TAGS: UV/EB Technology
- UV Inkjet Inks & their Benefits
- Components of UV-curable Inks
- UV-curable Inks – Major Issues & Their Solutions
- Overcome Challenges of UV Inkjet Ink Formulation with Resonac's Monomers
What are UV Inkjet Inks?
UV inkjet printing is a digital printing method that uses ultraviolet light for drying or curing the ink. The exposure to UV light triggers a chemical reaction that causes the ink to instantly polymerize. The method can be used to print on a wide variety of non-absorbing substrates including paper, aluminum, glass, wood, vinyl materials, foam board and acrylic.
Benefits of UV-curable Inks
UV curing is a VOC-free, green technology. Since no solvent needs to be evaporated, the curing process is faster and can be carried out at low temperatures. UV curable inks are not absorbed into the substrate which prevents smudging and removes the need for additional costly coatings.
In addition, UV curable inks offer the following performance benefits:
Let's explore the basic components of UV-curable inks and how to overcome the challenges faced by formulators while formulating UV curing inks.
Basic Components of a UV-curable Ink Formulation
A UV curable ink is made up of four main components:
- Monomers: A monomer is the basic building block of UV curable ink. It directly affects the viscosity, softness/hardness, flexibility and elongation characteristics of the ink.
- Oligomers: Oligomers are reactive resins used to improve adhesion to a wide range of substrates.
- Pigments: Finely ground pigments are added to the ink formulation to achieve the desired color.
- Photo-initiators & other additives: Photo-initiators in the ink formulation are critical to the UV curing process. When exposed to UV light, they generate the reactive species that starts the polymerization of monomers and oligomers. The photo-initiated reaction cures the ink without the need for heating or air-drying. Other additives such as dispersants and stabilizers may be added to improve stability and performance.
Major Issues in UV-curable Ink Formulation and their Solutions
Formulators face several challenges when formulating UV curing inks. Meeting all the challenges is a complex task since key ink properties are not independent of each other. Meeting one requirement, by adjusting the formulation, can affect other properties. A balance must therefore be struck between low viscosity, reactivity, stability, and performance characteristics depending on the specific application.
Achieving High Reactivity with Lowest Viscosity
An important objective in UV ink formulation is to achieve low ink viscosity. High viscosity inks generally produce a thicker film which is more susceptible to the inclusion of bubbles and pinholes. This objective can be achieved through the selection of suitable low viscosity components. Amongst monomers,
mono-functional monomers offer the lowest viscosity.
However, lower functionality reduces reactivity which means curing is slower and more time is needed to achieve full cure. The cured film is also softer due to lower cross-linked density. Soft films have lower scratch and smear resistance, but they may be useful for printing on flexible substrates. Formulators must therefore strike a balance between ink viscosity, ink reactivity and cured film properties to meet the needs of the application.
Generally, mono or di-functional monomers are a good choice for low-viscosity UV inkjet inks. For higher reactivity and enhanced physical properties of the cured ink film, higher functionality monomers and oligomers can be incorporated into the formulation.
- Acrylate monomers typically have a lower viscosity than acrylate oligomers. They are often used as the primary component in UV inkjet inks.
- Hyper-branched acrylate oligomers also have a lower viscosity than traditional oligomers of equivalent functionality. They can be used to achieve high reactivity at a low viscosity.
- DCPD (dicyclopentadienyl) monomers offer low viscosity in the range of 7 to 25 mPa.s at 25°C (77°F). They can be used as dilution monomers to improve handling.
Ensuring Stability with Good Reactivity
Impurities present in monomers and oligomers, such as peroxides or metals can cause premature ink polymerization and gelation. At temperatures higher than 30°C, the shelf life of the ink can be significantly reduced. High purity oligomers and monomers must, therefore, be used to achieve
better ink stability at elevated temperatures.
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Acrylic monomers produced through dehydration esterification show relatively poor color and acid-value stability which reduces the shelf life of the ink and its post-printing durability.
- DCPD acrylate monomers produced using a transesterification process, show better color and acid-value stability as compared to corresponding dehydration esterification products. DCPD acrylate monomers also help reduce cure shrinkage, due to the voluminous DCPD structure. This eventually improves the formulating stability of UV Inkjet products. The addition of secondary thiols can improve the reactivity and conversion ratio of double bonds on UV exposure.
Reactivity of UV inkjet inks can also be increased by increasing the photo-initiator concentration. Typically, the formulation contains 5-15% photo-initiator. During formulation, it is best to start at the lower end of this range and increase the concentration until an acceptable level of reactivity is reached.
Improving Ink Performance
Good adhesion can be difficult to achieve on some substrates such as PC, PMMA and ABS. The use of
DCPD monomers provides excellent adhesion to difficult substrates. The use of secondary thiols in UV inks can further improve adhesion to glass, stainless steel and PET.
The adhesion improves significantly with increasing thiol content.
A high
glass transition temperature (Tg) of the polymer improves the
hardness of cured ink. The alicyclic structure of DCPD monomers gives the polymer a high glass transition temperature (Tg). The Tg of polymers based on monomers with linear alkyl groups is low and generally decreases with the carbon number. DCPD monomers have a high Tg which gives the polymer improved hardness.
The effect of alicyclic structure for high Tg
To enhance
printed image brilliance and color gamut, fine pigments (average particle size 100 um) with a narrow particle size distribution should be used in ink formulation. Narrow particle size distribution also enables low viscosity and produces free-flowing inks with the reduced tendency of the dispersed particles to flocculate, re-agglomerate or settle. Improperly dispersed particles can cause inkjet nozzle blockage which leads to printing defects.
On the other hand,
small particle size can reduce image durability. The solution is to use high-performance pigments with high inherent light and weather stability; these include azo-pigments and polycyclic pigments.
High molecular weight dispersants can be added for improved dispersion, good wetting and deagglomeration.
Overcoming Oxygen Inhibition
Polymerization of acrylates can be affected by oxygen. Radical peroxides generated during polymerization can scavenge initiating and propagating radicals to stop the polymerization process. This problem is more prominent in low viscosity inks as oxygen diffuses more easily into low viscosity films.
Oxygen inhibition is particularly problematic on the surface of the curing film. It produces a tacky surface even if the bulk of the film has been properly cured. Following techniques can be used to
prevent oxygen inhibition:
- Print and cure under a nitrogen blanket; this can be expensive and may also be impractical especially in the case of scanning head UV inkjet platforms.
- Choose photo-initiators less susceptible to oxygen inhibition.
- Increase photo-initiator concentration.
- Use amine synergists to reduce oxygen inhibition; the tertiary amine function provides an active hydrogen donor site for the excited triplet state of the photo-initiator.
- Increased UV intensity or irradiance creates an overflow of free radicals which compensates for radicals consumed and neutralized by oxygen.
- Use secondary thiols that donate hydrogen to radical peroxides and themselves become free radicals that promote polymerization.
Overcome oxygen inhibition during polymerization using KarenzMT™ thiols
UV-LED Curability
Mercury UV lamps have conventionally been used for curing photo-curable resins. Products that use mercury are a risk to health and the environment. UV LEDs offer a safe alternate light source but the light output is generally insufficient for full curing. The use of secondary thiols
enhances the curability and improves the molding stability of UV inkjet products, even with acrylic monomers. Thiols act as a chain transfer agent and proceed with the next polymerization step while regenerating radicals. This mechanism leads to deep curing even in weak luminance.
Health & Safety Issues
Thiols are generally used as accelerators to enhance the curing of a UV system. There is growing concern about their impact on workers’ health due to the peculiar odor of sulfur. The odor in thiols is caused mainly by a small fraction of residual impurities. Thiols from which these impurities have been removed have a significantly reduced level of odor.
Acrylic monomers can cause skin irritation raising health concerns. Some DCPD acrylic monomers are specially produced to have lower skin irritation due to their special production methodology. Incorporation of these monomers in ink formulation can reduce skin irritation by up to 50%.
Talk to Terence Kenneth where he will help you overcome complex formulation challenges in UV inkjet inks like balancing low viscosity, UV stability, good jetting, etc. He will also guide you to select raw materials and troubleshoot problems with practical examples.
Overcome the Challenges of UV Inkjet Ink Formulation with Resonac’s Monomers
Benefits of Fancryl 500 Series Monomers
Resonac offers the
Fancryl 500 series of DCPD (dicyclopentadienyl) monomers. These monomers offer several useful features for inkjet applications due to the alicyclic DCPD chemical structure shown below:
Molecular structure of some Fancryl 500 series monomers
Fancryl monomers are produced using a transesterification method in which one ester is converted into another via an alkoxy moiety exchange. Compared to monomers produced using dehydration esterification, products of a transesterification process demonstrate very good stability as sulfur and other impurities are almost not present. The transesterification products have minimal ionic impurities, outstanding chemical stability, and lower skin irritancy.
Fancryl 500 series DCPD acrylic monomers show excellent color and acid-value stability when compared to acrylic monomers produced through dehydration esterification. These monomers
improve the adhesion and hardness of UV inkjet inks. Fancryl 500 series monomers have low viscosity which gives the ink good handling properties. In addition, Fancryl monomers offer the benefits of reduced skin irritation and lower cure shrinkage.
Benefits of KarenzMT™ Series Thiol Monomers
Resonac also offers the
KarenzMT™ series of secondary thiol monomers which help solve problems associated with the use of primary thiols. KarenzMT™ secondary thiols can help
overcome oxygen inhibition during the photo-polymerization process. They offer additional benefits such as UV-LED curability, improved reactivity, stronger adhesion, and low odor.
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Watch Video Tutorial – 5 Practical Tips for Obtaining Successful UV Inkjet Ink Formulation
Digital Inkjet Inks – A Comprehensive Guide