Residual monomer levels

Residual monomers in the latex are avoided either by effectively reacting the monomers to polymer or by physical or chemical removal. The use of tert-huty peroxypivalate as a second initiator toward the end of the polymeri2ation or the use of mixed initiator systems of K2S20g and tert-huty peroxyben2oate (56) effectively increases final conversion and decreases residual monomer levels. Spray devolatili2ation of hot latex under reduced pressure has been claimed to be effective (56). Residual acrylonitrile also can be reduced by postreaction with a number of agents such as monoamines (57) and dialkylamines (58), ammonium—alkali metal sulfites (59), unsaturated fatty acids or their glycerides (60,61), their aldehydes, esters of olefinic alcohols, cyanuric acid (62,63), andmyrcene (64). 

Kinetics of suspension PVC are identical to the kinetics of mass PVC, both increasing in rate with conversion (90). After polymerization to about 80—90% conversion, excess monomer is recovered, the slurry is steam-stripped in a column to a residual monomer level of about 0.0001% (10 ppm), excess water is centrifuged off, and the resin is dried with hot air. 

An example of the goodness of fit between measured residual monomer levels the optimized model predictions is shown in Figure 3. Model predicted values corresponding with measured residual monomer data for all five experimental runs are given in Table I. 

Further reduction in the residual monomer level for food packaging applications (Dow, BASF, etc.). 

Once calibrated, the NIR analyzer was used to investigate a number of factors expected to affect the polymerization kinetics, including reaction temperature, initiator type, and initiator concentration (relative to monomer concentration). These experiments, in addition to improving process understanding, also mimicked the effects of inadequate process control during a reaction. Figure 15.1 shows the effect of reaction temperature on kinetics. The reaction rate nearly doubles when the temperature is raised from 65 to 75 °C, and the concentration of unreacted monomer after 85 minutes is reduced from 1.1 to 0.5%. In-hne NIR monitoring allows unusual behavior in either reaction rates or residual monomer levels to be detected and corrected immediately. 

The latexes were prepared using a conventional semi-batch emulsion polymerization system modified for power-feed by the addition of a second monomer tank. Polymerization temperatures ranged from 30-85°C using either redox or thermal initiators. Samples were taken periodically during the polymerization and analyzed to determine residual monomer in order to assure a "starved-feed" condition. As used in this study this is a condition in which monomer feed rate and polymerization rate are identical and residual monomer levels are less than 5%. 

There has been continuous improvement in PS purification technology over the past 60 years as efforts continued to focus on getting the residual monomer levels lower and lower. These efforts continue even though it is generally... 

Some packaging applications may be particularly sensitive to styrene or other volatile components. SBC and crystal polystyrene blends, even with low residual monomer levels in the polystyrene, may be unacceptable for these applications. In this case one possible solution may be to co-extrude a thin food contact layer on the surface of the blended sheet. Studies have indicated that by co-extruding a thin layer of crystal polystyrene on the surface of an SBC and crystal polystyrene blend, the styrenic volatiles and the odor can be significantly reduced. However, the downside is that the added layer of crystal polystyrene can serve to make the blended part more brittle. Another option that could reduce odor, but not have a deleterious effect on impact, is a thin coextruded layer of PETG [20]. 

All the development data presented here are restricted to volatile contents of approximately 1% by weight or less. This represents a range of wide industrial importance. Within this range, also, various fluid properties such as film surface characteristics, molecular diffusivity, etc., would be expected to be relatively constant and independent of residual monomer level, thereby simplifying the system considerably. 

Based on the results of preliminary investigations it was concluded that capsules could be formed from emulsions containing about 2-10% (mass fraction) monomer and 0.5-2% (mass fraction) alginate subjected to radiation doses greater than about 7.5 x 10 2 M Rad to reduce residual monomer levels below toxic limits. Emulsions may be prepared in distilled water, saline, or buffered saline, but not in calcium containing solution. 

Residual monomer levels in the polystyrene and the alkane medium were determined using a Hewlett-Packard Model 5250 gas chromatograph equipped with a 6 foot Carbowax column. 

One pound of spray-dried poly(A/-vinylpyrrolidone) with 3.08% unsaturation, calculated as A/-vinylpyrrolidone, is dissolved in 6.61b of distilled water. To this solution is added 18 ml of 28% aqueous ammonia followed by 18 ml of 35% hydrogen peroxide. The mixture is immediately spray dried to produce a resin that is analyzed as having 1.1% unsaturation, calculated as A-vinylpyrrolidone. With repeated treatments the residual monomer level is said to be reducible to 0.65%. 

Free radical polymerization reactions are conducted under an inert atmosphere. Initiation with thermal or redox free-radical initiators is common [9,14], Industrial processes are designed to reduce residual monomer levels to below regulated limits [24, 25], This is achieved by pushing the reaction to completion by increasing the reaction temperature or increasing the initiator levels toward the end of the reaction. Also postpolymerization strategies have been employed, such as an enzymatic treatment with amidase [26]. 

To form a dispersion polymer, a starting solution that typically consists of monomers, water, acids and/or bases for pH adjustment, buffering agents, multivalent salts (ammonium sulfate and sodium sulfate are common), chain transfer agents, the prescribed polymeric dispersant, and chelant is added to the reactor. Polymerization is accomplished with azo and/or redox initiators. As the reaction begins, the solution becomes viscous. After a short period of time, a milky dispersion is formed. Near the end of the reaction, residual monomer levels are reduced to values below the specified limits by increasing the reaction temperature and/or initiator concentration. When the polymerization reaction is complete, additional stabilizing additives may be added to the mixture. 

The ability to follow comonomer concennation and copolymer composition in real-time allows operators to efficiently determine the end of the reaction. Residual monomer levels are regulated, and in a majority of cases are determined through the HPLC analysis of reactor samples. Reactor sampling, sample preparation, and analysis take time and reduce productivity, particularly if the batch is held in a reactor pending analytical clearance. Additional work that may be required would further reduce productivity. 

The small size of the latex particles allows the attainment of low residual monomer levels. 

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