Nucleation and reinforcement | additives feature
The nucleation and reinforcement of polypropylene is closely linked to orientation effects. Darin Dotson of Milliken provides some valuable insights into how they interact
New insights into the nucleation and reinforcement of PP
While the terms “nucleation” and “reinforcement” are often used in the polypropylene (PP) industry, the two terms are not interchangeable and can sometimes lead to confusion when describing the mechanism of action. In general terms, nucleating agents “reinforce” PP
by increasing the amount of crystallinity in the resin, as well as directing specific crystalline orientations within the polymer matrix. While modulus, HDT and tensile properties are usually increased with nucleation, the degree of change most often results from orientation differences in the PP articles. The nucleating particles themselves do not contribute to physical property enhancements. True reinforcing agents, however, increase the modulus and HDT of PP much more significantly by orienting themselves very strongly in the polymer matrix during processing, and these “stiff” particles actually increase the bending modulus of PP. Reinforc- ing agents are used at higher loadings than nucleators, and typically do not nucleate themselves, although there are exceptions. For example, talcum is a moder- ate nucleating agent for PP at low levels (<1%), but becomes a true reinforcing agent at higher loadings. Both nucleating agents and reinforcing agents have something in common: the particles must have enough aspect ratio to orient themselves strongly in polymer melt flows during processing.
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Nucleation insights Grades of PP that contain nucleating agents were
first introduced in the late 1960s. In the past, the primary benefits of using nucleating agents in PP were to increase the processing speed and to enhance physical properties, such as stiffness. Later “advanced” nucleators provided other benefits such as improved clarity and higher heat distortion temperatures (HDT). These benefits added value to PP resins for both producers and users. The original nucleating agents used in PP were
typically aromatic carboxylic acid salts, with sodium benzoate the most commonly used even until today. Some mineral fillers, like talc, and certain pigment colorants were also found to give a nucleation effect. Later “advanced” nucleators, such as the phosphate ester salts from Japan, further increased the crystalliza- tion speed and modulus of PP. One common disadvan- tage of these more powerful nucleating agents, however, was their tendency to induce very strong and anisotropic crystalline orientation to PP articles, particularly in the machine versus the transverse (MD/TD) directions of flat parts. This in turn led to post-mould warpage due to differential shrinkage and an unbalanced orientation which even further manifested itself in processes like solid phase pressure forming (SPPF). In 2002, 2006 and 2009, Milliken launched three
April 2012 | COMPOUNDING WORLD 33
Figure 1. PP spherulites
growing from the melt
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