Chapter 7, Coming together (Polymer Compatibility)
How do you get immiscible polymers working together? A good solvent for one will naturally be a bad solvent for the other so there is a genuine difficulty in finding a way of bringing them together. For polymers which aren’t too far apart, it’s still fairly easy to make a good guess and get a result. But what happens if they are seriously immiscible?
A good example is when you want to combine the properties of a silicone with those of another polymer. In order for the final system to be stable, there must be some reactive groups on both so that mutual cross-linking ensures that the phases can’t separate. But because it needs only a relatively small amount of cross linking we can assume that the polymer HSP are the standard, unmodified ones even though the real polymers would have a few percent of some reactive group such as an alcohol, an amine or a methacrylate.
A typical HSP test for a real-world silicone shows values around [17.1, 2.2, 3.1, 5.7]
The challenge is to compatibilise it with an epoxy [17.4, 10.5, 9, 7.9]
The distance between these two is 9.6 so their centres are each outside each other’s sphere. These are genuinely incompatible.
When the two polymers are entered into the Polymer table and selected, then the Solvents button clicked, the following appears:
You immediately find some helpful solvent suggestions. Something volatile such as MEK or THF will be ideal for generally allowing the polymers to mix, but you also tend to need some “tail” solvents to hang around whilst the reaction is finishing so something like Cyclohexanone would seem a good idea.
Readers might be a bit disappointed at how easy this seems. But the authors’ experience is that without the convenience of HSP thinking (and HSPiP) this sort of problem has involved many months of avoidable work by major corporations.
You can use the same sort of ideas to do some clever self-organising coatings. Suppose you want an acrylate polymer as the top surface of a dual-layer coating with an epoxy at the bottom. Of course you could make this a two-pass coating. But in some applications a one-pass coating, if feasible, would save lots of time and money. So what we would like is a spontaneous separation of the components when the solvent evaporates. But for this to happen the solvent must be rather poor for both of them. Hence the RED number for each polymer with the solvent should be in the 0.8-1.0 range. The polymer with the lower surface tension is expected to be at the air surface then becoming the topcoat if motion within the film allows this. Its further accumulation at the air surface to achieve a significant thickness is then enhanced by a reduced affinity for the other polymer, which then forms a primer. Clearly other surface active components in the coating can interfere with this.
The Polymer form provides an Acrylate [20.7, 4.1, 10.7, 11.5] and an Epoxy [18.5, 9, 8, 9.8]. If you select Friendly Solvents in the Solvent form then when you select the Acrylate and click Solvents you find that the RED number for Xylene is 0.89. Selecting the Epoxy and clicking Solvents gives a RED of 0.97 for Xylene.
This example is a simplified version of a real dual-layer coating. As the Xylene evaporates, the mutual incompatibility of the two polymers becomes evident and phase separation begins. The lower surface energy of the acrylate polymer brings it to the surface. The phase separation continues till you have an almost perfect dual-layer. The “almost” is important. In order to preserve adhesion, there must be some intermingling of the polymer chains at the interface. The kinetics of the system ensure that the phases don’t separate totally.
Again, this seems easy in retrospect, but if you tried to do this without the aid of HSP thinking it would take a very long time to get even the basic functionality working correctly.
Non-solvents coming together
One of the striking and unexpected predictions from HSP is that mixtures of non-solvents are perfectly capable of being excellent solvents. Hansen showed this back in 1967 when working on the polymer series for which we’ve provided up-to-date correlation data in files Polymer88xx. By choosing Polymer88E you find its HSP are [19.3 ,6.0, 10.4, 10.5]. If you check the data table you find that Diethyl ether [14.5, 2.9, 5.1] and Propylene carbonate [20.0, 18.0, 4.1] are both non-solvents. But a 50:50 mixture [17.3, 10.5, 4.6] is inside E’s sphere and is calculated to be, and was shown in practice by Hansen to be, a solvent. There are numerous examples of such mixtures of non-solvents being solvents and it is important for formulators to think outside the box (or, rather, outside the sphere) and start to get different solvency characteristics by mixing solvents they would otherwise have totally ignored. It’s worth saying once more that the Hildebrand solubility parameter simply cannot do this sort of thing because there is no coherent way of dealing with the issue that very different solvents can have very similar Hildebrand parameters.