Making it stick. Seite/Page: 31

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1 Making it stick Solvent-based acrylic pressure-sensitive adhesives with good shrinkage resistance Zbigniew Czech. When developing solvent-based acrylic pressure-sensitive adhesives (PSAs), tack, peel adhesion, shear strength and shrinkage are the main performance parameters. These are determined by the molecular weight of acrylic copolymer, polymerisation method and especially by the type and quantity of the crosslinking agents added to the PSA. In this work, several crosslinking agents are investigated. It has been concluded that unsaturated photoinitiators tend to generate the best shrinkage resistance. Since their introduction half a century ago, acrylic pressure-sensitive adhesives (PSAs) have been successfully applied in many fields. They are used in self-adhesive tapes, labels and protective films as well as in dermal dosage systems for pharmaceutical applications, in biomedical electrodes, the assembly of automotive parts, toys, and electronic circuits and keyboards. Four properties which are useful in characterizing the nature of PSAs are tack, peel adhesion (adhesion), shear strength (cohesion) and shrinkage. The first measures the adhesive's ability to adhere quickly, the second its ability to resist removal by peeling, and the third its ability to hold in position when shearing forces are exerted. Generally speaking the above mentioned performances of PSAs are to a large degree determined by the molecular weight of acrylic copolymer, polymerisation method and especially by the type and quantity of the crosslinking agents added to the PSA. Although acrylic polymers with low glass temperature have been used as PSAs in a variety of industries, a property inherent to all acrylic PSAs which negatively impacts tack and adhesion performance is shrinkage on PVC surfaces upon crosslinking. This phenomenon is attributed to the formation of a three-dimensional, covalently crosslinked network during crosslinking, which reduces intermolecular distances between the monomers used to form the crosslinked network. For example, before crosslinking, the molecules which comprise the acrylate PSA are separated by their characteristic van der Waal's radii. Upon crosslinking, these intermolecular distances are reduced due to the formation of covalent bonds between monomers which produces the desired highly-crosslinked PSA material. This reduction of intermolecular distances creates internal stress throughout the crosslinked network, which is manifested by reduced adhesion of the PSA adhesive to both the substrate and the object attached thereto [1]. Shrinkage is defined as the percentage or millimeter change of dimensions of the PVC foil covered with PSA after the adhesive has been crosslinked and attached to a glass substrate and kept for one week at a temperature of 70 C. If shrinkage is greater than 0.5% or 0.5mm the film is rejected [2-3]. High-performance solvent-based PSAs should have a very high shear strength (120N at 20 C, 40N at 70 C) as well as shrinkage lower than 0.5%, a high tack and a maximum peel adhesion. UV-induced crosslinking is a rapidly expanding technology in the PSA field because it is a solvent-free process, it is efficient and gives high-quality chemical crosslink bonding [4]. This crosslinking process has found interesting application for producing photoreactive PSA systems with high performance used in the coating industry for PVC surfaces. The idea of replacing the conventional room temperature crosslinked or two-part thermally crosslinked systems with single-component photoreactive pressure-sensitive adhesives has become very attractive to manufacturing industries [5]. The most important features of crosslinked polyacrylate PSAs, such as adhesion, cohesion and shrinkage can be controlled through the crosslinker type and amount. In the case of UV-crosslinkable adhesives the UV dosage determines these properties. The crosslinking of PSA with UV light can be done directly after its application. Normally polyacrylates absorb the entire light radiation below 300nm. In order to accomplish acceptable crosslinking with transparent PSAs, only UV wavelengths above 300nm are of importance and a UV wavelength range about 300 to 400nm is required. Increasing attention is being given to a new class of unsaturated copolymerisable photoinitiators. Polymerisation with other acrylic monomers and properties of synthesised PSAs containing these photoinitiators after UV crosslinking is also being investigated [6]. In order to develop solvent-based high-performance acrylic PSAs, selected metal chelates and selected propylene imines were used as crosslinking agents. Technical grade commercial crosslinking agents were purchased from the following suppliers: metal chelates titanium(iv) acetylacetonate (TiACA) and aluminum(iii) acetylacetonate (AlACA) from DuPont, propylene imines trismethylolpropan-tris-(n-methylaziridinyl) propionate ("Neocryl CX-100") from ICI and tris 1-(2-methyl) aziridinyl phosphineoxide (MAPO) from Arsynco. The crosslinking mechanism of UV photoreactive acrylic PSAs containing photoreactive benzophenone derivatives has been thoroughly investigated and it is presented schematically in Figure 1 [7]. AIACA is a better crosslinking agent than TiACA Figures 2 and 3 show how the examined metal acetylacetonates influence the main parameters of acrylic PSAs such as peel adhesion and shear strength. Preferred metal chelate crosslinkers aluminum acetylacetonate (AlACA) and titanium acetylacetonate (TiACA) were selected according to their solubility in common organic solvents, taking into account the preliminary values of cohesion. The metal chelates were used in concentrations between 0.1 and 1.0wt.% with respect to the acrylic polymer solids. After the addition of small concentrations of TiACA and AlACA an improvement of the PSA properties, like peel adhesion and shear strength, is observed. The maxima of peel adhesion for about 0.15wt.% of TiACA and AlACA are achieved. PSA layers crosslinked with TiACA are unusually tacky and aggressive. After exceeding a threshold of 0.2wt.% investigated metal chelates, the peel adhesion levels decrease. The cohesion results show that the TiACA and AlACA differ concerning ability to improve the shear strength of the crosslinked acrylic PSAs. Aluminum acetylacetonate (AlACA) is a better crosslinking agent than titanium acetylacetonate (TiACA). In order to reach the shear strength of 120N at RT and 40N at 70 C it is necessary to use about 0.5wt.% tested metal acetylacetonates. Propylene imine is a more useful crosslinker than MAPO

2 Crosslinking of solvent-based acrylic PSAs can be carried out by the use of a propylene imine crosslinker, a molecule that has two or more 2-methylaziridine groups capable (Figure 4) of reacting with the functional carboxylic groups on the polymer chain. The oxygen of the nucleophilic carboxylic group causes the opening of the tensioned propylene imine rings while the hydrogen atoms accompanying the carboxylic groups protonate the nitrogen atoms. Multifunctional propylene imines can also be used to crosslinking carboxylic functional acrylic PSAs. The crosslinking reaction takes place at room temperature. The influence of investigated propylene imine crosslinkers on peel adhesion and shear strength of crosslinked acrylic PSAs is illustrated in Figures 5 and 6. In a similar manner to the previously conducted experiments, for both investigated propylene imines the peel adhesion maximum for 0.1wt.% crosslinker was observed. An increase of propylene imine crosslinker concentration negatively influences peel adhesion. In opposition to the adhesive properties, the shear strength level attains high values. The highest cohesion value was observed for the small amounts (0.3wt.%) of the propylene imine "Neocryl CX-100". As can be seen from Figures 5 and 6, propylene imine "Neocryl CX-100" is a more useful crosslinker than MAPO with its central phosphorus heteroatom. As illustrated, "Neocryl CX-100" gives the best balance between adhesion and cohesion among the selected propylene imine crosslinked acrylic PSAs. Unsaturated photoinitiators give best shrinkage resistance Copolymerising the unsaturated photoinitiator into the backbone of the acrylic PSA copolymer allows crosslinking of the acrylic PSA with ultraviolet radiation, after formation of the copolymer. The most typical directions, however, are in the development of functionalised acrylated or vinylated of type I and type II photoinitiators (see Figure 7). Such functionalised chromophores will themselves either co-react with the crosslinked acrylic PSA or are capable of copolymerization to produce acrylic self-adhesive copolymers, which are not extractable. The investigated unsaturated photoinitiators were synthesized by Dr. Milker Klebstoff GmbH Horhausen (Germany) and at the Technical University in Szczecin (Poland). The influence of the concentration of the evaluated metal chelates (AIACA, TiACA, "Neocryl CX-100" and MAPO) on shrinkage of the synthesized solvent-based acrylic PSAs is presented in Figure 8. The same phenomenon is shown for investigated unsaturated photoinitiators of type I or type II (Figure 9). It can be seen that the acceptable shrinkage values under 0.5% are obtained using the evaluated propylene imine "Neocryl CX-100" and metal chelate AlACA and TiACA as crosslinkers in amounts above 0.3wt.%. By using 0.3wt.% of "Neocryl CX-100" an excellent lowest shrinkage level of 0.35% was observed. Generally speaking, it has been concluded that unsaturated photoinitiators tend to generate the overall best shrinkage resistance. It is interesting to note that the shrinkage of solvent-based acrylic PSAs after UV exposure tremendously decreases, suggesting continued crosslinking activity under UV exposure. Acceptable shrinkage values under 0.5% were noticed in the case of 4-acryloyloxy benzophenone (ABP) and 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-prop anone ("ZLI 3331") for 0.3wt.% of both photoinitiators. Above 0.3 wt.% "ZLI 3331" and 0.3wt.% ABP, the shrinkage values correspond to less than 0.45% and 0.35%. The best copolymerisable photoinitiator, referring to very low shrinkage of UV-crosslinked solvent-based acrylic PSAs, was ABP. The development of solvent-based acrylic PSAs with high performance and excellent shrinkage resistance has recently attracted considerable attention, due to their practical and potential application in various industrial branches to comprehensive self-adhesive technical and medical products. Solvent-based acrylic PSAs with high performance, good peel adhesion, high shear strength and high shrinkage resistance, can be formed by crosslinking with selected metal chelates and selected propylene imines. The unsaturated photoinitiators are predominantly used as a photoreactive crosslinking agent for solvent-based acrylic PSAs with excellent shear performance and low shrinkage for PVC sign and marking films. References [1] R. Milker, Z. Czech, 9. Münchener Klebstoff- und Veredelungsseminar, München, Germany (1984). [2] Z. Czech, J. Appl. Polym. Sci. (2001), 81, [3] Z. Czech, Adv. Polym. Techn. (2001), 20, [4] M. Bisges, Adhes. Age (2002), 11, 34 [5] C. Matijasic, Adhes. Age (2002), 12, 29 [6] Z. Czech, Polimery (2003), 5, 375 [7] Z. Czech,"Vernetzung von Haftklebstoffen auf Polyacrylatbasis", Wydawnictwo Politechnika Szczecinska, Szczecin (1999) ISBN: Results at a glance - Solvent-based acrylic pressure sensitive adhesives (PSAs) have many applications in self-adhesive technical and medical products. - Important properties of PSAs include tack, peel adhesion (adhesion), shear strength (cohesion) and shrinkage. - Shrinkage is a problem on PVC surfaces and is caused by crosslinking. - In order to improve these properties, several metal chelates and selected propylene imines were investigated as crosslinking agents - "Neocryl CX-100" gives the best balance between adhesion and cohesion among the selected propylene imine crosslinked acrylic PSAs. - Aluminum acetylacetonate (AlACA) is a better crosslinking agent than titanium acetylacetonate (TiACA). - Unsaturated photoinitiators tend to generate the overall best shrinkage resistance. The author: -> Dr Zbigniew Czech is head of research and development for pressure-sensitive adhesive synthesis, technology and application at Szczecin Technical University's Polymer Institute in Poland. Since completing his PhD in 1983, he has held senior positions at Lohmann (Germany) and UCB (Drogenbos/Belgium) and worked as a technical consultant in acrylic PSAs synthesis before starting his current role in 2003.

3 Figure 1: Photocrosslinking by use of unsaturated photoinitiators incorporated into polymer chain. Figure 2: Effect of metal acetylacetonate concentration on peel adhesion. Figure 3: Effect of metal acetylacetonate concentration on shear strength.. Figure 4: Investigated commercially available propylene imines.

4 Figure 5: Effect of the propylene imine crosslinkers on peel adhesion. Figure 6: Effect of the propylene imine crosslinkers on shear strength.. Figure 7: Unsaturated photoinitiators used by UV-initiated crosslinking. Figure 8: Influence of the crosslinking agent kind and amount on shrinkage.

5 Figure 9: Shrinkage of acrylic PSAs containing copolymerizable photoinitiators.