Effects of Processing Aids on Polymer Melt
Processing aids in general are a group of different kinds of materials that manufacturers can use to make processing and handling high-molecular-weight polymers easier and more efficient. When using processing aids to this end, you will generally see the benefits realized during the melt stage of your host polymer. Processing aids tend to affect the Melt Flow Index of a polymer.
What is Melt Flow Index?
When plastic is melted, you can measure its rate of flow or Melt Flow Index (MFI) to understand better how to process and handle the melted plastic. This measurement is generally used as a means for quality control in thermoplastics. The method used to measure a plastic’s flow rate is guided by the standards ISO 1133 and ASTM D1238. Using a machine (the Melt Flow Indexer) made of a heated barrel and a piston that is actuated with a certain weight, usually 2.16 kg or 5 kg, you can measure the flow of melted plastic. You place about 5 grams of the melted or softened plastic in the heated barrel, where it is pushed out by the actuated piston through a small capillary. After 10 minutes, the weight in grams of plastic that has flowed through the capillary is the plastic’s MFI.
The higher a plastic’s MFI number, the lower material viscosity it has. Conversely, a lower MFI number means that the plastic has less branching or higher molecular weight.
Two kinds of processing aids
There are two main kinds of processing aids that have an effect on polymer melt, and they are the groups of lubricants and fluoropolymer-based additives. Each group has its own effect on polymer melt, and is useful in its own ways.
Generally referred to as fluoroelastomers, these additives tend to be copolymers of hexafluoropropylene and vinylidene fluoride. The main effect of fluoropolymer-based additives is that they eliminate melt fracturing during polymer extrusion.
Lubricants, as the name suggests, help to keep polymers from sticking to any metal surfaces. They can also lower a polymer’s melt viscosity, improving the melt flow. These internal lubricants lower a polymer’s melt viscosity by working on an intermolecular level. With the lubricants working, it’s easier for polymer chains to slip by one another, allowing the polymer as a whole to flow through areas with more ease.
Plastic Service Center | Melt flow aids, processing aids, plastic polymers
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