«Viscoelastic flow effects in multilayer polymer coextrusion Dooley, J. DOI: 10.6100/IR555718 Published: 01/01/2002 Document Version Publisher’s ...»
As was discussed earlier, when resins with significantly different viscosities are coextruded into a multilayer structure, a phenomenon known as “viscous encapsulation” can take place. This phenomenon is illustrated in Figure 3-1 that shows a two-layer structure flowing down a tube. In this figure, the viscosity of layer “A” is less than layer “B”. This figure shows how the less viscous layer (A) tends to move to the highest area of stress (near the tube walls) and so encapsulate the layer with the lower viscosity (B).
In this part of the study, two-layer coextruded structures were made using different polystyrene resins in each layer with different colored pigments added to each layer to determine the location of the interface. A series of experiments were conducted that showed that the addition of the pigments at the loadings used in these experiments did not affect the flow properties of the resins. These two-layer structures were extruded through a circular channel and the encapsulation velocity was measured experimentally.
The resins chosen for this study were four high impact polystyrene resins (STYRON* 482, 421, 484, and 495), all manufactured by The Dow Chemical Company. For simplicity, these resins will hereafter be referred to as Polystyrene A, B, C, and D, respectively. The rheological properties of these resins at 204°C are shown in Figure 3-2. This figure shows that these resins have significantly different viscosities over the shear rate range tested. These particular polystyrene resins were chosen for this part of the study in order to produce as large a viscosity ratio between the layers in the coextruded structures as possible.
Viscosity (Poise) The primary coextrusion line used in this study for the two-layer experiments consisted of a 31.75 mm diameter, 24:1 length-to-diameter ratio (L/D) single screw extruder for the substrate resin and a 19.05 mm diameter, 24:1 L/D single screw extruder for the cap resin. These extruders were attached to a feedblock that was designed to produce a two-layer structure consisting of a 20% cap layer and 80% substrate layer. Attached to the exit of the feedblock was a die containing a circular cross-section. A schematic diagram of the primary coextrusion line used in these experiments is shown in Figure 3-3.
The circular die channel was designed to have a cross-sectional area of approximately
0.91 cm2 so as to be comparable to other channels that will be discussed later. The axial length of the die channel was 60.96 cm. This die channel was fabricated in two halves and bolted together so that it could easily be split apart for removal of the polymer sample.
For a typical experiment, the coextrusion line was run for a minimum of thirty minutes to ensure that steady-state conditions had been reached. The normal extrusion rate was approximately 3.4 kg/hr for the polystyrene resins that would give a wall shear rate in the range of 1 to 10 reciprocal seconds. The coextruded structures were extruded at approximately 204oC.
Variable-depth thermocouples were placed in the melt streams from each extruder just prior to the entry into the feedblock to ensure that the temperatures of the materials were the same prior to being joined together. When steady state was reached, the extruders were stopped simultaneously and the coextruded material was cooled while still in the die channel. After it had cooled to room temperature, the frozen polymer "heel" was removed from the die and examined. This procedure allowed the major deformations of the interface to be analyzed.
Of the four polystyrene resins studied, Polystyrene D was the most viscous (see Figure 3Since Polystyrene D resin was the most viscous, it was used as the substrate layer in a series of experiments in which each of the four polystyrene resins (A, B, C, and D) was coextruded in a two-layer structure as a 20% cap layer over a substrate of Polystyrene D. At a shear rate of 10 reciprocal seconds, the viscosity ratios of the structures were 2.5, 2, 1.4, and 1 for cap layers of polystyrene resins A, B, C, and D, respectively. These viscosity ratios are defined as the viscosity of the substrate divided by the viscosity of the cap layer. In these experiments, the substrate layer of Polystyrene D was always pigmented black while the cap layer containing the different polystyrene resins was always pigmented white.
Figure 3-4 shows the solidified samples removed from the circular die channel for the experiments described above. This figure shows all of the samples viewed from the bottom of the channel so that it is easier to observe the white cap layer flowing around and encapsulating the black substrate layer. The samples are labeled as the cap layer over the substrate layer, i.e., A/D represents a cap layer of Polystyrene A resin coextruded over a substrate layer of Polystyrene D resin.
The first observation to note from Figure 3-4 is that the D/D sample shows no sign of viscous encapsulation. This can be determined by the fact that only the black substrate layer is visible down the entire length of the die channel and so no white cap layer has moved to encapsulate the substrate. This is to be expected since each layer is composed of the same resin processed at the same temperature.
The second important observation from Figure 3-4 is that the higher the viscosity difference between the cap layer and substrate layer, the faster the viscous encapsulation occurs.
The samples listed in order of the speed of encapsulation from fastest to slowest are A/D, B/D, C/D, and D/D which have viscosity ratios of 2.5, 2, 1.4, and 1, respectively. Note that complete encapsulation of the A/D sample takes place in approximately 13 cm (~ 5 inches) while the C/D sample is not completely encapsulated even after 20 cm (~ 8 inches).
Figure 3-4. Samples removed from the circular die channel. Viscosity ratio decreases from 2.5 to 1 from top to bottom Figure 3-5 shows the A/D sample after it has been cut into 2.54 cm sections. This figure shows the sample cross sections beginning near the channel entry and then at distances of approximately 2.5, 5.1, 7.6, 10.2, and 15.2 cm from the channel entrance. Note how the white cap layer is flowing around and encapsulating the black substrate layer. This figure shows that the white cap layer completely encapsulates the black substrate somewhere between 10 and 15 cm downstream from the entry.
Figure 3-5. Cross-sectional images of the A/D sample beginning near the channel entry and then at distances of approximately 2.5, 5.1, 7.6, 10.2, and 15.2 cm from the channel entrance
3.3 Determination of encapsulation velocity The movement of the cap layer encapsulating the substrate layer can be measured in order to calculate an encapsulation velocity. The technique used to do this is illustrated in Figure 3-6.
The image labeled (a) represents a cross-sectional cut near the entry of the channel while (b) is farther downstream. In the figure, the angle that is defined by the ends of the cap layer is indicated by θ1 and θ2. The angles θ1 and θ2 define arcs on the channel wall, which have a set length. Measurement of these arc lengths along the distance downstream at which the samples were cut can be used to calculate the encapsulation velocity.
Figure 3-7 shows a plot of the arc length as a function of the channel distance for the various coextruded samples produced. This figure shows how the arc length gets smaller as the structure flows down the channel indicating the encapsulation of the substrate by the cap layer. Also notice how the order in which the structures reach zero arc distance (or full encapsulation) follows the trend of viscosity ratio. The structure with the largest viscosity ratio, A/D, is fully encapsulated at the shortest distance while the structure with the smallest viscosity ratio, C/D, is fully encapsulated at the longest distance.
The information shown in Figure 3-7 can be used to calculate an encapsulation velocity. The encapsulation velocity is calculated by measuring the distance the cap layer moves divided by the time it takes for that movement. The cap layer movement distance can be taken directly from Figure 3-7. The time related to that distance is calculated based on the average downstream velocity of the cap layer. This can be approximated from the channel flow rate, the resin viscosity, and the channel geometry. Using these data, Figure 3-8 was developed.
Figure 3-8 shows the encapsulation velocity as a function of the distance down the channel.
The curves in this figure were generated based on a least squares method to produce the best linear fit to the data. Note that the encapsulation velocities also follow the same trend of faster encapsulation with a larger viscosity ratio. For example, the A/D sample that has the largest viscosity ratio of 2.5 also has the highest initial encapsulation velocity and the shortest distance to full encapsulation.
This plot also shows that the encapsulation velocity is the highest near the entry of the channel and it decreases as the structure flows down the channel. This happens because the driving force for encapsulation is reduced as the structure flows down the channel since the less viscous material is occupying a larger percentage of the high stress region near the channel wall.
The other important point to note from Figure 3-8 is the magnitude of the encapsulation velocities near the entry of the channel, which is on the order of 0.15 cm/s. This compares to the average downstream velocity of the coextruded structure of 1 cm/s. This implies that the magnitude of the highest encapsulation velocity in this study is approximately 15% of the average downstream velocity.
Figure 3-8. Encapsulation velocity as a function of downstream distance
3.4 Conclusions This chapter has shown that a difference in viscosity between the layers in a coextruded structure can lead to a phenomenon known as viscous encapsulation. This is a very important effect that is encountered in many industrial processes. Designing coextruded structures with similar viscosities in the layers is a well understood technique to control layer deformation. The next chapter will deal with coextruded structures in which the layers have similar viscosities but layer deformation is still observed.
Chapter 4 Layer uniformity in coextrusion for structures with layers with similar viscosities: elastic layer rearrangement 4 Layer uniformity in coextrusion for structures with layers with similar viscosities
4.1 Introduction Viscous encapsulation occurs when layers of different viscosities are coextruded to form a multilayered structure. Designing the structure with similar viscosities in the layers, or “viscosity matching”, can minimize this effect. However, even with well matched viscosities, coextruded structures have been processed in which layer deformation still occurs. This chapter will discuss layer uniformity in coextruded structures with similar viscosities.
The resins chosen for this part of the study were a high impact polystyrene resin (STYRON 484), a low-density polyethylene resin (LDPE 641I), and a polycarbonate resin (CALIBRE* 300-22) all manufactured by The Dow Chemical Company. The rheological properties of these resins are shown in Figures 4-1 and 4-2. These figures show that these resins have significantly different viscous and elastic flow properties. Figure 4-1 shows that the polystyrene and polyethylene resins are both shear thinning while the polycarbonate resin is more Newtonian in viscosity. Figure 4-2 shows the differences in elasticity between the resins based on their storage moduli. The polystyrene resin appears to be the most elastic (based on storage modulus values) followed by the polyethylene resin and then the polycarbonate resin.
Figure 4-2. Storage modulus comparison of polystyrene, polyethylene, and polycarbonate resins Two-layer coextruded structures were made using the same polymer in each layer with different colored pigments added to each layer to determine the location of the interface. A series of experiments were conducted that showed that the addition of the pigments at the loadings used in these experiments did not affect the flow properties of the resins.
4.3 Experimental set-up
The primary coextrusion line used in this study for the two-layer experiments again consisted of a 31.75 mm diameter, 24:1 length-to-diameter ratio (L/D) single screw extruder for the substrate resin and a 19.05 mm diameter, 24:1 L/D single screw extruder for the cap resin.
These extruders were attached to a feedblock that was designed to produce a two-layer structure consisting of a 20% cap layer and 80% substrate layer. Attached to the exit of the feedblock was a die containing one of the different channel geometries studied. A schematic diagram of the primary coextrusion line used in these experiments was shown previously in Figure 3-3.
Experiments were run on several different lines with different extruder sizes and feedblock designs, all producing similar results.
Five different die channel geometries were used in this study; a square channel, a rectangular channel, a teardrop shaped channel, a circular channel, and a rectangular manifold channel. The square, teardrop, circular, and rectangular geometries were chosen since they are common shapes used in the design of feedblocks, dies, and transfer lines while the rectangular manifold channel is meant to simulate half of a coat-hanger style die. The square channel had sides that were 0.95 cm long. The teardrop and circular channels were designed to have approximately the same cross-sectional area (0.91 cm2) as the square channel. The rectangular channel had a 4:1 width to height aspect ratio with a width of 2.54 cm and height of 0.635 cm.
The rectangular manifold channel was designed based on principles used in the design of coathanger style die manifolds to produce uniform output across the die exit (65). The rectangular distribution manifold maintains a constant 4:1 aspect ratio down its entire length and starts with dimensions identical to those of the rectangular channel. This allows direct comparisons to be made between the two channels. The axial length of each of the die channels was 60.96 cm.
These die channels were fabricated in two halves and bolted together so that they could easily be split apart for removal of the polymer sample.