Routes to improve the strengthening of paper with aminoalkylalkoxysilanes

Aminoalkylalkoxysilane copolymers (co-AAAS) have the ability to simultaneously deacidify and strengthen paper documents. Nevertheless, despite enhancing the tensile strength, they generally fail to improve the folding endurance of the degraded groundwood pulp-rich papers. Focusing on that specific type of papers, several ways to overcome their lack of reinforcement were investigated. Promoting the polymerization of AAAS using catalysts and thermal steps was one of them. A monitoring with FTIR spectroscopy showed that the thermal step was the most efficient in speeding up the hydrolysis and polycondensation of 3-aminopropylmethyldiethoxysilane (AM) monomer in aqueous solutions. The progress of the two-steps reaction in terms of degree of polymerization of AAAS polymers was then studied in-situ after heating the paper, using Cross Polarization—Magic Angle Spinning 29 Si solid-state NMR, and was shown to be enhanced as well. The other optimization route explored was to stabilize the paper before treatment. Our previous research had shown that the presence of oxidized groups in paper hampered the strengthening. Sodium borohydride, a known bleaching agent in paper conservation, was used prior to the AAASs treatment to decrease the amount of carbonyls in paper. The reduction and the thermal step were compared in terms of the mechanical properties (folding endurance and tensile strength) upon AAASs treatment of several lignocellulosic papers, including three newsprints dated 1911–1923. The use of sodium borohydride led to a significant improvement of the folding endurance of the samples, an unprecedented result. It enabled at the same time to counteract the yellowing induced by the AAAS and to build a larger alkaline reserve.