Polypropylene is one of the most widely used types of general plastics. It has the advantages of low density, good rigidity, resistance to flexing, chemical corrosion, and good insulation. Its shortcomings are poor low-temperature impact performance, easy aging, and large molding

Polypropylene is one of the most commonly used varieties of general plastics. It has the advantages of low density, good rigidity, resistance to flexing, resistance to chemical corrosion, and good insulation. Its shortcomings are poor low-temperature impact performance, easy aging, and large molding shrinkage.

The low-temperature impact properties, molding shrinkage and heat aging properties of polypropylene can be improved through modification. If the shrinkage rate can be controlled well, it will be of great significance to the promotion and use of polypropylene modified materials, and it is also an important aspect to ensure product quality. In particular, the shrinkage rate is very important when using modified polypropylene to replace the traditional engineering plastic .

Polypropylene modification already has mature technology in China, but the analysis of shrinkage issues is relatively scattered. Chain Plastic Network integrates existing information and provides some elaboration on this issue, hoping to provide ideas to everyone in the industry.

Causes

The large molding shrinkage of polypropylene is a major shortcoming of polypropylene itself, which is mainly due to the high crystallinity of polypropylene. After crystallization, the specific gravity of polypropylene increases and the volume decreases. When the crystallinity is 0% and 100%, its specific gravity is 0.851 and 0.936 respectively. Therefore, the molding shrinkage of pure PP is generally between 1.7-2.2.

Controlling the molding shrinkage of polypropylene is mainly to control the crystallinity of the raw material during molding: the smaller the crystallinity, the smaller the molding shrinkage; conversely, the higher the crystallinity, the greater the molding shrinkage.

In polypropylene modified plastic , the addition of various modifiers destroys the original crystallinity of polypropylene to varying degrees, thus changing the original molding shrinkage of polypropylene.

1. The influence of rubber on the shrinkage rate of polypropylene

As the rubber content increases, the molding shrinkage rate shows a downward trend. This is mainly because the addition of rubber destroys the crystallinity of polypropylene itself, resulting in a decrease in molding shrinkage. Moreover, the three elastomers POE, EPDM, and SBS also have different effects on the molding shrinkage.

When the elastomer content is less than 5%, the effects of various types of elastomers on the shrinkage of PP are basically the same. However, after the elastomer content exceeds 5%, the influence of different elastomer varieties diverges. The order of influence on shrinkage is POEEPDMSBS, which is consistent with the toughening effect of elastomers on PP.

is mixed with inorganic fillers and elastomers in PP, which has a better shrinkage reduction effect.

2. The influence of mineral filling on the molding shrinkage of polypropylene modified materials

The mineral additives used in polypropylene mainly include calcium carbonate , talc powder , mica powder , etc. There are three main reasons why mineral additives affect the molding shrinkage of polypropylene modified materials:

First, the mineral additives themselves do not shrink, and their addition reduces the shrinkage rate of the polypropylene modified materials in overall proportion; second, mineral additives The addition of polypropylene will inevitably affect the crystallinity of polypropylene, thereby affecting the shrinkage rate; third, after the addition of fine mineral agents, it acts as a nucleating agent, changing the structural state of polypropylene and preventing the formation of large spherulites Formation also affects the molding shrinkage of polypropylene.

) Filler shape

Different shapes of inorganic fillers have different effects on shrinkage. The specific order of impact is flaky, needle-shaped, spherical, and spherical.

) Filler particle size

Different particle sizes of the same filler have different effects on the shrinkage rate. The smaller the particle size, the greater the reduction in shrinkage rate. Taking talc-filled PP (20% talc + 8% POE) as an example, the effects of different particle sizes of talc on shrinkage are as follows.

) Surface treatment of fillers

The same filler will have different effects on the shrinkage rate whether it is surface treated when compounded with resin. Surface-treated fillers have a greater impact on shrinkage.

) The amount of filler

Different amounts of the same filler have different effects on the shrinkage rate. The greater the amount added, the greater the decrease in shrinkage rate.

3. The influence of fibers on the molding shrinkage of polypropylene modified materials

The fibers referred to here include various types of inorganic fibers and organic fibers. Taking short glass fiber reinforcement as an example, adding different contents of glass fibers to PP resin can greatly reduce its molding shrinkage.

Compared with short glass fibers, the advantages of long glass fiber reinforced polymers are lower shrinkage and consistent longitudinal and transverse shrinkage. Whisker-reinforced polymers shrink less than glass fiber-reinforced polymers.

Glass fiber has the greatest impact on the molding shrinkage of polypropylene modified materials. When the glass fiber content reaches more than 30%, the molding shrinkage of the polypropylene modified material drops from 1.8 to 0.5, and the surface-treated glass fiber has a greater impact on the molding shrinkage than untreated glass fiber.

The addition of glass fiber destroys the crystallinity of polypropylene and affects the shrinkage rate. More importantly, the glass fiber limits the crystallization shrinkage of polypropylene.

4. The impact of the addition of polyethylene on the molding shrinkage of polypropylene

The addition of polyethylene also affects the molding shrinkage of polypropylene modified materials. Although polyethylene is also a highly crystalline plastic with a large molding shrinkage, adding it to polypropylene destroys their respective crystallinities to varying degrees, causing the overall molding shrinkage to decrease.

can be blended with other resins, or chemically cross-linked and grafted.

5. The influence of changes in polypropylene's own MFI on the molding shrinkage

The molding shrinkage of polypropylene is affected by its crystallinity, and the crystallinity is affected by its own molecular weight. When MFI increases, the molecular weight decreases, its crystallization speed increases, and the molding shrinkage increases.

Comparison of the effects of modifiers on the molding shrinkage of polypropylene

The effects of various modifiers on the molding shrinkage of polypropylene modified materials are shown in the table. From the table, it can be seen that the effects of various modifiers on polypropylene modification The effects of molding shrinkage of plastic materials are different, with glass fiber having the greatest impact. The order is roughly: glass fiber>mineral>elastomer>polyethylene.

When several modifiers are used together, the shrinkage effect of polypropylene is different, and there are many influencing factors. Such as the degree of dispersion of the modifier in polypropylene, the particle size of the mineral modifier itself, the surface treatment of various mineral additives, etc., all have an impact on the molding shrinkage rate.

In addition, during the molding process, the molding process of injection molding machine also has a great impact on the molding shrinkage rate. For example, changes in temperature and injection pressure of injection molding have an impact on the molding shrinkage rate.

Common material shrinkage

Source of the article: Plastic Vision