Ethylene copolymers, radiation crosslinking

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.

Properties of Crosslinked Films. Our purpose was to determine which commercially useful property improvements result from radiation crosslinking of ethylene-ethyl acrylate copolymers. Table III illustrates... 

Figure I. Effect of radiation crosslinking on stiffness of 85/15 ethylene-ethyl acrylate copolymer over the temperature range 25°-300°C.

Ethylene-propylene copolymers, EPR, and ter-polymers containing diene monomers, EPDM, crosslink on irradiation [Bradley, 1984 Aoshima et al, 1992]. The crosslinking can be enhanced by the use of crosslinking agents for example, EPDM containing 5-ethylidene-2-norborene, ENB, may be efficiently radiation crosslinked. Analogues of reaction (R-30) contribute to such crosslinking. 

B. Wang, M. Wang, Z. Xing, H. Zeng, G. Wu, Preparation of radiation crosslinked foams from low-density polyethylene/ethylene vinyl acetate (LDPE/EVA) copolymer blend with a supercritical carbon dioxide approach. J. Appl. Polym. Sci. 127, 912-918 (2013)... 

Zhang, W. X., Liu, Y. T., Sun, J. Z., The relationship between sol fraction and radiation-dose in radiation crosslinking of low-density polyethylene (LDPE) ethylen-evinylacetate copolymer (EVA) blend. Radiation Physics and Chemistry 1990,35(1-3), 163-166. 

Polar copolymers of ethylene, such as ethylene-vinyl acetate (EVA) and ethylene-ethyl acrylate (EEA), are readily crosslinked upon exposure to high energy irradiation [88]. In fact, the melt index of EVA can be controlled by the use of low doses (<50 kGy) of irradiation [89]. The presence in polar ethylene copolymers of comonomer units such as vinyl acetate or alkyl acrylates (methyl, ethyl and n-butyl) proportionately reduces the level of crystallinity, and since the majority of radiation responses of interest take place in the amorphous phase, the responses are more uniform throughout the polymer mass. When the irradiation is done at room temperature, the physical properties after irradiation follow the same trend as polyethylene [90]. 

Seguchi, T., Hayakawa, N., Yoshida, K., and Tamura, N., Fast neutron irradiation effect. II. Crosslinking of polyethylene, ethylene-propylene copolymer, and tetra-fluoroethylene-propylene copolymer, Radiat. Phys. Chem., 26, 221-225 (1985). Keller, A., and Ungar, G., Radiation effects and crystallinity in polyethylene, Radiat. Phys. Chem., 22, 155-181 (1983). 

A large volume of literature exists for PO blends with easily crosslinkable elastomers, but the information on radiation processing and degradation of HTPB is scarce. For dimensionally recoverable applications a blend of LCP with PVDF or PE was radiation crosslinked. Similarly, PSF or PES was blended with water-soluble PVP and crosslinked to make it water insoluble for medical or food applications. For the use in cable jackets and heat-shrinkable applications PVDF or a copolymer of tetrafluoroethylene and ethylene (ETFE) could be compounded with a thermoplastic elastomer, formed and radiation crosslinked. Adjusting composition and irradiation dose produced a series of materials with good balance of tensile strength and elongation. ... 

PVC, another widely used polymer for wire and cable insulation, crosslinks under irradiation in an inert atmosphere. When irradiated in air, scission predominates.To make cross-linking dominant, multifunctional monomers, such as trifunctional acrylates and methacrylates, must be added. Fluoropolymers, such as copol5miers of ethylene and tetrafluoroethylene (ETFE), or polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), are widely used in wire and cable insulations. They are relatively easy to process and have excellent chemical and thermal resistance, but tend to creep, crack, and possess low mechanical stress at temperatures near their melting points. Radiation has been found to improve their mechanical properties and crack resistance. Ethylene propylene rubber (EPR) has also been used for wire and cable insulation. When blended with thermoplastic polyefins, such as low density polyethylene (LDPE), its processibility improves significantly. The typical addition of LDPE is 10%. Ethylene propylene copolymers and terpolymers with high PE content can be cross-linked by irradiation. ... 

Recently, a versatile class of poly(ethylene propylene)/poly(ethylene oxide) block copolymer micelles were introduced they were stable due to a combination of high block incompatibility, kinetically frozen core, and high interfacial tension between core and solvent [53, 58]. Moreover, by using a co-solvent of varying composition, the aggregation number was controlled and soft spheres from star-like to micelle-like could be obtained. Another way is core stabilization via chemical crosslinking, say by UV radiation [59-64]. 

The use of these monomers for radiation cross-linking of polyethylene has been suggested [66]. With benzophenone as a photosensitizer, atactic as well as isotactic polypropylene is crosslinked with allyl acrylate by UV radiation. In this process both types of double bonds react [67]. Elastomers such as ethylene-vinyl acetate copolymer have been cross-linked with this monomer on a roller mill at 150°C using dicumyl peroxide as the initiator. Such cross-linked elastomers exhibit little or no swelling with aromatic solvents or chloroform after 24 hr at 30°C, conditions under which the uncured elastomers ordinarily dissolve [68]. Despite these interesting applications for such monomers, the bulk of the commercially produced allyl methacrylate finds application as a synthetic intermediate rather than as a monomer. 

Copolymerization of ethylene and propylene produces an elastomeric polymer that is virtually inert because of the absence of carbon-carbon double bonds (EPM). Such polymers thus tend to be crossUnked with peroxides or by radiation. To improve the reactivity of ethylene-propylene copolymers, 1-10% of a third monomer can be added to give a terpolymer or ethylene-propylene-diene monomer (EPDM). The primary diene monomers used in EPDM are 1,4-hexadiene, dicyclopentadiene, and ethyUdene norbomene. Introduction of an unsaturated monomer such as ethylidene norbomene will enable use of sulfur-based crosslinking systems. 

Deng, P.Y., Liu, M.H., Zhang, W.X., Sun, J.Z. Preparation and physical properties of enhanced radiation induced crosslinking of ethylene-vinyl alcohol copolymer (EVOH). Nucl. Instrum. Meth. Phys. Res. B 258, 357-361 (2007)... 

The modified fillers were used in two matrices with different polarity the ethylene-propylene copolymer EPM and hydrogenated acrylonitrile-butadiene rubber HNBR. Elastomers were crosslinked with dicumyl peroxide DCP. The influence of the variously modified fillers on the cross-linking density of the vulcanizates, rheometric and mechanical properties of filled systems were investigated. The ageing studies (thermal, atmospheric and under UV radiations) were conducted. 

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