The Effect of Extrusion Processing Conditions on the Properties of Blown and Cast Polyolefin Packaging Films.код для вставкиСкачать
Dev. Chem. Eng. Mineral Process., 1I (1/2), pp. 137-146, 2003. The Effect of Extrusion Processing Conditions on the Properties of Blown and Cast Polyolefin Packaging Films M. Billham, A.H. Clarke, G. Garrett, G.M. McNally" and W.R. Murphy Polymer Processing Research Centre, The Queen 's University of Belfast, Stranmillis Road, Belfast BTP 5AH, Northern Ireland, UK Thin mono-layerfilms of metallocene catalysed polyethylene, linear low density and conventional low density polyethylene, as well as polypropylene, were producedfrom a 38mm extruder through a 75mm diameter blownplm die and a 6OOmm castfilm die. By using the same die gap on each die to achieve equal draw-down ratios, the influence of orientationj?om the two processes on the mechanical properties of each firm was investigated. Tensile strength at break, Young's Modulus, percentage elongation at break, tear propagation resistance, in both machine and transverse direction all gave significant differences in properties with cast film when compared to different blow-up ratios for blown film. Di@erentiaI Scanning Calorimetry was used to measure the percentage of ctystallinity in eachfilm. Differences were found to show that the cooling process as the melt exits the die has a signtficant effect on the percentage of crystallinity. Introduction Over the past decade or more the use of linear low density polyethylene (LLDPE) and metallocene-catalysed polyethylenes (mLLDPE) have been used to replace conventional low density polyethylene (LDPE) in thin film packaging applications due to their higher tensile strength and suitability for down-gauging due to better draw-down. This has led to considerable reduction in the weight of polyethylene, up to 30% in some packaging film applications. Blown film and cast film extrusion are the two most common techniques used worldwide for producing flexible thin films. * Authorfor correspondence. 137 M. Billham, A . H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy In the blown film process, polymer melt is extruded through an annular die and the melt is drawn upwards and cooled by passing through an air cooling ring. During this drawing and cooling process, the melt tube formed from the die is normally inflated by air to four or five times the diameter of the annular die. The degree of bubble inflation is referred to as the Blow-up Ratio and this inflation process enables transverse orientation of the polymer melt during cooling. In the cast film process, the polymer melt is extruded through a straight die and on exiting the lip of the die the polymer melt is quenched by contacting a temperature controlled chill roll system. Although there are many reports on the effect of extrusion conditions for blown film [1-3] and cast film (4, 51 extrusion, very little information is currently available on the comparison of the two processes. Therefore there is an urgent need to quantify the difference in the mechanical performance of the different polyolefins (polyethylenes, polypropylenes) when processed into blown film and flat cast film. Many improvements have been made in the cooling efficiency on blown film extrusion lines however the rapid quench rate of cast film offers a more significant and controllable advantage to the film properties affected by rapid quenching. Although it is widely recognised in the industry that cast LDPE film, for example, produces a slightly stiffer and clearer film when compared to blown film, the relative difference for a particular polyolefin, and in comparison to other polyolefins is not well quantified for the two film extrusion techniques. The main aim of this investigation is therefore to quantify the effect of extrusion processing systems and conditions on the mechanical performance, crystallinity and optical properties  of a range of polyolefins including conventional low density polyethylene (LDPE), linear low density polyethylene (LLDPE), metallocene catalysed polyethylene (mPE) and polypropylene (PP). Experimental Details (a) Marerials Samples of polymer systems were supplied by Exxon Chemicals, Dow Plastics and Quarain. The material properties including density and melt flow indices are shown below. Table 1. List of materials and suppliers. 138 Dens@ Melt Flow Index (g/cm7 (gll Omin) Escorene 0.923 1.2 Dow Plastics Elite 0.925 0.85 LLDPE Exxon Chemicals Escorene 0.919 1 .o PP Quarain 0.910 2.1 Material Producer Trakname LDPE Exxon Chemicals mLLDPE Efect of Extrusion Processing Conditions on Properties of Polyolefins The polyethylene based resins were chosen for having similar densities and similar Melt Flow Indices (MFI) with a polypropylene resin as a comparable polyolefin. (6) Thinfirm preparation Cast films were produced from all resins using on a Killion KN150 38 mm extruder, fitted with a general purpose PE screw (LID 30, 3: 1 compression ratio) attached to a 600 mm wide coat-hanger type flat sheet die. During the trials the die gap was pre-set to 800 pm and the extruder was run at a constant screw speed of 30 rpm. On exiting the die the melt was quenched on a temperature controlled polished chill roll maintained at a temperature of 32°C and this was located at a distance of 110 mm from the die lips. The extruded film was then passed through a pair of nip rolls onto a winder. The speed of the chill roll and nip rolls were adjusted to ensure constant film thicknesses of 50 pm. The barrel temperatures were profiled from 180°C at the feed end up to 210°C at the die for the polyethylenes. Since the shear viscosity of PP is higher than for the polyethylenes, the barrel temperatures were 200°C to 220°C respectively for polypropylene. Blown films were produced from all resins using the same Killion extruder attached to a 75 mm diameter spiral mandrel type blown film die. An annular die gap was 800 pm and the extruder was set to a constant screw speed of 20 rpm. The melt was cooled on exiting the die using a single-orifice cooling ring. The haul-off rate was adjusted to produce 50 pm films at four Blow-up Ratios of 1.1, 1.5, 2 and 2.5. The extruder barrel and die temperatures were profiled once again from 185°C at the feed end up to 210°C at the die for the polyethylenes and 205°C to 220°C for the polypropylene. Table 2 below shows variation in the recorded melt temperatures, melt pressures and frost line height (FLH) during the extrusion trials for each film at all BUR’S. Table 2. Variation in recorded melt temperatures, melt pressures andfrost line height (FLH)for eachfilm at all BURS. 139 M. Billham, A.H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy (c) Film tensile analysis All tensile tests were performed using an Instron 441 1 Universal Tensile Tester according to BS2782, using a crosshead speed of 500 mm/min and an initial gauge length of 50 mm. Ten samples were tested in both machine and transverse directions. Tensile strength at break, Young's Modulus and percentage elongation at break were recorded. (a) Tear strength analysis All tear tests were performed using an Instron 441 1 Universal Tensile Tester according to the Tear Propagation Resistance standard ASTM D1938-94, using a crosshead speed of 250 mm/min and the single-tear method (trouser tear) test piece. Six samples were tested in each direction, the mean tear propagation strength was recorded. (e) Differential scanning calorimetric analysis In order to investigate the effect of extrusion processing conditions, polymer density and polymer type on the overall crystallinity of the various films, samples of the films were prepared for differential scanning calorimetric analysis. Tests were performed using a Perkin Elmer DSC-6thermal analyser, samples were heated from 40°C to 200°C at a temperature increase of 10°C mK'. The crystalline melting temperature was determined and the degree of crystallinity was calculated using a AH value of 198 M g-' for 100% polyethylene and 86 kJ g-' for polypropylene. (f) Shrinkage analysis Many polyethylene films are used for shrink wrap applications and in order to investigate the effect of process Shrinkage tests were performed on a Ray-Ran Hot Plate Heat Shrinkage Apparatus according to the manufacturers test procedure and the hot plate was set to a temperature of 130°C for the polyethylenes. Four samples of each film were tested and the shrinkage of the 50 mm diameter discs in both the machine and transverse direction was recorded. (g) Haze and gloss analysis Aesthetic properties such as clarity and gloss in packaging films are now regarded as extremely important in the marketing of products. Gloss was measured at angles of 20" and 60" using a BYK-Gardner micro-TRI-gloss reflectometer in accordance with ASTM D523. Observations were also recorded with regard to the clarity and haze of each film. 140 Efect of Extrusion Processing Conditions on Properties of Polyolefins Results and Discussion (i) Tensile Properties The effects of extrusion processing conditions on the tensile stress at break, in both machine and transverse direction, of cast film and blown films at different blow-up ratios for the different polyolefins are shown in Figure 1. The increase in blow up ratio in LDPE, LLDPE and PP showed a gradual decrease in the tensile strength at break in the machine direction due to the reduction in molecular orientation. The tensile strength of mPE was not influenced by B.U.R. in the MD, but showed a gradual decrease in the TD tensile strength as B.U.R.was increased. With mPE the higher frost line height at a BUR of 2.5 and the slower haul off speed, (see Table 1) resulted in a greater relaxation time between the die and the frost line resulting in chain disentanglement and relaxation to give less orientation and a lower tensile strength. Overall the tensile strength of mPE and LLDPE was significantly higher than LDPE due to the closer chain packing ability of the shorter branched polymers. PP had comparable tensile properties to mPE and LLDPE. The cast films have the highest tensile strength in the TD when compared to the blown films especially the mPE and LLDPE, apart from the lowest BUR in LDPE. Machine Direction 50 , Transverse Direction I I Figure 1. The effect of extrusion processing conditions on the tensile stress at break, in both machine and transverse direction, of cast film and blown films at dgerent blow-up ratiosfor the different po&olefins. The effect of extrusion processing conditions on Young’s Modulus, in both machine and transverse direction, of cast film and blown films at different blow-up ratios for the different polyolefins are shown in Figure 2. The cast films have the lowest Young’s Modulus in both MD and TD than the blown films on all polyolefins tested. There was very little difference in modulus, i.e. stiffness in either direction. LDPE has comparable modulus to mPE and slightly higher than LLDPE. Only the modulus of LDPE is influenced by BUR as it is decreased in both directions with increase in BUR. The modulus of PP is typically more than twice the modulus of the polyethylenes. I41 M. Billham, A. H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy Machine Direction Transverse Direction 600 &OO v -2400 $300 P200 F a100 > 0 LDF€ mpE LLOPE LDF€ FP mFE L L E PP Figure 2. The efsect of processing conditions on the Young's Modulus, in both machine and transverse direction, of cast film and blown films at dflerent blow-up ratiosfor the difserentpolyolefins. The effect of extrusion processing conditions on elongation at break, in both machine and transverse direction, of cast film and blown films at different blow-up ratios for the different polyolefins is shown in Figure 3. The elongation of LLDPE is approximately 20% higher than mPE and up to double that of LDPE, PP is very notch sensitive during sample preparation and is difficult to stretch at room temperature. Only the cast film of PP stretched suggesting a low orientation and crystallinity is required to allow any stress thinning. Elongation was greater in all polyethylene films in the TD with less orientation due to less draw-down from the die gap in this direction. The cast films with no orientation in the TD gave the highest percentage orientation compared to the blown film. Machine Direction Transverse Direction 1200 1200 rlOOO ClOOO .Q .Q c ( Y % 800 % 800 4 600 3 600 S 400 &? 400 200 0 200 0 LDFE mpE LLDF€ FP LDPE mpE LLDPE FP Figure 3. The effect of extrusion processing conditions on the percentage elongation, in borh machine and transverse direction, of cast film and blown fdms at different blow-up ratiosfor the different polyolefins. I42 Effect of Extrusion Processing Conditions on Properties of Polyolefins (ii) Tearproperties The effect of extrusion processing conditions on tear propagation resistance, in both machine and transverse direction, of cast film and blown films at different blow-up ratios for the different polyolefins are shown in Figure 4. The tear strength of LLDPE was generally higher than that of mPE, which was higher than LDPE. The tear strength in the TD was higher in all the films than that in the MD. Apart from PP the tear strength of all the cast films was always lowest in both directions, with the blown film at a BUR of 2.5 giving the highest tear strength in the MD for mPE and LLDPE. The mPE and the LLDPE gave a tough tear with a tendency to stretch along the tear in both directions whereas LDPE gave a straight ‘splitty’ tear in the TD and tended to tear across the film in the MD. Cast PP gave a comparable tear strength to LDPE in the MD but the PP blown films resulted in a very poor tear resistance. The tear resistance of PP in the TD was good for the cast film and the BUR of 1.1 but dropped significantly as the blow up ratio is increased. Figure 4. The eflect of extrusion processing conditions on the fear strength, in both machine and transverse direction, of cast film and blownfilms at diferent blow-up ratiosfor the duerent polyolefins. (iii) Shrinkage The effect of extrusion processing conditions on film shrinkage at 130°C,in both MD and TD, of cast film and blown films at different blow-up ratios for the different polyolefins are shown in Figure 5 . The results show that as the BUR is increased the film shrinkage decreases in the MD and increases in the TD. This is most evident with LDPE due to a lower crystalline melting temperature (T,) than mPE and LLDPE. Table 3 shows that LLDPE has a lower T, than the mPE, the heat shrinkage results show no significant difference. I43 M. Billham, A. H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy Figure 5. The efect of extrusionprocessing conditions on the heat shrinkage, in both machine and transverse direction, of castfilm and blownfilms at diferent blow-up ratiosfor the differentpolyolejns. (d) Crystallinity Figure 6 shows the effect of extrusion conditions on percentage crystallinity for each film, as measured using DSC analysis. The percentage crystallinity of mPE, at between 38% and 47%, was higher than that of LLDPE and LDPE at around 32% to 38%. The cast film compared to the blown films had the lowest level of crystallinity due to the temperature of the chill roll, better contact with the film surface and a more efficient cooling medium than air cooling. BUR of 2.5 for LDPE, LLDPE and PP were shown to have the highest levels of crystallinity. The crystallinity of mPE was not seen to be affected by BUR in blown film even with an increase in the frost line height. There is a significant increase, from 32% to 38%, in the crystallinity of LLDPE at a BUR of 2.5 from the cast and lower BUR respectively. PP shows a general trend of increase in crystallinity with increase in BUR.This is due to keeping a constant FLH and the haul off running slower to maintain 5 0 ~ mthere was more time for crystalline development in the melt, between exiting the die and cooling to below the crystallisation temperature. (e) Gloss and clarity. The effect of extrusion processing conditions on the gloss and clarity of cast film and blown films at different blow-up ratios for the different polyolefins are shown in Table 3. Both cast films of mPE and LLDPE exhibited excellent gloss and clarity. The gloss and clarity of the mPe and LLDPE was greatly reduced even at the lowest BUR and further reduced as the BUR was increased. This can be correlated with the increase in crystallinity from the cast to the highest BUR and concluded that the crystallinity of mPE and LLDPE is highly dependent upon the cooling rate on exiting the die and is directly related to the optical properties. The gloss and clarity of LDPE is superior in the cast film than blown film but is not significantly affected, unlike mPE, LDPE and PP, by increasing BUR even though the crystallinity is highest in the highest BUR. PP is clear in the cast film but even at the lowest BUR is very hazy and does not appear to deteriorate at higher BUR. This is because even at the low BUR I44 Esfect of Extrusion Processing Conditions on Properties of Polyolefins minimum cooling was necessary to achieve a stable bubble, therefore allowing the film to cool at a slow rate and crystallinity to develop. Figure 6. The eflect of extrusion processing conditions on the degree of crystallinity of castjilm and blown films at dgerent blow-up ratios for the diflerent polyolefins as measure by DSC analysis. Conclusions This work investigates the effect of processing conditions during blown film extrusion and cast film extrusion on the mechanical performance, related crystallinity and optical properties for a range of polyolefins including LDPE, LLDPE, mPE and PP. The results show that the Young’s Modulus of films manufactured using the cast film extrusion process were much lower than for films manufactured using the blown film process. DSC analysis showed that these films generally had lower crystallinity, which would tend to c o n f m this recorded reduction in modulus. The mPE films were shown to have the highest stress at break, highest Young’s Modulus and relatively high elongation (800%) in comparison to the other conventional polyethylenes. PP was shown to have the overall highest modulus in both MD and TD, which would tend to indicate that this was a much stiffer film, and this is reflected in the relatively high crystallinities observed for this polymer especially at higher blow-up ratios. The conventional LLDPE was shown to have the highest tear strength of all the polymers studied especially in the MD. The tear strength of PP was shown to be much lower than the polyethylenes in the MD, especially in blown films. The results also show that the MD tear strength for polyethylenes is dependent on BUR,with tear strengths increasing with higher BUR for the mPE and LLDPE, and a decrease in MD tear strength with increasing BUR being recorded for LDPE. The LDPE resin was shown to have the highest MD shrinkage values of all the polymers investigated, with slight decrease in MD shrinkage being recorded for increase in BUR. M.Billham, A.H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy The effect of extrusion processing conditions showed that the films manufactured using the cast film technique exhibited more clarity and gloss than films manufactured using the blown film technique. The gloss and clarity of the mPE and LLDPE was greatly reduced in blown film even at the lowest BUR. and reduced further as the BUR was increased. These optical properties are seen to be closely related to the changes in percentage crystallinity observed for all the films. Table 3. The efSect of extrusion processing conditions on the gloss and clarity of cast film and blownfilmsat diferent blow-up ratiosfor the different polyolefins. References 1. 2. 3. 4. 5. 6. 146 Beagan, C.M., McNally, G.M.. and Murphy, W.R. 1998. Metallocene Catalysed Polyethylene in Blown Film Applications. Antec, pp. 128-133. Beagan, C.M., McNally, G.M,. and Murphy, W.R. 1998. The Blending and Co-extrusion of Metallocene-catalysed Polyethylene in Blown Film Applications. Anfec, pp 90-94. Snell, P. 1982. Comparison of Properties of LDPE, LLDPE, HDPE. Film Extrusion Conference Proceedings, TAPPl Press, Atlanta, pp.5-10. Canning, K., and Baigui, B. 1999. Film Casting Low Density Polyethylene Melt. Anrec, pp.386-390. Degroot, A.T., Doughty, K., and Stewart, B. 1994. Effects of Cast film Fabrication Variables on Structure Development and Key Stretch Film Properties. Journal of Applied Polymer Science, Volume 52, pp.365-376. Johnson, M.B., Wilkes, G.L., Sukhadia, A.M., and Rohlfing, D.C. 2000.Optical Properties of Blown and Cast Polyethylene Films: Surface Versus Bulk Structural Considerations. Journal of Applied Polymer Science. Volume 77, pp.2845-2864.