Two roll-mixing process

The mathematical modelling of the two roll-mixing process of the PVC-benzidine mixture, based on the above mentioned variables afforded the following function of tensile strength variation ... 

The influence of the two roll-mixing process parameters on the mechanical properties of the product resulting by the simultaneous processing of PVC with an elastomer, was thoroughly studied with a PU containing dibenzyl diisocyanate-ethylene glycol and adipic acid as an elastic component [712],... 

The interesting fact deserves mention that by the two roll-mixing processing of this polymer mixture the tensile strength increases concomitantly with the impact strength although these two properties are known to show an opposite variation, Figure 3.240. 

The conclusion can be drawn that due to the activation of some reactions as a consequence of formation of free radicals, the two roll-mixing process of polymers could be a simple and efficient method for performing chemical modifications of the mac-romolecular compounds as well as for synthesizing new compounds showing properties superior to the starting ones. 

There are at least theoretical solutions to these two problems. The large volume is less of a problem in dosage forms such as instant or effervescent granules of ibuprofen [505], while the use of large quantities of solvents can sometimes be avoided by using a solvent-free production process such as the roll-mixing process for triturations. 

Two-roll mixing is another traditional method of mixing which relies on somewhat primitive equipment and on the skills of the operator. It has never been possible to establish standard procedure for this process and batch-to-batch results vary widely. 

Recently, a new technology for the devulcanization of sulfur-cured scrap elastomers was reported [36] using a material termed Delink [37]. Such technique of devulcanization was designated as Delink process. In this process, 100 parts of 40-mesh or finer crumb is mixed with 2 to 6 parts of Delink reactant in an open two-roll mixing mill. Delink reactant is a proprietary material, and its nature and composition was not disclosed. 

Usually, elastomers are processed through extrusion, forming in matrices, mixing. The first observations on the cold degradation of rubber by two roll-mixing, in the last century, refers to the fact that, under prolonged processing, natural rubber becomes sticky [703-705],... 

Natural rubber s processing by two roll-mixing, in the presence of styrene, at temperatures ranging within the 75-130 C interval permits a rapid polymerization of the monomer. The reaction is influenced by the elastomer/styrene weight ratio, being observed that its decrease causes increase of the polymerization reaction rate, which comes to exceed the one corresponding to styrene s thermal polymerization. 

In this manner, natural rubber modified with maleic anhydride was obtained, two roll-mixing being applied as the processing method [710],... 

Mechanochemical reactions during polymer processing by two roll-mixing... 

An important effect for the practical processing of PVC is noticed in case of its two roll-mixing in the presence of benzidine which could partially substitutes the classical thermal stabilizer... 

Compounding may be carried out by either a wet or a dry process. In the wet process, now obsolescent, the ingredients are mixed as a viscous solution in acetone in a dough mixer. The resulting dough is then rolled on a hot two-roll mill to evaporate the bulk of the solvent. It is then necessary to season the resulting hides until the solvent content is reduced to a tolerably low level. 

As an alternative to the wet process described above, moulding compositions may be made by mixing a powdered resin or a methylol derivative with other ingredients on a two-roll mill or in an internal mixer. The condensation reaction proceeds during this process and when deemed sufficiently advanced, the composition is sheeted off and disintegrated to the desired particle size. This dry process is not known to be used in any current commercial operation. 

The two-roll mill represents the earliest form of rubber processing machine, used by the rubber manufacturer being developed from the masticator of Hancock. Over the years the emphasis of the role of the two-roll mill has moved from being that of the prime means of compound preparation for the majority of the rubber industry, to that of secondary usage. In the modem factory conventional two-roll mills are used for compound blending and for sheeting off of compound mixed by other means. In addition they are used to warm-up compounds from store, and for a number of blending purposes to achieve compound uniformity for other processes. 

Processes which melt mix polymer and filler are capable of generating the high shear stresses necessary to cause agglomerate break up, together with re-distribution of the primary filler particles. Since its conception in 1835, the two-roll mill has proved to be an effective means of mixing additives into plastics and rubber and is still in widespread use today, principally for laboratory purposes, but to some extent in large-scale industrial applications. 

Two-roll mills have been analyzed in terms of the pressure distribution and velocity profiles created between the rolls [95], the shear imposed on fluid elements exposed to these conditions in the nip region [129] and their resulting efficiency as dispersive mixing devices [130,131]. An earlier mathematical model was proposed to describe the dispersive mixing process of carbon black in rubber on roll mills, through consideration of agglomerate size distribution and... 

Costabilizers. In most cases the alkyltin stabilizers are particularly efficient heat stabilizers for PVC without the addition of costabilizers. Many of the traditional coadditives, such as antioxidants, epoxy compounds, and phosphites, used with the mixed metal stabilizer systems, afford only minimal benefits when used with the alkyltin mercaptides. Mercaptans are quite effective costabilizers for some of the alkyltin mercaptides, particulady those based on mercaptoethyl ester technology (23). Combinations of mercaptan and alkyltin mercaptide are currently the most efficient stabilizers for PVC extrusion processes. The level of tin metal in the stabilizer composition can be reduced by up to 50% while maintaining equivalent performance. Figure 2 shows the two-roll mill performance of some methyltin stabilizers in a PVC pipe formulation as a function of the tin content and the mercaptide groups at 200°C. The test formulation contains 100 parts of PVC (Fikentscher K = 65), 1.2 parts of paraffin wax, 0.6 parts of calcium stearate, and 0.4 parts of methyltin-based stabilizers. 

Many methods for the conversion of acid copolymers to ionomers have been described by Du Pont (27,28). The chemistry involved is simple when cations such as sodium or potassium are involved, but conditions must be controlled to obtain uniform products. Solutions of sodium hydroxide or methoxide can be fed to the acid copolymer melt, using a high shear device such as a two-roll mill to achieve uniformity. All volatile by-products are easily removed during the conversion, which is run at about 150°C. A continuous process has been described, using two extruders, the first designed to plasticate the feed polymer and mix it rapidly with the metal compound, eg, zinc oxide, at 160°C (28). Acetic acid is pumped into the melt to function as an activator. Volatiles are removed in an extraction-extruder which follows the reactor-extmder, and the anhydrous melt emerges through a die-plate as strands which are cut into pellets. 

The apparatus used for the first tests included a 3/4-inch diameter, 20 1 L/D laboratory extruder. The second series of evaluations was run on a Brabender torque rheometer equipped with mixing head. The third tests utilized large scale processing equipment including a two-roll,... 

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