CO-ROTATING FULLY INTERMESHING TWIN-SCREW COMPOUNDING: ADVANCEMENTS FOR IMPROVED PERFORMANCE AND PRODUCTIVITY

Transcription

1 CO-ROTATING FULLY INTERMESHING TWIN-SCREW COMPOUNDING: ADVANCEMENTS FOR IMPROVED PERFORMANCE AND PRODUCTIVITY Paul G. Andersen, Coperion Corporation, Ramsey, NJ Frank Lecner, Coperion GmbH, Stuttgart, Germany Abstract Te co-rotating fully intermesing twin-screw extruder is te primary production unit for compounding of polymer based materials. It also as ad a long term presence in processing material in te cemical and food industry and more recently in parmaceuticals. Wile tis equipment celebrated its 5 t anniversary several years ago and migt be considered a mature tecnology, it as not experienced a decline in new developments as migt be expected, but rater a significant number of advancements continue to evolve. Tis paper will igligt several significant developments of te past 1 to 15 years. Tese are te implementation of ig torque (power) designs, te use of increased rpm in conjunction wit ig torque for improved operating flexibility and productivity, and finally a tecnology breaktroug for feeding difficult to andle low bulk density materials. Introduction Wile several initial concepts for co-rotating twinscrew devices were patented in te early 19 s by Wuensce [1] and Easton [2, 3], te co-rotating design used as te basis for essentially all twin-screw compounding systems marketed today is based on te selfwiping element geometry know as te Erdmenger profile. Te initial design and development of tis self-wiping element profile is described in German Patent 862,668 granted to W. Meskat and R. Erdmenger in 1952 wit a priority date of 1944 [No US patent filed]. Te objective of te design at tat time was for mixing ig viscosity liquids already in te fluid state, suc as postpolymerization reaction products. Te above noted patent along wit te numerous related patents wic followed (all issued to Erdmenger or one of is colleagues at Bayer) defined te base design parameters for te eventual development and commercialization in te late 195 s by Werner and Pfleiderer of te ZSK twin-screw extruder, as well as te many copies introduced during te intervening 5 plus years. Te key feature of te design is te self-wiping caracteristic of one screw wit respect to te oter. Tis eliminates stagnation and eventual degradation of material as it is transported along te lengt of te compounding extruder. As mentioned, te overall importance of te invention of tis self-wiping screw geometry is tat it is te basic patent related to te co-rotating twin-screw compounding system predominantly used today in te plastics, food and cemical industry. (For additional information related to te early development advances please see te ANTEC 9 paper by Andersen et al. [4] and Wite s 1991 book on Twin Screw Extrusion [5].) Since te development of te basic principles for corotating twin-screw extruder tere ave been a significant number of incremental improvements to te tecnology. Tese include numerous new screw element geometries as noted by Bierdel [6], te two-lobe element profile for increased internal free volume (te initial profile described in te first Erdmenger patent was based on a low free volume 3 lobe geometry), new screw saft geometries for improved power transmission, and new process applications for te system [7]. However one of te most significant steps forward was acieved wit te identification of te fundamentals of ig rpm / ig torque compounding tecnology [8]. Tis is te basis for US Patent 6,42,26 granted to Heidemeyer et al. on Marc 28,. Hig torque, ig rpm co-rotating twin-screw compounding tecnology Since te introduction of te first ig torque, ig available rpm ZSK MegaCompounder (Mc) in te mid 9 s, new advances in power transmission tecnology (gearbox as well as screw saft design and material of construction) ave permitted an additional 5% increase in torque capacity from te Mc specific torque or power volume factor (PVF: Md/a 3 [Md = torque/saft (Nm), a = centerline distance (cm)]) of 11.3 to te Mc 18 PVF of 18. Te impact of tis advancement in power transmission capacity is a resultant significant increase in productivity (production rates), efficiency and system flexibility for compounders. Te key to te success of tis tecnology is te increase in te power (torque) transmission capacity in combination wit increased screw rpm. A system tat simply runs at iger rpm will at some juncture impart enoug additional energy to te material being processed to cause degradation. Figure 1 illustrates tis latter point.

2 It sows tat te average sear rate (energy input) increases linearly wit screw rpm for any screw Do/Di (outer diameter to inner diameter ratio). Terefore te resultant material discarge temperature will increase proportionately. However, since te twin-screw compounder runs primarily in a starve feed mode, te iger power transmission capability provides te compounding unit te ability to process at a iger fill factor and terefore rate per rpm (i.e. Figure 2 comparison of lower fill degree top grapic vs. te iger fill degree bottom grapic). In turn, tis fill factor increase as a positive impact on lowering material temperature. average sear rate for all te material and consequently te total energy input (i.e. resultant discarge temperature) per kg of product produced. Terefore te processor as te flexibility to run te extruder at iger rpm witout exceeding material temperature limits. As an example, Figure 3 sows a comparison of 3 generations of twinscrew compounding units based on a ZSK 45 geometry processing 3% glass filled nylon 6, te ZSK Mc (power volume factor of 11.3), te ZSK Mc Plus (power volume factor 13.6) and te ZSK Mc 18 (power volume factor 18). Do/Di = 1.55 v u Do / Di = barrel ma gamma = v u / 3-2-fligted 2-fligted 2-fligted (Hig torque) (Hig volume) γ 1 > γ 2 > γ 3 > γ 4 Do/Di = 1.55 v u [circumferential velocity (v)] Sear rate γ ~ [cannel dept ()] ma Average Sear Rate [sec^-1] D o /D i = 1.25 D o /D i = Screw speed [rpm] Figure 1: Impact of Do/Di & RPM on Sear Rate As sown in te screw cannel for te lower portion of Figure 2, te additional material is added to te screw profile in te deeper (lower sear rate) middle section of te element geometry profile. Tis in turn reduces te. D o /D i = 1.55 D o /D i = 1.8 Figure 2: Impact of degree of fill on average sear rate In te top portion of Figure 3, te trougput vs. rpm is sown for te 3 generations. As would be expected, te unit wit te greater power volume factor (ZSK Mc 18 ) as te greatest trougput rate as a function of rpm. However, as sown in te lower portion of te figure, it also as te lowest specific energy input. By combining tese two results (iger rate at lower SEI), tis data sows tat tere is a double economic advantage for using te igest power volume factor equipment available. First, because of te lower SEI (Specific Energy Input also known as Sme: Specific mecanical energy), te iger PVF unit can produce an increased trougput rate wic is disproportionately greater tan te percent increase in te power volume factor for one macine generation to te oter. (In tis particular example, te rate increase is between 7 and 8% wile te PVF increase is just over 5 %.) A general guide for rate increase is: New Rate = Old Rate x (PVF Hig Power/PVF Low Power) x (SEI Low Power/SEI Hig Power). Second, tere is an absolute energy saving per Kg of product produced. An additional point needs to be stressed about ig torque ig rpm compounding extruders. Tese macines do not ave to be run, or even designed to run, at maximum rpm. As Figure 3 as sown, tere is rate

3 increase and energy savings advantage at any rpm. However, tere is anoter power/rpm synergy tat permits a second disproportionate increase in rate and terefore production economics. Trougput [kg/] Specific Energy Input [kw/kg] ,22,,18,16, Screw Speed [1/min] ZSK Mc 18 ZSK McPlus ZSK Mc Screw Speed [1/min] ZSK Mc ZSK McPlus ZSK Mc 18 Figure 3: Comparison of rate and SEI for 3% glass filled nylon 6 vs. rpm for tree generations of extruders based on te ZSK 45 geometry An example of tis relationsip [8] can be seen in Figure 4 were trougput rate is plotted against screw speed for tree torque utilization values. SEI is also sown as a field parameter. Te data comes from an ABS (Acrylonitrile-Butadiene-Styrene) graft co-polymer compounding process on a ZSK 58 Mc (D o /D i = 1.55, torque = 125 N-m/saft, PVF = 11.3). Te lines for 69% and 9% torque compare respectively 9% torque conditions on a lower power ZSK 58mm SuperCompounder (Sc) extruder (D o /D i = 1.55, 96 N- m/saft, PVF 8.7) vs. 9% on te Mc (125 N-m/saft). Tis is a torque difference of 3% between te two macines. For tis example, a constant screw speed of 7 rpm was selected. At 69% torque (9% on te 96 N- m/saft extruder) a trougput rate of 66 kg/, wit a SEI of.19 kw-/kg and a melt temperature of 29 C was obtained. Increasing te rate to 9% torque led to a reduction of SEI from.19 to.175 kw-/kg. Tis resulted in a 4% rate increase to 93 kg/r, not just te 3% increase as one migt ave expected. At te same time te melt temperature dropped 15 C down to 275 C. Tis is especially advantageous for eat and sear sensitive materials. Tey can be run at increased rates but lower temperatures. Trougput [kg/] 1,6 1,4 1,2 1, SEI kw/kg =.16 M d = 9 % M d = 69 % M d =5 % T m = 275 T m = 29 n = , rpm [ min -1 ] Figure 4: Utilization of increased torque Trougput [kg/] 1,6 T m = 29 C 1,5 SEI kw/kg =.16 1, 1, 1, T m = 275 C M d = 9 % M d = 69 % M d =5 % T m = 29 C n = rpm [ min -1 ] Figure 5: Utilization of increased torque and rpm , n = 1, 1,2 1, However, if te original temperature of 29 C is satisfactory, ten Figure 5 illustrates te really significant impact of combining ig torque wit ig rpm. Te rpm can be increased to 1 wit an associated rate of 15

4 kg/r and a material discarge temperature of 29 C, te same as te lower torque operating system. Tis is a rate increase of more tan 15% from te original 66 kg/r. Te productivity and economic impact of increasing trougput by more tan 15% is significant. However, tere is anoter potential option for te company looking at installing a new line. If you do not need to produce 15 kg/r, but only te original lower rate of 66 kg/r, ten you may be able to purcase a smaller diameter extruder. As example, te new ZSK 45 Mc 18, as more tan 1% greater KW vs. rpm tan te ZSK 5 Mc and as only sligtly lower KW tan te ZSK 5Mc +, Figure 6. However, as sown in Figure 7, it can actually produce an equivalent or even greater output tan te larger diameter unit. KW Screw Speed (1/min) ZSK 5 Mc + ZSK 5 Mc ZSK 45 Mc 18 Figure 6: Comparison of available power for ZSK 45 Mc18 vs. previous generations of te larger diameter ZSK 5 compounding extruder. As sown in Figure 3, te ZSK 45 Mc 18 can produce approximately 6 kg/r of 3% glass filled nylon 6 at 6 rpm, and 97 kg/r at 11 rpm. Making te assumption tat te SEI obtained wen running te ZSK 45 at Mc Plus conditions (.18 kw/kg at 6 rpm and.22 kw/kg at 11 rpm) translates to te larger ZSK 5 Mc Plus, ten te ZSK 5 Mc Plus would produce approximately 58 kg/r. at 6 rpm, rougly te equivalent of te ZSK 45 Mc 18. At 11 rpm, te ZSK 5 Mc Plus would produce approximately 95 kg/r., again, te same or sligtly less tan te ZSK 45 Mc 18 (Figure 7). Feed Enancement Tecnology (FET) Hig torque extrusion tecnology is only an economically viable manufacturing process wen te process takes advantage of all te available power. However, many compounds produced today contain ig levels of low bulk density material, suc as sub-micron, non compacted talc. Tese materials are difficult to feed into te extruder because of te significant volume of air wic must be removed. Additionally as bulk density decreases, te materials tend to fluidize more easily. Fluidization lowers te effective bulk density even furter and exacerbates feeding issues. Typical unit operations witin te compounding process were material is more susceptible to fluidization are: transfer from storage vessel to feeder, from feeder to twin-screw extruder and witin te feed zone conveying section of te twin-screw extruder. Wile tere are metods to minimize te potential for fluidization suc as dense pase conveying from storage to feeder, minimization of te feeder eigt above te extruder feed opening, incorporating a vent into te feed opper, extending te lengt of te conveying zone in te extruder feed section, te process eventually reaces a feed volume limitation, wic more often tan not is well below an economically viable production rate. Trougput (kg/r) Screw Speed (1/min) ZSK 45 Mc 18 ZSK 5 Mc + Figure 7: Rate as function of rpm for ZSK 45 Mc 18 vs. te larger, more powerful ZSK 5 Mc + (3% glass filled N6). Te FET tecnology as been presented [9], [1] in detail. However, as background, a brief description of te principle is presented below. Te objective of FET is to increase te feed intake / feed zone trougput capacity for difficult to feed materials. Tis is accomplised by improving te conveying efficiency troug an increase in te coefficient of friction between te feed and barrel wall i.e. minimize/eliminate wall slip. Te conveying efficiency / coefficient of friction increase is acieved by adering a layer of feedstock material to a portion of te barrel wall troug te application of vacuum to a specially designed section of te barrel wall in te feed zone wic is porous and permeable to te gas, but not to te feed product. Terefore te pore size of te porous section of te barrel wall relative to te particle size of te powder is very crucial. Additionally te optimum vacuum level applied to te device depends on particle size and sape of te feedstock. If particles were to penetrate te pores, te efficacy of te process would be reduced. However, if powder were to penetrate te pores it could be back flused out by applying a pressure troug te vacuum line(s). Wile powder infiltrating te porous barrel wall could be problematic, even more critical would be te presence of polymer melt or oter fluid. Bot of tese

5 materials would smear over te porous surface or even penetrate te pores and clog te porous structure. Te working principle of FET is illustrated in Figure 8. By applying te vacuum troug te porous material, air surrounding te polymer or filler is evacuated as it passes te FET barrel section insert. As te air is sucked toward te insert, it entrains and carries te particles toward te insert surface. Te air goes troug but te material remains beind to coat te surface. Tis coating, or filter cake, of densified polymer powder as te effect of increasing te coefficient of friction between te wall surface and te bulk of te material. Te layer of material adering to te barrel wall due to te vacuum is continuously renewed by te rotating screws. Additionally, te bulk density of te powder is increased as it passes te insert. Tese two effects combine to improve te conveying efficiency. Air FET insert vacuum Figure 8: FET operating principle Filter cake: densified Effects: air is removed iger bulk density friction is canged in te area of insert It as been demonstrated tat te overall production rate could be increased by incorporating FET [9]. However, tere are oter impacts of te tecnology. Similar to te advantages detailed previously in tis paper of using a iger torque capacity compounding unit, increasing te rate of te igly filled polymer compounding line wile all else remains te same, results in a lower overall energy consumption per unit of product produced. Lower unit energy translates into lower product temperature, wic in turn would mean less potential for material degradation or stabilizer package consumption. Figure 9, illustrates tis point. Tis data is for 4% talc (Luzenac 1445) filled PP run on te new generation Coperion Mc 18 ZSK 45 mm twin-screw compounding extruder. Witout te FET tecnology, te compounder can not take advantage of te iger torque capacity of te extruder. However, by implementing te FET, te system runs at full torque (~85%), te trougput as been increased more tan 5% and te discarge temperature lowered significantly. Trougput [kg/] Wit FET Torque= 85% 1 Specific Energy =.129 kw/kg 8 6 W/O FET Torque= 54% Specific Energy =.134 kw/kg Screw speed [1/min] Figure 9: Impact of improved feed intake on rate and material temperature Summary Significantly iger trougput rates are acieved wen polymers can be processed at ig rpm. However, for most systems simply increasing te rpm of an existing extruder will not accomplis te desired results. Wile rates will be increased, product properties may fall below acceptable levels. On te oter and, by combining ig rpm wit increased torque capability, polymer processing economics can be significantly improved witout deterioration of product properties. Also, wile compounders will continue to ave issues wit andling low bulk density feed materials; wit FET tey now ave an additional tool to elp tem utilize te full flexibility of te twin-screw compounding extruder. References Trougput increase: 5+ % T = minus 15 C 1. A. Wunsce, German Patent 131,392 (191) 2. R. W. Easton, Britis Patent 19,663 (1916) 3. R. W. Easton, U.S. Patent 1,468,379 (1923) 4. P.G. Andersen, Ci-Kai Si, Mark A. Spalding, Mark Wetzel, Tim Womer, SPE-ANTEC Tec. Papers, 55, (9) 5. J. L. Wite, Twin Screw Extrusion: Tecnology and Principles (1991) 6. M. Bierdel, Co-Rotating Twin-screw Extruders: Fundamentals, Tecnology, and Applications (7) K. Kolgrueber & W. Wiedmann Editors 7 P.G. Andersen, Plastics Compounding, D.B. Todd ed., (1998) 8. P.G. Andersen, E. Haering, K. Kapfer, SPE-ANTEC Tec. Papers, 43, (1997) 9. P.G. Andersen, M. Hoelzel, T. Stirner, SPE-ANTEC Tec. Papers, 57, (211) 1. P.G. Andersen, M. Hoelzel, T. Stirner, SPE- EUROTEC 211 Key Words: Twin-screw, compounder, Hig torque, Hig rpm, feed enancement, powder filler