Clarifi ers & nucleators | additives
Nucleators can improve the mechanical performance of PP while enhancing processibility, sometimes allowing major cycle and energy savings. Peter Mapleston explores the latest developments
Making progress in nucleation
Suppliers of speciality nucleating additives that modify the crystallinity of polypropylene continue to make measured progress in developing better functioning and more cost-effective products. Nucleating additives provide an increased number of sites for the initiation of crystallization during cooling of the polypropylene resin, so spherulites are smaller and more abundant than in non-nucleated polymer. The result can be signifi cant improvements in mechanical properties such as stiffness, enhanced heat resistance, more controllable shrinkage, and higher clarity (smaller crystals scatter light less). There are also potential processing gains; nucleation raises the crystallisation temperature and the rate of crystallisation of the polymer, which in turn makes it possible to reduce cooling times. The trend in nucleating agent development in recent
years has been to develop higher performance products that further improve stiffness and clarity in particular, says Philip Jacoby, who set up a consultancy in 2014 after spending many years as technology VP with additive producer Mayzo. “Stiffness improvement has been of particular importance in automotive applica- tions since this facilitates wall thickness reduction leading to lighter weight components and improved fuel effi ciency of the vehicle,” he points out. Down-gauging is also an important trend in packaging applications. Purpose-designed nucleating agents have made
considerable progress over the years. Starting with additives such as talc and whisker-like sodium
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benzoate, as well as other particulates (pigments have a nucleating effect too), and passing through a second generation - still current - that includes phosphate ester salts (also with whisker-shaped crystals) and sorbitols. So-called third generation types differentiate themselves with a more plate-like crystalline structure that induces a different type of orientation in the polymer lamellae.
Targeting shrinkage Milliken says its Hyperform HPN-20E induces lower and more isotropic shrinkage in PP than earlier generations since the polymer lamellae can grow in more directions. This gives improved aesthetics and required addition rates are much lower (six times less than with sodium benzoate, and 30 times less than talc). Hyperform HPN-20E also provides a better balance of stiffness and impact strength, the company says. The main downside is that the crystalline melting point, Tc, is not the highest achievable. Milliken says HPN-20E is intended for injection moulding materials; the company also offers Hyperform HPN-600ei, which promotes similar crystal growth but is aimed at polymers for thermoformable sheet. In addition, Milliken offers Hyperform HPN-68L, which offers a very high Tc, but shrinkage is higher (although isotropic) and improvements in transparency are less marked. This additive tends to be used more in masterbatches, to override the nucleation effect of
Main image: Milliken’s NX8000 clarifi er technology creates a fi brous network of crystals in PP to achieve outstanding clarity
May 2016 | COMPOUNDING WORLD 61
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