Various standards have been devised within the adhesive industry – ASTM, FINAT, AFERA, PSTC – for measuring the properties of tape-type products, including adhesive bandages, masking and packaging tapes and labels. Until recently, these have usually necessitated the use of several instruments and, in many cases, been prone to a degree of inaccuracy, a lack of objectivity and/or repeatability, especially where human involvement is significant.
The development of Texture Analysis has opened up a wealth of opportunities for adhesive testing. The TA.XTPlus Texture Analyser’s design and software is ideal for the measurement of adhesiveness and tensile strength. Its salient feature is the horizontal arm, to the underside of which probes, clamps etc can be attached. This arm moves vertically at a speed and to distances programmed by the user according to the test to be conducted. It contains a loadcell, which allows the highly accurate measurement of force and achieves data capture at up to 2000 points per second.
Typical Texture Analysis Tests for Adhesive Products
How to Measure Tack
‘Tack’ is the measure of initial grab or stickiness of an adhesive tape without the application of pressure. It can be the most important property; determining the success or failure of an adhesive bond. The loop tack test (FINAT method 9) measures the force required to separate a loop of tape material (adhesive outermost) which has been brought into contact with a specified area of a standard surface (usually glass or stainless steel). The higher the force, the tackier the sample. The tack force is the maximum force measured at the moment just prior to the tape loop being completely separated from the glass slide. ‘Quick-Stick’ tack (in N) is expressed as the average maximum value.
Loop Test System for ‘Quick-Stick’ tack measurement and typical graphs
Alternatively, the Avery Adhesive Test (AAT) using the TA.XTplus Texture Analyser has eliminated the disadvantages of traditional probe testers and modified an existing concept to develop a new technique for the measurement of tack. The major concept change involved recording and analysing the entire stress-strain behaviour of a probe test rather than the single value reported by earlier methods. Other improvements over the traditional testers include the use of a spherical probe to ensure contact consistency and the use of double-side tape to mount the test sample such that the effect of facestock stiffness on test data is minimised.
The advantage of the AAT is that the method can be used to specify adhesive performance by analysing the multiple parameters extracted from the force-distance AAT profile. A second advantage is that the test can then be used for QA/QC by profile pattern recognition software. Finally, the method can also be used for R&D by analysing the relationship between PSA molecular structure and the AAT profile.
Avery Adhesion Test using 1” Stainless Steel ball probe and typical graphs
How to Measure Peel Strength
Peel strength testing is especially common in the packaging industry but is also an important factor in the development of pharmaceutical tapes – in both cases, the adhesion strength chosen according to the function of the tape. The T peel and 90º and 180º peel tests not only provide an indication of the adhesiveness of the product, but also their strength. A 90º Peel Rig provides the correct peel angle to be maintained by the use of a cord and pulley system. A strip of tape is applied to the test bed or other standard surface of interest (according to customer specifications). The tape is peeled from the panel at a 90º angle at a specified rate, during which time the force required to effect peel is measured. The test is commonly used for quality assurance purposes where the minimum or maximum peel values expected for a particular tape can form an acceptance criterion.
An 180º Peel Test Rig is commonly used to determine the comparative peeling or stripping characteristics of adhesive bonds, essential for the application of adhesive bandages. Would dressings and adhesive strips are required to have qualities such as ‘low-tack’, water- or moisture-resistance; not only is the selection of the strip material and surface important, but also the properties of the adhesive.
Adhesive Indexing and 90° Peel Rig; 180° Peel Test Rig
One of the most useful attributes of modern instruments for tack testing is the capacity for indexing. This allows ten tests to be conducted on one sample, which highlights qualitative variations but also enables the user to calculate average readings to avoid potential distortions arising from exceptional abnormalities and increasing sample throughput. For the measurement of a tape’s adhesion to its release lining material or alternative substrate a Flexible Substrate Clamp incorporates a multi-slot plate and clamping fixture.
Indexing provides Efficiency – Multiple Indexing Plate and Flexible Substrate Clamp
How to Measure Unwind Adhesion
For the measurement of unwind adhesion a roll of tape is mounted on a mandrel (Tape Unwinding Rig) so that it can turn without appreciable friction. The mandrel is fixed on the bed of the Texture Analyser and the free end of the tape is fixed in the other clamp and the unrolling force is measured upon the texture analyser movement upwards. Both AFERA test method no. 4013 and PSTC 8 are performed in this way.
Adhesive Tape Unwinding Rig and typical graphs
For a full summary of typical texture analysis tests that can be performed on adhesive products:
This information has been sourced, reviewed and adapted from materials provided by Stable Micro Systems Ltd.
For more information on this source, please visit Stable Micro Systems Ltd.