Monomer inhibitors

In this pyrolysis, sub atmospheric partial pressures are achieved by employing a diluent such as steam. Because of the corrosive nature of the acids (HE and HCl) formed, the reactor design should include a platinum-lined tubular reactor made of nickel to allow atmospheric pressure reactions to be mn in the presence of a diluent. Because the pyrolysate contains numerous by-products that adversely affect polymerization, the TFE must be purified. Refinement of TFE is an extremely complex process, which contributes to the high cost of the monomer. Inhibitors are added to the purified monomer to avoid polymerization during storage terpenes such as t7-limonene and terpene B are effective (10). 

Uses. In the manufacture of rubber antioxidants and monomer inhibitors to stop radical polymerization in dyes, as a photographic developer in formulations for pharmaceuticals, perfumes, inks, and insecticides... 

Storage Store in cool, dry, well-ventilated area, out of direct sunlighL away from heat and ignition sources and work areas hygroscopic Uses Mfg. of antioxidants, pharmaceuticals, plasticizers, dyestuffs stabilizer for chlorinated hydrocarbons, ethylcellulose, styrene monomer UV inhibitor inhibitor for acrylic and vin monomers and acrylonitrile antioxidant for fats, oils, vitamins, and cosmetics monomer inhibitor in mfg. of paper/paperboard in contact with aq./fatty foods antioxidant in ac ic food pkg. 

Synonyms Hydroquinone monoethyl ether Uses Stabilizer in butyl acrylate monomer inhibitor in paper/paperboard in contact with aq./fatty foods in food-pkg. adhesives trace constituent in anise... 

Diethylaminoethyl methacrylate acrylic monomer inhibitor Hydroquinone monomethyl ether acrylic plastic, food packaging Acrylamide/sodium acrylate copolymer Polymethyl methacrylate Styrene/acrylates/acrylonitrile copolymer Styrene/acrylates/ammonium methacrylate copolymer... 

Hydroxypropyl methacrylate monomer inhibitor, paper/paperboard aq./fatty food-contact... 

Linear Blends The PPO solution and styrene monomer (inhibitor removed) were mixed with 1% azobisisobutyronitrile (AIBN) catalyst. The mixture was poured between glass plates with a Teflon spacer and subsequently polymerized at 70 C for 24 hours. The glass plate mold was kept in a horizontal position so that an even thickness sheet could be obtained. Combinations of 75%, 50% and 25% PPO by weight were made. 

Unsaturated, thermosetting polyesters are produced by addition polymerization reactions. Polyester resins can be formulated to have a range of physical properties from brittle and hard to tough and resistant to soft and flexible. Viscosities at room temperature may range from 50 to more than 25,000 cP. The polyester resin formulation usually consists of resin, monomer, inhibitors, catalyst or promoters, and appropriate fillers, additives, etc. 

This method is capable of determining styrene monomer inhibitors in styrene in amounts down to 10 ppm with an accuracy of 5%. To obtain quantitative results it is necessary to know the identity of the particular inhibitor present in the styrene monomer sample. 

All polymers in this study were synthesized by free radical solution polymerization. Ii ibitors may be removed by column purification through neutral or basic alumina, or by distillation from calcium hydride, although experience dictates that for most methacrylate monomers, inhibitor levels are too low to interfere... 

Place 25 g. of methyl methacrylate polymer (G.B. Diakon (powder). Perspex (sheet) U.S.A. Lucite, Plexiglass) in a 100 ml. Claisen flask, attach an efficient condenser e.g., of the double smface type) and distil with a small luminous flame move the flame to and fro around the sides of the flask. At about 300° the polymer softens and undergoes rapid depolymerisation to the monomer, methyl methacrylate, which distils over into the receiver. Continue the distillation until only a small black residue (3-4 g.) remains. Redistil the hquid it passes over at 100-110°, mainly at 100-102°. The yield of methyl methacrylate (monomer) is 20 g. If the monomer is to be kept for any period, add 0 -1 g. of hydro quinone to act as a stabiUser or inhibitor of polymerisation. 

We conclude this section by noting an extreme case of chain transfer, a reaction which produces radicals of such low reactivity that polymerization is effectively suppressed. Reagents that accomplish this are added to commercial monomers to prevent their premature polymerization during storage. These substances are called either retarders or inhibitors, depending on the degree of protection they afford. Such chemicals must be removed from monomers prior to use, and failure to achieve complete purification can considerably affect the polymerization reaction. 

Inhibitors and retarders differ in the extent to which they interfere with polymerization, and not in their essential activity. An inhibitor is defined as a substance which blocks polymerization completely until it is either removed or consumed. Thus failure to totally eliminate an inhibitor from purified monomer will result in an induction period in which the inhibitor is first converted to an inert form before polymerization can begin. A retarder is less efficient and merely slows down the polymerization process by competing for radicals. 

Inhibitors are characterized by inhibition constants which are defined as the ratio of the rate constant for transfer to inhibitor to the propagation constant for the monomer in analogy with Eq. (6.87) for chain transfer constants. For styrene at 50°C the inhibition constant of p-benzoquinone is 518, and that for O2 is 1.5 X 10. The Polymer Handbook (Ref. 3) is an excellent source for these and most other rate constants discussed in this chapter. 

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

The polymeric products can be made to vary widely in physical properties through controlled variation in the ratios of monomers employed in thek preparation, cross-linking, and control of molecular weight. They share common quaHties of high resistance to chemical and environmental attack, excellent clarity, and attractive strength properties (see Acrylic ester polymers). In addition to acryHc acid itself, methyl, ethyl, butyl, isobutyl, and 2-ethylhexyl acrylates are manufactured on a large scale and are available in better than 98—99% purity (4). They usually contain 10—200 ppm of hydroquinone monomethyl ether as polymerization inhibitor. 

Pure dry reactants are needed to prevent catalyst deactivation effective inhibitor systems are also desirable as weU as high reaction rates, since many of the specialty monomers are less stable than the lower alkyl acrylates. The alcohol—ester azeotrope (8) should be removed rapidly from the reaction mixture and an efficient column used to minimize reactant loss to the distillate. After the reaction is completed, the catalyst may be removed and the mixture distilled to obtain the ester. The method is particularly useful for the preparation of functional monomers which caimot be prepared by direct esterification. 

Acryhc acid and esters are stabilized with minimum amounts of inhibitors consistent with stabihty and safety. The acryhc monomers must be stable and there should be no polymer formation for prolonged periods with normal storage and shipping (4,106). The monomethyl ether of hydroquinone (MEHQ) is frequentiy used as inhibitor and low inhibitor grades of the acrylate monomers are available for bulk handling. MEHQ at 10—15 ppm is generally... 

The effectiveness of phenoHc inhibitors is dependent on the presence of oxygen and the monomers must be stored under air rather than an inert atmosphere. Temperatures must be kept low to minimise formation of peroxides and other products. Moisture may cause mst-initiated polymerization. 

In normal practice, inhibitors such as hydroquinone (HQ) [123-31 -9] or the monomethyl ether of hydroquinone (MEHQ) [150-76-5] are added to acrylic monomers to stabilize them during shipment and storage. Uninhibited acrylic monomers should be used prompdy or stored at 10°C or below for no longer than a few weeks. Improperly iahibited monomers have the potential for violent polymerizations. HQ and MEHQ require the presence of oxygen to be effective inhibitors therefore, these monomers should be stored in contact with air and not under inert atmosphere. Because of the low concentration of inhibitors present in most commercial grades of acrylic monomers (generally less than 100 ppm), removal before use is not normally required. However, procedures for removal of inhibitors are available (67). 

Hie common acrylic ester monomers are combustible liquids. Commercially, acrylic monomers are shipped with DOT red labels in bulk quantities, tank cars, or tank tmcks. Mild steel is the usual material of choice for the constmction of bulk storage facilities for acrylic monomers. Moisture must be excluded to avoid msting of the tanks and contamination of the monomers. Copper or copper alloys must not be allowed to contact acrylic monomers intended for use in polymerization because copper is an inhibitor (67). 

Since acrylic polymerizations liberate considerable heat, violent or mnaway reactions are avoided by gradual addition of the reactants to the kettie. Usually the monomers are added by a gravity feed from weighing or measuring tanks situated close to the kettie. The rate of monomer addition is adjusted to permit removal of heat with full flow of water in the condenser and a partial flow in the cooling jacket. Flow in the jacket can be increased to control the polymerization in cases of erroneous feed rates or other unexpected circumstances. A supply of inhibitor is kept on hand to stop the polymerization if the cooling becomes inadequate. 

Initiators, usually from 0.02 to 2.0 wt % of the monomer of organic peroxides or azo compounds, are dissolved in the reaction solvents and fed separately to the kettie. Since oxygen is often an inhibitor of acryUc polymerizations, its presence is undesirable. When the polymerization is carried out below reflux temperatures, low oxygen levels are obtained by an initial purge with an inert gas such as carbon dioxide or nitrogen. A blanket of the inert gas is then maintained over the polymerization mixture. The duration of the polymerization is usually 24 h (95). 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Eor most polymer applications the removal of the inhibitors from the monomer is unnecessary. Should it be requited, the phenolic inhibitors can be removed by an alkaline wash or by treatment with a suitable ion-exchange resia. Uninhibited MMA is sufftcientiy stable to be shipped under carehiUy controlled temperature and time restrictions. Uninhibited monomers should be monitored carehiUy and used promptiy. 

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