Assessing the Stable Process Time of Formulations Employing Torque Rheometry
Torque rheometry has proven itself a valuable tool for studying multicomponent thermoplastic systems to assess processing behavior. Torque rheometry measures the viscosity-related torque generated by the resistance of a material to the shearing action of the plasticating process. Typical analyses include the study of processing behavior, influence of additives, thermal sensitivity, shear sensitivity, compounding behavior, and others. Torque rheometry is therefore an excellent tool in helping determine the most process-stable formulation (resin, color concentrate, and additives) for your process. It is also often used successfully in trouble-shooting unwanted degradation and gel formation problems.
Let’s examine, in some depth, how this technique works, and how it can be used for your production needs:
In Figure 1, a standard processing curve is shown. This curve type gives information on melt behavior, stable processing time, and degradation behavior. Several points along the curve are used for evaluation. The loading point, L, shows when the mixer is completely filled and closed. This point is used as a time base for calculation only. The stable torque point, S, is when the torque reaches a stable value, usually taken at 10% above the minimum point. The minimum point, M, occurs when the material has reached its lowest viscosity. The onset of degradation point, O, shows when the material starts to degrade by cross-linking. This point is typically taken at 10% above the minimum point. The distance between the S and O is the stable processing time and is used to evaluate the stability of the material being tested. Finally, the degradation point, D, shows when the material has been degraded, after which chain scission predominates. This point can be used to calculate a degradation (or cross-linking) rate.
Figure 1. Standard curve for PVC and thermoplastics.
Figure 2. Rheology curve torque versus time for a 50-gram load, 60 rpms, and 250 C.
However, whether or not the measurement is automated, a considerable reduction in percent error can be accomplished by increasing the stable time by changing run parameters. For example, a 30-second error at each demarcation point translates to a 50% error for a two-minute stable time, but only a 5% error for a 20-minute stable time. This can be accomplished by optimizing the three parameters: 1) loading, 2) mixer rpms, and 3) temperature.
Ampacet has studied the degree of influence on these various process variables. The Pareto chart in Figure 3 shows that for stable time, the major influence in decreasing the stable time is, by far, the temperature. While the second order temperature term shows some inflection, the second most influential term that causes a decrease in stable time is rpms (which of course determines the mean shear rate). These three terms account for about 84% of the effect on the length of stable time.
Figure 3. Pareto chart showing factor effects on stable time.
The Pareto chart in Figure 4 also shows that for the degradation point, although the individual coefficient values are different, the total influence of the temperature, temperature squared, and rpms terms is around 84%. Remember that for the degradation point we are not looking for a maxima or minima, but that it is clearly observable, and for it to occur in a reasonable amount of time. That is, on the order of 25 to 30 minutes.
Figure 4. Pareto chart showing factor effects on degradation point.
A screening model and least squares models were constructed to predict the conditions at which the error in measuring the stable time is minimized, and the degradation point is still observable in the rheology curve. The models showed excellent correlation with R2 values of 0.978 and 0.976 for the stable time and degradation point respectively.
The screening model predicted a reasonable stable and degradation time with a 45-gram load, a rotor speed of 90 rpms, and a temperature of 190 C. The rheology curve shown in Figure 5 was run under those conditions (same 50% TiO2 LDPE concentrate) and produced a curve with a 13-minute stable time, and the degradation point occurring at about 27.5 minutes. This reduces the error from 50 to 7.7% for stable time. Clearly, these conditions represent a significantly improved way to determine the comparative stability of two equivalently loaded white concentrates. In the same manner, processing conditions can be found for other color concentrates, as well as for the base resins employed in the formulation in question.
Figure 5. Rheology curve torque versus time for a 45-gram load, 90 rpms, and 190 C.
Once suitable values have been established for your system, torque rheometry provides an excellent laboratory method for establishing relative thermal stability and shear sensitivity among various formulations. Additionally, torque rheometry is an excellent method of comparing the effectiveness of different anti-oxidant (AO) packages, which act to increase the stable process time. Ampacet offers a variety of standard AO concentrates.