Using Rubber Extrusion to Drive Quality in EPDM Manufacture

Since it was first introduced to
the industry in the late 1970s,
roof consultants have come to
recognize the advantages of
EPDM single-ply membrane
for low-slope commercial roofs.
The technical term for this highly durable
and weather-resistant material is “ethylene
propylene diene monomer.” However, it is
typically known by its acronym: EPDM.
Johns Manville (JM) has manufactured
roofing materials since 1858, and it began
selling and marketing EPDM in 1982.
CALENDERING: A TRADITIONAL
MANUFACTURING TECHNIQUE
The process of using a “calender” to
flatten rubber into thin sheets has been
around for more than a century. In this
process, heat-softened rubber polymer is
forced through two or more rollers to create
a sheet, and the resulting roof membrane—
the EPDM—is then rolled, cured, packaged,
and transported to project sites for installation
(Figures 2 and 3). While this technique
and the resulting membrane were acceptable
for the construction industry for many
years, over time, limitations inherent to
the calendering process
have become evident.
According to the
online research site
Appropedia, “To begin
the process, the polymer
must go through
blending and fluxing
before it goes through
the calender. Blending
is a process that creates
the desired polymer and
fluxing heats, and works
the blended polymer to
make it a consistency
easier for the calender
to handle. The polymer
is then sent through the
calender, and its thickness
is dependent on
the gap between the last
two rollers.”
Though the calendering equipment has
improved since the introduction of EPDM,
the process does produce inconsistencies
OC T O B E R 2 0 1 6 I N T E R F A C E • 2 1
Figure 1 – EPDM pellets dropping into the extruder.
Figure 2 – Simplified
diagrams showing
traditional calendering
(above) with a rolling
bank and material under
pressure in a roller-die
extruder (below).
in the finished sheets, especially in very
thick and very thin gauges. According to
the aforementioned article, “If the thickness
is below 0.006 inches, then there is a tendency
for pinholes and voids to appear in
the sheets. If the thickness is greater than
about 0.06 inches, though, there is a risk of
air entrapment in the sheet.”
EXTRUSION: MODERNIZING
THE PROCESS
In 2012, JM opened its own manufacturing
facility for EPDM in Milan, Ohio,
which is still the newest wide-sheet facility
in the world. Instead of traditional calendering,
the plant uses state-of-the-art roller die
extrusion equipment.
This technique has been
tested and recalibrated many
times in order to ensure the
highest standards of membrane
quality. There are,
indeed, calender rolls on an
extrusion line, but they are
post-blending. For the purpose
of this paper, “calendering”
is used when describing
how the polymer and formulation
are blended prior to moving
to the sheeting process.
The process starts by creating
the formulation from raw materials.
Once the various ingredients are mixed
together into uncured rubber, it is ready to
be processed into uncured sheets. EPDM
is often made by producing two thinner
sheets that are cured together to make one;
this redundancy creates a higher-quality
product. During this process, two separate
extruders are used
(Figure 4).
After mixing, the
hoppers at the extruders
are loaded with
material that can be
in the form of pellets
or slab. From there,
the material enters
the feed throat of the
extruder and goes
through a scraper
that removes any
material that sticks
to the screw or starts
to reverse. Inside the
screw itself, there
are multiple rows of
pins that are spaced
throughout the
extruder. The pins
are used to combine
and masticate the
material together and
can be configured
to adjust where the
most intense mixing
occurs.
This point of the
process is controlled
using temperature
control units (TCUs)
on the screw in the
extruder, as well on
the walls along the
various zones in the
extruder barrel. The
2 2 • I N T E R F A C E OC T O B E R 2 0 1 6
Figure 3 – Overview of a roller-die
extruder feeding a two-roll calender.
The JM EPDM manufacturing facility opened in May 2012 in Milan, Ohio, employing roughly 100
people. “It was a tremendous investment for us, as EPDM is extremely capital-intensive,” said Director
of Single-Ply Marketing Jennifer Ford-Smith. Ford-Smith noted, “The support from the organization’s
leadership and the commitment from every employee involved was what led the successful
implementation.”
Director of Operations
Brian Olson said, “The
company had tremendous
support of the local Milan
community. We were
able to build on our
established reputation in
the state of Ohio because
of our success with our
other manufacturing locations
in Defiance and
Waterville, Ohio.”
JM Employs Approximately 100 at Its
EPDM Facility in Milan, Ohio.
Milan Plant Is the Newest Wide-Sheet Facility in the World
TCUs are used to maintain a temperature at which the rubber
flows without adding unnecessary heat to the mix. The goal is to
blend the material together while avoiding curing it. In addition,
there is a differential temperature between the wall and the screw
to aid in the material flow process. The material then passes over
a temperature sensor at the end of the extruder that monitors
the rubber temperature as it is leaving the screw and entering
the die. The die is configured with temperature control on the top
and bottom, again to avoid curing and to maintain flow.
At this point in the process, the material is fully blended, and
OC T O B E R 2 0 1 6 I N T E R F A C E • 2 3
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Figure 4 – Extruder in an EPDM roofing plant. Two
extruders are utilized—each making a separate
sheet that will become one homogenous membrane.
Figure 5 – EPDM material
exiting the lip of the die.
the goal is to use the die
to expand the compound
in preparation to make
a sheet. As the material
leaves the die (Figure
5), it goes through a
set of lips that set the
shape of the material
before the final sheeting
(post-blending calender)
step of the process. The
die lip design influences
that particular compound’s
flow characteristics.
The screw speed
can be adjusted in order
to maintain a constant
feed rate—which, in
turn, maintains a more
consistent thickness
profile, eliminating surging
(common with traditional
calendering).
The material leaving
the extruder is kept to a one-to-one ratio
with the adjacent sheeting rolls (aka calender
rolls), which eliminates a rolling material
bank. It is simply pushed out through the
die in the form required. Traditional calendering
requires a large rolling material bank
behind the rollers to keep the sheet from
having voids or run-outs. The downside
of this approach is that it can potentially
create a significant number of pockmarks
or entrapped air. In addition, rolling banks
increase unnecessary head cycles to the
material, resulting in hot spots and/or
scorching. Because the material may not
turnover into the nip of the rolls, inconsistencies
or cured lumps in the calendered
sheet can result. Also, a calender is typically
fed by one or more mills, which are used to
heat up the rubber by working it between
two large rollers. The mills also entrap air in
the rubber as it is processed. Adding extra
temperature cycles and/or overworking the
rubber material can cause premature heat
aging, which potentially reduces the service
life of the finished product. Calendered
material also relies on operator involvement
in blending the rubber, which introduces
2 4 • I N T E R F A C E OC T O B E R 2 0 1 6
Figure 6 – Extruded
membrane prior
to entering the
doubling roll that
will combine two
layers into one
membrane.
Figure 7 –
Extruded
membranes
have
exceptional
surface
finish and
consistency.
other variables as a result of individuals’
varying techniques.
A main advantage of the extrusion process
is that it eliminates air gaps in the
sheet (pockmarks) and minimizes precured
lumps. It also allows for better temperature
control of the uncured EPDM. The extruder
has multiple areas of temperature control:
screw, feed throat area, end of screw area,
and die (top and bottom). All of these areas
can be heated or cooled as necessary to
minimize scorching of the material, thereby
reducing cured lumps. That is not easily
done with traditional warm-up mills.
The calender rolls are also fitted with
temperature control to cool the rubber
quickly and to minimize the rubber sticking
to the rollers. The material is trimmed
to width after it goes through the calender
rolls and is processed through a doubling
roll that combines the two half-mill sheets
(Figure 6). This roller is kept at a fixed temperature
and pressure to ensure consistent
lamination. Throughout the remainder of
the process, the rubber is kept in nearly
constant contact with a surface, with the
goal of minimizing stretching and deformation.
The sheet is monitored via thickness
gauges. These gauges blow air on the sheet
and monitor the backpressure and distance
between the axes of the rollers to calculate
the thickness. The sheet is also monitored
with a grayscale visual defect system. The
system works by looking at the sheet and
taking a reference image to compare against
every five minutes. If a difference is noticed
on the sheet, it will alert the operator to the
issue. As a result, extrusion delivers tighter
uniformity of thickness across the sheet
to provide a subtle but critical difference
from calendering. This difference leads to
enhanced seam performance and increased
longevity of the membrane.
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Figure 8 – Traditional calendered EPDM (A and B) alongside an extruded sheet (C).
OC T O B E R 2 0 1 6 I N T E R F A C E • 2 5
LESS LABOR WITH HIGHER QUALITY
Since the finished material is more uniform
and has little or no surface porosity or
air pockets within the EPDM, less backing
material or parting agent is needed to keep
the sheets on the rolls from sticking together
during the vulcanizing process. This means
there is less employee time required to prepare
the membrane for installation, and the
contractor has a much better chance of getting
a stronger seam, removing some margin
of error. Less time spent preparing the membrane
means potentially lower labor costs.
During a time when the building
trades—especially roofing—are experiencing
a critical shortage of labor, having a membrane
that is easier and quicker to install
and allows a crew to do more work in less
time is of paramount importance to a contractor’s
bottom line.
GREATER CONSISTENCY FOR
LONGER LIFE
Along with seam performance, a loss in
mil thickness over time, which leads to thin
spots or holes, is the most common failure
mode for an EPDM membrane. As previously
noted, extruded sheets exhibit a more
consistent, smoother surface—free of pockmarks,
voids, or entrapped air, and yielding
the full thickness consistently across the
sheet (Figure 7).
Once exposed to the tough rooftop
stresses of an ever-changing environment,
any voids or pockmarks where there is a
loss in thickness will be the most likely
place for a failure to begin. Ultimately, this
will result in a leak. By ensuring a consistent
thickness across the sheet, an extruded
membrane eliminates these potential problems,
helping to extend the life of the roof
system.
Furthermore, the consistent thickness
results in a smoother sheet, leading to
better lay-flat qualities. This enables easier
installation on adhered systems, and
appearance is enhanced due to lack of wrinkles
or “waviness.” In comparing extruded
sheets (C in Figure 8) to more traditional
calendered products, the Milan-produced
membrane was found to have 40% less
thickness variation across the width.
With building owners’ maintenance
budgets constantly under pressure, using
a membrane with fewer surface defects on
the day of installation can lead to a longer
service life and lower lifecycle costs.
2 6 • I N T E R F A C E OC T O B E R 2 0 1 6
Rick Gustin became
the EPDM product
manager for Johns
Manville in 2013
and played a pivotal
role in developing
their pretaped
FIT product. Rick
started his career
in the contractor
community before
coming to Johns
Manville in 1998,
where he held various roles, including Six
Sigma Black Belt and application engineer
before assuming responsibility as manager
of Guarantee Services. Rick graduated from
Rensselaer Polytechnic Institute in 1996 and
holds a degree in mechanical engineering.
Rick Gustin
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