Multifunctional propyleneimines-new generation of crosslinkers for ...

504
ARTICLE IN PRESS
Z. Czech / International Journal of Adhesion & Adhesives 24 (2004) 503–511
properties of the PSA coat, such as tack and peel
adhesion decrease (Fig. 1).
The time span, which can be utilized after the
crosslinking agent containing PSAs is set up, is called
pot life. After a second time span the so-called gel point
is reached, and the PSA can no longer be modified. This
process is illustrated below (Fig. 2).
The curve in general applies to each crosslinking
agent containing PSA system. That means that a fast
crosslinking systemhas actually a short pot life, and a
slow crosslinking systemhas a long pot life. Fast
crosslinking PSA systems for industrial use with short
pot life are often used with two-component applicationequipment,
which mix and add the reactive components
right before the coating [4].
In order to develope the solvent-based PSA crosslinking,
various multifunctional chemical compounds
with respectively reactive groups are used as the
following list illustrates:
* metal acidesters [5],
* metal chelates [6],
* metal salts [7],
tack, adhesion
25
20
15
10
5
0
tack
cohesion
crosslinker amount
adhesion
Fig. 1. Crosslinkage density influence on tack, peel, adhesion and
cohesion [2].
viscosity (Pas)
12
10
8
6
4
2
coating level
pot life
gel point
0
0 1 2 3 4 5 6 7
time (h)
Fig. 2. Viscosity dependence of time by crosslinker containing PSAs
[3].
cohesion
* polyfunctional isocyanates [8],
* organofunctional silanes [9],
* polycarbodiimides [10],
* polyfunctional ethyleneimines [11],
* polyfunctional propyleneimines [12],
* amino resins [13],
* epoxy resins [14].
1.1. Multifunctional propyleneimine derivatives
In order to achieve optimum crosslinking efficiency,
the known terrain of crosslinkers was widened by the
introduction of a new generation of crosslinkers, multifunctional
propyleneimines derived from 2-methylaziridine
containing saturated unstable rings with nitrogen
atoms.
Such compounds react with the carboxyl groups or
isocyanate groups of the polymer by opening the ring.
This classifies themas crosslinkers.
The crosslinking of carboxyl group containing pressure
sensitive adhesives is carried out under room
temperature, an increase of temperature accelerates the
crosslinking process. Because of the high reactivity of
polyaziridines the pot life of propyleneimine containing
PSA formulations is quite limited and lies in the range of
2– 8 h which is lower than the pot life of polyisocyanate
containing adhesives. This problemcan be overcome by
mixing the adhvesive mass and the crosslinker directly
prior to the coating step. In comparison to metal acid
esters or metal chelates the crosslinking yield of multifunctional
propyleneimines is so high that no afterreactions
like brittleness or a decline of peel resistance
due to ongoing crosslinking occur.
1.2. Reaction mechanism of crosslinking
The crosslinking of pressure sensitive adhesives with
multifunctional propyleneimines is mainly based on the
carboxyl groups offered by the vinylcarbonic acids
within the polymeric chain. The oxygen of the nucleophilic
carboxyl group causes the opening of the tense
propyleneimine rings while the hydrogen atoms accompanying
the carboxyl groups protonate the nitrogen
atoms (Fig. 3).
Such a crosslinking variation involves the following
consideration (Fig. 4).
Concidering the carbon-cation-structure the variant
2 seems to be the most realistic path backing the
reaction mechanism shown above.
General multifunctional propyleneimine groups containing
compounds are known for over 35 years. Along
with the progress in the field of organic synthesis the
chemical structure of propyleneimine crosslinkers was
adapted to the developments in polymer chemistry. The
application of such crosslinkers is product oriented and
only mentioned for special formulations. In the early