Addition of rosin acid during thermoplastic polyurethane synthesis to improve its immediate adhesion to PVC

MDI (diphenylmethane-4,4′-diisocyanate) and different mixtures of rosin acid and polyadipate of 1,4-butanediol (Mw=2400) (5-20 equivalent % with respect to the equivalents of OH groups in the macroglycol) were reacted to produce a urethane prepolymer; 1,4-butanediol was added as chain extender. The specific feature of this manufacturing procedure was the use of rosin acid as an internal tackifier in thermoplastic polyurethanes (TPUs), to provide high immediate adhesion to PVC. The TPUs obtained were characterized using gel permeation chromatography, FTIR and NMR-1H spectroscopy, differential scanning calorimetry, dynamic mechanical thermal analysis and plate-plate stress-controlled rheology. The TPUs were used as raw materials to prepare solvent-based polyurethane adhesives, whose adhesion properties were obtained from T-peel tests of solvent-wiped PVC/TPU adhesive joints. The increase in the amount of rosin acid in the prepolymer led to an increase in average molecular weight and the viscosity of TPU solutions, improved rheological properties, reduced crystallinity and slower kinetic of crystallization. On the other hand, the immediate T-peel strength and the ageing resistance of PVC/TPU joints were improved. The carboxylic acid functionality in the rosin structure reacted with the isocyanate to produce imide-urea moieties that were responsible for the improved properties of the TPUs containing rosin acid as an internal tackifier.