Page 40 - Plastics News February 2020

FeAtures

What You Need to Know to Get into Coextrusion

Jim Frankland

Key considerations for coextrusion range from determining the right extruder sizes to tooling, matching ma-
terial viscosities for when multiple plastics are combined, the result can yield properties distinct from those
of a single material

oextrusion requires far more polymer and extrusion layer. I suggest you evaluate and select from experienced
Cknowledge than monolayer extrusion, so operating coextrusion machine builders so they can provide you
personnel need to be properly prepared. And there’s lots all the technical information necessary for the polymer
to consider. First, it’s necessary to select polymers that selection, structure, and equipment design for a proposed
meet a host of performance requirements in the finished coextrusion line.
extrusion and will also bond to form a strong structure. However, the actual daily operation of the line depends
Bonding is largely dependent on the respective surface heavily on the melt viscosity exiting the extruders, which
energies of the polymers. If the polymers selected for the
depends on the melt temperature and shear rates in the
downstream tooling as the various layers are combined.
Mismatching viscosities can result in numerous quality
issues with a new coextruded product—even if everything
was designed properly based on a similar coextruded
product. This part of the process depends on the on-site
operating personnel and their understanding of polymer
viscosity with varying temperatures and shear rates.
Polymer shear-rate/viscosity curves of different resins in a
coex structure can actually cross over each other, resulting
in a very narrow operating window, so the slopes of the
curves or their power-law indexes should be considered
when selecting layers. The glass-transition temperatures
of the various polymers also need to be considered, as
trying to match viscosities to avoid interfacial instabilities
performance requirements of the finished coextrusion do and layer distortion with a polymer that melts at 500 F
not provide adequate bonding strength, then it will be layered with one that melts at 265 F is very difficult.
necessary to use tie layers, which will require additional Also important is sizing extruders relative to their
extruders. As we know Coextrusion is the process of requirement in the structure, so they can develop similar
pressing two or more materials through the same die melt temperatures without the need for special screws
to produce a single piece. When multiple plastics are for every structure. I recently ran thin HDPE cast film
combined, the result can yield properties distinct from on a line having three coextruders The three extruders
those of a single material.Among the many performance (sized 3.5-in., 4.5-in. and 6 in.)generated greatly varying
factors to consider—determined by the final use of the outputs—sometimes double. Because of this it was
finished product—are flexibility, chemical resistance, impossible to match the viscosities of the layers from all
permeability and ideal layer placement, to name a three extruders. And just about every kind of interfacial
few. For sheet and cast film applications in particular, instability was present except at very low outputs, where
processors must choose between a multi-manifold die the melt temperatures were relatively similar. Extruder
or a coextrusion feedblock. Coex feedblocks generally output stability is naturally essential, both with respect to
provide more flexibility as well as lower initial cost and melt temperature and output, although melt pumps can
less maintenance. Both have valves of various designs be used to eliminate output variation bypassing the tooling
to adjust the layer flow rates, but melt pumps are valves, which can have more effect on melt temperature.
typically used to precisely control the output of each
For a given screw design, melt temperature is controlled

Plastics News Februar y 2020 40