Hydrogenated Rosin and Terpene Resins: Analysing Advantages for UV and Water-based Inks

What is hydrogenation reaction on resin modification?

The reason unmodified rosin resin exhibits weak antioxidant properties is primarily due to the abundance of unsaturated double bonds (C=C double bonds) within its molecular structure, which serve as highly reactive sites readily susceptible to oxidation. In unsaturated double bonds, the π-electron cloud density shared between the carbon atoms is significantly higher than that of saturated single bonds, creating an electron-rich region. This elevated electron density renders the double bonds more prone to attraction and attack by electrophilic reagents, such as oxygen molecules in the air. Conversely, the electron cloud distribution between carbon atoms in saturated single bonds is more uniform, with electrons being more tightly bound, thus making them less likely to react with oxygen molecules. Moreover, the π-electrons in double bonds are more easily polarized and mobile compared to σ-electrons, further increasing their propensity to undergo chemical reactions with external oxygen molecules. Oxygen molecules, acting as typical electrophilic reagents, will readily attack the π-electrons in unsaturated double bonds, leading to addition reactions and the initial formation of unstable hydroperoxides. These hydroperoxides subsequently decompose or react with other molecules, ultimately yielding various oxidation products, including aldehydes and ketones.

Specifically regarding rosin, its primary components – rosin acids, such as abietic acid – contain multiple unsaturated double bonds within their molecular structures. When these double bonds are exposed to air, they are highly susceptible to the aforementioned oxidation reactions with oxygen, leading to the formation of peroxides and other complex oxidation products. This process results in the gradual deterioration of the rosin, most visibly manifested as a darkening in colour. Similarly, terpene resin, a natural resin produced from the polymerization of monoterpenes or sesquiterpenes, also contains a significant number of unsaturated double bonds in its molecular structure if it has not undergone modification treatments like hydrogenation. These unsaturated double bonds, much like those in rosin resin, are highly prone to oxidation by atmospheric oxygen, causing the performance of the terpene resin to degrade over time, with observable effects such as colour changes and an increase in viscosity.

To enhance the antioxidant properties of rosin resin and terpene resin, an effective approach is to perform hydrogenation modification. During hydrogenation, hydrogen molecules undergo addition reactions with the conjugated double bonds present in the resin molecules, leading to the saturation of the original double bonds and their conversion into more stable carbon-carbon single bonds (C-C single bonds). Within saturated carbon-carbon single bonds, each carbon atom is tightly bonded to other atoms via a single bond, sharing a pair of electrons. As a result, there are no longer unpaired electrons or π-electron clouds that are easily susceptible to electrophilic attack by oxygen molecules. Consequently, even in oxygen-rich environments, these saturated carbon-carbon single bonds are extremely resistant to oxidation reactions, thereby conferring excellent antioxidant properties to the hydrogenated rosin resin and terpene resin.

Hydrogenated Resins Solubility in Different Solvents

Following hydrogenation modification, hydrogenated rosin and hydrogenated terpene resins typically exhibit a marked improvement in their solubility across a range of organic solvents. This enhanced solubility facilitates their formulation into homogeneous solutions or stable dispersions, thereby providing greater convenience for downstream applications. Key factors influencing resin solubility include the polarity of the solvent, the molecular weight of the resin, and the ambient temperature. Owing to the molecular structural characteristics of hydrogenated rosin and hydrogenated terpene resins, they tend to display non-polar or weakly polar properties, resulting in good solubility in non-polar and weakly polar organic solvents. Conversely, in strongly polar solvents such as water, the solubility of these two hydrogenated resins is very limited, often to the point of being completely insoluble. It is noteworthy that an increase in temperature generally enhances the solubility of resins in solvents. In certain instances, appropriate heating can significantly improve the rate of dissolution and the final extent of dissolution, making it a commonly employed auxiliary technique in practical applications.

Hydrogenated Resins in Ink Formulation

Applicable to Water-based Inks, UV Curing Inks, Flexographic Printing formulations

Driven by increasing environmental demands, water-based inks are progressively replacing traditional solvent-based inks, such as those based on acrylic, alkyd, epoxy, and polyurethane resins, marking a green transition in the printing industry. In comparison to solvent-based inks, the most notable feature of water-based inks is their use of water as the primary solvent medium. This fundamental alteration in formulation significantly reduces the reliance on volatile organic solvents like toluene, xylene, and ethyl acetate, leading to a substantial decrease in the overall volatile organic compounds (VOCs) emissions from the ink formulation, and aligning more closely with the principles of sustainable development. Nevertheless, rosin-based and terpene-based derivatives are inherently hydrophobic resins, exhibiting poor solubility in water-based environments. This poses a challenge for their incorporation into water-based ink formulations, necessitating appropriate chemical modification and meticulous formulation design to overcome this hurdle.

To achieve stable dispersion of hydrophobic resins in aqueous media, emulsification technology has become an indispensable tool. This stable dispersibility is crucial for ensuring that water-based inks exhibit the desired printing properties. Given that the backbone structure of rosin resin or terpene resin is inherently hydrophobic, it is therefore essential to employ emulsifiers to effectively disperse them in water. In specific emulsification techniques, emulsifiers are typically used to disperse and encapsulate the resin into minute droplets, thereby enabling them to form a homogeneous and stable dispersion system in water. Furthermore, the addition of an appropriate amount of surfactant can significantly reduce the surface tension between the resin and water, further enhancing the stability of the dispersion and preventing the re-aggregation of resin particles. To obtain a finer and more stable emulsion, homogenisers or high-speed dispersers are also commonly utilised under conditions of high shear to break down the resin particles into even smaller droplets.

When selecting emulsifiers suitable for rosin and terpene derivatives, several key characteristics must be comprehensively considered [3, 4]. Firstly, the emulsifier must exhibit good affinity for the resin molecules to ensure thorough emulsification of the resin. Secondly, the emulsifier should possess good water dispersibility to guarantee the stability of the entire aqueous system. Thirdly, it is crucial to select an emulsifier with an appropriate Hydrophilic-Lipophilic Balance (HLB value typically between 8 and 12), as this directly influences the emulsification efficacy and the stability of the emulsion. Fourthly, the emulsifier should also be capable of imparting excellent film-forming properties and adhesion to the substrate to ensure the desired printing performance of the ink. Finally, the emulsifier needs to demonstrate good compatibility with other components in the ink formulation, as well as the pH of the system.

Currently, a wide variety of emulsifiers are available that are suitable for rosin and terpene derivatives. These mainly include non-ionic, anionic, and cationic emulsifiers. Practical experience has demonstrated that substances such as NP-10, NP-12, PEG-40, and anionic emulsifiers like sodium dodecyl sulphate (SDS) and acrylic emulsions all exhibit good affinity with rosin and terpene derivatives and are commonly selected as emulsifiers in water-based ink formulations.

Application in Flexographic Printing

In the realm of flexographic printing for packaging and labels, a high degree of resistance to brittleness is particularly required for the final product. Hydrogenated rosin resin and terpene resin play a pivotal role in ink formulations, effectively regulating the ink’s viscosity and thixotropy to ensure uniform distribution on the anilox roller. This significantly reduces the occurrence of banding or stacking during the printing process, ultimately enhancing the clarity and overall quality of the printed design. Considering that flexible packaging pouches often undergo various mechanical stresses such as bending and squeezing during production, transit, and use, the low glass transition temperature (Tg) characteristic inherent in hydrogenated rosin resin and terpene resin becomes especially important. This property allows the cured ink film to maintain good flexibility, effectively preventing labels or flexible packaging from cracking due to material brittleness when folded or bent, thereby significantly extending the product’s lifespan and preserving the integrity of its appearance.

Application in UV curing ink

The distinguishing feature of UV curable inks is their ability to achieve rapid curing through exposure to ultraviolet irradiation. These inks cleverly harness photochemical reactions, whereby photoinitiators within their formulation, when exposed to UV irradiation of a specific wavelength, efficiently absorb UV light energy. This then triggers the rapid polymerization of prepolymers and monomers present in the ink, forming a robust solid film in a very short timeframe, thus completing the drying and curing process. This method of drying differs significantly from traditional solvent-based or water-based inks. The drying process for conventional inks typically takes several minutes to even hours to complete, whereas UV curable inks often cure fully within a matter of seconds under UV lamp irradiation, which undoubtedly greatly enhances the efficiency of printing production. Furthermore, UV curable ink formulations typically contain no or only a minimal amount of solvent, a notable characteristic that gives them an advantage from an environmental perspective.

Rosin resin and terpene resin, owing to their excellent adhesion and good wetting properties, are frequently employed in UV curable inks as tackifiers or adhesion promoters. This is particularly the case when printing on substrates with challenging surfaces for adhesion, such as certain types of plastics, where their inclusion can significantly enhance the ink’s printing performance, ensuring that the ink film adheres firmly to the substrate surface. However, due to the absence of photoactive functional groups in the molecular structures of natural rosin resin and terpene resin that can respond to ultraviolet light to initiate polymerization reactions, such as the common acrylate groups, they cannot directly participate in UV photocuring reactions in their unmodified state. To overcome this limitation, it is common practice to use them in conjunction with other photocurable components (such as acrylate prepolymers) or to subject them to chemical modification reactions like esterification or maleation, thereby introducing functional groups that can be excited by ultraviolet light.

Modified rosin resin and terpene resin can serve as important components of the resin system in UV curable ink formulations. They not only provide good adhesion but also aid in pigment wetting and dispersion, and improve ink levelling, ensuring a smooth and even printed surface. When inks are required for printing on non-absorbent or specially treated material surfaces, such as plastics, metals, and glass, rosin resin or terpene resin can significantly enhance the bond between the ink and the substrate. This is particularly beneficial in solvent-free UV curable ink systems, where they can effectively compensate for any deficiencies in adhesion. For example, in the realm of flexible packaging printing, when printing is required on plastic films such as biaxially oriented polypropylene (BOPP) and polyethylene terephthalate (PET), the addition of an appropriate amount of rosin resin or terpene resin can significantly enhance the ink’s adhesion to these non-absorbent surfaces. Furthermore, the addition of terpene resin can also help to replace a portion of the synthetic resins used in the formulation, thereby reducing the ink’s VOCs emissions and effectively increasing the proportion of bio-based content in the formulation, making it more sustainable.

Following hydrogenation, the original unsaturated double bonds in rosin resin or terpene resin are fully saturated. This not only significantly diminishes their chemical reactivity but also greatly reduces their susceptibility to oxidation. Concurrently, the thermal stability of the resins is substantially enhanced, effectively preventing high-temperature decomposition or the occurrence of unwanted colour changes, even under the irradiation of UV curing lamps. This improved resistance to ultraviolet ageing also directly and considerably enhances the weather resistance of printed articles, with the anti-ageing benefits being particularly pronounced in printed products intended for outdoor use, ensuring the longevity of printed designs and colours.

It is worth noting that hydrogenated terpene resin typically exhibits superior non-yellowing properties compared to hydrogenated rosin resin. Terpene resin inherently possesses good resistance to yellowing, and this characteristic is further enhanced after hydrogenation modification. The colour of modified hydrogenated terpene resin can even approach colourless and transparent, making these excellent properties ideal for applications with extremely high requirements for ink transparency and stringent demands for colour stability. Furthermore, hydrogenated terpene resin can also further improve the flexibility of the cured ink film, effectively reducing brittleness issues resulting from excessive cross-linking, thereby preventing labels or flexible packaging from cracking when bent, and ensuring product quality and appearance.

Conclusion

In conclusion, both natural rosin and terpene resins, as well as their hydrogenated derivatives, hold a significant position in the modern ink industry due to their unique and excellent properties. They not only substantially enhance the adhesion of inks to a variety of substrates and impart an ideal gloss to printed materials, but also play a crucial role in regulating ink rheology and ensuring a smooth printing process. Of particular note is that hydrogenation modification significantly improves the non-yellowing and weather resistance of these resins, making them perform exceptionally well in applications demanding high colour durability, such as packaging, labels, and outdoor printed items. Furthermore, with increasing environmental awareness in the market, “bio-based content” has become an important metric for assessing the sustainability of inks. A growing number of ink manufacturers are actively seeking to increase the proportion of bio-based raw materials in their products to comply with stricter environmental regulations and meet consumer preferences for greener options. For instance, some companies have launched ink ranges that have achieved bio-based renewable content certification, highlighting their positive impact on reducing product carbon footprints. Through hydrogenation modification, the low-odour characteristic of terpene resin is further enhanced, and its use in UV curable inks also helps to lower VOCs emissions and increase the bio-based content of the formulation, aligning with growing environmental demands. From water-based inks to UV-cured flexographic printing inks, these naturally sourced or carefully modified resins and their derivatives, with their diverse functionalities and superior performance, continue to meet the stringent requirements for high-quality, high-performance inks across various printing sectors, particularly in the thriving packaging printing industry, where they are indispensable key components.