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Experimental study of melting of LDPE/PS polyblend in an intermeshing counter-rotating twin screw extruder.

INTRODUCTION

One of the fundamental tasks for extruder machines is the melting of the solid polymer whether in the form of pellets or powder. This is obviously true for both single screw extruders and twin screw extruders. Although melling in single screw extruders has been widely investigated and modeled, rather little information has been presented on the mechanism of melting that occurs in the twin screw extruders.

The single screw extruders have been broadly used for thermoplastic materials since the 1930s, and there have been many efforts to study the melting process since that time (1-9). Fundamental observations were made by Maddock (1) and Street (2). Their experiments were based on rapid cooling of an extruder, extraction of the screw, and the polymer strip which runs from hopper to die. These polymer strips were cross-sectioned in various places and the cross-sections were then analyzed. Generally, in the melting region a melt layer was observed along the barrel and in a pool close to the active flight. Tadmor et al. (5), (8), (9) first proposed a fundamental model for polymer melting in single screw extruders. This model argues that a melt layer is formed between the hot barrel and the solid which is scrapped off by the transverse flow in the screw channel and accumulates at the active flight. The solid bed is gradually decreased by the combined effects of heat conducted from the barrel and from viscous dissipation within the melt. There have been many efforts to improve this model (10-14). All the studies have been recently summarized by Rauwendaal (15).

Contrary to the melting in single screw extruders, experimental studies in twin screw extruders have appeared rather recently in the literature (16-22). These studies have almost completely involved modular self-wiping corotating twin screw extruders. Various models have been developed to analyze the melting process in these machines (19-22).

Our concern in this article is with intermeshing counter-rotating extrusion. The literature on melting behavior in counter-rotating extruders is very limited. It is well established that intermeshing counter-rotating twin screw extruders operate largely with a positive displacement pump mechanism (23-27) and exhibit very narrow residence lime distributions (28). Screw pumping characteristics have also been modeled (29), (30). There were limited observations of melting in these machines, which indicate that it occurs much more rapidly than in intermeshing corotating twin screw extruders (31), (32).

Recently, Wilczynski and White (33) have performed an extensive experimental study on melting in intermeshing counter-rotating twin screw extruders. They have revealed the mechanism of melting in these machines. This involves both mechanical working of pellets in the interscrew region and heal transfer from the barrel to the pellets. It has also been proved that the former mechanism is primary one. On the basis of these observations, models for melting in both those regions have later been proposed (34). And finally, the first composite model of solids conveying, melting and melt flow in a closely intermeshing counter-rotatine twin screw extruder has been developed [35]. Some aspects of that studies were also presented in (36), (37). All the experimental studies carried out by these researchers were performed on polyolefins: polypropylene (PP), low density polyethylene (LDPE) and high density polyethylene (HDPE). And, a "screw pulling-out" technique was used to investigate the polymer behavior in the extruder.

Recently, Wang and Min (38), (39) have investigated melting phenomena of polyvinyl chloride (PVC) compounds in an intermeshing counter-rotating twin screw extruder. They have applied both "screw pulling out" method and ultrasound in-line monitoring method. Ultrasound method provides a solution to overcome the time delay occurred in a "screw pulling out" method which is disadvantageous for studying of PVC melting process due to material degradation caused by extra heat history. It was found that calendering effect between two screws contributes to the melting of PVC particles, and results in dispersion or dissipation melting.

In this article, a further experimental studies of the melting mechanism in a starvefed closely intermeshing counter-rotating twin screw extruder of a modular Leis-tritz design are presented. Various polymeric materials, semicrystalline low density polyethylene (LDPE), amorphous polystyrene (PS) and (LDPE/PS) polyblend were investigated, at various operating conditions. A screw pulling-out method was used to characterize polymer behavior along the screw axis. In particular, the solid conveying, melting positions, the extent of starved character along the screws, and the fully filled regions were observed.

Up to the present, there is no reports on the melting behavior of polystyrene or any polyblend in an intermeshing counter-rotating twin screw extruder. Studying and modeling of the polyblend morphology development in these machines has also never been discussed. This is in evident contrast with corotating machines where a number of computational models have been proposed (40-43), and even with single screw machines (44), (45). The state-of-the-art in this field has recently been presented by Covas group (45).

EXPERIMENTAL

Apparatus

A Leistritz ZSE 27HP modular intermeshing counter-rotating twin screw extruder was used. It has 27 mmdiameter screws with a 22.5 mm distance between center-lines. Various screw elements were arranged on shafts to produce a particular screws configuration (see Fig. 1). These elements are thick flighted elements (FD), thinflighted elements (K.FD) which have large interflighi gaps, and shearing element (ZSS).

Material

The polymers used in the study were (i) a low density polyethylene LDPB (Basel Orlen, Purell 3020D), (ii) a polystyrene PS (BASF, Polystyrol 158K), and a LDPE/PS poly-blend of 85/15 composition (85% of LDPE and 15% of PS). The blend components were premixed mechanically. The low density polyethylene has density 0.927 [g/cm.sup.3], melt flow index MFI - 0.3 g/10min (190 [degrees], 2.16 kg), and a melting point of about 114 [degrees] The polystyrene has density 1.04 [g/cm.sup.3] melt flow index MFI = g/10min (200 [degrees], 5 kg), and a glass transition temperature of about 109 [degrees]. The low density polyethylene pellets were in the form of flat cylinders of about 3 mm diameter, and the polystyrene pellets of about 4 mm.

The viscous (low properties were determined at four different temperatures (140, 160, 180, and 200 C) using an Instron capillary rheometer working on the principle of a constant shear rate. The shear viscosity curves as a functions of shear rate and temperature are shown in Fig. 2 for the low density polyethylene, and in Fig. 3 for the LDPE/PS polyblend.

Procedure

The polymers were fed into the hopper at various rates: Q = 4 kg/h, Q = 8 kg/h and at the flood mode. The screw speed was set at N = 40 rpm and N = 80 rpm For the low density polyethylene and LDPE/PS polyblend, the barrel temperature was maintained at 180 [degrees] except near the hopper where it was set at 150 [degrees], and for the polystyrene the barrel temperature was maintained al 200 C except near the hopper where it was set at 150 [degrees].

A "screw pulling-out" technique [31] was used to investigate a polymer behavior along the screws axes. After the machine had reached a steady state, screws rotation was stopped and the barrel was quickly cooled to the room temperature. Then, when the barrel temperature slightly increased to the polymer melting point, the screws were pulled out from the barrel and the polymer from the screws channels was evaluated. For studying the behavior of the LDPE/PS polyblend in the machine, the polystyrene has additionally been colored.

In particular, the solid conveying, melting positions, the extent of starved character along the screws, and the fully filled regions were observed. To investigate the melting mechanism, polymer samples (carcasses) were stripped off from screws which were removed from the machine.

Photographs were made of the total screws which were pulled out from the extruder and of the polymer samples which were stripped off from the screws.

Pressure and polymer melt temperature in the die, and mechanical torque were also measured during an experiment.

RESULTS

General Observations

Generally, in the region near the hopper, there were solid pellets distributed above the screws that were freely transported along the screws as the screws rotated. However, the pellets were primarily transported in the bottom part of the barrel, and only small fraction of them was transported on the upper part of screws. The pellets were collected at the bottom part of the barrel adjacent to the pushing flights of screws. The pellets appeared to be both heated by the barrel and by being dragged into the calendering gap between the screws. In time, the pellets were converted into a homogeneous melt. Generally, the screws were fully filled for a short distance from the die only and were starved beyond it. There was an exception observed in the region of shearing elements where the screws were also fully filled with the material. In all the experimental runs, the material was completely molten until the shearing elements have been reached.

Polymer Behavior--Effect of Material

Low Density Polyethylene

Figures 4-7 show the photographs of the screws which were removed from the machine after extrusion of low density polyethylene at the various values feed rates and screw speeds.

The influence of flow rate (an increase from Q = 4 kg/h to Q = 8 kg/h) at a constant screw speed on the material distribution on the screws is exampled in Figs. 4 and 6 for a constant screw speed N--40 rpm, as well as in Figs. 5 and 7 for a constant screw speed N = 80 rpm.

We observed in all cases that the filled length increases with increasing the flow rate at a fixed screw speed. This results from that for higher flow rate higher die pressure is needed to squeeze more material through the die which leads to longer filled length for the higher pressure built-up in the extruder. Moreover, for higher flow rates melting seems to start earlier, and the part of the screw length which is needed for complete melting is longer.

The influence of screw speed (an increase from N = 40 rpm to N = 80 rpm) at a constant flow rate on the material distribution on the screws is exampled in Figs. 4 and 5 for a constant flow rate Q = 4 kg/h, as well as in Figs. 6 and 7 for a constant flow rate Q = 8 kg/h.

We observed in all cases that the filled length decreases with increasing the screw speed at a fixed flow rate. This results from that for higher screw speed leakage flow increase and the pressure drops per chamber, which leads to shorter tilled length for the constant total pressure drop in the screw channel that is equal to the die pressure. Moreover, for higher screw speed melting starts later, and the screw length which is needed for complete melting is shorter.

To investigate melting mechanism the polymer samples (carcasses) were stripped off from each screw removed from the machine and examined. In Fig. 8 a sample of low-density polyethylene is shown as an example. We

can see that polymer pellets form some kind of a pellet bed that decreases in length and width along the screws axes. The molten polymer is clearly seen on the surfaces which contacted with the barrel, active flights of the screw, and the calendering gap.

All of these observations are clearly consistent with experiments carried out by White and Wilczynski [33] for other screws configurations, and for the other type of polyethylene. The polyethylene used in that study was a low density polyethylene (Equistar, NA 204-000). Its density was 0.918 [g/cm.sup.3], melt flow index MFI = 5 g/10min, and a melting point of about 110[degrees].

Polystyrene

Polystyrene is an amorphous polymeric material without a melting point, but with some glass transition region and with some glass transition temperature. It has also no heat of fusion. Till now, there are no reports on the melting behavior of polystyrene in an intermeshing counter-rotating twin screw extruder.

Figures 9 and 10 show the photographs of the screws which were removed from the machine after extrusion of polystyrene at the utmost conditions of the experiment, that is at the feed rate Q = 4 kg/h and screw speed N = 80 rpm. and at the feed rate Q = 8 kg/h and screw speed N = 40 rpm.

The influence of flow rate and screw speed on the material distribution on the screws is clearly seen. For smaller flow rate and higher screw speed (see Fig. 9), melting starts later and the screw length that is needed for complete melting is shorter. Moreover, the fully filled region is smaller in this case. For higher flow rate and smaller screw speed (see Fig. 10), melting starts earlier and the screw length, which is needed for complete melting, is longer. Moreover, the fully filled region is longer in this case.

Melting behavior of the polystyrene is clearly seen from the photographs. It follows the mechanism described previously. The polymer pellets form some kind of a pellet bed which decreases in length and width along the screws axes. The granules of polystyrene are mainly collected at the bottom part of the barrel adjacent to the pushing flights of screws. At the beginning of melting region, the granules are distinctly seen, and when the melting progresses they get gradually softened to the homogeneous melt. The material has been completely molten until the shearing elements have been reached. In this region, the screws are also fully filled with the material.

LDPE/PS Polyblend

Till now, the melting behavior of polyblends in an intermeshing counter-rotating twin screw extruder has never been discussed in the literature. But, it is well known that the performance of the final polyblend compounds is strongly dependent on the morphology development in the machine. In turn, this is highly influenced by melting behavior of the material and thermomechanical history experienced by the molten polyblend during an extrusion process.

The LDPE/PS polyblend under study was an immiscible system of semicrystalline LDPE and amorphous PS of 85/15 composition (85% of LDPE and 15% of PS). For better visualization, the polystyrene has been colored.

Figures 11-14 show the photographs of the screws which were removed from the machine after extrusion of LDPE/PS polyblend at the various values feed rates and screw speeds.

The influence of flow rate (an increase from Q = 4 kg/h to Q = 8 kg/h) at a constant screw speed on the material distribution on the screws is exampled in Figs. 11 and 13 for a constant screw speed N = 40 rpm, as well as in Figs. 12 and 14 for a constant screw speed N = 80 rpm.

The inlluence of screw speed (an increase from N = 40 rpm to N = 80 rpm) at a constant flow rate on the material distribution on the screws is exampled in Figs. 11 and 12 for a constant flow rate Q = 4 kg/h, as well as in Figs. 13 and 14 for a constant flow rate Q = 8 kg/h.

We observed in all cases that the filled length increases with the flow rate at a fixed screw speed and decreases with the screw speed at a fixed flow rate. And also, in general, for higher flow rates at a fixed screw speed melting seems to start earlier, and the screw length which is needed for complete melting is longer. But, for higher screw speed at a fixed flow rate melting seems to start later, and the screw length which is needed for complete melting is shorter.

Figure 15 shows the photograph of the screws which were removed from the machine after extrusion of LDPE/ PS poly blend at the flood fed mode (in this case the measured flow rate was equal to Q = 11 kg/h), at the screw speed equal to N = 40 rpm. In this case the material has not been completely molten until the shearing elements have been reached and granules of an unmolten polystyrene could be observed beyond it.

Figures 16 and 17 show the LDPE/PS samples (carcasses) which were stripped off from screws after extrusion at the starve-fed mode and at the flood fed mode. A macroscopic study of melting of LDPE/PS polyblend has shown that minor component (PS) is dispersed in a matrix of major component (LDPE). Polyethylene has lower melting temperature and melts first, and the molten polymer (LDPE) encapsulates the polystyrene pellets (PS). Thus the softening and fusing of the PS granules in the LDPE/PS system is delayed to much greater distance along the screws channels. For the case of blended PS granules, fusion occurs more slowly than with an individual polymer. In general, it is rather seen that the molten polyethylene is mostly adjacent to the pushing flights of screws.

Process Characteristics

Pressure and polymer melt temperature in the die and mechanical torque were measured during an experiment. Some results of the measurements are shown in Figs. 18-23.

An influence of the flow rate on pressure and torque is clearly seen from Figs. 18-21. In all experimental runs, we observed that pressure and torque increase with increasing the flow rate. This result concludes that for higher flow rate higher die pressure is needed to squeeze more material through the die that leads to longer filled length for the higher pressure built-up in the extruder. And. since the screws are more filled with the material the mechanical torque must be obviously higher.

It is interesting that mechanical torque increases by increasing the screw speed for flood-fed extrusion but decreases by increasing the screw speed for starve-fed extrusion (Figs. 22 and 23). This explains that for higher screw speed a leakage flow increases and the pressure drop per chamber, which leads to shorter filled length for the constant total pressure drop in the screw channel that is equal to the die pressure. And, since the screws are less filled with the material the mechanical torque must be obviously smaller.

It is also worth mentioning that a relatively small level of starvation may result in high decrease in mechanical torque and consequently in smaller power consumption. It is seen from Fig. 23 that decreasing the flow rate, at the screw speed equal to N = 80 rpm, from Q=11 kg/h (flood feeding) to Q = 8 kg/h (starve feeding) results in decreasing the mechanical torque from 90 Nm to 50 Nm. It means that decreasing the flow rate of by 40% results in decreasing the mechanical torque of about 80%.

DISCUSSION

Flow Characteristics

There are certain observations of interest relating to material distribution on the screws which relate to pumping characteristics of screws. First, for the material distribution studies, for all the materials the machine was always fully filled near the die and in the region of shearing elements.

We observed for all the polymers that the filled length increases with the flow rate at a fixed screw speed. This explains that for higher flow rate higher die pressure is needed to squeeze more material through the die, which leads to longer filled length for the higher pressure built-up in the extruder.

We also observed that the filled length decreases with the screw speed at a fixed flow rate. When screw speed increases at a constant flow rate, the leakage flow must increase and the pressure drop per chamber also increases. In the result, the fully filled length decreases since the total pressure drop in the screw channel is constant, and it is equal to the die pressure which is still constant. In this analysis, we have omitted the influence of screw speed on the shear rate, as well as on the melt viscosity and melt temperature.

The analysis presented above can be expressed in a simple, well known analytical form [46]. The length of the fully filled region can be calculated by means of the number of fully filled chambers [n.sub.f],. For a melt of constant viscosity, the pressure developed per chamber [[DELTA]P.sub.C] is constant, which implies that the number of fully tilled chambers is equal to

[N.sub.f]=P.sub.die]/[DELTA]P.sub.c], (1)

where the die pressure Pdie is determined by the geometrical constant K (conductivity), flow rate Q, and melt viscosity [eta]

Pdie = Q[eta]/K. (2)

The flow rate Q in the counter-rotating extruder can be expressed as

Q = [[Q.sub.t]-Q.sub.l], (3)

where [Q.sub.t] is the theoretical throughput in the fully filled region, and [Q.sub.l] is the sum of all leakage flows over a cross section of the extruder

The theoretical throughput [Q.sub.1] is equal to the number of C-shaped, fully filled chambers transported per unit of time 1/N, where N is a screw speed, multiplied by the chamber volume [[V.sub.c]

Q.sub.t] = [2iNV.sub.c]. (4)

The total leakage flow, in general, can be expressed as a sum of drag flows and pressure flows in the following way

Ql = [alpha] N + [beta] [[DELTA]P.sub.c]/[eta] (5)

where [alpha] and [beta] are the geometrical constants.

Combining Eqs. 1-5, the number of fully filled chambers [n.sub.f], can be expressed as a function of the screw speed, (low rate, and geometrical constants as follows

N.sub.f] = [[beta]Q/K[(2iV.sub.c] -[alpha]) N - Q].(6)

Since the term [(2/V.sub.c] [alpha]) N is positive and higher than Q, this simple analysis reveals that the number of fully filled chambers [n.sub.f], increases when flow rate Q increases or when.screw speed N decreases. It has been proved by experiment.

Recently, screw pumping characteristics have been modeled by Hong and White [29, 30] for a non-Newtonian and nonisothermal flow in various types of screw elements of an intermeshing counter-rotating twin screw extruder. These are given in terms of the dimensionless groups

Q* = f([DELTA]p*), (7)

where Q* is the dimensionless flow rate and [DELTA]p* is the dimensionless pressure gradient,

Q* = [Q/2WHL.sub.c]N, (8)

[DELTA] p* = [H.sub.n+1] cos [phi] [[DELTA]p/12[eta]([pi]DN).sup.n] L(9)

where Q,. flow rate, tsp, pressure; W, screw channel width; Lc, chamber length; /V, screw speed; //, screw channel depth; D, screw diameter; L, screw length; L, power law index; k, consistency.

From these characteristics, we are able to calculate the length L of the fully filled region provided we know the polymer flow rate Q and the pressure drop of the screw [DELTA]p which is approximately equal to the die pressure [P.sub.die].The die pressure [P.sub.die] can be calculated for a given flow rate from Eq. 2.

Three-dimensional, non-Newtonian FEM simulations have also been performed (47), (49) to refine the melt flow description in the closely intermeshing counter-rotating extruder. The results related to pressure calculations (48), (49) were qualitatively consistent with the computations of Hong and White (29), (30). On the basis of these simulations screw pumping characteristics may also be developed. Simulations performed for different flow rates result in pressure gradient over the screw distance under study and these data may be presented in the dimensionless form given by Eqs. 8 and 9. And, these may be expressed by some approximate equations which may be implemented into the composite model of the process. This technique will be applied to develop a composite model of the counter-rotating twin screw extrusion which will be presented in the next article. The model will be verified with experimental data from these studies.

Melting

When screw speed increases the initiation of melting is delayed since the pellets are transported faster and the pellet bed is formed later. Since the C-chambers are less filled and the length of pellet bed is shorter, the length of the screws which is needed for complete melting is shorter. Also, higher screw speed result in higher viscous dissipation and faster melting.

When feed rate increases the melting seems to start earlier since formation of pellet bed is faster. As the C-chambers are more filled and the length of pellet bed is longer, the length of the screws which is needed for complete melting is longer.

The melting mechanism for pellets in an intermeshing counter-rotating twin screw extruder seems to involve both mechanical working of pellets in the interscrew region and heat transfer from the barrel to the pellets. The former mechanism is the primary one. It seems to be clear that melting starts and takes place mainly in the calender gap and at the barrel surface. The pellets are obviously dragged into the calendering gap where they are melted due to calendering action. Also, melting proceeds from the side of hot barrel, and melt layer is formed between the barrel and the pellet bed which is scrapped off by the screw flights and accumulates at the active flight. When melting proceeds the length of pellet bed decreases which is schematically shown in Fig. 24.

The melting mechanism revealed by White and Wilczynski (33) for polyolefins: polypropylene (PP), low-density polyethylene (LDPE), and high-density polyethylene (HDPE) has been confirmed for amorphous polystyrene (PS) and LDPE/PS polyblend.

CONCLUSIONS

Experimental studies have been carried out for starve-fed closely intermeshing counter-rotating twin screw extrusion of semicrystalline low density polyethylene (LDPE), amorphous polystyrene (PS), and (LDPE/PS) polyblend. The results have been compared with our previous studies on neat polyolefins. The general observations related to the polymer melting behavior and filling of the screw channel have been confirmed. We observed that the filled length increases with the flow rate at a fixed screw speed, and the filled length decreases with the screw speed at a fixed flow rate. We also observed that for higher flow rate, melting seems to start earlier and the screw length that is needed for complete melting is longer. And, for higher screw speed melting starts later and the screw length which is needed for melting is shorter. Melting mechanism and an importance of calendering action have also been confirmed. Starve-fed extrusion shows some advantages over the flood fed extrusion. The pressure built up along the screw is lower than in flood feeding and mechanical torque is smaller. Another benefit may be related to the melting action which is faster than in flood feeding. A relatively small level of starvation may result in high decrease in mechanical torque, and consequently in smaller power consumption. Intermeshing counter-rotating machines in comparison to corotating machines have an advantage in achieving faster melting. This seems due to high stress calendering deformations and flows between the screws.

Modeling of the starve-fed closely intermeshing counter-rotating twin screw extrusion of polyblend materials can be based on the presented above experimental observations. The composite modeling of extrusion processes usually includes melting and melt flow description. Modeling of the extrusion of polyblends additionally requires some evaluation of the morphology development. According to our observations, the LDPE/PS polyblend has been molten in the beginning part of the screws, and then material flowed as a melt in the fully filled region. Melting region of the dispersion of polystyrene (PS) in a polyethylene (LDPE) matrix and melt region of fully molten LDPE/PS polyblend will be discussed separately. As a melting models for two-component systems are currently not available for counter-rotating machines, melting of the major polymer might be taken into account only, at the first approximation. Polyblend melt flow may be considered as a flow of uniform material of respective viscosity or as a flow of two-component material of viscosity resulting from viscosities of components and their concentrations, according, for example, to the two-parameter logarithmic equation for immiscible blends proposed by Utracki [50]. For evaluation of the morphology development, "strong" and "weak" zones should be considered (40), (41). The strong zones are the fully filled regions, under pressure, in which complex flows and high shear rates are observed. The weak zones are the partially filled regions, where the molten material flows at low shear rates under atmospheric pressure.

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Krzysztof Wilczynski, Adrian Lewandowski, Krzysztof J. Wilczynski

Polymer Processing Department, Faculty of Production Engineering, Warsaw University of Technology, 02-524 Warsaw, Narbutta 85, Poland

Correspondence to: Krzysztof Wilczynski; e-mail: k.wilczynski@wip. pw.edu.pl

DOI 10.l002/pen.22103

Published online in Wiley Online Library (wileyonlinelibrary.com).

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Author:Wilczynski, Krzysztof; Lewandowski, Adrian; Wilczynski, Krzysztof J.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EXPO
Date:Feb 1, 2012
Words:5367
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