Inhibitors in styrene monomer

Many monomers used in polymer manufacture must be stabilised with a low concentration of an inhibitor, such as hydroquinone in order to prevent oxidation of the monomer during storage and consequent effects on its polymerization qualities. As the inhibitor would, itself, interfere in the polymerization, it is usually stripped from the monomer imm.ediately prior to use by processes such as flash distillation or washing with aqueous alkali solution. 

General methods are described in Methods 35 - 38 for the determination of inhibitors in monomers. 

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Determination of TBC in Styrene To determine the amount of inhibitor p-tert butyl catechol (TBC) present, the styrene monomer is extracted with aqueous NaOH and the aqueous layer containing the TBC as the coloured quinone is separated and the colour measured spectrophotmetrically at a wavelength of 485 run using NaOH of 1.0 N solution as a reference standeud. ASTM Method (D2120-82) is used for determining concentration of inhibitor, in styrene monomer. This method covers the measurement of the amount of residual TBC iii styrene monomer in the range from 0-100 ppm. 

METHOD 35 - TESTS FOR INHIBITORS IN STYRENE MONOMER IRON HI CHLORIDE/POTASSIUM FERRICYANIDE PROCEDURE. 

A cadiode-ray polarographic procedure is described which determines hydro-quinone, substituted benzoquinone and anthraquinone inhibitors in styrene monomer in amounts down to 5 ppm. 

This method is capable of determining down to 0.1 ppm p-tert butyl catechol inhibitor in styrene monomer with an accuracy of 5%. 

Rapid semi-quantitative tests for inhibitors in styrene monomer... 

Determination of p-tert-butyl catechol inhibitor in styrene monomer. 

One important factor to consider in the preparation of the organic phase is the presence of inhibitors in the monomers. Some formulae call for the removal of inhibitors, primarily TCB, from the monomers. The TCB inhibitor forms highly colored complexes with metallic salts rendering the final product colored. Styrene has about 50 ppm of TCB. DVB, being more reactive, contains about 1000 ppm of TCB. There are several options for the removal of inhibitors. Columns packed with DOWEX MSA-1 or DOWEX 11 ion-exchange resins (Dow Chemical Company) can be used. White drierite or activated alumina also works well. 

Styrene monomer was purified by vacuum distillation over CaHi. Inhibitor in styrene was removed using activated alumina. N-heptane was purified by distillation over sodium to remove tire trace of residual moisture. The purified slyreaie and n-heptane were stored over activated alumina under nitrogen blanket. Et[Ind]2ZrCl2 Strem Chem.), MAO (modified methylaluminoxane, type 3A, Akzo Novel) wee used without fiirtha" purification. 

Polyester resin coatings are synthesized with components that introduce unsaturation into the polymer chain (—C=C—). The paint is manufactured by mixing a dissolved polyester resin in styrene monomer with pigment and reaction inhibitor. Additional styrene and peroxide are packaged in a separate container and are mixed with the paint when applied using a dual-headed spray gun. Peroxide serves as a radical polymerization initiator for the polyester resin with monomeric styrene and cross-linking. Figure 13.5 shows the chemical structure of an isophthahc polyester resin. 

In the coating industry, polyesters are characterized by resins based on components that introduce unsaturation (—C=C—) directly into the polymer backbone. The most common polyester resins are polymerization products of maleic or isophthalic anhydride or their acids. In producing paint the polyester resin is dissolved in styrene monomer, together with pigment and small amounts of inhibitor. A free radical initiator, commonly a peroxide, and additional styrene are packaged in another container. When applied, the containers are mixed. Sometimes, because of the fast initiating reaction (short pot life) they are mixed in an... 

This spectrophotometric method determines inhibitors such as hydroquinone, p-tert butyl catechol or p-methoxy phenol in amounts down to 10 ppm in styrene monomer. 

This method is capable of determining styrene monomer inhibitors in styrene in amounts down to 1 ppm with an accuracy of 10%. To obtain quantitative results it is necessary to know the identity of the particular inhibitor present in the styrene monomer sample. 

The polymerization inhibitors in styrene, acrylonitrile and 2-ethyl hexyl acrylate were removed by passing the monomer over Amberlyst resins (Rohm and Haas) - A-27 resin for styrene and A-21 resin for acrylonitrile and 2-ethylhexylacrylate. 

In another study, uniform composite polymethyl-methacrylate/polystyrene (PMMA/PS) composite particles in the size range of 1-10 fim were prepared by the seeded emulsion polymerization of styrene [121]. The PMMA seed particles were initially prepared by the dispersion polymerization of MMA by using AIBN as the initiator. In this polymerization, poly(7V-vinyl pyrolli-done) and methyl tricaprylyl ammonium chloride were used as the stabilizer and the costabilizer, respectively, in the methanol medium. Seed particles were swollen with styrene monomer in a medium comprised of seed particles, styrene, water, poly(7V-vinyl pyrollidone), Polywet KX-3 and aeorosol MA emulsifiers, sodium bicarbonate, hydroquinone inhibitor, and azobis(2-methylbu-... 

However, other molecules exist which form free radicals of such high stability that they effectively stop the chain process. These molecules are called retarders or inhibitors the difference is one of degree, retarders merely slowing down the polymerisation reaction while inhibitors stop it completely. In practice vinyl monomers such as styrene and methyl methacrylate are stored with a trace of inhibitor in them to prevent any uncontrolled polymerisation before use. Prior to polymerisation these liquids must be freed from this inhibitor, often by aqueous extraction and/or distillation. 

Fig. 22.—Inhibition of the thermal polymerization of styrene at 90°C by benzoquinone. The log of the viscosity relative to that of pure monomer is here used as a measure of polymerization. The small induction period in the absence of quinone presumably was caused by spurious inhibitors present in the monomer. (Results of Foord. )...

Berlin [69] also confirmed the importance of the presence of OH radicals in his investigation of the polymerisation of polystyrene in the presence of styrene monomer when he found the addition of water to the reaction solvent (benzene) greatly enhanced the yield of polymer. However, latterly it has been argued for these systems that the appearance ofwater decomposition products (e. g. H2O2) led to oxidation of the various impurities, which previously, may have acted as inhibitors in the polymerisation process. 

By-products formed during their preparation (e.g., ethylbenzene and divinyl-benzenes in styrene acetaldehyde in vinyl acetate) added stabilizers (inhibitors) autoxidation and decomposition products of the monomers (e.g., perox-... 

Wash the styrene monomer twice with 25-ml portions of 25% aqueous sodium hydroxide in order to remove the inhibitor. Then wash twice with 25 ml of distilled water to remove any residual caustic reagent. 

EP-4 developed by ERDL is a very flexible polyester based on polyethylene glycol with molecular weight-200 (PEG-200), isophthalic acid (IPA) and maleic anhydride (MAn). Before its use, it is blended with styrene monomer (1 1) and cured at room temperature using cobalt naphthenate (as an accelerator) and methyl ethyl ketone (MEK) peroxide (as a catalyst). This meets the requirements of the main inhibitor and is used for inhibition of DB and CMDB propellants after the application of a barrier coat (generally a rigid polyester such as PR-3). However, it is observed during manufacture of EP-4 that there is a lot of batch-to-batch variation in properties in spite of the strict quality control measures adopted during its manufacture. 

It is a matter of general observation that with styrene, the polymerization-rate curves will exhibit three distinct phases, the nature of which can be determined by the polymerization conditions and the purity of the monomer (1) an initial slow period at the begin ning of the reaction, known as the induction period, which appears to be associated with the presence of an inhibitor or other impurity in the monomer (2) a period of relatively rapid polymerization, which persists almost to the end of the reaction, and for which the rate is exponentially dependent upon temperature and (3) a final slowing down in rate as the reaction approaches completion and the... 

I Materials, Double distilled water and absolute ethanol were used in all polymerisations Styrene monomer was washed with 10% w/w aqueous sodium hydroxide solution and then distilled under a nitrogen atmosphere with reduced pressure to remove inhibitor and impurities. 4.4f-Azobis (4-cyanovaleric acid) (ADIB), recrystallised from absolute ethanol to remove any peroxide impurities, and benzoyl peroxide (BzP) of reagent grade were utilised. 

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