Glassy-Eyed Over Glass Transition Temperature

“Glass”, to most people is either a windowpane, or preferably, something they hoist at their local watering hole. I’m pretty fond of both, but if you’re interested in polymers, “glass” has a more sophisticated and important meaning. To a materials scientist, anything that goes from a melt to a relatively solid state without crystallizing is called a glass.

It’s the crystallization thing that causes the confusion. The nature of non-crystalline (the lab guys call it “amorphous”) materials is that their molecules, the long chains of carbon atoms that make up resins at the atomic level, aren’t stacked neatly. In truly crystalline materials, like metals and graphite, they line up like Rockettes, with spacing so even that whacking them cold with a hammer can cause them to shear in neat planes.

Amorphous materials, like most common thermoplastics, aren’t so neat, organising like a random mess of spaghetti-like molecules. All this order and disorder matters when we try to do what we do in this business, namely change the resin from solid to a flowing state then back to solid again.

Highly crystalline materials like, say, ice, have neat patterns of atoms that break down when the temperature hits the well-defined melting point, often labelled “Tm”. Highly amorphous materials, like window glass, soften into a gooey mass as they slowly become more and more liquid with rising temperature. Common thermoplastic resins, like other glassy materials, don’t have a single “melting point” but instead are measured by the temperature at which the glass transitions to a rubbery state on its way to solidity. The lab rats unimaginatively call this the “glass transition temperature” or Tg.

But wait, you say, ‘I look at window glass all the time, and it’s a solid as rock’. In fact it isn’t, and is flowing all the time under the influence of gravity, just like water. In fact, some amorphous solids are called “supercooled liquids” by the white coat guys, which better describes what “solidity” is like for many amorphous materials at room temperature. Look at a windowpane from a home 100 to 150 years old, and you can see that it’s thicker at the bottom, as the glass flows downward, very, very slowly.

The key thing to remember as a processor is that you can’t think about processing temperatures as a “melting point” because unless you’re way above the Tg, which for most common commodity polymers leads to serious degradation of the material, the stuff inside the screw is far from liquid. Its behaviour is not only complex, it changes over a range of temperatures around the Tg. And other properties change, too. The ability to absorb heat (“heat capacity”) and density change around the Tg, as well as the thermal expansion coefficient. Adding to the complexity is the fact that the “melt” has a viscosity that also changes with shearing forces, so screw speed and design are issues, too.

If you’re using older presses for short run molding with a variety of materials, then working around a less than ideal screw profile isn’t new. If you have the luxury of a large L/D barrel and good zone control on heating, you can “sneak up” on the best set temperatures (independent of what’s really going on in the barrel) and relax for ten or twenty seconds. Compared with highly crystalline polymers like the classic nylon 6/6, knowing what temperature an amorphous commodity thermoplastic melts at in the lab is even less of an indication about good set points.

What’s the answer? Keep meticulous notes about operating conditions beyond what the “TV screen” tells you. Make sure that your transducers tell the truth throughout their operating range and if you need to tweak something a lot higher than the recommended temperature to get good performance, (c’mon, admit it!) write it down along with the time and ambient conditions. The good news is that amorphous resins like polystyrene have excellent processability. The bad news is that fewer customers today are willing to buy high value parts without the performance advantages of copolymers, alloys and blends of polymers of different crystallinity. Your best bet? Form a good relationship with the person that sells you the resin, and if he or she mentions that they sell the same stuff to “XYZ Plastics” down the road with no problems, find out what they’re doing. And take those material property sheets with a huge grain of salt.