Trace Metals in Polymers

Sources of trace metals in polymers are neutralising chemicals added to the final stages of manufacture to eliminate the effects of acidic catalyst remnants on polymer processing properties (e.g., hygroscopicity due to residual chloride ion). A case in point is high-density polyethylene (HOPE) and PP produced by the aluminium alkyl-titanium halide route which is treated with sodium hydroxide in the final stages of manufacture. 

A technique that involves combustion of the polymer under controlled conditions in a platinum crucible, followed by dissolution of the residual ash in a suitable aqueous reagent before final analysis by spectrophotometry, is of limited value. A quite complicated and lengthy ashing programme is necessary in this technique to avoid losses of alkali metal during ignition 0-1 hour from start heat to 200 C 1-2 

Sample By neutron activation By emission spectrography Original (ashed between 650 °C and 800 °C) Dope ash at 500 Diect ash at 500 C 

Henn [65] reported on a flameless atomic absorption technique with solid sampling for determining trace amonnts of chromium, copper and iron in polymers snch as polyacrylamide with a detection limit of approximately 0.01 ppm. 

AAS is a useful technique for the determination of traces of metals in polymers. In general, the polymer is ashed at a maximnm temperature of 450 C 0.1 hour from start heat to 200 °C 1-3 hours from start hold at 200 C 3-5 hours from start heat to 450 °C 5-8 hours from start hold at 450 °C. Ash is digested with warm nitric acid before spectrometric analysis. Detection limits for metals in polymers achievable by this procedure range from 0.02 ppm (zinc) to 0.57 ppm (iron). 

---

SEMI (2000) Test Method for Determining Trace Metals in Polymer Materials, SEMI F48-0600, SEMI, San Jose. 

Common sample types analyzed by ICP include trace elements in polymers, wear metals in oils, and numerous one-of-a-kind catalysts. 

A study of column extraction of metals was carried out using pre-packed columns available from Polymer Laboratories (now a part of Varian, Inc.) [6]. Trace metals in these monomers would have a detrimental effect on the stability of anaerobic adhesive products and must be absent prior to use. The columns used are specially coated macroporous polystyrene products that are compatible with polar, non-polar, protic and aprotic solvents. They are designed to remove metals from solvents and monomers. The metal removing SPE product is approximately 45 pm and based on a mono-dispersed macroporous polymeric material. 

Hoyer, B. and Florence, T.M. (1987). Application of polymer-coated glassy carbon electrodes to the direct determination of trace metals in body fluids by ASV, Anal. Chem. 59, 2839. 

The presence of trace metals in a polymer whether these are adventitious or deliberately added can have profound effects on their thermal stability. Thus, the presence of less than 1 ppm of adventitiously added copper can have profound effects on the heat stability of polypropylene. 

A direct reader ICP excels at the rapid analysis of multi-element samples. Common sample types analyzed by ICP include trace elements in polymers, wear metals in oils, and numerous one-of-a-kind catalysts. ICP instruments are limited to the analysis of liquids only. Solid samples require some sort of dissolution procedure prior to analysis. The final volume of solution should be at least 25 mL. The solvent can be either water, usually containing 10% acid, or a suitable organic solvent such as xylene. ICP offers good detection limits and a wide linear range for most elements. With a direct reading instrument multi-element analysis is extremely fast. 

This technique has been applied to determining the identity of oxygen absorbers in polymers [108] also to determine traces of metals in polymers. 

An AA spectrometer is also available with a graphite furnace and vapor generation accessories for the trace analysis of lead, antimony, arsenic, and mercury at parts-per-billion levels. AA is used for quantitative analysis of these metals in polymers as well as finished formulations. It has been used to determine the elemental composition of catalysts and plastic additives, polymer formulations, and composite materials. Samples may be rapidly acid digested prior to analysis using a microwave oven or similar techniques. Microwave furnaces are also available for dry ashing. 

Analytical techniques, such as mass spectroscopy, atomic absorption spectroscopy and atomic emission spectroscopy, for the detection of trace elements, mainly metal trace elements, in polymers, used particularly in food, are briefly considered. The disadvantages and suitability of such techniques are summarised. 10 refs. 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

---