Shear adhesion, or holding power, is one of the most important pressure sensitive adhesives (PSA) performances. Shear adhesion is the ability of a tape to resist the static forces applied in the same plane as the backing. It is usually expressed in the time required for a given weight to cause a given amount of tape to come loose from a vertical panel.
What is the determining molecular parameter that can significantly affect shear adhesion? Most adhesion researchers propose that Me (Molecular weight between two entanglements) is the controlling factor of shear adhesion. In practice, it is found that all the following parameters can affect shear adhesion to a certain extent.
1) Tg of adhesive
Shear adhesion increases as the Tg rises to a certain temperature lower than room temperature. According to rheological evaluation, Tan delta value at the test temperature- room temperature, rises as Tg increases. The higher the Tan delta value; the better the flow or wetting of the tested adhesive. However, when Tg is too high, the G’ will rise abruptly at glass transition zone. As a result, under light pressure, an adhesive may lose a certain degree of contact area to the bonding substrate and result in lower shear adhesion. More fundamental studies are needed to clearly verify this factor and precisely locate the Tg at which shear adhesion is the maximum.
As illustrated above, the relationship between Tg and test temperature is rather critical to locate the maximum shear adhesion. Experimentally, if a series of adhesive formulations can be designed to exhibit the same Gn0 but different Tg simply, perhaps, the formulation imparting the highest shear adhesion can be defined by a fixed temperature difference (ÄT) between the Tg and test temperature.
2) Tan delta minimum value of the adhesive
Empirically, it is found that shear adhesion improves as the Tan delta minimum value (at rubbery plateau zone) lowers to a certain value. This is because the action of disentanglement of molecular chains is easier to proceed for those adhesives exhibiting higher Tan delta values. However, the shear force is decreased again when the tan delta minimum is lower than a certain value. The adhesive becomes very elastic and loses the capability of dissipating shear stress within molecular chains by gradual disentanglement. As a result, the adhesive tends to delaminate from the bonded substrate more easily accompanied with an adhesive failure mode (no adhesive residues on a bonded surface) instead of presenting a cohesive failure mode. There may be a critical Tan delta minimum value that indicates the fracture mode change from cohesive failure to adhesive failure.
3) Rubbery plateau modulus
According to the most adhesion research findings, Me value is the most important parameter affecting shear adhesion. This is because the energy dissipation between entanglements will be higher for those having greater Me and therefore impart higher shear adhesion. This hypothesis may be based on those adhesives having very similar Tg and Tan delta minimum values. According to rheological measurements, when an adhesive possesses a greater Me, it will exhibit higher storage modus. Accordingly, for the same SBC (Styrenic Block Copolymer)-based formulations, if we simply select different SBCs having different styrene content and the same percentage of di-block, we may easily obtain different levels of rubbery plateau modulus (Gn0) without affecting Tg and Tan delta minimum. Through this series of formulations, we may be able to discover the highest shear adhesion which may present a certain value of Gn0.
4) Adhesive Polarity
Adhesive polarity is also an important factor which can affect all adhesion performances, such as tack, peel, and shear. Theoretically, all adhesion performances are improved as more polar ingredients are included in the formulation. This is because the polarity of adhesive can improve physical adsorption between adhesive and bonded substrates. Certainly, the incorporated polar ingredient will also change the rheological properties of adhesive. A detailed study is needed to verify the degree of contribution from adhesive polarity versus rheological properties.
5) Curing of adhesive
Many cured solvent and water-based PSAs exhibit very high shear adhesion disregarding Tg, Tan delta minimum value, and Gn0. Obviously, the infinite molecular weight of cured adhesives also affects shear adhesion significantly. However, this factor can not be applied to conventional SBC-based HMPSAs which are not curable. A separate study for curable adhesives is needed.
The behavior of shear adhesion is rather complicated. No simple molecular parameter can be confirmed as the only controlling factor of shear adhesion. A series of detailed fundamental studies are required in order to clearly understand the actual cause or contribution of the parameters described above.
For more information call or email Pierce Covert,
Glue Machinery Corporation