Monomer stabilization hydroquinone

Hydroquinone often is used as a monomer stabilizer. In the absence of oxygen it is not an inhibitor, however, and its action in the presence of oxygen is the result of its oxidation to quinone. See Ref. 98. 

VINYL ACETATE or VINYL ACETATE HQ or VINYL ACETATE MONOMER (108-05-4) C4HSO2 Forms explosive mixture with air [explosion limits in air (vol %) 2.6 to 13.4 fiashpoint 18°F/-7°C autoignition temp 756°F/402°C Fire Rating 3]. Polymerizes readily if not inhibited elevated temperatures and the influence of light, air, oxygen, water, or peroxides initiates violent reaction. Must be stabilized (hydroquinone or diphenylamine has been recommended) to prevent polymerization. Violent reaction with strong oxidizers, non-oxidizing mineral acids, 2-aminoethanol, butyl acrylate ... 

Industrially, hydroquinone is often added to monomers, although hydroquinone itself acts neither as an inhibitor nor as a retarding agent. If oxygen is present in the system, however, the hydroquinone will be oxidized to quinone. In a peroxide-initiated reaction, therefore, the hydroquinone acts in two ways directly as a reducing agent for the initiator, and indirectly (as benzoquinone) as an inhibitor. Therefore, hydroquinone is a stabilizer. 

Methyl methacrylate monomer (stabilizer, less than 0.02% hydroquinone) 610... 

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... 

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). 

Polymerization inhibitors are key additives which prevent premature gelation of the adhesive. The foimulator must carefully balance shelf stability and the required cure on demand. Due to its high propagation rate, MMA is difficult to inhibit. Some comments on specific inhibitors follow. The most common inhibitor to be found in component monomers is 4-methoxyphenol, which is also called the methyl ether of hydroquinone. This inhibitor is effective only in the presence of oxygen. A mechanism has been proposed, and is illustrated in Scheme 13 [128]. 

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-... 

Standard on Explosion Prevention Systems, National Fire Protection Association, 2002). For example, methyl acrylate and ethyl acrylate are commonly stabilized with hydroquinone monomethyl ether. Industry guidance recommends a minimum concentration of 5 vol % 02 in the atmosphere above the acrylate to prevent polymerization (Intercompany Committee for the Safety and Handling of Acrylic Monomers, Acrylate Esters, A Summary of Safety ana Handling, 3d ed., 2002). 

Most monomers can be stored unchanged under nitrogen only for short times (hours or days), even in the dark at low temperature. For long-term storage, a suitable stabilizer is therefore indispensable. Effective stabilizers (inhibitors) of radical polymerization are quinones, phenols, amines, nitro compounds, and some metals or metal compounds. The addition of 0.1 to 1 wt% of hydroquinone or 4-ferf-butylpyrocatechol results in sufficient stabilization of many monomers. 

The reactive vinyl monomers usually are stabilized against polymerization, while in storage, by addition of 0.1 to 1% of an inhibitor. 1,4-Benzenediol (hydroquinone), 2,6-di-terf-butyl-4-methylbenzenol, and 4- ert-butyl-l,2-ben-zenediol are used for this purpose. These substances are especially effective at scavenging RO- radicals, which are formed by oxidation of the monomer with atmospheric oxygen. 

TABLE 10.2 Addition Rate Constants kj of Various Radicals to Monomers Reaction Rate Constants kr of These Radicals with Oxygen, an Amine, a Stabilizer (HQME Hydroquinone-Methyl Ether) and a Spin Trap (TEMPO 2,2,6,6 Tetramethylpiperidine N-Oxyl). 

CHLOROSTYRENE or o-CHLOROSTYRENE (2039-87-4) Combustible liquid (flash point 143°F/62°C). Able to form unstable peroxides (may be stabilized with 0.1% hydroquinone). Contact with strong oxidizers may cause fire and explosions, strong acids. Incompatible with strong bases, butyl lithium, 2,5-dimethyl2,5-di(tert-butylperoxy)hexane, ferric chloride, halogens, sulfuric acid, xenon tetrafluoride. The uninhibited monomer vapor may block vents and confined spaces by forming a solid polymer material. 

Monomer, flake-like crystals from benzene, d ,1 1-122, mp 84.5 . bp2 87° hps 103° bp2) 125°. Solubilities In g/ (00 ml solvent at 30° water 215.5 methanol 155 ethanol 86.2 acetone 63,1 ethyl acetate 12.6 chloroform 2.66 benzene 0.346 heptane 0.0068. The solid may be stored in a cool, dark place. Readily polymerizes at the mp or under uv light. Commercial sol ns of the monomer may be stabilized with hydroquinone, terr-butylpyrocatechol, N-phenyl-2-naph thylamine or other antioxidants. LD, i.p. in mice 170... 

Stabilizers. All methacrylate monomers must contain some free-radical inhibitor if they are to be shipped and stored safely. Hydroquinone and p-methoxyphenol are most commonly used for this purpose. Most formulations will also contain benzoquinone, naphthoquinone, and similar stabilizers. Since the anaerobic compositions are strongly catalyzed by traces of metals, many formulators have found it advantageous to add chelators such as tetrasodium ethylenediaminetetraacetic acid (EDTA) (V). 

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