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Thixotropy, Creep, Sag and Slump
Tack
Loop Tack
Probe Tack
Rolling Ball
Quick Stick
Wet Strength
Wettability (Surface Energy/Tension)
Adhesives are usually supplied as liquids—that can vary in form between thin, watery fluids to thick, viscous pastes. Low viscosity adhesives are used in applications where the adhesive is required to flow easily (e.g. thread sealing where penetration into small gaps is needed). High viscosity adhesives are used in large gap joining and sealing operations where adhesive is required to remain in position after application. The flow or rheological characteristics control the dispensing, spreading, penetration and gap filling behaviour of the adhesive. This is often critical in adhesive selection and formulation of new products. It is also important for quality control (e.g. to test batch-to-batch variation or for shelf life). There are many methods for studying the rheology of adhesives ranging from simple tests providing qualitative properties to sophisticated techniques able to provide multi-dimensional data (covering rate, time, temperature and stress) required for flow process modelling [1, 2]. This section is concerned with rheological measurement methods for uncured adhesives, which range from thin fluids to visco-elastic pastes. Selection of the appropriate technique depends on the properties of the material being tested and the type of data required.
Rheological measurements are used to determine the behaviour of non-Newtonian liquids in simple flow situations using suitable material functions. The measurements provide correlations between molecular structure and material properties, and between material properties and behaviour in practical situations. These may also be used to predict material behaviour in complex flow situations. This requires sophisticated mathematical treatments using data obtained from simple rheological experiments. Flow regimes for visco-elastic fluids can be categorised as shear (where layers of fluid slide past each other) or extensional (where ‘slugs’ of fluid are drawn in tension). Most rheological measurement methods work in shear flow, but some techniques have been developed for extensional flow.
Viscosity and rheological measurements can be divided into three classes:
Steady Flow: For example, flow cup [3], tube [4–6] and capillary[7] viscometry, falling ball viscometry [8, 9] and rotational rheometry[10–12] techniques.
Oscillatory Testing: Specimen is subjected to an oscillating force or deformation. Tests can be performed using oscillatory rheometers [13] or dynamic mechanical testing[14] depending on the physical nature of the material.
Transient Condition Testing: The variation of deformation with time after a change, usually sudden, of applied stress or the variation in stress after a deformation [15]. The magnitude of the resultant deformation or stress, respectively, is dependent upon the visco-elastic properties of the test sample. These tests can be carried out on some equipment used for steady shear flow.
Properties of adhesives are likely to be sensitive to rate and temperature, and hence when selecting a rheological technique, consideration must be given to how rate and temperature are to be controlled/monitored and how the data relate to the application in question.
Standard flow characterisation methods for adhesives cover basic viscosity determination. ASTM D 1084 [16] describes four methods for characterising low viscosity or ‘free flowing’ adhesives. Brookfield viscosity results are the most commonly quoted viscosity measurements for adhesives.
Ford Flow Cup Method [3] determines Newtonian viscosities from the time for 50 ml of adhesive to flow from a vessel. Different orifice sizes are used to ensure test times are between 50 s and 100 s.
Brookfield Methods [10, 12] employ the principle of rotational viscometry; measuring viscosity by sensing the torque required to rotate a spindle at constant speed while immersed in the sample fluid—the resin in a liquid state. The torque is proportional to the viscous drag on the immersed spindle and thus to the viscosity of the fluid. Both Newtonian and non-Newtonian fluids can be measured. Most resins are non-Newtonian and the measured viscosity depends on the velocity gradient to which the product is subjected. For these viscometers, the velocity gradient is not the same for every point of the spindle, and hence the result is not strictly the ‘true velocity at a known velocity gradient’ and therefore is conventionally called ‘apparent viscosity.’ Different sizes of measurement geometries and instrument speed settings provide a range of shear rates for characterising the adhesive. These instruments cover viscosity ranges from 50 to 200,000 cP. Adhesive technical data sheets often provide a Brookfield viscosity value although this is normally a single point value.
Stormer Viscometer [17] uses a falling weight to rotate a paddle immersed in an adhesive. The time to produce 100 revolutions of the paddle is recorded. This method provides qualitative information on the adhesive viscosity for quality control purposes.
Zahn Viscosity Cups are designed to measure the time taken for a sample to flow through a calibrated orifice. The cup is immersed in the adhesive and withdrawn. The time between removing the cup from the reservoir until the flow breaks is measured and converted into a viscosity using conversion methods given in the standards.
Next: Viscometry
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