Page 83 - Handbook of Surface Improvement and Modification

78 Tackifiers

Figure 6.7. Schematic representations of the phase structure in the SIS/tackifier system. [Adapted, by permission,
from Sasaki, M; Fujita, K; Adachi, M; Fujii, S; Nakamura, Y; Urahama, Y, Int. J. Adh. Adh., 28, 372-81, 2008.]
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increase the autohesive tack of synthetic elastomers.
The inherent non-polar nature of the hydrocarbon resins restricts their utility in
applications where strong polar interactions are required between the joining surfaces.
Then, the polar tackifiers like rosins or phenolic resins are used. In some formulations,
blends of hydrocarbon resin and polar tackifiers are used to achieve a balance between the
compatibility and the adhesive performance. Grafting maleic anhydride onto the solid C5-
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aliphatic tackifier is another method of changing the polarity of tackifier. In addition to
polarity, other positive changes occur such as an increase in glass transition temperature
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and melting point (Figure 6.5). The glass transition temperature increases because chains
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have reduced mobility caused by the increased interaction of maleated species. Maleic
anhydride also acts as a physical barrier reducing oxygen diffusion which increases ther-
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mal stability of tackifier.
The tackifier based on a hydrogenated cycloaliphatic resin having poor compatibility
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with polystyrene significantly increased peel adhesion. Above 40 wt% tackifier gave the
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strongest adhesion. The nanometer-sized agglomerates of tackifier were increasing in
number with an increase in the tackifier content and the higher peel adhesion was obtained
when the larger amounts of agglomerates of tackifier were present in the polyisoprene
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matrix. The rosin phenolic resin tackifier had good compatibility with polystyrene, and
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therefore the peel adhesion increased effectively at lower tackifier content. Figure 6.6
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shows the chemical structure of tackifiers used in this study. Figure 6.7 shows phase
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structure in SIS/tackifier system. The molecules of hydrogenated cycloaliphatic tackifier
(polyisoprene compatible and polystyrene incompatible) dissolved well in the polyiso-
prene matrix, and the amount of dissolved tackifier increased with the tackifier content. 4