Crosslinking electron irradiation

Electron irradiation causes chain scission and crosslinking in polymers. Both of these phenomena directly affect the glass transition temperature (Tg) of the materials. Thermomechanical (TMA) and dynamic-mechanical analysis (DMA) provide information about the Tg region and its changes due to radiation damage. Therefore, DMA and TMA were performed on all irradiated materials. 

Table II shows the effect of various doses of electron irradiation on the solubility of ethylene-ethylacrylate copolymer. Extraction with boiling toluene for 64 hours revealed extensive crosslinking after absorption of a 5-megarep dose of radiation. Copolymers of ethylene with isobutyl acrylate and 2-ethylhexyl acrylate were examined briefly. A 29 wt.% isobutyl acrylate-ethylene copolymer of melt index 1.3 decigrams per minute was crosslinked to greater than 50% insolubility in boiling toluene by a radiation dose of 10 megareps. A copolymer containing 21% by weight of 2-ethylhexyl acrylate of melt index 8 decigrams per minute required a dose of 25 megareps to reach a 50% level of insolubility.

Rubbers and PTFE powder were first premixed in an internal mixer for 5 min at a temperature of 100°C and at a rotor speed of 50 rpm. Figure 10 shows two different crosslinking routes, i.e., thermally or using electron irradiation. In the case of EPDM, crosslinking was performed thermally and also with electron irradiation. 

Fig. 23. Optical micrographs of grid squares of PS films strained to 5%. The film on the left has been crosslinked by electron irradiation the one on the right is uncrosslinked...

Polyethylene oxide reactions were thoroughly investigated by Fisher etal. [376]. The G values for crosslinking, chain scission and volatile formation for electron irradiation at 14°C and a dose rate of 600 Mrad h 1 are given in Table 27. To account for these values, the mechanism proposed was... 

Crosslinking of PVC is known to occur on irradiation, with a low yield [Ohnishi et al., 1962]. More efficient crosslinking may be induced by irradiating PVC in the presence of multifunctional crosslinking agents [Salovey, 1973]. For example, plasticized PVC has been used as a wire and cable insulation. To enhance the crosslinking of PVC during electron irradiation, triallyl cyan-urate (TAC), trimethylolpropane trimethacrylate (TMPTMA), or trimethylolpropane triacrylate (TMPTA) have been used as the additives [Bradley, 1984]. 

Eldred [1974] electron-irradiated EPR and EPDM in air, in the presence or absence of the following crosslinking agents ethylene glycol dimethacrylate, EDMA, 1,4-hexadiene, HD,... 

The author argued that since LDPE requires 150-300 kGy doses for adequate crosslinking, whereas EPR and EPDM require 100-150 kGy, blends of EPDM with LDPE may respond well to radiation crosslinking. As the data in Table 11.26 show, LDPE blended with 30% VISTALON 3708, after a dose of 100 kGy had 80% gel fraction, whereas at that dose LDPE alone did not crosslink (this is unexpected though the samples were 2-mm-thick and electron-irradiated in air, at this dose the gel fraction of LDPE should be measurable, particularly since the author mentions that LDPE requires 150-300 kGy for adequate crosslinking see Chapiro [1962], Kammel and Wiedenmann [1976], Yu et al. [1992, 1994],... 

Even though the compositions of the blends varied from 30-70% of EVAc, and the rest of iPP (plus 0.15% stabihzer), the gel content remained between 50-63% at doses from 50 to 120 kGy. Unfortunately, detailed data are not given. It is obvious that as crosslinking of EVAc would increase with dose, scission of iPP will increase. There would also be a complex role played by the stabilizer the 50 kGy dose lower limit may well be the dose where the stabilizer gets used up. Further, these films could develop about 10-20% oxidation layer on their surface, with the electron irradiation used in this work (based on the data given by Kashiwabara and Seguchi, [1992]). Minkova and Nikolova [1989a,b] also report that the gel content... 

Meijer et al. [1988] and Elemans et al. [1988] investigated the potential of electron irradiation to stabilize the morphology of immiscible polymer blends. Their concept was that selective crosslinking of a dispersed phase in a matrix that remains unaffected, or degrades, should help fix the morphology of the blend. 

This is attributable to the protective effect of the PS in the blends. Further work on this interesting topic, using other crosslinking agents, mixtures of other plastics that need recycling, and electron irradiation, is warranted. 

Again, opportunities are illustrated. Specifically, electron irradiation can be used to crosslink polymeric systems. This can be accomplished by irradiating the polymer alone, usually at a temperature above Tg (and above Tm if pertinent) ). Or it can be accomplished by irradiating a polymer in the presence of a polymerizable monomer, as has been done for many years in the wire and cable industry e.g., polyvinyl chloride plus a multifunctional monomer like trimethylolpropane-trimethacrylate(3. Electron irradiation can also be used to form graft copolymers. In fact, this can be accomplished in... 

On this substructure a thin dense layer (in the range of 0.5 to 10 pm thick) is coated that has a very high separation capability. Different coating techniques are in use, most commonly a solution of the respective polymer in an appropriate solvent is spread onto the porous substructure. The solvent is evaporated, followed by further treatment to effect crosslinking of the polymer. Photosensitive, solvent-free prepolymers may be used for coatings that are later crosslinked by irradiation, e.g. with UV-light or electrons. 

Szymczak and Manson, 1974a,b White and Mann, 1967). As with electron-irradiated polyethylene, crosslinked poly(vinyl chloride) formulations (usually plasticized) are now of commercial interest as wire and cable coatings and insulation [see, for example, Nicholl (1969)]. In the following sections, typical properties will be described and discussed details of other standard polymer-monomer graft polymerizations are given by, e.g., Chapiro (1962) and Charlesby (1960). 

In the Longevity process, the UHMWPE bars are warmed, placed in a carrier on a conveyor, and are exposed to electron beam radiation, with a total dose of 100 kGy. The UHMWPE does not heat above the melt transition during the crosslinking. After irradiation, the UHMWPE is heated above the melt temperature (>135°C) for stabilization of free radicals. Components are then machined from the Longevity material, enclosed in gas-permeable packaging, and sterilized by gas plasma. 

Elastomers such as cis-l,4-polyisoprene (natural rubber), polybutadiene, polybutadiene-styrene (SBR), and poly-chloroprene have large amounts of unsaturatiOTi in the polymer backbone and aU undergo crosslinking upon irradiation with either electron beam or 7-irradiation. Table 52.3 gives some values for G(X) and the ratio of scission to crossUnk-ing G(S)/G(X) for several elastomers. The protective effect of the aromatic ring is shown by the decrease in yield as the percentage of styrene is increased for the SBR series. 

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