Performance Testing and Evaluation of UV Coating Cured Films

1. Surface dryness level
For certain formulations, after exposure to light, the underlying layer has undergone sufficient crosslinking and curing, yet the surface still exhibits adhesion and clearly shows fingerprint marks. In such cases, place a small ball of cotton on the coated surface and blow it away with your mouth. Then, inspect the coated surface to see whether cotton fibers have adhered to it. If a significant number of cotton fibers are found clinging to the surface, it indicates that the surface curing is inadequate and the coating lacks sufficient dryness. To address this issue, you can increase the intensity of the light source; or adjust the formulation by reducing the proportion of components with slower polymerization rates and increasing the amount of components that cure more rapidly. Additionally, adding antioxidant and anti-polymerization additives can also help overcome the problem of incomplete surface curing.

2. Hardness
The hardness of a cured film is an important and critical indicator for evaluating the quality of a coating. Methods for measuring the hardness of cured coatings include pendulum hardness, pencil hardness, and Shore hardness. Pendulum hardness is a relative measure of hardness and is frequently used in experimental studies. Pencil hardness is simple and easy to perform, making it widely adopted in industrial applications. Hardness is influenced by formulation components and photocuring conditions; for instance, using polyfunctional monomers and increasing reaction conversion rates and crosslinking degrees can both enhance the hardness of the coating.

3. Softness and smoothness
Many substrates that are coated exhibit a certain degree of deformability, which in turn demands that the coating itself possess corresponding flexibility—for example, paper, soft plastics, and leather. The quality of flexibility largely depends on the formulation itself; generally, polyurethane-acrylate components can enhance flexibility. The bend test of cured coatings is commonly used to characterize their flexibility: a steel rod of varying diameters is used as the axis around which the coated and cured material is folded. The test then assesses whether the coating cracks or peels off. If the coating has good flexibility but poor adhesion, the bend test may cause the coating to peel away from the substrate; conversely, if the coating has poor flexibility but good adhesion, the bend test might lead to cracking of the coating. Monofunctional acrylate monomers—especially reactive diluents like iso-octyl acrylate, which also function as internal plasticizers—can reduce the crosslinking density of the cured film, thereby improving its flexibility.

4. Tensile Properties
The tensile properties of a cured film are closely related to its flexibility. By applying increasingly stronger tensile forces to dumbbell-shaped cured films on a material testing machine, the elongation at break of the film layer is used to characterize its tensile performance. The tensile stress is converted into tensile strength. Higher elongation and tensile strength indicate that the cured film possesses better flexibility. Generally, film layers with good tensile properties also exhibit high flexibility; however, high flexibility does not necessarily imply high toughness. Flexibility is an important indicator for evaluating the mechanical and physical properties of cured coatings, while tensile strength is closely linked to the coating's resistance to mechanical damage.

5. Wear and scratch resistance
The abrasion resistance of UV-curable coatings is generally higher than that of conventional solvent-based coatings, because the former form a denser crosslinked network, whereas the latter mostly rely on solvent evaporation to cause resin particles to coalesce into a film, with a much lower degree of chemical crosslinking compared to UV-curable coatings. Floor coatings have particularly high abrasion resistance requirements; this property is typically measured using a wear-testing instrument. A UV-curable coating is applied onto a circular glass plate used for testing, then cured under UV light and placed on a wear-testing bench. After applying a specified load, the machine is started and rotated for a set number of revolutions. The rate of mass loss from the coating serves as an indicator of its abrasion resistance: the higher the wear rate, the poorer the coating's abrasion resistance. Abrasion resistance is closely related to the degree of crosslinking in the coating—increasing crosslinking enhances abrasion resistance. Therefore, UV-curable floor coatings often contain highly functional active diluents, such as TMPTA, pentaerythritol triacrylate, bis-trimethylolpropane tetraacrylate, and dipentaerythritol pentaacrylate.

In addition to abrasion resistance, the scratch resistance of a coating surface is sometimes also required to be quite high. Typically, scratch resistance is not measured using an abrasion tester but rather characterized by haze testing. This method reflects only the surface’s resistance to abrasion and does not fully correspond to the overall abrasion resistance of the entire film layer.

6. Adhesion
Adhesion is one of the most fundamental indicators for evaluating coating performance. In China, the circle-cutting method is commonly used for testing; specific procedures can be found in national coating inspection standards. In Europe and the U.S., the grid-cutting method is generally adopted: small 10mm×10mm squares are drawn on the coating surface, and the adhesion is assessed by measuring the rate at which the coating flakes off within these squares. In industrial testing, a simple tape test is often employed: a 600-grit adhesive tape is applied to the coating, and then peeled off at either a 90-degree or 180-degree angle to check whether the coating has detached from the substrate. The adhesion of a coating is primarily determined by the formulation properties. For porous or polar substrates, adhesion issues are relatively easy to address, and most conventional formulations can meet basic adhesion requirements. The cleanliness of the substrate surface significantly affects coating adhesion—especially when the substrate surface is contaminated with oil, paraffin wax, silicone oil, or other release agents and organosilicone additives. Such contaminants greatly weaken the surface polarity and hinder direct contact between the coating and the substrate, leading to a severe reduction in adhesion.

7. Glossiness
Glossiness is typically measured using a gloss meter, with measurement angles of either 30° or 60°. Generally, measurements taken at 60° yield higher values than those taken at 30°. Compared to solvent-borne coatings, UV-curable coatings tend to produce cured surfaces with higher gloss more easily. If leveling aids are added to the formulation, the gloss level can be even higher; commonly, the gloss of UV-curable coatings can readily reach 100% or above. In contrast, solvent-borne coatings undergo significant solvent evaporation during film formation and drying, which causes considerable disturbance to the coating surface, affecting its micro-level smoothness and making it prone to a reduction in gloss. As people’s aesthetic preferences continue to evolve, low-gloss finishes such as matte and satin finishes have become increasingly popular. Matte effects can be achieved by adding fine powders like microcrystalline wax or inorganic matting agents. Microcrystalline wax is typically synthetic polyethylene or polypropylene wax, dispersed within the UV-curable coating. During curing and film formation, due to its incompatibility with the coating system, the wax particles remain free and float on the surface of the cured film, creating a matte effect. This matte finish offers a soft hand feel and a certain waxy texture. However, since these matting agents merely float on the surface and have relatively low intrinsic strength, their scratch resistance is often unsatisfactory. Adding inorganic components such as fumed silica, silicon micropowder, or talc makes it difficult to achieve a simple matte-satin effect. In terms of inorganic matting, UV-curable coatings differ from traditional solvent-borne coatings: the latter contain substantial amounts of solvents, which evaporate extensively during film formation and drying, causing significant shrinkage of the film volume and allowing inorganic particles to easily emerge onto the dried film surface, thereby achieving a matting effect. By contrast, conventional UV-curable coatings essentially contain no volatile components, and their film-forming and curing mechanisms differ completely from those of solvent-borne coatings. Since there is no substantial solvent evaporation during drying, the film volume does not shrink significantly, and inorganic particles are less likely to emerge onto the film surface. When it becomes necessary to achieve a matte finish, an appropriate amount of low-toxicity, inert solvents is added to the formulation to increase the shrinkage rate of the film during curing, thus forcing the inorganic matting agents to expose themselves on the surface of the cured film.

Resins also influence the matting effect. Experience has shown that when gas-phase silica is used as a matting agent and added to ethoxylated polyfunctional monomers that contain absolutely no resin, the resulting coating film exhibits an excellent matte finish after curing. However, once propylene oxide acrylate is added, the matte and satin effects disappear—this may be related to the shrinkage rate of the resin during curing. Moreover, propylene oxide acrylate cures rapidly and demonstrates outstanding resistance to oxygen-induced polymerization inhibition; it achieves complete surface curing and readily forms coatings with high gloss. In contrast, polyurethane acrylates and polyester acrylates have relatively slower curing rates and are more susceptible to interference from oxygen-induced polymerization inhibition. As a result, the inner layers of the coating cure well, while the surface often remains incompletely cured and is easily disturbed by even small amounts of volatile components remaining within the film, leading to a deterioration in surface micro-smoothness.

8. Corrosion resistance
The corrosion resistance of UV-cured films includes their tolerance to dilute acids, dilute bases, and organic solvents. This resistance can be evaluated from two aspects: dissolution and swelling of the cured film. It reflects the film’s ability to withstand solvent-induced degradation. One method involves using a cotton swab dipped in solvent to perform bidirectional wiping on the film surface; the number of wipes required until the coating is completely worn through to the substrate serves as an indicator of solvent resistance. Alternatively, the swelling method can be used, with the weight gain percentage of the film under a fixed swelling time serving as the evaluation criterion. Since UV-cured coatings ultimately form a highly cross-linked network, they generally do not exhibit significant dissolution of the entire film layer. Instead, only a small portion of un-crosslinked components within the film may be leached out. Consequently, the gel content of UV-cured films often reaches over 90%.

9. Thermal Stability
Generally, decorative UV-curable coatings do not need to consider thermal stability issues. However, if they are used for coating heating appliances or components that generate heat, their long-term thermal resistance must be taken into account. Due to the higher degree of crosslinking in UV-curable coatings, their thermal stability is typically superior to that of solvent-based or thermally cured coatings. By using polyfunctional acrylate monomers, the thermal stability of the cured film can be further enhanced. The presence of aromatic ring structures in resin molecules can also improve thermal resistance; for instance, the cured film made from bisphenol A epoxy acrylate resin exhibits excellent thermal stability—after being exposed at 120°C for 200 hours, its infrared absorption spectrum and optical properties showed no significant changes. Phenolic epoxy acrylate resins, which contain a higher proportion of aromatic rings, demonstrate even better thermal resistance in their cured films.