How to improve paint and coating formulations: Molecular weight analysis

In this last post of our series of five on paint analysis, we will be highlighting the influence of the binder on coating properties – and in particular, the effect of molecular weight (MW) and molecular structure.

When considering the properties of paints and coatings, it is easy to focus attention on the all-important pigments and extenders. But of course, binders have an essential role to play too – after all, they quite literally hold it all together.

So, it is worth digging deep into the properties of the acrylic, polyurethane or epoxy resin that you are using in your paint formulation – and especially making sure that its molecular weight (MW) and molecular structure are optimized for the application.

How MW of polymers is measured

In the past, several methods were used to determine molecular weight, including end-group analysis and light-scattering methods, but these have now largely been superseded by gel permeation chromatography (GPC), otherwise known as size-exclusion chromatography (SEC). This works by passing the polymer solution through a column that contains beads with pores of various sizes. Smaller molecules get ‘delayed’ in more of the pores, so they take longer to pass through the column.

Using a suitable detector, you get a plot of elution time against abundance, which (if you like) you can flip left to right, to get a plot of MW against abundance. This profile not only tells you the average MW, but shows the distribution of weights too.

Maximizing information obtained from a single sample

Before I move on, I’d like to dwell for a moment on that point about choosing a ‘suitable’ detector. Many detectors used with GPC/SEC are based on refractive index measurement, but the problem is that unless you’re using calibration standards identical to the polymer being analyzed, it will only report relative molecular weight, which isn’t particularly useful.

A better approach is using light-scattering detectors, which allow generic standards to be employed, and absolute MW values and distributions to be determined. Not only that, but if you add a capillary viscometer to your system you can investigate polymer branching and molecular structure, while a UV detector will tell you about any chromophores present (and so provide information on the degree of copolymerization if one of the monomers is aromatic). So you can quite easily get a lot of useful information from a single sample.

Applications of GPC/SEC in the paint industry

So much for how to measure molecular weight distributions. What is this data useful for?

Helping to fine-tune coating performance

One of the main uses of MW information is helping to choose the right binder for the application. This can be a difficult balancing act. For example, with many paints, a higher-MW polymer may give a more durable finished surface, whereas lower-MW polymers may give a better gloss.

Either way, knowledge of the absolute MW value and the MW distribution is essential, and this information can be uncovered using GPC/SEC with a light-scattering detector and viscometer. And if you combine it with other measuring methods, it opens up options for determining the degree of chain branching, the behavior of the polymer in solution, and much more.

Controlling formulation viscosity

Viscosity is of course an essential characteristic of a paint or resin formulation, in which the physical properties of the polymer binder play a significant role. Once again, the absolute MW and the MW distribution, along with the degree of chain branching, are important metrics to understand.

Optimizing dispersion stability

To prevent pigment particles from agglomerating and settling out, particle size is of course an essential piece of information, with electrostatic potential also being important for polar solvents (such as water).
But in non-polar solvents, polymer-based dispersants can be vital for achieving good stability, by creating a film around each particle. To do this job well, the polymer’s characteristics need to be tightly defined, which is where GPC/SEC again has a role to play in the formulation scientist’s toolbox.

GPC/SEC – Helping formulation scientists to understand MW distributions

This has only been a brief overview, but I hope that it’s highlighted a couple of ways in which knowledge of MW values and distributions can be useful in optimizing the binder you’re using in a formulation.

Not only that, but it’s important to note that by using a GPC/SEC instrument with multiple measurement modalities, you can generate the maximum amount of useful information. This way (if you’ll forgive the pun) you can ensure that the binder is ‘pulling its weight in your paint formulation!

Interested in finding out more about the role of analytical methods to characterize paints and coatings?

Then check out our white paper: “Improving paint and coating formulations: Using nanostructural analysis to understand macroscopic properties”.

Stay tuned for more content next year by following our Advanced Materials specialists on LinkedIn.

You can also be the first to read our industry updates by subscribing to our Advanced Materials newsletter here.