Rubbers quantitative analysis

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

It is quite clear from Schemes 2.1-2.5 that in rubbers polymer identification and additive analysis are highly interlinked. This is at variance to procedures used in polymer/additive analysis. The methods for qualitative and quantitative analysis of the composition of rubber products are detailed in ASTM D 297 Rubber Products-Chemical Analysis [39]. 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

Table 10.32 is a shortlist of the characteristics of the ideal polymer/additive analysis technique. It is hoped that the ideal method of the future will be a reliable, cost-effective, qualitative and quantitative, in-polymer additive analysis technique. It may be useful to briefly compare the two general approaches to additive analysis, namely conventional and in-polymer methods. The classical methods range from inexpensive to expensive in terms of equipment they are well established and subject to continuous evolution and their strengths and deficiencies are well documented. We stressed the hyphenated methods for qualitative analysis and the dissolution methods for quantitative analysis. Lattimer and Harris [130] concluded in 1989 that there was no clear advantage for direct analysis (of rubbers) over extract analysis. Despite many instrumental advances in the last decade, this conclusion still largely holds true today. Direct analysis is experimentally somewhat faster and easier, but tends to require greater interpretative difficulties. Direct analysis avoids such common extraction difficulties as ... 

Thus quantitative analysis of elasticity is currently elusive, despite a great deal of work ongoing in this area. The extensive literature available on rubber elasticity by and large has not been adapted to hydrogels, and further work along these lines is necessary before quantitative predictions of swelling degree can be made from independent measurable polymer properties. 

J.M. Chalmers and N.J. Everall, Qualitative and quantitative analysis of plastics, polymers and rubbers by vibrational spectroscopy. In N.J. Everall, J.M. Chalmers and P.R. Griffiths (Eds.), Vibrational Spectroscopy of Polymers Principles and Practice, Wiley, Chichester, 2007, pp. 1-67. 

Gas chromatography (GC) and mass spectrometry (MS) can be coupled to the TGA instrument for online identification of the evolved gases during heating pyrolysis-GC/MS is a popular technique for the evaluation of the mechanism and the kinetics of thermal decomposition of polymers and rubbers. Moreover, it allows a reliable detection and (semi)quantitative analysis of volatile additives present in an unknown polymer sample. 

High resolution l3C NMR is also used in the determination of the composition of the dispersed phase in cured rubber modified epoxies in order to analyze the chemical structure of the mobile segments 152). In this case quantitative analysis is possible because the areas under each peak are approximately equal to the number of carbons contributing to the peak, and the intensities of the broad lines from the rigid phase are very low, almost indistinguishable from the baseline noise. The structure of crosslinked networks based on poly(3,4-pyrrolidinediethylene), synthesized by different methods, was determined from gels swollen in water and chloroform 153). 

TG-DTA and DSC are suitable for product quality control as exemplified by OIT measurements for polyethylene (PE) and quantitative analysis of the rubber phase in ABS and of a polymer/ softener/soot/mineral filler mixture [77]. 

A calibration curve is prepared, using absorbance versus concentration plot, so that the concentration of the unknown component can be determined. But quantitative analysis for a complex system like vulcanised rubber or a blend of two or three components, is not possible. The use of computers with the FTIR spectrometer, increases the rapid scanning capability, data processing for analysis of chemical or physical structural changes in polymers as a function of time over the entire mid-IR frequency. 

Like styrene content determination, acrylonitrile content in nitrile rubber can be determined with IR spectra following the quantitative analysis method. Butadiene units, adding up to an acrylonitrile unit in the polymer chain, show a strong tendency to add... 

Lattimer RP, Schur KM (1985) Quantitative analysis of rubber triblcnds by pyrolysis mass spectrometry J Anal Appl Pyrolysis 8 95-107... 

Charsley and Dunn (102) and Dunn (103) have applied TG to the characterization and quantitative determination of carbon black in rubber. The analysis involves volatilization of the oil and pyrolysis of the polymer in an inert atmosphere followed by oxidation of free carbon black in the rubber. 

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