(717i) Pressure-Sensitive Adhesives Based on Strain-Activated Crosslinking of Functional Groups

Pressure
sensitive adhesives (PSAs) bond materials during the application of physical
pressure. PSAs are widely applied in forms such as self-adhesive tapes, labels
and marking films, medical plasters and pads, dermal pharmaceutical dosage
forms, drapes, and biomedical electrodes. Conventionally, PSAs are manufactured
via solution casting or extrusion coating of low viscosity un-crosslinked
monomers or polymers onto a substrate, followed by post-crosslinking processes
such as on-web polymerization. These post-crosslinking processes typically
involve ultraviolet (UV) irradiation or heat induction [1], are capital-intensive,
are limited to environments with specialized equipment, and can produce
undesired breakdown products.

In response, we
have created novel crosslinking strategies that have the potential to greatly
simplify fabrication of PSAs. We have developed a method to cure crosslinkable
polymers by applying shear or compression, either during or immediately after
the adhesive coating process. Our polymer system comprised of a soluble and
flowable polymer mixture containing 2-(methacryloyloxy)ethyl acetoacetate
(AAEM) and aminoethyl methacrylate (AEMA). This polymer mixture is applied to a
substrate as a low-viscosity solution, and crosslinks spontaneously after
application of shear (Figure 1). This “force-sensitive” crosslinking method
allows us to create PSAs without UV light or heating, thus eliminating the need
for post-crosslinking processes. The dynamic moduli and viscosity of the
polymers increased with applied stress; in particular, up to 6 orders of
magnitude (OM) for storage modulus, 4 OM for loss modulus, and 5 OM for
viscosity within the operating shear strain of 1-20% and at constant shear rate
of 1 Hz (Figure 1). We have also formulated a mechanism to alter the
force-sensitive regime of the polymer network by adding in “molecular
shielders” (Figure 2). Non-reactive high molecular weight co-monomers were
grafted onto the polymers, significantly reducing the force sensitivity of the
PSA, and supporting our hypothesis that force is accelerating the crosslinking
of our polymers through forced physical interactions of the reactive end-groups.
In my presentation, I will also discuss other mechanisms to apply force to
these PSAs, such as thin diameter shear, shaking, and ultrasound. Furthermore,
I will discuss new mechanisms for molecular shield design, such as solvent quality.
In sum, our process of PSA processing eliminates the formation of undesired side-products,
and allows the formation of adhesive layers on temperature-sensitive
substrates. In addition to process simplification, the development of
strain-responsive adhesives would enable the use of UV-opaque adhesive
formulations and facilitates the incorporation of adhesives into porous or
complex substrates.

Figure 1. Change in
storage modulus, loss modulus, and viscosity under shearing of varying
concentration (mol%) of functional crosslinking sites. Oscillatory time sweep
tests were performed using a parallel plate rheometer (TA Instruments, model AR
2000) at 1000 µm gap, 1% shear strain, and 1 Hz shear rate
for a total of 28 h (10 h shearing, followed by 4 h pausing and another 14 h
shearing). Under shearing, polymers with higher functional groups resulted in
higher dynamic moduli and viscosity within shorter periods of time.

Figure 2. Change in
storage modulus, loss modulus, and viscosity under shearing of varying
concentration (mol%) of functional crosslinking sites, each with (light) or
without (dark) the presence of shielding groups (high molecular weight
co-monomer). Oscillatory time sweep tests were performed under similar
conditions as described in Fig. 1 except for 10% shear strain. Insignificant
increases in dynamic moduli and viscosity were observed in shielded polymers
which provided ‘screening’ of inter-chain crosslinking.

[1] Satas, D., Handbook of
pressure sensitive adhesive technology, 3^rd Ed. Springer. 1999.

I believe I will
benefit from the expertise in materials at this conference to deepen my
understanding of the role of material design in various industrial and medical
applications. I look forward to the opportunity to contribute my research results
and learn from peers and leaders in the field.