Adhesives manufacturers have to worry incessantly about viscosity. Users can be equally as frustrated by an adhesive that is too viscous to flow nicely into a joint, as by one with such a low viscosity that it runs everywhere. Fortunately, there are some tricks they can play. For many viscous adhesives, if you apply a shearing or sliding force, their viscosity decreases rapidly (this is called “shear thinning”), allowing them to flow in the gap between the substrates. Others are “thixotropic” which means that they become thin when stirred or sheared (so they are also shear thinning), making them easy to apply, then, after some time, become thick again, stopping them from “slumping” out of a joint.
The study of how things flow is called rheology and interested readers can find a friendly (but technical) eBook and set of apps on my Practical Rheology web pages, https://www.stevenabbott.co.uk/practical-rheology/.
Even if the manufacturer has created the perfect viscosity, there is still the problem that squeezing a joint together is not as simple as it sounds.
2.7 SQUEEZING GLUE
It may seem odd to be discussing the simple act of squeezing some glue between two surfaces. What can possibly be of interest in such a basic process? It turns out that many of our frustrations and problems arise because squeezing is far more difficult to control than we might imagine. There are times when we need a thick layer of adhesive. Other times we need as thin a layer as possible. It would be nice to put one drop or blob in the middle and squeeze it evenly to the edge – a large drop for a thick layer and a small drop for a thin one. Unfortunately in the thick case it is far too easy to over-squeeze and get an excess of glue that is messy to wipe off because, well, it's rather sticky. In the thin case it can prove too difficult to get the adhesive to reach the edge at all.
The frustration arises thanks to Stefan's law of squeezing (Figure 2.9). There are various ways of thinking about it, and interested readers can find out more in this app, https://www.stevenabbott.co.uk/practical-adhesion/drop-squeeze.php. Stefan's law tells us a number of things about how quickly the drop spreads and the thickness decreases:
The higher the viscosity, the slower the process. This is intuitively obvious. A “gel” superglue with 20× the viscosity will spread 20× slower than a “pure” superglue without additives. This, with perhaps some shear thinning behaviour, is under the control of the adhesive manufacturer and with things like superglue we can choose which viscosity to use.
As the drop expands it gets super-hard to expand further. If it takes 1 s to go from, say, 1 mm diameter to 2 mm, it will take 16 s to go from 2 mm to 4 mm. For those who like equations, the rate of increase of the radius, R, is proportional to 1/R4.
As the thickness of the drop decreases it gets super-hard to decrease it further. If it takes 1 s to go from 0.4 mm to 0.2 mm, it will take 8 s to go from 0.2 mm to 0.1 mm. In equation terms, the rate of decrease of height, H, is proportional to H3.
Figure 2.9 Stefan's squeeze law tells us that squeezing a cylinder of glue of radius R and height H, gets considerably harder as R increases and H decreases and it becomes near-impossible to obtain a very thin layer of glue.
https://youtu.be/M0z1Xq52-HQ The video shows this nicely – six individual drops squeeze out to almost 3× the area of one blob containing six drops.
Because drops with a large thickness are easy to squeeze, if we start a little too thick then we can easily over-squeeze. Because thin drops are hard to squeeze, if we start with a thin drop in the middle then it becomes very difficult to get the drop to come right to the edge of the joint.
If you are in the unfortunate position of having a thick layer of adhesive at one side of a joint and a thin layer at the other then, well, give up. It is super-difficult to get such a layer to re-adjust itself.
If we combine the previous paragraphs with Stefan's law, especially the bit about the difficulty of expanding R, the least bad way to achieve a good overall thin coverage is to place lots of little drops across the surface (Figure 2.10). They can easily expand (because R is small) and become self-adjusting in terms of thickness (if H of one is small, a thicker H can more readily reduce its thickness). All the demos I've seen of superglue holding up some heavy truck from a crane start with a bunch of dots of glue (on a super-polished metal surface). Whether they know of Stefan or not, clearly the technique has been found to be reliable. The “dot and dab” technique for plasterboard/drywall, the blobs of cement for paving slabs, and the tile adhesive comb technique are variations of this theme.
Figure 2.10 Getting around Stefan. Instead of one large drop, use a number of smaller ones.
2.8 GRIFFITH'S LAW
We all know that bits of junk can get in the way of good adhesion, so we like to ensure that surfaces are clean before sticking them together. One reason for this is described by Griffith's Law.
An otherwise strong joint can easily fail if the stresses at the interface encounter a weak zone, such as a bit of dirt or its opposite, a void from a bubble of air. Our instincts tell us, correctly, that while a very small particle or void is not a problem, a bigger one is more of problem. Griffith's law tells us that if the size of the void is a, the stress required for failure is proportional to 1/√a. As the size of the defect gets bigger, the stress required for failure becomes smaller. When we make an adhesive joint, any defect from junk or air can be the source of a Griffith's crack failure, with larger defects leading to earlier failure.
As we shall see, when we try to break an adhesive joint apart, stresses are seldom distributed evenly across the joint. If a particle or void happens to be located in an area of low stress, it won't cause a problem. Conversely, if it is at a point of high stress, failure is assured. One of the frustrations of adhesion is that things seem to break for no obvious reason. Sometimes the reason might be a bit of otherwise harmless junk that's in the wrong place at the wrong time.
Griffith's law tells us that if we regularly attend to keeping adhesive free of large dirt particles and air pockets, our chances of unwelcome surprises reduce accordingly. That extra bit of cleaning before you assemble a joint really is worthwhile.
2.9 CHEMISTRY ESSENTIALS
We need just a few chemistry essentials to understand most of adhesion. Don't panic if you didn't enjoy learning chemistry.
The hydrocarbons which contain just Carbon, C, and Hydrogen, H, are rather neutral molecules that, in ordinary circumstances, contribute nothing to adhesion. The exception is when the hydrocarbons contain Carbon–Carbon double bonds, CC, that can polymerize. In normal chemical drawings, the carbons are shown as simple lines. They actually have hydrogens attached to them but adding them to the images clutters things up unnecessarily. In Scheme 2.1 are the images of a few important groups, discussed below, to help visualize a few key ideas.
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