Plasticisers, analysis Solvent extraction

Plasticiser/oil in rubber is usually determined by solvent extraction (ISO 1407) and FTIR identification [57] TGA can usually provide good quantifications of plasticiser contents. Antidegradants in rubber compounds may be determined by HS-GC-MS for volatile species (e.g. BHT, IPPD), but usually solvent extraction is required, followed by GC-MS, HPLC, UV or DP-MS analysis. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out. The determination of antioxidants in rubbers by means of HPLC and TLC has been reviewed [58], The TLC technique for antidegradants in rubbers is described in ASTM D 3156 and ISO 4645.2 (1984). Direct probe EIMS was also used to analyse antioxidants (hindered phenols and aromatic amines) in rubber extracts [59]. ISO 11089 (1997) deals with the determination of /V-phenyl-/9-naphthylamine and poly-2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ) as well as other generic types of antiozonants such as IV-alkyl-AL-phenyl-p-phenylenediamines (e.g. IPPD and 6PPD) and A-aryl-AL-aryl-p-phenylenediamines (e.g. DPPD), by means of HPLC. 

Contaminated solvents and glassware are a very well known problem in analysis involving extraction. The major problem in the use of solvents is contamination with plasticisers, especially DEHP. After sample extraction usually enrichment of the analytes is required prior to the analysis. 

Applications Open-column chromatography was used for polymer/additive analysis mainly in the 1950-1970 period (cf. Vimalasiri et al. [160]). Examples are the application of CC to styrene-butadiene copoly-mer/(additives, low-MW compounds) [530] and rubbers accelerators, antioxidants) [531]. Column chromatography of nine plasticisers in PVC with various elution solvents has been reported [44], as well as the separation of CHCI3 solvent extracts of PE/(BHT, Santonox R) on an alumina column [532]. Similarly, Santonox R and Ionol CP were easily separated using benzene and Topanol CA and dilaurylthiodipropionate using cyclohexane ethyl acetate (9 1 v/v) [533]. CC on neutral alumina has been used for the separation of antioxidants, accelerators and plasticisers in rubber extracts [534]. Column chromatography of polymer additives has been reviewed [160,375,376]. 

Table 6.28 shows the solvent-extraction norms for plasticiser content. However, the participating laboratories were actually left free in the choice of their preferred method of analysis. 

Many commercial polymers contain not only the base material, but also a plethora of performance-enhancing additives such as anti-oxidants, stabilisers, plasticisers, fillers and pigments. Consequently, characterisation of such commercial materials can become extremely complex, requiring skills not only in analysis of the components, but also in their deformulation . In chapter 2, several examples are given of systematic approaches to this type of problem in different systems. It is fascinating to observe the combination of traditional separation techniques, such as solvent extraction, with modem chromatographic and spectroscopic methods. 

Analysis of the solvent extract obtained to prepare the plastic sample for an IR technique can be used to identify the plasticiser present in a compound. IR analysis of an ash of the compound obtained by heating at 550 °C can help to identify inorganic fillers. IR can also be used to identify specialist additives such as fire retardants. 

In PVC technology certain polymeric additives can be considered as process aids. These polymers have a similar composition to those used as impact modifiers in PVC formulations but are more compatible and so are primarily included to ensure more uniform flow and hence improve surface finish. Such process aids include ABS, chlorinated polyethylene, MBS, EVA-PVC graft polymers and acrylate-methacrylate copolymers. As these are usually found in unplasticised PVC, direct analysis of the product by IR will usually indicate the presence of those that have a distinctive spectmm as no masking by plasticiser will take place. However, even rigid PVC can contain a small amount of phthalate and so it is advisable to carry out a solvent extraction to clean up the matrix first. Where the process aid (e.g., chlorinated polyethylene) has a relatively bland spectmm, a technique such as NMR will be required to both detect and quantify it. NMR will usually be required to quantify the other types as well, unless the spectrum is very distinctive and standards of known composition are available. 

There are cases where stress cracking can occur due to the migration of a less volatile liquid (e.g., a plasticiser). An example of such a case is the stress cracking of ABS due to the migration of plasticiser from PVC, and this can be approached by the detection of phthalate in the ABS matrix by solvent extraction and then analysis of the extract using HPLC. 

Whereas the use of conventional fast atom bombardment (FAB) in the analysis of polymer/additive extracts has been reported (see Section 6.2.4), the need for a glycerol (or other polar) matrix might render FAB-MS analysis of a dissolved polymer/additive system rather unattractive (high chemical background, high level of matrix-, solvent- and polymer-related ions, complicated spectra). Yet, in selected cases the method has proved quite successful. Lay and Miller [53] have developed an alternative method to the use of sample extraction, cleanup, followed by GC in the quantitative analysis of PVC/DEHP with plasticiser levels as typically found in consumer products (ca. 30 %). The method relied on addition of the internal standard didecylphthalate (DDP) to a THF solution of the PVC sample with FAB-MS quantitation based on the relative signal levels of the [MH]+ ions of DEHP and DDP obtained from full-scan spectra, and on the use of a calibration curve (intensity ratio m/z 391/447 vs. mg DEHP/mg DDP). No FAB-matrix was added. No ions associated with the bulk of the PVC polymer were observed. It was... 

Robertson and Rowley [18] studied the extraction of plasticisers from PVC using different solvents prior to analysis by IR spectroscopy. 

Two general methods of plasticiser determination were distinguished between by Guiochon and Henniker [82] with and without preliminary extraction. Either may precede infrared spectrometry or GC. The most common method is to use ether to extract the plasticisers to be determined. If a quantitative analysis is required, the sample should be thin (0.1 mm or less) and should be extracted for several hours (usually 10 hours) to ensure that extraction is complete. If extraction is to be followed by spectrometry, care must be taken to eliminate all solvent by drying for 2-3 hours at 80 "C. If the analysis is to be done by chromatography, drying is unnecessary since the solvent is much more volatile than the plasticiser and will be well separated. 

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