Name: Jennifer

Country: England

Message: I am a chemistry student whose project involves investigating the effect of temperature on dyeing with Reactive Dyes. I have experimented with temperatures from 30°C to 80° with Procion Red MX. I understand that the higher temperatures (80°C etc) cause the hydrolysis of the dye with water to become more frequent (due to your site), but I am having some trouble trying to explain WHY the higher temperatures cause this to happen? My results from the experiment showed that 80°C caused the Cotton material to have a very poor ability to be dyed. However, I hypothesised that the higher temperatures would cause Cotton material to absorb dye more, because of the molecules having more kinetic energy, thus more collisions would occur between dye molecule and fibre, increasing the chance of a reaction taking place and bond formation... Do you have any idea where I have gone wrong in my thinking? Thank you very much for your time.

As you were thinking, hotter temperatures in general increase the rate of any reaction. The same thing occurs with dye—but also with the water that the dye is dissolved in. The dyes can react with either the cellulose fiber or with the water, the latter reaction being known as hydrolysis. The effect of the added energy is much greater on the dye reaction rate than on the ability of the dye to soak into the fiber. Increasing temperature too much causes the dye to react with the water before it ever gets into the fiber. In fact, it's better to let the dye soak into the fiber for some time before beginning your dye reaction by adding soda ash (or other pH increaser) and any heat.

The cellulose molecules in cotton material can dye well at 80°C, if the dye is already located in the fiber, adjacent to the cellulose molecule, before it reaches that temperature. It works fine to heat the dye reaction up excessively warm, if the dye is in place already. To make sure that this is true for some of your dye molecules, you must have already soaked your swatch of fabric or bundle of yarn in the dye, using either a high dye concentration or added salt to encourage the dye to approach the fiber, and including the soda ash or other high-pH chemical to assist the fiber in reacting with the dye, before heating the dye reaction. 

The problem occurs when the dye is not in the fiber, but only in the water, when it gets hot. The dye will react more quickly with both the fiber and with water, depending on which it is close to, when it gets warmer. What you did not allow for was the idea of competition for the dye between the water and the cellulose molecule. If the dye reacts with water before it can react with the cellulose, then it will not be able to react with the cellulose. To see the reaction that occurs between a dichlorotriazine dye molecule and cellulose fiber, see this page, the May 19, 2005 entry in my "All About Hand Dyeing Q&A" blog: "Chemical reaction for a dichlorotriazine dye with cellulose". The reaction between dye and water is similar. In the presence of a high pH, the dye molecule is attacked by either an activated cellulose molecule or by a hydroxide ion. As quoted there, "The attacking neutrophile can be either a cellulosate anion or a hydroxide ion, the former leading to fixation on the fibre and the latter resulting in hydrolysis of the reactive dye." 

Salt is important in this situation, even though it is not involved in the chemical reaction. If you have dissolved your dye in water, if the volume of the water is relatively high, then you will need to add salt (such as sodium chloride or sodium sulfate) in order to reduce the electronegativity of both the dye and the fiber. Otherwise, the fact that both the dye and the cellulose or other fiber have negative charges makes it difficult for the dye to approach the fiber closely enough to permit a reaction. Salt is not required for direct dye application methods, such as tie-dyeing or dye painting, because the relatively low amount of water used makes it more likely that a dye molecule will encounter a cellulose molecule in time to react with it.

By the way, you must find and use the correct name for your specific dye. All of the Procion MX dyes are dichlorotriazine dyes, but the different dyes within the Procion MX dye line are not equivalent. There are about five different reds available (in addition, of course, to all of the many mixtures of different dye colors sold by some dye retailers). Procion red MX-8B is far more reactive than Procion red MX-5B, which means that it reacts with both textiles and water more quickly. It is important also to find the generic name for your dye. It is better to be able to specify, for example "Colour Index reactive red 2" versus "Colour Index Reactive Red 11". For a chart showing the generic names for each of the Procion MX dyes in common use, see my page "Which Procion MX colors are pure, and which mixtures?". Another chart, this one showing the relative reactivity of a number of dichlorotriazine dyes, is located in the Dye Forum on my site. (The reactivity is expressed in the number of minutes required, at a specific temperature and pH, for the dye to hydrolyze, that is, to with water.) You can compare the chemical structures of various Procion MX type dyes, including several red ones, on my page "What is the chemical structure of Procion MX dye?". When describing your experiments, it is also important to specify not only the temperature of your dye reactions, but also the pH and the concentration of both dye and any salt that you may have added.

I hope that this will resolve the problem you've been having in understanding your dye's behavior.

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