How Do You Make an Azo Dye?
We don’t often think of color as something manufactured. The color of our favorite shirt or the paint on the wall seems inherent, but it’s actually a careful process to make the colors we see in products. Making dyes is difficult at best. If you follow accurate and detailed instructions, the process can be less daunting.
I believe that manufacturing dyes is just as much an art form as it is a science. Compare it to baking a cake. You follow the recipe exactly, but wind up with a sub-par cake. Maybe your oven was too hot or you waited too long between steps. Even when you follow the recipe, things can go wrong.
So how does this compare to dye making? Let’s look at a dye class and the steps that go into manufacturing it.
Creating an Azo Dye: Cracking Color from Oil
We’ll look at synthetic organic chemicals, in this case, Azo Dyes. These are the simplest of dyes to produce. We crack intermediate chemicals from crude oil, test them, and cause reactions. Sometimes we need multiple reactions to produce the dye product we want.
Dye manufacturing consists of five basic steps: diazotization, coupling, isolation-filtration, drying, grinding, and standardization. You must watch for one factor that influences all these processes: purity of intermediates. If you use impure or tainted intermediates, your result is contaminated dyes.
Like with baking a cake, if we have spoiled flour, sugar, or eggs, then you’re sure to end up with an inedible cake. The same holds true for manufacturing a dye. What you put in is what you get out.
Step One: Diazotization
In the first step, we produce a diazonium salt to react with a coupling component. The process involves nitrosation of primary aromatic amines. We do this by introducing a strong acid like HCL and Sodium Nitrite, with ice to control the temperature. After the reaction, we test the product with starch iodide paper and create a blue reaction. The more times we perform a diazo reaction, the different class of dye. These include dye products like disazo, trisaxo, and polyazo.
Step Two: Coupling
To complete the synthesis of an azo dye, the diazonium salt reacts as an electrophile with an electron-rich coupling component, like a phenol or an aniline. This is done through an electrophilic aromatic substitution mechanism. We must control this reaction by monitoring pH, volume, time, and temperature throughout the coupling reaction.
An end point test will show if the reaction completed. You achieve this by testing a salted sample of the coupling solution with a diazo or coupling. Normally, a slight excess of either the diazo or coupling exists. Whichever component was in excess will react after performing this test. Once you complete the test and the results look good, we can move to the next step.
Step Three: Isolation and Filtration
In the isolation-filtration process, you adjust the coupling liquid for pH, salt content, volume, and temperature. Then, filter the coupling. This process separates the liquid from the dyesalt crystals in the coupling liquid. Sometimes the product does not isolate and the coupling goes directly into the spray dryer, completing the drying phase.
The isolation-filtration stage produces a paste the consistency of thick mud. It contains at least 50% water, which we’ll remove by drying in the next step.
Step Four: Drying
A common way to dry the paste is to spread it onto trays and place it in a tray dryer. After spreading, the trays go on a rack dryer and dry at a specific temperature. We want to avoid charring during this process, so it’s critical to monitor this phase.
Once dry, we grind it to a specific particle size (usually the consistency of flour salt). If we used an alternative drying method, like spray drying, we can skip this step.
Step Five: Testing the Dye
The last step is to test the dye to ensure it meets specifications for the end product. These include concentration, shade, solubility, and often use-specific tests.
Once completed, the dye is standardized based on customers’ requests.
Testing is a moment of truth. The question is, will this batch meet the standard? It is the same question we have when making a cake, will we be able to enjoy it?
These five steps serve as a basis of what it takes to make one of the most common dye classes. Each dye family has its own set of processes and tests it undergoes to create the end color. Sometimes, the properties of the dye class are more important than the hues. Other times, vice versa. Having a clear understanding of what’s important for the customer will help guide the manufacturing process and result in the colors we see used every day.
To learn more about dye families and using dyes in your products, click below!