When an adhesive needs a new solvent blend, or you have to find ways that the components in an adhesive can be optimized to be “happy” together, or you need to find how to match the properties of the adhesive to those of a substrate, what should the adhesives formulator do?
To approach these issues from a science-based formulation method, we can use a powerful solubility and compatibility technique that uses
Hansen Solubility Parameters. Via HSP values for all our components we can rapidly identify which components like, or don’t like to be with each other.
Learn from Professor Steven Abbott what Hansen Solubility Parameters are and how they should become a go-to tool for science-based formulation.
Let's get started...
Better Adhesives via Science-based Formulation using Hansen Solubility Parameters
You have been asked to find a new solvent blend, or there is a requirement to incorporate some new ingredients into a formulation, or the compatibility between adhesive and substrate has to be maximized. How can you manage these requirements using the least amount of effort and expensive lab time?
If we try to use vague terms such as hydrophilic/hydrophobic or
polar/non-polar we quickly hit the problem that they are inadequate for describing the complexities of the molecules we use.
We need something better. And “better” means “with numbers” so we can formulate scientifically.
Focusing Specific Areas of Adhesives
We need a good measure of whether two molecules (or formulations) are “like” or “unlike” each other, via a “distance” between them. To do this, we must start with three numbers - The HSP, that describe each component within in our adhesive formulation. We need three numbers because we can’t capture chemistry in two numbers, and with four numbers things are too complex to handle.
Note, however, that the size of the molecule (via molecular weight or molar volume) plays an important role and is an implicit extra parameter.
We start by describing:
− What those three numbers are
− How you go about estimating or measuring them
− How you can use them in three specific areas of adhesion
The three specific areas of adhesion are:
− Coating an adhesive layer
− Ensuring compatibility between additives and the adhesive polymer
− Ensuring compatibility between adhesive and adherend
Hansen Solubility Parameters – The Three Numbers
− A solvent like hexane or a polymer like polyethylene is held together by general van der Waals, or Dispersive forces. Aromatics still only have Dispersion, but they have a broader electron cloud so have a larger amount of it. So, already we can see that one of the parameters must describe Dispersion, labeled
δD.
− Obviously, many solvents and polymers contain polar groups, so we need a number,
δP, to describe them.
− Finally, there is a difference between being Polar and having Hydrogen Bonds, so we need a Hydrogen Bonding parameter,
δH.
These three factors are the Hansen Solubility Parameters.
The table below lists the HSP of some common solvents:
| Solvent |
δD |
δP |
δH |
| Acetonitrile |
15.3 |
18 |
6.1 |
| Acetone |
15.5 |
10.4 |
7 |
| Benzene |
18.4 |
0 |
2 |
| Diethyl Ether |
14.5 |
2.9 |
4.6 |
| Dimethyl Sulfoxide |
18.4 |
16.4 |
10.2 |
| Hexane |
14.9 |
0 |
0 |
| Ethyl Acetate |
15.8 |
5.3 |
7.2 |
| Ethanol |
15.8 |
8.8 |
19.4 |
| Methylene Dichloride |
17 |
7.3 |
7.1 |
| N-Methyl-2-Pyrrolidone |
18 |
12.3 |
7.2 |
| Tetrahydrofuran |
16.8 |
5.7 |
8 |
| Water |
15.5 |
16 |
42.3 |
Looking at the list above, there is no problem making sense of the broad trend of the numbers.
− Acetonitrile, for example, is very polar, so its δP value is high, whereas its δH value is not so big as its ability to hydrogen-bond is modest. Ethanol is medium in terms of δP value, with a large δH value, as we would expect from such a hydrogen-bonded solvent. Each of them has a relatively small δD value.
− Benzene and DMSO both have higher δD values. Thanks to the large electron clouds around them!
− Hexane is a simple, relatively low δD solvent; acetone and ethyl acetate are just middle-of-the-road in their three parameters.
Determining How "Like" Molecules are
Now we are ready to find how “like” two molecules are. We just calculate the “distance”, D, in 3D space between any pair using the famous HSP formula (including a factor of 4 for the δD values)
D² = 4(δD1-δD2)²+ (δP1-δP2)²+ (δH1-δH2)²
− If the D value is less than ~ 4, then this is a reasonable match. If the value is greater than ~ 8, then this is a poor match.
− Given a target molecule plus a list of potentially interesting molecules, you simply calculate D between the target and each molecule, and after sorting them from low (good) to high (bad); you can choose which of the low D molecules meet your other requirements, such as volatility or greenness.
Typical examples involve a polymer as a target with the molecules being solvents. Equally, the target could be a specific portion of a resin and the molecules could be tackifiers. HSP also works well when the target is a filler or nanoparticle and the aim is to ensure maximum compatibility.
What if no molecule matches all your adhesive formulation requirements?
There is another key feature of HSP; when no single molecule meets all the requirements, you can work out a successful blend between two molecules which have high D values but which, for other reasons, you would like to use.
Suppose each molecule is reasonably matched to your target in terms of δD and δP but one has a high δH and the other has a low δH. The HSP of a blend is the weighted average of the respective parameters, so in this case, the δH can be tuned for a smaller distance to your target.
Therefore, you can blend two unusable molecules (D too large) into a usable formulation. This ability to turn poor starting materials into great blends is the reasons for HSP’s success over the past 50 years.
Given that to do these calculations you need the HSP values, where can you find them?
» Continue reading to learn how to find HSP of your own particle for creating a 'happy' formulation!