In recent years, the production and sales of transparent polypropylene in my country have shown a continuous upward trend. With the continuous emergence of new processes and new varieties, the production scale, production efficiency, product categories and application fields are constantly expanding, becoming one of the fastest growing varieties of polypropylene products.
Affected by the crystal size ofPP, the light transmittance and gloss of ordinary polypropylene limit its application in the fields of daily necessities and transparent packaging. However, the transparency and gloss of transparent polypropylene are comparable to typical transparent materials (PET ethylene terephthalate, PVC, PS polystyrene), so it can be widely used in household products, packaging and medical applications, etc. field. Its products have high toughness, rigidity, heat resistance and chemical resistance. In recent years, the production and sales of transparent polypropylene have shown a continuous upward trend.
There are many factors that affect the transparency of polymer materials, such as processing technology, molecular weight and distribution of polymers, etc., but the most important factors are as follows:
1.1 Refractive index
For large logarithmic polymers, they are not completely crystallized, that is, there are both crystalline regions and amorphous regions inside them, but when the refractive index of the two is different, the incident light will appear at the interface between the crystalline region and the amorphous region. Refraction and reflection occur, so that it cannot pass through directly, and it is milky white and opaque, such as PE and PA. On the contrary, when the refractive index of the crystalline region and the amorphous region are the same, the polymer is transparent, such as poly 4-methyl-1- pentene.
The higher the crystallinity of plastic products, the greater the anisotropy of the products and the lower the transparency. Therefore, when the crystallinity decreases, the transparency increases, such as those completely amorphous polymers, which are usually transparent, such as PMMA, PS, etc.
1.3 Crystal size
When the crystal size is smaller than the wavelength of visible light, light does not refract and reflect, so even if there is crystallization, it does not necessarily affect the transparency of the polymer. Therefore, whether it is a spherulite or a general crystal, the smaller the crystal size, that is, the finer the grain, the more conducive to the improvement of transparency.
1.4 Surface Roughness
For transparent plastics such as PE, when the sample is very thin, the surface roughness becomes the main factor affecting the light transmittance. If the surface is rough, there will be more incident light scattering loss, and the light transmittance will decrease.
Although there are many factors that affect the transparency of polymer materials, the research on the transparent modification of polymer transparent materials has been mainly focused on reducing the crystallinity and crystal size, and achieved good modification effects. Similarly, the transparent modification of PP is mainly studied from two aspects of crystallinity and spherulite size.
The commonly used PP is partially crystalline, with a crystallinity of about 50%-60%, so the aggregated structure of PP also has both crystalline and amorphous regions. The amorphous region is conducive to the passage of light, and when the light irradiates the crystalline region, because the crystal size is larger than the wavelength of visible light, the visible light is refracted and reflected, which is not conducive to the passage of light, thereby reducing the transparency. Therefore, improving the transparency of PP is basically carried out from two aspects. One is to reduce the crystallinity, that is, to increase the scope of the amorphous region; the other is to reduce the crystal size.
At present, the main methods to improve the transparency of PP include adding transparent agents, synthesizing random copolymers, metallocene-catalyzed synthesis of transparent PP, blending anti-reflection PP, and process control to improve the transparency of PP, etc.
The main method of PP transparency improvement
2.1 Add transparent agent★
Ordinary PP is usually crystallized into spherulites with larger crystal size. Since the diameter of spherulites is larger than the wavelength of visible light, the incident light is scattered, thereby reducing the light transmittance. After adding a clearing agent to PP, when PP melts and crystallizes, the clearing agent plays the role of crystal nucleation, making the original homogeneous nucleation into heterogeneous nucleation, increasing the number of crystal nuclei in the crystal system, and making the microcrystal The number of spherulites increases, the number of spherulites decreases, so that the crystal size becomes thinner and the transparency of the resin is improved.
At present, adding nucleating agents to PP is the simplest and most effective method for transparent modification of PP. According to the chemical structure and composition of nucleating agents, transparent nucleating agents are usually divided into inorganic nucleating agents and polymer nucleating agents. and organic nucleating agents.
Inorganic nucleating agents mainly include talcum powder, kaolin, calcium oxide, etc. Although they have a wide range of sources and low prices, due to their poor compatibility with resins and poor dispersion, the nucleating agents themselves will have turbidity and non-homogeneous effects, so the antireflection To a limited extent.
Polymer nucleating agents refer to some high-melting macromolecular compounds, mainly polyvinyl cyclosilanes, cellulose aromatic esters, polyurethanes, etc., but their blending properties with resins are not good, and the use process is not good. Mature, not yet a commercial variety.
Organic nucleating agents mainly include sorbitol, phosphate and rosin nucleating agents, which have a good effect of improving permeability. Sorbitol-based nucleating agents have the aggregation property of self-physical polymerization, and can be dissolved in molten PP to form a homogeneous solution. When the polymer is cooled, the transparent agent first forms a fibrous network through self-aggregation, which is not only uniformly dispersed, but also has a large surface area. With further cooling, PP first forms lamellar crystals under the action of orientation, and then other PP segments align and crystallize along the fiber axis. Therefore, the nucleation density of PP is increased, so that PP forms uniform and fine spherulites, reduces light scattering and refraction, and increases transparency.
For organophosphate nucleating agents, the alkylbenzenes in this type of compound have good affinity with PP resins, and through the interaction of PP skeleton chains and benzene rings, PP forms stable crystals with regular helical structures, which can significantly improve The mechanical properties of the material. For the rosin-type nucleating agent, because the molecule has a carboxyl group, it is unstable and prone to heterogeneous rearrangement or oxidation, which can make the PP crystal grains finer, increase the crystallization temperature, shorten the processing cycle, increase transparency, and is non-toxic tasteless.