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Nucleating Agents & Clarifiers Selection Tips for Polypropylene

Polypropylene is a widely used polymer employed in diverse applications due to its excellent combination of properties. Its properties, such as physical, mechanical and optical can further be enhanced with suitable use of nucleating agents and clarifying agents. These additives assist in the crystallization of PP during processing, thus enhancing the already acquired properties.

Understand how to use nucleating agents and clarifying agents as well as get selection tips to effectively increase production rate, modify structure and morphology, and reduce haze in your polypropylene formulations. Review:


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We would like to acknowledge Dr. Philip Jacoby for providing technical information needed to develop this article.

Role of Nucleating & Clarifying Agents in PP


Nucleators and Clarifiers for PPCrystallinity of semi-crystalline polymers is responsible for many of the characteristics, such as dimensional stability, clarity, and toughness.

For a defined part and process, the crystallinity is controlled by the polymer structure, the formulation, and the processing conditions that result in a specific balance of heat build-up and cooling. Consequently, crystallinity is often heterogeneous, the heat history being different for the skin and the core of the parts or goods.

Nucleating agents and clarifiers speed up and tune the crystallization allowing to adjust the end properties of semi-crystalline polymers to the functional requirements.

In polypropylene formulations, adding nucleating agents (also called nucleators) result in improved performance & processing properties, such as:

  • Improved clarity and reduced haze
  • Improved strength and stiffness
  • Improved Heat Deflection Temperature (HDT)
  • Reduced cycle time
  • Reduced warpage and more uniform shrinkage
  • Reduced pigment sensitivity regarding property changes with different colors
  • Improved processability in certain applications

Thus, nucleation is a powerful way to improve the physical, mechanical, and optical properties of polypropylene. Clarity, dimensional stability, warpage, shrinkage, CLTE, HDT, mechanical properties and barrier effect can be improved by the careful choice of nucleators or clarifiers.

Let's discover the crystallinity of polypropylene (PP) in detail, along with its nucleation process, and types of nucleators & clarifiers available to make the right selection.


Polypropylene and Its Crystallinity


Polypropylene is a widely used crystalline, commodity polymer made from the polymerization of propene monomer. Upon polymerization, PP can form three basic chain structures (atactic, isotactic, syndiotactic) depending on the position of the methyl groups. The crystallinity of the polymer is characterized by:

  • The shapes and sizes of the crystallites
  • The crystallinity ratios, and eventually
  • The orientation of crystallites

Isotactic polypropylene (iPP) is a semi-crystalline polymer. It is characterized by an excellent cost to performance ratio, making it very attractive in a wide range of applications like automotive, appliances, piping, packaging, etc.

Isotacticity index of iPP is directly linked to the degree of crystallinity which has a major impact on polymer performance. Isotacticity increases crystallization kinetics, flexural modulus, hardness & transparency, and decreases impact resistance & permeability.

Table below compares properties of two polypropylene homopolymers having a different isotacticity index.

Properties Standard PP1 PP2 Unit
Density ISO R 1183 0.904 0.915 g/cm3
Isotacticity Index NMR C 13 95 98 %
Flexural Modulus ISO 178 1700 2300 MPa
Deformation Temperature ISO 75 102 131 °C
Permeability ASTM D 1434 40000 30000 cm3-µm/m2-d-atm
Effect of Isotacticity Index on iPP Properties


Facts to Know Isotactic Polypropylene was discovered by G. Natta in the '50s. Spheripol technology developed in 1988 allows today's production of polypropylene with isotacticity index higher than 97%.


Crystallization of Polypropylene


Depending on the conditions, Isotactic Polypropylene can crystallize into four different phases denoted α, β, γ and mesomorphic smectic. The α and β phases are the most important.

Alpha phase

α Phase

  • This phase is the most stable and the most known.
  • The crystals are monoclinic.
Beta phase

β Phase

  • This phase is metastable, and the crystals are pseudo-hexagonal.
  • β-phase is mainly found in block PP copolymers and can be generated by addition of specific nucleating agents.
  • This form was discovered by Padden and Keith in 1953 and can be improved by crystallization between 130 and 132°C or by orientation with high shear or through addition of specific nucleating agents.
  • Presence of β-phase in PP homopolymer generally increase ductility in the finished parts. Maximum effect is observed at 65% of β-phase.
Gamma Phase

γ Phase

  • This phase is also metastable with triclinic crystals.
  • This form is not very familiar but appears mainly in low molecular weight polypropylene by crystallization at very high pressure and very low cooling rate.


Nucleation Process in Polypropylene


It is well-recognized that the start-up point of crystallization of polymers is small germs (little particles) naturally included in the melt-like catalyst residues, impurities, dust, etc. It is then possible to modify and control crystalline morphology by the addition of "artificial" germs introduced in the polymer melt. This operation is called Nucleation.

  • Nucleators or nucleating agents are employed that provide sites for the initiation of crystals.
  • Clarifiers are a subfamily of nucleators that provide smaller crystallites that scatter less light and, as a result, enhance the clarity for the same wall thickness of a part.

The role of these nucleating agents is to improve the physical and mechanical properties of finished parts.


How Do PP Nucleating Agents Work?


A nucleating agent is typically characterized as an insoluble particulate (discussed next) that increases the rate of crystallization.

When semi-crystalline polymers crystallize from the melt (typically during the cooling phase of a process), the lamellae organize from a primary nucleus to form complex macro-structures called spherulites. These spherulites continue to grow until they impinge on an adjacent spherulite at which point the growth ceases.

Properties of the polymers, including optical and physical characteristics depend on:

  • The end size of the spherulite structures
  • The crystalline orientation in the matrix

In nucleated polypropylene, crystallization occurs earlier in the cooling process and happens at a faster rate. This allows decreased cooling time of the polymer. Also, nucleation density is much higher and crystal spherulite size is much smaller.

Figure below shows an illustration of the heterogeneous nucleation process versus a non-nucleated resin for comparison:

Crystallization process
Illustration of the Crystallization Process in Non-Nucleated or Nucleated Matrix


Polypropylene is recognized as a relatively easy material to nucleate. This is because the rate of crystallization is low enough to allow the nucleating agent to have a direct impact on the nucleation density. Further, the effect of a nucleating agent depends on numerous parameters like:

  • The nature of the polypropylene (homopolymer, random copolymer, block copolymer)
  • The melt index
  • The polydispersity index
  • The processing conditions, and even
  • The polymerization process

Nucleated PP molding formulations are often used for the production of thin-walled injection molded parts (< 0.4 mm) where stiffness is required. In some cases, cycle-time can be shortened by 30%. Nucleating agents are also used as clarifier for films, sheets and molded parts, particularly for random PP copolymers.



Nucleators and Clarifiers: A Rich Panel of Additives


Some of the commonly used nucleating agents for polypropylene are:



Particulate Nucleating Agents


Particulate nucleating agents/ nucleants are typically high melting compounds which are dispersed in the polymer melt via compounding. These particles act as distinct “point nuclei” on which polymer crystal growth can commence.

  • The high concentration of nuclei leads to more rapid crystallization (shorter cycle times), and higher levels of crystallinity, which improves the strength, stiffness, and HDT of the PP.
  • The small size of the crystal aggregates (spherulites) leads to reduced light scattering and improved clarity.

The commonly used particulate nucleating agents include salts and minerals, such as talc, sodium benzoate, phosphate esters and other organic salts.

  • Talc and sodium benzoate are considered to be low performance, low cost nucleants, and provide a modest improvement in strength, stiffness, HDT, and cycle time.
  • The high performance, high cost nucleants, such as the phosphate esters and the bicycloheptane salts give better physical properties and some improvement in clarity.

Nucleating Agent Chemical Structure Strengths Weakness Cost & Loading
Sodium Benzoate
Sodium Benzoate
  • Low Cost
  • Only modest improvements in cycle time and stiffness
  • No improvement in haze
  • 600-1000 ppm
  • Low cost
Phosphate Esters NA-11 Phosphate Ester
  • Excellent improvements in stiffness, cycle time, and good clarity

  • Potential for warpage and non-uniform shrinkage
  • Pigment sensitivity

  • 600-1000 ppm
  • High cost
Bicycloheptane Salts Hyperform HPN-68L
  • Excellent stiffness & good clarity
  • Low warpage and uniform shrinkage
  • Less pigment sensitivity
  • Not as high a modulus as the Phosphate ester
  • 600 – 1000 ppm
  • High cost
Commonly Used Particulate Nucleating Agents


Soluble Nucleating Agents


Soluble nucleating agents, which are also referred to as “melt-sensitive”, typically have low melting points and dissolve in the molten PP.

  • As the polymer melt cools in the mold, these nucleants crystallize out first forming a finely distributed network with extremely high surface area.
  • As the temperature continues to drop the fibrils comprising this network function as nuclei to initiate the polymer crystallization.
  • The extremely high concentration of nuclei leads to very small PP crystal aggregates, which give the lowest level of light scattering and the best clarity.

All clarifiers are nucleants, but not all nucleants are good clarifiers.

Some common nucleants, such as sodium benzoate and talc, do not reduce spherulite size by a sufficient amount to give a low haze and high clarity molded part. The best clarity is generally achieved when soluble nucleants are used.

Soluble organic compounds which act as clarifiers include sorbitols, nonotols, trisamides.

Although these nucleants are mainly used to achieve high clarity and low haze, they also improve physical properties and reduce cycle time.

Nucleating Agent Chemical Structure Strengths Weakness Cost & Loading
Sorbitols
(Millad® 3988)
Millad 3988
  • Excellent clarity
  • Minimal plate-out
  • Very good organoleptics
  • High molding temperatures required
  • High loading levels
  • 1800 – 2500 ppm
  • High cost
Nonitols
(Millad® NX™ 8000)
Millad NX 8000
  • Highest possible clarity
  • Very good organoleptics
  • Minimal plate-out
  • Works with lower molding temperatures

  • Very high loadings required

  • 2000 – 4000 ppm
  • High Cost
Trisamides
(Irgaclear® XT 386)
Trisamides
  • Excellent clarity at low loading levels
  • Excellent thermal stability & ultra-low discoloration
  • No plate-out
  • Excellent organoleptics (low taste & odor)
  • Not as good clarity as the sorbitols and nonotols
  • 200 – 400 ppm
  • High cost of additive is tempered by the low addition levels needed
Commonly Used Soluble Clarifying Agents


Other Classification Aspects


Nucleators and clarifiers can also be classified according to their chemical family as well as crystal types.


Inorganic and Organic Nucleating Agents


Inorganic agents are, for example, talc or barium sulphate, nanoclays such as montmorillonite have a nucleating effect on polypropylene, thermoplastic polyesters, polyamide leading to faster nucleation rates and increased overall degrees of crystallization. Metal oxides, such as titanium dioxide or magnesium oxide, phosphates, carbonates or sulphates, of preferably alkaline earth metals, are also quoted.

Organic additives are very diversified from sorbitol derivatives up to mono- or polycarboxylic acids and the salts thereof, such as 4-tert-butylbenzoic acid, sodium or lithium benzoates, organophosphates or phosphate esters, adipic acid, diphenylacetic acid, sodium succinate or sodium benzoate; norbornane-carboxylic salt, polymeric compounds, such as ionic copolymers (ionomers), nitrogen and more or less complex molecules, for example, triphenodithiazine, dicyclohexyl-2,6-naphtalenedicarboxamide, pimelic acid with calcium stearate or quinacridone dye permanent red.


Classification According to the Favored Crystal Types: α, β, γ...


For example, the most effective α-nucleating agents are the sorbitol-based derivatives and the organic phosphates while the β-nucleating agents are among others triphenodithiazine, pimelic acid with calcium stearate or quinacridone dye permanent red.

Now, let's move our attention towards the factors to consider while selecting the suitable nucleating or clarifying agent in order to improve the performance of your polypropylene (PP) applications.


Factors Impacting Performance of Nucleants and Clarifiers


Particle Shape and Aspect Ratio


Nucleant particles with needle-like shapes (like NA-11) can lead to different shrinkage values in the machine and transverse directions. This shrinkage anisotropy can lead to warpage in the final part. Nucleant particles with a planer geometry, such as HPN-68L, can give more uniform shrinkage in the two directions leading to less warpage.


Particle Size & Particle Size Distribution


Smaller particle size leads to improved nucleation, but smaller particles can also be more difficult to disperse. Some nucleant particles, such as sodium benzoate, tend to re-agglomerate.


Acid Scavenger Used


Some acid scavengers, such as the fatty acid salts (e.g. calcium stearate) can be antagonistic towards certain nucleants, such as the phosphate esters and sodium benzoate. Dihydrotalcite (DHT-4A) should be used with these nucleants.

Never use calcium stearate with sodium benzoate since the calcium stearate will completely negate the nucleation of the sodium benzoate.


Degree of Dispersion & Presence of Undispersed Agglomerates


Sodium benzoate often forms agglomerates and is difficult to disperse properly.


Melt Temperature


Sorbitols require higher melt temperatures to give the best clarity, since they must fully dissolve in the polymer melt. Other soluble clarifiers, such as XT 386 dissolve in the PP at lower temperatures and are less melt-temperature sensitive.


How the Nucleant is Added?


Resin companies often incorporate the nucleant into the PP. If the pure nucleant powder is compounded in, it is best to use PP flake or powder, or use nucleant masterbatches.


Synergies and Antagonisms Between Nucleants and Other Additives


  • Acid scavengers can be synergistic or antagonistic. Fatty acid salts adversely affect the modulus of phosphate ester nucleated PP. DHT-4A (dihydrotalcite) improves the modulus of NA-11 nucleated PP and can allow lower addition levels of the nucleant to be used effectively.
  • In talc reinforced PP, NA-27 gives the highest modulus.
  • In pigmented PP, HPN-68L levels out the effect of pigment changes on modulus and reduces differential shrinkage and warpage.


Select the Right Nucleants and Clarifiers for PP


Before selecting the suitable nucleating or clarifying agent for your PP application, determine which property improvement you are most interested in:

  • If low haze and high clarity is important, then chose one of the soluble clarifiers.
  • For lower clarity requirements, the phosphate esters can be used.
  • If high modulus is of greatest importance, then chose one of the phosphate esters.
  • If low cost is of most importance, then chose sodium benzoate.
  • If low warpage and low pigment sensitivity is of most importance, then chose the bicycloheptane salt.

It is also imperative to decide how the nucleant will be incorporated into the PP resin. Always run appropriate tests to ensure that good dispersion and nucleation have been achieved.

  • Twin screw compounding is generally required unless well dispersed masterbatches of the nucleants are prepared first.
  • Run DSC on the nucleated PP resin. Improvements in cycle time generally correlate with increases in the crystallization temperature (Tc). Test properties of molded specimens.

Selecting Nucleating Agents for PP


Applications Scope: From Automotive to Consumer Goods


Nucleating agents increase several physical and mechanical properties of polypropylene, such as stiffness, impact properties, hardness, heat distortion temperature, etc. That leads to:

  • Downgauging, thin walling and weight reduction of the finished parts.
  • Reduction of cycle-times and increase of productivity.
  • Optimization of the dimensional stability solving shrinkage and warpage issues. So, in certain cases, post-machining can be eliminated.

Among the applications, some popular areas featuring nucleated/clarified polypropylene include (let us quote without claiming to be exhaustive):

  • Consumer products such as furniture, toys, sporting goods, lawn and garden furniture, flowerpots, planters, clothe hangers, toolboxes, tackle boxes, home/office/craft supplies, luggage, stationery...
  • Household applications such as refrigerator and dishwasher linings and parts, washing machine drums, small appliances such as blenders, toaster ovens, coffee pots, food processors, hair dryers, and electric can openers, waste containers, food storage containers, clothe, basket shelving, cooler linings, tableware, clothe storage containers, etc.
  • Automotive and transportation applications such as door panels, instrument handles, body panels, bumper fascias, battery cases, fan and radiator shields/shrouds, fluid containers, and others.
  • Medical applications such as syringes, labware, sharps containers, and other medical equipment and devices.

Nucleating agents and clarifiers speed up and tune crystallization allowing to adjust the end properties of semi-crystalline polymers to the functional requirements. Apart from the universal ratio quality/cost, nucleators and clarifiers progress toward a better efficiency allowing cost savings, better productivity, the improvement of organoleptic properties, the adaptation to bioplastics processing, the optimization of homo- and co-polymers of polypropylene to compete other thermoplastics such as PET, PVC and others.

How to Down-weight & Reduce Cost of Your PP Products?


Polymer ApplicationExplore the Latest News About Nucleating Agents

Polymer ApplicationCheck Out the Latest News About Clarifying Agents



About Dr. Philip Jacoby

Dr Philip JacobyDr. Phil Jacoby is the President of Jacoby polymer consulting. He received a Ph.D. in Physical Chemistry from the University of Wisconsin in Madison, Wisconsin. From 1975 – 2002 he worked for the Amoco Chemical company and BP Amoco as a Senior Research Associate in Polypropylene Product Development and R&D.

At BP Amoco, Dr. Jacoby helped to develop new polypropylene resin formulations for rigid packaging, film, and automotive applications. Dr. Jacoby holds 13 US patents and several international patents covering various polypropylene products, with emphasis on modifying the crystal structure of polypropylene. After leaving BP Amoco, he joined Mayzo Corporation as Vice President of Technology.

At Mayzo, Dr. Jacoby created a new business based on the use of Beta Nucleating agents in polypropylene. The beta nucleant mastebatches that Dr. Jacoby developed are used today in the production of microporous oriented PP films, thermoformed PP parts, and various other extruded and injection molded applications.

Dr. Jacoby is a past president of the Southern Section of the Society of Plastics Engineers (SPE), and a former board member of the Thermoforming Division of the SPE.

After retiring from Mayzo in March 2014, Dr. Jacoby started a consulting business (www.jacobypolymer.com) concentrating on Polypropylene, with particular emphasis on New Product Development, Problem Solving, and training & development courses.

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