Tack is a measure of how ‘sticky’ a substance is [28]. It is the property of a material that enables it to form a bond immediately on contact with another surface, which may be an adherend or another layer of adhesive, under light pressure. Although not all adhesives (e.g. film epoxies) are tacky at room temperature, tack is of great relevance to industries such as packaging and footwear where rapid processing is essential. Control of tack is important so that the right level of initial bond strength is achieved. If the tack is too low then the bond may rupture during manufacture or if the tack is too high then repositioning of substrates may be difficult.
Tack, normally determined from the force required to separate two components, is often measured as part of the quality control procedures of an adhesive manufacturer or user. The simplest ‘instrument’ for determining if something is tacky is the thumb. Obviously, this is rather subjective and, depending on the adhesive or processing conditions, is potentially hazardous to the operator. The measurement of tack is normally specific to a particular application. Different measurement standards and methods have evolved within industries [28]. Tack is not a fundamental material property, but depends on a wide range of factors including the method of testing. Therefore, the results from different tests are unlikely to be directly comparable.
To achieve high tack, the adhesive needs to wet the surface and spread to form a continuous interface between the surfaces. High wetting energies are associated with strong adhesion to the surface. High tack forces on separation are associated with effective wetting of the surface. Thus, tack gives an indication of how quickly an adhesive can wet and make intimate contact with a surface. The adhesive must fully wet the two surfaces to form a continuous film to achieve maximum tack strength. Therefore, the tack will depend on the pressure applied, the time history of the applied pressure and on the flow (or rheological) properties of the adhesive. These in turn imply that the rate at which the surfaces are brought together and separated will influence the measured tack force. Since the properties of the adhesive, substrate or interface may change with time, the dwell time will also be important. Results from different tack test measurements are rarely comparable [28].
The most widely used methods for measuring tack are:
Loop tack testing involves lowering a loop of tape being tested on to a rigid plate until it makes contact over a known area. The direction of travel is then reversed and the force required to separate the loop from the plate is measured. Depending on the method used, the adhesive can be applied to either the tape [29, 30] or the plate [31].
A widely used tack test method in the UK is FINAT Test Method Number 9 (FTM9) [30]. The loop is made from a strip of tape 25 mm wide and at least 175 mm long. The plate is made of “Float Process” glass 25 mm wide, so that the area of contact is 25 mm square. A tensile tester or similar machine is used. The crosshead speed setting is 300 mm per minute.
The probe tack test is based on an ASTM Standard [32]. The equipment was initially developed in the 1950s, and is commercially available [33]. The tape specimen is applied to an annular weight (to control force), and positioned above a cylindrical probe. The probe is raised to make contact with the specimen, held for a fixed time, and then the force required to cause separation is measured. The machine enables systematic studies of tack to be performed. Tack can be studied as a function of contact pressure, dwell time, probe speed and probe material.
The rolling ball method [34–36] is simple to carry out and the equipment is inexpensive. A ball is rolled down an incline on to the tape being tested. Tack is related to the inverse of the distance rolled before the tape stops the ball. The method becomes less accurate at longer travel. Choosing different incline angles, incline heights or type of ball, can vary the test to suit the type of adhesive being tested. However, results from different test equipment will not be comparable. The rolling ball test is intended primarily as a comparative method for quality control.
The backing material of the tape or the thickness of the adhesive layer can significantly influence the results from this test. A thick, ‘soft’ backing or a thick layer of adhesive will absorb a significant proportion of the kinetic energy of the ball and thus bring it to a stop earlier. Where the adhesive coating is thin, the area of contact between the ball and adhesive will be small (as the ball will not sink as deep) and the stopping distance will be longer.
Quick stick tests [37–39] are similar in many ways to tack tests. They characterise the ability of an adhesive to adhere instantly to a surface through the resistance to peel tape at a 90° angle from a standard surface. Tests are carried out in a tensile test machine using a special fixture to assure that the edge of the peel is always directly beneath the grips. The initial 25 mm of peel are ignored and the quick stick is measured from the force required to peel the remainder of the tape. Some quick stick tests specify pressing on the tapes with a standard 25 g roller and in all tests the dwell time between application and removal will be longer than those commonly employed in tack tests.
Next: Wet Strength
