Electron beam curing of EPDM.

Electron beam curing of EPDM

The radiation chemistry of polymers dates back to the end of the 1940s (refs. 1-3). In those days polyethylene and other macromolecular materials were exposed to the background radiation (radiation produced indirectly as a result of reactions between the original radiation and its environment) of nuclear reactors. It was established that these macromolecules changed in both a chemical and physical sense when exposed to high energy radiation (>[10.sup.2]eV). Every polymer was also found to be altered in its own specific way, with some polymers exhibiting the properties of crosslinked material and others being completely degraded. At the same time, some polymers were found to be relatively resistant to such radiation.

More extensive research has also established that the susceptibility of a polymer to high-energy radiation may be affected by all kinds of additives, and these findings justify a study of the effect of high-energy radiation on polymeric materials.

It was a long time before high-energy accelerators having sufficient capacity for industrial applications became available. It is now possible to generate any desired radiation dose needed to produce the required properties in the product to be irradiated. The dose can be increased or reduced simply by turning a potentiometer.

Normally EPDM rubbers are vulcanized by systems based on sulphur, resin or peroxide. The common feature of these systems is that they all require activator energy in the form of heat. The (extremely) high temperatures (approximately 180 [degrees] C) have the disadvantage that the final properties of the finished product may be affected in one way or another by a variety of uncontrolled side reactions which may occur.

Radiation curing, on the other hand, is a process which differs from those mentioned above in that the final curing is carried out at about 20 [degrees] C under closely controlled conditions (such as radiation dose, penetration depth, etc.), and this form of curing ultimately results in a more well-defined end product. In the rubber industry, this technique is used by large rubber processors (for example, in roof sheetting and cable production). Its widespread use is, however, impeded by the high investment costs. One way of avoiding these high costs is to arrange for the products to be irradiated by contractors.

Radiation curing

Theory of radiation curing Before dealing with the study itself, some frequently used concepts and definitions will be spelled out in greater detail. However, an in-depth description of the theoretical background of radiation chemistry falls outside the scope of this article.

Radiation curing, which is used to crosslink polymers or coatings, covers the entire spectrum of electromagnetic radiation energies responsible for chemical reactions, as shown in table 1 (refs. 4 and 5).

Electron beam radiation belongs to the high-energy type (energies of approximately [10.sup.2]eV and over). Four aspects of polymer crosslinking with high-energy radiation will be discussed in greater detail here. These are

* type of radiation and radiation source;

* the theoretically possible reactions and how they proceed;

* the type of polymer irradiated and its susceptibility to radiation;

* the chemical, physical and mechanical properties of the network formed

Types of radiation and radiation sources The types of radiation mentioned in table 1 can be subdivided into the following categories:

* noncorpuscular radiation such as microwaves, IR, sunlight (UV/VIS), X-rays and gamma-radiation, and

* corpuscular radiation such as alpha- and beta-radiation, high energy electrons, protons, deuterons and neutrons. All these types of radiation are classified as high-energy types, which also includes gamma-radiation.

The origin of high-energy radiation Alpha-, beta- and gamma-radiation are the result of the disintegration of radioactive elements, and in particular

* alpha-radiation is the emission of He nuclei having a positive charge of +2;

* beta-radiation is the emission of electrons, which have a negative charge;

* gamma-radiation is shortwave electromagnetic radiation (photons, <0.001 nm);

* protons, neutrons and dueterons are nuclear particles which are released during the disintegration of atomic nuclei as a result of radioactive decay or bombardment with other atomic particles.

Electron radiation, on the other hand, is generated in a machine and the amount of energy (that is to say, the velocity of the electrons) can be adjusted as desired. Many of the above-mentioned high-energy radiations have no commercial application but are used in fundamental research. It is only electrons and gamma-radiation which are used commercially.

Gamma-radiation Unlike electron radiation, gamma radiation is electromagnetic in nature (i.e., consists of photons). As a consequence, the depth to which the substrate to be irradiated is penetrated is different for the two types of radiation.

Gamma-radiation is generated in a [sup.60.Co] source (obtained by irradiating metallic cobalt with neutrons) and is the result of the following decay reaction which has a half life of 5.3 years:

[sup.60]Co + [right arrow] [sup.58.5]Ni + hv (1.17 - 1.33 MeV) + [Beta] - where h is Planck's constant and v is the frequency in Hz.

The radiation produced during the decay of [sup.60]Co has an energyquant of 1.17 and 1.33 MeV and it is mainly used for preserving foodstuffs, sterilizing medical products, germinating plants etc. Here the penetration depth has to be large since pre-packed small units are involved and in view of the high added value the role played by the cost price (irradiation time) is small.

Electron radiation (ref. 6) Electrons originating from an accelerator are incorrectly called beta-radiation. It is only electrons which are released as a result of the radioactive disintegration of elements which constitute beta-radiation.

In this century, Van der Graaff left a deep imprint on the development of electron accelerators, the generator named after him, in which a very high voltage is produced on an insulated metal cylinder, being well-known. Later developments by Van der Graaff's coworkers resulted in the modern accelerators in which a voltage of up to approximately [10.sup.7] (10 MeV) volts is produced on a tungsten cathode. The electrons are accelerated in high vacuum and leave the machine via a titanium window.

Theory of polymer crosslinking by electron radiation The reactions occurring on exposure to electron radiation can be subdivided into a number of types:

* initiation reactions;

* propagation reactions; and

* termination reactions.

The various types of reactions may result not only in crossinking but also in chain scission, and there is a clear parallel with peroxide curing reactions and with free-radical polymerization reactions.

The principal mechanisms which initiate crosslinking by interaction with high-energy radiation are shown in figures 1 and 2.

As is evident from the reaction mechanisms described, there is always competition between chain scission and crosslinking in high-energy irradiation of a macromolecular system. The backbone of the molecule may be broken or a side group (atom) (H, [CH.sub.3], etc.) detached, the chemical structure largely determining which process predominates. Table 2 shows the dissociation energy of some bonds and reveals, for example, that it requires less energy from rupture of a bond from a tertiary carbon atom than to a secondary carbon atom.

If it is assumed that the process is based on free-radicals only, scission leads to a reduction in molecular weight. However, the detachment of a side group/atom results in the formation of crosslinks if two macromolecules come into contact with each other during lifetime of their radicals. How can the relatively immobile macromolecules ever crosslink? A simple, but imprecise, answer to this question is that a free radical is not fixed in the macromolecule in terms of position and time but intramolecular and intermolocular radical movements take place.

Type of polymer irradiated and its behavior (refs. 8 and 9) Polymer systems react in their own specific way to electron radiation and over the years it has been found in practice that, in a multicomponent system such as, for example, a polymer blend or rubber compounds, every component will react individually and differently to the radiation. In principle, four categories can be distinguished:

* components which have a high sensitivity (such as ethylene norbornene [ENB], dicyclopentadiene [DCPD] and hexadiene [HD]) and compounds which contain displaceable unsaturated bonds, such as 1,2-vinyl compounds and methacrylates;

* components, such as polyethylene and polypropylene, which are relatively resistant to electron radiation compared with the compounds mentioned under 1;

* components, such as styrene, or aromatics in general, which are added to a polymer system to delay crosslinking;

* components, such as isobutylene and tetrafluoroethylene, which primarily result in degradation.

Category 1 - The vinyl monomers and oligomers (mono- or multifunctional) make an important contribution to the network finally produced since their specific chemical structure results in the relatively rapid and simple creation of reactive free radicals which may subsequently react with their environment. By vinyl monomers (monofunctional) are meant organic molecules having a molecular weight of approximately 100-500 g.

C[H.sub.2] = CHR [right arrow] non-crosslinked linear polymer (11)

This reaction depends on the chemical structure of the R group which largely determines the stability of the double bond. The process of crosslinking requires multifunctional oligomers (molecular weight approximately 200-1,000 g) which ultimately contribute to the formation of a complex network. Such compounds promote the so-called free-radical-induced propagation reactions. [Mathematical Expression Omitted]

Category 2 - After irradiation, thermoplastic vinyl polymers (molecular weight about [4.10.sup.4]-[10.sup.6] g), such as polyethylene, polypropylene, polyacrylates and, to a lesser extent, polyvinyl chloride, exhibit some improvement in physical and mechanical properties as a result of crosslinking. Network formation ultimately depends on two radicals "meeting" each other as a result of intermolecular or intramolecular displacements.

Category 3 - Compounds in this category are termed crosslink retarders. The addition of compounds (antirads) which capture free radicals, ions (radical stabilizers) or electrons to polymer materials may increase the resistance of the system to electron irradiation. Typical antirad agents include quinones, hydroquinones and aromatic amines. Also known as stabilizers, these ingredients are frequently applied in rubber compounding.

Aromatic compounds absorb a significant proportion of the electron radiation and, in addition, exhibit some degree of degradation.

Category 4 - Vinylidene polymers such as polyisobutylene, poly ([infinity] - methylstyrene), polymethacrylates and polyvinylidene chloride undergo degradation during irradiation. The structure of these polymers contains weak links with low bond energies and these can be easily activated by electron radiation and result, for example, in two more stable polymer fragments.

Chemical physical and mechanical properties of the network formed As stated previously, the polymer and the components present which are capable of being activated by electron radiation react with their environment. In this process, the component may react with itself and, as it were, polymerize, but it may also form bonds with other components.

In unvulcanized rubber, the molecules are scattered in a criss-cross manner, separate from each other. The stress required during deformation, for example stretching, will be low, that required for a particular deformation being dependent on the number of crosslinks or the crosslink density measured at a low deformation (ref. 10). The stress (P) and the crosslink density ([M.sub.c] - 1) are interrelated by the following equation: [Mathematical Expression Omitted] Where: P = stress * = density of the rubber R = gas constant T = absolute temperature [A.sub.o] = surface area of the testpiece before deformation [M.sub.c] = average molecular weight between the crosslinks I = deformation (elongation)

From this equation it can be concluded that the denser the network, the shorter the molecular segments between the crosslinks and the higher the stress.

Properties such as hardness, tensile strength, elongation at break, permanent set, tear strength, etc., change by different amounts during curing or during the subsequent buildup of a denser network.

Experimental section

Design of the study An average rubber formulation (this includes one for EPDM) can be split up into a number of basic elements, namely:

* polymer *

* fillers

* processing oils *

* activators

* accelerators

* vulcanizing agents

* stabilizers *

* coactivators * (when peroxides are used)

* miscellaneous ingredients. The ingredients marked with an asterisk are of importance in the study dealt with in this article, whereas the use of activators, accelerators and vulcanizing agents is irrelevant in electron beam curing. However, coactivators which are standard in peroxide curing can make a considerable contribution to crosslinking efficiency and some of these compounds have therefore been investigated (refs. 11-16).

Neither white nor black fillers normally play any significant part during radiation curing since they are relatively inert towards electron beams. They do have an indirect influence, though, since they have a substantial effect on the density of the compound which, in turn, affects the depth of penetration of the electron beams (refs. 17).

The group of miscellaneous ingredients includes specific ingredients of all kinds (such as processing aids, organic flame retarders etc.) which should each be investigated separately for effects on electron-beam curing. This group of compounds has not been included in the study. To summarize, this study covers the polymer, the processing oils, the coactivators and some stabilizers. The choice of the compounds was based on the following considerations:

Polymer A number of polymer variables are important in arriving at the most suitable EPDM type for an application. The variables studied are:

* molecular weight, [[Bar] M.sub.w] (types with different [[Bar] M.sub.L] (1+4) 125 [degrees] C);

* molecular weight distribution, [[Bar] M.sub.W]/[[Bar] M.sub.n] (narrow or broad distribution);

* type and quantity of termonomer (DCPD, HD, ENB or none);

* amorphous or crystalline ([C.sub.2]/[C.sub.3] ratio).

The study was based on commercially obtainable EPDM types. A summary of these types and their characteristics is given in table 3 (ref. 18).

Co-activators In the rubber industry coactivators are known to have a considerable influence on the ultimate crosslinking efficiency in peroxide curing. In view of the similar reactions which may occur during peroxide curing, such organic compounds may also be expected to have a considerable effect on electron-beam curing.

In principle, the coactivators (polyvalent compounds) most used for EPDM can be divided into two main groups, namely the allyl compounds and reactive acrylates.

The following were chosen from the allyl group of compounds:

* TAC - triallyl cyanurate (a very widely used coactivator for EPDM);

* TAIC - triallyl isocyanurate (a cheap alternative to TAC);

* 1,2-PB - polybutadiene with a varying cis content (in addition to acting as a coactivator, this compound has a considerable effect on the processability of EPDM and is highly soluble in this polymer).

The following acrylates were chosen:

* EDMA - ethylene dimethacrylate (apart from TAC, the coactivator most used in EPDM);

* TMPT - trimethylolpropane trimethacrylate (which is a very effective coactivator in peroxide curing and results in a considerable increase in the modulus);

* 1,3-BDDMA - 1,3-butanediol dimethacrylate;

* 1,6-HDDMA: 1,6-hexanediol dimethacrylate (both of these methacrylates are more soluble in EPDM than EDMA and TMPT).

Some of the properties of the above-mentioned compounds are presented in table 4.

Processing oils In general, only paraffinic and naphthenic oils are suitable for EPDM in view of their high solubility in EPDM. Aromatlc oils, on the other hand, can only be used in small quantities because of their limited solubility in EPDM. In view of the large number of oils to choose from, one type of oil has been investigated from each group, the choice being based on the viscosity gravity constant index. This choice resulted in a sufficiently large range of aromatic contents.

Stabilizers Since EPDM is already resistant to ozone and UV, it is mainly heat-resistant stabilizers which are generally of interest in relation to EPDM. An in-house DSM study has shown that, for high-temperature EPDM applications, the following stabilizers, in particular, are very effective:

* TMQ - polymerized 2,2,4-trimethyl-1,2-dihydroquinoline

* NDBC/Irganox 2002 - nickel dibutylparacresol/nickel alkyl-benzyl phosphoric acid ethyl ester

Determination of the most suitable radiation dose Before investigating the above-mentioned ingredients in more detail, the relationship is first established between the quantity of coactivator (TMPT) and the variation in a particular property as a function of the radiation dose, which was varied from 50 to 150 kGy (according to literature sources, the ideal dose for EPDM is approximately 100 kGy) (refs. 8-16).

Compound selection The formulation of the compound used (see table 5) is based in part on a study carried out by R.J. Eldred (ref. 13). It is a polymer-rich one because the main objective of this study is to determine the effect of the various polymer variables on the crosslinking efficiency during electron-beam curing.

Apparatus The compounds were produced on a Shaw K1 Mark IV internal mixer having a chamber capacity (water) of 5 liters and equipped with a data acquisition system part of DSM Elastomers Europe ELIMS (ref. 20). The following mixing procedure was adopted:

* 30 seconds breaking up of the polymer, followed by adding of all the ingredients. After approximately 120 sec. the ram is brushed off and mixing takes place until a constant energy level is reached.

A mixing curve of the type shown in Appendix 1 and also specifying the mixing conditions is plotted for every compound.

The procedure shown in table 6 is then used to press 2 mm plates from the compound made.

This procedure produces 2 mm thick plates (having a smooth surface) and is necessary because the compounds have a high polymer content and exhibit a pronounced "nervy" behavior.

The plates are then irradiated on two sides with an electron accelerator having a power of 3 MeV (100 kW). For a required radiation dose of, for example, 100 kGy, irradiation is always carried out with a dose of 50 kGy on each of the two sides.

Methods of measurement Table 7 lists all the methods of measurement used together with a reference to the relevant standards, with additional comments being added if necessary.

Results

Optimum radiation dose and amount of TMPT To determine the optimal radiation dose in relation to the effect of the amount of coactivator, EPDM type A was chosen (see table 3). The coactivator used for this part of the study is TMPT, analogous to the Eldred study (ref. 13), the content being 0, 5, 10, 15 and 20 phr. The radiation dose is varied from 50, 75, 100, 125 to 150 kGy.

The relationship between tensile strength, 300% modulus, elongation at break or hardness and amount of coactivator (TMPT) as a function of the radiation dose were plotted. From this graphical relationship it can be inferred that an optimum tensile strength is obtained at a radiation dose of approximately 100 kGy, after which it tends to decrease. For TMPT contents of up to approximately 10 phr, a slight increase in tensile strength can be observed over the entire range of radiation doses.

At a TMPT content of 10 phr, however, this product has reached its solubility limit in the compound, and after being stored for a few days, the TMPT migrates out of the compound. This is probably the reason why the tensile strength reaches a maximum at approximately 10 phr of TMPT.

The 300% modulus (figure 3) increases with radiation dose, reaching a plateau at, again, a TMPT content of approximately 10 phr (supersaturation). A striking feature is that the 300% modulus exhibits a relatively strong increase as a function of coactivator content and of radiation dose.

The 100% modulus has also been measured, but the variation in the values found is too small to be able to draw conclusions about the crosslink density. Consequently, the 300% modulus was used for this purpose in later parts of the study.

In the case of elongation at break, it is not an increase, but a decrease which is observed with increasing radiation dose. Most of this decrease takes place up to a radiation dose of approximately 100 kGy and a TMPT content of 10 phr. The decrease in the elongation at break is small both for higher radiation doses and for higher proportions of TMPT.

The hardness varies mainly as a result of the increase in coactivator content and has already reached a maximum at a radiation dose of approximately 75 kGy.

The main quality criteria chosen for the subsequent study were the 300% modulus and the elongation at break. This choice is based on the fact that these two criteria are the ones most markedly affected by the coactivator content and the radiation dose. Use of these criteria therefore renders it easier to make statements on the effect of the other variables investigated.

The network is built up particularly up to a radiation dose of 100 kGy. Between 50 and 150 kGy both the 300% modulus and the elongation at break change smoothly as a function of the dose applied. For this reason, the compounds were irradiated with a dose of 50, 100 and 150 kGy in the remainder of the study.

Effect of coactivator type The effect of the coactivator type was studied for various allyl compounds and for various acrylate derivatives. In both cases the crosslink density, derived from the 300% modulus and the elongation at break, was determined as a function of radiation dose.

From the 300% modulus it can be established that there is no or hardly any difference between the use of EDMA and of TMPT. Both compounds do have a greater effect than either 1,3-BDDMA or 1,6-HDDMA. Use of 1.3-BDDMA results in a still smaller increase in the 300% modulus than use of 1,6-HDDMA.

The decrease in the elongation at break reveals no difference between using EDMA and TMPT, while the other methacrylates investigated did not differ from one another in terms of elongation at break. As regards the allyl compounds, the following conclusions may be drawn:

* TAIC provides more rapid increase in the 300% modulus than TAC, but the differences are small. The use of a 1,2-BR with a high cis content (compare 50% with 85%) results in a more rapid increase in the 300% modulus.

* The decrease in the elongation at break reveals the above differences to a lesser extent. A striking feature in the case of 1,2-BR is that the decrease in elongation at break as a function of radiation dose is very small.

It may be concluded that the methacrylate derivatives are more effective than the allyl compounds, with EDMA having the same effectiveness as TMPT. Because of processing advantages (EDMA was available as a dry liquid) and its presumably higher solubility it compound, EDMA has been used as a coactivator in the further study.

Effect of stabilizer Figures 4 and 5 show the 300% modulus and the elongation at break alongside the type and amount of stabilizer as a function of radiation dose. A compound containing no added stabilizer is used as a reference.

At a low radiation dose (50 kGy) there is virtually no evidence of any effect due to the stabilizer, but at higher doses the values of the 300% modulus tend to be lower for higher doses of TMQ (1phr). The NDBC/Irganox 2002 system yields a clearly measurable reduction in the 300% modulus, which is most evident at the highest radiation doses (150 kGy).

The above phenomena are not observed in the case of elongation at break (figure 5), possibly because the effect of the spread in the measurements is too great.

Conclusion: stabilization of EPDM to be applied only if necessary; TMQ to be applied in the lowest possible dose.

Effect of type of oil The effect of the type of oil is clearly evident both from the elongation at break and from the 300% modulus.

Regardless of the level of the radiation dose, the aromatic components of the oil have an adverse effect on the crosslink density (antirad agent).

Conclusion: for electron-beam curing of EPDM only paraffinic oils shall be used, the content of aromatic components being as low as possible.

Effect of polymer variables Figures 6 and 7 show the effect of the molecular weight and the molecular weight distribution of ENB types on the degree of crosslinking in relation to the radiation dose. From these figures it can be seen that, as expected, for a virtually identical [[Bar] M.sub.w]/[[Bar] M.sub.n] the molecular weight (cf. EPDM D and F) reveals a difference between these two types both in terms of elongation at break and of the 300% modulus.

A comparison of two polymers, EPDM A and F, which are virtually identical in terms of molecular weight but differ in molecular weight distribution, reveals that an EPDM type having a broad distribution (F) has a higher 300% modulus level in absolute terms, while the level of the elongation at break is lower. Compared with the type with a broad distribution (F), the narrow-distribution type (A) yields a less rapid increase in the 300% modulus as a function of the radiation dose.

Conclusion: a higher molecular weight does not affect the crosslinking efficiency. An EPDM type with a broad molecular weight distribution yields a higher crosslink density than one with a narrow distribution.

Figures 8 and 9 show the effect of type and amount of termonomer in relation to the radiation dose.

It is evident both from the decrease in the elongation at break and from the 300% modulus that the HD type (I) displays an increase in crosslinks during irradiation that is identical to the increase observed in a copolymer (H).

Compared with the H copolymer, the HD type-(I) has a very high elongation at break in combination with a low 300% modulus. This is caused by the lower molecular weight and the lower [C.sub.2] content of I.

A striking feature is the equal increase in the 300% modulus (see figure 8) and the absolute level of the irradiated DCPD-type (E) compared with the ENB type (F). Both EPDM types with a narrow distribution (A and B) are found to have a lower modulus and a higher elongation than the types with a broader distribution (E and F). For identical molecular weight distributions, the increase in the ENB-content from 4 to 8.5% results in a higher crosslink density. For EPDM-types with a small distribution leads an increase in ENB-content to an increase in crosslink efficiency.

Conclusion: for EPDM types with a relatively broad distribution (E and F), the type of the third monomer (in case of equal amounts) does not affect the crosslinking efficiency. EPDM types with a broad distribution give a higher crosslinking efficiency than types with a narrow distribution. The crosslinking efficiency of an HD and a copolymer is lower than that of a DCPD or ENB type.

Figures 10 and 11 show the effect of the molecular weight as well as that of the [C.sub.2]/[C.sub.3] ratio (crystallinity). Both the increase in the 300% modulus and the elongation at break proves that EPDM F and G exhibit identical behavior, with a difference in the absolute level. The latter difference is caused by the higher [C.sub.2] content of G.

The EPDM type C displays a less steep increase in the 300% modulus as a function of the radiation dose compared with the other types (F and G). EPDM type C has a narrower molecular weight distribution. The higher absolute level of the 300% modulus of C is caused by the higher [C.sub.2] content and the higher molecular weight of C.

Therefore a higher [C.sub.2] content does not affect the crosslinking efficiency. Again, the molecular weight distribution appears to affect the crosslinking efficiency.

Discussion

The optimum radiation dose for EPDM is determined by the required pattern of properties. From this study it may be concluded that the network is primarily built up at a radiation dose of up to approximately 100 kGy. The degree to which it is built up depends partly on the coactivator used and the EPDM type used.

In choosing the coactivator, allowance has to be made for its solubility in EPDM. The type of oil chosen and any stabilizer additions will affect the crosslinking efficiency.

Contrary to studies published earlier (refs. 11 and 13), in this study it was found that when EDMA is used as a coactivator, no difference can be detected between a DCPD type (4%) and an ENB type (4%), provided both have an identical molecular weight distribution.

Increasing the ENB content has less effect on the final crosslink density than using a type having a broader molecular weight distribution. The study makes it possible to propose a number of formulations based on DCPD types (broad molecular weight distribution) and these will be compared in a subsequent study with sulphur, sulphur donor and peroxide curing, which will be reported elsewhere.

Table 4 - some data on co-activators

            Molar mass   Boiling point   Functionality   Category
               (g)        ([degrees] c/mbar
TAC             249          -                3            Allyl
TAIC            249          -                3            Allyl
1,2-BR        2,800          -                -            Allyl
EDMA           198          240               2           Acrylate
TMPT           338         185/5              3           Acrylate
1,3-BDDMA      226         110/4              2           Acrylate
1,6-HDDMA      254         >200               2           Acrylate

Table 5 - compound comparison

Ingredient             phr
Polymer                100
Carbon black N-550     40
Stearic acid           0.1
Paraffinic oil         15
Coactivator          Variable

Table 6 - pressing conditions

Press platen temperature   160 [degrees] C
Press platen dimensions    400 x 400 mm
Molding cycle:             3 Minutes, 0 kN
                           1 Minute, 10 kN
                           1 Minute, 50 kN
                           2 Minutes, 500 kN
Cooling:                   40 [degrees] C/min. under pressure
                           till room temperature

[Tabular 1 to 3 and 7 Omitted] [Figures 1 and 2 Omitted]

PHOTO : Figure 3 - relationship between tensile strength and amount of co-activator

PHOTO : Figure 4 - 300% modulus with type and amount of stabilizer as a function of radiation dose

PHOTO : Figure 5 - elongation with type and amount of stabilizer as a function of radiation dose

PHOTO : Figure 6 - effect of molecular weight and MW distribution on degree of crosslinking

PHOTO : Figure 7 - effect of molecular weight and MW distribution on degree of crosslinking

PHOTO : Figure 8 - effect of termonomer in relation to radiation dose

PHOTO : Figure 9 - effect of termonomer in relation to radiation dose

PHOTO : Figure 10 - effect of molecular weight and of the [C.sub.2]/[C.sub.3] ratio

PHOTO : Figure 11 - effect of molecular weight and of the [C.sub.2]/[C.sub.3] ratio

References

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