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-molecular 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 Beer-Lambert - light energy that is not absorbed in the upper layer of the film and 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.
UV lamp parameters
The effects of curing UV lamps can be fully and accurately linked by four characteristics: UV spectral distribution, irradiance, radiation dose and infrared radiation.
1. Spectral distribution It describes the phase radiant energy that is one of the functions of the lamp's emission wavelength.
The wavelength distribution of the radiant energy that arrives or reaches the surface. It is often expressed in a related standardized terminology. To show the distribution of UV energy, the spectral energy can be combined into 10 nm spectral bands to form a distribution table. This allows comparison between different UV lamps and easier calculation of spectral energy and power. Lamp manufacturers publish their spectral distribution data.
On-line detection uses a multispectral ray detector to characterize the spectral radiance or radiation. They obtain relative information useful for spectral distribution by sampling radiant energy in a relatively narrow (20-60 nm) frequency band. Due to the different construction of radiation detectors from different manufacturers, it is possible to compare them with each other, but it is difficult. There is no such standard to compare models and manufacturers.
2. Irradiance
Radiance is the radiated power that reaches the unit area of ​​the surface. The degree of radioactivity is expressed in watts per square centimeter or watts. It varies with lamp output power, efficiency, focusing of the reflective system, and distance to the surface. (It is a characteristic of the lamp and its geometry, so it is not related to speed.) The high-intensity, peak-focus power reference placed directly under the UV lamp is referred to as "peak radiation." Radiosity includes all factors related to power supply, efficiency, radiant output, reflectivity, focused bulb size, and geometry.
Due to the UV-curable material's absorption characteristics, less energy is reached below the surface than in the surface layer. Curing conditions in these areas may be significantly different. A material with a thick optical thickness (either high absorptivity, or a thick physical structure, or both) may reduce light efficiency, resulting in insufficient solidification of the material. In inks or coatings, higher surface radiances provide relatively high light energy. The depth of solidification is more affected by the radiation than the longer exposure time (irradiation). The influence of radioactivity is more important for highly absorbent (high opacity) films.
High radiation allows the use of less light triggers. The increase of photon density increases the collision of photon-light triggers, thus compensating for the decrease of phototrigger concentration. This works for thicker coatings because the photo-trigger in the skin absorbs and blocks the same wavelength from reaching deeper photo-trigger molecules.
3. UV radiation
Radiation energy reaching the unit area of ​​the surface. The amount of radiation represents the total amount of photons that reach the surface (and the rate of radiation is the rate of arrival). Under any given source, the amount of radiation is inversely proportional to speed and is proportional to the amount of exposure. The amount of radiation is the cumulative time of radiation, expressed in terms of Joules per square centimeter or miliJoules. (Unfortunately, there is no information about the radiometric or spectral content being replaced by the amount of radiation measured. It is simply the accumulation of energy at the exposed surface. Its significance is that it is the only feature that includes the speed parameter and the exposure time parameter.
4. Infrared radiation density
Infrared radiation is mainly infrared energy emitted by a quartz bubble of a UV source. Infrared energy and UV energy are collected together and focused on the work surface. This depends on the IR reflectivity and reflector efficiency. IR energy can be converted to radiation or radiometric units. But usually, the surface temperature it produces is important for attention. The heat it generates can be harmful and may also be beneficial.
There are many technologies that combine UV lamps to solve the relationship between temperature and IR. Can be divided into reduced emission, transmission and control of heat movement. The reduction in emission is achieved by using small diameter bulbs because it is the surface area of ​​hot quartz that emits almost all of the IR. The reduction in transfer can be achieved by using a cold mirror behind the lamp; or use a hot mirror between the lamp and the target. Heat movement reduces the temperature of the target - but only after the IR has caused a temperature rise - using cold air or heat sinks to control heat movement. The absorption of IR energy is determined by the material itself - ink, coating or substrate. The speed has a significant effect on the temperature caused by the incident IR energy and the energy absorbed by the work surface. The faster the process, the less IR energy is absorbed, causing the temperature to rise. It can speed up the production process by improving efficiency.
UV drying technology data
1. Most UV rays contain two UV wavelengths, both of which work simultaneously. Short waves work on the surface, longer waves act on the deep layer of ink or Lacquer. This is because short-wave energy is absorbed by the surface and cannot enter deep layers. Short-wavelength underexposure can cause the surface to become sticky, while insufficient long-wave energy can cause adhesion difficulties. 2. UV drying in CD production is used in two ways - protection of adhesive drying and printing ink drying.
A. Protective Glue: Coverage of protective glue is almost always carried out by spraying - spinning. Then exposed under UV. There are many ways of exposure, which can be roughly divided into: rotating or not rotating; focusing, defocusing or no focus.
B. Rotation mode: In this mode, the DISC is fixed under the UV lamp and rotated so that its surface is within a certain distance from the in-focus or out-of-focus UV lamp. Although the rotation method seems to be a good way to provide uniform exposure to the DISC surface