Pollutec Recycling

Rheology

Rheology is the study of deformation and flow. Consider the following examples of flow:

1. Squeezing toothpaste through a tube,
2. Tipping honey from a spoon, and
3. Pouring water into a glass.

The toothpaste, honey and water all have different viscosities. The toothpaste is the most viscous, the water the least. Viscosity means the resistance to flow.

In viscous flow, a material continues to deform as long as a stress is applied.

To put it more simply, in the case of the toothpaste tube, the toothpaste is squeezed out only for as long as we squeeze the tube.

Consider the toothpaste again, when it is squeezed and then released it does not return to the original shape. A rubber ball however, does return to its original shape when released due to its elastic properties, and likewise polymer melts will try to return to their original shape when stress is removed. So polymers combine both viscous and elastic properties and demonstrate what is called a viscoelastic response to stress.

Polymers are more viscous than most liquids. Polymers are more elastic than most solids.

The interaction between viscosity and elasticity frequently determines the success of any processing operation. Processing must take into account not only how the polymers flow in their molten state but also how they change as the temperature goes up and down and the polymers melt and solidify.

Two very common materials in commercial use are low density polyethylene (LDPE) and polyamide (PA).

As LDPE is heated, it turns from a solid, to a viscous gummy liquid and then to a mobile fluid as temperature is increased. PA on the other hand turns quite suddenly from a solid to a low viscosity (watery) fluid. It is important to understand that polymers may react differently to the heat and stress applied to them, in order to find the most suitable methods to form them.

A cheap and simple method of determining and comparing the flow of melts under standard conditions is a Melt Flow Indexer. This works in a similar way to squeezing toothpaste from a tube. A vertical load is applied to a piston and the polymer melt is squeezed through a die, this is shown in Figure 3.1. The amount of polymer that is extruded in a fixed time gives a measure of the flow as a melt flow index (MFI). A highly viscous material will have a low MFI, a low viscosity material will have a high MFI.

Figure 3.1 Basic parts of an MFI machine (a Ray-Ran machine is shown in this photograph)

We will see later that as a polymer degrades, its viscosity becomes lower. Charting the changes that occur during repeated recycling of material can give a measure of the degradation processes occurring. MFI can provide a cheap and simple method of measuring this change.