Sticking Together. Steven Abbott

“The feet of oxen, formerly esteemed, are now looked on as one of the bad materials that can be employed, & especially since the Butchers have begun to carefully remove a tendonous part of them, called the small nerve, or the shin nerve, that they sell by weight, & rather dearly for the production of a kind of oakum which is useful for caulking the panels of carriages, or to make suspension straps for carriages. When the feet are thus stripped of this tendonous part, they produce only a mucilaginous substance which is not suitable for making good glue; & if anyone makes use of them, it is because of their low price.”

      As M. du Monceau's book indicates, by the early 19th century, we enter the world of industrial adhesives that take up the rest of the book.

2.2 TECHNICAL BASICS

      Before we start, we need to make sure we have the right basic ideas and language to understand what makes a good, or poor, adhesive system. Be assured that what follows is rather gentle, yet at the end you will be able to understand what's really going on when we stick things together. We take it step by step, with none of the steps being especially difficult.

      The trickiest, final, section covers the science of the polymers used as adhesives. The emphasis is on the few important principles (e.g. what polymerization is, what a crosslink is) that anyone can grasp. Readers who dread chemistry need not worry.

2.3 UNITS

      We have to agree on a few measurement units and technical terms. Most readers will be familiar with them as they aren't too exotic.

      For length we will use metres, m, millimetres, mm, micrometres, µm and nanometres, nm, each 1000× smaller than the previous. The unit of centimetres, cm, doesn't fit into that nice scheme but is so common that it has to be included.

      For time we will go down to µs, ns and ps, micro, nano and picoseconds, one millionth, billionth and thousand billionth of a second.

      For weights and loads we will use kilograms, kg, grams, g and Newtons, N, which, if a weight is involved is just weight times gravity. For our purposes, gravity is 10 m s⁻¹ s⁻¹ so 1 kg is 10 N.

      Force per unit area has the units of N m⁻² which is also expressed as Pascals, Pa (Figure 2.4). One Pa is rather small, so we often have kPa, MPa and GPa for kilo, mega and giga, 1000, 1 million, 1 billion Pascals.

      Figure 2.4 Force, in N, is applied over an area in m². 1 N m⁻² is called a Pascal, Pa.

      If you pull, say, a piece of plastic which has a cross-sectional area of A with a force F, then you will get a fractional increase of length (the change in length divided by the original length), ε (Figure 2.5). The force per unit area is the stress in Pa. The fractional increase in length, strain, has no units. If you divide stress by strain you get modulus which gives an idea of the strength of the material. Modulus is also measured in Pa, as strain is dimensionless. Dividing by a small number gives a larger number, so the smaller ε for a given stress, the larger the modulus, which makes sense because a stronger material will stretch less. The modulus of typical polymers lies in the 1–4 GPa range, steel is 200 GPa. We will find later in the book that adhesion doesn't necessarily rely on brute strength; if the adhesive on a strong household tape has a modulus greater than 0.3 MPa, it does not work (it has no stickiness) – this really is strength through (a special type of) weakness.

      It is unfortunate that the words stress and strain start with the same three letters and in common language mean the same thing, but we are stuck with the terms and you just have to get used to remembering which is which.

      Figure 2.5 When we apply a Stress, Force/Area (F/A) in Pa we get an elongation, ΔL of the original length L. Their ratio is ε, which is the Strain, which is unitless. The ability to resist stresses is the Modulus, Stress/Strain, also in Pa.

      We often have to discuss work of adhesion and surface energy, each of which is measured as Joules per square metre, J m⁻². It happens that work is the same as energy, which is why the two measures have the same units and why “work of adhesion” is sometimes called “energy of adhesion”.

      The strength of an adhesive joint is often measured in force per unit length, i.e. N m⁻¹. If you sit down and do the sums (or if you go to my app page https://www.stevenabbott.co.uk/practical-adhesion/basics.php) you will find that a peel strength of 1 N m⁻¹ is the same as a work of adhesion of 1 J m⁻² (Figure 2.6).

      Figure 2.6 Classically, adhesion is measured as the force, N, per width, m in N m⁻¹, or as work, J, to create 1 m² of separated adhesive, in J m⁻². The two measures are exactly the same!

      It is tricky to know whether to add a space between a number and a unit. Some prefer 1N m⁻¹, others prefer 1 N m⁻¹. For clarity and readability I have standardized on using the space.

2.4 ADHESION TERMS

      The word for the stuff doing the sticking is “glue” or “adhesive”. What about the word for the thing being stuck? “Adherend” is the technical term, which can lead to phrases such as “… the adhesive attaches to the adherend…” This isn't particularly elegant, but there is no obviously better term to use.

      We often have to discuss adhesion in terms of peel (pulling vertically) and shear (pulling horizontally). There is a different sort of vertical pull, the butt pull. Figure 2.7 gives you a visual indication of what these terms mean.

      Figure 2.7 Whether a joint is tested in peel (vertical forces), shear (horizontal forces), or butt (whole sample vertical pull) makes a great difference to the effective strength.

2.5 SURFACE TENSION AND ENERGY

      If you go into your kitchen, take a random assortment of smooth, flat surfaces and place drops of either water, water with a dash of dishwashing liquid, or oil onto the surfaces you will see that the drops form different shapes, depending on both the liquid and the surface.

      The liquids have different surface tensions, that is, different forces pulling the liquid together at the surface. Why are there forces pulling the liquid together at the surface? Molecules in the liquid are attracted to each other (if they weren't the liquid would instantly vaporize). At the surface of a drop this self-attraction becomes visible because the way to maximize their self-attraction (or minimize the number of missing attractions at the surface) is to form the smallest-possible surface which is a sphere. Drops containing dishwashing liquid have much weaker attractions at the surface because the surface is covered by a monolayer of the surfactant/detergent with long hydrocarbon tails that interact rather weakly; similarly, the oil has a low surface tension because it is made up mostly of long tails.

      The surfaces have different surface energies. This arises for the same reasons – the molecules at the surface want to be with each other to a greater or lesser extent.

      If the molecules at the solid surface have a large surface energy (for example a metal surface) compared to the drop then the drop will prefer to spread out on the surface, gaining maximum surface-liquid interaction, rather than curl up on itself in air.