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Glossy paper

Release time:

2015-03-26 11:05

Definition of paper gloss coating:

Gloss coatings for paper are a type of varnish used for coating and decorating book and magazine covers, advertising posters, outer packaging cartons for goods, decorative paper bags, and other similar applications, thereby enhancing the aesthetic appeal of the coated surfaces. Previously, the conventional method of applying gloss coatings to paper involved adhering a layer of polyethylene or polypropylene film onto the paper surface. The application process included steps such as glue application and film lamination, followed by waiting for the interlayer adhesive to cure. This approach suffered from low overall production efficiency, higher technical requirements for application, and uncertain durability of the plastic film. Over time, especially when the product is used for extended periods, the film tended to peel off starting from the edges and corners.

In terms of construction technology requirements, production efficiency, and cost, UV-curable paper varnishes outperform plastic film coating processes and have become one of the highest-volume varieties in the field of UV-curing coatings. The most widely produced UV-curable paper varnish is the high-gloss clear coat, designed to enhance the glossiness of the coated surface. As people’s preferences evolve, a growing number of special-effect varnishes have also emerged, including matte, satin-matte, ultra-smooth, ice-flake, pearlescent, and gold-foil finishes.

The application of gloss coatings on paper typically targets paper substrates that have already been printed with ink. Paper materials are composed of cellulose and other additives, giving them a porous and polar surface characteristic. As a result, conventional UV-curable coatings exhibit strong adhesion to paper substrates; however, their adhesion to ink-printed areas depends on the specific properties of the ink.

Another varnishing process involves applying the varnish directly onto the wet ink surface. The ink is of a photocurable nature, and after the varnish is applied, both the varnish and the ink can be cured in one step using ultraviolet radiation. However, it’s important to note that wet ink and wet varnish may tend to penetrate and diffuse into each other, potentially damaging the typeface and printed patterns on the ink. In some cases, to prevent low-viscosity varnishes from penetrating deep into the paper substrate, a sealing primer coat is first applied prior to varnishing. The appropriate primer should possess sufficient polarity and plasticity to partially close the paper fibers without completely sealing them off; its surface should still retain a certain degree of roughness and numerous tiny micropores. This ensures that the UV topcoat achieves excellent adhesion to the substrate.

Depending on the intended application, the enhanced adhesion performance of the surface after varnishing and glossing of paper must be taken into account. When applying varnish to packaging cardboard boxes, large-area flat cardboard sheets are printed, varnished, and cured. After curing, they are cut and folded, and adhesive is applied to the areas intended for bonding before the pieces are joined together. If a silicone-based additive is used, the cured surface will have lower polarity and poorer affinity for most commonly used adhesives, resulting in poor bonding performance.

Odor issues are often a concern for users of clear coats. The odor generated after the clear coat forms a film primarily stems from photoinitiators, active amines, impurities, and certain monomers that may not have fully reacted. For example, iso-octyl acrylate itself has a relatively strong odor; if its residual rate remains high in the cured film, it can produce a noticeable, irritating smell. Active amines are typically used in excess, and after curing is complete, significant residues of these amines can remain, serving as another potential source of odor in the cured film. Selecting active amines that can participate in crosslinking can help eliminate the odor of the cured film.

Coating process:

According to their application processes, paper varnishes can be categorized into three main types: roll coating, offset printing, and screen printing. Among these, roll coating is the most widely used and accounts for the largest consumption of varnish. The latter two methods offer localized varnishing capabilities, making them suitable for selective decoration of specific areas on the printed surface. Different coating processes result in distinct properties of the coatings, particularly in terms of viscosity and rheological behavior.

Compared to roll-coated varnishes, screen-printed paper coatings have higher viscosity and often exhibit thixotropic properties to ensure printability. Due to their higher viscosity, these coatings require a longer time to level out after printing, which reduces coating efficiency. However, the screen-printing process itself already tends to lower production efficiency; therefore, screen-printing varnishing technology does not demand excessively high production efficiency. Its primary purpose is to achieve localized decorative effects on the substrate surface. Low-concentration leveling agents can impart re-adhesive properties to the cured film. Some experts also recommend using non-siloxane-based polyacrylate leveling agents, which can more reliably deliver excellent surface re-adhesion performance. Since screen-printed varnishes have relatively high viscosity, bubbles may form during application. If necessary, defoaming additives can be added to the formulation.

Offset varnish is suitable for offset printing systems. The metal offset plate is mounted on a rotating shaft and dipped into a reservoir of varnish. Based on the principle of oil affinity and water repellency, the varnish adheres only to the designated pattern areas on the offset plate. It is then transferred onto the substrate paper via an intermediate rubber roller, creating a localized gloss effect in specific regions. For packaging boxes, this process allows easy control over the glossed areas without compromising production efficiency—areas intended for gluing and bonding remain uncoated, ensuring optimal adhesion of PVA and other adhesive agents. Under these process conditions, the formulation of the varnish can be more flexible, eliminating the need to balance the conflicting demands between silicone-based leveling agents and the re-adhesion performance of the coating film. Offset printing boasts high production efficiency; a single offset press can handle multi-color overprinting of ink. By arranging the varnish application and UV curing units immediately after the multi-color ink-printing stations within the same press, it’s possible to integrate ink printing and varnish curing into a single, streamlined process. For UV-curable inks, increasing the viscosity of the overprint varnish can result in a thicker varnish film, producing a richer and more vibrant gloss effect. The formulation of offset varnish can draw upon the characteristics of the two previously mentioned types of varnishes, with appropriate adjustments made according to specific process requirements and quality standards.
In addition to the UV varnishes for papers produced using the above-mentioned processes, there are also UV varnishes that mimic intaglio and relief printing techniques.

Product Recommendation:

Glossy paper

Reference recipe:

Reference Formulation for Varnishing Coating on Coated Paper
Propylene glycol diacrylate containing 20% TPGDA	18.4%
TPGDA	55.4%
3EOTMPTA	10.0%
Benzophenone	5.0%
Tertiary amine monoacrylate	10.0%
Photoinitiator of the cleavage type	1.0%

Reference Formulation for Gloss Varnish on Screen-Printed Paper
Epoxy acrylic resin	46.0%
TPGDA	16.5%
3EOTMPTA	10.0%
Benzophenone	5.0%
Amine-modified acrylate	21.0%
Photoinitiator of the cleavage type	1.0%
Flow agent	0.5%

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UV-curable coating