UV cured PSA

Free-radical, radiation-curable systems generally consist of monomers, oligomers, photoactivators, other resins, and fillers or tackifiers. An ultraviolet source or electron-beam generator is used to cure the systems. As an illustration Stueben (37) describes a typical UV-cured PSA system containing acrylates and polyvinyl ether. McGinniss (38) discusses formulation design related to UV-curable systems. The cationic photoinitiators are shown in Fig. 1. 

Here are a few examples of different types of UV-cured PSAs developed in various applications ... 

Benefits of UV-cured PSAs 12 (removable labels) C-dose 33... 

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. 

In the past, electron beam radiation was applied to produce PSA exclusively however, recent improvements in UV curing technology (precise UV dose control, suitable photoinitiators) permit UV to be used to produce pressure-sensitive adhesives. PSA formulations can vary in consistency from low-viscosity liquids up to solids melting at 80°C (176°F). Therefore, applications may vary from screen printing to roll coating to melt extrusion. Coat weights for most PSA materials vary from 1 to 10 g/m. ... 

UV curing at low UV doses. But at UV doses above 630 mj cm, the relative C=C contents were almost the same. This may be due to the increased cross-linking density, which reduced the mobility in the PSAs. This phenomenon confirmed the result of many previous studies, that the maximum UV-curing rate was shifted to an earlier stage of UV irradiation when the photoinitiator content was increased. After this point, however, the increased crosslinking density reduced the molecular mobility and so the curing rate decreased [15-17]. 

Of the various methods used to manufacture UV-curable PSA, that of preparing a UV-crosslinkable acrylic PSA was adapted to evaluate the UV-curing behavior and adhesion performance at various UV doses. The UV-crosslinkable PSAs were copolymerized with acrylate monomers and an unsaturated photoinitiator, P-36, with varying contents of 2-HEMA and photoinitiator. 

UV cured silicone PSA s. Siloxanes containing epoxy groups cured using UV and catalysts such as bis(dodecyl-phenyl) iodonium hexafluoroantimonate. 

The reasons for using UV-curable PSAs, rather than solvent-based adhesives, remain essentially the same as for the other UV-curable systems and include high cure speed, reduced VOC emission, low energy consumption and enhanced product performance. The subject has been extensively investigated in recent... 

Radiation curing is very appropriate to cure PSAs, as the transferred energy is far higher than in thermal cross-linking. Both electron beam (EB) and ultraviolet (UV) radiation are applied. Their physical characteristics differ, as shown in Table 6 [263] ... 

PSA adhesives A significant use for radiation curing is the advancement or curing of pressure-sensitive adhesives. High solid EB and UV curing liquid adhesives are being formulated from a variety of resins and elastomers. Such adhesive compositions include acrylics, blends of acrylic monomers with elastomers, polyesters and other compositions. These products are primarily used as pressure-sensitive adhesives. 

These are typically 100% reactive Uquids cured to soKds. One substrate must be transparent for UV cure, except when used in coating applications. Absorption coefficient of the top substrate material affects penetration of the UV/EB irradiation. Curable formulations are used in laminating, PSA, and some medical applications. 

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

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