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PREPARATION OF HIGH MELT STRENGTH POLYPROPYLENE BY THE REACTIVE EXTRUSION PROCESS AND ITS FOAMING APPLICATION

Kun Cao, Zhan-quan Lu, Yan Li, Zhen-hua Chen, Zhi-cheng Shen, Zhen Yao, State Key Laboratory of Chemical Engineering(ZIU), Institute of Polymerization and Polymer Engineering, Department of

Chemical and Biological Engineering, Zhejiang Universiw, Hangzhou, 31 002 7, CHINA

Abstract

The reactive extrusion of maleic anhydride grafted polypropylene (PP-g-MAH) with ethylenediamine (EDA) as coupling agent was carried out in a co-rotating twin- screw extruder to produce long chain branched polypropylene (LCBPP). Part of PP-g-MAH was replaced by maleic anhydride grafted high-density polyethylene (HDPE-g-MAH) or linear low-density polyethylene (LLDPE-g-MAH) to obtain hybrid long chain branched (LCB) polymers. Compared with the linear PP, PE and their blends, the LCB polyolefins exhibit excellent dynamic shear and transient extensional rheological characteristics such as increased dynamic modulus, higher low-frequency complex viscosity, significantly enhanced melt strength and strain-hardening behaviors. LCB polymers also have higher tensile strength, tensile modulus, impact strength and lower elongation at break than linear polymer and their blends. Supercritical carbon dioxide (scC02) was introduced in the reactive extrusion process. With the presence of scC02, the motor current of twin extruder was decreased and LCB polyolefms with lower MFR, higher complex viscosity and increased tensile strength and modulus were obtained. This indicates that the application of scCO2 can reduce the viscosity of melt in extruder, enhance the diffusion of reactive species, facilitate the reaction between functional groups, and increase the LCB density. The foaming behavior of both linear and LCB polyolefms were studied. The results show that cellular materials produced from the LCB polyolefms have higher weight reduction, smaller cell size and better mechanical properties than those produced from the linear polymers.

Introduction

The melt strength and viscosity of commodity polypropylene (PP) drops rapidly as the temperature rises over its melting point. High melt strength is very important for thermofoming, extrusion coating, blow molding, and foaming processes etc. Grafting long chain branches onto PP backbone is a well-established technique to improve the melt strength of PP. Many methods have been developed to form the LCB structure, including in-reactor homopolymerization and copolymerization with special catalysts and agents [ 1-31.

and post-reactor reaction, such as electron beam irradiation [4-61 and peroxide curing [6-91. Recently, LCB polymers were prepared through chemical reactions between different functional groups on polymer chains [ll-131.

In our recent work[l4], PP with long chain structure were prepared by grafting PP, HDPE or LLDPE on its backbone in solution through reaction between functional groups. The creation of LCB structure resulted in higher storage modulus at low frequency, higher zero shear viscosity, reduced phase angle, enhanced shear sensitivities, and longer relaxation time. However, reaction in solution has many problems, such as toxic solvent harmful to humans and environment, long reaction time and low efficiency. The reactive extrusion process is favorable due to its advantages[lO] like fast reaction, continuous process and absence of solvent. To enhance the reaction, scCO2 can be fed to the extruder during reactive extrusion processes to reduce the viscosity of molten polymer, enhance the diffusion of reactive species, and facilitate the reaction.

In this paper, LCBPP was produced by the reaction between functional polyolefiens and EDA in a twin-screw extruder (TSE). The dynamic shear rheological properties, transient extensional rheological properties and mechanical properties of the produced LCB polymers were compared with various linear polyolefms and their blends. The effects of scCO2 had also been investigated. Both linear and LCB polymers were used to produce cellular polymers through injection molding with scC02 as the blow agent.

Materials

PP-g-MAH was obtained from Ningbo Nengzhiguang new material Co., Ltd, China (Mw: 130kg/mol, Mn: 43 kg/mol,Grafting degree: 0.32%). LLDPE-g-MAH was purchased from Nanjing Deba chemical Co., Ltd, China (Mw: 79 kg/mol, Mn: 20.7 kg/mol, Grafting degree: 0.39%). HDPEq-MAH was provided by Dalian Haizhou Co., Ltd, China (Mw: 80 kg/mol, Mn: 11.5 kg/mol, Grafting degree: 0.19Y0). Ethylenediamine (EDA), were acquired from Hangzhou

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Changqing Chemical Reagent Co., Ltd, China. All materials were used directly without any purification.

Synthesis of LCB Polymers and Foaming

The reactive extrusions were performed in a specially designed co-rotating TSE. The screw diameter is 20 mm and the ratio of length to diameter (LID) is 48. The large L/D ratio leads to long resident time and makes the TSE more suitable for reactive extrusion. The feeding arrangement is shown in Fig. 1.

TSE2O Modular Co-Rotating Twin-Screw Extruder

Fig. 1 Schematic diagram for the reactive extrusion process A: Polymers; B: EDA; C: scC02

The reactions occurred in the reactive extrusion process are shown in Fig. 2. First, the MAH group on PP- g-MAH chain is reacted with NH2 group of EDA to generate PP-g-NH2. PP-g-NH2 is further reacted with the residual PP-g-MAH to form LCBPP. Here, EDA acts as the bridge to link two polymer chains. When parts of PP- g-MAH are replaced with PE-g-MAH, the PP grafted with long PE chain is produced.

\ / A A A A L l l % ( B r i d s ) ~

---\A/\AA LCBPP

Fig. 2 The chemical reaction to form LCB polymer

Both HDPEg-MAH and LLDPEg-MAH were employed in this work. The weight ratio between PP and PE was 80:20. The extrusion temperature was fixed at 180°C in all runs. In Run 2, 4, 6 and 8, scC02 was fed to the TSE at 8MPa. The mass ratio between scC02 and polymers was controlled at 2%.

Cellular PP was prepared in the MuCell microcellular foam injection molding equipment supplied by Trexel (USA). ScC02 was used as the foaming agent. The weight fraction of scC02 was set at 5wt%. The melt temperature and mold temperature were 60 and 1 8O"C, respectively.

Characterization

The shear rheological behavior was measured using an ARES Rheometrics with parallel plates of 25mm in diameter and a gap of 1.5mm. Cylindrical samples of a thickness of 2mm and a diameter of 25mm were prepared by compression-molding at 180 "C. Dynamic fi-equency sweep measurements were carried out at 180 "C with fixed shear strain YO). The frequency range was varied from 0.1 to 100 rad/s. Transient extensional rheology was measured on an ARES Rheometrics with extensional clamps. Quadrate samples with a dimension of 2x lox 17mm were prepared by compression-molding at 180°C. Measurement was conducted at 180°C with various elongation rate. Melt flow index (MFR) data were measured at 230°C using a CEAST melt flow indexer with a load of 2.16 kg. The mechanical properties were measured through a Zwick 2020 universal tensile machine. Phillips XL-30 SEM was employed to observe the morphology of cellular PP.

Results and Discussion

Fig. 3 compares the MFR of starting polymers, their blends and products of reactive extrusion. It can be seen that the product of reactive extrusion between PPq-MAH and EDA (LCBPP) has significantly lower MFR than the starting polymer, PP-g-MAH. This suggests that the reaction shown in Fig. 2 has occurred and polymers with LCB structure have been generated. The MFR of the blend of PP-g-MAH and HDPE-g-MAH is lower than the MFR of PPg-MAH and higher than that of HDPE-g- MAH. The product of the reactive extrusion between PP- g-MAH, HDPEq-MAH and EDA (PP-g-HDPE) has a MFR lower than that of both PPg-MAH and HDPE-g- MAH. On the other hand, the MFR of PP-g-LLDPE (the product of reactive extrusion between PP-g-MAH, LLDPE-g-MAH and EDA) is lower than the MFR of PP- g-MAH but higher than that of LLDPEg-MAH.

The effects of scC02 are shown in Fig. 4. It can be found that the MFR of the LCB polymers obtained with the presence of scC02 are significantly lower than that of the LCB polymers obtained without scC02. The motor current of TSE was decreased when scC02 was introduced. This proves that using scC02 can reduce the viscosity of the melt in TSE, enhance the mobility of EDA and segments of polymer chain, and facilitate the reaction in the extrusion process.

471 /ANTEC 2010

40 1 0'

w e 9 cl 108

b

10 10'

0

Fig. 3 Melt flow rate of various polymers

a

7

6

5

2

1

0 LCBPP LCBPP PP-9-HDPE PP-g-HDPE PP-9-LLDPEPP-8-LL

withollt srcn:, ..,,.* = * C n O I... tL^... +*-r)* I... 1. LL-r)*I.II1*^I,l P-CnOl.ll.* =,

Fig. 4 Effects of scCOz on MFR and TSE motor current

Fig. 5 compares the storage modulus (G') and loss modulus (G") of the all samples. Seen from this figure, the reactive extrusion process has significantly changed G ' over the entire frequency range. The remarkable higher G ' at low shear rate is the results of higher levels of elasticity induced by the formation of LCB structure. The cross-over between G ' and G" is found for all samples with LCB structure, which is similar to the previously reported results[ 151. Loss angle, 6, defined as tans= G" / G ' is another parameter of melt elasticity: the higher elastic is, the lower loss angle would be. The loss angle of linear polymers, their blends and grafted polymers are plot as a function of frequency in Fig. 6 . Grafted polymers (LCBPP, PP-g-HDPE and PP-g- LLDPE) demonstrate lower loss angle than PPg-MAH and PP-g-MAH/PE-g-MAH blends. This is another indication of the presence of LCB structure in the grafted polymers.

The complex viscosities of PPq-MAH, PP-g- MAHIPE-g-MAH blends and grafted polymers are presented in Fig. 7. PP-g-MAH/PE-g-MAH blends have higher complex viscosities than PP-g-MAH over the entire frequency range studied. In the low frequency range, the complex viscosities of grafted polymers are significantly higher than that of PPg-MAH and PP-g- MAH/PEg-MAH blends. The differences between the complex viscosities of grafted polymers and linear polymers/blends are narrowed in the high frequency range.

1 10 10' m (radls)

PP-g-MAH 0 PP-g-MAH 0" LCEPP 0 LCBPP 0"

10'

10' m (rad/s) lo

I"

10'

I m(rrd/s)lo 10'

Fig. 5 The storage modulus and loss modulus of various polymers

10' PP-g-MAH LCBPP PPIHDPE Bleni PP-g-HDPE PPILLDPE Bler PP-g-LLDPE

10'

1 10 1 m , (radls)

Fig. 6 The loss angle of various polymers

I 0'

+i- E

1 0'

1 a, (radls)o

PPg-MAH LCBPP PPIHDPE Blend PPg-HDPE PPILLDPE Bleni PPg-LLDPE

100

Fig. 7 The comF-;x viscosity of variouspc-jmer

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Effects of scC02 on the complex viscosities of grafted polymers are shown in Figure 8. It can be seen that the complex viscosities of grafted polymers obtained with the presence of scC02 are higher than those of polymers obtained without scC02 in the all frequency range. Therefore, the application of scC02 results in high reaction degree.

Results of transient extensional rheology test are shown in Figs. 9 and 10. The extensional viscosity of PP- g-MAH is very low and decreased with the increase in time (strain), which is termed strain-softening phenomena. Blending PPq-MAH with PEq-MAH can improve the extensional viscosity but can't change their strain-softening behavior. On the other hand, all samples with LCB structure show very high extensional viscosity and strain-hardening behaviors. Melt strength can be defined as the largest elongation force before the polymer melt breaks [16]. Fig. 10 shows the time evolution of elongation forces during the transient extensional rheology tests for various polymers. The melt strength of starting PP-g-MAH is very low (0.45cN). The melt strength of LCBPP reaches 15.7cN, which is more than 30 times higher than the melt strength of linear PP. Blending PP-g-MAH with PEq-MAH can improve the melt strength slightly (0.52cN for PP/HDPE blend and 0.66cN for PP/LLDPE). Grafting LLDPE onto PP can improve the melt strength by 34 times while grafting HDPE onto PP can improve the melt strength by 11 times.

Mechanical properties of PP-g-MAH, PE-g-MAH, their blends and grafted polymers are compared. The tensile strength of LCBPP is more than 38MPa and increased more than 10% over the starting PP-g-MAH. The tensile modulus of LCBPP is close to 2000N/mm2, which is a 25% increased over linear PP. The notched impact strength of LCBPP, which reaches 3.7 kJ/m2, is increased 96% over that of PPg-MAH. However, the elongation at-break of LCBPP is lower than that of PP-g- MAH. Compared to PP, PE has lower tensile strength and modulus but higher impact strength and elongation at break. The tensile strength and modulus of PP/PE blends are higher than that of PE and lower than that of PP. The LCB polymers (PPq-HDPE and PPq-LLDPE) have higher tensile strength and modulus than blends of linear polymers. The impact strength and elongation at break of PP/PE blends are lower than that of PE and higher than that of PP. PP-g-HDPE and PPq-LLDPE have higher impact strength but lower elongation at break than PP/PE blends.

Fig. 11 shows the effects of scC02 on the mechanical properties. It can be seen that the tensile strength and modulus of the grafted polymers obtained with the presence of scC02 are significantly higher than those of the polymers obtained without scC02. The grafted

polymers obtained with scC02 have higher density of long chain branch as the result of enhanced reaction.

-m- LCBPP+ScCO2 -0 - LCBPP

Anguidr frequency, m !?rad/s) 1 0'

Fig. 8 Effects of scC02 on the melt rheology of LCBPP

1 0 6

PP-g -MAH LCBPP PP/HDPE Blend PP-!a -HDPE PP/LLDPE Blend

0 PP-g -LLDPE

0 6 10 16 20 26 3 Time (8)

Fig. 9 The melt elongation viscosity of various polymers

10

s B u o l c 0.1

-- - P P - W H -- - LCBPP

I-.- PP-gHDPE PP/HDPE Blend

PP/LLDPE Blend -4 - PP-gLLDPE

0 s 10 1s 20 26 30 3s

Tim (s) Fig. 10 Melt elongation force of various polymers

Fig. 12 shows the mechanical properties of foams produced from polymers with different structures. Both the measured properties and the modified properties are presented. To account for the weight reduction of cellular polymers, the modified properties is calculated as

Measured Properites 1 - Weight Reduction

It can be found that the measured tensile strength of LCBPP foam is similar to that of the linear PPg-MAH foam. However, the modified tensile strength of LCBPP foam is higher than that of PPq-MAH foam due to the higher weight reduction of LCBPP foam. Both measured and modified impact strength of linear PP foam are lower than that of LCBPP foam. The measured tensile strength of PP-g-HDPE and PPq-LLDPE foams are slightly lower than that of the foams produced from PPIHDPE and

473 /ANTEC 2010

PPiLLDPE blends, respectively. This is a result of the higher weight reduction of PPg-HDPE foams and PP-g- LLDPE foams. The modified tensile strength of PP-g- HDPE and PP-g-LLDPE foams are higher than that of foams produced from corresponding linear blends. The measured and modified impact strength of cellular PP-g- HDPE and PP-g-LLDPE are all higher than that of cellular PPiHDPE and PPiLLDPE blends. It can be concluded that LCBPP, PP-g-HDPE and PPq-LLDPE, can be used to produce cellular PP. The cellular materials produced from the polymers with long chain branches have better mechanical properties than the foams produced from linear PP, PE and their blends.

Morphologies of cells in the cellular materials produced from various polymers are shown in Fig. 13. Large cells are found in the linear foamed materials, including PP-g-MAH, PP/HDPE blends and PPiLLDPE blends. Their diameter is close to lmm. On the other hand, the cells in cellular materials produced from LCB polymers (LCBPP, PP-g-HDPE and PP-g-LLDPE) are much smaller due to the high melt strength and restrained cell rupture in the foaming process. The diameter of the cells in the foamed LCB polymers ranges from 20 to 200pm.

Conclusions

LCB polyolefms were obtained by the reaction between maleic anhydride grafted PP, HDPE or LLDPE and ethylenediamine in a twin-screw extruder. It has been found that the products of reactive extrusion have significantly lower MFR than the starting linear polymers and their blends. The shear and transient extensional rheological properties have been examined. Compared to the linear PP, PE and their blends, the LCB polymers possess enhanced dynamic modulus, increased low- frequency complex viscosity, and significantly higher melt strength. LCB polymers also exhibit strain-hardening behaviors instead of the strain-softening behaviors of linear polyolefms and their blends. The tensile strength, tensile modulus and notched impact strength of LCB polymers are considerably higher than those of linear PP and PPiPE blends. The elongation at-break of LCB polymers is lower than that of linear polymers.

scC02 has been introduced in the reactive extrusion process. It has been found that introducing scC02 can facilitate the reaction between the functional group grafted on polymer chains and resulted in lower MFR, higher complex viscosity and increased tensile strength and modulus.

Linear and LCB PP and PPiPE blends are used to produce cellular PP through injection molding with scCOz as the blow agent. It has been found that foams produced

from the LCB polymers have higher weight reduction, smaller cell size and better mechanical properties than the foams produced from linear polymers.

.-

- N

E E

2000 In

3 - H 1800 Q)

In - .-

' 1600

No 8cC01 Add ScCOz

25 LCBPP PP-g-HDPE PP-g-LLDPE

No ScCOn Add ScCOz

LCBPP PP-g-HDPE PP-g-LLDPE

Fig. 11 Effects of scC02 on tensile strength and modulus IIICII~UICIU I PllDllP 0 L ' ~ ' ' y L ' I \'""a, Modified Tensile Strength (MPa) Measured Impact Strength (MPa) Modifled Impact Strength (MPa) .- I r

30

20 e n .- c c y 10

= 6:

0 PP-g-MAH Foam LCBPP Foam

I Measured Tensile Strength (MPa) I Modifled Tensile Strength (MPa) I Measured Impact Strength (MPa) I Modlfled Impact Strength (MPa)

.- 8 40 r 3o

n - .P 20

Y 10 m c

I 6

0 PP/HDPE Blend Foam PP-g-HDPE Foam

MeasuredTenslle Strength (MPa) Modifled Tensile Strength (MPa) Measured Impact Strength (MPa) Modlfled Impact Strength (MPa)

.p 3o 2 s 20

m u E .- Q I 0

d s

0 . PPlLLDPE Blend Foam PP-g-LLDPE Foam

Fig. 12 Effects of polymer structures on mechanical properties of cellular polymers

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Acknowledgement

This study was supported by the National Natural Science Foundation of China (NSFC Major Project No.50390097 and NSFC Project Nos. 20576115 , 50773069 and 50873090) and 973 Program of China (No. 2009CB320603).

References

1.

2.

3.

4.

5.

6.

W. Weng, W. Hu, AH. Dekmezian, and C.J. Ruff, Macromolecules, 35,3838 (2002). Z. Ye, F. AlObaidi, and S . Zhu, Ind. Eng. Chem.

Res., 43,2860 (2004). J.A. Langston, R.H. Colby, T.C. Chung, F. Shimizu, T. Suzuki, and M. Aoki, Macromolecules, 40, 2712 (2007). D. Auhl, J. Stange, and H. MUnstedt, Macromolecules, 37,9465 (2004). F. Yushii, K. Makuuchi, S . Kikukawa, T. Tanaka, J. Saitoh, K. Koyama, Journal of Applied Polymer Science, 60,6 17-623, 1996. J. Gao, Y. Lu, G. Wei, X. Zhang, Y. Liu, and J. Qiao, J . Appl. Poly. Sci., 85, 1758 (2002).

7. R.P. Lagendijk, A.H. Hogt, A Buijtenhuijs, and A.D. Gotsis, Polymer, 42, 10035 (2001).

8. X. Wang, C. Tzoganakis, and G.L. Rempel, J. Appl. Poly. Sci., 61, 1395 (1996).

9. D. Graebling, Macromolecules, 35,4602 (2002). 10. G. Moad, Prog. Polym. Sci., 24, 81 (1999). 11. B. Lu, T.C. Chung, Macromolecules, 32, 8678

12. K.Y. Kim, and S.C. Kim, Macromol. Symp., 214,289

13. Q. Lu, C.W. Macosko, and J Horrion, J. Poly. Sci. A:

14. Z. Yao, Z. Lu, X. Zhao, B. Qu, Z. Shen, and K. Cao.

15. A.D. Gotsis, B. Zeevenhoven, and C. Tsenoglou, J.

16. S. Muke, I. Ivanov, N. Kao, and S.N. Bhattacharya,

(1999).

(2004).

Poly. Chem., 43,42 17 (2005).

J. Appl. Poly. Sci., 111,2553 (2009).

Rheol., 48, 895 (2004).

Polymer International, 50,5 15 (200 1).

Key Words: Polypropylene; Long Chain Branch; Reactive Extrusion; Supercritical Carbon Dioxide; Cellular Polymers

Fig. 13 Morphologies of cells in the foamed material produced from various polymers

475 /ANTEC 2010

AN EVALUATION OF THE PERFORMANCE OF DRIVE SYSTEM CONFIGURATIONS IN SINGLE SCREW EXTRUDERS

Jason C. Baird, Ph. D., Davis Standard LLC, Pawcatuck, CT John P. Christiano, Davis Standard LLC, Pawcatuck, CT

Abstract

A comprehensive study was conducted using three different types of extruder drive and motor configurations for a single screw extruder under identical conditions to compare the energy efficiency. Sound emission data was also measured and compared. Two systems of a conventional design used an AC or DC motor in combination with a mechanical gear reducer and were compared to a Direct Drive system using only a torque motor. The study showed that the Direct Drive system operated at higher energy efficiency and produced lower sound emissions compared to the AC & DC conventional systems. The results show that at the base speed of the drive train, the DC system consumed 10% more energy (kW) than the AC system, and the Direct Drive system consumed between 12 and 15% less energy than the AC system, depending upon the torque required.

Introduction

Equipment producers of single screw extruders are focusing on improving the energy efficiency of their equipment. The driving force for this trend is due to the rising cost of energy and public awareness concerning the environmental impact produced by the equipment over its lifetime. These impacts include contributions to green house gas, emissions due to the disposal of waste lubricating oil and noise pollution in the workplace.

Very little information has been published over the years to assist choosing the most energy efficient drive and motor configuration for a single screw extruder. Information comparing the performance of different drive and motor configurations on an extruder during actual operation using exactly the same extruder screw design, materials and operating conditions to compare energy efficiency is limited or nonexistent. Most of the available information has been limited to data abstracted from Manufacturer catalogs and reinterpreted for extrusion applications [ 1,2].

This study examined three different types of extruder drive and motor configurations for a single screw extruder and compared the energy efficiency between the systems. Sound emission data was also taken for comparison.

Two of the configurations are of conventional design, and utilize an AC or DC motor in combination with a mechanical gear reducer to multiply the torque produced by the motor required to rotate the extruder screw as shown in Figure 1 .

Figure 1 . Conventional extruder shown with an AC or DC motor, sheaves and gearbox

These units are compared to a Permanent Magnet Synchronous Motor, (PMSM), or torque motor as shown in Figure 2. This configuration has a very high power density motor and does not require the use of a mechanical gear reducer to multiply the torque. Both extruder configurations tested require the use of a thrust bearing to handle the high axial loads developed during extrusion.

Figure 2. Direct Drive Extruder with only a Permanent Magnet Synchronous Motor (PMSM)

In order to make an accurate comparison between the different drive and motor configurations, (system), each system was tested on extruders with the exactly the same processing section that included the feed section, barrel, screw and control system.

Two materials were selected for the study to investigate the performance over a range of torque levels.

ANTEC 201 0 / 476

A fractional melt HDPE was used for the high load study and PP was chosen for the low load study. Each system was tested over a wide screw speed range utilizing the same operating temperatures and a tube die with a fix orifice to provide backpressure.

The drive and motor performance was monitored during each test condition using a Power Quality Analyzer to record power consumption. Recorded parameters included three phase voltage, amperage and power consumption. All of the extruder operating conditions including throughput, melt temperature and head pressure and barrel heat flux were monitored to insure consistency during the experiments.

The data reported in this study comparing the energy efficiency of different drive and motor configurations is important. End users will be able to use this information to make intelligent choices concerning their equipment purchases in the future. Improving the efficiencies of their operations through intelligent equipment selection will help to reduce cost and impact to the environment over the life of the equipment.

Experimental

Extruder Experiments were conducted on a highly

instrumented 63.5mm (2 W’) F+30: 1 LID Davis-Standard extruder equipped with five electrically heated/water cooled barrel zones. A ThermaticB Temperature Control system was used to control the barrel temperatures and measure the heat flux in each zone of the extruder. Five pressure transducers were located along the axial length of the barrel at 5 L/D increments to evaluate the axial pressure generation along the screw and the cross channel gradients at each axial position as in Figure 3. A pressure transducer and exposed junction melt thermocouple were located in the discharge adapter of the extruder in order to evaluate the output and thermal process stability of the melt. A tubing die with a fixed pin and bushing was used to generate back pressure within the extruder.

Figure 3 - 63.5mm 30: 1 L/D Extruder Instrumentation

Materials Two different materials were trialed; Equistar

Petrothene LR732001 a 0.3 MI HDPE (a representative “high torque” material), and LyondellBasell Pro-Fax HD640H a 2.0 MFR PP (a representative “low torque” material).

Screw A DSBMB barrier screw of low intensity was

used for the trial. The design details are shown in Table 1. The screw featured a feed of 6.5 L/D with a barrier length of 15.5 LID, a spiral UCC (Maddock) mixing section of 2 LID and a final metering section of 5.5 LID. Depths shown are in mm.

L/D I F+5 I 1 I 15.5 I 2 I 5.5 h (mm) I 12.1 I - IU/C=1.02IU/C=l.O2I 4.8

Table 1 - Details of the experimental feedscrew

The barrel temperature profile, Zone 1 through 5, respectively, was set at 216, 227, 216, 210, and 204 “C for the PP and 177, 204, 191, 177, 177 “C for the HDPE during the entire study. The screw speeds used for each of the materials ranged from 25, 50, 75, 100, 125 to 150 RPM.

The data recorded at each operating condition included the extruder output, melt temperature in the center of the melt stream in the die adapter, motor power consumption, heat flux in each barrel zone, and pressure readings along the barrel. The pressure readings were recorded with a data acquisition system at a rate of 50 Hz.

Drive Configurations

for these experiments as follows: 1. DC Configuration

Three different drive configurations were used

Drive: 56 KW, (75 HP) Reliance FlexPak 3000 Motor: 56 KW, (75 HP), Reliance RPMIII

Motor, 1150/1725 RPM Sheaves: 1:1.5 Gear Box Ratio: 17.28:l Overall Ratio: 1 1.52: 1

Drive: 56 KW, (75 HP) Yaskawa F7 Motor: 56 KW, (75 HP) Baldor Reliance

Super-E XEX, 1800/2700 RPM Sheaves: 1.04: 1 Gear Box Ratio: 17.28: 1 Overall Ratio: 18: 1

Drive: 58 kW, Baumiiller BM4453 Motor: 58 kW, (77.8 HP), Baumiiller

DST315BX, 1001150 RPM, Sheaves: N/A Gear Ratio: N/A Overall Ratio: N/A

2. AC Configuration

3. Direct Drive (DD) Configuration

The DC and AC configurations utilized the same extruder and gearbox while the Direct Drive, (DD) configuration was an entirely separate although similar

477 /ANTEC 2010

extruder. presented in Figures 1 and 2.

Visual representations of the extruders are

Electrical Analysis Electrical measurements were made using a

Fluke 435 Power Quality Analyzer (PQA) with i200s current clamps set at a maximum of 200 Amps in a Delta configuration for 3-phase 480V power. The current and voltage clamps were connected to the same incoming leads to each of the drive configurations (e.g. mains in) and thus the measured electrical characteristics (e.g. power consumption and power factors, among others) were that of the drive system only and not the entire extruder. Logged data included the voltage and current for each leg, the total actual, active and reactive power for the system and the system’s power factor as a function of time.

Due to the inherent transient nature of the system, electrical data was logged at 0.5s intervals for 30 minutes of operation at each steady-state condition and averaged.

Sound Level Analysis Sound level measurements were taken using a

BrUel & Kjaer Type 2236A-007 Type 1 precision integrating sound level meter. Sound level measurements were taken at each extruder speed at 5 locations around the perimeter of each machine as illustrated in Figures 4 and 5.

Figures 4 and 5 - Sound level measurement locations for the AC and DC configurations (above) and DD

configuration (below) [figures not to scale].

Discussion

In order to interpret the results of these experiments, a brief overview of drive technologies must be presented.

DC & AC Drive Technologies In a typical DC brushed motor, such as the one

used in this experiment, a DC current (supplied by the drive) is applied to rotor andor stator windings via terminals or brushes and slip rings since the rotor must rotate. The stator of the motor may have permanent magnets or electromagnets depending upon its size (larger motors & power requirements will usually have electromagnets due to cost considerations). The rotor andor stator DC current are alternated via the use of a commutator, and repulsion between the magnetic fields of the rotor and stator results in rotation of the rotor within the stator. In this application, torque is proportional to current and RPM is proportional to voltage. Figure 6 illustrates the inner workings of a simple DC motor.

Typical Brushed Motor in Cross-section

/Commutator Rotor \

Windings

u u - Terminals

Magnet

’ Stator (case)

Figure 6 - TypicalDC brushed motor with permanent magnet stator

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DC motors were once popular due to their relatively low cost and ease of control, however they also tend to be very inefficient due to several factors:

AC/DC rectifier losses (“conversion” of plant three phase AC power to DC) Drive consumption (that is, power that the drive consumes to control the motor which doesn’t contribute to the overall power output of the extrusion system)

0 Bearing friction (resulting in a high idle current) 0 Resistance of rotor and alternator (voltage

losses) 0 Alternating magnetic field in rotor andor stator

(“iron losses”) 0 Brush friction & arcing (which also presents

maintenance issues)

0

0

Typically all of these inefficiencies manifest themselves as heat generation and noise, neither of which contribute to the extrusion of the resin.

An asynchronous AC motor such as the one used in this experiment operates in a similar fashion as the DC motor however the stator windings generate oscillating magnetic fields with AC power directly and thus do not suffer from the same large AC-to-DC rectifier losses that DC drives do. The oscillating current in the stator induces electrical currents and associated magnetic fields within the rotor which lag that of the stator. Thus these types of motors are termed “asynchronous”. AC motors are a bit more complicated to control, however. Here the motor RPM is proportional to the frequency of the AC current, and applied torque is proportional to current.

AC motors suffer from many of the same inefficiencies as DC motors except for rectifier and brush losses, however since these types of AC motors are inherently inductive, there are rather large losses (as heat) within the rotor.

The second half of the drawback of using traditional DC and AC motors is that they provide relatively low levels of torque and require the use of a gear box to multiply the torque need to rotate the screw. This, in turn, means that a second source of losses must be introduced to the drive system in order to use these motors for extrusion applications: a gearbox. Like any other application, a gearbox is subject to losses (inherently lossy) as it involves the movement of metal and other surfaces against one another and also has a great deal of rotational inertia. These losses (frictional) often manifest as heat and noise pollution in the immediate environment in addition to maintenance issues.

As with the traditional DC and AC drive systems, DD technologies also rely on the repulsion between magnetic fields for rotation of the rotor. A typical DD system such as the one used in this experiment utilizes very strong rare-earth magnets in the rotor and is thus termed “permanently excited”. A set of windings in the stator are magnetified directly by AC power and due to its permanent magnets, the rotor moves in synchronicity with the alternation of the stator’s magnetic field, thus the motor is also a “synchronous” motor. Ultimately this combination of enhancements results in far less electrical waste in the motor itself. Since the rotor is magnetified by very powerful permanent magnets, no losses occur in conveying electricity to the rotor or within the rotor itself. Also since the rotor moves in sync with the stator’s magnetic field, there are no “slip” losses between the two. Due to strong permanent magnets, Permanent Magnet Synchronous Motors (PMSMs) have high torque densities and can directly drive high-torque applications such as extrusion, without the need for a gearbox. While the PMSM itself may be a bit larger than its traditional counterpart, the overall footprint of the extruder can be smaller and more streamlined due to the lack of a gearbox arrangement.

Lastly, DD configurations can be significantly quieter than traditional gearbox arrangements due to their lack of a gearbox. Many DD configurations such as the one used in these experiments also utilize water to cool both the drive and motor, which is perhaps responsible for the largest reduction in system noise.

Experimental Validitv The goal of this study was to compare the overall

efficiency (via power consumption) of three different drive technologies: DC motor with gearbox & sheaves, AC motor with gearbox & sheaves, AC Direct-Drive motor (DD) without a gearbox or sheaves. Each experiment was carried out on the same size (bore and L/D) machine with the same screw, die and temperature profile (for PP and HDPE, respectively), and data was taken at a steady-state condition for each trial run. Since the process side of the energy balance for the entire system did not change from configuration-to- configuration, and data was only taken at steady-state conditions, the power consumption measured by the Fluke PQA genuinely reflected the differences between the drive configurations onlv, as the process was successfully eliminated as a variable. To c o n f m this, the performance curves for each of the runs (output vs. screw speed) have been overlaid in Figure 7 and the data provided in Table 2.

Direct Drive (DD) Technologies

479 /ANTEC 2010

160

140

120

25 50 75 100 125

Output vs. Screw Speed

26 26 27 24 27 26 53 43 52 41 53 41

74 55 76 55 76 56 94 I 68 98 I 69 95 I 70

130 I 86 127 I 86 129 I 84

......................................................................................................................................................................................................................................................

......................................................................................................................................................................................................................................................

......................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

............................................................................................................................................ ....................................... :..........................

M I 0

50 0

Power Consumed vs. RPM

E loo T 80 P

60

40

-400 5 1 f E 300

too

20

0 - 0 20 40 60 80 100 120 140 160

Extruder Speed (RPM)

Figure 7 - Output vs. screw speed for all drive configurations

Table 2 - Output vs. screw speed for all drive configurations

The tight groupings of each of the respective (PP below, HDPE above) performance curves indicates that the process-side of the energy balance has been eliminated as a variable in the overall energy balance for the machine.

Power Consumption The most striking results from this study are

presented in Figure 8, which overlays the power consumed vs. RPM for each of the drive systems and materials tested. In this and all curves in this paper, PP is represented by square symbols and HDPE by circles. The various drive systems are represented as follows: DC system, red; AC system, blue; DD system green. In Figure 8, the upper-most series of curves are for HDPE and the lower curves for PP. For each of the screw speeds and materials tested, one thing was consistent: the DC system consumed more power than did the AC system, which consumed more power than the DD system.

10 0

0 0 0 20 40 Mi 80

Eslruder Speed (RPW IW 120 140 IMi

Figure 8 - ConsumeL power vs. screw speed for all Live configurations and both materials

Considering that the current industry standard for extrusion, (and many other applications), is an AC drive system, the data in Figure 8 has been re-scaled such that the base power consumption is the AC system's power consumption at that speed for a specific material (e.g. DC and DD systems' power consumption is reported as a % deviation from the AC system's consumption), as in Figure 9.

Consumed Power Comparison vs. AC Geared/Belted Drive

70%

* MI% ,?

5 50%

$ 40% { 30% d

i 20%

'Z I 10%

t s 5 O%

-10%

-20%

_" "

EIlruder Speed (RPM)

Figure 9 - Consumed power for DC and DD drive systems as % deviation from the AC system.

Figure 9 clearly shows that the DD system retains an advantage over both conventional systems over the entire range of operating conditions. For the low- torque material (PP), the DD system consumed between 15 and 20% less power than the AC system, and the DC system consumed between 5 and 30% more power. Similarly for the high torque resin (HDPE), the DD system consumed between 10 and 15% less power than the AC system, and the DC system consumed between 7 and 25% more power. At a typical operating condition, and the base speed of all the motors used in the experiment, 100 RPM, the DC system consumed 10%

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more power than the AC system, and the DD system between 10 and 15% less than the AC system.

Noise Levels Yet another advantage to a DD system is an

extreme reduction in noise levels. Much of the noise generated by a traditional system (e.g. DC and AC drive with gearboxes) is due to the rotation of the gears and large cooling fan(s) which use forced air to cool both the stator and the rotor of the motor. With the two traditional systems used in this study, the DC system had an independent cooling fan which ran at the same rotational speed and air flow rate regardless of extruder speed, and the AC system had a built-in fan blade that spun with the motor itself. Figure 10 illustrates the differences in sound levels (averaged and ambient noise levels accounted for) measured for the three drive systems and two materials.

Sound Level (dB) YI. RPM (Arnbienl seeounled for)

60

Figure 10 - Sound level vs. extruder speed for each of the drive systems and materials.

To put these numbers in context, normal speech has a sound level of 60 - 70 dB, which is comparable to the DD configuration over its range of operation. The DC drive configuration is comparable to a telephone ring or being 5m away from the curb of a busy road at - 80 dB. On the higher end, the AC drive configuration, due to its built-in cooling fan, had a sound level of 3 5 dB which is comparable to a train whistle at 150m or heavy truck traffic. The level at which sustained exposure could result in permanent hearing loss is appx. 90 - 95 dB.

Other Considerations One other consideration (power consumption

and noise emissions being first and second, respectively) for DD systems is their relative simplicity. Traditional (geared and beltedsheaved) systems require periodic maintenance of the gearbox itself (oil changes, inspections, etc) whereas DD systems do not require any such maintenance.

Also, due to their relatively torque-dense nature, DD systems can have a much smaller footprint than their traditional counterparts, which may have implications for tight factory floors where space is at a premium.

Conclusions

A comprehensive experiment was performed to evaluate the differences in energy consumption between three different extruder drive systems (DC & AC geared & beltedsheaved, and Direct Drive (DD)) using two different materials (PP, HDPE).

The results show that at the base speed of the drive train (100 RPM in each case), the DC system consumed 10% more energy (kW) than the AC system, and the Direct Drive (DD) system consumed between 12 and 15% less energy than the AC system, depending upon the torque required. Sound level measurements showed that the DD system was 70 times quieter than the AC system at 100 RPM, and the DC configuration was 5 times quieter.

Clearly, Direct Drive systems have both an energy and sound advantage over AC & DC conventional systems. Due to their lack of a gearbox and beltshheaves, Direct Drive systems will also have lower maintenance costs over their lifetime.

There are, however, a couple drawbacks to Direct Drive systems: 0 Due to their complexity, PMSMs have a significantly

higher capital cost than conventional systems. 0 PMSMs are also limited in overspeeding to 15-20% of

the base speed of the motor. 0 Commercially available DD motors are currently

limited to a continuous torque rating of approximately 20,000 Nm (2.8 hp/RPM), which limits their use to extruders I 1 15mm in diameter (4 54”).

Acknowledgements

The authors would like to thank the following: 0

0

LyondellBassel for providing the materials (PP, HDPE) used in this study. Baumuller for providing the PMSM drive motor used in this study.

References

1. S. Barlow, SEANTEC T&. Pap4 55 1157

2. W.A. K r m , PEANTEC T&. P a p 4 45 268, (2009).

( 1 999).

481 /ANTEC 2010

Key Words: Direct Drive Extruder, Energy Efficiency, Torque Motor, Single Screw Extruder, Permanent Magnet

synchronous motor

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