Peroxide processing time

The majority of phosphate processes in use today are accelerated to obtain shorter treatment times and lower processing temperatures. The most common mode of acceleration is by the addition of oxidising agents such as nitrate, nitrite, chlorate and hydrogen peroxide. By this means, a processing time of 1 to 5 min can be obtained at temperatures of 43-71 °C. The resultant coatings are much smoother and thinner than those from unaccelerated processes, and, while the corrosion resistance is lower, they cause less reduction of paint gloss and are more suited to mass-production requirements. 

Organic bleaching bath stabilizer, effective over long process times with hydrogen peroxide permits reduction of water glass content. 

In practice cotton bleaching with hydrogen peroxide is carried out at 90-100°C, but the temperature may be increased to 120°C in the case of pressurised equipment with a corresponding reduction in process time. The rate of bleaching increases with the increase in temperature, but at the same time solution becomes unstable and degradation of cotton increases. Below 80 C, the evolution of perhydroxyl ion is very slow so also the rate of bleaching. 

Figure 13. Effect of processing time and temperature on the formation of peroxides in LDPE (numbers on curves indicate temperatures (°C) in open chamber, 150C indicates 150° in a closed chamber)...

Figure 17. Effect of processing time at 180°C on the peroxide and conjugated carbonyl concentration in polypropylene. (1) peroxide (open mixer) (2) peroxide (closed mixer) (3) conjugated carbonyl (open mixer) (4) conjugated carbonyl (closed mixer).

Due to its advanced technology and low cost the rubber industry often prefers the use of sulphur-vulcanized polyurethane elastomers even though some of their technical properties, e.g. resistance to thermal degradation, are inferior to the peroxide- and diisocyanate-cured grades which often have short processing times (i.e. they are scorchy) and whose cure may be adversely affected by the presence of moisture in the unvulcanized rubber mix water is present in rubber fillers, e.g. carbon blacks usually contain about 0-5-1% and some non-black fillers such as silicas and clays 2-10%. Also to maximize scorch time it is common practice to quench-cool the rubber after internal mixing by immersion in cold-water tanks or by cold-water spray application to the surface of the hot-milled sheet. 

Decker and Jenkins have also estimated the kinetic chain length of the peroxidation process for an epoxy acrylate. From the measured inhibition time and estimated rate of free-radical production, values of 2-8 were inferred. These did not chan considerably although the intensity was varied over a large range, from 1.5 to 800 pE cm s ... 

Process time s 5-10 (injection time) 30-90 (molding time for peroxide cured) ... 

Fig. 3.37. Effect of processing time on the formation of olefinic unsaturation and peroxides in poly(vinyl chloride) at different temperatures (O, ) peroxides and ( , ) unsaturation. (Reprinted from Scott, G., Polym, Plast. Technol. Eng., 11, 1 by courtesy of Marcel Dekker Inc, NY, 1978.)...

In poly(vinyl chloride) subjected to low temperature (170°C) processing or short processing times, mechano-chemically formed hydroperoxides are primarily responsible for the initiation of photo-oxidation [483,1939-1942]. Poly(vinyl chloride) processed for excessive times at high temperatures (210°C) appears to be more sensitive to carbonyl and olefin as well as peroxide photoactivators. Chemical removal of peroxides in processed poly(vinyl chloride) films effectively increases their initial photostability below that of a compression moulded poly(vinyl chloride) film but under these conditions the effect of carbonyl initiation can be seen. 

Propylene oxide is manufactured in the United States principally by Dow Chemical Company (chlorohydrin process) and ARCO Chemical Company, a Division of Atlantic Richfield (the peroxidation process based on both isobutane and ethylbenzene). In Western Europe, major producers include Dow Chemical Company, ARCO Chemical, Shell, BASF, and Bayer, through a subsidiary. For a time, a peracetic acid process for propylene oxide was operated in Japan however, competitive sources of by-product acetic acid forced discontinuation of this process. 

The mechanism of free radical polymerization of ethylene is outlined m Figure 6 17 Dissociation of a peroxide initiates the process m step 1 The resulting per oxy radical adds to the carbon-carbon double bond m step 2 giving a new radical which then adds to a second molecule of ethylene m step 3 The carbon-carbon bond forming process m step 3 can be repeated thousands of times to give long carbon chains... 

Thermally activated initiators (qv) such as azobisisobutyroaittile (AIBN), ammonium persulfate, or benzoyl peroxide can be used in solution polymeriza tion, but these initiators (qv) are slow acting at temperatures required for textile-grade polymer processes. Half-hves for this type of initiator are in the range of 10—20 h at 50—60°C (13). Therefore, these initiators are used mainly in batch or semibatch processes where the reaction is carried out over an extended period of time. 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

Pressures varied from 20 to 1500 Ibf/in (0.14 to 10.5 MPa) and reaction times were of the order of 5-35 hours. Reaction promoters included peroxides and salts of persulphuric and perphosphoric acids. Activators , accelerators and buffering agents were also discussed in the patent. The process of manufacture of Kel-F is understood to be based on this patent. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Calculating the Maximum Quantity of Lead and Lead Compounds. To calculate the maximum amount of lead and lead compounds present at your facility at any one time, you must consider types of metallic load and M types of lead compounds present at your facility, Including stockpiled raw materials, lead and lead oxide present in process equipment, the metallic lead and lead peroxide contained in finished batteries stored on-site, and stockpiled lead scrap. Since the reporting form is being prepared for lead compounds, the maximum amount reported is the total of the inventories of these materials. The maximum amount of metallic lead (2,305,000 pounds), lead oxide (205,000 pounds), and lead peroxide (625,000 pounds) present at your facility is 3,135,000 pounds, which is between 1,000,000 and 9,999,999 pounds. You would therefore report range 06 on Part III, Section 4, of the reporting form. 

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