information on sealants everyone should know
Most engineers and contractors have heard the complaint more than once: a duct system is leaking
and the problem seems to be the sealant; the same sealant that looked great in the sales literature,
even in the same laboratory and at the time the duct erected. What happened?
The sealant manufacturer may have relied on outdated specifications that do not begin to meet
today's technological requirements. Most of the current specifications were developed years ago for
other industry groups. It is not uncommon to find "Sigma, AMMA, federal, MIL, and ASTM'
specifications being used as guides for sealants in the sheet metal industry.
Many of these specifications call for the use of good test procedures but fail to cover the most critical
performance criteria. Instead, tests are needed that are designed specifically for the industry; tests
that will demonstrate how a material will perform under critical conditions in one, five, ten or even
twenty years after installation in a duct system.
It's possible for unscrupulous or unknowing chemists to cover up deficiencies with additives,
enabling a sealant to pass a lab test before being used in the field. As a result, many of today's
sealants have only short-term effectiveness. This, along with poor design and inadequate application
technique, causes leakage and energy loss, serious problems in duct construction.
Users are often tempted to simply ignore specifications and testings and instead purchase on impulse
and price. This isn't a solution to the problem. Users need to become more discerning in selecting
proper tests for prospective sealants.
It's important to have a basic understanding of key terms used in the industry. Understanding the
vocabulary used by sealant manufactures as well as learning how to perform some very simple tests
can help the user select the best product.
The following terms are widely used:
adhesion
Adhesion occurs when one material sticks to another. The key issue is how long the mating will last.
tack
Tack gives the sealant its quick binding capability. It's only good for a short time and is only
necessary until a permanent bonding occurs. Tack is achieved through a careful formulation of
elastomers, polymers and resins. These ingredients must be carefully selected to enable the sealant to
permanently bond later. Otherwise, the tack may be fine but other problems such as plasticizer
migration, cold flow, loss of tensile strength, long term ageing, and instability will crop up. All of
these can cause sealant deterioration relatively fast.
To achieve tack, some sealant manufactures resort to using, a lot of oils, plasticizers, by-products and
low cost hydrocarbon resins. When used to extreme these cause the problems listed; in moderation,
the imbalance will only effect reliability slightly. But the user will not necessarily be able to tell to
what extent the compound has been compromised.
specific adhesion
Specific adhesion is almost never identified in test procedures. It means, of course, that the sealant
has been formulated to adhere to a specific surface. To achieve the right mechanical or chemical
adhesion, a sealant manufacturer must work to blend the elastomers, polymers and resins with the
ultimate end in mind. Otherwise, the sealant can have tack and the appearance of adhesion and good
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quality but may deteriorate rapidly in the field. Specific adhesion creates the real bond between
particular substrates, allowing the sealant to work compatibly with them over a long period of time.
It's not, however, a static condition and should not be measured alone. Cold flow and creep must
also be measured in any relevant tests.
cold flow
Cold flow can occur at all temperatures. Cold flow is a material seeking its lowest level with respect
to gravity, such as water does. A thermoplastic material has hot flow when it's melted or is above its
softening point. Most thermoplastic materials cold flow below this softening point, especially when
they have been combined with plasticizers and oils. Some materials cold flow quickly; others take
years to develop this condition.
Generally, in testing for cold flow a material is placed in an oven at high temperature for a short time
to check for any deformities in the material. The problem with this test is that it can be made
ineffective by adding fillers such as asbestos and talc to the sealant. In the new material this can
make it appear that no cold flow is occurring when the opposite is true. An exception is when the
material that would cold flow is being totally absorbed by the asbestos and talc, in which case the
adhesion would age rapidly.
All cold flow is not bad. A lightly crosslinked polymer, when combined with proper fillers, will
result in a low force to compress material that will only slightly cold flow. such a material will wet
through dirty, oily and damp surfaces; a definite advantage. This material will have a lower tack
with a slower but more lasting bond to the surface. A sealant must be constantly monitored however,
to prevent too much cold flow. In testing for this, one must be careful to not confuse the desirable
low force to compress characteristics with what might appear to be cold flow.
polymer
Polymer is an ambiguous term often used to describe the backbone or primary structural member of a
sealing compound. The use o the term generally means the manufacturer has used some type of
thermoplastic material in the compound. Such materials can easily be affected by heat, cold and ultra
violet light. There are several "plastic-acrylic polymers" on the market today that are not always
cured to an elastomeric level.
These are perfect examples of this misuse of terms. Such products are vulnerable to a reversion of
cure, not only back to the level of the original polymer. They can easily be identified by a strong
odour, usually apparent after a prolonged exposure. These same problems frequently occur in other
so-called "polymers"made of non-cured butyl or butyl cured with resins. Remember that the term
"polymer" can be widely misused by sealant manufactures to disguise a lot of undesirable
proprietary materials.
elastomer
Elastomer describes the thousand of rubber polymers, both crosslinked and uncrosslinked, that are
used in the industry. In a curd state (crosslinked), most rubber elastomers age better by eliminating
the unsaturation in the compound. This creates a highly resilient lattice structure.
In the uncured state, some elastomers behave like a thermoplastic polymer, but this undesirable
action isn't always evident. An elastomer should be the mainstay of any sealant used in a dynamic
situation. It should meet the specific requirements of the job, although there is no single elastomer or
combination of elastomers that will meet all conditions.
plasticizer migration
Plasticizer migration occurs when the materials that make a compound flexible leave the compound
through bleeding, evaporation thermal decomposition or oxidation. The remaining material is less
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elastic than originally intended. The main concern is that the migrating material ends up as the
material that interfaces with the surface to which the sealant is bonded and becomes its weakest link.
This gives the sealant little or no tensile strength, leading to an eventual breakdown of the bond; in
other words, separation.
Plasticizer migration even affects silicones. It causes them to pick up dirt and lose adhesion over a
period of time. A well-formulated material will have NO migration or bleed at all. Gelling agents can
mask plasticizer migration, but a good ageing test will detect this. Long term exposure to the
elements will also reveal the use of such agents.
service and shelf life
A good sealant, when properly packaged, should have shelf life equal to its service life. (An
exception would be that of a crosslinking liquid sealant whose shelf life can only equal the shelf life
of the catalysing and curing agents in the product.) a compound with a solids content of one hundred
percent should never have a shelf life less than its service life.
Some manufacturers advertise a long service life but only a one-year shelf life. This is probably for
fear of a reversion of cure, oxidation, cold flow, or other dangers. Products on the shelf can be
readily scrutinised. Once installed in the field, though, they are not visible and the engineers or
contractors often overlook such problems.
The following simple tests can help the layman determine how effective a sealant may be. The only
requirements are time, refrigeration, water and oven :
Take a sample of the sealant currently being used and place it outside where it's exposed to the
elements, particularly to the sun. Keep duplicate samples inside that are packaged just as they were
when received from the manufacturer. Check both the stored and exposed samples in six months to
determine how much the material has bled, hardened, changed colour, developed an odour, become
soft, begun to cold flow, or just seems usable. Difference in appearance alone will indicate a great
deal about the sealant lasting qualities.
To test cold temperature formulations, place samples of both the sealant and the substrate to which it
will be bonded in a refrigerator at 0°C for twenty-four hours. Later, assemble them, as they would be
used in the field to check for ease of assembly. A material that can be assembled well at cold
temperatures is obviously the most desirable buy one must remember that such materials are
generally the most susceptible to failure at high temperatures. Another sample should therefore be
tested simultaneously at high temperatures.
heat is the acid test for any sealant It can cause migration, oxidation, cold flow or a reversion of
cure. Place sealant samples in an oven at 90°C. for seventy-two hours and observe how the sealant
performs. Look for discoloration, a gaseous condition or a mass with no body. A sealant that
performs extremely well will generally have some problems at the cold temperature level.
A high mark on any one test will not necessarily assure the user of having a good sealant. Generally,
the one exhibiting the least amount of deterioration overall in these visual tests will be the one that
performs the best. The objective, naturally, is to get one that performs well above average in all
categories.
percent of solids content
This property provides an indication of how much a sealer may shrink from its wet state to its final
dry state. The curing of most sealers and adhesives is either by solvent evaporation or polymerised
chemical crosslinking. As the sealer cures, the solvent materials evaporate and a matrix of solid
materials distributes itself across the covered area. Since less material is distributed over the same
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area, voids may appear and some of the sealer could pull away/ from various surfaces. The higher
percentage of solids content, the less shrinkage will occur however, one must take note of sealers and
adhesives that are filled with asbestos or talc. If the sealer has too much filler in them, it will age fast
and tend to crack off of the surface.
fire rating test
There is more demand today for fire hazard classification in sealants than ever before. However,
most of the manufactures have side-stepped this issue, especially the manufactures in the solvent-
based adhesives and sealants. These sealants are highly volatile in their wet state and it is difficult to
get a good fire hazard classification on them. Underwriters' Laboratories is by far the best criteria.
However, tests may be conducted with ASTM E-84 or NFPA 255 and as stated, the UL 723. The
optimum in fire hazard classification is to have sealers such as nitrile acrylic adhesives that do not
burn in the wet or dry states. It is the writer's opinion that the acrylic nitrile materials available today
have the best fire hazard classification. They present no fire hazard in the wet state as well as the dry
state.
Most engineers and contractors should be well acquainted with the source of adhesives and sealants
used and not hesitate to ask that the manufacturer of these various materials be prepared to warranty
their materials to do a specific job.
How can the buyer get a good sealant? Start by working with the sealant manufacturer. It's necessary
for a sealant to be compounded with a specific application or set of conditions in mind. Think of the
sealant as an integral part of the duct system and not just an afterthought to be purchased like an off
the shelf- item. If the sealant manufacturer understands what the duct designer is trying to
accomplish and what materials are to be used, it's much easier to formulate the correct sealant.
Establish tough internal specifications and instigate the necessary checks, balances and tests to see
that they are enforced. See that the supplier periodically shows proof of performance and also tests
internally for added protection.
Finally, insist that the purchasing department buys for performance and reliability, which means long
term, cost effectiveness and not just for price alone. Do this by arming them with a set of rigid
specifications and an internal test program. This allows them to determine if the supplier is meeting
the requirements on continuing basis and not just at the outset of the program.
Learn to recognise poor quality and help the sealant industry rid it self of those suppliers who offer
cosmetically attractive but functionally inferior products.
The most effective way to achieve these goals is to establish up to date stringent tests and
specifications for the sealant industry. Conscientious and competent sealant manufactures would
welcome the opportunity to have their products measured against such guidelines.
Written by:
Eric Overtoom
Carlisle Hardcast Europe B.V.
(publicatie SnipsMagazine 1988)
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