Tips and tricks for successful adhesion testing

The most frequently used test procedure for measuring the range of adhesive properties in foods is the probe test.  

In this test, a probe applies a force over the sample for a chosen period of time to achieve a good bond between the two surfaces before it is withdrawn until the sample completely separates (failure). 

A vessel and disc as used in a back extrusion test are also suitable; however the disc would be employed to test the surface of the material rather than venture into the sample. 

One thing is clear, however; for successful adhesion testing the sample needs to be held down in order to measure the force to separate the sample from the probe. If the whole sample is lifted up of the end of the probe when it attempts to withdraw, the force that is measured will be the weight of the sample. 

In the case of samples that are not self-supporting this is usually solved by simply testing a small quantity or ensuring the vessel in which the sample is contained is not lifted by holding it.

Firmness and stickiness can be measured using a relatively small cylinder probe which performs a penetration test to a chosen distance into the sample. Firmness is commonly the force to penetrate to the chosen distance whilst stickiness is the work/force necessary to overcome the attractive forces between the surface of the sample and the surface of the probe used for the measurement.

Again, in order to successfully measure the stickiness of a product, the sample needs to be held down to prevent it from being lifted on the end of the test probe upon its attempt to withdraw from the product. A Confectionery Holder is an example of one such means of supporting a sticky sample.
  
The Multi-hole Adhesion Plate provides the same solution whilst providing the ability to set-up and test multiples of products at the same time. The Flexible Substrate Clamp provides slots for multiple tests on the same clamped sample.

The Pasta Firmness/Stickiness Rig, on the other hand, provides a larger exposed rectangular region through which a rectangular compression plate can enter to measure firmness and stickiness. The pasta sheet is held down using a support block which is secured by screws at each side.

However, sometimes a compression test or adhesive test on the whole product surface is required. This then excludes the use of the Confectionery Holder or any other such device as the whole surface needs to be exposed for testing. We have seen several solutions to this over the years including the use of disposable plates (e.g. cardboard, plastic) onto which the sample is glued and can then be held down on the plate edges. We have also seen clever methods of gel adhesion testing which involves the setting of gels onto Velcro which serves as a suitable attachment method to allow the assessment of stickiness.

For the measurement of peel of adhesives from a substrate the problem requires the simple solution of adhering the substrate to the peel platform via double-sided tape. As long as the adhesion of the double-sided tape to the substrate is stronger than the peeling material the measurement will be successful.

RIGS FOR THE TENSILE TESTING OF SPECIFIC SAMPLES
 
The ‘tissue-box’ approach of the Warburtons Dough Stickiness Rig was considered the optimum approach to hold down large quantities of dough for their assessment of firmness and stickiness. A blade is able to penetrate through a slot in the sample holding plate and when withdrawn the sample is held down by the plate.

For smaller quantities of dough, the Chen/Hoseney Dough Stickiness Rig has long been the popular solution as it minimises surface drying and assists in the preparation of a new test sample by extrusion through small holes in the top of the rig.  The main body of the sample is held within the rig and the rig weight ensures the probe does not lift the sample upon withdrawal.

When measuring the force to remove the coating from a tablet, an adhesive surface needs to cling onto the tablet surface in order to pull it away from the tablet body. This is enabled by positioning double sided foam tape into the cavities of the Tablet Coating Adhesion Rig before mounting the tablet sample. The foam tape holds down the tablet body whilst the upper tape removes the upper coating surface of the tablet.
 

Sometimes adhesion of samples in situations that imitate their normal environment are required. The Mucoadhesion Rig is one such device which allows the testing of the adhesion force of tablets, pellets, granules or gels to a mucosa which is held in a clamp immersed in liquid similar to gastric fluid.

So what does the texture analysis data tell you?
There are three types of failure identified: if the material remains on the surface of the probe, cohesive failure occurs. If separation is clean (no material remains on the probe), adhesive failure occurs. In adhesive failure, the clean separation occurs at the surface and there is no legging or little necking deformation of the sticking material, whereas cohesive failure occurs within the material and leaves residues in both surfaces. The cohesive-adhesive failure is the transition between the complete cohesive and complete adhesive failure. For solid and semi-solid materials (such as gels, chewing gums, etc.) where the interfacial bonding balances against the internal mechanical strength of the material, either failure mechanisms could be possible, but for many viscous (or viscoelastic) fluid foods, cohesive failure is most likely the dominant mechanism.

Using the software adhesive test, the adhesive force (or stickiness) is taken as the peak force; the work of adhesion is taken as the area under the curve and represents the total work during the withdrawal of the probe. The area from the start of separation to the maximum force is considered the best measure of work of adhesion.  The cohesive force can be taken as the area between the maximum force and the point where force drops to near zero (the force will never return to zero if cohesive failure has occurred and residue remains on the probe at the end of the test). The stringinesss is the distance between the sample surface and the point where force drops to near zero where the product finally separates from the probe.

A major difference between texture analysis profiles is the way the force decreases after the maximum peak. Products that possess a ‘short’ texture (e.g. jam, chocolate spread) have their forces drop to zero or a value of negligible very quickly.  However, products that are ‘stringy’ produce a tailing curve which extends for a considerable distance before returning to zero (or near zero if sample remains on the probe at the point of separation). The distance that this tailing occurs can usually be termed stringiness and the area from the maximum force to this end point can be considered as the work of material stretching.

There is no doubt that the speed of probe separation has a great influence on the adhesive force and the work of separation and once chosen should remain constant for comparison purposes.

The graph from an adhesive test shown above highlights where two parameters of interest are obtained.

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The TA.XTplus 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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