TABLE OF CONTENTS We have already seen that thermosets cannot be reprocessed in the same manner as thermoplastics, because they do not re-melt. Therefore separation is needed of thermosets and thermoplastics. But what about thermoplastics: do they need to be separated from each other?
Can we simply just mix all the plastics together and reprocess them?
Well, if we could do this, plastics recycling would certainly be much easier. In some cases it may be possible to simply mix and reprocess but generally the properties of mixed plastics are not very good. Plastics tend not to be very compatible with each other, due to differences in their chemical compositions. The polymer molecules repel each other rather than mixing, much like a mixture of oil and water.
If two non-mixing polymers are put together, a process called phase separation occurs. A diagram of this system can be seen in Figure 4.1(b); compare this with the miscible system in Figure 4.1(a). Distinct regions of each polymer will be visible under a microscope or to the naked eye; no chemical bond exists between these materials. A way to imagine it is to think of an egg, where the yolk is inside but with no bond to the egg white. Now imagine an egg with lots of smaller yolks inside the white, this is what an immiscible polymer blend would look like. A commercial example of this kind of material is high impact polystyrene (HIPS). This is a blend of polystyrene and polybutadiene (a type of...
