Residual monomer analysis

Residual monomers in SAN have been a growing environmental concern and can be determined by a variety of methods. Monomer analysis can be achieved by polymer solution or directly from SAN emulsions (27) followed by "head space" gas chromatography (gc) (28,29). Liquid chromatography (Ic) is also effective (30). 

In this paper we present a meaningful analysis of the operation of a batch polymerization reactor in its final stages (i.e. high conversion levels) where MWD broadening is relatively unimportant. The ultimate objective is to minimize the residual monomer concentration as fast as possible, using the time-optimal problem formulation. Isothermal as well as nonisothermal policies are derived based on a mathematical model that also takes depropagation into account. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and time is studied. 

Apart from routine quality control actions, additive analysis is often called upon in relation to testing additive effectiveness as well as in connection with food packaging and medical plastics, where the identities and levels of potentially toxic substances must be accurately known and controlled. Food contact plastics are regulated by maximum concentrations allowable in the plastic, which applies to residual monomers and processing aids as well as additives [64-66]. Analytical measurements provide not only a method of quality control but also a means of establishing the loss of stabilisers as a function of material processing and product ageing. 

PyGC-MS/TG-MS Residual monomer/solvent identification, feedstock analysis... 

Extraction or dissolution almost invariably will cause low-MW material in a polymer to be present to some extent in the solution to be chromatographed. Solvent peaks interfere especially in trace analysis solvent impurities also may interfere. For identification or determination of residual solvents in polymers it is mandatory to use solventless methods of analysis so as not to confuse solvents in which the sample is dissolved for analysis with residual solvents in the sample. Gas chromatographic methods for the analysis of some low-boiling substances in the manufacture of polyester polymers have been reviewed [129]. The contents of residual solvents (CH2C12, CgHsCI) and monomers (bisphenol A, dichlorodiphenyl sulfone) in commercial polycarbonates and polysulfones were determined. Also residual monomers in PVAc latices were analysed by GC methods [130]. GC was also... 

Typical areas of application are identification of trace (ppm or ppb level) volatile organics in complex mixtures (e.g. olfactory principles) and monitoring of residual monomers in polymeric materials. Apart from HS-GC, analysis of volatiles can also be carried out by a variety of other methods, including hydrodistillation, SFE, US, adsorption trapping and SPME. 

Thermolysis-mass spectrometry is ideal for examining the amount of residual monomer and processing solvents present in polymers. In thermolysis, the polymer is heated from room temperature to 200-300 °C, and is then often held isothermally in order to drive off volatile components. Low-temperature pyrolysis (350-400 °C) of PP compounds in direct mass-spectral analysis has shown volatiles from PP at every carbon number to masses well above 1000 Da [37]. 

In this chapter, the main analytical techniques and the methods currently employed in industrial and research laboratories for the analysis of important classes of additives are reviewed. The use of both gas chromatographic and liquid chromatographic methods coupled with mass spectrometry features prominently. Such methodology enables the sensitive and specific detection of many types of organic additives in polymeric materials to parts per billion (jig/kg) levels. Much of the development of these methods has been undertaken as part of research into the migration or extraction of species from food-contact and medical materials [5-7], This chapter also includes some discussion on the analysis of residual monomers and solvents. 

Steichen RJ. 1976. Modified solution approach for the gas chromatographic determination of residual monomers by head-space analysis. Anal Chem 48 1398-1402. 

Headspace analysis involves examination of the vapours derived from a sample by warming in a pressurized partially filled and sealed container. After equilibration under controlled conditions, the proportions of volatile sample components in the vapours of the headspace are representative of those in the bulk sample. The system, which is usually automated to ensure satisfactory reproducibility, consists of a thermostatically heated compartment in which batches of samples can be equilibrated, and a means of introducing small volumes of the headspace vapours under positive pressure into the carrier-gas stream for injection into the chromatograph (Figure 4.25). The technique is particularly useful for samples that are mixtures of volatile and non-volatile components such as residual monomers in polymers, flavours and perfumes, and solvents or alcohol in blood samples. Sensitivity can be improved by combining headspace analysis with thermal desorption whereby the sample vapours are first passed through an adsorption tube to pre-concentrate them prior to analysis. 

Vinyl chloride monomer (VCM) is the main substrate for the manufacture of polymers used as packaging materials for food. Since VCM is considered by lARC to be a human carcinogen, monomer levels in PVC food packaging materials are strictly controlled. To ensure a safe product, the residual content of VCM in the finished material or article is limited to one mg per kg in the final product (Council Directive 78/142/EEC). Furthermore, VCM should not be detectable in foodstuflfs. Commission Directives 80/766/EEC and 81/432/ EEC give the method of analysis for official control of the VCM level in food packaging materials and in foods - gas-phase chromatography using the headspace method, after dissolution or suspension of samples in N,N-dimethylacetamide. Both residual monomer content of the polymer and... 

In principle the composition of a copolymer may also be determined by analyzing the composition of the residual monomer by a suitable method after polymerization. It will usually be necessary first to separate the copolymer by precipitation, followed by careful recovery of the filtrate containing the residual monomer, but the direct method of analysis of the copolymer will generally be preferred. Block and graft copolymers can be characterized in the same manner. However, consideration must be taken of the fact that they usually contain large... 

Since the late 1960 s a few papers have demonstrated compositional analysis of various polymer systans by Raman spectroscopy. For example, Boerio and Yuann (U) developed a method of analysis for copolymers of glycidyl methacrylate with methyl methacrylate and styrene. Sloane and Bramston-Cook (5) analyzed the terpolymer system poly(methyl methacrylate-co-butadiene-co-styrene). The composition of copolymers of styrene-ethylene dimethacrylate and styrene-divinylbenzene was determined by Stokr et (6). Finally, Water (7) demonstrated that Raman spectroscopy could determine the amount of residual monomer in poly(methyl methacrylate) to the % level. This was subsequently lowered to less than 0.1% (8). In spite of its many advantages, the potential of Raman spectroscopy for the analysis of polymer systems has never been fully exploited. 

Analysis of residual monomers in polymers (vinyl chloride in PVC). ... 

The films contained between 0.1 and 1% residual monomer according to gas chromatographic analysis. The transmittance and fluorescence spectra were measured with a Spectronic 505 spectrophotometer by Bausch and Lomb. To obtain correct fluorescence spectra with respect to energy, the above mentioned apparatus was calibrated using a wolfram-band lamp and an aperture driven by a wavelength-dependent correction... 

Many other spectroscopic techniques are used for the study of polymers, some having general applicability and others being used for a specific property characterization. For example, UV absorption spectroscopy has been used for the analysis of residual monomers and the presence of antioxidants in a polymer. Electron spin resonance,... 

Copolymer Analysis. Even though the overall copolymer composition can be determined by residual monomer analysis, it still is necessary to have reliable quantitative techniques for copolymer composition measurements on the actual copolymer, mainly because concentration detectors for SEC or HPLC are sensitive to composition and because the conversion histories are not always available. Some of the techniques used to determine copolymer composition are melt viscometry (46), chemical analysis, elemental analysis, infrared spectroscopy (IR), Nuclear Magnetic Resonance (NMR), ultra-violet spectroscopy (UV), etc. Melt viscometry, chemical and elemental analysis are general techniques that can be applied to almost any polymer. The spectroscopic techniques can be applied depending on the ability of the functional groups present to absorb at specific wavelengths. 

Sample Analysis, (i) Conversion. Measured samples (js20 ml) were taken from the reactor at regular intervals ( 30 min), and polymerisation quenshed by rapid cooling and the addition of a small quantity of t-butyl catechol. The polymer was isolated by rotary evaporation at reduced pressure to remove residual monomer and then reprecipitated several times from acetone solution with aqueous methanol. The product was finally dried under vacuum. Polymer yields were obtained by weighing. 

A final example involves the enantioselective polymerization of racemic 10-hydroxyundecanoic acid by CRL (Scheme 11.8) [31], The polymerization reaction was stopped when 50% monomer conversion was reached and polymers with a molecular weight of lkgmoT (PDI = 1.3) were isolated. Subsequent H-NMR analysis using the Mosher s acid derivatization procedure on the residual monomer and hydrolyzed monomer revealed an ee of 33 and 60%, respectively. Comparison with Mosher s esters of R-2-octanol and rao2-octanol showed that the S-monomer was preferentially incorporated in the polymer. 

Figure 1 also indicates the percent weight loss as a function of time for samples under desiccation. The effect of sample thickness is less pronounced. Again, a period of 10 days appears to be sufficient to attain nearly complete equilibrium. Other studies on previously dried samples indicate that at least 4 h must elapse before a significant increase in H2O content is observed in desiccated samples exposed to room air. Since these specimens were desiccated until just before testing, we consider that no moisture was present. Results of Figure 1 can be summarized as follows normal samples contained /0.6% H.O, dried samples had /0% H2O, and saturated samples had 2.2% H2O. Analysis by gas chromAography indicated that a small amount of residual monomer (F 0.6%) was present in these specimens. The analysis also indicated that this monomer was not removed by... 

The method enables one to evaluate the basic process parameters, such as the conversion and yield, without a prior knowledge of the composition of the mixture and without calibrating the instrument. The method significantly simplifies the material balance determination, reduces the analysis time and makes it possible to automate the analytical procedure. The method provides for a fuller utilization of computers and considerably improves the accuracy of the analysis. For example, it can help to control the entire process of producing monomers for synthetic rabber, including such analyses as the determination of the concentration of volatile components in solvents and absorbents, and the concentration.of residual monomers and other volatile components in polymers and their aqueous suspensions [176]. 

The polymerization kinetics were studied via time-conversion relationships at different dose rates (Figures 6-9). The radiation curing was carried out at room temperature in closed aluminum capsules. Percent monomer conversion was determined gravimetrically for N-vinylpyrrolidone. Evaporation was accomplished under vacuum in a ventilated oven at 50 °C until the weight was constant. For butyl methacrylate and polyester resins, the residual monomer content was determined by gas chromatographic analysis after immersion of the cured products in acetone. 

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