a t e
8 Fig.4.
Process diagrams of cable product i r r a d i a t i o n : a - m u l t i r o l l e r system; b - system with two drums of d i f f e r e n t construction.
Electron Beams in Cable Engineering
917
I t has been demonstrated experimentally and a n a l y t i c a l l y (5) that appropriate selection of electron energy and with a given insul a t i o n thickness in case of double-side i r r a d i a t i o n 80% of insulation volume has the absorbed dose within +20% of the optimum dose and in case of four-side i r r a d i a t i o n this v~lue reaches 94%. The indicated constructive decisions concerning the creation of processing equipment enabled the modification of cable and wire insulation with electron beam having the power up to 25 kW and above, the dose rate being 3-4 orders higher than that of isotope sources. However, in spite of the principal p o s s i b i l i t y of increasing the electron beam cross-section area u t i l i z a t i o n factor up to 60-70% by bringing together several r o l l e r systems under the scan horn, such systems, besides the additional operating d i f f i c u l t i e s with complicated wire charging, have some principal shortcomings: r e l a t i vely low electron beam cross-section area u t i l i z a t i o n factor due to the i m p o s s i b i l i t y of compact arrangement of successive wire turns; the necessity of replacing r o l l e r blocks when the i r r a d i a t e d products of one r e l a t i v e l y narrow group of diameters are changed for another; high tension force during product rewinding and hence deformation and breaking of the i r r a d i a t e d products especially when i r r a d i a t i n g conductors of small cross-section. To achieve higher technical-and-economic c h a r a c t e r i s t i c s nf the radiation-processing and to improve the q u a l i t y of i r r a d i a t e d cable products o r i g i n a l processing equipment free from the indicated above shortcomings (Fig. 4b) has been developed in the USSR. The system is designed for cable product transportation through electron beam with simultaneous formation of i r r a d i a t i o n zone and consists of two drums of d i f f e r e n t construction allowing by means of appropriate mechanisms to carry out long-distance control of w~re or cable layning pitch. This system allows to eliminate the drawbacks of the t r a d i t i o n a l r o l l e r system: there is no high tension force because every turn is set in r o t a t i o n independently; cable products of a wide diameter range may be rewinded (and hence i r r a d i a t e d ) on t h i s system with the r e a l i z a t i o n of electron beam cross-section area u t i l i z a t i o n factor close to I00% for any diameter product; this system allows to change the pitch of reel winding and thus to f u l f i l continuous t r a n s f e r from i r r a d i a t i o n of one diameter products to i r r a d i a t i o n of other diameter products without i n t e r r u p t i n g the i r r a d i a t i o n process. Depending on overall dimensions and design features of cable product the developed processing equipment ensures l i n e a r speeds of products movement in electron beam zone from 50 to 500 m/min and higher. The use of electron accelerators with 20 kW power in the beam and electron energy of 0,5-1,5 MeV combined with the developed highproduction equipment ensures high technical-and-economic effeciencv of wire and cable with i r r a d i a t i o n - m o d i f i e d polymer insulation manufacturing process. At present the electrotechnical industry of the USSR produces a wide v a r i e t y of wire and cable with i n s u l a t i o n and sheath based on i r r a d i a t i o n - m o d i f i e d polymers ( i n s t a l l a t i o n wire, control cable, shipboard cable, wire and cable for atomic stations, a i r c r a f t wire, computer wire, e t c . ) . All these products have increased heat resistance and r e l i a b i l i t y and are widely used in d i f f e r e n t f i e l d s of engineering (5, lO, l l ) . I t should be noted that in most cases the use of radiation enemy for a given production process is more e f f e c t i v e than t r a d i t i o n a l l y used types of energy, f o r example, thermal energy. That is why even
E. A. Abramyan and E. E. Finkel
918
comparison of purely economic factors of one and the same process which may be carried out, for example, both by thermochemical and i r r a d i a t i o n methods t e s t i f i e s in favour of the l a s t one. And one of the main factors predetermining the advantages of i r r a d i a t i o n - p r o duction process as compared to t r a d i t i o n a l methods of obtaining s i m i l a r products is the engineering level and perfect characterist i c s of the s p e c i f i c processing equipment including r a d i a t i o n source. Development and mastering of the r a d i a t i o n technology as applied to the problems of cable i n d u s t r y may be considered as appearance of a now s p e c i f i c f i e l d of i n s u l a t i n g and cable technique the promising future of which causes no doubt. REFERENCES I.
Radiation chemistry of Polymers (1973) Nauka, Moskow.
2. Radiation processing, Trans. of the F i r s t . I n t e r n . Meeting of r a d i a t i o n processing,Radiation physics and chemistrZ, 9, Nn I-R~1977) 3. I B.Peshknv, E.E.Finkel, Radiation modification of polymers is a progressive method of new i n s u l a t i n g materials creation, Paper No 3A 33, World Electrotechnical Congress, June 1977, Moscow. 4. E.E.Fihkel, et a l . (1977) S t a b i l i z a t i o n of i r r a d i a t i o n - m o d i f i e d Egl_hymers, Himija~ Moscow. 5. E.E.Finkel, R.P.Braginsky (1975) Heat-resistant wire and cable with i r r a d i a t i o n - m o d i f i e d i n s u l a t i o n , Energija, Moscow. 6. E.A.Abramyan, V.A.Gaponov, Source of accelerated p a r t i c l e s of equal energy, No 203144 (1968) B u l . i s o b r e t . , (USA patent N 3390303) 7. E.A.Abramyan. Heavy-Current electron accelerators, Radiation sources fQ~__ind_LLstria! processes (1969) 661, Intern.Atomic Energy, Agency, Vienna. 8. E.A.Abramyan, Radiation-chemical technology on electron accelerators in the USSR, Report on the present conference. 9. E.E.Finkel, et a l . , USA patent N 33860159. lO. E.E.Finkel et a l . (1968) Radiation chemistry and cable engineer i n g , Atomisdat, Moscow. I I . V.A.Privezentsev, l.B.Peshkov (1971) Windin9 and i n s t a l l a t i o n wires,Energija, Moscow.
ELECTRON BEAMS IN CABLE ENGINEERING E.A. Abramyan I n s t i t u t e of High Temperatures, Academy of Sciences, USSR E.E. Finkel, All-Union S c i e n t i f i c Research I n s t i t u t e of Cable Industry
SUMMARY The paper covers methods of radiation modification of polymeric cable insulation~ i t s properties and service conditions. Typical electron accelerators and conveyer devices used in cable industry are described.
Nowdays much attention is given to new s c i e n t i f i c trends the r e a l i zation of which in industry is capable of accelerating s c i e n t i f i c and technical progress~ in p a r t i c u l a r , creation of new materials and effective methods of t h e i r processing. As is apparent from the research and development results accumulated by the present time in the USSR (1) and abroad (2), polymer radiation chemistry is one of the most promising trends. Modern engineering is characterized by progressing complication of equipment and apparatus operating conditions which finds natural reflection in more severe requirements for the materials used including polymers. The problem of creating polymeric material with a given set of service characteristic is far from being always solved by synthesis of a new polymer; in a number of cases the more effective and sometimes the only possible way is the modification of polymers already manufactured by the industry. I t is j u s t in this respect that polymer radiation chemistry opens wide prospects. The formation of three-dimenshional network in polymer due to the effect of ionizing radiation of high energy s i g n i f i c a n t l y i n f l u ences all i t s properties and with the appropriate selection of absorbed radiation dose and i r r a d i a t i o n conditions results in appreciable improvement of a number of performance characteristics (the a b i l i t y to flow at temperatures exceeding melting temperature is l o s t , the e l a s t i c modulus and the resistance to cracking in stressed state increase, e l e c t r i c a l and mechanical properties at elevated temperatures improve, etc. (3). With a l l variety of application of i r r a d i a t i o n modified polymers, products of e l e c t r i c a l purpose have come f i r s t for many years. 911
912
E.A.
Abramyan and E. E. Finkel
The possibility of using irradiation modified polymers at elevated temperatures puts in the forefront the problem of their thermos t a b i l i z a t i o n , i . e . oxidation protection in operation. This is espec i a l l y c r i t i c a l for p o l y o l e f i n s . Though i r r a d i a t i o n of polymers including polyolefins increases t h e i r heat-resistance, i t at the same time leads to the reduction of t h e i r thermal s t a b i l i t y due to the accumulation of radiation defects in the structure. As a results of t h i s f a c t , the process of i r r a d i a t e d polyethylene oxidation in the a i r at a temperature exceeding melting temperature proceeds more i n t e n s i v e l y than i t happens with non-irradiated polymer, the speed of oxidation reaction increasing with radiation dose. In connection with these p r i n c i p a l l y new methods of c r y s t a l l i zing polymers s t a b i l i z a t i o n were developed (4) which allowed to r e a l i z e in f u l l measure the p o t e n t i a l i t i e s of such material as i r r a diated polyethylene. Common commercial polyethylene i r r a d i a t e d up to optimum radiation dose retains i t s perfoKmance c h a r a c t e r i s t i c s in the a i r at 20~ C for some hours, at 150vC - for dozens of hours and at lO0-110 C - for some hundreds of hours. D u r a b i l i t y of operation in the indicated above conditions is l i m i t e d by severe decrease of e l a s t i c i t y and increase of material b r i t t l e n e s s due to the process of oxidation occuring rather quickly at elevated temperatures. High effeciency of the developed thermostabilizing systems as compared to the conventional standard antioxidants is aparent from data in Fig. I . Service l i f e of i r r a d i a t e d polyethylene (estimated by the time of elongation reduction in the process of thermal ageing up to I00%) containing thermostabilizing systems formulated on the basis of structural-and-chemical priRciple of s t a b i l i z a t i o n ("A" composition) exceedes 7000 hours at 150vC and 200 hours at 200°C (versus several dozens of hours and some hours, respectively, for i r r a d i a t e d polyethylene with a standard a n t i o x i d a n t ) . bp~ 500
~,~
60C
J;,o J. ,o,.,... 20C
--
ial (
Fig.l.
P
N
~00 5000 TimO, ,~2
$4
1
Elongation as a functiRn of ageing tim 8 in the a i r at 200~C (a) and 150 C (b): l - "A" composition; 2 - "B" composition; 3 - "C" composition
I t is aparent that the p o s s i b i l i t y of such thermostabilized polyethylene longterm operation in the a i r at elevated temperatures s i g n i f i c a n t l y enlarges perspectives of usage and f i e l d s of application of t h i s material.
913
Electron Beams in Cable Engineering
The value of absorbed radiation dose at which the required set of radiation-modified polymer properties is achived has great pract i c a l importance. The reduction of the appropriate dose increases proportionally the capacity of the radiation - processing plant with a given radiation source power and leads to the decrease of concentration of radiation defects accumulated in the material. The work that had been carried out resulted in the creation of polyolefin-based compositions having the optimum set of properties which may be achieved with lower absorbed doses as compared to "A" composition. Fig.2 shows the dependence of gel-fraction content on the absorbed radiation dose. I t may be seen that while 70% - gelfraction content of "A" composition (curve l ) assuring the required set of material properties achieved at lO0 Mrad, the technological dose of "B" composition (curve 2) is reduced to 50 Mrad and for "C" composition which is capable of self-extinguishing when removed from flame° evolves l i t t l e smoke when burning and causes no metal corrosion (curve 3) the dose is reduced to lO-15 mrad. The service l i f e of "B" and "C" compositions is approximately on the same level as that "A" composition.
~00
-
~. 80 ,~. 5o
I
30 2o
'oJi 50
~00
200
3OO
~00
5O0
.~gso~ged dose, ~ d
Fig. 2. The accumulation of gel-fraction in d i f f e r e n t compositions based on polyolefins: ],4 - t y p e "A"; 2 - type "B"; 3 - type "C"; 5 - type "D" Early developments of polymer compositions for subsequent radiation modification involved mainly polyolefins. Alongside with these com. positions, PVC-based cross-linkable compositions are of substantial m~rest for insulating materials and cable technology. By the present time a number of formulations of i r r a d i a t i o n cross-linked PVC compounds have been developed including "E" type of improved p r o c e s s i b i l i t y (admits high-speed extrusio R and thin layer application) for operating temperature up to +70vC and "F" type for operating temperature up to +]05 C. Fig. 3 shows gel-fraction content dependence on i r r a d i a t i o n doze for i r r a d i a t i o n cross-linked PVC compound of "E" t y p e (curve l ) and the dependence of equilibrium deformation value of this compound
R.P.C. 14/3~5--PP
914
E.A.
Abramyan and E. E. Finkel
90' 80'
~
60'
! ¢~a ..40
3o P
3 to r~
~0
.OStorStd do~e ,
Fig.
MpoO
3. The absorbed dose dependence of gelfraction content and equilibrium deformation under the load of 0,5 kg at 150vC for PVC compound: 1,2 type "E"; 3 - type "F"
specimen on i r r a d i a t i o n dose in thermomechanical tests (curve 2). I t may be seen that in 15-20 Mrad range of absorbed dose rather high content of gel-fraction is achieve~ and the deformation of specimen under the strain of 0,5 kg at 150 C does not exceed 40% of i t s original thickness. From this figure i t is aparent that optimum properties of i r r a d i a t i o n cross-linked compound of "F" t y p e (curve 3) are attained at s i g n i f i c a n t l y lower absorbed radiation dose - about 5 Mrad. At present radiation processing is successfully applied to fluoropolymers (polyvinylidenefluoride, ethylene-tetrafluoroethylene copolymer, e t c . ) . Wide use of irradiation-modified polymers of d i f f e r e n t type for wire and cable insulation and sheath allows to improve t h e i r operating characteristics, manufacturing technology and realize not only valuable technical but also appreciable economiC effect (5). I t should be outlined that final properties of i r r a d i a t i o n modified polymeric material are unambigyously determined by absorbed radiation dose and by attaining the required uniform d i s t r i b u t i o n throughout the whole material volume, but p r a c t i c a l l y they do not depend on the kind of radiation (and accordingly on the type of radiation source employed in the conditions eliminating the possibil i t y of some other side processes which could effect the final proper ties of the product). At the same time technical expediency and economic factors of manufacture with radiation source of one or another type are determined by p e c u l i a r i t i e s of a particular manufacturing process. Radiation modification of wire and cable polymeric insulation and sheath is preferably carried out with the use of electron beams. High dose rates created in large space by modern heavy-current electron accelerators enable continuous i r r a d i a t i o n process by means of transportation (rewinding) of cable product in the electron beam
Electron Beams in Cable Engineering
915
zone and besides the inevitable processes of radiation oxidation are less c r i t i c a l due to short-term presence of the product in the radiation f i e l d (5). In the USSR for the organization of the f i r s t commercial production of wire and cable with irradiation-modified insulation electron accelerators of transformer ELT type have been developed and since 1968 used (6, 7). In the last few years on the basis of longterm operating experience of ELT plants a series of modified plants of ELV type has been created. Accelerators of both types are based on transformer method of voltage step-up. The principle difference between these plants is in the method of voltage r e c t i f i c a t i o n : in ELT an accelerating tube is used as a r e c t i f y i n g device assuring constant accelerating voltage due to the modulation of beam current; in ELV plant each section of the transformer multisection secondary winding contains a r e c t i f y i n g device similary to ICT generators. A number of main units of ELT and ELV accelerators are unificated and primary and secondary windings of ELT transformer, magnetic c i r c u i t parts~ accelerating tube, etc. are used with some modifications in ELV plant. Electric and design c i r c u i t of both plants are shown in the report of this conference (8). Parameters and overall dimensions of the accelerators used in cable industry are l i s t e d in the table. Maximum accelerator parameters attained in tests are given in brackets. Accele- Electron Average Housing Housing Supply E f f i c i - Date of rator e n e r g y , power, diameter height voltage e n c y , developtype MeV kW m m % ment ELT-I.5
0.7(1.5)
1 5 ( 2 5 ) 1.2
2
ELT-2.5 ELV-I
1.7(2.2) l
1 5 ( 2 0 ) 1.8 20 1.2
4.5 2
ELV-2
1.5
20
2
1.2
227 V 50 Hz -"380 V 400 Hz -"-
90
1964
90
1967
70 70
1973 1974
I t is l i k e l y that in the nearest coming years new accelerators for higher voltages and power (Electronica-500, ILU-6 and others) which are being under development w i l l be used (8). The development of commercial accelerators for cable industry may proceed in two other directions: the creation of 3-5 MeV energy plants with electron energy conversion in gamma-ray for large products i r r a d i a t i o n and the development of portable low energy accelerators with I-5 kW beam power for the i r r a d i a t i o n of small diameter cable insulation. Despite the evident advantages of electron accelerators as radiation sources for the organization of industrial radiation-processing of long-length (extended) product modification, the use of highpower electron beams for this aim is connected with some d i f f i c u l t i e s . High dose rate along with low heat conduction of polymers makes the problem of heat-removal from the irradiated product even more d i f f i c u l t . Specific p e c u l i a r i t i e s of cable products (combination of polymer and metal in the construction, c y l i n d r i c a l configuration, long-length with r e l a t i v e l y small diameter, etc.) predetermine considerable d i f f i c u l t i e s in attaining appropriate d i s t r i b u t i o n of the absorbed radiation dose over the whole insulation (sheath) volume. According to the p e c u l i a r i t i e s of irradiated cable products, primarily according to the maximum insulation (sheath) thickness and product diameter and hence the required maximum electron energy,
916
E.A.
Abramyan and E. E. Finkel
there are two variants of radiation-processing equipment arrangement: the use of local shielding incorporating the accelerator o u t l e t scan horn and the adjecent processing equipment forming the i r r a d i a t i o n zone or the arrangement of the accelerator and processing equipment mentioned above in a special vault the walls of which function as biological shielding. I t is well known that the d i s t r i b u t i o n of the absorbed dose over the depth of the i r r a d i a t e d material with f l a t geometry of the object and the d i r e c t i o n of incident beam of monoenergetic electrons being perpendicular to i t s surface is not uniform. The maximum of d i s t r i bution is at a depth of approximately one t h i r d of the maximum electron path and at a depth equal to two thirds of this value the dose again becomes equal to the dose on the surface. This leads to some d i f f i c u l t i e s in a t t a i n i n g the required uniformity of the absorbed dose over the whole volume of even f l a t object. The i r r a d i a t i o n of wire and cable is s t i l l more d i f f i c u l t due to the v a r i a t i o n of insulation thickness around the diameter in the plane perpendicular to wire axis, in the d i r e c t i o n of incident electron beam: as well as due to the screening e f f e c t of the conductor. One of the ways of overcoming the mentioned d i f f i c u l t i e s is the use of processing equipment in the form of a m u l t i r o l l e r system (Fig. 4a). This system allows continuous passing of the cable product under i r r a d i a t i o n through the zone of electron beam, scanned in Awo mutually perpendicular d i r e c t i o n s , with a revolution through 180v a f t e r each pass to eliminate the screening e f f e c t of the conductor. Multiple successive passing of wire or cable through the i r r a d i a t i o n zone allows to gain the required absorbed radiation dose by separate cycles~ the r e l a t i o n between the i r r a d i a t i o n cycle duration and the interval between successive cyles may be varied by processing equipment parameters in such a way that insulation would not be overheated above the permissible temperature in natural or forced cooling conditions. Such system of " f r a c t i o n a l " i r r a d i a t i o n provides reasonable heat-removal from the i r r a d i a t e d cable product at high
do~e r a t e
8 Fig.4.
Process diagrams of cable product i r r a d i a t i o n : a - m u l t i r o l l e r system; b - system with two drums of d i f f e r e n t construction.
Electron Beams in Cable Engineering
917
I t has been demonstrated experimentally and a n a l y t i c a l l y (5) that appropriate selection of electron energy and with a given insul a t i o n thickness in case of double-side i r r a d i a t i o n 80% of insulation volume has the absorbed dose within +20% of the optimum dose and in case of four-side i r r a d i a t i o n this v~lue reaches 94%. The indicated constructive decisions concerning the creation of processing equipment enabled the modification of cable and wire insulation with electron beam having the power up to 25 kW and above, the dose rate being 3-4 orders higher than that of isotope sources. However, in spite of the principal p o s s i b i l i t y of increasing the electron beam cross-section area u t i l i z a t i o n factor up to 60-70% by bringing together several r o l l e r systems under the scan horn, such systems, besides the additional operating d i f f i c u l t i e s with complicated wire charging, have some principal shortcomings: r e l a t i vely low electron beam cross-section area u t i l i z a t i o n factor due to the i m p o s s i b i l i t y of compact arrangement of successive wire turns; the necessity of replacing r o l l e r blocks when the i r r a d i a t e d products of one r e l a t i v e l y narrow group of diameters are changed for another; high tension force during product rewinding and hence deformation and breaking of the i r r a d i a t e d products especially when i r r a d i a t i n g conductors of small cross-section. To achieve higher technical-and-economic c h a r a c t e r i s t i c s nf the radiation-processing and to improve the q u a l i t y of i r r a d i a t e d cable products o r i g i n a l processing equipment free from the indicated above shortcomings (Fig. 4b) has been developed in the USSR. The system is designed for cable product transportation through electron beam with simultaneous formation of i r r a d i a t i o n zone and consists of two drums of d i f f e r e n t construction allowing by means of appropriate mechanisms to carry out long-distance control of w~re or cable layning pitch. This system allows to eliminate the drawbacks of the t r a d i t i o n a l r o l l e r system: there is no high tension force because every turn is set in r o t a t i o n independently; cable products of a wide diameter range may be rewinded (and hence i r r a d i a t e d ) on t h i s system with the r e a l i z a t i o n of electron beam cross-section area u t i l i z a t i o n factor close to I00% for any diameter product; this system allows to change the pitch of reel winding and thus to f u l f i l continuous t r a n s f e r from i r r a d i a t i o n of one diameter products to i r r a d i a t i o n of other diameter products without i n t e r r u p t i n g the i r r a d i a t i o n process. Depending on overall dimensions and design features of cable product the developed processing equipment ensures l i n e a r speeds of products movement in electron beam zone from 50 to 500 m/min and higher. The use of electron accelerators with 20 kW power in the beam and electron energy of 0,5-1,5 MeV combined with the developed highproduction equipment ensures high technical-and-economic effeciencv of wire and cable with i r r a d i a t i o n - m o d i f i e d polymer insulation manufacturing process. At present the electrotechnical industry of the USSR produces a wide v a r i e t y of wire and cable with i n s u l a t i o n and sheath based on i r r a d i a t i o n - m o d i f i e d polymers ( i n s t a l l a t i o n wire, control cable, shipboard cable, wire and cable for atomic stations, a i r c r a f t wire, computer wire, e t c . ) . All these products have increased heat resistance and r e l i a b i l i t y and are widely used in d i f f e r e n t f i e l d s of engineering (5, lO, l l ) . I t should be noted that in most cases the use of radiation enemy for a given production process is more e f f e c t i v e than t r a d i t i o n a l l y used types of energy, f o r example, thermal energy. That is why even
E. A. Abramyan and E. E. Finkel
918
comparison of purely economic factors of one and the same process which may be carried out, for example, both by thermochemical and i r r a d i a t i o n methods t e s t i f i e s in favour of the l a s t one. And one of the main factors predetermining the advantages of i r r a d i a t i o n - p r o duction process as compared to t r a d i t i o n a l methods of obtaining s i m i l a r products is the engineering level and perfect characterist i c s of the s p e c i f i c processing equipment including r a d i a t i o n source. Development and mastering of the r a d i a t i o n technology as applied to the problems of cable i n d u s t r y may be considered as appearance of a now s p e c i f i c f i e l d of i n s u l a t i n g and cable technique the promising future of which causes no doubt. REFERENCES I.
Radiation chemistry of Polymers (1973) Nauka, Moskow.
2. Radiation processing, Trans. of the F i r s t . I n t e r n . Meeting of r a d i a t i o n processing,Radiation physics and chemistrZ, 9, Nn I-R~1977) 3. I B.Peshknv, E.E.Finkel, Radiation modification of polymers is a progressive method of new i n s u l a t i n g materials creation, Paper No 3A 33, World Electrotechnical Congress, June 1977, Moscow. 4. E.E.Fihkel, et a l . (1977) S t a b i l i z a t i o n of i r r a d i a t i o n - m o d i f i e d Egl_hymers, Himija~ Moscow. 5. E.E.Finkel, R.P.Braginsky (1975) Heat-resistant wire and cable with i r r a d i a t i o n - m o d i f i e d i n s u l a t i o n , Energija, Moscow. 6. E.A.Abramyan, V.A.Gaponov, Source of accelerated p a r t i c l e s of equal energy, No 203144 (1968) B u l . i s o b r e t . , (USA patent N 3390303) 7. E.A.Abramyan. Heavy-Current electron accelerators, Radiation sources fQ~__ind_LLstria! processes (1969) 661, Intern.Atomic Energy, Agency, Vienna. 8. E.A.Abramyan, Radiation-chemical technology on electron accelerators in the USSR, Report on the present conference. 9. E.E.Finkel, et a l . , USA patent N 33860159. lO. E.E.Finkel et a l . (1968) Radiation chemistry and cable engineer i n g , Atomisdat, Moscow. I I . V.A.Privezentsev, l.B.Peshkov (1971) Windin9 and i n s t a l l a t i o n wires,Energija, Moscow.