Styrene, Residual

Fig. 7. Comparison of the thermal stabihty, ie, formation of monomer and loss of of FRPS and APS upon heating for 2.5 h at 285°C in glass tubes sealed at 0.67 or 13.3 kPa. A is FRPS residual styrene formed B, FRPS loss C, APS loss and D, APS residual styrene formed. To convert kPa to...

One of the key benefits of anionic PS is that it contains much lower levels of residual styrene monomer than free-radical PS (167). This is because free-radical polymerization processes only operate at 60—80% styrene conversion, whereas anionic processes operate at >99% styrene conversion. Removal of unreacted styrene monomer from free-radical PS is accompHshed using continuous devolatilization at high temperature (220—260°C) and vacuum. This process leaves about 200—800 ppm of styrene monomer in the product. Taking the styrene to a lower level requires special devolatilization procedures such as steam stripping (168). 

One particular growth area for polypropylene mouldings is for thin-wall packaging such as margarine tubs. This is largely at the expense of polystyrene and arises partly from economics and partly from the wish to have a produet free of residual styrene monomer. 

The particles contain residual styrene monomer which strongly absorbs light at 2 k and 280 nm wavelength. 

On the basis of the above observations it is concluded that at 350 nm the discrepancy between calculated and measured extinction coefficients may be attributed to the additives in the lat-ices and the size disparity of the particles in them. The presence of residual styrene monomer in the particles is strongly suspected. The data are inconclusive in this regard. Whether or not polystyrene latices absorb at 2 h nm can only be established once the contributions to the extinction coefficients from the additives and residual monomer if any, are established. A combination of the aforementioned are probably responsible for departure from theory at 2 k and 280 nm. 

HS-GC methods have equally been used for chromatographic analysis of residual volatile substances in PS [219]. In particular, various methods have been described for the determination of styrene monomer in PS by solution headspace analysis [204,220]. Residual styrene monomer in PS granules can be determined in about 100 min in DMF solution using n-butylbenzene as an internal standard for this monomer solid headspace sampling is considerably less suitable as over 20 h are required to reach equilibrium [204]. Shanks [221] has determined residual styrene and butadiene in polymers with an analytical sensitivity of 0.05 to 5 ppm by SHS analysis of polymer solutions. The method development for determination of residual styrene monomer in PS samples and of residual solvent (toluene) in a printed laminated plastic film by HS-GC was illustrated [207], Less volatile monomers such as styrene (b.p. 145 °C) and 2-ethylhexyl acrylate (b.p. 214 °C) may not be determined using headspace techniques with the same sensitivities realised for more volatile monomers. Steichen [216] has reported a 600-fold increase in headspace sensitivity for the analysis of residual 2-ethylhexyl acrylate by adding water to the solution in dimethylacetamide. 

Washing will be used to remove this residual styrene, since we have a water suspension. This makes the assumed amount of styrene very important, because it will determine the amount of wash water that must be added. Enough water must be present to dissolve all the styrene that remains. At 25°C the solubility of styrene in water is 0.032%. It should be greater at higher temperatures. Assume the reactor products will cool to 60°C in the wash tank. At this temperature the solubility of sfyrene in water is assumed to be 0.050%. This means that for each pound of styrene originally charged, 4 lb of water must be present in the wash solution to make it... 

Detailed studies led Gandini and Plesch to formulate the concept of pseudocationic polymerisations. These are reactions which show many of the characteristics of cationic polymerisations, but do not involve ions. Since they could see no other alternative compatible with general chemical knowledge, they formulated the reactive species as an ester, and they were able to support this view by direct experiments (formation of the ester in the styrene solution by metathesis). The evidence indicates that in the system styrene, perchloric acid, methylene dichloride, the poly(styryl perchlorate) ester requires four molecules of styrene for its stabilisation. When these are no longer available, the ester ionises, and the residual styrene is consumed by a very fast, truly cationic polymerisation ionisation of the ester is a complicated reaction which has been only partly elucidated. The initiation and propagation of the pseudocationic polymerisation can be represented thus ... 

Another, necessarily much less precise, method for determining n is available from the kinetic experiments [1]. At the end of these reactions, at the precise instant at which the reaction mixtures turned yellow, a very fast reaction took place (Figure 5). This represents the polymerisation of the residual styrene by a true cationic reaction caused by the ions formed from the ester at the point where the styrene concentration was reduced to a level no longer sufficient to stabilise the ester. From the very small temperature rise during this final fast reaction, the number of styrene molecules polymerised could be calculated it was always about four times the initial concentration of perchloric acid. This phenomenon was particularly evident in the reactions carried out at -19 °C in which relatively high acid concentrations were used so as to obtain reasonably fast polymerisations [1]. 

ISO 1622-1 1994 Plastics - Polystyrene (PS) moulding and extrusion materials - Part 1 Designation system and basis for specifications ISO 1622-2 1995 Plastics - Polystyrene (PS) moulding and extrusion materials - Part 2 Preparation of test specimens and determination of properties ISO 2561 1974 Plastics - Determination of residual styrene monomer in polystyrene by gas chromatography... 

Analytical Procedures. Figures for conversion are based on a method in which the graft copolymer was dissolved in o-dichlorobenzene and residual styrene monomer was determined by ultraviolet spectrophotometry. 

General purpose PS and HIPS are used in food packaging applications. In some packaging configurations, with no direct contact of a surface with the polymer the migration of residual styrene monomer via the vapor-phase with subsequent absorption into the food is believed to be a significant mechanism of accumulation. 

Fig. 8.10 Residual styrene concentration in PS extruded at 225°C. The open symbols refer to experiments without ultrasound, while the filled ones refer to experiments where ultrasound radiation was applied. The parameter is the absolute pressure in the chamber. Triangles 150 mmHg squares 50 mmHg, and circles 12 mmHg. [Reprinted by permission from A. Tukachinsky, Z. Tadmor, and Y. Talmon, Ultrasound-enhanced Devolatilization in Polymer Melt, AIChE J., 39, 359 (1993).]...

Content of residual styrene monomer in polystyrene containing food contact materials... 

The level of unpolymerised residual styrene monomer in commercial grades of polystyrene material has been reduced over the years from 1000 mg/kg (0.1 % w/w) to a target level of 500 mg/kg (0.05 % w/w) by more complete devolatilization after the polymerization step (Brighton 1982). The legal limits for styrene monomer in materials can be much higher (e.g. Australia 2500 mg/kg, AS 2070.3-1992). Hempel and Riidt (1988) carried out a survey of residual volatiles found in polystyrene and polystyrene copolymers whose results are summarized in Table 14-la. 

The results of a recent survey by the Inspection Health Protection/Food Inspection Department, Utrecht, Netherlands (van Lierop, Wildervanck 1996), shown in Table 14-lb, found an average residual styrene monomer content of 224 mg/kg in 31 different polystyrene containing food contact articles and packaging. The two highest contents found were 888 and 1459 mg/kg and in 14 articles less than 150 mg/kg was found. A comparison of the results of the two studies from 1988 and 1996 supports the stated industry objective of reducing styrene monomer contents and shows an overall downward trend. 

The residual styrene monomer remaining in the finished material can cause taints by transferring to the packed product in amounts that exceed the taste threshold concentration level in that particular food. Each food matrix has a characteristic styrene concentration (threshold concentration) above which the styrene taint becomes evident (Chapter 13). A series of sensory taste threshold concentrations taken from the literature for different foods are shown in Table 14-2. 

Example 14-1 Calculate the amount of styrene monomer that could migrate from a PS coffee creamer portion pack (7.5 g) with a residual styrene monomer content of 1000 ppm (mg/kg) into a coffee creamer containing 10 % fat. 

Given the acceptable threshold concentration for styrene in a given product, one can calculate a maximum acceptable initial concentration of residual styrene in the food contact material (QMcaic). Cp, is replaced with the threshold concentration (TC) in Eq. (14-4) to yield Eq. (14-8) or cf oc is replaced with TC in Eq. (14-3) to yield Eq. (14-9). 

Figures 14-1 and 14-2 show estimations of shelf life in a 7.5 g PS containing portion pack before two different taste threshold concentrations (2 and 0.1 mg/kg) of styrene are exceeded in the product. In each graph the diffusion coefficients from Linssen et al. (1992) for a 1 1 PS HIPS polymer blend at room temperature (23 °C) and refrigeration temperature (4 °C) are used. The estimation using Eq. (14-5) at 23 °C and 4 °C and an calculated apparent diffusion coefficient for PS/PE and PS/EVOH/PE structures (see Table 14-3) are used in Eq. (14-4) (see example 14-5) to calculate the days before a styrene taint is detected in the product. The shelf life is decreased by a factor of the square of the increase in the material s residual styrene content. As seen in Figures 14-1 and 14-2 a reduction in the taste threshold by a factor of ten means almost a 100 times decrease in the shelf life.

Figure 14-3 shows more clearly how the styrene content varies as a function of the residual styrene content in the material assuming a desired shelf life of 6 months. If one knows the threshold concentration in the product (e.g. 0.1 mg/kg for dairy creamers) then the necessary initial styrene concentration in the PS material can be selected to give a shelf life of 6 months before a styrene taint develops in the product. 

Styrene can cause off-flavors in food products packed in polystyrene packaging materials. Each food product has a different sensitivity to styrene off-flavors and thus each food package system needs to be evaluated individually. The formation of styrene off-flavor in a given package/product system can be estimated a priori given the styrene threshold concentration in the food, the initial residual styrene monomer concentration in the packaging material and the desired shelf life. If this information is not readily available then accelerated storage tests followed by a sensory comparison can be carried out to evaluate the potential of off-flavor formation in the product. 

Residual Styrene Determine as directed under Residual Styrene, Appendix IV. 

Calculate the content of residual styrene in the sample taken, in parts per million, by the formula... 

Chemical Tests and Determinations, 753 Chewing Gum Base, 782 Bound Styrene, 782 Molecular Weight, 783 Quinones, 784 Residual Styrene, 784 Total Unsaturation, 785 Chicle, 249... 

In a study recently carried out in Hong Kong on disposable plastic containers for take-away meals the migration of styrene oligomers, heavy metals and the overall migration from plastic containers and, where present, their lids were determined into food simulants under different test conditions. Results showed that all the disposable plastic container samples met the safety standards for heavy metals and residual styrene monomers. Hence, with the proper use of disposable plastic containers, they would be unlikely to cause a food... 

Figure 3.7 Residual styrene as a function of devolatilizer vaccum. Reprinted with permission from B. J. Meister and A. E. Platt, Ind. Eng. Chem. Res., 28, 1662 (1989). Copyright 1989 American Chemical Society...

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