Page 100 - Handbook of Adhesion Promoters

6

Polymer Modification

to Improve Adhesion

6.1 SELECTION OF CO-MONOMERS

The effect of the polar nature and the concentration of comonomer on tack, adhesion, and
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cohesion of pressure-sensitive adhesives has been studied. With this objective in mind,
the co- and ter-polymerizations of n-butyl acrylate with 2-hydroxypropyl methacrylate, 2-
hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and acrylic acid were carried out. 1
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Adhesion properties of tapes obtained from the co- and ter-polymers were evaluated. The
n-butyl acrylate-acrylic acid copolymer had greater cohesive and adhesive strength than
poly(n-butyl acrylate) due to the intermolecular dipole-dipole interactions with the partic-
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ipation of −COOH groups. Also, hydroxyl groups improved adhesive properties of tapes
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indicating the participation of hydrogen bonding. The secondary hydroxyl groups had
weaker intermolecular interactions and, thus they did not improve performance index
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(composite index based on adhesion, tack, and cohesion). Terpolymers showed higher
performance index than the copolymers most likely because of the increase in cohesion
and adhesion in the terpolymer due to the preferential intermolecular hydrogen bonding
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involving the −OH and −COOH groups.
Three monomers were used with the following results: 2-ethyl hexyl acrylate (pro-
vides tack and flexibility of the adhesive), acrylic acid (increases the T , enhances cohe-
g
sion and improves adhesion through hydrogen bonds) and t-butyl acrylate (increases the
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T , reduces the prepolymer viscosity). The peel values increase with increasing acrylic
g
2
acid amount and reach the maximum value at 15 wt% of acrylic acid.
The influence of the incorporation of hydroxyethyl acrylate, methyl methacrylate,
and acrylic acid, on the adhesion of statistical, uncrosslinked butyl acrylate-methyl acry-
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late copolymer to steel has been studied. The increasing polarity of comonomer leads to
an increase in shear modulus and consequently, to an increase in the stress peak, deforma-
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tion at the break, and tack values.
A diblock copolymer composed of poly(acrylic acid) and poly(n-butyl acrylate)
blocks was synthesized using reversible addition-fragmentation chain transfer polymeriza-
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tion and adsorbed onto soft acrylic latex particles prior to their film formation. When the
film formation occurs at pH=10, the ionomeric crosslinking occurs, and high tack adhe-
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sion is obtained in combination with high creep resistance. The illustration of carboxylic
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group interaction is given in Figure 6.1. At low pH, the neutral poly(acrylic acid) block is
not extended (it is collapsed on the particle surface because of its poor water solubility,
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Figure 6.1a). The hydrogen bonding interactions between poly(acrylic acid) blocks