Comparative Effect of Plasticizer Interchange on Flexible PVC Processing as

Measured by a Torque Rheometer SIMO T. BERGADO

Basic Organic Chemicals BASF Corporation

Kearny , N e w J e r s e y

Present testing methods of comparing dryblend and fusion characteristics of poly (vinyl chloride) address resin interchange, but not plasticizer interchange. The wide range of available plasticizers requires methodology to predict plasticizer performance from a processing viewpoint. Modification of the test parameters reported in this study makes it possible to predict how plasticizer interchange affects processing under dynamic conditions.

INTRODUCTION

he widespread use and availability of the torque T rheometer has made it a valuable tool in the poly (vinyl chloride) industry. Standard ASTM procedures have been established for the examination of PVC processing in the dryblend and fusion areas. Resin, being a major ingredient, has been the essential vari- able of these standard tests. Other components of the formulation have been constants. This study has followed ASTM procedures and altered them, where necessary, to make them applicable to plasticizer interchange. Three general purpose phthalate plas- ticizers and three PHR levels were selected to repre- sent the behavior and trends created by a change in plasticizer or its level.

The major key to plasticizer selection lies in its contribution to critical end use properties of a fin- ished article. However, economic constraints in- volved in producing any finished article often raises the question of processing ease to an equivalent level of importance. Contributions of the plasticizer to processing are attributable to its molecular weight, its solvating efficiency, and the amount used. To make the study meaningful, the plasticizers selected cover a range of molecular weight and solvating power. Di-2-ethylhexyl phthalate (DOP), di(hepty1, nonyl, undecyl) phthalate (a semi-linear 7-9-1 1) and diisodecyl phthalate (DIDP), were chosen to meet these requirements. The chosen PHR levels of plas- ticizer were 35, 52. and 70. The 52 PHR is the level specified in the ASTM D-2396 procedure for dry- blend. The other two levels are a nominal high and low bracket. In addition, these PHR levels cover a range well related to actual end use requirements.

DISCUSSION

To review dryblending, it can be described as the action of liquid plasticizers penetrating into and ab-

sorbing onto resin particles to form a free-flowing, relatively homogeneous mixture. The torque rheom- eter monitors the drying action through changes in torque and temperature during the process. A typical dryblend curve exhibits an increase in torque at plas- ticizer addition, a level section at initial mixing, a rise to a peak torque, and then a drop to a minimum (Fig. 1 ) . The peak torque portion of the curve indicates a stage where liquid plasticizer has wet the surface of the resin particles. Plasticizer, having been mixed with the resin in the heated Sigma mixer, becomes completely absorbed into the resin pores and is equally dispersed among those particles. Torque drops to a minimum where the dry point determina- tion can be made.

Considering that dryblending is a physical process, it is dependent upon the physical properties of the formulation components. In the case where resin is a variable, particle size, pore volume, and pore di- ameter are important factors in the dryblend process. In this study, resin is held constant.

For a liquid plasticizer, its viscosity and solvating power are the parameters involved in the dryblend mechanism. These properties enhance many of the actions involved in the dryblend process. For exam- ple, the action of the plasticizer is to adhere to the resin particles and become completely distributed among them. Another important factor is the rate of capillary action of the plasticizer penetrating the resin pores. This is dependent, primarily, upon the viscosity of the liquid. The last factor of plasticizer contribution to dryblend time is the concentration employed. The less liquid there is to be absorbed, the less time it takes to dryblend, other factors remaining constant.

In reviewing the torque rheometer representation of the fusion mechanism, three sectors of the torque- time curve are involved (Fig. 2). The first sector of the curve indicates fusion time, which shows up as

38 JOURNAL OF VINYL TECHNOLOGY, MARCH 1990. VOL. 12. NO. 1

Effect of Plasticizer Interchange on Flexible PVC

f ~ ~ ~ r n r n ~ ~ ~ m g ~ ~ ~ r n r n o , ~ ~ ~ ~ ~ ~ ~ ~ ~

Fig. 1. Typical dryblend curve.

Typical torque-f u s i o n curve Figure 2

'"""7 '""1 FUSION. POINT a00

TIME

Fig. 2. Typical torque fusion curve.

as a sharp increase in torque and gives an indication of the stability of the melt. The stability portion is not a consideration in this study. Our primary inter- est was to show how plasticizer interchange affects fusion characteristics such as time, torque, and tem- perature at the fusion point and at equilibrium. The recommended ASTM D-2538 conditions of tempera- ture and RPM ( 140°C and 31.5 RPM), did not ade- quately differentiate fusion times for different plas- ticizers in the general purpose range. After trials a t different temperatures, a temperature of 1 15°C was selected. After this temperature adjustment was made, definitive effects of plasticizer interchange on fusion time were noticed. This was not possible with the ASTM D-2538 procedure which specifies 140°C. In the present study, fusion time increased with in- creasing molecular weight of plasticizer. It was also observed, as expected, that increasing the amount of plasticizer decreases fusion time.

INSTRUMENTATION AND PROCEDURE a peak torque deflection. The second sector is the equilibrium portion which shows a virtually level The torque rheometer used for this study was a torque output. Observations of the melt characteris- Brabender Plasticorder of the mechanical type. The tics are found in this stage. The third sector is the powder-mix blend was processed in an oil-heated 650 crosslinking or degradation stage, which shows up mi Sigma mixer. Likewise, the heating medium for

JOURNAL OF VINYL TECHNOLOGY, MARCH 1990, VOL. 12, NO. 1 39

Sirno T. Bergado

the #5 roller-mixer fusion head was also an oil-heated type. This equipment, and others of similar nature, are well known and widely used in the laboratory. The standard ASTM procedures for powder-mix and fusion (D-2396, D-2538) served as guidelines. Table 1 outlines the base formulation and conditions of the dryblend test. Dry ingredients are mixed for 5 min- utes before plasticizer addition. Ten minutes after the dry point, the test is terminated. Evaluation of the dry point was accomplished by drawing best-fit, straight lines through two segments of the curve, which are the drop-off sector after peak torque and the sector following the minimum. Time was read to the nearest tenth of a minute at the intersection of the two lines. The corresponding temperature was also recorded.

The fusion portion of the study, following ASTM D-2538, utilized 47.0 gms of the previously mixed dryblend. In the method, the 47.0 gm charge is loaded into the heated mixer head and held for three minutes at the test temperature with the rotors off. When the mixer is turned on, the torque indicator rises to a peak and then drops off into the equilibrium stage. Fifteen minutes after peak torque, the run is stopped. In similar fashion to dry point determination, best- fit lines are drawn through the curve in the peak torque portion to measure fusion time. Peak torque, time to peak torque, and melt temperature are noted at this point. In the equilibrium section, the torque fifteen minutes past fusion time is defined a s the apparent melt viscosity (AMV). Temperature is noted at this point also.

EXPERIMENTAL RESULTS Dryblending was first evaluated at the standard 52

PHR level comparing DOP and semi-linear 7-9-11 phthalates. The drying time for both plasticizers were very close with the semi-linear only a matter of sec- onds longer. AT the 35 PHR and the 70 PHR level, times are similarly close, but in the same order. Introducing DIDP into the picture, its dryblend time was approximately 75% higher than DOP or the semi- linear 7-9-1 1 at all three PHR levels. Dryblend times increase with increasing PHR levels for all three

ing DIDP into the formulation, the incremental in- crease in molecular weight from the linear is about the same as that from DOP to the semi-linear. How- ever, the increase in viscosity and decrease in solvent power of the DIDP would be major factors in account- ing for its longer dryblend times.

In a brief examination of a higher molecular weight homologous series, trimellitates at the 52 PHR level, a relationship exists between the molecular weight and viscosity of the plasticizer and the dryblend time, similar to that exhibited by the phthalates. Compar- ing TOTM and a linear 7-9 trimellitate, (Table 3). there is little difference in dryblend times. The small increase in molecular weight of the linear is offset by a decrease in its viscosity. In changing to a linear 8- 10 trimellitate, the large increase in molecular weight outweights the lower viscosity and the dryblend time increases. The extreme case is exhibited by the phys- ical properties of TINTM. The high molecular weight and high viscosity combination produces a dryblend time nearly twice that of any other trimellitate in this formulation, indicating the necessity of closely mon- itoring its processing parameters.

Fig. 3. Dryblend times.

Table 2.

Dlasticizers. These results are shown in Fia. 3. Viscosity - Molecular (CPS

Plasticizer Weight 20%) Examination of the physical properties, molecular

weight and viscosity in Table 2, reveals the reasons behind the swings in dryblend time when switching DOP 391 81

Semi-Linear 7-9-1 1 41 8 58 DIDP 447 114

from one plasticizer to another. In comparing the DOP and the semi-linear 7-9- 11, there exists off- setting properties where increase in molecular weight is accompanied by a decrease in viscosity. Substitut-

Table 1.

Formulation Grams PHR

Resin (GP-4) 225.0 100 Clay 15.7 7 Lead Carbonate 22.5 10 Plasticizer 117.0 52

Table 3.

Dryblend Time Fusion

Visc. Plasticizer Mol Wt (2OOC) (minutes) (115OC)

TOTM 547 260 12.8 5.3 Linear 7-9TM 548 135 12.8 8.7 Linear 8-1 OTM 590 (est) 125 16.3 20.0 TINTM 588 430 27.5 34.3

40

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JOURNAL OF VINYL TECHNOLOGY, MARCH 1990, VOL. 12, NO. 1

Effect of Plasticizer Interchange o n Flexible PVC

For the fusion portion of the test, initial runs using ASTM D-2538 conditions of 140°C and 31.5 RPM were carried out. At the 52 PHR level, all three plas- ticizers produced fusion times of one minute, making differentiation between the three plasticizers impos- sible. A temperature adjustment seemed necessary for several reasons. First, it was felt that the change in conditions should be a simple one, and secondly, rotor speed was already on the low side (31.5 RPM). Further, in many processes, a temperature adjust- ment is the simplest form of control. To determine an optimum temperature, the basic 52 PHR formu- lation containing DOP and DIDP were run at temper- atures ranging from 90°C to 140°C. Results are shown in Fig. 4. The selection of 115°C as a working temperature was influenced by the fact that differ- ences in fusion time were measurable and that test- ing times would not be unreasonably long, even for some of the higher molecular weight phthalate plas- ticizers in current use. In comparing DOP and the semi-linear 7-9-1 1 at 52 PHR, the tests conducted at 115°C showed a significant difference in the fusion times, indicating that the higher molecular weight plasticizer does take longer to fuse under similar conditions. The viscosity factor does not appear to effect fusion time.

Substituting the DIDP formulation into the test, the effect of increased fusion time with increased molecular weight was observed. In the 70 PHR range, the relationship of molecular weight to fusion time remains the same. Results are shown in Fig. 5. At the 35 PHR level, there was a deviation from the previous trends. Fusion time for DOP was greater than either the semi-linear 7-9-1 1 or DIDP, with DIDP slightly longer than the linear. The data at this point, appear to contradict previous trends that are repre- sented at the 52 and 70 PHR levels. Causes of this unexpected phenomenon may not be readily appar- ent. This however, is not the subject of this study. The fact that this behavior at low PHR levels is revealed makes one thing clear. The proposed method, as applied to this formulation, performs the

20 19 - 18 5

6 - 5 - 4 -

3 - 2 - 1 -

90 110 130

TEMP dag C 0 DOP + DIDP

Fig. 4. Fusion t ime us. t emperature .

Fig. 5. Fusion t imes .

task it was intended to do. It reveals expected and unexpected behavior of the system. The individual processor, knowing the behavior of his own system before plasticizer interchange, can anticipate where a necessary adjustment, either in formulation or op- erating parameters, must be made in order to retain control of system performance.

CONCLUSION

Prediction of the effects of plasticizer interchange on the dryblending and processing of a flexible vinyl system is possible through the use of the torque rheometer. For dryblending purposes, the standard ASTM D-2396 for powder mix provides the necessary distinctions important to the processor, that is, how plasticizer interchange affects blender output. With regard to the fusion aspect of processing, distinction that was unobservable through the original ASTM D- 2538 method was made possible by lowering the temperature from 140°C to 1 15°C. The temperature of 115°C is specific only in the fact that it enables one to examine the range of phthalate plasticizers and their relative fusion times. A different tempera- ture may be appropriate for a system that contains ingredients that would cause a shift in the range of fusion times that need to be evaluated. Therefore, this approach of targeting an optimum temperature to investigate fusion characteristics over a broad range of formulation constraints will prove a useful tool to the processor in keeping competitive in a changing marketplace.

ACKNOWLEDGMENTS

The author wishes to thank BASF for permission to publish this work and thanks William H. Bauer for his guidance and direction.

BIBLIOGRAPHY 1 . ASTM D-2396-79, “Powder Mix Test of Poly (Vinyl

Chloride) Resins Using a Torque Rheometer.“ 2. ASTM D-2538-79, “Fusion Test of Poly (Vinyl Chloride)

Resins Using a Torque Rheometer.”

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Simo T. Bergado

3. J. A. Wingrave, “Behavior of Plasticizers in Poly (Vinyl Viny l Tech.. 1, 2 (1979). 6. L. I. Nass , Encyclopedia of PVC, Vol. 1.

4. M. T. Payne and J. A. Cannon, “Raw Materials in 7. F. C. Schutz, “Predicting Extruder Performance with Plasticized PVC,” SPE Journal (1960).

8. Walter Klesper. Plastics Design & Processing, Jan. 1972.

9. J. Darby, Fusion of PVC in a Banbury, 1957.

Chloride) Resins,’’ J. Vinyl Tech., 2. 3 (1980).

Flexible Vinyl Dry Blending and Extrusion”, J . Vinyl Tech., 4, 3 (1982).

5. D. H. Paul, “The Effect of the Plasticizer System on the Processability of Poly (Vinyl Chloride) Compounds,” J.

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