Pressure sensitive adhesives are a unique
form of adhesive that are permanently tacky at room temperature.
They adhere to substrates under pressure via polar attractive
forces rather than forming chemical bonds. The two most common
types are rubber and acrylic based. The rubber based adhesive
is more economic, but lacks the physical and chemical stability
of the acrylic based adhesive. There
are three distinct ways of applying PSAs:
• In solvent –
the adhesive ingredients are polymerized in solvent, then
cast onto the web
• As hot melt - hot
melt is coated onto the web then cooled before/during wind-up
• Emulsion based –
the adhesive ingredients are polymerized in water, cast on
the web,
and dried
PSAs can be single coated, double
coated, pattern coated and self wound:
Single coatings are bonded to one surface, referred to
as the face stock, typically film, PE foam, paper, tissue
or non-woven. A release liner, silicone coated, PET ,PE
or poly coated Kraft is applied to protect the adhesive
layer.
Double coatings are used to bond two substrates, different
PSAs can be used on each side of a carrier (typically plastic
film, tissue or non-woven material), the carrier add strength
to the adhesive configuration.
Transfer tape is unsupported adhesive film, it is coated
onto a release liner. Used in medical packaging, it is frequently
pattern coated.
Self wound systems comprise face stock coated with PSA
on one side, and silicone release on the other. Examples
include diaper tapes and short roll lengths of tape.
PSAs are used in the manufacture of specialty tapes, medical
packaging, surgical drapes and laminated film.
Measurement of coat weight is
important for 2 main reasons:
1. Adhesive coating needs to be uniform in the area to
which it is applied, to provide a high coefficient of adhesion
between the two substrates.
2. To minimize usage of costly acrylic, the aim is to apply
the minimum quantity of coat weight in order to achieve
adhesion.
Manufacturing Process
This varies according to the product being manufactured, but
essentially the solvent based and aqueous based adhesives
are applied through an excess application system such as a
meyer bar coater, or a metered system, such as a transfer
coater or gravure roller, hot melts are applied via an extrusion
coater.
Manufacturing Process
The possibilities vary according to the process, but if the
PSA is applied in aqueous soln, the most accurate measurement
is made wet-end, measuring water and inferring dry coat weight
from the solids ratio %.
Dry end measurements are made on hot melt PSAs, and on solvent
based systems to avoid the need for Nema 4 sensor housing
for the latter.
Solvent wet end is feasible, especially if coat weight is
low, but solvent ratios have to be maintained, and sampling
errors can be significant if not carefully controlled. |
