Polymer-Additive Mixtures

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

The literature reports various (multidimensional) chromatographic approaches involving SEC and LC operating on dissolved polymer/additive mixtures. Floyd [985] has used microbore (1 mm i.d.) SEC-RPLC for the quantitative analysis of Tinuvin P in a cellulose acetate solution in THF, after separation of the polymeric and additive fractions total analysis time about 30 min. Relative accuracy and precision of 3 % and 1.5% were quoted. SEC-RPLC was also used to determine the styrene level in polystyrene crystals [986]. Additives in copolymers have been separated in a SEC/C system [987]. Chlorohydrin mixtures may be analysed by RPLC, but not in the presence of polymer. Thus, SEC... 

UV spectrophotometry can be used for the analysis of polymer/additive mixtures in solution, provided that the following restrictions are taken into account ... 

Photophysical Processes in PET and Model Compounds. The photophysical processes in many polymer, copolymer, and polymer-additive mixtures have been studied (17. 18. 19). However, until recently, few investigations have been made concerning the photo-physical processes available to the aromatic esters in either monomeric or polymeric form. 

The discussion so far has focused on compatible materials of similar viscosity, i.e., simple mixing. In many cases, however, it is necessary to admix low-viscosity additives into a polymer melt. This can involve very high viscosity ratios, as much as ten million to one, and is very difficult for most static mixers. As the viscosity ratio increases, the length of the mixer required also increases. If the length of the mixer is insufficient, the low-viscosity component can separate in the form of splash-out. The splash-out limit, beyond which no further splash-out can be detected, is shown in Fig. 7.148 for SMX type mixing elements for several polymer/additive mixtures. 

Splash-out limit for SMX mixer for several polymer-additive mixtures... 

FD-MS has proved to be a particularly useful technique, since molecular ion abundances are high with respect to fragmented ions [39]. Electrospray ionisation MS (ESI-MS and ESI-MS-MS) has also been used for the analysis of polymer additive mixtures [40]. 

A polymer-additive mixture is out of thermodynamic equilibrium. Since the additive concentration in the environment is equal to zero, there is no equality between its chemical potentials in the environment and in the polymer. Additive molecules tend to migrate outside the polymer in order to reach an equilibrium. This migration is composed of two elementary steps (see Fig. 12.7) ... 

While RI detectors are used for isocratic HPLC, the other detectors, viscometer and light scattering, are not generally suitable for HPLC. As noted in See. II.D. absorbance detectors, either UV/visible or photodiode array (PDA), are much more useful. Table 5 and Fig. 13 show the HPLC of a polymer additive mixture. The reproducibility of 12 consecutive injections shown in Fig. 13 demonstrates exceptional reproducibility of the analysis, espeeially considering that both the solvent eomposition and solvent flow rates were programmed for this work. 

Flame Retardants. Bromine compounds make up an important segment of the market for flame retardants used in polymers. Additive flame retardants are added to polymers during processing reactive flame retardants react chemically to become part of the polymer chain itself. In addition to the compounds Hsted in Table 3, a number of proprietary mixtures and phosphoms—bromine-containing flame retardants are also sold (see Elame RETARDANTS, HALOGENATED, FLAAffi RETARDANTS). 

The advent of high-resolution capillary gas chromatography (HR-CGC) with on-column injection has resulted in improved GC analysis of polymer additives [92-94]. The solution of the additive mixture is injected directly into the cold end of the capillary column by means of a cold injector. Thus, sample discrimination, the instantaneous evaporation of the sample solvent, is avoided. The nonvaporising, on-column injection combined with very high resolution of the capillary columns allows accurate separation, identification and quantification of additives of complex mixtures. With the solvent venting technique, the sample is introduced into the column without splitting and sample concentrations... 

Applications Conventional GC is a workhorse in the qualitative and quantitative analysis of polymer additives in complex mixtures and has found numerous applications. Both GC and auxiliary techniques are particularly useful for characterisation of (semi)volatile constituents and additives ranging from gases to hydrocarbon waxes (fatty acids and their... 

David et al. [184] have shown that cool on-column injection and the use of deactivated thermally stable columns in CGC-FID and CGC-F1D-MS for quantitative determination of additives (antistatics, antifogging agents, UV and light stabilisers, antioxidants, etc.) in mixtures prevents thermal degradation of high-MW compounds. Perkins et al. [101] have reported development of an analysis method for 100 ppm polymer additives in a 500 p,L SEC fraction in DCM by means of at-column GC (total elution time 27 min repeatability 3-7 %). Requirements for the method were (i) on-line (ii) use of whole fraction (LVI) and (iii) determination of high-MW compounds (1200 Da) at low concentrations. Difficult matrix introduction (DMI) and selective extraction can be used for GC analysis of silicone oil contamination in paints and other complex analytical problems. 

Berger [340] has examined the use of pSFC in polymer/additive analysis. As many polymer additives are moderately polar and nonvolatile SFC is an appropriate separation technique at temperatures well below those at which additives decompose [300,341,342], SFC is also a method of choice for additives which hydrolyse easily. Consequently, Raynor et al. [343] and others [284,344] consider that SFC (especially in combination with SFE) is the method of choice for analysing polymer additives as a relatively fast and efficient sample preparation method. Characterisation of product mixtures of nonpolar to moderately polar components encompassing a wide range of molecular masses can be accomplished by cSFC-FID. Unknown polymer additives may be identified quite adequately by means of cSFC-FID by comparison with retention times of standards [343], However, identification by this method tends to be time-consuming and requires that all the candidate compounds are on hand. SFC-FID of some low-to-medium polarity additives on reversed-phase packed columns... 

Figure 4.6 cSFC-FID chromatogram of a synthetic mixture of polymer additives. 1-21, Topanol OC, Tinuvin P/292/320/326 /328, Chimassorb 81, erucamide, Tinuvin 770/440, Irgafos 168, Tinuvin 144, Irganox PS 800/1076/MD 1025/245/1035/3114/PS 802/1330/1010, in this order. For conditions see Raynor etal. [343]. Reprinted with permission from Raynor etal., Analytical Chemistry, 60, 427-433 (1988). Copyright (1988) American Chemical Society... 

SFC-FID is widely used for the analysis of (nonvolatile) textile finish components. An application of SFC in fuel product analysis is the determination of lubricating oil additives, which consist of complex mixtures of compounds such as zinc dialkylthiophosphates, organic sulfur compounds (e.g. nonylphenyl sulfides), hindered phenols (e.g. 2,6-di-f-butyl-4-methylphenol), hindered amines (e.g. dioctyldiphenylamines) and surfactants (sulfonic acid salts). Classical TLC, SEC and LC analysis are not satisfactory here because of the complexity of such mixtures of compounds, while their lability precludes GC determination. Both cSFC and pSFC enable analysis of most of these chemical classes [305]. Rather few examples have been reported of thermally unstable compounds analysed by SFC an example of thermally labile polymer additives are fire retardants [360]. pSFC has been used for the separation of a mixture of methylvinylsilicones and peroxides (thermally labile analytes) [361]. 

Apart from styrene oligomers [514], it appears that OPLC analysis of polymer additives has not been reported. However, the technique has been used for analysis of food antioxidants (BHA, BHT, NDGA and propyl, octyl, and dodecyl gallate) on silica with five different solvent mixtures and densitometric detection [479],... 

Applications Chromatography is a preferred technique for additive analysis as it allows both separation of additives in a mixture and subsequent quantitation. Despite the developments in GC, this technique cannot separate many polymer additives. Even with its lower efficiency in comparison to GC, HPLC is today one of the cornerstones in a polymer additive laboratory. Judging by the number of publications in recent years, HPLC is first among analytical methods for additives (confirmation/identification/quantification). Most additives may be analysed by HPLC if they can be dissolved in an HPLC solvent and absorb UV light. Typical polymer/additive analyses are carried out using LPE followed by HPLC with UV or RI detection [605-611]. Verification of the identity of an analyte is then based on a combination of retention time, UV and RI evidence. RPLC is used most frequently for polymer/additive analysis, but normal-phase and SEC are also used. Consequently, techniques for additive analysis by HPLC are legion. 

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