Colloidal Silica Processing Aid Slashes Molding Cycle Times

Ultra-fine synthetic silica particles have been shown in laboratory and commercial field trials to reduce injection molding cycle times by 20% to 30% in polypropylene, filled and unfilled nylon, PBT, and ABS. This processing advantage is accompanied by retention and even improvement of key physical properties. Other reported benefits are improved color dispersion
and reduced flow lines and surface defects. What’s more, the silica additive is said to mix easily with resin pellets in a blender, so melt pre-compounding is not required.

Nan-O-Sil ASD additive is high-purity, amorphous, colloidal silica in the form of a white powder. The spherical particles range in size from 0.02 to 0.55 microns (20 to 550 nanometers). Nan-O-Sil ASD is manufactured and marketed by Energy Strategy Associates, Inc., which has been working with technical consulting and R&D firm Rheo-Plast Associates to develop applications for this additive. Headed by Dr. Pravin L. Shah, Rheo-Plast specializes in polymer rheology and its application to extrusion, injection molding, and compounding of engineering plastics and blends/alloys. Rheo-Plast conducted laboratory testing of Nan-O-Sil in a variety of resins. Dr. Shah supplied the test results to Plastics Technology for this article.

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EXPERIMENTAL RESULTS

Injection molded sample bars were obtained by first tumbling resin pellets with Nan-O-Sil ASD for 15 min in a Conair blender. The additive coated the pellets well and uniformly, Shah reports. 

The most dramatic results with Nan-O-Sil ASD are the cycle-time reductions for PP, unfilled nylon 6, glass-filled nylon 66, unfilled PBT, and ABS (see Table 1). The additive also shows utility as a reinforcing filler that can raise the stiffness of PP and both stiffness and strength of nylon 6 and PBT (Tables 2 to 5). Depending on the resin and the amount of Nan-O-Sil used, there was little or no sacrifice of other properties, such as elongation or toughness.

In glass-filled nylons, using larger amounts of Nan-O-Sil ASD appears to have potential for reducing the amount of glass fiber required to improve strength and stiffness. One trial compared the physical properties of 13% glass or 10% Nan-O-Sil in nylon 6. As shown in Table 4, colloidal silica imparted a greater increase in tensile strength and flexural modulus than did the glass fiber. According to Shah, an in-depth study of combinations of Nan-O-Sil ASD and glass fiber is under way and results will be published next year.

HOW IT WORKS

Although the precise mechanism by which Nan-O-Sil ASD reduces cycle times and improves strength properties of molded plastics is still unclear, Shah says the additive has been shown to work at low addition levels of 0.4% to 0.8% as a heat sink to reduce the amount of cooling required during molding. Based on Brabender torque rheometer data, he also postulates that the additive works as a processing aid to reduce the shear gradient at the gate and facilitate melt flow orientation.

Also not clearly understood at present is the apparent melting-point depression observed in PP, though no such effect was evident in nylon. Further work is under way to determine the additive’s behavior in melt processing.