Analysis of UV Curing

UV curing analysis

The physical properties of UV-curable materials are essentially influenced by the drying system used to cure them. Achieving the desired performance—whether protective tape, ink, or adhesive—will depend on the parameters, design, and control methods of these tubes. The four key parameters of UV lamps are:

1. UV irradiance (or density)

2. Spectral Distribution (Wavelength)

3. Radiation Amount (or UV Energy)

4. Infrared radiation. Compared to maximum radiation or radiation amount, as well as different UV spectra, inks and protective adhesives exhibit very different characteristics. The ability to identify different UV lamp properties and match them to the optical properties of curable materials expands the scope of UV curing as a fast and efficient production process. There are many optical and physical properties of curing systems (besides their own composition) that affect the curing effect, resulting in differences in the appearance characteristics of UV curing materials.

Characteristics of the cured material

Th efficiency of a UV lamp depends on how easily photons are emitted into the curable material to initiate light-triggered molecules. UV curing is determined by the collision between photons and molecules. Light can trigger molecules to diffuse uniformly through materials, but photons are different. In addition to the characteristics of UV light sources, the cured film also has optical and thermodynamic properties. They interact with radiant energy, having a significant impact on the curing process.

Spectral absorption rate: Energy is the effect of a substance absorbing wavelength as its thickness gradually increases. The more energy absorbed near the surface, the less energy is obtained from the deep layers. However, this varies depending on the wavelength. The total spectral absorption rate includes all effects from phototriggers, monomers, oligomers, and additives including pigments.

Reflection and scattering: relative and absorbed, light energy is more often redirected by matter (or within matter); This is generally caused by matrix materials and/or pigments in the curable material. These factors reduce UV energy reaching deep layers but improve curing efficiency at the reaction site.

Optical density: similar to absorption, it consists of two factors: "opacity" and the thickness of the film; Including the dilution effects of absorption and scattering; It is represented by a single number, rather than as a spectral distribution.

Diffusivity: A thermodynamic property contains specific heat, conductivity, and density; The material's ability to "diffuse" and absorb heat; It affects the rise in temperature of the thin film and matrix caused by the sudden infiltration of infrared energy from the surface.

Infrared absorption: temperature has a significant impact on the curing reaction rate; Although the temperature rise in the reaction also affects temperature, the radiation (radiant IR) from the UV lamp is the fundamental source of surface heat (not heat transferred from the surrounding air or atmosphere). Excessive temperature rise is one of the key limiting factors affecting the curing process.

Optical thickness coatings and inks

Because opacity or color intensity are the characteristics we need, ink and pigment coatings present special issues. Adhesives usually also provide relatively thick films. Not the same as one ...

Because opacity or color intensity are the characteristics we need, ink and pigment coatings present special issues. Adhesives usually also provide relatively thick films. Unlike the physical thickness of a film, its optical thickness is extremely important. When light energy passes through or passes through a material, its reduction is described by Beer-Lambert—light energy in the upper layer of the film that is neither absorbed nor reflected will be transmitted and reach the bottom layer of the film. The meaning of spectral absorption: The absorbency of substances varies depending on the wavelength. Obviously, short UV wavelengths (200~300nm) are absorbed on the surface and cannot reach the bottom layer at all. Generally speaking, the thickness of the film is limited, and for the substrate, adhesive strength is the primary characteristic to have. Even phototriggers absorb the wavelength energy they are sensitive to, thereby preventing those wavelengths from reaching the deeper light-triggering molecules. One light-triggering agent is suitable for varnish coatings but may not be the appropriate choice for inks. For inks, light triggers with longer wavelengths are the better choice. Besides physical thickness, another role of spectral absorption is optical thickness. A film cannot have an optical thickness thick at one wavelength and thin at another. Even the optical thickness of varnish coatings at short wavelengths (200~300nm) tends to be thicker. When the cured product contains a layer of "transparent" material on top of the UV-curable material, its absorption limits light energy. These are commonly used by lamination, lens bonding, pharmaceutical assembly, and of course, DVD bonding. Understanding the spectral propagation characteristics of "transparent" materials is important to select the most effective spectra for curing through them. Generally, the selection of long-wavelength UV lamps, combined with long-wavelength light triggers, is key to successful curing with materials like PC. The important role of wavelength: Most UV curing involves two ranges of wavelengths working simultaneously (if IR is included, three are available). Short wavelengths operate on the surface, while long wavelengths work on the deeper layers of ink or coatings. This theorem is because short wavelengths are absorbed at the surface and cannot reach deeper layers. Insufficient shortwave exposure can cause the surface to become sticky; Insufficient longwave energy leads to poor adhesion. Each formulation and film thickness benefits from an appropriate short, long wavelength energy rate. The most basic mercury lamp emits energy in both ranges, but its strong emission at short wavelengths makes it especially suitable for coatings and thin ink layers. High-absorbent materials, such as adhesives and screen inks, are formulated better suited for long-wave curing with longwave optical triggers. The lamps used to cure these materials contain additives and mercury, and these lamps emit more UV under long-wave UV exposure. These longwave lamps also emit some shortwave energy, which is sufficient to cope with surface curing. Many highly specialized applications, such as curing materials with large amounts of titanium oxide as a pigment additive, or needing to cure through plastic or glass, require longwave curing because these materials almost completely block shortwave technology. Parameters of UV lamps ...