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Test methods for pressure sensitive adhesives

Pressure-sensitive adhesives (PSAs) have great importance in industry and are the main components of labels, tapes, and a wide variety of other products. Main features are:

  • 100% solid
  • Instantaneous adhesion with light pressure
  • Quick and easy to apply
  • Large panel of adhesion capacities (force, substrates)

Hot-melt adhesives are applied in a molten state and must flow on the surfaces to ensure wetting and adhesion.

Measuring physical properties such as viscosity, softening point or modulus is relevant and important for characterizing adhesives, providing information on the behavior of polymers, as well as the effects that plasticizers and other components may play in the adhesive.

Rheology provides a basic grounding, through the rheological properties it is possible to predict the performance of a PSA, while optimizing a formulation for a given application But adhesion properties such as tack, peel strength and holding power become handy quality control tests. PSA is a complex system so there is no single test which shows one specific product is fit for purpose, but all together they become an important tool for R&D and Quality Departments.

Adhesion properties depend on experimental parameters such as temperature, pressure, rate of debonding and, of course, nature of the adhesive and substrate as well.

Related standards are published by groups such as ASTM (American Society of Testing Materials) or associations like FINAT (European Association of the Self-Adhesive, Labeling Industry) or PSTC (Pressure sensitive tape council). Normalized tests are well defined and require equipment that is easily available.

Main PSA testing methods are:

Tack

El “Tack” is often defined as the initial grab or instantaneous holding power of an adhesive.

There are different ways to measure the tack depending on the application. Methods like loop tack, quick tack or rolling ball tack are common tests used in this field.

Loop tack measures the force required to separate a loop of PSA coated material of a standard surface while rolling ball tack measures the distance a ball, placed at the top of an inclined track, travel along an adhesive tape. Rolling ball test also provide information on softness, being the distance inversely proportional to tackiness and softness.

Peel strength

Peel adhesion is defined as the force required to remove pressure sensitive coated material from a standard plate at a specified angle and speed.

The scope of test method is to quantify the permanence of adhesion or peelability of self-adhesive pressure sensitive material. It is important to report the mode of bond failure, adhesion failure (without leaving residue on the plate) or cohesion failure (leaving residue). Surfaces like plate glass, aluminum, stainless steel or polyester film may be used but must be clearly stated alongside the results.

The equipment required is the same as that for the loop tack, but in this case a better reproducibility is achieved because of a better control of contact conditions and peel angles.

Shear adhesion or holding power

Shear is a force applied in the same plane as the label stock. Resistance to shear is related to the internal cohesion of the adhesive.

Several methods have been developed over the years to test shear resistance of PSAs from a Standard Surface (eg. plate glass, stainless steel or aluminum): The standard is the PSTC/FINAT/ASTM shear test. This test uses a standard test surface at a one-degree angle at room temperature. The samples are assembled using a load and placed into the test chamber. Time taken for the pressure sensitive coated material to part from the plate is noted.

A modification of the shear-resistance test is the shear adhesion failure temperature test (SAFT), a specific test method for measuring heat-fail temperature in shear of hot-melt adhesives. In this case the temperature of the chamber starts at ambient and is ramped upward at a prescribed rate. The temperature at which the adhesive layer fails is noted as the shear adhesion failure temperature (SAFT) and serves as a guide to the high-temperature performance of the adhesive.

In this method it is important to be able to determine as accurately as possible the temperature of the adhesive layer, rather than the air temperature, since the adhering components (test sample and test surface) will take some time to achieve the temperature of the air in the chamber.

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