One of the most popular general-purpose polymers is polypropylene. Low density, excellent stiffness, flex resistance, chemical corrosion resistance, and good insulation are some of its benefits. Poor low-temperature impact performance, quick aging, and significant molding shrinkage are some of its drawbacks.

Modifications can enhance polypropylene's low temperature impact performance, mold shrinkage, and heat aging characteristics. It is crucial to the usage and marketing of polypropylene modified materials if the shrinkage rate can be kept under control. It is also a crucial factor in ensuring product quality. The shrinkage rate is crucial, especially when modified polypropylene is used in place of conventional technical plastics.

Cause

A significant drawback of polypropylene is its significant molding shrinkage, which is mostly caused by the material's high crystallinity. Polypropylene's specific gravity rises and its volume decreases during crystallization. Its specific gravity is 0.851 and 0.936 at 0% and 100% crystallinity, respectively. As a result, the molding shrinkage of pure PP typically ranges from 1.7 to 2.2.

Controlling the crystallinity of polypropylene's raw materials during molding is crucial for reducing molding shrinkage; on the other hand, a higher crystallinity results in a bigger molding shrinkage.

In polypropylene modified plastics, the inclusion of different modifiers alters the initial molding shrinkage of polypropylene by modifying the degree to which the crystallinity of polypropylene is destroyed.

1. Rubber's impact on polypropylene's shrinkage

The molding shrinkage reduced as the rubber content rose. This is mostly due to the fact that adding rubber causes polypropylene to lose some of its crystallinity, which reduces molding shrinkage. Additionally, the three elastomers POE, EPDM, and SBS each have a distinct impact on molding shrinkage.

The effects of different elastomers on the shrinkage of PP are essentially the same when the elastomer concentration is less than 5%. The sequence of the effects on shrinkage is POE>EPDM>SBS, which is consistent with the toughening effect of elastomers on PP, only until the elastomer content approaches 5%. Before that point, the degree of influence of various elastomer kinds diverges.

When the elastomer and inorganic filler are combined in PP, the shrinkage rate is better minimized.

2. How mineral filling affects the molding shrinkage of materials treated with polypropylene

The most common mineral additions for polypropylene include calcium carbonate, talc, mica, etc. The impact of mineral additives on the molding shrinkage of materials treated with polypropylene can be attributed to three basic factors:

The addition of the mineral additive will unavoidably affect the crystallinity of polypropylene, affecting the shrinkage rate; the third is the fine mineral, which itself does not shrink and thus reduces the shrinkage rate of the polypropylene modified material from the overall ratio. The agent serves as a nucleating agent after being introduced, alters the structural state of polypropylene, prevents the production of big spherulites, and also influences polypropylene's molding shrinkage.

1) The filler's form

The shrinkage rate is affected differently by different inorganic filler forms, and the order of particular impact size is flake > needle > particle > spherical.

2) The filler's particle size

Shrinkage is affected differently by different particle sizes of the same filler, and shrinkage is reduced more dramatically the smaller the particle size. The effects of various talc powder particle sizes on shrinkage are as follows, using talc powder filled PP (20% talc powder + 8% POE) as an example.

3) Filler surface treatment

The shrinkage rate depends on whether the same filler is surface-treated or not when it is combined with the resin. Fillers with surface treatment have a bigger impact on shrinking.

4) The quantity of filler

The shrinkage is affected differently by different amounts of the same filler, and the addition has a stronger impact on the reduction of shrinkage.

3. Fiber's impact on the material modified with polypropylene's molding shrinkage

Here, both biological and inorganic fibers of various kinds are referred to as fibers. As an illustration, adding various amounts of short glass fiber reinforcement to PP resin will significantly decrease the molding shrinkage of the resin.

Long glass fiber reinforced polymers have lesser shrinkage and uniform longitudinal and transverse shrinkage than short glass fiber reinforced polymers. Compared to glass fiber reinforced polymers, whisker reinforced polymers shrink less.

The molding shrinkage of polypropylene modified materials is most affected by glass fiber. The molding shrinkage of the polypropylene modified material decreases from 1.8 to 0.5 when the glass fiber content exceeds 30%, and the surface-treated glass fiber has a bigger impact on the molding shrinkage than the untreated glass fiber.

Glass fiber decreases the crystallinity of polypropylene and slows the rate of shrinkage. Glass fiber is more significant since it reduces polypropylene's crystallization shrinkage.

4. The impact of polyethylene addition on polypropylene molding shrinkage

The molding shrinkage of materials treated with polypropylene is similarly impacted by the addition of polyethylene. Despite the fact that polyethylene is also a material with high crystallinity and a high molding shrinkage rate, when added to polypropylene, the crystallinity of each material is partially or completely dissolved, which lowers the total molding shrinkage rate.

It can be grafted, chemically cross-linked, or combined with other resins.

5. The impact of molding shrinkage on the modification of polypropylene's own MFI

The crystallinity of polypropylene, which is influenced by its own molecular weight, affects the molding shrinkage of the material. The molecular weight falls, the crystallization rate accelerates, and the molding shrinkage rises as the MFI rises.

Comparison of Modifier Effects on Polypropylene Molding Shrinkage

The table illustrates the impact of several modifiers on the molding shrinkage of materials treated with polypropylene. The table shows that different modifiers have varying impacts on the molding shrinkage of polypropylene modified materials. Effects of glass fiber Glass fiber is the biggest, followed by mineral, elastomer, polyethylene, and then glass fiber.

The shrinkage effect of polypropylene varies when multiple modifiers are employed combined, and there are several affecting factors. For instance, the amount of mineral additive dispersion in polypropylene, the size of the mineral additive's own particles, the surface treatment of different mineral additives, etc., all have an effect on molding shrinkage.

The molding shrinkage rate during the molding process is also significantly influenced by the injection molding machine's molding procedure. For instance, changes to injection molding pressure and temperature might affect manufacturing shrinkage.