Polyolefins elemental analysis

Most commercial polymers are substantially linear. They have a single chain of mers that forms the backbone of the molecule. Side-chains can occur and can have a major affect on physical properties. An elemental analysis of any polyolefin, (e.g., polyethylene, polypropylene, poly(l-butene), etc.) gives the same empirical formula, CH2, and it is only the nature of the side-chains that distinguishes between the polyolefins. Polypropylene has methyl side-chains on every other carbon atom along the backbone. Side-chains at random locations are called branches. Branching and other polymer structures can be deduced using analytical techniques such as NMR. 

Haslam et al. [32] reported the determination of Al in polyolefins by AAS. Typical AAS tests on rubber compounds involve several steps. The sample is combusted, and the resulting ash is dissolved in distilled de-ionised water. The solution is then used for AAS [126]. AAS or EDS can also be used for element analysis of filler particles. In order to determine the uniformity of tin compounds in polychloroprene after milling and pressing, Hornsby et al. [127] have ashed various pieces from one composition. After fusion of the residue with sodium peroxide and dissolution in HC1, the Sn content was determined by means of AAS. Typical industrial AAS measurements concern the determination of Ca in Ca stearate, Zn in Zn stearate, Ca- and Zn stearate in PE, Ca and Ti in PE film or Al and V in rubbers. 

Major industrial areas as the cement, ferro, non-ferro, petrochemical, textile or food industry, dispose of numerous Certified Reference Materials (organic and inorganic). For example, only the ferro-industry has already more than 300 CRMs and RMs listed in COMAR, the international database (jointly operated by LNE, BAM and NPL) which lists more than 10285 RMs (as of June 1998) of more than 400 producers [42]. Notwithstanding the size of the polymer industry (total production capacity for commodity thermoplastics is equal to over 140 Mt/a, of which about 50% of polyolefinic nature) it is surprising to note the scarcity of suitable polymer reference materials for elemental and molecular analysis. CRMs made from a polymer material and designed for molecular analysis are lacking totally, while those for elemental analysis are rare. In fact, until quite recently, for elemental analysis of polymers, only one set of four CRMs did exist, namely... 

Applications Basic methods for the determination of halogens in polymers are fusion with sodium carbonate (followed by determination of the sodium halide), oxygen flask combustion and XRF. Crompton [21] has reported fusion with sodium bicarbonate for the determination of traces of chlorine in PE (down to 5 ppm), fusion with sodium bisulfate for the analysis of titanium, iron and aluminium in low-pressure polyolefins (at 1 ppm level), and fusion with sodium peroxide for the complexometric determination using EDTA of traces of bromine in PS (down to 100ppm). Determination of halogens in plastics by ICP-MS can be achieved using a carbonate fusion procedure, but this will result in poor recoveries for a number of elements [88]. A sodium peroxide fusion-titration procedure is capable of determining total sulfur in polymers in amounts down to 500 ppm with an accuracy of 5% [89]. 

The advantages of atomic absorption techniques as opposed to spectrophotometric analysis for the determination of metals are that the former are amenable to multielement analysis and can be automated. There is some evidence that ashing polymers in silica crucibles rather than platinum can lead to up to 10% losses of elements such as copper by adsorption within the silica matrix to produce a compound that is not extractible by subsequent acid leaching. This does not occur when ashing is carried out in platinum. If silica crucibles are used then a magnesium oxide ashing aid should be employed as is demonstrated in the method for determining down to 0.1 ppm of copper in polyolefins, (Method 72). 

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