Exothermic polymerization

Acrylate and methacrylate polymerizations are accompanied by the Hberation of a considerable amount of heat and a substantial decrease in volume. Both of these factors strongly influence most manufacturing processes. Excess heat must be dissipated to avoid uncontrolled exothermic polymerizations. In general, the percentage of shrinkage decreases as the size of the alcohol substituent increases on a molar basis, the shrinkage is relatively constant (77). 

Both the hquid and cured 2-cyanoacryhc esters support combustion. These adhesives should not be used near sparks, heat, or open flame, or ia areas of acute fire ha2ard. Highly exothermic polymerization can occur from direct addition of catalytic substances such as water, alcohols, and bases such as amines, ammonia, or caustics, or from contamination with any of the available surface activator solutions. 

In order to faciUtate heat transfer of the exothermic polymerization reaction, and to control polymerizate viscosity, percent reactives are adjusted through the use of inert aromatic or aUphatic diluents, such as toluene or heptane, or higher boiling mixed aromatic or mixed aUphatic diluents. Process feed streams are typically adjusted to 30—50% polymerizable monomers. 

Polymerization. The polymerization of aziridines takes place ia the presence of catalytic amounts of acid at elevated temperatures. The molecular weight can be controlled by the monomer—catalyst ratio, the addition of amines as stoppers, or the use of bifimctional initiators. In order to prevent a vigorous reaction, the heat Hberated during the highly exothermic polymerization must be removed by various measures, ie, suitable dilution, controlled metering of the aziridine component, or external cooling after the reaction has started. 

THE can be polymerized by many strongly acidic catalysts, but not all of them produce the requked bitimctional polyether glycol with a minimum of by-products. Several large-scale commercial polymerization processes are based on fluorosulfonic acid, HESO, catalysis, which meets all these requkements. The catalyst is added to THE at low temperatures and an exothermic polymerization occurs readily. The polymerization products are poly(tetramethylene ether) chains with sulfate ester groups (8). 

Relatively high concentrations of organic peroxide or azo initiators are needed to obtain complete polymerization. After the reaction peak exotherm, polymerization slows down. Initiator concentrations must be high enough to complete conversion. Polymerization is inhibited by oxygen and copper, lead, and sulfur compounds (11). 

Butadiene reacts readily with oxygen to form polymeric peroxides, which are not very soluble in Hquid butadiene and tend to setde at the bottom of the container because of their higher density. The peroxides are shock sensitive therefore it is imperative to exclude any source of oxygen from butadiene. Addition of antioxidants like /-butylcatechol (TBC) or butylated hydroxy toluene (BHT) removes free radicals that can cause rapid exothermic polymerizations. Butadiene shipments now routinely contain about 100 ppm TBC. Before use, the inhibitor can easily be removed (247,248). Inert gas, such as nitrogen, can also be used to blanket contained butadiene (249). 

Handling and Storage. Cyanamide solution dimerizes to dicyandiamide and urea with the evolution of heat and a gradual increase in alkalinity accelerating the reaction. Storage above 30°C without pH stabilizer leads to excessive dimerization and can result in violent exothermic polymerization. Cyanamide should be stored under refrigeration and the pH tested periodically. Stabilized cyanamide can be kept at ambient temperature for a few weeks. 

Chemical Reactivity - Reactivity with Water No reaction unless in the presence of acids and caustics Reactivity with Common Materials Slow decomposition occurs, but generally the reactions are not hazardous Stability During Transport Stable if cool Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Violent, exothermic polymerization occurs at about 225 of. Acid fumes will also cause polymerization at ordinary temperatures Inhibitor of Polymerization None reported. 

In degree 2 only reactivity degrees are treated vis- i-vis exothermic polymerization in particular and addition reactions on the double bond (ethylene, butadiene, styrene, propylene), easy peroxidation (isopropyl oxide, acetaldehyde), hydrolysis (acetic anhydride). Possibly only propionitrile and substances with code 0 have an actual NFPA stability code. Every time one has to deal with the NFPA code one has to interpret it after carefully reading the paragraphs in Part Two. 

Graft copolymers and other polymers are prepared in a way that is common in polymerization techniques [1894]. For example, they are made by providing a foamed, aqueous solution of water-soluble monomeric material, initiating polymerization by adding an initiator, exothermically polymerizing... 

Exothermic chemical reactions, 25 299-301 catalytic converter, 10 45 formaldehyde manufacture by, 12 115 temperature-dependent enthalpy changes for, 25 303-305 Exothermic polymerization, 10 709 Exotic radioactive decays, 21 305-306 Expandable polystyrene (EPS),... 

Water offers a number of important properties as a solvent for polymerization reactions. As well as its high polarity, which gives a markedly different miscibility with many monomers and polymers compared to organic solvents, it is nonflammable, nontoxic and cheap. Water also has a very high heat capacity that sustains heat exchanges in a number of very exothermic polymerizations. Largely because of these factors, polymerizations are now widely carried out in aqueous media, and, for example, more than 50% of industrial radical polymerizations are carried out in water [19]. 

Foamed-in-place polyurethane is prepared by allowing a polyol [po y(ethy ene glycol), polyester alcohols, etc.] to react with a diisocyanate in the presence of an amine catalyst. The gas which creates the foam may be a dissolved material, such as a Freon, which volatilizes during the exothermic polymerization reaction.7 A second method involves the use of water in the reaction mixture this hydrolyzes part of the isocyanate to produce an amine and C02 gas. The Freon-formed material is preferred for the insulation of low-temperature apparatus because the thermal conductivity of the foam is greatly reduced at low temperatures by the condensation of the Freon in the cells. It is probable that the longterm effectiveness of this phenomenon must be maintained by surrounding the foamed plastic with an airtight enclosure which will prevent diffusion of air into and Freon out of the cells. 

One of the main problems in the selection of a cure cycle is to achieve control of the exothermic polymerization reaction, particularly for the case of large parts. The exothermic character of the polymerization reaction arises from the evolution of the Gibbs free energy ... 

Acetaldehyde. Exothermic polymerization initiated by acetic acid.3... 

Alkenes. Highly exothermic polymerization of alkenes, such as isobutene over aluminum chloride, may become uncontrollable.3... 

Cyano-2-propanol. Vigorous reaction due to exothermic polymerization of the dehydration product, methacrylonitrile.10... 

Dimethyl sulfoxide. Violent or explosive reaction occurs due to the formation and exothermic polymerization of formaldehyde.5 6... 

Two types of polymerization units are designed by Universal Oil Products Company. The U.O.P. Reactor-type unit contains the catalyst in tubes which are surrounded by water in a jacket for the purpose of removing the heat liberated by the exothermic polymerization reaction. The steam generated in the water jacket normally is used to preheat the feed. A feed-to-products heat exchanger furnishes the remaining heat requirements. Conventional depropanizer and debutanizer columns are used to fractionate the product. Figure 3 shows a flow diagram of a reactor type of polymerization unit. 

The time to tQ is the time for the wood-monomer mass to reach oven or curing temperature at T5. During the period of constant temperature, the induction period, the inhibitor is being removed by reaction with the free radicals. Once the inhibitor is eliminated from the monomer and wood, the temperature rises to a maximum which corresponds to the peak of the exothermic polymerization reaction. Polymerization continues to completion although at a decreased rate and the temperature returns to that of the curing chamber. The time to the peak temperature depends upon the amount of catalyst present, the type of monomer, the type of crosslinker, and the ratio of the mass of monomer to that of the wood. The wood mass acts as a heat sink. Figure 4 illustrates the effect of increased Vazo catalyst on the decrease in time to the peak temperature, and the increase in the peak temperature(10)... 

The reaction mass thickened with the onset of polymerization, and most reaction mixtures were too thick to stir by the end of the irradiation period. Reaction temperature reached 40-50°C due to the exothermic polymerizations. The reaction mass was allowed to stand for 2 hr at ambient temperature, and the polymer was isolated by filtration, washed with water and with ethanol, and vacuum dried at 60°C. Ungrafted PAN was... 

Much data is available about the relationships between chemical structure and the processability window (Tp-Tm) determined by DSC. Tp is the temperature onset of exothermic polymerization and Tm is the maximum of the melt-... 

Besides toxicity, HCN presents other hazards. HCN undergoes an exothermic polymerization at a pH of 5 to 11 to form solid black compounds. This polymerization can become explosively violent, especially if it is confined. The polymerization reaction is between HCN and cyanide ions, and the presence of water and heat contribute to the onset of polymerization. Therefore stored HCN should contain less than 1.0 wt.% water it should be kept cool and it should be inhibited with sulfuric, phosphoric or acetic acid129. 

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