Understanding how a Texture Analyser works – Adhesive testing

Adhesiveness (often referred to by consumers as ‘stickiness’ or ‘tackiness’) is key to perceived product quality, performance and effectiveness of such products as sealing tapes, curing glues, resins, waxes, adhesive plasters and labels used in the pharmaceutical, food, medical and packaging sectors. 

Testing adhesives is a very sophisticated process since it combines the Texture Analyser's ability to perform both compression and tension testing and measurement of force and distance. The sequence of events performed by the program are: 

Animation of how an adhesive test works to control a TA.XTplus Texture Analyser.

In conjunction with a Video Capture & Synchronisation System you can zoom in and replay all of the steps that occur during an adhesive test to understand the full characteristics of your sample.

Employing the assistance of a Video Capture & Synchronisation System to understand what occurs

Your adhesive test settings

You will need to choose the speed that your probe approaches the sample (pre-test speed) and the force (trigger force) that you wish the Texture Analyser to recognise in order to realise it has reached the sample surface.  This speed should be low in order to avoid overshooting the low force target that is being looked for.  The probe will then switch to a test speed which should also be low as it then seeks the applied force that you wish it to hold for the chosen contact time.  After this time which should be enough to achieve a good bond between the probe and the sample, the probe will withdraw at the post-test speed (which should be fast in order to encourage separation) to the required distance (which should be great enough for probe:sample separation to have occurred before the test finishes.

The first part of the curve usually shows the probe achieving the force that it has been specified to find and then attempting to hold that force for the chosen period of time. You can use the delay acquisition option on this test to delay the capture of this effective sample preparation steps of the data until the probe pulls away from the product.  This is useful if you want to hold for a long time.

Example: Adhesive test collecting all data (including initial compression period)

Example: A comparative adhesive test using the 'Delay Acquisition' feature.

This test is useful for testing products whose adhesive properties are of interest which may be dependent upon initial compressive loads, for example pressure sensitive or contact adhesive, double sided sticky tape etc.

Since the probe is left at the test distance from the trigger point at the end of the program, it is suggested that it be re-positioned near to the test sample before the program is re-run.

PID Tuning

For a test where you are going to a force the test settings require the choice of suitable PID settings which vary widely depending upon the sample type:

Choosing the proper values for P, I, and D is called "PID Tuning". The correct PID settings should give a smooth hold period with small oscillations, and the target force should be accurate with no offset. 

Choosing the correct PID settings for an application is usually a case of trial and error, altering each value to achieve the correct combination of rise time, steady state error and overshoot (as well as other properties) for the given application. These values depend on test settings and sample properties. In some cases, it might be appropriate to set one of the values to zero, and the following section outlines the properties of those test settings.

The following graph has good PID settings (this is an adhesive test):

To adjust the P, I and D values:

P – proportional gain

If the proportional gain is too high, the system can become unstable. In contrast, a small P results in a small output response to a large input error, and a less responsive controller. If the proportional gain is too low, the control action may be too small when responding to system disturbances.

Using the same settings as above but a very large P:

I – integral gain

A large I value results in large oscillations. A low I value results in a sluggish system.

Using the same settings as above but a very large I: 

D – differential gain

The amount subtracted from the output based on the rate of change of the error – this value determines how much to change the output due to a change in direction of the error.

Same settings as above but a large D value:

Max tracking speed

As well as adjustments to the P, I and D parameters, TA settings has a "max tracking speed" field. This is primarily a safety feature to prevent sudden probe movements in the case of a product breaking, for instance. Although it is the force that is being controlled, this is brought about by probe movements. The max tracking speed puts a limit on the speed at which the probe can move during the PID control. 5mm/s is a good compromise in most cases.

Why not request ‘Tips and tricks for successful adhesion testing' user guide:

and the Avery adhesive test article which helps understand how to analyse different shape curves produced from adhesive tape testing.

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplusC Texture Analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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