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.
We are familiar with water as H2O which we can show as H–OH, with one hydrogen atom attached to an oxygen+hydrogen group. I write it this way because much of adhesion depends on the availability of –OH (or “hydroxyl”) groups where the oxygen plus hydrogen group is part of a larger molecule. If the –OH is attached to an ethyl group containing two carbons and five hydrogens, we have ethanol, ordinary alcohol. There are two important things about –OH groups.
1 They like to associate with other –OH groups via a loose connection called a hydrogen bond. If a surface has some –OH groups and an adhesive also has some –OH groups then this loose association can encourage adhesion. However, water, with its H–OH can also associate just as well, so these loose associations are easily swamped by the presence of water.
2 They can react with a number of other groups to form stable chemical bonds that can increase adhesion. A carboxylic acid has a –CO2H group which can react with an –OH to form a stable ester bond. The ester in the image is the result of ethanol reacting with acetic acid. The –OH groups can also react with silanes, as discussed shortly.
Nitrogen atoms can form –NH2 groups (“amines”) which, like –OH groups, can self-associate and also react with things like carboxylic acids. They especially like to react with the key components of epoxy and urethane adhesives as well as the acrylates in ultra-violet (UV) adhesives.
In addition to forming full chemical bonds, the carboxylic acids with their –CO2H groups are also happy to form relatively strong associations with surfaces containing –OH and –NH2 groups as well as with metal ions. As we saw in Chapter 1, adhesives based on milk proteins work well in the presence of calcium (Ca) ions which interact strongly with the numerous –CO2H groups within the protein. Ions are the charged form of atoms and molecules. Calcium ions have two positive charges and are shown as Ca2+ and carboxylic acids form mono-charged negative ions called carboxylates, –CO2−. In the milk protein adhesives, on average each Ca2+ will be associated with two –CO2− in order for the charges to balance. These relatively strong interactions make the adhesive quite resistant to water, even though the water is happy to interact with isolated carboxylic acids or calcium ions.
We need to discuss a rather more complicated bit of chemistry; without it, the success of many practical adhesives makes no sense. The complication is not so much the chemistry itself, but the fact that we have to distinguish between different “Sil”s:
Silicon, the element, on which we make silicon chips, and which can be reacted with oxygen and other molecules to create:Silica (the mineral)Silicates (more minerals)Silicone release paper to which nothing sticksSilicones used in cosmetics for their slippery feelSilicones or siloxanes (bathroom sealants) which stick well, andSilanes
Each of these reacted forms of silicon has the equivalent of four chemical bonds around them. For a silane, one bond is to a carbon atom (which itself is part of a medium-size molecule or a polymer chain), and the other three are to alcohol groups such as methanol or ethanol. The silanes are important because they are happy to swap one or more of the alcohol groups with –OH groups from, say, the surface to which we want to stick something. This means that we can get strong bonds to any –OH containing surface, using an adhesive that contains silanes. We can also start by reacting a silane with a surface and use the group at the other end (for example an amine) to react into the rest of the system. When silanes react with themselves this produces the polymerization (“curing”) of the siloxane adhesives. Finally, the silanes are part of the curing system of the modern “silane hybrid” adhesives and sealants which are short, ordinary polymer chains stuck together via these silane reactions.
The reason that silanes are so important is that they can react with, and provide adhesion to, so many surfaces. Ordinary materials such as bricks or cements are themselves made of silicates which have –OH groups able to react with silanes. Similarly, although glass can be thought of as being relatively unreactive silica, there can be plenty of silicate groups at the surface. At the same time, most metal surfaces have a thin layer of “oxide” at the surface. Sticking to aluminium, Al, is not sticking to the metal but to an “aluminium oxide” surface. which loves to react with silanes to create “alumino-silicates”. However, “oxide” surfaces can be complex and many of them do not contain many free –OH groups. This means that for many systems the surface has to be treated so that it contains enough –OH groups to react with silanes.
How can silicones at the same time be super slippery as in cosmetics and release papers, and super-good adhesives? If you have pure siloxanes, with no silane groups, then they are remarkably wriggly materials that can flex and twist very easily, giving them their special slippery feel and release properties. As we will discover in Chapter 4, it needs just a few reactive groups (the silanes) in the right place to convert a system to strong adhesion, making it possible to convert the slippery molecules into good adhesives. Silicone systems can also be made rigid by adding yet another variant of silicon, the delightfully named cube-shaped silsesquioxanes.
There is one more variant of the silicon-based adhesives. Because silicates are the basis of many rocks, why not make a rock-like adhesive from silicates? Sodium silicate solutions can indeed act as adhesives in special applications, though the upside of their strength comes with the downside of being brittle.