Determination of additives

Polymeric compounds, particularly rubber compounds, contain a number of functional additives. TGA can be used to identify the presence of these additives, to evaluate the effect that they have on certain properties of the product (e.g. oxidative stability in the case of antioxidants) and, in the case of the major additives (e.g. plasticisers and fillers), it can be used to quantify them. 

These types of additives include man-made compounds such as phthalates and adipates, and naphthenic hydrocarbon mineral oils. Being viscous liquids they have relatively low molecular weights and so are usually volatile within the temperature range 150-300°C. This means that they are lost from a sample during a TGA analysis by volatilisation, rather than pyrolysis or oxidation, and this mode of loss is the same irrespective of the atmosphere type being used. 

Due to this behaviour, it is possible from the peak of the weight loss derivative to obtain some qualitative information on the type of plasticiser present. As always with TGA, the rate of heating will affect the precise temperature obtained for a given compound - the faster the rate of heating the higher the temperature - but the type of data that can be obtained are given in Table 6.1 using a series of monomeric ester plasticisers. 

It can be seen that the order is independent of molecular weight the ease with which the plasticiser volatilises being of overriding significance. 

Often the quantification of plasticisers by TGA is more accurate than a quantitative solvent extraction step. This is because a solvent is never completely selective in what it extracts from a polymer and any extract will always contain a certain amount of other material, particularly low molecular weight oligomers from the polymer, and so the extract value obtained is always higher than the actual plasticiser level that is present. 

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Spriggs J. Krc, Industrial Engineering Study on the Determination of Additives to Eliminate Cracking of Cast Explosive Charges , ARF Rept 5 (Final), Project C 114 (Sept 1958), AD 203745 32) W.R. Tomlinson O.E. Sheffield, PATR 1740 (Rev 1) (1958), 337 33) T. Urbanski et... 

Dossi, N., Simultaneous RP-LC determination of additives in soft drinks, Chro-matographia, 63, 557, 2006. 

Boyce, M.C., Determination of additives in food by capillary electrophoresis. Electrophoresis, 22, 1447, 2001. 

Environmental monitoring of chloroacetanilides requires methods that have the capability to distinguish between complex arrays of related residues. The two example methods detailed here for water monitoring meet this requirement, but the method for metabolites requires sophisticated mass spectral equipment for the detection of directly injected water samples. In the near term, some laboratories may need to modify this method by incorporation of an extraction/concentration step, such as SPE, that would allow for concentration of the sample, so that a less sensitive and, correspondingly, less expensive, mass spectral detector can be used. However, laboratories may want to consider purchasing a sensitive instrument rather than spending time on additional wet chemistry procedures. In the future, sensitive instrumentation may be less expensive and available to all laboratories. Work is under way to expand the existing multi-residue methods to include determination of additional chloroacetanilides and their metabolites in both water and soil samples. 

Table 1.13 Scientific publications on analytical methods for the qualitative and quantitative determination of additives and additive classes of Table 1.12...

Most methods for the determination of additives in plastics come essentially under two headings, namely with or without sample preparation. The following eight analytical categories are thus distinguished ... 

Analytical techniques for the quantitative determination of additives in polymers generally fall into two classes indirect (or destructive) and direct (or nondestructive). Destructive methods require an irreversible alteration to the sample so that the additive can be removed from the plastic material for subsequent detention. This chapter separates the additive wheat from the polymer chaff , and deals with sample preparation techniques for indirect analysis. 

Applications The majority of SFE applications involves the extraction of dry solid matrices. Supercritical fluid extraction has demonstrated great utility for the extraction of organic analytes from a wide variety of solid matrices. The combination of fast extractions and easy solvent evaporation has resulted in numerous applications for SFE. Important areas of analytical SFE are environmental analysis (41 %), food analysis (38 %) and polymer characterisation (11%) [292], Determination of additives in polymers is considered attractive by SFE because (i) the SCF can more quickly permeate throughout the polymer matrix compared to conventional solvents, resulting in a rapid extraction (ii) the polymer matrix is (generally) not soluble in SCFs, so that polymer dissolution and subsequent precipitation are not necessary and (iii) organic solvents are not required, or are used only in very small quantities, reducing preparation time and disposal costs [359]. 

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. 

Applications Conventional TLC was the most successful separation technique in the 1960s and early 1970s for identification of components in plastics. Amos [409] has published a comprehensive review on the use of TLC for various additive types (antioxidants, stabilisers, plasticisers, curing agents, antistatic agents, peroxides) in polymers and rubber vulcanisates (1973 status). More recently, Freitag [429] has reviewed TLC applications in additive analysis. TLC has been extensively applied to the determination of additives in polymer extracts [444,445]. 

LDPE or HDPE extracts has been determined colorimet-rically at 430 nm by oxidation with H202 in the presence of H2S04 [66]. p-Phenylenediamine derivatives such as Flexzone 3C, used as antiozonants in rubber products, have been determined colorimetrically after oxidation to the corresponding Wurster salts [67]. A wide range of amine AOs in polyolefins has been determined by the p-nitroaniline spectrophotometric procedure [68]. Monoethanolamine (MEA) in a slip agent in PE film has been determined as a salicylaldehyde derivative by spectrophotometric quantification at 385 nm [69]. Table 5.6 contains additional examples of the use of 1JV/VIS spectrophotometry for the determination of additives in polymers. 

Composition and structure of newly developed additives are commonly examined by IR, NMR, MS and elemental analysis, e.g. recently developed higher MW antioxidants [115]. Infrared spectroscopy is also well suited to the direct verification of compound composition and quantitative determination of additives in polymers. Gray and Neri [116] have used Soxhlet... 

In principle, measurement of the phosphorescence characteristics of samples obtained after extraction of polymers with organic solvents may also yield useful information regarding the nature and concentration of the additives present. Parker and Hatchard [157] have examined the possibilities of phosphorescence measurements for V-phenyl-2-naphthylamine. Although it should be possible to determine various analytes simultaneously by correct choice of ex and em wavelengths and phosphorescence decay, no pertinent reports are available. Phosphorescence finds limited application for the direct determination of additives in polymers (without prior extraction). 

DOSY is a technique that may prove successful in the determination of additives in mixtures [279]. Using different field gradients it is possible to distinguish components in a mixture on the basis of their diffusion coefficients. Morris and Johnson [271] have developed diffusion-ordered 2D NMR experiments for the analysis of mixtures. PFG-NMR can thus be used to identify those components in a mixture that have similar (or overlapping) chemical shifts but different diffusional properties. Multivariate curve resolution (MCR) analysis of DOSY data allows generation of pure spectra of the individual components for identification. The pure spin-echo diffusion decays that are obtained for the individual components may be used to determine the diffusion coefficient/distribution [281]. Mixtures of molecules of very similar sizes can readily be analysed by DOSY. Diffusion-ordered spectroscopy [273,282], which does not require prior separation, is a viable competitor for techniques such as HPLC-NMR that are based on chemical separation. 

Also, direct determination of additives by means of laser desorption in solid polymeric materials rather than in polymer extracts has been reported [266], Takayama et al. [267] have described the direct detection of additives on the surface of LLDPE/(Chimassorb 944 LD and Irgafos P-EPQ) after matrix (THAP)-coating. As shown in Scheme 7.13, direct inlet mass spectrometry is also applicable to transfer TLC-MS and TLC-MS/MS analyses without the need for prior analysis. For direct sample introduction a small amount of the selected... 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

Applications SFE-SFC solves problems in such diverse areas as polymers/monomers, oils/lubricants, foods, pharmaceuticals, natural products, specialty chemicals, coatings, surfactants and others. Off-line SFE-SFC survives alongside on-line determinations of additives, because of the need for representative sample sizes. Off-line SFE-SFC was used for extraction of AOs from PP [102]. In cases where the analyst wishes to perform further analysis on the extracted species, it is useful to be able to isolate the extract from the solvent. The ability to remove the solvent easily is particularly important when SFE is coupled on-line to chromatographic techniques, but is equally important for trace analysis when it is useful to concentrate... 

Table 8.23 collects together some typical ETAAS analyses of polymer formulations see also ref. [141a], GFAAS has also been applied for the determination of additive elements in lubricating oils [52]. Solidsampling GFAAS and NAA are preferred analytical tools for the analysis of mg samples, also in relation to RM production. 

In the field of RM certification, NAA represents a major analytical technique. It possesses unique quality assurance and self-verification aspects. Not surprisingly, therefore, NAA has been used to certify NIST standard reference materials [470]. By analogy, NAA has also been instrumental in analysing the EC polymer reference materials within the framework of the PERM project [1]. NAA was also used to validate a TXRF procedure for the determination of additives containing Ti, Zn, Br, Cd, Sn, Sb and Pb [56],... 

Laser desorption methods (such as LD-ITMS) are indicated as cost-saving real-time techniques for the near future. In a single laser shot, the LDI technique coupled with Fourier-transform mass spectrometry (FTMS) can provide detailed chemical information on the polymeric molecular structure, and is a tool for direct determination of additives and contaminants in polymers. This offers new analytical capabilities to solve problems in research, development, engineering, production, technical support, competitor product analysis, and defect analysis. Laser desorption techniques are limited to surface analysis and do not allow quantitation, but exhibit superior analyte selectivity. 

Determination of Additive Effects on the Decomposition of tert-Butyl Hydroperoxide and Hydrogen Peroxide. Solutions of tert-butyl hydroperoxide (1.0 mmol) and 30% aqueous hydrogen peroxide (1.32 mmol) in 5 mL of tert-butyl alcohol with the various additives (Tables 9 and 10) were held at 80°C for 24 hr. Peroxide analyses were obtained by sodium iodide/0.05N sodium thiosulfate titration. 

HPLC has had considerable success in separating compounds as diverse as steroids, carbohydrates, vitamins, dyestuffs, pesticides and polymers. It is used routinely for the assay of pharmaceutical products, the monitoring of drugs and metabolites in body fluids and for other biomedical, biochemical and forensic applications, such as the detection of drugs of abuse. The determination of additives in foodstuffs and beverages including sugars,... 

In the method, a weighed portion of a sample of coke dried at 110°C (230°F) and crushed to pass a No. 200-mesh sieve, mixed with stearic acid, and then milled and compressed into a smooth pellet. The pellet is irradiated with an x-ray beam and the characteristic x-rays of the elements analyzed are excited, separated, and detected by the spectrometer. The measured x-ray intensities are converted to elemental concentration by using a cahbration equation derived from the analysis of the standard materials. The K spectral lines are used for aU the elements determined by this test method. This test method is also apphcable to the determination of additional elements provided that appropriate standards are available for use and comparison. 

Refs 1)R.J.Heredia, "Significance of Cracks in HE Shell and Effect of Interior Coating on Crack Formation, PATR2269 (1956) 2) Armour Research Foundation Rept No 5 (Final Rept), "industrial Engineering Study on the Determination of Additives to Eliminate Cracking of Cast Explosives Chicago, 111, Sept 8, 1958 3)D.H.Johnson, "Study of Crack... 

M Jimidar, TP Hamoir, A Foriers, DL Massart. Comparison of capillary zone electrophoresis with high-performance liquid chromatography for the determination of additives in foodstuffs. J Chromatogr 636 179-186, 1993. 

W Flak, L Pilsbacher. HPLC determination of additives and preservatives. In L Nollet, ed. Food Analysis by HPLC. New York Marcel Dekker, 1992, pp 421-456. 

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