For UV curing

Raw materials for cationic curing The cationic curing process is preferred to the free radical curing process when there is a need for providing superior adhesion property across surfaces. Cationic curing offers the following benefits in comparison to the free radical curing process  

However, the development of cationic curing systems has been constrained by the limited availability of raw materials, which can be used for the process. 

Trimethylolpropane oxetane p MPO) The properties of TMPO which make it suitable for use as a reactive diluent raw material are  

Hydroxyl functionality Nominal hydroxyl and epoiqr equivalent weight (resp. HEW and EEW) Molecular weight (Mw. g/mol) Viscosity (mPas, 23 °C) 

FIGURE 3.2 Maximum curing speed (m/min) for nail scratch free and viscosity (mPas) as a function ofTMPO content. Curing conditions T pass under a 160W/cm Hg lamp (12 jm film on aluminium substrate) 

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

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]. 

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. 

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"... 

Commonly used monomers for UV curing include acrylates (7), styrene/unsaturated polyesters (8,9), and thiol-ene compositions (10-12). Currently, acrylate-functional systems constitute a major share of the UV curable polymers market, mainly due to their rapid curing via free radical chain polymerization. 

The most frequently used lamp for UV curing processes is medium-pressure mercury lamp. Its emission spectrum can be used to excite the commonly used photoinitiators. Moreover, this type of lamp has a relatively simple design, is inexpensive, can be easily retrofitted to a production line, and is available in lengths up to 8 ft (2.5 m). Power levels in common use are in the range 40 to 240 W/cm, and even higher levels are available for special applications. ... 

The robotic array is besf suifed for complex geometries. It uses the same type of robof fhaf applied fhe coafing, and the system has a single source of radiafion for each robof. An example of using robotics for UV cure is in Figure 7.1. 

The reactivity of cycloaliphatic epoxy monomers with cations is higher than that of glycidyl ether monomers. For this reason, cycloaliphatic epoxies are used for UV-cure coatings. The most efficient initiators are complex aromatic salts of Lewis acids such as diaryl iodonium, triarylsul-fonium, or arene diazonium (Table 2.23) (Crivello, 1999). 

McGinniss, V. C., "Photoinitiator of Acrylate Systems for UV Curing," 3rd International Radiation Curing Conference, Proc. of SME Radiation Curing Conference, FC76-486 (1976). 

Major uses for UV curing (room temperature) include use of flat stock-fillers in particle boards and hardwood flooring, and for high-gloss (no-wax) protective coatings for floor tiles, wood finishes, and other specialty uses. 

The dielectric strength of polyethylene has been measured as a function of additive concentration while in stabilised polymers the depth of degradation was found to be uniform . HALS have been found to be effective stabilisers for UV cured coatings and do not influence the cure rate . Bleached wood pulps can be effectively stabilised by ascorbic acid and wool by hydroxy-benzotriazoles and HALS . Hydroxybenzotriazole stabilisers also protect wood pulp and polyurethanes by co-reaction . The spectroscopic properties of monomeric and polymeric benzotriazoles have also been compared . UV absorbers are effective in clear coats and when grafted to wood . Dihydroxybenzophenone stabilisers inhibit the chain scission in the photodegradation of poly(methoxyacrylophenone) while tin stabilisers have also been found to photostabilise PVC . 

Therefore, VOC and SVOC emission profiles may differ depending on the factors mentioned as is demonstrated in Fig. 7 for moderate decay, fast decay, constant emission and increasing emission. All the studies were performed at 23 °C and 50% relative humidity. Even for the same type of products, the emission may be different over several orders of magnitude as is demonstrated in Fig. 8 for UV-cured furniture coatings. Here, chamber concentrations (2 VOC) were measured under identical conditions after 24 h and 672 h, respectively (see Ref. [43] for details). 

Photostabilization processes have not received much attention in the last year other than a multitude of review and specialist industrial articles. These include stabilization of polypropylene fibres, new stabilizer developments, sorting of stabilizers for polyethylene, trends in UV absorbers, hindered piperidines (hindered amine stabilizers (HAS)) for polyacrylics and urethanes, new stabilizers for polyolefins, enhanced service life applications, stabilizer mas-terbatches for polyester, stabilizers for UV cured acrylic overcoats for PVC, ° stabilization of clearcoats and various commercial packages. ... 

B. TOGETHER WITH REACTIVE DILUENTS AND PHOTOINITIATOR FOR UV CURE... 

Cyracure. [Unbn Carbide] Cycloaliphatic epoxy resins diluents and flexibilizers for uv-cured coatings and adhesives. 

Degacure . [Degussa] Binder for uv-curable coatings and inks photoinida-tor for uv-curing of binder systems. 

C. Roffey, Photogeneration of Reactive Species for UV Curing, Wiley, Chichester, 1997, pp. 417-418. 

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