Milling, neoprene

Turbines Temper mills Neoprene production lines... 

For example, milling Neoprene AC for 5 min [stock temperature -50 C (122°F)] reduces initial 100 C (212 F) peel strength from 7 lb/ in. to less than 1 Ib/in. After a bond aging period of one month, both have 12 Ib/in. peel strength at 100 C (212 F) as a result of partial curing with zinc oxide. 

Processing ndProperties. Neoprene has a variety of uses, both in latex and dry mbber form. The uses of the latex for dipping and coating have already been indicated. The dry mbber can be handled in the usual equipment, ie, mbber mills and Banbury mixers, to prepare various compounds. In addition to its excellent solvent resistance, polychloroprene is also much more resistant to oxidation or ozone attack than natural mbber. It is also more resistant to chemicals and has the additional property of flame resistance from the chlorine atoms. It exhibits good resiUence at room temperature, but has poor low temperature properties (crystallization). An interesting feature is its high density (1.23) resulting from the presence of chlorine in the chain this increases the price on a volume basis. 

Samples for 180° peel tests were prepared according to standard neoprene recipes (9). Neoprene AC or a graft copolymer prepared from it was mixed on an open mill at room temperature for 20-25 min with the following compounding ingredients in succession ... 

Below 71 °C it exists in what is called the elastic phase. In this phase Neoprene coheres tightly so that strong running bands are obtainable on a mixing mill. 

Between approximately 71 °C to 93 °C it exists in a granular phase5. In this condition Neoprene loses cohesion to it but tends to stick tightly to the mill rolls. 

Above 93 °C the Neoprene enters a plastic phase in which it is very soft and has only weak cohesion to itself, showing very little tendency to stick to the mill rolls and has almost no nerve. 

The uncured Hypalon compound behaves much like a thermoplastic resin. It is tough and nervy at room temperature, but its viscosity is reduced rapidly as it is warmed by working on a two roll mill. By contrast Neoprene is not as tough as Hypalon at lower temperatures. 

Hypalon does not break down during mastication in the sense that natural rubber and Neoprene do. These rubbers undergo an irreversible chemical change (i.e., a reduction in molecular weight) when they are worked in the mixing mill. 

Which roll will the stock stick to—front or back Most mills are arranged with a speed differential (friction ratio) between front and back rolls so one roll runs faster than the other. In general, natural rubbers and SBRs stick to the slower roll and EPDM, neoprene, etc., stick to the faster roll, but there are exceptions. 

Thermoplastic rubbers without premastication have lower solution viscosities than do conventional elastomers such as SBR, natural rubber, and neoprene after milling. This is because thermoplastic rubbers have low, precisely-controlled molecular weights. Figure 12 compares the viscosities of a typical S-B-S polymer with two conventional elastomers in toluene solution. 

Brief milling of Neoprene AF, on a cool mill, is suggested to break down a light, gel-like association which builds up as the polymer ages and prevents preparation of a smooth solution under low shear conditions. Overmilling (past five minutes) will lead to increased solution viscosity and poor solution stability. 

Mastics based on Neoprene AG can be prepared in a sigma blade mixer from polymer which has first been preswollen in solvent. Milling is not required to produce smooth, non-stringy products. 

Magnesium oxide serves a second function as a processing stabilizer. It is mill mixed with Neoprene before adding zinc oxide to retard scorch during processing. 

The procedures used to prepare Neoprene adhesive cements can affect end use properties such as hot cohesive strength and spray ability. Techniques used to prepare cements include milling, direct dissolving, and direct dissolving... 

Spraying. A low Mooney, low nerve Neoprene is preferred. Milling is required for the best spray characteristics. The viscosity should be below 250 cps. The solvent blend should contain predominantly fast-evaporating solvents which individually could not dissolve Neoprene. 

Resins are incorporated in Neoprene latex as solvent-cut emulsions, solventless pebble-milled dispersions, or sometimes as solvent-free emulsions prepared using invert emulsification techniques. In the latter case a resin with a melting point of 80°C (176 F) or lower is melted. Water and surfactants are added to the molten resin and the temperature of the mixture is decreased. Upon reaching a certain temperature, known as the phase inversion temperature, the water in molten resin emulsion spontaneously inverts to form a resin in water emulsion suitable for use in latex adhesives. A resin dispersion which can be prepared in this manner is shown in Table 16. This particular resin dispersion can be used to produce adhesives with moderate hot strength and good open time using the following recipe ... 

Two-roll mill blend Neoprene AC Neozone A antioxidant MgO ZnO... 

Solvent cements are generally produced by removing the milled elastomer from the mill and dissolving it in solvent. This cutting of the rubber into a solvent is accomplished in low-speed mixing equipment known as chums or in a variety of high-speed, heavy-duty mixers. For natural rubber and SBR, solvents such as toluene, hexane, or naphtha are commonly used. For nitrile, neoprene, and other polar polymers, polar solvents such as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), or chlorinated solvents may be used, alone or in blends with the nonpolar solvents. The blend of solvents finally used by the adhesive producer will depend not only on solvent power, but on the relative rates of evaporation which control drying time. The dissolved rubber can then be blended with whatever additives are needed to obtain the desired adhesive performance. 

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