Polyethylene, neutron activation

FT-IR has been used to determine residual catalyst support in commercial polyethylene at the level of 100 parts per million 841. The method is based on the use of the 1118 or 470 cm-1 bands to determine the quantity of silica support dispersed in the polymer. The band at 2020 cm-1 was used as an internal standard for the amount of polyethylene. Neutron activation analysis was used to calibrate the weight percent of silica present in each polymer sample. 

The concrete block walls of the cell housing the generator tube and associated components are 1.7 meters thick. The facility also includes a Kaman Nuclear dual-axis rotator assembly for simultaneous transfer and irradiation of reference and unknown sample, and a dual Na iodide (Nal) scintillation detector system designed for simultaneous counting of activated samples. Automatic transfer of samples between load station to the rotator assembly in front of the target, and back to the count station, is accomplished pneumatically by means of two 1.2cm (i.d.) polyethylene tubes which loop down at both ends of the system and pass underneath the concrete shielding thru a pipe duct. Total one-way traverse distance for the samples is approx 9 meters. In performing quantitative analysis for a particular element by neutron activation, the usual approach is to compare the count rates of an unknown sample with that of a reference standard of known compn irradiated under identical conditions... 

With regard to the hazard concern from physical and mechanical handling, the expl nature of the materials can pose a special problem. For example, during pneumatic transfer of samples in fast neutron activation, the polyethylene vials containing the expl approach speeds of 15m/sec and come to rest against a metal stop at both irradiation and count stations. However at PicArsn (Ref 13), in over 1000 irradiations and pneumatic transfers with up to 2.3g of shock-resistant secondary expls such as TNT, HMX,... 

The neutron activation method for the determination of arsenic and antimony in seawater has been described by Ryabin et al. [66]. After coprecipitation of arsenic acid and antimony in a 100 ml sample of water by adding a solution of ferric iron (10 mg iron per litre) followed by aqueous ammonia to give a pH of 8.4, the precipitate is filtered off and, together with the filter paper, is wrapped in a polyethylene and aluminium foil. It is then irradiated in a silica ampoule in a neutron flux of 1.8 x 1013 neutrons cm-2 s 1 for 1 - 2 h. Two days after irradiation, the y-ray activity at 0.56 MeV is measured with use of a Nal (Tl) spectrometer coupled with a multichannel pulse-height analyser, and compared with that of standards. 

In neutron activation analysis, the sample in a suitable container, often a pure polyethylene tube, is bombarded with slow neutrons for a fixed time together with standards. Transmutations convert analyte elements into radioactive elements, which are either different elements or isotopes of the original analyte. 

The majority of the applications of 14 MeV neutron activation analysis involve the use of short-lived indicator radionuclides. Therefore, it is essential that the sample be returned quickly to the counting station following irradiation. Pneumatic sample transfer systems employing compressed nitrogen, or a vacuum are most commonly used 34-35>. An inexpensive system may be constructed from ordinary low density polyethylene tubing 18>. Irradiation, delay and counting times are ordinarily controlled by means of preset timing circuits. Completely automated control and transfer systems are available commercially. 

S) Ehmann, W. D., and D. M. McKown Heat-Sealed Polyethylene Sample Containers for Neutron Activation Analysis. Anal. Chem. 40, 1758 (1968). 

Neutron activation has been used to determine the concentrations of trace elements in polyethylene. A procedure has been optimized which involves three irradiations with a SICWFOKE nuclear reactor and four counts with a gamma-ray spectrometer. Phosphorus is determined with beta-ray spectrometry. The detection limits, most of which are below one ppm, have been determined for 42 elements. The merits of the method are discussed in terms of sensitivity, accuracy, ease of use, interferences, and freedom from contamination. 

In many laboratories that have access to a nuclear reactor, neutron activation is used for the chemical analysis of rocks, minerals, petroleum, biological tissues, alloys, etc., and the technique is well suited for the determination of the concentrations of trace elements in polymers. Neutron activation analysis was used by Given et al. (1) in their studies of water tree growth in polymeric insulation and by Wu and Chen (2) in their studies of dopant-polymer interactions in MoCl5-dcped polyacetylene films. In this work the principles of the method are described and the possibilities are illustrated by means of measurements carried out on polyethylene. 

In studies where a knowledge of the diffusion of metallic ions in polymers is important, one often wishes to measure a profile of the concentration as a function of depth. Neutron activation cannot be used to measure these profiles directly, but if the sample can be cut into thin slices with a microtome, these can be analysed individually to construct the profile. In our laboratory this technique is used extensively to study the migration of ions into the polyethylene insulation of high-voltage cables (10). These impurities contribute to the degradation with use of the electrical properties of the cable. 

When INAA is used and the concentration of Na in the sample is above 1%, the Na y-rays mask other useful y-ray peaks. In particular, arsenic and zinc abundances are not as easily determined in the shales as they are in coal by INAA (28). The oil and water appear to present no leakage problems at fluxes of 2 X 10 n cm" sec" for irradiation periods of 2 hr. These liquids were irradiated in sealed polyethylene vials inside of sealed polyethylene bags. The neutron activation analyses were performed at the U.S. Geological Survey in Denver, Golorado. 

Neutron activation analysis of mercury in petroleum should always be carried out in quartz vials in place of the conventional snap-cap polyethylene vials. As noted in the section on Storage and Stability, the latter frequently lead to serious mercury losses after irradiation (see Chapter 2). 

METHOD 68 - DETERMINATION OF SILICA CATALYST SUPPORT IN HIGH DENSITY POLYETHYLENE. INFRARED SPECTROSCOPY AND NEUTRON ACTIVATION ANALYSIS. "... 

Three methods are described for the determination of down to 50 ppm of silica catalyst supports in high density polyethylene. These methods involve direct ashing and weighing, infrared spectroscopy and neutron activation analysis. 

Approximately 1 > 2 g of polyethylene powder is irradiated with neutrons obtained with a 500 ke V neutron activator according to the following reaction H(D,n)%e. Silicon is activated by the reaction Si(N,p) Al, and the concentration of silicon is then measured by counting the 1.78 MeV gamma-ray emission fiom the decay of A1. A Conostan 5000 ppm Si standard is used for instrument calibration. Two 3 in sodium iodide detectors are used to measure the 1.78 MeV gamma-rays. 

Results obtained for polyethylene samples containing residual silica support of three different catalysts by infrared (470 cm method), neutron activation analysis, ashing and weight are shown in Table 7.36. Infrared results differ from neutron activation analysis results by 0 - 5% while ashing and weighing techniques differ from neutron activation analysis results by 5 - 21% and 5 - 28% respectively. 

Of the elements of current concern the analysis for mercury has probably received the most attention. The low levels of mercury involved in biological and environmental samples usually means that chemical separation of the neutron-activated mercury is necessary and a variety of separation techniques have been published. Examples include volatilization, electrodeposition alone and after preliminary precipitation, ion exchange, and Ge (Li) spectrometry after a preliminary one-step anion-exchange separation. The analysis for Hg is complicated by the volatility of the element, which can lead to losses by absorption on to, and by diffusion through, polyethylene... 

Normal-mode analyses for polymers other than polyethylene (and the n-paraffins), for which neutron scattering data are available, have not been carried out in sufficient detail to yield complete phase-frequency relations. Calculations of the optically active phases of isotactic polypropylene have been completed by Miyazawa and co-workers 19) and by ScHACHTSCHNEiDER and Snyder 31), but these treatments neglected intermolecular forces, which could have a significant effect on the low-frequency modes observed by neutron scattering. The situation is similar for polytetrafluoroethylene, for which calculations are available for isolated chains in a planar zig-zag, rather than a helical, conformation (/5). 

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