THERMAL VOLATILISATION

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

The applications of simultaneous TG-FTIR to elastomeric materials have been reviewed in the past. Manley [32] has described thermal methods of analysis of rubbers and plastics, including TGA, DTA, DSC, TMA, Thermal volatilisation analysis (TVA), TG-FTIR and TG-MS and has indicated vulcanisation as an important application. Carangelo and coworkers [31] have reviewed the applications of the combination of TG and evolved gas analysis by FTIR. The authors report TG-FTIR analysis of evolved products (C02, NH3, CHjCOOH and olefins) from a polyethylene with rubber additive. The TG-FTIR system performs quantitative measurements, and preserves and monitors very high molecular weight condensibles. The technique has proven useful for many applications (Table 1.6). Mittleman and co-workers [30] have addressed the role of TG-FTIR in the determination of polymer degradation pathways. 

Thermoplastic polyester elastomers Tripropylene glycol diacrylate Triphenyl phosphate Thermoplastic polyurethane Trimethylolpropanetrimethacrylate Thermal volatilisation analysis Transmitter signal... 

A. Martin, R.L. Blanchard, The thermal volatilisation of Caesium-137, Polonium-210 and Lead-210 from in vivo labelled samples. Analyst, 1969, 94, 441. 

Introducing samples to the plasma via liquids reduces sensitivity because the concentration of the analyte is limited to the volume of solvent that the plasma can tolerate. An electro-thermal method seems an obvious choice to increase the detection limit as it will vaporise entirely most neat samples or using an increased concentration of sample in a suitable solvent. The sample is placed on a suitable open graphite rod in an enclosed compartment and heated rapidly (Figure 2.15). The electronics required for ICP-OES-ETV (inductively coupled plasma-optical emission spectroscopy-electro-thermal volatilisation) is similar to that for A AS and detection limits are better than ICP-AES. 

During an investigation of the thermal degradation of poly(vinylchloride - vinylacetate) blends, McNeill (14) observed acetyl chloride in the degradation products. We investigated this aspect by FTIR-EGA and found that the amount of acetyl chloride in the effluent decreased with increased residence time in the hot zone. This suggested that acetyl chloride is a primary decomposition product, rather than a product of reaction between HCl and acetic acid from pyrolysis of the copolymer. McNeill s work was performed by thermal volatilisation analysis and did not provide on-the-fly identification of the pyrolysis products. 

Turnbull L, Liggat JJ, MacDonald WA. Thermal degradation chemistry of poly(ethylene naph-thalate) - a study by thermal volatilisation analysis. PolymDegrad Stab 2013 98(11) 2244-58. 

In 1991, McNeill and Bounekhel [38] studied the thermal degradation of PET using thermal volatilisation analysis (TVA), and subsequently made rather controversial assertions based on the results. 

Studies of the thermal and chemical stability of polymers are of paramount importance and instrumentation used in these studies discussed in Chapter 9 include thermogravimetric analysis, differential thermal analysis, differential scanning calorimetry, thermal volatilisation analysis and evolved gas analysis. Monitoring of resin cure is another important parameter in polymer processing in which dynamic mechanical analysis, dielectric thermal analysis and differential scanning calorimetry is used (Chapter 10). 

In this technique, in a continuously evacuated system the volatile products are passed from a heated sample to the cold surface of a trap some distance away. A small pressure develops which varies with the rate of volatilisation of the sample. If this pressure is recorded as the sample temperature is increased in a linear manner, a thermal volatilisation analysis thermogram showing one or more peaks is produced. The trace obtained is somewhat dependent on heating rate, which therefore should be standardised. 

Table 9.2 Thermal measurement possible with commercial analysers (excluding evolved gas analysis and thermal volatilisation analysis) ...

Table 2.42. Characterisation of thermal volatilisation and desorption techniques ...

In general terms, absolute quantification by means of TD-GC-MS or thermal volatilisation techniques is a doubtful exercise because total desorption of the analyte(s) at a given temperature is not assured, internal standards are difficult to use and mass spectrometry is not exactly weU known for its quantitative excellence. No reports are available on the use of direct TD-CIS-GC-MS for quantification purposes. 

Thermal scanning microscopy Temperature-time profile Time/temperature resolved pyrolysis mass spectrometry Thermal ultraviolet Thermal volatilisation analysis Thermal wave infrared imaging Transmission X-ray microscopy Total-reflection X-ray fluorescence (c/r. TRXRF) Ultrasonic force microscopy Ultraviolet photoelectron spectroscopy Ultrasound... 

Other thermally based techniques, which have been developed in recent years include thermal volatilisation (Chapter 5), dynamic mechanical analysis (Chapter 8), thermomechanical analysis (Chapter 9), microthermal analysis (Chapter 10) and dielectric thermal analysis (Chapter 12). These have been used in phase transition studies, resin cure, modulus measurement, stress-strain studies, viscoelastic and rheological properties, morphology, topography, mechanical and thermal properties. 

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