Abstract
Pressure-sensitive adhesives (PSAs) adhere to nearly any surface under the application
of light pressure, with applications including tapes, labels, and aircraft joints.
Understanding surfactant and colloidal particle depth distribution during and after
the drying of waterborne PSAs is key to controlling adhesive properties.
Surfactant can have negative effects on appearance, wetting and adhesion, so there
is a desire to reduce them on PSA surfaces. I investigated the effects of particle
deformation and evaporation rate on surfactant stratification using a combination of
ion beam analysis and atomic force microscopy. Surfactant stratified at the surface
in layers ca. 150 nm thick for films with particles partially deformed via capillary
action. Stratification in this regime increased with evaporation rate. Films of nondeformable
or coalesced particles showed very little surfactant stratification, with
a coalesced particle barrier inhibiting stratification, especially at faster evaporation
rates.
Additionally, I investigated the effect of colloidal stratification on adhesion. To
achieve good adhesion of PSAs, there needs to be a balance of polymer viscosity
and elasticity. There is evidence in the literature that two-layered PSAs can lead to
superior properties, achieved through successive depositions. Unfortunately, this
process can be time consuming and expensive for manufacturers. To achieve a layered
structure in a single step, I stratified mixtures of adhesive particles and smaller
poly(butyl acrylate) particles in a colloidal dispersion. During film formation, small
particles stratified at the top surface, with layers up to 1 μm thick, according to ion
beam analysis. Stratification increased with evaporation rate, with films dried slowest
exhibiting no stratification. Adhesive properties were shown to transition from
brittle and solid-like to viscoelastic and tacky when elastic particles stratified at the
surface.
The ability to exert control over surfactant and particle stratification in PSAs will aid
in the design of new high-performance, multi-layered films, whilst avoiding detrimental
effects from surface stratified surfactant.