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One performance requirement of paste adhesives is often that they resist ‘slump’ or ‘sag’ (i.e. once applied they should retain their position and shape under their own weight—in the paint business this would be referred to as non-drip). Many adhesives available on the market are described as ‘low-slump’ or ‘non-slump’ systems although there are no precise definitions.
There are several standard tests (see [26, 27]) for characterising slump behaviour. These are subjective, assessing:
In rheological terms, slump resistance requires a high yield (or pseudo-yield) stress and low creep rates. The visco-elastic nature of polymers means that the response of an adhesive paste to an impulse, such as an applied deformation or pressure, will depend on the rate of application and the duration of the impulse. Thus, an adhesive with a high yield stress may flow or ‘slump’ given sufficient time.
Slump behaviour can be studied through creep measurements of the long-term flow of the paste under small but constant stress. Controlled stress rheometers are capable of applying constant loads needed for creep measurements on pastes or soft solids and such instruments are capable of measuring displacement to the high resolution needed to resolve low creep rates. Although paste adhesives can be fully characterised using modern rheological instruments as described there are no standards for determining slump performance using these tests, partly as they require expensive instruments and trained operators.
Slump resistant adhesives often take advantage of thixotropy—“a decrease of viscosity under constant shear stress or shear rate, followed by a time dependant recovery when the shear is removed.” Thisis the property that makes the viscosity of a liquid change with time and may be responsible for flow related problems such as sagging, sedimentation and pipeline blockages. Thixotropy is often confused with shear thinning or pseudo-plasticity. Shear thinning materials show a decrease in viscosity under increasing shear loads, but they do not show a time dependant recovery of viscosity unless they are also thixotropic. Hence a thixotropic material is shear thinning, but a shear thinning material is not necessarily thixotropic. In adhesive use, thixotropy can be a benefit. The lowering of viscosity as the adhesive is strained will aid spreading but the subsequent recovery in structure and thickness will help retain adhesive where it is desired.
Thixotropy can be characterised using rotational rheometers. However, many industries have developed their own “home made” tests to characterise thixotropy. For example, in the ketchup industry some companies characterise their materials by loading the sample into a small reservoir and allowing it to flow down a graded ramp, measuring the distance that the sample flows. This can be done after different intervals between ‘shaking’ the ketchup and opening the reservoir in order to study recovery. The resulting number is related to both the viscosity and the thixotropy of the sample.
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