Cell nucleating agents

A material, such as (a) a terpolymer of propylene, ethylene and butene-1, (b) a polyolefin composition, which includes about 31 to 39% of a copolymer of propylene and ethylene and about 58 to 72% of a terpolymer of propylene, ethylene and butene-1 or (c) a polyolefin composition, which includes about 30 to 65% of a copolymer of propylene and butene-1 and about 35 to 70% of a copolymer of propylene and ethylene, is irradiated and extruded through a die in the presence of a physical expanding agent and a cell nucleating agent to produce a structure having a density, which is at least 10 times less than the initial density of the material. The foam articles exhibit improved flexibility and low temperature toughness compared to conventional propylene polymer materials. 

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. 

Another commercial application is in PS foams, where it is used as a cell-nucleating agent. In rubber blends, talc is used as an antitack agent to prevent newly formed goods from sticking together. In some specialty elastomers, it reduces air and fluid permeability [5]. 

The cell structure of the polyolefin foam can be further influenced by adding cell nucleation agents into the polymer melt. Highly dispersed fillers and primary gas generators which build small voids (cavities) in the interface, or in the homogeneous melt, act as nucleation agents for further cell formation. 

The relative effectiveness of nucleating agents in a polymer can be determined by measuring recrystallization exotherms of samples molded at different temperatures (105). The effect of catalyst concentration and filler content has been determined on unsaturated polyesters by using dynamic thermal techniques (124). Effects of formulation change on the heat of mbber vulcanization can be determined by dsc pressurized cells may be needed to reduce volatilization during the cure process (125). 

Nucleating agents, such as fumed silica and kaolin, are usually added to the spin mix to facilitate spin agent flashing and to obtain uniform small size cells (12). 

Polyethylene. This is essentially a closed-cell insulation manufactured at 448 =t 2 K by an extrusion process. A blowing agent and nucleating agent are employed to control the cell size, and primary use is in insulating pipelines for hot and chilled water lines, air conditioning, and processing... 

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