Methods of Coextruding Films

There are two fundamentally different methods for coextruding films, namely, the coextrusion blown-film and coextrusion flat-film processes. The capital and operating costs for a coextrusion blown film process and a flat-die, chill-roll casting process depend on the product mix and utilization. Equipment suppliers provide comparative economic evaluations for specific products. The extruder used before the die and the take-off equipment used afterwards are standard equipment applied to the single-layer film manufacture of both blown and flat films.

The coextrusion blown-film process is based on the principle of two or more

polymers that have been plasticized and homogenized in separated extruders, fed into a coextrusion blown-film die to form a tubular structure, blown into a bubble, cooled and passed through the collapse frame, and collected by a winder. Figure 1-2 depicts the setup of a coextrusion blown-film process. The design of coextrusion blown-film die used most commonly today is that of a multimanifold spiral mandrel (Figure 1-3).

This die consists of several concentric manifolds, one located within another. The manifolds are supported and secured through the base of the die. Each manifold consists of a flow channel that spirals around the mandrel, which allows the melt polymer to flow down the channel or leak across a land area to the next channel. This flow pattern smoothes out the flow of the polymer and minimizes any weld lines in the final film. Another type of coextrusion blown film die is the stackable plate die (Figure 1-4). In this type of die, each layer is spread uniformly and formed into a tube in a single plate. The plates are then stacked on top of one another and the layers are added sequentially. This type of die is becoming popular for specific applications because the number of layers can be adjusted by simply changing the number of plates in the die. The major disadvantage for this type of die is that a large separating force exists between the plates and, thus, many die bolts are required to hold the plates together. This situation means that the plates must have rather large diameters to maintain structural integrity and this requirement can produce longer flow paths and temperature differentials that can be detrimental to thermally sensitive polymers.

The coextrusion flat-film process is based on the principle of shaping two or more polymers, which have been plasticized and homogenized in separated extruders, into a planar structure in a coextrusion flat die, cooling and stabilizing this structure by means of roll contact, and then winding it up to a trimmed working width. Figure 1-5 displays the setup of a coextrusion flat-film process. Two basic types of die used in flat-die coextrusion system are the multimanifold and feedblock/single-manifold dies.

1-6

A hybrid-type die combines feedblocks with a multimanifold die.

In a multimanifold die (Figure 1-6), each layer incorporates individual manifolds that extend to the full width of the die. Each manifold is designed to distribute its polymer layer uniformly before combining it with the other layers. The major advantage of a multimanifold die is the ability to coextrude polymers that have very different viscosities because each layer is spread independently prior to being combined. A significant disadvantage of wide multimanifold dies is the difficulty in coextruding very thin layers, such as thin cap or thin tie layers. When using a wide die, it is difficult to obtain uniformity when the extrusion rate per width is very low;

additionally, it is expensive and requires the attention of skilled operators. The feedblock/single-manifold dies of a flat-die coextrusion system use a feedblock before a conventional single manifold (Figure 1-7). A layered melt stream, which is prearranged ahead of the die inlet by the feedblock, is extended to the width of the die as it is reduced in thickness (Figure 1-8). Polymer melts from each extruder can be subdivided into as many layers as desired in the final product. One limitation of the use of feedblocks is that polymer viscosities must be matched fairly closely because the combined melt stream must spread uniformly within the die. A severe viscosity mismatch results in lay nonuniformity; the lower viscosity material tends to flow to the die edges. A crude rule of thumb is that polymer viscosities must be matched to within a factor of three or four, which is a reasonably broad range for many commercially important coextrusion processes. Often, to avoid flow instabilities, polymers are intentionally selected that have mismatched viscosities. The layer nonuniformity expected to arise from the mismatch is compensated by varying the melt temperature, to eliminate the viscosity mismatch, or by using a feedport that has a shaped geometry. Combinations of feedblocks and a multimanifold die are also used commercially. The multimanifold die can incorporate the same design principles as

the feedblock: i.e., vanes separating individual manifolds within the die. In a sense, the multimanifold die is a wide feedblock. A feedblock may be attached to one or more manifold inlets, as indicated in Figure 1-9. With this system, it is possible to coextrude polymers that have widely different viscosities and processing temperatures.