How is adhesive strength quantified?

When choosing a pressure-sensitive adhesive (PSA), you need to have confidence that it’s going to stand up to the rigours of the application. But with PSA–substrate interactions being so complex, it’s wise to back-up any decision with some hard numbers, rather than rely solely on previous experience or qualitative judgments.

To do that, of course, you need to look at the adhesive strength that a particular system can deliver. But what are the ways in which adhesive strength can be measured, and when are they most relevant? In this blog post we delve into this topic, and aim to give you a clearer idea of the metrics you should be thinking about when considering adhesive strength.

The first thing to say is that ‘adhesive strength’ is not a single number, for the simple reason that PSAs, in contact with a substrate, are structurally complex, anisotropic systems, and so can have different ‘strengths’ in different directions (and of course under different conditions). 

For R&D purposes, we might want to look at the intrinsic tensile strength of a particular component, because that provides us with a detailed understanding of when it will fail. But when helping customers choose a PSA, we invariably consider the conditions under which the entire system will fail – which, after all, is what ultimately matters to them. (We also look at exactly how it’s failed, but that’s a topic for another day).

The way we quantify this ‘point of failure’ depends very much on the reason for needing to know the strength in the first place. So we’ve divided up the discussion below into three questions that you might be wanting to ask. 

PSAs, because they’re based on viscoelastic adhesives, take a defined time to achieve their optimum strength. A long dwell time is fine if you’re prepared to wait, or if you might need to reposition your PSA after initial application. But in fast-moving environments such as production lines, you need to be confident that a component joined with a PSA in stage 1 isn’t going to fall off in stage 2.

To do this, you need to consider the tack strength (or more commonly, just ‘tack’), which is the strength of the bond achieved immediately after contact, usually after light pressure has been applied. This can be quantified in numerous ways, but for PSAs the most commonplace method is the ‘loop tack test’, involving slowly lowering a loop of PSA down onto a surface such as glass, and measuring the force required to quickly pull it off.

Importantly for PSA applications, the initial ‘tack’ of an adhesive does not indicate the final adhesive strength, which we’ll come to below.

Although the strength of bulk materials is measured in terms of force per unit area, this doesn’t apply to PSAs. This is partly because PSAs, being two-dimensional, allow the scope of the measurement to be reduced to the line where the join is breaking, or even to a whole construct of defined width. This results in a measurement in terms of force per unit length or just force, respectively. But in many cases, it’s simply more convenient to use a more readily measurable metric, such as the mass of an applied weight or the time taken for the joint to fail.

For those unfamiliar with the field, matters are further complicated by the units themselves. To take an example, peel strength is commonly given in metric units of N/m or N/cm. But because tapes are sometimes produced in Imperial widths, it’s not unusual to encounter ‘hybrid’ measurements such as N/inch, which demands that extra care is taken with conversions.

Maximum strength in a PSA is achieved when the adhesive has completely wetted the substrate’s surface, and the van der Waals forces have had the time to develop fully. 

This optimum degree of adhesion between the adhesive and the substrate is described using the peel strength, which involves applying a PSA to a surface, waiting the requisite time, and then measuring the force needed to pull off the tape, starting from one end. In principle, this can be done at any angle, but most commonly the free end is either raised so that it’s perpendicular to the surface (the 90-degree test), or it is folded back so that it’s alongside the adhered end (the 180-degree test).

Although for permanent joins the peel strength needs to be maximised, a lower value is desirable when the PSA is intended to be removable, or when the join in question involves a liner.

Complementing an understanding of the adhesive force between the adhesive and the substrate is obtaining a value for the cohesive force within the adhesive itself. In many cases where a PSA is intended for permanent use, this isn’t relevant, because PSAs are usually designed so that the weakest point of the whole system is the substrate.

But where the adhesive itself will be the weakest point – for example, where the substrates are mechanically strong, where you need the PSA to be removable, or where you’re looking at liners – then this cohesive strength becomes important, and should be quantified. This is done by measuring the shear strength, which is carried out in a similar manner to a peel strength measurement, but at 0 degrees – that is, where the tape is pulled off in the same direction as its free end.

There are two main ways of measuring shear force, and both require a defined area of PSA to be applied to a surface: 

• Dynamic shear strength is measured by increasing the force applied to the joint until it breaks, by moving the jaws of the instrument apart at a constant velocity.
• Static shear strength is measured by applying a standard mass to the free end of the PSA and measuring how long the PSA takes to detach. The exact value of the mass is important, because it influences the speed of the test, and thus the extent to which ‘elastic’ vs. ‘glassy’ characteristics of the adhesive are displayed.

A variation on static shear strength is SAFT (shear adhesion failure temperature), which involves gradually increasing the temperature of the system until it fails. This is most relevant to measuring the fail temperature of PSAs destined for use under conditions of elevated temperature.

There are various ways of defining PSA strength, and each of them is relevant to particular circumstances. By understanding the differences between them, you can ensure you’re considering the right characteristics for your application, and therefore ensure that the PSA you’ve chosen won’t let you down when in service.

If you need to discuss adhesive strength measurements, then please get in touch with an Avery Dennison specialist at andrew.christie@eu.averydennison.com. Alternatively, if you just need to identify the best PSA for your application, then you might like to try our Product Selector.