Printing on plastic with UV-curable inks brings a unique set of challenges—chief among them is achieving reliable adhesion on non-porous materials like polyethylene terephthalate (PET) and polypropylene (PP). These plastics are popular for packaging, labeling, and other industrial applications thanks to their toughness and resistance to chemicals. However, their inherently low surface energy can repel ink, resulting in problems like poor print coverage, ink flaking, and even total print failure.
One of the more effective solutions involves incorporating rosin emulsion—a naturally derived additive—into the UV ink formulation. Based on both lab data and field use, it can significantly boost ink adhesion on these tricky surfaces, offering a practical and sustainable edge for manufacturers.
Wetting Improvement on PET/PP Substrates
The first step to better adhesion is improving how well the ink wets the surface of the plastic. Wetting is all about how easily a liquid can spread and stay in contact with a solid—critical when working with low-energy surfaces like PET or PP.
Rosin emulsion helps by reducing the ink’s surface tension, allowing it to smoothly coat the plastic and reach into subtle surface irregularities. This broader contact area is essential for better physical anchoring and adhesive bonding.
1.The Science Behind Rosin Emulsion’s Wetting Properties
Rosin, sourced from pine resin, is composed mainly of resin acids such as abietic acid. These compounds contain both hydrophilic and hydrophobic regions, making them uniquely suited for bridging the gap between oil-based UV inks and plastic substrates.
When formulated as an emulsion, the hydrophobic parts of the molecule bond with the plastic surface, while the hydrophilic parts interact with the ink. This dual affinity helps promote more uniform ink film formation, resulting in dramatically improved surface coverage and bonding.
In some early tests using rosin emulsion on treated PP sheets, significantly less ink beading and greater uniformity were noticed, even before curing—an early visual cue of enhanced wetting.
2.Quantifying Wetting Improvement
To quantify this improvement, contact angle measurements are commonly used. A smaller contact angle means better wetting.
Several studies have recorded contact angle drops of 20–30 degrees when rosin emulsion is added to UV ink formulations. A reduction of just 15 degrees can already translate into visibly better ink holdout and adhesion, especially on untreated PP sheeting.
Compatibility Testing with UV-Curable Resins
While rosin emulsion brings big benefits in wetting and adhesion, it must be properly matched with the resin system in your ink. Mismatches can cause curing issues, phase separation, or a compromised finish.
1.Choosing the Right Rosin Emulsion
Not all rosin emulsions are created equal. Differences in particle size, surfactant systems, and solid content can all influence performance. Matching the emulsion’s characteristics with the chosen photopolymer resin is essential to maintain both stability and reactivity of the ink.
For example, high-solid emulsions can enhance adhesion but may raise viscosity—affecting flow through finer nozzles or printing plates. Trialing 2–3 variations during ink development can help pinpoint the ideal balance of adhesion, consistency, and curing performance.
2.Conducting Compatibility Tests
Compatibility tests usually start with small ink batches formulated with incremental levels of the rosin emulsion. Performance is then evaluated across several parameters:
Viscosity (to ensure proper print flow)
Cure speed under UV lamps
Surface gloss or matte appearance
Adhesion strength after full cure
One tip: monitor the ink stability over 48 hours post-blending—sometimes phase separation signals long-term incompatibility that isn’t immediately obvious.
Practical Printability Evaluation
Bench testing is crucial, but real-world validation often reveals challenges that lab settings miss. **Practical printability tests** involve running the enhanced ink formulation on industrial-scale PET and PP substrates using the intended printing system—whether flexo, screen, or inkjet.
1.Assessing Adhesion Performance
A range of standard tests can be used to evaluate how well the ink adheres in actual production:
Tape peel tests (ASTM D3359)
Cross-hatch adhesion tests
Rub and abrasion resistance (especially important for packaging subjected to handling)
These tests imitate wear and stress conditions, allowing you to ensure the ink won’t flake, smear, or degrade during the product’s lifecycle. A cross-hatch test often gives a fast and visual indication of whether further ink tweaks are needed.
2.Real-World Examples
A major beverage company implemented rosin emulsion in their UV flexo system for PET bottle labels. Before the switch, they struggled with ink delamination during bottle shrinking. Post-implementation, adhesion metrics improved by nearly 50%, eliminating rework and reducing print waste by 40%.
Similarly, a label converter working with PP-based films adopted rosin emulsion in their pigment UV ink. They reported significantly better resistance to scuffing during transit, reducing complaints from logistics partners and improving overall product presentation.
These case studies speak volumes about how the additive can impact not just print quality, but broader operational efficiency.
Rosin emulsion offers a robust way to overcome adhesion issues when printing on plastics with UV inks. By tackling the root challenges—poor wetting and surface incompatibility—it enables formulators and printers to elevate both quality and durability across various applications.
Combined with thorough compatibility testing and real-world validation, the use of rosin emulsion isn’t just a technical improvement. It’s a smart, cost-effective move toward more reliable, sustainable plastic printing.
Post time: Jul-14-2025