Rheological parameters are the 'invisible hand' in adhesive (especially pressure-sensitive and structural adhesives) R&D—they determine the feel during application, initial tack, holding power, and sag resistance of adhesives. But for engineers without a background in rheology, concepts like viscoelasticity, thixotropy, and yield stress can often be abstract and hard to grasp.

Viscoelasticity—adhesives are both liquid and solid. All polymer adhesives are viscoelastic materials: they flow like a liquid under slow deformation and respond elastically like a solid under rapid deformation. Oscillation tests on a rotational rheometer can quantitatively separate these behaviors: the storage modulus G' represents the elastic part, while the loss modulus G'' represents the viscous part. For pressure-sensitive adhesives, the Dahlquist criterion is directly related to G': only when G' is below 3×10⁵ Pa does it have enough wetting ability to develop initial tack.

Thixotropy—thin when applied, thick when left alone. Thixotropy is the property of an adhesive to decrease in viscosity under shear and recover viscosity when at rest. This is crucial for coating processes: the shear from the coating roller thins the adhesive for easy application, and once on the substrate, the viscosity recovers to prevent sagging. Rheometers can quantify this property using a 'thixotropy loop' test.

Yield stress—you need some force to make it flow. Yield stress is the minimum stress required for an adhesive to start flowing. Adhesives with yield stress behave like solids when at rest (no sag), but flow once the applied stress exceeds the yield value (so they can be spread).

Dehuisci (www.dehuisci.com) distributes TA Instruments rotational rheometers and can provide rheology method development services for adhesive customers. Phone: 0512-67071902.