Canadian Plastics

Crystal Clear: Clarifying Polypropylene

By Jim Anderton, technical editor   



There's a reason why polypropylene is the leading commodity plastic resin. It's cheap, easy to mold and form, and has just the right physical properties for thousands of useful consumer goods. When your kid breaks the garage door window playing st...

There’s a reason why polypropylene is the leading commodity plastic resin. It’s cheap, easy to mold and form, and has just the right physical properties for thousands of useful consumer goods. When your kid breaks the garage door window playing street hockey, however, you’re not going down to the Home Depot to pick up a sheet of PP. That’s because polypropylene isn’t water-clear the way “acrylics” like PMMA (polymethylmethacrylate) are. Or are they?

Addition of the appropriate additive can produce polypropylene parts with outstanding clarity, without unusual molding parameters or special equipment.

The key to figuring out how to make PP clear is to understand what makes it opaque or hazy in its natural state. Conventional commodity isotactic polypropylene is a semi-crystalline solid. “Crystalline” in this case means that the giant molecules of carbon chains line up in a more or less orderly fashion. For simple molecules like sodium chloride, table salt, the high degree of order makes a crystalline solid that’s so ordered it forms neat little cubes and can be highly transparent to visible light.

Polymers are a lot more complex, however, with regions of lesser and greater order called spherulites. It’s a name for regions of crystallinity within an amorphous bulk mass and they can be seen in everything from glass to volcanic rocks and minerals. Like all spherulites, in polypropylene they form around a “discontinuity” in he amorphous region, either naturally occurring or artificially introduced. The key to transparency is to grow enough spherulites in the bulk resin (as it freezes in the mold) so that they bump into each other, limiting their size. Keep them really small, billionths-of-a metre small, and they’ll form tiny crystals of the same order of magnitude as the wavelengths of visible light, and voila, transparency.

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Since the size of the spherulites is crucial, and you need lots of “nucleation sites” to keep them from growing too big, PP clarifier additives act like a controlled impurity, providing billions of “artificial defects” for the crystal to grow on. To work well, the additive has to be insoluble in the melt (crystal growth needs a tiny solid particle for nucleation), a very small particle size, on the order of 10 microns or less, and be very well dispersed in the polymer.

What makes nucleation happen is complicated. Melt index and polydispersity, processing conditions and the type of polymer (homopolymer, random copolymer or block copolymer) influences the process. The most efficient PP clarifiers are complex organic compounds called sorbitol dibenzyl acetals. Early versions were commonly DBS or 1,2,3,4-dibenzylidene sorbital. Second generation additives like (MDBS or 1,2,3,4 – di-para-methylbenzylidene sorbitol are more effective, but require careful processing conditions, typically 220-230 C and moderate shear rates, acid neutralizers and careful storage. The latest generation clarifiers like DMDBS or 1,2,3,4 – di-meta, para-methylbenzylidene sorbitol are even more efficient.

What sort of processing conditions do you need? According to PP clarifier manufacturer Milliken Chemical, for injection molding, melt temperatures of 200-270 C are recommended, with mold surface temperatures between 20 C to 40 C. Milliken recommends smooth, polished molds, SPI 1 or 2, but in many processes SPI 3 is acceptable.

Can you make money with clarified isotactic polypropylene? With applications in thin wall packaging, food product “squeeze” bottles, toys and medical products, the markets are out there. And with reasonable processing parameters and the availability of compounded resin, it shouldn’t be necessary to reinvent the wheel, either.

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