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Additives: Nucleating and Clarifying Agents

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As polypropylene (PP) cools from the moltenstate, individual chains begin to form crystallinestructures around nucleation sites. Thesenucleation sites can either be imperfections orinconsistencies in a polymer chain or foreignparticles in the melt. As the polymer continuesto cool, more chains crystallize around the

nucleation site, forming spherulites.

When PP cools completely, it is made up ofapproximately 60% crystalline and 40%amorphous, or non-crystalline, areas. Thecrystallization rate of PP is increased by thepresence of foreign particles in the melt aroundwhich the polypropylene chains can crystallize.

While it is cooling, PP also undergoessupercooling. Supercooling refers to thedifference between the melting temperature andthe temperature at which crystallization of thepolymer begins. This temperature is called the

"onset of crystallization temperature." Forexample, PP random copolymers (without anucleating agent) melt at about 145C (293F) butbegin to crystallize at about 112C (234F). Thistemperature difference means that molded partsmust be cooled well below the melting point ofPP before the resin begins to crystallize (seeFigure 1).

Nucleating agents (nucleators) and clarifyingagents (clarifiers) work by altering the way thePP chains crystallize and agglomerate in themolten state. Nucleators and clarifiers alsoreduce the degree of supercooling in the resins

by increasing the onset of crystallizationtemperature (see Figure 2). These agentsprovide numerous nucleating sites around whichthe PP chains can crystallize. The numerousnucleating sites also increase the amount ofcrystallinity in the resin and decrease theaverage size and narrow the size range of thespherulites (Figures 3 and 4). Nucleators andclarifiers also increase the stiffness of the endproduct and slightly reduce the impact strength.

COOLING CURVE

108.3C88.8 J/g

102.7C3

2

1

0

-1

-2

20

Heat Flow (W/g)

70 120 170

Temperature (C)

Figure 1:Differential Scanning Calorimetry (DSC)cooling curve for a non-nucleated randomcopolymer PP. Note the degree of super coolingfrom a melt temperature of 147C to the onset ofcrystallization at 108.3C

Heat Flow (W/g)

116.9C

120.6C

84.7 J/gCOOLING CURVE

40 90 140 190

3

2

1

0

-1

-2

Temperature (C)

Figure 2:DSC cooling curve for a nucleatedrandom copolymer PP showing the increase inonset of cooling temperature (120.6C) andtherefore a decrease in the degree ofsupercooling

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Additives: Nucleating and Clarifying Agents(continued)

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Figure 3:Optical microscopy slide showing thecrystal structure of a PP resin without anucleating agent.Photo Courtesy of Milliken Chemicals) NUCLE TING GENTSNucleating agents are generally inorganic

materials with a small average particle size and ahigh melting point. When a nucleated resin ismelted in an extruder, the nucleating agentremains solid and provides a site around whichthe PP spherulites can form.

Figure 4:Optical microscopy slide showing thecrystal structure of a PP resin with a nucleatingagent. Note the smaller and more uniformspherulite structure as compared to Figure 3.Photo Courtesy of Milliken Chem icals)

The most effective nucleating agents arechemical derivatives of benzoic acid. Because ofits strong nucleating ability and low cost, themost widely used nucleating agent is sodiumbenzoate, a synthetic, white, opaque powder.Other nucleating agents include naturallyoccurring minerals such as kaolin and talc.

Adding a nucleating agent to PP increases crystalgrowth, and this growth occurs faster and at ahigher temperature (15 to 20C) than without theadditive. Because the degree of supercooling isreduced, parts made from nucleated PP resinscan be removed from the mold at highertemperatures and as a result, cycle times aredecreased and output raised.

However, in multi-cavity and thin-wall injectionmolds, nucleating agents may cause the resin tosolidify before the mold cavity is completelyfilled, resulting in short shots. Another potentialdownside is that sodium benzoate is known to

interfere with red pigments, causing them to turnorange. Nucleating agents are also not typicallyused to stiffen random copolymer PP resinsbecause clarity decreases.

CL RIFYING GENTSClarifying agents (or clarifiers) are usuallyorganic, non-polymeric molecules. All clarifyingagents also act as nucleating agents, butnucleating agents are not necessarily clarifyingagents. Most clarifiers on the market today arechemical derivatives of dibenzylidene sorbitol(DBS) and have melting temperatures within theprocessing window of PP resins.

Clarifying agents melt during processing anddisperse in the polymer matrix. When the moltenpolymer cools, the clarifier solidifies at the sametemperature at which it melted.

At this point, PP crystallization starts at ahigher temperature than it would have withoutthe clarifier. So, as with nucleating agents, partscan be removed from the mold sooner and cycletimes o down.

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The information on this document is, to our knowledge, true and accurate. However, since the particular uses and the actualconditions of use of our products are beyond our control, establishing satisfactory performance of our products for the intendedapplication is the customer's sole responsibility. All uses of Equistar products and any written or oral information, suggestions or

technical advice from Equistar are without warranty, express or implied, and are not an inducement to use any process or product inconflict with any patent.

Equistar materials are not designed or manufactured for use in implantation in the human body or in contact with internal body fluidsor tissues. Equistar makes no representation, promise, express warranty or implied warranty concerning the suitability of thesematerials for use in implantation in the human body or in contact with internal body tissues or fluids.

More detailed safety and disposal information on our products is contained in the Material Safety Data Sheet (MSDS). All users of ourproducts are urged to retain and use the MSDS. A MSDS is automatically distributed upon purchase/order execution. You mayrequest an advance or replacement copy by calling our MSDS Hotline at (800) 700-0946.

Lyondell Chemical Company1221 McKinney, Suite 700P.O. Box 2583Houston, Texas 77252-2583(800) 615-8999http://www.Lyondell.com

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Additives: Nucleating and Clarifying Agents(continued)

Clarifiers also increase the clarity of PP byreducing the size of the spherulites. Smallerspherulites allow more light through thepolymer, which decreases the haze of the part.Unlike nucleating agents, clarifiers aretransparent, which also helps decrease hazevalues.

Care must be taken in processing clarifiedrandom copolymer PP resins because if too highof a melt temperature is used, the clarifierdegrades, and haze values go up. Parts moldedfrom clarified resins may have a slight, cherry oralmond-like odor, but a very strong smell nearthe molding machine can be a sign of clarifierdegradation.

Too little mixing (low back pressure) or a lowmelt temperature may keep the clarifying agentfrom properly dispersing. White specks in themolded part are usually an indication of poor

clarifier melting and dispersion. Therecommended melt temperature processingrange for clarified PP is between 193 and 232C(380 and 450F).

To maximize the clarity of the pad, the melttemperature should be the lowest possibletemperature at which the clarifier dispersesproperly. A high gloss mold is also necessary to

achieve the best clarity. In injection molding,reduce the packing pressure, since highpressures can cause the polymer chains to"pack," resulting in higher haze values. Toachieve 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 blowmolding machines are even less clear,comparatively.

Two problems may arise from the use of aclarified resin. First, clarifiers perform bestwhen processed within the normal processingwindow for PP. If the resin is processed at toohigh a temperature, unwanted odor and tastecharacteristics (organoleptics) occur in the endproduct. The clarity of the part also decreases ifthe processing temperatures are too high. If theresin is processed at too low a temperature, poordispersion and a decrease in clarity may result.

Second, because the clarifying agent causes thepart to set up at a higher temperature, ejectingthe part from the mold may be difficult. Poorejection can usually be prevented by increasingthe mold temperature or decreasing the coolingcycle so that parts eject at a higher temperature.Anti-static agents are usually added to theclarified resins to enhance mold release and toaid in clarifier dispersion.

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