2.8. Determination of the Degree of Polymerization (DP)

The polymerization degree for each of the different paper sample types, as in the case of the colorimetric measurement (untreated, deacidified, untreated and aged, deacidified and aged), was determined according to the UNI 8282 standard test [46] by measuring the viscosity limit of cellulose solutions in bis-(ethylenediamine)-copper (II) hydroxide (CED) 0.5 M. The CED solution was prepared by diluting the Aldrich^® 1M CED solution in water. A 10 mg amount of paper for each paper sample was dissolved in 20 mL of CED at 25 °C by stirring the system for at least 30 min. In order to obtain the required limit viscosities, 5 solutions for each of the investigated paper samples were analyzed: the base solutions, obtained as described above, and 4 solutions, obtained by diluting the base solution with 0.5 M CED. Four dilution ratios, 2, 4, 8 and 16, were used. Viscosity measurements were carried out at room temperature (25 °C) using an RFS III shear-strain-controlled rheometer (Rheometrics Co. Ltd., Piscataway, NJ, USA) equipped with parallel plate geometry (gap, 1.000 ± 0.001 mm; diameter, 50 mm) for the samples analyzed and a Peltier system (±0.1 °C) for temperature control. Steady flow experiments were performed, and the viscosity was measured at different shear rate values. The variation of viscosity versus shear rates for all samples was measured. It was observed that all samples had Newtonian rheological behaviors, which meant the viscosity was independent of the shear rates. Specific [η] viscosity was obtained according to Equation (1):

where c is the solution concentration, and ηsp = (η − ηs)/ηs, where ηs is the viscosity of the CED solvent by determining the vertical intercept of the ηsp plot versus the concentration. Finally, the degree of polymerization (DP) was calculated according to the equation: DP = 1.33[η]^0.905, according to UNI 8282:1994 [46]. Mean values, obtained by averaging those measured from Pieces 41/42, 43/44, 45/46, 107/108, 109/110 and 111/112 of untreated and untreated-aged book pages and those measured from Pieces 1/2, 3/4, 5/6, 91/92, 93/94 and 95/96 of deacidified and deacidified-aged book pages, were reported.