UV energy

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

The alkyl ethers of benzoin undergo dkect photofragmentation upon absorption of uv energy at ca 360 nm to produce two free-radical intermediates. 

As an example stereolithography is a 3-D rapid process that produces automatically simple to very complex shaped models in plastic. Basically it is a method of building successive layers across sections of pho-topolymerized plastics on top of each other until all the thin printed layers can be joined together to form a whole product. The chemical key to the process, photopolymerization, is a well established technology in which a photo initiator absorbs UV energy to form free radicals that then initiate the polymerization of the liquid monomers. The degree... 

Three types of light sources for artificial weathering are in common use (1) enclosed UV carbon arc [7.5 UV energy output, approx. (x sunlight)], (2) open-flame sunshine carbon, and (3) water-cooled xenon arc. Selection of the light source involves many conditions and circumstances, such as the type of material being tested, product service conditions, previous testing experience, or the type of information desired. 

A chromophore is a group which when attached to a saturated hydrocarbon produces a molecule that absorbs a maximum of visible of UV energy at some specific wavelength. 

Typical studies may be set up as detailed in Table 5. Samples set np as indicated should be sampled and analyzed intermittently over the conrse of approximately 12 weeks, and the degradation prodncts identified. To evaluate the influence of light, DS and DP should be exposed to a light source that produces an output similar to the D65/ID65 emission standard. Samples should ideally be exposed at 50%, 100%, and 200% of the ICH specified 1.2 million Inx honrs at an integrated near-UV energy of not less than 200Wh/m. ... 

Irradiation of MgO at room temperature in the presence of oxygen leads to formation of OJ and OJ (338, 355,407), which can be readily observed by EPR. This process occurs at UV energies much less than the bulk band gap, showing that surface lattice ions are involved which have a reduced band gap (1,356). On TiOz, OJ is the only stable species at room temperature, but irradiation in the presence of oxygen at 77 K (66,88,205,207,208) leads to Oj as well as O . The situation on Ti02 is very complicated (Section IV,B,1) and depends on the degree of hydroxylation of the surface (204, 205, 207, 408). 

Lamp (or bulb). The electrical energy supplied to the bulb is converted into UV energy inside the bulb. 

Infrared radiance is the amount of infrared energy primarily emitted by the quartz envelope of the UV source. This energy is collected and focused with the UV energy on the surface of the substrate to the extent depending on the IR reflectivity and efficiency of the reflector. Infrared radiafion can be expressed in exposure or irradiance units, but most commonly, the surface temperature it generates is of primary interest. The heat it produces may be a benefit or a nuisance. ... 

Laminating adhesives generally bond clear laminate plastic films. As long as the films are clear, the adhesive can be cured by UV energy if they are opaque, electron beam radiation is a better choice. 

Radiometers for three-dimensional cure are used for simultaneous multipoint measurements, for setup and process verification of the lamp system. They can be used with UV lamps mounted on a fixed bank or a robotic arm. The collected exposure data (irradiance and total UV energy) are displayed on a computer for each sensor position. A SDCure radiometer is shown in Figure 9.7, and an example of a screen display from a 3D radiometer in Figure 9.8. 

Instrument manufacturers offer a variety of devices, including monitoring systems for standard process or for spot curing technology and systems for process control. Online UV radiometers can measure and display the peak intensity and total UV energy used in the process. More-advanced systems map the entire process and evaluate the lamp focus, reflector focus and reflector efficiency within a specific... 

Although the herbicide, molinate (S-ethyl hexahydro-111-azepine-l-carbothioate), does not absorb appreciable sunlight UV energy, its vapor still was photooxidized by atmospheric ozone (Figure 8) (30). We, and others, have observed the same phenomenon with a number of other non-absorbing chemicals such as... 

---