Polyolefins analytical measurements

Analytical and test methods for the characterization of polyethylene and PP are also used for PB, PMP, and polymers of other higher a-olefins. The C-nmr method as well as k and Raman spectroscopic methods are all used to study the chemical stmcture and stereoregularity of polyolefin resins. In industry, polyolefin stereoregularity is usually estimated by the solvent—extraction method similar to that used for isotactic PP. Intrinsic viscosity measurements of dilute solutions in decahn and tetraHn at elevated temperatures can provide the basis for the molecular weight estimation of PB and PMP with the Mark-Houwiok equation, [rj] = KM. The constants K and d for several polyolefins are given in Table 8. 

Applications Quantitative dry ashing (typically at 800 °C to 1200°C for at least 8h), followed by acid dissolution and subsequent measurement of metals in an aqueous solution, is often a difficult task, as such treatment frequently results in loss of analyte (e.g. in the cases of Cd, Zn and P because of their volatility). Nagourney and Madan [20] have compared the ashing/acid dissolution and direct organic solubilisation procedures for stabiliser analysis for the determination of phosphorous in tri-(2,4-di-t-butylphenyl)phosphite. Dry ashing is of limited value for polymer analysis. Crompton [21] has reported the analysis of Li, Na, V and Cu in polyolefins. Similarly, for the determination of A1 and V catalyst residues in polyalkenes and polyalkene copolymers, the sample was ignited and the ash dissolved in acids V5+ was determined photo-absorptiometrically and Al3+ by complexometric titration [22]. 

This chapter summarizes the book with special emphasis on the future of migration modeling procedures for polyolefins and non-polyolefins in mono- and multilayer structures combined with modern analytical methods for measuring mass transfer in new polymer-migrant systems. 

The solvents used in analytical TREF are limited to chlorinated solvents, mainly ort/io-dichlorobenzene and 1,2,4 tri-chlorobenzene (perchloroethylene and a-chloronaphtalene have also been used), which can dissolve the polyolefins at high temperature and are transparent enough in the IR region of measurement. These solvents are the same as used in GPC/SEC analysis of polyolefins and are also appropriate for detection by refractive index, although this detector has not... 

Similarly to GPC, the amount of information obtained with TREF and Crystaf can be increased by adding more detectors to the system. For instance, LS and VlSC detectors have been used to determine molecular weight averages as a function of crystallization/elution temperature or comonomer content in the copolymer. The analytical results shown in Figure 2.4, for instance, were measured with a TREF/IR-LS system. Another TREF/IR-LS profile is depicted in Figure 2.12 for a rather complex trimodal polyolefin resin. 

UVA IS spectrophotometry is a well developed routine technique, which is used extensively in QA/QC laboratories, but not so frequently as a process analytical technique because of lack of selectivity of the spectra (exceptions are monoaromatic hydrocarbons). Visible and near-ultraviolet spectra often do not contain so much industrially useful information. However, the method is suited for the determination of components that can be readily distinguished from the sample matrix (e.g. UV absorbers in polyolefins, but not in EPs colour measurements of ABS). Provided that the analyte has a UV chromophore, UV absorbance measurements provide much greater sensitivity than NIR measurements. This enables antioxidants and other additives to be determined at the low ppm level. Polyolefins are transparent in the UV, which enables calibrations for the analysis of antioxidants to be matrix independent. 

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