10/04/2022 | pH | 10 MINUTE READ
How pH Monitoring Can Improve Print and Color Quality of Water-Based Ink
Ink Stability Depends on pH
Ink is comprised of extremely small particles and the stability of those particles in a solution are dependent on the solution pH. A larger gradient between the solution pH and the surface charge result in more stable particles. Most commercial water based inks are stable at an alkaline pH. Inks that have particles stable under neutral pH conditions exist, but are costlier.
Amines, a relative of ammonia, are an additive that works to address several ink control parameters; amines are added to increase and maintain pH for stability, to control ink drying time through amine choice, and to increase solubility of other ink additives such as resins. Different amines and different pH values result in different drying times; types of amines used for ink formulations are often referred to as faster and slower amines.
Viscosity Tuning via pH
Previous ink formulations required stringent pH monitoring and knowledge of chemistry to keep inks stable during a printing process. Modern inks are less subject to pH shift, however, monitoring is not only important for ink stability. In water based inks, pH is strongly tied to viscosity, which affects the color and texture of printed material. Color consistency relies on maintaining a constant viscosity because thicker ink will deliver more colorant and thinner ink less colorant. Viscosity also affects solvent retention and thus plays a role in drying rates.
Even small viscosity shifts can result in printing variation, thus pH monitoring can result in a more consistent printing run. Lower pH increases the rate of evaporation and thus can result in a thicker ink. Conversely, higher pH results in a thinner ink than expected due to less evaporation. Effects are significant when large volumes of ink are used. Intuitively, using large volumes may seem to ensure color consistency, but it may also leave the process at risk for chemical shifts.
Though small pH changes can affect viscosity, fortunately, pH adjustment and correction is relatively simple. While viscosity can be adjusted by the addition of water, tuning by water addition only helps after viscosity has shifted. Instead, by monitoring and maintaining pH, viscosity shifts can be avoided. The more monitoring and correcting, the better the printing result. The result of improved ink stability and improved process control means that printing today is easier than ever to maintain consistent print and color quality.
Water-based Ink pH Material Science
To understand why water-based printing ink is a favorite in the industry, you should know about the pH material science behind using this type of ink. Water is able to disassociate into hydroxide and hydrogen ions. Even though these ions are only present in small concentrations, these concentrations define the pH scale that ranges from 0-14.
At a temperature of 25 degrees Celsius, the water’s pH is around 7.0. In the event that there are a higher concentration of H30+ ions in the water, the solution is considered to be acidic. A lower concentration of these ions leads to alkaline water. On the pH scale, everything below 7.0 is considered acidic. All readings above 7.0 are alkaline.
Most of the components in water-based inks contribute to the pH that the ink has. For instance, the acrylic resins that water-based inks consist of are considered to be acidic. Keep in mind that acrylic resins aren’t soluble in water on their own. However, adding alkaline materials and substances to the water makes these resins soluble.
Once acidic resins are dissolved in water that’s otherwise alkaline, these resins will be neutralized, which results in the formation of soluble salts. The overall solubility depends on the current pH levels. In the event that the pH levels fall, the resins typically become less soluble.
Pigments are known to be insoluble. However, the pH of the solution dictates what the fine particle stability of pigments is. Dispersing agents and resins produce salts that surround the pigment particle, which leads to the pigment becoming more stable. The majority of water-based inks use ammonium hydroxide as well as volatile amines to make sure that the ink’s alkaline pH is properly maintained.
However, all of the ink components will still need to be tested to ensure optimal performance. Most of these inks have an alkaline pH that measures at 8.0-9.5. Ammonium hydroxide, however, can be volatile, which means that the pH levels could drop lower unless you take steps to enhance the pH range.
In the event that the pH drops to lower levels, the ink will become more viscous, which may lead to a build up of the substance on the printing plate as well as the production of high amounts of foam. The ink could also become highly odorous.
It’s common for pH levels to drop too far and cause a reduction in print quality. In this situation, the edge of the print may appear to have spots around it. The edge could also be more ragged. Modern technologies have provided ink formulators with the means of developing various water-based inks that are able to be used at a lower pH without any of the aforementioned issues of foaming. The eventual goal is to produce water-based inks that can be used at a neutral pH of 7.0.
The main challenge that must be overcome with lower-pH inks is the balance of water-resistance with the final printed product. The inks will need to deliver the same amount of water-resistance and adhesion while also using resin binders that use a lower amount of ammonia. The newest water-based inks that are pH neutral are still alkaline soluble. The only difference is that they are able to function when at a more neutral pH.
One reason why ink formulators want to solve these issues is because of the cost reduction. When working as intended, these inks should be able to run on the press without requiring a pH adjustment during the printing process. The press should also remain cleaner since a lower amount of foam will be produced.
Water-based vs. Plastisol: Which Ink is Better?
The two primary types of inks that are used in screen printing include water-based inks and plastisol. Plastisol printing ink is considerable cheaper and more commonly used. However, it’s largely outclassed by water-based ink.
Plastisol is long-lasting, always-available, and cheap, which leads some screen printers to wonder why they should use water-based inks for their screen-printing needs. In truth, water-based inks have a far higher quality than plastisol inks and offer many additional benefits, which include everything from being better for the environment to feeling more comfortable.
What is Plastisol Ink?
Plastisol ink is also known as polyvinyl chloride, which is the same substance that pipes are made from. This type of ink consists of liquid plastic as well. This is the most common ink available in screen-printing stores as a result of its affordability, accessibility, ease-of-use, and durability. You can leave this ink on your shelf for years and still use it without issue. While there are some advantages that come with using plastisol ink, there are also some notable downsides.
What is Water-based Ink?
Water-based inks consist of water as the primary solvent that carries the pigment. These inks consist of either acrylic resins or a hybrid blend of polyurethanes and acrylic resin. This type of ink is able to penetrate deeper into the fabric when compared to plastisol, which typically results in a softer and more comfortable print. The resins that this ink is made of are soft. All of the water in the ink will evaporate while the curing process is ongoing.
Advantages of Screen Printing with Water-based Inks
There are numerous advantages associated with using water-based inks for screen printing.
Water-based inks have a softer hand
In the screen-printing industry, the word “hand” refers to how a print feels as well as the print’s weight. A hand can be light and soft or heavy and rough. When it comes to clothing, it’s important for the print to have a light and soft feel. Water-based inks are able to produce exceedingly light and soft prints that aren’t available with plastisol inks.
Plastisol is a type of liquid plastic that forms into a solid once the liquid is heated at a very high temperature. The plastisol print essentially consists of two layers. The print will lay on top of a thin layer of solid plastic, which itself is positioned on top of the piece of clothing or other type of fabric.
Water-based inks are largely water-soluble despite consisting of a small amount of plastic. Since these inks are water-soluble, they can be absorbed directly into the fabric. Once the ink has been cured, the solvents in the ink will evaporate, which leaves behind pigmented binder compounds. Because of how soft the hand can be with water-based prints, these prints oftentimes look like dyed fabric.
Water-based inks are breathable
Water-based inks are also considerably more breathable than their plastisol counterparts. Breathability is the ability that moisture has of moving through a piece of fabric. Since water-based inks are directly absorbed into the fabric, most of the openings that are present between the fabric fibers remain open. Plastisol prints are known to close most of these gaps.
Water-based inks last longer
Water-based inks last much longer than plastisol inks. When you place a plastisol print on a shirt, you can expect the print for last for upwards of 100 washes before it starts to crinkle and fade. Eventually, the print will begin to chip off of the piece of clothing. Water-based inks are absorbed into the garment and may never fade.
You can achieve sharper detail with water-based inks
It’s possible to obtain sharper details with water-based ink. Even though you can have a high-detail print when using plastisol, the thinness of water-based inks means that they can be used with exceedingly high mesh counts to produce crystal-clear details.
Water-based inks are more environmentally friendly
Water-based inks are somewhat more environmentally friendly than plastisol inks, which is why many brands have moved towards using water-based inks to reduce their use of PVC. Keep in mind that each ink differs with how environmentally friendly it is. While water-based inks are typically more beneficial to the environment than plastisol inks, there are still some downsides to using water-based inks when taking the environment into account.
As mentioned previously, water-based inks contain a small amount of plastic via acrylics and other types of binders. Even though water-based inks don’t contain any PVC, they still use solvents that will evaporate during the curing process. While water is the main solvent used during the process, additional solvents like alcohol and formaldehyde are also used from time to time, both of which can be hazardous to the environment.
Water-based inks use more energy during the curing process. These inks also have a somewhat lower yield when compared to plastisol, which leads to more waste. If this waste isn’t safely removed from the facility in question, it could harm the environment.
If you work in the screen-printing or clothing industry, water-based inks are oftentimes preferable if you want to obtain a sharp and long-lasting print that provides a softer and more comfortable feel. If you want to effectively improve the color quality and print of water-based ink, it’s highly recommended that you perform regular pH monitoring with a pH sensor, which will allow you to avoid foaming and similar printing issues.
Posted by Dominic O'Donnell on October 4, 2022
Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.