Foam particle blending

The seventh trend is the increasing use of novel processing methods. For example, there is growing use of supercritical fluids (e.g., supercritical carbon dioxide and nitrogen gases) to foam polyolefin blends for density reduction. There is use of ultrasound to, for example, devulcanize cross-linked rubber. There is use of solid-state shear mechanical processing to break the polyolefin blend material into submicron particles to make environment friendly (water-based) polyolefin dispersions. There is use of electrospinning technique to make polyolefin fibers and in particular nanofibers. 

Supercritical C02-assisted extrusion applications mainly involve polymer blending, microcellular foaming, particle production, and reactive extrusion. Of course, supercritical CO2 can also be used as an interfacial agent, foaming agent, or plasticizer in other appUcations. 

There are two principal PVC resins for producing vinyl foams suspension resin and dispersion resin. The suspension resin is prepared by suspension polymerization with a relatively large particle size in the 30—250 p.m range and the dispersion resin is prepared by emulsion polymerization with a fine particle size in the 0.2—2 p.m range (245). The latter is used in the manufacture of vinyl plastisols which can be fused without the appHcation of pressure. In addition, plastisol blending resins, which are fine particle size suspension resins, can be used as a partial replacement for the dispersion resin in a plastisol system to reduce the resin costs. 

The number of PPE particles dispersed in the SAN matrix, i.e., the potential nucleation density for foam cells, is a result of the competing mechanisms of dispersion and coalescence. Dispersion dominates only at rather small contents of the dispersed blend phase, up to the so-called percolation limit which again depends on the particular blend system. The size of the dispersed phase is controlled by the processing history and physical characteristics of the two blend phases, such as the viscosity ratio, the interfacial tension and the viscoelastic behavior. While a continuous increase in nucleation density with PPE content is found below the percolation limit, the phase size and in turn the nucleation density reduces again at elevated contents. Experimentally, it was found that the particle size of immiscible blends, d, follows the relation d --6 I Cdispersed phase and C is a material constant depending on the blend system. Subsequently, the theoretical nucleation density, N , is given by... 

Fig. 12 Nucleation density of foamed PPE/SAN blends vs number of theoretically available nucleation sites (=particle density of the PPE phase). The dotted lines represent the theoretical nucleation density at different phase sizes of PPE (reprinted from [47])...

For evaluating the efficiency of the nanostructured interface for cell nucleation, the particle density of PPE, as a measure for the number of nucleating sites available for nucleation, is plotted versus the nucleation density observed for the foam (Fig. 21). For comparison, the previously found values of the uncompatibilized PPE/SAN blend are added. For PPE/SAN, even the relatively high number of PPE particles of around 5 x 10ncm-3 only leads to nucleation of approximately 2.5 x 1010 cells cm-3, i.e., only 1/20 of the potentially available PPE particles act as cell nucleating agents. Via compatibilization, however, not only the particle density of PPE and the nucleation density can be increased, but also the efficiency is strongly enhanced. While the number of cells directly scales with particle density, more than two foam cells are nucleated by one PPE particle. 

Isotactic Polystyrene. The familiar steam molding of pre-expanded particles has so far not been applied successfully to isotactic polystyrene. However, the polymer has been foamed, according to three disclosed methods. For example, finely divided acetone-insoluble polymer, with a melting point in excess of 200°C., is blended with a liquid selected from methylene chloride, aromatic hydrocarbons, or halogenated aromatic hydrocarbons. This blend is then heated (84). A mixture of molten polymer and methyl chloride, propane, or butane is suddenly depressurized (8). Foam may also be generated in a continuous manner directly from a butyllithium-initiated polymerization conducted in the presence of a 4/1 blend of benzene and petroleum ether (15). 

Polyimide-Based Syntactic Foam (9). Three-phase syntactic foams were made using a polyimide solution (22% PI-2080 in DMF) and hollow glass microspheres (Type B-30-B, 3M Co.) which have a particle density of 0.25 to 0.30 cm and a bulk density of 0.182 g/cm . The solution and glass spheres were hand mixed and packed into a 5" x 5" mold and compacted under pressure. Variations of foam density were obtained by molding specific quantities of blend into different volumes ranging from Vi to 1" in thickness. Greater densities required higher pressures with the maximum density obtained at a pressure of about 100 psi. 

The moisture content in air-dry wood fibers ranges from 6% to 7%, but the processes for plastics manufacturing tolerate little or no water. Even 1% or 2% moisture is considered too high [1, 6]. Removal of water is critical because any moisture remaining in the wood-plastic blend turns to steam and manifests itself in the form of foam, disrupting processes, resulting in poor surface quality, weak wood-plastic interface, and voids that are unacceptable for final sale [3, 25]. As a result, particles must be predried for blending. 

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