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Screw design for reclaiming barrier coextruded sheet scrap.

Screw Design for Reclaiming Barrier Coextruded Sheet Scrap

Thermoformed products present the inherent problem of generating high percentages of scrap. Only a few years ago it was common practice to assume that the process would produce 30% to 40% scrap to be recycled. That amount has increased to 50%, with some producers successfully recycling 60%. Recycling scrap into the product is the subject of this article. But there are many barrier materials and combinations of materials, so this discussion is limited to recycling polyvinylidene chloride (PVdC) and ethylene-vinyl alcohol copolymer (EVOH) resins, focusing on the role of screw design.


Several problems surface when these high levels of regrind are introduced into the structure. One is the reduction of percentage left for cap layers. For example, with 60% regrind, 10% barrier resin, and 12% in glue layers, only 18% is left to divide between the cap layers (Fig. 1, top). This situation may require adding pigments to the cap layer to hide undesirable visual effects, such as discoloration of the regrind layer when several-pass regrind is used or any dirt that may have been picked up during the grinding operation.

Another problem arises as the boundary layer between the regrind and cap layers moves closer to the high shear area near the channel wall. It is generally not necessary to use a glue layer at this interface, but in the higher shear area, the flow instability is difficult to control. The barrier resins are shear sensitive and gain excess heat when exposed to higher shear. In the case of reground PVdC, the result is more discoloration as the number of passes increases - due to the higher heat history.

If the regrind is put into one layer, forming a nonsysmmetrical structure (Fig. 1, center), or if a split barrier is used with the regrind in the center (Fig. 1, bottom), another problem arises. The virgin barrier layer is forced into the high shear area, and its heat history is increased, with similar potential for degradation and discoloration.

More than flow instability, problems associated with PVdC and polypropylene (PP) stem from mixing; in order to overcome these problems, the materials must be mixed without overheating the PVdC. While EVOH and PP are chemically incompatible, the EVOH molecules have an affinity for any unreacted catalyst in the PP. The materials combine to form a gel that is extremely difficult to disperse. When the proper amount of high shear is applied to disperse the gel, excess heat usually results. These gels occur more often when the EVOH is not thoroughly distributed throughout the PP in the regrind extruder. They also occur more often when the total amount of EVOH approaches 15%. The author has seen test results showing that 13% EVOH can cause gel formation. Some of the problem is located at the boundary of the regrind layer and the PP cap. Most solutions deal with reducing the exposure of the two materials to each other.


Many measures have been taken in the effort to solve regrind problems, with varying degrees of success. The most common approach is to keep track of the number of times the regrind is passed through the extruder. For PVdC, it is important to reduce the number of passes so that discoloration is minimized. For EVOH, more passes result in an increased percentage of EVOH in the mixture. If the structure starts with 10%, it can easily reach 15%.

Many methods reduce the percentage of regrind by blending virgin PP into the mixture. Some processors use ratio loaders to blend virgin and regrind materials in the regrind extruder. Ratios as high as 50/50 are common. Other processors bleed the regrind off into a less critical product, such as a monolayer package with no barrier requirement. The material is blended with virgin, and the regrind from that product is blended into the multilayer structure, resulting in a lower EVOH percentage.

Several methods are used in an attempt to reduce the incompatibility of EVOH and PP. Compatibilizers are commonly used to reduce the attraction of EVOH molecules to the unreacted catalyst. Some processors separately repelletize the regrind prior to recycling into the structure. If a good mixing screw is used in the repelletizing step, the improved distribution of EVOH raises the general melt quality of the recycled structure. There is some thought that the improved mixing aids in preventing gel formation, although this remains to be proven.

Melt index (MI) blending is another method of reducing incompatibility. This procedure is based on observations indicating that the MI of PP increases and the MI of EVOH decreases with successive extruder passes. By selectively blending two grades of EVOH and matching a grade of PP, as the two polymers change, the processor can cause their respective MIs to approach each other by the second or third extruder pass. For example, a 1.6-MI EVOH can be blended with a 5.5-MI EVOH to give a composite with a 3.5 MI to be used with a 2.0-MI PP. After perhaps two extruder passes, the two polymers will approach an MI of 2.5 from opposite directions. This method is less precise than it appears because it is difficult to know how much of each polymer is present in successive passes.

Another method reduces the affinity of EVOH for the PP catalyst by limiting the amount of time they are in contact with each other. Screw designs should permit as short a residence time as possible. However, it is also important to thoroughly mix the regrind, and at current throughput rates, a long screw is required. Feedblock and die makers minimize exposure time by designing their equipment so that PP cap layers and regrind layers come into contact as far downstream as practicable.

Screw Design - A Major

Part of the Solution

Clearly, the role of the screw is to melt and mix these materials with the lowest possible increase in temperature. Several designs have been evaluated. A Union Carbide or Maddock mixing section (Fig. 2) was tried, but as expected, too much heat was created. The Advancing Spiral Barrier (Fig. 2) was also tried. This screw is similar to the Maillefer design, the forerunner of all barrier screws. It worked fairly well, but was very sensitive to the clearance over the barrier flight. When the melt temperature approached 400[degrees]F, it did not mix well. When a tighter barrier clearance was used, the mixing improved and the melting was complete - but melt temperature increased to approximately 460[degrees]F. This was too hot for PVdC, and some burning occurred.

A Max Melt (Fig. 2), a barrier type screw similar to the Barr screw, was the next design used in this study. It did an adequate job of melting at a low temperature, but it has some of the same characteristics as the Maillefer screw. When the barrier was tightly spaced for good mixing, the temperature rose. This design could be successful if a distributive mixing device were added to it.

These three screw designs were tested on both PVdC and EVOH regrind. The results and comparisons were basically the same - even though EVOH can tolerate higher temperatures than PVdC, it has a stronger mixing requirement.

The screw that did the best all-around job of mixing and maintaining low melt temperature was the Twin Channel Wave, offered as the DOUBLE WAVE by HPM, shown in Fig. 3. Its method of melting is the reason for its success. The material is passed through a series of high and low shear areas. Intermittent periods of relaxation of the shear rate of the polymer has been shown to be the ideal way to melt polyolefin materials. Because the melt stream is divided and rejoined several times in the twin channels, distributive mixing is provided.

With the addition of talc fillers in the cap layers, the distributive mixing job of the screw has become more demanding, and an add-on mixer is needed. A popular device is the Sexton, Dulmadge type head (Fig. 4) with large, smooth radii in the grooves for self cleaning. This device acts in a similar manner to static mixers on the market in that it provides several separations and rejoinings of the melt stream. Since it is attached to the end of the screw and turns with it, the head assists in generating pumping pressure.

PHOTO : FIGURE 1. Multilayer constructions incorporating high levels of regrind.

PHOTO : FIGURE 2. Three of the screw designs evaluated

PHOTO : FIGURE 3. The Twin Channel Wave screw design was the most successful because the material passes through areas of high and low shear.

PHOTO : FIGURE 4. An add-on mixer, such as this Sexton, Dulmadge head, is needed for talc-filled cap layers.
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Author:Calland, William N.
Publication:Plastics Engineering
Date:Apr 1, 1990
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