UV-curing

A careful choice of formulation and chemistry is needed in this process. It is also possible to increase the amount of UV energy by increasing the number and power of the lamps used. The amount of UV energy used can be regulated based on the condition 

Overview of radiation-curable adhesives in packaging applications 

Cationic systems Cationic systems typically use epoxy and/or vinyl ether materials as compared to acrylic components used by free radical systems. This limits the monomers and oligomers available for use in these systems, and thus reduces the versatility in achieving the desired physical properties. 

Curing in cationic systems continues even when the light source is removed, but the effect is minimal and thermal energy is required for the cure to be effective. Cationic systems permit the use of relatively toxic photoinitiators with corrosive residues. While cationic systems are not impeded by oxygen inhibition, they can be poisoned by high humidity with alkali surfaces. 

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Table 10. Comparison of Characteristic Properties of CD-Modified BPA-Polycarbonate with UV-Curing Duromer...

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. 

S. G. Wentink and S. D. Koch, ed., UV Curing in Screen Printingfor Printed Circuits and the Graphics Arts, Technology Marketing Corp., Norwalk, Coim., 1981. 

Inks. Refined kaolin is a common ingredient in a large variety of printing inks (qv). In addition to extending the more expensive polymers present, ka olin also contributes to improved color strength, limits the penetration of the ink into the paper, controls rheology, and improves adhesion. Kaolin for this appHcation must usually be as white as possible and free from oversize particles. Surface treated clays are used to improve compatibiHty with oil-based ink. Clays can also be an ingredient in the newer water-based or uv-cured inks. 

Uses that have developed for uv curing reflect the special advantages of the system rather than replacement to reduce VOC emissions or energy... 

The UV cure system contains an epoxy or a vinyl ether functionalized PDMS polymer and a photo catalyst [36]. This latter, a diaryliodonium salt is photolyti-cally decomposed to form an active acid that polymerizes the epoxy or vinyl ether groups and crosslinks the network. 

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Silicone with vinyl-terminal groups is commonly UV-cured for release coatings. These materials have been combined with silicate tackifiers and acrylate monomers to make PSA compositions as well [55]. 

Hot melt aerylie polymers for UV cure are generally of lower moleeular weight than their solution counterparts in order to maintain an aceeptable application viscosity. Consequently more crosslinks are needed to produce a complete network and thus peel and tack suffer to some degree. Nonetheless, several UV curable materials have been commercialized for PSA tapes and labels. 

To improve on penetration and heat resistance (properties normally in opposition when formulating hot melts), reactive hot melt systems are being developed. Moisture curing urethanes have already gained wide acceptance in a number of wood and automotive applications. UV cure is slowly gaining acceptance in PSA, labeling, and other markets. 

The benzoin ethers (75, R-alkyl R H) and the ot-alkyl benzoin derivatives (75, R=H, alkyl R =alkyl) undergo a-scission with sufficient facility that it is not quenched by oxygen or conventional triplet quenchers.276 This means that the initiators might be used for UV-curing in air. Unfortunately, it does not mitigate the usual effects of air as an inhibitor (Section 5.3.2). The products of a-scission (Scheme 3.53) are a benzoyl radical (13) and an ( -substituted benzyl radical (76) both of which may, in principle, initiate polymerization, 76 2"... 

Pappas, S.P., UV Curing Science and Technology, Vols. 1 and 2, Technology Marketing Corporation, Connecticut, 1978, 1985. 

Wenting, S.G. and Koch, S.D., UV Curing in Screen Printing for Printed Circuit and the Graphic Arts, Technology Marketing Corporation, Norwalk, Connecticut, 981, 70. 

DIP-ToFMS is theoretically another option for the separation of additives from polymer in dissolutions using a probe temperature ramp. However, the technique also allows direct handling of solid substrate material, which is even more convenient. The technique has profitably been used for the analysis of non-UV cured ink, revealing diluent, photo-initiator and polymer [54]. 

During the last few years, new developments in polymer photochemistry have made it possible to graft various functional monomers onto surfaces of inert polymers like polyethylene, polypropylene and polyethyleneterephthalate. In the first attempts, initiator and monomer were transferred in vapor phase into a "UV Cure" irradiator containing the polymer sheet to be surface grafted. 

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