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Both clarity & color push PP packaging: a synergy of colorants and additives brings clarified polypropylene into more container and bottle applications.

At least two forces appear to be pushing the penetration of polypropylene d other materials in the packaging arena: the "wow" factor needed to differentiate products on store shelves, and lower costs. In particular, taking advantage of polypropylene's attributes, resin and additive suppliers have been improving the aesthetics of clarified PP (cPP). But nowadays, transparency alone is not enough to make a bottle or container stand out--hence the greater interest in both clarity and color.

See-Through Attention

With the right clarifying agent, cPP's aesthetics and production rates are competitive with those of PET, when used on the same equipment for injection stretch blow molding (ISBM), and other processes. For example, Milliken Chemical touts its Millad 3988 clarifying agent as speeding crystallization for ISBM pre-forms of beverage containers for fresh juice and dairy products. Other applications that can potentially use this additive include cosmetic bottles, food-storage containers, and optical-storage media (DVDs).

The Millad 3988 clarifying agent was recently demonstrated in a line of "grippy" concept bottles, molded in Marlex resin from Phillips Sumika Polypropylene Co. These "aggressively tactile" bottles, designed to provide a firm grip for products in the personal-care market, are made by extrusion blow molding (EBM). They provide a see-through clarity that transmits the color of the liquid inside, and they have a higher drop-height impact resistance than standard EBM resins, says Phillips Sumika.

This packaging is intended to stand out on a shelf, says Milliken technical account manager Kathryn Neal. "In North America, the demand for clear product packaging continues to grow, as research has shown that consumers consistently select clear packages over products in opaque containers." As an example of the trend, Milliken cites research indicating that more than 60% of consumers prefer see-through plastic bottlecaps to opaque caps.

Along with transparency, consumers like color, driving the development of colorants for cPP. Milliken's ClearTint colorants, for use with the Millad clarifying agent, reportedly use pigments that are non-migrating and FDA-compliant. In addition to food and beverage closures, colored cPP is targeted at detergent, shampoo, and pharmaceutical bottles.

Color vs. Clarity

But how do colorants interfere with the desired transparency of cPP? To varying extents, different pigments block and scatter light, causing haze. The resulting give-and-take between color and clarity has been studied by Axel Grimm of BASF's GBU Performance Chemicals (Ludwigshafen, Germany).

"Most pigments have a negative influence on the transparency of the PP bottle; therefore a wise selection of the appropriate pigments needs to be made," says Grimm. To understand the effects of different pigments, he and co-researcher Juergen Weiser looked at 39 organic and inorganic colorants, in various colors and with a wide range of pigment particle sizes. They molded the colorants into 2-mm plaques of random copolymer and metallocene cPP, and measured the plaques' haze and clarity.

Haze measurements were critical since they relate to what the human eye interprets as "transparency," says Grimm. In more technical terms, the researchers defined haze as the percentage of light that, in passing through a sample, deviates from the incident light beam by more than 2.5[degrees] (as per ASTM D1003).

They found that in general, colored random PP provides better (lower) haze values than metallocene-based resin, and that larger pigment particle sizes diminish transparency. In particular, inorganic pigments perform poorly, owing to relatively large mean particle sizes (generally greater than 500 nm).

However, the results showed that it's wrong to assume that smaller pigment particle sizes ensure good haze numbers, "especially when the investigated pigment is hard to disperse," says Grimm. Completely dispersed pigments improve clarity, while colorants introduced into the resin as powders show poorer haze values. In contrast to dispersed powders, the best haze values result from fully dispersed pigments in carrier resins, as with BASF's Eupolen pigment preparations, which contain no agglomerates. "These agglomerates, as a result of poor dispersion, act like large pigment particles and reduce the transparency of the material dramatically," he adds.

The researchers also found that pigment particle shape can affect transparency more than particle size. For example, for two organic yellow pigment types that have needle-shaped particles, the type with over twice the mean particle size (1310 nm vs. 571 nm) gave haze measurements similar to those of the sample with the smaller mean particle size. This is because the needle diameters were almost the same, even though the needle lengths were much different.

The researchers' future work may determine differences in haze caused by pigment chemistry (comparing phthalocyanine and isoindoline pigments, for example). Moreover, pigment concentration is also a key haze factor for some pigments, but not for others. "Whether an increase of the concentration will eventually increase the haze significantly will be a subject for further investigations."

cPP vs. PET?

Despite the high clarity possible with tinted cPP, the competition between PET and PP in colored bottles is still heavily determined by resin properties. "In contrast to PET, PP cannot be colored with dyes, but requires pigments for coloration in order to prevent migration from the semicrystalline PP material," says Grimm. PP also has gas-barrier limitations, although it is better for hot-fill bottle applications because of its higher heat stability. And it has density and cost advantages that make it suitable for cleaning-product and shampoo bottles, where "there is a move away from PET toward PP due to pricing of the raw materials," Grimm adds.

More cPP will be seen on store shelves, given the continuing double-digit annual growth expected for PP in food and non-carbonated beverage packaging. But will this growth be at the expense of PET? Not all PET packaging specialists seem particularly concerned, as indicated by discussions at a 2005 industry forum hosted by German information source PETnology GmbH. Improvements in cPP are seen "less as a direct competitive threat to PET and much more as a chance to support the success of two-stage stretch blow molding in the market," states PETnology's summary of the conference. Moreover, not placing all bets on PET, packaging specialists are carefully analyzing the costs of PP bottles for each potential packaging application.
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Title Annotation:NORTH AMERICA
Comment:Both clarity & color push PP packaging: a synergy of colorants and additives brings clarified polypropylene into more container and bottle applications.(NORTH AMERICA)
Author:Tolinski, Michael
Publication:Plastics Engineering
Geographic Code:1USA
Date:Jan 1, 2007
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