Residual monomer content

A common procedure for the preparation of vinylated alkyds is as foUows. A base alkyd resin is brought to the desired endpoint. The resin is then cooled to about 160°C and often diluted with aromatic thinner. The desired monomer is added, usually at about 20 —60% based on the final product, foUowed by an appropriate amount of a free radical initiator. Alternatively, a premix of the monomer and the initiator is added at a controUed rate over most of the reaction. The reaction is brought to monomer reflux, until the residual monomer content has fallen below a specified level. Residual monomer, if any, is stripped away before the product is diluted in a solvent, filtered, and packaged. 

After polymerization, excess monomer is stripped and recycled. The residual monomer content of the stripped emulsion does not represent an acute hazard. Worker exposure to monomer is monitored, and sources of exposure identified and corrected. 

In the case of photoinitiated polymerization, an oxygen-free aqueous solution of acrylamide with a concentration of about 50% mixed with a photosensibilizer and other required additives is passed through a column-type apparatus with exterior water-cooling. A thin layer of the solution is exposed to a mercury lamp, acquires the consistency of a plastic film, which then can be passed through a second exposure zone, and is crushed and dried. Acrylamide polymers produced by this method are easily soluble and have a low residual monomer content. 

To accelerate the polymerization process, some water-soluble salts of heavy metals (Fe, Co, Ni, Pb) are added to the reaction system (0.01-1% with respect to the monomer mass). These additions facilitate the reaction heat removal and allow the reaction to be carried out at lower temperatures. To reduce the coagulate formation and deposits of polymers on the reactor walls, the additions of water-soluble salts (borates, phosphates, and silicates of alkali metals) are introduced into the reaction mixture. The residual monomer content in the emulsion can be decreased by hydrogenizing the double bond in the presence of catalysts (Raney Ni, and salts of Ru, Co, Fe, Pd, Pt, Ir, Ro, and Co on alumina). The same purpose can be achieved by adding amidase to the emulsion. 

To prepare (true) latices, the monomers are emulsified in water with stirring and addition of emulsifiers, which help stabilize the monomer droplets. The molecular weight of the polymer molecules in the resultant latex can be controlled by the concentration and decomposition rate of added polymerization initiators. The residual monomer content can be reduced by optimizing the polymerization conditions and can also later be eliminated by steam distillation. 

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... 

At present all commercial polystyrene (with average molecular weights between 100,000 and 400,000) is manufactured by radical polymerization, which yields atactic polymers.476 Peroxides and azo compounds are commonly used initiators. The suspension process (usually as a batch process in water at 80-140°C) produces a product with relatively high residual monomer content.223 More important is the continuous solution process (usually in ethylbenzene solvent at 90-180°C), which yields high-purity product. Styrene can be copolymerized with numerous other monomers.477 One of these copolymers, the styrene-divinylbenzene copolymer produced by free-radical polymerization, has a crosslinked stucture and is used in... 

Although the problem has become less severe due to the trend towards reducing residual monomer content in materials there is still potential for taint problems to occur in products. Total styrene content in current materials varies within a range of 250 to 350 ppm (even for laminate materials). Of the commonly polystyrene containing portion pack materials mono-material PS has the greatest migration followed by PS/PE and the material with the lowest migration is from PS/EVOH/PE. 

Vinyl acetate is polymerized in aqueous emulsion and used widely in surface coating and in adhesives. Copolymerized with vinyl esters of branched carboxylic acids and small quantities of acrylic acid, it gives paint latices of excellent performance characteristics. G. C. Vegter found that a coagulum-free latex of very low residual monomer content can be produced from a mixture of an anionic and a nonionic emulsifier according to a specific operating procedure. The freeze/thaw stability of polymeric latices has been investigated by H. Naidus and R. Hanzes. 

Vinyl esters of trialkylacetic acid copolymerize randomly with vinyl acetate. Paint latices with a very low residual monomer content, based on vinyl acetate-W 911 copolymers, can be prepared reproducibly via a variant of the monomer emulsion addition technique, when a relatively large part of the anionic emulsifier is present in the initial reactor charge. 

Other Procedures. Residual monomer content, solids content, viscosities, F/T stability, and surface tension were determined as described... 

A polymerization reaction is normally followed by two principal separation processes elimination of monomers or diluents, and separation of solid polymers. The residual monomer content is often removed by the devolatilization. When this is important, extruders, tower, and wiped-film reactors are used (e.g., for certain step-growth polymerizations to very high conversion). 

Baueriedel, H., Adhesives Based on Polyurethane Prepolymers Having a Low Residual Monomer Content, U.S. Patent No. 4,623,709, 1986. 

The residual monomer content may change the inherent attractive forces between the particles. These forces depend on the nature of the polymer material... 

The residual monomer content will by external plasticization cause a considerable lowering of the polymer glass transition temperature. A correlation between stability and softness of the polymer particles may exist. The hydrophobic part of the emulsifier molecules may partly penetrate the particle surface and thus be anchored to the surface to some extent. The resistance to deformation of such a stabilizing layer, when subjected to mechanical shear, is assumed to be dependent on the polymer particle softness. With soft particles polymer chain entanglement may also occur on particle-to-particle contact, making redispersion of agglomerates more unlikely. 

It is perhaps appropriate at this point to review briefly some of the problems and technology involved in reducing the residual monomer content of plastic materials. 

Poly( inyl chloride), poly(acrylonitrile) and the high acrylonitrile copolymers have presented the major problems with respect to reducing the residual monomer content to extremely low levels. These are in the glas state tmder the conditions where monomer removal must be carried out. In principle, the temperature should be raised above the glass temperature to facilitate monomer removal. In practice, however, the systems are usually lattices or slurries of suspension polymer and coagulation could become a problem. In any case, both the rubbery and glassy states must be considered in any discussion of the monomer removal problem. The basic principles of the transport of gases in both situations have been presented briefly and with appropriate literature references in the introductory section of this review. 

Small organic molecules such as monomers will obey the same laws when present in the very low concentrations which are involved in practice. It is interesting to compare solubilities, for example under one atmosphere of nitrc en, a typical rubber will dissolve about 60 ppm, most polymers dissolve similar amounts. This is actually within the range of residual monomer contents under consideration in most toxic monomer problems. 

Mth problems concerned with the reduction of residual monomer content desorption kinetics are relevant. Fortunately these are normally Fickian but with concentration dependent diffusion constants. The com cations of so-called Case II, relaxation controlled diffusion often found in glassy polymers at high concentration gradients " are also not normally observed in desorption processes. 

Polymers which meet the definition of a polymer of low concern are eligible for the new Synthetic Polymers of Low Concern Category. A synthetic polymer of low concern is one which has a Mn >10(X), low charge density, low residual monomer content, does not dissociate readily, has low water solubility, if solid has a particle-size distribution such that <1% of particles have a diameter of <70 pm, is stable under conditions of use, and does not contain reactive functional groups. For these polymers less information is required and applications will be processed within 50 d. 

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. 

The effect of oxygen on the polymerization characteristics of the monomer is quite dramatic. In bulk polymerization of reasonably purified vinyl acetate, the process is autoaccelerated from the start and goes nearly to completion with a residual monomer content of 2-4%. When the monomer distillation is carried out in contact with air, inhibiting impurities form quite rapidly. These lead to dead-end polymerizations with 30-40% unreacted monomer left in the product [17]. 

In a suitable solvent, solution polymerization involving the delayed addition of the monomer may be carried out. In this procedure, all of the solvent and 10-25% of the monomer are heated to reflux in the presence of an initiator. As the polymerization proceeds, additional monomer is gradually added. To complete the polymerization, the reaction solution is heated for a prolonged period after the monomer addition has been completed. Even so, the residual monomer content may be as high as 2% [95]. 

In equipment similar to that used in Procedure 5-3 with provisions for maintaining a nitrogen atmosphere in the system, 210.75 gm of deionized water, 0.75 gm of Tergitol-7 (sodium heptadecyl sulfate, an anionic surfactant from Union Carbide), 12.5 gm of Cellosize WP-09 (a protective colloid from Union Carbide) and 1 gm of potassium persulfate is blended under nitrogen with agitation. Then 25 gm of vinyl acetate is added and the mixture is heated to 70°C. When the polymerization has started, with stirring, over a 2- to 3-hr period, 250 gm of vinyl acetate is added. The temperature is maintained at 70°-80°C. After the addition has been completed, heating at 70°-80°C is continued for an additional 30 min. The residual monomer content of this latex is less than l . ... 

Section 15.4). Vinyl chloride in PVC, styrene in PS, and acrylonitrile in PAN are well known examples of migration of residual monomer that were the object of intense investigations in the 1970s. As a result, these resins, as well as all plastics for food applications, are commercially produced with residual monomer contents below the maximum allowed by the FDA. 

Carcinogenicity Cancer-inducing effect (whereby the carcinogen has in most cases acted over a longer period). Carcinogenic monomers include in particular vinyl chloride and acrylonitrile. The permissible residual monomer content levels in plastics are very low. 

Makromolekulare Chemie 194, No.6, June 1993, p.1527-36 NON-DESTRUCTIVE DETERMINATION OF THE RESIDUAL MONOMER CONTENT EV POLYMERS BY RAMAN SPECTROSCOPY Dywan F Hartmann B Klauer S Lechner M D Rupp R A Woehlecke M Osnabruck,University... 

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