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Articles
by Chris A. Paschke, CPF GCF
"The Mystery of Dry Mount Adhesives"
September 1998
The mystery of
adhesives in many cases is more of a fear of the unknown. Over the past ten
years, I’ve observed many new products, new companies, and numerous versions of
established products reemerge as new. Many times these are developed to help
expedite our jobs and just make life simpler. The best way to keep up with the
new products and their applications is to be familiar with them. That means
read and test.
The computer
industry has been both a blessing and a beast. It has simplified our lives by
making sales, marketing, and pricing, more quick and efficient, but has also
given us the headaches of digital papers and inks we know not how to handle. What’s
a framer to do?
Even if you
understand ALL the adhesives and are comfortable with ALL the techniques…what
should you do with the project you are unfamiliar with? In some cases even
testing it can damage it. When insecure about the item to be mounted, it is
usually best to hinge or apply some approved noninvasive conservation technique.
All other forms of mounting require an absorption, moisture or heat that may
effect the unknown inkjet, vellum project, or whatever.
THE SCIENCE OF ADHESIVES
An adhesive is a
substance capable of holding two surfaces together in a strong bond. This close
bond is between the surface molecules of the material being adhered and the
substrate. The closer the two surfaces
fit together, the stronger the bond. Therefore a thin adhesive bond is often
stronger than a thick one.
Two main adhesive
types are natural, substances coming
from animal and vegetable sources; and synthetic,
being compounded from simple chemicals, many of which are polymers. Vegetable
glues come from starches and dextrins extracted from corn, potatoes, rice or
wheat. Natural gums, although vegetable in nature, are often blended with
synthetic rubbers to create adhesives often used in pressure-sensitive
cellophane and masking tapes.
Synthetic adhesives
fall into two categories: thermoplastic and thermosetting. Thermoplastic adhesives can be resoftened any number of
times by reapplying heat and will once again adhere and bond when cooled. They
are also soluble in selected solvents. Natural adhesives are predominantly
thermoplastic, the most widely used being vinyl resin adhesives, more commonly known
polyvinyl acetate (PVA) or white glue.
Thermosetting adhesives undergo an irreversible chemical
change when they harden, the result of a catalyst. Once hard, they do not melt
or resoften when heated and are considered insoluble in common solvents. Thermosetting adhesives include epoxies,
polyesters, and urethanes such as used with fiberglass. These adhesives adhere
well to most materials, porous and nonporous.
FRAMING ADHESIVES
Thermoplastic
adhesives are generally the ones found in picture framing. These adhesives used
in mounting artwork to substrates include wet, spray, pressure-sensitive and
heat-activated materials. The natural or synthetic base of an adhesive will
categorize and in turn determine its appropriateness for any given type of
mounting. Some adapt best to mass
production, where speed and permanence is desired, while others allow for
specialized attention during a slower mounting process.
WET GLUES
Thermoplastic in nature, wet glues are water-based adhesives
that include, vegetable starch and PVA. Nonremovable polyvinyl acetate
glues (PVA) are water-soluble when wet, but like any acrylic are quite
permanent once dry. Wet glues are bottled liquids that are applied to a
substrate with a brush, rubber roller, or airbrush. They require the pressure of a weight or cold
vacuum frame to create the initial bond, which will hold for an indefinite
period. Vegetable starch glues remain removable with distilled water even after
long-term mounting.
SPRAYS
These are air-drying
adhesives that are generally thermoplastics. They convert to a solid state by
evaporation of the solvent, and bond by mechanical adhesion (the adhesive
strength given by interlocking molecules) and mutual attraction. Sprays are packaged in aerosol cans and
require proper application and sufficient pressure to create an adequate bond,
see May 1998, "Spray Adhesives".
Since partial adhesive saturation is required to ensure the best bond,
when mounting some high gloss papers, RC photos and polyester fabrics, a truly permanent bond is difficult
to achieve.
PRESSURE-SENSITIVES
These are
predominantly thermoplastics, and require no moisture or heat prior for application.
Pressure-sensitive adhesives are available in rolls, precut sheets and
premounted to various substrates with release liners applied to protect them
from bonding until desired to do so. The adhesive has a release paper backing,
is tacky to the touch, and best results are derived from direct pressure of a
roller press, squeegee applicator, or weighting during the specified bonding
period.
HEAT ACTIVATED ADHESIVES
Dry mounting
adhesives are easiest to understand when broken into and defined as specific
categories. By taking the time to analyze the various available tissues, you
will be better prepared to select the proper adhesive to fit your needs. All heat-activated adhesives may be placed
into specific categories of:
s
Type
of bond (permanent or removable)
s
Physical
composition (tissue-core or film)
s
Degree
of porosity (breathable or nonbreathable)
s
Acidity
level (tissue pH)
TYPE OF BOND (PERMANENT
OR REMOVABLE)
One of the keys to
successful dry mounting is remembering where the bonding actually occurs. A
permanent adhesive bonds within the press. All layers of the mounting package
(top release material, art, adhesive, substrate and bottom release material)
must reach the required bonding temperature and remain there during the
required time allotment to set the adhesives. When removed they will be bonded.
A removable adhesive
bonds once removed from the press as it cools under a weight. It becomes
removable through the reapplication of heat, which reactivates the adhesive,
making the art separable from the mounting substrate. All mounted items should be placed under a weight when removed from
the press, regardless of whether permanent or removable, to expedite the
cooling and help reflatten bowed substrates. This weighting step remains the
suggested procedure regardless of where the adhesive bond occurs.
PHYSICAL COMPOSITION
(TISSUE OR FILM)
Dry mount adhesives
are available in both roll and precut sheets, and come in two basic
compositions; tissue-core, and film.
Tissues have a center core or carrier of either porous tissue (ColorMount, TM-2, Trimount, "Super" Unimount,
Promount...) or nonporous glassine-type sheet (MT-5, TM-1, Postermount...), with adhesive applied to either
side of the carrier for mounting.
Since both sides are identical, there is no top or bottom. They
are clean, dry, nontacky, relatively opaque white in color, and are extremely
time effective for production use. Tissues adapt extremely well to oversized
mountings, float mounting or multiple bite procedures. Pure film adhesives (Fusion 4000, TM-3, Flobond, Acid-Free Mounting Film, Versamount...) are 100% adhesive with no carrier or
tissue in the center. This makes them
translucent when unmounted and clear when mounted. Some brands of film may be pieced or overlapped
because of the lack of central carrier paper, which allows for greater use of
scraps.
DEGREE OF POROSITY (BREATHABLE OR NONBREATHABLE)
Porosity is the level of which an adhesive is permeable by
moisture or air. This is an extremely important designation when selecting a
tissue for compatibility with all selected mounting materials. If a nonporous/nonbreathable material, such
as a photograph or heavily lacquered print is to be mounted, the adhesive must
remain breathable to allow for air or steam to be forced out and/or through the
mounting layers. If this is not allowed the project can suffocate.
By using a
nonbreathable adhesive with a nonbreathable photo, there is much greater
potential for air to be trapped between the two nonporous items creating bubbles in the completed
mounting.
If a breathable item
is to be mounted to a breathable substrate, essentially any tissue may be used
to mount it simply because air will always be able to be compressed out through
and around the porous art and substrate.
In any mounting or laminating package, only one layer within
the sandwich between release materials on top and bottom may be nonporous. If two exist, such as photo
and adhesive, or photo and nonperforated laminate, air is likely to remain
trapped between the layers in the completed mounting. In other words, the
project will suffocate. Only one layer in any mounting or laminating package
may be nonporous, all others must be allowed to breathe.
NOTE: The
technique of perforating a nonporous laminate temporarily allows it the
porosity necessary to be used over a nonbreathable photo. This prevents
two-layer suffocation.
ACIDITY LEVEL OR pH
Since most dry mount
adhesives are inert, meaning they are incapable of stimulating a chemical or
molecular reaction, it is actually the carrier sheet that needs to be checked
for pH levels. Many manufacturers have developed tissues using acid-free or archival carrier papers in conjunction
with dry mounting adhesives and have named them accordingly. These tissues
mount at lower temperatures, are breathable, removable and neutral pH. They are
considered more delicate, but do not meet conservation standards.
Using heat activated
adhesives can never be considered archival because the very act of dry mounting
art to a substrate breaks all conservation guidelines. Adhesives travel toward the heat
as they are mounted, and the very act of dry mounting encourages a certain
percentage of nonreversible adhesive to penetrate the back of the artwork, even
with a removable tissue.
NOTE: Dry mount
adhesives are available from a number of manufacturers and distributors as both
name brands and private label brands. Adhesive prices do reflect the thickness
of tissues and the amount of applied adhesive per side, so price comparisons
need to reflect an understanding of variations in the physical nature of the
products.
THE
80/20 RULE
There is a
theorem, or standard, found within the framing industry called the 80/20 rule.
It states that regardless of your choice of mounting method of wet, spray,
pressure-sensitive (all considered cold) mounting, and dry (heat) mounting,
there will be one basic process used for mounting 80% of the items to be
mounted. The remaining 20% of the projects will require an alternative method
of mounting. This may be the result of a heat sensitivity, conservation
requirement, or simply not being certain of the mounting tolerances in the
first place.
This 80/20 concept
is applicable to both the overall mounting process as well as the two basic
brands of adhesives used within that process. For example, your 80/20 process
may be dry mounting/hinging; while your 80/20 dry mount adhesives may be permanent
tissue/removable film. It doesn’t appear to be the 80% of the mounting that
gives us the headaches, it’s the 20% of those unknown or unusual projects that
does. Pay attention to TTPM and be able to identify when a project is mounting
worthy or not.
Mounting has
always been the scariest part of framing, even for me. Though I may tell you
how to use adhesives, I’m not going to tell you what to use or when, there are
too many variables. I will say however, think
ahead, mount defensively, and always remember…"what if it doesn’t act
as I expect, when mounted?". That way you may be able to prevent
problems. By better understanding the
make-up and application of adhesives you should be better armed to select the
proper combination of materials to know what to do.
END
For more articles
on design see the Design Series under Articles by Subject.
Additional
information on mounting basics is found in The Mounting and Laminating
Handbook, Second Edition, 2002, and The Mounting And
Laminating Handbook, Third Edition, 2008. Creative Mounting,
Wrapping, And Laminating, 2000 will teach you everything you need
to know about getting the most from your dry mount equipment and materials as
an innovative frame designer. All books are available from Designs Ink
Publishing through this website.
For live
consultations with Chris Paschke, CPF GCF call Designs Ink, 661.821.2188. A
flat fee of $25 will be charged for each new technical problem. Unlimited calls
or emails are allowed for each established mounting problem.
Chris A Paschke,
CPF GCF
Designs Ink
Designs Ink
Publishing
785 Tucker Road,
Suite G-183
Tehachapi,
CA 93561
661.821.2188
info@designsinkart.com