UV-crosslinkable photoreactive pressure-sensitive adhesives synthesized from butyl acrylate and 4-acryloyloxy benzophenone

The manuscript describes synthesized of photoreactive UV-crosslinkable solvent-borne acrylic pressure-sensitive adhesives (PSA) based on butyl acrylate (BA) and copolymerizable photoinitiator 4-acryloyloxy benzophenone (ABP) characterized by molecular mass in the range of 180 000 to 480 000 Dalton. These copolymers were tacky but possessed insufficient cohesive strength after UV-crosslinking to be useful as PSA. They resulted in materials having a balance of cohesive and adhesive characteristics required of good PSA. Some of the parameters affecting the pressure-sensitive adhesive properties of the copolymer are: amount of the 4-acryloyloxy benzophenone, molecular mass of the polymeric components, UV-reactivity and such important properties like tack, peel adhesion and shear strength.


INTRODUCTION
Conventional solvent-borne acrylic pressure-sensitive adhesives (PSA) are generally copolymers of C 4 -C 8 alkyl acrylates and polar monomers, such as acrylic acid or hydroxyacrylates. Optionally, modifying monomers like methyl or ethyl acrylate and vinyl acetate may also be incorporated in the copolymer structure. Optimum cohesive and adhesive properties of the copolymers are attained by a proper balance of its molecular mass (usually very high), polarity, and the glass transition temperature ranging from -25 to -70 ºC. The acrylic PSA are generally applied onto the desired substrates as solvent-borne or water-borne coatings and subsequently dried. However, on account of solvent and energy costs, time consumption in the drying process, and constraints imposed by environmental pollution regulations, the relevant role plays the solvent-borne pressure-sensitive adhesives.
Solvent-borne acrylic pressure-sensitive adhesives are synthesized in organic solvents as viscoelastic polymers with permanent tack (initial adhesion) and the balance of two properties adhesion (peel adhesion) and cohesion (shear strength) [1]. In the long history of technology, pressure-sensitive adhesives (PSA) and self-adhesive articles as we know them are a fairly recent concept. The history of PSA was described by Villa [2]. The diverse crosslinking methods of acrylic PSA have been discussed in [3]. Ultraviolet-crosslinked solvent-borne acrylic PSA are one component systems. UV-crosslinked acrylic PSA systems were described in [4][5][6][7][8][9]. Photo-inducted crosslinking is a rapidly expanding technology on PSA area resulting from new properties and quality of chemical crosslinking bonding. This crosslinking process and a new class of UV-crosslinkable acrylic PSA founded interesting application for production of self-adhesive tapes, labels, protective films, sign and marking films, masking tapes, dental and medical materials.
The crosslinking mechanism of UV photoreactive PSA acrylics containing 2ethylhexyl acrylate and 4-acryloyloxy benzophenone has been thoroughly investigated and it is presented schematically in Fig. 1  The behavior of any pressure-sensitive adhesive can be reduced to three fundamental and interconnected physical properties: tack, peel adhesion and cohesion [11]. Tack of PSA is not an exactly defined, physical characteristic; it may be defined as separation energy. Nevertheless tack is still considered and rated by many as how well a self-adhesive sticks to the finger following only slight pressure and short dwell time. Peel adhesion is the force required to remove a PSA-coated material from a specified test surface. Shear strength is a real measure of the internal structural resistance of the polymer. Generally, the mechanical and physical (tack, peel adhesion) properties of acrylic PSA are dependent on its cohesion.

1. Materials
The following experiments were conducted to study the influence of amount of 4-acryloyloxy benzophenone on the following properties of the synthesized acrylic PSA such as viscosity, molecular mass, tack, peel adhesion and shear strength. The investigated PSA were synthesized with 0.1 to 3.0 wt.% of 4-acryloyloxy benzophenone and rest of butyl acrylate by polymerization in a typical organic solvents like ethyl acetate and acetone in rate 80 : 20 with 0.1 wt.% of thermal radical initiator AIBN. The polymer content during the polymerization was about 50 wt.%. butyl acrylate, ethyl acetate, acetone and AIBN are available from BASF (Germany). The unsaturated photoinitiator 4-acryloyloxy benzophenone

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ILCPA Volume 8 is purchased from ChemCycle (Germany). The photoreactive acrylic PSAs were coated with 60 g/m 2 dry polymer directly on a 36 m polyester foil and crosslinked after drying 10 min at 105 ºC with UV lamp of type U 350-M-I-DL from IST company by the UV dose of 700 mJ/cm 2 at various crosslinking times.

Measurements
The molecular mass studies were performed in tetrahydrofurane with a gel chromatography LaChrom system: RI Detector L-7490 and LaChrom UV Detector L-7400 from Merck-Hitachi, equipped with a PLgel 10 6 Å column from Hewlett-Packard. Viscosities of the solvent-borne copolymers were determined on Brookfield Synchro-Lectric Viscometer Model RVT with Thermosel attachment, using spindle # 27 at 2.5 rpm. The UV-exposure was measured using an integrating radiometer Dynachem TM Model 500, available from Dynachem Corporation, 2631 Michelle Drive, Tustin, CA 92680. The performance of the pressuresensitive adhesive acrylics were tested according to A.F.E.R.A. 4015 (tack), 4001 (peel adhesion) and 4012 (shear strength).

RESULTS AND DISCUSSION
It has been previously shown that copolymers of a butyl acrylate with a 4-acryloyloxy benzophenone containing a small amount of photoreactive groups exhibit well adhesively performance. The n-butyl acrylate-4-acryloyloxy benzophenone copolymers, which were prepared by solution polymerization of the monomers in the presence of AIBN had GPC polystyrene equivalent peak molecular weights ( w M ) in the range of 180 000 to 480 000 Dalton. These copolymers were clear, tacky, and cohesively weak viscous fluids. The data of Table 1 illustrate the effect of the 4-acryloyloxy benzophenone content on the molecular mass, viscosity, tack, peel adhesion and cohesion of PSA acrylic after UV-crosslinking using 700 mJ/cm 2 UV dose and crosslinking time of 60 s. The results of the Table 1  With the increasing of photoinitiator concentration there has been observed a decreasing of tack of investigated photoreactive solvent-borne acrylic PSAs.

International Letters of Chemistry, Physics and Astronomy Vol. 8
In the area of between 1.0 to 3.0 wt.% 4-acryloyloxy benzophenone it was observed that the tack very fast decreases. This phenomenon is typical for crosslinked adhesives.
The highest value of PSAs peel adhesion without cohesive failure (cf) was obtained by using between 0.7 to 1.0 wt.% of 4-acryloyloxy benzophenone.
The results of peel adhesion for amount of photoinitiator under 0.8 wt.% brought the highest adhesive performance, however with cohesion failure.
These types of results, characterized by insufficient cohesion of acrylic PSA, are unacceptable for their technological use. The increase of the concentration of the 4-acryloyloxy benzophenone used for the synthesis of UV-crosslinkable PSA acrylics causes an increase of their cohesion.
A very high cohesion value of 120 N order of magnitude was possible to be obtained by applying of 3.0 wt.% 4-acryloyloxy benzophenone in the acrylic copolymer chain. With 1.0 and 1.5 wt.% of unsaturated photoinitiator the acceptable cohesion level of 30 and 40 N was achieved.
The phenomenon of performance of UV-crosslinkable acrylic PSAs is a result of the UV-crosslinking, measured as UV-crosslinking time, which takes place when the PSA coated film is exposed to UV radiation.
In Fig. 7  In general, it can be said that the use of unsaturated photoinitiator 4-acryloyloxy benzophenone in the amount of 1.5 wt.-% in acrylic PSA copolymers gave the best balance of tack, peel adhesion and shear strength after the UV-crossliniking time between 60 and 180 seconds. For about 90 s of crosslinking time, the optimum of tack, peel adhesion and cohesion of UV-crosslinked acrylic PSAs values by incorporation into polymer chain of the investigated photoinitiator 4-acryloyloxy benzophenone was observed.

CONCLUSIONS
UV-crosslinkable acrylic PSAs designed to react with UV radiation offer a good alternative to other crosslinked solvent-borne adhesives.
From the evaluation of the experiments discussed in this publication, it can be concluded that:  The very interesting performances of UV-crosslinked solvent-borne acrylic pressuresensitive adhesives based on butyl acrylate were achieved by the incorporation into polymer backbone of 4-acryoyloxy benzophenone during the polymerization.
 Increasing of the amount of copolymerizable 4-acryloyloxy benzophenone increases viscosity and molecular mass of synthesized solvent-borne acrylic PSAs.
 From the investigated copolymerizable photoinitiator concentration the best results of relevant properties of pressure-sensitive adhesives, such as tack, peel adhesion and shear strength were given by about between 1.0 and 1.5 wt.% of 4-acryloyloxy benzophenone.
 After UV-crosslinking time between 60 and 90 s the investigated performance of acrylic PSAs containing 4-acryloyloxy benzophenone achieves a very high level.
International Letters of Chemistry, Physics and Astronomy Vol. 8  Generally, the properties values of synthesized UV-crosslinkable acrylic PSAs containing in the structure 4-acryloyloxy benzophenone are after UV crossliniking on very high level. Their shear strength shows an excellent level.