Page 95 - Handbook of Surface Improvement and Modification

90 Tackifiers

A pressure-sensitive adhesive composition is produced from a first polymer that is
acrylate polymer or copolymer comprising polymerized monomers selected from acrylic
acid, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, octyl acrylate, styrene,
and vinyl acetate, a second polymer selected from the group consisting of ethylene-acrylic
acid copolymer, oxidized polyethylene, oxidized ethylene-vinyl acetate copolymer, and
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maleated polyolefin. The composition is water-based and adapted for pressure-sensitive
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adhesion to substrates. The adhesive composition may include a tackifier resin selected
from a group consisting of rosin ester resin, rosin acid resin, synthetic hydrocarbon resin,
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synthetic terpenic resin and their combinations. The acrylate polymers are commonly
used to improve cohesion of the pressure-sensitive adhesives and their resistance to
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shear. Tackifiers, on the other hand, are commonly used to improve adhesion. The term
tackifier refers to a low molecular weight resin with a molecular weight in the range
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between about 270 and 1400. Most commercially available tackifiers have a molecular
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weight that is in the range between about 800 and 1200.
Amorphous poly-α-olefins (APAO) are produced by the copolymerization of α-ole-
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fins, e.g., propylene or 1-butane in the presence of Ziegler-Natta catalysts. The copoly-
mers have an amorphous structure which makes them useful for the production of hot-
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melt adhesives, either in neat or formulated form. Ethylene-co-propylene-co-butene-1
terpolymers, made either with or without an in-reactor-added organosilicone external
donor, were used in the formulation of improved-performance, APAO-based, hot-melt
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adhesives. A partially or fully hydrogenated synthetic hydrocarbon was used as tacki-
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fier. The tackifier was grafted with maleic anhydride. The adhesive also contained
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polyethylene or metallocene wax. The externally-added organosilicone donor was either
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cyclohexylmethyl dimethoxy silane or phenyl triethoxysilane. The inclusion of organo-
silicone imparts some unique characteristics to the physical and mechanical properties of
the terpolymer, not achievable without the presence of the donor in the reaction medium,
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properties that may favorably affect the final performance of the hot-melt adhesive.
An adhesive contains an isobutylene copolymer having pendant anhydride groups, a
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polyamine photobase generator, a tackifier. The pressure-sensitive adhesive has an
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exceptional adhesion to low surface-energy substrates. Polyisobutylene is an attractive
material for low surface energy bonding applications due to its excellent adhering proper-
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ties to olefin-based thermoplastics. Also, the excellent moisture and oxygen barrier
properties of PIB suggest that PIB-based materials have potential use in electronics and
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photovoltaic encapsulation applications. Conventional tackified pressure-sensitive adhe-
sives can also appear cloudy, demonstrating a loss of the characteristic transparency found
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in many conventional pressure-sensitive adhesive compositions.
The cloudiness is an indication of limited or incomplete compatibility of the tackifier
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and the polymers. The reduced compatibility may lead to a degradation of adhesive
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properties on aging, as evidenced by a loss of tack or reduced peel adhesion. In some
cases, the addition of a tackifier to an adhesive composition can be clear and appear to be
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compatible. However, after removing the solvent, curing the adhesive, or on aging, the
adhesive may become cloudy, indicating some incompatibility between the tackifier and
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acrylic base polymer. The tackifier is preferably selected from a material that is essen-
tially free of any ethylenically or acetylenically unsaturated bonds, such has hydrogenated
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rosin resins.