Degradation, radiation-caused

Thus, radiation causes degradation, chain scission and further crosslinking. To distinguish between chain scission, crosslinking and degradation products, sorption/desorption studies were conducted. Selected specimens of 73/27 and 80/20 TGDDM/DDS were allowed to absorb acetonitrile to equilibrium. The acetonitrile was desorbed, and the samples were tested. 

The energy associated with UV light is in competence with the chemical bond energies associated with any two atoms [104]. Hence, UV radiation often has the potential for retention of the properties of monomers and polymers while the other surface grafting techniques, which use ionisation radiation, cause damage to the substrate polymer due to excessive degradation. 

The effects of exposure of FD C Red No. 3 solutions to fluorescent, longwave, and shortwave UV sources were studied by Asker and Jackson (94). They found that exposure to fluorescent lighting (from cool white fluorescent tubes) was more detrimental to the stability of the dye solution than either of the other two UV sources studied. Similarly, the same kind of fluorescent radiation caused a higher degree of degradation of adriamycin solutions than either short wave or long wave UV radiation (47). The results of this study are presented in Table 3. 

Diltiazem undergoes hydrolysis to desacetyl diltiazem in aqueous buffer solutions (pH 1-7) (35). Diltiazem is most stable at pH 5. The decomposition follows pseudo-first order kinetics. The extrapolated room temperature shelf-life was 42 days or 15.8 days at pH 5 or 2, respectively. Exposure of aqueous buffered solutions of diltiazem at pH 2 or 7 to UV radiation caused more degradation as compared to the same solutions protected from light, demonstrating the light sensivity of the compound in solution (34). The stability of diltiazem hydrochloride in aqueous sugar solutions (fructose, dextrose, sucrose, sorbitol and mannitol) was determined to be superior to that in pH 5 aqueous buffer (36). 

Figure 19.29 shows the comparative shielding efficiency data for various materials. Rubber filled with lead oxides comes very close in performance to lead and is superior to concrete and aluminum. Exposure of these shields to radiation causes degradation of mechanical properties (hardness, in particular, is increased) but it does not affect shielding efficiency. 

Another field of application is actually emerging in the polymer engineering, where the radiation-caused degradation is used in a controlled manner to degrade the undesired high molecular mass. This field may be subdivided into ... 

They investigated the effect of radiation on tensile strength and elongation at break of EPDM/PP blend. Fresh and waste PPs were separately compounded with EPDM. In spite of the improvement in their gel content at 150 kGy, the degraded component causes alternation in mechanical properties. 

Ethylene/carbon monoxide copolymers containing 2 or 3 wt. o carbon monoxide are photo degradable polymers with the same general processing properties as high pressure, low density polyethylene. Exposure to UV radiation causes decomposition. The polymer is essentially a low density polyethylene with an environmental feature. Commercialization of these materials took place in the late 1960s. 

The results obtained to date have led to the conclusion that microwave radiation causes no degradation of the extracted compounds, unless an excessively high temperature arises in the vessel. However, a specific effect of microwaves on plant material has been found. Microwaves interact selectively with the free water molecules present in the gland and vascular systems, leading to rapid heating and temperature increase, followed by rupture of the walls and release of the essential oils into the solvent. Similar mechanisms are suspected in soils and sediments, where strong, localized heating should lead to an increase in pressure and subsequent destruction of the matrix macrostmcture. 

In principle, high-energy radiation causes aging when the polymer material can absorb the energy and the absorbed energy is sufficient to split chemical bonds (radiation-induced degradation) [17]. 

The aromatic or aliphatic diisocyanates have profound effects on the PUs properties [47-58]. The structural rigidity of aromatic HS generally produce elastomers with high tensile strength and modulus and enhanced thermal stability [55]. However, these aromatic units are susceptible to attack by ultraviolet radiation, causing degradation and yellowing without the use of stabilizers. 

It has previously been reported that the processes induced in the fibre by 248 nm radiation are different from those induced by 193 nm radiation." In addition, it is thought that the exposure of optical fibre to UV increases the stress in the core. The results reinforce these concepts they suggest that the 248 nm mechanism increases the internal core stress of the fibre significantly more than the 193 nm mechanism, such that 248 nm radiation causes a larger degradation of the fibre strength. 

Polystyrene exhibits the strongest absorption in the wavelength range of 250 — 280 nm, which is irrelevant for natural weathering. Shortwave UV radiation causes rapid degradation, in particular in the presence of oxygen. 

Energetic radiation causes gas liberation both in crosslinking plastics and in those that tend to degrade. Table 5.21 and Table 5.28. The amount of liberated gas depends on the radiation conditions (s. Section 5.3.3). 

Above a certain degree of crosslinking, strain at break and impact strength decrease again, and the plastic becomes hard and brittle. Radiation-chemical degradation also causes a decrease in mechanical properties with increasing radiation... 

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