Mixing additives into polymers

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. 

Gale, G.M. (1991) Mixing of solid and liquid additives into polymers using single screw extruders. Proceedings of the 49th SPE ANTEC, 37, pp. 95-98. 

Dispersive mixing is required for the mixing of solid additives into polymers. Most additives are solids and for effective performance need to be used as fine powders and well mixed into the polymer. Additives with a particle size of less than 100 pm tend to naturally agglomerate. These agglomerates will often be present, no matter what precautions are taken, whilst the mixing processes used can sometimes contribute to the problem. 

Two important terms used to describe additives in polymeric mixtures are distribution and "dispersion . When mixing polymers with additives, we want to create a system in which the additive is both well distributed and dispersed. Distribution refers to the even placement of the additive throughout the polymer. For example, a well made batch of chocolate chip cookies has good distribution of the chips if every bite has a chocolate chip in it. A poorly distributed cookie would have all its chips on one side. Dispersion, on the other hand, refers to the separation of the individual components of a solid additive into its smallest parts. Figure 10.6 illustrates both good and bad distribution and dispersion. 

Finely divided fillers used as additives in polymers have a tendency to agglomerate into larger structures due to strong, inter-particle attractive forces. As mentioned earher, it is the purpose of the dispersive mixing process to reduce the size of these agglomerates through the application of a controlled shear stress. 

Reactive polymer processing modifies or functionalizes the macromolecular structure of reactor polymers, via chemical reactions, which take place in polymer processing equipment after the polymer is brought to its molten state. The processing equipment then takes on an additional attribute, that of a reactor, which is natural since such equipment is uniquely able to rapidly and efficiently melt and distributively mix reactants into the very viscous molten polymers. The operation is shown schematically in Fig. 1.3. 

A compounding step is used to mix various stabilizers and additives into the polymer melt, which is finally chilled and pelletized. 

Hou (1993) reported that for a petroleum sulfonate-HPAM-mixed alcohol (isopropanol isobutanol = 8 1) system, the addition of polymer did not change the three types of phase behavior, but the upper phase and lower phase volumes were increased very slightly and the middle phase volume was decreased accordingly. This volume changes were caused by the interaction of the alcohol with HPAM. HPAM brought some of alcohol from the middle phase into the aqueous phase, resulting in the decrease in the middle phase volume. As polymer concentration was increased, the aqueous phase viscosity was increased while the middle phase viscosity remained almost unchanged because very little polymer would go to the middle phase. Therefore, HPAM had little effect on the middle phase properties. 

Pure resins are rarely processed into final products without the addition of selected compounds, called additives, which are incorporated during the process of extrusion and molding of a plastic resin or applied externally on the formed material. Compounding refers to the process of uniformly mixing the additives into the resin. A blend, by definition, is formed from two or more polymer resins which have been mixed together. 

Density tests. Referring to Table 2 below, the densities of all the trial mixes ranged from just above 1000 kg/m to 1073 kg/m. The addition of polymers into the trial mixes had a negligible effect on the densities of the concrete. The control mix, CL had a density of 1034 kg/m which is very close to the densities of the other trial mixes thus the density of the concrete is affected more by the lightweight aggregate and cement binder content rather than by the quantity or type of polymer addition. 

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