Definition of nucleating agent
Nucleating agent is a new functional additive applied to polyethylene, polypropylene and other incompletely crystallized plastics to shorten molding cycle, improve transparency, tensile strength, rigidity, toughness and other physical and mechanical properties of products by changing the crystallization behavior of the resin, accelerating the crystallization rate, increasing the crystallization density and promoting the microfabrication of grain size.
Nucleating agent mechanism of action (PP products)
When the nucleating agent is added to the plastic, the nucleating agent particles play the role of crystal nuclei, that is, more nuclei are added externally, which provides a good hotbed for the crystallization of plastics, thus promoting crystallization.
In general, nucleating agent particles must have sufficient aspect ratio. However, nucleating agents in general tend to induce a strong and anisotropic crystallization orientation inside the material, especially in the longitudinal versus transverse direction (MD/TD) of flat parts. This different shrinkage and unbalanced orientation can be further revealed in some processes such as solid phase pressure forming (SPPF), which can lead to warpage after molding.
Therefore, a good nucleating agent should be able to achieve a balanced crystalline orientation in the longitudinal and transverse planes of the PP part, thus providing excellent dimensional stability and reducing warpage and shrinkage problems.
Classification of common nucleating agents
(1) Alpha crystalline nucleating agent:
Mainly improve the transparency, surface gloss, rigidity, heat deflection temperature of the product, and also known as transparent agent, permeability enhancer, rigidity enhancer. Mainly including bifurcated sorbitol (DBS) and its derivatives, aromatic phosphate salts, substitution of benzoic acid salts, etc., especially the application of DBS nucleating transparent agent is the most common.
Alpha crystalline nucleating agents can be divided into inorganic and organic according to the structure.
(1) Inorganic class
Inorganic nucleating agents mainly include talc, calcium oxide, carbon black, calcium carbonate, mica, inorganic pigments, kaolin and catalyst residues. These are the earliest developed inexpensive and practical nucleating agents, the most researched and applied are talc, mica, etc.
(2) Organic
(a) carboxylic acid metal salts: such as sodium succinate, sodium glutarate, sodium hexanoate potassium benzoate, lithium benzoate, sodium cinnamate, sodium β-naphthoate, etc.. Among them, benzoic acid alkali metal or aluminum salt, tert-butyl benzoic acid aluminum salt, etc. The effect is better, the history of use is longer, but the transparency is poor.
(b) phosphate metal salts: organic phosphates mainly include phosphate ester metal salts and phosphate ester alkali metal substances and their complexes, etc.. This type of nucleating agent is characterized by transparency, rigidity, crystallization speed, etc., but poor dispersion.
(c) sorbitol benzyl fork derivatives: the transparency, surface gloss, rigidity and other thermomechanical properties of the products have a significant improvement effect, and have good compatibility with PP, is a class of transparent nucleating agent is currently under in-depth study. Its good performance, low price, has become the most active development at home and abroad, the most varieties, the largest production and sales of a class of nucleating agents. There are mainly dibenzyl fork sorbitol (DBS), two (to a methyl benzyl fork) sorbitol (P-M-DBS), etc.
(d) high melting point polymer type nucleating agent: at present, there are mainly polyvinyl cyclohexane, polyvinyl pentane, ethylene/acrylate copolymer, etc.. It is poorly miscible with polyolefin resin and has good dispersion.
(2) β-crystal nucleating agent
They are designed to obtain polypropylene products with high β-crystalline content, and have the advantage of improving the impact resistance of the products without lowering or even raising the heat deflection temperature of the products, so that the two conflicting aspects of impact resistance and heat deflection can be balanced.
One group is a small number of thick ring compounds with quasi-planar structures.
The other group is composed of certain dicarboxylic acids with oxides, hydroxides and salts of metals of group IIA of the periodic table. It can modify PP by changing the ratio of different crystalline forms in the polymer.
Adding nucleating agent has the following effects
1、Shorten PP molding cycle
Adding nucleating agent can increase the recrystallization temperature of polypropylene, speed up the crystallization rate and complete the crystallization in a short cooling time. Adding 0.2% of RQT-CH can shorten the molding cycle of PP by 7s and increase the working efficiency by 14%, which is very meaningful for large-scale injection molding products.
2、Improve PP mechanical properties
The density of crystalline polypropylene crystalline phase is higher than that of non-crystalline phase and has excellent strength. Without nucleating agent, crystalline polymers generate crystals when cooling in the molten state, which is automatic crystallization, this ball crystal is not uniform, incomplete, so when the force between the crystalline part of the ball crystal interface and the non-crystalline part, the gap between the broken grains is the first to be destroyed; when adding nucleating agent, the growth of the ball crystal can be controlled, so that the nucleus increases, crystallization is more perfect, the force is more uniform, so you can improve the yield strength of the polymer Therefore, it can improve the yield strength, impact strength and surface strength of the polymer, and improve the mechanical properties of polypropylene.
3、Increase the transparency and surface gloss of PP. As PP in the melting and cooling process, the number of spontaneous crystallization nuclei is very small, the crystallization rate is slow, easy to form large spherical crystals, the diameter of these spherical crystals is much larger than the visible wavelength, and the refractive index difference between the crystal and amorphous areas is large, resulting in the scattering of surface light, resulting in a decrease in transparency, so most of the polypropylene products made under normal processing conditions are translucent. A product with good transparency must meet the requirements of high crystallinity, neat crystalline orientation, and a crystalline size smaller than the wavelength of visible light. To achieve the required microfine spherical crystallization of polypropylene, the addition of nucleating agents is an extremely competitive method [8].
4. Effect on thermal properties
The addition of nucleating agents increases the crystallization temperature speeds up the crystallization, enhances the degree of crystallization, increases the density, and increases the heat deflection temperature, which can improve the heat resistance of the resin to some extent.