Radiation crosslinking of polymers-status, current issues, trends and challenges

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RADIATION CROSSLINKING OF POLYMERSSTATUS, CURRENT ISSUES, TRENDS AND CHALLENGES DipI.-lng. J. Gehnng and Dr. A. Zyball BGS BETA-GAMMA-SERVICE Dr. Wiesner GmbH & Co. D-51 674 Wiehl

ABSTRACT Radiation crosslinking improves mechanical, thermal and chemical properties in products made of thermoplastics, elastomers and recently also thermoplastic elastomers. A large number of polymers are radiation crosslinkable, some however only when additives are added. This paper will show the present application areas (cable, wires, tubing, foam, moulded parts) where future developments can be expected an potential areas of application i.e. developments of radiation crosslinking. KEYWORDS Radiation crosslinking, Thermoplastics, Elastomers. Thermoplastic Elastomers, Cable, Piping, Foam, Moulded Parts, Trend, Developments. 7. Introduction A paper written by Prof. A. Charlesby in 1954 IS entitled: How radiation affects long-chain polymers. In the subtitle he writes: ,,Data showing effects of radiation-induced crosslinking or degradation of polymers indicate that the effects can be used in polymer research and for industrial processes. Rough cost estimates define commercial feasibility.” As already mentioned, this article was publrshed 40 years ago; however, I feel these statements are still relevant today. Based upon this, I would like to deal with the jndustrial processes“ mentioned by Prof. Charlesby in this discussion of radiation crosslrnking. 2. Radiation Cross/inking It is general knowledge that polyethylene belongs to the group of polymers which can be crosslinked. As easy as an explanation of crosslinking may Initially seem, a deeper investigation shows that some serious questions arise that have not been able to be clarified up to this day such as, - how are radrcals created near each other to induce crosslinking - how does the morphology influence the crosslinking reactions - over which time-period are radicals stable - which influence do additives have - which influence do polymer end groups or branches have and many other intricate questions are included in these aspects. The answers are also of interest to service centres in the sector of radiation technology in order to develop new applications. $1:I WC 46.4,6(*,-e

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3. Property Changes by means of Cross/inking When radiation crosslinking is used in industrial applications the question immediately arises, which property improvements are achieved in my product through crosslinking. Will this technology solve the applications technical problem when viewed economically? The polymer groups that are here under consideration are: thermoplastics elastomers and thermoplastic elastomers (TPE) Especially in the last years, the TPE-sector has had a series of interesting new product developments and the prognosis is that this area will continue to grow in significance. In the field of polymers - thermoplastics. elastomers and thermoplastic elastomeres - the followrng property improvements can be achieved by means of crosslinking (Fig. 1). In many discussions, especially as an employee in a service centre, one experiences that a potential user of radiation technology e.g. expects his polymer product to have a considerably higher heat distortion temperature after its being crosslinked. It is difficult to sufficiently explain how the crosslinking works for e.g. the reflection in a curve demonstrating the shear modulus against temperature. The task is easier when speaking with processors of elastomeres or thermoplastic elastomers. These people work with polymers with a very low shear modulus. This group is delighted that the crosslinking induces an increase tn the elasticity in shear modulus of elasticity. They appreciate that the TPE can be used with similar properties at higher temperatures, as a molecular crosslinking occurs in the hard or elastomeric segments, Processors of thermoplastics are continually surprised at the results which are similar to those Prof. Charlesby published in 1954. A specimen which can be radiation crosslinked or an injection moulded part which can be radiation crosslinked retains its form even above the melt or glass temperature. This is termed as an improvement of the dimensional stability under heat. The elasticity in shear achieved by the crosslinking holds the part together to such an extent, that the part retains its form. However, the part cannot retain Its form under high temperatures, if the mechanical influences are very strong In many areas of application the improvement of the thermal dimensional stability is sufficient and it IS possible for e.g. to make a part formerly made of duroplastics with all its difficulties in the making from a thermoplastic, radiation crosslinkable polymer. The processing of the part is therefore significantly easier. The radiation crosslinking represents process technical, applications technical and economical benefits. 4. Crosslinkable polymers The number of crosslinkable polymers tn the already mentioned series of polymers is large. Various polymers however. require additives in order to be effectively crosslinked. Due to the large number of inquiries for such additives, many commercial manufacturers offer such in a masterbatch. The mixing of such directly into the production process greatly facilitates the procedure for the manufacturer of polymer parts. There are various theories about the reaction of such additives in the crosslinking process, which however we will not go into detail here 5. Application possibilities with radiation cross/inking The present most important application areas for the radiation crosslinking are listed in Fig. 2. These examples should demonstrate in following whrch property changes were made in order to achieve the required production propertres in the radiation crosslinked polymer used. 5.1 Cross/inking of cable insulation A typical example for cables made of crosslinked PVC is the cable on an iron. Further applications for such cables are all heating instruments, trip line in the area of welding robots or cables in the motor area of automobrles These cable insulations are expected to withstand

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a temperature of 250’ C for a short time period having the typical properties of a soft PVC; the cable must be flexible and inexpensive. PVC is only crosslinkable with polymerisable monomers. What causes the problem is usually the required flexibility of the cable which should not be influenced. A difficult task which can only in rare cases be satisfactorily resolved. A crosslinking additive and perhaps additional materials have to be found for the PVC which will guarantee that the flexibility will be retained even after the additives have been polymerised. In other words, an additive has to be found which will have elastomeric properties at room temperature after being crosslinked. An important subject which has been under discussion in the cable industry, is that of halogen-free, flame-retardent and inexpensive cable insulations. An example is shown in Fig. 3. 5.2 Piping for surface heating and warm water supply A technology that is widely used in Germany is surface heating using crosslinked PE-piping. Large factory halls, auditoriums or conference rooms, as well as large open areas e.g. curves that are prone to icing in winter use such piping as a heating method. In Germany alone, 80 million m of PE-piping are crosslinked per year. In comparison to non-crosslinked PE-piping (thermal stability approx. 70” C), crosslinked piping can be used in areas where temperatures reach 110” C and they therefore can be used under higher inside pressure for a longer time. In this area developments continue. The start of this technology was the XLPE-piping for surface heating, then XLPE-piping for warm water supply and today piping which is made of a combination of polymer and metal. The last mentioned piping are made of aluminium with wall-thicknesses of 0,l mm up to approx. 1 mm and PE as coating within as well as outside of the aluminium piping. The entire construction consists of 5 layers as between the PE and the Al a coupling agent is required. Extensive new applications for the piping types with various material combinations can be expected on the market in the near future. Here too, developments will continue. The following trends can be observed: 1. The manufacture of piping with large drameters for the warm water supply. Then only piping pieces up to 6 meters has to be irradiated and no longer piping delivered on drums. This will demand special handling systems in irradiation facilities in order to process such products on a large scale economically. 2. PE-piping has at present an oxygen diffusion that is too high. This diffusion has been reduced by coating the tubing with ethylenvinylalcohol after having been crosslinked. This serves as a diffusion blockage. Economically it would be better to integrate this diffusion blockage directly into the manufacturing process, i.e. at extrusion and prior to its being crosslinked. Patents in this area have been attempted. These however, have not been able to be successfully applied in the industry. The molecular hydrogen which is produced during the crosslinking process dissolves the diffusion blockage. New possibilities are being sought out. Is the grafting of suitable monomers on PE in one working process with radiation crosslinking a solution? 5.2.1 Corrugated piping The need for flexible corrugated piping for the most diversified application areas has significantly increase in the last years. Due to positive experiences with radiation crosslinking of standard piping i e. tubing, attempts have been made to use the same technology on corrugated piping. In tests a very flexible corrugated pipe made of EVA was tested for alternate bending fatigue limit. Normally it can withstand 50.000 cycles, however after a crosslinking took place, the cycles could be increased to 250.000 without any problems No tear was evident. 5.3 Shrinkable products Although already used industrially significance. It is still fascinating.

for more than 30 years

this technique

has not lost its

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In the 30 years since this technology has been used, new applications besides those used in the electro technology have been found for use in the sector of connectors in various industry branches. Although this shrink effect or memory effect is commonly known in the industry world wide, further applications are still expected. Heat-shrinkable products are manufactured from products other than PE. The shrinkage is already achieved at such low temperatures as 60” C to 70” C. Perhaps a broken leg will not be set in a cast but rather by means of a heat shrinkable product in the near future. 5.4 foamed material In the next few years, we can expect in Germany to pass a new regulation on heat conservation. The goal of this regulation is not only to achieve heat insulation but also to minimise the loss of heat, thereby conserving energy. The requirements are to conserve energy sources thereby relieving the environment to some extent e.g. that during burning less CO, is released into the atmosphere. Where does the radiation crosslinking come in where such requirements are demanded? By crosslinking for e.g. a PE-matrix. we can achieve a closed-cell foam. This has the advantage of having a better insulation effect than open-celled foams. Up to now we have spoken of insulation materials made of PE for heat insulating purposes. These insulating materials are also used for sound proofing. In some areas both requirements must be met as insulation as well as sound proofing. New application areas are insulating materials for the automobile sector. Especially interesting are the closed-cell foam materials made of crosslinked PP In order to achieve this, additional research has to be done. 5.5 Moulded parts We will now discuss an important application for the radiation crosslinking: this is the crosslinking of moulded parts. Whereby the first mentioned applications mainly apply to crosslinking by means of an electron beam accelerator, the crosslinking of moulded parts can also be done in a gamma facility. There are moulded parts with large wall-thicknesses or complicated geometries that can only be crosslinked via gamma rays. However, it is generally known, that the results arrived at with an electron beam accelerator cannot automatically be transferred to a gamma facility. When using radiation emitting from gamma facilities, one can generally expect a degradation in the surface, if the part is not being iradiated in the presence of an inert gas: a procedure that is only in exceptional cases realisable in industrial work shops. Crosslinking of moulded parts made of PE or other polyolefins is a regular procedure in industrial technology. In the last few years, the crosslinking of moulded parts made of PA has been actively pursued. This is especially true for all moulded parts for the electro technology. The parts must be able to withstand high temperatures for short periods that can occur for e.g. when they are soldered (e.g. at 350” C for approx. 5 sec.) or an arc discharge. For these reasons these parts were formerly made of duroplastics. Today, crosslinked polyamide fulfils the requirements. The manufacture of such parts is significantly easier. The job now is to make this technology known in the polymer manufacturing industry. Today, as many parts are still manufactured from duroplastics, a great potential exists for furthering this technology. 6. Outlook In the preceding chapter we mentioned the application of radiation crosslinked polymers and the therby used polymers mainly made of PE, other polyolefins, PVC or PA was reported on. Most of the polymers have not been checked for crosslinkability to the extent that PE has. This presents us with a great potential of having these polymers tested as to their crosslinkability perhaps with suitable additives. This is especially true for PU, EVA and PBT as well as the area of TPE’s.

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TPE’s are stall a relatively new group of materials which combine the benefits of thermoplastics and vulcanised / crosslinked rubber blends The outstanding characteristic of TPE is their thermoplastic processability. This, however, imposes limits on their heat resistance and their resistance to chemicals so that they cannot be used in certain fields of application of classic vulcanisates. Radiation crosslinking, in particular, here offers an opportunity to considerably improve the range of material properties whilst retaining all the advantages of the thermoplastic processability. As an example Fig. 4 shows the Improvement of the compression set by means of radiation crosslinking a TPE compound. These results have already led to many new and interesting applications in the sealing technology. Another area is the crosslinking of copolymers. We would here only like to mention the crosslinking of EPDM-copolymers. If one has dealt with steel, one knows that there are large variations in the area of alloys. In the past, alloys have also been made of polymers and these are being technically used. We feel that in this field a grafting or crosslinking by means of irradiation can be achieved in order to improve the properties of the polymer alloys As in the case of classrcal applrcatrons In radiation crosslinking, achieved by closely working together with the manufacturers polymer industry, their users and irradiation centres

new applications can only be of starting materials in the

Literatur Chapiro.

A

Radration Chemrstry of Polymeric Intersoence. New York 1962

Systems,

Charlesby.

A

How Radratron Affects Long-Charn Polymers Nucieonm Vol 16 No 6 June 1954. p 18-25

Charlesby,

A

and S H Pinner Analysrs of the Solubrlity Behaviour irradiated Polyethylene and other Polymers. Proc Ray Sot. A 249 (1959). p 367 f f

of

The Rad/ahon Chemrstry of Macromolecules Vol 1 Fundamental Processes and Theory, Academic. New York 1992 Vol 2 Radiatron Chemrstry of Substrtuted Vinyl Polymers. Academrc New York 1973

Dole. M

Strahienvernetzung von technischen Poiymeren Plastverarbeiter 42. Jg 1991 Nr 8 S 56-58

Stenglin,

U.

Valdiseri.

L. L and G V Reed : Polymer medification Rubber World. August 1974. p 40-47

by

und Formteiien

Coagent

Zyball.

A.,

lrradratron cross/inking of polyethylene rn the presence poiymenzable additives Kunststoffe. German Mast/c 67 (1977) 8p 461 f f

Zyball.

A.,

Radiation cross-linking of poly(vrnyl chloride) in the presence of poiymerizable monomeres Kunststoffe, German Plastic 72 (1982) 8 p 487 f f

assisted crosslinking.

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Properties

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A. Zyball

Polymers

D Thermal properties - Improvement of the dimension stability under heat - Polnted setting of the hot modules - Improvement of the pressure and tensile deformation - Increase of the resistance against thermal pressure - Increase of the resistance against wire heating - Improved aging resistance

o

Mechanical properties - Modules increase - Increase of strength - Decrease of ultimate elongation - Improvement of alternative bending stability - Improvement of the weld line strength - Increase of hardness - Improvement of the abrasion resistance - Improvement of long-time behaviour

D Chemical properties - Solubility reduction - Improvement of the swelling resistance - Improvement of the stress cracking resistance

Applications o o o II ID 0

of Crosslinked

Cables and wires Installationand undetfloor tubes Heat shrinkable tubes Films Foam matrix Profiles

D Blow moulded - Bottles - Tins

Polymers

heating

D Injection moulded parts - Heat shrinkable end caps - Cartridges - Electronic components - Automotive parts - Machinery parts - Gaskets

parts FIG. 2

Crosslinking of a halogen retarded PE (HFFR-PE)

80 Dose

FIG. 3

FIG. 1

120 [kGy]

free

flame

Compression (Gammaflex)

set of a crosslinked

Temperature

FIG. 4

[“Cl

TPE