TABLE OF CONTENTS We now know what plastics are made of, some of the things virgin materials are used for and the differences between thermoplastic and thermoset materials. It is here that we begin to concentrate on thermoplastics and examine what happens to them when they are processed. Methods are described in detail in Chapter 6. Processing in its simplest form is the act of melting, forming and then solidifying the melt. During this procedure three things need to be considered [1].
Deformation processes, which enable the product to be formed.
Heat and heat transfer, which enable the polymer to melt, flow and then solidify.
Changes to structure and properties as a result of processing.
We will begin with a study of deformation processes and for this, we need to consider rheology.
Rheology is the study of deformation and flow. Consider the following examples of flow:
Squeezing toothpaste through a tube,
Tipping honey from a spoon, and
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...
