Nucleating and Clarifying Agents

tech.topicEquistar Technical Tip Additives: Nucleating and Clarifying Agents which the PP chains can crystallize. The numerous nucleatingsites also increase the amount of crystallinity in the resin and decrease the average size and narrow the size range of the spherulites (Figures 3 and 4). Nucleators and clarifiers also increase the stiffness of the end product and slightly reduce the impact strength.Figure 2: DSC cooling curve for a nucleated random copolymer PP showing the increase in onset of cooling temperature (120.6°C) and therefore a decrease in the degree of supercooling Figure 3: Optical microscopy slide showing the crystal structure of a PP resin without a nucleating agent.(Photo Courtesy of Milliken Chemicals)n NUCLEATING AGENTS Nucleating agents are generally inorganic materials with a small average particle size and a high melting point. When a nucleated resin is melted in an extruder, the nucleating agent remains solid and provides a site around which the PPspherulites can form.(Continued on Page 2)Heat Flow (W/g)116.9°C 120.6°C 84.7 J/g COOLING CURVE 40901401903210-1-2Temperature (°C)As polypropylene (PP) cools from the molten state, individual chains begin to form crystalline structures around nucleationsites. These nucleation sites can either be imperfections or inconsistencies in a polymer chain or foreign particles in themelt. As the polymer continues to cool, more chains crystallize around the nucleation site, forming spherulites.When PP cools completely, it is made up of approximately60% crystalline and 40% amorphous, or non-crystalline,areas. The crystallization rate of PP is increased by thepresence of foreign particles in the melt around which the polypropylene chains can crystallize.While it is cooling, PP also undergoes supercooling.Supercooling refers to the difference between the melting temperature and the temperature at which crystallization ofthe polymer begins. This temperature is called the "onset of crystallization temperature." For example, PP randomcopolymers (without a nucleating agent) melt at about 145°C(293ºF) but begin to crystallize at about 112°C (234°F). This temperature difference means that molded parts must becooled well below the melting point of PP before the resin begins to crystallize (see Figure 1).Figure 1: Differential Scanning Calorimetry (DSC) cooling curvefor a non-nucleated random copolymer PP. Note the degree ofsuper cooling from a melt temperature of 147°C to the onset ofcrystallization at 108.3°CNucleating agents (nucleators) and clarifying agents(clarifiers) work by altering the way the PP chains crystallizeand agglomerate in the molten state. Nucleators and clarifiers also reduce the degree of supercooling in the resins byincreasing the onset of crystallization temperature (see Figure2). These agents provide numerous nucleating sites around COOLING CURVE108.3°C88.8 J/g102.7°C3210-1-220Heat Flow (W/g)70120170Temperature (°C)Equistar Technical Tip Additives: Nucleating and Clarifying Agents (Continued)Figure 4: Optical microscopy slide showing the crystal structure of a PP resin with a nucleating agent. Note the smaller and more uniform spherulite structure as compared to Figure 3.(Photo Courtesy of Milliken Chemicals)The most effective nucleating agents are chemical derivatives of benzoic acid. Because of its strong nucleating ability and low cost, the most widely used nucleating agent is sodium benzoate, a synthetic, white, opaque powder. Other nucleating agents include naturally occurring minerals such as kaolin and talc.Adding a nucleating agent to PP increases crystal growth, and this growth occurs faster and at a higher temperature (15 to 20°C) than without the additive. Because the degree of supercooling is reduced, parts made from nucleated PP resins can be removed from the mold at higher temperatures and as a result, cycle times are decreased and output raised.However, in multi-cavity and thin-wall injection molds,nucleating agents may cause the resin to solidify before the mold cavity is completely filled, resulting in short shots.Another potential downside is that sodium benzoate is known to interfere with red pigments, causing them to turn orange.Nucleating agents are also not typically used to stiffen random copolymer PP resins because clarity decreases.n CLARIFYING AGENTS Clarifying agents (or clarifiers) are usually organic, non-polymeric molecules. All clarifying agents also act as nucleating agents, but nucleating agents are not necessarily clarifying agents. Most clarifiers on the market today are chemical derivatives of dibenzylidene sorbitol (DBS) and have melting temperatures within the processing window of PP resins.Clarifying agents melt during processing and disperse in the polymer matrix. When the molten polymer cools, the clarifier solidifies at the same temperature at which it melted.At this point, PP crystallization starts — at a higher temperature than it would have without the clarifier. So, as with nucleating agents, parts can be removed from the mold sooner and cycle times go down.Clarifiers also increase the clarity of PP by reducing the size of the spherulites. Smaller spherulites allow more light through the polymer, which decreases the haze of the part. Unlike nucleating agents, clarifiers are transparent, which also helps decrease haze values.Care must be taken in processing clarified random copolymer PP resins because if too high of a melt temperature is used,the clarifier degrades, and haze values go up. Parts molded from clarified resins may have a slight, cherry or almond-like odor, but a very strong smell near the molding machine can be a sign of clarifier degradation.Too little mixing (low back pressure) or a low melt temperature may keep the clarifying agent from properly dispersing. White specks in the molded part are usually an indication of poor clarifier melting and dispersion. The recommended melt temperature processing range for clarified PP is between 193 and 232°C (380 and 450°F).To maximize the clarity of the pad, the melt temperature should be the lowest possible temperature at which the clarifier disperses properly. A high gloss mold is also necessary to achieve the best clarity. In injection molding,reduce the packing pressure, since high pressures can cause the polymer chains to "pack," resulting in higher haze values.To achieve the best possible clarity in blow molding, use an injection stretch blow molding machine. Injection blow molding machines produce parts with lower clarity and parts from extrusion blow molding machines are even less clear,comparatively.Two problems may arise from the use of a clarified resin.First, clarifiers perform best when processed within the normal processing window for PP. If the resin is processed at too high a temperature, unwanted odor and taste characteristics (organoleptics) occur in the end product. The clarity of the part also decreases if the processing temperatures are too high. If the resin is processed at too low a temperature, poor dispersion and a decrease in clarity may result.Second, because the clarifying agent causes the part to set up at a higher temperature, ejecting the part from the mold may be difficult. Poor ejection can usually be prevented by increasing the mold temperature or decreasing the cooling cycle so that parts eject at a higher temperature. Anti-static agents are usually added to the clarified resins to enhance mold release and to aid in clarifier dispersion.。