Ink odor is a complex consideration in ink development and the printing process. This article explores the technical aspects of ink odor in UV inkjet printing, investigating the range of factors influencing odor and considerations for developing low odor inks.
Ink odor
While the odor of UV ink is better than most solvent inks, it may not be as good as aqueous ink as they are formulated mostly with water. The curious feature of an ink’s odor is that it varies at different stages of the printing process. There is a vast difference between ink when the ink container has just been opened compared to when it has been cured on the substrate.
If we take these four examples, ink will have a different odor at each stage, and odor may or may not be an issue:
- Wet ink when the bottle is open
- During the actual printing
- The samples immediately after printing
- The printed samples after being left in the air for more than 24 hours
What causes an odor in UV inkjet inks?
As mentioned, throughout the lifecycle of ink, its smell will vary. When a container of fresh ink is opened – regardless of the formulation – there will be an odor. The second type of smell occurs during the printing stage. The final odor, and one which might be noticed by end users, is when the ink has been cured on the substrate. This normally occurs just after it is printed and can become unnoticeable when left in the air for more than 24 hours. Regardless of the type of ink, there is one characteristic that all UV inks share: there will always be an initial odor from wet ink which comes from the chemical formulation.
UV inkjet inks contain monomers, photo-initiators, dispersants, and pigments, with the main ingredient being monomers. Monomers are essential to the formulation of UV inkjet inks as they are vital for viscosity, adhesion and flexibility and folding resistance, however, they have an odor, and depending on the type of monomer, the odor level is different. Photo-initiators can also contribute to the odor. During the curing process, chemical bonds are broken to create reactive molecules and by-products of the reaction can have an odor.
Apart from the ink itself, there are many factors that can contribute to an odor during printing, such as ozone, system set-up and substrates. For example, mercury UV lamps can generate ozone, which is a source of odor, along with the ink. Normally, after sufficient curing, photo-initiators and unreacted monomers dissipate immediately after the printing process, and the odor can become unnoticeable after being left in the air for 24 hours. But if the ink doesn’t cure properly, these unreacted monomers can cause an odor issue on the prints, even after air drying.
An important point to consider is that substrate can have odor too. Substrates sometimes have coatings, and within certain temperature ranges of UV curing lamps, the substrate changes, which can release odor. As UV ink has an odor, it is easy to assume that any odor on the finished product is a result of the ink. Not only is it important to determine how strong the ink odor is but to rule out that there isn’t a smell coming from the substrate’s reaction to the curing lamps in the curing phase.
Formulating low odor UV inkjet ink
There are two important factors to consider when formulating low odor UV inkjet ink: raw materials selection and curing. We can select raw materials that are not volatile and odoriferous to achieve a low initial odor for wet ink. Also, we need to ensure that the curing of ink is sufficient to prevent a chemical release from the printed product.
Another aspect is that odor is very subjective. Apart from the intensity of the odor, our human perception also plays an important role, and the awareness of an odor varies widely among individuals. Different guidelines have tried to quantify or rate odor such as DIN EN 1230, BS EN 13725, and ASTM E544-99.
At Fujifilm, we have an internal standard based on DIN EN 1230-1. This European Standard specifies the test method for assessment of an odor released by a paper or board sample which will come into direct or indirect contact with foodstuffs. We use an “odor panel” of people who, in a controlled manner, assess the perceived odor and give realistic results as to its strength.
Lastly, the odor requirement varies between different applications. Some sectors such as Indirect Food Contact (IDFC) packaging require low-odor ink as an off-putting odor on food packaging would not be suitable. As Regulation (EC) No 1935/2004 states “the requirement articles and materials shall not transfer their components into the food in quantities that could endanger human health or bring about an unacceptable change to the composition, smell, or taste of the food”. Therefore, no trace of an odor from a UV inkjet ink must be transferred to the food for IDFC packaging printing.
The challenge of low odor inks
If a customer is content with every aspect of their UV inkjet ink apart from its odor, unfortunately, it is not a simple case of replacing ingredients with less pungent ones. Ink development is a complex process and each component in the formulation has a precise role. In many ways, changing a monomer or a photo-initiator is in effect creating a new ink which might impact key elements of the printing process such as the choice of printhead, curing lamps and substrate.
As odor varies at different stages of printing and is highly subjective, it’s important to clarify and agree on the test method and criteria to rate odor. Ideally, customers supply a “reference” ink or prints that have an acceptable odor level and one which they are satisfied with. Once criteria and expectations are clarified, then the process to formulate the low odor ink can get underway with confidence.
Also, there may be a cost implication with low odor inks. New ingredients that reduce the odor level may be more expensive and lead to higher ink costs which may question the long-term viability of the low odor ink. Furthermore, there may be a compliance issue with the new ingredients. Depending on where the ink will be supplied and used, different territories have different guidelines for ink formulations. Any changes in formulation would be assessed by our Regulatory and Compliance Team to ensure that it is permitted in the geographical areas planned for its intended use.
Summary
At Fujifilm, we consider odor during the design stage of ink formulation. However, it is important to realize that odor will vary at different stages during the printing process, from the initial smell of wet ink to the odor from the printed samples.
By using less volatile and purer raw materials, we can formulate ink with a lower initial smell. However, odor does not just concern the intensity of odor itself as the subjective nature of human perception plays an important role.
As mentioned, failure to consider the smell can lead to issues at a later stage when the ink is being used in a production environment.
Fujifilm has the experience to quantify and measure odor which will provide realistic results for our clients and the end users of their products.