Modification of Rheological Properties of LDPE for Coating

The rheological properties of the modified materials were measured, and no significant differences in their behavior under shear deformation were dete...
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MATERIALS AND INTERFACES Modification of Rheological Properties of LDPE for Coating Applications Karen Xiao,† Costas Tzoganakis,* and Hector Budman Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

The effect of shear modification and blending on the coating performance of a commercial LDPE resin was studied. Shear modification of the virgin resin as well as of its blends with another LDPE was performed in single- and twin-screw extruders. The rheological properties of the modified materials were measured, and no significant differences in their behavior under shear deformation were detected. On the other hand, changes in the entrance pressure drop and extensional viscosity were observed. These changes in extensional behavior were found to affect coating properties. Overall, neck-in improved through shear modification, whereas it was not sensitive to blending with a higher-molecular-weight resin. On the other hand, blending had an effect on the draw-down speed, and it can be used to achieve a balanced coating performance. Introduction Low-density polyethylene (LDPE) is a commodity polymer used extensively in extrusion operations such as coating, blown film, blow molding, and foaming. Extrusion coating with polyethylene has been widely used commercially on substrates such as paper, other plastic films, cloth, and glass fiber. The advantages of coating with polyethylene include increased tear and crease resistance; a good barrier against moisture, grease, and oil; flexibility; nontoxicity; and low coating cost. The coating process parameters that indicate how well the polyethylene of interest will coat onto the extrudate are the neck-in and draw-down speed. Neckin is defined as the difference between the width of the die surface and that of the coated part of the substrate, as shown in Figure 1. Draw-down speed is the speed at which the coated substrate can be pulled without breaking. It has been shown that a high draw-down speed is accompanied by an increase in neck-in, which is undesirable.1 In an extrusion process, it is desirable to minimize the neck-in and maximize the draw-down speed.2 Therefore, a compromise must be made between high draw-down speed and low neck-in. This can be achieved by modifying the rheological properties of the polymer used. Several researchers have found that shear modification of LDPEs results in a decrease in melt elasticity.4-8 Also, it has been found that the attainable degree of modification depends to some extent on the molecular weight distribution (MWD) and the branching frequency and that highly branched materials are most susceptible * Corresponding author: Prof. C. Tzoganakis, Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1. E-mail: [email protected]. Tel.: 519-888-4567 ext. 3442. Fax: 519-746-4979. † Current address: Brampton Engineering, Brampton, Ontario, Canada.

Figure 1. Schematic diagram of an extrusion coating process and definition of neck-in.

to shear modification treatment.9 Shear modification effects in a single-screw extruder were found to be most prominent at lower processing temperatures, increased screw speeds, and higher screw compression ratios.10 Modification of rheological properties by blending has also been studied extensively,11-13 and it has been shown that LDPEs and high-density polyethylenes (HDPEs) are miscible and that their viscosity follows a logarithmic mixing rule. The melt rheology of miscible blends of LDPE, HDPE, and linear low-density polyethylene (LLDPE) has been investigated recently,14 and it has been reported that the melt strength of the blends increased as the molecular weight of the LDPE used was increased. It has been observed that melt strength is affected by comonomer type and MWD modality,15 and increased melt strength was observed to lead to reduced neck-in performance. Finally, relating polymer molecular structures to rheological properties is very important, and several studies have addressed this topic.16-19 Such relationships are helpful in selecting materials as well as in inferring their performance in extrusion coating processes. In this research work, a commodity coating grade LDPE was modified through shearing and blending with another LDPE resin in an extruder. The main objectives

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Figure 2. Screw configuration of the twin-screw extruder used in the blending and the shear modification experiments.

Figure 3. Molecular weight distributions (MWDs) of the two virgin materials, LDPE1 and LDPE2.

of the work were (i) to improve the coating properties of the base resin and (ii) to identify rheological properties that have an impact on neck-in and draw-down speed. Experimental Section Materials. Two low-density polyethylene resins were used in this work: LDPE1, a coating grade resin, (Nova h z ) 352080 g/mol, PDI Chemicals, M h w ) 100000 g/mol, M ) 7) and LDPE2 (Nova Chemicals, M h w ) 120000 g/mol, M h z ) 521820 g/mol, PDI ) 11). Equipment and Experimental Procedures. The base resin, LDPE1, was modified by shearing as well as blending with small amounts of resin LDPE2. Experiments were carried out in a single-screw extruder (Haake Rheocord, Rheomix 3000) as well as in a twinscrew extruder (Leistritz LSM 30.34, L/D ) 35). The twin-screw configuration used in the shear modification experiments is shown in Figure 2. For the blending experiments, the materials were dry-blended before being fed into the extruder through a hopper, and the polymer strands coming out of the extruder were quenched in a cold water bath, pelletized, and collected. Blends containing 1-10% LDPE2 were investigated, and their MWD was characterized by gel permeation chromatography (GPC) using a Waters 150CV plus unit. The MWDs of the blends were found to be very similar, and this can be easily explained by the proximity of the MWDs of the two virgin materials (Figure 3). The melt flow index (MFI), the extrudate swell, and the entrance pressure drop of the resultant materials were all measured by capillary rheometry using a Kayeness Galaxy V capillary rheometer. Linear viscoelastic properties were measured in oscillatory shear experiments using a Rheometrics mechanical spectrometer. The

Figure 4. Effect of shearing and addition of LDPE2 on the entrance pressure drop for the single-screw extruder runs at various shear rates (b, 1000 s-1; 9, 700 s-1; 2, 400 s-1; 1, 100 s-1; (, 50 s-1). Open symbols correspond to the virgin LDPE1 material.

modified blends were subsequently evaluated in paper coating experiments (Nova Chemicals, Sarnia, Ontario, Canada), and their corresponding draw-down speed and neck-in were measured. The reproducibility of the experimental data was verified by repeating the blending and coating experiments at some of the blend compositions. Results and Discussions Rheological Properties. Addition of the highermolecular-weight resin LDPE2 resulted in an expected slight decrease in the MFI. Addition of LDPE2 in the range of 1-10% resulted in a decrease in the MFI from 3.81 to 3.35 for the single-screw extruder runs and from 3.81 to 3.15 for the twin-screw extruder runs. The effect of LDPE2 addition on extrudate swell was insignificant, while the effect on entrance pressure drop is shown graphically in Figures 4 and 5 for the single-screw and twin-screw extruder runs, respectively, at various shear rates. Shearing of the virgin LDPE1 material resulted in a reduction of the entrance pressure drop, while addition of LDPE2 did not seem to have a large effect. Inspection of the linear viscoelastic response of all of the materials revealed no major differences, as can be seen in Figures 6 and 7 for the single- and twin-screw experiments, respectively. Although the oscillatory shear data of the samples modified in the twin-screw extruder were very similar, their extensional viscosities revealed more differences between the virgin LDPE1 resin and

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Figure 5. Effect of shearing and addition of LDPE2 on the entrance pressure drop for the twin-screw extruder runs at various shear rates (b, 1000 s-1; 9, 700 s-1; 2, 400 s-1; 1, 100 s-1; (, 50 s-1). Open symbols correspond to the virgin LDPE1 material.

Figure 7. Storage and loss moduli of the virgin LDPE1 and the materials modified in the twin-screw (TSE) extruder.

Figure 8. Extensional viscosity of the virgin LDPE1 and the materials modified in the twin-screw (TSE) extruder. Figure 6. Storage and loss moduli of the virgin LDPE1 and the materials modified in the single-screw (SSE) extruder.

the shear-modified materials. The extensional viscosity of the materials modified in the twin-screw extruder was calculated from entrance pressure drop data using the method of Cogswell.20 Figure 8 shows the resulting extensional viscosities of the virgin LDPE1, of the shearmodified LDPE1, and of three shear-modified blends. These findings support the disentaglement process for the TSE-modified materials that underwent more intense shearing than the single-screw extruder modified ones. It can be seen that the extensional viscosities of all of the materials are very similar at high extensional rates. However, at low extensional rates, all of the modified materials exhibited reduced extensional viscosities compared to that of the starting LDPE1. This indicates that these materials can be stretched to higher total strains before breaking.21 Also, it can be seen that, at the lower rates, the extensional behavior of the virgin LDPE1 changes to strain hardening for the modified LDPE1 and blends. This is a fundamental difference

that is believed to affect the coating performance, as will be discussed next. It should be pointed out that the extensional viscosities of the single-screw-extrudermodified materials were calculated as well. However, no significant differences among these materials could be detected. The changes in extensional viscosity suggest subtle changes in the MWD (i.e., a change in branching or a change in the high-MW tail end of the MWD). Such changes could not be detected through GPC measurements, as the MWD of the modified samples were identical within experimental error. Coating Experiments. Both the single-screw- and twin-screw-extruder-modified blends were sent to Nova Chemicals for coating experiments. As mentioned before, the goal of the blending/shearing experiments was to achieve a lower neck-in and, at the same time, to maintain the draw-down speed at a desired level. Single-Screw Extruder. Figure 9 shows the results for the single-screw-extruder- (SSE) modified blends, 1-6%. The blends processed through the extruder showed a significant decrease in neck-in compared to that of the

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Figure 9. Extrusion coating results: neck-in and draw-down speed of the virgin LDPE1 and the materials modified in the single-screw (SSE) extruder.

Figure 10. Extrusion coating results: neck-in and draw-down speed of the virgin LDPE1 and the materials modified in the twinscrew (TSE) extruder.

virgin material before extrusion. As mentioned previously, a small neck-in and a high draw-down speed are desired in extrusion coating. As the material was processed, although neck-in performance showed a significant improvement, the draw-down speed decreased also, which was undesirable. One interesting point to note here is that the decrease in the neck-in seems to be independent of the amount of LDPE2 present in the blend. The neck-in after processing remained relatively constant for different percentages of LDPE2 in the blend. The percent of LDPE2, however, does seem to affect the draw-down speed performance during processing. The 2% blend showed a significant increase in the draw-down speed, while the neck-in remained relatively the same as that for the 1% blend. This observation can be tentatively explained by the fact that the improvement in neck-in is due to the disentanglement of the polymer chains, which causes a change in the extensional behavior of the material. However, this also means that the material cannot be stretched as much, and the draw-down speed decreases. By the addition of LDPE2, the molecular weight increases slightly, and the draw-down speed improves. Twin-Screw Extruder. Blends of various compositions, as well as the LDPE1 that was modified in the twinscrew extruder, were used in the coating experiments, and the results are summarized in Figure 10. The twinscrew-extruder-modified materials displayed the same trend as did the single-screw-extruder-modified materials in that the neck-in performance did not seem to be affected by the amount of LDPE2 present. However, the draw-down speed increased as LDPE2 was added to the system. This is seen by a sudden increase in the drawdown speed as 1% of LDPE2 was added to LPDE1 compared to that of the virgin LPDE1 processed in the TSE. The draw-down speed had a decreasing trend as the amount of LDPE2 in the blend increased. From these experimental results, the following observations can be made: A decrease in neck-in seems to be caused by the shear modification effect, which is most prominent in the twin-screw extruder. This is reflected by the fact that the TSE-modified materials show the lowest neck-in during processing. An increase in draw-down speed seems to be caused by the addition of the minor

component, LDPE2. This is reflected by the sudden increase in the draw-down speed after 1% of LDPE2 was added. The addition of LDPE2 did not seem to affect the neck-in performance but did affect the draw-down speed during processing. Comparing the coating results for the SSE- and TSE-modified materials one can see that, although there are no large differences between the twin- and single-screw neck-in data, there are significant differences in the draw-down speed data around the 1-2% LDPE2 range. As discussed previously, the change in the extensional behavior resulting from processing of these materials in the TSE seems to affect their coating performance. However, this effect cannot be fully explained since, in coating processes, the strain rates are high and, as can be seen in Figure 8, there are no large differences between the extensional viscosities of these materials at high strain rates. From a practical standpoint, the utility of extensional viscosity is limited, and instead the melt strength property is widely used to evaluate the resistance of a melt to extension.15 Preliminary measurements of the melt strength of some of our modified samples have indicated that melt strength increases with the LDPE2 percentage in the blends and that this property can be employed for process control to minimize material neck-in.22 Finally, it should be mentioned that addition of LDPE2 at the levels discussed in this work had no effect on the stability of the coating process. Concluding Remarks The coating performance of a commercial LDPE resin has been evaluated, and it has been found that shear modification and blending with another resin can be used to improve neck-in and draw-down speed. Materials modified in a twin-screw extruder (both the virgin material and the blends) showed a significant improvement in neck-in during a coating process. Neck-in decreased by as much as 40%. Because no variation in shear properties could be detected, the improved neckin results are attributed to changes in the extensional behavior of the material, as evidenced by changes in the entrance pressure data and extensional viscosity.

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Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada and from Nova Chemicals Inc. is greatly appreciated. The authors also thank Mr. Stuart Nield for helpful discussions and material donations and for arranging the coating experiments. Literature Cited (1) Honkanen, A.; Bergstrom, C.; Laiho, E. Influence of LowDensity Polyethylene Quality on Extrusion Coating Processability. Polym. Eng. Sci. 1978, 18, 985. (2) Tang, M. T.; Wasson, C.; Lin, S. V. Processability of Polyethylene Resins for Extrusion Coating. SPE Annu. Tech. Conf. 1993, 3147. (3) Nield, S. A.; Tzoganakis, C.; Budman, H. Control of a LDPE Reactive Extrusion Process. Control Eng. Pract. 2000, 8, 911. (4) Rokudai, M. Influence of Shearing History on the Rheological Properties and Processability of Branched Polymers. J. Appl. Polym. Sci. 1979, 23, 463. (5) Rudin, A.; Schreiber, H. P. Shear Modification of Polymers. Polym. Eng. Sci. 1983, 23, 422. (6) Ritzau, G.; Ram, A.; Izrailov, L. Effect of Shear Modification on the Rheological Behaviour of Two Low-Density Polyethylene (LDPE) Grades. Polym. Eng. Sci. 1989, 29, 214. (7) Baker, W. E.; Rudin, A. The Effect of Processing on Rheological and Molecular Characteristics of a Low-Density Polyethylene. Polym. Eng. Sci. 1993, 33, 377. (8) Rokudai, M.; Fujiki, T. Influence of Shearing History on the Rheological Properties and Processability of Branched Polymers. IV. Capillary Flow and Die Swell of Low-Density Polyethylene. J. Appl. Polym. Sci. 1981, 26, 1343. (9) Teh, J. W.; Rudin, A.; Schreiber, H. P. Plast. Rubber Proc. Appl. 1984, 4, 157. (10) Teh, J. W.; Rudin, A.; Schreiber, H. P. Shear Modification of Low-Density Polyethylene, J. Appl. Polym. Sci. 1985, 30, 1345.

(11) Dobrescu, V. In Rheology; Astarita, G., Marrucci, G., Nicolais, L., Eds.; Plenum Press: New York, 1980; Vol. 2, p 555. (12) Chuang, H. K.; Han, C. D. J. Appl. Polym. Sci. 1984, 29, 2205. (13) Utracki, L. A. Polymer Alloys and BlendssThermodynamics and Rheology; Hanser Publishers: New York, 1989. (14) Cho, K.; Lee, B. H.; Hwang, K. M.; Lee, H.; Choe, S. Rheological and Mechanical Properties in Polyethylene Blends. Polym. Eng. Sci. 1998, 38, 1969. (15) Goyal, S. K. Influence of Polymer Structure on the Melt Strength Behavior of Polyethylene Resins. SPE Annu. Tech. Conf. 1994, 1232. (16) Shida, M.; Cancio, L. V. Prediction of High-Density Polyethylene Processing Behavior from Rheological Measurements. Polym. Eng. Sci. 1971, 11, 124. (17) Bersted, B. H. On the Effects of Very Low Levels of Long Chain Branching on Rheological Behaviour in Polyethylene. J. Appl. Polym. Sci. 1985, 30, 3751. (18) Shroff, R.; Mavridis, H. New Measures of Polydispersity from Rheological Data on Polymer Melts. J. Appl. Polym. Sci. 1995, 57, 1605. (19) Vega, J. F.; Santamaria, A.; Munoz-Escalona, A.; Lafuente, P. Small-Amplitude Oscillatory Shear Flow Measurements as a Tool to Detect Very Low Amounts of Long Chain Branching in Polyethylenes. Macromolecules 1998, 31, 3639. (20) Cogswell, F. N. Converging Flow of Polymer Melts in Extrusion Dies. Polym. Eng. Sci. 1972, 12, 64. (21) Laun, H. M.; Schuch, H. Transient Elongational Viscosities and Drawability of Polymer Melts. J. Rheol. 1989, 33, 119. (22) Xiao, K. K.; Budman, H.; Tzoganakis, C. Control of Coating Properties of LDPE Through Melt Strength Measurements. Control Eng. Pract. 2000, manuscript submitted.

Received for review June 21, 2000 Revised manuscript received September 11, 2000 Accepted September 12, 2000 IE000599B