UV Curing of Styrenic Block Copolymer Based Pressure Sensitive Adhesives

Styrenic block copolymers (SBC) are widely
used in a variety of pressure sensitive adhesive
(PSA) applications both in tapes and labels.
The most commonly used styrenic block
copolymers for adhesive applications are tnblocks
containing an elastomeric mid-block and
two thermoplastic end-blocks (Figure 1). The
nature of the elastomenic mid-block can be
polybutadiene for the SBS series, polyisoprene
for the SIS series, polyethylene/butylene for
the SEBS series and polyethylene/propylene for
the SEP series of copolymers. SBC's combine
the properties of both thermoplastic materials
and elastomers and are also known as
Thermoplastic Elastomers (TPE).

The elastomeric mid-block and the polystyrene
(PS) end-block being specially designed to be
thermodynamically incompatible, tend to
segregate to form a two-phase morphology
(Figure 2). The styrenic domains act as
physical crosslinks in a three-dimensional
network. Above its glass transition
temperature ), the polystyrene starts to
soften, the domains lose their cohesion and
the polymer begins to flow. This phenomenon
is totally reversible; when the polymer melt is
cooled down, the polystyrene blocks resolidify
into domains and recover their initial strength.
A similar behaviour is observed when the
adhesives are compounded from solvent.

Considering the above, it is not surprising that
 SBC based adhesives are unsuitable for use in PSA's
which must withstand solvents or exposure to high
service temperatures, i.e., temperatures near
or above the Tg of the polystyrene end-blocks.