Download - Repositionable Adhesives
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Removable Pressure Sensitive Adhesive Products
Pressure-Sensitive Adhesive products ,
are usually bonded , firmly to applied
surface.
The PSA product peel -of or
delamination is not desired or Intended.
However, there are adhesive tapes where
in the , adhesive clean and easy removal
from substrates after use is desired.
A repositionable adhesive allows a label
to be repositioned without damage , to
either the label or the substrate. Many
applications require that the adhesive
holds the label firmly in place, yet does
not leave residue behind when the label
is removed.
This material holds items firmly, yet lets
you remove or reposition your artwork or
matting as many times as you want to,
even months later. Use on your
computerized mat cutters so your mats
won't slip. Also used to hold articles in
place that requiring stitching. It resists
bleeding, staining, or wrinkling. Should
only be used for temporary bonding. This
pressure sensitive material can be
removed from paper without lifting fibers
or delaminating it.
a range of removable adhesive tapes,
clean peel and perm/peel products. Both
single and double coated, controlled peel
products, designed for clean, residue free
removal, even after extended periods
History
In 1968, Dr. Spencer Silver, a scientist at
3M in the United States, was attempting
to develop a super-strong adhesive. But
he created a "low-tack", adhesive by
accident. It was found to be
reusable, pressure-sensitive adhesive
. For five years, Silver promoted his
invention within 3M, both informally and
through seminars, but without much
success.
In 1974, a colleague of his, Art Fry, who
had attended one of Silver's seminars,
came up with the idea of using the
adhesive to anchor his bookmarks.
A year later, in 1978, 3M issued free
samples to residents of Boise, Idaho, and
94 % of the people who tried them said
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that they would buy the product. On
April 6, 1980, the product debuted in US
stores as "Post-It Notes". In 1981, Post-
its were launched in Canada and Europe.
In 2003, the company came out with
Post-it Brand Super Sticky notes, with a
stronger glue that adheres better to
vertical and non-smooth surfaces.
Standard Post-it Brand notes have only
partial adhesive coating on the back,
along one edge. Similar products are
used for specialized purposes with full
adhesive coating; the US Post Office
uses such yellow address labels to
forward mail.
The yellow color was chosen by
accident; a lab next-door to the Post-it
team only had scrap yellow paper, which
the team initially used.
Introduction
Often we require a pressure sensitive
adhesive (PSA) bond that is not
permanent, and the substrates need to be
purposely "detached" at a certain time.
Uses for such adhesives include
nonstructural applications such as
removable labels, masking tape, surface
protection films, easily replaceable and
removable note paper, and bonding credit
cards to mailers. Applications can even
include structural applications such as
tooling placement, prototyping, and the
assembly of parts for eventual reuse or
recycling.
There are various types of removable
pressure sensitive adhesives available to
the end-user depending on the degree of
bond strength required. PSAs that are
filled with microspheres are claimed to
be the most repositionable of these
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systems. Here "repositionable" is defined
as the ability of the substrates to be
detached and then rebonded repeatedly.
This feature is desired when the bond is
meant to be repeatedly detached and
rebonded or when substrates are
commonly misapplied and need to be
repositioned.
Thus, repositionable adhesives differ
from what is referred to as "removable"
adhesives. Removable adhesives are
generally those providing an adhesive
bond that can withstand the rigors of
service but in which the substrates can be
purposefully detached leaving little or no
adhesive residue. The substrates
generally cannot be rebonded once they
are detached.
Mechanism
The concept of a repositionable PSA
made by incorporating microspheres into
a polymeric binder has been around for
several decades. For example, 3M
Company's famous Post-It™ Notes
product, commercialized in the 1980s,
consists of tacky, crosslinked acrylic
microspheres, which make "point
contact" with the paper substrate4.
These repositionable adhesives have a
unique composite-like structure in which
micro-sized spheres are held together by
a polymeric binder. The binder also
serves as the base polymer in the
adhesive formulation. Due to the
presence of the microspheres and the
discontinuous morphology , the adhesive
does not completely wet the substrate to
which it is applied. As a result, the
adhesive strength is less and does not
develop with time as in the case of
conventional pressure sensitive
adhesives.
Microspheres are tiny particles
measuring between 10 and 250 microns
in diameter. Although they are
microscopic in size, these spheres are
still much larger than the emulsion
particles found in conventional
adhesives. The size difference has a
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dramatic impact on adhesive tack. By
forming a discontinuous film,
microsphere adhesives limit physical
contact, resulting in low peel,
removability and stable tack over time.
Emulsion adhesives, on the other hand,
use smaller particles, which coalesce into
a continuous film and do not allow for
easy removal or repositioning.
A repositionable adhesive allows a label
to be repositioned without damage to
either the label or the substrate. Many
applications require that the adhesive
hold the label firmly in place, yet does
not leave residue behind when the label
is removed.
Repositionable adhesives have relatively
good coat ability but a low degree of tack
and peel strength. If the microspheres are
resilient and there is sufficient binder
concentration, the adhesive bond results
in an even peel strength during
separation with little adhesive transfer.
Table 1 shows the tack and peel strength
of representative permanent, removable,
and repositionable (microsphere)
adhesive system. Shear strength of a
microsphere PSA is very low and
generally not measured.
PSA Type Peel
Strength
gm/25
mm
Loop
Tack
gm/25
mm
Shear
Permanent >2000 1400 High
Removable 250-275 400 Med
Repositionable 90-100 250 Low
The Performance Challenges
To ensure perfect placement
should offer highest degree of
repositionability
To protect fragile substrate
clean removal from varies surfaces like ,
newsprint, paper, drywall and glass, with
no adhesive residue, transfer or ghosting.
Durability
Reusability
like paper tab markers to be used again
and again.
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Materials, Handling, and Application
Most repositionable microsphere PSAs
are made from acrylic emulsion base
polymers that serve as the "binder" as
well as the microspheres. It is important
that the microspheres (measuring 10-250
microns) are significantly larger than the
emulsion binder particles. The binder
concentration must be optimized to the
particle size of the microspheres in order
to provide the surface morphology that
will achieve the best adhesive
performance.
Microsphere adhesives can be produced
via one of two processes. Typically they
are formed by a water based free radical
suspension polymerization using two
acrylic monomers. In this case, the
microspheres are acrylic and form
together with the base polymer emulsion
particles. Another method of production
can be to form the base polymer and the
microspheres separately and then mix
them together in the formulating process.
This technique is used in specialized
applications as it provides somewhat
more control of the end-properties due to
the variety of microspheres that can be
produced.
The adhesive properties of microsphere
PSAs are controlled by a number of
factors. Most significant are coating
weight, particle size, concentration of
binder, and monomer characteristics such
as glass transition temperature (Tg) and
molecular weight. Table 2 provides
information on how these factors can be
used to control adhesive performance.
These factors must be balanced by the
formulator and / or converter to provide
adhesive properties that are optimized for
the specific application.
Factor Performance Contribution
Adhesive coating
weight
Peel adhesion strength is increased
by increasing coating weight. This
is not unlike other PSAs.
However, with microsphere
adhesives too much adhesive on a
smooth facestock can cause
layering of spheres on top of each
other leading to poor cohesive
strength.
Microsphere
particle size and
amount of binder
Large particle size microspheres
and low binder levels provide
rough coatings (lowest adhesion
level). High levels of binder and
small particle size will lead to
smoother coatings and better
wetting of the adhesive on the
substrate. Smoothest coatings give
the highest peel strength and peel
values that build over time. The
roughest surface will provide the
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lowest and most stable peel values.
Glass transition
temperature, Tg
Monomers that form low Tg
polymers (e.g., 2-ethylhexyl
acrylate provide good tack and
flexibility). Comonomers with
higher Tg (e.g., methyl or ethyl
acrylate) provide higher cohesion
and shear strength. This is not
unlike other PSAs.
Molecular weight Lower molecular weight
microspheres adhesive offer higher
tack, while higher molecular
weights provide better shear
strength. Very low molecular
weights can produce cohesive
strength problems.
In addition to lowering the tack and peel
strength, repositionable adhesives must
also provide a mechanism that
counteracts the peel strength
development with time noticed with all
PSAs. To tackle this, formulators have
replaced petrochemical based acrylate
esters with esters made from alcohols
found in vegetable oils.
In general, microsphere PSAs can be
used much like conventional PSAs.
However, certain Precautions should be
observed due to the composite nature of
the product.
General
Avoid high shear forces.
Small pumps operating at slow speed and
gentle mixing are preferred.
Microspheres rise to the top of their
container.
Gently stir before each use or transfer.
Microsphere adhesives are pH sensitive.
A pH of 7.9 to 9.5 is recommended.
pH can be adjusted with 21% aqueous
ammonia.
Once microspheres are coagulated, the
adhesive is unrecoverable.
Storage
Shelf life is six months.
Store between 10°- 30° C do not freeze.
Use partial drums quickly.
Pumping and Filtration
Be sure to eliminate air from adhesive
lines as this will create foam.
Do not use felt filter bags.
Fine filtration removes spheres;
Do not use filters finer than 60 mesh or
250 microns.
Change filter bags when pressure
exceeds 30 psi.
Handling
Add all formulating ingredients slowly
under gentle agitation.
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Defoamer can be added to minimize
foam (typically 0.1-0.2% if oil based and
0.1% if silicone based).
Premix defoamer at a ratio of 1:1 with
water.
Coating
Certain microsphere products are capable
of being coated by rod, roll, or slot-die
coated.
Avoid free-fall of the adhesive during the
recirculation process.
Polymeric microspheres are now helping
deliver enhanced performance in many of
these areas. Larger than particles found
in conventional emulsion adhesives,
these spheres have a big impact on
adhesive tack. They create a
discontinuous adhesive surface that
limits physical contact with a substrate,
making removability easier. The
microsphere adhesive holds labels firmly
in place on most substrates yet is
permanently repositionable.
Typically microsphere adhesives are
formed directly from raw materials by a
water-based free-radical suspension
polymerization using two acrylic
monomers. However, for specialized
applications the microspheres are
sometimes formed by dispersion
polymerization in an organic solvent,
and then added to a separately-formed
adhesive.
Component Amount
(g)
Total
proportion
Deionized water 242.3 60.57%
Crosslinked acrylic acid 8.4 2.10%
Sulfocarboxylic acid
anionic surfactant 4 1.00%
Ethoxylated oleil alcohol non-
ionic surfactant 3.2 0.80%
Ethyl acrylate/2-ethylhexyl
acrylate 141.6 35.39%
Dibenzoyl peroxide initiator 0.56 0.14%
Suspension polymerizations are heated
until the initiators thermally decompose
to generate a sufficient number of free
radicals to begin the reaction, with
nitrogen bubbled through the reaction
mixture to deoxygenate it. Monomers are
typically already present when the
reaction is initiated. Stirring time and
speed parameters can be used to control
monomer droplet size, with average
particle size tending to decrease with
increased and prolonged agitation of the
reaction mixture.
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This process yields a process that stable
aqueous suspension of microspheres can
be used immediately as an adhesive.
Alternatively it can be compounded with
tackifiers, plasticizers, rheology
modidiers, colourants, latex binders and
other additives.
Controlling microsphere adhesive
performance
The adhesive properties of microsphere
PSAs are most often modified by varying
glass transition temperature (Tg) using
different types and amounts of co-
monomers. Monomers that form low Tg
polymers like 2-ethylhexyl acrylate
provide adhesive tack and flexibility. Co-
monomers giving higher Tg polymers
like methyl or ethyl acrylate enhance the
adhesive's cohesion and shear strength.
Varying polymer molecular weight is
another way in which microsphere PSA
properties can be controlled, especially
tack and shear strengths. This can be
done through process control, for
example by changing polymerization
temperature, or by chemical control, by
changing initiator concentration or
adding chain transfer agents. Lower
molecular weight microsphere adhesives
show higher tack strength, while higher
polymer molecular weights provide
better shear strength. Low polymer
molecular weights can produce major
microsphere cohesivity problems when
larger amounts of chain transfer agents
are used.
The Ethyl Acrylate % ( Increase Ethyl
Acrylate and Decrease 2 Ethyl hexyl
Acrylate ) is useful in altering the
Adhesive Properties .
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While microsphere pressure sensitive
adhesives have proven to be
repositionable, the strength of their bonds
increases over time. To tackle this,
scientists have replaced the
petrochemical-based acrylate esters with
esters made from alcohols found in
vegetable oils. These are produced by
reacting acrylic acid with heptanol,
octanol or decacnol from sources like
coconut oil and palm kernel oil, or
animal fats such as tallow and lard.
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Suitable Applications
Carpet tape for Aero Plane.
The carpet tape is a double-sided,
differentially coated adhesive tape with
one clean peel removable side and one
very high adhering side. The modified
acrylic adhesive on the carpet side
provides very high levels of initial tack
and adhesion. The unique, clean peel
removable acrylic on the floor side
provides very secure adhesion to
composite floor panels and easy,
controlled peel removal with the carpet.
Masking of parts during powder
coating
Masking for Abrasive or Sand Blasting
During preparation, items are abrasive
blasted to remove oils, dirt, rust and mill
scale. Threaded portions of parts are
masked to prevent damage. Because of
the extreme pressure applied during
blasting, duct tape is used to mask for
this step. Duct tape is sticky and thick
enough to stand up to the heat caused by
the friction of blasting parts.
Masking for Curing Oven
Parts can then be masked to prepare for
baking in the curing oven using a high-
temperature tape. Temperature-resistant
tape comes in many sizes and
formulations. These tapes are made from
polyester or fiberglass with special
silicone-based adhesives able to
withstand extended periods at high
temperatures.
Protection of bright parts
Mounting of protection pads
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as an application tape for auto badges
Mounting of shop front and window
graphics
Temporary fixing in print finishing and
direct mail companies
Mounting of smart cards
Adhesive tape technology is seen as the
most flexible, clean and easy solution for
the smart card market’s manufacturing
process. Adhesive use for module
embedding guarantees both card quality
and long term resistance, which in turn
reinforce the Smart Card security.
More Applications
Ultra Removable Stickers and Labels