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.

I. Role of Nucleating Clarifying Agents in PP

Crystallinity 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.

II. 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.

III. 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.

α Phase

1. This phase is more stable and well-known.

2. These crystals belong to the monoclinic crystal system.

β Phase

1. This phase is metastable, and its crystals belong to the pseudo-hexagonal crystal system.

2. The β phase mainly exists in block copolymerized polypropylene and can be generated by adding specific nucleating agents.

3. This crystal form was discovered by Padden and Keith in 1953; it can be promoted by crystallization between 130°C and 132°C, high-shear orientation, or the addition of specific nucleating agents.

4. The presence of the β phase in polypropylene homopolymers usually improves the ductility of the finished product, and the effect is most significant when the β phase content reaches 65%.

γ Phase

1. This phase is also metastable, with triclinic crystals.

2. This crystal form is uncommon; it mainly appears in low-molecular-weight polypropylene and is formed by crystallization under extremely high pressure and extremely low cooling rates.

Ⅳ. 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.

Ⅴ. Nucleators and Clarifiers: A Rich Panel of Additives

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.

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.

Particle Shape and Aspect Ratio

Nucleant particles with needle-like shapes (like ADK STAB 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 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 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.

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.

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:

a.If low haze and high clarity is important, then choose one of the soluble clarifiers.

b.For lower clarity requirements, the phosphate esters can be used.

c.If high modulus is of greatest importance, then choose one of the phosphate esters.

d.If low cost is of most importance, then choose sodium benzoate.

e.If low warpage and low pigment sensitivity is of most importance, then choose 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.

Run DSC on the nucleated PP resin. Improvements in cycle time generally correlate with increases in the crystallization temperature (Tc). Test properties of molded specimen.

If you want to inquire about products related to nucleating agents, please feel free to contact us at any time.