Clarifying and Nucleating Agents

Clarifiers usually have some compatibility with the polymer. Some have sufficiently low melting points to melt in the polymer and form a network of nucleation sites, whereas others remain solid, providing a more limited number of sites. 

Examples of clarifiers include a number of sorbitol derivatives such as bis (3, 4 dimethyl benzylidene) sorbitol diacetal, and alumiitium hydroxy bis (4-tert-butyl) benzoate. 

The polypropylene industry is the main user of clarifying additives. Unclarified PP products tend to be hazy, whereas optical clarity is in great demand for containers and other packaging products. PVC, PET and polycarbonate can provide better visual appearance than unmodified polypropylene. 

The clarity achieved with these clarifiers in PP does not quite match that of PET. Incidental benefits include easier processing, although sometimes a change in crystallisation behaviour can introduce undesirable internal stresses and distortion. 

The other additives in the formulation need to be considered because of possible interactions. Benzoates react with stearic acid, reducing their effectiveness. 

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Early clarifying and nucleating agents sometimes resulted in difficult ejection from the mould, or gave rise to organoleptic problems. The suppliers have addressed these issues in recent years. 

Figure 2 Correlation of haze and nucleating efficiency for selected clarifiers and nucleating agents in a PP random copolymer (O, ) commercial nucleating agent, (A) talc, (X, +, ) clarifier of sorbitol typ>e.

Performance criteria of clarifying agents are clarity, organoleptics, processability and nucleation. A typical third generation clarifying agent is the sorbitol acetal Millad 3988 (3,4-dimethyl dibenzylidene sorbitol). 

There appears to be no physical (e.g., crystal habit or morphology) or chemical (e.g., surface groups) correlation to identify which nucleating agent works best with which polymer matrix. However, it is known that dibenzylidene sorbitol works best with PP (it is sometimes called a clarifier because it enhances transparency), and inorganic benzoate salts are successful with PET. 

Polypropylene can have its clarity and transparency improved by orientation, or by use of clarifying/nucleating agents. 

Nucleation and clarity 10 High crystallization and processing rates, high haze in cloudy PE and PP Nucleating agents for higher properties, clarifying nucleators for clearer PP containers... 

Strength and modulus Fundamental tests for strength and stiffness, along with HOT testing, reveal to what extent a clarifier also serves as a useful nucleating agent. 

Polypropylene blown film which has excellent optical and mechanical properties was obtained using nucleating agents. The polypropylene composition contains a clarifier selected from the group of aromatic phosphoric acid ester metal salts and almniniitm-hydroxy-bis[2,2 -methylene-bis(4,6-di-t-butylphenyl)phosphate] in the qirantities from 0.01 to 5 wt%. ... 

Polypropylene nonwovens or yams are produced by extruding a mixture of polypropylene and P-nucleating agent or certain clarifiers to form the improved polypropylene filaments." The P-nucleating agent comprises at least one of N, N -dicyclohexy 1-2,6 naphthalene dicarboxamide and NJ-Star NU-lOO from Rika."... 

The results presented above and Figure 1 indicate that transparency is closely related to the crystallization and the crystalline structure of the polymer. Any effect which influences these factors will change transparency as well. Several publications indicate that the introduction of heterogeneous nucleating agents or clarifiers into PP homo- and copolymers considerably improves the transparency of the product. 

Transparency of PP products can be efficiently improved both with traditional nucleating agents and clarifiers, which are various sorbitol derivatives. The improvement in the optical properties depends on the efficiency in destroying the spherulitic structure of PP or at least decreasing the size of the spherulites below a critical value. Strong nucleation... 

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