The Problem of Die Drool in the Extrusion of Recycled Polyethylene

Transcription

1 The Problem of Die Drool in the Extrusion of Recycled Polyethylene P. Brachet 1, E.L. Hinrichsen 1, Å. Larsen 1 *, L.K. Bjørndal 2, F. Brendryen 3, and T. Lia 4 1 SINTEF Materials and Chemistry, PO Box 124 Blindern, 0314 Oslo, Norway 2 Norfolier AS, PO Box 28, 2001 Lillestrøm, Norway 3 Folldal Gjenvinning AS, 2580 Folldal, Norway 4 Norfolier AS, PO Box 44, 3671 Notodden, Norway Received: 7 October 2008, Accepted: 11 February 2009 ABSTRACT Present understanding of the formation of die drool is reported in light of current developments in materials for blown film polyethylene (PE). Two series of samples were collected from a blown fi lm line, and the material from the die drool and the die channel, or blown fi lm, were analysed separately. The techniques used included thermal analysis, melt rheology, fi ltration of dissolved material, and X-ray diffraction. Die drool was found to consist of segregated material, with the segregation depending on the content of high-molecular-weight branched polyethylene or impurities. As the presence of these PE materials may increase, it might be advisable for processors of recycled PE fi lm to take prior actions to limit problems. INTRODUCTION Die drool is a generally occurring phenomenon in the melt extrusion of polymers and, as such, will also occur when processing recycled polymers. In recycling, however, batch-to-batch variations may be significant, and consequently problems with die drool will vary. The recycling of polymers complies with the need for reduced raw material and energy consumption, as generally 2 kg of crude oil is saved for every kg regranulated. An important source is mixed packaging from the industrial, * Corresponding author. Tel.: ; fax: age.larsen@sintef.no Smithers Rapra Technology,

2 P. Brachet, E.L. Hinrichsen, Å. Larsen, L.K. Bjørndal, F. Brendryen, and T. Lia household, and agricultural sectors. Polymer materials used in the films produced for these applications have excellent mechanical properties. If the material in the waste flow is sufficiently homogeneous, and if impurities are limited, regranulated materials will have many of the same advantages. Compared with the processing of virgin materials, the granulation and processing of recycled polymers entail extra costs related to inhomogeneous material and impurities. More impurities mean increased filter expenses during extrusion, and remaining impurities in the recycled material may lead to reduced surface quality as well as reduced mechanical properties, which may restrict the recycled materials to low-end applications. Additional complications related to recycled plastics come from everyday product development in the field of polymer processing. The continuous drive to improve product properties and reduce costs leads to changes in the material composition of the polymer waste stream. Multimodal materials, which may consist of low-molecular-weight (low-mw) linear fraction and highmolecular weight (high-mw) branched material, have been introduced with increased ductility, placing higher demands on the machinery for shredding. The use of additives to improve mechanical, barrier, and surface properties (such as PIB and EVA) is increasing, and multilayer films containing barrier materials with high melting temperatures are also becoming more common. Improved processability and mechanical properties allow the production of thinner films, and, as the total material consumption also steadily increases, the surface area of the recycled material increases even more. This presents challenges to the cleaning process which aims to remove surface impurities efficiently. Die drool (also known as die lip build-up, plate-out, die drip, or die peel) is the unwanted material accumulation on the open faces of extrusion dies. The die drool build-up occurring during extrusion affects all extrusion processes, including wire and cable coating, blown or cast film, and fibre spinning (1). This unwanted material affects significantly the engineering properties as well as the aesthetic qualities. It sticks on the external face of the die lips and breaks away from the melt during extrusion. Such accumulation of material may affect productivity, as it necessitates cleaning of the die, and therefore shutdown of the extruder. Die drool build-up is either continuous or intermittent or both, and may affect the geometry of the actual die at the exit, causing excessive surface roughness. Similarly, part of the drool can enter the die and mix with the material, altering the mechanical properties and producing local defects. 200

3 There is no single explanation of the die drool phenomenon. Causes identified include low-mw material and its structure, fillers, and volatiles. Die drool also depends on the die swell, which will vary according to polymer structure and processing conditions. Further, die drool may also result from pressure oscillations in the screw as well as degraded material. With the negative consequences, manufacturers will, of course, try to limit the amount of die drool. Die drool is found to be caused by the stresses present prior to the exit, and not only by their instant value at the die lips (2). Lower-surface-energy metals induce a lower level of die deposit, which is attributed to low adhesion between the polymer and the metal surface, thus reducing the effective shear (3). Flared die geometry (i.e. increasing the die channel width close to the die mouth) has been patented to suppress die drool (4,5), and this can cause as much as a 94% reduction in die drool (2). Increasing the land length and reducing the land entrance size can also reduce problems (6). Using additives blended into the base resin can significantly reduce die drool (7), especially fluoro-based additives (3,8). In a recent article, a comparison between experimental results and a theoretical analysis of the flow at the end of the die demonstrated the effect on die drool of the pressure in the die lip area (9). In the present study we report on experimental findings related to film blowing of recycled polyethylene. By thermal and rheological characterisation, we compare the composition of the material accumulated in the die drool with that of the material in the die channel or the blown film. MATERIAL AND PROCESSING The material used was a commercial-grade regranulated low-density polyethylene (LDPE) with a melt flow ratio (MFR) of g/10 min (190 C, 2.16 kg). Blown films were processed at Norfolier. The film blowing line was equipped with a die of 200 mm diameter, and the die gap was 1.5 mm. The melting temperature was 220 C and the yield was 120 kg/h. Two series of samples were collected for analyses on different occasions. The samples studied were: a. accumulated die drool and material from the die channel, collected from the machine at shutdown; 201

4 P. Brachet, E.L. Hinrichsen, Å. Larsen, L.K. Bjørndal, F. Brendryen, and T. Lia b. die drool, pellets, and blown film from the same material batch (mixedcolour material grade containing pellets of different colour depending on their origin). Die drool A appears as a smooth material, whereas B is more grainy. Figure 1 shows die drool similar to the series B samples. Figure 1. Section of film bubble at the die exit with accumulated die drool Characterisation and Sample Preparation Differential Scanning Calorimetry Each polymer has its own melting characteristics including melting temperature and heat of melting. These characteristics depend on the crystalline content of the polymer, which again depends on the polymer structure. Differential scanning calorimetry (DSC) is a method suited to indicating the constituent polymers of a sample and, in the case of blends, their relative amount. Samples from the die drool and the material removed from the die channel were analysed individually. DSC analyses were done on a Perkin Elmer Pyris 1. Samples were heated to 200 C and kept for 3 min to remove thermomechanical history before they were cooled to room temperature at 10 K/min. The samples were then heated at 10 K/min to 180 C. The thermograms shown in the following all represent this second heating step. Dynamic Mechanical Analysis In dynamic mechanical analysis (DMA) we apply an oscillating strain to the sample and measure the resulting stress in it. From these measurements we 202

5 derive the storage (or elastic) modulus, G, and the loss modulus, G, of the material as functions of the angular frequency, ω, of the sample strain. At given ω, G will generally be higher for high-mw materials, but it will also be influenced by crosslinking of the material. G represents the energy dissipated in viscous heating during deformation. From these quantities we derive what is called the damping factor: tan = G/ G (1) A low tan δ value means that the elastic modulus dominates. We also use the complex viscosity: * = 1 G 2 + G 2 (2) The function η*(ω) gives important information about the molecular weight distribution (MWD) of the material. From the DMA data, as for the examples discussed below, we can derive a crossover frequency and crossover modulus for each sample tested as the values corresponding to G = G = G c. Zeichner and Patel (10) were the first to suggest the polydispersity index PI = 10 5 /G c (3) as a measure of polymer polydispersity. This quantity is also measured by the MWD, which may be obtained by gel permeation chromatography. Samples for DMA were discs of 25 mm diameter and 1.6 mm thickness, cut from pressed plates. Frequency sweeps were performed at 190 C on a Physica MCR 300 instrument. Filtration We added 100 ml of xylene to 1 g of polymer and put this on reflux at 180 C until all was dissolved. The sample was then filtered, and the filtrate inspected by optical microscopy. Wide-Angle X-Ray Diffraction The filtrate from the dissolved material was analysed by wide-angle X-ray diffraction (WAXD), and the recorded diffractogram was compared with library spectra. Analyses were done on a Siemens D500 diffractometer. 203

6 P. Brachet, E.L. Hinrichsen, Å. Larsen, L.K. Bjørndal, F. Brendryen, and T. Lia RESULTS SERIES A Differential Scanning Calorimetry The melting thermograms are presented in Figure 2. The pronounced melting peaks in Figure 2 for the die channel material above 120 C would generally be associated with a linear polyethylene, such as a high-density polyethylene (HDPE), or with the linear fraction of bimodal linear low-density polyethylene (LLDPE). The wider melting peak around 110 C with a low-temperature shoulder represents either LDPE or LLDPE or a mix of the two. Integrating the thermograms in Figure 2, we find the heat of melting of the die drool to be 106 J/g, as opposed to 119 J/g for the die channel material. As the high-melting-temperature fraction of the die drool material is much lower, we conclude that the die drool contains a concentration of LDPE or LLDPE. This agrees with the differences in heat of melting. A lower heat of melting could, however, also be due to crosslinking of the material. Small quantities of polypropylene (melting temperature around 162 C), representing around 1% of the melting enthalpy, were identified in the channel material. Dynamic Mechanical Analysis Figure 3 shows G and G for the recycled PE from the die channel of series A. The curves with the crossover point between G and G are representative of the behaviour of PE melts. Figure 2. DSC analyses of material from die drool and die channel series A 204

7 Figure 4 shows the damping factor, tan δ, for parallel samples of the material from the die drool and the die channel. The samples from the die channel show fairly large scatter, whereas those representing the die drool are more consistent. This indicates a material that is inhomogeneous from the start, and shows that segregation takes place during the flow through the die. Particular Figure 3. Storage modulus G and loss modulus G for the die channel material from DMA at 190 C series A Figure 4. Damping factor, tan δ, versus angular frequency, ω, of samples from the die channel and the die drool series A 205

8 P. Brachet, E.L. Hinrichsen, Å. Larsen, L.K. Bjørndal, F. Brendryen, and T. Lia fractions are found in the die drool. The fact that die drool builds up over time but is still homogeneous indicates that a specific mechanism is at play. The lower damping factor in Figure 4 for the material from the die drool implies that it is more elastic, indicating that it contains either higher-mw or more branched material. In Figure 5, note the higher value of the complex viscosity, η*, of the die drool at low frequencies, which is explained by this material containing a fraction of higher-mw or more branched material compared with the material from the die channel. This also agrees with the damping factor results in Figure 4. At high frequencies, η* of the die drool drops below η* of the die channel, which can be caused by a material fraction of low MW (leading to what is termed shear thinning ). Table 1 presents G c and the derived PI for parallel samples tested. Note the significant difference between the samples from the die channel and those from the die drool, in spite of the significant variation between the individual samples from the die channel. The higher PI values in Table 1 agree with the results concerning complex viscosity in Figure 5, with the die drool containing fractions of both high- and low-mw material. Figure 5. Complex viscosity, η*, versus angular frequency, ω, of samples from the die channel and the die drool series A 206

9 Table 1. Data for series A samples from the crossover point of the DMA curves and the calculated polydispersity index Material Die channel 1 Die channel 2 Die channel 3 Die channel 4 Die drool 1 Die drool 2 Die drool 3 Crossover frequency (1/s) Crossover modulus/10 4 (Pa) PI RESULTS SERIES B Differential Scanning Calorimetry Small quantities of polypropylene, representing a maximum 2% of the melting enthalpy, were identified in both the die drool and the blown film. Figure 6 shows thermograms over a limited temperature range. Pellets of two different colours were analysed separately to see the effect of different material sources. For the dark pellet, the ratio between melting peaks in Figure 6 is close to that for the die drool. The thermogram for the die drool differs in Figure 6. DSC melting thermogram for light and dark pellets, die drool, and blown film second heating series B 207

10 P. Brachet, E.L. Hinrichsen, Å. Larsen, L.K. Bjørndal, F. Brendryen, and T. Lia that the total crystallinity is lower. In contrast to the die drool of series A in Figure 2, the die drool of series B also contains a significant fraction of linear PE (melting above 120 C). The specific heat of melting for the die drool is 78 J/g, as opposed to 110 J/g for the film. The difference is substantial, but it is not likely to be due to a less crystalline PE. We believe this can be explained by a concentration of impurities of a different nature to PE, and not contributing to the heat of melting, in the die drool. Dynamic Mechanical Analysis In Figure 7, the curves for the storage modulus and the loss modulus do not cross within the experimental range. This may be explained either by low MW or by crosslinking. A summary of the DMA results for the film material is given in Table 2. The values for the crossover frequency and moduli differ from those of the film material in series A. Figure 7. Storage modulus, loss modulus, and complex viscosity of the series-b die drool Table 2. Data for series B from the crossover point of the DMA curves and the calculated polydispersity index Material Crossover frequency (1/s) Crossover modulus/10 4 (Pa) Film Film PI 208

11 Figure 8. Complex viscosity of the film material and the die drool of series B The complex viscosity of the die drool in Figure 8 shows approximate powerlaw behaviour over the whole range of frequencies. This is associated with particulate flow, as is also observed for PVC, owing to the structure of this material (11). Filtration By optical microscopy we found the filtrate to consist predominantly of smaller particles. The typical size of these particles was around 100 μm and smaller. This means they may have passed through the extruder filter in the regranulation process. The content of fibres, as would be expected from residues of paper or wood, was negligible. Wide-Angle X-Ray Diffraction The diffractogram revealed the presence of quartz in the sample. This explains the lower specific heat of melting of the die drool in Figure 6. DISCUSSION AND CONCLUSIONS Our results show effects of segregation on the build-up of die drool, where both material composition and impurities play a role. For series A, the die drool of the received samples consists of segregated material fractions. Based on DSC and DMA, we see that the die drool material 209

12 P. Brachet, E.L. Hinrichsen, Å. Larsen, L.K. Bjørndal, F. Brendryen, and T. Lia contains a high fraction of branched polyethylene LDPE or LLDPE or crosslinked material. The branching may also result from crosslinking of degraded polyethylene. The fraction of linear polyethylene is strongly reduced. Also, the die drool material contains higher fractions of both high and low MW compared with the die channel material. The high-mw fraction may relate to modern bimodal LLDPE. These experimental findings are consistent with the literature. The identification of segregation of LLDPE or LDPE as well as high-mw material in the die drool should be seen in light of the constant change in the materials used in film blowing. The trend of increased use of LLDPE containing branched materials of high MW may thus lead to more die drool. Efficient blending of PE grades with large differences in MW places extra demands on the blending efficiency of the extruder and screw design. The observed segregation of high-mw material may result from insufficient blending in the compounding step. A recycler may have to opt for either more consistent material sorting or equipment with better blending performance. In series B, the die drool contains an accumulation of particulate impurities identified as quartz which, since their size was 100 μm and smaller, may have passed through the extruder filter in the regranulation process. The DMA results in Figure 7 indicate that the material is crosslinked. The quartz particles could possibly serve as crosslinking points, depending on the chemistry of the particle surface. As the die drool in this case contains a significant fraction of linear polyethylene, we conclude that, in the presence of impurities, the segregation mechanism observed for series A is not significant. The development towards thinner films gives increased surface area per volume of recycled polymer. This is expected to give more impurities in recycled materials unless the material is subjected to more efficient cleaning. A challenge for recyclers will be to obtain improved cleaning at a reasonable cost. From these findings, the processors of recycled materials may expect the problem with die drool to increase unless the recycling industry introduces improved washing and blending technology. The processors themselves may contribute by modifying the extrusion process. In this regard, die modification by flaring the die or by increasing the die land length is probably most relevant. Modification of the surface roughness of the die channel is a possible additional means. Adding fluoro-based additives for lubrication is also possible, but the costs must be considered. 210

13 ACKNOWLEDGEMENT This work was carried out as part of a project on the use of recycled polymers with financial support from the Norwegian Research Council. The authors are indebted to K. Windsland for performing the DSC analyses, and to B. Sommer for the DMA and WAXD. REFERENCES 1. Priester D.E. and Chapman G.R., Do not gamble with additives and modifiers, SPE RETEC, Atlantic City, NJ, (1996), Ding F., Zhao L., Giacomin A.J., and Gander J.D., Polym. Eng. Sci., 40 (2000) Chan C.M., Int. Polym. Proc., 3 (1985) Ohhata T., Tasaki H., Yamaguchi T., Shiina M., Fukuda M., and Ikeshita H., US Patent 5,417,907 (1995). 5. Rakestraw J.A. and Waggoner M.G., US Patent 5,458,836 (1995). 6. Klein I., Plastics World, May (1981) Chisholm P.S., Tikuisis T., Goyal S.K., Checknita D., and Bohnet N.K.K., US Patent 5,854,352 (1998). 8. Chapman G.R., Priester D.E., and Souffie R.D., What s new in polyolefins, SPE RETEC, Houston, TX, (1987), Chaloupková K. and Zatloukal M., Polym. Eng. Sci., 47 (2007) Zeichner G.R. and Patel P.D., Second World Congress of Chemical Engineering, Montreal, Canada, (1981), Hinrichsen E.L. and Thorsteinsen P., J. Vinyl and Add. Techn., 2 (1996)