EXTRUSION OF ELASTOMER FILM, EFFECTS OF ELASTOMER DESIGN ON CHILL ROLL STICKING

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1 EXTRUSION OF ELASTOMER FILM, EFFECTS OF ELASTOMER DESIGN ON CHILL ROLL STICKING Mary Ann Jones, Todd Hogan, and Sarah Gassner, The Dow Chemical Company, Midland, MI Abstract Sticking of plastic webs to roll surfaces during film casting or sheet calendaring may cause aesthetic defects as well as rate limitations. This study was undertaken to gain an understanding of the relative contributions of polymer density, crystallinity, and molecular weight to roll sticking. A design of experiments using ethylene-octene elastomers showed that the density of the polymer, and hence the crystallization temperature, had the most significant effect on the roll sticking performance. Within the range of polymers studied, the molecular weight did not have a significant contribution to the roll sticking. It was also shown that physical properties could be predicted by the combination of Mn and density. Introduction Sticking of the plastic to the roll surfaces can cause aesthetic defects as well as rate limitations when thermoplastic materials are cast onto chill rolls in cast film or cooling rolls in calendared sheet extrusion. Sticking can be particularly challenging with materials that have low glass transition or crystallization temperatures such as thermoplastic polyurethanes, polyolefin elastomers, and other materials. Cooling the chill rolls can eliminate this sticking but the cooling capacity of the chill rolls can be exceeded when running certain elastomer designs, resulting in production rate limitations.[1, 2] Roll design factors such as coolant temperature, coolant flow rate, roll diameter and shell thickness can impact cooling capacity.[3] Applying a release coating to the roll surface such as polytetrafluoroethylene (PTFE) or polydimethylsiloxane (PDMS) is also practiced. Material factors affecting sticking include additives, low molecular weight (oligomeric) species, and low crystallinity at the temperature of the rolls. This study was undertaken to gain a fundamental understanding of the relative contributions of polymer density, crystallinity, and molecular weight to roll sticking in film casting and sheet calendaring. A design of experiments was completed to understand and model the effect of ethylene-octene elastomer density, percent crystallinity, crystallization temperature, and molecular weight (Mw) on the temperature where sticking on the chill roll begins, as well as the optical and mechanical properties of the film. The choice of elastomer design is important to the end-use application, balancing properties such as heat resistance and tensile strength with the desire to minimize % haze. Materials Nine ethylene-octene elastomers were used in these experiments. All are ENGAGE polyolefin elastomers manufactured by The Dow Chemical Company. The basic physical properties for the elastomers are provided in Table 1. Table 1. Elastomer Design, Random Ethylene-octene Copolymer Sample Melt Index Density 1C/2.16 kg (g/cc) (dg/min) Elastomer A 1.8 Elastomer B Elastomer C 1.87 Elastomer D Elastomer E 3.2 Elastomer F Elastomer G 5.87 Elastomer H Elastomer I 15.8 It is known that there are additives that may contribute to roll sticking. To eliminate this potential variable, the only additives these materials contained were a low level of antioxidant so that additives would not be a factor in the roll sticking experiments. Experimental Approach A laboratory scale cast film line was used to measure chill roll sticking. The cast film line consisted of three, 25.4 mm (one-inch) diameter, 24 L/D extruders. The three extruders were connected to a co-extrusion feedblock where the individual flow streams were combined. The feedblock was connected to a 3 mm (8 inch) wide cast film die to form the molten web. The web contacted a 3 mm (8 inch) diameter temperature controlled chill roll. This chill roll had a diamond emboss surface texture and was chrome plated. The chill roll temperature could be easily controlled at this scale and analysis was possible on SPE ANTEC Anaheim 17 / 9

2 small quantities of sample. Neat elastomer film was extruded at.5 mm (.18 inch) thicknesses. The cast film line was operated at a screw speed of 3 rpm on each of the 3 extruders, which produced an output of approximately 6.8 kg/h (15 lb./h). The temperature profile for the extrusion system was as shown in Table 2. Table 2. Extrusion Line Temperatures Lab cast film line Temperature (deg C) Zone Zone Zone 3 1 Transfer Line 1 Feedblock 1 Die 1 Melt Temperature Range A ruler was placed to measure the amount of wraparound past the tangential release point due to sticking of the film to the chill roll as the chill roll temperature was varied. Both the chiller set-point and the roll surface temperature were recorded. Ratings for extent of sticking are listed below, normalized to the point on the ruler of no sticking. The highest temperature rated No Sticking and the lowest temperature rated Severe Sticking were recorded for analysis against the elastomer molecular design. JMP statistical software, available from SAS Institute, Inc., was used to analyze and model the data. Number average molecular weight data was measured via gel permeation chromatography (GPC), calculated based on elution curves from a concentration detector and the column calibration curve built by using 21 PS standards. The data was transformed into PE relative MW using a q-factor following the principal of Williams and Ward. The crystallization temperature, Tc, was determined via differential scanning calorimetry (DSC). A TA Q series DSC was run in a heat-cool-heat cycle from C to C at 1 C/min heating and cooling rate. Shear modulus curves were measured as a function of temperature, running a temperature ramp in cooling and evaluating modulus values at the measured sticking temperatures. The temperature ramps were run at a frequency of 1 Hz, 3 C/minute, and a 1% strain. Haze data were measured using the.5 mm thick film via a haze meter according to ASTM D3A. Tensile data were measured in both the machine and transverse direction, testing at mm/min according to ASTM D412. TMA analysis was run on a TA Instruments TMA 29, ramping from C to C at 5 C/minute and reporting the temperature at which the probe penetrates to µm. Discussion The temperature at which semi-crystalline materials stick to chill rolls is generally accepted to be directly related to the crystallization temperature (Tc). Above the Tc, the material will stick to the chill rolls and at a temperature below Tc the material will start to crystallize with the corresponding increase in modulus which allows the material to be peeled from the roll surface. Sticking performance and onset crystallization temperature data are compared in Table 4. The overall trend is that as the Tc decreases, the temperature at which sticking occurs also decreases. The elastomers chosen spanned a very broad range of crystallization temperatures and extent of crystallinity as is illustrated in Figure 2. Figure 1. Chill Roll Sticking Measurement Method Table 3. Ratings for Sticking Position on Ruler No Sticking Slight Sticking cm (.5-2. inches) Moderate Sticking cm ( inches) Severe Sticking > 11.4 cm (> 4.5 inches) Table 4. Tc and Chill Roll Sticking Temperatures Sample Onset Tc Highest Temperature Start Temperature via DSC with No Sticking of Severe Sticking (deg C) (deg C) (deg C) Elastomer A Elastomer B Elastomer C Elastomer D Elastomer E 53 Elastomer F Elastomer G Elastomer H 17 Elastomer I SPE ANTEC Anaheim 17 / 21

3 Heat Flow (W/g) Figure 2. DSC Cooling Curves, Elastomer Tc range The elastomer molecular design factors related to sticking split into two groupings. The first had to do with density as it drove the crystallization temperature (Tc). Figure 3 places the sticking temperatures onto a modulus versus temperature test for three elastomers of differing densities. We do not find a critical modulus value where sticking of the film no longer occurs, however the points are in the relatively same position to the point of modulus increase due to crystallization. Elastomer I was not tested as this low density sample stuck to the chill rolls to some extent at all temperatures measured. Storage Modulus (Pa) Temperature (degc) 1.E+8 1.E+7 1.E+6 1.E+5 Elastomer I 1.E+4 1 Elastomer G Elastomer A End of severe sticking zone Beginning of no sticking zone Elastomer C Elastomer A Figure 3. Example Storage Moduli versus Temperature on Cooling The Tc can be reflected by the elastomer density which is an elastomer property routinely measured and recorded on all published technical datasheets (figure 4).[4, 5, 6] Modeling against density is convenient in that the data across resins was readily accessible. However, the parameter thought most likely to control sticking was the Tc versus chill roll temperature. Onset Tc was recorded rather than peak Tc. Some of these elastomer crystallization curves were distinctly bimodal and choosing a representative peak value was difficult. Temperature (degc) Elastomer C Elastomer G Onset of Crystallization (degc) Figure 4. Onset Temperature of Crystallinity (Tc) versus Elastomer Density The highest chill roll temperature prior to the start of sticking and the onset temperature for severe sticking were plotted to define a performance range as a function of density (Figure 5). Sticking performance could be accurately predicted from density, where the polynomial line fit had an r2 of >.9. Chill Roll Sticking Zones (degc) Elastomer Density (g/cc) Elastomer Density (g/cc) Onset of Severe Sticking (degc) Highest Temperature Before Sticking (degc) Figure 5. Chill Roll Sticking Zone as a Function of Density The second factor of interest was the number average molecular weight of the elastomer. Figure 6 illustrates that the Mn does not predict sticking performance as a single primary factor. SPE ANTEC Anaheim 17 / 211

4 Chill Roll Sticking Zones (degc) Number Average Molecular Weight Onset of Severe Sticking (degc) Highest Temperature Before Sticking (degc) Figure 6. Chill Roll Sticking Zone as a Function of Mn Data was placed into the JMP software package, using a Response Surface Model to provide predictive capabilities. Models describing a balance of sticking with film performance were best when using both density and Mn as factors. In addition, properties other than sticking could be predicted only when adding Mn to the model. These properties included tensile strength and elongation, % haze, and heat resistance (TMA analysis). Ideally, a well designed design-of-experiments has corner and center points; however we needed to work with commercially available products resulting in a model produced from undesigned data. Grades did not exist in the ideal design pattern that also had the appropriate viscosity for the extrusion fabrication process. The JMP software provides a parameter estimate table and a plot comparing the actual data points to those of the model fit. P values allow the researcher to test the significance of each term in the model. Very small numbers indicate highly convincing significance. The p value or Prob>[t] must be <.5 to be significant as a term and is highly significant at <.1. Figure 7 contains the comparison of the actual to predicted severe sticking data in a response surface model with Density and Mn as the factors. All terms, as listed in Table 5, are significant with the exception of the squared term for number average molecular weight. Figure 7. JMP Software Model Fit, severe sticking start temperature (factors of density and Mn) Table 5. Response Surface Model p-values, Severe Sticking Temperature Term Prob> t Intercept <.1 Density (g/cc)(.857,.8).17* Measured Conv Mn (Kmol/g)(29,52).465* Density (g/cc)*density (g/cc).4* Density (g/cc)*measured Conv Mn (Kmol/g).71* Measured Conv Mn (Kmol/g)*Measured Conv Mn (Kmol/g).112 Molecular weight and the related property of melt index as sole factors do not predict performance in fabrication or in end use properties. However, molecular weight is a key factor in predicting film physical property performance when it is combined in a response surface model with density. Models of all responses improved when using Mn as a factor with density. While melt index is not fully predictive of molecular weight as illustrated in Figure 8, melt index can be used in place of molecular weight in this model with equally good results. In many cases, properties could be predicted only when combining one of these factors together with density. Table 6 and 7 contain a listing of r-squared for models with single factors in comparison with the combined factor models. Sticking at the chill roll and all basic film properties can be estimated from a response surface model. Both density and melt index are commonly reported properties for commercial materials of this type. SPE ANTEC Anaheim 17 / 212

5 Number Average Molecular Weight Melt Index, 1C/2.16kg (dg/min) Figure 8. Number Average Molecular Weight vs. Melt Index Table 6. R-squared values for density, Mn and melt index as single model factors Test Responses Single Factors Density Mn Melt Index Start Temperature, severe sticking % Haze Tensile Strength at Yield machine direction transverse direction % Elongation machine direction transverse direction Heat Resistance (TMA) Table 7. R-squared values for density with Mn and melt index as combined model factors Test Responses Response Surface r-squared values Density & Mn Density & Melt Index Start Temperature, severe sticking % Haze Tensile Strength at Yield machine direction transverse direction % Elongation machine direction transverse direction Heat Resistance (TMA) of ethylene-octene random copolymers in film casting and sheet calendaring. A design of experiments was completed to understand and model the effect of elastomer density, crystallization temperature and molecular weight (Mn) on the temperature where sticking on the chill roll begins. The density of the polymer, and hence the crystallization temperature, had the most significant effect on the roll sticking performance. Within the range of polymers studied, the molecular weight did not have a significant contribution to the roll sticking. Absent additives that contribute to sticking or alternatively, promote release, optimization of chill roll cooling and processing conditions are the most effective means to alleviate chill roll sticking. Elastomer selection is based not only on ease of processing but also on performance attributes such as tensile strength and elongation, % haze, and heat resistance. We successfully predicted both sticking at the chill roll and these added properties by the combination of Mn and density as model factors. Mn can be displaced as a factor by melt index with equally good results. Both processing and film performance can be predicted from commonly reported material properties. References 1. R. Palmer, Roll Design A Review of the Basics, SPE-ANTEC Tech. Papers, 45, 34 (1999). 2. J. Powers, Plastics Technology, 46 (August 1996) 3. J. Frankland, Plastics Technology, 26 (February 11) 4. K. Sehanobish, R. M. Patel, B. A. Croft, S.P. Chum and C. I. Kao, Journal of Applied Polymer Science, 51, 887 (January 1994) 5. S. Bensason, J. Minick, A. Moet, S. Chum, A. Hiltner, and E. Baer, Journal of Polymer Science: Part B: Polymer Physics, 34, 131 (1996) 6. J. Minick, A. Moet, A. Hiltner, E. Baer, and S. P. Chum, Journal of Applied Polymer Science, 58, 1371 (1995) This presentation is provided in good faith for informational purposes only. Dow assumes no obligation or liability. Trademark of The Dow Chemical Company ('Dow') or an affiliated company of Dow Conclusions This study was undertaken to gain a fundamental understanding of the relative contributions of polymer density, crystallinity, and molecular weight to roll sticking SPE ANTEC Anaheim 17 / 213