Pressure sensitive adhesives criteria

The dynamic shear modulus of pressure-sensitive adhesives at one second must be less than 3 x 10 dyn/cm, known as the Dahlquist criterion (92,96,97). The Dahlquist criterion is best met by employing a tackifier on an adhesive at 40 to 70°C above Tg. 

For example, Christenson et al. [3,19] performed a detailed study of polyisobutylene-based pressure-sensitive adhesives. Although these authors did not postulate a specific detachment criterion, they did extensive work characterizing the linear viscoelastic properties, the tensile stress-strain properties, and the peel force. In addition, they conducted detailed visualization of the deformation of the adhesive during peel and therefore, could assess the ability to predict the peel force from the mechanical properties of the adhesive and the visually observed detachment strain. In this work, the adhesive consisted of a blend of high and low molecular weight polyisobutylene. They showed that when they used the Giesekus model as the constitutive equation for the adhesive, they could accurately describe the stress-strain curves of the adhesive and the peel force was well predicted by the integral of the stress-strain curve up to the measured detachment strain. Their results are summarized in Table 1. 

In contrast to common rubbers, the entanglement molecular weight. Me, of most acrylics is sufficiently high that without dilution they meet the Dahlquist criterion discussed earlier (G <3x 10 dynes/cm, 10 Pa) and exhibit tacL With little or no cross-finking the neat polymers form a pressure-sensitive adhesive with some cohesion. To obtain adequate cohesion it is common to use polar monomers which can form hydrogen bonds, and usually also some level of cross-linking. (Acrylics are also discussed in O Chap. 14.)... 

An interesting observation is that, while geckos and some insects have adopted hairy tissues for robust and reversible adhesion, some other insects and tree frogs seem to have achieved this via smooth tissues. The main similarity of both designs is that the structured pad surfaces or particular properties of the pad materials guarantee a maximum real contact area with diverse substrates. Dahlquisfs criterion, which is based on empirical observations of pressure-sensitive adhesives, establishes an upper limit for Young s modulus ( 100 kPa) of materials with tack... 

After some early uncertainty in the literature about the nature of the pressure sensitive bond, Dahlquist [5,6] related modulus data to tack-temperature studies and observed that the compression modulus of the adhesive had to be less than about 3 X 10 dyne/cm (3 x lO Pa) before any adhesive tack was observed. This was explained as the highest modulus that still allowed the adhesive to be sufficiently compliant to wet out or come into molecular contact with the substrate and form dispersive bonds. As other investigators [7-9] accepted this requirement it was termed the Dahlquist Criterion . 

A second general criterion for pressure sensitivity is that the glass transition temperature of the adhesive be below the use temperature, which is usually room temperature. Broadly speaking, the To will be about 30-70°C below room temperature, depending on the base polymer and any added modifiers. 

Two sided pressure-sensitive tapes, as well as most PS labels, are interlined with release paper to prevent them from sticking together prior to use. The adhesive or antistick property requires that the release web have a surface energy significantly lower than the surface tension of the masscoat. Silicone coatings meet this criterion. 

Acrylic pressure-sensitive polymers consist mainly of a soft monomer with a low Tg and high Me. The three most common soft monomers are butyl acrylate, iso-octyl acrylate, and 2-ethylhexyl acrylate. All three have a Tg below —40°C and an Me above 15,000 Da see O Table 15.2. When polymerized on their own they meet the Dahlquist criterion at room temperature and well below. To increase peel, often a hard monomer is incorporated. These are high Tg monomers, and most are low Me. By moving up the Tg of the overall polymer, these hard monomers increase the viscous loss and thus the peel force, in much the same way as increasing the rate of peel. In addition, the lower Me hard monomers, such as MMA also increase the stiffness of the adhesive which further increases the peel force. Polar monomers are invariably incorporated to increase cohesive strength through intermolecular hydrogen... 

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