Polypropylene after irradiation

Several articles in the area of microwave-assisted parallel synthesis have described irradiation of 96-well filter-bottom polypropylene plates in conventional household microwave ovens for high-throughput synthesis. While some authors have not reported any difficulties in relation to the use of such equipment (see Scheme 4.24) [77], others have experienced problems in connection with the thermal instability of the polypropylene material itself [89], and with respect to the creation of temperature gradients between individual wells upon microwave heating [89, 90]. Figure 4.5 shows the temperature gradients after irradiation of a conventional 96-well plate for 1 min in a domestic microwave oven. For the particular chemistry involved (Scheme 7.45), the 20 °C difference between the inner and outer wells was, however, not critical. 

The mechanism of the decay reaction of the methyl free radicals at —196° is not known however, the y-ray irradiation of polypropylene at — 196°C. produces only methane and no ethane (36), as demonstrated by gas analysis after warming to room temperature after irradiation. It may be that the methyl free radicals abstract hydrogen atoms on warming to room temperature or that hot methyl radicals are produced during the radiolysis with sufficient excess energy to abstract hydrogen atoms at liquid nitrogen temperature. 

The ESR spectra of polyisobutylene after irradiation with ultraviolet light (6) are different from those obtained after irradiation with ionizing radiation. The spectra consists mainly of two components one, a sharp quartet which has a half life of 1% hours at liquid nitrogen temperature, has been attributed to free methyl radicals (XI), in analogy with ultraviolet-irradiated polypropylene (51). The broad component is composed of many superimposed lines and was interpreted as caused by three different radicals, all stable at liquid nitrogen temperature. One of these radicals (XV) is the counterpart to the methyl radical (XI) while the others are the two radicals (XIII and XVI) which can both be formed by hydrogen abstraction. 

The loss of Impact strength of polypropylene was followed from sheets stored In air at 25°C and 60°C after irradiation with electron beams. A marked difference in efficacy of phenolic and thioether-based stabilizers at the two temperatures was found, with the thioether active alone at 60°C but only synergistically at 25°C. This difference was also reflected qualitatively in differences in chemiluminescence emission from the samples. 

Tsuji et al. (77) observed that the component other than methyl radicals observed in the spectrum immediately after irradiation is mainly a four-line one. This spectrum changed into an apparent eightline spectrum after standing in the dark for several days at —196° C, and reverted to the original four-line spectrum on UV irradiation. These changes of spectra are identical to those reported by Iwasaki et al. (80) for polypropylene irradiated with ionizing radiation, and are attributed to radical conversions as follows. 

Figure 6. Comparison of the phosphorescence excitation spectrum of polypropylene film (--) before and after irradiation for 250 hr in a Xenotest-150...

The severe degradation of polypropylene following sterilizing doses of irradiation can be characterized mechanically by its failure to undergo the necessary work in practice. Embrittlement increases with time for an irradiated polypropylene, thus rendering an acceptable formulation totally unacceptable a few months after irradiation. Naturally, the decay of radicals can be accelerated by thermal annealing, limited by the geometrical distortion temperature. 

Polypropylene is the most widely used material for the manufacture of sterile disposable hypodermic syringes. However, the suitability of irradiation sterilization for polypropylene devices was until the 1980s severely restricted by yellowing and insidious embrittlement after irradiation. 

In polypropylene this process of autooxidation is long-lived, leading eventually to severe embrittlement, which may render a formulation that could have appeared to be acceptable (when assessed immediately after irradiation) totally unacceptable a few months later. 

R = polypropylene residue) was the formation of a radical with characteristic ESR spectrum after irradiation of polypropylene with the incorporated transformed anti-... 

Examination of the external appearance was made in liquid nitrogen after fast neutron irradiation of 1.7 x 10 nvt and a y dose of 4.5 x 10 R. The polypropylene changed in color from transparent to yellow and broke into small pieces. Polycarbonate and Mylar became very brittle. Nomex and Kapton were not brittle after irradiation. The tensile tests of polypropylene, polycarbonate, and Mylar were performed at a neutron doese of 9 x 10 nvt and a y dose of 2.4 x 10 R, since the specimens irradiated to a neutron... 

Stress-strain curves of polypropylene, polycarbonate, and Mylar at 77 K before and after irradiation of 9 X 10 nvt with a y dose of 2.4 x lO R at 5 K. The stress-strain curves of polycarbonate and Mylar before irradiation are in the hatched region. 

The effect of low-doses gamma radiation on the quenched forms of isotactic polypropylene was the subject of our previous work (1). In this work, we have studied the effects of gamma radiation on the monoclinic and hexagonal phase. The macrostructural states of iPP before and after irradiation were followed and the noticed effects were then related to its melting behaviour and molecular weight changes. 

Stress-strain curves of irradiated highly crystalline polypropylene exposed to gamma rays exhibit its ductile behavior with elongation values for the nonirradiated samples up to 80%. After irradiation, highly crystalline polypropylene becomes less ductile and tensile stress decreases as well. 

It is concluded that the thermal properties (melting temperature, melting enthalpy, and degree of crystallinity), thermal stability, and mechanical properties (elastic modulus tensile strength and extension at break) of highly crystalline polypropylene showed change after irradiation. 

In conclusion, it was found by Akin-Okten et al. [21] that the ultimate strength of perlite-filled polypropylene after a 25 kGy gamma radiation dose was better than that of unfilled irradiated polypropylene. 

The most biodegradable of the commodity polyolefins is polypropylene (PP). Pandey and Singh have shown that polypropylene, after removal of antioxidants by solvent extraction, biodegrades much more rapidly than polyethylene by mass loss in compost [33], PP lost over 60% mass in 6 months whereas LDPE lost about 10% in the same time. Ethylene-propylene (EP) co-polymers biodegraded at rates intermediate between PP and PE. As expected, prior UV irradiation (photooxidation) increased both the rate and extent of the bioassimilation. This is fully in accord with the rates of environmental peroxidation of these molecules [8] and it has been shown that PP acts as a sensitiser for the peroxidation of LDPE [34]. 

Polyethylene and polypropylene samples were defatted with acetone and hexane and stirred with a solution of N-(4-azido-2-nitrophenyl)-ll-aminoundecanoic acid (ANPAU) or p-azidobenzoic acid in ethanol. Polymer samples were removed and air dried. After irradiation with a mercury high pressure lamp at 260 run the polymer samples were washed with ethanol. 

PL can be used as a sensitive probe of oxidative photodegradation in polymers. After exposure to UV irradiation, materials such as polystyrene, polyethylene, polypropylene, and PTFE exhibit PL emission characteristic of oxidation products in these hosts. The effectiveness of stabilizer additives can be monitored by their effect on PL efficiency. 

Figure 16. Surface grafting of polypropylene film strips after 10 sec. irradiation measured as light absorption after dipping in aqueous crystal violet solution. The presoaking solutions contain 0.2 M benzophenone (all) and 1.3 M acrylamide (1), 0.8 M (2),...

Table 1. Change in the wettability of polypropylene film after exposure to ozone and ozone-UV irradiation...

Figure 8 Evolution of the spatial distribution of nitroxide radicals across polypropylene plaques of thickness 2 mm and containing HAS, after the indicated UV irradiation times. The arrow indicates the irradiated side. Adapted and reprinted from Lucarini et al. [56b]. Copyright 1996, with permission from Elsevier.

An 8000-member library of trisamino- and aminooxy-l,3,5-triazines has been prepared by use of highly effective, microwave-assisted nucleophilic substitution of polypropylene (PP) or cellulose membrane-bound monochlorotriazines. The key step relied on the microwave-promoted substitution of the chlorine atom in monochlorotriazines (Scheme 12.7) [35]. Whereas the conventional procedure required relatively harsh conditions such as 80 °C for 5 h or very long reaction times (4 days), all substitution reactions were found to proceed within 6 min, with both amines and solutions of cesium salts of phenols, and use of microwave irradiation in a domestic oven under atmospheric reaction conditions. The reactions were conducted by applying a SPOT-synthesis technique [36] on 18 x 26 cm cellulose membranes leading to a spatially addressed parallel assembly of the desired triazines after cleavage with TFA vapor. This concept was later also extended to other halogenated heterocycles, such as 2,4,6-trichloropyrimidine, 4,6-dichloro-5-nitropyrimidine, and 2,6,8-trichloro-7-methylpurine, and applied to the synthesis of macrocyclic peptidomimetics [37]. 

As the time of exposure to UV is increased, the magnitude of the synergistic effect between TMPTA and lithium nitrate is increased dramatically for the UV grafting of styrene to polypropylene (Table XI) such that after 16 hours of irradiation very large yields are obtained. Even with TFMA, the grafting yield in the 30% monomer solution is increased by almost one order of magnitude due to the synergistic effect. 

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