Multifunctional propyleneimines-new generation of crosslinkers for ...

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504 ARTICLE IN PRESS Z. Czech / International Journal of Adhesion & Adhesives 24 (2004) 503–511 properties of the PSA coat, such as tack and peel adhesion decrease (Fig. 1). The time span, which can be utilized after the crosslinking agent containing PSAs is set up, is called pot life. After a second time span the so-called gel point is reached, and the PSA can no longer be modified. This process is illustrated below (Fig. 2). The curve in general applies to each crosslinking agent containing PSA system. That means that a fast crosslinking systemhas actually a short pot life, and a slow crosslinking systemhas a long pot life. Fast crosslinking PSA systems for industrial use with short pot life are often used with two-component applicationequipment, which mix and add the reactive components right before the coating [4]. In order to develope the solvent-based PSA crosslinking, various multifunctional chemical compounds with respectively reactive groups are used as the following list illustrates: * metal acidesters [5], * metal chelates [6], * metal salts [7], tack, adhesion 25 20 15 10 5 0 tack cohesion crosslinker amount adhesion Fig. 1. Crosslinkage density influence on tack, peel, adhesion and cohesion [2]. viscosity (Pas) 12 10 8 6 4 2 coating level pot life gel point 0 0 1 2 3 4 5 6 7 time (h) Fig. 2. Viscosity dependence of time by crosslinker containing PSAs [3]. cohesion * polyfunctional isocyanates [8], * organofunctional silanes [9], * polycarbodiimides [10], * polyfunctional ethyleneimines [11], * polyfunctional propyleneimines [12], * amino resins [13], * epoxy resins [14]. 1.1. Multifunctional propyleneimine derivatives In order to achieve optimum crosslinking efficiency, the known terrain of crosslinkers was widened by the introduction of a new generation of crosslinkers, multifunctional propyleneimines derived from 2-methylaziridine containing saturated unstable rings with nitrogen atoms. Such compounds react with the carboxyl groups or isocyanate groups of the polymer by opening the ring. This classifies themas crosslinkers. The crosslinking of carboxyl group containing pressure sensitive adhesives is carried out under room temperature, an increase of temperature accelerates the crosslinking process. Because of the high reactivity of polyaziridines the pot life of propyleneimine containing PSA formulations is quite limited and lies in the range of 2– 8 h which is lower than the pot life of polyisocyanate containing adhesives. This problemcan be overcome by mixing the adhvesive mass and the crosslinker directly prior to the coating step. In comparison to metal acid esters or metal chelates the crosslinking yield of multifunctional propyleneimines is so high that no afterreactions like brittleness or a decline of peel resistance due to ongoing crosslinking occur. 1.2. Reaction mechanism of crosslinking The crosslinking of pressure sensitive adhesives with multifunctional propyleneimines is mainly based on the carboxyl groups offered by the vinylcarbonic acids within the polymeric chain. The oxygen of the nucleophilic carboxyl group causes the opening of the tense propyleneimine rings while the hydrogen atoms accompanying the carboxyl groups protonate the nitrogen atoms (Fig. 3). Such a crosslinking variation involves the following consideration (Fig. 4). Concidering the carbon-cation-structure the variant 2 seems to be the most realistic path backing the reaction mechanism shown above. General multifunctional propyleneimine groups containing compounds are known for over 35 years. Along with the progress in the field of organic synthesis the chemical structure of propyleneimine crosslinkers was adapted to the developments in polymer chemistry. The application of such crosslinkers is product oriented and only mentioned for special formulations. In the early
ARTICLE IN PRESS Z. Czech / International Journal of Adhesion & Adhesives 24 (2004) 503–511 505 O O Polymer C OH N R N HO C Polymer CH 3 CH 3 O H H O Polymer C O CH 3 N R N CH 3 O C Polymer H O O 2 C CH 2 N C R C N H 3 C HC CH CH 3 Fig. 5. Structure of bisamide crosslinking agents. O Polymer C O CH 3 CH CH 2 NH R NH CH 2 CH CH 3 O O C Polymer CH H 2 C 3 CH 3 CH N N 2 O C N CH CH HC 2 C CH 2 CH 2 CH 2 N C O CH CH CH 3 3 CH 3 O CH 3 CH 3 O Polymer C O CH CH 2 NH R NH CH 2 CH O C Polymer Fig. 3. Crosslinking of carboxylated PSA initiated by propyleneimine. O CH 3 CH 3 O H 2 C CH 2 N C NH CH 2 C CH 2 CH CH 2 CH 2 NH C N HC CH CH CH 3 3 CH 3 Fig. 6. Synthesis of multifunctional propyleneimine from isocyanates wit 2-methyl-aziridine. CH 2 1 CH 2 H 1 CH 3 CH NH R instable carbon cation N R 2 CH 2 CH 3 CH CH 2 NH R stable carbon-cation CH 3 Fig. 4. Crosslinking mechanism by propyleneimines [1]. Cl CH 2 3x HN + CH CH N 3 N Cl N Cl CH 3 CH 3 N N N N N N CH 3 Fig. 7. 2,4,6-Tris(1-methylaziridinyl)-1,3,5-triazine (tripropylenemelamine). 1980s, 3M was particularly active in this sector presenting new ideas about structure and application of polyfunctional propyleneimines [15]. Regarding their chemical structure the multifunctional propyleneimine crosslinkers can be prepared by the following reactions. Reaction products from multifunctional carboxylic acid chlorides and propyleneimine: This range of crosslinking agents is also known as bisamide crosslinking agents (BCA) (Fig. 5). Reaction products from multifunctional isocyanates and propyleneimine: This reaction is based upon the reactivity of the propyleneimine ring and the N¼C¼O-groups (Fig. 6). Propyleneimines based on s-triazine: As a result of the reaction between propyleneimine and cyanuric acid chloride or tris-(2-carboxyethyl)isocyanurate we find the following crosslinker (Fig. 7). Reaction products from multifunctional acrylates and propyleneimine: Propyleneimine reacts with the double bond of the acryloyl group (Fig. 8). Propyleneimines with central heteroatoms: Mainly phosphorus and sulfur serve as heteroatoms (Fig. 9). H 3 C H 3 C C 3 H 7 N N H 2 C CH 2. Experimental part O C O CH 2 CH 2 C O R O C CH The following experiments were conducted in order to study the influence of diverse propyleneimines such crosslinkers on other important properties of pressure sensitive adhesive such as tack, adhesion and cohesion. O CH 2 O O R O C CH 2 CH 2 N Fig. 8. Reaction of 2-methylaziridine with acryloyl group. O N P N CH 3 O O CH 3 N CH 2 S CH 2 C CH N 3 Fig. 9. Propyleneimines with heteroatom. C N CH 3 CH 3 CH 3
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