The lower the shrinkage of the polymer, the higher the dimensional accuracy of the product will be. Below is a brief description of several modification methods that effectively reduce the shrinkage of polymers.
1. All kinds of fibres are filled and modified
The fibres referred here include all kinds of inorganic and organic fibres. Take short glass fibre reinforcement as an example, adding different contents of glass fibre in PP resin can substantially reduce its forming shrinkage.
Long glass fibre reinforced polymers have the advantage of lower shrinkage and consistent shrinkage in both longitudinal and transverse directions compared to short glass fibres. Whisker reinforced polymers have lower shrinkage than glass fibre reinforced polymers, e.g. a comparison of shrinkage of glass fibre and calcium sulphate whisker reinforcedPP and ABS materialsis shown as below.
2. Inorganic filler filling modification
Inorganic fillers specifically include talc, calcium carbonate, barium sulphate, mica powder, wollastonite and montmorillonite, etc. Different fillers have different effects on the shrinkage rate.
(1) Shape of the filler
Different shapes of inorganic fillers have different effects on the shrinkage rate, the specific impact on the size of the order of flakes > needles > granules > spheres
(2) The 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. Take talc filled PP (20% talc + 8% POE) as an example, the effect of different particle sizes of talc on the shrinkage rate is as follows.
(3) Surface treatment of the filler
The same filler, whether or not it is surface treated when laminated with resin, has a different effect on the shrinkage rate. The surface treatment of the filler has a greater effect on the shrinkage rate.
(4) The amount of filler
The amount of the same filler is added differently to the shrinkage rate, the greater the amount added the greater the decrease in the shrinkage army.
3. Reducing crystallinity modification
(1) Add to reduce crystallization additives
Commonly used to reduce crystallization additives for small molecules, added to the molten state when the regularity of the large molecules broken, hindering the movement of large molecules, so as to reduce the purpose of crystallization.
(2) Add other resin blends
Such as a small amount of LDPE, HDPE resin blended in PP, can greatly damage the crystalline properties of PP in processing, to add PE resin in PP, for example, the impact of different content PE on PP shrinkage rate is shown in the table below. In addition it is also possible to add non-crystalline resins such as PS, ABS, PMMA and PC to PP, which can also play a role in reducing crystallisation.
The effect of adding other resins on shrinkage is far less than the effect of filling fibres and inorganic fillers, and generally only glass fibre reinforced PP can have an effect on shrinkage of around 50%.
4. Blended elastomer modification
Can be blended elastomers including POE, EPDM, SBS, etc., all kinds of elastomers on the plastic shrinkage rate is very close to the impact.
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 species diverges, and the order of influence on the shrinkage rate is POE>EPDM>SBS, which is consistent with the toughening effect of elastomers on PP.
The use of inorganic fillers in PP mixed with elastomers results in better shrinkage reduction.
5. Chemical cross-linking and grafting modification
Take PP resin as an example, graft modification can reduce the crystallinity of the resin, so as to achieve the purpose of reducing shrinkage. Take PE cross-linking as an example, cross-linking can significantly reduce the coefficient of linear expansion of the resin, thus achieving the purpose of reducing the shrinkage rate.