The role of van der Waals forces in pressure-sensitive adhesives

Pressure-sensitive adhesives (PSAs) – defined as those that adhere to a variety of substrates under light pressure – are a very widely used class of adhesives. As a component of laminated polymer tapes, they have been familiar for decades in the consumer market, but have also become increasingly widely used for non-structural industrial applications, including in the flooring, automotive, packaging, display and electronics sectors.

The success of PSAs lies in the fact that they do not require activation with heat, UV light or solvent, and because the design of the laminated system can be precisely tailored to achieve instantaneous bonding of almost any combination of surfaces, including polymers, paper, metal, glass and wood.

Underpinning this versatility are the electrostatic forces between the adhesive and the substrate in a PSA-bonded system – and in particular, the interactions known as van der Waals forces. In this article, we look at the role of van der Waals forces in the action of PSAs, and show why understanding them can provide insights into choosing the best adhesive system for the task at hand.

The mechanisms of adhesion

There are four main mechanisms by which adhesives operate:

• Chemical adhesion involves the sharing or transfer of electrons between atoms of the substrate and adhesive, to form strong covalent or ionic bonds.

• Mechanical adhesion involves the interlocking or ‘interdigitation’ of the adhesive into holes or pores in the substrate.

• Diffusive adhesion involves the diffusive penetration of long polymer chains in the adhesive into the body of the substrate.

• Electrostatic adhesion involves electrostatic attraction between the adhesive and the substrate, and is by far the most important mechanism involved in the action of PSAs.

The name ‘pressure-sensitive adhesive’, although universally used, is actually slightly misleading because the strength of the bond is not sensitive to varying degrees of pressure. Instead, it would be more correct to say that the adhesive is pressure-activated.

The purpose of applying this pressure is simple – to bring the adhesive into intimate contact with the substrate. In the majority of cases, a pressure of around 2 kg/cm2 is all that is needed to ensure maximum adhesion, and applying more pressure may not always provide further benefit. (Note that sometimes after application the adhesive requires time in which to flow and cover a substrate, and this can be as much as 72 hours).

This intimate contact is necessary because PSAs operate on the basis of dipole-based electrostatic interactions between the adhesive and substrate. These van der Waals forces operate only at very short distances, and to appreciate why there is this distance constraint, it is useful to describe these forces in more depth.

In the general sense of the term (see boxed text), van der Waals forces are found whenever two atoms or molecules come into proximity. Although they are often described as weak because they ‘only’ involve dipoles, in fact they may vary in strength considerably, depending primarily on the polarisability of the molecules involved (i.e. the ease with which charge separation can occur).

Named after Dutch physicist Johannes Diderik van der Waals following his work on the molecular nature of gases and liquids, the term ‘van der Waals forces’ is used in a general sense to refer to three types of electrostatic interactions between dipoles in neutral atoms or molecules:

The attraction between two permanent dipoles

The attraction between a permanent dipole and an induced dipole. This is shown in the image below

The London dispersion forces are the attraction between an instantaneous dipole (one that results from a temporary shift in the electron cloud relative to the nucleus) and an induced dipole. This is shown in the image below. In some texts, ‘van der Waals forces’ refers to these forces alone, but we use the term in its broader sense here.

As illustrated, any of these three forces may be involved in an electrostatic interaction between an adhesive and a substrate. However, the London dispersion forces are nearly always the largest contributor towards the attractive force between two molecules, largely because they apply to all atoms present at an interface, not just where permanent dipoles are present.

Van der Waals forces in general do not have a favoured direction, and the London dispersion forces increase with the number of atoms present. With the exception of Keesom forces, they are also independent of temperature, which means that – if other things are equal – there is no fundamental reason why PSAs cannot operate at high temperatures.

There is, however, one major constraint in the action of van der Waals forces – they are extremely short range, with the strength of Keesom, Debye and London dispersion each being proportional to 1/r6 (where r is the distance between the atoms or molecules involved). The fact that r is raised to such a high power means that van der Waals forces rapidly diminish with distance: a typical length over which they operate is 0.4–0.6 nm. Compared to the size of (for example) a molecule of benzene (about 0.5 nm), it is clear that two molecules need to approach fairly closely in order for van der Waals forces to take effect.

Having now gained an understanding of the electrostatic forces operating at the molecular level when a PSA contacts a substrate, we are in a position to describe how adhesion is affected by macroscopic considerations.

It will be obvious from what has been said above that the strength of the van der Waals forces between two surfaces will increase with the degree of close contact between them. Therefore, for an adhesive to work effectively, the surfaces must be brought into complete and intimate contact.

Achieving this depends upon two properties of a given substrate–adhesive system, as outlined below.

The first of these is a mechanical one – the flow of the adhesive, perhaps more accurately termed viscoelasticity. To achieve good coverage of a substrate (especially those with rough surfaces), the adhesive must deform easily under pressure. Adhesives are usually viscoelastic solids, meaning that a detailed understanding of their elasticity and viscosity is needed in order to ensure that they flow well under pressure, and so cover the surface.

The role of adhesive flow for performance of PSAs. (A) An adhesive with good flow properties will cover the entirety of a rough surface. (B) An adhesive with poor flow properties will not, and adhesion will be reduced as a result.

The second key system property is a chemical one – the surface energy (or surface tension) of the substrate, which can be viewed as the excess energy contained in the ‘unsatisfied bonds’ at a freshly cut surface. A surface with a high surface energy means that it will readily dissipate that energy by forming new bonds. If that is the case, then a flowable material in contact with it will spread to maximise the area of the interface. To put this another way, the adhesive will easily ‘wet out’ (or the surface will be easily ‘wetted’). 

The role of substrate surface energy for performance of PSAs. A substrate with low surface energy will resist the spreading of adhesive and this is represented by a wide contact angle. As the surface energy increases, the flow of the adhesive is encouraged until complete wetting is achieved with high surface energy substrates represented by the contact angle tending to zero (0).

Clearly, achieving intimate contact between an adhesive and a substrate requires both that the adhesive flows well, and that the adhesive formulation is matched to the surface energy of the substrate. Only then is the close contact necessary for the formation of van der Waals bonds achieved.

PSAs with the required properties can be designed for nearly all substrates and applications by considering carefully the physicochemical properties of the various components and the demands of the application. But, having done that, any change that affects either the flow or the surface energy needs to be carefully evaluated – a PSA that works well in one situation may not perform as desired in another apparently similar situation.

For example, any of the following could result in a reduction in the closeness of the substrate–adhesive interface, and consequently a reduction in adhesive strength:

• Adding modifiers to the substrate could lower its surface energy, and reduce its wettability.

• Using a substrate with a rougher surface morphology can result in incomplete flow of the adhesive, unless the adhesive layer thickness is increased.

•  Changing the environmental conditions (temperature, humidity) could change the flow characteristics of the adhesive.

In conclusion, ensuring maximum interface contact between substrate and adhesive is vital to maximise the number and strength of the short-range van der Waals forces that underpin the performance of PSAs. By appreciating this role, and understanding how it is affected by adhesive flow and substrate surface energy, effective, high-performance PSAs can be designed, and their application limitations understood.

If you need advice on identifying the best PSA for your application, then please try our Product Selector, or talk directly to an Avery Dennison specialist at andrew.christie@eu.averydennison.com.

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