Evaluation of PVC Comt>ound on the Brabender Torque iheometer

TOM HAWKINS Carlon, An Zndian Head Company

R b D Laboratory Oklahoma City, Oklahoma 73155

Increasing the rotor speed during the equilibrium portion of a Bra- bender torque rheometer test makes the test more sensitive to small changes in critical additives. The improved sensitivity of the instru- ment can assist processors in selecting the level of an additive for opti- mum performance and efficiency; in troubleshooting a processing or property problem; and in evaluating the performance ofadditives, for example, in the laboratory, before production trials.

INTRODUCTION th the continual improvement in PVC com-

processing performance, the need for better labora- tory techniques for evaluating PVC compounds is necessary. This paper discusses a minor change in technique on the Brabender torque rheometer, which has improved the sensitivity of the instru- ment to small changes in formulation and also shortened the time for testing.

DISCUSSION With the increased usage of fillers and the ad-

vent of improved resins, stabilizers, lubricants, pro- cessing aids, etc., it has become more and more difficult to measure the total effects of additive changes in a PVC compound. Additionally, be- cause of the requirement for improved product per- formance and higher production rates, there is a continuous need for new and improved methods to measure small changes in processing characteris- tics and formulations.

For years, Carlon has used the Brabender Plasticorder@ with the #6 mixer and measuring head for PVC compound development, in-plant quality control, and process troubleshooting. Re- cently, we modified our Brabender test procedure by increasing the rotor speed 40 to 60 rpm after fu- sion. This modification in procedure allows for testing the early processing characteristics of fu- sion and fluxing of the PVC powder at lower shear and temperature and the longer-term characteris- tics of equilibrium viscosity and stability at higher shear and temperature. This procedure accom- plishes two thines:

w pounds and the requirement for improved

0

0

110

u Makes the test more sensitive to the longer- term properties of equilibrium torque and sta- bility and, therefore, makes it easier to ob- serve the influence of small additive changes or additive differences. Reduces the test time.

INSTRUMENTATION Figure 1 pictures the equipment used for the

C. W. Brabender Plasticorder@ Model #PL- V151. This is a variable-speed unit with a speed range from 0 to 140 rpm. 3% type mixer-measuring head with remova- ble roller blades. The head is oil heated from a controlled-temperature bath. Honeywell Electronic 112 single-pen re- corder. The unit has a temperature range from 125" to 225°C.

All the research in this article was done on this unit, but it is not limited to this equipment. Elec- trically heated units should work equally well as long as the equipment is maintained and the proper procedures followed.

The temperature recorder is not always utilized with Brabender equipment, but for this work, it is very critical. The pictured recorder has proved sat- isfactory. The temperature span and the %' divi- sions meet the requirements for the accuracy of the procedure.

testing in this article. The components are:

F i g . 1 . l'orque rheometer.

JOURNAL OF VINYL TECHNOLOGY, SEPTEMBER 1982, VOL. 4, NO. 3

Evaluation of PVC Compound on the Brabender Torque Rheometer

OPERATING PROCEDURE The appendix has a general description of the

procedure used to conduct all the tests in this pa- per. Table l gives the conditions followed for this procedure when testing the formulation in Table 2. The Brabender conditions, primarily load and rpm, will be varied based on formulation.

The modification in the procedure given in the Appendix, and the basis for this article, is an in- crease in rotor speed after reaching a steady state in torque and temperature. For the formulation listed in Tab2e 2 and all the tests in this article, the starting speed was 80 rpm. After 5 min, a point where a steady state has been reached, the rpm were in- creased to 120 rpm for the remainder of the test. There is nothing magic about 120 rpm. It was se- lected because it is the maximum speed on some of Carlon's instruments and because it gave the de- sired results. Other speeds could be used ifthey ac- complish the same results.

THE BRABENDER PROCESS CURVE Before discussing the data in this article, a review

of the Brabender process curve is in order. Figure 2 is a typical Brabender process curve for

a rigid PVC compound with the critical points noted. Point 1 is the fusion peak. Torque, time, and temperature are noted at this point. Fusion is pri- marily influenced by lubricants and process aids. An external lubricant will increase fusion time and reduce torque and temperature, the reverse being true for internal and processing aid.

The second point is the portion of the curve referred to as equilibrium. Torque and temperature are monitored at equilibrium. At this period in the processing curve, the melt is homogeneous and in a melt state for fabrication to take place. Under these conditions, the torque should be smooth and the level and temperature stable. Equilibrium condi- tions can be affected by nearly anything in the formulation.

Point 3 is defined as the onset of crosslinking, or degradation, and determines the ultimate stabil-

Table 1. Operating Conditions

Cavity Temperature' "C

Rotor Speed, rpm

- 180 2 1

- See Report Charge, g - 68 -C 0.1

Scale, mg - 0 to 5,000

' Air space measured wlth thermocouple

Table 2. Control Formulation

Materlal PHR

Resin, 0.88 to 1.00 I.V. 100.00 Stabilizer 0.70 TiO, CaCO, Filler Wax Lubricant 'X' Calcium Stearate

2.00 25.00

1.0t01.4 0 to 0.6 1.6 to 2.4

TI... Ml""?.,

F i g . 2. Typical torque rheometer process curoe.

ity of the compound. This point is defined by the intersection of the slope of the upturning torque and the equilibrium torque. Stability time is the difference between degradation time (Point 3) and fusion time (Point 1). The stability of a compound is determined by the efficiency of the heat stabilizer and/or the stock temperature.

There are two other variables that are important in determining the overall processing characteris- tics of a PVC compound:

Stock temperature-Stock temperature is a reflection of all aspects of the formulation and should be monitored accurately throughout. The general shape and characteristics of the process curve-As you will see later in this ar- ticle, the stability and type of curve will indi- cate lubrication characteristics.

OLD VS. NEW PROCEDURE In order to demonstrate the differences between

the two Brabender procedures, two levels of wax were selected, 1.1 phr us. 1.3 phr, and process curves were run using both procedures.

Table 3 summarizes the results of these runs. There are no differences between results in the fu- sion portion because no change in procedure was made. The typical effects of increasing external lu- brication, an increase in fusion time and a reduc- tion in torque, can be noted. In the second portion of the curve, after the increase in rotor speed is made, the differences in the procedures are appar- ent. In both cases, as the wax is increased, there is a reduction in torque and an increase in stability time. What is apparent only in the new procedure is a reduction in stock temperature of 5"C, which ex- plains the lengthening of the stability. Using the old procedure, it could only be assumed that a re- duction in stock temperature caused the added stability.

The more dramatic aspects of the new procedure show up in the shapes ofthe torque and the temper- ature curves. Based on pipe production experience, it is known that 1.1 phr of wax in this formulation gives an underlubricated condition, high stock temperature and motor load, wavy I.D., and early yellowing. However, using the old procedure, this

JOURNAL OF VINYL TECHNOLOGY, SEPTEMBER 1982, VOL. 4, NO. 3 111

Tom Hawkins

1000

0 '

Table 3. Effects of Wax Level: New YS. Old Procedure

4 170

$60

Equilibrium Stability 80 rpm 120 rpm 80 rpm 120 rpm Fusion @80 rpm

Torque Temp. Torque Temp. Torque Temp. "C Time, mln Time. min mg "C mg "C mg Time, min

1175 1 98 1350 21 1 42.9 20.8 1.1 1.5 3100 188 1.3 1.7 2875 188 975 197 1150 206 53.1 27.4

would not have been apparent from the Brabender curve, (see Fig. 3A).

Figure 3 B uses the same compound and, compar- ing 1.1 and 1.3 phr of wax in the new procedure, the underlubricated condition is very visible. At 1.1 phr of wax, the stock temperature is very unstable and continues to climb. The torque never comes to equilibrium, and these are both signs of underlubrication. At the 1.3 phr level of wax, both temperarture and torque are stable.

DATA The remainder of this article will show how the

modified procedure can be used to detect the small changes in components of the control formu- lation.

EXTERNAL LUBRICANT-WAX In Table 4 , the wax (external lubricant) has been

varied from 1.0 to 1.4 phr in 0.1 part increments. The results of increasing the wax are: sow, , 210

1 I I I I 10 PO 10 4 0 x) 60

T l m - M8nul.s

F i g . 3A. Torque rheometer processing curve 6380 rpm

6000

I000

4000

: Ye A- c1

f 3000

2000

210

210

200

I90

I80

oy

f

Increase in fusion time with a corresponding

Reduction in stock temperature. A lengthening of stability time.

Figures 4A and B graphically show the effects that the wax has on the variables of stability and stock temperature. It should be expected that stock temperature would be reduced and stability lengthened with increasing wax. The higher rotor speeds during the equilibrium portion of the test clearly show this effect.

reduction in torque.

INTERNAL LUBRICANT-CALCIUM STEARATE

Table 5 shows the effect that the calcium stearate has on the processing of the control formulation at the level of 1.6 to 2.4 phr. The test and/or the formu- lation are not as sensitive to calcium stearate as it is to wax. Therefore, the increments of change were increased to 0.2 parts.

The first influence noted is the effects that cal- cium stearate has on fusion. As the level of calcium stearate is increased, the fusion time is faster and the torque higher. This is explainable by a more rapid increase in stock temperature and a lower fu- sion temperature. Figure 5 graphically displays this at the 1.8 and 2.2 phr levels. This phenomenon holds true for internal lubricants as well as pro- cessing aids.

In the equilibrium portion of the curve (at 120 rpm), as the calcium stearate increases, so does the stock temperature. This fact does not appear at the lower (80 rpm) speed.

Finally, as the calcium stearate is increased, the stability time lengthens. This is true even though the stock is hotter, which demonstrates the costa- bilizing nature of calcium stearate.

Lubricant 'X' Table 6 shows the effects of a lubricant 'X' from 0

to 0.6 phr. Lubricant 'X' is a proprietary lubricant that is very efficient in certain formulations.

Because lubricant 'X' has very little effect on fu- sion, it is necessary to optimize the level based on its influence on other aspects of processing. Using the technique of increased rotor speed, it was easy to see the results that the various levels have on the system and to select the optimum level of 0.4 phr. Below this level, the stock temperature is out of control; above this level, signs of overlubrication rapidly appear.

112 JOURNAL OF VINYL TECHNOLOGY, SEPTEMBER 1982, VOL. 4, NO. 3

Evaluation of PVC Compound on the Brabender Torque Rheometer

Table 4. Effects of Wax on the Brabender Process Curve

Fusion Equilibrium

PHR mg min mg "C min

1 .o 3225 1.3 1425 312.5 18.4 Unstable torque & temp. 1.5 1325 21 2.0 21.3 Unstable torque & temp. 1.1 31 00

1.2 3037 1.6 1200 209.5 25.8 Stable torque & temp. 1.3 2875 1.7 1150 208.0 27.4 Stable torque & temp. 1.4 2762 2.2 1150 208.0 >30. Chatter on torque line

Torque, Time, torque, Temp. Stability Comments

Table 5. Effects of Calcium Stearate on the Brabender Process Curve

I orque, Time, Torque, Temp., Stability Comments PHR mg min mg "C min

1.6 2800 1.8 1200 1.8 2850 1.7 1200 208.0 2.0 2950 1.4

2.2 3000 1.3 1250 2.4 3100 1.1

208.0 20.8 Stable torque & temp. 22.8 Stable torque & temp.

1225 209.0 23.4 Signs of instability in

21 0.0 24.2 Unstable torque & temp. 1300 21 1 .o 22.6 Unstable torque & temp.

torque and temperature

STABILIZER EVALUATION Table 7 is an example of data obtained when

evaluating a stabilizer candidate using the mod- ified Brabender procedure. This example is a com- parison of a stabilizer ofknown performance, a con- trol, us. a new candidate, stabilizer 'A'.

The evaluation consists of substituting stabilizer 'A' directly for the control and running a processing curve. The results of this run are reviewed, and a decision is made on what (if any) formulation ad-

14 I I I I

I I I I 10 15 20 25 30

0.9 ' Stability lime-Minuter

Fig. 4A. Effect of externul lubricant on stability time

I 4

\-

", \ \

I 0 \-

20b 208 210 212 214 Stork lempe.~ture-~C

Fig. 4 B . Effect of externul lubricunt on stock temperature.

justments should be made. In this particular case, stabilizer 'A' showed itself to be less lubricating. The last data line in Table 7 shows the results after the wax level was increased 0.1 hr. As can be seen,

once the adjustment is made. This is the first step used in evaluating a new sta-

bilizer candidate. Once the processing characteris- tics are adjusted, it is possible to compare the ef- ficiency of the stabilizer on its own merits. In some cases, a candidate will be eliminated at this point, In other cases, as with stabilizer 'A', the evaluation will move into the next phases of testing, which would include a production trial.

CONCLUSIONS The simple technique of increasing rotor speed

during the equilibrium portion of a Brabender pro- cessing test makes the test more sensitive to small changes in critical additives. This can assist in:

Selecting the level of an additive to give opti- mum performance and efficiency. Troubleshooting a processing or property problem.

stabilizer 'A' processes very c I/ ose to the control

200

0"

5 190

j x 1 180

m

170

I60 0 I 2 3 4 5

I$me-Mmutes

Fig. 5 . Effect of culciurn steurute on temperuture rise to fusion.

JOURNAL OF VINYL TECHNOLOGY, SEPTEMBER 1982, VOL. 4, NO. 3 113

Tom Hawkins Table 6. Effects of Lubricant ' X on the Brabender Process Curve

Fusion Equilibrium Torque, Time, Torque, Temp. Stability

PHR m9 min m9 "C min Comments

0 3050 1.4 1400 224.0 9.5 Temp. out of control 0.2 3000 1.5 1300 21 8.0 12.9 Temp. out of control 0.3 2925 1.6 1250 213.0 16.7 Unstable temp. 8 torque

1.4 1150 209.5 23.6 Stable temp. & torque 0.4 3050 0.5 2875 1.6 1200 207.5 27.1 Chatter on torque line 0.6 2900 1.5 1200 207.5 27.3 Chatter on torque line

Table 7. Stabilizer Evaluation on the Brabender

Fusion Equilibrium Torque Time Torque Temp Stability

Compound mg min mg "C min

Control 3050 1.5 1200 209.0 24.7 Stab. 'A' 3150 1.2 1350 212.0 20.4 Stab. 'A' 3050 1.4 1175 209.5 24.5

Adjusted

Evaluating their performance of like additives in the lab to make a final selection for produc- tion trials. Reducing the time of testing

ACKNOWLEDGMENTS The author wishes to thank Larry J. Hamilton for

his keen observations and numerous hours of per- forming tests, without which this article would not have existed. Also, I want to thank the many people at Carlon whose support and consultation were so important to the finished product.

APPENDIX The following is a general procedure for

Step 1. Allow cavity temperature to level out at 180" for 10 min and adjust speed to start- ing rpm.

Step 2. Put quick-loading chute into head and slowly pour powder into chute.

Step 3. Immediately after powder has been loaded, insert piston with 15 kg weight in place and slowly lower piston.

Step 4. Once piston has bottomed, turn on torque chart and temperature recorder.

Step 5. Just before reaching the fusion peak, re- move loading chute and swing loading arm into place.

Step 6. Stop test after crosslinking and thor- oughly clean rotors, bowl, and bushings.

Step 7. Lubricate bushings and wait 10 min after cavity temperature is up to 180°C before starting next test.

operating the Brabender Plasticorder:

114 JOURNAL OF VINYL TECHNOLOGY, SEPTEMBER 1982, VOL. 4, N O . 3