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What Adhesives Tell Forensic Scientists - and What that Can Tell Adhesive Formulators

You may have indirectly experienced the terror of being kidnapped and restrained by tape over the mouth, around the wrists and ankles through films and TV programs. When it happens in real-life, forensic scientists can use the pressure sensitive adhesives (PSA) found in the tape to gather clues. PSAs in drug packaging and explosive devices can also be analysed with techniques like atomic force microscopy (AFM) imaging and force mapping. In these studies, forensic scientists have learnt possibly even more about the structure of these adhesives than their manufacturers know. This begs the question: beyond helping solve crimes, can forensic science help make better adhesive products?

AFM uses a sharp probe tip at the end of a cantilever to scan specimen surfaces. The forces between the substrate and the tip deflect the cantilever according to Hooke's law, with the deflection measured by a laser. This can map the surface morphological and mechanical properties of the PSAs, and has been used to do so several times in the past¹. However, this year forensic scientists have conducted such an examination in an unprecedented level of detail.² They attached the non-adhesive side of one each of a cellulose, packaging, and electrical insulating tape, as well as three transparent OPP tapes to glass slides with double sided tapes. They then observed 5 µm x 5 µm patches of the adhesive at 30 nm resolution and assessed their roughness. The scientists also used the AFM to create force maps over similar size patches, this time taking measurements in 20 x 20 point grids. In this part of the experiment they measured maximum adhesive force of the particles forming the adhesive film to the probe tip (F_max), the maximum distance of deformation of these particles (d_max), and the adhesion energy (γ).

Figure 1: From the force on an atomic force microscope tip, and the distance the tip moves, the maximum adhesive force of the adhesive particles forming the film to the tip (Fmax), the maximum distance of deformation of these particles (dmax), and the adhesion energy (γ) can be measured

The first three tapes examined were those with clearly different appearances - and they also showed obvious distinctions on the nanometre-scale. Each adhesive film comprised of two phases - one hard or less energy dissipative and one soft, or more energy dissipative. In the cellophane tape, the scientists could distinguish individual softer adhesive polymer particles and harder molecules that they thought were possibly surfactants. By contrast, the adhesive molecules coalesced into a smoother film in the packaging adhesive, although surfactant or surfactant-enriched adhesive spots were still visible. The electrical insulation tape looked the least homogeneous, with the hard phase only present in very small amounts. Roughness measurements matched these morphological differences, with the electrical insulation tape being much rougher than the other two.

Figure 2: AFM roughness measurements indicate the morphology of adhesive films

Similarly, the three visually indistinguishable OPP tapes from different manufacturers all showed distinguishable morphologies under AFM imaging. They ranged from mainly homogeneous adhesive films with little surfactant, to films with very distinct hard phases, with the more homogeneous films again measured as less rough.

Force mapping showed that across each of the different tapes, hard phases had lower F_max, d_max and γ values. The softer, tackier, phases tended to demonstrate the kind of viscoelastic fibrillation expected with PSAs, while the harder phases deform very little. There was some variation in the hardness of each of the phases across the different adhesives, which was another factor in the difference between properties seen in different tapes. The forensic scientists attributed the presence of separate phases to poor compatibility between components used in the adhesive mixture. They noted the distribution patterns of hard and soft phases were different for each adhesive, providing another criteria for identification.

Typically PSAs are thought of as viscoelastic materials with properties somewhere between liquid and solid, depending upon speed of deformation. But what this means in practice is that their behavior varies on different length scales ranging from macro- to nanoscales. In particular, on the nanoscale it's clear that distinct phases exist with different adhesion force energy. The nanoscale phase distribution in various adhesives is observably different. This means that with AFM studies forensic scientists can "fingerprint" adhesives and, as these patterns are related to adhesives' performance, formulators can get clues too.

How do you think understanding the distribution of these nanoscale phases can help formulators?

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