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Test Methods to Evaluate Tack

Test Methods to Evaluate Tack

Tack is defined as the "stickiness" or "tackiness" of an adhesive in a liquid or semi-liquid state, but we often have difficulty in defining it explicitly or in absolute units. In order to optimize a formulation, it is necessary to use specific tests for the evaluation of TACK.

Learn the basic concept & importance of tack, and how it is measured. Review the various tests used to measure the tack.

Overview

Importance of Tack

Importance of Tack

Today, pressure-sensitive adhesives (PSAs) have great importance either in industry or in our everyday life. They are the main components of labels, tapes and decals. Their advantages such as:

  • Dry form
  • Instantaneous adhesion
  • Easy to apply, and
  • Large panel of adhesion capacities (force, substrates) are very important indeed

All these properties are explained by a large variety of formulations, but involving only a few types of components, like:


The art of the formulator is based on the selection of raw materials and additives, in order to find the best compromise between advantages and drawbacks given by each of these materials when they are blended and to improve the desired parameters and create synergies.

But, in order to optimize a formulation (and the efforts needed to obtain it), it is necessary to use specific tests for the evaluation of the particular property called TACK.

The word tack is often used but has a variety of meanings. Let’s discuss the elusive nature of its definition and the importance of this concept to the adhesives industry.


What is Tack?

What is Tack?

The word "tack" is unlike others in the adhesives industry. Most of us know what "tack" or "tackiness" refers to, but we have difficulty in defining it explicitly or in absolute units. The elusive nature of the word and the concept stems from the fact that "tack" results from a composite of several physical observations, parameters, and concepts. This is not necessarily a serious flaw, since indeed this is the case.

Tack is the property of an adhesive that allows it to adhere to another surface on immediate contact. It is the "stickiness" of the adhesive while in a fluid (e.g., paper cement) or semi-fluid (e.g., pressure sensitive adhesive) state. There are two stages that must be considered with this concept.

  • The first is the wetting stage where the tacky material must wet the substrate or a probe's surface - the most common probe being the human thumb. This initial stage is controlled by physical-chemical properties, such as critical surface tension, viscosity, adhesive thickness, etc.
  • The second stage is that of debonding the probe from the surface, and here rheological properties of the adhesive come into play.

According to the ASTM standard, tack has been defined as the property that enables an adhesive to form a bond of measurable strength with the surface of another material upon brief contact and under high pressure. Implicit is that the adhesive separates cleanly from the surface, without any macroscopic residue.

One can easily understand that the tack property is greatly influenced by the experimental parameters. It depends on:

  • The nature of the PSA
  • The adherend
  • The pressure and,
  • The time of contact

Moreover, because the response is a viscoelastic one, the temperature and the rate of debonding play a key role in the strength measured.

In other words, tack is defined by the test used to measure it. So, it is very important to know what kind of tests are used, what kind of information is given and their limits.

Related standards are published by national groups such as ASTM, and also by professional associations like European Pressure Sensitive Manufacturers' Association (EPSMA), or FINAT.

Normalized tests are well defined and require equipment that is easily available. They are a bridge between laboratories, users, distributors, and all the other partners involved in this industry. It is logical (and highly recommended) to use these methods if they are related to your problem.


Measuring Tack

Measuring Tack

Conditions to Obtain Maximum TackThere are many different ways to measure tack depending on the application. Tack is, however, simply the resistance to separation. Separation is rate- and temperature sensitive, and involves viscoelastic deformation of the bulk adhesive. The appropriate measurement is the work expended in separation rather than the force used.

Maximum tack occurs when the conditions of measurement and the properties of the tacky material combine to have a high energy loss within the adhesive material. This is the basis for the formulation and incorporation of tackifiers in adhesive formulations.

It seems that a whole dictionary of nomenclature related to tack has developed over the years. This would be suspected from a word with such broad meaning and application. For example, tack is sometimes referred to as "green strength" or the instantaneous holding power of an adhesive. This generally refers to the resistance to separation before the adhesive has had a chance to vulcanize or crosslink.

This characteristic may also be called "quick tack" or "aggressive tack". It may be one of the most important properties in determining the suitability of an adhesive, such as pressure sensitive tape, for a certain application.

Associated with tack is "dry tack" which is a property of certain adhesives to stick to one another even though they seem to be dry to the touch. Autohesive tack (or autohesion) is dry tack between materials having similar chemical composition. "Tack range" is the time that an adhesive will remain in a tacky condition.


Methods to Test Tack

Methods to Test Tack

Tack tests are designed specific for the application. The results will heavily depend not only on the conditions of the adhesive & adherend, but also on the way contact & separation is achieved. Tack measurements are also dependent on the time during which the substrates were joined.

Of course, such parameters are very application specific, and that is why many different tack tests have been constructed and are in use. The table below summarizes several common methods of measuring tack.

Organization
Test Method
Common Name
Note
ASTM
D2979
Probe Tack
 
ASTM
D3121
Rolling Ball Tack
Identical to PSTC 6
TLMI
LIB 1
LIB 2
Loop Tack
LIB1 uses a specially designed tester;
LIB2 uses a modified tensile tester
PSTC
5
Quick Stick
Similar to AFERA 4015
PSTC
6
Rolling Ball Tack
Identical to ASTM3121
FINAT
FTM
Loop Tack
Similar to TLMI methods except uses glass rather than stainless steel as the substrate
AFERA
4015
Quick Stick
Similar to PSTC 5
 ASTM - American Society of Testing and Materials
 TLMI - Tag and Label Manufacturers Institute
 PSTC - Pressure Sensitive Tape Council
 FINAT - European Association of the Self-Adhesive, Labeling Industry
 AFERA - Association des Fabricants Europeens de Rubans, Auto-Adhesifs

Tack, therefore, is not a true physical property of an adhesive, such as viscosity, modulus, or specific gravity. It is a composite property that has broad and somewhat qualitative meaning, but one that is very useful in practice. Quantitative meaning can only be defined through specific application.

Let’s discuss each method in detail...

#1. Rolling Ball Tack Test


In this procedure, a rolling object (a steel ball according to this standard, but other geometries and materials are also possible) is placed at the top of an inclined track pursued by a horizontal, upward-facing adhesive. The ball is rolling down, and the relevant measurement is the distance the ball travels along the adhesive tape.

This simple, but frequently used test is probably one of the oldest. Despite its simplicity, it gives a good idea of the adhesive behavior and is readily understood. This distance is inversely proportional to tackiness: the greater the distance, the less tacky the adhesive.

Rolling Ball Tack Test 

But this test can only be applied as an internal comparative test because there is no information about the adhesive surface or bulk parameters. And different attempts to summarize bibliographic data (using balls of different sizes, composition, texture) have not given (for instance) a good mechanical model, reliable measurement and adhesive properties.

Practically, the measurement needs to be performed several times to obtain average values and a significant result.

For many adhesives, a wide range of results are obtained; moreover, the maximum prescribed distance in the standard is often lower than the experimental one. The method by itself is low cost and easy to use but often needs to be adapted to the parameters of the real system (temperature, tack capacity). It is also a good quality control for tapes, for example.

Advantages Drawbacks Applications
  • Low cost
  • Easy to use
  • Easily understood
  • Low precision
  • Limited to aggresive tack
  • Impossible to vary each experimental parameter
  • Production control
  • Quick comparison of high-tack adhesives

#2. Loop Tack Test


In this test, tape samples are circular loops caught in the upper jaw of a tensile tester. This pear or teardrop is brought into contact with a horizontal surface (model or real system) for a short conventional time and then peeled off.

As for the rolling ball test, variations near this scheme are possible for example shape of the substrate & pressure applied. The main advantages of this test are:

  • It is easily and quickly performed
  • Does not require specific equipment (a standard tensile tester is sufficient), and
  • The reproducibility is in most cases acceptable

When equipped with an environmental chamber, one can also vary contact conditions, and in this way, study their influence. But drawbacks are originated from the method itself. Contact time and peel angle are difficult to reproduce, and the stiffness of the tape backing is also strongly influent.

Loop Tack Test

In summary, the loop tack test is a good means of measuring the tack of labels or tapes when the applications are confined to the experimental conditions.

Advantages Drawbacks Applications
  • Medium cost (requires a dynamometer)
  • Good precision and reproducibility
  • Easy to explain
  • Influence of backing
  • Care must be taken to reproduce pressure and area of contact
  • Choice of the adhesive, especially for labels or closed applications

#3. Peel Tack Test


Derived from the standard peel test (but time and pressure of contact are lower), it can be compared with the previous test (but in this case, the operator itself puts the adhesive on the adherend surface).

The equipment required is the same as that for the loop tack test, except that a reproducible pressure must be applied. If it is slightly less quick to carry out, a better reproducibility is achieved because of a better control of contact conditions and peel angles. It really becomes possible to compare peel measurements at different contact times & pressure and to extrapolate at zero time of contact to obtain an "absolute" value.

Peel Tack Test 

Nevertheless, the stiffness of the backing is also influent, and the results depend not only on adhesive tackiness but also on the tape modulus. To summarize, peel tack test can be interesting, but one can be far from a real tack test if pressure and time of contact are too high; as with other tests, one must be as near as possible to the usual conditions.

Advantages Drawbacks Applications
  • Medium cost (requires a dynamometer)
  • Good precision and reproducibility
  • Easy to explain
  • Influence of backing
  • Difficult to study low contact time
  • Choice of the adhesive, especially for labels or closed applications

#4. Probe Tack Test


The simplest equipment to evaluate tack properties is a thumb. Brought into contact with a slight pressure and, after a short delay, pulled away from the adhesive, the sensation of tackiness felt by the operator can be reported on a scale of, by example, 0 (no tack) to 5 (very aggressive tack). But it is obvious that this method is not reproducible - each operator has his own appreciation, varying according to the day or his own senses.

So, although more precise sensors and more reproducible probes are preferred, the idea is the same. Mechanical probe tack testers bring a probe (ball or cylinder of various material) into contact at controlled rate and pressure, wait a given delay, and measure the force needed to pull away at a specified rate. Most systems allow each individual parameter involved in the bond formation to vary (the ASTM D2979-00 is related to the first experimental equipment, called "Polyken Probe Tack").

Probe Test

One can obtain this figure, reporting the measured force during the experiment.

Measured Force During Experiment

It is very interesting to understand the phenomena observed during the separation between a tacky adhesive and an adherend.

  1. The stress increases linearly with the probe displacement
  2. The stress slowly increases and is no longer linear, because a nucleation phenomenon occurs (apparition of voids at the interface)
  3. The force goes up to a maximum, when the nucleation stops
  4. Cavities start to grow perpendicular to the interface, involving a decrease of the stress; at this stage, the shape of the curve depends on the rheological properties of the adhesive and on interfacial interactions between the adhesive and the probe; there are two possible ways for debonding to occur:
  5. Voids can grow until coalescence occurs: in this case the failure is adhesive and the stress decreases to zero
  6. Cavities can grow to reach a critical size, and fibrillation appears. Fibrils can lead to adhesive failure (f2) or cohesive failure (f1)

In this method, the experimental parameters are the same as during a real bonding process:

  • Contact pressure
  • Contact time
  • Temperature
  • Rate of separation
  • Probe used (shape, material), and
  • Adhesive system (thickness, backing, roughness)

Although the results are more difficult to analyze, this method is a precious one. The use of a dynamometer gives high precision and the parameters (forces at different contact times, but also energies of debonding) can lead to a better understanding of the PSA behavior.

Advantages Drawbacks Applications
  • High cost
  • Very versatile
  • Good sensibility and reproducibility
  • A lot of information is accesible
  • Possibility of varying each experiemental parameter individually
  • Careful preparation of samples
  • Needs time to be performed
  • Specific applications
  • Formulation, compounding
  • Research laboratory


Standard Test for PSAs


»  Explore the Test Methods to Measure Impact Strength of Adhesive Joints

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2 Comments on "Test Methods to Evaluate Tack"
Luis Varela de la R Jul 29, 2020
There is no mention about gravity. There is mention about specific gravity. Completely different subjects!
SANJAY B Nov 21, 2019
All of these test methods , having practised and scientifically analysed by me put a question " How could gravity affect test datas?" Being an admirer of Sir Albert Einstein , I feel , Rolling ball tack can be quantified if Moment of Inertia data of solid and hollow cylinders are taken into considerations . In loop tack , a reproducible loop is need of hour and it also can be done . None of test methods have taken polarity as defining parameter and that worries me . Same test data of 180 ° peel on stainless steel can have big difference in actual sticking to same materials ! I think Global Developments are more Copy Cats rather than Original Thinkings! I quote Sir Einstein " If you continue to take same way , result will be No Different " I feel Scientists are becoming Commercial rather than Original " Rgds Sanjay Bohra PSA Technologist +91 9830031707 snjybohra@hotmail.com

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