Progress of Polypropylene Transparent Modification
First, the main indicators to measure the transparency of polymers are:
1. Transmittance: The ratio of transmitted light intensity to human light intensity.
2. Transparency: ratio of directional transmittance to total light transmittance.
3. Haze: The ratio of the transmitted light intensity to the total transmitted light intensity.
In addition, there are some indicators such as: turbidity, cleanliness (degree of decomposition of the image by the standard printing size seen through plastic sheets), etc. are also used to evaluate the transparency.
2. PP is a crystalline polymer with a two-phase structure of crystalline and amorphous regions. The difference in density between the crystalline region and the amorphous region makes the crystalline phase have a higher refractive index than the amorphous phase. The difference in refractive index between the two phases causes light scattering and refraction at the phase interface. Therefore, the transparency of PP is closely related to its crystallization. .
Third, to improve the transparency of PP, we must control its crystallization, destroy the formation of large spherulite structure, methods are:
1. Reduce the crystal area (spherulite size) so that it is similar to the wavelength of light.
2. To disrupt the regularity of the molecular chain and reduce the crystallized crystals.
3. Make the refractive index of the crystalline phase and the amorphous phase similar.
4. Change the shape of the PP crystals, for example by stretching the spherulites into platelets.
Fourth, to improve the transparency of PP methods are roughly classified as:
1. Adding nucleating agent 2. Blending with other polymers 3. Grafting other monomers on the PP chain 4. Processing, such as quenching, stretching, etc. 5. Difference of PP transparency and its microscopic morphology ( The spherulite size is closely related, which is related to two parameters:
1. Spherulite growth rate 2. Number of crystal units per unit volume The polymer solidifies slowly, forming the characteristics of nucleation and crystal growth phase separation. Adding a nucleating agent can increase the nucleation rate and cause nucleation to occur at higher temperatures. Compared with the homogeneous nucleation, a higher crystal growth rate is obtained, so that a higher crystallinity and a smaller spherulite size are obtained. Therefore, adding a nucleating agent can not only improve the transparency of the polymerization (smaller spherulite size), improve the mechanical properties of the polymer (higher crystallinity), but also shorten the processing cycle of the polymer (faster crystallization rate) In addition, the addition of nucleating agents also helps to increase the dimensional stability of the PP.
6. To obtain a good nucleation effect, choose a suitable nucleating agent. A good nucleating agent should have the following characteristics:
1. Reduce the free energy of the included interface and have a good compatibility with the polymer.
2, melting above the melting point of the polymer, no decomposition.
3, non-toxic, stable, and the environment does not react.
4. It should have a crystal structure similar to that of a crystalline polymer.
Seven, PP transparent modified nucleating agent:
Organic categories:
1. Polysorbitol 2, organic phosphorus 3, organic amide inorganic:
1, metal oxides and their salts 2, silicon-containing 3, talc, carbon black
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