Seminar on

Role of Rheological Properties of UV Inks on Flexo Printability

By

Mr. Siddharth Raju Adagatla

Under the Guidance of

Prof. Dr. Akshay V. Joshi

 

Pune Vidyarthi Griha’s College of Engineering and Technology & G. K. Pate (Wani) Institute of Management, Pune, India 

Academic Year 2020-21

UV inks are a special type of inks that have been formulated to dry only when exposed to ultraviolet (UV) light. When the substrate  receives wet ink in printing press, it is immediately exposed to UV light so the ink does not have the opportunity to seep or spread. Hence the printed image will resemble an overall better dot and print effect, resulting in a high-quality finish. When UV ink is exposed under UV light, they absorb UV light and start a Polymerization reaction that employs the use of free electrons, or free radicals causing an instantaneous cross-linking of the resins. Photo-initiators generally react to wavelengths of between 200 - 400 nm. 

Fig 1: Polymerization reaction

Image copyright ©2008 Graphic Imaging Association (sgia.org) All Rights Reserved.

UV inks are widely used in Flexographic Printing and are most popular in the Outer-wrap food Packaging Industry and label application. UV inks are usually 100% non-volatile materials so there is no concern for the evaporation of any volatile materials like solvents or water-based inks. As 100% of the ink is applied to the substrate, UV inks have the potential to achieve greater densities than conventional inks. UV ink provides many desirable end-use qualities which includes excellent rub resistance, chemical resistance, exceptional color consistency, and superior gloss. Rheology is used to describe the deformation and flow behavior of a material. The rheology of UV inks is determined by the components that are added to the ink. Rheology is very important because the rheological properties are closely related to final print quality. The rheology of ink plays a central role in ink distribution and transfer in Flexographic printing processes. [1]

Fig 2: UV inks

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Ingredients of UV inks:

Monomers -

The basic unit for polymers is formed by a molecule of monomer. Monomers are used at 30-60 weight percent of the formulation in flexographic formulations. Choosing the right monomers is critical because a wrong type of monomer can lead to plate swell and create registration issues. Monomers provide lower viscosity and can influence rheology, cure rates, and adhesion of UV inks. [2] 

Oligomers -

Oligomer is the main component used as a binder in ink formulations. These are highly viscous and contain active groups for cross-linkage. Oligomers are essential intermediates of the polymerization reaction. Oligomers provide enhanced physical properties, cure speed, and adhesion. [3]

Additives -

Additives play a critical role in the formulation and production of inks, which adds necessary characteristics to the ink needs like to run more smoothly on press and also to protect the finished product. Additives re added to improve curing, increase viscosity and to prevent the ink from dropping off the screen, and improve adhesion. These additives come in many forms like waxes, surfactants, defoamers, or countless other products. [3]

Fig 3: Additives

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Colorant -

Colorant comprises pigment and dyes. The purpose of pigment or dye is to provide image contrast on a substrate. Pigments are small particles that are insoluble in ink vehicles and dyes are soluble in the ink.. Pigments are usually more opaque than dyes. Inks that contain pigments tend to bleed less, are resistant to chemicals and heat. Pigments are a good option for conventional printing because of their high color strength and lightfastness. [4]

Photoinitiators -

Photoinitiators are chemical substances in which the small molecules are sensitive to light. Upon light absorption, they undergo photochemical reaction to produce reactive species that are free radicals that will interact with the active components in the liquid formulations. A photoinitiator exhibits several properties which include high absorption at the exposure wavelength and high molar extinction coefficient, high quantum yield of formation of initiating species and high reactivity of the radical towards the monomer. [5]

It is important to understand the relationship between printability and the rheological behaviours induced by the interfacial interactions between different components within the ink. Because the rheological behaviour of ink directly affects printability. Adding different components to ink determines the rheology of inks which has a great impact on Flexo printability. Low viscosity ink gives a constant ink transfer at sufficiently high press speeds, but that the ink transfer always increases with increasing pressure at a given speed. Transfer characteristics are related to the viscoelastic properties of the ink. Due to Carbon black, UV ink formulations show shear thinning behaviour. The ink formulation with DPHA monomer is considered as the best formulation for the ink transfer between the Anilox roller and plate cylinder with higher density and clearer dot structure. Also for the ink transfer from a plate cylinder to a printed substrate, the transfer fraction of ink is more in the ink formulation with DPHA monomer due to the viscoelastic properties of the ink. [6]

            Mirror-coated paper is used in label printing, food packages, and other consumer goods. This substrate is printed with both water-based ink and UV ink with Flexographic Printing Press. Water-based ink used in the flexographic printing process is mainly composed of water, pigments as well as other additives like defoamers, and drying & adhesion agents, etc. Water-based ink is mainly used for absorbent substrates. Water-based ink is widely used in the corrugated packaging industry. The UV ink used in the flexographic printing process is mainly composed of monomers, oligomers, photoinitiators as well as pigments and has higher viscosity than convectional inks. So, UV ink is used for non-absorbent substrates. UV ink is widely used in the food packaging industry for outer wraps. Flexography press runs at a high speed using UV ink, as it quickly settles down completely when it passes through UV light. In flexographic printing more solid ink density is achieved by UV inks than water-based inks because UV ink is dried by polymerization process with negligible absorption. Whereas more print contrast is achieved by the water-based inks because water-based ink is dried by absorption and evaporation process and it penetrates in the pores of the substrate. In the case of 50% and 100% dot gain, UV inks have more dot gain than water-based inks due to their drying process that is polymerization. [7]

            Polyvinyl Alcohol is the largest volume of synthetic water-soluble resin that produced in the world. Polyvinyl alcohol films exhibits high tensile strength, abrasion resistance, and oxygen barrier properties which are superior to those of most polymers. The excellent chemical resistance and physical properties of PVA resins have resulted in broad industrial use as an excellent adhesive and possess solvent, oil, and grease resistance properties. The effect of ink and paper board characteristics on flexographic print quality is based on print density, solid content, pH, and particle diameter of the inks. Viscosities of the ink are increased with an increasing percentage of Polyvinyl Alcohol. For water-based UV inks, reduction of ink viscosity reduces the print quality in flexographic printing. On Newsprint and Bond paper, the ink formulation with Polyvinyl Alcohol used as a sole binder resembles some air bubbles on the printed image, whereas on Art paper, much air space with uneven pigment particles distribution occurs. Therefore 8% Polyvinyl Alcohol ink formulation is the best formulation with good printability on various substrates like paper printing, board, paper board, coated and uncoated paper, and other printing services involving the use of paper materials for the packing industry. Polyvinyl alcohol can be utilized as a sole binder in flexographic printing applications.  [8]

Fig 4: Micrographic Images with Varing Formulations on Different substrates

Image copyright ©2015 International Journal of Engineering and Applied Sciences. All rights reserved.

            Polymer film materials are used to print labels and packaging through the flexography printing process. Polyethylene and polypropylene have low surface energy because there is a lack of molecular interactions between polymer surfaces and inks, and the resulted images are created with low optical characteristics. Good surface wettability is  for ensuring good adhesive bonding. The contact angle of the liquid with the solid phase is called as wettability. UV-flexo inks have a higher viscosity than conventional inks so it requires high surface energy of polymer films. On higher surface energy and polarity polymer films, yellow inks spread is the best and black is the worst due to differing ability components of the ink structure formation. Decreasing the cosine of the contact angle reduces ink adhesion. The polarity of the surface has the presence of polar groups in intermolecular interactions provides an adequate adhesive bond. Maximum spreading and maximum adhesion of UV-inks are possible at high surface energy and a polar component of the surface film. [9]

            The rheological properties of ink suspensions depends on the degree of flocculation. The flocculated structure depends on the shape, size, and surface characteristics of the dispersed particles and the interaction between the dispersed medium and the particles. The interaction between pigment particles and the continuous phase creates the formation of a network of particles and aggregates within the ink, which can form a continuous structure over time. The rheological properties of pigment–polymer matrix systems gives information on the processing behaviors of composite materials. To understand and control the rheological properties of UV flexo inks it is necessary to accurately analyze the specific rheological properties produced by the chemical and physical interactions of the ink components. The viscosity of Prepolymer increases as an increase in the shear rate and Dynamic viscosity of Prepolymer is dependent on the molecular weight of oligomer. Dynamic viscosity of the Colorant is increased as the strain amplitude increased. These factors should be known by every ink manufacturer to achieve the desired ink formulation. [10]

References

1. 4 Types of Flexographic Inks & When to Use Them. Retrieved from https://blog.luminite.com/blog/4-types-of-flexographic-inks-when-to-use-them
2.    Monomer Definition and Examples. Retrieved from https://www.thoughtco.com/definition-of-monomer-605375#:~:text=A%20monomer%20is%20a%20molecule,natural%20or%20synthetic%20in%20origin
3.    UV CURING FLEXO INKS. Retrieved from  https://fdocuments.in/document/uv-curing-flexo-inks.html
4.   Ink. Retrieved from  http://printwiki.org/Ink
5.    What are Photoinitiators. Retrieved from  https://www.sglinc.com/articles/what-are-photoinitiators/
6. Kyoung .J. & Ken’ichi k.  (2007) Transfer characterization of UV flexographic ink: Relationships between printability and rheological properties, Nordic Pulp and Paper Research Journal Vol 22 no. 3/2007.  Retrieved from DOI: https://doi.org/10.3183/npprj-2007-22-03-p293-298
7.    Ravi T.  Ankit. B & Bijender  (2020) A Comparative Analysis of Print Quality with Water Based Ink and UV Ink Using Mirror Coated Substrate in Flexographic Printing Process,  International Journal of Advanced Science and Technology Vol. 29, No. 1, (2020), pp. 1754 - 1761. Retrieved from: http://sersc.org/journals/index.php/IJAST/article/view/34465    
8.  B. Magny. E. Pezron , Ph. Ciceron & A. Askienazy (Aug 1999) Key factors for UV curable pigment dispersions, Conference: RADTECH ASIA '99: 7. international conference on Radiation Curing, Kuala Lumpur (Malaysia), 24-26 Aug 1999  Retrieved from : https://www.osti.gov/etdeweb/biblio/20051967
9.    Vyacheslav R. (2013) Influence of Surface Energy of Polymer Films on Spreading and Adhesion of UV-Flexo Inks, Acta graphica 23(2013)3-4, 79-84  Retrieved from https://hrcak.srce.hr/index.php?show=clanak&id_clanak_jezik=203693
10.   Kyoung J. , Sun K., Jong W., Yong. L.  &  Ken K. (2017) Rheological Properties of UV-curable Ink  Influence of the Pre-polymer and Colorant, Journal of Photopolymer Science and Technology   Volume 30, Number 6 (2017)703 －7 0 8 Ⓒ 2017SPST. Retrieved from   DOIhttps://doi.org/10.2494/photopolymer.30.703