1. The matrix resin itself
The transparency of the matrix resin itself has a great influence on that of PP products. To improve the transparency of PP, we can add a small amount of polyacrylate resin with good compatibility and excellent transparency.
2. Nucleating agent
Significantly increase the crystallization velocity, the crystallization rate and crystal fine ratio.
(1) Inorganic nucleating agent is mainly ultrafine talc and SiO2. And it also includes calcium carbonate, mica powder, inorganic pigments and fillers, etc.. These are with low price and abundant sources. But it has a certain degree of influences on the gloss and transparency of products.
(2) Organic nucleating agent is a organic substance of low molecular weight with nucleation effect. It is represented by sorbitol and its derivatives. It has good compatibility with the matrix resin. And this can significantly improve the transparency of the product and surface gloss.
(3) The amount of nucleating agent for sorbitol, when its amount exceeds 0.3%, the transparency improvement effect is not obvious, or even decreases. But if it is less than 0.2%, the amount of nucleation is insufficient and the transparency isn’t enough. Therefore, the amount of nucleating agent is preferably 0.2-0.3%. In short, an appropriate nucleating agent content can better improve the transparency of PP.
3. Molding temperature
If the temperature is too high, the low-molecular-weight substances in the nucleating agent will decompose and volatilize. Then, it reduces the effective ingredients and the transparency improvement effect in the nucleating agent. The higher temperatures will destroy the original crystal nucleus in the PP and reduce the heterogeneous nucleation center, resulting in less modification effect. While the overly low temperature will cause the bad dispersibility of nucleating agent. Besides, the transparency effect is poor. Thus, choosing an appropriate molding temperature is particularly important to improve transparency.
4. Toughening agent
The toughening agent and PP are not mutually soluble with their different refractive index. It will be refracted at the mutual interface, thereby affecting the light transmittance of the article. As the toughening agent content increases, the area of the mutual interface will become obvious. While the light transmittance of the product will become worse.
5. Process parameters
The ideal processing temperature, cooling temperature, injection pressure and other process parameters also have a good influence on the transparency of PP.
Processing temperature: Under the processing conditions, the lower the processing temperature, the smaller the crystal size, and the better the transparency.
Cooling temperature: the lower the cooling temperature, the lower the crystallinity and the better the transparency.
Injection pressure, injection time, and dwell time will affect the orientation of molecules. The orientation will affect the crystallization of the molecules. So shortening the injection and holding time and reducing the pressure without affecting the performance and appearance of the product can improve its transparency to a certain extent.
6. Forming process
Comparing the products obtained from injection, drawing, and blowing processes, it is found that injection and blowing processes are the most beneficial to increase transparency.
The higher the finish of the mold, the better the transparency of the product.