Synthesis and characterization of polyurethane sealants containing rosin intended for sealing defect in annulus for disc regeneration

Thermoplastic polyurethanes containing rosin or mixtures of rosin and 1,4 butane diol in the chain extender were proposed as potential sealants for defects in disc regeneration surgery. The polyurethane sealants were prepared by using the prepolymer method and different mixtures of rosin and 1,4 butane diol were used as chain extenders. The existence of one carboxylic moiety in the rosin structure allowed the reaction with the isocyanate end groups in the prepolymer during polyurethane synthesis, creating additional urethane-amide hard segments. The polyurethanes were characterized by ATR-IR spectroscopy, differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMA), thermal gravimetric analysis (TGA), scanning electron microscopy, X-ray diffraction and laser confocal microscopy. The adhesion of the polyurethane sealants were tested by T-peel test of leather/polyurethane sealant/leather joints and by single lap-shear tests of aluminium/polyurethane sealant/aluminium joints. Depending on the rosin content in the chain extender the structure of the polyurethanes was different, i.e. more urethane and urethane-amide hard segments were created up to 50 eq% rosin in the chain extender and separation of domains was prevailing in the polyurethanes with higher rosin content. Furthermore, the addition of rosin caused an increase in the length of the polymer chains but a decrease in the storage modulus was produced (particularly in the polyurethane containing 50 eq% rosin), likely due to the bulky structure of the rosin as compared to the linear structure of 1,4 butane diol, allowing the separation of the linear polyurethane chains. On the other hand, the melting of the soft segments in the polyurethanes started at 40–57 °C and the addition of more than 50 eq% rosin in the chain extender decreased the melting enthalpy. Moreover, the crystallinity of the polyurethanes containing up to 50 eq% rosin showed lower number and smaller spherulites. Finally, the peel strength increased in the joints made with the polyurethane sealants containing rosin whereas the adhesive shear strength decreased when the polyurethane sealant contained 50 eq% rosin or less.