Radiation Crosslinked Thermoplastics

As stated above, plastics are polymerization, polycondensation, or polyaddition products of low-molecularweight monomers. The formation reactions are controlled by suitable reaction conditions such as temperature and pressure and by activating and inhibiting additives (e.g., catalysts, activators, or inhibitors) [8, 35]. 

Plastics can be crosslinked chemically, for example in the presence of peroxides, but also by radiation chemistry. Broadly speaking, radiation-induced crosslinking has a number of advantages over peroxide-induced crosslinking in terms of both process and quality, including the following [96]  

This explains the predominance of radiation-chemical reactions as a means of initiating crosslinking in thermoplastic materials. In principle the effect can be induced by electromagnetic waves such as X-rays or gamma rays or by corpuscular radiation (beta rays) such as accelerated electron beams, for example. The differences between 

FIGURE 2.9 Duration of irradiation required depending on type of radiation for absorption of a defined energy dose D = 100 kGy [71 ] 

Gamma rays generally have a low dose rate (lO to 10 Gy/min) hut high penetration range. A high dose rate necessitates sufficiently long irradiation over a period of several hours see Fig. 2.9. The use of this process is of interest for parts with complex structure. 

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Z. Ghazali, A. F. Johnson, K. Z. Dalhlan, Radiation crosslinked thermoplastics natural rubber (TPNR) foams, Radiation Physics and Chemistry, 1999, 55, 73-79... 

Uses Antioxidant, metal deactivator for polymers, PP, HDPE, LDPE, some thermoplastic elastomers used as primary insulation in wire/cable applies., EPDM, peroxide- and radiation-crosslinked PE, nylon, polyacetal, polybutene, PU, styrenic copolymers, unsat. rubber, PVC, PVB, hot-melt and sol n. adhesives, powd. coatings, metal coatings, rubber/plas-tic gaskets, plastic fabricated parts in contact with catalytic metals, oils/ lubricants in contact with metals, food pkg. 

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. ... 

Polyethylene, the most-used commodity thermoplastic, also shows wavelength-dependent behavior in photodegradation. Heacock [135] proposed UV radiation of about A = 257 nm to result in the formation of unsaturated and oxygenated structures in polyethylene, while the longer wavelength UV-A radiation was believed to cause mainly crosslinking reactions. Results from a monochromatic exposure study of low-density and high-density polyethylene... 

Chemical links/(the setting up of chemical links) between different molecular chains. A high amount of crosslinking can convert a thermoplastic into a thermoset, and this can be accomplished by chemical reaction, vulcanisation, degradation and radiation. 

For the same crosslinking degree and effort amplitude, the samples similarly characterised, but in conditions of accelerated ageing present a decrease of durability with about 30-40%, due to the UV radiation and temperature contribution to the diminish of energetic barrier required for thermo-fluctuating scission of overstressed chemical bonds. These results once again confirm the common mechanism, the radicalic chained one, which governs all the destructive processes. Table 3.116 [917]. In the case of thermoplastic elastomer, Estane, in the same conditions, the durability decay... 

Polystyrene is very difficult to cross-link by electron beam radiation or by peroxide without destroying its properties. As a result, applications have not been developed. The methyl groups of poly-PMS, by contrast, provide susceptible positions for such a reaction. PMS polymers can be cross-linked effectively by electron-beam radiation. This unique property of tpoly-PMS could open several new markets. Crosslinking turns a PMS-based thermoplastic into a thermoset resin, significantly improving its grease resistance and flammability behavior. 

This family of polymeric materials includes liquid polymers and thermoplastics that have been crosslinked. Preparation of thermosets follows the same compounding route used for thermoplastics however, a crosslinking agent or ionizing radiation are used to form chemical crosslinks. 

In principle, many of a thermoplastic material s properties can arise from radiation-chemistry crosslinking (Table 2.2). Even minute quantities of absorbed energy can produce considerabie changes in the properties of the plastic. Improved stability of shape at elevated temperature is a radiation-modified property that merits particular mention. The engineering thermoplastics that are meltable between 180 and 260 °C and are used for MID lose their thermoplasticity when crosslinked and become unmeltable. They can then sustain thermal loading spikes in excess of 400 C, which means that lead-free processes of assembly and connection technology can be used without difficulty [18,71]. 

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