How to control the shrinkage rate of modified PP?

PP has sufficient raw materials, low price, low density, good rigidity, flexural resistance, chemical corrosion resistance, and good insulation. Therefore, in recent years, PP modification has taken the first place in the plastic modification industry and has become the main variety of modified plastics, which has attracted more and more attention. However, the disadvantages of PP are poor low-temperature impact performance, easy aging, and large molding shrinkage.

The shrinkage control of PP modified material is an important aspect of PP modification. Good shrinkage control is of great significance to the popularization and use of PP modified materials. The following is a research on some issues about the shrinkage control of PP modified materials:

The large shrinkage molding of PP is a major disadvantage of PP itself, which is mainly due to the high crystallinity of PP. After crystallization, the specific gravity of PP increases and the volume decreases. When the crystallinity is 0% and 100%, the specific gravity is 0.851 and 0.936, respectively. Therefore, the molding shrinkage of pure PP is generally between 1.7 and 2.2. Controlling the molding shrinkage rate of PP is mainly to control the crystallinity of its raw material during molding: the smaller the crystallinity, the smaller the molding shrinkage rate; conversely, the higher the crystallinity, the greater the molding shrinkage rate.

In PP modified plastics, the addition of various modifiers destroys the original crystallinity of PP to varying degrees, thereby changing the original molding shrinkage rate of PP. To control the shrinkage rate of modified PP, the following types of modifiers are mainly used:

1. The influence of mineral filling on the shrinkage rate of PP modified material:

(1) The mineral additive itself does not shrink, and its addition reduces the shrinkage rate of the PP modified material from the overall ratio;

(2) The addition of mineral additives will inevitably affect the crystallinity of PP, thereby affecting the shrinkage rate;

(3) After the fine mineral agent is added, it acts as a nucleating agent, changing the structural state of PP, preventing the formation of large spherulites, and also affecting the molding shrinkage rate of PP.

2. The influence of glass fiber on the molding shrinkage rate of PP modified material: glass fiber has a greater influence on the molding shrinkage rate of PP modified material. When the content of glass fiber reaches more than 30%, the molding shrinkage rate of the PP modified material decreases from 1.8 to 0.5, and the surface-treated glass fiber has a greater impact on the molding shrinkage rate than untreated glass fiber. The addition of glass fiber destroys the crystallinity of PP and affects the shrinkage rate. More importantly, the glass fiber restricts the crystal shrinkage of PP.

3. The influence of the addition of polyethylene on the shrinkage rate of PP molding:

The addition of polyethylene also affects the molding shrinkage of PP modified materials. Although polyethylene is also a kind of plastic with high crystallinity, the molding shrinkage rate is also very large, but after adding PP to each other, the crystallinity of each other is destroyed to different degrees, so that the overall molding shrinkage rate is reduced.

The molding shrinkage rate of PP modified material is different due to the different modifiers added. In the experiment, we found that when several modifiers are used together, the effect is different and there are many influencing factors. Such as the degree of dispersion of the modifier in PP, the particle size of the mineral modifier itself, and the surface treatment of various mineral additives, all have an impact on the mold shrinkage. In addition, the molding process of the injection molding machine has a great influence on the molding shrinkage during the molding process. For example, changes in injection temperature and injection pressure have an effect on mold shrinkage.