UV UV Curing Analysis (I)

The physical properties of UV-cured materials are essentially affected by the drying system used to cure them. The expected performance gains, whether protective gels, inks, or adhesives, will depend on the parameters, design, and control of these lamps. The four key parameters of the UV lamp are:

1. UV radiation (or density)

2. Spectral distribution (wavelength)

3. Radiation (or UV energy)

4. Infrared radiation. With respect to the maximum radioactivity or amount of radiation, as well as different UV spectra, inks and protective adhesives will exhibit very different characteristics. The ability to identify different UV lamp properties and match them to the optical properties of the curable material expands the scope of UV curing as a fast and efficient manufacturing process. Many of the curing system's optical and physical properties (in addition to its own composition) affect the curing effect, resulting in differences in the UV cured material's performance.

Cured material properties

The efficiency of a UV lamp depends on how easy it is to launch photons into a curable material to initiate light triggering the molecule. UV curing is determined by photon-molecule collisions. Light can trigger molecules to spread evenly through the material, but the photons are different. In addition to the characteristics of the UV light source, the cured film has optical and thermodynamic properties. They interact with radiant energy and have a major impact on the curing process.

Spectral Absorption: Energy is the absorption of a substance into a wavelength in a gradually increasing thickness. The more energy absorbed near the surface, the less energy is obtained in the deep layer. However, this situation varies with the wavelength. The total spectral absorbance includes all effects from light triggers, single molecules, oligomers, and additives including pigments.

Reflection and Scattering: Relative to absorption, light energy is more often redirected by a substance (or within a substance); this is generally due to the matrix material and/or pigment in the curable material. These factors reduce the UV energy reaching deeper, but improve the curing efficiency at the reaction.

Optical density: Similar to absorption, it consists of two factors: "opacity" and the thickness of the film; including photodilution of absorption and scattering; expressed by a single number, not as a distribution of the spectrum.

Diffusion: A thermodynamic property that contains specific heat, conductivity, and density; the ability of a material to “diffuse” and accept heat; and the temperature of a film and substrate that is affected by infrared energy that suddenly enters the surface.

Infrared Absorption Rate: Temperature has a significant effect on the rate of curing reaction; although the temperature rise in the reaction also has an effect on temperature, the radiation from the UV lamp (radiant IR) is the fundamental source of surface heat (not from the surrounding The air or heat transferred in the atmosphere). Excessive temperature increase is one of the important limiting factors affecting the curing process.

Optical thickness coating and ink

Due to the fact that opacity or color intensity is a characteristic we need, inks and pigment coatings pose special problems. Adhesives generally also provide a relatively thick film. Unlike the physical thickness of a film, its optical thickness is very important. When light penetrates or passes through a material, its reduction is described by the Beer-Lambert—the light energy that is not absorbed by the upper layer of the film and that is not reflected will pass through to the bottom layer of the film.

The significance of spectral absorption

Absorption of substances varies with wavelength. Obviously, short UV wavelengths (200-300 nm) are absorbed at the surface and do not reach the bottom layer at all. In general, the thickness of the film is limited, and adhesion to the substrate is the primary characteristic it should have.

Even a phototrigger can absorb the wavelength energy it is sensitive to, preventing the wavelength from reaching the deep layer of phototriggerable molecules. A phototrigger is suitable for clearcoats, but may not be a suitable choice for inks. For inks, light triggers that correspond to longer wavelengths are a better choice. In addition to physical thickness, another function of spectral absorbance is optical thickness. It is impossible for a film to have a thick optical thickness at one wavelength and a thin one at another wavelength. Even the optical thickness of the clearcoat coating at short wavelengths (200-300 nm) tends to be thicker.

When the cured product contains a layer of "transparent" material over the UV-curable material, its absorbance prevents light energy. This is laminating, lens bonding, pharmaceutical assembly, and of course, DVD bonding, which is commonly used.

It is important to understand the spectral propagation properties of “transparent” materials in order to select the most effective spectrum for curing through them. Under normal circumstances, the selection of long-wavelength UV lamps, combined with long-wavelength phototrigger, is the key to successful cure through materials such as PCs.

The important role of wavelength

Most UV curing involves two ranges of wavelengths working simultaneously (if IR is included, 3). The short wavelengths work in the surface layer and the long wavelengths work in the deep layers of the ink or coating. This theorem is due to the fact that short wavelengths are absorbed in the surface layer and cannot reach deep layers. The lack of short-wavelength exposure can cause the surface to become sticky; the lack of long-wave energy can lead to poor adhesion. The thickness of each formulation and film will benefit from an appropriate short, long wavelength energy rate.

The most basic mercury lamp emits energy in these two ranges, but its strong emission at short wavelengths makes it particularly suitable for coatings and thin ink layers. Superabsorbent materials, such as adhesives and screen inks, are formulated for longer wave cures using long wavelength light triggers. The lamp used to cure these materials contains additives and mercury, which emits more UV under long-wave UV. These long wave lamps also radiate some short-wave energy, which is sufficient to cope with the solidification of the surface layer.

Many very specific applications, such as solidification of materials containing large amounts of pigment additives such as titanium oxide, or curing through plastic or glass, must be long wave cured because these materials almost completely hinder short waves.