UV-Curable De-Inking Oligomers Move the Industry Closer to Circular Plastics

By Endrit Shurdha, senior research scientist, Arkema

Sustainability is a Materials Challenge

Pressure to improve plastic recycling is increasing, yet progress remains limited. Globally, less than 10% of plastic waste is recycled, ¹ and in major markets, such as the United States, recycling rates are reported to be as low as 5–6%. ² Despite increased regulation and investment in recycling infrastructure, these numbers have changed little in recent years. They highlight a structural challenge: typical packaging formats, supply chains and polymer materials were not designed for circularity. As a result, the industry needs new solutions, such as next-generation UV-curable materials specifically designed to support circularity.

Regulatory timelines are accelerating this shift. The European Green Deal ³ requires that all plastic packaging circulated in the EU be recyclable by 2030. Meeting this target requires coordinated action across the value chain, from monomer and oligomer suppliers to converters, printers and brand owners. This effort requires rethinking how functional packaging materials interact with end-of-life recycling processes and how circularity can be achieved within existing recycling infrastructure.

The printing industry is critical to this transition. UV-curable inks and coatings enable high-performance labels, packaging and direct-to-object (DTO) inks providing adhesion, chemical resistance and fast curing that traditional systems struggle to match. At the same time, because UV-cured inks form crosslinked thermoset networks, they are difficult to remove during the wash stage of mechanical recycling. Effective de-inking of UV-cured ink films would enable circularity for printed plastic packaging.

This article examines how two UV raw materials, acidic acrylic and acidic aromatic (meth)acrylate UV oligomers, can be engineered to enable clean, rapid delamination under alkaline washing conditions, while preserving initial printability and adhesion during the use phase. A proof-of-concept approach is presented, demonstrating how these materials function as sacrificial primers or as ink components and maintain the adhesion and print performance required in flexographic and inkjet applications.

Why De-Inking Matters

Mechanical recycling typically follows a multi-step workflow:

1. Collection
2. Sorting
3. Washing and de-inking
4. Resizing
5. Separation
6. Compounding
7. Reprocessing

Among these steps, washing and de-inking are critical, as they are strongly influenced by raw material suppliers and ink formulators through innovative material design. Residual inks affect the quality of recycled plastics, influencing color, transparency, odor and thermal behavior. High-value recyclates, particularly food-grade or bottle-grade PET, have very low tolerance for ink contamination, as even small amounts of residual ink can noticeably increase color or haze relative to the total mass of the packaging.

UV-curable inks and coatings are engineered to remain on the substrate throughout the product’s life. Once cured, they form highly crosslinked thermoset networks that do not melt, dissolve or soften under conventional mechanical recycling conditions. In some UV ink and coating systems, adhesion is further enhanced through interpenetrating polymer networks (IPNs), in which the cured ink physically anchors into the substrate. Certain UV monomers can also swell polymer substrates, reinforcing mechanical interlocking and making ink removal even more difficult.

Addressing these challenges requires de-inking-enabled resin technologies that introduce controlled, trigger-responsive mechanisms, such as alkaline-responsive motifs, into the polymer backbone. These materials must preserve print performance and adhesion during use, while enabling efficient ink delamination only during the recycling wash stage.

Direct-to-Object Printing Requires Better De-Inking

Direct-to-object (DTO) printing applies UV-curable inks directly onto containers, eliminating the need for labels or adhesives. This application is expanding rapidly, with an expected market growth of 10.5% CAGR from 2025 to 2030. ⁴ Brands are adopting DTO due to several advantages:

1. Reduced materials (no labels or adhesives)
2. Customization and digital agility
3. High print resolution
4. Streamlined supply chains
5. Various item shapes and surfaces to print onto
6. Enhanced recyclability potential

The shift to DTO therefore accelerates the need for de-inking-ready UV chemistries, particularly in inkjet systems where viscosity and polarity demands are stringent.

Two Approaches to Enable De-Inking     

1. Primer Approach: Sacrificial UV Primer Layer
In this approach, a UV-curable primer is applied to the plastic substrate before printing with standard UV inks. During recycling, the primer is designed to selectively delaminate, carrying the cured ink film with it and enabling de-inking.

This strategy requires only one additional print station upstream in the printing line and minimizes disruption to existing processes. Because the primer is clear and pigment-free, its adhesion properties can be readily tuned, and a single primer formulation can be used across multiple ink systems, reducing formulation complexity and SKU proliferation. Similar sacrificial primer concepts are already used in PET shrink-sleeve applications for metal cans. ⁵

Limitations include the added cost of an extra print station and coating layer, as well as potential constraints on de-inking efficiency due to limited exposure of the primer to the alkaline wash, since it is sandwiched between the substrate and the UV ink and/or overprint varnish (OPV), as shown in Figure 1.

2. Ink Modification Approach: Built-In De-Inking Triggers
In this approach, the UV ink itself incorporates an acidic oligomer that triggers delamination from the plastic substrate when exposed to an alkaline de-inking solution. The benefit of this route is the large surface area of the ink film in contact with the de-inking solution, which can accelerate the delamination process. In addition, no extra printing station is needed, unlike the primer approach, since the triggerable oligomers are incorporated directly into the final ink formulation.

In some examples, acidic acrylic oligomers have shown better adhesion to PET compared to polyester oligomer formulations. Challenges with this approach include potential impacts on rheology and formulation stability, as well as the hydrophilic nature of the oligomers due to the trigger groups. Additional formulation considerations are discussed in the following section.

At a chemical level, the acidic backbone of the oligomers contributes directly to both adhesion and de-inking behavior. During printing and use, carboxylic acid groups interact with PET through polar interactions and hydrogen bonding, promoting good wetting and adhesion. Under alkaline de-inking conditions, these acid groups are neutralized to carboxylate salts, increasing ink hydrophilicity and weakening interfacial interactions with PET. This enables alkaline solution penetration and triggers ink delamination during washing, while maintaining performance during use.

Ink Formulation Comparison with the Acidic Oligomers

The two acidic oligomers evaluated in this article each offer distinct benefits and can be used in both flexographic and inkjet ink formulations at varying concentrations.

The acidic acrylic oligomer offers significant flexibility in backbone design. Its structure can be modified with a wide range of functionalities, including trigger groups, flexible or rigid components, aliphatic or polar moieties and either acrylate or inert functionality. This versatility makes the oligomer particularly attractive for formulation development. One limitation observed with this chemistry is thixotropy build, which requires formulation optimization.

The acidic aromatic oligomer can also be modified with various trigger groups and with either (meth)acrylate or inert functionality. This oligomer exhibits excellent chemical resistance and hardness. A limitation, however, is reduced light fastness due to the intrinsic aromatic nature of its backbone.

Figure 2 illustrates that, under neutral conditions (left), the cured UV ink remains strongly adhered to the PET substrate, while under alkaline washing conditions (right), the ink swells and delaminates from the surface.

Flexographic Ink Evaluation

The flexographic formulation tested consisted of 20% pigment, 30% oligomer, 20% tri- or tetra-functional monomers, 20% difunctional monomer, 1% dispersant/leveling aids and 9% photoinitiator package. Finished inks were printed on untreated PET substrates using a 3.7 bcm anilox hand proofer and cured under a 400 W H-bulb at 300 ft/min.

The cured samples were then placed in an alkaline de-inking solution for 10 minutes at 80–85° C under vigorous agitation. After the de-inking cycle, simulating recycling facility processes or APR recommendations, ⁶ the samples were removed and examined for residual ink. The results are shown in Figure 3.

The concentration of oligomers can be adjusted to achieve the desired ink formulation and viscosity. The objective of this study was to demonstrate the ability of acidic oligomers to initiate delamination compared to conventional oligomers without trigger groups. Final ink optimization will ultimately depend on the formulator’s specific application requirements and processing constraints.

Inkjet Ink Evaluation

The inkjet formulation tested consisted of 15% pigment, 15% oligomer, 60% monofunctional monomers, 1% dispersant/leveling aids and 9% photoinitiator package. Inks were printed on 20 µm untreated PET substrates and cured under 400 W H-bulb lamps at 50 ft/min.

Cured samples were immersed in an alkaline de-inking solution for 10 minutes at 80-85° C under vigorous agitation. After the de-inking cycle, simulating recycling facility conditions, the samples were removed and examined for residual ink.

During inkjet testing, differences emerged depending on the monofunctional monomers used. While isobornyl acrylate is a low-viscosity, high-Tg, low-polarity monomer, it did not facilitate de-inking. In contrast, tetrahydrofurfuryl acrylate (THFA) or 2-phenoxyethyl acrylate (PEA) monomers, with their high polarity and lower Tg, facilitate the delamination process due to the de-inking solution being more compatible with the cured ink. These results show that multiple formulation parameters must be balanced when designing de-inkable inks.

The Tg of the cured ink film must be high enough to provide abrasion and scuff resistance during use, yet low enough to allow penetration of the de-inking solution into the softened ink matrix. Similarly, ink polarity must be carefully controlled, as higher polarity promotes alkaline solution uptake, swelling the ink film and facilitating debonding from the PET substrate.

Crosslink density is another critical factor. While highly functional monomers promote rapid curing and high performance, excessive acrylate bond density can limit penetration of the alkaline solution and hinder de-inking.

Balancing Tg, polarity and crosslink density is therefore essential to achieving effective de-inking while maintaining end-use performance. In addition, UV curing conditions, such as energy density, peak irradiance and lamp type, can influence final network structure and accessibility to alkaline solutions. Successful implementation requires careful alignment of formulation design with application requirements, curing conditions and current recycling processes.

Conclusion

Plastic recycling is, and will continue to be, essential to a more sustainable circular economy. By incorporating trigger-enabled UV oligomers, ink formulators gain additional tools to design de-inkable UV ink systems that support the recovery of clear, high-quality recycled plastics. The acidic acrylic (meth)acrylate and acidic aromatic (meth)acrylate oligomers evaluated in this study demonstrate effective de-inking performance, giving formulators practical options to balance print performance with recyclability.

References

1. Houssini, K., Li, J., & Tan, Q. (2025). Complexities of the global plastics supply chain revealed in a trade-linked material flow analysis. Communications Earth & Environment, 6(1). https://doi.org/10.1038/s43247-025-02169-5
2. Rosane, O. (2022, November). Why most plastic isn’t recyclable, according to Greenpeace. World Economic Forum. https://www.weforum.org/stories/2022/11/greenpeace-report-most-plastic-not-recyclable
3. European Commission. (2019). The European Green Deal. European Commission; European Commission. https://commission.europa.eu/strategy-and-policy/priorities-2019-2024/european-green-deal_en
4. Direct To Shape Printer Market Size | Industry Report, 2030. (2024). Grandviewresearch.com. https://www.grandviewresearch.com/industry-analysis/direct-to-shape-printer-market-report
5. Premium Label and Packaging Solutions. (2024, October 11). Premium Label & Packaging Solutions. https://premiumlabelandpackaging.com/recyclable-shrink-sleeves-a-game-changer-for-eco-conscious-brands/
6. APR Design Guide Overview – Association of Plastic Recyclers (APR). (2024, November 24). Association of Plastic Recyclers (APR). https://plasticsrecycling.org/apr-design-hub/apr-design-guide-overview/

Endrit Shurdha is a senior research scientist at Arkema in the Sartomer division. Shurdha has been working in the Sartomer UV/LED/EB resin division for seven years, leading the graphic arts, advanced materials and electronic markets. He has more than 11 years of industrial experience in the ink industry, from formulation to raw material development. Endrit holds a Ph.D. in inorganic/materials chemistry from the University of Utah, specializing in structural property relationships of organometallic polymers. Shurdha can be reached at email: endrit.shurdha@arkema.com, www.sartomer.arkema.com.