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Polypropylene shrinkage difficult to control; What are the influencing factors?

2022-07-18 Page view : 22 views

Polypropylene is one of the most widely used general purpose plastics, with the advantages of low density, good rigidity, resistance to flexing, chemical resistance and good insulation. Its shortcomings are poor low-temperature impact performance, easy ageing and large moulding shrinkage.

Modification can improve the low temperature impact properties, moulding shrinkage and thermal ageing properties of polypropylene. Shrinkage, if well controlled, is important for the promotion of polypropylene modifiers and is also an important aspect of ensuring product quality. In particular, shrinkage is very important when using modified polypropylene to replace traditional engineering plastics.
Polypropylene modification has mature technology in China, but the analysis of shrinkage issues is more scattered. The chain plastic network integration of existing information, for this issue to provide some elaboration, I hope to give you in the industry to provide ideas.

Causes

The high shrinkage of polypropylene is a major disadvantage of polypropylene itself, mainly due to its high crystallinity. After crystallization, the specific gravity of polypropylene increases and its volume decreases. The specific gravity is 0.851 and 0.936 at 0% and 100% crystallinity respectively, so the forming shrinkage of pure PP is generally between 1.7 and 2.2.

The main control of the shrinkage of polypropylene is the crystallinity of the raw material: the smaller the crystallinity, the smaller the shrinkage; conversely, the higher the crystallinity, the greater the shrinkage.

In polypropylene modified plastics, the addition of various modifiers has, to varying degrees, damaged the original crystallinity of the polypropylene, thus altering the original shrinkage of the polypropylene for moulding.

I. Effect of rubber on the shrinkage of polypropylene

As the rubber content increases, the moulding shrinkage tends to decrease. This is mainly due to the fact that the addition of rubber destroys the crystallinity of the polypropylene itself, which leads to a decrease in moulding shrinkage, and the three elastomers POE, EPDM and SBS have a different effect on moulding shrinkage.

When the elastomer content is below 5%, the effect of each type of elastomer on the shrinkage of PP is basically the same. Only after the elastomer content exceeds 5%, the degree of influence of different elastomer varieties diverges, and the order of influence on shrinkage is POE>EPDM>SBS, which is consistent with the toughening effect of elastomers on PP.

The shrinkage reduction is better when inorganic fillers are added to PP in a mixture with elastomers.

II. Effect of mineral filling on the shrinkage of polypropylene modifiers in molding

The main mineral additives for polypropylene are calcium carbonate, talc, mica powder, etc. There are three main reasons for the influence of mineral additives on the shrinkage of polypropylene modifiers for moulding.

Firstly, the mineral additive itself does not shrink, and its addition reduces the shrinkage of the polypropylene modifier from the overall ratio; secondly, the addition of the mineral additive inevitably affects the crystallinity of the polypropylene, which affects the shrinkage; thirdly, the addition of the microfine mineral agent plays the role of a nucleating agent, which changes the structural state of the polypropylene and prevents the formation of large spherical crystals, which also affects the shrinkage of the polypropylene forming.

1) The shape of the filler
The shape of the inorganic filler has a different effect on the shrinkage rate, the specific size of the effect is in the order of flakes > needles > granules > spheres.

2) Particle size of the filler
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 effect of different particle sizes of talc on shrinkage is as follows.

3) Surface treatment of fillers
The effect on shrinkage varies depending on whether the same filler is surface-treated when laminated with resin. Filler with surface treatment has a greater effect on shrinkage.

4) Amount of filler
The amount of the same filler added has a different effect on the shrinkage rate, the higher the amount added the greater the decrease in shrinkage rate.

III. Effect of fibres on the shrinkage of polypropylene modifiers in moulding
The fibres referred to here include all types of inorganic and organic fibres. In the case of short glass fibre reinforcement, for example, the addition of glass fibres of varying content to PP resin can substantially reduce its moulding shrinkage.

Long glass fibre reinforced polymers have the advantage of lower shrinkage compared to short glass fibres and consistent shrinkage in both longitudinal and transverse directions. Whisker reinforced polymers have a lower shrinkage rate than glass fibre reinforced polymers.

Glass fibres have the greatest effect on the moulding shrinkage of polypropylene modifiers. When the glass fibre content reaches 30% or more, the moulding shrinkage of the polypropylene modifier decreases from 1.8 to 0.5 and the surface treated glass fibre has a greater effect on the moulding shrinkage than the untreated glass fibre.

The addition of glass fibre firstly destroys the crystallinity of the polypropylene and affects the shrinkage, and more importantly the glass fibre limits the crystalline shrinkage of the polypropylene.

IV. Effect of polyethylene incorporation on the shrinkage of polypropylene molding
The addition of polyethylene also affects the moulding shrinkage of the polypropylene modifier. Although polyethylene is also a highly crystalline plastic with a large moulding shrinkage, the addition of polyethylene to polypropylene disrupts the respective crystallinity of each other to varying degrees, resulting in a reduction in overall moulding shrinkage.

Blending with other resins, or chemical cross-linking and grafting modifications can be used.

V. Influence of changes in polypropylene’s own MFI on moulding shrinkage
The moulding shrinkage of polypropylene is influenced by its crystallinity, which in turn is influenced by the size of its own molecular weight. When the MFI increases, the molecular weight decreases, its crystallinity increases and the moulding shrinkage increases.

Comparison of the effect of modifiers on shrinkage in polypropylene moulding

The effect of various modifiers on the shrinkage of polypropylene modifiers is shown in the table, which shows that the effect of various modifiers on the shrinkage of polypropylene modifiers is different, with glass fibre having the greatest effect, in the following order: glass fibre > mineral > elastomer > polyethylene.

When several modifiers are used, polypropylene shrinkage effect is not the same, the influence of many factors. For example, the degree of dispersion of the modifier in polypropylene, the particle size of the mineral modifier itself, the surface treatment of various mineral fillers, etc., all have an impact on the shrinkage of the moulding.

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

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