{"id":1916,"date":"2015-01-30T08:00:52","date_gmt":"2015-01-30T13:00:52","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=1916"},"modified":"2015-09-16T15:57:45","modified_gmt":"2015-09-16T20:57:45","slug":"pe-%ce%b2-nucleating-agents-polypropylene","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/1916\/pe-%ce%b2-nucleating-agents-polypropylene\/","title":{"rendered":"\u03b2 Nucleating Agents for Polypropylene"},"content":{"rendered":"

By Moris Amon<\/a><\/em><\/p>\n

Isotactic homo-polypropylene<\/a> (PP) is a semicrystalline polymer of great commercial importance. In its solid state, microscopic crystalline and amorphous domains intermingle. The crystalline unit cell is usually monoclinic. This crystal configuration is called the\u00a0\u03b1 (alpha) form. There is also a \u03b2 (beta) form with hexagonal unit cells, which is usually present in small quantities, if at all \u2013 unless special processing conditions or special nucleating agents described in this article are used. The physical properties of the \u03b1 and\u00a0\u03b2 forms differ, and this difference can be used to advantage in certain applications, as will be explained.<\/p>\n

How can\u00a0\u03b2 crystals be detected?<\/strong><\/h3>\n
\"Figure<\/a>
Figure 1: DSC scan of a purely \u03b1 crystalline PP (source: US Patent 7,407,699). Click for a larger image<\/strong>.<\/figcaption><\/figure>\n

The most accurate way of determining the\u00a0\u03b2 content of a PP sample is wide-angle x-ray diffraction (WAXD). The\u00a0\u03b2 phase shows a single strong diffraction peak due to the (300) crystal plane, while the\u00a0\u03b1 phase shows three strong peaks due to the (110), (130), and (040) planes. The fraction of\u00a0\u03b2 in the overall crystalline phase, called the \u201cK value\u201d can be calculated as the ratio of the intensity of the (300) peak to the sum of the intensities of all four peaks. When K=0, all the crystals are \u03b1; when K=1, they are all \u03b2.<\/p>\n

Not all analytical labs have WAXD instruments. A more commonly available technique is differential scanning calorimetry (DSC) whereby the temperature of a small sample is raised at a controlled, constant rate (usually expressed in \u00b0C \/ min), and the variable heat input (W\/g) needed to maintain this rate is measured. The heat input will show peaks (endotherms) when crystals melt. These peaks are broad because the melting points of individual crystals\u00a0depend on their size and their level of perfection, both of which exhibit a range in industrial\u00a0PP.<\/p>\n

\"Figure<\/a>
Figure 2: DSC of a mixed \u03b1+\u03b2 crystalline PP (source: US Patent 7,407,699). Click for larger image.<\/figcaption><\/figure>\n

However, the locations of the peak maxima along the temperature axis are distinct for the\u00a0\u03b1 and\u00a0\u03b2 phases: typical values are 165-168\u00b0C for the \u03b1\u00a0and 152-155\u00b0C for the \u03b2. The ratio of the area under the\u00a0\u03b2 peak to the total area under both peaks gives an alternative K value. Because the broad DSC peaks overlap, whereas the narrow WAXD peaks do not, the latter method is more accurate. Examples of DSC scans of purely\u00a0\u03b1 and mixed \u03b1+\u03b2 PP are shown in Figures 1 and 2, respectively.<\/p>\n

How can one generate more\u00a0\u03b2 crystals?<\/strong><\/h3>\n

In most common PP products, whether extruded, blown, spun or molded, the K value is below 0.1 and often close to zero. Researchers have found that it can be raised by crystallizing the polymer from the melt very slowly at high temperatures (above 120\u00b0C). This is not practical since processors usually want to run faster, not slower, for economic reasons. There is also evidence indicating that crystallization at high shear rate favors\u00a0\u03b2 crystals slightly, (e.g. where the polymer flows rapidly very close to a mold wall), but such shear rates cannot be obtained throughout the thickness of a product.<\/p>\n

The practical way to raise the K value is to use special nucleating agents. Nucleating agents are additives finely dispersed into the polymer at low concentrations to seed crystals during solidification from the melt. They increase the speed of crystallization, and often, the final crystallinity (i.e. the ratio of crystalline mass to the total of crystalline and amorphous masses.) The well-known nucleating agents, such as sodium benzoate and sorbitol derivatives, help the formation of\u00a0\u03b1 crystals only. The specific\u00a0\u03b2 nucleating agents reported in the literature include the following:<\/p>\n