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The influence of polymer structure on melt strength behavior of PE resins.

The melt strength of polyethylene (PE) resins is an important property, one that is believed to determine the processing characteristics of a material during product forming. In the film blowing process, for example, a crucial processability factor is bubble stability, which appears to be controlled by the melt strength behavior of polyethylenes.(1,2) In the case of pipe extrusion, extrusion blow molding, and thermo-forming, the sagging characteristics of molten polyethylenes are largely determined by melt strength behavior.(3) The melt strength property can also be used to evaluate extrusion coating resins, because it is generally found that a higher melt strength correlates with a low degree of neck-in.(4,5)

In simple terms, the melt strength of a polymer indicates the resistance of a melt to extension, or sag. Therefore, it is generally related to the "extensional" viscosity of the polymer, as the melt index and flow index properties are related to the "shear" viscosity of the polymer. Although the extensional viscosity of a polymer is a well-defined rheological property, its utility is limited in practical situations because extensional viscosity data are normally available under isothermal, uniform, and low strain conditions. On the other hand, in most polymer processing operations, neither strain rate nor temperature is uniform. Furthermore, strain rates are very high in many commercial operations, such as film blowing and extrusion coating. Therefore, it is difficult to use extensional viscosity information obtained at low strain rates to predict resin performance in practical/commercial applications. Under these circumstances, measurement of melt strength under nonisothermal and high strain rate conditions (which are closer to the actual processing situations) appears to be more useful.

Several studies, some of which are listed in the references,(6-10) have been done to measure the melt strength of polyethylene resins and their blends. However, most of the previous studies have focused on: a) establishing different techniques for measurement of melt strength; b) correlating effect of melt index (molecular weight) with melt strength; or c) classifying the differences in melt strength behavior of various kinds of polyethylenes, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). The aim of the study discussed in this article was to determine the effect of resin structure on melt strength behavior at various temperatures "within a class" of polyethylene.

Experimental

A microprocessor controlled Rehovis capillary rheometer with tensile module attachment was used in the study. Polyethylene samples were charged into the rheometer barrel, heated, and melted at a desired extrusion temperature. The melted polymer was extruded through a 1.0-mm (L/D = 20) tungsten carbide flat entry die. Extrudate was drawn down in atmospheric air by means of a motor-driven pulley system. Tensile force in the extrudate was determined by measuring vertical force on the rotating pulley [ILLUSTRATION OMITTED]. The test procedure involved increasing the windup speed and noting the steady value of the force level or when the filament broke. This value of the force level is called the "melt strength" of the polymer under given operating conditions.(1,9)

Extrusion was performed at a constant piston speed for all runs. Rotational speed of the drawing pulley was varied, and up to twelve discrete data points were collected for each sample. Melt strength at a particular temperature was determined by averaging values of the data points thus obtained. In most cases, the variability in data was less than 5% of the mean melt strength value. A typical tensile force vs. speed curve, obtained through this test procedure, is shown in Fig. 2.

Results and Discussion

Various LLDPE resins of different molecular weights (melt indices), densities, comonomer types, and molecular weight distributions (MWD) were tested to evaluate the effect of polymer structure on melt strength behavior at several extrusion temperatures.

Figure 3 shows the effect of melt index (or weight average molecular weight, Mw) on melt strength for three hexene copolymer LLDPE resins having similar polymer structures in terms of polydispersity, comonomer distribution, and density. It is clear that as the melt index of a hexene LLDPE resin increases (or as Mw decreases), melt strength decreases at any given test temperature. Similar results were obtained for butene copolymer LLDPE, high pressure LDPE, and HDPE resins. The results are in accordance with the previous studies.(8,9)

Two hexene copolymer LLDPE resins, which had short chain branches and the same melt index (0.5 g/10 min) but different densities (922 and 934 kg/[m.sup.3]), were tested to see the effect of resin density, or comonomer content, on melt strength behavior. Figure 4 shows that resin density in the aforementioned range has no significant effect on melt strength of the LLDPE resins. In order to determine if this were also true for LDPE with long chain branches, two samples were obtained with the same process technology. The LDPE resins had similar melt indices (2.1 g/10 min) but different densities (919 and 923 kg/[m.sup.3]). Figure 5 shows that in the case of high pressure LDPE resins, density has a significant effect on melt strength behavior. Melt strength increases with a decrease in resin density for LDPE resins having similar melt index. This difference in the behavior of high pressure LDPE and LLDPE resins seems to stem from the means by which density is altered in the two types of resins. In LLDPE resins, density is reduced by increasing the number of short chain branches (SCBs) through an increase in the amount of comonomer in the resin. The increase in in the same type of SCBs is not likely to induce a higher degree of entanglement in the polymer melt phase. On the other hand, in high pressure LDPE resins, density is reduced by increasing the number/size of the long chain branches (LCBs) and/or by increasing the degree of crosslinking. These methods, in turn, are more likely to induce a higher degree of entanglement in the macromolecular melt structure. To further investigate the effect of different branching topologies of macromolecules in high pressure LDPE and hexene LLDPE resins, melt strength data for the two resins with similar melt indices (1.0 g/10 min) and densities (919 kg/[m.sup.3]) were obtained. Figure 6 shows that the high pressure LDPE resin with LCBs has approximately four times higher melt strength than the hexene LLDPE resin with SCBs, even though the resins have similar melt indices and densities. This suggests that the length of the branches attached to the main polymer backbone can have a significant influence on melt strength behavior of polyethylenes.

To evaluate the effect of comonomer type on melt strength of various LLDPEs, three different copolymers of the same melt index (1.0 g/10 min) and density (919 kg/[m.sup.3]) but different comonomer type - butene, hexene, and octene - were selected. Figure 7 shows that the comonomer type in LLDPE has a significant effect on melt strength of the copolymer, especially at lower melt temperatures. The melt strengths of these LLDPE copolymers can be placed in the following descending order: octene copolymers [greater than] hexene copolymers [greater than] butene copolymers.

Similar results were obtained for the fractional melt index copolymers, but are not presented here. It is important to note that the effect of comonomer (alpha olefin) type is more pronounced at lower melt temperatures. This may be explained by the fact that at lower temperatures, the higher alpha olefin SCBs produce a relatively higher degree of entanglement, thus increasing melt strength. However, at higher temperatures, the polymer molecules disentangle to such an extent that the effect of comonomer type on melt strength is diminished.

In order to see the effect of comonomer distribution, melt strength data were obtained for three hexene copolymer LLDPEs having slightly different melt indices (0.8, 0.9, and 1.0 g/10 min). Temperature rising elution fractionation (TREF) analysis of the resins, seen in Fig. 8, showed that the resin with the melt index of 0.9 g/10 min had somewhat more uniform comonomer distribution than the other two samples. Figure 9 shows that the melt strength profile for the resin with more uniform comonomer distribution falls between the other two profiles for resins having less uniform comonomer distribution. This indicates that more uniform comonomer distribution in LLDPE has no major effect on melt strength, especially at higher melt temperatures.

To investigate the effect of molecular weight distribution (MWD) on melt strength of unimodal hexene copolymer LLDPEs, four broad MWD resin samples and four narrow MWD samples were tested. The broad MWD samples had melt flow ratios (MFR) in the range of 37.4 to 43.3, whereas the narrow MWD samples had MFR values in the range of 24 to 27. Thus the broad MWD resins represented up to 60% higher MFR values compared with the narrow MWD resins. Polydispersity (Mw/Mn) values of the broad MWD resins were 35% to 60% higher than those of narrow MWD resins. All the broad or narrow MWD samples had a log-normal MWD, i.e., the MWD was altered by modifying or shifting both ends (low and high molecular weight) of the MWD profile. The typical gel permeation chromatography (GPC) profiles for two samples - one narrow and the other broad MWD - are shown in Fig. 10. The melt indices of the eight resin samples ranged from 0.4 to 1.0 g/10 min. Figure 11 shows melt strength data for all eight samples as a function of melt index at three different extrusion temperatures. Figure 11 indicates that at any given temperature, data for both the narrow and broad MWD resin samples fall on the same melt strength curve. This indicates that it is the molecular weight or melt index of the resin that affects melt strength; the MWD (or MFR) does not seem to have any significant influence on melt strength of unimodal LLDPE resins with log-normal MWD profiles.

To investigate how the modality of the MWD affects melt strength, two high molecular weight HPDE resins with similar flow rates (10g/10min) were selected. One resin had an essentially unimodal MWD profile; the other had a bimodal MWD, shown in Fig. 12. Figure 13 shows that at a higher temperature (190 [degrees] C), the difference in melt strengths of the two resins is small. However, at lower temperatures, the unimodal resin has much higher melt strength compared with the bimodal resin. In a separate study, it was also found that the unimodal HDPE resin had much higher die swell than the bimodal HDPE resin, thus indicating a much higher degree of molecular entanglement in the unimodal resin as compared with the bimodal resin.

To evaluate the effect of resin additives on melt strength, two LLDPE film resins with similar melt indices (0.5 g/10 min) and densities (922 kg/[m.sup.3]) but different additive packages were selected. One resin contained only the antioxidants package, whereas the other resin contained antioxidants and commercial quantities of a slip agent, an antiblock agent, and a fluoroelastomer polymer processing aid (PPA). Figure 14 shows that the non-reactive type of additives - such as the slip agent, antiblock, and PPA - do not have any significant effect on melt strength of the resin. On the other hand, these additives - especially the PPA - can change the shear viscosity profiles very significantly.

Conclusion

The effect of polymer structure on melt strength behavior of polyethylene resins was studied at several different temperatures. Melt index (or weight average molecular weight) and extrusion temperature of polyethylenes have a significant effect on melt strength. Melt strength decreases with an increase in melt index and extrusion temperature. For LLDPE resins with SCBs, the density, or amount of comonomer in the resin, does not appear to have any significant effect on melt strength. However, for high pressure LDPE resins with LCBs, melt strength increases with a decrease in resin density.

The type of comonomer (alpha olefin) in LLDPE resins has a significant effect on melt strength behavior of a copolymer. In general, melt strength increases with an increase in length of comonomer. For LLDPE resins that have a log-normal molecular weight distribution, the broadening of MWD does not seem to have any significant influence on melt strength behavior. However, a change in modality of the MWD (unimodal vs. bimodal) can have a major effect on the melt strength behavior of a resin.

In usual commercial quantities, nonreactive additives such as slip agents, antiblocks, and fluoroelastomer polymer processing aids do not have any effect on melt strength of a resin, even though these additives can substantially alter the shear viscosity behavior of a resin.

Acknowledgments

Thanks are due to Messrs. M. Chow, N. Bohnet, and P. Gobin, and Ms. L. Nazarewycz for their assistance in the experimental work presented here. The author also wishes to thank Novacor Chemicals Ltd. for permission to publish this work.

References

1. J.M. Dealy and K.F. Wissbrun, Melt Rheology and Its Role in Plastics Processing, Van Nostrand Reinhold, New York (1990).

2. H.H. Winter, Pure and Appl. Chem., 55, 943 (1983).

3. M.R. Drickman and K.E. McHugh, J. Plastic Film & Sheeting, 9, 22 (1993).

4. E.J. Kaltenbacher, J.K. Lund, and R.A. Mendelson, SPE J., Nov. 1967, P. 55.

5. M.T. Tang, C. Wasson, and S.V. Lin, SPE ANTEC Tech. Papers, 39, 3147 (1993).

6. J. Meissner, Trans. Soc. Rheol., 16, 405 (1972).

7. F.P. LaMantia and D. Acierno, Polym. Eng. Sci., 25, 279 (1985).

8. D. Acierno, D. Curto, F.P. LaMantia, and A. Valenza, Polym. Eng. Sci., 26, 28 (1986).

9. A. Ghijsels, J.J.S.M. Ente, and J. Raadsen, Int. Polym. Process., 5, 284 (1990).

10. A. Ghijsels, J.J.S.M. Ente, and J. Raadsen, Int. Polym. Process., 7, 44 (1992).
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Title Annotation:polyethylene
Author:Goyal, Shivendra K.
Publication:Plastics Engineering
Date:Feb 1, 1995
Words:2300
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