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Recycled PE pelletizing, what should I do to solve the unstable melting?

June 25,2022

1. Melt Index

The melt index is often expressed in terms of MI and is now internationally known as the melt flow rate, expressed as MFR.

It is commonly used as a parameter for quality control of thermoplastic resins and for thermoplastic moulding process conditions. It is the weight value of the melt flowing through a standard capillary at a specified temperature and constant load, often expressed in g/10 minutes. The melt index is a parameter used to differentiate the fluidity of various thermoplastic materials in their molten state.

For the same resin, the melt index can be used to compare its molecular weight as a production quality control.

Generally speaking, for resins with a certain chemical structure, a. the smaller the melt index, the larger the molecular weight, the higher the fracture strength, hardness, toughness, ageing stability and other properties are improved; b. while the melt index is large, the molecular weight is small, and the flow ability is correspondingly better when forming.

Recycled plasticsdue to more impurities, unstable nature, resulting in stress concentration, the material will have more defects after molding, affecting product quality, how to control the melt finger output good products?

2. Different melting, need to differentiate

1) HDPE resin grades are available in injection moulding, blow moulding, rotational moulding and film, and their melt indices vary.
a. Blown and film grade HDPE will have a smaller melt finger, generally below 2;
b. The melt finger for injection moulding will be greater than 2 and may even reach several tens.
This requires differentiated recycling, adopting different requirements for each material processed.

3. Co-mingled low melt finger materials

Firstly, blow moulded ones can be directly injected, whereas injected ones cannot be used for blow moulding.
The only way to turn a high melt index HDPE into a low melt index HDPE is to find a way to increase the molecular weight, which is more difficult to do simply by increasing the molecular weight, which involves polymer chemistry and can be very difficult to operate.
The only simple solution would be to use HDPE material with a lower melt index than required and blend it in a certain proportion with the current high melt index HDPE, the apparent melt index would be the middle value of the two raw materials.

4. Polymer binders

There is an additive called a melt finger conditioner, also known as a polymeric binder.
It is a type of processing modifier for polyolefin products developed for the mechanism of the normal molecular structure of polyolefin products. It effectively aggregates polyolefin polymers through various bonding techniques, thus increasing the attraction between polymer chain segments (i.e. van der Waals forces).

This modification process restores the properties of partially degraded polyolefin products to their pre-breakage properties, with significant improvements in individual properties depending on the amount added.

Its function is both to modify the effect of grafting and to overcome the shortcomings of the traditional compatibilizer’s own mechanical properties and poor fluidity. POE is a short branched thermoplastic elastomer with a uniform molecular mass distribution synthesized by DOW Chemical Company in the USA using metallocenes as catalysts, and its main properties are outstanding, such as tensile strength, tear strength and other main mechanical properties are improved compared to traditional elastic materials.

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