The thermal properties of a plastics are important in understanding how a particular material behaves in an extruder Knowledge of the thermal properties allows the selection of the appropriate machine, setting of correct process conditions, and in analysing process problems. The most important thermal properties are: thermal conductivity, specific heat, thermal stability and induction time, density, melting point, and glass transition temperature. We will discuss these next.
Thermal Conductivity
Thermal conductivity is probably the most important thermal property. Thermal conductivity is the ability of a material to conduct heat. Plastics have a low thermal conductivity - they are considered to be thermal insulators. This means that heating and cooling plastics by conduction is a slow process. Heating occurs in the extruder and cooling occurs after the die. The low thermal conductivity often determines how fast a plastics can be processed. This is true not only in extrusion but also in injection molding and, in fact, most plastics processing operations.
Another aspect of the low thermal conductivity of plastics is that non-uniform plastics temperatures are likely to occur. For instance, if a plastics melt is introduced to an extrusion die with a high temperature region on one side of the channel, it will take considerable time for the melt temperatures to equalise by conduction. If the channel is 20 mm in diameter, it may take 5 to 10 minutes for the temperatures to equalise. A typical residence time in an extrusion die is only about 30 seconds. As a result, the residence is too short for the melt temperatures to equalise by conduction.
This means that the high temperature melt stream will persist all through the die and will cause non-uniform flow at the die exit; typically, this will result in a local thick spot in the extruded product .
Specific Heat and Enthalpy
The specific heat is the amount of heat necessary to increase the temperature of a material by one degree . In most cases, the specific heat of semi-crystalline plastics is higher than amorphous plastics.
The amount of heat necessary to raise the temperature of a material from a base temperature to a higher temperature is determined by the enthalpy difference between the two temperatures . If we use room temperature as the base temperature the enthalpy of different plastics can be plotted against temperature.
The enthalpy is expressed in kW.hr/kg or HP.hr/lb; it is a specific energy, in other words, energy per unit mass. Most of the energy required in the processing of plastics is needed to increase the temperature of the plastics. If we know the starting temperature, usually room temperature, and the discharge temperature, we can determine the minimum energy required to process the plastics. For instance, if we look at curve for PVC then we can see that the specific energy required to raise the temperature from room temperature to 150C is about 0.05 kW.hr/kg. Thus, for each kg/hr we require 0.05 kW. If we process PVC starting at room temperature up to 150C at 100 kg/hr (220 lbs/hr) the minimum power requirement is 5 kW (6.7 HP).
If we compare low density polyethylene (LDPE) to PVC, we see that LDPE requires about 0.15 kW.hr/kg to go from room temperature to 150C. Thus, the specific energy requirement for LDPE is much higher than for PVC. In general, semi-crystalline plastics have higher specific energy requirement than amorphous plastics. Obviously, this affects the cooling as well. It means that to cool LDPE from 150C to room temperature much more heat has to be removed than in cooling the same mass of PVC at 150C down to room temperature.
Thermal Stability and Induction Time
Plastics can degrade in the extrusion process. The main variables involved in degradation are temperature and the length of time that a plastics is subjected to high temperatures. Plastics degrade when exposed to high temperatures; the higher the temperature, the more rapid the degradation. Degradation can result in loss of mechanical properties, optical properties, appearance problems, degassing, burning, etc. Other variables can affect degradation, for instance the presence of oxygen.
The induction time is a measure of the thermal stability of a plastics; it is the time at elevated temperature that the plastics can survive without measurable degradation. The longer the induction time at a certain temperature, the better the thermal stability of the plastics. The induction time can be measured using various instruments, such as a TGA (thermogravimetric analyser), TMA (thermomechanical analyser), cone-and-plate rheometer, and other instruments.
When the induction time is measured at several temperatures, the induction time can be plotted against temperature as shown here (inductio.cvs) for HDPE, high density polyethylene and EAA, ethylene-acrylic acid. Two things are clear from the figure. One, the induction time reduces exponentially with temperature. Two, the induction time for HDPE is much longer than for EAA. The thermal stability of one plastics can be much different from another plastics.
The thermal stability and induction time of a plastics can be improved by adding thermal stabilisers [slide]. In fact, most plastics contain thermal stabilisers. Some plastics have such poor thermal stability that they would not be melt processable without thermal stabilisers; an example is rigid PVC.
