TECHNICAL
BULLETIN
A Brief Discussion of Gel
Background
Over the past decade the use of gel
materials to seal HEPA filters to housings,
holding frames and grids has gained in
popularity. Gel materials are soft, resulting
in filter installations that are easier to seal
and require lower clamping pressure than
typical foam gasket systems. Silicone gel
originally was the only suitable material
available for this application. Since then
polyurethane gel systems have gained
in popularity because of their lower
outgassing characteristics and lower cost.
Thermoplastic hydrocarbon based gels
are also available but have not yet gained
acceptance in the HEPA filter marketplace.
By nature, gel systems are lightly crosslinked thermoset elastomers. The systems
are highly plasticized by materials that
may or may not be chemically bonded
to the polymer backbone. The delicate
nature of these lightly cross-linked
systems is the primary reason that gel
materials are unforgiving in processing
and final characteristics may vary.
Silicone Gel vs. Polyurethane Gel
The two systems perform the same task
with respect to HEPA filter sealing. They
provide a compliant, air tight seal between
the filter frame and the air delivery
system. The physical properties in terms
of softness, surface tack, and elasticity
are generally similar. Beyond that the
similarities end.
Silicone gels are well known, produced
by three major corporations, and find their
way into a variety of electrical, electronic
(encapsulation), vibration dampening
and sealing applications. HEPA filter
sealing is just one small market for these
gels. Drawbacks of silicone gel include
outgassing (detrimental to microelectronic,
disk drive and optical applications), high
cost, and possible cure inhibition when
applied directly to incompatible substrates.
Advantages of silicone include, higher use
temperature and generally good chemical
resistance.
Polyurethane gels have become well
known and understood over the past
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15 years. The products suitable for the
HEPA filtration market are made primarily
by one or two small domestic (USA)
manufacturers. Other manufacturers of
polyurethane gel are regionally known
world-wide and the chemistry and
techniques are accessible to many in
the polyurethane industry. Polyurethane
gels have gained acceptance into other
markets including footwear, toys and
novelty items, however the gel systems
used for these items are generally not
considered suitable for cleanroom use.
Drawbacks of polyurethane gel include
high moisture sensitivity of the unreacted
components resulting in a limited raw
material shelf-life, and incompatibility
with certain substrates that may result
in cure inhibition. The advantages
of polyurethane gel include, lower
outgassing, less ability to adsorb onto
surfaces and remain there causing
changes to surface energy and wet-ability,
lower cost, low toxicity, and fairly good
chemical resistance.
The bottom line is that both Silicone
and Polyurethane gels will perform the
basic job of sealing HEPA filters to the
air delivery system. The specific choice
of gel and the filter industry move toward
polyurethane gel is dictated by the needs
of the end user.
Gel Concerns
Over the years a number of concerns
have arisen involving both Silicone
and Polyurethane gel. Only concerns
of the end user, not those of the filter
manufacturer will be mentioned below.
Concerns of gel in finished product
include:
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Exudation of liquid components
(blooming, wet surface, gel dripping)
Reversion (from gel back to liquid
state)
UV degradation
Chemical degradation, Chemical
compatibility
Migration of liquid test aerosols into
and through the gel Migration of unreacted components out of the gel
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Tearing, cutting
Clumping, clinging Surface tack
variability Skin formation
Variability of final gel properties
Outgassing
Insufficient Penetration of Knife Edge
into Gel
Color shift
These concerns may be assigned to five
basic categories:
1.
Normal gel behavior, normal
characteristic.
2.
Improper design of gel or choice of
gel for application.
3.
Improper handling and processing of
gel by filter manufacturer.
4.
Improper handling, use, or design by
filter installer or end user.
5.
Invalid or inconsequential concern.
Normal Gel Behavior, Normal
Characteristics
This category exists because many
users of gel are simply not familiar with
the normal properties and characteristics
of gel systems. Most people lack the
experience to recognize the difference
between normal gel and a gel system
that is failing. In some cases this has
led to misdiagnosis of root causes for
problems encountered in the clean- room
and ineffective, misguided and wasteful
corrective action.
Improper Design of Gel, or Improper
Choice of Gel for Application
This category exists because many
users of gel are simply not familiar with
the gel systems available, have little
understanding of end user needs and
application, have little experience in using
gels, or are driven by other factors (cost,
speed of cure etc.) above all other factors.
Resulting issues of concern include:
Exudation, Reversion, UV degradation,
chemical degradation, poor chemical
compatibility, tearing, skin formation,
unacceptable outgassing. Numerous
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TECHNICAL BULLETIN
A Brief Discussion of Gel
examples over recent years can be cited
where an improper gel was chosen for a
particular application. When the wrong
gel is used for HEPA filter applications the
problems that result in the industry are
usually widespread and may affect many
end users.
Improper Handling and Processing of
Gel by the Filter Manufacturer
This category exists because many
users of gel simply do not have the
expertise, equipment or internal controls
and maintenance to avoid processing
problems. As mentioned earlier, gel
chemistry is fragile and unforgiving. While
some variability in the physical properties
of the end product should be expected
and tolerated, gel systems should not fail
due to processing problems. Problems
related to manufacturing processing
problems include exudation, unacceptably
large variability of final gel properties, cure
inhibition (wet spots), and gel dripping.
These are usually related to using gel
that is not stored properly, is past its
shelf life, is not metered and/or mixed
properly, or is applied to an incompatible
or contaminated substrate. Numerous
examples over recent years can also
be cited where the filter manufacturer
experienced problems due to process
control issues. The subsequent problems
that result in the industry are usually
isolated to a single or small batch of filters,
but related problems may also be found
sporadically among filters over many
batches and over a widespread number of
customers and sites if no corrective action
(or ineffective corrective action) is taken
and the process remains out of control.
Improper Handling, Use or Design by
End User
This category exists because many end
users of gel simply do not understand
how the gel system should work and how
filters and systems containing gel should
be handled. Problems include insufficient
penetration of the knife edge into the
gel, cutting of the gel, tearing, clumping,
clinging, UV degradation, reversion, and
chemical incompatibility with the process.
Some of these issues can be avoided
by simply designing the sealing interface
properly or unpacking, installing and
removing filters properly. Other times the
combination of gel selection and end-user
handling and use result in issues. Care
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should be taken in design stages that the
knife edge is centered in the gel channel
and sufficiently penetrates the gel surface
when the gel surface is 1/16 inches (1.5
mm) to 3/16 inches
(4.8 mm) below the top of the gel channel.
It is never recommended to fill a gel
channel completely since this causes
manufacturing problems and can cause
the gel to bulge out of the channel when
the knife edge is inserted. Theoretical
design interface may allow the knife edge
to “bottom out” in the gel channel, but in
reality, the filter should not be clamped so
tightly that the knife edge cuts completely
through the gel. Exceeding the elastic
limits of the gel will result in splitting and
cutting of the gel. Below the elastic limits,
the gel will “snap back” and return to
nearly its original state when the filter is
removed. Since gel is tacky and since the
disperse phase of the gel is mobile within
the gel system the margins along a cut
in the gel have a tendency to stick back
together. This is sometimes (incorrectly)
referred to as “self-healing.” Once the
gel is cut or fractured, it may stick back
together and form a seal, but it will not
reform chemical bonds. Therefore the gel
will not “self-heal” any more than a rubber
band that has snapped or a tire that is
punctured will heal itself.
Some individuals have tried to blame
problems with gel on either improper gel
selection or improper installation or design
when the real problem has been improper
handling or processing of the gel by the
filter manufacturer. An illustration of this
was a recent case where an end user was
told that the gel used in their product could
not be installed in a vertical application or
it would run out of the track due to gravity.
This is nonsense. The fact is that gel is a
cross-linked thermoset system, incapable
of flowing. Most gel filters are, in fact,
shipped with the gel in a vertical position.
Many tens of thousands of gel seal filters
are installed today in air handlers with
vertical banks. The same gel that is used
for ceiling grids and horizontal applications
may be used in vertical applications. If
the gel flows out of the channel there is
something has gone wrong with the gel
and the cause needs to be identified.
Invalid or Inconsequential Issues
This category exists in part, because of
the problems and issues cited above,
some end users have become hypersensitive to problems and rightfully so,
when critical downstream processes are
at stake. There is normal variation in gel
properties that are acceptable. Batch to
batch the hardness, surface tack, color of
the gel may vary somewhat. It is important
for all segments from supplier, processor,
manufacturer, installer through end user
to be knowledgeable and aware of what is
acceptable and what constitutes a problem
for a given application. The use of gel
systems specifically recommended for
HEPA filtration applications is the first step
toward achieving this goal. Color change,
particularly a color shift clear to light amber
or from blue toward green, yellow or clear,
by itself, is also normally considered of
little consequence as long as all other
properties of the gel remain acceptable.
The pigments used to color gel are
normally organic, non-metal containing
pigments and are not necessarily color fast
and usually not chemically bound to the
gel. The (un-pigmented) gel material itself,
over time tends to shift from a water clear
color toward a yellow clear color. This is
a normal characteristic of many elastomer
systems. With pigmented gels, Camfil,
in conjunction with the Gel Supplier, has
controlled the level of blue pigment in the
gel in order to avoid noticeable color shift
over time during normal use.
Recent issues of Gel compatibility with
Aseptic Cleanroom Chemicals and
Process
Camfil, over the past year has become
aware of a number of situations where gel
failure has occurred in cleanrooms. While
most of the information regarding these
incidents has been “word of mouth,” Camfil
does have specific knowledge regarding
cases where gel in some filters repeatedly
exhibited the accumulation of a wet oily
substance. Working with suppliers,
end users, independent experts and
laboratories we have done a significant
amount of work to try to establish the facts
surrounding these incidents.
In cases where we have detailed
information and based on extensive
laboratory testing, the circumstances
and experimental evidence does not
support an early theory that antimicrobial
agents, specifically Spor-Klenz, or
sodium hypochlorite (bleach) solutions
are singularly to blame for the problem.
Experimental evidence under very severe
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TECHNICAL BULLETIN
A Brief Discussion of Gel
conditions does suggest that sequential
use of Spor-Klenz and bleach can lead
to reactions that can occur in the gel
and can cause the liquid content of the
gel to increase. Since normal conditions
are much less severe, we can only say
that these agents used in combination
may worsen or accelerate but not by
themselves cause gel failure. Evidence
also suggests that prolonged exposure
to acids (e.g. hydrochloric acid (HCl) and
acetic acid, can be detrimental to gel
systems and can contribute to problems
of gel dripping. The current thinking is
that a combination of factors is usually
to blame when a gel problem, especially
gel dripping, is observed. The quality
of the gel materials, the extent of the
curing reaction, the characteristics of
the gel system and the control of the
component mixing are all factors that
are also important and can influence
the likelihood and extent of a problem
with the gel. The temperature of the
use environment and the application of
excessive amounts of liquid oil aerosols
(PAO, DOP, DEHS, Mineral Oil, etc.) can
also worsen or accelerate a gel dripping
problem. It has been demonstrated that
with a driving force present it is possible
that liquid aerosol used to test the filters
(PAO) accumulated above the gel can
theoretically migrate through the gel and
then along with unreacted components
of the gel, accumulated downstream
often at the corner miter joints of the
filter especially if these joints are not
well sealed. Tests indicate that these oil
aerosols when applied in vast excess may
cause a small amount of swelling to the
gel or can extract components of the gel
but do not chemically react to the gel.
Camfil conducted cursory tests to
determine the compatibility of various
filter construction materials including gel,
with Spor-Klenz. Polyurethane Potting
compounds, Filter Media, Gasket Material,
Silicone Potting Compound, Silicone Gel
and Polyurethane Gel substrates were
all directly exposed to both concentrated
Spor-Klenz and to a 1% Spor-Klenz
solution (recommended solution for
disinfection of surfaces.) In this study
the Spore-Klenz solutions were directly
applied to the substrate surface using a
dropper. The pool of liquid Spore-Klenz
was allowed to sit on the surface at room
temperature overnight until it evaporated
the next day. After 24 hours, no change
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to any substrate, including both gels, was
observed for both the concentrated and
dilute Spore- Klenz solutions. Another
test was conducted where polyurethane
gel was directly exposed to concentrated
vapors of an acidified hypochlorite bleach
solution at room temperature. After one
month of exposure some “skinning” and
loss of surface tack were noted on the
polyurethane gel sample. The skin was a
frosty white color and was very thin. There
was a slight decrease in the penetration
(softness) of the gel probably due to the
formation of the skin. No liquefaction or
reversion was observed. The underlying
gel was unaffected, only the gel surface
showed the changes noted.
In another study silicone gel was
partially immersed in an acidified sodium
hypochlorite solution for 24 hours at room
temperature. Observations following
exposure indicated that the gel matrix
itself was unchanged, however the bleach
solution effectively removed the blue
pigment from the gel sample and the
sample shifted in color from blue to clear.
Camfil has a program of continual study
and characterization of gels in an effort to
further develop industry knowledge.
In summary, the issue of gel dripping
seems to be caused by a combination
of multiple factors including the
original choice of gel, the quality of the
components, the reliability of the mixing
process and the exposure of the gel to
oxidizing chemicals or other detrimental
environmental stressors. A significant
excess exposure to oil or elevated
temperature may exacerbate the problem.
Recent Investigation of Gel
Compatibility with PAO (polyalpha
olefin)
In other studies with gel, Camfil and others
were able to document a reduction in
the plasticizer level of polyurethane gel
when it was immersed in PAO at room
temperature. The test conducted was a
very severe test since PAO would never
be expected to be present in such large
quantities where gel filters are installed.
Polyurethane gel plasticizer was able to
migrate out of the gel into the PAO, and
PAO was not able to migrate into the
gel in sufficient quantities to replace the
plasticizer, resulting in shrinkage (reduced
bulk volume) of the polyurethane gel.
This is strictly a physical reaction having
to do with diffusion of liquid materials
into and out of the gel. There was no
noticeable indication of any chemical
reaction occurring between silicone gel or
polyurethane gel and PAO. Independent
investigation by multiple researchers
indicates slight swelling of silicone
gel when exposed to PAO. In a study
conducted by Camfil we quantitatively
measured on average 5.5% swelling of
silicone gel after
immersion in PAO for 7 days at room
temperature and about 9.5% swelling after
55 days of immersion. The gel specimens
appeared normal and we were unable
to visually detect swelling of immersed
samples of silicone gels in PAO. We
did notice that one type of silicone gel
containing a blue pigment became clear
when immersed in PAO, and the PAO took
on a blue tint. Obviously, the pigment
was extracted from the silicone gel and
into the PAO. In either case, these effects
may be of only academic interest, since
the quantity of PAO present as a result
of normal HEPA filter integrity testing, in
relation to the quantity of gel present in the
system is extremely small, and in actual
field installation the physical effect of PAO
on gel should be unnoticeable.
Migration of Gel Discrete Phase
Recently Camfil and a researcher from a
large pharmaceutical company conducted
similar experiments where small plugs
of gel were placed on filter paper at a
controlled temperature and the resulting
diffusion or migration of unreacted
components of the gel out onto the filter
paper was measured over time. A “bullseye” ring formed around the gel and grew
in size over time. The rate of migration
was measured and found to vary based on
type of gel, manufacturer and grade. For
a particular type of gel, the results were
reproducible and were and characteristic
of the gel. In general we also were able
to qualitatively relate the rate of migration
directly with the quantity of extractable
material in the gel (determined by Soxhlet
extraction) and inversely with increased
physical properties of the gel.
The Effect of Knife Edge Insertion into
Gel
Recently Camfil conducted a series of
controlled experiments where various
types of knife edges were inserted into
different types of gel. The knife edges
used were:
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TECHNICAL BULLETIN
A Brief Discussion of Gel
1.
2.
3.
Dull edged extruded anodized
aluminum
Dull edged 11 gauge stainless steel
Sharp 90° edged 11 gauge stainless
steel
In each case the knife edges were inserted
into cured gel by a depth of 7/16 inches
(11 mm) and the knife edge was never
allowed to approach the bottom of the
gel channel closer than about 1/8 inches
(3mm).
We found that certain gels have extremely
low toughness and tensile strength and
cut or split in reaction to the stress of knife
edge insertion regardless of the type of
knife edge. We found that other gels cut
and split to some degree depending upon
the type of knife. In some cases cuts and
splits took only minutes to appear and in
other cases it took days or weeks. Still
other types of gels are extremely tough
and resist cutting or splitting even when
the knife edge was left inserted for up
to 5 months. Upon removal of the knife
edge, some gels tended to release from
all knife edges easily. Some adhered
more to the stainless steel than the
anodized aluminum and still other gels
stuck tenuously to all materials. Additional
certain gels tended to tear upon knife edge
removal resulting in avulsion to the gel
with associated gel residue remaining on
the knife edge surface.
Basically, some cutting and splitting of
the gel is normal depending upon the
system design and the type of gel used.
Conclusions about the gel cannot be
made based solely upon field observations
regarding the presence or absence of cuts,
splits or avulsions in the gel after filter
removal from the system. We were able to
determine what the normal characteristics
of the gel are during and after knife edge
insertion.
Quality Assurance
In addition to the certificate of compliance
provided by the raw material supplier,
HEPA filter manufacturers who use gels in
their products are must have a robust and
comprehensive quality assurance process
in place that ensures the quality of each
lot of component materials received and
the quality of the mixed components being
applied to the filters. The program must
ensure that both the materials and the
process being used are acceptable to give
a good, consistent quality end product.
Proper mixing and curing of the gel must
be verified prior to every production run (at
least daily). Cured retain samples should
be maintained for an extended period of
time.
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Conclusion
Legal Disclaimer
By nature, gel systems, whether silicone
based or polyurethane based, offer many
advantages and many challenges to end
users and filter manufacturers. The use
of gel to achieve filter seals is a welldeveloped and effective technology when
done correctly. Due to the fragile nature of
the chemistry (lightly cross-linked system)
the proper choice of materials and very
good process control in manufacturing
of filters is required in order to achieve
consistent quality. Because of our
depth of knowledge, experience, quality
systems, process control systems, and
availability of technical resources, Camfil
has established the qualifications needed
to successfully implement this technology
while avoiding widespread field problems.
In many cases, our experiments and
observations have been duplicated by
independent parties interested in the
subject and we have been able to likewise
duplicate a number of experiments
conducted by others. Because of their
many advantages, Camfil continues
to offer high quality silicone gel seal
systems primarily targeted for life science
applications and polyurethane gel systems
primarily targeted to microelectronic,
aerospace, optical and surface coating
applications.
The information, recommendations or advice
contained herein is given in good faith;
supplier makes no warranty or guarantee,
express or implied, (1) that the results
described herein will be obtained under enduse conditions, or (2) as to the effectiveness
or safety of any design incorporating
supplier’s materials, products, services
recommendations or advice. Nothing in this
document or any other document shall alter,
vary supersede or operate as a waiver of any
of the supplier’s standard conditions of sale.
Each user bears the full responsibility for
making its own determination as to the
suitability of supplier’s materials, products,
services, recommendations or advice for
its own particular purpose. Each user must
identify and perform tests and analysis
sufficient to assure it that its finished parts
will be safe and suitable for use under enduse conditions. Because the actual use of
products by the user is beyond the control
of Supplier, such use is within the exclusive
responsibility of the user and supplier
cannot be held responsible for any loss
incurred through incorrect of faulty use of the
products. Further, no statement contained
herein concerning a possible or suggested
use of any material, product, service or
design is intended or should be construed to
grant any license under any patent or other
intellectual property right of supplier or any
of its subsidiaries or affiliated companies,
or as a recommendation for the use of such
material, product, and service or design
in the infringement of any patent or other
intellectual property right.
The information contained in this document
is truthful and factual to the extent that
the facts are known at this time. The
information is provided in good faith to help in
understanding the issues discussed. Specific
references to actual companies or individuals
have not been made and assumptions shall
be avoided. Similarities to actual events may
not be used as evidence that a particular
company, event, material or individual was
involved in the cases cited above.
This document may contain proprietary
and confidential information and is to be
treated as such by Camfil. and by the
reader. Reproduction and distribution of this
document by any means is prohibited without
the express written permission of Camfil.
Steve Devine
B.S. Engineering Chemistry
M.S. Polymer Science and Engineering
Vice President of Research and Development
Camfil
Revised 11/29/12
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