Photoinitiators PSAs

Polyalkylene oxides (PAO) are prone to chain transfer. This tendency was used by Union Carbide to graft acrylate side chains via UV exposure of a PAO/acrylic monomer(carbamyloxy alkyl acrylate)/photoinitiator blend [58]. The final product was a water-sensitive PSA. 

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. ... 

The photoinitiator (single or in a combination) is vital to the manner in which an adhesive cures. The desired property of the PSA is a balance of good tack and good adhesion. If the state of cure is too high, the tack will be too low, and if the state of cure is too low, the adhesive will stick to the substrate, but will be difficult or impossible to remove. 

The main areas of research into UV-polymerizable PSAs are type, molecular weight, polarity, glass transition temperature (Tg), functionahties, and blending ratio of oligomers and monomers. Photoinitiator type (type I or type II) and kinds of tackifiers are also considered in the manufacture of PSA. As a type I... 

The 2-ethylhexyl acrylate, vinyl acetate, acrylic acid, 2-hydroxyethyl methacrylate, and 2,2 -azobisisobutyronitrile (AIBN, as initiator for polymerization) used for preparing UV-crossHnkable acrylic PSAs were obtained from Junsei Chemicals Co. and used as received. The photoinitiator P-36 containing a double bond was obtained from SK UCB Co., Korea. The typical synthetic method was as follows (see Table 17.1). 2-EHA (120 g), VAc (22.5 g), AA (7.5 g), AIBN (0.3 g), and ethyl acetate (75 g) were aU mixed in a 500 mL four-necked flask equipped with a stirrer, dropping fuimel, and thermometer. The polymerization reaction was initiated at 70 °C. After this temperature had been maintained for 1 h, ethyl acetate (75 g) containing P-36 (1.5 g) was gradually added to the flask while it was stirred for 1 h, and then the polymerization was carried out at 70-75 °C for another 5 h. All of the syntheses were carried out in a flask wrapped in aluminum foil... 

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. 

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. 

To better understand the PSA chemistry, there is a need to define what is known as the optimal process conditions. This refers to the optimal energy range where the cured adhesive is able to achieve the best possible performance specifications at a desired level of thickness. The same processing conditions cannot be used for the same adhesive with twice the application thickness it becomes necessary in that case to define different process conditions for that thickness. The optimal process conditions also depend on variations in lamps, photoinitiators, and oxygen inhibition. 

Photoinitiators must be added for UV crosslinking. A-PSAs can act as their own photo cross linker if they possess chromophoric groups that undergo UV-induced chemical bonding with each other. The chromophoric groups may be in the pendant or polymer backbone. 

FIGURE 4.7 Modification of acrylic PSAs using copolymerisable and additionable photoinitiators... 

The major properties of UV-crossIinked A-PSAs are their tack, peel, adhesion, and shear strength. These can be controlled by adjustments in the photoinitiator concentration, UV-crosslinking time and UV dose. The UV dose can be controlled by changing the power of the lamps and/or the speed at which the substrate passes under the lamps during production. 

The concept of radiation-curable PSAs can be simply described as a rapid photoinitiated cross-linking producing a viscoelastic material in which the... 

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