From Mirror Gloss to Sand Touch

Executive summary

A UV coating is the last layer a customer sees and the first one they touch. Gloss, matt, satin, drip-off contrast, raised crystal, haptic sand and bubble textures are not decorative afterthoughts; each is a controlled physical effect built from film thickness, surface tension, particle size and cure. This paper explains the physics behind each effect and the formulation and press conditions that reproduce it, drawing on the UniCure range for sheet-fed offset, narrow-web flexo and screen application.

1. Gloss: a question of roughness

Gloss is specular reflection, and specular reflection needs a surface smoother than the wavelength of light. A 60° gloss of 90 GU or more requires a coating film that levels completely before it cures — which is why gloss UV coatings are low in viscosity, have long open time relative to the moment of cure, and are applied at a film weight (3 – 4 g/m² wet on sheet-fed, 2.5 – 3 g/m² on narrow web) that fills the micro-roughness of the ink and board. Lower film weight, absorbent stock, over-dilution or a hot pressroom that thins the coating all lower gloss for the same reason: the film no longer bridges the surface texture.

The “wet look” on rich colours is a second-order effect: a high-refractive-index, perfectly smooth film over a dark ink minimises diffuse scatter so that the colour appears deeper and more saturated than the uncoated print.

2. Matt: scattering by design

A matt coating scatters light with a matting agent — precipitated or fumed silica, polymer beads or wax — dispersed in the liquid. As the film shrinks on cure the particles protrude from the surface, creating micro-roughness that diffuses reflection. Three consequences follow. Film weight has a ceiling: a heavier film buries the particles and raises gloss. Rub resistance is inherently lower than gloss because the particles that create the effect are the first thing a rub tester touches; high-rub matt grades use harder, better-bound particles and a tougher binder. And matting agents settle: the coating must be shaken before use and kept agitated on long runs.

Figure 1 – Gloss rises with film weight while matt is lost above about 3 g/m² (illustrative).

3. Satin and controlled sheen

Between the extremes, satin coatings (40 – 60 GU) balance a reduced matting-agent loading with a binder that levels well. They are the natural choice for stampable and glueable grades, where a smooth, additive-free surface is needed for foil and adhesive to key, and for premium cosmetic and spirits labels where full gloss looks cheap and full matt marks too easily.

4. Drip-off: surface tension as a printing plate

The drip-off effect creates a textured matt / gloss contrast without a second coating unit. A matt base coat with low surface energy is applied from the offset inking unit only where texture is wanted; a gloss top coat is applied overall from the coating unit. Where the top coat lands on bare ink or board it wets and levels to full gloss. Where it lands on the base coat it cannot wet — the surface tension difference is deliberately large — so it reticulates into droplets, like water on an oily surface, and cures in that pattern. Droplet size is controlled by the top-coat viscosity and the time between base and top coat: UniCure 12RC gives larger drips, UniCure 20RC smaller. Any surfactant, silicone or solvent contamination lowers the surface-tension difference and kills the effect, which is why drip-off systems demand dedicated, clean equipment.

Figure 2 – Drip-off: the gloss top coat levels on ink and board but reticulates on the low-surface-energy matt base (schematic).

5. Raised and textured screen effects

Screen application deposits 6 – 12 g/m² in one pass, ten times the film of a roller coater, and that thickness is the effect. Crystal (raised gloss) coatings are very high in viscosity so that the deposit holds a sharp, domed edge before curing; the challenge is curing through the full depth, which needs high lamp power and radiometer-controlled dose. Haptic coatings suspend hard particles that cure into a sand-like surface for tactile contrast and grip. Bubble coatings use a foaming mechanism captured by cure to give a raised cellular texture. All thick deposits are rigid: they are kept clear of folds and creases, and they need their own troubleshooting — mesh marks, bubbles and under-cure at the bottom of the film.

6. The properties behind the effect

Every effect is delivered on top of the properties the pack needs: scuff and rub resistance, slip, chemical resistance, and — where specified — stampability, glueability, anti-static behaviour or low migration. These come from the binder network and the additive package rather than from the effect chemistry, and they interact with it: slip additives that lower coefficient of friction also raise scuff resistance, while additives that migrate to the surface can reduce gloss or foil adhesion. Uniform Industries rates every UniCure grade for slip, scuff, gloss or matt level and flexibility on a five-box scale, with one headline property for its end use, so that a converter can choose an effect without losing the performance the pack requires.

7. Choosing and running an effect coating

Specify the effect and the end-use property together — gloss and stampable, matt and high-rub, drip-off and glueable — and let the data sheet’s rating profile guide the choice.

Hold film weight at the recommended value; gloss falls below it, matt fails above it.

Cure to the darkest, heaviest-coverage area, and cure through thick deposits.

Keep effect coatings on dedicated, clean equipment; contamination is the most common cause of a failed effect.

Key takeaways

Gloss is levelling; matt is scattering; both are set by film weight.

Drip-off is a surface-tension mismatch between two coatings, printed like an image.

Screen effects are thick films that must be cured through.

The effect sits on top of the performance properties; specify both.

References: ISO 2813 (specular gloss); ISO 4287 (surface roughness); ASTM D5264 (Sutherland rub); ISO 8295 (COF).