In the 1980s, everyone was excited about the prospect of using water based inks for flexible packaging films.
We already had success printing water based inks on paper and corrugated board, but the thinking of the day was that water could replace solvent in more areas of printing and the world would be a much better place.
Of course, back then there wasn’t the kind of focused attention on sustainability, recycling and the environment that we see today. In fact, the term “environmentally friendly” was still a new concept.
Today, we have taken the lessons of the past twenty something years and we now have a host of new raw materials that allow us to push the envelope further than we ever have, and printers today are using water based inks to print on a wide variety of non-porous substrates.
In this article, we’ll look at a specific formulating technique that I’ve used to develop water-based inks for today’s packaging needs.
The Great Solvent Ink
A common approach to building the ideal water-based ink is to use the solvent ink as the model for success.
We first have to understand what makes solvent based inks so great – and let’s agree that solvent inks are great because they are so ubiquitous in our industry.
The solvent ink is the “ideal” because it does what we need in a way that’s commercially acceptable:
- It runs fast
- the print quality is great
- it adheres to many substrates and
- the necessary raw materials are readily available to the world (see infographic below).
But wait a minute.
If we examine the number of positive attributes of water based inks (infographic below), water seems to easily win out because there are many more positive things associated with water based inks than there are with solvent inks.
So what’s going on here?
The problem for the water based ink formulator is that the printing world still operates on the solvent ink paradigm – printing ink success is still skewed toward the attributes of solvent ink – not the water based ink. Put another way – we value the attributes of the solvent ink more than we do those of water based ink.
Comparing Solvent and Water based inks
What the infographic above also shows is a direct comparison between the formulation of a solvent ink versus the formulation of a water-based ink, and the reason this is significant is that you can see just how much solvent is replaced by switching to water-based ink.
- Anatomy of a Water-Based Ink Formulation
Every ink formula consists of three distinct parts: the Colorant, the ink Vehicle (or ink medium) and the diluent. Of course, we could further sub-divide these parts, but for all intents and purposes, this works for the scope of this paper. Careful attention to each part is critical to achieving optimum results, both in terms of on-press performance and meeting the needs of the finished package.
- The Colorant
The colorant, usually in the form of a pigment dispersion, often contributes 50% or more of the total ink formulation. For this reason, the pigment dispersion must be carefully formulated to take into account the final ink requirements. For packaging inks, I prefer to use resin based pigment dispersions as opposed to surfactant based products. Resinated dispersions have better press stability, show better printability and are compatible with a wider range of ink emulsions
Looking at the formula below, we start off with a high molecular weight (and high acid number) acrylic solution resin, which is ideal for optimum pigment wetting and has great stability. We use aqua ammonia to make the resin water-soluble and avoid any slow evaporating amines at this stage, particularly if our targeted end use is printing on flexible packaging where we need fast drying and good water resistance.
In the pigment dispersion, we assist the wetting mechanism with the addition of a small amount of an anionic surfactant/wetting agent, which helps during the grinding stage. Take care not to add too much, as an excessive surfactant can make foaming an issue. We finish off the formula with a small amount of a silicone-based defoamer, which is highly effective and well suited to foam control in pigment dispersions. Finally, some biocide is used to discourage the development of mold.
The manufacturing process begins by pre-mixing all of the components on a Cowls or similar mixer, which helps combine all of the ingredients uniformly before the real dispersing begins. This pre-mixing stage is usually accomplished in about 30 minutes in a laboratory setting, but may last longer in a factory environment. Next, the mixture is placed in a bead mill or similar media mill and processed until the fineness of grind is around 1-3 microns. The time required for milling pigments will vary depending on the type of pigment, the grinding media, and the type of equipment you’re using. The final milled product should be free flowing and not thixotropic or paste-like.
Acrylic Resin Solution
|Aqua Ammonia (28%)||8%|
Pigment Dispersion Formulation
|Acrylic resin Solution||32%||Pigment wetting Resolubility/ gloss|
|Wetting Agent||0.3%||Pigment Wetting|
|Silicone based defoamer||0.3%||Defoaming|
- The Ink Vehicle
The choices you make for the ink vehicle is just as important as the colorant. Here we must build in specific properties, which will be dictated by the end use of the finished package. These properties will include things like coefficient of friction (slip), heat resistance, water or alkali resistance, rub resistance and lamination bonding ability. If we’re printing for tobacco packaging, we’ll limit the use of certain solvents and chemical ingredients that could taint the product.
Ink Vehicle for Surface Printing on Polyolefin Films
Material Percentage (%) Function Total 100% Soft Emulsion 83.2% Low Tg (-23) emulsion for adhesion, water resistance, gloss and flexibility. Polyethylene Wax 4.0% Abrasion resistance. Mineral oil defoamer 0.5% Defoamer Surfactant 0.5% Surface wetting Silicone additive 0.8% Slip and mar resistance. Water 11.0% Adjust viscosity to shipping viscosity.
- pH and final viscosity
The pH of the ink vehicle should be in the range 8.5 – 9.5. If you need to raise the pH, the best option is to use aqua ammonia (28%). It is possible to use other amines, like monoethanolamine (MEA) or solutions of Dimethylamine (DMEA), but keep in mind that slower evaporating amines like these will affect the drying speed of the final ink and can create some odor issues.A soft emulsion is essential for printing on films. Hard emulsions are too brittle (even when coalesced) for this type of application. The silicone additive (often an emulsion) offers several benefits, including adding slip, rub resistance and also scotch tape release.
The choice of diluents will depend on several factors: drying capacity of the printing press, environmental conditions ( humidity) and local restrictions on solvents. Under ideal conditions, we can use 100% water to reduce the ink to printing viscosity, but if we need to increase the drying speed, a blend of water and ethyl alcohol (or iso propyl alcohol) is advised. Many printing environments are not climate controlled and this can cause problems especially during the summer months when the humidity climbs. Under these conditions, it may be necessary to add up to 5% of a glycol ether solvent like Dowanal DPM to prevent “blushing” or “blooming” and to improve printability.
- Special additives
Other additives can be used to achieve further modifications and enhancements if the resin/emulsion combination isn’t sufficient. Two examples would be crosslinking additives like polyfunctional Aziridines and some metal based crosslinking agents.
Putting it all together
A finished surface ink formulation will look like this.
Finished Surface Ink Formulation
|Add items in the following order:|
|Pigment dispersion||40%||Add up to 60% depending on color strength requirements.|
|Water||4%||Substitute alcohol or glycol ethers depending on drying speed requirements.|
We also have to consider a couple of other important factors that affect ink performance: film treatment and printing cylinders. Uncoated substrates should be treated to a minimum of 42 dynes/cm to allow adequate ink wetting and adhesion. In most cases the best results are obtained when in-line treaters are used on the printing press. This is one area where water-based inks are less forgiving than solvent inks, so if you’re having adhesion issues, the first place to look is at the film treatment levels.
When it comes to cylinders, many printers use the same cylinders for solvent and water based inks, but this isn’t recommended. Typically gravure cylinders and flexographic anilox rolls should have shallower engravings for water based inks. The main reason is that it’s easier to evaporate water from lower volumes of ink, regardless of the type of printing press and its drying capability. Common gravure engravings currently being used for water based inks range between 175 LPI – 200 LPI, and anilox specifications of 180 LPI to up to 1200 LPI for process printing. Actual cell volumes vary depending on the design and the required color strength.
SOLVENT OR WATER? THE BENEFITS OF USING WATER BASED INKS
- Low or no VOC component.
- Lower odor.
- Inks utilize sustainable resources.
- Non-flammable (therefore less regulated environmentally).
- Longer in-can ink stability than solvent ink.
- Easier to formulate inks for direct/in-direct food contact.
- Reduced potential for discharging toxic materials.
- Improves plant working conditions.
- Increases life of photopolymer plates (flexography).
- Less wear on gravure doctor blades, particularly with white (Ti02) ink.
- Reduced environmental costs.
- No/Reduced incinerator costs.
The conversion from solvent to water-based packaging inks is still a work in progress.
In modern printing, the biggest challenge for water based inks is to keep up with the high press speeds (in excess of 1000 ft/min) commonly in place for solvent inks.
Water will never dry as quickly as a solvent, but the benefits of water are clear. In the days ahead most companies are focusing on modifying existing printing equipment to make using water based inks easier and on the raw material side there is a keen interest in developing faster-drying emulsions and those with better adhesion characteristics.
Advancements in these areas will ensure that the transition from solvent inks to water based inks is something we will see sooner rather than later.
Now, I would like to ask you what are your thoughts on the importance of water inks? Are you converting your solvent inks to water inks? If so, have you implemented any of the ideas listed above (how did it go)?