A sticky problem…

Adhesives are used everywhere, but how do we know how good they are or which one to use? Jo Smewing outlines some of the methods now used to put tapes through their paces

The value of the world market for self-adhesive tapes is set to expand at an annual pace of around four percent over the next three years, reaching $30 billion by 20121. Opportunities abound across an array of industries, from packaging and non-residential construction to electronics manufacturing. Proven high performance will allow adhesive tapes to continue to compete against other joining, bonding and sealing technologies. However, as pressure on quality control departments steps up, it is essential to be able to assess accurately adhesive properties and strength to determine the ideal components for a particular tape and so optimise its performance.

The fact that there are now over 100 industry-approved test methods for adhesive tapes indicates the increasing emphasis on product standardisation, quality and performance. Although this phenomenon is common to most modern industries, it is of particular importance to the adhesive tape sector because of the diversity of applications and sophisticated requirements: resistance to UV, chemicals and extreme weather, waterproofing, fire-proofing and safe use on the human body, to name but a few.

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.

‘Tack’ is the measure of initial grab or stickiness without the application of pressure. It can be the most important property; determining the success or failure of an adhesive bond. Three methods of tack testing are commonplace: the self-explanatory and rudimentary thumb test; the rolling ball method (PSTC-6, ASTM D3121); and the inverted probe test (ASTM D2979, AFERA 4025)

The tests are simple to perform and understand. During the rolling ball test, the tack of the tape will slow down and eventually stop the ball rolling across its surface. The shorter the distance travelled by the ball, the tackier the tape. To conduct the probe test, which is significantly more accurate, a specially-designed probe makes contact with the adhesive surface of the tape and is immediately withdrawn. The user measures the force required to separate the probe from the surface, indicating the tack.

“‘Tack’ is the measure of initial grab or stickiness without the application of pressure. It can be the most important property; determining the success or failure of an adhesive bond”

An additional option, the loop tack test (FINAT2 method 9) also measures force, but in this case the force required to remove the tape itself from a stainless steel or glass surface. The higher the force, the tackier the sample.

There is now an instrument which can perform both probe and loop tack tests, enabling manufacturers or users to obtain more accurate test data, providing an extremely precise profile of the tackiness of the adhesive. Using a TA.XTPlus texture analyser, available from Stable Micro Systems, different tests can be performed simply by changing the attachments to a single machine.

The 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 500 points per second.

The development of texture analysers has opened up a wealth of opportunities for adhesive testing. One of the most useful attributes of modern instruments for tack testing is the capacity for indexing. This allows 10 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. Graph 1 shows results of such tests on several major adhesive bandage brands, which were conducted in the USA. It clearly shows that the Kid Care products possessed the highest adhesive strength; demonstrated by the height of the force curve and the total work done – the size of the area under the curve. 3M’s flexible foam bandages adhered to the probe for the longest distance, illustrating the extensibility of both material and adhesive during debonding, whilst Curad’s Sensitive Skin bandages were the least adhesive.

Peel strength testing is especially common in the packaging industry. The T peel and 90º and 180º peel tests not only provide an indication of the adhesiveness of the product, but also their strength. However, it is important to remember that results can be affected by structural damage to the carrier and that the maintenance of the precise 90 or 180º angle can be difficult. These shortcomings can be partly resolved by performing drum tests.

Tensile strength is an important characteristic in many modern adhesive products. Being able to assess this gives an indication of uniformity and quality, and the product’s ability to withstand stresses during application and use. AFERA test 5004 (Breaking Strength and Elongation of Pressure-Sensitive Tapes) has traditionally required the employment of a tensile testing machine. By measuring force, time and distance, however, a texture analyser can perform both tests with more accuracy and flexibility than most commonly-used instruments.

Most standard test methods for adhesive products specify temperature parameters, and tapes which are to be used in hot or cold environments must be tested in realistic conditions. Backing materials and adhesives perform and behave very differently at varying temperatures. Silicone adhesives, for example, can withstand temperatures between -70ºC and +380ºC, whereas rubber resins can be used only between -5ºC and +70ºC. For this reason, Stable Micro Systems supplies thermal cabinets and temperature chambers, which can condition samples and/or probes before testing, and Peltier cabinets and testing plates for use during testing. Peltier equipment ensures that the effects of temperature are minimised, or that they can be investigated. Controlled-temperature testing is essential for tapes used for the medical, automotive, construction and aerospace industries.

Today there is simply no scope for guesswork or approximations in the adhesives industry, and yet budgetary constraints are tighter than ever. Quality is becoming the key factor for success, so manufacturers will increasingly find that quick, simple, objective and repeatable product testing is a pre-requisite. For these reasons, using multi-purpose instrumentation is a popular option. It offers savings in terms of expenditure, space and training and allows unrivalled accuracy and repeatability. The potential to identify and analyse minute differences in adhesive and strength properties will help to create the ideal adhesive for any application; guaranteeing performance, reliability and, ultimately, brand loyalty and market share.