Processing mill sticking

Processing Aids. Stearic acid [57-11-4] or other fatty acids and/or metal soaps of fatty acids are added to reduce shear degradation and mill sticking during mixing. Sorbitan monostearate (ICI s Span 60) is one of the best processing aids to reduce mill sticking. 

Stearic acid. Improve processability and reduce mill sticking. Amounts of 0.5 to 1 phr can be added. 

Other modifiers Although they are not common, additional ingredients can be added to polychloroprene adhesive formulations to improve specific properties. Plasticizers can decrease the glass transition temperature, influence crystallization tendency and reduce cost. Highly aromatic mineral oils can be used when reduction in crystallization rate is required. Stearic acid, in concentrations of 0.5-1 phr, improve processability and reduce mill sticking. Resorcinol type resins improve adhesion to textiles and metals. 

External lubricants are deliberately chosen not to be very compatible with the polymer and do not significantly reduce the melt viscosity. They migrate to the surface, particularly at ambient temperatures, preventing adhesion of the polymer to mould surfaces and extruder barrels. This loss of adhesion is a problem when printing onto the product surface. External lubricants characteristically increase the mill stick time (the time to the point where sticking occurs) when a PVC formulation is processed on a two-roll mill. A few external lubricants, such as oxidised polyethylenes, allow the production of clear products, but the less compatible ones do not. 

The effects of mill sticking are often most serious with halogenated polymers, where mild corrosion of process equipment may provide a clean steel surface, thus promoting high bond strengths. Heat build-up exacerbates the problem severely. Within the clay minerals it is known that the total surface hydroxyl content (which is the product of surface area and hydroxyl concentration) influences mill sticking and other properties markedly [36]. Other aspects of processability are also known to be influenced by this (Table 7.5). 

Stearic acid is a usual processing aid in butyl rubber formulations, acting as lubricant and minimizing mill sticking apart from this action, it also plays the role of activator in the curing system. Hydrocarbon or phenolic tackifying resins are sometimes provided in compounds to assist the adhesion in confection or splicing. 

Butadiene Rubber (BR) added at levels of up to 10% to improve processing of S-grades (reduced mill sticking) however, a reduction in flex-fatigue life may be observed. BR also improves low-temperature brittleness. 

CR compounds, especially of low viscosity, mineral filled, or those based on S-types, show a tendency to mill sticking. To overcome this effect, low friction ratios and low-temperature processing are recommended. Process aids may also assist in providing better mill release. If the compound temperature exceeds 70°C, CR compounds become somewhat grainy in appearance, lose cohesive strength, and stick to metal surfaces. 

To ensure that none of the material that is being ground sticks to the walls of the mill during the grinding process, thereby producing larger pieces, the samples are typically dried to 100—110 °C prior to grinding to expel any water. 

Stickiness is the ability of a rubber compound to stick to a nonrubber surface such as a metal or textile fabric surface. Excessive stickiness to metal can result in poor release from mills and problems in process equipment. On the other hand, the destruction of compound stickiness can cause slippage of the compound against metal surfaces in extruders or the rotors of an internal mixer. Various compounding additives, such as mill release agent.s or external lubricants, are used to control the level of stickiness. Note, compound stickiness and tackiness are not the same property. 

The production rate of the ball mill process has been found to be proportional to the speed of the drum rotation. The energy needed for the production of 1 ton of lead oxide is fairly constant when the mill operates at between 55 and 90% of its critical rotation speed. The critical rotation speed is assumed to be the speed of drum rotation at which lead pieces stick to the drum walls under the action of centrifugal forces and do not rub against each other. 

Mill release 0-10 Prevent sticking to processing equipment... 

Many polymers have their own process characteristics. NR and SBR process very easily on a two-roll mill. Other polymers process differently. Nitrile rubbers are well known for bagging (non-adherence to the rolls), while other polymers, especially those that are chlorinated, tend to stick firmly to the mill roll. Consequently, specific solutions have been found for each polymer s processing idiosyncrasies. 

Rubbers using this cure-site monomer were not entirely satisfactory as they had to be used with cure systems that had poor processing safety and also gave problems such as sticking on to mill rolls and mould fouling. 

---