Incorporating finely divided solid particles into semi-crystalline polymers such as polyolefins and polyamides can induce nucleation of fine polymeric crystals. An effective nucleating agent provides an active surface for polymer adsorption, providing a higher degree of crystallinity and a more uniform crystalline structure as the melt is cooled.
The most effective solid-particle nucleating agents have a high surface energy as well as a high surface area, generally resulting from a finer particle size. A high surface energy and high specific surface area increases the number of nuclei or spherulites formed and increases polymer chain mobility at higher crystallization temperatures, thus allowing faster crystallization rates.
The advantages in increasing the polymer crystallinity by addition of a nucleating agent include:
- Improvement in the stiffness or flexural modulus of the composite
- Improvement in the heat deflection temperature
- Improvement in injection molding cycle times, where parts can be removed more quickly from the mold.
Talc as a Nucleating Agent
Talc is a commonly used nucleating agent in polypropylene and other polyolefins. As little as a few tenths of a percent of an ultrafine talc will provide effective nucleation, with up to 5 percent by weight of larger talc particles being used for the purpose.
Here is an example of using a range of low levels of MicroTuff® AG609, a 0.8 micron ultrafine talc, for nucleation of a polypropylene homopolymer. Crystallization was followed using differential scanning calorimetry (DSC). For the measurement of crystallization peak temperature, samples were heated to 230oC, then cooled at a rate of 10°C/min. Isothermal (125oC) peak time to crystallization was measured by heating the samples to 230oC, quenching to 125oC, and holding at 125oC.
The polypropylene has a higher crystallization peak temperature and shorter isothermal crystallization time. The addition of the MicroTuff® AG609 talc to the polypropylene allowed the polymer to crystallize at a higher temperature compared to the unfilled (barefoot) control. Under the isothermal conditions, the presence of the talc substantially reduced the time required for polypropylene crystallization. This translates to faster production with lower molding cycle times.
Higher Reinforcement of Talc-Nucleated Polypropylene
Reinforcement of the polypropylene is a further benefit resulting from improved crystallinity after addition of MicroTuff® AG609 talc. With 0.5 percent ultrafine talc, both the flexural modulus and the heat deflection temperature are increased.
Specialty Minerals Talcs For Polyolefin and Polyamide Nucleation
Specialty Minerals Inc. (SMI) has developed several grades of talcs that are highly effective nucleating agents for crystalline polymers. These talc-based nucleating agents have a fine particle size, a platy morphology, and a high surface area for enhanced nucleation. The MicroTuff® AG talc products are appearance grade, coated Montana talcs. For any given particle size, they have higher surface areas, which should produce greater nucleation. For white, pastel or other color-sensitive products, the FlexTalc® talc products, made from high-brightness Chinese ores, are recommended. The choice of talc will depend on costs and physical requirements of the final composite. The finest particle size and highest loadings will have the greatest effect on raising the crystallization temperature.
Click on a product name to download that product’s technical data sheet. Click here to download an information sheet on nucleation with SMI’s MicroTuff® AG talcs.
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